SEMICONDUCTOR MICROELECTRONICS

AND NANOELECTRONICS PROGRAMS

NISTIR 7321

July 2006

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. 

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

This document is 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 .....................................................9

Polymer Photoresist Fundamentals for Next-Generation Lithography ..............................15

Critical Dimension and Overlay Metrology Program ...................................................21

Wafer-Level and Mask Critical Dimension Metrology ......................................................22

 Scanning Electron Microscope-Based Dimensional Metrology ..................................23

 Scatterometry-Based Dimensional Metrology .............................................................29

 Scanning Probe Microscope-Based Dimensional Metrology ......................................33

 Small Angle X-Ray Scattering-Based Dimensional Metrology ...................................39

 Fabrication and Calibration Metrology for Single-Crystal CD Reference Materials ..43

 Optical-Based Photomask Dimensional Metrology .....................................................49

 Model-Based Linewidth Metrology .............................................................................53

 Atom-Based Dimensional Metrology ..........................................................................57

Wafer-Level and Overlay Metrology ..................................................................................61

Front-End Processing Metrology Program ....................................................................67

Wafer and Chuck Flatness Metrology .................................................................................69

Modeling, Measurements, and Standards for Wafer Surface Inspection ............................73

Front-End Materials Characterization .................................................................................77

Interconnect and Packaging Metrology Program ..........................................................87

Atomic Layer Deposition – Process Models and Metrology ..............................................89

Superconformal Deposition: Copper and Advanced Interconnect Materials .....................95

Interconnect Materials and Reliability Metrology ..............................................................98

 Basic Materials Properties ............................................................................................99

 Test Structures For Interconnect Metrology and Modeling .......................................107

Solders and Solderability Measurements for Microelectronics ........................................ 113

Process Metrology Program ........................................................................................... 117

Gas Property Data and Flow Standards for Improved Gas Delivery Systems ................. 119

Low Concentration of Humidity Standards ......................................................................123

Temperature Measurements and Standards for Semiconductor Processing .....................129

Plasma Process Metrology ................................................................................................133

Assessment of Measurements and Standards for Gas Phase Processes in 

Semiconductor Device Manufacturing .............................................................................139
iv National Institute of Standards and Technology 

Analysis Tools and Techniques Program ......................................................................141

Thin-Film X-Ray Metrology for Microelectronics ...........................................................143

Electron Microscope Tomography of Electronic Materials ..............................................149

High-Resolution Microcalorimeter X-Ray Spectrometer for Chemical Analysis ............151

Device Design and Characterization Program .............................................................157

Device Characterization and Reliability ...........................................................................159

Nanoelectronic Device Metrology ....................................................................................165

Power Semiconductor Device Metrology .........................................................................171

Organic Electronics Metrology .........................................................................................175

Micro- and Nano-Electro-Mechanical Technology Metrology ........................................179

NIST’s Center for Nanoscale Science and Technology Nanofab .....................................183

Metrology for Spintronic Devices ....................................................................................185

BioElectronics Metrology .................................................................................................191

System Design and Test Metrology Program ...............................................................195

Metrology for System-on-a-Chip .....................................................................................197

At-Speed Test of Digital Integrated Circuits .....................................................................201

Thermal Measurements and Packaging Reliability ..........................................................205

Manufacturing Support Program ................................................................................. 211

Factory Time Synchronization Standards Development for E-Manufacturing ................213

E-Diagnostics Security ......................................................................................................215

Engineering Chain Management in the Semiconductor Industry .....................................217

NIST/SEMATECH E-Handbook of Statistical Methods ..................................................219

Abbreviations and Acronyms .........................................................................................220

Technical Contacts ..........................................................................................................224
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 2006. 
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 semiconductor standards development work of the ASTM, the International National 

Electronics Manufacturers Initiative (iNEMI), the EIA, the International Organization for 

Standardization (ISO), and Semiconductor Equipment and Materials International (SEMI).

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 NIST as follows:

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. (Secretary) 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 industry at this point has moved into the deep ultraviolet (DUV) 

spectrum. Currently, exposure tools operating at 193 nm are in leading edge manufacturing 

facilities. The fi rst 193 nm immersion lithography tools have been shipped to leading edge 

manufacturers and are being exercised for manufacture. High index fl uids and lens materials 

for 193 nm tools are under intense exploration to develop high 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 will be 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, and 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

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 and 157 nm immersion lithographies 

require accurate measurements of the index properties of the potential 193 nm and 157 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 minimumdeviation-angle refractometer. To improve on this 

absolute accuracy, NIST has begun constructing 

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 will enable 

measurements to an accuracy of 1 ppm, and will 

be used to characterize high-index lens materials 

and immersion fl uids for 193 nm and 157 nm 

lithography systems.

#

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

Figure 1. Facility for detector VUV radiation 

damage study using synchrotron radiation from 

SURF and a 157 nm excimer laser.

DELIVERABLES: Index measurement capability with 

1 ppm uncertainty. 2Q 2006
4 National Institute of Standards and Technology 

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 

begun, with the support of SEMATECH, a survey 

of candidate materials. We have identifi ed several 

classes of oxide-based materials which have very 

high indices (near 2.0) at this wavelength and 

which can in principle be highly transparent at 

193 nm. We have begun a program of complete 

optical characterization measurements of these 

materials to identify the most promising candidates and to assess their potential for development 

for lithography optics.

DELIVERABLES: Fully characterize the 193 nm 

optical properties of leading candidate high-index 

materials. 2Q 2006

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. In addition, we can provide 

calibrations for F2

 (157 nm) excimer lasers. To 

#

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#

meet the laser metrology needs of the optical lithography community, we have developed primary 

standards and associated measurement systems at 

193 nm and 248 nm, and 157 nm. Figure 2 shows 

the excimer laser calibration facility.

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. 

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 photolithography. In addition to providing measurement services to several companies 

which are developing new fl uids for 157 nm and 

193 nm lithography, we have performed our own 

survey of fl uids at 193 nm. The results are shown 

in Fig. 3. As this fi gure shows, there are a number 

of fl uids which are transparent at 193 nm and 

have n larger than water, up to 1.62 for glycerol. 

However, the thermo-optic coeffi cient, dn/dT, is 

larger as well, which may make the use of these 

diffi cult in a commercial stepper system because 

of thermal stability issues.

#
Semiconductor Microelectronics and Nanoelectronics Programs 5

We have performed additional measurements 

of the effect of dissolved air on the refractive index of water at several wavelengths in the visible 

and UV and compared the results with predictions 

based on the equation of state of water and the 

known partial molar volumes and polarizabilities 

of the soluble components of air. The data and 

model calculations are in reasonable agreement, 

and demonstrate that the magnitude of the effect 

gets larger at shorter wavelength, up to –6.7x10-6

at 193 nm. While industry uses degassed water for 

immersion lithography, our results will enable any 

potential optical distortions due to air exposure to 

be modeled at the system level.

We have identifi ed four major classes of oxide-based materials that have potential to be used 

as high-index lens materials for enabling further 

feature-size reduction with immersion lithography. The four classes are: (1) Group II oxides, 

e.g., MgO, (2) crystalline spinel (MgAl2

O4) and 

its variants, (3) a ceramic form of spinel, and (4) 

aluminum garnets, e.g., YAG. We have made 

preliminary characterization measurements of 

the key 193 nm optical properties (see Fig. 4), 

including the intrinsic birefringence, of members 

#

#

of each of these classes needed to assess their 

suitability. As a result of these measurements, 

several potentially feasible candidates have been 

established.

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. 5) 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 

#

#

Figure 3. Measured index of refraction n and thermo 

coeffi cient dn/dT of a number of potential immersion 

fl uids for 193 nm lithography.

Figure 4. Measured intrinsic birefringence of 

crystalline spinel vs. wavelength.

Figure 5. Measurement system for detector damage 

study by a 157 nm pulsed excimer laser.
6 National Institute of Standards and Technology 

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 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 photodiode 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 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 

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Figure 6. 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.

incident pulse energy, range discontinuities associated with detector gain, and detector background 

noise (see Fig. 6).

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, 193, 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

Air Products and Chemicals, Inc., Bridgette Budhlall, immersion photolithography fl uid 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).

#
Semiconductor Microelectronics and Nanoelectronics Programs 7

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).

P. S. Shaw, D. Shear, R. J. Stamilio, U. Arp, H. W. Yoon, R. 

D. Sanders, A.C. Parr, and K.R. Lykke, “The new beamline 3 

at SURF III for source-based radiometry,” Rev. Sci. Instrum. 

73, 1576 (2002).

P. S. Shaw, T. C. Larason, R. Gupta, and K. R. Lykke, “Characterization of UV detectors at SURF III,” Rev. Sci. Instrum. 

73, 1625 (2002).

D. X. Chen, M. L. Dowell, C. L. Cromer, Z. F. Zhang, 

“Thermal Response and Inequivalence of Pulsed UltravioletLaser Calorimeters,” J. Thermophys. Heat Trans. 16 (January 

2002) 36.

M. L. Dowell, R. D. Jones, H. Laabs, C. L. Cromer, and R. 

Morton, “New developments in excimer laser metrology at 

157 nm,” Proc. SPIE Metrology, Inspection, and Process 

Control for Microlithography XVI, D. J. C. Herr, Editor, 

(March 2002) 63.

P. S. Shaw, R. Gupta, and K. R. Lykke, “Characterization of 

an ultraviolet and a vacuum-ultraviolet irradiance meter with 

synchrotron radiation,” Appl. Opt. 41, 7173 (2002).
8 National Institute of Standards and Technology 
Semiconductor Microelectronics and Nanoelectronics Programs 9

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 underway to install EUVL into commercial production 

in 2011 at the 32 nm node. As part of that effort, 

ASML will deliver two alpha-generation steppers 

for initial testing later this year.

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 present 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 performed recently 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. 

Figure 1. Refl ectivity vs. wavelength of a typical 

MoSi multilayer mirror. The measurement was made 

at 5° from normal incidence.

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 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 

characteristics of EUV sensitive Si photodiodes 

using 532 nm radiation as a proxy for 13.5 nm; the 

Technical Contacts:

T. Lucatorto

C. Tarrio

S. Grantham

U. Griesmann

S. Hill
10 National Institute of Standards and Technology 

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 

diffusion current that decreases 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 will participate in an international 

intercomparison of EUV detector responsivity. 

This intercomparison will establish the worldwide 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 EUV lithography community.

DELIVERABLES:

EUV detector responsivity international intercomparison. 3Q 2006

EUV CCD responsivity measurement. 3Q 2006

EUV spectrograph measurement. 4Q 2006

3. EUV DAMAGE CHARACTERIZATION

One of the potential showstoppers for commercialization of extreme ultraviolet lithography (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. Various capping layers are currently 

being developed to extend the lifetime of the 

multilayer mirrors towards the required goal for 

commercialization of only a few per-cent refl ectivity loss over 30000 hours (h) of use.

To study the effectiveness of new capping layers 

and to better understand the underlying processes 

responsible for mirror degradation, NIST has 

commissioned a new beamline 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 successful capping layer available to the community 

has been ~ 2 nm of ruthenium. Our exposure 

facility has demonstrated that multilayers with 

this capping layer suffer approximately one-tenth 

the refl ectivity loss of bare Si-capped multilayers 

when exposed for ≈ 100 h under rather aggressive 

conditions of 1×10-4 Pa of water vapor.

Clearly, testing multiple capping layers for the 

required 30000 h optic-lifetime is not feasible. 

We have therefore developed a program for accelerated lifetime testing in which multilayers are 

exposed to various EUV intensities under various 

high-vacuum environments. The ultimate goal 

of this program is to develop a model of mirror 

damage that will enable the extrapolation of the 

years-long-lifetime under normal operating conditions from a few hundred hours of exposure under 

appropriately chosen accelerated exposure conditions. This effort requires an understanding of the 

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Semiconductor Microelectronics and Nanoelectronics Programs 11

underlying damage mechanisms that are being 

studied by an iterative series of controlled exposures and analysis. 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. Although this effort 

is just beginning, we have already discovered 

that small levels of hydrocarbons (1×10-7 Pa) 

can completely determine the rate of mirror damage, even in the presence of much larger partial 

pressures of water vapor (7×10-4 Pa). Thus, the 

vacuum environment must be carefully controlled 

and well characterized for any accelerated testing 

procedure to produce meaningful results. 

Figure 2. Spatial mapping of the refl ectivity of a 

multilayer mirror following exposure to EUV under 

aggressive environmental conditions.

To respond to the growing need for optic-lifetime 

characterization we have continued to expand our 

capabilities. The original lifetime testing chamber was developed with support from Sematech 

and Sandia National Labs. Recently, a second 

beamline with a lifetime testing chamber has been 

constructed and commissioned with industry support. In addition, with intramural support from the 

NIST Advanced Technologies Program, we are installing a pulsed source so that mirror degradation 

caused by the quasi cw irradiation of the SURF 

synchrotron can be compared with that caused by 

pulsed irradiation. 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:

Commission new lifetime testing beamline. 

2Q 2006 

•

Work with industrial collaborators to evaluate titanium dioxide as a possible capping layer. 4Q 2006

Install pulsed EUV source for testing under pulsed 

irradiation. 4Q 2006

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. 

•

•

Figure 3. A view of the XCALIBIR interferometer.

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 

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. 
12 National Institute of Standards and Technology 

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). A (statistical) measurement uncertainty of approximately 

0.2 nm rms is evident.

Figure 4. Topography of a 300 mm diameter 

XCALIBIR reference fl at.

Figure 5. Distribution of rms deviations from best 

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

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 6. Deviation from perfect spherical form of a 

1 kg precision silicon sphere.

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
Semiconductor Microelectronics and Nanoelectronics Programs 13

Figure 7. Geometry Measuring Machine (GEMM) 

for form measurements of free-form precision 

surfaces.

COLLABORATIONS

VNL at Lawrence Livermore National Laboratory, Saša Bajt, 

EUV Multilayer Development and Coating Team.

RECENT PUBLICATIONS

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).

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,” Appl. Opt. 2006 (in press).

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).
14 National Institute of Standards and Technology 
Semiconductor Microelectronics and Nanoelectronics Programs 15

POLYMER PHOTORESIST FUNDAMENTALS FOR

NEXT-GENERATION LITHOGRAPHY

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. The understanding developed in this 

program will provide 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 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), nuclear magnetic resonance (NMR), 

atomic force microscopy (AFM), fl uorescence 

correlation spectroscopy (FCS) and combinatorial 

methods. Our efforts focus on the fundamentals 

of polymeric materials and processes that control 

the resolution of the photolithography process 

including: (1) the physical properties of and 

polymer chain conformation within sub 50 nm 

structures; (2) the spatial segregation and distribution of 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 2005 International Technology 

Roadmaps for Semiconductors on page 9, Lithography Section, “Photoresists need to be developed 

that provide good pattern fi delity, good line width 

control, low line width roughness, and few defects. 

As feature sizes get smaller, defects and monomers 

will have comparable dimensions with implications for the fi ltering of resists.”

Technical Contacts: 

V. M. Prabhu

E. K. Lin 

W. L. Wu 

“[This work] opens a 

window of opportunity 

to construct structureproperty relationships 

between chemical 

transport mechanisms 

and ultimate resist 

resolution. It may also 

lead to insights into 

the ultimate, intrinsic 

resolution limits and 

critical dimension 

control of polymer-based 

imaging materials.”

E. Reichmanis and 

O. Nalamasu, Bell Labs, Lucent 

Technologies, Science, 297, 

349 (2002).
16 National Institute of Standards and Technology 

TECHNICAL STRATEGY

In this project, we use model photoresist materials 

to validate the new measurement methods. Model 

photoresist materials (248 nm, 193 nm, 157 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. We 

also are adapting the application of combinatorial 

methods as a tool to determine rapidly important 

lithographic parameters and to identify material 

factors impacting feature resolution. 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.

DELIVERABLES:

Develop model thin fi lms to measure the transport 

and kinetics of photoresist components and the 

deprotection reaction as a function of PAG size and 

resist copolymer chemistry. 2Q 2006

Conclude protection degree effect on the dissolution and swelling behavior and the relationship to 

surface roughness for 193 nm photoresist polymers. 2Q 2006

Utilize reaction-front bilayer geometry to identify 

and quantify the effects of developer solution 

parameters (base concentration, ionic strength) on 

the fi nal resolution of lithographic materials. 

3Q 2006

Quantify surface segregation, surface deprotection 

chemistry, distribution of photoresist components 

(resist, photoacid, base additive) in 193 nm resists, 

using NEXAFS. 4Q 2006

Conclude PAG loading effect on the dissolution and 

swelling behavior and the relationship to surface 

roughness for model EUV photoresist polymers. 

4Q 2006

Quantify the effect of dose on the form and spatial 

extent of photoacid diffusion at model lithographic 

line edges for EUV photoresist. 2Q 2006

ACCOMPLISHMENTS

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 in 

the 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 solidliquid interfaces. The aqueous base profi le, shown 

in Fig. 1, illustrates the penetration of the small 

base molecule throughout the thin fi lm as a function of developer strength. The swelling, due to 

polyelectrolyte effects, was predicted in FY03. 

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 1. 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.

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 

#
Semiconductor Microelectronics and Nanoelectronics Programs 17

processes. The fi rst order reaction rate constant 

decreases as a 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. 2. This composition-dependent reaction 

constant indicates a strong interaction of the acid 

with the increasing polar resist matrix that drastically reduces the acid transport rate. The severely 

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; 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.

Figure 2. 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.

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. 3. These experimental data provide 

a rational design of next-generation photoresist 

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

Figure 3. 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.

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 in Fig. 4. 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. 

#

Figure 4. Schematic diagram of the NEXAFS 

measurement geometry. Spectra are obtained from 

the fi lm surface and bulk simultaneously.
18 National Institute of Standards and Technology 

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.

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 

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. 5. Additionally, the effects of critical top coats are also investigated to understand 

the segregation and retention of PAG additives. 

#

Figure 5. 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.

The development step selectively removes 

UV exposed photoresist material and represents 

the last step in the fabrication of nanostructures 

prior to semiconductor etch and deposition. With 

dimensions shrinking to sub-50 nm, control of 

line-edge roughness becomes more important and 

contributions to roughness from the development 

step requires an improved framework. In the development step, the aqueous base TMAH developer shifts the local chemical equilibrium from an 

unionized form to the ionized form, for instance 

in the 248 nm material poly(hydroxystyrene). 

SANS data, in Fig. 6, demonstrate the origin of 

the miscibility in aqueous base is due to the ionization of the photoresist leading to polyelectrolyte 

behavior. The identifi cation of the presence of 

polyelectrolyte behavior during the development 

process provides an improved framework to 

understand the roles of added electrolytes, such 

as low molecular weight organic (tetramethylammonium chloride) and inorganic salts (NaCl, 

KCl). The addition of salts reduce the infl uence 

of polyelectrolyte behavior. Current experiments 

with different developing and rinsing protocols 

demonstrate that the pH of the rinse step is very 

important. This suggests the surface layer may 

contain polyelectrolyte effects even after development as demonstrated by an increase in surface 

RMS roughness for the development of bilayer 

samples with 0.26N TMAH followed by water 

rinse and 0.01 M HCl rinse. 

#
Semiconductor Microelectronics and Nanoelectronics Programs 19

Characterization of local bulk scale mixing 

is necessary for understanding future photoresist 

materials as feature dimensions are reduced to 

sub-65 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 PAG concentration on the dispersion and phase 

separation within bulk blends was found to depend strongly on the photoresist chemistry (see 

Fig. 7). In model 248 nm materials PFOS was 

found to mix on the molecular scale for loadings 

between 9 % and 45 %; hence the two components 

are thermodynamically miscible in this range. 

#

Figure 6. SANS data from a model photoresist 

polymer in organic solvent (PGMEA) and the 

developer base solution (TMAH). The scattering 

peak in TMAH solution is representative of the 

polyelectrolyte behavior.

Figure 7. Model 248 nm and 157 nm photoresist 

materials used to probe the intimacy of mixing of 

photoacid generator in blends by NMR methods.

These results were extended to a challenging 157 

nm formulation and revealed that, while PFOS is 

miscible within the two photoresists, the ternary 

system exhibits phase separation into domains 

exceeding 30 nm.

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. 8. The deprotection fronts exhibits 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. 8. 

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.

#

Figure 8. Two contrasting architectures of photoresist 

materials, but with similar chemistry: molecular 

glass and polymeric chemically amplifi ed resist. 
20 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. Figure 9 shows a CFM image 

of an EUV-exposed PMMA thin fi lm with 30 nm 

half pitch lines that has not been developed. The 

lighter areas are the unexposed PMMA.

#

Figure 9. Chemical Force Microscopy image on an 

EUV imaged PMMA thin fi lm with 30 nm half-pitch 

lines.

COLLABORATIONS

Polymers Division, NIST – Bryan D. Vogt, Shuhui Kang, 

Ashwin Rao, David VanderHart, Kristopher Lavery

Ceramics Division, NIST – Sharadha Sambasivan, Daniel 

A. Fischer

Center for Neutron Research, NIST – Sushil K. Satija

Optical Technology Division, NIST – John Woodward, 

Jeeseong Hwang

SEMATECH – Karen Turnquest

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

Thompson

IBM T. J. Watson Research Center – Dario L. Goldfarb

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

Cornell University – Christopher Ober

SEMATECH – Agreement 309841 OF

Intel – CRADA 1893

RECENT PUBLICATIONS

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

K. Turnquest, “Copolymer fraction effect on acid catalyzed 

deprotection reaction kinetics in model 193 nm photoresists,”

Proceedings of the SPIE, Vol 6153, (2006).

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

K. Satija, K. Turnquest, and W.-L. Wu. “The deprotection 

reaction front profi le in model 193 nm methacrylate-based 

chemically amplifi ed photoresists,” Proceedings of the SPIE, 

Vol 6153, (2006).

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

W.-L. Wu, S. K. Satija, M. J. Leeson, H. B. Cao, G. Thompson, 

D. S. Fryer “Fundamentals of the Reaction-Diffusion Process 

in Model EUV Photoresists,” Proceedings of the SPIE, Vol 

6153, (2006).

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

Lin, W.-L. Wu, K. Turnquest, and W. D. Hinsberg, “Dissolution 

Fundamentals of 193 nm Methacrylate Based Photoresist,” 

Proceedings of the SPIE, Vol 6153, (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,” accepted, App. Surf. Sci.(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).

B. D. Vogt, V. M. Prabhu, C. L. Soles, S. K. Satija, E. K. Lin, 

W.-L. Wu, “Control of moisture at buried polymer/alumina 

interfaces through substrate surface modifi cation,” Langmuir 

21, 2460-2464 (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).

V. M. Prabhu and E. K. Lin, “Advanced Measurements for 

Photoresist Fundamentals,” Semiconductor Fabtech International Quarterly, 24th Edition, 99 (2004). 

D. L. VanderHart, V. M. Prabhu, E. K. Lin, “Proton NMR 

Determination of Miscibility in a Bulk Model Photoresist 

System: Poly(4-hydroxystyrene) and the Photoacid Generator, Di(t-butylphenyl) Iodonium Perfl uorooctane Sulfonate,” 

Chemistry of Materials, 16(16), 3074-3084 (2004).

V. M. Prabhu and E. K. Lin, “Advanced Measurements for 

Photoresist Fundamentals,” Semiconductor Fabtech International Quarterly, 24, 99-103 (2004).

R. L. Jones, V. M. Prabhu, D. L. Goldfarb, E. K. Lin, C. L. 

Soles, J. L. Lenhart, W. Wu, and M. Angelopoulos, “Correlation of the Reaction Front with Roughness in Chemically 

Amplifi ed Photoresists,” ACS Symposium Series No. 874, 

“Polymers for Micro- and Nano-electronics,” 86, (2004). 
Semiconductor Microelectronics and Nanoelectronics Programs 21

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. 
22 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:

Scanning Electron Microscope-Based Dimensional Metrology

Scatterometry-Based Dimensional Metrology

Scanning Probe Microscope-Based Dimensional Metrology

Small Angle X-Ray Scattering-Based Dimensional Metrology

Fabrication and Calibration Metrology for Single-Crystal CD Reference Materials

Optical-Based Photomask Dimensional Metrology

Model-Based Linewidth Metrology

Atom-Based Dimensional Metrology

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Semiconductor Microelectronics and Nanoelectronics Programs 23

SCANNING ELECTRON MICROSCOPE-BASED

DIMENSIONAL METROLOGY

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 gun, 

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. 

