NBS SPECIAL PUBLICATION 400-19

U.S. DEPARTMENT OF COMMERCE / National Bureau of Standards

Measurement Technology

lOO

19 7 lb

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January 1 to June 30, 1975
NATIONAL BUREAU OF STANDARDS

The National Bureau of Standards^ was established by an act of Congress March 3, 1901. The Bureau's overall goal is to strengthen and advance the Nation's science and technology

and facilitate their effective application for public benefit. To this end, the Bureau conducts

research and provides: (1) a basis for the Nation's physical measurement system, (2) scientific and technological services for industry and government, (3) a technical basis for equity in trade, and (4) technical services to promote public safety. The Bureau consists of the Institute for Basic Standards, the Institute for Materials Research, the Institute for Applied Technology,

the Institute for Computer Sciences and Technology, and the Office for Information Programs.

THE INSTITUTE FOR BASIC STANDARDS provides the central basis within the United

States of a complete and consistent system of physical measurement; coordinates that system

with measurement systems of othef nations; and furnishes essential services leading to accurate and uniform physical measurements throughout the Nation's scientific community, industry,

and commerce. The Institute consists of the Office of Measurement Services, the Office of Radiation Measurement and the following Center and divisions:

Applied Mathematics — Electricity — Mechanics — Heat — Optical Physics — Center

for Radiation Research: Nuclear Sciences; Applied Radiation — Laboratory Astrophysics

— Cryogenics " — Electromagnetics " — Time and Frequency

THE INSTITUTE FOR MATERIALS RESEARCH conducts materials research leading to improved methods of measurement, standards, and data on the properties of well-characterized materials needed by industry, commerce, educational institutions, and Government; provides

advisory and research services to other Government agencies; and develops, produces, and

distributes standard reference materials. The Institute consists of the Office of Standard

Reference Materials, the Office of Air and Water Measurement, and the following divisions:

Analytical Chemistry — Polymers — Metallurgy — Inorganic Materials — Reactor

Radiation — Physical Chemistry.

THE INSTITUTE FOR APPLIED TECHNOLOGY provides technical services to promote

the use of available technology and to facilitate technological innovation in industry and Government; cooperates with public and private organizations leading to the development of

technological standards (including mandatory safety standards), codes and methods of test; and provides technical advice and services to Government agencies upon request. The Insti- tute consists of the following divisions and Centers: Standards Application and Analysis — Electronic Technology — Center for Consumer

Product Technology: Product Systems Analysis; Product Engineering — Center for Building

Technology: Structures, Materials, and Life Safety; Building Environment; Technical Evaluation and Application — Center for Fire Research: Fire Science; Fire Safety Engineering.

THE INSTITUTE FOR COMPUTER SCIENCES AND TECHNOLOGY conducts research and provides technical services designed to aid Government agencies in improving cost effec- tiveness in the conduct of their programs through the selection, acquisition, and effective utilization of automatic data processing equipment; and serves as the principal focus within

the executive branch for the development of Federal standards for automatic data processing

equipment, techniques, and computer languages. The Institute consists of the following

divisions: Computer Services — Systems and Software — Computer Systems Engineering — Information Technology.

THE OFFICE FOR INFORMATION PROGRAMS promotes optimum dissemination and

accessibility of scientific information generated within NBS and other agencies of the Federal Government; promotes the development of the National Standard Reference Data System and

a system of information analysis centers dealing with the broader aspects of the National Measurement System; provides appropriate services to ensure that the NBS staff has optimum

accessibility to the scientific information of the world. The OflRce consists of the following

organizational units: Office of Standard Reference Data — Office of Information Activities — Office of Technical

Publications — Library — Office of International Relations — Office of International Standards.

1 Headquarters and Laboratories at Gaithersburg. Maryland, unless otherwise noted; mailing address Washington, D C. 20234.

- Located at Boulder. Colorado 80302.
R 5 1976 Semiconductor Measurement Technology

^(^^ Progress Report, January 1 to June 30, 1975

C W. Murray Bullis, Editor

Electronic Technology Division

Institute for Applied Technology

National Bureau of Standards

Washington, D.C. 20234

Jointly Supported by:

The National Bureau of Standards,

The Navy Strategic Systems Project OflSce, The Defense Nuclear Agency, and

The Defense Advanced Research Projects Agency

U.S. DEPARTMENT OF COMMERCE, Elliot L Richardson, Secretory

James A. Baker, III, Under Secrefary

Dr. Betsy Ancker-Johnson, Assisiani Secretary for Science and Technology

U ; NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Acting Director

Issued April 1976
Kational Bureau of Standards Special Publication 400-19

Nat. Bur. Stand. (U.S.), Spec. Publ. 400-19, 95 pages (Apr. 1976) CODEN: XNBSAV
TABLE OF CONTENTS

SEMICONDUCTOR MEASUREMENT TECHNOLOGY

PAGE

Preface vlil

1. Introduction 2

2. Highlights 3

3. Resistivity 7 3.1. Standard Reference Materials 7 3.2. Spreading Resistance Measurements on Graded Structures 10

3.3. High-Speed Spreading Resistance Probe 10 3.4. Ionization of Dopant Density in Silicon 13

4. Surface Analysis Methods 17 4.1. Comparative Study of Surface Analysis Techniques 17 4.2. X-Ray Photoelectron Spectroscopy 18

4.3. Calibration Standards for Ion Microprobe Mass Analysis 20

4.4. Optical Test for Surface Quality of Sapphire Substrates 22

5. Test Structure Applications 25

5.1. Reevaluation of Irvin's Curves 25 5.2. Mathematical Models of Dopant Profiles 25 5.3. Dynamic MOS C-V Method 26

5.4. Epitaxial Layer Thickness 27 5.5. Bias-Temperature Stress Test 29 5.6. Extended Range MIS C-V Method ' 31

6. Materials and Procedures for Wafer Processing 33 6.1. Ion Implantation Parameters 33 6.2. Passivation Integrity 33

7. Photolithography 34 7.1. Optical Imaging for Photomask Metrology 34

7.2. Calibration Standards for Photomask Metrology 41

7.3. Line-Width Measurements 41

8. Test Patterns 44

8.1. Sheet Resistor Test Structures 44

8.2. Charge-Coupled Device Test Pattern 44

8.3. Test Pattern Design and Analysis for SOS/LSI 47

9. Interconnection Bonding 48

9.1. Non-Destructive Test for Beam-Lead Bonds 48

9.2. Wire Bond Pull and Shear Tests 51

10. Hermeticity 52 10.1. Helium Mass Spectrometer Method 52 10.2. Correlation of Moisture Infusion, Leak Size, and Device Reliability 52

iii
TABLE OF CONTENTS

PAGE

11. Device Inspection and Test 55

11.1. Automated Scanning Low-Energy Electron Probe 55

11.2. Scanning Electron Microscopy — Electron Beam Induced Current Mode 56 11.3. Scanning Electron Microscopy — Electron Beam Induced Damage 59

11.4. Flying-Spot Scanner 5O

12. Thermal Properties of Devices 55 12.1. Thermal Resistance — Darlington Pairs 66 12.2. Transient Thermal Response ..... 67

13. References 59

Appendix A Semiconductor Technology Program Staff 73

Appendix B Semiconductor Technology Program Publications 74

Appendix C Workshop and Symposium Schedule ' 77

Appendix D Standards Committee Activities 78

Appendix E Solid-State Technology & Fabrication Services 80

Appendix F Automated Integrated Circuit Processing and Assembly . .

.' 81

Index 85

iv
LIST OF FIGURES

PAGE

1. Two-point spreading resistance probe as modified for high-speed operation .... 11

2. Spreading resistance as measured with four different loads on a tungsten

carbide tip of radius 6 . 4 um 12

3. Spreading resistance as a function of load for three tungsten carbide

tips of different radius 12

4. Scanning electron micrographs of tracks left by a tungsten carbide probe

tip of radius 6.4 pm 12

5. Ionization of phosphorus in silicon at 300 K 14

6. Ionization of boron in silicon at 300 K 14

7. Yield as a function of energy of backscattered 2.0 MeV '^He'*' ions from

zinc-implanted silicon wafer 17

8. Angular dependence of x-ray photoelectron spectra from clean silicon surfaces . . 19

9. Photomicrograph of photomask repeat pattern 21

10. Photomicrograph of oxidized silicon wafer showing implantation microvolumes in a portion of the repeat pattern 21

11. Infrared reflectance spectra of sapphire surfaces 23

12. Reflection x-ray topograph of specially pre_pared (1012) sapphire surface

obtained with copper radiation using the (3030) reflection plane 23

13. Dif f ractiometer traces of specially prepared (1012) sapphire surfaces

obtained in the 0:29 mode with copper radiation using the (0224)

reflection plane 24

14. Capacitance-voltage characteristics of a p-type MOS capacitor with

N" = 10^^ cm~3 X = 120 nm, and C = 2.88 x 10"^ F/cra^ 27 A o ' o 15. Error curves for minim.um profiling depth as a function of background

dopant density 27

16. Dopant density profiles calculated from the deep depletion C-V characteristics of an MOS capacitor on four epitaxial layers 28

17. Dopant density profiles calculated from Wafer 2353 by signal averaging

R repetitive deep depletion C-V characteristics 28

18. A summary of flat-band voltage measurements on MOS capacitors at room

temperature following various treatments 30

19. Image intensity profiles of an opaque line on a transparent background

for incoherent and coherent illumination 35

20. Image intensity profiles of a single edge viewed by light of wavelength

A = 500 nm 37

21. Image intensity profiles of a single edge viewed by coherent illumination

with varying defocus 37

22. Image intensity profiles of a single edge viewed by coherent illumination

with IX spherical aberration and varying defocus 38

23. Line edge location with a filar eyepiece 38

24. Line edge location with an image shearing micrometer eyepiece 40

25. Interim chromium-on-glass line-width measurement artifact with transparent

lines on an opaque background 41

26. Polarizing interferometer 42

27. Schematic diagram of orthogonal pinv;heel van der Pauw sheet resistor structure . 45

28. Equivalent geometrical model of orthogonal pinwheel 45

V
LIST OF FIGURES

PAGE

29. Calculated measurement error due to finite contact width for orthogonal

pinwheels with four-fold rotational symmetry 45

30. Calculated measurement error due to finite contact width for orthogonal

quadrate crosses 45

31. Acoustic emission detector and force probe 49

32. Combination wobble-force probe and acoustic emission detector 49

33. Vacuum chuck for pulling beam-lead devices 49

34. Typical acoustic emission wave form 50

35. Scanning electron micrographs of a poorly bonded beam lead , 50

36. Data from bond pull test and bond shear test 51

37. Helium outgassing rate, R, of sealed borosilicate glass capsules as

a function of dwell time between pressurization and measurement 53

38. Dwell time necessary for helium outgassing rate of sealed borosilicate

glass capsules to fall to specified maximum allowable rates, R , as a

f ^- c _ • I • max function or pressurization time 53

39. Circuit for testing electron gun beam current 55

40. ASLEEP image of aluminum grid 56

41. ASLEEP image of MOS capacitor array 56

42. ASLEEP scan of 30-mil (0.76-mm) diameter MOS capacitors on 35-mil (0.89-mm)

centers 56

43. Energy dissipation as a function of penetration depth for several electron

energies in material with atomic number between 10 and 15 calculated according to Everhart and Hoff 56

44. Examination of silicon p-n junction in the electron beam induced current mode . . 57

45. Current gain, I^/Ig, for several absorber thicknesses as a function of

electron beam energy 58

46. Current gain, I^/Ig, for several beam energies as a function of absorber

thickness, t 58

47. Nomograph to convert alum.inum, silicon dioxide, or aluminum plus silicon

dioxide thickness in micrometers to absorber thickness in micrograms per

square centimeter 58

48. Comparison of measured electron beam induced currents with the results

of the elementary calculation 58

49. Pv.atio of backscattered to incident electrons, n, as a function of beam energy . . 60

50. Fractional mean energy backscattered, from aluminum as a function

of beam energy. Eg 60

51. Fraction of incident energy backscattered, f^, from aluminum as a function

of beam energy ' . . . 60

52. Photomicrograph of silicon interdigitated UHF transistor showing surface

topology 61

53. Photograph of the scanner display screen showing enhancement of 1.15 pm photoresponse due to hot-spot operation 61

54. Collector-base characteristic of UHF transistor connected in a commonemitter circuit 62

55. Photographs of the scanner display screen showing electrical nonlinearity

in the UHF transistor at certain of the operating points indicated by arrows on the collector-base characteristic 63 vi
LIST OF FIGURES

PAGE

56. Photographs of the scanner display screen showing a portion of a p-MOS

shift register 64

57. Photograph of the scanner display screen showing a portion of a p-MOS

shift register in the same static condition as in figure 56b except

that a logical one was changed to a logical zero 65

58. Modified circuit for measuring thermal resistance set up as a grounded

base, emltter-and-collector switching circuit using the collector-base

junction voltage of the input transistor as the temperature sensitive

parameter 67

59. Desired current path when measuring the thermal resistance of the input

Darlington transistor using the grounded-base , emltter-and-collector

switching method with the collector-base junction voltage as the tem- perature sensitive parameter 67

60. Desired current path when measuring the thermal resistance of the output

Darlington transistor using the grounded-emitter j em>itter-and-collector

switching method with the collector-base junction voltage as the tem- perature sensitive parameter 67

61. Safe operating area plots 68

LIST OF TABLES

PAGE

1. Summary and Analysis of Four-Probe Resistivity Measurements on Resident Wafers . 8

2. Four-Probe Resistivity Measurements on Circulating Wafers 9

3. Number and Sizes of Microvolumes in Repeat Patterns of Implantation Photomask . 20

4. Preliminary Resistivity-Carrier Density Data for Phosphorus-Doped Silicon

at 300 K 25

5. Object Line Widths and Corresponding RMS Differences Determined from the Curves

of Figure 19 36

6. Line Width Measurement Errors for a Transparent Line (20 ym or Wider) on an Opaque Background Viewed with 550 nm Illumination 40

7. Measured Values for Dimensions A, B, and C of Interim Measurement Artifact ... 42

8. MOS Capacitor Parameters 46

9. MOS Transistor Parameters 46

10. Smallest and Largest Detectable Leak Sizes for Borosilicate Glass Capillary

Test Leaks 53

vii
PREFACE

The Semiconductor Technology Program serves

to focus NBS efforts to enhance the performance, interchangeability , and reliability

of discrete semiconductor devices and integrated circuits through improvements in measurement technology for use in specifying

materials and devices in national and international commerce and for use by industry in

controlling device fabrication processes.

Its major thrusts are the development of

carefully evaluated and well documented test procedures and associated technology and the

dissemination of such information to the

electronics community. Application of the

output by industry will contribute to higher

yields, lower cost, and higher reliability

of semiconductor devices. The output provides a common basis for the purchase specifications of government agencies which will

lead to greater economy in government procurement. In addition, improved measurement

technology will provide a basis for con- trolled improvements in fabrication processes and in essential device characteristics.

periodically scheduled symposia and workshops. In addition, progress reports, such

as this one, are regularly prepared for is- suance in the NBS Special Publication 400

sub-series. More detailed reports such as state-of-the-art reviews, literature compilations, and summaries of technical efforts

conducted within the Program are issued as these activities are completed. Reports of

this type which are published by NBS also

appear in the Special Publication 400 subseries. Announcements of availability of

all publications in this sub-series are sent

by the Government Printing Office to those

who have requested this service. A request

form for this purpose may be found at the

end of this report.

The Program receives direct financial support principally from three major sponsors:

the Defense Advanced Research Projects Agency (ARPA) , the Defense Nuclear Agency

+ (DNA) , and the National Bureau of Standards

(NBS).*' In addition, the Program receives

support from the U. S. Navy Strategic Systems

§ Project Office. The ARPA-supported portion

of the Program, Advancement of Reliability,

Processing, and Automation for Integrated

Circuits with the National Bureau of Standards (ARPA/IC/NBS) , addresses critical

Defense Department problems in the yield,

reliability, and availability of integrated

circuits. The DNA-supported portion of the

Program emphasizes aspects of the work which

relate to radiation response of electron devices for use in military systems. There is considerable overlap between the interests of DNA and ARPA. Measurement oriented activity

appropriate to the mission of NBS is a critical element In the achievement of the objectives of both other agencies.

Essential assistance to the Program is also

received from the semiconductor industry

through cooperative experiments and technical exchanges. NBS interacts with industrial users and suppliers of semiconductor devices through participation in standardizing

organizations; through direct consultations

with device and material suppliers, government agencies, and other users; and through

Through ARPA Order 2397, Program Code 5D10

(NBS Cost Center 1+259555). All contract

work was funded from this source.

4-'Through Inter-Agency Cost Reimbursement

Order 75-816 (NBS Cost Center U259522).

^'Through Scientific and Technical Research

Services Cost Centers 4251126, 4252128,

and 4254115.

§ Code SP-23, through pro]ect order

N0016475PC70030 administered by Naval Am- munition Depot, Crane, Ind.iane (NBS Cost

Center 4251533) and Code SP-27, through

IPR SP6-75-4 (NBS Cost Center 4251547).

viii
SEMICONDUCTOR MEASUREMENT TECHNOLOGY

PROGRESS REPORT

January 1 to June 30, 1975

Abstract: This progress report describes NBS activities directed

toward the development of methods of measurement for semiconductor

materials, process control, and devices. Both in-house and contract

efforts are included. The emphasis is on silicon device technologies.

Principal accomplishments during this reporting period included (1)

completion and analysis of an interlaboratory evaluation of standard

reference wafers for resistivity measurement by the four-probe method,

(2) analysis of the effect of finite contact size on sheet resistance

as measured with a van der Pauw structure, (3) calculation of errors introduced in measuring line width with typical microscope systems, (4)

development of procedures for predicting the magnitude of electron beam

induced current in silicon device structures, (5) application of the

optical flying-spot scanner to observation of hot spots and non- linearities in UHF power transistors and of logic patterns in an MOS

shift register, and (6) determination of a more accurate electrical

method, based on peak junction temperature measurement, to establish

safe operating area curves for medium power transistors. Also re- ported are the results of work on spreading resistance measurements,

ionization of dopant impurities in silicon, Rutherford backscattering

measurements. X-ray photoelectron spectroscopy, ion microprobe mass

analysis, tests for determining the surface quality of sapphire substrates, reevaluation of Irvin's curves, mathematical models of dopant

profiles, deep depletion measurements of dopant profiles and epitaxial

layer thickness, bias-temperature stress tests on MOS capacitors, a high voltage capacitance-voltage method for measuring characteristics

of thick insulator films, ion implantation parameters, methods for

determining integrity of passivation overcoats, optical imaging and

calibration standards for photomask metrology, line-width measurements,

charge-coupled device test structures, a test pattern for silicon-onsapphire MOS device technologies, a nondestructive acoustic emission

test for beam-lead bonds, pull and shear tests for wire bonds, leak

detection by helium mass spectrometry, correlation of moisture infusion in semiconductor packages with leak size and device reliability,

an automated scanning low-energy electron probe, electron beam induced

damage in silicon device structures, and thermal resistance measurements

on Darlington transistors. Supplementary data concerning staff, publications, workshops and symposia, standards committee activities, and

technical services are also included as appendices. A sixth appendix

summarizes the results of an assessment of the impact of automation of

integrated circuit processing and assembly on future measurement re- quirements in the industry.

Key Words: Acoustic emission; beam-lead bonds; bias-temperature

stress test; boron redistribution; capacitance-voltage methods; chargecoupled devices; Darlington pairs; deep depletion; dopant profiles;

electrical properties; electron beam induced current; electron beam induced damage; electronics; epitaxial layer thickness; hermeticity; leak

tests; measurement methods; microelectronics; optical flying-spot scanner; passivation overcoats; photomask metrology; pull test; resistivity;

Rutherford backscattering; scanning electron microscope; scanning low

energy electron probe; semiconductor devices; semiconductor materials;

semiconductor process control; shear test; sheet resistors; silicon;

sllicon-on-sapphire ; spreading resistance; test patterns; thermal re- sistance; thermal response; transistors; wire bonds; X-ray photoelectron

spectroscopy

.

1
1 . I N T R 0 D LI C T I 0 M

This is a report to the sponsors of the

Semiconductor Technology Program on work

during the twenty-seventh and twenty-eighth

quarters of the Program. It summarizes work

on a wide variety of measurement methods for

semiconductor materials, process control, and

devices that are being studied at the National Bureau of Standards. The Program, which

emphasizes silicon-based device technologies,

is a continuing one, and the results and

conclusions reported here are subject to modification and refinement.

The work of the Program is divided into a number of tasks, each directed toward the

study of a particular material or device

property or measurement technique. This

report is subdivided according to these

tasks. Highlights of activity during the

quarters are given in section 2. Subsequent sections deal with each specific task

area. References cited are listed in the

final section of the report.

The report of each task includes a narrative

description of progress made during this re- porting period. Additional information con- cerning the material reported may be obtained directly from individual staff members identified with the task in the report.

Considerable reorganization of the Program

occurred at the end of this reporting period.

This report is organized along the new organizational structure which is listed together

with staff telephone numbers in Appendix A.

Background material on the Program and individual tasks may be found in earlier progress reports as listed in Appendix B. From

time to time, publications are prepared that

describe some aspect of the program in

greater detail. Current publications of

this type are also listed in Appendix B. Reprints or copies of such publications are usually available on request to the author.

In addition tutorial videotapes are being

prepared on selected measurement topics for

dissemination to the electronics community.

Currently available videotapes and procedures

for obtaining them on loan are also listed in

Appendix B.

Communication with the electronics community

is a critical aspect both as input for guidance in planning future program activities

and in disseminating the results of the work

to potential users. Formal channels for

such communication occur in the form of

workshops and symposia sponsored or co- sponsored by NBS. Currently scheduled seminars and workshops are listed in Appendix C. In addition, the availability of proceedings

from past workshops and seminars is indicated in the appendix.

An important part of the work that frequently goes beyond the task structure is participation in the activities of various technical

standardizing committees. The list of personnel involved with this work given in Appendix D suggests the extent of this participation. In most cases, details of standardization efforts are reported in connection

with the work of a particular task.

Technical services in areas of competence

are provided to other NBS activities and

other government agencies as they are requested. Usually these are short-term,

specialized services that cannot be obtained through normal commercial channels

.

To indicate the kinds of technology available to the Program, such services provided

during the period covered by this report are listed in Appendix E.

During this period, a study of the impact

of automation in the semiconductor industry

on measurement technology requirements during

the next five to ten years was conducted.

The findings and conclusions from this study

are summarized in Appendix F.

2
2 . !' I n I' LIGHTS

Highlights of progress in the various technical task areas of the program are listed

in this section. Unless otherwise identified

the work was performed at the National Bureau

of Standards.

Particularly significant accomplishments

during this reporting period included (1) com- pletion and analysis of an interlaboratory

evaluation of standard reference wafers for

resistivity measurement by the four-probe

method, (2) analysis of the effect of finite

contact size on sheet resistance as measured

with a van der Pauw structure, (3) calculation of errors introduced in measuring line

width with typical microscope systems, (4)

development of procedures for predicting the

magnitude of the electron beam induced current in silicon device structures, (5) application of the optical flying-spot scanner to observation of hot spots and • nonlinearities

in UHF power transistors and of logic patterns

in an MOS shift register, and (6) determination of a m.ore accurate electrical method,

based on peak junction temperature measurement, to establish safe operating area curves

for medium power transistors.

Resistivity — Analysis of the results of

the interlaboratory evaluation of standard

reference material SRM 1520, boron-doped

silicon for four-probe resistivity measurements, was completed; the standard wafers

were shown to be stable and the 95 percent

confidence estimate of reproducibility for

two-laboratory referee measurements was found

to be less than 2 percent.

A study of the effect of surface preparation

and other measurement param.eters on the re- sistivity of silicon wafers as determined

by the spreading resistance technique is nearing completion. Theoretical and experimental

development of a simplified algorithm for

analysis of spreading resistance measurements

on graded structures was begun at Solecon

Laboratories. The algorithm is based on a relationship similar to that employed in

analyzing differential sheet resistance measurements. Preliminary measurements at RCA

Laboratories suggest the feasibility of a high-speed spreading resistance probe which

operates without lifting the probe tips from

the wafer surface while they are moved across

the surface. At present, readings may be

taken 4.5 to 7 times faster than with con- ventional equipm.ent.

As one phase of the work to reevaluate the

relationship between resistivity and impurity

density in n- and p-type silicon, several

models were used to analyze the ionization

state of dopant impurities as a function of

dopant density. None of the models studied

gave satisfactory results in the inteirmediate

density range around 10^^ cm~^.

Surface Analysis Methods — Analysis of

Rutherford backscattering measurements on a heavily zinc implanted silicon specimen,

made as part of a comparative study of electron, ion, and photon beam surface analysis

measurement technologies, illustrated some of

the advantages and limitations of this technique for profiling impurity densities.

Additional study of the angular dependence

of x-ray photoelectron spectra from silicon

revealed crystal orientation effects and

suggest that the bulk plasmon peak results

from an extrinsic process which occurs during

the path of the electron out of the solid.

Work was undertaken at Texas Instruments to develop m.ethods for preparation of standard

specimens for empirically calibrating an ion microprobe mass analyzer so that quantitative measurements can be made of selected impurities in silicon and silicon dioxide re- gions of semiconductor devices. A mask was designed and fabricated for use in defining

a pattern of small areas in which the impurity of interest can be ion implanted.

A rapid nondestructive infrared reflectance

technique to determine the surface quality

of sapphire substrates is being investigated

at RCA Laboratories. Correlation has been

observed between the results of reflectance

measurements and other analytical methods

such as x-ray dif f ractometry , x-ray surface

and transmission topography, and chemical

etch tests.

A study of techniques for determining and

controlling the quality of furnace environments used in growing and subsequent annealing of gate oxides for '^'OS devices was undertaken. Attempts to measure trace am.ounts of

sodium in the furnace gas by means of a flame

emission spectroscopy system attached to the

exit port of the furnace tube were unsuccessful. Preliminary measurements sviggest that resonance fluorescence spectroscopy can be

used to detect extremely low levels of free

sodium in the furnace tube, and further investigation of this technique is in progress.

Surface analysis for silicon devices was

the topic of the fourth ARPA/NBS Workshop
HIGHLIGHTS

held at NBS Galthersburg in April. The workshop was attended by 146 representatives from

industry, government, and universities. Highlighting the workshop were three approaches

to overcoming the problem of sodium migration

during distribution profiling measurements

and debates about the existence and extent of

the non-stoichiometric transition region at the silicon dioxide-silicon interface. Another area of active discussion by speakers

and participants involved consideration of

the detection limits and capability for m.ak- ing quantitative determinations by beam probe

techniques. It was pointed out that no single

beam spectroscopy can provide all the answers

and that before beam techniques can be used

to provide reliable and quantitative determinations, both standard reference materials

and standard measurement procedures must be

developed

.

Test Structure Appliaations — Additional

data on n-type silicon have been collected

in the study of the resistivity-dopant density relationship with the use of the collector four-probe resistor and the basecollector diode on Test Pattern NBS-3. Electron mobilities found from these measurements

were in good agreement with mobility values

calculated from an empirical fit to Irvin's

curve for n-type silicon.

The computer program developed to solve the

boron redistribution problem by means of a finite difference algorithm was extended to include a radiative-type boundary condition

at the oxygen-oxide interface.

Theoretical analysis of the MOS deep depletion method for profiling dopant density

distributions showed that dopant densities

can be calculated to within about 5 percent

at depths as small as two Debye lengths from

the surface when the depletion model is used.

This measurement technique was also applied

to the determination of epitaxial layer

thickness

.

Preliminary bias-temperature stress measurements made on a group of MOS capacitors fabricated on w-type silicon wafers showed flat

band voltage shifts of 1 V or less. These

capacitors also showed sim.ilarly small voltage shifts under irradiation (unbiased) to

lO'* Gy (10^ rad(Si)) in the scanning electron

microscope

.

Several instrumental improvements were made

in the extended range capacitance-voltage

technique being developed at RCA Laboratories

for application to measurements on metalinsulator-semiconductor structures with thick

insulator layers

.

Materials and Procedures for Wafer Process - ing — As part of a study being conducted

at Hughes Research Laboratories on critical

parameters associated with ion implantation,

preliminary range and range straggle measurements of boron and phosphorus implants into

opposite- type silicon wafers were m.ade by means of an automatic capacitance-voltage

method

.

In work undertaken at RCA Laboratories to develop techniques for evaluating the integrity of passivation overcoats on metallized

integrated circuits, three different techniques are being investigated and compared.

Photo li thography — A theoretical investigation was begun to determine quantitatively

the effects of coherence of the illumination

on the image of two parallel opaque lines on

a transparent background and on the image of two parallel transparent lines on an opaque

background. The results of these calculations yield a quantitative measure of the

image quality based on root-mean-square differences between the spectra of the image

intensity profile and the ideal intensity

profile.

Calculations were also made of the effect

of the degree of coherence of the illumination at the object plane of a microscope

with diffraction limited lenses and its

effect on the intensity profile of a single

edge. Effects of spherical aberration and

defocussing were considered, and the measurement errors which might be encountered in

using filar and image shearing eyepieces were estimated.

Because of difficulties in fabricating the

originally designed measurement artifact

intended for use in establishing standards

for line-width measurements in the 1 to 10 ym range, a substitute interim artifact was

obtained. Comparative measurements were made

on this artifact with filar and image shearing eyepieces . The results of these measurements agreed well with the predictions of the

above theoretical calculations.

Test Patterns — Finite difference methods

were used to solve Laplace's equation in two dimensions in order to calculate cor- rection factors to account for the finite

length and width of contact arms on sheet

4
HIGHLIGHTS

resistor test structures of the van der Pauw

type. Cross structures with minimum process

stripe widths were found to be feasible.

The investigation at the Naval Electronics

Laboratory Center of the applicability of

the charge-coupled device as a test structure for use in semiconductor process control was concluded with additional correlation of parameters measured on various

charge-coupled device structures and those

measured on more conventional structures

such as MOS capacitors, MOS transistors,

and gated diodes.

A task to develop a test pattern array for

complementary MOS on sapphire integrated

circuits was initiated at RCA Laboratories

with the selection of design rules and five

classes of test structures to be included

in the pattern.

