On the cover (top to bottom): 

Joseph Hodges operating frequency-stabilized cavity ring-down spectroscopy (CRDS) 

for measuring very low concentration contaminants in process gases.

Time-resolved vertical distribution of optical emission at 750nm from a dual-frequency 

capacitively-coupled plasma.

Critical Dimension Small Angle X-Ray Scattering (CD-SAXS) detector image resulting 

from a dense array of 60 nm vias.

Curt Richter loads a molecular electronic sample for electrical characterization. 

Photo copyright Robert Rathe.
SEMICONDUCTOR MICROELECTRONICS

AND NANOELECTRONICS PROGRAMS

NISTIR 7426

July 2007

U.S. DEPARTMENT OF COMMERCE

Carlos M. Gutierrez, Secretary

Technology Administration

Robert Cresanti, Under Secretary of Commerce for Technology

National Institute of Standards and Technology

William Jeffrey, Director
DISCLAIMER

Disclaimer: Certain commercial equipment and/or software are identifi ed in this report to 

adequately describe the experimental procedure. Such identifi cation does not imply recommendation or endorsement by the National Institute of Standards and Technology, nor does 

it imply that the equipment and/or software identifi ed is necessarily the best available for 

the purpose.

References: References made to the International Technology Roadmap for Semiconductors

(ITRS) apply to the most recent edition, dated 2005 or the ITRS 2006 Update.

Semiconductor Industry Association. The International Technology Roadmap for Semiconductors, 2005 edition. SEMATECH: Austin, TX, 2005.

These documents are available on-line at URL: http://public.itrs.net or in printed copy by 

contacting SEMATECH, 2706 Montopolis Drive, Austin, TX 78741, ITRS department 

860-008, phone: (512) 356-3500.

The reader will notice that there are acronyms and abbreviations throughout this document 

that are not spelled out due to space limitations. We have listed the acronyms and abbreviations in an appendix at the end of this document.
Semiconductor Microelectronics and Nanoelectronics Programs iii

CONTENTS

Welcome and Introduction ....................................................................................................v

Offi ce of Microelectronics Programs Organization ............................................................vii

Lithography Metrology Program ......................................................................................1

Metrology Supporting Deep Ultraviolet Lithography ..........................................................3

Metrology Supporting Extreme Ultraviolet Lithography .....................................................7

Polymer Photoresist Fundamentals for Next-Generation Lithography ..............................13

Critical Dimension and Overlay Metrology Program ...................................................19

Wafer-Level and Mask Critical Dimension Metrology ......................................................20

 Model-Based Linewidth Metrology .............................................................................21

 Scanning Electron Microscope-Based Dimensional Metrology ..................................25

 Scanning Probe Microscope-Based Dimensional Metrology ......................................31

 Optical-Based Photomask Dimensional Metrology .....................................................37

 Scatterometry-Based Dimensional Metrology ............................................................41

 Small Angle X-Ray Scattering-Based Dimensional Metrology ...................................45

 Dimensional Metrology with Grazing Incident X-Ray Scattering ..............................49

 Atom-Based Dimensional Metrology ..........................................................................51

 Fabrication and Calibration Metrology for Single-Crystal CD Reference Materials ..57

Wafer-Level and Overlay Metrology ..................................................................................63

Front-End Processing Metrology Program ....................................................................69

Wafer and Chuck Flatness Metrology ................................................................................71

Modeling, Measurements, and Standards for Wafer Surface Inspection ............................75

Front-End Materials Characterization .................................................................................79

Plasma Process Metrology ..................................................................................................89

Interconnect and Packaging Metrology Program ..........................................................93

Atomic Layer Deposition – Process Models and Metrology ..............................................95

Advanced Nanoscale and Mesoscale Interconnects ........................................................101

Nanoporous Thin-Film Metrology for Low-κ Dielectric Materials..................................105

Interconnect Materials and Reliability Metrology ............................................................ 110

 Basic Materials Properties .......................................................................................... 111

 Test Structures For Interconnect Metrology and Modeling ....................................... 119

Pb-Free Surface Finishes for Electronic Components: Sn Whisker Growth ....................125

Process Metrology Program ...........................................................................................129

Gas Property Data and Flow Standards for Improved Gas Delivery Systems .................131

Low Concentration of Humidity Standards ......................................................................135

Temperature Measurements and Standards for Semiconductor Processing .....................141

Physical Properties of Liquid Precursors ..........................................................................145
iv National Institute of Standards and Technology 

Analysis Tools and Techniques Program ......................................................................147

Thin-Film X-Ray Metrology for Microelectronics ...........................................................149

Electron Microscope Tomography of Electronic Materials ..............................................155

High-Resolution Microcalorimeter X-Ray Spectrometer for Chemical Analysis ............157

Device Design and Characterization Program .............................................................163

Device Characterization and Reliability ...........................................................................165

Nanoelectronic Device Metrology ....................................................................................169

Power Semiconductor Device Metrology ........................................................................179

Organic Electronics Metrology .........................................................................................185

Micro- and Nano-Electro-Mechanical Technology Metrology ........................................189

NIST’s Center for Nanoscale Science and Technology Nanofab ....................................195

Metrology for Spintronic Devices ....................................................................................197

BioElectronics Metrology .................................................................................................203

System Design and Test Metrology Program ...............................................................207

Metrology for System-on-a-Chip (SoC) ...........................................................................209

At-Speed Test of Digital Integrated Circuits .....................................................................215

Thermal Measurements and Packaging Reliability .........................................................219

Manufacturing Support Program .................................................................................227

Factory Time Synchronization Standards Development for E-Manufacturing ................229

E-Diagnostics Security ......................................................................................................233

Engineering Chain Management in the Semiconductor Industry .....................................235

NIST/SEMATECH e-Handbook of Statistical Methods ...................................................237

Abbreviations and Acronyms .........................................................................................239

Technical Contacts ..........................................................................................................242
Semiconductor Microelectronics and Nanoelectronics Programs v

WELCOME AND INTRODUCTION

WELCOME

The microelectronics industry supplies vital components to the electronics industry and to 

the U.S. economy, enabling rapid improvements in productivity and in new high technology 

growth industries such as electronic commerce and biotechnology. The National Institute of 

Standards and Technology, NIST, in fulfi lling its mission of strengthening the U.S. economy, 

works with industry to develop and apply technology, measurements and standards; and applies substantial efforts on behalf of the semiconductor industry and its infrastructure. This 

report describes the many projects being conducted at NIST that constitute that effort.

HISTORICAL PERSPECTIVE

NIST’s predecessor, the National Bureau of Standards (NBS), began work in the mid-1950s 

to meet the measurement needs of the infant semiconductor industry. While this was initially 

focused on transistor applications in other government agencies, in the early 1960s the Bureau 

sought industry guidance from the American Society for Testing and Materials (ASTM) and 

the U.S. Electronic Industries Association (EIA). ASTM’s top priority was the accurate measurement of silicon resistivity. NBS scientists developed a practical nondestructive method 

ten times more precise than previous destructive methods. The method is the basis for fi ve 

industrial standards and for resistivity standard reference materials widely used to calibrate 

the industry’s measurement instruments. The second project, recommended by a panel of EIA 

experts, addressed the “second breakdown” failure mechanism of transistors. The results of 

this project have been widely applied, including solving a problem in main engine control 

responsible for delaying the launch of a space shuttle.

From these beginnings, by 1980 the semiconductor metrology program had grown to employ 

a staff of 60 with a $6 million budget, mostly from a variety of other government agencies. 

Congressional funding in that year gave NBS the internal means to maintain its semiconductor 

metrology work. Meeting industrial needs remained the most important guide for managing 

the program.

INDUSTRIAL METROLOGY NEEDS

By the late 1980s, NBS (now NIST) recognized that the semiconductor industry was applying 

a much wider range of science and engineering technology than the existing NIST program 

was designed to cover. The necessary expertise existed at NIST, but in other parts of the 

organization. In 1991, NIST established the Offi ce of Microelectronics Programs (OMP) to 

coordinate and fund metrological research and development across the agency, and to provide 

the industry with easy single point access to NIST’s widespread projects. Roadmaps developed by the U.S. Semiconductor Industry Association (SIA) have independently identifi ed 

the broad technological coverage and growing industrial needs for NIST’s semiconductor 

metrology developments. As the available funding and the scope of the activities grew, the 

collective name became the National Semiconductor Metrology Program (NSMP), operated 

by the OMP.

The NSMP has stimulated a greater interest in semiconductor metrology, motivating most of 

NIST’s laboratories to launch additional projects of their own and to cost-share OMP-funded 

projects. The projects described in this book represent this broader portfolio of microelectronics projects. Most, but not all, of the projects described are partially funded by the NSMP, 

which is providing a $12 million budget in fi scal year 2007. 
vi National Institute of Standards and Technology 

FOSTERING NIST’S RELATIONSHIPS WITH THE INDUSTRY

NIST’s relationships with the SIA, SEMATECH and its subsidiary, International SEMATECH Manufacturing Initiative (ISMI), and the Semiconductor Research Corporation (SRC) 

are also coordinated through the OMP. Staff from OMP and NIST Laboratories represents 

NIST on the SIA committees that develop the International Technology Roadmap for Semiconductors (ITRS), as well as on numerous SRC Technical Advisory Boards. NIST staff is 

also active in the International National Electronics Manufacturers Initiative (iNEMI), the 

EIA, the International Organization for Standardization (ISO), and Semiconductor Equipment 

and Materials International (SEMI). NIST supports the United States National Committee 

Technical Advisory Group for the International Electrotechnical Commission Technical Committee TC113 on Nanotechnology Standardization for Electrical and Electronic Products and 

Systems (Technical Advisor, USNC TAG for IEC TC 113 on nanotechnology) by funding the 

Technical Advisor to that organization.

LEARN MORE ABOUT SEMICONDUCTOR METROLOGY AT NIST

This publication provides summaries of NIST’s metrology projects for the silicon semiconductor industry and their suppliers of materials and manufacturing equipment. Each project 

responds to one or more metrology requirements identifi ed by the industry in sources such 

as the ITRS. NIST is committed to listening to the needs of industry, working with industry 

representatives to establish priorities, and responding where resources permit with effective 

measurement technology and services. For further information, please contact:

Offi ce of Microelectronic Programs

National Institute of Standards and Technology

100 Bureau Drive

Building 225, Room A317, Mail Stop 8101

Gaithersburg, MD 20899-8101

Telephone: (301) 975-4400

Fax: (301) 975-6513 

e-mail: nsmp@nist.gov 

Internet: http://www.eeel.nist.gov/omp
Semiconductor Microelectronics and Nanoelectronics Programs vii

OFFICE OF MICROELECTRONICS PROGRAMS

ORGANIZATION

KNIGHT, Stephen (Director) stephen.knight@nist.gov

(301) 975-2871 

MARTINEZ DE PINILLOS, Joaquin V. (Senior Scientist) jack.martinez@nist.gov

(301) 975-8125 

BUCKLEY, Michele L. (Offi ce Manager) michele.buckley@nist.gov

(301) 975-4400

From left to right: Joaquin Martinez de Pinillos, Stephen Knight, and Michele Buckley.

Semiconductor Microelectronics and Nanoelectronics Programs 1

LITHOGRAPHY METROLOGY PROGRAM

Advances in lithography have largely driven the spectacular productivity improvements of 

the integrated circuit industry, a steady quadrupling of active components per chip every 

three years over the past several decades. This continual scaling down of transistor dimensions has allowed more and more components on a chip, lowered the power consumption 

per transistor, and increased the speed of the circuitry. The shrinking of device dimensions 

has been accomplished by shortening the wavelength of the radiation used by the lithography 

exposure tools. The workhorse tools at this point operate at 193 nm. In order to further shrink 

dimensions liquid immersion tools with water as the immersion fl uid are being introduced. 

Further size reduction through the use of high index fl uids, lens materials and photoresist 

for 193 nm tools are under intense exploration for even higher numerical aperture systems. 

Looking beyond the deep ultraviolet, extreme ultraviolet radiation (EUV) at 13 nm is being 

investigated, and demonstration tools are being designed and assembled. At least three alpha 

tools were shipped to development consortia in 2006. The overall goal of this task is to support these developments in DUV and EUV. The areas of emphasis are characterization of 

lens materials, immersion fl uids, laser calorimetry, radiation detector sensitivity and damage, 

EUV lens metrology, and metrology for the development of advanced photoresist materials 

for both DUV and EUV.
2 National Institute of Standards and Technology 
Semiconductor Microelectronics and Nanoelectronics Programs 3

METROLOGY SUPPORTING DEEP ULTRAVIOLET

LITHOGRAPHY

Technical Contacts:

J. H. Burnett

M. Dowell

“It’s an excellent service 

NIST has performed for 

the entire industry. The 

kind of thing NIST is 

there for – to identify 

issues before the train 

wreck takes place.”

Mordechai Rothschild, 

Massachusetts Institute 

of Technology’s 

Lincoln Laboratory

GOALS

Develop solutions to key optical metrology issues confronting the semiconductor lithography 

industry. These include development of measurement methods and standards for characterizing 

deep ultraviolet (DUV) laser sources, detectors, 

and materials. One focus is on delivering highaccuracy measurements of DUV detector parameters and materials properties of immediate need 

by the industry. There is ongoing activity in the 

following areas: standards development, calibration services, characterization of optical materials, 

sources, and detectors, in addition to advising 

customers on in-house measurements.

CUSTOMER NEEDS

Increasing information technology requirements 

have created a strong demand for faster logic 

circuits and higher-density memory chips. This 

demand has led to the introduction of DUV 

laser-based lithographic tools for semiconductor 

manufacturing. These tools, which employ KrF 

(248 nm) and ArF (193 nm) excimer lasers, have 

led to an increased demand for accurate optical 

measurements at DUV laser wavelengths.

A new lithography technology, immersion lithography, depends on incorporating a high-index fl uid 

between the optical system and the wafer and 

possibly also incorporating a high-index material 

as the last lens element. Design and development 

of 193 nm immersion lithography requires accurate measurements of the index properties of the 

potential 193 nm fl uids and materials.

To support these efforts, the National Institute 

of Standards and Technology (NIST), with 

SEMATECH, has developed a DUV metrology 

program focusing on the characterization of DUV 

optical materials, sources, and detectors. 

The potential challenges for lithographic development are discussed in the 2005 International 

Technology Roadmap for Semiconductors. Page 1 

of the Lithography section states: “Signifi cant 

challenges exist in extending optical projection lithography at 193 nm wavelength using 

immersion lenses....” The need for advancing 

metrology in lithography is discussed on page 1 

of the Metrology section: “Metrology continues to 

enable research, development, and manufacture 

of integrated circuits. The pace of feature size 

reduction and the introduction of new materials 

and structures challenge existing measurement 

capability.”

TECHNICAL STRATEGY

High-accuracy measurements of the index properties of UV materials are required for the design of 

DUV lithography systems. NIST has been providing absolute index measurements at 193 nm and 

157 nm with an accuracy of about 5 ppm to the 

industry using its DUV minimum-deviation-angle 

refractometer. To improve on this absolute accuracy, NIST has constructed a new state-of-the-art 

minimum-deviation system and separately developed another system based on a vacuum ultraviolet 

(VUV) FT spectrometer and synchrotron radiation 

as a continuum source (see Fig. 1). Both of these 

systems enable measurements to an accuracy of 

1 ppm, and are used to characterize high-index 

lens materials and immersion fl uids for 193 nm 

lithography systems.

Figure 1. Facility for detector VUV radiation 

damage study using synchrotron radiation from 

SURF and a 157 nm excimer laser.

DELIVERABLES: Measurement of the refractive index 

of fused silica, CaF2

, and water, as well as high-index 

fl uids and high-index lens materials, with an accuracy 

of 1 ppm. 4Q 2007

Taking full advantage of the potential resolution 

gain with immersion lithography may require using high-index materials as the last lens element, 

though as yet no such material has been demonstrated at 193 nm. To address this need we have, 

with the support of SEMATECH, undertaken and 
4 National Institute of Standards and Technology 

completed a survey of candidate materials. As a 

result of this work we have identifi ed two very 

promising materials, ceramic spinel and lutetium 

aluminum garnet (LuAG). Both materials have 

indices near 2.0 at 193 nm and have the potential 

for high optical quality at this wavelength. In addition, the intrinsic birefringence is low for LuAG 

and negligible for ceramic spinel. We are now 

characterizing the complete UV optical properties 

of these materials.

DELIVERABLES: Fully characterize the 193 nm optical properties of LuAG and ceramic spinel. 4Q 2007

An absolute light source in the DUV range based 

on synchrotron radiation using NIST’s Synchrotron Ultraviolet Radiation Facility (SURF III) 

has been established using a dedicated beamline 

at SURF III. The fl ux of the DUV radiation at 

this beamline can be known to very high accuracy 

through the well established equations governing 

the behavior of the synchrotron radiation. The 

beamline is designed for customer calibration 

of a variety of DUV instruments to assist the 

development of the DUV lithography such as 

monochromators, discharge lamps, and irradiance meters. The spectral range covers all of the 

current important wavelengths for semiconductor 

industry such at 248 nm, 193 nm, 157 nm and even 

down to 13 nm. The uncertainty of such calibration is better than 1 % in the case of deuterium 

lamp calibration.

DELIVERABLES: Provide customer DUV calibration 

for discharge lamps, monochromators, and irradiance 

meters using SURF III source-based beamline with 

highest accuracy. Ongoing

Beginning with the fi rst edition of the National 

Technology Roadmap for Semiconductors 

(NTRS) in 1992, the semiconductor industry has 

made an organized, concentrated effort to reduce 

the feature sizes of integrated circuits. As a result, 

there has been a continual shift towards shorter 

exposure wavelengths in the optical lithography 

process. Because of their inherent characteristics, 

deep ultraviolet (DUV) lasers, and specifi cally 

KrF (248 nm) and ArF (193 nm) excimer lasers, 

are the preferred sources for high-resolution 

lithography at this time. To meet the laser metrology needs of the optical lithography community, 

we have developed primary standards and associated measurement systems at 193 nm, 248 nm, 

and 157 nm. Figure 2 shows the excimer laser 

calibration facility.

DELIVERABLES: Proven high-quality calibration 

services, and supporting measurements for excimer 

laser power and energy meters to the Semiconductor 

Industry at 248 nm, 193 nm, and 157 nm. Ongoing

ACCOMPLISHMENTS

We have used our Hilger-Chance refractometer system to assist the industry in the search 

for appropriate high-index fl uids (with n greater 

than water, 1.4366 at 193 nm) for possible use 

in immersion lithography resolution extension. 

Several promising fl uids have been developed by 

the industry, and we have characterized their UV 

optical properties. Recently we have worked with 

the suppliers to demonstrate that several of these 

candidate fl uids have stable optical properties over 

multiple exposure runs.

As a result of our high-index materials survey, 

we have identifi ed two very promising high-index 

193 nm transmitting materials, that can potentially 

enable immersion lithography extension. These 

candidates are ceramic spinel and lutetium aluminum garnet (LuAG). We have demonstrated that 

both materials have good 193 nm optical properties, including suffi ciently low spatial-dispersioninduced (“intrinsic”) birefringence (see Fig. 3). 

Due to the results of our measurements, LuAG 

and ceramic spinel are now being developed by 

the industry for this application.

#

#

Figure 2. Laboratory for excimer laser energy and 

power meter calibrations, with measurement systems 

for 248 nm, 193 nm, and 157 nm. The excimer 

lasers are along the top right and the enclosures for 

nitrogen gas purging are in the foreground. 
Semiconductor Microelectronics and Nanoelectronics Programs 5

Figure 3. Measurement of the intrinsic birefringence 

of LuAG vs. wavelength.

We have constructed a radiometric facility 

tailored for the DUV range using a beamline at 

NIST’s SURF III with the radiation measurement 

scale derived from a high-accuracy cryogenic 

radiometer. The beamline is designed for very 

general-propose high-accuracy measurements. 

We have used this facility to measure DUV general material properties such as transmission and 

refl ectance. Examples of such measurements 

include DUV mirrors, windows, fi lters, and also 

the transmission and absorption of liquids that 

could be used for immersion lithography. On the 

detector side, we have calibrated and characterized a variety of DUV detectors such as solid state 

photodetectors, solar-blind detectors, photoconductive detectors, and pyroelectric detectors. We 

also performed irradiance calibrations for DUV 

irradiance meters. 

We have built a facility at SURF III that allows simultaneous exposure of photodiodes to 

excimer radiation (see Fig. 4) and synchrotron 

radiation. Measurements of the spectral responsivity can be made in the spectral range from 

130 nm to 320 nm with a standard uncertainty of 

less than 1 %. The intense, pulsed laser radiation 

was used to expose the photodiodes for varying 

amounts of accumulated irradiation whereas the 

low intensity, continuously-tunable cw radiation 

from the synchrotron source was used to characterize the photodiodes. The changes in the 

spectral responsivity of different kinds of diodes 

such as UV silicon, GaP, GaAsP, PtSi, diamond, 

and GaN were measured for a large range of total 

accumulated dose from an F2

 excimer laser operating at 157 nm. Differing amounts of changes 

were seen in different diodes depending on the 

total excimer irradiation dose and they showed 

different spectral changes in the responsivity as 

well. This yields important information about 

#

#

Figure 4. Measurement system for detector damage 

study by a 157 nm pulsed excimer laser.

the mechanism responsible for the degradation of 

photodiodes. For example, we have determined 

that for silicon photodiodes under irradiation with 

a 157 nm excimer laser, an important mechanism 

for the degradation is the formation of trap states at 

the interface of the silicon-silicon dioxide induced 

by the damaging radiation. These trap states act 

as recombination centers and reduce the yield of 

electric current generated by incident radiation. 

A model was developed to simulate the change 

in response for photodiodes irradiated by 157 nm 

radiation.

During the last 15 years we have developed 

a suite of laser calorimeter standards for 248 nm, 

193 nm, and 157 nm excimer laser energy and 

power measurements traceable to SI units. The 

248 nm and 193 nm calorimeters use a specially 

designed absorbing cavity with a volume absorbing glass to reduce potential damage to the cavity 

by the high peak power in the UV laser pulses. 

The 157 nm calorimeter is a fundamentally new 

type of laser calorimeter standard that uses a thinwalled SiC absorbing cavity, which is designed to 

completely absorb and spread the incoming laser 

energy through multiple refl ections. All of these 

calorimeters are calibrated using an imbedded 

electrical heater that allows for traceability to SI 

units through electrical standards of resistance and 

voltage. Calibrations for industry customers are 

accomplished for each wavelength with appropriate measurement systems that involve purging of 

oxygen to eliminate atmospheric absorption of 

the laser radiation. 

As a further extension of our excimer laser 

services we have developed the capability to 

directly measure UV irradiance or “dose” at 

248 nm and 193 nm, which involves homogenizing the beam profi le and measuring the energy 

#

#
6 National Institute of Standards and Technology 

transmitted through a calibrated aperture. This 

capability can improve accuracy for customers 

who need to measure the energy absorbed at a 

surface such as at the wafer plane.

We have also developed the capability to characterize the nonlinear response of 193 nm and 

248 nm excimer laser detectors based on the 

correlation method. The method and system 

solves measurement diffi culties associated with 

the temporal and spatial fl uctuations of excimer 

laser pulse energy. Using this system, one can 

easily determine problems such as those due to 

the incident pulse energy, range discontinuities 

associated with detector gain, and detector background noise (see Fig. 5).

Figure 5. Nonlinearity measurement result of a 

193 nm pulsed laser energy detector. CF is the 

correction for the detector’s nonlinear response. 

The response is measured in four spans, covering 

2.5 meter settings. The large degree of nonlinearity at 

the low end of the meter range is due to background 

effects.

We completed the fi rst internal comparison 

of the NIST UV excimer laser calorimeters. This 

work includes measurements taken over the course 

of a two-year period in which the performance of 

the NIST 157 nm, 193 nm, and 248 nm excimer 

laser calorimeters was monitored at the design 

wavelengths as well as at the other excimer laser 

wavelengths. The results show good agreement 

among the transfer standards and excellent stability over time. From these data, we determined that 

the responsivity of the NIST UV laser calorimeters 

all agree within their stated uncertainties. In all but 

one case, the calorimeters’ responsivities agree to 

better than 0.3 %. The comparison between the 

DUV (193 nm) and UV (248 nm) calorimeters at 

248 nm uncovered a 1 percent difference between 

the calorimeters’ responsivities. This difference 

is due to partial transmission of the 248 nm 

radiation through the absorbing glass of the DUV 

calorimeter which, reduces the calorimeter’s 

absorptance and alters its response.

#

COLLABORATIONS

DuPont, Roger French, immersion photolithography fl uid 

development. Schott Lithotec, Lutz Partier, high index lithography materials development.

RECENT PUBLICATIONS

S. G. Kaplan and J. H. Burnett, “Optical properties of fl uids 

for 248 nm and 193 nm immersion photolithography,” Applied 

Optics 45, 1721 (2006).

A.H. Harvey, S.G. Kaplan, and J.H. Burnett, “Effect of dissolved air on the density and refractive index of water,” submitted to International Journal of Thermophysics (2005).

P. S. Shaw, R. Gupta, K. R. Lykke, “Stability of photodiodes 

under irradiation with a 157 nm pulsed excimer laser,” Appl. 

Opt. 44, 197 (2005).

S. Yang, D. A. Keenan, M. L. Dowell, “Nonlinearity Measurements of the Response of Silicon Photodiodes at 193 nm,” 

Council for Optical Radiation Measurement (CORM) 2005, 

May 10-12, 2005, Boulder, Colorado, (Approved by ERB). 

H. Laabs, D. A. Keenan, S. Yang, M. L. Dowell, “Measurement 

of detector nonlinearity at 193 nm,” Appl. Opt., Vol. 44, No. 

6, pp.841-848 (20-FEB-2005).

J. H. Burnett and S. G. Kaplan, “Measurement of the refractive index and thermo-optic coeffi cient of water near 193nm,” 

Journal of Microlithography, Microfabrication, and Microsystems 3, pp. 68-72 (2004).

J. H. Burnett and S. Kaplan, “Measurement of the refractive 

index and thermo-optic coeffi cient of water near 193nm,” in 

Proceedings of SPIE Vol. 5040 Optical Microlithography 

XVI, edited by Anthony Yen, (SPIE, Bellingham, WA, 2003) 

1142-1149.

Z. H. Levine, J. H. Burnett, and E. L. Shirley, “The photoelastic 

and elastic properties of the fl uorite structure materials, LiF, 

and Si,” Phys. Rev. B 68, 155120 (2003).

P. S. Shaw, U. Arp, H. W. Yoon, R. D. Sanders, A. C. Parr, and 

K.R. Lykke, “A SURF beamline for synchrotron source-based 

absolute radiometry,” Metrologia, 40, S124 (2003).

D. A. Keenan, H. Laabs, S. Yang, M. L. Dowell, “A 193 nm 

Laser Detector Nonlinearity Measurement System,” Proc., 

Measurement Science Conference, January 13-17, 2003, 

Anaheim, California, (01-JAN-2003).

C. L. Cromer, M. L. Dowell, R. D. Jones, D. A. Keenan, 

S. Yang, “A Primary Standard for 157 nm Excimer Laser 

Measurements,” 2003 Int’l Conf. on Characterization and 

Metrology for ULSI Tech. March 25-27, 2003, Austin, Texas, 

Vol. 683, pp.409-412 (01-MAR-2003).
Semiconductor Microelectronics and Nanoelectronics Programs 7

Technical Contacts:

T. Lucatorto

C. Tarrio

S. Grantham

U. Griesmann

S. Hill

R. Vest

Figure 1. Refl ectivity vs. wavelength of a typical 

MoSi multilayer mirror. The measurement was made 

at 5° from normal incidence.

METROLOGY SUPPORTING EXTREME ULTRAVIOLET

LITHOGRAPHY

GOALS

Provide leading-edge metrology for the development and characterization of sources, optical 

components, and dosimeters used in Extreme 

Ultraviolet Lithography (EUVL). (EUVL utilizes 

radiation at 13.4 nm.)

CUSTOMER NEEDS

An intense international effort is presently under 

way to install EUVL into commercial production 

in 2011 at the 32 nm node. ASML has delivered 

two alpha-generation steppers for initial testing, 

one to SEMATECH in Albany and one to IMEC 

in Belgium.

Several signifi cant challenges to commercialization of EUVL remain, including source power, 

optics lifetime, and optics and mask fabrication. 

The associated metrological challenges include 

the development of: (1) highly precise extreme 

ultraviolet (EUV) reflectometry; (2) accurate 

pulsed EUV radiometry for source comparisons 

and wafer-plane dosimetry; (3) accelerated testing 

techniques for optics lifetime characterization; 

and (4) nanometer-level optical fi gure measurement. 

TECHNICAL STRATEGY

1. PRECISE EUV REFLECTOMETRY

The NIST/DARPA EUV Refl ectometry Facility is 

located on a multipurpose beamline on the NIST 

Synchrotron Ultraviolet Radiation Facility (SURF 

III) storage ring. Currently the NIST/DARPA 

facility is the only one in the U.S. large enough 

to measure optics up to 40 cm in diameter and 

40 kg in mass. The facility has a demonstrated 

refl ectivity accuracy of 0.3 % and wavelength 

accuracy of 0.001 nm, with plans under-way to 

improve each accuracy by a factor of two in the 

near future.

Although primarily designed to serve the EUVL 

community by providing accurate measurements 

of multilayer mirror refl ectivities, this beamline 

with its associated sample chamber has also 

been used for many other types of measurements 

since the beamline’s commissioning in early 

1993. Among the other measurements in support 

of EUVL are the radiometric calibration of the 

“Flying Circus II” and “E-Mon” radiometers 

used for the comparison of source outputs, resist 

dosimetry, and determination of EUV optical 

constants through angle dependent refl ectance 

measurements. The refl ectivity of a typical mirror designed for use in a EUVL stepper is shown 

in Fig. 1. 

DELIVERABLES: Full refl ectivity maps of EUV mirrors up to 40 cm in diameter and 45 kg in mass on an 

as needed basis for the EUVL community. Many other 

types of EUV measurements including transmission, 

resist dosimetry and other testing, and cw radiometric 

calibrations of fully assembled fi lter radiometers used 

in source comparisons.

2. EUV DOSIMETRY

NIST is the primary national source for the radiometric calibration of detectors from the infrared to 

the soft X-ray regions of the spectrum. The Photon 

Physics Group is responsible for maintaining the 

spectral responsivity standards in the far- and 

extreme-ultraviolet spectral regions, including 

13.5 nm, the EUVL wavelength of interest. We 

operate several beamlines at the SURF III synchrotron radiation facility, a quasi-cw source, as 

well as a laser-produced plasma source, which is 

pulsed with a 10-ns pulse length. With these facilities, we can calibrate EUV detectors and dosimeter 

packages under either cw or pulsed conditions. 

A major concern when using solid state photodiodes for detection of short pulse length radiation 

is that the detector may saturate under the high 

peak power, even though the average power 

is moderate. We have measured the saturation 
8 National Institute of Standards and Technology 

characteristics of EUV sensitive Si photodiodes 

using 532 nm radiation as a proxy for 13.5 nm; 

the absorption characteristics in Si are nearly 

identical. This proxy procedure has been validated by correcting a 13.5 nm pulsed calibration 

for saturation in both the sample radiometer and 

the standard photodiode using saturation data 

obtained at 532 nm. The corrected results were in 

good agreement with individual component calibrations, while the uncorrected results were not.

We have found that the photodiode responsivity is 

an inherently non-linear function of pulse energy, 

but the responsivity can be fi t by a calibration 

function having two constant parameters. Use 

of the non-linear calibration function allows the 

photodiode to be used with reasonable uncertainty 

even when the responsivity has decreased due to 

saturation effects by as much as a factor of two. 

We have developed a model of the physical processes that are responsible for saturation. The loss 

of total collected charge is modeled as a result of 

recombination processes in the device during the 

electronic readout time. The model correctly predicts the behavior of both calibration parameters 

as a function of reverse bias in the range from 0 V 

to 150 V and spot sizes greater than 0.01 mm2

. 

We have found no dependence on pulse length 

from 10 ns to 1 μs, indicating that the effects of 

saturation are the same for laser-produced plasma 

sources and for the pinched plasma sources with 

longer pulse lengths. There is substantial deviation from the expected saturation behavior as the 

illuminated area decreases (i.e., the energy density 

increases) to very small spots sizes (less than 

0.01 mm2). The measured saturation loss is less 

than our model predicts, probably due to lateral 

Figure 2. Detailed study of time and intensity dependence of degradation 

at a single pressure of 7 x 10-4 Pascals (5 x 10-6 Torr). Four regions noted: 

1) very low fl uence – improved refl ectivity; 2) low to intermediate fl uence – 

decreased refl ectivity; 3) intermediate fl uence – recovered refl ectivity; and 

4) high fl uence – steadily decreasing refl ectivity.

diffusion current that reduces the local carrier 

density. It has been reported previously that the 

low-power limit of the pulsed responsivity is equal 

to the responsivity measured with a low-power, 

cw source. Combining these results, it is possible 

to transfer a continuous wave (cw) calibration to 

a pulsed application with low uncertainty.

In 2006, we participated in an international intercomparison of EUV detector responsivity. This 

intercomparison will establish the world-wide 

equivalence of detector responsivity measurements from the major National Metrology Institutes performing these calibrations. We will also 

work to expand our EUV measurement capabilities to include measurements of EUV spectrograph 

and CCD responsivity to further aid the EUVL 

community.

DELIVERABLES:

EUV detector responsivity international intercomparison; results expected. 3Q 2007

New type of power meter for pulsed source. 

2Q 2007

Photodiodes and other EUV radiometric devices on 

customer as needed basis.

3. EUV OPTICS DAMAGE

CHARACTERIZATION

One of the potential showstoppers for commercialization of EUVL is the degradation of the 

multilayer-mirror stepper optics. The mirrors lose 

refl ectivity because adsorbed contaminant gases 

such as hydrocarbons and water are cracked by 

the energetic (13.5 nm) photons. This leads to 

growth of an amorphous carbon layer on the optics 

surfaces or to oxidation of the optics themselves. 

The former effect is largely reversible; however, 

the latter is not. Unfortunately, these ambient 

contaminants cannot be eliminated by baking 

because the alignment of the mirror stack must be 

maintained to submicron tolerances and because 

of the presence of the outgassing from the resist 

coated wafer as it is being exposed. 

The oxidation and carbonization are actually competing processes, and preliminary demonstrations 

have shown that the addition of a hydrocarbon 

can result in the deposition of a sacrifi cial layer 

of carbon that inhibits the oxidation of the cap 

layer that is used to protect the optic. The present 

strategy by the stepper manufacturers is to attempt 

to fi nd a point in the oxidation-carbonization balance that leaves an oxidation-resistant cap layer, 

such as ruthenium, undamaged.

•

•

•
Semiconductor Microelectronics and Nanoelectronics Programs 9

To study the effectiveness of such a strategy and 

to better understand the underlying processes 

responsible for mirror degradation, NIST has 

installed two beamlines at the Synchrotron Ultraviolet Radiation Facility (SURF III) that can 

expose capped-multilayer samples to ≈ 6 mW/

mm2

 of 13.5 nm radiation in an environment of 

controlled water or hydrocarbon partial pressures 

up to 6.7×10-4 Pa. To date the most oxidationresistant capping layers available have been 

~ 2 nm of ruthenium or ~ 2 nm of titanium dioxide. Our exposure facility has demonstrated that 

multilayers with these capping layer suffer approximately one-tenth the refl ectivity loss of bare 

Si-capped multilayers when exposed for ≈ 100 h 

under rather aggressive oxidation conditions of 

1×10-4 Pa of water vapor. In addition to measuring the refl ectivity loss of exposed multilayers 

(Figure 2), the damage will be characterized using 

a range of surface analysis techniques including 

micro XPS. 

In addition to expanding our facilities, we have 

also established very fruitful collaborations with 

experts in surface science both within and outside 

the NIST community.

DELIVERABLES:

Work with industrial collaborators to evaluate 

various hydrocarbon-oxidation balance mitigation 

schemes. 4Q 2007

Develop with industrial collaborators standard testing for resist outgassing. 3Q 2007

4. SUB-NM UNCERTAINTY OPTICAL

FIGURE MEASUREMENT

The commissioning of the “eXtremely accurate 

CALIBration InterferometeR” (XCALIBIR) 

at NIST is now complete and the instrument is 

fully functional (Fig. 3). The XCALIBIR interferometer is a multi-confi guration precision 

phase-measuring interferometer for optical fi gure 

measurements of fl at, spherical and aspheric optics that can achieve the very low measurement 

uncertainties that are required for the measurement of EUVL optics. 

The XCALIBIR interferometer may be operated 

in either Twyman-Green or Fizeau confi gurations. 

A beam expander in the test arm of the interferometer provides a collimated test beam with 300 mm 

diameter. Transmission spheres are used to realize 

a spherical Fizeau interferometer for the testing 

of spherical and aspheric surfaces. The part under 

test is mounted on a remotely controlled 5+1-axis 

mount that can be moved on air bearings along 

•

•

Figure 3. A view of the XCALIBIR interferometer.

Figure 4. Topography of a 300 mm diameter 

XCALIBIR reference fl at.

a precision slideway in the direction of the optical axis of the interferometer. A system of three 

laser-interferometers tracks the movement of the 

test mount in the direction of the optical axis. A 

single-mode external cavity diode laser (ECDL) is 

used as the light source in XCALIBIR. The laser 

frequency can be modulated to vary the effective 

temporal coherence over a wide range. Optical 

fi bers with different core diameters are used to 

couple the light into the interferometer. The spatial 

coherence of the light source can thus be varied by 

using fi bers with different core diameter. 

The 300 mm diameter reference fl ats for the fl at 

Fizeau confi guration of the interferometer were 

calibrated with a 3-fl at, 6-position self-calibration 

test. Figure 4 shows the topography of one of the 

reference fl ats. A large number of 3-fl at measurements were made to estimate the measurement 

uncertainty. For each of the fl ats A, B, and C the 

rms of the difference between the averaged fl at 

solutions and the individual measurements was 

plotted in a histogram (Fig. 5, next page). A (statistical) measurement uncertainty of approximately 

0.2 nm rms is evident.
10 National Institute of Standards and Technology 

When measurements of aspheric optics without 

null-optics are made, it is frequently the case 

that only a part, or subaperture, of a surface can 

be measured at once. For the fi gure measurement of the entire aspheric surface a number of 

overlapping subaperture measurements must then 

be combined, or “stitched” together. We have 

implemented fl exible and robust algorithms for 

the stitching of subaperture measurements. To 

demonstrate the power of the stitching algorithm 

a precision silicon sphere with 96.4 mm diameter 

(a 1 kg mass sphere) was set up on a rotary table 

in XCALIBIR and the surface fi gure error was 

measured with an F/1.3 transmission sphere. 

138 surface error measurements were made at 

10° intervals. As shown in Fig. 6, the individual, 

overlapping, surface error measurements were 

then stitched together to form a map of the form 

error of the silicon sphere. 

Figure 5. Distribution of rms deviations from best 

estimates surface (mean) for three fl ats A, B, and C.

Figure 6. Deviation from perfect spherical form of a 

1 kg precision silicon sphere.

Figure 7. Geometry Measuring Machine (GEMM) 

for form measurements of free-form precision 

surfaces.

In addition to the XCALIBIR interferometer, 

we are developing a new metrology tool for the 

measurement of aspheric and free-form precision surfaces, the Geometry Measuring Machine 

(GEMM). In its current form, GEMM is a 

profi lometer for free-form surfaces. A profi le is 

reconstructed from the local curvature of a test 

part surface, measured at several locations along 

a line. For profi le measurements of free-form 

surfaces, methods based on local part curvature 

sensing have strong appeal. Unlike full-aperture 

interferometry they do not require customized null 

optics. The uncertainty of a reconstructed profi le 

is critically dependent upon the uncertainty of the 

curvature measurement and, to a lesser extent, on 

curvature sensor positioning accuracy. The new 

instrument, shown in Fig. 7, provides an alternative means for validating the measurements made 

with XCALIBIR. 

DELIVERABLES: Validation of stitching method for 

form error measurements of aspheric surfaces with 

low uncertainty. This will be done with an industrial 

collaborator. 4Q 2006

COLLABORATIONS

VNL at Lawrence Livermore National Laboratory, Saša Bajt, 

EUV Multilayer Development and Coating Team.

RECENT PUBLICATIONS

U. Griesmann, Q. Wang, and J. Soons, “Three-Flat Tests Including Mounting-Induced Distortions,” Optical Engineering 

2007 (in press).

S. Grantham, C. Tarrio, R. E. Vest, and T. B. Lucatorto, 

“Metrology for EUVL Sources and Tools, in EUV Sources 

for Lithography,” V. Bakshi, ed. (SPIE Press, Bellingham, 

WA, 2006).

R. E. Vest, Y. Barad, M. L. Furst, S. Grantham, C. Tarrio, 

and P. S. Shaw, “NIST VUV Metrology Programs to Support 

Space-Based Research,” Advances in Space Research 37, 

283-296 (2006).
Semiconductor Microelectronics and Nanoelectronics Programs 11

R. E. Vest, S. Hill, and S. Grantham, “Saturation effects in 

solid-state photodiodes and impact on EUVL pulse energy 

measurements,” Metrologia 43, S84-S88 (2006).

S. M. Owens, J. S. Gum, C. Tarrio, S. Grantham, J. Dvorak, 

B. Kjornrattanawanich, R. Keski-Kuha, R. J. Thomas, C. C. 

Kankelborg, “Narrow-band EUV multilayer coating for the 

MOSES sounding rocket,” in Proc. SPIE Vol. 5900, Optics 

for EUV, X-Ray, and Gamma-Ray Astronomy II, pp. 5-13 

(2006).

U. Griesmann, “Three-Flat Test Solutions Based on Simple 

Mirror Symmetry,” Applied Optics 45, 5856-5865 (2006).

N. Machkour-Deshayes, J. Soons, U. Griesmann, R. S. 

Polvani, J. Stoup, and Z. Q. Lu, “Form-profi ling of optics 

using the Geometry Measuring Machine and the M-48 CMM 

at NIST,” Accepted for publication in J. Res. NIST (2006).

S. Grantham, C. Tarrio, S. B. Hill, and T. B. Lucatorto, “Optics 

go to extremes in EUV Lithography,” Laser Focus World, Vol. 

41, no. 11, pp. 117-122 (2005).

C. Tarrio and S. Grantham, “A synchrotron beamline for extreme-ultraviolet multilayer mirror endurance testing,” Rev. 

Sci. Instrum. 76, 056101 (2005).

C. Tarrio, S. Grantham, R. E. Vest, and K. Liu, “A simple 

transfer-optics system for an extreme-ultraviolet synchrotron 

beamline,” Rev. Sci. Instrum. 76, 046105 (2005).

U. Griesmann, N. Machcour-Deshayes, J. Soons, B. C. Kim, 

Q. Wang, J. R. Stoup, and L. Assoufi d, “Uncertainties in 

aspheric profi le measurements with the Geometry Measuring 

Machine at NIST,” in Advanced Characterization Techniques 

for Optics, Semiconductors, and Nanotechnologies II; Angela 

Duparre, Bhanwar Singh, Zu-Han Gu; Eds., Proc. SPIE 5878, 

112-124 (2005).

U. Griesmann, Q. Wang, J. Soons, and R. Carakos, “A simple 

ball averager for reference sphere calibrations,” Optical 

Manufacturing and Testing VI; H. Philip Stahl; Ed., Proc. 

SPIE 5869, 189-196 (2005).

U. Griesmann, J. Soons, and Q. Wang, “Measuring Form and 

Radius of Spheres with Interferometry,” Annals of the CIRP 

53, 451-454 (2004).

W. M. Clift, L. E. Klebanoff, C. Tarrio, S. Grantham, O. R. 

Wood II, and N.V. Edwards, “Scaling studies of capping 

layer oxidation by water exposure with EUV radiation and 

electrons,” SPIE vol. 5374, Emerging Lithographic Technologies VIII p. 666 (2004).
12 National Institute of Standards and Technology 
Semiconductor Microelectronics and Nanoelectronics Programs 13

POLYMER PHOTORESIST FUNDAMENTALS FOR

NEXT-GENERATION LITHOGRAPHY

Technical Contacts: 

V. M. Prabhu

E. K. Lin 

W. L. Wu 

“[This team has ] made 

seminal contributions to 

the fi eld of lithography 

in elucidating the effects 

that contribute to line 

edge roughness using 

the unique technical 

capabilities and knowhow of NIST. Their 

successful cooperation 

with industry partners 

and research 

organizations has 

been both scientifi cally 

outstanding as well 

as a model of how to 

implement such joint 

research efforts.”

Ralph Dammel, AZ Electronics

GOALS

In this project, we are developing an integrated 

program involving fundamental studies of photoresist materials to be correlated with resist 

performance metrics impacting next generation 

photolithography. We work closely with industrial 

collaborators to develop and apply high spatial 

resolution and chemically specifi c measurements 

to understand varying material properties and process kinetics at nanometer scales and to provide 

high quality data needed in advanced modeling 

programs. This program provides a detailed 

foundation for the rational design of materials and 

processing strategies for the fabrication of sub 50 

nm structures. The unique measurement methods 

we apply include specular and off-specular X-ray 

and neutron refl ectivity (XR, NR), small angle 

neutron scattering (SANS), near-edge X-ray absorption fi ne structure (NEXAFS) spectroscopy, 

quartz crystal microbalance (QCM), solid state 

nuclear magnetic resonance (NMR), atomic 

force microscopy (AFM), infrared variable angle 

spectroscopic ellipsometry (IR-VASE), and fl uorescence correlation spectroscopy (FCS). Our 

efforts focus on the fundamentals of polymeric 

materials and processes that control the resolution of the photolithography process including: 

(1) the photoresist polymer chain architecture 

and conformation within sub 50 nm structures; 

(2) the spatial segregation and distribution of 

critical photoresist components; (3) the transport 

and kinetics of photoresist components, and the 

deprotection reaction interface over nanometer 

distances; (4) the material sources of line-edge 

roughness (LER), a measure of the ultimate 

resolution of the lithographic process; (5) the 

polymer physics of the developer solution and the 

dissolution process; and (6) infl uence of moisture 

on the thermo-physical properties of interfaces 

as applicable to immersion lithography. These 

data are needed to meet the future lithographic 

requirements of sub 50 nm imaging layers and 

critical dimensions. 

CUSTOMER NEEDS

Photolithography remains the driving and enabling technology in the semiconductor industry 

to fabricate integrated circuits with ever decreasing feature sizes. Today, current fabrication 

facilities use chemically-amplifi ed photoresists, 

complex and highly tuned formulations of a 

polymer fi lm loaded with photoacid generators 

(PAGs) and other additives. Upon exposure of the 

photoresist fi lm through a mask, the PAG creates 

acidic protons. A post-exposure bake is then applied and the acid protons diffuse and catalyze a 

deprotection reaction on the polymer that alters its 

solubility for development in an aqueous base solution. These reaction-diffusion, and development 

processes must be understood and controlled at 

the nanometer length scale to fabricate effectively 

integrated circuits. Chemically amplifi ed resists 

are also deposited onto bottom anti-refl ection 

coatings (BARCs). Interactions and component 

transport between the BARC and resist layer can 

lead to loss of profi le control or pattern collapse. 

Detailed studies of these interaction and transport 

mechanisms are needed to design materials for the 

successful fabrication of sub 50 nm structures.

There are signifi cant challenges in extending this 

technology to fabricate the smaller feature sizes 

(sub 50 nm) needed to continue performance 

increases in integrated circuits. First, new radiation sources with shorter wavelengths (193 nm or 

EUV) require photoresist fi lms nearing 100 nm 

thick to ensure optical transparency and uniform 

illumination. In these ultrathin fi lms, confi nement 

can induce deviations in several key materials 

parameters such as the macromolecular chain 

conformation, glass transition temperature, component distribution, or transport properties. Furthermore, the required resolution for a sub 50 nm 

feature will be on the order of 2 nm, approaching 

the macromolecular dimensions of the photoresist 

polymers. It is not yet clear how deviations due to 

confi nement will affect the ultimate resolution in 

these ultra-thin photoresist fi lms. Additionally, the 

material sources of feature resolution (line-edge 

and sidewall roughness) and profi le control need 

to be identifi ed and understood to ensure the success of needed patterning technologies.

The requirements for advanced photoresists are 

discussed in the updated 2006 International Technology Roadmaps for Semiconductors on page 

5, Lithography Section. Table 75 on “Lithography Diffi cult Challenges” for resist materials 

at sub- 32 nm indicates three issues (1) Resist 

and antirefl ective coating materials composed 

of alternatives to PFAS compounds, (2) Limits of 

chemically amplifi ed resist sensitivity for < 32 nm 
14 National Institute of Standards and Technology 

“[This team has ] 

made ground breaking 

contributions to 

the fundamental 

understanding of resist 

line edge roughness — 

a technological 

bottleneck that threatens 

to derail the continuing 

miniaturization of the 

semiconductor devices.”

Qinghuang Lin, IBM

“... I just fi nished a 

presentation for AMD 

on your work. Every 

time I look at it, the 

framework that has been 

established seems more 

and more valuable for 

future work. ... AMD is a 

huge proponent of what 

you have done.”

Karen Turnquest, AMD, 

SEMATECH, Lithography

half-pitch due to acid diffusion length, and (3) 

materials with improved dimensional control and 

LWR control.

TECHNICAL STRATEGY

In this project, we use model photoresist materials 

to validate the new measurement methods. Model 

photoresist materials (248 nm, 193 nm, and EUV) 

have been used initially to address several important fundamental questions including the thermal 

properties of ultrathin fi lms as a function of fi lm 

thickness and substrate type, the conformation 

of polymer chains confi ned in ultrathin fi lms, the 

surface concentration of PAGs, the diffusion and 

the deprotection reaction kinetics, and the physics of the development process. These results 

provide a basis for understanding the material 

property changes that may affect the development 

of lithography for sub-50 nm structures using 

thin photoresist imaging layers. The interaction 

between model photoresists and BARC materials 

also requires detailed experimental investigation to optimize the materials factors impacting 

lithographic performance. Each process step 

requires an interdisciplinary array of experimental 

techniques to measure the polymer chemistry and 

physics in thin fi lms as shown in Fig. 1. 

Figure 1. Key lithographic process steps studied 

for materials fundamentals. A model 193-nm resist 

under investigation is shown with the acid-catalyzed 

deprotection reaction.

DELIVERABLES: 

Develop reaction kinetics models to analyze experimental data as a function of dose, PEB time, and 

component concentration. 1Q 2007

Measure the in situ spatial extent of the residual 

swelling fraction by neutron refl ectivity. 2Q 2007

Measure the effect of salt valence on the spatial 

extent of the residual swelling fraction by neutron 

refl ectivity. 3Q 2007

Develop a method to analyze the depth profi ling 

of deprotection by variable-angle spectroscopic 

ellipsometry (IR-VASE). 3Q 2007

•

•

•

•

Quantify the lateral length evolution in a model lineedge photoresist system by off-specular neutron 

refl ectivity. 3Q 2007

Assess development effects and infl uence of PAG 

loading on LER using the EUV exposure tool at 

the Advanced Light Source, Lawrence Berkeley 

National Laboratory. 4Q 2007 

Advance solid state NMR characterization of photoresist/photoacid generator dispersion to thin fi lms 

using unique substrates. 4Q 2007

ACCOMPLISHMENTS

EUV photoresist polymers are expected for 

imaging at the 32 nm node and smaller. Similar 

to 193 nm the deprotection reaction front profi le 

is a critical factor. However, in particular to EUV 

photoresists high PAG loading as well as lower 

EUV doses are expected. We applied neutron 

refl ectivity to understand the effect of dose using 

model deuterated polymeric resists as shown in 

Fig. 2. The deprotection fronts exhibit two different length scales; a slow front that initiates high 

degrees of deprotection near the interface and 

a fast front that propagates into the resist with 

reaction-diffusion lengths consistent with those reported in the literature. However, the deprotection 

level and diffusion-length scale are exposure dose 

(photoacid concentration) dependent. The origin 

of the fast-diffusion front dependence on dose was 

hypothesized due to the increase in copolymer 

composition polarity as the reaction proceeds 

thereby limiting the spatial-extent. The evolving 

copolymer composition appears central in future 

modeling of latent image profi les. Neutron refl ectivity was demonstrated to have suffi cient chemical sensitivity and spatial resolution to measure 

the interfacial structure on sub-nm length scales. 

This approach can be extended to understand 

the effects of additives, such as photodegradable 

bases, as well as different architecture such as 

the molecular glass photoresists shown in Fig. 2. 

•

•

•

#

Figure 2. Two contrasting architectures of 

photoresist materials, but with similar chemistry: 

molecular glass and polymeric chemically amplifi ed 

resist. 
Semiconductor Microelectronics and Nanoelectronics Programs 15

The effect of architecture or “pixel size” may also 

play a crucial role to higher fi delity imaging. We 

develop dissolution fundamentals to help quantify 

the effects of swelling and dissolution rate on the 

effect of resist architecture.

The molecular origin of dimensional changes 

within ultrathin fi lms when exposed to developer 

solutions was measured using neutron refl ectivity. 

A model photoresist material provided needed 

fundamentals of material sources to line-edge 

roughness. Quartz crystal microbalance measurements complement these measurements with the 

added ability to measure the kinetics of swelling, 

however, the profi le and chemical specifi city are 

exclusively obtained with NR. 

These new measurement methods, applicable 

to immersion lithography, demonstrate that swelling and aqueous base penetration must be considered to improve dissolution models involving 

solid-liquid interfaces. The aqueous base profi le, 

shown in Fig. 3, illustrates the penetration of the 

small deuterium labeled base molecule throughout 

the thin fi lm as a function of developer strength. 

The infl uence of moisture and interfacial energy 

are also probed using NR, XR, and quartz crystal 

microbalance techniques, allowing complete equilibrium and kinetics measurement methods.

#

#

Figure 3. Developer Fundamentals for LER. Direct 

measure of the base concentration dependence of 

swelling and deuterated tetramethyl ammonium 

ion profi le throughout the thickness, of a model 

157 nm photoresist using liquid immersion neutron 

refl ectivity.

Insight into the mechanism of development, 

rinse, and drying at a gradient line edge was measured by neutron refl ectivity with nm resolution. 

Direct measurements highlight a residual swelling 

#

fraction during the development process steps 

with a lower composition limit for swelling at the 

model line edge. The fi nal surface roughness is 

formed by the collapse of this highly associated 

and surface swollen phase during the drying process. A mechanism of a simple transfer of roughness from the latent image to the developed image 

is challenged by these data due to the swelling and 

associating polymer layer. Extending these concepts to previous latent image analysis imply the 

minimization of the residual fraction provides the 

lowest surface roughness. This can be achieved 

through resist and optical design to provide the 

highest latent image profi le slope. However, an 

ideal collapse and complete elimination of surface roughness was not observed in agreement 

with lithographic studies. Future simulations 

and models should include both percolation and 

penetration of developer and an associative polymer behavior due to the heterogeneity and large 

chemical mismatch of resist components. 

The kinetics of an acid-catalyzed deprotection reaction in model photoresist materials was 

studied as a function of copolymer composition 

with Fourier transform infrared (FTIR) spectroscopy. Three methacrylate-based terpolymers 

with varying compositions of acid-labile and 

non-reactive (lactone) monomers were studied. 

A mathematical model was developed to analyze 

the acid catalyzed deprotection kinetics with respect to coupled reaction rate and acid-diffusion 

processes. The fi rst order reaction rate constant 

decreases as the non-reactive comonomer content 

is increased. Additionally, the extent of reaction 

appears self-limiting as verifi ed by a slowing 

down necessitating an acid-trapping chemical 

equation to model the data. An example is shown 

in Fig. 4. This composition-dependent reaction 

constant indicates a strong interaction of the 

acid with the increasing polar resist matrix that 

#

Figure 4. Wafer after exposure to varied DUV dose 

and fi xed baking time the color change represents 

slight changes in fi lm thickness with extent of 

reaction. Quantifi cation of the extent of reaction 

(deprotection fraction) versus post-exposure bake 

time for model 193 nm resist for varying dose.
16 National Institute of Standards and Technology 

drastically reduces the acid transport rate. The 

reduced acid transport is consistent with hydrogen 

bonding between photoacid and methacrylic acid 

product. These results demonstrate a correlation 

between polymer microstructure and acid catalyzed 

kinetics. These are necessary measurements for 

analysis of coupled reaction-diffusion processes. 

Finally, the models were applied to understand 

the limiting spatial extent of photoacid diffusion 

at the model line edge determined by neutron 

refl ectivity.

The deprotection reaction front profi le was 

measured with nanometer resolution by combining neutron refl ectivity and FTIR on a bilayer 

structure prepared with model 193 nm photoresists. The upper layer of the structure is loaded 

with the photoacid generator. Upon exposure and 

baking, the acid diffuses into the lower layer and 

catalyzes the deprotection reaction. The protecting group partially leaves the fi lm, quantifi ed by 

FTIR, upon reaction. The contrast to neutrons 

results from the reaction allowing for observation 

of the reaction front. By comparing the reaction 

front to the developed fi lm profi le, we obtain 

important insight into both the spatial extent of 

the reaction and the development process itself. 

These data are the fi rst available with this spatial 

resolution and are critically needed for the development of process control over nanometer length 

scales. We fi nd that the reaction front broadens 

with time while the surface roughness of the 

developed structure remains relatively sharp. Additionally, we fi nd that the reaction front width is 

dependent upon resist chemistry and PAG size as 

shown in Fig. 5. These experimental data provide 

a rational design of next-generation photoresist 

component from the resist chemistry to the reaction-diffusion process. 

#

NEXAFS measurements were used to measure the surface concentration and depth profi le of 

photoresist and BARC components and the surface 

reaction kinetics in model photoresist polymers as 

a function of common processing conditions. A 

signifi cant advantage of the NEXAFS measurement is the capability of separating interfacial and 

bulk signals within the same sample and experiment as shown in Fig. 6. NEXAFS measurements 

of interfacial chemistry are possible because of 

the limited escape depth of produced secondary 

electrons. By separately observing the electron 

and fl uorescence yield, the chemistry at the surface 

(2 nm) and bulk (200 nm) may be determined. Different chemistries may be observed by examining 

the near-edge X-ray spectra of light elements such 

as carbon, oxygen, fl uorine, and nitrogen. In this 

way, changes in the surface chemistry relative to 

the bulk fi lm can be investigated as a function of 

lithographic processing steps such as exposure and 

heating. We have found that fl uorinated PAG molecules preferentially segregate to the fi lm surface. 

The relative amount of segregation is dependent 

upon the specifi c polymer chemistry and PAG 

size. In addition, NEXAFS analysis of residual 

layers arising from BARC-resist component transport and interactions enable detailed analysis of 

potential mechanisms leading to loss of profi le 

control. UV exposure, post-exposure bake, and a 

novel atmosphere controlled chamber have been 

developed to test environmental stability against 

model airborne contaminants and infl uence on 

in situ processing.

#

Figure 5. Neutron refl ectivity results of the 

nanometer scale deprotection profi le shape 

dependence on photoacid generator size: 

TPS-Tf < TPS-PFBS < DTBPI-PFOS. These highresolution experimental data help verify current 

advanced reaction-diffusion models.

Figure 6. Schematic diagram of the NEXAFS 

measurement geometry. Spectra are obtained from 

the fi lm surface and bulk simultaneously.

NEXAFS measurements were used to measure the surface concentration and depth profi le 

of photoacid generators for advanced 193 nm 

photoresist materials for immersion lithography. 

These measurements quantify the infl uence of 

water immersion on the loss of these critical components. NEXAFS on wafer analysis combined 

#
Semiconductor Microelectronics and Nanoelectronics Programs 17

with LC/MS demonstrate that equilibrium water 

solubility of PAGs with varying perfl uoroalkyl 

length does not serve as the appropriate criteria 

for selecting PAGs for immersion lithography; 

rather the segregation of PAGs to the top few 

nanometers provides the majority of leaching as 

shown in Fig. 7. Additionally, the effects of critical top coats are also investigated to understand 

the segregation and retention of PAG additives. 

Characterization of local bulk scale mixing 

is necessary for understanding future photoresist 

materials as feature dimensions are reduced to 

sub-32 nm. The intimacy of mixing of PAG 

and photoresist was probed by solid state proton 

NMR methods based on inversion-recovery, 

solid-echo-spin-diffusion, and chemical-shiftbased-spin-diffusion pulse sequences. The effect 

of resist architecture for EUV lithography has 

been investigated as a function of different molecular glass core structures (see Fig. 8). The PAG 

miscibility in several protected and deprotected 

versions have been discovered with these new 

classes of resist materials. 

Current extreme ultraviolet photoresist 

materials do not yet meet requirements on exposure-dose sensitivity, line-width roughness, and 

resolution. Fundamental studies are required to 

quantify the trade-offs in materials properties and 

processing steps for EUV photoresist-specifi c 

problems such as high photoacid generator loadings and the use of very thin fi lms. Furthermore, 

new processing strategies such as changes in the 

developer strength and composition may enable 

increased resolution. In this work, model photoresists (Fig. 7) formulated without base quenchers 

are used to investigate the infl uence of photoacid 

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Figure 7. NEXAFS fl uorine-edge experimental results quantifying the extent of PAG loss from the surface due 

to water immersion. The series of PAGs tested for immersion lithography is shown as a function of equilibrium 

water solubility, extraction from thin fi lms, and NEXAFS surface composition analysis.

Figure 8. Model molecular glass with two different 

core architectures and photoacid generator studied 

for intimacy of mixing by NMR methods.

generator loading and developer strength on EUV 

lithographically printed images performed at the 

Advanced Light Source of Lawrence Berkeley 

National Laboratory. Measurements of LWR 

and developed line-space patterns highlight a 

combined PAG loading (Fig. 9) and developer 

strength dependence that reduce LWR in a nonoptimized photoresist.

Figure 9. SEM image of EUV patterned model 

photoresists with nested 1:3 line/space developed in 

0.1 mol/L TMAH.
18 National Institute of Standards and Technology 

The Optical Technology Division is developing chemical force microscopy (CFM) to measure 

the surface spatial distribution of chemical species 

in the imaged resist. Using CFM, the homogeneity of the resist can be imaged with chemical 

contrast using specially prepared probe tips. This 

information complements the height images obtained with AFM. The distribution of PAG after 

exposure and deprotected polymer after PEB 

are of interest. The distribution of deprotected 

polymer is of particular interest to quantify LER 

by non-destructive imaging techniques at the 

nanometer length scale. The latent image measured by CFM in Fig. 10 shows an image of an 

EUV-exposed model photoresist polymer (Fig. 7) 

resolved before development for two PAG loadings and varied 1:1 nested line/space.

#

Figure 10. Chemical force microscopy scans of the latent images resolved 

before TMAH development of 1:1 nested line/space at 5 mJ/cm2

 (a) 3 % 

and (b) 15 % PAG loading.

RECENT PUBLICATIONS

K.-W. Choi, V. M. Prabhu, K. Lavery, E. K. Lin, W.-l. Wu, 

J. T. Woodward, M. J. Leeson, H. B. Cao, M. Chandhok, G. 

Thompson, “Effect of photoacid generator concentration and 

developer strength on the patterning capabilities of a model 

EUV photoresist” Proc. of SPIE, 6519, 651943 (2007).

V. M. Prabhu, B. D. Vogt, S. Kang, A. Rao, E. K. Lin, S. K. 

Satija, and K. Turnquest, “Direct Measurement of the in situ 

Developed Latent Image: the Residual Swelling Fraction”

Proc. of SPIE, 6519, 651943 (2007).

W.-l. Wu,V. M. Prabhu, E. K. Lin, “Identifying materials 

limits of chemically amplifi ed photoresists,” Proc. of SPIE, 

6519, 651902 (2007).

S. Kang, B. D. Vogt, W. Wu, V. M. Prabhu, D. L. VanderHart, 

A. Rao, E. K. Lin, and K. Turnquest. “Characterization of 

compositional heterogeneity in chemically amplifi ed photoresist polymer thin fi lms with infrared spectroscopy.” Macromolecules 40,1497-1503 (2007). 

B. D. Vogt, S.Kang, V. M. Prabhu, A. Rao, E. K. Lin, W.-l. Wu, 

S. K. Satija, and K. Turnquest, “Infl uence of base additives 

on the reaction diffusion front of model chemically amplifi ed 

photoresists,” Journal of Vacuum Science and Technology B, 

25,175-182 (2007).

K. Lavery, B. D. Vogt, V. M. Prabhu, K.-W. Choi, E. K. Lin, 

W.-l. Wu, S. Satija, “Exposure dose effects on the reaction-diffusion process in model EUV photoresists,” Journal of Vacuum 

Science and Technology B 24, 3044-3048 (2006).

B. D. Vogt, S. Kang, V. M. Prabhu, E. K. Lin, S. K. Satija, K. 

Turnquest, and W. Wu, “Measurements of the reaction-diffusion front of model chemically amplifi ed photoresists with varying photoacid size,” Macromolecules 39, 8311-8317 (2006).

A. Rao, S. Kang, B. D. Vogt, V. M. Prabhu, E. K. Lin, W. 

Wu, and M. Muthukumar, “Effect of Deprotection Extent on 

Swelling and Dissolution Regimes of Thin Polymer Films,”

Langmuir 22, 10009-10015 (2006).

S. Kang, V. M. Prabhu, B. D. Vogt, E. K. Lin, W.-L. Wu, 

K. Turnquest, “Copolymer fraction effect on acid catalyzed 

deprotection reaction kinetics in model 193 nm photoresists,”

Polymer, 47(18),6293-6302 (2006).

V. M. Prabhu, S. Sambasivan, D. A. Fischer, L. Sundberg, and 

R. D. Allen, “Quantitative depth profi ling of photoacid generators in photoresist materials by near-edge X-ray absorption 

fi ne structure spectroscopy,” Applied Surface Science 253(2), 

1010-1014 (2006).

B. D. Vogt, C. L. Soles, C.-Y. Wang, V. M. Prabhu, P. M. 

McGuiggan, J. F. Douglas, E. K. Lin, W.-L. Wu, S. K. Satija, 

D. L. Goldfarb, M. Angelopoulos “Water immersion of model 

photoresists: interfacial infl uences on water concentration and 

surface morphology,” Journal of Microlithography, Microfabrication, and Microsystems, 4(1), 013003 (2005).

V. M. Prabhu, B. D. Vogt, W.-L. Wu, E. K. Lin, J. F. Douglas, 

S. K. Satija, D. L. Goldfarb, and H. Ito, “Direct measurement 

of the counterion distribution within swollen polyelectrolyte 

fi lms,” Langmuir 74, 6647 (2005).

COLLABORATIONS

Polymers Division, NIST – Shuhui Kang, Kristopher Lavery, 

David VanderHart

Ceramics Division, NIST – Sharadha Sambasivan, Daniel 

A. Fischer

Center for Neutron Research, NIST – Sushil K. Satija

Optical Technology Division, NIST – John Woodward, 

Jeeseong Hwang

Intel – Kwang-Woo Choi (Assignee to NIST), George 

Thompson

Cornell University – Christopher Ober

SEMATECH – Karen Turnquest

IBM Almaden Research Center – Hiroshi Ito, Robert D. Allen, 

William D. Hinsberg

AZ Electronics – Ralph Dammel, Frank Houlihan

DuPont Electronic Polymers L.P. – Jim Sounik, Michael T. 

Sheehan

Intel – CRADA

SEMATECH – Agreement 309841 OF (completed 11/ 06)
Semiconductor Microelectronics and Nanoelectronics Programs 19

CRITICAL DIMENSION AND OVERLAY

METROLOGY PROGRAM

The principal productivity driver for the semiconductor manufacturing industry has been 

the ability to shrink linear dimensions. A key element of lithography is the ability to create 

reproducible undistorted images, both for masks and the images projected by these masks 

onto semiconductor structures. Lithography as a whole, fabricating the masks, printing and 

developing the images, and measuring the results, currently constitutes ≈35 % of wafer processing costs. The overall task of the Critical Dimension and Overlay Program is to assist 

the industry in providing the necessary metrology support for current and future generations 

of lithography technology. These goals include advances in modeling, the provision of next 

generation critical dimension and overlay artifacts, development of advanced critical dimension and overlay techniques, and comparisons of different critical dimension and overlay 

measurement techniques.

Currently, critical dimension and overlay measurement improvements have barely kept up 

with lithography capabilities. To maintain cost effectiveness, continued advances need to be 

made. 
20 National Institute of Standards and Technology 

WAFER-LEVEL AND MASK CRITICAL DIMENSION

METROLOGY

This project is the single largest project at NIST supporting the semiconductor industry. It 

involves metrology and artifact development across a broad range of techniques. For this 

reason, the project is presented in a number of sub-sections, each focusing on a single technology. These are:

Model-Based Linewidth Metrology

Scanning Electron Microscope-Based Dimensional Metrology

Scanning Probe Microscope-Based Dimensional Metrology

Optical-Based Photomask Dimensional Metrology

Scatterometry-Based Dimensional Metrology

Small Angle X-Ray Scattering-Based Dimensional Metrology

Dimensional Metrology with Grazing Incident X-Ray Scattering

Atom-Based Dimensional Metrology

Fabrication and Calibration Metrology for Single-Crystal CD Reference Materials

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Semiconductor Microelectronics and Nanoelectronics Programs 21

MODEL-BASED LINEWIDTH METROLOGY

Technical Contact: 

J. Villarrubia

“Stack materials, surface 

condition, line shape 

and even layout in the 

line vicinity may affect 

CD-SEM waveform and, 

therefore, extracted line 

CD. These effects, unless 

they are accurately 

modeled and corrected, 

increase measurement 

variation and total 

uncertainty of CD SEM 

measurements.” 

International Technology 

Roadmap for Semiconductors, 

Metrology Section, p. 8 (2005).

“Due to the changing 

aspect ratios of IC 

features, besides the 

traditional lateral feature 

size (for example, 

linewidth measurement) 

full three-dimensional 

shape measurements 

are gaining importance 

and should be available 

inline.” 

International Technology 

Roadmap for Semiconductors, 

Metrology Section, p. 6 (2005).

GOALS

The goal of this project is to address the metrology 

needs of industry, particularly the U.S. semiconductor industry, for linewidth and line edge roughness metrology with uncertainties on the order of 

1 nm as required by the industry roadmap.

CUSTOMER NEEDS

A feature’s width is one of its fundamental dimensional characteristics. Width measurement 

is important in a number of industries including 

the semiconductor electronics industry, with projected worldwide sales of $273.8 billion in 2007 

[Semiconductor Industry Association projection, 

Feb. 2, 2007]. As a measure of its importance 

in that industry, consider that the term “critical 

dimension” or “CD” is used there nearly interchangeably with linewidth or gate width.

To support present and future semiconductor 

technologies, industry needs to measure gate 

electrode widths with total uncertainties, as identifi ed in the International Technology Roadmap 

for Semiconductors (ITRS), of less than 2.5 nm 

and with measurement precision (3 standard deviation repeatability) better than 0.5 nm. Neither 

NIST nor any other national laboratory presently 

offers a wafer linewidth measurement service or 

SRM with uncertainty at this level. In addition to 

measuring linewidths, the semiconductor industry 

has needs for measuring linewidth variation, that 

is, linewidth roughness (LWR). LWR in transistor 

gates has been linked to increased off-state leakage current and to threshold voltage variation. The 

2006 ITRS update specifi es that LWR, measured 

as three standard deviations of the CD, must be 

less than 2 nm in 2007 and be measured with 

better than 0.4 nm precision.

A line’s width must generally be determined from 

its image. However, the image is not an exact replica of the line. The scanning electron microscope 

(SEM), scanning probe microscope (SPM), and 

optical microscope all have image artifacts that 

are important at the relevant size scales. Physical linewidth determination therefore requires 

modeling of the probe/sample interaction in order 

to correct image artifacts and identify edge locations. Barriers to accurate linewidth determination 

include inadequate confi dence in existing models, 

the complexity and consequent expense of using 

some models, inadequately quantifi ed methods 

divergence, and ignorance of best measurement 

practices. Barriers to accurate LWR measurement 

include random errors due to noise or sampling 

and poorly understood measurement artifacts such 

as measurement bias that comes from treating 

random edge assignment errors as a component 

of roughness (a false “noise roughness”).

TECHNICAL STRATEGY

The scope of the model-based linewidth metrology 

project includes the development and improvement of computational models to simulate the 

artifacts introduced by measuring instruments, 

inversion of these models (to the extent possible) 

to deduce the sample geometry that produced a 

measured image, validation of models by appropriate experiments, development and testing of 

measurement processes that provide the necessary 

inputs for model-based deduction of sample width 

and shape, estimation of uncertainties for the measurement process, assistance to industry linewidth 

measurement by communication of best practices, 

and laying of the necessary research groundwork 

for a future linewidth and/or line shape Standard 

Reference Material. 

We have developed a model-based library method 

of determining linewidth and line shape from topdown SEM images. The top-down measurement 

confi guration is the one employed by industry 

CD-SEMs. Edge locations tell us the line’s width 

(the “CD” desired by industry). However, lines 

with different sidewall geometries appear to have 

different widths when measured using algorithms 

that are standard today on CD-SEMs. That is, 

sidewall variation masquerades as width variation. Accordingly, our method is a model-based 

algorithm that explicitly accounts for the physics 

of the interaction of the electron beam with the 

sample and the effect of sidewall geometry. 

The method works by describing edge geometry 

with a set of parameters (e.g., sidewall angle, corner radius, edge positions). The expected images 

for a range of parameter choices are calculated 

using a Monte Carlo algorithm (called MONSEL) 

that simulates electron trajectories. The resulting 

actual shape/calculated image pairs form a library, 

or database. To determine the shape of an unknown sample, its measured image is compared to 

computed images in the database to fi nd the closest 

match (Fig. 1, next page). The corresponding line 

shape is assigned to the unknown. In practice there 

may be more than two parameters, and the library 

is interpolated.
22 National Institute of Standards and Technology 

In recent years we have reported encouraging 

results for this method. Results for polycrystalline 

Si are shown in Fig. 2a and Fig. 2b. Measurements 

like these on poly-Si are industrially relevant after 

etch, and are also important as a prerequisite for 

the calibration of wafer linewidth standards. Such 

standards are likely to be fabricated in Si rather 

than resist because resist geometry is too unstable 

for a long-term standard. However, a capability to 

measure resist would also be industrially useful 

for pre-etch process control measurements. The 

effects of contamination, charging, and shrinkage 

when imaging nonconducting resists may result 

in poorer accuracy for resist samples than for Si 

samples. Results for a UV resist are shown in 

Fig. 2c. 

These prior accomplishments establish a direct 

link between the quality of SEM models and the 

accuracy of metrology that can be performed 

using them. In 2007, we are improving the capabilities of the underlying modeling tools used to 

generate libraries. The existing simulation codes 

are limited to certain classes of sample shapes, essentially lines uniform along their length and with 

cross sections characterized by a small number 

of geometrical parameters, e.g., width, sidewall 

angle, and corner radius. Improvements to the 

modeling code will permit simulation of other 

industrially important sample shapes, such as 

rough-edged lines, contact holes, and line footing 

(see, e.g., Fig. 3). To that end, we are translating 

the existing FORTRAN version of MONSEL 

Figure 1. Concept of metrology using a modelbased library. The measured left and right edges are 

compared to a library of images calculated for a range 

of possible edge shapes. The shape of the unknown 

structure that gave rise to the measured image is 

assigned to be the line shape corresponding to the 

image that most closely matches the measured one.

Figure 2. Agreement between MBL and crosssection measurements (a) for isolated polycrystalline 

(“poly”) Si lines in which top-down measurements 

were performed with a laboratory SEM, (b) for dense 

poly lines measured with a commercial CD-SEM, 

and (c) for dense UV resist lines measured with a 

commercial CD-SEM. In all cases the red lines are 

the cross sections predicted by the MBL technique 

based upon top-down measurements, and the blue 

lines are edges assigned based upon the cross 

sections.

into Java and revising the sample geometrical 

description. Instead of the previous cross-sectional description, samples will be described as 

a series of 3-dimensional regions, each of which 

is specifi ed by primitive shapes (plane-bounded 

objects, cylinders, spheres, etc.) or combinations 

of these. Once the 3-D version is compared and 

agrees with the existing version for shapes in both 

their repertoires, we will revisit the physics models 

for SE generation with a view to improving the 

simulation accuracy. We are encouraging further 

adoption of this method by CD-SEM suppliers by 

publication of the method, by making modeling 

software freely available to CD-SEM manufacturers, and by collaboration with their engineers.

DELIVERABLES:

Translate existing scattering physics module into 

Java framework. 4Q 2006

Complete the merger of existing MONSEL scattering physics into new 3-dimensional code. 1Q 2007

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Semiconductor Microelectronics and Nanoelectronics Programs 23

Develop alternative SE models (e.g., Gryzinski 

semi-classical SE generation) for inclusion in the 

new simulator. 2Q 2007

Investigate sensitivity of model results to choice of 

model. 3Q 2007

In Atomic Force Microscopy (AFM) the issues 

with width determination are similar, and an 

analogous solution using models of the tip/sample 

interaction to reconstruct the sample shape is possible. We made considerable progress on this a 

number of years ago, developing a mathematical 

morphological description of the sample/probe 

interaction in terms of dilation, and of sample 

reconstruction in terms of erosion. These previous 

developments were limited to samples, images, 

and tips that could be described by single-valued 

surfaces (e.g., images in the form of grayscale 

height maps). Although dilation and erosion 

similarly described image formation and sample 

reconstruction for re-entrant surfaces (e.g., undercut lines) we were unable to perform calculations for such surfaces because of limitations in 

our software implementation. In the summer of 

2006 we overcame that limitation by development of a “dexel” (depth element) representation 

to replace the pixel representation for images of 

such objects. We continue the development this 

year with publication of the result and demonstrations of its applicability to samples of interest in 

semiconductor electronics applications.

DELIVERABLES: 

Complete coauthored paper on 3-D image simulation and surface reconstruction using a dexel representation . Submit to Ultramicroscopy. 4Q 2006

Paper on applications of Image simulation and surface reconstruction with dexels for SPIE Advanced 

Lithography meeting. 1Q 2007

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Figure 3. An overview of a FinFET model showing 

Si source and drain (left and right boxes), a thin 

connecting Si fi n with HfO2 gate oxide, and a metal 

(TiN) gate electrode with a crossing fi n-like contact. 

The substrate (not shown, but coincident with the 

bottom of the pictured structures) is Si.

ACCOMPLISHMENTS

The 3-D geometry portions of the new Monte 

Carlo SEM simulator were completed in 2006. 

Scattering codes were completed and merged with 

the geometry parts early this year on schedule according to the milestones listed above. As a result, 

the planned 3-D simulator is currently working 

and under test.

We have improved our understanding of several physical processes, including work function 

barrier penetration by electrons, electron transport 

through dilute gases (relevant to the use of variable 

pressure SEM, proposed for mask metrology), and 

the effect of target electron binding energy upon 

scattering cross sections. This work is in preparation for including this improved physics in future 

models of SEM performance. 

In collaboration with Applied Materials, we 

developed an unbiased linewidth roughness estimator suitable for use in resist measurements, 

where the resist shrinks during measurement. The 

results were published in Proceedings of the SPIE 

Microlithography conference.

The work has generated considerable interest 

in semiconductor and nanotechnology applications areas, leading to an invited presentation at 

the Micro and Nano Technology Measurement 

Club meeting, NPL in Teddington, England, a 

tutorial on “Interconnect-relevant Characterization and Metrology Techniques” at the Advanced 

Metallization Conference, and a colloquium at 

the College of Nanoscale Science & Engineering 

(SUNY Albany). 

COLLABORATIONS

International SEMATECH, Benjamin Bunday, Michael 

Bishop.

Hitachi, Ltd., Maki Tanaka.

Applied Materials, R. Katz, C. D. Chase, R. Kris, R. Peltinov 

Illinois Institute of Technology, Prof. Xiaoping Qian

Intel Corp

RECENT PUBLICATIONS

X. Qian and J. S. Villarrubia, “General Three-Dimensional 

Image Simulation and Surface Reconstruction in Scanning 

Probe Microscopy using a Dexel Representation,” Ultramicroscopy, in press.

“Monte Carlo modeling of secondary electron imaging in three 

dimensions,” J. S. Villarrubia, N. W. M. Ritchie, and J. R. 

Lowney, submitted to Proc. SPIE 6518 (February 2007).

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24 National Institute of Standards and Technology 

“Image Simulation and Surface Reconstruction of Undercut 

Features in Atomic Force Microscopy,” Xiaoping Qian, John 

Villarrubia, Fenglei Tian, and Ronald Dixson, submitted to 

Proc. SPIE 6518 (February 2007).

 “Line-edge roughness and cross-sectional characterization of 

sub-50-nm structures using CD-SAXS,” R. L. Jones, C. Wang, 

E. K. Lin, W. Wu, J. S. Villarrubia, K. Choi, J. S. Clarke, and B. 

D. Bunday, submitted to Proc. SPIE 6518 (February 2007).

R. Katz, C. D. Chase, R. Kris, R. Peltinov, J. Villarrubia, and B. 

Bunday, “Bias reduction in roughness measurement through 

SEM noise removal,” Proc. SPIE 6152, 61524L, (2006)

J. S. Villarrubia and B. D. Bunday, “Unbiased Estimation of 

Linewidth Roughness,” Proc. SPIE 5752, p. 480 (2005).

J. S. Villarrubia, A. E. Vladár, and M. T. Postek, “Scanning 

electron microscope dimensional metrology using a modelbased library,” Surf. Interface Anal., 37, p. 951 (2005).

M. Tanaka, J. S. Villarrubia, and A. E. Vladár, “Infl uence of 

Focus Variation on Linewidth Measurements,” Proc. SPIE 

5752, p. 144 (2005).

J. S. Villarrubia, “Tip Characterization for Dimensional 

Nanometrology,” in Applied Scanning Probe Methods, eds. B. 

Bhushan, H. Fuchs, and S. Hosaka, p. 147 (Springer-Verlag, 

Berlin Heidelberg 2004). 
Semiconductor Microelectronics and Nanoelectronics Programs 25

SCANNING ELECTRON MICROSCOPE-BASED

DIMENSIONAL METROLOGY

Technical Contacts:

A. E. Vladar

J. S. Villarrubia 

M. T. Postek

“Scanning Electron 

Microscopy (SEM) – 

continues to provide 

at-line and in-line 

imaging ... and CD 

measurements. 

Improvements are 

needed ... at or beyond 

the 45 nm generation ... 

Determination of the real 

3-D shape...will require 

continuing advances in 

existing microscopy ...”

International Technology 

Roadmap for Semiconductors, 

2007

GOALS

Provide the microelectronics industry with highly 

accurate scanning electron microscope (SEM) 

measurement and modeling methods for shapesensitive measurements and relevant calibration 

standards with nanometer-level resolution. 

Carry out SEM metrology instrumentation development, including improvements in electron and 

ion optical system, detection, sample stage, and 

vacuum system. 

Conduct research and development of new metrology techniques using digital imaging and 

networked measurement tools solutions to key 

metrology issues confronting the semiconductor 

lithography industry.

CUSTOMER NEEDS

The scanning electron microscope is used extensively in many types of industry, including the 

more than $200 billion semiconductor industry 

in the manufacture and quality control of semiconductor devices. The International Technology 

Roadmap for Semiconductors (2005) states that 

“Scanning Electron Microscopy continues to 

provide at-line and inline imaging for characterization of cross-sectional samples, particle and 

defect analysis, inline defect imaging (defect 

review), and critical dimension (CD) measurements. Improvements are needed for effective 

CD and defect review (and SEM detection in 

pilot lines) at or beyond the 45 nm generation.” 

The semiconductor industry needs SEM standard 

artifacts, specifi cally those related to instrument 

magnification calibration, performance and 

the measurement of linewidth. This entails a 

multidimensional program including: detailed 

research of the signal generation in the SEM, 

electron beam-sample interaction modeling, 

developing NIST metrology instruments for the 

certifi cation of standards, and developing the 

necessary artifacts and calibration procedures. 

The manufacturing of present-day integrated 

circuits requires that certain measurements be 

made of structures with dimensions of 65 nm or 

less with a very high degree of precision. The accuracy of these measurements is also important, 

but more so in the development and pilot lines. 

The measurements of the minimum feature size, 

known as CD, are made to ensure proper device 

operation. The U.S. industry needs high-precision, 

accurate, shape-sensitive dimension measurement 

methods and relevant calibration standards. The 

SEM Metrology Project supports all aspects of 

this need since scanning electron microscopy is 

key microscopic technique used for this sub-100 

nm metrology.

TECHNICAL STRATEGY

The Scanning Electron Microscope Metrology 

Project, a multidimensional project, is being executed through several thrusts fully supported by 

the semiconductor industry. 

1. SEM Magnifi cation Calibration Artifacts: 

Essential to SEM dimensional metrology is the 

calibration of the magnifi cation of the instrument. 

Standard Reference Material (SRM) 2120 is an 

SEM magnifi cation standard that will function at 

the low beam voltages used in the semiconductor 

industry and high beam voltages used in other 

forms of microscopy (Fig. 1). Conventional optical lithography provides a chance for large amount 

of good quality samples produced inexpensively. 

Because of the improvements of this technology, 

it is now possible to produce the fi nest lines with 

close to 100 nm width and with 200 nm pitch values. The largest pitch is 1500 μm. In order to make 

this artifact available (while the fi nal certifi cation 

Figure 1. SEM image of the complete RM 8090 

Magnifi cation calibration standard reference 

material.
26 National Institute of Standards and Technology 

details are being completed) the artifact will be 

released as Reference Material (RM) 8090A.

DELIVERABLES:

Development of a new metrology SEM based on a 

variable pressure instrument. This instrument will 

be able to work with full size wafers and photomask. 4Q 2007

Preparation of an assessment of the error budget 

for the fully functional metrology system. Preparation of customized recipes for various versions of 

magnifi cation calibration samples. 4Q 2007

Upon availability of suitable quality samples, quality 

assessment and delivery of a batch of RM 8120. 

4Q 2007

Upon availability of suitable quality samples, 

completion calibration and delivery of a batch of 

SRM 2120 samples. 4Q 2007

2. SEM Performance Measurement Artifacts 

and Software Solutions: This effort included 

the development of the Reference Material 8091 

(Fig. 2.) and evaluation procedures suitable for 

correctly measuring image sharpness of scanning 

electron microscopes, especially of those that are 

used in the semiconductor industry. Currently we 

are working with ISO to develop robust software 

solutions that will allow for resolution performance tracking of SEMs. The resolution performance characteristics of the SEM are particularly 

important to precise and accurate measurements 

needed for semiconductor processing. As a part 

of this effort the NIST SEM Resolution Measurement Reference Image Set was worked out to test 

the resolution measurement software themselves. 

This Reference Image Set contains a large number of artifi cial images that were made by taking 

onto account the amount and type of de-focusing, 

noise, vibration and drift and electron landing 

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energies. Suitable samples are being sought to 

further improve this type of metrology (Fig. 3). 

Several samples have been purchased by leading 

semiconductor manufacturing companies and 

these measurement artifacts are used in the benchmarking efforts of International SEMATECH. The 

existence of these samples fosters better understanding, objective measurement and enforcement 

of SEM spatial resolution performance. 

Figure 3. Images that illustrate the image simulation 

steps for the NIST SEM Resolution Measurement 

Reference Image Set.

Figure 2. The RM 8091 Sharpness Reference 

Material.

3. SEM Linewidth Measurement Artifacts: 

Artifacts that are characterized and calibrated 

to the required small levels of uncertainty were 

and are in the focal point of the IC industry’s dimensional metrology needs. Therefore, at NIST, 

it has been a longer-term goal to develop and 

deliver appropriate samples. For a long time, the 

possibilities were limited by the lack of various 

technologies available, especially the lack of 

accurate modeling methods. NIST, through several years of systematic efforts, developed Monte 

Carlo simulation-based modeling methods that can 

deliver excellent results. These new methods can 

deduce the shape of integrated circuit structures 

from top-down view images through modeling 

and library-based measurement techniques with 

a few nm of accuracy. In several publications, 

NIST demonstrated the possibilities and described 

power of this measuring approach. Based on the 

newest results, now it is becoming possible to 

start to develop the long-awaited relevant line 

width standard for the semiconductor industry. 

Reference Material 8120 line width samples will 

be a relevant sample on a 200 mm and 300 mm Si 

wafer with polySi features with sizes from 1 mm to 

down to 50 nm. Standard Reference Material 2120 

is going to be the calibrated, traceable version for 

line width measurements. This work is being carried out in cooperation with SEMATECH. 

A new effort is under way that is aimed at the 

development of two new Reference SEMs. One 

with full-size wafer and mask capability is based 

on an environmental SEM (ESEM), which is 

very advantageous with charging samples such 

as quartz masks. The other Reference instrument 

based on a dual-beam SEM, and will be capable 
Semiconductor Microelectronics and Nanoelectronics Programs 27

to work with mask- and smaller size samples. 

Both microscopes will use the same type of 38 pm 

resolution laser interferometry, which provides 

traceability and it allows for the compensation for 

stage drift and vibration at the nm level. 

DELIVERABLES:

Design and preparation of line width metrology immerse litho artifact suitable for calibration on NIST 

and external measuring systems for certifi cation of 

wafer format line width samples. 3Q 2007 

Completion of preliminary measurements on immerse litho samples made by SEMATECH with the 

new “NISTMAG” metrology mask. 4Q 2007

4. Helium Ion Microscopy – a Promising 

New Technique for Semiconductor Metrology: The Helium Ion Microscope (HIM) offers a new, potentially disruptive technique for 

nano-metrology. This methodology presents an 

approach to measurements for nanotechnology 

and nano-manufacturing which has several potential advantages over the traditional scanning 

electron microscope (SEM) currently in use in 

integrated circuit research and manufacturing 

facilities across the world. Due to the very small, 

essentially one atom size, very high brightness 

source, and the shorter wavelength of the helium 

ions, it is theoretically possible to focus the ion 

beam into a smaller probe size relative to that of 

an electron beam of current SEMs. Hence higher 

resolution is theoretically achievable. In contrast 

to the SEM, when the helium ion beam interacts 

with the sample, it generates signifi cantly smaller 

excitation volume and thus the image collected 

is more surface sensitive. Similarly to the SEM, 

the HIM also produces topographic, material, 

crystallographic, and potential contrast, and offers ways for investigating new properties of 

the sample through the use of various detectors. 

Compared to an SEM, the secondary electron 

yield is quite high, allowing for imaging at very 

low beam currents, thus resulting in less sample 

damage. Additionally, due to their low mass, 

the helium ions themselves do not signifi cantly 

alter the sample, which would be common, for 

example, for gallium ions that are regularly used 

for ion milling. This development work is at its 

beginning the fi rst-in-the-world HIM arrived at 

NIST in May 2007. 

DELIVERABLES: 

Installation of the new HIM, design and preparation 

of measurement plan that addresses all important 

imaging and metrology parameters of the HIM. 

3Q 2007 

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•

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Completion of preliminary measurements and quality assessment of the imaging capability of the HIM 

using amorphous Si immerse litho samples made 

by SEMATECH. 4Q 2007

ACCOMPLISHMENTS

SEM Magnification Calibration Artifacts – Samples for Reference Material 8090 

have been made in the past successfully, but later 

several attempts with e-beam lithography yielded 

no further useful samples. We delivered to NIST 

Offi ce of Reference Materials 100 pieces of RM 

9080A, which were fabricated at International 

SEMATECH using 193 nm UV light lithography. 

The fi nest features are 100 nm wide with a 200 nm 

pitch. The largest pitch is 1500 μm. There are a 

large number of 250 nm wide crosses and grids for 

distortion and other measurements. Scatterometry 

patterns with varying pitches will also be available. The features on the conductive Si wafer will 

be formed from polySi material. These samples 

give suitable contrast in any SEM to allow the user 

to set the magnifi cation of the instrument from the 

smallest to the highest magnifi cations. The fi rst 

wafers containing the fi nal design were fabricated 

by International SEMATECH and found to be 

useful. The fabrication of the fi rst large batch of 

samples has been fi nished in the summer of 2006. 

With the arrival of the new metrology SEM and 

its laser interferometer sample stage these will 

be calibrated.

SEM Performance Measurements – After 

comprehensive studies and experiments a plasmaetching Si called “grass” was chosen for Reference 

Material 8091 (Fig. 2). This sample has 5 nm to 

25 nm size structures as is illustrated in the fi gure 

below. There have been 75 samples delivered to 

the Offi ce of Standard Reference Materials, NIST. 

A sharpness standard and evaluation procedure 

have been developed to monitor (or compare) 

SEM image quality. A NIST kurtosis method, 

Spectel Company’s user-friendly analysis system 

called SEM Monitor, and University of Tennessee’s SMART algorithm can be used with RM 

8091. An effort is under way to produce more of 

these samples. These samples are now available to 

the public, and many of them are already in use.

SEM Linewidth Measurement Artifacts –

For correct linewidth measurements accurate 

modeling methods are indispensable. The existing 

NIST methods have shown excellent results on 

polySi samples and they are under further development for higher accuracy. From a top-down view 

and using our high-accuracy modeling and fi tting 

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28 National Institute of Standards and Technology 

methods, a cross section of the lines can be determined with few nm uncertainties and discrepancies. The match is so good that this method may be 

capable of eliminating the costly and destructive 

cross sectional SEM measurements. The candidate 

sample for linewidth artifact must be relevant 

to state-of-the-art IC technologies, should have 

chips on 200 mm and later 300 mm wafers with 

all process variation and include a meaningful 

focus-exposure matrix (FEM). The design and 

the fabrication of the SEMATECH/NIST mask 

has been successfully completed. After CD-SEM 

measurements at SEMATECH, cross sectional 

measurements will be made at NIST. All of these 

measurements will yield an excellent database for 

a decision on how to proceed with this wafer type 

linewidth reference sample. It is conceivable that 

by the end of the year 2007 the Reference Material 

8120 line width samples will be available. This 

work is being carried out in close cooperation 

with SEMATECH.

