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XI Im9I National Institute of Standards and Technology

NAT'l INST. OF STAND & TECH R.I.C.

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NIST

PUBLICATIONS

NIST SPECIAL PUBLICATION 801

Laser Induced Damage

in Optical Materials:

1989

BOULDER DAMAGE SYMPOSIUM

Iqq SPIE Volume 1438 STP 1117

#801

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NIST SPECIAL PUBLICATION 801 [T^

Laser Induced Damage

in Optical Materials: 1989

Proceedings of the Boulder Damage Symposium, November 1-3. 1989

Sponsored by

National Institute of Standards and Technology

American Society for Testing and Materials

International Society for Optical Engineering

Defense Advanced Research Project Agency

Department of Energy

BOULDER DAMAGE SYMPOSIUM

October 1 990

U.S. Deprtment of Commerce

Robert A. Mosbacher, Secretary

National Institute of Standards and Technology

John W. Lyons, Director

Edited by

Harold E. Bennett

Naval Weapons Center

China Lake, California 93555

Lloyd L. Chase

Lawrence Livermore National Laboratory

Livermore, California 94550

Arthur H. Guenther

Los Alamos National Laboratory

Los Alamos, New Mexico 87545

Brian E. Newnam

Los Alamos National Laboratory

Los Alamos, New Mexico 87545

M.J. Soileau

University of Central Florida

Orlando, Florida 32816
National Institute of Standards and Technology

Special Publication 801 , 670 pages (Oct. 1990)

CODEN: NBSAV

U.S. GOVERNMENT PRINTING OFFICE

WASHINGTON: 1990

For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, DC 20402-9325
FOREWORD

The Proceedings contain the papers presented at the Twenty-First

Symposium on Optical Materials for High-Power Lasers held at the National

Institute of Standards and Technology in Boulder, Colorado, on November 1-3,

1989. The Symposium was sponsored jointly by the National Institute of

Standards and Technology, the American Society for Testing Materials, the

International Society for Optical Engineering, the Defense Advanced Research

Project Agency, and the Department of Energy. The Symposium was attended by

approximately 200 scientists from the United States, Canada, the United

Kingdom, Japan, France, and the Federal Republic of Germany. It was divided

into sessions devoted to the following topics: Materials and Measurements,

Mirrors and Surfaces, Thin Films, and, finally, Fundamental Mechanisms. The

Symposium Co -Chairmen were Harold E. Bennett of the Naval Weapons Center,

Arthur H. Guenther of the Los Alamos National Laboratory, Lloyd L. Chase of

the Lawrence Livermore National Laboratory, Brian E. Newnam of the Los Alamos

National Laboratory, and M. J. Soileau of the University of Central Florida.

They also served as editors of the proceedings.

The editors assume full responsibility for the summary, conclusions, and

recommendations contained in the report, and for the summaries of discussion

found at the end of each paper. The manuscripts of the papers presented at

the Symposium have been prepared by the designated authors, and questions

pertaining to their content should be addressed to those authors. The

interested reader is referred to the bibliography at the end of the summary

article for general references to the literature of laser damage studies. The

Twenty-Second Annual Symposium on this topic will be held in Boulder,

Colorado, October 24-26, 1990. A concerted effort will be made to ensure

closer liaison between the practitioners of high-peak power and high-average

power

.

The principal topics to be considered as contributed papers in 1990 do

not differ drastically from those enumerated above. We expect to hear more

about improved scaling relations as a function of pulse duration, area, and

wavelength, and to see a continuing transfer of information from research

activities to industrial practice. New sources at shorter wavelengths

continue to be developed, and a corresponding shift in emphasis to short

wavelength and repetitively pulsed damage problems is anticipated.

Fabrication and test procedures will continue to be developed, particularly in

the diamond- turned optics and thin- film areas. It has been our intention to

pause and reflect on progress over the past twenty years to the Symposium on Optical Materials for High Power Lasers. It will be our pleasure to present

the last (Thin Film, the Second Decade) in a comprehensive array of tutorial

lectures by distinguished workers in the field of laser induced damage in

optical materials.

The purpose of these symposia is to exchange information about optical

materials for high-power lasers. The editors will welcome comments and

criticism from all interested readers relevant to this purpose.

H. E. Bennett, A. H. Guenther, L. L. Chase,

B. E. Newnam, and M. J. Soileau,

Co-Chairmen

iii
DISCLAIMER

Certain papers contributed to this publication have been prepared by

non-NIST authors. These papers have not been reviewed or edited by NIST;

therefore, the National Institute of Standards and Technology accepts no responsibility for their accuracy, nor for their comments or recommendations.

Certain commercial equipment, instruments, and materials are identified

in this publication in order to explain the experimental procedure adequately.

Such identification in no way implies approval, recommendation, or endorsement

by the National Institute of Standards and Technology, nor does it imply that

the equipment, instruments, or materials identified are necessarily the best

available for the purpose.

iv
CONTENTS

FOREWORD

H.E. Bennett, A.H. Guenther, L.L. Chase, B.E. Newnam, and M.J. Soileau

Page

iii

DISCLAIMER iv

SYMPOSIUM WELCOME xii

M.J. Soileau

SEARCH FOR TECHNOLOGY TRANSFER IN HIGH POWER OPTICS xvi

C. Martin Stickley

SUMMARY OF MEETING 1

H. E. Bennett, L.L. Chase, A.H. Guenther, B.E. Newnam, and M.J. Soileau

I . Introduction 1

2. Overview 3

3 . Acknowledgments 5

4 . References 6

1989 INDEX - BOULDER DAMAGE SYMPOSIUM

MATERIALS AND MEASUREMENTS

Set of Standard Definitions for Laser Damage in Parameters and Procedures 9

J.W. Arenberg

Optical Characterization of Transparent Materials Using Ellipsometry 10

5. F. Nee and H.E. Bennett

Microindentation as a Technique For Assessing Subsurface Damage In Optics 25

R.S. Polvani and C. Evans

v
Automated Damage Testing Facility for Excimer Laser Optics 39

K. Mann and H. Gerhardt

Expanded Damage Test Facilities at LLNL 47

A.J. Morgan, F. Rainer, F.P. DeMarco, R.P. Gonzales,

M.R. Kozlowski, and M.C. Staggs

Laser Damage Database at 1064 nm 58

F. Rainer, R.P. Gonzales, and A.J. Morgan

Damage Measurements on Optical Materials for Use in High- Peak- Power Lasers 74

F. Rainer, R.M. Brusasco, J.H. Campbell, F.P. DeMarco,

R.P. Gonzales, M.R. Kozlowski, F.P. Milanovich,

A.J. Morgan, M.S. Scrivener, M.C. Staggs, I.M. Thomas,

S.P. Velsko, and C.R. Wolfe

Laser Induced Damage to Thallium Arsenic Selenide (TAS) 84

A. A. Said, M. Sheik-Bahae, M.J. Soileau, E.W. Van Stryland

N.B. Singh, and T. Henningsen

Laser Induced Damage in Schott's OG-550 Optical Absorption Glass 88

T. Whittaker, R. Goedert, and D. Templeton

Effects of Laser Damage Processes on Microwave Propagation 96

R.S. Eng, M.D. Abouzahra, N.W. Harris, D.R. Cohn, and P.P. Woskov

Laser Induced Damage to Silicon Photosensor Arrays 115

Chen-Zhi Zhang, Thierry Benchetrit, Steve E. Watkins

,

Rodger M. Walser, and Michael F. Becker

Sensitive n2 Measurements Using A Single Beam 126

M. Sheik-Bahae, A. A. Said, T.H. Wei, D.J. Hagen,

E.W. Van Stryland, and M.J. Soileau

vi
Optical Breakdown in Particle Suspension 136

Kamjou Mansour, M.J. Soileau and E.W. Van Stryland

Annealing of Induced Damage in Flouride Glass Components 151

Osama H. El-Bayoumi, M.J. Suscavage, and L.P. De Rochemont

The Effect of Laser Annealing on Laser Induced Damage Threshold 164

N.C. Kerr and D.C. Emmony

Shockwave Detection, An Efficient Way to Determine Multiple-Pulse Damage Thresholds 180

S. Petzoldt, A. P. Elg, J. Reif, and E. Matthias

Application of the Ronchi Ruling Beam Profiling Method to Axially

Symmetric Laser Beams 187

Robert M. O'Connell and Cheng-Hao Chen

Prediction of Laser Induced Damage by Comparison of Laser Fluence Profile

to Damage Spot Radius 200

R. Goedert, T. Whittaker, and D. Templeton

IR Laser Beam Profiling Using Quenched Fluorescence 214

N.C. Kerr, S.E. Clark and D.C. Emmony

Laser-Induced Failure in Biased Silicon Avalanche Photodiodes 220

Steve E. Watkins , Chen-Zhi Zhang, Rodger M. Walser,

and Michael F. Becker

SURFACES AND MIRRORS

The Effect of Subsurface Defects on "Incipient" (Below Threshold)

Laser Damage Nucleation In Fused Silica Optical Flats 2 30

T.J. Magee, C.S. Leung, F.D. Orazio, J.D. Boyer, B.R. Mauro , and V.E. Sanders

Quantitative Analysis of Surface Trace Metal Contamination on Substrates and Films by TXRF 239

R. S. Hockett

vii
An Error Analysis of the Wyko TOPO Noncontact Surface Profiler 254

Van A. Hodgkin

Ultra-Precision Grinding of LHG-8 Laser Glass and Laser Damage Thresholds 255

Y. Namba, K. Yoshida, H. Yoshida, and S. Nakai

Low Scatter Surfaces on Silicon Carbide 256

Roger A. Paquin and Matthew B. Magida

Physical Limits on Ultra-High Albedo Diffuse Reflectors 263

Perry Miles

Optical Damage on Si02 Cavity Mirrors Produced by High-Power VUV Laser Irradiation 267

Y. Takigawa, K. Kurosawa, M. Okuda, W. Sasaki,

K. Yoshida, E. Fujiwara, Y. Kato , and Y. Inoue

Damage Assessment and Possible Damage Mechanisms to 1-Meter Diameter

Nova Turning Mirrors ...278

G. Edwards, J. Campbell, R. Wolfe, E. Lindsey

Thermal Transport Studies of Optical Coatings, Interfaces and Surfaces

by Thermal Diffusion Wave Interferometry 291

Randall T. Swimm and Gary Wiemokly

Thin Films

Investigation of Thin Films Using Total Internal Reflection Microscopy 299

F.L. Williams, C.K. Carniglia, B.J. Pond, and W.K. Stowell

Scattering Characterization of Materials in Thin Film Form 309

C . Amra

Optical Properties and Laser Damage Measurements of Inorganic Polymer Films 324

Gregory J. Exarhos and Kevin M. Crosby

Int erfacial Stability in Optical Coatings 335

K.L. Lewis, A.M. Pitt, A.G. Cullis, K. Welford, and I.T. Muirhead

viii
Surface Analytical Methods for the Assessment of Damage in Optical Thin Films

James R. Hoenigman

350

Laser Conditioning of Optical Thin Films 360

C. R. Wolfe, M.R. Kozlowski, J.H. Campbell, F. Rainer,

A. J. Morgan, and R. P. Gonzales

Large Area Laser Conditioning of Dielectric Thin Film Mirrors 376

M.R. Kozlowski, C.R. Wolfe, M.C. Staggs , and J.H. Campbell

Population Distribution of Conditioned Damage Thresholds on AR Coated BK-7 Glass

With Varing Laser Spot Size 393

D. W. Mordaunt and J.W. Arenberg

Damage Threshold Measurements of Reflective, and Transmissive Optics at 130 nm 403

C.H. Muller, III and C.E. Hamilton

Laser Induced Damage Thresholds of Dielectric Coatings at 193 nm and Correlations to

Optical Constants and Process Parameters 404

J. Kolbe, H. Muller, H. Schink, H. Welling, and J. Ebert

Angular Dependence of Thin-Film Dielectric Coating Damage Thresholds Revisited 417

J. D. Boyer, S. R. Foltyn, B. R. Mauro, and V. E. Sanders

Pulse-width Dependence of Optical Coating Damage at 1052 nm 425

K. Yoshida, S. Ikunishi, H. Yoshida, Y. Kato , and S. Nakai

Damage Resistant Optical Coatings Prepared Using High Temperature, Plasma

Chemical -Vapor Deposition 426

J.H. Campbell, J.L. Emmett, R.M. Brusasco, F. Rainer, R.Th. Kersten,

V. Paquet, and H.W. Etzkorn

A High Temperature, Plasma-Assisted Chemical Vapor Deposition System 4^

R.M. Brusasco, J. A. Britten, C.B. Thorsness, M.S. Scrivener,

W.G. Unites, J.H. Campbell, and W.L. Johnson

ix
The Evolution of Molecular Beam Deposition (MBD) from Laboratory to Production Usage 455

C.C.H. Hale, I.T. Muirhead, S.P. Fisher, G.J.H. Mathew

Investigation and Modelling of Laser Damage Properties of Fabry-Perot Filters 471

A. Mclnnes, CM. MacDonald, D.R. Gibson and A.D. Wilson

High Damage Threshold A10.0H-SiO2 HR Coatings Prepared by the Sol-Gel Process 484

Ian M. Thomas

1-on-l and n-on-1 Laser Strength of Binder Aided Zr02 and Zr02 -Si02 Reflective

Sol-Gel Coatings 490

H.G. Floch and J.J. Priotton

Structural Modification of D20/H20-Dosed CaF2 Optical Thin Films 509

J . B . Franck

FUNDAMENTAL MECHANISMS

Non-Avalanche Dielectric Breakdown in Wide-Band-Gap Insulators

at DC and Optical Frequencies 528

P. Braunlich, S.C. Jones, X.A. Shen, R.T. Casper, E. Cartier,

D.J. DiMaria, M.V. Fischetti, and P. Kelly

UV Seeding of IR Laser Induced Damage 541

N.C. Kerr, S.E. Clark and D.C. Emmony

Measurements of uv Induced Absorption in Dielectric Coatings 551

M.H. Bakshi, M. Cecere, D.A.G. Deacon, and A.M. Fauchet

The Response of Multilayer Dielectric Coatings to Low Fluence Ultraviolet

Light Exposure 561

V.E. Sanders, J.W. Early, and W. Leamon

Radiation Damage in Single Crystal Csl (Tl) and Polycrystal Csl 568

0. Barnouin, A. Procoli, H. Chung, and G.H. Miley

x
Effects of Thermal Conductivity and Index of Refraction Variation on the

Inclusion Dominated Model of Laser-Induced Damage 576

M.Z. Fuka, J.K. Mclver, and A. H . Guenther

Theoretical Determination of the Nonlinear Optical Properties of Inorganic Polymers 584

Steven M. Risser and Kim F. Ferris

Relation Between n2 and Two-Photon Absorption 591

M. Sheik-Bahae, D.J. Hagan, E.W. Van Stryland, T.H. Wei

A. A. Said, E. Canto, and A. Miller

Photoconductivity of ZnS and ZnSe 598

B. E. Mason and CD. Marrs

Formation of a Pregiven Reflecting Surface Topography by Elastic Deformation of the

Mirror Substrate. New Concept of Adaptive Optical System 606

V.V. Apollonov, S.A. Chetkin, E.A. Ivanova, A.M. Prokhorov, G.V. Vdovin

Application of Ultrasonic Capillary Effect in Elements of Power Optics Cooled

by Means of a Heat Pipe 618

V.V. Apollonov, S.A. Chetkin, V.N. Kharchenko , V.N. Motorin,

and A.M. Prokhorov

Appendix I. List of Attendees 631

xi
WELCOME FOR 21st BOULDER DAMAGE SYMPOSIUM

M. J. Soileau

Professor of Electrical Engineering and Physics

Director, Center for Research in Electro-Optics and Lasers

University of Central Florida

Orlando, Florida

It is my pleasure and duty to call this year's meeting to order. I was abroad when the final program was put together by my co- chairs and did not

learn of this honor until a couple of weeks ago. I had no time to prepare

until this weekend, so this will be a bit rough and without the benefit of

elegantly prepared slides.

I don't mean to take a stab at my co-chairs, but I must admit that as I began my preparation I couldn't remember a single thing about previous opening

remarks by my esteemed colleagues! Did I miss something? Surely these

distinguished leaders of national laboratories must have said something

profound and prophetic! So I spent part of the day Saturday reviewing the

past utterances which have launched this meeting.

It is true that the memory is the second thing to go, because as I read

the opening remarks for the past 20 meetings, I found many, profound

statements - -a few of which are listed below:

This is a quote from the next speaker from 1972:

"It is a pleasure to be here this afternoon."

Martin Stickley, 1972.

"Whatever turns you off." (Alex's definition of damage)

Alex Glass, 1974

"A name which invokes images of people cracking rocks."

Alex Glass, 1976. (In response to Martin Stickley' s call for a more positive sounding name for the conference.)

"Our onion unfortunately, exists in Hilbert Space." (Alex

borrowing from an ancient philosopher's description that

learning is like peeling an onion- -each layer exposes another.)

"Who cares?" (Alex on why study damage?)

Alex Glass, 1976.

"It is my annual hope that each year's symposium will be the

last." (Alex the failed prophet.)

Alex Glass, 1978.

xii
"The key... is terawatts per megabuck.

Alex Glass, 1978.

"Power Optics." (A supplier of megabucks at the time in comparing laser

optics to electronics.)

Harry Winsor, 1978.

"Aside from a gain medium, lasers require mirrors." (Insightful words

from the great visionary.)

Harry Winsor, 1978.

"Progress has been made, but I am confident that in 10 years we will

celebrate the 20th and Art and Alex will still be running it!" "...we

owe a debt to these 2 young men." (Note the term young .

)

Martin Stickley, 1978.

To this point I've spared you the profound utterance of my current co-chairs

such as

"Welcome to the Tenth Anniversary Damage Symposium." (Art Guenther gave

this greeting at the 11th meeting.)

Art Guenther, 1979.

Then there were profound things said regarding the international

participation.

"...countries represented include the British Isles. Canada, England . France, Japan, Scotland , and West Germany."

Brian Newnam, 1980.

"International contributors ... have come a long way..."

Hal Bennett, 1985.

It should be clear to all how difficult it is for me since these guys

have used up all the good stuff! Further review of these openings presented

me with a good outline for my remarks. I will follow the trail blazed by my

predecessors

Outline

1. Welcome participants.

2. Acknowledge sponsorship of NIST, ASTM, and others.

3. Count the papers for this year's meeting and comment on the statistical

distribution.

4. Observe that thin films are a major problem.

5. Profound and prophetic statement about the future of the meeting.

I do welcome you to the 20th Anniversary Boulder Damage Symposium.

Those of you who are still asleep or partially hung over, may be confused by

the fact that last year was our 20th Anniversary celebration. The resolution

of this dilemma is the fact that last year was the 20th meeting and this is

xiii
the 20th Anniversary (but the 21st meeting) . One might say that this meeting

is old enough to drink - but you will also note that there is no wine and

cheese this year. This situation has resulted from rulings by NIST

accountants. In fact, the accountants have made it difficult for us to

conduct business as usual, so you can expect further changes next year.

We do want to thank NIST and the ASTM for their continued sponsorship

and all the helpful folks at Boulder who make the meeting run smoothly.

There are about 70 papers this year with contributions from 6 countries

,

in addition to the U.S. The talks from abroad constitute 27% of the papers,

about the same percentage as from Livermore and Los Alamos. About an equal

number of papers come from U.S. aerospace companies (15%) and U.S.

universities (16%) . About 9% are from DoD labs and the remaining 8% from

various other sources.

Our next speaker (Martin Stickley) has suggested on a number of

occasions that the name of the conference be changed to something more

positive. I want to finish this presentation by noting that there are many

positive things being done with phenomena studied by participants in these

meetings. Here is a list of a few:

"Spin Offs" of LID Phenomena

1. Laser marking, cutting and drilling of materials.

2. Laser medicine

a. laser scalpel

b. laser- induced breakdown for eye treatments and plaque removal.

3. Laser disc storage - the first application of LID to consumer products.

4. Photorefractive information' storage , processing, and phase conjugation

interconnects

.

5. Nonlinear refraction and nonlinear absorption for limiters, switches,

and optoelectronic computing.

6. Electro-absorptive switches using UHV manufactured quantum well devices

(SEED'S).

So I issue a challenge to us all to keep the name Damage Symposium, but

continue to seek positive applications of the phenomena we study. A wise

professor once told me that phenomena are neither good or bad- -they exist and

it's up to us to find ways to make good things using phenomena given by

nature

.

Before I left Orlando, I looked into by crystal ball for a glimpse of

future laser damage or optical materials problems - -here is my short list:

Future Topics for Optical Materials Research

1. Thin films for all applications. Two talks viewing 20 years of thin

film work.

xiv
2. New laser host and nonlinear optical materials for compact, efficient,

tunable and solid state lasers for many old and many new applications.

3. X-ray optics for lasers (remember Harry Winsor said that lasers have

mirrors...), and optics for x-ray microscopy and x-ray lithography.

4. Materials for x-ray lithography.

I'm sure that these and other topics will keep optical materials people

busy for some time- -the only question is where will the funding come from?

DoD funding will surely decline as peace is breaking out all over. DOE

funding usually finds its way to the national labs --but not much finds its way

out. DARPA, under Martin Stickley's leadership, provided much of the spark

that lead to modern day materials and much of the research reported at this

conference. Materials research tends to be too long-term for present day

funding. We need to all do our part in ensuring continued research support

for the foundation of the technology food chain- -materials research.

Since we don't have an official wine and cheese gathering, I hereby

propose that we forgo small gatherings of old friends for dinner, etc. and

meet instead at the Dark Horse for an informal, pay as you go, social hour.

xv
Manuscript received

2-14-90

SEARCH FOR TECHNOLOGY TRANSFER IN HIGH POWER OPTICS

C. Martin Stickley

BDM International, Inc.

1300 N. 17th St. , Suite 950

Arlington, VA 22209

In the late 60 's and through the 70' s, DARPA supported a broad program

to develop optics for use with high power cw C0 5 lasers and new laser and

nonlinear optical materials. This paper summarizes those efforts and asks

anyone who knows in what defense systems these have been used to contact

the author.

In the late 1960's and through the 1970's, the Materials Sciences Office (now the Defense

Sciences Office) of the Defense Advanced Research Projects Agency (DARPA) funded a broad

program to develop optics for high power continuous development of approaches to fabricate

large area windows with high transparency (absorption of 10" ^ per cm), optical surface

preparation techniques which leave very little residual absorption, and techniques for

depositing antireflection as well as reflecting coatings for transparent windows and mirrors.

Some development of new laser and nonlinear optical materials was also funded. Approximately

$22.9 M dollars were spent between 1967 and 1978.

The Defense Sciences Office is now searching for examples of where optical parts fabricated

using these techniques and the laser and NL0 materials have been used in defense systems.

While defense uses are of primary importance, NASA and industrial uses are also of interest.

Technologies which may have had the best chance of being utilized include, for windows:

casting of fluorides, reactive atmosphere processing of halides, forging of halides, and

CdTe growth, distortion and damage studies; for surfaces and coatings: polymers and surface

preparation of halides, fluorides, and selenides; for lasers: erbuim glass, and holmium

and erbium in YLF; for mirrors: beryllium optics; and for laser damage: platinum removal

and surface preparation of Owens-Illinois glass, and bulk and surface damage of ruby.

The tables which follow summarize the activities which were supported in each major area. Included are the specific technology developed (e.g. fluoride fusion casting), the contractor,

the amount of funding provided, and at the time period where the work was done.

If the reader knows of areas where any of these technologies may have been used in defense

systems (as well as for NASA and the commercial sector), please contact the author.

xv i

L
BDM INTERNATIONAL, INC.

LASER WINDOW MATERIALS

Fluorides

Fluoride Fusion Casting

Scaling of Fluoride Casting

Halides

Raytheon

Raytheon

166K

790K

76-77

74-76

Halide Superalloys for High Power Windows Raytheon 191K 72--73

Halide OH Removal Hughes Res. Labs 174K 72

Reactive Atmosphere Processing Hughes Res. Labs 215K 76--78

RAP of KBR Hughes Res. Labs 244K 74--76

Extrusion and Cross-Rolling Honeywell 1 95K 72--73

Forging of Alkali Halides Honeywell 1483K 73--78

Radiation Hardening Oklahoma 113K 72--74

Press Forging Halides NRL 521K 72--75 RAP of KBR - Characterization NRL 80K 76

BDM INTERNATIONAL, INC.

LASER WINDOW MATERIALS

II-VI Materials

# II-VI Distortion and Damage

CVD of Cadmium Telluride

CVD of CdTe and GaP

Characterization of II-VI Materials

use

Raytheon

Raytheon

MIT

1336K

192K

196K

308K

72-77

73- 75

75-77

73-76

Chalcogenides • Oxygen-Free Chalcogenides • Structure-Dependent Absorption

Catholic University 283K 72-74

Texas Instruments 72K 73-74

Other • Covalent Carbon Window Materials • Copper-Leaded CERVIT

UCLA

Owens Illinois

114K

27K

75-76

76

xvii
BDM INTERNATIONAL, INC.

LASER WINDOW MATERIALS

Measurement Techniques

Spectroscopy of Halides

Emissivity Facility

Multiple Wavelength Calorimetry

Chemical Laser Window Absorption

CO2 Laser Window Evaluation

Proof Testing

Mechanical Properties

Cornell

Block Eng.

Alabama

Raytheon

AVCO

MIT

MIT

209K

250K

58K

60K

279K

168K

320K

75-76

73-75

74- 76

74- 75

75-76

75-76

72-75

Theory

Theory of Coatings and UV Materials Xonics 1342K 72-78

BDM INTERNATIONAL, INC.

LASER COATINGS AND SURFACES

Polymer Protective Coatings UC Berkeley 169K 75--77

Polymer Protective Coatings Rockwell 149K 73--74

Surface and Coating Technology Hughes Res. Lab 1076K 73--76

Window Polishing and Characterization NWC 1696K 72--77

Passive Characterization/HF Coatings NWC 1042K 76--77

Coating Stress Measurements Perkin Elmer 200K 76--77

Coating Growth and Stress AFCRL 255K 73--75

Transparent Abrasives Raytheon 207K 73- 75

xviii
BDM INTERNATIONAL, INC.

LASER MATERIALS

Erbium Glass American Optical 191K 69-71

Holmium in YLF Sanders Associates 203K 71-73

Near Visible Laser Materials Sanders Associates 270K 72-75

Ceramics for Lasers Union Carbide 193K 1977

Wide Linewidth Materials Texas Instruments 225K 72-73

MIRRORS AND SURFACES

Beryllium for Optics

Ion Beam Optical Figuring

Perkin Elmer

Kollsman

1434K

146K

Prior to 69

Prior to 69

BDM INTERNATIONAL, INC.

NONLINEAR OPTICAL MATERIALS

Cinnabar

Chalcopyrite

High Damage Threshold Materials

Chalcogenides

IR Laser Components

IR Quantum Counter

Tyco

Stanford

Isomet

Westinghouse

Westinghouse

Purdue

190K

279K

98K

230K

201K

70-71

70-74

1973

72-75

Before 1969

xix
BDM INTERNATIONAL, INC.

DAMAGE TO LASER MATERIALS

Glass

# Laser Glass Damage

# Glass Surface Treatment

Ruby

# Bulk and Surface Damage

Measurement Techniques

# Laser Measurements

# Laser Damage Measurements

Mirrors

# RF Sputtered Metal Mirrors

Research

O Miscellaneous Topics

Owens Illinois

Owens Illinois

NBS

NBS

Battelle NW

Harvard, NWC,

USC, AFCRL,

Raytheon, Bendix

616K

291K

2369K

161K

69-71

72-73

Hughes Res. Labs 400K 69-71

Prior to 69

72-73

450K 72-73

749K 71-76
BDM INTERNATIONAL, INC.

LASER WINDOW MATERIALS

Fluorides

Fluoride Fusion Casting

Scaling of Fluoride Casting

Halides

Raytheon

Raytheon

166K 76-77

790K 74-76

Halide Superalloys for High Power Windows Raytheon 191K 72--73

Halide OH Removal Hughes Res. Labs 174K 72

Reactive Atmosphere Processing Hughes Res. Labs 215K 76--78

RAP of KBR Hughes Res. Labs 244K 74--76

Extrusion and Cross-Rolling Honeywell 1 95K 72--73

Forging of Alkali Halides Honeywell 1483K 73--78

Radiation Hardening Oklahoma 113K 72--74

Press Forging Halides NRL 521K 72--75

RAP of KBR - Characterization NRL 80K 76

BDM INTERNATIONAL, INC.

LASER WINDOW MATERIALS

II-VI Materials

# II-VI Distortion and Damage

# CVD of Cadmium Telluride

# CVD of CdTe and GaP

# Characterization of II-VI Materials

use

Raytheon

Raytheon

MIT

1336K

192K

196K

308K

72-77

73-75

75-77

73-76

Chalcogenides

# Oxygen-Free Chalcogenides • Structure-Dependent Absorption

Catholic University 283K 72-74

Texas Instruments 72K 73-74

Other

# Covalent Carbon Window Materials • Copper-Leaded CERVIT

UCLA

Owens Illinois

114K

27K

75-76

76

:xi
BDM INTERNATIONAL, INC.

LASER WINDOW MATERIALS

Measurement Techniques

Spectroscopy of Halides

Emissivity Facility

Multiple Wavelength Calorimetry

Chemical Laser Window Absorption

CO2 Laser Window Evaluation

Proof Testing

Mechanical Properties

Cornell

Block Eng.

Alabama

Raytheon

AVCO

MIT

MIT

209K

250K

58K

60K

279K

168K

320K

75-76

73-75

74-76

74-75

75-76

75-76

72-75

Theory

# Theory of Coatings and UV Materials Xonics 1342K 72-78

BDM INTERNATIONAL, INC.

LASER COATINGS AND SURFACES

Polymer Protective Coatings UC Berkeley 169K 75--77

Polymer Protective Coatings Rockwell 149K 73--74

Surface and Coating Technology Hughes Res. Lab 1076K 73--76

Window Polishing and Characterization NWC 1696K 72--77

Passive Characterization/HF Coatings NWC 1042K 76--77

Coating Stress Measurements Perkin Elmer 200K 76--77

Coating Growth and Stress AFCRL 255K 73--75

Transparent Abrasives Raytheon 207K 73--75

xxii

1
BDM INTERNATIONAL, INC.

LASER MATERIALS

Erbium Glass American Optical 191K 69-71

Holmium in YLF Sanders Associates 203K 71-73

Near Visible Laser Materials Sanders Associates 270K 72-75

Ceramics for Lasers Union Carbide 193K 1977

Wide Linewidth Materials Texas Instruments 225K 72-73

MIRRORS AND SURFACES

Beryllium for Optics

Ion Beam Optical Figuring

Perkin Elmer

Kollsman

1434K

146K

Prior to 69

Prior to 69

BDM INTERNATIONAL, INC.

NONLINEAR OPTICAL MATERIALS

Cinnabar

Chalcopyrite

High Damage Threshold Materials

Chalcogenides

IR Laser Components

IR Quantum Counter

Tyco

Stanford

Isomet

Westinghouse

Westinghouse

Purdue

190K

279K

98K

230K

201K

70-71

70-74

1973

72-75

Before 1969

xxiii
BDM INTERNATIONAL, INC.

DAMAGE TO LASER MATERIALS

Glass

# Laser Glass Damage

# Glass Surface Treatment

Ruby

# Bulk and Surface Damage

Measurement Techniques

# Laser Measurements

# Laser Damage Measurements

Mirrors

# RF Sputtered Metal Mirrors

Research

# Miscellaneous Topics

Owens Illinois

Owens Illinois

NBS

NBS

Battelle NW

Harvard, NWC,

USC, AFCRL,

Raytheon, Bendix

616K

291K

2369K

161K

69-71

72-73

Hughes Res. Labs 400K 69-71

Prior to 69

72-73

450K 72-73

749K 71-76

xx iv
Laser Induced Damage in Optical Materials

Twenty-First ASTM Symposium

November 1-3, 1989

The Twenty-First Annual Symposium on Optical Materials

for High-Power Lasers (Boulder Damage Symposium) was held at the National Institute of Standards and

Technology in Boulder, Colorado, November 1-3, 1989.

The Symposium was sponsored jointly by the National

Institute of Standards and Technology, the American

Society for Testing and Materials, the International

Society for Optical Engineering, the Defense Advanced

Research Project Agency, and the Department of Energy.

Approximately 200 scientists, including representatives

of the United Kingdom, France, Japan, Canada, and the

Federal Republic of Germany, attended the Symposium.

The Symposium was divided into sessions concerning

Materials and Measurements, Mirrors and Surfaces, Thin

Films, and, finally, Fundamental Mechanisms. As in

previous years, the emphasis of the papers presented at

the Symposium was directed toward new frontiers and new

developments. Particular emphasis was given to

materials for high power apparatus. The wavelength

range of the prime interest was from 10.6 yum to the uv

region. Highlights included surface characterization,

thin film substrate boundaries, and advances in

fundamental laser-matter threshold interactions and

mechanisms. The scaling of damage thresholds with pulse

duration, focal area, and wavelength was discussed in

detail. Harold E. Bennett of the Naval Weapons Center,

Arthur H. Guenther of the Los Alamos National

Laboratory, Lloyd L. Chase of the Lawrence Livermore

National Laboratory, Brian E. Newnam of the Los Alamos

National Laboratory, and M.J. Soileau of the University

of Central Florida were co-chairmen of the Symposium.

The Twenty- Second Annual Symposium is scheduled for

October 24-26, 1990, at the National Institute of

Standards and Technology, Boulder, Colorado.

Key words: laser damage; laser interaction; optical

components; optical fabrication; optical materials and

properties; thin film coatings.

1. Introduction

The Twenty-First Annual Symposium on Optical Materials for High-Power

Lasers (Boulder Damage Symposium) was held, as in previous years, at the

National Institute of Standards and Technology in Boulder, Colorado, November

1-3, 1989. The Symposium was held under the auspices of the ASTM with the

joint sponsorship of NIST, and the Department of Energy. Approximately 200

scientists, including representatives of the United Kingdom, France, Japan,

1
Canada, and the Federal Republic of Germany, attended the symposium. The

Symposium was divided into sessions concerning Materials and Measurements,

Mirrors and Surfaces, Thin Films, and, finally, Fundamental Mechanisms. In

all, approximately 70 technical presentations were made. Harold E. Bennett of

the Naval Weapons Center, Arthur H. Guenther, and Brian E. Newnam of the Los

Alamos National Laboratory, Lloyd L. Chase of the Lawrence Livermore National

Laboratory, and M. J. Soileau of the University of Central Florida were co- chairmen of the Symposium. Aaron A. Sanders of the National Institute of

Standards and Technology acts as conference Coordinator.

The purpose of these symposia is to exchange information about optical

materials for high power lasers. The authors welcome comments and criticism

from all interested readers relevant to this purpose and particularly relative

to our plans for the Twenty- second Annual Symposium, scheduled for October 24-

26, 1990, at the National Institute of Standards and Technology, Boulder,

Colorado

2
2 . Overview

The following comments by the Symposium co-chairmen represent their

impression of significant advances that were discussed immediately after the

close of the meeting. This is not meant to be a thorough review of the

conference, but only a brief glimpse of some of the highlights.

The largest single group of papers in the Fundamental Mechanisms session

dealt with the effects of high photon energies on laser- induced damage (LID)

at lower photon energies. As one example, short -wavelength harmonic radiation

generated within free -electron lasers can potentially reduce the LID at the

fundamental lasing wavelength. Another case involved the effect of

ultraviolet radiation on the damage resistance at 10.6 pm. Yet another paper

dealt with the use of photoconductivity as a diagnostic to identify defects

within the bandgap and to monitor the onset of LID.

One area covered at this meeting for the first time was the nonlinear

properties of inorganic polymers as well as those of organic polymers. In

addition, a theoretical paper described the contribution of conjugated

electronic systems to the nonlinear polarizability of such materials which

have potential use as photoelectronic devices.

Always of interest to theoreticians are the possible relationships of

apparently different classes of materials properties. This year, one group

studied the relationship between the nonlinear refractive index and twophoton absorption coefficient using a model that assumed that a Kramers - Kronig relationship exists between the two.

One paper provoked a great deal of discussion and controversy, and

suggested that avalanche breakdown is not a fundamental mechanism even for

intrinsically pure materials, as generally accepted. Data which they

believed supported their claim that damage occurs first by multiphoton

absorption followed by nonlinear free-carrier absorption and heating that then

leads to damage were presented. To account for earlier observation by other

experimenters, it was asserted that avalanche processes do take place after

breakdown is initiated. Nevertheless, indications of avalanche before

breakdown were ascribed to extrinsic processes and/or impure materials. These

conclusion challenge the laser damage community to propose a set of

independent experiments that can confirm or discount their proposed model.

Concerning surfaces and mirrors, perhaps one of the most encouraging

developments, after many years of continuous effort, was the progress reported

for the polishing of silicon carbide. SiC and Be are among the most

attractive mirror materials now being considered for use in space missions.

One study reported on two different techniques for producing SiC mirrors which

could be finished to varying degrees of surface polish and which were stable.

The most attractive technique was physical-vapor deposition finishing of SiC

by ion-beam sputtering very smooth coatings with a low level of stress. The

rms surface roughness on these elements was less than 0.5 nm(5 A), and stable

with time.

3
Another exciting development was that ultra-precise grinding of glass

surfaces with diamond had attained 0.5 nm(5 A) roughness, which is remarkable.

A number of papers concentrated on mirrors for the vacuum ultraviolet, a new

thrust this year. There was some concentration on production problems of

working with a system with 1 m diameter mirrors subjected to very high laser

fluences. Systematic errors in surface topography using noncontact

interferometers were discussed. We also heard a novel suggestion for laser

protection of surfaces by developing small spherical particles that would

diffusely reflect incoming laser beans . The particles must have absorption

levels of -10"5 to 1CT 6 cm" 1

. If this becomes possible, then mirrors with very

high reflectance of 99.999% or better could result.

In the area of thin film coatings, degradation by ultraviolet and

vacuum-ultraviolet wavelengths was emphasized. One parametric study concerned

degradation to Zr02 films caused by 25-eV synchrotron radiation.

Surprisingly, after a short period, the initial damage apparently healed,

possibly by the material evolving to a different crystal structure. Another

paper described the much higher resistance of Hf02 films to color-center

formation caused by a 248 -ran KrF laser when the usual 3% Zr02 impurity content

was reduced to a few tenths of a percent. We also heard a report on much

higher damage thresholds being attained for multilayer reflectors prepared by

the sol-gel process as a result of altering the coating process to decrease

the concentration of absorbing impurities.

4
3 . Acknowl ed gmen t s

The editors acknowledge the invaluable assistance of Aaron A. Sanders

and the other involved staff members of the National Institute of Standards

and Technology in Boulder, Colorado, for their interest, support, and untiring

efforts in the professional operation of the symposium. Particular thanks to

Susie Rivera of NIST for her lead in the preparation and publication of the

proceedings, as well as Margalene Hartman of NIST. Thanks, also, to Pat

Whited of the Air Force Weapons Laboratory and Delma Oberbeck of NIST for

conference coordination.

5
4. References

[I] Glass, A.J.; Guenther, A.H. , eds . Damage in Laser Glass, ASTM Spec.

Tech. Pub. 469, ASTM, Philadelphia. PA; 1969.

[2] Glass, A.J.; Guenther, A.H., eds. Damage in Laser Materials, Nat. Bur.

Stand. (U.S.) Spec. Publ. 341; 1970.

[3] Bloembergen, N. Fundamentals of Damage in Laser Glass, National

Materials Advisory Board Publ. NMAB-271, National Academy of Sciences;

1970.

[4] Glass, A.J.; Guenther, A.H., eds. Damage in Laser Materials: 1971, Nat.

Bur. Stand. (U.S.) Spec. Publ. 356; 1971.

[5] Bloembergen, N. High Power Infrared Laser Windows. National Materials

Advisory Board Publ. NMAB-356; 1971.

[6] Glass, A.J.; Guenther, A.H., eds. Laser Induced Damage in Optical

Materials: 1972, Nat. Bur. Stand. (U.S.) Spec. Publ. 372; 1972.

[7] Glass, A.J.; Guenther, A.H. , eds. Laser Induced Damage in Optical

Materials: 1973, Nat. Bur. Stand. (U.S.) Spec. Publ. 387; 1973.

[8] Glass, A.J.; Guenther, A. H. Laser Induced Damage in Optical Materials:

A Conference Report. Appl. Opt. 13 (1): 74-88; 1974.

[9] Glass, A.J.; Guenther, A.H., eds. Laser Induced Damage in Optical

Materials: 1974, Nat. Bur. Stand. (U.S.) Spec. Publ. 414; 1974.

[10] Glass, A.J.; Guenther, A.H. Laser Induced Damage in Optical Materials:

6th ASTM Symposium. Appl. Opt. 14 (3): 698-715; 1975.

[II] Glass, A.J.; Guenther, A.H. , eds. Laser Induced Damage in Optical

Materials: 1975, Nat. Bur. Stand. (U.S.) Spec. Publ. 435; 1975.

[12] Glass, A.J.; Guenther, A.H. Laser Induced Damage in Optical Materials:

7th ASTM Symposium. Appl. Opt. 15 (6): 1510-1529; 1976.

[13] Glass, A.J.; Guenther, A.H., eds. Laser Induced Damage in Optical

Materials: 1976. Nat. Bur. Stand. (U.S.) Spec. Publ. 462; 1976.

[14] Glass, A.J.; Guenther, A.H. Laser Induced Damage in Optical Materials:

8th ASTM Symposium, Appl. Opt. 16 (5): 1214-1231; 1977.

[15] Glass, A.J.; Guenther, A.H., eds. Laser Induced Damage in Optical

materials: 1977, Nat. Bur. Stand. (U.S.) Spec. Publ. 509; 1977.

[16] Glass, A.J.; Guenther, A.H. Laser Induced Damage in Optical Materials:

9th ASTM Symposium, Appl. Opt. 17 (15): 2386-2411; 1978.

[17] Glass, A.J.; Guenther, A.H. Laser Induced Damage in Optical Materials:

1978, Nat. Bur. Stand. (U.S.) Spec. Publ. 541; 1978.

6
[18] Glass, A.J.; Guenther, A.H. , eds . Laser Induced Damage in Optical

Materials: 10th ASTM Symposium, Appl. Opt. 18 (13): 2212-2229; 1979.

[19] Bennett, H.E.; Glass, A.J.; Guenther, A.H. ; Newnam, B.E. Laser Induced

Damage in Optical Materials: 1979, Nat. Bur. Stand. (U.S.) Spec. Publ

568; 1979.

[20] Bennett, H.E.; Glass, A.J.; Guenther, A.H. ; Newnam, B.E. Laser Induced

Damage in Optical Materials: 11th ASTM Symposium, Appl. Opt. 19 (14):

23375-2397; 1980.

[21] Bennett, H.E.; Glass. A.J.; Guenther, A.H. ; Newnam, B.E. Laser Induced

Damage in Optical Materials: 1980, Nat. Bur. Stand. (U.S.) Spec. Publ

620; 1981.

[22] Bennett, H.E.; Glass, A.J.; Guenther, A.H. ; Newnam, B.E. Laser Induced

Damage in Optical Materials: 12th ASTM Symposium, Appl. Opt. 20 (17):

3003-3019; 1981.

[23] Bennett, H.E.; Guenther, A.H. ; Milam, D. ; Newnam, B.E. Laser Induced

Damage in Optical Materials: 1981, Nat. Bur. Stand. (U.S.) Spec. Publ

638; 1983.

[24] Bennett, H.E.; Guenther, A.H. ; Milam, D. ; Newnam, B.E. Laser Induced

Damage in Optical Materials: 13th ASTM Symposium, Appl. Opt. 22 (20):

3276-3296; 1983.

[25] Bennett, H.E.; Guenther, A.H. ; Milam, D. ; Newnam, B.E. Laser Induced

Damage in Optical Materials: 1982, Nat. Bur. Stand. (U.S.) Spec. Publ

669; 1984.

[26] Bennett, H.E.; Guenther, A.H. ; Milam, D. ; Newnam, B.E. Laser Induced

Damage in Optical Materials: 14th ASTM Symposium, Appl. Opt. 23 (21):

3782-3795; 1984.

[27] Bennett, H.E.; Guenther, A.H. ; Milam, D. ; Newnam, B.E. Laser Induced

Damage in Optical Materials: 1983, Nat. Bur. Stand. (U.S.) Spec. Publ

688; 1985.

[28] Bennett, H.E.; Guenther, A.H.; Milam, D. ; Newnam, B.E. Laser Induced

Damage in Optical Materials: 15th ASTM Symposium, Appl. Opt. 25 (2):

258-275; 1986.

[29] Bennett, H.E.; Guenther, A.H.; Milam, D.; Newnam, B.E. Laser Induced

Damage in Optical Materials: 1984, Nat. Bur. Stand. (U.S.) Spec. Publ

272; 1986.

[30] Bennett, H.E.; Guenther, A.H. ; Milam, D. ; Newnam, B.E. Laser Induced

Damage in Optical Materials: 16th ASTM Symposium, Appl. Opt. 26 (5):

813-827; 1987.

7
[31] Bennett, H.E.; Guenther, A.H. ; Milam, D. ; Newnam, B.E. Laser Induced

Damage in Optical Materials: 1985, Nat. Bur. Stand. (U.S.) Spec. Publ.

746; 1987.

[32] Bennett, H.E.; Guenther, A.H.; Milam, D.; Newnam, B.E. Laser Induced

Damage in Optical Materials: 1986, Nat. Inst. Stand, and Tech. (U.S.)

Spec. Publ. 752; 1987.

[33] Bennett, H.E.; Guenther, A.H. ; Milam, D. ; Newnam, B.E.; Soileau, M.J.;

Laser Induced Damage in Optical Materials: 1987, NIST (U.S.) Spec.

Publ. 756; 1988.

[34] Bennett, H.E.; Guenther, A.H. ; Newnam, B.E.; Soileau, M.J.; Laser

Induced Damage in Optical Materials: 1988, NIST (U.S.) Spec. Publ. 775;

1989.

8
MANUSCRIPT NOT RECEIVED

A Set of Standard Definitions for Laser Damage Parameters and Procedures

J.W. Arenberg

Advanced Technology Department

TRW Space and Technology Group

One Space Park Drive O1/1210

Redondo Beach, CA 90278

ABSTRACT

It is an essential aspect of science, technology and commerce to be able to communicate in a common language of units, procedure and phenomenology. Presently, there exist no such standards

for the field of damage testing, leading to the present problems in scientific communication,

comparison of test results and commercial activities. A necessary precursor to the standardization and

proper cataloging of laser damage test data is a common nomenclature. There are many cases where

parameters, phenomenology and test results are denoted by identical names. Moreover, there are cases where a single idea is known by multiple labels. This lack of standardization in the definitions and

nomenclature leads to difficult, tedious and frequently unfruitful literature searches. This paper is intended to be the first step in the standardization of the field of laser induced damage by reviewing

the literature and assigning in one review paper, a,set of exclusive definitions or labels to the various

definitions of damage, threshold determination, spot size and pulse width. In making these

assignments, a vehicle is provided for the necessary seminal discussions leading to eventual adoption

of accepted standards of procedure and definition for laser damage testing.
Manuscript received

1-17-90

Optical Characterization of Transparent Materials Using Ellipsometry

S. F. Nee and H. E. Bennett

Physics Division, Research Department

Naval Weapons Center, China Lake, CA 93555-6001

Refractive indices of transparent materials can be measured with high

accuracy on small laboratory samples using null ellipsometry. Measurement

precision in both n and k obtained ellipsometrically is +0.0004 for semitransparent samples. Systematic errors in ellipsometric characterization of

optical constants for transparent materials can result from back-surface

reflection as well as from front-surface scattering caused by surface

roughness. An analysis of the contribution of these errors and the methods

of eliminating them are discussed. We conclude that careful ellipsometric

characterization can give indices of refraction to three decimal places in the

infrared for materials with low k.

Key words: ellipsometry; error analysis; extinction coefficients; optical

measurements; polarizations; reflectometry; refractive indices; roughness

effects; transparent materials.

1. Introduction

The index of refraction of a transparent material can be determined with high

accuracy by making the material into a prism and determining the minimum angle of

deviation [1]. For general characterization of newly developed materials, the minimum

deviation method is not convenient, since a relatively large homogeneous sample is required. Often large pieces of developmental materials are not available. Other

techniques for measuring the index of refraction n and the extinction coefficient k for

glasses in the low k region are ellipsometry and the Kramer-Kronig (KK) analysis of

reflectance data augmented by absorption measurements [2,3]. The KK technique is usually of lower accuracy for n than the minimum deviation method, and the k values

determined from the reflection data are not reliable in the low k region. Additional

absorption measurements using calorimetry or other techniques are required. However,

if data are available, consistent index values can be determined over an extended

wavelength range using a KK analysis. Ellipsometric measurements can be made on

small developmental samples of material and can achieve accuracies approaching those

obtainable using minimum deviation techniques, as will be shown. Often ellipsometric

measurements are not that accurate because of systematic errors resulting from

instrumentation or sample configuration. An analysis of some of these sources of error

will be given. Most ellipsometric measurements have been made in the visible region of

the spectrum. In the infrared region, contamination problems of the surface involving

monatomic layers are less severe than in the visible region, so it is an even more

attractive region in which to work. An attempt to characterize transparent materials by

ellipsometry in the infrared region was described in a previous paper [4]. In that paper,

the n and k for a smooth quartz sample were measured, and the change in \)/ and A due

to surface roughness as well as that due to a small difference in n and in k were

10
numerically computed. With these data, we could predict the maximum roughness

allowed for reliable measurement and the accuracies of n and k obtainable by

ellipsometry. Effects of roughness and texture were also investigated for

semitransparent materials [5-7]. This paper presents a systematic analysis for the errors in ellipsometric characterization and discusses methods to eliminate errors. Comparison of the measured results with theoretical predictions and the Handbook

results shows that reflection ellipsometry can offer accurate measurements of optical

constants in the infrared region to three decimal places in both n and k.

2. Basic Analysis for Reflection Ellipsometry

Ellipsometry measures the relative change of amplitude and phase between p- and

s-waves after light is output from a sample. For reflection ellipsometry, the

ellipsometric parameters, \\r and A, are defined through the complex ratio p as

r

p iA

p = — = tan\|/ e

(1)

where rp and rs are the coefficients of reflection at a given incident angle 0 for light

initially polarized in the p- and s-directions, respectively. If the complex refractive

index (n,k) of a sample is known, rp and r$ can be computed from Fresnel equations;

hence, \|/ and A for a single interface system are only functions of n, k, and 9. Inversely, n and k can be solved from the measured \j/ and A through Fresnel equations. Multiple

angle measurements provide more data sets to increase the precision. The ideal

conditions for measuring a perfect two-phase system are: (1) no back-surface reflection

contributes to the measured front-surface reflection beam; (2) the front surface is smooth enough so that the scattering from surface roughness is negligible; (3) the

sample is pure enough so that no observable volume scattering results from voids and

impurities; (4) the surfaces are clean enough to prevent confusing contributions caused

by surface contaminants. Condition (4) is particularly hard to satisfy in the visible

region, where submonatomic layers can contribute to the ellipsometric change.

Conditions (l)-(4) cannot always be satisfied. Thus, the effects of different imperfect

conditions on the ellipsometric measurement results will be analyzed.

Let the coefficients of reflection for an idealized system be rpo and rso , those for a

real system be rp

and rs , and the small deviations of these coefficients for a real system

from the ideal condition be rp

' and rs

'. The relations among these variables are

rp - rpo + rp' and rs - rso + rs' (2)

The p for a real system given by eq (1) can be expressed as

(x*'p x'\ __

i s

V

rpo rso / (3)

where p Dis the complex ratio for the ideal condition. The deviations in \f and A caused

by the deviation Dp (= p - p G ) are 11
Dy = 2

sin 2\\r Re(Dp/p0)

DA = Im(Dp/p0 ).

It is obvious from eq (3) that the relative deviation in p is

(4)

(5)

Dp ^ V rs

'

Po rpo rso (6)

Notice that rp0 is small near the Brewster angle and the above deviation will be large. It

is better to avoid the Brewster region when determining optical constants. Based on the

above principle, the effects on the optical characterization due to small absorption,

roughness, scattering, and other imperfect conditions are analyzed in the following

sections.

3. Effect of Instrumental Errors on Measurement of n and k

The complex index n+ik is used in the Fresnel equations to compute rpand rs of

absorbing materials. For semitransparent materials (k « 1), k can be treated as a small

perturbation on the system with k = 0. In this case, rpo and rso can be defined as the

reflection coefficients for a nonabsorbing medium with index n. Expansions of the

Fresnel equations for complex rpand rs to the first order of perturbation k give

1+i

2nk cos0

2 I 2 2

(n - 1) n - sin 0

(7)

(

1-i

2ri k cosG

(n

2 - l)(n2cos2

0 - sin

20) *\J n

2 - sin2

0

(8)

where 0 is the angle of incidence. The relative deviation of p obtained from eqs (6)-(8)

is

Dp -i2nk cos0

Po a 2 i \ f~2 ~1Z

(n - 1) V n - sin 0

1 +

n

n

2cos2

0 - sin

20 J

(9)

By eqs (4), (5), and (9), we see that the first-order effect of k changes only the value of

A but not \\f . Thus, data of \|/ determine n and data of A determine k for transparent

materials. The Brewster angle 8b is a singularity in eqs (8) and (9). DA increases as 9

approaches 0b . Certainly, eqs (8) and (9) do not hold at 0 = 0b • DA approaches 0 as 0

12
approaches 90°. Figure 1 shows the exact computed DA for the difference in A between

the real system with nonvanishing k and the ideal system with k = 0. Behaviors implied

by eq (9) agree with those exhibited by the exact curves of DA.

In ellipsometry, errors in \\r and A are limited by the signal intensity and the angle

precisions of the analyzer and polarizer. To determine k, 0 is chosen such that it gives

large enough DA as well as large enough signal. Signal is very small near 0b- The

regions about 10° away from either 90° or 0b are good choices of the angle 0. The error

in k can be estimated from

2 2 2 I 2 2

(n - 1) cosG (n - tan 0) v n - sin 0

5k ~ 5A

2n3 (cos

2

0 + cos

2

0') (10)

where 8A is the instrumental precision in A and 0' is the refraction angle. 8A depends on

the precision of the polarizer as well as the signal intensity. It is obvious that 8k is large

for grazing incidence and for large n. For n = 1.9, 0 = 45°, and 8A = 0.04°, 8k is estimated

to be about 3.2 x 10" 4 . Equation (10) gives the limit of ellipsometry on the

determination of k. In general, ellipsometric characterization for k is good only for k >

0.001.

To determine n only, data of y alone are enough if k ~ 0. Data of \\r can be obtained

either by ellipsometry or by reflectance measurements of both s- and p-polarizations.

Following the same procedure of deriving 8k, the error 8n for ellipsometry can be

obtained as

Sn - 5k -

2S\|f

sin2\|/ 5A ' (11)

5\|/

(n

2 - 1) cos0 (n

2 - tan2

0) «J n - sin

20

n

3 sin2\j/ (cos

20 + cos

2©') (12)

Equation (12) works also for data of \|/ from reflectometry. For our ellipsometer, 8\j/ =

0.01° and 8A = 0.04° for \|/ > 10°; then 0.5 8k < 8n < 1.5 8k for normal operation conditions.

Thus, errors in n and k due to the instrumental errors of an ellipsometer are about the same. Figure 2 shows the deviation of \\r for a system with a deviation of 8n relative to a system with an index n. The inverse relation of eq (12) is the approximation of these curves for small 8n. For a given 8vj/, 8n is smaller for 0 nearer to 0b than farther from 0b .

It is obvious from the figure that data for 0 > 0b will give better accuracy than data for

0< 0 B .

The uncertainties 8y in reflectometry are subjected to the signal noise as well as the

alignment of the field direction of the p- and s-polarizations. The errors from the

reflectance measurements are much larger and more unpredictable than the

13
ellipsometric errors. Even though the errors are large, measurements at multiple angles

of measurements can improve both accuracy and precision. Notice that the factor of

(n 2 - tan2 9) in eq (12) changes sign at the opposite sides of 9b; thus, by taking about an equal number of measurements at both sides, the accuracy can be dramatically

improved.

4. Sources of Systematic Error

Possible sources of error may arise from surface roughness effects or from the

detection of unwanted radiation from (1) back-surface reflection and scattering, (2)

volume scattering from impurities and voids, or (3) scattering from surface

contaminants. From eq (2), we see that detection of any radiation other than the smooth

front-surface reflection will contribute to errors in ellipsometry. Scattered light is usually not randomly distributed; if it were, it could be considered as random noise. For

a random noise source, the real and imaginary parts of Dp/p 0 by eq (6) are about equal,

and D\|/ = sin2\j/ DA/2 by eqs (4) and (5). The corresponding errors of 8n and 5k will be

about the same by eq (11). Significant scattered light may result in 5n and 8k of the

order of 10~ 3

. This error is serious in the characterization of k, but it is still in the third

decimal place for n. For k < 0.001, other methods such as calorimetry can give much

higher accuracy in k than can ellipsometry. Absorption coefficients as low as 10"4/cm,

which at a wavelength of 4 u.m corresponds to k = 6 x 10-9

, have been measured

calorimetrically.

The roughness effect on ellipsometry is a result of the polarization effect of the

rough layer [5, 8-11]. The rough layer can be treated as having an effective thickness d,

which is about 2V2 times the rms roughness [5], and an effective complex index, which

can be derived from effective medium theory [8-11]. For smooth surfaces with dA « 1,

the expansion of p to the first order of d/X is [12,13]

P = Po 1 + 4^- F(0,eb

,es) (13)

where

F(0,eb,£s

) =

na cos6 eb (eb - es) (es - ea)

es(eb - ea)(eb cot

2

0 - ea) (14)

Here p 0

is the correspondent p for the two-phase system with pure bulk; e a , e s , and e b are the complex dielectric constants for the ambient, surface layer, and bulk material,

respectively, and n a is the index of refraction for the ambient. The imaginary parts of

both eb and e s are very small for transparent materials, since they are directly

proportional to k. Thus, Re(F) » Im(F) by eq (14) and DA » Dy by eqs (4), (5), and (13).

The major effect of roughness on ellipsometry for transparent material is the change of

A from the ideal condition. The roughness effect might confuse the determination of k

but not n. However, DA(9) caused by k * 0 is quite different from DA(9) caused by the

roughness effect. A three-phase model should be used to reduce the ellipsometric data

when the roughness effect is not negligible.

Figure 3 shows D\j/ for a rough sample of quartz at a wavelength X of 5 u.m relative to

an ideal sample for 0.1 nm < d < 10 (im. Errors in x\f are negligible for roughnesses below

14
10 nm. The extinction coefficients for transparent materials are usually much smaller

than k ~ 0.004 used in figure 3, so that the are much smaller than the one shown in

figure 3. Thus, the determination of n is immune from the roughness effect, since the

roughnesses of transparent samples are usually below 2 nm. Figure 4 shows DA(0) for a rough sample of quartz at X = 5 u.m relative to the ideal condition for d being (A) 0.707

nm, (B) 2.828 nm, and (C) 14.14 nm. For case (B) where the rms roughness is about 1 nm, DA is larger than the noise level of the instrument. Thus, roughness effect is not

negligible for determining k for roughness larger than 1 nm.

5. Experimental

The null ellipsometer [14] used to measure the ellipsometric parameters can also be

used to measure the reflectances of both p- and s-waves. The apparatus and methods of

data reduction for ellipsometry are described in detail in previous papers [4,5]. For

reflectometry, data of reflectances of both p- and s-waves are converted to y first, and

then the existing computer programs were used to reduce the data. Some previous data

[4] are used for comparison purposes. These data are for a quartz sample with parallel

front and back surfaces. The quartz sample has a 1-cm thickness, a 3.86-cm diameter,

and a very rough back surface. New samples with small wedge angles between smooth

front and back surfaces have been tried; data of only one such sample are reported here.

The ellipsometric parameters for a fluoride glass sample were measured at X - 3.8 p,m at multiple incident angles using the null method. The fluoride glass sample is thin and

smooth on both sides with a 0.3° wedge angle and a 2.54-cm diameter. The vacuum

chuck holder was used for this sample. The back-surface reflection was completely

blocked out for all incident angles of measurements.

Elimination or suppression of the unwanted radiation detected by the ellipsometer is essential for accurate characterization measurements. Volume scatterings from

impurities and voids are characteristics of the sample itself; nothing can be done

optically except to make a purer sample. Scattering from contaminants can be avoided

by carefully cleaning the front surface. The roughness effect can be avoided by making

the front surface roughness smaller than 1 nm. Back-surface reflection and scattering

may give severe errors in ellipsometric characterization of transparent materials. If the

back-surface reflection beam can be guided away, then the back-surface scattering can

be reduced by making the back surface also very smooth. Methods to eliminate backsurface reflection are outlined below.

To prevent the back-surface reflection, a thick sample will provide large

displacement between the reflection beams from front and back surfaces. The

separation 8 between the two parallel reflection beams is

where t is the thickness of the sample. Figure 5 shows the separation 8(9)/t for

different indices of refraction. Samples with lower n have larger displacement. The

separation is large for angles between 30 and 70° and small for large 0. Conventional

sample holders, whose frames might block part of the reflected light for large angles of

incidence, fit nicely for thick samples. These holders allow the transmitted beam to go

away, so that the back-surface scattering is minimized. The quartz sample was mounted

5 =

tsin20

(15)

15
on this kind of holder. For the 1 -cm-thick quartz sample, 8 is larger than 6 mm for n =

1.4. The two beams can be separated easily by rectangular 3-mm-wide apertures.

Thick and large samples of the correct size for the sample holder are usually not

available for measurements. When a sample is thin, the back-surface reflection cannot

be separated from the front-surface beam. A wedge angle of about 0.5° between the

front and back surfaces can successfully separate the front-surface reflection from the

back-surface reflection. If a vacuum chuck holds the sample in the sample holder, the

spurious back-surface reflection beam from the holder plate can sometimes have a higher intensity than the front-surface reflection beam. The vacuum chuck holder has

the convenience of holding a sample of any size and shape and making measurements

for large incident angles; however, a wedge sample is necessary when using the vacuum

chuck holder. Even though the back-surface reflection beam can be separated, the backsurface scattering can still be severe if the holder plate or the sample back surface is rough. A smooth back surface is better than a rough back surface for a wedge sample.

A wedge angle larger than 1° in our setup can efficiently prevent the detection of near- angle scattering from the holder surface. Large angles of incidence also can reduce the

back-surface scattering appreciably. A smooth holding plate will be ideal for the

vacuum chuck holder.

The reflection angle 0" for the back-surface reflection beam for a wedge sample with

wedge angle a is

0" = sin

1 (sin0 cos2a + sin2a y/ n

2 - sin

2

0 ; : < : (16)

where

0C = sin

1 (cos2a - sin2a Vn2 - 1

)

(17)

At 0 = 0 C , 0" is 90°. For 0 > 0 C , total internal reflection occurs for the back-surface

reflection beam. For n = 2 and a = 2.5°, 0 C = 57.7°. 0 C

is smaller for larger n and a. For

very small a, the deflection angle 50 = 0" - 0 can be approximated as

80 = 2a sec0 Vn

2 - sin2

0

(18)

Figure 6 shows the exact 50(0) obtained from eq (16) for a = 1° and different n. The

deflection is larger for larger n and for larger 0. Thus, if the sample has a high index of

refraction, a wedge sample is more effective for beam separation than a thick sample.

For large angles of incidence, the reflection intensity is high and the measurements are much faster than for low-incident angles.

6. Results

Figures 7 and 8 show data of \\f at multiple incident angles for a quartz sample

measured at X - 3.846 and 5.0 \im, correspondingly. About the same number of

measurements were taken at both sides of 0 B . Data obtained from null ellipsometry are indicated by crosses; reflectometry data are indicated by octagons. The solid lines are

16
the computed \\f using the Handbook values [3] of (n,k) as (1.395, 4.96 x 10" 5

) at 3.846

um and (1.342, 0.00398) at X - 5.0 (im. It is obvious that the crosses fall directly on the

theoretical curves and most of the octagons are above the solid curves.

To see how well the data fit to the theoretical curves, figure 9 plots the deviations 5\|/

of the data with respect to their best-fit theoretical curves for both ellipsometric and

reflectance measurements at X = 3.846 um. The best-fit values of n and k for the

ellipsometric data give a minimum rms deviation of both \\r and A. The best-fit value of n for the reflectance data gives a minimum rms deviation of \\r . The least-square-fit

method of reducing the ellipsometric data was described in detail in previous work [4,5].

Results of the best-fit values for different sets of data and their rms deviations are

listed in table 1. The rms deviation for reflectometry is much larger than Oy for

ellipsometry. Instrumental errors occurring in reflectometry are higher than those in

ellipsometry. However, reflectance measurement is quick, while precise ellipsometric

measurement using the null method is slow.

Table 1. Ellipsometric results of optical constants obtained by

direct average of n and k for different 9 and by the leastsquare-fit method. Abbreviation E represents ellipsometry,

and R represents reflectometry. The Handbook values at the

corresponding wavelengths are also quoted.

Wavelength (um) 3.846 3.846 5.00 5.00

method E R E R

By direct average: n 1.3965 1.3958 1.3407 1.337

an ±5.6x10-4 ±0.017 ±5.5x10-4 ±0.016

k 2.1xl0" 4 0.00396

©k ±6.6xl0- 4 ±4xl0- 4

By least-square-fit: n 1.3965 1.3917 1.3408 1.337

k 2.3x10-4 0.00379

a¥

(deg) 0.0143 0.373 0.015 0.502

aA (deg) 0.049 0.063

N 8 14 10 14

Handbook values: n 1.395 1.342

k 4.96x10-5 3.98x10-3

To see more clearly the deviations for ellipsometric measurements, figure 10 plots

the deviations 5\|/ of the measured data with respect to their best-fit theoretical curves

for both wavelengths. The deviations are randomly distributed about the base and are close to the instrumental error. Thus, the uncertainty in the measurement is at the level

of the instrumental noise in this case. From table 1, the rms deviation o-y is about

0.015°. This number gives an rms deviation of n of o n = 2.1 x 10" 4

, as calculated from

eq (12) for n = 1.3965 for the data set of X. = 3.846 p. The standard deviation o n

17
computed from individual n values using the Fresnel equations is 5.6 x 10~4 , as listed in

table 1. Thus, the n reduced from the least-square-fit program gives better accuracy

than the n averaged from individual n values obtained using Fresnel equations.

Figure 11 shows the refractive indices computed directly from the ellipsometric and

reflectance data for X = 3.846 |im using the Fresnel equations. The reference line is set

at the Handbook value [3] of n at the corresponding wavelength. By comparing figures 9

and 11 for X = 3.846 u.m, we see that the deviations of n from the reference line for

reflectometry switches sides for 0 > 0b . If the same mechanism causes large 8\|/ values

with the same sign, then 8n for 0 > 0b has the opposite sign against 5n for 0 < 0b . As

these values of n are averaged, the systematic errors are mostly cancelled out. Thus, by

taking about the same number of measurements on both sides of 0b, the reflectance

measurements can give a rough estimate of n for a transparent material. The n computed from the more precise ellipsometric data stays at about the same value, and

the standard deviation is much smaller than the reflectance data.

Table 2 shows the ellipsometric data and the computed optical constants for a wedge

sample of fluoride glass. A vaccum chuck sample holder is used for this sample. The

values of A shown are at the level of the instrumental errors. Thus, the back-surface

reflection and scattering from the sample holder plate have been eliminated using a wedge sample. Since A is at the noise level, the value of k is expected to be of the order

of or smaller than 1CH. The deviations of n and k in table 2 are at about the same level as those shown in table 1. The use of wedge samples for measuring at high angles of

incidence is much faster than measuring near Brewster angle, since the signal is much

more intense.

Table 2. Ellipsometric data and the computed optical constants

for a wedge fluoride glass sample at multiple angles of

incidence and at 3.8-u.m wavelength, n and k are computed

directly from \|/ and A using Fresnel equations.

0 (deg) \|/ (deg) A (deg) n k (0.0001)

65.0 15.72 -0.01 1.4174 0.894

67.5 19.25 0.0 1.4181 0.0

70.0 22.64 0.0 1.4181 0.0

72.5 25.87 -0.02 1.4179 3.437

75.0 28.93 -0.02 1.4182 4.225

77.5 31.85 0.0 1.4183 0.0

Average 1.4180 1.426

Standard deviation 3.225xl0"4 1.911

Least-squares fit to Fresnel equations:

Best-fit value: 1.4177 0.995

rms error: 0.010° 0.011°

18
7. Conclusions

For semitransparent materials, characterization of n depends only on and

characterization of k depends only on A. The reflectance measurements with about the

same number of measurements at both sides of Brewster angle can also be used to

characterize n only. However, ellipsometry gives much higher accuracy and precision

than reflectometry. The errors 5n and 8k for ellipsometry are derived and proved to be

at the level of ±5 x 10" 4 for normal operation. The estimated values agree with the

observed uncertainty in the experimental results. The roughness effect affects mostly A

but not \|/, so that characterization of n is more or less immune to surface roughness. A

random noise source gives about the same magnitude of error in both n and k; these

errors seriously affect the percentage accuracy of k but not of n. Samples with

thicknesses larger than 1 cm or with wedge angles larger than 0.5° can largely reduce

the back-surface reflection and scattering. We conclude that, if care is taken,

ellipsometric measurements can give accurate as well as precise values of refractive

indices to the third decimal place for transparent materials in the infrared region of the

spectrum. The success of ellipsometric characterization of optical constants, roughness,

and texture for semitransparent materials has set a solid ground for further

investigations of subsurface damage, coatings, and other problems of optical materials

using ellipsometry.

8. References

[1] Malitson, I. H. "Interspecimen comparison of the refractive index of fused silica," J. Opt. Soc. Am. 55; 1205; 1965.

[2] Philipp, H. R. "The infrared optical properties of Si02 and Si02 layers on silicon," J. Appl. Phys. 50; 1053; 1979.

[3] Palik, E. D., ed. Handbook of optical constants of solids . New York; Academic Press;

1985. Pp. 749-763.

[4] Nee, S. F.; Bennett, H. E. "Ellipsometric characterization of optical constants for

transparent materials," Proc. SPIE 970 : 62-69; 1988.

[5] Nee, Soe-Mie F. "Ellipsometric analysis for surface roughness and texture," Appl.

Opt. 27 (14); 2819-2831; 1988.

[6] Nee, S. F.; Bennett, H. E. "Nondestructive evaluation of surface roughness in the

0.01- to 1.0-m range using infrared ellipsometry," Proc. SPIE 675 : 260-269; 1986.

[7] Nee, S. F.; Bennett, H. E. "Quantitative characterization of rough Si02 surfaces by

infrared ellipsometry," Proc. SPIE 818 : 34-45; 1987.

[8] Fenstermaker, C. F.; McCrackin, F. L. "Errors arising from surface roughness in

ellipsometric measurement of the refractive index of a surface," Surf. Sci. 16.; 85-96;

1969.

[9] Aspnes, D. E.; Theeten, J. B.; Hottier, F. "Investigation of effective medium models of

microscopic surface roughness by spectroscopic ellipsometry," Phys. Rev. B20 : 3292;

1979.

[10] Aspnes, D. E. "Optical properties of thin films," Thin Solid Films 89; 249; 1982.

19
[11] Marton, J. P.; Chang, E. C. "Surface roughness interpretation of ellipsometer

measurements using the Maxwell-Garnett theory," J. Appl. Phys. 45; 5008; 1974.

[12] Aspnes, D. E. "The accurate determination of optical properties by ellipsometry", in

Handbook of optical constants of solids . Palik, E. D., ed. New York; Academic Press;

1985. Pp. 89-112.

[13] Aspnes, D. E. "Spectroscopic ellipsometry of solids", in Optical properties of solids:

new developments. Seraphin, B. O., ed. Amsterdam; North-Holland; 1976.

[14] Leonard, T. A.; Loomis, J.; Haarding, K. G.; Scott, M. "Design and construction of three

infrared ellipsometers for thin film research," Opt. Eng. 21 (6): 971-975; 1982; also,

UDRI-TR-84-128.

A

X (/im) = 3.846

n(b) = 1.395 k(b)= 0.000

6k(b) = 1.000*10"" (a)

6"k(b) = lOOOtlO" 3 (b) 6k(b)= 5.000*10"' («

-i 1 1 1 1 1 1 r

24 32 40 48 56 64 72 BO

Incident Angle (degrees)

Figure 1. Difference function DA(9) for

5k = (A) 0.0001, (B) 0.001, and (C)

0.005; all are evaluated at n = 1.395

and k = 0. The deviations in A are not

negligible for 5k = 0.001 for angles

about ±3° away from the Brewster.

X (Mm) = 3.846 n(b)= 1.395

k(b) = 0.000

<5n(b) = 5.000*10" 3 (*)

B^

fin(b) = LOOOtlO"

3 (B)

——

'^

/////^^

— U- ID | | | | | | | | 24 32 40 48 56 64 72 80 88

Incident Angle (degrees)

Figure 2. Difference function D\j/(6)

for 5n = (A) 0.005 and (B) = 0.001;

both are evaluated at n = 1.395

and k = 0. The deviations in \|/ for 9

from 40 to 70° for curve B are about 0.02 to 0.03°. Since the

instrumen- tal error of \|/ is 0.01°,

the precision in n is expected to be

better than 0.001.

20
0.01 01 1

EFFECTIVE THICKNESS (MICRONS)

Figure 3. The relative change of y

due to roughness with effective

thickness of the rough surface ranging

from 0.1 to 104 nm. Errors in y

induced by roughness below 10 nm

rms are less than 0.01°.

E

id in\

n - 1 .4

55 68 65 70 Incident Rngle (degrees)

Figure 5. Separation 5(0) between the

parallel reflection beams from front and

back surfaces per unit thickness of the

sample for different refractive indices n. The separation is smaller for larger n.

X (fj.m) = 5.000

1 nfhl = 1 M?

/

k(b) = ^ 9R0»10~ 5

Layer No. ly = 1 n(ly)= 1.167

k(ly) = 1.902'IQ"

3 /

d (/im) = 0.000

a ' id (fim) = 7.070.10"'

b : 6~d (^m) = 2.828'10" 3 3

c: <5d (H = 1.414-10"'

35 40 45 50 55 60 65 70 75 80 85

Incident Angle (degrees)

Figure 4. Difference function DA(6)

for quartz samples at 5-u.m wavelength for effective thickness

deviations of (A) 0.707, (B) 2.828,

and (C) 14.14 nm, equivalent to rms

roughnesses of (A) 0.25, (B) 1, and

(C) 5 nm. The deviations in A for a 1-nm rms roughness are about at

the noise level of the measurements.

Wedge 1 dog

n - 1 .8

'

h -

• 1 • 1

. 4

i . . ^0 25 30 35 40 45 50 55 68 55 70 Incident Angle (degrees)

Figure 6. Deflection angle 69(6) between

the reflection beams from front and back

surfaces for 1° wedge angle and different

n. The deflection is larger for larger n.

21
0 I i

1

I I I

I I i i 30 35 40 45 50 55 60 65 70 75 80

Incident Angle (degrees)

Figure 7. \|/(6) for quartz at 3.846-um

wavelength. The solid curve is computed

from the Fresnel equations for Handbook

values of n = 1.395 and k = 4.96 x 10" 5 . The experimental ellipsometry data of y

are indicated by crosses, and reflectometry data are indicated by octagons.

0 I i i i i i I I i i 35 40 45 50 55 60 65 70 75 80 1 85

Incident Angle (degrees)

Figure 8. V|/(6) for quartz at 5 u,m

wavelength. The solid curve is computed

from the Fresnel equations for n = 1.342

and k = 0.00398. The experimental

ellipsometry data of \\f are indicated by

crosses, and reflectometry data are

indicated by octagons.

— •

WW

Quartz B

X = 3.846 |1

Reflectometry

Ellipsometry Base

30 40 50 60 70 80

Incident Angle (deg)

Figure 9. Deviations 5 \j/ of the

experimental data of figure 7 with

respect to their best-fit theoretical

curves at X = 3.846 um for both

ellipsometric and reflectance

measurements. The best-fit values of n and k are listed in table 1.

22
0.03

0.02 Wavelength a 5.0 • 3.846 B Base

40 50 60 70

Incident Angle (deg)

80

Figure 10. Deviations 8 \\f of the

ellipsometric data with respect to their

best-fit theoretical curves for both

wavelengths for the quartz sample. The

systematic errors have been reduced to

the level of the random instrumental

errors.

1.44

1.42

v

OS 1.40

a

S

1.38

1.36

a * *

Reflectometry • Ellipsometry Reference

• • • •

Quartz

X = 3.846 |i

30 40 50 60

Incident Angle (deg)

70 80

Figure 11. Refractive indices computed

directly from data of figure 7 by

ellipsometry and reflectometry for X =

3.846 (im using the Fresnel equations.

The reference line is set at the Handbook

[3] value of n = 1.395.

23
COMMENTS

Question: These experiments were done in air?

Answer: Yes.

Question: Did you look at the time dependence of damage as measured by

observing the pulse wave form?

Answer: No, not in this particular set of experiments. We've done that

somewhere else but no not here. I would suggest that that's a very important thing to look at, for example, it would bear on the

great controversy about avalanche and non-avalanche processes. If

you are near a threshold then you should damage at the peak of

pulse. If you have multiphoton effects dominating, damage should

occur at the end of the pulse. We are working on developing such

a system. The problem is that we are down on the nanosecond

oscilloscope range and it is difficult. That is one of the areas

that we plan to concentrate on.

Question: Wouldn't it be nice to put a piece of salt in there and observe

the charging. You can have in one case a semiconductor that is

totally absorbed in the UV, in the other glass, which is totally

absorbed in the C02 region. It would be nice to have something in

there which is mostly transparent to both of those regions.

Answer: Good idea.

24
Manuscript received

12-29-89

Microindentation as a Technique For Assessing

Subsurface Damage In Optics

R. S. Polvani and C. Evans

National Institute of Standards and Technology

Precision Engineering Division

Gaithersburg, Md 20899

ABSTRACT: Efforts to optimize precision machining emphasize form and surface

finish as the primary objectives. Minimizing subsurface damage is no less important

for high power optics. This paper reports our usage of instrumented

microindentation for subsurface damage assessments. Our instrument is an example of

a new class of diagnostic tool. All apply controllable loadings to an indenter and

continuously measure the load and resulting penetration. The utility of

microindentation depends on appropriate interpretations of load-depth curves. The

technique provides insight into the physical nature of the subsurface and is a means to obtain a variety of engineering properties from the subsurface region. We use it to profile flow stress and elastic modulus with depth, and to gauge creep

and anelastic behaviors. Instrumented microindentation promises rapid and

sensitive property assessments. We will use a range of examples to show this

potential is realizable.

KEYWORDS: glasses; metals; microhardness ; mechanical properties; microindentation;

precision machining; semiconductors; single point diamond turning; subsurface

damage; and Young's modulus

INTRODUCTION

Increasingly, optics are components combining

precision machined surfaces and minimum

levels of subsurface damage. Optical

birefringence, X-ray diffraction (XRD),

metallography, penetrant dyes, eddy current

and many other techniques are available to assess damage. However, a need remains for

better means of testing. The existing

techniques have limitations: they may apply

to one class of material, be time consuming

and destructive, and too expensive to perform

routinely. For example, XRD residual stress

measurements can be made in metals but not

glass optics, and X-ray topography is

limited to single crystals, or very coarse grained material. Metallography can be used

with all materials, but requires destroying

the object. This paper will indicate a range

of applications where subsurface damage is a critical factor, and describe a varied usage

of microindentation testing. Our need is to

determine the nature, extent and intensity of

subsurface damage. The technique promises

rapid, sensitive subsurface property

measurements. We will try to show this

promise is realizable with a range of

examples.

A useful, but too simple model relates the

damage to total depth of the layer of

modified material, and the degree of

modification. Degree is the difference

between surface values and a representative

for the sample's interior or bulk. A

saturated, homogenous or monolithic layer is

unlikely. A transition region made up of

several discrete and possibly different

layers is more likely. With depth below the

surface, a property gradient would appear in our results, and that is exactly what we see.

The physical form of the damage depends on the type of material. In brittle materialsceramics, glasses or semiconductors - damage

is a combination of residual stresses and

structural faults. These can be crazing,

incipient crack sites, cracks or voids. Even

in well polished glass optics, the faults can penetrate deep and cause a measurable

depression of the local modulus. In ductile

materials, the near surface is more likely to

be work hardened. Figure 1 offers a schematic view of the surface and interior

grain structure. Surface grains are more

twinned, and smaller. They may be strongly

aligned along a crystal direction, and have a high aspect ratio. Mechanically, a worked

subsurface would have a higher yield

strength. Lastly, the energy input from

cutting can lead to the grains

recrystal 1 izing and higher concentrations of

oxides. Both effects are seen in copper.

25
POTENTIAL APPLICATIONS: Modern single point

diamond turning of metal infrared optics is

a relatively mature technology [1]. The near surface damage levels, caused in fabricating

thermal imaging systems or scanner mirrors,

is considered functionally unimportant. That

premise is questionable. Subsurface damage

does affect the long term dimensional

stability. It may also cause veiling glare in

transmissive IR systems. For high power

laser systems subsurface damage has immediate

and striking effects. Wood surveyed materials

defects which lower the laser damage

threshold [2]. He included voids, pits,

dislocations, residual stresses, low angle

grain boundaries, and second phase

inclusions. Further, Hurt provides graphic

evidence of the severity of conditions

immediately below the surface in diamond

turned copper [3]. In some cases, the grains

recrystal 1 ize either during or very soon after the part is fabricated. Hurt and his

China Lake colleagues also found

relationships between subsurface damage,

optical scatter, reflectance and the laser

damage threshold [4-6].

Damage thresholds in high power transmissive

laser systems are also limited by subsurface

damage. For assessments of glass optics,

Brown and his colleagues pioneered taper

polishing techniques [7,8]. They emphasize

use of numerical damage indices. Two indices

are "eighty (80) percent of the cracks

shorter than" and basing damage penetration

on a ratio with the deepest crack detected.

Both can guide fabricators or process'

development, but taper polishing is slow,

labor intensive, and destructive.

The IC industry also needs low damage

transmission optics. Optical step and repeat

systems are keeping pace with decreasing IC

feature sizes by moving to ever shorter

wavelengths. Visible wavelengths are being

supplanted by the UV band. New steppers will

require large numerical aperture aberration

free lens systems, which will be made up of

aspheric elements. Near UV optical systems

will need significantly better surface

finishs and subsurface damage levels, than

systems operating in the visible. Stepper

lens fabricators are likely to harness an emerging technique, ductile regime grinding,

for rapid production of aspheric elements.

The need is for aspherics with minimal damage

and made without post polishing. GCA

Corporation* has already developed an excimer

laser based stepper system.

Excimer laser systems are of interest for a range of material removal processes [10].

* Specific commerical items are identified

only to provide a complete description and

does not imply an endorsement.

Ablative processing offers high precision

material removal without the heat affected zone of conventional laser processes.

Different materials show different fluence

thresholds for ablation, allowing

differential processing. An example is

polymer films on metal substrates. Laser

crystals and semiconductors are still more applications sharing a need for undamaged

surfaces

For all, process optimization could be

speeded if techniques for rapid damage

assessment were available. Instrumented

microindentation is a useful and versatile

diagnostic tool. To demonstrate the

capabilities, we will start by describing our experimental approach and then use a range of

examples.

EXPERIMENTAL

Our test strategy emphasizes: complementing

indentation assessments with other diagnostic

methods, drawing conclusions from comparative

property measurements, and placing little

emphasis on the immediate surface. Other

methods are used to complement rather than

simply verify the microindentation testing.

XRD and X-ray topography are used to gauge

the residual stress levels in metals and

ceramics. Metallography is used extensively

to image cracking and grain morphology.

Prof ilometry, differential interference

contrast ( D I C ) and scanning electron

microscopy (SEM) are used to characterize

surface finish and form. Our evaluations try

to reflect a consensus from several

viewpoints. The general operating mode is to use the microindenter as a comparator. By

making comparisons, reproducible results

become the necessary and sufficient

condition. Further, comparisons mask the

large differences inherent in real

material s

Although the opportunities and need are fewer, we make and use absolute measurements.

Moduli measurements are an example of

absolute values. Measurements made within

0.025 micrometer of the surface, critically

reflect: displacement resolution, surface

condition, and an exact knowledge of the

indenter geometry. A thin gold plate on steel substrate is a model system we use to

estimate the error in the property

measurements. The question is how large an error is acceptable? Characterizing layers

less than 0.025 micrometers is not a current

goal. Rather, it is to provide and use comparative measurements to aid precision

machining research and process development.

INSTRUMENT: Several instrumented

microindentation instruments are reported in

the literature [10-13]. They were developed,

in general, for use within a laboratory to

26
meet a specific purpose. Ours started with a need to measure the effects of strain rate on the dynamic indentation fracture properties

of glass. Some current capabilities of our instrument are largely refined carry overs from this start. Both its capabilities and

limitations have shaped our test strategy.

The Nanoindenter* was an IBM* laboratory

development which is now a commercial product

[13]. The unit developed at NIST, National

Institute of Standards and Technology, is a flexible micromechanical properties test

apparatus.

The instrument, method, and the analytical

procedures used to obtain properties has been

described in detail elsewhere [14-15]. The

four functional subassemblies are computer,

electromagnetic driver, main test frame, and

electronic instrumentation. See figure 2.

Originally, tests were fully computer

controlled. An HP-85* generated the indenter

loading, controlled the different electrical

subassemblies, and processed the data. Now a Compaq Model 386/20e* is used, but with a role limited to data acquisition and

processing. The electromagnetic driver is

rated for loads to 10 Newtons. The main

frame is a modified commercial stamping

press. A translation stage with X-Y drives

positions the specimen. Specimens may be

attached to the translation stage with

silicone grease, wax or cellulose adhesive,

but adhesive is preferred. Specific sample

features may be tracked or avoided with the

use of a targeting microscope.

The instrument uses a load waveform which is a modification of the hardness tester

loading. See figure 3a. The essentially

square loading waveform was retained, so results would be compareable to hardness

numbers. Modifications are using haver sine

sections for up and down loadings, and

holding a 10 percent post impression load.

Haver sines segments minimize impact effects

and the working bandwidth of the

instrumentation. The Lyco hardness tester*,

a cam driven machine, also uses sinusoidal up

and down load segments. Our instrument uses

the minor loading to prevent depth recovery.

We do this to have comparability between our results and hardness numbers.

An indent is a three dimensional object. This

instrument measures the impression size using

the depth. Hardness numbers reflect entirely

the projected surface dimensions of a recovered impression. In metals, recovery

primarily reduces the impression depth.

Recovery has little to no effect on surface

dimensions. Using an unrecovered depth in

the calculation, we gain comparability. For

dynamic regime testing, a sinusoidal impulse

is used. The impulse reduces the upper limit

of the instrumentation bandwidth to the test

frequency or reciprocal loading time. The

driver signal is the amplified output from a Wavetek Model 75* arbitrary function

generator. The waveform duration - up load to

post dwell - is a test factor. Loadings of

milliseconds to several hours are used.

Unless specified, a 0.4 second duration was

used. Two ways are used to measure load. At

low loading rates, the current through the

electromagnetic drive coil is used. At

dynamic rates, a piezoelectric load cell is

used. A full four arm bridge of eddycurrent

transducers is used to measure displacement.

Bridge resolution is better than 0.02

micrometers.

Instrumentation design avoids test frame and

indenter compliance errors. The load and

displacement measurements are made between

the back of the indenter and sample surface.

Both the placement and type of transducer

reflect this design feature. A properly

brazed indenter tip eliminates indenter

compliance. This can be shown with three

arguments. Apply Hooke's Law to the indenter,

and compare the relative compressions of the

diamond tip and shank. Second, use the Hertz

Equation to calculate penetration of a ball

indenter into an elastic sample, and compare

the calculation with an experiment. With our instrument we obtain agreement. Third, our modulus measurements are within 15 percent of

handbook values.

METHOD: A full range of indenter geometries

are used. Vickers microindenters are used

for investigations of glass and ceramic

properties. For metals and where workable

with glasses and ceramics, ball indenters are preferred because:

1. The test volume has a high surface area

to penetration depth ratio.

2. Large, easily measured loads give small

penetration depths. This is important

for dynamic measurements where quartz

cells are used.

3. Analytical and finite element solutions

for ball indentations are available. We

perform our routine calibrations using

the Hertz Equation.

Ball size is an effective control variable.

For elastic measurements, the hardest and

largest ball diameter is used. For fracture

threshold measurements, small balls are used.

For plastic indentations, the size depends

on three factors: need for an impression

free test surface, relative hardness of the

test material, and inhomogeneity of the

sample. The lower limit to diameters is the

difficulty of properly shaping diamond, and

the upper limit is the 10 Newton driver

capacity. Large diameter balls give better

averaging. Size can minimize the contribution

of crystal orientation or surface features to

27
the scatter. For example, to assess the

effect of diamond turning an indent should

include several tool marks. Despite the poor

surface finish, a 400 micrometer diamond

indenter is frequently used with ceramics

materials. We are developing 25 and 100

micrometer chemically polished diamond

indenters. These will be used to determine

the fracture threshold for several optical

glasses. The results presented here were

obtained using an 1600 micrometer diameter

silicon carbide ball. For a 1 micrometer

depth, the sampled surface areas is 5,023

micrometers^ or 80 micrometer in diameter.

Results are, typically, averages of twelve

(12) repeat runs, each load-depth curve has

2048 data pairs. The number of repeats is

not fixed. As many as sixty four (64) repeats

have been necessary to evaluate the bulk or baseline property of a hot pressed ceramic. A

95 percent confidence limit is the criteria

used to decide the number of replications.

For diamond turned, metallographic sections

or optically polished surfaces, run to run variation is low, so averaging over twelve

runs is sufficient. Because the instrument

is computer interfaced, high numbers of

replications are practical.

To speed processing by the HP-85* and conserve storage, the full 2047 data array

was not used. For plastic indentations, we tested the practice, and found no loss.

Within the accuracy of Lotus/123*, results

from either full or reduced data arrays

coincided. With the Compaq 386/20e*, the full

2048 array is used. We plan to convert to a 8,192 data array. Attempts to measure

individual fracture events in glass need the

enhanced resolution.

PROPERTIES: Results are graphically

presented in four ways: load and depth

versus time, load-depth, flow pressuredepth and delta curves. Data are initially

reported with a load and depth versus time

curve, see figures 3a. In two steps, load and

depth are cross plotted to obtain more sensitive results. First, a load-depth curve

is obtained, see 3b. Second, after using the

expressions described below, the data are transformed into a flow pressure-depth curve,

see figure 4. Load-depth curves reflect the

overall mechanical state. Flow pressure-depth

curves summarize a local probing of the

surface material. The Delta curve compares a reference to a set of surface values. Because

it is a relative measurement, the curve

scales to the differences. By masking out

the absolute value, the Delta curve permits

magnified displays. The reference for the

Delta curves is a fifth order polynomial

model of the indentation behavior. To obtain

this model, the sample interior is exposed by

metallographic or chemical polishing. A

pattern of indentations is made using the

same conditions that will be used to probe

the test surface. The polynomial is obtained

using linear regression.

Figure 3b gives load-depth curves for

synthetic sapphire, diamond turned copper,

and polymethylmethacrylate (PMMA). Sapphire

behaves as a perfectly elastic material. The

high value of the modulus, 390 GPa, causes a shallow depth, and dictates high slopes for

the up load and down load segments. The

elastic signature of alumina is the lack of a residual depth, or full closure over the

loading cycle. PMMA is the other extreme of

deformation behavior; it is a viscoelastic

material exhibiting elastic, plastic, creep

and anelastic behavior. The low Young's

modulus of 1.35 GPa, causes large penetration

depths and low slopes for the up and down

load segments. The viscous or creep part of

the deformation is shown by the continuing

penetration at constant load, see the depth

time curve in figure 3a. The plasticity is

given by the closure failure of the cycle.

Numerically, it is the difference between the

actual final depth, and depth projection of

the initial tangent to the down load curve. Diamond turned 0FHC copper exhibits

intermediate micromechanical behavior. Copper

has a 120 GPa modulus.

Because the initial recovery on down loading

is entirely elastic, load-depth curves are used to determine moduli [16]. Anelastic

recovery also occurs, but later in the down

loading. By regression, a straight line is

fitted, but only to the initial portion of

the segment. For a Vickers indenter and

Poisson's ratio equal to 0.33, the modulus is

given by the expression:

E = 0.163(dP/dh)(l/dm )

The tangent to the down load curve is dP/dh,

and typically coincides over the upper half.

The maximum plastic penetration depth is dm. Its value is intercept of the down load

tangent line and the depth axis of the loaddepth curve. The modulus has the units GPa

when the load is given in Newtons and the

depth in micrometers. We have developed an expression for the modulus from ball

indentations [15]. In addition to the

overall value, the modulus can be profiled

against the penetration depth. A series of

load-depth curves are obtained using

different peak loadings.

Hardness number is based on the residual

impression size. Flow pressure, the

instantaneous indentation resistance, is

related to the penetration depth, d, applied

load, P and type of indenter by the

relations:

a = 0.0001253DP/d Ball

28
a = 37.84P/d 2 Vickers

a = 15.81P/d 2 Knoop

D is the diameter of a ball indenter in

micrometers. The units are Newtons for load,

and micrometers for depth.

Flow pressure combines the elastic, plastic,

creep and anelastic deformation components,

but test strategy can separate the

components. Time, shape and direction of

loading are the important factors. By loading

very quickly, creep can be neglected. By

using identical waveforms for the up and down

loadings, the plastic component can be

decanted.

Flow pressure-depth profiles show the

variation of the mechanical properties in near surface regions. It is a subtle

alternative to the load-displacement curve. The underlying assumption is that the near

surface region is not monolithic, but rather

a multilayered or varying structure. If the

measurements assess the state of a material,

flow pressure should have two properties. It

should be a function of depth, and

independent of load. Stated differently,

properties measured from indentations at

different peak loads should lie on a single

master flow pressure-depth curve, and they

do. Figure 4 shows superimposed flow

pressure-depth curves for polished fused

silica. Measurements were made at peak loads

of 0.25, 0.5, 1 and 4N. The three low

loadings are a subset of the peak loading.

RESULTS and DISCUSSION

By example, we will show microindentation is an effective diagnostic tool and useful with

all three material classes. Real time

measurements, and freedom from a residual

impresion are two clear, but still largely

unexplored, advantages. Surface compliance is one example of a real time measurement. We

find we can use these values to gauge the

extent of cracking in brittle surfaces.

BRITTLE MATERIALS: Taylor and Syn, and

Blake and Scattergood have shown that

thickness determines the extent of

subsurface damage in precision machined

semiconductor materials, and so effective

chip thickness is a critical factor [17,18].

Figure 5a show the effect of light and deep

cuts into GaAs. Optical inspection and

profilometry showed no significant difference

between the two. The indentation behaviors

are different. The light cut is a less

compliant more fractured surface. The slopes

of the up and down load segments are steeper, and there is less creep. Figure 6a

reports the mechanical effects of sawing

rather than chemically polishing the surface.

Consider the sensitivity to processing in a second semiconductor. Figure 6b shows loaddepth curves for etched, diamond turned, and

as sawn germanium wafers. Taper sections were

made, and examined with optical and electron

microscopy. Using optical microscopy, no visible differences were found. Immediately

below the diamond turned surface, the

scanning electron micrographs indicated a higher defect density. The mechanical

differences in the germanium were quantified

with moduli measurements. See table 1.

Table 1 Summary of moduli (GPa) of [100]

axis single crystal germanium.

Applied load (N) 1 4

Etched 135 126

Diamond Turned 117 112

Sawed 61 55

Handbook 126 126

Both GaAs and germanium indicate a potential

capability. Differences unseen by

profilometry or optical inspection are detectable with microindentation.

METALS: The observation, that more damage

occurs from finishing rather than roughing

cuts, can be explained using the Ikawa and

Shimida model. It relates extent of plastic

work in ductile materials to the cutting

conditons [19,20]. Ignoring factors such as friction and strain rate, the depth of damage

can be equated to tool sharpness by:

<p = R^exp(a - 0.5)

The ratio of flow pressure to tensile yield

stress is a. R-j is the tool edge sharpness

or radius, and <p is the radius of the plastic

work front about a common center. Consider a new tool making very small depths of cut. The

character of subsurface damage will reflect a change in feed rate. We found inverting the

idea to be a way of evaluating tool wear. Using slow feed rates, 6061 T6 aluminum and

0FHC copper samples were rough and finish

diamond turned. We expected the flow

pressure-depth profiles would indicate the

sharpness. Figure 7a shows the affect of wear on a 0.625 mm nose radius tool. Using

identically machined witness samples, surface

conditions are reported for the tool as delivered and after machining a batch of

nine 62.5 mm diameter OFHC copper mirrors.

Figure 7b shows profiles for a 2.4 mm nose radius tool for the as delivered, and after

cutting 50 miles and 150 miles of 6061

aluminum.

Both tools continued to produce perfectly

acceptable surfaces. The tools appeared free

of edge nicks or other types of catastrophic

29
damage. To inspect the 0.625 mm nose radius

tool, we used Talystep traces across plunge

cuts. The tool edge recession was less than

25 nanometers. Diamond tool break-in, and

perhaps, wear is benign until the edge

starts chipping. Here wear occurs as a small

negative rake land forming between the

clearance and rake surfaces [21, 22]. A

negative rake land would cause increased

subsurface damage. Negative rake would also

increase the feed rate sensitivity. Figure 8

shows the effect of changing surface speed on subsurface damage of diamond turned 6061

aluminum. No effect was found in simultaneous

tests with OFHC copper. This can be explained

by the higher strain rate sensitivity of the

aluminum. Yield strength increases, and

ductility decreases with increasing rate.

GLASS OPTICS: The utility and sensitivity of

microindentation to the damage levels typical

of commercial optics were tested. Three

aspects were considered. First, the effect of

loading rate on the strength of soda lime

glass was found. The results are reported in

figure 9. Because glasses are viscoelastic

materials, their flow properties are strongly

rate sensitive. A modest increase in the

loading rate causes a noticeable increase in

the deformation resistance of the glass.

This sensitivity of glass has implications

for the fabrication of optics.

Second, the sensitivity of microindentation

to the surface condition was probed. For

this, our test piece was an optical flat. The

type of glass is unknown. Figure 10 shows

load-depth curves for the flat as-delivered,

after swab etching in 25% hydrofluoric acid,

and after submersion for 1 hour in the acid.

Photomicrography revealed swab etching

removes surface defects. Indentation showed

swab etching caused a more compliant

surface. After a 1 hour immersion, the

visible defects increased.

Microindentation gave a less compliant

surface. The result is fully consistent with

the finding of Dabbs and Lawn [23]. Who

found brief acid exposures of subcritical

flaws causes a sharp reduction in the crack

initiation load. Initially, the acid enlarges

the cracks. Long term immersion leads to

large but blunt tipped cracks, and the

observed strengthening.

Third, experiments are underway to evaluate

the rate of removal of subsurface damage

during the polishing of fused silica. Figure

11 shows flow pressure-depth profiles for as- ground and polishing times of 2, 8, and 14

hours. Using DIC at 500 magnification, we

found a higher density of visible scratches

in the 2 h sample, but after 2 hours, there

was no visible difference among the samples.

Talystep measurements revealed no systematic

differences among the polishing times.

CONCLUSIONS

Microindentation is a quick convenient means

of measuring the properties of the near surface of precision machined components.

However, further comparisons with other

methods of assessing subsurface damage are

still required.

Instrumented microindentation shows both

ductile and brittle materials are sensitive

to the effects of cutting parameters.

Experiments with glass show strain rate

effects and chemical effects. Acid etching

can cause both weakening or strengthening.

Tool wear and feed rate affect the intensity

of sub-surface damage from single point

diamond turning. A similar effect for depth

of cut may exist. Surface speed effects in

diamond turning of 6061 Aluminum have been

demonstrated.

ACKNOWLEDGEMENTS

The authors are grateful to many colleagues

at the National Institute for Standards and

Technology for helpful comments and

suggestions. In particular thanks are due to

C. Brady (metallurgical preparations), C. Giaque (surface prof ilometry) , and M. Postek

(scanning electron microscopy). Germanium

samples were supplied by T. Dow of North

Carolina State University. F. Garcia at Los

Alamos National Laboratory machined the

aluminum samples on which Fig 7 is based.

Fused silica samples were supplied by L. Lund

of GCA Tropel Corporation.

REFERENCES

[1] Evans C. "Precision Engineering: An

Evolutionary Perspective", M.Sc.

Thesis, Cranfield Institute of

Technology, March 1987.

[2] Wood R.M. "Laser damage in optical

materials" Adam Hilger, 1987.

[3] Hurt H.H. "A Metallurgical

Investigation of the Diamond Turned

Optical Surface" PhD Thesis, University

of Southern California, 1984.

[4] Hurt H.H. "Defects induced in optical

surfaces by the diamond turning

process" in Proc SPIE Vol 525,

Measurement and Effects of Surface

Defects and Quality of Polish, 1985.

[5] Decker D.L. "Tailoring of optical

properties of diamond machined optical

surfaces" Proc SPIE Vol 433 pp 96-103.

[6] Hurt H.H. and Franck J.B. "The effects

of defects on laser damage performance

30
of metal mirror surfaces" 16th Annual

Symposium on Optical Materials for High

Power Lasers, Boulder, CO, 15-17 Oct

1984.

[7] Brown N.J., Fuchs B.A., Hed P.P. and

Stowers I.F. "The Response of Isotropic

Materials to Abrasive Processes" in Proc

ASPE Spring Conference on Sub-Surface

Damage in Glass, Tucson, AZ, April 1989,

pp 66-87.

[8] Hed P.P., Edwards D.F., Davis J.B.

"Subsurface Damage in Optical Materials:

Origin, Measurement and Removal", in

Proc ASPE Spring Conference on SubSurface Damage in Glass, Tucson, AZ,

April 1989, pp 66-87.

[9] Znotins T.A. et al "Excimer lasers an emerging technology in materials

processing", Laser Focus/El ectrooptics, May 1987, pp 54-70.

[10] Bhushan B., Williams V.S. and Shack R.V.

"In-Situ Nanoindentation Hardness

Apparatus for Mechanical

Characterization of Extremely Thin

Films" Journal of Tribology, July 1988,

Vol. 110, pp 563-571.

[11] Yanagisawa M. and Motomura Y. "An

Ultramicro Indentation Hardness Tester

and Its Application to Thin Films"

Lubrication Engineering Jan. 1987, pp

52-56.

[12] Berdikov V.F., Pushkarev 0.1. and

Nazarenho V. A. "Micrqhardness Tester

With Automatic Recording of Indentation

or Scratching Diagram", translated from

Zavodskaya, Vol. 46, No. 5, May 1980,

pp 459-462.

[13] Pethica J.B., Hutchings R. and Oliver

W.C. "Hardness measurements at

penetration depths as small as 20 nm"

Phil Mag A, 1983, Vol 48 No. 4, pp 593-

606.

[14] Polvani, R.S., Ruff A.W. Jr., and

Whitenton E.P. "A Dynamic

M i c r o i nden t a t i on Apparatus for

Materials Characterization", Journal of

Testing and Evaluation JTEVA, Vol 16,

No. 1, Jan. 1988, pp 12-16.

[15] Polvani R.S. and Evans C.J.

"Microindentation Assessment of Near

Surface Material Properties and

Subsurface Damage", Proceedings of the

International Congress for

Ultraprecision Technology, May 1988,

Aachen, FRG. Springer-Verl ag, New York,

NY.

[16] Loubet J.L., Georges J.M. and Meille

G., "Vickers Indentation Curves of

Elastoplastic Materials"

Microindentation Techniques in

Materials Science and Engineering, ASTM

889, P.J. Blau and B.R. Lawn Eds.

American Society for Testing and

Materials 1986 pp 72-89.

[17] Taylor J., Syn C.K. and Donaldson R.R.

"Observations of the brittle-toductile cutting mode transition during

diamond turning tests of single crystal

silicon" 2nd Annual Meeting of the

American Society for Precision

Engineering, Columbus, OH, Nov 1987

[18] Blake P.N. and Scattergood R. "Single

point diamond turning of silicon and

germanium" 2nd Annual Meeting of the

American Society for Precision

Engineering, Columbus, OH, Nov 1987

[19] Evans C, Polvani R. , Postek M., and

Rhorer R. "Some observations on tool

sharpness and sub-surface damage in

single point diamond turning" SPIE Vol

802 Mn-process optical metrology for

precision machining' (1987) pp 52-66.

[20] Ikawa N. and Shimida S. "Cutting tool

for ultraprecision machining"

Proceedings of the 3rd International

Conference on Production Engineering,

Kyoto, July 1977

[21] Donaldson R.R., Taylor J. and Syn C. "Diamond tool wear versus cutting

distance on electroless nickel mirrors:

SPIE Vol 676, "Ultraprecision machining

and automated fabrication of optics",

San Diego, August 1986

'

[22] Hurt H.H. and Showman G. "Wear of a pre-selected diamond tool" SPIE Vol

676, "Ultraprecision machining and

automated fabrication of optics", San

Diego, August 1986.

[23] Dabbs T.P. and Lawn B.R. "Acid enhanced

crack initiation in glass" Journal of

the American Ceramic Society, Vol 65, No

3, March 1982.

31
Figure 1. A simple model for the microstructure in a subsurface damaged metal

OPERATING CAPABILITIES

Applied Load . . . 10" 3 to 5 N

Duration . . . 10"3 to >10+4 s

Depth Resolution .. <0.03 um

Load Resolution .. <0.0003 N

Temperature ... 0 to 100 °C

Indenters ... Vickers, Knoop

and Microball

DRIVER

33f

DE PTH GAUGE

^ Fjg=5

LOAD CELL

INOENTER

SAMPLE

X-Y !rSTAGE

OLDER

MAIN FRAME

Figure 2. A schematic diagram of the main frame showing the

placement of the instrumentation and sample.

32
0-1 0.2 0.3 0.4

Time (seconds)

LOAD + DEPTH

Figure 3a. Load and depth versus time curves for

polymethylmethacrylate (PMMA).

0 2 4 6 8 10

PENETRATION DEPTH (MICRONS)

ALUMINA + COPPER PLEXIGLASS

Figure 3b. Load-depth curves for synthetic sapphire, PPMA and

OFHC copper.

33
-I 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2

PENETRATION DEPTH (MICRONS)

4 N + 1 N 1/2 N A 1/4 N

Figure 4. Flow pressure-depth curves at different peak loads

for polished fused silica.

PENETRATION DEPTH (microns)

DEEP CUT + LIGHT CUT

Figure 5. Load-depth curves for light and deep depths of cut

in gall ium arsenide.

34
CHEMICAL POLISH SAW CUT

W**\ i 'i i I l l I l l I I I I I I I I I 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2

PENETRATION DEPTH (microns)

CHEMICAL POLISH + SAW CUT

Figure 6a. Flow pressure-depth curves for gallium arsenide

prepared by chemical polishing and saw cutting.

ETCHED DIAMOND TURNED SAWED

0 0.4 0.8 1.2 1.6 2 2.4

PENETRATION DEPTH (microns)

ETCHED + DIAMOND TURNED O AS-SAWN

Figure 6b. Load-depth curves for germanium surfaces prepared

with different processes.

35
0.1 -

o H—i—i—i—i—i—i—i—i—i—i—i—i—i—i—i—i—i—i—i

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2

PENETRATION DEPTH (MICRONS)

NEW TOOL + WORN TOOL

Figure 7a. Tool wear effects in the surfaces of diamond turned

copper after machining one, and then eight more pieces.

u "I 1 1 1 1 1 1 1 1 1 1 1

i

1 1 1 1 1 1 1

1 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2

PENETRATION DEPTH (MICRONS)

"NEW" TOOL + < 50 MILES O > 150 MILES

Figure 7b. Tool wear in diamond turned copper witness blanks

after cutting 6061 aluminum for the indicated distances.

36
1 1 1 1 1 1 1 1 1 0 12 3 4

PENETRATION DEPTH (urn)

O 0.5 m/s + 0.075 m/s O 0.045 m/s

Figure 8. Effect of cutting rate on the surface condition of

6061 T6 aluminum samples.

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1

1 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2

PENETRATION DEPTH (MICRONS)

2600 s + 26 s A .026 s

Figure 9. Effect of the loading rate on the strength of

glass. The testing was performed on samples under water.

37
5

AS—RECEIVED

Figure 10. Effects of surface conditioning on the load-depth

curves for an optical flat.

u ~l 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2

PENETRATION DEPTH (MICRONS)

FINISH GRIND +2h 4 h A8h X 14

Figure 11. Effect of polishing time on the flow pressure-depth

curves for fused silica.

38
Manuscript received

2-1-90

AUTOMATED DAMAGE TESTING FACILITY FOR EXCIMER LASER OPTICS

K. Mann and H. Gerhard

t

Laser-Laboratorium Gottingen

3400 Gottingen, FRG

A set-up for automated measurement of damage thresholds on UV optical components

is presented, being part of the EUREKA program "High Power Excimer Lasers". It includes on-line monitoring of probe beam parameters as well as sample condition,

using digital image processing techniques for both laser beam profiling and high

sensitivity damage detection. The latter is performed with a video microscopy system

by pixel-to-pixel comparison of the video frames taken before and after the test

laser pulse.

Along with a description of the experimental details for routine automated data

acquisition, results of damage threshold measurements on bare UV substrates at 248

nm are presented, showing a clear correlation with the amount of absorbed radiation

at this wavelength. On dielectric coatings strong evidence for impurity dominated

defect generation is found.

Key words: UV laser damage, damage testing, threshold determination, spatial beam

profiling, video microscopy, image processing, fluoride crystals, dielectric coatings

1 . Introduction

Since laser induced damage of optical components has become a severe problem for the

development of high power lasers during the last years, great efforts are made to improve the

quality of the employed optics. This requires, however, the use of controlled damage testing

experiments, which not only allow the accurate determination of damage thresholds, but can

also give insight into the fundamental damaging processes [ 1 ]

.

The threshold determination requires the repetitive execution of a sequence of various

measurements and computations, in order to characterize test laser pulse and sample

condition. Hence, an automation of this procedure is advantageous, since it not only minimizes

the possible sources of error but also provides a rapid and unbiased assessment of a sample.

At the Laser-Laboratorium Gottingen such an automated damage testing facility has been

installed for routine measurements on UV optical components. The basic outline of this fully

automated arrangement is described in this paper, together with the results of damage

threshold measurements on various fluoride crystals and high reflection dielectric coatings at

248 nm. In addition, a procedure for a functional damage test on HR coatings is presented.

39
temporal

energy Prol11*

spatial

profile

variable

attenuator

video

microscopy

rc Digital

"** image

Processing

reflectivity

xy-translatlon

He-Ne

scattering

Fig. 1: Experimental arrangement

2 . Experimental

2.1 Probe beam characterization

In Fig. 1 the experimental set-up used for damage threshold determination is shown

schematically, including a sketch of the computer controlled data acquisition system.

An essential part of any damage testing experiment using pulsed laser radiation is the

continuous variation of the laser fluence on the sample, followed by an inspection whether or not the respective pulse has caused damage. In our set-up the fluence can be varied by

means of a dielectric filter with angle -dependent transmission. By rotating this attenuator

with the help of a computer controlled stepper motor the output energy of the employed

excimer laser (EMG 202 MSC ILC, Lambda Physik) and hence the energy density on the

sample can be varied over almost two orders of magnitude. The necessary fluence values for

controlled damage are obtained by focussing the beam onto the sample with a spherical lens.

As indicated in Fig. 1, three diagnostic beams are derived for characterization of the test

laser pulse, i.e. measurement of

40
0 1 2 [mm] 0

Fig. 2: 3D intensity profile of a typical test laser pulse at 248 nm

- pulse energy with a pyroelectric detector

- temporal waveform with a vacuum photodiode

- spatial profile

for each irradiating laser pulse. The spatial intensity distribution in a plane equivalent to the

sample surface is measured with an UV laser beam profiling system, consisting of a special UV

sensitive video camera and a PC-based digital image processing board (frame grabber).

Details of this system are described elsewhere [2]. Fig. 2 shows a 3D plot of the pulse profile

at 248 nm as monitored with the camera. The relatively steep edges are obtained by imaging a

circular aperture onto the camera plane (and hence the target surface) , yielding a rather

flat-topped distribution without occurence of "hot spots". During the measurements the

intensity profile of each pulse is processed in order to calculate the correct fluence on the

sample, taking into account pulse-to-pulse fluctuations of the beam.

2.2 Damage recognition

In previous examinations it has been demonstrated, that from the various techniques applied

for damage detection the microscopic methods are the most sensitive ones [ 3 ] . Hence , in

addition to high magnification Nomarski microscopy (off-line) and He-Ne scattering (which is

used for comparison) in our set-up a video microscopy system is employed for standard online damage detection (cf. Fig. 1): By means of a CCD camera connected to a stereo

microscope the sample surface can be viewed in real time, i.e. during the damaging pulse, on

a video monitor.

Since it has been shown that a further enhancement of the detection sensitivity is possible by

appropriate methods of digital image processing [4], our video frame grabber is used also for

digitization and analysis of the microscopic image. This is accomplished by storage of the

images before and after the damaging pulse is applied, followed by a pixel-to-pixel comparison

and pseudo colour processing of these two data sets. In this way even faintest laser induced

changes on the sample are easily detectable.

41
2.3 Measurement sequence for threshold determination

Damage thresholds are defined as the average of the lowest damaging and the highest nondamaging fluence. The actual values are determined under computer control by the use of a complex measuring program. The various steps of this automated measuring cycle are listed

below:

1. Positioning the sample by a motor driven x-y-translation stage interfaced to the PC,

using cursor keys; simultaneously the site to be tested is viewed on the video monitor

by means of the video microscope

2. Storing the microscopic image in frame memory

3. Adjusting the attenuator transmission (i.e. pulse energy) with a PC triggered stepper

motor

4. Triggering the excimer laser from the PC

5. Measurement of pulse energy, temporal waveform and spatial intensity profile of the

excimer pulse; data transfer to the PC by A/D conversion and IEEE interface

6. Computation of the laser fluence and power density

7. Comparison of the actual microscopic image with the previous image; colouring of altered

pixels

8. Decision damage/no damage

9. Storage of all data

The sequence 1.-9. is then repeated for a new sample position and varied pulse energy, until

the damage threshold has been determined by an iteration procedure.

3. Results

3.1 Fluoride crystals

Since the UV transmission of many fluoride crystals is high, they are commonly used as

optical components for excimer lasers, e.g. as resonator windows. For this reason a set of 6

commercial UV grade fluoride crystals has been tested at 248 nm iff = 25 ns), together with

two chloride crystals and a quartz substrate for comparison. The measurements were

performed in the single pulse ("1 on 1") mode.

In Fig. 3 the results for rear, bulk as well as front surface damage are compiled, showing

that in most cases damage starts on the rear surface. This known effect can be explained by

the higher intensity on this surface due to constructive interference [5]. The highest damage

thresholds are observed for CaF2 as well as MgF2 crystals (F^, = 16-17 J/cm2 ), being in good

agreement with results of Rainer and Hildum (CaF2 : FD = 13 J/cm 2

; MgF2 : Fn = 19 J/cm2

[5]) and Scott (MgF2 : FD= 17.4 J/cm2 [6]) for the same wavelength and similar pulse

duration. On the other hand, Scott has measured for CaF2 a threshold of only 3.3 J/cm2 .

Since for a second CaF2 crystal from another vendor we have also obtained a much lower

value (6.8 J/cm2 ), this deviation is conceivable, indicating a large scatter in quality for the

various samples.

The discrepancy in the CaF2 results was the motivation for a further investigation: Since the

absorption of laser radiation is a preliminary condition for the subsequent damaging process,

42
damage threshold (J/cm*

CaF2 CaF2 MgF2 LIF BaF2 NaF SrF2 S102 NaCl KC1

I rear I I bulk front

Fig. 3: Rear, bulk and front surface damage thresholds measured for various

substrates at 248 nm

20

15

10

Damage threshold (J/cm2

)

CaFa M{jP

a MgFa A A

A

"X 248 nm

A

M0F

2 (used) A

MgFa (used) LIFA

CaF

a A

NaF

A

SrF

a A

BaPa A

A

NaCl

2 9 4 6 6 7

Io- T - R (%) (Absorption)

Fig. 4: Overall damage thresholds of Fig. 3, as a function of absorbed

radiation (cf. text)

43
the dependence between damage threshold F0 and absorption A has been examined. Fig. 4

shows the overall damage threshold of the various fluoride crystals as a function of A, as determined indirectly by subtracting the transmission T (measured at 248 nm with an UVSpectrometer) and the reflectivity R (calculated from Fresnels formula for given index of

refraction) from 100 % (= I 0 )- Obviously, a clear correlation between damage and absorption

is obtained, although in this crude model e.g. scattering losses are neglected. In particular,

the lower damage threshold of one of the CaF2 crystals coincides well with a much higher

portion of absorbed radiation, possibly due to a higher amount of impurities.

Fig. 5: Micrographs of high reflection laser mirror before and after irradiation witn

50 pulses of 1.5 J /cm2 at 248 nm

44
Another interesting result is also compiled in Fig. 4, i.e. the thresholds of 4 different MgF2 crystals. Whereas two new crystals show very similar values (= 16 J/ cm2 ), for two other

samples, which had been used already as resonator windows in a KrF laser, considerably

smaller thresholds are obtained, demonstrating the adverse influence of the fluorine environment.

3.2 HR coatings

Single pulse damage thresholds have also been measured for variuos HR coatings at 248 nm

and 193 nm. The actual values are considerably smaller than for the employed bare Suprasil

substrates (i.e. 1-5 J/cm 2 at 248 nm, < 2 J/cm2 at 193 nm [7]), in good agreement with

results of other authors [4,5].

In any case, damage is dominated by impurities in the dielectric coating, as can be readily

seen from a comparison of the video microscopic images. This is illustrated in Fig. 5, showing

micrographs of the same site of a HR coating before and after irradiation with 50 pulses of

1.5 J/cm2 248 nm) . Clearly, damage starts at defect or impurity sites, which are visible

already before testing. They absorb enough energy from the radiation field, that in their

vicinity the coating is disrupted (inclusion heating model [4,8]). However, although there is

no doubt about the cause of damage in this case, a complete identification (including elemental

analysis) of the damaging (and non-damaging) inclusions is not yet possible. Further work in

this direction is in progress.

4. Functional testing of HR coatings

In a few cases of HR dielectric coatings it has been observed that laser induced changes on

the sample, as seen by video microscopy, do not necessarily correlate with functional failure

of the component. Hence an alternative technique is applied for damage recognition, which is

of somewhat more practical importance for the performance of high reflecting optics, i.e.

measurement of the change in reflectivity of the sample on the laser irradiated site.

This is performed in the following way (cf. insert of Fig. 6): With the help of a beam splitter

the UV radiation reflected from the sample at normal incidence is measured with a pyroelectric

detector. For any laser pulse this signal is related to the corresponding incident energy,

yielding a relative measure for the reflectivity R of the sample. The measurement is performed

at a fluence small compared to the damage threshold, once before and once after the damaging

pulse is applied. Thus the loss in reflectivity ^R on the tested site can be determined. By

proper evaluation of the pyroelectric signals with the help of a digital storage oscilloscope as

well as averaging over many pulses a sensitivity < 0.2 % is achievable.

Fig. 6 shows an example of this technique for a commercial HR coating at 248 nm with an

initial reflectivity > 99 %. Above the damage threshold F^, as measured by video microscopy,

the reflectivity drops drastically with increasing fluence, since increasing portions of the HR

coating are ablated. Obviously, this functional test can be performed in a similar way for the

transmitted signal, e.g. in case of substrates or AR coatings.

45
A R [%]

61

0

6

-

~X 248nm

• 1 on 1

-10 -

16 -

-26 -

20 -

v

0 t 1

30 I I I I

0,1 10

Fluence [J/cm2

]

Fig. 6: Loss in reflectivity of HR coating on laser irradiated site

5 . Conclusion

An automated damage testing facility for UV optics has been presented, which makes use of

digital image processing techniques for both laser beam profiling and damage detection.

Damage thresholds measured at 248 nm for various fluoride crystals show a strong dependence

from the amount of absorbed radiation determined at this wavelength. In addition, by

measuring the loss in reflectivity on the laser-irradiated site, a functional damage test for HR

optics has been proposed.

[1] see e.g. "Laser Induced Damage in Optical Materials : 1987", NIST Spec. Publ. 756

(1987)

[2] K. Mann, H. Gerhardt in: "Laser und Optoelektronik" 4 (1989)

[3] K. Mann, H. Gerhardt in: "Laser Induced Damage in Optical Materials : 1988", to be

published

[4] S.E. Clark, D.C. Emmony, ibid.

[5] F. Rainer, E. A. Hildum in: "Excimer Lasers and Optics", SPIE Vol. 710 (1986)

[6] M.L. Scott, in: "Laser Induced Damage in Optical Materials: 1983", NBS Spec. Publ.

688 (1983)

[7] J. Kolbe, H. Miiller, H. Schink, H. Welling, J. Ebert in "Laser Induced Damage in

Optical Materials: 1989", to be published

[8] T.W. Walker, A.H. Guenther, P.E. Nielsen: IEEE Journ. Quantum Electr. QE-17, 2053

References

(1981)

46
Manuscript Received

5-4-90

Expanded Damage Test Facilities at LLNL*

A. J. Morgan, F. Rainer, F. P. De Marco, R. P. Gonzales, M. R. Kozlowski, and M. C. Staggs

Lawrence Livermore National Laboratory

P.O. Box 5508, L-490

Livermore, CA 94550

We are completing a thorough expansion of the laser-induced damage-test capabilities at LLNL which allow us to conduct tests under a variety of parameter conditions. We

have nine different laser systems which cover the following parameter space:

Laser Wavelength

(nm)

Max. PRF

(Hz)

Duration

(ns)

Peak Fluence

(J/cm 2

) Reptile 1 1064 30 10 65

2/4 532, 266 10 10 40, 25

3 355 10 10 40

Chameleon 3 1064, 532, 355, 266 10 3 >50

10 1064, 532, 355, 266 10 10 >50

Raster Blaster 1064 17.8 8 60

Variable Pulse Laser (VPLJ 1064 Single shot 1-100 >70

Kilroy 1064 6000 65 10

Felix 350, 248 100 20 30

We describe details of the physical configurations of the individual systems, laser and

sample diagnostics, and general testing procedures and capabilities. Damage test results are presented in a companion paper at this conference — "Laser Damage Database at 1064 nm".

Key words: damage; excimer lasers; frequency conversion; laser-induced damage; YAG

lasers.

1. Introduction

Over the course of the last fifteen years, the Lawrence Livermore National Laboratory (LLNL) has

established at least 12 different laser systems with the specific function of conducting studies in laserinduced damage to optical materials. Results from all of these facilities have been reported at the Boulder

Damage Symposium for many years. These systems have included:

• Various Nd:glass or Nd:YAG systems operating at 1064 nm and possibly at one or more of

the second, third and fourth harmonics of the fundamental wavelength;

• Excimer lasers operated at 351 or 248 nm;

• Copper vapor system operated at 500-650 nm (Laser Isotope Separation Laser Demonstration Facility).

* Work performed under the auspices of the U. S. Department of Energy by Lawrence Livermore National

Laboratory under Contract No. W-7405-ENG-48.

47
In addition we have also conducted damage experiments on LLNL's 120-kJ Nova laser (1 to 3 ns at 1053

and 351 nm) and supported the establishment of laser-damage test facilities at Optical Coating Laboratory,

Inc. (OCLI), Spectra-Physics, Inc., and Montana Laser Optics, Inc.

The facilities at LLNL have come and gone as funding and programmatic requirements have

changed. We describe in this paper the facilities that we have utilized or constructed at LLNL in the past

year. Specific test results from these lasers are described in several companion papers at these proceedings.

2. Laser diagnostics

Although each system incorporates slightly different laser diagnostics we have in general standardized our techniques for most of the facilities. In order to fully characterize the interaction of laser beams

with optical materials we need to measure or know the conditions of the following laser parameters at the

sample plane: wavelength, pulse-repetition frequency (PRF), pulse-irradiation sequence, polarization,

angle of incidence, pulse energy, spatial beam profile, peak fluence, pulse duration, and temporal pulse

shape. In figure 1 we show a generalized schematic of a typical damage-test facility with all of the possible

laser and sample diagnostic components. Not all components are necessarily used in each facility. We

shall describe the components in the sequence in which the laser pulse propagates from the laser to the test sample. We shall also inject some of the general philosophies that we have adopted in designing damagetest facilities in recent years.

Polarizer-Waveplate Laser

Attenuator

Figure 1. A typical layout of a laser damage test facility. Not all components are included in each

system. The optical distances AB, AC and ADEF must all be equal in order to diagnose the

laser parameters at the equivalent plane of the test sample.

2.1 Wavelength and PRF

The wavelength and PRF used for a particular experiment are usually determined by the laser system

supplying the irradiating pulses. For the YAG lasers the wavelengths include the fundamental and possible

second, third and fourth harmonics as determined by the frequency conversion crystals we utilize; for the

48
excimer lasers the gas mixture, KrF or XeF, determines which wavelength is obtained. In the case of

frequency conversion we typically employ several multi-harmonic turning mirrors to not only steer the

beam and physically decouple it from the laser but also to discriminate against the propagation of any

unwanted harmonics. We usually maintain separate beam lines for the different wavelength beam paths

until the final mirror position which directs the beam to the focusing lens, diagnostics and sample. At this

final mirror position we mount the appropriate wavelength mirror on a kinematic mount so that we can change wavelengths with a minimum of realignment. The PRF is usually determined by the maximum

operating rep-rate of the laser. This can range from single shots in the case of VPL, to 10 to 30 Hz for most

of our lasers, up to 6000 Hz or more in the case of Kilroy. For most of the rep-rated lasers we can also

operate at lower-than-maximum PRF, or with just single shots, by controlling either the rate at which the Qswitch Pockels' cell is triggered or by simply dialing in a rep-rate. This PRF is confirmed by the pulse

duration diode at low oscilloscope sweep speeds.

2.2 Pulse irradiation sequence

The sequence of irradiating pulses, as well as the PRF, may affect the results of laser damage. Prior

to the development of high power rep-rated lasers we conducted all of our tests with single shots. When

done on a previously unirradiated site such tests were termed as 1-on-l (alternately 1:1 or one shot per one

site). Repeated irradiations in this single shot mode on the same site were termed N-on-1 (N:l). Typically

each shot was done with successively higher fluences which contributed to higher bulk damage thresholds

of frequency conversion crystals. [1] Variations of this improvement in damage thresholds have been

called annealing, pre-conditioning, laser-conditioning, or simply conditioning. [2, 3 and related references

therein]. In the single-shot mode this was a long, painstaking procedure which was improved upon with the use of rep-rated lasers. In a PRF mode we further distinguish our tests by the designation S-on-1 (S:l) if we

irradiate a given site with many shots at the same fluence or by R-on-1 (R:l) if we ramp the fluence up from

near zero to some designated maximum limit in a smooth step-wise fashion. The latter is another means of

conditioning the sample in a matter of seconds rather than hours. Two other means of conditioning the

sample are to scan the sample in one dimension or raster it in two dimensions through a rep-rated beam.

The conditioning is then done by the wings of a typically gaussian-shaped beam wherein the sample is moved through successively higher fluence portions of the beam with each successive shot.

2.3 Polarization and energy control

The laser energy, and hence the laser fluence on the sample, can be controlled by a variety of

techniques such as: (1) changing the number or pumping levels of the laser amplifiers, (2) adding

absorbing or reflecting attenuators to the output beam, (3) changing the spot size on the sample, or (4) using

polarizers and waveplates to discard some fraction of the beam. Each technique has some drawbacks in

application or can change other parameters as well as fluence (pulse duration or shape change, spot size or shape change, loss of too much energy, uniformity of fluence increments, realignment of the irradiating or diagnostic beams, etc.). If our beam quality is good enough we usually employ the polarizer-waveplate

technique. The polarized output of the laser is put through a halfwave plate designed for that specific

wavelength. Rotation of the plate about the beam axis by a controlled amount rotates the polarization by

twice the angle so that the following polarizer then discards some fraction of the beam energy and transmits

a linearly polarized beam (typically P-polarization) of the remainder. This may be followed by another

halfwave plate which can rotate the output beam to any desired linear polarization (e.g. to test sample

polarizers for damage with either P- or S-polarized beams). This technique is easy to employ, provides

continuously variable attenuation, and in the case of the above mentioned R:l irradiation, provides a smooth way to ramp and condition a sample site. The technique has some disadvantages, however. If the

laser is unpolarized then one must first use another polarizer which immediately throws away half the

usable energy. Near maximum extinction and for poorly collimated beams the beam profile usually

degrades into a multi-lobed beam which may prove difficult to analyze for peak fluence. If we do in fact

wish to work with very low fluences we may use an 80% normal reflector in our beam line to discard most

of the energy and thus operate the waveplate-polarizer pair in a lower extinction mode. Alternatively we

may move our focusing lens closer to the sample to increase the spot size and lower the fluence on the

sample. We mount our lens holders on rails or adjacent to guide bars which allow us to move the lens along

the beam axis at will for changing spot sizes without major system realignments. The lens can also be

49
moved away from the sample to raise the fluence to higher levels but with the caveat in mind that small spot

sizes (< 0.6 mm 1/e2

) can lead to inaccurate fluence measurements (intrinsic damage of defect-free

materials rather than actual macroscopic damage in larger areas). The polarization dependence of damage

may also be subject to the incident angle of the laser beam to the sample surface. Typically we mount our samples at 10° incidence to avoid reflecting the beam back into the laser. This usually also provides

sufficient offset so that, for a transmissive sample, the reflection from the rear surface will not overlap with

the incident beam on the front surface. Such possible overlap can in fact produce pronounced interference

effects and higher fluences on the front surface. Our sample stage has a graded rotation platform so that we

can set the incident angle at any desired position from normal incidence to near grazing incidence. Thus,

samples such as 45°-incident mirrors or Brewster-angle optics can be tested at their specified angles. The

laser fluence, however, is always specified as that normal to the beam direction.

2.4 Energy or average power measurement

After our focusing lens we split off a portion of the laser beam with a bare polished, 3°-wedge-angle,

fused silica splitter to diagnose the beam. The splitter is usually 1/3 to 1/2 the distance from the lens to the

sample. This keeps the splitter in a low enough fluence range so that it does not get damaged and yet it still allows sufficient room to accommodate the diagnostic hardware. As shown in figure 1, we set up the

experiment so that the optical distance from the front surface of the splitter (A) to the sample plane (B) is exactly equal to the optical distance from (A) to the calorimeter pinhole (C) and also from (A) to the profiler camera (F). Note that in the latter case the optical distance in question follows the path from (A) to the rear surface of the diagnostic splitter (D) to a second splitter (E) to the camera plane (F). This assures that the

calorimeter pinhole and camera planes are in fact the equivalent planes of the sample. Whenever possible

we orient the two diagnostic beams so that they reflect at equal angles on either side of the incident beam

from the lens. This allows for maximum clearance around the lens and also keeps the angles of the

diagnostic reflections near-normal incidence. Near-normal incidence reduces the effects due to polarization and simplifies calculations for the various harmonics. Knowing the index of refraction of the

diagnostic splitter at the desired wavelength, we calculate the ratio of the energy transmitted to the sample

at (B) and the energy at the calorimeter pinhole (C). We specifically utilize a pinhole at this equivalent

plane in order to truncate our nominally gaussian spatial beam profile near the wings of the profile. We

employ several pinholes of graded sizes in a stainless steel plate which can be translated to assure that the

pinhole size does in fact satisfy this criterion. By measuring only the energy that is transmitted through the

known-size pinhole (referred to as apertured calorimetry) we assure ourselves that our peak fluence

calculation will not be greatly affected by ambiguities between true video signals and noise in the wings of

the spatial profile as recorded by the video beam profiler (see below). The calorimeter may be used either

as a single shot calorimeter or as an average power meter in the case of PRF irradiations. We utilize both inhouse designed calorimeters and commercial power-energy meters. The commercial units employ a black

absorbing surface which is relatively independent of wavelength so that it may be used for all the harmonics

of a particular laser system. We use a lens before the calorimeter (taking into account reflection and

absorption losses) in order to expand the beam to a size which will not damage the absorbing surface and

yet assure that no portion of the beam is clipped by the calorimeter aperture.

2.5 Spatial beam profile

The second diagnostic beam is directed to the beam profile diagnostic. In the past we utilized a variety of techniques developed at LLNL in order to characterize the exact nature of the spatial profile at the sample equivalent plane. These included both exposed film and electronic array beam profilers. [4- 7]

Currently we utilize commercially available beam profilers which provide not only accurate profile data but an assurance of uniformity between our laser systems. We have found that these commercial units do in

fact agree with our prior beam profile techniques. The diagnostic beam is directed to the camera via a second bare fused silica wedge in order to further lower the beam intensity. We then use neutral density

(ND) and/or band-pass filters to lower the intensity to a level high enough to provide sufficient light to the camera without saturating it. We use two types of filters to attenuate the beam over a wide range of

intensities. The first consists of absorbing glass filters mounted in a filter wheel so that we can dial in the

appropriate filter without handling it. These filters were fabricated in neutral-density increments of 0.125

from ND 1 to 1.875 for a fine adjust filter wheel and increments of 0.5 from ND 1 to 3.5 for a course adjust

50
filter wheel. The two wheels together provide a wide ND range in small increments. We avoid rear surface

reflections from the filters by using filters no smaller than ND 1 and take care to avoid interference between

other filter faces. The second type of filter consists of two opposed, linearly-graded, metallic, reflective

plates each ranging from ND 1 to 4 and with rear surface AR coatings. By opposing the direction of the

plates the net attenuation is uniform across the full width of the beam. The beam profiler captures an image

of the beam cross section and then uses the apertured-calorimeter data to calculate the beam profile and

peak fluence in units of J/cm2

. Although the profiler sees and displays the entire beam cross section, it uses only the information from the camera pixels that fall within the defined pinhole size. The calculation does

not depend on knowledge of the transmission of the filters in front of the camera. As long as the filters are not changed the calibration remains fixed. We, however, recalibrate the system with the first shot in each

sequence of irradiations.

2.6 Temporal profile

Finally, we measure the temporal beam profile of the laser pulse with a fast photo-diode. Unlike the

spatial information, the pulse duration is not dependent on which plane it is measured in. Therefore it may

be picked off from one of several different locations. Unless otherwise noted our temporal beam profiles are nominally gaussian in shape although with our multi-mode lasers we may in fact have modulation

within that envelope. We have found that our measurements of damage thresholds agree between different

laser systems independent of modulation for a given pulse duration.

3. Test sample diagnostics

We define damage to a site to be any detectable change in the optical sample after irradiation as compared to that site before irradiation. We utilize a variety of techniques to determine such a change

without prejudging whether the change in the sample has rendered it unusable as an optical component in a laser system. Often damage may indeed be very subtle and have minimal impact on the sample's

usefulness. Usually we try to measure absolute damage thresholds and characterize the damage morphology both at the threshold level and at fluences above that level. We distinguish damage that may occur on

the front or rear surfaces of the sample as well as in the bulk material. In some cases damage in one of these

regions may in fact preclude making an absolute determination of damage in another.

3.1 Nomarski microscopy

Our primary tool in assessing damage is Nomarski microscopy. We have designed and built a sample stage which allows the sample to be mounted at the appropriate angle of incidence for irradiation by

the laser and then be translated directly into the field of view of the Nomarski microscope. The microscope

is typically housed in a clean hood with filtered laminar air flow to keep the sample clean and expel any

emitted debris from ablated coating or substrate materials. We take Polaroid photographs of the site both

before and after irradiation and compare them for any changes. We may also make visual notes of the

appearance of the sample in the microscope since there is some loss of resolution in the photographs.

Typically, we utilize a magnification of lOOx since this provides a photograph which essentially covers the

entire irradiated region of a 1-mm diameter laser beam. We can detect changes but not necessarily resolve

artifacts down to a few |im in size. We also have the capability of examining the sample under 50x, 200x,

500x and lOOOx magnification. Much of our work entails looking at damage to the front surface of the

sample. However, in some circumstances we may extend the Nomarski microscopy to look at bulk damage

or rear surface damage through the front surface with some loss in resolution. Usually this involves a visual

observation rather than site-to-site photography. At lOOx magnification we have a depth of field of about

10 mm. We usually limit any assessment of bulk or rear surface damage to samples with no greater than this

thickness. Our irradiating beam is gradually focused to a position 100 - 500 mm after the sample, typically

using a 2- to 5-m focal length lens. We feel that self focusing and uncertainties in the beam fluence inside

the sample much beyond 10 mm preclude making an accurate assessment of the damaging fluences. Rear

surface damage may also be noted by looking at the irradiated sites by turning the sample around after

completion of the test. Only more obvious damage can be detected this way since we would have no preirradiation photographs to distinguish possible pre-irradiation artifacts from actual damage. Finally we

also utilize the microscope in a non-Nomarski mode by illuminating the sample from on top or by

51
backlighting. Backlighting proves to be an effective tool to detect forward scattered light from small bulk

defects.

3.2 Visual observations of damage

Visual observations are made of each site before and after irradiation using either a bright white

light source or scatter from a He-Ne laser. Subtle changes in a sample which entail no amplitude changes in a surface can in fact be nearly invisible under Nomarski microscopy and yet show up as a general "fogging"

to the naked eye under bright light illumination. We also make visual observations of the sample while

wearing appropriate laser goggles while we are irradiating the sample. Light emission from the sample can

be an indication of damage but it can also indicate the expulsion of volatile components from a porous

surface without any physical change or damage to the surface itself. We note the emission of light but do

not use it as a criterion for damage. An after-the-fact test often used to indicate possible changes in a sample is a "breath test". Water vapor condensation on the irradiation sites can produce pronounced differ- ences in damaged versus undamaged regions that are visible under bright light illumination. The very act of

fogging the sample with one's breath can, however, contaminate the sample for future tests.

3.3 Recorded real-time damage diagnostics

We utilize two other tools to a lesser degree to examine changes in a sample during irradiation and to record the results on video tape for replay. We observe scattered light from a bright light source or He-Ne

laser with a short distance telescope and record this with a CCD camera. The resolution of the video image

is less than that of the Nomarski microscope but it does provide the capability of observing the sample

under magnification during irradiation. Similarly, we have observed the thermal loading of absorptive

surfaces with an IR camera. This is particularly useful under high-PRF irradiations where a sample may not

fail on the first few shots but may fail after prolonged irradiation and heating.

4. Laser facilities

4.1 Reptile

The Reptile facility was so named because of its PRF capabilities and has been our primary system

for conducting laser damage studies since 1986. It was originally intended to provide insights into laser

damage at PRF's up to 120 Hz. Most of our work prior to Reptile was conducted in a single shot mode. We

constructed Reptile with four separate commercial YAG laser systems each of which ran multi-mode at 30 Hz with 16-ns pulses at 1064 nm. We multiplexed the four lasers together into one beam line using

synchronized rotating wave plates to yield a beam at 120 Hz. [7] The system ran in this fashion for six

months until we completed our first round of tests. We determined that except for obvious defectdominated samples there was no difference in damage thresholds measured at 120 Hz as was obtainable

with just one laser at 30 Hz or even even with single shots. Damage may have been more pronounced under

the PRF-irradiation compared to single shot tests, but this stemmed from the number of shots rather than the

PRF. Therefore, the facility was used primarily as a one-laser, 30-Hz facility.

We have recently upgraded Reptile to expand its capabilities at the other harmonics of YAG. This

configuration is shown in figure 2. Two of the lasers each had an amplifier and Faraday rotator installed to yield higher energies for the purpose of providing more efficient frequency conversion. One laser has been

successfully operated as a dedicated third harmonic (355 nm) laser capable of yielding fluences exceeding

45 J/cm2 at the sample plane with a nominal test-spot size of 1-mm diameter. We have also begun tests with

the other laser at the second harmonic (532 nm) with comparable peak fluence outputs. This laser is also

designed to double the second harmonic pulses with a second KD*P crystal to provide us with a fourth

harmonic (266 nm) testing capability. The laser has been modified to yield separate beam lines so that only one kinematically mounted green mirror needs to be moved in order to select the desired wavelength. In order to accomplish sufficient frequency conversion these new lasers are operated at a reduced maximum

PRF of 10 Hz. All of the lasers have been modified to yield pulse durations of 10 ns at the first three

harmonics so that we can conduct wavelength-dependent studies at the same pulse duration. Prior to this

upgrade the Reptile facility was operated at 16 ns.

52
Figure 2. The Reptile damage-test facility utilizes three lasers to generate pulses at the first four harmonics

of Nd:YAG (1064 nm, 532 nm, 355 nm and 266 nm). Most tests are conducted at 10 Hz with 10- ns pulses.

4.2 Chameleon

Chameleon was so named since it was one laser that could readily changes its color to any of the

four first harmonics of Nd:YAG (1064 nm, 532 nm, 355 nm and 266 nm). Figure 3 shows the general

layout of the facility. The system was designed to augment and improve upon the capabilities of the

Reptile facility. The laser is a one-of-a-kind, system designed and built by Quantel International (Continuum) to yield nearly diffraction limited beams with nominally smooth temporal and spatial gaussian beam

profiles. [8] The laser utilizes two separate oscillators to yield both short and long pulse durations. These

are adjustable enough so that each harmonic can be tuned to yield nominally equal pulse durations both at 3 ns and 10 ns. This will allow us the added capability of conducting wavelength scaling and at least a minimal degree of pulse duration scaling. At 10 ns and 10 Hz the laser yields 1064-nm pulse energies > 2 J. We can conduct damage studies at all harmonics with fluences exceeding 50 J/cm2 without having to resort

to spot sizes below 1 mm. We have already begun damage tests with Chameleon at 1064 nm and are prepared to conduct tests at 532 nm and 355 nm as required.

4.3 Raster Blaster

The Raster Blaster laser was originally constructed as a prototype facility for the purpose of

examining Nova laser glass for the presence of damage-causing platinum inclusions. [9] The system was

then replicated so that laser-glass vendors could conduct these tests at their own facilities. The laser is a commercial Nd:YAG unit operating at 1064 nm with 8-ns pulses at a PRF of 17.8 Hz. The system name

stemmed from its initial application when the large Nova glass disks were mounted in a large translation

stage which was then rastered through the laser beam. The laser has subsequently been reconfigured as shown in figure 4. Its primary function is to be used as a resource facility for programmatic damage efforts

without tying up our dedicated damage test facilities. To this end we have recently used the Raster Blaster

in a concentrated study of the improvement in damage thresholds of optical coatings by "laser-conditioning". [2]

4.4 Variable Pulse Laser (VPL)

The Variable Pulse Laser was one of the original damage test facilities built at LLNL. In the course

of 14 years we have used VPL to conduct 3200 damage tests. The system utilized a Nd:YAG oscillator and

preamplifiers and Nd:glass amplifiers to produce very clean, high energy pulses at 1064 nm. The oscillator

generated a nominal 100-ns pulse. The central portion of the pulse was then gated out with a spark-gap

53
1-, 2-, 3- and 4-o)

YAG laser with

3- and 10-ns

oscillators

Kinematic

|

HR

r

Diode

Power Meter

Sample in Nomarski

microscope stage

1

Figure 3. Chameleon employs one laser system with two oscillators to generate 3- and 10-ns pulses at the first four harmonics of Nd:YAG (1064 nm, 532 nm, 355 nmand 266 nm). The laser operates at 10 Hz and

can generate peak fluences > 50 J/cm2 with test-spot diameters > 1 mm at all four wavelengths.

Attenuator Lens

Figure 4. The Raster Blaster is a Nd:YAG laser facility which operates at 1064 nm with 8-ns pulses at 17.8 Hz.

It is currently used to study laser conditioning of optical coatings.

54
9-mm

Pre-amps Attenuator

25-mm

Amplifier

25-mm

Amplifier

Oscillator

Faraday

rotator

<>

Faraday

rotator

Beam

Profiler Calorimeter

Sample in

Nomarski

microscope stage

Figure 5. The Variable Pulse Laser (VPL) was a single shot Nd:YAG-glass laser system which was used for most

of the earlier laser damage work conducted at LLNL. It could generate clean spatial profiles at pulse

durations ranging from 1 to 100 ns. The facility was decommissioned at the end of 1989.

/

Kilroy 6-kHz,1o)

welder laser

)

Attenuator

1

Felix XeF 1

and KrF | excimer |

laser

I *

la Power

^ || Meter

0

Lens

Diode

CD Beam

Profiler

#

IR

Camera

Video

Sample in

Nomarski

microscope

stage

1

Figure 6. Kilroy is a Q-switchedNd:YAG laser used for high PRF damage and absorption measurements at 1064 nm. It can yield fluences up to 10 J/cm2 with 65-ns pulses at 6 kHz. Felix is an excimer laser used for

UV damage measurements at 351 or 248 nm. It can run at 100 Hz with 20-ns pulses.
driven Pockels' cell to yield pulses 1 to 50 ns in duration. Figure 5 shows the laser in its common

configuration although we have at times also added a 50-mm amplifier for higher energies. Since the larger

amplifiers were Ndrglass, we were restricted to operate at a rate of only one shot every 5 to 10 minutes to allow for sufficient cooling. Many of the components were more than 15 years old and required significant

maintenance. The final damage tests on VPL were conducted in December 1989 and the facility was then

decommissioned.

4.5 Kilroy

We constructed the Kilroy facility to extend the range of threshold data for 1064-nm pulses from a PRF of 120 Hz available from the Reptile faclity into the kilohertz regime. This high-PRF work was in

support of LLNL's free electron laser (FEL) program. Rather than multiplexing many lasers together we

opted to purchase a cw Nd:YAG welder laser with three heads. The laser cavity contains four acoustooptical Q-switches which can be synchronized to generate pulses at 500 Hz to 40 kHz. The pulse energy

and pulse duration are a function of the rep-rate. We standardized on 6 kHz as being closest to meeting

programmatic objectives. Figure 6 shows a schematic of the facility which is shared with the Felix excimer

laser. Since Kilroy operates multi-mode we find it necessary to up-collimate the beam by a factor of ten in

order to be able to ultimately focus it down to usable spot sizes ranging from 0.3 to 1 mm. We can achieve

peak fluences up to 10 J/cm2 with the smallest spot sizes. The pulse duration is highly modulated and at this

PRF has an envelope width of 65 ns full-width half-maximum (FWHM). In order to measure peak fluences

at the sample plane we need to gate out most of the pulses going to the beam profiler with a rotating aperture

disk. The disk, beam profiler and laser are synchronized to sample 1% of the pulses at a rep-rate of 60 Hz.

When operated as a cw laser Kilroy can be used as an effective tool in measuring absorption of optical

components at 1064 nm.

4.6 Felix

The Felix laser is a successor to several excimer systems that we have utilized at LLNL to study

laser damage phenomena in the UV regime. The laser can readily be switched from XeF to KrF operation

which allow us to conduct damage tests at 351 and 248 nm respectively. We operate the laser at 100 Hz and

can achieve peak fluences exceeding 30 J/cm2 with 20-ns pulses. The laser shares the facility with Kilroy

and is shown schematically in figure 6.

5. Conclusions

We have nine separate lasers in five facilities at LLNL for conducting measurements of laserinduced damage to optical materials. These systems cover the spectrum from 1064 nm to 248 nm, at pulse

durations ranging from 1 to 100 ns, and at PRF's ranging from single shots up to 6 kHz. The facilities

support the efforts of the Inertial Confinement Fusion (ICF) and Advanced Application programs at LLNL

for the purpose of continuing with our efforts to develop damage resistant optical materials for both

existing facilities and those being designed for future use.

6. References

[1] Swain, J.E.; Stokowski, S.E.; Rainer, F.; Milam, D. Improving the bulk laser damage resistance of

potassium dihydrogen phosphate crystals by pulsed laser irradiation. Appl. Phys. Lett. 40(4): 350-

352; 1982 February 15.

[2] Kozlowski, M.R.; Staggs, M.C.; Wolfe, C.R.; Campbell, J.H. Large area laser preconditioning of

dielectric thin film mirrors. Nat. Inst. Stand. & Tech. (U.S.) Spec. Publ.; (Laser Induced Damage in

Optical Materials: 1989) - to be published.

[3] Wolfe, C.R.; Kozlowski, M.R.; Campbell, J.H.; Rainer, F.; Morgan, A.J.; Gonzales, R.P. Laser preconditioning of optical thin films. Nat. Inst. Stand. & Tech. (U.S.) Spec. Publ.; (Laser Induced

Damage in Optical Materials: 1989) - to be published.

56
Milam, D. Laser-induced damage at 1064 nm, 125 psec. Appl. Opt. 16(5): 1204-1213; 1977 May.

Milam, D. Fluence in 1064-nm laser beams: its determination by photography with Polaroid film.

Appl. Opt. 20(2): 169-171; 1981 January 15.

Smith, W.L.; DeGroot, A.J.; Weber, M.J. Silicon vidicon system for measuring laser intensity

profiles. Appl. Opt. 17(24): 3938-3944; 1978 December 15.

Hildum, E. A.; Rainer, F.; Milam, D. A new average-power damage test facility at LLNL. Nat. Inst. Stand. & Tech. (U.S.) Spec. Publ. 756; 1987 October. 123 p.

Forrest, G.T. Pulsed YAG emits four wavelengths. Laser Focus World: 27; 1990 January.

Weinzapfel, C.L.; Greiner, G.J.; Walmer, C.D.; Hayden, J.S.; Komiya, K.; Kitayama, T. Large scale

damage testing in a production environment. Nat. Inst. Stand. & Tech. (U.S.) Spec. Publ. 756; 1987

October. 112 p.

COMMENTS

Question: I appreciate what you are saying about mechanical properties that

interface with chemical stability and the effects that they have

on optical properties. What can you say about the thermal

characteristics that cross" those boundaries, particularly the

thermal impedance and the heat flow.

Answer: One of the. points that one must begin by talking about is tbat

you've got to look at each particular layer separately. In fact

using our MBE method we do get very enhanced lattice ordering.

What you notice is that these layers are in fact crystalline.

Therefore you are getting a true thermal conductivity in each of

those particular layers. Now if we are getting reasonably good

growth onto each layer, i.e., we have a good interface without to

much dirt there, then we ought to be getting reasonably good

thermal conductivity at that point also. We know at this point

(and in fact a topic that I raised at Boulder about 2 years back)

that we did in fact perform laser damage measurements on a complex

digital structure of that type and compared it with what happened

if we just mixed it and they were both equal. So it looks as though one does have the sort of basic thermal properties.

Question: Did you have an explanation of why the upper layers always

separated in that one electron microgram from the lower layer?

Answer: It looks as though when you grow a II-VI compound on top of, in

fact, anything, be it glass or be it anything else you've got,

you've got S2 dimers which go down on to that particular

interface. These have to move around to meet. Your S2 dimer then

has to actually split up. That chemical process is different for

that particular interface than it is where you have BaF2 molecules

as a whole going down. I think it is because of the basic

difference in the way that the two species nucleate that's the cause of the separation.

57
Manuscript received

4-11-90

Laser Damage Database at 1064 nm*

F. Rainer, R. P. Gonzales, and A. J. Morgan

Lawrence Livermore National Laboratory

P.O. Box 5508, L-490

Livermore, CA 94550

In conjunction with our diversification of laser damage testing capabilities (see

"Expanded Damage Test Facilities at LLNL" this conference), we have expanded upon a database of threshold measurements and parameter variations at 1064 nm. This includes all

tests at low pulse-repetition frequencies (PRF) ranging from single shots to 120 Hz. These

tests were conducted on the Reptile laser facility since 1987 and the Variable Pulse Laser

(VPL) facility since 1988. Pulse durations ranged from 1 to 16 ns. The table below summarizes the test data scaled to 10-ns pulses.

Sample type Number

of tests

Damage thresholds (J/cm 2

) scaled to 10 ns at 1064 nm

Min. Average Max.