Figure 1. The fi nal design of the SRM 2120 Magnifi cation calibration 

standard reference material.

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 65 nm generation ... 

Determination of the real 

3-D shape...will require 

continuing advances in 

existing microscopy ...”

International Technology Roadmap for Semiconductors, 2003
24 National Institute of Standards and Technology 

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 

details are being completed) the artifact will be 

released as Reference Material (RM) 8120.

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:

This effort included the development of the Reference Material 8091 and evaluation procedures 

suitable for correctly measuring image sharpness 

of scanning electron microscopes, especially of 

those that are used in the semiconductor industry. 

The resolution performance characteristics of the 

SEM are particularly important to precise and accurate measurements needed for semiconductor 

processing. New methods and suitable samples 

are being sought to further improve this type 

of metrology. 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.

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 

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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.

DELIVERABLES: 

Design and preparation of line width metrology 

artifact suitable for calibration on NIST and external 

measuring systems for certifi cation of wafer format 

line width samples. 4Q 2006 

Completion of preliminary measurements on 

samples made by SEMATECH with the new “NISTMAG” metrology mask. 4Q 2006

ACCOMPLISHMENTS

SEM Magnification Calibration Artifacts 

– Samples for Reference Material 8090 have 

been made in the past once successfully, but later 

several attempts with e-beam lithography yielded 

no further useful samples. We now have a new, 

fi nal mask designed and fabricated at International 

SEMATECH to get samples made by 193 nm UV 

light lithography. The use of this conventional 

lithography and a somewhat modified design 

provides a chance for large amount of good quality 

samples produced inexpensively. Because 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. There 

are a large number of 250 nm wide crosses 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 

will 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 will be fi nished in the summer of 2006. 

With the arrival of the new metrology SEM these 

will be calibrated.

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#
Semiconductor Microelectronics and Nanoelectronics Programs 25

SEM Performance Measurements – After comprehensive studies and experiments a 

plasma-etching 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 underway to produce 

more of these samples. These samples are now 

available to the public, and many of them are 

already in use.

#

Figure 2. The RM 8091 Sharpness Reference 

Material.

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 

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 3 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 by 2 nm for 

each 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. 

#

Figure 3. 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 by 

2 nm fi eld of views.
26 National Institute of Standards and Technology 

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 Nanotip Electron Gun for CD-SEMs – The source diameter and the brightness of the electron beam are 

two of the major factors limiting the performance 

of CD-SEMs in the semiconductor production environment. Thus, alternative solutions to improve 

performance are being sought as the metrology 

for sub-100 nm lithography is being pushed to 

its limit. One possible alternative approach is the 

application of nano-tips as an electron source. 

Nano-tips, by comparison to conventional cold 

fi eld, offer a substantial increase in brightness 

(10 x to 100 x) and a large reduction (5 x to 10 x) 

in source size. Figure 4 proves that the nano-tip 

electron gun can deliver 6 nm resolution images 

while the original gun’s specifi cation for the test 

microscope was only 15 nm. Therefore, in an 

optimized electron optical column, substitution 

#

#

Figure 4. Nano-tip electron gun image showing 6 nm 

resolution taken on a gold-on-carbon resolution 

sample at 900 V accelerating voltage. The SEM 

original specifi cation resolution is 15 nm. The fi eld of 

view is 1.75 μm.

of a CFE source by a nano-tip could be expected 

to produce:

– Higher beam currents into a spot of given 

size,

– Better signal-to-noise ratio and resolution, 

and

– Faster scan rate and better charge control.

This work has proved that nano-tips indeed improve the resolution performance of SEMs and 

can be used for longer periods of time. Currently 

further experiments are conducted to assess the 

optimal setting of the electron gun and the electron 

optical column and life time and stability parameters of the nano-tips.

COLLABORATIONS

International SEMATECH, Advanced Metrology Advisory 

Group

International Technology Roadmap for Semiconductors; 

Microscopy and Metrology Sections

RECENT PUBLICATIONS

A. E. Vladár, Z. Radi, M. T. Postek and D. C. Joy, “Nano-tip 

Electron Gun for the Scanning Electron Microscope,” 2006 

(in press).

A. E. Vladár, 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.Vladár, 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. Vladár, 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. Vladár, W. J. Keery, 

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

Stage Vibrations,” 2004. SPIE 2004. 

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

of Handling Dimensional Measurement Results for Integrated 

Circuit Technology,” Proc. SPIE 5038, 2003, pp. 508-517.

J. S. Villarrubia, A. E. Vladár, and M. T. Postek, “Test of CDSEM Shape-Sensitive Linewidth Measurement,” Final Report 

to International SEMATECH (2003).

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

and Extending the Limits of SEMs’ Resolution,” Final Report 

to International SEMATECH (November 2003).

M. T. Postek, A. E. Vladár, “New Application of High Pressure/Environmental Microscopy to Semiconductor Inspection 

and Metrology,” SCANNING (2003). 

M. T. Postek, A. E. Vladár, “Is Low Accelerating Voltage Always Required for Semiconductor Inspection and Metrology?”

Microscopy Microanalysis IX pp. 978-979 (2003).
Semiconductor Microelectronics and Nanoelectronics Programs 27

Z. Radi, A. E. Vladár, and M. T. Postek, “Information Transfer 

Capability and Signal Processing Performance of Modern 

Scanning Electron Microscopes,” Microscopes and Microanalysis IX pp. 762-763 (2003).

M. T. Postek, A. E. Vladár, M. H. Bennett, “Photomask 

Dimensional Metrology in the SEM Part I: Has Anything 

Really Changed?” Journal of Microlith., Microfab and Microsystems (2003).

M. T. Postek, A. E. Vladár, M. H. Bennett, T. Rice, and R. 

Knowles, “Photomask Dimensional Metrology in the SEM 

Part II: High Pressure/Environmental SEM,” Journal of 

Microlith., Microfab and Microsystems (2003).

M. T. Postek, and A. Vladár, “E.Application of High Pressure/

Environmental Scanning Electron Microscopy to Photomask 

Dimensional Metrology,” Characterization and Metrology 

for ULSI Technology (D. Seiler et al. eds) AIP Press, New 

York (2003).
28 National Institute of Standards and Technology 
Semiconductor Microelectronics and Nanoelectronics Programs 29

SCATTEROMETRY-BASED DIMENSIONAL METROLOGY

GOALS

Our goals are: (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, 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 lineedge and line-width roughness and material anisotropy, as well as to perform RCW calculations 

on three-dimensional structures. 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 2006

Publication of two-dimensional RCW code in the 

SCATMECH library. 1Q 2007

Publication of three-dimensional RCW code in the 

SCATMECH library. 1Q 2008

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 

•

•

•

Technical Contact:

T. Germer
30 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.

DELIVERABLES:

Perform study of the limits of scatterometry precision for simple gate and gate-resist gratings. 

1Q 2006

Assess effects of line-edge and line-width roughness using random 2-D gratings. 3Q 2006

Perform study of the limits of scatterometry precision for metallic lines and complex sidewall profi les. 

1Q 2007

Develop method for assessing uncertainty arising 

from uncertainty in material optical constants. 

1Q 2007

3. Scatterometry Measurements – We are 

developing a new instrument, based upon the 

currently existing 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 capability will include angle-scanned scatterometry 

at a number of discrete laser wavelengths. This 

instrument will have 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. 

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DELIVERABLES: 

Perform initial scatterometry measurements using 

current instrumentation. 1Q 2006

Complete construction of new scatterometer and 

benchmark its performance. 4Q 2006

Perform measurements on potential reference scatterometry target. 4Q 2006

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.

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•

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#

Figure 1. The Goniometric Optical Scatter 

Instrument (GOSI) is a reference refl ectometer for 

diffuse and specular scattering measurements.
Semiconductor Microelectronics and Nanoelectronics Programs 31

In the area of scatterometry measurement, we 

have demonstrated angle-resolved scatterometry 

using back-focal-plane imaging in a microscope 

(see Fig. 3). By using suitable selection of the illumination angles, we measured the scatterometry 

signatures of targets with only specular refl ectance 

(grating pitch 300 nm) and those with both specular and higher-order diffraction (grating pitch 600 

nm). Targets with line widths of 146 nm to 158 

nm (as measured by SEM) were investigated, and 

the measurement was shown to have nanometerlevel sensitivity to line width. Initial generation of 

RCW libraries and extraction of target parameters 

from the libraries is underway, as is comparison 

of these parameters with measurements made by 

SEM and AFM.

#

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°.

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 and Egon Marx, “Simulations of Optical 

Microscope Images,” in Metrology, Inspection, and Process 

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

6152, in press (2006).

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, in press (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).

Figure 3. Image of the back focal plane of a 

microscope with a tilted slit illumination, showing 

angle-resolved specular (zero order, longer line) and 

diffracted (fi rst order, shorter side lines) effi ciencies.
32 National Institute of Standards and Technology 
Semiconductor Microelectronics and Nanoelectronics Programs 33

SCANNING PROBE MICROSCOPE-BASED

DIMENSIONAL METROLOGY

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 2005 edition, the goal in 2006 for 

critical dimension (CD) measurement precision 

for isolated lines was ± 0.58 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 2005 edition 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 in-house 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 2-dimensional pitch measurements 

using C-AFM as NIST participation in NANO5 – an 

International Supplementary Key Comparison in 

Nanometrology. 1Q 2006

Performed extensive CD-AFM measurements, 

using the SXM320 at NIST, on next generation 

single crystal critical dimension reference material 

(SCCDRM) samples for phase 1 of the process 

optimization experiment in collaboration with EEEL 

and ITL. 2Q 2006

•

•

Technical Contacts:

R. Dixson

G. Orji

T. Vorburger

J. Fu
34 National Institute of Standards and Technology 

Presented two papers on AFM dimensional metrology at SPIE Microlithography meeting – covering 

all three of the project thrusts: the C-AFM, the CDAFM at NIST, and the CD-AFM reference measurement system (RMS) at SEMATECH. 2Q 2006

Using the SXM320 at NIST, perform measurements 

on fi nal phase next generation SCCDRM samples 

as prototypes for a potential SRM. 4Q 2006

Perform additional SXM trial measurements of 

pitch and height on OMAG3L wafer to determine 

the SXM performance as a comparator for support 

the prototype SRM2089 wafer. 4Q 2006

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 

(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:

Implement new SPC monitoring procedure for CDAFM RMS at SEMATECH using secondary sites 

on the SCCDRM L4 chip that serves as system 

monitor. 1Q 2006

Complete a detailed study of higher order tip 

effects in CD-AFM metrology and publish peer 

reviewed journal article on the results. 3Q 2006

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ACCOMPLISHMENTS

The NIST/SEMATECH partnership in advancing AFM metrology in semiconductor manufacturing continues. George Orji is now in the 

second 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 there. George has continued the NIST 

engagement with SEMATECH by supporting their 

SEM 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 to develop linewidth standards. 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 

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 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.
Semiconductor Microelectronics and Nanoelectronics Programs 35

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.

#

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.

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.

of linewidth measurements is currently being 

planned and will be led by NIST.

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 

#

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 

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 (page 36). Currently, we are assessing 

the uncertainties and continuing to develop this 

approach. 

#

#
36 National Institute of Standards and Technology 

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 

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 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.

COLLABORATIONS

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

School of Mechatronic Engineering, Harbin Institute of Technology, Harbin, China.

RECENT PUBLICATIONS

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

“Traceable Atomic Force Microscope Dimensional Metrology 

at NIST,” to be published in Proc. SPIE 5752, (2006). 

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

Implementation of a CD-AFM based Reference Measurement 

System,” to be published in SPIE Proceedings Vol. 6152 

(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).

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.
Semiconductor Microelectronics and Nanoelectronics Programs 37

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,” Proc. SPIE 5375, (2004).

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

generation CD-AFM,” Proc. SPIE 5375, 633 (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,” Proc. SPIE 5375, 363 

(2004).
38 National Institute of Standards and Technology 
Semiconductor Microelectronics and Nanoelectronics Programs 39

SMALL ANGLE X-RAY SCATTERING-BASED

DIMENSIONAL METROLOGY

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 (see Fig. 1) 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. 

DELIVERABLES: Evaluate laboratory scale CD-SAXS 

instrument. 4Q 2006

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.

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 

“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 

Technical Contacts:

R. L. Jones

E. K. Lin

W. L. Wu
40 National Institute of Standards and Technology 

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. 

4. As circuit designs increase in complexity, 

next generation measurement methods must be 

capable of characterizing patterns that deviate 

substantially from line/space patterns. As an example, a capability to precisely characterize dense 

arrays of high aspect ratio vias non-destructively is 

needed. The use of a 2-D collimated beam and a 

2-D detector in the CD-SAXS technique provides 

a natural capability to characterize patterns such 

as vias, posts, and via pads. While the capability 

to observe such structures has previously been 

demonstrated, routine analysis of this class of 

structures has not been achieved. Using a series 

of patterns including vias of diameter less than 60 

nm, we are developing specifi c experimental protocols, extending those developed for line/space 

patterns, to provide a data set capable of describing 

a precise pattern shape in 2-D arrays of patterns.

DELIVERABLES: Provide measurements and model 

describing the measurement of 2-D arrays of 60 nm 

via patterns. 4Q 2006

5. CD-SAXS measurements can be applied 

towards several other important problems for 

both semiconductor and next-generation nanofabrication technologies. CD-SAXS measurements 

are being evaluated as a potential measurement 

method capable of determining the densifi cation of 

the sidewall of plasma etched, patterned nanoporous low-κ dielectric fi lms. Electron microscopy 

has been used, but has been unable to measure the 

densifi cation profi le with suffi cient resolution. In 

addition, CD-SAXS measurements can be used to 

evaluate the fi delity of pattern transfer with the 

nanoimprint lithography (NIL). In NIL, the pattern from a hard, master mold is transferred into an 

underlying organic material through mechanical 

pressure and elevated temperature or curing with 

ultraviolet light. CD-SAXS measurements of both 

the mold and the resulting pattern can be compared 

to evaluate the quality of the NIL structure.

DELIVERABLES: Demonstrate SAXS measurement methods for sidewall damage characterization 

in plasma etched patterned low-κ dielectric materials. 

3Q 2006

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 

#

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.

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: Quantify sidewall roughness originating from standing waves during photoresist imaging 

with CD-SAXS. 3Q 2006

DELIVERABLES: Provide measurement of model 

LER model structures with CD-SAXS. 4Q 2006
Semiconductor Microelectronics and Nanoelectronics Programs 41

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.

Demonstrated a capability to measure correlated fl uctuations in line edge position using 

CD-SAXS measurements of a series of line/space 

patterns in a “193 nm” photoresist produced at 

IBM T. J. Watson Research Center (A. Mahorawala) (see Fig. 3). These samples are believed 

to possess systematic, small shifts in the pattern. 

These shifts provide insight into measurements of 

line edge roughness (LER) where the roughness 

is highly correlated. The resulting data shows 

distinct streaks of intensity emanating from the 

diffraction peaks, particularly strong in the lower 

order peaks. These represent the fi rst evidence 

of a capability of CD-SAXS to measure different 

types of defects related to the overall line edge 

roughness. 

#

a plastic substrate at the IBM Almaden research 

center (M. Sanchez). The result demonstrates 

the capability of CD-SAXS to probe dense vias 

of sub-100 nm size. Ongoing work will attempt 

to provide a protocol for the evaluation of the 

full 3-D via shape as demonstrated for line/space 

patterns.

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.

Demonstrated a capability to measure a dense 

array of vias where the diameter of the vias were 

etched into a plastic substrate are less than 60 nm 

(see Fig. 4). Vias were etched into a produced into 

#

Figure 4. CD-SAXS detector image resulting from a 

dense array of 60 nm vias. The array of diffraction 

patterns suggests an approximately hexagonal 

packing of the vias.

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 

#

#
42 National Institute of Standards and Technology 

(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, Bryan Rice, 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.

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

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,” Nano 

Lett., in press (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).
Semiconductor Microelectronics and Nanoelectronics Programs 43

FABRICATION AND CALIBRATION METROLOGY FOR

SINGLE-CRYSTAL CD REFERENCE MATERIALS

GOALS

The goal of this project is to develop teststructure-based reference materials with emphasis 

on supplying road-map-compliant physical standards for critical-dimension (CD) metrology-tool 

development and calibration and to contribute 

to organizations supporting the development of 

CD metrology standards for the semiconductor 

industry. The specifi c near-term goal is fabricating a quantity of CD reference-features with 

nominal CDs in the range 20 nm to 160 nm and 

having 2σ (expanded uncertainties) of less than 

2 nm by July 2006.

CUSTOMER NEEDS

The Semiconductor Industry Association’s 

International Technology Roadmap for Semiconductors 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 

the quasi-atomically planar sidewall smoothness 

can be further extended to reference-feature segment lengths of up to 2 micrometers. Essential 

elements of the implementation include starting 

silicon SOI wafers 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 micro-machining. However, the diffi culty 

of obtaining satisfactory SOI material in larger 

diameters is driving us towards 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, in this case, 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 (page 44). To maintain 

maximum possible accuracy in the transfer-metrology operation, an elaborate reference-feature 

selection protocol has been established to identify 

Technical Contact:

M. W. Cresswell
44 National Institute of Standards and Technology 

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 dual-beam FIBand-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.

Since 200 mm (110) starting material is 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-toolcompatible at an acceptable cost. The chips that 

were delivered in January 2005 were mounted in 

200 mm carrier wafers for distribution.

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 the end 

user convenience,

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.
Semiconductor Microelectronics and Nanoelectronics Programs 45

• 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. 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.

DELIVERABLES: 

Conclusion and reporting of the screening experiment to identify which combinations of six 

pattern-transfer process factors drive down reference feature CDs and their uncertainties. 4Q 2006

Patterning and metrology of trenches patterned into 

a (110) SOI device layers for their evaluation as 

micro fl uidic channels for MEMS-based bio-applications having otherwise unavailable requirements. 

4Q 2006

Devising a hybrid optical-EB-direct-write lithography-process to pattern a selection of 200 mm bulksilicon (110) wafers with SCCDRM test structures 

with CDs at or below the 20 nm level. 1Q 2007

Applying 193 nm lithography to the patterning of a 

selection of 200 mm bulk-silicon (110) wafers with 

a SCCDRM test structures having referencefeature CDs at or below the 20 nm level. 3Q 2007

Extend measurements, analyses, and evaluation 

of the application of SEM-CD measurements as a 

SCCDRM transfer metrology. 3Q 2006

Fabricate and evaluate a selection of OCD grids 

patterned on SCCDRM wafers as an optical-CD 

reference material. 3Q 2006

Initiate the implementation of a plan to fabricate 

and calibrate a selection of isolated-line CD-SRMs. 

4Q 2006

•

•

•

•

•

•

•

DELIVERABLES: Reduce reference-feature sidewall 

asperities by developing separate processes that 

target sidewall contamination by organic materials and 

by residual moisture. These processes may include 

immersion of the patterned chips in ultra-sonically agitated solvents, extensive pre-cleaning of quartz-ware, 

and vacuum baking. 2Q 2006

ACCOMPLISHMENTS

This project, in collaboration 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. 

An early selection of AFM measurements has now 

identifi ed features having CDs at the 20 nm level. 

The uniformity of individual reference-feature 

CDs exceeds those of the earlier delivery. It is 

thus anticipated that the uncertainties that will be 

ascribed to them at the completion of the experiment in FY2006 will be below the ± 1.5 nm to 

± 3 nm range of prior deliveries in FY2005.

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, for the wafer lots to be run 

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 

project drafted and delivered a comprehensive 

business plan for SEMATECH to use in communicating the scope of this venture to member companies and other potential external clients. 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.

#

#
46 National Institute of Standards and Technology 

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 an application for 

which the reference materials are well suited and 

are becoming widely recognized. However, an 

emerging application of interest in the industry is 

SEM tool calibration. In this application, regular 

reference material cleaning is very 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 worst-case 

conditions was assessed. This work was done to 

deal with the expected questions on the subject 

when the 200 mm wafer based artifacts become 

available.

During the investigations of hydrocarbon 

contamination by SEM tools, and devising the 

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 submitted to an IEEE 

Transactions journal.

The emerging metrology known as optical-CD 

(OCD) scatterometry translates broadband light, 

diffracted from an on-wafer grating patterned 

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 

suitable for this purpose. 

#

#

Recently, the project published a full-length report 

on the fabrication and calibration of a selection of 

SCCDRM Reference Materials entitled “Report of 

Investigation of RM 8111: Single-Crystal Critical 

Dimension Prototype Reference Materials.” Editors of the NIST Journal of Scientifi c Research 

have invited, and have been provided with, a 

version appropriate for publication in that journal 

later this year. Several other papers on special 

aspects of the fabrication and calibration, such as 

test-chip design, AFM metrology, and HRTEM 

imaging, have been presented at the SPIE Spring 

Symposium of February, 2006, and the IEEE 

International Conference on Microelectronic Test 

Structures in March, 2006. A paper on the subject of etch-process optimization for uncertainty 

management has been accepted for presentation 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 written and submitted to 

an IEEE Transactions journal.

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 have 

in operation 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 people 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 

MEL and ITL Laboratories (http://www.mel.

nist.gov/ and http://www.itl.nist.gov/) with both 

of whom we of EEEL share an intramural ATP 

program award to reduce the certifi ed CDs and 

uncertainties of SCCDRMs through fabrication 

refi nements.
Semiconductor Microelectronics and Nanoelectronics Programs 47

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

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.

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. Inpress. 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).

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, “Junction-Isolated 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).
48 National Institute of Standards and Technology 
Semiconductor Microelectronics and Nanoelectronics Programs 49

OPTICAL-BASED PHOTOMASK DIMENSIONAL

METROLOGY

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. 

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. 

Figure 1. Modeling results for linewidth photomask 

samples showing CDs down to 0.25 μm.

Position, μm

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

past decade. Chrome-on-quartz photomasks with 

linewidth and pitch features in the range of 0.5 μm 

to 30 μm have been certifi ed on the previous NIST 

green light optical calibration system. Linewidth 

uncertainties have been reduced to 20 nm, k=2. 

The next generation in this line of standards is 

SRM 2059 (Fig. 2, page 50), printed on a standard 

size 1.4 × 10-4 cm3

 substrate with calibrated linewidths and space-widths ranging from nominally 

0.25 μm to 32 μm, and pitch patterns from 0.5 μm 

to 250 μm.

In response to customers’ needs for more accurate 

photomask feature size measurements, NIST has 

worked extensively to improve mask metrology 

Technical Contacts:

R. Silver

J. Potzick

T. Doiron
50 National Institute of Standards and Technology 

through 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. Modeling 

the effects of all of the relevant feature properties 

in both the mask metrology process and in 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. 

DELIVERABLES:

Compare and improve the accuracy of the NIST 

optical scattering code with new scattering models 

developed by Spectel and Panoramic Technologies 

for use in transmission. 2Q 2006

Complete the second round of calibrations of SRM 

2059 and complete the related documentation. Deliver the accompanying documentation to the Offi ce 

of Standard Reference Materials for distribution to 

customers. 4Q 2005

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.

Figure 2. The new linewidth photomask standard.
Semiconductor Microelectronics and Nanoelectronics Programs 51

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 2006

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 2006

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 2006

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. 