Development of High Accuracy Laser 

Interferometer Sample Stage for SEMs – The 

development of a very fast, very accurate laser 

stage measurement system facilitates a new 

method to enhance the image and line scan resolution of SEMs. This method, allows for fast signal 

intensity and displacement measurements, and 

can report hundreds of thousands of measurement points in just a few seconds. It is possible 

then, to account for the stage position in almost 

real time with a resolution of 0.04 nm. The extent 

and direction of the stage motion reveal important 

characteristics of the stage vibration and drift, 

and helps minimize them. Figure 4 illustrates 

the short-term motion of the sample stage, the 

left side of the fi gure depicts the location of the 

stage and the right side is the density distribution of the location of the stage during 1 minute 

of dwell time. The fi eld-of-views are 2 nm for 

all images. The high accuracy and speed also 

allow for a convenient and effective technique 

for diminishing these problems by correlating 

instantaneous position and imaging intensity. The 

new measurement technique developed recently 

gives a possibility for signifi cantly improving 

SEM-based dimensional measurement quality. 

#

Important new discoveries made it possible to 

correctly understand the motion of the stage at 

the nm level, and the characterization of various 

settings and fi ne tuning of the sample stage, which 

is critical for high-resolution work. 

Development of Line Edge Roughness 

Metrology for Integrated Circuit Technology 

– The measurement of line-edge roughness (LER) 

has become an important topic in the metrology 

needed for the semiconductor industry. NIST has 

successfully developed various LER metrics and 

methods to make reliable and statically sound 

LER measurements. The fi ndings have impact 

on the ITRS as they call for longer lengths for 

LER evaluation and on LER metrology in general 

because the new methods offer signifi cantly better 

metrology than what was available previously. The 

fi nal report of a year-long study was delivered on 

time to SEMATECH.

Development of Ultra-High Resolution He 

ion Microscope – The HIM technology is not yet 

as optimized, developed or as mature as the SEM. 

As a new technique, HIM (Fig. 5) is just beginning 

to show promise and the plethora of potentially 

advantageous applications for integrated circuit 

and nanotechnology have yet to be exploited. 

Now that commercial instrumentation is available, further work can be done on the fundamental 

science of helium ion beam generation, helium 

ion beam-specimen interactions, and the signal 

generation and contrast mechanisms defi ning the 

image.

In addition to these areas of work, modeling needs 

to be developed to correctly interpret the signal 

generation mechanisms and to understand the 

imaging mechanisms. These are indispensable 

for accurate nanometer-level metrology. HIM and 

SEM have some overlapping territory, but they 

remain complementary techniques. Helium ion 

beam microscopy is forging into new scientifi c 

#

#

Figure 4. Short-term motion of the SEM sample 

stage, the stage location (left) and its density 

distribution during a 1 minute of dwell time, 2 nm 

fi eld-of-views. Figure 5. Alis/Zeiss He ion microscope.
Semiconductor Microelectronics and Nanoelectronics Programs 29

and technology territories, and this new and innovative technology will develop new science and 

contribute to the progress in integrated circuit and 

nanotechnology. This development work is at its 

beginning, the fi rst-in-the-world HIM has arrived 

at NIST in May 2007. 

COLLABORATIONS

International SEMATECH, Advanced Metrology Advisory 

Group

International Technology Roadmap for Semiconductors; 

Microscopy and Metrology Sections

Zeiss/ALIS Corp

FEI Co. 

ISO

RECENT PUBLICATIONS

M. T. Postek, A. E. Vladar, J. Kramar, W. Ward, L. A. Stern, 

J. Notte and S. McVey, “Helium Ion Microscopy: a New 

Technique for Semiconductor Metrology and Nanotechnology” Scanning 2007.

A. E. Vladar, Z. Radi, M. T. Postek and D. C. Joy, “Nanotip Electron Gun for the Scanning Electron Microscope,”

Scanning 2006.

A. E. Vladar, R. Silver, J. Potzick, J. S. Villarrubia and M.T. 

Postek “2004 Development of Photomask Artifacts and Metrology Methodology for the 65 nm Device Generation with 

Extensibility Beyond” SEMATECH Final Report.

A. E.Vladar, J. S. Villarrubia, R. Dixson., T. Vorburger., N. G. 

Orji., B. D. Bunday, and M. Bishop, “Line Edge Roughness 

Metrology for Integrated Circuit Technology,” Final Report 

to International SEMATECH (March 2004).

A. E. Vladar, E. C. Jayewardene, B. N. Damazo, W. J. 

Keery, and M. T. Postek, “Laser Sample Stage-based Image 

Resolution Enhancement Method for SEMs,” SPIE 2004 

5375-181.

B. N. Damazo, E. C. Jayewardene, A. E. Vladar, W. J. Keery, 

and M. T. Postek, “New Method for the Measurement of SEM 

Stage Vibrations,” 2004. SPIE 2004. 
30 National Institute of Standards and Technology 
Semiconductor Microelectronics and Nanoelectronics Programs 31

SCANNING PROBE MICROSCOPE-BASED

DIMENSIONAL METROLOGY

Technical Contacts:

R. Dixson

G. Orji

J. Fu

GOALS

Improve the measurement uncertainty of criticaldimension measurements in the semiconductor 

industry through improvements in scanning probe 

microscope-based measurements. The International Technology Roadmap for Semiconductors 

(ITRS) identifi es dimensional metrology as a 

key enabling technology for the development of 

next-generation integrated circuits. For example, 

according to the 2006 update, the goal in 2007 for 

critical dimension (CD) measurement precision 

for isolated lines was ± 0.52 nm; this demand 

tightens to ± 0.29 nm by 2012.

Although most in-line metrology is performed 

using scanning electron microscope (SEM) and 

scatterometry, these instruments are not presently 

capable of fi rst-principles accuracy. That is, they 

must be calibrated using reference measurements 

from a tool or combination of tools which is capable of intrinsic accuracy. Such a tool is now 

referred to as a reference measurement system 

(RMS), and the 2006 update of the ITRS highlights 

the growing importance of an RMS. The use of 

atomic force microscope (AFM) and transmission 

electron microscope (TEM) cross section for this 

purpose – often in combination – is now a fairly 

common practice in the industry. 

The technical focus of this project, development 

and implementation of scanned probe microscope 

instrumentation for traceable dimensional metrology, is thus driven by the anticipated industry 

needs for reduced measurement uncertainty for 

in-line metrology tools such as the SEM and 

scatterometer – since these in turn rely on reduced 

measurement uncertainty for techniques such as 

AFM that are often implemented as an RMS. 

CUSTOMER NEEDS

The SEM is still the current tool of choice for 

inspection and metrology of sub-micrometer 

features in the semiconductor industry. Scatterometry or optical critical dimension (OCD) 

metrology is also rapidly gaining acceptance as 

an in-line process metrology tool. Scanning probe 

microscopes (SPMs) possess unique capabilities, 

which may signifi cantly enhance the performance 

of SEMs for in-line CD measurements, and are 

also emerging as CD measurement tools in their 

own right. A creative strategy, which successfully 

harnesses the strong points of both techniques in 

order to reduce the measurement uncertainty of 

sub-micrometer features, will help NIST meet 

the expectations of the semiconductor industry 

expressed in the current ITRS. As is the case 

with SEMs, the magnifi cation or scale of an SPM 

must be calibrated in order to perform accurate 

measurements. Although many SPMs are commercially available, appropriate calibration standards have lagged. In particular, the availability 

of traceable pitch, height, and width standards in 

this regime is limited.

TECHNICAL STRATEGY

The SPM dimensional metrology program consists 

of three inter-related thrusts: The fi rst two thrusts 

address SPM dimensional metrology with two inhouse research instruments at NIST, and the third 

thrust involves a partnership with SEMATECH to 

maintain traceability on a commercially available 

in-line SPM housed in the manufacturing facility 

at SEMATECH. The two instruments housed at 

NIST are a calibrated atomic force microscope 

(C-AFM) for measurement of pitch and step height 

and a critical dimension atomic force microscope 

(CD-AFM) for measurement of line width.

The C-AFM is a custom built instrument that has 

metrology traceable to the wavelength of light for 

all three axes of motion, and it has provided calibrated pitch and height measurements for a variety 

of nano-scale applications. Pitch measurements in 

the micrometer regime and below can currently be 

performed with relative standard uncertainties as 

low as 5 × 10-4, and step height measurements up 

to several hundred nanometers can be performed 

with a relative standard uncertainty approaching 

1 × 10-3. The C-AFM has participated in two international comparisons of sub-micrometer pitch 

measurements and one comparison of step height 

measurements.

DELIVERABLES: 

Completed refi nement of uncertainties – to reduce 

by approximately a factor of two - and amended 

report on two-dimensional pitch measurements 

using C-AFM as NIST participation in NANO5 – an 

International Supplementary Key Comparison in 

Nanometrology. 4Q 2006

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32 National Institute of Standards and Technology 

Performed CD-AFM measurements, using the 

SXM320 at NIST, on trial chips from 200 mm bulk 

Si (110) wafer to evaluate potential as an alternative starting material for the single crystal critical 

dimension reference material (SCCDRM) project. 

4Q 2006

Performed CD-AFM measurements, using the 

SXM320 at NIST, on next generation single crystal 

critical dimension reference material (SCCDRM) 

samples for phase 2 of the process optimization 

(confi rmation and refi nement) experiment in collaboration with EEEL and ITL. 1Q 2007

Presented paper describing results of the process 

optimization experiment for the “next generation” of 

single crystal critical dimension reference materials 

(SCCDRM) at SPIE Advanced Lithography Meeting. 2Q 2007

Presented paper describing a comparison between 

the calibration of the SCCDRM samples and similar 

commercially available linewidth standard at SPIE 

Advanced Lithography Meeting. 2Q 2007

Presented paper comparing two methods of TEM 

reference metrology – HRTEM and HAADF-STEM 

- for CD-AFM tip width calibration at SPIE Advanced Lithography Meeting. 2Q 2007

Organized, hosted, and ran an evening workshop 

on the history and future of CD standards at the 

SPIE Advanced Lithography Meeting. 2Q 2007

Participate in joint NIST-SEMATECH workshop on 

OCD standards – in regard to traceability and AFM 

reference metrology – at SEMICON West. 

3Q 2007

The second and third thrusts involve the most 

commonly used AFM-based method of linewidth 

metrology in industry: CD-AFM. This type of 

instrument is more sophisticated than conventional AFM and used a fl ared shape probe and 

two-dimensional feedback to image the sidewalls 

of near-vertical structures. The SXM320 is a prior 

generation commercially available CD-AFM. Initially, this instrument was used to implement the 

CD-AFM RMS at SEMATECH during the tenure 

of Ronald Dixson as the fi rst NIST Guest Scientist 

there. Now that the instrument is housed at NIST, 

the uncertainties have been further refi ned. Currently, pitch measurements can be performed with 

a relative standard uncertainty of approximately 

2 × 10-3. Step height measurements have a relative standard uncertainty 4 × 10-3, and linewidth 

measurements can have standard uncertainties as 

low as 1 nm. This is a result of the most current 

release of NIST single crystal critical dimension 

reference materials (SCCDRM) that was completed in 2005.

The third thrust involves collaboration with 

SEMATECH to maintain a traceable CD-AFM 

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(RMS) in the manufacturing facility there. This 

thrust is currently overseen by George Orji who is 

now the second NIST Guest Scientist at SEMATECH. A current generation CD-AFM, the Veeco 

Dimension X3D, is now being used to implement 

the RMS. As is true for the SXM at NIST, the 

SCCDRM project has resulted in the ability to 

perform linewidth measurements with a standard 

uncertainty of 1 nm. The relative uncertainties in 

pitch and height measurements have been recently 

reduced to 1 × 10-3 and 2 × 10-3, respectively.

DELIVERABLES:

Presented paper comparing two methods of TEM 

reference metrology – HRTEM and HAADF-STEM - 

for CD-AFM tip width calibration at SPIE Advanced 

Lithography Meeting. 2Q 2007

Publish a joint paper with major semiconductor 

manufacturer on use of CD-AFM reference metrology to evaluate the performance of in line FIB cross 

sections at EIPBN conference. 2Q 2007

Complete a report on the refi nements and extensions of the CD-AFM reference measurement 

system (RMS) for SEMATECH. 3Q 2007

ACCOMPLISHMENTS

The NIST/SEMATECH partnership in 

advancing AFM metrology in semiconductor 

manufacturing continues. George Orji is now in 

the third year of his tenure as the second NIST 

Guest Scientist at SEMATECH, following a three 

year tenure of Ronald Dixson as the fi rst NIST 

Guest Scientist. George has continued the NIST 

engagement with SEMATECH by supporting their 

SEM and scatterometer benchmarking activities 

with the RMS, and he has overseen the reduction 

of pitch and height uncertainties by approximately 

a factor of two. 

We are collaborating with other NIST 

researchers and external partners such as SEMATECH, Intel, and IBM to develop linewidth 

standards and CD reference metrology. A major 

component of this effort has been the single crystal 

critical dimension reference materials (SCCDRM) 

project initiated by NIST researchers in EEEL 

(Cresswell and Allen). In late 2004, we completed 

a release of SCCDRMs to SEMATECH and its 

member companies, and CD-AFM dimensional 

metrology played a central role. The measurements on the SCCDRM samples were performed 

by NIST scientist Ronald Dixson using the X3D 

at SEMATECH which had been implemented as a 

CD-AFM reference measurement system (RMS) 

for traceable measurements of pitch, height, and 

width. High resolution transmission electron 

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Semiconductor Microelectronics and Nanoelectronics Programs 33

Figure 1. Regression of CD-AFM width values on 

HRTEM values. The observed slope is consistent 

with unity – indicating that the two methods have 

consistent scale calibration. The average offset 

between the results was used to correct the tip width 

calibration.

microscopy (HRTEM) was used as an indirect 

method of tip width calibration for the SCCDRM 

samples. The AFM and HRTEM results that were 

used for the tip calibration are shown in Fig. 1. 

Features on the distributed samples ranged in 

width from approximately 50 nm to 250 nm with 

expanded uncertainties of about 2 nm (k = 2) on 

most features. 

Figure 2. CD-AFM image of a NIST45 SCCDRM 

specimen which can be used for tip width calibration. 

Measurements performed by R. Dixson, sample 

developed by M. Cresswell and R. Allen.

As a result of this project, CD-AFM linewidth 

measurements can now be performed with a 

1 nm (k = 1) standard uncertainty. This standard 

for width calibration is now being used on both 

the X3D and the SXM at NIST. An image of an 

SCCDRM taken on the SXM320 is shown in 

Fig. 2. In 2005, we launched the “next generation” of the SCCDRM project and are currently 

performing a series of designed experiments to 

further improve the performance characteristics of 

the SCCDRMs – including linewidth uniformity. 

#

In the fi rst phase of these experiments, we have 

observed feature widths as low as 20 nm with 

uniformity at the 1 nm level.

We have used the C-AFM to participate in a 

series of International Supplementary Comparisons in nanometrology coordinated by the CIPM 

(Comité International des Poids et Mesures) CCL 

(Coordinating Committee for Length). Successful 

participation in these Comparisons will facilitate 

international recognition of traceable measurements of dimensional quantities important to the 

semiconductor industry. We have previously 

participated in comparisons of step height and one 

dimensional pitch measurements. The results of 

these comparisons have now been published under the Mutual Recognition Arrangement (MRA) 

through which the NMIs recognize each other’s 

measurement capability, thus helping to eliminate technical barriers to trade. During 2005, we 

participated in a comparison of two dimensional 

pitch measurements: the C-AFM was used to 

measure both a 300 nm and 1000 nm grating. 

An example of a C-AFM image on the 1000 nm 

grating is shown in Fig. 3. A similar comparison 

of linewidth measurements is currently being 

planned and will be led by NIST.

#

Figure 3. An image of a 1000 nm pitch grating 

taken using the C-AFM as part of an international 

comparison of two dimensional pitch measurements.

Another effort involves the study of single 

atomic Si steps as fundamental height standards 

in the sub-nanometer regime. During 2004 we 

developed a draft standard for AFM z-calibration 

using the single atom steps for the ASTM Subcommittee E42.14 on STM/AFM. This draft standard 

is still being revised and undergoing review within 

the subcommittee. 

We are also developing a technique for AFM 

linewidth metrology based on image stitching. 

This involves the acquisition of paired images 

using a carbon nanotube probe. The nanotube tip 

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34 National Institute of Standards and Technology 

enables data acquisition at high resolution on one 

side of the line in each image, and the specimen 

is rotated 180 degrees between the two measurements. Stitching these two images into a composite offers the potential of accurate linewidth 

metrology. We have completed two generations 

of an experiment in which the stitching result is 

compared with CD-AFM. The composite image 

used in one of our published comparisons between 

image stitching and CD-AFM is shown in Fig. 4. 

Currently, we are assessing the uncertainties and 

continuing to develop this approach. 

Figure 4. Stitched composite slope image from two 

images of a linewidth sample taken with a carbon 

nanotube tip. Measurements performed by J. Fu; 

sample developed by M. Cresswell and R. Allen; 

probe developed by C. Nguyen of ELORET/NASA 

Ames.

We continue to refi ne the CD-AFM RMS at 

SEMATECH and bring its traceable measurement 

capabilities to bear on semiconductor manufacturing applications. During the tenure of both NIST 

Guest Scientists at SEMATECH, we have used 

this RMS system to provide reference metrology 

for both SEM and scatterometer benchmarking, 

measurements of SEM-induced shrinkage of 193 

nm photoresist, and to provide reference metrology for evaluation of several new optical methods 

for measurement of photomasks. 

In the SEM benchmarking applications, our 

basic methodology is to perform both CD-AFM 

and SEM measurements on a series of features 

on a focus exposure matrix that spans a process 

window. This approach means that both tools 

see features that have a range of widths as well 

as secondary characteristics such as sidewall 

angle and corner rounding. A regression of the 

#

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AFM/SEM data allows the SEM to be checked for 

bias, scale calibration, and linearity of response 

across the FEM. An example of a SEM/AFM 

regression using measurements of isolated polysilicon lines is shown in Fig. 5. SEMATECH and 

its members place high value on SEMATECH’s 

tool benchmarking activities and the support 

these activities receive from the NIST-supported 

CD-AFM RMS.

Figure 5. An example of an AFM/SEM regression 

performed as part of a SEMATECH benchmarking 

activity during the tenure of Ronald Dixson as the 

fi rst NIST Guest Scientist at SEMATECH. The 

measurements were performed on isolated etched 

polysilicon lines that spanned a wide process window 

focus exposure matrix. The Mandel regression allows 

for uncertainty in both variables and is the most 

appropriate way to compare a tool under test with the 

reference measurement system. In this example, the 

SEM performance is very good and exhibits a bias of 

only 3 nm and linearity within the uncertainties of the 

comparison.

COLLABORATIONS

Intel Mask Operations, Santa Clara, CA

Intel, Hilllsboro, OR

Intel, Santa Clara, CA

SEMATECH, Austin, TX

Veeco Metrology, Santa Barbara, CA

IBM Burlington, VT 

IBM Almaden Research Center, San Jose, CA

ELORET Corp./NASA Ames Research Center, Moffett 

Field, CA 

Departments of Mechanical Engineering and Chemistry, 

University of North Carolina, Charlotte, NC

School of Mechatronic Engineering, Harbin Institute of Technology, Harbin, China
Semiconductor Microelectronics and Nanoelectronics Programs 35

RECENT PUBLICATIONS

N. G. Orji, R. G. Dixson, “Higher Order Tip Effects in Traceable CD-AFM Based Linewidth Measurements,” Meas. Sci. 

and Technol. Vol. 18, 448-455 (2007).

N. G. Orji, R. Dixson, B. D. Bunday, J. A. Allgair, “TEM 

Calibration Methods for Critical Dimension Standards,” SPIE 

Proceedings Vol. 6518, 651810 (2007).

B. Bunday, J. Allgair, E. Soleck, C. Archie, N. G. Orji, J. 

Beach, O. Adan, R. Peltinov, M. Bar-zvi, J. Swyers, “The 

coming of age of tilt CD-SEM,” SPIE Proceedings Vol. 6518, 

65181S (2007).

R. Dixson and N. G. Orji, “Comparison and uncertainties 

of standards for critical dimension atomic force microscope 

tip width calibration,” SPIE Proceedings Vol. 6518, 651816 

(2007).

R. Dixson, W. F. Guthrie, M. W. Cresswell, R. A. Allen, 

Ndubuisi G. Orji, “Single crystal critical dimension reference materials (SCCDRM): process optimization for the 

next generation of standards,” SPIE Proceedings Vol. 6518, 

651815 (2007).

B. C. Park, J. Choi, S. J. Ahn, D-H Kim, J. Lyou, R. Dixson, N. 

G. Orji, J. Fu, T. V. Vorburger, “Application of carbon nanotube probes in a critical dimension atomic force microscope,”

SPIE Proceedings Vol. 6518, 651819 (2007).

R. Dixson, N.G. Orji, J. Fu, M. Cresswell, R. Allen, W. 

Guthrie, “Traceable Atomic Force Microscope Dimensional 

Metrology at NIST,” SPIE Proceedings Vol. 6152, 61520P1-11 (2006). 

N. Orji, A. Martinez, R. Dixson, J. Allgair, “Progress on 

Implementation of a CD-AFM based Reference Measurement System,” SPIE Proceedings Vol. 6152, 61520O-1-12 

(2006).

R. G. Dixson, R. A. Allen, W. F. Guthrie, and M. W. Cresswell, 

“Traceable Calibration of Critical-Dimension Atomic Force 

Microscope Linewidth Measurements with Nanometer Uncertainty,” J. Vac. Sci. Technol. B Vol. 23, 3028-3032 (2005).

J. Fu, R. Dixson, N. Orji, T. Vorburger, C. Nguyen, “Linewidth 

Measurement from a Stitched AFM Image,” AIP Conference 

Proceedings, Vol. 788, 421-426 (2005).

R. Dixson, J. Fu, N. Orji, W. Guthrie, R. Allen, M. Cresswell, 

“CD-AFM Reference Metrology at NIST and SEMATECH,”

SPIE Proceedings Vol. 5752, 324-336 (2005).

M. Cresswell, R. Dixson, W. Guthrie, R. Allen, C. Murabito, B. 

Park, J. Martinez de Pinillos, and A. Hunt, “Critical Dimension 

Reference Features with Sub-Five Nanometer Uncertainty,” 

SPIE Proceedings Vol. 5752, 288-303 (2005).

N. G. Orji, M.I. Sanchez, J. Raja, and T. Vorburger, “AFM 

Characterization of Semiconductor Line Edge Roughness,” 

in Applied Scanning Probe Methods, edited by B. Bhushan, 

H. Fuchs, and S. Hasaka (Springer Verlag, Berlin, 2004) p. 

273. 

B. D. Bunday, M. Bishop, D. McCormack, J. S. Villarrubia, 

A. E. Vladar, R. Dixson, T. Vorburger, N. G. Orji, J. A. Allgair, “Determination of Optimal Parameters for CD-SEM 

Measurement of Line Edge Roughness,” SPIE Proceedings 

Vol.. 5375 (2004).

R. Dixson and A. Guerry, “Reference Metrology Using a 

Next generation CD-AFM,” SPIE Proceedings Vol. 5375, 

633-646 (2004).

X. Zhao, J. Fu, W. Chu, C. Nguyen, and T. Vorburger, “An 

Image Stitching Method to Eliminate the Distortion of the 

Sidewall in Linewidth Measurement,” SPIE Proceedings Vol. 

5375, 363 -373 (2004).
36 National Institute of Standards and Technology 
Semiconductor Microelectronics and Nanoelectronics Programs 37

OPTICAL-BASED PHOTOMASK DIMENSIONAL

METROLOGY

Technical Contacts:

J. Potzick

R. Silver

T. Doiron

GOALS

Provide technological leadership to semiconductor and equipment manufacturers and other 

government agencies by developing and evaluating the methods, tools, and artifacts needed 

to apply optical techniques to the metrology 

needs of semiconductor microlithography. One 

specifi c goal is to provide the customer with the 

techniques and standards needed to make traceable dimensional measurements on photomasks 

and wafers, where appropriate, at the customer’s 

facility. The industry focus areas of this project 

are primarily the optical based methods used in 

overlay metrology, photomask critical dimension 

metrology, and high-accuracy two-dimensional 

placement metrology. 

CUSTOMER NEEDS

Tighter tolerances on CD measurements in photomask and wafer production place increasing 

demands on photomask linewidth accuracy. NIST 

has a comprehensive program to both support 

and advance the optical techniques needed to 

make these photomask critical dimension measurements. 

Accurate feature size metrology on binary 

photomasks (constituting 85 % of current mask 

production) becomes increasingly diffi cult as 

critical features shrink and their optical proximity 

correction cousins proliferate. Phase shift and 

EUV masks present additional challenges.

Improved two-dimensional overlay measurement 

techniques and standards are needed for measuring and controlling overlay errors on the wafer 

and accurately measuring image placement of 

features on the photomasks. Overlay control is 

listed in multiple sections of Table 116 of the 2005 

SIA ITRS as a diffi cult challenge for <32 nm processes. Overlay metrology and photomask feature 

placement metrology have not kept pace with 

resolution improvements and in-die correlation 

requirements and will be inadequate for ground 

rules less than 45 nm. In fact, Table 119a shows 

that there are current image placement and overlay 

control challenges with no known solutions for 

photomask image placement metrology beyond 

the 65 nm node. As shown in Table 119b, the 

problems are more acute for long term photomask 

CD metrology where the industry will soon be 

encountering metrology problems without known 

manufacturing solutions. 

TECHNICAL STRATEGY

There are two main strategic technical components 

of this project. 

1. Photomask linewidth measurements using the 

ultraviolet transmission microscope, calibration 

of NIST Photomask Linewidth Standard SRM 

2059, and the development of calibration methods to obtain photomask linewidth measurement 

uncertainties adequate to meet industry needs. 

The technical strategy for photomask linewidth 

standards is divided into two segments: (1) instrumentation and model development and (2) 

design and calibration of standard artifacts. An 

ultraviolet transmission microscope (Fig. 1) has 

been constructed which uses a unique geometry 

(a Stewart platform) as the main rigid structure. 

This microscope platform has good vibration 

characteristics and presents an open mechanical 

architecture. Higher image resolution, reduced 

transmission of UV light through the chrome, 

and reduced instrument vibration offer improved 

linewidth measurement uncertainties. 

NIST has supplied a substantial number of photomask linewidth standards worldwide over the 

past decade. NIST’s current chrome-on-quartz 

photomask linewidth standard, SRM 2059, 

(Fig. 2, next page) contains isolated linewidth 

and spacewidth features in the range of 0.25 μm 

Figure 1. Modeling results for isolated binary photomask lines from 4μm to 0.125μm.
38 National Institute of Standards and Technology 

to 32 μm, whose widths have been measured and 

certifi ed to an uncertainty of ~20 nm at the 95 % 

confi dence level.

Figure 2. NIST SRM 2059 Photomask Linewidth 

Standard. Isolated linewidths and spacewidths range 

form 0.250 um to 312 um.

In response to customers’ needs for more accurate 

photomask feature size measurements, NIST has 

worked extensively to improve mask metrology 

through optical image modeling and improved 

optical microscope characterization methods. The 

features on even the highest quality masks exhibit 

roughness and runout at the chrome edges, compromising the defi nition of edge and linewidth. 

In response to these problems, NIST introduced 

the Neolithography concept 1997: Modeling the 

effects of all of the relevant feature properties in 

both the mask metrology process and the wafer 

exposure and development processes, using existing and new software tools, can improve feature 

size accuracy by establishing accurate simulation 

results and improving the relationship between 

mask-feature metrology and the corresponding 

wafer-feature sizes. This concept (at least in part) 

can be seen today in the design for manufacturing 

(DFM) sector of the industry.

DELIVERABLES: Compare and improve the accuracy 

of the NIST optical scattering code with new scattering 

models developed by Spectel and JCMwave for use in 

transmission. 4Q 2007

2. The second major component is the development of two-dimensional grid calibration 

standards and associated measurement techniques, 

including statistical analysis and charge-coupled 

device (CCD) characterization as used by industry. 

These individual technical strategies for these 

components are described next.

We are approaching the problem of two-dimensional measurements from a couple of directions. 

The fi rst is the calibration of an artifact standard 

which can be used to bring all of the two-dimensional based inspection instruments to the same 

metric. This work has developed a standard grid 

which is now available as a NIST Standard Reference Material, # 5001. The artifact can be used 

by the semiconductor industry to standardize 2-D 

measurements. The SRM 5001 was developed as 

an industry consensus standard grid and calibrated 

with measurements on a state-of-the-art industry 

machine followed by verifi cation of the measurements using NIST Linescale Interferometer 

capabilities which resulted in traceable two-dimensional measurements. 

A new generation of grid for the SRMs has been 

fabricated and another round of measurements 

is in process. Each measurement of the grid has 

data in each of at least two orientations. Rotating 

the grid 90° between measurements samples a 

number of the geometric errors of the machine. 

The remaining geometric sources of uncertainty 

are the scale and some components of the linearity 

of each machine axis travel. 

Verifi cation of the industry-based grid measurements is done at NIST. The overall scale of the 

grid is checked with the NIST linescale interferometer, an instrument that is known to provide the 

most accurate 1-D measurements available in the 

world. Two sources of uncertainty not captured 

by these measurements include components of the 

straightness and effects of the plate bending when 

fi xtured. Work is now focused on the new Nikon 

5i two-dimensional metrology tool at NIST. The 

Nikon 5i has been fully characterized and is now a 

calibration tool for grid calibrations. In response 

to industry needs, this tool is now under development for use as a wafer calibration tool for large 

distance position-to-position wafer calibration.

To strengthen the foundation of NIST’s linewidth 

measurement traceability and to support the Bureau International des Poids et Mesures (BIPM) 

Mutual Recognition Arrangement, NIST has 

become the pilot laboratory for an international 

intercomparison of submicrometer linewidth measurements. National metrology institutes in nine 

countries around the world are participating.
Semiconductor Microelectronics and Nanoelectronics Programs 39

DELIVERABLES: 

Develop a new set of comparator algorithms to 

enable the SRM 5001 calibrations to be completed 

with the Nikon 5i tool in collaboration with the 

industry Ipro system. Complete the comprehensive analysis capabilities for centerline and edge 

detection methods with a complete uncertainty 

statement. 2Q 2007

Implement the standard scale correction and 

new mapping software for the Nikon 5I system. 

Measure the new set of two-dimensional calibration 

grids with a complete uncertainty. 1Q 2007

Complete the second round of calibrations of SRM 

5001 and complete the related documentation. Deliver the accompanying documentation to the Offi ce 

of Standard Reference Materials for distribution to 

customers. 2Q 2007

ACCOMPLISHMENTS

SRM 2800 Microscope Magnifi cation Standard is a standard-size microscope slide with 

calibrated pitch features ranging from 1 μm to 

1 cm (see Fig. 3). It contains a lithographically 

produced chrome pitch pattern consisting of a 

single array of parallel lines with calibrated center-to-center spacings. It contains no linewidth 

structures. This SRM is intended to be used for 

the calibration of reticles and scales for optical or 

other microscopes at the user’s desired magnifi cation. The SRM may be used in either transmission 

or refl ection mode optical microscopes, SEMs, 

or SPMs. Calibration is traceable to the meter 

through NIST Line Scale Interferometer. Fiftysix units have been calibrated and delivered to the 

NIST Offi ce of Standard Reference Materials and 

nearly half of current stock has been sold. 

•

•

•

#

Figure 3. The two dimensional grid photomask 

standard, now available through the SRM offi ce.

Figure 4 shows tool repeatability and mapping data for the two-dimensional mask 

calibration procedure.

The fi rst set of two-dimensional grid artifacts, 

known as SRM 5001, were delivered to the SRM 

offi ce and are selling well. These 6-inch photomasks have been measured on a state-of-the-art Ipro metrology system by the photomask manufacturer. A second set of re-designed 6-inch feature 

placement standards has now been measured and 

fully calibrated in the close collaboration between 

NIST and Photronics. The collaboration employs 

the industry tool and the traceability of the NIST 

line scale interferometer with appropriate statistical analysis (see Fig. 4). The uncertainty budget 

for grid measurements and all documentation has 

been delivered to the SRM offi ce and the second 

set of calibrated photomask grids will now be 

made available as a calibrated SRM.

#

In close collaboration with SEMATECH, 

we have completed a comprehensive report using optical methods for the calibration of phase 

shifting photomasks. This includes modeling and 

measurements in transmission and refl ection. This 

document is available through SEMATECH as a 

tech transfer document.

A comprehensive suite of two dimensional 

calibration methods for the calibration of optical 

systems and illumination systems is being developed based on the SRM 5001 grid calibration 

methodology. Some of these results were recently 

published at SPIE Microlithography. These methods are enabling a substantially improved optical 

calibration and alignment sequence as well as 

improved modeling inputs for more accurate 

linewidth measurements.

#

#
40 National Institute of Standards and Technology 

COLLABORATIONS

ISMT, IBM, IVS Schlumberger, KLA-Tencor, Intel, Motorola, AMD and several other leading manufacturers or tool 

vendors.

Dr. Mark Davidson, Spectel Research Corp.

RECENT PUBLICATIONS

J.Potzick, E.Marx, M.Davidson, “Accuracy in Optical Image 

Modeling,” Proceedings of SPIE Metrology, Inspection, and 

Process Control Conference for Microlithography XXI, vol. 

6518-39 (2007).

J.Potzick, E.Marx, M.Davidson, Parametric Uncertainty in 

Optical Image Modeling, Proceedings of BACUS Symposium 

on Photomask Technology, vol. 6349-187 (2006).

J.Potzick, A benefi t/cost model for metrology in manufacturing, Proc. ISMI Symposium on Manufacturing Effectiveness, 

SEMATECH, (2005)

Egon Marx and James Potzick, “Simulation of optical microscope images for photomask feature size measurements,” 

2005 Digest of the IEEE Antennas and Propagation Society 

International Symposium, Pages 2116-2119 (2005).

SEMI Standard P35-0704, Terminology for Microlithography 

Metrology, SEMI International Standards, 3081 Zanker Rd., 

San Jose, California 95134

J. Potzick, “Optical photomask CD metrology at NIST,” 

Proceedings of the 188th PTB Seminar on CD Metrology; 

H. Bosse, B. Bodermann, W. Mirande` Eds.; PhysikalischTechnische Bundesanstalt (PTB), Braunschweig, Germany, 

(Nov. 20, 2003)

J. Potzick, “Photomask feature metrology,” Ch. 21 of A 

Handbook on Mask Making Technology, Syed A. Rizvi, Ed., 

Marcel Dekker Inc. New York (2005).

J.M. Pedulla, J. Potzick, R. Silver, “Improving the uncertainty 

of photomask linewidth measurements,” Proceedings of SPIE 

29th International Symposium on Microlithography, vol. 

5375-31, (2004)

SRM 2059 Certifi cate (2006), J. Potzick

SRM 5001 Certifi cate (2006). D. Everett, L. Howard, R. M. 

Silver, and T. Doiron.

R. M. Silver, B. Barnes, R. Attota, J. Jun, J. Filliben, J. Soto, 

M. Stocker, P. Lipscomb, E. Marx, H. Patrick, R. Dixson, and 

R. Larrabee, “The Limits of Image-based Optical Metrology,” 

Proc. SPIE Intl. Soc. Opt. Eng. Vol. 6152, 61520Z (2006).

R. Attota, R. M. Silver, M. Bishop and R. Dixson, “Optical 

Critical Dimension Measurement and Illumination Analysis 

using the Through-focus Focus-metric,” Proc. SPIE Intl. Soc. 

Opt. Eng. Vol. 6152, 61520K (2006).

Y. J. Sohn, B. Barnes, L. Howard, R. Silver, R. Attota, and 

M. Stocker, “Kohler Illumination in High Resolution Optical Metrology,” Proc. SPIE Intl. Soc. Opt. Eng. Vol. 6152, 

61523S (2006).

P. Lipscomb, J. Allgair, B. Bunday, M. Bishop, R. Silver, 

R. Attota, and M. Stocker, “Calibration Optical Overlay 

Measurements,” Proc. SPIE Intl. Soc. Opt. Eng. Vol. 6152, 

615211 (2006).

R. M. Silver and A. Vladar, “Photomask Metrology,” Photomask Fabrication Technology, McGraw Hill, pp. 281-342, 

2005.

“Development of Photomask Standards and Metrology Methodology for the 65 nm Device Generation and Beyond,” A. 

Vladar, and R. M. Silver, SEMATECH Final Report, Tech 

Doc. # (2004).

J. M. Pedulla, J. Potzick, R. Silver, and R. Larrabee, “Improving the Uncertainty of Photomask Linewidth Measurements,” 

SPIE Vol. 5375, p. 317, 2004.

J. Potzick, J.M. Pedulla, M. Stocker, “Updated NIST Photomask Linewidth Standard,” Proceedings of SPIE 28th 

International Symposium on Microlithography, vol. 5038-34 

(2003).

R. M. Silver, R. Attota, M. Stocker, J. Jun, E. Marx, R. Larrabee, B. Russo, and M. Davidson, “Comparison of Measured 

Optical Image Profi les of Silicon Lines with Two Different 

Theoretical Models,” Metrology, Inspection and Process 

Control for Microlithography XVI, Proc. SPIE, Vol. 4689, 

p. 409, 2002.

R. M. Silver, J. Jun, E. Kornegay and R. Morton, “Comparison 

of Edge Detection Methods using a Standard Prototype Overlay 

Calibration Artifact,” Proc. SPIE Vol. 4344, p. 515 (2001).

J. Potzick, “The problem with submicrometer linewidth 

standards and a proposed solution,” Proceedings of SPIE 

26th International Symposium on Microlithography, vol. 

4344-20 (2001).

S. Fox, “Focus, edge detection and CCD camera characterization for development of an optical calibration standard,” 

University of Maryland, PhD Thesis 2000.

T. Doiron and R. Silver, “Two-dimensional grid Standards,” 

Proc. Measurement Sciences Conf. (2000). 

S. Fox, E. Kornegay, and R. Silver, “Characterization of Imaging Optics and CCD Cameras for Metrology Applications,” 

Characterization and Metrology for ULSI Technology, D. 

Seiler, et. al., eds, A.I.P. Press, 2000, pp. 378-382.

S. Fox, R. Silver Thesis Advisor, “Focus, Edge Detection, 

and CCD Camera Characterization for Development of an 

Optical Overlay Calibration Standard,” Graduate Thesis, 

Univ. of MD (2000).

J. Potzick, “The Neolithography Consortium—a progress 

report,” Proceedings of BACUS Symposium on Photomask 

Technology, vol. 4186-73 (2000).

J. Potzick, “The Neolithography Consortium,” Proceedings 

of SPIE 25th International Symposium on Microlithography, 

vol. 3998-54 (2000).

M. Schroeck, and T. Doiron, “Probing of two-dimensional 

grid patterns by means of camera based image processing,”

accepted by SPIE for January 2000. 

K. Meissner, and T. Wegener, “Robust Regression for Two 

Dimensional Coordinate Measurements,” Proceedings of the 

International Symposium on Laser Metrology for Precision 

Measurement and Inspection in Industry, Florianoplis, Brazil, 

October 13-15, 1999.

R. Silver, T. Doiron, S. Fox, E. Kornegay, M. Takac, and S. 

Rathjen, Two dimensional Grid Standards, Proceedings of 

the SPIE, The International Society for Optical Engineering, 

Metrology, Inspection, and Process Control for Microlithography XIII (1999).
Semiconductor Microelectronics and Nanoelectronics Programs 41

SCATTEROMETRY-BASED DIMENSIONAL METROLOGY

Technical Contact:

T. Germer

GOALS

Our goals are to: (1) increase the effectiveness of 

scatterometry and other optical critical dimension 

(OCD) methods by providing industry with new 

measurement techniques, improved modeling, 

and standards; (2) provide assessments of accuracy and sensitivity of various OCD methods; and 

(3) develop facilities to accurately assess OCD 

targets. Develop improved OCD methods for assessing critical dimension and overlay. 

CUSTOMER NEEDS

Scatterometry is increasingly becoming a preferred method for online critical dimension (CD) 

metrology. The method relies upon measurements 

of the refl ectance or diffraction of small test grating structures as functions of angle, wavelengths, 

and/or polarization. By comparing measurement 

results with an extensive library of theoretical 

simulations or by performing a real-time regression analysis, tools can extract such line profi le 

parameters as critical dimension, sidewall angle, 

and height, as well as more detailed descriptions 

of the sidewall shape. 

While scatterometry has gained signifi cant acceptance, it is continuing to grow in utility. However, 

there are many issues that remain that prevent it 

from having absolute accuracy. For example, the 

effects of fi nite illumination, fi nite target array 

size, line-edge and line-width roughness, uncertainties in the optical properties of the materials 

in the structure, neglect of surface oxides or other 

layers, radiometric accuracy, and the integrity of 

the theoretical model all contribute to the fi nal 

measurement uncertainty in ways that are at this 

time poorly understood. 

Ultimately, the industry needs reference artifacts 

that can test the validity of the results obtained by 

scatterometry tools. Such an artifact might consist 

of a set of gratings that have been characterized by 

a variety of techniques, including scatterometry, 

scanning electron microscopy (CD-SEM), atomic 

force microscopy (CD-AFM), and transmission 

electron microscopy (TEM). Scatterometry 

provides an independent method compared to 

the others, and if a comprehensive uncertainty 

budget were developed for the method, it would 

substantially improve the overall state of dimensional metrology.

TECHNICAL STRATEGY

There are three major strategies for improving 

the effectiveness of scatterometry. One strategy 

is to develop effi cient models for the diffraction 

of light by structures on surfaces, so that NIST 

has state-of-the-art capabilities to perform scatterometry measurements and analysis as well as 

to provide standard data for a variety of model 

structures. The second strategy is to assess the 

sensitivity and accuracy of scatterometry methods 

to different structures, in order to provide industry 

with an understanding of what determines the 

ultimate sensitivity and accuracy of the methods. 

Finally, the third strategy is to develop in-house 

measurement capabilities with the long-term 

goal to perform scatterometry measurements on 

reference materials, to perform inter-laboratory 

comparisons, and to further develop the scatterometry technique. 

Specifi c project elements are defi ned below:

1. Theoretical Scatterometry Modeling –

Rigorous coupled wave (RCW) based theories 

are the most common methods used to analyze 

scatterometry data. We have developed in-house 

capability to perform RCW calculations from 

arbitrary two-dimensional periodic structures 

consisting of isotropic materials. These codes 

are being used to generate libraries for profi le 

extraction as well as test calculations to assess the 

sensitivity of scatterometry to changes in model 

parameters or to non-ideal target profi les. Extensions to this capability are required to assess the 

effects of line-edge and line-width roughness and 

material anisotropy, as well as to perform RCW 

calculations on three-dimensional structures, such 

as contact holes. Building upon the ScatMech 

library of codes that have been made available 

on the web for diffuse scattering calculations, we 

intend to publish these codes.

DELIVERABLES: 

Publication of theory for anisotropic lines. 3Q 2007

Publication of two-dimensional RCW code in the 

ScatMech library. 3Q 2007

2. Assessment of Accuracy and Precision of 

Scatterometry – Scatterometry relies heavily on 

prior knowledge of the specifi c structure being 

examined. For example, optical properties of 

all incorporated materials are required for the 

•

•
42 National Institute of Standards and Technology 

simulations. Reasonable parameterization of 

sidewall profi les are required to yield meaningful 

profi les. Two dimensional structures are assumed 

to not exhibit line-edge or line-width roughness. 

In this program element, we are assessing the 

impact that each model parameter has on the 

outcome of the measurement. The goal is to 

establish an independent uncertainty budget that 

includes all sources of random and systematic 

uncertainties. Furthermore, we are assessing the 

sensitivity limits of scatterometry, to determine 

how far into the future scatterometry tools can 

provide critical dimension metrology. A report 

was made to SEMATECH outlining the methods 

used to determine the sensitivity and applying that 

methodology to two simple structures. 

DELIVERABLES:

Perform study of the limits of scatterometry precision and accuracy for several process-relevant 

gratings. 3Q 2007

Develop method for assessing systematic uncertainties arising in scatterometry. 3Q 2007

Publish computer program for assessing sensitivity 

and uncertainty in arbitrary scatterometers. 

2Q 2008

3. Scatterometry Measurements – We have 

recently upgraded our laser-based Goniometric 

Optical Scatter Instrument (GOSI), Fig. 1, used 

for diffuse scatter measurements, to perform 

scatterometry measurements on industry-relevant 

targets on 300 mm wafers. The measurement 

capabilities include angle-scanned scatterometry 

at a number of discrete laser wavelengths. This 

instrument has the capability to perform conical 

scatterometry measurements and will be used 

to perform traditional measurements as well as 

measurements of higher-order, non-specular 

diffraction and diffuse scatter. A long term goal 

is to develop reference scatterometry targets, 

measure them with this instrument, provide accurate determinations of their dimensions and 

profi les, and have an uncertainty placed on the 

results. Another long term goal is to develop 

novel measurement modalities that improve the 

utility of scatterometry. One method we have 

demonstrated is microscope-based scatterometry 

using back focal plane imaging. This technique 

enables collection of multiple diffraction order 

scatterometry signatures in a single image, can be 

confi gured for both dense and isolated targets, and 

allows scatterometry and image-based metrology 

to be performed on the same tool. Future projects 

will include the development of spectroscopic 

scatterometry instruments.

•

•

•

Figure 1. The new Goniometric Optical Scatter 

Instrument (GOSI) is a reference refl ectometer for 

diffuse and specular scattering measurements having 

300 mm sample capability.

DELIVERABLES: 

Perform measurements on potential reference scatterometry target. 1Q 2007

Construction of a spectroscopic scatterometry 

capability at NIST. 4Q 2008

ACCOMPLISHMENTS

We have developed effi cient rigorous coupled 

wave (RCW) software for two-dimensional periodic structures in the conical geometry. Results of 

the in-house code have been compared with fi nite 

difference time domain, surface integral equation, 

and other RCW implementations. This software is 

also confi gured to run on a large cluster computer, 

so that library generation is possible. 

As part of our effort in assessing the precision 

and accuracy of scatterometry, we have performed 

a study of the sensitivity of scatterometry to CD 

and sidewall angle for a large number of different 

measurement modalities, including angle-scanned 

and wavelength-scanned refl ectometry and ellipsometry, for amorphous silicon gate and gateresist structures. This study included parameters 

appropriate from the 45 nm half-pitch node to the 

18 nm half-pitch node. The results (see Fig. 2) 

demonstrated that scatterometry can achieve 

the necessary sensitivity to measure dense gratings at these nodes. Future work, however, will 

be necessary to achieve suffi cient sensitivity to 

isolated features.

•

•

#

#
Semiconductor Microelectronics and Nanoelectronics Programs 43

Figure 2. Estimated bottom dimension sensitivity, 

normalized by the 2 % of bottom dimension 

specifi cation, calculated for gate gratings measured 

by angle scans of the refl ected Stokes parameters 

with angles less than 70 degrees.

In the area of scatterometry-based optical 

critical dimension, (OCD) measurement, we 

have recently measured OCD linewidth of lines 

in grating targets fabricated using the singlecrystal critical dimension reference materials 

(SCCDRM) process. The SCCDRM implementation, developed by a multi-OU collaboration 

at NIST, provides lines with known geometries 

– typically vertical sidewalls – defi ned by the 

silicon lattice, and has led to development of a 

prototype linewidth standard for isolated lines 

designed for use in AFM calibration. We have 

shown that the linewidth obtained from the OCD 

technique for these targets is linearly related to 

linewidth obtained from SEM, with a slope near 

unity and zero offset, Fig. 3. Continuing efforts 

to reduce linewidth roughness of the target, to 

#

Figure 3. CD linewidth extracted from OCD, wOCD , 

versus linewidth measured by SEM, wSEM , for six 

targets on an SCCDRM chip.

analyze uncertainties in the OCD measurement, 

and to evaluate the suitability of these targets for 

OCD reference materials, are ongoing.

We have also extracted OCD linewidth from 

scatterometry signatures of silicon-on-silicon gratings obtained using back-focal-plane imaging in 

a microscope. We investigated targets with only 

specular refl ectance (grating pitch 300 nm) and 

those with both specular and higher-order diffraction (grating pitch 600 nm). Linewidths of 131 

nm to 140 nm were obtained, with the back-focalplane signatures demonstrating nanometer-level 

sensitivity to linewidth, and a linear relationship 

of linewidth obtained from scatterfi eld microscopy 

to linewidth measured by SEM was shown. 

COLLABORATIONS

International Sematech Manufacturing Initiative, Benjamin 

Bunday, Limits of Scatterometry Study. 

Department of Electrical Engineering, Texas A&M University, 

Professor Krzysztof Michalski, Modeling of Scattering by 

Lines Having Anisotropic Optical Properties.

RECENT PUBLICATIONS

Thomas A. Germer, “Effect of line and trench profi le variation 

on specular and diffuse refl ectance from a periodic structure,” 

J. Opt. Soc. Am. A 24(3), 696–701 (March 2007).

Richard Silver, Bryan Barnes, Ravikiran Attota, Jay Jun, Michael Stocker, Egon Marx, and Heather Patrick, “Scatterfi eld 

Microscopy for Extending the Limits of Image-based Optical 

Metrology,” Applied Optics, in press (2007).

Heather J. Patrick, Ravikiran Attota, Bryan M. Barnes, Thomas 

A. Germer, Michael T. Stocker, Richard M. Silver, and Michael 

R. Bishop, “Optical Critical Dimension Measurement of Subresolution Silicon Grating Targets using Back Focal Plane Imaging Scatterfi eld Microscopy,” Journal of Microlithography, 

Microfabrication, and Microsystems, submitted (2007).

Richard A. Allen, Heather J. Patrick, Michael Bishop, Thomas 

A. Germer, and Michael W. Cresswell, “Study of Test Structures for Use as Reference Material in Optical Critical Dimension Applications,” in Proceedings of the 20th International 

Conference on Microelectronic Test Structures, held March 

19-22, 2007 in Tokyo, Japan, in press (2007).

Thomas A. Germer, “Modeling the effect of line profi le variation on optical critical dimension metrology,” in Metrology, 

Inspection, and Process Control for Microlithography XXI, 

Proc. SPIE 6518, in press (2007).

Heather J. Patrick, Thomas A. Germer, Michael W. Cresswell, 

Richard A. Allen, Ronald G. Dixson, and Michael Bishop, 

“Modeling and Analysis of Scatterometry Signatures for Optical Critical Dimension Reference Material Applications,” in 

Proceedings of the 2007 International Conference on Frontiers 

of Characterization and Metrology for Nanoelectronics, held 

March 27-29, 2007 in Gaithersburg, MD, submitted (2007).

Thomas A. Germer and Egon Marx, “Simulations of Optical 

Microscope Images,” in Metrology, Inspection, and Process 

Control for Microlithography XX, C.N. Archie, Ed. Proc. SPIE 

6152, 61520I1–11 (2006).

#
44 National Institute of Standards and Technology 

Heather J. Patrick, Ravikirin Attota, Thomas A. Germer, 

Michael Stocker, Richard M. Silver, “Scatterfi eld microscopy 

using conventional and back focal plane imaging with an 

engineered illumination fi eld,” in Metrology, Inspection, and 

Process Control for Microlithography XX, C.N. Archie, Ed. 

Proc. SPIE 6152, 61520J1–10 (2006).

R. M. Silver, R. Attota, M. Stocker, M. Bishop, L. Howard, 

T. Germer, E. Marx, M. Davidson, and R. Larrabee, “Highresolution optical metrology,” in Metrology, Inspection, and 

Process Control for Microlithography XIX, R.M. Silver, ed., 

Proc. SPIE 5752, 67–79 (2005).

Ravikiran Attota, Richard M. Silver, Thomas A. Germer, 

Michael Bishop, Robert Larrabee, Michael T. Stocker, and 

Lowell Howard, “Application of through-focus focus-metric 

analysis in high resolution optical metrology,” in Metrology, 

Inspection, and Process Control for Microlithography XIX, 

R.M. Silver, ed., Proc. SPIE 5752, 1441–1449 (2005).
Semiconductor Microelectronics and Nanoelectronics Programs 45

SMALL ANGLE X-RAY SCATTERING-BASED

DIMENSIONAL METROLOGY

Technical Contacts:

R. L. Jones

E. K. Lin

W. L. Wu

GOALS

To develop a small angle X-ray scattering 

(SAXS)-based methodology to quickly, quantitatively, and non-destructively measure critical 

dimensions (CD) and feature shape with subnanometer resolution on production scale test 

samples. The focus is on delivering a technique 

capable of routine measurement of pattern shape, 

including critical dimension, sidewall angle, 

line width fl uctuations, and line edge roughness, 

and statistical deviations across large areas in 

dense high aspect ratio patterns. This method is 

expected to provide a potential solution for the 

metrology needs of future semiconductor technology nodes. Further, the wavelengths utilized 

by SAXS based measurements well complement 

current metrology tools based on optical scatterometry, SEM, and AFM. 

CUSTOMER NEEDS

The drive to reduce feature sizes to sub-50 nm 

technology nodes continues to challenge metrology techniques for pattern characterization. As 

outlined in the International Technology Roadmap 

for Semiconductors, existing techniques such as 

CD-SEM face signifi cant technical hurdles in 

quantifying parameters such as Line Edge Roughness (LER) as pattern sizes decrease. Emerging 

methods based on techniques such as atomic force 

microscopy are being developed and evaluated. 

However, uncertainties remain in defi ning suitable metrology for standardized measurements of 

both organic and inorganic structures. To address 

these issues, NIST is evaluating the potential 

application of small angle X-ray scattering as a 

measurement tool for both process development 

and the production of standards for current industrial solutions such as CD-SEM.

TECHNICAL STRATEGY

1. Exposure systems capable of sub-50 nm patterning are expected by 2009, requiring control of 

CD on the level of nanometers and in some cases 

sub-nanometer precision. These requirements will 

challenge traditional methods including CD-SEM 

and optical scatterometry. We are developing a 

transmission scattering based method capable of 

Angstrom level precision in critical dimension 

evaluation over large (50 μm x 50 μm) arrays 

of periodic structures (see Fig. 1). In contrast to 

optical scatterometry, SAXS is performed in transmission using a sub-Angstrom wavelength. The 

high energy of the X-ray source allows the beam 

to pass through a production quality silicon wafer, 

and could become amenable to process line characterization. The measurements are performed in 

ambient conditions, minimizing time required for 

sample preparation. The current capabilities of 

commercially available X-ray sources and detectors are suffi cient to implement a laboratory scale 

device capable of high precision measurements. 

This year, NIST has designed and installed the 

world’s fi rst laboratory scale CD-SAXS device. 

Figure 1. Schematic of the SAXS transmission 

geometry. Shown is the X-ray beam as it passes 

through the patterned sample and scattered at an 

angle 2θ. For a precisely aligned sample with known 

composition, a 3-d lineshape is obtained in one 

transmission measurement. Unknown samples can 

also be characterized through measurements at a 

variety of sample angles.

DELIVERABLES: 

Publish evaluation results from laboratory scale 

CD-SAXS instrument. 3Q 2007

Perform cross-sectional measurements using 

laboratory scale CD-SAXS instrument on patterns 

produced with EUV lithography as part of roundrobin study with SEMATECH and Intel. 4Q 2007

2. Characterization of pattern quality includes 

shape factors such as the sidewall angle and curvature. Measurements taken at a series of angles 

of incidence allows the reconstruction of the line 

shape. We have performed tests using multiple 

angles on a test grating and determined an ability to measure sidewall angle to within 1 degree. 

Ongoing analysis and technique refinement 

will provide additional shape factors, allowing 

determination of more complex shape information such as sidewall curvature. In addition to a 

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46 National Institute of Standards and Technology 

“high speed,” model dependent characterization 

performed in a single measurement, a second 

method will be developed based on measurements 

at multiple sample orientations. This method provides a more precise measurement of the entire 

pattern cross section in a non-destructive and 

model independent manner (see Fig. 2). In the 

current technology node, this measurement can be 

used to evaluate optical scatterometry models for 

line shape, while providing a general capability 

to measure a pattern of arbitrary cross section in 

future technology nodes. 

Figure 2. Data from measurements of a polysilicon 

line grating at varying sample angles. Shown is a 

contour plot of diffracted intensity as a function of 

scattering vectors parallel to the substrate, Qx , and 

normal to the substrate, Qz

. The sidewall angle, β, is 

the half angle of the prominent ridges.

DELIVERABLES: Develop and apply refi ned CDSAXS models to incorporate the effects of top and 

sidewall curvature. 2Q 2007

3. Current LER and CD specifi cation of LER 

in CD budgets requires high precision metrology for both quality control by vendors as well 

as quality assessment by customers. Techniques 

such as CD-SEM, a top down technique, measure qualitatively different quantities compared 

to that of optical scatterometry and AFM. Our 

approach is to provide metrology of sidewall 

roughness, where sidewall roughness is defi ned 

by deviations from the average sidewall plane on 

length scales smaller than the CD. In cooperation 

with the Advanced Metrology Working Group 

at SEMATECH, NIST has designed a series of 

structures to model different types of roughness. 

CD-SAXS data from these structures will help 

further develop modeling protocols.

DELIVERABLES:

Develop and publish refi ned models of line edge 

roughness scattering to fi t fi rst round of measurements of line edge roughness on model photoresist 

patterns produced by EUV lithography. 3Q 2007

Participate in round-robin measurements involving Intel and SEMATECH to compare line edge 

roughness measurements between CD-SAXS and 

CD-SEM in oxide line/space patterns created by 

EUV lithography and pattern transfer, identifying 

key differences between measurable quantities. 

4Q 2007

Provide CD-SAXS measurements of cross section 

in round robin study with SEMATECH and Intel to 

evaluate the potential capability of CD-SAXS as an 

industrial calibration method. 4Q 2007

ACCOMPLISHMENTS

We have provided the fi rst measurements of 

sidewall angle using small angle X-ray scattering 

(SAXS). Using a series of photoresist gratings 

produced at the IBM T. J. Watson Research Center (Q. Lin), CD-SAXS was performed using at 

the Advanced Photon Source (Argonne National 

Laboratory) to measure photoresist patterns with 

well-defined sidewall angles. The protocol 

involves measurements of the sample over 20 

degrees of incident angles, and reconstructing the 

Fourier representation of the pattern cross section. For a pattern with trapezoidal cross section, 

ridges of intensity propagate at twice the sidewall 

angle (see Fig. 2). The existence of the ridges is 

a model independent check on the validity of the 

trapezoidal model, while different shapes, such as 

a T-topped line, will produce different scattering 

patterns. The protocol can be adopted for other 

pattern shapes.

Our group has also demonstrated a capability 

to measure correlated fl uctuations in line edge 

position as a measure of line edge roughness. 

CD-SAXS measurements of a series of line/space 

patterns with model LER were produced using 

193 nm lithography in collaboration with the 

Advanced Metrology Advisory Group (AMAG) 

coordinated by SEMATECH. This initial study 

provided key data on the sensitivity of CD-SAXS 

to LER, including the development of models for 

LER and LWR contributions, in lines possessing 

periodic sidewall roughness with amplitudes of 

3 nm (see Fig. 3). In addition, a methodology 

and experimental data was published describing a route to use CD-SAXS as a measure of 

roughness propagating normal to the substrate. 

Samples of photoresist line/space patterns were 

provided by the IBM T. J. Watson Research Center 

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Semiconductor Microelectronics and Nanoelectronics Programs 47

(A. Mahorawala) (see Fig. 4). This type of roughness is commonly observed in photoresists due 

to imprecise tuning of the underlying anti-refl ective coating. Given the ability of CD-SAXS to 

extract the periodic component of roughness from 

non-periodic, the technique is capable of quantitatively extracting this component of roughness 

even when the amplitude is small compared to 

Figure 3. Detector image of etched polysilicon lines 

with designed line edge roughness. In addition to 

the typical diffraction spots characterizing pitch and 

linewidth, satellite peaks of intensity are observed 

symmetrically above and below the main diffraction 

axis. The additional peaks are a result of correlated 

sidewall roughness.

Figure 4. CD-SAXS intensity plotted in the inverse 

space plane Qx

-Qz, where Qx and Qz are a vectors 

parallel and normal to the substrate respectively. 

The main diffraction band in the center results from 

the line/space pattern, however the parallel intensity 

bands at Qz = +/- 0.08 result from roughness 

following a periodic wave normal to the substrate. 

A simulated model of the roughness provides the 

capability to extract quantitative information (inset).

the random component. These studies are being 

extended into sub-50 nm dense patterns produced 

by EUV lithography in collaboration with Intel 

and SEMATECH.

Initial feasibility studies have demonstrated 

the potential of CD-SAXS to detect and to estimate the extent of sidewall damage of nanoporous 

low-κ materials patterned into line/space patterns. 

Previously, NIST had demonstrated that blanket 

low-κ fi lms exposed to a plasma etch can have 

a skin layer with increased density and less hydrogen that may refl ect the collapse of the pore 

structure. Since plasma etch effects can lead to 

an increase in κ, it is important to measure the 

sidewall structure (porosity, electron density, etc.) 

of patterned low-κ fi lms. To address this challenge, SEMATECH provided line gratings etched 

in a candidate low-κ material, then backfi lled the 

trenches with the same candidate low-κ material. 

This backfi lled sample simplifi es modeling efforts 

and highlights the damaged regions to X-rays. 

The real time shape evolution of nanoimprinted polymer patterns were measured as a 

function of annealing time and temperature using 

Critical Dimension Small Angle X-ray Scattering 

(CD-SAXS). Periodicity, linewidth, line height, 

and sidewall angle were determined with nanometer resolution for parallel line/space patterns in 

poly(methyl methacrylate) (PMMA) both below 

and above the bulk glass transition temperature 

(TG). Heating these patterns below TG does not 

produce signifi cant thermal expansion, at least to 

within the resolution of the measurement. However, above TG, the fast rate of pattern melting at 

early time transitions to a slowed rate in longer 

time regimes. The time dependent rate of pattern 

melting was consistent with a shape dependent 

thermal stability, where sharp corners possessing 

large Laplace pressures accelerate pattern dynamics at early times. 

COLLABORATIONS

Polymers Division, MSEL, Chengqing Wang, Derek L. Ho, 

Christopher L. Soles, Hyunwook Ro, Yifu Ding

Intel Corporation, Kwang-Woo Choi, James Clarke, George 

Thompson, Melissa Shell, sub-50 nm structures.

SEMATECH, Ben Bunday, Pattern production and correlation 

to optical scatterometry (also through Advanced Metrology 

Advisory Group).

SEMATECH, Youfan Liu, Metrology of low-κ patterns using 

CD-SAXS.

Advanced Photon Source, Argonne National Laboratory, Steven Weigand and Denis Keane, Small Angle X-ray Scattering 

Instrumentation Development.

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48 National Institute of Standards and Technology 

University of Michigan, Stella Pang, Nanoimprinted polymeric 

structures.

Molecular Imprints, Doug Resnick. Characterization of sub-50 

nm structures including dense arrays of posts.

IBM Yorktown Heights, Qinghuan Lin, Development of pattern shape and sidewall angle metrology.

RECENT PUBLICATIONS

C. Wang, R. L. Jones, E. K. Lin, W.-L. Wu, B. J. Rice, K. Choi, 

G. M. Thompson, S. J. Weigand, D. T. Keane, “Characterization of Correlated Line Edge Roughness of Nanoscale Line 

Gratings using Small Angle X-Ray Scattering,” accepted to 

J. Appl. Phys. (2007).

E. K. Lin, W.-L. Wu, R. L. Jones, C. Wang, K. Choi, G. M. 

Thompson, B. J. Rice, “Critical Dimension Metrology with 

Small Angle X-ray Scattering,” accepted to FabTech (2007).

C. Wang, R. L. Jones, E. K. Lin, W.-L. Wu, J. S. Villarrubia, 

K. Choi, J. S. Clarke, B. J. Rice, M. Leeson, J. Roberts, R. 

Bristol, B. Bunday, G. Orji, “Line Edge Roughness and Cross 

Sectional Characterization of Sub-50 nm Structures Using 

Critical Dimension Small Angle X-ray Scattering,” accepted 

to Proc. Frontiers Adv. Metrol. (2007).

C. Wang, R. L. Jones, E. K. Lin, W.-L. Wu, J. Leu, “Standing 

Wave Roughness Characterization in Photoresists by Small 

Angle X-Ray Scattering,” Appl. Phys. Lett. 90, 193122 

(2007).

Y. Ding, H. W. Ro, J. F. Douglas, R. L. Jones, D. R. Hines, 

A. Karim, C. L. Soles “Polymer Viscoelasticity and Residual 

Stress Effects on Nanoimprint Lithography,” Adv. Mater. 19, 

1377 (2007).

C. Wang, R. L. Jones, E. K. Lin, W.-L. Wu, J. S. Villarrubia, 

K. Choi, J. S. Clarke, B. J. Rice, M. Leeson, J. Roberts, R. 

Bristol, B. Bunday, G. Orji, “Line edge roughness characterization of sub-50nm structures using CD-SAXS: Round-Robin 

benchmark results,” Proc. SPIE 6518, (2007).

R. L. Jones, C. L. Soles, E. K. Lin, W. Hu, R. M. Reano, S. 

W. Pang, “Pattern Fidelity in Nanoimprinted Films using 

CD-SAXS,” J. Microlith. Microfab. Microsyst. 05, 013001 

(2006).

R. L. Jones, W.-L. Wu, C. Wang, E. K. Lin, K.-W. Choi, B. J. 

Rice, G. M. Thompson, S. J. Weigand, D. T. Keane “Characterization of Line Edge Roughness using CD-SAXS,” Proc. 

SPIE 6152, 215 (2006).

H. W. Ro, Y. Ding, H. J. Lee, D. R. Hines, R. L. Jones, E. K. 

Lin, A. Karim, W.-L. Wu, C. L. Soles, “The Role of Stress in 

Nanoimprint Lithography,” Proc. SPIE 6151, 348 (2006).

R. L. Jones, T. Hu, C. L. Soles, E. K. Lin, R. M. Reano, S. 

W. Pang, D. M. Casa, “Real Time Shape Evolution of Nanoimprinted Polymer Structures during Thermal Annealing,” 

6, 1723 (2006).

R. L. Jones, E. K. Lin, W.-L. Wu, Q. Lin, S. J. Weigand, D. T. 

Keane, J. P. Quintana, “Cross Sectional Metrology of Dense, 

High Aspect Ratio Patterns using CD-SAXS,” Proc. SPIE 

5752, 404 (2005).

H.-J. Lee, C. L. Soles, H. W. Ro, R. L. Jones, E. K. Lin, 

W.-L. Wu, D. R. Hines, “Nanoimprint pattern transfer quality from specular X-ray refl ectivity,” Appl. Phys. Lett. 87, 

263111 (2005).

T. Hu., R. L. Jones, E. K. Lin, W.-L. Wu, Q. Lin, D. Keane, S. 

Weigand, J. Quintana “Small angle X-ray scattering metrology of side wall angle and cross section in nanometer scale 

gratings,” J. Appl. Phys. 96, 1983 (2004).

R. L. Jones, T. Hu, E. K. Lin, W. L. Wu, R. Kolb, D. M. Casa, 

P. J. Bolton, and G. G. Barclay, “Small angle X-ray scattering 

for sub-100 nm pattern characterization,” Appl. Phys. Lett., 

83, 4059 (2003).
Semiconductor Microelectronics and Nanoelectronics Programs 49

DIMENSIONAL METROLOGY WITH GRAZING INCIDENT

X-RAY SCATTERING

Technical Contacts:

W. L. Wu

E. K. Lin

C. L. Soles

GOALS

To develop a grazing incident X-ray scattering (GIXS)-based methodology to quickly, 

quantitatively, and non-destructively measure 

critical dimensions (CD) and feature shape with 

sub-nanometer resolution on test samples. The 

focus is to develop a technique capable of routine 

measurement of pattern shape, including critical 

dimension, sidewall angle, line width fl uctuations, and line edge roughness, and statistical 

deviations across large areas in dense high aspect 

ratio patterns. This method is expected to provide 

a potential solution for the metrology needs of 

future semiconductor technology nodes beyond 

30 nm nodes. 

CUSTOMER NEEDS

The drive to reduce feature sizes to sub-30 nm 

technology nodes continues to challenge metrology techniques for pattern characterization. As 

outlined in the International Technology Roadmap for Semiconductors, existing techniques 

such as CD-SEM and optical scatterometry (OS) 

face signifi cant technical hurdles in quantifying 

parameters such as Line Edge Roughness (LER) 

and sidewall angle as pattern sizes decrease. 

CD-SAXS measurements do offer an attractive 

solution for CD metrology but the transmission 

geometry requires high fl ux X-ray sources that are 

not yet available for laboratory or fab line instruments. However, a grazing incidence refl ection 

geometry for X-rays is fi nding increased use in 

semiconductor metrology and can utilize current 

lower fl ux X-ray sources. In this project, NIST is 

evaluating the feasibility of grazing incident X-ray 

scattering as a measurement tool to complement 

the transmission based CD-SAXS metrology currently under development in NIST.

TECHNICAL STRATEGY

1. We are developing a grazing incident X-ray 

scattering based method capable of Angstrom 

level precision in critical dimension evaluation 

to complement on-going effort of the transmission based CD-SAXS. In comparison GIXS has 

the following advantage and disadvantages over 

CD-SAXS;

The incident beam intensity can be enhanced 

by a factor of 4 once the incident angle is set at 

#

the critical angle of the substrate which is a silicon 

wafer for most of the cases. In contrast, there is 

an over 50 % loss of incident intensity in transmission geometry by going through the substrate. 

The overall difference is an 8x increase in incident beam intensity. This difference is signifi cant 

because low beam fl ux is the major drawback for 

laboratory scale X-ray systems.

For the case of a silicon substrate the critical 

angle with copper Kα

 line the incident angle is 

near 0.2° and this imposes a large footprint on 

the sample. For example, with an incident beam 

diameter of 100 μm the footprint on the sample 

will be near 3 cm along the refl ection direction. 

Such a large footprint is unacceptable for semiconductor applications. 

To overcome the footprint problem, the beam 

size at the sample position must be reduced. Current advancements in X-ray optics, however, have 

enabled the focusing of X-ray beam to 5 μm or 

less. We will work with industry and university 

partners to pursue and to evaluate optical components most suitable for CD measurements.

The shortcoming of using focused incident Xray beam is the loss in the angular divergence. The 

current CD-SAXS system at NIST is designed to 

have a high resolution such that it can resolve samples with a 400 nm period. However, X-ray based 

metrology will be needed by the semiconductor 

industry with the production of 30 nm structures. 

At these sizes, a lower resolution X-ray instrument 

will be adequate. Therefore, the current stringent 

requirements in angular divergence can be relaxed 

and focusing optics will become a viable route to 

shrink to beam size at the sample. 

DELIVERABLES: 

Evaluate X-ray focusing optics. 3Q 2007

Acquire X-ray focusing optics. 1Q 2008

Integrate X-ray focusing optics to the existing CDSAXS instrument. 3Q 2008

Evaluate samples with 60 nm or smaller repeat with 

the GIXS confi guration. 4 Q 2008

Conduct GIXS measurements with model samples 

for line width, line height, side wall angle and designed sidewall roughness. 2Q 2009

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50 National Institute of Standards and Technology 

2. The other challenge in the development of 

GIXS is in the analysis of the experimental data. 

For transmission based CD-SAXS the structure 

and the scattering intensity are related through 

simple Fourier transforms. This simple relation 

is no longer valid for GIXS data. Considerable 

efforts have to be devoted in the development of 

data analysis algorithm.

DELIVERABLES: 

Develop GIXS analysis algorithm and software for 

line width calculation. 1Q 2008

Develop GIXS analysis algorithm and software for 

line height and sidewall angle calculations. 

3Q 2008

Develop GIXS analysis algorithm and software to 

sidewall roughness calculation. 1Q 2009

We have successfully modifi ed the sample stage of 

the current CD-SAXS instrument to enable some 

preliminary GIXS measurements. A recent result 

from an ordered organic thin fi lm is given in Fig. 1 

and the footprint on this sample is over 5 cm. With 

a successful implementation of focusing optics 

this footprint can be reduced considerably. 

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R. L. Jones, C. L. Soles, E. K. Lin, W. Hu, R. M. Reano, S. 

W. Pang, “Pattern Fidelity in Nanoimprinted Films using 

CD-SAXS,” J. Microlith. Microfab. Microsyst. 05, 013001 

(2006).

R. L. Jones, W.-L. Wu, C. Wang, E. K. Lin, K.-W. Choi, B. 

J. Rice, G. M. Thompson, S. J. Weigand, D. T. Keane “Characterization of Line Edge Roughness using CD-SAXS,” Proc. 

SPIE 6152, 215 (2006).

H. W. Ro, Y. Ding, H. J. Lee, D. R. Hines, R. L. Jones, E. K. 

Lin, A. Karim, W.-L. Wu, C. L. Soles, “The Role of Stress in 

Nanoimprint Lithography,” Proc. SPIE 6151, 348 (2006).

R. L. Jones, E. K. Lin, W.-L. Wu, Q. Lin, S. J. Weigand, D. T. 

Keane, J. P. Quintana, “Cross Sectional Metrology of Dense, 

High Aspect Ratio Patterns using CD-SAXS,” Proc. SPIE 

5752, 404 (2005).

H.-J. Lee, C. L. Soles, H. W. Ro, R. L. Jones, E. K. Lin, W.-L. 

Wu, D. R. Hines, “Nanoimprint pattern transfer quality from 

specular X-ray refl ectivity,” Appl. Phys. Lett. 87, 263111 

(2005).

T. Hu., R. L. Jones, E. K. Lin, W.-L. Wu, Q. Lin, D. Keane, S. 

Weigand, J. Quintana “Small angle X-ray scattering metrology of side wall angle and cross section in nanometer scale 

gratings,” J. Appl. Phys. 96, 1983 (2004).

R. L. Jones, T. Hu, E. K. Lin, W. L. Wu, R. Kolb, D. M. Casa, 

P. J. Bolton, and G. G. Barclay, “Small angle X-ray scattering 

for sub-100 nm pattern characterization,” Appl. Phys. Lett., 

83, 4059 (2003).

W. L. Wu, E. K. Lin, Q. H. Lin, and M. Angelopoulos, “Small 

Angle Neutron Scattering Measurements of Nanoscale Lithographic Features,” J. Appl. Phys., 88, 7298 (2000).

Figure 1. GIXS result from an ordered organic thin 

fi lm supported on silicon wafer. Data collected on 

the NIST CD-SAXS instrument using Cu Kα X-ray 

beam.

COLLABORATIONS

Polymers Division, MSEL, R. Joseph Kline, Chengqing 

Wang, Derek L. Ho, Christopher L. Soles, Hyunwook Ro, 

Yifu Ding

Intel Corporation, Kwang-Woo Choi, Bryan Rice, sub-50 

nm structures.

RECENT PUBLICATIONS

M. L. Chabinyc ML, M. F. Toney, R. J. Kline, I. McCulloch, 

M. Heeney, “X-ray scattering study of thin fi lms of poly(2,5-

bis(3-alkylthiophen-2-yl)thieno[3,2-b]thiophene),” JACS, 

129, 3226 (2007).
Semiconductor Microelectronics and Nanoelectronics Programs 51

ATOM-BASED DIMENSIONAL METROLOGY

Technical Contact:

R. Silver

GOALS

Provide technological leadership to semiconductor and equipment manufacturers and other 

government agencies by developing the methods, 

tools, and artifacts needed to apply leading edge, 

high-resolution atom-based dimensional measurement methods to meet the metrology needs of 

semiconductor microlithography. One specifi c 

goal is to provide the customer with the techniques 

and standards needed to make traceable dimensional measurements on wafers with nanometer 

accuracy. We are developing nanometer-scale 

three-dimensional structures of controlled geometry whose dimensions can be measured and 

traced directly to the intrinsic crystal lattice. 

These samples are intended to be dimensionally 

stable and allow transfer to other measurement 

tools which can measure the artifacts with dimensions known on the nanometer scale.

CUSTOMER NEEDS

This project responds to the U.S. industry need 

for length intensive measurement capabilities 

and calibration standards in the nanometer scale 

regime. The new class of scanned probes have 

unparalleled resolution and offer tremendous 

promise for meeting these future measurement, 

test artifact, and calibration standards needs of 

the microelectronics industry. One important application of the high-resolution SPM methods is 

in the development of test artifacts and linewidth 

standards whose dimensions can be measured and 

traced through the crystal lattice from which they 

are made. In addition, these high-resolution tools 

can be coupled directly to a unique NIST-designed 

picometer resolution interferometer (Fig. 1).

The work funded in this project is for the development of atom-based test artifacts and linewidth 

standards to assist in the development of highresolution imaging techniques and calibration of 

linewidth metrology tools. We are also developing 

unique high-resolution interferometry capabilities 

which can be used in conjunction with accurately 

measured tips to measure feature critical dimensions at the nanometer scale. One focus of the 

research is to enable the accurate counting of atom 

spacings across a feature in a controlled environment and to subsequently transfer that artifact to 

other measuring instruments as a structure with 

atomically known dimensions. 

An essential element of this project is the fabrication of test artifacts and structures for the development of high resolution imaging methods. It 

is imperative to enable fabrication methods for 

sub-10 nm sized features. Several recent developments in optical microscopy, scatterometry and 

SEM metrology require test samples with critical 

dimensions below 10 nm. These test structures are 

simply not available at this time. In this project 

we are developing the methods for fabrication of 

sub-10 nm sized features and etching methods to 

transfer these patterns into the silicon substrates 

(see Fig. 2, next page). 

As critical dimensions continue to shrink, the detailed atomic structure, such as edge roughness or 

sidewall undercut, of the features to be measured 

represents a larger portion of the measurement 

uncertainty. Furthermore, particularly with SEMs 

and optical CD tools, the instrument response 

and the uncertainty in the edge location within an 

intensity pattern becomes a signifi cant issue due 

to the increased sensitivity to detailed elements 

of the edge detection model. The complexity of 

these physics-based scattering models and large 

computer resources required for each individual 

computation make the concept of having samples 

Figure 1. A new STM structure design has been 

implemented to improve the atomic scale imaging 

and structural characteristics. This system will 

provide improved nanometer scale interferometerbased imaging and measurement capabilities.
52 National Institute of Standards and Technology 

of known geometry and width essential. This 

project is developing samples of known geometry 

and atomic surface structure which yield well 

defi ned dimensional measurements. One goal is a 

measurement which results in a specifi c number of 

atoms across the line feature or between features. 

The process being developed allows for samples 

to be measured in the UHV environment and then 

stabilized and subsequently transferred to other 

instruments (Fig. 3). 

These methods of atom counting and high-resolution interferometry, as outlined in this project 

description, are non-destructive and are intended 

to yield samples which can be measured by various instruments such as an SEM optical CD tool 

with subsequent re-measurement on the atomic 

scale. This is a unique and important element of 

this work since there are no other known methods 

Figure 2. Key design elements for the instrument used to fabricate 

features with sub-10 nm dimensions. This tool utilizes advanced 

design tools to optimize vibration sensitivity and enable access to 

1 cm x 1 cm of sample area.

that allow this kind of atomic dimensional measurement without being destructive. In addition, 

this new method opens up the possibilities of 

basing the measurement metric on the intrinsic 

crystal lattice.

TECHNICAL STRATEGY

The technical work is focused into four thrust 

areas. 

1. The development of methods to prepare photolithographically patterned three-dimensional 

structures in semiconductor materials. These 

structures are being prepared in silicon to allow 

the atomic surface order which is commensurate 

with the underlying crystal lattice. This involves 

either using conventional photolithography methods for sample production or using the STM itself 

to fabricate very small nanometer scale features 

as shown in Fig. 2.

DELIVERABLES:

Write features in silicon with critical dimensions 

smaller than 7 nm. Apply the pattern writing 

process and etch features for use as optical scatterfi eld test structures using the new, Scanning 

Tunneling Microscope (STM). 4Q 2007

Work with EEEL and ISMT to develop improved 

methods for etching nanostructures written in silicon. Use RIE etching techniques to etch features 

with sub-10 nm dimensions in silicon. 4Q 2007

2. The development of techniques for the preparation of SPM tips with reproducible geometries 

and the direct characterization of the SPM tip geometry and dimensions on the atomic scale. These 

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Figure 3. A demonstration of the nanofabrication 

process with an RIE process used to transfer the 

patterns into the silicon substrate.
Semiconductor Microelectronics and Nanoelectronics Programs 53

well characterized tip probes can then be used to 

measure the samples with photolithographically 

defi ned and canonically ordered surfaces on the 

sub-nanometer length scale. We are developing the 

SPM tip etching, fi eld evaporation, and cleaning 

procedures which reliably yield stable W tips and 

produce atomic resolution on Si (7x7) surfaces. 

These tips are also useful in a collaboration with 

the SEM project for development as nanotips as 

SEM fi eld emitters. Our tip preparation methods 

leave us uniquely qualifi ed in this arena.

DELIVERABLES: Develop the rigorous and robust 

preparation techniques to enable routine atomic 

resolution using W (111) single crystal tungsten tips. 

Determine the stability of these W tips for use in STM 

imaging. 3Q 2007

3. Development of artifacts that can be atom 

counted and then measured in different metrology tools such as SEM and AFM. The integrity 

of the line geometry, such as side wall angle, is 

crucial to having useful artifacts for linewidth 

specimens. These edge geometry requirements 

are not as stringent for the magnifi cation standards 

since feature symmetry is the most crucial element 

in these measurements. The work on linewidth 

artifacts, therefore, is focused on using new 

silicon processing methods to reduce the process 

temperatures required for atomic reconstructions. 

Fully operational wet chemical processing has 

been demonstrated on atomically ordered Si surfaces at signifi cantly reduced temperatures. For 

sample preparation, we are utilizing the existing 

in situ processing apparatus and techniques from 

the UHV STM and concentrating on the reproducible production of atomically ordered Si surfaces 

and Si (111) step and terrace structures (Fig. 4). 

DELIVERABLES: Implement UHV sample and tip 

heating and preparation capability in the Omicron STM 

system. Enable robust alignment and interface with 

the system transfer mechanism for sample transfer 

into the UHV environment. The new STM and UHV 

preparation facility will allow improved Si sample prep 

and atomic surface reconstructions. 4Q 2007

4. Develop the new UHV Omicron system and 

the in-house designed facility for improved robust 

atomic resolution imaging. The new imaging 

capabilities are being developed and will next 

be applied to etched silicon features provided by 

Sematech on Si (111) wafers when they become 

available. The intent is to provide atomically resolved and precisely imaged quantitative results 

on etched silicon substrates. These nano-meterscale standard artifacts with atomically ordered 

surfaces will then act as linewidth or magnifi cation 

calibration samples. 

ACCOMPLISHMENTS

The ability to prepare atomically sharp tips 

in W (111) has been demonstrated. We are now 

implementing a robust methodology for repeatable 

UHV tip preparation. This is based on the new tip 

processing capability in the UHV STM system and 

will be developed to allow atomic tip preparation 

without the need for the FIFEM. 

A workshop from the SPIE Nanotechnology 

Working Group on nanoimprint technology for 

semiconductor manufacturing was held at Microlithography 2007. The workshop was organized 

by Rick Silver, Chris Soles and Mike Postek 

and included presentations and a panel discussion involving several industry and University 

leaders. The Nanotechnology Working Group is 

chaired by Rick Silver and Mike Postek and has 

served as a forum for developing and applying 

nanotechnology related manufacturing and fabrication elements which will directly benefi t the 

semiconductor manufacturing community.

In the continuing collaboration between the 

NIST atom-based dimensional metrology project 

and the Department of Mechanical Engineering at 

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Figure 4. Time resolved KMC simulation for surfaces 

with 0.02° miscut angle and 60° miscut orientation 

on the left and compared with experimental data on 

the right. All images are 4000 × 2000 nm2

.
54 National Institute of Standards and Technology 

the George Washington University a presentation 

was given at the ASPE conference on the dynamic 

analysis and eddy current damping effects on 

sensitive tunneling instruments. The work was 

based on solid model Pro E and Pro Mechanica 

modeling of structures for use in high resolution 

imaging tools such as the UHV STM. The actual tool modeled is housed in the AML and the 

mechanical structure used for this measurement 

instrument was based directly on the modeling 

results. The modeling results were further verifi ed 

using accelerometer measurements to match the 

theoretical modeling to experiment. The publication focused on this quantitative comparison 

including a full modal analysis. 

Collaboration between the NIST atom-based 

dimensional metrology project and the Department of Mechanical Engineering at the George 

Washington University the Graduate student 

Sumanth Chikkamaranahalli has demonstrated 

the operation of the new STM. This new instrument is expected to make important contributions 

to the advancement of nanometer and atomic 

scale standard artifact fabrication and metrology 

methods capable of sub-nanometer measurement. 

This new STM has been moved into the previous UHV vacuum facility used by the Burleigh 

instrument. The new improved metrology tool 

has replaced the previous instrument which has 

been the workhorse for several recent publications in Applied Physics Letters and the Journal 

of Chemical Physics. This project continues to be 

the basis for substantive successful collaborations 

with the University of Maryland and the George 

Washington University. 

An in depth paper was published in the 

Journal of Physical Chemistry. The paper is a 

leading comprehensive set of experimental and 

theoretical analysis on the preparation of silicon 

surfaces. This work was carried out in large part 

by Post Doc Hui Zhou and in collaboration with 

the University of Maryland, Department of Physics. This paper follows on a paper published in 

late 2005 in the Journal of Physical Chemistry 

titled “The Infl uence of defects on the morphology 

of Si(111) Etched in NHF”. In this new paper we 

have extended the Kinetic Monte-Carlo simulation method to study the etching dynamics of Si 

(111) surfaces in NH4F in a time-resolved basis. 

We have examined the step-fl ow dynamics of 

Si(111) etching using various simulation window 

sizes for variety of miscut angles and miscut orientations as well as those parameters which affect 

the formation of etch pits. The simulation results 

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have been compared with published experimental 

data to derive an absolute time scale.

We have prepared atomically fl at surfaces 

and obtained atomic order on the wet chemical 

prepared surfaces. The routine imaging of these 

surfaces on the atomic scale is expected to be 

enhanced with the new STM. These results have 

been recently published in leading chemical 

physics journals and presentation forums. These 

results are a substantial step forward in repeatable silicon surface preparation for atomic scale 

metrology and are now being supported in collaboration with SEMATECH and current industry 

requirements. The results, seen in Fig. 4, yield a 

new, more comprehensive understanding of the 

physical processes involved in making atomically 

fl at surfaces in silicon as required for much of the 

work in this project.

The silicon etching process has been developed and demonstrated for patterns written on 

silicon. Patterns with features as small as 10 nm 

have been written in hydrogen terminated silicon 

surfaces with subsequent pattern transfer into 

the silicon substrates. A publication recently 

appeared on this material. The importance of 

etching structures in Si (111) is now being supported by SEMATECH and a recent collaboration between NIST and SEMATECH to develop 

lithography methods and plasma etch method at 

SEMATECH’s fab is now under way.

A paper was presented to the ASPE special 

topics conference by Summanth Chikkamaranahalli, a graduate student from the George Washington University. The paper is an excellent analysis 

of damping and precision structures for use in 

STMs and atomic scale imaging. The analysis 

and modeling have been used in the development 

of the NIST STM for the atom-based dimensional 

metrology project. The tool has recently taken 

very nice high resolution atomic scale images. 

The scanning tunneling microscope was designed 

by the NIST team in collaboration with Prof. Vallance at the George Washington University. The 

FEA static and dynamics analysis using promechanica and preE packages gave the researchers 

quantitative tools to evaluate and develop the STM 

imaging head, and larger mechanical structure.

We have used the FIM techniques to analyze 

nanotubes and their structure. The nanotubes were 

directly characterized for use as SPM tips with 

dimensional analysis on the atomic scale. We are 

now applying these ideas to FIM tips used in SEM 

metrology (Fig. 5). 

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Semiconductor Microelectronics and Nanoelectronics Programs 55

COLLABORATIONS

Sematech, IBM, University of Maryland, Dept. of Physics, 

George Washington University, Dept. of Mech. Eng.

RECENT PUBLICATIONS

S. Chikkamaranahalli, R. Vallance, B. Damazo, and R. M. 

Silver, “Damping Mechanisms for Precision Applications in 

UHV Environment,” Proc. ASPE Conf., Pitts. PA (2006).

H. Zhou, J. Fu, and R. M. Silver, “A Time-resolved Kinetic 

Monte-carlo Simulation Study on Si (111) Etching,” Accepted 

to Journal of Physical Chemistry (2006).

S. Chikkamaranahalli, R. Vallance, B. Damazo, R. M. Silver, 

and J. Gilsinn, “Dynamic Modeling and Vibration Analysis of 

a UHV Scanning Tunneling Microscope,” Proc. ASPE Conf., 

Norfolk VA (Oct. 2005).

H. Zhou J. Fu, and R. M. Silver “The Infl uence of Defects 

on the Morphology of Si (111) Etched in NH4

F,” J Chemical 

Physics B, 109, 23386 (2005).

J. Gilsinn, H. Zhou, B. Damazo, J. Fu, R. Silver, “NanoLithography in Ultra-High Vacuum (UHV) for Real World 

Applications.” Proc. NanoTech (2004).

J. A. Dagata, C.A. Richter, R.M. Silver, E. Vogel, J.V. Martinez 

de Pinillos, “Metrology Development for the Nanoelectronics 

Industry at the National Institute of Standards and Technology,” Proc. NanoTech (2004).

H. Zhou, “Study of wet chemically prepared hydrogen 

terminated Si (111) surfaces and a novel implementation 

of a high resolution interferometer,” Thesis, University of 

Maryland, 2004.

Figure 5. Upper left shows an FIM image of a W 

(110) tip. On the upper right is a simulation of a 

FIM image of a W (110) tip with base radius 

r = 14.9 nm and n = 1.82. The lower panels show 

simulations of an FIM image of a W (110) tip with 

base radius r = 14.9 nm and n = 2 and a ball model 

with same parameters.

R. M. Silver, S. Gonda, J. Jun, L. Howard, and C. Jensen, 

“Atomic Resolution Measurements with a New Tunable Diode 

Laser-Based Interferometer,” Optical Engineering, Vol. 43 

No. 1 (Jan. 2004).

P. V. M. Rao, C. P. Jensen, and R. M. Silver “An enhanced 

model for STM tip geometry measured with FIM,” JVST B 

(Mar/Apr 2004).
56 National Institute of Standards and Technology 
Semiconductor Microelectronics and Nanoelectronics Programs 57

FABRICATION AND CALIBRATION METROLOGY FOR

SINGLE-CRYSTAL CD REFERENCE MATERIALS

Technical Contact:

M. W. Cresswell

GOALS

The goal of this project is to develop test-structure-based reference materials with emphasis on 

supplying road-map-compliant physical standards 

for critical-dimension (CD) metrology-tool development and calibration. The specifi c near-term 

goal is fabricating a quantity of CD referencefeatures with nominal CDs in the range 20 nm to 

160 nm and having 2σ (expanded uncertainties) of 

less than 1.5 nm by July 2007. Their application 

is primarily for AFM-tip calibration. A longerterm goal is to fabricate and calibrate gratings for 

use in optical critical-dimension metrology that is 

becoming widely used in advanced semiconductor 

manufacturing.

CUSTOMER NEEDS

The Semiconductor Industry Association’s 

International Technology Roadmap for Semiconductors (ITRS) 2005 p.36 under “Reference 

Materials,” states that it is critically important to 

have suitable reference materials available when 

a measurement is fi rst applied to a technology 

generation, especially during early materials- and 

process-equipment development. This project is 

concerned specifi cally with ensuring a source of 

such reference materials to satisfy the stated need 

throughout the near-term years.

Each generation of ICs is characterized by the 

transistor gate length whose control to specifi cations during IC fabrication is a primary determinant of manufacturing success. The roadmap 

projects the decrease of gate microprocessor unit 

(MPU) physical gate lengths used in state-of-theart IC manufacturing from present levels of 28 nm 

to 13 nm during the near-term years. Scanning 

electron microscopes (SEMs) and other systems 

used for traditional linewidth metrology exhibit 

measurement uncertainties exceeding specifi cations for these applications. It is widely believed 

that the situation can be at least partially managed 

through the use of reference materials with linewidths traceable to nanometer-level uncertainties. 

Until now, such reference materials have been 

unavailable because the technology needed for 

their fabrication and certifi cation has not been 

available. The technology that the project has 

developed for fabricating CD reference materials 

is known as the Single-Crystal CD ReferenceMaterial (SCCDRM) implementation.

Further details of customer needs that have been 

identified since the SCCDRM distribution to 

SEMATECH Member Companies in January 

2005, and now impact the project’s responding 

technical strategy, are described in the next section.

TECHNICAL STRATEGY

The fundamental SCCDRM technical strategy is 

to pattern Silicon on Insulator (SOI) device layers with lattice-plane selective etches of the kind 

used in silicon micro machining, which provides 

reference features with quasi-atomically-planar 

sidewalls. This unique attribute is highly desirable for the intended applications, particularly if 

sidewall nano-planarity can be further extended 

to reference-feature segment lengths of up to 2 

micrometers. Essential elements of the implementation include starting silicon SOI wafers with the 

device layer having a (110) orientation; alignment 

of the reference features to specifi c lattice vectors; 

and lithographic patterning with lattice-plane 

selective etches of the kind used in silicon micromachining. However, the diffi culty of obtaining 

satisfactory SOI material in larger diameters has 

driven us towards parallel evaluation of a bulk-wafer starting-material strategy. The roadmap states 

that measurement and certifi cation of reference 

materials must be carried out using standardized or 

well-documented test procedures. The traceability 

path for dimensional certifi cation of the project’s 

SCCDRMs is responsive to this requirement and 

originates with measurement of a selection of 

reference-feature CDs with both Atomic-Force 

Microscopy (AFM) and high-resolution transmission electron microscopy (HRTEM) imaging. The former technique is highly repeatable 

and of manageable cost while the latter enables a 

lattice-plane count that allows expression of the 

each CD in terms of a traceable distance, which 

is the periodicity of silicon (111) lattice planes. 