AR coatings 164 0.8 19 >56

HR coatings 283 0.7 18 56

Polarizers 47 0.8 8 41

Layers (1 or more, 1 material) 169 0.7 12 34

Metals (bare & enhanced) 49 0.4 6 40

Bare surfaces 226 1.6 26 61

Bulk material 175 0.8 21 61

Key words: anti-reflective (AR) coatings; bare substrates; bulk damage; damage; highly

reflective (HR) coatings; laser-induced damage; metallic coatings; polarizers; reflectors; solgel coatings; thin films.

1 . Introduction

For over fifteen years, the Lawrence Livermore National Laboratory (LLNL) has been actively

involved in the development of damage-resistant optical coatings and materials and in the measurement of

their laser-induced damage thresholds. In the course of that time we have conducted over 10,000 damage

measurements, the results of which have been reported extensively at proceedings of the Boulder Damage

Symposium as well as in technical journals. Typically, these reports have concentrated on a specific topic

with data culled from a large database. In recent years more extensive publication of general databases

have been made available in order to provide an overview of damage measurements covering a wide variety

of materials, fabricated by numerous vendors, utilizing many different fabrication processes, and tested

under countless different laser-parameter conditions. [1], [2]

* Work performed under the auspices of the U. S. Department of Energy by Lawrence Livermore National

Laboratory under Contract No. W-7405-ENG-48.

58
This variety lends itself to the usefulness of a computer-based database but at the same time sets certain limitations to the user. In general there are often more caveats that must be appended to laser

damage measurements than are practical within a tabular database. Hence, results of specific experiments

are reported and elaborated on in journals and proceedings based on a compilation of data. The database

will not of itself allow one to simply select the best optical component meeting a particular requirement

without adherence to many of these hidden caveats. Moreover, the proprietary nature of much of the work

by commercial vendors often prevents a total dissemination of the necessary information required in order

to design or select a particular optical component.

2. Database parameters and conventions

Because of these limitations, the data we present here provide, to a first degree, only an index of

measurements that have been conducted at LLNL during the past two or more years. The database is being

enlarged on a daily basis as measurements are currently being conducted as well as relevant past measure- ments are added as time and program demands require. It must be strongly emphasized that these

measurements do not necessarily represent the state-of-the-art nor necessarily a cross-section of what is achievable in terms of damage thresholds for a particular type of optical component. In virtually all cases,

the data show a high preponderance of thresholds grouped near the lower end of the threshold range. Since

the database does in fact list ail measurements within a particular category, many of the research samples

will naturally show poor performance. Not even the median thresholds should be construed as representing

what one should expect within a particular category. Once we, or the vendors under contract to us, have

developed a product that has achieved acceptably high damage thresholds we typically conduct sufficient

tests to verify that the results are repeatable. Highlights of measurements taken at LLNL in support of highpeak-power lasers are presented in the companion paper at these proceedings. [3]

Comparing measurements taken under a variety of laser parameter conditions is difficult unless one

takes into consideration the effects that the parameters and irradiation conditions may have on damage

thresholds. The data presented here consist solely of recent measurements conducted at 1064 nm with pulse

durations ranging from 1 to 16 ns. The data are always listed with the pulse durations used in the tests. Nearly all samples we have tested show a pulse-length scaling of between 0.2 and 0.5. In order to provide

some capability of comparing these data we have also scaled the thresholds to 10-ns values by a value of

0.35 which is nominally the average temporal scaling factor according to the following relationship:

Damage threshold = k (pulse duration) ° 35.

In table 1 we list the typical test-laser parameters which are based on laser capabilities and

experimental requirements. Descriptions of the laser systems are presented in a companion paper at these

proceedings. [4] Besides absolute damage threshold and laser parameters we also note the the type of

irradiation and damage morphology at each site location as detailed in table 2.

Table 1 . Laser parameters recorded with damage measurements

Pulse duration (ns) 1 - 65 depending on laser

Wavelength (nm) 1064,532, 355,351,266,248

PRF (Hz) 0 (single shot), 1, 6, 10, 15, 18, 30, 60, 100, 120, 6000

Polarization Typically P; also S or mixed

Incident angle 0° to grazing; typically 10°, 45°, 57°

Spot diameter (1/e 2) (mm) 0.3 - 3 depending on required fluence; typically about 1

An abbreviated sample of the computer database is shown in the appendix in table 3. Each test result

is usually printed on one line of a large table in a reduced type format. To display this sample database in a readable fashion we have broken it up into four segments. In tables 4a and 4b we provide detailed information to the user about the variety of samples, techniques and vendors that have already been included in

59
Table 2. Sample irradiation conditions and observations

Site location Front or incident surface

Rear or exit surface

Bulk material within the first 10 mm

Irradiation per site 1-on-l 1 shot only

N-on-1 N shots with increasing fluence, usually on a single

shot basis

S-on-1 S shots in PRF mode at the same fluence

R-on-1 R shots in PRF mode ramped from near zero to the

oesirea maxunum nuence

Scan Sample moved through a PRF beam

Damage morphology Description of damage at threshold at each applicable location

(front, rear, bulk)

Comparable morphology typically at fluences exceeding threshold

to characterize damage growth

the current database. We also supply information on how to interpret the data.

3. Overview of testing at 1064 nm

During the past two years we have have conducted over 1300 laser damage measurements, mostly at 1064 nm with 1- to 16-ns pulses at pulse repetition frequencies (PRF's) ranging from single shots up to 120.

200

E

AR HR Polarizer Layers of 1 Bare Bulk Metal

Material Surface Material

Figure 1. Two-year summary of laser damage tests at 1064 nm with 1- to 16-ns pulses.

An overview of these tests is shown in figure 1. The major portion of our efforts have been concentrated on the development of high threshold solgel coatings (AR's, HR's and single layers), HR's fabricated by physical vapor deposition (PVD), and

optically polished bare surfaces and bulk materials of non-linear crystals and substrate materials. We list below summaries of our tests conducted on seven broad categories of optical materials. For each category

we show a general histogram of all laser damage tests conducted at 1064 nm with pulse durations scaled to 10 ns. Depending on the category type we also list applicable details such as substrate materials, coating

60
materials, process types, and vendors of coatings, bulk materials and surface processing. Since these tests cover such a wide variety of optical components, material combinations, processes and proprietary data, we

do not itemize these details in most histograms. Related figures for each category show pertinent important

highlights that are germane to the programmatic laser development efforts at LLNL.

4.1 Bare polished surfaces

We record bare surface damage thresholds of most substrate materials that we test for bulk damage

as well as for samples which may have an AR coating on one surface but not the other. We observe rear surface damage if the results are not obscured by either front surface or bulk material damage. Typically

well-polished bare surfaces have among the highest thresholds that we measure and are usually only

exceeded by bulk damage to some materials. In figure 2 we show the aggregate test results of all bare

surface measurements in recent years for over 20 different substrate types from over 20 different vendors.

The vendor supplying the substrate was, however, not always the one who actually performed the surface

processing. Most samples were lap polished but we also list the other surfacing techniques that we have

investigated.

We have found that for several of the more commonly used substrate materials with refractive

indices near 1.5, the optimized polishing techniques have yielded 1064-nm damage thresholds which scale

relatively independent of the material types by a pulse-duration scaling factor of x

04

. This is demonstrated

in figure 3 for six materials (fused silica, BK-7, CVD glass, ULE glass, LG-750, and fluorophosphate glass)

at seven different pulse durations ranging from 150 ps up to 40 ns. These measurements are average values

of the best results obtained over a span of more than 12 years from at least six different laser systems.

4.2 Bulk materials

In figure 4 we summarize the measurements we have conducted on over twenty different types of

bulk materials including non-linear crystals, laser host materials and substrates. Many of the entries at the

higher fluence ranges actually represent lower limits of the thresholds. This was either because we ceased

irradiation when massive damage occurred to the bare or coated surfaces or because we could not extract

higher fluences from the laser. Our laser irradiation in the bulk material is conducted with a gently focusing

beam using 2.5- to 5-m focal length lenses. We typically limit ourselves to a test volume over which the

beam fluence changes < 1%. We also find it useful to work within a depth of field which can be readily

examined by Nomarski microscopy. Finally, within the constraints of the available laser fluence, we

attempt to utilize as large a cross-section beam as possible. By this means we determine the threshold of a macroscopic volume of material with potential microscopic defects rather than the intrinsic threshold of a defect-free material. We irradiate with beams on the order of 1-mm diameter for a depth not exceeding 10

mm.

Of particular interest to us at LLNL is the improvement in damage thresholds of frequency

conversion crystals to levels approaching those of substrate materials such as fused silica. We have

pursued the development of a variety of high threshold materials such as deuterated and undeuterated

potassium-dihydrogen phosphate (KDP) and £-arginine phosphate (LAP). In 1981 we reported increases

in bulk thresholds of KDP crystals by pre-irradiation of the material with a succession of sub-threshold

laser shots. [5] This laser conditioning has been reported at these proceedings for several years both for

non-linear crystals and optical coatings. Typical results of continued improvement are shown on the right

half of figure 5.

4.3 High reflective (HR) coatings

Figure 6 summarizes the tests that we have conducted in recent years for a variety of HR coating

techniques from 15 different vendors We list the assortment of materials used both for substrates and the

coatings. All of the coatings consisted of muli-layer stacks of two or more materials. We list only the

unique individual materials since the variety of material combinations is too numerous to elaborate upon.

In this figure we combine the results of tests on single wavelength, multi-wavelength, and partially

reflecting mirrors. 61
1064 nm

10-ns pulses

T

0 35 scale factor

40 -I

35

30 -

25 -

Number 20

of tests 15

SURFACE SUBSTRATE SUBSTRATE

PROCESS MATERIALS VENDORS

CVD 7940 LAP Airtron Diamond turn AI203 LG-750 AT&T

Drawn tube B203 Li 1 03 China

Ductile grind BK-7 LiNb03 Cleveland Crystal Etch CaF2 Quartz Coherent

Ion mill DLAP SlC Sl02 Corning

Lap polish Ge02 SrF2 Crystal Technology

PCVD Glass Suprasil CVD

Heralux TGG Deposition Sciences

wmmm KD*P ULE France TUo Harshaw

KTP Zr02 Heraeus Hoya

' Inrad LLNL

Optovac

Osaka

Schort Union Carbide

Zygo

Unspecified

10 20 30 40 50 60

Laser damage threshold (J/cm )

Figure 2. Summary of laser damage tests conducted on bare polished surfaces.

100

• Fused Silica (S,M,R)

o BK-7 (S,M)

CVD (R)

ULE (R)

LG-750 (R)

A Fluorophosphate (S)

Laser damage

threshold (J/cm 2

)

10

1064 nm

&5

i

0.4

(S) S. Stokowski, NBS 541 , 99 (1 978)

(M) D.Milam, NBS 541, 541,168 (1978)

(R) F. Rainer, NIST (1989)

10 100

Pulse duration (ns)

Figure 3. Damage thresholds of optimized polished surfaces scale as x

04 between 150 ps and 40 ns and are independent of glass type for a variety of common laser glasses having refractive

indices near 1.5. 62
1064 nm

10-ns pulses

T

0 35 scale factor

30

25

20

Number 15 J

of tests

10

5 -

SUBSTRATE MATERIALS SUBSTRATE VENDORS

7940 LAP Airtron Harshaw

B203 LG-750 AT&T Heraeus BK-7 LM03 China Hoya

CaF2 LiNb03 Cleveland Crystal Inrad DLAP M-16 Coherent Kigre

Ge02 Quartz Continental LLNL

Glass Si02 Corning Optovac

Heralux SrF2 Crystal Technology Osaka

KD*P Suprasil cn Schott

KDP TGG Deposition Sciences Union Carbide

KTP YOS France Unspecified

10 20 30

Laser damage threshold (J/cm )

Figure 4. Summary of laser damage tests conducted on bulk materials.

60

50 -

40 -

Damage

Threshold 30

(J/cm 2

) at 1064 nm

20 -

10 -

E-beam high reflectors

16-ns pulses

Titania

Silica

Hafnia

Silica

Zirconia

Silica

Nonlinear crystals

10-ns pulses

HI After conditioning

|~| Before conditioning

KDP

Bulk

DLAP

Bulk

Figure 5. Pre-irradiation of PVD-fabricated HR coatings and non-linear crystals with sub-threshold laser pulses (conditioning) increases their damage thresholds above unconditioned

thresholds.

63
We are investigating three major options for improved damage thresholds in HR coatings. These

include laser-conditioned PVD coatings, sol-gel coatings, and plasma assisted chemical-vapor-deposition

(PCVD) coatings. The latter two techniques and their results are reported upon in companion papers at this

conference. [6], [7], [8] Examples of typical improvements in damage thresholds by laser conditioning to PVD HR coatings are shown on the left side of figure 5. Improvement usually has been found to range

from 1.5 times to greater than 3 times unconditioned thresholds. We have found that the degree of improvement was often dependent upon the number and sequence of sub-threshold irradiation shots. Once conditioned the samples were found to retain their elevated thresholds permanently. [9], [10]

Number

of tests

1064 nm

10-ns pulses

X

0 35 scale factor

70 -I

60

50

40

30

20

10

VENDORS

Continental

CVI

Deposition Sciences

Laser Optics

Laser Power Optics

LLNL

Limeil

Matra

NRL

OCLI

Schott

Spectra Physics

Spindler/Hoyer

TecOptics

Trans World Optics

PROCESSES

CVD

E-beam PVD

PCVD

Plasma PVD

Solgel

Sputter PVD

Thermal PVD

SUBSTRATES

BK-7

Glass

Heralux

Phosphate glass

Quartz

SiC

Si02

Zerodur

Unspecified

10 20 30 40

Laser damage threshold (J/cm )

Figure 6. Summary of laser damage tests conducted on HR coatings.

MATERIALS

AI203

AIF2

CaS04

Ge02

Hf02

PbF2

Si02

Si02(F)

Si02(Ge)

SiOxNy

Ta205

Ti02

Y203

Zr02

Unspecified

i 60

4.4 Anti-reflective (AR) coatings

Figure 7 summarizes the extent of our recent AR coating testing. The bulk of this represented

testing of research samples both for LLNL as well as outside vendors. Hence, many of the early results are concentrated at the lower threshold range. As with the case of the HR coatings we list both the variety of

substrate and coating materials but do not elaborate on all of their combinations.

The major emphasis in this category has come from the development and implementation of the solgel process to produce damage-resistant AR coatings for fused silica substrates and lenses ranging up to 1- meter diameter and for large area arrays of KDP crystals for frequency conversion. Both of these

applications have been extensively implemented on LLNL's Nova laser system. The extent of improvement in this technology in just the last few years is demonstrated in figure 8. The shaded portions of these

histograms show all high threshold test results conducted in 1989 versus the best results (unshaded)

obtained for the previous year. Blocks of tests with an arrow in them indicate that the sample thresholds were at least as great as the levels shown in the figure. Few tests were conducted at 355 nm since that

testing capability had just been recently brought on line. Production of sol-gel AR coatings has become

routine and reliable enough so that very little future testing of them is anticipated.

64
1064 nm

10-ns pulses

X

0 35 scale factor

Etch

E-beam PVD

Solgel

Number

of tests 15

10

VENDORS MATERIALS SUBSTRATES

Cleveland Crystal AI203 BK-7

CTI CaF2 CaF2

Inrad Hf02 DLAP

Limeil Methyl FR5

LLNL Silicone KD*P

OCLI - MgF2 KDP

Union Carbide Silicone LAP

Virgo Siloxane LiNb03

Si02 M-16

Ta205 Quartz

Ti02 Si02

Zr02 YOS

Unspecified

10 20 30 40 50 60

Laser damage threshold (J/cm )

Figure 7. Summary of laser damage tests conducted on AR coatings.

Number

of tests

Figure 8. We have made significant improvements in laser damage thresholds of the best sol-gel

AR coatings measured during the past year compared to those of the previous year.
1064 nm

10-ns pulses

X

035 scale factor

30 i

25

20

Number 15

of tests

10

5

10 20 30

VENDORS

Burleigh

Continental

CVI

LLNL

Matra

OCLI

Spectra

Physics

TecOptics

ZC&R

Unspecified

40

Laser damage threshold (J/cm )

Figure 9. Summary of laser damage tests conducted on polarizers.

50

PROCESSES

E-beam PVD

Solgel

Sputter PVD

MATERIALS

Hf02

Si02

Ta205

Ti02

Zr02

Unspecified

60

60

50

40

Laser damage

threshold (J/cm 2

) 30

20

10

H After laser conditioning

Before laser conditioning

B C D

Test sample

Figure 10. Laser conditioning of commercial PVD-fabricated polarizers improves their damage

thresholds by an average factor of two.

66
4.5 Polarizers

Multi-layer stack polarizers are typically fabricated with more complex coating designs using more

layers than those used for comparable HR coatings. Unlike HR coatings, the coating-substrate interface of

polarizers sees the full intensity of the transmitted laser beam. By their very nature, polarizers will also

have greater angular sensitivity to each polarized component of the laser beam. These characteristics have

usually combined to yield among the lowest thresholds for multi-layer coatings as is shown in figure 9. We

have not conducted as many tests on polarizers in recent years as on AR and HR coatings but the

distribution of thresholds at the lower fluence range of this figure is typical of polarizers. We have,

however, recently also conducted conditioning tests on a variety of samples from several vendors. In figure

10 we show that, as with HR coatings, we can expect to find a significant improvement in thresholds by

implementing laser-conditioning. From a limited database of conditioned polarizer tests we have observed

average rises in thresholds of about a factor of two.

4.6 Single material coatings

Much of the research that we and many of our vendors have done in the development of multi-layer

coatings began with testing of single layers or multiple layers of the same material by all of the standard

deposition processes. In figure 1 1 we show the aggregate result of these tests on 25 different materials

applied by PVD, CVD or sol-gel processes. These coatings were often half-wave thick layers at 1064 nm

but not exclusively so. We list most of these materials specifically in figure 12 to illustrate the spread that one can expect in damage thresholds. One cannot specifically use these data in order to pick optimum

material combinations for fabricating high-threshold multi-layer AR's, HR's or polarizers. This database

represents a compilation of many research and development samples with a great variety of deposition

parameters including process type, thickness, number of layers, cleanliness, substrate type, and deposition

conditions.

4.7 Metal mirrors

Of all of the commonly used optical components in the laser industry, metal mirrors have consistently yielded the lowest damage thresholds. From a limited database we had reported thresholds no higher

than 4 J/cm2 for 16-ns pulses at 1064 nm. [1] The higher thresholds were usually obtained by enhancing the

bare metal substrate or plated metal coating with a dielectric overcoat or a multi-layer dielectric HR stack.

This produced a rise in threshold by a factor of two at best. A summary of those earlier tests and more

recent tests shows a small database in figure 13. We have seen some minor improvement in bare metal

thresholds but in general they still lie below 5 J/cm2 at 1064 nm for 10-ns pulses. We attributed the failure

of HR-overcoated metal mirrors to two factors. First of all, possible pinholes in the dielectric HR would

allow energy to propagate through the HR stack to the metal itself. Secondly, defects on the metal could

propagate corn growth on subsequent dielectric layers so that they do not behave as true HR's where the

defects print through. With dielectric substrates any leaked energy through the HR stack would be

dissipated by transmission through the substrate. A metal coating or substrate would, however, trap this

leaked energy at this interface and cause catastrophic failure to the metal. Recent coatings that have been

fabricated for us appear to have yielded superior dielectric HR stacks as is shown in figure 14. Although

damage thresholds have improved for bare molybdenum substrates, print-through of defects from the

substrates through the HR still caused thresholds to be low. However, when Mo coatings were plated on

defect-free silicon or fused silica we found thresholds to equal those of the same HR's deposited on fused

silica alone.

5. Conclusions

We have expanded upon our database of laser-damage tests at LLNL to include over 1 300 tests conducted at 1064 nm in the last few years. This represents a small fraction of over 10,000 tests that we

have conducted during the past 15 years. We have used a variety of laser systems to enable us to study the

effects of spot size, pulse duration, PRF, polarization and incident angle. As time permits this database will

be expanded to include relevant past data as well as all current testing. In addition, the database is currently

being expanded to include tests at the second, third and fourth harmonic of 1064 nm.

67
1064 nm

10-ns pulses x

0 35 scale factor

45 i

40

35

30

25

Number

of tests 20 •

15

10

VENDOR MATERIALS

Arizona State U. AI203 Nb205

Deposition AIF2 PbF2

Sciences BaF2 PbO

Limeil BiF3 Siloxane

1 1 Ml CaF2 SiN

OCLI CaS04 Si02

Papifif* • Bvl 1 lw W^Qfprn Ifvdlwl 1

CeF3 Si02(doDed)

Schott DyF3 SiOxNy

GE02 Ta205

Hf02 Ti02

PROCESSES LaF Triton -X

LiF Zr02

CVD MgF2

E-beam PVD

PCVD

Solgel

Thermal PVD

10 20 30 40

Laser damage threshold (J/cm )

50 60

Figure 1 1 . Summary of laser damage tests conducted on single or multiple layers of the same

material.

(0

<n

E

3

25

20

15

10

M AIF2 M AI203 H BaF2 BiF3 H CaF2 0 CaS04

m CeF3 0 DyF3 S Hf02 CO LaF LiF S MgF2

S Nb205 0 PbF2 S PbO Q] Si02 5 Si02(B) B Si02(Ge)

SiOxNy M Ta205 Ti02 Zr02

D H[|D|jQi!ggII

1064 nm

10-ns pulses

1

0 35 scale factor

rrrrrrrrrrri Lji'-' i Ea ril==li

l==l

i i i i n i i i i i i i

12 16 20 24 28 32 36 40

Laser damage threshold (J/cm )

44 48 52 56 60

Figure 12. Ranking of laser damage thresholds for a variety of single material coatings fabricated

by PVD, CVD and sol-gel processes (except single-layer silica sol-gel AR's which are

listed under the AR category).

68
Number

of tests

1064 nm

10-ns pulses

T

0 35 scale factor

SURFACE

PROCESS

Diamond turn

Dip

E-beam PVD

Electroplate

Epitaxy

Lap polish

VENDORS

CVI

Deacon

Epner Tech

LaserPower

LLNL

Newport

OCLI

SPAWR

Spectra Physics

UTOS

Wisotzki

MATERIALS

Ag

Al

Au

Cu

Hf02

MgF2

Mo

Si02

Ta205

Ta205

Ti02

Zr02

SUBSTRATES

Cu

Mo

SiC

Silicon

Si02

Zerodur

10 20 30 40 50 60

Laser damage threshold (J/cm )

Figure 13. Summary of laser damage tests conducted on bare and enhanced metal mirrors.

40

30.

Damage

Threshold 20

J/cm 2

HR COATING DESIGNS

Zirconia/silica

[gj Titania/silica/hafnia

m Tantala/silica

Mo Mo

plus

Mo precoat

Silicon

plus

Mo precoat

Fused silica

plus

Mo precoat

Fused silica

SUBSTRATE

Figure 14. Metal mirrors enhanced with pinhole-free dielectric HR stacks can have thresholds

comparable to those of the stacks on dielectric substrates. The arrows indicate that the

thresholds for those particular samples were less than the minimum test fluences shown.

69
We have broken our tests down into seven broad categories of optical samples including AR's,

HR's, polarizers, single and multiple layers, metals, bare surfaces, and bulk material. We, or our vendors,

have achieved 10-ns-normalized thresholds that reach or exceed 40 J/cm2 in all of these categories . This

has been accomplished by a variety of techniques worked on by LLNL and by our vendors including laser

conditioning, sol-gel coatings and PCVD coatings.

The database provides us with an effective tool in being able to cull information from many test covering a large parameter space of sample fabrication and laser testing techniques. It is, to a first degree, an interactive computer tool rather than just a printed list of test data. To that extent it has a somewhat

limited use when simply printed as a whole. The value of the database stems from our ability to be able to sort and search from a high volume of data. Unfortunately we are currently not at liberty to be able to publish a detailed database for general external use. To first order the database represents primarily

research in optical component development by LLNL and its vendors. It does not guarantee to include all vendors and processes or even the best ones. At a later date we may be able to provide for distribution a cross-referenced database which approaches such a goal but which must of necessity be limited by

proprietary information from our vendors.

6. References

[1] Rainer,F.; Hildum, E. A.; Milam, D. Database of average-power damage thresholds at 1064 nm.

Nat. Inst. Stand & Tech. (U.S.) Spec. Publ. 756; 1987 October. 410 p.

[2] Seitel, S.C. Laser damage test handbook and database of Nd: YAG laser optics. Bozeman, Montana:

Montana Laser Optics, Inc.; 1988.

[3] Rainer, F.; Brusasco, R. M.; Campbell, J. H.; De Marco, F.P.; Gonzales, R.P.; Kozlowski, M.R.;

Milanovich, F. P.; Morgan, A. J.; Scrivener, M. S.; Staggs, M. C; Thomas, I. M.; Velsko, S. P.; Wolfe, C. R. Damage measurements on optical materials for use in high-peak-power lasers. Nat.

Inst. Stand. & Tech. (U.S.) Spec. Publ.; (Laser Induced Damage in Optical Materials: 1989) - to be

published.

[4] Morgan, A.J.; Rainer, F.; De Marco, F.P.; Gonzales, R.P.; Kozlowski, M.R.; Staggs, M.C.

Expanded damage test facilities at LLNL. Nat. Inst. Stand. & Tech. (U.S.) Spec. Publ.; (Laser

Induced Damage in Optical Materials: 1989) - to be published.

[5] Swain, J.E.; Stokowski, S.E.; Milam, D.; Rainer, F. Improving the bulk laser-damage resistance of

KDP by baking and pulsed laser irradiation. Nat. Bur. Stand. (U.S.) Spec. Publ. 638; 1981

November. 119 p.

[6] Thomas, I.M. High damage threshold A1203 »H3 0-Si02 HR coatings prepared by the sol-gel process.

Nat. Inst. Stand. & Tech. (U.S.) Spec. Publ.; (Laser Induced Damage in Optical Materials: 1989) -

to be published.

[7] Campbell, J.H; Emmett, J.L.; Brusasco, R.M.; Rainer, F. Deposition of damage resistant optical

coatings using high temperature, plasma chemical-vapor-deposition. Nat. Inst. Stand. & Tech.

(U.S.) Spec. Publ.; (Laser Induced Damage in Optical Materials: 1989) - to be published.

[8] Brusasco, R.M.; Britten, J.A.; Thorsness, C.B.; Scrivener, M.S.; Unites, W.G.; Campbell, J.H. A

high-temperature, plasma-assisted chemical-vapor-deposition system. Nat. Inst. Stand. & Tech.

(U.S.) Spec. Publ.; (Laser Induced Damage in Optical Materials: 1989) - to be published.

[9] Wolfe, C.R.; Kozlowski, M.R.; Campbell, J.H.; Rainer, F.; Morgan, A.J.; Gonzales, R.P. Laser preconditioning of optical thin films. Nat. Inst. Stand. & Tech. (U.S.) Spec. Publ.; (Laser Induced

Damage in Optical Materials: 1989) - to be published.

[10] Kozlowski, M.R.; Staggs, M.C; Wolfe, C.R.; Campbell, J.H. Large area laser preconditioning of

70
dielectric thin film mirrors. Nat. Inst. Stand. & Tech. (U.S.) Spec. Publ.; (Laser Induced Damage in

Optical Materials: 1989) - to be published.

7. Appendix

Table 3. Sample of a computer-generated database table. The table has been

broken into four parts to aid in presentation

1\

I TEST

#

8| TEST

Date

SAMPLE OTHER

# ID #

COATING (or Bare, or Bulk)

Vendor Type Process Materials

R

R

R

R

R

R

508 A

508 B

508 C

508 D

508 E

508 F

7/13/89

7/13/89

7/13/89

7/13/89

7/13/89

7/13/89

A279B

A279B

A279B

A279B

A279B

A279B

1-YOS-4#2

1-YOS-4#2

1-YOS-4#2

1-YOS-4#2

1-YOS-4#2

1-YOS-4#2

L-Thomas

L-Thomas

L-Thomas

L-Thomas

L-Thomas

L-Thomas

AR1

AR1

BU LK

AR1

AR1

BULK

Solgel

Solgel

Solgel

Solgel

Solgel

Solgel

AIO(OH)

AIO(OH)

AIO(OH)

AIO(OH)

AIO(OH)

AIO(OH)

SUBSTRATE LxW(or D)xT LASER PARAMETERS

Type Source Polish by Process mm nm ns Hz pol o mm

YOS Vendor L-Prochnow Lap 23dx7 1064 10 10 P 10 1.1 YOS Vendor L-Prochnow Lap 23dx7 1064 10 10 P 10 1.1 YOS Vendor L-Prochnow Lap 23dx7 1064 10 10 P 10 1.1 YOS Vendor L-Prochnow Lap 23dx7 1064 10 10 P 10 1.1 YOS Vendor L-Prochnow Lap 23dx7 1064 10 10 P 10 1.1 YOS Vendor L-Prochnow Lap 23dx7 1064 10 10 P 10 1.1

1 i

THRESHOLD

Loc J/cm2

MORPH. (thresh />thresh) Shots/Site

Meth. #

REPORTS

M \i # M u # @

F 38.2 ±5.7

R > 20.9

B 17.9 ±2.7

F 43.1 ±6.5

R > 47.1

B 43.6 ±6.5

M 999 1 M 999 1 A ? 2

A 5 1 N

A ? 2

M 999 1 34

M 999 1 21

N 47

N 47

A ? 1 47

S/1 30

S/1 600

S/1 600

R/1 600

R/1 600

R/1 600

LDG89-117

LDG89-117

LDG89-117

LDG89-117

LDG89-117

LDG89-117

HH

SAMPLE DESCRIPTION & COMMENTS

Before test 0 After test

10-ns

thresh

Nd3+:Y2Si05 crystal 0 Massive R&B dmg @ lower fluences

Nd3+:Y2Si05 crystal 0 Massive R&B dmg @ lower fluences

Nd3+:Y2Si05 crystal 0 Massive R&B dmg @ lower fluences

Nd3+:Y2Si05 crystal 0 Slight improve w/anneal

Nd3+:Y2Si05 crystal 0 No dmg

Nd3+:Y2Si05 crystal 0 Clean B areas survived above thresh

38.2 > 20.9

17.9

43.1 > 47.1

43.6

71
Table 4a. Explanation of database table entries

1 TEST numbers are prefixed by a LETTER which signifies the test laser or facility: A — Air Force XeF Excimer

B — Raster Blaster C — Cyclops

D — Comparative Damage

F — Felix XeF Excimer

H — Chameleon

I — Isotope Separation

K — Kilroy L — ILS M — Montana Laser

N

O

R

V

X

Nova

OCLI

Reptile

VPL

KrF Excimer

TEST NUMBERS follow chronologically for each laser.

LETTER SUFFIXES distinguish separate results obtained under the same test number such as location: front, rear surface, or bulk material; irradiation type: conditioned or unconditioned; etc. DATE when test was begun — entries are in chronological order. 3 SAMPLE NUMBER (hopefully unique) assigned by us or the materials development group.

Any OTHER ID number that came with the sample (often not unique). Very long numbers may be partially hidden in print-outs but are extractable from the computer database.

Coating VENDOR; further numbers identify subtasks or runs from a particular vendor. If the "vendor" is an LLNL

employee his name is preceeded by L-. For a bare surface, the vendor who polished or treated the surface is listed. For

bulk material tests we list the front surface parameters in this section. The following surface vendors have been

catalogued to date:

Airtron Deacon Laser Optics

Ariz.U. Dep.Sci Laser Power

Burleigh EpnerTech Limeil Cleveland France LLNL

Continental Harshaw Matra

Corning Hoya NBS

Crystal Tech Inrad Newport

CVD Kodak OCLI

CVI Labsphere Optovac

PacificWestS.

Rochester

Schott

Shandong

SPAWR

Spectra Physics

Spindler/Hoyer

TecOptics

Thin Film Coat

Tinsley

Trans World Optics

UnionCarbide

UTOS

UofNM

Virgo

Wisotzki

ZC& R

Zygo

Optic TYPES are listed by the following general categories:

AR — antireflective coating — following numbers indicate for which harmonics the design applies:

1=1w (1064,1053 nm), 2=2w (532, 527 nm), 3=3w (355, 351 nm) 4=4w (266, 263, 248 nm)

BARE — bare surfaces which may have been polished, turned, extruded, etched, cleaved, etc., but without any

applied coating.

BULK — bulk material as opposed to bare surface or coating.

HR — high reflector or partial reflector — following numbers indicate harmonics (see AR above).

LA — layer(s) of only one material — following numbers indicate number of such layers

MET — bare or deposited metal surface — following HR or OC indicates the metal is covered by a dielectric HR

stack or a dielectric overcoat respectively.

MISC — miscellaneous such as liquid, powder.paint.cement.

POL — polarizer—following numbers indicate for which harmonics design applies (see AR above).

Coating PROCESSES cataloged to date are listed below. For bare surfaces some of these entries are the same as in the substrate section. CVD Epitaxy PCVD PVD-plasma

Dip Misc Powder PVD-sputter

Electroplate Paint PVD-ebeam PVD-thermal

Solgel

MATERIAL combinations have not all been catalogued consistently. In general, multiple layers are

listed with a /between them. Dopants or mixtures are represented by { }. For cataloguing purposes a common material such as Si02 is always listed last. UC = undercoat, OC = overcoat. Ag Cu MgF2/AI Si02{l}/Si02{h} Ti02/Triton-x Al/Si02 DLAP Mo Si02{N} Ti02/Hf02/Si02

Al/Ti02/Si02 DyF3 Nb205 Si02{N}/Si02{N'} Ti02/Hf02/Si02/Mo

AI203 Ge02/P205/Si02 Nd3:Y2Si05 Si02{P} Ti02/Siloxane

AI203/AIF2 Ge02/Si02 Opal Si02{Ti} Ti02/Si02

AI203/Si02 Glass PbF2 Si02{Ti,F}/Si02{F} Ti02/Si02/OC

AI203/Ti02/Si02 Heralux WG PbF2/AIF2 Si02{silicone} Ti02/Si02/UC

AIF2 Hf02 PbF2/CaS04 Si02/silicone Ti02/Si02/Hf02

AIO(OH) Hf02/Si02 PbF2/Si02 Si02{siloxane} Ti02/Si02/Zr02

AIO(OH){Si02} KD*P PbO Spectralon ULE

AIO(OH)/Si02 KDP Phosphate SrF2 Zr02

AIO(OH)/Ti02/Si02 KTP Quartz Ta205 Zr02/MgF2/Si02

Au LAAC Si3N4+d Ta205/Si02 Zr02/Si02

B203/Si02 LaF SiC Ta205/Si02/UC ZrO2/SiO2/M0

BaF2 LAP Silicone Ta205/Si02/Mo Zr02/Si02/Hf02

BaS04 LG-750 SiN TFF1 Zr02/Y203

BiF3 Li-formate Si02 TGG

BK-7 LiF Si02{B} Ti02

CaF2 LM03 Si02{F}/Si02 Ti02(anatase)

CaS04 LiNb03 Si02{Ge} Ti02(rutile)

CeF3 Methyl silicone Si02{Ge}/Si02 Ti02/siloxane

CH30H MgF2 Si02{Ge}/Si02{F} Ti02/siloxane/Triton-x

72
Table 4b. Explanation of database table entries

SUBSTRATE materials catalogued in the database to date are listed as follows: 7940 FR5 LAP Phos.APG TFF1

Al Ge02{P} LG-750 Quartz TGG

AI203 Ge02/Si02 Li-formate SiC Ti02

B203/Si02 Glass Li 103 Silicon ULE

BK-7 Heralux LiNb03 SiN YOS

CaF2 Heralux WG M-16 Si02 Zerodur

Ceramic KD*P Mo Spectralon Zr02

CH30H KDP Nb205 SrF2

Cu KTP None Suprasil

DLAP LAAC Opal Suprasil F3

3TRATE AND POLISHING VENDORS OR SOURCES are listed as follows: Airtron CVI Kodak OCLI UTOS

Applied Optics Deposition Sciences Labsphere Optovac Wisotzki

AT&T France Laser Power Osaka Zygo

China Fujian Limeil Schott

Cleveland Harshaw LLNL Shandong

Coherent Heraeus Matra Spectra Physics

Continental Hoya NBS Spindler/Hoyer

Corning Inrad Newport Tinsley

Crystal Technology Kigre NRL Union Carbide

SH or surface treatment PROCESSES used:

Beadblast Drawn Tube Fused None Super

Cleaved Ductile Ion mill PACE Unknown

Diam.T. Etch Lap Replicate

Substrate dimensions are in mm as length x width (or diameter if followed by d) x thickness.

Six laser parameters are specified:

WAVELENGTH in nm (1064, 1053, 532, 527, 355, 351, 266, 263, 248)

PULSE DURATION in ns (0.1 — 100).

Pulse repetition frequency (PRF) in Hz (0 — 8.6 k). All entries are in integers except high and low PRF values. Single

shot is designated by 0. POLARIZATION is given as P or S. Mixed or alternation'polarization is listed as PS. Elaborations may follow in the

comments section. ANGLE OF INCIDENCE in degrees (0° — 85°) (fractions may be rounded off but stored in database).

SPOT DIAMETER (1/eA2) in mm.

Laser damage threshold LOCATIONS are: F — at the front or incident surface of the sample

R — at the rear or exit surface of the sample. The entry is indented 1 space to aid in locating it. B — within the bulk material of the sample (extensive surface damage may cause damage to propagate into the

bulk material and vice versa). The entry is indented 2 spaces to aid in locating.

Thresholds are designated > or < if a full determination was not pursued because of lack of fluence, not of interest, severe damage elsewhere, or inability to measure. THRESHOLDS and ERRORS are in J/cm2 to 0.1 J/cm2 but not necessarily accurate to that degree.

THRESHOLD COMMENTS may follow with R for retest, ? for doubtful measurement, etc. Damage MORPHOLOGY comments are given in two sets of columns, the first at threshold, the second at a higher

fluence (lower if no higher tests were taken) — listed under @ in J/cm2.

MORPHOLOGY under columns M is coded as: A — artifact enhancement M — massive damage T

B — bulk damage N — no damage V —

C — crack or fracture P — pinpoint damage

D — delamination R — coating removal ? —

F — foggy appearance S — scald from plasma

SIZE of largest damage at a site is given in nm in the \l columns (999 means > 1 mm).

The NUMBER of observed damage phenomena is listed under #; note that many small damage points may be listed as 1 point when damage spreads to massive proportions (99 means > 99).

trail of bubbles or points

visual change (not seen by microscope)

Not noted or unknown

IRRADIATION TYPE is coded as follows:

1/1 — ONE shot per site. N/1 — NUMEROUS shots per site, 1 at a time with increasing fluences (conditioning or annealing).

S/1 — SEVERAL shots per site, each shot nominally at the SAME fluence with PRF irradiation. R/1 — many shots per site, beginning near zero fluence and RAMPED up to highest stated fluence in PRF mode

(conditioning). scan — sample SCANNED along line through a PRF beam (conditioning by wings of beam),

rast — sample RASTERED in 2 dimensions through a PRF beam (conditioning by wings of beam).

The NUMBER OF SHOTS on the site which defined threshold is listed under #. Identifying number of the REPORT in which the test results were written up. This is usually a Laser Damage Group Memo

(LDG) (a + sign means more reports are listed but hidden in the print-out)

H H Abbreviated further details (sample description before the 0, test results after). Abbreviations used for location and

morphology apply. Coating stack described as [T/S]5 means 5 layer pairs of Ti02/Si02. M F dmg @ 30 means

massive front surface damage at 30 J/cm2.

If tests were conducted with other than 1 0-ns pulses, t, the threshold, T, is scaled to a 1 0-ns value by T=tA 0.35. This is only a rough comparison aid. Scaling usually ranges between tA .2 and tA

.5 if at all.
Manuscript Received

5-4-90

Damage Measurements on Optical Materials

for Use in High-Peak-Power Lasers*

F. Rainer, R. M. Brusasco, J. H. Campbell, F. P. De Marco, R. P. Gonzales, M. R. Kozlowski, F. P. Milanovich, A. J. Morgan, M. S. Scrivener, M. C. Staggs, I. M. Thomas, S. P. Velsko, and C. R. Wolfe

Lawrence Livermore National Laboratory

P.O. Box 5508, L-490

Livermore, CA 94550

Recently, we have expanded our efforts to develop state-of-the-art optical components for use in large-scale, high-peak-power, solid state lasers. Laser-induced damage to many of these components sets critical constraints on construction costs and limits the peak

powers attainable in current and proposed devices. Increasing the damage threshold whenever possible by improving materials fabrication technology is therefore the ultimate goal of our work. The optical components and technological developments we are investigating are:

Highly reflective mirrors

Anti-reflective coatings

Non-linear crystals

Substrates and laser hosts

Polarizers

Kilo-layer coatings fabricated by high-temperature,

plasma chemical vapor deposition (PCVD)

Laser conditioned physical-vapor-deposition (PVD)

coatings

Multi-layer sol-gel coatings

Single layer sol-gel coatings deposited on substrates

and non-linear crystals

High purity KDP, LAP and related frequency conversion crystals

High threshold surface preparation techniques

High threshold bulk materials

Advanced PVD multi-layer coatings

Our research has resulted in an extensive database of laser-damage measurements.

We present summaries of these measurements with particular emphasis on those taken at 1064 nm and 355 nm.

Key words: antireflective (AR) coatings; bare substrates; bulk damage; conditioning;

damage; frequency conversion crystals; highly reflective (HR) coatings; laser-induced damage; plasma chemical vapor deposition (PCVD); polarizers; reflectors; sol-gel coatings; thin

films.

1 . Introduction

The damage-test capabilities at the Lawrence Livermore National Laboratory (LLNL) support and

work in conjunction with a wide variety of laser programs. We conduct and contract out to the commercial

sector research and development efforts to produce optical components with high damage thresholds for use in both high-peak-power and high-average-power laser systems. The high-peak-power work is in

support of LLNL's Inertial Confinement Fusion (ICF) program which involves (1) the existing Nova laser

* Work performed under the auspices of the U. S. Department of Energy by Lawrence Livermore National

Laboratory under Contract No. W-7405-ENG-48.

74
system, (2) smaller ICF development laboratories, and (3) the Advanced Driver program for developing a successor to Nova. Much of this work has also overlapped with research conducted for the development of

high-average-power laser systems including (1) the Medium Power Solid State Laser (MPSSL), (2) the free

electron laser (FEL) program, and (3) the Laser Isotope Separation (LIS) program.

2. Laser damage-test capabilities

During the past year we have operated six dedicated damage-test facilities at LLNL, conducted full- aperture damage tests with the Nova laser, and also supported damage testing capabilities at several commercial locations. [1] Most of our lasers are single-shot or rep-rated NdrYAG lasers operating at the

fundamental wavelength, 1064 nm (lw), or at one or more of its harmonics: 532 nm (2co), 355 nm (3co), and

266 nm (4co). We also utilize excimer lasers for UV testing at 350 nm (XeF) and 248 nm (KrF). We operate

with pulse durations ranging from 1 to 100 ns and at pulse-repetition frequencies (PRF) ranging from single

shots up to 6000 Hz depending on the particular laser. Typically we conduct our tests with test-spot

diameters of 1 to 2 mm, although we may resort to small spot measurements for very high fluences or also

irradiate full-sized optics with one shot on LLNL's Nova laser. We conduct tests at a given site with one or more shots at a constant fluence or with successive shots at gradually increasing fluence (laser conditioning). This conditioning can raise damage thresholds of optical materials by factors of two to three. [2, 3]

3. Damage database

We have maintained and are expanding an extensive database of damage test results which cover 15

years of work at LLNL. [4, 5] In figure 1 we show a summary of these results for the past two years which

represent 1300 tests conducted at 1064 nm with 1- to 16-ns pulses at PRF's ranging up to 120 Hz. The

major portion of our efforts have concentrated on:

(1) antireflective (AR) coatings fabricated by the sol-gel process;

(2) highly reflective (HR) coatings fabricated by physical vapor deposition (PVD), plasma assisted

chemical vapor deposition (PCVD), and the sol-gel process;

(3) polarizers fabricated by PVD;

(4) frequency conversion crystals; and

(5) laser host and substrate materials.

200

AR HR Polarizer Layers of 1 Bare Bulk Metal

Material Surface Material

Figure 1. We have conducted over 1300 damage measurements in the past two years, most at 1064 nm with

1- to 16-ns pulses at PRF's ranging from single shots to 120 Hz.

75
During the past year we have re-established testing capabilities at the harmonics of Nd:YAG and are resuming testing of many of these same types of samples at these wavelengths also.

4. High-peak-power laser requirements

We show in figure 2 the schematic of a typical beam transport system for a proposed large-aperture,

high-peak-power, solid state laser system for future ICF research. For 10-ns pulses the various components

in such a beam line must be able to survive fluences of 35 J/cm2 if they are subject to lco irradiation (laser

glass, HR coatings, polarizers, frequency conversion crystals) and 25 J/cm2 if they are subject to 3co

irradiation (frequency conversion crystals, focusing lenses, debris shields). We show in figures 3 and 4 an overview of the the current ranges of damage thresholds that we have achieved for these general categories

at lco and 3co respectively. These figures also provide information about specific materials, fabrication

processes and irradiation conditions that allow us to achieve the desired design goals at each wavelength.

Component: Laser

glass

Mirror Polarizer Frequency

Converter

Focus

lens

Debris

shield

Coating: None HR Polarizer AR AR AR

Frequency: 1co 1co 1co 1,2,3co 3co 3co

Damage threshold

specification at 35 35 35 35-1co

25-3co

25 -25

10 ns (J/cm2

):

Figure 2. A schematic diagram of a typical beam line for a large-aperture laser system shows the representative components. We are designing a system with components which must survive fluences of

35 J/cm2 at 1064 nm (lco) and 25 J/cm2 at 355 nm (3co) for 10-ns pulses.

5. Specific threshold achievements

5.1 Highly reflective (HR) coatings

A significant portion of our efforts has concentrated on developing high damage threshold HR

coatings by a variety of techniques (conditioned PVD coatings, kilo-layer PCVD coatings, and multi-layer

sol-gel coatings). We have in fact achieved design goal thresholds with each technique as shown in

figure 5. However, except for PVD coatings, much of our work is in the research stage and has not yet been

scaled up to full-sized optics with dimensions ranging from 0.3 to 1 m. The damage thresholds of PVD HR

coatings have gradually risen to levels at which they just barely meet our design goals. For several years

numerous papers have reported on the improvement of laser damage thresholds of optical coatings by laser

conditioning or "annealing" of the coatings. These past results have, however, reported only temporary

improvement of damage thresholds to HR coatings. Recently we have found that Hf02/Si0

2 HR coatings

have yielded permanent threshold increases by factors of two to three to levels exceeding 50 J/cm2 for

76
BARE GLASS SURFACES

AR COATS

BK-7 F-phosphate

ULE LG-750 Fused Silica

Sol-Gel

HR COATS

KDP (Bulk)

DLAP (Bulk)

POLARIZERS

E-beam coats — unconditioned E-beain coats — conditioned

'

'

i

1 „„„

1

1 CVD

Conditioned

Unconditioned

^Unconditioned^ Conditioned

Unconditioned Conditioned

1

H I-

-J

10 20 30 40

Laser Damage Threshold (J/cm

2

)

50 60

Figure 3. Our damage threshold goal for a large-aperture, high-peak-power, fusion laser system is 35 J/cm2

at 1064 nm (lco) for 10-ns pulses (grey vertical bar). The horizontal bars show our measured

range of damage thresholds for various laser materials; laser "conditioning" of many of these

materials places them near or above the threshold goal.

Fused Silica

Sol-Gel

I 1

Unconditioned Conditioned

I

1 I 1

Conditioned

HI 1 Unconditioned

BARE GLASS SURFACES

AR COATS

KDP (Bulk)

DLAP (Bulk)

10 20 30 40 50 60

Laser Damage Threshold (J/cm )

Figure 4. Our damage threshold goal for a large-aperture laser system is 25 J/cm2 at 355 nm (3(0) for 10-ns

pulses as indicated by the grey vertical bar. The horizontal bars show our measured range of

damage thresholds for various laser materials at 355 nm with 10-ns pulses. As in figure 3, conditioning significantly (and permanently) improves the damage thresholds of some materials.

77
10-ns pulses and which in turn do not degrade with time. [3] Moreover we have shown that this improvement in threshold can be extended to full-sized optics. [2] The summaries shown in figure 5 show both the

conditioned and unconditioned thresholds measured at 1064 nm for all of our highest threshold samples. A

few tests were conducted with 1- or 16-ns pulse durations. We have determined that these coatings scale

nominally as T

0 35 and have scaled the results accordingly to 10-ns values. [3] In some instances the damage

thresholds were greater than the maximum fluence used for a particular test so that we only established a lower bound for these samples. These are indicated by arrows.

20

16

12 -

8 -

4

v> r o

<D n

E 8 r- 3

4 -

PVD

Coatings

Data scaled to 10-ns pulses by T

0.35

Laser conditioned

|

J Not conditioned

-* Threshold > test fluence

JUL

Kilo-layer

CVD

* 1 1

i

1 i.i i i J—!==! ! —i. J I I- ...J (

Solgel

'

i _1 L I

1 J 1 _l I 10 20 30 40 50 60

Laser damage threshold (J/cm )

Figure 5. Over the past two years we have developed higher threshold HR coatings at 1064 nm by using

the three coating methods given above. The CVD and solgel techniques are still in a research

stage (i.e. small aperture); PVD coatings have been fabricated for full 1-m applications. The

data show the number of coating samples tested that gave the damage thresholds shown on the

abscissa. Most tests were conducted with 10-ns pulses but some 1- and 16-ns tests were scaled to 10-ns thresholds by x

035

.

A promising technique we are investigating is the fabrication of PCVD HR coatings. [6] In this

process alternate layers of slightly doped Si02 are deposited from the vapor phase plasma. The difference

in index of refraction between alternating layers is relatively small requiring 1000 or more layers to produce an effective HR coating. Research samples produced by this process as well as a modified CVD

process have yielded damage thresholds exceeding 60 J/cm2 for 16-ns pulses at 1064 nm. We have plotted

these data in figure 5 after scaling them to 10-ns thresholds. Most of the PCVD HR coatings were deposited on the inside of tubes nominally 1 cm in diameter. Threshold measurements were often limited by the poor

optical quality of the drawn tubes rather than the actual HR coatings which are approaching thresholds

close to that of fused silica substrates. We are presently completing the construction of a coating chamber

which will allow us to fabricate PCVD HR coatings on flat fused silica substrates up to 6" in diameter. [7]

We have successfully employed the sol-gel process for producing single layer AR coatings for many

years and have scaled their applicability up to optics exceeding 1 m in diameter. We have extended this

78
technology to produce multi-layer A10(OH)/Si02 HR coatings on 2"-diameter substrates. In the near future

we will apply this technology to 8"-diameter substrates. We have measured damage thresholds of these

coatings that exceed 40 J/cm2 for 16-ns pulses at 1064 nm. To conform with the other data in figure 5 we

have scaled these results to 10-ns values.

5.2 Antireflective (AR) coatings

Virtually all of the work that we have engaged in recently in the development of AR coatings has

revolved around the deposition of single layers of SiOz by the sol-gel process on fused silica, KDP, and

other substrates. In figure 6 we show the results of tests conducted during the past two years at lco and 3co. The tests represent a mixture of primarily 10- and 16-ns tests scaled to 10-ns values by x

035

. Almost all samples we have tested in 1989 have comparable or higher damage thresholds than the best sol-gel AR

coatings tested in the previous year. Moreover, in many of the recent tests we determined only lower

bounds to the thresholds because we were either not able to achieve higher laser fluences for damage testing or because of massive damage to the blemished, uncoated rear surfaces of the substrates. It should be noted

that most of the recent tests with thresholds < 35 J/cm2 were for research coatings which do not in fact

represent the state-of-the art of production technology. Routinely any sol-gel AR coating we fabricate at LLNL with known technology for high thresholds do in fact have thresholds exceeding 45 J/cm2

. Only a limited number of tests were conducted so far at 355 nm with all exceeding the design goal of 25 J/cm2 for

10-ns pulses. We were unable to damage two of the samples at fluences exceeding 38 J/cm2

.

Number

of tests

Laser damage threshold (J/cm 2

)

Figure 6. Damage thresholds of recent sol-gel AR coatings routinely exceed design goals for both 1064-

and 355-nm tests. Earlier tests were conducted with 16-ns pulses which were scaled to 10-ns

thresholds by x

035

; more recent tests were conducted with 10-ns pulses.

5.3 Frequency conversion crystals

The conversion of laser beams from 1064 nm to shorter wavelengths is crucial for ICF applications.

Frequency conversion crystals have always presented a significant problem from a damage standpoint in the construction of large-aperture laser systems. The crystals must first be grown to relatively large dimensions and yet be inclusion free to prevent bulk damage. The surfaces must then be figured and provided

with AR coatings which are environmentally sound and yet keep losses and damage to a minimum. The

latter problems have been routinely addressed with diamond turning and sol-gel AR coatings which can be

79
70

60

E 50

o

2

o

o>

reE

re T>

i_a>w

re

40

30

20

10

1064 nm 1064 nm

LAP KDP

Figure 7. Both LAP and KDP crystals can be grown to yield bulk damage thresholds > 60 J/cm2 at 1064

nm with 10-ns pulses. The samples meet or exceed the design goal of 35 J/cm2 at 1064 nm and

25 J/cm2 at 355 nm.

1000

100

0) c O) E

co o

E 3

a> £

ro a)

10

i i

>

i—i—i—r—r- —ii i—i—if—j- • Fused Silica

o BK-7

CVD

ULE

LG-750

A Fluorophosphate

.1

1064 nm

V

0.4

As

8

" "Qk

10 100

Pulse duration (ns)

Figure 8. Average damage thresholds for many bare polished substrates with indices of refraction near 1.5

are independent of glass type and scale as t

04

. The data are from references [4, 10, 11].

80
applied directly to the crystal surfaces at low temperatures. We have addressed the bulk damage issue by

adhering to higher standards of cleanliness and the use of ultra-filtration of the constituent solutions used in growing conversion crystals. [8] We have conducted numerous damage tests on both deuterated and

undeuterated versions of C-arginine phosphate (LAP) and potassium dihydrogen phosphate (KDP). In figure 7 we show the results of damage tests on crystals which have been laser-conditioned using relatively

large test-spot sizes (> 0.8 mm diameter). We have now standardized the technique to grow clean KDP

crystals which have virtually no observable bulk inclusions and we are working on extending it to the

growth of LAP. For both types of crystals we have measured 10-ns bulk damage thresholds exceeding

60 J/cm2 at 1064 nm. At 355 nm these crystals meet or exceed the design goal of 25 J/cm2 for 10-ns pulses.

5.4 Bulk materials and bare polished surfaces

Most elements in the typical beam line of a large-aperture laser systems have some type of optical

coatings deposited on them (AR, HR or polarizer). Essentially the only exceptions are the actual laser glass

disks, mounted at Brewster's angle, and occasional lenses, beam splitters or windows where reflections or

losses are not critical or where fluences exceed those that coatings can tolerate. At LLNL bulk damage

issues for laser glasses have have been addressed by an active campaign to reduce the inclusion of damagecausing platinum. [9] Other than frequency conversion crystals, the only other bulk materials typically

included in the beam line are fused silica substrates or Faraday rotator materials. With the absence of

inclusions, bulk damage to all these materials comfortably exceed 50 J/cm2 and measurements often are in- determinate because of damage to surfaces at lower fluences. Recently we have conducted only isolated

tests on bare surface samples. [4, 5] In figure 8 we show a compilation of the average thresholds for a variety of common substrate materials with nominal indices of refraction near 1.5. This shows that bare

surface thresholds scale as x

04 and are independent of material type. Since these are average values, they

are in fact relatively conservative because a wide variety of substrate types and polishing techniques are included. Many of our typical fused silica measurements are from rear surface tests conducted on bowlfeed polished (super-polished) substrates. These results are ancillary to the primary intent of measuring

thresholds of good quality AR coatings which had been deposited on the front surfaces of these same substrates. Rear-surface damage to bare surfaces is usually easier to observe than front-surface damage and

sets a conservative limit on the sample since the effective fluences at the rear are higher than at the

front. [12]

5.5 Polarizers

Polarizers consisting of multi-layer stacks of dielectric materials are typically fabricated with more

complex coating designs using more layers than those used for comparable HR coatings. Unlike with an HR coating, the coating-substrate interface of a polarizer sees the full intensity of the transmitted laser

beam. By their very nature, polarizers will also have greater angular sensitivity to each polarized

component of the laser beam. These characteristics have usually combined to yield among the lowest

thresholds for multi-layer coatings. We have recently conducted conditioning tests on a variety of HfOJ

Si02 polarizers from several vendors. In figure 9 we show that, as with HR coatings, we can expect to find a significant improvement in thresholds by implementing laser-conditioning. From a limited database of

conditioned polarizer tests we have observed an increase in threshold of about a factor of two to fluences >

40 J/cm2

.

6 Conclusions

We have constructed and utilized damage-test facilities at LLNL with which we have measured the

damage thresholds for all crucial optical components necessary for the design of a multi-megajoule, highpeak-power, solid state laser system. Our results show that these materials can meet the damage specifications of 35 J/cm2 at lco and 25 J/cm2 at 3d) with 10-ns pulses. We have verified that the laser conditioning

of multi-layer PVD HR's and polarizers can permanently raise 1064-nm damage thresholds above design

goals and be scaled up to full-sized optics. Research-sized HR's can be fabricated by both PCVD and the

sol-gel processes to exceed these goals and work is under way at LLNL to allow us to scale up this

technology. Full-scale and research versions of sol-gel AR coatings on KDP crystals and fused silica

substrates have reliably been operated and tested at fluences far in excess of design goals at both lco and 3co. 81
Careful attention to clean growth conditions has yielded KDP crystals with bulk thresholds comparable to those of fused silica. Bare and sol-gel-AR coated surfaces of fused silica and KDP have comparable

thresholds at their respective harmonics and also exceed the laser design goals at both harmonics.

60

50

40 CM 2 c

co o

E

CO 3

o 2 30

>- o 4>

(A

co

to

CD a 20

10

H After laser conditioning

Before laser conditioning

B C D

Test sample

Figure 9. Laser conditioning of commercial PVD-fabricated polarizers improves their damage thresholds

by an average factor of two to fluences > 40 J/cm2 for 10-ns pulses at 1064 nm.

7. References

[1] Morgan, A.J.; Rainer, F.; De Marco, F.P.; Gonzales, R.P.; Kozlowski, M.R.; Staggs, M.C. Expanded damage test facilities at LLNL. Nat. Inst. Stand. & Tech. (U.S.) Spec. Publ.; (Laser Induced

Damage in Optical Materials: 1989) - these proceedings.

[2] Kozlowski, M.R.; Staggs, M.C; Wolfe, C.R.; Campbell, J.H. Large area laser preconditioning of

dielectric thin film mirrors. Nat. Inst. Stand. & Tech. (U.S.) Spec. Publ.; (Laser Induced Damage in

Optical Materials: 1989) - these proceedings.

[3] Wolfe, C.R.; Kozlowski, M.R.; Campbell, J.H.; Rainer, F.; Morgan, A.J.; Gonzales, R.P. Laser preconditioning of optical thin films. Nat. Inst. Stand. & Tech. (U.S.) Spec. Publ.; (Laser Induced

Damage in Optical Materials: 1989) - these proceedings.

[4] Rainer, F.; Gonzales, R. P.; Morgan, A.J. Laser damage database at 1064 nm. Nat. Inst. Stand. &

Tech. (U.S.) Spec. Publ.; (Laser Induced Damage in Optical Materials: 1989) -these proceedings.

[5] Rainer, F.; Hildum, E. A.; Milam, D. Database of average-power damage thresholds at 1064 nm.

Nat. Inst. Stand. & Tech. (U.S.) Spec. Publ. 756; 1987 October. 410 p.

[6] Campbell, J.H.; Emmett, J.L.; Brusasco, R.M.; Rainer, F.; Gonzales, R. P.; Morgan, A.J.Kersten,

R.T.; Paquet, V.; Etzkorn, H.-W. Damage resistant optical coatings prepared using high temperature plasma chemical-vapor-deposition. Nat. Inst. Stand. & Tech. (U.S.) Spec. Publ.; (Laser

Induced Damage in Optical Materials: 1989) - these proceedings.

82
[7] Brusasco, R.M.; Britten, J.A.; Thorsness, C.B.; Scrivener, M.S.; Unites, W.G.; Campbell, J.H.; Johnson, W.L. A high-temperature, plasma-assisted chemical-vapor-deposition system. Nat. Inst. Stand. & Tech. (U.S.) Spec. Publ.; (Laser Induced Damage in Optical Materials: 1989) - these

proceedings.

[8] Montgomery, K.E.; Milanovich, F.P. High-laser-damage threshold potassium dihydrogen phosphate. Submitted for publication J. Appl. Phys.

[9] Campbell. J.H.; Wallerstein, E.P.; Hayden, J.S.; Sapak, D.L.; Warrington, D.; Marker, A. J.; Toratani, H.; Meissner, H.; Nakajima, S.; Izumitani, T. Elimination of platinum inclusions in phosphate

laser glasses. LLNL report UCRL 53932; 1989.

[10] Stokowski, S.E.; Milam, D.; Weber, M.J. Laser induced damage in fluoride glasses: a status report.

Nat. Bur. Stand. (U.S.) Spec. Publ. 541; 1978 October. 99 p.

[11] Milam, D. 1064-nm laser damage thresholds of polished glass surfaces as a function of pulse

duration and surface roughness. Nat. Bur. Stand. (U.S.) Spec. Publ. 541; 1978 October. 164 p.

[12] Crisp, M.D.; Boling, N.L.; Dube, G. Importance of Fresnel reflections in laser surface damage of

transparent dielectrics. Appl. Phys. Lett. 21(8): 364-366; 1972 October 15.

83
Manuscript received

3-8-90

Laser Induced Damage

to Thallium Arsenic Selenide (TAS)

A. A. Said, M. Sheik-Bahae, M. J. Soileau, E. W. Van Stryland

Center for Research in Electro-Optics and Lasers (CREOL)

University of Central Florida

12424 Research Parkway, Orlando, FL 32826

and

N. B. Singh, T. Henningsen

Crystal and Device Research and Development Center

Westinghouse Electric Corp.

Pittsburgh, PA 15235

We present results of a study of the laser induced damage thresholds of Tl3AsSe3 at 10 jzm using nanosecond

and picosecond pulses. The damage threshold was found to be =d0 J/cm2 with 130 ns (FWHM) and no damage was

observed using 60 ps pulses.

Key words: damage threshold; C02 laser; nonlinear crystals; TAS; SHG.

Introduction:

Thalium Arsenic Selenide, Tl3 AsSe3

, or TAS is an important optical material for frequency conversion in the infrared region. It is an efficient material for harmonic generation and optical parametric oscillation. TAS posesses second order optical susceptibilities that are nearly three times larger than those of Ag3AsS2 (Proustite) and a transparency range from 1.26 to 18 /im.[l] This material has a low absorption coefficient of 8.2 m"1 which was

determined by Barnes el al[2] by using a calorimetric technique.

In this paper, we present the results of a study of laser induced damage thresholds of two samples of

Thallium Arsenic Selenide grown at Westinghouse.

ATTENUATOR BEAM SPUTTER L1 SAMPLE

D

ENERGY MONITOR

Fig.l. The experimental set-up. The 10.6 pulse was tightly focused on the front surface of the sample.

84
Experiment and Results:

Of the two samples one was doped with silver. It was cylindrical with a 2.5 cm diameter and 1.5 cm length. Only

one surface was polished on which the damage measurements were taken. The other sample was undoped with a cone shape. The polished surface had a 2 cm diameter. The measurements were performed using linearly polarized,

normally incident pulses from a TEA C02 laser system equipped with an optical free- induction decay system that can produce pulses from 30 ps to 300 ns. The laser beam was focused on the first surface of the sample by lens Lx as depicted in Figure 1. The continuously variable attenuator consisted of two wire grid polarizers. Rotation of the

first polarizer varied the input energy keeping the polarization at the sample constant. Detector D, which monitored

the input energy, was calibrated using various Gentec energy detectors. The laser was operated in the TEM00 mode

and single longitudinal mode with pulsewidths of 130 ns (FWHM) and 60 ps (FWHM). Figure 2 shows the temporal

shape of the nanosecond pulse and the second order autocorrelation function of the picosecond pulse. This

autocorrelation function was obtained using a pump-probe method in another TAS crystal to produce 5 /*m light

dependent on the temporal overlap of the pump and probe.

Fig.2. The temporal shape of the nanosecond (130 ns FWHM) 10.6 /mi pulse (a) and autocorrelation scan of the OFID picosecond pulse (b). A pulsewidth of 60 ps (FWHM) was measured.

Pinhole size spatial beam scans were taken for each pulsewidth at the focal position of a 6.35 cm focal length lens Lr

These are shown in Figure 3 for nanosecond and picosecond pulses respectively. The beam spot size w0 measured was

57 /mi (HWl/e2M in the irradiance). This value agrees to witin 5% with the calculated w0 using the diffraction

equation w0=Af/7rw, where w and w0 are the beam spot size before and after focusing respectively, A is the wavelength

and f is the focal length of the focusing lens. Each site was irradiated only once. The damage was visually detected

through a long working distance microscope. With the nanosecond pulses, a damage threshold of 9.2 J/cm2 (30

MW/cm2

) was measured for the doped sample when the beam waist was 57 /mi and 11 J/cm2 (37 MW/cm2

) with a

calculated beam waist of 114 /tm which was obtained by scaling the previously measured w0 to a 12.7 cm focal length

lens. These thresholds do not show a significant difference. The damage thresholds for the undoped sample were 8.8 J/cm2 (29 MW/cm2

) and 12 J/cm2 (41 MW/cm2

) respectively. Figures 4 and 5 show the data from which the above

damage thresholds were extracted. With 60 ps pulses, no damage was observed on either sample. The maximum

fluence that could be reached was approximately 1.56 J/cm2 (19 GW/cm2

). Note that this maximum irradiance is =s

500 times higher than the nanosecond threshold. At this maximum fluence, one site of each sample was irradiated

for 5 minutes at 2 pulses per second without any damage observed.

85
1.2

(a)

-200 -100 0 100 200

r (microns)

Fig.3. Beam scan of the nanosecond pulse.(a) A spot size of 57 fim (HWl/e2M) in the irradiance was measured.

Beam scan of the picosecond pulse.(b) The high background is due to the EMI noise arising from the 40 KV

discharge of the laser amplifier.

Conclusion:

The damage threshold of TAS as obtained from the above measurements is deduced to be =;10 J/cm2. This value is comparable to the damage thresholds of GaSe and ZnGeP2 [3]. The damage threshold did not change upon doping

TAS with silver. Moreover, changing the spot size by a factor of two did not make a significant change on the damage threshold value. While frequency doubling the 10.6 yxm radiation an energy efficiency of 7% was obtained

from a 3.8 cm long TAS crystal using 60 ps pulses with an input irradiance of 150 MW/cm2 and a beam radius of .22 mm (l/e2M in the irradiance).

86
References:

[1] J. D. Feichtner and G. W. Roland, Optical properties of a nonlinear optical material: n3AsSe3

, Appl. Opt., vol.

11, 993-998, 1972.

[2] N. P. Barnes, Roger C. Eckhardt, D. J. Gettemy and L. B. Edgett, IEEE JQE, vol. QE-15, NO. 10, 1979.

[3] G. B. Abdullaev, K. R. Allakhverdiev, M. E. Karasev, V. I. Konov, L. A. Kulevskii, N. B. Mustafaev, P. P. Pashinin, A. M. Prokhorov, Yu. M. Starodumov and N. I. Chapleiv, Sov. J. Quantum Electron. 19 (4), 1989.

(a)

DAMAGE

NO DAMAGE

(b)

DAMAGE

NO DAMAGE

8 9 10 11

Fluence (J/cm2

)

12 0 12 18 24 30

Fluence (J/cm )

Fig.4. Damage data on doped sample using 130 ns (FWHM) pulses with 57 /xm (a) and 114 /xm (b) incident spot size

at 10.6 urn.

(a)

DAMAGE * • •<

NO DAMAGE

J L

(b)

DAMAGE •«•• mm •

i i i

DAMAGE

l

0 4 8 12 16 20 0 6 12 18 24 30

Fluence (J/cm ) Fluence (J/cm )

Fig.5. Damage data on undoped sample using 130 ns pulses with 57 itm (a) and 1 14 urn (b) incident spot size at 10.6 ^m.

87
Manuscript received

1-29-90

Laser Induced Damage in Schott's OG-550 Optical Absorption Glass

T. Whittaker, R. Goedert, and D. Templeton

U.S. Army Tank-Automotive Command

Attn: AMSTA-RSC

Warren, MI 48397-5000

Single and multiple shot laser induced damage threshold (LIDT) values were determined

for Schott's OG-550 optical absorption gl ass using Q—switched 5^32 nm laser radiation.

Bulk discoloration occurred for a single shot LIDT of 55 mJ/cm while surface pitting

resulted at a single shot LIDT of 5.6 J/cm . Multiple shot LIDTs were determined at

10, 5, and 2 Hz. For all three repetition rates, the multiple shot LIDTs decreased

exponentially from the single shot LIDT, and level off at 35% of that value.

Key Words: laser induced damage threshold (LIDT); single shot (SS); multiple shot (MS); surface

pitting; bulk discoloration.

1. Introduction

Today, lasers are used in the battlefield as rangef inders , target designators, and

illuminators. These lasers have become so common in modern warfare that the eyes of soldiers

are now endangered by accidental as well as intentional exposure.

Because of this relatively new and growing threat to the soldier's eye sight, the U.S. Army

has initiated an Optical Improvement Program. The program's objective is to provide ocular

protection from laser hazards for combat vehicle crewmen. An Optically Improved periscope

incorporates a laser protection filter that is designed to protect the crewman's eyes from

harmful laser radiation that may strike the periscope. Laser protection filters are classified

into three types: Type I - Multilayer interference filter, Type II - Absorption filter, and

Type III - Hybrid filter. In order to satisfactorily protect the crewman's eyes, each type of

filter must withstand certain levels of laser radiation before damage occurs. This paper presents the results of our preliminary investigation into the laser damage

resistance of, OG-550 glass, an "off-the-shelf" optical absorption filter (Type II) produced by

Schott. Both single and multiple shot laser induced damage thresholds (LIDTs) were measured.

2. Damage Determination

In this investigation, we define damage as any non-reversible change in a material's

structure. This definition encompasses both surface damage and bulk damage.

2.1. Single Shot

For the single shot (SS) LIDT tests, we used two methods to determine if damage occurred:

visual inspection and microscopic inspection. Visual inspection involved placing the orange

OG-550 test sample on a white sheet of paper, illuminating it with white light, and inspecting

the exposed sites for damage. Bright-field microscopic inspection was performed at lOOx

magnification.

2.2. Multiple Shot

For the multiple shot (MS) LIDT tests, we used a plasma spark to determine when surface

damage occurred. When a plasma spark formed a flash of light was radiated away from the front

surface of the test sample. A silicon energy detector was used to detect this emitted energy

and to signify the pulse at which surface damage occurred.

3. Test Setups

3.1. Single Shot

88
Our SS LIDT test setup is shown in figure 1. In this experimental setup, the pulse of a linearly polarized, Q-switched Nd:YAG laser (532 nm wavelength, 8-10 nsec pulse duration, output

energy up to 2 J, ^EMqo mode, PRF 1 - 10 Hz), made by Quantel, was used to generate damage at

the surface and in the bulk of the glass specimens. In order to cause surface damage, a positive, double-convex lens was placed in the optical path of the laser to focus the beam on to

the sample (focal length of lens f = 17.5 cm, sample 3.2 cm away from focus). The lens was removed to produce bulk damage only. The test sample was placed on a stage that was vertically

and horizontally adjustable (by increments of 10 um). The normal to the sample was rotated by 3

degrees relative to the axis of the beam to prevent laser light, reflected by the surface of the

sample, from returning into the laser. Two BK-7 glass beamsplitters, a pyroelectric energy

detector, and an energy meter were used to monitor the energy at the sample site through a calculated conversion factor. Two beam stops blocked the beamsplitters' second surface

reflections. Variable attenuation was provided by different combinations of KG-3 and float

glass filters. Last, an iris aperture was used to suppress multiple reflections from the

attenuation filters.

3.2. Multiple Shot

Our MS LIDT test setup is also shown in figure 1. This setup is the same as the SS LIDT

setup, with the addition of a silicon energy detector and another meter. In the setup, the

silicon detector's energy meter counted the number of pulses that occurred after surface damage,

and the pyroelectric detector's energy meter counted the total number of pulses incident on the

test sample.

4. Spot Size Determination

2 During the LIDT tests, the energy and 1/e spot size of the beam, at the sample site, were determined so that the fluence could be calculated. For each pulse, the energy was found by

measuring a fraction of the incident energy at the sample and adjusting this value by a conversion factor. The spot size (w) was determined by the slit method described below. The

spot size was not measured for each pulse, but it was measured for each test.

A 120 um wide slit was placed on the test Sample X, Y stage (where the test sample was normally located). A pyroelectric energy detector was also placed behind the slit and in the

path of the laser beam. To avoid damaging the detector, several KG-3 filters were used to

attenuate the beam. Starting from position zero, the slit was translated across the beam- in 120 um increments. The average energy transmitted through the slit was recorded at each position

after 100 shots had been fired.

Figure 2 shows the spatial energy distribution of a focused beam at the test sample. As the

figure shows, the spatial energy distribution of the beam is approximately Gaussian in shape.

To obtain the spot diameter (2w), the peak energy value is multiplied by 1/e (0.1353), the

two positions corresponding to this new energy level are read from the graph, and the smaller

position value is subtracted from the larger position value.

5. Test Procedures

5.1. Single Shot

Two SS LIDT tests were conducted. The first test used surface damage to define the damage

threshold, while the second test used bulk damage. The procedures that were developed for these

tests are described below.

For the first test, a 3.8 mm thick sample of OG-550 absorption glass was placed on the test

sample X, Y stage, and 50 to 1602sites were illuminated once at each of a series of fluence

levels ranging from 1 to 25 J/cm . The fluence level was recorded for each pulse and was

varied by inserting different combinations of KG-3 and float glass filters into the optical path

of the laser. For each sample, visual and microscopic inspection were conducted after all sites

had been exposed. The SS LIDT was then determined by averaging the lowest laser fluence causing

surface damage, F (D), with the highest laser fluence producing no surface damage,

F (ND). min

For the second test, a 3.8 mm thick sample of OG-550 glass was placed on the test sample X,

Y stage and 40 to 502sites were illuminated once at each of a series of fluence levels ranging

from 20 to 200 mJ/cm . Once again, the fluence level was recorded for each pulse and was varied by inserting various combinations of KG-3 and float filters into the optical path of the

laser. For each sample, visual inspection was performed after all sites had been exposed. In

this case, the SS LIDT was determined by averaging the lowest laser fluence causing bulk damage

with the highest laser fluence producing no bulk damage.

89
5.2. Multiple Shot

A 3.8 mm thick sample of OG-550 optical absorption glass was placed on the test sample X, Y

stage and 51 sites were illuminated, with multiple shots until surface damage occurred, at each

of a series of fluence levels ranging from 2 to 12 J/cm . First of all, a fluence level slightly above the SS LIDT value was chosen by placing the

proper combination of KG-3 and float glass filters into the optical path of the laser beam. This

fluence level was determined, without exposing the test sample to laser radiation, by placing a beam stop in-between the lens and beamsplitter, firing 100 shots, and dividing the average

energy at the sample site by the area of the beam. Next, the beam stop was removed and a laser

pulse was repeatedly fired, at a PRF of 10 Hz, at the site until the silicon detector was triggered by the plasma spark. The fluence level, the number of shots that occurred after

surface damage, and the total number of shots fired at the sample were then recorded. To obtain

the number of pulses that were needed to cause damage, the number of pulses that occurred after

the initial surface damage were subtracted from the total number of shots fired at the sample.

Finally, each new site was exposed to a known lower fluence and the process was repeated.

This test was then repeated at a PRF of 5 Hz and 2 Hz. On each sample, 51 sites were exposed to multiple shots at each PRF.

6. Results

6.1. Damage Morphology

After exposing several sites to different fluence levels, two types of damage were observed:

surface pitting and bulk discoloration. The first type of damage, surface pitting, occurred at

a fairly high fluence. This type of surface damage occurred when the laser beam was was focused

down to a small spot diameter and a little, crater-like pit (or pits) appeared on the front

(entrance) surface of the OG-550 glass. When the pit was large enough it was easily seen with

the naked eye. On the other hand, if the pit was very small a microscope had to be used to see

it. We also noticed that for large pits a plasma spark would form. The second type of damage,

bulk discoloration, occurred at a much lower fluence. This type of damage occurred when the

laser beam was unfocused and a small, circular region slightly below the front surface of the

OG-550 sample became discolored. This discolored region appeared black or grey and its diameter

was about the same size as the incident spot diameter. This discolored region could be seen with the naked eye, but it could not be seen when viewed under a microscope. We also noticed

that bulk discoloration always accompanied surface pitting.

6.2. Single shot

Table 1 gives a summary of the SS LIDT test's results. These results will be explained in

the following paragraphs.

For the first SS LIDT test, six samples of OG-550 absorption glass were damaged at the

surface. Two damage criteria were used to define the occurrence of surface pitting. Depending

on t^e damage criterion used, the average SS LIDT values ranged from 5.6 J/cm to 7.4

J/cn^. Visual inspection, with the naked eye, provided the highest average SS LID^, 7.4

J/cm . Using this criterion, the SS LIDTs were found to vary from 6.0 to 9.0 J/cm . Microscopic inspection, on the other hand, gave the lowest average SS LIDT with a value of 5.6

J/cm . In this case, the SS LIDTs ranged from 4.9 to 6.5 J/cm . Because the microscopic SS LIDT did not vary much from sample to sample and because

microscopic inspection provided the lowest average SS LIDT value it was determined to be the

best (and most accurate) damage criterion. Visual inspection, on the other hand, was determined

to be an acceptable alternative because it was a much quicker method that provided an average SS

LIDT value that was close to the lower microscopic SS LIDT value.

Figure 3 shows the typical SS LIDT test results for a 3.8 mm sample of OG-550 glass using

microscopic inspection as the surface damage criterion. As the figure shows, there is a range

of fluence values where surface damage may or may not occur. This occurs because some of the

sites are weaker than others due to scratches, pits, or absorbing inclusions in the glass [1].

This range, A F, is the difference between the maximum fluence producing no damage and the

minimum fluence causing damage. For both criterions used, the average AF was approximately 3

J/cm2

. After testing several samples, we observed that a plasma spark formed at fluences near and

above the average visual SS LIDT. This flash of light formed at the front surface of the test

sample whenever a pit was detected by visual inspection. At fluences well above the visual SS

LIDT a plasma spark was easily seen. At fluences near the visual SS LIDT it was harder to see. This new observation would be used for the multiple shot LIDT tests.

90
For the second SS LIDT test, three samples of OG-550 absorption glass were damaged in the

bulk of the material. Visual inspection, which was the only damage criterion u^sed, provided an average SS LIDT value of 55 mJ/cm and a range of SS LIDTs from 47 to 60 mJ/cm . Figure 4

shows the typical SS LIDT test results for bulk discoloration.

After inspecting the three samples for bulk discoloration, we noted that the discolored

region was extremely hard to see within a rather large range of fluence levels above the SS LIDT

value. Because the discoloration was so slight, there was no way to determine when bulk damage

actually occurred when more than one shot was fired without removing and inspecting the test

sample between shots. Thus, we concluded that multiple shot tests using bulk discoloration to

define the damage threshold were impractical.

6.3. Multiple shot

Figure 5 shows the typical MS LIDT test results. The results show that damage occurred well

below the visual SS LIDT level when multiple shots were fired at a site. The results also

indicated that for each sample and for each PRF, the fluence level required for damage decreased

exponentially with the number of pulses fired at a site from a maximum level to a minimum

level. The maximum fluence level occurred after one shot, while the minimum level occurred

after many shots. In each case, an equation was developed that simulated the results. The

fluence level, in general, was represented mathematically by the equation:

F = (F - F )exp[(l-N)/a] + F . max min min

where F was the maximum fluence required for damage, F . was the minimum fluence

requirecPr'or damage, N was the number of shots to damage, and "a" was a real constant. The

actual fluence was determined by mathematically manipulating this equation and performing a linear regression on the data. For each PRF, the equation for the fluence, that best fit the

data, took the form:

F = exp(b - mN) + F . r min

where N was the number of shots to damage, F ^ was the minimum fluence required for damage,

and b and m were constants that were determined1 by performing a linear regression on the

function ln(F - F^^). Table 2 shows the fluence equations found for each sample at each PRF.

Typically, the minimum fluence level required for damage increases with decreasing PRFs

[1]. For OG-550 glass, this did not occur for the PRFs tested. As the PRF was increased,

F increased some of the time but decreased others. In each case, the minimum fluence level

cnanged by less than 0.6 J/cm . This variance was due to the fact that for the same number of

shots there was usually a range of fluence values were damage occurred and was not due to the

PRF effect.

For OG-550 absorption glass, the average minimum fluence level required to produce surface

damage was 2.6 J/cm . After firing 25 or more shots the average fluence required to produce

surface damage had dropped down to its minimum level. At this point, only a fluence value equal

to 35% of the visual SS LIDT value (7.4 J/cm ) was needed to generate surface damage. Even

after 1000 shots, damage typically wouldn't occur at fluences below this value.

7. Summary

In conclusion, single shot LIDT tests and multiple shot LIDT tests were performed on 12

samples of OG-550 optical absorption glass filters. Over 1000 sites were exposed to laser

pulses produced by a linearly polarized, Q-switched Nd:YAG laser operating at a wavelength of

532 nm. Two types of damage resulted and three damage criterion were used to define the occurrence of damage [2],

8. References

[1] Wood, R.M., Laser Damage in Optical Materials, IOP Publishing

Limited, Accord, 1986, pp. 29-36.

[2] Whittaker, T.A. , Laser Induced Damage in Optical Absorption Glass,

Thesis, GMI Engineering & Management Institute, 1989.

91
TYPE OF DAMAGE: Surface Pitting

SS LIDT ( J/cm2 )

DAMAGE C 3 I TER I ON

SAMPLE # SPOT SIZE ( urn

)

Vis Ll a i Micro

432 6.0 4. 9

5 442 6. 7 6. 0

9

c- -—, cr 9.7 6.5

1 0 525 7. 9 5.5

1 1

t=- '—iCZ

«_l 7.3 t=r. O !_J

12 525 7 . 0 4.9

AVERAGE 7.4 5 . 6

TYPE GF DAMAGE: Bulk Discoloration

SS LIDT (mJ/cm*'-) 1

DAMAGE C R I TER I ON

SAMPLE # SPOT SIZE (mm) Vi sua l M icro

6 2 . 98 60

/ 2. 98 c=:-7

g 2. 98 47

A VERA 3E 55

Table 1. SS LIDT Test Results

SAMPLE // PRF FLUENCE EQUATION

13 10 F exp(2.12 - 0.23N) + 2.80

13 5 F exp(2.00 - 0.23N) + 2.20

13 2 F exp(2.17 - 0.27N) + 2.50

14 10 F exp(2.20 - 0.18N) + 2.45

14 5 F exp(2.17 - 0.23N) + 2.30

14 2 F exp(2.07 - 0.23N) + 2.90

15 10 F exp(1.71 - 0.15N) + 2.65

15 5 F exp(1.92 - 0.23N) + 2.40

15 2 F exp(1.80 - 0.23N) + 2.80

Table 2. MS LIDT Equations

92
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Figure 1. LIDT Test Setups

93
SPATIAL ENERGY DISTRIBUTION OF BEAM

Sample #10 Spot Size Determination

E max

= 1AOmJ

*^%ax) " °- 15 mJ

Spot Dia = 1020 um

Spot Size = 510 um

C3 - -a

e3

1

SPOT DIAMETER

• 3... —Q H H B B B E 3

i—1—i

1—r1 11—i

1 i1

i ^ i

1

i

1

i

1

i

1

i 240 450 720 9 60 1200 1440 1680 1920 2160 2400 2640 2SSC

SLIT POSITION(um) Slit Width = 120um

Figure 2. Spatial Energy Distribution of a Focused Beam

SINGLE SHOT LIDT OG-550 GLASS. 532nm

CRITERION: Surface Pitting, Microscopic

SS LIDT = ^max

(ND) i Fm

. n (D)] / 2

= (7.5 + 4.1) / 2

= 5.8 J/cm2

-HW- mi ii ii ii ill i iii h i mi i hh H i m i i ++ -h- i i n i n i

+ -HH-+ + + -H- + +

"i r I 4

"i r

p.

i

1 1 1 r 10

FLUENCE (j/cm2) SAMPLE #11 + SITES 1-119

Figure 3. Typical OG-550 SS LIDT Test Results

12 14

94
SINGLE SHOT LIDT OG-550 GLASS. 532nm

CRITERION: Bulk Discoloration, Visual

SS LIDT = ^max

(ND) i F—m I 2

= (64 + 56) / 2

= 60 mJ/cm

4- -H- 4- 4- 4t- 4-4-4-4-

-H- 4- -H- -H- 4- 4H- 4-4- 4- 4-

1

1 I I I

1 1 20 40 BO SO 100

FLUENCE (mJ/cm2) SAMPLE #6

4-- SITES 1 - 45

Figure 4. Typical OG-550 SS LIDT Test Results

MULTIPLE SHOT LIDT TEST OG-550 GLASS

CRITERION: Surface pitting, Plasma spark

100

NUMBER OF SHOTS TO DAMAGE SAMPLE #15

FRF = 10 Hz 1— PRF = 5 Hz PRF = 2 H:

Figure 5. Typical OG-550 MS LIDT Test Results

95
Manuscript received

12-21-89

Effects of Laser Damage Processes on Microwave Propagation

R. S. Eng, M. D. Abouzahra, N. W. Harris

Massachusetts Institute of Technology

Lincoln Laboratory,

P. 0. Box 73, Lexington, Massachusetts 02173

D. R. Cohn and P. P. Woskov

Massachusetts Institute of Technology

Plasma Fusion Laboratory

167 Albany Street, Cambridge MA 02139

Preliminary analysis has indicated that laser radiation may alter microwave propagation in transparent solids by both electron avalanche and thermally induced plasmas. These effects may

occur at fluence levels below that at which visible damage occurs. Temperature rises well below

the melting point may create sufficiently dense plasmas to affect propagation. The dependence

of propagation effects upon plasma density and size has been modeled. Effects of varying

the initial temperature have been studied. Irreversible changes in microwave absorption are

also considered. Effects are analyzed for a CdTe microwave modulator for CO2 laser radiation.

Plasmas with densities 10 15 — 10 16 cm 3 are generated at temperatures 500-700 °C in CdTe. At

these densities, microwave propagation may be significantly affected. Microwave measurements

of simulated plasmas have been carried out using metallic and resistive materials of different

dimensions Comparison is made with theoretical models. An experimental setup using a high

energy pulsed CO2 laser and a sensitive microwave reflectometer for measuring laser induced

effects will be discussed.

Key Words: C02 laser induced damage, plasmas in sohds, microwave transmission, temperature effects, cadmium telluride, pulsed laser heating.

1. Introduction

The propagation of microwaves in a semiconducting medium may be influenced by the heating of

the medium by incident laser pulses. Depending on the laser fluence, the temperature of the medium

can become significantly higher than the ambient temperature and optical damage can occur [1][2]. In microwave modulation of a high power CO2 laser beam using an electro-optic (EO) modulator with

semiconducting crystal such as CdTe, in addition to optical damage, [3][4], it is also necessary to consider

the possibility that microwave propagation can be affected before the optical damage threshold is reached.

The pulse energy threshold for optical damage threshold is in general assumed to be that needed

for melting. The melting point of CdTe is about 1100 °C. Because CdTe is a semiconducting material, free carrier generation by thermal excitation can be significant at temperatures well below the melting point.

For example, at about 520 °C, the intrinsic free carrier (electron) density for CdTe is about 1.0 X 10 1B cm-3

. It is thus possible to have a poorly conducting metallic sheet in crystal locations where there is high optical absorption that can result in damage (this absorption may be due to inclusions [5][fi]). This

medium can reflect as well as absorb microwave radiation.

96
In this paper we present theoretical analysis and experimental results concerning this possible interaction between an optical beam and a microwave signal. The experimental results include measurements

of simulated media using sheets of different conductivities placed inside microwave waveguides. Our pre- liminary results have further increased the understanding of the damage processes and optical-microwave

interactions in devices such as electro-optic modulators.

2. Electro-Optic Modulator Configuration

Figure 1 is a schematic diagram of an electro-optic modulator for microwave modulation of a CO2

laser beam. A pulsed laser beam enters the microwave waveguide and impinges on the front face of the

first of three 5.4 mm x 5.4 mm square CdTe crystals. After the optical beam passes through the modulator

crystals, it then exits at the other end of the microwave guide. The microwave signal is led in through a bent square hollow waveguide; it is impedance matched to the dielectric-filled waveguide represented by

the crystals which are gold-plated on all sides. Three or more crystals are needed to provide sufficient

sideband conversion efficiency at moderate microwave drive levels. To minimize optical reflection losses,

all crystal faces are anti-reflection coated. To generate a high sideband power, it is desirable to use a high

level of optical fluence at the input to the modulator. The end faces of each crystal have been subjected

to various optical fabrication processes and these are the places where high concentrations of impurity

inclusions are likely to be present. High optical fluences coupled with strong absorption centers at these

locations can cause the crystal temperature to rise significantly above the ambient temperature during the

passage of an optical pulse.

, SQUARE

CdTe CRYSTALS

GOLD-PLATED

LASER

IN LjlHHKr—

MICROWAVE

IN

-WtHU

\

RF MATCH

RF MATCH

LASER

OUT

LOAD

c

Figure 1. Schematic diagram of a CdTe single sideband electro-optic modulator

In the following paragraphs, an estimate of the temperature rise and the depth of heat penetration

during the passage of the optical pulse will be made.

97
Consider a slab of semiconductor material with a thickness L which is heated by an impinging

pulsed laser beam with a pulse width of r and a power density of J(r,t). The absorption coefficient is a. The energy absorbed over an area A(r) for a time t is given by

Ea (r,t) = J(r,t)A(r)(l - ea^L

) (1)

where both the power density and the absorption coefficient are functions of radial distance from the beam

axis. For simplicity, however, the optical beam will be assumed to be uniform. Also, it is simpler to assume

that the laser pulse is rectangular in shape. Finally, the absorption coefficient is in general a function of

radial distance to account for any nonuniform distribution of absorption centers in the semiconductor

medium. However, a detailed analysis of this general case is beyond the scope of this investigation; the

objective here is to find some closed- form expressions that would yield a reasonable estimate of the effect of

optical heating on microwave propagation characteristics. Therefore, it is assumed in the following analysis

that the absorption coefficient is uniform over the laser beam cross section and is limited to a small axial

distance L of a few //m. The assumption of absorption and resulting temperature increase over an area on

the order of the laser beam cross section is consistent with one of the often observed patterns of damage

13][4].

Using the above assumption, eq (I) becomes:

Ea (t) = J0 A{1 - ea" L

)t (2)

In laser pulse heating of a highly absorbing surface of a medium, the heat penetrates from the

surface to a depth of penetration by thermal diffusion. The penetration depth is [7]:

s = V2kt (3)

where n is the thermal diffusivity and t is the laser pulse width. The thermal diffusivity of CdTe has been

measured by Roussain and Le Bodn [8] for temperature from 100 K to 700 K. Table 1 lists the diffusivity

of CdTe.

At room temperature, the diffusivity of CdTe is about 0.046 cm2-s _1 and the penetration depth for a 30 fis laser pulse is about 17 fim. This decreases to about 10 fiva at about 700 K. Since the absorption

is not confined to the surface and the charge carriers can transport the thermal energy away from the

heated surface, an effective penetration depth of 30 fim appears to be a more suitable number to use for temperature rise calculations in the laser heated CdTe crystalline medium.

Having determined on the depth of thermal penetration, the next step is to obtain an estimate of

the temperature rise of the heated semiconductor. The temperature rise is assumed to be governed by the

heat capacity of the medium with [2]

AT = -A- =

Ea

pcAV pcnwz Al

where

• Ea is the absorbed laser energy and it has been defined in eq (2);

• p is the density of CdTe;

• c is the specific heat (at constant volume);

• w is the beam spot radius;

• AV is the effective volume of heat penetration; and

• Al is the penetration depth.

(4)

98
Table 1. Thermal parameters of CdTe

T (K) Diffusivity (crn^F1

) Conductivity (J-cm-^K-1) pc (J-cm^-K^s)

TOO

150

200

250

300

350

400

450

500

550

600

650

700

0.240

0.160

0.110

0.074

0.046

0.033

0.025

0.022

0.020

0.018

0.016

0.015

0.014

TuTo

0.184

0.127

0.095

0.075

0.064

0.056

0.050

0.045

0.041

0.038

0.035

0.032

1.20

1.23

1.25

1.28

1.63

1.94

2.25

2.26

2.27

2.28

2.30

2.31

2.32

The effective volume of heat penetration is the product of the laser beam cross section measured at the medium and the effective penetration depth. Using D — k/(pc), where D is the thermal diffusivity and

k is the thermal conductivity; the product of the density and the heat capacity pc can be found. Table 1 also lists the calculated values of pc from known values of thermal conductivity k [9] and thermal diffusivity

Using the following parameters, we obtain a temperature rise of about 500 °C:

• Incident energy=.Eo = 2 J;

• Fraction of incident energy absorbed=0.15

• Beam spot size=2w=0.3 cm;

• Depth of penetration=30 x 10~4 cm;

• pc = 2.3 3 cmrz K-x

The 2 J incident, energy is approximately the single shot damage threshold for CdTe for a 0.3 cm 1/e2 beam spot diameter extrapolated from damage test results using small 1/c 2 beam diameters of

approximately 0.1 cm [3] [4].

The 0.15 fraction of energy absorbed may seem high when compared with known calorimetric

measurement data using a cw laser beam. However, damage has occurred in pulsed laser testing on ARcoated CdTe surfaces and inside these surfaces [3] [4]. The occurrence of damage implies that relatively

high absorption has occurred by some mechanism, such as inclusions, and is roughly consistent with the

15% absorption discussed above.

2.1. Metallic Properties Induced by Laser Heating

Due to the energy absorbed, the temperature of the semiconductor rises. Associated with the

temperature rise are the following changes:

1. Free carrier densities (electron and holes) increase in the conduction and valence bands, respectively;

D[8}.

99
2. The charge carrier drift mobility (=sHall mobility) decreases due to higher collision rates at higher

temperatures;

3. The change in the free carrier density alters the medium complex permittivity.

The complex permittivity is derived as follows [10]: The equation of motion for an electron in a semiconductor is: ,d2 x m* dx „

where for simplicity we have limited the charge carrier motion to one dimension along the x direction, m*

is the effective mass, r is the time between collisions, Ex is the electric field in the semiconductor medium,

and e is the electronic charge.

Let the motion be a simple harmonic type with x = X0 e^ wt and Es = E0 e^ut

. The displacement

x(t) can be solved immediately and we obtain the amplitude Xo:

x° - -rr4r (6)

The displacement Xo of a charge e represents a dipole per unit volume and the overall volume effect is a

polarization P which can be represented by:

P = -neX0 = ~nf

E°

(7)

where n is the free carrier density. By definition we have

P = eoXfE (8)

where f0 is the free space permittivity, Xf is the susceptibility due to free charges (as opposed to bound

charges); \f ls then given by

ne2. .

The relative complex permittivity er is related to the complex index nT and the permittivity due to the

lattice e^:

eT = nT = eL + Xf = *L + ~,

, , —\ ( 10 )

Letting

com r-i (12)

where u>p

is the plasma frequency and v is the collision frequency. The relative complex permittivity

becomes w

2 V U)2

I II P P /I o\

eT = e -je = eL YT~l ~ 3 2~7~2 ( 13 )

The real and imaginary parts of nr = ^/ejT are n and k which are given below:

nT = n — j K = ^iA + ^V^fB^-j]/-^A + \VA2 + B2 (14)

100
where

u>

2 A = eL - -Tj—2 (15)

v u)

2

u>u>

2 + i/ 2

From eq (13) it is seen that in order to find the relative permittivity, we must first determine the

plasma frequency u>p and the collision frequency v , which depend on the free carrier density n and the time

between collisions, r, respectively.

First, in a CdTe crystal, the intrinsic carrier density for electrons, ne , is given by [11]:

ne = v/30x 101B Tie-°"xlo4 / T

(17)

where T is in °K and ne is in cm-3

. The carrier density is plotted in figure 2 as a function of medium

temperature in °C. Next, for the time between collisions, r, the general expression is:

Figure 2. Carrier density as a function of CdTe medium temperature

r = a

r as)

where a is the drift mobility. Because there are two kinds of charge carriers, we must treat the electrons

(subscript e) and holes (subscript h) separately and use separate effective masses m* and m*h and mobilities

He and Uh as follows:

Te = (19)

101
rh = ^

(20)

Values of the electron and hole effective masses are m* = 0.1 mo and m£ = 0.8 mo, respectively, where mo

is the electron rest mass. Measured values for the Hall mobilities were summarized in Ref. [12]. In the

temperature range from room temperature to about 1200 K, the mobilities can be approximated by

/xe = 1402 - 1.66T + 0.0005T2 cm^V^-s- 1 (21)

^ = 85(^)14 cnfV-1 ," 1 (22)

In the above equations, we have assumed that the drift mobilities /ze and fih for electrons and holes

are equal to their respective Hall mobilities. The complete expression for the relative permittivity er in eq

(13) contains the individual contributions from electrons and holes as follows:

"I _

ne2 t

2 ne e2 re2 nh e

2 r

2

u>

2 + u2 €0 m*l + w2 r2 m«e0 (l +w2 r

2

) m^l + u>

2 r

2

)

1 1

v ^1 _ ne2 t ^ne e

2 Te ^ nh e2 Th w u)

2 + v2 W60 m* 1 + w2 t2 u;m*e0 (l + w2 r

2

) wm^e0 (l + u2 r£)

The intrinsic densities due to doping are small and have been neglected above.

Figures 3 and 4 show the real and imaginary parts of the refractive index nT as functions of the

plasma density ne (or n/,) at 16 GHz and 5 GHz, respectively. It can be seen as the carrier density reaches

1 X 10 15 cm-3

, the semiconductor behaves like a metal with n w k; that is, the effective conductivity

becomes greater than the effective permittivity (u>e0

e ') as shown below. From Maxwell's equations,

assuming that there is no source of conductivity other than that from the thermally induced carriers as represented by eq (13)

VxH = D (25)

where H and D are the field quantities and D includes an effective conductivity term. D is given by

D = ju>D = j'wer

6 0 E = ju;£o[e' - je"]E = j'we0 e'E + e0u>e"E (26)

and the term eo<^"E is equivalent to <Te y/E, where ae ff is the effective conductivity.

As shown in eq (13),

2 2

I II wp • " WP

er =e-je = eL —— ——

The effective conductivity <7e /y is

Ul + V

At microwave frequencies, for v > w

<reff = e0«f" = e0v— p (27)

<7e ff = eo— = ne e — = ne e\x (28) v m"

102
100

10 -

* 1 cX

UJ

0.1

0.01

16 GHz

0.001

10 12

I I I I 1 I I I I

10 13

. ..I

10 1

_l I I

1 L_

10 15

I I I I I I I I

10

I I I I I I I I 16 10 17

CARRIER DENSITY (cm"3

)

Figure 3. Complex index of CdTe as a function of plasma density at 16 GHz

100

0.001

CARRIER DENSITY (cm"j

)

Figure 4. Complex index of CdTe as a function of plasma density at 5 GHz

103
The significance of eq (28) is that the effective microwave conductivity is equal to the dc conductivity. Using

this value of conductivity, we can go back to Maxwell's equations and derive the propagation constant and

impedance using the transmission line theory.

Table 2 lists the complex index and surface impedance for a 30 fim thick resistive sheet of heated

CdTe at 5 GHz and 16 GHz. The surface impedance Z, is defined as the inverse of the product of the

"complex" conductivity and the conducting sheet thickness:

Z, = 7(^—) (29)

here eo and eT are the vacuum and relative (complex) permittivities, respectively. In a highly conductive

medium, the "complex" conductivity jwtotT —*wtot" = ce // and we have the more familiar expression for surface resistance R, = l/(t X it). The impedances in table 2 are in fi/n and are for sheet thickness=30

fim.

Table 2. Complex Index (nr ) and Surface Impedance (Z,) for CdTe

Carrier density 5 GHz 16 GHz

(10

16 cm" 3

) nr = n — jn Z, = r, - jx t nr = n - jn Z. = r, - jx,

0.100 4.28 - j'2.82 41S2- j'1800 3.41 -ji.il 1714- j2354

0.316 6.17 - j5.26 1800 - ;288.5 4.07 - j'2.49 1464 - j'749

1.000 9.43 - j'8.87 713.7 -- j 43.

5.64 - j4.63 689.8 - j 137.0

3.160 14.34 -;13.98 298.7 - j'7.6 8.25 - j'7.60 296.6 - j'24.4

10.00 20.92 - j'20.67 138.6 -- j 1.67 11.89 - j'11.41 138.6 - j'1.67

3. Microwave Reflection Measurements of Metallic Disks Inside Waveguides

To estimate the microwave reflection from a laser heated semiconductor, simulations of a plasma

using disks of different surface resistance inside a waveguide were performed. These measurements were

carried out at 5 GHz using a 5.47 cm x 2.17 cm C-band waveguide. Different diameter copper disks were

used at first. Figure 5 shows the position of a metallic disk (a), its equivalent circuit (b), and the one port

equivalent circuit for microwave reflection. Reflection measurements were made with a microwave source

incident from the left end of the waveguide with the right end of the waveguide terminated in a waveguide

load. Figure 6 is a plot of 10 times the common logarithm of the voltage reflection coefficient r vs. disk

diameter in cm (squares). For a disk diameter of about 8.4 mm, the voltage reflection coefficient is about

-10 dB, which translates to a power reflection of about 1 %. The solid line in the figure represents the

calculated absolute values of the reflection coefficient r (eq (32) in the following) using an expression for the

shunt susceptance B to represent the disk inside a waveguide [13]. The shunt susceptance B normalized

to the waveguide admittance Yo is

B> = ^&f±l (30)

3 ab Aq

where r0is the disk radius, a and b are the waveguide transverse dimensions, Ao and Xg are the free space

and guide wavelengths, respectively.

The normalized reactance X' is given by: X' = X/Zn = -l/B', where X is the reactance and ZQ

is the waveguide impedance. Therefore, the input impedance Z{ for the combination of the shunt and the

terminated waveguide is the shunt impedance Z = jX in parallel with the waveguide impedance Zo with

Z' = Z/Z0 = jX/Zo = jX' = _ZoZ_

= Zo^

(31) Z0 + Z 1 + Z'

K

'

104
CROSS-SECTION LONGITUDINAL EQUIVALENT

VIEW VIEW CIRCUIT

(a) (b)

REFLECTION

REPRESENTATION

(C) .

Figure 5. Position of a metallic disk in a microwave waveguide (a), its equivalent circuit (b), and microwave

reflection representation (c)

Next, the reflection coefficient r is obtained

ZL -_Zo = _-l_

Zi + Zo 2Z' + 1

y '

Note that the reflection coefficient is a function of the normalized impedance Z' only. Since the normalized

impedance is implicitly given by eq (30), the effect of the waveguide medium on the reflection coefficient is then a function of the relative permittivity given in eq (30). The relative permittivity was not present in the original expression in [13]; we have included it in this memorandum for the CdTe dielectric waveguide

application.

Next the reflection coefficients for four different resistive sheets were also measured. These are plotted in figure 6 with the triangles, crosses, diamonds, and stars for the surface resistances of 100, 200,

377, and 1000 fi/d, respectively. As expected, the more conductive the sheet, the closer is the reflection

coefficient to that of the metallic disk. The measured reflection coefficients obtained for small resistive

disks are higher than that of the metallic disk. This is probably due to uncertainties in measurements at small reflection coefficients. Figure 7 shows the voltage reflection as a function of the surface resistance

for a 1.4 cm diameter disk. The lowest value of surface resistance is that of copper, less than 0.1 fi/D.

It represents the maximum reflection for this size of disk. Next, we extended our calculations to the 16

GHz regime and used the actual size of the 5.4 mm x 5.4 mm CdTe-filled waveguide modulator. Figure 8

shows the results for a disk with the resistivity of copper. Note that for a 3-mm diameter disk, the voltage

reflection is between —2 and —3 dB, which translates to a power reflection greater than 25 %. This is much greater than that shown in figure 6 for the observed data for the C-band rectangular waveguide.

105
0 + 1000Q/SQ.

0.0 0.2 0.4 0.6 0.8 1.0 DISK RADIUS (cm)

136296-1

Figure 6. Voltage reflections from circular disks in an air-filled C-band waveguide

0 100 200 300 400 500 600 700 800 900 1000 SURFACE RESISTANCE (ft/Sq.)

134909-2

Figure 7. Voltage reflection at 5 GHz as a function of the surface resistance for a 1.4 cm diameter disk

106
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 DISK RADIUS (mm)

Figure 8. Voltage reflection at 16 GHz as a function of copper disk diameter

4. Estimate of Microwave Reflection from Heated CdTe Modulator Crystal

In Section 2, we discussed how a heated semiconductor medium could have a very high free carrier

density and that under this circumstance the semiconductor could be characterized as a metallic-like

medium with a corresponding complex relative permittivity. In Section 3, we have shown that when an

electrically conducting sheet or resistive sheet is placed inside a rectangular waveguide to simulate such a plasma, microwave reflection is present; the reflections were measured for different degrees of conductivity

and were accurately predicted for the purely metallic case. In this section we will attempt to combine the

preceding two sections and estimate the magnitude of microwave reflection from a heated semiconducting

sheet placed inside a waveguide. Although we were able to obtain a closed form expression in the case of a

metallic disk suspended inside a rectangular waveguide, as far as we know there is no method available for the resistive sheet. In the following, A transmission line method is used to estimate the microwave power

reflection from a semiconductive sheet placed inside a waveguide.

4.1. Transmission Line Method

In measurements of surface resistance described in Ref. [14], a formula was used for determining

the surface resistance by measuring the microwave voltage reflection from a thin sheet of resistive material

placed inside a waveguide. This formula relating the microwave reflection R to a thin sheet of resistive

material with surface impedance Z, located at a quarter of a waveguide wavelength from a waveguide short

is given by:

-^Si-i (33)

Since the waveguide impedance Zgis

107
120tt , . o = I I 34

solving for the reflection R from eq (33) leads to:

In electro-optic modulator applications, the waveguide is terminated in its own characteristic impedance

Zg

; as a result, at the location of the sheet impedance, the incident microwave field sees not just a shunt

impedance Zt , but Ze in parallel with Zg. We denote this parallel combination of impedance by Zp

. Therefore, eq (35) becomes:

=

zg -zp =

Zg (Zg + Z,)-Z,Zg =Zg =1 1

(

v Zg + Zp Zg {Zg + Z,) + Z,Zg Zg + 2Z, 1 + 2% 1 + 2Z',

K

'

where Z'e = Z,jZg

is the normalized sheet impedance.

To evaluate this approach, we first assume a resistive sheet with surface impedance of about 713.7

fi (corresponding to a carrier density of about 1 x 10 16 cm-3as shown in table 2) filling the cross section

of a 4.76 cm x 2.21 cm C band waveguide at 5 GHz as will be subsequently discussed. The waveguide

impedance is given by:

120*" „ , , Zg = -j = —:- = 486 fi (37)

V

1 ~~ [(2)(4.76)1

2

Using eq (36), we next obtain a voltage reflection of about

R = -!— = 0.254 (38)

1 + 2(Z,)/486 v ;

The power reflection is then about 6.4 %.

Since the resistive sheet does not cover the entire cross section of the guide, we must multiply the

6.4 % by the ratio of the effective area of the slieet optical power impinging on to that across the whole

waveguide cross section. This factor is calculated as follows: The sheet disk radius w is taken to be 0.7 cm;

so the area of the disk is :r(0.7) 2 cm2

. The C-band waveguide internal dimension is 4.76 cm x 2.21 cm.

So we have

Correction Ratio = 4 / v w2 — x

2 cos

2

( —x) dx [ I dy I sin2( —x) dx]~ l (39)

Jo o- Jo Jo ol

where a and b are the rectangular waveguide internal dimensions and the electric field distribution is that

of a TEjo mode. Substituting the numerical values into eq (39) and performing the integration yield the

correction ratio

4(0.365) , % Correction Ratio = ~ '—^ = 0.278 (40)

2.21 * (4.76/2)

v '

The effective power reflection is 0.064 x 0.278 =1.77 %. This value is in fair agreement with the measured

voltage reflection of approximately —10 dB (see Fig. 6), which corresponds to a power reflection of about

1 %, for a 1.4 cm diameter 713 fi/D resistance disk suspended inside a C-band square waveguide. The

discrepancy is probably caused by neglecting the effect of diffraction in obtaining the correction factor.

Having achieved satisfactory agreement between the calculated and the experimental results for a re- sistive slieet partially covering a C-band rectangular waveguide, we want to extend the present calculations

to a dielectric-filled (CdTe) 5.4 mm x 5.4 mm waveguide at 16 GHz.

108
We assume that the resistive sheet is a plasma sheet of dense free carriers excited thermally by a pulsed laser beam. In particular, at a carrier density of 1 x It) 18 cm-3 occurring at a medium temperature

of 694 °C, the plasma sheet as an electric circuit can be characterized as:

nr - n- )k = 5.64 - j'4.63 (41)

The corresponding surface impedance Z, for a 30 /im thick sheet is:

Z, = 689.8 - j'137.0 fi (42)

The only other required parameter is the waveguide impedance, which is Zg = 139 fi; the value of guide

impedance is lower because the permittivity is about 1U.4.

Next, we substitute numerical values for Zg and Z, in eq (36) to determine the microwave voltage

reflection R as follows:

& = —r-=—7777^7 = 0.090 (43)

1 + 2(Z.)/139 V '

The power reflection is then about 0.0081. For a 1/e2 optical beam diameter of 3 mm impinging on the

modulator crystal inside a 5.4 mm x 5.4 nun square waveguide, the reduction in power reflection using an

expression similar to eq (40):

Correction Ratio = - =r^7TT7rt = 0.40 (44)

0.54 * (0.54/2)

v ;

Therefore, the effective power reflection is about 0.32 % at 16 GHz for a carrier density of about 1 x 10 16 cm-3

. Extending the calculations to other cases, we obtain the results shown in tables 3 and 4 for

the calculated power reflections at 5 GHz and 16 GHz, respectively for various carrier densities. In table

3, the observed reflections are also given for comparison purposes.

Table 3. Microwave power reflection at 5 GHz

Temperature Carrier density Calc. power reflection Power reflection from figure 7 (°C) (10

16 cm" 3

) 528 0.100 0.0007 not extrapolated

604 0.316 0.0038 not extrapolated

694 1.000 0.0177 0.0080

804 3.160 0.056 0.0190

940 10.000 0.113 0.0450

5. Microwave Absorption

The imaginary part of the refractive index, k, listed in table 2 is a measure of the microwave

attenuation. The microwave absorption coefficient, a, is given by

47TK a (53)

109
Table 4. Microwave power reflection at 16 GHz

Temperature Carrier density Calc. power reflection

( °C ) (10

16 cm-3

)

~1>28" (TTOO 0.00022

604 0.316 0.00065

694 1.000 0.00320

804 3.160 0.01400

940 10.000 0.04400

The microwave transmission Tr is then

TT = e~

al (54)

where I is the thickness of the plasma sheet. For small values of al, the loss is simply al. Table 5 lists the

microwave losses, al, for different carrier densities at 16 GHz. In calculating the losses, we used the same

correction factor that was used previously for the microwave transmission, namely, 0.40 at 16 GHz.

Table 5. Microwave absorption at 16 GHz

Temperature Carrier density Frac. power absorbed

(°C) (10 16 cm"3

) 528 0.100 0.0087

604 0.316 0.020

694 1.000 0.036

804 3.160 0.060

940 10.000 0.090

In a CdTe modulator housing containing three CdTe crystals, there are six end faces where possible

high absorptions occur. Therefore, one percent absorption per surface is perhaps too high for proper

modulator operation. Based on the results presented in table 5, it appears that microwave absorption is a more severe problem than microwave reflection. For detection of optical microwave interaction effects in CdTe, a method based on microwave absorption is perhaps more sensitive. For example, if the detection

sensitivity for microwave absorption is as high as 0.1 %, a free carrier density of 1 x 10 14 cm~3 can then

be detected which translates to a temperature of about 400 °C or about 1/3 of the melting point of CdTe.

6. Experiments with Electro-Optic Modulator and Discussions

The effects of optical beam heating on microwave propagation in CdTe are being investigated

experimentally in our laboratories. In particular, methods of reflection and absorption measurement have

been devised which appear to be sensitive enough for detecting the effects that have been estimated.

The CO2 MOPA system in the MIT Plasma Fusion Center has been reactivated for the present optical

microwave interaction study. This MOPA system is a TEA CO2 laser system which is capable of delivering more than tens of joules. Because of the presence of a leading edge spike in the output pulse shape, a time delay technique will be used to minimize this effect and create a pulse of a few microseconds by

amplifying the tail of the oscillation. Figure 9 is a schematic diagram of the TEA CO2 laser MOPA system.

All components are standard manufactured items from Lumonics Inc. Modifications have been made to produce single frequency operation for far IR pumping applications. The laser beam profile in general is slightly elliptical.

110
AMPLIFIER AMPLIFIER 3-PASS

AMPLIFIER

B rw\ M B '

>2 601 601 601 -co, M

HCOOH M

ISOLATOR R B 2-PASS PREAMPLIFIER V

ETALON

OSCILLATOR

M MIRROR

B BREWSTER ANGLE WINDOW

I IRIS G GRATING

R INVAR RODS

Figure 9. Schematic diagram of a TEA CO2 laser MOPA system

So far we have considered only the microwave reflection and attenuation problems. Examining table

2 shows that the microwave phase shift can also be affected because of the change in the index from the

room temperature value. For example, for a change in the refractive index value of 5 occurring at a free carrier density of about 3 x 1016 cm

-3

, the total change in phase is about 0.12 radian for a modulator

section containing three CdTe crystals with six possible absorption segments each 30 fim thick at the ends

of the crystals. Because a proper microwave phase is required for effective interaction with the optical

beam, the effect of a small perhaps not negligible phase shift on the modulator conversion efficiency is deemed complicated and must be investigated carefully using the modulator interaction theory. We plan

to pursued this topic in future work.

Finally, we have not considered the effect of laser beam heating on the semiconductor compensation

doping; it is possible that additional free carriers due to the doping ions are generated which may further

reduce the medium impedance.

7. Summary and Conclusions

Based on theoretical analysis and plasma simulation using conducting disks inside a C-band waveguide, it appears that the microwave propagation in CdTe can be affected at temperatures lower than the

melting temperature. The degree of degradation in microwave propagation depends on the maximum temperature reached and the plasma sheet thickness assumed. Based on the known value of thermal diffusion, a sheet thickness of 30 /xm is a reasonable estimate. Assuming this thickness and that the hot region has

about the same radial extent as the laser beam, it appears that a temperature of about 730 °C will give rise to a reflection of about 1 % and about 4 % at 940 °C and an absorption on the order of 1 % would occur at a temperature of about 550 °C and about 10 % at 940 °C. Measurements of microwave transmission can be

used as a diagnostic to study the damage process. For example, a measurement of the change of microwave

111
reflectivity as a function of number of pulses for fixed laser pulse energy might be used to study possible

changes in thermal conductivity. Other effects to be investigated are the microwave losses associated with

continuous heating at moderate temperatures [15].

8. Acknowledgment

The authors would like to thank L. J. Sullivan, W. E. Keicher, B. E. Edwards, and B. Lax for helpful discussions. This research was supported by the Department of the Navy for SDIO under contract F19628-85-C-0002.

9. References

[I] J. R. Myer, F. J. Bartoli, and M. R. Kruer, "Optical heating in semiconductors," Phys. Rev. 21B,

1559(1980).

[2] J. R. Mver, M. R. Kruer, and F. J. Bartoli, "Optical heating in semiconductors: Laser damage in Ge,

Si, InSb, and GaAs," J. Appl. Phys. 51, 5513(1980).

[3] R. S. Eng, J. G. Grimm, J. Greene, J. A. Daley, and N. W. Harris, "Measurements of Ultra-wide

Pulse Damage Thresholds of Anti-reflection Coated IR Materials at 10.6 /tm," Paper presented at the

Eighteenth Annual Symposium on Optical Materials for High Power Lasers," Boulder, Colorado(Oct.

26-28, 1987).

[4] J. G. Grimm, R. S. Eng, C. Freed, and R. G. O'Donnell, "Measurements of Pulse Damage Thresholds

of AR Coated CdTe at 10.6 /mi," Paper presented at the Nineteenth Annual Symposium on Optical

Materials for High Power Lasers," Boulder, Colorado(Oct. 26-28, 1988).

[5] T. W. Walker, A. H. Guenther, and P. Nielsen, "Pulsed Laser-induced Damage to Thin-Film Optical

Coatings-Part 1: Experimental," IEEE J. Quantum Electron. QE-17, 2041(1981); T. W. Walker, A.

H. Guenther, and P. Nielsen, "Pulsed Laser-induced Damage to Thin-Film Optical Coatings-Part II: Theory," IEEE J. Quantum ELectron. QE-17, 2053(1981);

[6] H. S. Bennett and CD. Cantrell, "Absorbing precipitates in cadmium telluride: estimates for catas- trophic laser-damage thresholds," Appl. Opt. 16, 2931(1977).

[7] M. Kruer, L. Esterowitz, R. Allen, and F. Bartoli, "Thermal models for laser damage in InSb photovoltaic and photoconductive detectors," Infrared Phys. 16, 375(1976).

[8] R. Doussain and H. P. LeBodo, "Thermal diflusivity of germanium, gallium arsenide, and cadmium

telluride," in Laser Induced Damage in Optical Materials: 1975, A. J. Glass and A. II. Guenther, eds. (NBS Special Publication 435), p. 98.

[9] G. A. Slack, "Thermal Conductivity of II-VI Compounds and Phonon Scattering by Fe2+ Lnpurities,"

Phys. Rev. B, 3791(1972).

[10] C. H. Lee, P. S. Mak, and A. P. DeFonzo, "Optical Control of Millimeter-Wave Propagation in

Dielectric Waveguides," IEEE J. Quantum Electron. QE-16, 277(1980).

[II] A. J. Strauss, "The physical Properties of Cadmium Telluride," Rev. Phys. Appl. 12, 167(1977).

[12] R. K. Willardson and A. C. Beer, Semiconductors and Semimetals , Vol. 13, Cadmium Telluride, K.

Zanio, ed. Academic Press, New York, 1978, Chapter 2.

[13] A. A. Oliner, "Equivalent Circuits for Small Symmetrical Longitudinal Apertures and Obstacles,"

IRE Trans, on Microwave Theory and Techniques, Vol. MTT-6, 72(1960).

112
COMMENTS

Question: A recent article I saw on silicon carbide concluded that, although

its a nice material, it's still too expensive even for government

work. Would you comment on the cost?

Answer: The reaction bonding process is a cheap process. You make up a slip; you cast it into a mold of whatever shape you want; you can

include leachable cords to make closed-back mirrors ; you can get

very precise shapes. This process in itself is very cheap

compared to, say, the CVD process where you do have a large

chamber and depositions take place over a long period of time. So

the facilities are not as expensive. The starting materials are relatively cheap; the silicon carbide powder itself for the

highest purity material is only about $25.00 a pound. It's a low

cost material. The expense primarily comes in the polishing, and

we're looking to reduce that with these polishing methods. We

think that we can bring the price down substantially.

Question: In your IBS process, you used a beta silicon carbide target, if I remember. Did you try an alpha-mixed target; did you notice any

difference in the target as far as a deposit?

Answer: With the IBS process, we only use the beta because it was available in a fully dense sheet form so we could get a decent

sized target. We did not try using an alpha target for the IBS.

With the ion-assisted deposition, we used alpha and beta as a source material from the E-beam gun. and found no difference in the

deposit, which was amorphous. We feel that since we did an amorphous deposit with the IBS process as well, it probably won't

make a difference in the source material.

Question: Is there any difference in the thermal expansion with

thermocycling? Is there any hysteresis effect in the multiphase

material?

Answer: We haven't seen anything at room temperature. We've cycled up to

400 °C and down to liquid nitrogen and have only made the

measurement at room temperature. We see no change. We're going

to be doing some cryogenic testing of bare and coated 6 inch

spheres where we'll be doing interferometry at about 100 K and

perhaps some intermediate temperatures.

Question: Is the thermoexpansion coefficient just a weighted average of the

silicon and the silicon carbide?

Answer

:

To a first order, yes

.

113
[14] M. D. Abouzahra, "Automatic Wide-Band Surface Resistivity Measurements of Resistive Sheets,

IEEE Trans. Instru. and Measurement, IM-36, 1031(1987).

[15] D. M. Wong, private communications.

Question: In many situations where you have multiple phases or different

crystal orientations in a polycrystalline substrate, you can sometimes even out the removal rates by the addition of chemical

agents in a mechanical - chemical process. Have you studied that at

all in these reaction bonded systems?

Answer: No, we haven't. We've looked strictly at diamond removal with

essentially a fixed- in hard lap.

Question: We've done some polishing of silicon carbide and also noticed a difference between a mechanical profile versus an optical

measurement, which we attributed in part due to the difference in

optical properties between the various phases that were present.

I would assume that if you were to use a Wyko or any kind of

interferometric method that this will show up and contribute to

the differences in height which you see, unless you were to silver

the part of metallize it first. And of course the scattering

results, maybe that was the other topic or other paper that you

were referring to. When you look at scattering, you see a scatter

difference that doesn't correlate with a profile measurement.

Answer: Yes. What we have seen is that when we aluminize the two-phase

material, we get the same Wyko profiles and roughness, but the

BRDF changes drastically.

Question: Following up on the earlier question on cost, you're probably in a unique position to comment on the relative cost for beryllium

versus silicon carbide optics. Of course it's a function of the

construction; lightweight or solid. If you made a lightweight

face sheet or mirror and were making one out of silicon carbide

and one out of beryllium, could you give a relative sense based on your own gut feeling as to where things are today? The difference

between silicon carbide and beryllium today, is it roughly like 2 to 1 in cost?

Answer: Today, because the maturity of the silicon carbide is not there,

it's more expensive than the beryllium. It's just not a mature

process as yet. But with very little development, I think silicon

carbide should be comparable in price or less than beryllium.

114
Manuscript received

4-2-90

Laser-Induced Damage to Silicon Photosensor Arrays

Chen-Zhi Zhang, Thierry Benchetrit, Steve E. Watkins, Rodger M. Walser,

and Michael F.Becker

Center for Materials Science and Engineering and

Department of Electrical and Computer Engineering

The University of Texas at Austin

Austin, Texas 78712

Laser-induced damage in two types of silicon photosensor array has been studied The samples were MOS CCD time delay integration (TDI) sensors with a 2048x96 element array of pixels and CID photodiode arrays of 512x1 pixels. The laser source was a Q-switched 1064 nm Nd YAG laser (10

u ep^' 10 nS pulses with a 250 ^m sPot radius)- Tests for morphological and electrical damage

to the CCD arrays have been reported previously. In new experiments, the micro-damage morphology

is examined and correlated with both the observed electrical degradation and newly observed stress

effects. We report the observation of surface deformation and lattice defects due to laser-induced

stresses in the Si02 and poly-silicon thin films on the silicon substrate. Measurements of damage for the CID arrays show them to be more resistant to laser damage than MOS structures such as CCD arrays. In addition, electrical degradation of these arrays was observed which affected the video

output signal from the devices.

Key words: laser damage; silicon photosensor arrays; laser-induced electrical degradation

1. Introduction

In this work, we have carried out a series of laser damage and diagnostic tests on CCD and photodiode array imaging devices, including the detection of performance degradation by sensitive electrical measurements and the

observation of morphological or microstructural changes using Normarski microscopy, SEM and TEM techniques. So

far, the previous data seemed to be insufficient to analyze the possible effects of laser interaction and to clarify the laser

damage mechanisms of these devices [1-3]. Solid-state sensors of this type are widely used in video cameras because of

their high-performance and long lifetime. Recently, these devices are of great interest in laser-related applications, such as

inter- satellite laser communications and laser-related imaging measurements.

The most commonly used solid-state imagers fall into two major categories according to their basic structure and

physical principals: charge-coupled devices (CCDs) and photodiode arrays. CCDs have low-noise levels and are

qualified for use in low light level imaging. Photodiodes, on the other hand, have the advantages of high quantum

efficiency, high dynamic range, and no smearing between pixels [4]. Since they are both used in laser related

applications, it is necessary to examine laser-induced performance changes and find their tolerance to laser-induced

degradation or damage. Better understanding of the damage mechanisms of these sensors will allow improvements in

their damage resistance and related systems' reliability.

Two main objectives of this paper are (1) to present the experimental results from this work to gain insight into

laser-induced damage processes and possible physical mechanisms; and (2) to compare the related laser damage

resistances and mechanisms between these two devices and to correlate the morphological damage with the electrical

performance degradation for each kind of device. The next section describes the samples tested and the experimental

setup and procedures. Section 3 gives the results obtained from the measurements of electrical performance and the

micro-evaluation of laser-induced morphological damage to these two different devices. In section 3 we also correlate

electrical degradation with morphological damage thresholds and discusses the damage mechanisms. Finally, in the

fourth section, several conclusions are drawn from this work.

2. Samples and Experimental Procedures

The CCD samples used in these experiments were 2048x96 pixel, n-channel TDI CCD arrays with 13x13 |im

pixel size, as shown in figure 1. The devices were fabricated on 100 mm diameter wafers by standard MOS-LSI

processes [5], including two level poly-silicon gates with individual thicknesses of 0.5 to 0.6 u.m, field oxide of 1.25

um, and thin gate oxide of 0.1 pm. Figure 2a shows a schematic cross-section view of the CCD active area. Another

type of sample tested, photodiode arrays, were packaged devices (EG&G RL-512S), containing 512x1 pixels with 25

Urn x 2.5 mm pixel size and 3 |im thick cap oxide, as shown in figure 2b.

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The laser source for these tests was a 1.06 (im Q-switched Nd:YAG laser with a pulse length of 10 ns (FWHM) at a 10 Hz repetition rate. The incident energy on the samples was controlled by an attenuator consisting of a rotating

halfwave plate followed by a fixed thin-film polarizer. A 76 cm focusing lens was used to obtain Gaussian spot sizes on

the sample with a radii about 250 (im. The computer calibrated the detectors, recorded the beam spot size, recorded the

energy of each incident pulse, and controlled the shutter which blocked the laser beam after the desired number of pulses.

During the testing process, the samples were positioned using a coaxial HeNe beam and a 20x alignment microscope

which was used to check for gross surface damage.

Following laser irradiation, both CCDs and photodiode arrays were arranged to be tested in the following

sequence: first electrical measurements, and then morphological observations, some of which were destructive to the

samples. For CCDs, the leakage currents and breakdown voltages between the two adjacent poly-silicon clock lines were measured with a HP 4145B semiconductor parameter analyzer and a probe station. Since these two parameters are very

sensitive to laser-induced degradation as compared to those obtained from the other on-wafer test patterns, such as

resistor bars, MOS capacitors and MOSFETs [6], the extent of electrical damage can best be described in terms of them.

After electrical evaluation, careful morphological examination of the poly-silicon lines was done under a 50-lOOOx

Nomarski optical microscope. As in our previous work, morphological damage has been defined as any visible change on the sample, especially color change within the laser spot observed under the Normarski microscope. In order to observe the microstructural changes of poly-silicon films before and after laser irradiation, some samples were

investigated by SEM and TEM. Those samples were prepared by conventional methods. The major procedures were: (1)

for SEM samples, polishing a cross-section of the devices at a 30° angle and removing the top and isolating oxides in a buffered HF solution; (2) for TEM observations, etching the silicon substrate from the backside of the CCD active area as well as removing the top oxide as described above.

The performance of the photodiode arrays was tested at dark, half- and saturation illumination levels using an EG&G RC1000/RC1001 evaluation board. Electrical degradation of the devices was characterized by the dark leakage

current in each junction element. The detailed experimental setup and typical video signal waveforms measured at each

test point on the evaluation board are shown in figure 3. The dark leakage current corresponding to each pixel element was determined from these video signals. The unprocessed video test point on the evaluation board gave the best

correspondence to the degree of electrical damage as it is most closely correlated to the raw recharge current of the pixel.

After signal processing (integrating the recharge current, sampling, and holding), the change in the video signal output

due to laser-induced leakage in each pixel was recorded. Operation of the imagers at half- and saturated illumination was

then carried out to characterize the their functional degradation in terms of photo response. These techniques are appropriate for determining local pattern noise and for correlating dark-current increase with physical defects on a pixelby-pixel basis.

3. Results and Discussion

3.1 CCD Arrays

It is apparent that the electrical isolation between two adjacent poly-silicon lines is sensitive to laser irradiation.

Shown in figures 4a and 4b, respectively, are the leakage current and breakdown voltage for adjacent poly-silicon clock

lines in the CCD active area as a function of laser fluence. The onset of changes of both parameters started at laser

fluences as low as 0.3 J/cm2. Both high leakage and low breakdown voltage occurred at fluences above 0.4 J/cm2. Such

degradation would usually lead to electrical failure since during normal operation the peak-to-peak voltage between the

lines is about 12 V. The electrical damage threshold for the clock lines can be defined as the laser fluence at which the

leakage current is above 1 (J.A or the breakdown voltage is below 25 V, which are drawn with the broken straight lines in figures 4a and 4b at about 0.55 J/cm2 in both cases.

Figure 5 gives a typical SEM micrograph for a sample irradiated at 0.7 J/cm2. It is clear that the edges of the poly-2

lines were broken and very ragged, while the same parts for undamaged areas look straight and smooth. Such minor

morphological changes indicate that melting has occurred and deformed those edges These changes in turn appear to be

related to the increase of leakage current and the decrease of breakdown voltage.

After laser irradiation at the morphological damage threshold, the poly-silicon grain size changes are remarkably

different in the four sections of poly-silicon lines, as shown in figure 6a. For unirradiated regions, the average grain sizes

of both poly-silicon lines were nearly the same (about 0.8 to 1 urn). Over the thick oxide or field oxide the grain size of

poly-Si 2 (5 |J.m wide) was changed to very fine particles 20 to 30 nm in size, and the grain structure of poly-Si 1(8 [im

wide) became long horizontal cells or columnar grains 0.2 x lum in size. Over the thin oxide or gate oxide, very slight

melting at grain boundaries was observed, but no significant grain size change occurred. Figures 6b-6d illustrate typical

TEM diffraction rings corresponding to the changes in grain structure in the different regions. In figure 6d for example,

the diffraction pattern looks much denser due to the fine grain size. In figure 6a, changes are also visible in the oxide

between the clock lines. Many poly-silicon inclusions have been formed in the oxide by the laser irradiation. These

inclusions, which may be responsible for the electrical leakage between clock lines, were most probably formed when the

edges of the poly-silicon lines melted.

116
The above phenomena can be explained in terms of local temperature extremes in different parts of the devices

induced by transient laser heating. These temperature rises depend strongly on the thermal parameters and device

structure. The absorption length at 1.06 (im has been found to be about several |J.m for heavily doped or implanted silicon

and poly-silicon, while it is about 300 (im for high resistivity single crystal silicon [7-8]. Our results show that strong

absorption in the heavily doped poly-silicon films results in a higher surface temperature response. Since the thermal

conductivity of Si is about one order of magnitude higher than that of SiC>2, thicker oxide isolation of the poly-silicon

from the substrate gave less heat conduction, which in turn led to a higher transient temperature response. For the narrower poly-silicon lines (poly-Si 2) on the thicker oxide, the highest temperature peak was expected. In this case, a grain fine structure was produced during rapid quenching following fast melting. Within the wider poly-Si 1 line on the

field oxide, the major heat flux flowed along the line direction, which resulted in the observed columnar grain structure

parallel to the line. The poly-silicon lines on the gate oxide suffered less extreme heating because the thinner oxide

conducted much more heat into silicon substrate. Computer simulations of transient heating in this structure are in progress to further analyze the effects of materials, structures, and variable laser energy absorption mechanisms.

In addition to thermal properties, process related stress induced by the intra-granular oxidation of the poly-silicon

films may play a role in the morphological damage threshold. Since the volume increases by a factor of two during the

growth of silicon oxide on silicon, a compressive stress may be generated along the grain boundaries during oxidation

[9]. When transient laser heating relived surface stress, the poly-silicon surface appeared significantly rougher and more

uneven under the cap oxide when observed using an optical microscope. The degree of roughness was reduced for a

series of samples which were prepared by etching the cap oxide to different thicknesses in a buffered HF solution before

laser irradiation. This phenomena was probably related to the relief of process-induced stress at the poly-silicon film

surface.

3.2 Photodiode Arrays

The electrical degradation of the photodiode arrays was investigated by studying the video signals obtained from

test points on the evaluation circuit card. Signals from damaged and undamaged pixels at dark-, half-, and saturatedillumination are shown in figures 7a-c, where the top curve is the raw video signal from the test point immediately

following the pre-amplifier, the middle curve is after charge integration, and the bottom one is the final sample-and-held

video output signal. This site was irradiated at 2.2 J/cm2 which was just above the onset of electrical damage but well

below the onset of visible morphology change. For those pixels located near the center of the laser spot, significant dark

leakage was measured, and no photo response was present. The video output signal from these damaged pixels was at the saturated white level. In figure 7d, the raw video signal from the same damage site is examined for two different

charge integration times, 10 ms and 40 ms. Both the peak recharge current voltage drop, AVrecharge (negative spike) and

the equilibrium recharge current voltage drop, AVjeakage (plateau after the spike) gradually increased (negative signal)

from the edge to the center of the laser irradiation spot. These two signal features are related to the loss of stored charge

in each damaged pixel during the integration time and to the voltage drop due to leakage during the recharge period

respectively. Note that the longer 40 ms integration time curve in figure 7d revealed a slight increase in the recharge spike

for the third pixel which was not evident for the shorter integration time. Although other sites were irradiated at 1.8, 1.9,

2.0 J/cm2 and higher fluences, only those sites irradiated at 2.0 J/cm2 or higher showed electrical damage similar to that in figure 7. All of the sites irradiated at fluences below 2.0 J/cm2 showed no degradation, and none were observed to have

any small leakage increases of the type observed at the edges of the clearly damaged sites, even when observed at long

integration times. Thus, the electrical damage threshold was abrupt at 2.0 J/cm2.

In order to compare the magnitudes of these changes in electrical parameters and quickly assess the degree of

damage, a bar chart representing the two leakage components described above was prepared as shown in figure 8. The

results on the left sides of the damage sites show that ,at the onset of electrical performance changes, the recharge current peak is the most sensitive parameter to laser-induced change. This is only clear when the scan direction (from left to right

in the figures) is taken into account. On the right side of the damage sites, a finite recovery time for the recharge circuitry

is observed in the form of a finite AVieakage at one or more undamaged pixels. Furthermore, those pixels showing

morphological changes are denoted in figure 7 by a darker shading on their AVrecharge bars. For the heavily damaged

central pixels at site 3 there was no recharge peak visible, and presumably those sites are showing more resistor-like than

capacitive, diode-like behavior and are practically shorted.

A sequence of micrographs showing laser-induced morphological changes on photodiode arrays as a function of

increasing laser fluence are shown in figures 9a-9c. This device has a 3 (im thick cap oxide and showed damage behavior

which was dependent on the heating behavior of the silicon below it. The examples shown in figure 9 include: (a)

plastically deformed bump at F = 2.9 J/cm2

; (b) cracks at the spot center at F = 3.0 J/cm2

; and (c) fractures and ruptures at F = 3.5 J/cm2 and above. From these results we deduce that a very high vapor pressure existed under the thick cap oxide

during transient heating. At 2.9 J/cm2 the resulting high temperature and pressure induced plastic deformation of the cap

oxide which resulted in a permanent bump after cooling. At higher fluences, the pressure was high enough to break the

oxide, while some evidence of plastic deformation remained around the edge of oxide opening. However, for multi-shot

damage, all evidence of plastically deformed edges around the oxide window have been removed as shown in figure 9d.

Perhaps the multiple temperature cycles propagated cracks and removed the deformed oxide area.

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To explain the above electrical and morphological damage behavior, the following damage mechanism is considered

to be responsible for the electrical damage. At laser fluences near the electrical damage threshold, a limited number of

defects are introduced into the surface junction region due to the rapid heating and cooling rates. In a previous study of

PIN photodiodes, those defects consisted of point defects which acted as deep levels within the energy gap [2]. For

higher fluences, shallow melting of the silicon surface occurred, and rapid resolidification left a large number of" defects at the junction termination. The presence of these defects greatly increased the the leakage and resulted in saturated video

output. In severe damage cases, surface melting or even boiling at the silicon surface resulted in an electrical connection

between adjacent pixels as observed in [10]. In such cases, it was impossible to detect individual pixel damage because

leakage from adjacent pixels could smear together and increase the total leakage current.

As a summary of this section, table I gives the results of 1-on-l laser-induced damage for all samples. It also

shows the trend of sheet resistance change in poly-silicon films by laser heating [6], Both types of imaging devices were

found to have lower thresholds for electrical damage than for morphological damage. These data are consistent with our proposed damage models: (1) for CCDs, poly-silicon films have a large optical absorption coefficient so that the isolating

oxide between poly-silicon lines was easily damaged; and (2) for photodiodes, the surface termination of the p-n junction was degraded and resulted in increased leakage current.

4. Conclusions

We have studied laser-induced damage effects on two kinds of silicon imaging arrays and have investigated both

electrical performance degradation and morphology changes. The strong optical absorption in and thermal isolation of the

poly-silicon films leads to much lower damage thresholds for CCD arrays than for bulk silicon photodiodes. Electrical

degradation of the isolation oxide between adjacent CCD poly-silicon clock lines was due to laser-induced heating and

quenching cycles and to changes which these cycles caused in the microstructure of the poly-silicon lines. These effects were related to the thermal isolation of the lines and built-in stress from the fabrication process. For photodiode arrays, a

detection and test technique for evaluating dark leakage current has been described. It was found that the electrical

degradation threshold for photodiode arrays is somewhat lower than the onset of visible morphological damage but

considerably higher than the onset of electrical degradation in CCDs. The electrical damage onset for photodiode arrays was also found to be lower than that in discrete photodiodes in which the surface junction termination was not irradiated.

These results have further verified that the laser-induced defects in the p-n junction region are responsible for the increase

of the dark leakage current in many types of photodiode devices.

We acknowledge the assistance of Bob Maddoux of EG&G Reticon in providing us with the photodiode array samples.

5. References

[1] D. L. Parker, F.-Y. Lin, S.-J. Zhu, D.-K. Zhang, and W. A. Porter, "A comparison of Nd:YAG fundamental and

second-harmonic Q-switched laser-beam lifetime doping in single-crystal silicon," IEEE Trans, on Elect Dev ED30, 1322 (1983).

'

[2] S. E. Watkins, C.-Z. Zhang, R.M. Walser, and M.F. Becker, "Laser-induced electrical parameter degradation in

silicon photodiodes," Proceedings of the 20th Annual Symposium on Optical Materials for Higher Power Lasers

NIST (NBS) Special Publication #775, 288, Boulder CO (1989).

[3] C.-Z. Zhang, S.E. Watkins, R.M. Walser, and M.F. Becker, "Laser-induced changes in the electrical performance of

silicon MOS device structures," Proceedings of the 20th Annual Symposium on Optical Materials for Higher Power

Lasers, NIST (NBS) Special Publication #775, 105, Boulder CO (1989).

[4] H.-F. Tseng, J. R. Ambrose, and M. Fattahi, "Evaluation of the solid-state image sensor," J. Imaging Sci. Vol. 29,

[5] IP. Ellul, H.-Y. Tsoi, J.J. White, M.I. King, and W.C. Bradley, "State-of-the-art imaging arrays and their

applications," Proceedings of SPIE 501, 117 (1984).

[6] M. F. Becker, C.-Z. Zhang, S. E. Watkins, and R. M. Walser, "Laser-induced damage to silicon imaging sensors

" SPIE 1105 Materials for Optical Switches, Isolators, and Limiters, 68 (1989).

[7] M. F. Becker, F. E. Domann, A. F. Stewart, and A. H. Guenther, "Charge emission and related precursor events

associated with laser damage," Fifteenth Annual Symposium: Optical Materials for High Power Lasers, NBS Special

Publication #688 (1984); and F. E. Domann, M. F. Becker, A. H. Guenther, and A. F. Stewart, "Charged particle

emission related to laser damage," Applied Optics, Vol.25, 1371 (1986).

[8] J. D. Chlipala, L. M. Scarfone and C.-Y. Lu, "Computer simulated explosion of poly-silicide links in laser- programmable redundancy for VLSI memory repair," IEEE Trans, on Elect. Dev. ED-36, 2433 (1989).

118
[9] E. A. Irene, E. Tierney, and D. W. Dong, "Silicon oxidation studies: morphological aspects of the oxidation of

polycrystalline silicon," J. Electrochem. Soc. 127, 705 (1980).

[10] S.S. Cohen, P.W Wyatt, G.H. Chapman, and J.M. Canter, "Laser-induced diode linking for wafer-scale

integration," IEEE Trans, on Elect. Dev. ED-35, 1533 (1988).

Table I: Summary of 1-on-l Laser-induced Damage

Device Fluence

(J/cm2

)

Electrical Change Morphological Change

<0.2 No Change No Change

0.2 - 0.3 Poly-Si: sheet R decrease;

Clock lines: breakdown V decrease and leakage I increase

No Change

CCD 0.5 - 0.6 Electrical failure (clock

lines becoming shorted)

Grain surfaces melted

0.7 - 0.9 Electrical failure

^ciock. unes snoricuj

change in color (surface

neigni increase^,

oxide damage pits

1.0 - 1.2 Electrical failure (clock and Al lines melted,

open circuit)

Top oxide broken

< 1.9 No Change No Change

Photodiode1.9 - 2.0 Junction leakage increases, no photo responseNo Change

2.9 - 3.0 Severe leakage Oxide bump & crack

Si surface melting

> 3.5 Melting and total pn junction failure

Open oxide window

Poly-Si 2 p0, y

. Sj ! (0.6 am) (0.7am)

Figure 1. Projection view of the CCD active area.

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Layer Thickness:

I I Cap CVD oxide (1 \im)

VZZft Thermal-grown oxide on poly-Si (0.4 (im)

VMM Poly-silicon line(0.6 urn)

Thermal-grown oxide (0.1 p.m)

^Silicon Dioxide

rap25

A A A A A ft i

p+ p+

n-Si Subsrate

Figure 2. Cross-section views of the test devices: a. CCD imaging array; b. photodiode array.

EG&G RC1000/RC1001 Evaluation Board

MSTR Device Under Test Video Pre-Amp

TfT RL512S

Diff. Amp & Integrator

Jill

Sample & Hold —

TV Video

Monitor

Digt. Camera

Card Computer

Digt Camera

C1001

SYNC

Programmable

Oscilloscope

Tektronix 1 1302

Two-Channel

Amp 11A32

Graphics

Printer

Figure 3. Block diagram of the measurement setup for photodiode arrays, where MSTR is a pulse signal starting a scan

operation, fl and f2 are two properly timed clock phase signals, and SYNC is an oscilloscope synchronizing signal.

120
c

CD

10

10

d 10

CD

CO

CD

10

-9 .

V = 1 volt

.** .1"

.»--*"'

/

:)

**

»" **

.,-r

9 1 i i i 0.0 0.2 0.4 0.6 0.8 1.0

Fluence (J/cm 2

)

a.

0.0 0.2 0.4 0.6 0.8

Fluence (J/cm 2

)

b.

1.0

Figure 4. Measured clock line electrical parameters vs. laser fluence: a. leakage current between adjacent poly-silicon

lines measured at 1 V applied voltage; b. breakdown voltage between adjacent poly-silicon lines. For fluences below 0.3

J/cm2

, the breakdown voltage was greater than 35 V.

Figure 5. SEM micrograph of the CCD active area after irradiation at 0.7 J/cm2

. The cap oxide has been etched away;

and in the upper-right half of the photo, the sample has been polished at a 30° angle.

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Figure 6. TEM micrographs of CCD poly-silicon lines: a. different grain size changes within the four regions

irradiation at 0.9 J/cm2

; b-d. typical TEM diffraction rings corresponding to three regions shown in figure 6a.

122
Figure 7. Video signals form photodiode pixels irradiated at 2.2 J/cm2 : a-c. correspond to the conditions of dark-, half-,

and saturated-illumination respectively, where the top curve is the raw video signal from the test point immediately

following the preamplifier, the middle curve is after charge integration, and the bottom curve is the final sample-and-held

output signal; d. an expanded view of the dark leakage video signal for integration times of 10 and 40 ms.

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normal pixel

I

AV(recharge)

-unit: x1 V

damage center

I

AV(leakage)

- unit: x100 mV

2.5-1

-Visible surface damaged pixels (x1V) 1 4 ~ 1 5V

Saturated level

1/2 illumination

Dark level

Site 1 (F=2.0 J/cm2)

Site 2

(F=2.9 J/cm2)

Site 3

(F=3.5 J/cm2)

Figure 8. A bar chart representing the two leakage components, AVrecharge and AVieakage, from pixels at three damage

sites of increasing laser fluence.

124
125
Manuscript received

3-8-90

SENSITIVE n2 MEASUREMENTS USING A SINGLE BEAM

M. Sheik-Bahae, A.A. Said, T.H. Wei,

D.J. Hagan, E.W. Van Stryland and M.J. Soileau

CREOL

Center for Research in Electro Optics and Lasers University of Central Florida, Orlando, FL 32826

We present a sensitive single beam technique for measuring nonlinear refraction in a variety of materials that offers

simplicity, sensitivity and speed. The transmittance of a sample is measured through a finite aperture in the far-field as the sample is moved along the propagation path (z) of a focused Gaussian beam. The sign and magnitude of the nonlinearity is easily deduced from such a transmittance curve (Z-scan). Employing this technique a sensitivity of better than A/300 wavefront distortion is achieved in n 2 measurements of BaF2 using picosecond visible laser pulses.

Key words: BaF2 ; CS2

; experimental technique; Kerr effect; nonlinear refraction; Z-scan

1. INTRODUCTION

We have recently reported a single beam method, which we refer to as a Z-scan, for measuring the sign and magnitude

of the nonlinear refractive index n 2.[l] In practice we have found that this method has a sensitivity comparable to interferometric methods. Here we describe this method in detail and demonstrate how it can be applied and analyzed

for a variety of materials. We also present a simple method to minimize parasitic effects due to the presence of linear sample inhomogeneities.

Previous measurements of nonlinear refraction have used a variety of techniques including nonlinear interferometry [2],

[3], degenerate four-wave mixing [4], nearly-degenerate three-wave mixing [5], ellipse rotation [6], beam distortion

measurements [7], [8], and our recently reported Z-scan technique. The first three methods, namely nonlinear

interferometry and wave mixing are potentially sensitive techniques but require a relatively complex experimental

apparatus. Beam distortion measurements, on the other hand, are relatively insensitive and require detailed wave

propagation analysis. The Z-scan technique is based on the principles of spatial beam distortion but offers simplicity as well as very high sensitivity.

We will describe this simple technique in Section II. Theoretical analyses of Z-scan measurements are given in Section

III for a "thin" nonlinear medium. It will be shown that for many practical cases, nonlinear refraction and its sign can be

obtained from a simple linear relationship between the observed transmittance changes and the induced phase distortion

without the need for performing detailed calculations. In Section IV we present measurements of nonlinear refraction in a number of materials such as CS2

, and transparent dielectrics at wavelengths of 532 nm, 1.06 /^m'and 10.6 ^m. In CS2 at 10 /xm, for example, both thermo-optical and reorientational Kerr effects were identified using nanosecond and

picosecond pulses respectively. We also describe how effects of linear sample inhomogeneities (eg. bulk index variations) can be effectively removed from the experimental data.

2. THE Z-SCAN TECHNIQUE

Using a single Gaussian laser beam in a tight focus geometry, as depicted in Fig. 1, we measure the transmittance of a nonlinear medium through a finite aperture in the far field as a function of the sample position z measured with respect

to the focal plane. The following example will qualitatively elucidate how such a trace (Z-scan) is related to the nonlinear refraction of the sample. Assume, for instance, a material with a negative nonlinear refractive index and a thickness smaller than the diffraction length of the focused beam (a thin medium). This can be regarded as a thin lens of variable focal length. Starting the scan from a distance far away from the focus (negative z) the beam irradiance is low and negligible nonlinear refraction occurs; hence, the transmittance (D2 /Dx in Fig. 1) remains relatively constant. As the sample is brought closer to focus, the beam irradiance increases leading to self-lensing in the sample. A negative

self-lensing prior to focus will tend to collimate the beam, causing a beam narrowing at the aperture which results in an

increase in the measured transmittance. As the scan in z continues and the sample passes the focal plane to the right

126
(positive z), the same self-defocusing increases the beam divergence leading to beam broadening at the aperture and,

thus, a decrease in transmittance. This suggests that there is a null as the sample crosses the focal plane. This is analogous to placing a thin lens at or near the focus, resulting in a minimal change of the far field pattern of the beam.

The Z-scan is completed as the sample is moved away from focus (positive z) such that the transmittance becomes linear since the irradiance is again low. Induced beam broadening and narrowing of this type have been previously observed

and explained during nonlinear refraction measurements of some semiconductors.[9],[10] A similar technique was also previously used to measure thermally induced beam distortion of chemicals in solvents.[l 1]

SAMPLE APERTURE

D1

Fig.l The Z-scan experimental apparatus in which the ratio D2/D1 is recorded as a function of the sample

position z.

A pre-focal transmittance maximum (peak) followed by a post-focal transmittance minimum (valley) is, therefore, the Z-scan signature of a negative refractive nonlinearity. Positive nonlinear refraction, following the same analogy, gives

rise to an opposite valley-peak configuration. It is an extremely useful feature of the Z-scan method that the sign of the nonlinear index is immediately obvious from the data, and as we will show in the following section the magnitude can

also be easily estimated using a simple analysis for a thin medium.

In the above picture describing the Z-scan, one must bear in mind that a purely refractive nonlinearity was considered

assuming that no absorptive nonlinearities (such as multiphoton or saturation of absorption) are present. Qualitatively,

multiphoton absorption suppresses the peak and enhances the valley, while saturation produces the opposite effect. The

sensitivity to nonlinear refraction is entirely due to the aperture, and removal of the aperture completely eliminates the

effect. However, in this case the Z-scan will still be sensitive to nonlinear absorption. Nonlinear absorption coefficients

could be extracted from such "open" aperture experiments.

3. THEORY

Much work has been done in investigating the propagation of intense laser beams inside a nonlinear material and the ensuing self-refraction [12], [13]. Considering the geometry given in Fig. 1, we will formulate and discuss a simple

method for analyzing the Z-scan data based on modifications of existing theories.

In general, various order nonlinearities can be considered; however, for simplicity, we first examine only a cubic

nonlinearity where the index of refraction n is expressed in terms of nonlinear indices n 2(esu) or 7(m2/W) through:

n = n0 + ^|E|2 = n0 + TI 0)

where n 0

is the linear index of refraction, E is the peak electric field (cgs), and I denotes the irradiance (MK.S) of the

laser beam within the sample. [n

2 and K are related through the conversion formula, n 2(esu)=(cn 0/407r)7(m J /VV), where c (m/sec) is the speed of light in vacuum]. Assuming a TEM00 Gaussian beam of beam waist radius w0 traveling in the +z

direction, we can write E as:

127
E(z,r,t) = E0

(t) -^expf-^-^j <*M . (2)

where w2 (z)=w0

2 (l+z2/z0

2

) is the beam radius, R(z)=z(l+z0

2/z2

) is the radius of curvature of the wavefront at z, z

0=kw0

2/2 is the diffraction length of the beam, k=2^/A is the wave vector and A is the laser wavelength, all in free space. E0

(t) denotes the radiation electric field at the focus and .contains the temporal envelope of the laser pulse. The

e

-i^(«,t) term contains all the radially uniform phase variations. As we are only concerned with calculating the radial phase variations A$r), the slowly varying envelope approximation (SVEA) applies, and all other phase changes that are uniform in r are ignored.