Interconnectior. Bonding — Procedures have

been developed to bond beam-lead devices

with one or two controlled weak bonds for use as test vehicles in the study of acoustic emission as a nondestructive test for

weak beam-lead bonds. Instrumental changes were made to improve the reproducibility

and sensitivity of the apparatus to the

very weak acoustic emission of poorly bonded

beam leads.

An interlaboratory evaluation of the destructive, double-bond pull test is being

conducted in cooperation with ASTM Committee

F-1 on Electronics. In addition, a two- laboratory comparison of the bond pull test with the bond shear test was conducted to determine their significance for evaluating

aluminum-aluminum ultrasonic bonds. The

shear test was found not to be sensitive to the weakest part of the connection, the heel

of the first bond. The pull test, on the

other hand, is particularly sensitive to the

weakening of the bond heel.

Hermetiaity — Several schemes are being

investigated for quantitative measurement of

gross leaks in hermetic packages. The rapid

cycle dry gas gross leak test was analyzed

further to include the case of repeated testing without repressurization in helium in or- der to pick out quickly leakers from a batch

of parts.

The effect of helium absorption on the test specimens being used in an interlaboratory

evaluation of the back-pressurization technique for leak detection with a helium mass

spectrometer, being conducted in cooperation

with ASTM Committee F-1 on Electronics, was further studied. It was decided that the

interlaboratory test should be deferred until

test specimens which are less susceptible to helium absorption can be obtained.

A study has been initiated at MartinMarietta Aerospace to derive a quantitative

relationship between leak size in hermetic

packages and moisture infusion. The initial

phase of the work involved the calibration

of an in-situ moisture sensor, development

of microventing procedures, and selection

of a moisture sensitive integrated circuit.

device Inspection and Test — Work on the

automated scanning low energy electron

probe at the Naval Research Laboratory

continued with assembly of the system and

demonstration of its ability to observe

variations in dielectric uniformity on an oxidized silicon wafer.

To facilitate application of the scanning

electron microscope operating in the electron beam induced current (EBIC) mode for

the diagnostic examination of silicon devices,

a procedure was developed for predicting the

magnitude of the EBIC signal. The procedure

is based on published depth-dose curves and

an analysis of the value for the fraction of

energy backscattered which was shown to be

essentially constant over the energy range

of interest. Despite the fact that several

simplifying assumptions were made, reasonable agreement was achieved with an experimental result.

The dual-laser flying-spot scanner was used

to examine several device types. Hot-spot

formation was observed in UHF power tran- sistors of a type used extensively in phased

array radar transmitters. The devices were

operating well within rated conditions when

the hot spots formed. The internal logic states in a p-MOS shift register were also

observed with the scanner. The logic pattern

was displayed nondestructively , and it was possible to change states within the device

selectively and reversibly by increasing the

incident light intensity.

Thermal Properties of Devices — Additional

measurements were made on a specially fabricated four-terminal Darlinaton transistor

to confirm the previously reported procedure

for obtaining the thermal resistance of the

output transistor from Indirect electrical measvremer "rs .

5
HIGHLIGHTS

Electrical techniques for estimating the peak

junction temperature of power transistors were further developed. By comparing electrically determined peak junction temperatures with those determined by infrared microradiometry , it was demonstrated that

these techniques could be used to determine more accurately the safe operating area limits of such transistors.

6
3 . RESISTIVITY

3.1. Standard Reference Materials

The multipass interlaboratory test (NBS

Spec. Publ. 400-17, pp. 8-9) of SRM 1520 [1] was completed. The purpose of the test was

to establish both the stability of this SRM

over a reasonable period of time and the

level of agreement to be expected by any two laboratories using the SRM for referee purposes .

Each of the six participating laboratories,

including NBS, was required to measure the room temperature resistivity of both wafers

of its resident SRM set once a month for a twelve-month period. The measurements were

made according to the standard four-probe

method [2], except that only two repetitive

measurements on each wafer were required instead of the normal ten. Raw data were submitted directly to NBS for calculation and

reduction. This part of the experiment

served both as a data base from which to es- timate the time stability of the SRM sets,

and also as a screen against sudden changes

in any laboratory's measurement response

which would influence the second part of the

study.

In the second part of the test, two complete

SRM sets, two silicon wafers at each of the

two resistivity levels, were circulated to each of the participating laboratories with

a frequency of one laboratory per month.

Two complete cycles of the laboratories were

made during the study. The estimate of precision to be expected in referee use of the

SRM's was generated from these data. The

resistivity was again measured according to the standard four-probe method [2] but only

five repetitions were required per wafer.

The use of two complete sets in this part

served both to enlarge the data base and to provide allowance for possible specimen

breakage during the study.

The data reported by one participant showed

a statistically significant high-side bias

for all measurements. Since it was determined that this participant failed to follow

the measurement procedure with respect to probe specifications and correction for temperature, these data were excluded from the

final analysis of the results of the experiment .

The stability was evaluated by analyzing the

drift of the resistivity values obtained on

the ten resident specimens during the course

of the experiment. The results are summarized in table 1. The data from each resident specimen were fitted to individual

linear regressions [3] as a function of

month number. The slopes of the individual

regressions, if statistically significant

above measurement scatter, indicate the extent of drift for that specimen-laboratory

combination over the course of twelve

months

.

Individual estimates of slope and intercept

with their estimated standard deviation are shown in the table. While seven of the ten estimated slopes were negative, indicating

slight decrease of resistivity with time on balance, only one of the calculated slopes

was larger than its estimated standard

error. In addition, the small values of

correlation coefficient suggest that the re- sistivity changes are not linear with time.

With separate analyses for high and low re- sistivity levels, pooled estimates indicate

that only slopes greater than 0.018 and

0.00018 ohm centimeters per month, respectively, would have been statistically significant at the 95 percent confidence level.

The point of interest is the typical drift, if any, of all the specimens at either the low

resistivity or the high resistivity level,

taken as a whole for each level. Since there was only one slope in either category which

exceeded its respective significance limit,

this is well short of the 95 percent yield

expected from the analyses performed if

the hypothesis of detectable drift for material of either resistivity level were true. Hence, as a whole, this hypothesis must be

rej ected

.

A statement of the precision to be expected

in various referee measurement uses was derived from analysis of the measurements on

the circulating sets of wafers using the

calculated values of within laboratory and

between laboratory variation for each speci- men used [4] . Table 2 summarizes the results of the measurements and gives estimates from these results of the repeatability for replicate measurements within a lab

and the reproducibility of measurements between two labs which could be expected with

95 percent confidence if the same specimens were used for referee measurement. The es- timates of repeatability and reproducibility

[4] are broken into three situations, namely

those where the two laboratories involved

would measure the specimen one, five, or ten

7
RESISTIVITY

times each before comparing results.

Although these estimates show noticeable

variation from specimen to specimen, it is

judged that the data base is sufficiently

large to pool the present results and apply

the projected values to any SRM slices generated from the same starting material. The

resulting 95 percent confidence estimates

of reproducibility for two laboratory referee measurements are 1.9 percent, 1.7 percent,

and 1.67 percent for the cases of one, five,

and ten measurements per laboratory, respectively. (J. Mandel,* J. R. Ehrstein,

and F. H. Brewer)

NBS Institute for Materials Research.

Table 1 - Summary and Analysis of Four-Probe Resistivity Measurements on Resident Wafers

Laboratory 1 Laboratory 2 Laboratory 3 Laboratory 5 Laboratory 6

A B A B A B A B A B

1 nil Qco 11 275 0 1 1 7T9 \ \ 1 oc. 1 1 1 oU 0,11954 1

1

oOo u

11771; 1 1 nan uyu u 1 c.\jo\y n 095

2 0.12020 11 294 0. 11814 11 421 0 11987 11 313 0 11800 n 075

3 0.12010 11 329 0. 12173 11 675 0 11972 n 238 0 11780 11 050 0 12006 11 088

4 0.12002 11 288 0. 11904 11 298 0 11908 11 242 0 11970 11 135 0 11922 11 160

5 0.12008 11 328 0. 11824 11 158 0 11912 11 208 0 11973 n 204

6 0.12000 11 286 0. 11850 11 368 0 11917 n 268 0 11975 n 260 0 12084 11 012

7 0.12032 n 324 0. 11979 11 599 0 11958 11 333 0 11885 11 015 0 11928 11 042

8 0.12006 11 304 0. 11790 11 246 0 11898 n 257 0 11659 10 815 0 11987 11 096

9 0.11998 11 312 0. 11830 11 306 0 11898 11 290 0 11675 10 885 0 12048 11 120

10 0.12005 11 305 0. 12031 11 489 0 11966 11 296 0 11896 11 046 0 11988 11 102

11 0.11982 11 274 0. 11955 11 509 0 11967 n 250 0 11778 10 998 0 12004 11 .163

12 0.11984 11 288 0. 11934 n 441 0 11892 11 291 0 11738 10 912 0 11960 11 113

bo 0.120035 11 3032 0. 118522 11 3250 0 119574 n 2888 0 118774 11 1617 0 120094 11 0778

bo 0.000117 0 0125 0 000767 0 1016 0 000209 0 0276 0 000701 0 0706 0 000393 0 0350

bi -0.000004 -0 0004 0. 000075 0 0101 -0 000033 -0 0016 -0 000080 -0 0187 -0 000018 0 0030

bi 0.000016 0.0017 0. 000104 0 0138 0 000028 0 0037 0 000095 0 0096 0 000050 0.0044

r 0.089 0 077 0. 223 0 226 0 348 0 131 0 256 0 524 0 130 0 232

Notes: bo = Estimated intercept

S, = Estimated standard deviation of intercept

bo

b]: = Estim.ated slope

S, = Estimated standard deviation of slope

V - Correlation coefficient

All values (except r) in ohm centimeters

8
RESISTIVITY Table 2 - Four-Probe Resistivity Measurements on Circulating Wafers

A. EXPERIMENT

Laboratory

A-13 A-17 B-84 B-85

p, n*cm s. p i

, Q-cm s, % P

.

Q-cm s, p

.

Q'Cm s.

Round 1

1 0.11925 0,.40 0.,11977 0.13 11 .108 0,,13 11 .175 0,,10

2 0.11941 0.,20 0.,11967 0.07 11 .054 0,,60 11 .067 1,,06

3 0.11900 0..15 0,,11970 0.39 11 .140 0,,54 11 .418 0,,83

5 0.11962 0.,21 0.,12010 0.21 11 .168 0.,10 11 .252 0,,12

6 0.11826 0.,21 0.,11913 0.30 n .049 0,,16 n .103 0,,31

Round 2

1 0.11940 0,,40 0,,11984 0.22 11 .112 0.,07 11 .228 0,.32

2 0.11897 0.,13 0.,11954 0.25 11 .103 0.,53 11 .302 0,,52

3 0.11984 0,,39 0.,12076 0.54 11 .163 0,,16 11 .346 0,,56

5 0.11805 0,,47 0.,11909 0.24 11 .034 0,,29 11 .113 0,.36

6 0.11743 0.,20 0.,11813 0.10 11 .051 0,,31 11 .192 0,.17

B. ANALYSIS

A-13 A-17 B-84 B-85

p, Q'cm 0.11892 1 .11957 11 .1003 M .1980

6, % 0.297 0.280 0.308 0.426

0.633 0.564 0.406 0.782

R, , % bio

0.824 0.776 0.854 1.180

0.369 0.347 0.382 0.528

0.261 0.246 0.270 0.373

1 .94 1 .75 1 .41 2.47

1 .79 1 .60 1.19 2.23

1.77 1 .58 1.15 2.20

Notes : p = average of five measurements

s = relative sample standard deviation (coefficient of variation) of five

measurements

p = grand average of reported averages

6 = calculated coefficient of variation from within lab error

5 = calculated coefficient of variation from between lab error

Li R^^ = predicted repeatability within lab - i measurements

R^^ = predicted repeatability between labs - i measurements

9
RESISTIVITY

3.2. Spreading Resistance Measurements

on Graded Structures

appears in the more general equation for

a(x).

This task was undertaken to develop a simplified calculational procedure for the an- alysis of spreading resistance measurements

on nonuniform structures such as diffused

layers, for the purpose of computing profiles of resistivity with position in the

layer. Thickness correction factors have

been developed for layers of uniform resistivity [5-8], but their extension to non- uniform layers requires excessive computer

time [5-7] or is of questionable validity

[8].

In the present work it is intended to demonstrate that a differential relationship suggested earlier [8] can, with the use of a relatively simple correction factor, be used

to generate resistivity profiles, utilizing

only the difference between successive

spreading resistance measurements and the

thickness dependence of the correction factor.

Formally, for a pair of circular contacts

on one surface of an infinite slab of material with uniform conductivity, o, the con- ductance, G, between the contacts is

G = 2aaH(t), (1)

where a is the contact radius and H(t) is a dimensionless factor which contains a cor- rection for slab thickness, t. By applying

the principle of superposition, eq (1) can

be generalized to an n-layer structure with

the result that

G = 2aX](f7i_l - cfi)H(t._^), (2)

i=l

where a = 0

.

n After changing eq (2) to an integral form,

and performing considerable mathematical manipulation, one finds that, in the limit for

thin layers,

a(x) = C dG

1 dx (3)

provided that the limiting form of H(t) is linear in thickness for sm.all t. Use of

eq (3) permits one to calculate conductivity

(or resistivity) profiles from measurements

of conductance as a function of distance into

the film, starting from the deepest part of

the layer. Under some circumstances, however, it may be necessary to know the second

derivative of H(t) with respect to x which

In order to evaluate H(t), the potential

distribution problem defined by two circular equipotential contacts on the surface of the two layer structure must be

solved. Because it has mixed boundary

conditions, the problem has no known

analytic solution for a layer of finite

thickness. It is expected however that

the method used by Schumann and Gardner

[5], based on the solution for a semi- infinite layer, can be applied. In using

this formulation, a current distribution

is assumed under the contact, and the

resulting potential averaged over the

contact interface in order to evaluate

the factor H. The effects on the factor H of several assumed current distributions and

weighted admixtures of such current distributions are being considered in order to more realistically approximate geometries

which are not semi-infinite. (D. H. Dickey")

Four sets of boron diffused wafers were prepared for use in studying the validity of

the simplified calculational procedure described above. Boron was diffused into

10 fi'cm phosphorus-doped wafers under contions designed to yield a surface density of

10^^ cm~^ or lower and a junction depth of

1, 2, 5, or 10 pm. Each set consisted of

four wafers with the same junction depth

diffused at the same time. Sheet resistance of each layer has been determined by

the four-probe method [9] and two wafers of

each set were prepared for profile determinations by spreading resistance both at NBS and at Solecon Laboratories. An additional wafer from each set has been re- served for profiling by the incremental

sheet resistance method [10] at NBS.

(Y. M. Liu)

3.3. High-Speed Spreading Resistance Probe

One of the limitations of the spreading re- sistance technique is the time required to collect data. This task was undertaken to define the mechanical and electrical conditions required to record the spreading resis- tance of two-point probe-to-silicon contact

at substantially faster rates than the usual

maximum rate of 10 to 15 data points per

minute and to provide means for digital reVfork performed at Solecon Laboratories under j NBS Contract No. 5-35881. NBS contact for ,| additional information: J. R. Ehrstein.

10
RESISTIVITY

cording of the spreading resistance in a form suitable for subsequent computer analysis .

The approach selected was to modify a previously built two-point probe to permit

readings to be taken while the probe points

are dragged over the wafer surface. A view

of the apparatus is shown in figure 1. The

probe points are mounted on separate arms, each of which is pivoted on ball bearings.

The force at the tip is varied by loading

the other end of the arm with weights (A) via

a pulley system (B) . Brass weights permit

the load to be varied from about 5 g to about 250 g in A. 5 g increments. The pivot

for each arm is attached to an x-y micrometer stage (G) so that the separation and

alignment of the probe points can be adjusted by viewing the points from the rear with the use of the microscope (B) and

prism (C) . The minimum spacing that can be

obtained between the points is 200 ym. With

some modifications of the holders, spacing

down to about 100 pm should be achievable.

The probes are fixed; the wafer is placed

on a table (F) driven by a stepping motor,

capable of 200 steps per second, driven by

a preset indexer (E) . The indexer can be

set so that the wafer moves under the

probes at speeds between 9.4 and 850 ym/s.

The resistance is determined by measuring

the current which results when a potential

of 10.0 ± 0.1 mV is applied between the two probe points. A voltage proportional to

the logarithm of the current drives the yaxis of an x-y recorder operated as a point

plotter (H) while a signal obtained from a ramp generator drives the x-axis. An internal timing circuit is used to trigger

the recording of the two signals; at presA Weights

B Pulley

C Prism

D Microscope

E Stepping motor drive

F Motor driven table

G Arm micrometer stage

H Point plotter

Figure 1. Two-point spreading resistance

probe as modified for high-speed operation.

ent readings may be taken as rapidly as 69 times per minute, 4.5 to 7 times faster

than with conventional equipment.

The objectives of the initial experiments were to establish operating regions and to delineate the limits of usefulness. Major

emphasis was placed on the choice of point

geometry and loading; the criteria of selection were mainly the signal-to-noise ratio

of the recorded trace, the reproducibility

of the calibration graph, and the sensitivity

to resistance change with load and speed.

These experiments were conducted with tung- sten carbide probe tips with 25-mil (0.64-mm)

diameter shafts.

In a typical set of experiments a new pair

of points is inserted into the holders, adjusted for equal length, and locked with a set-screw. Fine adjustment is made for

spacing and alignment so that the line joining the probe tips is perpendicular to the

direction of travel. Most runs were made

with a spacing of 200 to 400 pm between

points. The wafer is placed on the table and

held down by vacuum supplied by a small pump. The arm assembly is lowered manually until

the points touch the wafer and the arms are gently lifted off their rest stops. It is

this action that applies the preselected

load. Unlike the conventional spreading re- sistance probe, the lowering of the points

onto the wafer is not a critical step. Sets

of runs are then made using the same weight

and a series of resistivity standards, followed by the next run at a greater loading,

all under the same speed conditions and in

ambient light. The temperature in the measurement room can fluctuate by about 8°C and

is recorded. All scans are made in the same

direction, such that the angle between the

11
RESISTIVITY

(AE

UJo

<

cn

UJ

z

Q

<

UJ

a:a.

TIME ( seconds

)

7.5 15.0

10'

10'

^ ^-ipo"^ f°y'^'^°^'"'^g 9 .5 g

49g

o<w!P8«»*rt?v°w^ 75 g I02g

720 1440 2160

DISTANCE (;im)

Figure 2. Spreading resistance as measured

with four different loads on a tungsten

carbide tip of radius 6.4 pm.

LOAD (grams)

Figure 3. Spreading resistance as a function of load for three tungsten carbide

tips of different radius. (Curve 1, 25 pm; curve 2, 10 pm; curve 3, 6.4 ym; the ver- tical arrows indicate the load belov; which

the signal-to-noise ratio is unacceptably

low . )

b. Load: U9

c. Load: 129 g.

Figure 4. Scanning electron micrographs

of tracks left by a tungsten carbide probe

tip of radius 6.4 ym.

.

12
RESISTIVITY

shaft and the wafer surface is about 85 deg;

the wafer is moved in the direction of the acute angle.

With tungsten carbide points the trace is noisy when the load is light and becomes

reasonable after a sufficient force has been

applied. Figure 2 shows a set of typical

results on the same wafer with progressively

greater probe loadings. The probe loading

also affects the measured resistance. Figure

3 is a plot of measured resistance as a function of loading for probes of three different tip radii. All three curves in the

figure show an inflection point. Experimentally, it is found that the signal-to-noise

ratio increases as the loading is increased

and is generally not acceptable at loadings

less than the value indicated by the vertical arrows. From scanning electron micrographs of probe tracks, such as those

illustrated in figure 4, it is clear that the contact area is smaller with the light load

and that the point does not maintain uniform

contact with the surface. At very heavy

loads, the surface tends to chip and fissures

appear; the resistance decreases as load is

increased but the noise level does not

change

.

These observations lead to the tentative

conclusion that the points make contact over

the entire width of the track mark. This

agrees with the experimental results that

greater loading and larger radius give lower

spreading resistance values roughly in proportion to the width of the track mark. It

is clear that the silicon in contact with the

point is highly disturbed. However, the

disturbed layer is probably only a small

fraction of the volume of silicon sampled

by the probe point. The disturbed layer may,

in fact, help to minimize surface effects by

increasing the surface recombination velocity .

Tests were conducted over the entire speed

range using tungsten carbide points on n-type

silicon wafers with room temperature resistivity ranging from 0.01 to 70 n-cm. The

probe tips used had radii of 0.25, 0.4, and

1.0 mil (6.4, 10, and 25 ym) and were loaded

with about 40, 60, and 150 g, respectively.

No substantial difference in measured resistance or noise was observed as a function of

the speed with which the probe point tra- versed the wafer.

Initial calibration studies were carried out

using n-type silicon wafers in the above re- sistivity range. The wafers v/ere chemmechanically polished and the resistivity

measured by the four-probe method [2]. The

spreading resistance was measured using tungsten carbide points with a radius of 0.25 mil

(6.4 ym) , a load of 49 g. and a traverse

speed of 63.5 mm/s. The initial calibration

plot of log spreading resistance against log

resistivity exhibited considerable scatter

suggesting that many of the wafers may have

substantial non-uniformities. Data obtained

after the wafer surfaces had been cleaned in

a mixture of 5 parts sodium hydroxide, 1 part

water and 1 part hydrogen peroxide followed

by a mixture of 5 parts hydrochloric acid,

1 part water, and 1 part hydrogen peroxide,

and dried in air at 150° C for over 15 min

did not show significant differences.

A few tests were run with a drop of high

pressure cutting oil on the surface. The

effect of this film was to raise the measured

resistance by about 30 percent without decreasing the amount of noise. Examination of

the track marks using phase contrast microscopy at 720 X indicated that the amount of

debris close to the track was reduced.

(A. Mayer"'-' and N. Goldsmith")

3.4. Ionization of Dopant Density in Silicon

In semiconductor studies it is often of importance to know the relationship between the

density of majority carriers and density of

dopant impurities in uncompensated material.

In particular, in theoretical calculation of

carrier mobility it is necessary to know the

relative amounts of ionized and neutral atoms

to com.pute their scattering contributions.

For lightly doped, but extrinsic, uncompensated silicon at room temperature the Fermi

energy lies between the impurity energy and

the center of the band gap; hence, essentially all the substitutional dopant atoms are ionized and the majority carrier and dopant

densities are equal. As the density of dopant atoms is increased, the Fermi energy

approaches and passes through the impurity

energy and the fraction of ionized dopant

atoms is decreased; hence, the majority carrier density may be less than the dopant

density. At still higher dopant densities

the system becomes metallic and the dopant

atoms are again fully ionized. The unre- solved question is whether there is a significant region of dopant density which lies

Work performed at RCA Laboratories under NBS

Contract No. 5-35914. NBS contact for additional information: J. R. Ehrstein.

13
I

RESISTIVITY

100

80

60

40

20

1

1 1 1

1\ a" -

A

1 1 1

1 10' 10' 10^ PHOSPHORUS DOPANT DENSITY Icm ')

Figure 5. Ionization of phosphorus in

silicon at 300 K. (Theoretical: 1,

constant ionization energy; 2, eq (5);

3, eq (6); 4, Berg [18]. Experimental:

, Gardner et al. [19]; O, Crowder and

Fairfield [20]; #, Nakanuma [21]; ,

Esaki and Miyahara [22]; A, Irvin [24]

( arsenic ) .

)

lO'" 10" 10'° 10" BORON DOPANT DENSITY (cm

Figure 6. Ionization of boron in silicon

at 300 K. (Theoretical: 1, constant

ionization energy; 2, eq (5); 3, eq (6).

Experimental: , Pearson and Bardeen

[12]; A, Irvin [24]; Q, Hofker et al

,

[25].)

between the two regions of essentially full

ionization. To begin to answer this question

calculations were made of the fraction of

ionized dopant atoms according to various

models and the results compared with available experimental data.

The calculation of the ionized dopant density was made using a digital computer to solve, by iteration, for the Fermi energy

in the charge balance equation. Since the

minority carrier density can be neglected

in extrinsic silicon, the majority carrier

density equals the density of ionized dopant

atoms. Therefore, the charge balance equation becomes for n-type material

N {exp[(E -F)/kT] + 0.27}-! =

c c N^{1 + g^"lexp[(F-E^+e^)/kT] }-l

, (4a)

and for p-type material

N^{exp[(F-E^)/kT] + 0,27}~1 =

N {1 + g exp[(e +E -F)/kT]}~^ (4b)

a ^a a V

14
RESISTIVITY

where N or N is the effective density of

c V states in the conduction or valence band, E

c or E is the energy of the conduction or va- V

lence band edge, F is the Fermi energy; k is Boltzmann's constant; T is absolute temperature; N, or N is the donor or acceptor dod a pant density, g^ or g^ is the degeneracy factor of the donor or acceptor impurity, and

e , or e is the ionization energy of the dod a nor or acceptor impurity. A value of 0.5 was

used for g^ while g^ was taken as 0.25. The

carrier density on the left hand side of

these equations includes an approximation

given by Blakemore [11] for the Fermi integral which gives results within ±3 percent

of those given by the Fermi integral whenever

F/kT < 1.3. This limit was not exceeded in

the present calculations so that the results

are statistically correct for degenerate as well as non-degenerate conditions. For electrons, a density-of-states effective mass of

1.18 m [12] was used giving N = 3.22 x lO^^

o c 3 cm at 300 K. For holes, a mass of 0.81 m

[12] was used resulting in N = 1.82 x 10

cm~3 at 300 K.

19

Figures 5 and 6 show percentage of dopant

density ionized as a function of total density for phosphorus-doped and boron-doped

silicon, respectively, at 300 K for several

models. The curve for a constant ionization

energy of 0.045 eV (appropriate to both impurities) shows a rapid decrease in percent

ionization for dopant densities greater than

10^^ cm~^. Because of both effective mass

and degeneracy factor differences, the fraction of ionized atoms is less for boron-doped

silicon than for phosphorus-doped with the

same dopant density.

Experimental evidence exists which shows that

the ionization energy is not constant, but

decreases with increasing dopant density

[13, 14]. Penin [14] found that the equation

'dO

Se do(i ^ 7) (5)

with an ionization energy e 0.045 eV

for lightly-doped material, an average

distance r^ = [3/(4TrN^) ] ^/^ betxreen impurity atoms, and an effective Bohr radius

a* = 21 X 10~^ cm gave a calculated ionization energy in good agreement with his experimental results for phosphorus-doped silicon.

For boron-doped silicon the same equation

with identical parameters was used as e^^

and a* are essentially the same as in phosphorus-doped material. As seen in the figures, the use of this equation results in a higher fraction of ionized atoms at large

dopant densities. At 3 x 10^^ cm~^ the ionization energy has decreased to zero. This

is also the dopant density for which the theoretical calculations of Kleppinger and Lindholm [15] predict a disappearance of the ionization energy due to significant overlapping

of the impurity and conduction bands. The

assumption is made that all impurity atoms are ionized at higher dopant densities. This

transition is indicated by a dashed vertical

line. Complete ionization at high dopant densities is in agreement with Hall coefficient

measurements in heavily doped n- and p-type

silicon from 4 to 300 K which show no evidence

of an ionization energy at impurity densities

greater than 3 x 10^^ cm~^ [16] . Furthermore

Fistul' [17] argues that heavily doped semiconductors do not have shallow impurity

levels and consequently the dopant is

completely ionized as in the case of

metallic conduction.

The second model for the ionization energy

follows Pearson and Bardeen [13] in that the

energy is taken inversely proportional to the average distance of separation between

the dopant atoms:

e - aN 1/3,

do d where the constant a was ass

3.1 X 10-8 cm'eV for both ph

boron-doped silicon to give

energy at a dopant density o for comparison with eq (5)

.

ionization energy begins to doping densities than for eq of the more gradual decrease

ized density decreases monot

trast to the scallop in the

1 and 3 model

.

10^

(6)

igned the value

osphorus- and

zero ionization

f 3 X 10I8 cm-3

With eq (6) the

decrease at lower

(5) and because

in e . the iond onically in con- curve between

ed with Penin's

The above calculat

single energy leve

and a conduction o states appropriate

ial. It is known

density increases

into a band and th

sity of states of edge. To some ext tion energy which

ions were made ass

1 for the impurity

r valence band den

for lightly doped

[14] that as the d the impurity level

ere is tailing in

the nearby intrlns

ent the use of an depends on doping

ummg a

s tates sity of

materoping

broadens

the denic band

ionizadensity

15
RESISTIVITY

incorporates these changes. However for densities greater than 10^^ cin"^ the above equations begin to lose their validity. Consequently calculations were also made using a model developed for gallium arsenide by Berg

[18]. This model takes into account the

broadening of the impurity level and the

tailing of the conduction band. The density

of states in the broadened impurity level was assumed to be Gaussian in nature as formulated by Dyakonov et a.l . [19]. Berg

found that the model gave good computer fits

to his data provided that the tailing of the

conduction band edge was increased by the

factor 1.6 over the theoretical value. This

same factor was also used in the present calculations for n-type silicon. As seen in

figure 5 the percent ionization is similar

to that obtained from the Pearson and Bardeen

model. Ionization is considered to be com- plete when the tail of the conduction band crosses the middle of the impurity band,

which remains centered at 0.045 eV . This

transition to complete ionization is indicated by the vertical line at a density of

about 5 X 10^^ cm~3 . The model was not es- tended to calculations on p-type silicon but

the results are expected to show similar

trends

.

Experimental data on percentage ionization

are also shown in figures 5 and 6. For phosphorus-doped silicon, Gardner et al . [20]

compared carrier density derived from plasma

resonance with total density obtained from

neutron activation analysis. On a phosphorus-implanted specimen Crowder and Fairfield

[21] measured essentially the same impurity

profile by neutron activation analysis as they obtained by differential Hall effect

measurements. Nakanuma [22] studied the incorporation of phosphorus in epitaxial silicon by radiotracer studies and compared the

phosphorus content with that obtained by Hall

effect and conductivity measurements. He found no existence of electrically inactive

phosphorus in the layers up to densities of

3 X lO'^^ cm ^. Using colorimetric analysis

and Hall effect measurements, Esaki and

Miyahara [23] determined that the percentage

of ionized phosphorus in silicon at room tem- perature decreases gradually from nearly 100

to 60 percent with increasing phosphorus density in the range from 0.68 to 2.6 x 10^'^ cm~^ . Mousty et at. [24] compared neutron

activation analysis and Hall effect measurements on silicon doped with phosphorus in the

range 10^'' to 10^^ cm~^ . With the assumption

of complete ionization, they calculated the

Hall scattering factor and found that it increased from 1.0 at 10^^ cm"^ to 1.3 at 10^^

cm~^ and then decreased to 1.0 at 10^^ cm~^

.