•

•

•

#

NIST currently has completed the uncertainty 

analysis and documentation for SRM 2059 Photomask Linewidth Standard, which will enable 

customers to make traceable measurements of 

the dimensions of features on integrated circuit 

photomasks. The SRM 2059 photomask linewidth 

standard has been delivered and completed and is 

now for sale in the SRM offi ce. This represents the 

only calibrated SRM linewidth standard currently 

available from NIST.

The fi rst set of two-dimensional grid artifacts, 

known as SRM 5001, delivered to the SRM offi ce and is selling well. These 6-inch photomasks 

have been measured on a state-of-the-art I-pro 

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.

#

#

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.

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 

#

#
52 National Institute of Standards and Technology 

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.

COLLABORATIONS

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

vendors.

The 14 members of The Neolithography Consortium.

RECENT PUBLICATIONS

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 53

MODEL-BASED LINEWIDTH METROLOGY

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 at 

the order of 1 nm level 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 $254.7 billion in 

2006 [Reuters, April 7 2006, reporting on iSuppli forecast]. As a measure of its importance 

in that industry, consider that the term “critical 

dimension” or “CD” is used there nearly interchangeably with “linewidth,” and semiconductor 

device generations are known according to the 

characteristic width of the features, as in “the 65 

nm technology node.”

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 3 nm 

and with measurement precision (3 standard deviation repeatability) better than 0.6 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 2005 ITRS 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 

“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).

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 like this: A set of parameters 

for describing edge geometry is chosen. These 

parameters might be, for example, sidewall angle 

and corner radius. For a given set of parameter 

values, the expected image is calculated using 

a Monte Carlo algorithm that simulates electron 

trajectories. This calculation is repeated for other 

choices of edge parameters at discrete intervals 

representative of the range of shapes that one is 

“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).

Technical Contact: 

J. Villarrubia
54 National Institute of Standards and Technology 

likely to encounter in a measurement. 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). The corresponding line 

shape is assigned to the unknown. In practice 

there may be more than two parameters, and the 

library may be interpolated.

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.

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. 

In 2006, 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 would 

permit simulation of other industrially important 

sample shapes, such as rough-edged lines, contact 

holes, and line footing. We are also encouraging 

further adoption of this method by CD-SEM 

suppliers by further publication of the method, 

by making modeling software freely available 

to CD-SEM manufacturers, and by collaboration 

with their engineers.

DELIVERABLES:

Make a detailed plan for how to handle 3-dimensional geometrical descriptions for SEM modeling. Write software that implements and test the 

description. 3Q 2006

•

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.
Semiconductor Microelectronics and Nanoelectronics Programs 55

Write a software module that implements scattering 

physics. Merge scattering code and geometrical 

code into a simulation code for 3-dimensional 

samples. 4Q 2006

With regard to LWR measurement issues, last 

year we published a means of estimating LWR 

that measures and corrects for the false noise 

component of roughness. The result is an unbiased estimate of LWR (Fig. 3). However, in the 

limit when actual roughness is comparable to or 

smaller than the noise roughness term, random 

measurement errors can cause the correction term 

to sometimes be over-estimated, with the result 

that the estimated R2 is negative. We are currently 

investigating Bayesian roughness estimators that 

do not have this undesirable property.

DELIVERABLES: Derive a Bayesian probability distribution function for linewidth roughness variance given 

a set of measurements. Use the function to determine 

reasonable estimates of LWR and uncertainty. Submit 

the results to an appropriate journal or conference 

proceedings. 3Q 2005

•

Figure 3. Measured LWR vs. pixel integration 

time for two different LWR metrics. Lower pixel 

integration time corresponds to higher noise in the 

image. Vertical bars are ±1 standard deviation of 

the observed repeatability. These were repeated 

measurements at the same location—therefore all 

measured values ought to be the same and any 

observed dependence of LWR upon pixel integration 

time must be a measurement artifact. 

ACCOMPLISHMENTS

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. 

#

We showed that NIST’s model-based library 

metrology method is less sensitive to SEM misfocus than standard methods. There nevertheless 

was some measurable sensitivity, which we attributed to breakdown of our beam shape model 

at large defocus. This indicates a need for future 

model software to include more accurate beam 

shape information.

We were awarded a Dept. of Commerce Silver 

Medal and a Nano50 2005 Technology Award, for 

the model-based library technique, one of the “top 

50 innovators, techniques, and products impacting the commercialization of nanotechnology to 

mainstream markets.”

We completed work on an unbiased linewidth 

roughness estimator and published the results in 

Proceedings of the SPIE Microlithography conference. This publication was awarded the Diana 

Nyyssonen “Metrology Best Paper of 2005” award 

by conference organizers.

COLLABORATIONS

International SEMATECH, Benjamin Bunday, Michael 

Bishop.

Hitachi, Ltd., Maki Tanaka.

RECENT PUBLICATIONS

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, “Issues in Line Edge and Linewidth Roughness Metrology,” Characterization and metrology for ULSI 

Technology: 2005, AIP Conf. Proc. Vol. 788, edited by D. G. 

Seiler, A. C. Diebold, R. McDonald, C. R. Ayre, R. P. Khosla, 

S. Zollner, and E. M. Secula, p. 386 (AIP Press, New York, 

2005).

J. S. Villarrubia, A. E. Vladár, B. D. Bunday, and M. Bishop, 

“Dimensional Metrology of Resist Lines using a SEM 

Model-Based Library Approach,” Proc. SPIE 5375, 199-209 

(2004).

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). 

#

#

#
56 National Institute of Standards and Technology 
Semiconductor Microelectronics and Nanoelectronics Programs 57

ATOM-BASED DIMENSIONAL METROLOGY

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 nanometerscale 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 

(Fig. 2, page 58). 

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.

Technical Contact:

R. Silver
58 National Institute of Standards and Technology 

Figure 3. A demonstration of the nanofabrication 

process with an RIE process used to transfer the 

patterns into the silicon substrate.

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 

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 10 nm. Apply the pattern writing 

process and etch features for use as optical scatterfi eld test structures using the new STM. 

3Q 2006

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 2006

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 

•

•

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.
Semiconductor Microelectronics and Nanoelectronics Programs 59

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: Investigate alternative W tip 

crystalline structures and high temperature preparation methods for use in high resolution imaging. 

Determine the stability of these W tips for use as STM 

imaging and fabrication tips. 3Q 2006

3. Focus on the development of artifacts that can 

be atom counted and then measured in a number of 

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 reducing the process temperatures required 

for atomic reconstructions. We have made wet 

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

.

chemical processing fully operational and have 

also demonstrated 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: A new STM which has improved 

atomic scale imaging capability for atom-based 

dimensional metrology is now operational. We are 

implementing a UHV sample and tip heating and 

preparation capability which interfaces 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 2006

4. Develop and publish on the objective for 

in-situ stabilized, atomically ordered surfaces that 

can be transferred to other measurement instruments such as scanning electron microscopes, 

AFMs, or optical metrology tools. These nanometer-scale standard artifacts with atomically 

ordered surfaces will then act as linewidth or 

magnifi cation calibration samples. These samples 

require extensive surface preparation and modeling to achieve repeatable Si surfaces for metrology 

and fabrication purposes.

ACCOMPLISHMENTS

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 should be enhanced 

with the new STM currently being implemented. 

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 ability to prepare atomically sharp tips in W 

(111) has been demonstrated. The details of this 

methodology and the new models we have developed for analyzing sharpness have been published 

as an archived journal article. We are now implementing a robust and repeatable tip processing 

capability in both UHV STM systems. 

Develop the advanced modeling requirements 

to fully simulate the new STM structure. We have 

completed fi nite element analysis (FEA) simulations which test the dynamic modes of the STM 

and interferometer structure. These results have 

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60 National Institute of Standards and Technology 

signifi cantly improved the structural designs. 

Publications in the area of STM structural modeling and STM structural damping for improved 

dynamics have been instrumental in recent tool 

improvements and in enhancing our collaboration 

with the George Washington University.

The results, seen in Fig. 4, yield a new, more 

comprehensive understanding of the physical 

processes involved in making atomically flat 

surfaces in silicon as required for much of the 

work in this project.

The 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 underway.

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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We have measured directly the surface atom 

spacings based on an interferometer measurement. We have fi tted our UHV STM with a high 

accuracy sub angstrom resolution interferometer. 

We have closed the loop and made atomic resolution measurements with full interferometer length 

basis. The successful completion of this aspect 

has enabled direct distance determination with 

simple atomic counting.

The results for the first demonstration of 

direct interferometer measurements of surface 

atom spacings with a complete unbroken uncertainty have been published in Optical Engineering. 

These results were also presented at the ASPE conference and SPIE to make the new interferometry 

methods available to the industry. 

COLLABORATIONS

ISMT, IBM, University of Maryland, Dept. of Physics, 

University of Purdue, Dept. of Physics., George Washington 

University. 

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,” Submitted 

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.

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).

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#

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.
Semiconductor Microelectronics and Nanoelectronics Programs 61

WAFER-LEVEL AND OVERLAY METROLOGY

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 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 right. 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.

Technical Contacts:

R. Silver

T. Doiron
62 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 measureFigure 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.

ment 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 
Semiconductor Microelectronics and Nanoelectronics Programs 63

sized features. A variation of these targets allows 

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:

Publish an improved set of alignment and characterization tests for the overlay microscope, optics, 

and the x-y metrology stage. Use these new 

qualifi cation techniques in the fi nal uncertainties 

and tabulated combined uncertainty values for the 

latest set of SRM calibrations. This includes the 

characterization and calibration of complex optical 

alignment effects on overlay output values. 

2Q 2006

Use the OMAG 4 metrology reticles from the 

SEMATECH collaboration to fabricate a new set 

of overlay calibration targets/wafers. Work with 

SEMATECH and the Overlay Metrology Advisory 

•

•

Figure 4. New proposed overlay targets which 

occupy less than 2 μm x 2 μm in total space. This is 

designed to be an in chip target.

Group (OMAG) to develop and publish a new set of 

calibration techniques verifi ed for calibration of industry tools. This includes measurement protocols 

for evaluating wafer induced shift (WIS) and asymmetry components from the wafer. 2Q 2006

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 2006

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 in refl ection mode 

and transmission mode. Develop a standard set 

of simulation values and make them available for 

comparison and verifi cation by industry users. 

3Q 2006

•

•

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.
64 National Institute of Standards and Technology 

Develop new calculation methods for analyzing 

asymmetric overlay targets. Compare with modeling results of more standard double-etched silicon 

structures. 1Q 2006

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 50 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 are showing promise as 

potential solutions to these signifi cant metrology 

challenges. As a part of this project, we have developed 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 set of techniques to 

accurately measure optical illumination fi elds and 

publish results. Develop similar characterization 

methods to measure the collection optics set of 

aberrations and to perform optimum optical alignment. 2Q 2006

Develop fully automated focus and positioning control systems for the new scatterfi eld microscope. 

Complete the optical component design and layout 

for the new tool and assemble the illumination 

optics in an open architecture confi guration. Test 

the new optical tool to verify performance using 

scatterfi eld optical techniques. 4Q 2006

ACCOMPLISHMENTS

Researchers from the overlay metrology 

project have submitted a CD 240 for the disclo-

•

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#

sure of recent potential breakthrough material in 

high resolution optical metrology. As a part of 

this disclosure, a new method was proposed and 

subsequently verifi ed in the laboratory. This new 

approach is capable of measuring features at least 

as small as 20 nm in dimension with densities to 

one to one lone/space ratio.

NIST electromagnetic scattering code: Further 

successful applications of the NIST electromagnetic scattering code developed by E. Marx and 

two other industry codes have resulted in detailed 

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 optics successful: 

The optics project which was successful as champion for the Scatterfi eld competence proposal has 

proved to be very productive. The cross laboratory 

proposal has produced several recent results which 

were published and presented at the recent SPIE 

Microlithography meeting. The collaboration between the Manufacturing Engineering (MEL) and 

Physics (PL) laboratories has published results this 

year on the evaluation and results of high resolution optical methods involving scatterometry and 

the new scatterfi eld methods being developed 

within Precision Engineering Division (PED). 

The work also involves technical evaluation of 

the key issues associated with optical modeling 

and the n and k input parameters.

Signifi cant industry interest in new highresolution optical methods being developed under 

the Scatterfi eld Microscopy title. Following invited presentations at SEMATECH OMAG and 

Advanced Metrology Advisory Group (AMAG) 

metrology workshops, and discussions at SPIE, 

there was much interest in the surprising results 

of 40 nm sized features. These results obtained by 

optical methods are some of the smallest features 

shown to these audiences and go well beyond the 

expected capabilities of these tools. 

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 mate-

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Semiconductor Microelectronics and Nanoelectronics Programs 65

rial 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.

New scatterfield optical tool operational. 

The new scatterfi eld metrology tool has been assembled 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. 

Refl ection mode optical measurements of 

phase shifting photomasks. The fi rst round of 

measurements and model comparisons between 

profi les from phase shifting reticles has been 

completed. These results obtained in refl ection 

mode have compared experimental measurements 

with modeling results obtained using the Egon 

Marx electromagnetic scattering code. These 

results were documented and summarized in a 

SEMATECH fi nal report.

The NIST Overlay Metrology project leader 

has played a signifi cant role in the (OMAG) of 

SEMATECH. This group is developing a comprehensive set of measurement guidelines, test 

methods, and tool performance measures to be 

adopted by the semiconductor manufacturing 

industry. The group is made up of more than 15 

international semiconductor manufacturers and 

tool suppliers. The OMAG has strong interest in 

adopting several new methods developed in the 

NIST Overlay Metrology Project, in particular, 

the recently published methods for evaluating 

in-chip and device sized overlay targets. In 

addition, CCD array performance and overall 

optical system characterization and calibration 

performance measures developed at NIST have 

been adopted. 

Members of the optical metrology project 

met with several leading optical metrology tool 

manufacturers and international measurement 

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laboratories 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.

COLLABORATIONS

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

vendors.

The 14 members of The Neolithography Consortium.

RECENT PUBLICATIONS

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).

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

M. Davidson, T. Germer, and R. Larrabee, “High-resolution 

Optical Metrology,” SPIE Vol. 5752, (2005).

R. Attota, R. M. Silver, T. Germer, M. Bishop, R. Larrabee, 

M. Stocker, L. Howard, “Application of Through-focus Focus 

metric Analysis in High-resolution Optical Metrology,” SPIE 

Vol. 5752 (2005).
66 National Institute of Standards and Technology 

R. M. Silver, R. Attota, M. Stocker, M. Bishop, J. Jun, E. Marx, 

M. Davidson, and R. Larrabee, “High-resolution Optical 

Overlay Metrology,” SPIE Vol. 5375 P. 78 (2004).

R. Attota, R. M. Silver, M. Bishop, E. Marx, J. Jun, M. Stocker, 

M. Davidson, R. Larrabee, “Evaluation of new in-chip and 

arrayed line overlay target designs,” SPIE Vol. 5375 P. 395 

(2004).

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

SPIE Vol. 5375 P. 317 (2004).
Semiconductor Microelectronics and Nanoelectronics Programs 67

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 gate dielectrics, 

traditionally SiO2

 and SiON, will soon no longer be viable. 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.
68 National Institute of Standards and Technology 
Semiconductor Microelectronics and Nanoelectronics Programs 69

WAFER AND CHUCK FLATNESS METROLOGY

Technical Contacts:

U. Griesmann 

R. Polvani

Figure 1a. NIST’s infrared interferometer (IR2

). 

The main components of the interferometer are: 

collimator lens (CL), polarizing beam splitter (BS), 

phase-shifting reference mirror mount (RM), diverger 

lens (DL), zoom lens system (ZL), and camera (CA).

Figure 1b. Test end of the IR2 setup for TTV 

measurements of 300 mm wafers. The picture shows 

the collimator lens together with a silicon wafer and 

a return fl at.

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

Decreasing line-widths, the accompanying reduction in depth of focus, larger wafer diameters 

for current stepper lithography applications, and 

the advent of immersion lithography place ever 

increasing restrictions on wafer fl atness and the 

required measurement uncertainty. The International Technology Roadmap for Semiconductors 

(ITRS) 2005 edition projects a site fl atness of ≤ 45 

nm in 2010 for DRAM contacted and MPU/ASIC 

Metal 1 1⁄2 pitch of 45 nm, Front End Processes 

Section, Table 67a, page 7. We are focused on 

meeting customer requirements for calibrated 

thickness variation maps of free form wafers and 

fl atness measurements of chucked wafers. We are 

addressing the need for standard 300 mm wafers 

with calibrated thickness variation with Improved 

Infrared Interferometer (IR3

) at NIST. These 

independent, traceable wafer thickness calibrations 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). XCALIBIR has a 300 mm aperture for fl at 

measurements and provides a way of verifying 

models of wafer/chuck interactions.

TECHNICAL STRATEGY

1. The IR3 interferometer is an infrared phaseshifting interferometer, operating at a wavelength 

of 1550 nm which measures the thickness of lowdoped silicon wafers up to 300 mm diameter in 

a single measurement (see Fig. 1a and Fig. 1b). 

The interferometer may be used in several confi gurations with collimated and spherical test 

wave-fronts. The collimated wave-front 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 wave-fronts 

produces fringes and, by wavelength phase shifting, allows calculation of the wafer thickness 

variation. Figure 2 (page 70) shows the thickness 

variation map for a 300 mm silicon wafer. 

DELIVERABLES: 

Install interferometer in clean-room for calibration 

measurements, 4Q 2005. 

Develop complete uncertainty analysis for thickness and thickness variation measurements. 

Ongoing

Install laser with wide tuning range to enable measurements of very thin wafers. Ongoing

•

•

•

“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-chuck 

interaction studies are 

enabling advances in 

the state-of-the-art of 

wafer chucking that are 

essential to fully realize 

the potential of short 

wavelength lithography. 

WFSI’s ability to 

collaborate with NIST in 

this area is critical to us.”

T. D. Raymond

Wavefront Sciences, Inc., 

Albuquerque, NM
70 National Institute of Standards and Technology 

Figure 3. XCALIBIR interferometer confi gured for 

fl atness measurements. The main components of the 

interferometer, from left to right, are: breadboard 

with source and imaging optics, diverger lens 

(DL), collimator lens (CL), reference fl at (RF), 

and test mount (TM ). For silicon wafer fl atness 

measurements a wafer chuck is mounted on the test 

mount.

Figure 2. Thickness variation, with piston term 

subtracted, of a 300 mm double-side polished silicon 

wafer measured with IR2

. The height unit is nm, the 

coordinates in x- and y-direction are detector pixel 

numbers. 

2. A component in the evaluation of chucked 

wafer non-fl atness is the characterization of interactions between the vacuum chuck and wafer. 

We are collaborating with Wavefront Sciences, 

Inc. (WFSI), Albuquerque, New Mexico and potentially one or more lithographic stepper manufacturers to help understand these interactions. 

WFSI is carrying out numerical analyses of the 

chuck/wafer interface for various chuck geometries. Figure 3 shows a model of XCALIBIR, a 

general purpose 300 mm aperture phase measuring interferometer developed at NIST, which is 

used to measure the fl atness of chucked wafers. 

These can then be 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 being moved to a 

clean-room which will allow us to make calibration measurements of wafers supplied by industry 

customers.

The fl atness of 200 mm and 300 mm diameter 

wafers in the chucked condition was explored using the XCALIBIR interferometer.

COLLABORATIONS

During the course of this project, we have worked 

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 metrology tool validation.

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 

wafers.

RECENT PUBLICATIONS

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).

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Semiconductor Microelectronics and Nanoelectronics Programs 71

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).
72 National Institute of Standards and Technology 
Semiconductor Microelectronics and Nanoelectronics Programs 73

MODELING, MEASUREMENTS, AND STANDARDS FOR

WAFER SURFACE INSPECTION

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 currently 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. In order 

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 %. It 

is anticipated that accurate calibration particles as 

small as 30 nm will be needed soon.

By 2010, at the 45 nm node, particles having diameters 22.5 nm must be detectable on bare silicon 

and nonmetallic fi lms, with 36 nm on metallic 

fi lms. No known solutions exist at this time. [2005 

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 

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.
74 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) (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.

Figure 1. The Goniometric Optical Scatter 

Instrument is a state-of-the-art laser scattering 

facility.

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 2006

3. Size Distribution Measurements – Differential mobility analysis (DMA) has been shown to 

be capable of making accurate size measurements 

for mean particle size below 100 nm monosize 

polystyrene spheres. There are promising results 

for the measurement of the size distribution for 

broader size distributions; however, the results are 

not quantitative. Work is in progress to quantify 

the uncertainty in the size distribution measurement and to extend the method to smaller particle 

sizes.

DELIVERABLES: Deliver a 100 nm particle size 

SRM to replace SRM1963 and a 60 nm particle size 

SRM. 2Q 2006

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. 3Q 2006

ACCOMPLISHMENTS

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.

DELIVERABLES: Perform measurements of scatter # from various fi lms to assess the particle detection 

limits on those fi lms. 3Q 2006

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 
Semiconductor Microelectronics and Nanoelectronics Programs 75

Developed a NIST Calibration Facility for 

sizing monodisperse spheres suspended in water 

in the size range of 50 nm to 400 nm with an 

expanded uncertainty of 1.5 % of the peak size. 

This facility has been used for two customers 

providing calibration particles to the semiconductor community.

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 

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 3300 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.

#

#

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 %.

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 

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.

#
76 National Institute of Standards and Technology 

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

Thomas A. Germer, “Measuring Interfacial Roughness by Polarized Optical Scattering,” in Light Scattering and Nanoscale 

Light Scattering, Alexei A. Maradudin, Ed., (Elsevier, New 

York, 2006) in press.

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).

Thomas A. Germer and Michael J. Fasolka, “Characterizing 

surface roughness of thin fi lms by polarized light scattering,” 

in Advanced Characterization Techniques for Optics, Semiconductors, and Nanotechnologies, A. Duparré and B. Singh, 

Eds., Proc. SPIE 5188, 264–275 (2003).

Jung Hyeun Kim, Thomas A. Germer, Valeri I. Babushok, 

George W. Mulholland, and Sheryl H. Ehrman, “Co-Solvent 

Assisted Spray Pyrolysis for the Generation of Metal Particles,” J. Materials Research, 18(7), 1614–1622 (2003).

Thomas A. Germer, “Scattering by slightly non-spherical 

particles on surfaces,” in Seventh Conference on Electromagnetic and Light Scattering by Nonspherical Particles: Theory, 

Measurements, and Applications, T. Wriedt, Ed. (Universität 

Bremen, Bremen, 2003), pp. 93–96.
Semiconductor Microelectronics and Nanoelectronics Programs 77

FRONT-END MATERIALS CHARACTERIZATION

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 2004 

Update to the 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 profiling, this project defines optimum 

procedures for ultra-high depth resolution by 

Secondary Ion Mass Spectrometry (SIMS), 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 structures of high-κ combinatorial 

metal electrode stacks systems by implementing 

a combination of techniques including Scanning 

Kelvin Probe Microscopy (SKPM), internal photoemission (IPE), 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) 

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. 

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 

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 at-line 

2-D dopant profi le concentration measurements 

Technical Contacts:

G. Gillen: SIMS

D. Simons: SIMS

J. Small: TEM, 

 X-ray detectors 

J. Suehle: Electrical 

 Characterization

N. Nguyen: IPE, SE
78 National Institute of Standards and Technology 

with spatial resolution of 3.5 nm and precision 

of 4 % in 2005, increasing to spatial resolution 

of 2.8 nm or less and 2 % precision for the 2010 

through 2018 timeframe. Complete specifi cations 

are given for the short term in Table 120a on pages 

21–22, and for the long term in Table 120b on 

pages 23–24 of the 2005 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 2005 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 4, 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. 3Q 2006 

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: Produce prototype trace metal 

standards on silicon by ink-jet printing and evaluate by 

SIMS measurements. 3Q 2006 

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 energy 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 the depth of penetration 

of the primary ion. This process may potentially 
Semiconductor Microelectronics and Nanoelectronics Programs 79

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. 

3Q 2006

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 Si structure to determine band offsets. 