If the sample length is small enough that changes in the beam diameter within the sample due to either diffraction or nonlinear refraction can be neglected, the medium is regarded as "thin", in which case the self-refraction process is referred to as "external self-action". Such an assumption simplifies the problem considerably, and the amplitude VT and

phase <f> of the electric field as a function of z' are now governed in the SVEA by a pair of simple equations:

dz'

and

dl_ dz'

^= An(I)k, (3)

-a I , (4)

where z' is the propagation depth in the sample and a is the linear absorption coefficient. Note that z' should not be

confused with the sample position z. In the case of a cubic nonlinearity, Eqns. 3 and 4 are solved to give the phase shift A0 at the exit surface of the sample, which simply follows the radial variation of the incident irradiance at a given

position of the sample z. Thus,

A#z, r, t) = A0o(z, t) exp - , (5-a)

with

A$„(t)

A$0(t), the on-axis phase shift at the focus, is defined as,

A$„(t) = kAn0(t) , (6)

where L is the sample length, and An0 = 7l0

(t) with I 0(t) being the on-axis irradiance at focus (ie.z=0). Again we take

I 0(t) as the irradiance within the sample to account for Fresnel reflection losses.

The complex electric field after the sample, E', now contains the nonlinear phase distortion,

E' = E(z,r,t) e-aL/2 e

iA^r- t

) . (7)

By virtue of Huygen's principle one can obtain the far field pattern of the beam at the aperture plane through a zeroth

order Hankel transformation of E'.[14] We will follow a more convenient treatment applicable to Gaussian input beams

which we refer to as the "Gaussian Decomposition" (GD) method given by Weaire et. al.[15], in which they decompose

the complex electric field at the exit plane of the sample into a summation of Gaussian beams through a Taylor series expansion of the nonlinear phase term e

iA*( E ' r' 1

) in Eq. 7. That is,

oo y

- -- - - -- v : m=0

,iA*(«,r,t) = V [^o(M)r e

. 2mr2/w2(E)

/ . m!

m=0

Each Gaussian beam can now be simply propagated to the aperture plane where they will be resummed to reconstruct

128
the beam. When including the initial beam curvature for the focused beam, we derive the resultant electric field pattern

at the aperture as:

oo Ea (r,t) = E(z,r=0,t)e-°W2 V (iA^0(t)r

m!

m=0

g

z+ d

2

j

exp f- wz

ikr2 2Rm

+ iff. (9)

where d is the propagation distance in free space from the sample to the aperture plane, and g=l+d/R , R=R(z) being

the beam radius of curvature at the sample. As long as the far field condition is met, d can be considered independent

of the sample position z resulting in symmetric Z-scans. The remaining parameters in Eq. 9 are expressed as:

_ w2(z)

2m+l

w2 = w: mO g

z+ d

2

R,

g

2 +d2/d 2 and 6m = tan -1 d/dr

The expression given by Eq. 9 is a general case of that derived in Ref. [15] where they considered a collimated beam

(R=oo) for which g=l. We find that this GD method is very useful for the small phase distortions detected with the Zscan method since only a few terms of the sum in Eq. 9 are needed. The method is also easily extended to higher order

nonlinearities.

The transmitted power through the aperture is obtained by spatially integrating Ea

(r, t) up to the aperture radius r a , giving,

PT(A<l> 0

(t)) ce

1 Jo

Ea(r,t)| 2 rdr (10)

Including the pulse temporal variation, the normalized Z-scan transmittance T(z) can be calculated as:

1

oo PT (A$0(t)) dt

T(z) =

i:

(ii)

S | Pi(t)dt

-oo

where P^t) = 7rw0

2

I 0(t)/2 is the instantanous input power (within the sample) and S is the aperture transmittance in the

linear regime.

We first consider an instantaneous nonlinearity and a temporally square pulse to illustrate the general features of the Zscan. This is equivalent to assuming cw radiation and the nonlinearity has reached the steady state. The normalized

transmittance, T(z), in the far field, is shown in Fig. 2 for A$0 = ± 0.1 and a small aperture (S=0.01). They exhibit the expected features, namely a valley-peak (v-p) for the positive nonlinearity and a peak-valley (p-v) for the negative one. For a given A$0

, the magnitude and shape of T(z) do not depend on the wavelength or geometry as long as the far field condition for the aperture plane is satisfied. The aperture size S, however, is an important parameter since a large

aperture reduces the variations in T(z). This reduction is more prominent in the peak where beam narrowing occurs and

can result in a peak transmittance which cannot exceed (1-S). Needless to say, for very large aperture or no aperture

(S=l), the effect vanishes and T(z) = 1 for all z and A<3> 0

. For small |A$0 |, the peak and valley occur at the same

distance with respect to focus, and for a cubic nonlinearity this distance is found to be ^0.85zo

. With larger phase

distortions (|A$0 |>1) this symmetry no longer holds and both peak and valley move toward ±z for the corresponding

sign of nonlinearity (±A$0

) such that their separation remains relatively constant given by,

AZp

. v ~ 1.7z0 (12)

129
1.08 A* 0 =±0.25

-6 -3 0 3 6

Z/ZO

Fig.2 Calculated Z-scan transmittance curves for a cubic nonlinearity with either polarity and a small aperture

(S=0.01).

We can define an easily measurable quantity ATp

_ v as the difference between the normalized peak and valley

transmittance: Tp

- Tv

. The variation of this quantity as a function of |A$0 |, as calculated for various aperture sizes is illustrated in Fig. 3. These curves exhibit some useful features. First, for a given order of nonlinearity, they can be

considered universal. In other words, they are independent of the laser wavelength, geometry (as long as the far field condition is met) and the sign of nonlinearity. Second, for all aperture sizes, the variation of ATp

_ v

is found to be

linearly dependent on |A$0 |. Particularly for on axis (S^O) we find,

ATp

. v — 0.406 |A*0

| for |A<E>0 | < n , (13-a)

to be accurate to within 0.5 percent. As shown in Fig. 3, for larger apertures, the linear coefficient 0.405 decreases such

that with S=0.5 it becomes 0.34 and at S=0.7 it reduces to 0.29. Based on a numerical fitting, the following relationship can be used to include such variations within a ±2% accuracy;

ATp

. v ~ 0.406(1 -S)° 26

| A$0 | for |A$0

| < n . (13-b)

The implications of Eqns. 13-a and 13-b are quite promising in that they can be used to readily estimate the nonlinear

index (n2

) with good accuracy after a Z-scan is performed. What is most intriguing about these expressions is that they

reveal the highly sensitive nature of the Z-scan technique. For example, if our experimental apparatus and data

acquisition systems are capable of resolving transmission changes ATp

. v of =d°/o, we will be able to measure phase

changes corresponding to less than A/250 wavefront distortion. Achieving such sensitivity, however, requires relatively

good optical quality of the sample under study. We describe in the experimental section IV a means to minimize

problems arising from poor optical quality samples.

We can now easily extend the steady state cw results to include transient effects along with pulsed radiation by using the time averaged index change (An0

(t)) where,

I

oo An0

(t) I 0(t)dt -oo (An0(t))= , (14)

I.

oo

I 0(t)dt

The time averaged (A$0

(t)) is related to (An0

(t)) through Eq. 6. With a nonlinearity having instantaneous response and

decay times relative to the pulsewidth of the laser, one obtains for a temporally Gaussian pulse:

130
1.20

H i/2 J« Phase Change (red.)

Fig.3 Calculated ATp

. v as a function of the phase shift at the focus (A$0

). The sensitivity, as indicated by the

slope of the curves, decreases slowly for larger aperture sizes (S>0).

(An0

(t)) = An0/v/2 , (15)

where An0 now represents the peak-on-axis index change at the focus. For a cumulative nonlinearity having a decay

time much longer than the pulsewidth (eg. thermal), the instantaneous index change is given by the following integral:

where A a constant which depends on the nature of the nonlinearity. If we substitute Eq. 16 into Eq. 14 we obtain a fluence averaging factor of 1/2. That is,

where F is the pulse fluence at focus within the sample. Interestingly, the factor of 1/2 is independent of the temporal

pulse shape.

4. EXPERIMENTAL RESULTS

We examined the nonlinear refraction of a number of materials using the Z-scan technique. Fig. 4 shows a Z-scan of a

1 mm thick cuvette with NaCl windows filled with CS2 using 300 ns TEA C02 laser pulses having an energy of 0.85 mJ. The peak-valley configuration of this Z-scan is indicative of a negative (self-defocusing) nonlinearity. The solid line in Fig. 4 is the calculated result using (A$o)=-0.6 which gives an index change of (An0)~-lxl0~ 3

. As mentioned earlier such detailed theoretical fitting is not neccessary for obtaining (An0) (only ATp

. v

is needed). The defocusing effect shown in Fig. 4 is attributed to a thermal nonlinearity resulting from linear absorption of CS2 (a ~ 0.22 cm-1

at 10.6

fim). The risetime of a thermal lens in a liquid is determined by the acoustic transit time, r=w0/vB

, where vs is the

velocity of sound in the liquid [17]. For CS2 with v8 ~ 1.5 x 10 5 cm/sec and having wQ~60 /im, we obtain a risetime of

~40 ns which is almost an order of magnitude smaller than the TEA laser pulsewidth. Furthermore, the relaxation of

the thermal lens, governed by thermal diffusion, is of the order of 100 ms.[17] Therefore, we regard the nonuniform

heating caused by the 300 ns pulses as quasi-steady state, in which case, from Eq. 17, the average on-axis nonlinear

index change at focus can be determined in terms of the thermo-optic coefficient, dn/dT, as:

(16)

(An0(t)) = i A F , (17)
where F0

is the fluence, p is the density, Cv

is the specific heat and 1/2 denotes the fluence averaging factor. With the known value of pCv ~ 1.3 J/°Kcm3 for CS2

, we deduce dn/dT ~ -(8.3 +1.0)xl0" 4 °C_1 which is in good agreement with

the reported value of -8xl0~ 4 0C

_1

.[16]

Fig. 4 Measured Z-scan of a 1mm thick CS2 cell using 300 ns pulses at A=10.6 fim indicating thermal self- defocusing. The solid line is the calculated result with A$o=-0.6 and 60% aperture (S=0.6).

With ultrashort pulses, nonlocal nonlinearities such as thermal or electrostriction are no longer significant. Particularly,

in CS2

, the molecular reorientational Kerr effect becomes the dominant mechanism for nonlinear refraction. CS2is frequently used as a standard reference nonlinear material.[18, 19] We have used picosecond pulses at 10.6 /xm, 1.06 /xm

and at 0.53 /jm to measure n 2 in CS2

. We obtain the same value of n2

, within errors, at all three wavelengths,

(1.5±0.6)xl0_n esu at 10.6 /im, (1.3±0.3)xl0-11 esu at 1.06 nm and (I.2±0.2)xl0-11 esu at 0.53 iim. The external self- focusing arising from the Kerr effect in CS2

is shown in Fig. 5, where a Z-scan of a 1mm cell using 27 ps (FWHM)

pulses focused to a beam waist w0 of 26 fim from a frequency doubled Nd:YAG laser is illustrated. Its valley-peak

configuration indicates the positive sign of n2

. With ATp

. v = 0.24, and using Eq. 13-b with a 40 percent aperture (S =

0.4), one readily obtains a (An0

) = 5.6 x 10" B

. Using the peak irradiance of 2.6 GW/cm2

, this value of (An 0) corresponds to an n 2 ~ (1.2 ± 0.2)x 10~ n esu.

I. IS

-13.0 o.o Z (mm)

13.0 28.0

Fig. 5 Measured Z-scan of a 1mm thick CS2 cell using 27 ps pulses at A=532 nm. It depicts the self-focusing

effect due to the reorientational Kerr effect.

]32
The main source of uncertainty in the value of n2

is the absolute measurement of the irradiance. A plot of ATp

. v versus peak laser irradiance as measured from various Z-scans on the same CS2 cell is shown in Fig. 6. The linear behavior of

this plot follows Eq. 13 as derived for a cubic nonlinearity.

32

000 090 1.80 2 70 3 60 Irradiance (GW/cm2

)

Fig.6 Tp

. v

in percent as a function of the peak irradiance from the Z-scan data of CS2 at 532 nm, indicative of

the reorientational Kerr effect.

Transparent dielectric window materials have relatively small nonlinear indices. Recently, Adair et. al. [20] have

performed a careful study of the nonlinear index of refraction of a large number of such materials in a nearly

degenerate-three-wave-mixing scheme at A~1.06 fxm. Using the Z-scan technique, we examined some of these materials

at 532 nm. For example, the result for BaF2 (2.4mm thick) is shown in Fig. 7, using the same beam parameters as for CS2

. This Z-scan was obtained by purposely lowering the pulse energy to 2 /iJ in order to observe the resolution and the

sensitivity of this measurement. With a ATp

. v ~ 0.035, this Z-scan corresponds to a A/75 induced phase distortion. For

a unity signal-to-noise-ratio for our particular laser system, it is seen from Fig. 7 that the sensitivity to phase distortion

is better than A/300. For laser systems having better amplitude and pulsewidth stability, the sensitivity should be

correspondingly increased.

1.02

o

Z (mm)

Fig. 7 Measured Z-scan of a 2.4 mm thick BaF2 sample using 20 ps pulses at A=532 nm indicating the self- focusing due to the electronic Kerr effect. The solid line is the calculated result with A$o=0.085

corresponding to ~A/75 total phase distortion. The error bar shown corresponds to approximately A/480

induced phase distortion.

133
Aside from the statistical fluctuations of the laser irradiance, surface imperfections or wedge in the sample may lead to systematic transmittance changes with z that could mask the effect of nonlinear refraction. We found, however, that such "parasitic" effects may be substantially reduced by subtracting a low irradiance background Z-scan from the high

irradiance scan. A simple computer simulation of this process assuming that the surface imperfections do not disturb the circular symmetry of the beam or cause any beam steering, indicated that background subtraction indeed recovers the

original ATp

_ v arising from the nonlinear refraction effect even for quite large surface disturbances A<£, of up to jr.

Returning to the Z-scan of Fig.7, we obtain n2 ~ (0.81.15) xlO-13 esu for BaF2 at 532 nm, which is in close agreement

with the reported values of 0.7xl0" 13 esu [20] and l.OxlO-13 esu [3] as measured at 1.06 nm using more complex

techniques of nearly degenerate-three-wave-mixing and time-resolved-nonlinear-interferometry, respectively. Similarly

for MgF2

, we measure n2~0.25xl0~ 13 esu at 532 nm as compared to the reported value of 0.32xl0-13 esu at 1.06 /im

for this material as given in [20]. Dispersion in n2 for these materials between 1 and 0.5 nm is expected to be minimal.

It should be noted that the n2 values extracted from the Z-scans are absolute rather than relative measurements. If the beam parameters are not accurately known, however, it should be possible to calibrate the system by using a standard

nonlinear material such as CS2

.

5. CONCLUSION

We have demonstrated a simple single beam technique that is sensitive to less than A/300 nonlinearly induced phase

distortion. Using the Z-scan data the magnitude and sign of the nonlinear refraction can be simply determined. We

have derived relations that allow the refractive index to be obtained directly from the Z-scan data without resorting to computer fits. We have applied this technique to several materials displaying a variety of nonlinearities on different

time scales. It is expected that this method will be a valuable tool for experimenters searching for highly nonlinear

materials.

6. ACKNOWLEDGEMENT

We gratefully acknowledge the support of the National Science Foundation grant #ECS-86 17066, and the support of

DARPA/CNVEO and the Florida High Technology and Industry Council. We also wish to thank Alan Miller for helpful discussions.

7. REFERENCES

1. Sheik-Bahae, M.; Said, A. A.; Van Stryland, E. W. "High Sensitivity Single Beam n 2 Measurement," Opt. Lett. 14,955-957(1989)

2. Weber, M.J.; Milam, D.; Smith, W.L. "Nonlinear Refractive Index of Glasses and Crystals," Optical Engineering,

17^463-469 (1978).

3. Moran, M.J.; She, C.Y.; Carman, R.L. "Interferometric Measurements of Nonlinear Refractive-Index

Coefficient Relative to CS2 in Laser-System-Related Materials," IEEE J. Quantum Electron. QE-1

, 259-263

(1975).

4. Friberg, S.R.; Smith, P.W. "Nonlinear Optical Glasses for Ultrafast Optical Switches," IEEE J. Quantum Electron. QE-23 , 2089-2094 (1987).

5. Adair, R.; Chase, L.L.; Payne, S.A. "Nonlinear refractive index measurement of glasses using three- wave

frequency mixing," J. Opt. Soc. Am. B_4, 875-881 (1987).

6. Owyoung, A. "Ellipse Rotations Studies in Laser Host Materials," IEEE J. Quantum Electron. QE-9, 1064-1069

(1973).

7. Williams, W.E.; Soileau, M.J.; Van Stryland, E.W. "Optical Switching and n 2 Measurements in CS2

," Opt. Comm.

50,256-260 (1984).

8. Williams, W.E.; Soileau, M.J.; Van Stryland, E.W. "Simple direct measurements of n2

," Proc. 1 5th Annual Symp.

on Optical Materials for High Power Lasers, Boulder, CO (1983).

9. Hill, J.R.; Parry, G.; Miller, A. "Non-linear Refraction Index Changes in CdHgTe at 175K with 1.06 /im

Radiation," Opt. Commun., 43, 151-156 (1982).

10. Boggess, T.F.; Moss, SC.; Boyd, I.W.; Smirl, A.L. "Picosecond Nonlinear-Optical Limiting in Silicon," in Ultrafast Phenomena IV, edited by D.H. Huston and K.B. Eisenthal (Spriner-Verlag), (New York, 1984). pp. 202.

11. Harris, J.M.; Dovichi, N.J. "Thermal Lens Calorimetry," Analytical Chem. 52, 695-700(1980).

12. Akhmanov, S.A.; Sukhorokov, A.D.; Khokhlov, R.V. "Self-focusing and Diffraction of Light in a Nonlinear

Medium," Sov. Phys. Vspekhi; English Translation, K), 609 (1968).

13. Smith, W.L.; Bechtel, J.H.; Bloembergen, N. "Dielectric-Breakdown Threshold and Nonlinear-refractive Index

Measurements with Picosecond Laser Pulses, Phys. Rev. B L2, 706-714 (1975).

14. Gaskill, J.D. Linear Systems, Fourier Transforms, and Optics, John Wiley & Sons, New York, 1978.

134
15. Weaire, D.; Wherrett, B.S.; Miller, D.A.B.; Smith, S.D. "Effect of Low-Power Nonlinear refraction on laser beam propagation in InSb," Opt. Lett., 4, 331-333 (1974).

16. Raman, V.; Venkataraman, K.S. "Determination of the adiabatic piezo-optic coefficient of liquids." Proc. Roy.

Soc. A, 171, 137 (1939).

17. Hayes, J.N. "Thermal Blooming of Laser Beams in Fluids," Appl. Opt.^, 455-461 (1972).

18. Ho, P.P.; Alfano, R.R. "Optical Kerr effects in Liquids," Phys. Rev. A, 20, 2070-2087 (1979).

19. Thomas, P.; Jares, A.; Stoicheff, B.P. "Nonlinear Refractive Index and "DC Kerr Constant of Liquid CS2

," IEEE J. Quantum Electron. QE-10, 493-494(1974).

20. Adair, R.; Chase, L.L.; Payne, A. "Nonlinear Refractive Index of Optical Crystals," Phys. Rev. B, 39,

3337-3350(1989).

COMMENTS

Question: Are your results affected by surface nonuniformity?

Answer: If the scatter is homogeneous then it's no problem. If it's

grossly inhomogeneous or if the surface is corrugated or something, then it really effects the technique quite a bit. In

fact, you can use it to measure surface quality as well if you

turn the radiance down to very low values . We have done

experiments where we had rough surfaces and just took data from

lower radiance values. Then we took the data from higher radiance

values and just divided it up. We still get good signal, but it

does degrade as the quality of the optics degrade

135
Manuscript received

8-20-89

OPTICAL BREAKDOWN IN PARTICLE SUSPENSION

Kamjou Mansour, M.J. Soileau and E.W. Van Stryland

Center for Research in Electro-Optics & Lasers (CREOL)

University of Central Florida

12424 Research Parkway, Orlando, FL 32826

We have characterized the nonlinearities observed in suspensions of carbon black particles in liquids (CBS). We

have developed a preliminary explanation of the optical limiting characteristic of the CBS that qualitatively explains the low limiting thresholds. We have found that the limiting depends primarily on the input optical fluence (J/cm2

) rather than irradiance (W/cm 2

). We have monitored transmission, side scattered light, and the photoacoustic response of the CBS simultaneously. The nonlinear scattered light appears to be the dominant nonlinearity. Additionally, we have

observed that the nonlinearities disappear after repeated laser firings. Thus, in essense, we are performing a laser induced damage experiment, and we have prepared a material with a low damage threshold. These data have led us to the following model. The carbon first linearly absorbs the input light efficiently. The carbon is rapidly heated,

vaporizes and ionizes to form a rapidly expanding microplasma. This plasma absorbs and scatters subsequent light, thus

limiting the transmittance.

Key words: carbon black suspension; nonlinear absorption; nonlinear scattering; nonlinear transmission; nsec pulses;

optical limiting; psec pulses; .53 /im, 1.06 /im.

1. INTRODUCTION

Recently there has been a growing interest in the use of microparticle suspensions as nonlinear media. Here, we

demonstrate and review the dynamics of limiting in a suspension of absorbing microparticles that exhibit primarily

nonlinear scattering and nonlinear absorption. The absorbing microparticles of interest are carbon black particles which

are ~ 35 nm in diameter with agglomerates up to 500 nm in diameter. We demonstrate an optical limiting threshold as low as 80 watts peak power for 0.532 /im and 160 Watts for 1.06 /im using pulses of 14 nsec and 20 nsec (FWHM),

respectively.

Because of the structure of these dispersions, different mechanisms including nonlinear absorption, nonlinear

refraction (self-focusing, self-defocusing), nonlinear scattering and combinations of these mechanisms could be the source of the optical nonlinearity leading to such a low optical limiting threshold.fi] In order to investigate the relative contributions of these mechanisms we perform a series of experiments. We first perform optical limiting experiments

in the CBS and in the carbon black particles which are deposited on the surface of a glass substrate at 0.532 /im and

1.06 /im for nanosecond pulses. We present transmission measurements which directly show the fluence dependence of

limiting in CBS. We also perform a measurement that monitors the absorptance, transmittance and fraction of side scattered light simultaneously. As will be demonstrated, we are performing a laser-induced-damage experiment on a material with an extremely low damage threshold. This leads us to discuss and show the repetition rate dependence of

the CBS, and carbon black particles deposited on the glass substrate.

2. OPTICAL LIMITING

The optical geometry often used as a screening test, as it is sensitive to both nonlinear absorption and nonlinear

refraction, is shown in Figure l.[l] We used this arrangement with pulses from an Nd:YAG laser operated in the TEM00 mode of 14 ns (FWHM) duration at 0.532 /im and 20 ns at 1.064 /im. These pulses were focused by either a 40mm (.532/tm) or 50mm (1.064/im) "best form" lens, shown as Lr

This results in calculated beam radii of 3.5 /im at a wavelength of .532 /im and 7 /im at 1.064 /im inside the 1 cm cuvette. The spot radii quoted in this paper are halfwidths at the 1/e 2 maximum in irradiance. The transmitted pulse was then collected by a 102 mm focal length "best

form" lens L2 placed behind the sample. The laser beam then passed through a 400 /im aperture placed in front of a

silicon photodiode. This system had an overall low input transmittance of 70 percent for both wavelengths.

Using this optical geometry we have looked at optical limiting in various concentrations of CBS and a known

standard sample, CS2

, which has been previously used as an optical limiting material [2]. Optical limiting in CS2

is

primarily due to molecular reorientation of the molecules athough a small contribution from electrostriction may be

13G
present for nanosecond pulses and tight focusing geometries. The optical limiting of CBS and CS2 in a 1 cm thick cuvette is shown in Figure 2 for nanosecond laser pulses at 0.532 /im. In order to resolve the onset of limiting, the measurement was performed for lower input powers as shown in Figure 3. The CBS begins clamping the transmitted

fluence above approximately 80 watts input power and continues clamping for ~5 orders of magnitude of increased

input power. It is important to note that each laser firing irradiates "new" material as discussed in Section 5. The

limiting of the CBS at 1.064 /im is shown in Figures 4 and 5. As is shown, the onset of limiting for 1.064 /im laser pulses is approximately 160 watts. The optical limiting in a film of carbon black particles deposited on a glass substrate

at 1.06 /im is shown in Figure 6. The sample was moved to a new site for each laser firing. As is shown, the onset of

limiting for 1.064 /im laser pulses is approximately 600 watts.

The observed optical limiting using the geometry shown in Figure 1, could be due to several mechanisms including

nonlinear absorption, nonlinear refraction and nonlinear scattering. Below we describe a series of experiments

performed to determine the dominant nonlinearity.

3. NONLINEAR TRANSMITTANCE MEASUREMENT

We have studied the fluence dependence of the transmittance of CBS at 0.532 /im and 1.064 /zm. The optical

goemetry used is shown in Figure 7. Both a one meter and 50 centimeter "best form" lens were used to focus the nsec pulsed input beams at the two wavelengths. This resulted in measured spot sizes of 108 /im and 254 /zm for 0.532 /zm

light. The transmitted signal was collected by a large area uniform response Si photodiode located very close to the cell. The diameter of the detector is 1 centimeter and the beam radius at the detector is approximately 1mm in diameter.

The difference between this geometry and the optical limiting geometry of Figure 1 is that we are not using tight

focusing and the aperture has been removed so that this experiment is insensitive to nonlinear refraction. The results of

the transmittance measurements are plotted as a function of incident fluence and incident power in Figures 8, and 9 for 0.532 /zm. For incident fluences above 200 mj/cm2 for 0.532 /im, the transmission becomes increasingly nonlinear and

is essentially identical for both beam radii at a given wavelength as shown in Figure 8. However, the onset of a change

in transmittance appears at two different incident power levels for these beam radii as shown in Figure 9. Similar

results are observed for 1.06 /im laser pulses. We also observed that the onset of limiting for both nanosecond and

picosecond measurements occurs approximately at the same fluence. The experiments indicate that the nonlinear

transmittance observed is fluence dependent.

4. MEASUREMENT OF TRANSMITTANCE, SCATTERING AND ABSORBTANCE

In order to understand the nature of this fluence dependent nonlinear transmittance, we have set up an experiment

which enables us to monitor for a single pulse , the absorptance and the fraction of side scattered light simultaneously

with the transmittance measurement. This simultaneous measurement was conducted using the Q-switched Nd:YAG

laser. The laser light was focused by a 1 meter focal length "best form" lens L2 into a one centimeter cuvette cell. (See

Figure 7) The transmitted light was monitored as previously described. The absorption of the incident light was

directly monitored by a sensitive piezoelectric transducer which was placed in contact with the bottom of the cuvette with silicone grease. Photoacoustic measurements have been used to directly detect very small changes in absorption.[3]

The side scattered light was collected by a two-inch diameter, 98 mm focal length lens and detected by a Si photodiode.

The optical geometry for this measurement is shown in Figure 11.

The results of the simultaneous measurement of absorptance, transmittance, and the fraction of side scattered light

(side scattering light divided by incident light) are plotted as a function of incident fluence for 1.064 /zm laser pulses in Figure 11. For very low input fluences the CBS behaves linearly and there is no change in the three quantities

monitored. However, for input fluences close to 360 mj/cm2 for 1.064 /zm light, the transmittance begins to decrease

nonlinearly as is shown by squares in Figure 11. The absorptance increases nonlinearly for incident fluence close to this threshold value (i.e., onset of change of transmittance) and levels off at a higher value even for input fluences two

orders of magnitude above threshold. The scattered fraction increases nonlinearly as the transmittance decreases as shown by circles in Figure 11. This trend continues for at least two orders of magnitude above threshold. Similar

results observed for .532 /zm light. From these results we see that blocking of the light by the CBS above threshold is primarily due to nonlinear scattering.

5. NONLINEAR SCATTERING

The blocking of the incident light by scattering can readily be photographed as shown in Figure 12. These

pictures were taken by looking at the side window of the cuvette perpendicular to the direction of the incident light.

For low input fluences, top picture, the bright region in the photograph represents the input pulse which is linearly scattered (Mie scattering) by carbon black particles. However, for incident fluences above 200 mj/cm2

, the light is nonlinearly scattered to the sides and thus, blocks the incident light from being transmitted. This is shown in the lower

picture.

The dynamics of the transmittance loss is studied by time resolved transmission measurements conducted using

nanosecond pulses. By delaying the transmitted pulse with respect to the input pulse we are able to monitor both the incident and transmitted pulses shown in Figure 13. For an incident fluence above 200 mj/cm 2

, there is a sharp cut off

in time of the transmitted pulse (shown in graph b). The transmitted laser pulse is most strongly attenuated during the

137
later portions of the pulse, thus, the pulse appears advanced in time. This experiment again suggests that the attenuation

of the laser beam is produced by a laser-induced-breakdown plasma.

Under this assumption, the attenuation is dominated by scattering losses of the laser light by the plasma as seen in Figures 11. Thus, we are performing a laser-induced-damage experiment, and we have prepared a material with an

extremely low damage threshold. This is consistent with having high linear absorption, and is also consistent with the disappearance of the nonlinearities after repeated laser irradiation as discussed in the next section.

6. REPETITION RATE DEPENDENCE

Recently, we reported the repetition rate dependence of the CBS.[4] In that report we indicated that the onset of

the limiting threshold for 1 and 10 Hz repetition rate laser pulses is considerably different and is higher for the 10 Hz

repetition rate. This repetition rate dependence was removed by flowing the material at such a rate that virgin material was exposed to each laser pulse.

This bleaching or reduction of limiting of the laser light in the nonflowing liquid after repeated irradiation on the same region of the sample can be observed by monitoring the transmitted signal and scattered signal at a fixed fluence

above the single shot threshold for limiting. Using the geometry shown in Figure 1 1 and setting the incident fluence at 350 mj/cm2 for 0.532 /xm laser pulses and a 1 Hz repetition rate, we can observe that the transmitted signal increases as the number of the incident pulses irradiating the sample increases. The transmittance levels off to a value that is less than the linear transmittance of the solvent as shown in Figure 14. The scattered signal reduces and levels off after repeated laser firings. This is shown as circles in Figure 14. For flowing liquid, stirred liquid or a long elapse time

between the laser pulses irradiating the liquid, we have observed that the transmitted signal and scattered signal remain

fixed as shown in Figure 15. The maximum limiting capability is for the first laser firing and recovery of the irradiated

region of the sample is due to convection in the liquid at room temperature which for the spot size used is a slow

process in comparison with the repetition rate of 1 Hz. Note that using a tight focusing geometry and a 1 Hz repetition

rate that we do not see these cumulative effects. We observed that this recovery time is proportional to the radius of

the laser beam in the cell, consistent with convection.

The bleaching of CBS can also be monitored as shown in Figure 16 where we have used a very large spot size of more than 254 fj,m. Here, we have used a HeNe laser beam expanded to illuminate the region close to the front surface

of the cell. The scattering of the HeNe beam by the suspended microparticles is readily observable. We irradiated the CBS using an incident fluence of 350 mj/cm2 and photographed the scattered light after 10 seconds. This is shown in Figure 17. The dark spot appearing in the middle of the bright screen is the irradiated region showing that the

scattering centers have been "removed" by the laser pulse. We find that this excited volume is slowly replaced by fresh CBS after approximately two minutes due to convection. The conjecture is that the microparticles have been

considerably reduced in size or "atomized" so that they no longer efficiently scatter the HeNe light. The carbon particles

deposited on glass are blown off the surface with repetitive firings and therefore, limiting also disappears.

7. NONLINEAR REFRACTION

In order to investigate contributions of phase distortion due to a thermooptic effect, electrostriction, or other

nonlinear refractive mechanism, we monitored the far field profile of the transmitted beam using a vidicon tube with an

optical multichannel analyzer. For incident fluences below threshold and up to an order of magnitude higher than

threshold, we have not observed any broadening of the spatial profile of the transmitted beam. Therefore, the change

in index of refraction of the CBS is very small and the phase distortion due to this index change is less than 0.2 A which

is the sensitivity of our detection system. From this measurement we conclude that the contribution of nonlinear

refraction is small in comparison with the nonlinear scattering observed. In order to further validate this point we

removed the aperture in the original limiting geometry shown in Figure 1 and observed no change in limiting threshold.

8. MODEL

It appears from our experimental results that the dominant mechanism in the CBS is nonlinear scattering. The

nature of this nonlinear scattering is consistent with the formation of microplasmas by laser-induced-breakdown of the carbon particles. Therefore, the dynamics of limiting in CBS can be described by a simple model. The carbon linearly

absorbs the incident light very efficiently. The carbon is rapidly heated to several thousand degrees Celcius. A simple

calculation of the particle temperature at threshold gives a few thousand degrees Celcius. The carbon then vaporizes and

ionizes to form a rapidly expanding microplasma which absorbs and scatters later portions of the laser light and blocks

the transmission. This model is also consistent with the disappearance of the nonlinearities after repeated laser firings as the carbon particles will have undergone vaporization.

9. CONCLUSION

We have characterized the nonlinearities observed in a suspension of carbon black particles (CBS) in liquid. We

have developed a qualitative model based on laser-induced breakdown to explain the low limiting threshold of the CBS.

From our results, we conclude that nonlinear scattering due to rapidly expanding microplasmas is the dominate

mechanism leading to limiting. While the evidence obtained is insufficient to give details of this damage process, the

138
mechanism leading to limiting. While the evidence obtained is insufficient to give details of this damage process, the predictions closely match our observations. We note that the broad-band nature of the nonlinearity is primarily due to the fact that carbon is black, i.e., it absorbs all wavelengths.

10. ACKNOWLEDGEMENTS

This research was supported with funds from the DARPA/CNVEO and the Florida High Technology and Industry

Council.

11. REFERENCES

1. E.W. Van Stryland, Y.Y. Wu, D.J. Hagan, M.J. Soileau, and K. Mansour, JOSAB, Vol. 5, 1980, September 1988.

2. M.J. Soileau, Williams E. Williams, and Eric W. Van Stryland, IEEE J. Quantum Electronics, QE 19, No. 4, 731,

April 1983.

3. A.C. Tarn and C.K.N. Patel, Appl. Opt. 18(19), 3348, 1979.

4. M.J. Soileau, Kamjou Mansour, E.W. Van Stryland, E. Canto, and David J. Hagan, Joint U.S. - U.K. Conference on Sensor Protection, Malvern, England, 1987.

139
BS Li L 2 Pin Hole

Sample Transmitted

Energy Detector

Input Energy Detector

Figure 1. Optical limiting geometry.

2.0

1.6

CO

•*— c

-Q

<

0

O

c

CD

LL 0.8

~o

CD

1.2

E

COc

03

0.4

0.0

0.0

X- 532nm

t P

= 14 ns (FWHM)

€

/ • • • # •

ack Suspension

2.4 4.8 7.2 9.6 12.0

Power (Kwatts)

Figure 2. Plot of the limiting of CS2 (circles) and CBS (squares) as a function of input power for 14 ns (FWHM), 0.532 /urn pulses.

140
-t—

'c

X)

<

CDO

c

CD

u.

"O

CD

E

w

c

03

8.0

6.4

4.8

3.2 -

1.6

0.0

• CS 2 Carbon Black Suspension

X - 632nm

t P

- 14nsec(FWHM)

_L

0.00 240 480 720

Power (Watts)

960 1200

Figure 3. Plot of onset of limiting of CS2 (circles) and CBS (squares) as a function of input power for 14 ns (FWHM), 0.53 pulses.

12.00

in ~ 10.00

-0<

01o

c

D

8.00

6.00

Z 4.00 |-

1

inc

? 2.00 h

0.00

CS 2 Carbon Black Suspension

X = 1.06 fim

tp = 20ns (FWHM)

i

i i

i 0.00 4.00 8.00 12.00 16.00 20.00 24.00

Power (K WATTS)

Figure 4. Plot of limiting of CS2 (circles) and CBS (squares) as a function of

input power for 20 ns (FWHM), 1.06 pm pulses.

141
r

= cs 2

= Carbon Black

Suspension

Ip

1.06 ^un 20ns (FWHM)

0.00 0.20 0.40 0.60 0.80

Power (K WATTS)

1.00 1.20

Figure 5. Plot of on set of limiting of CS2 (circles) and CBS (squares) as a

function of input power for 20 ns (FWHM), 1.06 ixm pulses.

12.00

- 10.00

8.00

6.00

Z 4.00

2.00 -

0.00

• Glass

= Carbon Film

X = 1 .06 fim

lp e 20ns (FWHM)

•

• •

•

.

• :

.

• /

i i i i i 0.00 0.3 0.6 0.0 1.2 1.5 Power (K WATTS)

1.8

Figure 6. Plot of limiting of Carbon Particle on glass (squares) and glass as a function of input power for 20 ns (FWHM), 1.06 /zm pulses.

142
BS L1

A

FOCAL LENGTH

1 meter

SAMPLE

OUTPUT DETECTOR

INPUT DETECTOR

Figure 7. Optical geometry for transmission measurement.

0.76

Q. 0.71

c

Q.

D 0.66

O<^

CD

o

§ 0.62

E

CO

CO

0.52

X = 532nm

t P

- 14nsec(FWHM)

0

1-

-v.

±

160 320 480 640

Fluence (mJ/cm2

)

800

Figure 8. Measured CBS transmittance as a function of incident fluence

(mj/cm2

) for 108 /jm (circles) and 254 /im (squares) spot sizes at 0.532 /xm.

143
0.76

0.71

0.66

0.62

0.57

0.52

X = 532nm

t P

= 14ns (FWHM)

4

1

1

0.0 9.6 19.2 28.8

Power (Kwatts)

38.4 48.0

Figure 9. Measured CBS transmittance as a function of incident power

(KW) for 108 /im (circles) and 254 /zm (squares) spot sizes at 0.532 fim.

BS

Variable

Attenuator

Liquid Cell Transmitted

Lens Energy

Monitor

Input Energy

Monitor \r

Transducer

Scattered Energy

Monitor

Figure 10. Optical geometry used to monitor the transmittance, absorptance

and side scattered light.

144
1.0

— 0.6

o

CO

<

0.8 -

0.4

0.2

0.0

X - 1 064nm •

tp 2 0ns • ••

• •

t

Transmlttance 9 • Scattering Fraction

A Absorptance •

AA A Aa A a AAA AA A A AAA A

aaA^

• AM4AAAi

^ •

i i i

1 0 240 480 720 960 1200

Fluence (mJ/cm2

)

Figure 11. Plot of change of transmittance (squares), absorptance (triangle)

and scattering fraction (circles) as a function of incident fluence (mj/cm2

) for 1.06 fjtm, 20 ns pulses.

1

145
X =532 nm

tp = 1 4 nsec

Low Intensity

Front of the Cell^ 1 CM +Back of the Cell

High Intensity

Figure 12. Photographs A and B represent the input pulse which is scattered by CBS for low and high incident fluences respectively.

146
1.00

0.78

0.56

< 0.34

0.12

0.10

A = 532nm

tp = 14ns (FWHM)

Incident Pulse

(b)

Transmitted Pulse

18 36 54 72 90

Time (ns)

Figure 13. Graphs A and B represent the incident and transmitted pulse for

fluence levels below and above the threshold. Incident pulse in graph B has

been attenuated by a factor of 10 in respect to the transmitted pulse.

14 7
1.20

0.96

0.72

0)

03O

CO

2 0.48

0.24

0.00

k 1 064nm

tp • 20ns • Scattered Signal

Transmitted Signal

• | ."»

0 6 12 18 24 30

Number of Pulses (Fixed Energy)

Figure 14. Plot of fraction of scattered light (circles) and transmittance

(squares) as a function of the number consecutive laser pulses for 1.06 nm,

20 ns pulses at a fluence of 450 mj/cm2

.

1.20

0.96

— 0.72

o

CO

0.48

0.24

0.00

X - 1 064nm

tp - 20ns (FWHM) • Scattered Signal

Transmitted Signal

Flowing Carbon Black Suspension

_1_ 0 6 12 18 24 30

Number of Pulses (Fixed Energy)

Figure 15. Plot of fraction of scattered light (circles) and transmittance

(squares) in flowing CBS as a function of the number of consecutive laser pulses for 1.06 /im, 20 ns pulses at a fluence of 450 mj/cm2

.

148
Scattered HeNe

by carbon black particle

\

\

s

s

\\

Camera

Pulse Laser Light

532 nm

HeNe Light

632 nm

Figure 16. Optical geometry to monitor "bleaching" of carbon particles in CBS.

149
Exposed Area Scattered HeNe Light

Film Exposure after 1 0 Seconds

(b)

Film Exposure after 2 Minutes

Figure 17. Photographs A and B represent the "bleaching" of carbon

particles within the irradiated region and replacement of the irradiated

region with fresh liquid, respectively.

150
Manuscript received

ANNEALING OF INDUCED DAMAGE IN 3-21-90

FLUORIDE GLASS COMPONENTS

OSAMA H. EL-BAYOUMI AND MICHAEL J. SUSCAVAGE

Solid State Sciences Directorate

Rome Air Development Center

Hanscom AFB, MA 01731

AND

L. PIERRE DE ROCHEMONT

SpecTran Corporation

Sturbridge, MA

Heavy metal fluoride (HMF) glasses are of considerable interest for use

as multi spectral optical components. HMF glass samples of different

compositions were prepared and subjected to different doses of gamma

radiation at room temperature. Optical absorption measurements showed

that strong damage occurred in the UV and visible regions and no significant losses were detected in the infrared region.

1.0 INTRODUCTION

Heavy metal fluoride (HMF) glasses are of considerable interest for use as multi spectral

mid-infrared/visible shared aperature optical components [1 & 2], The primary characteristics

are continuous transparency from the mid-ir (-8.5 urn) to near-uv (-0.25 um), a low

refractive index, (< or = 1.5), low dispersion over both the ir and visible (Abbe # = 80), low

rayleigh scattering (less than silica glasses), low thermal distortion, low non-linear

coefficient and resistance to chemical attack by gases and halide acids as reported by

El-Bayoumi, et al. (3). An envisioned application for this glass family is in ionizing radiation environment. HMF glasses offer potentially lower intrinsic loses and lower susceptibilities to nuclear radiation than do silica-based glasses. (4).

Impurities and imperfections exist in all optical components and can contribute significantly

to intrinsic optical losses. For some applications, a certain level of imperfections can be

accepted under quiescent conditions which may become absorbing centers when subjected to

ionizing radiation. The susceptibility of silica-based bulk glasses and fibers to nuclear

radiation has been extensively studied and reviewed [5]. In oxide glasses, loses observed in

the near-ir are due to the long radiation-induced absorption tails that are centered in the

visible and near-uv spectral regions. The formation of the color-centers depends primarily on the total radiation dosage, dose rate, temperature, glass composition, and exposure time. These

losses, which usually have both transient and permanent characteristics, can si yni f i cantly

compromise the performance of optical components in nuclear environments. HMF glasses, on the

other hand, may be less susceptible to ionizing radiation because the induced absorption bands

usually peak at shorter wavelengths and have tails which decrease rapidly at longer wavelengths.

In this paper, we report the effect of ionizing gamma radiation on the optical properties of

bulk HMF glasses in the spectral region 0.2um to 9.Uum. The concentration of I nF 3 on

NaF and, consequently, the content of fluorine ions on the radiation induced damage in the

fluorozi rconate glass family (ZBLA) is examined. Irradiated glass samples were annealed at

100°C to increase the fluorine ion mobility without grossly affecting the glass structure and

at temperatures near the glass transition temperature (Tg) where structural relaxation can occur. However, within two hours after irradiation, annealing at room temperature also became

significant in the visible region. The primary objective of this work is to determine parameter

required to anneal some or all of the radiation induced damage without compromising the

integrity of the optical component.

2.0 EXPERIMENTAL

In this study, four compositions based on fl uorozi rconate (ZBLA) glass having a molar ratio

57ZrF4 - 36BaF2 - 3LaF 3 - 4A1F 3 were selected. This particular ZBLA glass was selected because

of its stability and it has been extensively characterized [6-8]. The glass compositions

and the modulated fluorine ion content of the base glass (a) are listed in table 1. Glass

151
transition tempertures (Tg) and crystallization temperatures (Tx) were determined using a Perkin-Elmer 7500 differential scanning calorimeter (DSC) at a heating rate of 10°C/min in an argon atmosphere.

Commercially available anhydrous fluorides were used as starting materials: ZrF 4 (99.0% or better, Cerac) was sublimed in-house to remove carbon -and oxide impurities. The sublimation

procedure is reported elsewhere [9]. BaF2 and NaF ("Optipur" grade, EM Laboratories), LaF

(99.9%, Rare Earth Division, American Potash and Chemical Corporation), AIF3 and InF 3 ("Optical

grade, 99.5% Cerac) where melted without further refining. All chemicals were handled, weighed

and mixed in an argon filled glovebox, with less than 20 ppm oxygen and water.

GLASS GLASS COMPOSITION (MOLE%) MOLE %

SAMPLE

I.D.

ZrF4 BaF2 LaF3 A1E InF3 NaE

F

A 57 36 3 4 0

1402 57 36 3 3 1 0

1430 53 20 4 3 20 28

D 57 36 3 4 2 -2

Tg = (265 - 320) C

Tx = (315 - 400) C

Table 1. Composition, Fluorine ion Variation, Glass Transition (Tg), Crystallization (Tx)

Temperature Range.

Fluoride glass batches were melted in vitreous carbon crucibles using a radio frequency heated

assembly. The furnace is sealed to the bottom of the glovebox allowing the top to be opened to

an atmosphere controlled area as shown in figure 1. All glass batches were melted for two

hours at 770°C. At the end of the melting cycle, the molten glass was stirred and allowed to

cool to 575°C before being poured into a brass mold preheated to Tg-15°C (glasses A&D) or crucible cooled (glasses 1402 & 1430). All glasses were annealed at Tg for two hours and

cooled to room temperature over four hours. The ylasses were cut and polished to a 1.0 micron

finish usiny diamond paste.

Glass samples were irradiated at Rome Air Development Center's radiation facilities at Hanscom

AFB, MA usiny cobalt-60 yamma source at room temperature for roughly 20 minutes, depending upon

the dose required. All absorption measurments started within ten minutes of the irradiation to

minimize the effect of room temperature annealing. A seperate experiment, discussed later in

this text, was conducted to measure the extent of room temperature annealing. The optical

absorption measurements in the spectral reyion 190nm to 3200nm were conducted on each sample

before and after irradiation using Perkin-Elmer Lambda 9 UV/VIS/NIK spectrophotometer. The

infrared transmission spectra of all samples in the optical region 2.0iim to lOpm were measured on a Perkin-Elmer 1600 series FTIR. A typical optical transmission spectra of ZBLA

and thorium fluoride based families are shown in figure 2 compared with the optical transmission of fused silica glass.

Polished samples were measured for baseline absorption prior to irradiation. These test

samples were then gamma irradiated with 1.0/Mrad. Trial irradiation runs showed that no induced optical damage could be detected beyond 1500nm. Irradiated samples were annealed at 100, 150, 200, 230, and 270°C for twenty four hours in a tube furnace under argon temperature.

152
RETRACTION

VIT CARS

CRUCIBLZ

INDUCTION COIL

THERMOCOUPLE

BN SUPPORT

SUSCEPTOR

GRAPHITE

SUPPORTS

LOWER RETRACTION

Figure 1. Fluoride Glass Melting Furnace Assembly

C/3

<nz

<

cr

100

80

60

40

20

I i I I I I I I

| I I I I I I I I

| - /

/

J

I

I

I

I J

I

/ ^ \

: ZrF4 -BaF2 -LaF3 -AIF3

-^1^^^

"

I : ~ i i - / f

I I:

BaF_ -ZnF, -LuF_ -ThF "1

2 2 3 4

1

1

I //

1 1 1 1 I I ! 1 1 1 l\ 1 1 1 1

0.1 0.2 0.5 1.0 2.0

WAVELENGTH (microns)

5.0 10

Comparison of UV-VIS-IK Transmission Windows in Silica and Heavy net a l Fluoride

Gl asses.

Fiyure 2

3.0 RESULTS AND DISCUSSION

The glass compositions, listed in table 1, were chosen to study the effect of I nF 3 and NaF c the radiation induced damage in a f 1 uorozi rconate glass family. The effect of NaF addition

153
the optical transmission of the base glass ZBLA (A) prior to irradiation is a slight shift in

the infrared absorption edge towards shorter wavelength. Doping the base glass with I nF 3 has

no effect on the optical transmission spectra prior to irradiation.

Figure 3a is the optical transmission as a function of wavelength covering the range 2. U-lU.Uym

for sample A before and after 1.0 Mrad gamma irradiation. It shows that this dose of radiation

did not affect the optical transmission of this glass over this spectral region. This infrared

hardness was observed in all fluoride glasses irradiated to l.U Mrad. Since no significant

color-center peaks were observed over the spectral range 85Unm and 32U0nm in any of these

glasses, as shown in figure 3b, discussion will focus on results obtained over the 2UU-85Unm

window, figure 3c.

Figure 3a.

— 4000 3000

100

FREQUENCY (CM ') 2000 1500 1 000

o

CO

CO

CO<

50-

0.00

AFTER

BEFORE

3.0 5.0 7.0 9.0

WAVELENGTH (MICRONS)

ZBLA Infrared window before and after l.U Mrad Irradiation

4.8n

A 3.1-f B

S

0 2.2-1 R

P

T 1.3H

I0

M 8.4

57ZrF4-36BaF2-4LaF3-3A 1F3

A

\<—AFTER IRRADIATION

\

BEFORE IRRADIATION

-8.5

288 788 1288 1788 2288

UAUELENGTH NH

2788 3288

Figure 3b. ZBLA UV-VIS-NIR Window before and after l.U Mrad Irradiation.

154
4.8-

3.

If

57ZrF4-363aF2-4LaF3-3AlF3

A

3

S

0

R

P

T

I0

N 8.4

-8.5

2.2

1.3-

38SNM

236NM J,

<— AFTER IRRADIATION

BEFORE IRRADIATION

258.8 358.8 458.8 558.8

UAUELENGTH NM

658.8 753.8 858.8

Figure 3c. Ultraviolet Color-Centers at 236 and 309nm Induced by 1.0 Mrad Irradiation in ZBLA

Fluoride Glasses.

After 1.0 Mrad irradiation, the base glass ZBLA, and the sodium fluoride doped glasses ZBLAN,

had dark brown and light brown tints respectively, whereas the indium fluoride doped glass

ZBLAI, was nearly colorless. The optical absorption over the 200-85Unm region in 1.0 Mrad

damaged ZBLA and ZBLAN had similar profiles, as shown in figure 4a. Both glasses exhibited two

strong absorption bands centered at 309 and 236 nm, which most closely match the assignments

for CI ^ FCI prescribed by Tanimura, et . al [10]. The uv edge shifted slightly towards longer

wavelength and absorption below 206nm increased. The glass containing 1.0 mole% InF 3 behaved

far differently from ZBLA or ZBLAN glass shown in figure 4a. Induced absorption peaks were not

observed at 309 or 236nm. However, growth of a color-center peak either in or close to vacuumuv occurred as shown in figure 4b. The proximity of this absorption to the ultraviolet cut-off

of the spectrometer precluded assessing damage in terms of shifts of the uv edge to longer

wavel engths

-e.E

2.6

A 1.5

B

S

0 1.8 4

p

p

T 8.5 \

I0n e.e

ZBLA

ZBLAN

BEFORE IRNADlATiOfl

ZSt.b 396.8 35P..e 40e.e

UAUELENSTH NM

458 .

8

588.8

Figure 4a. Comparison of UV Color-Centers Induced in ZBLA and ZBLAN (2.U mole% NaF )

155
4.08

A 3.86

B

S

0 2.8e

R

P

T 1.88

I0N 8.86

-1.8

38K.8 358.8 488.8 458.8 588.8

UAUELENSTH MM

Figure 4b. Induced Damage in ZBLAI (1.0 mole% I nF 3) Showing the UV Edge Shift.

3.1 LIFE TIME ANNEALING

Damage induced in all glasses by exposure to gamma irradiation could not always be completely

annealed. However, room temperture annealing, (life time annealing), does start to occur as soon as the samples are removed from irradiation chamber. Life time annealing of the induced

damage was performed on a ZBLAN glass sample irradiated to 1.0 Mrad as a test to determine a suitable heat treatment schedule. The absorption spectra in the 190-3200nm range was measured

every ten minutes for two hours. The absorption spectra was scanned again four hours after

irradiation. All fourteen scans are shown in figure 5. The most significant life time annealing of the induced damage over two hours occurred below 350nm.

Figure 6 is a plot of the percent change in absorption before and after 1.0 Mrad gamma irradiation as a function of life time annealing of the induced damage starting immediately after

removal of the sample from the irradiation chamber. Seven traces are shown in figure 6

representing different wavelenghts in the UV and visible optical region. It is clear from

this figure that induced damage is more significant at shorter wavelengths in the UV region.

As expected, a decrease in the absorption of the induced color centers was measured with

increasing annealing time. After one hour, significant annealing occurred at shorter wavelenghts. This annealing was less pronounced in the visible region. However, after two hours,

room temperature annealing also became significant at longer wavelength.

As it was mentioned early in this text, the time required to measure a spectra over the region

190nin-320U was ten mniutes. Life time annealing of induced damage occured after ten minutes

from the time the sampe was removed from the irradiation chamber. This effect, however small,

can be calculated for the purpose of precise qualitative analysis.

3.2 ROOM TEMPERATURE ANNEALING

The stability of the 309nm and 236nm induced absorption in ZBLA and ZBLAN glasses due to

l.OMrad gamma irradiation was monitored as a function of room tempertaure annealing time ranging from 30 minutes to 31 days. Figure 7 is the optical absorption spectra of ZBLAN glass

#1430 annealed at room temperature. The eleven traces represent the spectra of the glass

sample before irradiation (bottom), immediately after irradiation (top) and after annealing at room temperature for 30 min, 60 min, 120 min, 180 min, 24 hrs, 48 hrs, 9 days and 31 days. It is clear that the 236nm absorption peaks has almost disappeared after 3 hours of room temperature annealing. The difference between 9 and 31 days annealing was negligible. This may

suggest that the 236nm band is related to fluorine ion mobility and the 309nm band is due

structural damage. In the case of ZBLAI glass, 50% of the induced damage due to 1.0 Mrad gamma

irradiation was annealed after 31 days at room temperature as shown in figure 8.

\

\\

\\

AFTER IRRADIATION

BEFORE IRRADIATION

156
lMrad GAMMA IRRADIATED Z3LAN GLASdo

1.7-

A 1.4

B

S

0 1.1

R

P

T 8.8-

I0

N 8.5i

<— IMMEDIATELY AFTER

IRRADIATION

4 HRS LIFE TIME ->

8.2

253.8 380.8 358.8 488.8

UAUELENGTH MM

458.8 588.8

Figure 5. Four Hours Room Temperature Anneal iny of Induced Damaye in l.U Mrad Gamma Irradiated

ZBLAN Glass (2U.U rnole% NaF )

ANNEALING TIME IN HOURS

1 2 3 4 XNM

-10 -

-20 -

o

r—XZ

o

hCL<ro

CD<

± -30 -

LU(3

z

<

X

o

h-Z

LUo

<rLU

Q.

-40 -

-50 -

-60 -

Figure 6. Room Temperature Life Time Annelaing of Induced Damage in ZBLAN Glass in the Visible

and UV Regions.

157
2.

53ZrF4-28BaF2-4LaF3-3A !F3-28NaF

<— IMMEDIATELY AFTER IRRADIATION

259.8 308.8 358.8 488.8

UAUELENGTH NM

458.8 588.8

Figure 7. Thirty One Days Room Temperature Annealiny of Induced Damage in ZBLAN (2U.U mole% NaF

A

B

S

0

R

P

T

I0

N

5.8n

lMrad GAMMA IRRADIATED ZBLAI GLASS

3 .9J

2.8-

1.7-

8.6-

-8.5

\ \\

V-\ <~ IMMEDIATELY AFTER IRRADIATION

- X>

31 DAYS TOTAL ANNEALING TIME

BEFORE IRRADIATION"

388.8 358.8 488.8 458.8 538.8

NM

Figure 8. Thirty One Days Room Temperature Annealing of Induced Damage in ZBLAI Glass.

3.3 HIGH TEMPERATURE ANNEALING

The ZBLAN and ZBLAI glasses, after having been annealed for 31 days at room temperature, were

heated for 24 hours at 100, 15U, 200, 230, and 270°C. The absorption spectra of these glasses

after heat treatment are shown in fiyures 9 and 10. Also shown in the same figures, for com- parison, are the spectra before gamma irradiation and after annealing at room temperature for

31 days. It is clear that the induced damage was not completely annealed even when the glass

samples were heated at or near the glass transition temperature where structural relaxation

occurs

153
i j

; —p- , , 258.3 300.3 353.3 400.3 453.3 500.3

WAVELENGTH NM

Figure 9. Twenty Four Hours Annealing of Induced Damage at Different Temperatures in ZBLAN

Glass.

2.8

A

B

S

0

RP

T

I0

N

T\.

1.5

1.8-

\ \

\ \<- ANNEALED FOR 31 DAYS AT RT

\ \ \

8.5-

8.8

T°C

180

153

288

238

278

BEFORE IRRADIATION

-8.5-

388.8 358.8 488.8

UAUELENGTH NM

458.8 530.3

Figure 10. Twenty Four Hours Annealing of Induced Damage at Different Temperatures in ZBLAI.

The results in figure 9 and ID are summarized in figure 11 where the percentage change in

absorption before and after irradiation are plotted as function annealiny temperature. The

induced damage observed in ZBLAN glass was annealed to 15% of the original damage when the

glass was heated at 1UU°C. The annealiny process slowed markedly even after heating at the

high temperature. In contrast, the recovery of the original absorption before irradiation of

ZBLAI glass was yradual . Most of the damaye was annealed only after heatiny the glass at

270°C, below the ylass transition temperature.

4.U CONCLUSIONS

Although the limited results presented in this work are not sufficient to present a complete

characterization of radiation effect on the optical properties of heavy metal fluroide ylasses,

some interesting relationships can be hiyhliyhted. Induced damaye due to l.UMrad gamma

irradiation was not observed in the region 1.0-8.0)im. However, the tail of the UV and

visible damage affected the absorption in the near infrared region ( 1 .0-1 . 2 nm ) . Strong

induced damage was observed at 309 and 24bnm in ZBLA glass and 309 and 236nm in ZBLAN containing 20.0 mole% NaF. The ultraviolet absorption edge was shifted towards longer wavelength as in the ZBLAI glass containing 1.0 mole% InF 3

159
1 0 « " 1 1 1 20 60 100 140

PERCENT CHANGE IN ABSORPTION X10 2

Figure 11. High Temperature Annealing of Induced Damage in ZBLAN and ZBLAI Glasses

In all these glasses, damage observed in the spectral region 1.0-1. 2pm was completely

recovered after a short room temperature anneal. When the glasses were annealed at 100°C for

24 hours, 90% of the damage was annealed in ZBL-A and ZBLAN samples and only 60% of the transmission was recovered in ZBLAI glass. Most of the induced damage was recovered in ZBLAI glass.

Most of the induced damage was recovered in ZBLA, ZBLAN- and ZBLAI glasses when the samples were heat treated at near the glass transition temperature.

In conclusion, heavy metal fluoride glasses in the ZBLA family are susceptible to l.OMrad gamma

irradiation in the ultraviolet and visible spectral region. Induced damage can be significantly

reduced by annealing. High temperature annealing is effective for damage that occurs in the

ultraviolet and visible region. Further work is necessary to identify sources and mechanisms

of induced damage and to identify dopants that may eliminate or reduce damage and enhance

transmission recovery below 100°C.

The authors would like to express their appreciation to Bruce L. Gilpin and John R. Cappelli

for irradiating the glass samples and Steven D. Mittleman for valuable suggestions.

REFERENCES

[1] Drexhage, M.G.; Quinlan, K.P.; Moynihan, C.T.; Sal eh-Boul os , M. Advances in Ceramics. Vol

2. Edited by Bendow, B. and Mitra, S., The American Ceramic Society, Columbus, OH, 57; 1981.

[2] Gannon, J.R. Proc. SPIE, (266): 63: 1981.

[3] El-Bayoumi, O.H.; Suscavage, M.J.; Bendow, B.J., Non-Crystalline Solids. 73 (1-3): 613; 1985.

[4] Gannon, J.R. J., Non-Crystalline Solids. (43): 239; 1981.

[5] Frieble, F.J., Opt. Tng. (18): 552; 1979.

[6] Drexhage, M.G.; El-Bayoumi, O.H.; Moynihan, C.T., Proc. SPIE (320): 27; 1982.

[7] Polain, M.; Lucas, J. Verres refract. (32): 505; 1978.

[8] Miyashita, T.; Manabe, T., IEEE J. Quan. Elec. (QE-18): 1432; 1982.

[9] Hutta, J. J.; Suscavage, M.J.; Drexhage, M.G.; El-Bayoumi, O.H., Proc. SPIE (484): 83; 1984.

160
[10] Tanimura, K. ; Ali, M. ; Feuerhelm, L.F.; Sibley, S.M.; Sibley, W.A.; J. Non-Crystalline

Solids. (70): 397; 1985.

COMMENTS

Question: Some place between 10 and 15 years ago, an experiment was performed at the Air Force Weapons Laboratory to study subsurface

influences on the damage of fused silica surfaces. Let me briefly

describe what was done. We started off with a rather rough course grind and, as you know, the general rule of thumb is that

subsurface fractures are produced that of three times what the

grit size is. After grinding the samples we would remove a sample

and continue the process with the next finer grit size, removing

three times the previous grit size, and then remove a sample until

we got down to about a 3 micron grit. We then took all of those

samples which had variout: levels of subsurface damage and polished

all of them in the same way. We obtained surfaces, if I recall, on the order of 7 angstroms RMS, certainly not a super polish but

not bad. After performing the laser damage experiments, we saw no difference in the damage threshold with all these varying degrees

of subsurface fracture. The damage measurements were done at 1 micron and 30 nanoseconds, just for the audience's interest.

Answer: In those days when people were doing subsurface damage studies,

there was no technique that really revealed what the surface

conditions were. The only criteria that they were using was a stamped-on particle size and they were assuming by the "rule of

thumb" to remove a certain amount of material. What we are saying

here is that for brittle material, and especially optical material

and semiconductor material, which is quite fragile, when you don't remove all the subsurface damage during polishing, there are crack

propagations in the surface which drive deeper and deeper beneath

the surface, resulting in this sense in surface damage. So in some of the earlier studies, what we think is that it made no difference because whatever polish was used, it did not eliminate

this subsurface damage layer. The damage threshold was basically

saturated in terms. of subsurface damage. So you didn't see any

difference no matter what you did. We are proposing here that the

technique described in this paper is the only time that we have

had a nondestructive technique that allows you to qualify and to

characterize what the surface looks like before you try to damage

it. It should now be possible to get some meaningful data and see

if there is any difference caused by subsurface damage on other

levels

.

Question: Could you tell me how you concluded that there are impurity

clusters in the defects?

Answer: Well, we have done Secondary Ion Mass Spectroscopy.

Question: On fused silica?

161
Not on fused silica; there's a lot of charging on fused silica.

On fused silica, the impurity cluster is looked at by some sort of

a chemical etch technique to delineate the etch and look at the

strain patterns using high resolution electron microscopy to see what the strain looks like. We don't know, certainly, what kind

of contaminants we have in those impurity clusters.

So you don't have any direct evidence that there are impurities in

those defects?

Right.

I think you need to tell us a little more about your experiment.

If I understand from your abstract, the damage measurements were made in the ultraviolet at 2480 angstroms, which might have some effect on the sort of damage that you see. The other thing is

what you mean by "standard polish" and "super polish." Quite

honestly, I have never seen this sort of "pits" in the standard

polish samples that you seem to be seeing. When looking at it,

say for example, under Nomarski microscopy, the defects look like

etch pits

The surface that you look at has been subjected or has been

exposed to a delineation etch. You don't see anything on the

Nomarski microscope until after you etch it. Then you expose all

the pits and scratches.

We etch the surface for a specific time to remove a specific

amount of material, and then we look at the surface using a Nomarski microscope and see what type of features are brought out

or decorated by the etch.

So all the pictures you showed were after etching?

They have been etched.

Can you tell us a little bit about your etch?

The etch is different from the HF base etch. This is another area

of the topic that I presented some data on at the ASPE meeting.

It's a very, very elaborate chemical screen to look at the

kinetics. By using different proportions of chemical oxidizing

agent and complexing agent to desoften the material one can bring

out this subtle damage. People using the HF alone might see some damage, but they don't really have the high resolution of the etch

that we have developed. And I will not reveal the etch

formulation at this time.

I see. It makes it difficult to check your work.

That's fine. We are not here trying to exchange information; we are more doing business. We will not reveal the etch.

This is a scientific meeting by the way.

162
Answer: We are building a photon backscattering system from a commercial

point of view. Once we build that, we will reveal all the

information we have found for the past eight years. That is

because of people who steal the data and don't give us credit.

Comment: I guess I would disagree most vigorously. We are here to exchange

information. That's the nature of scientific meetings. Second

thing I would say is that criminology, first and only, doesn't

leave much maneuvering room. I congratulate you in patenting

photon backscattering, but there have been many people at this

meeting that have reported various optical techniques for

examining the quality of optical surfaces prior to damage.

"Frustrated Total Internal Reflection" by Paul Temple is one example that I think stands out. I think the people at Rochester

had developed a very elegant system also for using small scale

photo acoustic techniques for mapping out surfaces. Finally, I would note that the terminology of super polish and nonsuper

polish was one, too, that about 15 years ago we decided was not

very quantitative. One man's super polish is often another man's

fine grind.

Answer: True. That's why we developed the KBS system. The super polish

is a commercially available optic. It's not what we're doing. So

we're here just to bring out to you that this work has been done

extensively in the semiconductor industry. The optical industry

is not really open to evaluating this technique, and they're

trying to invent other techniques that are not really sensitive

enough to determine what the surface characteristics are on the

surface they're looking at.

163
Manuscript received

2-12-90

The Effect of Laser Annealing on Laser Induced Damage Threshold

N.C.Kerr and D.C.Emmony

Department of Physics

Loughborough University of Technology

Loughborough

Leicestershire

U.K.

A significant enhancement of the single shot Laser Induced

Damage Threshold of CaF2 and fused silica and a moderate enhancement

for GaAs and Al has been observed as the result of laser annealing

using an excimer laser operating at 248 nm. This phenomenon is

primarily attributed to a reduction of the residual surface roughness

of the samples.

Key words: Excimer laser; Laser beam annealing; Laser induced damage

1. Introduction

In the context of Laser Induced Damage (LID) laser annealing refers to irradiating

samples at non damaging fluences with the intention of raising their Laser Induced Damage

Threshold (LIDT).

This effect has been reported by several authors. Swain et al [1] reported upto a factor of 2 increase in the single shot threshold of colloidal silica antiref lection (AR)

coatings if they are processed by typically 10 non damaging pulses. Temple et al [2,3]

have reported more than a factor of 2 increase in the LIDT on samples of fused silica

processed by a cw CO2 laser and then damage tested by a pulsed 1.06um wavelength laser.

Two mechanisms are thought to be responsible for this effect. Temple et al have

proposed that the cracks and scratches left on the surface after mechanical polishing are reduced by the heating effect of the laser whereas, Swain et al have suggested that the

heating effect removes surface contaminants. Both of these mechanisms result in a more

"ideal" surface and thus will increase the surface damage threshold.

Qualitative enhancement of the LIDT of a range of UV and IR materials; such as fused

silica, CaF2» GaAs, Si, Ge, and MgF2 by laser annealing have been observed by the authors

whilst using the damage test facility at Loughborough University of Technology [4]. To

determine if either of the proposed mechanisms dominates the annealing process we

endeavoured firstly to quantify the increase in LIDT for a variety of annealing regimes

and then to make careful observations of how the surface composition, morphology and

structure had been altered by the annealing process.

2. Experimental

The damage facility centres around a Lambda Physik model EMG 200 excimer laser.

This is operated with a Krypton and Fluorine gas mix and produces a nominal 1J pulse of

164
30ns duration at a wavelength of 248nm. A 20cm focal length fused silica lens was used to

focus the beam such that the full width at the l/e^ points in the horizontal and vertical

directions was respectively 1.5mm and 0.5mm. Control of the pulse energy was achieved by means of a liquid dye cell attenuator, filled with rhodamine 6G in distilled water, whilst

measurement of beam energy was performed using a Gentek joulemeter calibrated using a Laser Instrumentation Calorimeter. The spatial profile of the excimer beam and hence the

on-target fluence levels were measured using a computer-controlled video framestore system

[5] that essentially images the fluorescence that the excimer beam produces in a thin

piece of absorbing glass. The beam which had no localised hot spots had an approximately

gaussian profile in the vertical direction but was almost rectangular in the horizontal

direction. Peak fluence in the beam is quoted throughout this paper.

The surface morphology of the samples was investigated using a Scanning Electron

Microscope (SEM), optical microscopy, and following Temple [6] Total Internal Reflection

Microscopy (TIRM). The surface structure was investigated using a Scanning Transmission

Electron Microscope (STEM) for electron beam diffraction whilst the surface composition

was determined using Secondary Ion Mass Spectroscopy (SIMS) and Laser Induced Mass

Analysis (LIMA).

A High Resolution Schlieren imaging system utilising a computer controlled video

framestore and camera [7] was used to detect the onset of damage. The apparatus was arranged as shown schematically in figure (1). A 5mW HeNe laser was used as the source of

the surface probe. Damage is defined as a significant measurable change between an image

in the framestore of the surface before an incident excimer pulse and an image taken after

the pulse.

Measurements of the single shot LIDT were made on samples of Ge, Al, Si, GaAs, fused

silica and CaF2- Prior to testing these samples underwent no special treatment except to

have their surfaces wiped with methanol to remove dust etc. Two types of Si were used

during this work. The first consisted simply of boron doped single crystal wafers used

for the initial damage threshold measurements. At a later stage thin sputtered amorphous

Si films were used for the electron diffraction work. These films were formed by dc

planar magnetron sputtering onto ICI 'o' grade 50um thick polyethylene tetraphthalate.

This flexible polymer was handled in the vacuum web coater of Ridge et al [8] and coated

from the 100mm exposed pole magnetron described by Spencer et al [9]. The film thickness

was approximately 60nm. The sputtering was performed by Dr R.Spencer and R.Howson of the

University Vacuum Coating Group [10].

Firstly the samples were damage tested to determine their initial, unannealed

single shot damage thresholds. The excimer fluence incident upon the sample surface was gradually increased and at each fluence 6 individual sites on the sample surface were

damage tested. Usually for an accurate determination of damage threshold more surface

sites are irradiated but because in this experiment large changes in threshold level were expected fewer sites could be used. The damage threshold was defined as the fluence which

produced damage on 50% of the sites tested at that fluence.

The same process was then repeated, but prior to irradiation the area being tested

was given a series of annealing laser pulses. These were varied in intensity from just

below the untreated surfaces damage threshold to less than a tenth of the value and in

number from one to a hundred. The damage threshold was again set at the 50% survivability

level for each of the different annealing regimes used.

3 . Results

Modification of the LIDT for fused silica and Al are shown graphically in figure (2)

and figure (3). CaF2 showed results similar to those of fused silica whilst GaAs showed

results similar to those of Al. Annealing Si and Ge gave no measurable increase in LIDT

but a clear decrease in the amount of damage caused by the excimer pulses. This effect is

165
more clearly seen on dielectric surfaces were the annealing pulses were of very low

fluence, see figure (4). Annealing pulses with fluences in the range of 60- 80% of the

untreated single shot LIDT produced the most significant changes in LIDT. At fluences

much below 50% of this level no significant rise in the single shot LIDT was observed for

annealing regimes involving up to several hundred pulses. The relative change in LIDT

for the Al and GaAs samples was not as large as for dielectric materials.

Analysis of single crystal wafer samples of Si before and after annealing using SIMS

showed an order of magnitude change for the sodium line at mass number 23, figure (5).

Typical LIMA results are shown in figure (6).

Using optical microscopy to examine the morphology of the annealed surfaces showed

that for every surface which had been annealed the degree of roughness upon it was

reduced. In some cases it appears that nearly all surface marks have been totally removed

over the area of interaction of the laser beam whereas in others the roughness has been

greatly reduced but not entirely removed, see figure (7).

The very smooth CaF2 and fused silica surfaces showed no detail using dark field

microscopy but using TIRM it is clear that laser annealing has removed surface marks,

figure (8).

Investigations of annealed, specially prepared amorphous silicon surfaces using

electron beam diffraction showed that the crystal structure of the sample had changed

significantly during the annealing process, see figure (9).

A. Discussion

Inspection of dark field images for an annealed Al surface such as that in figure

(7) would indicate by the reduced number of bright spots in the image that the sharp edges

on the sample surface have been removed, however care must be taken in the interpretation

of these photographs. Using interference contrast microscopy, figure (10) it is clear

that rather than removing the two scratches running across the laser interaction area the

annealing has simply rounded off their sharp edges. Note also the "doughnut" shaped

structures around points in the annealed area. These are characteristic of melting and

subsequent resolidif ication of a localised region on the surface of a material where the

density of the liquid phase is less than the density of the solid [11]. These regions

appear localised around what we assume to have been sharp points on the unannealed samples

surface. It should be noted that dark field images are obtained using a type of Schlieren

process which stops off the zeroth order, and some of the lower orders in the light

forming the image of the surface. For this reason dark field imaging shows up the high

frequencies due to sharp points on the surface but not the more smoothly varying contours.

The theory by Bloembergen [12] demonstrates that sharp edged cracks or defects on a surface lead to the largest enhancement of the local surface electric field and

consequently a reduction of the surface LIDT over the bulk. The removal of such sharp

edges will effect the magnitude of this field enhancement and therefore the damage

threshold of the surface.

It is the authors' contention that the reduced effect of annealing on metal or semiconductor surfaces compared to that on dielectrics can be explained in terms of their

differing interactions with the incident radiation. In a dielectric the absorption of the

incident energy is small with the result that the small increase in absorption due to the

enhanced electric fields around local surface roughness can be a significant fraction of

the total absorption. Thus a small change in surface roughness can cause a large change

in LIDT. In the case of a metal nearly all of the laser energy is absorbed within lOOnm

166
of the surface (the skin depth). The increased absorption due to the enhanced electric

fields at the surface are thus relatively insignificant compared to the total absorption

of energy in the sample and hence annealing has little effect on the surface LIDT.

Removal of contaminants by any method can certainly be expected to raise the damage

threshold of a surface. Clark [7] notes that the removal of absorbed contaminants from

the atmosphere in Sol-Gel coatings by cleaning with Methanol can greatly increase their

damage threshold. To this end analysis using LIMA was performed at low laser fluences and

at a wavelength of 266nm, nearly equal to that of the excimer laser. Initially it was hoped that the low fluence pulses would simulate the situation encountered during laser

annealing with the excimer laser. The resulting mass analysis would then indicate the

species being evolved from the sample during the annealing process. However, for LIMA to

function it must always vapourize and ionize a small volume of the sample, hence damaging

the surface. The results would seem to indicate that common surface contaminants,

absorbed from the atmosphere, can be expected to be evolved during the annealing process,

at the typical laser fluences used.

Similarly, our results from SIMS analysis can be interpreted to demonstrate that

surface contaminants in the form of sodium are being burnt off the surface by low fluence

laser pulses.

Liu [13] and Leamy [14] have demonstrated that it is possible to change

polycrystalline Si to single crystal material by laser annealing. Intuitatively we feel

that such an annealing process could be used to raise the surface damage threshold of

semiconductors, this is confirmed by Cullis [15]. It is noted that any process which will

produce a single crystal layer on a sample surface will result in a larger resistance to

laser induced damage. However after reading the papers by Liu and Leamy it is clear that

the fluences they were using to produce their changes were well above the damage threshold

level of the samples. It is however pointed out by Vitali [16] that even at low laser

fluences, well below the surface damage threshold, as used here changes in crystal

structure can be expected such as from amorphous to polycrystalline, or polycrystalline to

single crystal depending upon the exact fluence used and the number of laser pulses. The

results reported herein would certainly confirm this observation. The amorphous silicon

structure as indicated by the diffuse ring pattern in figure (9a) has clearly been

transformed by laser annealing into a polycrystalline pattern as indicated by the well

defined rings in figure (9b).

This transformation was induced by multiple laser pulses with fluences of 0.25Jcm"^,

well below the melting threshold for Si of 0.65Jcm"^ at 248nm quoted by Jellison [17]. It

should be noted however that Vitali clearly demonstrates that structural transitions such

as the above can gradually occur without the material being constrained to melt by

irradiating with successive laser pulses which lead to a cumulative effect.

5. Conclusions

Laser annealing can significantly improve the LIDT of a sample surface. The

dominant process leading to this would appear to be removal or reduction of the severity

of surface roughness. Strong indications of the removal of surface contaminants and

changes to the surface crystal structure means that these mechanisms cannot be ruled out

as contributing processes in raising the LIDT.

167
Acknowledgements

N.C.K. acknowledges the financial support of SERC and BDH Ltd, Poole, Dorset. We are indebted to Dr. Alison Chew and D.Sykes of Loughborough Consultants Ltd. for the use of

their SIMS. We would also like to thank J.Bates, Angela Kerr and Anne Waddilove for

helpful discussions and technical assistance.

References

[1] J.E.Swain, W.H.Lowdermilk, D.Milam, "Raising the Surface

Damage Threshold of Neutral Solution Processed BK-7 by

Pulsed Laser Irradiation," Natl. Bur Stand. (U.S.)

Spec. Pub. 669 (1982), pp292

[2] P. A. Temple, W.H.Lowdermilk, D.Milam, "Carbon Dioxide

Laser Polishing of Fused Silica Surfaces for Increased

Laser-Damage Resistance at 1064 ran," Applied Optics,

Vol. 21, No 18, pp3249, (1982).

[3] P. A. Temple, W.H.Lowdermilk, D.Milam, "C02-Laser Polishing

of Fused Silica Surfaces For Increased Laser Damage

Resistance at 1.06um," Natl. Bur. Stand. (U.S.) Spec.

Pub. 568 (1979) pp229.

[4] S.E.Clark, I.Laidler, D.C.Emmony, B.A.Omar,

M. J.Shaw,"U.V. Laser Beam Diagnostics using a Computer

Controlled Video Framestore System," Natl. Bur. Stand.

(U.S.) Spec. Pub. 746 (1985), pp35.

[5] B.A.Omar, S.E.Clark, D.C.Emmony, D.B.Hollis,

M.J. Shaw, "The Application of Doped Glass Fluorescers to

the Recording of Pulsed Ultra-Violet Laser Beam

Profiles," Optics and Laser Technology, Vol. 20, No 4,

pP 193, (1988).

[6] P. A. Temple, "Total Internal Reflection Microscopy: A

Surface Inspection Technique," Applied Optics, Vol. 20,

No 15, pp2656, (1981).

[7] S.E. Clark, "Excimer Laser Induced Modification of

Optical Surfaces," Ph.D. Thesis, Loughborough

University, England, (1988).

[8] M.I. Ridge, R.P.Howson, C. A. Bishop, Proc. SPIE, Vol. 325,

Optical thin films, pp46, (1982).

[9] A.G.Spencer, C. A. Bishop, R.P.Howson, Vacuum, 37,

p P 363, (1987).

[10] Vacuum Coating Group, Loughborough Consultants Ltd,

University of Technology, Loughborough, Leicestershire,

(U.K.).

168
[11] D.C.Emmony, N.J.Phillips, J.H.Toyer, L. J .Willis , "The

Topography of Laser-Irradiated Germanium," J. Phys. D: Appl. Phys., Vol. 8, ppl472, (1975).

[12] N. Bloembergen, "Role of Cracks, Pores and Absorbing

Inclusions on Laser Induced Damage Threshold at

Surfaces of Transparent Dielectrics," Applied Optics,

Vol. 12, No 4, pp661, (1973).

[13] P.L.Liu, R.Yen, N. Bloembergen, R.T.Hodgson, "Picosecond

Laser-Induced Melting and Resolidif ication Morphology

on Si," Appl. Phys. Lett., Vol. 34, No 12, pp864,

(1979).

[14] H.J.Leamy, G.A.Rozgonyi, T.T.Sheng, G.K. Celler, "Periodic

Regrowth Phenomena Produced by Laser Annealing of

Ion-Implanted Silicon," Appl. Phys. Lett., Vol. 32,

No 9, pp535, (1978).

[15] A. G . Cullis , "Transient Annealing of Semiconductors by

Laser, Electron Beam and Radiant Heating Techniques,"

Rep. Prog. Phys., 48, (1985), pll55-1233.

[16] G.Vitali, "On some less evident aspects of laser

annealing," Phys. Letts., vol. 78A, No 4, pp387-390,

(1980).

[17] G.E. Jellison, D.H.Lowndes, D.N.Mashburn, R.F.Wood,

"Time resolved reflectivity measurements on silicon

and germanium using a pulsed excimer KrF laser heating

beam", Physical Review B, Vol. 34, No 4, pp2407-2415,

(1986).

169
Figure captions.

Figure (1).

Experimental arrangement used to measure Laser Induced Damage Thresholds for a metal

or semiconductor surface. For a transparent dielectric sample the surface was viewed in

transmission.

Figure (2).

Laser Induced Damage Thresholds for Fused Silica.

(a) Unannealed surface.

(b) Surface annealed with five 2.8 J/cm^ laser

pulses

(c) Surface annealed with one hundred 2.8 J/cm^

laser pulses.

Figure (3).

Laser Induced Damage Thresholds for Al.

(a) Unannealed surface.

(b) Surface annealed with twenty 2.8 j/cm^ laser

pulses

Figure (4).

An example of how laser annealing can decrease the amount of damage caused by the

excimer laser pulse.

(a) Damage caused on an unannealed CaF2

surface by one incident excimer pulse

with a fluence of 6 j/cm^*

(b) Damage caused on the same sample by a 6 J/cm^

excimer pulse incident after 1000 annealing

pulses of 0.15 J/cm^ each.

Figure (5).

The results of a SIMS analysis of a Si sample.

(a) SIMS trace before annealing.

(b) SIMS trace after annealing, showing very little

change

Figure (6).

The results of LIMA performed on Al and Ge samples showing

the evolution of common surface contaminants.

Figure (7).

Typical dark field images taken of an Al and Ge surface after annealing. Note the

discontinuity of scratches across the centrally located black annealed region.

Figure (8).

TIRM images taken on a CaF2 surface.

(a) Video image of the surface before annealing.

(b) Video image of the same area after annealing.

Note the reduction of surface scattering points

in the region which has been annealed

(c) Computer processed image of (b)-(a). This

shows more clearly that only areas on the

surface irradiated by the annealing pulses have

had their scatter reduced.

Figure (9).

The results of STEM and electron diffraction measurements on silicon both (a) before

and (b) after laser annealing. The insets show the respective electron diffraction

patterns. Clearly the unannealed silicon surface was amorphous in nature as indicated by

its diffuse ring type diffraction pattern. After annealing the diffraction pattern is

made up of discrete well defined rings indicating a polycrystalline structure.

170
Figure (10).

An interference contrast image showing the annealed region of Figure (7a) where the

two scratches cross it at higher magnification. Note that the scratches do appear to have

been erased in some parts but more generally have become rounded off. At the foot of the

photograph corresponding to the edge of the annealed region the scratches are still well

defined.

Figure 1

171
LASER INDUCED DAMAGE THRESHOLD FOR FUSED SIUCA.

Figure 2

LASER INDUCED DAMAGE THRESHOLD FOR Al.

LASER FLUENCE. (J/cm2

)

Figure 3

172
igure 4a Figure 4b

Figure 5a Before annealing

3

2

111, ill d+J ^ :

: ,

: 130 140 150 160 170 180 190 200 210 220 230 240 "C

Figure 5b After annealing

130 140 150 160 170 180 190 200 210 220 230 240 ^°

173
~i 1 r ~i 1 r

-P

C

QJ

C

c_D

U

Al

K

C

O

CD>

i—

ro

i—

QJ

Na

JUL

_i i i_ 20

JUL

40

AIO

/z

60

Figure 6a Aluminium

JL

_i i i_ _i i i_ 80 100

-i 1 r "i 1 r ~i i i r

1 r

-

-t-» c

QJ

C_

c_Z!U

c

o Ge

QJ

>

+-> ro

i—

QJ

CE Na

Al

_l I L.

Ca

_i i u _i i i_ _i i i_ 20 40 60

/z

80 100

Figure 6b Germanium

174
Figure 7b Germanium

175
176
Figure 9 (a) STEM image

Figure 9 (a) Electron diffraction pattern

177
Figure 9 (b) STEM image

Figure 9 (b) Electron diffraction pattern

178
Figure 10

179
Manuscript received

2-27-90

SHOCKWAVE DETECTION, AN EFFICIENT WAY TO DETERMINE MULTIPLE-PULSE DAMAGE THRESHOLDS

S. Petzoldt, A. P. Elg, J. Reif, and E. Matthias

Fachbereich Physik, Freie Universitat Berlin

Arnimallee 14, D-1000 Berlin 33, FRG

The detection of damage associated shock pulses via the deflection of a probe laser

beam is exploited to study multiple-shot damage thresholds in CaF 2 , PMMA, copper, and

aluminium. We find a reduction of threshold intensity as compared to single-shot conditions. The experimental results point towards deviations from the previously postuS-1 lated fluence dependence F^ = F t N [see Ref.4]. They are, however, compatible with

a simple model relating the threshold reduction to defect accumulation via non-Hnear

energy absorption.

Key words: Multi-shot damage, probe beam deflection, defect accumulation

Optical systems for high-power pulsed lasers are limited by damage to the optical components. In

practical applications, the maximum usable, intensity is most often determined by the multipleshot damage threshold. Due to accumulative effects [1,2], this threshold is frequently considerably lower than the single-shot threshold, in contrast to observations in multiple dielectric

layers [3]. For a classification of different materials, it is important to know the number of

pulses at a given intensity, that can be applied before damage occurs. In other words: The relation between the N-pulse damage threshold and the intensity of the laser beam is of great interest for a reliable prediction of the longevity of optical components. Presently, several laboratories are working on that question. E.g., M.F. Becker et al. [4] have measured single- and multiple-pulse damage thresholds of copper to investigate damage morphologies and damage accumulation, and to determine the damage mechanism. From their data, they derived the cumulative equation

F

N

= f/"1 (1)

where is the single-shot damage threshold fluence, F

N is the threshold for N laser pulses,

and S is a material specific parameter derived from the experimental data.

In Fig. 1 a schematic of such accumulative curves is represented. The total energy (number

of pulses times threshold fluence) is plotted in a log-log scale as a function of the number of

shots. In this plot the data should represent a straight line with slope S, according to Eq.(1).

A slope of S=1 is standing for the absence of incubation effects. Normally, the measured data

provide a slope smaller than 1. The difference between slope 1 and slope S is the reduction of

the total threshold energy, and should be a measure for incubation. Becker et al. explain the

incubation for metals as a storage cycle of thermal stress-strain energy, induced by the laser

pulses.

180
Number of shots (N)

Figure 1: Schematic of the incubation effect according to Eq. (1): The total absorbed energy is

plotted versus the number of pulses (log-log coordinates).

In the following we report on measurements of the multiple-shot damage thresholds of PMMA,

CaF 2 , Cu, and Al for ns pulses of green (530 nm) laser light and present a simple phenomenological model to account for the accumulation effect. In our experiments we use the probe beam deflection technique [5] to detect the Shockwave connected with surface damage. As shown previously [5,6], this is quite appropriate to rapidly yield laser damage thresholds. The method utilizes a probe laser beam passing at short distance parallel to the sample surface (see Fig. 2).

In case of damage, a shock pulse emerges from the sample, and the associated transient change of

refractive index causes the deflection of the probe beam. This is monitored via the intensity

variation behind a narrow slit placed on one slope of the spatial beam profile. The shock pulse

energy derived from this intensity variation was shown to be a function of the degree of damage.

DYE LASER

ILE

Figure 2: Principle setup for the beam deflection technique

181
In a preliminary experiment, we measured the intensity dependence of the damage thresholds

on CaF., and PMMA for comparison, which are shown in Fig. 3. In part a), of both cases, 1-on-1

measurements are displayed. In part b) the data of measurements with overlapping spots are plotted. Here each laser shot hit a predamaged area. For CaF 2 there is a big difference in threshold

and slope between isolated and overlapping spots. The change in slope indicates, that the pro- cess is changing from avalanche breakdown to multiphoton ionization [7]. In the case of PMMA,

however, the threshold is decreasing but the slope above threshold is not changing very much

pointing to only one process responsible for single- and muliple-shot damage.

0.01 0.10 1.00 10.0 0.01 0.10 1.00 10.0

INTENSITY [ GW/cm2

] INTENSITY [ GW/cm2

]

Figure 3: Damage thresholds for CaF,. (left) and PMMA (right)

(a): isolated spots (1-on-1); (b): overlapping spots (predamaged).

The accumulation effect was studied by measuring multi-shot damage thresholds. For this

purpose, we illuminated the sample with a repetition rate of 3 Hz at fixed laser intensity, and

counted the number of shots needed for a first damage event to occur, i.e., the counter was

stopped by the first detectable deflection signal. This experiment is fully computer controlled,

and so we are able to rapidly collect substantial quantities of data, quite in contrast to conventional techniques like, e.g., microscopic inspection. Therefore we have the possibility to do

these measurements for a high number of different intensities and to use relatively low intensities where up to 10,000 shots are needed before damage occurs. In Fig. 4 the data of CaF 2 , PMMA,

copper, and aluminium are shown. Along the lines of Ref. [4], the accumulative energies N-F

N are

plotted as a function of the number of pulses N. The solid line is a fit of Eq.(1) to these data. The parameter S was determined to be 0.88 for copper, 0.91 for aluminium, 0.84 for PMMA, and

0.95 for CaF 2 . Obviously, for high values of N this kind of data analysis becomes highly insensitive. Therefore, in Fig. 5 the actual threshold intensity is plotted as a function of the

number of laser pulses to damage.

182
inc

QJ 4JC

c

0) n

EZ

100

10 . 0

1 . 00

PMMA

Slope = .84

1.00 10.0 100. 1000

Number of Shots

c

0) a

£Z

100 .

10 . 0

1 .00

CaF,

Slope = .95

1.00 10.0 100. 1000

Number of Shots

100

5 i°-0

L

CU a

EcZ

1 . 00

100 .

1.00 10.0 100. 1000

Number of Shots

inc

cu 5 io-o

in

<uE

-Pc

0)EZ

1 . 00

Copper

Slope = .88

1.00 10.0 100. 1000

Number of Shots

Figure 4: Analysis of incubation effect according to Eq. (1) for PMMA, CaF 2 , AT, and Cu.

ID<

>> P

•rl cnc

CD +JcM

1 .00

.800

. BOO

•• PMMA

. 400

200

+ ++ ++

.

+

+

1.00 10.0 100. 1000

Number of Shots

<

COc

cu +jc

1 .00

. 800

.600

. 400

. 200

CaFa

+ ++ + + + ++ +

+ ++

++

+ +

+ + +

1.00 10.0 100. 1000

Number of Shots

<

cnc

cu

1 .00

. 800

.600 " .400

. 200

Aluminium

1.00 10.0 100. 1000

Number of Shots

— 1 .00

d - 800

.600 <— .400 •

>.

-p

inc

cu

•4-> cM

. 200

Copper

1.00 10.0 100. 1000

Number of Shots

Figure 5: Log-log plot of damage threshold vs. number of shots. Note the deviation from straight

lines, which are expected from Eq. (1).

183
According to Eq. (1), this should yield straight lines again, with slopes of (S-1). Closer

inspection of Fig. 5, however, shows deviations from this expectation. For CaF 2 and copper there

is a change to greater S-values for higher numbers of shots and smaller intensities. That points

to a decreasing incubation for lower intensities. The data of PMMA contrarily provide a decrease

of the S-value for a greater number of shots. That looks like an increasing incubation effect.

The differences between the species may be the result of different incubation processes. If incubation is interpreted by defect creation [1,8], in the case of CaF 2 and copper a decay rate of

defects would explain the observed behavior. In the other case of PMMA, permanent defects and an increase of the defect creation cross-section with the density of defects would be an explanation for the deviation from the straight line. So, Eq. (1) is a description of the coarse behavior only, but there are deviations in the fine structure. These deviations, we think, depend

on the different materials.

In the following, we try to present a simple phenomenological model to account for a more detailed analysis of the effect of energy accumulation leading to reduced damage thresholds.

For this purpose, let us assume the energy absorption in the material to occur via k individual absorbers, e.g. defects. Then, we have the absorbed energy per pulse

Eabs

= k - (2)

where the energy per absorber

e = (°

n

l n" 1 )F (3)

may depend non linearly on intensity I and fluence F. Further, we postulate that the absorbed

energy primarily leads to the generation of additional absorbers, i.e., after the i-th pulse we

have

k

i+1

= k.+ r(Ok.e. (4)

So, the increment in number of absorbers per number of pulses is proportional to the energy absorbed

^ = r(e)ke.' (5)

Integration of Eq. (5) yields

In k + const = r(e.)ei. (6)

Defining damage to occur when the energy absorbed during one pulse equals the threshold energy

E 0 , the number of absorbers at threshold is given by

k

th= Eo/e. (7)

184
Combining Eqs. (6) and (7), we obtain the number of shots N, required for damage at single absorber energy e:

N = const + In E 0 - In £

7W~i (8)

In order to evaluate the absorber creation rate /"(O let us think of the following simple model:

In the simplest case, r(e) = r 0 would be a constant. If we allow, however, for a portion of

energy to be trapped at the absorber, as would be done by a barrier for energy transfer, we try

the ansatz of a reduced rate

r(e) = r 0 -1/at

We can refine this by replacing

1-1/(aer 0 ) * exp[-

and obtain from Eq. (8):

(9)

(10)

Li C - In 6. N = =— exp

* 0

fc (11)

<

toc

CD -MC

<

>

P

•rl COc

cu 4JcM

10 .0

8.00

6.00

A .00

2.00

PMMA

+ + + \-4.

+ +\

+ \\

n

' =

1 .00

Log Number of Shots

10.0

1 .00

Log Number of Shots

10 . 0

<

+J

•rl COc

cu

<

COc

cu +JcM

10 .0 a. oo 6.00

A . 00

2 . 00

1 .00

.800

.600

. 400

200

• CaF2

— ^+

+ + + ++^^-

n' = -1/5

1 . 00 10

Log Number of Shots

Copper

+

-1/2

1 . 00

Log Number of Shots

10 . 0

Figure 6: Analysis of incubation effect along the lines of the defect creation model

(log [threshold intensity] vs. log[log(number of shots)]). For large N, all data may be represented by Eq. (12). The slopes n' are obtained by assuming integer orders of nonlinear energy

input.

185
If we keep, as a limit at low intensities, only the exponential contribution

N » const exp(1/bt) (12)

we can indeed compare this with our data, as shown in Fig. 6, by assuming very simple integer

nonl inearities n in Eq. (3). These are Indicated in Fig. 6 by straight lines with slopes 1/n. It

should be remembered here, however, that this model does not yield any insight into the microscopic processes of energy dissipation and defect creation, as is tried in [1] for alkali halide

crystals. Instead, it is intended to provide a phenomenologlcal parameterization of the Incubation effect.

In conclusion, the probe beam deflection technique is a useful tool to investigate multishot damage thresholds, providing us with the possibility of rapidly collecting statistically

S-1 significant quantities of data. The function FN

= F^ of Ref.[4] is fitting our data but

closer inspection provides some deviations. A simple model, relating the accumulation effect to

defect creation via non-linear energy input, is compatible with our experimental results.

We gratefully acknowledge support from the Deutsche Forschungsgemeinschaft, Sfb 337, and

the BMFT, contract No. 13N5627.

References:

[1] S.C. Jones, P. Braunlich, R.T. Casper, X.-A. Shen, and P. Kelly;

Opt. Eng. 28, 1039 (1989)

[2] S. Kuper and M. Stuke; Appl.Phys. A49, 221 (1989)

[3] C.R. Wolfe, M.R. Kozlowski, J.H. Campbell, F. Rainer, A. Morgan, and R. Gonzales; "Laser

Pre-Conditioning of Optical Thin Films", this volume

[4] Y. Jee, M.F. Becker, and R.W. Walser

J.Opt.Soc.Am. B5(3), 648 (1988)

[5] S. Petzoldt, A. P. Elg, M. Reichling, J. Reif, and E. Matthias;

Appl.Phys. Lett. 53, 2005 (1988)

[6] E. Matthias, S. Petzoldt, A. P. Elg, P.J. West, and J. Reif;

NIST Special Publication 756, 217 (1987)

[7] S. Petzoldt, A. P. Elg, J. Reif, and E. Matthias;

NIST Special Publication 1988, 1n press

[8] R.T. Williams; Opt. Eng. 28, 1024 (1989)

186
Manuscript received

2-20-90

Application of the

Ronchi Ruling Beam Profiling Method to Axially Symmetric Laser Beams

Robert M. O'Connell

and

Cheng-Hao Chen

Electrical and Computer Engineering

University of Missouri-Columbia

Columbia, MO 65211

The application of the Ronchi ruling beam characterization method to axially symmetric

optical beams is analyzed. Specific results are derived for the Airy and focused annulus diffraction

patterns. Plots of the ratio of minimum to maximum transmitted optical power versus the first null

radius of the beam functions show that for the Airy pattern and other focused annuli with

obscuration ratios smaller than approximately 0.30, the method should be just as useful as with

Gaussian beams.

Key words: Airy pattern; axially symmetric beam; beam profiling; focused annulus;

Gaussian beam; Ronchi ruling.

1. Introduction

Although many of the lasers used in laser damage studies emit non-Gaussian beams, they are often

assumed to be Gaussian for beam profiling purposes because it is very easy to apply the standard beam

profiling methods (e.g., pinhole, slit, knife-edge) to a Gaussian beam. Unfortunately, this approach can lead

to significantly inaccurate profiles (and, thereby, to incorrect damage thresholds) when, for example, the beam

contains substantial sidelobes. Such is the case when the beam in question is the focused output of a laser

with a confocal unstable resonator, i.e., a focused annulus.

While such axially symmetric beams can be profiled with the knife-edge method [1], doing so requires a large number of accurately positioned data readings over a wide range of power levels, which can introduce

significant measurement error. Another approach is to use the Ronchi ruling method, which has previously

been applied successfully to Gaussian beams [2-4]. In this paper, we consider its application to axially

symmetric beams in general and to the focused annulus beam function in particular. We review the

application of the method to Gaussian beams and then derive analogous results for the axially symmetric

focused annulus, a special case of which is the Airy pattern. The results will show that the method appears

to be most useful for focused annuli with obscuration ratios smaller than approximately 0.3.

2. The Ronchi Ruling Method: Application to Gaussian Beams

2.1. Analysis

The Ronchi ruling method can be most easily understood in terms of its application to a specific beam

function. As applied to a Gaussian beam, the method consists of moving a ruling (reflective or transmissive)

of known optical characteristics transversely through the beam and measuring the ratio K of the minimum

total power Pmin reflected or transmitted to the maximum total power Pmax reflected or transmitted. The 1/e

2 beam radius r

Q can then be obtained from a graphical relationship between K and Tq that results from an

analysis of the optical power reflected or transmitted when a Gaussian laser beam illuminates a Ronchi ruling

with specific optical characteristics (period, duty cycle, and reflectance or transmittance).

The appropriate graphical relationship for an ideal Ronchi ruling and a Gaussian beam can be derived

with the aid of figure 1. As shown in the figure, the bars and spaces in the ruling are assumed to be infinitely

long, of equal width L, and to have 100% reflectance and transmittance, respectively. Assuming that the bars

187
and spaces of the ruling are oriented parallel to the y-axis, expressions for the transmitted powers Pmin and

Pmax can be obtained from parts a) and b) of the figure, respectively.

Assuming that the Gaussian beam has been focused and the measurement is to be made in the focal

plane, the intensity I(x,y) of the beam is expressed as

I(x,y) = Iq e

•2(x

2 + y2

)/^

2

(1)

where I0is the intensity at the center of the beam. The optical power P in any portion of the focused beam

can be obtained by integrating eq (1), i.e.,

P = Ir e-2x2/

r0

2

dx dy . (2)

The limits on x in eq (2) are different for Pmin and Pmax, but the limits on y are from -oo to +oo in both cases. The integral on y is thus

e'2y2/r

»

2 dy = r0

-J tt/2 (3)

and eq (2) becomes

P = I0r0 ^ f o„2 , 2

-2x /rn e ' 0 dx , (4)

which can be used to determine expressions for both Pmin and Pmax.

Using figure la and eq (4), it is evident that

3L/2 7L/2

Pmin = 2W «/2 e-2

^

dx + e-2x2/

r2

0 dx

L/2 5L/2

11L/2

e-2x2/

r2

0 dx +

9L/2

188
3L/2 L/2

= 2I0

r0

J */2

2/JZ

e"2x /

r 0 dx - 2 /.2 e"2x /r 0 dx

7L/2 5L/2

e-^O dx e"2x2/

r2

0 dx +

0

Similarly, figure lb and eq (4) yield

L/2 5L/2

Pma* = 2I0

r0

4 V2

2x2A

2

o dx +

-2x

2

/r

2

0 c

3L/2

+

9L/2

7L/2

e-2

^

dx +

= 2I0

r0

4 */2

L/2

0

5L/2

2x2/r2

0 dx +

2/,2

e"2x >

X 0 dx

3L/2 9L/2

-2x

2/r2

n

. , f -2x

2

/r

2

. e ' 0 dx + e ' 0 dx

0 0

7L/2

J e

-2x

2

/r

2

0 dx +

0

If the variable change u = 2x/rQ

is made, eq (5) and (6) become, respectively,

3L/r0 2

Pmin " 7rI0r 0

u

z/2

L/

f0 -u 2

/2 r e

'

(2tt) 1/2

du

(2r) 1/2

du

189
7L/

r0 -u

2

/2

e

' +

5LA0 -u

2

/2

Car)

1/2

du -

0

(2*0

1/2

du + (7)

and max

L/r0 „2u72

5L/r0 -u2

/2

(201/2

du + (20V2du

3L/

r0 -u

2

/2

9LAo .u

2

/2

(201/2

du +

7L/r0 .u2/2

(201/2

du - (201/2du (8)

The integrals in eq (7) and (8) are related to the well-known normal distribution function, for which tabulated

values are available [5]. Defining

ARG

$ (ARG) = (2tt)" 1/2 -u

2

/2 A e ' du , (9)

eq (7) and (8) become, respectively,

Pmin ~ 7rr 0*0 $(3L/r0

)

- $(L/r0

) + $(7L/r0)

- *(5L/rQ

) +

*( (4m - 1) L/r0)

- *( (4m - 3) L/rQ ) m=l

(10)

and Pmax = *r 0l0 *(L/r0

) + <K5L/r0)

- *(3L/r0

) + *(9L/r„) +

•(LAo) + *((4m + 1) L/r

0

)

- $((4m - 1) L/r0) m= l

(11)

Finally, using eq (10) and (11), the ratio K of Pmin to P is obtained as

190
M-oo

»((4m - 1) L/r0

)

- »((4m - 3) L/r0) m=l

K = (12)

m—oo

»(L/r0

) + J

*((4m + 1) L/r0

)

- •((4m - 1) L/r0) m=l

where $( ) is defined in eq (9) and the quantity M has been introduced into the summations in order to limit

the sums in computations.

2.2. Results

A computer-generated plot of K versus rQ/L obtained with eq (12) is shown in figure 2. Clearly, if L

is known, a single measurement of K will determine rQ from such a plot. Repeated trials showed that most

of the curve in figure (2) can be produced with values of M = 3 or 4 in eq (12). Larger M are needed only

to show that the curve approaches 1 for large r0/L. Also, increasing M in eq (12) causes no detrimental

effects to the curve at small rQ/L. As will be seen below, in the case of axially symmetric beams, increasing

M to improve one end of the curve will reduce the accuracy of the other end. Finally, note that the curve in

figure (2) is approximately linear from rQ/L = 0.6 to 1.45. Using a 400 line per inch ruling, it should thus be

possible to accurately measure Gaussian beam 1/e2

radii in the range from approximately 19 /xm to 46 fim.

Of course, larger radii can be accurately measured with rulings have larger L.

3. Application to Any Axially Symmetric Beam Function

As with a Gaussian beam, characterizing any beam with the Ronchi ruling method requires a graphical

relationship between the optical power ratio K and some property that characterizes the beam (e.g., the 1/e2

beam radius in the Gaussian case). A unique relationship has to be derived for each ruling - optical beam

function combination. For an ideal Ronchi ruling and any axially symmetric beam, an expression for K can be obtained with the aid of figure 3. The circles in the figure represent the beam, whose optical intensity

function and arbitrary radial extent are I(r) and R, respectively.

To calculate Pmin and Pmax, note first that the beam power P in the area A to the right of any vertical

line and above the x-axis in figure 3 can be obtained from the expression

P = I(r) dA (13)

Using polar coordinates, in which dA = rdrdfl, and referring to figure 3, eq (13) can be rewritten as

R cos

_1 (x/r) R

P = rl(r) dr de rI(r)cos"

1 (x/r) dr

0

191
Since the last form is a function of x, we define it as 4(x), i.e.,

R

rI(r)cos"

1 (x/r) dr (14)

Referring once again to figure 3, it can be seen that Pmin and Pmax can respectively be expressed in terms of

<f>(x) as

pmin = 4 WL/2) - rf(3L/2) + *(5L/2) - *(7L/2) + ...]

4 2 W(4m - 3)L/2) - *((4m - l)L/2)] , m=l

and Pmax =4 4(0) - 4(L/2) + 0(3L/2) - 0(5L/2) + 0(7L/2) + ..

= 4 4(0) - 4(L/2) + 2 W(4m " !)L/2) - 4((4m + l)L/2)

m=l

Using these forms, the ratio K = Pmm/Pmax becomes

fo((4m - 3)L/2) - 0((4m - l)L/2)]

m= l K =

4(0) - 0(L/2) + ^ [4((4m - l)L/2) - 4((4m + l)L/2)]

m= l

(15)

where <f>( ) is defined in eq (14).

Equations (14) and (15) are valid for any axially symmetric beam and an ideal Ronchi ruling. To be

practically useful, however, they must be applied to a specific beam intensity function I(r) and expressed in

terms of some parameter characteristic of that function. This is done specifically for the Airy diffraction

pattern and the focused annulus in the following section. Since the Airy pattern is just a special case of

focused annulus, the analysis will be done for the more general focused annulus, and the results simplified

appropriately for the Airy pattern.

192
4. Application to the Focused Annulus and Airy Diffraction Patterns

4.1. Analysis

The output of a laser based on the confocal unstable resonator design [6] is a uniform beam in the

shape of an annulus [7]. As illustrated in the inset of figure 4, the annulus consists of a circular aperture of

radius a and a central circular obstruction of radius ea, where e is defined as the obscuration ratio. When an

annular beam is focused, the intensity function of the resulting Fraunhofer diffraction pattern can be written

as [8]

2x2

i(r) = i0/(i - o

2J1 (rc) ^(erc)

e —

rc ere

(16)

where IQ

is the intensity at the center of the beam, J

x (

) is the Bessel function of the first kind, first order, and

c is a shorthand constant for the expression

c - 2itot/{\ (17)

In eq (17) a is the radius of the diffracting aperture, f is the lens focal length, and A is the wavelength of the

diffracted light. Note that when e = 0 the diffracting aperture shown in figure 4 becomes a simple circle for

which the diffraction pattern is the well-known Airy pattern. Accordingly, eq (16) reduces, as expected, to

I(r) = I0[2J

1 (rc)/rc]

: (18)

The Airy pattern is an important special case because it occurs frequently in most laser systems due to the

common use of circular apertures and lenses.

Figure 4 shows normalized plots of eq (16) for,the cases e = 0.0 (the Airy pattern) and 0.5. Note that

as e increases, an increasing fraction of the total energy in the beam appears in the sidelobes. Thus, the error inherent in characterizing this beam function by assuming it to be approximately Gaussian becomes more

significant with increasing e.

Application of eq (14) and (15) to the focused annulus begins by using eq (16) in eq (14) and defining a new variable y = rc. Equation (14) thus becomes

2\2„2

4>(xc) = 4I0/(l-e

z

) zc

y = Rc-c

y=xc

-i 2

Ji(y) - «Ji(«y) (l/y)cos"

1 (xc/y) dy (19)

The upper limit of eq (19) is written R-oo because, although the Airy pattern has infinite radial extent, the

integral has to be terminated at some finite point for purposes of computation. Since the argument of the

function <t>( ) in eq (14) has been changed from x to xc in eq (19), all the arguments of 4>{ ) in eq (15) must

also be multiplied by the shorthand constant c. Also, it is evident that when eq (19) is used in eq (15) the

factor 4I

Q/(l-e

2

)2

c

2

will be common to every term and thereby cancel. It is convenient, therefore, to define a new function to be used in eq (15), i.e.,

193
Rc-oo

0(Z) =

<f>(z = xc)

4I0/(l-e 2

) 2

c2 Ji(y) - eJi(«y) (l/y)cos-1 (z/y) dy . (20)

In terms of eq (20), eq (15) can be rewritten as

M-c

e((4m - 3)Lc/2) - e((4m - l)Lc/2)

m=l

K = (21)

oo

7r(l-e

2

)/4 - 6(Lc/2) + 6((4m - l)Lc/2) - e((4m + l)Lc/2)

m=l

where the quantity M has been introduced in the summations for computational purposes and the first term

in the denominator has been replaced by its actual value, i.e.,

e(0) = -

2 J

Ji(y) - «Ji(«y) (l/y) dy

0

Ji

2

(y)

dy + e' dy - 2e

r Ji(y)Ji(«y)

dy

0 0

= n/2 (1/2 + e2/2 - e2

) = tt(1 - e z)/4

where the tabulated integral [9]

(

j n(at)Jn

(bt) (b/a)n

dt =

2n

0

was used to evaluate the three integrals in the calculation.

The final step in applying eq (15) to the focused annulus is to replace the quantity c in eq (21) with a parameter that characterizes the beam function. In analogy with the use of the 1/e beam radius rQ

in the case of Gaussian beams, the radial distance t

1 to the first null of the focused annulus is defined here as that

194
characteristic parameter. It is evident in figure 4 that the first zero of the focused annulus depends on the

value of e. We therefore define g(e) as the value of rc corresponding to the first null of the intensity function

in eq (16), i.e., g(e) = ^c, so that the form of c to be used in eq (21) is

c = g(<0Ai (22)

From eq (16) it can be seen that g(«) is the smallest solution of the transcendental equation

Jj(rc) - eJ

1 (frc) = 0 , (23)

which can be solved iteratively. For example, for e = 0.0 and 0.5, solution of eq (23) yields g(e) = 3.833 or

1.2207T and 3.145 or I.OOI71-, respectively. Substitution of eq (22) into eq (21) yields the required result for K,

i.e.,

m=l

e((4m - 3)g(e)L/2r1

)

- e((4m - l)g(e)L/2ri)

K = (24)

e((4m - l)g(£ )L/2ri )

- e((4m + l)g(e)L/2ri) m= l

7r(l-S 2)/4 - 0(g(e)L/2ri) +

where ©( ) is defined in eq (20).

4.2. Results

Equation (24) provides the desired relation between the measured ratio K and the radial distance ^

characteristic of a specific focused annulus. For comparison with the results shown in figure 2 for Gaussian

beams, eq (20) and (24) were used to obtain plots of K versus r

1 /L. The repeated integrations of eq (20)

required in eq (24) were done numerically using Simpson's one-third rule [10]. To ensure that the upper limit

Rc of every integral was always larger than the lower limit, it was necessary to require that Rc be at least as large as the largest argument in eq (24), which is (4M+l)g(e)L/2r1

. This leads to the condition

rj/L > (4M + 1) g(£ ) / 2Rc (25)

which reveals a computational tradeoff between the minimum r^L for which eq (24) can be evaluated and

the number of terms M used in the summations. Note that no such tradeoff was present in the Gaussian beam

case. Fortunately, the difficulty imposed by this tradeoff could be essentially eliminated by a sufficiently large

choice of the upper limit Rc (200), which also guaranteed that most of the energy in the beam was accounted

for in the computations.

In reference [8], it is shown that the fraction of the total power or energy of the Airy pattern (the e =

0 case) contained in the axial circle of radius R is given by the expression

P(R) = 1 - J

0

2(Rc) - J

x

2(Rc) (26)

195
where J

0( ) and J

x ( ) are Bessel functions of the first kind, orders zero and one, respectively. Using large

argument formulas [11] for J

0( ) and Jx ( ) in eq (26), it was found that for Rc = 200, P(R) = 99.68%. For

values of e other than zero, P(R) is slightly less, as can be seen in the normalized curves of figure 4, where

the sidelobe energy content increases with e. The fraction of total energy contained within a finite radius of a focused annulus with arbitrary e can be calculated by numerically integrating eq (20) with z = 0. This has

been done previously [12-14], and the results of reference [14] show that for Rc = 200, at least 99% of the

beam energy is included if e < 0.5 and at least 98% is included if e < 0.8. Furthermore, with Rc = 200 in eq

(20) and as few as three terms (M > 3) in the summations of eq (24), plots of K versus rJ~L were found to be virtually identical for values of r

x/L up to approximately 2.5. At that point K ~ 0.99, which is well beyond

the useful portion of a given plot.

Figure 5 shows plots of K versus r

1(/L for e = 0.0, 0.25, and 0.50, which were obtained with Rc = 200

in eq (20) and M = 5 in eq (24). The lower ends of the curves terminate at r-j/L ~ 0.20 due to the limitation

imposed by eq (25). Fewer terms (M < 5) in the summations would allow the curves to extend to smaller

values of r^L. On the other hand, larger M are needed to show how the upper ends of the curves approach

1.0 asymptotically with increasing r

1 /L. Therefore, the use of M = 5 represents an acceptable compromise

that yields the useful portion of the K versus r-|/L relationship.

As is the case with Gaussian beams, a K versus rj/L Ronchi ruling plot is most useful if it varies

smoothly, monotonically, and approximately linearly over as wide a range of (r-j/L, K) values as possible. The

curves in figure 5 (and others not shown here) show that, beginning with e ~ 0.45, a portion of the K versus rj/L data exhibits a non-monotonic behavior that becomes more pronounced with increasing e. Thus, the

method is most useful for smaller e, i.e., e < 0.45. For example, Curve (a) in figure 5 for the Airy pattern (e = 0) is smooth and reasonably linear over the approximate range of (r-j/L, K) values from (0.75, 0.1) to (2.25,

0.9). Thus, using a 400 line per inch Ronchi ruling (L = 31.8 jjm), it should be possible to accurately measure

first nulls of Airy patterns ranging in size from approximately 24 /xm to 71 /xm. This result is similar to that

discussed earlier for Gaussian beams (see Section 3). Also, since the behavior of Curve (b) in figure 5 for

e = 0.25 can be seen to be similar to that of Curve (a), similar comments can be made about it and other

curves corresponding to relatively small e. Thus, the Ronchi ruling method should be at least as useful with

those beams as with Gaussian beams.

Curves of K versus r^h for larger e can also be used, but in a somewhat restricted manner. For

example, Curve (c) in figure 5 for e = 0.50 is monotonic and roughly linear over two relatively small ranges

of (xJU K), i.e., from approximately (0.29, 0.12) to (0.50, 0.42) and from approximately (1.4, 0.55) to (2.0,

0.97). In terms of a 400 line per inch ruling, these two ranges would enable measurement of first null radii

ranging from approximately 9.2 /zm to 15.9 nm and from approximately 44.5 to 63.6 ym.

Determining the t

1 of a focused annulus from a plot of K versus ^/L depends upon whether the value

of e is known. If e is known, the procedure is the same as in the case of Gaussian beams. A single

measurement of K with a Ronchi ruling of known L allows t

1 to immediately be read from the appropriate

plot of K versus r

1 /L. If the value of e is unknown (the more likely situation), measurements of K with two

different Ronchi rulings of known bar and space widths (L1 and 1^), are needed to determine rj. The first measurement of K with the known Lx will determine a set of possible r

1 values, one from each of a set of curves of K versus r

x /L for different e. With the known L^, a set of r

1 /L2

ratios can then be formed and used

with the set of K versus r

1 /L curves to predict the value of K to be measured with L^. The second

measurement should agree with only one of those predictions, thereby establishing both e and r

1

.

Note, finally, that a measurement of r

1 also determines the outer radius a of the diffracting aperture.

This becomes evident when eq (17) and (22) are compared, which shows that

a = g(0&/2*ri , (27)

where the lens focal length f and the wavelength A of the diffracted light are presumably known.

196
5. Summary and Conclusions

We have reviewed the application of the Ronchi ruling method to Gaussian beams and analyzed its application to axially symmetric beams. Equations (14) and (15) express the ratio K of minimum to maximum

transmitted optical powers in terms of the ruling parameter L and the beam function I(r) of an arbitrary

axisymmetric beam. Equations (14) and (15) are converted to eq (20) and (24), respectively, when applied

to the focused annulus beam function (eq (16)), a special case of which is the Airy pattern (eq (18)).

In analogy with previous Ronchi ruling analyses of Gaussian beams [1-3], eq (20) and (24) were used

to generate plots of K versus r-j/L, where r

x

is the first null of the specific beam function. Typical examples

are given in figure 5. The results suggest that the Ronchi ruling method is most useful for focused annuli with

relatively small obscuration ratios i.e., e < 0.30 (approximately). In such cases the K versus r^/L plots are

similar in behavior to the results obtained for Gaussian beams, i.e., monotonic and roughly linear over a

relatively wide range of (r-j/L, K) values.

It should be possible to quickly and accurately characterize beams with first nulls larger than or equal

to approximately 25 jim. If e is known, one measurement of K with a ruling of known L determines both t

1 (from a K versus r-j/L plot) and the outer radius a of the diffracting aperture (from eq (27)). If e is unknown,

a second measurement with a different known ruling will determine e, r^ and a.

6. References

[1] R. M. O'Connell and R. A. Vogel, "Abel inversion of knife-edge data from radially symmetric pulsed

laser beams", Appl. Opt. 26, 2528 (1987).

[2] L. D. Dickson, "Ronchi ruling method for measuring Gaussian beam diameter", Opt. Eng. 18, 70 (1979).

[3] E. C. Broockman, L. D. Dickson, and R. S. Fortenberry, "Generalization of the Ronchi ruling method

for measuring Gaussian beam diameter", Opt. Eng. 22, 643 (1983).

[4] M. A. Karim, "Measurement of Gaussian beam diameter using Ronchi rulings", Electron. Lett. 21, 427

(1985).

[5] Standard Mathematical Tables (The Chemical Rubber Co., Cleveland, 1965), p.258.

[6] W. F. Krupke and W. R. Sooy, "Properties of an unstable confocal resonator C02 laser system", IEEE

J. Quantum Electron. QE-5, 575 (1969).

[7] J. T. Verdeyen, Laser Electronics (Prentice-Hall, Englewood Cliffs, 1981), p. 106.

[8] M. Born and E. Wolf, Principles of Optics (Pergamon, Oxford, 1975), pp.395-418.

[9] G. N. Watson, A Treatise on the Theory of Bessel Functions (Cambridge, New York, 1944), p.405.

v.

[10] R. G. Stanton. Numerical Methods for Science and Engineering (Prentice-Hall, Englewood Cliffs, 1961),

p.116.

[11] Standard Mathematical Tables (The Chemical Rubber Co., Cleveland, 1965), p. 350.

[12] W. T. Welford, "Use of annular apertures to increase focal depth", J. Opt. Soc. Am. 50, 749 (1960).

[13] B. L. Mehta, "Total illumination in an aberration-free annular aperture", Appl. Opt. 13, 736 (1974).

[14] H.F.A. Tschunko, "Imaging performance of annular apertures", Appl. Opt. 13, 1820 (1974).

197
(a) MIN 1

, M - 1 .1

II

1

L/2

"J

1

BAR

SPACE

i_x

(b)

1 MAX

M = 1

4 »

Figure 1. Geometry used to analyze the characterization of a Gaussian beam with an ideal Ronchi ruling: (a) for the

calculation of Pmin ; (b) for the calculation of Pmax .

ii

Figure 2. Plot of K versus r

0 / L for a Gaussian beam

(Equation (12)).

198
Transparent space

(a)

Opaque

L U2

(b)

r /

1 R

V

1 X

Figure 3. Geometry used to analyze the

characterization of an arbitrary axially symmetric beam

with an ideal Ronchi ruling: (a) for the calculation of

mm ; (b) for the calculation of Pmax*

Figure 4. Geometry of an annular

aperture (inset) and normalized cross- sections of two axially symmetric beam

functions: (a) the Airy pattern (Equation

(18)) or focused annulus (Equation (16))

with e = 0.0; (b) the focused annulus

(Equation (16)) with e = 0.5.

Figure 5. Plots of K versus r

1 / L obtained using Rc = 200

in Equation (20) and M = 5 in Equation (24) for various

focused annuli: (a) e = 0.0 (the Airy pattern); (b) e = 0.25;

(c) e = 0.50.

199
Manuscript received

1-29-90

Prediction Of Laser Induced Damage By Comparison Of Laser Fluence

Profile To Damage Spot Radius

R. Goedert, T. Whittaker and D. Templeton

U.S. Army Tank-Automotive Command

Countermeasures Branch

Warren, MI 48397-5000

In this method, the laser induced damage threshold for a material is obtained

by determining the fluence within the damaging beam at a radial distance equal to

the radius of the damage spot. The energy profile of the laser pulse is determined by translating a slit across the beam and reading the energy passing through

the slit with2Position . The laser profile is then quantified into fluence squares

of area 400um in a two dimensional fluence grid. The radius of the damage site

is then applied to the fluence grid, indicating the fluence square that corresponds

to the edge of damage and it is this value that is the predicted damage threshold.

Key Words: Laser Induced Damage, Profile, Fluence Grid, Surface Pitting

1. Introduction

Damage threshold measurements are often very time, consuming and expensive, but for many

applications, all that is needed is a good approximation of the threshold for a given set of

parameters. This paper presents preliminary results of a method for determining the damage

threshold of an optical glass by relating the size of the damage site to the energy profile of

the damaging pulse of light. It is the goal of this work to develop and narrow the process

sufficiently such that very few damage sites will be needed to make an accurate prediction of

the single shot damage threshold of the glass.

The optical glass used in this investigation was Schott Glass Corporation's 0G-550

absorption glass. This glass is a cut-on filter with its 50% transmission point at 550nm

while it absorbs very strongly at 532nm. Figure 1 is a transmission curve of 0G-550. The

single shot and multiple pulse laser induced damage thresholds of OG-550 to Q-switched 532nm

radiation have been determined by this laboratory, Whittaker et . al

[1], and presented at the

1989 Symposium on Optical Materials for High Power Lasers, National Institute of Standards and

Technology in Boulder, Colorado in a paper entitled, "Laser Induced Damage In Schott' s OG-550

Optical Absorption Glass." The results of that investigation are summarized in table 1 and

figure 2. The primary hypothesis of this work is that each particular portion of the damaging beam

is mostly responsible for the damage done to the target over that area which it impacts.

Heating of other areas of the target by other portions of the beam may contribute to the

damaging of any particular area, but this aspect is ignored for the single shot damage

threshold determination (heat propagation does not appear to be significant until about the

sixth pulse of multiple shot irradiance at 10 Hz for the energy levels utilized). Figure 3 pictorially displays the concept. Filbert et . al [2], presented work showing a direct

correlation between the shape of damage within a damage site and the shape of the fluence

distribution within the laser spot. They used a 100x100 pixel array camera to capture the

multimode fluence distribution of the laser spot. This distribution contour plot was then

compared with an enlarged picture of the damage site. Their example showed that sections of common fluence within the contour plot had the same shape as sections of damage within the

damage site.

2. Damage Requirement

Since the method requires that the average radius of the damage site be measured, energy

levels must be set to cause damage that is roughly circular. In addition, since the method

200
relies on matching tb,e damage site to the energy profile of the beam, damage sites with radii

larger than the 1/e radius of the beam are not desirable. The type of damage referred to

in this investigation is surface pitting. Figure 4 displays photographs of a typical damage

site

.

3. Experimental Setup

Figure 5 shows the laser damage optical setup. The laser source is a Quantel

International model 581 Q-switched Nd:YAG with doubler lasing at 532nm with a pulse length of

approximately lOnsec. The pulse energy is measured using a Laser Precision Corporation

silicon detector, model RJP-765. The f/2 lens is used to bring the laser spot size to a diameter on the order of 600um at the sample surface.

4. Fluence Within the Laser Pulse

Two separate processes were developed which use common one-dimensional single slit energy

profile data and translate it into two dimensional fluence data. The laser beam profile is

obtained by translating a 120um slit perpendicular to the beam (x-direction) in increments of

60um or 120ura and measuring the energy passing through the slit. Figures 6 and 7 show the

energy profile obtained with the slit method for our data. The first method, 'The Common Area

Method', yields a fluence value for the common area between the slit and a beam of diameter

1/e , for the slit at any location. The second method, the 'Fluence Grid Process',

quantizes the single slit profile data into a two dimensional grid of fluence squares of the same area. The Common Area Method assumes only that the beam is circular while the Fluence

Grid Process assumes that it is gaussian. The Fluence Grid Process is preferred, but the

Common Area Method deserves some discussion since it does not require a gaussian beam and for

comparison between the two methods.

4.1 Fluence Grid Process

Very simply, the method quantifies the laser spot into a grid of squares of varying

fluences. One then measures the diameter of the damage site and matches the point on the

glass at the diameter of the damage to the square within the beam that caused the damage.

That particular square has a fluence associated with it and that fluence is considered the

damage threshold of the glass.

Again, the beam is characterized by translating a 120um slit across the beam in 60um or 120um increments and the energy passing through the slit recorded. This data is then

interpolated so that we have a value for the energy passing through a 120um slit for every

20um increment across the beam (note that this adds extra data points such that when you add

them up you have a value larger than the actual total energy; the solution is simply to divide

the total by the number of added data points within the 120um range). The beam is assumed to

be gaussian and therefore we can find a best fit gaussian curve through our data. Figures 6

and 7 show the gaussian best fit through our slit profile data. Using a computer spread

sheet, we construct a two dimensional array with a data point (cell) every 20um in both the x and y directions (the beam travels in the z direction). The values in the cells represent a rotation of the gaussian best fit curve. Each cell contains the equation for a gaussian in

the form:

Cell = C

2 , e -((x

2 + y2 )/2*sigma 2

) , ^2

Energy 2*PI*sigma

where, x & y indicate the cell position

within the array &

2

C = 2*PI*sigma *(Peak Energy)

Area

eq (1)

The total energy of the damaging laser pulse is known and the interpolated energy profile is

adjusted such that the total energy under its curve is equal to this value. Then C is

adjusted until the total of all the elements in the array equals the energy of the damaging

pulse. At this point the values in the fluence grid are the correct values for the given

damaging pulse, and the damage threshold can be found using the damage site radius. Figure 8

is a schematic diagram of the process. Note that the choice to interpolate the data every

20um, and therefore have a fluence square area of 400um , is completely arbitrary; our

201
limiting factor is the memory of our computer.

4. 2 The Common Area Method

In this method the objective is to determine the fluence in the slice of the beam that

intersects with the edge of the damage site; it is this fluence that we assume to be the

damage threshold for the glass. That is, if our profiling slit is of width 120um and our damage site has radius 200um, then we want the fluence in the slice of width 120um centered at

200um from the center of the laser spot. After determining the energy profile of the beam by

the slit translation method, we interpolate this data to obtain a curve of energy through the

slit with position. A BASIC program written at the Tank-Automotive Command entitled,

"Profiles" is utilized to calculate the fjommon area between the slit and the beam for the slit

at a given position within a beam of 1/e radius. Since we have the total amount of energy

passing through the slit at any position across the beam, we can determine the fluence for any

slice of the beam. However, there are two boundary conditions that apply. The first is that

damage sites used in this method must not have a radius larger than 90% of the beam spot

radius. This is because the area calculation tends quickly to zero after this point, since we

are not allowing the inclusion of any area outside2the 1/e radius. Although there is

energy in the wings of the gaussian beyond the 1/e point, a logical limit had to be placed

on the common area calculation. The second boundary is that a damage site radius can not be

any smaller than the slit width. As the damage site gets smaller, tending to zero, the common

area calculation tends to its maximum value (that of the slit positioned at the center of the

beam). This causes a diverging discrepancy between the immediate area around the edge of the

damage site (the area over which we are interested in the fluence) and the area calculated by

the program. This divergence needs a limit then, which should probably be larger than the

slit width, but this limit is at least obvious.

5. Results

When comparing the results of this investigation to the damage threshold values stated by

Whittaker et . al

in table 1, compare with the values given for microscopic damage.

5. 1 Results of Fluence Grid Process

There are two data sets presented here, each using a different beam radius. The beam

radius for data set A was 325um and for data set B it was 295um. Figures 9 and 10 show the

damage thresholds achieved for each beam diameter using the accepted method of decreasing the

fluence until no damage is seen. While the threshold achieved for data set A agrees well with

the data presented in table 1, it is not understood why the threshold achieved for data set B

is higher.

Table 2 shows the predicted single shot damage thresholds achieved using the Fluence Grid

Process, along with the radius of the damage site used to obtain the value and the number of

laser pulses that caused the damage. Note that while2there is considerable variation within

the data, the average value, for data set B, 6.16 J/cm , is very close to the accepted value

for data set A of 6.3 J/cm . Also the average value of data set A, 7.97 J/cm , is

reasonably close to the accepted value for data set B of 9.9 J/cm . Single shot and two shot data are presented in the table. It was noted that two shots

yielded a more uniform damage site in most cases and therefore this data is presented to

examine the influence of the second shot on the prediction of the damage threshold. It appears that the two shot data yields a threshold value close to that obtained for the single

shot data. While it may be desirable to use two shot data (since the damage is more uniform),

it is still unclear how to handle two shots of slightly varying energies and for this reason

the use of two shot data requires further study.

Figure 11 is a plot of the total fluence on site versus damage radius. This demonstrates

that there is a region over which the diameter of the damage spot increases linearly with

increasing fluence on site. The data obtained with the 325um radius beam, data set A, appears

better behaved than that from data set B with a beam diameter of 295um. Figure 12 is a plot

of the predicted single shot damage threshold versus the damage spot radius. This plot seems

to demonstrate that our premise that the edge of the damage sight will point directly to the

threshold fluence is flawed. There is, however, a definite linear relationship that is worth

exploring. We will examine this relationship further to see if the y intercept is some function of the threshold value for several different materials.

5.2 Results of Common Area Method

Figures 13 and 14 present the predicted damage threshold values versus damage spot radius

202
for data sets A and B using the Common Area Method. The data within the boxes is considered

'allowed' because it meets the boundary conditions that the damage site cannot have radius

larger than 90% of the beam spot radius or smaller than the slit width. The average predicted

damage threshold for data set A is 16.98 J/cm^ and for data set B is 7.66 J^/cm . Again,

these values are to be compared with 6.3 J/cm for data set A and 9.9 J/cm for data set

B. While the value of 7.66 J/cm is considered a good approximation, it is not understood

why the threshold obtained for data set A is roughly twice as large as the accepted value.

Note that the threshold data within the allowed regions of figures 13 and 14 does not increase

or decrease with increasing damage site radius. This is the result that is desired. This

shows that while the fluence within the beam changes quickly with increasing radius from the

center of the beam (as it does with a gaussian beam), the threshold value determined with this

method remains fairly constant.

6. Conclusions

While it is too early yet to draw a definite conclusion on the utility of the methods,

they appear to yield, on average, good approximations to the accepted damage threshold value.

The goal of the method, however, is to closely approximate the threshold value using very few

data points, and this has not yet been accomplished. Figure 12 indicates that the initial

hypothesis is not correct, at least for the glass used in this work. It remains to be seen if

this method will apply to coated optics and metalized mirrors. Other hypotheses should be

tested with the data in figure 13, from the Common Area Method, since this data behaves as expected

.

Since it is expected that the large spread within the damage threshold region (the overlap

between 'damage' & 'no damage' in figures 9 & 10) is inherent to the material and that this

parameter cannot be changed, other aspects of the experiment such as beam quality must be

known to a high degree. It also appears that this same variation may be manifesting itself

within each damage site resulting in less than circular damage. We are planning to measure

multiple radii for each damage site and then average these values to see if this may be a method to obtain a reasonable average value from only a few damage sites. We are also

planning to utilize a 256X256 array camera and framegrabber system to capture the shape of

each damaging pulse. It is suspected that hot spots within the laser beam may be contributing

to the variation in our data and the f ramegrabbing system should help to expose this problem.

The camera system also displays a fluence value for each pixel. If the laser spot and damage

site can be imaged together, fluence values could be given that literally outline the damage

site. It is expected that the average of these many values for a single damage site will

yield a close approximation to the true threshold value, and thus only a few damage sites will

be needed for an accurate threshold determination.

Other additional work will include the application of this method to other optical

glasses, thin film coatings and metallized mirrors. It is clear that if this process can be

refined such that only a few damage sites are needed for a damage threshold determination, the

method will find great utility in areas such as testing of one-of-a-kind parts, testing

expensive materials or coatings, or a quick quality control measure.

Special thanks to Dr. John Bennett who helped us develop the fluence grid process, and to

Mazin Kashat and Gerald Koenig who wrote the Profiles program which calculates the common area between a beam of 1/e radius and a slit of width x at any given position.

7. References

[1] Whittaker T.A. , Goedert R. , Templeton D., "Laser Induced Damage in Schott's OG-550

Optical Absorption Glass," National Inst, of Standards & Tech. Laser Damage Symposium, 1989.

[2] Filbert P.C. , Guttman J.L., Navoda C.L., Watson B.A., "Application of a Computer-Automated IR Sensitive Camera to the Determination of Pulsed Laser Surface Damage

Thresholds," Natl. Bur. Stand. (U.S.) Spec. Publ. 727, pp. 77-84, 1984.

203
a

o

w2

ws

OS

EOG-550 LONGPASS FILTER

Spectral Transmittance

4-50 510 540 570 600 630

WAVELENGTH (nm)

660 690 720 750

Figure 1. Spectral transmission of OG-550 absorption glass.

TABLE 1. TYPE OF DAMAGE: Surface pitting

SINGLE SHOT DAMAGE THRESHOLD (J/cm

LASER DAMAGE CRITERION

SAMPLE # SPOT SIZE Visual Micro

(RADIUS am) (100X)

3 432 6.0 4.9

6 442 8.7 6.0

9 625 9.7 6.5

10 625 7.9 5.6

1 1 626 7.3 6.8

12 626 7.0 4.9

A' ERAGE 7.4 6.6

204
0\

<->

U

Z

H

3

4

b

MULTIPLE SHOT LIDT TEST OG-550 GLASS

CRITERION: Surface pitting.Plasma spark

100

»— PRF = 10 Hz

Figure

NUMBER OF SHOTS TO DAMAGE —1— PRE = 5 Hz SAMPLE #15

PRE = 2 Hz

2. Multiple shot laser induced damage threshold (LIDT)

results presented by Whittaker et. al.

Figure 3. The drawing displays the concept that particular

portions of the beam are responsible for particular

portions of damage within the damage site.

205
THE SETUP;

Figure 5. The laser damage optical setup.

206
Gaussian Fit To Energy Profile

120um Slit Translation, Beam Rad.=325um

0.04-5 -i

2

-460 -380 -300 -220 -140 -60 20 100 160 260 340 420

position perpendicular to beam (um) —•— Gaussian Pit Slit Profile

Figure 6. Gaussian, fit to the energy profile, derived from the

slit translation method, for data set A.

Fit To Gaussian

Sample 9-25, Data Set B

0.05 -i—

hi

-450 -260 -60 140 340

position (um)

Profile —•— Gaussian fit

Figure 7. Gaussian fit to the energy profile for data set B.

207
o

CD

CD

CO

CO

111

o

O

cc

g

o

LU

o

z

LU

o

CD

cd

o

o

cm

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CO

CD

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o

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cc

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CD

CD

CD

O

CM

o

crLU

<

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- LU

|< JO I- o => m

cc < m ^ c m

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QLLI

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O

o o

CM f

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Figure 8. A schematic diagram of the Fluence Grid Process.

208
Q

SI

nD

0

SINGLE SHOT LIDT OG-550 GLASS, 532nm

CRITERION: pitting, microscopic

am n i in in ii i

an d nnrn m a dcdd nnn en

~

i r

10

~i 1 1 r

14 ie 13

PLUBNCB (J/cmXtCE) SAMPLE "9-25"

SITES 32-67 + ss LIDT = 6.3 J/cm2

Figured. Single shot damage threshold results for data set A

using the standard method. Threshold is 6.3 J/CM^.

i

HQ

0

z

SINGLE SHOT LIDT OG-550 GLASS, 532nm

CRITBRION: pitting, microscopic

rrm o HJ QO CD

nn o ODD

d

~~\ r ~i 1 1 r

10

i

14- 16

1 IB

SITES 173-226

FLUENCB (0/cmX*2) SAMPLE "9-25"

SS LIDT = 9.9 .J/cm2

Figure 10. Single shot damage threshold results for data set B using the standard method. Threshold is 9.9 J/CM^.

209
TABLE 2. PREDICTED SS LIDT

DATA SET A BEAM RADIUS-32S urn LIDT DAMG. RAD. * OF

8.313 45

7.790 80

16. 13 130

14.38 140

1 1.96 160

12.79 160

1 1.(3 160

1 1.98 185

11.98 190

1 1.42 200

9.010 213

6.583 250

4.594 275

5.375 275

3.200 295

3.250 300

3.667 300

3.135 305

3.177 305

2.870 315

1.302 325

3.496 325

2.380 32 5 AVERAGE - 7.414 J/CM*

AVERAGE OF

SS DATA ALONE * 7 97 J/CM

DATA SET B BEAM RADIUS-295 urn LIDT

18.04

13.21 8.540

6.690

6.790

6.040

5.170

6.580

5.980

4.12S

4.790

3.420

3.670

3.170

3.530

3.750

3.630

6.125

4.080

3.396

3.730

2.750

2.SQ0

AVERAGE

DAMG. BAD * OF SHOTS

90

175

2 10

2 15 225

235

240

240

245

250

250

260

260

280

264

270

270

275

280

285

285

290

300

5.640 J/CM "

AVERAGE OF

SS DATA ALONE ' 6.16 J/CM

"otal Fluence on Sight vs Damage Radius

Sample 9-25, All Single Shot Data

Damage Spot Radius (urn) Data set B + Data set A

Figure 11. The total fluence (over the entire l/e2 radius of the

beam) on site versus the damage site radius.

210
Predicted SS LIDT vs Damage Spot Radius

Sm.pl. 9-25, All Single Shot Data

Damage Spot Radius (-urn) Data set B + Data set A

Figure 12. The predicted damage threshold versus the damage site

radius using the Fluence Grid Process.

Fluence vs Damage Radius

The Common Area Method: Data Set A

Fluence (J/cm2)

40 80 120 160 200 240

Damage Spot Radius (um)

280 320

Data Set A

Figure 13. The predicted damage threshold versus damage site radius

for data set A using the Common Area Method.

211
Fluence vs Damage Radius

The Common Area Method: Data Set B

Fluence (J/cm2)

90 110 130 150 170 190 210 230 250

Damage Spot Radius (um)

270 290

Data Set B

Figure 14. Th<- predicted damage threshold versus damage site

radius for data set B using the Common Area Method.

212
COMMENTS

Question: In about 1980, somewhere in that time frame, OCLI provided

Livermore with silica titania reflectors that had thresholds in excess of 20 joules per square centimeter at 1 nanosecond. These

parts should be in the archives somewhere, I suppose. Have any

parts with that high a damage threshold initially been conditioned

to see if you get even a higher threshold?

Answer: I don't believe our initial coatings were quite that high, but

recently we have made good titania coatings and conditioned them.

Our problem with titania is that over a period of a couple of

years we get good and bad results from titania. It's not a very

controllable material, at least not as controllable as the other

dielectrics I've mentioned.

Question: There are applications where optics are sold and they go on the

shelf for three or four years before they're put into systems in

field rework. I wonder if you have any data or if you've looked

at any data carefully enough to extrapolate your claim that this

enhancement is permanent to longer time frames than I've heard

mentioned so far? It has important economic consequences, if this

is possible. Have you got data like that? Are you looking at

that? Will you be mapping it over a period of years.

Answer: No. At this point, we've been following this conditioning in a low absorption stoichiometric films for maybe nine months, and we

see it as an important thing. I believe there's a good chance

that it is permanent for ,longer times, but the coatings have to be

kept in a very controlled environment if they are going to be

stored. I think they'd have to be kept clean, in a generic sense.

Question: Have you given any thought to comparing your laser annealing to

just baking or thermal annealing, which is maybe not practical for

the 1 meter optics, but a lot cheaper.

Answer: Yes. We've thought about it, and that is something that would be

very revealing to do. We haven't done that experiment yet. We're

also attempting other kinds of annealing. The laser annealing may

be the most awkward way to do it. We've tried flash lamp

annealing, for example, and you'll hear a little more about that,

I think, from the next speaker. But we're certainly interested in

other techniques. The laser annealing may not be the best way to

do this at all.

213
Manuscript received

2-12-90

IR Laser Beam Profiling Using Quenched Fluorescence

N.C.Kerr, S.E.Clark and D.C.Emmony

Department of Physics

Loughborough University of Technology

Loughborough

Leicestershire

U.K.

The construction and performance of a system for the spatial

characterization of a CO2 pulsed laser beam is described. Using

thermally quenched fluorescence a direct imaging technique has been

developed for single shot CO2 laser pulse beam profiling. A CCD

video camera and framestore interfaced to a PC computer are used to

image the spatial intensity distribution produced by a CO2 pulse

incident upon a temperature dependent luminescent screen. The

resolution is similar to much more expensive pyroelectric detector

array techniques with all the advantages of two dimensional beam

processing

Key words: beam profiling, CO2 laser, pulsed.

1. Introduction

Laser beam profiling is a problem as old as the laser itself, with many methods of

varying degrees of success having been reported. Methods such as photographic film,

scanning pinhole and linear array techniques have been used. Whilst references can be

found in the paper by Rose et al [1] it is worth while to note that all of these methods

suffer from the long time scale required to perform the measurements. Using a CCD video

camera, a 2-dimensional image of the laser beam profile can be obtained quickly with only

a single laser pulse.

A fast, single shot, focal zone beam profiling system has previously been built for use with UV, visible and near infrared lasers at the Loughborough University laser damage

facility by Clark et al [2]. These authors based their technique on a CCD video camera

linked to a computer controlled video framestore. In cases where the laser wavelength is

within the response band of the camera (40-1100nm) the beam is imaged directly, but at

shorter wavelengths, fluorescent glasses which produce visible light if excited by UV

light are used. To extend this technique to longer wavelengths e.g. 10.6 um, a mechanism

to linearly convert the IR to visible radiation must be found.

To enable near linear conversion of the IR to visible, thermal imaging plates, such

as those marketed by Optical Engineering were used. These plates make use of what is

known as thermal quenching of induced fluorescence. In this process, the plates are made

to flouresce by illumination with a cw UV light. If the plates are heated, e.g. by a CO2

pulse, the fluorescence decreases in proportion to the local temperature, i.e. the local

fluence and hence in proportion to the beam intensity.

214
2. Experimental

Figure 1 shows a schematic layout of the beam profiling/interaction zone. The pulsed

output of a Laser Applications TEA CO2 laser operating at 10.6um is focused in the plane

X-X using a 15cm focal length CaF2 lens. This plane is then imaged onto the luminescent

screen using a second CaF2 lens. The two lens combination is used both to provide a suitable magnification of the profile for imaging and to arrive at a suitable energy

density for the luminescent screen.

The luminescent screen [3] has a normal sensitive range of 0.4-2. A W/cm^, a damage

threshold of 24 W/cm^ (under cw illumination) and a resolution of 4 lines/mm. By

magnifying the image incident upon the screen (using the second CaF2 lens), the resolution

in the luminescent image is effectively increased by a factor equal to the magnification.

A fourfold magnification was used to give the system a resolution of about 16 lines/mm in

the damage zone, which is a figure comparable to that stated for much more sophisticated

and expensive CO2 beam profiling systems using arrays of pyroelectric detectors. The

degree of quenching, i.e. darkening of the screen is linearly dependent upon the incident

CO2 intensity to within 10% over the stated ranges.

To be sure which plane through the focus of the first lens was being imaged, a graticule was placed at X-X and illuminated using a HeNe laser and the image focused on the luminescent screen. As well determining which plane is in focus, this process enables

the physical extent of the image as seen on the video monitor to be calibrated.

The CCD camera imaged the luminescent screen using a 50mm camera lens at as near normal incidence as possible to reduce depth-of -field problems. The signal from the camera was sent to an Eltime Image III framestore where it was digitised and stored. The

resulting video images which are made up of 256x256 pixels each having 6 bit (64

greylevel) resolution, can be displayed on a video monitor and/or processed using a PC

computer. Ideally, to specify the beam profile to an accuracy of say 5%, at least 160

grey levels are required. Such 8 bit frame grabbers are available but we were constrained

by the Eltime, which was the only frame grabber available to us.

The luminescent screen is flood-illuminated using a 125W UV lamp that covers the

peak excitation wavelength (360 nm) of the screen. The fluorescence which is in the

yellow/green region of the spectrum is easily detected by the CCD camera, however the

decrease in brightness caused by the heating due to the CO2 laser pulse corresponded to a change of at best 4 greylevels compared to the constant background signal. Such a small

change does not produce enough dynamic range to perform beam profiling measurements such

as determining the l/e^ points.

Simple amplification of the video signal is not satisfactory as it would also

amplify the background. It was therefore necessary to build a circuit which selectively

amplified the signal of interest, the block diagram of which is illustrated in figure 2.

The dc component in the video signal associated with the background illumination is

removed from the signal due to the quenching of the fluorescence by firstly amplifying the

whole signal. The output of this amplifier stage is then passed to the base of a fast

transistor. Only signals above the 0.7v turn on level of the transistor are passed to its

output. By careful adjustment of the amplifier gain and dc offset it is ensured that only

the component of the signal corresponding to the quenched fluorescence is above this

level. The amplification is variable upto xlO and has the effect of increasing the

contrast of the resultant video image. This circuit also accommodated the laser and

framestore trigger and synchronised the capture of a video image to the start of a video

field from the CCD camera.

215
Using the computer and suitable processing routines it is possible to rapidly

process images in the framestore. Subtraction of images can be used to remove fixed

pattern noise in the video image, this is due to dark current non-uniformities in the

sensitive element of the camera. Further, it is possible to average the signals for any

number of frames, which reduces the random electronic noise introduced by the selective

amplifier circuit.

Laser beam profiling can simply involve the recording of a single frame of the

quenched fluorescence and subsequent computer processing. However because of the speed of

operation of the system as a whole, the routine method adopted is to average the video

signal over 16 laser pulses. Figure 3 shows a typical processed video image after it has

been intensity inverted during the processing. This inversion allows the image to be

processed directly by profiling routines previously written for use with UV beam images,

as well as giving a more acceptable picture of the laser beam.

Figure 4 shows the intensity along the cursor drawn horizontally in the previous

figure. For comparison, the intensity profile obtained in an identical image plane using a scanning pinhole in conjunction with an integrating pyroelectric detector is also shown.

The two profiles are normalised to the same peak intensity. Excellent agreement is

clearly evident between the two sets of data. It should be noted that figure 3 contains

all of the 2-dimensional information necessary to obtain any desired horizontal or vertical profile across the CO2 pulse. The time taken to obtain this data, including

image processing time, was less than 5 minutes. To obtain a similar resolution and number

of profiles using a scanning pinhole would take in excess of 6 hours and several thousands

of laser pulses. The Laser Applications laser has particularly good shot to shot

reproducibility otherwise the scanning pinhole technique would not even be applicable .

3. Conclusion

Using thermally quenched fluorescence, a direct imaging technique has been developed

for single shot CO2 laser pulse beam profiling. The resolution is similar to much more expensive pyroelectric detector array techniques with all the advantages of two

dimensional beam processing.

Acknowledgements

The authors would like to acknowledge the technical assistance given by Mr M. Pancholi and Mr R. Pancholi of the Physics Department Electronics Workshop and Angela Anne

Kerr in the preparation of this manuscript. NCK would especially like to acknowledge the

financial support of BDH Ltd and SERC.

4. References

[1] Rose A., Nie Y.-X. and Gupta R. Laser Beam Profiling Measurement by Photothermal

Deflection Technique. Appl. Opt., 25 1738-1741 (1986)

[2] Clark S.E. Excimer Laser Induced Modifications of Optical Surfaces, Ph.D. Thesis,

Loughborough University, 1988.

1 [3] Optical Engineering Inc., Santa Rosa, CA 95401, U.S.A.,

model 23-7 CO2 beam probe.

216
Laser

Trigger

pulse

Selective

amplifier

UV lamp

Trigger

pulse

Fluorescent

screen

50mm camera lens

Video camera

Video

Framestore Computer Video

monitor

Figure 1. Experimental arrangement of apparatus.

Video

input

Amplifier

stage xlO

variable

Video signal

removal

stage

Discriminator

stage

Contrast enhanced

signal

sync pulses

Adder

circuit

Find

start of

field

Trigger

circuit

Delay

To framestore

To C0

2 laser

Figure 2. Block diagram of the selective amplifier

and trigger circuits.

217
Figure 3. Intensity inverted video picture of CO2 laser

quenched fluorescence. The rectangular focal

zone is approximately 5mm X 5mm.

HORIZONTAL POSITION (mm).

Figure 4. Intensity profile across the focal zone corresponding to the curser in figure 3. For

comparison a scan using a pinhole was made

along the same line. The pinhole scan points are each the average of 5 laser pulses.

218
COMMENTS

Comment: Annealing glass certainly can remove color center damage; however,

some studies we've done previously have shown that residual

structural damage still persists in the glass. For example, in

the case of heavily irradiated yttrium phosphate glasses, we have

done some vibrational spectroscopy and have shown that, even after

annealing, changes in the chemical bond angle between the atoms assumes a wider distribution then in the undamaged glass. I suspect the same may be occurring in your fluoride glass. We are not talking about the mechanism, but it could be also due to

impurities. Remember this is a fluoride glass and it's a biconic

glass and the fluorine is immobile, so that we can have color

centered fluorine. One of the things that we would like to do is

to take the samples and reradiate them again and measure and see

if there is permanent damage that happens to them. We are interested in looking at mechanisms.

Questions: What was the energy of your gamma?

Answer: One megahertz in cobalt 60.

Question: Do you think that the energy of the gammas has the effect of just

producing electron hole pairs or is it doing knock on type of

damage in your material?

Answer: I am not sure. We did not take extra precautions to minimize the

impurities like earth and iron in these glasses, so we are not

sure about the source of color centers.

Question: Can you give us some idea about the optical quality of your glass?

I notice that you are not stirring your melt or are you liable to

have striae inhomogeneities , seeds and other things?

Answer: We supplied large samples of these for laser testing and we did

take an extra precaution for stones and things. This fluoride

glass is very fluid. In this experiment we cast them but in

general we don't cast them, we leave them in the crystal to be

annealed inside the crystal. Stones are not common in fluoride

glasses

.