There is the possibility that the peak in

the scattering factor is due in part to incomplete ionization. Irvin [25] gives re- sults on two specimens of arsenic-doped silcon on which both neutron activation analysis and Hall effect measurements were made

and on one boron-doped specimen for which

the total boron content was determined by a photometric technique for comparison with

the Hall effect result. Based on the amount

of boron impurity added to the melt and subsequent Hall effect measurements on the grown

crystals, Pearson and Bardeen [13] concluded

that each boron atom gave one charge carrier

(within ±20 percent) except for one very

heavily doped crystal in which only part of

the boron went into solid solution. Hofker

et al . [26] profiled a boron-implanted wafer

by secondary ion mass spectrometry and com- pared it with Hall effect measurements on

the same wafer.

The sharp decrease in electrical activity

of dopant atoms at densities greater than

about 10"^ cm~^ appears to occur because many

of the dopant atoms do not occupy substitutional sites at these high densities rather

than because of incomplete ionization. Unfortunately, the available experimental data

are insufficient to resolve the question of

whether there is an intermediate range of

dopant densities, near 10^^ cm~^, for which

ionization may not be complete. Therefore

it is not possible at this time to establish

the validity of any of the models used in the

calculations. (W. R. Thurber)

The parameters for arsenic-doped silicon are very similar to those of phosphorus-doped

silicon

.

16
il . SURFACE A rj A L Y S I S f1 E T H 0 D S

4.1. Comparative Study of Surface Analysis

Techniques

As part of the comparative study of surface

analysis techniques (NBS Spec. Publ. 400-12,

pp. 17-18) a Rutherford backscattering ex- periment [27, 28] was performed.* Only one

of the four specimens included in the study

was suitable for analysis by this technique.

The atomic number of the impurity in each

of the other three specimens was below the

atomic number of the host matrix, and there

was an insufficient amount of the impurity

present for it to be observed over the background signal due to the host. The specimen

which was measured, a silicon wafer implanted

with 30 keV zinc ions to a dose of 5 x 10^

cm" MeV '*He'' ions ove dose of 10 yC.

was exposed to a 40 nA current of 2,0

^„„o r^,;er a 1 mm^ area, to a total

The experiment is conducted by recording the

number of particles backscattered from the

specimen at a fixed angle as a function of

the energy of the backscattered particles

using a silicon surface barrier detector and

multi-channel pulse—height analyzer [27]. Two measurements were made, one with normal

incidence and one with the beam incident at

The measurements were made by Prof. M. A, Nicolet at the van de Graaf accelerator

facility of the California Institute of

Technology, Pasadena, California.

45 deg. The results are presented in figure

7. The energy scale was established by measuring the '*He"'" backscattered energies from

aluminum (1.102 MeV) and gold (1.845 MeV)

surfaces; the channel width was determined

to be 3.46 keV.

The projected range of the implanted particles may be estimated from the energy differ

ence between the energy of the peak response

obtained with normal incidence and the en- ergy of a backscattered helium atom just

after collision with a zinc atom. This en- ergy loss corresponds to a range of about

37.5 nm which is somewhat larger than the

value, 22.4 nm, predicted by LSS theory [29]

If the detector had sufficient resolution,

the distribution profile of the implanted

ions could be inferred from the shape of the

response. However, the resolution of the

detector used was only 'vl5 keV. Since the

expected range straggling, defined as the

half width of the distribution profile at l//e of the maximum density, corresponds

to an energy difference of only about 4 keV,

it is clear that the width of the response

peak is due primarily to the detector resolution, and no information regarding the

shape of the implanted distribution can be

inferred. Although detecting systems with

significantly better resolution are available, they are much less sensitive than

silicon detectors and their use requires

much greater time to complete the measurement .

0:2000

DETECTOR

2 MeV ^He'

,8=172°

Si M-Rp

200 300 CHANNEL NUMBER

Si

400

Zn

Normal incidence.

1125 1565 BACKSCATTERED ENERGY (MeV)

?noo —

\l

* •

300 400 NUMBER

Si Zn

1.125 1.565 BACKSCATTERED ENERGY (MeV) Angle of incidence, 45 deg,

Figure 7. Yield as a function of energy of backscattered 2.0 MeV '^He ions from zincimplanted silicon vjafer. 17
SURFACE ANALYSIS METHODS

It is also possible to determine the total

zinc implantation dose, D„ , from the ratio

Zn

of the area of the response peak to the area

under a portion of the silicon plateau [28],

If this is done for the two measurements in

figure 7, one obtains D_ = 4.99 x 10^^ cm"^

Zn and 5.52 x 10^^ cm~^ . These values are in

reasonable agreement with the value of

D„ (5 X 10^^ cm~^) determined at the time

Zn

of implantation.

Although this technique is a rapid, non- destructive method for establishing identity,

location, and density of impurity atoms, it

has several important limitations. First,

it is most sensitive to impurities of high

atomic number; the bulk detection limit is proportional to the square of the atomic

number. Furthermore, for elements of lower

atomic number than the host matrix, the peak

appears superimposed on the host plateau,

which further degrades the detection limit.

Second, the method lacks lateral resolution. If the beam diameter were reduced

to increase lateral resolution, the duration of the measurement would have to be

substantially increased or else the detection

limits would suffer significantly. Third,

the position of the peak depends on both the

location and identity of the impurity atoms

and also on the orientation of the crystal.

This can complicate the analysis and may re- quire that one have considerable knowledge

of the specimen to sort out the various parameters of interest. Finally, the technique

requires rather large amounts of the impurity

to be present if it is to be seen; hence it

is more frequently used for analysis of thin

surface layers than for profiling trace impurity distributions. (A. G. Lieberman)

4.2. X-Ray Photoel ectron Spectroscopy

Previous studies of the angular dependence

of x-ray photoelectron spectra from dirty

surfaces, which revealed depth profile information about carbon and oxygen overlayers

on silicon (NBS Spec. Publ. 400-12, pp. 15-

17), were extended to clean silicon surfaces.

The angular dependence of the Si(2p) photoline at about 1160 eV, its associated first

surface (Sp) and bulk (Bp) plasmon peaks at about 1150 and 1145 eV, respectively, the

Si (2s) photoline at about 1110 eV, the background signal at about 1000 eV, and the KLL

Auger complex near 1600 eV were measured.

The results are shown in figure 8.

The background region shows only a smooth

variation with angle. At small and negative

angles, the intensity falls off due to shadowing of the x-ray beam by the specimen surface. At angles near 90 deg, the specimen

subtends a small solid angle at the analyzer . slits. Also, microscopic surface irregularities tend to reduce the electron intensity.

The behavior of the other spectral regions

is much more interesting, showing a number

of sharp peaks with angular widths as small

as 2 or 3 deg. Similar effects have been

reported previously on single crystals of

sodium chloride and gold, although not with

such sharp angular resolution. The explanation for these regions of enhanced emission

has been attributed to internal electron

diffraction or channeling effects. While

a channeling description is apparently valid

for heavy particles and high ("^ 100 keV)

energy electrons, there is some doubt that

this description is valid for electrons in

the 1 keV energy range. However, data re- ported for emission of 6- particles (with

energy up to 450 eV) from radioactive ^^^Yb

implanted in a silicon single crystal [30]

appear to substantiate the validity of the

channeling concept at low energies.

Ratios of certain pairs of intensities are

also shown in figure 8. The Si(2s) to Si(2p)

ratio (not plotted) is essentially constant.

The bulk plasmon (Bp) to Si(2p) ratio, however, shows a number of peaks and valleys.

In particular, at angles where the Si(2p)

intensity goes through a maximum, the Bp intensity does not increase proportionately as much. This has some profoundly interesting

implications. One of the most important,

but also most difficult, questions faced by

j the ESCA Task Force of the Surface Analysis

Subcommittee of ASTM Committee E-2 on Emission Spectroscopy is the question of how

should one measure XPS peak intensities to derive quantitative elemental ratios. The

difficulty arises in part because one has not

known to what extent plasmon and other energy

loss peaks should be included with the main

photopeak intensity. Previously, there has

been no good way of distinguishing whether

the plasmon peaks seen are an intrinsic pro- i cess associated with the primary photo-ioni- j zation event or an extrinsic process produced - during the path of the electron out of the

solid. The existing data are contradictory

and inconclusive. For magnesium, aluminum,

and sodium, one group maintains that plasmon •' creation is entirely an extrinsic process ^ [31], while for graphite, another group maintains that the process is entirely intrinsic

[32].
SURFACE ANALYSIS METHODS

If, in silicon, the bulk plasmon creation

were entirely an intrinsic process, one might assume that the effective origin of

the emitted electron would be associated with

its original silicon lattice site, in which

case the Bp intensity should scale with the

Si(2p) line intensity. If the plasmon were

created during the passage of this electron

from the solid, its effective origin would

be expected to correspond to a non-substitutional location and the Bp/Si (2p) intensity

ratio should then decrease along channeling

directions. This is precisely what is observed to happen in the vicinity of the <111>

channeling direction at 9 = 0 deg. This

may be the first solid experimental evidence

and method for deciding between the intrinsic

versus extrinsic character of a plasmon excitation.

The surface plasmon (Sp) to Si(2p) ratio is

quite scattered due to poor statistics on the

Sp count rate, but it also appears to show

a dip near the <111> direction at 9 = 0 deg.

The more interesting behavior is the increase

in the ratio at large angles where the electrons are leaving nearly parallel to the

specimen surface. It is under these angular

conditions that the surface tends to be em- phasized in the spectra so the Sp count rate

tends to become proportionally larger.

The KLL/Si(2p) ratio shows still another interesting behavior. At small angles, the

x-ray beam strikes the specimen at near grazing incidence and eventually approaches the

critical angle for total reflection. Under

these conditions, the x-ray penetration depth

is limited to approximately the wavelength

of the radiation which is in the order of

1 nm. Since the escape depth of the Auger

electrons at about 1600 eV is greater than

that of the Si(2p) photoelectrons at about

1160 eV, the limited penetration depth of the

x-rays selectively decreases the effective

depth from which the Auger electrons originate and hence causes the KLL/Si(2p) ratio

to decrease near grazing incidence.

20 40 60 6 [degrees!

a. Relative intensities for the KLL Auger

complex, curve 1; the Si(2p) and Si (2s)

photolines, curves 2 and 3, respectively;

the bulk (Bp) and surface (Sp) plasmons,

curves 4 and 5, respectively; and the

background spectrum, curve 6,

20 40 60 80 B [degrees]

b. Intensity ratios: Bp/Si(2p), curve 1; Sp/Si(2p), curve 2; and KLL/Si(2p), curve 3,

SPECTROMETER

X-RAY SOURCE N-VWW

SPECIMEN

c. Geometric arrangement

Figure 8. Angular dependence of x-ray photoelectron spectra from clean silicon surfaces.

19
SURFACE ANALYSIS METHODS

Due to randomizing collisions of the emitted

photoelectrons , anisotropies of the valence

band structure were thought to be removed

from the XPS spectra. However, a possible

consequence of the dependence of the escape

probability of an electron upon electron site

origin is that the apparent valence band density of states might indeed depend on the

angle of observation. From observations of

the valence band spectrum of silicon at sev- eral different angles, it was deduced that

the apparent density of states does indeed

change

.

It has been tacitly assumed by most XPS ex- perimentalists that crystal orientation would

have little or no effect on valence band

spectra provided one stayed away from grazing electron take-off angles where the surface is strongly emphasized. These results

show this assumption is not valid.

(N. E. Erickson", A. G. Lieberman,

T. E. Madey- and J. T. Yates, Jr.-)

4.3. Calibration Standards for Ion Microprobe Mass Analysis

This task was undertaken to develop methods

for preparation of standard specimens for em- pirically calibrating an ion microprobe mass

analyzer (IMMA) so that quantitative measurements can be made of selected impurities in

silicon and silicon dioxide regions of semiconductor devices. Implicit in this objective is the evaluation of the applicability

of the IMMA for this type of analysis and

the determination of the instrumental parameters best suited for these analyses.

The microvolume photomasks have been designed

and fabricated. The repeat pattern is 100

by 100 mils (2.54 by 2.54 mm) and contains

regions ranging in size from 1/2 by 1/2 mil

(13 by 13 um) to 30 by 30 mils (0.76 by

0.76 mm). Figure 9 shows a photomicrograph

of the photomask repeat pattern. The numbers

and sizes of the implanted regions are listed in table 3. The metal locator photomask has an identical pattern V7ith each

opening slightly larger than the implant

opening. The regions outside the implant

openings are protected from implantation by

the photoresist film. Both are correct positive photomasks to facilitate alignment

without special alignment marks that could

necessitate extra processing steps. Figure

10 is a photomicrograph of an oxide layer

implanted with phosphorus using this set

of masks.

Initial material selected for the study is a group of 190 2-in. (50-mm) diameter wafers

cut from the same dislocation-free, floatzone p-type silicon crystal with room temper- ature resistivity in the range 3 to 25 fi'cm. Twelve wafers were chosen randomly for evaluation of substrate impurity content. All

12 wafers were cleaned according to a standard procedure; six were oxidized in dry

oxygen at 1100°C to obtain a 0.12 ym thick

1

Table 3 — Number and Sizes of Microvolumes in Repeat Patterns of Implantation

Photomask

The plan for preparation of the standards is

to ion implant the impurity of interest into

silicon or silicon dioxide matrices in small,

well-defined surface areas. Since the depths

of the implants are controlled by the implant energy, the standard sample will be

essentially a microvolume containing a known total amount of impurity but with a concentration that ranges from several thousand parts per million atomic to less than 1 part per billion atomic. The microvolumes

range in size from small enough to be entirely consumed during analysis to large enough

to be considered an infinite surface plane.

Aluminum metallization is used to locate the

implanted region and, in the case of silicon dioxide, to act as the necessary surface

conductive layer.

Size, mils (pm) Number

NBS Surface Processes and Catalysis Section,

Physicel Chemistry Division.

1,z X '13 X 13) 5

1 X 1 '25 X 25) 5

2 X 2 '51 X 51) 9

2 X 3 '51 X 76) 9

3 X 4 '76 X 100) 9

4 X 5 '100 X 130) 5

5 X 6 ;i30 X 150) 4

6 X 7 '150 X 180) 4

7 X 8 '180 X 200) 1

8 X 9 '200 X 230) 1

9 X 10 (230 X 250) 1 30 X 30 (760 X 760) 1

20
SURFACE ANALYSIS METHODS

Figure 9. Photomicrograph of photomask

repeat pattern. (Magnification: 'v27X.)

Figure 10. Photomicrograph of oxidized

silicon wafer shov/ing implantation microvolumes in a portion of the repeat pattern.

(Magnification: '^leX.)

I 21
SURFACE ANALYSIS METHODS

oxide. To minimize the introduction of con- tamination during oxidation the furnace had

double quartz tubes with dry nitrogen flowing between the walls of the inner and outer tubes. All 12 wafers are being analyzed

by neutron activation analysis and ion microprobe mass analysis (IMMA) . Long-term

point counts are being taken on the IMMA at masses 23, 30, 31, 52, 75, and 197, corres- ponding to sodium, silicon, phosphorus,

chromium, arsenic, and gold, to determine a statistical background level relative to the

silicon matrix to be expected in the analysis

of the implanted standards.

The linearity of the IMMA secondary ion pickup and detection system is being determined

as a function of raster size by monitoring

the silicon ion count at 10-second intervals

over a one- to two-hour period. The initial

parameters chosen_^for the study were a silicon matrix; a ^^62, 20 keV, 10 nA primary

sputter beam with a 15 ym spot; and a 20-mil

(0.5-mm) resolution slit. The raster size

is being varied from 25 by 20 pm to 300 by

240 ym in 25 by 20 pm steps. This study has

already shown that the pickup can vary as much as 50 percent from raster site to raster

site. However, the variation does not appear

to be a predictable function of raster size.

The variation is minimized by tuning the primary magnet and primary column for symmetrical ion current at the opposite extremes of

both the X-axis and y-axls sweeps. Also, the

secondary magnet must be tuned for the best

symmetrical signal at the opposite ends of

the X-axis sweep.

Characterization of the secondary ion signal

as a function of raster size and electronic

aperture size has been initiated. The Si

signal is being collected in 10-s increments

using 200 by 160 ym, 150 by 120 ym, 100 by

80 ym, and 50 by 40 ym rasters with electronic aperture ratios of 1 by 1, 3/4 by 3/4,

1/2 by 1/2, and 1/4 by 1/4 on each raster

size. The first results suggest that the

signal is a function only of the electronic

aperture acceptance ratio and is independent

of the raster size. This shows that constant

sputtered volumes do produce a constant sec- ondary ion signal.

The sputter rate for 20-keV ^^0^ on silicon

as a function of raster size has been determined by measuring the depth of the craters sputtered during the raster size linearity

study. The depths were measured by both optical interf erometry and surface profilometry. The sputter rates obtained from the

10 nA beam ranged from 0.017 to 4.16 nm/s

and varied linearly with surface area sputtered. (R. Dobrott" and G. B. Larrabee'"')

4.4. Optical Test for Surface Quality of

Sapphire Substrates

This task was undertaken to develop a fast

nondestructive optical test for the surface

quality of sapphire substrates and to cor- relate optical data with data from conventional analytical techniques and with measurements of the electrical properties of

heteroepitaxial silicon films in siliconon-sapphire (SOS) structures. Barker [33] has

demonstrated that forbidden vibrational modes

may be introduced into the infrared reflection spectrum of sapphire as a result of

crystallographic strain and work damage.

This observation is being utilized in the

development of a process to examine non- destructively the surface perfection of

sapphire substrates on which the silicon

is to be heteroepitaxially grown. The

reflection technique is ideally suited

for surface analysis, particularly in the case of sapphire, since the surface penetration depth in the spectral region of

interest is estimated not to exceed a : few hundred angstroms. This is a region

of vital importance to the nucleation

and growth of silicon films, and thus significantly affects the properties of semiconductor devices. The techniques under investigation can be used to study the surface

perfection of sapphire substrates both before

and after silicon epitaxy, since silicon is

transparent in the spectral region of the

measurement

.

Sapphire substrates from several different

crystal growth and polishing vendors have

been examined by the infrared reflectance

technique, and the results compared with ^ control specimens. Examples of the reflec- i tance spectra of both "good" and "poor"

surfaces are given in figure 11. The ] specimens were supplied by different

| vendors for use in silicon epitaxy; the

spectra were obtained on the surfaces as re- ] ceived. Curve a displays a high degree of i surface perfection similar to the spectrum

that would be obtained on a sapphire surface

after annealing or chemical etching to remove

Work performed at the Central Research 5 Laboratories of Texas Instruments Incor- ] porated under NBS Contract No. 5-35917. [. NBS contact for additional information: [ A. G. Lieberman

.

22
SURFACE ANALYSIS METHODS

polishing work damage. Curve b displays

surface damage similar to that left by diamond polishing grit of particle size from

3 to 6 ym. In addition, Barker's surface

analysis technique has been extended to multiple reflection methods to greatly accentuate small differences in the surface quality

of the substrates. This is essential in order to understand uncontrolled variations in

crystallinity and electrical properties of

epitaxial silicon films.

Other analytical methods such as x-ray diffractometry (line broadening) , x-ray surface

and transmission topography, and chemical

etch tests have also been used to characterize sapphire surfaces. It was found that

there is a correlation between the results

of reflectance measurements and the other

analytical tests performed. For example,

the x-ray surface topograph in figure 12 shows two distinct regions. The dark region

which was polished with 6 ym diamond shows

considerable lattice damage as indicated by

the features in that portion of the topograph. The light region which was polished

with a suspension of colloidal silica to achieve a high degree of surface perfection

shows a featureless topograph consistent

with the characteristics of a defect-free

surface. The infrared reflectance spectra

WAVELENGTH (/im)

10 12 14 16 18 20 25 30

. 1 ,1.1,1.1.1,1,1 .1.1 .... I ....I,,.

of these regions were similar to the traces shown in figure 11 and thus consistent with

x-ray data.

Dif f ractometry traces (6:20 mode) from the same two regions are shown in figure 13. The half width for the "good" surface was

0.035 deg and for the damaged surface 0.045

deg. These values are typical for several

specimens prepared in the above manner. The

line broadening is 0.01 deg due to polishing

damage if it is assumed that the surface

polished with colloidal silica is a perfect

surface. Photomicrographs of surfaces preferentially etched in potassium hydroxide

show characteristics similar in appearance

to the x-ray topographs, revealing decoration

of damage lines on the crystal surface. Characterization of very small differences in

surface perfection is being explored. As a result of present work, infrared surface analysis based on multiple reflectance appears

to be potentially more sensitive to surface

perfection than any of the other nondestructive tests currently in use.

Either absolute or relative values of the

optical constants [refractive index (n) and

the absorption index (k) ] are the parameters

for quantifying the magnitude of the observed

forbidden modes. Absolute values of the con- stants are obtained from numerical analysis

of reflectance data using Kramers-Kronig re- lations following the general treatment of

Roessler [34] and the related computer program of Klucker and Nielsen [35]. Initial

sapphire surfaces. (Curve a, surface Figure 12. Reflection x-ray topograph of

polished with a suspension of colloidal specially prepared (1012) sapphire surface

silica; curve b, surface polished with 5 ym obtained with copper radiation using the

':o 3 ym diamond.) (3030 ) reflection plane.

23
SURFACE ANALYSIS METHODS

Figure 13. Diffr£ctionieter traces of specially prepared (1012) sapphire surfaces

obtained in the 6 :_2e mode with copper radiation using the (022U) reflection plane.

(Surface polished with a suspension of colloidal silica, curve A; surface polished

with 6 ym to 3 pm diamond, curve B.)

activity has centered on obtaining and testing the program with standard test data supplied with the program. This phase of testing is now complete.

The general applicability of multiple re- flectance measurements to dielectrics and

semiconductors is being examined. The wavelength region of interest depends on the absorption characteristics of the material in

question. Preliminary measurements indicate

that the deformation of symmetry states (as

in sapphire) is not essential to these measurements. Epitaxial silicon films grown on work-damaged surfaces (as revealed by infrared reflectance) are degraded electrically

relative to films grown simultaneously on control substrates. Optical reflectance

measurements made on the silicon films in

the ultraviolet region near the absorption

edge of silicon have shown higher absorption

coefficient values at a given wavelength for

the silicon on the degraded sapphire than for

the films grown simultaneously on the control

substrates. Thus, it may also be possible

to monitor the quality of the epitaxial film

optically. (M. T. Duffy-,

P. J. Zanzucchi", and G. W. Cullen")

Work performed at RCA Laboratories under NBS -i Contract No. 5-35915. NBS contact for additional technical information: K. F. Galloway .

24
5 . TEST STRUCTURE APPLICATIONS

5.1. Reevaluation of Irvin's Curves

Resistivity and carrier density measurements

were made on additional phosphorus-doped silicon wafers as part of the continuing work on

the experimental redetermination of the re- sistivity-dopant density relation using previously reported procedures (NBS Spec. Publ.

400-17, pp. 13-14). Bulk resistivity was determined from measurements on test structure

3.17, collector four-probe resistor. Electron density was obtained from measurements

on test structure 3.8, the MOS capacitor over

collector, by the deep depletion method (NBS

Spec. Publ. 400-17, pp. 8, 10-11) and from

measurements on test structure 3.10, basecollector diode, by the junction capacitancevoltage (C-V) method (NBS Tech. Note 788, pp. 9-11). Electron density data from the

junction C-V method were used for calculating

the mobility as this method appears to be more reproducible at the present time than

the MOS procedure.

The four-probe resistor and the base-collector diode are about 3 mm apart on the test pattern; this can be a source of error in

wafers with large resistivity variations. In order to minimize the effects of these variations, mobility is computed from the electron density measured on a diode and the

average of the resistivities measured on the

two collector resistors on either side of the

diode. The procedure is repeated for typically four good diodes in the same general

area of the wafer, usually near the center,

to arrive at an average mobility as given in

table 4. Wafers were screened for radial re- sistivity gradients by four-probe measurements prior to processing. The maximum variation found for the wafers in the table was 6 percent between center and half-radius positions. With the averaging technique employed,

the error in the mobility due to gross resistivity gradients is estimated to be no greater than about 1 percent.

The experimental mobility values were com- pared with those calculated from the measured

electron-density by the equation of Caughey

and Thomas [36] which closely fits the Irvin

curve [24] for n-type silicon. These results

further suggest that the Caughey-Thomas equation for mobility gives values in agreement

with experiment to within ±5 percent in the

resistivity range studied. (W. R. Thurber,

R. L. Mattis, Y. M. Liu, and M. G. Buehler)

5.2. Mathematical Models of Dopant Profiles

The computer program developed to solve the

boron redistribution problem using a finite

difference algorithm (NBS Spec. Publ. 400-17,

pp. 11-12) was extended to include a radiative-type boundary condition at the oxygenoxide interface (z = 0)

:

9 C I -Di -r-^l = - C [Ci(0,t) - C ] , (7) 3z lz=z P u

' o where Dj and are the diffusion coefficient

and concentration of the impurity in the oxide, C and C are specified positive conp u stants, z is the coordinate of the oxideo silicon interface in the moving coordinate

system (NBS Spec. Publ. 400-1, pp. 9-11) associated with the oxide. Equation (7) is

believed to be more appropriate in certain

Table 4 - Preliminary Resistivity-Carrier Density Data for Phosphorus-Doped Silicon at 300 K

Slice No. p

»

n(MOS),

cm"3

n(jcn),

cm"3

p(exp),

cm^/V •$

p(calc),

cm2/V-s

percent

diff^

A0.27Ph-2 0.276±G.005 (2.20±0.05)xl0it' (2.21+0.05)xl0i6 1023±26 983 4.1 B0.47Ph-l 0.499+0.011 (1 .134±0.026)xlOi5 (1 .147±0.045)xl0^6 1093±49 1088 0.5

A2.0Ph-2 2.24±0.07 (2.37±0.05)xlOi5 (2.24±0.03)xlOi5^ 1244±13 1244 0.0

B3.9Ph-l 4.02±0.03 (1 .270±0.048)xl0i5 (1 . 237+0. 007)xl0i5 1255±12 1272 -1 .3 B12Ph-l 13.35±0.45 (3.87+0. 16)xl0i'* (3.60±0.14)xl0i'+ 1301 ±36 1306 -0.4

^100x[y(exp)-y(calc)]/ij(calc)

25
TEST STRUCTURE APPLICATIONS

cases of redistribution than the boundary

conditions employed previously (NBS Spec.

Publ. 400-1, p. 11, eq (8)).

Ku [37] has found a closed-form solution to the uniform redistribution problem with the

assumption that eq (7), with C =0, holds at

P the oxygen-oxide interface. His work suggests that the solution to the uniform re- distribution problem in the case of zero flux

between the oxide and air should possess two qualitative features: (i) that the value of

the concentration in the silicon at the moving front be independent of time and (ii)

that the concentration in the oxide be spatially uniform and independent of time. The

computer program reproduced both these features to several significant figures, except

for a very brief period of time near t = 0. The total time taken until these qualitative

features set in can be shortened by using

smaller time increments, At. (S. R. Kraft")

5.3. Dynamic MOS C-V Method

In a previous discussion of the MOS deep depletion method for profiling dopant density

(NBS Spec. Publ. 400-17, pp. 8, 10-11), it was observed that the model for extracting

the profile breaks down in the region near

the surface due to the departure of the ex- perimental capacitance-voltage curve from

the ideal depletion curve. An analysis was carried out to determine how close to the

surface one can make a profile measurement

before the depletion model breaks down.

An experimental high frequency deep depletion

curve, an ideal high frequency deep depletion

curve, and a theoretical low frequency curve

are illustrated in figure 14 for the case in

which the flat band voltage occurs at zero applied gate voltage (i. e., work function

differences, oxide charge, and interface

state charge are neglected) . Although a uniformly doped p-type substrate is assumed for

purposes of illustration, the results are ap- plicable to n-type substrates and can be ex- tended to the case of nonuniform (with depth)

dopant distributions.

The experimental deep depletion curve deviates

from the ideal in the accumulation region, at the flat-band condition, and partly into the

depletion region. However, in these regions.

the experimental curve coincides with the low

frequency curve. The analysis involves use

of the theoretical expression [38] for the

low frequency depletion capacitance based on

the assumption of uniform doping in the semiconductor and calculation of the total MOS

capacitance, assuming an oxide thickness, X . 0 The apparent dopant density profile is then

calculated using the depletion model.

The apparent profile is described by two para- metric equations

N(X) ^ a3 (8) N. sinh [2b^ cosh (U. - U ) - a^]

A f f s

and

X = - /2 L„ - , (9)

where is the assumed constant dopant density = 2 n_j^ sinh U^, is the Fermi potential,

U is the surface potential, a = sinh (U^ - U ) s

r » ' f s sinh U^, b = [cosh (U^ cosh U,

NBS Mathematical Analysis Section, Applied

Mathematics Division

U^) + U sinh U.]-'-/^, n. is the intrinsic carrier

s f 1 density, and L^^ is the intrinsic Debye length.

The dimensionless potentials and U are

f s normalized to kT/q, where k is Boltzmann's

constant, T is absolute temperature, and q is the electronic charge, and are referenced

to the intrinsic Fermi potential.

In carrying out the analysis, eq (8) is

solved iteratively to determine the surface

potential which corresponds to the maximum

permissible error in N(X). The value of X for this value of surface potential, calculated from eq (9) , gives the minimum depth

(X . ) which corresponds to the given error; mm ^ ^ the profile is more accurate at larger values

of X. It turns out that, for sufficiently

extrinsic material (N^ ^ lO^'* cm~^), the

ratio N(X)/K^ is essentially independent of

N, , and that X . is inversely proportional

A mm J c- f to the square root of N.

.

A

The results of this analysis are plotted in

figure 15 which presents error curves for the

minimum profiling depth as a function of the

background dopant leve] . For example, with

a background dopant density of 10-^^ cm~^, one

can determine the profile with error approaching 1 percent for depths near 0.14 pm; the

26
TEST STRUCTURE APPLICATIONS

0 1.1 Vg IVOLTSI

Figure 14. Capacitance-voltage characteristics of a p-type MOS capacitor with N = 10^5 cm"3, X = 120 nm, and C = 2.88 x 10~^ F/cm^. (Curve A, high frequency,

deep depletion, experimental; Curve B, high frequency, deep depletion, ideal;

Curve C, low frequency, theoretical.)

error approaches 10 percent for depths near

0.104 ym.