However, HRTEM is totally destructive and thus 

is not useful for supplying reference features to 

end users. The project’s traceability strategy 

thus features state-of-the-art AFM as a transfer 

metrology to deal with this constraint. Transfer 

metrology relates CDs extracted by AFM to be 

traced to SI units through the construction of a 

so-called calibration curve. An example of such 

a curve is shown in Fig. 1 (next page). To maintain maximum possible accuracy in the transfer-
58 National Institute of Standards and Technology 

metrology operation, an elaborate reference-feature selection protocol has been established to 

identify reference features that qualify by virtue 

of their CD-uniformity, as contributors to the construction of the calibration curve, or for delivery to 

end users. Multiple reference features on a large 

set of as-patterned test chips are identifi ed initially 

by high-power optical inspection. This procedure 

checks primarily for continuity, cosmetics, and 

apparent uniformity of the narrowest-drawn sets 

of six features that are incorporated into test structures, which are called HRTEM targets. Drawn 

feature linewidths range from 350 nm to 600 nm 

and the “process bias” typically decreases these to 

etched CDs of between 50 nm and 300 nm. The 

“best” 10 % of the AFM targets passing optical 

inspection, and having estimated replicated CDs 

in the range 50 nm to 200 nm, are then SEM-imaged at 20K magnifi cation to narrow the selection 

process further. Digitized profi les of the CDs of 

top-down SEM images are then extracted at 25 

nm intervals. The measurements are transferred 

to a database, which is then interrogated to identify chips, and AFM targets on them, that have 

more uniform SEM-CD profi les at the narrower 

CDs — typically less than 150 nm. Candidate 

AFM targets so identifi ed on each chip are then 

CD-profi led by AFM. Chips having AFM targets 

with all six features having superior uniformity 

are then partitioned into a calibration sub-set and 

a product subset. All six features of the selected 

targets on the chips on the calibration sub-set 

are then subjected to HRTEM imaging. These 

are the chips whose designated AFM targets 

are to be used as contributors to the calibration 

curve. The design of all HRTEM targets enables 

the capture of six HRTEM images in a single 

Figure 1. Transfer metrology relates CDs extracted 

by AFM to be traced to values SI units through the 

construction of a so-called calibration curve. 

Figure 2. The technical strategy has been to dice 

each 150 mm (110) wafer after lithography and to 

mount selected chips in micro-machined standard 

200 mm carrier wafers to accommodate the product 

reference-feature chips.

dual-beam FIB-and-thinning operation. Since 

such operations are very costly, this capability is 

economically advantageous. Moreover, since the 

features on each target are designed such that they 

are systematically staggered in CD by increments 

of 30 nm, HRTEM inspection of a single target 

enables the generation of a 6-point calibration 

curve spanning a 150 nm range.

Because 200 mm (110) starting material until 

recently has been unobtainable at an acceptable 

cost, this project’s technical strategy has been to 

dice each 150 mm (110) wafer after lithography 

and to mount the separate chips in micro-machined 

standard 200 mm carrier wafers to accommodate 

the product reference-feature chips. The scheme is 

shown in Fig. 2. The result is that fi nished units are 

rendered metrology-tool-compatible at an acceptable cost. The Project’s technical strategy is now 

evolving in a way that is addressed in the material 

that follows below. In summary, it is responding 

to industry push to implement measures that make 

the SCCDRMs more compatible with end-user 

requirements. These measures include:

• replacing the carrier-wafer with a monolithic 

200 mm wafer implementation,

• replacing the buried oxide of SOI wafers with 

a buried boron diffusion having an epitaxial 

silicon layer deposited over it,

• further reducing the CDs of calibrated features to 20 nm and the uncertainties of the 

calibrated CD to less than 1.0 nm,

• improving the reference feature’s CD uniformity to enable certifying the CD of an 

extended length,

• improved on-wafer navigation for end-user 

convenience,
Semiconductor Microelectronics and Nanoelectronics Programs 59

• calibrating a selection of OCD gratings that 

are replicated at the same time as the isolated 

lines that have been supplied so far, and

• improved management of the organic residues that sometimes impair the cosmetic appearance of the reference features and their 

environment on the wafer and to some extent 

adversely affect the uncertainty values of the 

delivered product.

Implementing these aggressive measures requires 

wafer-processing facilities that require innovative 

teaming with other laboratories. Our strategy 

takes a page from the roadmap that explicitly 

states that standards institutions need rapid access 

to state of the art development and manufacturing 

capability to fabricate relevant reference materials 

(ITRS) 2005 p. 36 under “Reference Materials.” 

Likewise, the roadmap states that metrology, 

process, and standards research institutes, standards organizations, metrology tool suppliers, 

and the university community should continue to 

cooperate on standardization and improvement of 

methods and on production of reference materials 

(ITRS) 2005 p. 36 under “Scope.” In response 

to this mandate, we have developed a unique 

relationship with the Microelectronics Research 

Centre (MRC) with the University of Texas in 

Austin. Our collaboration has resulted in the extraordinary OCD grating section shown in Fig. 3 

which exhibits 36-nm lines at 180-nm pitch and 

having a height of 430 nm. The lithography in 

this case was done by MRC’s JEOL direct-write 

e-Beam system. In a related collaboration we are 

working with both MRC and the Scottish Microelectronics Centre (SMC) at the University of 

Edinburgh to implement a hybrid optical-Ebeam 

lithography process to diversify our portfolio of 

reference materials for semiconductor-manufacturing applications. 

DELIVERABLES: 

Repeat of the screening experiment to confi rm and 

refi ne the prior identifi cation of which combinations 

of six pattern-transfer process factors drive down 

reference feature CDs and their uncertainties. 

4Q 2007

Demonstrate a hybrid optical-EB-direct-write lithography process to pattern a selection of 200 mm 

(110) wafers with SCCDRM reference features with 

CDs at or below the 20 nm level. 1Q 2008

In collaboration with SEMATECH, apply193 nm 

lithography to the patterning of a selection of 200 

mm bulk-silicon (110) wafers with a SCCDRM 

test structures having reference-feature CDs at or 

below the 20 nm level. 2Q 2008

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•

Figure 3. OCD grating section fabricated with SCCDRM technology which exhibits 

36-nm lines at 180-nm pitch and having a height of 430 nm. The lithography was 

performed by collaborating UT-Austin staff using their direct-write e-Beam system. 

Publish measurements and analyses on the application of SEM-CD metrology as a SCCDRM 

transfer metrology. 3Q 2007

Fabricate, evaluate, and publish a selection of 

preliminary measurements made on OCD gratings 

patterned on SCCDRM wafers to serve as an optical-CD reference material. 2Q 2007

Implement a plan to fabricate and calibrate a selection of isolated-line CD-SRMs. 4Q 2007

ACCOMPLISHMENTS

This project, in collaboration with the NIST 

Precision Engineering and Statistical Engineering 

Divisions, has recently designed and implemented 

a screening experiment to identify which combinations of six pattern-transfer process factors drive 

down reference feature CDs and their uncertainties. A 26-2 fractional factorial experiment design 

was implemented to enable an effi cient study of 

this relatively large number of factors and to obtain 

information on the effects from their interactions. 

Extensive AFM measurements that have now been 

made have identifi ed numerous features having 

CDs at the 20-nm level. The uniformity of individual reference-feature CDs exceeds those of the 

earlier delivery. As a consequence, it is anticipated that the uncertainties that will be ascribed to 

them at the completion of the experiment will be 

well below the prior 1.5 nm to 3 nm range.

In preparation for implementing the fi ndings 

of the process-optimization exercise referenced 

in the previous paragraph, and responding to the 

customer needs described earlier, chip layouts 

have been designed for three new SCCDRM 

fabrication ventures. Two of them are for 200 

mm-wafer monolithic implementations. The fi rst 

of these is an exercise to evaluate some available 200 mm (110) material, which is diffi cult to 

obtain but which we have on hand, to facilitate 

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60 National Institute of Standards and Technology 

running wafer lots on the 193 nm lithography 

tool at SEMATECH. The evaluation lithography 

is being accomplished with an i-line tool at the 

University of Edinburgh. The second is a design 

that is being incorporated onto a new reticle for 

lithography by the 193 nm Step-and-Scan tool on 

line in SEMATECH. The third venture for which 

new CAD has been designed is a new complex 

optical/e-beam-direct-write process, which is 

being applied to 100 mm (110) starting material. 

The lithographic processing for which the CAD 

has been designed will be carried out jointly by 

the Microelectronics Research Center, of the 

University of Texas at Austin, and the Institute 

for Integrated Micro and Nano Systems at the 

University of Edinburgh.

One of the main drivers of the customers’ 

need to replace the carrier wafer with a monolithic 

implementation is that the former is vulnerable to 

contamination originating during post-assembly 

cleaning. Ordinarily this is not a leading concern in AFM-tip calibration, although it cannot 

be dismissed for SCCDRM use in ultra-clean 

facilities. Otherwise, tip calibration is becoming 

widely recognized as an application for which the 

reference materials are well suited.. However, an 

emerging application of interest in the industry is 

SEM tool calibration. In this application, regular 

reference-material cleaning is necessary due to 

the nature of the metrology. During the current 

year, a new cleaning procedure was devised and 

evaluated at the chip level. At the same time, 

the rate of hydrocarbon deposition during worstcase conditions was assessed. This work was 

performed to answer anticipated questions on the 

subject when our 200-mm wafer-based artifacts 

become available.

During the investigations of hydrocarbon 

contamination by SEM tools, and devising a 

cleaning procedure to deal with it, we took the 

opportunity to look at the issue of reference material calibration using HRTEM as the primary metrology and SEM as the transfer metrology. The 

attractiveness of this approach to calibration is 

that SEM is a much more readily available facility 

and is generally faster than AFM even though one 

can expect uncertainty infl ation. The results were 

technically useful and have been incorporated into 

a manuscript that has been accepted for publication in an IEEE Transactions journal.

The emerging metrology known as optical-CD 

(OCD) scatterometry translates broadband light, 

diffracted from an on-wafer grating patterned 

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into the resist or fi lm, into accurate profi les of the 

grating’s features from which key parameters, 

such as CD, can be extracted. Whereas currently 

favored metrology tools such as CD scanning 

electron microscopes and AFMs require a vacuum 

wafer environment, OCD metrology does not, 

and is fast and non-invasive. The possibilities of 

OCD extend to characterizing the sidewall angle 

and height of critical features. Until physical 

standards are available, the full promise of OCD 

control scatterometry may not be met. Since the 

SCCDRM implementation is well suited to the 

fabrication of calibrated reference materials for 

this new application, we have now accomplished 

the fi rst-ever fabrication and inspection of gratings 

for this purpose. A paper on the very encouraging 

fi rst results, which were obtained by collaborating 

staff in NIST’s Physics Laboratory and at SEMATECH have been presented at an international 

conference in Tokyo, Japan.

Recently, the project published a comprehensive full-length report on the fabrication and calibration of SCCDRM Reference Materials entitled 

RM 8111: Development of Prototype Linewidth 

Standard in the NIST Journal of Scientifi c Research. Several other papers on special aspects of 

the fabrication and calibration, such as test-chip 

design, AFM metrology, and HRTEM imaging, 

were presented at the SPIE Spring Symposium 

of February, 2007, and the IEEE International 

Conference on Microelectronic Test Structures in 

March, 2007. A paper on the subject of etch-process optimization for uncertainty management was 

presented at the International Electron, Ion, and 

Photon Beam Technology and Nanofabrication 

Conference in June, 2006. A manuscript on the 

comparison of SEM-CD measurements and traceable-AFM CD measurements has been accepted 

for publication in an IEEE Transactions journal 

in June 2007. An important fi rst description of 

fabrication of SCCDRMs on 200-mm bulk wafers 

which was performed in collaboration with the 

Scottish Microelectronics Centre was presented at 

the Frontiers of Characterization and Metrology 

for Nanoelectronics in March 2007.

COLLABORATIONS

The project is now actively collaborating with the 

Microelectronics Research Center of the University of Texas at Austin (http://www.mrc.utexas.

edu/amrc/publications.html), and the Institute 

for Integrated Micro and Nano Systems at the 

University of Edinburgh in Scotland (http://www.

see.ed.ac.uk/IMNS/). Both these organizations 

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Semiconductor Microelectronics and Nanoelectronics Programs 61

operate the advanced wafer processing tools that 

we will be using to address the customer needs 

referenced in the sections above. Both institutions 

have highly skilled staff with both of which we 

have published recently. We are interacting with 

the CAD and reticle staff the ISMI Subsidiary of 

SEMATECH, (http://ismi.sematech.org/) who 

have invited us to share space on a new reticle 

for their SVGL 193 nm Step-and-Scan lithography tool to fabricate an advanced generation 

of SCCDRMs for distribution to the member 

companies, as well as for possible calibration and 

distribution from NIST as SRMs. 

We also interact regularly and closely with 

NIST’s Physics, MEL and ITL Laboratories 

(http://www.mel.nist.gov/ and http://www.itl.nist.

gov/) with whom we of EEEL share an intramural 

ATP program award to reduce the certifi ed CDs 

and uncertainties of SCCDRMs through fabrication refi nements.

STANDARDS COMMITTEE

PARTICIPATION

Electrical Test Structures Task Force, Co-Chair 

(Richard A. Allen)

SEMI International Standards Micro-lithography 

Committee, member (Richard A. Allen)

RECENT PUBLICATIONS

B. J. Shulver, A. S. Bunting, A. Gundlach, L. I. Haworth, A. W. 

Ross, A. J. Smith, A. J. Snell, J. M. Stevenson, A. Walton, M. 

W. Cresswell, R. A. Allen, ‘Extraction of Sheet Resistance and 

Linewidth from All-Copper ECD Test-Structures Fabricated 

from Silicon Preforms,’ 2007 International Conference on 

Microlectronic Test Structures, pp.14-19 (22-MAR-2007)

R. A. Allen, R. Dixson, M. W. Cresswell, W. Gutherie, B. 

J. Shulver, A. S. Bunting, J. T. Stevenson, and A. Walton, 

“CD Reference Materials Fabricated on Monolithic 200 mm 

Wafers for Automated Metrology Tool Applications,” 2007 

International Conference on Frontiers of Characterization and 

Metrology for Nanoelectronics, Mar 27-29, 2007, Gaithersburg, MD (01-SEP-2007)

B. Li, L. Shi, P. S. Ho, J. Zhou, R. A. Allen, M. W. 

Cresswell,’Test Structures for Study of Electron Transport in 

Nickel Silicide Features with Linewidths between 40 nm and 

100 nm,’ IEEE ICMTS International Conference on Microelectronic Test Structures, March 06-09, 2006, Austin, Texas, 

pp.18-23 (01-APR-2006)

B. J. Shulver, A. S. Bunting, A. Gundlach, L. I. Haworth, A. 

W. Ross, A. J. Snell, J. M. Stevenson, A. Walton, R. A. Allen, M. W. Cresswell, ‘Design and Fabrication of a Copper 

Test Structure as a Electrical Critical Dimension Reference,’ 

Proc., IEEE International Conference on Microelectronic Test 

Structures, March 06-09, 2006, Austin, Texas, pp.124-129 

(01-APR-2006)

C. Nwokoye, M. E. Zaghloul, M. W. Cresswell, R. A. Allen, 

C. E. Murabito, ‘A New Critical Dimension Metrology for 

Chrome-on-Glass Substrates Based ons-Parameter Measurements Extracted from Coplanar Waveguide Test Structures,’ 

SPIE, Bay Area Chrome Users’ Symposium, September 18-22, 

2006, Monterey, California, 12 pp. (01-OCT-2006)

M. W. Cresswell, W. Gutherie, R. Dixon, R. A. Allen, C. E. 

Murabito, J. V. Martinez, ‘RM 8111: Development of a Prototype Linewidth Standard,’ Journal of Research (NIST JRES), 

Vol. 111, No. 3, pp.187-203 (01-MAY-2006)

A. E. Yarimbiyik, H. A. Schafft, R. A. Allen, M. E. Zaghloul, 

and D. L. Blackburn, “Implementation of Simulation Program 

for Modeling the Effective Resistivity of Nanometer Scale Film 

and Line Interconnects,” NISTIR 7234, February, 2006.

JEDEC Standard JESD202, “Method for Characterizing the 

Electromigration Failure Time Distribution of Interconnects 

Under Constant-Current and Temperature Stress,” March 2006. 

(Written by H. A. Schafft).

A. E. Yarimbiyik, H. A. Schafft, R. A. Allen, M. E. Zaghloul, 

and D. L. Blackburn, “Resistivity of Nanometer-Scale Films 

and Interconnects: Model and Simulation,” 2005 IEEE International Integrated Reliability Workshop Final Report, pp. 139-

140 (open poster). IEEE Catalog Number 05TH8799. ISBM 

0-7803-8992-1. Library of Congress Number 2005920974.

Dixson, R. G., Allen, R. A., Guthrie, W. F., Cresswell, M. W, 

Traceable Calibration of Critical-dimension Atomic Force 

Microscope Linewidth Measurements with Nanometer Uncertainty J. Vacuum Sci. Technol. B Vol 23 Issue 6 pp. 3028-3032 

November 30, 2005. 

Cresswell, M. W., Dixson, R. G., Guthrie, W. F., Allen, R. A., 

Murabito, C. E., Park, B., Martinez, J. V., Hunt, A. Critical 

Dimension Reference Features with Sub-Five Nanometer 

Uncertainty SPIE 30th International Symposium Microlithography Metrology, Inspection, and Process Control San Jose, 

CA Vol 5752 pp. 288-303 May 30, 2005. 

Allen, R. A., Hunt, A., Murabito, C. E., Park, B., Guthrie, W. F., 

Cresswell, M. W Extraction of Critical Dimension Reference 

Feature CDs from New Test Structure Using HRTEM Imaging 

ICMTS IEEE International Conference on Microelectronic 

Test Structures Leuven, Belgium April 18, 2005.

Bin Li, Li Shi, JiPing Zhou, Paul S. Ho, Richard A. Allen, 

Michael W. Cresswell Test Structures for Study of Electron 

Transport in Nickel Silicide Features with Linewidths between 

40 nm and 160 nm ICMTS IEEE International Conference 

on Microelectronic Test Structures Leuven, Belgium April 

18, 2005.

CD-AFM reference metrology at NIST and SEMATECH 

Ronald Dixson, Joseph Fu, Ndubuisi Orji, William Guthrie, 

Richard Allen, and Michael Cresswell Proceedings of SPIE 

– Volume 5752 Metrology, Inspection, and Process Control 

for Microlithography XIX, Richard M. Silver, Editor, May 

2005, pp. 324-336.

W. F. Guthrie, R. G. Dixson, R. A. Allen, M. W. Cresswell, C. 

E. Murabito, B. Park, “Establishing Traceability of Linewidth 

Reference Materials Using CD-AFM and HRTEM,” 2005 

International Conference on Characterization and Metrology for ULSI Technology, March 15-18, 2005, University of 

Texas at Dallas.
62 National Institute of Standards and Technology 

R. A. Allen, A. Hunt, C. E. Murabito, B. Park, W. F. Guthrie, 

and M. W. Cresswell, “Extraction of Critical Dimension Reference Feature CDs from New Test Structure Using HRTEM 

Imaging,” Proceedings of the 2005 IEEE International Conference on Microelectronic Test Structures, pp. 5-10, March 

2005, Leuven, Belgium.

“Report of Investigation of RM 8111: Single-Crystal Critical 

Dimension Prototype Reference Materials,” National Institute of Standards and Technology, Gaithersburg, Maryland, 

March 2, 2005.

M. W. Cresswell, R. G. Dixson, W. F. Guthrie, R. A. Allen, C. 

E. Murabito, B. Park, J. V. Martinez de Pinillos, and A. Hunt 

“CD Reference Features with Sub-Five Nanometer Uncertainty” published in SPIE Proceedings Vol. 5752 (2005).

A. E. Yarimbiyik, H. A. Schafft, R. A. Allen, M. E. Zaghloul, 

and D. L. Blackburn, “Modeling and Simulation of Resistivity 

of Nanometer Scale Copper,” Microelectronics Reliability. 

In-press. On-line publication date: December 19, 2005. 

R. A. Allen, R. I. Patel, M. W. Cresswell, C. E. Murabito, B. 

Park, M. D. Edelstein, L. W. Linholm, “Recent Developments 

in Producing Test-structures for Use as Critical Dimension 

Reference Materials,” ICMTS IEEE International Conference 

on Microelectronic Test Structures, Mar. 22-25, 2004, Awaji, 

Japan, pp. 35-40 (01-MAR-2004).
Semiconductor Microelectronics and Nanoelectronics Programs 63

WAFER-LEVEL AND OVERLAY METROLOGY

Technical Contacts:

R. Silver

T. Doiron

GOALS

Provide technological leadership to semiconductor and equipment manufacturers, and other government agencies by developing and evaluating 

the methods, tools, and artifacts needed to apply 

optical techniques to the metrology needs of semiconductor microlithography. One specifi c goal is 

to provide the customer with the techniques and 

standards needed to make traceable dimensional 

measurements on wafers at the customer’s facility. The industry focus areas of this project are 

primarily optical based methods used in overlay 

metrology and optical, wafer level critical-dimension (CD) metrology. 

CUSTOMER NEEDS

Tighter tolerances on CD measurements in wafer 

production place increasing demands on linewidth 

accuracy and on overlay tolerances. A recent 

industry focus on high throughput, lower cost of 

ownership metrology tools, which enable more 

dense sampling strategies has lead to a comprehensive program at NIST to both support and advance the optical techniques needed to make these 

high throughput overlay and photomask/wafer 

critical dimension measurements (see Fig. 1). 

Improved two-dimensional overlay measurement techniques and standards are needed for 

measuring and controlling overlay capabilities 

of steppers and separating out the contributions 

from the photomask. Overlay is listed in multiple 

sections of Table 116 of the 2005 ITRS as a diffi cult 

challenge for both >32 nm and <32 nm processes. 

Overlay measurements have not kept pace with 

resolution improvements and in-die correlation 

requirements and will be inadequate for ground 

rules less than 45 nm. In fact, Table 118a shows 

that no known solutions for overlay output metrology exist beyond the 65 nm node. As shown in 

Table 118b, the problems are more acute for long 

term CD metrology where the industry is currently 

encountering metrology problems without known 

manufacturing solutions. 

TECHNICAL STRATEGY

There are two main strategic technical components 

of this project. 

1. NIST has developed an overlay metrology 

tool that has undergone continuous development 

of the mechanical hardware, optical components, 

and measurement algorithms to obtain uncertainties comparable to or better than the best industry 

overlay tools. This optical 

metrology tool is now used 

to calibrate standards and 

to support the development 

of improved measurement 

algorithms and alignment 

techniques. The technical 

strategy for overlay metrology is divided into two 

segments: (a) instrumentation development and the 

advance of overlay metrology techniques, and (b) the 

design and calibration of 

standard artifacts. Pattern 

placement and overlay of 

the various lithographic 

levels is monitored with a 

series of targets, each in a 

different plane. The overlay 

offset is then obtained by 

optical measurements with 

a determination of the relative target centerlines. Any 

misalignment in the overlay metrology system will 

Figure 1. The image is a CCD camera image from an array of 39 nm CD 

features etched in polysilicon. The panels in the center column each show 

sets of profi les from the peaks and valleys of the through-focus focus metric 

curves, seen on the left. The upper two panels are from 50 nm CD linewidth 

arrays while the lower two are from the 39 nm linewidth array. These data 

were acquired with 0.4 illumination NA, 0.8 collection NA, and 436 nm 

wavelength.
64 National Institute of Standards and Technology 

translate into an artifi cial overlay offset, referred 

to as tool induced shift (TIS). Additionally, there 

are residual errors caused by asymmetries in the 

target edges or covering layers (resist) known as 

wafer induced shift (WIS) (Fig. 2). 

Figure 2. An example of reversal methods applied to 

determine WIS and the asymmetry of the target itself.

A set of standard artifacts and procedures, developed at NIST and published, has been implemented to assist in aligning overlay measurement 

systems and minimizing TIS. After alignment, the 

tool must then be calibrated with standard artifacts 

to yield accurate overlay offsets. The measurement system used for this component is an optical 

refl ection mode instrument, typically operated in 

a bright fi eld mode. The hardware includes high 

resolution image capture with a full fi eld CCD data 

acquisition system which has been fully characterized and calibrated. This instrument has enabled 

a detailed study of CCD array performance and 

characterization. In this work, several CCD 

acquisition systems have been evaluated and improved edge detection and CCD array calibration 

procedures have been implemented. These same 

methods for two-dimensional CCD array analysis 

are now being applied to optical component alignment error and aberration analysis. 

Standard overlay artifacts have been fabricated 

in 200 mm and 300 mm wafers and calibrated for 

SRM distribution. These overlay artifacts are for 

the calibration of industrial optical overlay tools. 

The artifacts have been fabricated in single crystal silicon and provide an array of etched silicon 

three-dimensional targets. These wafers additionally have an extensive set of characterization 

targets and research structures developed in close 

collaboration with SEMATECH and leading semiconductor manufacturers, for example, Fig. 3.

We have also developed a series of new overlay 

targets and linewidth targets intended to enable the 

measurement of overlay and linewidth with device 

sized features. A variation of these targets allows 

Figure 3. A schematic of the target designs is shown in (a). This is one example of several variations of this 

design. The lower part of the fi gure shows an image and a set of profi les for a target which refl ects higherorder optical content.
Semiconductor Microelectronics and Nanoelectronics Programs 65

in-chip targets to be placed throughout the active 

area of a die. These results have been published 

recently and collaborative work is progressing to 

develop the commercial applications to measure 

overlay using device sized features in targets of 

small overall dimension. There is also a comprehensive effort developing new high resolution 

overlay targets intended to enable the continued 

use of optical overlay measurements for the 65 

nm node and beyond. An example of the latest 

high resolution targets is shown in Fig. 4. 

DELIVERABLES: 

New alignment techniques and optical characterization tests for the overlay microscope, optics, 

and the x-y metrology stage have been recently 

published. These new techniques are used to 

determine the fi nal uncertainties and tabulated 

combined uncertainty values for the latest set of 

SRM calibrations. These techniques include the 

characterization and calibration of complex optical 

alignment effects and illumination alignment and 

design errors and their effects on overlay output 

values. 2Q 2007

Use the OMAG 4 metrology wafers from the 

SEMATECH collaboration for an evaluation of 

overlay calibration targets/wafers. Work with 

SEMATECH and the Overlay Metrology Advisory 

Group (OMAG) to develop a new set of calibration 

•

•

Figure 4. New overlay targets, which occupy less 

than 2 μm x 2 μm in total space. This is designed to 

be an in chip target.

Figure 5. TM polarization is on top and TE is shown on the right. The vertical axes 

are units of normalized intensity and the horizontal axes are illumination angle in 

degrees. The data show good sensitivity to nm changes in linewidth. The image has 

been normalized to the background. A fourth order fi t is used to analyze the data. The 

dynamic range is in part the result of the background normalization.

structures and techniques for calibration of industry 

tools. Apply these measurement protocols to optical system alignment, overlay errors, and evaluation of wafer and tool induced measurement errors. 

3Q 2007

Work with SEMI and SEMATECH in the development of new target designs and standards 

specifi cations for overlay metrology. This new 

specifi cation is focused on bringing some standardization back to the overlay target designs and a 

move away from the growing number of proprietary 

overlay target designs, for calibration purposes. 

4Q 2007

Modeling the effects of all of the relevant feature 

properties encountered in overlay measurements 

and optical critical dimension measurements using 

existing and new software tools, can yield signifi -

cant improvements in measurement accuracy, as 

in Fig. 5. NIST has a world class effort in optical modeling. This includes the comprehensive 

comparison of two rigorous coupled wave guide 

scattering models, a fi nite difference time domainbased model and the NIST-developed exact integral equation solver method. The latter method 

has become recognized as the most accurate in 

existence today.

DELIVERABLES: 

Compare and test the accuracy of new scattering models for line width evaluation. Develop a 

standard set of simulation values and make them 

available for comparison and verifi cation by industry users. Publish results for the semiconductor 

industry. 2Q 2007

•

•
66 National Institute of Standards and Technology 

Apply the through-focus focus-metric technique to 

illumination alignment and inhomogeneity. Compare experimental results with modeling results and 

publish results. 4Q 2007

2. The second component of this project is the 

development of new, advanced high-resolution 

optical metrology techniques. A new class of 

optical measurement techniques known as scatterfi eld microscopy is being developed at NIST. 

This methodology has demonstrated the possibility of using optical methods for line width 

and overlay metrology with targets composed of 

features smaller than 40 nm critical dimensions. 

This new approach utilizes structured illumination in parallel with engineered target designs. 

The technique is well suited for high resolution 

microscopy of metrology targets as encountered 

in semiconductor manufacturing.

A key element of this approach is to develop 

optical methods which can be suitably applied 

to device-sized features. Current metrology 

requirements are demanding higher throughput, 

non-destructive measurements with nanometer 

sensitivity to enable tighter process control as 

well as closed-loop integrated process control. 

The current class of scatterometry and scatterfi eld optical techniques have shown promise as 

potential solutions to these signifi cant metrology 

challenges. As a part of this project, we have constructed a new optical tool specifi cally intended to 

make this type of scatterfi eld measurement. This 

effort includes comprehensive optics modeling 

as well as a new optical confi guration designed 

in-house. The optical design work includes a 

thorough examination of illumination effects and 

accurate methods to prepare optical illumination 

fi elds.

DELIVERABLES: 

Develop a comprehensive more accurate set of 

techniques to accurately measure optical illumination fi elds and publish results. Develop characterization methods to measure the collection optics 

set of aberrations and to perform optimum optical 

alignment. 3Q 2007

Develop the capability to measure dense features 

which are smaller than 40 nm CD. Demonstrate 

the sensitivity to 1 nm changes in linewidth for 

densely positioned silicon structures, and develop 

modeling techniques for the accurate interpretation 

of measurements. 4Q 2007

ACCOMPLISHMENTS

Researchers from the overlay metrology project have submitted an invention disclosure of a 

new potential breakthrough in high resolution op-

•

•

•

#

tical metrology. As a part of this disclosure, a new 

method has been proposed to measure overlay on 

targets which are very dense and only refl ect zero 

order optical content. This new, image-based approach is capable of measuring overlay on features 

as small as 20 nm in dimension with densities to 

one to two line/space ratio. Sematech has worked 

jointly on this exciting new technique.

Staff from the OMP funded Precision Engineering Division (PED) optical metrology project 

met with several leading optical metrology tool 

manufacturers regarding recent advances in the 

scatterfi eld microscopy technique. The individual 

discussions included details about the new high 

resolution microscopy techniques being developed 

at NIST and their potential industrial application 

and implementation. Research in the area of high 

resolution optical methods is now being pursued 

at several companies with clear applications in 

overlay metrology and potential applications in 

optical based critical dimension metrology. The 

discussions largely focused on potential technology transfer between the NIST optical projects and 

development scientists at the optical metrology 

companies. Details on recent techniques for optical aberration measurements and on methods for 

evaluating Kohler illumination were covered. 

NIST electromagnetic scattering code has 

been compared successfully in new applications 

of two other industry codes resulting in detailed 

published comparisons and study of arrayed 

targets. These model extensions are providing 

simulations results and guidance in optical tool 

design and metrology target designs for features 

currently down to 50 nm. The models are now 

capable of simulating targets in array formats as 

well as illumination at a single angle for analysis 

and development of scatterfi eld methods.

High resolution scatterfi eld optical techniques 

continue to be successful. The optics project 

funded as a competence has proved to be very 

productive. The cross laboratory proposal has 

produced several recent results which were published and presented at the recent SPIE Advanced 

Lithography meeting. The collaboration between 

the Manufacturing Engineering (MEL) and 

Physics (PL) laboratories has resulted in several 

publications this year including a paper in Applied 

Optics and a very substantial report to Sematech 

on the Limits of optical Critical Dimension metrology. This collaboration has developed back 

focal plane imaging and Scatterfi eld illumination 

techniques which have now resulted in improved 

experiment to theory agreement. 

#

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#
Semiconductor Microelectronics and Nanoelectronics Programs 67

Signifi cant industry interest in new high-resolution optical overlay target designs developed 

under the overlay metrology project. Following 

invited presentations at SEMATECH OMAG and 

Advanced Metrology Advisory Group (AMAG) 

metrology workshops, and discussions at SPIE, 

there was much interest in the new target designs 

and NIST is working closely with Sematech to 

implement the new designs on a metrology reticle 

and new set of test wafers. 

NIST researchers have made model comparisons between the E. Marx developed optical 

scattering code, the Spectel company Metrologia 

metrology modeling package, a recently NIST 

developed code by T. Germer and the Panaramic 

Technologies FDTD code. Different material 

systems were compared as well as detailed material comparisons some of which were published 

recently. New results, based on the full integration 

of the NIST scattering code and Spectel optical 

microscope model, show very good agreement 

based on recent code enhancements and improvements. This is an important step in the effort 

to provide industry the quantitative ability to 

determine sample-dependent effects on overlay 

tool performance. Results have been presented 

at SPIE Microlithography.

A paper titled “The Limits of Optical Critical 

Dimension Metrology” was presented by Rick 

Silver of the PED at the SPIE Advanced Lithography symposium in San Jose. The paper described 

an in depth study carried out by jointly PED 

optics researchers and Thom Germer of Physics 

Lab. The research was a summary of the results 

written up in an extensive study under contract 

to Sematech. This paper gave an overview of 

the optical critical dimension (OCD) limits from 

a fundamental perspective based on extensive 

modeling and statistical analysis. This paper was 

considered a benchmark for the future of OCD and 

its capabilities. The paper laid the groundwork for 

future studies on the practical limitations as the 

fundamental limits are well beyond those limits 

imposed by the variation of the targets themselves 

at the nanometer scale. Multiple electromagnetic 

models were compared and their stability and 

integrity verifi ed prior to the analysis of the fundamental limits. The study gave measurement 

uncertainties for the different techniques in their 

fundamental application and evaluation of sensitivity to changes at the nanometer scale. 

New scatterfield optical tool operational. 

The new scatterfi eld metrology tool has been as-

#

#

#

#

sembled and is now operational in the Advanced 

Metrology Laboratories. This tool is specifi cally 

designed for performing sophisticated illumination engineering and scanning and has already 

produced important results. The new tool is 

fully computer controlled and based on a NIST 

designed optical column to enable access to a 

large conjugate back focal plane for illumination 

engineering. The new tool has been instrumental 

in investigating recent uncovered polarization dependent transmission effects in the optical train. 

A paper was presented at SPIE Microlithography on new aspects of optical tool design 

focused on designing the 193 nm next generation 

Scatterfi eld microscope. A key aspect of this 

paper was a new treatment of the propagation of 

the electromagnetic scattered intensity profi les 

through the optical train. This approach is expected to give more accurate evaluation of the 

optical measurements by including the optical 

system aberration and its effects directly on them 

measured profi les. The paper included a further 

breakdown and analysis of Kohler illumination 

as used in many bright fi eld microscopes and its 

application to the new 193 nm optical tool. The 

analysis has allowed NIST staff to identify key 

problems in the illumination such as polarization 

dependent transmission through the optics. 

COLLABORATIONS

SEMATECH, IBM, Intel, KLA-Tencor, Nanometrics, Applied Materials, Motorola, AMD, and several other leading 

manufacturers and tool vendors.

RECENT PUBLICATIONS

R. M. Silver, B. Barnes, R. Attota, J. Jun, M. Stocker, E. Marx, 

and H. Patrick, “Scatterfi eld Microscopy to Extend the Limits 

of Image-based Optical Metrology,” To appear in Applied 

Optics, (2007).

R. M. Silver, T. Germer, R. Attota, B. Barnes, B. Bunday, J. 

Allgair, E. Marx, and J. Jun, “Fundamental Limits Optical 

Critical Dimension Metrology: A Simulation Study,” SPIE 

Proc. Feb. Vol. 6518 (2007).

Y. J. Sohn and R. Silver, “Kohler Illumination Analysis for 

High Resolution Optical Metrology using 193 nm Light,” Proc. 

SPIE Intl. Soc. Opt. Eng. Vol. 6518, (2007).

Bryan M. Barnes, Lowell P. Howard, Jay Jun, Pete Lipscomb, 

and Richard M. Silver, “Zero-order imaging of device-sized 

overlay targets using scatterfi eld microscopy,” Proc. SPIE 

6518, 65180F (2007).

R. M. Silver, R. Attota, B. Barnes, and H. Zhou, “A Theoretical 

and Experimental Evaluation of Scatterfi eld Microscopy for 

use in Patterned Defect Inspection,” Sematech Tech Transfer 

Doc, Feb. (2007).

#
68 National Institute of Standards and Technology 

R. Attota and R. M. Silver, “Optical Illumination and Critical 

Dimension Analysis using the Through-focus Focus-metric 

Method,” SPIE Vol. 6289, p. 6289OQ, 2006

R. Attota and R. M. Silver, “Optical Illumination and Critical 

Dimension Analysis using the Through-focus Focus-metric 

Method,” SPIE Vol. 6289, p. 6289OQ, 2006.

B. Barnes, L. Howard, and R. M. Silver, “Illumination Optimization for Optical Semiconductor Metrology,” SPIE Vol. 

6289, p. 6289OP, 2006.

R. M. Silver, B. Barnes, R. Attota, J. Jun, J. Filliben, J. Soto, 

M. Stocker, P. Lipscomb, E. Marx, H. Patrick, R. Dixson, and 

R. Larrabee, “The Limits of Image-based Optical Metrology,” 

Proc. SPIE Intl. Soc. Opt. Eng. Vol. 6152, 61520Z (2006).

R . Attota, R. M. Silver, M. Bishop and R. Dixson, “Optical 

Critical Dimension Measurement and Illumination Analysis 

using the Through-focus Focus-metric,” Proc. SPIE Intl. Soc. 

Opt. Eng. Vol. 6152, 61520K (2006).

Y. J. Sohn, B. Barnes, L. Howard, R. Silver, R. Attota, and 

M. Stocker, “Kohler Illumination in High Resolution Optical Metrology,” Proc. SPIE Intl. Soc. Opt. Eng. Vol. 6152, 

61523S (2006).

H. Patrick, R. Attota, B. Barnes, T. Germer, M. Stocker, R. M. 

Silver, and M. Bishop “Scatterfi eld Microscopy using back 

focal plane imaging with an engineered illumination fi eld,”

Proc. SPIE Intl. Soc. Opt. Eng. Vol. 6152, 61520J (2006).

P. Lipscomb, J. Allgair, B. Bunday, M. Bishop, R. Silver, 

R. Attota, and M. Stocker, “Calibration Optical Overlay 

Measurements,” Proc. SPIE Intl. Soc. Opt. Eng. Vol. 6152, 

615211 (2006).

D. Kandel, M. Adel, A. Frommer, V. Levinski, A. Rapoport, 

and R. M. Silver, “Imaging Simulations of Optimized Overlay 

Marks with Deep Sub-resolution Features,” Proc. SPIE Intl. 

Soc. Opt. Eng. Vol. 6152, 61522X (2006).
Semiconductor Microelectronics and Nanoelectronics Programs 69

FRONT-END PROCESSING METROLOGY

PROGRAM

The dimensions of the active transistor areas are approaching the spacing between dopant 

atoms, the stochastic regime, complicating both modeling and doping gradient measurements. 

Thin dielectric and conducting fi lms are approaching monolayer thicknesses.

As device dimensions continue to shrink, junctions and critical fi lm thicknesses approach the 

realm of several atoms thick, challenging gradient, thickness and wafer fl atness and roughness metrology as well as electrical and reliability characteristics. The current gate stacks, 

currently poly silicon over SiO2

 and SiON dielectrics, are being replaced by high κ – metal 

gate stacks. The overall task is to provide starting wafer dimensional and defect metrology, 

suitable metrology and reference materials for their dielectrics and junctions, including electrical characterization, gradient, thickness and roughness metrology, and overall reliability 

metrology.
70 National Institute of Standards and Technology 
Semiconductor Microelectronics and Nanoelectronics Programs 71

WAFER AND CHUCK FLATNESS METROLOGY

Technical Contacts:

U. Griesmann 

Q. Wang

J. Soons 

“NIST continues to 

support the effort to 

bring quantitative 

standards and 

measurement practices 

to the semiconductor 

wafer metrology area. 

Their support in the 

area of wafer thickness 

metrology has led to 

unprecedented levels 

of thickness correction 

on industry standard 

300 mm Si wafers, 

when combined with 

deterministic fi nishing 

technologies such as 

MRF. The collaboration 

with NIST has been a 

signifi cant and crucial 

component to the 

success of this effort.”

Paul Dumas and Marc Tricard 

QED Technologies – 

A subsidiary of Cabot 

Microelectronics Corp, 

Rochester, NY

GOALS

Develop measurement support for 300 mm 

diameter silicon wafers used in lithography applications. This project provides measurement 

and technology infrastructure to support the 

measurement of wafer thickness variation of 

300 mm silicon wafers, and surface fl atness of 

chucked wafers.

CUSTOMER NEEDS

With the evolution of exposure tools for optical 

lithography towards larger numerical apertures, 

the semiconductor industry expects continued 

demand for improved wafer fl atness at the exposure site. The allowable site fl atness for 300mm 

wafers is expected to be less than 45 nm by 2010 

and it may be as low as 25 nm by 2015 according to the International Technology Roadmap 

for Semiconductors (ITRS 2006). This requires 

wafers with low thickness variation and presents 

a challenge for both wafer polishing and metrology tools, which must be capable of meeting 

the specifi cations. We are addressing the need 

for standard 300 mm wafers with calibrated 

thickness variation with the Improved Infrared 

Interferometer (IR3

) at NIST. The interferometer 

is used for independent, traceable wafer thickness calibrations, which enable manufacturers of 

wafers and wafer metrology instruments to certify 

the performance of their metrology instruments. 

In addition, thickness variation measurements of 

silicon wafers can be combined with models of 

wafer/chuck interactions to determine the fl atness of low surface area wafer vacuum chucks, 

which is diffi cult to measure directly. The surface 

fl atness of chucked wafers can be measured using NIST’s “eXtremely accurate CALIBration 

InterferometeR” (XCALIBIR).

TECHNICAL STRATEGY

1. The Improved Infrared Interferometer (IR3) 

was developed at NIST for the characterization 

of the thickness variation of silicon wafers with 

diameters up to 300 mm. The IR3 interferometer is 

an infrared phase-shifting interferometer, operating at a wavelength of 1550 nm which measures 

the thickness of low-doped silicon wafers up to 

300 mm diameter in a single measurement (see 

Fig. 1 and Fig. 2). The interferometer may be 

used in several confi gurations with collimated and 

spherical test wave-fronts. The collimated wavefront mode is the current focus of the project. In 

this method, the planar infrared wave-front is normally incident on the wafer. A portion of the beam 

is refl ected from the front wafer surface, while the 

rest passes through the wafer and refl ects from the 

rear surface. The interference of these two wavefronts produces fringes and, by wavelength phase 

shifting, allows calculation of the wafer thickness 

variation.

Figure 2. Test arm of the IR3 interferometer. shown 

are the collimator lens and the beam expander 

together with a 300 mm silicon wafer. The 

insensitivity of the measurement to vibration permits 

the use of a simple wafer mount.

Figure 1. Solid model of NIST’s improved infrared 

interferometer (IR3

). The main components of the 

interferometer are indicated: collimator lens (CL), 

polarizing beam splitter (BS), λ /4-plates (LP), 

reference mirror for the Twyman-Green mode (RM), 

diverger lens (DL), polarizer (PO), zoom lens 

(ZL), motion controllers for the zoom lens (MC), 

and camera (CA). The size of the base plate is 

approximately 20 cm × 30 cm.

DELIVERABLES: 

Develop complete uncertainty analysis for thickness and thickness variation measurements. 

4Q 2007

•
72 National Institute of Standards and Technology 

Install laser with wide tuning range to enable measurements of very thin wafers. 3Q 2007

Calibrate the thickness variation of a set of wafers 

as NIST Standard Reference Materials (SRM), 

which will be made available for purchase. 4Q 2007

2. Measurements of the fl atness of chucked wafers are made with XCALIBIR, a general purpose, 

300 mm aperture phase measuring interferometer operating with visible light at 633 nm. The 

interferometer is housed in a clean room, which 

eliminates dust particles between chuck and wafer. 

The results are used to evaluate the infl uence of 

wafer-chuck interactions on the chucked wafer 

fl atness. 

ACCOMPLISHMENTS

IR3 has undergone a major upgrade that enables us 

to address the metrology needs for 300 mm diameter wafers. A collimator lens has been installed 

that can illuminate the entire surface of a 300 mm 

wafer and allows us to make a measurement of the 

wafer’s thickness variation in a single measurement. The imaging system of the interferometer 

now can measure wafers with larger slopes and the 

spatial resolution of the detector was doubled to 

2 pixels/mm. Further improvements will be aimed 

at reducing the noise level and at improving the 

measurement uncertainty. In addition, a new laser 

with wide tuning range will make it possible to 

measure very thin wafers. The optical components 

in the IR3

 interferometer were improved to reduce 

the measurement noise. Wavelength phase-shifting has been implemented and a TTV map repeatability of 5 nm peak to valley has been achieved 

for 300 mm wafers. The IR3 interferometer is now 

housed in a clean room, which enables us to make 

calibration measurements of wafers supplied by 

industry customers.

The fl atness of 200 mm and 300 mm diameter 

wafers on pin chucks was explored using the 

XCALIBIR interferometer in a collaboration with 

Wavefront Sciences Inc., Albuquerque, NM.

IR3

 was used to develop a sub-aperture magnetorheological polishing process for the fi nishing of 

300 mm wafers with ultra-low thickness variation 

in collaboration with QED Technologies. Figure 3 

shows that a total thickness variation (TTV) 

of about 40 nm could be achieved. The SFQR 

resulting from the thickness variation (assuming 

an ideal chuck) is shown in Fig. 4. These results 

demonstrate that a suitable sub-aperture fi nishing 

process can achieve the exposure site fl atness 

expected by the ITRS for 2015.

•

•

Figure 3. Wafer thickness variation of a 300 mm 

silicon wafer before and after sub-aperture polishing. 

The total thickness variation was reduced from 

238 nm to 42 nm over a 292 mm aperture (4 mm edge 

exclusion).

Figure 4. SFQR of the wafer shown in Fig. 3 after 

sub-aperture fi nishing for 25 mm x 25 mm sites with 

the same edge exclusion (4 mm) as in Fig. 3.

COLLABORATIONS

During the course of this project, we have interacted with several companies on problems 

relating to wafer fl atness metrology and chucked 

wafer fl atness.

1. WaveFront Sciences: Flatness measurements 

of free form and chucked wafers for the 

validation of a metrology tool developed by 

WaveFront Sciences.

2. MEMC Electronic Materials: Wafer thickness 

standard development.

3. Siltronic: Wafer thickness standard development.

4. Intel: Development of very thin silicon thickness standards.

5. QED Technologies: Development of ultra-fl at 

300 mm silicon wafers.
Semiconductor Microelectronics and Nanoelectronics Programs 73

6. Lumetrics: Evaluation of Lumetrics thickness gauging technology for wafer thickness 

metrology.

RECENT PUBLICATIONS

U. Griesmann, Q. Wang, M. Tricard, P. Dumas, and C. Hall, 

“Manufacture and Metrology of 300 mm Silicon Wafers 

with Ultra-Low Thickness Variation,” AIP Proceedings of 

the 2007 Conference on Characterization and Metrology for 

Nanoelectronics, Gaithersburg, MD, USA (2007)

Q. Wang, U. Griesmann, and R. S. Polvani, “Interferometric 

Thickness Calibration of 300 mm Silicon Wafers,” Proceedings of the ASPE Topical Meeting on Precision Interferometric 

Metrology, Middletown, CT (2005).

U. Griesmann, Q. Wang, and T. D. Raymond, “Optical Flatness Metrology for 300 mm Silicon Wafers,” AIP Proceedings 

of the 2005 Conference on Characterization and Metrology 

for ULSI Technology, University of Texas, Richardson, TX, 

USA (2005).

D. R. Neal, P. Pulaski, T. D. Raymond, D. A. Neal, Q. Wang, 

and U. Griesmann, “Testing highly aberrated large optics 

with a Shack-Hartmann wavefront sensor,” Proc. SPIE 5162, 

129-138 (2003).

T. L. Schmitz, A. Davies, C. J. Evans, and R. E. Parks, “Silicon 

wafer thickness variation measurements using the National 

Institute of Standards and Technology infrared interferometer,” 

Opt. Eng. 42, 2281-90 (2003).
74 National Institute of Standards and Technology 
Semiconductor Microelectronics and Nanoelectronics Programs 75

MODELING, MEASUREMENTS, AND STANDARDS FOR

WAFER SURFACE INSPECTION

Technical Contact:

T. A. Germer

”I would like to thank you 

and NIST for the support 

that you have provided 

to VLSI Standards in the 

sizing of polystyrene 

latex spheres through 

the work of Dr. George 

Mulholland. We are 

very pleased with the 

measurements that 

Dr. Mulholland has 

performed, and with the 

level of technical support 

that the NIST staff has 

provided to us. This 

work is of technical and 

economic importance 

to the semiconductor 

industry and to VLSI 

Standards, because the 

ability to correctly size 

ever smaller particulate 

contaminants on silicon 

substrates is key to the 

manufacturing yield of 

silicon chips. We look 

forward to a continuing 

technical relationship 

between VLSI Standards 

and NIST.”

Marco Tortonese, Ph.D.

VLSI Standards, Inc.

GOALS

Our goals are: (1) provide industry with models, 

measurements, and standards for particles and 

other defects in order to improve the inspection of wafer surfaces; (2) develop facilities to 

accurately measure particle size and to deposit 

monosize particles on calibration artifacts to 

reduce the uncertainty in the sizes of particles 

used by the semiconductor industry to calibrate 

scanning surface inspection systems (SSIS); and 

(3) investigate theoretically and experimentally 

the behavior of light scattering from particles, 

defects, and roughness on wafer surfaces.

CUSTOMER NEEDS

The Semiconductor Industry Association’s (SIA) 

International Technology Roadmap for Semiconductors identifi es the detection and characterization of defects and particles on wafers to 

be a potentially show-stopping barrier to device 

miniaturization. The roadmap specifi es polystyrene latex (PSL) equivalent diameter particles that 

must be detectable on bare silicon, nonmetallic 

fi lms, metallic fi lms, and wafer backsides each 

year. Currently, no solutions to this inspection 

problem exist for particles on bare silicon, on 

non-metallic fi lms, and on wafer backsides, while 

it is anticipated that no acceptable solutions will 

exist for metallic fi lms in 2010. While the detection sensitivity for defects must be increased, the 

ability to characterize defects in terms of size, 

shape, composition, etc., is critical for yield-learning. Defects must be characterized independent 

of defect location and topology. 

With the need to detect smaller defects, the costs 

of inspecting wafers are skyrocketing. For new 

advances to be implemented in production environments, improvements in sensitivity must be 

achieved without suffering a tradeoff in throughput and must be cost-effective. The drive towards 

in situ sensors for production tools requires techniques which can be effectively miniaturized.

In order that wafer manufacturers and device 

manufacturers have a common basis for comparing specifi cations of particle contamination, 

improved standards for particles are needed. A 

recent comparison of the measurements of calibration wafers by 13 different Scanning Surface 

Inspection System (SSIS) indicated unacceptably 

large deviation between the SSIS results and 

the actual particle sizes. This study involved six 

particle sizes ranging from 88 nm to 290 nm and 

included the NIST SRM 1963 and two other sizes 

measured by NIST. For the two smallest particle 

sizes, 88 nm and 100.7 nm, the scanners systematically underestimated the size by about 8 %. 

The SIA roadmap specifi es the need for accurate 

calibration particles to size critical semiconductor components scaling to 32 nm or smaller by as 

early as 2008.

By 2010, at the 45 nm node, particles having diameters 22.5 nm must be detectable on bare silicon 

and nonmetallic fi lms, with 54 nm on metallic 

fi lms. No known solutions exist at this time. [2006 

ITRS, Yield Enhancement, Table 113a]

TECHNICAL STRATEGY

There are two major strategies for improving the 

performance of scanning surface inspection systems. One strategy is to develop a fundamental 

understanding of optical scattering at surfaces so 

that tool manufacturers can optimize the performance of their instrumentation, in terms of defect 

detection limits and discrimination capabilities, 

to characterize the response of instrumentation to 

different types of defects, and to develop and calibrate particles of well-defi ned size and material. 

Recent work by this group has demonstrated that 

the polarization of light scattered by particulate 

contaminants, subsurface defects, and microroughness has a unique signature that can be used 

to identify the source of scatter. In particular, it 

was found that small amounts of roughness do not 

depolarize scattered light. This fi nding has enabled 

the development of instrumentation which can collect light over most of the scattering hemisphere, 

while being blind to microroughness. That instrumentation, for which a patent has been awarded, 

should result in a factor of two improvements in 

minimum detectable defect size.

A second strategy is to provide leadership in 

the development of low uncertainty calibration 

particles for use in calibrating surface scanners. 

A major focus has been development of the differential mobility analysis (DMA) method for 

accurately sizing monosize polystyrene spheres. 

This work together with a SSIS round robin has 

provided evidence that current SSIS measurements have an unacceptably large uncertainty for 
76 National Institute of Standards and Technology 

particle sizes in the 90 nm to 100 nm size range. 

The technical focus of our future work will be 

applying the DMA for accurately sizing calibration particle sizes as small as 30 nm, developing 

methods for generating other types of monosize 

particles, and developing laser surface scattering 

methods for quantitative particle sizing.

Specifi c project elements are defi ned below:

1. Polarized Light Scattering Measurements – The Goniometric Optical Scatter Instrument (GOSI) enables accurate measurements of 

the intensity and polarization of scattered light 

with a wide dynamic range, high angular accuracy, 

and multiple incident wavelengths (visible and 

UV) on 300 mm wafers (see Fig. 1). We measure 

the light scattering properties of well-characterized samples exhibiting interfacial roughness, 

deposited particles, subsurface defects, dielectric 

layers, or patterns. The emphasis is on providing 

accurate data, which can be used to guide the 

development of light scattering instruments, and 

to test theoretical models.

DELIVERABLES: Perform measurements of scatter 

at 266 nm from various fi lms to assess the particle 

detection limits on those fi lms. 2Q 2007

Figure 1. The Goniometric Optical Scatter 

Instrument is a state-of-the-art laser scattering 

facility.

2. Theoretical Light Scattering Calculations – The focus of our theoretical work is on: (a) 

developing models that accurately predict the polarization and intensity of scattered light, and (b) 

determining what information can be effi ciently 

and accurately extracted from light scattering 

measurements. Approximate theories are used in 

conjunction with more complex techniques to gain 

an understanding of which parameters affect the 

light scattering process. Particular cases that are 

being analyzed include: (a) scattering by defects 

and roughness associated with dielectric layers, 

(b) scattering by particulate contamination on bare 

and oxidized wafers, and (c) scattering by periodic 

structures.

DELIVERABLES: Publish response curves for a 

variety of scattering geometries for particles on wafers 

with blanket fi lms. 3Q 2007

3. Size Distribution Measurements – Differential mobility analysis (DMA) has been shown 

to be capable of making accurate size measurements for mean particle diameters as small as 

50 nm. However, discrepancies have been noticed 

between PSL measurements using the DMA, and 

measurements using light scattering on the surface 

of a wafer. These discrepancies are possibly due 

to the PSL particles deforming on the bottom as 

they adhere to the wafer. This problem will be 

investigated by comparing measurements of the 

PSL particles to measurements of silica particles, 

which are less likely to deform when they contact 

the wafer surface. Measurements will be performed using both the DMA and light scattering 

instruments. 

DELIVERABLES: Compare measurements of silica 

particles to PSL particles. 4Q 2007

4. Resource on Particle Science – Over the past 

fi ve years, the particle-related work has included 

projects with SEMATECH and particle suppliers 

to the semiconductor industry. A number of needs 

by particle related companies were expressed at 

a NIST particle workshop including redoing the 

uncertainty assessments of existing particle SRMs 

and offering a particle sizing calibration service. 

Providing support for particle needs critical to 

the semiconductor industry will continue to be a 

priority.

DELIVERABLES: Provide technical support to the 

SEMI Advanced Wafer Surface Inspection System task 

force. 4Q 2007

ACCOMPLISHMENTS

Completed and certifi ed the measurements for 

new NIST Standard Reference Materials (SRM). 

Measured and certifi ed SRM 1964, particles with 

a nominal diameter of 60 nm. Also measured and 

certifi ed SRM 1963a, with a nominal diameter 

#
Semiconductor Microelectronics and Nanoelectronics Programs 77

of 100 nm, to replace the previous 100 nm SRM 

1963, which was corrupted due to agglomeration. 

SRM 1963a and SRM 1964 are currently available for purchase. 

Completed initial screening process and 

preliminary measurements for development of a 

30 nm SRM. Identifi ed primary and secondary 

candidate samples for the 30 nm SRM, based on 

diameter and distribution measurements. Researched measurement uncertainty for particles 

smaller than 50 nm and devised strategies for 

reducing the uncertainty. 

Developed and improved the NIST Calibration Facility, which uses Differential Mobility 

Analysis for sizing monodisperse spheres in the 

size range of 50 nm to 400 nm. Reduced the expanded uncertainty to 1.0 % of the particle size 

by correlating the slip correction to the measured 

particle size. Increased resolution and accuracy 

of measurements through improved equipment 

and intermediate measurements.

Determined that the electrospray technique 

can produce an aerosol having characteristics 

optimal for transferring particles in a liquid 

suspension onto wafers for particle diameters as 

small as 25 nm. This signifi cant fi nding enables 

improved wafer depositions by reducing the 

number of contaminant residue particles, the 

number of doublets, and the amount of residue 

on the particles.

Coordinated the experimental design and 

uncertainty analysis with the University of Minnesota for the fi rst measurement of slip correction 

of particles smaller than 300 nm. These measurements are critical to improving the accuracy of 

particle size by the DMA in the nanometer size 

range.

In collaboration with the University of Maryland, developed a method for generating pure 

copper spheres with diameters ranging from 100 

nm to 200 nm. These spheres, which mimic realworld particles better than polystyrene, were used 

to validate particle scattering theories in conditions for which models have a higher degree of 

uncertainty. Measured polarization and intensity 

of light scattered from the copper spheres and 

found good agreement with the Bobbert-Vlieger 

theory for light scattering from a sphere above 

a surface.

Developed a method, based upon scattering 

ellipsometry, for quantifying scatter from two 

sources and demonstrated its use by characterizing 

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the roughness of both interfaces of an SiO2

/silicon system. This fi nding establishes the validity 

of the light scattering models for roughness in a 

dielectric fi lm, which in turn limits the detection 

sensitivity of SSIS instruments. The method was 

also used to characterize scattering from steel 

surfaces, demonstrating capability to distinguish 

between scattering from surface roughness and 

material inhomogeneity. The method was further 

used to study the scatter from an anti-conformal 

polymer fi lm, helping to establish the limits of 

validity of the scattering theory.

Developed the SCATMECH library of C++ 

routines for light scattering. Published the SCATMECH library, providing a means for distributing 

scattering models and polarized light calculations 

to others. From the time of its public availability 

in March 2000, over 4200 copies of the library 

have been downloaded from the web. The Modeled Integrated Scatter Tool (MIST), a Windows 

application for calculating integrated scatter, was 

released in June 2004. 

Extended the theory of scattering of a sphere 

on a surface to axially-symmetric non-spherical 

particles. Demonstrated that the scattering by a 

metal particle on a surface is extremely sensitive 

to the shape of the particle in the region where 

the particle contacts the surface. This unusual 

sensitivity to shape must be considered when 

light scattering tools classify particles for material and size.

Performed a light-scattering-based diameter 

measurement of the NIST 100 nm PSL sphere 

standard (SRM 1963) deposited onto a silicon wafer. The measurement results included a thorough 

assessment of the uncertainties that arise in such 

measurements. It was found that the uncertainty 

was dominated by the uncertainty in the shape of 

the particle on the surface. Results for smaller PSL 

particles suggest that surface-induced deformation 

of the particles must be considered.

Assisted in revising SEMI M53, a practice 

for calibrating scanning surface inspection systems, by developing a model-based calibration 

scheme that matches measured signals from PSL 

spheres to the predictions of a theoretical model. 

The accepted model for scattering by the spheres 

is specifi ed in the standard as that provided by 

the MIST program. The new method has several 

advantages over the previous method, including: less sensitivity to changes in availability of 

specifi c size standards, improved accuracy, less 

variability between instruments, and an ability to 

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78 National Institute of Standards and Technology 

extract a quantitative accuracy from the calibration. The expanded uncertainty found by applying 

the calibration to a commercial instrument was 

better than 1 % of the diameter, Fig. 2.

Figure 2. Sample calibration of a commercial wafer 

scanner using the new SEMI M53 method. The 

relative expanded uncertainty in particle diameter 

has been reduced to less than 1 %.

COLLABORATIONS

Department of Chemical Engineering, University of Maryland, 

Professor Sheryl H. Ehrman, Validation of Scattering Theory 

Using Novel Monodisperse Particles.

Department of Mechanical Engineering, University of Minnesota, Professor David Pui, Generation and Measurement of 

Nanosize Particles. 

National Metrology Institute of Japan, Dr. Kensei Ehara, 

Nanoparticle Metrology.

RECENT PUBLICATIONS

Donnelly, M. K., Yang, J. C., “Screening Candidates for 30 

nm Spheres,” NISTIR 7345, August 2006.

Mulholland, G. W., Donnelly, M. K., Hagwood, C., Kukuck, 

S. R., and Hackley, V. A., “Measurement of 100 nm and 60 

nm Particle Standards by Differential Mobility Analysis,” J. 

Research NIST 111, 257–312, 2006.

Thomas A. Germer, “Measuring Interfacial Roughness by Polarized Optical Scattering,” in Light Scattering and Nanoscale 

Light Scattering, Alexei A. Maradudin, Ed., (Elsevier, New 

York, 2007) Chapter 10, pp. 259–284.

Thomas A. Germer and George W. Mulholland, “Particle 

Size Metrology: Comparison Between Aerosol Electrical 

Mobility and Laser Surface Light Scattering Techniques,” in 

Characterization and Metrology for ULSI Technology 2005, 

D.G. Seiler, A.C. Diebold, R. McDonald, C.R. Ayre, R.P. 

Khosla, S. Zollner, and E.M. Secula, eds., Proc. AIP 788, 

579–583 (2005).

Jung Hyeun Kim, Sheryl H. Ehrman, George W. Mulholland, 

and Thomas A. Germer, “Polarized light scattering by dielectric and metallic spheres on oxidized silicon surfaces,” Appl. 

Opt. 43(3), 585–591 (2004).

J. H. Kim, S. H. Ehrman, and T. A. Germer, “Infl uence of 

particle oxide coating on light scattering by submicron metal 

particles on silicon wafers,” Appl. Phys. Lett. 84(8) 1278-

1280 (2004).

J. H. Kim, S. H. Ehrman, and T. A. Germer, “Infl uence of 

particle oxide coating on light scattering by submicron metal 

particles on silicon wafers,” Appl. Phys. Lett. 84(8) 1278-

1280 (2004).
Semiconductor Microelectronics and Nanoelectronics Programs 79

FRONT-END MATERIALS CHARACTERIZATION

Technical Contacts:

G. Gillen: SIMS

D. Simons: SIMS

J. Small: TEM, 

 X-ray detectors 

J. Suehle: Electrical 

 Characterization

N. Nguyen: IPE, SE

GOALS

To provide industry with new and improved 

measurements, models, data, and measurement 

traceability/transfer mechanisms to enable the 

more useful and more accurate metrology infrastructure needed for select silicon CMOS frontend materials characterization. Major focus is 

placed on metrology requirements from the 2005 

International Technical Roadmap for Semiconductors (ITRS) expressed as diffi cult challenges: 

(1) structural and elemental analysis at the device 

level, including SIMS and HRTEM, and (2) 

metrology for advanced gate stacks and other thin 

fi lms. Additional work will be undertaken as possible on additional important thin fi lms and bulk 

material properties for silicon and other emerging 

semiconductors.

(1) To improve capabilities for compositional 

depth profi ling, this project develops new methods for depth-profi ling polymeric materials by 

Secondary Ion Mass Spectrometry (SIMS), 

defi nes optimum procedures for ultra-high depth 

resolution, develops depth-profi ling reference 

materials needed by U.S. industry, and improves 

the uncertainty of implant dose measurements by 

SIMS. 

(2) To address needs in composition and thickness measurements for thin fi lms and interfaces, 

this project develops new optical and physical 

characterization methods, as well as, characterizes 

the accuracy and reliability of existing methods. 

Materials of interest include high-κ and low-κ

materials, polymers, silicon-on-insulator (confi ned silicon), and strained silicon-germanium. 

High Resolution Transmission Electron Microscopy (HRTEM) is being developed as a chemical 

tomography tool for determining 3-D elemental 

distributions in advanced materials.

(3) Determine the work function, band offset, 

and interfacial defect structures of high-κ combinatorial metal electrode stacks systems by implementing a combination of techniques including 

Scanning Kelvin Probe Microscopy (SKPM), 

internal photoemission (IPE), ellipsometry (SE), 

backside FTIR, and external photoemission (Soft 

XPS and Inverse photoemission), and theoretical 

modeling.

CUSTOMER NEEDS

The Front-End Materials Characterization project 

addresses key material characterization problems 

associated with the integrated circuits industry’s 

front-end process, particularly new gate stack processes and materials, and metrology for structural 

and element characterization at the device level, 

particularly ultra-shallow junctions. Front-end 

processing requires the growth, deposition, etching, and doping of high quality, uniform, defectfree fi lms. These fi lms may be insulators, conductors, or semiconductors. The 2005 International 

Technology Roadmap for Semiconductors (ITRS) 

and 2006 Update near-term (through 2009) diffi cult challenges for front-end processes include: 

metrology issues associated with gate dielectrics 

fi lm thickness and gate stack electrical and materials characterization, introduction of metal 

gate electrodes with appropriate workfunctions, 

and metrology issues associated with 2-D dopant 

profi ling. Metrology needs for Thermal/thin fi lms, 

Doping Technology, SOI, and strained-silicon are 

discussed in the Metrology section of the 2005 

ITRS and 2006 Update. 

Since source/drain dopant profi les are a critical 

factor determining the performance of a transistor, 

dopant profi ling has always been needed by the 

silicon integrated circuit industry. One-dimensional dopant profi les from SIMS or electrical 

techniques remain an important process control 

tool. As transistors are scaled to ever-smaller 

dimensions, the variation of dopant profi les in 

two- and three-dimensions also begin to infl uence 

device operation. Two- and three-dimensional 

dopant profi les are now needed to validate models 

of the processes used to produce ultra-shallow 

junctions and for accurate device simulations.

SIMS is most likely to provide the solution to precision requirements for 1-D dopant concentration 

measurements. These goals can be achieved by 

careful control of SIMS depth-profi ling conditions and by developing and making available 

implant reference materials for common dopant 

elements. 

The ITRS identifies structural and elemental 

analysis at devices dimensions (for example 3-D 

dopant profi ling) as one of the diffi cult challenges 

beyond 2009. Offl ine secondary ion mass spectroscopy has been shown to provide the needed 
80 National Institute of Standards and Technology 

precision for current generations including ultrashallow junctions. Two- and preferably threedimensional profi ling is essential for achieving 

future technology generations. Activated dopant 

profi les and related TCAD modeling and defect 

profi les are necessary for developing new doping 

technology. The ITRS requirements are for atline dopant profi le concentration measurements 

with precision of 4 % in 2005, decreasing to 2 % 

precision for the 2010 through 2018 timeframe. 

The lateral/depth resolutions for 2-D/3-D dopant 

profi ling decrease from 3.5 nm in 2005 to 1.5 nm 

in 2012. Complete specifi cations are given for the 

short term in Table 120a on pages 12-13, and for 

the long term in Table 120b on pages 14-15 of the 

2006 Update to the Metrology section. The need 

for advances in image and spectral modeling for 

TEM and STEM applied to 3-D atomic imaging 

and spectroscopy is discussed under Emerging 

Research Materials and Devices in the Metrology 

section of the 2005 ITRS.

TECHNICAL STRATEGY

The 2006 Update to the ITRS expressed as diffi cult challenges: “starting materials metrology 

and manufacturing metrology are impacted by 

the introduction of new substrates such as SOI. 

Impurity detection (especially particles) at levels 

of interest for starting materials and reduced edge 

exclusion for metrology tools. CD, fi lm thickness, and defect detection are impacted by thin 

SOI optical properties and charging by electron 

and ion beams,” and “measurement of complex 

material stacks and interfacial properties including physical and electrical properties” (page 2, 

Table 116, Metrology Section). Our focus areas 

include development of refi ned metrology methods and standards for SIMS and TEM, developing 

improved X-ray detection capabilities for SEMs 

and electron microprobes, and the characterization 

of high-κ metal gate interfaces, including band 

offsets and barrier heights.

STRUCTURAL AND ELEMENTAL ANALYSIS

AT THE DEVICE LEVEL, INCLUDING

SIMS AND HRTEM

Secondary ion mass spectrometry (SIMS) has 

demonstrated the capability to meet the ITRS 

dopant profiling requirements for B, As, and 

P. However, the detailed analytical protocols 

required to achieve these goals have not been 

completely specifi ed. We have organized an 

international round robin study through ISO 

committee TC201 to investigate the parameters 

that must be controlled to make highly repeatable 

dose measurements of As with SIMS instruments. 

In addition to improved repeatability for dopants, 

the ITRS roadmap also requires increased SIMS 

detection limits for trace metal and organic contamination analysis of semiconductor devices. We 

are also working on novel methods to enhance 

detection limits for common metal contaminants 

by increasing the ionization effi ciency during the 

SIMS sputtering process. 

DELIVERABLE: Measure detection limits of selected 

trace metals on silicon using high transmission/high 

mass resolution SIMS system. 2Q 2007 

Analysis of trace metal and organic contamination on silicon surfaces is a high priority of the 

ITRS roadmap. To utilize effectively tools such 

as secondary ion mass spectrometry for trace 

contamination on silicon surfaces, suitable trace 

standards must be developed. Over the past 20 

years, piezoelectric drop-on-demand ink-jet printing has evolved into a precision microdispensing 

technology with a diverse range of applications. 

Examples of applications include desktop color 

printers, printing and synthesis of DNA arrays and 

printing of molten solder for use as electrical interconnects on integrated circuits. We are exploring 

the possibility of using ink-jet technology to print 

elemental and organic contamination standards 

on silicon. Piezoelectric printers are capable of 

printing single microdrops of fl uid at the rate of 

thousands of drops per second. Each drop contains 

a known concentration of the material of interest. 

Large concentration ranges are possible simply by 

varying the number of drops printed. Our fi rst attempts will explore ink-jet printing of organic test 

dyes on silicon with subsequent characterization 

by SIMS. Once standard operating procedures 

are developed for ink-jet printing, it should be 

feasible to produce standards, (both organic and 

trace metal) for quality control and calibration of 

a variety of analytical techniques including SIMS, 

XPS, AES, EPMA, TXRF and others. 

DELIVERABLES: Develop methods for depositing 

known amounts of organic and inorganic reference 

materials on silicon surfaces using inkjet printing technology. 2Q 2007 

As integrated circuit dimensions shrink to the 

sub-micrometer regime, there is continued need 

for accurate and quantitative dopant depth profi ling with ultra-high-depth resolution. To probe 

shallow dopant profi les in Si and other materials, 

SIMS instruments typically utilize very low en-
Semiconductor Microelectronics and Nanoelectronics Programs 81

ergy primary ion beams to bombard the sample 

surface. In this case, it is diffi cult to obtain a 

well-focused and high current density beam, 

especially in a magnetic sector SIMS instrument. 

Recently, there has been growing interest in using 

molecular ion beams for depth profi ling. When a 

molecular primary ion beam impacts the surface, 

it dissociates into its constituent atoms with each 

atom retaining a fraction of the initial energy of 

the cluster. This process can lead to impact energies on the order of a few 10’s of electron volts 

and a corresponding reduction in the depth of 

penetration of the primary ion. This process may 

potentially allow for ultra high resolution depth 

profi ling. In this project, we will utilize C60

+

 and 

Bi3

+

 cluster primary ion beam sources at NIST to 

sputter depth profi le Si, GaAs, SiC, and multiple 

delta-layers test materials. 

Some thin-fi lm materials such as metals do not 

sputter as uniformly as silicon under ion bombardment. In these cases, the achievable depth resolution is limited not by the penetration depth of the 

primary ion but by the topography induced by the 

sputtering process itself. We will also explore the 

use of cluster bombardment SIMS for reduction 

of sputter induced topography in metal fi lms. This 

approach will be applied to study depth profi ling 

analysis of gold diffusion in copper and the depth 

distribution of blanket metals fi lms (copper metallization on silicon).

DELIVERABLES: Evaluate possible applications of a 

Bi cluster ion source for ultrashallow depth profi ling. 

2Q 2007

Measurement of the barrier height and determination of the band structure is not straightforward. 

Recent work by numerous researchers has shown 

that the measured band offset between a metal gate 

electrode and high-κ gate dielectric is dependent 

on numerous factors including composition, structure, and thickness of both the metal gate electrode 

and the high-κ dielectric. Because of limitations 

associated with any single technique, we believe 

that determination of band offsets and work functions requires the use of an array of techniques 

and the broad expertise available at NIST. We will 

focus our efforts on the following measurements: 

standard Capacitance-Voltage (C-V) and tunneling current-voltage (I-V) measurements, internal 

photoemission, Kelvin probe, and soft X-ray and 

inverse photoelectron spectroscopy. 

DELIVERABLES: Improve effi ciency of internal photo 

emission system and demonstrate performance on 

metal/high-κ GaAs structure to determine band offsets. 

2Q 2007

New technologies being considered when silicon 

technology can no longer be scaled include alternative high mobility substrates including GaAs 

and InGaAs. Unfortunately, there is no high quality gate dielectric like silicon dioxide that can be 

directly grown on the substrate. Research is being 

conducted to study materials systems using a highκ gate dielectric material deposited directly on the 

GaAs or InGaAs substrate. Innovative interfacial 

chemistry is required to provide a good interface 

free from defects and trapping states. We are 

using internal photo emission and spectroscopic 

ellipsometry to study optically active defects at 

the interface after various chemical treatments. 

Electrical test structures are being fabricated to 

correlate the optical results with electrical characterization.

DELIVERABLES: Prepare GaAs surface and deposit 

high-κ fi lms by NIST-CSTL ALD processes and conduct optical characterization. 4Q 2007

ACCOMPLISHMENTS

IMPLEMENTATION OF C60

+ CLUSTER ION

SIMS CAPABILITY 

Previous efforts with cluster ion sources used for 

analyzing both organic and inorganic materials 

have been very successful. Minimization of beaminduced damage in organic materials has allowed 

depth profi ling of polymers such as photoresists 

and enhanced ion yields for high-molecular weight 

fragments. Inorganic material analysis has benefi tted in the area of ultra-shallow depth-profi ling as 

well as for analysis of some particularly diffi cult 

systems such as metal multi-layers stacks. We 

have investigated the use of a commercially available C60

+

 ion source on the NIST magnetic sector 

SIMS instrument. We have produced stable ion 

beams of C60

+

 and C60

2+ with typical currents approaching 20 nA under conditions that allow for 

several hundred hours of operation. The beam 

can be focused into a spot size of ~1 μm allowing 

micrometer spatial scale mapping of patterned wafers. Optimal experimental conditions have been 

defi ned to allow for depth profi ling analysis of 

silicon wafer samples, delta doped structures and 

metal multilayers. One of the critical issues for 

optimization of this source for analysis of silicon 

wafer samples is the primary ion impact energy 

of the C60

+

. As shown in Fig. 1 (next page), we 

have found that at impact energies below ~12 keV, 
82 National Institute of Standards and Technology 

carbon deposition is the dominant process during 

bombardment precluding the acquisition of depth 

profi les from the wafer sample. Successful depth 

profi les are only achieved at the impact energies 

exceeding 12 keV. The deposition effect may be 

useful for lithographic applications as it allows 

direct ion beam writing of a conductive carbon 

layer on silicon. 

Figure 1. Sputtering or deposition rate for C60

+ on Si 

as a function of impact energy (angle of incidence in 

parentheses).

DEPTH PROFILING OF ORGANIC

OVERLAYERS USING SIMS

Organic photoresists and low-κ dielectric materials are key components for front end semiconductor processing. There is also a growing interest 

in the use of organic semiconductor materials for 

organic light emitting diodes and organic thin fi lm 

transistors. In anticipation of a growing need for 

metrology tools to characterize these types of 

materials, we are developing new approaches 

to characterize the chemical composition and 

in-depth distribution of organic thin fi lms on 

silicon. Typically, the use of ion beam sputtering 

techniques, such as secondary ion mass spectrometry (SIMS), results in extensive chemical degradation of organic thin fi lms such as photoresists 

or organic light emitting diodes. However, we 

have found that cluster primary ion bombardment SIMS can minimize this degradation allowing for intact characteristic ions to be obtained 

throughout the depth of the fi lm. Furthermore, it 

appears that analyzing these organic materials at 

cryogenic temperatures provides further reduction 

in beam-induced damage. In this project, an SF5

+

polyatomic primary ion source was used to depthprofi le poly(methyl methacrylate) (PMMA) photoresist by SIMS at a series of temperatures from 

-75 °C to 150 °C. The depth profi le characteristics 

(e.g., interface widths, sputter rates, damage cross 

sections, and overall secondary ion stability) 

were monitored as a function of temperature for 

atactic, syndiotactic and isotactic PMMA. At 

low temperatures it was found that the secondary 

ion stability increased considerably. In addition, 

the interfacial widths were signifi cantly smaller. 

Examples of this increased stability at low temperatures for syndiotactic and isotactic PMMA 

are illustrated in Fig. 2 for the PMMA fragment at 

m/z = 69. Corresponding AFM images indicated 

that there was also decreased sputter-induced topography formation at these lower temperatures 

as illustrated in Fig. 3. Higher temperatures were 

typically correlated with increased sputter rates. 

However the improvements in interfacial widths 

and overall secondary ion stability were not as 

prevalent as was observed at low temperatures. 

The importance of the glass transition temperature 

(Tg

) on the depth profi le characteristics was also 

apparent. The sputter properties of isotactic and 

syndiotactic PMMA differed greatly. Isotactic 

Figure 2. Intensity of m/z = 69 (fragment 

characteristic of PMMA) as a function of increasing 

SF5

+ dose for syndiotactic and isotactic PMMA at 

three different temperatures.
Semiconductor Microelectronics and Nanoelectronics Programs 83

PMMA showed sharper interface widths and better depth profi le characteristics from 0 °C to 65 °C 

over those observed for syndiotactic or atactic 

PMMA. The results of this study demonstrate that 

it is possible to monitor the chemical composition 

of photoresist thin layers on silicon by analyzing 

the samples at cryogenic temperatures. This work 

has resulted in collaborations with International 

Sematech to prepare thin fi lms of a PMMA photoresist on silicon that will be used as standards 

for compositional depth profi ling. 

Figure 3. Atomic Force Microscopy (AFM) 

topography images (1 μm x 1 μm area) of SF5

+

sputtered crater bottoms at different temperatures: 

(a) PMMA surface, Rrms = ~0.85 nm, (b) 25 °C, Rrms

= ~11.37 nm, (c) -75 °C, Rrms = ~0.274 nm, and 

(d) 125 °C, Rrms = ~1.00 nm.

ment repeatability compared with Si2

-

; and (c) the 

observation that point-by-point normalization can 

extend the linearity of SIMS response for arsenic 

in silicon beyond 1x1016/cm2

.

Fifteen SIMS laboratories participated in this 

study, as well as two laboratories that performed 

Low-energy Electron-induced X-Ray Emission Spectrometry (LEXES) and one that made 

measurements by Instrumental Neutron Activation Analysis (INAA). The labs were asked to 

determine the implanted arsenic doses in three 

unknown samples using as a comparator Standard 

Reference Material 2134, with a certifi ed dose 

of 7.33x1014 atoms/cm2

. The use of a common 

reference material by all laboratories resulted in 

much better interlaboratory agreement than was 

seen in the previous round-robin that lacked a 

common comparator. The relative standard deviation among laboratories was less than 4 % for 

the medium-dose sample, and somewhat larger 

for the low- and high-dose samples (see Fig. 4). 

The high-dose sample showed a signifi cant difference between point-by-point and average matrix 

normalization because the matrix signal decreased 

in the vicinity of the implant peak, as previously 

observed. The average dose from point-by-point 

normalization was in close agreement with that 

determined by INAA, indicating that the SIMS 

relative sensitivity factor approach is valid for 

arsenic concentrations in silicon as high as 4 

atom percent.

Figure 4. As round robin results for medium dose 

sample with point-by-point normalization to matrix 

signal. SIMS lab results shown in red with error 

bars indicating within-lab repeatability. Relative 

standard deviation among labs is 3.9 %. Also shown 

are LEXES results (blue dots), average of SIMS lab 

results (red line), and NAA result (black dashed line).

ROUND-ROBIN STUDY OF ARSENIC

IMPLANT DOSE MEASUREMENT IN

SILICON BY SIMS

An international round-robin study was undertaken under the auspices of SIMS subcommittee 

SC 6 of International Organization for Standardization Technical Committee TC 201 on Surface 

Chemical Analysis. The purpose of the study 

was to determine the best analytical conditions 

and the level of interlaboratory agreement for 

the determination of the implantation dose of 

arsenic in silicon by SIMS. Motivations for this 

study were: (a) the relatively poor interlaboratory 

agreement that was observed in a previous roundrobin study before a certifi ed reference material 

had become available; (b) the observation that 

the use of Si3

-

 as a matrix species combined with 

AsSi- detection may result in improved measure-
84 National Institute of Standards and Technology 

ENHANCED PERFORMANCE OF

MICROCALORIMETER X-RAY DETECTOR

The NIST transition edge sensor (TES) microcalorimeter X-ray detector is a versatile instrument 

that combines the high resolution of wavelengthdispersive detectors with the extended range of 

energy-dispersive detectors. The detector is a 

small calorimeter operated at a low temperature 

(70 mK). The temperature change caused by the 

absorption of single X-ray photons is registered 

by a current through the transition edge sensor 

— a thin metal fi lm that undergoes a superconducting-normal transition at the operating point. 

Our present detector has demonstrated a FWHM 

resolution of 4.4 eV at 5.9 keV. The technology 

of microcalorimeters is more complicated than 

most X-ray detectors, so that a new set of operating considerations has emerged in the process of 

developing it as an analytical instrument. 

We report here the fi rst operation of the NIST 

TES microcalorimeter X-ray detector to perform 

quantitative analysis of a sample with an electron 

probe. The detector element is mounted at the 

end of a probe projecting from the dewar into 

the electron microscope. The probe extends 

horizontally, which requires a sample mounting 

angle of 45° to optimize the yield with a vertical 

electron beam. The sample was analyzed using 

a microbeam probe with a beam current of 7 nA 

at an energy of 12 keV. 

The detector is a microcalorimeter element 

400 μm square, located approximately 30 mm 

from the source of the radiation. Its energy range 

is 400 eV to 7 keV. Count rates of 50/s to 170/s 

were registered while taking spectra of the sample 

and related standards. Spectra were processed 

using conventional analog pulse amplifi ers and 

pulse-pileup circuitry modifi ed for the relatively 

long period of the microcalorimeter pulses. The 

pulses were recorded with a 16K channel multichannel analyzer board with an approximate 

resolution of 0.5 eV/channel. All spectra were 

obtained in 1000s live time.

The sample was the K-411 glass which is part 

of the NIST Standard Reference Material 470, 

Mineral Glasses for Microanalysis, a standard 

intended for use with electron probe microanalysis 

and SIMS. It contains previously analyzed quantities of SiO2

, MgO, FeO, and CaO. Spectra were 

also taken of reference standards consisting of Fe, 

SiO2, MgO, and chloroapatite [Ca5(PO4)3Cl]. A 

spectrum of K-411 glass used in the quantitative 

analysis is shown in Fig. 5.

Figure 5. Spectrum of the K-411 Standard 

Reference Material glass taken with the NIST TES 

microcalorimeter X-ray detector excited by an 

electron beam.

Calculations were carried out using a Monte Carlo 

procedure which varied the composition of the 

respective elements. The maximum deviation 

from the concentrations certifi ed in the NIST SRM 

in the fi rst round of analysis is 2 %. The work 

demonstrates that the NIST TES microcalorimeter 

detector has considerable potential for the analysis 

of X-ray fl uorescence lines which would overlap 

in current energy-dispersive detectors or would be 

too low in energy for most wavelength-dispersive 

detectors.

INKJET PRINTING FOR TRACE METAL

CONTAMINATION STANDARDS

The feasibility of using piezoelectric drop-ondemand inkjet printing technology to produce 

trace metal contamination standards on silicon wafers has been demonstrated. Prototype standards 

of Fe and Cu contamination with surface concentrations between 1013 atoms/cm2

 and 1015 atoms/

cm2 have been produced. Figure 6 shows SIMS 

secondary ion images of prototype standards 

for iron produced by printing different numbers 

of drops at a single spot. Concentrations range 

from approximately 1013 atoms/cm2 for one drop 

Figure 6. Secondary ion images from 1, 5, 10, 25, 50, 

and 100 drops of Fe solution in a spot. Scale bar is 

250 μm.
Semiconductor Microelectronics and Nanoelectronics Programs 85

Figure 7. LACSBI with EFTEM imaging of a FinFET. 

See text for details.

per spot to approximately 1015 atoms/cm2 for 100 

drops per spot. SIMS measurements show that 

the secondary ion intensity is linear with respect 

to concentration over the concentration ranges 

investigated in this initial study. The success 

of these preliminary experiments has prompted 

further work to prepare other elements in addition 

to Fe and Cu and at concentrations ranging from 

107

 atoms/cm2 to 1015 atoms/cm2.

LACSBI: A TECHNIQUE FOR REMOVING

UNWANTED DIFFRACTION EFFECTS FROM

TEM-BASED CHEMICAL IMAGES

As front end devices move from the familiar structures of conventional CMOS toward new materials and complex 3-D geometries, there will be an 

increasing need for fl exible metrology solutions at 

the nanometer length scale. Energy-fi ltered transmission electron microscopy (EFTEM) provides 

a method for rapid spectroscopic characterization 

with nanometer resolution, suitable for both conventional 2-D and tomographic (3-D) chemical 

imaging. However, the extraction of quantitative chemical images with EFTEM is limited by 

the susceptibility of all convention transmission 

electron microscopy (CTEM) to diffraction effects in crystalline specimens: coherent scattering 

contrast that complicates the interpretation of the 

[incoherent] chemical contrast. 

This effect is illustrated in Fig. 7 with an EFTEM 

spectral image reconstruction of a FinFET device 

structure. Here, the distinct colors correspond 

to amorphous silicon oxide (red), silicon nitride 

(green) and hafnium oxide (blue) phases, as well 

as the single-crystal silicon fi n (cyan). Only the 

silicon phase will exhibit diffraction effects. At 

conventional fi xed beam imaging conditions (a), 

the variation in intensity in the fi n from its base 

(bright) to its tip (dark) is actually the superposition of a <011> diffraction pattern on the chemical 

image. Effectively, the measured silicon concentration would be high in the bright regions and low 

in the dark regions, relative to that measured in the 

amorphous phases. A comparable image with the 

microscope operated in large angular convergence 

scanned beam illumination (LACSBI) mode (b), 

fi lters out this diffraction contrast. Since all other 

phases are amorphous, for which no diffraction 

effects are expected, this application provides a 

“null” experiment for identifi cation of potential 

artifacts with LACSBI. No visible loss in spatial 

resolution is evident in the comparison of Fig. 7 

(a), (b), and (c), and a line trace from the LACSBI 

image (c) shows a spatial resolution of ~2 nm and 

normalized phase fractions that sum to 99.4 % ± 

1.1 % along the line trace.

LACSBI mitigates diffraction contrast by averaging the data acquisition over many incident beam 

orientations, thus providing a largely incoherent 

image, as required for quantitative analysis. As 

important as this signal incoherence is for quantitative 2-D imaging and compositional profi les, 

it is essential for tomographic reconstruction of 

3-D chemical images, since the incoherent signal 

obeys the “projection” requirement for all existing 3-D reconstruction methods from tilt series of 

images. Thus LACSBI provides a breakthrough 
86 National Institute of Standards and Technology 

in nm-scale metrology for the complex 3-D 

architectures that are expected for nonconventional CMOS and post-CMOS devices.

This study is a collaboration with Brendan Foran 

of Aerospace Corp.

CHARACTERIZATION OF THE HIGH-κ

METAL GATE BARRIER HEIGHT USING

INTERNAL PHOTO EMISSION (IPE)

Metal gates on high-κ dielectrics are being actively searched to replace the traditional poly-Si 

gate / SiO2 / Si (MOS) structure for the next 

advanced CMOS device generations. In the selection of an appropriate metal gate and high-κ

dielectric, one of the most important properties 

of the selected materials that needs to be considered is the barrier heights (Φ) at their interfaces. 

Internal Photoemission (IPE) is a direct method 

to measure Φ. In addition, IPE can give insights 

into other properties of the fi lms such as internal 

fi eld effects, interface local charges, fi xed charge 

scattering, trappings.

Recently, in a collaboration with SEMATECH, 

we employed IPE to determine the barrier heights 

of two ternary metals of great interest for next 

generation of advanced CMOS: TaSiN and TaN. 

These metals were deposited on a single dielectric SiO2

 or HfO2 layer or a stack of SiO2/HfO2

with various SiO2 thicknesses. For the single 

dielectric layer, it was found that Ф0 = 3.36 

eV and 3.54 eV for TaSiN/SiO2 and TaN/SiO2

interfaces, respectively, and Ф0 = 2.24 eV and 

2.47eV for TaSiN/HfO2 and TaN/HfO2 interfaces, 

respectively. It is observed that the difference in 

the barrier heights of both metals is about 0.2 eV 

on either of the dielectrics. This indicates that no 

Fermi pinning is evident at the HfO2 interfaces. 

These results are in a very good agreement with 

the work function determined from electrical CV 

measurements.

For the more complex structure, TaSiN or TaN 

on SiO2

/HfO2 stacks, however, Fermi level pinning is observed when SiO2

 is inserted between 

HfO2 and Si substrate. Figure 8 shows a typical 

photoemission yield for TaN / (HfO2-SiO2 stack) 

structure biased at +3.0 and -3.0 Volt . Clearly, at 

these biases, the effective barrier heights at TaN / 

(HfO2-SiO2 stack) interface and (HfO2-SiO2 stac) / 

Si interface are 2.49 eV and 3.18 eV, respectively. 

In addition to the barrier height, IPE yield also 

provide other optical features such as critical 

interband transitions, i.e., E1

 and E2 of Si. The 

barrier height at zero internal fi eld (Ф0) is thus 

determined by using Schottky plot of the linear 

relationship of the barrier height and square-root 

of internal fi eld (F1/2). Figure 9 shows the Shottky 

plots for various SiO2 thicknesses. As a result, Ф0

at the metal gate and HfO2-SiO2 stack was found to 

pin at 2.5 eV for both metals. At the interface of 

HfO2-SiO2 stack and silicon substrate, Ф0 = 4.5 eV. 

Furthermore, between both metal gates and the 

dielectric stack, the barrier heights appear to be insensitive to the applied fi eld strength (see Fig. 9). 

It’s speculated that it could possibly caused by the 

Figure 9. Schottky plots of TaSiN/ HfO2-SiO2 /Si 

and TaN/ HfO2-SiO2 /Si stacks for various SiO2

thicknesses. Ф0’s at both the TaSiN and TaN gates 

and HfO2-SiO2 interfaces are found to pin at 2.5 eV 

for both metals, bottom curve. At the interface of 

HfO2

-SiO2 stack and silicon substrate, Ф0 = 4.5 eV, 

top curve. Between both metal gates and the 

dielectric stack, the barrier heights appear to be 

insensitive to the applied fi eld strength. It is possible 

that a fi xed plane charges existing near the interface 

give rise to a Ф1

 potential which is independent of the 

external applied fi eld (inset) or it is due to the higher 

permittivity of the HfO2 layer.

Figure 8. Barrier heights at TaSiN/HfO2

-SiO2 (red 

curve) and HfO2-SiO2/Si (blue curve) interfaces 

determined by intertal photoemission at a typical bias 

of 3.0 V and -3.0 V, respectively.
Semiconductor Microelectronics and Nanoelectronics Programs 87

fi xed plane charges near the interface. It’s been 

theoretically simulated that the barrier height 

could be controlled by the Ф1 potential of charged 

plane located at a distance x1

 from the interface 

(see Fig. 9 inset).

FILM THICKNESS DEPENDENCE OF SUBBANDGAP DEFECT STATES OBSERVED IN

POLYCRYSTALLINE HAFNIUM OXIDE

In collaboration with IBM we compare hafniumbased high-κ dielectric fi lms grown by MOCVD 

and ALD. MOCVD-grown HfO2 fi lms are mostly 

monoclinic, while HfSiO fi lms are amorphous. 

Thin ALD-grown HfO2 fi lms are amorphous, 

while thick fi lms are monoclinic, with traces of 

orthorhombic or tetragonal phases present. We 

note that sub-bandgap states may be the underlying cause for gate leakage via Frenkel-Poole 

hopping. The addition of Si to HfO2

 reduces the 

tendency for crystallization, mitigating such issues. However, such sub-bandgap states likely 

will not be a limiting factor in high-κ based CMOS 

technologies, since they line up close to the band 

edge and are therefore not accessible at the low 

gate voltages employed. 

COLLABORATIONS

International SEMATECH, ATDF – Thin oxide depth-profi ling by SIMS, backside depth profi ling of patterned PMOS 

wafers

Ionoptika – Development of a C60

+

 primary ion source for 

advanced semiconductor technology

MicroFAB Inc – Advanced inkjet printing technology for 

deposition of trace metal standards on silicon surfaces

Peabody Scientifi c – Ion source development for Semiconductor SIMS

Micron Semiconductor – trace organic detection

Aerospace Corporation – TEM-based chemical imaging

SEMATECH, Characterization of metal gate/high-κ systems. 

University of Maryland, College Park, Ultra-thin gate oxide 

reliability.

MSEL, Characterization of metal gate/high-κ systems.

IBM, Electrical characterization of high-κ systems.

Intel, Electrical characterization of high-κ systems.

Texas Instruments, Electrical characterization of high-κ

systems.

Rutgers University, Characterization of metal gate/high-κ

systems.

Micron, Characterization of metal gate/high-κ systems.