219
Manuscript received

3-6-90

Laser-Induced Failure in Biased Silicon

Avalanche Photodiodes

Steve E. Watkins*, Chen-Zhi Zhang, Rodger M. Walser, and

Michael F. Becker

Center for Materials Science and Engineering and

Department of Electrical and Computer Engineering

The University of Texas at Austin

Austin, Texas 78712

The characteristics of laser-induced electrical failure in biased silicon avalanche photodiodes

have been observed. The samples were RCA reach-through avalanche photodiodes with

antireflection coatings. They were biased at typical operating voltages during irradiation. The laser source was a Q-switched 1064 nm Nd:YAG pulsed laser operating at 10 Hz with a 10 ns pulse

length and with a 300 |im spot radius.

The current-voltage characteristics were monitored for permanent change as a function of laser

fluence and the degradation thresholds were found. Two types of change were observed. The first type was a large increase in bulk leakage current. It may be modeled by the introduction of defects

into the depletion region by deep melting transients. The second type was catastrophic failure in

which the devices were electrically shorted after irradiation. It may be modeled by excessive current density in the photodiode junction. The type of failure was determined by the parameters of the

biasing circuit.

Key words: avalanche photodiodes; electrical degradation; laser damage; and silicon photodiodes.

1. Introduction

Laser damage to photodetectors is an important concern. Photodetectors are particularly susceptible to damage

as compared to other optical system elements since they are highly absorbing at their operating wavelengths and are often subjected to focused irradiation. Laser-induced phase changes have been modeled thermally for PbS, PbSe,

and HgCdTe infrared detectors [1,2]. Laser damage in silicon solar cells has caused electrical degradation without

the observation of a surface phase transformation [3]. Laser-induced changes in current characteristics, breakdown

voltage, light spot profiles, and noise characteristics have been observed in silicon photo-transistors, p+ n photodiodes, and avalanche photodiodes [4, 5, 6]. Fluence thresholds for microscopic damage and loss of

responsivity as a function of irradiation pulse length have been reported [7]. In our previous work, laser-induced

electrical degradation of silicon high performance PIN and avalanche photodiodes was characterized for nanosecond

optical pulses [8, 9, 10].

The author's address is currently the Department of Electrical Engineering, University of Missouri- Rolla, Rolla,

Missouri 65401.

220
In this paper, we examine the electrical degradation mechanisms in silicon avalanche photodiodes for irradiation

by nanosecond pulses. The photodiodes were irradiated while under typical bias voltages by a near band gap, laser source in order to approximate typical operating conditions. Two types of electrical failure were observed. The

first type was a large increase in bulk leakage current. The second type was catastrophic failure in which the

detectors were electrically shorted after irradiation. Models which include the* effects of biasing are proposed for

the degradation.

2. Experimental Procedures

The experimental configuration for irradiating the samples is shown in figure 1. The laser source was a 1064

nm, stable resonator Q-switched Nd:YAG laser operating at a repetition rate of 10 Hz and with a pulse length of 10

ns (FWHM). The incident energy on the samples was controlled by an attenuator consisting of a rotating half-wave

plate followed by a fixed, thin-film polarizer and monitored by a reference energy meter. The standard deviation of

the pulse-to-pulse energies was two percent or less.

The spatial profile of the Gaussian beam was measured by the scanning slit method and the FWHM was

determined. The l/e^ beam-spot radius w0 was obtained using the equation 2wo = FWHM (^2/2)1/2. a 76 cm

focusing lens was used to achieve spot radii on the samples of about 300 (im. The peak-on-axis irradiation

fluences were calculated using F = 2E/tcw0

2, where E is the pulse energy. The reference energy meter was

calibrated and the spot size was determined during each test session.

In the damage testing process, the samples were positioned using a HeNe alignment laser and a 20X alignment

microscope. The computer recorded the energy of each incident pulse, controlled the shutter which blocked the

beam after the desired number of pulses, and calculated the peak-on-axis fluence. Detailed morphological

examinations and micro-photography were done subsequently with a 50-500X optical Nomarski microscope.

Morphological damage was defined as any visual change in the surface as observed under the Nomarski

microscope. Reverse saturation current and breakdown voltage were the electrical parameters monitored for

permanent change. A Keithley Model 617 Electrometer, a Tektronix Model 577 Curve Tracer, a Fluke Model 77

Multimeter, and a custom current measurement circuit were used to make the measurements.

During irradiation, the avalanche photodiodes were biased at a typical operating point of about fifty volts below

the breakdown voltage. This bias voltage gives a multiplication gain of about one hundred. The biasing circuits are shown in figure 2. For both circuits, the photodiode is in series with a 50 Q load resistance. Circuit # 1 has a 0.01 |iF capacitor in parallel with the photodiode and load resistance series combination; circuit # 2 has a 1010 pF

capacitance in parallel with the photodiode and load. ' Also, stray inductance was minimized in circuit # 2 by using

stripline construction and low inductance components.

The test samples were supplied by RCA and were silicon avalanche photodiodes of type C30817. The device

cross section is shown in figure 3. The detectors had the standard antireflection coating for 1.06 |im. Their active area was 0.5 mm^.

3. Electrical Degradation and Morphological Damage

The avalanche photodiodes were irradiated over a range of laser fluences at single and multiple pulses per

device while under typical bias voltage. The reverse-biased current-voltage characteristics were measured before

and after irradiation. One set of detectors used circuit # 1 in figure 2 and was biased at 336 V which is approximately 50 V below the breakdown voltage. The second set of detectors was biased using circuit # 2 in figure 2. The bias voltage was set individually to be 50 V below the breakdown voltages for each photodiode.

The irradiation fluences and leakage currents for the sample set using bias circuit # 1 are shown in table 1

.

Devices A, B, C, and D were irradiated with a single pulse per device, i.e. 1-on-l tests, at 2.1 J/cm^, 5.7 J/cm^,

6.5 J/cm^, and 9.7 J/cm^, respectively. The current-voltage characteristics are shown in figure 4. The predamage

characteristic, as shown by the initial curve, has an almost constant leakage current at bias voltages below the

breakdown voltage. The current characteristics were not significantly changed for devices A and B, but were

greatly changed for devices C and D. The leakage current at the operating voltage increased by almost three orders

of magnitude for device C and by more than five orders of magnitude for device D (table 1). The threshold for

electrical change is then between 5.7 J/cm^ and 6.5 J/cm^. Electrical degradation of identical avalanche

photodiodes which were unbiased during irradiation occurred at similar fluences, i.e. greater than 6.5 J/cm^, but the

221
change in the leakage current was much less severe [8,9]. The characteristic for unbiased photodiodes irradiated

between 6.5 J/cm2 and 9.5 J/cm2 is also included in figure 4.

The damage morphologies of devices A, B, C, and D are shown in the micrographs of figure 5. At these

fluences, the antireflection coating has been shattered in the irradiated area. The damage spots show melting and

boiling surface morphologies which are identical to those obtained for unbiased photodiodes. Note that the surface

may have severe physical damage (see fig. 5.a. and b.) without any associated change in the current characteristic.

Table 1. Electrical Degradation Test Results for Bias Circuit # 1

Device 1-on-l Irradiation Leakage Current at 336 V

Designation Fluence Predamage Post da mage

A 2.1 J/cm2 77 nA 77 nA

B 5.7 J/cm2 77 nA 80 nA

C 6.5 J/cm2 77 nA 5.5 ^A

D 9.7 J/cm2 72 nA >20nA

The fluence thresholds and damage characteristics for the sample set using bias circuit # 2 are shown in table 2. This set contained twenty photodiodes which were subjected to tests of one pulse per device and thirty thousand

pulses per device. All devices irradiated below 0.4 J/cm2 were not morphologically damaged or electrically

degraded. All detectors irradiated above 0.5 J/cm2 were electrically shorted, i.e. the reverse-biased photodiodes

passed a current on the order of milliamperes with a very small voltage drop. Some of the photodiodes, but not all, irradiated between 0.4 J/cm2 and 0.5 J/cm2 were degraded. All photodiodes were either electrically unchanged or

shorted. For the single-pulse tests, no morphological damage was observed in the irradiated area below 1.0 J/cm2. However, a melting morphology occurred at the edge of the active area for every shorted detector and only for

shorted detectors. This morphology is shown in figure 6.a. At higher fluences, the damage morphologies also

included the typical melting and boiling spot, a separation of the antireflection coating from the surface in a

dentritical pattern, and a melt channel (see fig. 6.b.). For the tests of thirty thousand pulses per device, pitting was

observed without any electrical degradation below 0.4 J/cm2. Edge melting morphologies, the typical center melt

spot, and a melt channel were observed above this degradation threshold as shown in figure 6.c.

Table 2. Electrical Degradation Test Results for Bias Circuit # 2

N-on-1 Test Fluence Threshold Damage Characteristics

1-on-l ~ 0.4 J/cm2 Electrical Short

Junction Spike

Edge Melt Morphology

30,000-on- 1 ~ 0.4 J/cm2 Electrical Short

Junction Spike

Edge and Surface Morphology

222
Several of the avalanche photodiodes in both sample sets were disassembled, etched to remove the contact

metalizations, and polished to give cross sections. No unusual morphologies were observed on the devices for

which degradation consisted of an increase in leakage current. However, every diode which failed catastrophically

had a deep hole on the back, or junction side (see fig. 6.d.). The cross section of such a photodiode is depicted in

figure 7. The hole was three to five microns in depth and was filled with metal before the etch. This junction

spike was presumably the reason the devices were shorted. Typically, it was directly beneath the melting

morphology at the edge of the front metalization. Although for the higher fluences as in figure 6.b., it was below

the center boiling spot.

4. Discussion

The degradation of electrical performance in avalanche photodiodes was clearly a function of biasing voltage

and biasing circuit parameters. The degradation was more severe for detectors which were biased during irradiation

than for unbiased detectors. All photodiodes in which electrical change was observed were rendered useless by the

degree of the degradation. The type of electrical change was dependent upon the circuit parameters. The devices

suffered either degraded current characteristics or catastrophic failure.

The former type of degradation was an increase in the leakage current at high bias voltages. The data suggests

that this increase can be attributed to the introduction of deep level defects in the depletion region of the photodiodes.

Rapid regrowth of the crystal during deep melting transients leaves defects in this electrically sensitive region and

thereby increases the bulk leakage current. Morphological change is initially confined to the thin electricallyinactive surface layer by nonlinear limiting.

The fluence threshold for current degradation and the damage behavior of the biased detectors were roughly

equivalent to those of unbiased detectors. The proposed model was originally developed for unbiased photodiodes

[8]. The much lower morphological damage threshold and the extensive surface melting at the degradation

threshold indicate that deep laser-induced melting is required to cause this type of change in the electrical

performance. The degraded current characteristics (see figure 4) are indicative of an increase in bulk leakage

current due to defects in the p- depletion region of figure 3. [9, 10]. At bias voltages below about 170 V, the p- region is not depleted near the surface and defects located there will not contribute to the leakage current [11].

Hence, the current characteristics are unchanged at low bias voltages and degraded at high bias voltages. The

behavior of the biased photodiodes only differed from that of the unbiased photodiodes in the much larger current

increase induced at a given fluence above threshold. The responsible mechanism is not clear.

The other type of degradation was catastrophic failure in which the photodiodes were electrically shorted after

irradiation. This type of failure was not observed for unbiased avalanche photodiodes or for PIN photodiodes [9].

It only occurred for biased avalanche photodiodes using circuit # 2. This failure may be modeled by excessive

current densities in the photodiode junction. Crystal defects and nonuniformities in the diode structure can lead to localized concentrations of current, i.e. current filaments. For a large laser-induced current transient, excessive

current density may result and cause localized heating. The local temperature may enter the intrinsic range of the

doped silicon in which the carrier concentration depends on the temperature and not the doping density. Then,

additional carriers are generated by the temperature increase and the current density is increased further. The higher

current density gives more local heating which generates more carriers. Positive thermal feedback is thereby

established and the devices eventually fails [12]. This proposed mechanism is similar to the failure mechanisms for

transistors under some conditions and for avalanche photodiodes which are biased above their breakdown voltage

[12, 13, 14].

The metal spike from the back contact metalization was apparently the immediate cause of failure. The spike

probably formed when local heating of a current filament melted the silicon and the adjacent contact metalization.

The junction was thereby shorted. The melt morphology at the edge of the front contact metalization was probably

caused by the large steady-state current occurring after the device shorted. The separation of the antireflection

coating and the melt channel only occurred for high fluences. It is unclear whether these morphologies are directly

related to the failure mechanism or are incidental effects. The junction spike was always located opposite the edge

of the front surface metalization for fluences near the threshold. The physical origin of this behavior is unclear. At

high fluences, the spike was beneath the irradiated area and may have been photo-initiated.

Only biasing circuit # 2, which had the fastest rise time due to its stripline design and low-inductance

components, allowed catastrophic failure. Because of its slower response, circuit # 1 presumably prevented the

peak of the current transient from reaching the excessive levels necessary for catastrophic failure.

223
5. Summary

The electrical degradation of biased avalanche photodiodes is more severe than for unbiased photodiodes. The

biased devices may fail from laser-induced degradation of bulk leakage current characteristics or from laser-induced

shorting of the device, i.e. catastrophic failure. The type of failure appears to be determined by the rise time of the

biasing circuit. The fluence threshold for degraded current -characteristics was about 6 J/cm^ which is approximately equal to the threshold for similar degraded current characteristics in unbiased detectors. This current increase may be modeled by the introduction of defects in the depletion region during melting transients. The

fluence for catastrophic failure was about 0.4 J/cm^ which is slightly less than the threshold for single-pulse surface

morphological damage of 1 .0 J/cm^. This failure may be modeled by excessive current density transients which

induce junction spikes.

6. References

[I] M. Kruer, L. Esterowitz, F. Bartoli, and R. Allen, "Thermal Analysis of Laser Damage in Thin-Film

Photoconductors," J. Appl. Phys. 42, 2867 (1976).

[2] F. Bartoli, L. Esterowitz, R. Allen, and M. Kruer, "A Generalized Thermal Model for Laser Damage in

Infrared Detectors," J. Appl. Phys. 47_, 2875 (1976).

[3] Y. Matsuoka and A. Usami, "Normal Laser Damage of Silicon Solar Cells without Phase Change," Appl.

Phys. Lett. 25_, 574 (1974).

[4] J.F. Giuliani and C.L. Marquardt, "Electrical Effects in Laser-Damaged Phototransistors," J. Appl. Phys.

45, 4993 (1974).

[5] D.L. Parker, F. Lin, S. Zhu, D. Zhang, and W.A. Porter, "Selective Lifetime Doping in Silicon by Laser

Scanning," IEEE Trans. Electron Devices 29_, 1718 (1982).

[6] M.A. Acharekar, "Laser Damage in Silicon Avalanche Photodiode," Laser Induced Damage in Optical

Materials: 1987, Nat. Inst, of Standards and Technology (US) Spec. Publ. 756, 50, (1988).

[7] M. Kruer, R. Allen, L. Esterowitz, and F. Bartoli, "Laser Damage in Silicon Photodiodes," Optical and

Quantum Electronics 8_, 453 (1976).

[8] S.E. Watkins, C. Zhang, R.M. Walser, and M.F. Becker, "Laser-Induced Electrical Parameter Degradation

in Silicon Photodiodes," 20th Symposium on Optical Materials for High Power Lasers, Boulder, Colorado,

25-28 October 1988 (Proceedings to be published by NIST).

[9] S.E. Watkins, C. Zhang, R.M. Walser, and M.F. Becker, "Electrical Performance of Laser Damaged

Silicon Photodiodes," Appl. Opt. 29(6), 827 (1990).

[10] S.E. Watkins, "Electrical Parameter Degradation of Silicon Photodiodes Induced by Nanosecond 1.06 urn Laser Pulses," Ph.D. Dissertation, The University of Texas at Austin, 1989.

[II] P.P. Webb, RJ. Mclntyre, and J. Conradi, "Properties of Avalanche Photodiodes," RCA Review 3_5_, 234

(1974).

[12] A. Blicher, Field-Effect and Bipolar Power Transistor Physics, Academic Press, New York, 186-193

(1981).

[13] R.A. Sunshine and M.A. Lampert, "Second-Breakdown Phenomena in Avalanching Silicon-on-Sapphire

Diodes," IEEE Trans. Electron. Devices ED- 19 (7), 873 ( 1972).

[14] M.W. Muller, "Current Filaments in Avalanching PIN Diodes," Appl. Phys. Lett. 12 (6), 218 (1968).

224
Variable

Attenuator

Energy

Probe &

Meter

PR

Shutter Lens (f = 76 cm)

=0=

Sample Stage

& Controller

Spot Radius ~ 300 microns

I

Microcomputer

Trigger

Photodiode

PR

Q-switched NdrYAG Laser

(1064 nm, 10 nsec pulse)

Figure 1. The experimental configuration for irradiating the photodiodes. During

the tests, the devices were biased. M and PR indicate mirrors and partial reflectors,

respectively.

100 kQ

Power =

Supply o.Ol jiF

-=2Z

-O—OPhotodiode

50 Q —o

a. Biasing Circuit # 1

10 —o—oPhotodiode

50 a

o

Power =

Supply 1010 pF

Z2Z

b. Biasing Circuit #2

Figure 2. Biasing circuit # 1 (diagram a.) and biasing circuit # 2 (diagram b.) for the

avalanche photodiodes. Biasing circuit # 2 was of stripline design.

225
AR SiO Coating

Metal Contact

Light

1 micron p+

100 microns pSi

Back Metal Contact

Figure 3. Cross section of the RCA avalanche photodiode type C30817 with reachthrough structure. The devices had antireflection coatings.

Reverse-Biased Current-Voltage Characteristics

of RCA Avalanche Photodiodes

<

c

500

c

9 400

300 -

3

o

c

g

(0

B 200 1

(0

C/)o

tn

>_o

>

a> DC

100 -

Device D

Device C

Unbiased

Devices

-Initial

100 200

Voltage (V)

300 400 500

Figure 4. Reverse-biased current-voltage characteristics of irradiated photodiodes.

The initial curve is the characteristic before irradiation and degradation. Devices A,

B, C, and D were irradiated while biased using circuit #1. The characteristics of

devices A and B were unchanged, i.e. the initial curve. The postdamage

characteristics of devices C and D are shown. The typical postdamage characteristic

of unbiased devices irradiated between 6.5 and 9.5 J/cm2 is also shown.

226
Figure 5.

227
22S
Figure 5. Damage morphologies of RCA avalanche photodiodes which show deep

melting. Biasing circuit #1 was used during irradiation. The current characteristics

were not changed for devices A and B (a. and b.). For device C (c), the leakage

current at 336 Volts increased from 77 nA to 5.5 (J.A. For device D (d.), the

leakage current increased from 72 nA to over 20 mA. Nomarski micrographs,

a. Device A 1-shot at 2.1 J/cm2 b. Device B 1-shot at 5.7 J/cm2

c. Device C 1-shot at 6.4 J/cm2 d. Device D 1-shot at 9.7 J/cm2

Figure 6. Damage morphologies of RCA avalanche photodiodes which failed

catastrophically. Biasing circuit #2 was used during irradiation. Micrographs b.,

c, and d. have the same scale. Nomarski micrographs.

a. Edge morphology for 1-shot at 0.4 J/cm2 b. 1-shot at 3.9 J/cm2

c. 3000-shots at 0.4 J/cm2 d. Back side of shorted device

Volume Absorbing Li 9

nt Pulse

Back Metal Contact

Figure 7. Cross section of a shorted avalanche photodiode. This catastrophic failure

only occurred for devices biased using circuit # 2. The morphologies resulting from

near-threshold fluences are the junction spike from the back contact metalization and

the melt morphology at the edge of the active area. Neither of these morphologies

are located in the volume which absorbs the optical energy.

229
Manuscript received

1-29-90

The Effect of Subsurface Defects on "Incipient" (Below Threshold)

Laser Damage Nucleation In Fused Silica Optical Flats

T . J . Magee and C . S . Leung

XMR Inc.

Santa Clara, California 95054

F. D. Orazio, Jr.

VTI Inc.

Dayton, Ohio 45431

J. D. Boyer, B. R. Mauro, and V. E. Sanders

Los Alamos National Laboratory

Los Alamos, New Mexico 87545

The development of subsurface defects in optical components

has been extensively investigated by the authors and others over

the past few years. Investigation of optical components using

nondestructive photon backscattering (PBSTM)

, delineation etch/

optical microscopy and charge decoration/delineation techniques

has shown laterally variable spatial distributions of subsurface

defects in the form of lineated remnant polishing traces, voids,

and microscopic impurity clusters introduced during the surface

finishing process. In this paper, we report on the correlation

between nondestructive PBSTM mapping and delineation etch/optical

microscopy techniques for detection of subsurface defects, as

well as the effect of subsurface defects on the initiation of

microscopic or "incipient" laser damage ( \ = 248 nm) at levels

below the point of macroscopic or catastrophic failure observed

at the surface. The detection of laser induced microscopic

cracking on nucleation sites within nondestructively identified

areas provides additional insight into the progressive development of failure sites in distributed subsurface regions of low

RMS, optically finished surfaces.

Key Words: fused silica; incipient laser damage nucleation;

microscopic impurity clusters; subsurface defects

1. Introduction

Damage to optical materials under intense photon irradiation has always

been a major problem [1]. The proceedings of the annual Boulder Damage

Symposium document 2 0 years of progress in resolving various key fundamental

issues in understanding the laser induced damage mechanism [2]. Although

tremendous research effort has been made to understand the laser damage

mechanism in optical materials, defining the state-of-the-art laser

resistant material is often difficult. It has been shown that it is

difficult to conduct laser damage tests and to predict laser induced damage

thresholds for the same materials from sample to sample [3,4]. In addition,

the "apparent" laser damage threshold may vary spatially across the surface.

The occurrence of incipient damage beneath the surface at laser energy

levels far below the observable macroscopic "damage" point has been

identified recently by the authors for various optical materials such as

fused silica, alexandrite rod, and potassium titanyl phosphate (KTP)

crystals. Although the role of cracks, pores, and absorbing inclusions on

laser induced damage thresholds has been discussed previously [5], it should

be noted that the highly localized subsurface damage regions present in a well-polished optical surface has a more dramatic effect on avalanche

breakdown by the high power laser pulse. The subsurface defect commonly

found in the otherwise optically smooth surface of optical materials is

230
introduced by conventional lapping/polishing processes through propagation

of the damage into near-surface regions of a super smooth surface depending

on the fabrication schemes as well as the characteristics of the material

[6] .

One of the major problems in predicting laser damage thresholds is the

lack of nondestructive techniques to characterize the test surface

extensively prior to the damage test procedure as well as the criteria of

laser-induced damage threshold that are based on phenomenological observations and statistical data at the test sites. Nondestructive mapping

techniques are needed for measuring spatial surface quality and subsurface

defects on the test parts, otherwise anomalous effects are seen in

determining surface damage threshold of optical materials that leads to

erroneous assumptions in laser damage mechanism modeling.

The objective of this paper is to report on the correlation between

nondestructive photon backscattering mapping and selective etch delineation

techniques for detection of subsurface defects, as well as the effect of

subsurface defects on the initiation of microscopic or "incipient" laser

induced damage in fused silica. Although the relation of subsurface

structure in fused silica and laser damage threshold has been reported

previously [7,8,9], with some form of controlled grinding/polishing

procedures, the lack of surface and subsurface defects characterization

techniques on these test parts makes predicting damage thresholds erratic.

Conventional grinding/polishing procedures can produce severe subsurface

damage, impurity clusters, as well as other subsurface imperfections in the

near-surface region of the processed parts. Nondestructive techniques are

needed to identify the nature of subsurface defect structure that has eluded

investigators for a long time. Recently, a nondestructive photon

backscattering technique has been develop to map subsurface defects found in

optical blanks such as fused silica and sapphire [10]. Spatially varying

subsurface damage from a super-polished surface has densities far below that

of a standard polish. Using a selective delineation etch, all the UV laser

exposure spots (248 nm) on a standard polished surface can be revealed at

energy f luences ranging from 4.7 - 9.5 J/ cm2 and the catastrophic damage can

be found to be associated with the high scatter regions of the PBS map.

2. Results and Discussion

The fused silica optical flat used in the nondestructive testing photon

backscattering (NDT-PBSTM) mapping was a one-inch diameter flat with a super

polish on one side and a standard polish on the other. The fused silica

optical flat was subjected to NDT-PBSTM measurements on both sides. The

PBSTM instrumentation used is a very sensitive patented technique that

measures scattering of HeNe laser light as a function of angle and produces

a plot of scattering level as a function of position on the optical flat.

The PBS maps of the super polished and standard polished surface of the

fused silica optical flat are shown in figures 1 and 2. In the super

polished surface of the optical flat, very few features can be found in the

PBSTM map with a low average scatter level (.0252). In comparison, in the

standard polished surface of the optical flat, the lineated features of

high-scatter intensity are distributed across the test area with a high

average scatter level (1.0221). These features are caused by the buried

spatially varied subsurface damage produced by the conventional polishing

process. The subsurface defects have very distinct characteristics,

dependent upon material properties and the specific fabrication procedures

used to produce the ultra smooth surface.

Following the NDT-PBSTM mapping, the fused silica flat was subjected to

laser damage tests at Los Alamos National Laboratory. Damage threshold

measurements after exposure to 23 ns, 248 nm laser pulses were made on both

sides of the one-inch diameter fused silica blank. The super-polished side

was tested with a laser energy f luences ranging from 6.4 - 16.2 J/cm2 while

the fluence on the standard finished side ranged from 4.7 - 12.0 J/cm2

. Each site was exposed to 100 shots at a specific fluence using a laser pulse

repetition rate of 50 Hz. The spot size was approximately 0.20 x 0.75 mm

231
for all sites. Damage was detected by visual examination of the test sites

using a microscope equipped with white-light, bright-field illumination at

100X. Using the observable surface change as a criteria for laser damage,

the damage thresholds were 8.0 and 6.3 J/ cm2 for the super-polished and

standard finished sides, respectively.

After the laser damage threshold measurements, the fused silica optical

flat was subjected to a sensitive etch delineation procedure. In this etch

procedure for fused silica, approximately 2 urn of material was removed at a faster rate in the matrix material having a high density of defects.

Nomarski micrographs showing the representative etch delineated subsurface

defect features found on the super polished and standard polished surface

are shown in figure 3. As shown in figure 3, it can be seen that the

density of subsurface defects (voids and scratches) on the standard polished

surface is higher than the super polished surface.

Of particular interest in the sensitive etch delineation procedure is

the delineation of all laser damage sites on the standard polished surface

with energy fluences exposure as low as 4.7 J/cm2

. On the contrary, except

for the catastrophic damage sites, no etch delineated test sites can be seen

on the super-polished surface even with higher energy fluence exposures.

Upon careful examination of the etch delineated laser damage test sites on

the standard polished surface, it is apparent that incipient laser damage

has been induced in the subsurface defect regions on the standard polished

surface. The "incipient" laser damage induced in the subsurface regions at

various laser fluences after exposure to selective etch is shown in figure

4. Delineated laser induced subsurface defects are seen at all laser

fluences on the standard polished surface.

These results clearly demonstrate the occurrence of "incipient" damage

beneath the surface at energy levels far below the macroscopic/catastrophic

failure point. In this respect, both the definition of "laser damage"

threshold and the understanding of damage' nucleation at subsurface sites

will require serious reconsideration. The approach to predict laser damage

threshold by considering statistical data and phenomenological observations

is limited by the lack of availability of controlled experimental data such

as sensitive nondestructive test techniques on the surface quality of the

test samples. Most laser damage tests have been performed without any

extensive characterization of the spatially varying subsurface defects.

The influence of subsurface defects on laser damage is illustrated in

figure 5 for fused silica at a specific site. In this case, the laser

damage initiated at a site containing a relatively high density of absorbing

macroscopic inclusions and subsurface damage, resulting in conchoidal

fracture generated by the thermal gradients within the matrix material

Depending on the characteristic of these absorbing inclusions and subsurface

damage, the laser damage threshold can change drastically since the initial

interaction of laser pulses with these defects will have a direct impact on

various material properties upon exposure to subsequent laser pulses.

3 . References

[1] Guenther, A.H. A chronology of laser damage research. Lasers &

Applications. 3(5): 87-92; 1984.

[2] Proceedings of the Annual Symposium on Optical Materials for High Power

Lasers (Boulder Damage Symposium) : available at National Bureau of

Standards Special Application.

[3] Seitel, S.C. Planning a laser damage test. Photonics Spectra. 19(8):

84-92; 1985.

[4] Soileau, M.J. Laser induced damage. Photonics Spectra. 21 (11) : 109-114

1987.

232
[5] Bloemberger, N. Role of cracks, pores, and absorbing inclusions on

laser induced damage threshold at surfaces of transparent dielectrics.

Applied Optics. 12 (4 ): 661-664

.

[6] Magee, T.J. ; Leung, C.S. Overview of subsurface damage in brittle

materials - evolution of concepts and current status. Park, R.E., ed.

Proceedings of ASPE spring conference on subsurface damage in glass.

1989 April 25-27; Tucson, AZ . 15-55.

[7] House II, R.A. ; Bettis, J.R. ; Guenther, A.H. Subsurface structure and

laser damage threshold. IEEE J. of Quant. Elect., EQ-12, No. 5:363-364.

[8] Temple, P. A.; Lowdermilk, W.H. ; Milam, D. Carbon dioxide laser

polishing of fused silica surfaces for increased laser-damage

resistance at 1064 nm. Applied Optics 21 (18) : 3249-3255 (1982).

[9] House, R.A. ; Bettis, J.R. ; Guenther, A.H. Preparation techniques and

hydroxyl concentration vs. surface damage threshold. NBS Special

Publication 462, Laser induced damage in optical materials, p. 310

(1976)

.

[10] Magee, T.J. ; Leung, C.S.; Orazio Jr., F. Nondestructive and destructive

analysis of spatially distributed subsurface damage in quartz and

sapphire optical flats. Parks, R.E. ed. Proceedings of ASPE Spring

Conference on subsurface damage in glass; 1989 April 25-27; Tucson, AZ

147-173.

233
6739

Y-AXI5

5739

2.0330

( PPM-'SR >

PBSTM nap

NAME: XD1

I FIRST RUN'5CPlM:

3' 1 LAST RUN'SCAN:

3' HQ 1 RUN'SCAN 'IHCR: U 1

AUERRGE: 0.B5SE

5718 X-ftXIS 6718

Figure 1. Photon Backscattering (PBSTM) map of super-polished fused silica

optical flat (Original map is on multicolor plot)

6748

Y-AXIS

s?ie

i . 0633

(PPM-'SRj

FBS T " M0p

NAME: X01 FIRST RUH'SCAH:

B' 1

'

LAST RUN'SCAN: y sal RUH'SCAN IHCR: V 1

X-AXIS

1 .Girl

5.USRB

Figure 2. Photon Backscattering (PBSTM) map of standard polished fused

silica optical flat (Original map is on multicolor plot)

234
(a) Super Polish

Subsurface

damage

(b) Standard Polish

Voids Subsurface

damage

5 ofim

Figure 3. Nomarski micrographs showing representative subsurface defect on

fused silica surface after exposure to selective delineation etch

(a) super polish, (b) standard polish

235
5.4 5 J/ cm2

4.71 J/ cm2

Laser exposure

areas

20011m

Figure 4. Nomarski micrographs showing "incipient" laser damage in subsurface regions (etch delineated after laser exposure) at various

laser fluences for standard polish surface

236
10.08 J/ cm'

Absorbing

macro-inclusion

Conchoidal

fracture generated

by thermal

gradients

5 o fim

5.4 5 J/cm2

Incipient laser

damage in regions

of microscopic

impurity clusters

on subsurface

damage

so fi m

Figure 5. Nomarski micrographs showing "incipient" damage initiated at (a)

macroscopic absorbing inclusion sites and (b) lineated subsurface

damage sites

237
COMMENTS

Question: If I understand correctly, your characterization of the films does

not allow for a variation in crystallite size as the film grows or

as you get additional films. Now in fact for many films you do

get an increase in structure as you go to thicker films. Doesn't

that require that one extend this theory to consider that effect?

Answer: We do not detect interference from variations in microstructure

It Is due to the range of measurable special frequencies. Even if

you measure at 80°, the peak defects are ten times higher than the

thickness of the layer. Maybe, if we used longer wavelengths it

would be possible to detect possible correlation or interference.

238
Manuscript received

12-29-89

Quantitative Analysis of Surface Trace Metal Contamination

on Substrates and Films by TXRF

R. S. Hockett

Charles Evans & Associates

301 Chesapeake Drive

Redwood City, CA 94063

The damage to optics from high power laser radiation depends in part

upon the surface films on the system optics. Approaches to hardening

these surfaces to these types of radiation have focused on layered film

compositions and upon the reduction of structural defects in the films

and the substrates. The importance of heavy metal contamination at

trace levels (less than one thousandths of a monolayer) in the top few

monolayers of the surfaces, or at the film interfaces, has been

minimized because of insufficient analytical technology to detect these

impurities. These localized heavy metals may absorb radiation, ionize

to high positive states, and become the source of defects leading to

damage. This paper will describe a new analytical technique, Total

Reflection X-Ray Fluorescence (TXRF) , which is capable of quantitatively

detecting heavy metals (Z>11) on the surface (top few nm's) of the

substrates or films, with detection limits down to 10 11 atoms/cm2 (in

most cases several orders of magnitude better than ESCA or AES , and

quantitative, in contrast to SIMS.)

Key Words: Contamination; Damage; Defects; Films; Metals; Surfaces;

Total Reflection X-Ray Fluorescence

1. Introduction

The damage to optics from high power laser radiation depends in part

upon the surface films on the system optics. Approaches to hardening these

surfaces to these types of radiation have focused on layered film compositions

and upon the reduction of structural defects in the films and the substrates

[1] . The importance of heavy metal contamination at trace levels (less than

one thousandths of a monolayer) in the top few monolayers of the films, or at

the film interfaces, has been suspected by not studied because of insufficient

analytical technology to detect these impurities. These localized heavy

metals may absorb radiation, ionize to high positive states, and become the source of defects leading to damage. These heavy metals may even be part of

the explanation for the major discrepancy between bulk thermal conductivities

and theoretical damage thresholds in thin film coatings [2].

The effect of bulk trace impurities in some films has been studied [3-

5], as well as the effect of surface cleaning [6], but the effect of surface

(top few monolayers) impurities, at levels below the detection capability of

Auger Electron Spectroscopy (AES) or Electron Spectroscopy for Chemical

Analysis (ESCA), upon the damage threshold has not been studied. The reason

is until recently there has been no analytical instrument capable of

auanti tatWely detecting these highly localized (in depth) impurities. The

theory of small metallic inclusions (1-20 nm diameter) in the bulk of the

films has also been addressed [7], but the effect of aggregated surface

impurities in the top few monolayers has not.

The difficulty in quantitatively detecting trace metals localized in the

surface (top few monolayers) has been great. AES and ESCA, which can provide

semi-quantitative measurements, are sensitive to these depths, but only have

detection limits on the order of 1 to 0.1 atomic % at best. This is about

10 to 10 13 atoms/cm2

, or roughly one tenth to one hundreth of a monolayer,

respectively. Other analytical techniques, such as laser ionization mass

239
spectrometry , electron probe, secondary electron microscopy equipped with

energy dispersive x-ray analysis, and Rutherford backscattering spectrometry

do not have the depth resolution. Secondary ion mass spectrometry has the

depth resolution, but the non-linear ion yields from the sputtering process in

the first few monolayers makes quantification not possible. The same problem

(i.e., the difficulty in quantitatively measuring the trace impurities in the

top few monolayers) has also existed in the semiconductor industry for

processes which polish and clean substrates and thin films.

2. Total Reflection X-Ray Fluorescence

Recently in 1987 a new surface sensitive analytical technique was developed [8] to meet this need in the semiconductor industry. The technique

called Total Reflection X-Ray Fluorescence (TXRF) has detection limits, when

using a Molybdenum anode, on the order of 10 1 atoms/cm2 for heavy metals on silicon and SiO surfaces, as shown in Figure 1. This is about two orders of

magnitude more sensitive than AES or ESCA for the heavy metals.

In the TXRF technique x-rays from a conventional x-ray anode strike the

substrate surface at grazing incidence and totally reflect from the substrate

surface. The atoms in the top few monolayers emit fluorescence x-rays

depending upon the incident x-ray energy and the energy levels of the surface

atoms. The fluorescence radiation is detected then by an energy dispersive x- ray spectrometer, a conventional Li-drifted silicon detector which is cooled

by liquid nitrogen.

The TXRF technique has been shown to detect heavy metals even on ultraclean commercial quality silicon substrates [8,9] and has been applied to

cleaning process development of silicon substrates by integrated circuit (IC)

manufacturers [10,11]. Figure 2 shows a TXRF spectra (detected fluorescence

x-rays versus the fluorescence x-ray energy) for a commercial silicon wafer.

The substrate was not intentionally contaminated with the detected Fe , Cu and

Zn at levels in the mid 10 11 to low 10 12 atoms/cm2 range. These impurities

arise from the cleanrng process, are localized in the top few monolayers of

the substrate, and are not detectable by AES or ESCA. Figure 3 shows a TXRF

spectra of an Si0

2 layer on a silicon substrate where the Si0

2 was cleaned

using semiconductor grade processing equipment; here we see Cr, Cu and Zn at

levels of 1011 atoms/cm2

. Figure 4 shows a TXRF spectra of a commercial ill?GaAs

semiconductor substrate with Fe and Cu surface levels in the 10 to 10

atoms/cm2 range also. The technique is also capable of detecting metal

contamination on diamond films [12] . The TXRF technique is beim rapidly

accepted in the semiconductor and IC industry and is expected to become a standard technique [13] for surface analytical measurements.

2.1 Quantification

Quantification is achieved using a silicon substrate which has been

intentionally contaminated with Ni of a known areal density using a diluted

wet chemistry standard. Once the calibration for Ni is completed, the

calibration for the other metals is completed using sensitivity factors which

were measured once when the instrument was manufactured.

Correlation of TXRF with nitrogen-beam Rutherford backscattering

spectrometry (RBS) and vapor phase decomposition followed by atomic absorption

spectroscopy (VPD/AAS) has validated the quantification approach. The

correlation between TXRF and VPD/AAS is shown in Figure 5 for Fe, Ni, Cu and

Zn on silicon wafer surfaces. The multi-point TXRF measurements were made

using XSA-8000 instruments at Charles Evans & Associates on one set of wafers

and at GeMeTec (Munich, West Germany) on another set of wafers. The

intentionally contaminated wafers and VPD/AAS measurements were provided by

Kyushu Electronic Metals Co. Ltd., Japan. A BLANK was measured at Charles

Evans & Associates for 5.6 hours to check for instrumental background; only Cu

at 7xl0 10 atoms/cm2 was detected. The correlation between TXRF, RBS (nitrogen

beam) , and VPD/AAS for the sum of the metal contamination is shown in Figure

6. The reason for summing the metals is RBS cannot mass resolve these metals,

but detects them as one mass.

240
2.2 Stability

Analytical statistical process control (SPC) is completed using periodic

TXRF measurements on two 100 mm diameter silicon samples. The SPC

measurements are made daily, since a within- day time study revealed the

measurements were stable over a one day time period. The first sample is the

calibration standard which contains Ni and Bi. The Ni is used to derive the

calibration constant which gives the quantification. A drop in this value

outside of SPC may indicate the x-ray tube is losing power. The Bi is

monitored for SPC of the high energy calibration. The second sample is a commercial wafer with Cu, Fe and Zn plated from its original cleaning. (The

TXRF signal versus angle variation reveals the metals are plated.) The Cu is

monitored for SPC of the angle calibration, the Fe for SPC of the detection

limit, and the Zn for SPC of the energy resolution (the ability to separate

the Zn k- alpha line from the Cu k-beta line)

.

Figure 7 shows a plot of the Ni calibration (counts/sec) and Bi

measurement over a 3 month period. Figure 8 shows a plot of the measurement

of Cu, Fe and Zn over the same period of time. The measurements on the Cu, Fe

and Zn contaminated sample were terminated after this period of time when the

Cu, Fe and Zn measurements went out of SPC control, and an angle scan revealed

particulate contamination had been introduced to the sample, i.e., a special

cause could be assigned to the out-of-SPC measurement. The Cu readings in

Figure 8 shows a drop with time that is also out of SPC control, while the Fe

and Zn do not. The drop has a slope of 0.35% per day, and may be detectable

for the Cu because of its high signal level, while the lower levels of Fe and

Zn result in too much variability to identify a statistically valid drop.

This same drop can be created by inserting a similar wafer into the instrument

for a series of 50-90 sequential measurements in one day without removing the

wafer between measurements. A similar experiment was tried where a Co

contaminated 125 mm diameter wafer was sequentially measured for 49 times in one day without removal from the clamp. The Co was localized on the surface

and in the subsurface; in this case there was no drop. Recent radiotracer

studies have shown Cu can be activated at room temperature to diffuse into

crystalline silicon. In any case a drop of only 0.35% per day (or per

measurement) is not a practical problem for the measurement precision itself.

Precisions (one relative standard deviation) depend upon the specific

element and its amount. For the SPC contaminated wafer the precisions are 5%

for Cu at 4xl0 12 atoms/cm2 (linear drop removed); 29% for Fe at 0.5xl0 12 atoms/cm2

, and 18% for Zn at 0.4xl0 12 atoms/cm2

. The precisions for the Ni

counts/sec and Bi reading are 3.8% and 6.4% respectively.

3. Applications

We anticipate that since there is heavy metal contamination at these

levels on commercial silicon, SiO , and GaAs substrates manufactured for the

IC industry, then there will most likely also be heavy metal contamination at

these levels, or higher, on substrates and films used for high power laser

optics . There is the opportunity then to apply this new TXRF technique to

high power laser optical structures in order to reduce the heavy metal

contamination from processing and to show a reduction in optics damage. The

two generic application areas are:

(1) the polishing and cleaning process development for high power laser

optical substrates before film growth, and

(2) the cleaning process development of the film surfaces after film

growth

The approach works for silicon and GaAs substrates and films. However,

development work is required in order to apply TXRF to other critically

important high power laser optical substrates and films. The reason is there

are some differences between the semiconductor applications and the non- silicon optical materials applications, i.e., the matrix signals, the optimum

excitation source, the critical angle, matrix interferences, optimum angle for

signal-to-background, and quantification.

241
Group I - TXRF Immediate Applications

1.1 Silicon

1.2 Fused Silica

LI Silicon Carbide

Group II - TXRF Expected Applications

11. 1 Be on Silicon Carbide

11. 2 Be on Fused Silica

II. 3 Low Z Dielectric Stacks with and without Al Baselayer on ULE, Silicon

Carbide and Silicon.

I I. 4 Low Z Coatings

II. 4 NaCl on Al

Group III - TXRF Anticipated Applications

III. l Heavy Metal Fluorides

111. 2 MgO and AlN coatings on Heavy Metal Fluorides

111. 3 Zinc Selenide

111. 4 Mo and Au films on Al

111. 5 Ag on Silicon

111. 6 Au on Be on Fused Silica

3.1 Development

Group II Samples: Be on Silicon Carbide; Be on Fused Silica; Low Z Dielectric Stacks with and without Al Baselayer on ULE, Silicon Carbide

and Silicon; Low Z Coatings; and NaCl on Al

The Group II samples are different from Group I samples in that

the surfaces are low Z and therefore do not provide matrix signals

with which to experimentally determine the critical angle for

total reflectance. There are two approaches to overcoming this

difficulty

One approach is to calculate the critical angle for the low Z surface ; a table of calculated critical angles for a range of low

Z materials already exists. Then the approximate angle of

incidence can be found using a high Z sample (like a silicon

substrate) which is already well understood. The low Z sample can then be inserted into the TXRF instrument and the angle setting

from the high Z sample calibration will be used. The drawback

from this approach is the repeatability of the measurement will

not be as good as if we could directly measure the critical angle

for the low Z surface

A second approach is to deposit a high Z coating (less than one monolayer) onto the low Z sample surface and use the high Z coating to in situ calibrate the angle settings and then use the

calculated angle setting for the low Z matrix. The drawback here

is that the deposition process may itself add surface

contamination. We have at Charles Evans & Associates an electron

beam heated silicon deposition system by which we can attempt to

add very low levels of silicon to the low Z surface. Previous

studies have indicated that this can be done without adding heavy

242
metal contamination, but the rate control may be difficult for a submono layer deposition.

Group III Samples: Heavy Metal Fluorides; MgO and AlN coatings on Heavy

Metal Fluorides; Zinc Selenide; Mo and Au films on Al ; Ag on Silicon;

and Au on Be on Fused Silica.

The Group III samples are different from the Group II samples in

that they will produce a desirable matrix signal for critical

angle calibration (as with the Group I samples), but the matrix

signal may increase the background levels and introduce

interferences. This in effect is what happens with GaAs

.

Observation of Figure 4 indicates there are major Ga and As

fluorescence peaks in the spectra and these interferences restrict

the heavy metals which can be detected on GaAs . Some of this

problem can be reduced by using different energy x-rays for the

incident x-rays, if the excitation of the matrix can be avoided or dampened.

4. References

[I] Abstracts from the High Power Laser Optical Components Topical Meeting,

sponsored by SDIO, NWC and AFWL, October 24-25, 1988, Boulder, CO.

[2] Arthur H. Guenther, Laser Induced Damage in Optical Materials: 1986 . NIST

Special Publication 752, ASTM STP 1028, (U.S. Department of Commerce/National

Institute of Standards and Technology) , Proceedings of the Symposium, November

3-5, 1986, Boulder, CO (September 1988), p. xiii.

[3] S. C. Seitel and L. G. DeShazer, ibid., p. 65.

[4] Ian Laidler and David C. Emmony, ibid., p. 151.

[5] Larry D. Merkel and Pradip K. Bandyopadhyay , ibid., p. 677.

[6] D. J. Gallant, Marilyn Law and Brad Pond, ibid., p. 159.

[7] W. Becker, J. K. Mclver, and A. H. Guenther, ibid., p. 603.

'"I Peter Eichinger, Heinz J. Rath, and Heinrich Schwenke , Semiconduc tor

i, orication: Technology and Metrology. ASTM STP 990 . Dinesh C. Gupta, edit

/^erican Society for Testing and Materials, 1988.

[9] R. S. Hockett, S. M. Baumann, and E. Schemmel, Diagnostic Techniques for

Semiconductor Materials and Devices . ECS Vol. 88-20, edited by Thomas J. Shaffner and Dieter K. Schroder, published by the Electrochemical Society

(Pennington, NY), p. 113 (1988); "Comparison of wafer cleaning processes using

total reflection x-ray fluorescence (TXRF) , " R. S. Hockett and William Katz

,

J. ECS, November 1989.

[10] "An evaluation of wafer cleaning: suppression of oxidation induced

stacking faults and control of oxide thickness," A. Hariri of Siliconix, Inc.,

and R. S. Hockett, Semiconductor International, August 1989.

[II] V. Penka and W. Hub of Siemens Ag, "Detection of metallic trace

impurities on Si (100) surfaces with total reflection x-ray fluorescence

(TXRF)," Fresenius Z. Anal. Chem. 333, 586 (1989); "Application of total

reflection x-ray fluorescence in semiconductor surface analysis,"

Spectrochimica Acta 44B, 483 (1989).

[12] R. S. Hockett and James Knowles, "Characterization of heavy metal

contamination on diamond films using SIMS and Total Reflection X-Ray

Fluorescence (TXRF) , " to be presented at the Fall Meeting of the Materials

Research Society, Boston, 1989 (abstract F3.2).

[13] "The application of TXRF to semiconductor surface analysis," R. S. Hockett, invited presentation for the ASTM F104 annual workshop, February 1,

1989, San Diego, CA.

243
o

0>

( 0

,(H») Z WO/SWO±V

244
tn 4JC

3

oo

0 Fluorescence X-ray Energy (keV) 20.48

Figure 2. TXRF Spectra for a Commercial Silicon Wafer

03 U

c

O

u

0.000 20.480

Fluorescence X-ray Energy (keV)

Figure 3. TXRF Spectra for an Si0

2

Film on Silicon after Cleaning

245
t

Fe 1.9 X 10 12 atoms/cm2 Cu 0.6 X 10 12 atoms/cm2

\

0.000 20.480

Fluorescence X-ray Energy (keV)

Figure 4. TXRF Spectra for a Commercial GaAs Wafer

246
13.5 -

13 -

" 12.5 -

12

11.5 -

11 -

10.5

10

10

TXRF (CE&A) VERSUS VP.D/AAS

A +

A +

« + A

(a)

11 -1 r

12

-i r

13

VPD/AAS (LOO AT0MS/CM2)

FE + Nl O CU A ZN

TXRF (GEMETEC) VERSUS VPD/AAS

1«

N

2Il/l 2

13.5 -

12.5 -

11.5 -

10.5 -

FZ VPD/AAS (LOO AT0MS/CM2) + Nl * CU A ZN

Fig. 5 Correlation between TXRF and VPD/AAS (Kyushu Electronic Metals Co.,

Japan) for Fe , Ni, Cu and Zn on spin-coated silicon wafers: (a) TXRF and

wafers at Charles Evans & Associates, (b) TXRF and wafers at GeMeTec (Munich,

West Germany) . The spin coating and VPD/AAS were done by Kyushu Electronic

Metals Co., Japan. The TXRF instruments were Perkin-Elmer XSA-8000.

247
13J -

12-5 -

a%

a£

k.O

2

3M

k.O

w 11J -

10i -

TXRF AND RBS VS VPD/AAS

sim or re. nl cu and zn on sbjgon

TXRF CEAA

VPO/AAS (LOO OF SUVI OF AT0US/CU2) + TXRF OQ4ETEC RBS

Fig. 6 Correlation between TXRF (Charles Evans & Associates and GeMeTec)

RBS (GeMeTec), and VPD/AAS (Kyushu Electronic Metals Co.) for summed Fe, Ni,

Cu, and Zn contamination on spin-coated silicon wafers.

2.48
Q

Z

oo

LJ

1/1 s

w

I-z

Do

o

2oN

to2o

U

60

59

58 -

57

56 -

55 -

54

53

52 -

51

50

49

48

20 40

SEQUENTIAL MEASUREMENT DAY

60

SEQUENTIAL MEASUREMENT DAY

Fig. 7 TXRF signals versus measurement day over 3 months for residuecontaminated calibration wafer: (5a) counts/sec for Ni standard (1 ng Ni)

tracks x-ray power; (5b) measured Bi atoms/cm2 tracks high energy calibration.

(50 sec integration, 30 kV, 60 mA, air, 200 micron Mo filter)

240
Fig- » XXRF areal density (atoms/cm2

) versus measurement day over 3 months for prime silicon wafer:

(6a) Cu tracks angle calibration; (6b) Fe tracks

detection limit; (6c) Zn tracks energy resolution.

(700 sec integration, 30 kV, 60 mA, air, 50 micron Mo filter)

4 3 -

4.2 -

2.6 -

2.5 H 1 1 1 1 1 r

0 2(1 40 60 SEQUENTIAL MEASUREMENT DAY

09 t

0 8

-

0 20 40 60 SEQUENTIAL MEASUREMENT DAY

250
COMMENTS

Question: When you were running along the raster of preconditioning, did you

ever encounter anomalously weak sites that failed in the

preconditioning process itself?

Answer: No.

Question: First question is how many samples of each species did you test?

Answer: The data was from the average of two to three samples for each

raster program, and the results were comparable to within about

15%

Question: The other thing is, have you done any preconditioning in other

than an air environment, say in vacuum or nitrogen or hydrogen or forming gas or anything like that?

Answer: No.

Question: Do you have a rough idea of how high the temperature gets at the

surface when you do the conditioning? Is it like 200°C is it like

500°C?

Answer: From looking at the older NOAA films which have damage, you can

see that the particles back down onto the surface of the mirrors.

The particles are in some cases spheres, which indicates that the

films did in fact melt. And Fluoresced, too, which is the lower

melting component of the films, I think that melting temperature

is about 1800°C.

Question: In your electronic defects model I assume that the defect states

before and after preconditioning are the same - you don't create

any new ones or you don't destroy any old ones? The only thing

changing is the population of the deep levels, right?

Answer: Yes. We're not expecting that the preconditioning illumination is

strong enough to create or destroy states. It's been shown that

with UV illumination you could change the actual defects involved,

but not a lot of work has been done to study these defects at

1 micron.

Question: With regard to your postulated deep level defects, if you are changing the charge state of these defects, then you might expect

a metastable situation. How long would you expect preconditioning

to last before the defect states might return to the original

charge state?

Answer: Well, I think that the metastable state exists when the defects

are charged. It's been shown by, I believe, Schwartz and Hughes at

the University of Houston. He's used EPR to look at Si02 films

put down by several different techniques, and the less equilibrium

251
techniques, like E-beam deposition or R sputtering, show that

these states were filled. The equilibrium techniques, like hightemperature CVD, showed that those defects were empty. So I think

that the films that we're looking at were nonequilibrium to start

with, and the conditioning is moving them toward equilibrium. So

they shouldn't revert back.

Question: It is well-known that for wide-band gap uncoated materials, the same action of multiple - shot laser radiation leads to reduction of

the damage threshold instead of the increase that you observe for

your thin films. One possible idea is that you are creating

electronic states which promote the damage threshold.

Answer: I think maybe the key difference here is, again, the wavelength of

the illumination. What you're talking about, is that at 1 micron

or at some higher energy photon?

Question: Well, observations like this have been observed at 1 micron as well as higher photon energies. So for example, we measure at

half the wavelengths of 500 nanometers, which is not so short

wavelengths, anyway.

Answer: You can't just generalize. We're not proposing that the mechanism

that we're proposing here is a general conditioning mechanism for

all materials.

Question: But the uncoated materials are just behaving the opposite way.

Answer: Uh-huh. Well, it's definitely possible that we might be also

creating further defects, but with these dielectric films, we're

not seeing a decrease in the threshold. We're seeing only an increase

Comment: M. J. Soileau, University of Central Florida: I might say that it

is not a universally observed fact that preconditioning of wideband gap dielectrics lowers the threshold. I've done countless

experiments myself where we've seen the same effects reported in

this paper with uncoated dielectric surfaces. You radiate at low

intensities and then you have an increase in damage threshold.

Its not universally observed that the damage threshold decreases

In fact, there are good examples where the same results reported

here are seen.

Question: Is the fact that you didn't see any conditioning with the xenon

lamp somewhat inconsistent with your hypothesis of electronic

defects?

Answer: I think the key difference there is probably the fact that you're

also hitting the sample with UV photons at the same time. It's

been shown for silica films that if you illuminate them with UV

radiation, you fill those defects. And so, we're trying to

condition with both UV and IR radiation at the same time. And

we're not going to see any effect.

252
Comment: I might say we did some experiments on bulk zirconia, where we

were, by multiphoton absorption, populating defects. And there were lots of them so you could measure their absorption and then

could easily remove them at lower radiance, multiple shots at

lower radiance, removing carriers from those defects. I think

there are some close parallels to the model that you're proposing

there. I might just add, it's a very complicated process. You've

got a multiphoton process to generate defects and perhaps s ingle

photon process to remove them. And with a wide-band source, you

could have lots of things going on simultaneously.

253
MANUSCRIPT NOT RECEIVED

An Error Analysis of the Wyko TOPO Noncontact Surface Profiler

Van A. Hodgkin

Physics Division, Research Department

Naval Weapons Center, China Lake, CA 93555-6001

ABSTRACT

An increasingly important tool in the characterization and measurement of optical materials and

components in the Wyko TOPO Noncontact Surface Profiler. Optically, it consists of a low- to mediumpower microscope in which the objective, depending upon the power, is a visible light interferometer of

either Michelson, Mireau, or Linnik design. By integrating a detector array into the viewing system and

attaching the reference arm of the interferometer to a piezoelectric transducer, the technique of surface

profiling by phase shifting is accomplished by computer.

The potential utility of this system to the investigator interested in the fundamental mechanisms of

laser-induced damage is enormous, provided that the user is aware of its proper use and limitations. This

paper is intended to act as a user's guide to the TOPO system in such cases and to document its precision

and accuracy limitations given a variety of surface types and measurement configurations.

254
MANUSCRIPT NOT RECEIVED

Ultra- Precision Grinding of LHG-8 Laser Glass

and Laser Damage Thresholds

Y. Namba

Chubu University

1200 Matsumotocho, Kasugai

Aichi 487, Japan

K. Yoshida, H. Yoshida, and S. Nakai

Institute of Laser Engineering

Osaka University

2-6 Yamadaoka, Suita

Osaka 565, Japan

ABSTRACT

It has been very important to reduce the machining cost of optical materials in the laser fusion

program. LHG-8 laser glass was ground with the ultra-precision surface grinder having a glass-ceramic

spindle of zero-thermal expansion in order to remove or shorten the optical polishing process in manufacturing high power laser optics. The laser damage thresholds were measured at 1 ns and 30 ns in pulse width and 1.053 nm in wavelength on ground surfaces as well as optically-polished surfaces. 0.5 nm

rms in surface roughness was measured on the ground surface with WYKO's TOPO-3D. The laser damage

thresholds on the ground surface were very similar to that on the optically-polished one.

255
Manuscript received

3-2-90

Low Scatter Surfaces on Silicon Carbide

Roger A. Paquin and Matthew B. Magida

Hughes Danbury Optical Systems, Inc.

Danbury, CT 06810-7509

Reaction bonded silicon carbide (RB SiC) can be

readily fabricated to near net shape and mirror blanks

produced by this method can potentially be less costly

than those fabricated by chemical vapor deposition (CVD)

However, RB SiC is two phase, SiC and up to 30% silicon

(Si) , and can not normally be directly polished to low

scatter and roughness levels due to the difference in

hardness of the two phases. We have investigated the

polishability of RB SiC as a function of Si content and

microstructure. Our results show that with a favorable

microstructure, RB SiC can be polished to less than 10A

rms. For reduced roughness and lower scatter surfaces, we

have developed low temperature deposition techniques to

apply polishable, single phase coatings to the figured two

phase surfaces. Scatter and roughness measurements show

levels are comparable to CVD SiC.

KEY WORDS: BRDF; coatings; finishing; grinding; ion assisted

deposition; ion beam sputtering; low scatter; microstructure;

mirrors; polishing; reaction bonded silicon carbide; silicon;

silicon carbide; surface roughness.

1 . Introduction

Silicon carbide (SiC) is an excellent material for laser mirrors

primarily due to its superior properties relative to other often used mirror

materials. Table 1 shows that the various forms of SiC have low thermal

expansion, high thermal conductivity, very high mechanical properties and

high transition temperature. These properties lead to distortion coefficients that compare favorably with the low expansion dielectrics, while fair

exceeding them in fracture strength. For cooled mirrors, SiC is superior to

molybdenum (Mo) in both thermal and mechanical properties.

As shown in the table and in figure 1, there are a number of types of

SiC. Chemically vapor deposited (CVD) SiC is a pure form of the cubic beta

phase that is grown on a mandrel and has a columnar grain structure. It has

been polished to super low levels of scatter and roughness. There are no

fully dense and pure forms of hexagonal alpha SiC, since it requires about

two percent of additives such as boron and aluminum to achieve full density.

Both CVD and alpha are difficult to form into complex shapes. Reaction

bonded (RB) SiC is a polycrystal 1 ine two phase material consisting of

interpenetrating networks of silicon (Si) and alpha SiC. The Si content can

range from twelve to forty percent. Reaction bonded SiC can readily be

formed into complex mirror shapes and can be polished to reasonably low

roughness and scatter levels. The major reason that RB SiC has not been

used more extensively is that the mirror surface, when polished by conventional methods, exhibits a topologically differential microstructure. This

256
is shown in figure 2, where the Si phase is 300 to 700A below the level of

the SiC.

Table 1. Selected Mirror Material properties* at 20°C

a k E P F T a/k

Material 10" 6 /°c W/cm-°C GPa g/cc MPa °C 10" 6 cmA

CVD SiC 2 . 5 1 . 10 445 3.21 >200 1300 2 . 3

Alpha SiC 3 . 2 1.40 435 3.22 >300 1800 2.3

RBSiC (30%Si) 2 . 64 1.58 324 2.93 >200 1400 1.7

RBSiC (12%Si) 2 . 68 1.75 386 3 . 11 >200 1400 1.5

ULE 0. 008 0. 013 67 2.21 7 800 0.6

Zerodur 0. 015 0.016 92 2.53 10 500 0.9

Molybdenum 5.0 1.40 324 10.21 400 500 3 . 6

Beryllium 11.4 1.93 287 1.85 280 500 5.9

*a-coeff icient of thermal expansion; k-thermal conductivity; E-elastic modulus;

p-density; F- fracture strength: modulus of rupture except 0.2% yield strength

for Mo and Be; T- transition temperature: highest usable temperature for optics,

in protective atmosphere and exclusive of optical coatings.

Figure 1. Silicon carbide substrates, formed by three different

processes, shown polished: (a) sguare hot pressed alpha SiC; (b)

lightweight isoformed reaction bonded SiC (alpha SiC + Si) ; (c)

four squares of CVD beta SiC.

Given this limitation, we decided to investigate this promising new

mirror material because of the following factors: (1) near net shape capability of RB SiC fabrication methods that allows the formation of internal

cavities, such as used in heat exchanger mirrors, (2) the potential for

257
modifying the microstructure for better polishability and (3) the potential

for ultra-low scatter by overcoating with a polishable SiC coating to bury

the two phase microstructure, as we have successfully demonstrated with

beryllium [1]

Figure 2. Microstructure and Wyko surface profile for polished

reaction bonded silicon carbide mirror specimen showing step

heights of 300 to 700A at phase boundaries.

2 . Experimental Approach

We concentrated our investigation in two areas: (1) substrate finishing, including substrate material effects and processing approaches, and (2)

coatings for ultra-low scatter. The technigues developed were used to demonstrate low scatter surface finishing. These topics are described in the

following sections.

2.1. Substrate Finishing Development

During our investigation we worked with a number of different microstructures. We varied the Si content and at a given Si level, varied the

grain size and grain size distribution of the SiC as described previously

[2]. Typical microstructures are shown in figure 3 for a monomodal SiC grain

size in a 30% Si material, and a multimodal distribution with 12% Si. Our

optical fabrication experiments were carried out using a number of lap materials, grinding and polishing abrasive grits and fluid media, while varying

the applied pressure and relative velocities.

Figure 3. Microstructures of reaction bonded silicon carbide

(SiC) : (a) 30% silicon (Si) with monomodal SiC grain size distribution, (b) 12% Si with multimodal SiC distribution.

258
2.2. Silicon Carbide Coating Development

Coatings that are normally applied to RB SiC to obtain low scatter

finishes include CVD SiC and PVD Si. The CVD SiC is deposited at elevated

temperatures, in excess of 1000 °C, and yields a polycrystalline beta (cubic)

material with fine columnar grains. The Si can be deposited using any of

several physical deposition methods such as evaporation from thermal or

electron beam sources. The Si can be either polycrystalline or amorphous

depending on deposition conditions. The Si can be finished to very low

scatter levels but has a mismatch in properties with SiC.

We chose to investigate processes that utilize ion beams for higher

energy deposition to deposit pure SiC at or near room temperature. The

higher energy results in better adhesion, lower defect concentration, higher

packing density for near bulk properties and reduced residual stress, and

broad substrate applicability. We investigated two of these methods, shown

schematically in figure 4, ion assisted deposition (IAD) and ion beam sputtering (IBS) . For the former, an electron beam is used to evaporate pure

alpha SiC chunks while an argon ion source bombards the substrate. For the

latter, the argon ion beam sputters SiC from a CVD target onto the substrate.

In IAD, the ions add energy to the arriving atoms, while with IBS, the sputtering mechanism results in SiC with high kinetic energy.

SUBSTRATE

argon ion assisted electron beam evaporation, (b) argon ion beam

sputtering.

2.3. Polishing Demonstration

To demonstrate the low scatter polishing methods developed during the

program, bare and SiC coated 76mm flat and 152mm spherical mirrors were

prepared in addition to the 25 and 38mm diameter flat mirrors. Some of these

mirrors are shown in figure 5.

3 . Results

We developed a baseline optical finishing process similar to that of

Fuchs and Brown of LLNL [3], in which grinding and figuring are carried out

on metal/epoxy composite laps with successively finer diamond grits with

water and diamond extender, while polishing utilizes pitch with sub-micron

diamond. We were able to achieve figures of better than 0.1 visible wave

without difficulty on all of the 25 and 38mm flats. Achievable surface

259
roughness, as measured by Wyko, was dependent on the microstructure. On 3 0%

Si material, we were able to achieve 2 0-3 0A rms with large grain monomodal

grains, 15A with fine monomodal grains, as shown in figure 3a, and 15A with a multimodal grain size distribution. When 12-15% Si RB SiC was polished, the

multimodal grain distribution yielded surfaces with roughness less than 10A

rms. This investigation shows that low scatter and roughness (<10A rms)

mirror surfaces can be produced on RB SiC with tailored microstructures . A

preferred microstructure with reduced Si content and fine multimodal grain

size has been shown for RB SiC.

Figure 5. Silicon carbide substrates of various sizes and shapes

which have been optically finished to demonstrate our low scatter

finishing process. Shown are 25, 38, 76mm diameter flats, 152mm

diameter spheres and a 254mm oval shaped targeting mirror blank.

Coating deposition experiments were carried out to determine optimum

parameters for the two techniques and to evaluate differences between coatings produced by the two methods. We found that in our initial approach

involving IAD, we were unable to identify a combination of parameters that

would repeatedly produce smooth, stoichiometric SiC. Parameter ranges investigated included: coating thicknesses from <1 to >5 microns; ion assist

levels from zero to the sputter regime; substrate temperature from 30 to

200 °C; and source material of high purity alpha chunks to a CVD plate.

Although good adhesion was achieved, the coatings contained high defect

densities (associated with spatter) and were typically of f-stoichiometry

,

being Si deficient.

In our alternate approach, ion beam sputtering, we were more successful. Defect free, stoichiometric, amorphous coatings up to six microns thick

260
]were deposited. While the coatings were readily polishable to very low

roughness and scatter levels, some compressive stress was observed in the

coatings. We are continuing our investigation in an effort to eliminate this

stress

.

The results of our polishing efforts on bare and coated samples are

shown in table 2 and figure 6. The data represent the best sample mirrors

measured to date. The 12% Si material, called reaction bonded in the figure,

has significantly smaller surface roughness and lower scatter than the 30% Si

material. The BRDF at one degree is very low for this two phase material.

While not described peviously, we polished some CVD SiC which has much lower

BRDF than the best RB SiC, but note that the Wyko roughness falls between the

two bare materials. The IBS coating however has the lowest numbers for both

scatter and roughness, and in fact, is the lowest scatter surface ever measured in our laboratory. The variance between roughness and scatter data is

similar to that observed for beryllium, and is the subject of investigations

in our laboratory and others. If overcoated with an opaque metal film, the

scatter, but not necessarily the roughness, should drop. The two phase RB

SiC should show the greatest change. This study is a subject for future

presentations

.

Table 2. Summary of SiC Surface Quality Results

for Bare and Coated Surfaces

Material

Roughness*

A rms

BRDF Slope

1° to 10°

BRDF at 1°

from slope

30% Si

12% Si

IBS Coating

CVD Coating

9.4

5.8

2.9

8 . 2

•2 . 4

•1.1

•1.8

1.3

1.3 E 0

8.4 E-2

3.3 E-3

7.4 E-3

* Roughness measured by Wyko

i i i i i i 1 1

1

i i ii i i i i i i 1

1

0001 001 01 I

BETA - BETA 0

Figure 6. Bidirectional reflectance distribution function (BRDF)

measurements at 633nm comparing polished bare and coated silicon

carbide. Siliconized is 30% Si and reaction bonded is 12% Si.

261
The polishing and IBS coating processes have been applied to 152mm

diameter spherical mirrors with radius of curvature of 610mm, fabricated from

12% Si reaction bonded SiC. To date, one-tenth wave figure has been achieved

on bare surfaces, and two mirrors have been IBS coated with approximately

four microns of amorphous SiC. Surface roughness and scatter measurements

have not yet been performed on the bare mirrors and the coated ones are not

yet polished.

4 . Conclusions

In this experimental program we have shown that reaction bonded silicon

carbide is a viable laser mirror material when the silicon content is low

(22-15%) and a multimodal, small, SiC grain distribution is employed. This

preferred material is polishable to 10A rms with one-tenth wave or better

figure using diamond slurries on pitch laps after grinding and figuring with

diamond on composite laps.

We have developed an ion beam sputtering process for overcoating the

polished RB SiC with amorphous SiC that can be polished to ultra-low scatter

and roughness (<5A rms) levels.

The processes described in this paper are scaleable to complex shapes,

aspheric surfaces and to very large sizes.

This work was performed with Hughes Danbury (formerly Perkin-Elmer)

IR&D funding. We acknowledge the continuing support of Gerry Weaver, Carborundum Specialty Products, in the development of the SiC materials investigated in this program. We would also like to thank Max Bruckenthal of Hughes

Danbury who performed all of the optical finishing.

5 . References

[1] Paquin, Roger A. ; Gardopee, George J. ; Peterson, James "Hot Isostatic

Pressed Beryllium Mirrors," in Proceedings of the Topical Meeting on

High Power Laser Optical Components , Hopkins, Alan, ed. 1987 October

29-30; Boulder, CO. Weapons Laboratory, AFWL-CP-88-01 , 1988 May. Pp

23-32

[2] Magida, Matthew B; Paquin, Roger A. "Silicon Carbide Mirror Surface

Technology," in Proceedings of the Topical Meeting on High Power Laser

Optical Components , Stanford, James L. , ed. 1988 October 24-25; Boulder, CO. Naval Weapons Center, NWC TP 7 017 Part 1, 1989 July. Pp 52-

58.

[3] Fuchs, B. A. "Removal Rates, Polishing and Subsurface Damage of Chemical Deposited Silicon Carbide," presented at the OSA Workshop or? Optical Fabrication and Testing , 1987 October 20-23; Rochester, NY.

262
Manuscript received

12-29-89

PHYSICAL LIMITS ON ULTRA-HIGH ALBEDO DIFFUSE REFLECTORS

Perry Miles, R&D Associates

Marina del Rey, CA

1.0 INTRODUCTION

While the theoretical possibility of achieving near-perfect diffuse reflectance in a purely scattering material is longstanding' 1

' , no attempt seems to have been made to develop

scatterers with albedos of 0.999 and beyond. Such ultra-high albedo diffuse scatterers are now of interest because they offer the possibility for the development of shields against

high levels of laser radiation<2)

.

The simple two-stream model <3) for a thickness, L, of such a material gives an albedo:

A - k* L(1 - R) + 2R

- ka L

ks L(l - R) + 2

1 + 2R

ks L(l - R) + 2 .

k„ « kc

where ka describes attenuation by absorptive processes and ks describes the total scatter,

assumed to be equal, forward and reverse, per unit length (volume) of material, and where,

for the sake of simplicity, we have ignored the Fresnel reflection at the front surface, but

have included a rear surface reflection R to take into account the effect of any opaque

substrate material such as a metal plate. For all practical values of substrate reflectance

(R < 0.95), and for ks L > 10 3

, we may use the approximation

A - 1 - 2- ka L

ks L

Accordingly, the achievement of A = 0.9999, in a 1 cm thick plate, for example, calls for a scatter coefficient, ks , greater than 2X10A cm" 1 together with a residual absorption

coefficient, ka

, less than 10~ 5 cm

-1

. The issues for this paper are first, whether such

levels are achievable, even at low irradiance levels below 10 3 W/cm2

, and second, whether

this performance can be preserved at time average irradiance levels above 10'' W/cm2 in

pulsed wavetrains where the instantaneous irradiance levels may exceed 10 7 W/cm2

.

2.0 CANDIDATE MATERIALS

At first sight, these requirements seem attainable with careful attention to

cleanliness in the preparation of the matrix material and in its treatment to develop a porous or other inhomogeneous structure. As we pointed out in Reference 2, inorganic

glasses have already been developed for use in fiber optic transmission lines (Figure 1)

with attenuation constants less than 1 db/km (2.3 X 10" 6 cm" 1 ).

In some spectral ranges, these attenuation levels are dominated by scatter losses,

which are not a limiting factor in the present context. Even OH impurity generated

absorption can be kept to below 10" 5 cm

-1

, while the intrinsic (linear) electronic and

Stokes, G.G., Proc. Roy. Soc. (London) 11, 545 (1862)

2Miles , P., "Conceptual Bases for Ultimate Hardening Against CW and RP

Lasers," AIAA/DNA Laser Effects and Target Response Conference, March

1988

3 Schuster, A., Astrophys. J. 21, 1 (1905)

263
Figure 1

(U) LOSS BREAKDOWNFOR AN ULTIMATE LOW -LOSS SINGLE -MODE FIBER

02

00

•o

to

Q 0.1

005

003

0 02

INFRARED

; ABSORPTION

| LOSS

(From Miya, T., Terunuma, Y., Hosaka,

T., & Miyashita, T., Electron. Lett.,

15, 106, 1978)

ULTRAVIOLET

A8SORPTION LOSS / LOSS DUE

TO WAVEGUIDE

., IMPERFECTIONS

I _ ]MPERFECT(ONS_

1 J I I I 1

.2 1.3 14 15

WAVELENGTH , Mm

18 20

UNCLASSIFIED

264
lattice phonon excitations may give absorption coefficients two orders of magnitude lower

still- -at least at room temperature.

At higher levels of heating, we are concerned both with changes in the optical

properties of the matrix and with the possibility of thermally induced fractures. These are

the same concerns that, in the past, have faced the designers and developers of window and

lens elements for high power laser optical trains. It is highly likely, therefore, that the

best materials for the UHA protective shields may well be scattering forms of materials

already developed for those same high power laser weapon and IR optical components. Alkali

halides, CaF2 , ZnSe, fluoride glasses, and possibly sapphire, spinel, fused silica, high

purity forms of SiC, as well as the host glasses for Nd: glass laser components are all

likely candidates.

Over the period 1970-1975, the absorption coefficients of fully dense forms of many of

these materials were reduced by careful processing techniques' 4

' to levels so low that

conventional transmission spectroscopy was unable to provide accurate measurements of

absorption in the transparent spectral range between the visible/UV and IR absorption edges.

In its place, a variety of vacuum calorimetric techniques were developed with sensitivities

down to 10" 6 fractional loss, both linear and non- linear, along with photo acoustic

techniques with even greater sensitivities' 5,6 '

. These same techniques will be needed to assess the quality of UHA materials

.

3.0 LIMITS ON PERFORMANCE

Beyond the fundamental limitation imposed by the finite thickness of the scatter plate,

the most obvious problem comes from residual absorption throughout the material. In fact,

heating of the scattering material, either as a linear process or one accelerated by in- creases in the absorption coefficient itself, may set the limits on its ability to withstand

high levels of incident laser radiation. Thus, an alumina scattering shield with an areal

density 2 gm/cm , a specific heat of 1 J/gm/K and an albedo of 0.9999 would heat up at the

rate of 5 X 10" 5 I 0 K/s . For I 0 = 10 5 W/cm2 this is 5 K/s or 200 K in 40 s . At I 0 = lO 4 W/cm , it would take 400 s to reach 200 K--even longer, if a deliberate attempt were made to

transfer heat out of the irradiated area.

For pulsed irradiation, on the other hand, much higher temperature excursions would

occur at absorbing impurity centers and in their immediate surroundings. For absorbing

centers less than 1 ^im in diameter, however, significant heat diffusion could extend to ten

diameters from the center during pulse lengths greater than 10 ns , thereby reducing the

maximum temperature excursion at the site itself. Apart from the well-known case of Pt

inclusions in laser glasses <7)

, the specific origin of the damage threshold under pulsed

irradiation of highly transparent materials has been an unresolved issue for many years.

Recent work in the field <8> is shedding more light on the intrinsic nonlinear free carrier

absorption and heating that precedes detectable damage in wide bandgap materials, but while

intrinsic multiphoton excitation of carriers has been observed at UV and visible

*Miles, P., Opt. Eng. 15, 451 (1976)

5Hordvik, A., App . Opt. 16, 2827 (1977)

6 Tam, A.C., Rev. Mod. Phys . 58, 381 (1986)

7 Hopper, R.W. , Uhlmann, D.R., J. App. Phys. 41, 4023 (1970)

8 Chen, X.A. , Jones, S.C., Braunlich, P.F., Phys. Rev. Letts. 62, 2711 (1989)

265
wavelengths' 9,10 ', the initiation of damage in such materials at infrared wavelengths is still

likely to be associated with impurities and structural defects.

The single pulse damage level for wide bandgap materials is of the order of 20 J/cm2 in

a 1 ns pulse, that is, at instantaneous irradiance levels of 2 X 10 10 W/cm2

. Whether these

levels will be achievable in scatterers is an open question.

The possible augmentation of the internal radiation field strength in the vicinity of

micropores and microcracks - -particularly in high index materials - -was also pointed out long

ago ( 11 )

, and in the present case, where pores and scattering discontinuities are an explicit

aspect of the material design, this effect can be expected to be of great importance. The

preparation of external optical surfaces with fractional loss factors below 1CT 6 is rare and

these residual surface losses are also assumed, though without proof, to be due to

structural imperfections and the accumulation of absorptive impurities. It remains to be seen whether internal surfaces can be made any better. If they cannot, a material with a 1CT 6 loss every micrometer of path would result in a limiting albedo of the order of 0.99--

the apparent present state-of -the-art (12)

, interestingly enough- -and of no use as a high

power laser shield.

Quite apart from the geometric intensification of the field at crack tips that might

also be impurity sites, the question arises whether the internal surfaces or discontinuities

themselves exhibit nonlinear behavior to a far greater degree than the bulk material.

Indeed, the careful study of ultra-high albedo materials where the surface to volume ratio

has been increased by many orders of magnitude over comparable solid, non-porous, materials

may lead to new understanding of the optical properties of surfaces and interfaces per se.

4 . 0 SUMMARY

Past efforts to develop low loss, damage resistant window, lens and optical fiber

materials provide the starting point for the development of near-zero absorption scatter

reflective coatings. Designs can be envisioned with absorption coefficients less than lCT*1 cm" 1 and with external albedos greater than 0.999. Achievable limits of absorption will

presumably depend on the same factors that determine the damage threshold for transparent

laser optical components - -namely residual impurities, intrinsic absorption mechanisms and

non-linear responses to pulsed radiation. In particular, the linear and nonlinear optical

properties of internal surfaces and interfaces may turn out to be the most important

limiting factor for strongly scattering materials. Explicit 'studies of the properties of

surface discontinuities may become essential.

5 . 0 ACKNOWLEDGEMENT

This work was carried out under the Defense Nuclear Agency's Contract Number DNA001-88-

C-0046.

9Nathan, V., Guenther, A.H., Mitra, S.S., JOSA, B2 , 294 (1985)

10 Jones, S.C., Chen, X.A., Braunlich, P.F., Kelly, P., Epifanov, A.S., Phys

Rev. B 35, 894 (1987)

nBloembergen, N . , App . Optics 12, 661 (1973)

12Weidner, V.R., Hsia, J.J., JOSA 71, 856 (1981)

266
Manuscript received

2-8-90

Optical Damage on Si02 Cavity Mirrors

Produced by High-Power VUV Laser Irradiation

Y. Takigawa

Department of Solid State Electronics, Osaka Electro-Communication University,

Neyagawa, Osaka 572 Japan.

K. Kurosawa and M. Okuda

Department of Electronics, University of Osaka Prefecture,

Sakai, Osaka 591, Japan.

W. Sasaki

Department of Electrical Engineering, Miyazaki University,

Miyazaki 889-21 Japan.

K. Yoshida, E. Fujiwara and Y. Kato

Institute of Laser Engineering, Osaka University,

Suita, Osaka 565 Japan.

Y. Inoue

R/D Engineering Department, Scientific Equipment Division,

Shimadzu Co., Kyoto 604 Japan

We report observation results about damages on Si02 mirror surfaces. The

damages were made when the mirrors were used as a cavity reflector for the Ar and Kr

excimer lasers. The surface profile, transmission and reflectance spectra, and X-ray

photoelectron spectra show that bulk Si is isolated in the surface layer which was exposed to 9.8eV photons from the Ar excimer laser. The Kr excimer laser, whose

photon energy is 8.5eV, does not induce such a phenomenon. The Ar excimer laser

photons, surmounting the fundamental band gap of Si02, 9eV, are considered to

create high density excitons that induce the Si-0 bond breaking and Si isolation.

Key words: VUV laser, rare gas excimer laser, quartz glass mirror, mirror

damage, x-ray photoelectron spectroscopy.

267
1. Introduction

High-purity synthetic Si02 glasses are the most promising materials for mirrors, windows

and lens in a wide spectral region from visible to ultraviolet (UV) wavelength. It has

been observed, recently, that UV excimer lasers whose-photon energies are higher than 5eV

generate absorbing color centers in Si02 [1-2]. In addition to a fact that, in cases of

ArF and KrF excimer lasers, two photons could surmount the fundamental band gap energy

of 9eV [3], the resultant number of color centers follows a (power) 2 dependence, so that

excitons that are created via a two-photon generation process is capable of creating color

centers. Since the color centers created by the sub-band gap excimer laser radiation are

the same types as those created by ionizing radiation such as X-rays, r -rays or E-beams

[4-5], the generating mechanisms are inferred to be the same. We have been developing high power argon (Ar) and krypton (Kr) excimer lasers for

applications to plasma diagnostics, photo-chemistry, lithography, and others [6-8]. On the

course of the project, Si02 glass plate surfaces which were used as the laser cavity

reflector, suffered damages (beam patterns) even at relatively low output power [9].

Apparently the surface temperature rises up beyond its melting point and the surfaces are observed to be roughened. In addition to the thermal effect, isolated Si was detected,

when the mirrors were used for the Ar excimer laser [10]. The Si isolation is considered to

originate from electronic excitation, because the photons of 9.8 eV must excite electrons

from the valence band to the conduction band and generate efficiently electron-hole pairs.

In this paper, we show observation results of the surface damages, which are induced by

the Ar and Kr excimer lasers, with surface topography, transmission and reflectance

spectra, and X-ray photoelectron spectroscopy. Discussion about the SI isolation mechanism

is also described.

2. Electron Beam Pumped Rare Gas Excimer Lasers

Prior to the description about the laser induced damages, we should note the high-power

rare-gas excimer lasers as illustrated in figure 1. High purity Ar or Kr gas was filled in a

stainless steel pipe of 5 mm diameter, 80 cm length and 80 u m thickness. This pipe acted as an anode for an electron beam accerelated at 700 kV. The laser cavity, located at both ends

of the anode pipe, was composed of a Si02 glass reflector and a MgF2 output coupler. The

output mirror had 63% transmittance and 8% reflectance at 126 nm. The output energy of the

laser beam of 5 mm in diameter, which was came out through the output mirror and window of

MgF2 , was measured in vacuum. The surfaces of the Si02 reflector were observed to be

suffered damages even when the laser output energy was relatively low, around 5 mJ. The

energy stored in the cavity was calculated from the laser output energy by taking account of

the transmittance of the output mirror and window. The energy was considered to correspond

to that in front of the mirror surface. It should be noted, furthermore, that clear beam

patterns with 5 mm diameter were observed even by only one shot, and that the patterns

became clearer by multiple shots. In this paper, we observed the beam patterns made by 2

-

5 shots of the Ar or Kr excimer laser, which provides the output energy of about 10 mJ.

268
3. Observations of Mirror Damages

3.1 Surface topographic observations

The beam patterns were observed clearly with naked eyes and also with an optical

microscope. The scattered light by the surface roughness probably enables us to see the

beam patterns, but since their contrast was very faint, we could not show you clear photos.

The beam patterns were more clearly seen by using a surface topographic method, as shown in

figure 2. These were taken with an optical interferometric profilemeter (Wyko Co.,

NCP-1000). The peculiar feature of the beam patterns is wrinkles along the peripheral, that

is, remarkable ripples near the edges are observed. Such features are observed for both of

the laser radiation. The surface temperature was estimated to be far beyond the melting

temperature by the 10 mJ laser radiation. Therefore, it seems us that the molten layer moves

toward the peripheral of the beam pattern and the wrinkles were formed along the edges.

3.2 Optical properties

In order to have more detailed insight into the observed damages, we have taken

transmission and reflectance spectra of the Si02 mirrors in the visible to VUV regions using

synchrotron radiation [11]. Figure 3 shows the transmission spectra, in which curve (a) is

taken from a non-irradiated part of the mirror, curve (b) from the Ar excimer laser induced

damage, and curve (c) from the Kr excimer laser induced damage. The tramsmittance just

above the optical band gap wavelength decreases significantly for both of the laser induced

damages. This is probably partly because of the surface roughness enhanced by the laser

radiation.

Figure 4 shows the reflectance spectra of the mirror, in which curve (a) is taken from a non-irradiated part, curve (b) from the Ar excimer laser induced damage, and curve (c) from

the Kr excimer laser induced damage. In curve (a), there appear four distinct peaks, at

10.4, 11.7, 14.0, and 17.3eV, which are assigned to transitions between s-p hybrid orbitals

with Si-0 bonds [12]. In curve (c) four peaks still appear besides for slight decreasing of the

reflectance. However, in curve (b), their peaks become very weak and almost disappear. This

result indicates that Si-0 bonds are broken in the surface layer of the Si02 mirror due to

high power Ar excimer laser irradiation. In addition to the peak disappearance, the

reflectance spectrum from the Ar excimer laser induced damage (curve (b)) has found to

have a peculiar feature. The reflectivity in the UV region becomes significantly enhanced

and has a broad peak around 200nm. With respect to the profile of the spectrum is concerned,

the spectrum resembles that of bulk Si [13]. If bulk Si is isolated in the surface layer of the

Ar excimer laser induced damage, it is explained well that the reflectance peaks disappear

and also that the reflectivity increases in the UV region. It should be pointed out here

that such a phenomenon has not been observed in the Kr excimer laser induced damages.

3.3 X-ray photoelectron spectroscopic studies

For studying such phenomena, one of the most promising methods is X-ray

photo-electron spectroscopy (XPS), which provides an information about chemical bonding

269
nature. An ordinary XPS analysis offers only an average value over the exposed area by

X-ray radiation. Sometimes spatially resolved spectra are required. Several techniques for

this purpose have been offered and realized. Yamaguchi et al. have developed an electromagnetic lens system which collects the photoelectron and also which makes focused

beam onto an entrance slit of the photoelectron energy analyzer [14]. By using an appropriate aperture at the slit, photoelectron coming from a selected area is

analyzed. The smallest area analyzable is about 0.20mm in diameter onto the sample

surface. It is interesting furthermore that a precisely controlled X-Y stage for the sample

holder is installed and thus two-dimensional images have become possibly obtained. In this

paper, we used MgK a emission at 1253.6 eV for the excitation and the electron energy was analyzed by a 180 ° hemispherical electron energy analyzer.

X-ray photoelectron spectra were taken from several different points at intervals of

1mm along a line including two beam patterns induced by the Ar and Kr excimer lasers. In

figure 5 are shown the respective XPS spectra. Curves 4 to 9 are taken from points in the Ar

excimer laser induced beam pattern, and curves 11 to 16 from points in the Kr excimer

laser induced beam pattern. The others are from non-irradiated area. The strong peaks,

which are seen in all of the spectra, are assigned as the Si-2p emission in SiOs glass, because

of the binding energy of 103.4 eV. In addition to their peaks, a relatively small peak

appears only in the spectra 4 to 9. Its binding energy is 99 eV and thus it is identified as the Si-2p emission of bulk Si. It is worthwhile at this stage that Si-0 bonds are broken and

thus bulk Si appears macroscopically in the surface layer of the Si02 glass due to the strong

Ar excimer laser irradiation but not due to the strong Kr excimer laser irradiation.

With the precisely controlled translational stage, two dimensional distribution of

photoelectrons having a given binding energy is obtained. Figure 6(a) shows such an

image taken with 103.4 eV emission of Si-2p in Si02 glass. The brighter spots in the figure

indicate the higher density of the photoelectrons emitted from them. On the other hand,

the darker spots correspond to them with the lower density. In this figure, all surface area

except for the circular area in the left hand side of this figure is seen in white, which

indicates that Si02 is the predominant species in this area. On the other hand, in the

circular area, relatively small amount of Si02 is included. Figure 6(b) shows another image

taken with 99 eV emission of Si-2p in bulk Si. This figure shows that bulk Si is included only

in the circular area which just corresponds to the beam pattern exposed to the Ar

excimer laser irradiation. It should be noticed that the bulk Si appears only in the beam

pattern made by the Ar excimer laser radiation, and further that is isolated uniformly in

the beam pattern, at least with a spatial resolution of 200 p. m. The Si isolation could not be

observed in the beam pattern irradiated by the Kr excimer laser.

In general, glasses usually include much more defects and impurities than crystals. We

are very interested in whether the Si isolation in Si02 is a peculiar phenomenon in glasses or not. In order to this point, we observed the beam patterns of a crystalline Si02. Figures 7(a) and (b) show two dimensional distribution images with 103. 4eV photoelectrons

(a) and with 99eV photoelectrons (b). In these figures, two beam patterns are included,

one in the left hand side induced by one shot of the Ar excimer laser and one in the right

hand side induced with five shots. These images show that both of the patterns include bulk

Si isolated and deficient Si02. Nevertheless, the five shots induce much larger amount of

bulk Si than the one shot. It can be concluded here that the Si isolation by 9.8eV photons is a

270
common phenomenon in both of amorphous and crystalline Si02, namely, that the Si isolation

is a fundamental property in Si02.

4. Discussion

The penetration depth of the Ar excimer laser radiation Is calculated to be 52 nm, from

data about the optical constants of Si02 glass [12]. The laser radiation with 200 mJ/cm2

fluence onto the mirror surface, corresponding 10 mJ output energy, include 1.2 x 10 1T

photons/cm2

. Taking account of reflectivity of 14% at 126nm, we can estimate that these

photons hit the small volume of Si02 . If one photon excites one electron from the valence band

to the conduction band, an extremely high density of electrons, about 1022/cm3, are

generated. Even though a part of the photons are required to rise up the mirror surface

temperature, still a large amount of electrons are present in the conduction band. In the

case of the Kr excimer laser, the intrinsic Si02 property shows almost transparent at

146nm [12], but the Si02 glass usually show strong absorption around 146 nm. In fact,

the optical band edge shown in figure 3 was 165 nm, 7.5 eV. The SiOs glass mirror used here

shows strong absorption even around 146 nm, and thus the damage patterns were observed for

the Kr excimer laser irradiation. Therefore, if high density of electrons excited have a capability to break Si-0 bonds and isolate bulk Si, the Kr excimer laser photons could induce

the same phenomenon of Si isolation as the Ar excimer laser. It should be noted here that the

difference of the photon energies is remarkable, that is, 9.8 eV photons break the Si-0

bonds but 8.5 eV photons not. Presumably the electron excitation directly from the valence

band to the conduction band and thus creation of excitons is of importance for this

phenomenon.

In summary, with selected area X-ray photoelectron spectra, Si-0 bonds have been

observed to be broken in Si02 glass by the Ar excimer laser radiation, but not by the Kr

excimer laser radiation. The two dimensional distribution images with given energies show the

uniformly distributed pattern of the Si isolation within the beam pattern induced by the

Ar excimer laser irradiation. The Si isolation, that is, Si-0 bond breaking, is considered to

be caused by excitons created by the Ar excimer laser photons, in addition to the

thermal effect appearing as the wrinkles along the beam pattern periphery. This point has

not een confirmed yet. The detailed mechanism on the Si isolation will be discussed in future.

5. References

[1] Arai, K.; Imai, H.; Hosono, H.; Abe, Y.; Imagawa, H. Two-photon processes in defect

formation by excimer lasers in synthetic silica glass. Appl. Phys. Lett. 53(20): 1891-1893;

1988.

[2] Tsai, T. E.; Griscom, D. L.; Friebele, E. J. Mechanism of Intrinsic Si E'-Center

Photogeneration in High-Purity Silica. Phys. Rev. Lett. 61(4): 444-446; 1988.

[3] DiStefano, T. H.; Eastman, D. E. The band edge of amorphous Si02 by photoinjection and

photoconductivity measurements. Solid State Commun.; 9(24): 2259-2261; 1971.

[4] Griscom, D.L. Defect Structure of Glasses. J. Non-Cryst. Solids. 73(1): 51-77; 1985.

271
[5] Feigl, F. J.; Fowler, W. R.; Yip, K. L. Oxygen vacancy model for the ET center in Si02.

Solid State Commun. 14(3): 225-229; 1974.

[6] Sasaki, W; Uehara, Y.; Kurosawa, K. ; Fujiwara, E.; Kato, Y.; Yamanaka, M. High power

tunable coherent radiation around 126nm with an argon excimer laser. Rev. Laser Engineer.

14(5): 370-377; 1986.

[7] Sasaki, W.; Kurosawa, K. ; Herman, P. R.; Yoshida, K.; Kato, Y. "Intense coherent radiation in

the VUV and XUV region with electron beam pumped rare gas excimer lasers," in Proceedings

of the OSA topical meeting on Short Wavelength Coherent Radiation: Generation and

Applications, R. W. Falcone; Kirz, J., ed. 1988 Sep. 26-29; North Falmouth; Cape Cod, MA.

pp. 184-188.

[8] Kurosawa, K; Sasaki, W. ; Fujiwara, E.; Kato, Y. High power narrow band operation and

Raman frequency conversion of an electron beam pumped krypton excimer laser. IEEE J. Quantum Electron. 24(9): 1908-1914; 1988.

[9] Takeuchi, K. ; Kurosawa, K. ; Sasaki, W. ; Takigawa, Y. ; Yoshida, K. ; Fujiwara, E.; Kato, Y. Output characteristics of Ar excimer laser with various cavity mirrors. Rev. Laser

Engineer. 16(10): 691-698; 1988. (in Japanese)

[10] Takigawa, Y.; Kurosawa, K.; Sasaki, W. ; Fujiwara, E.; Yoshida, K.; Kato, Y. Si-0 bond

breaking in Si02 by vacuum ultraviolet laser radiation. J. Non-Crystal. Solids (in press).

[11] This reseasrch was supported by the Joint Studies Program of the Institute for Molecular

Science.

[12] Phillipp, H. R. Optical properties of non-crystalline Si, SiO, SiO x and Si02. J. Phys.

Chem. Solids 32(8): 1935-1945; 1971.

[13] Philipp, H.R. Silicon dioxide (glass), in Handbook Of Optical Constants of Solids.

E.D.Palik, ed. Orlando, Fl: Academic press; 985. 749-763.

[14] Yamaguchi, M.; Goto, H.; Mitamura, S.; Kato, I.; Hattori, T.; Shimizu, H.; Kajikawa, T. ; Nagaso, T. Development of X-ray photoelectron spectroscopy ESCA-1000.

J. Shimadzu Scientific Instruments 1(2): 62-66; 1989. (in Japanese)

272
Figure 1 Schematic illustration of the electron beam pumped rare gas excimer laser.

RMS: 1 1 . 1 nm P-V:77.4nm

DISTANCE (mm)

RMS:13.1nm P-V:85.9nm

DISTANCE (mm)

Figure 2 Surface topographic images of the beam patterns induced by the Ar excimer laser (a)

and by the Kr excimer laser (b).

273
PHOTON ENERGY (eV)

30 15 10 8 7 6

100

IDO

^ 50

z

<

a:

2.5

1 1 1 1 1 1 i i r a

1 1 100 200 300 400 500 600

WAVELENGTH (nm)

100

PHOTON ENERGY (eV)

30 15 10 8 7

LUo

< Crt - 2

</) z

<

100 200 300 400

WAVELENGTH (nm)

500 600

PHOTON ENERGY (eV)

8 7 6 5 4

T

100 200 300

WAVELENGTH (nm)

400 500 600

Figure 3 Transmission spectra of the Si02 glass mirror: curve (a) is taken from a non-irradiated

part of the mirror surface, curve (b) from a beam pattern induced by the Ar excimer laser,

and curve (c) from a beam pattern induced by the Kr excimer laser.

2 74
PHOTON ENERGY (eV)

100 200 300 400

WAVELENGTH (nm)

500 600

30

T

15

1

PHOTON ENERGY (eV)

10 8 7 6 5 4

T

200 300 400

WAVELENGTH (nm)

600

15

PHOTON ENERGY (eV)

10 8 7 6 5 4 2.5

100 200 300 400

WAVELENGTH (nm)

500 600

Figure 4 Reflectance spectra of the Si02 glass mirror: curve (a) is taken from a non-irradiated part of the mirror surface, curve (b) from a beam pattern induced by the Ar

excimer laser, and curve (c) from a beam pattern induced by the Kr excimer laser.

275
I

I

I I I I I

I I I I no 100

BINDING ENERGY (eV)

I |

I I I I

|

I I | | 110 100

BINDING ENERGY (eV)

Figure 5 XPS specrta of Si-2p emission from several points on the Si02 glass mirror surface.

Figure 6 Two dimensional images of the Si02 glass mirror surface taken with 103. 4eV Si-2p

emission of Si02 (a) and with 99eV Si-2p emission of bulk SI (b).

276
Figure 7 Two dimensional images of the crystalline Si02 mirror surface taken with 103. 4eV

Si-2p emission of Si02 (a) and with 99eV Si-2p emission of bulk Si (b).

277
Manuscript received

3-20-90

Damage Assessment and Possible

Damage Mechanisms to 1 -Meter Diameter Nova Turning Mirrors*

G. Edwards, J. Campbell, R. Wolfe, E. Lindsey

University of California

Lawrence Livermore National Laboratory

P.O. Box 5508, L-490

Livermore, California 94550

A major concern in the operation of the LLNL 120 kJ Nova laser system is damage to the

nominal 1-meter diameter mirrors (i.e. HR coatings). The damage appears to originate from

microscopic defects that vary in size up to approximately 30 urn in diameter. These

defects produce damage sites up to 250 \im and, in some cases, covering 201 or more of

the mirror surface. Defects due to particulates that are in or under the coating are highly unlikely due to the fact that coating thickness is only 5 urn thick and particles

up to 30 urn would produce surface roughness that could be easily detected.

Consequently, the defects must be plate-like (i.e. "2-D") in nature. A by-product of the

damage is a haze left on the surface of the mirror. This haze is analyzed and shown to be

blown-off particles of ZrOg/S^ coating material. The particle morphology suggests

nucleation from a vapor phase which implies localized heating in the coating to

temperatures on the order of 3000°K. Some possible damage mechanisms are also explored.

1 . Introduction /Background

Since the early 1970' s, LLNL has designed, constructed and operated a succession of

six increasingly powerful Nd:glass laser systems to support fusion energy research. The

latest of these is the Nova laser facility, a nominal 120-TW, 120-kJ system that became

operational in 1985 [1]. A continuing problem with all large, high-peak-power laser

systems is damage to key optical elements. The high reflectivity, dielectric coatings

that are used on Nova to steer the output laser beams to the target chamber are perhaps

the weakest elements (in terms of damage) on the system. In this paper we assess the

damage to these mirrors and possible damage mechanisms.

*Work performed under the auspices of the U.S. Department of Energy by Lawrence

Livermore National Laboratory under Contract No. W-7405-ENG-48.

278
The Nova HR coatings receiving the highest fluences are located at the mirror denoted

by the letters "C" and "E" in figure 1 [2]. For example, for a nominal 125 KJ, 1-ns shot,

o the average fluence on these mirrors is about 4 J/cm . Assuming a modulation of the

2

beam intensity of 1.5:1 this corresponds to peak fluences of roughly 6 J/cm .

"E" "C"

Characteristic Mirror Mirror

1. Nominal

reflectance 1064 nm 99% 96.5%

532 nm 99 50 355 nm 45 50

2. Coating Zr0

2 /Si02 ZKtySiOj

materials:

3. Layer design 1:1/2:1 1:1/2:1

4. # Layers 36 24

Schematic diagram of a The mirrors in the high f

Target room

Figure 1 mirrors.

their key characteristics are summarized in the insert.

C" and "E" and

The "C" and "E" mirrors both have the same HR coating materials. However, the "C"

mirrors serve a diagnostic function that allows some light transmi ttance and consequently

they have fewer layers.

Figure 2. Photograph of the 1.0 m mirror on Nova beamline 4 (a) before, and (b)

after four years of service on the laser system. The beam imprint is clearly seen

as the "hazy" region that is left after damage.

279
Recently an "E" mirror that had been badly damaged (fig. 2) was removed from the

system. The measured reflectivity across the mirror surface is shown in figure 3. In the most heavily damaged regions the reflectivity had dropped to roughly 66%. In addition, the mirror had a uniform "haze" layer over the area of the beam imprint. This

layer could be easily removed by wiping and contributed about 5% to the overall drop in

reflection. Microscopic inspection of undamaged areas of the HR coating (that is, areas outside of the beam path) showed a large number of "particles" with diameters up to

30 jam.

-1.0 m

Figure 3. Measured reflectivity at 1.06 vim at various locations on the mirror

surface[2]. This is for the same mirror as shown in figure 2. Wiping the haze

from the surface increases the reflectivity about 5% above the values shown. The

small diamonds indicate areas of heavy damage.

Inspections of other mirrors on Nova showed similar results although the other

mirrors do not seem to be as badly damaged. Furthermore, microscopic inspections of the

other HR coatings show that "particles" are always present in the coatings. Consequently

we assume that the assessment given here of the damage to the one of the Nova mirrors is a

fair representation of that expected for the other high fluence mirrors on Nova.

2. Analysis of Nova HR Coating Damage

2.1 Density and morphology of coating defects

Microscopic inspection of undamaged HR coating regions shows a large number of

visible defects (fig. 4a). Based on the photomicrographs of the coating, we estimate

there are about 80 to 100 per mm

2 in the 5-30 pm range. From the shape of the defects

(i.e. circular) one naturally draws the conclusion that these are small particles embedded

280
in the coating. But are they? We suggest that they are not particles simply because they

are too large relative to the coating thickness and thus would not give the observed

smooth coating surface. This point is illustrated in figure 5 where the total coating

thickness (about 5 ym) is shown relative to an embedded 30 \im particle. Note that if

many particles of the 30-yim size were embedded in the coating one should be able to

easily see them when looking at the coating surface from an oblique angle. Since the

coating surface is very smooth (less than 25-50 A rms surface roughness) then it is

assumed that the defects are not particles (i.e. "spheres") but are more nearly

two-dimensional (i.e. plate-like).

(a) (b)

Figure 4. Photomicrographs of the (a) undamaged and (b) damaged region of the Nova

mirror HR coating.

Of course the next obvious question is what are they if they are not particles? So

far that remains unknown; all our attempts to quantitatively analyze such defects have

fai led.

2.2 Correlation of damage with observed defects

Figure 4b is a photomicrograph taken of the damaged region of the mirror coating.

This is to the same scale as the photograph in figure 4a allowing a one-to-one visual

comparison of damaged and undamaged regions of the coating. It is clear that the average

damage "spot" is larger than the average defect and that in many instances the damage at

one or more adjacent sites coalesced into a larger area. It is also apparent that the

density of damage spots is greater than the density of defects. For example, the density

2 of damage spots in figure 4b is about 300 per mm or about three times the defect

density shown in figure 4a.

The fact that one does not observe any defects remaining undamaged in figure 4b leads

one to conclude that visually observed defects do indeed cause damage. However, they

don't seem to occur in sufficient numbers to account for all the damage. This latter

observation has to be tempered with two facts: first the photos are of two different

regions on the mirror surface and the defect density may have been slightly different at

281
the two sites; second, a number of "barely visible" defects (< 2 to 4 ym) can be seen in

the undamaged coating that were not included in the original (undamaged) defect-density

estimate.

(a) 3-D "particle" model

30 pm

5 pm

(b) 2-d "plate-like" defect model

5 pm 10 pm 30 pm ——

(—_- 1 5 pm

Figure 5. Schematic diagrams showing the effects of (a) spherical or (b) plate-like

defects on a nominally 5ym-thick Nova HR coating. Note the large amount of surface

roughing that would be easily visible for the case of the spherical (3-D) defects.

2.3 Source of the "haze" on the mirror surface

Analysis of the particles that produced the haze on the surface of the mirror show

they are Zr0

2 and Si02

, the same materials as the coating (fig. 6a). Thus the coating

material blown off during damage created particles of Zr0

2

/Si0

2

that litter the

damaged surface.* SEM images of the particles show that they are less than about 1 ]im

in diameter and have both jagged and smooth surface morphologies (fig. 6b). The smooth

particles probably nucleated and grew from a vapor phase whereas the jagged particles are

simply ejected solid debris, i.e. not vaporized.

One possible scenario for the damage process is shown in figure 7. Before damage,

the coating contains a number of defects that, when irradiated, heat to a temperature high

enough to vaporize the surrounding Zr0

2

/Si0

2 coating. Based on the normal boiling

points of these materials this has to be greater than 3000°K. The ejected material 1

* We analyzed a number of particles and were unable to detect other elements that might be

associated with some source of external contamination.

282
consists of a mixture of solid fragments and molten or vaporized Si0

2

and Zr0

2

- The

vapor cools, nucleates and forms either single or agglomerated particles.

Figure 6. Energy Dispersive X-ray (EDX) analysis and SEM photographs of the particle

debris removed from the damaged Nova HR coated surface.

283
Based on the extent of damage (about 20-30% of the coating area) and the original

coating thickness of about 5 y.m, one can estimate the thickness of the debris layer.

This is about 2 \im assuming a porosity of 50%.

The observation that the vaporized Si0

2

and Zr0

2 nucleates, grows and

agglomerates is not surprising. Simple thermodynamic theory [3-5] shows that growth will occur once a molecular cluster exceeds a critical radius, r , given by:

r

c = 2GV/(RTln[p/pe

]) (1)

2 3

where G is the surface energy (erg/cm ), V is the molar volume (cm /mole), R the molar

gas constant (erg/mol K) and T the temperature (K). The driving force for nucleation and

growth is the departure from equilibrium represented by the ratio of measured and

equilibrium vapor pressures, i.e. P/Pe

> Assuming Si0

2 vaporization at 3000K and

subsequent cooling to 1000K, it is easy to show that the critical cluster radius is less

than the Si0

2 molecular dimension. This unphysical result simply implies that the

system is so far from equilibrium under these conditions that nucleation occurs almost

instantaneously. The point here is that simple theory predicts that the Si0

2 (or

Zr0

2

) vaporized during damage rapidly nucleates and grows forming many of the small

particles that comprise the debris we see littered on the mirror surface. Moreover the

debris particle size distribution (< 1.0 ]im) and morphology is consistent with this

observation.

3. Damage Mechanism in the Nova HR Coatings

It is clear from the data presented above that damage is caused by localized heating

at defect sites in the coating. The question that always remains is: exactly how is the

laser energy coupled into the material? One mechanism that is generally accepted for thin

films is the so-called impurity model that simply assumes bulk absorption of the energy by

some extrinsic defect, e.g. "dirt" buried in the coating. In the case of e-beam

evaporated coatings this "dirt" model is often invoked because of the observed particulate

spatter that occurs from the source material in the evaporation crucible. A number of

papers have been published on modeling spherical impurities embedded in thin films or bulk

material and the associated analytic (or numerical) solutions to the heat transport

equations in this geometry. There is little that we can add to this literature. However,

one question that is rarely asked in relation to the impurity model is what could the

impurity be? In other words, what materials have a bulk absorption high enough to give

the temperatures required for vaporization (i.e. damage) and is it likely they would be in

the coating?

To assist in our search for such materials, we used a simple 1-D heating model in

figure 8a to help bracket the probable range of absorption coefficients. This model

assumes an absorption profile through the defect given by

-j— = exp (-ax)

o 284

(2)
(a) Before

Coating

(b) During

Laser beam

7v^<r~ Vaporized

N

\ material

*- Coating

y L y j y l y j f y / y / y / y

Fig. 7. Simple physical picture of the proposed process for damage to Nova HR coatings

(a) before, (b) during and (c) after damage.

where I Q (J/cm ) is the incident laser fluence, I is the fluence at position x (cm),

and a (cm

-1

) is the absorption coefficient of the impurity. It is straight-forward to

show that the corresponding temperature rise through the absorbing region is:

AT(x) = exp(-ax) (3)

P P

with a maximum at the front surface (x = 0):

AT(O) = -f- (4)

V

P

285
where p is the material density (g/cnr) and C is the specific heat (J/g»K). Thus

for a given laser fluence one can get a reasonable idea of the impurity absorption

required to heat the sample to the boiling point of the coating materials (i.e., Si0

2 and Zr0

2

) and then, in turn, bracket possible impurity materials that might be present.*

(a)

Absorbing

region

Light intensity

distribution

Temperature

distribution

Absorption coeff (cm 1

)

Figure 8. (a) Simple l-D model of laser heating at an absorbing impurity and (b)

predicted temperature rise at the incident surface of an impurity vs. the impurity

absorption coefficient. The calculated temperature rise is per joule/cm2 of

absorbed incident laser energy. Thus, for example, for 10 J/cm2 of absorbed

incident fluence, simply multiply the temperature rise by 10.

Before proceeding there is one thing to note about equation (4), that is, it is pulse

length independent. This is valid only if the characteristic absorption depth (i.e. 1/a)

is long compared to the characteristic thermal diffusion distance x:

x = 2( K t)"^ (5)

where k is the thermal diffusivity (cm

2 /s) and t is the laser pulse width (s). This

is a very important point because it is well known that damage thresholds generally seem

*For these "back-of-the-envelope" estimates we have neglected the enthalpy change

associated with melting (i.e. heat of fusion).

286
to scale with laser pulse width somewhere between t

* to t

' . In table 1 is a

list of common coating materials with their associated thermal physical properties;

these data are from Lang, et. al.[6] The calculated characteristic thermal transport

distance (x) for a 1-ns pulse is given in the far right column; for these dielectrics

the thermal transport is typically less than 0.1 to 0.2 pm. Consequently if the 5-1

absorption coefficients are significantly less than 1/x (i.e. < 10 cm ), the damage

threshold should scale only weakly with pulse length in the nanosecond-or-less region. As

will be shown below, even relatively strongly absorbing dielectrics have absorption

coefficients less than this at 1.06 \m. Thus, one would expect a very small

pulse-length-dependence for the damage threshold if this impurity-heating model is valid.

(In the case of the original Nova HR coatings we have found a negligible pulse length

scaling of the 1.06 ym damage threshold between 1 and 10-ns [10].

For comparison, we've also included a list of metals in table 1 with their

thermal-physical properties and characteristic diffusion depth in 1-ns. Note that for

metals the optical absorption depth (< 10 yim) is much less than the thermal

diffusion distance (~ 1 jam). Thus one would expect damage due to metal particles to

0 5

scale as approximately t

' in the ns region (for particles > 1 >im) . This is indeed

observed, as in the case of the Pt-inclusion damage in glasses [7,8].

Table 1. Summary of thermal-physical properties reported by Lange et al . for a number of crystalline dielectrics often used in optical thin films and, for comparison, several metals.

Material Index

k

(J/cm«s«K)

C

P(J/g«K)

P (g/cm

3

)

T

(melting)

(K)

T

(Boi 1 i ng)

(K)

K

(cm

2 /s)

X

(um)

I. Dielectrics

A1

20

3

1 72 0 346 0 774 4 2325 3250 0 112 0.21 Hf0

2 2 25 -0.017 0 168 8.68 3137 -5600 0 012 0.068

LaF

3

T 59 0 0508 0.462 5.94 1763 -2600 0 019 0.086

Li F 1 37 -0.017 1 61 2.635 1121 1950 0 004 0.040

MgF

2

1 43 0 116 0 394 3.176 1536 2512 0 093 0.193 MgO 1 83 0 6 0 92 3.77 3098 3870 0 173 0.263 Si0

2

1 44 0 14 0 737 2.32 1696 2500 0 082 0.181 ThF

4

1 59 0. 1 0 359 6.32 1380 -2000 0 044 0.133 Th0

2

1 90 0. 142 0 234 10 3540 4670 0 061 0.156

Y2°3 2 10 0. 0246 0 451 5.01 2683 0 on 0.066 Zr0

2 2 25 0. on 0 46 5.7 2950 4570 0 0042 0.041

II. Metals

Au 2. 97 0 131 19.3 1336 3243 1 17 0.69

Pt 0. 69 0 131 21 .45 2042 4803 0 24 0.31

Cu 3. 94 0 380 8.96 1356 2868 1 15 0.68

Ag 4. 18 0 234 10.49 1233 2483 1 70 0.83

Al 2. 04 0 896 2.71 8 42 0.58

We return now to the question of what dielectrics have absorption coefficients high

enough to cause thermal heating to ~ 2000-3000 K? Figure 8b is a plot of maximum

2

temperature rise (per J/cm absorbed) vs. absorption coefficient. Since the Nova

2 mirrors damage at roughly 7-8 J/cm then the impurity absorption would have to be

287
> 1000 cm" to give the observed thermal heating. So, what are some possibilities? One

is SiO, a suboxide of silica that is often proposed to be present in e-beam Si0o _i 2 coatings. SiO has an absorption coefficient of < 100 cm at 1 \im [9] so it would not

give the required temperature rise. Some other common dielectrics that are also strong

absorbers at 1.0 ym include FeO, Fe

2 0

3

, Cu0

2

, „Cu0, and NiO. Based on absorptions

at 1.0 ym for these materials when doped in glass, we have estimated bulk absorptions

(table 2). Note that almost all of these have absorptions greater than 1000 cm

-1

. So one could propose that the coating is contaminated by numerous inclusions of materials of

this type. But is that likely? Remember that the defect density is about 100/mm2 so there would have to be a fairly large contamination problem within the coating chamber.

Moreover, the contaminants would have to be flat-plates (not particles) to agree with the

observation that the defects are largely 2-dimensional , producing negligible surface

roughening.

This leads us to two main conclusions regarding the mechanism for damage to Nova HR

coatings. First, a damage mechanism based solely on absorbing dielectric particles (i.e.

dirt) is possible but unlikely, even though it does predict a negligible pulselength

dependence as is reported for Nova. However, it requires plate-like (2-D) particles that

have very high absorption coefficients. The possible candidates for these impurities are probably not suboxides of the coating material (e.g. SiO) but rather some external

impurities. In addition, the very high absorption values that are required eliminate all

but the most strongly absorbing dielectrics as impurities. The concentration of defects

in the Nova coatings is so high it is difficult to believe there is an outside

contamination problem of this magnitude.

Table 2. Estimated absorption coefficients at 1.0 pm for several possible dielectrics

contami nants

Material Absorption Coefficient (cm- l)

SiO < 100

FeO 5200

Fe 2°3 960

Cu

2

0 2180

CuO 3490

Ni 2640

288
Second, it is clear from the data that visible defects do lead to damage so they must

be absorbing energy by some mechanism. We believe it may be due to electron donor (or

acceptor) states lying close enough to the conduction (or valence) band to undergo

excitation by linear absorption. High densities of these states may be associated either

with the bulk or boundaries of the observed defects leading to sufficient absorption to

produce the heating observed. Thus one would expect the damage to be associated with

observed physical defects. Notice that the scale length of the region containing these

absorbing states; would need to be > 0.1 to 0.2 jam to explain the negligible pulse

length scaling that is observed. This is on the order of a single-layer coating thickness

suggesting a plate-like defect region equal to the coating thickness and up to about

30 yim in diameter.

Acknowledgments

The authors gratefully acknowledge the efforts of the late George Watchmaker (TID

Photo Department) in taking the photographs of the damaged and undamaged Nova mirror

coatings. Also, the assistance of George Greiner on several aspects of this effort, are greatly appreciated.

References :

1. H. T. Powell, J. H. Campbell, J. T. Hunt, W. H. Lowdermilk, J. R. Murray, and D. R.

Speck, "High Power Nd:Glass Lasers as Drivers for Inertial Confinement Fusion",

Lawrence Livermore National Laboratory report UCRL-99517 in Proceedings of the

Workshop on Inertial Confinement Fusion, 1988 (in press).

2. G. R. Wirtenson, "Coatings for High Energy Applications - The Nova Laser", Lawrence

Livermore National Laboratory report UCRL-99517 in Proceedings of SPIE

International Society of Optical Engineering Meeting, Jan. 19-24, 1986.

3. G. D. Ulrich, "Theory of particle formation and growth in oxide synthesis flames",

Combustion Science and Technology, 4, pp. 47-57, (1971).

4. J. Bloem, "Nucleation and growth of silicon by CVD", J. of Crystal Growth, 50, pp.

581-644 (1980).

5. A. P. George, R. D. Murley, and E. R. Place, "Formation of Ti0

2

aerosol from the

combustion supported reaction of Ti CI

4

and 0

2

", Faraday Symposium of the Chemical

Society on Fogs and Smokes, London, England, p. 63, 1973.

6. M. R. Lange, 3. Mclver, A. Guenther, Laser Induced Damage in Optical Materials: 1983,

NBS special publication 688, pg. 454.

289
7. R. Gonzales and D. Milam, "Evolution during multiple-shot irradiation of damage

surrounding isolated Pt-inclusions in phosphate laser glass", Proceedings of the

Boulder Damage Symposium, Oct. 1985, NBS special publication 746, p. 128.

8. J. Pitts, "Modeling laser damage caused by Pt-inclusions in laser glass", Proceedings

of the Boulder Damage Symposium, NBS special publication 746, p. 537.

9. Handbook of Optical Constants of Solids . E. D. Pal i k, ed; Academic Press, Inc. (1985)

p. 767.

10. D. Milam, Lawrence Livermore National Laboratory, private communication (1989).

290
Manuscript received

1-30-90

Thermal Transport Studies of Optical Coatings, Interfaces

and Surfaces by Thermal Diffusion Wave Interferometry

Randall T. Swimm and Gary Wiemokly

Center for Laser Studies

University of Southern California

Los Angeles, CA 90089-1 1 12

Thermal transport data on thin metal coatings are presented. The data illustrate

the ability of the measurement technique to characterize the thermal transport properties

of single samples. The method offers insight on coating survivability.

keywords: adhesion; coatings; damage; delamination; laser hardness; laser-induced damage; nondestructive evaluation; nuclear hardness; optical coatings; thermal conductivity;

thermal diffusivity; thermal properties; thermal transport; thin films.

Introduction

The unifying characteristic of thin-film coating studies is the need to separate the surface and

interface properties of the coating from the bulk properties of the coating. Frequently this separation

is accomplished by varying a characteristic length of the system. Methods of doing so fall into at least

two classes. The first class of measurement methods is based on varying the sample thickness. By

plotting the measured value of an appropriately chosen property of the samples as a function of sample thickness, the sum of the surface and interface contributions may be extracted as the ordinate

intercept of the data at zero thickness, and the bulk contribution may be obtained as the slope or the

reciprocal of the slope of the data. A tacit assumption of this approach is that the sample properties

are independent of coating thickness. An additional requirement is that the material properties be

repeatable among samples. Finally, the signal to noise ratio and the data reproducibility must be adequate. In practice, it may be difficult to satisfy all of these conditions, especially with regard to material properties repeatability. As a consequence, data are occasionally reported which show nonphysical results such as negative slopes or negative intercepts. Unless the substrate polishing and coating

deposition processes are under sufficient control to yield repeatable coating properties, the attempt to

obtain meaningful results by utilizing measurement techniques based on multiple samples of different

coating thickness is futile.

An alternative class of measurement methods is based on varying a characteristic length imposed by the measurement process, and obtaining separate surface, bulk and interface contributions

for a single coated sample. Such techniques may include those based on varying a spot size, the physical offset between pumped and probed regions, a wavelength or a diffusion length. The present work

falls into this latter class of measurement methods. The goal of this paper is to show that a great deal

may be learned about the thermal transport properties of a single sample.

291
Background

The measurement and analysis techniques have been described in earlier proceedings 1 -2.

Briefly, the physics of the measurement is as follows. Under steady-state conditions of sinusoidally

modulated surface heating, thermal waves generated at the sample surface diffuse through the sample

and partially reflect at abrupt discontinuities of the thermal effusivity (equal to the thermal conductivity divided by the square root of the thermal diffusivity3). Reflected thermal waves that return to

the sample surface interfere with the instantaneous thermal waves being generated at the surface,

causing the surface temperature response to be out of phase with the response that would occur in the

absence of reflected thermal waves. In the present work, a metal coating or a bare metal substrate is used to ensure the required condition that the thermal waves be generated in a depth that is small

compared to the local thermal diffusion length in order to preserve interference contrast. The thermal

waves are generated by heating the sample using a sinusoidally- modulated argon ion laser beam. The

temperature response is detected using an unmodulated helium neon laser beam reflected from the

center of the heated region, based on the fact that the metal reflectivity is temperature dependent.

The weak intensity modulation induced on the HeNe laser beam is measured using a silicon detector

and lock -in detection. The system electronic phase is separately calibrated by directing the pump

beam into the probe detector. The measured phase of the temperature response constitutes the data.

The data are modelled using the boundary value solution for the presumed sample structure, in which

the material properties are included as adjustable parameters. In the present modelling, the piecewise constant approximation is used, and the thermal contact resistance is included. A key consideration is the uniqueness of the solution. This can be inferred only by individually studying a sufficient

range of different sample types so as to exclude alternative possibilities.

The present measurement system is an upgraded version of that described in the earlier proceedings 1>2. A Coherent, Inc. model 90-6 Innova argon ion laser is used as the pump laser. A second

probe beam line has been added to allow the temperature response at the coating-to-substrate interface to be determined for samples with a transparent substrate. Various other refinements have been

added to avoid systematic errors. These include improved detector shielding, optical baffling to minimize light scattered into the detectors, and careful optical alignment to avoid stray reflections. A custom-built Lasermetrics, Inc. electro-optic modulator with reduced crystal spacing is used to avoid

thermal lensing in the index matching fluid at high pump-beam power (3 W at 514.5 nm). Data are recorded using an automated system.

Recent related work by other authors is listed in the references'*- 1 1.

Results and Discussion

Results for a 1 n.m rf sputtered Ni coating deposited on fused silica are presented in Fig. 1. The

sample was sinusoidally heated at the coating surface. The phase of the sinusoidal temperature

response is plotted as a function of the logarithm of the modulation frequency. The circles show the

response at the coating surface. The squares show the response at the coating-to-substrate interface.

The upper curve shows the results of a zero-parameter fit (i.e. no adjustable parameters), based on a one-dimensional heat flow model assuming the handbook values for thermal conductivity and thermal

diffusivity of Ni in bulk format, and also assuming a thermal contact resistance equal to zero. The

292
lower curve shows the theoretical response at the interface under identical assumptions. Several

conclusions may be drawn from the data and the model agreement.

V)

a>L

05

0) "O

in

+

0)

CD

Q.

log (f [Hz])

Fig. 1. 1 [im RF Sputtered Ni Coating on Fused Silica

First consider the near -equality of the surface and interface data at low modulation frequencies.

The phase of the temperature response at the surface and the interface is expected to be equal at

modulation frequencies for which the coating thickness is much less than the thermal diffusion length

in the coating. The observed near-equality is a Strong indicator of whether the data are due to the

temperature modulation of the coating reflectance as assumed. Because of the small values typical of

the thermal coefficient of the reflectance, numerous synchronous background signals giving rise to

systematic errors had to be eliminated before the observed agreement of surface and interface

response was attained.

Attempts to fully account for the data in terms of a one-dimensional heat flow model with a non-zero thermal contact resistance failed. However, the one-dimensional model is known to fail at frequencies for which the thermal diffusion length is not small compared to the pump spot diameter.

The tentative interpretation of the data is that the Ni coating is displaying essentially bulk-like thermal transport behavior, with negligible thermal contact resistance, and with three-dimensional heat

flow exhibited at low modulation frequencies. In order to verify the signature of three-dimensional

heat flow in the simplest possible case, a solid metal mirror was studied.

Results for a solid Mo mirror, using a 1.90 mm diameter gaussian pump spot are considered

next. The surface of the mirror was visibly oxidized. Mo is known to form sub -oxide surface layers

several hundred angstroms thick 12. The model calculation assumes no oxide layer, and uses handbook values for the Mo thermal transport parameters. The theoretical derivation follows that of

Carslaw and Jaeger^, except that the temporally modulated gaussian pump intensity distribution is used for the heat flux instead of a constant flux over a circular region as treated by Carslaw and

293
Jaeger. Consider the cylindrically symmetric heat diffusion equation for a homogeneous isotropic

medium (assuming small temperature excursions, and therefore ignoring temperature dependence of

thermal transport coefficients), for the region z 2 0

2-r 2 T

d T d T 1 8T

dr

2 dz 2

IC dt

(1)

where T equals temperature, k equals thermal diffusivity, z equals depth, r equals radial distance and

t equals time. For a solution of the form T = T e

iwt

. where T is temperature amplitude, i is the square

root of minus one and w is angular frequency, a Fourier transform yields the steady-state equation:

2„ 2„ . —d T a T i(o Y

+ —r - —T = 0. dr

2 dz 2 k

(2)

For a gaussian pump beam, Eq. (2) is to be solved subject to the following boundary conditions:

dT

- K

dz - In e (3)

z-0

where K equals thermal conductivity, Io equals intensity and ro equals the 1/e2 radius of the pump

beam. Applying a Hankel transform of order zero to Eq. (2) yields the equation:

dz 2 v.-£v.-o (4)

where

V0 = C rT(r,z)J 0 (or)dr. (5)

and o is the transform variable conjugate to r. Eq. (4) is subject to the following transformed bound

ary conditions.

- K

dz z-0

= I 0 I re

v oy

J 0(or)dr (6)

It follows by inspection that the following is a solution to Eq. (4), subject to Eq. (6) and vanishing as

-21^-

vo = v ( re

* WJ 0(or ) dr e L=L

K I / 2 io

0 + —

(7)

294
and therefore by inversion that the following is a solution to Eq. (2), subject to Eq. (3) and vanishing as

This integral may be simplified as follows 14 .

T(r,z) = (9)

Eq. (9) expresses the temperature at a general point in a semi-infinite solid with gaussian heating at

the surface. In the experiments, the temperature is measured at the center of the heated spot.

Therefore, the point of interest has coordinates r=0, z=0. Consequently, the temperature response

simplifies to the following expression (after converting the integral to dimensionless variables) 15;

where s = oVk/w, and b = (ro/2)"\/w/2K:. The phase 0 of the temperature response relative to the sinusoidal pump-beam modulation may be obtained as the arctangent of the quotient of the imaginary

component divided by the real component, as follows.

Calculations based on this expression (with no adjustable parameters) are presented in Fig. 2.

Reference to Fig. 2 shows that most of the measured phase variation is accounted for by the

model. On the basis of one-dimensional calculations, the departure of the data from the model is con295
sistent with that expected, due to the presence of the surface oxide layer. The form of the threedimensional numerical calculations also supports the hypothesis that the low-frequency departure of

the measured phase from the one-dimensional model presented in Fig. 1 is due to the onset of threedimensional heat flow.

0)

ine

log (f [Hz])

Fig. 2. Solid Mo Mirror

In an effort to obtain data free from three-dimensional heat flow effects, a metal coating of

lower thermal diffusivity was studied. Results for a 1 [im rf sputtered Ti coating deposited on fused

silica are presented in Fig. 3. The phase of the sinusoidal temperature response is plotted as a function

of the logarithm of the modulation frequency. The circles show the response at the coating surface.

The squares show the response at the coating-to-substrate interface. The upper curve shows the

results of a zero-parameter fit (i.e. no adjustable parameters), based on a one-dimensional heat flow

model assuming the handbook values for thermal conductivity and thermal diffusivity of Ti in bulk

format, and also assuming a thermal contact resistance equal to zero. The lower curve shows the

theoretical response at the interface under identical assumptions.

The near equality of the of the surface and interface data at low modulation frequencies again

confirms the absence of a large class of potential systematic errors. Note also that the low modulation

frequency data are in agreement with the one-dimensional heat flow model calculations. The gradual

increase of the phase difference between the temperature response at the surface and the interface is

expected, on the basis of thermal transit-time effects. The agreement of the experimental and theoretical differences between surface and interface response per se is fair. Unfortunately, the data show

additional, unexpected structure in the modulation-frequency range above 10 kHz. The authors originally suspected that this structure may have been due to the presence of an additional stratified layer

with an abrupt interface. Possible explanations in terms of a thermally-thick interface layer, a coating

surface oxide layer, and a sub-surface substrate polishing damage layer were all examined via modelling. However, additional measurements were performed on a number of samples, including a Ni

296
coating e-beam deposited onto a sapphire substrate, a Ti coating dc magnetron sputtered on to a fused

silica substrate that had been argon-ion beam milled after polishing to remove a graded layer two to

10

o

L.

0)

+

-25 -

-15:

-ID-

'S -

-30

0 2 3 4 5 6

log (f [Hz])

Fig. 3. 1 ^m RF Sputtered Ti Coating on Fused Silica

ten micrometers thick 16, and the Mo mirror at a finer grid of modulation frequencies. In every case,

structure was observed in the same modulation frequency interval. Since the unexplained data

structure occurs at the same modulation frequency in many dissimilar samples, it is quite likely to be

the result of a systematic error. Further work to identify the cause is under way. Notwithstanding the

presence of the unexplained structure, the tentative interpretation of the data is that the Ti coating is

displaying essentially bulk -like thermal transport behavior, with negligible thermal contact resistance

(much less than 10~ 2 K/(W/cm2 )), and with one-dimensional heat flow exhibited at all modulation

frequencies employed.

Thermal-wave interferometry can produce a wealth of thermal transport information on a single sample. Onset of three-dimensional heat flow, contributions due to thermal contact resistance, evidence of transit time effects, and the presence of unanticipated stratified layers are some of the phenomena that may be seen. As the preceding results imply, the present agreement with plausible models based on handbook transport data suggests that quantitative modelling using adjustable parameters should also be possible, provided that the physical origin of the observed data is understood. The

natural consequence of the sensitivity to the detailed transport physics that is shown is that great care must be taken while interpreting the data. However, the revealed detail ensures that essential effects are not overlooked.

Conclusion

297
Acknowledgments

Productive discussions with Alan Stewart (Battelle PNL), Kent Stowell (Weapons Laboratory)

and Patricia White (Battelle PNL) are gratefully acknowledged. This research was funded by grant

AFOSR-88-0038 from the Air Force Office of Scientific Research.

iR.T. Swimm and L. Hou, NIST Special Publication 752, Laser Induced Damage in Optical Materials:

1986 (Government Printing Office, Washington, DC, 1988), p. 251.

2r.T. Swimm, NIST Special Publication 756, Laser Induced Damage in Optical Materials: 1987

(Government Printing Office, Washington, DC, 1988), p.328.

3R.T. Swimm, Appl. Phys. Lett. 42, 955 (1983).

4

J. Baumann and R. Tilgner, J. Appl. Phys. 58, 1982 (1985).

5C.A. Paddock and G.L. Eesley, J. Appl. Phys. 60, 285 (1986).

^Akira Ono, Tetsuya Baba, Hiroyuki Funomoto and Akira Nishikawa, Jpn. J. Appl. Phys. 25, L808

(1986).

7L.J. Bortner, R.S. Newrock and D.J. Resnick, J. Appl. Phys. 61, 4452 (1987).

8E.T. Swartz and R.O. Pohl, Appl. Phys. Lett. 51, 2200 (1987).

9B.M. Clemens, G.L. Eesley and C.A. Paddock, Phys. Rev. B 37, 1085 (1988).

10H.P.R. Frederikse and XT. Ying, Appl. Opt. 27, 4672 (1988).

1 1J.C. Lambropoulos, M.R. Jolly, C.A. Amsden. S.E. Gilman, M.J. Sinicropi, D. Diakomihalis and S.D. Jacobs,

J. Appl. Phys. 66, 4230(1989).

l^Ray Bell (Lockheed, Inc.), private communication.

13h.S. Carslaw and J.C. Jaeger, in Conduction of Heat in Solids (Oxford University Press, Oxford, 1959),

Chap. XVII, p. 461.

l^M. Abramowitz and I.A. Stegun (ed.), in Handbook of Mathematical Functions. U.S. Dept. of Commerce, Nat. Bur. of Stand., Appl. Math. Series 55 (Government Printing Office, Washington, DC, 1964),

Eq. 11.4.29, p. 486.

15The exp(-o2ro 2 /8) factor in the integrand of Eq.(9) keeps the integral and its derivatives with respect to r and z well behaved as r—0 and z-»0.

16 In collaboration with Patricia White.

Erratum

A systematic error that entered during the system phase calibration was discovered in the data

presented in 12]. Consequently the data and data analysis in that paper are invalid.

298
r

Investigation of Thin Films

Using Total Internal Reflection Microscopy

F.L. Williams, C.K. Carniglia and B.J. Pond

S. Systems Corporation

Developmental Optics Facility

P.O. Box 9316

Albuquerque, NM 87119

W.K. Stowell

Weapons Laboratory

WL/ARCB

Kirtland AFB, NM 87117

The application of total internal reflection microscopy (TIRM) to the

inspection of subwavelength optical features on surfaces has been well

documented. 1-3 This paper discusses recent experiments testing the feasibility of

using TIRM to study dielectric coatings in the nucleation stage. Samples were

coated with thin films of different thicknesses to allow optical scattering levels

to be determined as a function of coating thickness. An increase was observed in

scatter levels with increasing coating thickness, and is assumed to be related to

thin film microstructure. Thus, TIRM provides a means of detecting film defects in

the early stages of coating growth, as well as identifying possible precursors to

nodules that are often observed in thin and thick films.

Key Words: surface inspection technique, thin film defects, optical scatter

1. Introduction

Total internal reflection microscopy (TIRM) has been shown to be an effective technique

for observing subwavelength features on surfaces. 1-3 Scattered light is easily detectable

from scratches and pits on an optical surface, as well as from subsurface microcracks. In addition, scattering from optical coatings can be viewed. 2 The purpose of this study is to

investigate the practicality of examining scattered light from thin films using TIRM.

The basic configuration for TIRM is illustrated in figure 1. A laser beam is directed

into a prism by means of an adjustable mirror. The beam is then coupled into an optic

through a thin layer of index matching fluid, and is incident on the top surface of the

optic at an angle slightly greater than the critical angle ec for total internal

reflection. Thus, there is no specular beam in the region above the optic. However, light

scattered by surface and subsurface defects can escape from the optic and be observed by an optical microscope focused on the surface from above. For visual observation, a laser with

several hundred mW of power is required for inspecting most surfaces.

Figure 2 illustrates a plot of the intensity (i.e., the square of the electric field) at

the surface of the optic for a beam polarized in the transverse or s direction and with an incident angle slightly greater than ec

. The standing wave present in the optic has a maximum intensity which is four times the incident intensity. The evanescent field extends

a few wavelengths into the region on the other side of the interface. Note that for this

and other intensity plots, a wavelength of 514 nm is used.

299
For a very thin film on the surface, the intensity profile is essentially unchanged, as shown in figure 3a. Here a film with a thickness of 100 A and a refractive index n = 1.85

has been added to the surface of a fused silica optic with index n s = 1.45. If the film is

thicker, the intensity profile is altered and depends on the film thickness, as illustrated

in figure 3b.

Scalar scattering theory applied to surfaces predicts that for s polarization, the total

integrated scatter is proportional to the electric-field -intensity. 4 Therefore, the scatter

as a function of film thickness may be estimated by integrating the intensity over the

thickness of the film. Figure 4 shows the estimated relative scatter as a function of film

thickness for a ZK>2 film having a refractive index of 1.85 on a silica substrate. The

incident angle is slightly greater than ec and the wavelength is 514 nm. As expected, the

scatter increases with increasing film thickness, but there are oscillations in the curve resulting from the standing-wave nature of the electric field. For very thin films

(thickness < 200 A), however, the intensity is nearly unchanged, so the scatter is

proportional to the film thickness. This calculation does not include effects due to

variations in scattering-site density within the coating, which have been observed in

dielectric films. 5 » 6

2. Experiment

Thin films of Zr02 were simultaneously deposited on 2-in-diam fused-silica substrates of

"standard-grade" (SG) surface quality and "super-polished" (SP) surface quality. All

substrates were cleaned using detergent, deionized water and organic solvents before being

loaded in the coating chamber.

Electron-beam evaporation was used to produce the thin films analyzed in this study.

The coatings were produced in an ultra-high vacuum chamber that was brought to a pressure of

~1 x 10~9 torr prior to deposition. The films were deposited at a rate of -0.2 A s _1 in an atmosphere of ~6 x 10~ 5 torr 03. Coatings havi-ng nominal uniform thicknesses of 50 A, 250

A, 500 A and 1000 A were produced. The substrates were masked so only half the surface

became coated during deposition. This allowed characterization of both the coated and

uncoated surface of each sample generated in this study.

After being coated, the substrates were removed from the vacuum chamber and analyzed by

TIRM. The TIRM apparatus was located directly under a high-efficiency particle air (HEPA)

filter to minimize surface contamination during characterization. An argon-ion laser

operating at 514 nm was used as the illumination source, and the laser was light-monitored

to produce a stable output of 500 mW. A cube polarizer was situated in the beam just prior

to the input mirror to insure that only s-polarized light illuminated the sample. A standard microscope with a camera attachment was used to produce micrographs of the surfaces

using TIRM. The exposure time for all micrographs was fixed at 30 sees.

It is important to note that it is very difficult to characterize by photography the

nature of scatter images that can be observed using TIRM. Exposure time, laser power, the

type of photographic film used and the photographic development process all have dramatic

effects on the recorded image quality. These parameters must be carefully optimized to

achieve even an approximate reproduction of the quality of TIRM images that are easily

observed by the naked eye. Consequently, TIRM is an inspection technique that must be

personally experienced to be fully appreciated.

3. Results

Figure 5a is a TIRM micrograph of an SG optic that has half of its surface coated with

-50 A of Zr02- The figure is a photograph taken near the edge of the coating such that the

left half of the micrograph is an image of the coated portion of the optic, while the right

half depicts the uncoated surface. All TIRM micrographs presented in this report were

produced in the same manner.

Figure 5b is a micrograph produced by Nomarski microscopy of the same region

characterized in figure 5a. While the TIRM micrograph of this region reveals the presence

of scattering defects, there is virtually no detail observable in the Nomarski micrograph.

300
This comparison emphasizes the sensitivity of TIRM and its utility in examining surface and

subsurface imperfections.

The samples examined in this study exhibit two general types of optical scatter

observable by TIRM:

a) "Background scatter," which appears as a form of diffuse graininess that is not

localized and occupies the entire field of the image.

b) "Point scatter," which is visible as distinct bright points of light.

Figures 6a and 6b are TIRM micrographs obtained from optics that have been coated with

-50 A of Zr02- For these and the rest of the photos presented in this report, the

micrographs designated by "a" are taken from SG optics while "b" micrographs are taken from

SP optics. In both figures 6a and 6b, it can be seen that the coated side exhibits more

point scatter than does the uncoated side. This is especially true for the SP optic, which

is practically devoid of point scatter on the uncoated side. In addition, the SP optic

displays considerably less background scatter compared to the SG optic.

These trends continue and become more pronounced as film thickness increases. In figures 7a and 7b, TIRM micrographs of SG and SP optics coated with -250 A of ZrO? are presented. Figures 8a and 8b are TIRM micrographs taken from optics coated with -500 A of

Zr02, and figures 9a and 9b show TIRM micrographs taken from optics coated with -1000 A of

Zr02- Inspection of these micrographs reveals that an increase in film thickness is

accompanied by an increase in the number and photodensity of point scatter sites in the

coated region. The SG optics also appear to generate point scatter sites at a faster rate

than SP optics.

The background scatter does not behave in the same manner. In fact, when using

s-polarized illumination, the background scatter level of the coated region generally

decreases as film thickness increases.

4. Discussion

It is a well-known phenomenon that coating defects increase in size as thin film growth

progresses. 7 These defects in dielectric fi-lms are usually in the form of nodules. 8 ' 9 The

point scatter observed in this study most likely originates from such nodules.

Several researchers have reported the result that nodules in general begin at the

substrate/film interface and grow in size as the film thickness increases

7 ~ 9 As described

above, however, the TIRM point scatter, which is presumed to be evidence of film nodules, is seen to increase with increasing film thickness. This fact appears to indicate that, while

nodule growth may begin only at the substrate/film interface, the size of nodules generated

is non-uniform. This could yield the observed result that point scatter level increases

with increasing film thickness, since nodules not detectable at the onset of film growth

could expand to detectable size as the film becomes thicker. This conclusion is supported

by Liao et al. (1988), who observed that nodules in thin films are indeed nonuniform in

size. 9

It has been theorized that nodule growth originates only from surface contaminants such

as dust, residual polishing compounds and evaporant spatter. 9 These particles act as seeds

located at the substrate/film interface. In the current study, coatings deposited on SG

optics generate point scatter sites at a greater rate compared to SP optics. Since SP

optics exhibit less surface defects compared to SG optics, we conclude that nodule

generation in this study depends on the surface quality of the optic. It is not known at

this time, however, if nodule generation is occurring directly from surface defects acting

as seeds, or if the poorer surface quality of SG optics makes them more prone to accumulate

nodule seeds in the form of surface contamination.

The dependence of background scatter on whether the surface is coated or not may be

explained by considering the intensity near the substrate/film interface. Figure 10 shows

the value of the s-polarized intensity determined at the interface as a function of film

thickness. As discussed above, the intensity is four times the incident value for an uncoated substrate. By depositing film material on the optic, however, the value of the

301
intensity at the interface is seen to decrease. Thus, there is a direct correlation between

the interface intensity and the background scatter level observed from coated surfaces

analyzed in this study. This fact leads us to conclude that the background scatter detected

by TIRM originates from substrate defects at and just interior to the optic surface. The

fact that SP optics exhibit considerably less background scatter than do SG optics supports

this finding.

5. Conclusion

Total internal reflection microscopy has been used to characterize thin films of ZrC>2

deposited on SG and SP optics. Point scatter sites are presumed to represent evidence of

thin film nodules in the coating, while background scatter most likely originates from

surface and subsurface defects in the optic. Both forms of TIRM scatter are seen to depend

on film thickness and on the surface quality of the optic. Consequently, TIRM has been

shown to be a sensitive method of surface analysis capable of detecting the effects of

coating process parameters and optical surface quality in thin films.

6. Acknowledgements

This effort was sponsored by the Weapons Laboratory, Kirtland AFB, NM, 87117, and was funded by the Air Force Office of Scientific Research.

Some of the figures were generated using a program provided by Thin Film Designer

Software.®

7. References

1) M. Hercher, "Virometer - an instrument for the measurement of the size of viruses using

an optical microscope," in Clever Optics . Proc. SPIE 126, 17 (1977).

2) P. A. Temple, "Examination of laser damage sites of transparent surfaces and films using

total internal reflection microscopy," in Laser Induced Damage in Optical Materials:

1979 , Natl. Bur. Stand. Spec. Publ . 568, 333 (1980).

3) M. Law, J. Bender and C.K. Carniglia, "Characterization of calcium fluoride optical

surfaces," in Laser Induced Damage in Optical Materials: 1986 , Natl. Inst. Stand.

Technol. Spec. Publ. 752, 532 (1988).

4) C.K. Carniglia, "Scalar scattering theory for multilayer optical coatings," Opt. Eng.

18, 104 (1979).

5) R.E. Klinger and P. Swab, "Evolution of surface roughness and scatter in evaporated

zirconia/silica multilayer coatings," in Optical Thin Films II: New Developments , Proc.

SPIE 678, 41 (1986).

6) K.H. Guenther, H.L. Gruber and H.K. Pulker, "Morphology and light scattering of

dielectric multilayer systems," Thin Solid Films 34, 363 (1976).

7) R. Messier, "Toward quantification of thin film morphology," J. Vac. Sci. Technol. A 4,

490 (1986).

8) K.H. Guenther, "Nodular defects in dielectric multilayers and thick single layers,"

Appl. Opt. 20, 1034 (1981).

9) B. Liao, D.J. Smith and B. Mclntyre, "The formation and development of nodular defects

in optical coatings," in Laser Induced Damage in Optical Materials: 1985 , Natl. Bur.

Stand. Spec. Publ. 746, 305 (1988).

302
MICROSCOPE

SAMPLE

Figure 1. Total Internal Reflection Microscopy

Figure 2. Intensity near the surface of the optic for total internal

reflection

303
I

I

Op t i c Ri r

Fi lm

(a). 50-A-thick coating

Opt i c RI r

Film

(b). 1000-A-thick coating

Figure 3. Intensity near the surface of a coated optic for different film

thicknesses
Figure 4. Estimated relative TIRM scatter from a Zr02 thin film as a function of film thickness

(a) (b)

Figure 5. Micrographs of the same surface taken by (a) TIRM and (b) Nomarski

microscopy

303
306
307
308
Manuscript received

2-12-90

Scattering Characterization of Materials in Thin Film Form

C. Amra

Laboratoire d'Opiique des Surfaces et des Couches Minces - Unite Associee au CNRS - Ecole Nationale Superieure

de Physique de Marseille - Domaine Universitaire de St Jerome - 13397 Marseille Cedex 13 - France

The "scattering characterization" that was developed in Marseilles

enables us today, with the help of complementary tools (causal model,

isotropy degree, ...), to point out the origin of micro-roughness in optical

coatings without any ambiguity. Each material can be associated to a key

parameter that characterizes its scattering. These parameters can be

determined through measurements performed on one single layer, and

then can be used to predict scattering in more complex systems.

Experimental results (IAD, IP) are presented and show that in most

cases, scattering measurement of substrate is sufficient to predict

scattering from any coating with a high accuracy. However, at very low

scattering levels (under 10*6), this prediction is limited by the presence

of local defects in the coatings.

key words : cross-correlation laws; Ion Assisted Deposition; isotropy

degree; local defects; microroughness; optical coatings; scattering; thin

films;

1. Introduction

It is well known today that if we want to establish an accurate energy balance

in multilayer optical coatings, we have to take account of numerous factors that arc connected more or less directly with the microstructure of materials in thin film

form [1, 2, 3]. We will be interested here in surface scattering, and will say first

that, at low loss levels, it is not easy to separate the effects of intrinsic absorption A

(connected with extinction coefficients of materials) from the effects of scattering

(due to roughness). Indeed, according to the method which is involved to measure

the absorption (spectrophotometric measurements [4] or photodeflection technique

[5] ...), we will have respectively access to intrinsic absorption plus external

scattering (which is the quantity of scattered light that is measurable between 0°

and 90°), or to intrinsic absorption plus other losses connected with internal

scattering (which is the quantity of scattered light embedded inside the coating). For

instance, if we consider a half-wave layer made of a material which extinction

coefficient is 10~ 4

, a roughness of 1.8 nm at the two interfaces will give rise to more

309
scattering than absorption, and therefore we will have access to an apparent

absorption A

1 which is in fact equal to the scattering D (A* = A + D = D). The

consequence is that in some cases, we will observe unexpected variations of

absorption versus wavelength [6], since we know that scattering can vary in phase

or in phase cancellation with reflection, according to the correlation between

interfaces [7, 8].

The distribution of scattered light from an optical multilayer obviously

depends on the design of the coating, but also on the roughness of each interface of

the stack and on cross-correlation laws between these roughnesses [9, 10]. It is then possible, provided that we use adequate experimental and theoretical tools, to

extract from scattering measurements numerous information (two dimensional

roughness spectrum and autocorrelation function, isotropy degree variations ...) concerning each interface defect. This information is closely connected with

material microstructure (at the scale of optical frequencies), and therefore

scattering investigation appears to be an accurate method to characterize materials

in thin film form.

The "scattering characterization" that was developed in Marseilles enables us

today, with the help of complementary tools (causal model [8] for cross-correlation

laws, isotropy degree variations [11], ...) to point out the origin of micro-roughness

in optical coatings without any ambiguity relative to the choice of the parameters

(roughness and cross-correlation coefficients). Each material can be associated to a key parameter that characterizes its scattering. These parameters can be determined

through measurements performed on a single layer, and then can be used to predict

scattering in more complex systems.

We present here our methods of investigation and give the results for

coatings produced by Ion Assisted Deposition (IAD) [12] and Ion Plating [13] (TP).

2. Cross-correlation laws

2.1. Some recalls about the theory

According to the vector theories [9, 10], the scattered light from an optical

coating can be written as:

where

BRDF.cos0(9,0) is the scattered flux per unit of surface and solid angle,

normalized to the incident flux.

- 0 and 0 are the angles that characterize a scattering direction (0 from the

sample normal, 0 polar angle).

(1)

i.J
- The ideal coefficients Q [10] depend only on the design of the coating, and

on illumination and observation conditions. They take account of the magnitude of

the scattering sources at each rough interface, as well as of the multiple reflections

of each scattered wave inside the multilayer.

- Yjj is the roughness spectrum of interface j and contains all information

relative to the defects of this surface: it is the Fourier Transform of the

autocorrelation function of the profile of surface (j). Its integral over the range of

measurable spatial frequencies gives the square of the roughness 8j of this surface.

- ocij is the cross-correlation coefficient between interfaces (i) and (j), and

takes account of interference phenomena between the waves scattered by these two

surfaces. It can be seen as a degree of coherence between the two scattering sources

(i) and (j), or as a degree of similitude between the two interface profiles (i) and (j).

Usually the roughness spectrum is taken as the sum of the Hankel Transform

(for isotropic surfaces) of a gaussian and an exponential function [11], which

requires 4 parameters. Since most of the time cross-correlation coefficients and

roughnesses are considered as independent parameters, therefore prediction of

scattering from a p-layer stack requires the knowledge of 5p+4 parameters. In these

conditions, there are always several solutions to fit the experimental results, which

explains the limited number of non-ambiguous conclusions that can be found in the

literature [7, 14, 15]. For instance, even in the case of a single layer, one can

explain the distribution of scattered light (in the whole angular range) by

considering correlated or uncorrelated surfaces (fig. 1).

Now we will show how the use of a causal model together with isotropy

variations of scattering enables us to eliminate any ambiguity relative to the choice

of the parameters.

2.2. A causal model

Correlation is certainly the key parameter of scattering since it enables

destructive interferences to occur between the scattering sources inside the stack

(provided that the design is adequate), which permits reduction of scattering losses.

If we have uncorrelated roughnesses for instance, scattering will practically always

increase with the number of layers of the stack. On the contrary case (perfectly

correlated roughnesses), scattering can strongly be reduced after coating

(antiscattering effect [15]). In a more general way, scattering in narrow-band

filters will vary by a factor 50 according to the value of correlation between

interfaces (with a high sensitivity to these parameters).

Usually the correlation is considered to be a real constant number between 0

and 1. In fact we have to take account of a more realistic model and write that the

profiles of interfaces (i) and (i-1) are connected -by the following relationship:

hj = a^.i * h i_ i + gi

where (*) is a convolution product. This means that the defects of interface (i) are due to two different phenomena, that arc reproduction (perfect or partial) of

311
interface (i-1) defects and addition of residual roughness that characterizes the

grain size of the material in thin film form. Therefore the "filtering" function ajj.i

describes the action of the material at the interface, and gi is due to the material

microstructure (notice that these two functions can be connected).

The question is to know whether one or the other term will be predominant in

scattering phenomena; for this we will first consider coatings produced on rather

rough substrates (of the order of 1.5 nm) so that we can be sure that residual

roughness is negligible compared to reproduction of substrate defects (experiment

will prove this situation). In these conditions correlation is the Fourier Transform

of the filtering function and varies with spatial frequency. Consequently the

parameters are no more independent, since cross-correlation laws and roughness

spectra are connected by:

Yii = I ay | 2

. Yjj

Therefore, and provided that the substrate is measured before coating, prediction of

scattering from a p-layer stack (with two H and L materials) requires only the

knowledge of two functions cch(ct) and ocl(ct) [a is the spatial frequency]. Indeed, if we look at expression (1), we can notice that:

- The ideal coefficients Ci are given by the theory when the design of the

coating is known.

- The roughness spectrum yjj of interface (j) is given as a function of the

roughness spectrum ys of the substrate, provided that correlation is known:

Yjj = f (Ys)

- The roughness spectrum ys of the substrate can be measured before coating.

- The correlation ajj is given as a function of the successive cross-correlation

coefficients otk,k-i:

i-j-1

aij = IT oci-k,i-k-l (j < 0

k=0

Since we can assume that the same material will have the same effect on interfaces

whatever the layer of the stack, we can write: oc2k,2k-l = O-L and a 2k-l,2k-2 = och»

where an(o) and aj^(a) are relative to the high and low index materials that

constitute the multilayer.