A commonly used limit for the depletion model

is defined as 2A, where A is the extrinsic

Debye length [39]. The dashed line on figure

15 indicated the error line for the 2X limit. For any dopant density, the 2X line corresponds to a 5.34 percent error at the indicated X ^ . (R. Y. Koyama) min '

5.4. Epitaxial Layer Thickness

The MOS capacitance method for epitaxial layer thickness measurement was raised to a higher level of sophistication by employing

the deep depletion MOS capacitance-voltage

(C-V) method (NBS Spec. Publ. 400-17, pp. 8,

10-11) . The method is basically the rampvoltage method as it has been described (NBS

Spec. Publ. 400-4, p. 51) except that instead

of merely locating the capacitance, C^, at which a break in the C-V curve occurs, the

entire dopant density profile is calculated

from the C-V curve. Consequently, the deep

depletion method is applicable over the wider

range associated with the ramp-voltage method

(NBS Spec. Publ. 400-17, pp. 39-41).

N,(cni-3)

Figure 15. Error curves for minimum profiling depth as a function of background dopant density. (See text for discussion .

)

27
TEST STRUCTURE APPLICATIONS

_ 10' E

1 ! a

1 1 1

2.52f I

0 051015 2 0 2530 0 040812162024

0 05 10 15 20 25 30 0 04 08 12 16 20 24

DISTANCE INTO SPECIMEN |^m)

Figure 15. Dopant density profiles calculated from the deep depletion C-V characteristics of an MOS capacitor on four epitaxial layers. (a, Wafer 2303, thermal

oxide dielectric; b, Wafer 2353, thermal

oxide dielectric; c. Wafer 2302, negative

photoresist dielectric; d. Wafer 2351,

negative photoresist dielectric. The

vertical bar marks the layer thickness as measured by the step-relaxation method.)

0 04 08 12 16 20 24 DISTANCE INTO

0 04 08 12 16 20 24 SPECIMEN [fim]

Figure 17. Dopant density profiles calculated from Wafer 2353 by signal averaging R repetitive deep depletion C-V characteristics, (a, R = 1000; b, R = 100; c, R = 10;

d, R = 1.)

Typical dopant density profiles are shown for

devices on four wafers in figure 16. For

three of the specimens, the epitaxial thickness was measured by the step-relaxation

method (NBS Spec. Publ. 400-4, p. 51) and

is indicated by a vertical line which, in

each case, lies within the transition re- gion which constitutes the layer-substrate

interface. The fourth specimen was not measurable by the step-relaxation method

for a voltage step from -HlOO V to -100 V. The high dopant density calculated for

shallow depths is an artifact of the measurement (sec. 5.3) and does not represent a real dopant density variation.

In two of the wafers studied, the dielectric

was thermally grown silicon dioxide; in the

other two, the dielectric was negative photoresist (NBS Spec. Publ. 400-12, p. 26). In obtaining these profiles, a triangular voltage waveform was applied to the devices.

The capacitance of the device for the positive-going voltage could therefore be observed in addition to the capacitance for

negative-going voltage. For the wafers having thermal oxide dielectric the capacitance

was nearly a single-valued function of voltage regardless of the voltage slope. For the

wafers having negative photoresist dielectric, however, there was a significant dependence on the slope. With the negative photoresist dielectric there was also an additional anomaly in the C-V curve at shallow

depths. Because of these effects, the profiles measured with oxide dielectric are believed to be more accurate than those measured with photoresist dielectric, but the

problems do not appear to affect the determination of the epitaxial layer thickness.

Because the equation for calculating the dopant density includes a derivative, precautions must be taken to eliminate excessive

scatter from the profiles. To obtain the

profiles shown in figure 16, the derivative

was calculated from a formula which effectively averages over a range of 16 data

points. In addition, signal averaging was carried out such that the C-V curve which was

analyzed represented the average of 1000

repetitive responses. The need for signal

averaging was noted by calculating profiles

in which 1000, 100, 10, and 1 responses were

used to construct the average C-V curve. Such profiles are shown in figure 17 for the

specimen with the profile shown in figure 16b.

Note that the portions of the profile in the

transition region and substrate suffer the

greatest degradation as averages of fewer re- sponses are taken. The non-random nature of

28
TEST STRUCTURE APPLICATIONS

the scatter for the case of no signal averaging (single response) is a result of the

10-bit resolution of the capacitance measurement and the manner in which the computer

operates on arrays

.

Examination of these profiles graphically

illustrates the problem which has been en- countered in attempting to correlate measurements of epitaxial layer thickness by different methods. The transition region has

a finite width and different methods identify

different points in the transition region as the location of the boundary between layer

and substrate. Hence, the agreement between

methods depends on the nature of the tran- sition region, especially for the case of

very thin layers where the width of the tran- sition region may be comparable with the

layer thickness. The present procedure may

be of use in developing suitable criteria for

definition of the location of the boundary or

in controlling the profile shape in the tran- sition region. (R. L. Mattis)

5.5. Bias-Temperature Stress Test

A preliminary set of measurements was carried

out as the initial phase of an effort to develop improved understanding of the application of the bias-temperature stress test to the characterization of oxide films. The

oxides used in this phase of the study were

thermally grown on 2.0-in. (51-mm) diameter

n-type silicon wafers with room temperature

resistivity in the range 5 to 10 fi'cm and

<100> surfaces. Oxidation at 1000°C for 100

min in dry oxygen resulted in an oxide film

approximately 80 nm thick. Following the

oxidation the wafers were annealed in dry

nitrogen at 1000°C for 20 min. Aluminum was

e-beam evaporated over the oxide to a thickness of about 500 nm, further annealed in

dry nitrogen at 500°C for 15 min, and patterned with the Metal mask of Test Pattern

NBS-3 (NBS Spec. Publ. 400-12, pp. 19-22).

The entire backside of the wafer was metallized with antimony-doped gold.

The test pattern mask contains four 15-mil

(0.38-mm) diameter capacitor gate electrodes

(test structures 3.2, 3.3, 3.8, and 3.19);

the test pattern is repeated every 200 mils

(5.1 mm) in both directions across the wafer,

j. The primary measured quantity for the invesI tigation is the flat-band voltage, V , of

F B MOS capacitors determined by the high frequency capacitance-voltage (C-V) technique

(NBS Spec. Publ. 400-4, pp. 34-37). The

general procedure for the study of a given

29

wafer was to measure the capacitance as a function of voltage for a number of 3.8 test structures in the "as-processed" condition;

at the least, the 16 structures in the central 4 by 4 array of patterns were measured.

Following this, the wafer was subjected to one or more of the following treatments:

positive hias-tem-pevatvcoe (BT) stress — heat

wafer to 300°C on hot-cold stage (NBS Spec.

Publ. 400-4, pp. 39-40); contact the grounded

probe to the gate electrode; apply +10 V to probe for 10 min; cool wafer to room temperature with the grounded probe still contacting

the gate electrode.

negative bias-temperature (BT) stress — heat

wafer to 300°C on hot-cold stage; contact the

grounded probe to the gate electrode; apply

-10 V to probe for 10 min; cool wafer to room temperature with the grounded probe still

contacting the gate electrode.

heat treatment — heat wafer to 300°C on hotcold stage for 20 min in dry nitrogen ambient

with no probe contact.

electron irradiation — expose, without applying bias, a portion of the wafer to a dose

of lO'* Gy (10^ rads(Si)) in the scanning

electron microscope using a beam energy of

20 keV, a beam current of 0.25 nA, and an ex- posure time of 10^ s.

Six lots of several slices each have been

fabricated by the processing facility with

no special treatment of the furnace. As an example of preliminary results, figure 18 summarizes the flat-band voltage measurements

on the four wafers of the fifth lot. Each

bar represents the span of the flat-band

voltages for the measured number of devices

shown in parentheses and is labeled with the

condition of the device. To begin with, the

flat-band voltages of the as-processed wafers

fell in the region from -0.21 to -0.27 V. Three of the four wafers had flat-band voltages for the 16 measured devices tightly

grouped within ±10 mV; these were subjected

to additional study. It should be noted here

that since the gate metal is aluminum, the

intrinsic shift of the flat-band voltage due

to the metal-semiconductor contact potential

difference is about -0.3 V [40] for the specimens studied. Therefore, in the as-processed

condition, these devices appear to have a net negative charge in the interface or oxide re- gion of the capacitors.

Wafer 5A was subjected to negative BT stress,

positive BT stress, and heat treatment. The
TEST STRUCTURE APPLICATIONS

POSITIVE IT STDESS

1321 c=:i US PDOtESSED

1121 <==! MEGtTIVE BT STDESS

HEtT TDEtTED

I «S PBOCESSED H 5B

=1 SOI imtDltTIOH

HEtT TDEtTED

POSITIVE IT STRESS

•S PROCESSEO

I POSITIVE OT STIIESS

III)

81 = HUT TREATED* =3 aEMTIVE >T STRESS REMTIVE BT STRESS*

1151 c

I HE<T TREITED =1 HEAT TREATEO*

> POSITIVE BT STRESS''

|1«l

|16| <=o «S PROCESSEO = RESATIVE BT STRESS^

HEAT TREATEO^

- 50

-0 6 -0.4 -0 3

(voltsl

Figure 18. A summary of flat-band voltage measurements on MOS capacitors at room temperature following various treatments. (See text for explanation.)

presence of positive mobile ion species is

readily revealed by the negative shift of the

flat-band voltage under positive BT stress.

In this case there was a slight negative

shift with the negative BT stress as well.

The net flat-band voltage shift (AV„^) indir D cates mobile ion contamination for these devices ranging between 6.7 and 8.7 x 10^'^ cm~^ . Subsequent heat treatment of the wafer

caused V^^ to drift back toward the as-processed condition but without full recovery.

Wafer 5C was subjected to electron irradiation, heat treatment, negative and positive

BT stress, and a second heat treatment. There are two separate subsets of devices.

One subset contained 16 devices which were measured in the as-processed condition in

the usual way. The second subset contained

16 devices v;hich were not measured in the

as-processed condition; the devices in this

subset, consequently, had not been probed

prior to the electron irradiation. The data

from the second subset are marked with an asterisk in the figure. Electron irradiation

caused a large negative shift of the flatband voltage and simultaneously introduced a high density of interface states in the subset measured after irradiation. Heat treat- ment after irradiation appeared to remove the

interface states; the flat-band voltage for

previously measured devices shifted to slightly more negative values whereas those measured

for the first time after processing, irradiation, and heat treatm.ent formed a tighter

group at smaller flat-band voltages. With

negative and positive BT stress, both sets

of devices behaved similarly although the

second subset of devices was affected less

and always formed a tighter grouping. The

net flat-band voltage shift resulting from

negative and positive BT stress indicate

ionic contamination ranging from 2.9 to 10.2 X 10l° cm-2 and from 1.7 to 5.0 x 10^°

cm~^ for the first and second subsets, re- spectively. After subsequent heat treat- ment of the wafer the flat-band voltage re- laxed toward the as-processed value.

Wafer 5D was treated similarly to wafer 5A,

but the devices measured after BT stressing were different from those measured in the

as-processed condition. Although the flatband voltage showed larger spreads, the

behavior was essentially the same as that

of wafer 5A.

These results are presented only to be indicative of the kind of inform.ation which might

be expected from study of the BT stress test; they are too preliminary to warrant detailed

30
TEST STRUCTURE APPLICATIONS

analysis and definitive conclusions. It should be noted that the use of the BT stress

test as an indicator of mobile ion contamination assumes that the mobile ions merely shift

the C-V curve in an amount proportional to the net charge moved to the interface. In general, however, this assumption is an oversimplification. Interface states can distort the C-V characteristic which can cause error in

the determination of the flat-band voltage.

This is particularly true of devices which

have been electron irradiated and those which

have high mobile ion contamination.

(R. Y. Koyama and D. A. Maxwell)

5.6. Extended Range MIS C-V Method

The metal-insulator-semiconductor (MIS) ca-

! pacitance-voltage (C-V) technique is widely

used to determine such parameters as semiconductor dopant density, flat-band voltage,

and interface state density. A basic limitation of the technique has been the accuracy with which the semiconductor depletion

capacitance, C^, could be determined. Generally for reasonable accuracy, it was necessary that the ratio of C to C,., the insus I

' lator capacitance, be ten or less. Recently,

I modified instrumentation has been developed

I [41] so that accurate measurements can be made

when the ratio C fC^ is 100 or more. The inI herent limit no longer depends on this ratio

I but on the noise level of the capacitance mea1 surement. This modified measuring instrument

j then allows the use of the C-V technique on

I MIS structures with thick insulator layers.

Among the potential applications of the modified instrument are the characterization of

I the silicon-glass interface under a thick

1(50 ym) layer of glass, the determination of

the flat-band voltage under a thick field

oxide on an LSI chip, and the characterizaI tion of silicon on sapphire and the silicon-

[ sapphire interface by using the sapphire substrate as the insulator in an MIS capacitor.

The original modified instrument [41], al-

„j though suitable for demonstration purposes

i| and laboratory application, requires improveII ment and additional modification before it

.'I will be suitable for general use. This task

I was undertaken to develop Improved bias proI tection circuitry to prevent damage to the

I measurement equipment in case of catastrophic

I dielectric failure of the specimen under high

bias, to develop a high-voltage power supply

I || with a linear sweep capability to facilitate

;;i \ use of the instrument in applications where

the capacitance is a function not only of the

applied voltage but of the sweep rate as well,

to develop an improved specimen holder with

an integral safety interlock system, and to further demonstrate the applicability of the

method.

The original modified instrument [41] had a potential problem when very high bias voltages were applied to the specimen under test.

If the specimen capacitor developed a short

circuit, a large part of the applied bias

voltage was transferred to the capacitance

meter, thus damaging it. To prevent this

damage, a bias protection circuit was developed that prevents voltage excursions beyond

the ±200 V limit of the capacitance meter.

However, the use of the circuit limits the

range of capacitance that may be measured

without introducing excessive added error. The first version of the circuit limited the

measured capacitance to the range 0 to 40 pF

if the added error introduced by the circuit

was to be kept below 1 percent. To overcome

this limitation, the values of the components

in the previous version of the circuit were

optimized. The resulting improved bias protection circuit has been tested to ±9600 V under repeated breakdown conditions. The

added error in the range of measured capacitance 0 to 140 pF is less than 1 percent.

The modified technique [41] often requires

the application of high bias voltage to the

capacitor sample to be measured. When this

bias voltage exceeds some threshold value

(typically 3 to 5 kV) , there exists the danger of breakdown along the insulator surface

or at an electrode edge. One method for

prevention of this breakdown [42] is to coat

the surface and electrode edge with a greaselike silicone material; this is very effective

but is messy, time-consuming, and inconvenient. It has been found that if an insulating silicone rubber toroid, having appropriate properties, is pressed over the specimen

surface including the electrode edge, breakdown can be effectively prevented at applied

bias voltages up to ±10 kV. The surface of

the rubber in contact with the specimen must

be very smooth, and the rubber itself must

be very resilient. It is also important

that the rubber contain no voids in the vicinity of the surface (in contact with the

insulator) and that it have both a very high

resistivity and a low dissipation factor.

Other instrumental improvements, including

the power supply with linear sweep capability

interlock, are being constructed. Initial

evaluations have been made of the measurement

II
TEST STRUCTURE APPLICATIONS

technique as a diagnostic tool for characterizing the silicon-sapphire interface and for

monitoring changes at or near the interface

resulting from high-temperature processing

steps and high-energy irradiation in connection with other projects. In these studies

changes in the charge density at the siliconsapphire interface due to oxidation and an- nealing steps and to electron irradiation were observed directly. (A. M. Goodman")

32

Work performed at RCA Laboratories under

NBS Contract No. 5-35912. NBS contact for

additional information: P. Y. Koyama.
6 . MATERIALS A fl D PROCEDURES

FOR WAFER P R 0 C E S S I [J G

6.1. Ion Implantation Parameters

This task was undertaken to develop and disseminate to the semiconductor device and integrated circuit industry practical information and experimental data to assist in improving the design and fabrication of ionimplanted doping profiles. Some of the

important aspects of profile control still

needed are accurate knowledge and control of

dose and of ion range and straggle (or standard deviation) . This control of implanted

doping profiles in production is important

because implantation is known to be capable

of much greater control of doping profiles

than diffusion, and it is possible to con- struct implanted profiles which cannot be

obtained by diffusion. However, this potential superiority of implantation can be lost

to the practical world of device and circuit

fabrication unless adequate knowledge and

control of dose and range and straggle are provided. Initial measurements have been

made of range and range straggle of boron

and phosphorus implanted into 100 Q'cm silicon of the opposite conducting type in a production-type implantation system. Doses were approximately 1.5 x 10^^ cm~^ and implantation energies were varied from a lower

limit that gives a range of about 150 nm to an upper limit of 600 keV. After implantation, the wafers were annealed at 900°C for

30 min in argon. The results for profiles

implanted with the beam tilted 7 deg from the

<100> direction generally agreed with the results of calculations [29,43] based on LSS

theory

.

6.2. Passivation Integrity

This task was undertaken to develop techniques for evaluating the integrity of passivation overcoats on metallized integrated

circuits, specifically practical techniques

to detect localized structural defects and

to measure their population density. The

techniques are intended to be suitable for

routine quality control by manufacturers, to be applicable both to IC devices in wafer

form and to individual pellets, and to allow

estimation or quantization of the number of

localized structural defects (such as pinholes and microcracks) per unit area in the

oxide, glass, or nitride overcoat. Essentially nondestructive methods, which do not damage areas that are defect-free, are preferred .

Three types of promising methods are being

explored, developed, and assessed to allow

comparison: (1) selective chemical etching

to visualize defects, (2) electrophoretic

defect decoration, and (3) electrostatic

corona charge decoration of defects. In the

initial phase of the work test structures

have been designed and fabricated, specialized instrumental equipment has been set up, and investigation of the three methods has

been started.

T T (W. Kern' and R. B. Comizzoli )

The range, R^, and range straggle, AR^ , were

determined from measurements of the dopant

profile made on reverse-biased Schottky barrier diodes by means of an automatic capacitance-voltage technique described by Gordon

et al. [44]. Schottky barriers were employed

because with them contact can be made to a wafer without disturbing the implanted profile. The barrier is formed by evaporating

a metal film (aluminum for p-type implanted

surfaces and gold for n-type) about 100 nm

thick through a metal mask which defines dots

varying in size from 0.1 to 0.15 mm in diameter on the front (im^planted) surface of the

wafer. A low resistance contact is made on

the backside of the wafer by low energy implantation of a suitable dopant.

(R. G. Wilson'"')

Work performed at Hughes Research Laboratories under NBS Contract No. 5-35891. NBS

contact for additional information: D. C. Lewis

.

^Work performed at RCA Laboratories under

NBS Contract No. 5-35913. NBS contact for

additional information: T. F. Leedy.

33
7 . P H 0 T 0 L I T H 0 G R A P n Y

7.1. Optical Imaging for Photomask Metrology

Line widths in the micrometer range are com- monly measured with a microscope equipped

with a micrometer eyepiece. In this measurement it is not the physical object or geometrical line width which is measured, but instead it is the magnified image of the line.

Therefore, the accuracy of these measurements

is strongly dependent on the quality of the

line image. This quality depends on the aberrations of the imaging lens, the focus or amount of defocus present, and the spatial

coherence of the light [45]. The spatial

coherence is a measure of the phase correlation of the light in a plane perpendicular

to the direction of propagation. The degree

of correlation is a measure of the ability

of the light to manifest interference effects. It ranges from a value of 1 for co- herent light where the phase at each point

in the perpendicular plane is correlated

with the phase at any other point and interference occurs, to a value of 0 for incoherent light where the phase at any point is completely independent and interference is

absent. The degree of correlation is a function of the type of illumination, the

path length, and the numerical aperture of

the focusing lens element in the optical

system.

A theoretical investigation was begun to determine quantitatively the effects of

coherence on the image of two parallel opaque

lines on a transparent background and its

inverse. This is the simplest statement of

the problem that must be solved to determine

the effects of coherence on line width measurements. The first part of this investigation considers a single line and a diffraction limited lens system. It addresses two questions

:

What is the minimum size of a single opaque

line on a transparent background that can

be recognizably imaged as a line at any

given quality level? and

How can this quality level be specified?

mally written [46] for the incoherent case

as

:

=j^h(x,

"it ^ y) h (x,y) U (x,y) U (x,y) dxdy,

6 g

and for the coherent case as:

I = l//h(x,y) U (x,y) dxdy

where U (x,y) is the complex amplitude of

radiation from the object, and h(x,y) is the

complex response of the optical system.

Expressions for these functions were derived

with the assumption that the lens was diffraction-limited with an impulse response of

sin x/x for the one dimensional case. The

characteristic width of this function, taken

to be the distance between the first node

points on each side of the line x = 0,

is a measure of the smallest dimension that can be resolved by the lens. The expressions

for the intensity profiles for both illumination cases were derived in terms of the

ratio. A, of object line width, 2b, to the

width of the impulse response; the numerical

aperature, NA; the wavelength of the light,

X; the magnification of the lens, m; and the

image coordinate, 6.

The expression for the image intensity profile for the coherent case is:

I

Si[2^(5 + |)]- Si[2^(C - |)]^

and for the incoherent case is:

KC) = 1 - )] - Si[4^(C - f)]

l-cos[4^(C + |)] 1-cos[4tt(C - |)])

4Tr(C + |) 4^(C -

The first of these questions was approached

by calculating the intensity profile of the

image of an ideal line for both the case of

spatially incoherent light and the case of

spatially coherent light. Here an ideal

opaque line is defined as having zero percent

transmittance across its width and a 100 percent transmittance elsewhere. The intensity

profiles of the image of an object are forv.here C = 6 NA/Am and Si[x] =JJ^ (sin v/v)dv.

These functions are plotted in figure 19 as

a function of the parameter E, for the indicated values of A. In all calculations the

wavelength was taken as 500 nm. For illustrative purposes this figure is divided ver- tically through the center, with the edges

of the ideal image intensity profile (the

34
PHOTOLITHOGRAPHY

image profile that is an exact magnification

of the object profile) located at ^ = ±1.0.

The intensity profiles for incoherent light

are plotted on the left side of the figure

and the profiles for coherent light on the

right side. Since the profiles are symmetrical about the center of the figure, the

complete line profile may be visualized by

mirroring any particular profile through the

figure center. The profiles I-l and C-1 in

the figure show a badly degraded image and

are indicative of the images that result when

a line whose width is somewhat smaller than

the resolution limit of the lens is imaged.

The other curves of this figure represent

image intensity profiles for wider lines and

illustrate the image edge profiles becoming

sharper as the line widths become appreciably

larger than the impulse response of the lens.

With the exception

in figure 19, all o files cross the ide

the same point for

For the incoherent

proximately 50 perc

is a relatively eas tinguish visually,

tensity profiles fo

of the curves I-l and C-1

f the image intensity pro- al image edge at nearly

each illumination case, case this point is at apent transmittance . This

y intensity level to disHowever, the image inr the coherent case cross

the ideal image edge at 25 percent transmittance, and this cannot be consistently detected visually.

Which profiles are actually present when measuring a line width with a microscope? It

is not uncommon to observe "ringing" near the

line image edge in a microscope (bright and

dark bands near the image edge) . Such "ringing" is shown for the case of coherent illumination by the oscillations of the image intensity profiles about 100 percent transmittance, while this phenomenon is absent for

the incoherent case. Thus, it may be con- cluded that an appreciable amount of illumination coherence is present under normal

microscope operating conditions.

To establish a quantitative measure of the

image quality, the RMS differences between

the spectra of the image intensity profiles

and the ideal intensity profiles were evaluated. This difference is arbitrarily defined

as

D

= {f[I (01 - F[I^(0]PdC

where F[I^(C)] is the spectrum of the ideal

Figure 19 . Image intensity profiles of an opaque line on a transparent background for

incoherent and coherent illuniination . (See test for discussion of curves.)

35
PHOTOLITHOGRAPHY Table 5 - Object Line Widths and Corresponding RMS Differences

Determined from the Curves of Figure 19.

Curve A % RMS Object Line Width for NA of

0.25 0.5 0.65 0.85

Coherent Illumination:

C-1 0.4 10 0.7 ym 0.4 ym 0.3 pm 0.2 pm C-2 2.2 5 4.4 2.2 1.7 1.3

C-3 6.3 2 12.5 6.3 4.8 3.7

C-4 12.3 1 24.7 12.3 9.5 7.2

Incoherent Illumination:

I-l 0 4 10 0 9 pm 0 4 pm 0 3 pm 0.2 pm

1-2 1 5 5 2 9 1 5 1 1 0.9

1-3 4 9 2 9 7 4 9 3 7 2.9

1-4 9 8 1 19 5 9 8 6 3 5.7

Impulse Response Width:

2.0 pm 1 .0 pm 0.76 pm 0.59 pm

image intensity profile, F[I^(5)], is the

spectrum of the calculated or actual image

profile, and P is the interval of integration.

The spectrum of the image, F[I(5)], is a function in the spatial frequency domain

(line cycles per unit length) . Since a lens

forms an image by transmitting information

of different spatial frequencies, a comparison of the spectrum of the image with that of

the object should provide a quantitative measure of the image quality. Values of the

percent RMS differences are listed in table 5 as functions of values of the parameter A. For each of several numerical apertures com- monly found in microscope objectives, table

5 also lists the approximate width of the impulse response and the corresponding object

line widths. (R. E. Swing")

An investigation of the degree of coherence

of the illumination at the object plane of a microscope with diffraction limited lenses

and its effect on the intensity profile of

a single edge was begun. The degree of coherence is a function of the illumination source

and of both the condenser and objective nu- merical apertures. Light produced by a large

incandescent source and focused by a lens is

NBS Optics and Micrometrology Section,

Mechanics Division.

completely incoherent only when the sourceto-lens distance is very small and the angle

from the source subtended by the lens diameter is very large. As the source-to-lens

distance increases, the coherence of the

light increases. The degree of coherence of

the illiominatlon affects the shape of the

image intensity profile and its displacement

from the step function edge position that

would be predicted by geometric optical con- siderations only.

Figure 20 (after Welford [47]) compares several theoretically calculated edge image intensity profiles. Curve A represents the

edge image intensity profile for a large incandescent source and uniformly filled,

matched condenser and objective lenses with

numerical apertures of 0.65. Curve B represents the profile for incoherent illumination corresponding to an infinitely large

condenser aperture. Curve C represents the

profile for coherent illumination corresponding to an infinitely small condenser aperture

The ideal edge is shoxm by Curve D. These

curves are all normalized to converge to the

100 percent intensity level at large distances from the edge.

Several phenomena are illustrated in this

figure. The edge image intensity profile for

incoherent illumination (Curve B) has zero displacement from the ideal edge position at the 50 percent intensity level. This is the

36
PHOTOLITHOGRAPHY

-0.8 -0.6 -0.4 -0.2 0 0 2 0.4 0.6 0.8 DISTANCE [pm]

Figure 20. Image intensity profiles of a single edge viev/ed by light of wavelength

A = 500 nm. (A, matched numerical apertures

of 0.65; B, incoherent illumination; C, co- herent illumination; D, ideal edge.)

intensity level at which the edge is usually

located by visual observations. On the other

hand a displacement of 0.08 pm is observed at

the 50 percent intensity level for the edge

profile obtained for matched numerical apertures (curve A) . The displacement distance,

X, from the ideal edge may be found for

pairs of matched apertures from the relation

^ 0.65

^ NA ""f

' where is the x-coordinate appropriate to curve A in figure 20.

Ringing is shown in curve A by the oscillations between the 90 and 100 percent intensity levels. These appear in the microscope

image as alternate light and dark bands parallel to the edge image. This observed ringing indicates that the illumination has a high degree of coherence. Also, the edge

intensity profile produced with matched nu- merical apertures is closer to the intensity

profile for coherent illumination than it is

to the intensity profile for incoherent illumination.

Focus affects the shape of the edge intensity

profile and is subjectively determined by

selecting the object-to-lens distance which

produces the sharpest image. Theoretical

calculations of the effect of defocusing on intensity profiles were made for coherent illumination. The amount of defocus, A, is the

longitudinal displacement of the actual object

plane from the object plane predicted by geo181 \ 1 r

1,6 -

1.4 -

-1 0 0 1.0 2,0 3.0 X|NA|/X

Figure 21. Image intensity profiles of a single edge viewed by coherent illumination

with varying defocus. (A, no defocus; B, h\ defocus; c, lA defocus; D, ideal edge.)

metric optics (the geometric focus) . In making the calculations it is more convenient to consider the quantity nA , the maximum deviation, in multiples of wavelength, of the actual wave front from the ideal spherical V7ave front, which is related to A by

nA = A (NA)2/2.

Figure 21 shows image intensity profiles for

various amounts of defocus. It may be seen from this figure that, due to the ringing,

the contrast between the edge and the first

dark band is greater for small amounts of

defocus than for the proper focus. Consequently, an observer may easily choose a defocused microscope setting as the correct or best focus because of the Increased sharpness produced by a small amount of defocusing. If this should occur, the location of

the edge would be chosen from, an image intensity profile that Is appreciably degraded and

would be displaced significantly from the

geometric image. The tendency for an ob- server to choose an incorrect focus and degraded edge profile can be further appreciated by noting that a lA defocus for A = 0.5 um and an objective lens with NA = 0.65

corresponds to a defocus displacement A of

only about 2.4 ym (or about 0.1 mil).

The image quality is also degraded by spherical aberration of the objective lens. This

aberration causes light rays passing through

the outer portions of the lens to be focused

at a different distance from the lens than

37
PHOTOLITHOGRAPHY

Figure 22. Image intensity profiles of a single edge viewed by coherent illumination

with IX spherical aberration and varying de- focus. (A, no defocus; B, -lA defocus; C,

-2X defocus; D, ideal edge.)

lOOr

- 50-

DISTANCE

a. Image intensity profiles: A, single

edge as viewed by incoherent illumination;

B, ideal edge; C, filar eyepiece crosshair.