1Q 2006

Combinatorial “libraries” of candidate metal gate 

electrode fi lms (on high-κ layers supplied by 

SEMATECH or member companies) are fi lms 

in which the composition has been intentionally 

graded in the plane of the fi lm, thus allowing 

one to explore many fi lm compositions simultaneously. The combinatorial libraries will be 

subjected to a variety of thermal treatments to 

understand the stability of the interface. Using 

these libraries, we will be able to screen a wide 

variety of candidate metal gate electrodes. Many 

metals possess the appropriate work functions for 

use as NMOS (φm ~ 4 eV: Al, Ta, Mo, Zr, Hf, V, 

Ti) or PMOS (φm ~ 5 eV: Co, Pd, Ni, Re, Ir, Ru, 

Pt) metal gate electrodes. However, in their pure 

form, most suffer from other problems such as 

reactivity and/or adhesion issues with the high-κ

gate dielectric. Binary or ternary alloys of these 

metals might be more suitable. Therefore we will 

investigate combinatorial libraries of alloys in 

the systems, for example, Ru-Hf, Ru-Ta, Ru-Mo, 

Pt-Al, Pt-Ni, Ta-Ru-Pt, W-Re-Pt, Al-Ta-Ru, and 

Co-Ni-Ti. Further, oxides, nitrides, oxynitrides 

and silicides of these metals and alloys (e.g., 

RuO2

, ReO2, HfN, TiNx, TaNx, Ta-Si-N) might also 

possess the proper work function and interfacial 

stability with the high-κ gate dielectric, and they 

will similarly be investigated. These alloys will 

be fabricated in blanket form which for physical 

characterization such as internal photo emission 

(IPE), spectroscopic ellipsometry, as well as with 

shadow masks to form capacitors for electrical 

characterization. 

DELIVERABLES: Determine Optical constants for 

Ti-Ni-Pt ternary system using vacuum ultraviolet spectroscopic ellipsometry. 3Q 2006

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 C602+ 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 

#
80 National Institute of Standards and Technology 

energy of the C60+. As shown in Fig. 1, we have 

found that at impact energies below ~12 keV, 

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 impact energies of 

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 SIMS depth profile Poly(methyl 

methacrylate) (PMMA) photo resist at a series of 

temperatures from -75 °C to 125 °C where the primary glass transition for PMMA occurs at 105 °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. It was found that 

at low temperatures, the secondary ion stability 

increased considerably. In addition, the interfacial widths were signifi cantly lower. Examples 

of this increased stability at low temperatures 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 (page 81). 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 results of this study demonstrate that 

it is possible to monitor the chemical composition 

of thin photoresist layers on silicon by analyzing 

the samples at cryogenic temperatures. 

Figure 2. Positive ion intensities of characteristic 

fi ngerprint ion of PMMA m/z = 69 plotted as a 

function of depth for a PMMA fi lm on Si (≈160 nm) 

measured at varying temperatures (at and below 

room temperature).
Semiconductor Microelectronics and Nanoelectronics Programs 81

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.

SI-GE COMPOSITION STANDARDS

A single-crystal boule with a nominal formula of Si0.86Ge0.14 was determined by wavelength dispersive electron probe microanalysis 

(WD-EPMA) to have a micro-scale expanded 

uncertainty no greater that 1.5 % relative mass 

fraction for Ge and considerably less for Si. 

Bulk specimens of this material were analyzed 

with instrumental neutron activation analysis 

(INAA) and inductively coupled optical emission spectroscopy (ICP-OES). The consensus 

mean of the two analyses using a bound on bias 

statistical method was 30.228 % mass fraction 

with a relative expanded uncertainty of 0.41 %. 

The quantifi cation and heterogeneity uncertainties combined in quadrature resulted in a relative 

microanalysis uncertainty of 1.56 %. WD-EPMA 

quantifi cation of the boule using pure Si and Ge 

wafer standards resulted in an average value of 

30.36 % mass fraction Ge from determinations 

at 15, 20, and 25 kV with a maximum expanded 

uncertainty of 1.44 % relative. This specimen can 

now be used as a reliable reference standard for 

EMPA quantifi cation of Six

Ge1-x fi lms on Si that 

are needed by industry as reference standards for 

their Si and Ge compositions.

Two fi lms of Six

Ge1-x on Si were prepared by the 

ASM America using chemical vapor deposition. 

The nominal compositions were Si0.90Ge0.10 and 

Si0.75Ge0.25 with approximate thicknesses of 4 μm 

#

and 5 μm, respectively. The purpose was to characterize the fi lms for the extent of heterogeneity 

and composition and to distribute them to industry 

as reference composition standards. The extent 

of heterogeneity was determined with micrometer 

step traverses and with multiple point, multiple 

sample, and duplicate data acquisitions using WDEPMA. The overall relative expanded heterogeneity uncertainty, including the between-specimens, 

between-points, and experimental components, 

was 1.04 % and 0.94 % for Ge in Si0.90Ge0.10 and 

Si0.75Ge0.25 respectively. Quantitative analysis of 

the two fi lms with WD-EPMA at 20 kV, using 

the Si0.86Ge0.14 boule cited above as the standard, 

resulted in Ge concentrations of 22.72 % mass 

fraction ± 1.23 % for Si0.90Ge0.10 and 43.53 % mass 

fraction ± 1.06 % for Si0.75Ge0.25. These relative 

expanded uncertainties include the heterogeneity component. The overall uncertainty for the 

analysis, combining in quadrature the uncertainty 

observed for each fi lm with that determined for the 

boule results in a relative expanded uncertainty of 

1.98 % for the Si0.90Ge0.10 fi lm and 1.89 % for the 

Si0.75Ge0.25 fi lm. Five samples of each fi lm will be 

packaged and made available to industry as NIST 

Research Materials before the end of FY 2006.

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 measurement 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 

#
82 National Institute of Standards and Technology 

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).

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 wavelength-dispersive 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 (page 83).

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.
Semiconductor Microelectronics and Nanoelectronics Programs 83

METHODS FOR DEPTH-SENSITIVE

IMAGING BY SCANNING TRANSMISSION

ELECTRON MICROSCOPY

As the use of data acquisition hardware 

capable of acquiring spectrum images becomes 

widespread, many analysts are employing pixel 

classifi cation techniques to process their microanalysis datasets. Before the advent of spectrum 

imaging, analysts classifi ed structural features in 

their samples using elemental X-ray maps. Today, 

many commercial microanalysis packages come 

equipped with some form of classifi cation procedure for assigning pixels or larger spatial features 

to one of a small number of categories or classes. 

These class labels are presented to the analyst to 

provide insight into the chemical heterogeneity of 

the sample, or sometimes even its spatial distribution of “phases.” 

This proliferation of classifi cation techniques 

within the microanalysis community has led to 

the need for accurate measures of performance 

of the various classifi cation algorithms. Different 

pixel classifi cation methods vary in their accuracy, 

precision, and specifi city; they exhibit different 

strengths and weaknesses depending on the situation and what information is to be extracted from 

the data. The methods also exhibit different levels 

of sensitivity to problems in the data such as the 

presence of noise or badly overlapped spectral 

peaks. Thus, the goal of post-classification 

performance metrics is to illuminate how these 

methods meet the required analytical challenges 

and how gracefully their performance degrades 

when applied to poor or unusual data.

The best measures of performance for classifi ers 

of all types are based on comparison of their 

#

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.

predicted class assignments with known class 

membership. With real samples this is nearly 

impossible since the spectrum image is often 

the only source of phase information available 

at the relevant length scales. The calculation of 

synthetic spectrum images from computer models 

(Fig. 6a-e) obviates this problem, since both the 

composition and microstructural geometry are 

precisely known for the phantom samples. This 

allows the creation of a “ground truth” image revealing the true class membership of each pixel. 

Using this ground truth image as a key, the performance of any given classifi er can be assessed, 

including measurement of error rates (both errors 

of commission and omission). When the classifi er 

returns the same number of classes as the ground 

truth (Fig. 6f), all errors can be tabulated in a 

confusion matrix; when the number of classes 

is variable (Fig. 6g) error assessment is not as 

straightforward. Classifi ers with thresholds as free 

parameters (such as Bayes classifi ers) benefi t from 

Figure 6. (a) ground truth image showing 

arrangement of the four Raney Nickel phases 

simulated by 3-D Monte Carlo; a 64 x 64 pixel 

spectrum image (SI) was generated with an XEDS 

spectrum at each pixel (b) Al K X-ray map derived 

from the SI (c) Ni K X-ray map (d) Fe K X-ray map 

(e) RGB overlay of the three elemental maps with 

red=Al, green=Fe, and blue=Ni (f) unsupervised 

pixel classifi cation result using iterative K means 

and four pixel categories to classify the fi rst 20 

principal components of the SI; the poor quality 

of the classifi cation is deliberate to illustrate the 

utility of post-classifi cation performance metrics 

(g) unsupervised pixel classifi cation result using 

the ISODATA iterative algorithm to classify the fi rst 

20 principal components; the number of classes is 

a free parameter in this algorithm, complicating 

measurement of classifi cation accuracy.
84 National Institute of Standards and Technology 

analysis of their receiver-operator characteristic 

(ROC) curves, which reveals the relationship of 

false negative vs. false positive classifi cation error 

rates as a function of the cut-off threshold.

CHARACTERIZATION OF THE HIGH-κ

METAL GATE BARRIER HEIGHT USING

INTERNAL PHOTO EMISSION (IPE)

Metal gates have been intensively searched 

in the past few years to replace the traditional 

polysilicon for the next generations of MOSFET 

devices. One of the most important parameters 

used to select an appropriate metal is the barrier height (Ф0

) at the metal gate and the gate 

dielectric. Two important ternary metals: TaSiN 

and TaCN on either SiO2 and HfO2 were studied. 

Using internal photoemission, we were able to 

determine the barrier heights between these metals and dielectrics. As a result, Ф0

 = 3.36 eV and 

3.55 eV for TaSiN/SiO2 and TaCN/SiO2 interfaces, respectively. Figure 7 shows a plot of barrier 

height vs the square root of electric fi eld for both 

metal on the HfO2 gate dielectric. It is observed 

that Ф0 = 2.24 eV and 2.47eV for TaSiN/HfO2

and TaCN/ 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. 

#

Figure 7. Plot of barrier height vs the square 

root of electric fi eld obtained from internal photo 

emission experiments. The barrier heights differ by 

approximately 0.2 eV.

FILM THICKNESS DEPENDENCE OF SUBBANDGAP DEFECT STATES OBSERVED IN

POLYCRYSTALLINE HAFNIUM OXIDE

In collaboration with IBM we compare 

hafnium-based high-κ dielectric films 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 phase present. Fig. 8 shows that sub-bandgap absorption is 

observed in polycrystalline, but not in amorphous, 

fi lms. We note that sub-bandgap states may be the 

underlying cause for gate leakage via FrenkelPoole 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. 

Figure 8. The imaginary part of the dielectric 

function, determined from VUV-SE data, for 5, 

10, 20, 40 nm thick ALD HfO2 fi lms. All MOCVD 

HfO2 spectra (not shown) are similar to that of the 

polycrystalline 40 nm ALD HfO2 sample. All MOCVD 

HfSiO spectra (not shown) are similar to that of the 

amorphous 5 nm ALD HfO2

 sample.

COLLABORATIONS

International SEMATECH, Joe Bennett – Thin oxide depthprofi 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
Semiconductor Microelectronics and Nanoelectronics Programs 85

Peabody Scientifi c – Ion source development for Semiconductor SIMS

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 et al., “Round Robin Study of Arsenic Implant 

Dose Measurement by SIMS,” Appl. Surf. Sci. (2006), in 

press.

G. Gillen et al., “Depth Profi ling using C60

+ SIMS- Deposition and Topography Development During Bombardment of 

Silicon,” Appl. Surf. Sci. (2006), in press.

G. Gillen, C. Mahoney, M. Wagner and A. Fahey, “3D Molecular Imaging SIMS,” Appl. Surf. Sci. (2006), in press.

A. Fahey, C. Mahoney and J. Hudgens, “Applications of the 

Hadamard Transform to Time of Flight SIMS,” Appl. Surf. 

Sci. (2006), in press.

A. Fahey, G. Gillen, P. Chi and C. Mahoney, “Performance 

of a C60 Ion Source on a Dynamic SIMS Instrument,” Appl. 

Surf. Sci. (2006), in press.

C. M. Mahoney, A. J. Fahey and G. Gillen, “TemperatureControlled Depth Profi ling in Polymeric Materials using 

Cluster Secondary Ion Mass Spectrometry (SIMS),” Appl. 

Surf. Sci. (2006), in press.

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. 

(2006), submitted for review.

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. (2006), submitted for review.

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.

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).
86 National Institute of Standards and Technology 
Semiconductor Microelectronics and Nanoelectronics Programs 87

INTERCONNECT AND PACKAGING METROLOGY

PROGRAM

Advances in interconnect and packaging technologies have introduced rapid successions 

of new materials and processes. Environmental pressures are leading to the reduction and 

eventual 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 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.
88 National Institute of Standards and Technology 
Semiconductor Microelectronics and Nanoelectronics Programs 89

ATOMIC LAYER DEPOSITION – PROCESS MODELS

AND METROLOGY

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, copper seed 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 identifi ed in the 2005 

International Technology Roadmap for Semiconductors (ITRS) as “one of the few enabling 

methodologies that can reduce development 

cycle times and costs.” [2005 ITRS, Modeling 

and Simulation, page 1] A TCAD topical area 

identifi ed in the 2005 ITRS 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.” [2005 ITRS, 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” [2005 ITRS, 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. [2005 ITRS, 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.” [2005 ITRS, 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 ultra-thin fi lms.” 

[2005 ITRS, 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 associated 

in-situ metrologies for ALD processes.

TECHNICAL STRATEGY

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 

custom-built, research-grade ALD reactors with 

optimized optical accessibility and benchmarking the numerical results with experimental data. 

These data are obtained using various measurement techniques, including vibrational spectroscopies and mass spectrometry. HfO2

 ALD using 

tetrakis(ethylmethylamino) hafnium (TEMAH) 

and water has been selected as the chemical system for primary investigation.

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 the 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.

1. The fi rst step in providing validated ALD 

process models is evaluating the suitability of 

Technical Contacts:

J. E. Maslar

D. R. Burgess, Jr.

W. S. Hurst
90 National Institute of Standards and Technology 

diagnostics that are sensitive to ALD chemistry. 

Mass spectrometry and optical spectroscopic techniques are of particular interest because of their 

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. Research-grade 

ALD reactors with optimized accessibility for the 

various gas-phase-sensitive and surface-sensitive 

techniques have been designed and constructed.

DELIVERABLES:

Complete evaluation of fi ber-optic coupled, polarized, in-situ Raman spectroscopic measurements 

for probing ALD surface processes. 3Q 2006 

Complete evaluation of time-resolved, in-situ FTIR 

measurements for probing ALD gas phase processes. 3Q 2006 

Interface mass spectrometer with ALD chamber. 

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:

Compilation of bibliography pertaining to ALD and 

CVD systems. Make this information available 

through the Standard Reference Data website. 

Ongoing

Perform high level ab initio quantum calculations 

for HfO2 precursors and derivatives to benchmark 

heats of formations and bond dissociation energies. 

3Q 2006

Develop simple model for HfO2 ALD from TEMAH 

and water. 4Q 2006

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, and ultraviolet Raman spectroscopy. 

The data provided by these measurements, spatially resolved when possible, would include such 

fi lm characteristics as thickness, stoichiometry, 

HfO2 phases present, and degree and type of impurity incorporation.

DELIVERABLES: Investigate the relationship between ALD process parameters, reactor gas fl ow and 

temperature fi elds, gas phase species identities and 

concentrations, and resulting ALD fi lm characteristics. 

Ongoing

ACCOMPLISHMENTS

ALD Reactor Modeling — Three dimensional CFD models for gas fl ow and temperature 

fi elds have been developed for various reactor 

geometries, including optically-accessible reactor 

designs, and used to model fl ow and temperature 

under various process conditions, as shown in 

Fig. 1. 

•

•

•

#
Semiconductor Microelectronics and Nanoelectronics Programs 91

Figure 1. A two dimensional gas velocity profi le 

slice obtained from a full three dimensional reactor 

model.

In addition, time-resolved precursor distributions 

have been modeled, as shown in Fig. 2. 

Figure 2. A three dimensional mass fraction profi le 

showing water mass fraction distribution 0.1 sec 

after water injection into the reactor.

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.

HfO2

 ALD — Research-grade, optically-accessible ALD reactors have been designed and 

constructed with full optical access for surface 

and gas-phase Raman and FTIR spectroscopic 

measurements. An associated pulsed gas delivery system has been designed and constructed. 

Using this deposition equipment, ALD HfO2

fi lms have been deposited under a variety of 

process parameters using TEMAH and water or 

tetrakis(dimethylamino) hafnium (TDMAH) and 

water. Films have been characterized with a number of techniques, including VUV-SE and XPS, as 

shown in Fig. 3 and Fig. 4, respectively.

#

Figure 3. 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).

Figure 4. X-ray photoelectron spectrum of HfO2

 ALD 

fi lm (data courtesy of K.L. Steffens).

4.0 4.5 5.0 5.5 6.0 6.5 7.0 7.5 8.0 8.5 9.0

0

1000

2000

3000

4000

5000

6000

[n(E)a(E)E]1/2

Photon Energy, E (eV)

Eg = 5.68 eV

80x103

60

40

20

0

Intensity (cps)

600 400 200 0

 Binding Energy (eV)

O 1s

Hf 4p1/2

Hf 4d Hf 4f

C 1s

Hf 5p

Hf 4p3/2

Si 2p Si 2s

In-Situ Optical 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. 5 (page 92). 

# [n(E)•

α(E)•E]1/2
92 National Institute of Standards and Technology 

In addition, in-situ FTIR measurements during 

TEMAH injection pulses were shown to be sensitive to TEMAH decomposition in the delivery 

system. In-situ surface Raman spectroscopic 

measurements are on-going. Excitation wavelengths are being varied and polarization analysis 

employed to determine maximum sensitivity to 

ALD species.

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.

For Al2O3 ALD, prototypical small cluster (AlxOyHz

) species have been utilized to represent the surface layer. Rate expressions based on calculated 

structures and energies have been derived using 

transition state theory, as illustrated in Fig. 6. 

#

Figure 5. Gas phase FTIR spectra as a function of 

purge time following a water pulse. 

6x10-3

4

2

0

Absorbance

3000 2000 1000

Wavenumber (cm-1)

19 s PWater

TEMAH

15 s PWater

10 s PWater

5 s PWater

0.5 s PWater

0.1 s PWater

(CO2) (H2O)

C-H N-C, C-C, Hf-N

Figure 6. Calculated potential energy surface for 

reaction of trimethyl aluminum with a hydroxylated 

aluminum oxide cluster.

A detailed chemical kinetic model for ALD of 

Al2O3 from TMA and water has been constructed 

based on these rate expressions. This model is 

being refined based on further reactor model 

simulations, comparison with experimental observables, and supplemented with additional 

quantum calculations, where necessary. High 

level ab initio calculations, up to CCSD(T)/augcc-pVnZ (n=2-4) have been done for small species to benchmark heats of formation and bond 

dissociation energies for AlHnX species (n = 0-2, 

X = H, F, Cl, OH, NH2, CH3). Additional calculations will be done to provide higher level corrections (e.g., core-valence, relativistic, etc).

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 Density 

Functional Theory using the B3LYP/LANL2DZ 

method. The calculated frequencies were compared to experimental infrared spectra obtained 

in the ALD reactor and tentatively identifi ed as 

TEMAH and MEA, as shown in Fig. 7. 
Semiconductor Microelectronics and Nanoelectronics Programs 93

Figure 7. Comparison between calculated and 

experimental spectra of TEMAH and MEA.

12x10-3

10

8

6

4

2

0

Absorbance

3000 2000 1000

Wavenumber (cm-1)

TEMAH

MEA

Calculated TEMAH

Calculated MEA

Hf-N

N-C

N-C

C-H

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://srdata.

nist.gov/ckmechx/ (external) and http://h105097.

nist.gov/ckmechx (internal) has been made available through the NIST Standard Reference Data 

website. This site currently 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 hydrocarbonbased reactions. 

RECENT PUBLICATIONS

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,” 

Electrochemical Society Transactions, 2006, in press.

D. R. Burgess, Jr., J. E. Maslar, W. S. Hurst, E. F. Moore, W. A. 

Kimes, and N. V. Nguyen, “Atomic Layer Deposition Process 

Models and Metrologies,” Characterization and Metrology for 

ULSI Technology: 2005 International Conference, edited by 

D. G. Seiler et al., AIP Conference Proceedings 788, Melville, 

NY: American Institute of Physics, pp. 141-146, 2005.

#
94 National Institute of Standards and Technology 
Semiconductor Microelectronics and Nanoelectronics Programs 95

Technical Contacts:

D. Josell

T. P. Moffat

SUPERCONFORMAL DEPOSITION: COPPER AND

ADVANCED INTERCONNECT MATERIALS 

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, examining the generality of the 

superconformal fi lling mechanism and exploring processes utilizing novel barriers and/or seed 

geometries.

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 ever 

smaller 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. To overcome 

these hurdles the National Institute of Standards 

and Technology (NIST) is enhancing existing copper technology through improved understanding 

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 2005 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. We 

have also developed electrolytes and processes for 

superconformal deposition of advanced interconnect materials such as silver (the only metal with 

a higher conductivity than copper) and gold (a 

metallization for wide-bandgap semiconductors) 

as well as alternative processing schemes such as 

chemical vapor deposition. Research results for 

this year are summarized below.

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 ruthenium, eliminating the need for the PVD 

copper seed layer and the corrosion problems that 

come with it. 

Our work has explained the sensitivity of fi lling 

of ruthenium coated trenches to the manner in 

which the ruthenium surface is treated prior to 

plating, demonstrating correspondence between 

the surface state and the wetting and fi lling of 

ruthenium coated trenches (Fig. 1). 

Figure 1. Nucleation and growth of copper on ruthenium is sensitive to the initial surface state. Three 

limiting behaviors are shown: (A) Exemplifi es the 

desired result of early coalescence of the electrodeposited copper followed by bottom-up superfi lling 

that is characteristic of growth on a conventional 

copper seed-layer, (B) poor trench fi lling characteristic of Volmer-Weber growth mode on poorly 

wetting, oxidized ruthenium and (C) selective growth 

within the feature that is observed sporadically in a 

few specimens.
96 National Institute of Standards and Technology 

We have used voltammetry to demonstrate that 

the absence of a two-dimensional wetting layer 

of “underpotential deposited copper” on oxidized 

ruthenium surfaces is associated with copper 

deposition by the undesirable Volmer-Weber 

growth mode and poor fi lling visible in Fig. 1B. 

Voltammetry and ellipsometry have been used to 

fully explore adsorption on ruthenium of additives required for copper superfi ll in conventional 

structures. The understanding of the competitive 

dynamic of multicomponent adsorption and 

consumption and its connection with microstructural and morphological evolution provided by 

this work will facilitate establishment of a robust 

manufacturing process. Metrology for assessing surface quality that is compatible with the 

electrochemical process used for interconnect 

fabrication (Fig. 2) and techniques for repairing 

oxidized surfaces are also detailed. 

Figure 2. Deposition on oxidized and oxide-free 

ruthenium surfaces yields substantially different 

deposition signatures in superfi lling electrolyte. 

Competition between transport-limited adsorption of 

suppressing additive and rapidly increasing surface 

area of the discrete copper nuclei that form on the 

oxide-coated surface yields a bump in the currentvoltage trace that is a unique signal of the poor 

wetting visible in Fig. 1B. A smooth trace essentially 

identical to that for deposition on a copper surface 

is obtained for deposition on activated ruthenium 

surfaces.

DELIVERABLES: Publications detailing (1) voltammetry for assessing wettability of ruthenium surfaces, 

including the role of additives during superfi lling of patterned features, and (2) ellipsometry to independently 

quantify the adsorbate coverages for comparison with 

the results of the electroanalytical measurements. 