Moreover, we must not forget that with an incident wavelength of 0.6 u.m ,

scattering in the direction 0 = 5° is due to the presence, on the rough interfaces, of

sinusoidal gratings of period C = 7 (im. It is then very probable that, whatever the

material microstructure, the layer will perfectly reproduce a grating of period

nearly one hundred times its thickness (for visible applications)(fig.2). For this

reason, we will assume that correlation is perfect at least for large period defects

(that is, to explain scattering at small angles), and that it decreases with decreasing

period defects. Therefore the materials act as low-pass filters, and the filtering

functions an and ccl can be characterized with only one parameter which is a cutoff frequency. Briefly, scattering from a p-layer stack can be predicted with only

two cut-off frequencies (oh and ox) if the substrate is measured before coating.

312
BRDF.CQS (8)

\ 01 = 0 and 50/5i = 0.55

\ d = 0.5 and 50/5i = 1 /

9 (Deg .)

30 60 90 120 150 180

Figure 1:

Scattering from a single

XqII high index layer. The

angular distribution can be

explained in the whole range

(0° -> 180°) assuming one or

the other of the following cases:

- The two roughnesses are identical and correlation is

partial, that is: 8 0 /5i = 1 and ocio = 0.5.

The roughnesses are

different and correlation is zero,

that is: 80 /5] = 0.55 and ocio = 0.

Note that Xq is the wavelength

of incident light.

For 9 = 5°

Figure 2:

Schematic representation

of the ratio C(9)/e, where C(6)

is the grating period

responsible for scattering in

the direction 0 . The layer

thickness is e. For 9 = 60 c

10

10

10

10

10

10

10

10

BRDF cos (6)

c = 1 ^m

" 1 G

e(deg.)

Figure 3:

Influence of cut-off

frequency o c on the angular

scattering curve of a single

layer.

30 60 90 120 150 180

313
These parameters can be determined through measurements performed on single

layers, and then can be used to predict scattering from more complex optical

systems.

Fig.3 shows how the angular distribution of scattering is modified when the

cut-off frequency is varied. The case where a c

is infinite is that of perfect

correlation for all period defects (no cut-off frequency).

Experimental results are in complete agreement with this causal model (see

section IV) in the case of coatings produced on rather rough substrates (1.5 nm). In

a second stage, we will have to decrease the substrate roughness in order to see whether this model remains valid or not (negligible residual roughness?). But in

order to obtain a very detailed investigation and eliminate any ambiguity on the

origin of micro-roughness, we will first present a complementary tool that is

isotropy properties of scattering.

Notice here that we have worked with classical quarter-wave coatings (all

layers of same material have identical thicknesses), for which reason we did not

investigate dependence of correlation with thickness.

3. Isotropy variations of scattering

With the preceding causal model, we have seen that the substrate is

privilegied since its defects are responsible for the roughness of each interface. In

fact, many substrates present privilegied directions that give rise to non-isotropic

scattering, which can be pointed out through level maps (fig. 4). This information

can be very helpful for investigation of microroughness.

3.1. Uncoated surfaces

Instead of limiting ourselves to the qualitative view of fig. 4, we can define a quantitative value of anisotropy through considerations on the angular

autocorrelation. Starting from the measurements of scattering in whole space, wc

can extract the bidimensional roughness spectrum and calculate the normalized

angular autocorrelation F(a) of Autocorrelation function of surface defects. In the

case of an isotropic surface, the angular function F(a) is constant and we say that

the isotropy degree is equal to unity. In the contrary case (fig. 5), F(ot) is first

decreasing, and we define the isotropy degree as the minimum of the curve. In

other words, isotropy degree is analogous to autocorrelation length but from a point

of view of rotation.

If we want to obtain more information and work with each spatial frequency,

we can perform the same calculation for each period defect and plot the variations

of isotropy degree with spatial frequency (or scattering angle) (Fig. 6). For this,

the surface profile is seen as an infinite sum of sinusoidal gratings, and we calculate

the isotropy for all gratings that have same period but different orientations. This is

a real advantage against the preceding angular function F(a) which integrates all

314
Figure 4:

Level maps of scattering from an isotropic (A) and a non isotropic (B)

substrate. Departure from perfect circles is due to the presence of privilegied

directions on the surface.

F(oc)/F(0)

Figure 5:

Angular function F(oc) calculated from the measurements of

scattering. Notice the perfect

symmetry of the curve at 180°. The

isotropy degree is near 0.6 in this case. The presence of local extrema

is due to the existence of several

directions on the surface.

Isotropy degree

l.o

Figure 6:

Variations of isotropy

degree versus spatial

frequency a (or scattering

angle). Large period defects

show a more pronounced

anisotropy.

.60 .

.20

0

6 (Deg.)

30 60 90

315
period defects simultaneously (as an analogy, the roughness spectrum gives more

detailed information than only the roughness).

3.2. Optical coatings

Isotropy variations of scattering is a real signature of the substrate roughness

and can be used to investigate the origin of microroughness in optical coatings. In

fact, the preceding angular function F(a) cannot be directly used here since it is

relative to the defects of only one surface. For this reason, we use a different

angular function F'(cc) related to scattering and not to roughness spectrum.

Therefore the method of investigation is the following: starting from a substrate

which scattering has been measured in whole space, we extract the bidimensional

roughness spectrum and therefore we calculate (using the causal model) the

distribution of scattered light in whole space from any coating produced on this

substrate. In this way, we have access to the angular function F'(oc) which is relative

to the predicted scattering. At the same time, the coating is produced and its

scattering is measured in whole space, so that we can compare directly the

calculated and measured angular functions F'(a).

The use of isotropy degree variations is more convenient since we can show

that it is independent of the design of the coating, provided that these coatings are produced under quasi-normal incidence deposition [8]. This means that, due to the

fact that all functions are normalized, the isotropy degree variations calculated with

the substrate scattering are identical to those calculated with the scattering of the

coating (assuming that residual roughness is negligible). This presents two real

advantages: first, it is not necessary to calculate the scattering from the coating

(measured isotropy degree variations can be directly compared before and after

coating and must be the same if residual roughness is negligible, whatever the

values of cut-off frequencies). Secondly, since isotropy variations are independent

of the design of the coating, the comparison of the curves before and after coating

is valid even if there are errors on the layer thicknesses. As a summary, we will say

that isotropy of scattering is a precious complementary tool, since any difference

observed between the curves measured before and after coating will point out the

limit of the causal model. This tool will be very useful to investigate the

experimental results of section IV.

As for a remark, we will also say that this tool permits an eventual columnar

growth of materials to be detected [8]. Indeed, for a coating produced at oblique

incidence deposition, the substrate defects are reproduced with a lateral shift, which

modifies the interference phenomena between the scattered waves (in this case, the

correlation has a phase term). The consequence is that the isotropy variations will

be modified and can therefore be easily detected (fig. 7): even if all surfaces of the

stack are isotropic, scattering will not be isotropic.

316
4. Application to experiment

Experimental results are relative here to Ti02 and Si02 materials obtained with

IAD. We will not present the results for Ta205 and Si02 materials obtained by I. P.,

since the conclusions are exactly the same (no difference between these two

techniques and materials for the scattering behavior).

4. 1 Classical stacks

Fig. 8 shows that the agreement between calculation and measurements is

excellent for a single layer, and this in the whole range of angular scattering. The

predicted angular scattering curve was calculated with the measured roughness

spectrum of the substrate (using the causal model), and the superimposition to

experiment is obtained for an infinite cut-off frequency (no cut-off frequency):

this means that we can explain the experimental results assuming a perfect

correlation for all period defects.

This is first confirmed using the angular autocorrelations of fig. 9, since

there is no difference between calculation and measurements. At last, if we observe

the variations of isotropy degree before and after coating (fig. 10), we have

perfect agreement between the two curves at low scattering angles (large period

defects) and a slight difference at large angles. It is not obvious that such difference

is due to the limit of the causal model because, at this stage of accuracy, we could

implicate some errors due to the linearity of the detector (notice that we have a

linear scale). Anyway, these three curves (fig. 8, 9, 10) prove that correlation is

quasi-perfect in the whole range of measurable spatial frequencies and that

residual roughness can be neglected here.

Fig. 11 and 12 show that these conclusions remain valid in the case of

multilayer stacks (19-layer mirror and 9-layer Fabry-Perot filters). Therefore

measurement of substrate is sufficient to predict scattering from these coatings with

a high accuracy.

4.2. Specific stacks

The preceding coatings were produced on rather rough substrates (near 1.5

nm). Now we can wonder whether the causal model remains valid or not in the case

of very low roughnesses; in other words, will the reproduction of substrate defects

be predominant if we use highly polished substrates? Moreover, we could say that

some coatings are more or less sensitive to the correlation (depending on their

design), and that it is the reason why we do not detect shifts for perfect correlation

at large scattering angles. In order to solve these two problems, we have produced

specific stacks that are very sensitive to correlation [16] (a variation of correlation

of 1% near unity increases the total scattering losses by a factor two), and these

stacks were produced on very different substrate qualities (1.5 nm, 0.5 nm, 0.1 nm -

317
Figure 7:

Variations of isotropy degree

calculated for a layer produced at

normal (P = 0) and oblique

incidence deposition ((3 = 20°). The

calculation is performed using the

causal model together with the

measurements of a non isotropic

substrate.

BRDF . cos (8

)

9(Deg.)

Figure 8:

Calculation and

measurements of angular

scattering curve of a single

6Xo/4 layer of SiC>2

produced by I.A.D.

Isotropy degree

0 30 60 90

F'(a)/F(0)

0.2

0 180 360

OC (Deg.)

Figure 9:

Calculation and measurement of

angular function F'(oc) relative to the

scattering from a (6L) Si02 layer.

Isotropy degree After

^Before

—i 1 9 (Deg.)

0 30 60 90

Figure 10:

Isotropy degree variations

before and after coating by a (6L)

Si02 layer.

318
the case of 0.1 nm is relative to a silicon wafer substrate). For these coatings, the

theory predicts that the two angular scattering curves measured before and after

coating must be the same if correlation is perfect in the whole range of scattering.

The results are presented in fig. 13, and must be discussed now.

Whatever the substrate roughness, we can first notice that for the three

coatings (Fig. 13), the curves before and after coating are superimposed at low

scattering angles, which proves that correlation is perfect at least for large period

defects. Concerning the disagreement observed at large angles, this could be

explained a priori by involving scattering due to residual roughness or even to the

volume of materials, but in these cases the scattering would be isotropic for these

large angles. In fact, for fig. 13a and 13b, we can verify that the isotropy degree

is very similar before and after coating (and different of unity) whatever the

scattering angle, which proves that the disagreement is not due to residual

roughness or bulk scattering. On the other hand, we demonstrate that the

disagreement is due to a 1.5% error on the thickness of the layers (fig. 14), which

is indeed the accuracy limit obtained with optical monitoring (for this stack). This

error thickness was completely verified using spectrophotometric measurements

[4] and guided wave techniques [17]: thickness given by scattering measurements

is as accurate as thickness given by these two techniques. This accuracy is due to the

fact that scattered light can see the layer thickness at all directions.

However, such reason cannot explain the disagreement observed in fig. 13c.

In this case (0.1 nm substrate roughness), scattering levels are so low that the

scattering due to local defects in the layers becomes predominant (compared to the

scattering due to reproduction of substrate roughness). Usually, such presence of

pits in the coating (that can be seen with Nomarsky microscope) has no influence

on the scattering if the BRDF.cos 0 is greater than 10"^, which is not the case for the

stack produced on the silicon wafer substrate. Nomarsky microscope reveals the

presence of one pit (diameter lu.m) in a 350 u.m diameter microscope field: in these

conditions we have approximately 100 pits in the illuminated region (diameter 0=2

mm) of the sample. Fig. 15 shows how the scattering from local defects can be

quantified on a substrate. Unfortunately it is very difficult, even if we start from a

perfectly clean substrate, to eliminate the presence of local defects in the volume of

the layers: the presence of pits occurs during deposition, and their number

increases with deposition time (or number of layers).

As a summary, we will say that reproduction of substrate defects is

predominant for a substrate roughness of 1.5 nm or 0.5 nm. For lower substrate

roughnesses, an eventual residual roughness (due to the grain size of materials) is

not easy to detect since the scattering is due to local defects in the volume of the

layers.

319
BRDF COS (9)

2.10"

10 20 30 40

THETA

50 60 70 80 90

Figure 12:

Calculation and measurements of

the angular scattering curve from a 9-

layer Fabry-Perot filter (— = 0).

oc

Figure 11:

Calculation and measurement of

the angular scattering curve from a 19-

layer quarterwave mirror produced by

I.A.D. Materials are SiC>2 and TiC>2.

Calculation is performed with an infinite

cut-off frequency (— = 0) for both

materials.

BRDF COS (9)

10"

10"

10"

2.10"

10 20 30 40 50 60 70 80 90

10 -2

BRDF COS (9)

10

10

10

10

-3

10

10

-7 Before

30 90

9 (Deg.)

BRDF COS (9)

10

10

10

10

10

-7

30 60 90

9 (Deg.)

BRDF COS (9)

10

10

10 -5

10

10

10

-6

7

Before

30 60

B

90

9 (Deg.)

Figure 13:

Angular scattering curves before and

after coating by an absentee layer stack (2H

2L 2H) produced on a substrate which

roughness is 1.5 nm (A), 0.5 nm (B) and 0.1

nm (C).

320
BRDF.cos (6)

Figure 14:

Influence of thickness errors on the

angular scattering curve of a single layer.

90

6 (Peg.)

Figure 15:

Scattering from a perfectly clean

silicon wafer substrate and from the same

substrate with one defect (1 |im diameter)

in a 350-u.m diameter microscope field.

10

10

10

10

10

10

10

-5

BRDF.cos (0)

30 60 90

0(Deg.)

5. Conclusion

With materials such as Ti02 and Si02 obtained by IAD, as well as Ta205 and Si02

obtained by IP, scattering phenomena in optical coatings are well understood and

measurements of the substrate is enough to predict scattering from a coating of any

design with high accuracy. Residual roughness due to the materials appears to be

negligible and therefore correlation is perfect in the whole range of measurable

spatial frequencies (0.6 U-m" 1 -> 20 p.m

_1

).

Isotropy variations of scattering constitute a precious tool to study the origin

of microroughness in optical coatings, and leads to elimination of any ambiguity

relative to the choice of the parameters (roughness and cross-corelation laws). The

conjoint use of isotropy and causal model gives also access to the thickness of the

layer.

Investigation of residual roughness requires the use of substrates with

roughnesses near 0.1 nm or less. But in these conditions, scattering levels are so low

that scattering due to local defects in the coatings (depending on materials and

deposition technique) becomes predominant.

321
References

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(1982)

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inhomogeneity and film structure", Thin Solid Films, 57, 173 (1979)

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Assisted Deposition techniques: study of uniformity," Applied Optics, 28, 2952-

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322
COMMENTS

Comment: We shouldn't forget Alex Glass's comment that whatever turns you

off is the definition of laser damage.

Comment: I would like to make a remark concerning damage thresholds. I think what we should, in any case possible, measure predamage

phenomena. They should finally lead up to damage and we should

measure them as a function of photon flux or whatever unit you

want to use that gives not only the appreciation of what the

mechanism is, but will also help to clearly define what the damage

threshold means at that point at which one observes some changes.

Secondly, I would want to warn people about using high energy

electrons particularly in scanning electron microscopy to search

for damage on surfaces. This pertains mostly to halides. You

will create primarily defects starting with excitons which have a severe photochemical reaction following and you will simply alter

the surface with an electron beam of kinetic energy larger than

the bandgap, this process is usually extremely efficient. So you

will damage your surface by looking for damage

Answer: Thank you. I think that shows up a good point here. There is the

dichotomy of bifurcation of science and engineering, in regard to

laser damage. The engineering definition is whatever turns you

off, or we need to use the part until it is broke, or can I deliver hardware with these optics. However, a threshold needs to

be defined through science and I think that fits nicely with your

comments

Comment: I do really hope that our co-workers from China do show up here

because on a recent visit there, earlier this year, where they were using photothermal deflection in a different way then I have

observed any place else, I think I observed for the first time a 100% reliable indicator of precatastrophic damage.

323
Manuscript received

12-29-89

Optical Properties and Laser Damage Measurements of Inorganic Polymer Films

Gregory J. ExarhosPacific Northwest Laboratory

Richland, WA 99352

Kevin M. Crosby

Bel oi t College

Beloit, WI 53511

Linear phosphorus-nitrogen based polymers have been synthesized and deposited

as thin films from solution on silica or silicon substrates using dip-coating or spin-casting methods. Most materials transmit well into the ultraviolet region of

the spectrum and have refractive indices (1.5 to 1.7 at 500 nm) which are controlled by the functionality of chemical substituents introduced at the

phosphorus atom. Films exposed to pulsed laser irradiation (1064 nm , 8ns

pulsewidth) exhibit damage morphologies ranging from severe cratering to

delamination. The chloro-substituted polymer shows a spheroidal surface morphology

after exposure to a 25 J/cm2 pulse suggesting melting or condensation of ablated

material. The relative stability of these materials to laser irradiation will be

discussed in terms of chemical bond ionicity which is influenced by atom

electronegativity and the nature of substituent groups. Preliminary measurements

of the non-linear optical response observed in several of these materials (SHG)

will also be discussed.

Key Words: inorganic polymers; laser damage; optical polymers; phosphazene; second

harmonic generation; thin film.

1.0 Introduction

Fundamental studies of composition/structure/property relationships in linear highmolecular-weight polymers which contain inorganic atoms in their backbone chains can lead to

development of new materials for optical coating applications. Such inorganic polymers as the isoelectronic and isostructural polysiloxanes, -[0-SiR2]xi and polyphosphazenes, - [N=PR2]xi where R signifies a particular chemical functional group, offer improved physical

properties over fully organic based polymer materials. This is in part due to the presence

of electronegative atoms such as nitrogen or oxygen in the backbone which tends to perturb

chemical bonding along the chain. Alcock discusses some of the new inorganic polymer systems

and current materials applications in a recent review article [1]. These range from new elastomers which do not embrittle at low temperatures used for pipeline applications, and

fast ion conducting polymers for solid state batteries, to bioactive polymers designed as drug delivery systems. While many of these materials can be deposited as thin films, their

optical properties are not well known.

A number of factors suggest that phosphazene polymers may be attractive alternatives for

optical coating applications. Polyphosphazenes exhibit good solubility properties in both

hydrophilic and hydrophobic solvents depending on the nature of the substitutional group on the phosphorus atom. This not only facilitates sample purification and solution deposition

of thin films, but simplifies polymer derivitization. Since many of these materials exhibit

enhanced thermal stability over organic polymer analogs, their tendency to degrade under high

laser fluences may also be proportionally higher.

Chemical bonding along the backbone chain involves overlap between the phosphorus d and

nitrogen p electronic orbitals. As a result of the symmetries of the participating orbitals

and the high electronegativity of nitrogen, electron density is localized at the nitrogen

atom centers. Electron delocal ization along the polyphosphazene chain is frustrated, in

contrast to the organic polyacetylene system, and the polymer becomes a wide bandgap

insulator. A zwitterionic model has been proposed to describe the chemical bonding in this

system [2]. Two consequences of this bonding model are apparent: phosphazene polymers should

324
exhibit good transmission properties in the ultraviolet region of the spectrum; and the chain

nitrogen atom basicity should promote attachment of phosphazene polymers to hydroxylated

surfaces through a hydrogen bonding mechanism. The latter prediction has been verified by

3 lp magnetic resonance measurements of phosphazene molecules adsorbed on colloidal alumina

surfaces [3]. In addition, the pol arizabi 1 ity of the -P=N- bond is controlled by the nature

of chemical groups substituted on the phosphorus atom [4]. This suggests the possibility for

non-linear optical effects in these materials driven by the electron withdrawing power of the

substitutional group.

Recent work in this laboratory has been directed toward the design, synthesis, and

characterization of phosphazene polymers, and the development of techniques for solution

deposition of polymer films. The real and imaginary parts of the refractive index for

several polyphosphazene films deposited on silica substrates have been determined from

measured transmission spectra. Polymer films exposed to high fluence pulsed irradiation or high intensity CW irradiation exhibit catastrophic damage morphologies characterized by severe cratering, film delamination, and melting. Results for five polymer systems will be

discussed. Initial observations of non-linear optical behavior in several systems will also

be presented.

2.0 Synthesis of Phosphazene Polymers

Preparation of phosphazene-based polymers and cyclic ring polymer precursors has been an active area of research for the past twenty five years. Principal work has focused on development of new synthetic methods, characterization of chemical bonding, and kinetic

measurements of the polymerization reaction [5]. Synthetic routes for the polymerization of

phosphazenes contrast procedures used for the preparation of fully organic based polymers.

Figure 1 illustrates the ring-opening polymerization mechanism commonly used to prepare high

molecular weight inorganic polymers. Usual precursors include the cyclic bisdichlorophosphazene trimer or tetramer. At moderate temperatures, 500 K, an equilibrium

between ring and chain moieties is established in the melt. Upon cooling, the solidified

mixture can be separated into high molecular weight polymers and a low molecular weight

fraction which consists of short oligomers and small cyclic rings.

INORGANIC POLYMER THIN FILM

ci .ci

CI

N'

II

(III, D

i CI N

.CI

CI

heat

CI

I ..' P= N

ICI -1

REACTIVE

POLYMER

-R

P= NI R

-R , -Ar

-R =|-OR,-OAr

-NHR,-NHAr

Figure 1. Thermally induced polymerization route to the synthesis of phosphazene polymers.

Owing to the reactivity of the phosphorus-chlorine bond and the ease of dissolution of

the polymer in non-polar solvents, nucleophilic substitution reactions readily occur. This

allows preparation of well characterized derivatives in good yield. The nature of the

substituted chemical group determines the physical properties of these polymers. Amine and

alkoxy derivatives of poly bis-dichlorophosphazene have been prepared and can be deposited as thin films from solution. The five materials chosen for this investigation are listed in

table 1. 325
Table 1. PHOSPHAZENE POLYMERS, -[N=PR2] X -. AND SOLVENT COMPATIBILITY

Name Chemical Formula

of R Group

Designation

Chlorophosphazene

(toluene, tetrahydrofuran)

Methyl ami nophosphazene

(water, trifluoroethanol)

Phenyl ami nophosphazene

(tetrahydrofuran)

Pi peri di nophosphazene

(chloroform, benzene)

Tri f 1 uoroethoxyphosphazene

(acetone)

-CI

-N(CH3 ) 2

-NHC6H 5

-NC5H10

-OCH2CF3

CP

MAP

PAP

PP

TFEP

3.0 Experimental

Polymers were prepared by nucleophilic substitution of poly bis-dichlorophosphazene

using published methods [5]. The initial chloro-polymer was purified by sublimation of lower

molecular weight species followed by successive recrystallization. Derivatized materials were also subjected to multiple recrystal 1 izations from appropriate solvents to insure

highest purity as determined from High Performance Liquid Chromatography (HPLC) and 31p

magnetic resonance measurements. All materials studied were fully substituted high molecular

weight homopolymers as indicated by a single 3 1 p line in the magnetic resonance spectrum, and were thermally stable to 580 K.

Films were deposited onto cleaned silica substrates using dip-coating or spin casting

methods. Polymer solutions were filtered through 0.2 micrometer Gelman f luoropolymer

membranes prior to use. Substrate surfaces were cleaned and hydroxylated by treatment with

NaOH/methanol followed by a hydrochloric acid and deionized water rinse to remove any sodium

chloride that might be present on the surface. The dip-coating procedure involved immersing

cleaned substrates in the appropriate polymer solution for 30 seconds and then withdrawing

the substrate from solution at a rate of 1 mm/sec. Spin coated films were prepared using an Integrated Technologies Inc. Model P6204 system. Spin parameters included a 2 sec acceleration time followed by a hold at 4000 rpm for 150 seconds and a 4 second deceleration

time. Polymer concentrations averaged approximately 0.3 normal for deposition of 1 micrometer thick films.

Transmission spectra were recorded over the wavelength range from 190 to 900 nm. Analysis of measured spectra using the methods of Manifacier, et.al. [6] allowed

determination of film thickness and refractive index.

Films were subjected to pulsed laser radiation from a Quantel Model 581C Nd:YAG laser

(1064 nm, 8 nsec pulsewidth, 0.65 J/pulse, 30 Hz). A 50 mm diameter 700 mm focal length

laser focusing singlet (Melles Griot 01LFS055) optimized for 1064 nm radiation was used to

image the beam onto the sample. Spot sizes on the polymer coatings ranged from 1 mm to 5 mm

in diameter. Following irradiation, samples were examined using an optical microscope. The

effect of CW irradiation on polymer coatings was also evaluated using a Raman microprobe

(Spex Model 1482). A 488 nm probe beam from an argon ion laser at power levels up to 2 watts was directed through the microscope optics and imaged directly onto the coating. Spot sizes

averaged 5 micrometers in diameter and irradiations were conducted for fixed time intervals.

The observation of green emission during polymer film irradiation with 1064 nm pulsed

irradiation prompted preliminary measurements of non-linear effects in these materials.

Several phosphazene polymers and polymer precursors were mechanically powdered (5 micrometer

particle size), compressed to 2.0 cm diameter pellets under a pressure of 0.1 GPa, and

evaluated for their ability to generate second harmonic light. The powder technique

originally developed by Kurtz and Perry [7] was used. This method has been successful in the

326
evaluation of nonlinear optical properties of organic polymer systems [8]. A 0.25 m focal

length monochromator and diode array detector replaced the photomultipl ier and optical

filters used in the original experiment.

4.0 Optical Properties of Polyphosphazene Films

The optical transmission spectra of several phosphazene polymers deposited on cleaned

silica substrates are shown in figure 2. Most of these materials, even when exposed to the

ambient laboratory atmosphere for one month, exhibited negligible change in measured optical

transmission. However, the chloro-substituted polymer film began to degrade after a period

of two weeks due to reaction with oxygen and water vapor in the atmosphere. This instability

is attributed to the high reactivity of the phosphorus-chlorine bond resulting in hydroxyl

addition to the chain and eventual crossl inking of the network. While the dimethylamine

polymer is water soluble, the other materials are all hydrophobic, and show no measurable

reactivity with water. Optical measurements of dried films following immersion in water

reveal no changes in measured transmission spectra.

r--

CTi

R f

—i

1 1

V

A

* k /V

f1

1

TFEP

i i \3Z nm 8 2jar i9@ 82Q

~i r

PAP

J u

1 9 0 1 9 0

Figure 2. Transmission spectra of several phosphazene polymers deposited on cleaned silica

substrates.

The fringe patterns observed in the transmission spectra attest to the uniformity of

film thickness over the 1 cm2 measurement area. Four polymer films, (CP, MAP, PP, TFEP)

transmit significantly at wavelengths as short as 250 nm, while PAP begins to absorb strongly

at 300 nm due to the presence of the conjugated phenyl groups in the polymer. The optical

properties of these films were determined by analysis of the interference fringes according

to the method described by Manifacier, et.al. [&]. The optical constants for these materials

are reported in table 2, and the dispersion in refractive index as a function of wavelength

for the phenyl -substituted polymer is displayed in figure 3.

327
Table 2. Real (n) and Imaginary (k) Parts of the Refractive Index

Measured at 500 nm for Polyphosphazene Films

POLYMER n k THICKNESS (nm)

CP 1.584 1.34E-3 2492

CP 1.625 0.78E-3 1604

CP 1.642 0.80E-3 233

PAP 1.632 1.16E-3 756

MAP 1.494 1.26E-3 379

PP 1.542 1.32E-3 1221

TFEP 1.574 0.67E-3 1083

n

1. 78 H

1. 74

1. 70

1. 66-

1. 62-

350. . 450.

A nm

550. 650. 750.

3. 60

2. 80 -

1

2. 00-

1. 20

. 400

i •

350. , 450.

A nm

550. 650. 750.

Figure 3. Wavelength dependence of the real and imaginary parts of the refractive index for

the phenylamine-substituted phosphazene polymer.

5.0 Laser Damage in Phosphazene Polymer Films

5.1 Pulsed Irradiation Damage

Polymer films deposited on silica substrates were exposed at normal incidence to pulsed

1064 nm radiation at fluences from 1 - 25 J/cm2. Single and multiple shot (30 second

exposure) irradiation at fluences less than 3 J/cm2 resulted in no observable microscopic

damage. Figure 4 exhibits three distinct damage morphologies for the phenylamine, piperidine,

and chloro substituted polymers at higher fluences.

Catastrophic damage in PAP results in the formation of a large ablated region surrounded

by a series of irregular but distinctly bounded shelves. Debris from ablated material

appears randomly distributed over the damaged area. The damage region for PP consists of.

similarly cratered region. However, a street-like fracturing of the undamaged region away

from the periphery of the damage crater is also evident. The fracturing may indicate a greater degree of polymer chain orientation present in the PP dip-coated film. A distinct

damage morphology is also observed for CP dip-coated films. The periphery of the damage

crater consists of a random arrangement of well-formed spheres which apparently condensed

following ablation. Further measurements are needed to determine the nature of these

decomposition species. It is likely that the spheres may be smaller phosphazene chain or ring oligomers formed by laser-induced depolymerization of the film as suggested by laser

Raman characterization of the damage region.

328
Figure 4. Catastrophic single pulse irradiation damage to several polyphosphazene films

deposited on silica. (A) PAP, 6 J/cm2; (B) PP, 9 J/cm2; (C) CP, 25 J/cm2.

5.2 CW Laser Irradiation Damage

The three polymer films discussed above were also subjected to CW laser irradiation

(488 nm) at relatively high power densities. The damage morphology depicted in figure 5 for

the PAP polymer film is representative of that observed in all films that failed. A 5 micrometer crater was seen in PAP, and PP films exposed to CW irradiation for periods of 60

seconds at power densities of 0.24 and 0.29 MW/cm2. No damage was observed for polymer films

irradiated for less than 60 seconds at these measured power densities. In addition, the

chloropolymer (CP) failed to show any sign of damage at power densities approaching 1 MW/cm2

and exposure times up to 300 seconds. The stability of this material attests to the high

purity of the polymer and absence of chromophores which might absorb at 488 nm leading to

localized heating.

*

*

Figure 5. CW laser damage to a PAP film irradiated for 60 seconds at a power density of

0.24 MW/cm2.

329
6.0 Second Harmonic Generation in Phosphazenes

Several phosphazene polymers and polymer precursors were observed to emit green light

when irradiated with 1064 nm pulsed radiation from a Nd:YAG laser (ca. 1 mJ/pulse at 30 Hz).

Figure 6 shows the intensity dependence of SHG on excitation laser pulse energy for the bisdichloro phosphazene trimer. Emission measured at 532 nm and the observed quadratic power

dependence confirm the non-linear optical response of this material. Continuing measurements

on derivatized phosphazenes indicate that the magnitude of the SHG effect is controlled by

the electron withdrawing power of the substituent groups on the phosphorus atom as well as the molecular conformation.

Figure 6. Second Harmonic Generation in the bisdichlorophosphazene trimer under pulsed

1064 nm excitation.

7.0 Conclusions

Optical properties of substituted polyphosphazenes are determined by the nature of the

d-p pi bonding along the inorganic polymer chain, which tends to localize electron density

around nitrogen centers. Three-center bonds are formed at the nitrogen atoms,' and the

symmetry of the molecular orbitals precludes electron conjugation along the chain. The

nature of chemical functional groups which can be substituted on phosphorus perturbs the

charge distribution along the chain and modifies the electronic structure of the polymer.

Lowest energy allowed electronic transitions probably occur between filled pi bonding

orbitals and empty pi antibonding orbitals, although weaker transitions originating from the

nitrogen nonbonding orbitals are also expected. These constraints suggest that

polyphosphazene materials should behave like wide band-gap insulators, and offer an explanation for the relatively high ultraviolet transparency. However, electronic

transitions localized on the substitutional groups can also modify the electronic structure

of the polymer as seen in the phenylamine substituted material where absorbtion at 300 nm is

significant. These concepts suggest that perturbation of polyphosphazene optical properties

can be achieved through suitable derivatization, and provide an avenue for design of new material s

in

t

e

n

B

ty

5

Nanometers

330
A contrast in chemical bonding and associated electronic structure between organic

polymers and inorganic polyphosphazenes has been demonstrated in a recent theoretical paper

[?]. Preliminary measurements of inorganic polymer optical properties are reported in this

paper and suggest their use in dielectric film applications. The solubility of phosphazene

polymers facilitates purification and deposition from solution as uniform films. Laser damage

appears in polyphosphazene films at thresholds comparable with organic polymer materials.

Improvement in damage susceptibility can be expected with increased attention to polymer

purity and deposition procedures. The first observation of non-linear behavior in

phosphazenes is reported and warrants further study from both experimental and theoretical

modeling studies.

8.0 Acknowledgments

This work has been supported by the Materials Sciences Division of the Office of Basic

Energy Sciences, U. S. Department of Energy. Inorganic polymer and polymer precursors have

been synthesized by Dr. W. D. Samuels whose assistance is gratefully acknowledged. The

authors also acknowledge the contribution of Mr. Joel Drews who assisted in the development

of solution deposition procedures and measurement of polymer film optical properties.

9.0 References

[1] H. R. Allcock, "Inorganic Macromolecules" , Chem. Engin. News 63(11) , 22-36 (1985).

[2] K. F. Ferris, P. M. Friedman, and D. M. Friedrich, "Electronic Structure of the

Phosphonitri 1 ic Trimer - Role of d Orbitals in Chemical Bonding", Int. J. Quantum Chem:

Quantum Chemistry Symposium 22, 207-218 (1988).

[3] G. J. Exarhos, K. F. Ferris, D. M. Friedrich, and W. D. Samuels, "31p NMR Studies of

Aqueous Colloidal Dispersions Stabilized by Polyphosphazene Adsorption", J. Am. Ceramic

Soc. 71(9) , C406-407 (1988).

[4] L. S. Dake, D. R. Baer, and D. M. Friedrich, "Auger Parameter Measurements of Phosphorus

Compounds for Characterization of Phosphazenes", J. Vac. Sci. Technology A 7(3) , 1634-1638 (1988).

[5] H. R. Allcock, Phosphorus-Nitrogen Compounds, Cyclic, Linear, and High Polymeric

Systems , Academic Press, New York, pp. 73-87 (1972).

[6] J. C. Manifacier, J. Gasiot, and J. P. Fillard, "A Simple Method for the Determination

of the Optical Constants n, k, and the Thickness of a Weakly Absorbing Thin Film",

J. Phys. E 9, 4002-4004 (1976).

[7] S. K. Kurtz, and T. T. Perry, "A Powder Technique for the Evaluation of Nonlinear

Optical Materials", J. Appl . Phys. 39(8) , 3798-3813 (1968).

[8] D. Chen, N. Okamoto, and R. Matsushima, "Second Harmonic Generation from Organic

Nonlinear Materials of N-Alkyl-4-Nitroaniline", Optics Commun. 69(5,6) , 425-428 1989).

331
COMMENTS

Question: Pages of questions; start with some simple ones. What about the

rather voluminous amount of data on temperature dependence of

laser induced damage in wide band gap dielectrics and also

frequency dependence of laser induced damage in wide band gap

dielectrics that are absolutely totally inconsistent with

multiphoton initiated damage?

Answer: That's what you think. We have measured temperature dependence

and it's a difficult experiment. When you have avalanche you get

a very steep temperature dependence; Manenkov has done that and

has calculated that. When you use intrinsic properties and study

intrinsic damage, the steep temperature dependence is not there.

They have been able to do between 50 K and 300 K. These

experiments have actually been published in one of the Boulder

Conferences. We have done more work since then. Frequency

dependence there was the basic thing. We have two frequency

dependencies in this kind of physics and the multiphoton cross section is a drastic function of frequency. Free carrier heating

is a function of rms electric field strengths and thus laser field

strengths. When you increase the wave length you get more efficient free carrier heating.

Question: Is that observation consistent with other free carrier absorptions

measurements in other materials?

Answer: We have measured the cross section of that and we feel we are right. The problem in this business is that people draw

conclusions by measuring damage thresholds alone and they have no idea whether they are chasing an extrinsic or intrinsic phenomena.

You should never trust any of these experiments unless they have

proven through subthreshold damage experiments what the flux

dependence is that leads up to damage. I'm firmly convinced that

aside from the data which you singled out, that Manenkov data is

intrinsic. Nobody here in this business, in the colloquium has ever reported intrinsic damage events except our prebreakdown

experiments that finally get led up to intrinsic damage. So don't

believe the volume dependencies, the wavelengths dependencies, the

temperature dependencies, which people have measured with

extrinsic material and have interpreted them with intrinsic

theories. It simply doesn't work and 99.9% of all the data is in

this category.

Question: There has been a lot of early data that supports avalanche

breakdown at DC in dielectrics as a mechanism for breakdown.

Quite extensive work that dates back decades. How do you

reconcile that? Finally, what is the breakdown in Si02 ? You must

have some multiplication of electrons or else it is still an insulator?

332
Ok, that's a loaded question. What my talk implies and what the

people from IBM also believe is that we have to throw out

literally 60 years of work of DC breakdown experiments. That's

what that means, nothing less. The IBM people are in the same

boat. Now when do they see damage? When they increase their

voltage. Sometimes at certain points, at certain voltages, the

sample simply goes because of electron photon collisions.

I would like to comment on your question. Most of the work we did

was on Si02. The situation was like this, that the DC breakdown

field increased exponentially with the improvement of technology.

The breakdown field is now like 25 megavolts per centimeter. At

the time when Frohlich was doing his model, which is a nice model,

breakdown fields were something like 6 megavolts per centimeter.

You could explain these results within his model. Then the

breakdown field increased and you had to add new stuff to the

model, so that's the situation now. I think something else is

going on in laser physics here. I am not an expert in laser

physics but I think that the purer the material gets and the more

care you take results in the breakdown field being pushed higher

and higher. It seems that for Si02 the avalanche breakdown field

actually is very high and that we haven't seen it with our DC

experiments, which go up to 25 megavolts. If we use our theory

and extrapolate to higher fields, we would expect to avalanche at

higher values. The avalanche then still exists. It just doesn't

some into play in our experiments. It comes in at far higher

field strengths so that it may exist when you shoot the laser with

pulse that is a factor of ten above the damage threshold. You

have avalanche even when you have discharge in gases. That's not

what we want to dispute, we want to simply tell you that we haven't found it and th'e reason is that the acoustic phonon

scattering has stabilized.

How much of your data is on crystalline quartz and how much on amorphous silica and are they different?

We have only looked at the amorphous silica.

It seems to me that about seven or eight years ago there was a Russian group that did a two frequency experiment, or something

like it, and came to completely opposite conclusions. Do you have

any comment on their data and the interpretation?

Well that's the same thing I always say, you have to prove first

that you have intrinsic materials in front of you because

otherwise you are beset by problems due to imperfect materials.

This extrinsic damage is a totally different picture.

Can we extrapolate your results to other materials?

Yes, we have thought about it. We were in the process of writing

a proposal but you know how that goes. We thought it would be

marvelous to take these experimental techniques and study relevant

optical materials such as calcium fluoride. The Harshaw people
told us that they have a huge contract from SDI to make the

biggest most perfect calcium fluoride crystal. I don't know

whether it ever happened but I think some of these crystals now

are getting so good that you can reach the intrinsic damage limit.

If you don't know what you have you may already have the materials

perfected to the point where you have reached the intrinsic limit

and you may still plough millions into make more perfect material.

Our techniques should be carried over to other materials,

particularly to those which we know how to make in a very perfect

way. Those which have practical applications or practical

importance are barium fluoride, calcium fluoride, magnesium

fluoride, the oxides, maybe even materials which are used in

dielectric coatings. For coatings the physics if far more complicated because we have lots of failures due to film mismatch

and all these things, but at least we could find out what is the

temperature that you dump into you coating.

I think in this area the physics has just begun. We should come

to the conclusion that taking something and firing at it and

measuring a histogram of how it damages is simply no good, We

don't understand at all what's going on. It may be good for

engineers that want to get better coatings but if you want to get

to the limit of what you can do this is not good enough.

Question: Does your theory support the material variation throughout the

alkali halides in damage threshold?

Answer: We have not done all alkali halides - these experiments are very

involved and it is very difficult to get clean samples. We didn't

have the time to measure them all but I have not the slightest

doubt that given a perfect material, they will all behave in a similar way.

Question: Well actually I didn't mean the measurement,

predict a variation in alkali halides?

Will your theory

Answer: It flatters me that you consider me a theoretician. We are experimentialists . The theories are from other people; Epifanov;

Sparks, and Fischetti are the main players. All we try to do is

do clean experiments which demonstrate predamage energy

deposition. That gives you a chance to compare experiment with

theory

Question: Have you talked with Nicolaas Bloembergen about your results to

find out what he sees?

Answer: No, his articles claim that since they studied damage in say,

covalent materials (in diamond) our theory must be wrong.

Nicolaas Bloembergen does not believe in this and I simply can not

help him.

334
Manuscript received

2—22-90 Interfacial Stability in Optical Coatings

K L Lewis, A M Pitt, A G Cullis and K Welford

Royal Signals and Radar Establishment

Malvern, Worcestershire WR14 3PS

England

I T Muirhead

OCLI Optical Coatings

Ridgeway, Hillend Industrial Park,

Fife, Dunfermline KY11 5FR

Scotland

As optical coatings become more complex, partly due to improved deposition

processes and more stringent design criteria, the number of layers in the

coating increases, with a corresponding risk of interfacial instability.

This can lead to partial film de 1 ami nat i on , arising from residual stress

effects, from interdif fusion of different chemical species across the film

boundaries, or from the presence of impurity species at the interface

itself. These effects are very relevant to the application of such

coatings in laser systems, especially where the newer digital techniques

are used to synthesise graded designs. One useful way of exploring the

effect is to incorporate a large number of interfaces within a Fabry-Perot

etalon cavity, and to temperature cycle the structure, whilst measuring its

spectral response. The changes observed can be correlated with information

derived from X-ray photoelectron spectroscopic examination of discrete

interfaces specifically fabricated to simulate those present in the etalon

cavi ty

INTRODUCTION

The problem of interfacial stability is becoming of increasing importance

as optical coatings become more complex, with greater numbers of component

layers. Several factors can influence the behaviour of such interface

regions under high flux levels. Firstly, these are regions at which

impurities or particulates often segregate because of interruptions in the

fabrication process, or movement of furniture (eg masks) within the coatingchambers. This can then result in regions of porosity as the fallowing

layer tries to accommodate the impurity or inclusion, with risk of increase

in stress levels. A second characteristic is that most interfaces are

atomically rough as a result of the polycrystal 1 ine nature of the component

materials. This can serve to weaken the interfacial bond in certain cases

and increase the risk of de 1 ami nat i on . A third factor that arises is

related to the possibility of material diffusion across the interface, with

formation of new phases with different optical constants. This can modify

the optical characteristics of the component, perhaps degrading its

performance under critical conditions.

In previous work C13, this latter process was identified as a possible

explanation for reduction in reflection band intensity in distributed Bragg

reflectors. Here, evidence was provided for the formation of a separate

interface species from the results of depth profiling X-ray photoelectron

spectroscopy, and from reverse analysis of the reflector design. The

relative magnitude of such chemical diffusion processes will be associated

with the temperature at which the films were deposited, and on the

different materials present.

Some new techniques are proposed to explore the fundamental issues involved

in determining the stability of interfaces, emphasising the different types

of failure to be expected. The first addresses the mechanical stability of

the interface, whilst the second explores chemical reaction processes by

335
directly observing the changes occuring at an engineered surface interface.

The remainder of the work is concerned with an assessment of the optical

stability of the interface expected and observed as a result of thermal

cycling.

MECHANICAL STABILITY OF INTERFACES

Many of the conventional tests applied for film adhesion are concerned with

the bond between coating and substrate. For example, in the case of the

Scotch tape test, failure mostly results in separation at the coatingsubstrate interface, and only occasionally within a given multilayer. More

quantitative techniques, such as the scratch test, involve drawing a stylus

across the film under successively increasing loads. Although the flakes

of material delaminated in this test can be microscopically examined, their

form is not amenable to cross-sectional transmission electron microscopy

(XTEM) to reveal the origins of failure within the coating.

In this work, as-deposited multilayers have been cross-sectioned by

cleaving, epoxy mounting and abrasive thinning to lOOum in thickness.

Higher than usual levels of mechanical stress used during these operations

can serve to introduce a degree of controlled microcracki ng in the

multilayer. Thin foils are then produced by further thinning of the crosssection by argon ion milling, culminating in the use of reactive iodine

etching to remove extrinsic dislocation loops and other ion-beam damage

artefacts.

Figure (1) shows a cross-sectional transmission electron micrograph of a digital BaFiVZnS multilayer produced by molecular beam evaporation as

described previously 121. This technique ensured freedom from background

impurity species at all stages during film deposition, with partial

pressures of water vapour in the 10 1 :E mbar range. The level of stress

introduced during foil preparation has led to fracture of the film and

delamination at interfaces at different levels in the film. Close

examination of regions of the micrograph -in figure (1) show that the

separation always occurs beneath the darker contrast layers in the coating.

These are the BaF.a layers, as more clearly shown in figure (2). This is a somewhat surprising observation, since it would be expected that the

chemical interaction occurring at all BaF:;;-/ZnS interfaces would be

identical. Clearly this is not the case and there is a significant

difference between the way in which BaF,; nucleates onto fresh ZnS surfaces,

and the reverse process of ZnS nucleating onto EaF:;: .

Examination of the cross-section in figure (2) at sufficiently high

magnification to resolve the individual lattice planes does not reveal any

major differences between the two types of interface.

DIRECT ASSESSMENT OF INTERFACE REACTIVITY

It is difficult to explore the chemical processes occurring at interfaces

without the risk of the probe technique used influencing the interaction

taking place. The interfaces of greatest interest are frequently those

within a multilayer structure and these are only accessible by profiling or

cross-sectioning. In order to avoid any uncertainty, a new approach has

been taken, in which the interface chemistry has been reconstructed at an

accessible surface. This involves covering a freshly grown surface of one

material with a monolayer of a second in ultra-high vacuum and following

the subsequent reactions using a surface sensitive technique such as X-ray

photoelectron spectroscopy (XPS) . The molecular beam facility previously

described [3] is ideally suited for such studies, since it allows the

deposition of high purity films in any desired configuration, and their

subsequent analysis at any stage in situ without exposure to the

atmosphere

336
The engineered interface is shown schematically in figure (3), highlighting

the various possibilities for material reaction and/or transport. On an

atomistic scale, real surfaces are far from smooth, incorporating ledges,

kinks and hillocks which act as favourable sites for material

incorporation. Deposition of a second material to the extent of a monolayer or so is not going to achieve complete coverage of the surface,

and there will always be sites where the underlying material is exposed.

Material transport can occur as a result of desorption, surface diffusion,

chemical reaction or diffusion into the underlying film material. Although

XPS is a surface sensitive technique, it actually provides information on

the constituents in the top few atomic layers. Further detail can obtained

by angle resolution if required. Such interfaces can easily be studied for

different materials combinations, but most attention to date has been

focussed on the BiFa./ZnS system, using films deposited from molecular beams

of BiFs, Zn and Sa. The latter species were obtained as a result of the

congruent evaporation of high purity ZnS.

Figures (4a) and (4b) compare the spectrum of a fresh ZnS surface with

that obtained when that same surface is covered with a nominal monolayer of

BiF-3. The major changes apparent are the reduction in intensity of the Zn

and S related signals, and the appearance of Bi and F related features. On

standing in UHV, the F related peaks gradually decay over a period of

several hours, with no change in the intensities of the other peaks, and no

absorption of other background species such as oxygen (peak at 532eV) . In

2.7 hours, the integrated area for the F Is peak had reduced by 14%, whilst

in 48 hours, the reduction was considerably greater to give the spectrum

shown in figure (4c). A slight increase in the Zn Auger signal is apparent

in this latter case, but of more significance is the complete change in the

form of the doublet near 160eV. This spectral region is expanded in figure

(5) clearly highlighting the changes occurring. It is notable that there

is no change here in the intensities of the Zn Auger peaks at 240.50 and

263.75eV, nor for that matter in the S 2s peak at 225.75eV. This implies

that the Zn and S concentrations in the analysed volume do not change with

time. The characteristic 2p signal for sulphur at 162eV virtually

coincides with the Bi 4f peaks, but this still allows the evolution in the

surface chemistry to be followed clearly as a function of time. When

freshly deposited, the Bi 4f y/;; peak overlaps the sulphur peak. After 160

minutes, a gradual shift in the Bi binding energy occurs, revealing the S

2p signal as a more defined shoulder. Within 2 days standing, the Bi 4f

7/2 peak has moved to 156.75eV, some 4eV lower than its original position,

whilst the Bi 4f >=,/:;; peak is now coincident with the S 2p signal. This

explains the overall trend in general appearance of the doublet.

The changes observed are entirely attributable to a shift in the chemical

environment of the Bi from a configuration centred on Bi-F bonding to one

centred on Bi-S bonding. This is accompanied by the reduction in surface

fluorine coverage observed. It is believed that the fluorine reduction

occurs as result of diffusion into the underlying ZnS film. The formation

of bismuth sulphide is notable and has important implications from an

optical point of view, because of its difference in refractive index to

that of BiFs.

This concept of using a model interface is applicable to a wide range of

materials combinations, and can be used to select those likely to show the

greatest stability. It is limited only in cases where spectral features

are difficult to resolve due to line overlap. Temperature effects can be

explored at a fundamental level providing some insight into processes

likely to occur during laser irradiation.

EFFECT OF INTERFACE REACTIVITY ON STABILITY OF COMPLEX FILTER STRUCTURES

In the selection of materials combinations for complex filter designs, it

is important to address the effect of material interaction on the resulting

337
increased risk of significant change as a result of chemical interaction.

It is of interest to explore the magnitude of the changes likely to be

encountered and to compare the results of those studies with observed

behaviour. Chemical reaction effects can be accelerated by increase in

temperature, so allowing a candidate filter structure to be assessed for

any permanent changes in characteristic simply by heating and subsequent

cooling to its original temperature.

Previous work [43 has highlighted the use of etalons for assessing the

thermal stability of thin film structures. That study was concerned with

the behaviour of resonant cavities containing one uniform spacer material.

Such structures are ideal for measuring the temperature coefficient of

refractive index of thin films, simply by determining the transmission

spectra at different temperatures and calculating the value of dn/dT from

the spectral shifts observed. Any permanent changes in the spacer material

appear as permanent line shifts on cooling the filter back to room

temperature. By using an etalon in high order, the spectral shifts can be

determined over a range of wavelengths simultaneously, providing a measure

of the dispersion in dn/dT. The etalon fringes are of course

characteristic only of the spacer medium, and slight changes in the end

mirrors do not have a major effect on line position. It is not strictly

necessary to use an etalon with high reflectivity mirrors for these

measurements, although these serve to sharpen the interference maxima

observed. The fundamental interference processes still occur in single

layers of material giving rise to the well-known maxima and minima in

transmission spectra. Careful measurement of their position also allows a determination of refractive index and its temperature dependence. Some

results obtained on Sum thick films of ZnSe produced in UHV are presented

in figure (6), and clearly show the trend obtained with increase in

temperature. Values obtained from successive temperature cycling

experiments merely overlay on the curves shown, demonstrating the inherent

stability of such material even in thin film form.

The etalon technique is extended in this work to explore interfacial

stability by fabricating filters with spacer layers containing a large

number of interfaces. One configuration adopted contains a repeat

structure of 20 individual layer pairs within the etalon cavity and is

based on the design (HL ) */ (HHO . 24L) ::KO / (HL) " /sub . The periodic form of the

spacer gives a Bragg resonance peak at the centre design wavelength, but

the etalon fringes at longer wavelengths are characteristic of the average

index of the entire etalon cavity. The calculated transmission of such a structure using H = 2.5 (ZnSe) and L = 1.5 (BaF.. > is shown in figure (7a)

for a design cetred on 500nm. Sharp transmission maxima are clearly

delineated at 612.5 and 635. Onm. Increase in the value of refractive index

of the ZnSe component of the filter as a result of heating can be readily

expcred by substituting different values for H in the design, and rerunning the matrix calculation for the film ensemble. For- example, a change in H from 2.5 to 2.55 results in the above two etalon fringes

shifting to 625 and 647. 5nm respectively as shown in figure (7b).

Experimental results for a real filter of similar design, but centred at a slightly different wavelength are presented in figure (8). Here the

characteristic fringes at 561, 585, 606 and 631nm shift by 3nm to longer

wavelengths on heating the filter from 25°C to 150°C in air. This is

indicative of a change in refractive index An of the ZnSe of about 0.015, a value in good agreement from that obtained above on single films of ZnSe.

It also shows that the changes in transmission spectra are dominated by the

temperature dependant properties of the ZnSe component of the filter. On

cooling to 25'C, the filter characteristic returns to its original

position, with evidence of a slight decrease in absorption implied by

increase in intensity of the 561nm peak. This wavelength is close to the

absorption edge of the ZnSe, and the decrease in absorption suggests that

an annealing of lattice defects in the film may be occuring. A second

338
heating of the film to 200 "C as shown in figure (9) accentuates the changes

observed in the 150 °C case , but also highlights a larger permanent change

in characteristic on cooling to 25'C. This is indicative of a permanent

rsducT- 1 on in s vsrs^G i sirs^ti vs 3. ncLs 3£ ox "txis stds ^ s z~ i svs r . s. nci i s prssu ins q, to be due to the result of interface reactions since the effect is not

observable in the case of ZnSe alone. Clearly the shift in wavelength in

this case is quite small and for most applications insignificant.

It is difficult to model the peak shift without some a proiri knowledge of

the extent of material diffusion across interfaces, and the resulting phase

composition of the materials produced as a result of reaction. However,

the assessment technique is adequate for demonstrating whether a particular

interface configuration is stable or not, and for providing guidance in the

selection of materials for use in complex filter designs.

CONCLUSIONS

Three techniques have been explored for assessing interface stability. The

first reveals delamination processes induced by mechanical stress within

multilayer films by direct observation in cross sectional transmission

electron microscopy. The second addresses chemical reactivity by analysis

of an engineered interface accessible to surface analysis techniques in

UHV. The effect of such processes on the resulting optical characteristics

of complete filter structures is explored in the final technique, by

measurement of temperature induced shifts in etalon characteristics whose

spacer layers contain a large number of interfaces.

The interface between BiFs and ZnS appears to be unstable as a result of

diffusion of fluorine into the ZnS and formation of a bismuth sulphide

phase. In comparison, structures containing ZnSe and BaF.;S are

comparatively stable, with only slight changes observable after temperature

cycling in air to 200°C.

©HMSO, London 1989

REFERENCES

1. K L Lewis, I T Muirhead, A M Pitt, A G Cullis and G M Williams: Proc

1988 Boulder Damage Symposium

2. K L Lewis, A M Pitt, N G Chew, A G Cullis, T J Wyatt-Davies , L

Charlwood, 0 D Dosser and I T Muirhead: Proc 1986 Boulder Damage

Symposium, IJIST Sp Publ 752 365 (1988)

3. K L Lewis, I T Muirhead, A M Pitt, A G Cullis, G M Williams and T J

Wyatt-Davies: J Vac Sci Technol a7_ 1413 (1989)

4. K L Lev/is, A Miller, I T Muirhead and J Staroml ynska : Proc 1987 Boulder

Damage Symposium, NIST Sp Publ 756 338 (1988)

339
Figure 1. Cross sectional transmission electron micrograph (XTEM) showing

stress induced failure of BaF^/ZnS digital multilayer.

Figure 2. Detail of stressed BaFxr/ZnS digital multilayer showing occurrence

of delami nation beneath BaF..- layers.

340
AB -+- CD = ABCD

AB + CD = ABO -+- D

AB -t- CD = ABD -h C

AB -+- CD = ACD -+- B

AB -+- CD = BCD + A

AB + CD = AC -+- BD

AB -+- CD = AD + BC

Figure 3. Schematic diagram of interface constructed between two generic

binary materials AB and CD, using one monolayer of CD. Various

possibilities for chemical interaction/material transport are highlighted.

341
Mg XPS Maximum Plotted Count Rate = 110630 cps Analyser Energy » 50 eV Step Size = 0.2500 eV 1 Scans of 4000 Chans at 100 ms per chan

following a days stand at 25C

AP487 ZnS with B1F3 monolayer

deposited at 150C. analysed ASAP

0.00 100.00 200.00 300.00 400.00 500.00 600.00 700.00 Binding Energy [ eV ].00 900.00 1000.00

Figure 4. X-ray photoelectron spectra (XPS; of ZnS films (a) as grown, <b)

with a monolayer" cf BiF ... deposited at 150°C, CO as (b) after 48 hours in

UHV at 25 °C.

342
Figure 5. Detail of X-ray photoel ectron spectra of ZnS films (a; as grown,

Cb) with a monolayer of BiFs deposited at 150°C, (c) as (b) after 160

minutes in UHV at 25°C, (d) as (b) after 48 hours in UHV at 25°C.

343
1 1 1

0. 6 0. 7 0. 8

Wavelength (pm)

Figure 6. Temperature dependence of refractive index of ZnSe.

x - experimental, line is fitted to three parameter Sellmsier equation.

344
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COMMENTS

Question: Do you have any feeling for how long this condition effect lasts,

first of all? And secondly, if it does last, do you feel it is a useful process for improving an operational threshold?

Answer: Absolutely. You are going to get a lot more detail on this

tomorrow and I don't want to take away the thunder from the people

who are doing that. We have shown that conditioning does in fact

last. We've looked at an over annealed sample, for example,

annealed the entire substrate, and after 10 weeks still see no degradation in the damage threshold. We still have that sample

around and doing a lot of testing on it. The conditioning does

indeed last and we're certainly hoping that this is a good way to

be able to get these thresholds up higher because the improvement

is in the order of factors of 2 or 3. We've seen that on KDP

crystals where we've looked at the bulk. That's practically six,

seven, eight years ago, that we did that. It's only been in the

last few years that people have talked about coatings. The

thought was that it doesn't really last. Well, I think that Bob

Wolfe and Mark Kozlowski tomorrow will explain what we think is

really happening when we do this conditioning and indicate why we feel this is a permanent phenomena.

Question: In relation to this conditioning, what procedure do you go through

in cleaning the sample before you do these retesting.

Answer: The sample is cleaned the same way each time unless we have

specific requests from somebody to clean something in an exotic

fashion.

Question: What is that procedure?

Answer: We do an ethanol wipe with a lens tissue several times.

Question: That's it?

Answer: That's it.

Question: Is this independent of rep-rate even going into very high reprates?

Answer: This is something that we are working to determine. How many shots

do you need in order to condition and what rep -rate you need to do

that. Typically, in our single shot systems, it's very difficult

to put too many shots on to a system. On REPTILE where we typically are now firing at 10 Hz and we radiate for a minute, we will ramp up from zero fluence up to the maximum fluence where we want to go to in a continuous step-wise fashion. This doesn't mean that you have to put 600 shots on there.

348
Question:

Answer

Question:

Answer

Comment

Will the threshold possibly go down for very high rep-rates?

The highest rep-rates we have typically gone to are 120 Hz on a REPTILE laser when we used four lasers . We are not running at

120 Hz anymore , with one laser I can run at 30 Hz , so we have not

done any of that testing. There really has been no effect

observed of difference between 120 Hz testing and 30 Hz testing.

If there is an effect, it's not because the rep -rate but it's

because of the number of shots we put on it. For instance,

titania coatings are notorious in that you put more shots on there

and the damage will get more, get larger. This is not a rep -rate

effect but a shot-rate effect. On a kilohertz tester we have not

been able to test things to an extreme, but basically there is no rep-rate effect.

Do you understand the difference between LAP and KDP and its

performance?

I know that they are different crystals and one of them is a hydrocarbon. Other than showing you the chemical formula, again

I'm not a chemist and I would really not like to speculate on those differences.

This DLAP data has been corroborated by a Japan group here last

year. They reported very high damage thresholds for DLAP. We

grew some in our own lab first time out and also measured

extremely high damage threshold for that material.

349
Manuscript received

12-29-89

Surface Analytical Methods for the Assessment of Damage

in Optical Thin Films

James R. Hoenigman

University of Dayton Research Institute

Dayton, Ohio 45469

With the recent advances in surface analytical instrumentation, a methodology

now exists which enables optical thin films to be studied in considerable detail.

Scanning Auger spectroscopy with argon ion etching capabilities in conjunction with

scanning electron microscopy, backscatter electron microscopy, optical microscopy, and

light element energy dispersive x-ray spectroscopy have been successfully used at the

University of Dayton to study coating defects and radiation induced damage in a variety of specimens. A brief description of the capabilities and limitations of some of these techniques will be given. Ball cratering, a relatively obscure technique by

which underlying layers in an optical stack can be exposed, has proven to be a powerful tool which has extended the range of the above mentioned analytical techniques.

Some examples of adhesion failures, crack phenomena, substrate and film failures, and

Auger depth profiling will be presented.

Key words: Auger depth profiling; Auger spectroscopy; Ball cratering; Dielectric

films; Energy dispersive x-ray spectroscopy; Optical coatings; Radiation damage.

1. Introduction

Over the past 25 years instrumentation for Auger electron spectroscopy (AES) has evolved

from infancy to its present fourth generation state. AES has contributed significantly to

advances which have occurred in most thin film technologies over the same period of time. A

synergistic relationship has developed between AES and the microelectronics industry. The

result has been that as more sophisticated digital electronic microcircuitry has been developed

it has been incorporated into each new generation of AES instrumentation.

A typical state-of-the-art scanning Auger microprobe is designed to be an Auger

spectrometer as well as a scanning electron microscope with secondary and backscatter electron

imaging capabilities. Allowances are made for the addition of an energy dispersive x-ray

detector which extends its analytical capabilities to include bulk analysis. A differentially

pumped ion gun, which enables Auger depth profiles to be acquired, is an essential part of an Auger microprobe. These components are enclosed in an ultrahigh vacuum chamber with a rapid

sample insertion interlock which enables specimens to changed in a few minutes. Finally,

computer control has become an integral part of this instrumentation. Hardware and software

has been developed which enables complete instrument control with full data acquisition and

data processing capabilities.

Auger spectroscopy has traditionally been used on conductive or semiconducti ve specimens.

Lower operational beam currents, however, which can be used in state-of-the-art Auger

microprobes, now permit nonconductors to be successfully studied in many cases. We have taken

advantage of this to study dielectric thin films on glass, silicon, and metal substrates. The

objective of this paper is to demonstrate how the complementary analytical capabilities of a state-of-the-art scanning Auger microprobe can be used effectively to study damage mechanisms

in optical thin films.

350
2. Analytical Techniques

The fundamental principles relating to Auger spectroscopy, scanning electron microscopy

(SEM), and energy dispersive x-ray spectroscopy (EDS) are adequately described in the literature [1-4]. One point, however, which often is not emphasized enough is the complementary

nature of these techniques, especially as they apply to the study of damage in optical thin

films.

When an energetic beam of electrons is focused on a solid surface of interest, several

processes occur, each of which provides the analyst with specific information about the

specimen. Figure 1 shows the well known "teardrop" model which is commonly used to describe

the interaction of an energetic electron beam with a solid surface. It is immediately apparent

that different sampling depths are associated with each process. The information generated in

Figure 1. Teardrop model for electron beam initiated processes within solid

specimens. Each process is restricted to a specific sampled volume.

each instance is in some way dependent upon the primary beam energy and is limited to a specific sampled volume. Table 1 summarizes the type of information and the approximate depth

resolutions of each of these processes for typical primary beam energies. An Auger microprobe

equipped with the appropriate detectors enables the analyst to sequentially probe deeper into a specimen by selecting different detectors.

Table 1. Depth restrictions and type of information which results

from electron beam initiated processes

Auger 0 to ~5 nm Elemental (Li to U), qualitative and quantiative

Secondary electrons 0 to ~50nm Compositional contrast, imaging and morphology

Backscatter electrons 0 to -300 nm Compositional contrast, imaging and morphology

X-rays 0 to -1 nm Elemental (B to U), qualitative and quantitative

351
In the discussion which follows, the term "surface" is used in conjunction only with information generated by the Auger process. In dielectric films this is on the order of 5 nm or

less into the specimen. The surface of a specimen can be etched with an argon ion beam exposing new surface deeper into the specimen. Using suitable thin film standards of known

thickness, etch rates can be determined for dielectric films of interest enabling a composition versus depth profile to be constructed. Etch rates on the order of 20 nm/min are achievable. An example of an Auger depth profile will be shown later.

The analytical techniques described thus far are applied to exposed exterior surfaces of a specimen. With most multilayer optical stacks, Auger depth profiling is a very time consuming

process to use to probe buried layers and interfaces, especially if the substrate interface is

of interest. We have successfully employed a mechanical erosion technique to expose buried

layers and interfaces in optical stacks. The method is called "ball cratering" and is

described in detail elsewhere [5,6]. Briefly, a polished spherical metal ball is mechanically

rotated at a point of interest on a specimen. A slurry, made from diamond paste, is prepared

and applied at the contact point of the ball with the specimen before rotation begins. The

result is a spherical crater in the specimen which appears as a set of concentric rings in a two dimensional SEM image or optical photograph as will be shown later. If the diameter of the

ball is known, the ring diameters can be measured from a photograph of known magnification and

the thickness of each layer calculated from the spherical geometry. By exposing successive

layers in an optical stack, access is gained to each layer and the interfaces between them.

Craters can be located to intersect damage sites and in undamaged regions enabling comparative

measurements to be made. With optical microscopy and SEM imaging, missing coatings, film

delaminations, and crack penetration can be readily observed at cratered damage sites.

A new generation of thin window or windowless EDS detectors for ultrahigh vacuum applications has enabled light element detection down to boron in some cases. Quantification,

however, is difficult because of x-ray overlap problems and difficulties associated with determining true peak intensities using the reduced-beam energies which are required to enhance

light element detection. Valuable qualitative information can be gained, however, in high

purity thin films. Since Auger is more sensitive to light elements, it can be used effectively

in conjunction with EDS to quantify elemental compositions in such films even though it samples

only a few monolayers compared to perhaps 0.5 /im for light element EDS.

3. Strengths and Limitations

The characterization of optical films by nonoptical methods was reviewed by K. H. Guenther

in 1981 [7]. Most of what was said at that time is still true regarding the relative strengths

and weaknesses of many of the methods reviewed. The lower operational current densities which

are possible in state-of-the-art Auger microprobes, however, have resulted in reducing some of

the specimen charging problems associated with acquiring Auger spectra from uncoated dielectric

films. Further, Auger spatial resolution has increased dramatically in recent years. Auger

spectra can be acquired routinely from submicron features under certain circumstances.

Specimen charging, however, remains the principal problem in using electron excitation techniques to evaluate dielectric films.

Ball cratering can expose buried layers and interfaces within an optical stack for subsequent Auger and EDS analyses. Craters on the order of 1 mm in diameter are required. It is

difficult, however, to accurately position craters to intersect regions of interest. Further,

specimens must be cleaned after cratering to remove the abrasive slurry. This, in some cases,

can introduce contamination and produce delamination in fragile films. By locating craters at

both damaged and undamaged sites on a specimen, cleaning induced damage can usually be distinguished from radiation induced damage by comparing photographs of the craters. By its nature,

ball cratering is destructive and should be used only on post mortem specimens. Ball cratering

used in conjunction with AES, EDS, and SEM imaging provides a powerful analytical approach for

evaluating damage deep within dielectric stacks.

The methodology we have employed for evaluating radiation damage in post mortem optical

specimens consists of using Auger spectroscopy, light element EDS, and SEM and backscatter

352
imagining in conjunction with ball cratering and optical microscopy. Craters are made at

damaged sites and in undamaged regions where comparative studies have proven useful in identifying damage mechanisms. Ball cratering in conjunction with Auger depth profiling and light

element EDS, has enabled the detailed study of substrate/stack interfaces and the bulk compositions of subsurface dielectric films to be determined directly.

4. Crack Damage

A specimen which consisted of several alternating thin films of Si,N. and AKO, was ball

cratered at a radiation damage site and at an undamaged site. Figure 2 Shows images of both

B

Figure 2. SEM image of a ball crater at a radiation induced site (A)

and in an undamaged region (B). It is clearly evident that damage

occurred within the optical stack.

353
sites. It is immediately apparent that cracking and delamination has occurred at the radiation

damage site, whereas, the undamaged site, which experienced the same cratering and cleaning

procedures, does not show evidence of cracking. In this example the cracking and subsequent

delamination is typical of what occurs as a result of stress relief cracking in thin films.

The exact layer or interface at which the damage occurred can be located by counting the rings

and using Auger spectroscopy to identify the exposed surface. Figure 3 shows a crater from a different specimen where substrate failure is clearly evident. Hence, ball cratering together

with optical microscopy and/or SEM imaging can be used effectively to determine whether cracks

terminate within dielectric stacks or penetrate through to the substrate and to identify where

delamination occurs.

Figure 3. An SEM image of a ball crater acquired at a radiation

damage site. Cracks are observed which penetrate the optical

stack into the glass substrate. A large wedge of glass (lower

portion of image) was removed either during cratering or in the

cleaning process which followed. A large amount of wear debris

is also observed within the crater.

5. Substrate Interfaces

In another example, a specimen was ball cratered at a radiation damage site. The specimen

consisted of a stack of alternating SUN./Al^Cu layers on a 100 nm film of Al on a glass

substrate. Information regarding the Al oCu/Al/gl ass region was of interest. Auger spectroscopy was used in conjunction with ion etching to produce a depth profile (Figure 4) of the last

two interfaces after exposure by ball cratering. For this profile, the ion beam was turned on for 30 sec, then turned off while the Auger spectra were acquired. This sequence was repeated

until the profile was completed. Each pair of data points, one each from the oxygen peak and

the aluminum peak, corresponds to a single sputter time. Peak shapes at selected times for

this profile are shown in Figure 5. The stable position of the oxygen peak (-510 eV) indicates

that specimen charging did not occur. The Al peak shape change from oxide to metal is evident

in the Al KLL peak (-1380 eV) as the A1 9 CL/A1 interface is traversed.

Interfacial broadening as a result of radiation induced damage can be measured by comparing

depth profiles using identical ion beam parameters from a damaged and undamaged site on the

same specimen, although this was not done in this example. This can be considered a first

order approach to study radiation damage induced interfacial broadening. It is assumed that

354
0 10 20 30 40 50

Sputtar Tim* Cm in.)

Figure 4. An Auger depth profile acquired from an Al

2 03/Al/gl ass region after ball

cratering. The interfaces are quite sharp and the Al region is almost devoid of

oxygen.

KitcTic ocwr (»v)

Figure 5. Auger peak shapes at specific times during the depth profile in Figure 4.

The aluminum peak shape changes as expected as the metal oxide to metal interface

is penetrated.

355
ion beam induced effects in both profiles are constant and any observed differences result from

radiation induced effects. More basic research is required to enable ion beam induced effects

to be measured directly in different dielectric films so that radiation induced interdiffusion

at interfaces can be more accurately measured and modeled.

In another example, Auger spectroscopy was used to study an Al/glass interface after depth

profiling. Figure 6 shows the sputtered region of interest, the textured region at the left

Figure 6. An SEM image acquired from a ball cratered optical stack after ion

sputtering. The innermost ring near the left edge shows evidence of ion

beam texturing which is not observed in other dielectric films in this

stack. This suggests a sputtering anomaly which could be related to

compositional differences.

center of the photograph. An Auger spectrum acquired within this region (Figure 7) showed the

presence of carbide formation. The formation of metal carbides by low energy ion bombardment

5000

z

X

-5000

500 1000 KINETIC ENEMY

1500

(•V) 2000

Figure 7. An Auger spectrum acquired from the textured region in Figure 6.

The carbon peak shape indicates the formation of a carbide. Carbon was

not observed outside the textured ring.

356
has been reported in the literature [8] and has been observed in our laboratory with clean

titanium surfaces. Whether the observed carbide, in this example, formed as a result of ion

bombardment or radiation induced damage is not known. These data do suggest, however, that

efforts should be made to eliminate carbon contamination from substrate surfaces before the

deposition of metal films and subsequent dielectric films. Even small amounts of carbon (a few

monolayers) trapped at the metal/substrate interface on a poor thermally conductive substrate

could result in carbide formation if elevated temperatures are anticipated during the service

life of the specimen. Power densities of just a few watts/cm are capable of initiating car- bide formation if the absorbed thermal energy is not rapidly dissipated in the substrate.

6. EDS of Thin Films

Ordinarily EDS is considered a bulk analytical technique which can sample at least a micron

into a specimen under typical operating conditions. For dielectric stacks with layers of 0.5

/jm or less, the technique can be modified to produce valuable qualitative data at both damaged

and undamaged sites on a specimen. This is accomplished by selecting a thin film of interest

in a dielectric stack and reducing the primary electron beam excitation energy while observing

the resulting EDS spectrum. When x-rays from atomic species are no longer observed from an underlying layer, the resulting EDS spectrum is being generated entirely within the selected

layer of the stack. If a ball crater is made in a stack of films, each successive layer can be

probed by adjusting the primary beam energy in the manner described. We have applied this

technique to an optical stack which allegedly contained alternating boron nitride and alumina

films each about 0.5 nm thick to demonstrate its usefulness. The specimen was ball cratered.

Figure 8 shows an EDS spectrum which was acquired from the first alumina film. Aluminum and

oxygen are clearly observed. An EDS spectrum acquired from the second alumina film was

identical

a 000 VFS = 2048 5.120

Figure 8. Light element EDS spectrum from the first alumina film in a dielectric

stack after adjusting the primary electron beam energy so that only the alumina

layer is sampled.

Figure 9 shows an EDS spectrum acquired from the outermost boron nitride film. Boron

cannot be observed with our EDS detector, although a subsequent Auger spectrum confirmed its

presence. The nitrogen signal from the boron nitride film is clearly evident. In addition,

silicon and oxygen are observed. An EDS spectrum from the second boron nitride film was identical indicating that the boron nitride films in this stack had a silicon/oxygen bulk

component. Auger spectra acquired from the boron nitride films along the ball cratered surface

confirmed the presence of silicon and oxygen in the boron nitride. Hence, the usefulness of

light element EDS for evaluating the bulk composition of thin dielectric films has been

357
N

I

i

A

si i 7

0 000 VFS = 512 5 120

Figure 9. Light element EDS spectrum from the outermost boron nitride film,

film. Silicon and oxygen are observed in this film in addition to

nitrogen from the boron nitride.

demonstrated. Together with ball cratering and AES, a powerful methodology has been developed

for probing bulk compositions of individual layers deep within optical stacks. The approach we have described has to be modified slightly if one or both of the films in the stack contain

heavier elements which cannot be excited with reduced primary beam energies such as were used

in the above example. We have also used light element EDS to determine the composition of

micron size particulates within dielectric films, to identify surface debris, and to analyze

ring defects in optical films. Further, in conjunction with ball cratering and AES, valuable

qualitative data from radiation induced bulk damage sites has been obtained.

7. Summary

The usefulness of AES, EDS, SEM, and Auger depth profiling in evaluating dielectric thin

films has been demonstrated. Further, ball cratering allows these techniques to applied se- quentially to underlying layers deep within an optical stack. Stress relief cracking,

substrate failure, and film delamination can be studied by ball cratering at damaged and un- damaged sites and comparing the results using optical microscopy and scanning electron

microscopy. Collectively, these analytical techniques when used in conjunction with ball

cratering enable a powerful methodology for the study of radiation induced damage in optical

thin films. Much more basic research needs to be done on dielectric thin films to enable fully

quantitative results to be achieved using a combination of these techniques.

The author wishes to acknowledge the financial support of the Naval Weapons Center, China

Lake, California for this work and the enthusiastic support given the author by T. M. Donovan.

8. References

[1] Joshi, A.; Davis, L. E.; Palmberg, P. W. Auger electron spectroscopy, chapter 5 in

Methods of surface analysis, A. W. Czanderna, ed. Amsterdam: Elsevier Scientific

Publishing Company; 1975. 159-222.

[2] Briggs, D; Seah, M. P. Practical surface analysis by Auger and x-ray photoelectron

spectroscopy. New York, NY: John Wiley & Sons; 1983.

358
[3] Murr, L. E. Electron and ion microscopy and microanalysis. New York, NY: Marcel Dekker,

Inc; 1982.

[4] Reed, S.J.B. Energy-dispersive x-ray spectrometer, chapter 4 in Quantitative electronprobe microanalysis, Scott, V. D.; Love, G., ed. Chichester, England: Ellis Horwood

Limited; 1983. 81-100.

[5] Walls, J. M.; Brown, I. K.; Hall, D. D. The application of taper-sectioning techniques

for depth profiling using Auger electron spectroscopy. Appl . Surf. Sci. 15(1-4): 93-107;

1983 april

[6] Walls, J. M. The application of surface analytical techniques to thin films and surface

coatings. Thin Solid Films 80: 213-220; 1981.

[7] Guenther, K. H. Nonoptical characterization of optical coatings. Appl. Optics 20(20):

3487-3502; 1981 October 15.

[8] Okuyama F.; Fujimoto, Y. Direct evidence for a thermal effect of Ar

+ ion bombardment in a conventional sputtering mode. J. Vac. Sci. Technol . A4(2): 237-238; 1986 March/April.

COMMENTS

Question: Can you say anything about the cost of the facility?

Answer: The capital equipment cost to field the coater is about $350,000.

The cost does not include the costs of any personnel working on

it. The facilities have run about $50,000 so I'd say that the

total cost is roughly about $400,000 to $500,000.

359
Manuscript Received

5-8-90

Laser Conditioning of Optical Thin Films*

C. R. Wolfe, M. R. Kozlowski, J. H. Campbell, F. Rainer,

A.J. Morgan, and R.P Gonzales

University of California

Lawrence Livermore National Laboratory

P.O. Box 5508, L-490

Livermore, CA 94550

Results are presented that show the damage thresholds of e-beam deposited multilayer Hf02/Si02 thin films can be permanently increased by a factor of 2 to 3 by illumination

with subthreshold fluences of laser light. This sub-threshold illumination procedure is referred to as "laser conditioning". The films used in this study were prepared by three

different physical-vapor-deposition techniques: ion-beam sputtering, plasma plating and e- beam evaporation. Only the e-beam deposited films showed consistent and significant

improvement with laser conditioning. Of the material pairs examined (Hf02/Si02 , Zr02/Si02 and TiO^SiO^, HfO^S^ gave the greatest and most consistent damage improvement with

conditioning. The number of layers and the reflective or transmissive characteristics of the

Hf02/Si02 films were found to have little' impact on laser conditioning of the film. The

results show that the damage thresholds of a wide range of e-beam deposited coatings (e.g.

HR's, polarizers, etc.) can be improved by laser conditioning. Several possible conditioning

mechanisms are examined.

Key words: "annealing", band-gap states, conditioning, damage, damage thresholds, dielectric oxides, laser conditioning, laser damage, optical thin films, paramagnetic states, point

defects.

1. Introduction

At LLNL we are improving processes for fabricating damage resistant optical coatings for use on high-peak-power lasers such as those used in inertial confinement fusion (ICF) research. The coating

fabrication processes under study are physical vapor deposition (PVD), plasma assisted chemical vapor

deposition (PCVD), and sol-gel processing. Each alternative has certain advantages. However at the

present time only PVD involving e-beam evaporation in vacuum is a fully scaled and practical process

capable of fabricating the high-quality, large-aperture optical coatings needed for ICF research.

A significant limitation on the operating fluence (or peak power) of high-peak-power lasers is the

laser damage threshold of the optical components. Over the last two decades laser conditioning, or "annealing", using sub-threshold illumination has been reported as a method to increase the damage

threshold of optical thin films and bulk optical materials [1-10]. In most cases, details of the process and

materials involved have been sketchy. In addition, little effort has been made to apply such a process to improve the threshold of optical components installed in operating laser systems. In general, two mechanisms have been proposed as responsible for the observed conditioning effects. The first mechanism

involves "laser cleaning", whereby volatile contaminants (especially atmospheric water) are removed

either from the surface [1-4, 7, 8] or from the bulk of the material [5, 9]. It has been proposed that

* Work performed under the auspices of the U. S. Department of Energy by Lawrence Livermore National

Laboratory under Contract No. W-7405-ENG-48.

360
absorption by these contaminants leads to laser damage. This conditioning process appears to be reversible

for surfaces [3] but permanent for bulk materials [9]. The second mechanism involves laser heating of the

material until a change in crystallinity is observed [6] or polishing damage is removed [10]. The

mechanism applicable to a particular system appears to be dependent on the optical material and the laser

conditioning wavelength.

It has been shown by Wilder and Thomas that dielectric multilayer coatings similar to those used as turning mirrors in the LLNL Nova laser system show a laser conditioning effect [5]. In our study a number

of different optical coatings were prepared in an attempt to identify the importance of different film parameters on both conditioned and unconditioned damage thresholds for materials used on the Nova laser.

These film parameters are: dielectric material, deposition method and multi-layer film design. In the

following pages we examine each of these three parameters. In addition we examine possible changes in

the coating material that may lead to laser conditioning. Specifically we present evidence that absorption/

desorption of contaminants and laser induced phase changes are not responsible for the conditioning effect

observed in the films studied here. Also we observed that the conditioning effect is permanent for these

films. Finally in a concluding section we propose an electronic defect mechanism to explain the laser

conditioning effect in the dielectric films.

We intend to employ conditioning in the near future to increase the damage threshold of one meter

diameter replacement mirrors for the Nova laser. Fabrication of these new mirrors with Hf02/Si02 highly

reflective (HR) coatings is scheduled to begin during the 1990 fiscal year.

2. Damage testing

The "figure-of-merit" for coating serviceability continues to be damage threshold. Damage thresholds measured at 1064 nm on three different facilities at LLNL are reported here. The repeatability on

different lasers adds to the credibility of our conclusions. The Nd:YAG laser systems used are:

i. A variable pulselength laser which operates in a single shot mode with pulselengths

varying from 1 to 16 ns.

ii. A rep-rated laser that operates at pulse-repetition frequencies (PRF) of 10, 15 or 30

Hz with a pulselength of 10 or 16 ns.

iii. A rep-rated laser that operates at 18 Hz with a pulselength of 8 ns and is capable of

rastering large aperture optics automatically.

The laser fluence is determined using calorimetry measurements and the measured beam profile

recorded with a CCD video camera. Commercial software is used to calculate the peak fluence from these

data. The laser spot size is usually greater than 1 mm diameter (1/e2

). Damage is defined as any visual

change in the sample after laser irradiation when viewed by 100X Nomarski microscopy. To observe the

on-set of microscopic damage, photographs of the test area are taken before and after testing and compared

for evidence of damage to the surface. A detailed description of damage testing apparatus and procedures

is presented in an accompanying paper in this symposium by Rainer et al. [11].

The damage thresholds reported here were determined by one or more of the four test sequences

shown in figure 1. The four test sequences differ in the number of shots, the time between shots, and the

range of fluences used. The damage threshold measured by 1:1 and S:l shot programs are reported as "unconditioned" thresholds; these shot programs expose each site to only one fluence level. The 1:1 test uses a single shot per site. The S:l test uses a series of constant-fluence shots on each fresh (i.e. unexposed)

site with the time between shots being short (equal to 1/PRF). Conditioned thresholds are measured by R:l

and N:l tests. The R:l test uses a series of shots separated by short intervals as in S:l, but the fluence is varied from zero to a pre-set upper bound in a linear ramp. The N:l test uses a series of single shots, the

fluence of each shot is increased step-wise until a pre-set upper bound is reached. The time between shots

for N:l testing can be rather long, generally several minutes. Uncertainty in the threshold measurement is typically ± 15%.

361
Figure 1. Four methods used to measure damage threshold: (a) single shot per site (1:1), (b) multiple shots

per site with large increments in fluence between shots (N:l); (c) multiple shots per site at constant fluence (S:l), and (d) multiple shots per site with a ramped increase in fluence (R:l).

Note that N:l and R:l tests give "conditioned" damage thresholds.

3. Results and discussion

3. 1 Damage threshold dependence on film materials

Si02 was used as the low-index dielectric for all multilayers prepared in the course of this work.

High-index dielectrics considered were: Hf02, Zr02 , Ti02 and Ta205

. Ta205 was rejected because of the

need to bake coated parts in 02 after deposition to achieve full oxidation and low absorption. Baking at elevated temperatures is not practical for massive (250 kg) Nova-scale optics.

Hf02, Zr02 and Ti02 paired with Si02 were directly compared in simple"quarterwave" HR designs,

the optical performance of the HfO^SiC^ coating is shown in figure 2. Inconsistent damage thresholds were obtained from TiCySiOj mirrors fabricated both during this study and in the past. The formation of

highly absorbing sub-oxides of Ti02 [12] is difficult to avoid during high vacuum deposition and may be

the reason for the inconsistent damage thresholds. Therefore Ti02 was also eliminated as a potential film

material, leaving only Hf02 and Zr02 as high-index materials of interest.

E-beam evaporated HR coatings of both Hf02/Si0

2 and ZrCySiC^ were prepared in order to study

the influence of laser conditioning on the damage threshold. Upon conditioning, both material combinations produced a marked improvement in damage threshold for 1-ns pulses, as shown by the data in figure

3. These results indicate that Hf02/Si02 and Zr02/Si02 have essentially equivalent damage resistance; the

unconditioned thresholds are 6-8 J/cm2 and conditioned thresholds are as much as 3 times greater.

362
100

400 600 800 1000

Wavelength nm

1200 1400

Figure 2. Measured reflectance vs. wavelength for a simple Hf02/Si02 quarterwave stack that was prepared as part of this study. The reflectance at 1064 nm is > 99.5%.

25

20

| 15

2

o

&10

nQ

Unconditioned

threshold

Conditioned

threshold

Hf02 /Si02 Zr02 /Si0 2

Figure 3. Comparison of unconditioned and laser-conditioned damage thresholds

for Hf02/Si02 and ZrCySiC^ HR quarterwave stacks for 1-ns pulses,

1064 nm. The unconditioned values are from 1:1 tests and the conditioned values are for N:l measurements.

363
100 r 1 1 1 1 1—i—i

r—\ i r

1 10 100

Pulse duration (ns)

Figure 4. Measured pulselength scaling for conditioned and unconditioned damage thresholds of

HfCySiOj quarterwave HR coatings at 1064 nm. The data are from measurements on the three

different laser systems described in section 2 and the solid line represents a least squares fit to the data.

The HfCySiOj coating thresholds are very reproducible and show about the same factor of improvement due to conditioning over a broad pulselength range, as shown in figure 4. The reproducibility of the

threshold measurements was remarkably consistent considering three different laser-damage-test systems were used in the study. Based on the data in figure 4 for the HfCySiC^ coatings, the damage threshold

scales with pulse-length as approximately:

unconditioned: D ~ 7 t 035

> — p

laser conditioned: D ~ 19 t 0 30

t — P

where Dt is the damage threshold in J/cm2 and t is the laser pulselength in ns. This pulselength scaling of

the damage threshold falls within the range typically reported by others studying damage to thin films (i.e. between about tp

0 25 and tp

04

) [see for example reference 13].

In contrast to the HfCySiOj thresholds, we found that the ZrCySiC^ coatings showed significant

site-to-site variability in threshold measurements. In some tests the ZrOj/SiOj showed marked improvement with conditioning (as in figure 3) while in other tests they showed no improvement.

The reason for the difference in damage performance of the Zr02

- vs. Hf02-based coatings used in

this study is unclear. When the Zr02/Si02 films did condition, the threshold improvement was comparable

364
to that observed for the HfO^SiOj films. We therefore suggest that Hf02/Si02 and Zr02/Si02 have similar

intrinsic film properties. However, in the case of Zr02

, the damage threshold is often controlled by

extrinsic properties such as visible defects. These extrinsic defects lead to the site-to-site variability

observed in the damage threshold. Using Nomarski microscopy, we found that the Zr02

- and Hf02-based

films have about the same density of visible defects; however, the defects tend to be somewhat larger in the

Zr02/Si02 films. As was pointed out by Kozlowski et al. [14], it is unlikely that modest differences in size or density of defects, per se, leads to different damage threshold. Rather these differences probably

indicate different types of defects that may be more or less susceptible to either laser damage or conditioning or both.

3.2 Damage threshold vs. film deposition method

During the course of this study we prepared HR films of Hf02/Si02 by three different deposition

techniques: (1) conventional e-beam evaporation, (2) ion-beam sputtering (IBS), and (3) plasma plating

(PP). In addition, for the case of e-beam evaporation, we examined the effects of scale-up by preparing

coatings in a small (1.2-m diameter) research -scale chamber as well as the large 3-m diameter productionscale chamber used to fabricate the current 1-m diameter, 250-kg Nova mirrors.

60

50 -

40 -

E

o2

o

in 30

0

O)aE

(a a

20

10

111

Unconditioned

threshold

Conditioned

threshold

e-beam evaporation

(Production scale) (R&D scale)

Plasma

plating

Ion beam

sputtered

Figure 5. Unconditioned (S:l) and conditioned (R:l) damage thresholds (16 ns, 1064 nm) for HfO,/Si02 HR coatings prepared by e-beam evaporation, plasma plating, and ion-beam sputtering. The e- beam data are for coatings prepared on both the small scale R&D coater and the large scale (3-

m diameter) production coater. 365
The choice of IBS and PP as coating methods was based on the observation that coatings produced

by these methods have less porosity (i.e. higher density) and therefore have greater mechanical strength and

environmental durability [15-19]. This latter quality was thought to be particularly important because of

previous reports of film conditioning (and laser damage) being related to water-vapor absorption [1,3, 4].

However, our measurements show that HR films of Hf02/Si02 deposited by IBS and PP have lower damage

thresholds than those made by conventional e-beam evaporation. Figure 5 compares the measured damage

thresholds of coatings made by e-beam evaporation, IBS and PP. Laser conditioning does not significantly

increase the damage threshold of the PP coatings. In addition, threshold measurements of IBS films were

very inconsistent and the conditioning effect was not always observed, consequently no laser conditioned

value is reported in figure 5. In this study laser conditioning was consistently observed only in e-beamdeposited coatings. In addition we note that the film absorption at 1064 nm, measured by calorimetry [20], was 500-5000 ppm for PP, 100-250 ppm for IBS and -100 ppm for e-beam deposition.

Figure 5 also compares measured damage thresholds for coatings made in a small and a large e-beam

coating chamber. The results show that conditioning is observed for the films produced in these two

chambers; therefore the conditioning characteristics of the HfCySiOj films are preserved in scale-up. The

conditioned and unconditioned thresholds for the coatings produced in the small coater are 30-40 % higher

than those made in the production coater, however.

3.3 Damage threshold dependence on coating design

The data in the previous section describes laser conditioning effects in HR coatings consisting of a simple optical quarterwave stack of high- and low-index materials. We also found that the conditioning

effect occurs in both transmissive and reflective coatings and in single as well as multilayer films. In other

words, the conditioning effect appears independent of the coating design for HfCySiC)2 film materials.

Figure 6 shows the unconditioned and conditioned damage thresholds for: (a) single-wavelength

(1064 nm) HR coatings each consisting of a 27-layer quarterwave stack, (b) complex, 64-layer trichroic HR

60

50

E

£ 40

o| 30

|

20

a

Q

10

Unconditioned

threshold

Conditioned

threshold

Single

wavelength

mirrors

Multiple

wavelength

mirrors

Polarizers Evanescent

wave

coatings

Number of layers: 27 64 32

Figure 6. Comparison of conditioned and unconditioned damage thresholds for Hf02/Si02 multilayer

HR's, polarizer and single-layer evanescent wave coatings; all data are for 1064-nm laser

irradiation. The HR data are for pulse lengths of 16 ns and the polarizer and evanescent wave

coatings are for 10-ns pulses. _ ,

,
coatings designed to reflect at 1064, 532 and 355 nm, (c) 1064-nm polarizers with a rejection ratio of about

300 to 1, and (d) a single-layer Si02 evanescent wave coating 1400 nm thick. Note that for this series of

coatings there appears to be no correlation between the threshold improvement via conditioning and the

coating design. In addition, the degree of improvement by conditioning is also largely independent of the

number of layers (or interfaces) within the coating for these HfCySiC^ coatings.

3.4 Effects of material changes on laser conditioning

Laser conditioning of thin films has previously been linked to the removal of adsorbed water or

other atmospheric contamination [1, 3, 21-23]. When such a "cleaning" mechanism was reportedly involved, the enhancement of the damage threshold by conditioning was often found to be temporary [3].

Experiments performed at LLNL on HfCySiC^ mirrors indicate that the laser conditioning effect is permanent for these films and not related to the presence, or removal, of atmospheric water. To demonstrate this, HfCySiOj mirrors made by conventional e-beam deposition were allowed to equilibrate with

ambient air (20°C, relative humidity -50%) and were then damage tested at 1064 nm. Both the unconditioned and conditioned damage thresholds were determined. The coatings were then desiccated for 10

weeks over P2Os to remove physically absorbed water [24]. The removal of water was observed as a shift in reflectance to shorter wavelengths (table 1). The occurrence of spectral shifts in optical filters upon drying

is well known [25]. The conditioned and unconditioned thresholds of the dried coatings were then

measured and compared to the thresholds of the same samples before they were dried. No change in threshold was observed (fig. 7). Furthermore, the reflectance shift caused by drying was found to be

reversible. When the dried coatings were re-exposed to moist air their reflectance shifted back to longer

wavelengths and the original performance was recovered. Figure 8 shows the gradual shift in the

reflectance at 950 nm of one of the dried mirrors as it was re-equilibrated with ambient air. The optical

absorption at 950 nm was used to monitor the slight shift in coating performance because a pronounced

minimum in reflectance occurred at this wavelength (fig. 2). This structure allowed the slight reflectance

shift to be accurately monitored. The time scale for equilibration is about 4 weeks. (Measurement of water

absorption at 3350 cm'1 verified that the change in reflectivity was due to changes in the water content.)

Table 1. Measured Short Wavelength Shift of Hf02/Si02 HR

Coatings After Drying Over P205 for 10 weeks

Deposition method Spectral Shift (nm)

E-beam evaporation: R&D-scale chamber 25

E-beam evaporation: Production-scale chamber 5

Ion beam sputtering 7

Plasma plating 3

The role of photolytic vaporization of contaminants in conditioning was investigated also. We

attempted to "clean" and thus condition HfCySiC^ mirrors by exposure to broadband flashlamp light [26].

The lamp output fluence is about 10 J/cm2 in 0.5-ms pulses, although it is over a very broad spectral range

(~ 350 to 2000 nm). No increase in damage threshold was observed for the mirrors processed in this way,

we therefore tentatively rule out "photolytic cleaning" of contaminants as a conditioning mechanism. Additional discussion of these experiments as they relate to electronic defects is given by Kozlowski et al.

[14].

X-ray analyses of the HfCySiC^ HR coatings also indicate that conditioning does not involve any

obvious material phase change. A diffractometer scan of an unconditioned mirror (using Cu, Ka x-rays

367
100

80

60

40

Unconditioned

S:1

Conditioned

R:1

20

Control

(not dried)

Dried over

Figure 7. Conditioned (R:l) and unconditioned (S:l) damage thresholds (1064 nm, 16 ns) for HfCySiO.,

HR coatings equilibrated in ambient air and desiccated over P2Os

.

Figure 8. Time dependence of the spectral shift, to short wavelength, of a desiccated Hf02/Si02 HR upon

exposure to air. ~,

fl
with wavelength of 1.5405 A) is shown in fig. 9a. Peak assignment indicates that the Hf02 is partially

crystalline. The crystalline fraction appears as mostly the monoclinic phase and a trace of cubic present on

a high amorphous background. TEM analysis has shown that the Hf02 layers are initially amorphous, but

as the deposition continues, the crystal size increases so that at about halfway through the deposition

columnar growth is observed. In contrast, Si02 is entirely amorphous. Diffractometer scans of the

conditioned and unconditioned coatings showed no detectable change (± 5-10%) in the quantities or types

45 63

29 degrees

Figure 9 X-ray diffractometer signal vs. 29 for (a) an unconditioned and (b) a conditioned Hf02/Si02 HR coating. 'M' and 'C identify peaks associated

with the monoclinic and cubic phases of crystalline Hf02

, respectively; (c)

the difference between the two spectra as determined by subtracting (b)

from (a). -,

q
of phases present (fig. 9). Therefore we conclude that no phase change, that is measurable by this

technique, accompanies laser conditioning of these coatings.

As a final note we report that the enhancement of damage threshold by laser conditioning is permanent in the HfCySiC^ e-beam deposited films. To determine this, a large area (~ 4-cm diameter) of an HR coating was conditioned using the Nova laser. The conditioned threshold of this sample was then

monitored by subsequent small-aperture tests over a period of about 10 weeks. Details of this experiment

and the results are given in the companion paper by Kozlowski et al. [14]. The results indicate that the preconditioned threshold was increased > 2 times over the unconditioned threshold and remained unchanged,

within experimental error, over a period of 10 weeks. As shown above (fig. 8), the time scale for complete

reabsorption of water in a desiccated film is only about 4 weeks. Thus film conditioning caused by laser

induced water (or other contaminant) desorption, followed by reabsorption can be eliminated. We conclude

that for all practical purposes the conditioning effect is permanent.

3.5 Thoughts on the mechanism for laser conditioning

Before proceeding to describe what may cause the conditioning effect, it is worthwhile to state what

it does not depend on (at least for the films discussed in this work). The results in the preceding sections

indicate the conditioning effect is:

i. noi associated with the removal of absorbed moisture or other atmospheric contamination,

ii. noi clearly dependent on the number of coating layers or the transmissive or reflective

characteristics of the optical design,

iii. noi associated with measurable recrystallization within the film.

The results therefore indicate that the often proposed "laser cleaning" or phase change mechanisms are not

the dominant mechanisms responsible for the conditioning phenomena occurring in the dielectric films of

interest here. In a paper by Edwards et al. [27] we further show that linear absorption by contaminating

particles [5] does not cause damage. We instead propose below that the laser conditioning effect observed

in high quality optical thin films is associated with intrinsic electronic defects in the films.

Of the materials we have tested, conditioning is only consistently observed in HfO,/Si02 films

deposited by conventional e-beam evaporation. One significant characteristic of e-beam coatings, that

distinguishes them from films made by other processes, is the low energy of the material being deposited

(« 1 eV). In addition, the condensation process is one of extremely rapid quenching from a high

temperature vapor to a much cooler substrate ~100-300°C; this can be thought of as "splat cooling". This

rapid quenching can result in porous films dominated by intrinsic defects [28]. We believe that the "nonequilibrium" state of these e-beam films is associated with their laser conditioning properties. In the

following paragraphs we speculate on the nature of these intrinsic defects and on how they may be

associated with the laser conditioning process.

We propose that the laser damage of unconditioned films deposited by e-beam evaporation occurs by a four step process:

i. photo-excitation of electrons from shallow electronic gap states into the conduction

band,

ii. excitation of the free carriers to high energy by acceleration under the optical electric

field or by free carrier absorption,

iii. subsequent transfer of the excess energy to the lattice via avalanche [29] or an electron-phonon interaction (lattice heating) [30],

370
iv. heating of the film to some critical damage temperature such as the melting or boiling

point of the dielectric material.

The importance of conduction band electrons to optical damage is not a novel concept. In fact, the

dependence of the laser damage threshold on the presence of conduction electrons has been demonstrated

for bulk Si02 [30], as well as for silicon and glass [31]. The novel concept is the connection between the

conduction band electrons and the conditioning effect. We suggest that laser conditioning occurs due to the

removal of the source of conduction band electrons in step (i) above. That is, when the dielectric material

is illuminated at a low fluence, the electrons in the defect levels are excited to the conduction band. Since

the optical electric field is not large enough to cause damage (steps (ii) to (iv)) the electrons decay into deep

levels from which they cannot be easily excited into the conduction band on subsequent illuminations.

When the 'conditioned' dielectric is subsequently illuminated at intensities above the unconditioned

damage threshold, the number of electrons available for transfer to the conduction band is low and,

therefore, the net energy transferred to the lattice is too low to cause damage; hence the observed laser

conditioning effect.

E-beam deposited dielectric coatings are known to have a high concentration of electronic defects in

the band-gap [32]. Point defects in amorphous and crystalline Si02 are well characterized because of their

importance to solid state electronic technology [33-35]. Very little is known however about similar defects

in other wide band gap dielectrics such as Hf02 , Zr02

, and Ti02

. As shown schematically in figure 10,

energy bands in amorphous solids can be thought of as distorted by local variations in bonding. In general,

this produces discrete states in the band gap that are distributed randomly in the amorphous material [36].

Wide band gap dielectrics deposited by e-beam evaporation are highly disordered and electronically

metastable. A high concentration of occupied gap states located above the Fermi level may result from this

type of disorder. An example of such defects, paramagnetic point defects, have been observed in

amorphous Si02 [37]. It has been demonstrated that these states can be filled by electrons as a result of UV

illumination and then subsequently emptied by thermal treatment [38].

>»D>

i_a>c

LU

C

o

1_ 4—

o

a> LU

E

c

E

F

Distance Density of states

(a) (b)

Figure 10. (a) Schematic diagram showing the model representation of the energy bands of an amorphous

dielectric solid including gap states. Ev , Ef and Ec refer to the energies associated with the

valence band, Fermi level and conduction band, respectively; (b) schematic representation of the

corresponding density of states. The shaded regions represent occupied levels.

The independence of the laser conditioning effect on film design and number of layers suggests that

the electronic defects of interest are bulk defects and not associated with interfaces. Further discussion of

371
the nature of the electronic defects and the mechanism by which energy is transferred to the crystal lattice is given by Kozlowski et al. [14].

As shown above, "non-equilibrated" films produced by e-beam deposition produced consistent

conditioning effects. In contrast, films of Si02 prepared by high' temperature PCVD [39, 40] have very high

unconditioned thresholds, comparable to those for conditioned films of e-beam deposited SiOr We

believe that the PCVD process produces inherently more equilibrated films, which are not susceptible to the

damage/conditioning mechanisms important to unconditioned e-beam coatings. Unfortunately the PCVD

coating technology is not yet developed to the scale required to fabricate large-aperture optical components

for fusion research lasers.

Because of their non-equilibrium deposition conditions, the plasma plating and ion-beam sputtering

processes examined in this study might be expected to produce electronically highly defective films with a high concentration of electronic defects similar to those produced by e-beam deposition. The IBS and PP

films, however, showed negligible improvement in the damage threshold upon laser conditioning. The low

unconditioned threshold and lack of a conditioning effect suggests that a damage mechanism other than the

electronic defect mechanism described above might be dominant for these films.

4. Conclusions

The results of experiments reported here show that optical thin films of Si02 or Hf02/Si02 deposited by conventional e-beam processes can be laser conditioned to significantly increase their laser

damage threshold. This is accomplished by illuminating the coating at fluences below the damage fluence.

Our results indicate that:

i. the damage threshold can be increased by factors as great as 2-3x,

ii. the conditioning effect is permanent,

iii. the conditioning effect is dependent on the dielectric material and the deposition

technique, but shows little dependence on coating design or the size of the deposition

chamber

v. conditioning may be related to the removal of electrons from intrinsic defects energetically close to the conduction bands in these materials.

Through the use of the conditioning process the damage threshold of e-beam deposited optical

coatings can be made to exceed the operating fluences required by current and future fusion laser systems.

Currently, e-beam deposition is the only process by which large-aperture, high-quality, optical coatings can

be made. Results presented here show that the conditioning properties of e-beam deposited films are not

influenced by scale-up to a production scale deposition chamber. In a paper by Kozlowski et al. [14] we

further show that the laser conditioning process can be used to increase the damage threshold over entire

large area optical components. These laser conditioned e-beam coatings will comprise the next generation

of large aperture optical thin films used on the LLNL Nova laser system.

5. Acknowledgements

The coatings studied in the course of this work were prepared under contract to the Advanced

Products Division and Research Department of Optical Coatings Laboratory, Inc. (OCLI) in Santa Rosa,

Ca. and Spectra- Physics Optics Co. in Mountain View, Ca. The authors gratefully acknowledge the assistance of Dr. M. Shinn in measuring the water absorption spectra, Dr. M. Sattler in performing the TEM

analysis, and L. Summers in making the x-ray measurements on the HfO^S^ films. The efforts by A.

Clasen in preparing this manuscript for publication are also deeply appreciated.

372
6. References

[I] Porteus, J. O.; Faith, W. N.; Allen, S. D. Laser desorption analysis of H20 and other contaminants

from optical surfaces. Nat. Bur. Stand. (U.S.) Spec. Publ. 638; 1981 November. 273 p.

[2] Swain, J. E.; Lowdermilk, W. H.; Milam, D. Raising the surface damage threshold of neutral

solution processed BK-7 by pulse laser irradiation. Nat. Bur. Stand. (U.S.) Spec. Pub. 669; 1982

November. 292 p.

[3] Frink, M. E.; Arenberg, J. W.; Mordaunt, D. W.; Seitel, S. C; Babb, M. T.; Teppo, E. A. Temporary

laser damage threshold enhancement by laser conditioning of anti-reflection coated glass. Appl.

Phys. Lett. 11(1987) 415.

[4] Arenberg, J. W.; Mordaunt, D. W. Experimental investigation on the role of wavelength in the laser

conditioning effect. Nat. Inst. Stand. & Tech. (U.S.) Spec. Pub. 775; 1988 October. 516 p.

[5] Wilder, J. G.; Thomas, I. M. Effect of n on 1 laser treatment on damage threshold of selected optical

coatings. Nat. Inst. Stand. & Tech. (U.S.) Spec. Pub. 756; 1987 October. 369 p.

[6] Stewart, A. F.; Guenther, A. H.; Domann, F. E. The properties of laser annealed dielectric films.

Nat. Inst. Stand. & Tech. (U.S.) Spec. Pub. 775; 1988 October. 259 p.

[7] Wang, V.; Rudisill, J. E.; Giuliano, C. R.; Braunstein, M.; Braunstein, A. Pulsed C02 laser damage in windows, reflectors and coatings. Nat. Bur. Stand. (U.S.) Spec. Publ. 414; 1974 May. 59 p.

[8] Foltyn, S. R.; Newnam, B. E. Multiple-shot laser damage thresholds of ultraviolet reflectors at 248

and 308 nanometers. Nat. Bur. Stand. (U.S.) Spec. Publ. 620; 1980 September. 265 p.

[9] Swain, J. E.; Stokowski, S. E.; Milam, D.; Kennedy, G. The effect of baking and pulsed laser

irradiation on the damage threshold of potassium dihydrogen phosphate crystals. Appl. Phys. Lett. 41 (1982) 12.

[10] Temple, P. A.; Lowdermilk, W. H.; Milam, D. Carbon dioxide laser polishing of fused silica

surfaces for increased laser-damage resistance at 1064 nm. Appl. Opt._21 (1982) 3249.

[II] Morgan, A. J.; Rainer, F.; De Marco, F. P.; Gonzales, R. P.; Kozlowski, M. R.; Staggs, M. C. Expanded damage test facilities at LLNL. Nat. Inst. Stand. & Tech. (U.S.) Spec. Publ.; (Laser Induced

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[12] Sorensen, O. T., ed. Non-stoichiometric oxides . Chapters 1, 2 and 7; Academic Press, New York,

1981.

[13] Rainer, F.; Vercimak, C. L.; Milam, D.; Carniglia, C. K.; Tuttle-Hart, T. Measurements of the

dependence of damage thresholds on laser wavelength, pulse duration and film thickness. Nat. Bur.

Stand. (U.S.) Spec. Publ.688; 1983 November. 268 p.

[14] Kozlowski, M. R.; Staggs, M. C; Wolfe, C. R.; Campbell, J. H. Large area laser preconditioning of

dielectric thin film mirrors. Nat. Inst. Stand. & Tech. (U.S.) Spec. Publ.; (Laser Induced Damage in Optical Materials: 1989) - these proceedings.

[15] Iscoff, R. Ion assisted deposition. Lasers and Optronics Z (1988) 53.

[16] McNeil, J. R.; Al-Jumaily, G.; Jungling, K. C; Barron, A. Properties of Ti02 and Si02 thin films

deposited by ion assisted deposition. Appl. Opt. 24 (1985) 490.

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[17] Martin, P. J. Review - Ion based methods for optical thin film deposition: Journal of Materials

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[18] Allen, T. H. Reactive ion beam sputtered coatings: Society of Vacuum Coaters, Proceedings of the

30th Annual Technical Conference, April 27-May 1, 1987, Boston, MA.

[19] Rancourt, J. D. "Future Prospects", Chapter 7 in Optical Thin Films Users Handbook. Macmillan,

New York, 1987.

[20] Rancourt, J. D. Optical Thin Films Users' Handbook. Section 5.1.10 "Measuring Absorption by

Calorimetry", p. 153. MacMillan, New York, 1987.

[21] Tarn, A. C; Schroeder, H. Laser induced thermal desorption of moisture from a surface in atmospheric conditions. Appl. Phys._M (1988) 3667.

[22] Kovalev, V. L; Faizullov, F. Influence of adsorbed water on the optical strength of infrared optics

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[23] Brauns, B.; Schafer, D.; Wolf, R.; Zscherpe, G. Effect of the substrate preparation with C02 laser

radiation on the laser resistance of optical layers. Thin Solid Films 138 (1986) 157.

[24] Voorthuyzen, J. A.; Keskin, K.; Bergveld, P. Investigations of the surface conductivity of silicon

dioxide and methods to reduce it. Surface Science 187 (1987) 201.

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Resistance"; Chapter 9, especially p. 398, "Factors Affecting Layer Properties", MacMillan, ed.,

1986.

[26] Wolfe, C. R.; Campbell, J. H.; Lyon, R. E.; Pitts, J. H.; Powell, H. T. Optical damage in epoxy

polymers by millisecond light pulses. Nat. Bur. Stand. (U.S.) Spec. Publ. 752; 1986 September.

194 p.

[27] Edwards, G.; Campbell, J. H.; Wolfe, C. R.; Lindsey, E. F. Damage assessment and possible damage mechanisms to 1-meter diameter Nova turning mirrors. Nat. Inst. Stand. & Tech. (U.S.) Spec.

Publ.; (Laser Induced Damage in Optical Materials: 1989) - these proceedings.

[28] Beauchamp, W. T.; Hichawa, B. P.; Innus, M. H. Systematic design approach leads to better optical

coatings. Laser Focus/Electro-Optics, 24(5) (1988) 109.

[29] Bloembergen, N. Laser induced electric breakdown in solids. IEEE J. Quantum Electron., OE-10

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[30] Jones, S. C; Braunlich, P.; Casper, R.; Shen, Xiao-An; Kelly, P. Recent progress on laser induced

modifications and intrinsic bulk damage of wide band-gap optical materials. Opt. Eng. 28 (1989)

1039.

[31] Kerr, N. C; Clark, S. E.; Emmony, D. C. U.V. seeding of I.R. laser induced damage. Nat. Inst. Stand. & Tech. (U.S.) Spec. Publ.; (Laser Induced Damage in Optical Materials: 1989) - these

proceedings.

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of intrinsic bonding defects and impurities in Si02 thin solid films: in Defects in Glasses. Mat. Res.

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[33] DiMaria, D. J. Defects and impurities in thermal Si02

: The physics of SiQ2 and its interfaces.

Proceedings of the international topical conferences, Pantelides, S. T., ed. Perganon Press 1978,

Chapter 4.

[34] Griscom, D. L. Defects in noncrystalline oxides: in Defects and their structure in non-metallic

solids . Henderson, B.; Hughes, A. E., eds. Plenum Publishing Co., New York, 1976, p. 323.

[35] Griscom, D. L. Point defects in amorphous Si02

: What have we learned from 30 years of experimentation?, Materials Res. Soc. Sym. Proc. 6J_ (1986) 213.

[36] Fuhs, W. Transport properties of amorphous semiconductors, in Physics of structurally disordered

solids . Mitra, S. S., ed. Plenum Press, New York, 1976, p. 41 1.

[37] Stathis, J. H. Identification of native defects in amorphous-Si02

: in The Physics and Technology of

Amorphous SiOrDevin, R. ed. Plenum Publ. Co., New York, 1988, p. 141.

[38] Stathis, J. H.; Kastner, M. A. Photoinduced paramagnetic defects in amorphous silicon dioxide.

Materials Res. Soc. Sym. Proc, 6_I (1986) 161.

[39] Campbell, J. H.; Emmett, J. L.; Brusasco, R. M.; Rainer, F.; Gonzales, R. P.; Morgan, A. J.; Kersten,

R. T.; Paquet, V.; Etzkorn, H.-W. Damage resistant optical coatings prepared using high temperature plasma chemical-vapor-deposition. Nat. Inst. Stand. & Tech. (U.S.) Spec. Publ.; (Laser

Induced Damage in Optical Materials: 1989) - these proceedings.

[40] Paquet, V.; Etzkorn, H.-W.; Kersten, R. Th.; Emmet, J. L.; Campbell, J. H.; Brusasco, R. M.; Rainer,

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375
Manuscript received

3-20-90

Large Area Laser Conditioning of Dielectric Thin Film Mirrors

M.R. Kozlowski, C.R. Wolfe, M.C. Staggs, J.H. Campbell

Lawrence Livermore National Laboratory

University of California

P.O. Box 5508

Livermore CA 94550

The laser conditioning of dielectric thin film HR coatings has been studied as a practical method for the

improvement of the damage thresholds of large area (1.1m dia.) high power 1064 nm laser mirrors on the LLNL

120 kJ, 100 TW Nova laser system. Both HfCySiC^ and ZrO./Si02 HR coatings were conditioned by rastering with

a small (-0.2 mm) diameter beam from a pulsed (18 Hz, 8 ns) Nd-YAG laser (1064 nm). The samples were rastered

at various fluences below the unconditioned damage threshold and subsequently damage tested. Large area conditioning studies were also performed using a large aperture beam ofthe Nova laser. The laser conditioning effect was found to be permanent. Improvements in damage threshold due to conditioning were as high as a factor of 2.7

and were dependent on the conditioning parameters. A model for the conditioning effect is proposed based on the

emptying of electronic defect levels within the bandgap of the dielectric materials.

Key Words: electronic defects; halfnia; HR coatings; laser conditioning; laser damage; silica, zirconia.

1 . Introduction

The cost of optical components for high power laser systems, such as those used in fusion energy research,

increases dramatically with the size of the optic. In order to keep the power delivered by the laser high while keeping

the cost of the system low, high damage threshold optical materials are needed. Presently, increases in the output

of the 120 kJ Nova laser system used for inertial confinement fusion research at Lawrence Livermore National

Laboratory are limited in part by the damage threshold of the dielectric multilayer turning mirrors. These mirrors

have diameters of 96 and 109 cm and are deposited on 16 cm thick BK-7 glass substrates.

Over the last two decades many researchers have reported that the laser damage threshold of both bulk [1-

5] and thin film [4-10] optical materials can be increased by first illuminating the material at a sub- threshold fluence.

Threshold increases have been observed for photon wavelengths ranging from 10.6 |jm [5] to 0.248 (im [6]. This

sub-threshold illumination has been referred to as "laser conditioning" or "laser annealing". Increases in damage

thresholds by a factor of three are typical but increases as high as a factor of 10 have also been reported [5]. A few

studies have attempted to determine which conditioning parameters provide the largest increase in the damage

threshold. Arenberg et al. [9] have studied the laser conditioning effect as a function of the conditioning fluence

for 1064 nm/532 nm antireflection coatings of undisclosed composition. Laser conditioning at 1064 nm increased

the 1064 nm damage threshold by 40% but did not influence the 532 nm threshold. The probability of damage

occurring at fluences slightly above the unconditioned damage threshold was found to decrease as the conditioning

fluence approached the unconditioned damage threshold. The conditioning effect in Arenberg's study was also

found to last only 4-5 days. The 532 nm damage threshold was not influenced by conditioning at either 532 nm or

*Work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National

Laboratory under contract number W-7405-ENG-48.

376
1064 nm. The authors suggested that the conditioning effect was due to removal of water from the films. Stewart

et al. [8] have studied the effect of rastering single layer dielectric films with a CW C02 laser. For fluences ranging

from 25 W/cm2 to 100 W/cm2 they observed indications, from both optical and structural measurements, of

recrystallization of the dielectric materials. They reported a correlation between the crystallization and a small (<

40%) increase in the damage threshold for thin films of rf-sputtered Ta2Os and sol-gel TiOr Areas conditioned in

these earlier studies were typically the size of the damage test laser beam, no larger than a few mm in diameter. To

our knowledge, the only direct test of the practicallity of laser conditioning large area optical materials was a study

of KDPcrystals [3] . It was shown that the 1064 nm damage threshold of KDP could be increased by rastering a large

area of the crystal with a small area beam from a rep-rated laser. The conditioning effect was observed for subthreshold fluences of either 1064 or 350 nm light. In contrast to Arenberg's AR coatings, the conditioning effect on KDP has been shown to be permanent over more than a month [2]. Clearly the laser conditioning phenomena

is complex and generalizations must be made carefully.

This paper presents the preliminary results of a study to determine how the laser conditioning effect could

be used to increase the damage threshold of the Nova turning mirrors. Laser conditioning of 1064 nm dielectric

high reflective coatings was first reported by Wilder and Thomas [10]. In a companion paper in the present volume,

Wolfe et al. [11] has reported on the influence of materials and coating design on the increase in the laser damage

threshold due to laser conditioning. Here we concentrate on large area conditioning studies on two types of HR

coatings: a) the present Nova mirrors: ZrO/Si02 reflectors designed for 1064, 532, and 355 nm light, and b) R&D

HfO/SiOj reflectors designed for 1064 nm light. Both coatings were deposited by OCLI* using electron beam

deposition in a 3-m diameter planetary coater. This production coater is designed to handle up to three 1 -m diameter

substrates and was purchased and installed by LLNL during the construction of the Nova laser. The ZrOj/SiOj

coatings were made in 1983 while the HfO/SiOj coatings were made in 1989. The two principal large area conditioning methods examined were i) rastering a small area beam back and forth across the sample surface, and

ii) illuminating a large area optic using a large aperture laser beam such as that available on the Nova laser. Laser

conditioning parameters examined included the illumination fluence and the number ofillumination pulses. Wealso

performed preliminary conditioning tests using broadband flashlamps.

Several mechanisms have been proposed in the literature to explain the conditioning phenomenon observed

for bulk materials, surfaces, and thin films. These mechanisms include the desorption of water or other contaminants

from the surface [7,9], melting or recrystallization of structural defects [8], and the slow volatilization of absorbing

inclusions from within the films [10]. Wolfe et al. [11] has experimentally demonstrated that for the HR coatings

of interest here the first two of these mechanisms do hot seem to apply. Edwards et al. [12] has similarly presented

arguments against the applicability of the absorbing inclusion model to damage in the Nova mirrors of interest here.

We instead propose that the conditioning effect in the dielectric HR coatings is related to the emptying of electronic

defect states in the bandgap. These electronic defects serve as the source of conduction band electrons required to produce damage in the dielectric layers. We will discuss this damage/conditioning mechanism in detail in Section

6.

2. Standard damage testing of present and R&D Nova mirror coatings:

All damage tests were made using a 1064 nm Nd:YAG laser with a beam diameter of 0.2 mm at 80% of the

peak fluence. The pulse length was 8 ns and the repetition rate was 18 Hz. The damage threshold was chosen as the

lowest fluence which caused a light flash at the coating surface and a visible change in the oxide surface properties,

as determined by the sensitive "breath test" [13]. The breath test relies on water vapor condensation patterns to

identify damage areas. The damage thresholds obtained in this way are in good agreement with those obtained using

xlOO Nomarski microscopy. Two types of damage tests were performed using the 18 Hz laser: S-on-1: multiple shots of the same fluence at a single site. R-on-1: multiple shots of increasing (ramped) fluence at a single site.

For the R-on- 1 tests the laser fluence was increased at -0.2 J/sec until damage was observed. This rate corresponds

to a fluence increment per shot of -0.013 J/cm2

. For S-on-1 tests the samples were illuminated for about 30 seconds

(-530 shots) unless damage was observed, in which case illumination was stopped immediately. In nearly all Son-1 tests where damage was observed it occurred during the first couple of pulses. The damage threshold values

reported are ±15%. Each reported threshold represents the average of 1 to 4 tests.

'Optical Coatings Laboratories Inc., Santa Rosa, Ca

377
S-on- 1 damage tests showed that the two HRcoatings studied had similar thresholds (fig. 1 ). The thresholds

at 8 ns were 12 J/cm2 for the present Nova coating and 16 J/cm2 for the R&D coating. These values are essentially

the same given the accuracy of the measurement. The R-on-1 tests, however, produced quite different results for

the two coatings. For the present Nova coatings the R-on-1 threshold was 18.5 J/cm2

, a factor of 1.5 increase over

the S-on-1 (i.e. unconditioned) threshold. For the new coatings the R-on-1 threshold was 44 J/cm2

, corresponding

to a factor of 2.7 increase in the damage threshold. In most cases the damage in these coatings is observed to occur

at microscopic (< 50 |im) defects in the coatings. (The8nsR-on-l andS-on-1 thresholds reported here for the R&D

coatings agree well with values of 1 6 (S-on- 1

) and 37 (R-on- 1 ) J/cm2 predicted from data obtained at different pulse

lengths on other LLNL damage test systems [1 1]).

Unconditioned

50 — Ramp

conditioned

2o

40

30

20

10

Present Nova

2r02 /Si02

R&D

Hf02 /Si02

Figure 1 : Unconditioned and ramp conditioned 1064 nm damage thresholds

(18 Hz, x =8ns) of Nova ZrCySiO, and R&D HfO/SiO, HR coatings.

Conditioning performed using damage test laser.

The R-on- 1 tests clearly showed that laser conditioning had a larger effect on the R&D coatings than on the

present Nova coatings. There are three major differences between the R&D and current Nova HR coatings that may

cause the differences in their damage performance: i) change of high index material (Hf02 vs. Zr02), ii) change of

high reflectivity design wavelength, and iii) decreases in the size, density, and perhaps type of film defects due to advances in coating technology since the present Nova coatings were made. Damage measurements on R&D WOJ

Si02 and ZrO/SiOjOf various designs indicate that the material or design dependence of the conditioning effect is small [11]. Therefore we conclude that items (i) and (ii) above have a negligible effect on the conditioning of the

films used in this study. This leaves item (iii), differences in defects, as the likely explanation for the conditioning

effect differences. This also agrees with our observation that damage in both films (R&D and Nova) typically

originate at the site of visible defects.

Measurements were made of the film defect sizes using dark field, bright field, and Nomarski microscopy.

All three techniques showed that the defects in the R&D coatings were smaller than those in the Nova coatings:

nominally 5 to 1 0 |imvs. 1 0 to 30 (im, respectively. The R&D coatings also had a slightly lower defect density (about

30 to 80 per mm2

) compared to the current Nova coatings (about 100 per mm2

). It is unlikely that these modest

changes in size or areal density of the defects, per se, influences the damage threshold. Rather, these differences

probably indicate different types of defects, with one type of defect being more susceptible to conditioning than the

other.

378
Differences in the performance of the two coatings is also indicated by the dependence of their damage

threshold on laser pulse length. For the present Nova mirrors no pulse length dependence is observed [14]. For the

HfOj/SiOj R&D coatings, however, the damage threshold, DT (J/cm2

) is dependent on the pulse length, x

p(ns) as [1 1] DT = 7.1 V

355

- (1)

The differences in the pulse length dependence and the extent of threshold enhancement due to conditioning for the

two coatings may indicate a change in the laser damage mechanism. It should be emphasized that without

conditioning the damage thresholds of the two films studied are approximately the same at 8 ns. The threshold

improvements for the R&D coating is apparent only after conditioning.

3. Large Area Conditioning via Raster-scanning:

In the R-on-1 tests described above an area only the size of the test beam (-0.2 mm) was conditioned. We

are interested, however, in laser conditioning coatings the size of the Nova mirrors (1.1 m), therefore other, more

practical, methods of laser preconditioning were studied. Here we present the results of conditioning studies

performed by rastering a large coating surface with a small area beam (-0.2 mm) at laser fluences below the S-on1 damage threshold. The rastering was done using the damage test laser and a programmable x-y stage. The x-y

stage velocity was chosen such that the sample was shot every 0. 1 mm in both the x and y directions. This scan rate corresponds to four shots/site for a 0.2 mm diameter beam. Areas of -4 cm2 were rastered with various fluences

below the S-on-1 damage thresholds of both coatings. Four types of pre-conditioning programs were examined in

this study:

a) raster at 10% of the S-on-1 threshold

b) raster at 63% or 55% of the S-on-1 threshold

c) raster at 85% of the S-on-1 threshold

d) consecutive rasterings at five fluences increasing from 37 to 85% of the

S-on- 1 damage threshold (hereafter referred to as "step conditioning").

These conditioning programs, along with that for ramp conditioning, are shown graphically in fig. 2. For the 5-step

conditioning the time between individual illuminations was approximately 1 hr. This is in contrast to the ramp

conditioning where the time between shots is -0.1 s. Note also that the increment in fluence between shots in the

ramp is only -0.05% of the S-on-1 damage threshold.

Figure 2: Laser conditioning program used in raster conditioning

and ramp conditioning experiments.

379
For both the R&D and present Nova coatings, single or multiple fluence raster conditioning resulted in an

increase in the S-on- 1 threshold. The average damage thresholds measured for the different conditioning programs

are shown in fig. 3. Conditioning increased the damage threshold of the present Nova mirror by a factor of 1.2 to

1.3. For the R&D coatings, however, conditioning increased the threshold by a factor of 1.2 to 2.4. It is not clear

at this time which type of raster conditioning program would provide the largest increase in the damage threshold.

It appears, however, that no clear advantage is gained by step conditioning. The most important conclusion reached was that for both coatings all raster conditioning programs resulted in conditioning factors less than that obtained

by the ramped fluence technique (i.e. R-on-1).

R&D Hf0

2 /Si02 Nova ZrCySiQ,

10% 55% 85% 5-step 63% 85% 5-step

Condition program

Figure 3: Conditioned 1064 nm damage thresholds (18 Hz, x = 8ns) of

Nova ZrO/Si02 and R&D HfCySiOj HR coatings for various raster conditioning programs. Unconditioned and conditioned thresholds

are included for reference.

In all tests we defined the damage threshold as the lowest fluence at which damage occurred. Below that threshold we assume that there is zero probability of damage. As the fluence is increased above the threshold the

probability of damage increases until a fluence is reached at which any location would damage. We observed that

the range of fluences over which the damage probability changed from 0% to 100% was dependent on the

conditioning history of the sample. For S-on-1 thresholds the 0-100% damage probability transition range was -10

J/cm2, while for the R-on- 1 tests the range was > 40J/cm2

. For raster conditioned samples the range was intermediate

between the S-on- 1 and R-on- 1 cases. The change in the abruptness of the damage threshold indicates that the film

properties which control the damage threshold after conditioning are not uniform across the sample. It should be

noted that since the damage threshold is not abrupt for the conditioned samples, the choice of the lowest damage

fluence as the reported damage threshold results in a conservative value for the conditioned damage threshold. In some areas R-on- 1 testing increased the damage threshold by more than a factor of four.

4. Large Aperture Nova Conditioning

Using the beam size and raster rate used above, it would take nearly two months to raster a 109 cm diameter

mirror. Obviously a more practical large area conditioning technique is required. We therefore next examined the

effectiveness of using a large aperture beam from the Nova laser (Ins, 1064 nm) for the conditioning illumination.

380
If this method is effective, Nova mirrors can be conditioned in-situ. Two 5-cm samples were examined in this study:

a HfO/Si02 1064 nm HR and a ZrO/Si02 1064 nm HR. It has been shown that the 1-on-l (single pulse at a given

site) damage thresholds for these two coatings at 1064 nm are both ~7 J/cm2 at 1 ns [11]. During testing the coating

sumples were mounted down line from a condensing lens which focussed a mid-chain Nova beam down to a 4 cmdiameter spot. Shot energies were measured using a calorimeter and the beam profile was recorded on film.

Densitometer traces of the developed film were used to measure beam modulation and the average fluence. After

each shot the sample was removed from the holder and visually inspected under a microscope at magnifications up

to 40X using bright-light illumination. The whole coating surface was also photographed for macroscopic changes

using a 35-mm close-up camera. At the completion ofinspection, the sample was drag-wipe-cleaned using methanol

and remounted in the test system. A fiducial mark was used to insure the sample alignment remained the same from

one shot to the next. At the completion of the tests on Nova, the samples were again photographed at magnifications

up to 400X using both Nomarski and bright and dark field illumination.

A total of seven laser shots were fired at the HfCySiC^ and one at the ZrCySiOj coating. Figure 4 shows

the illumination history in terms of the average fluence for each shot and fig. 5 shows a typical fluence profile for

the Nova beam. Note that the peak-to average modulation of the beam intensity is about 1 .3-to-l. In the case of the

HfCySiC^ coating, we slowly increased the laser fluence from a value of about 3.5 J/cm2 up to about 12 J/cm2

; 12

J/cm2 is about 1 .5 to 2 times the 1 -on- 1 damage threshold. Microscopic and large area inspection of the HfCySiC^

coating after each shot showed no change. Furthermore, the "breath-test" also showed no change in the sample. The

final Nova shot was fired on the Zr02/Si0

2 coating. This single shot, having a mean fluence of about 10.6 J/cm2 was

significantly above the single-shot damage threshold (7 J/cm2

). As expected, the sample damaged; this was clearly seen by both our microscopic inspection and the "breath-test". These Nova test results show that large area optics can be conditioned using the Nova laser beam.

14

Eo

0)o

c

0)3

E

reo

n

c

rea> 2

12 —

10

No damage

Damage

Sample 1 Sample 2

6 —

Unconditioned

damage

threshold

5 6 7

Shot number

Figure 4: Beam fluence vs. shot number for large aperture

Nova conditioning experiment. X=1064nm, x = 1 ns.

Previous small-area damage tests had shown that the conditioned damage threshold for the HfCySiOj

coating should be about 18-20 J/cm2 (at 1-ns) vs. an unconditioned (1-on-l) threshold of 6-8 J/cm2

. We decided not

to test the HfCX/SiC^ sample on Nova above about 12 J/cm2

. This was for two reasons: first, it provided us with

a conditioned, yet undamaged large area sample that could subsequently be tested to determine if the conditioning

effect was permanent. Second, because of beam modulation the peak fluence seen by the test sample may have been

as much as 1 .3-times the mean fluence. Thus at a fluence of 1 2 J/cm2

, some regions for the sample could be subjected

to fluences near the maximum damage threshold expected from conditioning.

381
Figure 5 : Example oftypical Nova arm-9 beam profile showing degree

of modulation. The split down middle of beam is due to an apodizer.

In order to compare the damage thresholds obtained by Nova and raster conditioning we damage tested, at 8 ns, the Nova conditioned sample. Figure 6 shows that the Nova conditioned sample has a damage threshold in the range obtained by raster conditioning. Note once again that all the large-area conditioned thresholds are lower

than that obtained by ramp conditioning.

50

40 —

30

E

oS

oc

V)

<d 20

D)

tOE

Q

10

x2.7

x2.4

Ramp

conditioned

x1.2

x2.2

x2.1

x 1.4

Unconditioned

10% 55% 85% 5-step Step

I I i Raster Nova

Figure 6: 1 064 nm damage thresholds ( 1 8 Hz, T

p=8ns) of R&D HfO/

Si02 HR coatings conditioned by raster scanning and large aperture

Nova illumination.

382
The HfCySiOj coating that was conditioned on Nova was further damage tested to determine if the effect

waspermanent. These damage tests were done at 1064nmand 10 ns; the results are shown in fig. 7. Tests conducted

over a period of about 10 weeks showed no drop in the conditioned damage threshold.

40

E

o

2 30

o

c(AO

S, 20

coE

<o Q

10

I

I

Unconditioned

threshold

20 40 60

Days since Nova irradiation

80

Figure 7: 1064 nm damage thresholds (x = 10 ns) vs. time after

conditioning for the R&D HfO/Si02 HR coatings illuminated on Nova.

Independent conditioning tests performed by Floch (at Commissariat a l'Energie Atomique, France) using

Hf02/Si0

2 coatings prepared by Matra (France) showed no change in damage threshold for up to six weeks [15].

Damage threshold improvements of a factor of 2-3 were observed in the French study.

5. Flashlamp Conditioning Experiments

Besides the raster and Nova conditioning methods, we also attempted to condition the thin film dielectrics

by broadband flashlamp illumination. Broadband illumination is commonly used to decompose or outgas

contaminants from vacuum systems. Similarly, we intended this process to remove organic contaminants from our

thin films. The flashlamp study consisted of illuminating a HfO/SiC^ HR sample with 20 flashes of a Xenon arc lamp. The illumination intensity was about 10 J/cm2 for each shot and the pulse length was 0.5 ms. The spectral

output of the lamp is given in fig. 8. Subsequent S-on-1 damage tests to the sample showed that flashlamp

illumination did not change the damage threshold of the HR coatings. Two obvious differences between the

flashlamp and the laser illumination are the range of photon wavelengths involved and the magnitude of the electric

field produced in the sample. The long pulse length of the flashlamp results in a low electric field since

E=[2FL/T

p ny]°5 (2)

where E is the electric field (V/cm), FL

is the fluence (J/cm2

), X

p is the pulse length (sec), n is the refractive index

(1.45 for Si02), and y is the admittance in free space (2.66xl0 3 F/s). For FL=10 J/cm2 and x= 5X104 sec, the electric

field is 3xl03 V/cm. This field is less than 1% of the field associated with an 8 ns laser pulse of the same fluence.

Possible connections between illumination wavelength, electric field, and the conditioning effect are discussed in Section 6.

383
Wavelength (nm)

Figure 8: Time integrated Xe flashlamp output spectra [36]. Outut

below 350 nm is suppressed by Ce dopant contained in the flashlamp

envelope.

6. Electron Defect Model

Several different mechanisms for laser conditioning have been suggested in the literature (film crystallization, water desorption, etc.). Different mechanisms may apply for different combinations of materials, deposition

methods, illumination wavelength, etc.. A complete review of the conditioning mechanism literature is outside the

scope of this text. We are interested here primarily in the laser conditioning of 1064 nm dielectric HR films of HfO/

Si02 deposited by e-beam evaporation.

We are presently examining a possible mechanism for laser induced optical damage in which conduction

band electrons are heated by interaction with the laser beam and subsequently transfer energy to the crystal lattice. Damage occurs when the film temperature reaches some critical value such as the melting point of the dielectric

material. There are two conditions that must be met for film damage to occur: (i) electrons must be available in the

conduction band, and (ii) the laser intensity must be high enough to transfer sufficient energy to the lattice to cause damage. We propose that the source of conduction band electrons is the photo-excitation of electrons from shallow

defect levels located below the conduction band edge. We further propose that laser conditioning occurs due to the

removal of this source of conduction band electrons. When the dielectric material is illuminated at a low fluence,

the electrons in the defect levels are excited to the conduction band. Since the optical electric field is not large enough

to cause damage the electrons decay into deep levels from which they cannot be easily excited into the conduction

band on subsequent illuminations. A diagram of the proposed band structure for the unconditioned and conditioned

dielectrics are shown in fig. 9. When the 'conditioned' dielectric is subsequently illuminated at intensities above

the unconditioned damage threshold, the number of electrons available for transfer to the conduction band is low

and, therefore, the net energy transfered to the lattice is too low to cause damage. This increase in the damage

threshold is the observed laser conditioning effect.

The importance of conduction band electrons to optical damage to bulk dielectric samples has been

demonstrated by other investigators. Jones et al. [16] have shown that no heating of a-Si0

2 occurred by 1064 nm

illumination unless the sample was simultaneously illuminated with 266 nm light. The 266 nm light provided

384
p /////////////////////////////////////// mmmmmmmm/

Not

conditioned Conditioned

Figure 9: Schematic energy band diagram of the location of electrons

(•) in traps (-) for unconditioned and laser conditioned dielectric films.

conduction band electrons by multiphoton excitation from the valance band. Once these electrons were present in the conduction band, they could be heated by the 1064 nm photons (i.e. free carrier absorption). Similarly, Kerr et

al. [17] has shown that the 10.6 (im damage threshold for silicon and glass is decreased by simultaneous illumination

by UV light. Also, Soileau et al. [18] has shown that the damage threshold of bulk fused Si02 can be decreased by

inducing electronic defects in the material by gamma irradiation. These defect levels serve as a source of conduction

band electrons leading to laser damage. In that study the electrons were excited to the conduction band by two photon

absorption of 532 nm photons. In the following paragraphs we consider the nature of the defect levels in the HR

films of interest to Nova and discuss the mechanisms by which energy is transferred from the laser pulse to the lattice via the conduction band electrons.

6.1. Free carrier generation

Assuming a one photon (1064 nm) excitation process, the defect states contributing electrons to the

conduction band ofour dielectric films could be up to 1 .2 eV below the band edge. In the presence of the large optical

electric field (~106 V/cm2

) produced by the laser pulse, activation barrier lowering associated with the Poole-Frenkel

effect may occur however [19]. The magnitude the the barrier lowering, AEb (eV), is given by

AEb = [e3E/7te

oe] 1/2 (3)

where E is the magnitude of the electric field (V/cm), e is the charge on an electron, and e is the dielectric constant,

and eo is the permitivity of vacuum (F/cm). The resulting decrease in the activation energy of the trap-to-conduction

band transition would be as much as 0.6 eV for the materials of interest here. The Poole-Frenkel effect would

therefore allow a 1064 nm photon to excite electrons from states nearly 2 eV below the conduction band edge.

Electron paramagnetic resonance has been used to demonstrate that occupied defect levels may be present

between the Fermi level and conduction band in amorphous Si02 [20]. If the defect levels are occupied by only one

electron they are paramagnetic and produce an EPR signal. Stathis et al. [21] has shown that these defect levels in high quality a-Si02 are normally empty but can be filled by illumination with UV photons. One particular type of

385
defect, generally associated with a peroxide radical, can be made paramagnetic by illumination with 7.6 eV photons.

If the electrons excited into the defects by the 7.6 eV photons come from the valance band, these defect levels must

be about 1-2 eV below the conduction band of the ~ 9 eV bandgap Si02. It should therefore be possible to excite

the electrons from these paramagnetic states into the conduction band by the absorption of a single 1064 nm photon

(1.2 eV).

In its most stable state high purity a-Si02 should not have any occupied electronic levels above the Fermi

level. Occupied states can be induced in this material by UV, x-ray, or charged particle irradiation. Schwartz et al.

[22] has shown, however, that paramagnetic bonding defects are present in some as-deposited thin film SiOr

Since

e-beam and rf-sputter deposition techniques are accompanied by soft x-rays and energetic particles, electrons and

holes can be generated during deposition. These charge carriers could then become trapped at appropriate precursor

sites resulting in the formation of paramagnetic point defects. For techniques such as hot wall low pressure chemical

vapor deposition (LPCVD) and plasma enhanced chemical vapor deposition (PECVD), which do not have attendant

soft x-rays or energetic particles, no paramagnetic states are observed. Cold wall LPCVD, which produces nonequilibrium films, have paramagnetic states but they can be annealed out by heating the samples to 300°C. Optical

absorption measurements on rf-sputtered films of Si02 have also shown that the electronic properties of thin films

are dependent on deposition parameters [23]. Paramagnetic defect densities on the order of 1016/cm2 have been

measured [21].

It has often been suggested that damage in dielectric multilayers occurs first in the high index material.

Studies of single layer materials have shown that the damage threshold of Hf02 and Zr02 are typically below that of Si02 [24]. Unfortunately there is little information available in the literature regarding defect levels in Hf02 and

Zr02

. It has been shown, however, that structural defects produced by ion implantation can reduce the bandgap of

Hf02 by as much as 2 eV [25]. In general, the electronic band structure of materials are more sensitive to long range

structural disorder the smaller their bandgap [26]. Therefore the electronic properties of Hf02 and Zr02 are more

likely to be influenced by structural defects than those of Si02

. For the HfO/Si02 films studied here, x-ray diffraction

studies have shown that the Si02 layer is amorphous. The Hf02 layer, however, is amorphous at the Si02 surface

but as the Hf02 layer thickens, oxide with a predominantly monoclinic structure is produced [27]. This same type

of structural transition has been reported for thin films of Zr02 [28].

The model presented here assumes that charged defects are the primary source of conduction band electrons

in the dielectric films. There are, however, four other sources of conduction electrons which we must consider:

1 . Thermal detrapping from shallow levels in the dielectric . Since the large bandgap insulators of interest have

very low conductivities, this source of electrons is unlikely.

2. Multiphoton absorption by valance band electrons. For the 1064 nm irradiation 5 photons would be needed

for ionization in Hf02 and 8 photons would be required for Si02. It has been suggested in the literature that multiphoton ionization is unlikely for n>4 [ 16] . Photoacoustic measurements by Jones et al.[ 1 6] have indicated that no multiphoton ionization occurs in high purity a-Si02 for 1064 nm photons. Three photon absorption is observed

however for 266 nm light. The number of photons required for multiphoton absorption in Si02 and Fif02 given above

are based on the bandgaps of the near perfect crystalline materials. These bandgaps are 9.0 eV and 5.1 eV,

respectively. For highly defective thin films, however, bandgaps several eV lower have been measured for both Hf02 [25] and Si02 [23]. For these lower bandgap films multiphoton processes may become more significant.

Photoacoustic measurements are planned which will determine if multiphoton absorption is important at 1064 nm

for the e-beam films of interest here.

3. Electron-impact ionization and subsequent avalanche formation. Recent photoacoustic measurements [16]

indicate that avalanche ionization does not occur in KBr, NaCl, KI, or a-Si02

. Furthermore, experimental and

computer modeling studies by DiMaria et al. [29,30] have shown that the distribution of energies of conduction

electrons in Si02 is not high enough to allow impact ionization to occur. No comparable measurements have yet

been reported on Hf02 or Zr02

, however.

4. Electrons produced by laser induced defect formation (i.e. F-centers). It is believed that if F-centers were

produced by the laser pulse illumination, an accumulation effect would be apparent, i.e. single shot damage

thresholds would be higher than multiple shot thresholds [16]. Such an accumulation effect has not been observed

for the dielectric stacks of interest here, but has been observed in sol-gel processed Ti02 coatings [31].

386
Based on the available literature, it therefore seems likely that laser damage in our dielectric materials is associated

with conduction band electrons and that the source of these electrons is shallow defect levels.

6.2.. Energy transfer

There are three commonly discussed mechanisms for transfer of energy between the laser beam and the

crystal lattice of a dielectric material. In the classic electron-avalanche model of Bloembergen and Yablanovitch

[32], the lattice heating mechanism was taken from the Drude model for ACconductivity in metals [33]. In the Drude

model, electrons are accelerated by the optical electric field and then lose energy to the lattice by electron-phonon

interaction (i.e. Joule heating). The other two mechanisms for lattice heating: (a) free-electron heating and (b)

polaron heating, involve the direct absorption of photons and subsequent transfer of energy to the lattice by electronphonon interactions. Free electron heating is more likely for the ionic materials and the short wavelengths of interest

here [16]. The three mechanisms can be experimentally differentiated by the relationship between the energy

absorbed and the laser pulse energy. For example, the energy absorbed by the free-electron model has a non-linear

dependence on pulse energy while for the polaron mechanism the dependence is linear. Jones et al. [ 1 6] have shown

that for Si02

, and for many alkali-halides, lattice heating occurs by the electron-heating mechanism.

7. Discussion of Results

The laser conditioning experiments reported here demonstrated that the damage thresholds of the dielectric

multilayer stacks could be increased as a result of as few as one subthreshold illumination pulse. Based on the simple

laser conditioning mechanism discussed above, it might be expected that the effect of conditioning should increase

with fluence or number ofpulses until the cumulative photon flux has removed all the electrons from the defect levels near the conduction band. Further illumination should have little effect. Some of our data suggests this picture is

correct whereas other results suggest that the conditioning mechanism is more complex than this. Clearly more data

is required in order to understand the relationship between the conditioning program and the increase in the damage

threshold fluence.

We have observed that damage in the dielectric stacks tends to occur at the sites of film defects visible by

Nomarski microscopy. The defect driven damage is likely the result of either of two mechanisms: (i) the defects

are regions of high concentrations of occupied defect levels which serve as the source of conduction band electrons, or (ii) the defect areas are regions of enhanced electric fields, as described by Bloembergen [34] , and therefore allow

damage to occur at lower fluences than that of a perfect material.

An interesting observation reported by Wolfe et al. [1 1] is that the pulse length dependence of the damage

threshold for both unconditioned and ramp conditioned materials are the same. The thresholds are proportioned to T

p

* where x is approximately 0.3. This result may suggest that the damage mechanism remains the same after conditioning.

Our attempts to condition the HR films by flash lamp illumination were not successful. There are three

possible explanations for this lack of a damage threshold increase. The first explanation is related to the broadband

nature of the flashlamp. The optical film is simultaneously being irradiated with both UV and IR irradiation. Stathis

[35] has shown that paramagnetic states can be generated by UV illumination. The UV photon may therefore counter

the emptying effect of the IR illumination. The second explanation is related to the low electric field produced by

the long pulse length (~1 ms) flashlamp output The field produced by the flashlamp is a factor of 103 lower than that of the 8 ns laser pulses. If barrier lowering by the Poole-Frenkel mechanism is required for the excitation of trapped

electrons, then the flashlamp illumination would not result in a conditioning effect. Third, the flashlamp output

produces a much lower energy density over a given wavelength interval than does a laser. Consequently, if the

absorption cross section associated with the conditioning process is small (and wavelength specific) then the effect

of the flashlamp would be expected to be negligible. In addtion, flashlamp illumination has also been shown not to

alter the damage threshold of laser conditionable KDP [2].

It may be possible to empty the paramagnetic defects by thermal excitation as well as optical excitation.

387
Several paramagnetic states in a Si02 can be emptied by thermal treatments at temperatures below 700°C [35]. No

information is available on thermal detrapping from states in Hf02

. For dielectric multilayers thermal conditioning

is limited by thermal stress build up and may not be practical for thin films prepared by physical vapor deposition

methods.

8. Conclusions

The damage threshold of HfOj/SiOj high reflection coatings can be increased by a factor of about 2.7 by

ramped-fluence laser conditioning (i.e. R-on-1 damage tests). This laser conditioning effect can be used to permanently increase the damage threshold of large area optics. Large areas were conditioned by either rastering a surface with a small beam or by illumination with a large aperture beam such as that available on the Nova laser.

In order to be practical, these large area conditioning methods require illumination in fairly large incremental steps

in laser fluence. Such discrete illumination techniques produced smaller increases in damage threshold than can be

obtained by the small step, ramped-fluence techniques. Increases of a factor of about 2 can be expected for discrete

step, large area conditioning. Further study is needed to determine the optimum discrete fluence illumination

program. In the case of practical conditioning of Nova turning mirrors, the large aperture conditioning procedure

is the most attractive since the mirrors can be conditioned in-situ on Nova. In contrast, raster conditioning of a 1.1 m diameter mirror with the damage test beam would take several weeks and a dedicated facility.

The mechanism proposed here for the laser conditioning phenomenon is based on the presence of subbandgap electronic defect levels that are intrinsic to the e-beam deposition process. This is in contrast to extrinsic

mechanisms, such as absorbing inclusions and water contamination, that have been suggested in the past. The move

toward intrinsic mechanisms may indicate that thin film processing techniques have improved over the years, as should be expected. We are currently involved in using the EPR technique to identify the electronic defects that are present in the dielectric films and to determine if the charge state of the defects are altered by the sub-threshold

conditioning illumination. If the electronic defect model rs correct, further improvements in film properties might

be gained by using deposition techniques that are more "equilibrated" (in a structural or electronic sense). Plasmaassisted CVD is one such possible coating method.

References:

1. B. Newnam, A. Nowak, and D. Gill, " Short pulse C02-laser damage studies of NaCl and KC1 windows".Laser

Induced Damage in Optical Materials: 1979. NBS Spec. Pub. 620, p. 209.

2. J. Swain, S. Stokowski, D. Milam, and G. Kennedy, The effect of baking and pulsed laser irradiation on the

damage threshold of potassium dihydrogen phosphate crystals", Appl. Phys. Lett. 41 (1982) 12.

3. R. Gonzales, M. Staggs, M. Singleton, D. George, C. Weinzapfel, S. Weinzapfel, "Variations with laserpulse

duration of the thresholds at 350 nm and 1064 nm for bulk damage in crystals of KDP", UCLR-95284, presented

at the Boulder Damage Conference, 1986.

4. V. Wang, J. Rudisill, C. Giuliano, M. Braunstein and A.Braunstein, "Pulsed C02 laser damage in windows,

reflectors and coatings", Laser Induced Damage in Optical Materials: 1974. NBS Spec. Pub. 414, p. 59

5. B. Brauns, D. Schafer, R. Wolf, and G. Zscherpe, "Effect of the substrate preparation with C02 laser radiation on the laser resistance of optical layers", Thin Solid Films, 138 (1986) 157.

6. F. Rainer, D. Milam, and W. Lowdermilk, "Laser damage thresholds of thin film optical coatings at 248

nm".Laser Induced Damage in Optical Materials: 1981 . NBS Spec. Pub. 638, p. 339.

7. J.E. Swain, W.H. Lowdermilk, and D. Milam, "Raising damage thresholds of gradient -index antireflecting

surfaces by pulsed laser irradiation" Appl. Phys. Lett. 41 (1982) 782.

388
8. A. Stewart, A. Guenther, F. Domann, "The properties oflaser annealed dielectric films". Laser Induced Damage

of Optical Materials: 1987. NBS Spec. Pub. 756, p. 369.

9. J. W. Arenberg and D. Mordaunt, "Experimental investigation of the role of wavelength in the laser

conditioning effect", Laser Induced Damage in Optical Materials: 1988 . NBS Spec. Pub. (to be published).

10. J. Wilder and I. Thomas, "Effect of n-on-1 laser treatment on damage threshold of selected optical coatings",

Laser Induced Damage of Optical Materials: 1988. NBS Spec. Pub. (to be published).

11. C. R. Wolfe, M. Kozlowski, J. Campbell, F. Rainer, A. Morgan, and R. Gonzales, "Laser pre-conditioning of

optical thin films", Laser Induced Damage of Optical Materials: 1989. NBS Spec. Pub. (to be published).

12. G. Edwards, J. Campbell, R. Wolfe, and E. Lindsey, "Damage Assessment and possible damage mechanisms

to 1-meter diameter Nova turning mirrors" Laser Induced Damage of Optical Materials: 1989. NBS Spec. Pub. (to

be published).

13. T.J. Baker, "Breath figures", Phil. Mag., 44 (1922) 752.

14. G.R. Wirtenson and F. Rainer, LLNL intermal memorandum SOC 88-005 (Feb. 1988).

15. Herve Floch, Commissariat a l'Energie Atomique, Villeneuve Saint Georges, France, private communication.

16. S.Jones, P. Braunlich,R.T. Casper, X. -A. Shen, and P. Kelly,"Recent progress on laser-induced modifications

and intrinsic bulk damage of wide-gap optical materials", Optical Engineering 28 (1989) 1039.

17. N. C. Kerr, S. E. Clark, and D. C. Emmony, "UV seeding of IR laser induced damage" abstracts to the Laser

Induced Damage of Optical Materials: 1989. Boulder CO., Oct. 1989.

18. M. J. Soileau, N. Mansour, E. Carlo, and D. L. Griscom, "Effects of radiation induced defects on laser-induced

breakdown in Si02", Mat. Res. Soc. Svmp. Proa. Vol. 61 (1986) 205.

19. A. R. Newark and U. Stimming, "Photoinduced electron transfer involving localized electronic states",

Electrochimica Acta, 32.(1987) 1217.

20. E.P. O'Reilly and J. Robertson, "Theory ofDefects in Vitreous Silicon Dioxide", Phys. Rev. B, 27 (1983) 3780.

21. J. H. Stathis and M. A. Kastner, "Photoinduced Paramagnetic Defects in Amorphous Silicon Dioxide", Mat- Res. Soc. Svmp. Proc . Vol. 61 (1986) 161.