BACKGROUND LINE ioor

DISTANCE

rays passing through the center portions of

the lens and is present to some degree in all

optical systems. Spherical aberration is

greater when microscope objectives are used

at the wrong tube length or when objectives

and eyepieces of different types or manufacture are used together. The effects of this

aberration may be partially compensated by

Introducing an equivalent amount of defocus;

when this is done a sharper image is produced. Spherical aberration is also described in terms of nA , where nA now refers

to the maximum difference, in multiples of

wavelength, between the aberrated wavefront

and the ideal spherical wavefront produced

by a diffraction limited lens. Figure 22 shows calculated edge image intensity profiles for coherent illumination and an objective lens with a spherical aberration

of lA and different focus settings. This

figure shows that a sharp image intensity

profile is produced when the amount of defocus suitably compensates the spherical

aberration; this focus is usually called the

best focus. This figure also shows that the

contrast between the edge image and the bands

produced by ringing increases as the defocus

is increased further. This Illustrates the

difficulty of visually choosing the correct

b. Superposition of the edge and filar

eyepiece crosshair im.age intensity profiles

showing the apparent location of the edge

(A) and the ideal edge (B) as the crosshair

is traversed from a dark region to a bright

region

.

100

LINE BACKGRDUND

DISTANCE

c. Superposition of the edge and filar

eyepiece crossnair image intensity profiles

showing the apparent location of the edge

(A) and the ideal edge (B) as the crosshair

is traversed from a bright region to a dark

region

.

Figure 23,

eyepiece

.

Line edge location with a filar

38
PHOTOLITHOGRAPHY

focus because of the temptation to adjust

the microscope for the highest contrast.

These edge image intensity profiles were

used in a preliminary analysis of line width

measurement errors that one might expect when

using microscopes equipped with a filar or image shearing eyepiece or using equipment

that locates the edge by electronically

processing the edge image intensity profile.

Calculations were made for a transparent line

on an opaque background, viewed by 0.55-ym

transmitted light. Image intensity profiles

for incoherent illumination, coherent illumination, matched numerical apertures, defocus

with coherent illumination, and spherical

aberration with defocus and coherent illumination were considered. The optical system

consisted of a 12. 5X eyepiece and a 40X, 0.65

numerical aperture objective. Since the

model is based on the image intensity profile

of a single edge, the results are applicable

only for lines 20 ym or more wide. For nar- rower lines viewed with this system, it may

not be assumed with confidence that the diffraction effects occurring at one edge do not interfere with those at the other edge.

Consider first measurements with a filar eyepiece. The intensity profile of an ideal edge

for incoherent illumination and the image

intensity profile that results in a filar

eyepiece with the cross hair located in the

image plane are illustrated in figure 23a.

When the cross hair is traversed from

the dark region to the bright region, the

edge position is taken as the center of the cross hair image when the edge of the cross hair nearer to the bright region just begins

to cover the bright region as illustrated in

figure 23b. For illustrative purposes, the

threshold visibility point is arbitrarily

taken at the 10-percent intensity level.

Thus, the edge is located by the filar eyepiece at the value of the coordinate corres- ponding to the 10-percent intensity level

minus one-half the cross hair width. When

the cross hair is traversed from the bright

region to the dark region, a bright line begins to appear on the dark side of the cross hair image as shown in figure 23c. The edge

position is taken as the position of the center of the cross hair image when this bright

line is just no longer visible. In this case

the edge is also located by the filar eyepiece at the value of the coordinate at the

10-percent intensity level minus one-half the cross hair width. If, when the measurement

is made, the image of the cross hair is tra- versed across the line in one direction (as

is usually the case) , the measured line width

is equal to the difference of the coordinates

of the two 10-percent intensity level positions .

An image shearing micrometer eyepiece positions the images of the two edges of a line

side by side so that one of the edge images

can be traversed across the other. The image

intensity profiles of a left and right edge

with the ideal edge located at the 50 percent

intensity level are shown in figure 24a for

the case of ideal symmetric edges. As these

two images are superimposed by the image

shearing eyepiece, the intensity profiles

add. As the images cross, the bright region between them (1) narrows to a bright

line, as shown in figure 24b, which corresponds to the two edge profiles crossing

at the 75-percent intensity levels, (2) disappears when the two edge profiles cross at

the 50-percent intensity levels, and (3)

finally changes to a narrow dark line, as shown in figure 25c, which corresponds to the two edge profiles crossing at the 25-

percent intensity levels. The line edge location is generally picked when the two edge

images touch with neither a bright line nor

a dark line existing between the two images. For symmetric edges this criterion results

in locating the edge at the 50-percent intensity level and no measurement error results

if the illumination is incoherent. If the

ideal edge location corresponds to some other intensity level, as would be the case

for coherent or partially coherent illumination, there will be some error in locating

the edge.

Very explicit criteria may be applied consistently to determine the edge location with

electronic processing and analysis of the

image intensity profile. One of these criteria is to locate the edge at the 50-percent image intensity level which results

in locating the edge at the same position as

the image shearing micrometer eyepiece.

Another of these criteria is to locate the

edge at the position of maximum image intensity profile slope. This criterion results

in locating the edge position at the 50-percent intensity level only for the case of

incoherent illumination.

Results of calculations of measurement error, based on the above criteria, are listed in

table 6. The apparent width is greater than

the actual width when the error is positive

and smaller when the error is negative. If the illumination is reflected rather than

39
PHOTOLITHOGRAPHY

Table 6 — Line Width Measurement Errors for a Transparent Line (20 ym or Wider)

on an Opaque Background Viewed with 550 nm Illumination

Illumination Condition

Image Shearing

Filar Eyepiece or Maximum

Eyepiece 50 percent Slope

Intensity Level

Incoherent Illumination — Geometric Focus +0 56 ym 0. 0 ym 0.0 ym

Matched Numerical Apertures — Geometric Focus +0 27 -0. 17 -0.23

Coherent Illumination — Geometric Focus +0 16 -0. 19 -0.25 — ±0.65 ym from Geometric Focus (+x/4 Defocus) +0 25 -0. 34 -0.46 — ±1.3 ym from Geometric Focus (±x/2 Defocus) +0 59 -0, 66 -1.00 - +X/4 Spherical Aberration +0 17 -0. 14 -0.28 — +X/4 Spherical Aberration, -A/4 Defocus +0 14 -0. 17 -0.30 — +X/4 Spherical Aberration, -A/2 Defocus +0 20 -0. 09 -0.30 — +A/2 Spherical Aberration +0 16 -0. 19 -0.25 - +A/2 Spherical Aberration, -A/2 Defocus +0 30 +0. 02 -0.68 — +A/2 Spherical Aberration, -A Defocus +0 41 +0. 14 -0.97

BACKGROUND LINE BACKGROUND

a. Image intensity profiles of opposite line edges viewed with incoherent illumination

illustrating the images prior to superpositioning them over each other with the image

shearing eyepiece: A, left edge; B, right edge; Cj, left ideal edge; Cz, right ideal edge.

DISTANCE b. Overlap of image intensity profiJes A and

B with a bright band between the two light

images corresponding to locating the line

edge at the 75-percent intensity level.

160

140

120

100

80

60

40

DISTANCE c. Overlap of image intensity profiles A and B with a dark band between the two light

images corresponding to locating the line

edge at the 25-percent intensity level.

Figure 24. Line edge location with an image shearing micrometer eyepiece.

40
PHOTOLITHOGRAPHY

transmitted or if the line is opaque on a transparent background, the sign of the error

is reversed.

It should be stressed that these results are

for the case of wide lines where the two edges may be treated independently. It should

also be noted that in practice higher numerical aperture objectives are frequently used

in line-width measurements although they are not used to full capacity due to a lower nu- merical aperture of the condenser. At best

they are illuminated non-uniformly which

reduces their resolving power. This may also

be true of the 0.65 numerical aperture con- sidered here. Many other factors which affect line-width measurement by narrowing or broadening the line image were not considered.

These factors include the properties of the

photomask materials, electronic clipping and

edge enhancement, and the variability of the

human eye. The method of calibration of the

measuring instrument also was not considered.

(D. Nyyssonen '

)

230

A—

5 —

»

^30j „ 60 Jl 60

f^tn] P /inn ^ | /im

5

B —

-lOfim

T

30 /Am

60

n 50

*~/im ~^

\ K-5/xni

30«,m

-10/Am

30 1 1 60_

fitn fim mm 3 0M^m

Figure 25. Interim chromium-on-glass linewidth measurement artifact with transparent

lines on an opaque background. (All dimensions are nominal, ±0.25 pm .

)

7.2. Calibration Standards for Photomask

Metrol ogy

Attempts to fabricate the previously described artifact (NBS Spec. Publ. 400-17,

pp. 37-38), intended for use in establishing standards for line width measurements in

the 1- to 10-pm range, have been thus far un- successful. This measurement artifact was to resemble a hard surface photomask. The pattern was to be etched in a chromium film deposited on a glass substrate coated with a tin oxide film. The tin oxide film is optically transparent and was to provide a con- ductive substrate in order to enable the

artifact to be used in a scanning electron

microscope (SEM) . On every attempt to fabricate this artifact, a reaction occurred between the chromium and tin oxide resulting in

an unacceptable loss of opacity in the chromium. The reaction has not been identified.

An alternative approach to producing a measurement artifact was evaluated. A mask of

the design shown in figure 25 consisting of

transparent lines on an opaque background was

obtained. These lines were scribed on a glass plate coated with chromium to a thickness of 50 to 100 nm. The patterned artifact

was then overcoated with approximately 50 nm

of carbon to enable its use in the SEM. This

carbon layer does not affect the transparency

NBS Optical Physics Division

of the artifact and provides the necessary

conductive coating.

(J. M. Jerke" and D. B. Novotny)

7.3. Line-Width Measurements

A series of measurements was made on the dimensions A, B, and C on the artifact illustrated in figure 25. These measurements were made to obtain additional data to assist in

developing a basis for comparison of measurements made with microscopes equipped with

different eyepieces. All of these measurements were made using a narrow-pass filter with

the band center at 486 nm. The center-tocenter spacing between two lines (dimension

C) was measured on the NBS line-standard interferometer [48] as 34.924 ± 0.0025 ym. The

uncertainty in this measurement is the 3-

sigma value and represents the range in which

99 percent of the mean values of any set of measurem.ents of this dimension will fall when

made with the NBS line-standard interferometer. The results of the measurements made

with various microscope eyepiece combinations

are listed in table 7; the uncertainties re- present the sample standard deviation obtained

for each set of measurements. The measurements were normalized to the mean value of

dimension C. The standard deviation associated with dimension C represents calibration

uncertainty.

41
PHOTOLITHOGRAPHY

Table 7 — Measured Values for Dimensions A, B, and C of Interim Measurement Artifact

Illumination

Filar Eyepiece Image Shearing Eyepiece

Condition

Transmitted Light,

Bright Field 5 25±0 09 5 22±0 04 34 92±0 11 4 87±0 06 4 88+0 02 34 92+0 03 Reflected Light,

Bright Field 4 94±0 92 4 92±0 01 34 92±0 02 5 11±0 03 5 09+0 04 34 92±0 09

Reflected Light,

Dark Field 5 09±0 09 5 15±0 06 34 92±0 07 5 27±0 10 5 24±0 10 34 92±0 19

S M

a. Photograph of the interferometer

mounted on a scanning electron microscope

stage

.

b. Schematic drawing of the configuration

of the interferometer and associated optical

components

.

A Beam-splitting polarizing cube M Stage mirror

B Corner cube P. Prisms

C. Quarter-wave retardation plates -r ^

' ^ S Test specimen

Figure 26. Polarizing interferometer.

42
PHOTOLITHOGRAPHY

Differences between values measured with filar

and image shearing eyepieces for the brightfield, transmitted-light case agree in sign

with the predictions of the calculations sum- marized in table 6 for matched numerical a- pertures. The differences for the brightfield, ref lected-light case are opposite in

sign as predicted from the discussion of

section 7.1. Caution, however, should be

used in comparing the differences in table

7 with the errors predicted in table 6. The

model used to predict the errors assumed no interaction between the two edge profiles,

an assumption probably not valid for S-ym

wide lines.

(F. W, Rosberry" and D. B. Ncvotny)

A polarizing interferometer with a 1 nm displacement resolution was constructed to make

in-situ measurements of the movement of a scanning electron microscope (SEM) stage

without interference from subtle changes in

the optical path length caused by flexing of

the vacuum system walls. A photograph of

^ this interferometer mounted on an SEM stage

i is shown in figure 26; the configuration of

j its components is also shown schematically

' in the figure. The principles of operation

of this interferometer are similar to those

previously described (NBS Spec. Publ. 400-17,

p. 36). The relative sizes of the interferj ometer and other components shown in the

photograph may be estimated by comparing them

with the diameter of the specimen, S, which

is 12 mm. The interferometer consists of one beam-splitting polarizing cube. A; one corner cube, B; three quarter-wave retardation plates, (the one at the bottom of the

interferometer is not shown) ; and a reference

mirror cemented to one of the retardation

plates, C2. The path of the laser beam is

shown by a dashed line. The beam enters the

SEM chamber through a side port and is directed to the interferometer and to the stage

mirror, M, by the system of prisms, P^.

This circuitous beam path was necessary to minimize the blockage of secondary electrons

by the interferometer and optical components.

Failure to minimize this blockage was shown

to result in an unacceptably low signal-tonoise ratio. The positions of the interferometer and its components were determined by

placing objects simulating their sizes into

the SEM chamber in various possible alignment

configurations. All of the configurations

which lent themselves to easy optical alignments blocked 80 to 90 percent of the emitted

secondary electrons which would normally be

detected. The illustrated configuration results in a decrease of only 20 percent in the

normal signal-to-noise ratio.

(A. W. Hartman*)

*NBS Optics and Micrometrology Section,

Mechanics Division.

43
8 . TEST PATTERNS

8.1. Sheet Resistor Test Structures

The equation for the sheet resistance of a symmetrical van der Pauw structure [49]

R (vdP) =

s _n_ AV

Inl I' (10)

where I is the current and AV is the measured

potential difference, assumes infinitely nar- row contacts along the perimeter of the device. In the past this assumption was usually appropriate since most van der Pauw test structures could be designed with large body

areas and small contact arms. However, the

need to measure the sheet resistance of very

small devices produces an uncertainty in the

validity of this assumption since the width

of the contact arms must be a significant

fraction of the perimeter of the device.

A computer program was developed to solve for

the error in measuring the sheet resistance

of a van der Pauw structure designed with

orthogonal boundaries and finite size contacts. This program uses a nine-point finite

difference approximation to Laplace's equation and a successive over-relaxation solution [50]. This solution is used to set up

the indefinite admittance miatrix for a four

terminal network which is solved for the van

der Pauw voltage [51]. Checks for residue

oscillation are made and the over-relaxation

factor adjusted accordingly. The program

starts with a coarse grid which is refined

in steps until the desired accuracy is obtained. At present this program can operate

on several geom.etries ranging from a square

to a cross, including all orthogonal "pinwheels" having rotational symmetry.

metric, orthogonal quadrate crosses in figure

30. These figures can also be used to obtain

the true sheet resistance from

R (true) =

s

R (vdP)

s

1 - E This analysis demonstrates that many novel

van der Pauw designs are possible. For example, a cross constructed with minimum process stripe widths (D/S = A/S = 1.0) is found

to have negligible error. (J. M. David)

8.2. Charge-Coupled Device Test Pattern

This study was undertaken to investigate the

applicability of the charge-coupled device

(CCD) as a test structure for use in semiconductor process control. The study was concluded with additional correlation of

parameters as measured from various CCD

structures and those measured using more con- ventional structures such as MOS capacitors,

MOS transistors, and gated diodes.

Additional measurements were made of oxide

thickness, dopant density, flat band voltage,

and mobile charge density on two wafers using

both the 32-bit and 128-bit circular CCDs

(operated as capacitors) and various MOS ca- pacitors (NBS Spec. Publ. 400-17, pp. 28-31).

The results are summarized in table 8.

Additional measurements of transistor parameters were also made on a wafer using several MOS field effect transistors and the 32-

bit and 128-bit circular CCDs operated as transistors (NBS Spec. Pubis. 400-12, pp. 23,

25, and 400-17, pp. 30-31). The results of

these measurements are summarized in table 9. A typical van der Pauw sheet resistor of this

type is shown in figure 27. This structure

is included in test pattern NBS-3 (NBS Spec.

Publ. 400-12, pp. 19-22). This geometry was modeled for the computer program by replacing the extended pad area and metallized contacts by shorts at the ends of the contact

arms as sho^«7n in figure 28. This represents

the worst case condition.

The normalized error

E = [R (true)

s R (vdP)]/R (true),s s

'

where R (true) is the true sheet resistance

and R (vdP) is the sheet resistance obtained

s from eq (10), is plotted for symmetric, or- thogonal pinwheels in figure 29 and for symSurface state densities were determined on gated diodes by measuring the leakage current

as a function of gate voltage. When this is

done the fraction of the leakage current due

to the surface states can be extracted. This

fraction, I , is given by

gen,s

I gen.s

qA n . s 12,

s 1 o

where q is the electronic charge, A^ is the area of the gate, n^ is the intrinsic carrier density, and s is the surface recombio nation velocity which is given by

s^ = av^^.(kT/q)N^^,

44
TEST PATTERNS

Figure 27. Schematic diagram of orthogonal

pinwheel van der Pauw sheet resistor Figure 28. Equivalent geometrical model of

structure. orthogonal pinwheel.

45
TEST PATTERNS

Table 8 — MOS Capacitor Parameters

Wafer Device

Thin Thick Substrate

Cell Oxide Oxide Dopant

No. Thickness, Thickness, Density,

nm nm cin"^

Flat-Band

Voltage,

V

Mobi le Charge

Densi ty,

cm"^

I

CCD 242 MOSCAP 13 110 8±15.1 (1.74± )xl0i5 -1 00±0 12 (3.1±2.4)xl0l0

MOSCAP 20 112 5±14.0 1067±126 (1.45± )xlOi5 -0 99±0 10 (2.9±0.9)xl0i°

MOSCAP 21 1078±141 (1 .52±0.45)xl0i5 -2 16±0 33

32-bit CCD 2 109 7±20.2 1026±226 (3.8±3.7)xl0i5 -1 54±0 26 (5.6±3.4)xlOio

128-bit CCD 1 109 2±20.0 1131±183 (4.9±3.0)xl0i5 -1 30±0 07 (7.2+3.2)xlOio

CCD 245 MOSCAP 13 115 2±9.5 (1 .43±0.01 )xl0i5 -1 .00±0 06 (1 .9±1 .1 )xlOio

32-bit CCD 2 105 2±5.2 910±47 (5.0±0.9)xl0i5 -1 .12±0 15 (4.5±3.2)xlOio

128-bit CCD 1 111 8±19.1 (4.2±2.0)xl0i5 -1 .44±0 20 (1 .l±0.5)xlOio

Table 9 - MOS Transistor Parameters

Wafer Device Cell

No.

VI /I

Linear Region Saturation Region

MOSFET 26 31 1 -0 088±0.001 1080±62 -0 203±0.001 856+77

MOSFET 23 3 39 -0 143±0.002 1072±59 -0 208+0.001 792±65

MOSFET 28 1 55 -0 lOOtO.OOl 1054+24 -0 159±0.001 672±94

23-bit CCD 2 0 0080 -0 35±0.26 1673±181 -0 69±0.19 676±257

128-bit CCD 1 0 0029 -0 25+0.03 1768±286 -0 50±0.20 544±100

46
TEST PATTERNS

where a is the capture cross section for minority carriers (taken as 10-'^ cm~^ [521),

V , is the thermal velocity of minority earth

riers (taken as 10 cm/s at room temperature), k is Boltzmann's constant, T is the

absolute temperature, and N^^ is the number

of generation-recombination centers per unit area per electron volt near the surface of

the silicon (surface state density) . Values

for N measured on four gated diodes on

ss wafer 242 ranged from 1.4 x 10 1° to 2.1 x

10^'' cm~^'eV~'^. These results were compared

with measurements of surface state density

by the double-pulse, charge transfer loss

method (NBS Spec. Publ. 400-12, p. 25) [53] on a sampling of 128-bit circular CCD's on

the same wafer which yielded value between

1.4 X 10l° and 8.8 x lO^O cm-^-eV"! for N . ss

(I. Lagnado")

8.3. Test Pattern Design and Analysis for

SOS/LSI

This task was undertaken to produce a set of

overlay masks containing test devices and structures suitable for characterizing a large scale integration (LSI), silicon-onsapphire (SOS) process (or other mask com- patible processes) and to develop testing

procedures for these structures.

The pattern is comprised of five classes of test structures, each intended to test a different aspect:

1. structures to test design and process parameters,

2. structures to test spatial variations ,

3. structures to test geometrical variations ,

4. redundant arrays to test series

strings of transistors, crossovers,

and contact openings, and

5. circuit elements, and dimensional

and alignment structures.

(or equivalent) was chosen to be pads 4 mils

(0.1 mm) on a side, minimum, on 6-mil

(0.15-mm) centers, minimum. These dimensions

have been used previously and cause little

problem during automatic probing. This

choice of pad center and size has a first

order effect on the overall pattern size.

The type 2 pattern consists of four devices:

1, Probe resistance test and coding

function,

2, n-channel MOS transistor, 0.3 mil

by 0.6 mil (8 pm by 15 pm)

,

3, p-channel MOS transistor, 0.3 mil

V. by 0.6 mil (8 ym by 15 ym) , and

4, MOS capacitor with both types of

diffusion.

The first device consists of two pads shortcircuited together. It serves to test both

the presence of a suitable pattern and the

resistance of the contacting probes. Information regarding the wear of probe tips and

variation in probe resistance across a wafer

can be obtained from this structure The

next two devices provide high information

density and were chosen with nominal dimensions. These dimensions are flexible and may

be adjusted to suit particular conditions of

greatest interest. The fourth device is an MOS capacitor. Since it has both types of

peripheral diffusion, the type of starting

material is irrelevant, and both sides of the

C-V curve are available. It would be desirable to use more devices but the pattern size

with only these devices is approximately 40

mils (1 mm) long and 6 mils (0.15 mm) wide.

This allows a maximum of 40 devices of each

type across the pellet in either direction.

Consideration is being given to adding an- other capacitor to allow for the possibility

of two types of starting material or to ex- amine the so-called "well" in bulk CMOS processes. This would add approximately 7 mils

(0.18 mm) to the length. ^ (W. E. Ham' and J. M. David)

Classes 1, 3, 4, and 5 occupy quadrants of

the pattern while class 2 structures are ar- ranged in a cross between the quadrants.

The design of the type 2 pattern was completed. It was done first since it largely

determines the area remaining in the quadrants. The design rule for this and all

other patterns to be tested with probe cards

Work conducted at Naval Electronics Laboratory Center under NBS Order No. 502498. NBS contact for additional information: M. G. Buehler

.

'''vJork conducted at RCA Laboratories under

NBS Contract 5-35916.

47
9 . I N T E R C 0 fJ rj E C T I 0 fl R 0 N D HI G

9.1. Non-Destructive Test for Beam-Lead

Bonds

In earlier work on the acoustic emission test

on bonded beam-lead devices (NBS Spec. Publ.

400-12, pp. 31-32), the devices were stressed

by applying a downward force on the die. Although some tests were very successful in

detecting weakly bonded beam leads, the re- sults of other tests were more ambiguous.

Therefore, to establish a base line for measurement techniques and equipment sensitivity,

bonds made by bonding 0.001-in. (25-pm) diameter aluminum wire bonds to gold metallization on both ceramic and silicon substrates were monitored for acoustic emission during

pull testing. Emission from partial lift-ups

and pre-break cracks, comparable with the ex- pected emission from weak beams, was easily

detected. The actual wire-bond breaks registered much higher counts. To avoid any

ringing of the broken wire, the entire bond

system, including the pulling hook, was covered with a thick acoustic coupling compound

knoxm as "sticky goop." This damped all but

the acoustic emission from the direct break.

Since this emission was a single high-amplitude peak, it was more easily observed on an oscilloscope than on the normal acoustic

emission averaging counter.

In an effort to improve reproducibility and

sensitivity to the very weak acoustic emission from poorly bonded beam leads, different types of detector configurations and

extended frequency ranges were investigated. A special detector probe was prepared* to convert the force probe into a combined force

probe and detector as shown in figure 31. Preliminary measurements indicate that this

method of mounting the detector is more

effective than the conventional method of

mounting the detector under the substrate.

However, coincidence systems involving both

types of mounted detectors may be even more

sensitive than either one alone. The emission picked up from the die has higher frequency components than that through the sub- strate so that the detector electronics must

have higher frequency response. The best

measurements thus far have resulted from

stressing the device near the sides rather

than in the middle as previously described.

This suggested that a rocking or wobbling

force might be better than a constant one.

This probe was prepared by Seymour Edelman

,

NBS Bulk Properties Section, Polymers Division .

A combination wobble-detector-probe is

pictured in figure 32.

In the above work it was found that the

acoustic emission from poorly bonded beams

often consisted of a few short peaks above

the noise. These may not be observed in

typical acoustic-emission count totalizing

equipment, where the discriminator is typically adjusted for a background noise level

of 5 to 10 counts per interval. Therefore,

digital pulse capturing equipment has been

used to record the bursts which could then

be observed on an oscilloscope. Tunable

frequency filters also can be used to separate the signal from the noise, since the

signal tends to have a frequency appropriate

to the geometrical resonance of the chip or substrate, depending on the position of the

detector

.

The primary method of stressing beam-lead

devices has been to apply a force downward

onto the die. However, since it would be desirable to find a practical method of lifting the die upward, thus applying a direct

peel force on the beam-lead bonds as well as on the beam anchors , a vacuum chuck was made

to pull a bonded beam-lead device upward. A

silicon rubber chuck, molded with the imprint

of a 14-beam die, was bonded to the end of a hypodermic needle, and the needle hole was

cut through the rubber as shown in figure 33. This unit can replace the force probe detector on the force gauge in the test apparatus

(fig. 32). Tests with this apparatus yielded

a maximum pull force of about 50 mN (5 gf)

using house vacuum and about 60 mN (6 gf)

using a mechanical pump vacuum. This may be

sufficient to stress beams for acoustic

emission tests. However, because air leaks

generate noise at frequencies up to several

hundred kilohertz, use of the chuck must be

reevaluated in the 1 to 3 MHz range.

(G. G. Harman and W. A. Cullins)

Initial procedures for contaminating bonding

substrates to produce test specimens with one

or two weak beams sometim.es resulted in good

bonds. A new method was found to impair beam

lead bondability in a reliable manner. In this method, chrome-gold coated ceramic substrates are heated to 300°C for 2 h to diffuse the chrome to the surface where it oxidizes [54, 55]. The chrome oxide is removed

by etching with eerie ammonium nitrate in re- gions where good bonds are desired. Bonds

made to regions where the chrome oxide re- mains are weak and unreliable. Photomasks

48
INTERCONNECTION BONDING

A Acoustic waveguide and force probe

B Sensitive piezoelectric detector

C Adaptor coupling to force gauge

Figure 31. Acoustic emission detector and

force probe

.

A Force gauge

B Wobble control

C Acoustic waveguide and emission

detector

D Substrate with beam-lead devices

Figure 32. Combination wobble-force probe

and acoustic emission detector.

M

4 1 M

A Molded silicone rubber chuck for

gripping die

B Vacuum port

C Adaptor coupling to force gauge

Figure 33. Vacuum chuck for pulling beamlead devices .

4A

49
a. Depression in the substrate remaining

after beam was peeled back. The arrow denotes the largest broken welded area. Magnification: '\'580X.)

were designed to facilitate etch removal

of the chrome oxide in designated patterns.

An investigation was carried out to establish

that the chrome-oxide method of producing

unreliable bonds still left a few areas that were welded and therefore could produce valid

acoustic emission signals when the device

was stressed. Devices were bonded with various bonding schedules to substrates that had

chrome oxide on the surface. The devices were stressed with the wobble-force-detector

apparatus (fig. 32) and acoustic emission

signals were recorded on digital pulse capturing equipment. The oscilloscope trace of

a typical signal is shown in figure 34. The

poorly bonded beams were then peeled back

and examined for evidence of bonded areas and

acoustic emission point sources. Examination

of the beam-lead depression in the substrate

with a scanning electron microscope, revealed

tiny broken welded areas around the perimeter

as shown in the scanning electron micrograph

in figure 35a. The largest of these is indicated by the arrow. These broken welded areas

are also shown as tiny white dots in a circle near the perimeter in the scanning electron

micrograph of the beam itself in figure 35b.

The fact that the welded areas lie around the

perimeter is in agreement with deformation

theory of thermal compression bonding first

put forth by Tylecote [56]. (G. G. Harman)

b. Peeled beam lead. (Magnification;

^580X.)

Figure 35. Scanning electron micrographs of

a poorly bonded beam lead.

50
INTERCONNECTION BONDING

9.2. Wire Bond Pull and Shear Tests

The bond pull test and the bond shear test were compared in an experiment conducted

jointly with Sandia Laboratories to determine their significance for aluminum-aluminum ultrasonic bonds. Three groups of wire

bonds were made on an 800 nm thick aluminum

film electron-beam evaporated on an oxidized

silicon wafer. The aluminum film was patterned into squares 5.0 mil (0.127 mm) on a side on 10.0 mil ('0.254 mm) centers. Each

group consisted of approximately 40 loops of

1.0 mil (25 ym) diameter aluminum wire ultrasonically bonded at each end. Different con- ditions were used in bonding each of the

groups so that the bonds in each group had a significantly different deformation. All of

the bond pairs were designed to break at the

heel of the first bond. Thus, the entire

welded portion of the bond remained on the

bonding pad after pull testing and the bond

could be subsequently shear tested. The

ratio of the loop height to the bond-to-bond

spacing was set to be 0.3 so that when the

bond pairs were pulled midway between the two bonds, the force in the direction of pull was

equal to the force in the wire. Therefore,

the pull strength and shear force data could

be compared directly.

First, half the bond pairs in each group were

tested by pulling in a direction normal to the substrate midway between the bonds.

Then, a newly developed precision shear tester [57] was used to obtain bond shear forces

on both the pulled and unpulled bonds.* The

results of these measurements are summarized

in figure 36.

These results may be understood if the metallurgical nature of the bond is considered.

For aluminum ultrasonic bonds the bond heel

becomes metallurgically overworked and weak

as the bond deformation increases, but the amount of the welded area increases. The

pull test is particularly sensitive to the

weakening of the bond heel but not to the

total weld area unless the latter is so small

that the bond lifts off during the test. Therefore, the pull force decreases as the

deformation increases.