Yale University, Electrical characterization of high-κ systems. 

RECENT PUBLICATIONS

D. S. Simons, R. G. Downing, G. P. Lamaze, R. M. Lindstrom, R. R. Greenberg, R. L. Paul, S. B. Schiller and W. F. 

Guthrie, “Development of Certifi ed Reference Materials of 

Ion-Implanted Dopants in Silicon for Calibration of Secondary Ion Mass Spectrometers,” J. Vac. Sci. Technol. (2007), 

submitted for review.

K. J. Kim, D. W. Moon, C. J. Park, D. Simons, G. Gillen, H. 

Jin and H. J. Kang, “Quantitative Surface Analysis of Fe-Ni 

Alloy Films by XPS, AES and SIMS,” Surf. .Interface Anal. 

(2007), accepted for publication.

K. J. Kim, D. Simons and G. Gillen, “Quantitative Depth 

Profi ling of an Alternating Pt/Co Multilayer and a Pt-Co Alloy Multilayer by SIMS Using a Buckminsterfullerene (C60 ) 

Source,” Appl. Surf. Sci. 253 (2007) 6000-6005.

C. M. Mahoney, A. J. Fahey and G. Gillen, “Temperature-controlled Depth Profi ling of Poly(methylmethacrylate) (PMMA) 

Using Cluster Secondary Ion Mass Spectrometry (SIMS): I. 

Investigation of depth profi le characteristics,” Anal. Chem. 

79, 828-836 (2007).

C. M. Mahoney, A. J. Fahey, G. Gillen, C. Xu and J. D. 

Batteas, “Temperature-controlled Depth Profiling of 

Poly(methylmethacrylate) Using Cluster Secondary Ion Mass 

Spectrometry (SIMS): II. An investigation of sputter-induced 

topography, chemical damage and depolymerization effects,”

Anal. Chem. 79, 837-845 (2007).

T. Jach, N. Ritchie, J. Ullom, and J. A. Beall, “Quantitative 

Analysis with the Transition Edge Sensor Microcalorimeter 

X-ray Detector,” Advances in X-ray Analysis 50 (2007), accepted for publication.

D. S. Simons et al., “Round Robin Study of Arsenic Implant 

Dose Measurement by SIMS,” Appl. Surf. Sci. 252, 7232-

7235 (2006).

G. Gillen et al., “Depth Profi ling using C60

+ SIMS- Deposition and Topography Development During Bombardment of 

Silicon,” Appl. Surf. Sci.252, 6521-6525 (2006).

G. Gillen, C. Mahoney, M. Wagner and A. Fahey, “3-D 

Molecular Imaging SIMS,” Appl. Surf. Sci. 252, 6537-6541 

(2006).

A. Fahey, G. Gillen, P. Chi and C. Mahoney, “Performance 

of a C60 Ion Source on a Dynamic SIMS Instrument,” Appl. 

Surf. Sci. 252, 7312-7314 (2006).

C. M. Mahoney, A. J. Fahey and G. Gillen, “Temperature-Controlled Depth Profi ling in Polymeric Materials using Cluster 

Secondary Ion Mass Spectrometry (SIMS),” Appl. Surf. Sci. 

252, 6502-6505 (2006).

D. S. Simons, “Summary of ISO/TC 201 Standard: XIII. ISO 

18114:2003 — Surface chemical analysis — Secondary-ion 

mass spectrometry — Determination of relative sensitivity factors from ion-implanted reference materials,” Surf. Interface 

Anal. 38, 171-172 (2006).

J. Ehrstein, C. Richter, D. Chandler-Horowitz, E. Vogel, 

C. Young, S. Shah, D. Maher, B. Foran, P. Y. Hung, and A. 

Diebold, “A Comparison of Thickness Values for Very Thin 

SiO2

 Films by Using Ellipsometric, Capacitance-Voltage, and 

HRTEM Measurements,” Journal of the Electrochemical Soc., 

vol. 153, p. F12, 2006.
88 National Institute of Standards and Technology 

T. Jach, J. A. Small, and D. E. Newbury, “Improving Energy 

Stability in the NIST Microcalorimeter X-ray Detector,” Powder Diffraction 20, 134 (2005).

E. Windsor, G. Gillen, D. Bright, P. Chi, A. Fahey and J. 

Batteas, “Techniques for Improving SIMS Depth Resolution: 

C60

+ Primary Ions and Backside Depth Profi le Analysis,” 

Characterization and Metrology for ULSI Technology 2005, 

15-18 March 2005, Richardson, TX, USA, AIP Conference 

Proceedings no.788: 349-55.

K. J. Kim, D. W. Moon, P. Chi and D. Simons, “Development and Applications of Multiple Delta-Layer Reference 

Materials for Semiconductor Analysis,” J. Surf. Anal. 12, 

83-87 (2005).

D. S. Simons, P. H. Chi and K. J. Kim, “Quantitative Measurement of Arsenic Implant Dose by SIMS,” J. Surf. Anal. 

12, 157-160 (2005).

K. J. Kim, D. W. Moon, P. Chi and D. Simons, “Development 

of B-doped Si Multiple Delta-Layer Reference Materials for 

SIMS Profi ling,” Surf. Interface Anal. 37, 802-808 (2005).

M. S. Wagner, “Degradation of poly(acrylates) under SF5

+ 

primary ion bombardment studied using time-of-fl ight secondary ion mass spectrometry. 1. Effect of main chain and pendant 

methyl groups,” Surf. Interface Anal. 37, 42-52 (2005).

M. S. Wagner, “Degradation of poly(acrylates) under SF5

+ 

primary ion bombardment studied using time-of-fl ight secondary ion mass spectrometry. 2. Poly(n-alkyl methacrylates),” 

Surf. Interface Anal. 37, 53-61 (2005).

M. S. Wagner, “Degradation of poly(acrylates) under SF5

+ 

primary ion bombardment studied using time-of-fl ight secondary ion mass spectrometry. 3. Poly(hydroxyethyl methacrylate) 

with chemical derivatization,” Surf. Interface Anal. 37, 62-70 

(2005).

S. Sayan, N. V. Nguyen, and J. Ehrstein, J. J. Chambers, M. 

R. Visokay, M. A. Quevedo-Lopez, L. Colombo, D. Yoder, 

I. Levin, and D. A. Fischer, M. Paunescu, O. Celik, and E. 

Garfunkel, “Effect of nitrogen on band alignment in HfSiON 

gate dielectrics,” Appl. Phys. Lett. 87, 212905 (2005).

N. V. Nguyen, D. Chandler-Horowitz, A. Davydov, C. Hacker, 

S. Park, H. Xiong, J. Ehrstein, J. Kopanski, J. Suehle, E. Vogel 

and M. Frank, “Crystallinity and Defects in Hafnium Oxide 

and Hafnium Silicate Films,” High Dielectric Constant Gate 

Stacks III of the 208th Meeting of The Electrochemical Society 

in October 16-21, 2005.

N. V. Nguyen, Albert V. Davydov, Deane Chandler-Horowitz, 

Martin M. Frank, “Sub-bandgap defect states in polycrystalline hafnium oxide and their suppression by admixture of 

silicon,” App. Phys. Lett. 87 (2005).

N. V. Nguyen, S. Sayan, I. Levin, J. R. Ehrstein, I. J. R. Baumvol, C. Diremeier, C. Krug, L. Wielunski, P. Y. Hung, and Alain 

Diebold “Optical band gaps and composition dependence of 

hafnium-aluminate thin fi lms grown by atomic layer chemical 

vapor deposition,” J. Vac. Sci. Technol. A 23 (2005).

S. Sayan, N. V. Nguyen, and J. Ehrstein, T. Emge and E. 

Garfunkel, I. Levin, E. P. Gusev, Hyoungsub Kim and P. J. 

McIntyre, “Structural, Electronic, and dielectric properties 

of ultrathin zirconia fi lms on silicon,” Applied Physics Letter 

86, 152902 (2005).
Semiconductor Microelectronics and Nanoelectronics Programs 89

PLASMA PROCESS METROLOGY

GOALS

To provide advanced measurement techniques, 

data, and models needed to characterize plasma 

etching and deposition processes important to 

the semiconductor industry, enabling continued 

progress in model-based reactor design, process 

development, and process control.

CUSTOMER NEEDS

To fabricate future generations of devices, the 

semiconductor industry requires improvements in 

plasma etching and deposition processes. Plasma 

processes and equipment face increasingly stringent requirements due to the need to maintain 

high device yields at decreasing feature sizes, the 

introduction of new dielectric materials, and the 

constant pressure to keep production effi ciency 

high. To meet these challenges, the International 

Technology Roadmap for Semiconductors (ITRS, 

2006 update) identifi es a need for better, more 

predictive modeling of the impact of equipment 

on process results (Modeling and Simulation 

section, pages 6-7, Table 123a). To obtain more 

reliable predictions of the chemical, physical, 

and electrical properties of processing plasmas, 

the dependence of these properties on processing 

equipment, and the effect of these properties on 

process results, further progress in model development and validation is required. The ITRS also 

identifi es a need for development of robust sensors and process controllers (Metrology section, 

page 2, Table 116) which are able to convert large 

quantities of raw data into information useful for 

improving manufacturability and yield.

TECHNICAL STRATEGY

Our multifaceted program provides numerous outputs to assist our customers, including advanced 

measurement methods, high-quality experimental 

and fundamental data, and reliable, well-tested 

models of plasma behavior.

First, we develop and evaluate a variety of 

measurement techniques that provide industry 

and academia with methods to characterize the 

chemical, physical, and electrical properties of 

plasmas. The techniques we develop include 

improved laboratory diagnostic measurements for 

use in research and development, as well as more 

robust, non-perturbing measurements for use in 

process monitoring and control in manufacturing 

applications.

In addition to measurement techniques, we also 

provide data necessary for gaining an understanding of complex plasma properties and for testing 

and validating plasma models. The data help 

semiconductor manufacturers and plasma equipment manufacturers to better understand and control existing processes and tools and help them to 

develop new ones. The experimental data we provide are measured under well-defi ned conditions 

in highly-characterized standard plasma reactors. 

Our reactors include capacitively coupled cells as 

well as inductively coupled, high-density plasma 

reactors, one of which is shown in Fig. 1.

Figure 1. One of the inductive, high-density plasma 

reactors used in our experimental studies.

Finally, we are engaged in the development and 

validation of plasma models. Such efforts concentrate on modeling of plasma sheaths, the thin 

regions at the boundary of the plasma. Sheaths 

play a dominant role in determining discharge 

electrical properties and the properties of the 

highly energetic ions that are necessary for plasma 

etching. More accurate sheath models are needed 

to better predict and optimize discharge electrical 

characteristics and ion kinetic energies. Sheath 

models are also used to develop new types of 

process monitoring techniques based on radiofrequency electrical measurements.

Our ongoing and planned efforts focus on measurement, data, and modeling challenges in the 

following specifi c areas: 

1. Our recent efforts have focused on an 

electrical measurement technique developed at 

NIST for use in process monitoring and control 

Technical Contacts:

M. Sobolewski

K. Steffens

E. Benck

 

“NIST is one of the 

leaders in plasma 

processing related 

research in the U.S. 

They have capability to 

thoroughly understand 

plasma behavior using 

a variety of diagnostics 

tools.”

Peter Ventzek

TEL, Inc.

“The NIST plasma 

process metrology 

group has helped us 

to understand the 

fundamental physical 

and chemical processes 

that are important to 

electronics materials 

and semiconductor 

processing industries.”

Bing Ji,

Air Products and 

Chemicals, Inc.
90 National Institute of Standards and Technology 

applications. This technique relies on noninvasive, nonperturbing measurements of the 

radio-frequency (rf) current and voltage applied 

to plasma reactors. The rf measurements are 

compatible with commercial reactors and they 

contain valuable information about the fl ux and 

energy of the ions that bombard wafers during 

processing. Values for the total ion fl ux and ion 

energies are obtained by analyzing the current 

and voltage signals using electrical models of the 

plasma and its sheaths. To validate the technique, 

experiments in an rf-biased, inductively coupled 

plasma reactor have been performed both with 

and without silicon wafers loaded in the reactor. 

Plasma potentials, sheath voltages, total ion fl ux, 

and ion energy distributions obtained from the rf 

measurements have been compared against independent measurements and shown to be in good 

agreement. The technique has been used to monitor long-term drift in ion energy and total ion fl ux. 

We have also monitored the more rapid changes 

that occur when the pressure, power, and gas 

fl ow are perturbed in ways that mimic equipment 

faults. Small changes in ion energy and total ion 

fl ux that occur over the course of a normal oxide 

etch have also been monitored. Future efforts are 

directed toward demonstration of the usefulness 

of the technique in industrial plasma reactors, 

which may differ from our research reactors in 

several ways. First, industrial reactors are often 

equipped with electrostatic chucks, which contain 

an insulating layer that may have an electrical 

impedance large enough to signifi cantly affect 

discharge electrical characteristics. Second, to 

accommodate 300 mm wafers, industrial reactors 

require large electrodes, which draw rf currents 

that are signifi cantly larger than can be handled 

by our present rf current probes. Third, in many 

plasma etching applications, industrial inductively coupled plasma sources are not operated 

in the high-density, purely inductively coupled 

mode, but are instead operated in a lower-density 

mode that couples power into the plasma both 

inductively and capacitively. These issues will 

be investigated fi rst by experiments performed 

at NIST, in which we will modify our research 

reactors to more closely resemble industrial reactors. Additional tests, undertaken in collaboration 

with industrial partners, will then be performed in 

actual commercial reactors.

DELIVERABLES: 

Evaluate the validity and utility of rf measurement techniques, electrical models, and analysis 

techniques in a laboratory reactor equipped with an 

insulating chuck. 3Q 2007

•

Evaluate the accuracy of commercial high-current 

probes and their use in extending rf-based ion fl ux 

and ion energy monitoring to industrial-scale reactors. 1Q 2008

Perform validation tests of noninvasive, rf-based 

ion fl ux and ion energy monitoring in an inductively 

coupled reactor operated in the low-density, mixed 

capacitive-inductive mode. 2Q 2008

Perform validation tests of noninvasive, rf-based 

ion fl ux and ion energy monitoring in industrial, 

inductively coupled plasma reactors. 

4Q 2008

2. Dual-frequency, capacitively-coupled plasma 

reactors are becoming increasingly important in 

semiconductor manufacturing processes, however, 

there appear to exist only limited amounts of published data on these systems. Most experimental 

papers concentrate solely on the etching characteristics, and numerous theory papers on dual-frequency capacitive systems show few comparisons 

with actual experimental data. Several important 

fundamental questions about the operation of these 

systems are not completely understood. How 

does applying the two rf frequencies to a single 

electrode compare with applying the frequencies 

to separate electrodes? How does varying the two 

frequencies modify the plasma? How independent 

is the ion energy control and plasma production? 

These questions will be investigated using a 

combination of optical measurements (such as 

time-resolved optical emission spectroscopy), 

invasive probes (Langmuir probes, wave cutoff 

probes, and mass spectrometers equipped with 

ion energy analysis) and noninvasive electrical 

measurements. Efforts will be directed at obtaining a well-validated, complete model for the 

physical and electrical behavior of dual frequency 

discharges. In addition to providing useful insight 

and fundamental understanding of the operation of 

such systems, it would also allow more accurate 

computer simulations of prototype dual-frequency 

equipment and would enable the noninvasive rfbased process monitoring technique to be used 

in dual-frequency capacitively coupled plasma 

reactors.

DELIVERABLES: 

Develop model for ion fl ux and electrical characteristics of dual-frequency capacitively coupled 

plasmas and validate by optical and electrical 

measurements. 2Q 2009

Develop model for ion energy distributions in 

dual-frequency capacitively coupled plasmas and 

validate by mass spectrometer measurements. 

4Q 2009.

•

•

•

•

•
Semiconductor Microelectronics and Nanoelectronics Programs 91

ACCOMPLISHMENTS

The NIST-developed, noninvasive, modelbased electrical technique for monitoring ion energy and total ion fl ux has recently been validated 

in actual etching conditions in CF4

/Ar plasmas 

(Fig. 2). Unlike previous validations, performed 

with no wafer present, the recent validations were 

performed with silicon wafers — oxidized and 

bare — loaded into the reactor. The wafer, as well 

as the contact between the wafer and the electrode 

on which it rests, both contribute an electrical impedance which, if unaccounted for in the model, 

can cause errors in the ion energy distributions 

and total ion fl ux obtained from the noninvasive 

technique. At low rf bias frequencies < 100 kHz, 

the contributed impedance was large, resulting 

in substantial errors in the noninvasive results. 

Nevertheless, at bias frequencies of 1 MHz or 

higher, which are more typical of semiconductor 

manufacturing, the wafer and wafer contact contribute only a few ohms of impedance, resulting in 

an uncertainty in noninvasive ion energies of only 

a few electron volts. The speed of the analysis algorithms has also recently been greatly increased, 

making it possible to monitor changes in ion energy and total ion fl ux with a time resolution on 

the order of 1 second. In recent demonstrations, 

the speeded-up technique has been used to monitor small changes in ion energy and total ion fl ux 

that occur over the course of a “normal” oxide 

etch, as well as larger changes that occur when 

the pressure, power, and gas fl ow were perturbed 

in ways that simulate equipment faults. 

#

Figure 2. Ion energy distributions from noninvasive 

electrical measurements, determined in real-time 

during an oxide etch in an Ar/CF4

 plasma.

0 20 40 60 80 100

0

1

2

ion energy (eV)

ion flux (arbitrary units)

CF3+

Ar+

CF2+

CF+

In a collaboration with KRISS, the Korea 

Research Institute of Standards and Science, a new 

method for measuring electron number density in 

plasmas, the wave cut-off method, has recently 

been implemented in NIST laboratories. Unlike 

Langmuir probe measurements, which are commonly used for measuring electron density, cut-off 

measurements do not suffer from problems with 

rf compensation or deposition of insulator layers. 

Comparisons performed in one of our inductively 

coupled plasma reactors showed good agreement 

between electron densities measured by the cut-off 

probe and by a Langmuir probe. The accuracy of 

the cut-off probe was shown to be greater than the 

Langmuir probe. The cut-off probe has been used 

to characterize the spatial variation of the electron 

density in the inductively coupled reactor and the 

effect of rf substrate bias on the electron density. 

Interest in the bias effect has been stimulated by 

recently proposed, new methods for rf biasing, 

such as double-frequency bias and nonsinusoidal 

bias, which may contain Fourier components at 

frequencies higher than those previously used for 

biasing. Our measurements showed that even at 

bias frequencies as high as 30 MHz, the effect of 

bias on electron density is small, and therefore 

that nearly independent control of ion energy 

and ion fl ux can be maintained even at such high 

frequencies. Simple analytic models that describe 

the effect of bias on electron density, which would 

be useful for estimating such effects under other 

experimental conditions in other plasma reactors, 

have been derived and validated by the cut-off 

measurements. A manuscript describing these 

results has been written and will appear in print 

soon.

Fundamental data continue to be distributed to 

plasma modelers throughout industry and academia via the Web-based NIST “Electron Interactions with Plasma Processing Gases” database. 

This Web site has experienced tens of thousands 

of hits throughout its history.

RECENT PUBLICATIONS

M. A. Sobolewski, “Real-time, noninvasive monitoring of ion 

energy and ion current at a wafer surface during plasma etching,” J. Vac. Sci. Technol. A 24, 1892-1905 (2006).

M. A. Sobolewski, “Effects of wafer impedance on the monitoring and control of ion energy in plasma reactors,” J. Appl. 

Phys. 100, article #063310, 1-13 (2006).

M. A. Sobolewski, “Noninvasive monitoring of ion energy 

drift in an inductively coupled plasma reactor,” J. Appl. Phys. 

97, article #033301, 1-12 (2005).

#
92 National Institute of Standards and Technology 

K. L. Steffens and M. A. Sobolewski, “2-D imaging of 

temperature in CF4 plasmas,” IEEE Transactions on Plasma 

Science, 33, 370-371 (2005).

K. L. Steffens, M. A. Sobolewski, “2-D Temperature Mapping in Fluorocarbon Plasmas,” AIP Conference Proceedings, Vol. 788: Characterization and Metrology for ULSI 

Technology, (American Institute of Physics, Melville, NY, 

2005) pp. 333-337.

M. A. Sobolewski, “Noninvasive monitoring of ion energy 

in an inductively coupled plasma reactor,” AIP Conference 

Proceedings, Vol. 788: Characterization and Metrology for 

ULSI Technology, (American Institute of Physics, Melville, 

NY, 2005) pp. 319-323.

M. A. Sobolewski, “Monitoring sheath voltages and ion 

energies in high-density plasmas using noninvasive radiofrequency current and voltage measurements,” J. Appl. Phys. 

95, 4593-4604 (2004).

K. L. Steffens and M. A. Sobolewski, “A technique for 

temperature mapping in fl uorocarbon plasmas using planar 

laser-induced fl uorescence of CF,” J. Appl. Phys. 96, 71-81 

(2004).

M. Edamura and E. C. Benck, “Effects of voltage distribution along an induction coil and discharge frequency in 

inductively coupled plasmas,” J. Vac. Sci. Technol. A 22, 

293-301 (2004).

E. C. Benck, G. Y. Golubiatnikov, G. T. Fraser, B. Ji, S. A. Motika, and E. J. Karwacki, “Submillimeter-wavelength plasma 

chemical diagnostics for semiconductor manufacturing,” J. 

Vac. Sci. Technol. B 21, 2067-2075 (2003).

K. L. Steffens and M. A. Sobolewski, “Effect of changing the 

electrode gap on the spatial and electrical properties of O2

/CF4

plasmas,” J. Vac. Sci. Technol. A 21, 241 (2003).

E. C. Benck, Y. Wang, and A. Goyette, “Ion energy distribution 

and optical measurements in high-density, inductively coupled 

C4F6 discharges,” J. Appl. Phys. 94, 1382-1389 (2003).

M. Edamura and E. C. Benck, “Transition from E to H mode 

discharge in pulse-modulated inductively coupled plasmas,” 

J. Vac. Sci. Technol. A 21, 470-475 (2003).

M. A. Sobolewski, “Monitoring sheath voltages and ion 

energies in high-density plasmas using noninvasive radiofrequency current and voltage measurements,” in AIP Conf. 

Proc. 683, Proceedings of the 2003 International Conference 

on Characterization and Metrology for ULSI Technology, 

195-199 (2003).

E. C. Benck, G. Y. Golubiatnikov, G. T. Fraser, D. Pluesquelic, 

R. Lavrich, B. Ji, S. A. Motika, and E. J. Karwacki, “Submillimeter-wavelength plasma diagnostics for semiconductor 

manufacturing,” in AIP Conf. Proc. 683, Proceedings of the 

2003 International Conference on Characterization and Metrology for ULSI Technology, 190-194 (2003).
Semiconductor Microelectronics and Nanoelectronics Programs 93

INTERCONNECT AND PACKAGING METROLOGY

PROGRAM

Advances in interconnect and packaging technologies have introduced rapid successions of 

new materials and processes. A major new technology thrust over the past several years is 

the move to three dimensional integration. Environmental pressures have led to the reduction 

and elimination of lead in solder used for attaching chips to packages and packages to circuit 

boards. The overall task of this program is to provide critical metrology and methodology 

for mechanical, chemical, metallurgical, electrical, thermal, and reliability evaluations of 

interconnect and packaging technologies.

The function of packaging is to connect the integrated circuit to the system or subsystem 

platform, such as circuit board, and to protect the integrated circuit from the environment. 

The increasing number of input/output (I/O) on circuits with vastly larger scale of integration 

is forcing ever smaller I/O pitches, the stacking of chips with via hole interconnect, the use of 

fl ip chip bonding, and the use of intermediary platforms called interposers. The integration of 

sensors and actuators onto integrated circuits through MEMS technology and the increasing 

use of low cost integrated circuits in harsh environments is increasing the complexity of the 

packaging task. Environmental concerns are forcing the need for development of reliable 

lead-free solder and other low environmental impact packaging materials.

System reliability requirements demand modeling, testing methods, and failure analysis of 

the integrated circuits before and after packaging. Metrology is a signifi cant component of 

reliability evaluation.
94 National Institute of Standards and Technology 
Semiconductor Microelectronics and Nanoelectronics Programs 95

ATOMIC LAYER DEPOSITION – PROCESS MODELS

AND METROLOGY

Technical Contacts:

J. E. Maslar

D. R. Burgess, Jr.

W. S. Hurst

GOALS

Develop validated, predictive process models and 

in situ metrologies for atomic layer deposition 

processes.

CUSTOMER NEEDS

Atomic layer deposition (ALD) is increasingly 

being utilized as a method of depositing the thin 

(nanometer-scale), conformal layers required for 

many microelectronics applications, including 

high-κ gate dielectric layers, diffusion barrier 

layers, and DRAM dielectric layers. However, 

signifi cant developmental issues remain for many 

of these applications.

One potential solution to some ALD developmental issues is technology computer-aided 

design (TCAD). TCAD has been identified 

in the International Technology Roadmap for 

Semiconductors (ITRS) 2005 Edition as “one of 

the few enabling methodologies that can reduce 

development cycle times and costs.” [ITRS 2005 

Edition, Modeling and Simulation, page 1] A 

TCAD topical area identifi ed in the ITRS 2005 

Edition is “Equipment/feature scale modeling—

hierarchy of models which allows the simulation 

of the local infl uence of the equipment (except 

lithography) on each point on the wafer, starting from the equipment geometry and settings.” 

[ITRS 2005 Edition, Modeling and Simulation, 

page 1] A diffi cult challenge related to this topical area is “Integrated modeling of equipment, 

materials, feature scale processes and infl uence 

on devices” [ITRS 2006 Update, Modeling and 

Simulation, Table 122, page 2] with associated issues including “Fundamental physical data (e.g., 

rate constants, cross sections, surface chemistry 

for ULK, photoresists and high-κ metal gate); 

reaction mechanisms, and simplifi ed but physical 

models for complex chemistry” and ALD deposition modeling. [ITRS 2006 Update, Modeling and 

Simulation, Table 122, page 2] In addition, the 

2005 ITRS notes that “a key diffi cult challenge 

across all modeling areas is that of experimental 

validation.” [ITRS 2005 Edition, Modeling and 

Simulation, page 1] Further, with respect to experimental validation, “One of the major efforts 

required for better model validation is sensor 

development and metrology, especially for models 

predicting the fabrication and behavior of ultrathin fi lms.” [ITRS 2005 Edition, Modeling and 

Simulation, page 17] This project is an attempt to 

assist in solving some ALD developmental issues 

by developing validated, predictive process models and in situ metrologies for ALD processes.

TECHNICAL STRATEGY

This project involves two general directions of 

investigation: development of in situ metrologies 

sensitive to ALD chemistry and development of 

ALD chemical reaction mechanisms. These two 

directions are seen as mutually-supporting. It is 

expected that experimental results that elucidate 

ALD chemistry will aid in chemical mechanism 

development and ultimately in process model 

validation. Further, it is expected that important 

reaction species will be identifi ed as the understanding of a particular ALD reaction improves, 

thus facilitating the design of improved process 

metrologies. While these two directions will be 

closely coupled for development of a specifi c 

ALD chemical reaction mechanism, this will not 

preclude exploration of non-mutually-supporting aspects of the metrology development and 

model development directions. For example, 

fundamental thermochemical and chemical 

kinetic properties of numerous organometallic 

compounds potentially suitable for ALD are under 

investigation. However, it would not be feasible to 

simultaneously provide experimentally validated 

ALD process models for all compounds.

Various in situ diagnostics are being evaluated 

for use in characterizing gas phase and/or surface 

processes. Gas phase diagnostic development has 

focused on metrologies that are sensitive to gas 

phase chemistry, particularly gas phase chemistry 

that can be related to fi lm properties. Diagnostics 

that are sensitive to gas phase chemistry can be 

used to help optimize gas injection conditions 

rather than simply monitor precursor delivery. 

In addition, the results from such diagnostics 

will be more useful for process model development. ALD process models are being developed 

by incorporating the detailed chemical reaction 

mechanisms developed in the course of this project 

into commercially available computational fl uid 

dynamics (CFD) codes that simulate the gas fl ow 

and temperature fi elds in an ALD reactor. Experimental validation of the overall process model is 

accomplished by modeling the performance of a 
96 National Institute of Standards and Technology 

custom-built, research-grade ALD reactor with 

optimized optical accessibility and benchmarking the numerical results with experimental data. 

HfO2

 ALD using tetrakis(ethylmethylamino) hafnium (TEMAH) and water has been selected as the 

chemical system for primary investigation.

1. A number of diagnostics are being evaluated 

for sensitivity to ALD chemistry and integration 

into deposition systems. Mass spectrometry and 

optical spectroscopic techniques are of particular interest because of their proven potential for 

in situ monitoring. While sensors that are sensitive to gas phase species (e.g., mass spectrometry 

and Fourier-Transform infrared (FTIR) spectroscopy) are easier to integrate into commercial reactors (e.g., in the gas exhaust line), these techniques 

are only sensitive to volatile species. Hence, it is 

sometimes diffi cult to relate the species detected 

with such techniques to mechanisms of interest on 

the growth surface. Hence, both gas-phase-sensitive and surface-sensitive techniques are being 

evaluated to probe ALD chemistry. The suitability 

of Raman spectroscopy and FTIR spectroscopy 

for probing ALD surface processes under actual 

deposition conditions is being investigated. In 

addition, the suitability of mass spectrometry 

and FTIR spectroscopy for probing gas phase 

ALD processes and how best to relate gas phase 

species to important surface processes is being 

investigated. A research-grade ALD reactor with 

optimized accessibility for the various gas-phasesensitive and surface-sensitive techniques has 

been designed and constructed.

DELIVERABLES:

Complete evaluation of time-resolved, in situ FTIR 

measurements for probing HfO2

 ALD gas phase 

processes. 3Q 2007

Complete evaluation of mass spectrometry 

measurements for probing HfO2

 ALD gas phase 

processes. 4Q 2007

Optimize quantum cascade laser system for probing HfO2

 ALD gas phase processes. 4Q 2006

2. The calculation, estimation, and dissemination of fundamental thermochemical and chemical 

kinetic properties of organometallic compounds 

with potential application to ALD and chemical 

vapor deposition (CVD) is an important aspect 

of this project and is an ongoing process. The 

thermochemical properties and reaction kinetics 

of most useful organometallic compounds and 

related molecular precursors are poorly characterized. This project obtains these properties 

through three activities involving theoretical 

•

•

•

estimations and modeling studies. The fi rst data 

activity compiles and evaluates currently available 

thermochemical and chemical kinetic data for organometallic compounds and related precursors. 

The second activity supplements available data 

by using ab initio and semi-empirical quantum 

calculations coupled with transition state calculations to develop detailed chemical kinetic models 

from computed molecular structures, thermodynamic properties and spectroscopic properties of 

relevant compounds. The third activity utilizes 

the experimental and computed thermochemical 

and chemical kinetic data to develop detailed 

chemical kinetic models for the decomposition 

of organometallic precursors and deposition processes leading to thin fi lm growth.

DELIVERABLES: 

Perform high level ab initio and Density functional 

theory (DFT) quantum calculations for ALD and 

CVD organometallic and metal halide precursors 

and derivatives containing Al, Hf, Zr, Ga, and Ti 

atoms. Benchmark heats of formations using correlations with available experimental data in the 

literature. 2Q 2007

Develop an ALD mechanism for HfO2

 deposition 

from TEMAH and water and incorporate the mechanism into a two-dimensional CFD reactor model. 

4Q 2007

3. An effort is also being made to investigate 

the relationship between ALD reactor conditions 

and concomitant HfO2 fi lm properties. This relationship is being investigated by correlating the 

results of a variety of ex situ fi lm characterization 

measurements to reactor conditions as determined 

by in situ measurements and numerical modeling 

of the temperature and fl ow fi elds in the reactor. 

In situ measurement techniques include those 

techniques being developed for model validation. 

Ex situ measurement techniques include vacuum 

ultraviolet spectroscopic ellipsometry (VUV-SE), 

FTIR spectroscopy, X-ray photoelectron spectroscopy (XPS), X-ray diffractometry, atomic force 

microscopy, ultraviolet Raman spectroscopy, and 

various electrical measurements. The data provided by these measurements, spatially resolved 

when possible, include such fi lm characteristics 

as thickness, stoichiometry, HfO2

 phases present, 

degree and type of impurity incorporation, leakage 

current, and dielectric constant.

DELIVERABLES: Investigate the relationship between 

HfO2

 ALD process parameters, reactor gas fl ow and 

temperature fi elds, gas phase species identities and 

concentrations, and resulting ALD fi lm characteristics. 

Ongoing

•

•
Semiconductor Microelectronics and Nanoelectronics Programs 97

ACCOMPLISHMENTS

ALD Reactor Design — An ALD reactor with 

diagnostic access near the wafer surface that is 

also capable of growing high-quality, reproducible HfO2 fi lms was designed and fabricated. Gas 

fl ow and temperature profi les in this reactor were 

simulated using three dimensional CFD modeling, 

as shown in Fig. 1. Diagnostic access does not 

signifi cantly perturb gas fl ow, especially near the 

wafer surface. Hence, reproducible, high-quality fi lm deposition and in situ diagnostics can be 

performed simultaneously. This reactor is being 

used to provide data to validate process models 

and develop metrologies. 

#

Figure 1. A cross section of the helium carrier gas 

velocity distribution in the diagnostic-accessible 

reactor under typical deposition conditions (obtained 

from a full three dimensional simulation).

HfO2 Film Deposition — ALD HfO2 fi lms are 

being deposited under a variety of process conditions using tetrakis(ethylmethylamino) hafnium 

(TEMAH) and water or tetrakis(dimethylamino) 

hafnium (TDMAH) and water. Films have been 

characterized with a number of techniques, 

including VUV-SE, as shown in Fig. 2, XPS, 

FTIR, Raman spectroscopy, X-ray diffractometry, 

atomic force microscopy, and current-voltage 

measurements. 

#

Figure 2. Imaginary part of the complex dielectric 

function as a function of photon energy for HfO2

ALD fi lm as determined using VUV-SE. The inset 

illustrates determination of band gap energy by 

extrapolation of the expression [n(E)•α(E)•E]1/2 to 

zero (data courtesy of N.V. Nguyen).

ALD Reactor Modeling — Three dimensional 

CFD models for gas fl ow and temperature fi elds 

have been developed for this reactor. In addition, 

time-resolved precursor distributions have been 

modeled, as shown in Fig. 3. The results from 

these models have been used to help optimize 

reactor designs, especially optical window design, 

and deposition conditions. Chemical reaction 

mechanism models for HfO2 ALD are being 

developed for use with these three dimensional 

fl ow and temperature models to simulate the entire 

ALD process.

#

Figure 3. A three dimensional mass fraction profi le 

showing TEMAH mass fraction distribution after 

5 sec of helium purge following a 3 sec TEMAH 

injection into the reactor. 
98 National Institute of Standards and Technology 

In situ Measurements — In situ gas phase 

FTIR measurements have been performed during 

HfO2 fi lm deposition and have been shown to be 

sensitive to deposition reactants, TEMAH, and 

products, N-methylethylamine, as illustrated in 

Fig. 4. In addition, in situ FTIR measurements 

during TEMAH injection pulses were shown to 

be sensitive to TEMAH decomposition in the 

delivery system. A quadrupole mass spectrometric sampling system has been designed and 

constructed for gas phase sampling near the wafer 

surface.

#

Figure 4. Gas phase FTIR spectra as a function of 

purge time following a water pulse. 

Chemical Properties Calculations — Molecular structures and energies for precursors, 

adsorbates, intermediates, and transition states 

have been calculated using ab initio and density 

functional theory quantum calculations for ALD 

of Al2O3 from TMA and water and HfO2 from 

TEMAH or TDMAH and water. Although the 

primary focus of this work is on HfO2

 ALD, Al2O3

ALD is also being investigated as a model system 

for two main reasons. First, there has been signifi -

cant work on Al2O3 ALD, and consequently, there 

are adequate experimental data for use in chemical 

mechanism validation. Second, aluminum is more 

amenable to quantum calculations than hafnium, a 

transition metal. It is expected that lessons learned 

in developing an ALD model for Al2O3 ALD 

will be useful in development of a HfO2 ALD 

model. High level ab initio calculations, up to 

CCSD(T)/aug-cc-pVnZ (n=2-5) have been done 

for small species to benchmark heats of formation 

and bond dissociation energies for AlHn

X species 

(n = 0-2, X = H, F, Cl, OH, NH2, CH3). Additional 

#

calculations have been also done to provide higher 

level corrections for core-valence, relativistic, and 

basis set defi ciencies. DFT calculations have been 

utilized to compute heats of formation for ALD 

precursors, related molecules, and references 

molecules. The DFT calculations used the B3LYP 

method with LANL2DZ+ECP basis sets for the 

metals and 6-311G(d,p) basis sets for other atoms. 

Calculations were done on about 130 different 

organometallic and metal halide species containing aluminum (Al), hafnium, zirconium, gallium, 

and titanium with alkyl, alkoxy, and amino ligands 

- M(R)n

, M(OR)n, M(NR2)

n

 as well as about 50 

reference hydrocarbons, oxidized hydrocarbons, 

and nitrogen-substituted hydrocarbon species. 

Empirical corrections to the DFT energies will be 

determined by benchmarking and comparison to 

reference compounds with available values in the 

literature. A detailed chemical kinetic model for 

ALD of HfO2 from TDMAH and water has been 

constructed based on these rate expressions. This 

model is being refi ned based on further reactor 

model simulations, comparison with experimental 

observables, and supplemented with additional 

quantum calculations, where necessary. 

Reference Spectra Development — Adequate 

reference spectra could not be located for TEMAH 

or N-methyl-ethanamine (MEA), a gas phase 

product of the TEMAH and water ALD reaction. 

Hence, infrared and Raman vibrational frequencies were calculated with DFT using B3LYP 

theory with LANL2DZ basis sets. The calculated 

frequencies were compared to measured infrared 

spectra obtained in the ALD reactor and tentatively 

identifi ed as TEMAH and MEA, as shown in 

Fig. 5. In the infrared spectra, vibrational modes 

in the CH wavenumber range were scaled by 0.95 

and the modes in the lower wavenumber range 

were scaled by 0.961.

Database Website — A Website http://kinetics.

nist.gov/CKMech has been made available. This 

site contains bibliographic and thermochemical 

information of silicon hydrides, halocarbons, and 

organometallic compounds important to semiconductor processes, including information pertaining 

to ALD and CVD of aluminum, Al2

O3, and other 

related ALD systems (e.g., Zr, Hf, etc.), as well 

as a signifi cant amount of information pertaining 

to hydrocarbon-based reactions. 

#

#
Semiconductor Microelectronics and Nanoelectronics Programs 99

Figure 5. Comparison between calculated and 

measured spectra of TEMAH and MEA.

RECENT PUBLICATIONS

J. E. Maslar, W. S. Hurst, D. R. Burgess, Jr., W. A. Kimes, 

N.V. Nguyen, and E.F. Moore, “In Situ Monitoring of Hafnium Oxide Atomic Layer Deposition,” 2007 International 

Conference on Frontiers of Characterization and Metrology 

for Nanoelectronics, 2007, submitted.

J. E. Maslar, W. S. Hurst, D. R. Burgess, Jr., W. A. Kimes, and 

N. V. Nguyen, “In Situ Characterization Of Gas-Phase Species 

Present During Hafnium Oxide Atomic Layer Deposition,” 

ECS Transactions 2, 133-143 (2006).
100 National Institute of Standards and Technology 
Semiconductor Microelectronics and Nanoelectronics Programs 101

ADVANCED NANOSCALE AND MESOSCALE

INTERCONNECTS

Technical Contacts:

T. P. Moffat

D. Josell

GOALS

This project is developing solutions to metrology 

issues confronting integrated circuit manufacturers in the area of interconnect metallization. 

Present efforts include determining the essential 

process requirements for superconformal fabrication of high aspect ratio, low resistivity metallizations for both on-chip and chip stack applications, 

examining the generality of the superconformal 

fi lling mechanism and exploring processes utilizing novel barriers and/or seed geometries. This 

is complemented by surface chemistry studies 

aimed at understanding and optimizing the feature 

fi lling process.

CUSTOMER NEEDS

Increasing information technology requirements 

have yielded a strong demand for faster logic circuits and higher-density memory chips. The low 

electrical resistivity of copper and the ability of 

electrodeposition to “superconformally” fi ll high 

aspect ratio features has made electrodeposited 

copper the interconnect material of choice in 

silicon technology. However, the move to eversmaller dimensions has led to the rise of new 

challenges, including fabrication of ever-thinner 

copper seeds, which are required for the copper 

superfi ll process, and increased resistivities of 

the metallizations due to size effects. At the same 

time the movement towards 3-D integration of 

integrated circuits is under way. This involves 

the replacement of long horizontal conductors by 

short vertical interconnections; in addition to an 

improved RC response, the new architecture enables integrations of heterogeneous devices. Many 

challenges to the fabrication through-silicon-vias 

(TSV) remain that require an understanding of the 

surface chemistry of copper. To help overcome 

these hurdles the National Institute of Standards 

and Technology (NIST) is enhancing existing copper technology through improved understanding 

of the surface chemistry and metrologies for the 

superfi ll process, examining new interconnect 

materials, and pursuing new fabrication techniques such as seedless processing and atomic 

layer deposition.

Interconnect metallization issues are discussed 

in the Interconnect section of the 2006 update 

of the International Technology Roadmap for 

Semiconductors.

TECHNICAL STRATEGY

To meet future industrial needs, we have, over the 

life of this project, developed metrology and fully 

disclosed copper electrolytes that permit characterization of the ability of generic electrolytes to 

fi ll fi ne features. The derived Curvature Enhanced 

Accelerator Coverage (CEAC) mechanism has 

served as a platform for this understanding. An 

important consequence of bottom-up superfi lling 

is the phenomena of “momentum plating” that 

can lead to bump formation above fi lled features. These bumps greatly hamper subsequent 

planarization processes. However, the overshoot 

phenomena may be controlled by the addition of 

certain cationic surfactants to the copper plating 

baths as shown in Fig. 1 (next page). In the past 

year, we undertook a variety of electroanalytical 

and feature fi lling experiments that help establish 

suitable strategies to help quantify, understand, 

and control bump formation. Through this activity the CEAC model was successfully extended 

to explain and predict the fi lling behavior during 

deposition from complex plating baths containing any combination of suppressors, accelerators 

and levelers. 

DELIVERABLES: Publications detailing the experimental demonstration of the use of cationic surfactants 

to control undesired momentum plating during the 

superfi lling process. Publications detailing the extension of the NIST CEAC superfi lling model to quantitatively describe both superconformal feature fi lling and 

bump control. 1Q 2007

In parallel with this effort a variety of surface 

analytical studies, e.g. ellipsometry, XPS, and 

in situ STM, are under way to directly probe the 

competitive adsorption dynamics between multiple additives and the copper surface in order to 

provide molecular level insight into additive function and design. Particular attention is given to the 

role of potential-dependent adsorption processes 

that are central to the performance of the copper 

pulse plating method used to form TSV.

DELIVERABLES: Publications using surface analytical probes to independently quantify adsorbate 

coverage’s relevant to superconformal feature fi lling. 

1Q 2007, 3Q 2007, and 4Q 2007

The generality of the bottom-up superfi lling process to the application of other material systems 

in ULSI and MEMS remains a subject of great 
102 National Institute of Standards and Technology 

Figure 1a. The effect of the cationic surfactant, 

DTAC, on the Cu deposition rate in the presence of 

PEG-Cl-SPS surperfi lling additives. Note quenching 

of the hysteretic voltammetric response and increased 

inhibition with increasing DTAC concentration. 

Figure 1b. Cross sectional FE-SEM images of Cu 

deposition on an array of trenches as a function of 

DTAC concentration. Attenuation of overfi ll bumping 

is evident as the DTAC concentration increases to 

60 μmol/L, beyond this value DTAC interferes with 

superfi lling.

(b)

interest. In addition to silver (the only metal with 

a higher conductivity than copper) and gold (a 

metallization for wide-bandgap semiconductors) 

the prospect for deposition of iron group metals 

(for magneto-electronic circuitry) has been examined and the preliminary results, as shown in 

Fig. 2, are encouraging. 

DELIVERABLES: Publications detailing the fi rst example of deposition of conducting magnetic materials 

for Damascene metallization. 3Q 2007 and 4Q 2007

Figure 2. Superconformal Ni deposition from a 

NiSO4 + NiCl2 +H3BO3 electrolyte containing 

polyethyleneimine. Note the preferential fi lling of 

the smallest features and the most densely patterned 

region.

We’ve also built upon last year’s demonstration 

of gold superfi lling by exploring the impact of 

catalyst surface diffusion on the feature fi lling 

process as detailed in Fig. 3. Likewise, we continue to explore the utility of the CEAC as model 

as a design tool for alternative processing schemes 

such as chemical vapor deposition.

Figure 3. Predicted effect of surface diffusion on Au 

superfi lling of a trench. The dimensionless diffusion 

coeffi cient was systematically increased over three 

orders of magnitude in going from left to right. The 

smaller values lead to rounding of the corners, 

while intermediate values cause cusplike fi lling 

similar to that often reported for leveling. Filling is 

nearly conformal for the highest values of catalyst 

surface diffusion whence the CEAC geometry driven 

enrichment process is nullifi ed.

DELIVERABLES: Publication exploring the importance of catalyst surface diffusion on shape evolution 

during superfi lling. Including a comparison of observations and CEAC predictions. 1Q 2007

In addition to the electrodeposited copper conductor itself, current metallization technology 

employs a barrier metal and a PVD copper seed. 

As feature sizes continue to shrink, the resistive 

barrier materials negatively impacts electrical performance and deposition of, and on, the PVD seed 

becomes problematic. These diffi culties are driving a search for alternative barrier/wetting layer 

materials and processes. Ruthenium is one focus 

because its electrical and thermal conductivities 

are approximately twice those of conventional 

tantalum barriers, it is immiscible with copper, 

and copper can be electroplated directly on it, 

eliminating the need for the PVD copper seed 

layer and the corrosion problems that come with 

it. In the past we have demonstrated direct copper 

superfi lling on Ru, Os, and Ir barrier/adhesion 
Semiconductor Microelectronics and Nanoelectronics Programs 103

seed layers as well as appropriate metrological 

aspects required for process control. This work 

continues with the evaluation of state-of-the-art 

ALD barrier materials.

DELIVERABLES: Publications detailing seedless 

superfi lling of copper on PVD and ALD layers. 

4Q 2007

ACCOMPLISHMENTS

The existing CEAC model of superfi lling was 

extended to include the effect of leveling additives 

along with the metrology required for assessing 

the kinetics of the competitive adsorption process. 

The impact of leveling additives on overfi ll bump 

formation during trench fi lling was demonstrated 

using prototypical cationic surfactants. 

Bottom-up superfi lling of trenches with gold, 

that might fi nd use in the formation of contacts 

and metallizations for compound semiconductors, 

was demonstrated. The importance of catalyst 

surface diffusion in superfi lling processes was 

revealed and shown to be particularly relevant to 

the gold system.

A range of electroanalytical and surface 

analytical studies are well under way to quantify 

the competitive and co-adsorption processes associated with copper superfi lling additives. In 

particular, ellipsometry was used to reveal a.) 

difference in adsorption behavior between rate accelerating thiols versus disulfi des, b.) irreversible 

adsorption of thiols and disulfi des versus reversible adsorption of halide and PEG, c.) inhibition 

of PEG adsorption by a pre-adsorbed sulfonateterminated thiol or disulfi de monolayer fi lm, and 

d.) displacement of the deposition rate inhibiting 

PEG layer by adsorption of the sulfonate-terminated thiol or disulfi de accelerator.

COLLABORATIONS

D. Wheeler, J.E. Bonevich, L.J. Richter, M.L. Walker, P.J. 

Chen, W.F. Egelhoff: NIST.

Christian Witt: AMD

R. Gordon, Harvard University

T. Aaltonen, M. Leskelä and M. Ritala: University of Helsinki.

RECENT PUBLICATIONS

T. P. Moffat, D. Wheeler and D. Josell, “Superconformal Film 

Growth,” pg 96-114 in “Electrocrystallization in Nanotechnology, Ed. G. Staikov, Wiley-VCH (2007).

#

#

#

M. Walker, L. J. Richter, and T. P. Moffat, “Potential Dependence of Competitive Adsorption of PEG, Cl-, and SPS/MPS 

on Cu: An In Situ Ellipsometric Study,” Journal of the Electrochemical Society 154(5), D277-D282 (2007).

D. Josell, T. P. Moffat and D. Wheeler, “Superfi lling When 

Adsorbed Accelerators Are Mobile,” Journal of the Electrochemical Society, 154(4), D208-D214 (2007).

T. P. Moffat, D. Wheeler, S. K. Kim and D. Josell, “Curvature 

Enhanced Adsorbate Coverage Mechanism for bottom-up 

superfi lling and bump control in damascene processing,”

Electrochimica Acta, in press, (2007).

S.-K. Kim, D. Josell and T. P. Moffat, “Cationic Surfactants 

for the Control of Overfi ll Bumps in Cu Superfi lling,” J. Electrochemical Society, 153 (12), C826-C833 (2006).

S.-K. Kim, D. Josell and T. P. Moffat, “Electrodeposition of Cu 

in the PEI-PEG-CL-SPS Additive System: Reduction of Overfi ll 

Bump Formation During Superfi lling,” J. Electrochemical 

Society, 153 (9), C616-C622 (2006).

M. Walker, L. J. Richter, and T. P. Moffat, “Competitive 

Adsorption of PEG, Cl-, and SPS/MPS on Cu: An In Situ 

Ellipsometric Study,” Journal of the Electrochemical Society 

153(8), C557-C561 (2006).

T. P. Moffat, D. Wheeler, S.-K. Km and D. Josell, “Curvature 

Enhanced Adsorbate Coverage Model for Electrodeposition,”

Journal of The Electrochemical Society, 153(2) C127-C132 

(2006).

D. Josell, D. Wheeler and T. P. Moffat, “Gold Superfi ll in Submicrometer Trenches: Experiment and Prediction,” Journal of 

the Electrochemical Society, 153(1) C11-C18 (2006).
104 National Institute of Standards and Technology 
Semiconductor Microelectronics and Nanoelectronics Programs 105

NANOPOROUS THIN-FILM METROLOGY FOR LOW-κ

DIELECTRIC MATERIALS

GOALS

This project addresses critical development and 

integration issues with the interlayer dielectric 

(ILD) insulator materials needed for semiconductor interconnect applications by developing 

measurement methodologies to quantify their 

porous structures and related properties. This is 

accomplished by working closely with industry 

and academia to develop and apply these methods on state-of-the-art materials that are leading 

candidates for next generation integrated circuit 

applications. Our unique measurement infrastructure is based on a synergistic and complimentary 

packaging of specular X-ray refl ectivity (SXR), 

X-ray porosimetry (XRP), small angle scattering using both neutrons and X-rays (SANS 

and SAXS), and a suite of ion beam scattering 

techniques including Rutherford backscattering 

spectroscopy (RBS), grazing angle backscattering 

(GBS), and forward recoil elastic spectroscopy 

(FRES). With this measurement platform we can 

provide high quality data and measurements of 

the chemical composition, fi lm thickness, density, 

coeffi cient of thermal expansion (CTE), moisture 

uptake, porosity, pore size distribution, wall density within a porous matrix, and even the shape or 

geometry with which periodically patterned pores 

are arranged. These measurements can be applied 

to quantify not only the as-applied, virgin material 

to aide in materials development, but also after the 

various ashing, etching, deposition, or chemical 

mechanical polishing (CMP) processes that tend 

to alter or damage the fi nal structure of the porous 

material. This year there is a greater emphasis 

on the process induced damage over the materials development to refl ect the current industrial 

focus. Integration issues are currently the greatest 

concern when trying to implement porous ILD 

materials into functional CMOS devices.

CUSTOMER NEEDS

The continued scaling or shrinking of the integrated circuit (IC) devices require new, more 

effi cient ILD materials to combat the growing 

problems of a rapidly increasing power density 

on the chip, greater signal propagation delays due 

to the RC time constant, and increased cross-talk 

between the adjacent interconnects that continue 

to be squeezed closer together. The solution has 

been to develop low dielectric constant (low-κ or 

Technical Contact:

C. Soles

ultralow-κ) materials that incorporate nanometer 

scale pores to lower their effective dielectric 

constant. While this recognized approach is 

effective in meeting the ultralow-κ dielectric 

constant target, introducing porosity simultaneously compromises many of the other properties 

that are equally as important such as mechanical 

strength, thermal conductivity, thermal expansion, 

and the crucial barrier properties (i.e., resistance 

to Cu ion migration or moisture uptake) of these 

highly porous material. Likewise the integration process exposes these materials to harsh 

conditions, including mechanical stresses under 

a caustic environment in CMP polishing, ashing 

and etching induced degradation of the material, 

and thermally generated stresses resulting from 

the CTE mismatch of dissimilar materials and the 

large and rapid temperature changes in the back 

end of the line processes. All of these processes 

tend to damage or degrade the porous structures. 

Developing a material that is optimized to meet 

all of these conditions has been an enormous challenge. There is still currently no clear consensus 

among IC chip manufacturers for the selection 

of a class of material or a processing method 

of nanoporous fi lms. Candidates have included 

silica-based fi lms, organic polymers, inorganic 

spin-on materials, chemical vapor deposited 

materials, and several others. Irrespective of the 

candidate materials, there is a strong need for 

high quality structural data to understand correlations between applied processing conditions 

and the fi nal physical properties. Only through 

detailed characterization techniques will materials 

engineers have the proper feedback to know how 

a process affects the porous material and to then 

rationally design new materials that are suitable 

to withstand the harsh integration processes associated with CMOS fabrication.

TECHNICAL STRATEGY

While the fi eld of porosimetry is broad and generally well-established, the small sample sizes (typically fi lms less than 1 μm thick) and the desire to 

characterize materials in situ, directly on actual 

silicon wafers, greatly narrows the number of suitable measurement methods. Therefore a suite of 

measurement techniques, depicted in Fig. 1 (next 

page), that are especially sensitive to samples in 

the thin fi lm geometry have been developed at 
106 National Institute of Standards and Technology 

NIST. When combined, these techniques can fully 

characterize the properties that are critical for 

ILD applications. The measurement techniques 

include various combinations of our refl ectivity 

techniques (SXR and XRP), small angle scattering (SANS and SAXS), and ion beam scattering 

methods (RBS, GBS, and FRES) to determine 

important structural and physical property information in porous fi lms less than 1 μm thick. These 

measurements are performed directly on fi lms 

supported on thick silicon substrates, meaning that 

actual processing effects can be investigated. 

The elemental composition of the fi lms are needed 

to convert the scattering length densities obtained 

from our X-ray and neutron techniques into a 

meaningful physical density. For the case of 

unknown or proprietary ILD materials, we have 

developed RBS & GBS ion beam techniques to 

determine the Si, C, and O (or other elements other 

than H) content of the fi lm while the FRES ion 

beam technique quantifi es the H content. In all 

of these techniques a beam of high-energy ions is 

directed toward the sample surface. The number 

of scattered particles is counted as a function 

of their energy and angle. We have developed a 

method to fi t the spectra and then compute the 

absolute ratios of the C, O, Si, and H in the fi lm. 

From chemical composition, we can convert the 

scattering length densities into the physical or 

mass density of the fi lm.

Figure 1. A chart indicating which techniques are 

needed to measure the various critical properties of a 

porous low-κ material.

SANS and SAXS are transmission scattering 

techniques well-suited for characterizing critical 

length scales of the porous networks. Inclusions 

in the fi lm of a different scattering length density 

than the matrix lead to small angle scattering 

when their length scales are on the order of 5 to 

500 nm. By modeling the diffracted intensity as 

a function of the scattering vector, one can determine parameters such as the pores size, the pore 

size distribution, the inter-pore spacing, and the 

periodicity with which the pores are distributed. 

For ordered porous structures this reveals the 

symmetry or periodic lattice on which the pores 

are distributed. In the case of SANS, a powerful contrast matching technique can be used by 

mixing hydrogenated and deuterium substituted 

organic condensate vapors (such as toluene). At a 

critical concentration the hydrogenated-deuterated 

mixture of solvents will have exactly the same 

scattering length density as the matrix. By controlling the partial pressure of this mixture, the vapor 

can be selectively condensed inside the pores of 

a given radius (governed by the critical radius 

for capillary condensation), thereby rendering 

the condensed pores “invisible” to the scattering 

technique. This is an extremely powerful method 

to specifi c size dependent information about the 

population of pores.

High-resolution XR is an established, powerful 

technique to accurately measure the structure 

of thin fi lms in the direction normal to the fi lm 

surface. In particular, the fi lm thickness, fi lm 

quality (roughness and uniformity) and average fi lm density can be determined with a high 

degree of precision. In addition, changes in the 

density profi le from processing or post-application treatments can be determined. The CTE is 

determined from measurements of the fi lm thickness at different temperatures. More recently, we 

have been extending the characterization with XR 

measurements to the lateral or in-plane dimensions 

of nanopatterned structures using an effective medium approximation. The density measurements 

are averaged over lateral length scale larger than 

the coherence length of the X-ray source. In the 

limit of periodic patterns, the density profi le as 

a function of height conveys information about 

the average line to space ratio (or the equivalent 

patterned volume to unpatterned volume ratio 

for non-linear structures) as a function of pattern height. When these line-to-space ratios are 

coupled with a precise measure of pattern pitch or 

periodicity from a SAXS or SANS measurement, 

one can characterize the entire pattern cross section as a function of height.

DELIVERABLES: Measure and report on the way in 

which integration processes impact the porous structure in nanoporous ILD materials. 2Q 2007

The number of companies trying to develop ILD candidate materials has signifi cantly #
Semiconductor Microelectronics and Nanoelectronics Programs 107

decreased. The industry as a whole is moving 

closer to integrating nanoporous ILD into next 

generation devices. The remaining selection of 

candidate materials can be largely divided by their 

different deposition processes (i.e., CVD versus 

spin-on). The major issue now is how these different classes of materials withstand the integration 

processes. Therefore, we have switched from a 

large scale materials screening effort to a more 

focused characterization of how the different 

integration processes, such as ashing, etching, 

plasma treatment, and CMP polishing, affect the 

critical properties of the different classes of porous 

materials. These integrating processes tend to 

damage the pore structure and thereby reduce the 

effect κ (dielectric constant after integration) of 

the dielectric stack. Measurement techniques are 

needed to quantify and depth profi le the process 

induce damage to the porous materials.

DELIVERABLES: Extend X-ray porosity characterization techniques from planar fi lms to patterned nanoporous structures. 3Q 2007

Our porosity characterization measurements 

thus far have focused on planar, non-patterned 

low-κ fi lms. While this has been successful in 

the early stage evaluation of candidate materials, our techniques are starting to loose traction 

for the integration issues where patterned low-κ

structures are of interest. It is therefore critical 

to extend our measurements to patterned low-κ

materials. Recently, we have shown that the 

effective medium approximation is valid for the 

nanoscale periodic patterns of the size scale that 

are of interest to the semiconductor industry. 

This means that we can characterize the average 

density as a function of height through a nanoscale 

pattern. In periodic linear grating patterns, this 

average density defi nes the line-to-space ratio as 

a function of height. When coupled with an accurate knowledge of the pattern periodicity, we 

can defi ne the average pattern width as a function 

of pattern height. Now that we have shown that 

the average density within the patterned material 

can be characterized, the next step is to see how 

the density changes when a probe molecule is condensed inside the pores of the patterned material. 

This will allow us to determine the porosity and 

wall density of the patterned material as a function 

of pattern height, extending the use of XRP from 

planar to patterned structures.

DELIVERABLES: Characterize the fi delity and porosity of nanoporous material patterned by nanoimprint 

lithography. 4Q 2007

#

Recently there is a growing interest within 

the community to directly pattern spin-on 

organosilicate ILD materials by nanoimprint 

lithography (NIL). It has been shown that by 

imprinting the ILD material with a multi-level 

nanoimprint template to directly fabricate a 

T-gate type structure, one can greatly reduce the 

number of lithography, deposition, and etching 

steps in the back end of the line process. This 

could signifi cantly reduce manufacturing costs. 

Efforts this year will synergistically couple our 

expertise in thin fi lm nanoporous X-ray metrology 

with our simultaneous efforts in NIL metrology 

development. Specifi cally, we will adapt our XR 

shape metrology to quantify the fi delity of the 

nanoimprint process to evaluate the quality of 

the patterned ILD materials. We will also adapt 

our XRP measurements on patterned samples to 

quantify how the NIL process itself affects the 

pore structure. These measurement techniques 

will quantify the feasibility of NIL processes 

for back end of the line interconnect fabrication 

schemes. This is a topic that all of the major NIL 

tool companies are actively engaged in.

ACCOMPLISHMENTS

We have fi nally completed our contract work 

with the SEMATECH (ISMT) Interconnect program. Over the past 7 years, we have evaluated 

over 180 candidate fi lms for ISMT, provided to 

them by their member companies. These fi lms 

have been characterized in terms of their thickness, CTE, moisture uptake, fi lm connectivity, 

pore volume, pore size, and matrix density / 

chemical composition by XR, SANS, RBS, and 

FRES. Quarterly reports were delivered on the 

results and distributed to member companies. 

Additional measurements were performed on specifi c samples for further analysis using methods 

such as additional SANS confi gurations or X-ray 

porosimetry. These measurements were critical 

in helping the industry rationally narrow the wide 

fi eld of candidate materials for next generation 

ILD materials.

A NIST Recommended Practice Guide, Special Publication 960-13, entitled “Pore Characterization in Low-κ Dielectric Films Using X-ray 

Refl ectivity: X-ray Porosimetry” was released to 

provide the details and recommendations for the 

use X-ray porosimetry to characterize nanoporous 

thin fi lms. This document describes how to build 

an X-ray Porosimetry Device, how to properly use 

and align the X-ray optics, how to control the partial pressure of the adsorbate molecules, and how 

#

#

#
108 National Institute of Standards and Technology 

to interpret the data. Hard copies of this publication (Fig. 2) were mailed to all of our contacts in 

the low-κ dielectric community and made available for free download through our website: http://

polymers.msel.nist.gov/PDF/X-ray_Porosimetry_

Recommended_Practice.pdf.

Figure 2. The cover image from our Recommended 

Practice Guide on characterizing low-κ dielectric 

fi lms with X-ray porosimetry.

The detailed pores structures in a series of 

highly ordered nanoporous were fully characterized using our measurement techniques. Well 

controlled, ordered porous structures have the 

potential to control the degree of interconnectivity 

through porous network, possibly maximizing the 

barrier properties of the material. Therefore it is 

very important to have measurements methods 

to completely characterize both the size and arrangement of the pores in these ordered systems. 

Working with our collaborators at the University 

of Massachusetts, Amherst, a series of organosilicate fi lms with a periodic, self-assembled pore 

structures were studied in detail. From these 

measurements we were able to quantify the total 

porosity, the pore size distribution, the periodicity or spacing of the ordered pores, the degree of 

interconnectivity between adjacent pores, and the 

density of the wall material between the pores. 

Of particular interest we found that the A-B-A 

triblock copolymer used as the pore generating material lead to a dual distribution of pore 

sizes. The B block phase segregated into regular 

spherical pores with a radius of approximately 

#

(4 to 5) nm on a cubic lattice of a periodicity of 

approximately (13 to 15) nm. The second population of pores was much smaller in size and located 

within the wall material, presumably due to the 

decomposition of the A blocks which do not phase 

segregate from the organosilicate marix, between 

the pores generated by the B blocks. While extremely small, these A block pores accounted for 

a signifi cant fraction, approximately (30 to 50) % 

by volume, of the total porosity. This non-trivial 

fraction of the total porosity has implications on 

the barrier properties of these materials, suggesting that triblock copolymers may not be the most 

effective porogen materials for creating isolated, 

non-interconnected pores. 

COLLABORATIONS

Polymers Division, NIST – Hae-Jeong Lee, Bryan D. Vogt, 

Christopher L. Soles, Hyunwook Ro, Da-Wei Liu, Ronald L. 

Jones, Wen-li Wu, Eric Lin; Center for Neutron Research, 

NIST – John Barker, Charles J. Glinka, Derek L. Ho

International SEMATECH – Sanjit Daas, Youfan Liu

University of Massachusetts-Amherst – James Watkins

Dow Chemical – Brian Landes, John Lyons, Brandon Kerr, 

Jason Niu, Tom Kalantar

LG Chemicals – Minjin Ko

IBM T. J. Watson Research Center – Alfred Grill, Qinghuang 

Lin

Seoul National University – Kookheon Char, Do Yoon

Technion – Michael S. Silverstein, Miche Shach-Caplan

University of Michigan - David Gidley

Axcelis Technologies, Incorporated – Carlo Waldfried

Rohm and Haas Company - Michael Gallagher

Dow Corning – Byung Keun Hwang

RECENT PUBLICATIONS

Q. Lin, S. A. Cohen, L. Gignac, B. Herbst, D. Klaus, E. Simonyi, J. Hedrick, J. Warlaumont, H. J. Lee and W. L. Wu, 

“Low Dielectric Constant Nanocomposite Thin Films Based 

on Silica Nanoparticle and Organic Thermosets” J. Polym. Sci 

B., 2007, 45(12), 1482.

H. W. Ro, K. Char, E. C. Jeon, H. J. Kim, D. K., H. J. Lee, J. 

K. Lee, H. W. Rhee, C. L. Soles, and D. Y. Yoon Adv. Mater. 

2007, 19, 705–710.

H. J. Lee, C. L. Soles, D. W. Liu, B. J. Bauer, E. K. Lin, W. L. 

Wu, and M. Gallagher, “Structural Characteristics of Methylsilsesquioxane Based Porous Low-κ Thin Films Fabricated 

with Increasing Crosslinked Particle Porogen Loading” J. Appl. 

Phys., 2006, 100 (6), 064104.

M. S. Silverstein, B. J. Bauer, R. C. Hedden, H. J. Lee, and B. 

G. Landes, “SANS and XRR Porosimetry of a Polyphenylene 

Low-κ Dielectric” Macromolecules, 2006, 39(8), 2998.
Semiconductor Microelectronics and Nanoelectronics Programs 109

B. D. Vogt, H. J. Lee, W. L. Wu, and Y. Liu, “Specular X-ray 

Refl ectivity and Small Angle Neutron Scattering for Structure 

Determination of Ordered Mesoporous Dielectric Films”, J. 

Phys. Chem. B 2005, 109, 18445.

M. S. Silverstein, M. S. Caplan, B. J. Bauer, R. C. Hedden, 

H. J. Lee, and B. G. Landes, “Nanopore Formation in a 

Polyphenylene Low-κ Dielectric” Macromolecules, 2005, 

38(10), 4301.

B. D. Vogt, R. A. Pai, H. J. Lee, R. C. Hedden, C. L. Soles, W.-l. 

Wu, E. K. Lin, B. J. Bauer, and J. J. Watkins. “Characterization of Ordered Mesoporous Silica Films Using Small-Angle 

Neutron Scattering and X-ray Porosimetry.” Chemistry of 

Materials 2005, 17(6) 1398-1408.

Soles, C. L., Lee, H. J., Lin, E. K., and Wu, W. L., “Pore 

Characterization in low-κ Dielectric Films using X-ray 

Refl ectivity: X-ray Porosimetry” NIST Special Publication 

960-13 (2004).
110 National Institute of Standards and Technology 

INTERCONNECT MATERIALS AND RELIABILITY

METROLOGY

This is a large project that involves parts that are not reasonably combined in a single document. For this reason, the project is presented in two sub-sections, each focusing on a single 

aspect. These are:

Basic Materials Properties

Test Structures for Interconnect Metrology and Modeling

•

•
Semiconductor Microelectronics and Nanoelectronics Programs 111

BASIC MATERIALS PROPERTIES

Technical Contacts:

D. Read

N. Barbosa III

B. Keller

GOALS

The objectives of this project are: (1) to develop 

experimental techniques to measure the material 

properties that control the reliability of thin conducting and insulating fi lms in interconnects under 

mechanical, thermal, and electrical stresses; these 

include basic tensile properties, elastic modulus 

by both static and dynamic means, residual stresses and strains, fatigue and fracture resistance, and 

thermomechanical response. (2) to advance the 

ability to anticipate and meet interconnect reliability challenges by relating thin fi lm reliability 

to microstructure and by developing understanding of the relationships between various modes of 

failure in nanoscale interconnect structures. 

CUSTOMER NEEDS

Thin fi lms are an essential component of all advanced electronic devices. Interconnect structures 

built up on ULSI microchips consist of 11 thinfi lm layers now, and will reach 14 layers in the 

long term (International Technology Roadmap for 

Semiconductors, 2006, Interconnect, Tables 80ab). These structures are fabricated using adjacent 

layers of materials with very different thermal 

expansion coeffi cients, exotic materials such as 

nanoporous low-κ dielectric, and operate at ever 

higher temperatures. Both experimental measurements and modeling and simulation of material 

behavior are needed, and these efforts need to 

be complementary. According to the Roadmap 

(2005 Edition), Interconnect, p. 41, Cost effective 

fi rst pass design success requires computer-aided 

design (CAD) tools that incorporate contextual 

reliability considerations in the design of new 

products and technologies. It is essential that 

advances in failure mechanism understanding and 

modeling, which result from the use of improved 

modeling and test methodologies, be used to 

provide input data for these new CAD tools. With 

these data and smart reliability CAD tools, the 

impact on product reliability of design selections 

can be evaluated. The 2005 iNEMI (International 

National Electronics Manufacturing Initiative)

Research Priorities document reports a similar 

need. The top fi ve research areas listed by iNEMI 

include Materials & Reliability and Design. Under Design, the top fi ve research priorities include 

Mechanical and Reliability Modeling, p. 16. Appendix C of the document has a detailed section on 

Reliability, Mechanical Analysis, and Simulation, 

p. 44, which emphasizes the need for material 

characterization and reliability prediction. Key 

considerations in this paragraph and elsewhere 

in the document are effects of temperature and 

of cyclic loading and effects of size scaling. The 

iNEMI document specifi cally calls out the need to 

predict and understand failures in MEMS devices, 

p. 39. The message is clear: understanding and 

modeling of mechanical performance and potential failure modes in these devices require knowledge of the mechanical behavior of the fi lms. This 

issue of mechanical modeling is likely to increase 

in signifi cance with the growing integration of 

interconnect between the chip and the package, 

with their disparate material sets, and with the 

push into exotic nanoscale materials.

Because the fi lms are formed by physical vapor 

deposition, electrodeposition, or spin-on deposition, their microstructures, and hence, their 

mechanical properties, are different from those 

of bulk materials of the same chemical composition. While the general principles of conventional 

mechanical testing are applicable to thin fi lms, 

conventional test equipment and techniques are 

not. Because vapor-deposited fi lms are of the order 

of 1 μm thick, the failure loads are of the order of 

gram-forces or less, and the specimens cannot be 

handled directly. So, techniques specifi c to fi lms 

on silicon substrates are needed. Developing test 

methods must eventually become applicable to 

test structures that can be included in production 

or development wafers, so that applicability to 

‘real’ materials can be demonstrated. The intent 

of our goal of testing specimens similar in size 

to structures on actual production devices is to 

maximize the relevance of our results. Industry 

needs test methods that are effi cient and integrable 

in the fab environment. We see nanoindentation 

and high-amplitude a.c. measurements as candidates to become routine, or perhaps even on-line, 

methods. Compared to these, the microtensile test 

is more laborious, but it provides unambiguous 

results. We will continue to promote and offer 

the microtensile test as a reference method, while 

developing methods to extract needed material 

property information from the more integrable 

methods such as nanoindentation and high-amplitude a.c. testing. 

We are also working on TEM- and SEM-based 

techniques for measurement of local strains, to 

provide detailed information about the nature 

of deformation and stress-strain response in the 
112 National Institute of Standards and Technology 

dimensionally-constrained materials that make up 

microelectronic systems. The techniques are based 

on electron diffraction measurements of lattice 

parameters and have spatial resolutions that are 

compatible with state of the art device dimensions. 

Electron backscatter diffraction (EBSD) offers a 

means by which such strains may potentially be 

mapped with near-20 nm resolution, since the 

technique is integrated with the scanning action 

of a SEM. Convergent-beam electron diffraction 

(CBED) is a TEM technique that can potentially 

measure strains with better than 5 nm spatial 

resolution, but requires specimen thinning, and 

is therefore subject to artifacts. We have efforts 

under way to advance both approaches. 

Radically different materials and material technologies are being considered for future ULSI 

devices, as the further development of leadingedge lithography increases in cost and complexity. An example of a radically new material is the 

carbon nanotube. An example of a new material 

technology is self-assembly. Effective use of these 

new materials systems will require signifi cant 

extension of the reliability metrology and analysis 

toolset, to understand and address new kinds of 

reliability issues. The NIST laboratory research 

programs, which back up the near-term metrology 

developments described here, are beginning to 

develop experimental and analytical techniques 

to address these challenges; NIST management 

is encouraging these developments through the 

new strategic focus area in nanotechnology. We 

held a workshop entitled Reliability Issues in 

Nanomaterials, 17–19 August 2004, to gauge 

customer interest in the available tools for characterization of nanomaterials, which, of course, 

encompasses advanced interconnect structures at 

both current and foreseen size scales. Microelectronics manufacturers were represented, along 

with universities, national laboratories, and more 

general commercial interests. The referenced report gives the details, but two key messages for 

this research project were the widespread adoption 

of nanoindentation for material characterization, 

and the serious consideration given to atomicscale materials engineering. We are planning 

the second such workshop, entitled “Materials 

Characterization for Nanoscale Reliability,” to 

be held in Boulder in August, 2007, to gauge 

progress in development of reliability metrology 

applicable to microelectronics and nanomaterials, and to update metrology needs that could be 

addressed by NIST.

TECHNICAL STRATEGY

We are developing a variety of measurement 

techniques to provide material property data on 

interconnect materials. In testing and exercising these techniques, we develop data that are 

valuable in themselves, and we also develop our 

understanding of the relationship between the 

observed behavior and the microstructure, as infl uenced by processing conditions specifi c to the 

specimen material at hand. We study individual 

thin fi lms and multilayer interconnect structures 

on silicon substrates, both obtained from industry 

and fabricated by researchers within NIST and 

elsewhere. Specimens of CMOS structures have 

been obtained through the MOSIS service run by 

UCLA (Fig. 1). Occasionally, wafers or fabricated specimen geometries are received directly 

from our counterparts in industry.

Figure 1. Test chip produced in the 1.2 μm AMI 

CMOS process available through the MOSIS service.

Measurement capabilities operating within this 

project include microtensile testing, d.c. and a.c. 

thermomechanical fatigue and micro- and nanoscale characterization of industrially relevant 

materials. We have developed the silicon-frame 

tensile specimen and the piezo-actuated tensile 

tester, which operate successfully for specimens 

100 μm wide and larger. Because problems 

were encountered with specimens narrower than 

100 μm, a new technique, called the force-probe 

tensile test technique, has been developed. The 

apparatus includes a tensile loading system operable within the SEM. This system has now been 

used on specimens as small as 2 μm wide. It is 

anticipated that the magnifi cation of the SEM will 

allow testing even narrower specimens.
Semiconductor Microelectronics and Nanoelectronics Programs 113

We have begun an effort on electrodeposited 

copper, including specimens from industrial colleagues and produced in-house, to characterize 

the variability of the mechanical behavior of 

electrodeposited copper with deposition conditions, fi lm thickness, annealing, and other relevant 

variables.

DELIVERABLES: A one-day short course consisting 

of 6 lectures by NIST staff on Interconnect-Relevant 

Characterization & Metrology was given in conjunction 

with the 2006 Advanced Metallization Conference. A 

repeat of this short course at the 2007 conference is 

under consideration. 4Q 2007

Our measurements involving alternating current 

stressing of chip-level interconnects are being 

used to explore the relationships between electrical, thermal, and mechanical reliability. High current density a.c. signals are run at low frequencies 

through Al- and Cu-based electromigration structures in either passivated or unpassivated states. 

Joule self-heating results in a cyclic strain due to 

thermal expansion mismatch between the metal 

lines and their substrates. The associated deformation then leads to severe topographic distortions 

in unconstrained lines, and can eventually cause 

open circuit failure. We investigate the effects 

of current density, frequency, crystallographic 

orientation, and encapsulant material. The use of 

advanced analytical techniques including EBSD, 

SEM, and TEM has revealed surprisingly similar 

microstructural damage and failure mechanisms 

in a.c.-stress and microtensile tests. This indicates at least the possibility that electrical tests 

may be exploited to produce information about 

the mechanical characteristics of thin fi lms. If 

this electrical-mechanical test can be made to 

produce relevant data, it will be of signifi cant 

benefi t because of the experimental convenience 

of electrical stressing on specimens of a wide 

range of sizes, including very small.

We have recently presented the fi rst set of experimental data comparing the results of high 

amplitude a.c. testing and tensile testing on the 

same material.

DELIVERABLES: Technical publication comparing 

results of high amplitude a.c. and microtensile testing 

on e-beam-deposited aluminum, in press. 3Q 2007

The initial ultimate strength value deduced from 

the a.c. test by extrapolating the results back to 

one loading cycle, and correcting for the residual 

stress present in the line-on-substrate specimen, 

was approximately equal to the value measured 

in the tensile test. However, physical differences 

between the tests include: thermomechanical vs 

mechanical stressing; elevated and variable temperature in the a.c. test; residual stresses in the fi lm 

on substrate vs none in the free-standing tensile 

specimen; substrate constraint causing biaxial 

loading as well as additional strengthening in the 

a.c. test; and cyclic loading vs monotonic strain to 

failure. By more realistic treatment of these differences, in measurements on specimens of different 

materials with different grain sizes and different 

line widths, we will quantify the uncertainty with 

which static mechanical properties can be deduced 

from the a.c. test. By comparison, the rule for 

obtaining the ultimate tensile strength from a 

nanoindentation test typically overestimates the 

microtensile result by a similar amount. 

DELIVERABLES: We are planning the 2nd NIST 

Workshop on Reliability Issues in Nanomaterials. 

3Q 2007

We are also developing non-contact optical methods to measure mechanical properties of thin fi lms 

and interconnects using MEMS test structures that 

are compatible with the CMOS process. Such 

non-contact methods may be useful in monitoring the variations of mechanical properties in the 

production environment. A method has already 

been developed to measure the residual strain 

in the passivation and interconnect fi lms using 

fi xed-fi xed beams. We are currently developing 

resonant measurements for cantilever beams to 

characterize the elastic modulus of each layer 

(Fig. 2). 

Figure 2. Resonant frequency response of 

micromachined CMOS cantilever test structures.

The method is based on the fact that the resonant 

frequency of the cantilevers is related to their 

elastic modulus, density, and the geometry. They 

can be fabricated with different combinations of 
114 National Institute of Standards and Technology 

layers and then comparisons of the resonant frequency can be used to extract the modulus of the 

individual layer. The cantilever test structures can 

be fabricated simultaneously with the fi xed-fi xed 

beams and measurements of residual strain and 

strain gradient can be made. With the residual 

strains, from the fi xed-fi xed beam method, the 

elastic constant, from the resonant method, and 

the strain gradient, from the curvature of cantilevers, the residual stress and its gradient can be 

calculated.

DELIVERABLES: Our results are being disseminated in conference presentations and peer-reviewed 

articles in archival journals, as well as in presentations 

and written reports to the organizations that have supplied specimens. 4Q 2007

ACCOMPLISHMENTS

Progress in recent years on measurements 

of the mechanical behavior of thin fi lms has put 

us in the position of starting to be able to make 

various kinds of critical comparisons among our 

results: results for the same materials in different 

laboratories; results for similar materials from 

different sources; results for the same property by 

different measurement methods; and experimental results versus numerical simulations. These 

comparisons are necessary to reach our goals of 

providing accurate and believable measurement 

techniques and an understanding of the results. 

Microstructural characterization is critical for defi ning the applicability of test results for different 

materials and test techniques. Currently, we are 

improving our measurements for characterizing 

the microstructure of electrodeposited copper in 

order to better understand the infl uence of microstructure on reliability. Figure 3 shows the grain 

structure of a commercially produced copper fi lm, 

as mapped out by electron back scatter diffraction. 

The colors represent local crystallographic orientation, according to the inset. Regions of constant 

color are metallographic grains. The long, straight, 

parallel boundaries are twin boundaries. 

#

Figure 3. EBSD map of electrodeposited copper fi lm 

specimen. The colors indicate the surface-normal 

crystallographic orientation of each pixel, according 

to the key, inset. Regions of constant color are 

metallographic grains. The long, straight, parallel 

boundaries are twin boundaries.

Based on input received at the workshop on 

Reliability Issues in Nanomaterials, discussed 

above, an international round robin to assess 

interlaboratory reproducibility of measurements 

of hardness and elastic modulus by instrumented 

indentation, on thin fi lm materials typical of those 

used in interconnect structures in microelectronic 

devices, has been conducted; a technical publication reporting the results is in review.

DELIVERABLES: Paper reporting round robin has 

been accepted for an upcoming special issue of Metallurgical Transactions A. 3Q 2007

A specimen fi lm selected after consultation 

with a group of experts in the fi eld, consisting of 

a copper fi lm with a platinum passivation layer, 

was contributed by Sandia National Laboratory. 

Open participation was solicited by e-mail to 

over 100 laboratories. Fragments of the specimen wafer were distributed to approximately 30 

laboratories around the world. The goal was to 

obtain results representative of the experience of 

a “typical customer” using a testing laboratory. 

Approximately 25 laboratories conducted tests 

and contributed reports. The results have been 

analyzed with assistance from the NIST Statistical Engineering Division. The scatter in modulus 

and hardness are much larger from laboratory to 

laboratory than within laboratories; Fig. 4 shows 

the hardness data. 

#

#
Semiconductor Microelectronics and Nanoelectronics Programs 115

Figure 4. Reported results for indentation hardness 

of the round robin specimen material, a Cu fi lm on a 

silicon substrate. The error bars at each data point 

span a total range of 4 times the standard deviation 

reported for multiple indentations. The average of 

the reported hardness values is 2.59 GPa.

This result indicates that comparisons of instrumented indentation results from different 

laboratories should be treated with caution, and 

that further investigation is needed to identify the 

sources of the scatter. 

DELIVERABLES: Papers based on two conference 

presentations on the comparison of electrical and 

mechanical stress effects have been accepted for the 

respective technical conference proceedings, to appear in 3Q 2007.

We have been working to demonstrate the 

applicability of our small-scale mechanical testing 

techniques to materials recently introduced in the 

microelectronics industry, specifi cally copper, in 

the form of both sputtered thin fi lms and thick 

electrodeposits. A microtensile specimen of electro deposited copper is shown in Fig. 5. 

#

Figure 5. Microtensile specimen of electrodeposited 

copper produced at NIST. The slender strip on the 

left is the test section; the tab with the hold is used 

for loading. The grip section is 10 μm wide by about 

200 μm long.

We typically fi nd that the strength values are far 

above the handbook values for pure annealed bulk 

copper, and the elongations are much lower than 

the handbook values. Increasing the fi lm thickness from, for example, 1 μm to 10 μm, increases 

the ductility from around 1 % or less to 5 % or 

more. These data were obtained in microtensile 

tests of a new variety of electrodeposited copper 

supplied by an industry collaborator. Annealing 

also changes the tensile properties markedly. We 

have extended our microtensile test capability 

to temperatures up to 150 °C. Figure 6 shows 

recent results for contact Al-Si, from MOSIS, and 

electrodeposited copper, made at NIST. 

Figure 6. Tensile strength plotted against 

temperature for microtensile specimens of aluminum 

contact metal obtained through MOSIS, and 

electrodeposited copper made at NIST.

The demonstrated applicability of the force-probe 

technique to a variety of specimen materials and 

temperatures leads us to think that this technique 

and its complementary specimen geometry may 

become a standard method for microtensile testing.

Figure 7a (next page) shows a high-resolution 

SEM image of the surface of an electrodeposited 

(ED) copper specimen made at NIST. The distinctive morphology, an array of joined spheres, 

may not be representative of normal production 

material. But it may represent, in an exaggerated 

form, microstructural features commonly present in ED copper. Its crystallographic symmetry, 

lattice parameter, and mechanical properties all 

appeared normal when measured by standard 

techniques. Figure 7b shows an atomistic model 

that simulates the mechanical behavior of this 

morphology, though at a smaller scale. The atoms shown as solid black are in regions of low 

face-centered-cubic symmetry, while the atoms 

shown in color are surrounded by near-perfect 

fcc structure. This simulation presents a possible 
116 National Institute of Standards and Technology 

explanation for our measured value of the elastic 

constant of ED copper, which is lower than the 

bulk value. We have extended our atomistic modeling to the mechanical behavior of quantum dot 

structures such as germanium in silicon and indium arsenide in gallium arsenide, specifi cally, the 

strain in and around the heterogeneous inclusion. 

Strains surrounding the quantum dot structures 

play an important role in creating the confi ned 

electron states and in providing a driving force 

for self-assembly of arrays of dots. Our recent 

paper on GeSi quantum dots, listed below, modeled strains around a quantum dot and predicted 

surface distortions above buried dots; these might 

be usable to deduce characteristics of buried dots, 

such their size and depth beneath the surface, from 

measurements at the surface.

Figure 7. (a) High-resolution SEM image of 

electrodeposited copper specimen made at NIST. The 

original magnifi cation was 500,000. The copper 

“balls” are 30–50 nm in diameter. (b) Atomistic 

model simulating the morphology and mechanical 

behavior of this copper electrodeposit. The atoms 

shown as copper-colored have near-normal facecentered-cubic crystal environments; the atoms 

shown as black are also copper atoms, but their local 

crystallographic symmetry is lower. Through the use 

of periodic boundary conditions, this 23000 atom 

model simulates a specimen of indefi nite size.

Our a.c. tests continue to reveal damage phenomena drastically different from that observed in 

conventional d.c. electromigration tests. To understand the results, we have pursued crystallographic 

mapping experiments within a field emission 

SEM, due to the generous amount of information 

that can be obtained with such an approach. What 

has become apparent is the tremendous variation 

in behavior from grain to grain. We have observed 

signifi cant growth of selected grains in a polycrystalline interconnect, which subsequently become 

more susceptible to surface offset formation with 

further cycling (Fig. 8). 

#

Figure 8. A.C. testing sequence in Al-1Si, showing 

development of surface offsets, grain size and shape 

changes, and grain orientation changes, at 0, 40, 

320, and 697 seconds of A.C. cycling at about 

12 MA/cm2

. Upper images: surface topography 

by SEM; lower images: same locations, color 

indicates grain orientation; drawing: changes in 

crystallographic orientation of grains as indicated.
Semiconductor Microelectronics and Nanoelectronics Programs 117

Accompanying such grain growth is a re-orienting 

of some grains, into a more accommodating orientation for slip activity. In other words, grain reorienting seems to provide a larger resolved shear 

stress on a particular grain. Many factors interact 

to control the processes leading to catastrophic 

failure by open circuit, at a site where the interconnect cross section has decreased very signifi cantly 

due to deformation. We are attempting to construct 

a model describing the roles of these factors, 

including strength of individual grains (grain 

size), ease of slip within individual grains (grain 

orientation), and prevalence of dislocation sources 

(grain boundary structures, surrounding grain 

constraints). The end goal is to offer a scheme 

for predicting where an open circuit should be 

expected, given the initial grain structure of the 

interconnect. We are exploring the applicability 

of the a.c. test to commercial damascene copper 

structures. In these initial experiments we have 

plotted the data as lifetime to open circuit vs cyclic 

temperature range in Fig. 9. 

Figure 9. Comparison of lifetimes under high 

amplitude a.c. of Cu lines and Kelvin vias in 

damascene structures made with oxide and low-κ

dielectric, plotted as cyclic temperature vs lifetime. 

When plotted in these coordinates, the vias fail at 

much lower temperatures than the lines in both 

structures, but the dielectric material does not affect 

the lifetimes.

On this basis, we found a big difference between 

vias and lines, but no difference between oxide 

and low-κ dielectric structures. The preliminary 

conclusion from these tests is that the combined 

effects of thermal, electrical, and mechanical 

stresses need to be accounted for in designing 

reliable interconnect systems, and that with the 

a.c. test we can explore material response to all 

of these stresses.

It is sometimes impossible to get fi lms of individual materials with the interconnect stack in a 

chip that goes through a normal manufacturing 

process. In the Micro- and Nano-Electro-Mechanical Technology (MNT) Metrology Project, 

we are studying measurement methods that use 

composite laminate specimens that include several 

materials, such as metal and dielectric. The elastic 

modulus of the individual fi lms in the laminate 

is to be determined by differences between the 

resonant frequencies of beams with different 

combinations of metal, dielectric, and polysilicon. 

More information on this effort can be found on 

that project’s report. 

COLLABORATIONS

Tom Mountsier, Novellus Inc.

Michael Lane, IBM, Yorktown Heights, NY

Ju-Young Kim, Seoul National University, Korea

University of Karlsruhe, Germany, Prof. Eduard Arzt, Dr. 

Cynthia Volkert

Intel Corp., Hillsboro, OR, Dr. Sadasivan Shankar, Dr. Brad 

Sun

Motorola, Inc., AISL, Tempe, AZ, Betty Yeung, and MATC, 

Schaumburg, IL, Dr. Andrew Skipor

MOSIS Integrated Circuit Fabrication Service, 4676 Admiralty 

Way, Marina del Rey, CA, Dr. Tom Veriner.

RECENT PUBLICATIONS

R. R. Keller, N. Barbosa III,, R. H. Geiss, and D. T. Read, An 

Electrical Method for Measuring Fatigue and Tensile Properties of Thin Films on Substrates, Key Engineering Materials, 

Vols. 345-346 (2007) pp. 1115-1120 (online at http://www.

scientifi c.net).

D. T. Read and V. K. Tewary, Multiscale model of near-spherical germanium quantum dots in silicon, Nanotechnology 18 

(10) 105402, 2007.

R. H. Geiss and D. T. Read, Need for Standardization of EBSD 

Measurements for Microstructural Characterization of Thin 

Film Structures, accepted for publication in Proceedings of the 

2007 International Conference on Frontiers of Characterization and Metrology for Nanoelectronics, to be published by 

the American Institute of Physics, 2007.

D. T. Read and R. H. Geiss, High Amplitude AC Tests of 300-

nm Damascene Interconnect Structures, submitted to Materials 

Research Society Symposium Proceedings, 2007.

D. T. Read and A. A. Volinsky, “Thin Films for Microelectronics and Photonics: Physics, Mechanics, Characterization, 

and Reliability,” in Micro- and Opto-Electronic Materials and 

Structures: Physics, Mechanics, Design, Reliability, Packaging, edited by E. Suhir, C.P. Wong, Y.C. Lee, Springer, New 

York, 2007.

N. Barbosa III, R. R. Keller, D. T. Read, R. H. Geiss, and R. P. 

Vinci, “Comparison of Electrical and Microtensile Evaluation 

of Mechanical Properties of an Aluminum Film,” submitted 

to Metallurgical and Materials Transactions April 2006. Paper 

presented at 2006 TMS Annual Meeting.
118 National Institute of Standards and Technology 

R. R. Keller, R. H. Geiss, N. Barbosa III, A. J. Slifka, and 

D. T. Read, “Strain-Induced Grain Growth during Rapid 

Thermal Cycling of Aluminum Interconnects,” submitted to 

Metallurgical and Materials Transactions April 2006. Paper 

presented at 2006 TMS Annual Meeting.

D. T. Read, R. R. Keller, N. Barbosa III, and R. H. Geiss, “Nanoindentation Round Robin on Thin Film Copper on Silicon,”

submitted to Metallurgical and Materials Transactions April 

2006. Paper presented at 2006 TMS Annual Meeting.

R. R. Keller, R. H. Geiss, Y.-W. Cheng, and D. T. Read, 

“Microstructure Evolution during Electric Current Induced 

Thermomechanical Fatigue of Interconnects,” Mater. Res. 

Soc. Symp. Proc. Vol. 863, B9.5.1, (2005).

R. H. Geiss, R. R. Keller, D. T. Read, and Y.-W. Cheng, “TEMBased Analysis of Defects Induced by A.C. Thermomechanical 

versus Microtensile Deformation in Aluminum Thin Films,” 

Mater. Res. Soc. Symp. Proc. Vol. 863, B7.2.1, (2005).

R. R. Keller, C. A. Volkert, R. H. Geiss, A. J. Slifka, D. T. 

Read, N. Barbosa III, and R. Mönig, “Electrical Methods for 

Mechanical Characterization of Interconnect Thin Films,” 

2005 Advanced Metallization Conference, p. 643-648.

R. H. Geiss, D. T. Read, A. Roshko, K. A. Bertness, and R. R. 

Keller, “Applications of EBSD to the Study of Localized Deformation,” 2005 Microscopy and Microanalysis Meeting.

Robert R. Keller, David T. Read, and Roop Mahajan, NIST 

Special Publication 1043, Workshop on Reliability Issues 

in Nanomaterials, U.S. Department of Commerce, August 

2005. 

R. R. Keller, and D. T. Read, “The NIST Workshop on 

Reliability Issues in Nanomaterials, August 17-19, 2004,” 

Proceedings, Government Microcircuit Applications & 

Critical Technology Conference 2005, Las Vegas, NV, April 

4-7, 2005.

R. R. Keller, R. H. Geiss, Y.-W. Cheng, and D. T. Read, 

“Microstructure Evolution during Electric Current Induced 

Thermomechanical Fatigue of Interconnects,” Materials, 

Technology, and Reliability of Advanced Interconnects: 

MRS Symposium Proceedings, March 28-April 4, 2005, San 

Francisco, CA.

R. H. Geiss, R. R. Keller, D. T. Read, and Y.-W. Cheng, 

“TEM Analysis of Deformation in Thin Films of Aluminum 

after both A.C. Thermomechanical and Tensile Deformation,” Thin Films – Stresses and Mechanical Properties XI: 

MRS Symposium Proceedings, March 28-April 4, 2005, San 

Francisco, CA.

R. R. Keller, R. H. Geiss, Y.-W. Cheng, and D. T. Read, 

“Electric Current Induced Thermomechanical Fatigue Testing of Interconnects,” AIP Conference Proceedings 788, 

Characterization and Metrology for ULSI Technology, pp. 