22. R. N. Schwartz, M. D. Clark, W. Chamulitrat, and L. Kevan, "Electron Papamgnetic Resonance Studies of

Intrinsic Bonding Defects and Impurities In Si02 Thin Solid Films" in 'Defects in Glasses', Mat. Res. Soc. Svmp.

Vol. 61 (1986), 359.

23. T. Hickmott and J. Baglin, "Stoichiometry and atomic defects in rf-sputtered Si02", J. Appl. Phys., 5_Q (1979)

317.

24. A. Guenther and T. Humphreys, "Physical aspects of laser-induced damage of optical materials" , Proc. SPIE. Vol. 401 (1983) p. 247.

25. J. W. Schultze, B. Danzfuss, O. Meyer, and U. Stimming, "Electrochemical investigations of ion-implanted

oxide films", Mat. Sci. Eng. 69 (1985) 273.

26. J. Tauc, "Optical properties of non-crystalline solids" in Optical Properties of Solids. F. Abeles ed.,North

Holland Pubs., Amsterdam, 1972.

389
27. Tom Allen, Optical Coatings Laboratory Inc., private communication. (1989).

28. N.V. Krizosheen, N.N. Zapleshko, and N.F. Fedorov, "Interrelationship between the structure and optical

properties of zirconium dioxide thin films", Inorg. Mater., 19 (1983) 825.

29. D. J. DiMaria and M. V. Fischetti, "Hot electrons in silicon dioxide: Balistic to steady-state transport", Appl.

Surface Sci., 2Q (1987) 278.

30. D. J. DiMaria, M. V. Fishetti, M. Arienzo, and E. Tierney, "Electron heating studies in silicon dioxide: Low

fields and thick films", J. Appl. Phys. 60 (1986) 1719.

31. I.M. Thomas, J.G. Wilder, and R.P. Gonzales, "HR coatings prepared from colloidal suspensions", Laser

Induced Damage of Optical Materials: 1987 . NBS Spec. Pub. 756, p. 286.

32. E. Yablonovitch and N. Bloembergen, "Avalanche Ionization and the limiting diameter of filaments induced

by light pulses in transparent media", Phys. Rev. Lett. 29 (1972) 907.

33. N.W. Ashcroft and N.D. Mermin, Solid State Phyics. Saunders College, Philadelphia (1976).

34. N. Bloembergen, "Role of Cracks,Pores, and Absorbing inclusions on laser induced damage thresholds at surfaces of transparent dielectrics", Applied Optics, 12 (1973) 661.

35. Stathis, J. H., "Identification of native defects in a-Si02" in 'The Physics and Technology of Amorphous Si02

',

Edited by R. Devin (Plenum Publ. Corp., 1988), p. 141

36. C.R. Wolfe, J.H. Campbell, R.E. Lyon, J.H. Pitts, H.T. Powell, "Optical damage in epoxy polymers by

millisecond light pulses", Laser Induced Damage in Optical Materials: 1986. NBS Spec. Pub. 752, p. 194.

390
COMMENTS

Question: You said you did the abrasion with the diamond slurry. Did you

ever find any carbon from the diamond? Are you concerned about

the imbedding of the diamond in the film or the pick up of water

by the films?

Answer: There are several questions there. Do we see diamond paste or diamond particles in the crater itself. We were unable in these

craters to pick up carbon. The analysis goes down to parts per

thousand so that if there is a particle there we should see a half

micron or the tenth micron size particle. We should see a significant carbon signal if we hit one of those particles.

Probing around I have never seen the evidence of diamond imbedded

in that way. I would expect perhaps to see diamond in the ULE

glass simply because it is softer. In answer to the second part

of your question, we do not clean these specimens with water after

we crater them. We use organic solvents and essentially what we

do is to use the organic solvent to clean the specimen and then we

go back and lightly etch with perhaps a 1000 angstrom argon ion

beam to clean that up. I would not believe what I see at an interface that was cleaned in that fashion. I would step back and

depth profile away from that maybe a thousand angstrom. I would

have to go through a thousand angstroms of the layer above in

order to get to the interface. I tried to stay away for the

exposed interfaces and then move back, if you understand what I mean.

Question: There is a danger in coming back to that same point. If you are using a steel ball, might you not be getting carbon from the high

carbon steel embedded in the process.

Answer: If that were true we should see it all over the crater and we

don't. In the example I showed you where we had ion beam

texturing, if the carbon would have been uniform all over the

crater we should have seen that texturing all over the crater. It

did not appear all over the crater, it appeared at the interface.

Question: What is the energy resolution of your Auger spectrometer and can you distinguish different crystals in phases in the material?

Answer: No, we can not distinguish crystals in phases. Energy resolution,

that question is best answered this way. Peaks are broad, we are looking at 15 or 20 electron volts wide, the energy resolution of

this spectrometer is usually way below the width of the peak. So we are somewhere, I really forget the spec on the instrument, but

the peak shifts that we would expect to see are much, much larger

than the energy resolution of the spectrometer. However, I would

say this: we very seldom try to correlate peak shift information

in Auger data with chemical state. If you want to do that, go to

XPS.

391
Question: You mentioned that you didn't see any charging on this particular

film; is that a general result do you think or will it depend

strongly on the materials?

Answer: No that's magic, straight and clear. There are a number of tricks

one can use to get rid of the charging on these films. First, we

do these specimens at perhaps 60° to 70° degrees incidence. So

the charge tends to deflect off. Second, in the first generation

Auger instruments beam currents on the order of seven or eight

microamps were common. These spectra were taken at perhaps thirty

nanoamps so the beam current has been reduced significantly.

Finally, if you want to pull some other tricks what you can do is

to broaden out the spot and defocus it slightly. It doesn't

affect the peak shapes at all. What it does is to decrease the

energy density, which gets red of the charging. But every

material is different.

392
Manuscript received

2-14-90

Population Distribution of Conditioned Damage Thresholds

on AR Coated BK-7 Glass with Varying Laser Spot Size

D. W. Mordaunt and J. W. Arenberg*

Hughes Aircraft Company

P. O. Box 902, Bldg. E1/MS B129

El Segundo, CA 90245

In this paper we report on an empirical study of the population distributions of

conditioned laser damage thresholds on anti reflection (AR) coated BK-7 glass while

varying the laser spot size. Two different 1 .06 u.m AR coating designs were tested. All damage tests were conducted at the design wavelength of 1 .06 u.m. To obtain the

population distribution, 20 determinations of the conditioned damage threshold were

performed. This procedure was repeated for each spot size on each of the samples.

For control purposes, the unconditioned damage threshold was also measured for

each spot size and coating design. The data was analyzed statistically by describing

the population distribution and testing for differences as a function of spot size and

coating design.

Key Words: laser damage; damage test; laser conditioning; preconditioning;

prepulsing

1. Introduction

This paper reports an empirical study of the population distributions of conditioned

laser damage thresholds on antireflection (AR) coated BK-7 glass while varying the

laser spot size. The phenomenon of laser conditioning consists of an enhancement in the laser induced damage threshold due to exposure of the sample to subthreshold

fluences and has been well documented in the literature [1-8]. This conditioning effect

(also referred to as preconditioning) has been attributed to various effects including

the desorption of adsorbed contaminants in thin film coatings, the annealing of surface

defects and the annealing of molecular defects such as electron traps in a crystalline

substrate. The conditioning effect is particularly interesting because of the possibility

of permanently increasing the damage threshold of critical laser optics and thereby

obtaining an increase in the lifetime and reliability of the laser system. In this report we

continue our previous efforts to understand and quantify the laser conditioning effect.

We investigated the conditioning of two different designs and report the dependence

of the conditioning on the laser spot size. The results were analyzed to provide

information about the distribution and density throughout the coatings of the main actor

in the laser conditioning effect.

2. Experimental Results

The damage test facility is shown in figure 1 . It consists of a Nd:YAG oscillator and

two Nd:YAG amplifier heads which are capable of generating up to 1 .5 J of near

diffraction limited energy at the 1.06 u,m Nd:YAG fundamental. The energy is polarization outcoupled and focused onto the target plane where the sample is located. A wedge is used to generate two equivalent target planes where the beam

energy and spatial profile are measured. The sample is observed on line with a 25X

magnification microscope and video system and can be illuminated with up to 25 mW

of He-Ne laser power to observe changes in the surface scatter. We typically used

about 2.5 mW of He-Ne power in our experiments. The experimental test parameters

are summarized in table 1 . All of our testing was performed at 1 064 nm and damage

* Current address: TRW Space and Technology Group, One Space Park Drive, 01/1210,

Redondo Beach, CA 90278.

393
was defined as the first observable change that occurred when the sample was

observed on line at 25X magnification under illumination by a 2.5 mW He-Ne laser. All of the test samples consisted of multilayer dielectric thin film coatings deposited on BK7 glass using electron beam evaporation. Sample A was a MgF2/Al2C>3 multilayer

antireflection (AR) coating deposited on unheated substrates while sample B was a

Al203/ZrC>2 multilayer AR coating for which the substrates were heated prior to deposition.

Figure 1 . Layout of the damage test facility.

The unconditioned and conditioned thresholds were measured for both samples.

For the unconditioned test procedure, each site on the surface was exposed to only a

single fluence. The test sites were separated by at least three times the diameter of

the laser spot and each site was exposed to 25 shots or until damage was observed.

For the conditioned test procedure, each site was initially exposed to 1/2 the

unconditioned threshold and then the fluence was slowly ramped up stepwise until

damage was observed. The sites were exposed to 25 shots at each fluence level

before increasing to the next higher level. All threshold measurements are accurate to ±10%.

The results of the unconditioned threshold measurements are presented in table 2. The unconditioned threshold shows some dependence on spot size, particularly for

the smallest spot size of 0.25 mm. These results were fairly typical of off-the-shelf

production laser optics, with sample B being considerably more durable than sample

A. The unconditioned damage test results are presented in figures 2 and 3 as a plot of

the test result (damage or no damage) versus fluence. The unconditioned testing

showed considerable overlap between damaging and non-damaging fluences.

394
3

w

a> DC

(A

Damage

No Damage

1

1

1

1

. 1 1 0 50 100 150

Fluence (J/cm2)

Figure 2a. Unconditioned results for sample A with a 0.25 mm spot size.

(A

0> DC

V)

Damage

No Damage

OB 03

I

1 1 ' 1 ' 1

"

1

' 1 15 25 35 45 55 65

Fluence (J/cm2)

Figure 2b. Unconditioned results for sample A with a 0.5 mm spot size.

~ Damage

3

CO

<D QC

a) No Damage

B

G

I

'

1

' 1—I

1

1 1

1

1

1 1 <

1 20 30 40 50 60 70 80 90

Fluence (J/cm2)

Figure 2c. Unconditioned results for sample A with a 1 .00 mm spot size.

395
~ Damage

tn0

DC

a> No Damage

am

I

1

' 1 1 0 50 100

Fluence (J/cm2)

Figure 3a. Unconditioned results for sample B with a 0.25 mm spot size.

3

(A

a> DC

Damage m m Q Q B3

CO

<d No Damage m - d

1

1

1 11 11 1 ' 0 10 20 30 40

Fluence (J/cm2)

Figure 3b. Unconditioned results for sample B with a 0.5 mm spot size.

*; Damage

3

(00

DC

CO J0 No Damage QBE

r

' 11 11 1

• 1 0 5 10 15 20

Fluence (J/cm2)

Figure 3c. Unconditioned results for sample B with a 1 .00 mm spot size.

396
The results of the conditioned damage threshold measurements are presented in the

histograms of figures 4 and 5 for samples A and B, respectively. Approximately 20

spots per sample were tested at each spot size to allow a statistical analysis of the

defect distribution. A comparison of the conditioned and unconditioned

measurements is provided in figure 6. The conditioning increased the damage

threshold for the smaller spot sizes but provided little improvement at the 1.0 mm

diameter spot size. The standard deviation of the conditioned threshold vs spot size is shown for both samples in figure 7. The deviation was largest for the smallest spot

size and also was largest for coating A, which had the lowest overall damage

threshold. These results are consistent with a defect dominated damage mechanism

in which the defect spacing is on the order of 1 mm. In this case, the smaller spot sizes

would exhibit a large variation in the conditioned threshold, depending on whether the

laser spot happened to intersect a coating defect. If the spacing is on the order of 1 mm, then as the spot size approaches the defect spacing, the measured thresholds

will become more tightly grouped, as was observed.

Table 1 . Laser test parameters.

Laser Parameter Value used for testing

Wavelength 1064 nm

Pulsewidth 15nsecFWHM

Temporal Profile Smooth with minimal mode beating

Pulse Repetition Frequency 1 Hz

Spot Size 0.25 to 1 .0 mm full width at 1/e2

Spatial Profile Approximately Gaussian (monotonically

increasing to a central on-axis peak)

Angle of Incidence Normal

Laser Polarization Linear

Sample Substrate BK-7 Glass

Coating Sample A Multilayer AR with MgF2/AI203

Coating Sample B MultHayer AR with AI203/ZrC>2

Irradiation History Both Conditioned and Unconditioned

Table 2. Unconditioned damage threshold measurements.

Spot size

(FWin mm at 1/e2

)

Sample A

(J/cm2

)

Sample B

(J/cm2

)

0.25 29.2 82.5

0.50 13.1 42.5

1.00 8.5 48.2

397
y'//.-v. :

'::

.

M:

:

:

:

:

:

:

: : m

}

1 r 1

, '{"" —,—

Fluence In J/sq. cm

Figure 4a. Histogram of conditioned failures for sample A with a spot size of 1 .0 mm.

Fluence In J/sq. cm

Figure 4b. Histogram of conditioned failures for sample A with a spot size of 0.5 mm.

1

?\

i

>\ rmm- mm:

'

i

Fluence in J/sq. cm

Figure 4c. Histogram of conditioned failures for sample A with a spot size of 0.25 mm.

398
Fluence in J/sq. cm

Figure 5a. Histogram of conditioned failures for sample B with a spot size of 1 .0 mm.

30 35 40 15 50 55 60 65 70 ,75

Fluence in J/sq. cm

Figure 5b. Histogram of conditioned failures for sample B with a spot size of 0.5 mm.

Fluence In J/sq. cm

Figure 5c. Histogram of conditioned failures for sample B with a spot size of 0.25 mm.

399
J I

I

1 1 I

1 1 1 I

' 1

0.0 0.2 0.4 0.6 0.8 1.0

Spot Size (FW in mm at 1/e2)

Figure 6b. Conditioned and unconditioned performance of sample B.

Figure 6. Comparison of the conditioned and unconditioned test results for the three

different spot sizes.

3. Conclusions

In summary, we have verified that the threshold enhancement due to conditioning is largest for small spot sizes and decreases as the spot size is increased. The variability

in the conditioned threshold measurements was also largest for the small spot size,

with the distribution becoming more tightly grouped as the spot size was increased.

400
50

0.2 0.4 0.6 0.8 1.0

Spot Size (FW in mm at 1/e2)

Figure 7. The standard deviation of the conditioned threshold was largest for the

smallest spot size.

This evidence indicates that the spacing of the coating defects that were responsible

for the damage was on the order of 1 mm.

These results demonstrate the variability in damage threshold and laser

conditioning with variations in the coating design, materials and deposition

parameters. Although both coatings were multilayer, dielectric coatings deposited on

BK-7 glass, the designs and materials were different for the two samples. A significant

change in deposition processes was that coating A was deposited on a cold substrate

while coating B was deposited on a preheated substrate. Our experience with

production laser optics indicates that preheating the substrates tends to yield a higher

density coating which is harder, more durable, and has a a higher damage threshold.

Because of the process variations used in the production of our two samples, we

expect that coating A will be lower density and therefore more porous than coating B.

Our results are consistent with this conclusion since sample A had a lower damage

threshold and exhibited a much higher level of conditioning than did sample B. This

empirical evidence supports previous efforts to establish a link between coating

porosity, adsorbed contaminants and the laser conditioning effect [ 3, 5-7 ].

4. References

[1] Wolfe, C. R.; Kozlowski, M. R.; Campbell, J. H.; Rainer, F.; Morgan, A.; Gonzales, R.

"Laser Preconditioning of Optical Thin Films", presented elsewhere in this symposium.

[2] Kozlowski, M. R.; Staggs, M.; Wolfe, C. R.; Campbell, J. H. "Large Area Laser

Preconditioning of Dielectric Thin Films", presented elsewhere in this symposium.

[3] Arenberg, J. W.; Mordaunt, D. W. "Experimental Investigation on the Role of

Wavelength in the Laser Conditioning Effect", presented at the 20th Symposium on

401
Optical Materials for High Power Lasers (Boulder Damage Symposium), Boulder, CO,

October, 1988, proceedings to be published.

[4] Wilder, J. G.; Thomas, I. K. "Effect of n on 1 Laser Damage Threshold of Selected

Optical Coatings", presented at the 20th Symposium on Optical Materials for High

Power Lasers (Boulder Damage Symposium), Boulder, CO, October, 1988,

proceedings to be published.

[5] Arenberg, J. W.; Frink, M. E. "On the Role of Water in the Laser Conditioning

Effect", Laser Induced Damage in Optical Materials: 1987, NIST Special Publication

756; October 1988, p. 430.

[6] Frink, M. E.; Arenberg, J. W.; Mordaunt, D. W.; Seitel, S. O; Babb, M. T.; Teppo, E. A. "Temporary Laser Damage threshold enhancement by laser conditioning of

antireflection-coated glass", Appl. Phys. Lett. 5J_, (6), 415 (1987).

[7] Frink, M. E.; Arenberg, J. W.; Mordaunt, D. W.; Seitel, S. O; Teppo, E. A.

"Quantification of the Magnitude and Duration of the Preconditioning Effect in AR

Coated BK-7 Glass", Laser Induced Damage in Optical Materials: 1986, NIST

Special Publication 752, September 1988, p. 279.

[8] Swain, J. E.; Lowdermilk, W. H.; Milam, D. Nat. Bur. Stand. (U. S.) Spec. Publ.

No. 669, 292 (1982).

COMMENTS

Question: In your germanium data, showing a linear plot of damage threshold

versus the square of the field, there was a roll off at the high

values . Do you have an explanation for that?

Answer: We might be changing the optical thickness through simply binding

the fields and that wouldn't be covered by the simple model.

Question: Can you give some information concerning scattering?

Answer: Scattering, I don't really know about. We didn't take that kind

of loss into account in the field calculations. A quick

calculation shows that it is not going to be important for this

model. But it probably will have an effect, a small effect. That

also might help to account for the roll off at the high energy end

of the scale

402
MANUSCRIPT NOT RECEIVED

Damage Threshold Measurements of Reflective

and Transmissive Optics at 130 nm

C.H. Muller, III and C.E. Hamilton

Spectra Technology, Inc. Bellevue, Washington 98004-1495

ABSTRACT

The GLORIA VUV facility at Spectra Technology, Inc. has been used to measure the damage

threshold and reflectivity of first surface Al+MgF2 mirrors and second surface MgF2 +Al+MgF2 mirrors. The mirrors were illuminated by high power VUV coherent radiation at 130 nm. Mirror

reflectivity was monitored for several thousand pulses for each optic studied. Two nanosecond long

pulses and energy densities as high as 20 mJ cm"2 were used for the measurements. In addition,

measurements of "color center" formation rates in transmissive optics have been obtained. The

GLORIA VUV facility, experimental measurement system, and damage results will be discussed in

detail.

Work supported by Sandia National Laboratory

403
Manuscript received

2-1-90

Laser Induced Damage Thresholds of Dielectric Coatings at 193 nra and Correlations to Optical Constants and Process Parameters

J. Kolbe, H. MUller, H. Schink, H. Welling

Institut flir Quantenoptik, Universitat Hannover,

Welfengarten 1, D-3000 Hannover 1, FRG

and

J. Ebert

Laseroptik GmbH, 3008 Garbsen 8, FRG

Laser-induced damage thresholds of dielectric single layers and

reflectors were measured at 193 nm. Layers of AI2O3 and Si02 were

prepared by electron beam evaporation and by ion beam sputtering;

layers of NaF , A1F 3 , MgF 2 , GdF 3 , LaF 3 , NdF 3 and YF 3 were prepared

by thermal evaporation. Spectrophotometry methods were used to

evaluate optical constants and inhomogeneity coefficients in the

spectral range between 150 nm and 250 nm. The dependence of

refractive indices and absorption coefficients on process parameters and deposition methods was analyzed in order to prepare

low loss reflectors for 193 nm and 157 nm.

Key words: fluoride layers; UV coatings; UV laser damage;

excimer laser mirrors; ion-beam sputtered UV coatings;

UV losses in dielectric layers.

1. Introduction

In the last few years, the number of laser applications in material

processing has increased by an appreciable amount [1]. It has turned out that

in many applications, the use of ultraviolet laser radiation has considerable

advantages in comparison to visible or infrared light. The interaction mechanism between light and matter is dominated by photoab 1 at ion rather than by

thermal processes; so the interaction is restricted to very thin surface layers, which is extremely useful, for example, in eye surgery. - In semiconductor applications, the shorter wavelength of ultraviolet light facilitates an

improved resolution in chip fabrication.

The KrF eximer laser which operates at 248 nm has already proven as a powerful tool to produce intense ultraviolet light. Nevertheless, the use of

even shorter wavelengths would be connected with further progresses in the

above mentioned applications. This affords, however, the availability of highquality optical components. For this reason, a study has been initiated to

develop dielectric mirrors with high reflectivities and damage thresholds at

193 nm (ArF excimer laser). Additionally, mirrors for 157 nm (F2 excimer laser) have been deposited and analyzed.

In the first step of our study, we deposited single layers of coating

materials with absorption edges below 250 nm. The optical properties of the

404
layers were measured and evaluated by spectrophotometr i c methods. The deposition processes and parameters were optimized in order to minimize absorption

and scatter losses. To get some insight into the mi crostructure of the layers,

their cosmetic appearance, water content, and inhomogeneity were investigated.

Finally, the damage thresholds at 193 nm were measured.

In the second step, high reflecting quarterwave stacks were deposited.

The reflection and transmission coefficients were measured and compared with

theoretical data which had been computed with the single layer values of the

refractive indices and extinction coefficients. The damage thresholds were

determined and compared with the results for the single layers.

2. Experimental Apparatus

Fluoride and oxide coatings were deposited by thermal and electron beam

evaporation. The vacuum chamber was equipped with an oil diffusion pump and a liquid nitrogen-cooled Meissner trap; the base pressure was smaller than

5*10-6 mbar; the substrates could be heated up to 400 °C; the layer thickness

was controlled by an optical monitoring system.

Additionally, oxide coatings were deposited by ion beam sputtering. This

was performed in a cryo-pumped coating plant. Two 5 cm Kaufman ion guns with

hollow cathodes and one common hollow cathode neutralizer were used to sputter

the targets. These consisted of the oxide materials to be coated; their diameter was 20 cm; the angle between the targets and the ion beams was 45°. Each

gun produced an argon ion beam of 130 mA with an ion energy of 1000 eV. The Ar

backpressure due to the gas flow through the ion guns and the hollow cathode

neutralizer was 10-* mbar; an amount of 10-^ mbar oxygen was added to

produce fully stoichiometric oxide coatings. The deposition rate was 0.4 nm/s

for silica and 0.3 nm/s for alumina.

The reflection and transmission measurements were made with a PerkinElmer Lambda 9 spectrophotometer (wavelengths above 190 nm) , and with a selfconstructed VUV-spectrophotometer (wavelengths between 140 nm and 240 nm)

The damage threshold measurements were performed by the " Laser-Laboratorium Gottingen e.V.". A Lambda Physik EMG 202 MSC excimer laser was used to

produce pulses at 193 nm with a pulse length of 25 ns . The beam profile was

approximately gaussian. The beam cross-section at the probe was elliptical and

had a l/e2 area of 0.65 mm2 . The energy densities given below are the mean

values within the l/e2 area; the peak energy densities are higher by a factor

of about 2.5. Damage was detected by in-s itu-microscopy in connection with

digital image processing. Further details of the arrangement are given in a recent paper by K. Mann [2].

3. Experimental results

3.1 Single layers

Only few data about the basic optical properties (refractive index and

extinction coefficient as functions of wavelength) of dielectric thin films

for the deep ultraviolet are available from the literature [3]... [10]. For

this reason, a series of suitable materials was selected for further investigations. Single layers were deposited by different processes and process

parameters in order to minimize their scatter and absorption losses. The wavelength-dependent refractive indices (n) and extinction coefficients (k) of the

405
I

j

i

optimized layers were evaluated by an envelope method described in ref. [21].

Finally, the damage thresholds were measured and compared with the extinction

coefficients

3.1.1 Oxide materials

Layers of silica and alumina with thicknesses of about 400 nm were deposited onto fused silica substrates by electron beam evaporation and by ion

beam sputtering. For the e-beam coatings, well established process parameters

(substrate temperature: 300 °C, deposition rate: 0.5 nm/s; oxygen backpressure: 2*10-'' mbar) were used. For the IBS coatings, the optimized parameters are given in section 2.

Figure 1 shows the evaluated n and k values of alumina as functions of

wavelength. The deviations of the values for n from those reported in refs.

[4] and [19] are smaller or equal to 0.02 for the evaporated layers and 0.01

for the ion-beam sputtered layers. The extinction coefficients are comparable

to the results given in ref. [19].

The e-beam deposited silica layers had a refractive index almost identical to the fused silica value; the index of the IBS-coatings was slightly

higher (see fig. 1), which agrees with the information given in ref. [20]. In

both cases, the extinction coefficient was smaller than 10-* for wavelengths

down to 190 nm.

The damage thresholds of the deposited oxide coatings are plotted in

figure 8. The low values of the alumina layers are obviously caused by their

high absorption losses. Generally, the thresholds of the IBS coatings are

slightly higher than those of the e-beam coatings, which may be attributed to

their higher packing density and better mechanical stability.

1.90 -y1.80

1.70

Opticol constants of alumina films

c

<o 1.60 >

"oo

•r.1.50

-+

M0

30

25

Ldo

1.30

20

15 S

o

10 u

200 250 300 wavelength (nm)

IB

Optical constants of silico films

o : ion beam sputtering + : fused silicia

ai 1.5 -

12

extinction coefficient less than 10E—

200 225 250 275 300 325 350 375 wavelength (nm)400

Fig. 1. Refractive indices and extinction coefficients of oxide films

deposited by electron-beam evaporation and by ion-beam sputtering.

The film thicknesses are about 400 nm.

406
3.1.2 Fluoride Materials

In the deposition of dielectric coatings consisting of fluoride materials, internal stresses as well as scatter losses arising from the microstructure of the layers impose severe limitations. To get a first insight into

the mechanical and optical properties of fluoride coatings, single layers of

different thicknesses (2, 8, 14, 20, 26 QWOT at about 200 nm) were deposited

onto substrates of BK-7 glass, fused silica, and calcium fluoride. This was

done for several substrate temperatures between 100°C and 400°C. The materials

were evaporated from molybdenum boats (deposition rates in nm/s: 3 {A1F3>, 0.8

{MgF2} , 0.25 {NdF3}, 0.5 {other materials}).

The layers were examined by visual inspection (with illumination by

intense white light) and by dark field microscopy (magnification 125 times).

The results are given in table 1 and can be summarized as follows:

- All single layers of NaF and YF3 had an inferior quality. While it is well

known that NaF is very hygroscopic, the reason for the results with YF3 is

not quite clear.

- In layers of MgF2 , GdF3 , LaF3 , and NdF3 which were deposited onto

substrates heated up to 400 °C, cracks have been observed in some cases.

In NdF3 layers, the maximum layer thickness without cracks strongly depends on the substrate material: while the thermal expansion coefficient

of the substrate (values are given in table 2) increases, the maximum

layer thickness increases, too. Similar tendencies can be observed in MgF2

and GdF3 layers.

- Layers of AIF3 have a good quality if deposited onto substrates of fused

silica or calcium fluoride at high substrate temperatures. The reason for

the inferior quality of the layers deposited on BK-7 glass is not understood yet.

- Thick Layers of GdF3 , LaF3 , and NdF3 show a hazy appearance if deposited

at a low substrate temperature (100 °C). As can be seen in figure 2, the

optical losses are almost proportional to the layer thickness. So the

change from "+" to "-" with increasing thickness just reveals the increase

of the losses; it is not connected with a qualitative change in the

structure of the layers.

A strong dependence on the substrate temperature is observed in several

properties of the single layers. The extinction losses drop significantly with

increasing substrate temperature (fig. 3). The inhomogene i ty as well as the

shift of optical thickness before/after venting with atmospheric air increase

significantly at low substrate temperatures (figs. 4, 5). Although the data

shown in these figures only refer to GdF3 layers, similar tendencies have been

observed in most of the other fluoride materials. These facts clearly demonstrate that the optical losses of the layers are closely connected with their

internal microstructure

The results presented above show that the deposition of fluoride coatings

with low optical losses can be performed only at high substrate temperatures.

These, however, may lead to cracking of the layers caused by internal stresses. The results in table 1 indicate that cracking can in some cases be

avoided by choosing a substrate material with an appropriate thermal expansion

coef f ic ient

407
Table 1. Morphology of evaporated fluoride single layers.

Layers marked with "+" showed no defects;

layers marked with "(+)" were damaged by rubbing with lens tissue and acetone;

layers marked with "-" showed one of the following defects:

- cracks (mean separation is given in brackets);

- extremely high pinhole densities;

- hazy appearance without any detectable fine structure.

MaterialLayer quality vs • Substrate Morpho 1 ogy

substrate 1 ayer thickness materi a

[explanations are

temperature [QWOT at 200 nm] [F . S . = given below]

fused

2 8 14 20 26 silica]

NaF ; 400 °c all pinholes

NaF100 °c — — — all pinho 1 e s A1F 3 ; 400 °c + Car 2 + — — — BK-7 pinholes after ageing > 10 days

+ + + + F . S . A A A A AIF3 ; 100 °c CaF 2 pinholes after ageing > 10 days

+ — BK-7 pinho 1 es

+ + — — r . b . pinholes after ageing > 1U days

MgF 2 ; 400 °c L at 2 + + + + + BK-7

+ + — — F . S . cracKs { dv n)

MgF 2 ; 100 °c ( + ) CaF2

+ + ( + ) ( + ) ( + ) BK-7

+ + cracks ( 1 U [i)

yf 3 ; 400 °c ( + ) -^11 all pinho 1 es 100 °c all pinho 1 e s GdF 3 ; 400 °c + CaF 2 + + + + BK-7

+ + F.S. cracks (10 11) GdF 3 ; 100 °c

+ + BK-7 hazy

+ + F.S. hazy

LaF 3 ; 400 °c CaF 2 cracks (5 pi) + + BK-7 cracks (10 p)

+ + F.S. cracks (5 u)

LaF 3 ; 100 °c CaF 2 pinho 1 es

+ + + BK-7 hazy

+ + + F.S. hazy

NdF 3 ; 400 °c + CaF 2 + + BK-7 cracks (20 /i) + F.S. cracks (10 u)

NdF 3 ; 100 °c CaF 2 hazy

+ + + Bk-7 hazy after ageing > 10 days

+ + + F.S. * * * * * *

I

I

9

I

I

408
Table 2. Optical properties and thermal expansion coefficients

of substrate and film materials.

The data have been taken from references [10] to [18] and refer to the bulk

materials; abbreviations: (o), (e): ordinary, extraordinary ray;

o.a. : optical axis

Material n (550nm) Band Edge

[nm]

Therma

coe f f

expans ion

[10-6 PK-i]

LiF 1 . 39 105 37

NaF 1 . 33 36

MgF 2 1 .38 (0) 113 9 _•_ o.a

1 .39 (e) 14 11 o.a.

CaF 2 1 .43 124 24

A1F 3 1 . 38

YF 3 1 . 55 17

LaF 3 1 . 60 135 14

NdF 3 1 . 60 14

GdF 3 1 . 58

Si0 2 (cryst.) 1 .55 (0) 148 13. 4 x 0 . a

1 .56 (e) 8. 0 H o.a.

Si0 2 (fused) 1 .46 160 0 . 55

A1 2 0 3 (cryst.

)

1 •77 (0) 141 5 . 0 _l 0 . a

1 .76 (e) 6 . 7 11 o.a.

BK-7 glass 1 .52 315 7 . 1

409
.t 4 .DO

tv/ -100 c

I- /

/ 400 X

0 " 'j 10 IS 20 2i Optical film thickness (QWOT ot 200 nm)

o

Substrate temperature (deg C)

I—

I

\

\

\

\

\

1

Fig. 2. Extinction losses at 200 nm versus

layer thickness in GdF3 single

layers evaporated at substrate

temperatures of 100 °C and 400 °C.

Fig. 3. Extinction losses at 200 nm versus substrate temperature in GdF3 single layers (thickness: 2 QWOT at

200 nm)

Fig. 4. Inhomogeneity I versus substrate

temperature for the high-index

fluoride materials.

I:=(n2 -ni)/n2 ;

Ti2 film refractive index at the

interface substrate-film;

ni : film refractive index at the

interface substrate-air.

Fig. 5. Shift S of optical film thickness

before/after venting with atmospheric air in 240 nm thick GdF3

films

S:={nd(after v. )-nd(before v.)}/nd

410
The evaluated refractive indices and extinction coefficients of fluoride

layers deposited at 400 °C are shown in figures 6 and 7. - In references [3],

[7] and [9], extinction coefficients of LaF3 layers are given for wavelengths

between 220 nm and 250 nm. The values given in [3] are almost identical to our

results, while those from refs. [7] and [9] are somewhat higher. The extinction coefficients of MgF2 and AIF3 given in ref. [5] are comparable to our

values; those from ref. [6] are higher by more than a factor of 2. In each

case, the layers with relatively high losses had been deposited by electron

beam evaporation. This is in accordance with our observations.

The damage thresholds of single layers are plotted in figure 8 together

with their extinction coefficients. It can be seen that layers with high

losses have low damage thresholds, and vice versa. Furthermore, it is observed

that the damage thresholds of the high-index fluoride materials are much

higher than those of the high-index oxide materials.

Fig. 6. Refractive indices and extinction coefficients of high-index fluoride single layers (thickness: about 400 nm) versus wavelength.

Fig. 7. Refractive indices and extinction coefficients of low-index fluoride single layers (thickness: about 400 nm) versus wavelength.

411
1.0

E

o\0.6

—>

~o 0.6 o

_C

<fi 0)

L. 0.4

sz

\—

CO

CTi 0.2 O

E

OQ 0.0

1.0

0.8

-0.6

0.4

0.2

0.0

Si02 Si02 AI203 AJ203 NaF AIF3 WgF2 GdF2 YF3 NdF3 LaF3 e- IBS e- IBS

beam beam

100 100

I 80 :

UJo 60-

c

o

40-

o

c

X

Ul 20-

0-

Si02 Si02 AI203 AI203 NaF AIF3 MgF2 GdF2 YF3 NdF3 La F3

e- IBS e- IBS

beam beam

Fig. 8. Damage thresholds and extinction coefficients at 193 nm

layers (thickness: typically 400 nm)

of single

412
3.2 High-reflecting quarterwave stacks

From a series of appropriate material combinations (see table 3), highreflecting quarterwave stacks were deposited. The systems (except the pure

oxide systems) were deposited simultaneously onto heated substrates of fused

silica, BK-7 glass, and calcium fluoride. The substrate temperature was between 300 °C and 400 °C. All systems (except the pure oxide systems) had cracks

when deposited onto fused silica, but it was not difficult to deposit coatings

without visible defects onto calcium fluoride and BK-7 substrates. These facts

support the hypothesis that the internal stresses in fluoride coatings are

strongly dependent on the thermal expansion coefficients of the substrates. - Figure 9 shows a typical transmi ttance and reflectance spectrum of a high

reflecting multilayer stack.

1 QQ I I '

I

1 ' ' ' '

I I

1 '

I

1 ' i i I i i i i i i . I i i i . i i i 1 i t t i I i i i . . i . i

OlllMIIIII|IIMIIIII|IIIIIIIM|lllllllll|'> l > | llll|''lMMII|IIIMIIII|IIIMMIir

160 170 180 190 200 210 220 230 240 wavelength (nm)

wavelength (nm)

Fig. 9. Reflectivity (R), transmittance (T) , and R+T versus wavelength

of a high-reflecting quarterwave stack consisting of 41 layers

NdF3 /AlF 3 .

413
In table 3, some optical data of the HR-coatings are listed. The measured

reflection and transmission values at the central wavelength are compared with

theoretical values computed from the single layer data for n and k. A good

agreement (differences less than 1 %) is observed in the combinations AI2O3/

S1O2 1 LaF3/MgF2, and LaF3/AlF3. In all other cases, however, the measured

reflectivities are significantly lower than the computed ones. Two possible

explanations can be given for this behaviour:

- Deviations of a real design from an ideal quarterwave stack may reduce its

reflectivity (while the transmittance increases). This is observed in the

systems made of LaF 3 /AlF 3 , GdF 3 /AlF 3 , NdF 3 /AlF 3 , and NdF 3 /MgF 2 .

- Interface losses which are relatively small in single layers, may contribute significantly to the total losses in a multilayer system. In this

case, the reflectivity as well as the transmittance are smaller than the

theoretical values. This may be caused either by the surface roughness of

the substrates which is probably reproduced at each interface, or by

interface defects caused by the chemi ca 1 /phy s i ca 1 properties of the layer

materia 1 s

In the deposited systems, both mechanisms seem to contribute significantly to the optical losses. Their better understanding as well as their reduction will be the subject of further investigations.

The damage thresholds of the HR coatings are shown in figure 10. It can

be seen that the thresholds of the coatings with AI2O3 as the high-index

material have relatively low damage thresholds. The best values were obtained

with the systems made of NdF 3 /MgF 2 , LaF 3 /AlF 3 , and LaF 3 /MgF 2 . These results

Table 3. Optical data of the deposited multilayer HR stacks.

In some cases, the central wavelength Lc is different from 193 nm. For

these coatings, the R and T data were taken at the individual values of Lc .

Material

L c No. of Re x p Rth Texp Tth Ed a d

(nm) 1 ayer

(%) (%) (%) (%) ( J /cm'-

A1 2 0 3 /Si0 2 193 61 92 . 7 93. 5 <0 . 1 0. 2 0 . 26

Al 2 0 3 /Si0 2 193 41 93 . 5 94 . 6 0.2 0 . 2 (IBS)

AI2O3/AIF3 193 39 92 . 2 97. 9 <0. 1 <0. 1 0 . 20

Al 2 03/MgF 2 199 41 96 . 2 98. 4 0. 1 <0 . 1 0.07

NdF 3 /AlF 3 193 41 97. 6 99. 0 0.3 <0. 1 0. 64

NdF 3 /MgF 2 187 39 96.5 98. 1 0.3 0. 1 1 . 14

LaF 3 /AlF 3 188 41 97 . 6 98. 3 0.5 0. 1 1 . 35

LaF 3 /MgF 2 193 41 97. 2 97. 6 0.5 0. 4 1 . 52

GdF 3 /AlF 3 200 41 80.4 98. 3 2.1 0. 2 0.71

414
0.0

2.0

-1.5

-1.0

0.5

0.0

AI203 AI203 AI203 AI203 NdF3 NdF3 LaF3 LaF3 GdF3

Si02 Si02 AIF3 MgF2 AIF3 MgF2 AIF3 MgF2 AIF3

IBS

Fig. 10. Laser induced damage thresholds at 193 nm of high-reflecting

multilayer stracks. - The dashed lines indicate the thresholds

of the corresponding single layer results.

are consistent with the fact that the single layer damage thresholds of the

high index fluoride materials are considerably higher than those of the alumina single layers. - The intermediate values for the NdFs/AlFs and GdF3/AlF3

systems do not correlate with the single layer results. We suppose that they

are caused by unfavorable deposition conditions. So it should be possible to

get increased values by further optimization of the process parameters.

On the basis of the results described above, high-reflecting quarterwave

stacks centered at 157 nm were deposited. They consisted of 41 layers of

LaF3/AlF3 and had measured peak reflectivities of about 95 %. Their environmental resistance against the discharge gas of a F2-excimer laser was tested

by LAMBDA PHYSIK, Gottingen (FRG). A very good durability of the coatings has

been observed there.

4. Conclusion

The purpose of the presented work has been to develop dielectric mirrors

with low optical losses and high damage thresholds at 193 nm. Coatings of

alumina and silica were deposited by electron beam evaporation and by ion beam

sputtering. It has been shown that the IBS coatings had absorption losses and

damage thresholds comparable to those of the e-beam coatings. The measured

reflectivities and damage thresholds of the deposited high-reflecting quarterwave stacks were 93 % and 0.3 J/cm2 , respectively. - Quarterwave stacks consisting of alumina as high-index material and AIF3 or MgF2 as low-index

material showed no significant improvements in comparison to the pure oxide

coat ings

415
The highest reflectivities (97.6 %) and damage thresholds (1.5 J/cm2)

were obtained with pure fluoride stacks. It has been shown that a decisive

requirement for the production of high quality fluoride coatings is their

deposition onto heated substrates. This, however, leads to high internal

stresses which often cause cracks or other severe defects in the layers. While

it seems to be impossible to deposit high reflecting fluoride stacks onto

fused silica substrates heated up to 300 °C or even more, good results have

been achieved with BK-7 and calcium fluoride substrates. This is attributed to

their higher thermal expansion coefficients which tend to reduce the internal

stresses in the coatings.

The measured peak reflectivities of the investigated fluoride stacks have

values up to 97.6 % at 193 nm , and 95 % at 157 nm . On the other side, significantly higher values were calculated from single layer data on n and k.

Present^', the reasons for these differences are not known exactly and will be

a subject of further investigations.

5. Acknowledgements

We gratefully acknowledge the technical assistance of Mr A. Starke in the

deposition of the ion-beam sputtered coatings, and the collaboration of Mr K.

Mann (Laser-Laboratorium Gottingen e.V.) in the measurement of the damage

thresholds. The work was supported by the BMFT (F&E-Vorhaben Nr. 13 N 5408).

6. References

T.

K.

G.

J . G.

A.

0.

D. Smith and

H.K. Pulker,

F. Rainer et

Znotins, SPIE 71_0_, 55 (1986)

Mann, to be published in: NIST (U

Hass, J.B. Ramsey and R. Thun , J.

T. Cox, G. Hass and. J.B. Ramsey, J

Hass, J.B. Ramsey and R. Thun, J.

S. Barriere and A. Lachter, Appl . Opt

R. Wood II et al., Appl. Opt. 23, 20,

S) Spec. Publ

Opt.- Soc. Am.

Physique 25 , Opt. Soc. Am.

16, 11, 2865

(1988)

49, 2, 116

250 (1964)

49, 2, 116

(1977)

(1959)

(1959)

3644 (1984)

P. Baumeister, Appl. Opt. 18, 1, 111 (1979)

Appl. Opt. 18., 12, 1969 (1979)

al., Appl. Opt. 24, 4, 496 (1985)

Lando 1 t-B6rnste in , Physikal isch-chemische Tabellen, I

, 912 (1923)

E.D. Palik, Handbook of Optical Constants of Solids, Academic Press,

Orlando (1985)

A.J. Moses, Optical Materials Properties, in: Handbook of Electronic

Materials Vol. 1, IFI/Plenum, New York (1971)

A. Smakula, Einkr i st a 1 1 e , Springer Verlag, Berlin (1962)

Gmelin Handbuch der anorgani schen Chemie, 8. Auflage, Teil C3,

Springer Verlag, Berlin (1976)

Mel les-Gri 1 lot , Optics Guide 3, p. 51 (1985)

M.L. Scott, NBS Spec. Publ. 688, 329 (1983)

J.S. Browder and S.S. Ballard, Appl. Opt. 1_6, 12, 3214

L.G. Van Uitert et al., Mat. Res. Bull. 1_2, 307 (1977)

B. Pond et al., NIST Spec. Publ. 752, 410 (1986)

T. Raj, J.S. Price and C.K. Carniglia, NBS Spec. Publ.

C. K. Carniglia, J. Opt. Soc. Am. A3 (13), P40 (1986)

(1977)

746, 325 (1985)

416
Angular Dependence of Thin-Film Dielectric Coating

Damage Thresholds Revisited

J. D. Boyer, S. R. Foltyn, B.R. Mauro, and V. E. Sanders

Los Alamos National Laboratory

Chemical and Laser Science Division

Los Alamos, New Mexico 87545

Newnam et al. [1] reported experiments showing that the angular dependence of 351-nm laser

damage thresholds in HfC»2/Si02 multilayer dielectric reflectors was much weaker than even the 1/cosG

expected from simple geometric fluence dilution. Several plausible explanations were suggested, but none were convincing. We propose a simple geometric model based on a cylindrical form for the

coating defect responsible for damage initiation. We have measured 248-nm damage thresholds for bare

fused silica, evaporated aluminium films, and HfOySiC^ and A^OySiC^ dielectric reflectors at angles

out to 85°. The measured data agree well with our simple model.

Key words: angular dependence; aluminium oxide; coating defects; evaporated aluminium; fused silica; hafnium

oxide; laser damage thresholds; multilayer dielectric reflectors; silicon dioxide.

1 . Introduction

Before the experiments of Newnam et al. [1], it was believed that the laser damage thresholds for an S-plane

polarized beam would increase with incident angle (0) due to simple geometric fluence dilution and also due to the

decrease in S-polarization electric field within the film. Geometric fluence dilution alone predicts a l/cos9 increase in damage threshold and peak electric field calculations predicted an even more dramatic increase. Figure 1 compares the

laser damage thresholds of Hf02/Si02 reflectors reported in ref. 1 with a l/cos0 angular dependence. If these results

are general for multilayer dielectric reflectors, we have an opportunity to extend our understanding of laser induced

damage.

We wished to verify the weak angular dependence and also utilize this behavior to gain further understanding of

damage phenomena. In addition to verification, several other questions beg answers. Is this phenomena unique to HfOySiG^ multilayers or will it also occur in others such as A^OySiC^? Does it occur in bulk material such as fused

silica? Is it sensitive to polarization? Of course the big question is, why do we not observe at least the simple l/cos0

increase in laser damage thresholds with increasing incident angle. In this paper we propose an extension of geometric

fluence dilution by considering the interaction of the diluted fluence with an absorbing defect. In addition we report

experiment results which answer some of the questions raised above. The role of absorbing defects in laser damage has an observational basis [2] and additional support from theoretical work [3,4].

2. Proposed Model

A model which accounts only for geometric fluence dilution at the surface or within the bulk cannot account for

the experimentally observed angular dependence of laser damage thresholds in some multilayer dielectric reflectors. It is apparent that the interaction cross-section does not decrease as rapidly as it would for a uniformly absorbing spherical

defect. A simple extension of the uniformly absorbing spherical defect model is that which considers a uniformly

absorbing cylindrical defect embedded in the film with the cylinder axis orientated normal to the film surface. Additional

assumptions in this model are that the total energy which the defect must absorb for damage to occur is a constant independent of incident angle and that the cylinder height is the thickness of the film.

417
We now consider not only fluence at the film surface, but in the medium as well. Refraction modifies the beam

geometric cross-section slightly. The following expression gives the ratio of the incident fluence to fluence in the

medium of refractive index n as a function of angle.

_ (l-sin26/n2)

1/2 B =

cose ( i

)

This modification of the l/cos0 behavior is modest ( about twenty percent for HfC>2 at 85° ) and is included primarily for

completeness. A cylindrical defect of radius r embedded in a film of index n and thickness t presents a cross-sectional

area

A = (2rt/n) sinO + 7tr2 cosO (2)

to the incoming beam. As the film is tilted the fluence in the medium is diluted and the defect area is modified. The area may increase or decrease depending on the aspect ratio of the cylinder. The aspect ratio therefore becomes the chief free parameter of this model. When we impose the condition that the absorbed energy required to initiate damage at the defect

is independent of angle, we arrive at an expression relating the damage threshold fluence at any angle F(8) to the normal

incidence damage threshold fluence Fq.

F(8) = ^

(2t/7tnrB) sinO + cos9 (3)

This model is compared with the laser damage thresholds of HfOySiO^ reflectors from reference [1] in figure 2. We note that for small aspect ratios this model overestimates the increase in thresholds at large angles, but not so much

as the 1/cosG model. For large aspect ratios the present model predicts a decrease in damage thresholds over a broad

range of angles before thresholds rise. Given the simplistic nature of many of our assumptions and also experimental

uncertainties in the experimental results, this model is quite successful in explaining the weak angular dependence

observed. We performed additional experimental tests of this model which comprise the remainder of this paper.

3 . Test Procedures and parameters

The test setup and procedure was the standard multiple-shot used at Los Alamos and described in some detail

previously. [1,5,7] Significant modifications include thin-film polarizers, a low intensity uv diagnostic illumination beam

picked off before the beam attenuator, two-dimensional beam profile characterization generated by scanning a reticon

array across the focal plane, computerized fluence measurement and control, and computerized data reduction. Table 1 below details specific test parameters.

Table 1 . Damage Test Parameters

Wavelength: 248 nm

Pulselength: 23 ns

Repetition Rate: 50 Hz

Spot Size: 0.2 x 0.8 mm at normal inc. Shots/Site: 100

Polarization Ratio: 1000:1 for S-polarization experiments

418
With the exception of the evaporated aluminium film test samples, the damage diagnostic was the telescope and

video camera with the uv diagnostic beam for illumination. The fluence in the diagnostic beam was 5-10 mJ and too

small to contribute to damage. The telescope was maintained at an angle of fifteen degrees with respect to the sample

normal in an effort to maintain constant diagnostic sensitivity. For the aluminium films we found that visual examination

with a low-power stereo microscope and white light provided the best reliability.

4. Test Samples

We tested four groups of samples. All samples were two inches in diameter. Bare evaporated aluminium films on BK7 were chosen to represent the case where damage is expected to occur at the surface due to bulk absorption.

Fused silica( Corning 7940) was chosen to represent a non-absorbing bulk material. The HfC^/SiC^ multilayer

reflectors were shipped by a coating vendor instead of the A^OySiC^ reflectors ordered. HfC>2 films are absorbing at 248 nm and do not represent defect dominated laser damage initiation. The HfOySiC^ reflectors were designed for

seventy-five degree angle of incidence and coated in a single run. The S-plane polarized reflectance band for this design

angle is broad enough that these coatings are good reflectors at all test angles. The A^OySiC^ multilayers were also

designed for seventy-five degrees, but were coated in several coating runs. The A^OySiC^ multilayer reflectors are expected to behave at 248 nm as the HfOySiC^ reflectors did at 351 nm in ref. [1].

5 . Experimental Results

The normalized laser damage threshold test results for the Coming 7940 fused silica are shown in figure 3. All

samples are from the same lot and normal incidence damage thresholds were obtained by measurements on the 45 and 60

degree test samples. Most of these data are for unpolarized light and only the simple 1/cosE geometric fluence dilution

enhancement of the damage threshold is expected. The damage thresholds obtained with unpolarized light follow a

l/cos9 scaling quite well. We also measured a few samples with S-polarized light to see if the stronger enhancement due

to decreased peak electric field was present.fl] The results do indicate some additional enhancement, but the present data

are insufficient to characterize this additional enhancement.

Figure 4 gives the normalized damage thresholds for evaporated aluminium on BK7. The normalization is the

normal incidence damage threshold for this lot (0.12 J/cm2). These results for the test with unpolarized light show at

least the 1/cosG geometric fluence dilution scaling. Tests with S-polarized light again show an additional enhancement.

We do not understand why the scatter for the test with polarization is so much greater.

Figure 5 displays normalized damage thresholds for both the HfOySiC^ and A^OySiC^ multilayer reflectors.

All of these results are for s-polarized light and the normalization is chosen to make the sixty degree points fall on the

1/cosG curve for comparison. We immediately see that the HfC>2 damage thresholds follow the 1/cosE curve quite well

at 248 nm. The A^OySiC^ damage threshold values, however, increase more slowly. We note that this implies that our normalization constant is probably too large and that the eighty five degree thresholds are more likely about four or

five times greater than the normal incidence thresholds. Figure 6 compares the multilayer dielectric damage thresholds

with spherical and cylindrical defect models. Here the A^OySiC^ damage thresholds are normalized so that the value at

sixty degrees agrees with our model and chosen aspect ratio and the average values at each angle are plotted for clarity.

6. Summary

We have verified that multilayer dielectric reflector laser damage thresholds may not scale with simple geometric

fluence dilution at the film surface. Figure 6 shows that the A^OySiC^ laser damage thresholds scale less rapidly than

l/cos0 and are in good agreement with our cylindrical model with r/t = 0.4. Our model and assumptions would imply a

defect radius of about 10 nm and that melting of the defect would require absorption of about one percent of the incident

energy. The results for aluminum films, bare fused silica, and the HfOySiC^ multilayers which are absorbing at 248 nm do scale with geometric fluence dilution at the sample surface. For the aluminium and HfC^/SiC^ multilayers, it

is probable that bulk absorption is more important than local defects. The l/cos0 scaling of the fused silica damage

thresholds implies that either the defects are more spherical or that the energy-absorption mechanism is different than in

multilayer dielectric films.

419
7 . References

[1] Newnam, Brian E.; Foltyn, Stephen R.; Gill, Dennis H.; Jolin, L. John Angular dependence of multilayer-reflector

damage thresholds. Nat. Bur. Stand. (U.S.) Spec. Publ. 727; 1984. 342-351.

[2] Gorshkov, B. G.; Danieliko, Y. K.; Nikolaev, U. N.; and Sidorin, A. V. Sov. J. Quant. Elect. 12 (3); 388; 1983.

[3] Lange, M. R.; Mclver, J. K.; and Guenther, Arthur H. An initial study of: The inclusion model for repetitively

pulsed laser damage. Nat. Bur. Stand. (U.S.) Spec. Publ. 727; 1984. 382-393.

[4] Becker, W.; Mclver, J. K.; and Guenther, A. H. Initiation of laser induced damage by small metallic particles. Nat. Bur. Stand. U.S.) Spec. Publ. 752; 1986. 603-610.

[5] Newnam, B. E.; Foltyn, S. R.; Jolin, L. J.; Carniglia, C. K. Multiple-shot ultraviolet laser damage resistance of

nonquarterwave reflector designs for 248 nm. Nat. Bur. Stand.(U.S.) Spec. Publ. 638; 1983. 363-371.

[6] Foltyn, S. R.; Jolin, L. J.; Newnam, B. E. Progress in ultraviolet damage testing at Los Alamos. Nat. Bur. Stand.(U-S.) Spec. Publ. 669; 1984. 266-273.

420
30 40 60 60 70 80 90

Anglo of Incidence

Figure 1. Comparison of the experimental damage threshold values from reference [1] with the 1/cosG expected from

simple geometric scaling

oA

I 3

A

H

V

I

a

o

z

Hf02 /SiOj

r/t 0.1

r/t 0.2

r/t - 0.4

30 60

ANGLE OF INCIDENCE (degrees)

90

Figure 2. Comparison of the experimental damage threshold values from reference [1] with the cylindrical defect extension to simple geometric scaling

421
16

oA

I

A

H

•o8

•Ba

**o

z

10

0

40

1/coa

(1/coa)

a random poL

0 i-poL

60 60 70

Anglo of Incidanoa

80 90

Figure 3. Laser damage threshold values for uncoated Corning 7940 fused silica with random polarization scale as l/cos6. The S-polarization results are further enhanced.

20

oA

I

A

H

V

8

3

a

o

Z

16

10

6

0

40

1/COO

(1/008^

/

/ / D / random pol. /

0

/

0 a-pol.

$ / 0 /

i i i i 60 60 70

Anglo of Inoldonco

80 90

^

lgU

u

e

?j ^

aS6r dama§e threshold values of evaporated aluminium with random polarization scale as 1/cosG The

thresholds for the S -polarization results are enhanced more than the uncoated fused silica.

422
2

O

a

l

A

H

tJ

a

**o

z

10

1/CO«

(1/CO«)

HfOj/SiOj

0 Afapj/Sipi

40 50 60 70

Angle of lnoid«no«

80 90

Figure 5. The laser damage threshold values for HfCtySiC^ and A^OySiC^ multilayer dielectric reflectors are compared

with l/cos0 and l/cos29.

12

TJOA

I

H

TJ8•»* "3a

**o

jj

HfO^ /SiOa 0 AljC^/SlO^

r/t - 0.4

»

/

—

^

1

0—

1

i

40 50 60 70 80

ANGLE OF INCIDENCE (degrees)

90

Figure 6. The laser damage threshold values for Hf02/Si02 and Al2OySi02 multilayer dielectric reflectors are compared

with l/cos9 and the cylindrical defect model with r/t = 0.4.

423
COMMENTS

Comments: Several years ago, when the defect-initiated damage modeling was

done, Mclver and his people looked at the cylindrical impurities

and, in fact, came up to the same conclusion you have, namely,

that it was the aspect ration that was important.

Answer: Thank you, I wasn't aware of that work.

Question: Do you expect any dependence on pulse length, width, shape?

looked like you were doing everything for 23 nanoseconds.

It

Answer: Right. The pulse- length dependence of damage thresholds, I think,

is entirely another question. And I do expect something. I think

results have shown that you can get a pulse length dependence all

the way from a basically nonpulse- length dependence on out to a square root of pulse -length dependence. And which you'll see depends on the particular wavelength and experimental setup.

424
MANUSCRIPT NOT RECEIVED

Pulse-width Dependence of Optical Coating Damage at 1052 nm

K. Yoshida, S. Ikunishi, H. Yoshida, Y. Kato, and S. Nakai

Institute of Laser Engineering

Osaka University

Suita, Osaka 565

Japan

ABSTRACT

Pulse- width dependence of optical coating damages at 1053 nm laser wavelength is studied.

Quarterwave-thick single layer films of high-index materials of A12 03 , Zr02 and Ti02

, and low-index

materials of MgF2 and Si02 were tested. Damage thresholds of the MgF2 and Si02 films follow the square

root dependence on the pulse-width r (yV scaling) over the pulse widths of 0.35-90 ns. Although the damage

thresholds of the A1203 , Zr02 and Ti02 films followed the \/t scaling from 0.35 to 1 ns, they become

independent of the pulse-width for the pulse-widths longer than 6 ns. That is, the pulse-width dependence

of the damage threshold is determined by the absorption property of the coating layer, which controls the

damage mechanism.

425
Manuscript received

7-9-90

Damage Resistant Optical Coatings Prepared

Using High Temperature,

Plasma Chemical-Vapor-Deposition*

J.H. Campbell, J.L. Emmett, R.M. Brusasco and F. Rainer

University of California

Lawrence Livermore National Laboratory

P.O. Box 5508, L-490

Livermore, California 94550

R. Th. Kersten, V. Paquet and H. -W. Etzkorn

Schott Glaswerke

D-6500 Mainz

West Germany

Multilayer dielectric optical coatings, nominally consisting of 1000 or more optical-quarterwave layers,

have been prepared by reacting SiCl

4 and a halogenated dopant (e.g. GeCl4

) with 02 in a microwave-driven plasma.

The dopant concentration is such that the index difference (An) between adjacent quarterwave layers is about 0.02

or less. The deposition is carried out at high substrate temperatures (850 to 1 100°C) producing a fully dense, fused

silica coating. Surface damage thresholds of high reflectivity (HR) coatings prepared by this plasma process are

comparable to those for fused silica. For example, at 1.06 p.m and 16-ns we measure surface damage thresholds

greater than 45 J/cm2 compared to about 45-60 J/cm2 for super-polished optical fused silica.

I. Introduction

The output of high-peak-power lasers is often limited by the damage threshold of the multi-layer dielectric

coatings that are deposited on various optical elements in the system. In this paper we discuss the results of

experiments using high temperature (T > 850°C) plasma-assisted chemical vapor deposition to produce damage

resistant optical coatings. The coatings typically consist of several thousand quarter-wave layers of fused Si02

lightly doped with a glass network modifier or network former (e.g. Ge02

, F) to give an interlayer index variation.

Currently, most multilayer dielectric optical coatings used in high power laser systems are prepared using

* Work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National

Laboratory under Contract No. W-7405-Eng-48.

426
physical vapor deposition techniques (e.g. e-beam evaporation) and typically consist of a stack of alternating layers

of metal oxides and/or fluorides. Although the exact mechanism for laser induced damage in these coatings is still

widely debated, there are a number of inherent weaknesses in the deposited structure that have been proposed to lead

to optical damage. These include (but are not limited to) the following:

• Thermal-mechanical and/or chemical incompatibility between layers [1].

• Bulk and localized regions of high optical absorption (often >105 times the natural, pure material)

due to process-induced defects in the coating materials [2,3,6,7].

Incorporation of light absorbing particulates (e.g. dust and other contaminates) in the coating during

deposition [4-9].

• The presence of voids or microstructural defects formed in the coating layer that enhance the local

electric field [8,9].

To achieve high damage thresholds in an optical coating one would like to develop a coating and an

associated deposition technique that is not plagued by the above weaknesses. It is well known that optical fusedsilica produced by a high temperature CVD process, such as that used by Corning, has high thresholds for both

surface and bulk laser damage [10-12]. In the Coming CVD process, Si02 is formed by vapor-phase hydrolysis of

SiCl

4 and then deposited layer- upon-layer until a boule of Si02 is formed [13]. Thus, in a general sense, the Si02

is a monolithic CVD "coating".

We believe the excellent damage resistance of optical fused silica is because it lacks the potential damage

causing weaknesses outlined above. First, the material is compositionally uniform. Consequently there are no

distinct material interfaces with thermal-mechanical or chemical incompatibility. Second, the chemical stoichiometry is uniformly controlled throughout the material by use of high-temperature, oxidizing deposition conditions.

Third, any dust, dirt or other particulate contamination is fully oxidized and either dissolved directly in the glass or

incorporated as an insigificant, low absorption, scattering center. Finally, the fused silica is a fully dense, amorphous

body without the high density of microstructural defects that are often produced by conventional physical vapor

deposition processes.

In principle then, if one could prepare an optical coating via the CVD technology used to prepare fused silica,

it should have excellent resistance to laser damage. Figure 1 illustrates one such coating concept. In brief, various

low concentrations of either network modifiers or network formers are added to alternating layers of the silica glass

matrix. Doping the Si02 with these materials produces the index difference needed to achieve the desired optical

characteristics, yet the dopant concentration is low enough that the bulk Si02 glass remains largely unaltered. Thus

there are no distinct material interfaces. The coating is produced under the same high temperature (>1000°C),

oxidizing conditions used for preparing bulk Si02 and therefore is amorphous, fully dense, with very low optical

absorption and essentially defect-free. Since the dopant concentration is low, a large number of layers are required

to produce the desired optical effect. This is shown in figure 2 where predicted reflectance curves are shown for

multilayer coatings containing small dopant concentrations (< 10%) of Ge02

, F, or B203 in the silica matrix. In summary then, the coating is a silicate glass monolith comprised of several thousand alternating optical-quarterwave

layers of slightly different glass composition (i.e. refractive index).

427
(Plus

Dopant)

SiCI 4 + 0 2 Si02 + Cl 2 4

Fused-silica substrate

Coating

- Doped

Si0 2 - Undoped

Si02

Figure 1 . Conceptual view of multilayer optical coatings prepared

by chemical vapor deposition of alternating layers of doped and

undoped fused Si02

.

2. Selection of a CVD coating process

The CVD processes used to prepare the preforms for silica optical fiber fabrication can, in principle, also be

used to prepare optical coatings such as those described above. Typical fiber-preform processes operate at high

temperatures under highly oxidizing conditions and at high deposition rates. Dopants are usually added to achieve

variable refractive index control for the required optical fiber performance.

The optical fiber CVD processes can generally be broken down into two types. One is the so-called "soot"

process in which the silica precursor (SiCl

4

) is reacted thermally with 02 to produce fine (0.1 to 1.0 u,m) particles

of Si02

; these particles then deposit on the substrate wall and are subsequently sintered to a fully dense, amorphous

Si02 monolith [14-16]. The second general deposition process is plasma driven. In this case the reaction of the

precursor and 02 are driven by a microwave or rf initiated plasma [17-19].

The plasma processes have the advantage that the thickness of a single deposition layer can be controlled

to the angstrom level. In contrast, the layer thickness resolution of the "soot" process is at best several Si02 particlediameters (a few tenths of a micron) and thus impractical for deposition of typical quarter-wave thick layers used

in optical coatings.

Of the current plasma processes, the one having the best thickness control at high deposition rate is the Schott

plasma-impulse chemical vapor deposition process (PICVD) [19-21]. Control of the deposition layer thickness (and

hence refractive index profile) to within a few angstroms is possible while at the same time achieving deposition

rates of a few microns per minute. This is the process we have selected for demonstration of the optical coating

concept outlined here. The coatings have been deposited on the inside of a 1.7 cm id Si02 tube since that is the

configuration ofthe presently available plasma CVD systems. Work is underway to adapt this process to large planar

substrates.

428
o

u

c

(0o

0)

a>u

c

(0o

0) H—O

CC

100

80

60

40

20

An = 0.015

>aaaaaAAAAA/1

250 layerpairs

1000 1020 1040 1060 1080

Wavelength (nm)

1100

100

80

60

40

— An = 0.004

1200 layerpairs

1040 1060

Wavelength (nm)

1080 1100

Figure 2. Examples of expected optical performance calculated for two HR coatings

comprised of (a) 500 and (b) 2400 alternating quarterwave layers of Si02 and Si02 containing small additions of a glass network modifier. The index difference between

quarterwave layers is 0.015 and 0.004, respectively.

3. Preparation of Plasma CVD Coatings

The optical coatings were prepared using the plasma-impulse-CVD process developed by Schott Glaswerke.

Further details of the process are described elsewhere [19-21]. The PICVD process is shown schematically in

figure 3. It consists of three major components: a gas supply system, the deposition unit and vacuum pump. All

three units are under computer control. Reactant gases are continuously supplied to a 1 .7 cm id x 120 cm longSi02 glass tube which serves as both the reaction vessel and deposition substrate. Immediately surrounding the quartz

tube is a coaxially positioned microwave cavity. This entire assembly is located in a tubular furnace that typically

heats the substrate to 800-1 100°C.

/.on
The output from a 2.45 GHz magnetron, triggered by a pulse generator, is used to drive the deposition

process. The deposition process consists of a sequence of four steps (fig. 4). First the Si02 tube is filled with gas

reactants (e.g. SiCl

4

,

2

), second, plasma is ignited and sustained by a pulse from the magnetron. The plasma ignition

occurs at the end of a short inner tube and the plasma then serves as a lossy inner conductor that enables the coaxial

propagation of the microwave down the length of the outer conductor. In the next step the plasma ignited inside the

tube drives a gas-phase reaction between SiCl

4 and 02 and ultimately produces a thin, amorphous deposit on the

tube wall. Previous studies have shown that there is gas phase deposition of SiO at the walls followed by

heterogenous reaction with 02 forming Si02 (versus particulate formation and then deposition). Therefore, excellent

thickness coating control is possible by controlling the reactant gas composition and pressure. The final step of the

process consists of pumping out product gas (e.g. Cl

2 and unreacted 02

) and since the flow is continuous, also

simultaneously refilling the tube. Experience has shown that the deposition efficiency is nearly 100%.

Typical deposition conditions used for these experiments are given in Table 1. Deposition rates were

generally about 2 fim/min over the length of the tube (-50 cm); this corresponds to single-pulse layer thickness of

about 30A at the nominal 100 Hz pulse repetition rate. The substrate temperature was generally kept about 1000°C

to reduce inclusion of product gases in the film and enhance the film quality.

The multilayer optical coating was prepared by periodically mixing Si02 precursors gas (i.e. SiCl4

) with a

halogenated dopant gas. Dopant gases where chosen that are known to give stable silicate glasses and produce the

desired index change without producing unacceptable thermomechanical stresses (fig.5). In most cases, co-dopants

of Ge and F were used to produce the desired index change.

Table 1 : Typical deposition conditions for preparing multilayer doped SiO coating using Schott's PICVD process.

Substrate:

Substrate geometry

diameter:

length:

Substrate temperature:

Pressure: 02 mass flow:

SiCl, mass flow:

Dopant flow:

Dopants:

Dopant Source:

Pulsed plasma repetition rate: Microwave frequency:

Microwave power (avg.):

Deposition rate:

Incremental thickness:

Index profile:

Quarterwave thickness:

(at 1.06 |im)

Si02

, Heralux WG

1.7 cm ID, 2.0cmOD

> 50 cm

850 to 1 100°C

3 mbar

200 seem

50 seem < 10 seem Ge02

, F

GeCl4, CC12F2 100 Hz

2.45 GHz

0.6 kW

~ 2 |im/min.

-30 A

sinusoidal or square wave

1800 A

430
Furnace

Waveguide

Figure 3. Schematic of the PICVD deposition system.

"=>

Gas inlet Microwave input

e)

Exhaust gas

Figure 4. Four main phases of the PICVD deposition process on the inside

of a tubular substrate: (a) filling reactant gases (eg. SiCl

4 + 02

), (b) ignition,

(c and d) propagation of the plasma causing reaction and then deposition of

the gas phase products, and (e) exhaust of residual gas to the pump system.

431
0 0.1 0.2 0 0.1 0.2

Dopant fraction Dopant fraction

Figure 5. Variation of (a) refractive index and (b) thermal

expansion coefficient vs. dopant ion concentration in SiOr The data are from ref. 22.

4. Characterization of Optical Coatings Prepared by PICVD

4. 1 Optical Characterization

The measured transmission of two typical PICVD coatings is shown in fig. 6. The peaks in the curves (i.e.

regions of low transmission) indicate reflection bands. The first coating (fig. 6a) consists of approximately 1000

optical quarterwave layers with a sinusoidal index variation and a maximum An of about 0.01 between adjacent

layers. The layer thickness was adjusted to give a single maximum reflectance at 1064 nm. The shoulder on the

short-wavelength side is due to small process variabilities. In general the measured bandwidth of the PICVD

coatings is about 2-3 times the theoretical value. This suggests that with future improvement in process control,

narrower bandwidth coatings with fewer layers should be possible.

The coating shown in fig. 6b consists of about 5000 quarterwave layers with roughly every 1000 layers

designed to reflect a different wavelength (hence 5 reflectance maxima). In this case, the tail on the short-wavelength

side is due to bulk absorption by the glass matrix and is not due to process variability. Since the deposition process

is fully automated it is possible to create coatings having any number of peaks either fully or partially reflecting.

Moreover by closely grouping the peaks such that they overlap, reflective coatings of any arbitrary bandwidth can

be achieved. In principle a "white light" HR coating (i.e. a coating reflective over the entire visible region) could

be fabricated with this technology.

Note that the use of a sinusoidal index profile produces a single reflectance band. Consequently reflections

at harmonic wavelength are strongly suppressed except for the small amount (< 5%) introduced by small

process variations.

432
900

«> 100 m

1000 1100 1200

200 400

Wavelength [nm]

600

Figure 6. Measured transmission versus wavelength for (a) 1000

and (b) 5000 quarter-wave-layer optical coatings prepared by the

PICVD process. The dopant profile was sinusoidal in both cases

with the amplitude of the index variation about 0.01.

The overall absorption loss in optical thin films produced by the PICVD process is near the theoretical limit

of that expected for the pure bulk material. The reason for this is clear; these coatings were made using the same

deposition process used to prepare the full-density optical preforms used to fabricate optical fibers. The reported

loss spectrum for the Ge02-doped Si02 material used to prepare these coatings is shown in fig. 7; the measurements

were made on a fiber drawn from the material. Note that the loss at 1 .0 (J.m is approximately 1 dB/km corresponding

to ~ 10"6 cm"1. This absorption loss is about 5 orders of magnitude lower than that for high quality thin-films produced

by current e-beam evaporation processes (fig. 8).

4.2 Compositional and Structural Characterization

Figure 9 is a SEM photograph showing the cross-section of a portion of an approximately 2000-layer coating

designed to reflect at 1064 nm. The line-to-line spacing (corresponding to two quarterwaves) is about 365 nm. The

deposited film is fully dense showing no visible defects.

433
1.0 1.2 1.4

Wavelength (ym)

1.6

Figure 7. Loss spectrum for a Ge-doped, graded index Si02 fiber

drawn from a preform made by the PICVD process (see ref. 19).

10'

10

-2

otoo

5

10~*

E

o

10"

10"

e-beam coating

(100 ppm)

PICVD

Figure 8. Comparison of typical optical absorption for Ge-doped Si02 made by high

temperature plasma CVD [19] with that typically reported for high quality thin films

produced by e-beam evaporation (see for example ref. 7 and 23).

434
Figure 9. SEM photograph showing the cross-section of a portion

of a 2000-layer Ge-doped Si02 coating deposited by the PICVD

process. Note the 2 Jim scale marker.

0.5 1.0 1.5

Depth (microns)

2.0

Figure 10. Results from SIMS analysis of the top 12-131ayersofaF,Ge

co-doped SiOz coating.

435
We have analyzed the dopant concentration profile through several of the PICVD layers using secondaryion-mass-spectrometry (SIMS). Figure 10 shows one example of SIMS data from the top approximately 12-13

layers of a 2000 layer Ge-F co-doped Si02 coating. The dopant concentration variation gave a peak-to-valley

refractive index differences of 0.016. The SIMS data were collected using an oxygen primary-ion beam of 7 keV

and current of 400 nA directed at the sample at a 60° angle of incidence. A 300 x 300 |im flat-bottomed crater was

formed by rastering the beam while the data were collected in a 30-p.m diameter spot at the crater center. Charging

of the substrate was eliminated using a charge neutralization technique. The depth resolution is about 4 nm near the

surface and becomes gradually poorer with increased sputter depth. Nevertheless the individual coating layers are

clearly resolved. The lobes seen on the leading and trailing edges of the first few Ge-doped layers are probably real

and a result of variability in the dopant mass flow controllers during this particular run. A calculated transmission

spectrum, using the layer thickness determined from the SIMS data, is in good qualitative agreement with the

measured transmission spectrum. The coating did show a reflection at the second harmonic that we believe is due

to the Ge02-doped layers being systemmatically thicker than required for a perfect optical quarterwave.

5. Damage Threshold Measurements

Most damage threshold measurements of the PICVD coatings were carried out at 1064 nm, with a 1 6-ns pulse

width and 30 Hz repetition rate. The details of the damage test system (termed REPTILE) are described in a companion paper at this conference [24]. The REPTILE damage test system is one of five systems currently in use

at LLNL to provide a data base of damage measurements to support our optical materials research efforts.

In brief, the damage measurements are carried out using the set-up shown schematically in figure 11. The

output from a commercial (Quantel) Nd:YAG laser is directed through an attenuator and long focal length lens

(~2 m) onto the test sample. The attenuator is used to vary the test fluence. The sample is located in a near field

region where the beam spot diameter is about 1 mm in diameter. The test sample is examined both before and after

laser irradiation using 100X Nomarski microscopy; we define damage as any observed change in the sample under

these viewing conditions. The beam profile and energy are recorded using a calorimeter and video camera as shown

in figure 1 1 a. The fluence profile of the beam was then calculated using a commercially available software package.

The PICVD coatings were deposited on the inside of a 17.2 mm fused silica tube as described in a previous

section. In order to properly test the coating, the tube was cut lengthwise into four pieces (fig. lib). The laser beam

was directed onto the inner coated surface of the sample at about a 10° angle of incidence.

Not all of the coatings that were damage tested were designed to reflect at 1064 nm leading to significant

transmission through the sample. This often caused damage at the output surface and significant rerroreflections.

Thus, we feel our measurements represent a lower bound or conservative value of the damage thresholds that can

be expected for the PICVD coatings.

Figure 12 compares the measured damage thresholds of the PICVD coatings* with the surface damage

thresholds of super-polished (Zygo) fused silica and Ge-doped fused silica prepared using very high temperature

(~ 1800°C) chemical vapor deposition. The undoped fused silica is the same high quality material used to fabricate

*bulk thresholds are much higher than the surface damage thresholds.

436
lenses, debris shields and other transmissive optical elements on the current 120 kJ LLNL Nova laser. Fused silica

has the highest surface damage threshold of any material or coating on the Nova laser and consequently we use it

as a "bench mark" to compare with new optical materials. The kilolayer PICVD coatings are nearly equivalent in

damage threshold to the surface threshold of super-polished fused silica. The pulse length scaling for damage to

fused Si02 is approximately:

DT = 19 t 0 4 T p

where DTis the damage threshold (J/cm2

) and t is the laser pulse width (ns).

Unfortunately we only tested one PICVD coating at 1 ns where it gave a damage threshold of about 1 3J/cm2

.

This is somewhat lower than fused silica but is probably inconclusive without further tests (i.e. more sample

statistics).

(a)

(b)

Figure 11. (a) Schematic representation of the 1064 nm laser damage test system used to test the PICVD coatings,

and (b) configuration of the damage test sample cut from the 17.2 mm ID Si02 tube used as a substrate for the coating

process.

437
1000

100

Laser damage

threshold (J/cm 2

)

10

—i—i—i—i—r- -i 1 iii 1

1 # Fused Silica

O PICVD [Si02 /Si02(F)(Ge)]

CVD [Si(VSi02 (Ge)]

0.4

O

-i i i i i -i—i—i i i i

.1 1 10

Pulse duration (ns)

Figure 12. Measured damage threshold at 1064 nm versus pulse width for super-polished, bare

fused silica and coatings prepared by either PICVD or very high temperature (~ 1800°C) CVD.

The data for the bare fused silica are from ref. 10, 11 and 12. The PICVD coating samples

consisted of 1000 or more quarter wave layers of doped Si02

.

100

6. Extending the PCVD Coating Process to Flat Substrates

There are three major technical advances that must be made before the high temperature PICVD process can

be used to prepare optical coatings on flat substrates:

1. Achieve uniform deposition over large, flat areas (> 10 cm diameter),

2. Maintain the same high deposition rates and efficiencies that were used in preparing coatings on the

inside of tubular substrates, and

3. Uniformly heat large areas to temperatures near 1000°C while maintaining a uniform reaction

plasma.

In a companion paper [25] we discuss each of these issues further in light of our recent progress in designing and

building a planar, high-temperature PCVD system.

438
7. Conclusions

High reflectivity (HR) coatings, nominally consisting of 1000 to 2000 optical-quarterwave layers, have

been prepared by reacting SiCl

4 and a halogenated dopant (e.g. GeCl4

) with 02 in a microwave-driven plasma. The

dopant concentration is such that the index difference (An) between adjacent quarterwave layers is about 0.02 or less. The deposition is carried out at high substrate temperatures (about 900 to 1 100°C) producing a fully dense, fused

silica coating. The motivation for using the high temperature conditions is the observation that commercially

available CVD deposited fused silica (used for making focusing lenses and other high fluence laser optical

components) has very high surface and bulk damage thresholds.

Measured damage thresholds of HR coatings prepared by this plasma CVD process are comparable to those

for super-polished fused silica surfaces. For example, at 1.064 nm and 16-ns we measure PICVD coating damage

thresholds of about 45 J/cm2 compared to about 45-60 J/cm2 for the surface of super-polished Corning optical fused

silica. The plasma CVD coatings have the highest N-on-1 damage threshold of any optical coating tested to date

by the LLNL Laser Program.

The present coating method is an adaptation of a process developed by Schott Glaswerke for producing

optical fiber preforms. Besides the advantage of operating at high temperature and under highly oxidizing

conditions, this process offers: (1) high deposition rates (about 2-3 (im/min.), (2) control of the refractive index

profile to within dopant diffusion limits, and (3) ultra-pure starting materials giving extremely low bulk absorptions.

Consequently, low-scatter, essentially defect-free coatings consisting of several thousand quarterwave layers can

be deposited in a few hours. Furthermore, the index profile can be easily tailored to be sinusoidal, square-wave, etc.

The major disadvantage of the present deposition process is that it was developed for coating the inside of

fused silica tubes about 2-cm in diameter (used in making optical fiber preforms). Therefore it must be modified

to coat flat substrates; extensive efforts are currently underway at LLNL and Schott to develop such a coating system.

Acknowledgements:

The authors gratefully acknowledge the efforts of Ms. A. Clasen in preparing this finished manuscript.

References:

[ 1 ] R. R. Austin, R. C. Michaud, A. H. Guenther, J. M. Putman and R. Harniman, "Influence of Structural Effects

on Laser Damage Thresholds of Discrete and Inhomogeneous Thin Films and Multilayers", Laser Induced Damage

in Optical Materials: 1972. NBS Pub. 372, pg. 135.

[2] M. R. Kozlowski, M. Staggs, C. R. Wolfe and J. H. Campbell, "Large Area Laser Conditioning of Dielectric

Thin Film Mirrors", in Laser Induced Damage in Optical Materials: 1989. (this proceedings).

[3] C. R. Wolfe, M. R. Kozlowski, J. H. Campbell, F. Rainer, A. Morgan, and R. Gonzales, "Laser Conditioning

of Optical Thin Films", in Laser Induced Damage in Optical Materials: 1989. (this proceedings).

[4] D. Milam and R. A. Bradbury, "Laser Damage Threshold for Dielectric Coatings as Determined by

Inclusions", Appl. Phys. Lett. 23_, 654 (1973).

439
[5] T. W. Walker, A. H. Guenther, and P. Nielsen, "Pulse Laser-Induced Damage to Thin-Film Optical

Coatings-Part I: Experimental" and "Part II: Theory", IEEE, J. Quantum Electron., OE-17. 2041 (1981).

[6] W. H. Lowdermilk, D. Milam, and F. Rainer, "Optical Coatings for Laser Fusion Applications", Thin Solid

Films. 22, (1980) 155-166.

[7] W. H. Lowdermilk and D. Milam, "Laser-Induced Surface and Coating Damage", J. Quantum Electron. QE17, 1888 (1981).

[8] D. M. Aikens and J. R. Taylor, "Cause of Damage in Multilayer Dielectric Coatings Exposed to High

Average Power Visible Laser Radiation", in Laser Induced Damage in Optical Materials: 1987 . NIST Spec. Pub.

756, p. 419.

[9] N. Bloembergen, "Role of Cracks, Pores and Absorbing Inclusions on Laser Induced Damage Thresholds

at Surfaces of Transparent Dielectrics", Applied Optics, 12 (1973), 661.

[10] S. Stokowski, D. Milam, and M. Weber, Laser Induced Damage in Optical Materials: 1978. NBS Spec. Pub.

541,(1978) p. 99-108.

[11] D. Milam, "1064 nm Laser Damage Thresholds of Polished Glass Surfaces as a Function of Pulse Duration

and Surface Roughness", in Laser Induced Damage in Optical Materials: 1978. NBS Spec. Publication 541 (1978),

p. 164-167.

[12] F. Rainer, R. P. Gonzales and A. J. Morgan, "Laser Damage Database at 1064 nm", in Laser Induced Damage

in Optical Materials: 1989. NIST Spec. Publication (this proceedings).

[13] J. F. Hyde, "Method of Making a Transparent Article of Silica", US patent 2,272,342 (Feb. 10, 1942).

[14] S. R. Nagel, J. B. MacChesney and K. L. Walker, "An Overview of the Modified Chemical VaporDeposition

(MCVD) Process and Performance", IEEE J. Of Quant. Elect. OE-18. (1982) p. 459-476.

[15] H. Suda, S. Hibuta, and M. Nakahara, "Multiflame VAD Process for High-Rate Fabrication of Optical Fiber

Preforms", Transactions of the IECE of Japan E69_ (1986) 1206-1202.

[16] P. C. Schultz, "Fabrication of Optical Waveguides by the Outside Vapor Deposition Process", Proceedings

of the IEEE 68 (1980), p. 1187-1190.

[17] H. Lydtin, "PCVD: A Technique Suitable for Large-Scale Fabrication of Optical Fibers", J. Lightwave

Techn. LT-4 (1986), p. 1034-1038.

[18] J. Irven and A. Robinson, "Optical Fibers Produced by Plasma Augmented Vapour Deposition", Phys. and

Chemistry of Glasses, 21 (1980), p. 47-52.

[19] Th. Hunlich, H. Bauch, R. Th. Kersten, V. Paquet, and G. F. Weidmann, "Fiber Preform Fabrication Using

Plasma Technology: A Review", J. Opt. Commun. £ (1987), p. 122-129.

440
[20] H. Bauch and V. Paquet, "Fibers with Pure Si02-Core Made by PICVD", J. Opt. Commun. £ (1987),

p.136139.

[21] H. Bauch, V. Paquet and W. Siefert, "Preparation of Optical FiberPreforms by Plasma-Impulse CVD", Conf

Proc. SPIE, Optical Fiber Characteristics and Standards, SPIE Vol. 584, p. 33-37; Cannes (1985).

[22] P. C. Schultz, "Recent Advances in Optical Fiber Materials", Proceedings of Second Interntional OttoSchott Colloquium, Jena, DDR, July 12-16, 1982, p. 215-225.

[23] D. Milam, W. H. Lowdermilk, F. Rainer, J. E. Swain, C. K. Carniglia and T. Tuttle Hart, "Influence of

Deposition Parameters on Laser Damage Thresholds of Silica-Tantala AR Coatings", Applied Optics 21 (1982), p. 3689-3694.

[24] A. Morgan, F. Rainer, F. P. DeMarco, R. P. Gonzales, M. R. Kozlowski and M. C. Staggs, "Expanded

Damage Test Facilities at LLNL," in Laser Induced Damage in Optical Materials: 1989. LLNL report

UCRL-101642 (these proceedings).

[25] R. Brusasco, J. A. Britten, C. B. Thorsness, M. S. Scrivener, W. G. Unites, J. H. Campbell and W. L. Johnson,

"A High Temperature, Plasma-Assisted Chemical Vapor Deposition System," in Laser Induced Damage in Optical

Materials: 1989. LLNL report UCRL- 101766 (these proceedings).

441
COMMENTS

Question

Answer

Question

Answer

Question

Answer

Question

Answer

Did you consider other mechanisms other than the Drude model, for

example band filling?

No, we just used a simple Drude model.

Certainly in the 10 micron region the overwhelming contribution to

the refractive index is not the Drude contribution but the

contribution due to band filling, so you might want to have a look

at that.

We shall look at that then, thank you.

Could you tell us a bit more about the deep level impurities, what

they might be , and how you diagnosed them?

We've done a number of tests on those in the discrete photodiode

devices through cd measurements . That is how we know that they

are deep levels about mid band and not shallow levels in the band

gap. These levels were induced by melting transients in the

discrete diodes. When we see this electrical damage onset that we have melted to the surface of the silicon.

How do you know that you have introduced new defects rather than

allowing the Fermi level to shift and change the charge state of an existing defect?

The cd measurements as a function of frequency will tell you that

you really have not changed the charge state, but that these are bulk not surface defects. In the discrete devices we are looking

at a junction that is below the surface, so we weren't really

looking at traps in thick surface states. The defect level of an undamaged device was very low as measured by cd measurement before

laser radiation.

442
Manuscript received

3-20-90

A High Temperature, Plasma-Assisted

Chemical Vapor Deposition System*

R. M. Brusasco, J. A. Britten, C. B. Thorsness,

M. S. Scrivener, W. G. Unites, and J. H. Campbell

University of California

Lawrence Livermore National Laboratory

P.O. Box 5508, L-490

Livermore, CA 94550

W.L. Johnson

Prototech Research, Inc. 106 South 54th Street

Chandler, AZ 85226

We have designed and built a high-temperature, plasma-assisted, chemical vapor deposition

system to deposit multilayer optical coatings of Si02 and doped-Si02 on flat substrates. The coater

concept and design is an outgrowth of our recent work with Schott Glaswerke demonstrating the use of

plasma assisted CVD to prepare very high damage threshold optical coatings. (That work is reported in

a companion paper at this Symposium).

The coater is designed to deposit up to several thousand alternating quarterwave layers of Si02 and doped Si02 on Si02 substrates at deposition rates up to several microns per minute. The substrate

is resistively heated to about 1 000°C during the deposition phase of the process. The plasma is driven

by a 1 3.56 MHz RF unit capable of producing power densities of up to 1 40 W cm 3 in the reaction zone. The coater is designed to be adaptable to microwave generated plasmas, as well as RF. Reactant gas

flow rates of up to 10slmcan beachievedatalOtorroperating pressure. Reactants consist of 02 , SiCI

4 and a volatile halogenated dopant (e.g. GeCI4 or CCI2 F2

). These gases react in the plasma volume

producing Si02 with dopant concentrations (e.g. Ge02

) of up to a few percent. A variable dopant

concentration is used to produce index differences (nominally about 0.01) between adjacent optical

layers.

*Work performed underthe auspices of the U.S. Department of Energy by Lawrence Livermore National

Laboratory under contract number W-7405-ENG-48.

443
Some of the major technical problems associated with the design and construction of this coater

are discussed. These include flow distribution and its effect on deposition uniformity, substrate heater

design and operation, choice of RF vs. microwave driven plasma and diagnostics design and placement.

To address many of these issues, we have relied heavily on numerical and experimental modeling of the

fluid flow, reaction kinetics, and heat transfer within the coater. Furthermore, preliminary experiments

carried out on prototype designs were used to guide our development efforts.

1.0 Introduction

High reflectivity (HR) coatings for the next generation laser fusion driver will require laser damage

thresholds not less than 40 J/cm2 at 1 .06 \i wavelength[1 ]. Considering that bulk fused silica possesses

a high laserdamage threshold and is made by high temperature chemical vapordeposition, an effort has

been initiated to apply a high temperature deposition method to the creation of a multilayer HR reflector.

This so-called "kilolayer" concept, and its success in a circular tube geometry, is described more fully in another paper in this publication[2]. The key processing issue now is to translate this process to one

suitable for flat substrates.

The task requires separation into four roughly concurrent efforts; preliminary experimentation with

the coating process, examination of heat and mass transport issues, investigation of fundamental plasma

characteristics and hardware production and evaluation. With respect to mass transport, Wilcock et al.[3]

and Law and Masliyah [4] have successfully used naphthalene sublimation in the analysis of mass

transport uniformity. Since, mathematically, naphthalene sublimation (removal) is equivalent to deposition in terms of uniformity, we constructed a test bed to model the mass transport in the proposed reaction

chamber geometry. We wished to determine whether the proposed geometry would lead to a uniform

deposition and what the critical parameters werelhat would assure uniform mass transport.

With respect to plasma characteristics, we have demonstrated deposition of up to =5000 quarter

wave layers inside a 3.0 cm diameter tube[2] using 2.45 GHz radiation to excite the plasma. However,

the abi lity to scale plasma applicators at 2.45 GHz uniformly for large flat substrates, the cost of such scaleup and possible use of other radiation frequencies remain unresolved issues. As an example of frequency

dependence, two literature studies concerning a photoresist stripping process can be used to compare

RF versus microwave plasmas. The papers by Dzioba et al.[5] and Degenkolb et al.[6,7], describe

experimental systems that are nearly identical, with similar reaction volumes, pressures, and oxygen flow

rates. However, the study utilizing 2.45 GHz exhibited a more highly-excited oxygen plasma environment,

inferred from the optical emission spectrum, and an order-of-magnitude increase in the stripping rate compared to the study which used 13.56 MHz radiation. The extent to which differences such as this

would affect our CVD coating apparatus is unknown at this time, but could have a large impact in terms

of hardware development and coating performance, particularly laser damage resistance. We have

constructed a plasma test bed to investigate the differences between RF and microwave plasmas and to

refine plasma diagnostic techniques.

Among the diagnostic techniques being considered for monitoring the coating environment are

residual gas analysis for mass balance and impurity monitoring and determination of the electron density

(n

e

) and electron temperature (T

e ) [8] with a Langmuir probe.

444
2.0 Experimental

Preliminary deposition studies were carried out in a horizontal silica tube reactor, shown

schematically in figure 1. The tube was 7.62 cm in diameter (expanded to 12 cm in the reaction zone)

and approximately 61 cm long. The tube was contained in a radiantly heated furnace with the heating zone

approximately 30 cm long. Athimble containing a fused silica substrate, a graphite susceptorand a boron

nitride insulator was inserted into the hot zone of the reaction tube. The graphite served as the absorber

in the radiant furnace and as the driven electrode for the application of RF at 1 3.56 MHz. Gas flows of

oxygen, silicon tetrachloride and germanium tetrachloride were established using mass flow controllers.

Pressures were monitored with Baratron gauges and controlled by a butterfly valve controller. Typical

operating parameter ranges are given in table I.

To Vacuum pump and

exhaust

RF Electrode

Thermocouple

Boron Nitride

Graphite Susceptor

O^.SiCU.GeCU

Inlet Gas

System

12 cm dia. tubular quartz

reactor

Quartz-halogen heating lamps

inside gold-plated furnace

Figure 1 . Schematic of radiantly heated plasma CVD reactor.

Mass transfer experiments were performed in a 1 :1 scale model of the proposed coater (fig. 2) with

a reaction volume of approximately 800 cm3 [9]. The substrate-to-injection plate spacing could be adjusted

with appropriately sized spacers. A Duo Seal 1398 vacuum pump allowed pumping rates as high as 5

slm through the reactor at reaction chamber pressures from 200 to 2000 Pa. A gas distribution plate

consisting of a square array of 237 holes of 0.0635 cm diameter on 1 cm centers or 72 holes of 0.229 cm

diameter on 1 cm centers was studied. Substrates were prepared by hot pressing naphthalene at

approximately 78°C and 68 MPa pressure into aluminum molds. The substrates were weighed before

and after the sublimation experiment to determine the mass of naphthalene that sublimed and from this

an average rate of removal was calculated. The substrate surface was also profiled before and after a

run on a Moore profilometer (resolution 3 X 10 4 cm) to determine uniformity of sublimation.

445
Table 1 . Summary of typical operating conditions used in our preliminary radiant heating plasma CVD

experiments

Pressure

Flow rate oxygen

Flow rate SiCL4 Flow rate GeCl4 RF power (CW exp.)

RF power (peak)

RF power (average)

Pulse width

Pulse Repetition Frequency

Temperature

Reaction time

Number of experimental runs

0.3-3.0 torr

1 00-500 seem

20-90 seem

0-10 seem

300 Watts

57-94 Watts

8.5-15 Watts

10-15 X 10"3 sec

10-30 Hz

1 050-1 250°C

60 min.

16

Air in

^^^^

Inlet Plenum

Gas Distribution Plate

Substrate

2.15cm

z

15cm dia.

Naphthalene

Substrate

t

To Vacuum

Figure 2. Schematic of the mass-transfer test system

446
Initial experiments on plasma characteristics were carried out in a silica reactor. Langmuir probe

experiments were done in a horizontal fused silica tube of 15 cm diameter in the reaction zone and an

overall length of 90 cm, as shown in figure 3. The plasma was excited by RF at 1 3.56 MHz using 2.5 cm

wide strips of metal as electrodes that were attached to the outside of the reactor tube in a clam-shell

arrangement. The Langmuir double probe was constructed as described by Chen [10] and placed as

close to the electrodes as possible and extended into the plasma region approximately 5 cm. Presently,

diagnostics experiments are conducted in a full scale mock-up of the proposed PCVD coater geometry

as shown in figure 4, with adiameter of 1 5 cm and a height adjustable from 1 to 7 cm. The plasma is excited

by either a parallel plate RF electrode arrangement, with the driven electrode at the bottom of the chamber,

or with a waveguide applicator for 2.45 GHz radiation. A residual gas analysis system using an Inficon

Quadrex 100 Residual Gas Analyzer (RGA) was attached to the exhaust port of the test chamber.

To Pump

;13.56 MHz)

Inlet Port

SiCI 4 02 Ar

Langmuir

Probe

Figure 3. Schematic of tubular plasma test bed

3.0 Results and Discussion

3.1 Preliminary Deposition Experiments

Preliminary deposition experiments using 13.56 MHz RF to drive the reactive plasma in the

horizontal, radiantly heated furnace pointed out several significant problem areas. First, the substrate

temperature was difficult to determine. The thermocouple monitor could only read the back of the

susceptor and was influenced by the driving RF electric field. Second, the deposited layers were uneven

and contained particulates. The existence of cold walls in contact with the process caused generation

of particulate silica, which found its way onto the growing film. Also, there was considerable leakage of

the plasma into undesirable areas of the reactor, such as directly behind the susceptor and along the walls

of the thimble. This leakage significantly reduced the plasma power density available in the reaction

volume. In addition, the total RF power available was only 1 kW.

447
Gas Flow Inlet Gas Plenum

RF or Microwave Antenna

Figure 4. Schematic of parallel-plate plasma test bed.

3.2 Mass Transport

Figure 5 shows the surface profile of a naphthalene disk before and after a sublimation run where

the disk was exposed to 2 slm flow for 5 hours to remove approximately 50% of the naphthalene material.

The increased surface roughness following sublimation is attributed to the crystalline nature of the

naphthalene causing preferential removal based on grain orientation. The mass removal rate was greater

at the outer edge of the disk than at the center due to edge effects associated with a finite disk size. An

analysis of the radial removal rate versus total amount of material removed, neglecting the outer 1 .5 cm

edge, shows the uniformity to be ±5%. Figure 6 compares the dimensionless mass transfer rate versus Reynolds number (Re) for experiment with that predicted by stagnation flowtheory assuming uniform gas

injection through the top plate. Here we define the Reynolds number as,

where H is the distance separating the gas injection plate and substrate (cm), V is the superficial axial

injection velocity (cm/sec) and r| is the kinematic gas viscosity (cm2/sec).The mass transfer rate is made

dimensionless by,

y' = (W)fJPj/_H_\ (2)

44 8
where:

w = Mass loss (gm)

t = time (s)

A = Naphthalene surface area (cm2

) P = Pressure (torr)

Psat = Naphthalene vapor pressure (torr)

H = Reactor height (cm)

£> = Naphthalene diffusivity in air (cm2/s)

P = Gas density (g/cm3

) M = Molecular weight naphthalene (gm)

14. cm

Figure 5. Plot of the naphthalene disk surface contour before and after evaporation.

For low Reynolds numbers, the mass transfer rate agrees with the stagnation flow theory [11] and the

removal of naphthalene is uniform. Above a Reynolds numberof about 3 the mass removal rate increases

sharply due to jet impingement as shown in figure 7. Here, jet impingement has caused locally high mass

removal directly under the inlet orifices, causing a dimpling of the naphthalene surface. These studies

have defined flow rate ranges, for a given reactor and gas distribution geometry, that lead to radially

uniform mass transfer in the reactor; further details of this work are given in ref. 9.

3.3 Plasma Characterization

Figure 8 shows some preliminary data taken from Residual Gas Analysis of a mixture of 02 and

SiCI

4 exposed to an RF plasma. A very low baseline of oxygen gas is compared with a mixture of 02 and

SiCI

4 without plasma, which reveals the fragmentation pattern of silicon tetrachloride. Upon striking the

plasma, the fragmentation pattern indicates a dramatic increase in the signal at mass 70, attributed to Cl

2

449
parent ion, with corresponding decreases in the masses of the lowersilicon halides. Residual gas analysis

will be useful in characterizing the stable volatile reaction products as well as monitoring the presence of

impurities in the gas streams. The major improvement here will be the calibration of the instrument to

provide a mass balance for the reactants and products and to increase the mass range such that all fragments of SiCI

4 and dopants can be monitored routinely.

Dlmensionless

Mass Transfer Rate

y'

Experiment

Theory

T

2 3

Reynolds Number

Figure 6. Dimensionless mass transfer rate versus Reynolds number

Figure 7. Photo of jet impingement on a naphthalene disk.

450
40 i

30 -

Ion Current

(PA) 20

10 -

IT

37 70 63

Mass Number

98

1 .2 Background

(no plasma)

2. S iCI4 + 02 (no plasma)

3. SICI4 + O2

(100W plasma)

Figure 8. Residual Gas Analysis of coater effluent with and without plasma.

Current-voltage data were taken with a Langmuir double probe in oxygen at 1 torr pressure at

several 13.56 MHz RF power levels. Figure 9 shows that with increasing RF power, the electron

temperature increases from about 4 eV at 50 watts to approximately 6 eV at 150 watts. Similarly, the

electron density increases from about 6.0 X 1

9 crrr3 at 50 watts to 1 .3 X 110 cm-3 at 1 50 watts. These

data are in qualitative agreement with data from Heidenreich et al. [1 2] investigating oxygen microwave

afterglows and Yamagishi et al. [13] investigating air-helium plasmas excited by several methods. The

key developments in Langmuir probe measurements will be to extend their operation at high power

densities and in the electronegative plasmas anticipated in the coater environment.

Langmuir Probe

Electron

Temperature 4

(eV)

3 -

1.40e+10

- 1.20e+10

- 1.00e+10

8.00e+9

- 6.00e+9

Electron Density

X1010 (cnr3

)

-1 1 1 ' 1

'

1

1

80 100 120 140 160

RF Power (Watts)

4.00e+9

Figure 9. Plot of electron temperature and electron density versus RF power for oxygen gas

in a 1 Torr pressure plasma as determined by Langmuir probe.

451
Exhaust Ports Gas Distribution Jets & Diagnostic Ports Fused Silica Substrate

Reaction Zone

RF Electrode &

Furnace Enclosure

Opaque Silica Radiation Barrier

Clear Silica Isolation Thimble

Figure 10. Schematic drawing of PCVD coater reaction chamber.

3.4 Hardware Development

Building on the results of the preliminary investigations, a coating system was specified by LLNL

and designed and built by Prototech Research Inc. A schematic of the coating chamber is shown in figure

1 0. It is capable of handling fused silica substrates up to 1 5 cm in diameter and 1 cm thick. The furnace

is a 3-zone back-heating system designed to heat the substrates to about 1 000°C. Some details regarding

the reaction environment are described in table 2. The "hot wall" reactor is formed by using closed-cell

porous silica glass to enclose the reaction volume and absorb radiant energy from the furnace. Gas is

injected through a top plate via an array of gas injection ports and exhausted out the top at the periphery

of the substrate. The containment vessel for the furnace acts as an electrode for a 24 kW 13.56 MHz RF

generator. Plasma confinement is achieved by isolating the walls of the electrode with a silica thimble

which is evacuated to less than 1 0

4 torr. The absorbing silica provides dielectric insulation to suppress

plasma breakdown outside of the reaction chamber. The coating apparatus is located in a clean box to

prevent particulate contamination. This coater is designed to be as flexible as possible to allow for design

changes to meet coating process demands.

Table 2. Summary of designed operating characteristics of the high temperature,

plasma CVD coating system

Substrate Size

Oxygen Flow Rate

SiCI. Flow Rate 4 GeCL Flow Rate

RF Power

Substrate Temperature

15 cm dia. X 1 cm thick

0-8 slm

0-2 slm

0-500 seem

24 kW maximum

1000°C nominal, 1200°C max.

452
4.0 Conclusions

We have recently shown that very high damage threshold dielectric coatings can be made using

plasma CVD at high temperatures (=1 000°C) [2]. In this paper we discussed our efforts at scaling this

process to large, flat substrates (> 1 0 to 50 cm diameter). There are four main technical issues that must

be solved to successfully implement this coating process:

1 . Uniform deposition over a large area (+ 1 -5%)

2. High deposition rates (1 -4 jxm min 1 )

3. Uniform substrate heating at temperatures up to about 1 000°C

4. Minimal dopant diffusion

To address these technical issues we have designed, built and operated a series of test bed

reactors to look at mass transport, plasma chemistry, and substrate heating issues. We have identified

a flow regime that provides uniform radial mass transfer and hence should give uniform deposition. In

this work we have used mathematical models to guide and interpret the experiments. In addition, work

is in progress to look at the effects of RF versus microwave driven plasmas on the deposition process and

we have begun to identify and refine plasma characterization techniques. Based on these results we have

built a large coating system designed to coat substrates up to about 15 cm in diameter with several

thousand quarter-wave layers of alternating doped and undoped Si02

. Details of the coater design

specifications have been formulated and given. The system is expected to be fully operational by mid1990.

References

[1] Powell, H.T., J.H. Campbell, J.T. Hunt, W.H. Lowdermilk, J.R. Murray and D.R. Speck, Proceedings of the Workshop on Inertial Confinement Fusion, Varenna, Italy (Sept. 1988). LLNL report

UCRL-99517. High Power Nd:Glass Lasers as Drivers for Inertial Confinement Fusion.

[2] Campbell, J.H., J.L Emmett, R.M. Brusasco, F. Rainer, R.Th. Kersten, V. Paquet and H.-W.

Etzkorn, Laser Induced Damage in Optical Materials, 1989. NTIS special publication (in press).

Damage Resistant Optical Coatings Prepared Using High Temperature Plasma-Chemical-VaporDeposition.

[3] Wilcock, D..C.C. Wright and B.L Button, Proc. 8th Int. Heat Transfer Conf., 3, 1 1 99, 1 986. Multijet

Inclined Heat Exchanger Modules: Mass/Heat Transfer and Pressure Loss Characteristics.

[4] Law, H.S. and J.H. Masliyah, Ind. Eng. Chem. Fundam., 23, 446, 1984. Mass Transfer Due to a

Confined Laminar Impinging Axisymmetric Jet.

[5] Dzioba, S. G. Este and H.M. Naguib, J. Electrochem. Soc, 129(11), 2537, 1982. Decapsulation

and Photoresist Stripping in Oxygen Microwave Plasmas.

453
[6] Degenkolb, E.O., C.J. Mogab, M.R. Goldrick, and J.E. Griffiths, Appl. Spectrosc.,30(5), 520, 1 976.

Spectroscopic Study of Radiofrequency Oxygen Plasma Stripping of Negative Photoresists. I.

Ultraviolet Spectrum.

[7] Griffiths, J.E. and E.O. Degenkolb, Appl. Spectrosc., 31 (2), 1 34, 1 977. Spectroscopic Study of Radiofrequency Oxygen Plasma Stripping of Negative Photoresists: II Visible Spectrum.

[8] Steinbruchel, Ch., B.J. Curtis, H.W. Lehmann and R. Widmer, IEEE Trans. Plasma Sci., PS-14(2),

137, 1986. Diagnostics of Low-Pressure Oxygen RF Plamsas and the Mechanism for Polymer

Etching: A Comparison of Reactive Sputter Etching and Magnetron Sputter Etching.

[9] Thorsness, OB. and J.A. Britten, LLNL Report, UCRL-102041, October 1989. Mass Transfer

Characteristics in Radial-Flow Multijet CVD Reactors.

[10] Chen, F.F., "Electric Probes", Plasma Diagnostic Techniques, Chapter 4, pp. 113 ff, Academic

Press, NY, 1965.

[11] Houtman, O, D.B. Graves and K.F. Jensen, J. Electrochem. Soc, 133, 961, 1986. CVD in

Stagnation Point Flow.

[12] Heidenreich, J.E., III, J.R. Paraszczak, M. Moisan and G.J. Sauve, J. Vac. Sci. Technol. B, 5(1),

347, 1987. Electrostatic probe analysis of microwave plasmas used for polymer etching.

[1 3] Yamagishi, M., K. Mizuno and J. De Groot, J. Quant. Spectrsc. Radiat. Transfer, 40(4), 51 9, 1 988.

Spectroscopic Measurements of Air Plasma.

454
Manuscript received

1-29-90

The Evolution of Molecular Beam Deposition (MBD)

from Laboratory to Production Usage

C C H Hale, I T Muirhead, S P Fisher, G J H Mathew

The use of molecular beam technology (MBD) under ultra high vacuum

conditions (UHV) has proved to be a highly controllable process capable of

producing dense films which are free of porosity, low in impurities and

having bulk like optical properties. These distinctions are requisite for

coatings which require a high laser damage threshold, low scatter and

stable spectral characteristics. The application also lends itself to the

formation of novel distributed Bragg reflectors (DBR) and rugate

structures which require a highly controllable deposition process to form

the many, often very thin, layers involved.

In order to further the research in this field and produce advanced

optical coatings for applications requiring stringent specification beyond

the capability of conventional technology, a VG Semicon V90H MBE system

has been installed at OCLI, Scotland, purchased and operated under a joint

venture between OCLI Ltd and Heriot Watt University. This facility can provide a thickness uniformity of better than +/- 0.5% over a 4" substrate.

This paper will review the current results from this unique

deposition facility and describe the advances made in depositing complex

multilayer coatings.

Key words: defect density; large substrates; low absorption and new

equipment

Chris Hale and Ian Muirhead are with OCLI Optical Coatings Ltd., Ridge

Way, Hillend Industrial Park, Fife, Scotland, KY11 5FR, United Kingdom.

Shari Powell Fisher is with Optical Coating Laboratory Inc., 2789

Northpoint Parkway, Santa Rosa, California 95407-7397, U.S.A.

Dr Gordon Mathew is with Heriot Watt University, Riccarton, Edinburgh,

Scotland, EH14 4AS, United Kingdom.

455
Introduction

The continually increasing expansion in the field of optics and

opto-electronics has placed great demands on thin film performance.

Many applications demand a combination of low absorption, low scatter,

high laser damage thresholds and enhanced environmental stability for

thin film structures, which play an ever more critical part in the

performance of optical or opto-electronic systems.

The requirement has lead to the development of various methods of

improving the quality of optical thin films beyond that obtainable

using conventional processes. Once such is to employ an ultra high vacuum (UHV) chamber in conjunction with Knudsen effusion cells to

deposit material under strictly controlled conditions in a low

contamination environment.

The technique was pioneered at the Royal Signals and Radar

Establishment (RSRE) in the U.K (6) and the approach builds on the

technology developed for molecular beam epitaxy (MBE) of crystalline

semiconductors which has proved to be quintessential in providing

ultra-pure material for advanced electronic devices. The approach

described in this paper differs mainly in that the substrates are not

lattice matched crystalline materials but rather polycrystalline or amorphous substances. As this is not, therefore, an epitaxial

technique, it is described as molecular beam deposition (MBD).

The equipment used at RSRE was only suitable for laboratory usage

but the results were sufficiently encouraging for the process to

evolve further with the procurement of a larger and specialised UHV

facility, which was situated at OCLI Optical Coatings Ltd, in the U.K

- Figure 1.

This new facility would allow the accelerated development of the

process plus provide a small to medium scale production facility to

manufacture advanced optical components for the market place which

would exhibit the same qualities previously obtained on a smaller

scale.

The UHV system was purchased largely from a grant from the

Science and Engineering Research Council (SERC) obtained on behalf of

Heriot Watt University, Edinburgh. The facility is operated under a joint venture between OCLI Optical Coatings Ltd and Heriot Watt

University.

456
2. The Molecular Beam Technique

In recent years there has been increasing interest in Molecular

Beam Deposition (MBD) and Ultra High Vacuum (UHV) thin film deposition

for optical applications. Previously, this work had been primarily

limited to electronic applications where extreme purity and a structure free of dislocations is paramount to enhanced electrical

properties. There are many aspects of the process, however, which can potentially lead to improved optical thin films. The low defect level

can reduce the amount of light scattered from the resulting film;

bulk-like densities increase the refractive index over conventional

thin films and a structure free of voids gives enhanced spectral

stability (2) and increased environmental stability.

These improvements are a result of the very low levels of

contaminants in the vacuum environment and of the evaporation

techniques used. The residual gas encountered at the nominal

pressures employed with conventional thermal evaporation (i.e. 10 E-6

mbar) is composed of a large fraction of water vapour, absorbed

oxygen, organics (oils, etc) and/or process gasses (e.g. Argon). In

comparison, in the UHV environment, the partial pressures of such sources of contamination are in the 10 E-ll mbar regime. Futhermore,

the degree of control available allows the potential of techniques for

microstructure control and refractive index synthesis to be realised

at a sub-microscopic (nanoscopic) level.

In any vacuum deposition process, the degree of contamination of

a surface by the residual environment of a coating chamber can be

estimated from the Kinetic Theory of Gasses from which the number of

molecules of species (i) striking and adhering to the substrate per

unit area is given by:

S.Pi 2.67.10 E22.Pi

R = = 2.pi.Mi.kT) (M-jT)

where P^ = Partial Pressure of i, M^ = Molecular Weight of i, S

Sticking Coefficient. S is generally around 0.1 for permanent gasses.

An ideal coating process would be configured so that the residual

flux is negligible compared with the incident flux of material to be

deposited e.g. by using a very high incident flux of coating material

or by decreasing the residual impurity flux by deposition under UHV

conditions. In conventional evaporators, the rate incidence of water

on a surface is equivalent to one monolayer every minute at 10 E-5

mbar. In the UHV environment (10 E-ll mbar) employed for MBD, this

drops to less than one monolayer over the total length of a typical

deposition run.

457
Description of Current Facility

The UHV/MBD facility installed at OCLI consists of the following

three chambers, separated from one another by gate valves. Figure 2.

3.1 Growth Chamber

This is the main section of the equipment capable of a base

pressure in the mid 10 E-ll mbar range. The substrate

holder/manipulator can accommodate a single four inch substrate

or a multiple of smaller, e.g. nine by one inch. The ultra-high vacuum provides an environment with residual active gas species

e.g. 02, CO, H20 and C02 in the sub 10 E-ll mbar range.

It is generally accepted that it is particularly important

for the deposition of optical thin films to take place in an environment with a minimum H20 partial pressure. In particular,

the absorption of moisture between columns of the thin film

structure affects the morphology. (6).

3.2 Preparation & Analytical Chambers

One of the advantages of working in the UHV regime is that

it allows access to a range of surface analytical facilities.

This chamber also operates at 10 E-ll mbar and offers X-ray

Photo-electron Spectroscopy. Further extension could provide

Secondary Ion Mass Spectrometry (SIMS) and Auger Electron

Spectroscopy (AES). Substrates can be studied at any stage in

the loading/cleaning/deposition process. Substrate cleaning is

achieved through the use of 0-3 KeV Ar ions from a de-focus sed

beam ion source, assuming uniform etching profiles.

A cassette of ten substrates can be stored under vacuum in

this chamber allowing the movement of samples from preparation

chamber to deposition chamber without having to open the system

to atmospheric pressure.

3.3 Load Chamber

The substrates are loaded into the system via rapid entry

load lock without disruption of work in the other chambers.

Substrates are cleaned chemically prior to loading into the

equipment. The load chamber is evacuated using a cryopump.

Pressures of around 10 E-8 mbar are attained before transferring

the substrate to the preparation chamber via an hydraulic valve.

458
3.4 Evaporation Sources

The requirement of the molecular beam generators is to

provide a stable flux of ultra-high purity beams which is of a uniform and appropriate intensity. This is provided by

Knudsen-cells of which there are four on the OCLI machine with

the capacity for four more. In a Knudsen cell, the solid or liquid source material is held in an inert crucible source and a thermocouple is used to provide temperature feedback. These

simple concepts belie the considerable design, development and

experimentation which has been invested in perfecting the

performance of such vital components in MBE equipment.

At typical Knudsen operating temperatures, the flux from the

cell to the substrate can be considered as operating in the

molecular flow regime as the mean free path of the constituent

particles is of the order of one metre and hence interactions

within the beam can be considered as negligible. The beams may

be effectively interrupted by the interposition of fast

mechanical shutters capable of reliable continuous cycling with

open/close times less than 0.1 seconds with minimal associated vacuum degradation in the growth environment.

3.5 Layer Thickness Control

As optical thin films may often have an individual layer

thickness of a few nanometres, monitoring thickness as the layer

grows is of primary importance. This is often done by monitoring

the change in reflection from a beam of narrow band-width light

reflecting of the growing surface. In the OCLI facility, many

wavelengths over the visible and near infrared spectrum is

monitored simultaneously using a broad band optical monitor. As

well as providing many wavelengths with which to monitor layer

growth it also allows the visualisation of the optical

performance of the device, as the layers are growing. A quartz

crystal rate monitor is also used whenever optical monitoring is

unsuitable.

459
Coating Materials and Film Microstructure

The choice of materials for optical coatings is wide and varied.

Knudsen cells, however, have a limited temperature range and hence are usually used for the higher vapour pressure materials. This,

nevertheless, permits the deposition of a considerable range of

materials. These include group II-VI compounds, chalcogenides and

flouride materials. Fluoride materials are of particular interest

because of their low refractive indices and their wide spectral

bandwidths. For example, PbF2 can be used in the 0.2 to 15 micron

range. Although most fluorides are partially soluble in water, the

actual moisture resistance of a film is dependent on the techniques

used for its deposition. A characteristic feature of material grown

under optimum conditions in the UHV/MBD system is high film density,

resulting in reduced moisture uptake and laser damage thresholds 3

times that of conventional coatings. (5)

The techniques employed have also led to a high degree of

interface perfection, the morphology of the films being then only

substrate limited. As many of the problems encountered with thin

films are related to the polycrystalline columnar microstructure, it

would be ideal to avoid this by depositing the films epitaxially on to

the substrate. Restrictions on the choice of substrate and thin film

materials with their associated lattice constants makes this a difficult goal. This aim, nevertheless, can be partially achieved by

the fabrication of "pseudo-amorphous" films using digital stratified

designs. These films are synthesised by repeating discrete layers of

different materials, the thickness of each component layer being

negligible compared with the wavelength under consideration. The fast

shuttering and stable fluxes afforded by MBD is ideal for this task.

(5)

The high degree of control obtained over the growth process

enables refractive index synthesis and the ability to deposit novel

structures such as digital graded index films and rugate filters. By

altering the mark space ratio of the two materials during deposition,

a graded index layer can be achieved. Such designs can be used to

avoid high electric field intensities at interfaces within the coating

structure

Unfiromity

Uniformity is an important parameter for optical thin film

coatings because of the sensitivity of interferometric structures to

thickness variations of even a fraction of a wavelength of light. In

this work, the materials are evaporated from crucibles, the geometry

of which corresponds to the axis of each K-cell being aligned such

that the focus of the full K-cell set lies approximately 50mm behind

the substrate plane.

460
The substrates that have been employed are glasses, fused silica

or sapphire for the visible and germanium, silicon, chalcogenide or alkali-halide for infra red applications. Substrate temperatures

ranged between 20°C to 350°C.

Deposition was performed using a single K-cell for each material

with growth rates of typically 1 um/hour.

The variation of optical thickness across a coated surface was measured using standard spectroscopic techniques. With single layer

films, the peaks and troughs in the reflection spectrum correspond to

optical thicknesses equal to integral numbers of quarter and half

wavelengths of the probe wavelength. Two spectroscopic instruments

have been used for this work; a Monolight 6800 series scanning grating

monochromator and a Bomen/Edinburgh - Instruments spatial scanning

fourier transform spectrometer. The latter provides spectra with a typical resolution of 0.2nm in the visible and a spatial resolution

of 1mm. Software has been developed for converting raw data into

optical thickness contour maps.

5.1 Uniformity Results

Tests were performed using ZnSe in granular form (1 - 2mm

size) and better than 99.9% purity level. Crucibles were filled

to near maximum capacity and effusion could be considered to occur from a circular area of 40mm. With this full-cell

configuration there is negligible focusing from the mouth of the

cell. Crucibles with increased cone angles of 1° were used to

determine whether better uniformity could be obtained over the

standard cells but similar results were obtained and it was concluded that the standard 70cc crucible is close to the optimum

design. Uniformity of +/- 1.2% across 100mm was routinely

obtained with this configuration.

Improved uniformity was obtained by fine tuning of the source geometry. This can be achieved by exploiting the fact

that the granular source material fuses in the crucible once the

cell has been heated to deposition temperature. This solid

surface is sloped due to the settling under gravity of the

granules when a full crucible is first put into the system. It was found that by adjusting the plane of the material surface,

with respect to the substrate plane, very high levels of

uniformity were obtained. A thickness variation of +/- 0.2% over

100 mm was achieved over several consecutive runs. Figure 3.

Similarly, complete interference filter structures have been

fabricated using ZnSe and BaF2 with the spatial variation in the

centre wavelength of the pass band indicative of only +/- 0.05%

variation of the central spacer layer over 80mm diameter. In

this case, the narrow interference fringes permit very accurate

measurement of small fluctuations in thickness.

461
It was previously thought that without the use of a planetary rotation system, commonly used in conventional coating

facilities, high levels of uniformity might be difficult to

achieve. This work has shown that by using thermally stable

Knudsen effusion cells, as opposed to heated boats used for

conventional thermal evaporation and by optimising

source/substrate geometries, excellent uniformity can be achieved.

Source Stability

Each Knudsen cell is heated by a series of resistive tantalum

foil strips and these are powered by Eurotherm controllers with

feedback from a thermocouple assembly which fits around the narrow end

of the conical crucible. The temperature stability of this

arrangement is +/- 0.1°C over the operating range of the cells (up to

1450 C°). The partial pressure of the coating flux from the cell can

be measured by positioning a monitoring ion gauge (MIG) between the source and substrate. The ouput of the ion gauge controller reads to

two decimal places.

Remarkable effusion stability has been recorded from the cells

under normal operating conditions. Stability of source flux as measured on the MIG is stable to two decimal places during lengthy

deposition times and remains constant for constant cell temperatures

during subsequent deposition runs.

This feature of the equipment allows many of the multilayer

designs to be fabricated using rate as the controlling factor. As an example of this, figures 4a and 4b show the theoretical and measured

performance of a distributed Bragg reflector formed by alternating

layers of ZnSe and BaF2 . The filter performance shown was achieved

from the first attempt, after individually calibrating the deposition

rate for the two materials. The comparison shows close agreement

between the theoretical and measured performance.

As an experiment to test the rate stability, the design was repeated with a slight variation in thickness in the first and last

layers. This has the effect in theory of moving the ripple from the

long wavelength side of the rejection band to the short. Figures 5a

and 5b show the theoretical and measured characteristics for this

test, which again show close agreement. The later deposition run was performed several days after the first with no further calibration of

the sources.

462
This stability is a fundamental feature of the system and is

vital for the production of devices requiring stringent

specification. The fabrication of, for example, a rugate filter,

requires an uncompromising process control but unless the coating

process is sufficiently stable to respond to control parameters then

the desired spectral performance of the filter will not be obtained.

7. Absorption

It has been shown previously that materials deposited under ultra

high vacuum have lower levels of absorption than that measured for the same material deposited by conventional thermal evaporation. This is

due to lower contamination levels and absence of porosity. The

absorption coefficient of ZnSe for MBD layers was found to vary

between 100 and 1000 1/cm between 676 and 521 nm. (1)

A thin film device which is highly sensitive to levels of

absorption is a Fabry-Perot etalon which usually takes the form of,

HLHL. . . (mHH) . . .LHLH where H indicates a quarter wave optical thickness

layer of a high index material, (ZnSe, n=2.7 at 514nm) and L is a quarter wave layer of low index material, (BaF2, n=1.45 at 514nm).

the integer, m, gives the etalon spacer thickness. (2)

Two etalons were fabricated, one using conventional thermal

evaporation and the other using MBD. The transmission spectra near

the ZnSe absorption edge is shown for each device in figure 6. It can

be seen that the MBD device has a higher transmission in the

transmissive region and that transmission extends further at the

fundamental absorption edge. The absorption edge roll-off is

dominated by impurities which affect the spectrum in a variety of ways

but all have the effect of reducing the energy gap between the

conduction and valence bands, which allows fundamental absorption to occur for longer wavelengths than would be the case for a pure

material. (4)

This demonstration shows qualitatively the lower absorption

coefficient for ZnSe prepared by MBD compared to the same prepared by

conventional thermal evaporation.

463
8. Surface Quality

Another feature of the thermally stable Knudsen cells is that the

emission of spatter is effectively eliminated. Spatter inclusions in

a growing layer usually results in the formation of large nodular

defects in the coating which can cause scatter and which are prime

sites for laser damage effects. (3)

In order to demonstrate the improvement in surface quality

between single layers produced by conventional thermal evaporation and

MBD, figures 7a and 7b show photographs of ZnSe surfaces produced by

the two techniques. The materials were of the same thickness, were

deposited at similar rates and with the same substrate temperature.

The photographs show a greater density of surface anomalies for the

surface prepared by conventional evaporation. The traces from a surface profileometer show the absence of surface anomalies on the MBD

layer and also reveals that the surface is smoother (the vertical

scale for the MBD layer is 0 to 800 A and 0 to 4000 A for the other).

The rms roughness for the MBD layer was 25 A whereas that for the

other layer was 90 A.

The reduction of scatter in coatings is of primary importance

when fabricating narrow band pass filters with very high optical

densities. If the light to be rejected experiences scatter then it

will have angle effects in the multilayer structure. For very narrow

rejection filters this will mean that the scattered light "misses" the

rejection band and is transmitted. This effect could reduce the

actual optical density of the filter by orders of magnitude.

9. Laser Damage Thresholds

Due to the short time that the facility has been in operation, no quantified assessment of laser induced damage thresholds have been

made although a test programme has been initiated. There is every reason to expect, however, that the laser damage thresholds should be as good, if not better, than those measured on films produced by MBD

at RSRE. (5)

10. Future MBD Developments

The V90 was commissioned in late February, 1989 and the system is

under constant test and development. A range of materials is being

studied to assess their optical properties and the analysis of

multilayer interfaces is in progress to select pairs of high and low

index materials which will not form poor interface boundaries or unwanted alloys. Materials being studied include zinc sulphide, zinc

selenide, barium fluoride, lead fluoride and a range of other fluoride

compounds. The combination of barium fluoride and zinc selenide has

consistently produced good results and is suitable for the

microstructure engineering of multiple, very thin layers (kilo

layers). Such engineering allows the synthesis of refractive indices

between the high and low limit and is applicable to graded index

coatings such as wide band AR and rugate designs.

464
The ability to produce tight tolerance filters introduces the

opportunity to form optically bistable devices and agile filters, by

the inclusion to the multilayer of non-linear materials. Optically

bistable Fabry-Perot devices have been fabricated which operate at a number of laser wavelengths. (2)

The low absorption, low defect density, absence of porosity and

high laser damage thresholds in MBD coatings makes the process

suitable for the manufacture high power density laser optics. (5) The

process is presently being used to fabricate laser mirrors for

wavelengths ranging from UV to 10.6 um.

Although the V90 H represents a great advance in UHV equipment

development for optical thin film fabrication, it still has

limitations. The system was an "off the shelf" item developed

primarily for the electronics industry and as such is limited, in this

case, to a 100mm substrate diameter. The thermal sources are adequate for electronic materials but limit the range of optical

materials which can be used. There is no reason, though, why the

equipment (pumps, sources etc) should not be incorporated into a custom designed chamber, incorporating large susbstrate handling

capabilities, load locks and the possibility of electron gun sources

for the deposition of high temperature materials such as oxides.

These possibilities are constantly under review.

Conclusion

A UHV facility for the manufacture of optical thin films by

molecular beam deposition on 100mm diamter substrates has been sited

at 0CLI Optical Coatings Ltd in the UK. It has proved suitable for

the production of thin film materials which exhibit low absorption,

low defect density and good uniformity over 100 mm. The technique has

been used to produce thin films with high laser damage thresholds.

The deposition process is highly stable and is capable of being

controlled for the formation of multiple, very thin layers.

Multilayer devices comprising BaF2 and ZnSe have been fabricated in

the system.

465
12 . References

[1] Optical Bistability in II-VI Compounds. A Miller, J

Staromlynska, I T Muirhead, K L Lewis, D Craig, G Steward.

II-VI-87, Monteray, July 1987.

[2] Optical Bistability in Molecular Beam Deposited ZnSe/BaF2

Multilayer Interference Filters. A Miller, I T Muirhead, K L

Lewis, J Staromlynska and D R Welford. Journal De Physique,

Colloque C2, Supplement au No 6. Tome 49, juin 1988.

[3] Nodular defects in dielectric multilayers and thick single

layers. K H Geunther, App. Op. Vol 20 No 6.

[4] Elementary Solid State Physics. Omar. Addison-Wesley. pp

292-300.

[5] Fabrication of Fluoride Thin Films Using Ultra High Vacuum

Techniques. K L Lewis, A M Pitt, N G Chew, A G Cullis, T J

Wyatt-Davies, L Charlwood, 0 D Dosser, I T Muirhead. Proc.

Boulder Damage Symposium, 1986.

[6] Assessment of Optical Coatings Prepared by Molecular Beam

Techniques. K L Lewis, J A Savage, A G Cullis, N G Chew, L

Charlwood, D W Craig. Proc. Boulder Damage Symposium, 1984.

466
Figure 2: Schematic representation

of the basic V90H

467
UNIFORMITY RESULTS

THEORETICAL PERFORMANCE

V J

MEASURED PERFORMANCE

Figure 4: (a) Theoretical and (b) measured

performance of DBR filter

produced using rate as the

controlling agent

THEORETICAL PERFORMANCE

\ /

(a)

MEASURED PERFORMANCE

(b)

niK'i.M i

Figure 5: Theoretical and (b) measured

performance of similar design

with slight alteration to move

ripple. Rate control again

was used.

468
CONVENTIONAL UHV

ABSORPTION

COMPARISON

FABRY-PEROT DESIGN

TOP REFLECTOR

BASE REFLECTOR

Figure 6: Transmission spectra for two etalons fabricated

by conventional thermal evaporation and MBD.

The MBD filter shows superior optical characteristics.

0 £k e 3C O A'. '5 yc_ e 8'i o

400 aoo

aoo

400

-GOO soo

-

WUyv

1 * idGi Pi'

4©s, a

3SOO 3000

2500 2000

1G00

lOOO SOO

Figure 7: (a) Surface of ZnSe deposited

by MBD plus surface profile

(b) Surface of ZnSe deposited

by conventional thermal

evaporation plus surface

profile

469
COMMENTS

Question: First question is the same as the last speaker. What is the price

tag?

Answer: The total price of the equipment you saw there including cleaning

facilities, XPS , is about three-fourths of one million pounds.

Question: Have you quantified the absorption and defect densities yet?

Answer: We haven't qualified them from this machine. We are going back to

the results from RSRE. The absorption could change from 1000 to

100 in inverse centimeters between wavelengths 647 ran to 521 nm.

Question: Can you get practical production throughput on normal reflectors

in such a device or does it take a couple of days

Answer: The deposition rates we have readily achieved are up to three

microns an hour. These rates aren't particularly fast but a fast

rate isn't necessary for the stability of the cells. We do have

the ability to store up to twenty index cassettes under

ultra-high vacuum. The system is thus designed with small scale

production in mind.

Question: One of the advantages of Keith Lewis's system was that he was able

to demonstrate that in a number of cases he did in fact get

optical constants, particularly absorption coefficient, which were equivalent to the bulk material. I believe that is the first time

that bulk optical properties had been achieved for some of these

thin films. You have shown us qualitatively that you are getting

significantly better results than those obtained for conventional

films. But have you gotten quantitative results. Do you know how

close you are to the bulk, optical properties of the materials?

Answer: We haven't quantified that as yet. That is why I showed only the

qualitative results. It was only a visual representation of the

difference in absorption. But we will get quantative results and

report them.

Question: What fraction of the conventional coating materials do you think

that you will ultimately be able to handle by this technique?

Answer: We bought an off the shelf system. The system is developed by VD

Semicon for the electronics industry and you just fit it with cell sources. You have a temperature range of around 1450°C,

considering the low pressures that we have. We can evaporate most

materials, for example, ionic fluorides. There is no reason at

all why we couldn't expand this system to take E-guns, larger

sources, larger substrate handling capabilities. It just needs

funding

470
Manuscript received

2-22-90

Investigation and Modelling of Laser -Damage Properties of

Fabry-Perot Filters

A. Mclnnes, CM. MacDonald, D.R. Gibson and A. D. Wilson

Applied Physics Group

Barr & Stroud Ltd

Pilkington Optronics

Anniesland

Glasgow G13 1HZ

Scotland, UK

The electric field distribution within a fabry-perot

etalon is readily variable by changing the angle of

incidence or the thickness of the etalon. The laser damage

threshold (LDT) of Fabry-Perot etalons was measured as a function of electric field distribution, monitored through

the transmittance. Two types of etalon were used - thin

film Fabry-Perot devices with high finesse were investigated

at 1.064/im, and a simple dielectric slab etalon (Germanium)

was investigated at 10.6/jm.

In the latter case the results show a correlation

between the LDT and the inverse square of the calculated

peak electric field (1/E2

) in the sample. For the thin film

Fabry-Perot, the correlation is between the LDT and 1/E2 at

the spacer boundaries. Subsequent sample analysis also

suggests that damage was initiated at spacer boundaries.

It is concluded that the laser damage properties of

such etalons are in agreement with theoretical expectations,

and that their damage behaviour can be described by a single

parameter.

Key words: Fabry-Perot etalons, Laser damage, CO2 laser,

Nd : YAG laser, electric field distribution.

1. INTRODUCTION

The laser damage behaviour of Fabry-Perot etalons (FPEs) of

two types - thin film type was studied with a Nd : YAG laser at

1.064/im, and a simple dielectric slab etalon of Germanium was

studied with a CO2 laser at 10.6/im. It is found that the LDTs

are highly variable, depending on the conditions of the

measurement, but the behaviour can be explained and agrees with

theoretical expectations.

FPEs are ideal devices to examine because their linear

behaviour is well understood and their transmittance and

reflectance can be changed by varying the angle of incidence, the

thickness or the thin film design. Small changes in any of these

parameters cause large changes in the electric field distribution

within the device, so that by, for example, tilting a FPE, the

peak fields can be varied in magnitude and position, and thus

used to probe the interior of the FPE.

471
2. GERMANIUM ETALON

The measurements on this were carried out using a dedicated

CO2 laser damage testing system, whose parameters are shown in

Table 1.

Table 1. CO2 Laser System Parameters

Wavelength (^m) 10.6

Pulselength 100ns spike + 2ps tail

Spot size (/jm) 210 1/e2 diameter

Mode quality TEMOQ

In these experiments, the LDT was measured as a function of

optical thickness. This was controlled by varying the sample

temperature, thus allowing it to be irradiated at normal

incidence throughout. Angle could have been used, but would have

introduced unnecessary complications such as beam walk-off. The

measurements were carried out with the sample at the beam waist,

where the beam is best collimated.

In order to find the internal electric field distribution we

need to know the optical thickness, which is obtained through

measurement of the device transmission.

Figure 1 shows transmission vs. temperature for the

Germanium FPE. Note the large modulation, due to the high

refractive index of Ge. Also, we see that it takes a temperature

change of «14°C to scan one complete free spectral range, which

corresponds to an optical thickness change of A/2. This means

that tight temperature control is not necessary. This figure

also illustrates the fact that measurements were carried out over

more than one free spectral range.

Figure 2 shows the electric field distributions within the

FPE in three different transmission states. High transmission

means high internal fields, and low transmission means low

internal fields. Other states are intermediate.

From earlier work [1], [2], [3] the LDT is expected to be

related to the field distribution, and indeed a strong

correlation is found between LDT and the inverse of the peak

field squared.

Figure 3 shows the results. Up to «35Jcm they fit a

straight line very well. This tells us that LDT x Yr for these

points is a constant. In other words, the value of E , the peak

fluence inside the sample, is constant at the onset of damage,

regardless of the combination of input fluence arid optical

thickness that give rise to it. This value is ll.5Jcm . This

means that if >11.5Jcm~2 exists within the sample, it will

damage

472
As for the two points not on the line, there are two

possible explanations. Both involve the fact that energy is

high. It could be that another process is beginning to limit the

LDT, such as a surface effect perhaps, or possibly we are seeing

a nonlinear change in refractive index, which leads to a change

in the optical thickness and hence increases the peak field from

the expected value.

There are few points to note here. Firstly, the range of

LDT values observed is approximately a factor of 4 . Bear in mind

that this was all carried out with one sample. This large range

is obtained purely by changing the optical thickness.

The next point is that it is the peak field sguared that is

important. It is known [4], [5] that damage in Ge is dominated

by the effect of a surface layer with higher absorption than the

bulk, due to polishing debris and absorbing defects. Yet it is

not the field at the surface that determines the LDT, but the

peak field which, as we see from Figure 2, can be up to A/4 =

0.66/jm inside the sample. This suggests that in this case,

damage is initiated inside the sample. It therefore also

suggests that the surface absorbing layer is at least 0.66pm

thick.

3 . THIN FILM ETALONS

These measurements were carried out using a dedicated NdrYAG

laser damage testing facility whose parameters are shown in Table

2.

Table 2. NdrYAG Laser System Parameters

Wavelength (pm) 1.064

Pulselength (ns) 15

Mode Quality TEMOQ

There were three different designs of thin film FPE, and

they are shown in Table 3 . All the "H" layers are ZnS, all the

"L" layers MgF2 , and all were quarter-waves at 1070nm. This was

chosen so that tilting the FPE by a few degrees would change it

from a low to a high transmission state.

Table 3. Design of Thin Film Fabry-Perot Filters

Design 1 (HL) 4 2H (LH) 4

Design 2 (HL) 3 4H (LH) 3

Design 3 (HL) 4 8H (LH) 4

The angular performance of the three filters is shown in

Figure 4. The broader transmission maximum of design 2 is due to

the fact that its mirrors have three repeat units rather than the

four of designs 1 and 3. Design 3 has the narrowest peak and the

473
greatest angular peak position because it has the thickest spacer

layer.

The effect of angle on the field distribution within design

1 is shown in Figure 5. On going from low to high transmission

there is a very large increase in the peak field sguared at the

spacer layer boundaries.

Figure 6 shows typical damage site morphology, and this, in

conjunction with the stylus profilometer trace, can be used to

elucidate the position of initiation. Material has been removed

from the beam area down to the depth of the spacer layer,

suggesting that the point of damage was at the spacer boundary,

which we would expect from the field distribution. Damage was

seen to occur at one or other, or both, spacer layer boundaries.

The results (fig. 7) show good correlation again, but this

time with the fluence at the spacer boundaries. Note that this

plot includes data for all 3 filter designs. Again the product

LDT x E = constant, in this case equal to 25Jcm . There is an

even larger range of LDT, due to the amplification of the field

at the spacer boundaries.

Thin film interaces within the multilayer are regions of

high defect and impurity density, and therefore regions of high

absorption [6]. Also, the interfaces are regions of mechanical

weakness. In combination with the high field, this results in

preferential damage at the spacer interfaces.

The two points not on the line may be explained as follows.

Both are design 3 data, and if we look at the field pattern for

this case as shown in Figure 8, we see that there is another peak

at the first layer boundary. This boundary, at the top of a 24

layer stack, is likely to be under stress, and indeed when we

look at the morphology we see a quite different pattern, (fig.

9) . The film has simply ruptured, and this is consistent with

damage just below the surface.

4. CONCLUSIONS

In both cases it has been shown that the LDT is highly

dependent on the exact measurement configuration. However, when

field effects are taken into account, the behaviour is found to

be as expected. In fact, a single number can be found to

describe the LDTs of each device type; 25Jcm~ 2 at the spacer

boundaries for the thin film FPE and 11.5Jcm-2 inside the sample

in the case of germanium.

This has the well-known corollary that if a high LDT coating

is desired then the peak fields should be kept away from the

interfaces

It also serves as a warning to be extra careful of

measurement conditions when testing samples or devices which

could act as etalons.

One last point is that there might be an application for the

Ge etalon as a variable reflectivity high LDT mirror which does

474
not need coatings or depend on polarisation for its variability.

It could possibly be used as an output coupler for high energy

CO2 lasers.

The permission of the management and directors of Barr & Stroud Ltd to publish this work is gratefully acknowledged.

5. REFERENCES.

[1] Apfel, J. H. ; Matteucci, J. S.; Newnam, B. E. ; Gill, D. H.

Symposium on Laser Induced Damage in Optical Materials. Nat.

Bur. Stand. (U.S.) Spec. Publ . 462; 1976 301p.

[2] Apfel, J. H. ; Symposium on Laser Induced Damage in Optical

Materials. Nat. Bur. Stand. (U.S.) Spec. Publ. 509; 1977

251p.

[3] Gill, D. H. ; Newnam, B. E. ; McLeod, J. Symposium on Laser

Induced Damage in Optical Materials. Nat. Bur. Stand. (U.S.)

Spec. Publ. 509; 1977 260p.

[4] Gibson, D. R. ; MacDonald, C. M.-; Wilson, A. D. Symposium on

Laser Induced Damage in Optical Materials. Nat. Bur. Stand.

(U.S.) Spec. Publ. 746; 1985 203p.

[5] McDonald, C. M. ; McGeoch, S. P.; Mclnnes, A. A.; Nelson, C.

H. ; Wilson, A. D. "Laser-induced damage mechanisms in model

optical materials." J. Phys. D. : Applied Physics (UK) 21 ; S85-S87; 1988.

[6] Ristau, D. ; Dang, X. C. ; Ebert, J. Symposium on Laser

Induced Damage in Optical Materials. Nat. Bur. Stand. (U.S.)

Spec. Publ. 727; 1984 298p.

475
1.Q0.8

Transmission

0.2

0

f. f

i /

. +.._.

i /

!

\

•1

- •+

\ \

'

20 25 30 35 40 45 50

Temperature (C)

Figure 1 Transmission vs temperature for the germanium etalon at

the temperatures used for the LDT measurements

(experimental)

476
IN

Thickness = 12 b

4 Transmission -1-0

— AIR

WWW

-2 2

IN

GERMANIUM

Thickness = 12-5 h

4 Transmission = 0-341

AIR

AIR

-2 2

GERMANIUM

Thickness- 13 X

- 4

Transmission =0-206

AIR

Figure 2 Time-averaged electric field distributions within the

germanium etalon in different transmission states.

477
LDT

(Jem

2

)

60

50

40

30

20

10

0 1.0 2.0 3.0 4.0 5.0

INVERSE PEAK AVERAGE FIELD SQUARED

INSIDE ETALON — Vb

Figure 3 LDT vs l/(peak time-averaged field squared)

germanium etalon.

inside

1.0

0.8

0.6

Transmission

0.4

0.2

0 2 4 6 8 10 12 14 16 18 20

Angle (degrees)

Figure 4 Transmission vs angle of incidence for the three thin film

designs, at 1.064/im.

478
6 = 0° T^0.05

e = io°

T^0.50

6 = 11.5"

T~ 0.90

S

P

Polarisation

Polarisation

AIR HLHLHLHL 2H LHLHLHLH GLASS

AVERAGE FIELD SQUARED vs. POSITION THROUGH A THIN FILM

FABRY-PEROT FILTER FOR DIFFERENT ANGLES

OF INCIDENCE

Figure 5 Time averaged field squared vs position through a FPE (design 1) at different angles of incidence.

thin film

479
LOADING 3 mg

STYLUS RAD. 2um

I 1 lOO^m

Figure 6 Stylus profilometer trace and Nomarski micrograph of a damage

site on a thin film FPE.

10

LDT 1.0

. (Jem

2

)

n

•

D •

•

•

• 2H

4H

8H

0.1 1.0 10

INVERSE PEAK AVERAGE FIELD SQUARED

AT SPACER BOUNDARIES — Y~>c max

Figure 7. LDT vs 1/ (peak time averaged field squared) at the spacer boundaries of the thin film FPEs)

480
(HL)

4 2H(LH)*

(HL)

1 4H(LH)3

(HL)' 8H(LH)4

/Uwv

Figure 8 Time averaged field squared vs position through thin film FPEs

of different designs all at normal incidence (low

transmission, high LDT)

LOADING 3 mg

STYLUS RAD. 2 urn

i \

i I

100/im

Figure 9 Stylus profilometer trace and Nomarski micrograph of a damage

site on a thin film FPE.

481
COMMENTS

Question: I'm confused about the composition of aluminum oxide. I saw that

boehmite and Al203 were both listed in your data.

Answer: Yes, I'm afraid I have been sort of lax. A boehmite is a crystallized form of dehydrated aluminum. It is quite a stable

hydrate. You would have to heat to 500° before it dehydrates and

it dehydrates to something like monohydrate alumina. There are

two ways of writing the monohydrate. You can either call it A100H

or call it AL2 03 H2 0. What we are doing now is we tend to call it

A100H because I think that, that makes it easier to understand

than when you see the H2 0 on the end of alumina. But the

monohydrate and boehmite are both recognized as fairly stable

crystalline hydrates.

Question: What kind of thickness control do you have when you look at the

different spin rates. Do you still get uniformity?

Answer: Yes, the thickness control is surprisingly good. We usually vary

the thickness by varying the concentration of the applied solution

rather than by varying the difference in spin rate. If you spin

faster you will tend to get thinner coats because it spins off

faster. So slow spinning gives you thick coats, faster spinning

gives you thin. We like to spin reasonably fast to get the stuff

right out to the side. You can vary the thickness also by varying

the concentration in the material that you are adding. If you

have a high concentration of course more stays on the substrate.

Spin coating is almost idiot proof for getting things uniform. I was told that by an expert who knows about these things

Question: Have you done any work at all with spin casting on curved

surfaces?

Answer: Yes, we have spin coated lenses convex and concave. Now, I don't

know exactly what the focal length was, but it was considerable

and I thing g forces , the centrifugal force out seems to be much

greater than lg. We have never had any problem. It is very easy

to see this because all these single coatings are colored so you

can see how good the uniformity is just by looking at it. The

human eye is pretty good for detecting nonuniformity . Things

always turn out nicely uniform.

Question: Did you characterize the silica gel size?

Answer: Yes, silica particles vary from eight nanometers to 20 nm and they

are roughly spherical. Our alumina particles are plates and they

are roughly 30 nm by 30 nm and about 5 nm thick.

482
Question: Do you do anything to keep the particles from becoming aggregates

in solution?

Answer: No, as long as you keep them in solution they are mainly

monodispersed . One of the problems we do have, especially with

the silica, is that if we dry it out and then try to resuspend it,

it aggregates. That is a problem we have especially in a dip

coating apparatus where the fluid goes up and down. A crude coat

tends to form around the outside and it doesn't redisperse. The

alumina does.

483
Manuscript received

3-20-90

High Damage Threshold A10.0H-Si0

2 HR Coatings Prepared by the Sol-Gel Process*

Ian M. Thomas

University of California

Lawrence Livemore National Laboratory

P.O. Box 5508, L-483

Livermore, CA. 94550

Our study of highly reflective (HR) dielectric coatings prepared by the sol-gel

process has indicated that hydrated alumina and silica are the materials of choice for the

high and low index components respectively.

Ne can now prepare 32-36 quarter-wave-layer samples from colloidal suspensions of the

relevant oxides by spin coating at room temperature. These have reflectivities of about

99% and damage thresholds in the range 30-40 J/cm at 1064 nm with 16 ns pulses at 30

Hz. This threefold improvement in threshold over that reported earlier for this system

was attributed to an improvement in the preparative technique particularly in regard to

cleanliness. Coatings have been prepared recently on substrates up to 8" in diameter and

scale-up development continues.

1 . Introduction

The work described in this paper is part of a continuing study on the preparation of

high damage threshold coatings suitable for use on the optical components in high power

laser systems. Earlier investigations with the same objective on porous silica

anti-reflective (AR) coatings [1], porous fluoride AR coatings [2], Ti0

2 and

Ti0

2 -Si02 high reflectivity (HR) coatings [3] and A1

2 0

3

.H

20 and

A1

2 0

3

.H

2 0-Si02 HR coatings [4] have been previously reported.

Our investigation throughout has been based on the use of colloidal suspensions as

coating media. These have the following advantages over other systems:

1. Liquid system which is easily applied at room temperature by conventional means.

2. No cure or heat required. Stress-free.

3. Rapid application

4. High purity product

*Work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore

National Laboratory under Contract No. W-7405-ENG-48.

484
5.

6.

7.

Especially good for large substrates.

Low capital equipment cost.

Coating easily removed and replaced with no damage to substrate.

In our work with HR coatings we have always used Si0

2 as the low index component.

Colloidal suspensions of Si0

2 are readily prepared and can be used to give high damage

threshold, single layer Si0

2 coatings [lj. We have used these as AR coatings on

substrates up to one meter in diameter. Ti0

2 , Zr02 , Hf02 , Ta2 05

, and A10.0H

were selected for investigation as the high index component. All but A100H were eventually rejected mainly because single coatings from each material had only low to

moderate damage thresholds. The preparation of suitable colloidal suspensions of Zr0

2 and Hf0

2 were also quite involved. These would have been difficult to prepare in

quantities suitable for application to large substrates.

Suspensions of A10.0H were found to be readily prepared and to give high damage

threshold single layer coatings. The A100H-Si0

2 system was therefore chosen for further

investigation. The initial work was carried out on 5 cm diameter substrates but our ultimate objective was to scale-up to much larger sizes.

2. Experimental Procedure

2. 1 Colloidal Suspensions

Suspensions of A100H and Si0

2 were prepared by the hydrolysis of sec-butoxide

aluminum and tetraethyl silicate respectively by methods previously described [1,4]. Both

alkoxides were distilled prior to use to ensure a high purity product.

The Si0

2 suspension was prepared at 3% concentration in ethanol and then diluted to

21 with ethanol prior to use. The A100H suspension was prepared at 1% concentration in

water. It was then adjusted to pH 5 with an ion exchange resin, evaporated under vacuum

to 16% and finally diluted to 4% with methanol prior to use.

2.2 Coating Procedure

All coating suspensions were filtered through a 0.2 jam membrane filter. 5 cm

diameter and 1 cm thick fused silica or BK-7 substrates were then coated using a spin

coater in a 0.2 mm filtered, forced air horizontal flow clean hood. The first coat

applied was always Si0

2 and this was followed with alternating A100H and Si0

2 coatings

allowing about 10-15 minutes drying time between coats. The final coat was A100H. All

coatings were prepared at an optical thickness of 266 nm corresponding to quarterwave at

1 .06 nm.

The preparation and processing of both suspensions is illustrated in figures 1, 2 and

3.

485
Reagents Coating suspension Coating

AI (OC4 H 9 ) 3

H 2 0

HCI

/ /

\ - ,. \

/ /

c S

/ \ \ "~7

represents AIO.OH

(crystalline boehmite, approx. 30 nm x 30 nm x 5 nm)

Figure 1. Preparation and application of colloidal AIO.OH.

Reagents Coating Suspension Coating

Si (OC2 H

5

) 4

C2 H

5 OH

H

2 0

NH0

O O O O O O

O O O O O O

O O O O O O

o o o o o o

o o o o o o

o o o o o o

o o o o o o

o o o o o o

00000003000

ooooooooooo ^ ooooooooooo

O represents Si0

2 particle

(approx. 20 nm in diameter)

Figure 2. Preparation and application of colloidal Si02

ooooooooooooooooooo

oooooooooooooooooooooooooooooooooooooo

ooooooooooooooooooo

ooooooooooooooooooo

ooooooooooooooooooo

Substrate

Metal oxide

Silica

Metal oxide

Silica

Figure 3. Multilayer HR coating.

486
2.3 Damage Threshold Measurements

Damage threshold measurements were carried out at 1064 nm using multishots at a pulse

length of 16 ns at 30 Hz. The beam spot size was approximately 1 sq. mm. Each site on

the sample was irradiated for 60 seconds (1800 shots) and then inspected for damage.

A new site was then selected and irradiation repeated. From a sequence of

irradiations, the damage threshold was defined as the average of the highest fluence which

caused no damage and the lowest fluence that did.

3 . Discussion of Results

The refractive indices of the Si0

2

and A10.0H prepared from colloidal suspensions

have been shown to be 1.22 and 1.44 respectively [4]. A large number of alternating

layers will therefore be required for high reflectance in a HR mirror system because of

the low index difference. Calculations indicated that about 30-36 layers would be

required to achieve reflectance greater than 99% on BK-7.

Table I shows the actual reflectances obtained for a variety of samples containing

from 26 to 40 layers. It can be seen that at least 34 layers are required for 99%

reflection. Figure 4 shows the transmission curve of a fused silica substrate coated with

36 layers. The reflectance of this sample is greater than 99% at 1064 nm. The narrow

band third harmonic reflectance of about 95%, which is characteristic for this type of

quarterwave stack is also present.

100- i

1

,

1

i

80 -

%

Transmission

o

o

20 -

j

0 i.i.i u 1 300 500 700 900 1100

X(nm)

[SiC-.ALO/ 0

Figure 4. Transmission spectrum of 36-layer HR coating.

Laser damage threshold measurements at 1064 nm (16 ns pulse, 30 Hz) are also listed in

2 - 2 Table I. These range from 21-50 J/cm with an average of 37 J/cm with no apparent

dependence on number of layers. This perhaps is not unexpected as the first few layers in

487
all samples are exposed to, and reflect most, of the fluence. These damage thresholds are much higher than we have reported previously [4]. While the reason for this is not known

for sure we suspect that improvement in technique, especially in regard to cleanliness,

has made a major contribution.

Table 1. Reflectance and damage threshold of several HR coatings.

Layers Reflection at 1.06 Damage 1064 nm/16 ns

26 95.0 % 34 J/cm 2

30 98.0 45

30 98.5 28

32 98.0 38

32 98.5 50

34 99.0 21 36 99.0 37

40 99.5 40

Our efforts are now directed towards scaling-up the coating process. We are currently

coating 8" diameter fused silica substrates and have encountered a few minor problems.