LOW OEFORMAtlOK MEDIUM DEFORMATION HIGH DEFORMATION

I 4 t.8 2.8 DEFORMATION (wire diametersl

Figure 36 . Data from bond pull test and

bond shear test. (Pull test, shear test before pulling, ; shear test after pulling,

O ; error bars represent one sample standard

deviation above and below the mean strength

of approximately 20 bonds . )

mation increases, indicating that the amount

of welded area increases with deformation.

However, from the relatively larger error

bars in the shear test data it can be concluded that, even though the bond heel

strength in the pull test is uniform, the

actual welded area under each bond displays

a random variation. This is consistent with

previous work on the formation of ultrasonic

bonds [58].

No comparison was made between the pull test and the shear test on gold wire. However,

past work with ultrasonically bonded gold

wire [59] has shown that the bond pull test

is relatively less sensitive to bond and heel

deformation of gold wire bonds than to that

of aluminum wire bonds for deformations up to about three wire diameters. The shear

strength of gold wire bonds, however, would

be expected to increase with bond deformation

in the same way as for aluminum wire bonds.

(G. G. Harman and C. A. Cannon)

The shear test, on the other hand, is com- pletely independent of the condition of the

'' heel; it is sensitive only to the actual

amount of welding. As such, it has been used

to evaluate the quality of welding in thermal

compression bonds [57] and is a very valuable

research tool. In figure 36 it is apparent

that the shear force increases as the deforThis portion of the experiment was conducted

at Sandia Laboratories by Wayne Vyne

.

51
10. N E R fl E T I C I T Y

10.1. Helium Mass Spectrometer Method

Anomalously high values of indicated leak

rate were obtained from the second stage of

the interlaboratory evaluation of the helium

mass spectrometer leak detector method [60]

for testing fine capillary leaks in large

volume ('\^1.5 to 2 cm^) containers which is

being conducted in cooperation with ASTM

Committee F-1 on Electronics (NBS Spec. Publ.

400-4, p. 67). Experimental data obtained

with sealed-off capsules of approximately the same dimensions as the test leak specimens

indicated that the high, uniform values of

measured leak rate were due to initial permeation of helium into the borosilicate

glass matrix followed by diffusive outgassing after removal of pressurization (NBS

Spec. Publ. 400-12, pp. 33-34). A fixture

was devised to allow exposure only of the

tip area of the leak to the leak detector

which reduced background outgassing by about

a factor of ten; however, such manipulation increased the possibility of damaging the fine

capillary tip.

At the January meeting of the Hermeticity

Section of Committee F-1, it was decided to defer continuation of the interlaboratory

test until it could be determined whether a modified pressurization schedule could be

used to attenuate outgassing without causing severe degradation of leak testing sensitivity. The possibility of conducting the pressurization step at reduced temperature, to decrease diffusion of helium into the glass, was also discussed but rejected as being too awkward to be practical.

To evaluate the effects of modification of

the pressurization schedule, the indicated

leak rates were determined for the sealed-off

capsules with an exposed surface area of

about 16 cm^ following pressurization in helium at 5 atm (absolute) (5 x 10^ Pa) for 0.2,

0.5, 1, and 20 h. Equivalent leak, or out- gassing, rate, R, was measured as a function

of time from a few minutes after pressurization until the rate fell off to about 10~^

atm-cm^/s (10~^ Pa*m^/s) or less. The re- sults are shown in figure 37. As expected,

initial outgassing rates were found to be

essentially independent of pressurization

time, being a function of helium concentration in the glass surface.

The minimum detectable leak rate is limited

by the outgassing rate. The dwell time necessary for the outgassing rate to fall to a

particular value can be derived from the data

of figure 37 as a function of pressurization

time with maximum allowable outgassing rate,

R , as a parameter. Some extrapolation was max

employed as indicated in the figure by the

broken portion of the curves. Typical dwell

time characteristics, which are of the form

xjhere tj is the dwell time m hours and t

, d . . . . , p IS the pressurization time in hours, are shown in figure 38. For all three characteristics the exponent a is about 0.65.

If the leak detector sensitivity, Q . , is min

taken as R , the smallest and largest de- ' max ° tectable leak sizes, L . and L , for giver mm max

values of t , and t can be determined accordd p ing to the procedures of ASTM Method F134

[60]. Examples are listed in table 10. It should be noted that about one fifth of the test leaks have leak sizes smaller than 8 x 10~S atm-cmVs (8 x IQ-^ Pa-m^/s) and about

two fifths have leak sizes smaller than 2 x 10"'^ atm'cm^/s (2 x 10~8 Pa-m^/s) . Only one

of the 50 test leaks has a leak size larger

than 3 x IQ-^ atm-cm^/s (3 x IQ-^ Pa-m^/s). I From these results it was concluded that the i interlaboratory evaluation should be deferred

until additional test specimens, less susceptible to helium absorption, can be obtained. (S. Ruthberg^

10.2. Correlation of Moisture Infusion, [' Leak Size, and Device Reliability

A study has been initiated to derive a quantitative relationship between leak size in

hermetic packages and moisture infusion along

with the consequent effect on device reliability. The approach is to incorporate a dew point moisture sensor and a moisturesusceptible integrated circuit into packages,

each with a prefabricated microchannel leak

of known size, which would then undergo leakf

testing and accelerated life test.

The first phase of this effort was undertakei

to evaluate the accuracy of the dew point

measurement procedure, fabrication procedure;

for and stability of the microvents, and the

moisture sensitivity of a suitable integratec

circuit

.

52
HERMETICITY

TIME (h)

Figure 37, Helium outgassing rate, R, of

sealed borosilicate glass capsules as a function of dwell time between pressurization

and measurement. (Pressurization times:

Curve a, 12 min; Curve b, 30 min; Curve c,

1 h; Curve d, 20 h.

)

= 1 1 1 1 1 1 II iiij 1 II 1 nil

-

~I00

_

^mox

otm cm^/g

1 X 10'^"=

TIME 5 XI0"8 ~

ELL

1

1 1 1 Hill 1 1 M nil! 1 1 1 1 lITT

I 10 100 PRESSURIZATION TIME (h)

Figure 38. Dwell time necessary for helium

outgassing rate of sealed borosilicate glass

capsules to fall to specified maximum allowable rates, R , as a function of pressuri-

.. ' max' zation time

.

Table 10— Smallest and Largest Detectable Leak Sizes

for Borosilicate Glass Capillary Test Leaks^

h ^d'

h atnrcm^/s

^min'

atm-cm^/s

^max

atm-cm^/s

1 4 5 X 10-8 9 X 10-7 6 X 10-^

1 10 1 X 10-8 4 X 10-7 3 X lO-'* 20 30 5 X 10-8 2 X 10-7 7 X lO-'* 20 75 1 X 10-8 9 X 10-8 3 X 10-5

pressur-ized at 5 atm for t hours;

package volume = 2 cm^ ^

53
HERMETICITY

Two types of moisture sensor are being utilized. One is an Interdigltated set of thick

film electrodes on an alumina substrate. The

second Is an electrode system formed between

the die attach area and the leads of a dualin-line package. In use, the leakage current

between the electrodes is measured while the

temperature of the package is reduced, by

means of a thermoprobe, until the dew point

is reached as indicated by an abrupt current

increase [61]. A set of 25 specimens was

fabricated; each specimen is an open microcircuit package with the two sensors attached.

To reduce electrical leakage each specimen was

coated with epoxy on all surfaces but the sen- sor and the area which would be contained

within the internal volume of the package if

it were sealed. To test the sensors, each

package was mounted on a temperature con- trolled probe and was placed in an environmental chamber which was first stabilized in

a dry mode and then set to give the desired

relative humidity in the range between 5 and

50 percent. The thermoprobe was stabilized

at an initial reference temperature and then

brought down in small temperature increments

while leakage current was monitored. The

relative humidity appropriate to the measured

dew points was compared with the relative

humidity of the chamber. Initial data indicate close agreement between the two sensors

and the psychrometric parameters.

The microvents are produced by laser drilling the case and back plating until a suitable leak size has been obtained. A matrix

experiment has been performed to determine

the suitability of procedures. A reasonable

yield with a range of leaks from 10"^ to 10~^

atm«cm3/s (10"*= to 10"^ Pa-m^/s) has been

obtained.

The 741 operational amplifier was selected

for trial as a suitable integrated circuit

test vehicle. This is a monolithic bipolar

device with an MOS capacitor. Two groups of

25 devices each were obtained. One group was

unpassivated; the second was passivated with

silane glass. All devices were pretested

thermally and electrically, mounted for direct exposure, and then submitted to a humidity step-stress test under dc bias at 85''C. The devices were operated for 168-h incremental periods at 20, 50, 70, 85, and 95

percent relative humidity until failure,

which was defined as drift of one or more

electrical parameters outside specified

limits. Two of the unpassivated devices

failed in the first step, 15 more in the

second step, 7 in the third step, and the

last in the fourth step. Of the glassivated

devices, one failed in the second step, one

in the third step, one in the fourth step,

and 4 in the fifth step. Each of the failed

circuits was examined microscopically to establish the cause of failure. All failures were determined to be moisture related as evidenced by the presence of corrosion on

the aluminum metallization runs. Based on these results the unpassivated 741 operational amplifier was selected for use in the

moisture sensitivity life test.

(S. ZatZ" and S Ruthberg)

Work performed at Martin-Marietta Aerospace,

Orlando Division under MBS Contract No. 535880,

54
11. DEVICE INSPECTION AND TEST

In.l Automated Scanning Low-Energy Electron

Probe

The initial electron gun structure (NBS Spec.

Publ. 400-12, p. 28) for the ASLEEP system

was completed and mounted in the vacuum system for testing and alignment. The gun was

placed in the circuit shown in figure 39 for

beam current tests. The target was an aluminum grid comprising 60 ym wide lines on 600

um centers over silicon dioxide (NBS Spec.

Publ. 400-17, p. 46) mounted on the specimen

manipulator. The 6-V battery was inserted

into the circuit during initial testing to overcome patch effects at the target. The

focus coil was adjusted to a magnetic induction of 70 gauss (7 mT) and the target current maximized by adjusting the various potentials. The maximum current collected bv

the target was 1.7 nA, obtained with the

heater current at 9.3 A, grid 1 at 30 V and

grid 3 at 500 V.

i The first electron gun developed a short be-

! tween the high voltage and the cathode necessitating a modification of the design. The

glass insulators used to shield the high voltage leads were fractured where they crossed

the low voltage elements of the gun. Notchj ing the grid and cathode holders to allow

clearance for stronger tubular ceramic insulators corrected this problem. The original

gun used three sapphire rods to hold the gun

elements in place. The rods were notched and

metal tabs were inserted in the notches and

spotwelded to the gun elements. This scheme

made alteration and repair of the gun difficult. The elements of the gun are now held

in their proper relationship by hollow ceramic spacers through which a bolt can be run

to hold the assembly under compression. The

modified gun has been tried and found to be

|inore successful.

The computer control system rasters the elec-

^ tron beam over the test grid. If the electrometer in the beam current test circuit

(fig. 39) is replaced by a low noise amplifier, an image may be form.ed on a cathode

jijray tube (CRT) by using the amplified target

1 current signal to control the CRT beam intensity. Figure 40 is a picture of one such

image taken of the aluminum grid during prelliminary system checkout. The narrow line

||at about one half the grid spacing near the

{edge of the picture is a scratch in the silicon dioxide surface. Figure 41 is an image

lof an array of MOS capacitors made of 30-mil

(0.75-mm) diameter dots of aluminum on silicon dioxide 100 nm thick. The capacitors were scanned with a beam of fixed energy

(approximately 3.5 eV) . The four white ca- pacitor dots in the center of the picture

are shorted to the substrate due to defects

in the oxide. Similarly, two rows of shorted

capacitors are located where the wafer was

scribed and broken.

Additional software has been generated for

presenting surface potential mapping information. The electron beam is scanned in the

x-y plane, the retarding voltage is ramped to

a preselected threshold current level, the

surface potential is scanned over any selected

area of the target, and the results are presented in a three dimensional display all under computer program control. In the presentation, the y-axis is somewhat elongated and

the z-axis deflection of the scan line is

proportional to the relative surface potential. Figure 42 shows a surface potential

scan of an array of capacitors on silicon

dioxide. Variations in height of the profiles are a direct measure of the dielectric

uniformity assuming the metal work function

to be constant. A high dot indicates a high

capacitance. This result qualitatively demonstrates the ASLEEP as a non-destructive com- puterized surface diagnostic technique.

(W. C. Jenkins- and G. P. Nelson-)

Work conducted at the Naval Research Laboratory under NBS Order No. 501718. NBS

contact for additional technical information: K. F. Galloway.

GRID I -GRID 2 GRID 3 pDRIFT TUBE

FOCUS COIL DIRECTLY HEATED CATHODE- TARGET

30 V 500 V 6

6VELECTROMETER +800V

Figure 39. Circuit for testing electron gun

beam current

.

55
DEVICE INSPECTION AND TEST

0,10

Figure 42. ASLEEP scan of 30-mil (0.76-inTii) diameter MOS capacitors on 35-inil (0.89-mm)

centers

.

0 05

1 5keV1 1 1 1

lOkeV

20keV

\ 1 iV 1 11 100 200 300 400 500

PENETRATION DEPTH (^g/cm') 600

Figure 43. Energy dissipation as a function

of penetration depth for several electron

energies in material with atomic number be- tween 10 and 15 calculated according to Everhart and Hoff [68].

11.2. Scanning Electron Microscopy —

Electron Beam Induced Current Mode

A first-order prediction of the magnitude of

electron beam induced current (EBIC) signals

in device structures resulting from exposure

to the electron beam in a scanning electron

microscope (SEM) can be obtained from elementary considerations. This problem has

been approached with varying degrees of so- phistication by several authors [62-66].

The EBIC signal can be predicted if the magnitude and spatial variation of the electronhole generation within the structure is known

Specific construction details of the device,

device biasing, and SEM operating parameters,

such as beam accelerating voltage and beam

current, must be considered. In many cases

the required specimen details are known or can be estimated to an acceptable accuracy.

Early work on the distribution of energy loss

as a function of penetration depth for kilovolt electrons, the information necessary for

calculating electron-hole generation, was

done by Griln [67]. He experimentally determined the electron energy absorption as a function of penetration depth in air and

demonstrated two important points. First,

he obtained a relationship between electron

56
DEVICE INSPECTION AND TEST

projected range, R^, and electron beam

energy, E^:

ber in the range 10 < Z < 15, the energy dissipation is given by

where R is expressed in micrograms per square

G centimeter* and E„ is expressed in kiloelectron

D volts, which was valid over the range 5 keV < E < 25 keV. Second, he showed that the shape

Bof the depth-dose curve is practically invariant if the penetration distance is normalized

to R„ and the energy is normalized to E . G D Everhart and Hoff [68] extended these general

conclusions to solids and obtained a generalized depth-dose curve for solid materials by

taking the steady-state electron beam induced

current through the insulating layer of a metal-oxide-semiconductor structure as a measure of the energy dissipation in that layer.

They found for structures of aluminum, silicon

dioxide, and silicon the projected range.

R, 4.0 E^l-^5

to be accurate for 5 keV < E < 25 keV. They

B also found that for elements with atomic numELECTRON BEAM

1

1

a. Schematic illustration of

junction showing absorber

thickness, t, above silicon

and incident electron beam.

p

: n (

b. Schematic illustration of

experimental arrangement for current measurement

.

Figure 44. Examination of silicon p-n junction in the electron beam induced current

mode

.

dE.i^:VjB_^

dx R^

(11)

where y = x/R , f is the fraction of inciG B dent energy backscattered, and X(y) = 0.60 +

6.21y - 12.40y2 + 5.69y3. Equation 11 is

plotted in figure 43 for several beam energies

with the assumption that f = 0.1 (sec. 11.3.)

a From these curves one can estimate the EBIC

signal in a silicon device.

Consider a silicon p-n junction covered by

aluminum or silicon dioxide or both to a thickness t as illustrated in figure 44a.

If one assumes that all the energy deposited

in the silicon goes into the creation of

electron-hole pairs, the number of pairs generated by each incident electron is

where

R^ dE , G — dx

dx

and Ep, the energy required to create one pair

is 3.64 eV [69] .

If one assumes 100 percent collection efficiency, the electron beam induced current,

^E = V'

where I is the SEM beam current, is regisB tered by the ammeter in figure 44b. The quantity N is therefore the current gain; N is

plotted as a function of electron beam energy

for several thicknesses of absorber above the

silicon in figure 45 and as a function of

absorber thickness for several beam energies

in figure 46. The nomograph in figure 47 can

be used to convert aluminum, silicon dioxide,

or aluminum plus silicon dioxide thickness

in micrometers to absorber thickness in micrograms per square centimeter.

As an example of the use of these graphs to make a quick estimate of the current gain to

The measure of length used here is the product of material density and thickness.

57
DEVICE INSPECTION AND TEST

t=0 t=125/ig/cm2

t=250/4g/cm^

10 20

BEAM ENERGY |keV|

30

Figure 45. Current gain, I^/I^, for several

absorber thicknesses as a function of electron beam energy.

0 too 200 300 400 500

t l/ig/cm^i

Figure 46. Current gain, 1^/1^, for several

beam energies as a function of absorber

thickness, t.

E

4.

- 15

- 10

- 05

CO

- 800

- 700

- 600

500

400

300

- 200

- 15

E

4.

10

0.5

0.0

Figure 47. Nomograph to convert aluminum,

silicon dioxide, or aluminum plus silicon

dioxide thickness in micrometers to absorber

thickness in micrograms per square centimeter. (To use, draw a line from the silicon dioxide thickness to the aluminum thickness and read the absorber thickness

directly .

)

10'

10^

10^

10'

10^

8—

B

- o

10 l-o- 0

Ig ^ 100 pA O Ig = 500 pA

10 20 30

BEAM ENERGY |keV|

Figure 48. Comparison of measured electron

beam induced currents with the results of

the elementary calculation.

58
DEVICE INSPECTION AND TEST

be expected when performing an EBIC examination, consider a 10-keV electron beam incident on a silicon structure with a 500-nm

thick surface film of silicon dioxide. From

figure 47, the total absorber thickness is

115 yg/cm^ and from figure 46 the current

gain is 800.

An experimental measurement was made for

comparison with the results of this calculation. The device chosen for study was a p-n

diode similar to the one shown in figure 44. The diode was fabricated in a wafer of phosphorus-doped silicon with room temperature

resistivity in the range 5 to 10 fi'cm and a <111> surface orientation. The p-region was

formed by diffusing boron to a depth of approximately 1.4 ym; the diffusion profile

was gaussian and the sheet resistance was

about 180 Q/U. The thickness of the oxide

layer above the junction was approximately

330 nm. The I-V characteristics of the diode were unchanged when measured after performing

the EBIC measurements.

The diode was exposed to both 100 and 500

pA SEM electron beams at energies from 2.5

to 30 keV. Although the active region of

the diode was 1.5 mm (0.06 in.) in diameter

only a small portion of the central part

(approximately 10~^ cm^) was scanned in order

to eliminate edge effects. The electron beam

induced current was measured with an ammeter

capable of measuring currents from 1 nA to 100 yA connected across the p-n junction.

The results are shown in figure 48 in which

I„/I^ (or N) is plotted as a function of beam

energy. The solid line represents the calculation for an absorber thickness of 330 nm.

It is apparent that there is reasonable

agreement between the experimental and theoretical results. Discrepancies at higher

beam energies may arise from failure to collect all the carriers produced while the disagreement below 5 keV may be attributed to the injection of energy (low energy x-rays,

electron straggling, etc.) or charge from the

absorber. Plotting the induced current normalized by the beam current produces a curve

that is essentially independent of beam current, as expected. Similar results were obtained for other similar diodes.

The simple calculation presented here seems

to give an adequate first order prediction

of the electron beam induced currents for

simple device structures. However, the validity of the assumptions used should be as- sessed for each specific application.

(K. F. Galloway, K. 0. Leedy,

D. E. Sawyer, and W. J. Keery)

11.3 Scanning Electron Microscopy —

Electron Beam Induced Damage

In order to utilize the energy deposited versus penetration results of Everhart and Hoff

[68] to calculate the energy deposited in

aluminum, silicon dioxide, and silicon structures by a low energy electron beam (NBS

Spec. Publ. 400-17, pp. 43-46), knowledge of

the fraction of incident energy backscattered from the specimen (f_) is necessary.

D This fraction is usually taken to be 0.1 from

the work of Bishop [70] at 30 keV. A study

was undertaken to examine the validity of

using this value at lower electron beam

energies

.

The fraction f depends on t], the fraction

B of incident electrons backscattered , and the

fractional mean energy of the backscattered

electrons

:

^B = ^ hcJS '

where E„ , is the mean energy backscattered

Bck

and E is the beam energy. Both E /E and

a Bck B

ri depend on the incident energy, specimen

composition, the incident beam angle, and the

scattering angle at which they are measured.

The data reviewed here are for normal incidence and are integrated over all possible

scattering angles.

There have been several experimental determinations of n using a variety of experimental techniques [71-75]. In the energy range

of interest here (usually E < 30 keV) , the

B fraction of electrons backscattered from

aluminum or silicon is almost independent

of the beam energy, E , as shown in figure 49. Data on the fractional mean energy backscattered are scarce [70,76,77]. _Figure 50

illustrates the variation of E„ , /E„ with Bck B beam energy for electrons backscattered from

aluminum. The values given by Thomas [76] were measured at 138 deg with respect to the

beam direction; the average value over all

backscattering angles would be greater.

values of f for an aluminum specimen can be

B calculated using these values of E„ , /E„ and

Bck B values of n from figure 49b interpolated

where necessary to obtain values at the same energies. The results, with error bars es- timated on the basis of scatter in the re- ported data, are shown in figure 51. It is

59
DEVICE INSPECTION AND TEST

0.2

10 20

BEAM ENERGY |keVl

30

a. Silicon. ^, Heinrich [71]; , Bishop

[73]; A, Drescher et al. [75].

10 20 30 BEAM ENERGY [keV]

b. Aluminum. i>, Heinrich [71]; O, Colby

et al. [72]; , Bishop [73]; 0> Weinryb

and Philibert [74]; A, Drescher et al. [75].

Figure 49. Ratio of backscattered to incident electrons, n , as a function of beam energy.

apparent that taking the value of f^ to be

0.1 in the range 5 to 30 keV makes no more

than a 2 percent contribution to the error

in calculating the energy deposited. This

contribution is small in comparison to the

other possible sources of error. To a first

approximation for silicon specimens, values

of fg can be taken to be the same as aluminum. The results are also expected to be

applicable in general to devices consisting

of silicon, silicon dioxide, and aluminum.

(K. F. Galloway, W. J. Keery, and K. 0. Leedy)

n.4. Flying-Spot Scanner

The dual-laser flying-spot scanner (NBS Spec.

Publ. 400-17, pp. 42-43) was applied to characterization of UHF power transistors and an MOS shift register. In the former it was re- vealed that the scanner could be used to map

temperature variations over the active re- gions and to detect portions of the device

" 0.6

0.4

10 20 BEAM ENERGY (keV|

Fractional mean energy backfrom aluminum as a Thomas

Figure 50. _

scattered , E„ , /E

£ uBck a _ function of beam energy, E

which operate in a nonlinear manner. In the

latter, the logic pattern was displayed non- destructively and it was possible to change

states within the device selectively and re- versibly

.

The UHF transistors studied were bipolar silicon devices designed to furnish 5 W at frequencies up to 1 GHz. The surface topology

is shown in the photomicrograph in figure 52. There are four in-line cells connected in

parallel with a total active area 1.2 mm

long and 0.15 mm wide which appears as a broad vertical stripe in the center of the

figure. The emitter and base contacts are interdigitated; the emitter fingers come in

from the right and the base fingers from the

left. The finger metallization is 2 ym wide,

and the metallization stripes are separated

by 8 pm. The separation allows access of the

laser to the active regions of the device.

0.11

0.10 -

0.09

0.08

0.07

10 20

BEAM ENERGY |keV|

Figure 51. Fraction of incident energy

backscattered, fg, from aluminum as a function of beam energy

.

[76]; B, Darlington [77].)

60
DEVICE INSPECTION AND TEST

Figure 52, Photomicrograph of silicon

interdigitated UHF transistor showing

surface topology. (Magnification: '^115X,)

The transistors were scanned while they were

operated in a dc-biased common-emitter con- figuration. The collector load was 60 Q and

the voltage drop across the resistor was used

to modulate the display screen. It was found

that localized regions of high temperature,

or hot spots, formed in several of the transistors within the operating range listed in

the manufacturer's data sheet. Figure 53 is

a photograph of the display screen for one

of the UHF transistors scanned with the 1.15

pm laser while being operated with a collector-emitter voltage, V^g, of 26 V and a collector current, I^, of 250 mA. The display

was formed by using intensity modulation to present a ref lected-light image and by usir.f: intensity modulation and vertical deflection

of the horizontal scan lines together to present the 1.15 ym transistor photoresponse

.

The hot-spot region is the bright area in the center of the photograph. The hot-spot location was confirmed using a conventional passive infrared microscope.

The enhanced photoresponse is due to the in- crease in absorption coefficient, a, with

temperature. The absorption of monochromatic

radiation varies with the distance x into

silicon as exp(-ax). For single-crystal

silicon at room temperature, a

~ 0.3 cm"-'- for 1.15 urn radiation [78]. However, as the

Figure 53 . Photograph of the scanner display screen showing enhancement of 1.15-ijm

photoresponse due to hot-spot operation.

(The hot-spot region is the bright region

approximately in the center.)

temperature is increased, the forbidden

energy gap becomes smaller [79] and a increases, resulting in an increase in the number of electron-hole pairs photogenerated in

the active volume of the device. Calculations relating the enhanced photoresponse to temperature- induced bandgap variations are

in order of magnitude agreement with the preliminary experimental observations

.

Although the results presented were obtained

with static operating conditions, similar re- sults with the transistor operating with rf drive and in its own circuit should be feasible. While these initial observations have

focussed on hot-spot observations, the use- fulness of the measurement technique is not restricted to hot-spots. For example, as the power dissipation was increased in the

range prior to the onset of hot-spots, the

normal temperature rise of the transistor

was readily detected by a uniform enhancement

of its photoresponse.

A transistor of the same type was used for

the characterization of regions of nonlinear

operation. During normal operation, lasers

produce optical radiation, not at just one discrete wavelength, but with a series of

discrete lines centered about the nominal

wavelength. Mixing of these lines produces

a partial modulation of the laser radiation

in multiples of a fundamental modulating frequency. For the 0.633 pm laser used for this

part of the investigation, the fundamental

frequency is 500 MHz, and this 500 MHz mod61
DEVICE INSPECTION AND TEST

BASE CURRENT (mA)

Figure 54. Collector-base characteristic

of UHF transistor connected in a commonemitter circuit. (The nonlinear region in

the center (D) is associated with hot-spot

operation.

)

ulation was used for the detection of nonlinearities. The transistor was connected in

the common-emitter mode with a supply voltage of 35 V. The base current, I , was

B adjusted to allow the transistor to be biased

anywhere between cutoff and a saturation current (I„) of 520 mA. The ch^.racterisC C D tic for a typical device is shown in figure

54. The deviation from linearity near the center was found to be associated with the

formation of a hot spot.

The mapping of operating nonlinearities was performed by scanning the transistor with the

modulated laser. A 470 MHz signal generator

was coupled to the base lead and a 30 MHz intermediate frequency amplifier and detector

were connected across the 60 Q. collector load

resistor. The detector output was used to modulate the display screen.

If square-law nonlinearities are present

within the transistor, frequency mixing of

the 470 MHz signal and the 500 MHz laser

modulation occurs and a 30 MHz difference

frequency output is produced. The internal

operating mechanism is believed to be the

following: The relationship between base

minority carrier density in excess of equilibrium, and the internal base current,

whether this current be due to an electrical

input or to photogeneration, can quite generally be represented by a power series. For devices, or device portions, which are operating linearly, the power series (exclusive of a possible constant) is a single term linear in the current and the characteristics

are described by a straight line. For this

case, there is no variation in the minority

carrier distribution within the device at frequencies other than the two input ones, and so only these two frequencies appear in

the electrical output. However, if the characteristics are not linear, frequency mixing

may be detected at the output. Specifically,

if the square term is present, and this is

the next possible term in the power series

after the linear one, frequencies which are

the sum and the difference of the two inputs

are produced.

If the UHF characteristics are similar to the

static one shown in figure 54, then it might

be anticipated that mixing would occur with

the transistor biased near saturation,

near cutoff (the characteristics near cutoff

resemble those of a mixer diode) , and perhaps

in the hot-spot regime. Mixing in these regions was found to occur as indicated by the

photographs of the display screen sho^im in

figure 55. These photographs show the display at the corresponding operating points in

dicated by arrows in figure 54. In A, the

transistor is in partial cutoff, and a strong

i-f output is obtained uniformly. In B, the first linear region has been entered, and

the signal has decreased. The hot-spot re- gion has been entered in C. There actually

are two hot spots, one on each side of a cell

boundary, and their locations as determined

with a passive infrared scanner are indicated

by arrows in D . It is interesting to see

that operation in the hot-spot regime can influence linearity in cells other than just

the one, or ones, operating in this regime.

Perhaps this is due to coupling through the

collector region common to all cells. In E, the operating point is outside of the hotspot region, and the characteristics again

are fairly linear and so little mixing occurs For the last case, F, the device is near

saturation, and again strong non-linearity

is observed on the display screen. Inspection of the display screen shows that the non linearity is more pronounced along the righthand emitter side. Perhaps this is so because the base contact is on the left, and

62
DEVICE INSPECTION AND TEST
DEVICE INSPECTION AND TEST

pulse was injected through the clock input

and a logical one injected via the data

input at the lower left of the figure

.

Figure 56. Photographs of the scanner display screen showing a portion of a p-MOS shift

register

.

a small amount of debiasing of the righthand emitter side is occuring due to the

dc voltage drop along the base fingers.

It was also shown that the scanner can be

used to observe logic information passing

through an MOS shift register without altering the characteristics of the device.