491-495 (2005).

Y.-W. Cheng, D. T. Read, J. D. McColskey, J. E. Wright, “A 

tensile-testing technique for micrometer-sized free-standing 

thin fi lms,” Thin Solid Films 484 (1-2), 426-432, 2005.

D. T. Read, “Atomistic simulation of modulus defi cit in an 

aggregate of metal spheres,” Journal of Applied Physics 97 

(1): Art. No. 013522, January 1, 2005.

R. R. Keller, R. H. Geiss, Y.-W. Cheng, and D. T. Read, “Microstructure Evolution During Alternating-Current-Induced 

Fatigue,” IMECE2004-61291, Proceedings of IMECE04, 

2004 ASME International Mechanical Engineering Congress 

and Exposition, November 13-20, 2004.

D. T. Read, Y.-W. Cheng, and R. Geiss, “Mechanical Behavior 

of Electrodeposited Copper Film at Elevated Temperatures,” 

IMECE2004-61320, Proceedings of IMECE04, 2004 ASME 

International Mechanical Engineering Congress and Exposition, November 13-20, 2004.

R. Monig, R. R. Keller, and C. A. Volkert, “Thermal fatigue 

testing of thin metal fi lms,” Review of Scientifi c Instruments 

75 (11): 4997-5004, November 2004.

D. T. Read, Y.-W. Cheng, and R. Geiss, “Morphology, 

Microstructure, and Mechanical Properties of a Copper 

Electrodeposit,” Microelectronic Engineering, 75 (1): 63-70, 

July, 2004.

D. Josell, C. Burkhard, Y. Li, Y.-W. Cheng, R. R. Keller, C. A. 

Witt, D. R. Kelley, J. E. Bonevich, B. C. Baker, B. C., and P. 

Moffat, “Electrical properties of superfi lled sub-micrometer 

silver metallizations,” Journal of Applied Physics, 96 (1): 

759-768, July 1, 2004.
Semiconductor Microelectronics and Nanoelectronics Programs 119

TEST STRUCTURES FOR INTERCONNECT

METROLOGY AND MODELING

Technical Contacts:

M. W. Cresswell

H. Schafft

GOALS

The overall goal is to make contributions to a 

test-structure infrastructure that is relevant to selected state-of-the-art interconnect-system needs 

as stated by the ITRS Interconnect Report 2005 

which states that the function of an interconnect 

system is to distribute clock and other signals 

and to provide power/ground, to and among, the 

various circuits/systems functions on a chip. The 

fundamental development requirement for interconnect is to meet the high-speed transmission 

needs of integrated circuits in the face of further 

scaling of feature sizes.

Test-structure metrology applications can be 

classifi ed into two categories: (1) optimal selection of materials and processes for developing 

and maintaining fabrication processes for new 

interconnect systems that comply with increasing 

device density and performance needs, and, (2) 

parameter extraction for modeling and predicting the performance of interconnect systems for 

particular fabrication architectures. In the fi rst 

category, the project’s test structures for process 

development target the extraction, by electrical 

means, of key dimensional parameters such as 

conductor CD. This is a key unsolved problem for 

cases in which copper conductors are embedded in 

barrier metals of different species. Additionally, 

the adverse interaction of skin effect with highresistance barrier metal alloys near the surfaces 

of conducting features, particularly those of the 

wider upper-level conductors, is a related and 

as-yet unresolved issue. In the second category, 

a near-term goal in materials studies is the fabrication of electrically testable, all copper, features 

having lateral dimensions in the 20- to 100 nm 

range. This project’s unique approach seeks to assess the impact of CD scaling on the fundamental 

physics of electron transport in features built from 

single metal species, such as copper, without the 

metrology complications resulting from having 

other metal species incorporated into interconnect 

features. The information is to generate a computational infrastructure to facilitate modeling the 

performance of features that are replicated with 

two or more metals and to aid the verifi cation of 

dimensional parameter extraction for processcontrol purposes. Among the test structures being 

implemented for modeling the performance of 

advanced interconnect implementations are strip 

lines that are to be rf-tested from d.c. to 10 GHz, 

consistent with projected clock speeds of integrated circuits over the next half decade.

CUSTOMER NEEDS

Interconnect is becoming the principal factor that 

determines the maximum performance that can be 

attained with emerging generations of integrated 

circuits. The dramatic reversal, from performance 

which is limited by transistor delay to that which 

is limited by interconnect delay, challenges the 

approach of continuing to scale the conventional 

metal/dielectric system to meet future interconnect 

requirements. Future advances in IC performance 

will be governed increasingly by the advances 

in interconnect technology, at least as much as 

by advances in active devices. As aluminum is 

replaced by hybrid copper/barrier-metal conductors, the initial benefi ts of the higher conductivity 

are becoming problematic as a result of the predominance of contributions of the barrier metal 

to the effective conductor resistance. There are 

two mechanisms. When barrier metal, with its 

higher resistivity, replaces more highly conducting copper, the proportion of the total CD, which 

is allocated to copper, is reduced dramatically 

as CD is scaled. Confounding the phenomenon, 

higher clock speeds favor conduction by the outer 

regions of the composite conductor, which render 

it adversely non-linear. Modeling the generalized 

binary-metal interconnect conductors at high frequencies is therefore a central issue. This is a very 

complex task on which no known reports exist in 

the technical literature. However, we have been 

able to make a start on providing a tool for dealing 

with this issue by drawing on NIST resources in 

other Divisions.

Whereas there is no simple global solution to the 

challenge of electrical metrology for extracting the 

CDs of binary-metal interconnect conductors at 

this time, advances being pursued by this project 

are likely to play a leading role. Finally, overlay 

is a metrology challenge that is attributable as 

much to scaling per se, rather than to the materials 

selected for interconnect implementation. Overlay 

metrology is being challenged by exacerbation 

of the tool- and wafer-induced shifts that are 

generally manageable for technology generations 

introduced prior to the present time.
120 National Institute of Standards and Technology 

TECHNICAL STRATEGY

The four-part current technical strategy is to build 

on opportunities afforded by the project’s unique 

SCCDRM experience. The same substrates that 

have been developed for CD reference-material 

applications are being applied to the fabrication of 

single-metal, initially copper-only, test structures 

with features having lateral dimensions in the 

20 nm to 200 nm range. The novel process-fl ow, 

which has been reported in a joint paper with the 

University of Edinburgh at recent conferences, 

is illustrated in Fig. 1. The approach shown there 

enables a defi nitive assessment of the impact of 

feature-dimension scaling on the fundamental 

physics of electron transport in narrow features 

patterned from copper, without the complications resulting from having other metal species 

that serve as barrier layers. The information so 

provided enables modeling the performance of 

features that are replicated with copper-cored 

binary-metal technology and will aid in the verifi cation of dimensional parameter extraction for 

process-control purposes. 

We are continuing a study of the interaction of 

skin effect with high-resistance barrier metal alloys near the surfaces of conducting features at 

frequencies that need to be extended to ~100 GHz. 

This higher upper limit is driven by near-term 

year clock speeds having rising and falling edge 

components that substantially exceed 20 GHz. As 

mentioned above, higher clock speeds favor conduction by the outer regions of the binary-metal 

interconnect conductor through the so-called “skin 

effect” phenomenon, which render it adversely 

non-linear. The technical strategy is to model 

the parameters of a selection of binary-metal 

interconnect-conductor architectures using the 

distributed resistance, inductance, conductance, 

and capacitance (RLGC) model over the sated 

range of frequencies using fi nite-element Maxwell 

software, which is available at NIST. This approach is quite ambitious and there are no known 

analytical solutions for generalized binary-metal 

interconnect conductors. Since there is no report 

of such analyses in the scientifi c literature, we 

have chosen fi rst to verify our Maxwell solutions 

for a selection of single-metal architectures for 

which analytical solutions do exist. 

A class of test structures for process maintenance, 

where the need for innovation is driven by scaling, 

is overlay standards. This project has successfully 

applied for patents on an electrically calibrated test 

structure to serve as a process- and tool-specifi c 

overlay standard that avoids all the limitations of 

other approaches. The strategy is to reduce the 

concept to practice by replicating the test structures with a bi-level polysilicon-metal process 

and conduct parametrical testing. Among the test 

structures that are being designed for modelingparameter extraction are strip lines that will be rftested from d.c. to 40 GHz, consistent with clock 

speeds of road-map integrated circuits over the 

next several years. The strategy is the computation of rf-impedance parameters from S-parameter 

measurements. An important spin-off from this 

activity is the application of rf-measurements to 

validating key dimensional parameters of masks 

which are patterned for interconnect fabrication, 

another application which has been sought by 

the mask-vendor and mask-user industries. This 

work will be done initially on binary masks but, if 

successful, will be extended to the examination of 

opportunities in dimensional metrology for more 

complex binary-mask applications. Further into 

the future, the experience gained here may be useful for development of a non-contact electrical-CD 

metrology, which is understood to be needed by 

the photo-mask industry.

Figure 1. The process fl ow which was devised for 

the fabrication of single-metal, copper-only, test 

structures with features having lateral dimensions in 

the 20- to 100-nm range.

The project’s tool-kit has a test structure that will 

allow the extraction, by electrical means, of key 

dimensional parameters for process-control purposes such as copper-cored interconnect-feature 

CD. This responds to an unsolved problem when 

copper conductors are embedded in barrier metals. 

The solution is sought after by companies marketing interconnect-system modeling software tools 

and their industry clients. Our strategy is based 

on a test structure we fi rst proposed and published 

several years ago.
Semiconductor Microelectronics and Nanoelectronics Programs 121

The ITRS Interconnect Report of 2005 relates 

how both grain-size and feature dimensions affect 

the effective resistivity of copper in the electrical 

wiring of integrated circuits. The consequences 

are of growing concern because new technologies require ever-smaller dimensions that lead 

to greater resistance and faster operation, which 

requires smaller resistance. This project strategy 

is to develop an easy-to-use, versatile effectiveresistivity simulation program that is superior to 

other approaches in use for studying size effects in 

collaboration with a graduate program at George 

Washington University. It shows how scattering of electrons by surfaces, grain boundaries, 

and impurities increases the effective resistivity 

of copper in thin fi lms and lines as dimensions 

approach and become shorter than the mean 

free path of electrons in bulk copper. In related 

experimental work, copper fi lms are fabricated, 

their sheet resistances and physical thicknesses are 

measured, and the predictions of the simulation 

program are compared to experimental results. 

Figure 2 shows the various scattering events that 

are included in the simulation program to simulate 

size effects in copper that was reported in a recent 

NIST news release. 

migration, assisting in preparing a new JEDEC 

stress test for stress voiding, and for preparing 

JEDEC guidelines for designing stress-voiding 

test structures.

DELIVERABLES: 

Report results of modeling the distributed resistance values of interconnect conductors that can 

be simulated by both analytical solutions and a 

commercial Maxwell solver. 1Q 2007 

Complete analysis of experimental measured 

increase in effective resistivity of copper fi lms with 

thicknesses decreasing from approximately 200 nm 

to 15 nm, and compare measurements with simulation results. 4Q 2006

Design, and verify by simulation, the performance 

of a new test chip for the fabrication of test structures that can be electrically calibrated to serve 

as an overlay reference material for high-density 

interconnect fabrication. 1Q 2007

Fabricate and test chemically stable test structures 

having copper-only features and make ECD measurements. 4Q 2007

Report initial electrical measurements made on 

second-generation SCCDRM test structures that 

have been converted to single-crystal nickel-silicide. 1Q 2007 

Design, model, fabricate, and test a selection of 

co-planar waveguide test structures on chromeon-glass photo-masks to evaluate the effi cacy of 

developing a non-contact ECD method for sampling 

printed features. 3Q 2007

ACCOMPLISHMENTS

As a result of close extended collaboration 

with the University of Edinburgh, the project has 

been able to develop and report a unique copper damascene process for the fabrication of a 

scaled electrical test structure having copper-only 

features. The purpose is to facilitate studies of 

electron transport in pure copper without having 

to correct for the complexities of the interaction 

of copper with barrier-metal fi lms. This implementation enables the separation of the effects of 

surface and grain-boundary scattering, as a function of the cross-section dimensions of the conductor by electrical testing. A second joint paper 

describing the accomplishment was presented at 

the ICMTS 2007 conference in Tokyo, Japan, on 

March 8, 2006.

In close collaboration with NIST operations in 

Boulder, Colorado, Maxwell software was applied 

to the calculation of RGLC parameters of cylindrical copper conductors with diameters ranging 

from 50 nm to 5000 nm, which were coated 

with TaN barrier metal of thicknesses typical of 

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•

•

•

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#

Figure 2. The scattering events that are included 

in the simulation program to simulate dimensional 

effects in copper.

The project is also providing technical and related support to the JEDEC Committee JC14.2 

on Wafer Level Reliability of the JEDEC Solid 

State Technology Association to enhance the 

reliability of in the new copper interconnect 

technologies. This support is for revising interconnect standards, which were originally written 

for aluminum interconnects, so that they will be 

applicable to copper metallization. The primary 

focus is in writing a new JEDEC standard for 

constant current and temperature stress testing for 

electromigration, assisting in revising the JEDEC 

standard for isothermal stress testing for electro-
122 National Institute of Standards and Technology 

current IC-interconnect applications. At frequencies above 10 GHz, signifi cant increases 

in resistance were observed. These results have 

been set aside to allow for verifi cation of the 

modeling by comparing those of a selection for 

copper-only wires that were obtained from exact 

solutions based on Maxwell’s equations. Initial 

results indicate close comparisons for the cases 

that have so far been compared.

A paper that describes a new simulation program and its use to study the effects of surface 

and grain-boundary scattering on the effective 

resistivity of copper in thin planar fi lms and small 

cross-section lines was completed. The paper was 

accepted for publication in a mid-2006 issue of 

the Microelectronics Reliability journal. 

A report was completed that describes a 

computer program, which simulates the impact 

of the size on the effective resistivity of line and 

fi lm conductors. Flow charts and the program 

code are provided as appendices. This work was 

published in February 2006 as a NISTIR.

In close collaboration with the Laboratory for 

Interconnect and packaging at the University of 

Texas at Austin, test structures for the investigation of the effect of linewidth scaling on electron 

transport in nickel mono-silicide features have 

been designed and fabricated. The features 

were patterned on (110) silicon-on-insulator 

wafers with i-line lithography that replicated 

test structures from which voltage/current (V/I) 

measurements could be extracted. Subsequently, 

the patterning of single-crystal features with 

direct-write electron-beam lithography has been 

developed in order to facilitate the future reduction of the linewidths of NiSi features having a 

highly controlled surface microstructure to linewidths below 40 nm. Figure 3 shows a selection 

of 40 nm features that were etched in (110) silicon 

after electron-beam lithography as reported in a 

joint paper with the University of Texas at Austin 

at a recent conference. 

Work has been initiated on a new CD metrology based on coplanar waveguide test structures. 

Extensive e-m fi eld modeling of characteristic 

impedance and distributed capacitance, which 

we have conducted in collaboration with the Department of Electrical & Computer Engineering 

at George Washington University, indicates that 

the extraction of these values from S-parameter 

measurements can be applied to sampling CDs of 

test-structure features that are printed on photo-

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#

masks. At this time, the design of a set of thru and 

de-embedding structures has been designed and a 

supplier of the substrate has been identifi ed.

A poster paper was delivered at the 2005 IEEE 

International Integrated Reliability Workshop to 

discuss the details of the model used to simulate 

size effect on the resistivity of copper, and to describe results of the simulation studies.

A new JEDEC standard JESD202 was completed and published in March 2006. The standard 

describes an accelerated stress test method for 

determining sample estimates and their confi -

dence limits of the median-time-to-failure, sigma, 

and early percentile of a lognormal distribution, 

which are used to characterize the electromigration failure-time distribution of equivalent metal 

lines subjected to a constant current-density and 

temperature stress. Procedures are provided 

to analyze complete and singly, right-censored 

failure-time data.

The design and fabrication of a new test 

chip for the fabrication of test structures that are 

electrically calibrated to serve as process-specifi c overlay reference materials for high-density 

interconnect fabrication has been completed in 

collaboration with staff at the Scottich Microelectronics Centre of the University of Edinburgh. 

Preliminary results were presented in a paper at the 

IEEE ICMTS 2007 conference in Tokyo, Japan, 

on March 8, 2006.

#

#

#

Figure 3. A selection of 40 nm features that 

were etched in (110) silicon after electron-beam 

lithography.
Semiconductor Microelectronics and Nanoelectronics Programs 123

COLLABORATIONS

The leading collaborators are semiconductor processing facilities and laboratories at three major Universities.

The Scottish Microelectronics Centre at the University of 

Edinburgh (SMC) is now providing a substantial part of the 

project’s wafer fabrication operations. More importantly, the 

staff there are highly and broadly experienced in interconnect 

fabrication materials, processes, and issues. Audio teleconferences are exchanged between Project and SMC staff on this 

and a second project regularly.

The Electrical-Engineering and Computer-Sciences Department at George Washington University has been funding 

the studies of Ph.D students and a Professor to work on this 

NIST Project for almost a decade. The present collaboration 

contributes special skills in rf parameter extraction and the 

physics of electron transport.

A new collaboration with the Microelectronics Research 

Center at the University of Texas at Austin has resulted on the 

transformation of narrow SCCDRM features to silicide material. They are also contributing very desirable ultra-narrow-line 

lithography for the next-generation SCCDRM fabrication.

This project has always benefi ted from an active collaboration 

with Litho-Metrology Operations at SEMATECH. Although 

this project’s focus has only recently shifted away from lithometrology to interconnect, we are actively pursuing a parallel 

shift in our collaboration. 

RECENT PUBLICATIONS

R. A. Allen, R. Dixson, M. W. Cresswell, W. Gutherie, B. 

J. Shulver, A. S. Bunting, J. T. Stevenson, and A. Walton, 

“CD Reference Materials Fabricated on Monolithic 200 mm 

Wafers for Automated Metrology Tool Applications,” 2007 

International Conference on Frontiers of Characterization and 

Metrology for Nanoelectronics, Mar 27-29, 2007, Gaithersburg, MD (01-SEP-2007).

B. J. Shulver, A. S. Bunting, A. Gundlach, L. I. Haworth, A. W. 

Ross, A. J. Smith, A. J. Snell, J. M. Stevenson, A. Walton, M. 

W. Cresswell, R. A. Allen, ‘Extraction of Sheet Resistance and 

Linewidth from All-Copper ECD Test-Structures Fabricated 

from Silicon Preforms’, 2007 International Conference on 

Microlectronic Test Structures, pp.14-19 (22-MAR-2007).

B. Li, L. Shi, P. S. Ho, J. Zhou, R. A. Allen, M. W. Cresswell, 

‘Test Structures for Study of Electron Transport in Nickel 

Silicide Features with Linewidths between 40 nm and 100 nm’, 

IEEE ICMTS International Conference on Microelectronic 

Test Structures, March 06-09, 2006, Austin, Texas, pp.18-23 

(01-APR-2006).

B. J. Shulver, A. S. Bunting, A. Gundlach, L. I. Haworth, A. 

W. Ross, A. J. Snell, J. M. Stevenson, A. Walton, R. A. Allen, M. W. Cresswell, ‘Design and Fabrication of a Copper 

Test Structure as a Electrical Critical Dimension Reference’, 

Proc., IEEE International Conference on Microelectronic Test 

Structures, March 06-09, 2006, Austin, Texas, pp.124-129 

(01-APR-2006).

C. Nwokoye, M. E. Zaghloul, M. W. Cresswell, R. A. Allen, 

C. E. Murabito, ‘A New Critical Dimension Metrology for 

Chrome-on-Glass Substrates Based ons-Parameter Measurements Extracted from Coplanar Waveguide Test Structures’, 

SPIE, Bay Area Chrome Users’ Symposium, September 18-22, 

2006, Monterey, California, 12 pp. (01-OCT-2006).

M. W. Cresswell, W. Gutherie, R. Dixon, R. A. Allen, C. E. 

Murabito, J. V. Martinez, ‘RM 8111: Development of a Prototype Linewidth Standard’, Journal of Research (NIST JRES), 

Vol. 111, No. 3, pp.187-203 (01-MAY-2006).

A. E. Yarimbiyik, H. A. Schafft, R. A. Allen, M. E. Zaghloul, 

and D. L. Blackburn, “Implementation of Simulation Program 

for Modeling the Effective Resistivity of Nanometer Scale Film 

and Line Interconnects,” NISTIR 7234, February, 2006.

JEDEC Standard JESD202, “Method for Characterizing the 

Electromigration Failure Time Distribution of Interconnects 

Under Constant-Current and Temperature Stress,” March 2006. 

(Written by H. A. Schafft).

A. Emre Yarimbiyik, Harry A. Schafft, Richard A. Allen, Mona 

E. Zaghloul, David L. Blackburn: Modeling and simulation 

of resistivity of nanometer scale copper. Microelectronics 

Reliability 46(7): 1050-1057 (2006).

W. F. Guthrie, R. G. Dixson, R. A. Allen, M. W. Cresswell, C. 

E. Murabito, B. Park, “Establishing Traceability of Linewidth 

Reference Materials Using CD-AFM and HRTEM,” 2005 

International Conference on Characterization and Metrology for ULSI Technology, March 15-18, 2005, University of 

Texas at Dallas.

R. A. Allen, A. Hunt, C. E. Murabito, B. Park, W. F. Guthrie, 

and M. W. Cresswell, “Extraction of Critical Dimension Reference Feature CDs from New Test Structure Using HRTEM 

Imaging,” Proceedings of the 2005 IEEE International Conference on Microelectronic Test Structures, pp. 5- 10, March 

2005, Leuven, Belgium.

“Report of Investigation of RM 8111: Single-Crystal Critical 

Dimension Prototype Reference Materials,” National Institute of Standards and Technology, Gaithersburg, Maryland, 

March 2, 2005.

M. W. Cresswell, R. G. Dixson, W. F. Guthrie, R. A. Allen, C. 

E. Murabito, B. Park, J. V. Martinez de Pinillos, and A. Hunt 

“CD Reference Features with Sub-Five Nanometer Uncertainty” published in SPIE Proceedings Vol. 5752 (2005).

A. E. Yarimbiyik, H. A. Schafft, R. A. Allen, M. E. Zaghloul, 

and D. L. Blackburn, “Resistivity of Nanometer-Scale Films 

and Interconnects: Model and Simulation,” 2005 IEEE International Integrated Reliability Workshop Final Report, pp. 139-

140 (open poster). IEEE Catalog Number 05TH8799. ISBM 

0-7803-8992-1. Library of Congress Number 2005920974.

Dixson, R. G., Allen, R. A., Guthrie, W. F., Cresswell, M. 

W., “Traceable Calibration of Critical-dimension Atomic 

Force Microscope Linewidth Measurements with Nanometer 

Uncertainty,” J. Vacuum Sci. Technol. B Vol 23 Issue 6 pp. 

3028-3032 November 30, 2005. 

Cresswell, M. W., Dixson, R. G., Guthrie, W. F., Allen, R. A., 

Murabito, C. E., Park, B., Martinez, J. V., Hunt, A., “Critical 

Dimension Reference Features with Sub-Five Nanometer 

Uncertainty SPIE 30th International Symposium Microlithography Metrology, Inspection, and Process Control,” San Jose, 

CA Vol 5752 pp. 288-303, May 30, 2005. 

Allen, R. A., Hunt, A., Murabito, C. E., Park, B. , Guthrie, W. 

F., Cresswell, M. W., “Extraction of Critical Dimension Reference Feature CDs from New Test Structure Using HRTEM 

Imaging ICMTS IEEE International Conference on Microelectronic Test Structures,” Leuven, Belgium April 18, 2005.
124 National Institute of Standards and Technology 

Bin Li, Li Shi, JiPing Zhou, Paul S. Ho, Richard A. Allen, 

Michael W. Cresswell, “Test Structures for Study of Electron 

Transport in Nickel Silicide Features with Linewidths between 

40 nm and 160 nm,” ICMTS IEEE International Conference 

on Microelectronic Test Structures Leuven, Belgium April 

18, 2005.

CD-AFM reference metrology at NIST and SEMATECH 

Ronald Dixson, Joseph Fu, Ndubuisi Orji, William Guthrie, 

Richard Allen, and Michael Cresswell Proceedings of SPIE 

– Volume 5752 “Metrology, Inspection, and Process Control 

for Microlithography XIX,” Richard M. Silver, Editor, May 

2005, pp. 324-336.

R. A. Allen, R. I. Patel, M. W. Cresswell, C. E. Murabito, B. 

Park, M. D. Edelstein, L. W. Linholm, “Recent Developments 

in Producing Test-structures for Use as Critical Dimension 

Reference Materials,” ICMTS IEEE International Conference 

on Microelectronic Test Structures, Mar 22-25, 2004, Awaji, 

Japan, pp. 35-40 (01-MAR-2004).

R. A. Allen , M. W. Cresswell, C. E. Murabito, R. Dixson, E. 

H. Bogardus, “Critical Dimension Calibration Standards for 

ULSI Metrology,” Characterization and Metrology for ULSI 

Technology: 2003, International Conference on Characterization and Metrology for ULSI Technology, Mar 24-28, 2003, 

Austin, Texas, pp. 421-428 (30-SEP-2003).

R. A. Allen, B. A. am Ende, M. W. Cresswell, C. E. Murabito, 

T. J. Headley, W. F. Guthrie, L. W. Linholm, C. H. Ellenwood, 

E. H. Bogardus, “Test Structures for Referencing Electrical 

Linewidth Measurements to Silicon Lattice Parameters Using 

HRTEM,” IEEE Transactions on Semiconductor Manufacturing, Vol. 16, No. 2, pp. 239-248 (01-MAY-2003).

R. A. Allen, M. W. Cresswell, L. W. Linholm, “JunctionIsolated Electrical Test Structures for Critical Dimension 

Calibration Standards,” 2003 ICMTS IEEE International 

Conference on Microelectronic Test Structures, Mar 17-20, 

2003, Monterey, California, pp. 3-7 (31-MAR-2003).

J. von Hagen, H. A. Schafft, “Temperature Determination 

Methods on Copper Material for Highly Accelerated Electromigration Tests (e.g., SWEAT),” 2002 IEEE International 

Integrated Reliability Workshop Final Report, 2002 Integrated 

Reliability Workshop, Oct 21-24, 2002, Lake Tahoe, California, pp. 45-49 (01-FEB-2003).

H. A. Schafft, J. R. Lloyd, “Electromigration Discussion Group 

Summary,” 2002 IEEE International Integrated Reliability 

Workshop Report, 2002 IEEE International Integrated Reliability Workshop, Oct 21-24, 2002, Lake Tahoe, California, 

pp. 204-206 (01-FEB-2003).

H. A. Schafft, L. M. Head, J. Gill, T. D. Sullivan, “Early Reliability Assessment Using Deep Censoring,” Microelectronics 

Reliability, Vol. 2003, No. 43, 16 pp. (01-JAN-2003).
Semiconductor Microelectronics and Nanoelectronics Programs 125

PB-FREE SURFACE FINISHES FOR ELECTRONIC

COMPONENTS: SN WHISKER GROWTH

Technical Contacts:

W. J. Boettinger

K.-W. Moon

G. R. Stafford

M. E. Williams

“NIST provides an 

invaluable service to the 

industrial community 

through

unparalleled commitment 

to understanding the 

fundamental scientifi c

principles and the basic 

metrology required to 

properly investigate a

metallurgical problem..”

Dr. Robert Hilty, 

Director of Materials Research, 

Tyco Electronics 

Technology Division

Middletown, PA

GOALS

Spurred on by European legislation and the 

“green movement” of the Pacifi c Rim countries, 

the worldwide electronics industry is being compelled to remove lead (Pb) from components. The 

formation of tin (Sn) whiskers on Pb-free surface 

fi nishes is a major reliability issue. Numerous 

failures due to shorting from whiskers have been 

reported, especially in the high reliability realm of 

aerospace, military, and medical device applications. The goal of this project is to provide data 

and materials measurements of critical importance 

in order to develop a fundamental understanding 

of Sn whisker growth and provide industry with 

information needed for mitigation strategies.

CUSTOMER NEEDS

For many years, electronic component leads and 

leadframes made of copper (Cu) or Cu alloys 

were “pretinned” with a protective layer of electrodeposited Sn-Pb prior to use or storage. This 

protective layer preserves the solderability of 

the component leads when assembled on circuit 

boards. Elimination of Pb has been mandated 

by the European Community (EC) directives 

on Waste Electrical and Electronic Equipment 

(WEEE) and the EC Restriction of the Use of 

Certain Hazardous Substances (RoHS) in Electrical and Electronic Equipment. Both took effect 

in July 2006. China’s version of RoHS was 

implemented in March 2007. U.S. manufacturers 

must comply with these requirements in order to 

compete in the global market.

It is well known that the use of pure Sn protective 

electrodeposits has serious problems. Sn whiskers 

(fi laments of metal typically 1 μm in diameter and 

several mm long) can grow from these deposits 

and cause electrical shorts and failures. Historically, Pb was added to Sn deposits to prevent 

whisker growth as well as to lower cost. Thus 

the development of Pb-free Sn alloy deposits to 

replace Pb-containing Sn deposits is considered 

important, and tests which ascertain the tendency 

to form whiskers are critical. 

The U.S. microelectronics industry has clearly 

articulated a need for research and measurement 

to predict and prevent Sn whiskers in the 2007 

International Electronics Manufacturing Initiative (iNEMI), and 2006–2007 IPC technology 

roadmaps. Specifi c industrial needs have been 

identified through NIST participation in the 

iNEMI technology working groups (TWG) on 

Sn whiskers. A series of industry and academia 

sponsored workshops have been held over the past 

six years to address the problem of Sn whisker 

growth. In order to prevent or mitigate whisker 

growth, these workshops have reached a consensus list of concerns: understand the infl uence of 

electroplating conditions, grain structure/shape/

orientation, compressive stress in the electrodeposits, intermetallic compound (IMC) formation 

at the SnCu interface, diffusion of Sn, and thermal 

cycling effects on electroplated components.

TECHNICAL STRATEGY

The current NIST research program has focused 

on three of industries concerns; viz., plating conditions/grain structure modifi cation, IMC formation, 

and stress measurements using X-ray diffraction. 

Many companies are also attacking the list of 

industry concerns; e.g., Boeing has studied Sn diffusion using Sn isotopes, Agere Systems has lead 

the way in uncovering thermal cycling effects, and 

Freescale has examined the relationship between 

grain orientation and whiskers.

The NIST work is examining the following. 

Grain size and shape comparison of deposits 

electroplated at different current densities with 

and without pulsing and with and without Bismuth 

(Bi) addition have been conducted. The infl uence 

of interfacial IMC growth was eliminated by using 

a substrate that does not react with the Sn based 

deposits to form IMC. The use of X-ray diffraction has been employed to measure the stress in 

electroplated Sn deposits. iNEMI, specifi cally 

requested NIST to, “.... take the leadership role 

in developing the measurement standards and the 

required degree of process specifi city.” 

DELIVERABLES: 

Determine feasibility of X-ray sin2ψ methods for 

measuring residual stress in Sn electrodeposits. 

3Q 2007

Determine methods to eliminate columnar grain 

structure in Sn electrodeposits. 3Q 2007

Complete Manuscript “The infl uence of pulse pulsing on the grain structure of Sn Electrodeposits.” 

4Q 2007

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•
126 National Institute of Standards and Technology 

ACCOMPLISHMENTS

Two separate studies involved modifying the 

Sn grain structure. Varying the electroplating 

current density through a wide range modifi ed 

both the grain size and structure of the Sn deposit 

(see Fig. 1). 

#

Figure 1. Surface microstructures and cross section 

FIB microstructures of bright Sn on Cu substrates 

depending on current density: a) 30 mA/cm2 b) 

90 mA/cm2, and c) 200 mA/cm2. As the current 

density increases, the columnar grain size decreases 

and the grain shape changes from wavy grain 

boundaries to facetted grain boundaries.

The addition of Bi combined with pulse plating disrupted the columnar structure resulting in 

equiaxed grains. Under certain values of plating 

current and levels of Bi solute addition, the fi lamentary whisker growth was eliminated. 

Intermetallic compounds (IMC) form at the 

interface between the Sn deposit and Cu substrates. Bright Sn and a bright Sn-Cu alloy were 

plated on Tungsten (W), a substrate material that 

does not form intermetallic compounds with Sn, 

and stored at room temperature. As with the 

electrodeposits on Cu, the deposits on W showed 

hillocks on pure Sn deposits and whiskers on 

the Sn-Cu alloy. Thus the absence of interfacial 

IMC had no effect on the formation whiskers. 

This is important because several authors have 

claimed that IMC formation is the unique cause 

of whisker growth. 

Residual stress in electroplated Sn deposits 

was measured using the sin2

ψ X-ray diffraction 

method. The anisotropic nature of the Sn crystal 

lattice (bct) and the low stress levels (~ 10 MPa) 

presented many challenges for this technique. A 

detailed analysis of errors in the measurement 

method as it applies to Sn has been performed and 

a best practice guide is being written.

NIST remains a co-chair of the iNEMI Sn 

Whisker Modeling Group (members include 

ChipPAC, Cookson, Delphi Delco, Freescale, 

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Hewlett Packard, IBM, Intel, Intersil, IPC, Motorola, Rohm & Haas Electronic Materials, Soldering Tech, Solectron, Sun Microsystems, Texas 

Instruments, and Tyco Electronics). NIST is also 

an active participant in the iNEMI Accelerated Sn 

Whisker Test Group.

COLLABORATIONS

International Electronics Manufacturing Initiative (iNEMI); 

Co-Chair of the Sn Whisker Modeling Group and a member 

of the Accelerated Sn Whisker Test Group. In an effort to 

develop a standard X-ray diffraction method to measure stress 

in Sn electrodeposits we are conducting round robin experiments with Dr. Robert Hilty of Tyco Electronics located in 

Middletown, PA.

Northrup-Grumman, Linthicum, MD; NIST provided fast 

growing whisker samples of Sn-Cu electrodeposits on Zn 

substrates for tests of acrylic conformal coatings. These 

samples can reduce the evaluation time of the coatings used 

by industry as a whisker mitigation method to prevent whisker 

related failures of electronic devices.

RECENT PUBLICATIONS

M. E. Williams, K.-W. Moon, C. E. Johnson, W. J. Boettinger, 

D. Josell, and A. Deal (Lehigh University), “Hillock and Whisker Growth on Sn and SnCu Electrodeposits on a Substrate 

Not Forming Interfacial Intermetallic Compounds,” Journal 

of Electronic Materials (March 2007).

G. R. Stafford, C. Beauchamp, U. Bertocci, E. Sandnes, and 

W. J. Boettinger, “Sn Whisker Formation: Stress Generation 

at the Sn-Cu Interface,” Spring MRS Meeting, Symposium E: 

Pb-Free and RoHS-Compliant Materials and Processes for Microelectronics, San Francisco, California, April 9-13, 2007.

G. R. Stafford, M. E. Williams, C. E. Johnson, K.-W. Moon, 

U. Bertocci, O. Kongstein, and W. J. Boettinger, “Whisker 

Formation in Pb-free Surface Finishes,” ECS Transactions, 

v 1, n 13, Green Electrodeposition, 2006, pp 71-85.

M. E. Williams (NIST), V. Schroeder (HP), P. Bush (SUNY 

Buffalo), N. Vo (Freescale), H. L. Reynolds (Sun Microsystems), “Tin Whisker Test Method Development,” IEEETransactions on Electronics Packaging Manufacturing, V29, 

4, 2006, pp 231-238.

W. J. Boettinger, U. R. Katttner, K.-W. Moon and J. H. Perepezko (U. of Wisconsin, Madison), “Recommended Practice 

Guide for Differential Thermal Analysis (DTA) and Heat 

Flux Differential Thermal Analysis (DSC) Measurements of 

Alloy Melting and Freezing,” NIST Special Publication Series 

960-15, Feb. 2007.

W. J. Boettinger, C. E. Johnson, L. A. Bendersky, K.-W. Moon, 

M. E. Williams and G. R. Stafford, “Whisker & Hillock Formation on Sn, Sn-Cu and Sn-Pb Electrodeposits,” Acta Materialia 

53 (2005) 5033–5050.

R. J. Schaefer, D. J. Lewis, “Directional Solidifi cation in a 

AgCuSn Eutectic Alloy,” Metallurgical Transactions A, 36A 

(10): (2005) pp 2775-2783.

K.-W. Moon, C. E. Johnson, M. E. Williams, O. Kongstein, 

G. R. Stafford, C. A. Handwerker, and W. J. Boettinger. “Observed correlation of Sn oxide fi lm to Sn whisker growth in SnCu electrodeposit for Pb-free Solders,” Journal of Electronic 

Materials, V34, 9, 2005, pp L31-33.
Semiconductor Microelectronics and Nanoelectronics Programs 127

C. A. Handwerker, “Transitioning to Pb-Free Assemblies,” 

Circuits Assembly, p. 39, March 2005.

C. A. Handwerker, F. W. Gayle, E. E. de Kluizenaar, K. 

Suganuma, “Major International Lead (Pb)-Free Solder 

Studies,” in Handbook of Lead-Free Solder Technology for 

Microelectronics Assemblies, ed. By K.J. Puttlitz and K.A. 

Stalter, Marcel Dekker, NY, pp 665-728 (2004).

K.-W. Moon, W. J. Boettinger, “Diffi culties in Determining 

the Eutectic Compositions: The Sn-Ag-Cu Ternary Eutectic,”

Journal of Metals 56 (4), pp. 22-27 (2004).

U. Bertocci, “An EQMB examination of Cu surface oxides 

in borate buffer,” Electrochemical Acta, 49 (11): pp.1831-

1841 (2004).

C. A. Handwerker, U. Kattner, K.-W. Moon, “Materials Science Concepts in Lead-Free Soldering,” Lead-Free Soldering 

in Electronics, eds. Suganuma, K., Dekker, M., New York pp. 

19-48 (2004).

S. L. Allen, M. R. Notis, R. R. Chromik, R. P. Vinci, D. J. 

Lewis, and R. J. Schaefer, Microstructural Evolution in PbFree Part 2: “Directional Solidifi ed Eutectic in SnAgCu, SnCu, 

and SnAg Alloys,” Journal of Materials, Research, 19 (5): pp. 

1425-1431 (May 2004).

A. Rae, C. A. Handwerker, “NEMI’s Lead-free Alloy; Applicable to Today’s Commercially Available Alloys,” Circuit 

Assembly, vol. 15 (4), pp. 20-25 (April 2004).

W. J. Boettinger, M. D. Vaudin, M. E. Williams and L. A. 

Bendersky, “EBSD and EDS Study of the Phase NiSn4 ,” J. 

Electronic Materials, 32, 511-515 (2003).

C. A. Handwerker, J. Bath, E. Benedetto, E. Bradley, R. 

Gedney, T. A. Siewert, P. Snugovksy, and J. Sohn. “NEMI 

Lead-Free Assembly Project: comparison between PbSn and 

SnAgCu reliability and microstructures,” SMTAI Conference 

Proceedings, Rosemont, IL, pp. 664-669 (Sept. 2003).

I. Boguslavsky, P. Bush, E. Kam-Lum, M. Kwoka, J. McCullen, K. Spalding, K. Vo, and M. E. Williams. “NEMI tin 

whisker test method standards,” SMTAI Conference Proceedings, Rosemont, IL, pp. 670-675 (Sept. 2003). 
128 National Institute of Standards and Technology 
Semiconductor Microelectronics and Nanoelectronics Programs 129

PROCESS METROLOGY PROGRAM

Device scaling has been the primary means by which the semiconductor industry has achieved 

unprecedented gains in productivity and performance quantifi ed by Moore’s Law. Until 

recently only modest changes in the materials used have been made. The industry was able 

to rely almost exclusively on the three most abundant elements on Earth — silicon, oxygen, 

and aluminum.

Copper is now predominantly the conductor of choice over aluminum because of its intrinsic 

higher conductivity. A variety of low-dielectric constant materials are being introduced to 

reduce parasitic capacitance, replacing silicon dioxide. As dimensions continue to shrink, 

the traditional silicon dioxide gate dielectric thickness has been reduced to the point where 

tunneling current has become signifi cant and is compromising the performance of the transistors. This is requiring the introduction of higher dielectric constant materials. Initially 

the addition of nitrogen to the gate material is suffi cient, but in the near future more exotic 

materials such as transition metal oxides, silicates, and aluminates are required. Additionally, the gate conductor, traditionally polysilicon, is being replaced by metal or metal silicide 

to eliminate the polysilicon depletion effect. With the replacement of the traditional silicon 

dioxide/polysilicon gate stack processes with materials capable of supporting ever shrinking 

geometries, the task of the industry becomes more diffi cult. The overall task represented by 

the projects below refl ects the need for analytical techniques with unparalleled spatial resolution, accuracy, robustness and ease of use.

Accurate metrology of process gases is essential for reproducible manufacture of semiconductor products. Critical physical parameters need to be measured on a wide variety of reactive 

and non-reactive process gases, allowing the accurate calibration of fl ow meters and residual 

gas analyzers. Water and other contaminants at extremely low levels in process gases present serious manufacturing diffi culties. Accurate calibration of water vapor at extremely low 

vapor pressures is required.

Atomic layer deposition processes are increasingly being used for high quality thin dielectrics 

and conductors. Techniques for understanding the deposition mechanisms and characterizing 

the compounds that are formed are being developed. Theoretical studies elucidating the thermodynamics and quantum mechanical properties of these compounds are being conducted.
130 National Institute of Standards and Technology 
Semiconductor Microelectronics and Nanoelectronics Programs 131

GAS PROPERTY DATA AND FLOW STANDARDS FOR

IMPROVED GAS DELIVERY SYSTEMS

Technical Contacts:

J. J. Hurly

M. R. Moldover 

“I was pleasantly 

surprised when I came 

across your database 

of thermophysical 

properties of gases used 

in the semiconductor 

industry. 

It was virtually exactly 

what I was searching 

for.”

Bob Rathfelder, 

Project Engineer, 

Parker Hannifi n 

GOALS

NIST will measure the thermophysical properties 

of the gases used in semiconductor processing. 

The property data will improve the modeling of 

chemical vapor deposition and the calibration 

of mass fl ow controllers (MFCs). As the data 

are acquired, they will be posted to an online 

database. 

NIST also will develop primary standards for gas 

fl ow in the range from 10-7 to 10-3 mol/s and transfer this fl ow measurement capability to the US 

semiconductor industry (10-6 mol/s ≈1.3 standard 

cubic centimeters per minute (sccm)).

CUSTOMER NEEDS

The 2005 International Technology Roadmap 

for Semiconductors (ITRS) emphasizes that the 

grand challenge for front-end processes is “material limited device scaling.” The Modeling and 

Simulations section reinforces this theme: “Modeling and simulation tools in equipment, process, 

device, package, patterning, and interconnect are 

only as good as the input materials parameters. 

In many cases, these parameters are not known. 

Databases ... are needed.” Meeting this challenge will require improvements in MFCs for the 

deposition and etching of diverse new materials. 

Thermal MFCs meter a wide variety of toxic, 

fl ammable, and corrosive gases over a large range 

of fl ow rates. Elements necessary for improved 

MFCs will be accurate models of gas properties 

and reliable physical standards for gas fl ow.

Participants at an industry workshop at NIST 

identifi ed the gases and properties of highest 

priority, and they recommended publishing the 

property values in a public, Web-based database. 

The gases include process gases, “surrogate” 

calibration gases, and binary mixtures of process 

and carrier gases. The identifi ed properties and 

required uncertainties include the following.

heat capacity at constant pressure ±0.1 %

equation of state (gas density) ±0.1 %

viscosity ±0.5 %

thermal conductivity ±0.5 %

The workshop also required the following uncertainties for physical standards for gas fl ow.

primary standards for gas fl ow ±0.025 %

transfer standards for gas fl ow ±0.1 %

TECHNICAL STRATEGY

We are measuring the speed of sound u(T, p) in 

process gases and in the surrogate gases that are 

often used for calibration. The speed-of-sound 

data have relative standard uncertainties of 0.01 %. 

Measurement conditions range as high as 425 K 

and 1500 kPa (or to 80 % of the vapor pressure for 

condensable gases). Figure 1 shows an example 

of such data. The speed-of-sound data are used 

to determine the ideal-gas heat- capacity Cp

0

(T) 

with the targeted uncertainty of 0.1 %. The pressure and temperature-dependences of u(T, p) are 

correlated with model two-body and three-body 

intermolecular potentials. These potentials are 

used to calculate the virial equation of state for 

the density ρ(T, p) and to get fi rst estimates of the 

viscosity η(T) and the thermal conductivity κ(T). 

For gases where reliable data exist, we verifi ed 

that results calculated in this way have uncertainties that are less than 0.1 % for density, 10 % for 

viscosity, and 10 % for thermal conductivity from 

200 K to 1000 K.

We also are developing novel acoustic techniques 

to measure the viscosity and thermal conductivity 

with uncertainties of less than 0.5 % as specifi ed 

Figure 1. Speed of sound in tungsten hexafl uoride as 

a function of pressure along isotherms.
132 National Institute of Standards and Technology 

by the industry workshop. Figure 2 shows a second-generation acoustic viscometer made from 

Monel. Throughout the project, the results will 

be made available to industry through publications in professional journals, presentations at 

professional meetings, and entries in an on-line 

database at http://properties.nist.gov/semiprop 

(see Fig. 3).

Figure 2. Second-generation acoustic viscometer 

made from Monel for use with corrosive gases.

Figure 3. Sample page from on-line database 

located at http://properties.nist.gov/semiprop/.

We have developed a diverse series of primary 

standards for gas fl ow. The fi rst was a constantvolume (pressure rate-of-rise) primary standard 

that we developed to measure fl ows up to 10-3 

mol/s with uncertainties of about 0.1 %. It has 

been replaced by a constant-pressure (variable 

volume) standard that can operate at pressures 

from 0.5–9 atmospheres. The third primary standard is gravimetric; fl ow measurements made by 

a transfer standard are integrated and compared 

to the weight change of a gas bottle. The second 

and third standards have a standard uncertainty 

of 0.02 %. Figure 4 demonstrates the accuracy 

of the fl ow standards.

Transfer standards allow the primary fl ow standards at NIST to be compared to fl ow meters 

at other locations, such as MFC manufacturers. 

Although a fl ow meter manufacturer often uses 

its own primary standard, comparisons with NIST 

allow the manufacturer to demonstrate profi ciency 

and, if necessary, provide traceability to NIST. For 

this purpose, we have developed a series of very 

stable transfer standards based on laminar fl ow 

through a thermostatted duct. The fi rst generation 

used a stainless steel, helical duct of rectangular 

cross-section. It was used to perform on-site 

profi ciency tests of industrial fl ow standards at 

fabrication facilities and MFC manufacturers. The 

second-generation transfer standard uses quartz 

capillaries with a circular cross-section, which are 

available commercially for gas chromatography. 

It has been used for comparisons with metrological 

institutes of other countries as well as manufacturers of fl ow meters. Its standard uncertainty is 

0.03 %. The third-generation standard improves 

convenience by combining the quartz capillary 

with commercial instrumentation to measure 

pressure and temperature.

DELIVERABLES: 

Install new spherical resonator for measuring the 

speed of sound in the hazardous gases facility. 

4Q 2007 

Calibrate resonator. 4Q 2007

A cylindrical resonator was used to study nine 

process gases, but it must be replaced due to 

contamination. A spherical resonator has been 

fabricated and will be installed in the hazardous 

gas handling facility. The spherical resonator will 

be capable of producing higher accuracy measurements than the cylindrical resonator.

•

•

Figure 4. Comparison of three primary fl ow meters. 

The transfer standard compared the constantpressure fl ow meter (circles) with the gravimetric 

fl ow meter (squares) and a constant-volume fl ow 

meter (triangles). (1 μmol/s ≈ 1.3 sccm.)
Semiconductor Microelectronics and Nanoelectronics Programs 133

DELIVERABLES: 

Develop improved model of acoustic viscometer.

2Q 2007

Develop model of acoustic thermal conductivity 

resonator. 3Q 2007

The second-generation Greenspan viscometer 

incorporates lessons learned from the previous 

device, thereby allowing an improved acoustic 

model. The model of the thermal conductivity 

acoustic resonator will need to be tested and further developed from calibration measurements. 

DELIVERABLES: Publish in an archival journal the 

measured viscosity in BCl3

, Cl2, HBr, C4F8, CO, CO2, 

NH3, and SiF4. 2Q 2007

The acoustic model for the Greenspan viscometer 

must be applied to each data set to calculate the 

viscosities of the gases as a function of temperature and pressure. 

DELIVERABLES: Update on-line database by with 

viscosity measurements in BCl3, Cl2, HBr, C4F8, CO, 

CO2, NH3, and SiF4. 4Q 2006 

The measurements will be disseminated through 

papers in professional journals, talks given at 

professional meetings, and the on-line database.

ACCOMPLISHMENTS

We designed and assembled a second-generation Greenspan viscometer. Its Monel construction 

allows the study of corrosive process gases.

We measured the speed of sound in the 

process gases Cl2, NF3, and N2O. Typically, the 

standard uncertainty of the speed of sound was 

less than 0.01 %. From these data the ideal-gas 

heat-capacity was determined to within 0.1 %, 

and an equation of state was developed to predict 

the gas densities to within 0.1 %. Viscosity was 

measured in these three gases plus CF4

 and C2F6

with an uncertainty of approximately 0.5 %. Viscosity was also measure in the gases BCl3

, Cl2, 

HBr, C4F8, CO, CO2, NH3, and SiF4. This data set 

is being prepared for publication and addition to 

the online database.

We continued to provide immediate access to 

our results by updating the database of gas properties at http://properties.nist.gov/semiprop/.

We verifi ed the accuracy of the primary fl ow 

meters by comparing the lower and upper ends 

of their ranges with other, overlapping NIST 

fl ow meters. Near both the lower end (0.3 sccm) 

•

•

#

#

#

#

and the upper end (1000 sccm) the agreement of 

0.03 % was within the mutual uncertainty of the 

comparison. 

We used the second-generation transfer standard and a prototype of the third-generation transfer standard to make a comparison of gas fl ows 

with a manufacturer of mass fl ow controllers. 

RECENT PUBLICATIONS

R. F. Berg, “Simple fl ow meter and viscometer of high accuracy for gases,” Metrologia, 42, 11-23 (2005).

R. F. Berg and S. A. Tison, “Two primary standards for 

low fl ows of gases,” J. Res. Natl. Inst. Stan., 109, 435-450 

(2004).

J. J. Hurly, “Viscosity and speed of sound of gaseous nitrous 

oxide and nitrogen trifl uoride measured with a greenspan 

viscometer,” Int. J. Thermophys, 25, 625-641 (2004).

#
134 National Institute of Standards and Technology 
Semiconductor Microelectronics and Nanoelectronics Programs 135

LOW CONCENTRATION OF HUMIDITY STANDARDS

Technical Contacts:

J. T. Hodges

D. Ripple

K. Bertness

“The LFPG is the ‘Gold’ 

standard for moisture 

generation and after the 

round robin experiments 

it is possible for the 

industry to talk about 

moisture measurements 

that can be compared to 

the standard.”

Suhas Ketkar, 

Air Products and Chemicals

GOALS

The primary objective is to establish quantitative 

standards enabling the accurate measurement of 

trace quantities of water vapor (< 1013 molecules 

cm-3). This effort supports the development and 

application of commercial humidity sensors used 

for gas purity measurements and inline monitoring 

and process control — functions that are relevant 

to minimizing wafer misprocessing.

CUSTOMER NEEDS

Measurement needs and technical challenges 

for airborne molecular contamination (AMC) 

in semiconductor wafer processing spanning 

the next 15 years are given in the 2005 International Technology Roadmap for Semiconductors 

(ITRS) http://public.itrs.net/. A key measure of 

technological progress defi ned in the ITRS is 

yield enhancement (YE) which is the process of 

improving baseline yield for a given technology 

generation from R&D yield level to mature yield. 

AMC affects YE and constitutes a major impediment to wafer environment contamination control 

particularly for 300 mm wafer manufacturing 

processes. An industry expert, G. Dan Hutcheson 

of VLSI Research Inc., estimates that “a 1 % yield 

increase equates to $1M per day additional profi ts 

for a modern 300 mm fabrication line,” The Chip 

Insider®. The ITRS also states “The impact of 

AMC on wafer processing can only be expected 

to become more deleterious as device dimensions 

decrease. There is a need for better AMC monitoring instrumentation ... to measure AMC at the part 

per trillion level ... low cost, routine monitoring 

may be required as devices approach molecular 

dimensions.” Target impurity levels relevant to 

wafer environmental contamination control for a 

number of AMC including H2O, THC, CO2 and 

other AMCs in various bulk gases are given in 

ITRS Table 115 and quantify the measurement 

needs in a variety of wafer production processes. 

Of these impurities, water vapor is one of the most 

ubiquitous and diffi cult to eliminate. Similarly, 

the Semiconductor Equipment Manufacturers 

International (SEMI) has standards describing 

the production and delivery and value assignment of ultra-high purity gases for semiconductor 

manufacturing http://downloads.semi.org/PUBS/

SEMIPUBS.NSF/webstandardsgases. These 

standards illustrate the stringent requirements 

for gas purity measurements in semiconductor 

processes. We also note a new emphasis (beginning with the 2004 ITRS) on epitaxial processes 

that use gases as source materials, including SiGe 

and III-V semiconductor requirements for power 

amplifi ers and extension of physical models to 

III-V semiconductors.

Although a variety of high sensitivity sensors of 

water vapor are available, most do not directly 

measure water in the gas phase. Rather they 

typically respond to moisture-induced changes 

in bulk or surface properties associated with 

the adsorption of water vapor. Consequently, a 

rigorous fi rst-principles determination of sensor 

response is often precluded, thus compromising 

accuracy. Moreover, since many such devices 

exhibit drift and poor reproducibility, frequent 

recalibration is required. Interpretation of these 

measurements is also complicated by complex 

physical interactions of water vapor with technical 

surfaces in transfer lines, in reaction chambers and 

in sensor housings.

TECHNICAL STRATEGY

The development of accurate and robust water 

vapor sensors requires well-characterized reference standards against which such devices can be 

evaluated. This should include a primary method 

of measurement for water vapor concentration and 

a complementary method yielding high-precision 

and stable sources of water vapor. By providing 

access and traceability to the unique capabilities at 

NIST discussed below, instrument manufacturers 

and sensor users can assess the overall performance and accuracy of their measurements. 

Our strategy is to establish complementary capabilities in high-precision generation and measurement of water vapor. To address these respective 

needs, we have developed a thermodynamically 

based humidity source and high-sensitivity optical absorption measurement methods discussed 

below. The thermodynamic humidity source, 

known as the Low Frost-Point Generator (LFPG) 

(see Fig. 1 and Fig. 2, next page), serves as the 

project cornerstone and is capable of delivering 

3 mmol to 3 nmol of water vapor per mole of dry 

gas. Here the water vapor concentration in a gas 

stream is precisely controlled by active regulation of the saturator temperature and pressure. 

As such, the LFPG is ideally suited for testing 

the performance of various sensing and humidity 

generation technologies. To date, it is has been 

used to characterize water vapor measurement and 

generation systems at the research and development stage as well as commercial devices.
136 National Institute of Standards and Technology 

1. A common technology used by the semiconductor industry for delivering controlled quantities of water vapor is based upon the controlled 

permeation of water vapor (called permeation tube 

generator (PTG)) through a material, followed by 

mixing and dilution with a dry gas of known fl ow 

rate. We have constructed a calibration system for 

water permeation tubes, comparing permeation 

tubes to the LFPG using a commercial water 

vapor sensor as a nulling device. This approach 

constitutes an effi cient and low-cost mechanism 

for the dissemination of NIST trace humidity standards. Experience with the system has revealed 

several limitations of traditional implementations, 

including deviations of the output from the equation generally used to predict the temperature 

dependence of the permeation rate. 

Figure 1. NIST Low Frost-Point (humidity) 

Generator.

Figure 2. Steady state response of saturator control 

thermometers in NIST Low Frost-Point Humidity 

Generator.

DELIVERABLES: 

Complete uncertainty analysis of permeation-tube 

calibration system and measure temperature dependence of permeation tubes. 3Q 2006

Complete documentation of system. 1Q 2007

2. In its standard mode of operation, the LFPG is 

currently limited to generating greater than 3 nmol 

mol-1 of water vapor in N2 based on the minimum 

achievable temperature of the saturator. We have 

recently demonstrated a new strategy for pmol 

mol-1-level humidity generation using dry-gas 

dilution of the water vapor/gas mixtures produced 

by the LFPG output stream and have validated the 

technique with a variety of self-consistency tests. 

Presently, we are establishing the uncertainties and 

optimal methodologies for this technique. Development of gas-handling manifolds with ultra-low 

adsorption/desorption of water will be a key focus 

of our work in the next year.

DELIVERABLES: 

Perform uncertainty analysis of dilution scheme. 

3Q 2006

Develop improved gas purifi cation scheme for the 

dry-gas source. 4Q 2006

Investigate attainable dry-drown rates using coated 

tubes for gas manifolds. 2Q 2007

4. To complement our established capability 

in precision generation of trace humidity levels, 

we are developing absolute techniques based 

upon the absorption of visible and near-infrared 

laser radiation. Water vapor has an absorption 

spectrum comprising thousands of distinct vibrational absorption transitions in this spectral 

region. Thus, the concentration of water vapor 

can be readily determined in terms of measurements of sample absorbance and independently 

determined absorption line intensities. Recent 

advances in source and detector technology, 

and new spectroscopic techniques that extend 

the sensitivity of laser absorption measurements 

now enable the precise sensing of water vapor at 

concentrations below 1010 molecules cm-3. To 

account for line broadening effects, and mitigate 

interference effects associated with absorption 

by other species the most precise absorption 

measurements require that individual transitions 

be spectrally resolved. This demands a technique 

having a frequency resolution much smaller than 

the characteristic widths of the absorption transitions, and requires that the frequency intervals in 

•

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•

•
Semiconductor Microelectronics and Nanoelectronics Programs 137

the measured spectrum be accurately determined. 

By combining high spectral resolution with high 

precision absorbance measurements, the water 

vapor concentration can be found independently 

of the composition of the carrier gas. Of the 

optical absorption methods, cavity ring-down 

spectroscopy (CRDS) is expected to be the most 

suitable for a primary method (Fig. 3). CRDS is a 

cavity-enhanced optical absorption technique that 

has high sensitivity, fast response, and probes a 

compact well-defi ned volume. It is important to 

emphasize that under certain conditions, CRDS 

can exhibit exceptional spectral resolution, enabling detailed measurements of absorption line 

shape. To this end, we have developed a refi ned 

version of CRDS called frequency-stabilized cavity ring-down spectroscopy (FS-CRDS). Here, the 

ring-down cavity is actively length stabilized, the 

probe laser is frequency locked to the ring-down 

cavity, and the frequency axis of the spectra is 

based upon the longitudinal mode spacing of the 

ring-down cavity. Taking advantage of the high 

spectral resolution afforded by FS-CRDS, various line shape effects such as speed-dependent 

pressure-broadening and collisional narrowing 

of these transition line shapes by various media 

can now be quantifi ed. 

Figure 3. Frequency-stabilized CRDS system.

DELIVERABLES:

Assemble and test portable FS-CRDS apparatus 

based on 1380 nm fi ber-optic DFB laser for realtime continuous monitoring of LFPG output. 

2Q 2006

Link H2

O transition line intensities in the 1380 nm 

spectral region to LFPG for water vapor concentration measurements in the range 1 μmol mol-1 to 

100 μmol mol-1 and measure line shape effects in 

N2 and other gases. 4Q 2006

Optimize the FS-CRDS apparatus for minimum water background, and compare FS-CRDS response 

to diluted gas streams from the LFPG, at concentrations of 10 nmol mol-1. 1Q 2007

5. Moisture contamination is a serious problem 

in phosphine, arsine, silane, ammonia, and similar 

gases used in the epitaxial growth of high-purity 

semiconductor layers. Gas manufacturers have 

been lacking adequate instruments to refi ne their 

products, and end-users are unable to measure 

contamination in transferring gases from cylinder 

to process tool. The critical concentrations of the 

impurities are not well known; however, it is believed that >10 nmol mol-1 oxygen or water in most 

process gases is undesirable. Optical methods for 

measuring the moisture impurity concentrations 

combine high sensitivity and straight-forward 

traceability through the LFPG absorption line 

strength measurements. Researchers in the NIST 

Electronics and Electrical Engineering Laboratory 

have developed a CRDS system similar to the 

ones described in the previous sections but linked 

with a semiconductor crystal growth system to 

measure H2O at very low concentrations in semiconductor source gases. The system was rebuilt 

in 2005-2006 to improve performance, to enable 

side-by-side tests with commercial instrumentation, and to allow greater fl exibility in correlating 

gas purity with semiconductor growth results. We 

continue to measure the lineshape, absorption 

coeffi cients, and frequency of optical transitions 

for water, phosphine, arsine, and ammonia in the 

vicinity of 940 nm and 1380 nm. This information 

is critical to facilitate the use of high-sensitivity spectroscopy techniques in these gases. The 

laboratory is equipped to allow safe handling of 

toxic gases such as phosphine and arsine, enabling 

collaborative experiments with industry on direct 

measurements of moisture in those gases. The 

CRDS capability should ultimately lead to improvements in semiconductor source gas purity, 

which will allow crystal growers to choose less 

expensive growth conditions without sacrifi cing 

optical emission effi ciency and yield in LEDs, 

semiconductor lasers, and photodetectors. 

•

•

•

Using the existing FS-CRDS apparatus and a 

portable fi ber-optics-based FS-CRDS system, we 

will make multiple independent measurements 

of H2O transition line shapes and line intensities 

in the vicinity of 1380 nm. We will use transfer 

standards for humidity generation and measurement as well direct measurement of humidifi ed 

gas samples delivered by the thermodynamicbased LFPG. 
138 National Institute of Standards and Technology 

DELIVERABLES:

Publish arsine and phosphine spectra with pressure broadening coeffi cients for line at 943.082 nm. 

1Q2008 

Test ammonia spectra in vicinity of 940 nm for possible overload with H2

O transition lines. 4Q 2007

Correlate water impurity concentrations in ammonia and nitrogen with gallium nitride semiconductor 

growth results. 4Q 2008

ACCOMPLISHMENTS

We have constructed a permeation tube calibration facility (see Fig. 4). The purpose of this 

system is to provide measurement traceability 

for industrial users of permeation tube humidity 

generators. The calibration system comprises 

a custom PTG, the LFPG and high-sensitivity 

quartz crystal microbalance (QCM). The watercontaining permeation tubes to be calibrated are 

placed inside the temperature-stabilized PTG 

oven. Water vapor diffuses from the tube surface 

into a precisely controlled stream of purifi ed N2. 

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Figure 4. New PTG calibration apparatus.

Figure 5. Calibration repeatability (top) and 

deviations from the fi tting function (bottom) of a 

water-vapor permeation tube in the NIST Permeation 

Tube Calibration Facility.

Figure 6. Dilution system for extending the LFPG 

operating range to pmol mol-1 levels.

A new strategy for pmol mol-1 (ppt)-level 

humidity generation has been successfully implemented. The approach, shown in Fig. 6, involves 

the controlled dilution of water vapor/gas mixtures 

produced by the LFPG, using a fl ow dilution 

system similar to that incorporated within the 

PTG calibration system described above. At mole 

fractions below 10-9, the uncertainty in water 

vapor mole fraction is dominated by water vapor 

background produced by adsorption/desorption of 

water on internal plumbing surfaces and by water 

vapor remaining in the diluent after purifi cation. 

We performed three sets of measurements to validate the method. First, the same nominal output 

concentration was produced while varying both 

the water vapor concentration produced by the 

Low Frost-Point Generator and the level of fl ow 

dilution. Deviations from a constant concentration 

were measured. Second, the calculated water vapor mole fraction of the diluted generator output 

#

“[The] trend towards 

higher gas purities is 

expected to continue 

in the future as devices 

shrink and become 

more complex, so 

it is important for 

Matheson Tri-Gas to 

stay at the forefront of 

analytical technology 

developments. In this 

regard Matheson TriGas’s collaboration with 

NIST has been most 

benefi cial. Use has 

been made of NIST’s 

specialized capabilities 

for handling hazardous 

electronic specialty 

gases as well as 

expertise in the cavity 

ring-down spectroscopy 

technique. The work 

will allow Matheson to 

potentially offer new 

grades of phosphine in 

the future as required 

by the microelectronics 

market.” 

William J. Kroll, 

Chairman and CEO, 

Matheson Tri-Gas

The diluent gas fl ow rate is adjusted so that the 

water vapor concentration produced by the PTG 

is equivalent to that produced by the LFPG, as 

measured by the QCM. From these measurements 

the water permeation rate of the tube under test 

can be determined in terms of the known LFPG 

output. As seen in Fig. 5, the calibrations derived 

from this system are repeatable to approximately 

1 %, which is signifi cantly superior to traditional 

gravimetric methods.
Semiconductor Microelectronics and Nanoelectronics Programs 139

Figure 7. APIMS measurement of LFPG background 

H2O concentration showing decay to steady state 

level.

Figure 8. High-resolution FSSM-CRDS spectrum of 

a pair of pressure-broadened water vapor absorption 

transitions. Symbols are experimental points, and 

lines are Voigt fi ts to the measured profi les.

was compared against the reading of a commercial 

hygrometer based on cavity ring-down spectroscopy. Third, we measured the water remaining in 

the diluent gas after purifi cation at various fl owrates both with and without an additional cryotrap. 

An approximate water background of 0.3 × 10-9 

was inferred from the data, and the results were 

consistent within the combined uncertainty of the 

LFPG output and the water background.

The LFPG uncertainty analysis is based on 

the uncertainties in temperature and pressure 

within the LFPG saturator, ice vapor pressure 

and the enhancement factor for mixtures of water 

vapor and air. However, this analysis neglects 

background effects associated with the transient 

adsorption and desorption of water vapor from 

internal surfaces in the fl ow manifold located 

downstream of the LFPG. For the lowest range 

considered, such processes may affect signifi -

cantly the water vapor concentration in the sample 

gas delivered by the LFPG to test instrumentation. In collaboration with Air Products Inc., 

we quantifi ed the magnitude of this water vapor 

background using atmospheric pressure ionization 

mass spectrometry (APIMS). Results, shown in 

Fig. 7, indicate that the background contribution 

to H2

O from system components downstream of 

the LFPG was less than 0.2 nmol/mol. These 

measurements also demonstrated that linearity 

deviations of the LFPG output H2O mole fraction 

are less than 0.1 nmol/mol.

We have successfully developed an FS-CRDS 

system with automated spectral scanning capability to measure the absorption coeffi cient of trace 

quantities of water vapor. In FY 2005 the FSCRDS method was used to probe H2

O absorption 

transitions in the 935 nm spectral region, and a 

#

#

spectral resolution of 50 kHz and reproducibility 

better than 0.25 % were demonstrated. In conjunction with humidity standards for determination of 

water vapor concentration, these spectroscopic 

measurements yielded relative uncertainties in line 

intensities less than 0.5 %. Figure 8 shows measured spectra (symbols) and theoretical spectra 

(solid lines) for a pair of overlapping water vapor 

absorption transitions, each case corresponding 

to a given total gas pressure (with N2

 as the buffer gas). These results illustrate that the spectral 

resolution and linearity of the FS-CRDS method 

enable precise quantifi cation of pressure broadening, collisional narrowing and asymmetries of the 

absorption line shape, thus minimizing systematic 

errors in the determination of number density and 

line intensity that typically arise from instrumental 

line broadening effects. Also, detection limits less 

than 10 nmol mol-1 of H2O in N2 have been demonstrated, using the relatively weak absorption 

lines near 940 nm. 

In collaboration with Matheson Tri-Gas and 

Tiger Optics, we demonstrated sensitivity of less 

than 10 nmol/mol for H2O in phosphine using 

commercially available instrumentation, the fi rst 

such measurement with this low sensitivity (see 

Fig. 9, next page). Additional experiments at 

NIST with arsine indicated that new wavelengths 

were needed to measure H2O in arsine to similar 

sensitivities. We have also used the NIST hazardous gas CRDS system to measure water vapor 

concentrations in the toxic gases phosphine and 

arsine in the 940 nm spectral region. Testing of 

both arsine and phosphine in this spectral region 

indicated that the least overlap with host gas lines, 

#
140 National Institute of Standards and Technology 

and hence the highest sensitivity to water contamination, is present for the H2

O line at 943.082 

nm. Future experiments will characterize this line 

for pressure broadening coeffi cients and ultimate 

sensitivity limits. Figure 10 shows a typical H2O 

spectrum in arsine. 

Figure 9. Response curve for commercial instrument 

measuring H2O in phosphine with sensitivity of 

1.3 nmol mol-1 .Zero offset of 9 nmol mol-1 is most 

likely from residual H2

O in phosphine. 

Figure 10. Absorbance spectra for H2

O in arsine 

near H2

O absorption line at 943.082 nm, showing 

feasibility of measuring H2O concentrations on the 

order of 50 nmol mol-1. 

Dow Chemical, Linh Le and J. D. Tate; CRDS measurements 

for process gas control RH Systems, Robert Hardy; Characterization of low range chilled-mirror hygrometers and humidity 

generators for standards laboratories. 

Southwest Sciences Inc, Chris Hovde.; Development of wavelength modulation laser hygrometer for trace H2

O sensing.

Tiger Optics, Calvin Krusen; evaluation of commercial CRDS 

technology.

Air Products and Chemicals Inc.; CRDS measurements of 

trace H2O in corrosive process gases.

RECENT PUBLICATIONS

D. Lisak and J.T. Hodges, “High-resolution cavity ring-down 

spectroscopy measurements of blended H2

O transitions,” Appl. 

Phys. B (in press).

H. H. Funke, M.W. Raynor, K.A. Bertness, and Y. Chen, “Detection of Trace Water Vapor in High-Purity Phosphine using 

Cavity Ring-down Spectroscopy,” Applied Spectroscopy, 61 

(2007): p. 419-423.

G. E. Scace, “Generating Parts per Trillion Levels of Water 

Vapor in Nitrogen,” in Proc. 5th International Symposium on 

Humidity and Moisture, Rio de Janiero, 2006 (in press).

J.T. Hodges and G.E. Scace, “Developing advanced humidity 

standards to measure trace water vapor in specialty gases,” 

Micro, 24, 59 (2006).

D. Lisak, J.T. Hodges and R. Ciurylo “Comparison of semiclassical line shape models to rovibrational H2

O spectra measured by frequency-stabilized cavity ring-down spectroscopy,”

Phys. Rev. A., 73, 012507 (2006).

V. Vorsa, S. Dheandhanoo, S. Ketkar, and J. T. Hodges, 

“Quantitative absorption spectroscopy of residual water vapor 

in high purity gases: Pressure-broadening of the 1.39253 μm 

H2

O transition by N2 , HCl, HBr, Cl2 and O2 , ” Appl. Optics, 

44, 611-619 (2005).

J. T. Hodges and R. Ciurylo, “Automated high-resolution 

frequency-stabilized cavity ring-down spectrometer,” Rev. 

Sci. Instrum., 76, 023112 (2005).

J. G. Cormier, J.T. Hodges, and J.R. Drummond, “Infrared 

water vapor continuum absorption at atmospheric temperatures,” Jour. Chem. Phys., 122, 114309 (2005).

S. Y. Lehmann, K. A. Bertness, and J. T. Hodges, “Optimal 

spectral region for real-time monitoring of sub-ppm levels of 

water in phosphine using cavity ring-down spectroscopy,” 

Jour. of Crys. Growth, 261, 225-230 (2004).

J. T. Hodges, “High-resolution cavity ring-down spectroscopy 

of H2

16O at 10687.36 cm-1,” ISBN 1-55752-765-2, Laser Applications to Chemical and Environmental Analysis, Annapolis, 

MD, February 9-11 (2004).

J. T. Hodges, H. P. Layer, W. M. Miller, and G. E. Scace, “Frequency-stabilized single-mode cavity ring-down apparatus for 

high-resolution absorption spectroscopy,” Rev. Sci. Instrum. 

75, 849-863 (2004).

COLLABORATIONS

Air Products and Chemicals Inc., Seksan Dheandhanoo; 

APIMS measurements of trace moisture and characterization 

of semiconductor gas purity.

Matheson Tri-Gas, Mark Raynor, Jun Feng, and Hans Funke; 

CRDS measurements of trace moisture in phosphine.

Tiger Optics, Yu Chen; CRDS measurements of trace moisture 

in phosphine.
Semiconductor Microelectronics and Nanoelectronics Programs 141

TEMPERATURE MEASUREMENTS AND STANDARDS

FOR SEMICONDUCTOR PROCESSING

Technical Contact:

D. Ripple

“On behalf of SensArray 

Corporation, I express 

our deep gratitude to 

NIST for signifi cantly 

contributing to advances 

in semiconductor 

lithography process 

optimization and control. 

NIST has contributed 

to improved CD 

control in lithography 

processes through 

the establishment of 

measurement methods 

and standards for wafer 

thermal bake cycle 

dynamics in resist 

processing. This support 

is just in time, as the 

next area of focus for 

photo resist processing 

optimization is bake 

cycle dynamics. Your 

work will be of value 

to every advanced 

semiconductor factory.”

Wayne Renken, President 

SensArray Corp.

GOALS

Our goal is to enable the improved accuracy of 

temperature measurements in semiconductor 

wafer processing as prescribed in the International Technology Roadmap for Semiconductor 

(ITRS). 

Our project, initiated in 1997, has resulted in 

improved calibration wafer technology based on 

thin-fi lm thermocouples (TFTCs) in conjunction 

with wire thermocouples (TCs) on test wafers for 

in-tool radiation thermometer (RT) calibration. 

We have also developed improved procedures for 

the calibration of lightpipe radiation thermometers 

(LPRT) and theoretical models for the relationship between the true wafer temperature and the 

indicated radiance temperature. 

With the completion of this work, we are now 

focusing on: (1) collaborating with the semiconductor industry in implementing new methods 

for reliable and traceable temperature measurements, (2) developing new resistance sensors for 

the range 300–600 °C, and (3) understanding 

measurement errors when calibration wafers are 

used in non-isothermal or high heat-fl ux environments.

CUSTOMER NEEDS

The measurement needs of the semiconductor 

manufacturing industry, as stated in the ITRS, 

continue to evolve. Examples of recently identifi ed challenges are:

1. The temperature dependence of present day 

resists limits the available critical dimension in 

the Post-Exposure Bake (PEB) process. Improvements will require more stringent temperature 

measurement and control, or improved resists 

(Lithography, 2006 ITRS update, p. 7).

2. Fabrication of high-density, non-volatile 

memory is very sensitive to process temperature 

(Process Integration, Devices, and Structures, 

2006 ITRS update, p. 2).

3. Accurate process models and control algorithms require characterization of the device thermal history during a variety of processing steps 

(Modeling and Simulation, 2005 ITRS, p. 2).

Current needs include better temperature measurement uncertainty in post exposure bake (PEB) 

processing of resists, in rapid thermal processing 

(RTP) of wafers including silicide formation in 

the temperature range of 300 °C to 600 °C, and 

in supporting development of instrumented calibration wafers for a variety of other processing 

steps. Understanding differences of temperature 

readings between different instrumented wafers 

is also a high priority.

Our customers are the device manufacturers and 

the suppliers of thermal processing equipment and 

temperature measurement instrumentation. 

TECHNICAL STRATEGY

Our research is focused on two projects that will 

enable the semiconductor industry to meet the 

roadmap requirements: (a) develop new, accurate sensors for the 300 °C to 600 °C range, 

where commercial sensors are inadequate, and 

(b) investigate effects on measurements of imperfect thermal environments with commercial 

instrumented-wafer technology. 

Industry instrumentation and process developers 

have also asked us to develop calibrated thin-fi lm 

resistors for wafer temperature measurement 

from 300 °C to 600 °C. These measurements are 

needed for more accurate control of the silicide 

anneal RTP. We have undertaken the development of precision platinum thin-fi lm resistance 

thermometers directly on the silicon wafers to 

improve the uncertainty of in situ wafer temperature measurement at these temperatures. We 

have developed reliable methods for producing 

durable sensors with good performance. We are 

presently extending this work to explore rhodium 

and iridium thin-fi lm thermometers. 

DELIVERABLES:

Publish paper on the fabrication methods, optimization, and attainable uncertainty of thin-fi lm Pt resistor thermometers directly on Si wafers. 2Q 2007

Explore feasibility of using iridium fi lms as resistance sensors up to 600 °C. 2Q 2007

Publish results for thin-fi lm Rh and Ir resistor thermometers directly on Si wafers. 3Q 2008

•

•

•
142 National Institute of Standards and Technology 

As a fi rst step in determining the sensitivity of 

a sensor on an instrumented wafer to a non-isothermal environment, it is necessary to measure 

the thermal resistance between the sensor and 

the silicon substrate. We have developed an apparatus in which the temporal response of a sensor 

to a nearly instantaneous heat pulse is measured. 

Simple theoretical models can be fi t to the data 

to ascertain the thermal resistance between sensor 

and wafer. This simple, non-destructive technique 

is envisioned as a useful quality-assurance tool 

for manufacturers of instrumented wafers. The 

same techniques may be extended to study the 

sensitivity of various sensor designs to high, nonisotropic heat fl uxes.

DELIVERABLES: 

Paper on measuring the thermal response time of 

commercial embedded sensors. 2Q 2007

Conduct experiments on sensitivity of sensors in 

instrumented wafers to non-isotropic heat fl ux. 

4Q 2007

Instrumented calibration wafers are now being 

commercially developed for temperature measurements in a variety of unusual processing 

environments, such as plasma processing. Modeling of the interaction of the instrumented wafer 

with its thermal environment is needed to meet 

constraints on the allowable temperature of the 

calibration wafers and to assist in interpretation 

of data obtained with the wafers. We continue to 

supply instrumentation manufacturers with heattransfer models that provide physical insight into 

the measurement errors and thermal management 

of the instrumented wafers.

DELIVERABLES: Assist manufacturers of instrumented wafers with heat transfer models in vicinity of 

sensors. 4Q 2007

ACCOMPLISHMENTS

DEVELOPMENT OF RESISTANCE SENSORS

FOR 300 °C TO 600 °C APPLICATIONS

We have explored the performance of platinum resistance thermometers deposited directly 

on oxide-coated silicon wafers. Our work has 

measured the effects of thickness and bond 

coat (Ti or Zr) of the Pt thin fi lms. We have 

also measured the effect of ambient atmosphere 

(air or nitrogen) on the hysteresis and thermal 

coeffi cient of resistivity, α. The value of α was 

not sensitive to the annealing temperature or Ti 

bond-coat thickness, but did depend on the Pt 

•

•

#

thickness. Metallurgical analysis has been completed of sensors fabricated both at NIST and by 

commercial vendors. The optimized Pt sensors 

have uncertainties of 1 °C in the range 200 °C 

to 600 °C, limited by thermal hysteresis related 

to the thermal expansion mismatch between the 

platinum and the silicon substrate.

TRANSIENT RESPONSE OF TEMPERATURE

SENSORS DURING THE POST EXPOSURE

BAKE PROCESS

Recent studies on dynamic temperature profi ling and lithographic performance modeling of 

the PEB process have demonstrated that the rate 

of heating and cooling may have an important 

influence on resist lithographic response. We 

conducted an experimental and analytical study 

to compare the transient response of commercial, 

embedded platinum resistance thermometer (PRT) 

sensors with surface-deposited TFTCs. A dual 

instrumented wafer for PEB evaluation is shown 

in Fig 1. Experiments were performed on a commercial module using wafers instrumented with 

calibrated type-E TFTCs and commercial PRTs.

#

Figure 1. Wafer instrumented with PRTs and type-E 

thin-fi lm thermocouples.

We measured the temperature of Si wafers in a 

commercial-type PEB module using both embedded PRTs and TFTCs through a typical thermal 

cycle from ambient, to 150 °C, and back to ambient. The transient response of the TFTCs led 

the PRT sensors, indicating a PRT lag (typically) 

of 2 °C on heating and up to 4 °C on cooling for 

several seconds. The wafer time constants for 

response were strongly affected by the air gap distance between the wafer and hot plate as expected. 

Thermal models were presented that showed 

estimates for heating time constants in good 

agreement with experimental data. Lithography 

simulation results were presented that showed the 

effects of transient and offset temperature profi les 

on CD variations.
Semiconductor Microelectronics and Nanoelectronics Programs 143

CALIBRATION OF PRT SENSORS FOR

INSTRUMENTED WAFERS

Calibration of PRTs that have been imbedded 

in instrumented wafers presents a challenge for 

the manufacturer: the wafers are much larger than 

commonly calibrated thermometers, the calibration process cannot contaminate wafers intended 

for use in a semiconductor-processing facility, and 

the uncertainty requirements for PEB applications 

are fairly demanding (standard uncertainty of approximately 0.01 °C). To validate the methods 

used in industry, a commercial instrumented 

wafer was calibrated both by NIST and by the 

manufacturer in the range 15 °C to 95 °C. The 

results were well within the stated manufacturing 

tolerance of the wafer and our expectations for the 

sensors used on the wafers.

THERMAL RESISTANCE OF PRT 

SENSORS FOR INSTRUMENTED WAFERS

We have successfully developed a method 

for a relatively inexpensive measurement of the 

thermal resistance between resistance sensors and 

the underlying silicon substrate of instrumented 

calibration wafers. The thermal resistance is 

predominantly determined by the properties of 

the adhesives and potting compounds used to 

secure the sensor—a low thermal resistance is 

needed for the sensor to give a true reading of the 

substrate temperature independent of the thermal 

environment of the wafer. The method is nondestructive, and may be performed on used as well 

as new instrumented wafers to ensure that sensor 

attachment to the wafer substrate is acceptable for 

the thermal environment of use. Presently, we are 

simplifying the method and data analysis to allow 

ready adoption of the technique by industry.

RECENT PUBLICATIONS

W. A. Kimes, K. G. Kreider, D. C. Ripple, and B. K. Tsai, 

“In Situ Calibration of Lightpipe Radiometers in Rapid 

Thermal Processing Between 300 °C and 700 °C,” in Proceedings of the 12th IEEE International Conference on Advanced 

Thermal Processing of Semiconductors, RTP 2004, J. Gelpey, 

et al., eds., Portland, OR, p. 156-162, Sept. 2004.

B. K. Tsai, D. P. DeWitt, K. G. Kreider, and W. A. Kimes, 

“Emissivity Compensated Pyrometry for Specular Silicon 

Surfaces on the NIST RTP Testbed,” in Proceedings of the 

12th IEEE International Conference on Advanced Thermal 

Processing of Semiconductors, RTP 2004, J. Gelpey, et al., 

eds., Portland, OR, p. 167-172, Sept. 2004.

K. G. Kreider, D. P. DeWitt, J. B. Fowler, J. E. Proctor, W. A. 

Kimes, D. C. Ripple and B. K. Tsai, “Comparing the Transient 

Response of a Resistive-Type Sensor with a Thin-Film Thermocouple during the Post-Exposure Bake Process,” Proceedings of SPIE 29th International Symposium Microlithography, 

Vol. 5378, 22-27, Santa Clara, CA, p. 81-92, Feb. 2004.

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B. K. Tsai, D. P. DeWitt, E. A. Early, L. M. Hanssen, K. G. 

Kredier, B. J. Lee, and Z. M. Zhang, “Emissivity Standards for 

Improved Radiation Thermometry during Thermal Processing 

of Silicon Materials,” Proceedings of TEMPMEKO 2004, 

Dubrovnik, Croatia, (to be published).

K. G. Kreider, D. H. Chen, D. P. DeWitt, W. A. Kimes, and 

B. K. Tsai, “Effects of Lightpipe Proximity on Si Wafer Temperature in Rapid Thermal Processing Tools,” Characterization 

and Metrology for ULSI Technology, D. Seiler et al., eds., AIP, 

NY, pp. 200-204, March 2003. 

K. G. Kreider, D. H. Chen, D. P. DeWitt, W. A. Kimes, and B. 

K. Tsai, “Lightpipe Proximity Effects on Si Wafer Temperature 

in Rapid Thermal Processing Tools,” Proceedings of the 11th 

IEEE International Conference on Advanced Thermal Processing - RTP 2003 Charleston, SC, 125-129, 2003.

B. K. Tsai, “Traceable Temperature Calibrations of Radiation 

Thermometers for Rapid Thermal Processing,” Proceedings of 

the 11th IEEE International Conference on Advanced Thermal 

Processing - RTP 2003 Charleston, SC, 101-106, 2003.

D. H. Chen, D. P. DeWitt, B. K. Tsai, K. G. Kreider, and W. A. 

Kimes, “Effects of Wafer Emissivity on Rapid Thermal Processing Temperature Measurement,” Proceedings of the 10th 

International Conference on Advanced Thermal Processing, J. 

Gelpey, et al., eds., Vancouver, BC, pp. 59-67, 2002.

D. H. Chen, D. P DeWitt, B. K Tsai, K. G. Kreider, and W. 

A. Kimes, “Effects of Wafer Emissivity on Rapid Thermal 

Processing Temperature Measurement,” Proceedings of 8th 

Symposium on Temperature: Its Measurement and Control In 

Science and Industry, D.C. Ripple et al., eds., AIP, NY, pp. 

735-740, October 2002.

K. G. Kreider, D. P. DeWitt, D. H. Chen. W. A. Kimes, C. W. 

Meyer, and B. K. Tsai, “Wafer Emissivity Effects on Lightpipe Radiometry in RTP Tools,” Rapid Thermal Processing 

Technology Symposium at the 201st Electrochemical Society 

Meeting, Philadelphia, PA, May 15, 2002, pp. 273-281.

K. G. Kreider, W. A. Kimes, C. W. Myer, D. C. Ripple, B. K. 

Tsai, D. C. Chen, and D. P. DeWitt, “Calibration of Radiation 

Thermometers in Rapid Thermal Processing Tools Using Si 

Wafers with Thin-fi lm Thermocouples,” Proceedings of 8th 

Symposium on Temperature: Its Measurement and Control in 

Science and Industry, D.C. Ripple et al., eds., AIP, NY, NY, 

pp. 1087-1092, October 2002.

B. K. Tsai and D. P. DeWitt, “Characterization and Calibration of Lightpipe Radiation Thermometers for use in Rapid 

Thermal Processing,” Proc. of 8th Symp. on Temperature: Its 

Measurement and Control In Science and Industry, D.C. Ripple 

et al., Eds, AIP, NY, pp. 441-446, October 2002.

B. K. Tsai and D. P. DeWitt, “Improving Temperature Accuracy 

for Rapid Thermal Processing at NIST,” Proceedings of the 

NCSLI 2002 Workshop & Symposium, 2002.
144 National Institute of Standards and Technology 
Semiconductor Microelectronics and Nanoelectronics Programs 145

PHYSICAL PROPERTIES OF LIQUID PRECURSORS

Technical Contact:

R. Berg

GOALS

New materials used in gas phase processes will 

require new measurements and standards for 

their effi cient delivery to the wafer surface. The 

National Institute of Standards and Technology 

(NIST) is supplementing its ongoing project to 

measure the thermophysical properties of semiconductor gases with a new effort to measure the 

relevant properties of liquid precursors. Vapor 

pressure is the most important property because 

it controls the behavior of vapor delivery devices. 

Therefore, the project’s fi rst goal is to produce 

accurate vapor pressure data for relevant liquid 

precursors from room temperature to as high as 

200 °C. Such data will improve the modeling 

of chemical vapor deposition processes and the 

design and use of bubblers and other devices that 

deliver precursor vapors. Other goals are to characterize the thermal stability of liquid precursors 

and to devise a method for measuring vapor pressure that is suitably accurate yet more convenient 

than existing methods.

CUSTOMER NEEDS

Precursors for chemical vapor deposition are 

frequently liquid compounds that, until recently, 

were either rare or nonexistent. Their vapors are 

delivered to the process chamber either by direct 

injection (fl ash evaporation) or by bubbling a 

carrier gas through the liquid held in a “bubbler.” 

Anecdotes from companies that either sell liquid 

precursors or sell the associated process equipment suggested an industrial need for improved 

property data of these specialty chemicals. Designers and users of mass fl ow controllers, bubblers, and other gas and liquid delivery devices 

could use such data to optimize the performance 

of those devices.

Vapor pressure determines the concentration of the 

precursor-carrier mixture produced by a bubbler, 

and it sets a lower limit on the wall temperature of 

the delivery line and process chamber. Although 

high vapor pressures are preferred, the acceptable range is wide. For example, when heated to 

100 °C, the vapor pressure of the widely used liquid TEOS (tetraethyl orthosilicate) is PV(100 °C) 

= 12 kPa. In contrast, a candidate precursor might 

be considered even if its vapor pressure were 100 

times lower, say PV(100 °C) = 0.1 kPa.

Other important properties include liquid viscosity 

and thermal stability. Viscosity can affect the performance of injection systems. Thermal stability 

is important because the precursor’s decomposition rate may limit the process’s upper operating 

temperature and thereby the vapor pressure.

TECHNICAL STRATEGY

NIST will identify and assess the quality of existing vapor pressure data of compounds such as Hf 

[(CH3)

2

N]4 (acronym TDMAH). NIST will also fi ll 

gaps in the existing data by developing apparatus 

capable of accurate vapor pressure measurements. 

The resulting accurate databases will be provided 

to industry. NIST also will investigate alternate 

methods to measure vapor pressure.

DELIVERABLES: Develop apparatus to measure the 

vapor pressures of metal-organic precursors. 1Q 2007

The new apparatus uses pressure gauges that operate at the same temperature as the sample. This 

direct method was chosen over indirect methods 

that rely on, for example, thermogravimetric 

analysis or nuclear magnetic resonance. Challenges include achieving appropriate control of 

the sample temperature and avoiding systematic 

errors due to the presence of decomposition products, dissolved gases, and other impurities. 

DELIVERABLES: Investigate alternate methods to 

measure vapor pressure. 2Q 2006

The initial investigation will include an estimate 

of the method’s uncertainty and the identifi cation of sources of systematic error. The leading 

candidate is speed-of-sound measurements of a 

mixture comprising the equilibrium vapor plus 

one atmosphere of an inert gas.

DELIVERABLES: Identify existing vapor pressure 

data. 3Q 2007

Existing data can be found in journal publications, 

the NIST Chemistry WebBook, NIST standard 

reference database 87, and other databases. The 

applicability of estimation techniques, such as 

group contribution methods, will be examined. 

DELIVERABLES: Develop a system to measure the 

decomposition rate of metal-organic vapors. 4Q 2007
146 National Institute of Standards and Technology 

Some metal-organic precursors are known to decompose at temperatures below 200 °C. The new 

system will use a mass spectrometer to identify the 

decomposition products as well as contaminants 

present in the original sample.

ACCOMPLISHMENTS

NIST organized a SEMI workshop at Semicon West 2006 with the goal of soliciting directions for research on the physical properties of 

gases and liquid precursors. The 25 participants 

discussed four questions: (1) Which gases should 

be measured next? (2) Which liquid precursors 

should be measured? (3) What properties of 

liquid precursors are most important? (4) Are 

the data effi ciently available? A report of the 

workshop is available at http://www.cstl.nist.

gov/div836/836.06/.

Although some precursors, such as TEOS, 

have been used for many years and have well 

known properties, others are poorly characterized 

because they were only recently created or identifi ed. As an example of the resulting diversity, 

precursors used recently by researchers to make 

hafnium oxide have included hafnium nitrate, hafnium chloride, tetrakis-dimethylamino hafnium, 

and tetrakis-diethylamino hafnium. The diversity 

inherent in research and the proprietary concerns 

of precursor manufacturers are considerations 

in selecting a precursor to characterize. Several 

workshop participants suggested that NIST fi rst 

measure the example precursors named by the 

SEMI task force on precursor specifications 

(Liquid Chemicals committee / Europe). The 

workshop report lists some of those examples, and 

a broader range of precursor examples is listed in 

the supplementary table of precursors attached to 

the ITRS. The workshop participants pointed out 

that, in addition to vapor pressure, other important 

properties include thermal stability, liquid viscosity, and materials compatibility.

The vapor pressure apparatus, which consists 

of an oven, a vacuum system, a novel thermoelectric temperature control scheme, and appropriate 

instrumentation, was assembled. Measurements 

that compared the readings of pressure gauges 

inside the oven with other gauges outside the 

oven were taken between 20 °C and 200 °C. 

The resulting calibration surface will allow the 

readings of the hot gauges to be corrected to the 

required accuracy. Temperature stability within 

the requirement of 20 mK was demonstrated up 

to 200 °C. An independent thermometer demon-

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strated that temperature uncertainty was less than 

20 mK up to the independent thermometer’s limit 

of 100 °C.

Requirements were identifi ed for a 500 amu 

mass spectrometer, which must include a capillary sampling system suitable for metal-organic 

vapors at 200 °C.

Gas fl ow measurement research at NIST in 

support of the semiconductor industry created a 

quartz capillary fl ow meter with a hydrodynamic 

model of unprecedented accuracy. A recent spinoff application is use of the model with a ratio 

viscometer that yielded values of unprecedented 

accuracy (< 0.1 %) for the viscosities of argon, 

hydrogen, methane, ethane, and xenon at temperatures from 200 K to 400 K. Another spin-off 

is a capillary fl ow meter that provides a 0.1 sccm 

reference gas fl ow for calibrating spinning rotor 

vacuum gauges.

RECENT PUBLICATIONS

E. F. May, R. F. Berg, and M. R. Moldover, “Reference viscosities of H2

, CH4, Ar and Xe at low densities,” International 

Journal of Thermophysics (accepted 2007).