1Q 2006 and 2Q 2006

As feature widths and seed-layer dimensions 

decrease, implementation of atomic layer deposition (ALD) is being considered for seed and/or 

barrier deposition. We focused our efforts on seedless superfi lling with ALD ruthenium and iridium 

coated features through a collaboration with the 

University of Helsinki (Fig. 3). We used PVD 

osmium coated features to further examine the 

impact of oxidation and processing on wettability 

and superfi lling in seedless processing. 

Figure 3. Trenches superfi lled with copper electrodeposited directly on an ALD iridium layer. The 

insert shows a higher magnifi cation view of the 

iridium layer; a thin alumina adhesion layer is visible 

underneath it.

DELIVERABLES: Publications detailing seedless 

superfi lling of copper on PVD and ALD layers. 

1Q 2006

We also extended our CEAC mechanism to probe 

the coupling of area change and the traditional diffusion-adsorption-consumption model of leveling. 

A code that includes competition of suppressor, 

catalyst, and leveler for surface sites in prediction 

of metal deposition and shape change (operating 

on the freeware Python platform) was used with 

experimentally derived kinetics to predict the 

impact of the leveling additive PEI on overfi ll 

bump formation (Fig. 4, page 97). The predictions 

capture the observed reduction of overfi ll bump 

height upon addition of leveler. Enhanced coverage of deposition-suppressing leveler through 

area-reduction within the CEAC mechanism also 

explains the observed inversion, upon addition 

of leveler, of relative bump heights over regions 

with high and low densities of superfi lled features 

(i.e., why a higher bump is observed over isolated 

features in the presence of leveler rather than over 
Semiconductor Microelectronics and Nanoelectronics Programs 97

grouped features as in its absence). These results 

have substantially improved understanding of 

the competitive dynamic that underlie empirical 

strategies of adding leveling additives to superfi lling electrolytes in order to reduce overfi ll bump 

formation. The work will benefi t post-plating 

planarization.

Figure 4. Simulations of feature fi lling as a 

function of the concentration of leveling additive 

in the electrolyte. The predictions are obtained 

using kinetics of the leveler PEI in a CEAC model 

that includes competitive adsorption of suppressor, accelerator, and leveler additives. Dilute 

leveler reduces the overfi ll bump height; higher 

concentrations destroy the superfi ll dynamic. 

DELIVERABLES: Publication detailing extended 

CEAC formalism for modeling of electrolytes containing leveling additives as well as metrology for assessing kinetics of adsorbate interactions. 1Q 2006 and 

4Q 2006 

We also built upon last year’s demonstration of 

gold superfi ll for fabrication of gold interconnects for III-V semiconductors such as GaAs and 

GaN. This effort included quantitative assessment 

and description of the kinetics of gold superfi ll 

with a lead accelerator. Experimental trench fi lling results compared favorably with the CEAC 

derived predictions.

DELIVERABLES: Publication detailing assessment of 

gold superfi ll kinetics with comparison of observations 

and CEAC predictions. 1Q 2006

ACCOMPLISHMENTS

We demonstrated the use of ruthenium, 

iridium and osmium, PVD and ALD, as interconnect surfaces compatible with superconformal 

electrodeposition of copper without the need for a 

copper seed layer. We also established metrology 

for assessing the state of the surface with regard to 

wettability and effective seedless superfi ll. 

#

We extended the existing CEAC model of 

superfi ll to include leveling additives in the electrolyte, describing metrology for assessing the 

kinetics of the competitive adsorption process. We 

also explored the impact of leveling additives on 

overfi ll bump formation during trench fi lling.

We quantified bottom-up superfilling of 

trenches with gold that might fi nd use in the formation of contacts and metallizations for compound 

semiconductors.

COLLABORATIONS

D. Wheeler, J.E. Bonevich, L.J. Richter, M.L. Walker, P.J. 

Chen, W.F. Egelhoff: NIST.

Christian Witt: Cookson-Enthone.

T. Aaltonen, M. Leskelä and M. Ritala: University of Helsinki.

RECENT PUBLICATIONS

M. Walker, L. J. Richter, D. Josell and T. P. Moffat, “An ellipsometric Study of the Cl-induced Adsorption of PEG on 

Ru and Cu UPD/Ru,” Journal of the Electrochemical Society 

153(4), C235-C241 (2006).

D. Josell, J. E. Bonevich, T. P. Moffat, T. Aaltonen, M. Ritala 

and M. Leskelä, “Iridium Barriers for Direct Copper Electrodeposition in Damascene Processing,” Electrochemical and 

Solid-State Letters 9(2), C48 (2006). 

D. Josell, C. Witt and T. P. Moffat, “Osmium Barriers for 

Direct Copper Electrodeposition in Damascene Processing,” Electrochemical and Solid-State Letters 9(2), C41-C43 

(2006).

T. P. Moffat, D. Wheeler, S. K. Kim and D. Josell, “Curvature 

Enhanced Adsorbate Coverage Model for Electrodeposition,”

Journal of the Electrochemical Society 153(2), C127-C132 

(2006).

T. P. Moffat, M. Walker, P. J. Chen, J. E. Bonevich, W.F. Egelhoff, L. Richter, C. Witt, T. Aaltonen, M. Ritala, M. Leskelä 

and D. Josell, “Electrodeposition of Cu on Ru Barrier Layers 

for Damascene Processing,” Journal of the Electrochemical 

Society, 153(1), C37-C50 (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).

T. P. Moffat and D. Josell, “Seedless Superfi lling: Opportunities and Challenge,” invited article in Semiconductor FabTech 

27th Edition, 133-136 (2005).

#

#
98 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 99

BASIC MATERIALS PROPERTIES

GOALS

The objectives of this project are: (1) to develop experimental techniques to measure the 

reliability-related properties of thin interconnect 

conducting and insulating fi lms, including basic 

tensile properties, elastic modulus by both static 

and dynamic means, residual stresses and strains, 

fatigue, fracture resistance, and thermomechanical 

response, in specimens fabricated and sized like 

materials used in actual commercial devices; and 

(2) to advance the ability to anticipate and meet 

thin interconnect reliability challenges by relating thin fi lm reliability to microstructure and by 

developing understanding of the relationships 

between various modes of thin fi lm failure. 

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, 2005, 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), Interconnect, p. 45, 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 2003 NEMI (National 

Electronics Manufacturing Initiative) Research 

Priorities document reports a similar need. In a 

section beginning on page 24, entitled Modeling, 

Simulation and Design Tools, Emerging Areas for 

Electronics Packaging, Table 3, Projected Development and Research Needs for Simulations in 

Emerging Areas, includes nanoscale modeling and 

simulation as an area, experimental tools capable 

of measuring electrical, thermal, and mechanical 

Technical Contacts:

D. Read

B. Keller

phenomena/material properties at smaller scale as 

a need, and “Issue: how is the property and behavior different from bulk behavior/macro-scale?” as 

a comment. 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.

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 highamplitude 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 

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. 
100 National Institute of Standards and Technology 

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 

underway 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 atomic-scale 

materials engineering. We are in the early stages 

of planning the second such workshop, 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.

Measurement capabilities operating within this 

project include microtensile testing, d.c. and a.c. 

thermomechanical fatigue and measurements. 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.

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.

Figure 1. Test chip produced in the 1.2 μm AMI 

CMOS process available through the MOSIS service.
Semiconductor Microelectronics and Nanoelectronics Programs 101

DELIVERABLES: We are organizing a one-day 

short course consisting of 6 lectures by NIST staff on 

Interconnect-Relevant Characterization & Metrology, 

in conjunction with the 2006 Advanced Metallization 

Conference. 4Q 2006

Our measurements involving alternating current 

stressing of chip-level interconnects are used to 

explore the relationships between electrical 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 the 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 review. 2Q 2006

The initial ultimate strength value deduced from 

the a.c. test by extrapolating the results back to 

one loading cycle was less than a factor of two 

higher than 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 

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.
102 National Institute of Standards and Technology 

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 2006

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. Currently 

we are placing major focus on the comparison of 

stress-to-failure a.c. electrical tests vs. microtensile tests. Figure 3 shows the effect of line width 

on current density at failure in ramp-to-failure 

tests. Such scale-related effects will have to be 

factored into analysis procedures for deducing 

material properties from a.c. tests.

#

Figure 3. Effect of line width on current density at 

failure in ramp-to-failure tests in PVD copper fi lms.

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 is in 

review for a special issue of a technical journal. 

2Q 2006

#

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. 

#

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 three conference 

presentations on the comparison of electrical and mechanical stress effects are in review for the respective 

technical conference proceedings, to appear in 

4Q 2006.

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. 

#
Semiconductor Microelectronics and Nanoelectronics Programs 103

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. 

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 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.

Figure 6. Tensile strength plotted against 

temperature for microtensile specimens of aluminum 

contact metal obtained through MOSIS, and 

electrodeposited copper made at NIST.

Figure 7a 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-centeredcubic symmetry, while the atoms shown in color 

are surrounded by near-perfect fcc structure. This 

simulation presents a possible explanation for 

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.
104 National Institute of Standards and Technology 

our measured values of the elastic constant of 

ED copper, which are 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 selfassembly of arrays of dots.

Our a.c. tests continue to reveal damage phenomena drastically different from that observed in 

conventional d.c. electromigration tests. Figure 8 

shows the large difference in lifetime seen under 

the a.c. and d.c. test conditions. We have pursued 

crystallographic mapping experiments within a 

fi eld 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. 9). Accompanying such grain growth is a 

re-orienting of some grains, into a more accommodating orientation for slip activity. In other 

words, grain re-orienting 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 

#

Figure 8. Time to open circuit under various 

electrical testing conditions. “A.C.” and “D.C. Cold” 

data obtained with specimen nominally at room 

temperature. “D.C. Hot” tested at nominal specimen 

temperature of 227 °C. “D.C. Hot” data from U. E. 

Möckl, M. Bauer, O. Kraft, J. E. Sanchez, Jr., and E. 

Artz, MRS Symp. Proc. Vol. 338, 373 (1994).

Figure 9. 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.

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.

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-ElectroMechanical Technology (MNT) Metrology Proj-
Semiconductor Microelectronics and Nanoelectronics Programs 105

ect, 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

Glenn Alers, 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. 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

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.

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 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, 2006.

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.

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.
106 National Institute of Standards and Technology 
Semiconductor Microelectronics and Nanoelectronics Programs 107

TEST STRUCTURES FOR INTERCONNECT

METROLOGY AND MODELING

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 

conducting-feature 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 high-resistance 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 process-control 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 performance 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/barriermetal 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 the 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, a metrology challenge that is attributable as much to scaling per se, rather than to the materials selected for 

interconnect implementation, is overlay. 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.

Technical Contacts:

M. W. Cresswell

H. Schafft
108 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- to 100 nm range. The novel process-fl ow, 

which has been reported in a joint paper with the 

University of Edinburgh at a recent conference, 

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. 

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.

We are initiating 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 

RLGC parameters of a selection of binary-metal 

interconnect-conductor architectures over the 

stated 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 binarymetal 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 a patent 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-measurement 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.
Semiconductor Microelectronics and Nanoelectronics Programs 109

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, is 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. 

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 electromigration, 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. 3Q 2006

Complete for publication a new JEDEC standard 

for electro-migration stress testing with constant 

current and temperature. 1Q 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. 3Q 2006

Fabricate and test chemically stable test structures 

having copper-only features and make ECD measurements. 4Q 2006

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. 4Q 2006

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 joint paper 

describing the accomplishment was presented at 

the ICMTS 2006 conference in Austin, Texas, on 

March 8, 2006.

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110 National Institute of Standards and Technology 

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 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 by the Microelectronics 

Reliability journal for appearance in mid-2006. 

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 an 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 De-

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Figure 3. A selection of 40 nm features that 

were etched in (110) silicon after electron-beam 

lithography.

partment 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 photomasks. 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 CAD 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 has been 

completed. A program to simulate and validate 

the calibration of the standard has been applied 

to the design and fabrication of a wafer-lot has 

been initiated.

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Semiconductor Microelectronics and Nanoelectronics Programs 111

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 Department 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

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).

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, “Modeling and Simulation of Resistivity 

of Nanometer Scale Copper,” Microelectronics Reliability. Inpress. On-line publication date: 19-December-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.

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, “Junction-Isolated 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).
112 National Institute of Standards and Technology 

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 113

SOLDERS AND SOLDERABILITY MEASUREMENTS FOR

MICROELECTRONICS

GOALS

Solders and solderability are increasingly tenuous links in the assembly of microelectronics as 

a consequence of ever shrinking chip and package dimensions, the broadening use of fl ip-chip 

technology, and the movement toward environmentally friendly lead-free (Pb-free) solders. To 

support needs in this area, the goal of this project 

is to provide data and materials measurements of 

critical importance to solder interconnect technology for microelectronics assembly.

CUSTOMER NEEDS

The U.S. microelectronics industry has clearly 

articulated measurement needs for solderability 

and assembly, especially for Pb-free solders. For 

example, the urgency for materials data for Pbfree solders has been specifi ed in the 2002 and 

2004 iNEMI, 2003 and 2004–2005 IPC, and 2003 

and 2005 ITRS Roadmaps. The European Community (EC) directives on Waste Electrical and 

Electronic Equipment (WEEE) and the Restriction of the Use of Certain Hazardous Substances 

(RoHS) in Electrical and Electronic Equipment 

take effect in July 2006. U.S. manufacturers must 

comply with these requirements in order to compete in the global market. These industrial needs 

are addressed under this NIST project.

Additional needs have been identifi ed through 

participation in the International Electronics 

Manufacturing Initiative (iNEMI) technology 

working group (TWG) on tin (Sn) Whiskers. 

The protective layer of Sn deposited on copper 

(Cu) is usually referred to as a “pretinned” coating and is required to maintain solderability of 

the component during storage prior to assembly. 

Pb-free coatings of nearly pure Sn tend to grow 

“whiskers,” however, which can cause shorts 

across leads (see Fig. 1). Thus the development 

of Pb-free Sn alloy platings to replace Pb-containing Sn layers is considered important, and tests 

which ascertain the tendency to form whiskers 

are much needed.

A series of industry sponsored workshops have 

been held over the past four years to address the 

problem of Sn whisker growth that has arisen 

due to worldwide conversion to Pb-free solder 

technology. These workshops have reached the 

consensus that residual compressive stress in the 

tin is the basic cause of whisker growth. The international organization representing these industries 

and organizing the workshops, iNEMI, has asked 

the Metallurgy Division (via a letter to F. Gayle, 

10/14/2005) to focus on the, “Establishment of 

Primary and Secondary Standards for sin2

Ψ stress 

determination in electroplated tin fi lms and to 

write a measurement technique standard for tin 

fi lm sin2Ψ.” Further they ask NIST to, “.... take 

the leadership role in developing the measurement standards and the required degree of process 

specifi city.”

TECHNICAL STRATEGY

We are providing the microelectronics industry 

with measurement tools and data to address solder 

interconnect problems. For example, a special 

NIST publication, “Recommended Practice Guide 

for Differential Thermal Analysis (DTA) and Heat 

Flux Differential Thermal Analysis (DSC) Measurements of Alloy Melting and Freezing” will 

Technical Contacts:

W. Boettinger

U. Kattner

K. W. Moon

M. Williams

“I consider your [NIST 

Metallurgy Division] 

group a key asset to 

industry in making 

these decisions on 

the soundest possible 

scientifi c basis. There is 

no comparable body of 

expertise anywhere in 

the world that matches 

that found in your 

group.”

Dr. George T. Galyon (IBM) 

-Chairman, NEMI Tin Whisker 

Modeling Committee

Figure 1. Top) Sn hillocks grew on the surface of 

pure Sn deposit. Bottom) Sn whiskers and contorted 

Sn hillocks grew on the surface of Sn-Cu alloy 

deposit. These surface disturbances formed even 

though W was used as a substrate for plating, which 

does not form an intermetallic compound with Sn.
114 National Institute of Standards and Technology 

give researchers a consistent method to interpret 

thermal analysis data of solders and other alloys. 

In addition, a thermodynamic database has been 

developed and publicly distributed for modeling 

the processing behavior of Pb-free solder systems. 

These databases for phase diagrams and thermodynamics critical to solder development and for 

mechanical properties of solder will continue to 

be expanded and distributed via the web. We also 

provide guidance for adoption of these solders into 

assembly processes through work with industrial 

standards organizations. 

It is well known that the use of pure Sn protective deposits has serious problems. Sn whiskers 

(fi lamentary whiskers typically 1 μm diameter and 

several mm long) can grow from the plating and 

cause electrical shorts and failure. Historically Pb 

was added to Sn plate to prevent whisker growth 

as well as to lower cost. In the current research 

program, it was decided to focus on Pb-free Snrich deposits with alloying additions that might 

retard whisker formation. The Sn-Cu system was 

selected for compatibility reasons, since Sn-CuAg is likely to be the Pb-free solder of choice 

for industrial application. The substitution of 

a different solute for Pb in the Sn-rich deposit 

was proposed to also retard whisker growth. A 

detailed microstructural comparison of deposits 

with high and low whisker formation tendency 

has been conducted. Sn grain size, shape, and 

residual stress have been measured and correlated 

to whisker growth.

DELIVERABLES:

Determine methods to eliminate columnar grain 

structure in Sn electrodeposits. 3Q 2007

Complete Manuscript “Examination of Sn Electrodeposit on Substrates Not Forming Intermetallic 

Compounds” to determine whisker tendency on 

tungsten (W) substrates that do not form intermetallic with Sn. 3Q 2006

Determine feasibility of X-ray sin2

Ψ methods for 

measuring residual stress in Sn electrodeposits. 

3Q 2007

ACCOMPLISHMENTS

“Recommended Practice Guide for Differential Thermal Analysis (DTA) and Heat Flux 

Differential Thermal Analysis (DSC) Measurements of Alloy Melting and Freezing” has been 

completed and will appear as a NIST Special 

Publication Series 960. This document provides 

a detailed description of proper technique and 

measurement pitfalls in the determination of alloy 

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liquidus and solidus temperatures; i.e., the freezing 

(or pasty) range. Such methods are important for 

the selection of Pb-free solder alloys that satisfy 

industry specifi cations.

NIST remains a co-chair of the iNEMI Sn 

Whisker Modeling Group (includes members 

ChipPAC, Cookson, Delphi Delco, Freescale, 

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.

Building on the work from the recently 

published paper, Acta Materialia 53(2005) 5033–

5050, entitled, “Hillock and Whisker Formation 

in Sn, Sn-Cu and Sn-Pb Electrodeposits,” three 

new experimental studies are underway. The 

fi rst two, in their early stages, focus on attempts 

to alter the columnar grain structure using pulse 

plating and/or addition of Bi which may poison the 

crystal growth and cause renucleation of Sn grains 

during the electrodeposition process. The third 

study focuses on the role of the reaction between 

the Sn and the substrate (typically Cu) to form 

intermetallic compound as a contributing source 

of residual stress. Bright Sn and 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 

Sn-Cu alloy. Thus changing the substrate had 

little discernable effect on the whisker defect 

formation.

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 particular extensive 

discussions with Dr. George Galyon, Poughkeepsie/IBM on 

the fundamental causes of whisker growth.

Northrup-Grumman, Linthicum, MD, Provided samples 

consisting of Sn-Cu electrodeposits on Zn substrates for 

their tests on the use of acrylic conformal coatings to prevent 

whisker growth.

RECENT PUBLICATIONS

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, 

submitted April 2006.

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Semiconductor Microelectronics and Nanoelectronics Programs 115

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 (Nov 2005) 5033–5050.

G. R. Stafford, M. E. Williams, C. E. Johnson, K.-W. Moon, 

and W. J. Boettinger, “Whisker Formation in Pb-Free Surface Finishes,” Proceedings for the Green Electrochemistry 

Symposium, 208 Meet. Electrochem. Soc., Los Angeles, 

California, Oct. 16-21, 2005.

R. J. Schaefer, D. J. Lewis, “Directional Solidifi cation in a 

AgCuSn Eutectic Alloy,” Metallurgical Transactions A, 36A 

(10): 2775-2783 Oct. 2005.

K. 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 Sn-Cu 

electrodeposit for Pb-free Solders,” Journal of Electronic 

Materials, V34, 9 Sept. 2005.

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. 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. 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).
116 National Institute of Standards and Technology 
Semiconductor Microelectronics and Nanoelectronics Programs 117

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.

Recently, however, copper has been introduced for interconnect conductivity, replacing aluminum alloys. 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 will be required. As dimensions are reduced, 

gate depletion effects and dopant diffusion through the gate dielectric are limiting transistor 

performance. 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 contamination at extremely low levels in process gases presents serious manufacturing diffi culties. Accurate calibration of water vapor at extremely low vapor 

pressures is required.

Accurate metrology of process gases is essential for reproducible manufacture of semiconductor products and a wide variety of metrology issues emerge in plasma, chemical vapor, 

and rapid thermal processing steps used in semiconductor manufacture.
118 National Institute of Standards and Technology 
Semiconductor Microelectronics and Nanoelectronics Programs 119

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 

GAS PROPERTY DATA AND FLOW STANDARDS FOR

IMPROVED GAS DELIVERY SYSTEMS

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.

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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.

#

#

Figure 1. Speed of sound in tungsten hexafl uoride as 

a function of pressure along isotherms.
120 National Institute of Standards and Technology 

We also are developing novel acoustic techniques 

to measure the viscosity and thermal conductivity 

with uncertainties of less than 0.5 % as specifi ed 

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).

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.

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 

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.)

DELIVERABLES:

Install new spherical resonator for measuring the 

speed of sound in the hazardous gases facility. 

2Q 2006 

Calibrate resonator. 3Q 2006

A cylindrical resonator was used to study nine 

process gases, but it must be replaced due to 

contamination. A spherical resonator has been 

•

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Semiconductor Microelectronics and Nanoelectronics Programs 121

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.

DELIVERABLES:

Develop improved model of acoustic viscometer.

2Q 2006

Develop model of acoustic thermal conductivity 

resonator. 3Q 2006

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: From the collected data calculate 

the transport properties in the semiconductor process 

gases identifi ed by the customer, gases include BCl3

, 

Cl2, HBr, C4F8, CO, CO2, NH3, and SiF4. 2Q 2006

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 

4Q 2004, publish viscosity measurements in BCl3, Cl2, 

HBr, C4

F8, 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.

DELIVERABLES: 

Publish in archival journal a paper on primary gas 

fl ow standards. 1Q 2006

Repackage transfer standard based on commercial 

measurement package. 2Q 2006

The third-generation transfer standard will use 

commercial instrumentation to measure the pressures and temperature of gas fl owing through a 

quartz capillary. The commercial instrumentation will improve the fl ow meter’s convenience, 

and the quartz capillary will be similar to that of 

the second-generation standard. Using the same 

capillary and model will ensure an uncertainty of 

0.1 %. An improved design based on preliminary 

tests will improve the reliability of the capillary 

package.

•

•

•

•

DELIVERABLES: Integrate static gravimetric fl ow 

standard for calibrating mole blocks. 1Q 2006

The static gravimetric fl ow standard is the only 

devices that measure directly mass fl ow rate instead of a secondary quantity such as velocity or 

heat loss. This standard is used to calibrate mole 

blocks as secondary standards for fl ows less than 

1000 sccm.

DELIVERABLES: Draft SP-250 for gas fl ow calibrations. 4Q 2006

The SP-250 document will be used to establish a 

routine calibration service at NIST for gas fl ows 

in the range from 10-7 to 10-3 mol/s (1 to 1000 

sccm).

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. 

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#
122 National Institute of Standards and Technology 

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).
Semiconductor Microelectronics and Nanoelectronics Programs 123

LOW CONCENTRATION OF HUMIDITY STANDARDS

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, page 124), 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 

“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

Technical Contacts:

J. T. Hodges

D. Ripple

K. Bertness
124 National Institute of Standards and Technology 

generation systems at the research and development stage as well as commercial devices.