These include an increase in the number of artifacts and "comets" in the coatings and, in

some cases, radial lines have appeared. The artifacts mean that we must improve our

cleanliness to an even greater extent and the radial lines are probably due to non-uniform

drying and suspension instability during coating. These factors are all under

investigation.

4. Conclusions

We have selected A10.0H - Si0

2 over several other candidates for continuing

evaluation as an HR coating system.

Our research on small 2" diameter samples has indicated that this system has high

laser damage threshold and adequate optical performance but requires a large number of

layers (32-36).

A coating system for 8" diameter substrates is under development.

488
5 . References :

[1] Thomas, I. M. "High Laser Damage Threshold Porous Silica Antireflective Coating,"

Appl. Opt. 25, 1481 (1986).

[2] Thomas, I. M. "Porous Fluoride Antireflective Coatings," Appl. Opt. 27, 3356 (1988).

[3] I. M. Thomas, "Single Layer Ti0

2

and Multilayer Ti0

2

-Si0

2 Optical Coatings

Prepared from Colloidal Suspensions," Appl. Opt. 26, 4688 (1987).

[4] Thomas, I. M. "Single Layer A1

2

0

3

.H 2

0 and Multilayer A1

2

0

3

.H

2 0-Si02 Optical Coatings Prepared from Colloidal Suspensions," Appl. Opt. 28, 4013 (1989).

489
Manuscript received

3-8-90

1-on-l And n-on-1 Laser Strength of Binder Aided

Zr02 and Zr02-Si02

Reflective Sol-Gel Coatings

Herve G. FLOCH and Jean Jacques PRIOTTON

Commissariat a l'Energie Atomique

Centre d'Etudes de Limeil-Valenton

94195 Villeneuve Saint Geroges Cedex

France

Abstract

In continuation to our search program on laser-damage resistant coatings and in synergy with the

Lawrence Livermore National Laboratory, we have prepared porous single layer coatings of ZrC^ on fused

silica and BK-7 substrates from suitable aqueous colloidal suspensions of crystalline zirconia at room

temperature. By adding judiciously a soluble inorganic binder to the suspension prior to application, it has

been possible to increase substantially both the coating refractive index and the abrasion-resistance. The

optimum binder concentration was about 30 % and at this level the optical properties and laser damage

threshold of the zirconia coating were satisfactory. Multilayer high reflectivity dielectric coatings were also

elaborated by laying down quaterwave-thick, alternating coats of this binder-aided zirconia with silica, also

prepared from colloidal suspension. To achieve 99 % reflectivity, 17-19 layers were required. Single shot (1-

on-1), laser damage tests at 1064-nm wavelength with a pulse length of 3-ns were carried out on both the

single layer and multilayer systems. The thresholds averaged 8.6 and 8.2 J/cm 2 respectively. With a laserannealing post-treatment, n-on-1 threshold values were higher than the 1-on-l figures by a factor of about

1.5.

Key words : Laser damage, ZxQj, zirconia-silica coatings,

sol, binder, HR-coatings, sol-gel coatings.

490
1 - INTRODUCTION

The sol-gel process is a versatile route for the preparation of films or dense

monolithic bodies of high purity glasses and ceramics. This field has been known for

many years and has created a real excitement in the last decade within the research

domain of materials. Indeed, the sol-gel process has allowed already to make better

useful materials and even authorized the synthesis of unusual non-crystalline solids

either by extending the normal glass formation limits or by obviating problem of phase

separation. The main winning cards of the sol-gel process is likely the fact it avoids

meltings of suitable precursors, and starts from solutions, therefore many "exotic" new

composite systems with unusual microstructure can also be obtained. Most of the sol-gel

innovations have been brought together through excellent books of papers submitted in

basic symposia / and through a recent book describing the principles, developments,

techniques and applications of sol-gel processing /8/.

The uniqueness of the so-called "sol-gel" process is to go all the way from the

precursor to the product, allowing better control of the whole process and the synthesis

of "tailor-made"materials. Such an approach is basically articulated in three steps

1. Preparation of a solution or colloidal suspension of the suitable(s) component(s). This

step gives the SOL.

2. Gellation of the SOL by polymerization or coagulation. This step gives the GEL.

3. Conversion of the wet GEL to a useful product, generally through a thermal

treatment.

So, at Limeil-laboratory we have prompted interest with the sol-gel technology

since 1985, the year where we acquired and activated our 20 KJ - 1 ns neodynium : glass

Phebus laser destinated to laser-fusion experiments (ICF-program), for high damage

threshold optical coatings. Additionnally, since this time we have maintained an official

collaboration with the Lawrence Livermore National Laboratory alike involved on ICFprogram, and particularly with Ian M. Thomas for laser-resistant sol-gel coatings. Doctor

Ian M. Thomas is probably the pioneer as for the promotion and the development of

highly damage resistant sol-gel coatings presently used in the most powerful lasersystems operating at Livermore and Limeil laboratories and elsewere.

491
Previous investigations, with the imperative to develop highly damage resistant

films on, porous silica antireflective (AR) coatings /9 , 10/, porous fluoride AR coatings

/ll/, Th02 and Th02-Si02high reflectivity (HR) coatings /12/, Ti02 and Ti02-Si02

HR coatings /13 , 14/ and more recently on AlOOH and A100H-Si02 HR coatings

/15/ have been essentially reported over these last five years by Thomas and Floch.

The work described in this paper is part of the continuing effort we maintain to

achieve the laser-strength HR-coatings must have to withstand the high peak fluences

delivered by current and future high-power lasers. It is now clearly established that

dielectric multilayered mirrors must be built up from colloidal media (sols) and not from

solutions as summarized and illustrated, in a recent survey /16/, primarily because the

colloidal method gives stress-free coatings. Others main advantages of the route using

colloids are the room temperature process, the low capital cost of equipment and the

low cost of processing.

The Ti02-Si02 HR system, while giving satisfactory optical performance, did not

have the high laser damage threshold that was expected. The Th02-Si02 HR system, in

spite of respectable results on both optical properties and laser-strength has been

abandoned for safety reasons. The major hazard of thoria is its strong toxicity when

organism-ingested and not its spontaneous radioactivity as often perceived. Moreover, to

date, the A100H-Si02 HR system only has fulfilled the expected damage resistance

requirement, but because of the relatively low refractive index difference between silica

(1.22) and bohemite alumina (1.44) coatings, about 35 total layers are demanded to

achieve reflectance greater than 99% onto BK-7. This last point is substantially

detrimental for clean and scatter-free mirror realization. Thus, in order to get round this

penalizing aspect, we decided to address new investigations on Zr02

high index

material, knowing it is a stoutly refractory material, with adequate redox and phase

diagrams. Such caracteristics could help to obtain damage resistant HR coatings,

associated with less layers than in the alumina-silica combination.

492
II - COLLOIDAL ZIRCONIA SUSPENSIONS

A. Standard sol

Aqueous colloidal suspensions of zirconium oxide (also called zirconia) have

received a consistent attention for many years and in many countries for the production

of a variety of ceramics utilized in nuclear field. One of the first patents describing the

synthesis of zirconia concentrated aqueous sols has been claimed by Alexander and

Bugosh in 1961 /ll/. Later in 1964 Clearfield precised chemical conditions to produce

different crystalline forms of hydrous zirconia /18/. In both cases, these processes

started from ionic precursors as oxychloride or oxynitrate salts which then were

hydrolyzed in temperature for a sufficient period of time to allow cationic

polymerization by an olation (or deprotonation) process until nucleation in colloidal

dimension was reached and to induce crystallization of the grown solid phase. In 1968,

Woodhead prepared zirconia aquasols by precipitating zirconium hydroxide from salts in

alkaline medium and by subsequently digesting the fresh slurry in dilute acid conditions

(peptization) /19/. Since these works, numerous detailed papers have been published

about zirconia sols, clarifying experimental conditions and characterizations of produced

powders, either with ionic /20 , 21/ and molecular /22 , 23/ precursors. The

prementioned references are by no means complete, and should be considered

exemplary only.

As a first evaluation, we decided to start our investigations on zirconia colloidal

coatings using aquasols made from ionic precursors. By another way, we selected the

zirconium oxychloride (zirconyl chloride) salt (ZrOC^) as the preparative source

because the major absorption band of likely residual chloride anion is at about 180 nm

and therefore is far away from the wavelength of our interest 1064 nm.

Before use, the zirconium oxychloride has been carefully recrystallized twice in

strongly acidic HC1 solutions, giving sparkling needle-like white prismatic crystals. The

chosen preparative method for producing zirconia aquasols was an adoption of the one

depicted by Clearfield /18/, and consisted in a controlled precipitation of the oxide

through an hydrothermal treatment. Such an hydrolysis was carried out using osmozed

deionized water and the zirconium solution was passed through a 0.2 /xm Millipore

teflon membrane to remove any suspended solid before processing.

493
The overall hydrolysis reaction can be represented by the following chemical

equation

ZrOCl2 + H20 I

100°,c Zr02 + 2HC1

The hydrolysis was conducted at reflux temperature to allow nucleation and

oxide germination, and on the other hand to help the precipitated oxide to crystallize.

An excellent paper adressed by Livage et al on the sol-gel chemistry of transition metal

oxides gives a more detailed assessment about the polycationic condensation mechanism

via olation /24/. Their quantitative analysis is based on the Partial Charge Model use

/25/.

A typical zirconia suspension was prepared as follows : Recrystallized zirconium

oxychloride octahydrate (65 g : 0.2 mole Alfa) was dissolved into stirred doubly distilled

water (300 g) at room temperature and then filtered at 0.2 jum through a teflon

membrane filter. This mixture was rapidly added into stirred deionized water (15 kg)

contained in a 20-liter glass reactor. The temperature was then raised to the boiling

point, establishing a smooth and regular reflux. Refluxing was continued over about 40

hours, until a slightly opalescent suspension was obtained. At this stage, the mixture was

spontaneously cooled down until room temperature and then concentrated HC1 (d =

1.18) was added in large excess inducing an agglomeration and a precipitation of

zirconia particles. After the decantation was completed, the acidic supernatant liquor was eliminated meticulously and the fine powder was easily redispersed in about 500

mliters water by dialysis. The dialysis was performed using a cellulosic molecularporous

membrane (Spectrapor 3, MWCO = 3500) in running water. In this way, the pH of the

suspension could be increased from an initial 0-0.5 to about 5 with no decrease in

colloidal stability or gelation appearance. The partially neutralized suspension was

concentrated under reduced pressure (20 mmHg) by distilling off the water, until the

product weighed 207g. This solution that corresponds to a colloidal suspension

containing the equivalent of 12 % ZrC»2 was readily filtered through an hydrophylic 1.0

Mm teflon membrane.

A scanning Electron Microscopy (SEM) of such a colloidal suspension using the

grid analysis method revealed that the zirconia was obtained in the form of

approximately spheroidal particles 300 - 500 A in diameter. The micrograph presented

hereafter illustrates the colloid morphology. See figure 1.

494

I
Fig. 1 : SEM of ZrC>2 particles obtained by forced - hydrolysis of a 1.3.10'° M zirconium

oxychloride solution.

Additionnally, an X-ray diffraction analysis of the zirconia agglomerates present

in suspension showed that the oxide consisted in monoclinic microcrystals (baddeleyite

allotropic variety).

B. Binder-aided suspension

The zirconia particles in the colloidal suspension prepared as described above

have likely a significant number of peripheral surface hydroxyl groups which are firmly

bound to the underlying zirconium atoms. These groups can be reacted with

hydrolyzable zirconium compounds such as the oxychloride salt for example in the

manner shown in figure 2. The zirconium dichloride oxide will then act as a binder to

increase the adhesion between adjacent particles and we would expect a stronger, more

abrasive-resistant coating to result. The degree of crosslinking will depend on the

amount of reagent added. At some level the zirconium salt reagent will have linked up

all the available particle hydroxyl groups and any excess will then go to fill up the air

space between zirconia colloids. We would then expect at this stage a decrease in

porosity and consequent increase in refractive index.

495
Fig. 2 : Surface hydroxyl groups can be linked by reaction with zirconium oxychloride.

This last point is particularly interesting owing to the fact that it implicates a

possible flexibility on the final refractive index of the coating. So, by judiciously

proportioning the level of binder added to the initial zirconia suspension, we could

substantially raise the index of the coating and therefore reduce appreciably the number

of total-layers in HR-films while maintaining high reflectivities. Moreover, such binderaided coatings could possess an enhanced overall strength.

Compositions containing the standard suspension with various quantities of

binder were then prepared by the addition of zirconium oxychloride to a sample of the

standard sol. A typical preparation of a composition containing 70 % particles and 30 %

inorganic binder was carried out as follows :

Recrystallized zirconium oxychloride (13.1g : 0.04 mole) was added into stirred

12 % zirconia standard aqueous suspension overnight. The pH of such a solution being

about 1, it was necessary to reduce its acidity by the addition of an anion exchange resin

as Amberlite IRA-93SP that corresponds to a secondary amine free-base. The mixture was partially neutralized by stirring with the anion exchange resin until the pH had been

raised to a maximum of 4 otherwise at higher value coagulation took place progressively.

496
The final binder-contained coating mixture included a total of 15 % zirconia, 70 % of

which was in the form of colloidal zirconia and the remaining 30 % in solution as a

soluble zirconium oxo-hydroxo polycation. The coating-mixture was then readily filtered

through a 1.0 j^m teflon filter. A sketch of the chemical preparation is presented in

figure 3.

REAGENTS

o o o o o

o o o o o

o o o o o

o o o o o

o o o o o

o o o o o

BINDER

COATING SUSPENSION COATING

o / o o o

o o o

\ / / o o o

- / /

O O O /

o

N O /O * o

O REPRESENTS Zr02 PARTICLES

\ REPRESENTS SOLUBLE BINDER

Fig. 3 : The preparation of binder-aided zirconia sols is basically simple

III - COLLOIDAL SILICA SUSPENSION

A typical silica sol was prepared by the base catalyzed hydrolysis of distilled

tetraethylsilicate in pure ethanol or methanol by the method previously described

/9,10/. In basic medium, hydrolysis of the silicate is prefered over condensation to

soluble organosiloxane polymers /26/. The material was prepared at 3.0 % silica

concentration, and consisted of monidispersed roughly spherical particles with a

diameter of about 20 nm.

497
IV - COATING PROCEDURE

Before use, all coating suspensions were filtered through a teflon membrane filter

of 1.0 /xm pore size for zirconia coating-sols and 0.2 /xm for silica. We have prefered to

spin-coat rather to dip-coat because it uses much less material. Coatings were deposited

on 5-cm diameter and 1-cm thick fused silica of BK-7 glass substrates using a spin-coater

in a 0.2 /xm filtered, forced air, clean hood. All coatings were prepared at an optical

thickness of 266 nm, corresponding to quaterwave at the use wavelength of 1064-nm. All

coating suspensions were applied at room temperature with a relative humidity in the

range 50-60 %. Before coating-operation all substrates were UV/O2/H2O2 cleaned

using an ozone-photoreactor (PR- 100 ; UVP.Inc), achieving hydrophilic surfaces.

Zirconia coatings were applied from various aqueous binder-aided sols

containing 15 % ZrC>2 in which the binder ratios were incleased to 10 %, 20 %, 30 %

and 40 %. We had no wetting problems with our aqueous mixtures because as mentioned before the UV-ozone treated surfaces remained very hydrophilic and this

within about 20 minutes. So, such a coating procedure allowed us to avoid the use of a

nonionic surfactant and therefore eliminated a risk of potential carbonaceous

contamination leading to laser damage, and on another hand to avoid dilution with

lower aliphatic alcohols, such as methanol or ethanol, which tend to thicken

progressively the sols until complete gelation after a few days storage. One coat at

1500 rpm was required to give a coating of the correct thickness.

Silica coatings were applied from a suspension in ethanol containing 3.0 % SiC>2

in basic medium (pH ~ 10). One application at 1300 rpm was required to give a coating

of the desired thickness. Such a coating was easily water-wetted contrary to n-propanoldiluted silica sols which were water-repellent. In the preparation of multicoated samples,

both silica and zirconia coatings were air dried at room temperature for 10-min prior to

the application of the next coat. Looking at the reflected color of the coated parts, we

could appreciated with unaided eyes the uniformity of the films. The majority of our

coatings had no discernible color variations.

498
V - LASER DAMAGE MEASUREMENTS

Damage threshold measurements were carried out on our W0

-facility at 1064-nm

using single shots with a pulse length of 3-ns. The beam spot size was about 2.0-mm

diameter. We conducted 1-on-l (one shot of known fluence onto a selected site) and non-1 (n shots of ramped fluence onto a selected site) damage tests. Each sample site was

inspected under a Vickers microscope and video-memorized before and after laser

irradiation to allow a simple and performant photographic comparison. In like manner,

we could attest if the sample damaged yes or no. Damage was defined to be any

evolutive 5-10 micron size range alteration observable by our technique. From a

sequence of irradiations, the damage threshold was defined as the average of the highest

fluence which caused no damage and the lowest fluence which did. While in general, we

do not wish to discount the significance of minute damage spots at low fluence levels, we

feel that it is important and logical to differentiate the notion of very minor and

nonevolutive damage from one of consistent and growing damage. Such an

interpretation of damage threshold seemed to us more adequate to judge the suitability

of a particular coating for practical application.

VI DISCUSSION OF RESULTS

A. Single Layer ZrOo Coatings

The refractive indices of the different binder-aided ZrC»2 coatings are shown in

table 1. Note that the standard sol, binder-free, gave very low coating index value

compared to the dense material. This indicates that coating was about 60 % porous. The

90/10 composition remained poorly affected by the binder presence just like its

abrasion-resistance. The 80/20 composition revealed already a notable increase in

refractive index and gave abrasion-resistant coating against an alcoholic drag-wipe. The

70/30 composition was the selected one for HR multicoated films because it has a

relatively high index value associated with abrasion and adhesive tape resistance.

Continuing to raise the binder-content, the refractive index still increased, but coatings

appeared significantly stressed when observed under microscope, exhibiting some fractal

cracks. Therefore such ZrC>2 coating-sols were useless for multilayered systems.

Althought the 70/30 composition gave slightly stressed zirconia coatings, we found that

this coating-formula was a good compromise for subsequent HR-coating build up. All

the binder-aided ZrC^ coatings exhibited a slight spectral evolution when heated at

100°C, consisting roughly in a 50-nm shift to shorter wavelengths.

499
This shrinkage phenomenon is likely due to water desorption associated with

completing polycondensation and drying of the binder. There was no change in the

optical spectrum when coatings were exposed for one day to varying relative humidity.

Particle - binder

ratio

Refractive

index

100 - 0 1.48

90 - 10 1.50

80 - 20 1.56

70 - 30 1.65

60 - 40 1.72

Table 1 : Refractive indices of various binder-aided ZrC>2 coatings

The refractive index values reported above have been deduced from

spectrophotometrically measured transmission spectra of respective binder-aided single

layer ZrC>2 coatings. Standard optics calculations indicated the corresponding refractive

index assuming an index of 1.46 for the fused silica substrate. As a remark, we noticed

that ZrC>2 single coat made from straight aqueous sol, binder-free, gave coating which

was weakly antireflective onto BK-7 substrate and furthermore with an index value

lower than the one reported by Thomas et al for the same material at the time of

preliminary investigations ; his figure being 1.55 /27/.

Laser damage thresholds in a 1-on-l mode ranged from a low of 6.3 J/cm2 to a

high of 11.7 J/cm2 with an average of 8.6 J/cm2. We think the wide range of damage

thresholds is probably attributed to local coating-contamination inherent to the process

itself and consequently favoring damage. Samples with low thresholds generally showed

only a few small defects in the beam area. Massive damage over the whole beam area

did not occur until fluences in the range 14-15 J/cm2 were applied. After fine

observations under Reicher stereomicroscope, we observed that in all cases damage

originated at visible artifacts. Clean areas appeared stonger to laser than those busy.

500
When laser-annealing was directed onto such sol-gel zirconia coatings, we

obtained at the first try a consistent enhancement of the laser-strength. The laserannealing or laser-conditioning consists to increase progressively the laser fluence, this

procedure is called "n-on-1" mode. Figure 4 summarizes through an histogram the laserresistance of both 1-on-l and n-on-1 tested ZrC>2 films. N-on-1 thresholds were in the

range 11.6-15.4 J/cm2 and averaged 13J/cm2. In this irradiation mode the massive

damage appeared at fluences up to 25 J/cm2

. Wilder and Thomas had recently

suspected and reported that some evaporated and sol-gel dielectric coatings were laserannealing sensitive /28/. Such an observation opened rapidly an interest and since this

time, laser-annealing has been adapted systematically in our lab and elsewere. Even

thought we do not know yet, what is the responsible mechanism involved in this

phenomenon, we know for sure that the induced enhancement of the laser resistance was durable over at least 6 months. Notable effort are presently in progress in our lab

and at LLNL in order to identify the reason of the observed damage improvement and

previews were already advanced by Kozlowski et al /29/, based on possible electronic

rearrangement in condensed dielectric materials. Regarding our sol-gel zirconia

coatings, which must be in good thermodynamic-equilibrium as confered by the

chemical nature of the sol-gel process itself, we think to date that the observed

enhancement in laser-strength, by a factor of about 1.5, might be likely due to a gentle

desorption of volatile absorbing contaminants through coating-porosity. A subsequente

emission spectrographic analysis of our ZrC>2 coatings revealed a consistent but

comprehensible amount of hafnium (< 5000 ppm), significant traces of iron (< 1000

ppm), silicon (<500 ppm) and aluminium (< 400 ppm). Most of these ionic impurities

absorb at 1064-nm and therefore are particularly prejudical to high laser damage

threshold achievement.

Fig.4 : 1-on-l and n-on-1 laser strength of binder-aided ZrC>2 coatings

501
B. Multilayer coatings

Multilayer coatings were readily prepared at room temperature by applying

alternating coats of 70/30 composition zirconia and silica. Layers were applied with

approximately 5-min drying time under an IR-lamp between applications and with the

substrate remaining in the spin coater. Occasional samples would contain comets as a

result of coating over small artifacts on the surface of the previous layer. In multilayer

applications the cleanliness criterion becomes drastically critical, so, stringently tidy

conditions during coating deposition must be considered.

Calculations using Fresnel laws indicated that, with refractive indices of 1.22 for

silica and 1.65 for binder-aided zirconia, about 17-19 layers would be required to achieve

reflectance close to 99 % on fused silica, and this was experimentally confirmed. This

coating-combination involved much less layers than within the reported alumina-silica

system /15/. For multicoat build up, we have not been faced to any problem concerning

the wettability of the silica coating by the aqueous zirconia sol. After about 5-7 layers were deposited onto well-polished substrates, coating surface aspect evidenced fine

radial-lines testifying of an apparent inhomogeneous flow motion of coating-solution.

Resulting HR-films including from 9 to 19 total layers were uniform in reflection

performances over the entire coated diameter but were slightly disrupted in surface by

the presence of such radial-lines. However, further relative-roughness measurements

made on our Reicher device gave surprisingly good figures in the range 15-20 A RMS.

We plan to adress in the future, investigations in order to reduce the radial-lines

phenomenon because it gave noticeably rise to laser-damage. All the 17 total layer

mirrors made with the described coating procedure and reflecting about 99 % of the

incident 1064-nm wavelength were unfortunately betrayed after microscope observation,

revealing some isolated fractal cracks inside coating-stacks. These cracks rose

significantly light scattering when exposed with an intense white light source. This last

point perhaps fixes the limits of such a sol-gel coating-version for highly reflective films

in the near-infrated spectral range. All made multilayered-mirrors were also abrasionresistant.

1-on-l laser-strength measurements gave thresholds varying from a low of

8.0 J/cm2 to a high of 8.8 J/cm2 with an average of 8.2 J/cm2

. In contrast of what we

observed with the others oxide systems / 12-15/, these figures are conservative to those

of the relevant single layers. We have so far no precise explanation for this noting,

except maybe the possibility that the binder by covering the zirconia particles reduced

substantially their specific area and consequently decreased their spontaneous chemical

adsorption that is suspected as detrimental for laser-resistance.

502
As for single layer coatings, the use of laser-annealing has been very effective. Non-1 thresholds, even though some mirrors were localy crazed, ranged from 10.5 J/cm2

to 13.7 J/cm2 with an average of 12.0 J/cm2

. At threshold values, damage consisted of

dispersed and tiny spots about 10-20 jum in size. In n-on-1 mode, massive damage was

attained at 25-30 J/cm2 with catastrophic failure about 300 Mm diameter. In order to

illustrate the laser -annealing efficiency onto a ZrC^-SiC^ coating, we present hereafter

the difference in damage aspect of selected sites irradiated with laser impacts of 15.7

J/cm2

, with 1-on-l (figure 5) and n-on-1 (Figure 5 bis) damage tests.

Fig. 5 : (Zr-Si) - Zr sol-gel coating

without laser-annealing post-treatment

60 urn

Fig. 5 bis : (Zr -Si) -Zr sol-gel coating

with laser-annealing post-treatment

503
VII - CONCLUSION

We have prepared using the sol-gel process, porous abrasion-resistant m-ZrC>2

coatings on fused silica or BK-7 substrates from binder-assisted aqueous suspensions.

These coatings were readily spin-applied at room temperature and required no

subsequent processing after water evaporation. They had a refractive index increasing

with binder adjunction. We found that the 70/30 colloidal-binder composition was the

best compromise for ulterior multilayered mirror realization. Such a mixture gave

zirconia coatings with an index of about 1.65 associated with 1-on-l and n-on-1 laser

damage thresholds of 8.6 and 13 J/cm2 respectively for 1064-nm wavelength pulses of

3-ns duration.

Multilayer HR-coatings were also elaborated by stacking alternate quarterwave

coatings of this zirconia with silica also prepared from a colloidal suspension. This

operation was also conducted at room temperature, with about 5-min drying time under

an IR-lamp between respective coats. About 17-19 total coats were required to give a

reflectivity of 99 %. The 1-on-l and n-on-1 thresholds of such mirrors averaged 8.2 and

12 J/cm2 respectively in the same laser-conditions. HR-coatings were also abrasionresistant to an alcohol drag-wipe operation but revealed some strain-induced fractal

cracks due to binder drying.

From the technical point of view, it is true that when attempt is made to prepare

large amount of zirconia coating-sol using such a cheminal method, we were confronted

to a limitative aspect because hydrolysis of relevant precursor needs very large quantity

of water and therefore clumsy devices. This is very penalizing when large size coated

components have to be considered.

In endind, we think that if it was possible to start from molecular precursors as

zirconium alkoxides rather than ionic salts for well defined coating-sol preparations, we

should have more chance to achieve high laser damage threshold figures compared to

the current moderate performances and that thing without having recourse to laserannealing post-treatment.

504
Acknowledgements

The authors are very grateful to C. CORDILLOT for all damage measurments

performed at Limeil-Laboratory. The author Herve G. FLOCH is particularly indebted

to Dr. Ian M. Thomas for his valuable and stimulating helping discussions and advices

which have consistently contributed to orientate and finish this study off.

505
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Ulrich (Mater. Res. Soc. Proc. 73, Palo Alto, California, 1986).

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Ulrich (Mater. Res. Soc. Proc. 121, Reno, Nevada, 1988).

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and D.R. Ulrich (John Wiley and Sons, Inc., 1984).

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Wiley and Sons, Inc., 1986).

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McCauley and R.A. Haber (American Ceramic Society Inc., 1987).

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Appl. Opt., 25, 1481 (1986).

10. H.G. Floch, J.J. Priotton, "Porous Silica Sol-Gel Coatings for Nd : Glass High-Power

Pulsed Laser Uses", The Physics and Technology of Amorphous SiC»2, Edited by

R.A. Devine, Plenum press, 561 (1988).

11. 1.M. Thomas, "Porous fluoride Antireflective Coatings", Appl. Opt., 27, 3356 (1988).

12. H.G. Floch, J.J. Priotton, J.F. Mengue and C. Cordillot, "1064-nm and 350-nm

Radiation Stability of Low Density Th02"Si02 High-Reflective Coatings

Deposited from Sols", 19 th Boulder Damage Symposium Proc, 290 (1987).

506
13. 1.M. Thomas, "Single-Layer Ti02 and Multilayer Ti02-Si02

Optical Coatings

Prepared from Colloidal Suspensions", Appl. Opt., 26, 4688 (1987).

14. H.G. Floch, J.J. Priotton and I.M. Thomas, "Ti02-Si02colloidal HR-films for HighPower Lasers", Le vide, Les Couches Minces, vol 44, 245, 33 (1989).

15. I.M. Thomas, "Single-Layer A1203-H20 and Multilayer Al203-H20-Si02

Optical

Coatings Prepared from Colloidal Suspensions", Appl. Opt., 28, 4013 (1989).

16. H.G. Floch, J.J. Priotton and I.M. Thomas, "Optical Coatings Prepared from

Colloidal Media", Thin Solid Films, 175, 173 - 178 (1989).

17. G.B. Alexander, J. Bugosh, "Concentrated Zirconia and Hafnia Aquasols and Their

Preparation", U.S. patent 2,984,628 (1961) assigned to Du Pont de Nemours and

Co.

18. A. Clearfield, "Crystalline Hydrous Zirconia", Inorg, Chem, 3, 146 (1964).

19. J.L. Woodhead, "Stabilized Zirconia Particles by Sol-Gel Processes", Science of

Ceramics 4, 4, 105 (1968).

20. A. Garg and E. Matijevic, "Preparation and Properties of Uniform Coated Inorganic

Colloidal Particles", J. Coll. & Interf. Sc., 126, 243 (1988).

21. A. Bleier and R.M. Cannon, "Nucleation and Growth of Uniform m-Zr02

", MRS

Symp. Proc, 73, 71 (1986).

22. T. Ikemoto, N. Mizutani, M. Kato and Y. Mitarai, "Synthesis of Monodispersed

Zirconia Fine Particles ", Yogyo Kyokaishi 93, 585 (1985).

23. T. Ogihara, N. Mizutani and M. Kato, "Processing of Monodispersed Zr02 Powders",

Ceram. Intern. 13, 35 (1987).

24. J. Livage, M. Henry and C. Sanchez, "Sol-Gel Chemistry of Transition Metal Oxides",

Prog. Solid St. Chem., 18, 259 (1988).

25. J. Livage and M. Henry, "Ultrastructure Processing of Advanced Ceramics", Edited

by J.D. Mackenzie and D.R. Ulrich, (Wiley, New York), 183 (1988).

507
26. B.E. Yoldas and D.P. Partlow, "Wide Spectrum Antireflective Coating for Fused

Silica and Other Glasses", Appl. Opt., 23, 1418 (1984).

27. 1.M. Thomas, J.G. Wilder and R.P. Gonzales, "HR Coatings Prepared from Colloidal

Suspensions" 20 th Boulder Damage Symposium (Co), 1988.

28. J.G. Wilder and I.M. Thomas, "Effect of n-on-1 Laser Treatment on Damage

Threshold of Selected Optical Coatings", 20 th Boulder Damage Symposium, (Co),

1988.

29. M.R. Kozlowski, M. Staggs, C.R. Wolfe and J.H. Campbell, "Large Area Laser

Conditioning of Dielectric Thin Film Mirrors", 21 st Boulder Damage

Symposium, (Co), 1989.

COMMENTS

Question: There has been quite a bit of work over the years, as I recall, in

this area performed by people who are trying to get total

scattering systems, people who are making magnesium oxide for

integrating spheres. Actually, some of it was done here at the

Bureau of Standards, I think. Have you looked into what sort or

numbers they get?

Answer- As I say, the only numbers I've seen in handbooks for scatterers

are at the 99% level. I'd appreciate any information that anyone

can give me about the real state of the art of scattering

surfaces

508
Manuscript Received

3-2-90

Structural Modification of D20/H 2 0-Dosed CaF2 Optical Thin Films

J. B. Franck

Physics Division, Research Department

Naval Weapons Center, China Lake, California 93555-6001

Unusual behavior has been reported for D20/H20-dosed CaF2 optical thin

films. This behavior includes resistance to laser-induced damage at 2.7-|j.m

radiation, improved damage morphology, and reduced optical absorption in the

water band for even H20-dosed CaF2 optical thin films. Analysis of the films

strongly suggests that the modification of film properties is not the result of

inclusion of the D2 0/H2 0 into the film matrix or chemical contamination. Rather,

the act of dosing has modified the structural properties of the films, thus

producing the effects observed.

Key words: adsorption; calcium fluoride; D2 0; desorption; H2 0; laser damage;

thin films.

Background

In work presented previously [1], it was reported that unexpected behavior had been

found for ~l-(j.m-thick CaF2 thin films produced by thermal evaporation in the presence of

copious amounts of either D2 0 or H2 0 (called dosing). The CaF2 films were evaporated onto a variety of substrates; however, most of the analysis was performed on films laid down onto

single-point diamond-machined (SPDM) oxygen-free high-conductivity (OFHC) Cu substrates

or SPDM ultrabright acid Cu (UBAC) plating on OFHC substrates. Several properties of these

films were modified when compared to undosed films of CaF2 . Resistance to laser-induced

damage at 2.7 |im was observed as well as improved damage morphology. Also observed

was a reduced optical absorption in the water band. This was seen for both D2 0 and H2 0

dosing. A corollary to the above phenomena was that no evidence of optical absorption was

found in Fourier transform infrared (FTIR) spectrometry for the heavy water band with D2 0

dosing.

D 2 0-dosed CaF2 films were made initially in an attempt to produce a controlled sample

set for laser-induced desorption experiments concerned with water contamination. The

purpose of the D2 0 dosing was to produce surfaces rich in a controlled desorbate. Dosing

with D2 0 resulted in films containing residual molecules behaving chemically in a manner

very similar to water; however, the abundance of D2 0 in nature is several orders of

magnitude less than for H2 0. Surprisingly, the number of residual molecules (either

adsorbed onto the surface or incorporated into the bulk) was too small to be seen in FTIR

spectrometry in the heavy-water band. It was assumed that optical absorption in the

water band would increase—not decrease. This was expected because the films were

introduced into the atmosphere, and the dynamics of the adsorption/desorption process [2]

strongly indicate that loosely bound surface D2 0 will be replaced rapidly by the atmospheric

H 2 0. Evidence will be presented to suggest strongly that the altered film characteristics are due solely to film-structure modification and that, as the absorption data suggest, there is

509
little D2O/H2O incorporated into the matrix of the CaF2 films. Rather, the residual D2O/H2O is due to the molecules being adsorbed onto the surface of the growth columns, which are the

artifacts found in most optical thin films manufactured nonepitaxially. The reduced optical

absorption, therefore, is a consequence of a reduced surface area for the film. X-ray

crystallography data suggest that the dosed films are composed of larger, more ordered

crystallites. For a given film area, the surface area of the film increases as the size of the

crystallites decrease. This is analogous to increasing the surface area of a sample by

crushing it into a powder.

Discussion

The absorption of H2O onto the surface of CaF2 has been studied by workers in the

field from the early 1930s [3-5] to the present. Ogura [6], de Boer [7], and Gregg [8] have

studied the adsorption/desorption process as well as the influence of microporosity on the

adsorption/desorption process in a dynamical sense. Porteus et al. [9,10] have studied the

dynamics of laser-induced desorption from CaF2 surfaces. The present investigation was a

direct result of an attempt to produce a sample set with controlled characteristics for laserinduced desorption study of CaF2 optical coatings. The coatings were produced in a bell jar

system by evaporation from a Mo boat. For each coating run, the coatings were deposited

onto unheated substrates and the coating thickness was monitored by a quartz crystal

monitor. After the coating run, a witness sample was measured by stylus profilometry to

determine the coating thickness. The base pressure for the diffusion-pumped system was

below 1 x 10" 6 Torr. Dosing was performed by attaching one of two glass sampling cylinders

to a leak valve. One cylinder contained H2O, while the other contained 99.9% D2O. The

cylinders were outgassed in the following manner. One at a time, they were attached to a vacuum system operating in the mid 10-7 Torr range; during evacuation, they were placed

in a dry-ice/methanol slurry. This caused the liquid in the sampling cylinder to slowly

solidify as the vacuum system removed the more volatile elements.

Comparison studies were performed on undosed, H2O-, and D20-dosed samples, and

coating runs were always performed in that order. After the samples and system were

cleaned, the runs were as follows: (1) undosed CaF2 films; (2) H20-dosed CaF2 films

(introduction of H2O into the system; since water is ubiquitous, it actually presented no

additional contamination); and (3) D20-dosed CaF2 films. Detailed coating-run information is

available elsewhere [2].

Figure 1 shows a FTIR spectroscopy plot of a NaCl window used as a substrate for a

test film. Figure 2 shows a FTIR plot of an Optovac CaF2 window that was to be used as a coating material. Figure 3 shows a FTIR plot of a l-|im-thick CaF2 film. Absorption due to

H2O (OH) is seen, but there is no evidence of absorption due to D2O (OD). This sample was

dosed with D2O, and the coating chamber was brought up to atmosphere with D2O and taken

immediately to the FTIR apparatus.

Laser-induced damage onset analysis at 2.7 u.m was performed on a variety of

samples. The data are presented in table 1. The highest damage onset was found for a D20-dosed CaF2 film on a quartz substrate; H20-dosed CaF2 on quartz was not tested. For a CaF2 film on SPDM UBAC, the highest damage onset was found for H2O dosing; D2O dosing

ranked next, and the lowest damage onset was with the undosed film. All Cu samples were

baked in a low-vacuum oven for 12 hours at 115°C, with the exception of the first undosed

sample that exhibited such a high residual stress that it self-fractured before baking.

Spectral reflectance at 2.9 Jim for three of the samples can be found in the fifth column of

the table, and the accompanying spectral plots are shown in figures 4 through 6. Shown in

510
Table 1. Laser damage onset and morphology

CaF2

doping Substrate

Damage

onset, 1 J/cm2

1-R

at 2.9(im

R

at 2.9um

Damage

morphology

Defect/

intrinsic

None SPDM

UBAC #12

run #14

5 10.4 89.6 fracture &

delamination

intrinsic

H2O SPDM

UBAC #13

run #15

30 7.5 92.5 ablation intrinsic

P2O SPDM

UBAC#15

run #17

22 ablation &

fract at high

fluence

intrinsic

D7O SPDM

UBAC #16

run #18

12 9.5 90.5 ablation intrinsic

E>20 SPDM

OFHC #3 run #6

12 ablation intrinsic

E>20 quartz #1

run #6

75 ablation defect

None quartz #4

run #10

20 fracture &

delamination

defect

figure 7 is a plot of laser damage onset values versus spectral reflectance at 2.9 urn. The

highest two additional points are for samples whose spectral reflectance is known but laser

damage onset is unknown. The predicted value (which is quite high) may not actually be

correct if failure is the result of defects at the higher fluence values. Defect-dominated

failure was not seen for any of the CaF2 films tested on the Cu substrates, as presented in

table 1. Shown in figures 8 through 10 are photographs of laser-induced damage, showing

the damage morphology for undosed CaF2 films. Figures 8 and 9 are SPDM Cu substrates;

figure 10 is a quartz substrate. Failure takes the form of fracture and delamination. For the

quartz sample in figure 10, the CaF2 film has been completely removed from the substrate

in some regions. Figures 11 through 13 are photographs of laser damage sites showing the

damage morphology for dosed CaF2 on SPDM Cu substrates. The damage does not appear to

be defect dominated. The damage morphology appears to be ablation and "orange-peellike" surface roughening. A laser-induced desorption-onset and adsorption-site (called

adsite for convenience) density analysis performed at 2.7 \xm [2] showed that adsites, like

laser damaging defects, tend to be distributed randomly about the surfaces under test. This

was found for CaF2 films on both dielectric and metal substrates. An attempt was made to

compare the adsite density (found in laser-induced desorption-onset analysis) with

damaging defect density (found in laser-induced damage-onset analysis) for CaF2 on SPDM

Cu. Surprisingly, it was found (table 1) that the films on SPDM Cu failed in a uniform

manner while desorbing in an adsite-dominated fashion. There are two simple explanations

for this phenomena: either the films are defect free or the defect density is so high that the

failure is uniform because a defect is always encountered. We have taken the liberty to

describe these surfaces as defect-domination free, since a defect is defined operationally as

the element on a surface that fails prematurely. Regardless of one's operational definition,

511
the main point here is that the damaging defects did not correlate with the desorbing

adsites.

Figures 14 and 15 are photographs of interference fringe patterns for undosed and

H20-dosed CaF2 on thin "stress measurement" samples, respectively. Fewer stress-induced

fringes—a result of the bowing of the thin SiC»2 sample—can be seen on the H20-dosed film.

Figures 16 through 19 are spectral reflectance plots for undosed (unbaked and baked);

and H2O dosed (unbaked and baked) to determine the modification of optical absorption at 2.9 urn as a function of dosing and baking. While it was found that baking had a more

pronounced improvement in reflectance at 2.9 (im, dosing also improved the reflectance.

Coating Analysis

A sample set was developed for the purpose of performing analytical tests. The CaF2

films on SDPM UBAC (samples #1 through 12) as well as the CaF2 films on graphite (samples

#1 through 4) are listed in table 2, where the samples are listed by run number, substrate,

type of dosing, thickness, and baking.

Table 2. CaF2 SPDM analysis sample parameters

Sample # Run # Substrate Dosing Thickness Baking

1 22 Cu un 0.6 um baked

2 22 Cu un 0.6 (im u n

3 24 Cu un 0.5 um u n 4 24 Cu un 0.5 um baked

5 25 Cu H20 0.34 um baked

6 25 Cu H20 0.34 ^m u n

7 25 Cu H20 0.34 um u n

8 25 Cu H20 0.34 \im baked

1 23 graphite none 800 A baked

2 23 graphite none 800 A u n

9 26 Cu D2O 0.4 |im baked

10 26 Cu D2O 0.4 u.m u n

11 26 Cu D2O 0.4 (im u n 12 26 Cu D2O 0.4 jim baked

3 26 graphite D2O 800 A u n 4 27 graphite DO 800 A baked

Shown in table 3 is a listing of the Auger data taken by A. Green of the Naval Weapons

Center (NWC). The samples tested were SPDM OFHC Cu #1 & 2, 5 & 6, and 9 & 10. The data

are tabulated by sample number, sputtering, dosing, baking, amplitude of the Ca peak, ratio

of the oxygen peak to the Ca peak, and ratio of the Cu peak to the Ca peak. While data exist

for the F peak, they are not presented in this table. This is because F is so mobile, that the

peak values may have no fixed relationship with the actual amount of F present in the

material.

512
Table 3. Auger parameters

Sample # Sputter Dosed Baked Ca # O/Ca Cu/Ca

Case

1 a yes 3.357 0.212 0.043

b 2 min yes 3.337 0.134 0.048

2 a ... 3.038 0.153 0.03

b 3 min 3.290 0.095 0.051

5 a H20 yes 2.843 0.177

b 3 min H20 yes 3.128 0.081 0.032

6 a H20 — 2.704 0.107 0.041

b 3 min H20 3.144 0.072 0.055

9 a E>20 yes 2.853 0.185

b 3 min E^O yes 3.165 0.101

10 a D2O 2.845 0.076

b 3 min 3.054 0.065

Oxygen is the first element that will be discussed. From table 3, the undosed/baked/

unsputtered #l(a) sample (#1) can be seen to have the highest value for the oxygen peak.

Since no sputtering has taken place, the surface remains unaltered; in fact, all of the

unsputtered samples showed a greater oxygen peak compared to the same sample after

sputtering has ablated the surface. The lowest oxygen peak was seen for the D2 0 dosed/

unbaked/sputtered coating #10(b). There are two points that come as a surprise: First, the

baked samples show a higher oxygen peak than those unbaked. If one assumes that the

presence of the oxygen peak is the result of adsorbed H2 0/D2 0, then baking should reduce

the H2 0 on the surface; hence, the oxygen peak should go down. The reverse was seen!

Second, all four of the dosed coatings showed a lower oxygen peak than the undosed, with

the exception of samples #5(a) and 9(a) (before sputtering)—as compared to sample #2(a)

(before sputtering).

The highest peak for Cu was found for the H2 0-dosed/unbaked/sputtered sample

#6(b), in contrast to the D2 0-dosed samples, which showed no demonstrable Cu peak. The

data for the undosed/baked samples are very similar to sample #6(b) above. One important

item is that the Cu peak always appeared to increase with sputtering depth into the coating.

Presented next will be the Auger data taken by R. Scheri of NWC. Two samples were

chosen for detailed analysis, specifically on the basis of previous laser-induced damage

performance tests on similar samples. The coatings that failed at the lowest fluence levels

were the unbaked/undosed CaF2 films, in contrast to the H2 0-dosed/baked CaF2 films that

withstood the highest fluences.

The peak heights found on the two samples were ratioed, as in the previous data, and

are presented in table 4. Listed in the three columns are the sputter time, ratio of oxygen to

Ca, and ratio of Cu to Ca. As in the previous results, the oxygen peak decreases with depth

and the Cu peak increases with sputtering depth. However, there is an anomaly in the data

points for both 20-min sputter times for the oxygen peaks. While the oxygen peak is

steadily decreasing for the 5- and 10-min sputter times, the 20-min sputter value increases

for both the undosed/unbaked and the H2 0-dosed/baked CaF2 coatings. From table 4, it can

be seen that this phenomenon is not seen for the Cu peak, which is monotonically increasing

513
for the undosed/unbaked film. At the same time, no evidence of the Cu peak is seen in the

dosed/baked coating until 20 min of sputtering has occurred. In contrast to these

anomalous data points, the previous Auger data saw no such effect for up to 10 min of

sputter time. The sputter-ion current was much higher for the previous Auger data.

Table 4. Auger parameters

Sputter time Oxvsen/Ca Cu/Ca

Sample #2. Undosed/T Jnhake.H

0 min 0.765 0.121

5 min 0.661 0.138

10 min 0.645 0.168

20 min 0.765 0.191

Sample #5. H?0 Dosed/Rake.rl

0 min 0.670

5 min 0.551

10 min 0.554

20 min 0.620 0.08

The x-ray analysis was particularly interesting. The samples tested were quartz #8,

10, and 12 and represented undosed, H2O-, and D20-dosed CaF2 on quartz, respectively.

Plotted data from sample #8 are shown in figure 20. For all three samples, the first broad

peak is seen as the result of the substrate. The first sharp peak at around 30 degrees is the

(111) crystal orientation peak for CaF2. The second sharp peak is the (220) crystal

orientation peak attributed to CaF2. For a powder sample, the (220) peak would be the

100% peak with the (111) at 92% of the (220) peak height. Clearly, the (111) peak is twice

as high as the (220) peak for sample #8.

The plotted data for sample #10 are shown in figure 21. The same broad peak can be

seen as in the previous data for the undosed CaF2 film. However, the first sharp peak

belonging to the (111) crystal orientation is approximately 11 times higher than the peak

belonging to the (220) crystal orientation. This indicates that there is a much stronger

preference for the (111) planes to be oriented parallel to the substrate for the H20-dosed

film than for the undosed film.

The plotted data for sample #12 are shown in figure 22. There is a significant

difference between this D20-dosed film and the H20-dosed film in that while the (111) is

approximately the same height, there is only a small vestige of the (220) peak.

The analysis performed by C. Lowe-Ma of NWC was as follows. The average unit cell

edge length for samples #8 and 12 were calculated to be 5.462 and 5.461 A, respectively. C.

Lowe-Ma cautioned that the F anion could be replaced by either an OH or OD anion in the

crystal without any significant modification in unit cell size. From the ratio of the peak

514
heights, one can form a rough estimate of the relative portion of the material in the (111)

versus the (220) crystal orientation. For sample #8, this means that about twice as much

material is in the (111) as the (220) orientation. For sample #10, the amount of material in

the (111) orientation is roughly a factor of 11 times that of the material in the (220)

orientation; for sample #12, the ratio is on the order of 30:1. No evidence of Ca(OH)2 or Ca(OD)2 crystallites was found. BaF2 powder was used as a standard for the determination

of the average "crystallite size." The method utilized was the Warren-Averbach Fourier

analysis approach. With this technique, an average size of about 284 A was calculated for

the crystallites of sample #8. For sample #12, an average crystallite size of 477 A was

calculated; for sample #10, it was 665 A. It is interesting to note that the order of the

average crystallite sizes parallels the order of optical absorption and laser damage.

Summary and Conclusions

The unusual properties observed for the dosed CaF2 thin films required a substantial

number of tests before a reasonable mechanism was determined. Evidence strongly points

to modification of the film structure induced by the presence of H2O or D2O during the

coating process. The low-temperature bakeout appears also to have played a role in some of

the film properties observed.

From the analytical data discussed previously, it was observed that there was no

evidence of residual D2O in any of the films produced in the presence of copious amounts of

D2O. Only the laser-induced desorption onset experiment had the necessary sensitivity to

consistently see the D2O adsorbed onto the surface of the CaF2 films randomly distributed in

adsites. One first questions why no trace of D2O was found, while substantial amounts of

H2O were present. Adsorption/desorption is, a dynamic process, de Boer [7] shows that

there are 1 x 1020 molecules/sec/cm2 of H2O bombarding a surface for even 10% humidity.

For samples carried around in typically 35 to 40% humidity, it may seem more surprising

that any residual D2O was found. Ogura [6] discusses the problem of removing residual H2O

molecules from the pores frequently found in thin-film structures. He found that for pore

sizes smaller than 13 A it was quite difficult to remove the H2O molecules. He also presents

a pore modeled in the shape of an ink bottle. Such a pore would be very difficult to empty

once filled. If such pores were filled with D2O during the coating process, the dynamics of

the adsorption/desorption process dictate that only the types of pore structures just

discussed would tend to retain the trace amounts of D2O found in the laser-induced

desorption experiment. It also seems quite reasonable that pulsed irradiation could provide

the stimulus to remove these trapped D2O molecules.

We may now have a clue as to why the CaF2 films made in the presence of H2O/D2O

showed a decreased optical absorption in the water band while not absorbing in the heavy

water band. While it is true that normal H2O replaced the heavy water in the D20-dosed

samples, the overall surface area would be reduced as well as the number of pores if the

thin-film growth columns were larger. Does evidence exist that this has taken place? The

answer is unequivocally, yes. From the x-ray crystallography data, it was determined that

the grain size of the CaF2 films on ultrabright acid Cu followed the optical absorption. The

H20-dosed film had the lowest optical absorption and the largest grain size, while the

undosed film had the highest optical absorption and the smallest grain size. The D20-dosed

films exhibited an intermediate grain size and adsorption. Electron micrographs showed

that the dosed CaF2 films exhibited a larger columnar structure than the undosed films. This

is probably the result of larger grain size. One concern that should be addressed here is the

question of contamination. The analysis of the x-ray crystallography data indicates that

Ca(OH)2 was not present in the H20-dosed films. It is possible that an OH- ion can replace a F- in the CaF2 with only a minor crystal distortion. The average unit cell edge length at 5.462 A for both the undosed and D20-dosed films indicates that the two films are quite

515
similar in their unit cell structure. When the undosed film was retested to compare with a H20-dosed film, it was found to have an average unit cell edge length of 5.463 versus 5.465

A for the H2O-dosed CaF2 film. Thus, the average unit cell edge length for all three samples

was found to be quite similar. Crystallite size appears to determine the properties seen for

the behavior of these films. Auger and secondary ion mass spectroscopy (SIMS) analyses

did show that trace amounts of contaminants were found in both the dosed and the undosed

CaF2 films. There did not appear to be a significant difference in the contaminant level in the

films as a function of dosing for most of the contaminants, but not for all of them. The

exceptions were Cu and oxygen. The Auger and SIMS analyses were discussed earlier. If

one assumes a more ordered system and, hence, larger crystallites and columnar structure

than as stated earlier, an overall reduced surface area would follow. It is possible that Cu

migrates up from the substrate along the surface of the growth columns toward the surface

of the CaF2 film. It is also possible that H2O and, hence, oxygen migrate down from the top

surface toward the substrate. Both of these phenomena should be impeded if the surface

area is reduced. And, this is in general what was observed. The dosed films exhibited a reduced Cu migration from the substrate toward the surface and a reduced oxygen

migration from the top surface toward the substrate.

We might wonder why dosing during the coating process caused the structural

modification of the thin film without incorporating into the film. The process may be

thought of as a "catalytic-like" one. If a simple model is permitted, figures 23 and 24 are cartoons showing CaF2 molecules pictured as large circles and H2O molecules pictured as smaller circles with a dot in the center. Figure 23(a) shows an ordered CaF2 surface with an incoming CaF2 molecule about to hit a corner region. Since the sticking coefficient is fairly

high and the kinetic energy fairly low, the molecule adheres where it struck with a little

additional movement, shown in figure 23(b). In contrast, the same CaF2 surface is shown in

figure 24 with two monolayers of H2O on the bottom step. Figure 24(a), like the previous

cartoon, shows an incoming CaF2 molecule. This time, however, the in-bound molecule

strikes both the CaF2 molecule on the corner of a step as well the H2O molecule in intimate

contact with it. Now, after striking this surface [shown in fig. 24(b)], the molecule is

attracted to both the CaF2 and the H2O molecule. With what little kinetic energy it has along

with the high mobility of the H2O molecule, the CaF2 molecule moves around until it finds

the most stable potential region, which happens to be the position in the surface that tends

toward a crystalline structure. Additional support for this model can be found in figures 20

through 22, which show the result of the x-ray crystallography analysis. From these figures,

it can be seen that one major effect of dosing the CaF2 caused a considerable increase in the

(111) crystal peak. This represents a set of crystalline planes "parallel" to the Cu substrate.

The small spot laser-induced technique provided information on the spatial

distribution of the adsorbed gases. For the dosed coatings, it was found that H2O/D2O

introduced during the coating process can act as a modifier of the structure of the film being

deposited. Operationally, there are localized regions randomly distributed about the

surface—adsites—where H2O/D2O aggregate.

From the conclusions drawn, one might expect that comparable results may be found

for fluoride coatings produced in a manner similar to that described in this paper. However,

often the coatings are deposited onto heated substrates. It seems likely that the increased

temperature of the substrate will reduce the adsorption of H2O/D2O onto the surface being

coated and thus reduce the action seen for unheated substrates. Also, since this study did

not find the optimum operating substrate temperature, gas pressure, or even gas type, this

work still remains to be performed.

516
References

[1] Franck, J. B.; Porteus, J. O.; Johnson, L. F.; Pentony, J. M.; Faith, W. N. "The consequence

of doping optical materials with D2O," presented at the 20th symposium on optical

materials for high power lasers; Boulder, CO; 1988 26-28 October. (Proceedings in

process.)

[2] Franck, J. B. "Laser-induced desorption and damage of water- and heavy-water-dosed

optical thin films," Ph.D. Dissertation. Tucson, AZ; Optical Sciences Center; University of

Arizona; 1989.

[3] de Boer, J. H.; Dipple, C. J. "Oberflachenbestimmung durch adsorption von

wassermolekulen," Z. Phys. Chem. B 25; 399-411; 1934.

[4] Barraclough, P. B.; Hall, P. G. "Adsorption of water vapor by calcium fluoride, barium

fluoride, and lead fluoride," J. Chem. Soc. Faraday Trans. 71; Part 11; 2266-2276; 1975.

[5] Porteus, J. O.; Franck, J. B.; Seitel, S. C; Allen, S. D. "Defect characteristics of optical

surfaces using pulsed laser damage methods," in Measurement and effects of surface

defects and quality of polish . Baker, L. R.; Bennett, H. E., ed. Proc. SPIE, 525: 43-50;

1985.

[6] Ogura, S. "Some features of the behavior of optical thin films: A study of packing

density, structure, and water sorption phenomena in optical thin films," Ph.D.

Dissertation. England; Department of Physics and Physical Electronics; Newcastle Upon

Tyne Polytechnic; 1975.

[7] de Boer, J. H. The dynamical character of adsorption . Oxford; Clarendon Press; 1968.

[8] Gregg, S. J.; Swing, K. S. W. Adsorption, surface area, and porosity, 2nd ed. San Diego,

CA; Academic Press; 1982.

[9] Porteus, J. O.; Faith, W. N.; Allen, S. D. "Laser desorption analysis of H2O and other

contaminates from optical surfaces," in Laser induced damage in optical materials:

1981 . Bennett, H. E.; Guenther, A. H.; Milam, D.; Newnam, B. E., ed. 1981 November 17-

18; Boulder, CO. Nat. Bur. Stand. (U.S.) Spec. Publ. 638; 1983 September. Pp. 273-279.

[10] Allen, S. D.; Porteus, J. O.; Faith, W. N.; Franck, J. B. "Contaminant and defect analysis of

optical surfaces by infrared laser induced desorption," Appl. Phys. Lett. 45_; 997-999;

1984.

517
FT1R SPECTRUM OF

NaCI SUBSTRATE

H

U

EH

EHs

10

MICRONS

Figure 1 substrate,

the NaCI.

FTIR transmission plot of a NaCI

The roll-off around 15 u.m is due to

FTIR SPECTRUM OF OPTOVAC CaF2 WINDOW

(COATING MATERIAL)

2.Z l7^0 2B.0 S.i MICRONS

Figure 2. FTIR transmission plot of an Optovac

window later broken for use as coating material.

The slope on the short wavelength side is an instrumental artifact. The important consideration is that no absorption bands associated with

either H2O or D2O can be seen.

518
103.0-

93.0-1 1 1

, 2.2 3.0 4.0 5.0 MICRONS

Figure 3. FTIR transmission plot of a D20-dosed

l-|im-thick CaF2 film on the NaCl substrate

presented in figure 2. The absorption band

attributed to H2O is seen with some fine

structure associated with hydrocarbons around

3.3 (im as well as CO2 around 4.3 (im. No

evidence of the D2O dosing can be seen at ~ 3.9

|im.

z

WftVELENGTH Cum)

Figure 4. Plot of reflectance versus wavelength shown

with wavelength decreasing from left to right for an undosed CaF2 film on SPDM Cu. The sample has gone

through a 12-hr bakeout. The reflectance minimum at 2.9 nm at around 89.5% is due to H2O. The additional

local minimum at 3.4 |im is due either to loosely bound

OH" or hydrocarbons. The bump around 4.3 jim is a calibration artifact.

519
IBB

39

99

- -

- - -

_ L.

\

1

97

96

95

j

- —

.JL - 1

~\

94

93

92

91

9B

5.4 5.2 5.B 4.8 4.6

WBVELENGTH Cum)

Figure 5. Plot of reflectance versus wavelength shown

with wavelength decreasing from left to right for a D2Odosed CaF2 film on SPDM Cu. The sample has gone

through a 12-hr bakeout. The reflectance minimum at 2.9 urn, up from the coating discussed in figure 4 at around 90.5%, is due to H2 0. The additional local

minimum at 3.4 p.m has increased dramatically over the

undosed film and has saturated the detector. No

evidence of absorption related to the D2 0 dosing is

evidenced.

5.4 5.2 5.0 4.B 4.6 4.4 4.2 4.0 3.8 3.6 3.4 3.2 3.0 2.6 2.6 2.4 2.2

HRVELENGTH Cum)

Figure 6. Plot of reflectance versus wavelength shown

with wavelength decreasing from left to right for a H2Odosed CaF2 film on SPDM Cu. The sample has gone

through a 12-hr bakeout. The reflectance minimum at 2.9 u.m, up from the reflectance of the undosed, dosed,

or the D2O dosed at around 92.5%, is due to H2O. The

additional local minimum at 3.4 u.m has also increased

dramatically over the undosed film and has saturated

the detector.

520
89 90 91 92 93 94 95

REFLECTANCE (X) @2.9um

Figure 7. Plot of laser damage onset value versus

spectral reflectance at 2.9 fim for the samples discussed

in figures 4 through 6. A best-fit straight line passes

through all three points: the hollow square, solid circle,

and hollow circle. Two additional points—the solid

triangle and hollow triangle—whose spectral reflectances

are known but damage onsets are not known are placed

on the line corresponding to their abscissa value for

spectral reflectance. The corresponding laser damage

onset value predicted in this fashion is quite high for

2.7 u.m. This is, of course, only a prediction, as the

straight line may roll off at higher fluences; however,

the prediction made in this manner is certainly

interesting.

Figure 8. Photograph of HF

laser-induced damage of

undosed CaF2 on SPDM Cu.

Damage morphology is

fracture and delamination;

360X magnification.

521
Figure 9. Photograph of HF laser-induced

damage of undosed CaF2 on SPDM Cu. Damage

morphology is fracture and delamination;

360X magnification.

Figure 10. Photograph of HF laser-induced

damage of undosed CaF2 on quartz. Damage

morphology is fracture and delamination;

360X magnification.

Figure 11. Photograph of HF laser-induced

damage of H2 0-dosed CaF2 on SPDM Cu.

Damage morphology is ablation and "orangepeel-like" surface roughening; 360X magnification.

522
Figure 12. Photograph of HF laser-induced

damage of H20-dosed CaF2 on SPDM Cu.

Damage morphology is ablation and "orangepeel-like" surface roughening; 360X magnification.

Figure 13. Photograph of HF laser-induced

damage of H20-dosed CaF2 on SPDM Cu.

Damage morphology is ablation and "orangepeel-like" surface roughening. Damage has

penetrated into the Cu without modifying the

damage morphology; 360X magnification.

Figure 14. Photograph of the

interference fringe pattern for undosed

CaF2 films on thin "stress measurement"

sample. If the film is stressed, it will

cause the thin, flat SiC»2 sample to bow

either positively or negatively

depending on whether the stress is

compressive or tensile. A few fringes

are visible, indicating some low residual

stress in the coatings.

Figure 15. Photograph similar to figure

14 with the exception that it is H2O

dosed. Even fewer fringes can be

observed with this sample, indicating

less residual stress than in the undosed

film.

523
RUN #19 Cu *17

lea

W 93

—

'— — —

V ———

bis —

— f- —

1

/

/

5.4 S.2 5. B 4.8 4.6 4.4 4.2 n

MHVELENGTH (urn)

Figure 16. Plot of spectral reflectance versus wavelength shown with wavelength decreasing

from left to right for an undosed CaF2 film on SPDM

Cu. The sample is unbaked. The reflectance

minimum at 2.9 \im reflectance is around 89.5%.

The additional local minimum at 3.4 urn can be

seen.

5.4 5.2 5.6 4.B 4.S 4.4 4.2 4.0 3.8 3.6 3.4 3.2 3.B 2.8 2.6 2.4 2.2

HflVELENGTH Cum)

Figure 17. Plot of reflectance versus wavelength

shown with wavelength decreasing from left to

right for an undosed CaF2 film on SPDM Cu. The

sample has gone through an 18-hr bakeout. The

reflectance minimum at 2.9 (xm is up from the

reflectance of the unbaked film at around 94.0%.

524
18

WflVELENGTH iumi

Figure 18. Plot of Reflectance versus wavelength

shown with wavelength decreasing from left to

right for a H2 0-dosed CaF2 film on SPDM Cu. The

sample is unbaked. The reflectance minimum at 2.9 urn is around 91.0%. The additional local

minimum at 3.4 jim can be seen.

RUN #20 Cu#18 Rfter bake

WHVELENGTH Cum)

Figure 19. Plot of reflectance versus wavelength

shown with wavelength decreasing from left to

right for a H20-dosed CaF2 film on SPDM Cu. The

sample has gone through an 18-hr bakeout. The

reflectance minimum at 2.9 [im is up to around

95.0%.

525
CPS 17. 6B 2.9B 1.67 1.20

0.0

DEGREES

Figure 20. X-ray crystallography data shown

plotted with counts per second for the ordinate

and angle in degrees for the abscissa. The

coating is an undosed/unbaked CaF2 film on

quartz. The broad peak around 22 deg is due to the substrate. There is a sharp peak around 29

deg associated with the (111) crystal orientation

and a second peak around 47 deg associated

with the (220) crystal orientation for CaF2.

DEGREES

Figure 21. X-ray crystallography data shown

plotted with counts per second for the ordinate

and angle in degrees for the abscissa. The

coating is a D20-dosed/unbaked CaF2 film on

quartz. The broad peak around 22 deg is due to the substrate. The sharp peak around 29 deg

associated with the (111) crystal orientation is much higher for this coating than for the

undosed coating. The second peak around 47

deg associated with the (220) crystal orientation

appears the same.

526
DEGREES

Figure 22. X-ray crystallography data shown

plotted with counts per second for the ordinate

and angle in degrees for the abscissa. The

coating is a H20-dosed/unbaked CaF2 film on

quartz. The broad peak around 22 deg is due to

the substrate. The sharp peak around 29 deg

associated with the (111) crystal orientation is

much higher for this coating than for the

undosed coating. The second peak around 47

deg associated with the (220) crystal orientation

appears much smaller than for the undosed or

the D20-dosed film.

(a)

(b)

Figure 23. Cartoon representing a CaF2 regular surface being impinged

upon by a CaF2 molecule shown in

(a). After the impact, the molecule

moves a little, but comes to rest in a manner such that a void is left

behind, shown in (b). After several

million iterations, the once ordered

surface builds up into a strongly

disordered surface.

O

(a)

(b)

Figure 24. Cartoon representing a CaF2 regular surface being impinged

upon by a CaF2 molecule shown in

(a). Unlike the case in figure 23, this

surface has H2O molecules adsorbed

onto it. After the impact, the

molecule moves more freely and has

the possibility of finding the lowest

potential, hence building a more

ordered surface, depicted in (b).

After several million iterations, the

once ordered surface builds up into a more ordered surface.

527
iuuscx-J.pt received

4-2-90

Non-Avalanche Dielectric Breakdown in Wide-Band-Gap Insulators

at DC and Optical Frequencies

P.Braunlich, S.C.Jones, X.A.Shen, and R.T.Casper

Dept. of Physics, Washington State University,

Pullman, WA 99164-2814

E.Cartier, D.J.DiMaria, and M.V.Fischetti

IBM T.J. Watson Research Center, P.O.Box 218,

Yorktown Heights, New York 10598

P.Kelly

Physics Division, National Research Council,

Ottawa, Canada K1A 0R6

It is the purpose of this paper to put recent novel experimental efforts to understand dielectric breakdown

in wide-band-gap materials in various fields into a common framework and outline significant changes in the

understanding of dielectric breakdown at optical frequencies and free electron heating under DC conditions.

New experimental techniques used to measure multiphoton absorption and energy deposition in wide-bandgap alkali halides in the prebreakdown regime have led to hard evidence refuting the avalanche model of

laser-induced damage at visible laser wavelength. The experiments show that virtually all lattice heating occurs via nonlinear absorption of laser photons by multiphoton excited free electrons. Direct measurements

of free electron heating by DC fields in thin Si02 -films and direct measurements of electron-phonon scat- tering rates of energetic free electrons impose a new understanding of carrier heating. The scattering of free electrons with non-polar acoustic phonons is found to be the dominant interaction in preventing the free

carriers from reaching energies high enough to cause impact ionization and initiate avalanche breakdown.

These results unambiguously show that the role of avalanche breakdown under DC conditions has been

overestimated in the past.

avalanche breakdown; multiphoton absorption; electron heating; lattice heating; electron-phonon inter- action; hot-electron transport.

1 . Introduction

For several decades the phenomenon of dielectric breakdown in wide-band-gap insulators has been the subject of

intense research in such different fields as the development of insulation in high power electrical equipment [1], of

passivation layers in electronic devices [2], and of high power laser optics [3]. Due to the great technical importance of

all these fields, the interest in the breakdown phenomena was kindled mainly by the desire to find materials which can

withstand the voltage-, electrical field- or power-levels required for the specific technical applications. In all fields,

breakdown manifests itself in a sudden, often totally destructive and irreversible material change. For a long period of

time it was not possible to detect any precursors which would actually announce the catastrophic event. Therefore it is not surprising that in all technically relevant fields a vast literature exists, in which some sort of experimentally determined damage threshold, as the only easily measurable quantity, is taken as a test for various existing breakdown models.

528
In retrospect, this approach, justified from an engineering point of view, has actually provided little insight into the

fundamental process leading to breakdown. By analyzing the literature, it is indeed possible to find theories - developed on the premise of a perfect intrinsic solid - which "predict" the highest, at the time available, breakdown strength as the

"intrinsic" limit of the insulator under consideration. As purer solids and more perfect device structures become available, we are now in a situation where extrinsic breakdown (not due to the behavior of the perfect, ideal solid, but triggered by impurities, structural imperfection, etc.) is eliminated to a large extent. Therefore it has become possible to study dielectric breakdown not only as a sudden, statistically predictable destructive event but to investigate precursors

to catastrophic breakdown such as electron heating [4-14] and lattice heating [3,15-19] well below the catastrophic

damage threshold. A second important aspect which led, as we will review in this paper, to a new understanding of

dielectric breakdown consists of the application of novel experimental techniques both in the field of DC [4-14] and laser [15-19] breakdown. Additionally, the development of new, more complete theories for the electron-phonon [20-23] and

photon-electron-phonon interaction [24] as well as the application of advanced Monte Carlo techniques [21-23] to the

electron transport problem in insulators have significantly contributed to this recent development.

The title of this contribution is intentionally provocative for the following reasons. The so called "avalanche breakdown" model in all its modifications has been, and still is, widely believed to be the ultimate and right answer to the

dielectric breakdown phenomena in each community involved in dielectric breakdown in wide-band-gap insulators. This

view point requires serious revision, since convincing experimental evidence has been gathered over the last years which

shows that avalanche formation is not necessarily the limiting process for the dielectric strength of wide-band-gap

insulators.

Laser experiments [3,15-19] have shown that in specially purified materials significant lattice heating can be detected

at power levels as low as half the power for catastrophic breakdown. With increasing pulse power, the measured lattice heating becomes so strong that the insulator actually heats up close to its melting point in the center of the focal volume

and thermo-mechanical destruction becomes inevitable [15-18]. Such a gradual energy deposition is totally incompatible

with the avalanche breakdown model but can be consistently explained if virtually all lattice heating occurs via nonlinear

absorption of laser photons by multiphoton excited free carriers. Measurements of free-carrier heating in a two-pulse

experiment [19] directly confirm that lattice heating is indeed due to photon absorption by free carriers and the subsequent release of the excess kinetic energy to the lattice by phonon excitations. Again, catastrophic failure occurs when

the temperature in the insulator approaches its melting point. These same results have been found in various ionic crystals

as well as in Si02. In the field of DC breakdown, the understanding of dielectric breakdown has been changed in a similarly dramatic

way [1,4-14]. In Si02 with MOS(metal-oxide-silicon)-structures no indication for avalanche formation can be found

up to DC fields as high as 1 6 MV/cm and breakdown has to be considered as an extrinsic event (mostly triggered by local

o imperfections of the MOS-structures), at least up to an oxide thickness of 500 A. Direct measurements of the hot

electron distribution at high electric fields [4-10] show that the electron distribution gets significantly heated at fields well below that for destructive breakdown. But the electron distribution is found to be stabilized well below the

ionization threshold and an "average" electron cannot ionize the solid. The experiments, however, do not exclude that

ionization and multiplication might indeed be the limiting factor in even more perfect MOS-structures or in bulk SiO,

samples under DC conditions. An estimate of such a limiting field can be obtained from independent measurements of

electron-phonon scattering rates [13] and subsequent simulations of electron heating using classical Monte Carlo techniques [21-22]. This procedure shows that the interaction of "hot" conduction band electrons with the Si02 lattice is so strong, that impact ionization is not expected for average electrons up to fields of 25 MV/cm, not only in thin oxide

films but also in bulk Si02

.

It is the purpose of this paper to review these recent novel experiments in different fields which have actually led in- dependently to the conclusion that the role of avalanche breakdown has been overestimated as a limiting factor for the

dielectric strength of wide-band-gap insulators both at optical frequences and under DC conditions.

However, we want to stress here that with the present bulk of experimental data it would be hasty to claim that avalanche breakdown is irrelevant for all wide-band-gap insulators. The critical field or power levels for avalanche breakdown under given experimental conditions (temperature, laser frequence, etc.) entirely depends on the properties of the

solid via the electron-phonon interaction. It is this interaction which prevents runaway to energies high enough for impact ionization below some critical field. The crucial question simply is whether this interaction is weak enough to allow

multiplication before some other fundamental or simply extrinsic process leads to breakdown. For DC breakdown, in possibly all wide-band-gap insulators studied, one can argue that there is no solid experimental proof that any published

breakdown fields represent an intrinsic threshold for avalanche formation. In laser breakdown a similar statement can

be made, but here, at least in a few cases, strong evidence for another fundamentally limiting process has now been es- tablished.

The paper is organized as follows. In chapter 2, the results of single pulse laser experiments in the prebreakdown re- gime will be presented. In chapter 3, we review the electron-phonon interaction concepts in wide-band-gap insulators

and show some results of Monte Carlo simulations to illustrate the consequences of the new understanding of free

electron heating under high field conditions with Si02 as an example. In chapter 4, an experimental technique to determine electron-phonon scattering rates in a zero field experiment and results for SiO? will be presented. Finally, we

529
summarize the experiments which give direct access to the hot electron distribution under high DC fields. We only briefly

outline the basic ideas of the experimental techniques and put more emphasis on the discussion of the results in order to outline the crucial role of a correctly chosen electron-phonon interaction scheme. The reader interested in experimental

and theoretical details will find references for more detailed publications in each chapter.

2. Interaction of single high-power laser pulses with wide-band-gap

insulators in the prebreakdown regime

The investigation of single-pulse laser damage discussed here has been ongoing for approximately the last 10 years, and the most significant results of the experimental program have been presented in detail at the 1986, 1987, and 1988

Boulder Laser Damage Symposia. We wish here to only briefly present the results as a unified discussion for purposes

of review. A comprehensive review is available in Ref. [3]. Experiments were performed in all cases in specially purified alkali halide crystals prepared by the University of Utah

Crystal Growth Laboratory. The laser system produced single pulses of approximately 80 psec (half width at 1/e) at 532 nm and 100 psec at 1064 nm. Data from photoacoustic and luminescence measurements were obtained only for

pulse energies below the damage threshold, i.e., only prebreakdown processes leading up to damage were monitored.

Two types of experiments were performed in the prebreakdown regime to measure the absorption of laser beam energy, and these measurements were compared with models including processes that are known to occur in alkali halides

following electron-hole pair production. Two alternative models for free-electron heating were investigated. The dif- ference in (theoretical) heating efficiency of these two lead to conclusions regarding the relative importance of primary

defects in single shot laser damage.

Calibrated photoacoustic measurements of prebreakdown energy absorption from 532 nm laser pulses in NaCl re- vealed that the primary process for the interaction is four-photon absorption. In figure 1 the peak temperature induced

in the beam focal volume by a single pulse of peak photon flux density, Fp

, is displayed. These results are compared with

three models of the interaction.

They account in detail for the creation and recombination of e-h pairs, creation and annihilation of primary defects

(Vk , STE, STH, F), and the resulting heating of the lattice. We allow for two possible pair generation processes, multiphoton absorption and impact ionization. The processes are depicted in figure 2. Generally, electrons are elevated

to the conduction band either from the valence band or from ionized defects (STE's, F-centers). Free valence band holes

are trapped rapidly, but may be freed again by hole absorption, then quickly retrapped. The net result is heat deposition

of per event. Vk centers may orient themselves into directions relatively inert to the polarized laser light, thus the

hole absorption does not occur for each Vk center throughout the duration of the laser pulse.

F-center formation is assumed to occur through the triplet STE state at high temperatures. This effect is included

or excluded as to its interest in the particular experiment modeled.

The two treatments of free-carrier heating are called (1) the free-electron model, due to Epifanov et al. [24] a nonlinear absorption of electromagnetic energy by fairly energetic conduction electrons and (2) the polaron model, wherein

low energy conduction electrons which are strongly coupled to lattice ions absorb photons linearly. The first is vastly more efficient theoretically, but there was no a priori reason to prefer one of these unproven models. For completeness,

we also compared the data with a model including electron-impact ionization avalanche production of e-h pairs. Full

details of these models are available in Ref. [15].

The calculations are superimposed on the data in figure 1. Both treatments of free carrier heating satisfactorily represent the measurements. The four-photon absorption cross section is ambiguous because the photoacoustic effect is a

result of total absorbed energy. Allowing for secondary absorption processes (in addition to four-photon absorption)

without a preference for one of the two alternatives, it cannot be determined which value of aw is most correct, since

both are satisfactory. We can see, however, that if model (1) is correct, then fewer electrons, and therefore, fewer primary defects are required to produce the effect. In fact, for model (1), « 92 % of all energy absorption is via free electron heating. In model (2) defect absorption, primarily Vk center absorption, accounts for most of the heating (>

50 %). Further, the avalanche process does not account for the data.

In order to discover which model is most correct, and thus the importance of primary defects, it is necessary to model

similar measurements with a known value of aw. This was accomplished using the self-trapped exciton recombination

luminescence or STERL technique.

The STERL technique of measuring multiphoton excitation takes advantage of this intrinsic radiative relaxation of

electron-hole (e-h) pairs in alkali halides. Since the process is intrinsic, the total light signal output is a measure of the

number of e-h pairs generated in an excitation event. This then represents an improvement in measuring multiphoton

cross sections over the photoacoustic method because, in a limited range of incident pulse intensities, the response of the

sample to incident ionizing radiation is proportional only to the total number of excited carriers. This proportionality

falls off when the crystal temperature exceeds that at which non-radiative recombination processes become important.

This effect was used as a thermometer to determine lattice heating in KBr when pulse energies were increased beyond

530
500

300

100

<

50

30

10

ii

ii *'

• iii

-

A *T

II •

ll

1

S m m

J

m

I i

1

It

II - II

1 IP

It

1

1 Ik

1*

II

1

1 10 20

Fp(10

29

photons/cm2 sec)

Figure 1. Composite double-logarithmic plot of the calculated temperature increase at the focal point vs incident energy

of 532 nm pulses for two NaCl samples. The solid line is that calculated with the four-photon-polaron model, using

a

14

' = 2x1 0

_1I3c»?8 sec

1

; the slope is « 3.7. The dashed line is that obtained with the four-photon-free carrier model,

using a'

4

' = 1 .5x\0- ]Ucm* sec1

; the slope is 3.93. The broken line was obtained with the avalanche-free carrier model,

using initial carrier density Hc0 «7jd0locm-\

the level where thermal quenching of the luminescence begins. Armed with the value of a

<41 obtained at low pulse intensities, the multiphoton-polaron and -free electron theories of energy absorption were compared with data, revealing

that the polaron-defect absorption is simply too weak to account for the observed temperature increase, leaving the

multiphoton-free-carrier model as a complete, satisfactory model of prebreakdown energy absorption. Further, temperature data indicate that the highest intensity nondamaging laser pulses induced heating in KBr to only a small amount

below the melting point. These measurements could all be explained and modeled without any necessity of including

avalanche generation. In fact, these measurements and calculations preclude the role of avalanche generation at least up to the melting point, even though electron densities greater than Wem-3 are present. These results were verified

in Kl (three photon band gap at 532 nm). Details of the measurements and computations pertaining to KBr are available

in Refs. [16- IX]. Results of the model calculations, compared with data, are presented in figure 3. Only the tr-band of

the STE emission was monitored due to interference of scattered laser light with the 77-band.

The conclusion reached in the STERL study was bolstered by a third experiment. Pulses of 266 nm laser light were

used to create e-h pairs in NaCl and SiO, (via 2-photon absorption), and photoacoustic signal was monitored as a function of coincident pulse energies of 1 064 nm laser light, shown in figure 4. Details are presented in Ref. | 1 9 1. Since

this wavelength is too long to create significant numbers of c-h pairs at these relatively low intensities, the only mechanism by which absorption could take place is via conduction carriers. The nonlinear nature of the resultant signal verified the correctness of the free-electron treatment of the conduction carrier absorption, thus relegating the participation

of defects to a minor or possibly insignificant role in the single shot damage process. Exaggerating their roles in the

model calculations showed that the models would cease to represent the data. Therefore, we conclude primary delects

play at most a minor role in single pulse laser damage of pure alkali halidcs.

531
Figure 2. Processes considered in the model are production of electron-hole pairs by four-photon absorption, trapping

of holes to form Vk centers, trapping of electrons by Vk centers to form self-trapped excitons (STE), and F-center production. Free carrier absorption, STE excitation, and defect ionization, and electron-hole recombination are also considered.

1000 F

inco

o

-C

a. ° 100 -

0)o

c

a>o

a>cE3

«

O

oO

10

29 2 Peak Flux (10 photons/cm sec)

Figure 3. Calculated (solid and dashed lines) and measured (A) spatially- and temporally-integrated a luminescence

emission from KBr as a function of the peak photon flux in 532 nm laser pulses of 100 psec duration (1/e intensity

halfwidth). The solid line is obtained using the free carrier heating model, while the dashed line is based on the polaron

heating model. This luminescence is induced by four-photon exciton generation with subsequent radiative recombination

of electrons and V. centers.

The principal conclusions reached in these experiments are that, by monitoring prebreakdown processes up to the

damage threshold, no generation of an electron impact ionization is observed, nor need it be invoked to explain the single

532
shot damage process. The principal avenue of energy absorption by the solid is photon absorption by multiphotongenerated free electrons. This was also observed in experiments at 1064 nm, where the conditions of high free-electron

density and high field amplitudes failed also to show any evidence for avalanche formation, while revealing the predicted

behavior for free electron absorption.

The free-electron theory of laser energy absorption by conduction band carriers in wide gap insulators, as developed

by Epifanov et al. [24], concerns primarily the role of acoustic phonons in the scattering processes required for an

electron to gain energy in an alternating electric field. We make no claims to have studied the details of this interaction

for high frequency alternating fields, but in the following it is shown that in dc electric fields the acoustic phonon scattering mechanism prevents the formation of electron avalanche in Si02 at electric field strengths beyond the breakdown

threshold. For purposes of comparison, we state measured breakdown threshold intensities [3] obtained in samples

which demonstrated multiphoton absorption up to damage (i.e., samples which showed no evidence of avalanche formation at damage):

KI IB = 15GW/cm2

, EB = 6 MV/cm

KBr IB = 245GW/cm2

, EB =\Q.9MV/cm

NaCl IB = 145GW/cm2

, EB = 19 MV/cm

Breakdown field values are the amplitudes, not rms values.

Figure 4. Photoacoustic signal as a function of the energy of the 1064 nm pulse obtained in NaCl and SiOn with a constant pump pulse (266 nm) energy. It clearly shows a nonlinear dependence on the energy of the heating pulse. The solid

line is the fit to the experimental results [19].

3. Theory of high field electron transport

In this section, we will shortly review the present theoretical understanding of hot electron transport in wide bandgap-insulators [20-23]. We will focus the discussion on Si02 because Si02 is at present the best understood wideband-gap insulator as far as hot electron transport goes and also because most of the experiments reviewed in this paper were obtained in this material.

Because of the wide band-gap of in Si02 (£'gap w9 eV), an electron in the conduction band only interacts with the

phonon modes over a wide energy range. Only at energies £kin > 9 eV can electronic excitation and electron-electron

scattering come into play. Consequently, any prediction about the behavior of a conduction band electron under the

action of an applied electric field crucially depends on the correct modeling of the electron-phonon interaction over the

whole energy range from near thermal energies up to typically 10 eV. Any prediction of an electron-avalanche theory

entirely depends on the validity of the chosen electron-phonon interaction theory. All relevant phonon modes and the correct energy dependence of these scattering processes are crucial. To illustrate this point, we give a short historic summary of the understanding of breakdown in SiO,. It is interesting,

that basically the same historic development can be seen in the field of laser breakdown. The present understanding of

533
hot electron transport in Si02 is indeed very similar to that proposed by Sparks et al. [20] in the context of avalanche

breakdown in alkali halides as early as 1981. In this sense, the following discussion is indeed very general.

Until 1982, transport in SiO, was believed to be dominated by interaction of the electrons with only the polar-phonon

modes of the lattice atoms through inelastic scattering [25,26]. The electrons were thought to lose the energy gained

from the field in large amounts (0.153 and 0.063 eV for the dominant optical phonon modes in Si02 ) through the cre- ation of these oscillations of the lattice atoms every 1-2 A. Electrons were believed to stay at thermal energies, near the

bottom of the conduction band, and to gain significant amounts of energy from the applied filed only near destructive

breakdown of SiOz which typically occurred around 8 MV/cm. Destructive breakdown was believed to occur when the

electrons gained enough energy to cause impact ionization which in turn caused the current density to become large

enough to trigger melting and vaporization of the oxide. However even in the 1970's, there were some problems with this picture. Both experimentally [27-29] and theoretically [30], evidence was brought up, that the LO-phonon scattering

length of 1-2 A might in reality be a factor of 10 larger. Consequently, the LO-phonons could stabilize the electron en- ergy distribution only up to 1.5 MV/cm and LO-phonon runaway would occur at this low field [30]. After 1982, our understanding of high-field steady state transport in Si02 began to change as direct studies of charge carrier energies

[4-14] became possible and improved Monte Carlo techniques [21-23] were applied to study the transport of conduction

electrons. From these studies a new picture for carrier transport in Si02 evolved. It became clear that the motion of

electrons in the oxide layer is completely controlled by their interaction with the lattice and not by electronic excitations or defect and/or trapping states in the forbidden gap. Energy gained by the field is still lost mostly in large amounts to the polar modes. However, the non-polar phonons (quasi-elastic, large-angle scattering) help to stabilize the electron

energy at high fields. They cause large angle scattering and therefore longer meandering paths characteristic of

dispersive transport. Electrons stay at near-thermal energies only at fields lower then « 1.5 MV/cm. At larger fields,

the distribution heats up to a steady state condition where the average energy can be as large as 6 eV at the highest fields attained before destructive breakdown occurs. Before the work of Sparks et al. [20] and Fischetti et al. [21-23], the importance of the non-polar modes at high fields had been mostly neglected in insulators. This resulted in the incorrect idea

that most electrons in SiO, and alkali halides were near thermal energies until breakdown, where the electrons accelerated to energies greater than that of the band-gap and caused impact ionization.

0 1 2 3 4 5 6

ELECTRON ENERGY ( eV )

Figure 5. Electron-phonon scattering rates in Si02 as a function of electron energy at room temperature. Notice the

strong increase of the acoustic-phonon scattering (nonpolar, TA, LA) rate with increasing electron energy. It is this interaction that stabilizes the electron energy distribution at high fields.

In figure 5, calculated electron scattering rates for the various phonon modes in a spherical-band model are shown.

These rates reflect the present understanding of electron-phonon interaction in Si02 and are believed to be qualitatively

true for polar inorganic and organic insulators in general [13]. Formal discussions of the calculation and the correct choice of electron-phonon coupling constants, the role of the conduction band structure, the electron-phonon interaction

scheme etc. can be found in Refs. [21-23]. See also Sparks et al. [20] as well as Porod and Ferry [31] and Zakharov and

Fiveisky [32]. All hot electron transport measurements presented below can be consistently explained with the scattering

rates shown in figure 5. The polar electron-phonon interaction (dashed line in fig. 5) is due to the coupling between the

electron and the dipole field with the LO-phonons of the crystal. Because of the Coulombic nature of the interaction, it shows a 1/k dependence (k is the electron wave vector) and is most important at the energy of the LO-phonon modes

[25]. In contrast, the scattering rates for the non-polar collisions with the acoustic phonons (transverse (TA) and longitudinal (LA) acoustic dotted line in fig. 5) and the transverse-optical (TO-)phonons (dashed-dotted line in fig. 5) show

534
a completely different energy dependence. Non-polar scattering becomes more important as the electron gains energy.

The role played by the two distinctly different scattering processes for high field transport can be seen most clearly in figure 6. In this figure, the electron trajectories using classical Monte Carlo simulations are shown. At low fields,

"streaming-type" transport is observed where most of the electronic paths are in the direction of the applied field. The energy distribution is stabilized near thermal energies by LO-phonon emission. At fields significantly above the

threshold for electron heating, dispersive transport dominates due to the large angle scattering caused by the acoustic

phonons. Although the scattering rate for the polar phonon modes is decreasing with increasing electron energy,

the distribution is stabilized against velocity runaway at energies of 2-6 eV by the rapid increase in the total path length

due to the increasing scattering rate of the non-polar phonon modes. In the following sections, various experimental

results which directly confirm this new understanding of electron transport in SiO, will be summarized.

0 10 20 30 40 50 0 10 20 30 40 50

DISTANCE ALONG THE FIELD (nm) DISTANCE ALONG THE FIELD (nm)

Figure 6. Trajectories of several electrons in Si02 in real space at two values of the electric field at room temperature.

The low-field (1 MV/cm) trajectories show that transport is dominated by forward polar-scattering with LO-phonons.

At high fields (8 MV/cm), the non-polar scattering at large angles with acoustic phonons randomizes the direction of

motion and effectively increases the actual total path length of the electron. However, the large energy losses of the hot

electrons are still dominated by polar, inelastic scattering with LO-phonons.

4. Experimental determination of energy dependent electron-phonon scattering rates

As outlined in the previous chapter, the prediction of any breakdown model depends crucially on the accurate knowledge of the energy dependent electron-phonon scattering rates for all relevant scattering processes. Over recent

years, a technique called Internal Photoemission for Transport Analyses (IPTA) has been developed and applied to various insulators in order to provide exactly this information [11-14,33]. In simple terms, the technique operates as follows. Electrons of a well defined energy EWm are injected via internal photoemission into thin dielectric overlayers on

metal substrates. (In practice, a broad energy distribution with a well defined maximum energy will result from this process. But the simplification of monoenergetic injection is irrelevant for the presented discussion.) The metal simply

acts as electron source and supports the thin films. The electron kinetic energy, £kin , can be varied via the photon energy

used for injection. During transport through the film, the injected hot electrons will be subject to phonon scattering

(change direction, gain and lose energy) and eventually a certain fraction is emitted into vacuum. The number of emitted

carriers as well as their energy distribution yields significant information on the electron-phonon interaction in the solid

[14]. In the experiment, the electron energy distributions of emitted electrons are measured as a function of overlayer

thickness and the scattering rates are extracted from the variations of the metal source distribution with thickness. This

is done by solving the Boltzmann transport equation for the substrate overlayer geometry with appropriate boundary

conditions either analytically [14] or with Monte Carlo techniques [21]. Independent of the details of the problem's

solution it turns out that the injected current density, J0, decreases exponentially with film thickness, d,

J(d,Ekin ) = J() exp( -d/\c ") . The effective scattering length, A

cfr (Ekin ) depends on both LO-phonon and acoustic

535
phonon scattering via 1/Acr( = l/A LO (l/A'° +1/Aac

) . Here A

LO and A

ac are the inelastic and elastic scattering lengths

at energy Ekin due to LO- and acoustic phonon scattering, respectively. These quantities are related to the scattering

rates, 1 /t, discussed in the previous section via 1/t=v/A, where v is the electron velocity.

0

1 1 1 1 10~ 1 10° 10 1 102

KINETIC ENERGY (eV)

Figure 7. Experimentally determined effective scattering lengths, A

eff

, in Si02 for various electron kinetic energies

[14,32]. The scattering lengths were measured with low energy electron transmission experiments through thin Si02 films. The scattering lengths for LO-phonon scattering, A

LO

, acoustic phonon scattering , A

ac (and electron-electron

scattering, A

c c ) as obtained from electron-phonon interaction theory are shown for comparison. The solid line shows

how the various scattering contributions combine to yield the effective scattering length, A

efr

, measured in the experiment.

Experimentally determined effective scattering lengths in SiOz are shown in figure 7 for electron energies ranging

from 0.1 eV to 100 eV. As can be seen, the effective scattering lengths show significant variations with energy and reflect the energy dependence of the various scattering processes discussed before. The solid line is calculated from the

electron-phonon scattering rates shown in figure 5 using the equations mentioned above. As can be seen, the experiment

confirms the calculation in detail. The various curves in figure 7 show how LO-phonon, acoustic phonon and electronelectron scattering contribute to A

eff at various energies. It can be seen that quasi-elastic, large-angle, acoustic phonon

scattering controls the transport of electrons in the energy range from 2-13 eV. Only at lower and higher energies do

the inelastic processes of LO-phonon emission and impact ionization (and other electronic excitations), respectively,

dominate. The observed strong increase of the acoustic scattering length above typically 13 eV energy is still subject to discussion. But it is most probably due to the fact that with increasing electron energy the electron scattering with

acoustic phonons becomes more and more forward directed and thus reduces the momentum transfer per collision

[23,33]. It can be shown that such a transition from isotropic to forward scattering yields an apparently smaller scat- tering rate in a transmission experiment [14].

5. Experimental determination of electron energy distributions at high fields

The most powerful techniques used to study carrier heating under the action of an electric field in Si02 are electroluminescence, carrier separation and vacuum emission [4-1 1]. Conceptually, all these experiments are very similar to the one described in the previous section. Electrons are injected (by Fowler-Nordheim tunneling) into thin layers

of SiO, on one side of a MOS-capacitor structure. Under the action of the applied field these electrons are then trans- ported to the opposite interface where there energy is measured in various ways. Electroluminescence and carrier separation measure only the average energy of the hot electron distribution. In contrast, vacuum emission measures the

energy distribution itself. Each technique has its limitations, but their combination yields a quantitatively consistent

picture for transport in Si02

. The electroluminescence [4,5] technique measures the spectra of light emitted from thin metal gates of MOS

capacitor structures. These photons are due to radiating surface-plasmon-modes stimulated in the metal layer by hot

electrons impinging on the oxide-metal interface after transport through the Si02 film. The information on the average

electron energy in the oxide can then be extracted from the photon energies.

536
Carrier separation [6,7,10] measures the number of electron-hole pairs produced in single-crystal silicon by hot

electrons injected after transport through the Si02 overlayer. The number of electron-hole pairs can be converted into

the average energy of the incoming hot electron energy distribution.

Vacuum emission [6,8,10] directly measures the hot electron distribution ejected into vacuum through the thin metal

gate of a MOS capacitor. Various types of standard electron energy analyzers can be used to collect and energy analyze

these hot electrons. Average energies can be directly calculated from the measured distributions.

Figure 8. Average electron energy versus anode field from the experimental data (squares: carrier separation, circles: vacuum emission) in comparison with the result of Monte Carlo simulations (solid line). At fields below 1.5 MV/cm,

the electron energy distribution can be stabilized close to thermal energies by LO-phonon scattering. Around 2 MV/cm

LO-phonon runaway occurs, but the energy distribution gets stabilized against runaway at 2-6 eV by large angle acoustic

phonon scattering via Umklapp processes.

In figure 8, average electron energies obtained by the three techniques mentioned above are shown as a function of

applied field. The solid line shows the result of a Monte Carlo simulation as described in chapter 3. As can be seen, the

Monte Carlo simulation reproduces the result in detail. Below 1 MV/cm the average electron energy is of the order of

kT. The energy distribution is efficiently stabilized by LO-phonon emission. At about 1.5 MV/cm the average electron

energy rapidly increases to about 2-4 eV. This phenomena known as "LO-phonon runaway" simply expresses the fact,

that the electrons gain more energy from the field then they loose to the lattice via LO-phonon excitation. However, the

average electron energy gets stabilized around 4 eV and only slightly increases towards higher fields. At all fields measured, the energy of an average electron stays well below the ionization threshold. As previously discussed, this stabili- zation is due to the rapid increase of the acoustic phonon scattering rate with increasing electron energy.

To conclude, let us present some results, which show that there are still some problems left to solve. Only the laser experiments were performed in bulk samples. All other experiments are done in thin film structures and it is not straight

forward to extrapolate results obtained in thin film experiments to breakdown results in the bulk oxide.

This is illustrated in figure 9, where full energy distributions as obtained by the vacuum emission technique are shown

for different electric fields and oxide thickness. As can be seen, the energy distributions start to develop a high energy

tail both with increasing fields and increasing oxide thickness. Both trends would at least qualitatively be consistent with

the ideas of avalanche breakdown. One would indeed expect, that energy distributions as shown in figure 9 lead to significant electron multiplication in a bulk sample. It is to date not entirely clear, whether these tails reflect an intrinsic property of Si02

. First, the high energy tails cannot be reproduced in classical Monte Carlo simulation [21,22] using

scattering rates compatible with the internal photoemission experiments presented above [13]. Second, above 20 eV o electron energy, the inelastic mean-free path is less than 10 A ( see fig. 7) and involves energy losses of at least 8 eV

per collision. These numbers show that the probability for electrons to have energies of 20 to 40 eV is extremely small. A field of 100 MV/cm (compared to the applied field of 10 MV/cm) would be required to keep an average electron

o o above 20 eV energy. A collision-free flight of 300 A (compared to a scattering length of 10 A) would be needed in order

to bring an electron out of the predicted Monte Carlo distribution up to 40 eV energy. Thirdly, it is believed that the

electrons evolving into vacuum are emitted through small holes (inter-grain boundary cracks, missing grains, etc.) in the

top metal contact of the MOS-capacitor. These imperfections in the MOS-structure lead to field distortions and allow some electrons to gain additional energy in vacuum. It can be shown that electrons emitted through the center of a

o o missing grain in the top metal contact (modeled as a circular hole with 500 A diameter on a 1500 A thick oxide with an

applied field of 10 MV/cm) can pick up an additional energy of typically 20 eV in vacuum. These arguments indicate

537
Energy (eV) Energy (eV)

Figure 9. Comparison of electron energy distributions for varying oxide thickness (500-1500 A) determined using vac- uum emission for several electric fields at room temperature. Although the average energies deduced from these dis- tributions are not strongly dependent on oxide thickness, a few "lucky" electrons out in a high energy tail are not

stabilized against run-away. It is, at present, not clear whether this tail reflects an intrinsic behavior of Si02 or is due to imperfections of the oxide films (channel type defects) or possibly a side effect of the experiment (see text).

that the high energy tail could be an artifact of the vacuum emission technique, but this still must be unequivocally shown

experimentally. Although we are cautious in drawing conclusions on the role of electron multiplication for breakdown

in bulk samples, the Monte Carlo results would still be valid even for very thick oxides.

6. Conclusions

It was the intention of this paper, to put into a common framework recent experimental and theoretical effort from

different fields but all related to the phenomena of dielectric breakdown in wide-band-gap insulators. Most of these

efforts were, at least in an early stage, to some extent stimulated by the search for a experimental proof of the

avalanche-breakdown model. Ironically, the experiments performed to achieve this goal now rather suggest that the

avalanche concept for breakdown might play a more minor roll for solid state breakdown in wide-band-gap insulators

than believed by most researchers for decades. In several ionic crystals and in SiOz

, the reviewed novel laser experiments

give strong evidence that single pulse induced destructive breakdown at 532 nm wavelength is not initiated by electron

multiplication and electron-avalanche formation, but is due to lattice heating by nonlinear photon absorption of free electrons generated by multiphoton absorption. In Si02

, the presented results lead to a new understanding of hot electron

transport. The existing experimental data conclusively eliminate the avalanche breakdown model for an oxide thickness

o smaller than 500 A. Classical Monte Carlo simulations using experimentally determined energy dependent scattering

rates for the various electron-phonon interactions do not predict any electron multiplication up to fields well above 20

MV/cm even in bulk SiOz.

In the field of DC breakdown, especially with respect to MOS technologies, this is actually good news, since these

new results clearly show that Si02 can indeed withstand the gradually increasing field strength inevitably occurring with

device miniaturization (if the device fabrication and oxide quality can be improved accordingly). In the field of laser breakdown however, the new results show for the first time that there exists a mechanism inherent to the laser-field- insulator interaction which is different from avalanche breakdown but nevertheless fundamentally limiting the power

density an optical dielectric can withstand.

7. References

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in; Festkorperprobleme (Advances in Solid State Physics), Volume 27, 223-240, P. Grosse (ed.), Vieweg,

Braunschweig 1987

[2] P. Solomon, Breakdown in silicon oxide - A review, J.Vac.Sci.Technol. 14, 1 122 (1977)

538
3] S. C. Jones, P. Braunlich, R. T. Casper, X. A. Shen, P. Kelly, Recent progress on laser-induced modifications and

intrinsic bulk damage of wide gap materials, Opt. Eng. 28, 1039 (1989).

4] T. N. Theis, J. R. Kirtley, D. J. DiMaria, D. W. Dong, Light emission from electron-injector structures,

Phys.Rev.Lett. 50, 750-754 (1983)

5] T. N. Theis, D. J. DiMaria, J. R. Kirtley, D. W. Dong, Strong electric field heating of conduction-band electrons in Si02 , Phys.Rev.Lett. 52, 1445-1448 (1984)

6] D. J. DiMaria, M. V. Fischetti, E. Tierney, S. D. Brorson, Direct observation of the threshold for electron heating

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7] D. J. DiMaria, T. N. Theis, J. R. Kirtley, F. L. Pesavento, D. W. Dong, S. D. Brorson, Electron heating in silicon dioxide and off-stoichionetric silicon dioxide films, J.Appl.Phys. 57, 1214-1238 (1985)

8] S. D. Brorson, D. J. DiMaria, M. V. Fischetti, F. L. Pesavento, P. M. Solomon, and D. W. Dong, Direct measurement

of the energy distribution of hot electrons in silicon dioxide, J.Appl.Phys. 58, 1302-1313 (1985)

9] M. V. Fischetti, D. J. DiMaria, L. Dori, J. Batey, E. Tierney, J. Stasiak, Ballistic electron transport in thin silicon dioxide films, Phys.Rev. B 35, 4404-4414 (1987)

10] D. J. DiMaria, M. V. Fischetti, M Arienzo, E. Tierney, Electron heating studies in silicon dioxide: Low fields and

thick films, J.Appl.Phys. 60, 1719-1726 (1986)

11] P. Pfluger, H. R. Zeller, J. Bernasconi, Hot-electron transport in polymeric dielectrics, Phys.Rev.Lett. 53, 94-97

(1984)

12] E. Cartier and P. Pfluger, Transport and relaxation of hot conduction electrons in an organic dielectric, Phys.Rev.

B34, 8822-8834 (1986)

13] E. Cartier and P. Pfluger, Experimental determination of energy dependent inelastic and elastic scattering rates of

hot electrons in large bandgap insulators, Physica Scripta T23, 235-241 (1988)

14] J. Bernasconi, E. Cartier, P. Pfluger, Hot-electron transport through thin dielectric films: Boltzmann theory and

electron spectroscopy, Phys.Rev. B38, 12567-12581 (1988)

15] S. C. Jones, A. H. Fischer, P. Braunlich, P. Kelly, Prebreakdown energy absorption from intense laser pulses at 532

nm in NaCl, Phys. Rev. B37, 755-770 (1988).

16] X. A. Shen, S. C. Jones, P. Braunlich, P. Kelly, Four-photon absorption cross section in potassium bromide at 532

nm, Phys. Rev. B36, 2831-2843 (1987).

17] X. A. Shen, P. Braunlich, S. C. Jones, P. Kelly, Investigation of intrinsic optical damage in potassium bromide at 532 nm Phys. Rev. B38, 3494-3504 (1988).

18] L. Simpson, X. A. Shen, S. C. Jones, P. Braunlich, P. Kelly, in Proc. 20th Annual Symposium on Optical Materials

for High Power Lasers (Boulder, CO, Oct. 1988) Natl. Inst. Stand. Technol., in press.

19] X. A. Shen, S. C. Jones, P. Braunlich, Laser heating of free electrons in wide-gap optical materials at 1064 nm,

Phys. Rev. Lett. 62, 2711 (1989).

20] M. Sparks, D. J. Mills, R. Warren, T. Holstein, A. A. Maradudin, L. J. Sham, E. Loh, Jr., D. F. King, Theory of

electron-avalanche breakdown in solids, Phys.Rev. B24, 3519-3536, (1981)

21] M. V. Fischetti, Monte Carlo solution to the problem of high-field electron heating in Si02

, Phys.Rev.Lett. 53,

1755-1758, (1984)

22] M. V. Fischetti, D. J. DiMaria, S. D. Brorson, T. N. Theis, J. R. Kirtley, Theory of high-field transport in silicon

dioxide, Phys.Rev. B 31, 8124-8132 (1985)

539
[23] M. V. Fischetti and D. J. DiMaria, Quantum Monte Carlo simulation of high-field electron transport: an application

to silicon dioxide, Phys.Rev.Lett. 55, 2475-2478 (1985)

[24] A. S. Epifanov, A. A. Manenkov, A. M. Prokhorov, Theory of avalanche ionization induced in transparent

dielectrics by an electromagnetic field, Zh. Eksp. Teor. Fiz. 70, 728 (1976) [ Sov. Phys. JETP 43, 377-382

(1976)].

[25] H. Frohlich, On the theory of dielectric breakdown in solids, Proc.R.Soc.London.Ser. A 188, 521-531 (1947)

[26] D. K. Ferry, Electron transport and breakdown in Si02

, J.Appl.Phys. 50, 1422-1427 (1979)

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, accepted in Phys.Rev. B (1990)

540
Manuscript received

2-12-90

UV Seeding of IR Laser Induced Damage

N.C.Kerr, S.E.Clark and D.C.Emmony

Department of Physics

Loughborough University of Technology

Loughborough

Leicestershire

U.K.

The values of the laser induced surface damage thresholds for

samples of silicon, fused quartz and soda lime glass at both 0.248um

(UV) and 10.6um (IR) are presented. Experiments are then described

where laser damage is induced using crossed UV-IR beams, with varying

time delays imposed between the arrivals of the two laser pulses at

the sample surface. The results of these crossed beam experiments

are then discussed. Order of magnitude reductions in the IR damage

thresholds of silicon and glass are observed when the beams are

crossed, no such reduction is observed for quartz. Further, only in

the case of glass samples is a reduction in the UV damage threshold

seen when the beams are crossed. The reduction in IR threshold on silicon has been seen with delays between the laser pulses of up to

100ns. The reduction in IR threshold for glass was seen for delays

up to 1.2ms. These results lead to a discussion of the role of

so-called seed electrons in the damage process and the mechanisms

operative at the different wavelengths.

Key words: excimer laser damage, CC2 laser damage, crossed laser

beams, seeding electrons.

1. Introduction

The laser induced damage of optical components is the basic limiting factor on the

output power which a laser can develop. With the continuing need for ever mdre powerful

yet compact lasers it is becoming critically important to find more damage resistant

optical materials. As part of the search for such materials it is important to try to

understand the mechanisms for laser induced damage.

Absorbing defects and inclusions, scratches and cracks are usually the dominant

sources of bulk and surface damage respectively and reduce the threshold intensity at

which damage begins. In practically all real situations these defects limit the laser

induced damage threshold.

Intrinsic mechanisms set the ultimate optical strength of a material. Research has

shown that the most probable mechanism for intrinsic breakdown is a mixture of both

avalanche and multiphoton ionization [1]. The lowering of the surface damage threshold

caused by such things as scratches and cracks is thought to be due mainly to the enhanced

electric fields around such structures [2].

541
It is generally accepted that seed electrons which are necessary for the development

of the electron avalanche due to collisional ionization arise from the photoionization of

impurities and defects or the multiphoton ionization of the host atoms in the crystal

lattice. The breakdown threshold is determined by the probability of the appearance of a seed electron within the electric field of the incident laser radiation.

The damage threshold due to avalanche ionization should not depend on the initial

electron concentration nQ as long as n0V>l, where V is the interaction volume. Under

these conditions the artificial injection of excess carriers which can arise from the

photoionization in the material caused by, say, auxilary UV irradiation should not effect

the breakdown threshold since the probability of there already being a starting electron

within the interaction volume is unity. However if nQV<l UV irradiation of the sample can exert a large influence on the damage threshold if the avalanche process is a dominant

damage mechanism since a starting electron is unlikely to be already present within the

interaction volume.

So-called crossed beam experiments, as outlined herein, are of interest in

determining the role of seeding electrons and in attempting to make clear the nature of

the damage mechanism operative at different wavelengths. Such knowledge is of interest

since it allows a fuller understanding of the laser induced damage process.

Previous work [3,4] using crossed beam configurations has centered on using very

tightly focused beams in an attempt to effectively get between defects and inclusions in

the bulk of samples so that intrinsic damage thresholds are being measured and to satisfy

the condition nQV<l. In the work reported here results have been obtained using large

spot sizes on the surfaces of materials. This is because of the constraints placed upon

us by the availability of apparatus and the divergence of the UV laser beam. In a large

spot size situation such as this, extrinsic damage mechanisms are likely to be dominant.

It is felt that such a situation is however a more realistic example of real laser damage

situations. If we consider that for an opaque sample the incident laser radiation

penetrates the sample to a certain skin depth then we can still visualize a focal volume

at the surface. For small focal volumes the starting electrons must be produced by

multiphoton excitation of defect states; whereas, for large focal volumes, such as exist

in this work, conduction band electrons initially present can initiate the avalanche

process

2. Experimental arrangement

The experimental arrangement of the apparatus is shown in figure 1. The damage

facility at Loughborough [5] is based on a Lambda Physik KrF excimer laser working at

248nm and producing nominal 1J pulses of 30ns duration. Control of the on-target excimer

energy is achieved by means of a liquid dye cell attenuator, whilst energy measurement is

by use of a Laser Instrumentation calorimeter. To reach the required fluences a quartz

lens was used to focus the beam, with spatial beam profiling being performed by means of a computer controlled video framestore system which images the fluorescence produced by

absorbing the excimer laser radiation on a suitable glass fluorescer [6]. Using this

system it is possible and is standard practice to quote the peak fluence in the excimer

beam. The laser spot size at the surface is 1.5mm X 1.0mm.

IR laser pulses were obtained from a Laser Applications TEA CO2 laser operating at

10.6um. This gives the usual 100ns pulse followed by the long tail associated with the N2

in the gas mixture. The nominal total laser pulse energy is 5J. The laser output is

focused using a 15cm focal length CaF2 lens and has a typical spot size on target of 3mm X

3mm. Control of the fluence at the target is obtained by using calibrated attenuating

plastic films of varying thicknesses.

542
Damage detection was performed by use of high resolution Schlieren imaging (i.e.

imaging with a suitable stop/filter in the focal plane of the imaging lens) with a CCD

video camera linked to a computer controlled video framestore to enable fast and accurate

processing of the images. The framestore is an Eltime Image III model linked to a Sperry

PC/IT for image processing. Hard copy of video images is obtained from a 3M Dry Silver

Copier. The target area is probed using a lOmW cw HeNe laser. The probe intensity was varied using a rotating polaroid filter.

Triggering pulses for the lasers are provided by a Lyons signal generator which has

two independent outputs. These can be delayed relative to each other and were tailored

to the requirements of the two lasers by a pair of fast transistor switches. The delay

between the trigger pulses are set at the signal generator which has a measured jitter of

less than 10ns. By modifying the CO2 laser trigger input circuitry and careful adjustment

of the thyratron reservoir voltage the jitter on the CO2 laser trigger was reduced to less

than 10ns. The jitter on the excimer laser trigger was so small as to be unmeasurable

using the facilities at our disposal.

The relative time of arrival of each laser pulse at the target was monitored using

beamsplitters and fast detectors suited to each wavelength used. For the IR pulses a silicon beamsplitter and integrating pyroelectric detector was used. For the UV a fused

silica beamsplitter and fast silicon photodiode was used.

The two beams were arranged so that the smaller UV spot was positioned in the center

of the larger IR spot. The imaging optics were arranged so that the IR spot just filled

the image field of the Schlieren imaging camera.

3. Experimental technique

Firstly, by irradiating a series of sites on the sample surface with single UV laser

pulses alone of increasing fluence the UV damage threshold was measured. The damage

threshold for IR laser pulses alone was then measured in a similar manner.

The UV fluence was reduced in measured steps from this damage threshold value and for

each fluence the corresponding IR damage threshold was remeasured. This process was performed on each sample with no delay between the end of the UV and the start of the IR

laser pulses. The fluences of the two pulses were then fixed at values which were seen to

have produced a marked decrease in damage threshold and the delay of the IR pulse was

increased until no effect was observed. The alternate case of delaying the UV pulse

relative to the IR was then investigated to see if any reduction in damage thresholds

could be measured.

Measurements of the magnitude and decay of fluorescence induced in the glass were

also made, the fused quartz exhibited no visible fluorescence. The fluorescence was monitored using a Hamamatsu IP28 photomultiplier tube. This covers the spectral range

185~650nm and has a response time of 2.2ns. The fluorescence was monitored through the

glass slide. The fluorescence spectrum was measured for the glass sample using a scanning

monochromator . The spectrum was constructed by shot to shot excitation of the glass

slide. The signal from the photomultiplier was displayed on a fast oscilloscope with the

input matched to the cable.

h . Results

Figure 2 shows a typical sequence of Schlieren images obtained on a sample of silicon

using crossed UV and IR beams with no delay between the arrival of the pulses at the

543
I

1

sample. Image processing consisted simply of subtraction of an initial image of the

surface and an image after the desired number of laser pulses had been incident at the

surface. Figure 2a and 2b are processed images showing the effect on the surface of 10 UV

and 10 IR pulses respectively. The fluences in each case were set at 75% of the

respective wavelength's single shot damage threshold. Although there are low contrast

artifacts associated with interlacing of video fields visible in these processed images no detectable damage can be seen. Figure 2c is the processed image obtained when the two

beams are crossed. Damage is restricted solely to the region of overlap of the two beams

in the center of the frame. Figure 2d is the processed image obtained if the UV fluence

is reduced to 50% of the surface UV damage threshold. Clearly the damage is still

localised to the region of overlap of the UV beam on the IR beam but the degree and extent

of the damage has decreased.

I The results for the variation of IR damage threshold with additional UV fluence are presented in figures 3, 4 and 5 for silicon, quartz and glass respectively. No time delay

existed between the end of the UV and start of the IR laser pulses. Order of magnitude

reductions in damage threshold are evident for the silicon and glass samples. No

reduction was observed in the damage threshold for the quartz sample.

f i

-

I Figures 6 and 7 show the time dependencies of these effects as the delay between the

end of the UV pulse and the start of the IR pulse is increased. For silicon the influence

of the UV pulse persisted for only 100ns at most before it had no effect in reducing the

IR damage threshold. In glass the effect persisted for between 500ys and 1ms.

•

I

If the IR pulse was incident before the UV pulse no reduction in the UV damage

threshold was observed on samples of silicon and quartz. On glass the damage threshold of

the sample was reduced with the effect persisting for times between the IR and UV pulse of

upto 350ns. Limitations in the trigger electronics meant that this was the largest delay

of its type available. i B j

Fluorescence measurements on glass samples irradiated with UV pulses showed that the

fluorescence was in the form of a wide band covering the wavelengths 375-570nm. The

fluorescence decay time was typically 1.0 to 1.5ms at all wavelengths, see figure 8.

j

5. Discussion

For silicon, as a consequence of k, the complex part of its refractive index, being non zero the UV radiation is totally absorbed. The bandgap of the sample (l.leV) is far

less than the photon energy of the incident light (5.0eV) and so the dominant damage

mechanism would be expected to be electron avalanche in the UV with the initial electrons

being produced by photoionization

At fluences below the UV damage threshold these electrons can only be expected to

exist in the silicon for a short time, a value of hundreds of nanoseconds would not be

unreasonable for heavily Boron doped samples such as those used [7]. If an IR pulse is

incident within this time then these electrons would be expected to seed the IR damage

process. Our results would tend to confirm this with the electrons produced by UV

irradiation existing for up to 100ns. The UV radiation creates a dense electron plasma at

the surface of the silicon. The plasma then absorbs the IR radiation by free carrier

absorption. In the absence of the UV the intrinsic absorption is very low, in the

presence of the plasma the coupling of the IR radiation to the surface is greatly

enhanced. The plasma, heated by the IR cools down and transfers energy to the lattice in

the form of phonons. For small delays between the pulses free carrier absorption probably

dominates due to the heavy absorption of the UV but as the delay increases the plasma

density decreases and the IR coupling decreases. The plasma density is increased for

larger temperature rises due to the increasing absorption coefficient with temperature,

544
and hence for increasing fluences.

For silicon relatively little of the IR radiation is absorbed (imaginary complex

refractive index equal to 1.27 x 10"^) and the photon energy of the IR radiation is only

0.12eV making multiphoton ionization of electrons improbable. The few electrons promoted

to the conduction band by either photoionization or thermal heating of impurity levels

would not be expected to last very long or to significantly influence the threshold for

subsequent UV pulses. These results confirm this since the UV threshold of silicon is not

dependent upon auxilary IR irradiation fluences or time delays. The damage mechanism in

the IR is probably thermal in origin for silicon in the large spot size condition.

For quartz the results clearly indicate that the damage mechanisms at each wavelength

are different. In the IR a thermal mechanism is expected since quartz strongly absorbs

IR. At wavelengths around 10.6um quartz has a resonant absorption region leading to its

strong absorption [8]. In the UV an avalanche mechanism must be invoked to explain the

damage process. The relatively low absorbtion of UV and the large band gap for quartz,

which is greater than 5eV, means that few electrons are expected to be produced by a low

fluence UV laser pulse and hence the seeding effect of UV on IR pulses will be minimal.

It is possible that the number of electrons produced by low fluence IR irradiation is too

low to seed UV damage for fluences up to practically the IR threshold itself.

Glass absorbs both UV and IR radiation. Glass is an extended molecular network

without symmetry or periodicity. The region of transmission for all types of glass is

bounded at short wavelengths by the fundamental absorption edge and at long wavelengths by

IR absorption bands. The UV irradiation obviously promotes a large number of electrons to

the conduction band where they can stay for a long time, as indicated by the long

fluorescence decay which is produced as they relax. Any seeding effect of the UV for IR

pulses would be expected to persist for a time at least as long as the decay time of the

fluorescence. This is confirmed experimentally.

Glass is very impure containing many metal impurities. These can be expected to be

thermally heated by IR pulses and to excite electrons for quite a long time. For this reason subsequent UV pulses have their damage threshold reduced since their damage

mechanism is already seeded by the previous IR pulse.

6. Conclusions

In silicon the damage mechanism for IR wavelengths is almost certainly thermal in

origin. IR radiation cannot directly photoionize electrons unless an improbable number of

steps are assumed. However the presence of a large number of seed electrons created by UV

induced bandgap ionization increases the absorption coefficient for IR radiation and

subsequent electron avalanche. The ability to modify the IR damage threshold by prior

irradiation with UV up to 100ns before the IR pulse indicates that the seeding process has

a lifetime around this value. For UV irradiation the damage mechanism is electron

avalanche since direct photoionization of electrons is possible at these wavelengths.

When the IR pulse preceeds the UV pulse it has no effect on the UV laser damage threshold

in silicon.

In quartz the IR damage mechanism is almost certainly thermal in nature. It also seems that the heating by the IR pulse absorption does not modify the UV damage threshold

up to the level where the IR heating itself causes permanent damage. The UV damage

threshold is almost certainly determined by electron avalanche after multiphoton

ionization

545
In glass the IR and UV damage mechanisms are almost certainly thermal and electron

avalanche in nature respectively for independent irradiation. As for silicon the large

number of electrons excited by a UV pulse significantly modifies the absorption of a subsequent IR laser pulse. Unlike silicon an IR laser pulse can lower the UV damage

threshold by reason of its heating effect at the glass surface.

Acknowledgements

NCK would like to acknowledge the financial assistance of BDH Ltd and SERC.

7. References

[1] A. Vaidyanathan, T. W. Walker, A. H. Guenther, The Relative Roles

of Avalanche Multiplication and Multiphoton Absorption in Laser

Induced Damage of Dielectrics, IEEE Journal of Quantum Electonics,

Vol. QE-16, no. 1, 89-93, (1980).

[2] N. Bloembergen, Laser- Induced Electric Breakdown in Solids,

IEEE Journal of Quantum Electronics, Vol. QE-10, no. 3, p375-386,

(1974).

[3] B. G. Gorshkov, Yu. K. Danilelko, A. S. Epifanov, A. A. Manenkov,

A. M. Prokhorov, A. V. Sidorin, Influence of Additional Ultraviolet

Illumination on Breakdown of Alkali Halide Crystals by CO2 Laser

Radiation, Sov. J. Quantum Electron, 11, (1), p85-86, (1981).

[4] B. G. Gorshkov, A. S. Epifanov, A. A. Manenkov, A. A. Panov,

Studies of Laser-Produced Damage to Transparent Optical

Material in the UV Region and in Crossed UV-IR Beams, NBS Spec.

Publ. 688, US Government Printing Offices, Washington DC, p76-86,

(1983).

[5] S. S. Wiseall, D. C. Emmony, A Laser Damage Facility in the

Ultraviolet, NBS Spec. Publ. 669, US Government Printing

Offices, Washington DC, pl02, (1984).

[6] B. A. Omar, S. E. Clark, D. C. Emmony, D. B. Hollis, M. J. Shaw,

The application of doped glass fluorescers to the recording of

pulsed ultra-violet laser beam profiles, Optics and Laser

Technology, vol. 20, no 4, pl93-198, (1988).

[7] Private communication with Dr R.Howson and Dr D.Sykes, Loughborough

University of Technology.

[8] F. Keilmann, Y. H. Bai, Periodic Surface Structures Frozen into CO2

Laser Melted Quartz, Appl. Phys. A, Vol 29, p9-18, (1982).

546

A sequence of schlieren images of a silicon surface

2a (top left) Processed image showing the effect of 10 UV laser

pulses with a fluence of 75% the single shot

damage threshold

2b (bottom left) Processed image showing the effect of 10 IR

laser pulses with a fluence of 75% the single

shot damage threshold

2c (top right) Processed image showing the effect of a single

pair of crossed UV-IR pulses with fluences as above

2d (bottom right) Processed image showing the effect of a single pair of crossed UV-IR pulses but with

the UV fluence reduced to 50% of the single

shot damage threshold

548
10

6

o

oAV)

CDu

XIu

Hi60

I

PSH

6o

o

siW

CD

>-l

0>60

cd6

cd

Pi

5-

0

0 0.5

Fig 3

uv fluence (J cm )

Variation of IR damage threshold for silicon

with additional UV irradiation

uv fluence (J cm )

Fig 4 Variation of IR damage threshold

for fused quartz with additional

UV irradiation

Fig 5 Variation of IR damage threshold for Fig 6 variation of the probability of dama

glass with additional UV irradiation for differ ing delays between the UV

and IR laser pulses on silicon

549
0 0.5 1 1.5

Time delay (ms)

Fig 7 Variation of the probabilty of damage

for differing delays between the UV

and IR laser pulses on glass

in

0 1.0 2.0

Time delay (ms)

Fig 8 The decay of UV laser induced fluorescence in a glass slide

550
Manuscript received

12-29-89

Measurements of UV Induced Absorption in Dielectric Coatings

M.H. Bakshi, M. Cecere, D.A.G. Deacon, A.M. Fauchet3 )

Deacon Research

2440 Embarcadero Way

Palo Alto CA 94304

(415) 493-6100

a )NSLS, Brookhaven National Lab, Upton NY 1197 3

We discuss the first results obtained from a new testing facility for measuring the induced absorption in multilayer dielectric coatings exposed to intense UV radiation. The absorption loss

is measured in-situ during exposure, as a function of time for various UV photon energies, intensities, and sample materials. The

sample is irradiated by the direct beam from the TOK undulator at

NSLS and measured in-vacuo. The undulator is typically run at K =

1 (so that the harmonic emission is low) to simplify the data analysis. The test chamber is separated from the undulator beamline by

a differentially pumped line which permits the introduction of carbonaceous gases into the chamber at pressures up to 10 -5 Torr. By

varying the current and energy of the stored electrons, one can adjust the intensity and photon energy of the undulator first

harmonic in the range up to 1.2 w/cm2

, and between about 5 eV and

35 eV.

Key words: damage, dielectric coatings, free electron laser,

harmonics, lossmeter, multilayer, optics, UV induced absorption

1. Introduction

Both free electron laser optics and space optics must perform well in the

presence of intense UV irradiation. Progress in the current state of the art

is needed for the applications planned in these fields. In this report, we describe the first results of an experiment designed to supply quantitative information on the rates and the scaling characteristics of these phenomena.

Spaceborne optics, thermal radiators, and detector surfaces are observed

to degrade significantly [1] as a result of solar UV exposure. The space optics problem appears to be caused by the appearance of a carbonaceous deposit

on the optical surface. Boiler et al. [2] measured the thickness of the carbon

layer which formed on metallic surfaces exposed to synchrotron radiation, and

developed a model for the deposition rate. This model postulates that photo

emission of electrons from the surface is primarily responsible for cracking

adsorbed carbonaceous molecules which leave behind the observed residue. The

model predicts different regions in parameter space in which the scaling of the

deposition rate is either linear in intensity, or in the pressure of the carbonaceous precursor, or in the product of the two parameters. However, the

rate of the process, and the boundaries between the different regions have not

yet been determined. Stewart et al. [3] have verified the UV dependence of the

mass deposition in space, and generated a quite similar model describing the

transport of the contaminant molecules and the deposition of residue. Although

information is available concerning the outgassing rates of many different

materials used in spacecraft [4], the rates of UV induced carbonaceous film

growth are unknown. As a result, it is not possible to calculate and optimize

the lifetime of optical components exposed to significant solar UV. The performance of optical systems for missions such as SOHO, CLUSTER, OSL, UARS,

ATLAS, ORFEUS, SUNLAB, and the Space Station depend on a design process which

must rely on rules of thumb until the film growth process is understood in more

detail

Free electron laser performance has been severely affected [5-7] in experiments where the average UV flux is large enough to cause absorption changes in

the resonator mirrors. Recent experiments [8,9] have demonstrated that the

551
harmonic generation process in FELs [10] can be described by a conversion efficiency for each harmonic. High average power FELs therefore produce a high

average flux of UV radiation which will strike the optics. Table 1 shows

values for the conversion efficiency measured and predicted for two types of

FEL systems. The experimental results [12] for the odd harmonics are close to

the theoretical predictions, and do not change their magnitude much for grossly

different wiggler designs, as can be seen from the table. High power FEL

Table 1. FEL Harmonic Power

Harmonic Relative Power Relative Power

Number Mark III FEL [11] 360 periods

(linear wiggler) (tapered wiggler)

Experimental Data [12] Theoretical Prediction [13]

1

2

3

4

5

6

7

2X10

8X10

7x10"

1x10

3X10

-3

-4

1

1X10" 6

-6

-7

3x10 8

1x10

1x10

9x10

2x10

-5

-8

-7

-10

5xl0~ 8

optics will be irradiated by intense beams of UV photons. Since the higher

power FELs also have a heat dissipation problem in the optics, UV induced absorption is of serious concern.

While carbonaceous films have been observed to form on FEL optics [5,6],

additional phenomena are also observed. Fast time-constant, partially reversible loss changes were observed in the virgin mirrors of the Orsay FEL experiment [14]. An additional loss creation mechanism seems to appear when higher

energy photons are present. Our experiment was developed to supply some additional information concerning the scaling and the mechanism of the various induced absorption processes. The previous work (which has been continued at Orsay [15]) was handicapped by the inability to measure the optical losses in

situ. Typically, optics had to be exposed in the vacuum, and removed for later

measurements in the air. This process had two large disadvantages. First, the

labor and down time needed for the vacuum breaks exceeded the available

resources. Second, the measurements were compromised by the fact that the

damaged optics could only be measured after being exposed to the additional

dust and hydrocarbons of the atmosphere. Some experiments were possible insitu [14], but the measurement could only be done in the presence of the damaging UV photons.

In our experiment, loss measurement diagnostics have been incorporated

into the vacuum chamber so that the optical absorption is measured independent

of the other parameters of the system. Here, we report the results of our initial shakedown run.

2 . Experiment

All measurements are done in an ultra-high vacuum system (10 -10 Torr) to

simulate the FEL and the space environments, and provide controlled surface

conditions. The samples are exposed to the tunable UV beam produced by the TOK

undulator [16] mounted on the UV storage ring at NSLS. The UV photon energy is

adjusted between 6 and 35 eV by changing the ring energy. The bandwidth is

determined primarily by the emittance of the ring, and varies between about 4%

and 8% depending on the parameters of the stored beam. The photon flux of the

fundamental depends on the stored current, but at 100 mA, varies from 200

mW/cm2 to 86, 41, 17, and 5 mW/cm2 for photon energies of 35 eV, 23, 16, 10,

and 6 eV, respectively. The peak power is 75 times higher than the average

552
powers quoted above. The optical absorption measurements are made using a

cavity ringdown approach [17]. A measurement is made of the photon lifetime in

a resonator which incorporates the sample as one of the mirrors. This approach

provides the desired loss coefficient, and is very sensitive due to the long

mean lifetime of the photons captured in the cavity. The experimental system

is described in more detail in reference [18].

Our samples in the first run were multilayer high reflectors made of a quarter wave stack of Si02 as the low index material and either Ti02 or Zr02 as

the high index material. In addition, most samples had a half wave of Si02

deposited on top to protect the high index material from direct exposure. The

samples were optimized for reflectivity at either 633 nm or 432nm.

The cavity ringdown approach sums up the contributions from all the dissipation channels and determines the total loss. In principle, an observed

change in loss could be due to changes in any combination of absorption, scatter, diffraction, and transmission of the sample. However, there is no known

mechanism by which either the diffraction or the transmission could be sufficiently modified by the UV to produce the effects we see, and the possibility

of a change in the scattering coefficient is very remote. In contrast with the

above, there are many known channels for the induction of absorption in the

coating material. The UV photons excite electrons into the conduction band,

where they are mobile until becoming trapped at a defect, forming an absorption

center. At the surface, adsorbed volatiles can be photolyzed into a permanent

absorbing carbonaceous film. In addition, hard photons can excite the material

in ways which lead to the creation of new defects, and therefore additional absorption. We have had scatter measurements made on samples exposed to UV

photons, and on unexposed samples. We find that the loss due to scatter is

only a few ppm, and that UV exposure does not change the scatter. The measured

loss, and the induced change in it are almost entirely due to absorption in the

present experiment.

Figure 1. Schematic drawing of the experimental setup. The UV photons

from the wiggler (1) pass through the differential isolation line (2) with

photon shutter (3) into the spherical exposure chamber (4). The sample transfer chamber (5) allows samples to be exchanged into the measurement point (11)

via the linear and rotary manipulator (13). A dye laser (14) is focussed (15)

through the resonator mirrors (16) to the detector (17) . Selected gasses can

be introduced through the controlled leak valve (12) . An RGA (10) and an XPS

system consisting of X-ray source (7) and electron analyzer (6) can be used to

determine the residual atmosphere and the surface constituents. An ion sputter

gun (9) and electron flood gun are available for cleaning the sample surfaces.

553
3. Results

Figure 2 shows the results of a typical UV exposure. This sample mirror

was fabricated with a total loss (at 432 nm) of less than 200 ppm per reflection. The UV photon beam was turned on for the first time at about 100 seconds

into the scan. A rapid rise in loss is observed, followed by a slower relaxation to an equilibrium level about 500 ppm above the initial loss. The UV was

turned off, on, and off again in this example, at about 410, 600, and 900 seconds, respectively. Each time the UV is turned off, the loss falls a small

fraction of its initial rise (about 150 ppm in this case) , with a similar time

constant behavior as for the initial rise. Turning on the UV again boosts the

losses back to the level attained earlier. Interestingly, the slow rise time

component is not present in the subsequent exposures, although a slow fall time

component is present in all the post-exposure relaxations. While the time constants can vary from sample to sample by about 50%, the fast and slow rise

times are typically 1 s and 70 s, independent of the photon energy or intensity.

Apparently, the absorption of these samples consists of three parts: the

initial loss (which is not pure absorption) , a long lived "static" UV induced

absorption, and a dynamic UV induced absorption which is present only during

exposure. After the UV is turned off the absorption quickly falls back to its

"static" level. Due to the lack of sufficient data, many questions remain concerning the dependence of the absorption on the exposure conditions. The next

goal of this experiment is to determine the scaling of the static and dynamic

components of the loss vs. UV photon energy and intensity, and as a function of

the material.

a

Oh

w

W

O

800

600

400

200-

i i i i i

i

1 1 i i i i i 500

i

I

i i i

' 1000

I I 1 I 1500

Time, s

Figure 2 . An example of the

measured changes in optical absorption as a function of exposure time.

The loss of an overcoated Si02 /Zr02

multilayer mirror is measured at 432

nm, the design wavelength of the

multilayer stack. A beam of 19 eV

photons with an intensity of 140

mW/cm2 (4.7il0 16 photons/s-cm2

) is

incident on the optic at the

measurement point. The UV beam is

turned on and off twice as described

in the text.

2600

W

o

i—3

1600

600

Si/ZrSi

I t Si/TiSi i

i i i i i i i i T \~V T 1 "I' 1 "

1 1 1 1 | 1 1 1 200

Time, s

400

Figure 3 . Comparison of UV induced absorption for two multilayer

samples fabricated from different

high index materials, Ti02 and Zr02 , and the same low index material,

Si02

. The UV photon energy is 23 eV

and the loss measurement is made at

633 nm. While the initial absorption is of little significance, the

magnitude of the UV induced rise for

the Zr02 /Si02 sample is seen to be a

factor of four larger than the

Ti02 /Si02 sample. Note that both

samples have a half wave Si02 overcoat; the difference in induced absorption implies a good mobility for

the solid state excitations which

mediate the effect.

554
Figure 3 shows the performance of silica-titania samples along with that

of silica-zirconia measured at 633 nm under similar exposure conditions. At

this photon energy (23 eV) , the initial rise of the loss is 240 ppm for the

titania and 800 for the zirconia. This relative performance persists at lower

UV photon energies. We plan to extend our measurements to additional materials

so that we can build up a database of the static and dynamic losses. It would

also be desirable to obtain this information as a function of the probe

wavelength.

Several curves of loss vs. time taken at different photon energies are superposed for comparison in figure 4. This data was taken on Si02 /Ti02 mirrors

with an Si02 overcoat. The form of the three curves is the same, but the

amplitude of the initial rise grows rapidly with increasing photon energy. An

independent measurement has shown that for our parameters, the magnitude of the

initial rise is almost independent of flux: the initial rise at 16 eV barely

doubled when the photon flux was increased by a factor of 20. Since the photon

fluxes in these curves differed roughly by factors of two, (5.6, 3.6, and 1.4 x

10 16 cm

-2

, from top to bottom) we therefore ascribe the trend in the initial

rise primarily to the energy change.

At least at the lower photon energies, as shown in figures 2 and 4, it

would appear that the curve saturates: longer exposure times result in no additional absorption. This data leaves open the question of how long the absorption will remain flat, particularly if the photon flux were to be substantially

higher than used here. Indeed, as the photon energy is increased as shown in

figure 5, we discover a different absorption growth characteristic which must

be due to a new mechanism.

Figure 5 shows absorption vs. time curves for three higher photon

energies. As in figure 2, the 25 eV curve is interrupted twice by turning off

the UV beam at 730 and 2150 seconds. The 29 eV curve is interrupted once at

1900 seconds, and the 35 eV beam once at 3200 seconds. Initially, these curves

have the same form as the curves of figures 2 and 4: the loss jumps up sharply

and flattens out. From there, the additional loss grows at an accelerating

rate and ultimately reaches high levels. The 35 eV curve is shown to reach.

1.6% absorption during its 50 min exposure. The initial absorption jump is

dwarfed by the strong new loss creation mechanism. This latter sample was exposed to an intensity which dropped slowly from 1 W/cm2 down to 800 mW/cm2 due

to the 4 hour lifetime of the beam during the experiment.

The key question remaining to be answered is the scaling of the new mechanism. It is clear that the growth rate drops rapidly with energy. But is

there a threshold? If the mechanism is active at all energies, the question

becomes: how many photons of a given energy does it take to produce, say, a 1%

loss change? This is a serious issue for applications such as the free electron laser which produce a high intensity of UV radiation.

Figure 6 shows an entirely unexpected phenomenon which occurs in Si02 /Ti02 mirrors when exposed to high K undulator radiation. When the magnetic field is

increased from K=l to its maximum at K=5.6, the on axis intensity grows (approximately) linearly in K, and the spectrum changes drastically. The spectrum

evolves from essentially a single peak at the undulator fundamental [19], to a series of overlapping peaks at all of the harmonics which approaches the spectrum of a bending magnet. The high K undulator therefore produces intense

broadband radiation between about 3 eV and about 220 eV.

As can be clearly seen from the solid curve (taken at 600 ma beam current) , the absorption loss is initially driven to higher levels, and then

bleached or annealed away almost entirely. In the 600 ma case, the residual

absorption is only 280 ppm (static losses, plus 300 ppm dynamic) above the unexposed value. If the exposure is stopped anywhere in the middle of the curve,

the loss stabilizes near the current value. The initial exposures drive the

losses up to more than 2% absorption. This gross absorption rise appears to be

independent of the intensity, although its rate of rise has some intensity dependence. The loss reduction takes over from the induced losses fairly quickly, but the rate of improvement at 290 ma is much lower than that of the 6 00 ma

data. Some of the bleaching effect may be permanent: in a reexposure 34 hours

555
1400 I

i i i i i i i i i | i i i ii i , m |

i i i i i i i i

i | i i i i i i i i i | 26 76 126 176 226

Time, s

Figure 4. Initial exposure

data is shown for overcoated

Si02 /Ti02 mirrors exposed to three

different photon energies and

measured at 63 3 nm. As described in

the text, the induced loss in this

region is almost insensitive to the

intensity, but shows an increase

with photon energy.

200 1200 2200 3200

Time, s

Figure 5. Exposure data is

shown for higher photon energies

where a new loss mechanism appears.

The induced absorption varies in a

nonlinear way with the fluence and

can reach extremely high levels.

The intensities were, from bottom to

top, 3 00, 27 6, and 1000 mW/cm2

respectively

30000

B

ft

w

o

20000

10000

100 mA

290 mA

50 100 150 200

Fluence, mA-hr

Figure 6. The solid curve

shows UV induced absorption in overcoated Ti02 /Si02 followed by a UV

induced reduction in the absorption

measured at 632 nm. Four data sets

taken under similar conditions are

superimposed

1

556
later, the maximum loss attained was only 25% of the initial maximum, and the

time required for bleaching was only 10% of the initial time.

This effect is apparently due to a direct photon mediated bleaching process [21], but it is difficult to explain the intensity dependence, i.e. the

fact that curves measured at different intensities do not show the same loss at

the same fluence. We believe it is unlikely that the drop in the losses is due

to thermal annealing, although the lower temperature rise produced by the lower

intensity beams is consistent with the smaller effect on the loss. A rough

calculation of the sample temperature indicates that the losses begin to drop

before the surface temperature reaches about 150 °C in the initial exposures.

In subsequent exposures, the estimated temperature at the time the losses begin

to drop is much lower. At present we do not have the equipment to measure the

sample temperatures during exposure.

4. Discussion

The reflected optical wave forms a standing wave in the multilayer stack

[22]. The amplitude decays with depth so that appreciable field is present in

only the first 5 or 6 layers. When illuminating the mirror at its design

wavelength, the standing wave forms nodes at the surface, and at every low-tohigh index interface within the stack. An antinode is formed in the middle of

the surface layer, and at every high-to-low index interface. The UV photons,

however, are absorbed exponentially at different rates depending on the layer

material and the photon energy. The excitations created by the UV photons may

also be mobile. The net absorption seen is related to the overlap of the local

UV induced absorption coefficient and the local standing wave intensity.

The absorption coefficients for fused silica and crystalline Ti02 (rutile)

are known [23]. However, the properties of the ion sputtered amorphous films

which make up our samples are somewhat different, since they are porous, may

have a different stoichiometry , and contain a much higher concentration of dangling bonds and other defects than the solid material. The absorption of the

higher energy photons should be similar to the bulk material, but the band edge

will be broadened and shifted to lower energy.

At 35 eV, the absorption coefficient in fused silica is 0.11, which implies a 1/e penetration depth of 500 Angstroms. Since the Si02 surface layer

thickness is 2170 (1470) Angstroms for the 633 (432) nm sample mirrors, only a few percent of the flux penetrates the surface layer into the next layer, which

is the high index material Ti02 or Zr02

. (Note that the physical thickness of

the layers is larger than quoted because of voids created during the coating

process. Since the optical thickness is accurately 1/2 wave, the effective

thickness quoted above is also accurate.) As the UV photon energy is reduced,

the 1/e penetration depth drops rapidly to 480, 360, 195, 172, and 168 Angstroms at 35, 29, 23, 16, and 10.4 eV, respectively. Below 10.4 eV, the absorption of bulk fused silica drops rapidly, becoming essentially transparent

under 6 eV. Thus, over the entire range of UV photon energies of interest here

the silica layer is absorbing. The fraction of UV flux which penetrates the

top Si02 layer is larger at higher UV photon energy and shorter probe

wavelength. The Ti02 layer underneath is highly absorbing [23] down to at

least 3.7 eV. Essentially all of the photons which pass through the Si02 coating are absorbed in the Ti02 layer. We do not have absorption data for Zr02 , but we expect its behavior is comparable to the Ti02

.

In the 6 eV to 2 5 eV range, the majority of the incident photons are absorbed in the Si02 top layer. The small fraction of the photon flux which penetrates the Si02 layer follows a strong scaling behavior (from 10 -6 to 10 -2

, depending on the parameters) which is not at all followed by the induced absorption. Since the loss creation mechanism is not completely saturated, we

conclude that the relevant excitations must be produced in the top layer. The

absorption event promotes an electron to the conduction band, where it can find

and get trapped at a localized state such as a dangling orbital near a vacancy.

Such a center in turn absorbs over a range of wavelengths through low-lying excitations .

557
One of the questions our work raises is: what is the mobility of the UV

induced excitations? Our results (Fig. 3) show that the size of the induced

absorption depends strongly on the second layer material. The excitation centers are created in the top layer, but the sample response seems to depend on

the nature of the buried layer. There are two factors which could lead to such

a behavior: impurities and excitation mobility. We had our samples analyzed by

charge-neutralized SIMS (Secondary Ion Mass Spectroscopy) . Silica typically

contains Na and K as impurities, which were found in the ppm range for all the

samples tested [24]. There was also Ti present in silica layer at about the 1%

level for the Si02/Ti02 sample, and about the same amount of Zr for the

Si02 /Zr02 sample. (The intermixing resulted from spurious sputtering of adjacent anodes in the sample deposition chamber.) It is possible that the differences observed in Fig. 3 are due to the different oscillator strength for

electrons trapped at the two different types of defects.

The other interesting possibility is that the excitations created by the

UV photons in the top layer migrate to the lower layer where they form absorption centers. In this scenario, the characteristics of the absorption center

depend on the lower layer material; the excitation is only required to transport energy to the center. It may be that the initial absorption rise is

mediated by excitations generated in Si02 but which can propagate thousands of

Angstroms to activate remote absorption centers.

At and above 25 eV, the growing penetration of higher energy photons into

the high index layer may be partially responsible for the appearance of the new

absorption mechanism, but it is not likely. The difference in penetration between the 25 and the 29 eV cases is a factor of 20, which is much larger than

the times-three change in the induced absorption (after 2 5 min) . Over the same

25 minute period, the change in penetration between the 29 and the 35 eV cases

(a factor of 5) is smaller than the change in induced absorption (a factor of

ten) . It would appear that a threshold is being crossed which suddenly turns

on the new mechanism visible in Figure 5.

When the UV induced excitation becomes trapped, the energy released

depends on the UV photon energy. The excess energy can trigger a number of additional processes. One of the processes which comes in to play at higher

photon energies is stimulated desorption [25] . For example, the threshold for

0

+ desorption from silica lies between 18 and 31 eV depending on the number of

0 atoms bonded to Si in the sample [26]. Desorption contributes to defects by

disrupting the local (ideally, tetrahedral) ordering. More sites become available than previously to trap an electron excited by UV photon absorption.

These provide new absorption channels contributing to increased absorption. In

view of the energy at which this new mechanism turns on, we conjecture that

photo desorption of oxygen may be responsible.

We wish to thank Olive Lee and Larry MacNeil for their experimental and

technical help during several of the night runs required to perform these experiments, and the NSLS staff for their efforts to help us interface the experiment with the beamline. This work was supported in part by the SDIO and

managed by the ONR and the Los Alamos National Laboratory under contracts

N00014-87-C-0154 , 9-X28-1472G-6 , and 9-LSH-1765G-1 , and in part through the UV

ring operating funds provided by the DOE through contract DE-AC02-76CH00016

5. References

[1] D.F. Hall, T.B. Stewart, and R.R. Hayes, "Photo-enhanced Spacecraft Contamination Enhancement", ESA SP 232 f 39 (1985).

[2] K. Boiler, R.P. Haelbich, H. Hogrefe, W. Jark, C. Kunz , "Investigation of

carbon contamination of mirror surfaces exposed to synchrotron radiation",

Nucl. Instr. & Meth., 208 273-279 (1983).

558
[3] T. B. Stewart, G. S. Arnold, D. F. Hall, and H. D. Marten, "Absolute Rates

of Vacuum-Ultraviolet Photochemical Deposition of Organic Films", J. Phys

Chem. 93, 2393 (1989)

[4] B. Campbell, NASA Ref. Pub. 1124, NASA Goddard Space Flight Center, Greenbelt MD 20771.

[5] M. Ambrosio, G.C. Barbarino, M. Castellano, N. Cavallo, F. Cevenini, M.R.

5Masullo, P. Patteri, "Dielectric mirrors damage due to the radiation of a high K undulator", Nucl. Instr. & Meth A250 289-292 (1986).

[6] M. Billardon, P. Elleaume, J.M. Ortega, C. Bazin, M. Bergher, M. Velghe,

D.A.G. Deacon, Y. Petroff, "Free electron laser experiment at Orsay: a review", IEEE J. Quant. Elect. OE-21) 805-823 (1985).

[7] G.A. Kornyukhin, G.N. Kulipanov, V.N. Litvinenko, N.A. Mesentsev, A.N.

Skrinsky, N.A. Vinokurov, P.D. Voblyi, "Status of the INP Optical

Klystron", Nucl. Instr. & Meth. A237 281-288 (1985).

[8] D.J. Bamford, D.A.G. Deacon, "Measurement of the coherent harmonic emission from a FEL" , Phys. Rev. Lett 62 1106-1109 (1989).

[9] B.A. Newnam, R.W. Warren, D.W. Feldman, W.E. Stein, "Harmonic generationstrength and mode shape", to be published in the Proceedings of the 11th

International Conference on FELs, Naples FL, August 1989.

[10] A. Gover, A. Friedman, A. Luccio, Nucl. Instr. & Meth. A259 163-176

(1987) .

[11] S. Benson, D.A.G. Deacon, J.N. Eckstein, J.M.J. Madey, K. Robinson, T.I.

Smith, and R. Taber, J. Phys. (Paris) , Colloq. 44, Cl-353 (1983)

[12] D.J. Bamford and D.A.G. Deacon, "Measurement of the Harmonics Emitted in

the MARK III Free Electron Laser", to be published in Nucl. Instr. & Meth.

(1989) ; in Proceeding of Tenth International Free Electron Laser Conference, Jerusalem, Israel, August, 1988.

[13] M.J. Schmitt, C.J. Elliott, and B.E. Newnam, "Harmonic Power Implications

on Free Electron Laser Mirror Design", Nucl. Instr. & Meth. A272 . 586(1988)

[14] P. Elleaume, M. Velghe, M. Billardon, J.M. Ortega, "Diagnostic techniques

and VUV induced degradation of the mirrors used in the Orsay storage ring

FEL", Nucl. Instr. & Meth., A237 263-267 (1985).

[15] M.F. Velghe, M.E. Couprie, M. Billardon, "Specific optical properties of

multilayer mirrors for FEL experiments", to be published in the Proceedings of the 11th International Conference on FELs, Naples FL, August 1989.

[16] A.M. Fauchet, G. Vignola, J.N. Galayda, C. Pellegrini, B.M. Kincaid, R.R.

Freeman, F. De Martini, to be published in the proceedings of the 10th International FEL Conference, Jerusalem (1988)

[17] A. O'Keefe, D.A.G. Deacon, "Cavity ring-down optical spectrometer for absorption measurements using pulsed laser sources", Rev. Sci. Instr. 59

2544-2551 (1988)

[18] M.H. Bakshi, M.A. Cecere, and D.A.G. Deacon, "UV Photon Induced Absorption

in Multilayer Dielectric Mirrors", submitted for publication in Nucl. Instr. & Meth. (199 0) ; in the proceedings of the 11th International FEL Conference, Naples, FL, August 28-September 1, 1989.

[19] W.B. Colson, "The nonlinear wave equation for higher harmonics in FELs",

IEEE Journ. Quant. Elect., OE-17 1417-1427 (1981).

559
[20] see for example, E. Ritter, "Dielectric Film Materials for Optical Absorptions", Phys. of Thin Films, 8, 1 (1975); P.W. Levy in Radiation Effects

in Optical Materials , SPIE 541 . 1 (1985).

[21] M. Born, E. Wolf, Principles of Optics . Pergamon Press, Oxford, (1975)

[22] E.D. Palik (ed.) , Handbook of optical constants of solids . Academic Press,

Orlando (1985) .

[23] Evans East, Inc., Plainsboro, NJ, private communication; Deacon Research,

unpublished results.

[24] M.L. Knotek, in Semiconductors and Insulators: Recombination Induced

Defect Formation in Crystals . Gordon & Breach, ed. by F.C. Brown and N.

Itoh, 5, 361 (1983) .

[25] M.L. Knotek and J.E. Houston, J. Vac. Sci. Technol . B 1 . 899 (1983).

560
Manuscript received

2-16-90

The Response of Multilayer Dielectric Coatings to Low Fluence

Ultraviolet Light Exposure

V. E. Sanders, J. W. Early, and W. Leamon

Chemical and Laser Sciences Division

Los Alamos National Laboratory

Los Alamos, New Mexico 87545

Multilayer dielectric high-reflectance coatings were investigated for potential application in a

high-average-power, 1-jim wavelength, free-electron laser oscillator. Of concern are the ultraviolet harmonics of the primary wavelength generated within the oscillator that tend to degrade

the dielectric coatings. These coatings are required to be high reflecting and low absorbing with

respect to the 1-u.m wavelength and resist degradation/damage from the ultraviolet harmonics.

A KrF excimer laser was used to generate the low fluence ultraviolet light at 248-nm wavelength, a wavelength that simulates the fourth harmonic of the 1-(im free-electron laser. A

variety of candidate dielectric oxide thin film materials are compared in this report. In each

multilayer dielectric high reflector measured, a candidate high-index dielectric material is used in combination with Si02 as the low-index material. Damage test measurements are reported for

the 248-nm wavelength with Hf02 being the high-index material with significantly the best results.

Key words: excimer laser damage; free-electron laser optics; laser damage; multilayer dielectric

coating; ultraviolet damage.

1 . Introduction

Both metal and dielectric coatings are being developed for high-reflectance mirrors to be used in a

1-(im wavelength, high-average-power, free-electron laser. Silver metal coatings have been demonstrated [1] as meeting the requirements for these mirrors when applied to advanced designs for large

cooled mirror substrates. Multilayer dielectric coatings have the potential for significant improvement

over metal coatings owing to a demonstrated [2] significantly improved reflectance factor at the 1-u.m

primary wavelength. In both cases, metal and dielectric, the major portion of the 1 -|im primary wavelength that is not reflected is absorbed in the coating and is transformed into heat. The heat may distort or destroy the mirror surface if not dissipated and carried away. Therefore, the potential improvement

offered by dielectric coatings translates into improved laser beam quality, smaller/higher quality substrates, and significantly reduced cooling requirements.

Each of the dielectric thin film materials considered in this study is to some extent vulnerable to

ultraviolet light. Therefore, the ultraviolet harmonics generated along with the 1-(im primary wavelength

within the free-electron laser oscillator are a potential threat to a dielectric coating. The ultra-violet

harmonic fluences relative to the primary wavelength are described/calculated in the free-electron laser

literature [3]. The threat/damage of concern is from generation of color-centers [4] in the dielectric

coating. This causes the absorption at 1-|im wavelength to increase, thereby negating the potential

advantage over a metal coating that is not degraded by the ultraviolet light. This ultraviolet-induced

degradation of dielectric mirrors in a free-electron laser has been observed in low-gain, low-power

devices [5] and is therefore of much concern in proposed high-average-power free-electron lasers.