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Semiconductor Microelectronics and Nanoelectronics Programs 147

ANALYSIS TOOLS AND TECHNIQUES PROGRAM

The continuing shrinking of device dimensions and the rapid inclusion of new materials into 

device fabrication demands the development of new analytical tools and techniques. The 

need for new analytical techniques and tools has increased as the components in the transistors approach the low nanometer level and the number of transistors per chip approaches 

1 billion. The development of tools capable of characterizing the structures produced in the 

laboratories as well as those needed to confi rm the manufacturing process require signifi cant 

improvements. Those used for production also require signifi cant speeds not to slow down 

the commercial manufacturing. This latter condition may require some sacrifi ces in the 

resolution and accuracy of those tools. In addition, more signifi cant modeling that allows us 

to bridge those areas where measurements can be done to those where knowledge is needed, 

but measurements cannot be done is critical. Signifi cant improvements in tools capable of 

analyzing properties of defect particles in the sub-30 nm size are urgently needed. Global 

and local stress measurement techniques need to be developed.
148 National Institute of Standards and Technology 
Semiconductor Microelectronics and Nanoelectronics Programs 149

THIN-FILM X-RAY METROLOGY FOR

MICROELECTRONICS

Technical Contacts:

J. P. Cline

D. Windover

GOALS

This multi-year collaborative effort between 

SEMATECH and NIST will provide the semiconductor community with the measurement 

methodology and calibration capability for accurate thin film characterization using X-ray 

refl ectometry (XRR) and high-resolution X-ray 

diffraction (HRXRD).

CUSTOMER NEEDS

In recent years, the semiconductor industry has 

driven scientifi c advancement towards processing 

nanometer-scale material coatings with unprecedented uniformity in thickness, composition 

control, and unique electrical and mechanical 

properties. Nanotechnology represents the fastest growth area of industry in the United States 

today. Simultaneous to this rapid advance in thin 

fi lm processing technologies, the X-ray diffraction 

user community and instrument manufacturers 

have collaboratively developed techniques such 

as XRR and HRXRD that permit the quantitative profi ling of thin fi lm characteristics, such as 

thickness, density profi le, composition, roughness, and strain fi elds. With the XRR method in 

particular, parameter modeling by conventional 

methods can be an intractable problem, involving deconvolution of instrument response, data 

model theory, model selection, and model refi nement/fi tting, which has prevented the realization 

of the technique’s potential in the characterization of nano-dimensional thin fi lm structures. 

This effort addresses the mounting industry call 

for accuracy in thin film characterization (in 

particular for thickness, density, and roughness 

determination). 

TECHNICAL STRATEGY

This fundamental XRR effort involves two parallel characterization projects being performed on 

identical, temporally stable, multilayer artifacts 

supplied by SEMATECH. The NIST project 

consists of in-house XRR and HRXRD characterization with International System of Units (SI) 

traceable measurement instrumentation and SI 

traceable, fi rst principles data modeling including Bayesian analysis providing refi nement of 

instrumental and model parameters as well as 

structural model selection. In parallel to this 

NIST measurement project, SEMATECH will 

measure or have measurements performed using 

commercial “in-line” XRR instrumentation and 

NIST will analyze these data with commercial 

software to study the limitations of commercial 

instrumentation and software modeling. Combining results from both studies will ultimately 

allow accuracy traceability and error estimation 

for commercial instrumentation. This work will 

also address theoretical XRR modeling limitations and compare software model refi nement 

and model selection approaches. The multi-year 

collaboration will provide the community with 

total error budget estimates for a given XRR 

structural analysis approach and instrumentation. 

This work will take approximately three years to 

address the issues discussed [FY 05, 06, and 07]. 

The progress in FY05 included a comprehensive, 

fi rst principles development of the XRR theory 

necessary for traceable modeling of data and a fi rst 

round of measurements on artifacts by commercial 

and NIST instrumentation. FY06 work explored 

the application of an approximate Bayesian model 

selection method to compare the relative probability of different structural models for measured 

XRR data. Preliminary results with simulated 

XRR data have shown success in determining 

the initial structural model used to generate the 

simulated data. FY07 deliverables will include the 

development of an XRR prototype SRM to clarify 

instrument response issues and provide calibration 

of instrumentation at SEMATECH.

NIST XRR Study – The NIST standard approach 

for reducing uncertainty in a measurement 

technique involves the generation of a Standard 

Reference Material (SRM). Figure 1 (next page) 

shows the three parallel research aspects needed 

for the development of an XRR SRM: (1) the 

manufacture of a robust thin film calibration 

artifact with a high degree of temporal stability 

and contamination and/or oxidation resistance, 

(2) the development of SI traceable measurement 

instrumentation, and (3) the creation of an SI 

traceable, fi rst principles data analysis approach. 

Figure 2 (next page) shows the current status of 

steps required to address these aspects for our 

XRR SRM project.
150 National Institute of Standards and Technology 

Figure 1. Aspects necessary for a NIST Standard 

Reference Material. All aspects are essential to 

provide structural parameter SI traceability.

Figure 2. NIST XRR SRM strategy. This fl owchart 

illustrates the key steps in the NIST XRR SRM 

development project. The light areas indicate work in 

progress. The outline colors indicate the appropriate 

aspect of the SRM development process for each step.

This collaborative effort with SEMATECH allows 

us to examine semiconductor industry relevant 

structures as potential candidates for an XRR 

calibration artifact. NIST has also engaged other 

national metrology institutes (NMIs) in an effort to 

assess the temporal stability of international NMI 

pre-standards. Both PTB of Germany and NMIJ/

AIST of Japan have provided us with structures 

for comparative XRR analysis. This cooperative 

study will help gauge which thin fi lms structures 

provide stable, well defi ned, refi nement parameters suitable for SI traceable modeling.

NIST has applied considerable resources to 

developing SI traceable X-ray Reflectometry 

instrumentation. Over the past seven years, NIST 

has constructed the Ceramics Division Parallel 

Beam Diffractometer (CDPBD) for SI traceable measurements of Powder XRD, Epitaxial 

HRXRD, and thin-film XRR artifacts for the 

Standard Reference Materials (SRM) program 

(see Fig. 3). Over a similar timeframe, the Physics Laboratory developed the Physics Laboratory 

Figure 3. Ceramics Division Parallel Beam 

Diffractometer (CDPBD). Showing a rotating 

anode X-ray source (center left), monochromator 

for generating parallel/monochromatic X-rays 

(upper center), angle-encoded rotation stages for 

measuring diffraction angles (lower right) and 

receiving optics/detector (center right). In order to 

achieve accuracy in angle measurements and X-ray 

wavelength stability, numerous design features 

of varying complexity are present ranging from a 

“fl oating” platform holding the X-ray source to a 

hanging second rotation stage (top center) designed 

specifi cally to rotate the cables for the diffraction 

rotation stages.
Semiconductor Microelectronics and Nanoelectronics Programs 151

X-Ray Refl ectometer (PLXRR) specifi cally for 

XRR characterization of semiconductor thin-fi lm 

artifacts. In FY07, we will be working with the 

Physics Laboratory to integrate the PLXRR into 

the XRR SRM effort allowing us cross-laboratory measurement comparison in our certifi cation 

process to move the PLXRR into our temperature 

controlled laboratory space.

SI traceability in either lattice parameter (XRD) 

or fi lm thickness (XRR), d, requires simultaneous 

traceability in X-ray wavelength, λ, and diffraction angle, θ, following Bragg’s law of diffraction: 

2d = nλ / sin (θ). The present NIST instrument 

development project involves addressing SI 

traceability aspects for both the diffraction angle 

and wavelength measurement on the CDPBD. 

Establishing SI traceability of the diffraction 

angle requires implementing optical encoding on 

the two rotation stages used to move the sample 

and detector. The optical encoder errors are then 

“mapped” using an external angle reference to 

generate SI traceability error bounds for each 

axis. The rotation stages presently have accuracy bounds of ± 2.0 μrads (± 0.4 arc seconds). 

Calibration experiments and collaboration with 

encoder manufacturers is currently under way to 

achieve an approximate one order of magnitude 

improvement in accuracy by year. Determining SI 

traceability in wavelength involves constructing a 

stable, well modeled optics assembly to convert 

angular and energy divergent radiation from an 

X-ray source into parallel, monochromatic radiation for use in diffraction. The CDPBD uses a 

monochromator with Si (220), 2/4-bounce channel-cut crystals to fi lter the direct source into a 

source of highly parallel, single energy X-rays. SI 

traceability in X-ray wavelength from our source 

will be performed using an SI traceable reference 

crystal (with measured relative standard uncertainty of 3 in 10-8). The X-ray wavelength for our 

instrumentation currently has a relative standard 

uncertainty of ≈1 in 10-5 (FY06). The accuracy 

error bounds and instrument parameters from the 

wavelength and angle determination studies will 

provide an overall instrument response function 

which will be incorporated into NIST XRR profi le 

modeling. The same instrumental parameters will 

establish guidelines for response profi le modeling 

of commercial instrumentation.

NIST is currently developing fi rst principles, 

SI traceable XRR software to analyze data and 

refi ne model and instrument response parameters. XRR analysis is essentially an “inverse 

problem” wherein we select input parameters 

for a “guessed” structural model. This model is 

then used to simulate data that is compared with 

measured data and “Goodness of Fit” parameters 

are determined. This process is repeated until 

a “best fi t” or best “Goodness of Fit” is found 

and the “best fi t” model parameters become the 

“refined” model parameters used to describe 

the real structure. Three major questions limit 

the effectiveness of this current XRR modeling 

approach: (1) How do we accurately simulate 

the data using a structural model? (2) How do 

we know which structural model describes the 

measured structure? (3) How do we accurately 

compare simulated and measured data? The 

NIST software development effort will attempt 

to answer each of these questions. 

To address the model data simulation issue, we 

have developed XRR theory from Maxwell’s 

equations explaining all approximations and constraints required for the XRR modeling method. 

This approach will in the future combine XRD, 

HRXRD and XRR in the same fundamental 

modeling theory and allow for SI traceability in 

derived refi ned parameters (FY08+). To address 

the question of model accuracy, we are implementing a Bayesian/Maximum Entropy analysis 

approach to determine the probability that a given 

structural model correctly describes a given data 

set. This model selection component is essential 

to determining the validity of initial structural 

assumptions in any XRR analysis. Addressing 

the third question is of considerable interest. 

Commercial refi nement approaches attempt to 

optimize a solution through a “chi squared” or 

model/simulation minimization criterion (e.g., 

genetic algorithms). This approach attains fast 

solution times most compatible with industrial 

need, at the loss of statistical data required for 

parameter uncertainty calculations. NIST, in collaboration with various companies is developing 

a statistical sampling based refi nement using a 

Markov Chain Monte Carlo (MCMC) formalism 

to generate SI-traceable uncertainty estimations. 

The MCMC approach is necessary for developing formal Bayesian model selection methods 

(FY08+) to compliment existing Approximate 

Bayesian approach.

NIST/SEMATECH XRR Study – The NIST/SEMATECH project consists of measurements and 

analysis from commercial instrumentation to 

allow comparison and calibration transfer from 

NIST SI traceable measurements and analysis.
152 National Institute of Standards and Technology 

The NIST/SEMATECH project combines measurements with commercial “in-line” XRR instrumentation with complimentary compositional 

analysis results as feedback for comparison with 

NIST measurements performed in parallel on the 

same artifacts. The measurement and modeling 

work at NIST will provide SI traceable XRR 

measurements and parameter analysis for artifacts 

and potential candidate SRMs. The collaboration 

work with SEMATECH allows interface and 

calibration transfer between NIST SI traceable 

measurements and “in-line” instrumentation. 

NIST will then use results from NIST SI traceable measurements, SEMATECH commercial 

measurements and modeling, and NIST SI traceable XRR modeling, to quantify error bounds and 

overall error budgets on the accuracy and precision possible from commercial instrumentation 

and commercial modeling software.

Calibration and accuracy determination of commercial instrumentation are the primary goals of 

this collaboration. To achieve these goals, we need 

to develop accurate instrument response functions 

for commercial instrumentation being calibrated. 

The instrument response function for commercial 

instrumentation may be the dominant term in the 

overall accuracy budget for XRR measurements. 

To address instrumentation effects on accuracy, 

work must be performed with different commercial XRR system geometries to discover the 

mechanical alignment parameters that dominate 

the error budgets of each system. This will require 

cooperation from instrument vendors to provide 

the necessary information or provide detailed 

instrument response functions directly. The 

incorporation of instrument response parameters 

into NIST modeling will allow for comparison 

between traceable measurements at NIST and 

measurements on commercial instrumentation in 

the fi eld. The NIST instrument response function 

information will be developed through our instrument traceability study discussed earlier. We can 

then combine specifi c instrument “corrections” 

to the accuracy error budget and use a calibration artifact such as a temporally stable thin fi lm 

structure measured on the commercial instruments 

and at NIST to provide instrument calibration and 

stability monitoring (FY07+ deliverable).

Project Deliverables – The fi nal project results 

available to SEMATECH will include XRR 

uncertainty estimations based on NIST XRR 

software and the calibration artifacts necessary 

for optimizing the performance of commercial 

“in-line” and laboratory XRR instruments. 

Theoretical uncertainty estimations provided by 

NIST software and simulated structural data will 

determine the limitations of XRR applicability for 

arbitrary multilayer systems on commercial XRR 

instrumentation (FY08+). Calibration artifacts 

measured on NIST SI traceable instrumentation 

will allow routine system monitoring, alignment 

calibration, and instrument response function 

comparisons to ensure commercial instrument precision and stability. These deliverables will dramatically improve the precision and accuracy of 

conventional XRR characterization of multilayer 

structures which exhibit well-established composition and uniformity (FY08+ deliverables).

DELIVERABLES: 

NIST analysis of SEMATECH measurements using 

new XRR modeling approaches. 2Q 2007

Approximate Bayesian interface selection method. 

2Q 2007

NIST analysis of SEMATECH & NIST measurements (SRM prototype). 4Q 2007

ACCOMPLISHMENTS

In FY05, the CDPBD moved to equipment 

space in the Advanced Measurements Laboratory (AML at NIST) which provides instrument 

temperature stability of ± 0.02 °C. Preliminary 

calibration of the angle measurement has been 

assessed for uncertainty determination (accuracy 

determination has been completed) and improvements using a new compensation approach (being 

developed through a joint National Metrology 

Laboratory effort with PTB) will be implemented 

(FY07). First principles XRR theory development 

with emphasis on justifying approximations present in commercial XRR software was completed 

(FY06) and a dynamical scattering based model 

has been implemented with Monte Carlo Markov 

Chain (MCMC) methods for structural parameter 

refinement and formal parameter uncertainty 

analysis (FY07+). A third/forth round of measurements on SEMATECH artifacts measured by both 

conventional and NIST traceable XRR systems 

will be completed this year (FY07). 

FY06/7 Technical transfer: Model Independent Structural Information – In FY06/7, 

we measured structures from SEMATECH and 

calibration standards from PTB to test a new approach to discovering structural information using 

XRR. We try multiple structural models using 

commercial optimization refi nement and compare the “best fi ts” of each model to discover any 

model independent, constant parameters across 

•

•

•

#

#
Semiconductor Microelectronics and Nanoelectronics Programs 153

the analysis set. In Fig. 4, we show this approach 

for 5 models of a Pulse Laser Deposition Pt layer 

on optical fl at (provided by PTB). The fi gure 

shows a cross section mass density profi le for the 

sample. Although the density of the surface layer 

is diffi cult to refi ne, the combined thickness of all 

the high density layers in every model attempted 

produces a constant, model independent structural 

parameter. This approach provides us with a way 

of locating what information the XRR method is 

most effective in measuring. 

Figure 4. Mass Density Profi le of Pt on optical fl at. 

We show Model Independent Structural Information 

for a 50 nm Pt fi lm on Optical fl at. The optimized 

results from fi ve different XRR models are shown. 

In each case, the total thickness of the Pt layer(s) is 

nearly identical providing us with information on the 

fundamental structure.

FY07 Technical transfer: Preliminary 

MCMC XRR Modeling – In FY07, we began 

testing a sampling based XRR refi nement approach for developing SI-traceable uncertainty 

estimates for model parameters. Figure 5 shows 

the uncertainty range, or posterior probability 

density for the total thickness of the Pt on Optical Flat specimen analyzed above. We see a 

very narrow uncertainty which shows excellent 

agreement between sampling and optimization 

approaches to XRR analysis. Figure 6 shows 

another advantage of the MCMC method; we can 

test which parameters correlate with each other. 

In this case, a two-layer model for the same Pt 

on Optical Flat sample has multiple solution 

regions, which causes complicated uncertainty 

evaluations and may cause optimization methods to be trapped in false minima. More work is 

required on interpretation and implementation of 

the MCMC method.

#

Figure 5. MCMC XRR posterior probability for Pt on optical 

fl at. Showing the total Pt thickness uncertainty estimation or 

“posterior probability density” for the fi lm used in Figure 4.

Figure 6. MCMC XRR 2-d histogram for Pt on optical fl at. Shows 

the highly correlated and complex nature of XRR analysis. Here, 

a two-layer model is being applied to the same sample, and 

several possible solution regimes appear. The axes are thickness 

for t1 , the Pt layer and t2 , an interface layer. These multimode 

cases can often trap optimization based XRR analysis methods.

COLLABORATIONS

SEMATECH – PY Hung

PTB, Precision Metrology – Peter Thomsen-Schmidt

LETI – Emmanuel Nolot

NMIJ/AIST – Toshiyuki Fujimoto

Bede Scientifi c Inc. – Matthew Wormington

Bruker AXS – Assunta Vigliante & Arnt Kern

Coruscavi – David L Gil

Jordan Valley Semiconductors – Dileep Agnihotri

Panalytical – Martijn Fransen

Rigaku MSC – Joe Formica

Technos International – Meredith Beebe
154 National Institute of Standards and Technology 

RECENT PUBLICATIONS AND

PRESENTATIONS

D. Windover, D. L. Gil, J. P. Cline, A. Henins, N. Armstrong, P. 

Y. Hung, S. C. Song, R. Jammy, A. Diebold, “X-ray refl ectometry determination of structural information from atomic layer 

deposition nanometer-scale hafnium oxide thin fi lms” – oral 

presentation & conference proceedings at Spring 2007 Materials Research Society, San Francisco, CA (April 2007).

D. L. Gil, D. Windover, J. P. Cline, A. Henins, N. Armstrong, 

P. Y. Hung, S. C. Song, R. Jammy, A. Diebold, “MCMC 

method for determining structural information using X-ray 

refl ectometry” – poster presentation at 2007 Frontiers of Characterization and Metrology for Nanoelectronics, Gaithersburg, 

MD (March 2007).

D. Windover, D. L. Gil, J. P. Cline, A. Henins, N. Armstrong, 

P. Y. Hung, S. C. Song, B. H. Lee, R. Jammy, A. Diebold, 

“NIST method for determining model-independent structural 

information by X-ray refl ectometry” – poster presentation & 

conference proceedings at 2007 Frontiers of Characterization and Metrology for Nanoelectronics, Gaithersburg, MD 

(March 2007).

D. Windover, D. L. Gil, A. Henins, N. Armstrong, J. P. Cline, 

“Prototype standards for X-ray refl ectometry: an international perspective revisited” – poster presentation at Spring 

International Centre for Diffraction Data, Newtown Square, 

PA, (March, 2007).

P. Xu, D. Windover, N. Armstrong, M. Wormington, A. Henins, 

J. P. Cline, “Progress on the NIST High Resolution X-Ray Diffraction Standards” – poster presentation at Denver X-Ray 

Conference, Denver, CO, (August, 2006).

D. Windover, N. Armstrong, A. Henins, J. P. Cline, “NIST 

X-Ray Reflectometry Metrological Analysis Approach” 

– poster presentation at Denver X-Ray Conference, Denver, 

CO, (August, 2006).

N. Armstrong, D. Windover, A. Henins, J. P. Cline, “NIST 

X-Ray Metrology and SRM 1979 for nanocrystallite size 

broadening” – invited presentation at CSIRO, Sydney, Australia (November 2005).

D. Windover, N. Armstrong, J. P. Cline, “Prototype Standards 

for X-Ray Refl ectometry: an international perspective” – invited colloquia at PTB, Braunschweig, Germany (September 

2005).

D. Windover, N. Armstrong, J. P. Cline, “Prototype Standards 

for X-Ray Reflectometry: an international perspective” 

– poster presentation at Denver X-Ray Conference, Denver, 

CO, (August, 2005).

N. Armstrong, J. P. Cline, J. Ritter, and J. Bonevich, “SRM 

1979: The NIST SRM for nanocrystallite size broadening” 

– invited presentation at Denver X-Ray Conference, Denver, 

CO (August 2005).

D. Windover, N. Armstrong, J. P. Cline, P. Y. Hung, A. Diebold, 

“Characterization of Atomic Layer Deposition using X-Ray 

Refl ectometry” – poster presentation & conference proceedings at 2005 International Conference on Characterization and 

Metrology for ULSI Technology, Dallas, TX (March 2005).

N. Armstrong, W. Kalceff, J. P. Cline, and J. Bonevich, “A 

Bayesian/Maximum Entropy method for certifi cation of a 

nanocrystallite-size NIST Standard Reference Material” – Ch. 

8 in Diffraction analysis of the microstructure of materials, pp. 

187-227, Ed. P. Scardi and E.J. Mittemeijer, Springer-Verlag, 

Berlin, ISBN: 3-540-40519-4 (2004).

N. Armstrong and P. Lynch, “Determining the dislocation 

contrast factor for X-ray line profi le analysis” – Ch. 10 in 

Diffraction analysis of the microstructure of materials, pp. 

249-307, Ed. P. Scardi and E.J. Mittemeijer, Springer-Verlag, 

Berlin, ISBN: 3-540-40519-4 (2004).

N. Armstrong, W. Kalceff, J. P. Cline, J. Bonevich, “Bayesian 

inference of nanoparticle-broadened X-ray line profi les” – J. 

NIST Research, 108(5), pg 155-178 (2004).

N. Armstrong, M. Leoni, M. Scardi, “Some concerns regarding 

the Wilkens’ model for dislocation line broadening” – invited 

presentation at Size-Strain 4: EPDIC 9, Prague, Czech Republic, (September 2004).

N. Armstrong, J. P. Cline, W. Kalceff, and A. Dowd, “Status 

of the NIST Nanocrystalline Size SRM–1979” – invited presentation at Size-Strain 4: EPDIC 9, Prague, Czech Republic 

(September 2004).

N. Armstrong, J. P. Cline, W. Kalceff, and A. Dowd, “Bayesian characterization of nanoparticles from line profile” 

– presentation at Denver X-Ray Conference, Denver, CO 

(August 2004).

N. Armstrong, W. Kalceff, J. P. Cline, J. Bonevich, P. A. Lynch, 

C. C. Tang, S. A. Thompson, “X-ray diffraction characterization of nanoparticle size and shape distributions: – Application 

to bimodal distributions” – Proceedings of the 28th Annual 

Condensed Matter & Material Meeting, Wagga, Australia 

(February 2004).
Semiconductor Microelectronics and Nanoelectronics Programs 155

ELECTRON MICROSCOPE TOMOGRAPHY OF

ELECTRONIC MATERIALS

Technical Contacts:

Z. H. Levine

J. H. Scott

GOALS

Enable the use of three-dimensional imaging for 

thick samples using commercial transmission 

electron microscopes (TEM) and scanning transmission electron microscopes (STEM). Typical 

samples include porous low-κ dielectrics, twolayer interconnect samples, and photonic band 

gap materials.

CUSTOMER NEEDS

The NTRS/ITRS has recognized the need for 

three-dimensional imaging of interconnects for 

several years. In this study, our principle objective is to determine the morphology of pores in 

low-κ dielectric material. Two aspects of the pore 

distribution are critical: (a) the largest pores may 

lead to failure of the dielectric (e.g., short circuits), 

and (b) the connectivity of the pores is important 

to understand the transport of chemicals during 

the fabrication of the interconnect. 

The potential solutions and major challenges for 

interconnect are discussed in the 2005 International Technology Roadmap for Semiconductors 

Update on pages 6 and 7 of the Interconnect 

Section. Three dimensional control of interconnect features (with its associated metrology) is 

required to achieve necessary circuit performance 

and reliability.

TECHNICAL STRATEGY

1. We have various theoretical capabilities in 

hand, including the ability to perform 3-D Monte 

Carlo simulations, the ability to perform tomographic reconstructions using an in-house 3-D 

Bayesian code for tomographic reconstruction 

which permits an arbitrary transmission-thickness 

relation, and a unique code which can determine 

the orientation of the tilt axis on the unit sphere 

from a tilt series of fi ducial reference marks, 

Fig. 1.

2. In parallel, we will develop theoretical expertise for the effects of coherence on the samples, 

including understanding when Bragg diffraction 

is signifi cant for tomography, and to develop 

simulations of coherent beam illumination, and 

strategies for beam rocking to reduce said coherence while preserving a suffi ciently straight beam 

for standard tomographic algorithms.

DELIVERABLES: Theoretical prediction of Energy 

Filtered Transmission Electron Microscopy (EFTEM) 

diffraction patterns of [111], [001], and [110] silicon 

slabs in the plasmon regime. 2Q 2007

ACCOMPLISHMENTS

Principle accomplishments include: creation 

of 3-D Bayesian code for tomographic reconstruction; completion of code for identifi cation 

of rotation axis on the unit sphere and alignment 

for tomography; further development of 3-D Java 

Monte Carlo simulation of these complex geometries, allowing understanding of how observed 

X-ray intensities into composition, Fig. 2 (next 

page).

COLLABORATIONS

International Sematech, Brendan Foran, preparation of low-κ

samples, electron microscopy.

Chris Soles and Hae-Jeong Lee, NIST, MSEL, Polymers 

Division; low-κ samples

Accurel, Inc. Preparation of focused ion beam sections from 

low-κ samples

Lucent Technologies, Shu Yang; preparation of photonic band 

gap material.

IBM, Frances Ross; preparation of Si:Au pillar sample.

#

Figure 1. Tomographic reconstruction of a photonic 

band gap sample. An interconnecting network 

of material and void is present in the center of 

the fi gure. The bands on the outside are from the 

platinum overlayer added to protect the sample 

during preparation by a focused ion beam.
156 National Institute of Standards and Technology 

RECENT PUBLICATIONS

Z. H. Levine and R. M. Dunstan, “Synthetic Incoherence for 

Electron Microscopy,” J. Opt. Soc. Am. A, in press (2007).

Z. H. Levine, A. Volkovitsky, and H. K .Hung, “Alignment of 

Fiducial Marks in a Tomographic Tilt Series with an Unknown 

Rotation Axis,” Comp. Phys. Comm., in press (2007).

Z. H. Levine, “Synthetic Incoherence Via Scanned Gaussian 

Beams,” J. Res. NIST 111, 429 (2006).

Z. H. Levine, A. J. Kearsley, and J. G. Hagedorn, “Bayesian 

Tomography for Projections with and Arbitrary Transmission 

Function with Applications in Electron Microscopys,” J. Res. 

NIST 111, 411 (2006).

I. M. Anderson, J. H. J. Scott, and Z. H. Levine, “3-D Chemical Nanoscale Imaging Via EFTEM Spectral Imaging,” Proc. 

Microscopy and Microanalysis (2006).

Z. H. Levine, “An X-Ray Tomography Primer,” Electronic 

Device Failure Analysis 7, 26 (2005).

Z. H. Levine, “Theory of Bright-Field Scanning Transmission 

Electron Microscopy,” J. Appl. Phys. 97, 033101 (2005).

Z. H. Levine, S. Grantham, D. J. Patterson, I. McNulty, I. C. 

Noyan, and T. M. Levin, “Imaging material components of 

an integrated circuit interconnect,” J. Appl. Phys. 95, 405 

(2004).

Z. H. Levine, “Tomography in the Multiple Scattering Regime 

of the Scanning Transmission Electron Microscope,” Appl. 

Phys. Lett. 82, 3943 (2003).

Z. H. Levine, J. A. Gao, S. Neogi, T. M. Levin, J. H. Scott, 

and S. Grantham, “Parallax measurements of an integrated 

circuit interconnect using a scanning transmission electron 

microscope,” J. Appl. Phys. 93, 2193 (2003).

Figure 2. 3-D Java Monte Carlo simulation of interconnect phantoms and observed X-ray emission (Cu=red 

Al=blue SiO2=green) from 0 degree to 180 degree tilt.
Semiconductor Microelectronics and Nanoelectronics Programs 157

HIGH-RESOLUTION MICROCALORIMETER X-RAY

SPECTROMETER FOR CHEMICAL ANALYSIS

Technical Contacts:

J. N. Ullom

J. A. Beall

W. B. Doriese

W. D. Duncan

G. C. Hilton 

K. D. Irwin

S. W. Nam

C. D. Reintsema

L. R. Vale

GOALS

We will develop new generations of X-ray spectroscopy tools to meet the materials analysis needs 

of the semiconductor manufacturing industry. 

Energy-Dispersive Spectrometers (EDS) based 

on microcalorimeters have the ability to detect 

photons with high energy resolution and nearunity quantum effi ciency. Using these tools, a 

wide range of materials analysis problems can 

be solved. In semiconductor manufacturing, improved X-ray materials analysis is needed to identify nanoscale contaminant particles on wafers and 

to analyze very thin layers of materials and minor 

constituents. Microcalorimeter EDS improves the 

spectral resolution by one to two orders of magnitude compared to the semiconductor Si-EDS, 

the existing industry standard. Such improved 

resolution combined with energy dispersive operation makes possible direct spectral separation 

of most overlapping peaks often encountered with 

Si-EDS in complex multi-element systems. The 

improved resolution of the microcalorimeter EDS 

also increases the peak-to-background ratio. Peak 

shape and shift can be studied to reveal chemical 

state information. 

Developing arrays of X-ray microcalorimeters 

will enable the acquisition of high statistics spectra in reduced time, improving the effi ciency and 

statistical quality of existing materials analysis 

applications. Further, large-format arrays (up to 

1000 pixels) will make it possible to chemically 

analyze smaller features and trace constituents, 

and to track rapidly evolving X-ray spectra for 

in-process and process-stream monitoring.

The microcalorimeter EDS detector invented at 

NIST consists of a superconducting thermometer (a superconducting transition-edge sensor 

(TES)) and an X-ray absorber fabricated on a 

micromachined Si3

N4 membrane, and cooled to 

cryogenic temperatures (0.1 K). When X-rays 

are absorbed in the detector, the resulting heat 

pulse in the microcalorimeter is measured by the 

TES thermometer. The change in the temperature 

of the thermometer is measured by a superconducting quantum interference device (SQUID) 

amplifi er. The temperature pulse height gives a 

measurement of the energy of the X-ray photon 

one to two orders of magnitude more sensitive 

than Si-EDS. The detector is cooled to 0.1 K by 

a compact adiabatic demagnetization refrigerator. 

We are presently simplifying this refrigerator to 

reduce system costs and increase the accessibility 

of microcalorimeter technology.

CUSTOMER NEEDS

Improved X-ray detector technology has been 

cited by SEMATECH’s Analytical Laboratory 

Managers Working Group (ALMWG, now Analytical Laboratory Managers Council (ALMC) 

as one of the most important metrology needs for 

the semiconductor industry. In the International 

Technology Roadmap for Semiconductors, improved X-ray detector technology is listed as a key 

capability that addresses analysis requirements for 

small particles and defects. The transition-edge 

sensor (TES) microcalorimeter X-ray detector 

developed at NIST has been identifi ed as a primary 

means of realizing these detector advances, which 

will greatly improve in-line and off-line metrology 

tools that currently use semiconductor energydispersive spectrometers (EDS). At present, these 

metrology tools fail to provide fast and unambiguous analysis for particles less than approximately 

0.1 μm to 0.3 μm in diameter. Improved EDS 

detectors such as the TES microcalorimeter are 

necessary to extend the capabilities of existing 

SEM-based instruments to meet the analytical 

requirements for future technology generations. 

With continued development, microcalorimeter 

EDS should be able to meet both the near-term and 

the longer-term requirements of the semiconductor 

industry for improved particle analysis.

“Prototype microcalorimeter energy dispersive 

spectrometers (EDS) and superconducting tunnel 

junction techniques have X-ray energy resolution capable of separating peaks that overlap 

and cannot be resolved with current generation 

lithium-drifted-silicon EDS detectors. Such new 

X-ray detectors will allow resolution of slight 

chemical shifts in X-ray peaks providing chemical 

information such as local bonding environments. 

These advances over traditional EDS and some 

wavelength dispersive spectrometers can enable 

particle and defect analysis on SEMs located in the 

clean room. Beta site systems are now beingtested. 

These detectors can also be implemented in micro 

XRF systems, using either an electron beam or 

a micro focus X-ray beam as excitation source. 

Both are currently in beta phase. XPS (X-ray 
158 National Institute of Standards and Technology 

photon electron spectroscopy) is also currently 

being developed as a means to determine thickness and composition of thin (up to 50 nm) fi lms.” 

2006 International Technology Roadmap for 

Semiconductors.

TECHNICAL STRATEGY

1. The usefulness of single-pixel X-ray microcalorimeter EDS in materials analysis has now 

been well established in a variety of demonstrations. Arrays of X-ray microcalorimeters for 

increased collection area and count rate have 

been demonstrated and their energy resolution is 

comparable to single sensors. To meet the needs 

of the semiconductor industry, it is necessary to 

make both single-pixel and array microcalorimeter systems more widely available. In addition 

to microcalorimeters, the system requires novel 

superconducting electronics to instrument the 

detectors, compact adiabatic demagnetization 

refrigerators to simplify cooling to milliKelvin 

operating temperatures, and custom roomtemperature electronics and software to process 

the output signals. Our goal is to develop new 

generations of detector systems, to transfer them 

to the Chemical Science and Technology Laboratory (CSTL) in NIST Gaithersburg, Maryland for 

collaborative use in studying problems of interest 

to the industry (Fig. 1), and to work with other 

partners in disseminating the technology. 

DELIVERABLES: Provide continued support and 

upgrades for the single-pixel microcalorimeter system 

transferred to CSTL. Prepare for the eventual transfer 

to CSTL of an array microcalorimeter system. Collaborate with CSTL on using the microcalorimeter to study 

problems of interest to the semiconductor industry. 

4Q 2007

2. The adiabatic demagnetization refrigerator 

that provides 0.1 K operating temperatures is a 

crucial part of a microcalorimeter system. To date, 

adiabatic demagnetization refrigerators have been 

precooled to 4 Kelvin with liquid nitrogen and 

helium. However, the use of liquid cryogens is an 

obstacle to potential microcalorimeter users. Consequently, it is desirable to precool the adiabatic 

demagnetization refrigerator with a push-button 

mechanical cryocooler whose only consumable is 

electricity. 

DELIVERABLES: Design and build an adiabatic 

demagnetization refrigerator cooled by a pulse tube 

mechanical cryocooler. Understand and minimize the 

electrical and mechanical effects of the cryocooler on 

microcalorimeter and electron microscope operation. 

2Q 2007

3. One of the barriers to widespread dissemination of X-ray microcalorimeter instruments is 

the complexity and cost of the adiabatic demagnetization refrigerator used to cool to 100 mK. 

The development of an on-chip solid-state microrefrigerator based on superconducting tunnel 

junctions to cool from 300 mK to 100 mK would 

greatly simplify the cryogenic system needed for 

microcalorimeter EDS. Adiabatic demagnetization 

refrigerators could be replaced by small, simple, 

and inexpensive 3He systems, which cool to 

300 mK, coupled to the solid-state tunnel-junction 

refrigerator cooling to 100 mK. 

DELIVERABLES: Demonstrate the cooling of a X-ray 

microcalorimeter using a tunnel-junction refrigerator 

that could be coupled to a simple 3

He refrigerator. 

3Q 2007

4. The operation of microcalorimeter arrays 

requires multiplexed SQUID readout. A timedomain SQUID multiplexer has previously been 

demonstrated and been used to successfully read 

out eight microcalorimeters under X-ray illumination. Signifi cant increases in multiplexer bandwidth and pixel-handling capacity are possible 

based on evolutionary design improvements.

DELIVERABLES: Demonstrate improved SQUID 

multiplexer and multiplexed operation of up to 128 

microcalorimeters under X-ray illumination. Characterize multiplexer performance. 4Q 2007

Figure 1. Single-pixel NIST X-ray microcalorimeter 

system on a scanning-electron microscope. The 

system was developed in the Electronics and 

Electrical Engineering Laboratory (EEEL) at NIST, 

Boulder, and transferred to the Chemical Science and 

Technology Laboratory (CSTL) at NIST Gaithersburg 

to be used to study problems of interest to the 

semiconductor industry.
Semiconductor Microelectronics and Nanoelectronics Programs 159

ACCOMPLISHMENTS

We designed and built an adiabatic demagnetization refrigerator that is precooled by a 

mechanical cryocooler. This simple and compact 

system is operated by the push of a button and 

requires no liquid cryogens. Because the cryocooler achieves a base temperature of 2.8 Kelvin, 

lower than that of liquid helium, the operating 

time of the demagnetization refrigerator below 

0.1 K is signifi cantly extended. In addition, we 

have demonstrated undegraded operation of a high 

resolution microcalorimeter in the electromagnetic environment of the cryocooler and its control 

electronics. We have also characterized the effects 

of the mechanical vibration of the cryocooler on 

other instruments such as a scanning electron 

microscope. Through a combination of vibration 

reduction measures, we have almost completely 

eliminated distortion of the microscope image due 

to the cryocooler. A photograph of the pulse tube 

cooled – demagnetization refrigerator undergoing 

vibration testing on an electron microscope is 

shown in Fig. 2. 

#

Figure 2. Cryogen-free adiabatic demagnetization 

refrigerator (left) undergoing vibration testing on a 

scanning electron microscope.

We developed a new generation of microcalorimeters that incorporate additional normal 

metal regions to suppress internal noise. In 

addition to being more stable and easier to bias, 

the energy resolution of these microcalorimeters 

is signifi cantly improved. We demonstrated an 

#

energy resolution of 2.4 eV FWHM at 5.9 keV. We 

also hold the world record for energy resolution 

for an EDS X-ray detector of 2.0 eV at 1.5 keV, 

which is over 30 times better than the best high 

resolution semiconductor-based detectors currently available.

We continue to support the microcalorimeter 

system installed on a CSTL scanning electron 

microscope in Gaithersburg, Maryland. As shown 

in Fig. 3, CSTL staff have demonstrated the ability of the microcalorimeter to do quantitative 

microanalysis.

#

Figure 3. Comparison of elemental weights 

determined by microcalorimeter with certifi cate 

values for NIST K411 standard reference glass. The 

solid line corresponds to perfect agreement between 

microcalorimeter and certifi cate values (data 

courtesy of Terry Jach).

We demonstrated the ability to read out eight 

TES microcalorimeters in a single multiplexed 

amplifi er channel with 3.8 eV FWHM energy 

resolution (Fig. 4). The detectors were fast, having 

a signal decay-time constant of about 150 microseconds. Combining this result with closed-form 

calculations and a new Monte-Carlo software 

package that performs a detailed simulation of our 

SQUID multiplexer, we can reliability predict the 

future performance of the multiplexer system. In 

particular, with only evolutionary improvements 

#

Figure 4. Photograph of an 8x8 array of 

microcalorimeters. Each microcalorimeter has a 

1.5-μm-thick Bi absorber. The middle chip is a fi lter 

chip. The chip on the right is a SQUID multiplexer.
160 National Institute of Standards and Technology 

to the basic architecture, our time-division SQUID 

multiplexer will be able to readout 32 detectors per 

channel with 4 eV resolution or better. Planned 

improvements include increased open-loop 

system bandwidth, well-matched pulse rise and 

fall times, lower SQUID noise, and optimization 

of the coupling between microcalorimeters and 

SQUIDs. We have built a test apparatus to house 

and read out up to 128 microcalorimeter pixels and 

are preparing to demonstrate multiplexed operation of a 4 x 32 high-resolution microcalorimeter 

array (Fig. 5). 

Figure 5. 128 pixel microcalorimeter array and 

multiplexed readout circuitry.

We have continued work on an on-chip solidstate refrigerator to cool X-ray microcalorimeters 

from 300 mK to 100 mK. If successful, this refrigerator could greatly simplify the cryogenic system 

needed for microcalorimeter EDS. Adiabatic 

demagnetization refrigerators could be replaced 

by small and inexpensive 3He systems coupled to 

the solid-state refrigerator. The device is a superconducting analog of a Peltier cooler. Cooling is 

produced in the devices by the tunneling of electrons through normal-insulator-superconductor 

junctions. Recently, we demonstrated refrigerators 

able to cool bulk material as well as electrically 

separate pieces of thin-fi lm electronics, such as a 

X-ray microcalorimeter (Fig. 6). The solid-state 

refrigerator is fabricated on a silicon wafer using 

conventional thin-film and photolithographic 

techniques, has no moving parts, and operates 

continuously. We recently cooled a X-ray microcalorimeter and demonstrated that no additional 

noise was introduced into the microcalorimeter 

output. This work has been featured twice on the 

cover of Applied Physics Letters and once on the 

cover of Physics Today.

#

We created a chemical shift map showing the 

chemical bonding state of Al in a sample containing both Al and Al2

O3 (see Fig. 7). An aluminum 

fi lm was deposited on part of a sapphire substrate. 

A microcalorimeter EDS was used to measure the 

X-ray spectrum as the electron beam was rastered 

to form the SEM image. The Al X-ray line position 

was shifted by a small amount (about 0.2 eV) in 

the regions containing Al2O3 as compared to the 

regions containing elemental Al. The high energy 

resolution of the microcalorimeter allowed the 

shift to be measured, resulting in the false-color 

image below. The map clearly demonstrates that 

microcalorimeter EDS can be used to discriminate 

the chemical bonding state using shifts in the positions of X-ray lines. The result also highlights 

the need for large-format arrays to increase the 

data collection rate. The image shown here was 

acquired over several hours. Images such as this 

could be acquired much more quickly and with 

much sharper position resolution using an array 

microcalorimeter.

#

Figure 6. Colorized micrograph of solid-state 

refrigerator. A cube of germanium (yellow) located 

on a suspended, micromachined membrane is cooled 

by four pairs of normal-insulator-superconductor 

tunnel junctions. The ratio of the volumes of the 

germanium and the junctions is the same as the 

ratio of the volumes of the Statue of Liberty and an 

ordinary person (about 11 000). Macroscopic wires 

(at upper left) contact the germanium to measure its 

resistance; these are cooled as well.
Semiconductor Microelectronics and Nanoelectronics Programs 161

Figure 7. SEM photo (top) of an aluminum fi lm 

partially covering a sapphire substrate and a 

false-color map (bottom) of the shift in the mean 

X-ray energy of the Al Kα peak taken using a 

microcalorimeter EDS system. The shift in energy is 

due to the chemical bonding of the aluminum with 

the oxygen. The average energy shift is ~0.2 eV.

COLLABORATIONS

Chemical Science and Technology Laboratory, NIST, Gaithersburg, Terry Jach, Lance King, Dale Newbury, John Small, 

and Eric Steel, the development of microcalorimeter EDS 

systems. 

RECENT PUBLICATIONS

K. D. Irwin, “Seeing with Superconductors,” Scientific 

American, Nov. (2006).

J. N. Ullom, J. A. Beall, W. B. Doriese, W. D. Duncan, L. 

Ferreira, G. C. Hilton, K. D. Irwin, G. C. O’Neil, C. D. 

Reintsema, L. R. Vale, B. L. Zink, “Optimization of TES 

Microcalorimeters,” Nuclear Instruments and Methods A 

559, 422 (2006).

W. B. Doriese, J. A. Beall, W. D. Duncan, L. Ferreira, G. C. 

Hilton, K. D. Irwin, C. D. Reintsema, J. N. Ullom, L. R. Vale, 

Y. Xu, “Toward Kilopixel Arrays: 3.8 eV Microcalorimeter 

Resolution in Eight-Channel SQUID Multiplexer,” Nuclear 

Instruments and Methods A 559, 808 (2006).

N. A. Miller, A. M. Clark, D. R. Schmidt, J. A. Beall, G. C. 

Hilton, K. D. Irwin, L. R. Vale, J. N. Ullom, “A Practical 

Electron Tunneling Refrigerator: Cooling of Low Temperature Detectors and Bulk Material,” Nuclear Instruments and 

Methods A 559, 633 (2006).

K. D. Irwin, “Noise Theory of Nonlinear, Nonequilibrium 

Calorimeters and Bolometers,” Nuclear Instruments and 

Methods A 559, 718 (2006).

K. D. Irwin, J. A. Beall, W. B. Doriese, W. D. Duncan, G. C. 

Hilton, J. A. B. Mates, C. D. Reintsema, D. R. Schmidt, J. N. 

Ullom, L. R. Vale, B. L. Zink, K. W. Lehnert, “Microwave 

SQUID Multiplexer,” Nuclear Instruments and Methods A 

559, 802 (2006).

D. E. Newbury, K. I., G. H., D. A. Wollman, J. A. Small 

and J. Martinis, “The Impact of the Transition Edge Sensor 

Microcalorimeter Energy Dispersive X-ray Spectrometer 

on Electron Probe Microanalysis,” a chapter in the book 

Cryogenic Particle Detection, C. Enss (Ed.), Springer, Berlin, 

Heidelberg (2005).

K. D. Irwin and G. C. Hilton, “Transition-Edge Sensors,”

a chapter in the book Cryogenic Particle Detection, C. Enss 

(Ed.), Springer, Berlin, Heidelberg (2005).

J. N. Ullom, J. A. Beall, W. B. Doriese, W. D. Duncan, L. 

Ferreira, G. C. Hilton, K. D. Irwin, C. D. Reintsema, L. R. 

Vale, “Optimized Transition-Edge X-ray Microcalorimeter 

with 2.4 eV Energy Resolution at 5.9 keV,” Appl. Phys. Lett. 

87, 194103 (2005).

A. M. Clark, N. A. Miller, A. Williams, S. T. Ruggiero, G. 

C. Hilton, L. R. Vale, J. A. Beall, K. D. Irwin, J. N. Ullom, 

“Cooling of Bulk Material by Electron-Tunneling Refrigerators,” Appl. Phys. Lett. 86, 173508 (2005).

W. B. Doriese, J. A. Beall, S. Deiker, W. D. Duncan, L. 

Ferreira, G. C. Hilton, K. D. Irwin, C. D. Reintsema, J. N. 

Ullom, L. R. Vale, and Y. Xu, “Time-division multiplexing of 

high-resolution TES X-ray microcalorimeters: Four pixels and 

beyond,” Appl. Phys. Lett. 86, 4762 (2004).

S. W. Deiker, W. B. Doriese, G. C. Hilton, K. D. Irwin, W. H. 

Rippard, J. N. Ullom, L. R. Vale, S. T. Ruggiero, A. Williams, 

and B. A. Young, “Transition edge sensor using dilute AlMn 

alloys,” Appl. Phys. Lett., 85, 2137 (2004).

K. D. Irwin, K. W. Lehnert, “Microwave SQUID Multiplexer,”

Appl. Phys. Lett. 85, 2107 (2004).

J. Pekola, R. Schoelkopf, J.N. Ullom, “Cryogenics on a Chip,”

Physics Today, May (2004).

J. N. Ullom, W. B. Doriese, G. C. Hilton, J. A. Beall, S. Deiker, 

W. D. Duncan, L. Ferreira, K. D. Irwin, C. D. Reintsema, L. 

R. Vale, “Characterization and Reduction of Unexplained 

Noise in Superconducting Transition-Edge Sensors,” Appl. 

Phys. Lett., 84, 4206 (2004).

A. M. Clark, A. Williams, S. Ruggiero, M. L. van den Berg, J. 

N. Ullom, “Practical Electron-Tunneling Refrigerator,” Appl. 

Phys. Lett. 84, 625 (2004).

S. Ruggiero, A. Williams, W. H. Rippard, A. Clark, S. Deiker, 

L. R. Vale, J.N. Ullom, “Dilute Al-Mn Alloys for Superconductor Device Applications,” J. Low Temp. Phys. 134, 973 

(2004).
162 National Institute of Standards and Technology 

S. Ruggiero, A. Williams, W. H. Rippard, A. Clark, S. Deiker, 

L. R. Vale, J.N. Ullom, “Dilute Al-Mn Alloys for Superconductor Device Applications,” Nucl. Instrum. & Methods in Phys. 

Res. A 520, 274 (2004).

K. D. Irwin, D. Audley, J. A. Beall, J. Beyer, S. Deiker, W. B. 

Doriese, W. Duncan, G. C. Hilton, W. Holland, J.N. Ullom, 

L. R. Vale, Y. Xu, “In-focal-plane SQUID multiplexer,” Nucl. 

Instrum. & Methods in Phys. Res. A 520, 544 (2004).

G. C. Hilton, J. A. Beall, S. Deiker, L. R. Vale, W. B. Doriese, 

J. Beyer, J.N. Ullom, C. D. Reintsema, Y. Xu, K. D. Irwin, 

“X-ray Microcalorimeter Arrays Fabricated by Surface Micromachining,” Nucl. Instrum. & Methods in Phys. Res. A 

520, 435 (2004).

W. B. Doriese, J. Beall, J. Beyer, S. Deiker, S. Ferreira, G. C. 

Hilton, K. D. Irwin, J. M. Martinis, S.W. Nam, C. D. Reintsema, J.N. Ullom, L. R. Vale, “A Time-Domain SQUID multiplexer for readout of X-ray microcalorimeter arrays,” Nucl. 

Instrum. & Methods in Phys. Res. A 520, 559 (2004). 

J. N. Ullom, W. B. Doriese, G. C. Hilton, J. A. Beall, S. Deiker, 

K. D. Irwin, C. D. Reintsema, L. R. Vale, Y. Xu, “Suppression 

of excess noise in transition-edge sensors using magnetic fi eld 

and geometry,” Nucl. Instrum. & Methods in Phys. Res. A 

520, 333 (2004).

N. A. Miller, J. N. Ullom, J. A. Beall, G. C. Hilton, S. Deiker, 

W. B. Doriese, K. D. Irwin, C. D. Reintsema, L. R. Vale, Y. 

Xu, “Robust infrared fi lters for X-ray spectroscopy,” Nucl. 

Instrum. & Methods in Phys. Res. A 520, 638-640 (2004). 

B. L. Zink, K. D. Irwin, G. C. Hilton, D. P. Pappas, J. N. Ullom, 

M. Huber, “Lithographically patterned magnetic calorimeter 

X-ray detectors with integrated SQUID readout,” Nucl. Instrum. & Methods in Phys. Res. A 520, 52 (2004).
Semiconductor Microelectronics and Nanoelectronics Programs 163

DEVICE DESIGN AND CHARACTERIZATION

PROGRAM

As microelectronics pushes into the nanoelectronics regime traditional CMOS reaches fundamental limits; new device structures such as Multi-Gate FETs (MUGFETs), fully depleted and 

partially depleted silicon-on-insulator, various alloys such as silicon-germanium and silicongermanium-carbon, strained layers and other exotica such as carbon nanotube, spintronic, 

and phase change and molectronic device structures need to be characterized. Another addition to the portfolio is the emerging fi eld of organic materials electronics. With all of these 

changes taking place, NIST has established the Center for Nanoscale Science and Technology 

(CNST), a multidisciplinary center composed of a research program and the Nanofab, a fee 

base, shared use user facility with advanced processing and metrology equipment suitable 

for advancing into the nanotechnology era.
164 National Institute of Standards and Technology 
Semiconductor Microelectronics and Nanoelectronics Programs 165

DEVICE CHARACTERIZATION AND RELIABILITY

Technical Contacts:

J. Suehle

M. Green

GOALS

The goal of the Advanced Device Characterization 

and Reliability Project is to improve and develop 

reliability and electrical characterization tools 

for advanced and emerging CMOS technologies. 

Deliverables include test methods, diagnostic 

procedures, reliability data, electrical characterization methods, physical models for wear-out, 

and methodologies to determine energy band 

diagrams and barrier heights for advanced gate 

dielectric stack systems..

A specifi c focus is to increase the understanding 

of the relationship between the gate dielectric 

material/interface properties and device electrical 

and reliability measurements. 

The Si microelectronics community is currently 

faced with major materials challenges to further 

scaling. The gate stack (i.e, the gate dielectric, 

SiO2, and the gate electrode, doped polycrystalline 

Si), which has served the industry for 35 years, 

must now be entirely replaced with one having a 

higher capacitance and lower power dissipation. 

Gate dielectrics having higher dielectric constants 

than SiO2 will replace SiO2, and metal electrodes 

will replace polycrystalline silicon. The enormous 

complexity of selecting the proper combinations 

of new gate dielectrics and gate metal electrodes 

can only be attacked using combinatorial materials methodologies. Therefore, we will be implementing this technology.

CUSTOMER NEEDS

The MOSFET (Metal Oxide Field Effect Transistor), which is the current basis of ULSI (UltraLarge-Scale Integration) circuits, is beginning 

to show fundamental limits associated with the 

laws of quantum mechanics and the limitations 

of fabrication techniques. The continual decrease 

of the gate dielectric fi lm thickness is identifi ed 

as a critical front-end technology issue in the 

Semiconductor Industry Association’s (SIA’s) 

International Technology Roadmap for Semiconductors (ITRS) with effective thickness values 

dropping to 1.0 nm or less by 2010.

Due to increased power consumption, intrinsic 

device reliability and circuit instabilities associated with SiO2

 of this thickness, a high permitivity gate dielectric (e.g., Si3

N4, HfSixOy, ZrO2) 

with low leakage current and at least equivalent 

capacitance, performance, and reliability will be 

required. The physics of failure and traditional 

reliability testing techniques must be reexamined 

for high-κ/SiOx

 stack dielectrics that exhibit polarity dependent defect generation and breakdown 

characteristics.

Electrical characterization of Metal Oxide Semiconductor (MOS) capacitors and Field Effect 

Transistors (FET) has historically been used to 

determine device and gate dielectric properties 

such as insulator thickness, defect densities, mobility, substrate doping, bandgap, and reliability. 

Electrical and reliability characterization methodologies need to be developed and enhanced to 

address issues associated with both ultra-thin SiO2

and alternate dielectrics including large leakage 

currents, quantum effects, and thickness dependent properties. As compared to SiO2

, very little 

is known about the physical or electrical properties of high dielectric constant gate dielectrics in 

MOS devices. The use of these fi lms in CMOS 

technology requires a fundamental understanding 

of the relationship between the gate dielectric 

material/interface and device electrical and reliability measurements.

TECHNICAL STRATEGY

There are two main focus areas for this project:

Developing electrical and reliability characterization techniques for ultra-thin SiO2

/oxynitrides 

and high dielectric constant gate dielectrics.

Develop combinatorial (fast, local) measurement techniques to measure appropriate electrical 

properties on gate stacks consisting of new gate 

dielectric and gate metal electrode materials.

The fi rst focus area is to develop robust electrical 

characterization techniques and methodologies to 

characterize charge trapping kinetics, threshold 

voltage, Vt, instability, defect generation rates, 

spatial and energetic defect profi les, and timedependent dielectric breakdown (TDDB) for both 

patterned device samples and blanket fi lms obtained from our collaborators. Many issues such 

as tunnel/leakage current and spatially dependent 

properties associated with metal oxide and silicate 

dielectrics are also present in ultra-thin oxide 

and oxide-nitride stacked dielectrics. Therefore, 

many of the characterization schemes will fi rst 

be developed on the simpler ultra-thin oxide and 

oxide-nitride dielectrics and then be applied to the 

metal oxide and silicate dielectrics for a variety 

#

#
166 National Institute of Standards and Technology 

of high-κ samples subject to different deposition and gate electrode processes. Studies will 

be conducted to determine the effect of multiple 

interfaces on stress-induced defect generation and 

wear-out. It will be determined what technique 

or combination of techniques provides the most 

consistent results for all fi lms. The electrical results will be used to validate simulation models 

and compared to studies from various analytical 

materials characterization. 

DELIVERABLES:

Study Gate-induced leakage current (GIDL) in 

metal gate/high-κ gate dielectric MOSFETS. 

2Q 2007

Use ultra-fast current voltage measurements to 

study fast trapping kinetics in SiC/SiO2 and metal 

gate/high-κ systems. 3Q 2007

Use 1/f noise analysis and charge pumping to 

profi le trap density in high-κ/SiO2

 stacks on Si and 

III-V substrates. 4Q 2007

Conduct time-dependent dielectric breakdown 

study of SiO2/SiC system. 4Q 2007

Combinatorial analysis of the work functions as a 

function of composition in the Ta-Al-N/HfO2 gate 

stack. 3Q 2007

ACCOMPLISHMENTS

CHARACTERIZATION OF GATE-INDUCED

DRAIN LEAKAGE CURRENT IN HIGH-Κ

GATE DIELECTRIC MOSFETS

Experiments conducted at NIST on transistors 

with hafi um oxide (HfO2) gate dielectrics have 

shown that although the gate leakage current is 

reduced, another leakage current associated with 

the drain of the device is signifi cantly increased. 

The NIST research has shown that this leakage 

current known as gate-induced drain leakage 

(GIDL) appears to be due to trap-assisted tunneling by defects or traps existing in the HfO2

, Fig. 1. 

Special ultra-fast measurements were developed 

to separate leakage components associated with 

transient electron trapping from traps not associated with the defects responsible for the enhanced 

GIDL. The results of this work are signifi cant in 

that they demonstrate how the thickness of the 

high-κ material affects the increased leakage current and suggests how the dielectric system can be 

engineered to mitigate it. Details of the study were 

presented at the 2006 IEEE International Electron 

Device Meeting in San Francisco CA. 

•

•

•

•

•

#

ULTRA-FAST CHARACTERIZATION OF

TRANSIENT GATE OXIDE TRAPPING IN

SIC MOSFETS

MOSFET threshold voltage (VTH) and drain 

current (I

D) instability in state-of-the-art 4H-SiC 

MOSFETs has been studied using conventional dc 

and fast I-V measurements, Fig. 2. This instability 

may be explained by trapping of channel electrons 

in the pre-existing traps at the SiC/SiO2 interface 

and at the carbon-rich transition layer extending 

several nanometers into the oxide. These traps are 

located in the upper half of the energy band gap. 

Because of the fast transient behavior of the trapping and de-trapping processes, conventional dc 

measurement techniques underestimate the sever-

#

Figure 1. Substrate current yield (IB /ID ) with GIDL 

current yield (IGIDL /ID ) as a function of VGS at 

VDS=2 V of 0.35 μm gate length SiO2 dielectric and 

high-κ dielectric n-MOSFETs.

Figure 2. ΔVTH of a SiC MOSFET measured as a 

function of time for the two different gate stress 

patterns. The stress bias (VSB ) was 7.5 V and stress 

interval was 100 ms. It is clear that measuring 

the threshold voltage using a dc sweep (~1 s 

measurement time) reveals only a fraction of the 

instability phenomenon.
Semiconductor Microelectronics and Nanoelectronics Programs 167

ity of this phenomenon. Post oxidation annealing 

in NO passivates the oxide traps and as a result the 

transient trapping is almost eliminated. A physical 

model explaining the role of the nitrogen in passivating the oxide defects was proposed.

SPATIAL PROBING OF TRAPS IN

NMOSFET WITH ALD HFO2/SIO2

STACKS USING LOW FREQUENCY NOISE

CHARACTERISTICS

Low frequency (LF) noise was studied in 

n-type Metal-Oxide-Semiconductor Field-EffectTransistors (nMOSFETs) with various HfO2

 or 

interfacial layer (IL) thicknesses and is found to 

be dominated by 1/f noise in the frequency range 

1 Hz ≤ f ≤ 1.6 kHz. LF noise magnitude increases 

with HfO2 thickness and decreases with IL SiO2

thickness. Traps at the channel and dielectric interface do not contribute to the 1/f noise or cannot 

be resolved from thermal noise. The LF noise correlates well with the hysteresis or Vth instability 

observed during DC measurements. The volume 

trap density calculated from 1/f noise analysis is 

more than one order of magnitude higher in 7 nm 

HfO2 than in 3 nm HfO2 devices. Qualitative 

trap spatial profi les can be obtained from the LF 

spectra, and the stress induced redistribution of 

trap distribution can be determined.

COMBINATORIAL WORK FUNCTION

EXTRACTION OF TA1-XALXNY

COMPOSITION SPREADS FOR THE

ADVANCED GATE STACK

We have systematically mapped the work 

functions (Φm) of AlxTa1-xNy composition spreads 

on HfO2 using combinatorial methodology for the 

fi rst time. A maximum of Al content (x = ~ 0.5) 

was confi rmed to maintain suffi cient conductivity for the metal gate application through the 

capacitance – voltage (C-V) measurement. The 

EOT maps suggested AlxTa1-xNy composition 

spreads thermally stable. Φm and Qf were extrapolated using Vfb vs. EOT. The values of Φm, from 

5.1 ~ 5.3 eV for the Al0.05Ta0.95Nx alloys to ~ 4.9 

eV for the Al0.5Ta0.5Nx alloys, suggested a good 

replacement for polysilicon in the advanced gate 

stacks, Fig. 3. The fi xed oxide charge density (Qf) 

map confi rmed the negative polarity induced the 

increase of Φm. Our results strongly suggested the 

potential of AlxTa1-xNy alloys ideal for the p-MOS 

application. 

#

#

STANDARDS COMMITTEE

PARTICIPATION

JEDEC JC14.2 Committee on Wafer-Level Reliability, Dielectric Working Group, Chairman 

(John S. Suehle).

PROFESSIONAL COMMITTEE

PARTICIPATION

• Technical Program Chair, 2006 IEEE International Reliability Physics Symposium (JSS)

• Technical Program Committee 2006 IEEE International Electron Device Meeting (JSS)

• Chairman of Dielectric Working Group, 

JEDEC JC14.2 Committee on WLR (JSS)

• Management Committee IEEE Integrated 

Reliability Workshop (JSS)

• Special Member of Graduate faculty, University of Maryland (JSS)

• Guest editor for Special Issue IEEE Tran. on 

Device and Materials Reliability (JSS)

• Editor, IEEE Transactions on Electron Devices. (JSS)

COLLABORATIONS

IBM, Alternative Gate Dielectrics 

Micron, Boise, ID, Characterization of metal gate dielectric 

systems.

ARL, Characterization of defects and reliability of SiC gate 

dielectric systems.

GE, Reliability characterization of SiC gate dielectric systems.

Intel, Characterization of high-κ/InGaAs systems

Intel, metal gate/SiO2

 combinatorial systems

IMEC, Ta-Al-N/HfO2 work functions, combinatorially

Figure 3. The extracted average work functions 

(Φm ) as a function of Al content for the Alx

Ta1-xNy

composition spread after FGA, 900°C, and 1000°C. 

Φm was extracted by extrapolating the line in the Vfb

vs. EOT plot as shown in the inset. An abnormal peak 

was observed with x < 0.1 after 900° anneal.
168 National Institute of Standards and Technology 

Lucent Technologies, Reliability Characterization of ultra-thin 

gate dielectrics.

SEMATECH Characterization of metal gate high-κ systems.

Rutgers University, Characterization of high-κ gate dielectrics

University of Maryland, College Park, ultrathin gate oxide 

reliability, combinatorial analysis of advanced gate stacks 

U. Texas at Austin, Optical properties of ZrO2

 and HfO2 for 

use as high-κ gate dielectrics 

U. Texas at Dallas, high-κ gate dielectric systems..

RECENT TALKS/PUBLICATIONS

J. J. Kopanski, Book Chapter: “Scanning Capacitance Microscopy for Electrical Characterization of Semiconductors and 

Dielectrics in Scanning Probe Microscopy: Electrical and 

Electromechanical Phenomena at the Nanoscale, SpringerVerlag (in press).

M. L. Green, K.-S. Chang, S. DeGendt, T. Schram, J. Hattrick-Simpers, “Application of combinatorial methodologies 

for work function engineering of metal gate/high-κ advanced 

gate stacks,” Microelectronics Eng., to be published 6/07.

Hao D. Xiong, Dawei Heh, Moshe Gurfi nkel, Qiliang Li, 

Yoram Shapira, Curt Richter, Gennadi Bersuker, Rino Choi, 

John S. Suehle, “Characterization of Electrically Active Defects in High-κ Gate Dielectrics By Using Low Frequency 

Noise, Charge Pumping, and Fast Id-Vg measurements,” 

accepted, 2007 15th biannual conference Insulating fi lms on 

semiconductor, Greece.

Wenyong Wang, Hao D. Xiong, Monica D. Edelstein, David 

Gundlach, John S. Suehle, Curt A. Richter, Woong-Ki Hong 

and Takhee Lee, “Low-frequency noise characterizations of 

ZnO nanowire fi eld effect transistors,” Journal of Applied 

physics 101, 044313 2007; Received 8 November 2006; 

accepted 25 December 2006; published online 28 February 

2007.

J. S. Suehle, “Issues in Reliability Projection of Advanced Gate 

Dielectrics,” University of Maryland, Dept. of Mechanical 

Engineering Seminar, Feb 9, 2006.

J. S. Suehle, “Reliability Challenges for Advanced Semiconductor Devices,” SEMICON West, San Francisco Ca on 

July 10, 2006.

J. J. Kopanski, “Scanning Probe Microscopes for the Characterization of Electronic Materials and Devices,” INTEL, 

Hillsboro, Oregon, January 9, 2006. Invited by Mike 

Gardner, Program Manager of Materials Research, External 

Programs.

J. J. Kopanski, M. Y. Afridi, S. Jeliazkov, H. Xiong, and T. 

Walker, “Limitations on the Accuracy and Spatial Resolution 

of Scanning Kelvin Force Microscopy for Work Function 

Measurements of Nanostructures,” Seeing at the Nanoscale IV: 

An International Conference Exploring Nanostructure Imaging, Characterization & Modifi cation using SPM & Related 

Techniques, p. 55, (University of Pennsylvania, Philadelphia, 

PA, July 17-20, 2006).

M. Gurfi nkel, J. Suehle, J. Bernstein, Y. Shapira, A.J. Lelis, 

D. Habersat, and N. Goldsman, “Ultra-fast measurements of 

Vt instability in SiC MOSFETS due to positive and negative 

constant bias stress,” Proceedings IEEE Integrated Reliability 

Workshop, Oct. 16-19, 2006.

H. D. Xiong, J. S. Suehle, “Spatial probing of traps in nMOSFET with ALD HfO2/SiO2 stacks using low frequency noise 

characteristics,” Proceedings IEEE Integrated Reliability 

Workshop, Oct. 16-19, 2006.

M. Gurfi nkel, J. Suehle, J. Bernstein, Yoram Shapira, “Enhanced Gate Induced Drain Leakage Current in HfO2 MOSFETs due to Remote Interface Trap -Assisted Tunneling,” Proc. 

IEEE IEDM, December 11-13, 2006.

Hao Xiong, Nhan Nguyen, Joseph Kopanski, John Suehle, 

and Eric Vogel, “Work function characterizationTaSiN and 

TaCN electrodes using CV, IV, IPE, and SKPM,” ECS Trans. 

Vol. 3, pp. 25-36 2006.

K.-S. Chang, M. Green, J. Suehle, E. Vogel, H. Xiong, J. 

Hattrick-Simpers, I. Takeuchi, O. Famodu, K. Ohmori, P. 

Ahmet, T. Chikyow, P. Majhi, B.-H. Lee, M. Gardner., 4th 

International Workshop on Combinatorial Materials Science 

& Technology, San Juan, Puerto Rico, 2006. “Combinatorial 

Study of Ni-Ti-Pt Ternary Metal Gate Electrodes on HfO2

 for 

the Advanced Gate Stack.”

K.-S. Chang, M. Green, J. Suehle, E. Vogel, H. Xiong, J. Hattrick-Simpers, I. Takeuchi, O. Famodu, K. Ohmori, P. Ahmet, 

T. Chikyow, P. Majhi, B.-H. Lee, M. Gardner, “Combinatorial 

Study of Metal Gate Electrodes on HfO2

 for the Advanced 

Gate Stack,” NIST Combinatorial Methods Center, NCMC-10 

workshop, Gaithersburg, MD (2006). 

Rao Mulpuri, Siddarth Sundaresan, Yong-lai Tian, Mark 

Ridgway, John Schreifels, and Joseph Kopanski, “Ultra-hightemperature microwave annealing of Al+- and P+- implanted 

4H-SiC,” Journal Applied Physics (submitted August 20, 

2006).

S.-E. Park, N. V. Nguyen, J. J. Kopanski, J. S. Suehle, and E. 

M. Vogel, “Comparison of scanning capacitance microscopy 

and scanning Kelvin probe microscopy in determining twodimensional doping profi les of Si homostructures,” J. Vac. Sci. 

Technol. B 24, 404-407 (2006).

K.-S. Chang, M. L. Green, J. Suehle, E. M. Vogel, H. Xiong, 

J. Hattrick-Simpers, I. Takeuchi, O. Famodu, K. Ohmori, P. 

Ahmet, T. Chikyow, P. Majhi, B.-H. Lee, and M. Gardner, 

“Combinatorial study of Ni-Ti-Pt ternary metal gate electrodes 

of HfO2 for the advanced gate stack,” Appl. Phys. Lett. vol. 

89, p.142108 (2006).

M. L. Green, K.-S. Chang, I. Takeuchi, and T. Chikyow, A 

Combinatorial Study of Metal Gate/HfO2

 MOSCAPS, Proc. 

ECS, Cancun, 11/06.

J. S. Suehle, “Characterization of Band Structure, Defects, and 

Reliability of High-κ/Metal Gate Stacks,” Micron Technology 

Boise ID, Nov. 11, 2005.

Albert V. Davydov, Deane Chandler-Horowitz, Martin M. 

Frank, “Sub-bandgap defect states in polycrystalline hafnium 

oxide and their suppression by admixture of silicon,” N. V. 

Nguyen, App. Phys. Lett. 87 (2005). Publication date: Nov. 

5, 2005.

N. V. Nguyen, S. Sayan, I. Levin, J. R. Ehrstein, I. J. R. Baumvol, C. Diremeier, C. Krug, L. Wielunski, P. Y. Hung, and Alain 

Diebold, “Optical band gaps and composition dependence of 

hafnium-aluminate thin fi lms grown by atomic layer chemical 

vapor deposition,” J. Vac. Sci. Technol. A 23 (2005). Publication date: Oct. 25, 2005.

S. Sayan, N. V. Nguyen, and J. Ehrstein, J. J. Chambers, M. 

R. Visokay, M. A. Quevedo-Lopez, L. Colombo, D. Yoder, I. 

Levin, and D. A. Fischer, M. Paunescu, O. Celik, and E. Garfunkel, “Effect of nitrogen on band alignment in HfSiON gate 

dielectrics,” Appl. Phys. Lett. 87, 212905 (2005). Publication 

Date: Nov 16, 2005.
Semiconductor Microelectronics and Nanoelectronics Programs 169

NANOELECTRONIC DEVICE METROLOGY

Technical Contacts:

C. A. Richter

N. Zimmerman

A. Davydov

J. Lehman

GOALS

The overall goal of the Nanoelectronic Device 

Metrology (NEDM) task is to develop the metrology that will help enable new nanotechnologies 

(such as Si-based quantum devices, molecular 

electronics, and carbon nanotubes (CNTs)) 

to supplement and/or supplant conventional 

Complementary Metal Oxide Semiconductor 

(CMOS) devices. This involves determining the 

critical metrology needs for these exploratory 

technologies. One specifi c goal is to develop the 

precise metrology and characterization methods 

required for the systematic characterization of Sibased nanoelectronic devices. Two related goals 

are the optimization of Si-based single-electron 

devices for one-electron logic, and the development of ultra-sensitive nanotransistor devices 

to observe charge reconfi gurations in biological 

systems. Another targeted goal is to develop test 

structures and methods to measure the electrical properties of small ensembles of molecules 

reliably (see Fig. 1). A fi nal targeted goal is 

to develop rapid identifi cation and purifi cation 

techniques for CNTs. 

CUSTOMER NEEDS

Mainstream CMOS, which is the current basis of 

ULSI (Ultra-Large-Scale Integration) circuits, is 

approaching fundamental limits associated with 

the laws of quantum mechanics and the limitations of fabrication techniques. The Semiconductor Industry Association’s (SIA’s) International 

Technology Roadmap for Semiconductors (ITRS) 

(http://public.itrs.net/) shows no known solutions 

in the short term for a variety of technological 

requirements including gate dielectric, gate leakage, and junction depth. Therefore, it is expected 

that entirely new device structures and computational paradigms will be required to augment 

and/or replace standard planar CMOS devices. 

Two promising beyond-CMOS technologies that 

each take a very different fabrication approach 

are molecular electronics and Si-based quantum 

electronic devices. Molecular electronics is based 

upon bottom-up fabrication paradigms, while 

Si-based nanoelectronics are based upon the 

logical continuation of the top-down fabrication 

approaches used in CMOS manufacturing. These 

two approaches bracket the possible manufacturing techniques that will be used to make future 

nanoelectronic devices. 

Molecular electronics (ME) is a fi eld that many 

predict will have important technological impacts 

on the computational and communication systems 

of the future. In ME systems, molecules perform 

the functions of electronic components. Alternatively, research and development for siliconbased nanoelectronics (e.g., Si-nanowire FETs, 

Si-based RTDs [resonant tunneling diodes], and 

silicon quantum dots) for the post-CMOS era are 

currently of interest due to their inherent compatibility with CMOS technology. Finally, there is a 

large potential set of customers for ultra-sensitive 

charge electrometry of biological systems. Many 

diseases result from changes in protein structure or 

folding. We will investigate whether such changes 

can be elucidated through capacitive coupling to 

nearby nanotransistors.

Before carbon nanotubes (Fig. 2, next page) can 

fulfi ll their promise as ultrastrong fi bers, electrical 

wires in molecular devices, or hydrogen storage 

components for fuel cells, better methods are 

needed for purifying and identifying raw nanotube 

materials. We are working towards this goal by 

developing simple methods of cleaning nanotubes by exposing them to carefully calibrated 

laser pulses. 

In all these cases, our project has the capability 

to offer early guidance to these emerging fi elds 

and to assist companies in pursuing productive 

areas and rejecting problematic ones. Because 

these fi elds are not yet mature, as our relatively 

moderate efforts progress, they can yield large 

payoffs for the customers.

Figure 1. Schematic overview of a suspended SixNy

membrane nanopore-based molecular electronic test 

structure.
170 National Institute of Standards and Technology 

TECHNICAL STRATEGY

Develop the electrical and physical metrology 

of Si-based nanoelectronics. The focus is on the 

basic building blocks of silicon quantum electronic devices (e.g., quantum layers, wires, and 

dots of silicon surrounded by silicon dioxide). By 

identifying and addressing the critical metrology 

issues associated with these basic building blocks, 

the basis of metrology for future Si-based ULSI 

nanotechnology will be defi ned.

DELIVERABLES: 

Complete development of a simple single nanowire 

manipulating system (SNMS) to precisely transfer 

and align individual nanowires into test structures 

for electrical characterization; prepare and publish 

manuscript. 1Q 2007 

Experimentally characterize the noise properties of 

nanowire fi eld effect transistor structures. Prepare 

and submit manuscript. 1Q 2007 

Design versatile Chemical Vapor Deposition (CVD) 

reactor capable of fabricating Si and SiGe alloy 

thin-fi lms and nanostructures 3Q 2007; start 

assembling CVD system. 4Q 2007 

Complete development of fabrication process for 

tunable-barrier Si single-electron devices. 4Q 2007

Assess Coulomb blockade behavior versus small 

dimensions of tunable-barrier devices – is it possible to achieve both high-temperature operation 

and reliable, low noise device performance? 

2Q 2008

Develop robust molecular test structures (see 

Fig. 1, for example) in order to use them to measure the electrical properties of molecules. The 

•

•

•

•

•

Figure 2. A high-resolution TEM micrograph 

showing a multi-walled carbon nanotube (MWNT) 

and the amorphous carbon layer on the outer 

surface. The parallel nanotube walls correspond to 

the walls of the MWNT. Photo courtesy of Gurpreet 

Singh of University of Colorado.

measured electrical properties will be correlated 

with systematic characterization studies by a variety of advanced analytical probes and the results 

used to determine charge conduction mechanisms 

and in the validation of predictive theoretical 

models. One specifi c aspect of this research is 

to develop geometries and architectures that 

maximize fl exibility and operating temperature 

of single-based single-electron devices.

Figure 3. Curt Richter loads a molecular electronic 

sample for electrical characterization. Copyright 

Robert Rathe.

DELIVERABLES: 

Present and publish the fi rst unambiguous experimental evidence of the existence of molecular 

species in molecular magnetic tunnel junctions – 

devices that combine features of moletronics and 

spintronics. 1Q 2007

Utilize backside FTIR-based technique to determine 

the chemical origin of the displacement of aliphatic 

molecules directly attached to silicon after metallization. Prepare and submit manuscript. 2Q 2007

Systematically utilize IETS (inelastic electron tunneling spectroscopy) measurements to characterize molecules in silicon-base METS. Prepare and 

submit manuscript describing results. 3Q 2007

Establish test-structures that enable the integration 

of molecular electronics with CMOS for the on-chip 

electrical characterization of hybrid Si/molecular 

electronic devices. 1Q 2008

Determine the ultra-sensitive charge electrometry 

capabilities of Si-nanotransistors through measuring the sensitivity and its relationship to device 

structure. A longer term strategy is to assess how 

the sensitivity is affected by biological structures 

in solution.

DELIVERABLES: Assess proof-of-principle by detection of DNA denaturation/renaturation transition using 

Si nanotransistor. 4Q 2007

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Semiconductor Microelectronics and Nanoelectronics Programs 171

Develop rapid, quantitative methods for identifying CNTs. Develop laser purifi cation methods 

for CNTs.

DELIVERABLE: Determine the limitations and best 

possible accuracy of the Scanning Kelvin Force 

Microscopy (SKFM) measurement of surface potential 

when applied to nanowires. Apply SCM and SKPM 

to the measurement of the electrical properties of 

nanostructures, such as dopant concentration, work 

function, and surface state density. 4Q 2006 

Develop laser processing methods for purifying 

CNTs. 

DELIVERABLE: Demonstrate pulsed laser treatment 

of an individual Multiwall Carbon Nanotube (MWCNT) 

induced selective exfoliation of amorphous carbon 

contamination layers. 2Q 2007 

ACCOMPLISHMENTS

Spin-polarized Inelastic Electron Tunneling 

Spectroscopy of a Molecular Spintronic Device 

Investigated by EEEL Researchers. Researchers in EEEL have fabricated and characterized 

molecular-monolayer magnetic tunnel devices. 

Molecular spintronic systems were fabricated 

by sandwiching a self-assembled monolayer of 

octanethiol between two ferromagnetic electrodes 

in a nanopore (Fig. 1), demonstrating that single 

molecules can be used as the ultimate building 

blocks for spintronic devices. By using inelastic 

electron tunneling spectroscopy (IETS), Wenyong 

Wang and Curt Richter have obtained the fi rst unambiguous experimental evidence of the existence 

of molecular species in such magnetic tunnel junctions. Tunneling spectroscopy was also utilized 

to investigate the spin-polarized inelastic electron 

tunneling processes in the molecular devices. The 

measurements revealed that inelastic scattering 

due to molecular vibrations is likely the main 

cause of an observed junction magnetoresistance 

bias-dependence. These results illustrate that such 

inelastic scattering events must be accounted for 

when predicting the performance of practical molecular spintronic devices. Molecular electronic 

devices with spin-dependent tunneling transport 

behavior offer an innovative and extremely enticing direction towards spin electronics, from 

both fundamental and technological points of 

view. Due to the weak spin-orbital and hyperfi ne 

interactions in molecules, the spin coherence over 

time and distance could be preserved much longer in molecular nanosystems than in traditional 

semiconductors, which makes them a suitable 

playground for spin manipulations.

#

Demonstration of homogeneity and uniformity in tunable-barrier Si single-electron devices. 

A very common problem in semiconductor-based 

single-electron devices is the lack of homogeneity in general, and the lack of ability to fabricate 

devices with uniform device parameters in particular. In a recent study of four nominally identical 

devices, we showed the enhanced fl exibility of 

these devices, and also the substantial improvement in uniformity. In particular, across these four 

devices we showed less than a 20 percent spread 

in a variety of gate capacitance values ranging 

from a few aF up to a few tens of aF.

Lack of charge offset drift in tunable-barrier 

Si single-electron devices demonstrated. We have 

demonstrated that the recently tested tunablebarrier single-electron devices (similar to the 

one in Fig. 4) exhibits the same advantage as the 

previously measured fi xed barrier ones: there is 

no time-dependent drift in the operating characteristics of these devices, in contrast to metal-based 

single-electron devices. This demonstration allows us to continue working in the tunable-barrier 

devices, which have both advantages of superior 

fl exibility and stability in time.

#

#

Figure 4. Cross-sectional micrograph of tunablebarrier devices fabricated by our project at the 

Cornell Nanofabrication Facility.
172 National Institute of Standards and Technology 

Investigation of new “barrier” capacitance. 

In the tunable-barrier devices, the barriers are 

produced electrostatically by small “fi nger” gates; 

these electrostatically-defi ned barriers, which are 

a few tens of nm in size, have not been previously 

investigated. As a fi rst step in developing the 

metrology of characterizing such small barriers, 

we have shown that through Coulomb blockade 

measurements, it is possible to measure the “barrier” capacitance across these quantum tunneling 

regions. These capacitances (Fig. 5) range in 

value down to just a few aF, and such a measurement is only possible with Coulomb blockade 

studies. We are beginning a collaboration in order 

to compare our results with simulations; our hope 

is that this metrology study will allow validation 

of the simulation models. 

#

Figure 5. Measurement of “barrier” capacitance 

produced by electrostatic control from a single lower 

gate.

neers modifi ed a standard probe station used to test 

individual components in microelectronic circuits. 

The station includes a high-resolution optical microscope and a system for precisely positioning 

work surfaces under a pair of customized titanium 

probes with tips less than 100 nanometers in diameter. In a two-step process, silicon nanowires 

suspended in a drop of water are deposited on a 

special staging wafer patterned with a grid of tiny 

posts, and dried. Resting on the tops of the posts, 

selected nanowires can be picked up by the two 

probe tips, which they cling to by static electricity. 

The test structure wafer is positioned under the 

probes, the nanowire is oriented by moving either 

the probe tips or the wafer, and then placed on the 

wafer in the desired position. The technique’s 

fi ne level of control allows the precise placement 

of single nanowires to create elaborate structures 

for testing nanowire properties. This capability 

has been demonstrated by building a multipleelectrical-contact test structure for measuring the 

resistance of a nanowire independent of contact 

resistance (Fig. 6b), and a simple electromechanical “switch” suitable for measuring the fl exibility 

of nanowires.

Figure 6. (a) Schematic of NIST single nanowire 

manipulation system. (b) Scanning electron microscope 

image shows a single silicon nanowire positioned in 

an etched trench using NIST’s nanowire manipulation 

technique. The trench helps keep the nanowire in 

position during the fabrication of the rest of the test 

structure, which measures metal/nanowire contact 

resistance. The scale bar is 20 micrometers long.

System Developed to Precisely Align Single 

Nanowires. A system (schematically shown in 

Fig. 6a) has been devised for manipulating and 

precisely positioning individual nanowires on 

semiconductor wafers. This technique, allows 

the fabrication of sophisticated test structures 

to explore the properties of nanowires, by using 

only optical microscopy and conventional photolithographic processing in lieu of advanced (and 

expensive) tools such as focused ion or electron 

beams. The smallest-diameter nanowires today are 

built in a “bottom-up” fashion, assembled atomby-atom through a chemical growth process such 

as chemical vapor deposition. This is essentially 

a bulk process; it produces haystacks of jumbled 

nanowires of varying lengths and diameters. To 

control the positioning of nanowires, NIST engi-

#
Semiconductor Microelectronics and Nanoelectronics Programs 173

Interfaces in Molecular-Monolayer/SiO2 

Based Molecular Junctions Characterized. 

The results of dc-current-voltage (IV) and accapacitance-voltage (CV) measurements have 

been correlated with vibrational spectroscopy 

of Au/monolayer/SiO2

/Si structures to establish 

an improved understanding of the interactions at 

the buried metal/monolayer and dielectric/silicon 

interfaces. Towards the goal of observing and 

characterizing electrical device behavior based 

upon intrinsic molecular effects, the role of test 

structures and their interfaces must be understood 

and eventually effectively controlled. A novel 

backside-incidence Fourier-transform infraredspectroscopy technique previously developed by 

members of the NIST research team was used 

to characterize buried metal/molecule interface 

to probe the interaction of the top-metallization 

with the organic monolayers. Both the spectroscopic and electrical results indicate that Au 

has a minimal interaction with alkane monolayers deposited on SiO2

 via silane chemistry. An 

intriguing negative-differential-resistance and 

hysteresis is observed in the IV measurements 

of Au/alkane/SiO2/Si devices. It is unlikely that 

this behavior is intrinsic to the simple alkane 

monolayers in these structures. Based on the 

results of extensive electrical characterization, 

the observed IV features are attributed to charge 

trapping and detrapping at both the alkane/SiO2

and the Si/SiO2 interfaces. These data illustrate 

that the dielectrics and other materials used when 

fabricating molecular devices must be made at 

the highest level of control to avoid impurities 

and defects which are likely to lead to spurious 

device behavior. 

Enhanced Inversion Mobility in Silicon 

Nanowire Field Effect Transistors Demonstrated. 

Dr. Sang-Mo Koo and colleagues have demonstrated that silicon nanowire (SiNW) fi eld effect 

transistors (FETs) fabricated by a standard ‘topdown’ approach exhibit substantially enhanced 

transport performance. A systematic study on the 

inversion electron transport properties of SiNWs 

with different channel geometries has shown 

that a SiNW device exhibits enhanced inversion 

channel current density: the extracted electron 

inversion mobility of the 20 nm width nanowire 

channel (1000 cm2

/Vs) is found to be two times 

higher than that of the reference MOSFET of large 

dimension (W >1 μm). The enhancement is attributed to the possible suppression of inter-valley 

phonon scattering due to strain in SiNW caused by 

the oxidation process. As the feature sizes of FETs 

are scaled downward, the semiconductor industry 

#

#

is working to meet the increasing challenges of 

nanoscale devices that are smaller and yet can 

be manufactured with minimal deviation from 

today’s standard manufacturing processes. These 

results strongly suggest that lithographically fabricated SiNW FETs, which are compatible with 

Si ULSI technology, can bring about signifi cant 

performance benefi ts in nanoscale electronics, preserving the basic silicon technology infrastructure 

upon which current industry relies.

Silicon nanowires as enhancement-mode 

Schottky-barrier fi eld-effect transistors. We have 

shown that SiNWs with Schottky contacts can be 

used as enhancement-mode FETs with an excellent 

on/off current ratio. The process does not require 

any source and drain doping or silicide formation, 

thereby allowing for a simple process without 

thermal annealing. Silicon nanowire fi eld-effect 

transistors (SiNWFETs) were fabricated with a 

highly simplifi ed integration scheme to function 

as Schottky barrier transistors with excellent enhancement-mode characteristics and a high on/off 

current ratio ~107

. SiNWFETs show signifi cant 

improvement in the thermal emission leakage 

(~6×10-13 A/μm) compared to reference FETs with 

a larger channel width (~7×10-10 A/μm). The drain 

current level depends substantially on the contact 

metal work function as determined by examining 

devices with different source-/drain- contacts of 

Ti (≈4.33 eV) and Cr (≈4.50 eV). The different 

conduction mechanisms for accumulation- and 

inversion-mode operation were determined and 

confi rmed by comparison with two-dimensional 

numerical simulation results. Schottky barrier 

FETs are of great interest in their own respect 

as an alternative to traditional doped source and 

drain device structure, because sub 100 nm range 

scaling encounters fundamental problems including high leakage current and parasitic resistance. 

Schottky barrier FETs have a number of advantages including simple and low-temperature processing, good suppression of short-channel effects, 

and the elimination of doping and subsequent 

activation steps. These features are particularly 

desirable for SiNW devices because they can 

circumvent diffi cult fabrication issues such as an 

accurate control of the doping type/level and the 

formation of reliable ohmic contacts. 

Two-dimensional Electrostatics to Enhance 

Channel Modulation in Dual-gated Silicon 

Nanowire Devices. We have experimentally observed enhanced channel modulation in dual-gated 

silicon nanowire (SiNW) fi eld-effect transistors 

(FETs). In this work, SiNW FETs were fabricated 

#

#
174 National Institute of Standards and Technology 

using electron-beam lithography to investigate the 

electrostatic control of current in semiconductor 

nanowire devices. These novel top- and bottomgated FETs are based upon simple top-down test 

structures that rely upon self-aligned Schottkycontacts to enable the electronic properties of 

SiNWs to be readily studied. Improved device 

performance is observed for the dual-gated SiNW 

FETs when compared to simultaneously fabricated 

large area control FETs. The SiNW devices (with 

widths down to approximately 60 nm) exhibit 

an on/off current ratio greater than 106

, which is 

more than three orders of magnitude higher than 

that of control devices prepared simultaneously 

having a large channel width (5 μm). In addition, the top gate is found to suppress ambipolar 

conduction effectively, which is one of the factors 

limiting the use of nanotube or nanowire FETs for 

complimentary logic applications. Two-dimensional numerical simulations have confi rmed an 

important physical insight illustrated by this work: 

due to the reduced dimensionality of SiNWs, 

electrostatic control is enhanced when compared 

to larger channel width devices. 

Comparison of charge offset drift in metalbased and Si-based SET transistors. We have 

completed a study comparing the charge offset 

drift in metal-based and Si-based SET transistors. 

To date, there has been no reproducible way to 

eliminate the charge offset drift in metal-based 

SET devices, while Si-based devices have been 

characterized that exhibited minimal charge offset 

drift. The long-term charge offset behavior appears to be correlated with an observation in the 

short-term noise: two-level fl uctuators in the Sibased devices are completely stable with respect 

to time, thermal cycling, etc.; in contrast, twolevel fl uctuators in the metal-based devices are 

notoriously “fragile.” We have devised a model 

that comprehensively explains these results: in 

the metal-based devices, the defects that produce 

the drift are interacting strongly, as if they are in 

a glassy environment. These interactions are the 

underlying source of the long-term drift, and of 

the lack of stability in individual two-level fl uctuators. This analysis leads us to suggest that a 

new fabrication process in which the metal fi lms 

are deposited with low stress will improve the 

performance of the metal-based devices.

Nano-gap capacitors. We have succeeded in 

devising an assembly process and measurement 

scheme that allows us to measure the electrical 

breakdown in air with greater accuracy than previously reported. This new capability depends 

#

#

crucially on the ability to accurately measure the 

gap, which we accomplish by a combination of 

capacitance measurements and simulations of 

capacitance versus separation. We have shown 

the ability to measure over a range of separations 

between 0.3 microns and 50 microns. These 

measurements of electrical breakdown in air at 

small separations are of increasing importance to 

the MEMS fi eld. 

We have also investigated the standard theory for 

the breakdown at small separations (below about 

5 microns). In this regime, the standard theory 

uses Fowler-Nordheim tunneling in vacuum, 

along with a surface roughness-induced field 

enhancement; this fi eld enhancement is typically 

a factor of about 100. By combining electrical 

breakdown measurements with AFM measurements of surface roughness, we have for the fi rst 

time been able to quantitatively test this theory. 

We fi nd that the roughness in our smooth Au fi lms 

is much smaller than necessary for the standard 

theory to be correct. We conclude that the standard 

theory cannot explain our experiments, and by 

extension is suspect in most of the measurements 

done to date.

Joint NIST/HP Research Progresses Toward 

Critical Molecular Electronics Measurements. 

Research at the NEDM and Hewlett Packard 

(HP) Laboratories is progressing toward reliable 

methods for measuring the electrical behavior 

of molecular electronic devices, an emerging 

nanotechnology eyed for future integrated circuits. 

By using a crossbar test structure consisting of 

a molecular monolayer sandwiched between a 

series of perpendicular metal wires, collaborators 

at separate facilities recorded nearly identical 

electrical measurements. This step, along with 

others taken to eliminate potential sources of error, ensures that the measured behavior is directly 

attributable to the device and not the experimental 

setup. Electrical (current-voltage, or IV) measurements of crossbar devices containing eicosanoic 

acid exhibit a controllable, two-state switching behavior that is due to the presence of the molecular 

layer. However, the molecular monolayer is not the 

sole cause. Rather, the switch-like behavior most 

likely arises from the interaction of the molecules 

with the electrodes. This example illustrates that 

the properties of molecular electronic devices 

are often determined not by the molecule alone, 

but by the entire device that consists of both the 

molecules and the attachment electrodes. This 

two-state behavior was independently measured 

in two separate laboratories, indicating that it 

#
Semiconductor Microelectronics and Nanoelectronics Programs 175

is not a measurement artifact and illustrating 

that these devices are robust enough to ship via 

conventional methods and remain active. In addition to IV measurements, what well may be the 

fi rst capacitance-voltage (CV) measurements of 

molecular monolayer-based devices were taken 

at NIST. These CV curves also show two-state 

behavior. 

Novel approach to investigate buried metalorganic interfaces for molecular electronics. We 

have developed and used a novel approach to 

investigate the top metal contact in metal-organic 

monolayer devices. In the emerging arena of 

molecular electronics, detailed characterization 

of organic monolayers encapsulated between two 

electrodes is necessary to correlate the electrical 

responses of molecular devices with the fundamental physical properties of the monolayers. The 

technique developed at NIST takes advantage of 

the natural infrared transparency of Si wafers to 

enable vibrational characterization of monolayer 

fi lms after deposition of a technologically relevant metal electrode. The samples as prepared 

encapsulate the organic layer in exactly the same 

manner as fully fabricated devices. IR and electrical samples were fabricated simultaneously, 

allowing direct comparison of the spectroscopic 

results with electrical device performance. Two 

different chemical processes were utilized to attach nm-thick alkane fi lms to the Si substrates: a 

conventional silanization process for fi lms on thin 

oxides and a novel process previously optimized 

at NIST for direct attachment to silicon. Molecules on silicon are of interest as active electronic 

layers and for surface engineering. Monolayers 

bound directly to silicon are expected to have 

less interfacial capacitance than those on oxides, 

be more amenable to further processing, and be 

resistant to degradation die to the nature of the 

strong covalent bond. Results were presented at 

the 7th International Molecular Electronics conference and published. 