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. 

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 

the measured spectrum be accurately determined. 

By combining high spectral resolution with high 

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•

Figure 1. NIST Low Frost-Point (humidity) 

Generator.

Figure 2. Steady state response of saturator control 

thermometers in NIST Low Frost-Point Humidity 

Generator.
Semiconductor Microelectronics and Nanoelectronics Programs 125

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. 

Using the existing FS-CRDS apparatus and a 

portable fi ber-optics-based FS-CRDS system, we 

will make multiple independent measurements 

of H2

O 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. 

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. Semiconductor device 

manufacturers have expressed frustration with 

the irreproducibility of source material purity 

from vendor lot to vendor lot. The critical concentrations of the impurities are not well known; 

however, it is believed that >10 nmol/mol 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. 

In collaboration with researchers in the NIST 

Chemical Science and Technology Laboratory, 

researchers in the NIST Electronics and Electrical 

Engineering Laboratory have developed a CRDS 

system linked with a semiconductor crystal growth 

system to measure H2

O at very low concentrations 

in semiconductor source gases and to correlate the 

process gas impurities with crystal properties. The 

system is being used to measure the lineshape, 

absorption coeffi cients, and frequency of optical 

transitions for water, phosphine, and ammonia 

in the vicinity of 935 nm and 1380 nm. This 

information is critical to facilitate the use of highsensitivity 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. 

DELIVERABLES:

Modify CRDS system for parallel tests with commercial instrumentation and conduct joint experiments on H2

O in phosphine. 3Q 2005

Measure phosphine absorption lines in vicinity of 

H2

O transition line at 943.082 nm and compare 

to previous H2

O lines explored for overlap with 

phosphine. If new line offers superior sensitivity, 

measure pressure broadening coeffi cients for H2

O 

in phosphine. 4Q 2005

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Figure 3. Frequency-stabilized CRDS system.
126 National Institute of Standards and Technology 

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 

#

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 

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 

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#

Figure 4. New PTG calibration apparatus.

placed inside the temperature-stabilized PTG 

oven. Water vapor diffuses from the tube surface 

into a precisely controlled stream of purifi ed N2. 

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.

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.
Semiconductor Microelectronics and Nanoelectronics Programs 127

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.

Figure 7. APIMS measurement of LFPG background 

H2

O concentration showing decay to steady state 

level.

We have successfully developed an FSCRDS system with automated spectral scanning 

capability to measure the absorption coeffi cient 

of trace quantities of water vapor. In FY 2005 

the FS-CRDS method was used to probe H2O 

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 

#

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.

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 935 nm. 

We have used a similar FS-CRDS system to measure water vapor concentrations in the toxic gas 

phosphine in the 935 nm spectral region. Testing 

of fi ve strong water absorption lines in this region 

indicated that the least overlap with phosphine 

lines, and hence the highest sensitivity to water 

contamination, is present for the line at 943.082 

nm. Future experiments will characterize this line 

for pressure broadening coeffi cients and ultimate 

sensitivity limits. Figure 9 shows a typical H2

O 

spectrum obtained with the automated FSSMCRDS system and comparison with previously 

published measurements. 

Figure 9. Survey spectrum of water vapor obtained 

with frequency stabilized CRDS apparatus. The 

numbers correspond to peaks of individual 

absorption transitions.
128 National Institute of Standards and Technology 

COLLABORATIONS

Air Products and Chemicals Inc., Seksan Dheandhanoo; 

APIMS measurements of trace moisture and characterization 

of semiconductor gas purity.

Matheson Tri-Gas, Mark Raynor and Hans Funke; CRDS 

measurements of trace moisture in phosphine.

Tiger Optics, Yu Chen; CRDS measurements of trace moisture 

in phosphine.

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

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, “Advanced humidity standards 

address new demand for measurements of trace water vapor 

in specialty gases,” (Micro, in press).

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 

H2O 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).
Semiconductor Microelectronics and Nanoelectronics Programs 129

TEMPERATURE MEASUREMENTS AND STANDARDS

FOR SEMICONDUCTOR PROCESSING

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, 2005 ITRS, p. 12).

2. Fabrication of high-density, non-volatile 

memory is very sensitive to process temperature 

(Process Integration, Devices, and Structures, 

2005 ITRS, 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. 31)

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 700 °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. 

This community forms our project’s Common 

Interest Group (CIG), 20 companies meeting annually since 1997. They serve as a bridge between 

research and practice, provide advice on shaping 

objectives, and generate opportunities for technology transfer. 

TECHNICAL STRATEGY

Our research is focused on four projects that will 

enable the semiconductor industry to meet the 

roadmap requirements: (a) support our industrial collaborators in the use of test wafers with 

improved thin-fi lm technology to demonstrate 

in-tool calibration of RTs traceable to the ITS-90, 

(b) develop new, accurate sensors for the 300 °C to 

700 °C range, where commercial sensors are inadequate, (c) investigate effects on measurements of 

imperfect thermal environments with commercial 

instrumented-wafer technology, and (d) develop 

silicon-wafer emittance standards for improved 

temperature and emittance measurements. 

Cooperative projects with Applied Materials and 

Atmel industries are investigating the use of the 

NIST calibration wafers in industrial RTP tools. 

These evaluations are critical for transferring the 

NIST developments to the semiconductor processing industry. The NIST TFTC calibration wafer 

has demonstrated unique capabilities in temperature measurements and in establishing traceability 

to the ITS-90. We have also distributed a document “Instructions for use of the NIST Thin-Film 

Thermocouple Calibration Wafer” in response to 

requests from our CIG. 

DELIVERABLES: NIST TFTC calibration wafers and 

instructions for application of NIST calibration wafer, 

and joint report with Applied Materials and Atmel on 

LPRT calibrations. 3Q 2006

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.
130 National Institute of Standards and Technology 

Members of our CIG 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. Work 

that remains is to calibrate with high accuracy the 

optimized sensors, to determine the best attainable 

hysteresis and uncertainty. 

DELIVERABLES:

Publish paper on the fabrication methods, optimization, and attainable uncertainty of thin-fi lm Pt resistor thermometers directly on Si wafers. 3Q 2006

Explore feasibility of using iridium fi lms as resistance sensors up to 600 °C. 2Q 2007

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. 3Q 2006

Conduct experiments on sensitivity of sensors in 

instrumented wafers to non-isotropic heat fl ux. 

3Q 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.

•

•

•

•

DELIVERABLES: Assist manufacturers of instrumented wafers with heat transfer models in vicinity of 

sensors. 4Q 2006

ACCOMPLISHMENTS

DEVELOPMENT OF RESISTANCE SENSORS

FOR 300 °C TO 700 °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.

CALIBRATION OF CLRTS IN THE RANGE

50 °C TO 150 °C

LPRTs have been become mainstream tools for 

temperature measurements in the RTP community. 

We have calibrated LPRTs for RTP applications 

using a sodium heat-pipe blackbody between 

700 °C and 900 °C with an uncertainty of approximately 0.3 °C (k=1) traceable to the ITS-90. Recently, cable-less LPRTs (CLRTs) offer a decisive 

advantage to enable radiometric measurements at 

lower temperatures than traditional LPRTs. The 

application of CLRTs can eliminate 2.0 °C or 

more uncertainty from the calibration scheme. We 

have used a Post-Exposure-Bake Test Bed at NIST 

to perform intercomparisons between TFTCs 

and the new low-temperature (50 °C to 150 °C) 

CLRTs. Our study has established calibration 

uncertainties for comparison of CLRTs against 

the TFTCs in-situ, and has established uncertainties for model-corrected CLRTs calibrated against 

blackbodies. 

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 
Semiconductor Microelectronics and Nanoelectronics Programs 131

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.

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 non-destructive, 

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.

RECENT PUBLICATIONS

W. A. Kimes, K. G. Kreider, D. C. Ripple, and B. K. Tsai, “Insitu 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.

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.

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.

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.
132 National Institute of Standards and Technology 

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.
Semiconductor Microelectronics and Nanoelectronics Programs 133

PLASMA PROCESS METROLOGY

GOALS

Our goal is 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 2005 

International Technology Roadmap for Semiconductors (ITRS) identifi es a need for better, more 

predictive modeling of the impact of equipment 

on process results (Modeling and Simulation section, pages 15-16, and Table 122). 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 4, 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. An electrical measurement technique developed at NIST for use in process monitoring and 

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

Motorola

“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.
134 National Institute of Standards and Technology 

control applications continues to be the subject of 

further testing and validation. 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 currents, 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. In particular, such reactors are 

often equipped with electrostatic chucks, which 

may have an electrical impedance large enough 

to signifi cantly affect plasma electrical characteristics. In one collaboration with an industrial 

company, we have characterized chuck electrical 

properties in a prototype commercial reactor. 

Additional characterization will be performed 

in industrial reactors and in a laboratory reactor that has been modifi ed to have an insulating 

chuck similar to the electrostatic chucks used in 

industry. Tests will be performed in both kinds 

of reactors to evaluate the validity and usefulness 

of the NIST-developed electrical measurements, 

models, and analysis techniques.

DELIVERABLES: Evaluate the validity and utility of 

rf measurement techniques, electrical models, and 

analysis techniques in a laboratory reactor equipped 

with an insulating chuck, and in commercial plasma 

reactors equipped with electrostatic chucks. 3Q 2007

2. Sub-millimeter wavelength absorption spectroscopy, a sensitive and versatile diagnostic technique, will be used to study the etching process 

of 193 nm photoresists. The etching process of 

193 nm photoresists is strongly correlated with 

line-edge-roughening (LER) of features produced 

using 193 nm photolithography. Sub-millimeter 

wavelength absorption spectroscopy couples to 

the rotational energy states of molecules and can 

be used to monitor a wide variety of molecular 

species in a plasma, including etching by-products. These measurements will be combined with 

the results of other research groups in order to 

determine methods of reducing LER in 193 nm 

photolithography. 

DELIVERABLES: Identify and measure densities of 

molecules produced by plasma etching of 193 nm 

photoresist-coated wafers using sub-mm wavelength 

absorption spectroscopy. 4Q 2006

3. Dual-frequency capacitively-coupled plasma (CCP) sources are becoming increasingly 

important in semiconductor manufacturing 

processes, however, there appear to exist only 

limited amounts of published data on these plasma 

sources. Most experimental papers concentrate 

solely on the source etching characteristics, and 

numerous theory papers on dual-frequency sources show few comparisons with actual experimental 

data. We will address several issues related to 

dual-frequency sources, concentrating on how the 

sheath is affected by the dual frequencies. 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? 

A new diagnostic technique will be developed to 

help address these issues. The plasma frequency 

cut-off probe described below will be modifi ed to 

enable it to make time-resolved electron density 

measurements throughout the rf cycle. Results 

should provide information on the nonlinear interaction of the lower frequency bias voltage with 

the plasma. 

DELIVERABLES: Evaluate the application of the 

plasma frequency cut-off probe for time-resolved electron density measurements in a dual-frequency CCP 

source. 1Q 2007

ACCOMPLISHMENTS

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 

#
Semiconductor Microelectronics and Nanoelectronics Programs 135

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, based on theoretical arguments, is believed 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.

In addition, we have combined the cut-off 

probe electron density measurements with two-dimensional (2-D) optical measurements in capacitively-coupled Ar/CF4

 plasmas (Fig. 2). Planar 

laser-induced fl uorescence (PLIF) was used to 

measure 2-D density maps of the reactive radical CF. The 2-D images of spontaneous plasma 

emission from excited species within the plasma 

were also measured. Radial and axial profi les of 

electron density, CF density, and plasma emission 

were measured in CF4/Ar plasmas and compared 

to determine the nature of the correlation of reactive species spatial distributions with electron 

density distributions. 

#

Figure 2. Measurements taken in 100 mTorr, Ar/CF4 

plasmas of a) CF density by PLIF and b) plasma 

density radial profi les by the cut-off probe technique.

Spatially and temporally resolved measurements of optical emission from a dual-frequency 

capacitively-coupled plasma source have been 

made. Although these types of sources are becoming increasingly important in semiconductor 

manufacturing processes, there appears to be 

only a limited amount of published data related 

to them. This investigation utilized an imaging spectrometer coupled to an intensifi ed CCD 

camera to image the vertical distribution of the 

optical emission from a single atomic transition 

as a function of time, synchronized with the lower 

frequency rf bias voltage. This provides insight 

into the non-linear coupling of the rf bias to the 

plasma production. It was found that electronegative feed gases (such as CF4

) signifi cantly alter 

the temporal behavior of the plasma emission, 

indicating a change in the electron heating or 

plasma production processes caused by the rf bias 

voltage. The temporal behavior of the optical 

emission was dependent on gas pressure. These 

results are being correlated with the voltage and 

current waveforms at the electrode surfaces which 

are simultaneously recorded.

#
136 National Institute of Standards and Technology 

Many industrial plasma etchers are equipped 

with optical emission spectrometers, which 

have proven useful for endpoint detection, fault 

detection and classifi cation, and automatic process control. We have performed experiments 

in which optical emission measurements are 

combined with noninvasive current and voltage 

measurements. Optical emission complements 

the electrical measurements by providing information about drift or other changes in the chemical species within the plasma—information that 

would be diffi cult or impossible to obtain solely 

from electrical measurements. We have made 

a series of measurements in CF4

/Ar plasmas to 

investigate the correlation between electrical and 

optical emission changes in the plasma which 

occur as a function of time during an oxide etch 

and during chamber conditioning. We measured 

emission from Ar, F, CF, CF2 and SiF. Changes 

are observed in the optical emission from many 

species and in electrical parameters as the etch 

endpoint is reached (Fig. 3). A particularly strong 

correlation was found between the plasma impedance and F emission.

Figure 3. Time-resolved vertical distribution of 

optical emission (750nm) from a dual-frequency 

capacitively-coupled plasma. The plasma was 

created using 26 W at 13.56 MHz applied to the 

upper electrode and 9 W at 2 MHz applied to the 

lower electrode. A gas mixture of CF4

:Ar:O2 was 

used with a ratio of 2.0:7.5:0.5. The solid horizontal 

lines indicate the edges of the electrodes.

Figure 4. 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+

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. 

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, “Noninvasive monitoring of ion energy drift 

in an inductively coupled plasma reactor,” J. Appl. Phys. 97, 

article #033301, 1-12 (2005).

K. L. Steffens and M. A. Sobolewski, “2-D imaging of 

temperature in CF4

 plasmas,” IEEE Transactions on Plasma 

Science, 33, 370-371 (2005).

#

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. 4). 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 

#
Semiconductor Microelectronics and Nanoelectronics Programs 137

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).
138 National Institute of Standards and Technology 
Semiconductor Microelectronics and Nanoelectronics Programs 139

ASSESSMENT OF MEASUREMENTS AND STANDARDS

FOR GAS PHASE PROCESSES IN SEMICONDUCTOR

DEVICE MANUFACTURING

GOALS

New materials used in gas phase processes will 

require new measurements and standards for the 

effi cient delivery of those materials to the wafer 

surface. The National Institute of Standards and 

Technology (NIST) will supplement its ongoing 

project to measure the thermophysical properties of semiconductor gases with a new effort to 

measure the relevant properties of liquid precursors. NIST will identify the properties of greatest 

interest, identify existing accurate data for those 

properties, and devise techniques to measure those 

properties where accurate data do not exist. The 

data will improve the modeling of chemical vapor 

deposition, the calibration of mass fl ow controllers, and the design and use of bubblers and other 

devices that deliver precursor vapors. This project 

will be coordinated with continued work on standards for gas fl ow at rates less than 1000 μmol/s 

(about 1 standard liter per minute).

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”. Accounts 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 

device performance. An ongoing project at NIST 

has produced accurate property data for semiconductor process gases such as boron trichloride and 

nitrogen trifl uoride. The project, which received 

guidance from a NIST-organized workshop in 

2000, has posted data for specifi c heat, second 

virial coeffi cient, viscosity, and other properties 

at properties.nist.gov/fl uidsci/semiprop/. The new 

effort will include liquid precursors, with likely 

measurements being vapor pressure and liquid 

viscosity. 

Vapor pressure is the most important physical 

property of liquid precursors. It 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, namely 

even if 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 the gaps between property 

needs and available data and then begin fi lling 

those gaps with new measurements and data compilation. Desired results include new or enhanced 

NIST databases, new NIST capabilities to measure 

the vapor pressure and liquid viscosity of liquid 

precursors, and the transfer of those capabilities 

to industry.

DELIVERABLES: Identify the most important physical 

properties of liquid precursors. 2Q 2006

The most important physical properties are vapor 

pressure, liquid viscosity, and thermal stability, 

followed by others, density and such as heat of 

vaporization. Ancillary concerns include materials 

compatibility and suspended particles.

DELIVERABLES: Identify existing property data and 

assess its quality. 2Q 2006

Existing data can be found in technical literature, 

the NIST Chemistry WebBook, NIST standard reference database 87, and other databases. The applicability of estimation techniques, such as group 

contribution methods, also will be examined.

DELIVERABLES: Organize and lead workshop 

at Semicon West to identify the gaps between the 

needed data and the available data for physical properties such as vapor pressure, viscosity, and specifi c 

heat. 3Q 2006

Technical Contact:

R. Berg
140 National Institute of Standards and Technology 

Workshop discussion topics:

(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?

DELIVERABLES: Identify suitable methods to measure vapor pressure and decomposition. Procure and 

assemble apparatus. 4Q 2006

Avoiding mercury and other incompatible materials eliminates many classic techniques. 

ACCOMPLISHMENTS

We continue to broaden the uses of the standards 

for gas fl ow that were developed recently at NIST. 

The model for the quartz capillary fl ow meter was 

applied to a ratio viscometer that yielded values 

of unprecedented accuracy (0.084 %) for the 

viscosity of argon at temperatures from 200 K to 

400 K. Similar results are expected for hydrogen, 

methane, ethane, and xenon. A similar fl ow meter will be used to provide a reference gas fl ow 

for calibrating spinning rotor pressure gauges at 

moderate vacuum.

RECENT PUBLICATIONS

E. F. May, M. R. Moldover, R. F. Berg, and J. J. Hurly, “Transport properties of argon at zero density from viscosity-ratio 

measurements,” Metrologia 43, 247-258 (2006).
Semiconductor Microelectronics and Nanoelectronics Programs 141

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.
142 National Institute of Standards and Technology 
Semiconductor Microelectronics and Nanoelectronics Programs 143

THIN-FILM X-RAY METROLOGY FOR

MICROELECTRONICS

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 fi lm characterization using X-Ray 

Refl ectometry (XRR).

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 High-Resolution X-Ray Diffraction (HRXRD) 

and XRR 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 

fi lm characterization (in particular; 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 Système International (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 using commercial software to address 

limitations to 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 estimations for a 

given XRR structural analysis approach and instrumentation. This work will take approximately 

three years to address the issues discussed [FYs 

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 deliverables will explore 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.

NIST XRR Study – The NIST ‘standard’ approach 

for improving accuracy and precision in a measurement technique involves the generation of a 

Standard Reference Material (SRM). Figure 1 

shows the three parallel research aspects needed 

for the development of an XRR SRM: (1) the 

Technical Contacts:

J. P. Cline

D. Windover

N. Armstrong,

University of 

 Technology Sydney

Australia

Figure 1. Aspects necessary for a NIST Standard 

Reference Material. All aspects are essential to 

provide structural parameter SI traceability.
144 National Institute of Standards and Technology 

manufacture of a robust thin fi lm calibration artifact with a high degree of temporal stability and 

contamination and/or oxidation resistance, (2) the 

development of SI traceable measurement instrumentation, (3) the creation of an SI traceable, fi rst 

principles data analysis approach. Figure 2 shows 

the current status of steps required to address these 

aspects for our XRR SRM project.

NIST has applied considerable resources into 

the development of SI traceable X-Ray Refl ectometry instrumentation. Over the past seven 

years, NIST/MSEL has constructed the Ceramics 

Division Parallel Beam Diffractometer (CDPBD) 

for SI traceable measurements of Powder XRD, 

Epitaxial HRXRD, and thin-fi lm XRR artifacts for 

the Standard Reference Materials (SRM) program 

(see Fig. 3). Over a similar timeframe, the Physics Laboratory developed the Physics Laboratory 

X-Ray Refl ectometer (PLXRR) specifi cally for 

XRR characterization of semiconductor thin-fi lm 

artifacts. As of FY06, we will be working in cooperation with the Physics Laboratory, to integrate 

the PLXRR into the XRR SRM effort allowing us 

cross-laboratory measurement comparison in our 

certifi cation process.

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 Laboratories in an effort to 

assess the temporal stability of international NML 

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.

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 145

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 underway to 

achieve an approximate one order of magnitude 

improvement in accuracy by next year (FY07). 

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 (FY07 deliverable).

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 (FY07+). 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 (approximate 

method in FY06). Addressing the third question 

is of considerable interest with current commercial 

refi nement approaches which often use dissimilar 

“chi squared” or model/simulation minimization 

criterion for overall refi nement “fi gures of merit” 

such that comparison between refi nement software 

packages is impossible. NIST will perform a rigorous analysis of current refi nement approaches 

(such as genetic algorithms which are popular in 

commercial software for their speed advantage, 

Monte Carlo Markov Chain, simulated annealing, etc.) and of applicable minimization criterion 

to develop a consistent refi nement approach for 

NIST modeling (FY07).

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.

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 
146 National Institute of Standards and Technology 

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 error 

budget estimations based on NIST XRR software 

and the calibration artifacts necessary for optimizing the performance of commercial “in-line” and 

laboratory XRR instruments. Accuracy and precision estimations provided by NIST software will 

determine the limitations of XRR applicability for 

arbitrary multilayer systems on commercial XRR 

instrumentation. 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 (FY07+ deliverables).

Our deliverables schedule for the calendar year 

3/2006-3/2007 time frame follow:

DELIVERABLES: 

NIST analysis of SEMATECH measurement. 

2Q 2006

Approximate Bayesian model selection method. 

2Q 2006

NIST analysis of SEMATECH measurements. 

4Q 2006

Approximate Bayesian interface selection method. 

2Q 2006

ACCOMPLISHMENTS

Last year, 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 in data 

modeling (current 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 in the future (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 refi nement and formal parameter uncertainty analysis 

(2Q 2006). A second round of measurements on 

SEMATECH artifacts measured by both conventional and NIST traceable XRR systems will be 

completed this year (2Q 2006). 

FY06 Technical transfer: Approximate Bayesian approach to model selection – Preliminary 

studies into the use of an approximate Bayesian 

approach for assigning relative probabilities between two structural models was completed last 

spring (FY05). This study was done with simulated XRR data sets. In FY06, this data analysis 

approach is being expanded to apply to data with 

included instrumental parameters. Initial results 

with this method stirred considerable interest due 

to its ease of implementation with conventional 

XRR packages. A requested deliverable in FY06 

is the recipe for this approach on both structural 

•

•

•

•

#

#
Semiconductor Microelectronics and Nanoelectronics Programs 147

model selection and interface type selection. In 

Fig. 4a, we show simulated XRR data from a 

two-layer structural model with included Poisson 

statistical noise. Two different structural models 

were used in data refi nement: a single-layer model 

(Fig. 4b) and a two-layer model (Fig. 4c). As 

expected, the two-layer model (Fig. 4c) showed 

to be most probable for each statistical noise level. 

The impressive feature in the study was that the 

probability ratio between the two models favored 

the two-layer case by many orders of magnitude 

for each level of statistical noise. This approximate method cannot give the absolute probability 

of a model. It can only provide a comparison of 

probabilities between two models, which is often 

a crucial piece of information (i.e., is there an 

interface or surface oxide present?). This recipe 

will be available to SEMATECH later this summer (2Q 2006).