Previously, the scanning electron microscope

has been used to examine MOS logic circuits

[80], but this requires the inconvenience of

inserting the circuit into and removing it

from a vacuum system. Furthermore, the electron beam can cause catastrophic damage to MOS circuits especially when they are exposed

under bias (NBS Spec. Publ. 400-17, pp. 43-

46) unless special precautions are taken

[811. The shift register vjas a static dual

128-bit p-MOS ion-implanted device. In order

to be able to observe the logic flow in the

device with the laser scanner, the package

leads were connected as appropriate for normal circuit operation and information which

describes the circuit operation was extracted

by monitoring variations in power supply cur- rent to the device.

Figure 56 shows the pictures of the displays

obtained by scanning the p-MOS shift register

with the 0.633 pm laser at low intensity.

Figure 56a shows about one third of one of

the 128-bit registers and some associated input/output circuitry. In the main part of

the register a random pattern of logical ones

and zeros v/hich appear as displaced dark and

light areas can be seen. The logic flows, in

this register, along a snake-like path which

begins in the left-hand column at the bottom

and flows up the column, then down the second column, up the third and down the

fourth, where it switches' the output circuitry at the bottom of the fourth column.

. 11 1 ^ ^. cult was operated with reduced power supply ; voltage, it was observed that some logic I cells did not always function properly.

It was also found while scanning this MOS

device that internal logic states can be

changed nondestructively by increasing the

j laser intensity with the polarizer-analyzer ; combination in the optical system. Logical

j ones can be selectively changed to zeros and

j vice versa by decreasing the laser scan [ raster to a region on the logic cell where a change is desired. The laser is aimed to one side of the logic cell to change a zero to a one, and to the other side to change a one to

a zero; aiming the laser is accomplished by

Figure 56b shows the same register after a single clock pulse has been injected through

the clock input and a logical 1 injected via

the data input. It can be easily seen upon

comparison of the two photographs how the

logical ones and zeros have shifted in accordance with the clock pulse. The output has

also switched states. Demonstrated here is

the operation of a good device, although improper internal operation on a defective device has also been observed. When the circuit was operated with reduced power supply

voltage, it was observed that some logic

cells did not always function properly.

64
DEVICE INSPECTION AND TEST

Figure 57. Photograph of the scanner display screen showing a portion of a p-MOS

shift register in the same static condition

as in figure 55b except that a logical one was changed to a logical zero (shown by the

arrow) by the laser beam.

moving the device. The laser intensity is

then increased to change the state of the

cell, and then returned to its previous intensity. The laser raster scan then can be

restored to its original size. The scan raster is decreased by electronically decreasing

the excursion of the laser deflection mirrors. This operation changes only the desired cell and does not affect the state of

any other cells in the register.

Figure 57 shows the register as it appeared

in figure 56b except that the laser was used

to change the state of the logic cell identified by the arrow. Note that no other logic

cell has been changed and no input pulses were

injected into the device. This ability to change internal logic states nondestructively

could have far-reaching benefits in testing

of LSI devices which have internal processing

blocks which are not externally accessible.

(D. E. Sawyer* and D. W. Berning)

Prof. H. P. D. Lanyon, Electrical Engineering

Department, Worcester Polytechnic Institute,

Worcester, Massachusetts, provided theoretical insight into effects that govern the

temperature dependence of the photoresponse

.

65
11 L T H E R L PROPERTIES OF DEVICES

12.1. Thermal Resistance - Darl ington Pairs

Previous work on the measurement of thermal

resistance of Darlington transistors showed

that the thermal resistance of the output

transistor, R , is given to a good approximation by

^6JC2 ^^eJC(l+2) R

6JC1 (12)

where R0jc(]^+2) ""^^ thermal resistance de- termined by measuring the series combination

of both emitter-base junction voltages and

R. is the thermal resistance of the input

6 JCl

transistor* (NBS Spec. Pubis. 400-12, pp. 35-37, and 400-17, pp. 52-55). The experimental results reported previously were made

on devices using external emitter-base shunt

resistors which had magnitudes which were nearly optimum for directly measuring Rgj^j

and R„,„^T,TS by the emitter-only switching

technique [82]. To test this relationship

further, measurements were made on a specially fabricated four-terminal Darlington

device^ with an input diode and integrated

emitter-base shunt resistors whose magnitudes and ratio were considerably less than

optimum [83]

.

It was found that eq (12) is still valid in

this instance, but that it was not possible

to measure Ro-rr-i by the emitter-only switchNote that for a Darlington transistor, RgjQ-]_

is defined as the rise in temperature of

the hottest spot on the input transistor

divided by the total power dissipation with

both transistors operating normally and Rqjq2

is defined as the rise in temperature of

the hottest spot of the output transistor divided by the total power dissipation with

both transistors operating normally. The

thermal resistance of the device as a whole

is taken as the larger of RgjQ-]_ or RgjQ2*

The quantity P-g ( i_.,.o

) is an average of

the thermal resistances of the input and

output transistors given by the ratio of the

apparent temperature rise indicated by the sum of the junction voltages to the total

power dissipation.

ing method using the emitter-base voltage as the temperature sensitive parameter (TSP) because the ratio of the input shunt resistance

to the output shunt resistance was too small

(2:1) to allow the input transistor to be

turned on while the output transistor was

turned off. Therefore, the collector-base

voltage of the input transistor was used as the TSP to determine '^^^^^ with the use of

the measuring circuit (NBS Spec. Publ. 400-

12, pp. 35-37 connected in the grounded-base,

emitter-and-collector switching mode with

switches Sj and S^ open, switch Sg closed,

and switch S3 set to C as shown in figure 58. During measurement, the series transistor

switch Q is turned off and the parallel

s transistor switch Q is turned on by a clock

P pulse which also activates a sample-and-hold

circuit (not shown) which senses the voltage

between terminal C and terminal Bj. In this

measuring configuration, the measuring current, Ij^, has two possible parallel paths:

via the collector-base junction of the tran- sistor or via the resistor Rj in series

with the collector-base junction of the

transistor Q2 . The desired path is via Qj, as indicated in the partial circuit diagram

in figure 59. However, if Rj is small and Q2

is, as is usual, hotter than Qj , a considerable amount of current may also pass through

Q2 . One way to reduce the effects of the

unwanted parallel paths is to make I^ larger

so that the voltage drop in Rj^ restricts

the percentage of I^, that is diverted from M

Qj. For the device studied, R. as mea0 J CI sured by this method reached a constant value for I,, > 3 mA. M

~

The thermal resistance of the input transistor was also measured by the emitter-only

switching method (with switch Sj open, switches S2 and closed, and switch S3 set

to E in the circuit of figure 58 by changing

the lead from terminal E2 to terminal Ba^l

which provides direct access to the emitterbase junction of Qj. As with discrete devices, this value of R^ ^r-i was found to be

0 J L

1

larger than that determined using the collector-base voltage as the TSP. Although

the emitter-only switching value is probably

the more accurate, it would not be measurable

for a three-terminal Darlington of the same internal configuration.

Type c device as defined in figure 26, NBS

Spec. Publ. 400-12, p. 35. To study the validity of directly measuring

the thermal resistance of the output transis66.
THERMAL PROPERTIES OF DEVICES

Figure 58. Modified circuit for measuring

thermal resistance set up as a grounded base,

emitter-and-collector switching circuit using

the collector-base junction voltage of the

input transistor as the temperature sensitive

parameter.

' -*>Rmm

Figure 59. Desired current path when measuring the thermal resistance of the input

Darlington transistor using the groundedbase, emitter-and-collector switching method

with the collector-base junction voltage as the temperature sensitive parameter.

Rmm

Figure 60. Desired current path when measuring the thermal resistance of the output

Darlington transistor using the groundedemitter, emitter-and-col] ector switching

method with the collector-base junction

voltage as the temperature sensitive parameter .

tor, Rgj(^2' using the collector-base voltage of Q2 as the TSP this measurement was made [83] on the same four-terminal Darlington device with a range of measuring currents

in the circuit of figure 58 connected in the

quasi-grounded-emltter , emitter-and-collector

switching mode with switches S2 and Si^ open,

switch Si closed, and switch S3 set to C. The measured value ranged from 2.39°C/W for

= 0.5 mA to 4.16''C/W for 1^^ = 24 mA. The

reason for this strong current dependence is

the multiplicity of current paths available

for the measuring current, I^^. Among the

possibilities are: the series combination

of the resistor R2 and the base-collector

junction of the transistor Q2 (the desired

path shown in figure 60; the series combination of the emitter-base and the basecollector junctions of Q2; the series com- bination of the resistors R2 and and the

base-collector junction of the transistor Q^;

the series combination of R2 and the emitterbase and base-collector junctions of Qj. Con- duction across the emitter-base junctions is

made possible when the junctions are reverse biased by current passing through the appropriate shunt resistor. The relative fractions of current in the various paths depends

on the magnitude of I^. If a commutating

diode were across the output, (as is often

the case) , the possibilities are even more

numerous. (S. Rubin)

12.2. Transient Thermal Response

An electrical technique for measuring the

peak junction temperature of power transistors has been developed (NBS Spec. Pubis.

400-12, pp. 37-40; 400-17, pp. 55-56). The

importance and utility of such a technique

for determining the peak junction temperature

can be seen by comparing the measured safe

operating area (SOA) limits of a device with

the manufacturer's published SOA.

The values of collector current, I^, and

collector-emitter voltage, V„„, for which the

measured junction temperature equalled the

specified maximum safe junction temperature were determined by three techniques, standard

electrical [82], electrical peak temperature

[84], and infrared microradiometry , for a number of devices. These values of I^., and

are compared with the manufacturer's

CE specified SOA limits for two different devices in figure 61. The percentage of the

67
THERMAL PROPERTIES OF DEVICES

^ 2.0

1.0

oo

0.1

1 1—Mill

I I I I I I I I

I I I I I II I

1 2 5 10 20 50 100

COLLECTOR-EMITTER VOLTAGE jV]

a. 25-W single diffused transistor

1.0

5 0.5

I—o

0.1

1 1 1 100 %

V OA

1 1 1 1

{

1 II

—

-

0 A 72 % _

- A 33%

o\ A 19% _ o\ A14 %

1 i 1 1 1 M 1 1 R 1 A8| % |—

10 20 50 100 200 500 COLLECTOR-EMITTER VOLTAGE (V]

b. 35-W triple diffused transistor

Figure 51. Safe operating area plots. (Solid curves, manufacturer's specified safe operating area limits; data points, current-voltage values at which the junction temperature as measured by the infrared (), standard electrical (A), or electrical peak (^) technique

equalled the specified maximum safe junction temperature. The percentage of available ac- tive area being used at steady state is shown for each standard electrical measurement.)

total active area of the device actually

dissipating power at steady-state is noted

for each point that was determined by the

standard electrical technique.

The steady-state current distribution, and

therefore the temperature distribution. Is essentially uniform over the entire thermally

limited operating range for the 25-W device.

This Is why the agreement between the three

techniques Is so good. Note that the electrical peak temperature values agree almost ex- actly with values determined by Infrared mlcroradlometry . The manufacturer has been

rather conservative In the thermal rating of

this device; the specified SOA thermal limits

restrict the power that can be applied to the

device to considerably less than that for

which the peak junction temperature equals

the maximum safe temperature (200°C)

.

The 35-W device exhibits severe current nonunlformltles , and therefore temperature non- unlformltles , over a significant portion of

the specified SOA. Note that both the Infrared mlcroradlometry and electrical peak tem- perature techniques show that the maximum

safe temperature (200°C) can be exceeded within the published SOA limits. This Is shown by

the points which lie to the left of the specified SOA for < 0.4 A. However, the standard electrical technique would lead one to believe that the specified SOA safely limits

the power over the entire range.

For both devices studied, use of the standard

electrical technique would appear to permit

operation at higher power than either of the

other two methods. However, It is well known

that the peak j unction . temperature cannot be

measured accurately by standard electrical

techniques. Consequently, when this method

is used to develop the SOA, the limits are normally shifted toward lower voltages. In spite of this, the resulting limits do not guarantee that the maximum safe temperature

will not be exceeded for low current-high

voltage operation when current constrictions

occur. On the other hand, for high currentlow voltage operation, both devices studied

tended to be underutilized In that more power

could safely be dissipated than the specified

SOA limits permit. The ability to measure the

peak junction temperature by a relatively simple electrical method should enable manufacturers to generate more realistic SOA limits

and also permit device users to verify more readily the specified SOA limits for particular devices and applications.

Although the sets of measurements reported were made on only one device of each type,

they are in each case representative of nu- merous measurements made on many similar devices. (D. L. Blackburn)

68
13. REFEREfJCES

1. Silicon Resistivity Standards, SRM-1520,

Office of Standard Reference Materials,

National Bureau of Standards, Washington, D. C. 20234

2. Standard Method for Measuring Resistivity of Silicon Slices with a Collinear

Four-Probe Array, ASTM Designation

F-84, Annual Book of ASTM Standards^

Part 43 (November 1975) . (Available as

a separate reprint from American Society for Testing and Materials, 1916 Race Street, Philadelphia, Pa. 19103).

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5-5.1.1, 5-5.1.2.2, 5-5.1.3, 5-5.1.5.

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Electronics 15_, 809-817, (1972).

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.

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Si lioon Device Processing, C . P

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.

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18. Berg, N. J., The Effects of Nuclear Radiation on Low Tem.perature Transport Properties of Epitaxially-Grown Gallium Arsenide, Ph. D. Thesis, Electrical Engineering Department, University of Maryland, 1974 (Unpublished, available from

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Mitchell, D. L., Magnetic Freeze-Out

of Electrons in Extrinsic Semiconductors, Phys. Rev. 18£, 813-818

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Buck, T. C, Amsel, G., and Eisen, F.,

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and Thin Films, Thin Solid Films 17^ 1-41 (1973).

29. Gibbons, J. F., Johnson, W. S., and

Myloric, S. W. , Projected Range Statistics, Semiconductors and Related Materials, 2nd Ed. (Dowden, Hutchinson,

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Single Crystals, Canadian J. Phys. 46 , 543-550 (1968).

31. Pardee, W. J., Mahan, G. D. Eastman,

D. E., Pollak, R. A., Ley, L., Feely,

F. R., Kowalczyk, S. P., and Shirley,

D. A., Analysis of Surface- and BulkPlasmon Contributions to X-ray Photoemission Spectra, Phys. Rev. B 11 , 3614-

3616 (1975).

32. Bradshaw, A. M., Cederbaum, S. L.,

Domcke, W. , and Kraus, U., Plasmon Coupling to Core-Hole Excitations in Carbon,

J. Phys. C. - Solid State Physics 1_, 4503-4512 (1974).

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Corundum, Phys. Eev. 132, 1474-1481

(1963)

.

34. Roessler, D. M. , Kramers-Kronig Analysis of Reflection Data, Brit. J. Appl.

Phys. 16, 119-123 (1965).

35. Klucker, R. , and Nielson, U., KramersKronig Analysis of Reflection Data,

Comput. Phys. Comm. 6, 187-193 (1973).

36. Caughey, D. M. , and Thomas, R. E.,

Carrier Mobilities in Silicon Empirically Related to Doping and Field,

Proc. IEEE 55_, 2192-2193 (1967).

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During Thermal Oxidation of Silicon,

J. Appl. Phys. 35, 3391-3397 (1964).

38. Grove, A. S., Deal, B. E., Snow, E. H.,

and Sah, C. T., Investigation of Thermally Oxidized Silicon Surfaces Using

Metal-Oxide-Semiconductor Structures

,

Solid-State Electronics 8^, 145-163

(1965)

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39. Nicollian, E. H., Hanes, M. H., and

Brews, J. R., Using the MIS Capacitor

for Doping Profile Measurements with

Minimal Interface State Error, IEEE

Trans. Electron Devices. ED-20 , 380-

389 (1973).

40. VJolf, H. F., silicon Semiconductor Data,

p. 306 (Pergamon Press, Oxford, 1969).

41. Goodman, A. M., A Useful Modification of

the Technique for Measuring Capacitance

as a Function of Voltage, IEEE Trans. Electron Devices ED-21 , 753-757 (1974)

.

42. Goodman, A. M., An Investigation of the

Silicon-Sapphire Interface Using the

MIS Capacitance Method, IEEE Trans. Electron Devices ED-22 , 63-65 (1975).

43. Brice, D. K. , Ion Implantation Range

and Energy Deposition Distribution,

Vol. 1 (IFI/Plenum, New York, 1975).

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44. Gordon, B. J., Stover, H. L., and

Harp, R, S., A New Impurity Profile

Plotter for Epitaxy and Devices,

Silicon Device Processing , C. P. Marsden, ed., NBS Spec. Publ. 337, pp. 273-284 (November 1970)

.

45. Considine, P. S., Effects of Coherence

on Imaging Systems, J. Opt. Soa. Am.

56, 1001-1009 (1966).

46. Goodman, J. W., Introduction to Fourier

Optics, p. 127 (McGraw-Hill Book Co.,

New York, 1968)

.

47. Welford, W. T., Length Measurements at the Optical Resolution Limit by Scanning

Microscopy, Colloquia of the International Commission for Optics, Bruxelles,

May 6-9, 1958, Optics in Metrology,

P. Mollet, ed., pp. 85-90. (Pergamon

Press, New York, 1960)

.

48. Cook, H. D., and Marzetta, L. A., An Automatic Fringe Counting Interferometer for use in the Calibration of

Line Scales, J. Res. Natl. Bur. Stand.

65C , 129-139 (1961).

49. van der Pauw, L. S., A Method of Measuring the Resistivity and Hall Coefficients on Lamella of Arbitrary Shape,

Philips Research Reports 13_, 1-9 (1958).

50. Gratzman, S., The Application of the

Relaxation Method to the Calculation of

the Potential Distribution in Hall

Plates, Solid-state Electronics 9_, 409-

416 (1966).

51. Zadek, L. A., Multipole Analysis of Networks, IRE Trans. Circuit Theory CT-4 , 97-105 (1957).

52. Nicollian, E. H., and Goetzherger, A.,

The Si-Si02 Interface - Electrical Properties as Determined by the Metal In- sulator-Silicon Conductance Technique,

Bell System Tech. J. 46_, 1055-1133

(1967).

53. Carnes, J. E., and Kosonocky, W. F.,

Fast-Interface-State Losses in ChargeCoupled Devices, Appl. Phys. Letters

20, 261-263 (1972).

54. Panousis, N. T., and Bonham, H. B.,

Bonding Degradation in Tantalum NitrideChrome-Cold Metallization System, 11th

Annual Proceedings IEEE Reliability

Physics 1973, Las Vegas, Nevada, April 3-

5, 1973, pp. 21-25.

55. Holloway, P. H. and Long, Jr., R. L.,

On Chemical Cleaning for Thermocompression Bonding, IEEE Trans. Parts, Hybrids,

and Packaging PHP-11 , 83-88 (1975).

56. Tylecote, R. F., Pressure Welding of

Bright Alloys Without Fusion, Trans.

Inst. Welding, 1_, 163-178 (November

1945)

.

57. Jellison, J. L., The Effect of Surface

Contamination on the Thermocompression

Bondability of Gold, Proc . 25th Electronic Components Conference, Washington,

D. C, May 12-14, 1975, pp. 271-277.

58. Harman, G. G., ed.. Semiconductor Measurement Technology: Microelectronic

Ultrasonic Bonding, NBS Spec. Publ.

400-2, pp. 73-76 (January 1974).

59. Harman, G. G. loc . ait., pp. 94-95

60. Tentative Recommended Practices for

Determining Hermeticity of Electron

Devices with a Helium Mass Spectrometer

Leak Detector, ASTM Designation

F 134, Annual Book of ASTM Standards,

Part 43 (November 1975) . (Available

as a separate reprint from American

Society for Testing and Materials,

1926 Race Street, Philadelphia, Pennsylvania 19103.)

61. Zatz, S., A New Simplified Method to Measure Moisture in Micro Enclosures,

Proc, 24th Annual Electronic Components

Conference, Washington, D. C, May 13-

15, 1974, pp. 29-33.

62. Czaja, W. , Response of Si and GaP p-n

Junctions to a 5- to 40- KeV Electron

Beam, J. Appl. Phys. _37, 4236-4247

(1966) .

63. Hoff, P, and Everhart, T. E., Carrier

Profiles and Collection Efficiency in

Gaussian p-n Junctions Under Electron

Beam Bombardment, IEEE Trans. Electron

Devices ED-17 , 458-465 (1970).

64. Thornton, P. P., Scanning Electron

Microscopy , pp. 217-243 (Chapman and

Hall Ltd., London, 1968).

65. Brese, J. F., Electron Beam Induced

Current in Silicon Planar p-n Junctions: Physical Model of Carrier Generation. Determination of Some Physi71
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cal Values in Silicon, Saanninq Eleotron

Miarosoopy/2972 (Part I), pp. 105-112

(IIT Research Institute, Chicago, 1972).

66. Gonzales, A. J., On the Electron Beam

Induced Current Analysis of Semiconductor Devices, Saanning Eleatron Miaroscopy/1974 (Part IV), pp. 941-948 (IIT

Research Institute, Chicago, 1974).

67. Grlln, A. E., Lumineszenz - Photometrische

Messungen Der Energieabsorption in

Strahlungsfeld von Elektronen Quellen

Eindimensionaler Fall in Luft, Z. Naturforsah. 12A, 89-95 (1957).

68. Everhart, T. E., and Hoff, P. H., Determination of Kilovolt Electron Energy

Dissipation vs. Penetration Depth in

Solid Materials, J. Appl. Phys. 42 , 5837-5846 (1971).

69. Fiebiger, J. R., and Muller, R. S., Pair

Production Energies in Silicon and Germanium Bombarded with Low-Energy Electrons, J. Appl. Phys. 43, 3202-3207

(1972).

70. Bishop, H. E., Electron Scattering in Thick Targets, Brit. J. Appl. Phys. 1_8, 703-715 (1967).

71. Heinrich, K. F. J., Electron Probe Microanalysis by Specimen Current Measurement, Proceedings of the Fourth International Congress of X-Ray Optics and

Microanalysis, R. Castaing, P. Deschamps,

and J. Philibert, eds. , pp. 159-167

(Hermann, Paris, 1966).

72. Colby, J. W., Wise, W. N. , and Conley,

D. K. , Quantitative Microprobe Analysis by Means of Target Current Measurements, Advan. X-Ray Anal. _10, 447-461

(1967).

73. Bishop, H. E., Electron Scattering and

X-ray Production, Ph.D. Dissertation,

University of Cambridge (1966) .

74. Weinryb, E. and Philibert, J., Mesure du

Coefficient de Retrodif fusion des Electrons de 5 a 30 keV, Compt. Rend, 258 , 4535-4538 (1964).

75. Drescher, R., Reimer, L. , and Seidel, H.,

Ruckstreukoef f izient und SekundSrelektronen-Ausbeute von 10-100 keV-Elketronen und Beziehungen zur Raster-Elektronenmikroskopie, Z. ang. Phys, 29_, 331-

336 (1970).

76. Thomas, R. N., Scattering of 5-20 keV

Electrons in Metallic Films, Ph.D. Dissertation, University of Cambridge (1961)

.

77. Darlington, E. H., Electron Backscattering at Low Energies, Ph.D. Dissertation,

University of Cambridge (1970)

.

78. Dash, W. C, and Newman, R., Intrinsic

Optical Absorption in Single-Crystal

Germanium and Silicon at 77° and 300°K,

Phys. Rev. 99, 1151-1155 (1955).

79. Smith, R. A., Semiconductors, pp. 204-

207 (Cambridge University Press, Cambridge, 1961).

80. Plows, G. S., and Nixon, W. C. , Operational Testing of LSI Arrays by Stroboscopic Scanning Electron Microscopy,

Microelectronics and Reliability 10 , 317-324 (1971).

81. Crosthwait, D. L., and Ivy, F. W. , Voltage Contrast Methods for Semiconductor

Device Failure 'Analysis , Scanning Electron Microscopy/1974 (Part IV), pp. 935-

940 (IIT Research Institute, Chicago,

1974)

.

82. Thermal Resistance Measurements of Conduction Cooled Power Transistors, EIA Recommended Standard RS-313-B (Revision

of RS-313-A), October 1975 (Available

from Electronics Industries Association,

2001 Eye Street, N. W., Washington, D. C. 20006)

.

83. Rubin, S., Thermal P^esistance Measurements on Monolithic and Hybrid Darlington Power Transistors, PESC '75 Record,

IEEE Power Electronics Specialists Conference 1975, Culver City, California,

June 9-11, 1975, pp. 252-261.

84. Blackburn, D. L., An Electrical Technique for the Measurement of the Peak

Junction Temperature of Power Transistors, IZth Annual Proceedings^ Reliability Physics 197 S, Las Vegas, Nevada,

April 1-3, 1975, pp. 142-150.

72
APPENDIX A SEMICONDUCTOR TECHNOLOGY PROGRAM STAFF

Electronic Technology Division

(301) 921-3357

J. C. French, Chief

Miss B. S. Hope+

C. P. Marsden^'"'" J. F. Mayo-Wells^

Dr. W. M. Bullis, Asst. Chief for

Semiconductor Technology'"'

R. I. Scace§

Mrs. M. L. Stream

Dr. A. H. Sher, Asst. Chief for

Operations*

R. L. Raybold?'^ Mrs. T. J. Talbot

t

Mrs. E. Y. Trager"*"*

Semiconductor Characterization Section

(301) 921-3625

F. H. Brewer

Dr. W. M. Bullis, Chief*

Miss F. C. Butler+

Mrs. E. C. Cohen^

Dr. J. R. Ehrstein

Dr. K. F. Galloway

W. J. Keery

Mrs. K. 0. Leedy

Dr. D. C. Lewis

Dr. A. G. Lieberman

Dr . S . Mayo

Mrs. B. A. Oravecif"* Miss D. R. Ricks

H. A. Schafft

Mrs. E. F. Smith^+

L. R. Williams

Semiconductor Processing Section

(301) 921-3541

Dr. M. G. Buehler

Mrs. C. A. Cannon

J. M. David

Miss D. L. Hines"'" Dr. R. Y. Koyama

J. Krawczyk

T. F. Leedy

Y. M. Liu

R. L. Mattis

D. A. Maxwell

Dr. W. E. Phillips

D. E. Sawyer

Dr. A. H. Sher, Chief*

L. M. Smith

I. Szabo

W. R. Thurber

Electron Devices Section

(301) 921-3621

Dr. J. H. Albers

D. W. Berning

D. L. Blackburn

G. W. Burdette^

W. A. Cullins

Mrs. K. E. Dodson+

H. E. Dyson

Mrs. A. D. Glover+

G. G. Harman

H. K. Kessler

Dr. D. B. Novotny

F. F. Oettinger, Chief

G. J. Rogers

S. Rubin

S. Ruthberg

+ Secretary

H Part Time

* Telephone: (301) 921-3786

§ Telephone: (301) 921-3625

t Telephone: (301) 921-3621

# Telephone: (301) 921-3541

X Telephone: (301) 921-3821

73
APPENDIX B SEMICONDUCTOR TECHNOLOGY PROGRAM PUBLICATIONS

B.l. Prior Reports

A review of the early work leading to this Program is given in Bullis, W, M., Measurement

Methods for the Semiconductor Device Industry — A Review of NBS Activity, NBS Tech. Note

511 (December 1969)

.

Progress reports covering the period July 1, 1968, through June 30, 1973, were published

as NBS Technical Notes with the title. Methods of Measurement for Semiconductor Materials,

Process Control, and Devices:

Quarter Ending NBS Tech. Note Date Issued NTIS Accession No

September 30, 1968 472 December 1968 AD 681330

December 31, 1968 475 February 1969 AD 683808

iicxLCn J-i-j ^yoy H O O ijQxy A.y \)y ' AD 692232

June 30, 1969 495 September 1969 AD 695820

September 30, 1969 520 March 1970 AD 702833

S9 7 May 1970 AD 710906

March 31, 19 70 555 September 1970 AD 718534

June jU, ly/u DOV November 1970 AD 719976

September 30, 1970 571 April 1971 AD 723671

December 31, 1970 592 August 1971 AD 728611

March 31, 1971 598 October 1971 AD 732553

June 30, 1971 702 November 1971 AD 734427

September 30, 1971 717 April 1972 AD 740674

December 31, 1971 727 June 1972 AD 744946

March 31, 1972 733 September 1972 AD 748640

June 30, 1972 743 December 1972 AD 753642

September 30, 1972 754 March 1973 AD 757244

December 31, 1972 773 May 1973 AD 762840

March 31, 1973 788 August 1973 AD 766918

June 30, 1973 806 November 1973 AD 771018

After July 1, 1973, progress reports were issued in the NBS Special Publication 400 subseries with the title. Semiconductor Measurement Technology

:

Quarter Ending NBS Spec. Publ. Date Issued NTIS Accession No

September 30, 1973 400-1 March 1974 AD 775919

December 31, 1973) 400-4 November 1974 March 31, 1974 f COM 74-51222

June 30, 1974 400-8 February 1975 AD/A 005669

September 30, 1974 400-12 May 1975 AD/A 011121

December 31, 1974 400-17 November 1975

B.2. Current Publications

As various phases of the work are completed, publications are prepared to summarize the

results or to describe the work in greater detail. Publications of this kind which have

been issued recently are listed below:

Rogers, G. J., Sawyer, D. E., and Jesch, R. L., Semiconductor Measurement Technology:

Measurement of Transistor Scattering Parameters, NBS Spec. Publ. 400-5 (January 1975).

Lewis, D. C, On the Determination of the Minority Carrier Lifetime from the Reverse Recovery Transient of przR Diodes, Solid-State Electronics 1_8, 87-91 (January 1975).

74
APPENDIX B

Sher, A. H., Semiaonduator Measurement Technology: Improved Infrared Response Technique

for Detecting Defects and Impurities in Germanium and Silicon p-i-n Diodes, NBS Spec.

Publ. AOO-13 (February 1975).

Jerke, J. M., Development of Dimensional Measurement Techniques from 1 to 10 Micrometers and Application to Optical Microscope Measurements, Winter Symposium '75 on Micro-Photofabrication. Society of Photographic Scientists and Engineers, Palo Alto,

California, February 6-7, 1975, Paper C2.

Ciarlo, D. R. , and Schultz, P. A., Automated/C Photomesh Inspection Technologies,

Winter Symposium '75 on Micro-Photofabrication, Society of Photographic Scientists and

Engineers, Palo Alto, California, February 6-7, 1975, Paper C4.