New reactor for growing doped Si, Ge, and 

SiGe nanowire heterostructures. A. Davydov 

and S. Krylyuk have designed a versatile CVD 

system for fabricating Si and SiGe alloy thinfi lms and nanostructures. The system includes 

hot-wall horizontal reactor with 4 independently 

controlled temperature zones and automated gas 

delivery system. The reactor is designed to be 

capable of growing Si and SiGe alloys on round 

or square 1” substrates at atmospheric or reduced 

(down to 1.3 Pascals (10 mtorr)) pressure. The 

#

#

system design utilizes SiH4

 gas as a Si source, 

solid Ge or germanium halides as Ge-sources 

and solid sources for p (boron-based) and n-type 

(phosphorus based) doping. To achieve rapid 

growth process interruption/restoration without 

exposing system to the outside atmosphere (e.g., 

for realization of abrupt interfaces in fabricated 

heterostructures, such as p/n junctions in Si, Ge, 

and SiGe multi-layers), the reactor design includes 

adjacent “preservation zone” to be maintained 

at the growth temperature and pressure in inert 

atmosphere. All parts of the system have been 

manufactured and/or purchased and the assembly 

of the system is in progress. The system is planned 

to be operational for growing Si nanowires and 

thin fi lms in 4Q of 2007. Capabilities for growing 

Ge and SiGe using GeH4

 gas as well as utilizing 

gaseous sources for n- and p-doping can be added 

as necessary in FY 2008 (funding permitted). The 

new CVD system will enhance the NEDM’s capability of fabricating Si and SiGe nanostructures 

with desired structural and electronic properties 

for nano-metrology needs.

Laser induced exfoliation of amorphous 

carbon layer on an individual multi-walled CNT 

(MWCNT). By applying a pulsed laser treatment 

of an individual MWCNT, we selectively induced 

exfoliation of the amorphous carbon contamination layer. The MWCNT was exposed to a 248 nm 

excimer laser. After the treatment, transmission 

electron microscopy (TEM) images (Fig. 7, next 

page) show that the amorphous layer has expanded 

and separated from the crystalline MWCNT walls. 

This interesting observation has implications for 

laser cleaning and possible thinning of MWCNTs 

to reduce the radial dimensions.

Rapid and inexpensive identifi cation of bulk 

carbon nanotubes. We estimated the volume 

fraction of metallic and semiconducting carbon 

single-wall nanotubes (SWNTs) for purifi ed laser 

vaporization SWNTs, from an effective medium 

approximation and the measured spectral responsivity of a LiTaO3

 pyroelectric detector covered 

with SWNT “bucky” paper. The detector spectral 

responsivity from 600 to 2000 nm was proportional to the expected absorption coeffi cient of the 

SWNTs, and variations near 700, 950, and 1750 

nm correlated with characteristic interband transitions and proportions of SWNTs consistent with 

20 % metal and 80 % semiconductor materials.

#

#
176 National Institute of Standards and Technology 

FY OUTPUTS

COLLABORATIONS

Hewlett-Packard, R. Stanley Williams et al., 

Interface properties of molecular electronic test 

structures 

NTT, Akira Fujiwara, Si-nanowire metrology

National Renewable Energy Laboratory, A. C. 

Dillon, optical properties of CNT materials.

NIST Divisions 836, 837, 838, Dr. Roger van Zee 

et al., NIST Molecular Electronics Team

Purdue University, Prof. David Janes, Hybrid Simolecular devices and test structures 

University of Notre Dame, Prof. Greg Snider, Si 

single-electron device fabrication

Virginia Polytechnic Institute and State University, R. L. Mahajan, laser treatments of CNT 

materials.

Yale University, Prof. M. A. Reed, Robust molecular electronic test structures 

RECENT PUBLICATIONS

Li, Q., Zhu, X., Xiong, H. D., Koo, S.-M., Ioannou, D. E., 

Kopanski, J. J., Suehle, J. S., Richter, C. A., “Silicon nanowire 

on oxide/nitride/oxide for memory application.” Nanotechnology 18, 235204 (May 2007).

Li, Q. L., Koo, S. M., Richter, C. A., Edelstein, M. D., 

Bonevich, J. E., Kopanski, J. J., Suehle, J. S., Vogel, E. M., 

“Precise alignment of single nanowires and fabrication of 

nanoelectromechanical switch and other test structures.”

IEEE Transactions on Nanotechnology 6, (2), 256-262, 

(MARCH 2007).

Gergel-Hackett, Nadine, Rose, G.S., Paliwoda, P., Hacker, 

C.A., Richter, C.A., “On-Chip Characterization of Molecular 

Electronic Devices using CMOS: The Design and Simulation 

of a Hybrid Circuit Based on Experimental Molecular Electronic Device Results,” Proceedings of the ACM Great Lakes 

Symposium on VLSI 2007, (MARCH 2007).

Wang, W. Y., Xiong, H. D., Edelstein, M. D., Gundlach, D., 

Suehle, J. S., Richter, C. A., Hong, W. K., Lee, T., “Low 

frequency noise characterizations of ZnO nanowire fi eld effect transistors.” Journal of Applied Physics 101, (4) 044313 

(FEB 2007).

Zimmerman, Neil M., Simonds, Brian J., Fujiwara, Akira, 

Ono, Yukinori, Takahashi, Yasuo, Inokawa, Hiroshi, “Charge 

offset stability in tunable-barrier Si single-electron tunneling 

devices,” Appl. Phys. Lett 90, 033507 (JAN 2007).

Hurst, K. E., A. C. Dillon, D. A. Keenan, and J. H. Lehman, 

“Cleaning of carbon nanotubes near the -plasmon resonance,” Chem. Phys. Lett 433, 301 (12-JAN-2007).

Hourdakis, Emmanouel, Bryant, Garnett W., and Zimmerman, 

Neil M., “Electrical breakdown in the microscale: Testing the 

standard theory,” J. Appl.. Phys. 100, 123306 (DEC 2006)

Wang, W. Y., Richter, C. A., “Spin-polarized inelastic electron 

tunneling spectroscopy of a molecular magnetic tunnel junction.” Applied Physics Letters 89, (15) 153105 (OCT 2006).

Zimmerman, Neil M., Fujiwara, Akira, Inokawa, Hiroshi, 

Takahashi, Yasuo, “Electrostatically-Gated Si Devices: Coulomb Blockade and Barrier Capacitance,” Appl. Phys. Lett 

89, 052102 (JULY 2006).

Richter, C. A., Hacker, C. A., Richter, L. J., Kirillov, O. A., 

Suehle, J. S., Vogel, E. M., “Interface characterization of 

molecular-monolayer/SiO2

 based molecular junctions.” SolidState Electronics 50, (6), 1088-1096 (JUNE 2006).

Gilbert, K. E. H., J. H. Lehman, A. C. Dillon, and J. L. 

Blackburn “Toward rapid and inexpensive identifi cation of 

bulk carbon nanotubes,” App. Phys. Lett. 88, 143122 (03-

APR-2006).

Theocharous, E., R. Deshpande, A. C. Dillon, and J. Lehman, 

“Evaluation of a pyroelectric detector with a carbon multiwalled nanotube black coating in the infrared,” App. Opt. 45, 

1093 (20-FEB-2006).

Fujiwara, A. F, I. Hiroshi, K. Yamazaki, H. Namatsu, Y. Takahashi, N. M. Zimmerman, S. B. Martin, “Single electron tunnelling transistor with tunable barriers using silicon nanowire 

MOSFET,” Appl. Phys. Lett. 88, 053121 (30-JAN-2006).

Figure 7. TEM micrographs showing the same 

region on the MWNT (a) immediately after welding 

to the TEM grid and (b) after laser treatment. The 

hemispherical protrusion was an intrinsic feature 

used to repeatedly locate the same region after 

treatment. The arrows indicate areas were the 

amorphous carbon layer separated from the MWNT. 

Photo courtesy of Gurpreet Singh of University of 

Colorado.
Semiconductor Microelectronics and Nanoelectronics Programs 177

Hourdakis, E., B. J. Simonds, N. M. Zimmerman “Sub-micron 

gap capacitor for measurement of breakdown voltage in air,”

Rev. Sci. Instr. 77, 034702 (MAR-2006).

Koo, S. M., Q. L. Li, M. D. Edelstein et al., “Enhanced channel modulation in dual-gated silicon nanowire transistors,”

Nano Letters 5 (12), 2519 (DEC-2005).

Richter, C. A., C. A. Hacker, and L. J. Richter, “Electrical 

and spectroscopic characterization of metal/monolayer/Si 

devices,” Journal of Physical Chemistry B 109 (46), 21836 

(24-NOV-2005).

Hacker, C. A., C. A. Richter, and L. J. Richter, “Spectroscopic 

Characterization of Buried Metal Interfaces Using Backside 

FTIR and Metal-Molecule-Silicon Samples,” Characterization 

and Metrology for ULSI Technology 2005, AIP Conference 

Proceedings no.788 : 610-14, (SEP-2005).

Koo, S. M., M. D. Edelstein, Li Qiliang et al., “Silicon 

nanowires as enhancement-mode Schottky barrier fi eld-effect 

transistors,” Nanotechnology 16 (9), 1482 (SEP-2005).

Ramachandran, G. K., M. D. Edelstein, D. L. Blackburn et al., 

“Nanometre gaps in gold wires are formed by thermal migration,” Nanotechnology 16 (8), 1294 (AUG-2005).

Richter, C. A., D. R. Stewart, D. A. A. Ohlberg, R. S. Williams, 

“Electrical Characterization of Al/AlOx /Molecule/Ti/Al Devices,” Appl. Phys. A 2005, 80, 1355–1362. (March 2005).

Hacker, C. A., Anderson, K. A., Richter, L. J., Richter, C. A., 

“Comparison of Si-O-C interfacial bonding of alcohols and 

aldehydes on Si(111) formed from dilute solution with ultraviolet irradiation,” Langmuir 21(3), 882-889 (2005).
178 National Institute of Standards and Technology 
Semiconductor Microelectronics and Nanoelectronics Programs 179

POWER SEMICONDUCTOR DEVICE METROLOGY

Technical Contact:

A. Hefner

GOALS

The goal of the project is to develop electrical 

and thermal measurement methods, measurement 

equipment, and theoretical models in support of 

the development and application of advanced 

power semiconductor devices.

CUSTOMER NEEDS

There are signifi cant technical requirements for 

more effi cient, higher voltage power semiconductor devices. The application needs range from 

more efficient power supplies for computers 

and consumer appliances, to electric automobile 

power converters, to more effi cient power generation, transmission and distribution. Rapid technical advances are occurring in the development of 

new power semiconductor materials, fabrication 

processes, and device designs to address these 

needs. With the introduction of these new materials and designs come new requirements for 

characterizing the performance and reliability of 

the fabricated devices. 

The most exciting, and potentially revolutionary, 

development in this area is the rapid progress in 

the development of wide band-gap semiconductor 

materials for power semiconductor devices. Wide 

band-gap semiconductors such as silicon-carbide 

(SiC) have long been envisioned as the material 

of choice for next-generation power devices. 

Recent advances in single crystal SiC material 

and fabrication technology have ushered in a 

new era of wide band-gap power semiconductor 

devices. This has led to the commercialization of 

SiC power Schottky diode products in the 400 V 

to 1200 V range and to the development of HighVoltage, High-Frequency (HV-HF) power devices 

with 10 kV, 20 kHz power switching capability. 

The emergence of HV-HF devices with such 

capability is expected to revolutionize industry 

and military power generation, transmission, and 

distribution by extending the use of switch-mode 

power conversion technology, with its superior 

effi ciency and control capability, to high voltage 

applications.

“In 2002, Dr. Calvin Carter of Cree Inc. received 

the US National Medal of Technology from 

President George W. Bush for: “his exceptional 

contributions to the development of Silicon Carbide wafers, leading to new industries in wide 

band-gap semiconductors and enabling other 

new industries in ... more effi cient/compact power 

supplies, and higher effi ciency power distribution/transmission systems.”

Several industry and government programs are 

currently under way to accelerate the development and application insertion of SiC power 

semiconductor devices. The goal of the DARPA 

Wide-Band-gap Semiconductor Technology High 

Power Electronics Program (WBST-HPE) is to develop half-bridge power semiconductor modules 

with 15 kV, 110 A, 20 kHz switching capability. The recently awarded WBST-HPE Phase 3 

effort (http://www.darpa.mil/baa/baa06-30.html) 

anticipates that this semiconductor technology 

will enable the HV-HF switching required for a 

Solid State Power Substation (SSPS). The Electric 

Power Research Institute (EPRI) also identifi ed 

the benefi ts of HV-HF semiconductor technology, 

which include advanced distribution automation 

using solid-state distribution transformers with 

signifi cant new functional capabilities and power 

quality enhancements, and the Department of 

Energy has identifi ed HV-HF power devices as 

an enabling technology for alternative energy 

sources and energy storage systems, as well as 

transmission and distribution systems. 

TECHNICAL STRATEGY

The strategy of the NIST project is to support the 

measurement infrastructure of the power semiconductor and energy systems technology industries 

by developing and evaluating measurement methods and techniques where suitable ones do not exist for characterizing critical electrical and thermal 

properties of power semiconductor devices. This 

includes electrical, thermal, and safe operating 

limit characterization, establishing performance 

metrics, and developing methods for extracting 

device model parameters. The NIST project also 

establishes the theoretical foundation required for 

development of advanced power semiconductor 

devices, and develops circuit simulation models 

for emerging power semiconductor devices to aid 

in the rapid adoption and effective utilization of 

new technologies. NIST is taking a lead role in 

developing the device metrology and performance 

metrics necessary for industry and government 

HV-HF semiconductor device development efforts 

and in the evaluation of potential future impacts 

power semiconductor technologies requiring 

future investment. 
180 National Institute of Standards and Technology 

METROLOGY HIGH-VOLTAGE HIGHFREQUENCY SWITCHING DEVICES

The emergence of HV-HF power devices presents 

unique challenges in metrology and specifi cation 

of device electrical and thermal requirements. 

NIST has recently developed unique world-class 

laboratory facilities for characterization of HV-HF 

SiC power switching devices including: a) 25 kV 

variable-pulse-width curve tracer, b) 15 kV 100 A 

50 ns inductive and resistive load switching tester, 

c) 4 kV, 50 A, 10 ns diode reverse recovery tester, d) pulsed and multi-channel long term diode 

forward current stress and monitoring systems, 

e) high-speed high-power Temperature Sensitive 

Parameter module package measurement system, 

and f) rapid thermal cycling/shock module package stress system. 

SUPPORT PROGRAMS TO DEVELOP

HV-HF SEMICONDUCTOR DEVICES

AND APPLICATIONS

A major driving force spearheading the development of HV-HF power devices is the ongoing 

DARPA WBST-HPE program focused on developing the technology deemed necessary to enable 

Solid State Power Substations (SSPS) for future 

Navy warships. Conventional distribution approaches being considered for the next generation 

of aircraft carriers employ a 13.8 kV a.c. power 

distribution that is stepped down to 465 V a.c. by 

using large (6 ton and 10 m3) 2.7 MW transformers. Substantial benefi ts in power quality enhancement, advanced functionality, size, and weight are 

anticipated by replacing this transformer with an 

all solid state design. NIST played a key role in 

WBST-HPE Phase 1 and has been selected to be 

the exclusive device and package evaluation and 

metrology lab for the Phase 2 and 3 programs for 

2005 through 2008. A new NIST/DOE program 

was recently established to analyze advanced 

power electronic component technologies needed 

for 300 MW Power Conversion Systems (PCS) to 

enable future Near Zero Emission Fuel Cell Based 

Power Plants Fueled with Coal-Derived Gas. The 

extremely large PCS is required to convert the 700 

V, 1000 A DC power output of one thousand fuel 

cell modules to the 300 kV AC power transmission level. 

DELIVERABLES:

Participate in planning future government and 

industry high power generation, transmission, and 

distribution programs enabled by emergence of 

HV-HF power semiconductor devices including 

WBST-HPE Phase 3 and the DOE Fuel Cell Power 

Plant program. 4Q 2007

Participate in coordinating the effective utilization of 

DARPA HPE Phase 2 devices in the SSPS power 

converter developed by the HPE Phase 3 contractor team. 2Q 2009

Perform analysis to identify advanced high-megawatt PCS technologies requiring investment to 

meet the goals of the DOE SECA and FutureGen 

programs for near-zero emission central station fuel 

cell power plants. 4Q 2008

Provide leadership in Chairing an Interagency Task 

Force and in establishing an industry roadmap for 

High Megawatt Power Converters. 1Q 2008

CHARACTERIZE ELECTRICAL

PERFORMANCE OF DARPA HPE 

DEVICE DELIVERABLES

NIST continues to serve as the exclusive device 

deliverable evaluation lab for the DARPA WBST 

HPE Phase 2 program providing data and analysis 

critically important to evaluate contractors and 

plan future programs. NIST evaluates device performance and provides feedback to contractor and 

DARPA program manager as well as the potential 

component users in government and industry. 

The NIST characterization data and analysis has 

continually identifi ed critical technology advancements required to meet DARPA program goals 

such as recommendations to improve surface 

passivation, reducing internal gate resistance, 

improving PiN power diode (diode made with 

p-type to intrinsic to n-type semiconductor junctions) speed, transitioning program goal from PiN 

to Schottky diodes, and specifi c design parameter 

targets for MOSFET and IGBT (insulated gate 

bipolar transistor) designs. The NIST HV-HF 

characterization results are routinely used in by 

DARPA in planning documents and by the device 

developers in publications. 

DELIVERABLES: Apply NIST HV-HF power device 

test systems to evaluate the performance of the 10 

kV, 100 A, 50 ns half-bridge power modules produced 

by the DARPA WBST-HPE program and asses the 

potential for enabling advanced commercial and military power generation, transmission, and distribution 

systems. 2Q 2008

•

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•

•
Semiconductor Microelectronics and Nanoelectronics Programs 181

SIC POWER DEVICE MODELS

FOR HV-HF POWER CONVERTER

SIMULATION

Accurate and robust circuit simulator models 

for HV-HF semiconductor devices are needed 

to evaluate the impact of the new semiconductor 

technology on power converter system performance. NIST develops the generic physics-based 

models for the SiC power semiconductor devices 

that are provided in commercial circuit and system simulation software. In addition, NIST also 

develops model parameter extraction methods 

needed to measure the physical and structural 

parameters of specifi c devices. The NIST model 

parameter extraction tools have recently been 

used to characterize SiC power MOSFETs and 

diodes introduced by the DARPA WBST-HPE 

Phase 2 program. These models were used as a 

virtual prototype to perform simulations of a 2.7 

MW Solid State Power Substation (SSPS) by the 

DARPA/ONR/Navsea WBST HPE Government 

Independent Panel providing the basis for the HPE 

Phase 3 Broad Area Announcement. The models 

are also to be used as part of the NIST/DOE advanced PCS technology evaluation. 

DELIVERABLES: Develop and apply models of 

advanced component technologies for high megawatt 

power conversion systems to determine technologies 

requiring substantial federal investment to meet the 

DOE SECA and FutureGen program goals for central 

station fuel cell power plant PCS requirements. 

4Q 2008

METROLOGY FOR SIC DEVICE

DEGRADATION AND RELIABILITY

Although signifi cant progress has been made in 

improving the quality of the SiC starting material and the fabricated devices, a major concern 

for devices with minority carrier injection is the 

degradation in the electrical characteristics after 

prolonged forward bias conduction. The degradation occurs from latent material defects such 

as Basel Plane Dislocations (BPDs) that result 

in the formation and growth of stacking faults 

activated by excess-carrier recombination. NIST 

has recently developed automated stress and degradation monitoring systems to assess degradation 

of SiC devices. The monitoring methods include 

forward conduction voltage drop, switching reverse recovery characteristics, and pulsed thermal 

imaging of current uniformity.

DELIVERABLES: Apply long term application-like 20-

kHz switching test system to evaluate HV-HF application reliability and use to evaluate reliability of DARPA 

WBST-HPE. 4Q 2007

ACCOMPLISHMENTS

NIST played a key role in planning and coordinating activities of the DARPA HPE program 

on high voltage SiC power devices including: 

Evaluated contractor performance for DARPA 

WBST HPE Phase 2. Participated in planning 

and writing Broad Area Announcement (BAA) 

for DARPA WBST HPE Phase-3 programs and 

presented status of SiC power devices at the Phase 

3 Industry Bidder Briefi ng day. Also presented the 

status of the DARPA HPE program at GomacTech 

in 2005 and 2006, and presented the status of HVHF semiconductor devices at the IEEE Industry 

Applications Society Meeting (this paper received 

the 2006 William M. Portnoy Award). Served as 

Member of DARPA/ONR Solid State Power Substation (SSPS) Government Independent Design 

Panel. Participated in planning ONR Mantech SiC 

Power Device manufacturability program. 

NIST leading industry and other Federal 

Agencies in coordinating activities in High Megawatt Power Conversion Systems. Planed and held 

the fi rst High Megawatt Converter workshop at 

NIST with participants from major government 

and industry programs requiring advanced power 

converter technologies for high megawatt power 

conversion systems. Hefner (NIST) appointed 

as the Chairman of the Interagency Task Force 

for High Megawatt Power Conversion systems. 

Served as panel member for DOE Program on 

SiC-based Inverters in Near Zero Emission Fuel 

Cell Based Power Plants Fueled with Coal-Derived Gas.

High voltage clamped inductive and resistive 

switching test bed developed. A low parasitic inductance 15 kV switching test system for clamped 

inductive and resistive SiC MOSFET switching 

characterization was developed and integrated into 

the 25 kV safety-interlocked curve-tracer system. 

A low parasitic capacitance temperature-controlled 

test fi xture with 20 kV voltage isolation and 350° 

C maximum controllable temperature was also 

included to enable investigation of HV-HF device 

characteristics at elevated operating temperature. 

The hardware is controlled by the NIST virtual 

curve-tracer instrumentation software.

#

#

#
182 National Institute of Standards and Technology 

NIST unique HV-HF device metrology 

demonstrated unprecedented performance of 

DARPA WBST-HPE: The NIST high voltage curve 

tracer and the NIST high-voltage high-frequency 

(HVHF) switching test systems were used to 

demonstrate the unprecedented performance of 

DARPA WBST-HPE MOSFETs, e.g., 10 kV, 

12 A, 50 ns inductive load switching shown in 

Fig. 1. Typical high voltage silicon devices require 

several microseconds to switch at 6.5 kV maximum. Signifi cant advances in materials, device 

design and HV-HF metrology were required for 

this achievement.

#

Figure 1. NIST measurement of the unprecedented 

performance of the SiC power MOSFETs switching 

at 10 kV, 12 A and 200 °C in less than 75 ns.

Figure 2. Comparison of reverse recovery time for 

two different 10 kV SiC PiN diodes, (a) and (b), and 

a 10 kV JBS SiC diode (c), all at 125 °C.

NIST contributed to device design required 

to improve 10 kV SiC rectifi er reverse recovery 

speed: NIST provided guidance on techniques for 

improving the reverse recovery time (switching 

speed) of 10 kV SiC rectifi ers developed by the 

HPE program. The guidance included structure 

changes to control the plasma distribution during reverse recovery in PiN diodes as well as the 

recommendation that the program be refocused 

toward Junction Barrier Schottky (JBS) diodes. 

NIST characterized the reverse recovery performance of the various diode designs that were 

produced using the NIST 4 kV, 50 A, 10 ns diode 

reverse recovery tester. Results indicate that the 

reverse recovery charge (area under the negative 

portion of the current waveform in Fig. 2 is reduced using plasma engineering but only the JBS 

diode is capable of 20 kHz operation. 

#

NIST HV-HF SiC models were used to 

simulate system performance: NIST’s metrology, 

device modeling, and parameter extraction tools 

have resulted in software models for the HV-HF 

devices that are being produced by the DARPA 

WBST HPE program. These models are being 

used by industry and government to simulate 

the performance of future power distribution 

and conversion systems enabled by the new HVHF semiconductor device technology. Figure 3 

shows the comparison of the model with measured 

results.

#

Figure 3. Comparison of measured (dashed) and 

simulated (solid) inductive-load switching turnoff waveforms for a SiC MOSFET. Both tests were 

performed at 25 °C with a clamp voltage of 5 kV 

and drain currents of 4 A (blue) and 8 A (red), 

respectively.

Extended capabilities of IMPACT parameter 

extraction software. The capabilities of the parameter extraction software, IMPACT, were extended 

to include MOSFETS (in addition to IGBTs) and 

three prototypes of SiC material (in addition to 

Si). The parameter extraction hardware was also 

extended to enable capacitance versus gate- and 

drain-voltage characterization up to 5 kV. 

#
Semiconductor Microelectronics and Nanoelectronics Programs 183

NIST determined that the SiC PiN diode 

forward bias degradation due to stacking fault 

growth is also accompanied by reduced reverse 

recovery charge and by substantial reduction in 

device conduction area. This is signifi cant because it was shown that a factor of ten reduction in 

conduction area and thus current handling capability occurs for what was previously thought to be 

a small increase on-state voltage. The conduction 

area is measured using the unique NIST highspeed pulsed thermal imaging system as shown 

in Fig. 4. The paper describing this work won 

the William M. Portnoy Award at the 2005 IEEE 

Industry Applications Society Meeting 2004. 

#

Figure 4. Pulsed thermal image of Low degradation 

rate SiC PiN diode showing 30 % area reduction 

after 500 hours of operation.

Demonstrated HV-HF Switch-Mode Power 

Conversion for the fi rst time. NIST demonstrated 

HV-HF Switch-Mode Power Conversion for the 

fi rst time using DARPA HPE 10 kV, 5 A SiC 

power devices in a specially designed power converter test system. The demonstration shows that 

the DARPA HPE SiC MOSFETs are capable of 

20 kHz at 5 kV with 2X voltage margin as shown 

in Fig. 5. By contrast, the fastest high-voltage 

silicon devices (6.5 kV IGBT) are only capable 

of less than 300 Hz at 3.2 kV without overheating. 

The DARPA HPE MOSFET performance meets 

important industry need and will enable many new 

HV-HF power conversion systems. 

#

Figure 5. Inductor current, drain current, and drain 

voltage for as SiC MOSFET operating in a 5 A, 

10 kV, 20 kHz switch-mode power converter. 

Developed physics-based, transient, 

electro-thermal simulation capability for highvoltage, high-current SiC bipolar power devices. 

A Physics-based TCAD simulations capability 

has been developed by NIST for high-voltage, 

high-current SiC power semiconductor devices. 

This capability has been applied to the electrothermal study of SiC power thyristors operating 

under high-current, pulsed-power conditions 

as shown in Fig. 6. The use of electro-thermal, 

physics-based device and circuit modeling is 

fundamental in the study of the operating limits 

of SiC power devices.

#

Figure 6. SiC thryristor anode voltage and anode 

current pulsed-power waveforms. 

COLLABORATIONS

Synopsys Inc., SiC power device modeling and parameter 

extraction for IGBT library component models

CREE, Characterization and application of SiC power devices

Northrop Grumman Corp., Characterization and application 

of SiC power devices

Virginia Tech., Silicon and SiC power device utilization

Powerex, Power semiconductor device packaging

DARPA/ONR/Navsea, SiC power devices for SSPS and other 

applications
184 National Institute of Standards and Technology 

GE CRD, SiC power devices for robust integrated power 

electronic systems 

University of Puerto Rico Mayagüez, Electro-thermal simulation of power electronic systems

University of Wisconsin Madison, SiC power system simulation

Purdue University/Carnegie Mellon University/Vanderbilt 

University/Auburn University, Development of process 

technologies for SiC power devices

Electric Power Research Institute, power semiconductor devices for solid state intelligent universal transformer

Department of Energy, electric vehicle power electronics and 

power converters for 300 MW fuel cell generation plant

Stanford University, numerical simulation of SiC power 

semiconductor devices.

Army Research lab, Pulsed power semiconductor devices.

RECENT PUBLICATIONS

L. M. Hillkirk, A.R. Hefner, R. W. Dutton, S. B. Bayne, and 

H. O’Brien, “Electro-Thermal, Transient, Mixed-Mode 2-D 

Simulation Study of SiC Power Thyristors Operating Under 

Pulsed-Power Conditions,” Proceedings of the 2007 IEEE 

International Conference on Simulation of Semiconductor 

Processes and Devices (SISPAD 2007), Sept. 2007, Vienna, 

Austria.

Tam H. Duong, David W. Berning, A. R. Hefner, Jr., and 

Keyue M. Smedley, “Long-Term Stability Test System for 

High-Voltage, High-Frequency SiC Power Devices,” Proceedings of the 2007 IEEE Applied Power Electronics Conference 

(APEC 2007), February 25–March 1, 2007, Anaheim, CA, 

pp. 1240-1246.

J. M. Ortiz-Rodríguez, T. Duong, A. Rivera-López, and A. R. 

Hefner, “High-Voltage, High-Frequency SiC Power MOSFETs 

Model Validation,” to be appearing in 2007 IEEE Power Electronics Specialists Conference (PESC), Orlando, FL.

Madelaine Hernández-Mora, Adwoa Akuffo, Colleen Hood, 

José M. Ortiz-Rodríguez, and Allen Hefner, “Experimental 

Evaluation of SiC PiN Diode Forward Bias Degradation and 

Long Term Stability,” to be appearing in 2007 IEEE Power 

Electronics Specialists Conference (PESC), Orlando, FL.

Allen R. Hefner and Sharon Beermann-Curtin, “Status of 

the DARPA WBST High Power Electronics Program in SiC 

Device Development and Technology Transition, “Proceedings of the Government Microcircuit Applications and Critical 

Technology Conference (GOMACTech) 2007, March 19-22, 

2007, Lake Buena Vista, FL. 

Hefner Jr. A. R., Sei-Hyung, R., Brett H., Berning D. W., Hood 

C. E., Ortiz-Rodriguez J. M., Rivera-Lopez A., Duong T., 

Akuffo A., Hernandez M., “Recent Advances in High-Voltage, 

High-Frequency Silicon-Carbide Power Device,” Proceedings 

of the 2006 IEEE Industry Applications Society (IAS) Annual 

Meeting, October 08-12, 2006, Tampa, FL, pp. 330-337. 

Berning D. W., Hefner Jr. A. R., Rodriguez J., Hood C. E., Rivera A., “Generalized Test Bed for High-Voltage, High-Power 

SiC Device Characterization,” Proceedings of the 2006 IEEE 

Industry Applications Society (IAS) Annual Meeting, October 

08-12, 2006, Tampa, FL, pp. 338-345. 

Duong T., Hefner Jr. A. R., and Berning D. W., “Automated 

Parameter Extraction Software For High-Voltage, High-Frequency SiC Power MOSFETs,” Proceedings of the 2006 IEEE 

Workshop on Computers in Power Electronics (COMPEL), 

July 16-19, 2006, Troy, NY, pp. 205-211.

Ortiz-Rodriguez J. M., Hefner Jr. A. R., Berning D. W., Hood 

C. E., and Olcum S., “Computer-Controlled Characterization of 

High-Voltage, High-Frequency SiC Devices,” Proceedings of 

the 2006 IEEE Workshop on Computers in Power Electronics 

(COMPEL), July 16-19, 2006, Troy, NY, pp. 300-305.

Hefner Jr. A. R., “Status of High-Voltage, High-Frequency 

Silicon-Carbide Power Devices,” Proceedings of the Government Microcircuit Applications and Critical Technology 

Conference (GOMACTech) 2006, San Diego, CA, March 

20-23, 2006, pp. 232-235. 

Reichl J.R., Lai J.S., Hefner A.R., McNutt T.R., Berning D., 

“Inverter Dynamic Electro-Thermal Modeling and Simulation 

with Experimental Verifi cation,” Proceedings of the 2005 IEEE 

Power Electronics Specialists Conference (PESC 2005), pp. 

2208-2215.

Lai, J.-S.; Yu, H.; Zhang, J.; Alexandrov, P.; Li, Y.; Zhao, 

J.H.; Sheng, K.; Hefner, A., “Characterization of normally-off 

SiC vertical JFET devices and inverter circuits,” Conference 

Record of the 2005 Fourtieth IEEE Industry Applications 

Society (IAS) Annual Meeting, Volume 1, 2-6 Oct. 2005, 

pp. 404-409. 

A. R. Hefner, D. Berning, and C. Ellenwood, “Metrology For 

High-Voltage, High-Speed Silicon Carbide Power Devices,” 

GOMACTech 2005, pp. 226-229 (2005).

T. R. McNutt, A. R. Hefner, H. A. Mantooth, Jr., J. Duliere, 

D.W. Berning, R. Singh, “Silicon carbide PiN and merged 

PiN Schottky power diode models implemented in the Saber 

circuit simulator,” IEEE Transactions on Power Electronics, 

Volume 19, Issue 3, pp. 573-581 (May 2004).

S. Ryu, S. Krishnaswami, M. O’Loughlin, J. Richmond, 

A. Agarwal, J. Palmour, A. R. Hefner, “10 kV, 123-m/spl 

Omega//spl middot/cm/sup 2/ 4H-SiC power DMOSFETs,” 

IEEE Electron Device Letters, Volume 25, Issue 8, pp. 556-

558 (Aug. 2004).

J. M. Ortiz-Rodriguez, A. R. Hefner, D. Berning, M. VolezReyes, M. Hernindez-Mora, G. Gonzalez, “Lumped-parameter 

thermal modeling of an IPEM using thermal component models,” IEEE Proceedings on Computers in Power Electronics 

Workshop, pp. 43-48 (Aug.2004).

J. Reichl, D. Berning, A. R. Hefner, J.-S. Lai, “Six-pack IGBT 

dynamic electro-thermal model: parameter extraction and 

validation,” IEEE Applied Power Electronics Conference and 

Exposition, Volume 1, pp. 246-251 (2004).

A. Hefner, T. McNutt, A. Akuffo, R. Singh, C. Ellenwood, D. 

Berning, M. K. Das, J. J. Sumakeris, R. Stahlbush, “Characterization of SiC PiN diode forward bias degradation,” IEEE 

Conference Record Industry Applications Society Annual 

Meeting Volume 2, pp. 1252-1260 (Oct. 2004).

R. Singh, A. R. Hefner, Jr., “Reliability of SiC MOS Devices,”

Journal of Solid-state Electronics, Vol. 48, No. 10-11, pp. 

1717-1720 (24-JUNE-2004).
Semiconductor Microelectronics and Nanoelectronics Programs 185

ORGANIC ELECTRONICS METROLOGY

Technical Contacts:

D. DeLongchamp

D. Gundlach

L. Richter

GOALS

Organic electronic devices are increasingly incorporated into commercial prototypes and are projected to revolutionize integrated circuits through 

new applications that take advantage of low-cost, 

high-volume manufacturing, nontraditional substrates, and designed functionality. A critical 

need has emerged for new diagnostic probes, 

tools, and methods to address new technological 

challenges. Organic electronics adoption will be 

advanced considerably by the development of an 

integrated and interdisciplinary suite of measurement methods to correlate device performance 

with the structure, properties, and chemistry of 

critical materials and interfaces. NIST will guide 

the development of standard test methods and 

provide the fundamental measurements needed 

for the rational and directed development of 

materials and processes to realize the potential 

of organic electronics.

CUSTOMER NEEDS

An exciting array of new devices and applications are now possible with the development of 

electronic devices using organic materials because 

they are amenable to low-cost, high volume manufacturing, incorporation on fl exible substrates, 

and designed functionality. Completely new 

technologies are under development including 

printable large-area displays, wearable electronics, paper-like electronic newspapers, low-cost 

photovoltaic cells, ubiquitous integrated sensors, 

and radio-frequency identifi cation tags. Market 

estimates range from $10–30 B globally by 

2010–2015, with applications in displays, logic, 

and lighting. Organic light emitting diodes for 

displays and lighting form the fi rst generation of 

products, projected to grow from approximately 

$0.5 B today to $3 B in 2010. Market expansions to $250 B by 2025 have been estimated 

should major technology and business barriers 

be overcome. At this stage, new materials and 

processes are evolving rapidly to optimize device performance, ease processing limitations, 

and demonstrate frontier applications. However, 

systematic progress in organic electronics is challenging because of the enormous range of potential materials (from polymers to nanocomposites) 

and manufacturing methods (roll-to-roll printing, 

fl uidic self-assembly, micro-contact printing, laser 

ablative printing, and ink-jet printing).

A key technical barrier is the lack of correlation 

between device performance and the structure, 

properties, and chemistry of critical materials 

and interfaces. Without this knowledge, guided 

improvements in materials and processing design 

are not possible and meaningful standard test 

protocols cannot be developed. Characterizing 

the chemically complex (mostly carbon-based) 

soft interfaces in organic devices is critical to 

proper interpretation of carrier transport behavior. 

Identifying and controlling specifi c contributions 

to performance variation requires metrology unavailable to device manufacturers and spanning 

multiple disciplines that include device physics, 

chemistry, and materials science.

TECHNICAL STRATEGY

The initial focus of the NIST program is on the 

organic fi eld effect transistor (OFET) because 

it is the basic building block of circuitry. The 

fundamental framework, characteristics, and 

issues that arise during OFET development are 

transferable to other organic electronic devices 

because of commonalities in architecture and 

interfaces. NIST is engaged in complementary 

activities to address the primary technical barriers 

facing the adoption of organic electronics: unique 

measurements of organic materials and interfaces 

for both structure and chemistry and electronic 

properties; and the development of an integrated 

measurement test platform to correlate device 

performance with the processing conditions, 

microstructure, and primary chemical structure 

of organic semiconductors. 

1. Interfacial structure and chemistry fundamentals: The basic OFET consists of thin layers 

(20 nm to 50 nm thick) of disparate materials 

including the organic semiconductor, dielectric, 

electrode, and substrate. OFET performance 

and operational stability critically depend upon 

charge transport and material interactions between 

inorganic and organic materials, particularly at 

semiconductor/dielectric and semiconductor/

metal interfaces. Detailed interfacial structure information (electronic structure, molecular orientation, interfacial roughness, interfacial chemistry), 

correlated with electronic properties is required to 

predict performance.

We are developing a suite of powerful measurement methods including X-ray, neutron, and 

optical probes to characterize critical organic 
186 National Institute of Standards and Technology 

interfaces. Changes in interfacial structure, 

chemistry, and orientation are correlated with 

OFET device evaluation. Near-edge X-Ray Absorption Fine Structure (NEXAFS) spectroscopy 

and nonlinear optical techniques are ideally suited 

for nondestructive characterization of organic interfaces for chemistry, orientation, and structure. 

NEXAFS can distinguish chemical bonding in 

the light elements and measure the orientation of 

interfacial molecules. Spectroscopic ellipsometry 

(SE) is a powerful tool to study the electronic 

state of molecules and evaluate how the transition dipoles of electronic excitations are oriented 

with respect to fi lm geometry. Polarized infrared 

absorption (IR) spectroscopy provides both confi guration and orientation information for many 

chemical moieties common to organic electronics 

materials. Specular and grazing X-ray diffraction (XRD) measurements measure long range 

positional order to describe unit cell confi guration 

and domain size information. Finally, scanning 

probe techniques such as atomic force microscopy 

provide surface morphology information to assess 

domain shape, size, and spacings.

DELIVERABLES:

Complete NEXAFS, XRD, and AFM measurements of liquid crystalline semiconducting polymer 

fi lms as a function of the surface roughness of the 

substrate and heat treatment. 2Q 2007

Complete NEXAFS, XRD, polarized IR, SE, and 

AFM measurements of liquid crystalline small molecule fi lms as a function of the side chain length. 

2Q 2007

Complete measurements of the effect of side chain 

attachment density on the thin fi lm morphology of 

liquid crystalline poly(thiophene) materials. 

3Q 2007

Complete measurements of the effect of crystal 

growth of solution processed small molecule fi lms 

on charge transport. 4Q 2007

2. In situ measurement platform: The highest charge carrier mobilities of semiconducting 

polymers are obtained only after the fi lm is heated 

through a mesophase transition. Detailed microstructure analysis of the polymer fi lm during the 

mesophase transition is critical to determining 

optimal chemical structures for future generations 

of polymer semiconductors.

We are developing methods for performing 

structure measurements described in section 1 on 

samples while they are heated through a mesophase transition. NEXAFS and polarized IR will 

monitor the confi guration and orientation of the 

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side chains. XRD will measure the thermal expansion of the lattice and the growth and orientation 

of ordered domains. Polarized optical microscopy 

will determine the degree of spatial reorganization 

of the fi lm through changes in the birefringence. 

NEXAFS and SE will monitor the orientation of 

the polymer backbone. Transistor measurements 

will assess the combined effects of thermal disorder, fi lm reorganization, and the thermal activation 

of charge carriers. The combination of each of 

these measurements will be used to determine the 

packing arrangement of the polymer in the mesophase and identify the key structural elements of 

the molecule responsible for the mesophase, and 

the mechanism whereby heating to the mesophase 

increases carrier mobility.

DELIVERABLES: 

Develop instrumentation to incorporate inert atmosphere hot stages on the NEXAFS, SE, and IR instruments and determine the effect of temperature 

and side chain attachment density on the structure 

of the side chains and backbone with temperature 

2Q 2007

Complete transistor tests to determine how polymer 

semiconductor carrier mobility changes with temperature. Assess potential impact of mesophase 

transitions near room temperature on electrical 

performance. 3Q 2007

Complete synchrotron XRD study of the change in 

diffraction peaks as a function of temperature and 

side chain attachment density. 3Q 2007

Obtain a variable temperature polarized optical 

microscope and determine the change in birefringence as a function of temperature. 3Q 2007

ACCOMPLISHMENTS

A strategy was developed to successfully resolve the detailed microstructure of the currently 

highest-performing polymer semiconductor, poly 

(2,5-bis(3-tetradecylthiophene-2-yl)thieno[3,2-

b]thiophene) (pBTTT-C14) in collaboration 

with Merck Chemicals. The strategy used a 

combination of polarized photon absorption 

spectroscopies NEXAFS, SE, and IR - along with 

spacing information from XRD and atomic force 

microscopy (AFM). The combination of these 

complementary techniques elucidated structure 

elements not possible by a single technique 

(Fig. 1). The microstructure model validated 

theoretical predictions of collaborators at the Palo 

Alto Research Center (PARC). The strategy was 

applied to other polymer semiconductors and allowed us to identify a signature structural motif 

that distinguishes higher-performing polymers 

from lower-performing. The highest-performing 

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Semiconductor Microelectronics and Nanoelectronics Programs 187

polymers have interdigitated side chains while 

others do not. Side chain interdigitation provides 

a mechanism for three-dimensional ordering 

and therefore results in larger domain sizes and 

improved charge transport. A consideration of 

primary chemical structure variation reveals that 

interdigitation is controlled by the attachment 

density of side chains to the polymer backbones. 

These results establish clear guidelines for side 

chain attachment in the synthetic design of high 

performance polymer semiconductors.

Figure 2. Gradient heat treatment of pBTTT 

transistors.

Systematic evaluation of transistor geometry 

revealed that polymer semiconductor charge carrier mobility varies with channel length, and the 

highest mobilities occur in the shortest channels. 

A detailed analysis of the temperature dependence 

of the current-voltage characteristics for transistors with different channel lengths showed that 

the transistor current was not contact limited for 

temperatures less than 300 K and proved that 

the mobility was fi eld-dependent, following a 

Poole-Frenkel-like behavior. The fi eld dependence 

was strongest for the most ordered fi lms. These 

results have important implications for channel 

and dielectric scaling, and may improve the accuracy of organic transistor models for robust 

circuit design.

COLLABORATIONS

Polymers Division, MSEL – Jan Obrzut, Youngsuk Jung, 

Tatiana Psurek, Leah Lucas, Eric Lin, Joseph Kline

Semiconductor Electronics Division, EEEL – Curt Richter, 

Oleg Kirillov, Behrang Hamadani, Oana Jurchescu 

Ceramics Division, MSEL – Daniel A. Fischer, Joseph 

Woicik

Surface and Microanalysis Science Division, CSTL – Andrew 

Moad

University of California, Berkeley – Kanan Puntambekar, 

Clayton Mauldin, Jean Fréchet, Vivek Subramanian

Stanford University – Mang-mang Ling, Zhenan Bao, Alberto 

Salleo

Palo Alto Research Center – Michael Chabinyc, John 

Northrup

#

Figure 1. Detailed packing structure of pBTTT.

A method was developed to optimize the 

heat treatment of polymer semiconductors in collaboration with the NIST Combinatorial Methods 

Center. A thermal gradient stage was used to anneal a substrate covered with polymer transistors 

over a ~70 °C range about the polymer mesophase 

transition. After cooling, a programmable probe 

station was used to map the performance of 

transistors that had been heated across the gradient. Transition to higher charge carrier mobility 

occurred just beyond the mesophase transition 

temperature (Fig. 2). The method provides a 

general means to optimize the heat treatment of 

organic semiconductors.

Variations in dielectric chemistry were found 

to impact polymer microstructure. Films deposited on hydrophobic surface preparations, such as 

octyltrichlorosilane, form large molecular terraces 

with high charge carrier mobility. Films deposited 

on bare oxide form much smaller terraces with 

one decade lower carrier mobility. The results fi t 

a nucleation model where crystals are more likely 

to nucleate from adhesive surfaces.

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188 National Institute of Standards and Technology 

Merck Chemicals – Iain McCulloch, Martin Heeney

Stanford Synchrotron Radiation Laboratory – Michael 

Toney

Corning – Sue Gasper, He Mingqian

Northwestern – Antonio Facchetti, Tobin Marks

University of Kentucky – John Anthony

Penn State University – Tom Jackson

RECENT PUBLICATIONS

B. H. Hamadani, R.J. Kline, I. McCulloch, M. Heeney, C. A. 

Richter, D. J. Gundlach, “Infl uence of source-drain electric 

fi eld on mobility and charge transport in organic fi eld-effect 

transistors,” Submitted to J. Appl. Phys.

R. J. Kline, D. M. DeLongchamp, D. A. Fischer, E. K. Lin, L. 

J. Richter, M. L. Chabinyc, M. F. Toney, M. Heeney, I. McCulloch, “The critical role of side-chain attachment density 

on the order and device performance of polythiophenes,” 

Submitted to Macromolecules.

R. J. Kline, D. M. DeLongchamp, D. F. Fischer, E. K. Lin, M. 

Heeney, I. McCulloch, M. F. Toney, 1. “Signifi cant dependence 

of morphology and charge carrier mobility on substrate surface 

chemistry in high performance polythiophene semiconductor 

fi lms,” Appl. Phys. Lett., 90, (2007) 062117.

M. L. Chabinyc, M. F. Toney, R. J. Kline, M. Heeney, I. McCulloch, “X-ray Scattering Study of Thin-Films of Poly(2,5-

bis(3-alkylthiophen-2-yl)thieno[3,2-b]thiophene,” J. Amer. 

Chem. Soc., 129, (2007) 3226-37.

D. M. DeLongchamp, R. J. Kline, E. K. Lin, D. A. Fischer, 

L. J. Richter, L. A. Lucas, M. Heeney, I. McCulloch, J. E. 

Northrup, “High Carrier Mobility Polythiophene Thin Films: 

Structure Determination by Experiment and Theory,” Adv. 

Mater. 19, (2007) 833-837.

M. C. Gurau, D. M. DeLongchamp, B. M. Vogel, E. K. Lin, D. 

A. Fischer, S. Sambasivan, L. J. Richter, “Measuring molecular 

order in poly(3-alkylthiophene) thin fi lms with polarizing 

spectroscopies,” Langmuir 23, (2007) 834-842.

L. A. Lucas, D. M. DeLongchamp, B .M. Vogel, E. K. Lin, 

M. J. Fasolka, D. A. Fischer, I. McCulloch, M. Heeney, G. E. 

Jabbour, “Combinatorial screening of the effect of temperature 

on the microstructure and mobility of a high performance 

polythiophene semiconductor,” Appl. Phys. Lett. 90 (2007) 

012112.

D. M. DeLongchamp, M. M. Ling, Y. Jung, D. A. Fischer, M. 

E. Roberts, E. K. Lin, Z. N. Bao, “Thickness dependence of 

microstructure in semiconducting fi lms of an oligofl uorene 

derivative,” J. Amer. Chem. Soc. 128 (2006) 16579-16586.

J. Obrzut, K. Kano, T. Psurek, C. K. Chiang, D. DeLongchamp 

“Non-linear charge transport in polythiophene under high AC 

fi eld,” MRS Proc. 889 (2006).

D. M. DeLongchamp, Y. Jung, D. A. Fischer, E. K. Lin, P. 

C. Chang, A. R. Murphy, J. M. J. Frechet, V. Subramanian, 

“Correlating Molecular Design to Microstructure in Thermally 

Convertible Oligothiophenes: The Effect of Branched vs. 

Linear End Groups.” J. Phys. Chem. B 110 (2006) 10645-

10650.

M. L. Chabinyc, F. Endicott, B.D. Vogt, D. M. DeLongchamp, 

E. K. Lin, Y. L. Wu, P. Liu, B. S. Ong, “Effects of humidity on 

unencapsulated poly(thiophene) thin-fi lm transistors,” Appl. 

Phys. Lett. 88 (2006) 113514.
Semiconductor Microelectronics and Nanoelectronics Programs 189

MICRO- AND NANO-ELECTRO-MECHANICAL

TECHNOLOGY METROLOGY

Technical Contacts:

J. Marshall

C. McGray

R. Allen

GOAL

The objective of this project is the development 

of test structures and test methodologies for the 

characterization of Micro-Electro-Mechanical and 

Nano-Electro-Mechanical Technology (MNT) 

fabrication processes. MNT is the extension of 

MEMS (Microelectromechanical Technology 

Systems) to include nanometer-scale technology. 

Full characterization of an MNT process requires 

accurate measurement of a large set of material 

properties, including Young’s modulus, residual 

stress, thermal coeffi cients, surface roughness, 

density, and Poisson’s ratio; dimensional metrology; and device parameters. Ideally these 

measurements should be quick, performed in 

the manufacturing line, with measurements at 

multiple locations across each wafer to allow 

characterization of process variation.

CUSTOMER NEEDS

Two NIST U.S. Measurement System (USMS) 

workshops have been held relating to MNT 

metrology.

The fi rst USMS Workshop was held in Pittsburgh, 

PA on September 22, 2005, immediately after the 

Metric 2005 Conference. Representatives from a 

number of companies with interest in MNT technologies attended and were asked to help defi ne 

and prioritize the metrological needs of the MNT 

community. From the gathered workshop inputs 

and subsequent discussions approximately seven 

measurement needs (MNs) were crafted. One 

need of particular interest to this project concerns 

material property measurements for “fabless” 

MEMS. The concept of fabless MEMS is one 

in which a company can produce devices via a 

foundry rather than their own, expensive (over 

$100 million), fabrication facility. This is similar 

to the foundry model, which has been successfully 

implemented in the semiconductor industry. A 

report describing the outputs of this USMS Workshop was published in the MEMS Industry Group 

(MIG) 5-year anniversary report [2].

The second USMS Workshop was held on March 

15, 2006, during Pittcon, in Orlando, Florida. 

This meeting was targeted on metrology needs for 

microfl uidics applications. Representatives from 

seven companies presented their metrology needs 

in this rapidly growing fi eld. During this meeting, a need for dimensional metrology was stated. 

Many of the proposed microfl uidic applications 

depend upon accurately knowing the dimensions 

of the measurement device.

TECHNICAL STRATEGY

Overview: The MNT Metrology Project is currently championing MNT standardization efforts 

in three venues: ASTM, SEMI, and NIST. Highlighting the need for MNT standardization is the 

number of companies and other organizations 

participating in these standards activities.

ASTM: The fi rst four MNT standards (on 

in-plane lengths, residual strain, strain gradient, 

and terminology) [3,5-7] originated in the ASTM 

E08.05.03 Task Group on Structural Films for 

MEMS and Electronic Applications. Also within 

this task group, a standard is being developed for 

determining Young’s modulus, ultimate strength, 

and fatigue for a surface micromachining process. 

SEMI: The MNT standardization efforts being actively pursued in SEMI’s North American 

MEMS Standards Committee include recentlypublished SEMI Standard MS1-0307 on wafer-to-wafer bonding alignment targets, SEMI 

MS2-0307 on step height measurements [1], and 

SEMI MS3-0307 on MEMS terminology. Other 

SEMI MNT standardization efforts include a draft 

standard on Young’s modulus (Doc 4330), a draft 

standard for wafer-wafer bond-interface strength 

(Doc 4419), a proposed guide for the design and 

materials for ultra high purity microfl uidic systems 

(Doc 4213), specifi cations for microfl uidic interfaces to electronic device packages (Doc 4214), 

and work on a draft standard for stiction. 

An additional activity in SEMI’s MNT standardization effort is the identifi cation of standards 

related to MNT technology. SEMI is attempting 

to identify these standards and incorporate them 

into SEMI’s “Living Standard.” This Living 

Standard, which will list MNT standards from 

all standardization bodies and be maintained by 

the North American MEMS Standards Committee, will serve to guide industry and researchers 

to existing standards and highlight areas where 

standards may be needed.

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190 National Institute of Standards and Technology 

DELIVERABLES: Draft “Living Standard” and easyto-access SEMI Standards Web page incorporating, 

for example, a compilation of all MNT standards and 

pertinent links for purchasing. 4Q 2007

NIST: Proposals have been crafted in the 

MNT Project for the development of two Standard 

Reference Materials (SRMs); the MNT 5-in-1 

SRM and the micro-chevron SRM.

MNT 5-in-1 SRM: The prototype MNT 5-in-1 

SRM is expected to double as the step-height 

and Young’s modulus round robin test chip. The 

fi ve parameters will be in-plane length, residual 

strain, strain gradient, step-height, and Young’s 

modulus. The fi rst three parameters have been 

standardized in ASTM as E 2244, E 2245, and 

E 2246. The fourth parameter recently became 

the MS2-0307 standard in SEMI. And, the draft 

standard for the fi fth parameter is being balloted 

in SEMI as Doc 4330.

The fi ve measurements will be taken at NIST on 

the 5-in-1 SRM and delivered to the customer in 

order for them to compare their in-house measurements with those taken at NIST.

Customers, including design companies, equipment manufacturers, and fabrication services, 

have expressed support for this SRM.

This is an interlaboratory effort, involving MSEL 

and MEL in addition to EEEL.

DELIVERABLES: Design of prototype 5-in-1 SRM 

complete and ready for fabrication. 4Q 2007

Micro-Chevron SRM: The micro-chevron SRM 

is proposed for the calibration of wafer bond 

strength. A micro-chevron test structure (see 

Fig. 1) would be used in this destructive test. 

Wafer-wafer bonding is a mainstay for MEMS 

design and fabrication. MEMS components, 

such as acceleration sensors, gyroscopes, 

micropumps, or microvalves that are increasingly found in smart automotive and navigation 

control systems or in medical devices typically 

use wafer bonding technologies. Due to being 

subjected to mechanical stresses, the industrial 

applications of these components require both 

a high mechanical strength and reliability of the 

wafer-bonded interface. For a knowledge of the 

strength determining factors (such as fatigue and 

stress corrosion) of wafer bonding, for quality 

control, and for the development of new bonding technologies, a method for determining the 

strength of such bonds is important to producers 

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and users of MEMS devices, of wafer bonding 

equipment, and of wafer materials.

In the area of dimensional metrology for microfl uidics, the MNT Metrology Project has undertaken development of dimensional metrology 

test structures and techniques. The goal of this 

work is standard test structures, methods, and 

analysis techniques such as exist for semiconductor devices.

DELIVERABLES: Prototype test chip for microfl uidics dimensional metrology. 4Q 2007

In the area of microrobotics, the MNT metrology project is organizing the initial nanogram 

demonstration competition at the 2007 RoboCup 

tournament to be held July 2007 in Atlanta, Georgia. This high-profi le activity, involving teams 

from universities in the U.S. and overseas, will 

highlight NIST’s MEMS activities. The MNT 

metrology project is also investigating the unique 

metrology issues associated with micro-robots 

with the goal of 

DELIVERABLES: Organize and hold the demonstration competition for nanogram-scale robotics at the 

2007 RoboCup tournament in Atlanta, Georgia. 

3Q 2007

Figure 1. Studs mounted to a micro-chevron test 

structure.

NIST’s role: Activities of the MNT Metrology 

Project include the following:

1. Keeping tabs on the progress of the various 

standardization efforts via attending standardization meetings (such as ASTM and SEMI) and 

maintaining the “Living Standard” for SEMI’s 

N.A. MEMS Standards Committee.
Semiconductor Microelectronics and Nanoelectronics Programs 191

2. Facilitating a collaborative research process 

between the different parties and acting as a neutral intermediary in shepherding the measurement 

techniques through the standardization process, if 

necessary. This could take the form of being the 

acting technical secretary for a standard.

3. Providing technical assistance to others during the standardization effort, if necessary (and 

given the appropriate resources).

4. Although the completion of each standard 

will have its own “character,” given NIST’s experience with the completion of the four ASTM 

MEMS standards, guidance can be provided as to 

what is involved in the completion of a standard 

test method and lessons learned along the way. 

5. Develop standard reference materials, as 

appropriate. 

6. Narrow in on the next material properties 

to work on at NIST (given the inputs received 

from the community and the USMS report [2]) 

and follow through with the work, if appropriate. 

Currently, the next parameters being considered 

for standardization are Young’s modulus, residual 

stress, residual stress gradient, and wafer bond 

strength as described below:

Young’s modulus: The draft standard (Doc 4330) 

being prepared for ballot obtains Young’s modulus 

from resonating cantilevers and fi xed-fi xed beams 

oscillating out-of-plane. It applies to fi lms, such 

as found in MEMS materials, which can be imaged using a non-contact optical vibrometer or 

comparable instrument.

DELIVERABLES: Young’s modulus draft standard 

(Doc 4330) submitted to ballot. 4Q 2007 

Residual stress and its gradient: The draft 

Young’s modulus standard (Doc 4330) also 

provides for residual stress and stress gradient 

calculations. These calculations require the 

Young’s modulus value obtained in the standard 

and values for residual strain (ASTM E 2245) and 

strain gradient (ASTM E 2246). High values of 

residual stress lead to failure mechanisms in ICs 

such as electromigration, stress migration, and 

delamination. Knowledge of the residual stress 

values can be used to improve the yield in CMOS 

fabrication processes. 

Wafer bond strength: This draft standard (Doc 

4419) uses micro-chevron test structures which 

consist of two materials, typically two silicon wafers bonded together. The main characteristic of 

the test structure is the wedge-like structure (which 

gives it its name) in the bond interface. Studs 

are glued to either side of the test structure and 

positioned in the material testing machine where 

the test structure is pulled apart (see Fig. 1). The 

maximum load is measured and the critical wafer 

bond toughness is calculated, which is a measure 

of the wafer bond strength. This draft standard 

can be used to determine a preferred bonding 

technique and it can be used to obtain on-wafer 

spatial distributions of wafer bond strength.

DELIVERABLES: Wafer bond strength draft standard 

(Doc 4419) submitted to ballot. 3Q 2007

ACCOMPLISHMENTS

MEMS Calculator: The SED MEMS Calculator Web site [10] includes data sheets on which 

calculations can be performed that go hand-inhand with the MEMS standards or standardsrelated work. Data Sheets K, L, and M are the 

most recent additions to this web site. Data Sheet 

K is for step height measurements, Data Sheet L 

is used to obtain the thicknesses of all the layers 

in a 1.5 μm CMOS process, and Data Sheet M is 

used to obtain Young’s modulus values.

DELIVERABLES: Data Sheets K and L incorporated 

on the SED Web site (2Q 2007). Data Sheet M updated on the SED Web site. 3Q 2007 

Thickness: SEMI Standard MS2-0307 on 

step heights was recently published [1]. Step 

height measurements are incorporated in an 

electro-physical technique to fi nd the thicknesses 

of all the layers in a 1.5 μm commercial CMOS 

foundry process. Figure 2, next page, shows a test 

chip design with thickness test structures along 

the top edge of the chip. A design rendition of a 

sample thickness test structure is given in Fig. 3a, 

next page, with its cross-section given in Fig. 3b.) 

Relatively low values for uc

 (between 0.00012 μm 

and 0.056 μm for a given processing run) have 

been found. In addition, an earlier version of 

this technique has been verifi ed via the successful 

optimization of the Young’s modulus values for 

the various layers in the process.

With this technique, a virtual transition region 

was found between the physical and the electrical 

approach, which is based upon the value of uc. 

For the presented data set, the transition region is 

between the uc values of 0.0073 μm and 0.013 μm, 

where the electrical approach tended to be used 

for the smaller values of uc (which can be viewed 

as those layers, such as the poly1-to-active area 

#

#
192 National Institute of Standards and Technology 

oxide and the poly1 layers, which tend to be fabricated earlier in the processing sequence) and the 

physical approach used for the larger values of 

uc

 (which can be viewed as those layers, such as 

the metal1 and the glass layers, which tend to be 

fabricated later in the processing sequence).

The electro-physical technique is detailed in a 35-

page paper, which is currently being reviewed. 

DELIVERABLES: Draft paper entitled “Electro-physical Technique for Post-fabrication Measurements of 

CMOS Process Layer Thicknesses,” receive ERB 

approval. 4Q 2007

Figure 2. CMOS test chip design incorporating 

thickness test structures, cantilevers, fi xed-fi xed 

beams, and tensile test structures.

Figure 3. For a thickness test structure: a) a design 

rendition and b) a cross-section.

Young’s modulus: The test chip design 

given in Fig. 2 includes cantilevers ranging in 

length from 100 μm to 400 μm. (A procedure 

was developed using the optical vibrometer to 

obtain the resonant frequencies of the shorter 

length cantilevers.) Plots of Young’s modulus 

versus length for each layer exhibit phenomenal 

results. The optimized Young’s modulus results 

are stable as a function of length and within the 

realm of acceptability! Figure 4 shows these plots 

for metal1 and metal2. The data points given 

at L=500 μm represent the averages from two 

previous chip submissions. The error bars on the 

data points represent a plus or minus one sigma 

variation. The minimum and maximum bounds 

represent values given in the literature. The line 

represented by ‘Eguess’ is the initial value used 

in the optimization. An initial result from tensile 

tests done by David Read at NIST-Boulder found 

an average metal1 and metal2 Young’s modulus 

value of 63 GPa, which falls nicely between the 

metal1 and metal2 lines in Fig. 4. 

The resonant frequency technique for fi nding the 

Young’s modulus values is presented in an article 

prepared for Electron Device Letters, which is 

currently being reviewed.

DELIVERABLES: Draft article entitled “Young’s Modulus Measurements in Standard IC CMOS Processes 

using MEMS Test Structures,” receive ERB approval. 

4Q 2007

#

Figure 4. Optimized Young’s modulus values for 

metal1 and metal2 versus length.

COLLABORATIONS

BayTech Group, 30 Winsor Way, Weston, MA, Winthrop A. 

Baylies.

MOSIS Integrated Circuit Fabrication Service, 4676 Admiralty 

Way, Marina del Rey, CA, Dr. P. Thomas Veriner.

NIST Boulder, Materials Science and Engineering Laboratory, 

Materials Reliability Division, Dr. David T. Read.

Klaros Corporation, Beallsville, MD Robert I. Scace.
Semiconductor Microelectronics and Nanoelectronics Programs 193

Others (current and/or future): Univ. of Wisconsin, Exponent, 

Pennsylvania State Univ., Fraunhofer Inst. (Germany), Univ. 

of Maryland, MEMSCAP, and others not yet identifi ed.

RECENT PUBLICATIONS

SEMI MS2-0307, “Test Method for Step-Height Measurements of Thin, Refl ecting Films Using an Optical Interferometer,” March 2007.

MEMS Industry Group (MIG) 5-Year Anniversary Report, 

“Standardization and the Study of the U.S. Measurement 

System for Micro Nano Technologies,” September 2006.

ASTM E 2444–05εl, “Standard Terminology Relating to Measurements Taken on Thin, Refl ecting Films,” January 2006.

J. C. Marshall, R. I. Scace, and W. A. Baylies, “MEMS Length 

and Strain Round Robin Results with Uncertainty Analysis,” 

NISTIR 7291, January 2006. 

ASTM E 2244–05, “Standard Test Method for In-Plane Length 

Measurements of Thin, Refl ecting Films Using an Optical 

Interferometer,” December 2005.

ASTM E 2245–05, “Standard Test Method for Residual Strain 

Measurements of Thin, Refl ecting Films Using an Optical 

Interferometer,” December 2005.

ASTM E 2246–05, “Standard Test Method for Strain Gradient 

Measurements of Thin, Refl ecting Films Using an Optical 

Interferometer,” December 2005.

J. C. Marshall, E. M. Secula, and J. Huang, “Round Robin for 

Standardization of MEMS Length and Strain Measurements,” 

SEMI Technical Symposium: Innovations in Semiconductor Manufacturing (STS: ISM), SEMICON West 2004, San 

Francisco, CA, July 12-14, 2004.

J. C. Marshall, “MEMS Length and Strain Measurements Using an Optical Interferometer,” NISTIR 6779, August 2001.

SED MEMS Calculator Web site: http://www.eeel.nist.

gov/812/test-structures/MEMSCalculator.htm.
194 National Institute of Standards and Technology 
Semiconductor Microelectronics and Nanoelectronics Programs 195

NIST’S CENTER FOR NANOSCALE SCIENCE

AND TECHNOLOGY NANOFAB

Technical Contacts:

R. J. Celotta

G. Henein

GOALS

The CNST Nanofab is a national, shared access, 

shared cost, user facility that provides for the measurement and fabrication of nanoscale structures, 

including, electronic devices, magnetic devices, 

MEMS, bio-devices, prototypical nanoscale test 

structures, measurement instruments, and standard reference materials. It provides:

Access to expensive nanoscale measurement 

and fabrication tools, technologies, and expertise 

in a shared-access, shared-cost environment to 

both NIST and external users.

A venue to foster collaboration in nanotechnology across NIST’s laboratories and with 

NIST’s external partners in industry, academia, 

and other government agencies.

A workplace that enables CNST and others 

to help remove the measurement barriers that are 

delaying or preventing the implementation of new 

products based on nanotechnology.

CUSTOMER NEEDS

To continue to respond to U.S. science and industry’s needs for more sophisticated measurements 

and standards in a variety of nanotechnologies in 

the face of heightened global competition, NIST 

has established the Center for Nanoscale Science and Technology (CNST). Its mission is to 

provide science and industry with the necessary 

measurement methods, standards, and technology 

to facilitate the development and productive use of 

nanotechnology from discovery to production. A 

key component of the CNST is the Nanofab, a national, shared access user facility that provides an 

economical means to access the expensive tools 

required to measure and fabricate nanostructures. 

The CNST Nanofab is located in two of the fi ve 

buildings (see Fig. 1) in the Advanced Measurement Laboratory (AML) at the Gaithersburg, 

Maryland campus. 

TECHNICAL STRATEGY

The AML location provides the CNST with superior vibration, temperature and humidity control, 

and air cleanliness. The Nanofab cleanroom has 

approximately 740 m2

 of Class 100, raised fl oor, 

bay and chase, clean room space. NIST has 

invested in a complete suite of new equipment 

#

#

#

(capable of processing 150 mm wafers) that is 

now fully operational (Fig. 2). This includes wet 

chemical wafer cleaning stations, furnaces (two 

banks of four tubes each), LPCVD (poly, nitride, 

LTO), rapid thermal annealer, four reactive ion 

etchers (SF6

/O2, Fl Metal, Cl Metal, Deep), one 

XeF2 isotropic silicon etcher, fi ve metal deposition 

tools (three thermal, one e-beam, one sputterer), 

contact lithography (front- and back-side alignment), critical point dryer, microwave plasma 

asher, nanoimprint lithography, converted SEM 

e-beam lithography, focused ion beam, and numerous monitoring tools (FESEM, spectroscopic 

ellipsometer, contact profi lometer, 4-point probe, 

microscope with image capture, AFM, etc.). A 

state-of-the-art e-beam lithography system with 

mask making capability has been installed and is 

scheduled for operation in July 2007.

Figure 1. Main entrance to the AML showing the 

CNST Nanofab on the left.

Figure 2. Oxidation and annealing furnace banks.

The Nanofab is operated as a shared access user 

facility. This means that the staff of NIST and its 

partners, subject to provisions, training, and user 

fees, are permitted to independently operate the 

equipment. As of May 1st, 2007, the CNST Nanofab is accessible to users from outside NIST, such 
196 National Institute of Standards and Technology 

as academia and industry. The tools are operated in 

a manner such that a wide variety of materials can 

be processed. Additionally, the CNST Nanofab is 

unique in that it is located in close proximity to the 

CNST Research Division, the NIST Laboratories, 

and the NIST Center for Neutron Research which 

contain both a wealth of measurement science 

expertise and a vast array of state-of-the-art and 

advanced prototype tools. .

DELIVERABLES: E-beam lithography system fully 

operational by 3Q 2007.
Semiconductor Microelectronics and Nanoelectronics Programs 197

METROLOGY FOR SPINTRONIC DEVICES

Technical Contacts:

J. Unguris

W. Rippard

“Imaging and metrology 

of magnetic structures 

are at the heart of the 

systematic development 

and characterization 

of magnetoelectronic 

materials and devices”

WTEC Panel on 

SPIN ELECTRONICS 

Final Report August 2003 (p.18)

GOALS

The overall goal of the Metrology for Spintronic 

Devices program is to develop the metrological tools that will enable the development of 

electronic devices that exploit the electron spin 

degree of freedom in addition to its charge. 

Present spintronic devices include magnetic 

random access memory (MRAM), which has the 

possibility of becoming a “universal” memory 

element, hard-disk drive read heads, and other 

highly-compact magnetic memory devices, such 

as those based on current induced domain-wall 

motion in magnetic wires. In addition, spintronic 

devices also present an avenue towards replacing 

conventional complementary metal oxide semiconductor (CMOS) technology as device dimensions shrink into the deep nanometer regime, 

development of entirely new device structures 

such as nanoscale microwave components for onchip spectral analysis, nanoscale timing elements, 

and microwireless communication architectures. 

In each case, a fundamental understanding of 

the interactions between a spin-polarized current 

and ferromagnetic metals and semiconductors is 

a necessity. The particular goal of this project is 

to develop the metrology and perform the fundamental measurements to enable the development 

of these spin-based devices.

An additional goal is to enable the development 

of magnetoelectronic and spintronic devices by 

using Scanning Electron Microscopy with Polarization Analysis (SEMPA) to directly image 

the nanoscale magnetic structure and correlate 

the magnetic structure with device performance.

CUSTOMER NEEDS

MRAM elements are currently being shipped to 

customers for incorporation into products which 

are expected to be commercially available by the 

end of 2006. The magnetic memory elements are 

expected to enter the marketplace at the 180 nm 

lithography node. Although no public roadmap 

for MRAM exists, companies such as IBM and 

Freescale Semiconductor expect that the present 

circuit architecture, which relies on switching 

(writing) using applied magnetic fi elds, will not 

be viable beyond the 65 nm lithography node. 

These limitations arise because the scaling laws 

associated with the stability of the magnetic 

structures used in MRAM devices: As the device 

dimensions are decreased, the magnetic elements 

are required to have increased stability against 

thermal fl uctuations. This, in turn, will require 

larger writing fi elds, which cannot be supported 

in the present device architecture. It is expected 

that a new switching mechanism will be required 

for scaling MRAM devices below the 65 nm node. 

One way to circumvent this scaling limitation is 

to switch the elements through the use of spin-polarized currents fl owing through the memory elements to switch (write) them via the spin transfer 

effect. While this switching mechanism is largely 

expected to allow continued scaling of MRAM 

elements, it has been experimentally demonstrated 

only within the last fi ve years, and the present 

understanding of the effect remains only qualitative. In order to enable the continued scaling of 

MRAM devices, the fundamental metrology and 

measurement of the intrinsic scaling laws associated with the spin transfer effect are a necessity. In 

particular, methods to defi nitively relate threshold 

switching currents to fundamental materials properties need to be developed, materials damping 

parameters need to be measured as a function of 

device size, and tunnel junction characteristics 

need to be understood at the atomic scale.

The performance, and ultimately the commercial 

viability, of magnetoelectronic devices such as 

Magnetic Random Access Memories (MRAM) 

or magnetic fi eld sensors depend on understanding and controlling their magnetic nanostructure. 

Nanoscale magnetic features such as domains, 

domain walls, anisotropy dispersion, edge effects, 

grain structure, and defects critically affect device 

performance. Imaging these magnetic nanostructures requires techniques with high spatial resolution and high magnetic sensitivity that do not 

disturb the tiny amount of magnetic material used 

in a device. SEMPA has 10 nm spatial resolution, 

1000 Fe atom sensitivity, and is nondestructive 

while imaging. Furthermore, SEMPA images 

the magnetization directly allowing direct comparisons with computer generated micromagnetics 

simulations of ideal devices.

More generally, many semiconductor devices 

encounter scaling limitations as the device 

dimensions shrink into the deep nanometer regime. For instance, present CMOS scaling laws 

indicate that the devices will encounter both 

quantum mechanical and fabrication limitations 

below the 10 nm length-scale. One potential 

avenue towards circumventing this roadblock is 
198 National Institute of Standards and Technology 

spin-based devices, which afford the possibility of 

fast operation, low power-dissipation, and general 

compatibility with CMOS-based device fabrication. Furthermore, novel effects which occur only 

at the nanoscale in hybrid ferromagnetic systems 

can also be exploited to provide new functionality in nanoscale devices. These devices point a 

way to creating new nanoscale systems such as 

on-chip spectral analysis, nanoscale microwave 

generators, as well as nanoscale microwave mixers, phase discriminators, and chip-to-chip wireless communication schemes. However, as with 

the spin-based memory devices, in order for these 

spin-enable devices to be commercially realized, 

the fundamental measurement techniques required 

to understand the interactions between spin polarized electrons and ferromagnetic materials need 

to be developed and implemented. 

TECHNICAL STRATEGY

The technical strategy is to develop the metrology and perform the fundamental measurements 

to characterize and understand the effects of 

spin-polarized currents in spin-based magnetic 

nanostructures. Particular attention will be paid to 

the intrinsic scaling laws associated with device 

switching times in both all-metallic and magnetic 

tunnel junction MRAM structures, as well as the 

thermal contributions to the critical current densities. These measurements will help to provide a 

basis for scaling of MRAM devices below the 65 

nm node. Our technical work will also focus on 

evaluating the potential of nanoscale spintronic 

devices for replacing/augmenting CMOS in the 

deep nanometer regime. One potential method 

to transmit information without charge is through 

the use of spin waves. Our work will focus on 

understanding the generation, transmission, and 

phase control of localized nanoscale spin wave 

oscillators.

1. Develop methods to measure damping in 

magnetic nanostructures, particularly as a function 

of oxidation.

DELIVERABLES: 

Measure damping in NiFe magnetic nanodot arrays

with sizes ranging from 200 nm to 50 nm. Measurements will be performed on both intentionally 

oxidized structures and ones capped to prevent 

oxidation. Compare results with continuous fi lms. 

Complete measurements in 1Q 2007 and submit 

results in 2Q 2007.

•

Develop metrology to electrically excite and detect 

FMR resonance in individual nanoscale devices 

made to continuous fi lms and patterned fi lms. 

Complete measurements in 2Q 2007 and submit 

results in 3Q 2007.

2. Perform magnetic imaging of tunnel junction 

device under active bias.

DELIVERABLES: Develop infrastructure and 

fabrication techniques to allow Lorentz imaging and 

simultaneous electrical characterization of magnetic 

tunnel junction devices under active bias. Complete 

measurements in 2Q FY 2007 and submit results in 

3Q 2007.

3. Determination of thermal contributions to 

linewidths of spin transfer oscillators.

DELIVERABLES: Develop infrastructure to measure 

the linewidth of spin transfer oscillators as a function of 

temperature (300 K – 10 K) and applied fi eld strength 

in order to directly determine thermal contributions to 

oscillator linewidth (phase noise). Complete infrastructure upgrade in 1Q 2007 and complete measurements 

in 3Q 2007.

SEMPA is already an established tool for imaging 

magnetic nanostructures in simple ferromagnetic 

structures. As structures are reduced in size to 

the nanoscale, the fabrication process can affect 

the magnetic properties, but to date there has not 

been a systematic investigation of this. Further, 

special protocols and instrumentation need to 

be developed for imaging the magnetization in 

samples that are operationally as close as possible 

to commercial magnetoelectronic devices. In particular, sample preparation techniques that expose 

the ferromagnetic element directly to the SEMPA 

probe need to be developed for a wide range of 

samples including insulating ferromagnetic oxides 

and delicate multilayers. Of course these cleaning 

procedures must not disturb the magnetic structure 

under investigation. Finally, since a critical question is the behavior of magnetic nanostructures 

in a functioning device, our recently developed 

instrumentation now makes possible magnetic 

imaging while the device is electrically active 

i.e., while applying magnetic fi elds or measuring 

magnetoresistance.

1. Investigate the effect of fabrication method 

on magnetic structure of nanoscale magnetic elements.

DELIVERABLES: SEMPA images of arrays of permalloy magnetic elements (lines, squares and ellipses) 

ranging from 50 nm to 2 micrometer prepared by FIB, 

dry etching, and liftoff. 3Q 2007

•
Semiconductor Microelectronics and Nanoelectronics Programs 199

2. Develop new sample cleaning capability 

to allow SEMPA imaging of a wider range of 

samples including insulating oxides and very thin 

fi lm multilayers.

DELIVERABLES: Integrate electron cyclotron plasma 

source with the SEMPA system to provide an atom 

source producing thermal energy neutral atoms, a 

downstream plasma mode providing low energy ions 

and neutrals, and a broad beam ion source. 4Q 2007

ACCOMPLISHMENTS

High-speed Current Induced Switching for 

MRAM Devices: In collaboration with Hitachi 

Global Storage Technologies (HGST) and Freescale Semiconductors we have demonstrated 

sub-nanosecond (290 ps) switching of nanoscale 

MRAM devices is possible via the spin transfer 

effect. To date this is the shortest switching time 

that has been reported in the literature. The inverse 

switching time (i.e., the switching frequency) 

at room temperature is linearly proportional to 

the current applied to the device. This fi nding is 

in accordance with the behavior expected from 

theoretical considerations and indicates that the 

measured room-temperature switching threshold is strongly affected by thermal fl uctuations 

in the nanoscale devices. By extrapolating the 

measured switching current to zero applied pulse 

width, the theoretical value of the intrinsic device 

critical current can be determined. However, this 

theoretical method of determining the intrinsic 

switching thresholds (i.e., those related to materials properties and device size) from stochastic, 

thermally activated switching events had not been 

experimentally verifi ed. By extensively comparing the thresholds for current driven switching at 

room temperature and 4 K for a range of applied 

fi eld values, we have been able to verify the 

extrapolation methods used to infer the intrinsic 

switching current, identify thermal contributions 

to the switching threshold, and determine the 

materials/device parameters that are responsible 

for setting the intrinsic switching threshold. These 

measurements will allow for improved device 

design and reliability for future MRAM at the 65 

nm node and beyond. 

Damping in Magnetic Nanostructures: One of the 

critical parameters in determining the threshold 

switching current in magnetic devices is the magnetic damping parameter; the larger the damping 

in the system, the larger the current required to 

switch the device. While the material dependent 

damping parameters have been routinely measured in macro/microscopic devices, very little 

#

is known about how damping is affected when 

device sizes are decreased to the nanometer length 

scale. As device dimensions are decreased, the 

surfaces of devices become increasingly important. This is particularly the case for damping in 

magnetic nanostructures. Nanomagnetic devices 

will typically have oxidation of the ferromagnetic 

materials on at least some of their sides, and ferromagnetic oxides are well known sources of 

damping. Recently some researchers have shown 

indirect evidence that the magnetic damping in 

magnetic nanostructures can be increased by more 

than a factor of ten relative to that of macroscopic 

devices. We have recently developed the metrology capabilities to directly measure the magnetic 

damping in a variety of magnetic systems over a 

wide range of device sizes. Our new optical method of measuring damping on arrays of devices has 

shown that, at room temperature, the oxidation 

has a negligible effect and shape variations in the 

nanoscale devices increases the apparent damping by only about a factor of two. We have also 

developed a new method to electrically measure 

the damping parameter in single devices having 

dimensions below 100 nm. These measurements, 

while still being perfected, corroborate those from 

the large arrays. 

Noise and Current Induced Switching in 

Magnetic Tunnel Junction Devices: Currently, 

typical spin transfer based MRAM structures are 

based on all-metallic structures. These structures 

have resistances on the order of 5–10 ohms and are 

poorly impedance matched to standard semiconductor circuitry. While this impedance mismatch 

will become less severe as device dimensions are 

decreased, it will likely be necessary to implement 

magnetic tunnel junction (MTJ) devices. However, in general, there is also an increase in noise, 

particularly at low frequency, when incorporating 

tunnel junctions into device structures, which can 

degrade device performance and read-out fi delity. The low-frequency noise is associated with 

defects and atomic scale variations in the barrier. 

We have successfully measured and characterized this excess noise in high-quality aluminum 

oxide magnetic tunnel junctions with dimensions 

on the order of several micrometers and used 

Lorentz transmission electron microscopy (TEM) 

imaging to investigate the correlation between 

electronic noise and the structural properties of 

the barrier. More recently we have upgraded the 

TEM instrumentation and fabricated devices so 

that the magnetic devices can now be imaged 

with Lorentz microscopy under active bias. This 

#
200 National Institute of Standards and Technology 

allows us to image directly the magnetic fl uctuations in a device while simultaneously measuring its electrical characteristics. Through these 

simultaneous measurements we can discriminate 

between intrinsic magnetic fl uctuations in the 

MTJ devices from those associated with electrical 

defects in the barriers. 

Nanoscale Generation of Spin Waves for 

Communication Architectures: At present, spintronic devices are being considered as potential replacements for future generation CMOS devices. 

The basic advantage of the spintronic devices is 

that they provide the potential to perform both 

communication and computation through spin 

instead of charge, and so may be able to perform 

these operations with comparatively low power 

dissipation. Several proposed communication and 

computation architectures are based on the local 

generation and subsequent propagation of spin 

waves, Fig. 1. Previously, we have shown that 

nanoscale magnetic devices are able to generate 

coherent, gigahertz frequency spin waves. However, the currents required to generate them was 

relatively high and their propagation away from 

their source was only theoretical. We have performed the metrology to show directly that in the 

correct geometry the spin waves can coherently 

propagate more than 0.5 μm away from their point 

of generation, putting fairly lenient geometrical 

constraints on potential device considerations. 

Furthermore, by incorporating CoFeB materials 

in these devices we have been able to lower the 

currents required for the creation of spin waves 

by roughly a factor of three. This represents a 

signifi cant advance towards viable device applications.

#

Figure 1. Micromagnetic simulation showing spin 

wave interactions between two closely spaced 

nanoscale oscillators.

Nanostructured Magnetic Arrays to Control 

Motion of Magnetic Flux Quanta: SEMPA images 

of a close-packed array of triangular magnetic elements contributed to a multi-institution research 

effort focused on the control of the vortex motion 

in superconducting materials. The magnetic ele-

#

ments form an array of asymmetric pinning potentials underlying the superconducting material, 

which creates a ratchet effect causing a rectifi ed 

vortex motion (in one direction) when excited by 

an ac current. This offers interesting possibilities 

for manipulating fl ux quanta in fl uxonics devices. 

Fig. 2 (a) shows an array of such triangles in the as 

grown state. In the lower part of Fig. 2, a higher 

resolution image of the as-grown magnetization of 

one of the triangles (b) is compared to the ordered 

state (c) after applying an in-plane fi eld. The 

magnetic state of the underlying periodic array 

of magnetic triangular structures determines the 

ratchet dynamics of the magnetic fl ux quanta in 

the superconductor. 

Figure 2. Magnetization (direction given by color 

wheel) of an array of triangular elements for control 

of magnetic fl ux quanta in a superconductor.

Spin Momentum Transfer Switching in Magnetic Nanowires: Current driven domain wall 

motion offers the exciting prospect of manipulating magnetization in magnetic devices without 

applying an external magnetic field. As the 

spin-polarized electrons of an electron current in 

a ferromagnet pass through a domain wall, there 

is a torque on the electrons tending to align their 

spin magnetic moments with the magnetization 

direction. Conservation of angular momentum 

requires a reaction torque, known as the spin trans-

#
Semiconductor Microelectronics and Nanoelectronics Programs 201

fer torque, from the electrons to the magnetization 

that displaces the domain wall in the direction 

of electron fl ow. We have constructed custom 

sample holders for SEMPA to allow us to image 

the magnetization in a nanowire while a current 

is flowing. SEMPA images showing current 

induced domain wall motion are shown in Fig 3. 

In the top two panels, (a) and (b), the vortex wall 

is “pinned” and there is no change on increasing 

the current from 0 to 4 mA. In Fig. 3 (c), when 

5 mA is applied, the wall is seen to have moved 

to the left in the direction of electron fl ow. The 

wall makes a large jump about 80 % of the way 

through the image which is scanned from top to 

bottom. The image (d) taken immediately after 

(c) shows that the wall has moved to the left to 

be stopped by the next pinning site. Not much is 

known about the pinning sites, but it is thought 

that pinning can be caused by structural defects or 

chemical inhomogeneities. At present, when pinning sites dominate the domain wall motion, the 

wall moves from one randomly located pinning 

site to the next. If the extrinsic defects that cause 

pinning can be eliminated or controlled such that 

they can be tailored to do a particular job, current 

induced domain wall motion could have wide 

applicability in spintronic devices.

Figure 3. SEMPA images showing the current 

induced displacement of a vortex domain wall.

Freescale Semiconductor, MRAM device imaging and incorporating magnetic tunnel junctions into MRAM devices.

Hitachi Global Storage Technologies, high-speed switching 

in all-metallic MRAM devices.

Seagate Technologies, phase-locking mechanisms in spintronic 

microwave oscillator arrays.

U. S. Army Research Laboratory, delivery of magnetic nanoscale spin wave oscillators.

U.S. Army Research Laboratory, imaging magnetic sensors.

FUTURE OPPORTUNITIES

Nanoscale spintronic devices also provide an opportunity for the development of dense non-volatile memory elements as well as new spin-enabled 

nanoscale microwave devices, such as microwave 

sources and mixers for timing, communications, 

and spectral analysis applications. In addition, 

they also provide one potential avenue towards 

replacing/augmenting conventional CMOS devices in the deep nanometer regime. As such, we 

expect that spintronic devices hold the potential to 

have a great impact on the semiconductor industry, 

particularly as device dimensions shrink into the 

nanoscale. The goal of this project is to provide 

the fundamental metrology and measurements 

to enable the development of this wide range of 

devices and applications.

RECENT PUBLICATIONS

M. R. Pufall, M. L. Schneider, W. H. Rippard, and S. E. Russek, 

“Low-fi eld current-hysteretic oscillations in spin-transfer 

nanocontacts,” 75 140404R (2007).

M. L. Schneider, M. R. Pufall, W. H. Rippard, S. E. Russek, 

and J. A. Katine, “Thermal effects on the critical current of spin 

torque switching in spin valve nanopillars,” Applied Physics 

Letters 90, 092504 (2007).

W. H. Rippard, M. R. Pufall, and S. E. Russek, “Comparison 

of frequency, linewidth, and output power in measurements 

of spin-transfer nanocontact oscillators,” Physical Review 

B 74, 224409 (2006).

M. R. Pufall, W. H. Rippard, S. E. Russek, S. Kaka, and J. A. 

Katine, “Electrical measurement of spin-wave interactions of 

proximate spin transfer nanooscillators,” Physical Review 

Letters 97, 087206 (2006).

A. V. Silhanek, W. Gillijns, V. V. Moshchalkov, V. Metlushko, 

F. Gozzini, B. Ilic, W. C. Uhlig and J. Unguris, “Manipulation 

of the Vortex Motion in Nanostructured Ferromagnetic/Superconductor Hybrids,” Applied Physics Letters 90, 182501 

(2007).

A. V. Goncharov, A. A. Zhukov, V. V. Metlushko, G. Bordignon, H. Fangohr, C. H. de Groot, J. Unguris, W. C. Uhlig, 

G. Karapetrov, B. Ilic, and P. A. J. de Groot, “Anisotropy of 

Magnetization Reversal and Magnetoresistance in Square 

Arrays of Permalloy Nano-Rings,” IEEE Transactions on 

Magnetics 42(10), 2948-2950 (2006).

COLLABORATIONS

IBM, MRAM device imaging and thermal induced noise in 

tunnel-junction based MRAM devices.
202 National Institute of Standards and Technology 

T. L. Monchesky and J. Unguris, “Magnetic Properties of 

Co/GaAs(110),” Physical Review B 74, 241301(R) (2006).

W. Yu, J. A. Bain, W. C. Uhlig, and J. Unguris, “The Effect of 

Stress Induced Anisotropy in Patterned FeCo Thin Film Structures,” Journal of Applied Physics 99(8), 08B706 (2006).

W. C. Uhlig and J. Unguris, “Size Dependent Transition from 

Shape to Exchange Dominated Magnetic Nanostructures in 

Patterned Zigzags,” Journal of Applied Physics 99, 08G302 

(2006).

W. C. Uhlig and J. Unguris, “Size Dependent Transition from 

Shape to Exchange Dominated Magnetic Nanostructures in 

Patterned Zigzags,” Journal of Applied Physics 99, 08G302 

(2006).

S. Kaka, M. R. Pufall, W. H. Rippard, T. J. Silva, S. E. Russek, 

and J. A. Katine, “Mutual phase-locking of microwave spin 

torque nano-oscillators,” Nature 437, 389-392 (2005).

W. H. Rippard, M. R. Pufall, S. Kaka, T. J. Silva, S. E. Russek, 

and J. A. Katine, “Injection locking and phase control of 

spin transfer nano-oscillators,” Physical Review Letters 95, 

067203 (2005).

S. Kaka, M. R. Pufall, W. H. Rippard, T. J. Silva, S. E. Russek, 

J. A. Katine, and M. Carey, “Spin transfer switching of spin 

valve nanopillars using nanosecond pulsed currents,” Journal 

of Magnetism and Magnetic Materials 286, 375-380 (2005).

S. E. Russek, S. Kaka, W. H. Rippard, M. R. Pufall, T. J. Silva, 

and M. Donahue, “Micromagnetic modeling of nano-contact 

spin-transfer oscillators,” San Jose, Magnetism and Magnetic 

Materials Conference, ES-25, 2005.

S. E. Russek, S. Kaka, W. H. Rippard, M. R. Pufall, and T. J. 

Silva, “Finite-temperature modeling of nanoscale spin-transfer oscillators,” Physical Review B 71, 104425 (2005).

M. R. Pufall, W. H. Rippard, S. Kaka, T. J. Silva, and S. E. 

Russek, “Frequency modulation of spin-transfer oscillators,”

Applied Physics Letters 86, 082506 (2005).

W. H. Rippard, M. R. Pufall, S. Kaka, T. J. Silva, and S. 

E. Russek, “Direct-current induced dynamics in Co90Fe10 /

Ni80Fe20 point contacts,” Physical Review Letters 92, 027201 

(2004).

F. C. S. da Silva, W. C. Uhlig, A. B. Kos, S. Schima, J. Aumentado, J. Unguris, and D. P. Pappas, “Zigzag Shaped Magnetic 

Sensors,” Applied Physics Letters 85, 6022 (2004).

C. A. F. Vaz, L. Lopez-Diaz, M. Kläui, J. A. C. Bland, T. L. 

Monchesky, J. Unguris, and Z. Cui, “Observation of a Geometrically Constrained Domain Wall in Epitaxial FCC Co 

Small Disks,” Journal of Magnetism and Magnetic Materials 

272-276(3), 1674-1675 (2004).

C. A. F. Vaz, T. L. Monchesky, J. Unguris, E. Bauer, S. Cherifi , 

S. Heun, A. Locatelli, L. J. Heyderman, Z. Cui, and J. A. C. 

Bland, “Spin Confi gurations and Classifi cation of Switching 

Processes in Mesoscopic Ferromagnetic Rings Down to Sub100 nm Dimensions,” Journal of Magnetism and Magnetic 

Materials 272-276(3), 1631-1636 (2004).
Semiconductor Microelectronics and Nanoelectronics Programs 203

BIOELECTRONICS METROLOGY

Technical Contacts:

D. R. Reyes

J. J. Kasianowicz

B. J. Polk

M. Gaitan

GOALS

There is rapidly growing interest in the application 

of microelectronics and integrated circuit-based 

fabrication methods to manipulate and monitor 

biological processes. This emerging fi eld of 

bioelectronics will require new competencies 

in the NIST laboratories to support metrology 

and standardization. Our goal over the next fi ve 

years is to develop an internal competency in 

this fi eld and to establish links with industrial 

partners in order to position NIST to coordinate 

the development of a bioelectronics metrology 

roadmap. Towards this end, we are developing 

a versatile bioelectronic platform with integrated 

electronic and microfl uidic components that will 

enable a broad range of quantitative cellular assays, ultimately at the single-cell level. We will 

characterize the performance of the platform 

and then use it to study single-cell response to 

various toxins. These measurement methods will 

also facilitate fundamental understanding of the 

behavior of heterogeneous cell populations and 

cellular interactions, and will enhance NIST’s 

capabilities to address emerging measurement 

needs for the medical and pharmaceutical industries. This effort will result in new metrology tools 

and test methods to support growing commercial 

industries that use high throughput methods for 

determining drug effi cacy and toxicity.

CUSTOMER NEEDS

Cell-based assays are a primary tool used by the 

pharmaceutical industry to measure therapeutic 

drug effi cacy and cytotoxicity. These time-consuming and labor-intensive assays currently 

involve millions of cells in each culture reservoir. 

This leads to some inherent measurement biases 

since the collective response only represents the 

average for the whole population and, for example, cells at different stages of development will 

respond differently to stimuli. A true paradigm 

shift is to utilize micro- and nano-fabrication technologies to perform these experiments on single 

cells or small cell clusters in continuous fl ow 

microfl uidic systems with integrated microsensors 

and MEMS. Single-cell assays will isolate the 

parameters affecting cell response and allow the 

various subpopulations present in bulk cultures to 

be distinguished. Single-cell assays, when multiplexed, will allow for more rapid identifi cation 

of drugs and drug targets.

TECHNICAL STRATEGY

Recent scientifi c reports describe the ability to 

stimulate electronically and probe single cell 

activity and to transport, sort, and position single 

cells. These capabilities have resulted in signifi -

cant advances in biological sciences and medicine. 

An excellent example is patch clamp technology 

that has revolutionized the fi eld of electrophysiology. Advances in microfabrication methods have 

recently led to the development of patch clamp 

arrays and other automated on-chip techniques; 

however, the widespread adoption of more complex integrated systems for biologically relevant 

measurements continues to face technological 

hurdles. These include complicated materials 

integration issues (maintaining a biocompatible 

environment for cells within the in-vitro measurement systems, developing stable and drift-free 

electrodes for accurate measurements, in varied 

buffer solutions, etc.), the diffi culty of accurately 

determining the electrical/electromagnetic response of integrated electronic/MEMS/fl uidic 

systems, and issues related to reproducible fabrication of integrated devices. By addressing these 

critical measurement infrastructure needs, NIST 

will accelerate the development of powerful new 

bioelectronic platforms and techniques.