COLLABORATIONS

SEMATECH – P.Y. Hung & Alain Diebold

PTB, Precision Metrology – Peter Thomsen-Schmidt

Bede Scientifi c Inc. – Keith Bowen & Matthew Wormington

Bruker AXS – Arnt Kern & Alan Coelho

Jordan Valley Semiconductors – Dileep Agnihotri

NMIJ/AIST – Toshiyuki Fujimoto

Rigaku MSC – Tom McNulty & Joe Formica

Technos International – Henry Yeung

RECENT PUBLICATIONS AND

PRESENTATIONS

N. Armstrong, D. Windover, A. Henins, J. P. Cline, “NIST 

X-Ray Metrology and SRM 1979 for naoncrystallite 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, and A. 

Diebold, “Characterization of Atomic Layer Deposition using 

X-Ray Refl ectometry” – poster/paper at 2005 International 

Conference on Characterization and Metrology for ULSI 

Technology, Dallas, TX (March 2005).

Figure 4. Example of the Bayesian Model selection 

method. The probability of different structural 

models can be compared to fi nd the most probable 

structure for given data. In this example, a twolayer structural model was used to simulate X-Ray 

Refl ectometry data (4a). A single-layer model (4b) 

and a two-layer model (4c) were used for data 

characterization and a Bayesian Model selection 

comparison approach was attempted. The two-layer 

model was shown to be the “correct” model for all 

two-layer simulated data at typical measurement 

statistical noise levels.

a)

b)

c)
148 National Institute of Standards and Technology 

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 149

ELECTRON MICROSCOPE TOMOGRAPHY OF

ELECTRONIC MATERIALS

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.

GOALS

Enable the use of three-dimensional imaging for 

thick samples using commercial 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 3D Monte 

Carlo simulations, the ability to perform tomographic reconstructions using an in-house 3D 

Bayesian code for tomographic reconstruction 

which permits an arbitrary transmission-thickness 

relation. Other capabilities are under development including a tomographic alignment program. 

An example is shown in Fig. 1.

DELIVERABLES: Tomographic alignment program 

which will report the tilt axis direction on the unit 

sphere. 2Q 2006

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.

ACCOMPLISHMENTS

Principle accomplishments include: creation 

of 3D Bayesian code for tomographic reconstruction; completion of prototype 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, 

shown in Fig. 2 (page 150).

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

#

Technical Contacts:

Z. H. Levine

J. H. Scott
150 National Institute of Standards and Technology 

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.

Accurel, Inc. Preparation of focused ion beam sections from 

low-κ samples

Lucent Technologies, Shu Yang; preparation of photonic band 

gap material.

RECENT PUBLICATIONS

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).
Semiconductor Microelectronics and Nanoelectronics Programs 151

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

HIGH-RESOLUTION MICROCALORIMETER X-RAY

SPECTROMETER FOR CHEMICAL ANALYSIS

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.

“Promising new technology such as high-energy 

resolution X-ray detectors must be rapidly commercialized. Prototype microcalorimeter energy 

dispersive spectrometers (EDS) and superconducting tunnel junction techniques have X-ray 

energy resolution capable of separating overlapping peaks and providing chemical information. 

These advances over traditional EDS and some 

wavelength dispersive spectrometers can enable 

particle and defect analysis on SEMS located in 

the clean room.” 2003 International Technology 

Roadmap for Semiconductors

TECHNICAL STRATEGY

1. The usefulness of single-pixel X-ray microcalorimeter EDS in materials analysis has now 
152 National Institute of Standards and Technology 

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 room-temperature 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. 

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.

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 2006

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 operation. 1Q 2006

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 3

He systems, which cool to 

300 mK, coupled to the solid-state tunnel-junction 

refrigerator cooling to 100 mK. 

DELIVERABLES: Demonstrate the cooling of an 

electrically-separate piece of thin-fi lm electronics using 

a tunnel-junction refrigerator that could be coupled to a 

simple 3He refrigerator. 2Q 2006

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 larger numbers of 

microcalorimeters under X-ray illumination. Characterize multiplexer performance and design next-generation system. 4Q 2006

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 

#
Semiconductor Microelectronics and Nanoelectronics Programs 153

environment of the cryocooler and its control 

electronics. We have also begun to characterize 

the effects of the mechanical vibration of the 

crycooler on other instruments such as a scanning 

electron microscope. 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 recently 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 a world 

record energy resolution of 2.4 eV FWHM at 

5.9 keV (see Fig. 3). 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. This year, 

we provided data analysis support and provided 

replacement feedback electronics. As shown in 

Fig. 4, CSTL staff have recently demonstrated the 

ability of the microcalorimeter to do quantitative 

microanalysis.

#

#

Figure 3. X-ray spectrum from improved NIST 

microcalorimeter. The FWHM resolution was 2.4 eV 

for 5.9 keV X-rays. 

We recently demonstrated the ability to read 

out eight TES microcalorimeters in a single multiplexed amplifi er channel with 3.8 eV FWHM 

energy resolution (Fig. 5). The detectors were fast, 

having a signal decay-time constant of about 150 

microseconds. Combining this result with closedform calculations and a new Monte-Carle software 

#

Figure 4. 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)

Figure 5. 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.
154 National Institute of Standards and Technology 

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 

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 3 x 32 high-resolution microcalorimeter array.

We have continued work on an on-chip 

solid-state 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 3

He 

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 normalinsulator-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 

#

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.

thin-fi lm and photolithographic techniques, has 

no moving parts, and operates continuously. We 

recently cooled a thin-fi lm payload from 320 mK 

to 225 mK. 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 

#

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.
Semiconductor Microelectronics and Nanoelectronics Programs 155

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.

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

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,” accepted, Nuclear Instruments and 

Methods A (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,” accepted, 

Nuclear Instruments and Methods A (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,” accepted, Nuclear Instruments 

and Methods A (2006).

K. D. Irwin, “Noise Theory of Nonlinear, Nonequilibrium 

Calorimeters and Bolometers,” accepted, Nuclear Instruments 

and Methods A (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,” accepted, Nuclear Instruments and 

Methods A (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).

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).
156 National Institute of Standards and Technology 
Semiconductor Microelectronics and Nanoelectronics Programs 157

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 and molectronic 

device structures need to be characterized. Another addition to the portfolio is the emerging 

fi eld of organic materials electronics. To this end we have initiated a new program, “Device 

Design and Characterization.” One project that has to be highlighted because it will impact 

all the projects in the portfolio over time is the Advanced Measurements Nanofabrication 

Facility Support project. With the completion of the Advanced Measurements Laboratory on 

the Gaithersburg, Maryland campus, we have populated a cleanroom facility with advanced 

processing and metrology equipment suitable for advancing into the nanotechnology era.
158 National Institute of Standards and Technology 
Semiconductor Microelectronics and Nanoelectronics Programs 159

DEVICE CHARACTERIZATION AND RELIABILITY

GOALS

The goal of the Advanced Device Characterization 

and Reliability Project is to improve and develop 

reliability and electrical characterization tools 

for advanced 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 for metal/

high-κ 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 evolving decrease 

of the gate dielectric fi lm thickness to an oxideequivalent value of 2 nm 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 0.8 nm or 

less by 2010. For effective gate dielectric thicknesses below ~ 2.0 nm, SiO2

 is being replaced, 

initially by oxynitrides or oxide/nitride stacks, and 

then by either metal-oxides or compounds such 

as metal silicates.

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 ultra-thin gate oxides that exhibit excessive 

tunneling currents and soft breakdown. 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 and 

time-dependent 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 

#

#

Technical Contacts:

J. Suehle

E. Vogel

M. Green
160 National Institute of Standards and Technology 

to the metal oxide and silicate dielectrics for a 

variety 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:

Develop ultra-fast current-voltage and “On-thefl y” characterization of charge trapping in High-κ

MOSFETs. 2Q 2006

Use scanning Kelvin probe microscopy (SKPM), 

CV, and IPE to compare metal work functions of 

high-κ stacks. 3Q 2006 

Study time-dependent dielectric breakdown in 

metal gate/high-κ gate dielectric systems. 4Q 2006

Compare metal work function measurements with 

measurements performed with other techniques 

including internal photo emission and scanning 

Kelvin probe microscopy on ternary metal systems 

produced by combinatorial techniques. 4Q 2006

ACCOMPLISHMENTS

FUNDAMENTAL UNDERSTANDING OF

METAL GATE WORKFUNCTION AND

BARRIER HEIGHT MEASUREMENTS USING

CV ANALYSIS

C-V measurements were performed on TaN/

TaSiC/HfO2/SiO2, TaN/TaCN/HfO2/SiO2, TaN/TaSiC/SiO2

, and TaN/TaCN/SiO2 stacks provided by 

SEMATECH. The HfO2 fi lms were deposited on 

a single wafer with a SiO2 interfacial layer that 

varied in thickness such that metal workfunction 

can be obtained by extrapolating the CV fl atband 

voltage to 0 equivalent oxide thickness (EOT). 

The C-V curves were modeled using the North 

Carolina State University CVC program, and 

the workfunction was obtained from the SiO2

thickness dependence. Figure 1 shows the fl atband voltage as a function of EOT on SiO2

. The 

effective workfunction of 4 nm TaSiN (4.13 eV) 

is approximately 0.25 eV smaller than that of 4 

nm TaCN (4.38 eV). The effective workfunction 

of these metals on HfO2 are observed to be ~0.3 

eV smaller than on SiO2

. We have performed 

IPE measurements on all wafers and different 

thicknesses of SiO2 stack. We observe numerous 

transitions and are in the process of performing 

•

•

•

•

#

modeling to understand the results. We have also 

performed Scanning Kelvin-Probe Microscopy 

(SKPM) and are in the process of analyzing these 

data.

FILM THICKNESS DEPENDENCE OF

Figure 1. Flatband voltage as a function of EOT to 

extract effective workfunction for TaN/TaSiC/HfO2/

SiO2, TaN/TaCN/HfO2/SiO2, TaN/TaSiC/SiO2, and 

TaN/TaCN/SiO2 stacks.

SUB-BANDGAP DEFECT STATES OBSERVED

IN POLYCRYSTALLINE HAFNIUM OXIDE

We compare hafnium-based high-κ dielectric fi lms grown by MOCVD and atomic layer 

deposition (ALD). MOCVD-grown HfO2 fi lms 

are mostly monoclinic, while HfSiO fi lms are 

amorphous. Thin ALD-grown HfO2 films are 

amorphous, while thick fi lms are monoclinic, with 

traces of orthorhombic or tetragonal phase present. 

Figure 2 (page 161) shows that sub-bandgap absorption is observed in polycrystalline, but not in 

amorphous, fi lms. 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.

#
Semiconductor Microelectronics and Nanoelectronics Programs 161

Figure 2. The imaginary part of the dielectric 

function, determined from VUV-SE data, for 5, 

10, 20, 40 nm thick ALD HfO2 fi lms. All MOCVD 

HfO2 spectra (not shown) are similar to that of 

the polycrystalline 40 nm ALD HfO2

 sample. All 

MOCVD HfSiO spectra (not shown) are similar to 

that of the amorphous 5 nm ALD HfO2 sample.

OPTIMIZATION OF SCANNING KELVIN

PROBE MICROSCOPY FOR DETERMINING

METAL WORK FUNCTION

A series of modifi cations and optimizations 

were performed with the SKPM methodology to 

improve the determination of metal work function 

for a variety of different metal samples prepared 

at NIST. The work function of the PtIr5 probe 

tip was corrected (4.9 eV) from contact potential 

measurements performed on specially prepared 

samples. The initial assumption for the work 

function of n+-Si was recalculated based on the 

actual SKPM tip resistivity. Finally, the cleaning 

of the metal samples was optimized using acetone, methanol, HF, and DI H2

O. The results are 

shown in Fig. 3. The fi gure shows the extracted 

metal work functions for a variety of metals and 

compared to literature values obtained from different contact potential difference techniques. The 

agreement is very good as indicated.

#

COMBINATORIAL STUDY OF NI-TI-PT

TERNARY METAL GATE ELECTRODES ON

HFO2 FOR ADVANCED GATE STACKS

Combinatorial techniques were used to study 

the Ni-Ti-Pt ternary metal gate system deposited on 

HfO2 dielectrics. The fl at-band voltage (ΔVfb) and 

leakage current (JL) were systematically studied 

through capacitance – voltage (C-V) and current 

– voltage (I-V) analysis. Both parameters offer a 

basis for understanding the gate stack properties 

since they are directly related to the effective work 

metal work function (Фm). A Si shadow-mask 

with hundreds of nominal 250 μm2 openings was 

placed directly on top of the wafer to make in-situ 

metal-oxide-silicon capacitors (MOSCAPS). Capacitance-voltage and current-voltage data were 

obtained by measuring a combinatorial array of 

several hundred capacitors. Figure 4 shows a 

two-dimensional map of ΔVfb (extracted from CV 

analysis) and JL. A more negative ΔVfb is observed 

close to the Ti-rich corner than close to the Ni- and 

Pt-rich corners. This observation implies a smaller 

#

Figure 3. Plot of metal work function determined 

by Scanning Kelvin Probe Microscopy for seven 

different metals samples prepared at NIST. The fi gure 

shows the comparison with literature values.

Figure 4. shows the map of the fl at-band voltage 

shift (ΔVfb ) extracted from C-V characteristics at 

1 kHz across the Ni-Ti-Pt libraries (left plot)and the 

leakage current density (JL ) map measured at an 

applied D.C. substrate bias of -1 V (right plot).
162 National Institute of Standards and Technology 

Фm near the Ti-rich corners and higher Фm near 

Ni- and Pt-rich corners. In addition the measured 

JL values are consistent with the observed ΔVfb

variations. 

STANDARDS COMMITTEE

PARTICIPATION

JEDEC JC14.2 Committee on Wafer-Level Reliability, Dielectric Working Group, Chairman 

(John S. Suehle).

PROFESSIONAL COMMITTEE

PARTICIPATION

Technical Chair, IEEE International Reliability 

Physics Symposium (J. Suehle)

Technical Chair, IEEE Interface Specialist Conference (E. Vogel)

Management Committee, IEEE International 

Integrated Reliability Workshop (J. Suehle)

Technical Committee, IEEE International Electron Device Meeting (J. Suehle)

SEMATECH/SIA working committees (E. Vogel)

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.

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 

U. Texas at Austin, Optical properties of ZrO2

 and HfO2 for 

use as high-κ gate dielectrics 

RECENT PUBLICATIONS

D. Heh, E. M. Vogel, J. B. Bernstein, C. D. Young, G. A. 

Brown, G. Bersuker, P. Y. Hung, and A. Diebold, “Electrical 

Characterization of Spatial Distributions of Trapping Centers 

in HfO2

/SiO2 Gate Stack,” submitted to IEEE Trans. Elec. Dev. 

(submitted for press).

S.-E. Park, J. S. Suehle, J. J. Kopanski, E. M. Vogel, and A. 

Davydov, “Determination of work functions for metals by 

scanning Kelvin probe microscopy,” submitted to J. Appl. 

Phys. (submitted for press).

D. Heh, E. M. Vogel, J. B. Bernstein, C. D. Young, G. A. 

Brown, P. Y. Hung, and A. Diebold, G. Bersuker, “Spatial 

Distributions of Trapping Centers in HfO2

/SiO2 Gate Stacks,”

accepted Appl. Phys. Lett. (submitted for press).

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).

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 

Tech. B, vol. 24, p. 404 (2006).

S.-E. Park, J. S. Suehle, J. J. Kopanski, and E. M. Vogel, 

“Calibration of work function for Pt0.95Ir0.05 fi lms with 

scanning Kelvin probe microscopy,” Appl. Phys. Lett., 88, 

on-line, (2006).

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.

D. R. Burgess, Jr., J. E. Maslar, W. S. Hurst, E. F. Moore, W. 

A. Kimes, R. R. Fink, N. V. Nguyen, “Atomic Layer Deposition – Process Models and Metrologies,” AIP Conference 

Proceedings 788, Characterization and Metrology for ULSI 

Metrology Technology, pp. 69-72, (2005).

S. Sayan, M. Croft, N.V. Nguyen, T. Emge, J. Ehrstein, I. 

Levin, J. Suehle, R.A. Bartynski, E. Garfunkel, “The relation 

between crystalline phase, electronic structure, and dielectric 

properties in high-κ gate stacks,” AIP Conference Proceedings 

788, Characterization and Metrology for ULSI Metrology 

Technology, pp. 92-101, (2005).

J.-P. Han, S. M. Koo, E. M. Vogel, E. P. Gusev, C. D’Emic, C. 

A. Richter, and J. S. Suehle, “Reverse Short Channel Effects 

in High-κ gated nMOSFETs,” Microelectronics Reliability, 

vol. 45, p. 783, (2005).

(Invited) J. S. Suehle, B. Zhu, Y. Chen, and J. B. Bernstein, 

“Detailed Study and projection of Hard Breakdown Evolution 

in Ultra-thin gate Oxides,” Microelectronics Reliability, 45, 

p. 419 (2005).

N. V. Nguyen, Albert V. Davydov, D. Chandler-Horowitz, 

M. 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 A. 

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).
Semiconductor Microelectronics and Nanoelectronics Programs 163

S. Sayan, N. V. Nguyen, and J. Ehrstein, T. Emge and E. 

Garfunkel, I. Levin, E. P. Gusev, H. Kim and P. J. McIntyre, 

“Structural, Electronic, and dielectric properties of ultrathin 

zirconia fi lms on silicon,” Applied Physics Letter 86, 152902 

(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).

Park, S., Kopanski, J. J., Kang, Y. , Robins, L. H., “Surface 

photovoltage spectroscopy of minority carrier diffusion 

lengths in undoped and Si-doped GaN epitaxial fi lms,” Physica 

Status Solidi, (24-MAR-2005).
164 National Institute of Standards and Technology 
Semiconductor Microelectronics and Nanoelectronics Programs 165

NANOELECTRONIC DEVICE METROLOGY

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 and molecular electronics) 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 Si-based 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). 

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.

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.

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 (see Fig. 2).

Figure 1. Curt Richter loads a molecular electronic 

sample for electrical characterization.

Technical Contacts:

C. Richter

N. Zimmerman

A. Davydov
166 National Institute of Standards and Technology 

DELIVERABLES:

Complete investigation of two-dimensional electrostatics in dual-gated Si-nanowire FETs nanofabricated from SOI wafers; prepare and submit 

manuscript. 4Q 2005

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 submit 

manuscript. 3Q 2006

•

•

Figure 2. A silicon nanowire grown by chemical 

vapor deposition integrated into a transfer length 

method test structure to determine nanowire contact 

resistance. 

Figure 3. Schematic overview of a suspended SixNy

membrane nanopore-based molecular electronic test 

structure.

Develop robust molecular test structures (see 

Fig. 3, for example) in order to use them to 

measure the electrical properties of molecules. 

The 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.

DELIVERABLES:

Develop, demonstrate, and disseminate a novel 

FTIR-based technique to characterize top-metal/

molecular interfaces. Investigate initial metal/molecule interfaces. Prepare and publish manuscript. 

4Q 2005

Utilize backside FTIR-based technique to characterize top-metal/molecular interfaces, and investigate metal/molecule interactions. Prepare and 

submit manuscript. 3Q 2006

Assess the ability of Six

Ny membrane nanoporebased molecular electronic test structures to 

characterize spin-dependent inelastic tunneling 

processes in molecular magnetic tunnel junctions 

– devices that combine features of moletronics and 

spintronics. Prepare and submit manuscript. 

3Q 2006

Develop geometries and architectures that 

maximize fl exibility and operating temperature 

of single-based single-electron devices.

DELIVERABLES: Complete and publish an experimental and theoretical study comparing charge offset 

drift in metal-based and Si-based SET transistors 

and/or the static charge offset problem in Si-based 

SET transistors. 4Q 2005

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: Develop a “nano-gap capacitor” 

to characterize electrical breakdown in air at small 

separations. 2Q 2006

Develop measurement methodology, models, and 

understanding needed to apply Scanning Capacitance Microscopy (SCM) and Scanning Kelvin 

Force Microscopy (SKFM) to the quantitative 

measurement of the local electrical properties 

of nanowires and other nanoelectronic devices. 

Probe shape, probe metal, operating conditions, 

and sample surface properties all infl uence the 

measured contact potential difference with SKFM. 

The accuracy of the measurement can be improved 

by careful control of the measurement properties, 

use of reference materials, and modeling (see 

Fig. 4).

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•
Semiconductor Microelectronics and Nanoelectronics Programs 167

Figure 4. Top: Simulated nanowire test 

structure consisting of four distinct metal lines in 

various repeat patterns. Bottom: Initial SKFM 

characterization of Kelvin Probe workfunction 

and resolution test structure. Measured work 

functions agree to within 90 mV of handbook values. 

Nanostructures introduce complicating geometry and 

surface effects.

DELIVERABLE: Determine the limitations and best 

possible accuracy of the 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

ACCOMPLISHMENTS

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 (see Fig. 5, 

page 168).

#
168 National Institute of Standards and Technology 

Figure 5. Assembly drawing and photograph of our nano-gap capacitor.

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 using electron-beam lithography 

to investigate the electrostatic control of current in semiconductor nanowire devices. These 

novel top- and bottom-gated FETs are based 

upon simple top-down test structures that rely 

upon self-aligned Schottky-contacts 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 nano-

#

#
Semiconductor Microelectronics and Nanoelectronics Programs 169

technology 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 

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. 

FY OUTPUTS

COLLABORATIONS

NTT, Akira Fujiwara, Si-nanowire metrology

Hewlett-Packard, R. Stanley Williams et al., 

Interface properties of molecular electronic test 

structures

NIST Divisions 836, 837, 838, Dr. Roger van 

Zee et al., Molecular Electronics Competence 

Project 

Yale University, Prof. M. A. Reed, Robust molecular electronic test structures 

RECENT PUBLICATIONS

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).

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).
170 National Institute of Standards and Technology 

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).

Koo S.-M., A.-F. Fujiwara, J.-P. Han, E. Vogel, C. Richter, 

and J. Bonevich, “High Inversion Current in Silicon Nanowire Field Effect Transistors,” Nano Lett., Vol. 4, 2107-2111, 

Nov. 2004.

Richter L. J., C. S. Yang, P. T. Wilson, C. Hacker, R. D. van 

Zee, J. J. Stapleton, D. L. Allara, J. M. Tour, “Optical Characterization of Oligo(phenylene-ethynlyene) Self-Assembled 

Monolayers on Gold,” J. Phys. Chem. B., Vol. 108, No. 33, 

pp. 12547-12559 (19-AUG-2004).

Hacker C. A., J. D. Batteas, J. C. Garno, M. Marquez, C. 

A. Richter, L. J. Richter, R. D. van Zee, C. D. Zangmeister, 

“Structure and Chemical Characterization of Self-Assembled 

Mono-Fluoro-Substituted Oligo(phenylene-ethynlyene) Monolayers on Gold,” Langmuir, Vol. 20, No. 15, pp. 6195-6205 

(22-JUN-2004).

Richter C. A., C. Hacker, L. J. Richter, E. M. Vogel, “Molecular Devices Formed by Direct Monolayer Attachment 

to Silicon,” Solid State Electronics, Vol. 48, pp. 1747-1752 

(17-JUN-2004).
Semiconductor Microelectronics and Nanoelectronics Programs 171

POWER SEMICONDUCTOR DEVICE METROLOGY

GOALS

The goal of the project is to develop electrical and 

thermal measurement methods and equipment 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 and 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 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 High-Voltage, 

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 distribution and conversion 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 underway 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 announced 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. In addition the Department 

of Energy has identifi ed HV-HF power devices 

as an enabling technology for alternative energy 

sources and energy storage systems. 

TECHNICAL STRATEGY

The NIST strategy is to support the measurement 

infrastructure of the semiconductor industry by 

developing and evaluating measurement methods 

and techniques where suitable ones do not exist 

for characterizing critical electrical and thermal 

properties of power devices and ICs. This includes 

electrical, thermal, and safe operating limit characterization, establishing performance metrics, 

and developing methods for extracting device 

model parameters to aid in application insertion. 

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. 