561
2. Experiment

A schematic diagram of the testing apparatus is shown in figure 1 . The device is based on a highprecision reflectivity measurement technique for low-loss laser mirrors [6]. The operation and calibration of this device are described in the report [7] on the initial phase of this coating development study

reported at the previous symposium on optical materials for high-power lasers. The candidate test mirror/coating is part of a continuous-wave helium-neon laser, lasing at 1 .1 u.m. This wavelength is near the nominal 1-u.m primary wavelength of interest to the proposed high-average-power free-electron laser. The 1 .1-u.m high-reflectance test mirror is simultaneously and coincidentally exposed to

ultraviolet laser radiation from an excimer laser. KrF lasing at 248 nm was used to simulate the fourth

harmonic. As color-centers are generated in the dielectric coating from the ultraviolet exposure, the

reflectivity at 1.1 urn decreases, causing the intracavity lasing power in the helium-neon laser to decrease.

The changing intracavity power is monitored by detecting a partial reflection from an internal window shown in figure 1 . Not shown in the figure is a means of monitoring the transmittance of the test mirror via a similar photodiode and recorder as used with the indicated window. In addition, scatter is observed with a visible HeNe laser. In each measurement reported, neither transmittance nor scatter

were observed to change significantly. Therefore, the measured change in reflectivity in each case is a result of changing absorption as expected from the creation of color-centers. As a result, the changing reflectivity (AR), i.e., changing absorption of the coating, is measured in real time. As demonstrated in reference 7, AR is measured to an accuracy near 0.01%.

Variable

Reflectance

Window

Test

Mirror

Laser

Output

InGaAs ^X^

Photodiode V

KrF (248 nm)

UV Excimer

Laser

1 .1 p.m Intracavity

HeNe Plasma

HeNe Laser Beam

Gain Tube

High

Reflectance

End

Mirror

Amplifier

Recorder

Computer

cc Data < i oE

Time

Figure 1 . uv-damage testing apparatus

Figure 2 is a generic example of data from this apparatus. As the ultraviolet exposure of the test coating is initiated, a corresponding change in reflectivity is measured. When the ultraviolet laser is turned off, it is noted in time and again results in a corresponding change in reflectivity.

562
0 20 40 60 80 100

uv Exposure Duration (min)

Figure 2. Generic data from testing apparatus

3. Results

The traditional lowest absorption, high-index dielectric thin film material for 1-jim wavelength

coatings, Ti02

, does not fair well in this test or in the free-electron laser [5]. The specific task of this

study has been to survey the alternate high-index coating materials that might make acceptable highreflectance, low-absorbing coatings near 1-(im wavelength.

In each case tested the material is fabricated into a multilayer, high-reflectance stack in combination with Si02

, the low-index material. Figure 3 shows a comparison of several high-index materials. The excimer laser parameters are indicated in the figure. The excimer laser spot on the test mirror is substantially larger, by a factor of three, than the 1 .1 -^m helium-neon spot providing a significant margin to consider a constant effect across the area sampled. These results indicate that pure

(0.9997) Hf02 is substantially the best choice at 248 nm. Note that this coating has degraded by only a factor of 0.03% after a one-hour exposure at the indicated fluence. In addition to the superior ultraviolet damage performance, the HfO./Si02 coating has demonstrated [2] reflectivity at 1 .06-(im wavelength in excess of 0.9999 and damage thresholds that far exceed the free-electron laser requirement.

563
0 20 40 60 80 100

uv Exposure Duration (min)

Figure 3. Comparison of high index materials in

uv-damage experiment

In addition to a comparison of the indicated high-index materials in figure 3, this study demonstrated an unexpected relation between ultraviolet power density and induced damage, i.e., increased

absorption at 1 .1-(im wavelength. Figures 4, 5, and 6 show the relation. Figure 4 shows the continuous change in reflectivity during a 65-minute exposure at two separate pulse repetition rates (100 Hz

and 8.4 Hz) associated with the excimer laser. In each case the individual pulse parameters are the

same, as indicated in the figure. The pulse repetition rates differ by a factor greater than 1 0, but the

rate at which absorption is being induced, i.e., the generation of color-centers, is nearly the same.

Figure 5 shows results from a single coating at separate sites on the coating. As indicated, the

accumulated damage after 65 minutes of exposure is independent of ultraviolet pulse repetition rate for

the given parameters.

i

Figure 6 indicates data from the same coating sample as represented in figures 4 and 5. In this

case the reflectivity change after 65 minutes of exposure is indicated as a function of individual ultraviolet pulse fluence.

II

i

564
0>c

.cO

u

tt>

cc

0.4

-0.3 -

t= -0.2 -

-0.1

-0.0

I I I I I Hf0o + 3% Zr09

- High Index Material

1

Si02

- Low Index Material —

uv Wavelength - 248 nm 3=O —

uv Pulsewidth - 12 ns >

uv Fluence - 30 mJ / cm 2

/ pulse

c

o

> 1 00-Hz Pulse Rep Rate

^^^^ +

m ^-—--^ *

/ —

>^ 8.4-Hz Pulse Rep Rate

*

1 1 1 1 1 1

10 20 30 40 50

uv Exposure Duration (min)

60 70

Figure 4. uv-induced reflectance change compared

at two separate repetition rates

& « O ft

<> c

E

> in o <f>

CC ,2-

-0.4

-0.3

-0.2

-0.1

0.0

Hf0

2

i i i i i

+ 3% Zr02

- High Index Material

Si02

- Low Index Material uv wavelength - 248 nm _

uv pulse width - 12 ns

2 uv fluence - 10 mJ / cm / pulse

X

x x

I

I

X

I ! I 20 40 60 uv Pulse Rate (Hz)

80 100 120

Figure 5. uv-induced reflectance change (after 65

minutes exposure) compared for several

repetition rates

565
D) O

C Q.

co O

xo

E

. E

> m

tt)

CD

"frhigh index material

uv wavelength - 248 nm

uv pulse rate - 35 Hz

uv pulse width - 12 ns

uv Fluence (mJ / cm / Pulse)

Figure 6. Coating reflectivity change as a function

of peak uv fluence

We conclude from figures 4, 5, and 6 that the rate of induced absorption change or color-center

generation is average power independent and peak power dependent. Each of the other materials

indicated in figure 3 showed this same characteristic.

4. Conclusions

From the above-reported results and from the initial results at 351 nm reported in reference 7, we

may conclude that the HfCySiCX, coating using pure Hf02 is a good candidate for an ultraviolet- resis- tant coating. More particularly and in addition, given the results reported in reference 2 associated with

0.9999 reflectance and damage thresholds at 1-u.m wavelength that far exceed the FEL system re- quirement, we conclude that this Hf02/Si02 coating is a good candidate for a high-average-power freeelectron laser. However, this study is not complete and will next obtain data at 193 nm using an ArF

laser to simulate the fifth harmonic of 1 u.m. Therefore, we consider our conclusions to be interim at

this time in the study with respect to free-electron laser application.

In addition, we conclude that the average versus peak power nature of induced absorption as

indicated in figures 4, 5, and 6 is significant. This relation has not been reported and is not resolved in the theoretical explanation of color-center generation in thin films. We conclude that this aspect of the

study needs further consideration both experimentally and theoretically.

5. References

[1] Sanders, V.; Jolin, L; Salazar, S. Damage to Silver Coatings from High Average Power 1-u.m

Laser, to be published in Laser Induced Damage in Optical Materials: 1988 (Boulder Damage

Symposium Proceedings).

566
[2] Sanders, V.; Jolin, L; Salazar, S. Laser Neutralizer Coating Validation Study, to be published in Proceedings of the Topical Meeting on High Power Laser Optical Components (1989).

[3] Schmitt, M.; Elliott, C; Newnam, B. Harmonic Power Implications on Free-Electron Laser Mirror

Design. Nuclear Instruments and Methods in Physics Research A272 (1988) 586-589.

[4] Levy, P. Overview of Nuclear Damage Processes; Phenomenological Features of Radiation

Damage in Crystals and Glasses; Radiation Effects in Optical Materials. Levy, P., ed. Proc. SPIE

541; 1985 March 2-27.

[5] Elleaume, P.; Velghe, M.; Billardon, M.; Ortega, J. Diagnostic Techniques and UV-lnduced

Degradation of the Mirrors Used in the Orsay Storage Ring Free-Electron Laser. Applied Optics,

Vol. 24, No. 17, September 1, 1985.

[6] Sanders, V. High Precision Reflectivity Measurement Technique for Low-Loss Laser Mirrors. Appl.

Opt. 16 (1): 19-20; 1977.

[7] Early, J.; Sanders, V.; Leamon, W. The Response of Multilayer Dielectric Coatings to Low Fluence

UV Light Exposure, to be published in Laser Induced Damage in Optical Materials: 1988 (Boulder

Damage Symposium Proceedings).

567
Manuscript received

12-29-89

Radiation Damage in Single Crystal CsI(Tl)

and Polycrystal Csl

O.Barnouin, A.Procoli, H.Chung and G.H.Miley

Department of Nuclear Engineering

University of Illinois at Urbana-Champaign

103 South Goodwin Avenue

Urbana, IL 61801

Radiation damage in single crystal CsI(Tl) and polycrystal

Csl was assessed by measuring the changes in radioluminescent

intensity caused by successive neutron and gamma ray pulses

from a TRIGA nuclear reactor. The radioluminescent intensity

from the single crystal decreases within nine pulses by 3 0% in

the near infrared range (0.7-4.5 ^m) and by 25% in the visible

range (0.2-1 ^m) before staying constant. For the polycrystal

Csl, the emission in the visible range decreases by 60% within 6

pulses and then remains constant, whereas no measurable emission

is observed in the infrared. Finally, a heating and cooling

cycle is shown to repair the damage for both crystals.

Key words: Radiation damage, infrared radioluminescence,

visible radioluminescence, cesium iodide, reactor irradiation,

gamma ray irradiation, infrared window material

1. Introduction

Csl is a material that can be used for infrared detector windows and

for making scintillators for high energy physics experiments (it is then

doped with Thallium) . This crystal may have to work in a harsh radiation

environment which can cause an undesired signal to be sent to the detector

or modify its calibration. It is therefore important to measure the

response of this crystal to a flux of neutrons and gamma rays and to study

the effects of dose accumulation. Such a study was conducted at the TRIGA

nuclear reactor of the University of Illinois.

Radiation damage is most often studied through absorption measurements

over the visible spectrum [1], rarely through radioluminescence

measurements. Radioluminescence occurs [2] when an electron excited to the

conduction band recombines radiatively, either directly to the valence

band, or to a defect state (indirect, radiative recombination). Indirect

recombination, however, can also occur without emission of a photon and the

energy of the electron is lost through heating of the crystal lattice. The

radioluminescent intensity then results from a competition between these

processes. High energy radiations, by exciting electrons into the valence

band and by creating new defects, modify the relative contribution of these

recombination processes to the global recombination and leave the material

in a state different from that before irradiation. One then speaks of

radiation damage.

The investigation of radiation damage on CsI(Tl) has already been

performed [3], although not extensively, with 1.25 MeV photons from a 60 Co

source as gamma-ray source. The radioluminescence was due to irradiation

568
by a Cs source. Although the total dose and dose rate were very much

lower than in the present study (-500 rads, 100 rads per minute) , a sizable

reduction of the radioluminescence was observed (3 5% decrease of the

emission at 500 rads) . The damage was also found to be permanent.

It is the purpose of this paper to present results on

radioluminescence measurements in both the visible and near infrared

regions during a high-dose, pulsed irradiation of single crystal CsI(Tl)

and polycrystal Csl by neutrons and gamma rays. It was found that, in both

crystals, the radioluminescence decreases at first with increasing dose,

but stabilizes after a few pulses. Also, heating the sample makes the

radioluminescence decrease more rapidly in the single crystal CsI(Tl) than

in the polycrystal. Finally, a cycle of heating and cooling is shown to

repair the damage.

2 . Experimental Setup

The samples underwent a series of pulses from the TRIGA pulsed nuclear

reactor of the University of Illinois. The main characteristics of the

pulse are shown in table 1 and the experimental setup is shown in figure 1.

Table 1. Pulse characteristics of the

University of Illinois 7 TRIGA nuclear reactor

Pulse width

Total power in a pulse

Total dose per pulse (SiC>2)

Dose components : neutrons

gammas

Peak dose rate

Pulse rate

12.5 ms

5.8 kWh

100 krads

5 krads

95 krads

6.7 Mrad/s

4 pulses/hour

The CsI(Tl) sample was manufactured by Harshaw Company. It was grown

from a solution and is a disk 25.4 mm diam. , 2 mm thick. The polycrystal

was made at the Oak Ridge National Laboratory and is made of a powder

compressed uniaxially at 100 MPa at a temperature of 150°C. It is 25.4 mm

in diameter and 10 mm in thickness.

The sample under test is placed in the middle of a nine-inch long

aluminum rod mounted on a carriage. This carriage is positioned in the

middle of a thruport adjacent to the reactor core (fig. 1) . The rod is

wrapped in a resistive heating tape used for raising the temperature of the

sample. The rod is long enough to ensure a uniform temperature profile on

the sample surface during the temperature rise. A thermocouple is attached

to the carriage and is in direct contact with the sample to monitor its

temperature carefully.

The light emitted by the sample impinges on a four-inch diameter,

focussing aluminum mirror located at the exit of the thruport. The light

is focussed onto an infrared, PbSe detector (detection range 0.7-4.5 /iin) and onto a visible range, silicon detector (detection range 0.2-1 pm) . A

light chopper was used to account for the effect of the radiation on the

detectors

A typical reactor pulse gives a 0.1 Mrad dose in a SiC>2 sample. 0.5%

of that dose (Table 1) comes from neutrons and 95% from gamma rays. The

absorbed dose was not measured again in Csl, but a guick look at the

569
FSL-89-23

focussing mirror

PbSe detector (0.7-4.5um)

to oscilloscope

ZnSe IR window

lead shield

v/

iron donuts

(4- I.D.)

Test Carriage

22' REACTOR

CORE

/ /

THRU-PORT

BEAM PORT PLUG

(NOTE: NOT TO SCALE)

Figure 1. Experimental setup

respective gamma absorption coefficients of Si02, Cs and I shows that the

dose in Csl should not be larger than that in Si02 by more than a factor of

two. Due to the higher atomic numbers of Cs and I with respect to SiC-2,

the role of gamma rays will still be accentuated in Csl, making the neutron

contribution to the radioluminescence and to the damage negligible.

3 . Experiments performed and Results

Repetitive pulsing was performed for two consecutive days on the

single crystal CsI(Tl) and for one day on the polycrystal. The pulses were

spaced 17 minutes apart (this minimum time is imposed by NRC regulations)

3.1 Single crystal CsI(Tl)

The sample of single crystal Csl that had been delivered was found to

contain Thallium impurities (20 to 30 ppm [4]). An emission spectrum was

obtained with an OMA and revealed an emission peak at around 0.54 /im which

is characteristic of the presence of Thallium [4,5] (whereas pure Csl only

emits around 0.305 /im [4]). Apart from shifting the bulk of the emission

to other wavelengths (as is desired in high-energy physics applications

[5]), the presence of an impurity like Thallium can have important

consequences for our measurements: it can "protect" against radiation

damage "by scavenging free carriers before they can find a more damaging

site" [6]. Further experiments will be needed with pure Csl to assess the

role played by Thallium in the present results (apart, again, from the

wavelength shift of the emission)

The change in radioluminescence for increasing dose in shown in figure

2 for the visible and infrared ranges (data taken at the peak of the

pulse). In both ranges, the radioluminescence from CsI(Tl) at the peak of

the pulse decreases and stabilizes after around nine pulses. The decreases

amount to - 30% of the initial signal in the infrared and - 26% in the

570
visible range for data taken the first day. Due to the scatter of the

data, a total of 16 pulses were taken to firmly establish the presence of

this saturation level before the temperature of the sample was raised. The

results as a function of temperature are shown in figure 3 for both

wavelength ranges. The emission decreases by factors of 30-40 for a temperature rise of 100-12 5 "C above room temperature.

500'

40(/'

10E

o^.300

E

«

enc

az 200

100

raL-89-ioe

o o • S •

8

' *

n20

*

f .

o o o o

o o

0.1 0.5 1.0 Dose (Mrad)

n

E

u

io8>c

o

a:

JO

1.5

Figure 2. Radiation damage in single crystal CsI(Tl) at

room temperature. Solid circles are data taken the

first day, open circles to data taken the next day.

Infrared data are at the top.

Data taken during the second day (fig. 2) show what may look like a recovery of the radioluminescence after the first day. The

radioluminescence during the first pulse is higher than the stabilization

level reached the day before. It then decreases for increasing dose until a constant level is reached that is slightly lower than observed the day

before. We will analyze this behavior below, after taking a look at the

results on polycrystal Csl.

3.2 Polycrystal Csl

This sample was first irradiated with a series of nine pulses. The

resulting radioluminescence change at the peak of the pulse is shown in

figure 4. a for the visible range emission. No measurable infrared emission

was observed. The drop in figure 4. a is around 60% of the original

emission and takes places within six pulses (compared with the 25% emission

drop within nine pulses for CsI(Tl)). After stabilization of the

radioluminescence (the scatter of the data is obviously less in this case

than for the single crystal) , the temperature was slowly increased to 170°C

571
raL-w-io7

o 1 u

c

V

u

0)«c

o

'6o

x Infrared • visible

X

X 50

T CC)

100 150

Figure 3. Radioluminescence intensity for increasing

temperature. Data taken right after the data at

room temperature.

and pulses were taken as the temperature was increased (fig. 4.b). The

decrease of the emission is then due to thermal quenching. In contrast

with the measurements on the single crystal, raising the temperature

decreases the emission (relatively to the stabilization level obtained

after 6 pulses) by only 60%. This represents a decrease by a factor of

2.5, whereas a diminution by 3 0 or 4 0 was observed for the single crystal.

The emission from the polycrystal also remains constant between 150°C and

170°C.

The temperature was then brought down and some pulses were taken

around room temperature. Obviously, the radioluminescence has recovered

from the first series of pulses. It however decreases again when the

irradiation goes on at room temperature (fig. 4.c) and it stabilizes at the

same level as that observed in figure 4. a.

As a final experiment, the temperature was increased rapidly (in about

15 minutes) to 135°C and then brought down again after a pulse was taken at

that temperature. The result is indicated by the two crosses in figure

4.b. Provided that there is no large temperature gradient at the surface

of the sample (as might arise when heating the sample so fast) , this

experiment shows that the radioluminescence at higher temperature depends

on the heating rate (fig. 4.b, lower cross). However, the cooling rate was

the same as during the first cycle, and the radioluminescence then

coincides with the previous data (fig. 4.b, upper cross).

4. Analysis

These experiments show that Csl, either single or polycrystal, does

suffer radiation damage when irradiated by a pulse of neutrons and gamma

rays. This radiation damage is evidenced by a decrease of the

radioluminescent intensity for increasing dose. However, two important

conclusions have been reached: 1) the radioluminescent intensity stabilizes

572
F3L-a>-104

I I I I I I I I I I I I I I I I I I I I I I I I

I i i i 0.1 0.S 1.0 90 1 00 190 2.0 2.5 2.8 Dose [Mrad (S102 )] Temperature fC) Dose (Mrod)

Figure 4. Radiation damage in polycrystal Csl.

Chronological order is from left to right.

after a few pulses and, 2) the damage can be repaired by heating and

cooling the sample, as is shown by the recovery of the radioluminescence

after cooling.

The initial decrease of the radioluminescence might be due to several

phenomena. First, an increase of the self-absorption during the

irradiation. Absorption experiments performed at 0.63 3 pm show that the

absorption at that wavelength increases during the whole pulse. However,

the fact that the radioluminescence pulse is symmetric with respect to the

peak of the pulse and is always proportional to the radiation pulse tells

us that self-absorption does not play a more important role in the second

half of the pulse than in the first half.

Second possibility, non-radiative recombinations can be favored by

irradiation at high-energy. Available literature [7] does not give more

precise explanations for the decrease of the radioluminescence for

increasing dose. This decrease, together with the asymptotic behavior

observed after 0.6-0.9 Mrad, can, however, be compared to the decrease and

stabilization of the infrared transmission spectrum recorded after

successive steady-state irradiations (0 Mrad (fresh sample), 1 Mrad,...) of

a similar sample (fig. 5) . The transmission decreases up to a dose of 1

Mrad (or less, since no data is available before 1 Mrad) , and then remains

constant for larger doses. Although the spectral range is different from

that of the radioluminescence data, both kinds of results nevertheless

evidence a saturation effect for the same total dose.

It is impossible for us to evidence the exact saturation mechanism

from the data presented above. Considering the small contribution of

neutrons to the total dose and the small kerma of Csl with respect to SiC>2,

neutron creation of defects is not a determining factor in our results.

However, traps can be filled by recombination of electrons located in the

conduction band after ionization by gamma-rays. They can also be emptied

by excitation. The number of electrons in traps influences the relative

rates for radiative and non-radiative recombinations, hence the total

radioluminescent emission. Whatever the real cause, figures 4 and 5 tell

us that, after ~ 6 pulses, a kind of balance is reached in which the color

centers would lose as many electrons by excitation as they would gain

573
FSL-B9-120

1 8 20 30 40

WAVELENGTH A (microns)

50

Figure 5. Transmission spectrum of polycrystal Csl before

and after several irradiation from the TRIGA reactor

at steady-state power.

through electronic capture. An increase in temperature alters that balance

by exciting electrons out of the traps (quenching)

The remark about the recovery of the radioluminescence intensity after

heating comes from the results on the polycrystal (fig. 4.c). The

temperature rise, apart from quenching the emission, also modifies the

number of electrons in traps, decreasing the possibilities for charge

carriers to recombine non-radiatively , thereby increasing the

radioluminescence at room temperature. This can also explain the recovery

of the radioluminescence in the single crystal at the beginning of the

second run, which took place after a temperature rise was performed at the

end of the first day. The fact that the stabilization level reached during

the second day is lower than that observed during the first day might be a real feature of radiation damage or be due to increased self absorption in

the sample (due in particular to moisture absorbed during the night)

Thermal quenching of the emission is not as effective in the

polycrystal as in the single crystal (fig. 3, 4.b). Furthermore, the

emission from the polycrystal stabilizes above 150°C (there is no data

above 170°C, however) . This might be a sign that shallow traps have been

emptied by the temperature rise and that the trap distribution is not

continuous, whereas it would be continuous in the case of the single

crystal

5. Conclusions

This work shows that radiation damage in polycrystal Csl is stronger

that in single crystal CsI(Tl). However, the presence of Thallium as a dopant in the single crystal clouds the issue. Further measurements on

pure Csl are needed to determine whether Thallium effectively protects the

crystal from radiation damage (or the opposite)

Several features of the radioluminescence exhibited by the samples are

noteworthy: 1) the damage, as evidenced by radioluminescence measurements,

stabilizes in either case after a dose of 0.6-0.9 Mrads provided at room

574
temperature; 2) a heating-cooling cycle repairs the damage by annealing

some of the defects and, 3) heating the sample 12 0°C above room temperature

decreases the emission in the polycrystal by only 60%, versus 98% in the

case of the single crystal. The repair of the damage by heating is

important since it means that the damage cannot be considered permanent, as

long as one can afford to heat it up.

This work was performed under contract with the Oak Ridge National

Laboratory and the U.S.A. Strategic Defense Command.

We thank the University of Illinois' TRIGA reactor crew for their

assistance in the experiment.

6. References

[1] For example, Kubo, K. "Radiation Effects in LiF Crystals," Journal of

the Physical Society of Japan 16; 2294; 1961.

[2] See, for example, McKeever, "Thermoluminescence of Solids", Cambridge

University Press 1985.

[3] Bobbink, G.J. ; Engler, A.; Kraemer, R.W. "Study of Radiation Damage to

long BGO Crystals," Carnegie Mellon University Report CMU-HEP 83-13;

1983. Cited by Ref.4.

[4] Brad Utts, Harshaw Company, Solon (OH), private communication,

September 21, 1989.

[5] Grassmann, H. ; Lorenz, E. ; Moser, H.G. "Properties of CsI(Tl)

Renaissance of an Old Scintillation Material," Nuclear Instruments and

Methods in Physics Research 228 ; 323; 1985.

[6] Williams, R.T. "Nature and Defects Generation in Optical Crystals,"

SPIE Radiation Effects in Optical Materials 541 ; 25; 1985.

[7] Levy, P. "Overview of Nuclear Radiation Damage Processes:

Phenomenological Features of Radiation Damage in Crystals and Glasses,"

SPIE Radiation Effects in Optical Materials 541 ; 2; 1985.

575
Manuscript Received

5-17-90

Effects of Thermal Conductivity and

Index of Refraction Variation on

the Inclusion Dominated Model of Laser-Induced Damage

M. Z. Fuka and J. K. Mclver

University of New Mexico Physics Department, Albuquerque, NM, 87131

A. H. Guenther

Los Alamos National Laboratory Los Alamos, NM, 87544

The inclusion-dominated model of laser-induced damage is reexamined in light of recent measurements of thin film thermal conductivities. In particular, the effects of varying the thermal

conductivity and imaginary part of the index of refraction are explored. Two different values of the

index of refraction are shown to be associated with a given damage threshold, and physical mechanisms are proposed for each value. Thermal conductivity is demonstrated to affect only weakly the

value of the imaginary part of the index of refraction corresponding to a particular damage threshold.

Key words: Key words: laser damage, thin films, thermal conductivity, inclusion-thermal modeling, refractive

index.

1. Introduction

The familiar [1] inclusion-dominated model of laser-induced damage in thin films incorporates a small

localized region in the film that absorbs more incident radiation than the surrounding host film. The energy

absorbed at the imperfections then diffuses into the main body of the film. Thus, the rate at which an inclusion can reach a critical damaging temperature depends in part on the cross section of the absorbing inclusion

presented to the incident radiation and on the ability of the host thin film to diffuse heat.

Previous work [2,3] has shown that this model successfully describes the scaling of single-shot damage

thresholds with respect to the laser pulse duration and the thermal properties of the film. In past laser

damage calculations using this model [3], thermal property parameters approximating accepted values for

bulk materials were used for the host and inclusion. Recent measurements [4], however, strongly suggest

that thermal conductivities of materials in thin film form can be orders of magnitude less than bulk values.

The first part of this present work considers the effects on laser damage thresholds of varying the thermal

conductivities of host and inclusion.

The second part of this work discusses the effects on predicted damage thresholds of varying the imaginary

part of the index of refraction of the inclusion. In particular, a given damage threshold can be achieved by

two physical mechanisms, surface and bulk absorption by the inclusion, each associated with very different

values of the imaginary part of the index of refraction. Since thus far comparisons of model predictions

to experimental data have been limited to those cases where it was assumed that the incident radiation is absorbed uniformly by the inclusion, using various values of the index of refraction may therefore provide a means to approximate different inclusion absorption mechanisms.

2. Theory

A general description of the model is illustrated in Figure 1. A host thin film contains a spherical inclusion

of radius a. The host film and inclusion are materials with different optical, thermal and mechanical properties.

It is assumed that only the inclusion absorbs the incident radiation, and that the size of the absorbing inclusion

is much smaller than the laser spot size. The absorption can thus be thought of as occurring in an optical

field of constant radial intensity.

As shown in the figure, optical properties of the inclusion are specified by the complex index of refraction, n = nR + irij, where nR and rij are the real and imaginary parts of the index of refraction, respectively. The

incident laser pulse is described by its intensity /, wavelength A, and pulse duration t x

. The thermal properties

of the system are described by the thermal conductivity, K, and the thermal diffusivity, D, of each region.

These properties in concert with the rate at which radiation is absorbed are taken to be independent of

the temperature. The system is considered damaged when the surface of the inclusion reaches some critical

temperature, for example, the melting temperature of the film.

The temperature distribution T of the system is described by the thermal diffusion equations 3 :

576
AMBIENT

ATMOSPHERE

FILM (HOST)

INTENSITY PROFILE

INTERFACE

*h, *%, C^fc

SUBSTRATE

INTERFACE

*s, \ C*s

I • LASER INTENSITY (W/cm2

) A - LASER WAVELENGTH (cm)

t p

- LASER PULSE WIDETH AT FWHM (sec)

n • INDEX OF REFRACTION

a • RADIUS OF SPHERICAL INCLUSION [cm)

K • THERMAL CONDUCTIVITY (W/c« - °K)

D • THERMAL DIFFUSIVITY (cnVsec)

C

p

- SPECIFIC HEAT AT CONSTANT PRESSURE (J/cm] • °K)

1 - FILM THICKNESS (cm)

SUBSCRIPTS

I • INCLUSION

h • HOST

s • SUBSTRATE

Figure 1. The inclusion-dominated breakdown model.

1 d .dTi A

r

2 —- H dr Ki

1 dTi

T*i dt ~ r

2 dr

1 dTh T^d_ 2 d_

l\~dT ~ r* d/ dr

0 < r < a

Th , r > a (1)

where the subscripts i and h refer to the inclusion and host, respectively. A is the source term. The interface

between the impurity and the host is described by the boundary conditions:

Ti(a) = Th (a),

Ki

dTi

~dr~

dTh

(2)

577
The solution for this system of equations can be found by using a Laplace (temporal) transform or an

integral (spatial) transform. The latter is more general, since it assumes no specific form of the temporal

absorption profile. The solution for the temperature of the inclusion subject to the initial conditions

Ti {r,t = 0) = Th {r,t = 0)=0, (3)

is given by [3]

^-^{if+s(i -a-?f^'w*}.

where

F(y) =

(siny-ycosy)sin(ry|a)

y

2 {(csiny) - ycosy) 2 + h? y

2 sin2 y}

a

2 7= A

c = 1 "

As in previous work, the source term A is written in terms of the incident intensity I

jf dr*A{r,t) = Q(^,n^I{t), (6)

where Q is the absorption cross section computed from Mie scattering theory [3] and rij is the imaginary part

of the index of refraction of the inclusion. The damage threshold is then defined as

E = I

' dtl{t) [Jem- 2

], Jo

(7)

where t p

is the pulse length required to reach the critical temperature Tc at the radius a of the absorbing

region. Tc tp = f(r,Q,E) is inverted to give E = g(Tc ,tp ,r, Q) in Joules per centimeter squared. For all calculations discussed in this work, the critical temperature is chosen to be 2000K, a typical value for the type

of materials commonly used in dielectric thin films. The exact value of Tc is not critical because of the large

value of dTc /dt involved in the damage interaction.

3. Effects of Thermal Conductivity Variation

Figure 2 shows the results of reducing the thermal conductivities of the host (left-hand column) and the

inclusion (right-hand column) in order of magnitude steps. Since conductivity and diffusivity are dependent on each other according to

K£=Pcp> (8)

578
50.00

40.00

30.00

20.00

10.00

0.00

0.00 50.00

40.00

egEo

30.00 o

SZw

0) JZH

20.00

03CO

caE

03 10.00 Q

0.00

0.00

50.00

40.00

30.00

20.00

10.00

0.00

0.50

0.50

1

k i - 0 142 J/cm-s Dj - 0.0594 cm2

/s

k h - 0.1 J/cm-s-K Dh = 0.05 cm 2

/s

1.00 1 50 200

k j = 0.0142 J/cm-s-K

Dj = 0.00594 cm 2

/s

k h =0.1 J/cm-s-K

Dh = 0.05 cm2s

1.00 —I1.50 2.00 T

k j =0.00142 J/cm-s-K

Dj =0.000594 cm 2/s kh-0.1 J/cm-s-K

Dh =• 0.05 cm 2

/s

0.00 0.50 1.00 1.50 Inclusion radius (microns)

2.00

25.00

20.00

15.00

10.00

5.00

0.00

k|DikhDh

0.142 J/cm-s-K 0 0594 cm

2/s

0.1 J/cm-s-K

0.05 cm

2

/s

0.00

25.00

^ 20.00 o

XIO 15.00 -C

in

<x>

0) 10.00

03E

03

5.00

0.00

0.00

25.00

20.00

15.00

10.00

5.00

0.00

050 1 00 1.50 2.00

k i =0.142 J/cm-s-K

Di = 0.0594 cm

2

/s

k h = 0.01 J/cm-s-K

Dh = 0.005 cm 2

/s

0.50 1.00 1.50 2.00

I I

Ki

I

= 0.142^ /cm-s-K

Di = 0.0594 cm 2/s

k h = 0.001 J/cm-s-K

Dh = 0.0005 cm

2

/s

i

1 nn 1 50 Inclusion radius (microns)

2.00

(a) (b)

Figure 2. Effects of varying thermal conductivity on damage threshold. In the left hand column (a), host

conductivity is held constant and inclusion conductivity and diffusivity are varied. In the right

hand column (b) inclusion conductivity is held constant while host conductivity and diffusivity are varied.

579
they are reduced proportionally. The damage threshold is plotted as a function of the inclusion radius.

As expected, damage thresholds decrease for decreasing host thermal conductivity. Conversely, thresholds

increase overall for decreasing inclusion thermal conductivity. Damage thresholds change by roughly a factor

of two for each order of magnitude change in conductivity. The damage threshold is therefore relatively

weakly affected by large changes in host/inclusion thermal conductivities. This suggests that the use of this

model remains appropriate for the much lower thermal conductivities recently observed. Note too that, as expected in the Mie absorption process, the minimum damage threshold occurs at the same inclusion radius

as the thermal conductivities are varied. The thermal impedance mismatch between host and inclusion is not

considered in these calculations since including the impedance would only complicate the calculations without

effecting the qualitative results presented here.

20.00

^ 15.00 o

T3O

-Co 10.00

k_ x:

a)o>

roE

g 5.00

0.00

-15.00 -10.00 -5.00 0.00 n, = Im (index of refraction) t

Figure 3. Damage Threshold as a function of nr

.

4. Variation of the Imaginary Part of the Index of Refraction

The damage threshold is shown in Figure 3 to be quite sensitive to changes in the imaginary part of the

index of refraction n7 , particularly for ri/ of magnitude much less than one. Normally in the use of this model

the imaginary part of the index of refraction is taken to be a free parameter to be fitted to experimental

damage thresholds. The sensitive dependence of the damage threshold to index of refraction variation may

therefore provide a means to compensate for the changes in thermal conductivity discussed in the previous

section. As can also be seen in the figure, a given damage threshold is predicted by two values of n7. These

values correspond in the model to two different physical mechanisms for damage one for rij much less than one and one for rij much greater than one. As shown in Tables 1 and 2 the values of rij that predict a given

damage threshold vary slowly for large thermal conductivity variations.

Table 1. Effects of varying inclusion thermal conductivity and diffusivity on the imaginary part of the refractive

index.

Kh = .l(J/cm-s- K), Dh =.15cm2 /s

K{ D{ n'(i)

0.142 0.0594 -0.011 -14.0

0.0142 0.00594 -0.015 -12.0

0.00142 0.000594 -0.034 -8.2

0.000142 0.0000594 -0.029 -8.8 = smallest magnitude root

^1(2) — largest magnitude root

k h =0.1 J/cm-s-K

580
Table 2. Effects of varying host conductivity and diffusivity on the imaginary part of the index of refraction.

Kh = .142( JI cm -s- K), D{ = .0594cm2 /s

Dh n/(i)

0.1 0.15 -0.011 -14.0

0.01 0.15 -0.0040 -23.0

0.001 0.0015 -0.025 -27.0

0.0001 0.00015 -0.0015 -35.0

Jij(i) = smallest magnitude root

ni{2) = largest magnitude root

25.00

i i

I Laser pulse

I

. Host

20.00

E

o

-g 15.00 O

w

03i_ ~ 10.00

0)CO

n3E

ro Q 5.00

0.00

I nj- -0.011 ^ I

k h =0.1 J/cm-s-K

Dh =0.15cm 2

/s

k | =0.0142 J/cm-s-K

D| = 0.0594 cm2/s

X = 1 .06 um

Re(n) =1.3

t ( =1.5x10'8

s

_L

0.00 0.50 1.00 1.50 Inclusion radius (microns)

2.00

00 (b)

Figure 4. Bulk absorption by the inclusion. The left hand side (a) illustrates the proposed mechanism. On

the right hand side (b), damage threshold is plotted as a function of inclusion radius for rij =

-.011.

In the first proposed mechanism, an rij of very small magnitude means that the skin depth 6 of the

inclusion is on the order of or greater than the radius of the inclusion r{ (Fig. 4a). The inclusion can therefore

be regarded as heating uniformly, absorbing some energy EA proportional to the inclusion volume ('bulk'

absorption). Energy diffuses some distance 1 (the diffusion length) into the host. The total energy diffused,

ED , is then proportional to (r{ + /) 3

. The damage threshold is then proportional to ED /EA . This ratio

approaches a constant for large inclusion radius, and infinity as the radius goes to zero (Fig. 4b).

For n7 of magnitude much larger than one, on the other hand, the skin depth 6 of the inclusion is much

less than the radius of the inclusion (Fig. 5a). The energy EA absorbed by the inclusion is thus proportional

to

EA <x 3r,

2 [t + 6) + 3r, (£2 - 62

) + £

3 + 63 (9)

581
0.00 0.50 1.00 1.50 2.00 Inclusion radius (microns)

(a) (b)

Figure 5. Surface absorption by the inclusion. The left hand side (a) illustrates the proposed mechanism.

On the right hand side (b) damage threshold is plotted as a function of inclusion radius for rij =

-14.0.

This is referred to as 'surface' absorption. So in the limit of large r\ (assuming 6 <C r, ) the ratio ED /

A that determines the damage now goes linearly in r,- (Fig. 5b). For very small inclusion radius, that is, for a skin depth of approximately r, , the bulk absorption picture once again holds, yielding a damage threshold

approaching infinity as r, goes to zero (Fig. 5b).

This work does not propose that a specific inclusion material should be considered to have two very

different indices of refraction. Rather it suggests that despite the fact that diffusion lengths for host and

inclusion are not explicitly included in the model, two very different physical mechanisms for absorption can

be investigated theoretically by using appropriate indices of refraction. Particularly interesting are the results

of plotting laser damage as function of inclusion radius for the two nj that yield a damage threshold of

13J/cm2 (Fig. 6).

As can be seen in the figure, using an index of refraction yielding a surface absorption mechanism for

the inclusion absorption gives qualitatively better agreement with experimental results [5]. On the other

hand, a value of nt chosen to model inclusion bulk absorption shows good agreement for large radii. For

values of the imaginary part of the index of refraction between these two extremes, the correct mechanism

represented by the model is most likely a composite of inclusion bulk and surface absorption. These results

suggest that both types of absorption must be considered to obtain the best possible theoretical picture of

laser damage. Changing the index of refraction, though nonphysical, provides a simple method to study the

qualitative differences in the two mechanisms using the inclusion dominated model. These results further

suggest that the inclusion dominated model could be adapted to make predictions of laser damage not only

for those experimental cases where uniform absorption can be assumed, but also to cases where the absorption

is nonuniform. Ideally, such an adaptation will include a more physically accurate method of including bulk

and surface absorption mechanisms for inclusion absorption.

5. Summary

In conclusion, this work shows that the damage threshold predictions of the inclusion-dominated model

for laser damage in thin films are relatively weakly affected by extremely large (order of magnitude) changes

in the thermal conductivities. On the other hand, predicted damage threshold energies are rather sensitive to the imaginary part of the index of refraction n7 in this model. A given damage threshold is predicted

by two different values of n./, one of much smaller magnitude than the other. Different damage mechanisms

can be associated with the two different indices of refraction. In the one, bulk absorption dominates, in the

other, surface absorption. Through a combination of these mechanisms, it appears that the model can be

adapted to yield realistic damage thresholds and indices of refraction for a variety of inclusion/host thermal

conductivities.

582
Figure 6. Comparison between experimental [5] data and model for ThF4 films.

6. References

[l] H. Goldenberg and J. C. Trantor, "Heat Flow in an Infinite Medium Heated by a Sphere," Brit. J. Appl.

Phys., 2; 296-298; 1952 November.

[2] T. W. Walker, A. H. Guenther and P. Nielsen, "Pulsed Laser-IOnduced Damage to Thin-Film Optical

Coatings Part II: Theory," IEEE J. Quant. Electr., QE-17; 2053-2065; 1981 October.

[3] M. R. Lange, J. K. Mclver and A. H. Guenther, "Pulsed Laser Damage in Thin-Film Optical Coatings:

Flourides and Oxides," Thin Solid Films, 125; 143-155; 1985 April.

[4] D. C. Lambropoulos, M. R. Jolly, C. A. Amsden, S. E. Gilman, M. J. Sinicropi, D. Diakomihalis and S. C.

Jacobs, "Thermal Conductivity of Dielectric Thin Films," J. Appl. Phys. 66; 4230-4242; 1989 November

1.

[5] T. W. Walker, A. H. Guenther and P. Nielsen, "Pulsed Laser-Induced Damage to Thin-Film Optical

Coatings Part I: Expeirmental," IEEE J. Quant. Electr., QE-17; 2041-2052; 1981 October.

583
Manuscript received

Theoretical Determination of the Nonlinear Optical Properties of Inorganic Polymers 2-15-90

Steven M. Risser and Kim F. Ferris

Pacific Northwest Laboratory1

, Materials and Chemical Sciences Center, Richland WA 99352

ABSTRACT

Third order hyperpolarizabilities were calculated using the Huckel Hamiltonian and

third order perturbation theory for a series of phosphonitrilic compounds, (X2P-NX1, as a

function of bond length alternation, ligand substitution and backbone conformation.

Phosphonitrilic compounds show hyperpolarizabilities comparable to those reported for

organic species, and are modulated by ligand group electronegativity. In contrast to organic

polyenes, the difference in tz orbital energy between phosphorus and nitrogen is critical to

determining the onset of saturation and the magnitude of the hyperpolarizability. Conformation

effects are smaller than those seen in polyenes.

Keywords: Nonlinear, hyperpolarizability, inorganic polymers, optical properties,

phosphazenes

1.0 INTRODUCTION

While the search for materials with large macroscopic nonlinear properties has often

focused on conjugated organic molecules, inorganic polymers feature many similar chemical

bonding interactions and should also be viewed as viable candidates. Recent experiments on

polyphosphazenes have shown that these polymers exhibit large bulk nonlinearities and high

laser damage thresholds2. These properties combined with their transparency in the visible

spectrum mark these compounds as potential alternatives for use in fabricating optical devices.

Most of the electronic properties of the polyenes, such as absorptions and both the

linear and nonlinear optical properties3

, are governed by the conjugated n bonding electrons.

Inorganic polymers such as the phosphonitrilic system feature two independent 7t-bonding

networks, the normal n network which is constructed from out of the plane pz orbitals, and a

second, less highly conjugated ri network lying in the plane of the molecule4. The interaction

of the two conjugated n electron systems has large effects on the electronic properties of the

polyphosphazenes, increasing the optical transition energies of the out-of-plane n electrons5 as compared to organic molecules, and making intrinsic contributions to the electronic

hyperpolarizabilities of the molecule.

The similarities in the chemical bonding between organic and inorganic polymers would

indicate that some features which affect the nonlinear optical properties of polyenes, such as

degree of bond alternation and chain conformation, should be considered for their potential

influences on inorganic polymers. In addition, characteristics such as the large

electronegativity differences between atoms along the polymer backbone which are unique to

the inorganic polymers, may also have large effects on the hyperpolarizabilities. This

584
electronegativity difference may restrict the delocalization of the k electrons and decrease the

magnitude of the hyperpolarizabilities.

R1\ P2 R\ .>-R

2 n v P N Pv 3 \P N/ \P- N/V3 ^ "R2 R1 /R2

Figure 1. Schematic of phosphazene tetramer

We have calculated third order hyperpolarizabilities for a series of phosphazenes,

(X2PN)n (tetramer shown in Figure 1), as a function of its molecular parameters, that is, ligand

substitutions, bond alternation and chain conformation. These effects of these parameters on

the hyperpolarizability are compared to similar calculations for polyenes, and are interpreted in

terms of the electronic structure and chemical properties of the phosphazene.

2.0 METHODS

The nonlinear optical properties of organic molecules are dominated by the conjugated

7t electrons, allowing them to be treated in a predictive sense by a % electron Hamiltonian for

the determination of their hyperpolarizabilities6

. The Huckel Hamiltonian was selected for this work based upon two features: 1) The chemical bonding of phosphonitrilic systems are

dominated by k bonding interactions, and 2) The simplicity of the Hamiltonian allows also for

calculation of the hyperpolarizabilities for a range of molecular parameters.

The Huckel Hamiltonian is a tight-binding k electron Hamiltonian allowing interactions

between near neighbor atoms. In second quantized form, this Hamiltonian can be written as:

A=y a.c.t c. + y m cj c, +C+C,)

i ,o i ft] o

where q (qt) is the destruction (creation) operator, which destroys (creates) a n electron on atom i. The second summation is restricted to bonded pairs of atoms i,j. Atomic pz orbitals are assumed to be orthogonal, and molecular eigen states are formed as linear combinations of the

atomic pz orbitals. The ground state of the molecule is formed from the Slater determinant of

the occupied molecular eigen states.

There are two sets of matrix elements which define the Hamiltonian, the Coulomb term

oq which is approximately equal to the ionization energy of a p electron atom i, and the hopping

term Py describing the transfer of an electron from atom i to atom j. Ligand group effects at the

phosphorus site were modelled by using a range of differences in the 7t orbital energy (Aa)

between the nitrogen and phosphorus centers. Structural parameters for the polyphosphazenes were taken from recent ab initio calculations for (F2PN)4

. Because of the large role bond

alternation plays in calculations of the hyperpolarizability for polyenes and the existence of

bond length alternation in short chain length phosphazenes4b- 7

, we have performed calculations

585
for a series of bond length alternations (0.00 Ppn = no bond alternation).

The third order hyperpolarizability can be determined by expanding the energy of the

molecule in a Taylor series, and using perturbation theory to calculate the coefficients of the

expansion. Thus, the third order hyperpolarizability can be shown to be:

YaPX8=P

^ifeo (Ei-E0 )(^E0)( Ek-^)

olpjixiluploxolu^ljxglpglo

" ifo (^-^)(^-^)(^-E0 )

<o|uJoxo |pp

|i><3 Im^IH lM$l 0>

-2E

U/o (^-E0 )(^-E0)(^-E0)

<o|^a

|oxo|np|o>o||i

x

|ixi|^

6|o>

+ f0 (Ej- E^- E^- E

Q

) /

where a, p, X, and 8 refer to Cartesian coordinates in the molecular reference frame, P being the

permutation operator for a, p, X, 8; \i the one electron dipole operator, 10) is the ground state

wavefunction, li) Ij) Ik) are excited state wavefunction, E0 the ground state energy and Ej the

excited state energy. Summations extend over all possible excited states. Because of the

nature of the dipole operator, it is possible to restrict the first summation to single and double

electron excitations. The latter three summations are restricted to single electron excitations.

3.0 RESULTS

The focus of the current investigation is to provide a theoretical basis for the nonlinear

optical properties of phosphonitrilic polymers. While there are a number of similarities in the

electronic properties between the organic and phosphonitrilic systems, one of the significant

differences is the charge distribution along the polymer backbone. Although polarized

chemical bonding situations are well known to exist in organic polymer systems, larger

electronegativity differences are still possible when second row elements are included in the

polymer backbone. The effects of these changes in the electron distribution and their

interactions with bond alternation contributions are examined in the following discussion on the

hyperpolarizability of the polyphosphonitrilic system.

Molecular Structure and Conformation

Crystallographic studies of the chloro-substituted polymer have shown the bonds to be

of equal length within the resolution of the measurements8

, while recent ab initio calculations

have shown the existence of bond length alternation in the small linear phosphonitrilic

compounds. The existence or non-existence of bond alternation may be vital, as bond

alternation has been firmly established as a principal factor in theoretical calculations of the

586
third order hyperpolarizabilities in polyenes. Previous theoretical calculations of the

hyperpolarizability of trans and cis polyenes have shown that the magnitude of bond alternation

plays a crucial role in deciding not only the magnitude but also the sign of the average

hyperpolarizability6 . The effects of possible bond alternation on the third order

hyperpolarizability for a fixed difference in orbital energies (Aa=0.6PpN)

^

shown in Figure

2. The average hyperpolarizability per repeat unit for the cis-trans conformation is shown for

five distinct amounts of bond alternation. The inclusion of bond length alternation has no

qualitative effects on the third order hyperpolarizability, and causes only a slight decrease in

magnitude. This is in contrast to the cis-trans polyenes, where greater degrees of bond

alternation have been shown to increase the average hyperpolarizability.

The large amount of flexibility in the backbone of inorganic polymers gives rise to more

than one possible conformation for the polymer. Both experimental and theoretical

determinations of the hyperpolarizability of polyenes have shown that the hyperpolarizability

COE

E

to

TOo

alt=0.00P PN alt=0.02P PN alt=0.04P PN alt=0.06PPN

alt=0.08PPN

0.5

Figure 2. Hyperpolarizability per repeat unit of cis-trans conformation vs. number of repeat

units. The energy difference between pz orbitals is held constant at Aoc=0.6Ppn.

per repeat unit is dependent on the conformation of the polyene, but the magnitudes of the

hyperpolarizability for the all-trans and cis-trans conformations are almost exactly identical per

unit length along the primary axis of the molecule. Since inorganic systems commonly assume

trans, cis-trans, and helical conformations due to their high backbone flexibility, the

phosphazene conformation may also be significant in determining the hyperpolarizability of

phosphazenes. To determine whether this same length dependence exists in phosphazenes, we

have also performed calculations on the trans conformation of the polyphosphazene. The

structural parameters were kept the same as for the cis-trans phosphazene. The

hyperpolarizability per repeat unit for the trans conformation with varying degrees of bond

587
5

o alt

alt

alt

alt

alt

2

0.5 0.7 0.9 log N

1.1 1.3 1.5

Figure 3. Hyperpolarizability per repeat unit of trans conformation vs. number of repeat units. The energy difference between pz orbitals is held constant at Acx=0.6Ppn.

alternation is shown in Figure 3. The difference in orbital energies was held constant at Aa=0.6ppN to allow for direct comparisons with the cis-trans results in Figure 2. The

magnitudes of the hyperpolarizability per repeat unit for the trans conformation are comparable

to those of the cis-trans conformation indicating that the length of the conjugated system, and

not the backbone conformation, is also the important feature for inorganic polymers.

The onset of saturation and the increase in nonlinearity per repeat unit determines the

bulk nonlinearity for large molecular weight polymers. The rapid increase in the

hyperpolarizability observed for small number of repeat units as shown in Figure 2 does not

continue indefinitely, but instead quickly reaches saturation, where the nonlinearity becomes

linearly dependent on the number of repeat units. Previous research in organic polymers has

shown that the nonlinear optical properties saturate at a small number of repeat units, with the

degree of bond alternation included in theoretical models being the key ingredient in

determining when the models show saturation. The similarity of the shapes of the curves in

Figures 2 and 3 indicate that bond alternation has no effect on the onset of saturation for the

phosphonitrilic system. The decrease in the magnitude of the curves when bond alternation is

included show that the existence of bond alternation in the polymer will decrease the magnitude

of the bulk nonlinearity for either conformation of the polymer.

Electronic Interactions

The chemical bonding in phosphazenes has distinctly different electronic properties

from organic polymers which may play a large role in determining their hyperpolarizabilities.

Perhaps most important of these is the interaction of the substituent groups attached to the

phosphorus with the in-plane and out-of-plane 7t bonding networks. The effects of varying

substituent groups on the hyperpolarizability of phosphazenes was represented through

variations in the energy of a pz electron on the phosphorus atoms. Figure 4 shows the

hyperpolarizability per repeat unit plotted against the number of repeat units for a number of

energy differences, Aa, between the nitrogen and phosphorus pz orbitals, with the bond

alternation held constant at 0.04pPN. Changing the energy difference between orbitals has a

588
very large effect on the hyperpolarizability. Decreasing the orbital energy difference increases

both the magnitude of the hyperpolarizability and its rate of growth with increasing number of

repeat units. This is most evident for the Aa=0.0 curve, where the hyperpolarizability curve

still exhibits significant curvature even for 60 repeat units. The effects of conformation on these electronic influences were investigated by

performing similar calculations on the trans conformation of the phosphazenes. Figure 5

shows the hyperpolarizability per repeat unit of the trans conformation plotted against the

number of repeat units for a number of values for Aa, with the bond alternation held constant at

a value of 0.04pPN. The curves appear very similar to those of Figure 4, with the exception of

for very small number of repeat units, where the Aa=0.0 curve drops noticeably below that for

the 0.5 eV curve.

Figure 4. Hyperpolarizability per repeat unit of cis-trans conformation polyphosphazene vs. number of repeat units for various energy differences between pz orbitals on nitrogen and

phosphorus. The bond alternation is held at 0.04Ppn.

Figure 5. Hyperpolarizability per repeat unit of trans conformation polyphosphazene vs. number of repeat units for various energy differences between pz orbitals on nitrogen and

phosphorus. The bond alternation is held at 0.04(3pn589
4.0 CONCLUSIONS

We have calculated the third order hyperpolarizability for phosphazenes of increasing

length using the Huckel Hamiltonian and third order perturbation theory, using a series of

possible bond alternations and representations of different substituent groups on the

phosphorus. These calculations have shown that bond alternation does not appear to play a

large role in determining the saturation of polyphosphazenes, in contrast to the results observed

for organically based polyenes. Substituent effects on the electronic structure play crucial

roles in determining the hyperpolarizability, with the largest magnitudes coming with weakly

electron drawing groups.

5.0 REFERENCES

[1] The Pacific Northwest Laboratory is operated by Battelle Memorial Institute for the U.S.

Department of Energy under contract DE-AC06 76RLO-1830.

[2] G.J. Exarhos and K. Crosby, Laser Induced Damage in Optical Materials: 1989, NBS Spec.

Publ., (submitted).

[3] B.F. Levine and C.G. Bethea, J. Chem. Phys. 63, 2666 (1975).

[4] K.F. Ferris, P. Friedman, and D.M. Friedrich, Int. J. Quant. Chem. S22, 207, (1988); K.F.

Ferris and C.B. Duke, Int. J. Quant. Chem. S23, (1989) in press.

[5] K.F. Ferris, S.M. Risser, A.K. Hanson, Proc. Mat. Res. Soc: Electrical, Optical and

Magnetic Properties of Organic Solid State Materials, ed. C. Liang, in press.

[6] S.M. Risser, S. Klemm, D.W. Allender and Michael A. Lee, Mol. Cryst. Liq. Cryst., 150b,

631 (1987).

[7] H.R. Allcock, N.M. Tollefson, R.A. Arcus, R.R. Whittle, J. Am. Chem. Soc. 107, 5166

(1985).

[8] H.R. Allcock in "Phosphorus-Nitrogen Compounds" New York: Academic Press (1972).

590
Manuscript received

3-8-90

RELATION BETWEEN n2 AND

TWO-PHOTON ABSORPTION

M. Sheik-Bahae, D.J. Hagan, E.W. Van Stryland,

T.H. Wei, A.A. Said, E. Canto, and A. Miller

Center for Research in Electro-Optics and Lasers and the Department of Physics

University of Central Florida

Orlando, FL 32816

We present evidence from nonlinear refractive index measurements, two-photon absorption measurements, and four- wave mixing measurements on semiconductors showing that the bound electronic nonlinearity can be calculated from

two-photon absorption dispersion via a simple Kramers-Kronig analysis. This analysis shows n2 changing from

positive to negative as the photon energy approaches the band-gap energy, consistent with observations. Additionally,

this simple calculation, which assumes two parabolic bands, gives good agreement with measured values of n2 in widegap dielectrics that are 2 to 3 orders of magnitude smaller than in semiconductors.

Key words: nonlinear refraction, two-photon absorption, Kramers-Kronig, four-wave mixing, semiconductors

Optical nonlinearities in semiconductors and transparent materials have been the subject of extensive studies during the

past two decades. Of particular interest have been the third order effects in semiconductors. This interest has

primarily dealt with the production of a real carrier plasma and the associated negative change in the refractive index.

In this letter we will discuss the smaller and "faster" nonlinearities due to bound electrons which is positive for photon

energies much less than the bandgap energy. The analogy in wide-gap dielectric materials is to self-focusing from the

positive nonlinear refractive index n2 (eg. self-focusing in glasses). Recently we found that measurements in semiconductors substantially above the two-photon-absorption (2PA) edge yield negative values for n 2 .[l] This

dramatic dispersion of n2 can be partially explained using a nonlinear Kramers-Kronig (KK) relation between n2 and

the 2PA coefficient /8, or between the real and imaginary parts of the third order nonlinear susceptibility x^

3^- The

magnitude and dispersion of n2is of interest because of its importance in nonlinear propagation in fibers, fast optical

switching, self-focusing and damage in optical materials, and optical limiting in semiconductors. Below, we describe our calculation of the bound electronic Kerr effect, n2

, and compare our results with data for a large number of

materials, including semiconductors and dielectrics.

This n2 arises from the real part of x^> and is defined through the refractive index change An, where

An(u/> = -y(u) Iw = ^|EJ*, (1)

with Iu and Eubeing the irradiance and electric field at frequency w respectively and n2=(27r/n0)Re{x(3

)}. The linear refractive index is n0

, and 7 and n2 are related by n2(esu)=(cn

oif/407r)(MKS) with c the speed of light.

Most theoretical calculations of n2 have been confined to the zero frequency limit.[2-6] Semi-empirical formulations

have been most successful in predicting the magnitude of n

2 .[5,6] For example, the formula obtained by Boling, Glass

and Owyoung (BGO) in relating n2 to the linear index (n0

) and the dispersion of n0 in terms of the Abbe number has

been successfully applied to a large class of transparent materials.[6, 7] Their theory predicts the low frequency

magnitude of n2

. We describe a KK method of calculation of n2 that predicts the dispersion as well as the magnitude

of n2

. This calculation assumes that 2PA gives the dominant contribution to n2 and that other contributions from

Raman and the AC Stark effect or virtual band-blocking are ignored. We will return to this assumption later.

591
Based on the principle of causality, the KK transformation states that a change in the refractive index (An) at u is associated with changes in the absorption coefficient (Aa) throughout the spectrum (a/) and vice versa. We write this

as:

An(w;£) =

J

AoM|i d^

(2)

where c is the velocity of light in vacuum and £ is a parameter (or parameters) denoting the "cause" of change in the absorption. The cause need not be of optical origin but of any external perturbation such as thermal excitation, etc. For cases where an electron-hole plasma is injected, the consequent change of absorption has been used to obtain the plasma contribution to the refractive index. In this case, the £ parameter is taken as the change in plasma density (AN)

regardlesss of the mechanism of generation of the plasma or the pump frequency.[8] In the case of 2PA the change is due to the presence of a pump field at fl (ie. £=ft). The corresponding nonlinear refraction is An(w, fl), which gives the dispersion of the index change with w. For the case of self-refraction, (j=Q, and this gives what is commonly referred

to as n2

. Van Vechten and Aspnes [4] obtained the low frequency limit of n2 from a similar KK transformation of the Franz- Keldysh electro-absorption effect where, in this case, £ is the DC field. The bound electronic contribution to

x(

3) can originate from various absorptive counterparts that are quadratic functions of the pump field. Effects of this order may include 2PA, the electronic Raman effect, and the optical Stark effect. Here we consider only 2PA.

A wealth of experimental and theoretical work regarding 2PA in semiconductors and crystalline materials exists. In accordance with the predictions derived from either second order perturbation theory [9, 10] or a Keldysh-type

formalism [11], the 2PA coefficients of the semiconductors studied in Ref. [12] were found to be in good agreement

with the theoretical expression given as:

VeT

/W) = K F2(2ftu//E

g

) , (3) n0 E

g

where K is a material independent constant and Ep (related to the Kane momentum parameter, a momentum matrix

element) is nearly material independent and posseses a value of =21 ev for most direct gap semiconductors. These data

ire shown in Fig. 1. Note /?=(47ra;/n 0)Im{x(3

)}. The function F2

is only a function of the ratio of the photon energy

huf to E

?

(ie. the optically coupled states). The functional form of F2 reflects the assumed band structure and the intermediate states considered in calculating the 2PA transition rate. The simplest model assumes a pair of isotropic

and parabolic bands and intermediate states that are degenerate to initial (valence) or final (conduction) states. Neglecting the Coulomb interaction, this simple formalism yields: [10]

F,(2x) = ( 2x~

1)

.

3/2 for 2x>l . (4)

(2x)

5

Figure 2 shows the dispersion of P as given by Eq. 4.

The best fit to the data of Ref. 12 using Eqs. 3 and 4 gave K=3.1xl03 (see the line in Fig. 1) in units such that Ep and

Eg were in eV and 0 was in cm/GW, while theory gave 5.2xl03 .[9] When nonparabolicity was included the average /? was 26% lower than theory; however, the frequency dependence of /5 changed very little. Interestingly, Eqs. 3 and 4

also give a fair estimate of p for a number of transparent materials measured using the 3rd and 4th harmonics of

picosecond N<± YAG laser pulses.[13, 14]

Equations 3 and 4 pertain to a degenerate case where the two photons involved are of the same frequency and source. For a KK transformation the nondegenerate 2PA coefficient for two distinct frequencies is needed (ie. fi the "cause" and uf the integration variable in Eq. 2). Extending the same simple model to obtain the non-degenerate 2PA

coefficient has led to dispersion functions that are afflicted with "infrared divergences".[15, 16] This has been a common problem originating from the use of the A p perturbation to calculate the bound electronic nonlinear

susceptibilities in solids.[15]

592
1 1 7C]—

GaAs !

_i i

i

i i 1

1.5 2.0 2.5 3.0 3.5 4.0

E,|ev)

Figure 1. A log-log plot of the scaled value of f) as a function of Eg to show the Eg dependence. The line is a fit

to the data using the F2 of Eq. 4.

0.10 .

,

, 1 1

1 0.09 -

0.08 -

0.07 -

0.06 -

2Ha;

Figure 2. A semilogarithmic plot of F2 as a function of 2fto;/Eg with the data of Fig. 1 superimposed.

Although special cases have been considered [17], a general theory that would rigorously address the proper scaling and

dispersion of the non-degenerate 2PA is yet to be developed. For this reason we assume that the dispersion function

F2 for the nondegenerate 2PA coefficient, /?(w',ft), can be given by Eqn. 4 modified with the substitution of 2huf by

fiui'+hQ; thus, F2(2x) is replaced by F2(x'+X), where x'=ftu)'/Eg and X=ftfl/E

g

, and x'+X>l. This substitution is strictly

valid only for x'=X, however, the predictions resulting from this substitution show remarkable agreement with the

data, as will be shown.

593
The change of the absorption spectrum (at u') induced by the presence of a strong pump at fi can be written for 2PA

as Aa(u/;fi) = £(w';fi)In , where I n

denotes the irradiance of the pump field. Similarly the change in refractive index at w induced by the presence of a strong pump at fi can be written as An(u>;fi) = 7(u>;fi)In

. Applying the KK

transformation Eq. 2 at this point yields a relation between 7(w;ft) and /9(o/,fl). Using Eq. 3 with F2(x'+X) in Eq.2 we

obtain for the degenerate case (<j=fi):

Tf = K —^G2(fe/E

g), (5)

2no2£

g

where the dispersion function G2(x) is given by,

r M _ -2+6x-3x2-x3-3/4x4 -3/4xs+2( 1 -2x) 3/2 6( 1 -2x)

with 6(x) being the unit step function.

Using the value of K obtained from the 2PA measurements, Ep =21 eV, and converting from 7 to n2

, we obtain the

final expression for n2as:

n2(esu) = K' 2' ' *'

, (7) noEg

where K'=3.4xl0~8 and Egis in ev. Equation 7 explicitly shows an E~4 band-gap dependence for the magnitude of n2

, and the sign and the frequency dispersion of this quantity are given by the simple closed form function G2

. We can now readily compare the predictions of this theory with experiment.

Utilizing a newly developed technique (Z-scan) for n2 measurements [1,18] that can determine its magnitude and sign, we have measured n2 for several materials at 1.06 and 0.53 pm. This simple technique has been shown to be an

accurate and sensitive tool for measuring n2 even in the presence of nonlinear absorption. We find, for example, in materials such as ZnSe at 0.53 pm, where 2PA is present, n2

is negative, but that the sign changes at 1 .06 ^m.

Picosecond degenerate-four- wave-mixing (DFWM) measurements show this third order response (time resolution

limited by the 30 ps pulsewidth), while at higher irradiances the slowly decaying 2PA generated free-carrier refraction

is seen.[19] Figure 3 shows the DFWM experimental geometry. The DFWM signal as a function of backward pump

delay is shown in Figure 4. This signal shows a "fast" third order nonlinearity followed by a slowly decaying "fifth" order nonlinearity.

Modelocked

Nd: YAG Laser V

\

input detector

output detector

Figure 3. A schematic of the experimental geometry of the degenerate-four- wave-mixing experiment.

The order of the nonlinearity was determined by fixing the temporal delay at a) zero delay, or at b) long delay, and

simultaneously varying the input energy of all three input beams. A log-log plot of the DFWM signal versus input

energy (or irradiance) shows straight lines having slopes of three at zero delay (a) and five at long delay (b) as shown

in Figure 5. The fast response of the third order nonlinearity at zero delay (time response limited by the 30 picosecond

laser pulsewidth) indicate its bound electronic origin. The higher order nonlinearity observed at long delays is the

594
plasma nonlinearity due to carriers produced by 2PA, a sequential Im{x(3)}:RE{xW} effect. This appears as an

effective fifth order nonlinearity.

24

20 -

Backward Pump Delay (ps)

Figure 4. A plot of degenerate-four-wave-mixing signal as a function of backward pump beam temporal delay

using the geometry of Figure 3.

10' -

10' -

20 60 100

Irradiance (MW/ cm2

)

Figure 5. A log-log plot of the DFWM signal at delays a) and b) of Fig. 4 as a function of input with the energy

of all three input beams varied simultaneously.

A graphical comparison of the data for n2is given in Figure 6 which shows the band-gap scaling law and in Figure 7 where the dispersion and sign of n2 are explicitly compared to the G2 of Eq. 7. Many of the experimental values for

large gap optical crystals are obtained from recent measurements by Adair et.al. using a "nearly degenerate-threewave-mixing" scheme.[7] We have also measured n2 in a number of these wide-gap dielectrics using the Z-scan

method and have obtained comparable results. The dispersive behavior of n2 in Figure 6 is seen to be most significant

within the range Eg /2<#u;<E

g where 2PA is present and relatively small in the low frequency limit hw«Eg

.

A noticeable difference between the magnitude of the measured and calculated values is seen near the one photon

absorption edge. Considering the simplicity of the model in deriving Eq. 7, such deviations are not unexpected. The

measured large negative values of n2 as compared to the calculated values near the fundamental absorption edge may

be attributed to the refraction due to the "optical Stark effect" which is also referred to as "virtual band-blocking" and

has been ignored in our calculations. The contribution of this mechanism to the electronic nonlinear susceptibility has

been shown to have a strong band-gap resonance and follows the same scaling as given in Eq. 7.[20] This effect,

which is negative for all frequencies below the band-gap, vanishes quickly for longer wavelengths (#w<E

g /2) and has

negligible contribution in the transparency region of the material.

595
10

Figure 6.

Figure 7.

I

I I I I | - CdTe

0.5 Se0.5

Eg (ev)

A log-log plot of the scaled n2 versus energy gap (E

g

) for a large class of optical materials. The solid line represents the theoretical result as obtained from Eq.(7) with no adjustable parameters and has a slope of

-4. The solid circles represent the data from ref.[7] all obtained at A=1.06 /mi. The remaining data are our measurements using the Z-scan technique taken at A=1.06 /xm (solid squares) and at A=.532 /im (open

squares).

o

0.15

0.10

0.05

0.00

-0.05

-0.10

-0.15

CdS

O CdS.-Se-.

0.8 0.9

CdTe o

I I I L

ZnSa „ J I I L

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

ftW/Eg

The calculated dispersion function G2 versus ftw/E

g

. The same n2 data as in Figure 6, here scaled as n2n0Eg/K', are compared with the theory. Only the semiconductor data within the highly dispersive

region are labeled for comparison.

596
G.D. Mahan, Theory of Two-Photon Spectroscopy in Solids," Phys. Rev. 170, 825-838(1968).

F.V. Bunkin, "Two Quantum Transitions in Optics," Sov. Phys. XETP, 23, 1121-1123(1966).

M. Sheik-bahae, A.A. Said, and E.W. Van Stryland, "High Sensitivity, Single Beam n2 Measurements," Opt. Lett. 14, 955-957(1989).

DJ. Hagan, E. Canto, E. Miesak, M.J. Soileau, and E.W. Van Stryland, "Picosecond Degenerate Four Wave

Mixing Studies in ZnSe," paper, TUX, pp.160, Technical Digest of the Conference on Lasers and Electro-Optics,

Anaheim, CA, OSA Technical Digest Series, No. 7, 1988.

B.S. Wherrett, A.C. Walker, and F.A.P. Tooley, "Nonlinear Refraction for CW Optical Bistability," in Optical

Nonlinearities and Instabilities in Semiconductors, H. Haug, eds. (Academic Press, Inc. 1988.) pp.239-272.

597
Manuscript received

3-20-90

Photoconductivity of ZnS and ZnSe

B. E. Mason and C. D. Marrs

Physics Division, Research Department

Naval Weapons Center, China Lake, California 93555-6001

ZnS and ZnSe are important materials for laser windows and optical thinfilm coatings. Understanding the laser-induced damage mechanism in optical

materials provides the ability to make improved damage-resistant materials.

Photoconductivity (PC) techniques have demonstrated the capability to provide

information on carrier production that can lead to electron avalanche by single or multiple photon absorption processes produced by a high-intensity laser

beam. A study of the linear PC using a monochromated Xe lamp and the

nonlinear PC using various lasers as excitation sources will be presented for

chemically vapor-deposited ZnS and ZnSe.

Key words: charge-production; CVD ZnS; CVD ZnSe; electron avalanche;

impurity absorption; laser-induced damage; multiphoton absorption;

photoconductivity

1. Introduction

ZnS and ZnSe are popular materials for optical components in high-power laser systems.

Owing to the 3.8 and 2.7 eV band gaps for ZnS and ZnSe, respectively, excellent

transmittance from the visible to IR and relatively high laser damage thresholds are observed. These properties make ZnS and ZnSe ideal materials for windows and coating

materials in high-power laser systems. However, laser damage in these materials is

observed by photons with less energy than the band gap energy. This damage may be due

to charge-producing crystalline defects, even if present in low concentrations.

Photoconductivity (PC) measurements are a sensitive and nondestructive method of

probing these charge-producing defects [1]. This paper presents and discusses the results

obtained from linear and nonlinear PC measurements.

2. Samples

The samples used in this study were chemically vapor-deposited (CVD) ZnS and ZnSe

and were supplied by CVD, Inc. The band gap energy of ZnS is 3.8 eV (326 nm), and the

band gap energy of ZnSe is 2.7 eV (459 nm).

Sample-purity information was not available; however, typical impurities found in

these CVD materials are Cu, Fe, Mg, Ni, Cr, Co, Al, Mn, CI, Br, and I. The concentrations of

these impurities range from sub-ppm to 3 ppm. The ZnS samples were 1 x 1 x 0.250 in.

with all surfaces polished. The ZnSe sample was cylindrical in shape with a diameter of 1.5

in. and a thickness of 0.375 in. Two flat surfaces were polished on the edges at 180° from

598
each other to allow laser radiation to exit and enter the sample without any focusing of the

beam by the sample.

3. Experimental Details

The experimental arrangement used in the PC measurements is illustrated in figure 1. The DC circuit is designed to measure photocurrents produced when photons are absorbed

by crystalline or impurity-related defects and produce free charge carriers. The wide band

gaps of ZnS and ZnSe crystals ensure that the electrodes are nonohmic, i.e., blocking

electrodes. The photocurrent measured is the displacement current associated with the

motion of photo-generated charge in the electric field produced by the bias voltage (VDC).

The magnitude of VDC varied from -30 V for ZnSe to -1000 V for ZnS.

The samples investigated were mounted in an aluminum vacuum chamber that was

typically evacuated to 10" 5 torr to prevent electrode-to-electrode breakdown along the

edges of the sample. The electrode arrangement was the same as that used by Lee et al. [2].

The ground electrode was brass, while the front or biased electrode was 28-mesh etched

nickel screen supported by an aluminum ring. The nickel mesh provided a uniform electric

field across the sample. Together, the sample holder and aluminum vacuum chamber

constituted a guard ring to minimize the effects of ground loops.

For the linear PC measurements, light from a 450-W UV-enhanced Xe arc lamp was

dispersed with a 2.6-nm bandpass McPherson 218 scanning monochromator before

entering the vacuum chamber through a fused-silica window. The monochromator used in

these measurements was a grating instrument, which necessitated the use of optical cut-off

filters to prevent false PC readings from higher order wavelengths. Monochromated light

from the arc lamp illuminated the ZnS and ZnSe samples through the nickel mesh electrode.

By applying the DC voltage with the negative potential on the nickel electrode, any

photoelectrons generated from the nickel mesh would be rejected, minimizing interference

in detecting small internally generated photocurrents. The photocurrent was measured

with a Keithley 642 electrometer. By interfacing the spectral position of the

monochromator to the x-axis of an x-y recorder and using the electrometer to drive the yaxis of the recorder, a spectral scan of the photocurrent vs wavelength could be generated.

The spectral scans presented and discussed have been normalized to the spectra of the Xe

arc lamp. This normalized photocurrent is the photoresponse of the sample.

The nonlinear PC measurements were made using a Nd:YAG laser that was capable of

emitting 1.064- and 0.532-|im radiation. The pulse width of the laser was 16.6 ns operating

at 1.064 |im and 16.2 ns operating at 0.532 nm. The beam was passed through the fused

silica window and focused to a spot size of 40 um in the center of the sample. By using

neutral density filters, the laser pulse energy was varied and monitored by a Gentek

pyroelectric energy meter. By varying the laser pulse energy and recording resulting peak

photocurrent, the peak photocurrent-vs-laser pulse energy curves were generated.

4. Discussion

4. 1 . Linear PC Data

Data presented here are in the form of linear x-y plots displaying normalized

photocurrent or photoresponse vs wavelength. The wavelength range is 300 to 800 nm for

the ZnS sample and 400 to 800 nm for the ZnSe sample.

599
The linear ZnS data are shown in figure 2. Two prominent peaks are located at 345 and

378 nm, followed by a broad spectral response from approximately 450 out to 800 nm. The

origin of the peak at 345 nm is unknown at the present time. Georgobiani et al. [3] suggest

the peak at 378 nm is due to an acceptor defect. This defect consists of a hole trapped at a Zn vacancy. The broad spectral response from 450 to 800 nm is associated with impurities,

since the photoresponse occurs at energies below the band edge of ZnS. The specific

impurities are unknown at the present time; however, Qidwai and Woods [4] suggest that Cu

may be a contributor to the absorption-carrier production in this region.

The linear ZnSe data are shown in figure 3. Two prominent peaks are observed at 475

and 515 nm, followed by a monotonically decreasing spectral response. Lewis and Arthur

[5] have attributed the peak at 475 nm to a transition from a Na acceptor defect to an unknown donor level. They observed this peak during photoluminescence excitation

spectrum (PES) studies of ZnSe. In Lewis' model, a Zn site is occupied by a substitutional Na

ion. However, Jones and Woods [6] report the same peak in their PES studies of ZnSe, which

they attribute to a transition from the valence band to an acceptor state below the

conduction band edge. It is suggested that the acceptor state may be associated with CI

ions. More work will be required to determine the correct model. The peak at 515 nm has

been observed also in the PES studies of Lewis and Arthur. These authors suggest that the

peak results from a deep acceptor site that consists of a substitutional Cu ion on a vacant Zn

site. The spectral region of the monotonically decreasing photoresponse corresponds to the

same region observed by Lewis in his PES studies and is attributed to the Cu-related defect.

4.2. Nonlinear PC Data

Nonlinear PC processes obey the equation

ip - EBm

until electron avalance or laser-induced damage occurs, where ip

is peak photocurrent, Eb is

laser pulse energy and m is the order of the absorption process. Therefore, data presented

here are in the form of log-log plots that display peak photocurrent vs laser pulse energy,

where m is the slope. Laser wavelengths of 1.064 and 0.532 |im were used with each

sample in the nonlinear study.

4.2.1 Nonlinear ZnS Data at 1.064 urn

The plotted data in figure 4 show the laser pulse energy covering three decades and

the peak photocurrent covering three decades. At pulse energies approximately less than

1.5 x 10"3

J, m = 0.61, indicating the presence of charge-producing defects that absorb

1.064-nm radiation. The nonlinear increase of photocurrent with increasing laser pulse

energy above this value, suggests the onset of multiphoton-induced photocurrent. The

band gap of ZnS is 3.8 eV, which corresponds to 3.3 times the photon energy of 1.064-(im

radiation. The near vertical slope (m = 8.9) also suggests the onset of electron avalanche.

At a laser pulse energy of approximately 2.2 x 10~ 3

J, laser-induced damage had occurred.

4.2.2. Nonlinear ZnS Data at 0.532 urn

The nonlinear PC data for ZnS is shown in figure 5. As for 1.064-)im excitation, the low

pulse energies produce a slope less than 1 (m = 0.73), indicating charge production by

defect states. The linear PC scans show significant photoresponse at 0.532 um (see fig. 2).

No nonlinear increase of photocurrent with increasing laser pulse energy is observed up to

the energy producing damage to the sample. The band gap of ZnS corresponds to 1.6

600
times the photon energy of 0.532-jim radiation. If electron avalanche occurs, no greater

than two-photon absorption would be required to generate valence-to-conduction-band

transitions. Thus, the onset to avalanche would not require a dramatic increase in slope.

This suggests that avalanche (damage) initiated from low-order absorption processes.

4.2.3. Nonlinear ZnSe Data at 1.064 urn

At low pulse energies, no observable PC signal was detected. This suggests that singlephoton absorption producing free carriers does not occur at 1.064 u.m. However, a PC signal

was observed at high pulse energies, but the sample had sustained catastrophic damage.

The fact that a PC signal and catastrophic damage were observed simultaneously supports

electron avalanche as a damage mechanism. Since the band gap energy of ZnSe is 2.3 times

the photon energy of 1.064-u.m radiation, multiphoton (i.e., two-photon) absorption would

be required to generate free carriers.

4.2.4. Nonlinear ZnSe Data at 0.532 |im

The linear PC of ZnSe shows a significant photoresponse at 0.532 u.m. The nonlinear

ZnSe data at 0.532 u.m are shown in figure 6. At low laser pulse energies, m = 0.84,

indicating free-carrier production from defects absorbing 0.532-(im radiation. With

increasing laser pulse energies, no nonlinear increase in photocurrent was observed up to

the energy producing damage to the sample. The band gap energy of ZnSe is 1.2 times the

photon energy of 0.532-u.m radiation. Analogous arguments to those presented for ZnS with

0.532-|im radiation incident also apply to ZnSe.

5. Summary

Linear and nonlinear photoconductivity have been observed in ZnS, while only linear

photoconductivity has been observed in ZnSe.

Linear photoconductivity in ZnS and ZnSe is attributed to impurity-related defects. Cu

is suggested as the prevalent impurity in both materials and the source of the 415- to 800-

nm broad response in ZnS and the 515-nm peak in ZnSe. The 378-nm peak in ZnS is

attributed to an acceptor defect associated with a Zn vacancy, while the origin of the peak at 345 nm is currently unknown. The literature suggests that the 475-nm peak in ZnSe may

be associated with a Na- or Cl-ion-related defect.

Nonlinear photoconductivity, where i p Eb"1

, shows charge generation at both 1.064

and 0.532 ^.m in ZnS and ZnSe. In ZnS at 1.064 |im, higher laser pulse energies yield a slope

(m) »1, indicating multiphoton avalanche charge production. For ZnS at 0.532 |im and ZnSe

at 1.064 and 0.532 u.m, data support the initiation of avalanche from low-order absorption

processes, where m 2.

6. Future Work

The direction of the future work will be to gain a better understanding of the

morphology of the laser damage process.

A comparison of the photoresponse of crystalline and CVD materials (doped and pure)

will be made to observe any changes in the linear damage thresholds PC spectra and the

shape/behavior of the nonlinear PC spectra as related to the laser-damage thresholds.

601
Photoluminescence studies will be performed to gain an insight into the absorption

mechanisms of the impurity related defects in crystalline and CVD (doped and pure) ZnS

and ZnSe.

The temperature range of the PC experiments will be extended from 95-300 K in order

to measure activation energies of the defects.

Time domain studies of carrier production will provide the time scale of the laser

damage process in crystalline at CVD (doped and pure) ZnS and ZnSe.

7. References

[1] O'Connell, R. M.; Marrs, C. D. "Photoconductivity study of charge-producing defects in

CaF2," presented at the 20th Symposium on optical materials for high power lasers,

Boulder, CO; 1988 October 26-28. (Proceedings in process.)

[2] Lee, R. S.; Merklin, J. F.; Marrs, C. D.; Richter, M. H. "Impurity photoconductivity of yirradiated LiF," Phys. Status Solidi (b) 103; 605; 1981.

[3] Georgobiani, A. N.; Maev, R. B.; Ozerov, Yu. V.; Strumban, E. E. "Deep traps in zinc sulfide

single crystals," Bull. Acad. Sci. USSR, Phys. Ser. 40; 181-184; 1976.

[4] Qidwai, A. A.; Woods, J. "Deep level in In and Ga doped ZnS," J. Cryst. Growth 59; No. 1-

2; 217-222; September 1982.

[5] Lewis, K. L.; Arthur, G. S. "Surface and free carrier absorption processes in CVD zinc

selenide," in Laser induced damage in optical materials: 1982. Bennett, H. E.; Guenther,

A. H.; Milam, D.; Newnam, B. E., ed. 1982 November 16-17; Boulder, CO. Nat. Bur. Stand.

(U.S.) Spec. Publ. 669; 1984 January. Pp. 86-101.

[6] Jones, G.; Woods, J. "Luminescence of self-activated and copper-doped zinc selenide," J. Cumin.; 9(5); 389-405; 1974.

VACUUM CHAMBER (10' 5

torr)

FROM 450W XENON LAMP

PHOTOCURRENT i

ELECTROMETER

FROM LASER

1.064 um OR 0.532 nm

Figure 1. Experimental arrangement

used in the photoconductivity

measurements. Excitation source was

either the xenon lamp or the Nd:YAG

laser.

500 600 WAVELENGTH (nm)

Figure 2. Linear PC spectrum from the CVD ZnS

sample, bias voltage - 1000 VDC.

602
o

X

WAVELENGTH (nanometers)

Figure 3. Linear PC spectrum from the CVD

ZnSe sample, bias voltage - 30 VDC.

LASER-INDUCED DAMAGE

LASER PULSE ENERGY (J)

Figure 5. Nonlinear PC data from the CVD

ZnS sample obtained with 0.532-|im

radiation focused inside the sample.

LASER-INDUCED DAMAGE

10" 1!T J

LASER PULSE ENERGY (J)

Figure 4. Nonlinear PC data from the CVD

ZnS sample obtained with 1.064-|im

radiation focused inside the sample.

LASER-INDUCED DAMAGE

LASER PULSE ENERGY (J)

Figure 6. Nonlinear PC data from the CVD

ZnSe sample obtained with 0.532-nm

radiation focused inside the sample.

603
COMMENTS

Could you say something about the long term environmental

degradation of the optical properties of these materials,

particularly the effect of solar radiation.

Right, let me point out two things. First of all, when we make

these polymers we worry about residual organic solvents that might

be deposited in the polymer films. For this reason we put the

polymers we take in an oven at between 50 °C and 100 °C for varying

amounts of time. We saw no change in the optical properties as a result of this heating. Secondly, we took some of these polymers

and exposed them to the laboratory atmosphere for a period of a month. We then measured their optical properties and found that

there was no discernable change in the fringe spacing over that

period. I indicated that some of these polymers are soluble in

water but they also form very nice films. One of the good

attributes of the polymers is that once you get them down it is

difficult for solvent molecules to grab onto them to pull them in' solution, which attests to their enhanced stability.

When you see the size of this polymer have you done the molecular

size distribution?

The only thing that we've done is high performance liquid

chromotrophy to try and get an idea of what the molecular weight

of the material is. So far the materials measured here have had

very high molecular weights with something like ten thousand

repeat units along the chain. We currently are investigating

techniques for making shorter chain materials using a boron

trichloride catalyst to knock the chain link down. We think those

materials will have different properties as well.

I'm a little concerned about stress and temperature changes. Have

you done any thermal cycling?

The only thermal cycling we have done is to put these in an oven

at between 50°C and 100°C and look at the optical properties after

we have withdrawn them. There appears to be no change. A lot of

the materials that I have illustrated here are elastomers so they can stretch with temperature. Some of the other materials like

the phenyl substituted material are quite brittle. That material

is stable up to about 350°C and then it decomposes. One of the

first slides I had, I showed a glass transition temperature of

let's say the trifluoroethylene as minus 60°C. That material

retains its elasticity to those temperatures. As a matter of fact

that was one of the first commercial applications of these

materials. It was used in the Alaskan pipeline as a gasketing

material

604
Question: I just wondered about the purpose of the second harmonic

qeneration measurements. Do these have a crystallized phase so they might be useful as harmonic generation materials?

Answer: These polymers can be crystallized and I think they can be

oriented. In an initial measurement we observed green emission

when we hit the polymer with IR. It would be interesting to see if indeed the wavelength emitted was 532 nm. One thing we find

out is when we substitute different groups on a polymer chain, for

example, the isocyclamate , we observe the intensity to go up by

over an order of magnitude. I think if we can affect some kind of

orientation in the polymer it might be an attractive non- linear

material

Question: I just want to make a comment about the previous question.

Organic polymers have been shown to have extremely high ratios

that are associated with the conjugative systems in those

molecules and you would expect that non- organic polymers could

have similar affects. They should then have a lot of applications

for waveguide structures.

605
Manuscript received

12-29-89

FORMATION OF A PREGIVEN REFLECTING SURFACE TOPOGRAPHY

BY ELASTIC DEFORMATION OF THE MIRROR SUBSTRATE. NEW

CONCEPT OF ADAPTIVE OPTICAL SYSTEM.

V.V. Apollonov, S.A.Chetkin, E.A.Ivanova, A.M.Prokhorov,

G.V.Vdovin.

General Physics Institute, USSR Academy of Sciences

117942, Moscow, Vavilov street, 38, USSR

ABSTRACT

Complete model of a multiactuator flexible mirror, made as a thin

plate with discrete piezo-actuators is described. Experimental results

validated the efficiency of proposed model are presented . New concept of

adaptive optical system with flexible mirror and linear wavefront sensor

is developed.

INTRODUCTION

Adaptive mirror (AM) with continuous reflecting surface and discrete

actuators have a series of advantages in comparison with other types of wavefront correctors. However, the control of the AM in adaptive optical system

(AOS) is rather difficult because its influence functions (IF) are not orthogonal, no localized. Besides, the model, describing operation of AM, does not

exist

The purpose of this work is a description of analytic model of the AM,

consisting of thin flexible substrate and an array of discrete actuators.

This model is used for determination of AM parameters and numerical simulation

of AM operation in AOS.

Effective algorithm of realization of phase conjugation in AOS with nonorthogonal influence functions is proposed.

1. NUMERICAL SIMULATION OF AM OPERATION.

Model of the AM

Theory of a thin plate is sufficient for description of elastic interaction of substrate and actuators. Within this theory the substrate deflection

WCr*) under the action of load P(r") satisfies the following equation:

AAW(r") =P(1T) /D (1)

with the boundary conditions, corresponding to the method of substrate

fixation. In (1) D is a cylindrical stiffness of the AM substrate [1].

We'll consider the circular AM with N actuators and clamped edge [2].

When the control signal is applied to the k-th actuator, it produces pointwise

force P and other actuators produce reaction pointwise forces P , where i

is the actuator number. In this case the solution of the equation (1) is

W (r,^)=PR2 /( 1677D) fft(r,lf) (2)

where R is the substrate radius, f ^

( r ,lf

) = dJ r ,lP) + JEq^oC^ r ,^ ) ,

<^(r,^) = (l-r 2

) (1-jjj) + {rZ+

$] _2r

J

)

<) cos(

H

? "'^j ) )ln( (r * +jj

-2.rj)jCOS-(tf-0j) )/

(1+r

\?,) ^rP.cos^-O;.))

606
(j)j,Oj) - coordinates of the j-th actuator. Reaction forces q L

, corresponding to the action of the unit force P, are defined from the system of linear

equations

N

I an qL = ( ( j =1 ' N;j#k) (3)

i*k

where =cL^( Oj ,0^ )+ S^jG, (JlJ- Kronecker symbol, G=R2 /( 1677d) K^. K-actuator stiffness. Force, produced by the k-th actuator is equal to

P=alo/(K'a +f

fe (j)fe ,0fe )R

2 /(16TTD) ) , (4)

where al0 is the elongation of the actuator under the action of the applied control signal at the free state (out of the mirror). The transverse piezoeffect

is ordinary used for the control of the AM reflecting surface shape by piezoelectric actuators. In this case Alois related to the value of control voltage

U by the equation:

a1„= dMl/d U, (5)

where dji is piezoelectric constant of the actuator material, 1 is the length, d

is transverse dimension of the actuator.

As the actuator influence on the AM surface is additive, prof ile r formed

when voltages are applied to all the actuators, is the following

// w(r,vj>)= d31 1/d Iff i (r,if)/(fi ( j>L ,#o)+G) Ut) (6)

Normalized IF of two AM, - the first with 5 piezoelectric and the second

with 19 magnetostriction actuators, found experimentally -(1,3) and calculated

by the equation (2) - (2,4) are presented in Fig.l and 2. Meansquare error of

approximation of experimental IF by the model ones is about 0.08 of meansquare

deviation of experimental IF from plane. Obtained accuracy proves the efficiency of the theory of a thin plate for description of the AM operation.

Efficiency of Actuator Operation

Efficiency of actuator operation in the AM is characterized by the ratio

of actuator elongation in AM to actuator elongation in free state and by coefficient of transformation of control signal energy to the energy of substrate

deformation. Actuator elongation in AM is equal to W^( 0^,0^), hence, as

following from (2) -(5):

Al/Al=l-G/[G+^( j? fe , )] (7)

a1/ i lo (G) dependencies for one-actuator mirror with different

positions of actuator relatively to the center of AM are presented in Fig.3A.

Increase of G leads to decreasing a1/a1o, radial shift of actuator from mirror

center also leads to decreasing a1/a1o» il/ilo (G) dependencies for actuators,

located in hexagonal array with distance between actuators b=0.3R are presented

in Fig.3B. For actuators, located at equal distances from AM center, the dependencies are identical. While the distance from the center increases, a1/a1o decreases as for the case of one actuator. Comparison of curves in Fig. 3 A and

Fig.3B shows that increasing the number of actuators leads to increasing of

effective substrate stiffness and, hence, to decreasing of displacement range

of individual actuator.

Efficiency of control signal energy transform to the energy of substrate

deformation is characterized by the value of energy transform coef f icient ( ETC )

,

607
equal to the ratio of substrate deformation energy to the summarized

energy of substrate and actuators:

1=V0 /( I VL +V0 ) (8)

where V 0 - energy of substrate elastic deformation, Vj, - potential energy of

the i-th actuator. Dependence of ^ on the parameters of AM in case of actuators based on transverse piezoeffect is the following:

A/

l* k

(9)

where k 3 ^ is coefficient of piezomechanical coupling of actuator material [4].

Thus, ETC depends on the displacement, number of actuators and two dimensionless parameters - relative stiffness G and coefficient k.51 . K[(G) dependencies for actuators of 19-element AM (hexagonal array, b=0.3R,

kj^=0.3) are presented in Fig. 4. Dependencies of maximum value of ETC on the

distance between actuators for the case of hexagonal array of 19 actuators are

presented in Fig.5A, and the values of relative stiffness, corresponding to

maximum ETC - in Fig.5B; k3 ^ =0 . 3 . The ETC dependencies on coefficient k 51 are

similar for actuators located at different distances from mirror center -

- when k31 increases, ETC increases too. As an example, ^(ks^ ) dependencies for

19-element mirror with hexagonal array of actuators are presented in Fig. 6.

(b=0.3R;G=0.2)

Numerical calculations have demonstrated that for effective operation of

actuators in the AM, it is necessary to optimize their displacement and adjust

their parameters with parameters of the substrate.

Correction of Seidel Aberrations BY AM

For effective correction of aberrations, the AM control system must

provide control signals on actuators, producing forces, that form reflecting

surface of given shape with certain accuracy. The degree of compensation can be

characterized by meansquare error:

2n A e-CJ

:wa -W^ )rdrdlf/( 77 A* W* A/ )] (10

0 0 where W a (r,^ ) =W M cu 'Y ( r , ^ ) - the shape of aberration, WT (r,if ) - shape of

AM reflecting surface, A - radius of operating aperture, that is the aperture

at wluch conjugation is accomplished. Equation for Wf is written in the form

WT = Z Qt °U ( r , If ), where Q t =R2 / ( 167TD) P L

, P{,is the force, produced by the i-th

actuator. Q ^1 >, providing minimum £ , are found from the system of linear equations

where jBtfc = / Kc( r ,ip

) otft( r ,<f ) rdrdf , =Wkax Yf r ,<f ) cCfe( r ,lf )rdrolf. The results of

numerical simulation of Seidel aberrations compensation are presented below

[2] .

608
Hexagonal array of actuators, defined by the distance between actuators b

and the number of actuators N, is typical for the AM. £ (b) dependencies for

correction of Seidel aberrations by the AM with N=37, A=0.8R are presented in

Fig. 7. The arrangement of actuators powerfully influences on the value of compensation error, although hexagonal array is kept, therefore, it is necessary

to optimize the actuators arrangement in order to provide effective correction

of phase distortions

£(A) dependencies for compensation of Seidel aberrations by the AM with

N=37, b=0.32R are presented in Fig. 8. While A decreases from R to 0.6R,£ decreases in 5-50 times depending on type of aberration, hence, this way of increasing of compensation accuracy is very efficient.

Histogram, characterizing £ (N) dependence for compensation of Seidel

aberrations by the AM with N=7,19,37,61 in hexagonal array, is presented in

Fig. 9; b is optimum in each case, A=0.8R. It is evident that the increase of

the number of actuators more than N=61 is not expedient when aberrations to be

corrected are rather smooth.

Determination of demands to the AM alignment in the AOS is important for

providing efficient compensation of phase distortions. Dependence of compensation error on the value of shift of beam axis relatively to the mirror center

for the case of compensation of defocus by the AM with N=19, b=0.3R, A=0.6R.

is illustrated in Fig. 10. Even a small shift of the beam axis causes the

critical error increase, therefore, the AM must be centered with high accuracy

in the AOS. Fig. 11 illustrates the dependence of accuracy of compensation of

defocus and astigmatism by AM with N=19, b=0.48R, A=0 . 6R on the value of

angle between the normal to the mirror surface and the axis of incident beam.

Dependencies demonstrate that maximum accuracy of compensation corresponds to

the case of normal incidence of radiation.

Results of numerical simulation of Seidel aberrations correction have demonstrated that compensation error for pregiven phase aberration can be essentially decreased by optimization of actuators arrangement, limitation of operating aperture of the AM in the AOS.

AOS under consideration includes linear wavefront sensor with M independent phase measuring channels and phase corrector with N control channels.

Using wavefront corrector with nonorthogonal IF provides a considerable goal

in correction quality, but it simultaneously complicates AM control in phase-

-conjugation AOS . Wavefront reconstruction from wavefront sensor response vector needs N

s computer operatins, and using of initial assumptions about wavefront to be corrected. These assumptions lead to reconstruction error increasing. At the first (calibration) stage M-dimension wavefront sensor response

vectors are measured for consecutive appling of unit signal to each control

channel of corrector in the presence of model wavefront. Then orthogonal basis

of vectors Wi is formed by mean of orthogonalization of response vectors

and each of computed vectors is presented as a linear combination of response

2. REALIZATION OF PHASE CONJUGATION BY MEANS OF

ORTHOGONALIZATION OF WAVEFRONT SENSOR BASIS.

AOS Control Algorithm.

vectors

(12)

609
Orthogonal basis IP: and coefficients <^ij are stored in control system memory. —

At second (working) stage, vector of jsensor response ^ on aberrated wa- vefront is decomposed into basis vectors 1^ : J

^~LH%, (13)

-+ —* Pk is defined as j5fc = ( ^ , Tj/fc ).

ution of (12) to (13) gives: ^ >v 1* <M>

where

Substitution of

here /rjfik defines the value of control signal to be applied to the j-th

control channel of corrector. The main advantage of proposed algorithm is that

its realization needs only N computer operations to determine all control

signals in N-channel phase-conjugation AOS.

Numerical Simulation of AOS Operation

We have considered phase conjugation AOS, including Hartmann-type wavefront sensor and mirror-like phase corrector with discrete actuators. Efficiency of proposed control algorithm can be estimated by comparison of compensation error, provided by AM controlled by proposed algorithm and optimally controlled. The degree of compensation we'll characterize by residual meansquare

error, devided by initial meansquare error of aberrated wavefront. Efficiency

of proposed algorithm for 19-channel AM (hexagonal array ,b=0 . 42R, A=0.8R) and

19-channel Hartmann-type sensor in comparison with optimal control in case of

Seidel aberrations compensation is illustrated in Fig. 12. The error of compensation for proposed algorithm is in 1.5 - 4 times high than that is for optimal

control. So, proposed algorithm of phase conjugation, excluding initial assumptions about controlled wavefront, is effective. Definition of all control

signals requires in this case only N

2 computer operations.

REFERENCES

1. Timoshenko S.P., Woinovsky-Kreiger S., Theory of Plates and Shells.

New York, 1959.

2. Apollonow V.V. , Chetkin S.A., Ivanova E.A., Prokhorov A.M., Materials

of Lasers '88 International Conference, Lake Tahoe , Nevada, pp. 298-306

3. Apollonov V.V. , Chetkin S.A., Ivanova E.A. , Prokhorov A.M., Temnov S.N.,

Experimental Technic der Physic 37 (1989) 1, pp. 41-60.

4. Physical Acoustics ed. by W.P.Mason, v.l., part A., Moskow: Mir, 1966

5. M.Born, E.Wolf, Principles of Optics, Oxford, England: Pergamon Press, 1965

610
i I

Fig. 1

Fig. 2. Experimental (1,3) and theoretical (2,4) IF of 19-element cooled AM

with magnetostriction actuators

611
612
613
0,4-

Fig.5

Fig.

A) Maximum value of ETC via the distance betveen actuators of 19-element AM with hexagonal array (kji=0.3)

B) Relative stiffness G corresponding to the maximum ETC via distance

betveen the actuators of 19-element AM with hexagonal array

(k 3j =0.3)

1 - J>=0; 2 -

J> =b; 3 -

J> = 1.7b; 4 -

J?

= 2b

ETC via coefficient of piezomechanical coupling k 3i for actuators

of 19-element AM with hexagonal array (b=0 . 3R ; G=0 . 2)

1 - j=Q; 2 - JJ = 0.3R; 3 - J)=0.52R; 4 - J)=0.6R

614
052

Fig. 7. Meansquare errors of Seidel aberrations compensation by AM with

hexagonal array of 37 actuators via distance betveen actuators

(A=0 .8R)

1 - defocus; 2 - spherical aberration; 3 - tilt;

4 - coma; 5 - astigmatism

F

05

0,2

04 /A /

^-jz———-7""^

0.6 0,8

A/R

Fig. 8. Meansquare errors of Seidel aberrations compensation by AM with

hexagonal array of 37 actuators (b=0.32R) via radius of operating

aperture

1 - defocus; 2 - spherical aberration; 3 - tilt;

4 - coma; 5 - astigmatism

615
Ho''

Fig. 9. Meansquare errors of Seidel aberrations compensation by AM with

hexagonal array of actuators via the number of actuators

a - N=7; b - N=19; c - N=37; d - N=61

1 - defocus; 2 - spherical aberration; 3 - tilt;

4 - coma; 5 - astigmatism

£10'

Fig. 10. Meansquare errors of defocus compensation by AM with N=19,

b=0.3R, A=0.6R, via shift of beam axis relatively to

the mirror center

616
Fig. 11. Meansquare errors of defocus (1) and astigmatism (2) compensation

via AM tilt (N=19 , b=0 . 43 R, A=0 . 6R)

610

6

a. a

4 7

I

t

i

i

2

<

6

i

&

n

i 2 3 4 5

Fig. 12. Errors of Seidel aberrations compensation by AM with hexagonal

array of 19 actuators (b=0.42R) in case of optimal cont.ro] (a)

and in case of control in AOS with 19-channel Hartmann-type sensor

(hexagonal array , b=0 . 26R) ( b)

1 - defocus; 2 - spherical aberration; 3 - tilt;

4 - coma; 5 - astigmatism

617
Manuscript received

4-2-90

APPLICATION OF ULTRASONIC CAPILLARY EFFECT IN ELEMENTS OF

POWER OPTICS COOLED BY MEANS OF A HEAT PIPE

V.V. Apollonov, S.A. Chetkin, V.N. Kharchenko , V.N. Motorin,

A.M. Prokhorov

Institute of General Physics of the USSR Academy of Sciences,

Vavilova 38, Moscow, 117942

INTRODUCTION

Development and creation of powerful technological lasers requires complex solution of problems of optics, quantum electronics and thermal physics. The achieving of stable in time

energetic parameters of technological lasers with cooled elements of power (EPO) and adaptive (EAO) optics can be achieved

only by a correct choice of their cooling modes, eliminating

or compensating negative effects of heating EPO by laser radiation because of principal difference of reflectance of the mirror surface from I. The convective regime of EPO or EAO cooling is mainly used in laser technique and carried out by pumping the coolant through a penetrable disperse compact heat exchanger, on the work surface of which a thermally thin layer,

separating radiation and the coolant liquid in heat changer,

is made /1,2/. The intensity of heat transfer and therefore,

the degree of thermostatting of the reflecting surface of EPO

are determined by a large number of design and technological

factors not always reproducible in the EPO manufacturing. Nevertheless, for EPO, made on the basis of powder porous structures, the level of removing heat fluxes reach 2 . 10 3 wt/cm 2 under the thermal deformation of the mirror surface less than

1 jum, the effective heat transfer factor for a heating mirror

surface equal to ~10^ wt/m 2. °C in this case /2/.

When solving the problems of thermos tating of extended reflecting surfaces, the usage of convective intraporous cooling is

not advisable since it requires high cooler consumption. Th'fe usage of heat pipes (HP) for thermostatting of extended optical surfaces is the most rational approach to creation of large-scale EPO. The choice of HP as a heat exchanger for EPO

is followed from HP fuctional possibilities and advantages on

a number of technical-and-economic indices. Among tham, first

of all, large equivalent thermal conductivity of HP, amounting

to *v- 10* wt/m . grad, sufficiently high efficiency of heat pick

up and, therefore, high degree of thermostatting of the heating

mirror surface, carried out by means of heat flux transformation into the latent heat of phase transition. Moreover, EPO

in this case has essentially lesser weight and overall dimensions of the whole cooling system, abscence of mobile parts

(t.g., pumps) and, due to this, operats without vibration and

noise; also the complete autonomy of EPO is reached in this

case /5/.

618
6

FIGURE 1. Design of an experimental

setup for investigating the ultrasonic

capillar effect

However, the traditional approaches to

creation of HP cannot completely satisfy the needs of high intensities laser

optics in removing still larger specific heat fluxes from the reflecting sur

face of EPO. Largely, this because of

the fact that for heat transferring devices, based on employing the HP methodology, the upper level of the removed

heat flux is determined by the transport possibilities of capillary-porous

structure of the HP wick. To remove

the indicated limitation, the processes

of the cooler transportation along the

capillars of the HP wick shoud be substantially intensified. The well-known

passive (not requiring the supply of

energy) and active (with the supply of

energy) methods are not able to completely eliminate the above limitations on

the cooler transport in the HP wick.

One of the promising and practically

non-investigated, with respect to HP,

methods - is the active way of intesifying the mass transfer in the HP wick,

based on ultrasonic capillar effect by

Konovalov (USCE) /4/. Using the process of liquid rise in an

isolated capillar, exposed to ultrasound, one can solve the

problem of intensifying the mass transfer in the HP wick, i.e.

practically completely eliminate the transport limitations on capillary-porous structure and thus substantially raise specific heat loadings on the reflecting surfaces of EPO. Therefore

from the scientific point of view, the investigation of thermophysical characteristics of HP for EPO, in which USCE is emploied to intensify the heat mass transfer, is of interest.

h,mm 2

FIGURE 2A. Dependence of the height of

water rise in a capillar on time (d=

=1,2 mm; f=41 kHz; N=25 wt) for different

values of clearance between the source's

face and the capillar's cut:

1 - A =0; 2 - A. =0.1 mm; 3 - A =0.2 mm; A - A=0.j mm; 5 - A =0.5 mm.

619
600

FIGURE 2B. The height of rise of transformer oil and ethyl alcohol in the capillar

against time, for different values of

the capillar s diameter and zero value of

clearance between the radiator and the capillars cut: 1 - 1.2 mm (ethyl alcohol);

2 - d=0.i31 mm (oil); 3 - d=3 mm (oil).

THE PHYSICAL MODEL OF ULTRASONIC

INTENSIFICATION OF HEAT MASS EXCHANGE

IN HP

4-s 3s ZO\ USCE, registered as invention in 1972, consists in raising heignt and velocity of liquid penetration into capillary channels under the action of

ultrasound. Application of USCE will allow to raise the consumption of the cooler and thus to eliminate capillary limitations, imposed on mass transfer in the HP wick.

It is well known from the analysis of currently available work,

that, the propagation of ultrasonic oscillation in liquids is

accompanied by emergence of the so called secondary phenomena:

sound pressure /5/, acoustic flows /6/, certain types of constant forces //, -/ affecting the element of the medium and its

adjacent surfaces, and also cavitation /\ /. Those mecanisms,

however, can not explain the experimental results on USCE.

With the purpose of finding out conditions, leading to the increase of capillary pressure under ultrasound, and developing

a physical model of the process, a setup was created, involving: the system 5 of measurements and registration of velocity and height of the liquid in the capillar /Fig. 1/, a generator of ultrasonic oscillations (GUO), magnetos trictive transducer I and ultrasonic vibrator 2 placed in a bath with the liquid under investigation 3. Glass cylindrical capillars 5 fixed in the bath 4 at a given height using fine adjustment

screw 7 and rod 8. For visual observation and taking photographs, the capillar was lighted with a lamp 9. The process

was observed and photographed through a peephole using microscope. The temperature of the liquid was measured by a thermocouple 10 and the height of liquid in the capillar was determined using rule 11. The velocity of the liquid rise in the

capillar under ultrasound was measured using contiguous photographing of its height. The experiments were carried out under

following conditions: power of GUO was W=25 wt, inner diameters of the capillars d were from 0.2 mm

to 3 mm. Water, alcohol and transformer oil

were used as liquid.

it, m

ft

0,8 z:

The ultrasonic effect on the capillar move- ment of the liquid was investigated at a frequency 41 kHz under acoustic power

FIGURE 3A. The equilibrium height of rise

d of distilled water in the ultrasonic field

against the capillar's diameter.

620
45"

/£

9e

I FIGURE 3B. The average velocity of water

rise in the capillar under ultrasound

against the capillar 's diameter. Exposure

time was 1 s.

12.5 wt, which corresponded to the intensity of ultrasonic field 1.6 . 10 wt/m J

. O Ofy 0,6 1,2 -1,6 £ The analysis of experimental data, showed

in Figure 2A, B, discovers, that the height of the water rise in

capillary channels of different diameters, with practically

zero distance from the radiating surface of the magnetostricter to the capillar's cut, in case of reading the height of

its rise from the height of the liquid rise in the capillar in

the absence of ultrasound, increases many times and depends on the capillar's diameter. For example, in a capillar of diameter 3 mm under f=41 kHz the height of water's rise in the ultrasonic field exceeded the equilibrium one 110 times. The plot

in figure 3A, shows that there exists an optimal size of the ca- pillar's diameter dependent on the frequency of ultrasound with

the velocity of rise of the liquid (water) is maximum. It

should be noticed that the kind of liquid causes no substantial

effect on the size of the optimal diameter of the capillar. If

the capillar's diameter increases, the equilibrium height of

the liquid's rise in the capillar decreases, together with

the acoustic impedancem that is equal to the ratio of the amplitude of sound pressure P^i to the amplitude of volume velocity (Vjfl.S). In thiscase, mass forces begin to play larger

role, since relatively large volume of liquid passes through

the capillar. In the case of small diameter of the capillar

(less than 1 mm), the equilibrium height of the liquid's rise

in it is substantial and amounts to 10-50 mm. In thiscase,

the impedance of the system is also large. However, sound-capillar pressure in this case decreases, since the resistance

force, increases, due to increase of specific surface of contact

of the liquid with the capillar's wall.

The observing of the liquid's behaviour over the ultrasonic radiator showed, that the area with developed acoustic cavitation

emerges over the radiator's surface. The cavitation was observed at the same point of the surface of the GUO source. A whi'te

mobile cloud of cavitation bubbles emerged over that point. In

the case of ultrasonic action at the transformer oil, the picture was some what different, i.e. cavitation was observed both in the center

of the radiator and in its peripheral

area. Because of viscosity of the transformer oil substantially exceeding the

viscosity of water, and of substantially lower surface tension that the water's, the cavitational cloud is unstable and bubbles have larger dimensions

FIGURE 4. Photograph of cavitation

picture near the capillar's face in

water

621
Pm

Po

o

FIGURE 5. A physical model of

ultrasonic capillar effect.

that in the water's case. When

setting un the capillar into

the cavitation cloud (Fig. 4),

the maximum velocity of the capillar rise was observed. When

setting up the capillar aside

from the area with developed

cavitation^ the increase of

velocity and height of capillary rise was insignificant.

It should be noticed that cavitation at the surface of the

GUO source progresses only under ultrasonic intensities exceeding the cavitation threshold. With the amplitude of ultrasonic pressure lower than

the cavitation threshold (P < 0.74 . 10 Pa), cavitation ceased and the water column lowed to the height of the equilibrium

capillary rise in the abscence of ultrasound. When carrying out

the experiment, it was noticed that the effect of cavitation

exposes itself more distinctly in the case of water, saturated

with gas, than in the case of using settled distilled water with

a lowered proportion of dissolved gases. Attempt to treat the

experimental results obtained^, using the Konovalov methodology

/10/, showed inconsistency of the physical model of USCE_,considered in this work. Nevertheless, the analysis of the experimental results suggests that the rise of the liquid in the capillar under ultrasound occurs due to a periodical force.

Sound sinusoidal pressure can not ensure the rise of the liquid

in the capillar. The presence of pressure gradient along the

capillar axis can emerge only in the case of asymmetrical, different from the sinusoidal for of ultrasound pressure (e.g.,

ultrasound with the pressure wave of the form showed in Figure 5 ensures the emergence of non-zero average pressure gradient along the capillar axis). Acoustic cavitation, for example, can ensure such transformation of harmonic ultrasonic

wave. If the pressure in the liquid becomes lower than the

threshold one for emergence of cavitation in the phase of rarefaction of the ultrasonic wave, a cavitation bubble emerges

on the capillar 's cut and closes it. In this case the colomn

of liquid in the capillar experiences the action of surface

tension force, the weight of the colomn of liquid and the pressure of saturated vapour of liquid inside the bubble P . This pressure is larger than the minimum pressure in the phase

of rarefaction of the ultrasonic wave. Thus, during the action

of the half-wave of rarefaction, the liquid flows from the carillar under gravitation and the difference of pressures

0,751

djmi

atm During the half-wave of comFIGURE 6. Water cavitation factor vs. the capillar's diameter for water tempera3 ture 20 °C.

622
FIGURE 7, Relative height of water

rise along a capillar vs. the acoustic

Reynolds number; • - data from /10/,

water, f=23.5 kHz; + - data of the pre

sent paper, water, f=41 kHz; V - data

from /ll/, water, f=40.4 kHz;0-

C

2

H

5 OH, f=40.4 kHz, from /10/; - water, f=19.4 kHz, from /10/; T - C 9 H c-0H

f=19.4 kHz, from /10/. z 0

pression the bubble collapses and the colomn of liquid experiences, apart from the above mentioned, the action of force

of ultrasonic pressure sucking the liquid into the capillar.

Hence, the acoustical cavitation is accompanied by distortion

of the harmonic form of pressure oscillation in the ultrasonic

wave and by emergence of some average in time force ensuring

additional rise of the liquid in the capillar. In the case of

capillars of sufficiently large diameters, a system of small

cavitational bubbles emerges that shut off the pass section of

the capillar. In such a situation the action of the system of

cavitational bubbles will be equivalent to the above considered action of a separate bubble.

Kinetics of capillar movement of, the liquid in the ultrasonic

field is described, in accordance with the considered physical model, by the equation

,d 2 h , ,dhN 2 2G , . . SJi f

, xdh P sound^ _ n

v hdT"2 + ( dT} " R^ +

§h + WJ ^ h+1

o

) ^f y

= 0 (1)

derived from the balance of forces, acting at the colomn of

liquid of variable mass under following assumptions: 1. the

flow of the liquid in the capillar is laminar and the Poiseuill's friction lav; holds: f=&4/Re; the equilibrium height,

if the liquid's rise in the capillar in the abscence of ultrasound^ is determined according to the Juren formula.. In equation (1) the following notation is adopted: h - is the height of the liquid's

rise in the capillar; G> - is the factor

of surface tension of the liquid; R - is the capillar' s radius; ^ - is the liFIGURE 8. Schematic design of experimental HP with a source of ultrasonic oscillations: 1 - pressurized body; 2 - magneto-strictive radiator; 3 - capillapy-porous wick; 4 - pipe-artery; 5 - cooler in

the condensation zone; 6 - upper cavity

for gathering the cooler in the heating

zone; 7 - opening for the cooler output;

8 - concentrator of ultrasonic magnetostictive radiator

623
; FIGURE 9. Experimentally obtained depen-

~T>«6f 1 dency of the height of water rise in a capillar of diameter 3 mm for determining

& Q £

: 1 ~ without ultrasonic action; 2

- under ultrasound of power 1.5 kwt.

quid's density; is its viscosity; h is the height of JX

the liquid's rise along the capillar without ultrasound ;

souncj~

is the pressure in the field of ultrasonic wave; "tii, - is the factor of cavitation. To solve equation (1), let us use the following assumptions, i.e. omit the two first summands because

of their small values and replace the .variable value of ultrasonic pressure by its average value: P. T is the period of oscillation; P.sound

sound

= (2 P

^sound^^'^'

9TP )/20f

max v

- |f + gh +

8iM (h+ l o )^/(R2 o) - P sound */

?

= 0 (2)

Taking into account the adopted assumptions, the equation as- sumes the form:

_ dh R ? ,2$ . P sound*s

V ~ dT

= 8^ (h+l

Q

)

( fR ' gh +

f

} (3)

From equation (3) with V=0 the expression for determining the ^ cavitation factor (Fig. 6) has form:

7

*" ^max-^/Psound (4)

v8 The developed theory allows to determine

the height of the liquid's rise against

time

FIGURE 10. The design of a setup for

reading consumption characteristics of an

ultrasonic pump: 1 - magnetostrictive radiator; 2 - bath with water; 3 - steel ca- pillar; 4 - compound glass capillar; 5

- attachement of the capillar; 6 - rule for

measuring height; 7 - discharging funnel;

3 - measuring capasity; 9 - stop-watch;

10 - termometer.

624
H, cum i FIGURE 11A. Experimental dependence of conI sumptiOB.,on the height of water in a capillar with T=22 °C, amplitude of GUO voltage

U=80 V and angle of inclination of HP

dL=90 grad.

0,40 0,551

h -h

max o max

h + h ] o

J (5)

which is a transcendental equation for determining the dependency h = f(¥).

The outcome of experimental study of USCE was a generalization

of the experimental results in the form of the dependence

(h*/h ) = f(Re£|? ), where h

x = h - h

g ^ is the excess of the

height of the liquid's rise in the capillar under ultrasonic

2 action over the equilibrium height; Recj = 4o)R /V - is the oscillatory Reynolds number, V - is the kinematical viscosity of

the liquid. The experimental results of different authors /10,

11/ are generaliwed by criterial equation /12/ (Fig. 7):

(h*/h ) 0,5

= 0.092 . Re 0,27

. fl» 0 ' 23

(6)

where - is dimensioneless time.

INVESTIGATION OF OPERATION OF A HEAT PIPE WITH A SOURCE OF

ULTRASONIC OSCILLATIONS

The mechanism of ultrasonic influence on the processes of heat

mass transfer in HP has not been studied , though the received

positive effect of such influence in other thermal devices is

sufficiently large /13/. Here, a cylindrical HP^actuated in

a vertical position with the evaporation zone located above,

was investigated. The HP consisted of a sealed body (Fig. 8;

in the internal space of which the face of a maghetos triction

radiator of ultrasonic oscillations, capillary-porous wick, a transport artery-pipe were situated. The expression for determining the maximum heat flux, transported by HP in the condensation zone, as is well-know, has the form

FIGURE 11B. Experimental dependence of water

S 7j consumption on temperature T with voltage amp-

- ' litude — of-— U=80 V,/ angle o of inclination

90 grad and height of a capillar pipe 220 mm

625
it

0,9

0,5

g ,9/3

!

FIGURE 11C. Experimental dependence of

water consumption through the artery on

the angle of inclination of HP with voltage amplitude on GUO U=90 V.

0 id ZD 4S &) 75

max

2 V

h • b#g

o

° max . sound (^XK)

-1

(7)

where X - is the length of the condensation zone, h - is

the difference of enthalpies of vapour h and liquid h^.

The ultrasonic influence reduces to increasing the maximum

max. sound height of wick's absorption by the value of j

max. withaut sound

Let us asses the ultrasonic influence on thermal processes in the HP

evaporation zone, assuming that only the surface tension factor changes

in the ultrasonic field. As a rule,(ois the function of many variables and depends on a number of factors, that don't lend

themselves to accurate analitical description. The assessment

of change in the value of the surface tension factor in ultrasonic field was carried out here on the basis of the wellknown formula, describing the velocity of liquid's rise in a capillar /14/:

dh _ R

2<? , jocose , x d~ "Sjfli

gn; (6)

The analysis of expression (o) allows to obtain the assessment

of alteration of the value of the surface tension factor in

the form:

h

<Oef _ R?2 i-l hef s:

AOD

"500 3M

ZOO

mo

Ci^^^^

(Jul

To

3£

K70O

§1

1

K

(S)

100 ZOO 1600 400

FIGURE 12. Distribution of water temperature along the IIP axis in a stationary

mode with different levels of heat loading.

626
FIGURE 13. Temperature in the

adiabatic zone of HP vs. time under different heater power.

1 The values h; h

gt ; K, entering

expression (9) are obtained from

. the experimental data using the

0 5s Ms ifsl velocity of the liquid's rise in

the capillar undes ultrasound

(Fig. 9), allowing to determine the value of K from the measured values of h and h £• Thus, the analysis, carried out,

showed, that the intensification of heat transfer in HP under

ultrasonic action is related with intensification of mass transfer of the cooler along the feeding arteries of HP in the ultrasonic field. Ultimate possibilities of ultrasonic intensification of capillary mass transfer were determined in our experiments using a setup with a design, shown in Fig. 10. The

employing in the setup of a compound capillar and mobile fun^

nel allowed to measure consumption depending on the height of

the artery's cut, its angle of inclination and water temperature. The frequency of ultrasonic radiation was fixed by dimensions of a concentrator of the ultrasonic radiator. The experiments were carried out using tap water settled during twenty-four hours.

The dependence of consumption of a capillary ultrasonic pump

(CUP) on the height of the artery cut is chown in Fig. 11A,

where the inverse dependence between the height of the liquid

rise in the capillar and the consumption ^provided by CUP, is

observed. To determine the relationship between consumption

and temperature, an electric heater was placed into a bath

with water. The results of determining the efficiency of CUP

operation against temperature are shown in Fig. 11, with the

height of water in the bath amounting to 45 mm and the period

of measurement equal to 50 s. Obviously, the consumption of water, ensured by CUP, increased with water temperature, since

viscosity and surface tension decreased. Fig. 11B shows the dependence of water consumption through the capillar on the angle

of inclination of its axis. Obviously, with the increase of

the angle of inclination of the capillar, the water consumption

through it under ultrasound increases since the projection of

water gravity on the capillar axis decreases.

The investigation of processes of heat mass transfer was carried out on HP, operating in a vertical position^ with a source

of ultrasound. When investigating heat exchange the temperature difference on the walls of the evaporator and condenser

FIGURE 14. Specific radial heat power

transmitted by HP operating with a source

of ultrasonic oscillations vs. saturation

temperature along the length of 300 mm: -

_ t" calculations according formula /?/;

0 25 so 75 foo experimental data for d = 3 mm, T=25 °C.

627
microprocessor; 12

transducer of thermal deformations; -14

rnal deformations.

FIGURE 15. Schematical block-diagram

of EPO based on HP with local control

of heat exchange intensity along the reflecting surface on the basis of CUP,

insuring correction of thermal deformations: 1 - reflecting surface; 2 - wickbase; 3 - capillary pipes of CUP; 5

- perphorated pipe boards, G - magnetostrictive radiators; 7 - channel (pin)

wick of condensation zone; S - EPO body;

9 - commuting device, 10 - ultrasonic

generator (USG) controlled on the vole

tage amplitude of output signal; 11 - comparator; 13 - digital converter of

transducer of therwere determined according to the formules: =

q »d/\ ; AT „ = q • 0 /I • • The measurement of "

" c UT, con. M con Aw temperature of tne HP

body allowed to restore the distribution of vapour temperature

along HP. Since the temperature distribution in HP was restored from measurements of the heat flow, the accuracy of such

measurements was determined by the value of heat losses. For

a given setup, the total neat losses amounted to 5 % of the nominal heat power, transr. i t ted by HP.

Thus, the distribution of temperature of KF walls along the

axis } in the stationary working mode against the value of

transmitted by HP heat power, are shown in Fig. 12. It follows

from the given data that HP is activated under heat loadings

exceeding 300 wt. The nature of temperature distribution in HP

under heat loading 700 wt shows that in this case HP is still

far from the limiting mode of operation that is characterised

by the abscence of the temperature gradient along the condensation zone.

The described setup allowed to investigate dynamics of temperature changes in the adia'oatic zone of HP with different powers of the heater. In this case the measurement error did not

exceed 0.25 % (fig. 15).

To compare the experimental data with the results of calculations Fig. 14 gives a generalizing dependence of specific

heat power^ transmitted by a radially water HP with a source

of ultrasound, at length 330 mm. The comparison ot theory and experimental data showed that CUP

nearly to an order of magnitude intensified the capillary mass

transfer, causing nearly to an order of magnitude the increase

of the limiting heat flow, transmitted by HP. Moreover, HP with

ultrasonic intensification of mass transfer possessed an operating ability in the vertical position, which the classical design of HP was not able to ensure.

628
THE ELEMENT OF POWER OPTICS OF A TECHNOLOGICAL LASER BASED

ON HP WITH A SOURCE OF ULTRASONIC OSCILLATIONS

As is well-known, the main factor, restricting the value of the

limiting heat flow, transmitted by HP, is the capillar pressure

of the HP wick and the sound restriction on the vapour tract

of HP. The power increase of technological lassers, the presence of local inhomogeneities in distribution of power in the

cross-section of the laser beam determine the importance of

the problem of creation of EPO in which a local control of

heat exchange intensity along the reflecting surface is realized. The possibility of creation of such a thermo-controlled

optical element . can be related with the usage of USCE. The

principle scheme of the proposed and developed thermo-control

led EAO is shown in Fig, 15. The proposed design of EAO differs from the known designs in that the control by thermodeformations of the mirror surface is conducted by thermophysical methods. The developed design allowes to control the consumption of the cooler along the reflecting surface of EAO locally, in accordance with the value of local density of the

heat flow. In addition in EAO, on the HP basis all constraints on capillary pressure of the wick are excluded, which

increases its heat transmitting capasity. In this case the

wick of the evaporation zone carries out the function of a uni

form redistribution of the cooler along the surface of the EAO

base, and the wick of the condensation zone serves to keep

constant some excess of the cooler.

The carried out investigations of USCE, the testing of a model

of HP with an ultrasound source corroborates the operating abi

lities and efficiency of EAO on the HP basis proposed here.

CONCLUSION

1. The physical model USCE is proposed and experimentally subs

tantiated. A half-empirical quantitative treatement of USCE is

proposed, consisting in emergence of ustrasonic cavitation of

liquid near the capiliar's cut.

2. An experimental HP with a source of ultrasound is developed and the limits of intensification of heat mass transfer if

HP during the operation of USCE are determined.

3. A design of a thermoregulatec EAO on the USCE basis is proposed .

REFERENCES

1. V.V. Apollonov, V.I. Borodin, I.V. Goncharenko, V.V. Ostanin, A.M. Prokhorov, V.Ju. Homich, E.V. Hristjan, S.A. Chetkin

Static and adaptive elements of power optics. // Izvestija of

the USSR Acad. Sci., physical ser., 19G4, vol. 48, no 8,

p. 1625-1645.

629
2. V.V. Apollonov, P.I. Bystrov, V.F. Goncharov et al. The prospects of using porous structures for cooling the- elements of

power optics. // Kvantovaja elektronika. - 1979, vol. 6, no 12,

p. 2533-2545.

3. V.G. Voronin, A.B. Revjakin, V.Ja. Sasin, V.S. Tarasov.

Lowtemperature heat pipes for flying vehicles. /Moscow, Mashinostroenie, 1976, 200p.

4. E.G. Konovalov. Ultrasonic capillar effect //Discoveries in

the USSR. - Moscow, CNIIPI, 1973, p. 16-18.

5. O.K. Keller, G.S. Kratysh, G.D. Lubjanizky. Ultrasonic cleaning. /Leningrad, Mashinos troenie , 1977, 184p.

6. A.V. Kortnev, T.V. Makarova. Effect of ultrasonic oscillations on diffusion processes in liquid//In:Akustika i zvuk. - Kiev, Technika, 1966, p. 2S-41.

7. C. Eckart. Vortices and stream caused by sound waves./

Phys. Rev. 1948, vol. 73, no 1, p. 68-76.

8. I.N. Kanevski. Constant forces emerging in acoustic field//

Akusticheskij jurnal, 1961, vol. 7, 1, p. 3-17.

9. Ju.P. Rosin, V.S. Ticho.nova, M.N. Kostjuchek. On anomalously large pressures in a capillar in immediate vicinity of

the source // Ukrainskij fizicheskij jurnal, 1975, vol. 20, no 2, p. 214-219.

10. E.G. Konovalov, I.N. Germanovich. Ultrasonic capillar effect. Doklady of the USSR Acad. Sci., 1962, vol. 6, no 8,

p. 492-493.

11. V.I. Bulgakov. Experimental and theoretical study of processes of wood impregnation and painting in ultrasonic field.

Thesis. Moscow, 1981, 222p.

12. G.N. Afanasjev, V.I. Bulgakov, A.N. Oblivin, A.K. Voskresenski. To the ultrasonic capillar effect // ^auchnye trudy

of MLTI, 1981, vyp. 130, p. 21-27.

13. V.G. Levich. Physical and chemical hydrodynamics. Moscow,

Acad of Sci. -of the USSR, 1952, 538p.

630
APPENDIX I

BOULDER DAMAGE SYMPOSIUM

November 1-3, 1989

List of attendees

Joseph Abate

Hampshire Instruments, Inc.

10 Carlson Road

P. 0. Box 10159

Rochester, NY 14610

Jonathan Arenberg

TRW

One Space Park, MS 01/1210

Redondo Beach, CA 90278

213 813-9551

Mamoru Adachi

Adachi New Industrial Co., Ltd.

1-14-20 Itachibori Nishi-Ku

Osaka, 550

JAPAN

06 858-6203

Donnie Aminou

Rocky Mountain Instrument

1501 S. Sunset St.

Longmont, CO 80501

303 651-2211

Claude Amra

CNRS

ENSP de Marseille

Universitaire de St. Jerome

13013 Marseilles Cedex 13,

FRANCE

33 91288070

William L. Andre

US Army Strat. Def. Command

106 Wynn Dr.

P. 0. Box 1500, CSSD-H-DT

Huntsville, AL 35807-3801

205 722-1666

Mark Babb

Montana Laser Optics

P. 0. Box 4151

Bozeman, MT 59772

406 586-5100

Steve Bandettini

OCLI

2789 Northpoint Pk.

Santa Rosa, CA 95407-7397

707 525-7160

Steve Barnes

Oxford Inst. Ltd.

Eynsham, Oxford 0X8 1TL,

ENGLAND

865 882 855 Ext. 321

Olivier Barnouin

Univ. of Illinois

214 Nuclear Engineering Lab

103 S. Goodwin Ave.

Urbana, IL 61801

217 333-1750

Philip Beck

Martin Marietta

P.O. Box 179, L8005

Denver, CO 80201

303 971-5075

631
Michael F. Becker

Univ. of Texas at Austin

ENS 143, ECE Dept.

Austin, TX 78712

512 471-3628

Robert Bieri

Lawrence Livermore Natl. Lab. /MIT

P. 0. Box 808, L-644

Livermore, CA 94550

415 423-0897

Denise Beeson

3-M Mountain View

420 N. Bearnard

Mountain View, CA 94043

415 969-5200

James D. Boyer

Los Alamos Nationl Lab.

P. 0. Box 1663, MS J564

Los Alamos, NM 87545

505 667-9222

Raymond Bell

Lockheed

Mail Stop 9720/202

3251 Hanover Street

Palo Alto, CA 94304-1191

415 424-3396

Bernard Bendow

TRW

01/1280

One Space Park

Redondo Beach, CA 92647

213 813-9540

Robert K. Bratton

Litton Laser Systems

P. 0. Box 547300

Orlando, FL 32854-7300

407 297-4522

Arthur Braundmeier

McDonnell -Douglas Elect . Sys . Co

Bldg. 101, Mail Code 1066267

P. 0. Box 516, Dept. Y440

St. Louis, MO 63166-0516

618 692-2359

Harold Bennett

Naval Weapons Center

Michelson Labs

Code 38101

China Lake, CA 93555

619 939-1440

Alain Bergeron

National Optics Inst.

369 Franquet St.

P. 0. Box 9970

Sainte-Foy, Quebec G1V4C5

CANADA

418 657-7006

Jerry R. Bettis

Rocketdyne-Div Rockwell

M/S FA03

6633 Canoga Ave.

Canoga Park, CA 91303

818 700-6166

Peter Braunlich

Washington State University

Department of Physics

Pullman, WA 99164-2814

509 332-3328

Ernie Braunschweig

R&D Associates

2600 Yale Boulevard, SE

Albuquerque, NM 87106

505 842-8911

Jerry Britten

Lawrence Livermore Nat'l. Lab.

P. 0. Box 5508, L-367

Livermore, CA 94550

415 423-7653

Raymond Brusasco

Lawrence Livermore Nat ' 1 . Lab

P. 0. Box 5508, L-490

Livermore, CA 94550

415 422-3111

632
Jack Campbell

Lawrence Livermore Nat'l. Lab.

P. 0. Box 5508, L-490

Livermore, CA 94550

415 422-6497

Charles Carniglia

S Systems Corp.

P. 0. Box 9316

Albuquerque, NM 87119

505 844-1064

Eduard Cartier

IBM

Watson Research Center

P. 0. Box 48

Yorktown Heights, NY 10598

914 945-3339

Jack Chambers

II-VI Inc.

375 Saxonburg Blvd.

Saxonburg, PA 16056

412 352-4455

Lloyd L. Chase

Lawrence Livermore Nat ' 1 . Lab

P. 0. Box 5508, L-490

Livermore, CA 94550

415 422-6151

Robert Chow

Lawrence Livermore Nar'l. Lab.

P. 0. Box 808, L-791

Livermore, CA 94551

415 422-7615

Charles Cockrum

Rocky Mountain Instrument

1501 S Sunset St.

Longmont, CO 80501

303 651-2211

Daniel Cohn

MIT Lincoln Lab

167 Albany St.

Cambridge, MA 02139

617 253-5524

Lisa Connally

Wright Patterson AFB

WRDC/MLPJ

WPAFB , OH 45433-6533

513 255-1957

Michael Creighton

Perkin-Elmer Corp.

Mail Station 420

761 Main Ave

Norwalk, CT 06859-0420

203 834-4511

John Daly

Litton Laser Systems

2707 S. Orange Blossom Tr.

Apopka, FL 32703

407 297-4457

Barbara Darnell

Spectral Systems

Ragged Hill Rd.

W. Brookfield, MA 01585

508 867-3321

Steven A. Davidson

MIT Lincoln Lab.

244 Wood St.

Lexington, MA 02173

617 981-3901

David Deacon

Deacon Research

900 Welch Rd. , Suite 203

Palo Alto, CA 94304

415 326-1520

Donald Decker

Naval Weapons Center

Code 3816

China Lake, CA 93555

619 939-3247

Frank Demarco

Lawrence Livermore Nat'l. Lab.

P. 0. Box 5508, L-491

Livermore, CA 94550

415 422-3022

633
John Detrio

Univ. of Dayton Res. Inst.

300 College Park Ave.

Dayton, OH 45469

513 229-3453

Gary Edwards

Lawrence Livermore Nat ' 1 . Lab

P. 0. Box 5508, L-486

Livermore, CA 94550

415 423-1921

Randle Dewees Osama El-Bayoumi

Naval Weapons Center U.S. Air Force

Code 3816 RADC/ESM

China Lake, CA 93555 HAFB, MA 01731

619 939-3168 617 377-3693

Manfred Dieckmann

Laser Zentrum Hannover e.V

Vahrenwalder Strasse 7 Welelfengarten 1 3000 Hannover 1,

WEST GERMANY

0511 762-4265

Jean Dijon

Commisiariat a l'Energie Atomique

D . LET1

CENG Ave des Martyrs

85X 38041

Grenoble, 38041

FRANCE

76884313

Terry Donovan

Naval Weapons Center

Code 3818

China Lake, CA 93555

619 446-2115

Fred Doss

Spectra-Physics Optics Corp.

1330 W. Middlefield Rd.

P. 0. Box 7013

Mountain View, CA 94039-7013

415 961-2550

George Dugdale

Perkin- Elmer Corp

761 Main Ave.

Norwalk, CT 06859-0420

203 834-6271

Phil Elizondo

Lockheed Research & Dev.

Dept. 97-20, Bldg. 202

3251 Hanover St.

Palo Alto, CA 94304

415 524-2315

David Emmony

Loughborough Univ. of Technology

Dept. of Physics

Loughborough, Leics LE11 3 TV

UNITED KINGDOM

509 223306

Richard Eng

MIT Lincoln Laboratory

244 Wood Street

Lexington, MA 02173

617 981-3695

Gregory J . Exarhos

Battelle Northwest

Battelle Boulevard

P. 0. Box 999, MS K2-44

Richland, WA 99352

509 375-2440

Bill Farmer

U.S.A. Strat. Def. Command

Attn: CSSD-H-D

P. 0. Box 1500

Huntsville, AL 35807-3801

205 895-4780

634
George Ferguson

Lockheed Palo Alto Lab.

0/9720 B/202

3251 Hanover St.

Palo Alto, CA 94304

415 424-2319

Kim Ferris

Pacific Northwest Laboratory

P. 0. Box 999

Richland, WA 99352

508 375-3754

Mary Fuka

Univ. of New Mexico

800 Yale Blvd. NE

Albuquerque, NM 87131

505 277-2616

Yoshiyuki Fukui

Shin-etsu Quartz Prod Co

Shinjuku San-ei Bldg. 1-22-2

Shinjuku-ku, Tokyo

JAPAN

0429 44 9289

Shari Powell Fisher

OCLI

Ridgeway

Hillend Industrial Park

Dun Fermi ine FIFE , KY11 SFR

UNITED KINGDON

0383 823-631

Herve Floch

Commissariat al'Energie

Centre d' Etudes de Limeil

BP27

Villeneuve St. Georg, 94190

FRANCE

Ronald Francis

Ball Aerospace

P. 0. Box 1062

Boulder, CO 80303

303 939-5556

Jerome B. Franck

Naval Weapons Center

China Lake, CA 93555

619 939-3827

Alan Frank

Lawrence Livermore Nat'l. Lab.-

P.O. Box 808, L-281

Livermore, CA 94550

415 422-7271

Andy Garay

US Army Strat. Def. Command

GBL PO

(CSSD-H-FB)

WSMR, NM 88002-1198

505 678-2383

Adolf Giesen

IFSW, Univ of Stuttgart

Pfaffenwaldring 43

7000 Stuttgart 80,

WEST GERMANY

Robert Goedert

US Army TACOM

AMSTA-RSC

Warren, MI 48397

313 574-5325

Johnny G rammer Lockheed Missiles & Space

1111 Lockheed Way

0/51-50, B/586

Sunnyvale, CA 94089-3504

408 756-4491

Richard Grauslys

Kollsman

220 Daniel Webster H

M/S 2-B12-2

Merrimack, NH 03054-4809

603 595-6093

635
Arthur H. Guenther

Los Alamos National Lab.

P. 0. Box 1663

Los Alamos, NM 87544

505 667-1331

Sandra R. Gyetvay

The Aerospace Corp.

P. 0. Box 92957, M2/241

Los Angeles, CA 90009

213 336-8239

Yubong Hahn

Rocky Mountain Instrument Co

1501 S. Sunset

Longmont, CO 80501

303 651-2211

Christopher Hale

Optical Coating Lab. Inc

Ridgeway

Hillend Industrial Park

Fife, KY11 5FR UK

SCOTLAND

44 383 823631

Chuck Hamilton

Spectra Tech

2755 Northup Way

Bellevue, WA 98004

206 827-0460

Richard S. Hockett

Charles Evans & Associates

301 Chesapeake Dr.

Redwood City, CA 94063

415 369-4567

Lloyd Hodge

LMSC

14055 Palomino Way

Saratoga, CA 95070

408 742-6232

James Hoenigman

University of Dayton

Research Institute KL562

Dayton, OH 45469

513 229-3119

Robert Hoffman

Ctr. Night Vision & Electro-Optics

AMSEL-RD-NV-L

Rm 2, Bldg 317

Ft. Belvoir, VA 27060

703 664-1431

Samuel Holmes

Northrop Research & Tech Center

One Research Park

M/S 345/T20

Palos Verdes Peninsula, CA 90274

213 544-5314

Alan Hopkins

W. J. Shafer Assoc.

5100 Springfield Pik

Suite 511

Dayton, OH 45431

513 253-9572

Hamilton Hunter

Martin Marietta Energy Sys

P. 0. Box 2008

Oak Ridge Natl. Lab.

Oak Ridge, TN 37831-6372

615 576-4295

Helmar Janee

Science Applications Int'l. Corp.

Pacifica Technology Division

11696 Sorrento Valley Rd. , Ste A. San Diego, CA 92121

619 453-2530

John Jewell

Naval Research Lab

Code 6505

4555 Overlook Ave., S.W.

Washington, DC 20375-5000

202-767-9325

Michael Jin

University of Rochester

250 E. River Rd.

Lab. for Laser Energetics

Rochester, NY 14623-1299

716 275-5101

636
Linda Johnson

Naval Weapons Center

Code 3818

China Lake, CA 93555

619 939-1422

Mark Kozlowski

Lawrence Livermore Nat'l.

P. 0. Box 5508, L-489

Livermore, CA 94550

415 422-2925

Lab.

L. John Jolin

Los Alamos National Lab.

MS J564

Los Alamos, NM 87545

505 667-7102

G. Edward Kuhl

W. J. Shafer Assoc.

5100 Springfield Pk.

Suite 511

Dayton, OH 45431

513 253-9572

George Jones

Contraves Goerz Corp

610 Epsilon Dr.

Pittsburgh, PA 15238

412 967-7733

Ramin Lalezari

PMS Electro-Optics

1855 South 57th Court

Boulder, CO 80301

303 443-7100

Mark A. Kahan

Optical Research Associates

945 Concord Street

Framingham, MA 01701

508 872-6001

Huai-Chuan Lee

Hoya Optics, Inc.

3400 Edison Way

Fremont, CA 94538

415 490-1880

Robert Keim

Eastman Kodak Co.

121 Lincoln Ave.

Rochester, NY 14653-8118

716 253-2375

Floyd Kinder

COMARCO

1201 N. China Lake Blvd.

Ridgecrest, CA

Margaret Kohin-Nitschelm

Litton/Itek Optical Sys

10 Maguire Rd.

Lexington, MA 02173

617 276-3332

Juergen Kolbe

Universitaet Hannover

Institut Fuer Quantenoptik

Welfengarten 1 D-3000 Hannover 1,

FEDERAL REPUBLIC OF GERMANY

049 511-762-3381

Olive Lee

Deacon Research

900 Welch Rd.

Suite 203

Palo Alto, CA 94304

415 326-1520

Charles Leung

XMR, Inc.

5403 Betsy Ross Dr.

Santa Clara, CA 95054

408 988-2426

Keith Lewis

Royal Signals & Radar Establishment

St. Andrews Rd.

Malvern, Worcs WR14 3PS

UNITED KINGDOM

44 684 895062

637
Matthew Magida

Perkin Elmer

100 Wooster Heights, MS848

Danbury, CT 06810

203 797-6355

Klaus Mann

Laser Laboratorium Grottingen eV

Robert-Bosch-Breite 10

Grottingen, 3400

WEST GERMANY

0551 61067

C. Denton Marrs

Naval Weapons Center

Code 3817

China Lake, CA 93555

619 939-2470

Bryan Mason

Naval Weapons Center

Code 3817

China Lake, CA 93555

619 939-2470

John Gordon Mathew

Heriot-Watt University

Physics Dept.

Riccarton Campus

Edinburgh, Scotland EH14 4AS

UNITED KINGDOM

031 451-3069

Eckart Matthias

Freie Universitaet Berlin

Fachbereich Physik

Arnimallee 14

1000 Berlin 33

WEST GERMANY

FRG-30-8383340

Alasdair Mclnnes

Barr & Stroud Ltd.

Applied Physics Group

Annies land

Glasgow, G13 1HZ

Scotland

UNITED KINGDOM

041 954 9601 x2527

Ted McMinn

McDonnell Douglas

P. 0. Box 516, MC 1066267

St. Louis, MO 63166

314 234-4321

Kent Meeks

Sandia Nat'l Lab

P. 0. Box 5800

Albuquerque, NM 87185

505 844-1040

John Midavaine

Eastman Kidak Co

D337

MS 38015

Rochester, NY 14653-8015

716 253-2462

Perry A. Miles

R&D Assoc

4640 Admiralty Way

Marina del Rey, CA 90295

213 822-1715

Kent Moncur

KMS Fusion

700 KMS Place

Ann Arbor, MI 48108

313 769-8500

Bryan Monosmith

NASA/Goddard

MS 723

Greenbelt, MD 20771

301 286-7017

Thomas Mooney

Barr Associates, Inc.

2 Lyberty Way

Westford, MA 01886

508 692-7513

Mark Moran

Naval Weapons Center

Code 3817

China Lake, CA 93555

619 939-3827

638
David Mordaunt

Hughes Aircraft Company

Bldg. E1/MSB129

P. 0. Box 902

El Segundo, CA 90245

213 616-5422

Hasegawa Niichiro

Shin-Etsu Quartz Product

22-2 Nishi-Shinjuku l-Chome

Shinjuka

Tokyo

,

JAPAN

03-348-1911

Amber J . Morgan

Lawrence Livermore Nat'l. Lab.

P. 0. Box 5508, L-249

Livermore, CA 94550

415 423-3277

Aspasia Sylvia Nasla

The Welding Institute

Abington Hall

Abington

Cambridge, CB16AL

ENGLAND

223 891162

Hiroshi Noguchi

Daido Steel Co . , Ltd.

2-30 Daido -Cho

Minami -Ku

Nagoya-City, Aichi-Ken 457

JAPAN

81 52-611-2511

Robert O'Connell

University of Missouri

Electrical & Computer Eng. Dept.

Columbia, MO 65211

314 552-8373

Soe-Mie Nee

Naval Weapons Center

Code 38101

China Lake, CA 93555

619 939-1425

Brian Neff

Thermo Electron Technologies

9550 Distribution Av

San Diego, CA 92121

619 578-5885

Brian E. Newnam

Los Alamos National Lab.

MS-J564

Los Alamos, NM 87545

505 667-7979

Davis Nichols

Boeing Aerospace & Electronics

Mail Stop 8H-29

P. 0. Box 3999

Seattle, WA 98124

206 773-8938

Elke Oertel

Laser Zentrum Hannover e.V.

Engelbosteler Damm 25

3000 Hannover 1 Hannover

GERMANY

0511 762-3381

Albert Ogloza

Naval Weapons Center

Code 3816

China Lake, CA 93555

619 939-3247

Takayuki Okamoto

Okamoto Optics Co.

8-34 Hayamachi

Isogo-ku

Yokohama

,

JAPAN

045 752 2233

639
Yoshiaki Okamoto

Okamoto Optics Co

8-34 Haramachi

Isogo-ku

Yokohama

JAPAN

045 752-2233

Nora Osborne

Univ. of Dayton Res. Inst.

300 College Pk

KL 165

Dayton, OH 45469

513 229-4508

Frank Padilla

USASDC

P. 0. Box 124

(CSSD-H-FB)

WSMR, NM 88002-1198

505 678-2441

Roger A. Paquin

Perkin Elmer

Electro-Optics Tech Div

100 Wooster Heights

Danbury, CT 06810

203 797-6130

Michael Pavia

Hampshire Instruments Inc.

10 Carlson Road

P. 0. Box 10159

Rochester, NY 14610

Emile Pelletier

CNRS

ENSP de Marseille

Domaine Universitaire de StJerome

13397 Marseilles Cedex 13, 13013

FRANCE

91 28 83 28

G. Paul Perryman

Texas Instruments Inc.

8505 Forest Lane

P. 0. Box 660246, MS 3122

Dallas, TX 75266

214 480-1157

Cathy Peterson

Hughes Santa Barbara Res. Cent.

M/S 54

75 Coromar Drive

Goleta, CA 93117

805 562-2913

Stefan Petzoldt

Freie Universitat Berlin

1000 Berlin 33

Arnimallee 14,

GERMANY

Robert Poirier

Corion Corp.

73 Jeffrey Ave

Holliston, MA 01746

508 429-5065

Robert S . Polvani

NIST

Bldg 220, Rm A 107

Gaithersburg, MD 20899

301 975-3487

Bradley Pond

S Systems Corp.

P. 0. Box 9316

Albuquerque, NM 87119

505 844-1064

James 0. Porteus

Naval Weapons Center

Code 3817

China Lake, CA 93555

619 939-3827

Jack Price

US Naval Surface Warfare

10901 N. Hampshire Ave.

Code R41, White Oak Lab

Silver Spring, MD 20903

202 394-2272

640
Manfred Rahe

Laser Zentrum Hannover e.V.

3000 Hannover 1

Weckenstr. 19

Hannover

,

FEDERAL REPUBLIC OF GERMANY

05 11 762-4265

Frank Rainer

Lawrence Llvermore Nat'l. Lab.

P. 0. Box 5508, L 490

Livermore, CA 94550

415 422-4376

Tilak Raj

Martin Marietta Energy Systems

P. O. Box 2003, MS7272

Oak Ridge, TN 37831-7272

615 574-4822

Juergen Reif

Freie Universitat Berlin

Fachbereich Physik

Arnimallee 14

1000 Berlin 33,

WEST GERMANY

49308387157

David Resendes

Physical Sciences, Inc.

Research Pk

P. 0. Box 3100

Andover, MA 01810-7100

508 475-9030

Steven Risser

Battelle Pacific Northwest Lab

Box 999, K2-44

Richland, WA 99352

509 375-2323

Dan Rogers

Honeywell

3660 Technology Dr.

P. 0. Box 1361 MM 65-2600

Minneapolis, MN 55440

Chris Rowan

EM Industries

5 Skyline Drive

Hawthorne, NY 10532

914 592-4660

Gary Saenz

Rockwell Int'l

Rocketdyne Div. FA03

6633 Canoga Ave

Canoga Park, CA 91303

Virgil Sanders

Los Alamos National Lab.

P. 0. Box 1663, MS J564

Los Alamos, NM 87545

505 667-7398

Paul Schall

JAYCOR

2951 28th St., Suite 3075

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I

644
4. TITLE AND SUBTITLE

Laser Induced Damage in Optical Materials: 1989

5. AUTHOR(S)

Harold E. Bennett (NWC) , Lloyd L. Chase (LLNL) , Arthur H. Guenther (LANL)

Brian E. Newnam (LANL), and M. J. Soileau (UCF)

6. PERFORMING ORGANIZATION (IF JOINT OR OTHER THAN NIST, SEE INSTRUCTIONS)

U.S. DEPARTMENT OF COMMERCE

NATIONAL INSTITUTE OF STANDARDS AND TECHNOLOGY

BOULDER, COLORADO 80303-3328

7. CONTRACT/GRANT NUMBER

8. TYPE OF REPORT AND PERIOD COVERED

BL-114A

(5-90) U.S. DEPARTMENT OF COMMERCE

NATIONAL INSTITUTE OF STANDARDS AND TECHNOLOGY

BIBLIOGRAPHIC DATA SHEET

PUBUCATION OR REPORT NUMBER

NIST/SP-S01

2. PERFORMING ORGANIZATION REPORT NUMBER

PUBUCATION DATE October 1990

9. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (STREET, CITY, STATE, ZIP) National Institute of Standards and Technology (NIST)

American Society for Testing and Materials (ASTM)

International Society for Optical Engineering (SPIE)

Defense Advanced Research Project Agency (DARPA)

Department of Energy (DOE)

10. SUPPLEMENTARY NOTES

11. ABSTRACT (A 200-WORD OR LESS FACTUAL SUMMARY OF MOST SIGNIFICANT INFORMATION. IF DOCUMENT INCLUDES A SIGNIFICANT BIBUOGRAPHY OR

LITERATURE SURVEY, MENTION IT HERE.)

The Twenty-First Annual Symposium on Optical Materials for High-Power Lasers (Boulder

Damage Symposium) was held at the National Institute of Standards and Technology in Boulder,

Colorado, November 1-3, 1989. The Symposium was sponsored jointly by the National Institute

of Standands and Technology, the American Society for Optical Engineering, the Defense

Advanced Research Project Agency, and the Department of Energy. Approximately 200 scientists

including representatives of the United Kingdom, France, Japan, Canada, and the Federal

Republic of Germany, attended the Symposium. The Symposium was divided into sessions

concerning Materials and Measurements, Mirrors and Surfaces, Thin Films, and, finally,

Fundamental Mechanisms. As in previous years, the emphasis of the papers presented at the

Symposium was directed toward new fromtiers and new developments. Particular emphasis was

given to materials for high power apparatus. The wavelength range of the prime interest

was from 10.6 fxa to the uv region. Highlights included surface characterization, thin film

substrate boundaries, and advances in fundamental laser-matter threshold interactions and

mechanisms. The scalling of damage thresholds with pulse duration, focal area, and

wavelength was discussed in detail. Harold E. Bennett of the Naval Weapons Center, Arthur

H. Guenther of the Los Alamos National laboratory, Lloyd L. Chase of the Lawrence Livermore

National Laboratory, Brian E. Newnam of the Los Alamos National Laboratory, and M. J. Soileau

of the University of Central Florida were co-chairmen of the Symposium. The Twenty-Second

Annual Symposium is scheduled for October 24-26, 1990, at the National Institute of Standards

and Technology, Boulder, Colorado.

12. KEY WORDS (6 TO 12 ENTRIES; ALPHABETICAL ORDER; CAPITALIZE ONLY PROPER NAMES; AND SEPARATE KEY WORDS BY SEMICOLONS)

laser damage; laser interaction; optical components; optical fabrication;

optical materials and properties; thin film coatings.

13. AVAILABILITY

UNLIMITED FOR OFFICIAL DISTRIBUTION. DO NOT RELEASE TO NATIONAL TECHNICAL INFORMATION SERVICE (NTIS).

X

X

ORDER FROM SUPERINTENDENT OF DOCUMENTS, U.S. GOVERNMENT PRINTING OFFICE,

WASHINGTON, DC 20402. ORDER FROM NATIONAL TECHNICAL INFORMATION SERVICE (NTIS), SPRINGFIELD, VA 22161.

14. NUMBER OF PRINTED PAGES

670

15. PRICE

ELECTRONIC FORM



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