Our technical approach is to integrate microelectronic, MEMS, and microfl uidic systems with cells 

in order to achieve a new level of control over the 

electronic/biological interfaces under study. We 

will develop methods to adhere and grow cells 

in defi ned patterns in a biological hybrid in-vitro

environment that incorporates microelectronic 

circuits, both electrochemical and RF, to stimulate 

and sense cell activity. Microchannel networks 

will be used to transport the biological specimens 

to exact locations and to deliver precise amounts 

of chemicals or drugs to the local cellular environment. These techniques will allow us to apply 

precise electrical, electromagnetic, and chemical 

stimulation to the cells and to measure their metabolic, electrical, and physiological responses. We 

focused our initial research efforts on the study 

of retinal (neuronal) cells, and currently, we are 

working with other mammalian cells such as 

fi broblasts (NIH-3T3), epithelial cells and sperm 

cells.
204 National Institute of Standards and Technology 

DELIVERABLES: 

Develop an array of micro-electrochemical cells 

and methods to pattern cells on the array for singlecell electrochemical measurements. 4Q 2007

Develop an array of microelectrodes for impedance-based measurements of cell growth and 

response. 3Q 2008

Investigate the feasibility of using microelectrodes 

for cytotoxicity measurements. 4Q 2008

Develop an electronic-based method, using microwave heating, for on-chip DNA amplifi cation. 

2Q 2008

Characterize low sensitive fi eld effect transistor (ISFET) -based measurements of pH in a microfl uidic 

network by referencing to a pH determination using 

a fl uorescent dye. 1Q 2008 

ACCOMPLISHMENTS

We developed a method to monitor NIH-3T3 

fi broblast cell growth on gold electrodes using 

impedance spectroscopy. In this measurement 

technique, individual cells are represented as 

resistive and capacitive elements, which increase 

the impedance of the system as the number of 

cells increase. The two-electrode system was 

fabricated using photolithographic techniques and 

is comprised of thin-fi lm gold electrodes with an 

underlying TiW adhesion layer. The system was 

then passivated with an oxide layer, which was 

etched through to defi ne and expose the electrode 

active areas. Fibroblast cells (NIH-3T3) were 

able to grow directly onto the gold surface, and as 

they adhered and multiplied, the impedance of the 

system increased. Cells preferentially grew on the 

bare gold surface and not on the oxide. However, 

the oxide surface can be rendered biocompatible with the use of polyelectrolyte multilayers 

(PEMs), leading to successful measurements 

of cell growth with impedance spectroscopy 

(Fig. 1). Thus, the fabricated cell confl uence 

•

•

•

•

•

#

Figure 1. (Left) Graph shows the impedance measurements taken over time as the cells 

grow on either a bare gold surface or a PEMs-treated surface. (Right panel) NIH-3T3 

fi broblast cells growing on a PEMs-modifi ed surface. The polycation used to promote 

cell adhesion is poly(allylamine hydrochloride).

detector demonstrates the ability to monitor cell 

adhesion and growth with impedance measurements. Future work will focus on the use of this 

system to monitor cytotoxic effects, of a variety 

of chemicals, using impedance spectroscopy to 

detect cell detachment from the surface.

We designed and fabricated a microchip, 

called mH+, which enables us to study the metrology problems associated with pH determination 

within planar microfl uidic networks, e.g. Lab-ona-Chip (LOC) systems. The work was motivated 

by the realization that LOC systems might enjoy 

more widespread commercialization if reliable 

and accurate electronic chemical sensors are more 

closely integrated with the microfl uidic networks. 

#

Figure 2. (Top panel) Schematic of mH+ Layout. 

Overall chip dimensions are as indicated however 

other features are not shown to scale. The gray lines 

indicate fl ow channels molded into PDMS, 280 μm 

wide by 50 μm high. Arrows indicate the direction 

of fl uid fl ow. a) test solution inlet, b) reference 

solution inlet, c) main outlet, d) blank inlet, e) blank 

outlet, am) alignment mark, RE) on-chip Ag/AgCl 

quasi-reference electrode, REC) reference electrode 

contact, DRC) electrical contact to ISFET drain, 

SOC) electrical contact to ISFET source. (Bottom 

panel) Photograph of sensing region of the mH+ 

device, scale as indicated. The ISFET is visible near 

the center of the fi gure, while the Ag/AgCl quasireference electrode appears in the lower right portion 

of the fi gure. Alignment marks and electrical contacts 

to the ISFET and reference electrode are made of 

gold.
Semiconductor Microelectronics and Nanoelectronics Programs 205

The mH+ chip comprised 1) an ion-sensitive 

fi eld-effect transistor (ISFET) fabricated on a 

silicon substrate, 2) an on-chip Ag/AgCl quasireference electrode (RE), and 3) a microfl uidic 

network molded in the transparent elastomer, 

poly(dimethylsiloxane) (PDMS) (Fig. 2). Thus, 

both ion-sensitive fi eld-effect transistor (ISFET)-

based and fl uorescence intensity (FI)-based pH 

measurements were conducted simultaneously 

with the mH+ chip. Since two independent pH 

measurements are used on the same device, the 

mH+ chip can be used to quantitate the accuracy 

and precision of microfl uidic pH determinations. 

The utility of the mH+ chip was demonstrated 

with comparisons of the dynamic range and fl ow 

rate dependence of the ISFET-based and FI-based 

pH measurements. Additionally, ISFET-based 

measurements clearly resolved step changes in 

pH as small as 0.26 pH unit in fl owing solution. 

Sample plugs of less than 100 microliters were 

needed for the measurements.

A method for fabricating Ag/AgCl planar 

microelectrodes for microfl uidic applications is 

presented. Micro-reference electrodes enable 

accurate potentiometric measurements with miniaturized chemical sensors, but such electrodes 

often exhibit very limited lifetimes. Our goal is 

to construct Ag/AgCl microelectrodes reliably 

with improved potential stability that are compatible with surface mounted microfl uidic channels. 

Electrodes with geometric surface areas greater 

than or equal to 100 square micrometers were 

fabricated individually and in an array format by 

electroplating silver, greater than one micrometer 

thickness, onto photolithographically patterned 

thin-fi lm metal electrodes (Fig. 3, left panel). The 

surface of the electroplated silver was chemically 

oxidized to silver chloride to form Ag/AgCl microreference electrodes. Characterization results 

#

Figure 3. Photographs (left panel) of the NIST micro reference electrode array (mREA) and close-up photo 

of an Ag/AgCl planar microelectrode, surface, and test. The right panel is a drawing of a fl uidic microwave 

heating assembly showing a gold coplanar transmission line patterned on glass with a poly(dimethylsiloxane) 

microchannel bonded over it.

showed that Ag/AgCl microelectrodes produced 

by this fabrication method exhibit increased stability compared with many devices previously reported. Electrochemical impedance spectroscopy 

allowed device specifi c parameters to be extracted 

from an equivalent circuit model, and these parameters were used to describe the performance of the 

microelectrodes in a microfl uidic channel. Thus, 

stable Ag/AgCl microelectrodes, fabricated with a 

combination of photolithographic techniques and 

electroplating, were demonstrated to have utility 

for electrochemical analysis within microfl uidic 

systems.

Rapid temperature cycling of fl uids is essential for increasing the throughput of chemical 

and biochemical processes and chemical synthesis such as polymerase chain reaction (PCR). In 

micro total analysis systems (μTAS), temperature 

cycling is typically implemented by external heating blocks or by integrated microresistive heating 

elements (microreactors). Our new approach for 

temperature cycling of microfl uidic systems is 

based on delivering microwave heating energy using a microwave transmission line. A microwave 

coplanar waveguide is integrated with a surface 

mounted elastomeric microfl uidic channel (Fig. 3, 

right panel). This approach will have application 

to microwave assisted chemistries, which have 

recently been shown (in macro-scale devices) to 

have several advantages over conventional resistive heating approaches. These advantages include 

the ability to deliver heat directly to the fl uid (and 

not to the surrounding medium), requiring lower 

temperatures to implement a chemical reaction 

and exhibition of more uniform heating profi les. 

Recently, we have demonstrated, in a different 

microchip format, the use of microwave heating 

for the amplifi cation of human DNA (PCR). We 

are currently working towards the integration 

#
206 National Institute of Standards and Technology 

of other processes, such as cell lysis and DNA 

extraction, into a microfl uidic system.

We have integrated electrodes within microfl uidic devices to carry out a.c. dielectrophoresis. 

In this trapping technique, electric fi elds polarize 

cells inducing electrostatic forces, which trap 

the cells against the electrode edges. We have 

been able to trap cells suspended from bulk 

cultures within microchannels when flowed 

past integrated, energized electrodes. We have 

observed that more than 85 % of cells passing 

the electrodes were trapped. On the other hand, 

when electrodes were deactivated, about 70 % of 

the cells were subsequently detached by solution 

fl ow. Deposition of PEMs on the electrodes renders an adhesion layer to further cell attachment. 

Trapping experiments carried out after PEMs 

were deposited over the electrodes showed that 

all immobilized cells remained adherent after the 

electrodes were deactivated. We have been using a.c. dielectrophoresis to array cells within a 

microfl uidic channel perpendicular to fl ow. 

RECENT PUBLICATIONS

J. W. F Robertson, C.G. Rodrigues, V. M. Stanford, K. Rubinson, O. V. Krasilnikov and J. J. Kasianowicz.. “Single molecule 

mass spectrometry in solution using solitary nanopores” Proc. 

Natl. Acad. Sci. (USA), 104, 8207-8211, 2007.

B. J. Polk “Design and Fabrication of a pH Sensitive FieldEffect Transistor for Microfl uidics with an Integral Reference 

Electrode” ECS Transactions, Vol. 3, in press.

D. R. Reyes, J. B. Hutchison, L. E. Locascio and M. Gaitan 

“Photopatterned Polyelectrolyte Multilayers for Cell Culture,” Proc. MicroTAS 2006 Conference, Tokyo, Japan, pp. 

260-262, 2006.

S. P. Forry, D. R. Reyes, M. Gaitan and L. E. Locascio “Cellular immobilization within microfl uidic microenvironments: 

Dielectrophoresis with polyelectrolyte multilayers” J. Amer. 

Chem. Soc. 128 (42): 13678-13679, 2006.

P.-A. Auroux, J. J. Shah, J. Booth, V. R. Mulpuri, L. E. Locascio and M. Gaitan, “Microfl uidic Method for Thermal Cycling 

by Microwave Dielectric Heating,” Proc. MicroTAS 2006 

Conference, Tokyo, Japan, pp. 1465-1467, 2006.

S. P. Forry, D. R. Reyes, M. Gaitan and L. E. Locascio “Facilitating the Culture of Mammalian Nerve Cells with Polyelectrolyte Multilayers,” Langmuir, 22 (13): 5770-5775, 2006.

B. J. Polk, A. Stelzenmuller, G. Mijares, W. MacCrehan, 

and M. Gaitan, “Ag/AgCl Microelectrodes with Improved 

Stability for Microfl uidics,” Sensors and Actuators B, 114: 

239-247, 2006.

B. Nablo, K. M. Halverson, R. G. Panchal, S. Bavari, T. 

L. Nguyen, R. Gussio, M. Misakian and J. J. Kasianowicz, 

“Biophysical properties of the Bacillus anthracis PA63 ion 

channel,” Biophysical Journal 90 (1): B179-B179 Suppl. 

S, (2006).

#

S. P. Forry, D. R. Reyes, B. J. Polk, M. Gaitan and L. E. Locascio, “Generating Cell Arrays in Microfl uidic Networks,” 

Proc. MicroTAS 2005 Conference, Boston, MA, pp. 1398-

1400, 2005.

S. Sundaresan, B. J. Polk, D. R. Reyes, V. R. Mulpuri and 

M. Gaitan, “Temperature Control of Microfl uidic Systems 

by Microwave Heating,” Proc. MicroTAS 2005 Conference, 

Boston, MA, pp. 657-659, 2005.

D. K. Shenoy, W. R. Barger, A. Singh, R. G. Panchal, M. 

Misakian, V. M. Stanford, and J. J. Kasianowicz, “Functional 

reconstitution of protein ion channels into polymerizable phospholipid membranes,” Biophysical Journal 88 (1): 552A-552A 

Part 2 Suppl. S, 2005.

K. M. Halverson, R. G. Panchal, T. L. Nguyen, R. Gussio, S. 

F. Little, M. Misakian, S. Bavari, and J. J. Kasianowicz, “Electrophysiological analysis of anthrax toxin interactions,” Biophysical Journal 88 (1): 476A-477A Part 2 Suppl. S, 2005.

J. J. Kasianowicz, “Nanopores - Flossing with DNA,” Nature 

Materials 3 (6): 355-356 JUN 2004.

B. J. Polk, M. Bernard, J. J. Kasianowicz, M. Misakian, and 

M. Gaitan, “Microelectroplating silver on sharp edges toward 

the fabrication of solid-state nanopores,” Journal of the Electrochemical Society 151 (9): C559-C566 2004.

D. R. Reyes, E. M. Perruccio, S. P. Becerra, L. E. Locascio, and 

M Gaitan, “Micropatterning neuronal cells on polyelectrolyte 

multilayers” Langmuir 20 (20): 8805-8811 2004.
Semiconductor Microelectronics and Nanoelectronics Programs 207

SYSTEM DESIGN AND TEST METROLOGY

PROGRAM

Lead counts of several thousand per chip and test frequencies in the microwave regime challenge current test methodologies. The addition of new functions to provide system on-chip 

solution poses additional testing challenges. The overall task is to develop test methodologies 

to address these new requirements.

Accurate at-speed test methodology of digital integrated circuits is a critical requirement. 

Traditional methods utilizing integrated circuit (IC) contact probing technology requires large 

contact pads incompatible with current IC designs. The development of alternative probing 

approaches through non-contact and intermittent probing techniques appear very promising. 

However, to implement these techniques, solving the at-speed test calibration issues is crucial. 

With the challenges facing designers and the rising costs of development, it is essential to 

develop accurate testing strategies.
208 National Institute of Standards and Technology 
Semiconductor Microelectronics and Nanoelectronics Programs 209

Technical Contacts:

A. R. Hefner, Jr.

M. Y. Afridi

“... SoC is the driver 

for convergence of 

multiple technologies 

not only in the same 

system package but also 

potentially in the same 

manufacturing process.”

The International Technology 

Roadmap for Semiconductors, 

2003. 

“... the trend 

toward digitisation 

notwithstanding, the 

analogue content 

of SOCs is likely to 

increase.”

Ron Wilson, Electronic Design 

News (EDN), 2006

METROLOGY FOR SYSTEM-ON-A-CHIP (SOC)

GOALS

One of the key metrology issues confronting the 

semiconductor System-on-a-Chip (SoC) industry 

is the develop-ment of measurement methods and 

standards for characterizing embedded-sensor 

(ES) Virtual Components (ES-VCs), a critical 

class of building blocks from which SoCs are 

developed. The goal of this project is to promote 

and support the development of hardware and 

software standards for specifying embeddedsensor (ES) virtual-components (VCs) compatible with the System-on-a-Chip (SoC) integration 

methodology used for digital IC design. 

This NIST effort will enable ES-VCs to be included in SoC CAD libraries and enable integration of ES-VCs with the existing digital VCs used 

ubiquitously by industry to design large ICs. The 

methods and standards developed as a result of 

this work will be essential for the realization of 

integrated, low-cost, smart homeland security and 

envi-ronmental sensor systems. One focus is on 

delivering standards to facilitate the incorporation of multi-technology (MT) VCs including 

MEMS-based (MicroElectroMechanical System) 

VCs into SoCs. 

The project activities include: multi-technology 

hardware description language (HDL) model 

development, VC interface standards, synthesis 

and scaling standards for ES-VCs compatible 

with digital methodologies, testing standards, 

verifi cation standards and high-level models of 

system components. The NIST MEMS-based 

integrated gas-sensing VC is used as a test bed 

to demonstrate the viability of these standards. 

In addition, the demonstration of general purpose gas-sensing VC methodologies is used to 

facilitate the adoption of these MT-VCs into new 

Home-land Security and industrial applications. 

Compatibility with the IEEE 1451 set of sensor 

network standards is supported by the Department 

of Homeland Security.

CUSTOMER NEEDS

Recent advances in high density CMOS integration and the ability to co-integrate MEMS-based 

sensor devices enables cost effective complex 

system designs fabricated on a single chip. The 

need for standards arises when the system-on-achip is designed using IP (Intellectual Property) 

cores from multiple vendors. These cores must be 

compatible for design success, thus demanding 

standards in the area of interoperable interfaces, 

models and verifica-tion strategies for multitechnology SoC designs. 

The SoC design challenges include managing increasing system complexity, achieving 

system-level verifi cation, and bridging the separate disciplines of system architecture and chip 

design. These challenges are being overcome with 

the use of platform-based design approaches that 

emphasize design reuse, i.e. the development of 

ES-VCs that can be used as cost-effective building 

blocks for SoC devices, and standards for ES-VC 

IP interoperability with the SoC design fl ow.

The direct customers for this infrastructure building will be the makers of system design software, 

ES-SoC IP de-signers, SoC manufacturers and 

systems designers. This is generally recognized 

by the chip designers, manufactur-ers, and EDA 

tool developers:

“What is the most recent development that promises to truly enable a system on a chip? It is the 

ability to combine CMOS and MEMS structures 

into one process fl ow.”

Randy Frank and Dave Zehrbach, Motorola, 

in Sensors Online, 1998

“Definitely, System-on-a-chip is the driving 

paradigm in our space, and there are some fundamental differences in culture and engineering 

mentality as well as some new technical skills that 

need to be developed in engineering. At the highest 

level, system-on-a-chip implies that you need to 

think like a system designer but implement like a 

chip designer, and those traditionally have been 

different disciplines...”

Shane Robison, Executive Vice President of 

Engineering, Cadence Design Systems, Inc. 

EDAcafe.com 

TECHNICAL STRATEGY

1. To successfully develop ES-VCs for SoC 

design methodology, the fi rst step in this multistep process is to de-velop the ability to make the 

ES-VC devices via a standard CMOS compatible process. To exercise this capability we have 

chosen a MEMS microhotplate based embedded gas-sensor, including operational amplifi ers, 

decoders, and an analog-to-digital converter 

(ADC), and a microcontroller for control and 

data processing.
210 National Institute of Standards and Technology 

2. The second step is to make ES-VCs compatible with the standard digital SoC design 

methodology. This ap-proach will require ES-VC 

to incorporate digital interface circuitry and to 

have the DFT/BIST (Design For Test/ Built-In 

Self Test) functionality required by SoC standards. To facilitate this approach we will develop 

methodolo-gies and standards for adding digital 

shells to ES-VCs and demonstrate them on the 

gas-sensor VC described above. 

DELIVERABLES: Design and fabricate an improved 

version of the previously fabricated fully digital gassensor VC in a standard CMOS 0.5- micron technology to demonstrate a standard digital interface and 

SoC Design-For-Test functionality. 2Q 2008

3. The predominant design approach used by 

industry for SoC devices is top-down design. 

This requires that high-level models (in SystemC/

HDL) exist for the VCs that are candidates for use 

in any particular system of interest. Compared 

to those for digital VCs, the methodology and 

standards for developing high-level models for 

ES-VCs is at best poorly developed. To address 

this need, high-level models are being developed 

for ES-VCs using Analog and Digital Hardware 

Description Languages and higher-level system 

description languages such as SystemC. We 

are also developing methodologies to validate 

these models. The digital systems industry has 

standards set by or-ganizations such as OCP-IP, 

VSIA and OSCI to foster large-scale interchange 

and interoperability of modular digi-tal IP, and 

we believe that such standards in ES-VC fi eld 

are a key factor for the growth of an ES-VC IP 

industry.

DELIVERABLES: Compare high-level model simulation results for microhotplate-based gas-sensor 

ES-VCs with measured data from a fully digital 

ES-VC. 1Q 2009

4. The synthesis process is well defi ned for the 

digital SoC design and is well supported by a large 

number of design libraries. Currently the libraries, 

methodology, and standards for ES-VC synthesis 

do not exist. We are developing standards and 

metrologies for ES-VCs that will be compatible 

with standard digital synthesis tools. 

DELIVERABLES: Develop methodology and standards to allow ES-VC to be syntheses by standard 

MT-synthesis tools and demonstrate their viability via 

our microhotplate gas-sensor VC. 2Q 2008

5. Scaling digital circuitry is a key capability 

used by digital designers to reduce costs and 

ensure compatibility with different fabrication 

technologies. Since most systems that would 

use ES-VCs will be predominantly digital, it 

is important that there be an equivalent scaling 

capability for the ES-VCs. To address the need for 

scaling ES-VCs, we are developing metrologies 

for digital-compatible scaling processes. 

DELIVERABLES: Develop methodologies and standards for an equivalent ES-VC scaling approach and 

demonstrate its viability via our microhotplate gassensor technology. 4Q 2007

6. The testability of ES-VCs represents another 

signifi cant challenge since standards and methodologies for non-digital circuits do not exist. The 

most promising approach to address testability is 

to use BIST techniques. To facilitate this approach 

we will develop methodologies and standards for 

adding BIST to ES-VCs and interface with them 

via the digital shell.

DELIVERABLES: Develop methodologies for built-in 

self test of ES-VC devices and demonstrate their 

viability via our microhotplate gas-sensor technology. 

4Q 2007

7. Beside a gas sensor VC, a gas-sensor SoC 

requires quantifi cation, classifi cation, communication, and control capability. These functional 

requirements can easily be implemented by an onchip microcontroller with appropriate algorithms 

and protocols.

DELIVERABLES: Demonstrate a gas-sensor VC 

interfaced with an 8051 microcontroller that is IEEE 

1451 sensor-network standard compliant. 4Q 2007

NIST is a natural home for this work because NIST 

has advanced measurement capabilities across the 

spectrum of sensor technologies.

ACCOMPLISHMENTS 

A monolithic micro-gas-sensor system 

was successfully designed and fabricated in a 

standard 1.5 μm CMOS process (Fig. 1). The 

gas-sensor system incorporated an array of four 

microhotplate-based gas-sensing structures. The 

system utilized a thin fi lm of tin oxide (SnO2) 

as a sensing material. Digital decoders selected 

individual elements of the sensor array and an 

operational amplifier monitored sensing film 

conductance. Detection of gas concentra-tions in 

the 100 parts-per-billion range was achieved. This 

represented an improvement in sensitivity of two 

orders of magnitude over existing MEMS-based 

microhotplate gas-sensors. 

#
Semiconductor Microelectronics and Nanoelectronics Programs 211

Investigated existing and emerging SoC 

design methodologies, and adapted digital SoC 

design tool-fl ow to enable integration of mixedsignal MEMS VCs. 

A four element gas-sensor VC was successfully designed, fabricated and electrically characterized to demon-strate that the design approach 

was compatible with SoC design methodology. 

The performance of the 8-bit ADC exceeded the 

gas-sensor VC design requirements (Fig. 2). 

#

#

Figure 1. Micrograph of gas-sensor system to be 

used as demonstration vehicle.

Figure 2. Layout of the four element gas-sensor VC 

with analog-to-digital control.

Figure 3. Layout of the four element gas-sensor 

VC with electrostatic discharge (EDS) protection 

circuitry.

Electrostatic discharge (ESD) protection 

structures were added to the gas-sensor and successfully tested These ESD test structures are 

based on multi-fi nger thyristor-type devices and 

are designed to achieve optimum perform-ance 

and reduced area (Fig. 3).

#

A new post-process etching technique was 

developed to integrate MEMS devices with 

standard submicron CMOS processes and a new 

microhotplate design that scales with standard 

CMOS structures and voltage levels (Fig. 4). 

This will enable co-integration of MEMS sensor 

devices with high density submicron digital 

systems using cost effective standard CMOS 

foundries. The submicron gas-sensor test chip 

was characterized. Characterization data showed 

the new scalable microhotplate will provide the 

temperature required for gas-sensor operation at 

3.3V.

#

Figure 4. New microhotplate design implemented in 

standard submicron CMOS technology.

Methodologies for designing digital interface 

shell functionality for ES-VCs were developed 

and demonstrated by designing the gas sensor 

SoC architecture.

#
212 National Institute of Standards and Technology 

A high-level model of the microhotplate gas 

sensor ES-VC was developed. A HDL-based 

microcontroller core was synthesized for a 0.25-

micron standard CMOS fabrication process. Highspeed SystemC models of the micro-controller 

and microhotplate were developed to facilitate 

SoC software design and protocol development. 

This is the fi rst time a MEMS device has been 

modeled in systemC and the results demonstrated 

the importance of including MEMS device models 

in high level system modeling

Microhotplate-based gas-sensor yield was 

improved by adopting a new chromium etchant. 

A computer-controlled gas-delivery system 

was designed, assembled, and tested. The gas 

delivery protocols for calibrating the microhotplate-based gas sensors were designed and 

implemented.

A submicron-microhotplate test-structure 

chip was designed to compare the performance of 

different CMOS compatible temperature sensors 

and to measure the contact resistance between 

different types of post-processed gas sensor 

electrodes. This chip also supports the extraction 

of more accurate microhotplate thermal-model 

parameters (Fig. 5). 

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Figure 5. Layout of new microhotplate test-structure 

chip with different types of temperature sensors and 

four point electrodes for gas-sensor fi lm contactresistance studies.

An 8051 IP core was synthesized and fabricated in the MOSIS-TSMC 0.25 micron standard CMOS process (Fig. 6). The chip design is 

compatible with the IEEE1451 sensor network 

communication standards. Bridge type new microhotplate test structures were also included on 

the same chip to test the compatibility of our current post process-ing steps with this sub-micron 

CMOS process.

#

Figure 6. Microcontroller and new microhotplate 

test-structures Layout.

A prototype gas-sensor VC platform was 

designed and fabricated for the fi rst time in the 

MOSIS-AMI 0.5 micron standard CMOS process 

(Fig. 7). The design includes bridge type microhotplate structures, 8-bit DAC, 10-bit ADC, heater 

drivers, analogue and digital multiplexers, and 

operational amplifi ers. The design has a fully digital interface to control and measure the analogue 

signals associated with gas sensor, and a complementary analogue interface for debugging.

#

Figure 7. Gas-sensor VC layout for submicron CMOS 

fabrication.
Semiconductor Microelectronics and Nanoelectronics Programs 213

Project personnel collaborated with MEL to 

incorporate IEEE 1451 network sensor standards 

compatibility into the gas-sensor SoC. Transducer 

Electronic Data Sheets (TEDS) were written for 

the TEDS that are required by the standard. A 

gas-sensor SoC Transducer Interface Module 

(TIM) was emulated on a laptop and verifi ed to 

comply with an essential subset of the IEEE 1451 

command set that were issued over an RS232 link 

to an MEL laptop. 

RECOGNITION

2005 GEORGE ABRAHAM OUTSTANDING

PAPER AWARD:

M. Y. Afridi, A. Hefner, C. Ellenwood, R. Cavicchi, and S. Semancik “Characterization System 

for Embedded Gas-sensor Systems-on-a-Chip” 

GOMACTECH 2005, pp. 94-97, 2005.

RECENT PUBLICATIONS

A. Varma, M. Y. Afridi, A. Akturk, P. Klein, A. Hefner and B. 

Jacob, “Modeling Heterogeneous Systems With SystemC: A 

Digital/MEMS Case Study,” in Proceedings of the ACM/IEEE 

International Conference on Compilers, Architecture, and 

Synthesis for Embedded Systems (CASES ‘06), pp 54-64, 

Seoul, Korea, October 2006.

J. Geist, M. Y. Afridi, A. Varma and A. Hefner, “Simple Thermal-Effi ciency model for CMOS-Microhotplate Design,” in 

Journal of Research at NIST, Vol 111(3), pp. 243, 2006.

M. Y. Afridi, A. Hefner, J. Geist, C. Ellenwood, A. Varma, B. 

Jacob, “Microhotplate-Based Sensor Platform for Submicron 

SoC Designs” IEEE International Semiconductor Device 

Research Symposium (ISDRS), pp. 500-501 2005.

Yaqub Afridi, Allen Hefner, Colleen Ellenwood, Richard 

Cavicchi, Stephen Semancik “Characterization System for 

Embedded Gas-sensor Systems-on-a-Chip” GOMACTECH 

2005, pp. 94-97, 2005.

Javier A. Salcedo, Juin J. Liou, Muhammad Y. Afridi, Allen R. Hefner, and Ankush Varma “Electrostatic Discharge 

Protection for Embedded-sensor System-on-a-Chip” (Abs. 

2587) 26th meeting of The Electrochemical Society (ECS) 

October, 2004.

M. Y. Afridi , A. R. Hefner Jr., D. W. Berning, C. H. Ellenwood, 

A. Varma, B. Jacob, S. Semancik, “MEMS-based embedded 

sensor virtual components for system-on-a-chip (SoC),” Solid 

State Electronics, Vol. 48, pp. 1777-1781 (24-JUN-2004).

M. Y. Afridi, A. Hefner, D. Berning, C. Ellenwood, A. Varma, 

B. Jacob, S. Semancik, “MEMS-Based Embedded Sensor Virtual Components for SoC” IEEE International Semiconductor 

Device Research Symposium (ISDRS), pp. 500-501 2003.

Muhammad Y. Afridi, John S. Suehle, Mona E. Zaghloul, 

David W. Berning, Allen R. Hefner, Richard E. Cavicchi, 

Steven Semancik, Christo-pher B. Montgomery, and Charles 

J. Taylor, “A Monolithic CMOS Microhotplate-Based Gas 

Sensor System,” IEEE Sensors Journal, vol. 2, no. 6, pp. 

644-655, Dec. 2002.

# M. Y. Afridi, J. S. Suehle, M. E. Zaghloul, D. W. Berning, A. 

R. Hefner, S. Semancik, R. E. Cavicchi, “A MONOLITHIC 

IMPLEMENTATION OF INTERFACE CIRCUITRY FOR 

CMOS COMPATIBLE GAS_SENSOR SYSTEM” Proceedings of the IEEE Internal Symposium on Circuit and Systems, 

pp. II 732 – 735, (May 2002).
214 National Institute of Standards and Technology 
Semiconductor Microelectronics and Nanoelectronics Programs 215

AT-SPEED TEST OF DIGITAL INTEGRATED CIRCUITS

Technical Contacts:

D. F. Williams

J. Moreland 

P. Kabos 

P. Hale

“As electronic devices 

shrink into the nanometer size scales and 

integrated circuits 

operate at multi-GHz 

clock rates, probing their 

internal characteristics 

is becoming both more 

critical and far more 

diffi cult than ever 

before.”

Travis M. Eiles, Ph.D. 

Intel Corp.

GOALS

Develop and demonstrate metrology for the atspeed test of digital integrated circuits. The program will resolve the essential metrology issues 

of at-speed digital integrated circuit test. It will 

apply its results to characterizing and calibrating 

high-impedance probes and developing scanning 

probe microscopes (SPMs) capable of precisely 

positioning fi eld probes above the surface of the 

integrated circuit.

CUSTOMER NEEDS

In the device debug and characterization world, 

a key challenge is the development of diagnostic 

tools, particularly for timing information. The 

limitations in the current PICA/time-resolved 

photoemission detector technology point to the 

need for radically different methodologies (2005 

ITRS Test and Test Equipment Section, page 9). 

Traditional forms of IC contact probing (fi nepitch probe cards and RF probes) technology 

require large contact pads incompatible with the 

operation and economic constraints of modern 

IC designs (2005 ITRS, Test and Test Equipment 

Section, page 24). The 2005 ITRS concludes that 

micro-positioner and atomic-force-microscopebased probing will be required at each roadmap 

generation to probe minimum sized transistors 

(2005 ITRS, Test and Test Equipment Section, 

page 9).

We are developing alternative probing approaches 

that use high-impedance probes, non-contact 

probes, and atomic-force microscopes that respond to either electric or magnetic fi elds near 

transmission lines in the circuits. However, while 

the uncalibrated fi eld measurements performed 

by these probing systems are suitable for fi eld 

mapping, they are a far cry from the precise 

measurements of voltages and currents required 

for electrical design.

Solving the critical at-speed test calibration issues 

will add enormous value to the probing systems 

currently being used or developed for high-performance digital integrated circuits. Developing 

characterization and calibration methods for highimpedance probes, whether of the conventional 

type or mounted on atomic-force microscopes, 

will help speed the development and implementation of these new measurement tools, and so create 

a new paradigm for the at-speed test of high-speed 

digital integrated circuits.

TECHNICAL STRATEGY

We will develop calibration artifacts with precisely known high-frequency voltages and circuits suitable for characterizing and calibrating 

high-impedance probes and samplers of all types. 

We will focus on fundamental calibration issues: 

transforming the response of the probes to the 

electric and magnetic fi elds above the integrated 

circuit into accurate voltages and currents inside 

the circuit. Figure 1 shows a result of the highspeed metrology we have developed, an on-chip 

waveform measurement to 200 GHz.

Figure 1. Three on-chip waveform measurements 

performed with our electro-optic sampling system.

We will fi rst apply the characterization and calibration procedures to conventional high-impedance probes. To facilitate the development and test 

of electric and magnetic probes with nanoscale 

resolution, we have constructed a universal SPM 

test bed for these probes. We are applying our 

characterization procedures to miniature SPM 

probes suspended on custom cantilevers designed for high frequency measurements on the 

nanoscale. Finally, we plan to tie our metrology 

back to fully characterized electro-optic sampling 

measurements.

The 2005 ITRS notes that 10s or 100s of picoseconds of delay are now critical, and that there is no 

instrument available that simultaneously satisfi es 

the noise fl oor, analog bandwidth, and test time requirements for high performance interfaces (2005 

ITRS, Test and Test Equipment Section, pages 7 

and 30). To address this, we are also working on 

methods for calibrating and correcting imperfections in waveform measurement equipment that 

cause distorted or blurred measurements. These 

effects include instrument response, impedance 
216 National Institute of Standards and Technology 

mismatch, multiple refl ections, dispersion, timebase distortion, jitter, and drift. After calibration 

and correction, the measurement has an improved 

fi delity and is traceable to fundamental physical 

principles. Traceability for frequency response is 

provided to 110 GHz (in coax) and beyond (on 

wafer) through the NIST electro-optic sampling 

system. Our calibration strategy is independent 

of the particular signals being measured and is 

applicable to digital, RF/microwave, and mixed 

signal systems, enabling the single platform measurement solutions called for in the 2005 ITRS, 

Test and Test Equipment Section, page 36.

DELIVERABLES: 

Fabricate and verify functionality to 50 GHz of generator circuits based on photoconductive switches 

suitable for the construction of portable and 

calibrateable on-chip pulse source for characterizing high-impedance probes at the micrometer and 

nanometer length scales. 4Q 2007

Construct a fi ber optic based high speed scanning 

probe system based on magneto-optic Kerr effect 

and demonstrate capability of micrometer spatial 

resolution. 4Q 2007

Develop a calibration for the time-domain insertion 

delay of a coaxial cable with sub-picosecond resolution (limited by connector repeatability). 4Q 2007 

ACCOMPLISHMENTS

We have demonstrated a novel frequencydomain method for measuring time-domain delay 

of dispersive calibration artifacts for differential 

delay. This frequency-domain approach can 

also account for pulse shape and source and load 

impedance effects. We have also demonstrated 

a method to propagate uncertainty in the NIST 

Timebase Calibration Software through to various 

pulse parameters and have demonstrated differential delay calibrations of a dispersive calibration 

artifact with < 1 ps standard uncertainty. 

We have developed, fabricated, and tested a 

noninvasive AFM scanning probe for measuring 

local microwave power.

We have developed a method of characterizing and calibrating conventional high-impedance 

probes for low-invasive waveform measurements.

We have applied our high-impedance-probe 

characterization method to a probe mounted on 

an atomic-force microscope. 

We have constructed an SPM universal test 

bed. 

•

•

•

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We have demonstrated a calibrated measurement of an on-wafer pulsed waveform up to 200 

GHz using electro-optic sampling. 

We have developed a procedure for testing 

high-impedance probes directly on our electrooptic sampling system. This system has a calibrated 

bandwidth of 200 GHz.

We have demonstrated a method for correcting oscilloscope timebase jitter, drift, and 

distortion.

We have fabricated fast photoconductive 

switches and demonstrated a rise time of better 

than 5 ps, largely exceeding our goal of constructing generator chips useable to 50 GHz. We will 

use these to construct a portable and calibrateable 

on-chip pulse source for characterizing high-impedance probes at the micrometer and nanometer 

length scales, Fig. 2.

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Figure 2. Measurement system used to verify the 

functionality of our photoconductive switches.

Figure 3. The Fiber-optic trench with integrated 

45-degree mirror.

We developed the fabrication process for creating the MEMS chips with integrated 45-degree 

mirrors and trenches for fi ber alignment, Fig. 3. 

#
Semiconductor Microelectronics and Nanoelectronics Programs 217

We assembled the fi ber optic interferometer 

for the measurement of the defl ection of the cantilevers including the thermal feedback control.

RECENT PUBLICATIONS

Simone Lee, T. Mitch Wallis, John Moreland, Pavel Kabos, 

and Y. C. Lee, “Asymmetric Dielectric Trilayer Cantilever 

Probe for Calorimetric High-Frequency Field Imaging,”

IEEE JMEMS 16, pp. 78 -86, (2007).

T. S. Clement, P. D. Hale, D. F. Williams, C. M. Wang, A. 

Dienstfrey, and D. A. Keenan, “Calibration of sampling oscilloscopes with high-speed diodes,” accepted for publication in 

IEEE Trans. Microwave Theory and Tech., 2006.

A. Dienstfrey, P. D. Hale, D. A. Keenan, T. S. Clement, and D. 

F. Williams, “Minimum-phase calibration of sampling oscilloscopes,” accepted for publication in IEEE Trans. Microwave 

Theory and Tech., 2006.

D. F. Williams, A. Lewandowski, T. S. Clement, C. M. Wang, 

P. D. Hale, J. M. Morgan, D. A. Keenan, and A. Dienstfrey, 

“Covariance-based uncertainty analysis of the NIST electrooptic sampling system,” IEEE Trans. Microwave Theory and 

Tech., Jan. 2006.

D. F. Williams, H. Khenissi, F. Ndagijimana, K. A. Remley, 

J. P. Dunsmore, P. D. Hale, C. M. Wang, and T. S. Clement, 

“Sampling-Oscilloscope Measurement of a Microwave Mixer 

with Single-Digit Phase Accuracy,” IEEE Transactions on 

Microwave Theory and Techniques, Vol. 54, No. 3, March 

2006.

P. D. Hale, C. M. Wang, D. F. Williams, K. A. Remley, and J. 

Wepman, “Compensation of random and systematic timing 

errors in sampling Oscilloscopes,” IEEE Transactions on 

Instrumentation and Measurement, Vol. 55, No.6, December 

2006.

S. Lee, Y. C. Lee, T. M. Wallis, J. Moreland, and P. Kabos, 

“Near fi eld imaging of high-frequency fi elds with calorimetric cantilever probes,” Journal of Applied Physics, 99, pp. 

08H306-1-08H306-3, (2006).

D. F. Williams, P. D. Hale, T. S. Clement, and J. M. Morgan, 

“Calibrated 200 GHz Waveform Measurement,” IEEE Trans. 

Microwave Theory and Tech., April, 2005.

T. M. Wallis, J. Moreland, W. Riddle, and P. Kabos, “Microwave power imaging with ferromagnetic calorimeter probes 

on bimaterial cantilevers,” Journ. of Mag. and Magn. Mater, 

vol 286, pp. 320-323 (2005).

#
218 National Institute of Standards and Technology 
Semiconductor Microelectronics and Nanoelectronics Programs 219

THERMAL MEASUREMENTS AND PACKAGING

RELIABILITY

Technical Contacts:

N. Barbosa

A. J. Slifka 

A. R. Hefner

“I am writing to let 

you know that NIST 

has been of great 

assistance to Cenymer 

Corporation, one of our 

portfolio companies. 

This portfolio company 

has developed a novel 

material. Dr. Andrew 

Slifka at NIST was 

able to provide the 

company with thermal 

measurements that 

are critically important 

to Cenymer as it 

approaches potential 

customers. It is great 

to have NIST in the 

community as a resource 

to companies like 

Cenymer.”

Tim Connor

Sequel Venture Partners

GOALS

Our goal is to provide the microelectronics industry with information, guidance, and tools through 

technology transfer to characterize the behavior 

of features and interfaces in packaging and onchip that are thermally stressed. Information 

and guidance are provided directly to individual 

companies and to consortia through collaborations 

whereby we are provided with specimens that 

present a reliability concern to the manufacturer 

or end-user, or a model system is fabricated inhouse. The results of the tests are reported to 

the provider, and are typically reported in the 

general literature or at technical meetings. This 

system contributes toward achieving our third and 

primary goal — providing the industry with the 

tools to characterize these thermomechanical behaviors. Review and evaluation of the techniques 

by our industrial collaborators allow us to refi ne 

the techniques to make them optimally benefi cial 

to industry, and to demonstrate to the industry at 

large the capabilities of the techniques on actual 

packages in development, which is essential for 

technology transfer.

Overcoming thermal limitations has recently 

become a major issue in CMOS-based microelectronic circuits because: (1) power levels in CPUs 

have reached the same levels as in power devices; 

(2) power density nonuniformities are leading to 

hot spots in microprocessors as well as power 

ICs; (3) new materials with different thermal 

properties are being introduced; (4) many new 

and future technologies (e.g., SOI, 3-D integration) tend to isolate power dissipating elements 

thermally; and (5) shrinking dimensions and increasing frequency of ICs are causing signifi cant 

power dissipation in the interconnects. To address 

these issues, reliable methods for measuring the 

temperature distribution in ICs and power devices 

are required.

A broad range of temperature measurement techniques are investigated by this project and several 

unique temperature measurement system requirements have been identifi ed and demonstrated by 

NIST. High speed transient thermal imaging methods are being developed for measuring localized 

heating effects at the semiconductor chip surface. 

This enables the measurement of transient heating 

events such as burst operation of IC Functional 

Unit Blocks (FUBs) and enables the measurement 

of transient current constriction failure events in 

RF and high power devices. Additionally, methods 

are being developed to evaluate the heat transfer 

across package layer interfaces using high-speed 

temperature sensitive parameter (TSP) measurements. This is important for in situ measurement 

of heat transfer performance degradation after 

various levels of thermal stress such as power 

cycling, thermal cycling and thermal shock. 

CUSTOMER NEEDS

The trend in electronics is toward components of 

higher density and smaller size using less expensive materials. Materials used in packaging and 

interconnects are many and display a variety of 

thermal responses that are not always compatible. 

This makes interconnect lines and interfaces particularly vulnerable to thermomechanical fatigue 

failures. The program seeks to offer support and 

verifi cation of models conducted or sponsored by 

the microelectronics industry. 

The need to measure operating temperature of 

a semiconductor device can be divided into the 

following four broad categories: (1) predicting 

reliability or operating life of device, (2) measuring material/device thermal properties in situ, (3) 

confi rming or determining the operating limits or 

thermal performance of a device, and (4) validating thermal models for device, chip, and system 

performance. Thermal measurements on small, 

<10 μm, structures are needed due to the high 

density of current and future interconnect and 

packaging designs. Heat transfer information at 

interfaces, such as those seen at solder/intermetallic interfaces and Direct Bonded Copper (DBC) 

isolation layer is needed for modeling of future 

package designs. 

Temperature measurements for microelectronic 

devices are more important today than they ever 

have been. It always has been true that extreme 

temperature places limits on the operating range 

of nearly all devices, but today, increasing power 

dissipation and power densities threaten to create temperatures that block continued progress 

according to “Moore’s Law.” Clearly, new and 

innovative methods for cooling chips and packages must be found along with new materials and 

circuit designs, and architectures for decreasing 
220 National Institute of Standards and Technology 

the power dissipation and operating temperature. Equally as clear, we must have accurate 

and well-understood methods for measuring the 

temperature of devices to aid in the development 

of these new techniques and materials. 

Measurement of localized and transient heating 

is also becoming increasingly important in high 

performance ICs as it is becoming prohibitive 

to remove the heat that would be dissipated if 

the entire IC was operated continuously at the 

full power density level. System techniques that 

dynamically operate all or part of the chip at reduced power, such as dynamic voltage scaling or 

clock gating, require experimental evaluation of 

the rapid transient heating and thermal diffusion 

from locally heated FUBs. System level power 

reduction strategies are generally recognized by 

the ITRS 2005 as necessary for the continual 

advancement of electronic systems at a rate consistent with “Moore’s law.”

“As the thermal resistances are made smaller and 

smaller for high power devices, thermal metrology 

with better resolution and capabilities is required. 

For example there is strong need to establish 

industry wide transient thermal measurement 

technique standard.” ITRS 2005.

“Power consumption is now the major technical 

problem facing the semiconductor industry. As 

feature sizes shrink below 0.1 micron, static power 

is posing new low-power design challenges. ITRS 

predicts a decrease in dynamic power per device 

over time. However, the doubling of on-chip 

devices every two years would force an increase 

of current leakage on a per-chip basis.” ITRS 

2005.

“New materials, structures, and processes create 

new chip reliability (electrical, thermal, and mechanical) exposure. Detecting, testing, modeling 

and control of failure mechanisms will be key.” 

2006 ITRS Update: one of the fi ve “Diffi cult 

Interconnect Challenges” for ≥ 32 mm, ITRS 

Interconnect, p. 2.

“As feature sizes shrink, interconnect processes 

must be compatible with device roadmaps and meet 

manufacturing targets at the specifi ed wafer size. 

Plasma damage, contamination, thermal budgets, 

cleaning of high A/R features, defect tolerant processes, elimination/reduction of control wafers are 

key concerns. Where appropriate, global wiring 

and packaging concerns will be addressed in an 

integrated fashion.” 2006 ITRS Update: one of 

the fi ve “Diffi cult Interconnect Challenges” for 

≥ 32 mm, ITRS Interconnect, p. 2.

TECHNICAL STRATEGY

1. We are developing techniques in scanning 

thermal microscopy, utilizing the AFM (atomic 

force microscope), which have much to offer 

the microelectronics industry. We have been 

approached by industry with requests for aid as 

simple as “How high are the temperatures in this 

MCM (multichip module) during service” to as 

challenging as “What is the interfacial thermal 

resistance of an alloy/intermetallic interface.” The 

fi rst was answered using the IR microscope; the 

second has yet to be answered. Determining temperature and temperature distribution at the micron 

and submicron size scales and additionally in the 

time domain will aid in determination of failure 

mechanisms in packages and interconnects.

DELIVERABLES: Evaluate commercially available 

Scanning Probe Microscope (SPM) tips for thermal 

SPM. Determine performance for measuring temperatures at and around on-chip interconnects. 4Q 2007

2. Carbon nanotubes have potential in heat-removal applications for microelectronics. Thermal properties show great promise in this area, 

if production issues can be addressed. We will 

continue to cooperate with industry by measuring 

the thermal conductance achieved by newly-developed thermal compounds that incorporate carbon 

nanotubes. 

DELIVERABLES: Measure interfacial thermal resistance of improved carbon nanotube-covered copper 

and baseline copper materials. 3Q 2007

3. Stress-voiding, electromigration, thermomechanical fatigue and other mechanisms that 

can cause failures in interconnect lines are all 

accelerated by temperature. Using scanned- thermal-probe microscopy (SThM), we are measuring 

temperatures of damaged locations on lines and 

temperature distribution on and around interconnect lines to elucidate failure mechanisms and 

predict lifetimes. Our current commercial SThM 

equipment is marginal for this application. We will 

determine whether any commercially-available 

sensors are adequate for the task. If so, we will 

use the technology in our research on commercial 

fi lms and lines; if not, we will defi ne the improvements needed in the sensors.

DELIVERABLES: Measure lines both temporally and 

spatially. Use results in our work on thermomechanical stressing and reliability of interconnect lines. 

1Q 2008

4. High speed temperature sensitive parameter 

(TSP) measurements are required for in situ evaluation of the heat transport through the interface 
Semiconductor Microelectronics and Nanoelectronics Programs 221

of multi-layer package systems. Recently NIST 

developed a high speed transient thermal impedance system using a TSP for multi-chip power 

modules. This enables assessment of die attach 

and DBC isolation attach degradation after thermal cycling and thermal shock stress. This system 

also enables validation of electro-thermal device 

models needed for electrical and thermal system 

design.

DELIVERABLES: Perform thermal cycling and thermal shock stress on DARPA Wide-Bandgap Semiconductor Technology High Power Electronics program 

(DARPA HPE) devices and American Competitiveness 

Institute ManTech devices. Use unique NIST high 

speed, high current TSP system to evaluate die attach 

and DBC attach integrity before and after thermal 

stress. 1Q 2008

5. A limitation of commercially available infrared (IR) thermal imaging systems is their inability to make high-speed transient thermal image 

measurements. NIST has modifi ed a commercial 

IR system to enable measurement of high speed 

temperature maps of the chip surface with 1-μs 

temporal resolution and 15 μm spatial resolutions. 

The NIST system permits the measurement of 

the chip heat source distribution before the heat 

diffuses to surrounding regions and enables the 

measurement of transient heating events such as 

reduced power IC FUBs (clock gating, dynamic 

voltage scaling, and Vdd gating), enables the 

measurement of transient current constriction 

failure events in RF and high power devices, and 

measurement of current uniformity in large area 

power devices. 

DELIVERABLES: Improve the spatial resolution of 

the NIST high-speed transient thermal imaging system 

from 15 μm to 3 μm while maintaining the 1-μs time 

resolution. Begin to transfer the NIST capability to 

commercial instrument manufacturer. 3Q 2008

6. There is a strong need to establish industry 

wide transient thermal measurement technique 

standards as described in the 2005 ITRS roadmap. NIST recently developed and tested a 

transient thermal calibration test structure with 

1-μs temporal resolution using micro hotplates 

with integrated temperature sensors. The set of 

calibration test structures is now being expanded 

to develop a prototype IR transient thermal imaging calibration test chip.

DELIVERABLES: Apply micro hotplate transient thermal calibration test structures to evaluate performance 

of NIST developed 3 μm, 1 μs high-speed transient 

thermal imaging system. 4Q 2008

7. Dynamic electro-thermal models of semiconductor packages are needed to evaluate the interaction of the electrical system with the thermal 

management system. NIST fi rst introduced the 

concept of package and module thermal network 

component models in the early 1990’s and has 

since used this methodology to develop models 

for several specifi c power module packages. Recently, the program manager and contractors of 

the DARPA HPE program has requested that NIST 

provide models for the 10 kV, 100 A SiC power 

models being developed by the DARPA HPE 

Phase 2 program (device phase) contractors so 

that the models can be used to design the thermal 

management and fault mitigation systems for the 

prototype future naval aircraft carrier Solid State 

Power Substation being developed by the DARPA 

HPE Phase 3 (system phase) contractor.

DELIVERABLES: Provide initial thermal network component models needed by DARPA HPE program to 

develop the prototype future naval aircraft carrier Solid 

State Power Substation. 4Q 2007

ACCOMPLISHMENTS

Since there is no commercially available 

device for calibration of a thermal atomic-force 

microscope (AFM) probe tip, we developed 6 

different designs and evaluated the electrical 

performance as a function of temperature. We 

also chose the best design for use with the AFM 

in both steady-state and transient thermal modes. 

Figure 1 shows the device. 

#

Figure 1. Calibration structure for scanned thermal 

microscopy. The resistive heater and temperature 

sensor were fabricated from 300 nm thick aluminum, 

which allowed for easy fabrication, but limited 

the maximum operating temperature to several 

hundred degrees Celsius. The design ensures that 

the calibration area, located on the sensor structure 

between the sensor voltage taps, remains at a uniform 

temperature.
222 National Institute of Standards and Technology 

A description of the device has been submitted for 

publication; the reference is listed below.

The change in resistance as a function of temperature of the sensor can be used to calibrate AFM 

probe tips. Figure 2 shows the calibration curve 

for one of the devices.

Figure 2. Calibration curve from a NIST microdevice showing device 

temperature as a function of sensor resistance.

Feasibility has been shown of measurements 

of electronic interconnect lines using the thermal 

SPM. We have attempted to calibrate the response 

of the probe tip to temperature, but the measurement is impractical with present equipment.

The measurement of thermomechanical 

fatigue is done in a transient fashion at a rate on 

the order of 100 Hz. The electronics that control 

the thermal scanned-probe microscope have 

specifi cations from the manufacturer that state 

that the thermal settling time constant is 350 microseconds due to the thermal mass of the probe 

tip. Therefore, transient measurements up to 2800 

Hz should be possible. To determine feasibility 

of measuring thermomechanical fatigue, we have 

made measurements using the thermal probe tip 

on an interconnect line that was being heated at 

100 Hz a.c. Figure 3 shows a plot of data from 

the thermal microscope in the form of thermal 

voltage as a function of time. 

A calibrated probe tip will allow us to present this 

data in the form of temperature as a function of 

time. Two papers were written that include some 

results: R. R. Keller, R. H. Geiss, N. Barbosa III, 

A. J. Slifka, and D. T. Read, “Strain-Induced Grain 

Growth during Rapid Thermal Cycling of Aluminum Interconnects,” presented in April 2006 and 

accepted for an upcoming issue of Metallurgical 

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Figure 3. Temperatures obtained at four locations on 

and near an 8 μm wide line heated by low frequency 

alternating current. Trace A is at the line center, B 

is on the line at 2 μm from the center, C is at the line 

edge, and D is 5 μm away from the line edge.

and Materials Transactions, and R. R. Keller, C. 

A. Volkert, R. H. Geiss, A. J. Slifka, D. T. Read, 

N. Barbosa III, and R. Mönig, “Electrical Methods 

for Mechanical Characterization of Interconnect 

Thin Films,” 2005 Advanced Metallization Conference proceedings. 

We have been making measurements of carbon nanotube mats grown on copper substrates as 

a potential replacement for copper-and-thermalgrease heat sinks. Measurements of two copper 

specimens with thermal grease between all three 

interfaces was compared with two carbon nanotube (CNT) specimens placed face-to-face, with 

thermal grease between the measurement plates 

and copper blank sides at low contact pressure 

(fi nger-tight, 555 psi). Figure 4 shows the results. 

Even at this low contact pressure, before the CNTs 

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Figure 4. Comparison of temperature differences 

across thermal-grease-and-copper interfaces and 

carbon nanotubes.
Semiconductor Microelectronics and Nanoelectronics Programs 223

begin developing large amounts of tube-to-tube 

contact, the CNT specimens outperform the copper with thermal grease.

Development of the high-speed (1 μs) transient thermal imaging system was completed. 

The acquisition and data analysis capabilities 

of this system have been extended to include a 

burst method and to allow frames from different 

transient movies to be compared more readily. 

An auto-scaling capability and user defi nable 

number of frames have also been added. A thin 

carbon-black chip coating process was developed 

to improve sample emissivity without altering the 

chip surface thermal response time. New thermal 

microscope hardware has been purchased and 

installed to enable future upgrade of the highspeed transient thermal imaging system to 3 μm 

spatial resolution from it’s present 15 μm spatial 

resolution.

NIST recently developed and tested a transient thermal calibration test structure with 1 μs 

temporal resolution using MEMS-based microhotplates with integrated temperature sensors. 

This specially designed temperature-reference 

microhotplate structure is calibrated and used 

to verify the results of the transient IR thermal 

imaging system. Figure 5 compares the single 

point IR transient response with the electrically 

measured transient response of a microhotplate 

structure for a 20 μs heating pulse. 

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Figure 5. Microhotplate thermal response to a 20 μs 

heating pulse (as indicated) measured with both IR 

and electrical methods.

relatively uniform before degradation and that 

only 1 % of the chip is conducting all of the current 

after substantial degradation. Degradation of SiC 

diodes remains was highly variable as of 2006, as 

demonstrated in Figs. 6a and 6b. Figures 6a and 6b 

are current uniformity images of two different SiC 

10 kV, 20 A diodes operated at full rated current for 

500 hours under forward bias. The diode shown 

in Fig. 6a demonstrates good current uniformity 

after 500 hours operation and the diode shown in 

Fig. 6b demonstrates highly non-uniform current 

As low-degradation rate SiC PiN (power 

diode made with p-type to intrinsic to n-type 

semiconductor junctions) diodes are beginning 

to emerge, the NIST transient thermal imaging 

system is being used to determine the current 

uniformity after various levels of stress up to 

several thousand hours of operation. The pulsed 

thermal image results show that the current is 

#

Figure 6a. SiC diode relative temperature uniformity 

map shows relatively uniform current conduction 

throughout the chip after 500 hours stress at full 

current. The four green areas at the lower part of 

the chip are caused by wire bond shadowing of the 

thermal radiation.

Figure 6b. Relative temperature uniformity map for a 

similar SiC diode shows highly non uniform current 

conduction after 500 hours stress at full current. 

Both of these diodes are mounted on a temperaturecontrolled heatsink and the background temperature 

is 75 °C.
224 National Institute of Standards and Technology 

conduction after 500 hours operation (only 50 % 

of device is conducting). NIST has evaluated the 

current uniformity degradation of numerous SiC 

PiN diodes samples from 2005 through 2007, 

providing essential information resulting in the 

development of “degradation free” SiC power 

device technology.

NIST has developed an “in situ” method to 

measure the high-speed heating response of Insulated Gate Bipolar Transistors (IGBTs) packaged 

in high power modules. The method uses the gatesource voltage, VGS, at a constant, relatively low 

current and at a high anode-cathode voltage as the 

temperature sensitive parameter (TSP). This TSP 

method is used to validate and extract short-time 

constant thermal parameters for an electro-thermal simulation model of the IGBT (including the 

details of the package module). 

The high-speed TSP method was applied to 

compare heating response measurements with 

the thermal models for a commercial high power 

IGBT half bridge module, a commercial six-pack 

IGBT module, and a prototype NSF Center for 

Power Electronic Systems Module. Electro-thermal simulations of a full three-phase inverter have 

been successfully performed and compared with 

measured results of the power converter temperature monitors. The NIST high-power, high-speed 

TSP method has been extended to measure the 

package cooling performance of 10 kV SiC power 

MOSFETs being developed by the DARPA HPE 

program and is being used to aid in the development of packages produced by that program.

The hardware construction and computer 

interfacing for the NIST high power rapid thermal 

cycling/shock test system were completed. This 

included necessary electronics instruments, fl ow 

controllers, high thermal impedance plumbing 

interfaces, and computer control of air fl ow rate, 

water fl ow rate, and heater power PID temperature 

controller. The system produces 300 °C thermal 

cycles with computer programmable temperature 

rise and fall times (as fast as 10 min rise and fall 

times are obtained). The rapid thermal cycling/

shock test systems are currently being applied 

to SiC power modules developed by the DARPA 

HPE program where the NIST high-speed TSP 

and thermal imaging systems are being used to 

monitor thermal performance degradation.

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COLLABORATIONS

University of Nebraska: Kevin Cole

Cenymer Corporation: Scott Joray

Cynthia Volkert, Forschungszentrum Karlsruhe, Germany

Reiner Mönig, MIT, Cambridge, MA

Tom Mountsier, Novellus Inc.

Michael Lane, IBM, Yorktown Heights, NY

Ju-Young Kim, Seoul National University, Korea

Roop Mahajan, University of Colorado

Amit Khosla, Motorola

Krishna Ramadurai, University of Colorado

University of Puerto Rico Mayagüez: Miguel Velez

University of Maryland: Bruce Jacob

University of Maryland CALCE Electronic Products and 

Systems Center: Patrick McCluskey

American Competitiveness Institute (ACI), Navy ManTech 

center: Barry Thaler

General Electric, Research, Raju, Ravisekhar.

Powerex, Scott Leslie.

RECENT PUBLICATIONS

N. Barbosa III and A. J. Slifka, “Spatially- and TemporallyResolved Thermal Imaging of Cyclically Heated Interconnects 

by Use of Scanning Thermal Microscopy (SThM),” submitted 

to Microscopy Research and Technique.

Allen R. Hefner and Sharon Beermann-Curtin, “Status of the 

DARPA WBST High Power Electronics Program in SiC Device 

Development and Technology Transition,” Proceedings of the 

Government Microcircuit Applications and Critical Technology Conference (GOMACTech) 2007, March 19-22, 2007

Madelaine Hernández-Mora, Adwoa Akuffo, Colleen Hood, 

José M. Ortiz-Rodríguez, and Allen Hefner, “Experimental 

Evaluation of SiC PiN Diode Forward Bias Degradation 

and Long Term Stability,” in 2007 IEEE Power Electronics 

Specialists Conference (PESC), June 2007.

R. R. Keller, R. H. Geiss, N. Barbosa III, A. J. Slifka, and D. 

T. Read, “Strain-Induced Grain Growth during Rapid Thermal 

Cycling of Aluminum Interconnects,” Paper presented at the 

2006 TMS Annual Meeting, April 2006. Metallurgical and 

Materials Transactions (approved publication).

Hefner Jr., A. R., Sei-Hyung, R., Brett, H., Berning, D. W., 

Hood, C. E., Ortiz-Rodriguez, J. M., Rivera-Lopez, A., Duong, 

T., Akuffo, A., Hernandez, M., “Recent Advances in HighVoltage, High-Frequency Silicon-Carbide Power Devices,”

IEEE Industry Applications Society (IAS) Annual Meeting, 

October 2006.

R. R. Keller, C. A. Volkert, R. H. Geiss, A. J. Slifka, D. T. 

Read, N. Barbosa III, and R. Mönig, “Electrical Methods for 

Mechanical Characterization of Interconnect Thin Films,”

2005 Advanced Metallization Conference Proceedings, p. 

643-648.
Semiconductor Microelectronics and Nanoelectronics Programs 225

J. R. Reichl, J.-S. Lai, A. R. Hefner, T. R. McNutt, D. Berning, 

D., “Inverter Dynamic Electro-Thermal Modeling and Simulation with Experimental Verifi cation,” IEEE Power Electronics 

Specialists Conference, pp. 2208-2215 (June 2005).

A. R. Hefner, T. R. McNutt, A. Akuffo, R. Singh, C. Ellenwood, D. Berning, M.K. Das, J. J. Sumakeris, and R. 

Stahlbush, “Characterization of SiC PiN Diode Forward 

Bias Degradation,” Industry Applications Conference, 2004, 

pp.1252-1260. 

J. Reichl, D. Berning, A. Hefner, and J-S. Lai, “Six-Pack IGBT 

Dynamic Electro-Thermal Model; Parameter Extraction and 

Validation,” IEEE Applied Power Electronics Conference, 

February 2004.

Blackburn, D. L., “Temperature Measurement of Semiconductor Devices – A Review,” Proceedings of Semiconductor 

Thermal Measurement and Management Symposium, pp. 

70-80 (March 2004).
226 National Institute of Standards and Technology 
Semiconductor Microelectronics and Nanoelectronics Programs 227

MANUFACTURING SUPPORT PROGRAM

Cross-cutting all manufacturing disciplines is the need for supporting information technology 

and processing capabilities. The transition to 300 mm fabs, single-wafer lots, and the extensive 

use of foundries has made effi cient information handling and automation indispensable to the 

production process. Information technology touches nearly every aspect of semiconductor 

production and distribution, including process control, tool to tool interconnect, automated 

material handling and tracking, reticle management, and information interchange across the 

supply chain. NIST is working with the International Semiconductor Industry Initiative 

(ISMI, a subsidiary of SEMATECH) to provide and maintain an on-line engineering statistical 

handbook to support process control, and is working with industry on standards required to 

support information fl ow across the “engineering chain,” and to provide standards for secure 

electronic diagnostic data and for coherent time synchronization on the factory fl oor.
228 National Institute of Standards and Technology 
Semiconductor Microelectronics and Nanoelectronics Programs 229

FACTORY TIME SYNCHRONIZATION STANDARDS

DEVELOPMENT FOR E-MANUFACTURING

Technical Contact:

Y. Li

GOALS

The project’s objective is to facilitate the development of standards and guidelines to achieve 

reliable clock synchronization and time stamping 

capabilities for supporting present and future eManufacturing needs. The project aims to educate 

the industry about the requirements, related issues, 

and potential solutions in regards to distributed 

precision clock synchronization.

CUSTOMER NEEDS

Increasing wafer sizes, decreasing critical dimensions, and new material introduction all challenge higher initial yield, lower manufacturing 

costs, and time to market demands. Among the 

key issues driving the challenges are monitoring 

and maintaining surface uniformity, decreasing 

process tolerance windows, as well as increasing 

process and equipment complexity. 

Advancing next-generation semiconductor 

manufacturing will require data to be collected 

and analyzed from a rising confl uence of data 

streams, due to narrowing tolerance windows, 

new material introduction and novel processing techniques. Some of the pertinent sources 

include process equipments and their operational 

subsystems, in-line metrology, and factory environmental conditions. The retrieved data is vital 

for designing new processes, maintaining optimal 

equipment and processing capabilities, controlling 

the equipment, providing information for rapid 

yield learning, and expediting handoff of process 

technology for volume production. 

As 300 mm factories mature, the fabrication 

facilities continue to face diffi cult challenges 

in managing the increasing factory complexity, 

which has resulted in an: 

“Explosive growth of data collection/analysis 

requirements driven by process and modeling 

needs.” 2006 ITRS Update Edition, Factory 

Integration, p. 2.

Managing the data proliferation will be critical in 

realizing the anticipated benefi ts of greater data 

accessibility. Extrapolating intelligent correlations from volumes of data in a timely manner 

requires a common, effi cient method of merging 

the data. 

“Device and process complexity make the ability 

to trace functional problems to specifi c process 

areas diffi cult.” 2006 ITRS Update Edition, Factory Integration, p. 3.

Quality time stamps ensure the ability to precisely 

pin-point problems and other types of cause-effect 

relationships. Accurate time stamps will become a 

key component in data quality for Advanced Process Control (APC) applications, especially in the 

realm of Fault Detection Classifi cation (FDC).

With the advent of the Equipment Data Acquisition (EDA) interface where data collection rates 

can increase to 10,000 data points per second, one 

millisecond time stamp accuracy will be required. 

An inaccurate time stamp potentially renders 

the data invalid for many data mining and other 

analysis applications, yet there continues to be a 

wide variation in time stamping accuracies among 

semiconductor manufacturing systems. A robust 

infrastructure to support accurate time stamps will 

provide the means to better exploit the increased 

data availability in next generation semiconductor manufacturing. Such an infrastructure will 

require clock synchronization of all the related 

local clocks in the manufacturing site including the 

clocks within the process and metrology tools. 

TECHNICAL STRATEGY

Through active participation in the SEMI International Equipment Engineering (IEE) TF, this 

project will continue to support standards development and legacy standards updates to ensure 

factory equipment and equipment components 

can reliably and consistently report time and 

time-stamp data. 

The fi rst approach is to continue to be aware of 

the latest developments in evolving distributed 

clock synchronization technologies, specifi cally 

in the mainstream protocols such as IEEE 1588, 

Precision Clock Synchronization Protocol for Networked Measurement and Control Systems, and 

NTP, Network Time Protocol. The semiconductor 

industry requirements have been presented to the 

IEEE 1588 group in order to leverage feedback 

on recommended practices.

DELIVERABLES: Document time synchronization and 

time stamping issues in networked control systems to 

present to the IEEE 1588 committee. 1Q 2007
230 National Institute of Standards and Technology 

To expedite integration of time synchronization 

protocols into the equipment, standards will be 

necessary to make the tasks as effi cient as possible. The SEMI Time Synchronization Working 

Group has recently developed a new standard for 

defi ning a clock object to ensure consistent reporting of time data along with contextual information 

to allow the user to determine the synchronization 

accuracy of the time. Additionally, legacy standards need to be updated in order to leverage the 

new time object. NIST also plans to document 

recommended methods for implementing the 

upcoming clock requirements.

DELIVERABLES: Continue to participate in SEMI 

standards efforts to update relevant time and date 

information in Information and Control Committee 

standards to ensure the ability to leverage the new 

clock defi nition without confl icts. 3Q 2007

To enable accurate time stamping, it is imperative 

to have a reliable and accurate clock synchronization methodology. Research on practical 

application and performance testing of the 

synchronization protocols would determine the 

impact of various computing factors (e.g., CPU 

usage, network traffi c, constraints of sensors and 

embedded systems, etc.) on distributed clock 

synchronization performance. The Engineering 

Research Center at University of Michigan has 

been collaborating with NIST in conducting studies on the accuracies achievable with softwarebased time synchronization protocols in industrial 

networks. To fully understand the intricacies, a 

factory simulation system is currently being developed. The work will have practical benefi ts in 

ensuring the potential solutions and new industry 

standards are feasible and effective in achieving 

accurate clock synchronization and data time 

stamping. Additionally, the factory simulation 

system can be extended to model the impact and 

cost-benefi ts of accurate time stamps on factory 

APC applications.

DELIVERABLES: Create a hybrid factory simulationsensor network environment with networked computers and embedded sensor and control systems to 

research the impact of the system on time synchronization and time stamping accuracy. 4Q 2007

ACCOMPLISHMENTS

A NIST internal report (NISTIR 7184) on 

“Semiconductor Factory and equipment Clock 

Synchronization for e-Manufacturing” was published in December 2004.

#

The presentation on “Running Out of Time: 

Improvements Required in Current Semiconductor and Equipment Clock Synchronization for 

Supporting Future Real-Time Data Collection” 

was given at AEC/APC Symposium in September 

2004.

“Intricacies of Time: Demystifying Clock 

Synchronization and Time Stamping for e-Manufacturing” was presented at AEC/APC Symposium in September 2005.

“In Search of the Key to the Lock: Clock 

Synchronization Requirements in Semiconductor 

Manufacturing” was presented at the IEEE 1588 

Conference in October 2005. 

“Using Network Time Protocol (NTP): 

Introduction and Recommended Practices” was 

published as an ISMI Technical Report in February 2006.

“Legacy Standards Update: SEMI Standards 

Analysis to Meet Factory Time Synchronization 

Requirements” was presented at the SEMI Spring 

IEE TF meeting in March 2006.

Poster presentation on “New Industry Requirements for Clock Synchronization and Time 

Stamping” was published at the European AEC/

APC Symposium in March 2006.

“Time Synchronization in Manufacturing 

Networks” at the Network Performance Workshop (University of Michigan) was presented in 

April 2006.

Presented the talk on “Advancing Towards 

Factory-Wide Data Quality for APC Applications, 

AEC/APC XVIII Proceedings, Westminster, CO, 

September 29-October 6, 2006.

Presented the talk on “Practical Aspects 

Impacting Time Synchronization Data Quality in 

Semiconductor Manufacturing,” Proceedings of 

the 2006 IEEE-1588 Conference, Gaithersburg, 

MD, October 2-4, 2006.

Published “Factory and Equipment Clock 

Synchronization and Time-Stamping Guidelines,” 

ISMI Technology Transfer, October 20, 2006.

Presented “Advancing Towards Factory-Wide 

Data Quality for APC Applications, AEC/APC 

Asia Symposium Proceedings, Taipei, Taiwan, 

November 30-December 1, 2006.

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Semiconductor Microelectronics and Nanoelectronics Programs 231

COLLABORATIONS

Harvey Wohlwend, Gino Crispieri, International SEMATECH 

Manufacturing Initiative

James Moyne, Naveen Kalappa, Jonathan Parrott Engineering 

Research Center, University of Michigan Ann Arbor

Alan Weber, Alan Weber and Associates

RECENT PUBLICATIONS

H. Wohlwend, Y. Li, “New Industry Requirements for Clock 

Synchronization and Time Stamping,” European AEC/APC 

Symposium, Mar 27-31, 2006, Aix En Provence, France.

N. Kalappa, J. Moyne, J. Parrott, Y. Li, “Practical Aspects 

Impacting Time Synchronization Data Quality in Semiconductor Manufacturing,” Proceedings of the 2006 IEEE1588 Conference, Conference on IEEE-1588 Standard for 

a Precision Clock Synchronization Protocol for Networked 

Measurement and Control Systems, Oct 02-04, 2006, Gaithersburg, Maryland.

J. Moyne, J. Parrott, N. Kalappa, Y. Li, “Practical Aspects 

Impacting Time Synchronization Data Quality in Semiconductor Manufacturing,” AEC/APC Symposium XVIII, Sep 30, 

2006 to Oct 05, 2006, Westminster, Colorado.

E. Simmon, Y. Li, J. V. Messina, “An External Stopwatch for 

Measuring the Timing of Events in a Computer or Distributed 

Computing Environment,” AEC/APC Symposium XVII, Sep 

24-29, 2005, Indian Wells, California.

E. Simmon, Y. Li, J. V. Messina, “Sands of Time: An External 

Stopwatch for Measuring the Timing of Events in a Computer 

or Distributed Computing Environment,” AEC/APC Symposium, Sep 26-29, 2005, Indian Wells, California.

Y. Li, H. Wohlwend, L. Rist, E. Simmon, “Intricacies of 

Time: Demystifying Factory Clock Synchronization and Time 

Stamping for E-Manufacturing,” AEC/APC Symposium, Sep 

26-29, 2005, Indian Wells, California.

Y. Li, “Using NTP: Introduction and Recommended Practices,” ISMI Technology Transfer.
232 National Institute of Standards and Technology 
Semiconductor Microelectronics and Nanoelectronics Programs 233

E-DIAGNOSTICS SECURITY

Technical Contacts:

Y. Li

E. Otema

J. Baboud

K. Brady

GOALS

The project strives to employ the latest web 

services security technologies for mitigating increasing security risks of potential Internet-based 

application systems, such as e-Diagnostics. As 

a proof of concept, the project aims to design a 

web-based framework that leverages available 

security technologies to ensure proprietary semiconductor process and equipment data would not 

be compromised.

CUSTOMER NEEDS

Optimizing factory and process capabilities and 

effi ciency will require more data to be shared 

among disparate partners in the engineering 

chain. The ability to share data among disparate 

partners has become a near-term issue in the 

International Technology Roadmap for Semiconductors (ITRS):

“Manufacturing knowledge and control information need to be shared as required among 

disparate factories.” 2006 ITRS Update, Factory 

Integration, p. 2.

However, a key concern is the security of the data 

and the ability to protect intellectual property and 

other forms of valuable proprietary information. 

One application where data sharing between the 

factory and the equipment supplier is necessary 

to expedite equipment repair is e-Diagnostics. 

E-Diagnostics enables equipment suppliers to 

remotely monitor, maintain, diagnose, and repair 

tools on the factory fl oor. Enabling a web-based 

application would provide greater fl exibility for 

equipment engineers to monitor or troubleshoot 

equipment on the manufacturing fl oor. With the 

convenience and economic advantages of being 

able to diagnose and resolve problems via the 

Internet come all the security challenges of the 

digital world. The challenges in managing Internet-based security have been a critical obstacle in 

using web services for e-Diagnostics. However, 

the web services community has been striving to 

resolve security issues by developing technologies to expedite integration and management of 

security. A web services security framework 

would provide an initial step towards realizing a 

web-based e-diagnostics system. A framework 

infrastructure promotes interoperability and rapid 

integration of new security technologies to ease 

the management of dynamic security policies with 

the proliferation. 

TECHNICAL STRATEGY

Companies have expressed the need for methods 

to manage security risks, but not at the expense 

of productivity. Therefore simplicity of use and 

security are the key objectives of this project. 

The web services community has been striving to 

resolve security issues by developing technologies 

to expedite integration and management of security. The eXtensible Markup Language (XML) 

security suite provides a standard paradigm to 

manage access control in order to restrict who 

may and may not control the equipment remotely, 

encryption and signatures in order to protect the 

data and ensure against malicious attackers, and 

key management to ensure the encryption used 

is safe. These features work with web services 

transport systems such as Simple Object Access 

Protocol (SOAP) to further ensure the safety of 

the data passed in transit. Leveraging the available standards in an effective manner can serve as 

a potential line of defense. However, employing 

XML-based standards alone are not suffi cient in 

implementing a secure framework. Security must 

be managed through careful interaction with factory systems, databases and networks, as well as 

the users accessing the factory systems. 

Robust identity management authentication and 

access control of legitimate users must be in 

place. The system should be readily managed to 

accommodate dynamic security policies. Simplifying security management is a key practice in 

avoiding security loopholes. Therefore, in facing 

the proliferation of security threats and system 

vulnerabilities, the industry can benefi t by leveraging mainstream security tools to provide a robust 

line of defense against potentially costly threats to 

intellectual property and the continuous operation 

of factory systems.

Based on the needs of industry, a framework will 

be designed and implemented that allows for 

fi ne-grain access control to restrict unauthorized 

users from infi ltrating the system, while ensuring legitimate users have convenient access to 

resources necessary to complete their tasks. In 

order to strengthen the system, we will incorporate 

the current web-services security technologies 

available to encrypt messages and secure transportation of all sensitive information. The result of 

the project would be a prototype to demonstrate a 

secure web services framework for e-Diagnostics 

and other web-based applications in the semiconductor industry.
234 National Institute of Standards and Technology 

DELIVERABLES: Complete an identity management 

and access control framework based on Security Assertion Markup Language (SAML), eXtensiblle Access 

Control Markup Language (XACML) and Lightweight 

Directory Access Protocol (LDAP). 4Q 2007

Lastly, the security of the system will need to be 

analyzed and verifi ed. Due to the rapidly evolving 

nature of security attacks, it is imperative to ensure 

systems are not only secure against current threats, 

but also mitigate potential risks in the future. The 

project will examine the latest types of attacks 

on identity management and access control, and 

design methods to evaluate the ability of a system 

to mitigate such risks.

DELIVERABLES: Evaluate possible threats to the 

web security framework and research methods to 

evaluate the security vulnerabilities of a system. 

4Q 2007

ACCOMPLISHMENTS

Draft white paper on “Survey of Web Services Security Technologies and Recommended 

Practices.”

Initial software undergoing development at 

NIST is available at: http://e-diagnostics.sourceforge.net/.

COLLABORATIONS

École Supérieure d’Informatique et d’Applications de Lorraine (ESIAL)

Harvey Wohlwend, Gino Crispieri, International SEMATECH 

Manufacturing Initiative

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Semiconductor Microelectronics and Nanoelectronics Programs 235

ENGINEERING CHAIN MANAGEMENT IN THE

SEMICONDUCTOR INDUSTRY

Technical Contacts:

J. Messina

K. Brady

GOALS

This project will investigate and document key 

issues related to electronic data exchange, supply 

chain communication, and other automation standardization needs confronting the semiconductor 

industry. The objective of this project is to facilitate the evolution of an integrated semiconductor 

“Engineering Chain” which provides each partner 

with the data needed to make business decisions 

in a timely manner.

CUSTOMER NEEDS

Increasing technological requirements has led the 

semiconductor industry to seek further means of 

cost savings by improving factory productivity, 

streamlining business models, and accelerating 

their supply chain. The growing complexity of 

product development and manufacturing models 

in parallel with shrinking product lifecycles further exacerbates the challenges the industry faces. 

Maximizing interoperability among automation 

systems in the factory and throughout the supply 

chain will become a greater issue for realizing the 

semiconductor industry’s requirements to reduce 

time, inventory, and therefore costs.

“Manufacturing knowledge and control information need to be shared as required among 

disparate factories” 2006 Update International 

Technology Roadmap for Semiconductors Factory Integration (ITRS) 2006 Update, Factory 

Integration, p.2.

The semiconductor industry is also seeking to 

control costs and improving semiconductor yields 

through the development of advanced Factory 

Information and Control Systems (FICS). Next 

generation FICS are expected to be able to collect information from equipment that will enable 

intelligent automated decision-making concerning 

factory resources. (This will require data quality assurance). Materials will be processed and 

dispatched to equipment in a way to minimize 

production equipment idle time thus maximizing factory output. In addition, FICS will allow 

factories to be more reconfi gurable as needed 

even to the point of being able to modifying wafer 

processes on a wafer-by-wafer basis. This is even 

more important with factories expected to use 

even few total wafers with the expected transition 

to 450mm. Developing these FICS will require a 

greater level of interoperability and data exchange 

within the semiconductor factory. 

“Process Control solutions must be able to support the ability to run different process parameters 

for each wafer, both between and with process 

runs.. 2005 ITRS Factory Integration, p. 16. 

An additional concern for the semiconductor industry is the growing trend towards environmental 

legislation that seeks to protect the environment 

from hazardous substances. An example of this 

new trend is the European Union’s Restriction 

of Hazardous Substances (RoHS) in electronics 

directive that seeks to reduce or eliminate six substances known to be hazardous to humans and the 

environment. This will result in a massive shift in 

the availability of parts as some components are 

removed from the marketplace while others are 

redesigned completely to remove the banned substances. In addition to forcing product changes, 

the only way for companies to comply with such 

legislation will be to support the exchange of 

material composition information throughout the 

entire supply chain. Complying with the new 

legislations will greatly add to the cost of design 

and can only be mitigated through the development of appropriate standards.

TECHNICAL STRATEGY

To help the industry achieve their goal of effective 

partnerships within an Engineering Chain, this 

project will engage in industry efforts that would 

benefi t from NIST’s neutrality, multi-industry 

expertise, and broad and detailed knowledge of 

the Information Technology (IT) standards development process. 

DELIVERABLES: Chair the Product Lifecycle Information Management (PLIM) TWG of the ITRS 2007 

Roadmapping Activity. 4Q 2007

Attendance at workshops, conferences, and standards meetings held by key organizations in the 

semiconductor community such as International 

SEMATECH (ISMT), Semiconductor Equipment 

Materials International (SEMI) and RosettaNet 

provides opportunities to assess current areas of 

IT standards development. As the industry moves 

towards utilization of mainstream computing 

technologies including eXtensible Markup Language (XML) and Simple Object Access Protocol 

(SOAP), the project also provides guidance in 
236 National Institute of Standards and Technology 

leveraging existing best practices from other 

industries and to promote collaborations among 

industry partners for mutual benefi t.

The investigation also includes an informal survey 

of semiconductor supply chain partners including 

designers, chip manufacturers, and equipment 

suppliers to gauge their existing practices, requirements and priorities. Site visits will provide 

an opportunity for key industry fi gures to share 

their vision of how IT standards would expedite 

advancement of new technologies and business 

models.

DELIVERABLES: Establish a working arrangement 

with SEMATECH and SEMI in order to identify top 

industry IT-standards related needs and develop 

potential solutions to the current challenges based on 

cross-industry solutions facing similar issues. 

3Q 2007

As appropriate, the survey also provides an opportunity for providing insights and guidance 

from similar industry efforts in supply chain and 

equipment communication standards to facilitate 

existing standards developments.

In response to industry needs, the survey will 

validate internal NIST IT projects against the 

semiconductor industry’s requirements. One potential project is the Infrastructure for Integrated 

Electronics Design and Manufacturing (IIEDM). 

IIEDM has facilitated neutral XML-based standards development in the electronic exchange 

of technical and business data in the electronic 

components supply chain. Similarities in challenges and technologies utilized may provide an 

opportunity for future collaborations. In addition, 

experience with standards development in various 

industries positions NIST to provide impartial 

cross-industry benchmarks for IT exchange 

strategies.

DELIVERABLES: Work with industry to create a material declaration standard designed to assist industry 

in complying with all the forthcoming environmental 

legislation being passed around the world. 4Q 2007

ACCOMPLISHMENTS

In conjunction with IPC, NIST developed IPC 

1752 material declaration standard that supports 

the exchange of RoHS environmental legislation 

through the engineering chain. 

Created a tutorial on how to improve the 

semiconductor standards development process 

through the use of XML and UML. This tutorial 

has been taught as a class at both NIST and various SEMI standards workshops. 

#

#

Assessed the current and future direction of 

IT use in semiconductor and mask manufacturing. 

Site visits conducted at four IC manufacturers, one 

design house, and one photomask supplier.

NIST co-led the Engineering Chain Management sub-team of the ITRS Factory Information 

and Control Systems team. Efforts were made to 

determine the scope and identify the challenges of 

creating an effective Engineering Chain.

Surveyed current standards development 

activities in semiconductor manufacturing and 

potential NIST roles in various task forces by 

participating in a variety of standards activities. Diagnostic Data Acquisition (DDA), XML, 

Process Control System (PCS), and Equipment 

Engineering Capabilities (EEC) are possible areas 

where NIST can play a future role. Leveraging use 

of mainstream technologies appears to be widely 

accepted. Data acquisition standards, including 

issues of data modeling, speed, bandwidth, quality 

and integrity, have become critical in advancing 

e-manufacturing efforts. Follow-up discussions 

with members of SEMI and ISMT indicated 

interest in enlisting NIST expertise and contacts 

to leverage best practices from other industries 

on specifi c issues such as XML-based standards 

development for manufacturing, timing and synchronization of equipment data, as well as data 

security issues.

COLLABORATIONS

Alan Weber, Alan Weber & Associates

ITRS Factory Integration Technical Working Group

SEMI XML Task Force

SEMATECH 

RECENT PUBLICATIONS

K. Botsford, J. V. Messina, E. Simmon, “North American 

Environmental Compliance Attitudes Towards Electronics,”

Jun 04-06, 2007, Singapore, Singapore.

E. Simmon, “RoHS Harmonization ? Progress Toward a 

Single Global Standard?,” Advanced Forum on Achieving 

and Maintaining Global RoHS Compliance, Jun 11-12, 2007, 

San Francisco, California.

H. Wohlwend, G. Crispieri, Y. Li, “Advancing Factory-Wide 

Data Quality for APC Applications,” AEC/APC SymposiumAsia, Nov 30, 2006 to Dec 01, 2006, Taipei, Taiwan.

H. Wohlwend, G. Crispieri, Y. Li, “Advancing Towards Factory-Wide Data Quality for APC Applications,” AEC/APC 

Symposium XVIII, Sep 30, 2006 to Oct 05, 2006, Westminster, 

Colorado.

A. Griesser, “Ensuring High Quality Data Transfer Standards,” AEC/APC Symposium XVI, Sep 18-24, 2004, Westminister, Colorado.

#

#

#
Semiconductor Microelectronics and Nanoelectronics Programs 237

NIST/SEMATECH -HANDBOOK OF STATISTICAL

METHODS

Technical Contacts:

W. F. Guthrie

A. Heckert

J. J. Filliben

“I reviewed the e-Handbook on the net and I am 

very impressed with the 

content, organization 

and consolidation of the 

statistical methods.”

Jack Lewis

Microchip Technology Inc.

GOALS

The goal of the NIST/SEMATECH e-Handbook 

of Statistical Methods project is to produce an 

online resource to help scientists and engineers 

incorporate statistical methods into their work 

more effi ciently. Electronic publication and a 

practical, example-driven format were chosen 

to make the e-Handbook readily accessible to its 

target audiences in industry, including the semiconductor industry in particular. 

CUSTOMER NEEDS

Semiconductor manufacturing requires extraordinary discipline in process control. For example, 

a typical integrated circuit manufacturing process 

involves several hundred steps. These steps may 

include as many as thirty lithographic levels, 

which require extremely precise alignment with 

respect to one another. Tight process control is 

essential to produce a functional circuit. SEMATECH has recognized the importance of statistical 

process control in semiconductor manufacturing 

and has developed and maintained expertise in this 

fi eld since its inception in 1988. Two of the more 

diffi cult issues impeding routine implementation 

of these techniques, however, include educating 

the users and making the tools easy to use.

TECHNICAL STRATEGY

NIST and SEMATECH formed a collaboration 

under the umbrella Cooperative Research and 

Development Agreement (CRADA), combining 

the general expertise in engineering statistics 

at NIST with the semiconductor manufacturing 

expertise resident at SEMATECH (Fig. 1).

DELIVERABLES: Update and maintain the 

e-Handbook on-line, and continue to distribute the 

e-Handbook on CD for off-line use. 4Q 2007

ACCOMPLISHMENTS

Since release of the fi nal version, work has 

focused on publicizing the e-Handbook, responding to user feedback, and developing a compact 

disk version for off-line use (Fig. 2). The web 

version of the e-Handbook averages approximately 1 million hits per month and over 8,500 

e-Handbook compact disks have been distributed 

to industrial, government, and academic users all 

over the world over the last two years. Publicity on 

the e-Handbook has appeared in Science, Quality 

Digest, MicroMagazine.com, States News Service, 

National Science Digital Library Report for Math, 

Engineering, and Technology, and American Statistician. In February 2007, a Division workshop 

was held to discuss additions to the e-Handbook. 

It was agreed there to seek proposals for additional 

material to be added to existing chapters, or for 

new chapters. Several potential topics for new 

chapters were identifi ed and discussed. 

#

Figure 1. NIST e-Handbook of Statistical Methods 

team (left to right, Alan Heckert, Mark Reeder, Will 

Guthrie, and Carroll Croarkin) reviewing a page 

from the chapter on product reliability.

Figure 2. Page from a case study in the process 

modeling chapter of the Handbook.

e
238 National Institute of Standards and Technology 

COLLABORATIONS

International SEMATECH, Paul Tobias, Jack Prins, Chelli 

Zey; project planning, organization, writing and editing.

AMD, Barry Hembree; project planning, organization, and 

writing.

Motorola, Pat Spagon; project planning, organization, and 

writing.

RECENT PUBLICATIONS

NIST/SEMATECH e-Handbook of Statistical Methods, M. 

Carroll Croarkin and Paul Tobias, editors, http://www.nist.

gov/stat.handbook/.
Semiconductor Microelectronics and Nanoelectronics Programs 239

ABBREVIATIONS AND ACRONYMS

A.C. alternating current

ADR adiabatic demagnetization refrigerator

AEM analytical electron microscopy

AES Auger-electron spectroscopy

AFM atomic force microscope

ALMWG Analytical Laboratory Managers Working Group (ISMT)

AMAG Advanced Metrology Advisory Group (ISMT)

ANSI American National Standards Institute

ARXPS angle resolved X-ray photoelectron spectroscopy

ASPE American Society of Professional Engineers

ATP Advanced Technology Program (NIST)

BCB benzocyclobutene

BESOI bond and etch-back silicon-on-insulator

BGA ball-grid array

BIPM Bureau International des Poids et Mésures

BIST built-in self-test

BST barium strontium titanate

C-AFM calibrated atomic force microscope (NIST)

C-V capacitance-voltage

CAD computer-aided design

CCD charge-coupled device

CD critical dimension

CMOS complementary metal oxide semiconductor

CMP chem-mechanical polishing

CPU Central Processing Unit

CRADA Cooperative Research and Development Agreement

CRDS cavity ring-down spectroscopy

CSP chip-scale package

CTCMS Center for Theoretical and Computational Materials Science (NIST)

CVD chemical vapor deposition

D.C. direct current

DFT design-for-test

DMA differential mobility analyzer

DRAM dynamic random-access memory

DSP digital signal processing

DUV deep ultraviolet

EBSD electron backscatter diffraction

EELS electron energy loss spectroscopy

EDC embedded decoupling capacitance

EDS energy-dispersive spectroscopy

EMC electromagnetic compatibility
240 National Institute of Standards and Technology 

ABBREVIATIONS AND ACRONYMS (CONT’D)

EMI electromagnetic interference

EPMA electron probe microanalysis

EUV extreme ultraviolet

FIFEM fi eld ion fi eld emission microscope

FIM fi eld ion microscope

FWHM full-width half-maximum

GIXR/SE grazing incidence X-ray refl ection/spectrascopic ellipsometry 

GIXPS grazing incidence X-ray photoelectron spectroscopy

HRTEM high resolution transmission electron microscope 

HSQ hydrogen silsesquoxane

I-V current-voltage

IC integrated circuit

IGBT insulated-gate bipolar transistor

IPC Association Connecting Electronics Industries

ISMT International SEMATECH

ISO International Organization for Standardization

ITRS International Technology Roadmap for Semiconductors

LEED low-energy electron diffraction

LER line-edge roughness

LFPG low frost-point generator

LOCOS LOCal Oxidation Of Silicon

LPP laser-produced plasma

LPRT light-pipe radiation thermometer

MBE molecular beam epitaxy

MEMS micro-electro-mechanical systems

MFC mass fl ow controller

MMIC millimeter and microwave integrated circuits

MOS metal-oxide-semiconductor

MOSFET metal-oxide-semiconductor fi eld-effect transistor

MPU MicroProcessor Unit

MUX multiplex

NCMS National Center for Manufacturing Sciences

NDP neutron depth profi ling

NGL next generation lithography

NEMI National Electronics Manufacturing Initiative

NIST National Institute of Standards and Technology

NSMP National Semiconductor Metrology Program

NLO non-linear optical

NSOM nearfi eld scanning optical microscopy

OMAG Overlay Metrology Advisory Group (ISMT)
Semiconductor Microelectronics and Nanoelectronics Programs 241

PED Precision Engineering Division (NIST)

PLIF planar laser-induced fl uorescence

PMI phase-measuring interferometer

PTB Physikalisch-Technische Bundesanstalt

PZT lead zirconium titanate

QM quantum mechanics

RAM Random-access memory

RGA residual gas analyzer

RLGC distributed resistance, inductance, conductance, and capacitance

RTA rapid thermal annealing

RTP rapid thermal processing

SANS small-angle neutron scattering

SBIR Small Business Innovative Research

SCM scanning capacitance microscope

SEM scanning electron microscope

SHG second harmonic generation

SIA Semiconductor Industry Association

SIMOX separation by implantation of oxygen

SIMS secondary-ion mass spectrometry

SoC system-on-a-chip

SOI silicon on insulator

SPM scanning probe microscope

SRC Semiconductor Research Corporation

SRM® Standard Reference Material

SSHG surface second-harmonic generation

SSIS surface-scanning inspection system

STM Scanning Tunneling Microscope

SURF III Synchrotron Ultraviolet Radiation Facility III

TCAD technology computer-aided design

TDDB time-dependent dielectric breakdown

TDR time-domain refl ectometry

TEM transmission electron microscope

TFTC thin-fi lm thermocouple

TOF time-of-fl ight

TMAH tetramethyl ammonium hydroxide

UHV ultra-high vacuum

UV ultraviolet

WMS wavelength modulation spectroscopy

VUV vacuum ultraviolet

XPS X-ray photoelectron spectroscopy

XRR X-Ray Refl ectometry
242 National Institute of Standards and Technology 

TECHNICAL CONTACTS
Semiconductor Microelectronics and Nanoelectronics Programs 243
244 National Institute of Standards and Technology