Blackburn, D. L., An Electrical Technique for the Measurement of the Peak Junction

Temperature of Power Transistors, 13th Annual Proceedings, Reliability Physics 1976,

Las Vegas, Nevada, April 1-3, 1975, pp. 142-150 (IEEE Cat. No. 75-CHO 931-6 PHY).

Blackburn, D. L., and Oettinger, F. F., Transient Thermal Response of Measurements of

Power Transistors, IEEE Trans. Industrial Electronics and Control Instrumentation

IECI-22 , 134-141 (May 1975)

.

Galloway, K. F., Keery, W. J., and Leedy, K. 0., Integrated Circuit Damage Resulting

from SEM Examination, Proc. 25th Annual Electronic Components Conference , Washington,

D. C, May 12-14, 1975, pp. 263-266.

Buehler, M. C, David, J. M. , Mattis, R. L., Phillips, W. E., and Thurber, W. R.

,

Planar Test Structures for Characterizing Impurities in Silicon, Extended Abstracts

of the Meeting of the Electrochemical Society, Toronto, Ontario, May 11-16, 1975,

pp. 403-404.

Rubin, S., Thermal Resistance Measurements on Monolithic and Hybrid Darlington Power

Transistors, PESC '75 Record, IEEE Power Electronics Specialists Conference 2975,

Culver City, California, June 9-11, 1975, pp. 252-261 (IEEE Cat. No. 75-CHO 965-4 (AES).

Kraft, R., Finite Difference Techniques for Diffusion and Redistribution Problems with

Segregation-type Boundary Conditions, Proceedings of the AICA International Symposium

on Computer Methods for Partial Differential Equations, Lehigh University, Bethlehem,

Pa., June 17-19, 1975, pp. 328-334.

Jerke, J. M., Semiconductor Measurement Technology: Optical and DimensionalMeasurement Problems with Photomasking in Microelectronics, NBS Spec. Publ. 400-20

(October 1975)

.

Lieberman, A. G., Problems in Using Surface Analysis Techniques for the Depth Profiling

of Microelectronic Materials, Technical Digest, 1975 International Electron Devices

Meeting, Washington, D.C., December 1-3, 1975, pp. 126-129 (IEEE Cat. No. 75-CH 1023 lED)

.

Sawyer, D. E., and Berning, D. W., Laser Scanning of Active Semiconductor Devices,

Technical Digest, 1975 International Electron Devices Meeting, Washington, D. C,

December 1-3, 1975, pp. 111-114 (IEEE Cat. No. 75-CH 1023 lED)

.

Buehler, M. G., David, J. M. , Mattis, R. L., Phillips, W, E., and Thurber, W. R.,

Semiconductor Measurement Technology: Planar Test Structures for Characterizing

Impurities in Silicon, NBS Spec. Publ. 400-21 (December 1975).

Pease, R. L., Galloway, K. F., and Stehlin, R. A., Radiation Damage to Integrated

Injection Logic Cells, IEEE Trans. Nucl. Phi/s . NS-22 . 2600-2604 (December 197S')

.

75
APPENDIX B

B.3. Availability of Publications

In most cases reprints of articles in technical journals may be obtained on request to the author. NBS Technical Notes and Special Publications are available from the Super

intendent of Documents, U.S. Government Printing Office, Washington, D. C. 20402, or the National Technical Information Service, Springfield, Virginia 22161, or both.

Current information regarding availability of all publications issued by the Program

is provided in the latest edition of NBS List of Publications No. 72 which can be

obtained on request to Mrs. E. C. Cohen, Room B346, Technology Building, National

Bureau of Standards, Washington, D. C. 2023A.

B.4. Videotapes

Color videotape cassette presentation on improvements in semiconductor measurement

technology are being prepared for the purpose of more effectively disseminating the

results of the work to the semiconductor industry. These videotapes are available

for distribution on loan without charge on request to Mrs. E. C. Cohen, Room B346,

Technology Building, National Bureau of Standards, Washington, D. C. 20234. Copies

of these videotapes may be made and retained by requestors. Two videotapes, Defects

in PN Junctions and MOS Capacitors Observed Using Thermally Stimulated Current and

Capacitance Measurements, by M. G. Buehler, and Laser Scanning of Active Semiconductor

Devices, by D. E. Sawyer and D. W. Berning, have been completed and released for

distribution. As an added feature, arrangements can be made for the authors to be

available for a telephone conference call to answer questions and provide more detailed information, following a prearranged showing of either of the videotapes.

76
APPENDIX C WORKSHOP AND SYMPOSIUM SCHEDULE

C.l. Proceedings or Reports of Past Everits :

Symposium on Silicon Device Processing, Gaithersburg, Maryland, June 2-3, 1970.

(Cosponsored by ASTM Committee F-1 and NBS) . Proceedings: NBS Spec. Publ. 337 (November 1970)

.

ARPA/NBS Workshop I. Measurement Problems in Integrated Circuit Processing and Assembly, Palo Alto, California, September 7, 1973. Report: NBS Spec. Publ. AOO-3

(January 1974 )

.

ARPA/NBS Workshop II. Hermeticity Testing for Integrated Circuits, Gaithersburg

i

Maryland, March 29, 1974. Report: NBS Spec. Publ. 400-9 (December 1974).

Spreading Resistance Symposium, Gaithersburg, Maryland, June 13-14, 1974. (Cosponsored by ASTM Committee F-1 and NBS). Proceedings: NBS Spec. Publ. 400-10

(December 1974).

ARPA/NBS Workshop III. Test Patterns, Scottsdale, Arizona, September 6* 1974i

Report: NBS Spec. Publ. 400-15 (February 1976).

ARPA/NBS Workshop IV. Surface Analysis for Silicon Devices, Gaithersburg, Maryland,

April 23-24, 1975. Proceedings: NBS Spec, Publ. 400-23 (March 1976).

Reliability Technology for Cardiac Pacemakers, Gaithersburg, Maryland^ July 28-29;

1975. (Cosponsored by Food and Drug Administration and NBS). Report: NBS

Spec. Publ. 400-28 (to appear).

77
APPENDIX D

STANDARDS COMMITTEE ACTIVITIES

ASTM Committee F-1 on Electronics

J. H. Albers, Secretary, Packaging Subcommittee; Hybrid Microelectronics Subconimittee

M. G. Buehler, Chairman, Task Force on Test Patterns, Process Controls Section;

Semiconductor Crystals, Semiconductor Processing Materials, Semiconductor

Measurements, and Quality and Hardness Assurance Subcommittees

*W. M. Bullis, Secretary; Editor, Semiconductor Crystals Subcommittee

J. R. Ehrstein, Chairman, Resistivity Section; Semiconductor Crystals, Semiconductor

Processing Materials, Semiconductor Measurements, and Quality and Hardness

Assurance Subcommittees

*J. C. French, Chairman, Editorial Subcommittee

G. G. Harman, Secretary, Interconnection Bonding Section; Hybrid Microelectronics, and

Packaging Subcommittees

K. 0. Leedy, Chairman, Packaging Subcommittee; Chairman, Interconnection Bonding

Section; Semiconductor Crystals, Semiconductor Processing Materials, Semiconductor Measurements, Hybrid Microelectronics, and Quality and Hardness

Assurance Subcommittees

T. F. Leedy, Semiconductor Crystals, Semiconductor Processing Materials, Semiconductor

Measurements, and Quality and Hardness Assurance Subcommittees

D. C. Lewis, Laser, Semiconductor Crystals, Semiconductor Processing Materials,

Semiconductor Measurements, Hybrid Microelectronics, Packaging, and Quality

and Hardness Assurance Subcommittees

*C. P. Marsden, Honorary Chairman

R. L. Mattis, Editor, Semiconductor Measurements Subcommittee; Semiconductor Crystals,

and Semiconductor Processing Materials Subcommittees

*J. F. Mayo-Wells, Secretary, Editorial Subcommittee

D. B. Novotny, Editor, Semiconductor Processing Materials Subcommittee; Semiconductor

Crystals and Packaging Subcommittees

W. E. Phillips, Chairman, Lifetime Section; Secretary, Semiconductor Crystals Subcommittee; Semiconductor Processing Materials, Semiconductor Measurements, and

Hybrid Microelectronics Subcommittees

G. J. Rogers, Lasers and Quality and Hardness Assurance Subcommittees

S. Ruthberg, Chairman, Hermeticity Section; Hybrid Microelectronics, and Packaging

Subcommittees

-'<R. I. Scace, First Vice-Chairman

-<H. A. Schafft, Publicity Officer

A. H. Sher, Semiconductor Crystals, Semiconductor Processing Materials, Semiconductor

Measurements, Hybrid Microelectronics, Packaging, and Quality and Hardness

Assurance Subcommittees

W. R. Thurber, Semiconductor Crystals and Semiconductor Measurements Subcommittees

ASTM Committee E-10 on Radioisotopes and Radiation Effects

W. M. Bullis, Subcommittee 7, Radiation Effects on Electronic Materials

J. C. French, Subcommittee 7, Radiation Effects on Electronic Materials

*A11 subcommittees.

78
APPENDIX D

D. C. Lewis, Subcommittee 7, Radiation Effects on Electronic Materials

R. I. Scace, Subcommittee 7, Radiation Effects on Electronic Materials

Electronic Industries Association: Solid State Products Division, Joint Electron Device

Engineering Council (JEDEC)

D. L. Blackburn, Task Group JC-25-5 on Thermal Characterization on Power Transistors,

and Committee JC-25 on Power Transistors

F. F. Oettinger, Chairman, Task Group JC-25-5 on Thermal Characterization on Power

Transistors, Committee JC-25 on Power Transistors; Technical Advisor, Thermal

Properties of Devices, Committees JC-13.1 on Government Liaison for Discrete

Semiconductor Devices, and JC-22 on Rectifier Diodes and Thyristors

S. Rubin, Chairman, Council Task Group on Galvanomagnetic Devices

D. E. Sawyer, Task Group JC-24-5 on Transistor Scattering Parameter Measurement

Standard, Committee JC-24 on Low Power Transistors

H. A. Schafft, Technical Advisor, Second Breakdown and Related Specifications

Committee JC-25 on Power Transistors

lEC TC47, Semiconductor Devices and Integrated Circuits

S. Rubin, Technical Expert, Galvanomagnetic Devices; U.S. Specialist for Working

Group 5 on Hall Devices and Magnetoresistive Devices

IEEE Electron Devices Group

J. C. French, Standards Committee

F. F. Oettinger, Standards Committee Task Force on Second Breakdown Measurement

Standards

H. A. Schafft, Standards Committee Task Force on Second Breakdown Measurement

Standards

IEEE Magnetics Group

S. Rubin, Chairman, Galvanomagnetic Standards Subcommittee

Semiconductor Equipment and Materials Institute

R. I. Scace, Standards Committee

Society of Automotive Engineers

W. M. Bullis, Planning Subcommittee of Committee H on Electronic Materials and

Processes

J. C. French, Subcommittee A-2N on Radiation Hardness and Nuclear Survivability

F. F. Oettinger, Steering Committee, Electronic Systems Committee

79

APPENDIX F

A U T 0 f I A T E D I fj T E G R A T E D CIRCUIT PROCESSING

A fm A S S E M B L Y

*

A study was conducted to review automated

procedures for integrated circuit (IC) processing and assembly, forecast future trends

or directions expected in this technology,

and identify economic and technological factors which at this time appear to inhibit

or delay the development of such automated

procedures. In this context the following

were examined:

1. the susceptibility of existing process technologies to automation;

2. the present status of automation at various stages of production within

the IC industry, here including both

device manufacturers and manufacturers of processing equipment;

3. the impact which emerging process

technologies are likely to have on automation in the future;

4. the status of development of the

measurement technology needed for

control of automated processes, and

the availability of equipment to perform such measurements;

5. the business, economic, and technological factors which may inhibit or delay the advent of automated processes; and

6. the priorities for automating various key process and assembly functions and the criteria appropriate

for developing these priorities.

The study identified certain areas of importance for further automation of IC produc- . tion. The primary emphasis of this study

of automation was its application to the processing of wafers for integrated circuits and

the assembly of chips from these wafers into

devices. Thus, other areas of automation also

applicable to semiconductors or the circuit

design of ICs were only given minor attention

or excluded.

Information for the study v/as obtained from

field visits, interviews, and analysis of

in-house data on the basis of which subjects

especially relevant to automated IC processing and assem.bly were identified. Discussions were restricted to information which

81

respondents felt was not proprietary and

not confidential. In this connection over

20 commercial IC manufacturers were visited

regarding the automation and measurements

used in IC production and how they intend

to apply automation in the areas under study

in the future; 15 other laboratory or pilot

IC fabrication facilities were visited to discuss their special needs for automation

and measurements in IC manufacture; and relevant equipment, such as processing equipment,

production automation systems and services,

m.easuring equipment, and test systems were

reviewed

.

Many IC manufacturers considered the automation of their production an important subject with certain details held proprietary.

Nevertheless, in general respondents were

quite willing to discuss information on measurements and automation which are normally sHown to interested customer-visitors

and where a description of capabilities on ongoing research might enhance their own marketing efforts, such as new research already published, processing equipment or systems for sale, controls included in detailed

purchase specifications, and trends in automation on a broad and general basis. The

helpful cooperation of many individuals in

the industry was essential to the conduct of

the study and is gratefully acknowledged.

Today, some 80 to 130 steps are used in manufacturing ICs. Many of these are repeated,

so that about 20 to 30 basic unit processes

or operations are used. The differing sus- ceptibility of various unit operations has led

to differing status in regard to the degree of

This study was conducted by Arthur D. Little,

Inc., Cam.bridge , Mass., under NBS Contract

No. 4-35807. The views and conclusions ex- pressed herein are those of the author and

should not be interpreted as necessarily

representing the official position, either

expressed or implied , of the Defense Advanced

Research Projects Agency or the National

Bureau of Standards. This summary is presented in preliminary form both to provide

feedback to the industry and to elicit com- ments, pro or con, from the industry. Com.- ments should be addressed to W. M. Bullis,

National Bureau of Standards, Washington,

D. C. 20234.
APPENDIX F

automation. It was found that automation has

only been applied to those unit operations

where positive results of control were demonstrable, technologies were feasible and

available, automation appeared necessary or desirable, economics were favorable, and

management interest and business climate

were positive.

Several IC manufacturers are now extensively

automating the collection and analysis of

electrical measurements from test pattern

chips measured at the completion of wafer

processing with the hope that results from

these measurements will provide improved

understanding and control of the processes as well as means for further automation.

Although many process steps now are extensively mechanized, only a few steps are automated to a high degree of control. The

increasing mechanization of handling has improved process consistency. This provides

an approach toward a smooth flow of product

wafers, chips, and devices and has aided efficient and rapid processing and facilitated

measurement and data collection on a similar

mechanized basis. The higher throughput per

equipment also has improved the economics of

measurement and processing.

There is a general trend towards automation

of individual unit processes. The least

automated process steps are etching and

cleaning steps which are still difficult to control at all. In other processes the processing conditions are kept constant with

sensitive analog measurements and feedback

control, such as for temperature, gas flow,

or bonding pressures. Here open-loop control

is the best solution at this time. This allows one to freeze the process or assembly

conditions with individual regulators or feedback controls. The final device characteristics cannot yet be measured accurately

enough or results are still too inconsistent

for use of parameter measurements in closedloop control. Only in a few instances has

closed-loop automation been applied to control some parameter of the IC crystal, wafer,

device, or contacts.

Automation is being applied to the sequencing

and control of the process variables, or to the data collection and analysis of status. Automation of time and program sequence

control is becoming prevalent, and a few

automated data collection systems are now

being introduced. At present, these two automation approaches have been coupled only

in the case of crystal growing. Although

there are only a few com.puter-controlled

systems in use, there is large and widespread interest in computer control and

many in the industry specify com.puter compatability for all new equipment.

It was found that highly automated electronic

test equipment is available for the measurement of IC and test pattern characteristics.

Other in-process parameters which are now

routinely measured on wafers include the

sheet resistance or resistivity and thickness

of silicon layers, the thickness of dielectric

layers, and the electrical characteristics of

simple transistors and MOS capacitors. The

impossibility of measuring the wafer, chip,

or device during many processes generally

rules out closed-loop control except via

post-process parameter measurements. The

latter are limited by problems of electrical

contacting, optical instrument complexity,

the minute dimensions involved, and inability to profile a wafer surface in depth nondestructively

.

A number of in-process parameter measurements

could become suitable for closed-loop automatic control vjith greater measurement accuracy or improved methods of instrumentation.

Among the parameters of interest are epitaxial layer thickness and composition, oxide

layer thickness and composition, transistor

base width, MOS transistor load resistors

and threshold voltage, photoresist exposure,

completion of etching of layers, and contact

and bond characteristics.

Inspection and defect analysis after many

photolithographic steps is presently carried

out visually by operators. This is highly

subjective, not consistent, and very costly.

Visual inspection at many IC manufacturers

requires as many as 20 percent of all direct

production workers.

Based on the above findings several conclusions were reached:

1. There ore significant needs and opportunities for further extending

and enhancing the automation of IC

production that go beyond the present

status. There is no overall automated production system yet in sight

to cover most processing or assembly

steps. Mutual compatability of unit

equipment and overall system integration will require considerable

further development and standardization. Among the technological factors 2ikelv to retard the adoption of

82
APPENDIX F

automation are the lack of correlation between some process variable

measurements and product parameters

and characteristics, present difficulties in measuring the thickness

of dielectric or oxide layers es- pecially in windows to be etched,

limited accuracy and range of the

measurement of desired process variables, long throughput time of wafer

and device production, which delays

the feedback of measurement results

for corrective action or closed-loop

control, lack of measurement equipment providing suitable accuracy and

reliability for use in a production

environment, lack of standardization

for sequence control and data acquisition, too long a delay between a process step and the measurements such

as on test patterns, and the potential

displacement by alternative emerging

processes

.

Economic factors likely to inhibit

automation in the future are the high

cost of most automation equipment,

computers, and their software. Other

factors are the cost and time required for the development of automation,

difficulties in evaluating possible

cost savings, and accounting and or- ganizational conditions. On the

other hand, business pressures con- tinue on the IC industry for increased and more flexible production

at lower cost and scrap. Thus investments in additional automation

are being considered and approved.

Measurements are important for automation. Each present improvement in

automation has resulted from significant progress in measurement and

control techniques. Present industry

emphasis on freezing process variables

focuses immediate attention on measuring process variables with requirements for greater accuracy of presently measured variables and new techniques for those now not routinely

measured

.

For a number of state-of-the-art

devices industry is using closedloop control of the most critical

product parameter, requiring inprocess measurement of such parameters as of MOS threshold voltages,

high resistivity for transistor

loads, narrow base widths, or line

widths of critical patterns. These

uses are generally rather limited; more widespread policies of measuring product parameters in-process

primarily for monitor purposes and

indications of error or alarm have

caused an unwarranted stagnation in

the interest for improved measuring

equipment for production use. However, it is important to note that

improved methods of measurements and

rapid evaluation of measurement data

are needed for process control in

future automated production and that

several emerging processes use different technologies and require other

physical principles of measurement

than were used in production heretofore. Among the quantities involved

are ion beam currents , dopant vapor

pressures, etching rates of new etching media, alignment errors, photoresist sensitivity, laser position,

and bonding pressure.

Many new measurement techniques now being developed by industry or at NBS

are applicable to research and development as contrasted with production of ICs . To be also applicable to the automation of production

processes, new measurement techniques

must provide non-destructive readings,

high accuracy and consistency, and

rapid availability of data, measured

on reliable equipment.

3. Industry's movements in two opposite

directions regarding measurements on

test patterns and monitor wafers are not incompatible . On the one hand,

appreciable effort is placed in the

development and use of test patterns

preceding wafer probe tests, to overcome deficiencies perceived with

present monitor wafer measurements.

On the other hand, respondents show

strong interest in improving methods

and techniques of measuring inprocess wafer parameters on product

or monitor wafers, to provide more

accuracy and versatility even in the

production environment.

In favor of test patterns are the

following

:

(1) contacting problems are avoided, with better accuracy;

83
APPENDIX F

(2) automated equipment can measure

many different parameters on appropriately designed test patterns;

(3) extensive data analysis facilities are available in the equipment;

(A) the computer system easily makes

corrections for calibration

curves, temperature, or other

influences; and

(5) there is considerable silicon

and labor saving if monitor

wafers are eliminated.

In favor of monitor wafer measurements are;

(1) smaller companies can afford the

less expensive equipment;

(2) with new processes, measurements can be made during a run instead of waiting;

(3) short enough turn around tim.e might never be attained to permit utilization of test pattern

measurements for process control and automation;

(4) monitor wafers will still be

needed to control errors immediately; and

(5) continuity with prior experience

and intuition is preserved.

Both approaches are necessary and

useful

.

Measurement^ of geometrical variations of parameters are irrportant

.

For IC and LSI processing, a number

of areas have become important which

relate to the distribution of defects on chips and wafers, and to the profiles of composition in depth

of the several layers. Thus there

is interest in Industry in such

topics as:

(1) measurement of defects now re- corded by microscopic visual

inspection, ranging from bare

wafers to those in process,

to bonded and metallized chips;

(2) recording and evaluating the

variations of the many types of

measurements over the geometry

of chips, wafers, and lots;

(3) utilizing data analysis, corre- lation techniques, and display

methods to treat the accumulation of measurements and sta- tistical data, especially to evaluate geometrical variations;

(4) developing non-contacting scanning methods for wafer and chip

evaluation;

(5) improving techniques of avoiding contact errors in conventional probe measurements, such

as of resistance and capacitance; and

(6) depth-profiling of parameters

by electrical and optical or other means, including capacitance-voltage profiling.

5. The requirements on control of autom.ation for production of ICs for

commercial and DOB use are sometimes

divergent. Commercial priority is

placed on high volume production for

lowest component cost. This approach involves use of automated

production to maximize yields and

minimize labor, material, and production costs. Priority for DOD

end use is the quick certain delivery of small quantities of very

reliable devices to minimize system,

maintenance and repair costs, with

considerable documentation of test results, lot production information, and inspections. These needs are served by automated production to minimize elapsed time and engineering

effort at both component and system

levels, retain all data to provide

traceability of information, and

facilitate prompt re-manufacture at an undetermined later time.

(H. G. Rudenberg*)

*Arthur D. Little, Inc.

84
INDEX

acoustic emission 48-50

ARPA/NBS Workshop on Surface Analysis for

Silicon Devices 4 ASTM Method F 84 7-9

ASTM Recommended Practice F 134 52-53

automation 81-84

beam-lead bonding 48-50

bias-temperature stress test 29-31

boron redistribution 25-26

capacitance-voltage methods 26-27; 27-29;

31-32

carbon contamination 16-18

charge-coupled device test pattern 44,

46-47

Darlington pairs 66-67

deep-depletion method 26-27; 27-29

dopant profiles 10; 25-26

edge definition 34-41

electron beam induced current mode 56-59

electron beam induced damage 59-60

electron mobility 25 electron spectroscopy for chemical analysis

18-20

emitter-only switching method 66-67

epitaxial layer thickness 27-29

filar eyepiece 38-43

flame emission spectroscopy 3 flying-spot scanner 60-65

four-probe method 7-9

gross leak tests 5

helium mass spectrometer leak test 52-53

hermeticity 52-54

hot-spot detection 60-61

image shearing micrometer eyepiece 39-43

infrared microradiometer 67-68

infrared reflectance method 22-24

insulator films 31-32

interf erometric measurements 42-43

ion implantation 33 ionized dopant density 13-16

ion microprobe mass analysis 20-22

irradiation, SEM 29-31

Irvin's curves 13-16; 25

junction temperature, peak 67-68

line-width measurements 34-43

line-width standards 41

metal-photoresist-semiconductor capacitor

microscopy, optical (theory) 34-41

moisture infusion 52, 54 MOS capacitor 26-31; 44, 46-47; 55-56

MOS shift register 64-65

MOS transistor 44, 46-47

non linear operation, transistor 61-65

oxidation furnace, test for 3 oxide films 29-31; 33

passivation overcoats 33 photomask measurement 34-43

pull test 5; 51

rapid gas cycling gross leak test 5 resonance fluorescence spectroscopy 2 Rutherford backscattering spectrometry

17-18

safe operating area, transistor 67-68

sapphire, silicon on 22-23; 47 scanning electron microscopy 56-60

scanning low energy electron probe 55-56

secondary ion mass spectrometry 20-22

shear test 51 sheet resistors 44-45

spreading resistance 10-13

SRM 1520 7-9

test patterns 44, 46-47; 47 thermal resistance 66-67

thernial response 67-68

UHF transistor 60-65

ultrasonic bonding 51

van der Pauw sheet resistor 44-45

wire bonds 51

X-ray photoelectron spectroscopy 18-20

85
M»S.1I4A (REV. 7-73) " U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET

1. PUIUJC ATION OR REPORT NO. NBS Spec. Publ. AOO-19

2. Gov't Accession

No.

3. Recipient's Accession No.

4. TITl.K AND SUBTITLH

Semiconductor Measurement iecnnoiogy. rrogress Report

January 1 to June 30, 1975

5. Publication Date April 1976

6. Performing Organization Code

y. AUTMOR(S) W. Murray Bullls, Editor

8. Performing Organ. Report No.

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project/Task/Work Unit No.

11. Confrarr /Grant No. ARPA Order Pfini. Code SDK

DNA lACRO 75-816; SSPO Order N0016475P070030, IPR SP6-75-4

12. Sponsorinp Organization Name and Complete Address (Street, City, State, ZIP)

NBS -Washington , D. C. 20234

SSPO - Washington, D. C. 20376

DNA - Washington, D. C. 20305

ARPA - 1400 Wilson Boulevard, Arlington, Va. 22209

13. Type of Report & Period Covered Interim". Jan.

1 to June 30, 1975

14. Sponsoring Agency Code

IS. SUPPLEMENTARY NOTES

It. ABSTRAC;T (A 200-word or less factual summary ot most sigpificant information. If document includes a significant

bibliography or literature survey, mention it here.)

This progress report describes ;IES activities directed tou.ird tlic development of methods of ireasurement for semiconductor materials, process rontrcl, and oVvicen. Botli in-Iimise and contract efforts are Included. The cmph.nsls Is on silicon device tech- nologies, Principal accoir'pllshments during this reporting period Included (1) completion and analysis of an interlaboratory

evaJuntlon of standard reference wafers for resistivity measurement by the four-probe method, (2) analysis of the effect of finite contact size on sheet rL'slst--jnce as measured with a van der I'auw structure, (3) calculation of errors introduced In ceSsurlni; line width with typical microscope systems, (A) development of procedures for predicting the magnitude of electron

bcaa induced current in silicon device structures, (5) application of the optical flying-spot scanner to observation of hot spots and ncn-linearities In rf power transistors and of logic patterns in an MOS shift register, and (6) determination of a Bcre accurate electrical method, based on peak junction temperature measurement, for establishing safe operating area curves

for mcdluir power transistors. Also reported arc the results of work on spreading resistance measurements, ionization of dopant Irpurlties In silicon, Rutherford backscatterlng measurements. X-ray photoelectron spectroscopy, Ion mlcrcprobc- mass analysis,

tests for determining the surface quality of sapphire substrates, reevaluatlon of Irvin's curves, mathematical models of dopant

profiles, deep depletion measurements of resistivity profiles, ireasurement of epitaxial layer thickness by the deep depletion

mctliod, fc las-t* "^.perature stress test measurements on MOS capacitors, a high voltage capacitance-voltage method for measuring

characteristics of thick insulator films, ion implantation parameters, methods for determining integrity of passivation overcoat*, optical imaging and calibration standards for photomask m.etrology, line-width measurements, charge-coupled device test Structures, test pattern design and analysis for silicon on sapphire MOS device technologies, nondestructive acoustic emission

test for tear.-lead bonds, pull and shear tests for wire bonds, a rapid cycle dry gas gross leak test, leak detection by helium

nass spectrometry, correlation of moisture Infusion in semiconductor packages with leak size and device reliability, an autoDot'd sr^rifilng low-energy electron probe, electron beam induced damage In silicon device structures, and thermal resistance

neasurer.tnts on Darlington transistors. Supplementary data concerning staff, publications, workshops and symposia, standards

coTsnlttoe activities, and technical services are also included as appendices. A sixth appendix is included to summarize the results if a study which was carried out to assess the impact of automation of integrated circuit processing and assembly

future reasurcnent requirements in the industry.

17. KEY WORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper name; separated by semicolons)

Acoustic emission; beam-lead bonds; bias-temperature stress test; boron redistribution; capacitance-voltage methods; chargecoupled device structures; Darlington pairs; deep depletion; dopant profiles; electrical properties ;• electron beam Induced

current; electron beam induced damage; electronics; epitaxial layer thickness; four-probe method; hermetlclty; leak tests; measurenent methods; microelectron' ts; moisture Infusion; optical flying-spot scanner; oxide films; passivation overcoats; photomask metrology; pull test; resistivity; Rutherford backscatterlng; scanning electron microscope; scanning low energy

electron probe; semiconductor devices; semiconductor materials; semiconductor process control; shear test; sheet resistors;

silicon; silicon on sapphire; spreading resistance; test patterns; thermal resistance; thermal response; transistors; ultrasonic bonding; wire bonds; X-ray photoelectron spectroscopy.

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all concerned interests with a basis for common understanding of the characteristics of the products. NBS

administers this program as a supplement to the activities of the private sector standardizing organizations. Federal Information Processing Standards Publications (FIPS PUBS)—Publications in this series collectively constitute the Federal Information Processing Standards Register. Register serves as the official source of information in the Federal Government regarding stand- ards issued by NBS pursuant to the Federal Property

and Administrative Services Act of 1949 as amended,

Public Law 89-306 (79 Stat. 1127), and as implemented

by Executive Order 11717 (38 FR 12315, dated May H,

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Consumer Information Series—Practical information, based on NBS research and experience, covering areas

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NBS Interagency Reports (NBSIR) —A special series of interim or final reports on work performed by NBS for outside sponsors (both government and non-government). In general, initial distribution is handled by the sponsor; public distribution is by the National Technical Information Service (Springfield, Va. 22161) in paper

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The following current-awareness and literature-survey

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Cryogenic Data (Center Current Awareness Service A literature survey issued biweekly. Annual subscription: Domestic, $20.00; foreign, $25.00.

Liquefied Natural Gas. A literature survey issued quarterly. Annual subscription: $20.00.

Superconducting Devices and Materials. A literature

survey issued quarterly. Annual subscription : $20.00.

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Division, National Bureau of Standards, Boulder,

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