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 

Technical Contact:

A. Hefner
172 National Institute of Standards and Technology 

using large (6 ton and 10 m3) 2.7 MVA 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. 

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, EPRI Intelligent Universal Transformer Program, and the DOE Fuel Cell 

Power Plant program. 3Q 2007 

METROLOGY HIGH-VOLTAGE HIGHFREQUENCY SWITCHING DEVICE

PERFORMANCE

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) 3 kV 30 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. 

DELIVERABLES: Apply NIST high-voltage, highfrequency power device test systems to evaluate 

the performance the 10 kV, 100 A, 50 ns half-bridge 

power modules produced by the DARPA WBST-HPE 

program and asses the potential for enabling advance 

commercial and military power distribution systems. 

1Q 2007

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 bipolar-type devices (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 

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:

Use NIST forward-bias stress and degradation 

monitoring systems to assess progress of WBSTHPE program in producing degradation-free PiN 

diodes using Low Basel Plane Dislocation Density 

epitaxial layer technology. 3Q 2006

Develop a long term application-like 20-kHz switching test system to evaluate HV-HF application 

reliability and use to evaluate reliability of DARPA 

WBST-HPE. 1Q 2007

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 resulting in models that were used to 

predict the performance of the Phase 3 SSPS. 

DELIVERABLES: Collaborate with industry and government programs to simulate the performance of HVHF devices in power conversion system applications. 

4Q 2007

ACCOMPLISHMENTS

NIST played a key role in planning and coordinating government and industry programs 

and activities on high voltage SiC power devices 

including: Evaluated contractor performance for 

DARPA WBST HPE Phase 2. Participated in 

planning and writing BAA for DARPA WBST 

HPE Phase-3 programs and presented status of 

SiC power devices at the Phase 3 Industry Bidder 

Briefi ng day. Served as Member of DARPA/ONR 

SSPS Government Independent Design Panel. 

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Semiconductor Microelectronics and Nanoelectronics Programs 173

Participated in planning ONR Mantech SiC Power 

Device manufacturability program. 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. Participated in planning U.S. Army TACOM program 

on Reliable 4H-SiC MOSFET for Hybrid Electric 

Vehicle Power Systems. Served as the Moderator for the SiC Devices and Materials Focus at 

the DARPA/ONR Future of Power Electronics 

Workshop and assisted in determining the focus 

for a future DARPA SiC power device program. 

Served as the Chairman for the Industrial News 

Session at the International Conference on Silicon Carbide and Related Materials. Served as a 

reviewer for the DOE FreedomCAR and Vehicle 

Technologies Programs.

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. Typical 

high voltage silicon devices require several 

microseconds to switch. Signifi cant advances in 

materials, device design and HV-HF metrology 

were required for this achievement. 

#

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 HV-HF 

semiconductor device technology. 

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 temperaturecontrolled test fi xture with 20 kV voltage isolation 

and 350° C maximum controllable temperature 

was also included to enable investigation of HVHF device characteristics at elevated operating 

temperature. 

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. 

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 paper describing this work won the William M. Portnoy Award 

at the IEEE 39th Annual Meeting of the Industry 

Applications Society in October 2004. 

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

GE CRD, SiC power devices for robust integrated power 

electronic systems 

#

#

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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 100 ns. 

High voltage silicon power devices require several 

microseconds to switch even at half the voltage and 

125 °C. These devices were produced by Cree Inc 

under the DARPA WBST-HPE Phase II program. 

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 

#
174 National Institute of Standards and Technology 

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

RECENT PUBLICATIONS

A. R. Hefner, “Status of High-Voltage, High-Frequency Silicon-Carbide Power Devices,” GomacTech 2006, pp. 232-235, 

(22-MAR-2006).

A. R. Hefner, D. Berning, and C. Ellenwood, “Metrology For 

High-Voltage, High-Speed Silicon Carbide Power Devices,” 

GOMACTech 2005, pp. 226-229 (2005).

J.-S., Lai, H. Yu, J. Zhang, P. Alexandrov, Y. Li, J. H. Zhao, K. 

Sheng, K., A. Hefner, “Characterization of normally-off SiC 

vertical JFET devices and inverter circuits,” IEEE Conference 

Record Industry Applications Society Annual Meeting Volume 

1, Oct. 2005, pp. 404-409 (2005).

J. R. Reichl, J.-S. Lai, A. R. Hefner, T. R. McNutt, D. Berning, 

“Inverter Dynamic Electro-Thermal Modeling and Simulation 

with Experimental Verifi cation,” IEEE Power Electronics 

Specialists Conference, pp. 2208-2215 (June 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 175

ORGANIC ELECTRONICS METROLOGY

GOALS

Organic electronic devices are increasingly being incorporated into electronics packaging 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 exists for new diagnostic probes, 

tools, and methods to address new technological 

challenges. Their 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 provides 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 (Fig. 1). 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 represent the fi rst 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 found in organic devices is critical to 

proper interpretation of charge transport through 

molecules rather than single crystal structures. 

Identifying individual contributions to performance variations requires metrology unavailable 

to device manufacturers and spanning multiple 

disciplines: device physics, material electronic 

structure, chemistry, and materials science.

TECHNICAL STRATEGY

The initial focus of the NIST program is on the 

organic fi eld effect transistor (OFET) because, as 

in the silicon industry, the transistor is the basic 

building block for active devices. 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: (1) unique measurements 

of organic materials and interfaces for both (a) 

structure and chemistry and (b) electronic properties; and (2) the development of an integrated 

measurement test platform to correlate device 

performance with the structure and properties of 

active organic materials. 

Figure 1. Schematic representation of potential 

organic electronics applications.

Technical Contacts:

D. DeLongchamp

D. Gundlach

L. Richter

E. Lin
176 National Institute of Standards and Technology 

1. Interfacial structure and chemistry fundamentals: The basic OFET consists of thin layers 

(30 nm to 60 nm thick) of disparate materials 

including the organic semiconductor, dielectric, 

interconnect, 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 capable of in-situ nondestructive 

characterization of critical organic 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, measure the orientation of interfacial 

molecules, and quantify fi lm defect levels. Second 

order nonlinear optical spectroscopies, such as 

sum frequency generation (SFG) or coherent antiStokes Raman spectroscopy (CARS), are particularly appropriate for probing the buried semiconductor/dielectric interface. Interfacial structure 

(width, roughness) between OFET layers must 

also be measured in order to interpret performance 

variations. X-ray and neutron refl ectometry are 

powerful thin fi lm characterization methods for 

determination of the interfacial profi le between 

layers. Additional information about the materials 

is obtained using atomic force microscopy (AFM), 

spectroscopic ellipsometry, and polarized infrared 

absorption measurements (IR).

DELIVERABLES:

Complete NEXAFS measurements of evaporated 

small molecules onto silicon surfaces as a function 

of fi lm thickness and deposition temperature. 

4Q 2005

NEXAFS measurements of chemical conversion, molecular orientation, and defect density of 

thermally convertible oligothiophenes with varying 

number of repeat units. 1Q 2006

Complete measurements of annealing temperature 

effects on the thin fi lm morphology of liquid crystalline poly(thiophene) materials. 3Q 2006

•

•

•

Complete correlation of interfacial structure 

and transistor performance of liquid crystalline 

poly(thiophene) materials. 4Q 2006

2. Electronic property fundamentals: The 

response of organic electronic materials to electrical fi elds must also be measured to separate 

device architecture artifacts and intrinsic material 

properties. Model sandwich test structures have 

been designed to individually test the frequency 

response of semiconducting and dielectric materials employed within organic electronic ensembles. 

Capacitance and conductance measurements 

will be performed at frequencies up to 12 GHz 

to determine the dielectric properties of these 

materials. These results will be compared with 

traditional metrics such as current or capacitance 

versus voltage curves as functions of temperature, layer thickness, or contact metals. With 

new information (electronic and interfacial), we 

will elucidate mechanisms underlying the many 

anomalous phenomena observed in OFETs, such 

as permanent bias instability and the true nature 

of carrier mobility distribution, both of which are 

opaque to traditional metrics.

DELIVERABLES:

Construct samples and evaluate feasibility of frequency measurements of poly(3-hexyl thiophene) 

(P3HT) thin fi lms. 1Q 2006

Complete measurements of the dielectric response 

of P3HT as a function of frequency and temperature. 3Q 2006

3. Integrated measurement platform: To directly 

compare OFET performance with its interfacial 

structure and chemistry, test structures have been 

fabricated on a single substrate that will include 

both active devices and pre-defi ned measurement 

regions. We will identify motifs that develop after 

processing and during operation that critically 

affect OFET performance. The use of an integrated substrate removes variations that may affect 

measurements performed separately. Specifi c test 

structures optimized to transfer information to 

industrial laboratories will be developed. We plan 

to provide electrical measurement protocols that 

will identify electronic signatures representative 

of performance-infl uencing structural changes and 

meaningful standard test methods. 

DELIVERABLES: 

Measure fi eld effect hole mobility, threshold voltage, 

and on/off ratios for P3HT OFETs spin-cast onto 

the test structure with varying spin-coating speeds. 

4Q 2005

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Semiconductor Microelectronics and Nanoelectronics Programs 177

Evaluate correlations between structural measurements and device performance of liquid crystalline 

polythiophene thin fi lms. 4Q 2006.

ACCOMPLISHMENTS

Near-edge X-ray absorption fi ne structure 

(NEXAFS) spectroscopy was applied to several 

classes of organic electronics materials to investigate the electronic structure, chemistry, and 

orientation of these molecules near a supporting 

substrate. In collaboration with the University of 

California-Berkeley, NEXAFS spectroscopy was 

used successfully to quantify the simultaneous 

chemical conversion, molecular ordering (Fig. 2), 

and defect formation of soluble oligothiophene 

precursor fi lms for application in organic fi eld 

effect transistors. Variations in fi eld-effect hole 

mobility with thermal processing are directly 

correlated to the orientation and distribution of 

•

#

molecules within 3 nm to 20 nm thick fi lms. Further, NEXAFS measurements on a series of oligothiophene precursors showed that the orientation 

development is strongly affected by the number 

of units in the core of the molecule and the change 

in the structure of the solubilizing group.

A variety of polarized optical spectroscopies: 

infrared (IR) absorption, variable angle spectroscopic ellipsometry (SE), and near-edge X-ray 

absorption fi ne structure (NEXAFS) were applied 

in concert to investigate the structural evolution 

of poly(3-alkylthiophene) thin fi lms induced by 

thermal processing. The combined spectroscopies 

determine the orientation distribution of the main 

chain axis (SE and IR), the conjugated π system 

normal (NEXAFS), and the side chain axis (IR). 

We fi nd signifi cant improvement in the backbone 

order of the fi lms after heating above the melting 

point. Less aggressive thermal treatments are 

less effective. Interestingly, IR studies show that 

the changes in backbone structure occur without 

signifi cant alteration of the structure of the alkyl 

side chains. The data indicate that the side chains 

exhibit signifi cant disorder for all fi lms regardless 

of annealing conditions.

Variations in the spin coating speed were used 

to systematically vary the solvent evaporation 

rate and the P3HT conjugated plane orientation 

was measured with NEXAFS spectroscopy. We 

fi nd that the P3HT conjugated plane varies from 

an edge-on to a plane-on orientation as a function of the spin speed or solvent evaporation 

rate employed. This variation does not result in 

changes in surface morphology as observed by 

AFM, but it does correlate closely to the saturation 

hole mobility measured in fi eld effect transistors. 

These results provide important insight into the 

structure formation in solution-processed conjugated molecules. Further, most additive patterning 

processes envisioned for solution-processable 

semiconductors will include a drying step, and 

the drying rate appears to be a critical variable in 

process design.

COLLABORATIONS

Polymers Division, MSEL – Jan Obrzut, C. K. Chiang, 

Brandon M. Vogel, Youngsuk Jung, R. Joseph Kline, Tatiana 

Psurek, Leah Lucas

Semiconductor Electronics Division, EEEL – Curt Richter, 

Oleg Kirillov, Eric Vogel

Ceramics Division, NIST – Daniel A. Fischer, Sharadha 

Sambasivan

Surface and Microanalysis Science Division, CSTL – Marc 

Gurau, Zachary Schultz

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Figure 2. a, NEXAFS carbon K-edge spectra 

collected at changing incident angles from a fi lm 

annealed at 200 °C. The green area indicates the 

oligothiophene resonance used to determine 

planar orientation while the pink area indicates 

the resonance used to determine long axis 

orientation. b, Image depicting the orientation and 

spatial orientations of its primary K-edge carbon 

resonances. Blue arrows indicate incident polarized 

soft X-rays with electric fi eld vectors extending 

normal to the plane of photon polarization.

 a

b
178 National Institute of Standards and Technology 

University of California, Berkeley – Amanda Murphy, Paul 

Chang, Jean Frechet, Vivek Subramanian

Stanford University – Mang-mang Ling, Zhenan Bao

Xerox – Liang Li, Beng Ong, Michael Chabinyc

Merck Chemicals – Iain McCulloch, Martin Heeney

RECENT PUBLICATIONS

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.” In press J. Phys. Chem. B (2006).

M. C. Gurau, D. M. DeLongchamp, B. M. Vogel, E. K. Lin, D. 

A. Fischer, S. Sambasivan, L. J. Richter, “Ordering in Poly(3-

alkylthiophene) Thin Films Determined with Polarized Optical 

Spectroscopies,” submitted to J. Chem. Phys. (2005).

M. L. Chabinyc, F. Endicott, B. D. Vogt, D. M. DeLongchamp, 

E. K. Lin, Y. Wu, B. S. Ong, “Effects of Humidity on Unencapsulated Poly(thiophene) Thin Film Transistors,” (2005).

D. M. DeLongchamp, B. M. Vogel, Y. Jung, M. C. Gurau, C. 

A. Richter, O. Kirillov, J. Obrzut, D. A. Fischer, S. Sambasivan, L. J. Richter, E. K. Lin “Variations in semiconducting 

polymer microstructure and hole mobility with spin coating 

speed” Chem. Mater. 17, 5610 (2005).

A. R. Murphy P. C. Chang P. VanDyke J. S. Liu, J. M. J. Frechet, V. Subramanian, D. M. DeLongchamp, S. Sambasivan, 

D. A. Fischer, E. K. Lin, “Self-assembly, molecular ordering, 

and charge mobility in solution-processed ultrathin oligothiophene fi lms.” Chem. Mater. 17(24) 6033-6041 (2005).

D. M. DeLongchamp, S. Sambasivan, D. A. Fischer, E. K. 

Lin, P. Chang, A. R. Murphy, J. M. J. Fréchet, V. Subramanian, 

“Direct correlation of organic semiconductor fi lm structure to 

fi eld-effect mobility,” Advanced Materials, 17, 2340 (2005).

J. Obrzut and K. Kano, “Impedance and Non-Linear Dielectric 

Testing at High A.C. Voltages Using Waveforms,” IEEE Transactions on Instrumentation and Measurement (2005).

D. M. DeLongchamp, B. D. Vogt, C. M. Brooks, K. Kano, 

J. Obrzut, C. A. Richter, O. Kirillov, E. K. Lin, “Infl uence 

of a water rinse on the structure and electrical properties of 

poly(3,4-ethylene dioxythiophene):poly(styrene sulfonic acid) 

fi lms,” Langmuir 21, 11480 (2005).

J. Obrzut and A. Anopchenko, “Input Impedance of a Coaxial 

Line Terminated with a Complex Gap Capacitance - Numerical 

and Experimental Analysis” IEEE Transactions on Instrumentation and Measurement, 53: 1197-1201 (2004).
Semiconductor Microelectronics and Nanoelectronics Programs 179

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 recently 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” 

microelectromechanical systems (or 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 is in preparation and will be published in the MEMS Industry 

Group (MIG) 5-year anniversary report.

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 two venues: ASTM and SEMI. Highlighting 

the need for MNT standardization is the number 

of companies and other organizations participating 

in these standards activities.

DELIVERABLES: MEMS Industry Group report, 

“Standardization and the Study of the U.S. Measurement System for Micro Nano Technologies.”

The fi rst four MNT standards (on in-plane lengths, 

residual strain, strain gradient, and terminology) 

[1-4] originated in the ASTM E08.05.03 Task 

Group on Structural Films for MEMS and Electronic Applications. Also within this task group, 

standards for determining Young’s modulus, ultimate strength, and fatigue for a surface micromachining process are being pursued via grants. 

The MNT standardization efforts being actively 

pursued in SEMI’s North American MEMS 

Standards Committee include recently-published 

SEMI Standard MS1-0306 on wafer-to-wafer 

bonding alignment targets. Other SEMI MNT 

standardization efforts include a proposed guide 

(PR9-0705) on microfl uidic interfaces; a proposed 

standard (PR11-1105) on MEMS terminology; 

and work on stiction, wafer specifi cations, wafer 

bond inspection techniques, wafer bond strength, 

and RF MEMS qualifi cation.

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 

N.A. MEMS Standards Committee, will serve to 

guide industry and researchers to existing standards and highlight areas where standards may 

be needed.

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. 3Q 2006
180 National Institute of Standards and Technology 

Figure 1. CMOS test chip design incorporating 

thickness test structures, cantilevers, fi xed-fi xed 

beams, and tensile test structures.

Figure 2. For a thickness test structure: a) a design 

rendition and b) a cross-section.

a)

b)

Finally, the MNT Metrology project has undertaken development of dimensional metrology 

test structures and techniques for microfl uidics 

applications. The goal of this work is standard 

test structures, methods, and analysis techniques 

such as exist for semiconductor devices.

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.

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.

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. Narrow in on the next material properties 

to work on at NIST and follow through with the 

work, if appropriate. Currently, the next parameters being considered for standardization are 

thickness, Young’s modulus, residual stress, and 

residual stress gradient as described below:

Thickness: To measure the 35 thicknesses for 

the 48 p-well, CMP designs over fi eld oxide and 

over n-doped active area for a 1.5 μm commercial 

CMOS process, an electro-physical technique is 

used which combines a physical approach and an 

electrical approach. The physical approach obtains 

thicknesses from step height measurements on 

thickness test structures using a non-contact optical interferometer. (Figure 1 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. 2a, with its 

cross-section given in Fig. 2b.) The electrical approach obtains thicknesses from MOSIS-supplied 

capacitances, MOSIS-supplied sheet resistances 

and MOSIS-supplied resistivities, crystalline 

lattice calculations, and the equating of similar 

oxides between platforms. The thickness from 

the approach that results in the lowest value for 

the combined standard uncertainty, uc

, is the reported value.

For standardization, the above method will require 

simplifi cation in order to apply to more than one 

process.

Young’s modulus: The resonant frequencies of 

16 independently designed CMOS cantilevers as 

measured with an optical vibrometer are used to 

fi nd the Young’s modulus values of the various 

layers in the process. These frequencies in addition 

to the thicknesses as obtained using the electrophysical technique described above are the inputs 

to an optimization program, which outputs the 

Young’s modulus values.
Semiconductor Microelectronics and Nanoelectronics Programs 181

Figure 3. Optimized Young’s modulus values for 

metal1 and metal2 versus length.

For standardization, the above method will require 

simplifi cation in order to apply to more than one 

process. It is envisioned that this method will be 

benefi cial to the nano-world when studying such 

things as nanoscale cantilever-based physical, 

chemical, and biological sensors.

Residual stress and its gradient: As an extension 

of the above mentioned thickness and Young’s 

modulus work, CMOS cantilevers and fi xedfi xed beams can be fabricated on the same chip 

(as given in Fig. 1). The cantilevers can be used 

to fi nd the strain gradient and Young’s modulus 

values and the fi xed-fi xed beams can be used to 

fi nd residual strain. Given these measurements, 

the residual stress and its gradient can be calculated. 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. 

DELIVERABLES: The results of our work will be 

presented at the appropriate standards organization 

and in articles for publication. 1Q 2007

ACCOMPLISHMENTS

Thickness: The electro-physical technique 

proved successful in fi nding the thicknesses of 

all the layers in the 1.5 μm commercial CMOS 

foundry process with relatively low values for uc 

(between 0.00012 μm and 0.056 μm for a given 

processing run). 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.

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 which 

tend to be fabricated earlier in the processing 

sequence) and the physical approach used for the 

larger values of uc.

The electro-physical technique is detailed in a 30-

page paper, which is currently being reviewed. 

Young’s modulus: The test chip design 

given in Fig. 1 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 3 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. 3. 

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 by a co-author.

COLLABORATIONS

MOSIS Integrated Circuit Fabrication Service, 4676 Admiralty 

Way, Marina del Rey, CA, Dr. P. Thomas Veriner.

Klaros Corporation, Beallsville, MD Robert I. Scace.

BayTech Group, 30 Winsor Way, Weston, MA, Winthrop A. 

Baylies.

NIST Boulder, Materials Science and Engineering Laboratory, 

Materials Reliability Division, Dr. David T. Read.

Others (current and/or future): Coventor, Inc., Pennsylvania 

State Univ., Fraunhofer Inst. (Germany), MEMSCAP, Sandia, 

and others not yet identifi ed.

RECENT PUBLICATIONS

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. 
182 National Institute of Standards and Technology 

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.

MEMS Length and Strain Calculator Web site: http://www.

eeel.nist.gov/812/test-structures/MEMSCalculator.htm.
Semiconductor Microelectronics and Nanoelectronics Programs 183

NIST’S CENTER FOR NANOSCALE SCIENCE AND

TECHNOLOGY NANOFAB

GOALS

The NIST Center for Nanoscale Science and 

Technology (CNST) Nanofab will:

Enable fabrication of prototypical nanoscale 

test structures, measurement instruments, standard reference materials, electronic devices, magnetic devices, MEMS, and bio-devices critical to 

NIST’s Strategic Focus Areas (Nanotechnology, 

Homeland Security, Healthcare) and the nation’s 

Nanotechnology Needs

Provide access to expensive nanofabrication 

tools, technologies and expertise in a shared-access, shared-cost environment to NIST and its 

partners

Foster internal collaboration in Nanotechnology across NIST’s Laboratories

Foster external collaboration in Nanotechnology with NIST’s partners. 

CUSTOMER NEEDS

To continue to respond to U.S. science and industry’s needs for more sophisticated measurements 

and standards in the face of heightened global 

competition, NIST has constructed one of the 

most technologically advanced facilities in the 

world — the Advanced Measurement Laboratory, 

or AML. The CNST Nanofab (Fig. 1) is one of 

fi ve buildings in the AML at the Gaithersburg, 

MD campus. The Nanofab provides researchers 

at NIST working on a variety of semiconductor 

and other nanotechnology research the ability to 

fabricate prototypical nanoscale test structures, 

measurement instruments, standard reference 

materials, and electronic devices.

#

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#

#

TECHNICAL STRATEGY

The AML contains two above ground instrument 

buildings, two completely below ground metrology buildings, and one Class 100 clean room 

building housing the CNST Nanofab. The AML 

provides NIST with superior vibration, temperature and humidity control, and air cleanliness. The 

CNST Nanofab has approximately 1000 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), 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, 

etc.). A state-of-the-art e-beam lithography system with mask making capability is scheduled for 

installation during the Fall of 2006.

Figure 1. Main entrance to the AML showing the 

CNST Nanofab on the left.

Figure 2. Oxidation and annealing furnaces banks.

Technical Contacts:

R. Celotta

G. Henein

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. The tools are operated in a manner 

such that a wide variety of materials can be processed. The facility is part of NIST’s Center for 

Nanoscale Science and Technology. Unlike other 

nanofabs, the NIST nanofab is unique in that it is 
184 National Institute of Standards and Technology 

located next to the most advanced metrology tools 

in the world (AML), and its focus is the fabrication 

of nanoscale structures necessary for metrology 

and standards in support of the semiconductor 

industry, nanotechnology, biotechnology, and 

homeland security. 

DELIVERABLES: E-beam lithography system fully 

operational. 4Q 2006
Semiconductor Microelectronics and Nanoelectronics Programs 185

METROLOGY FOR SPINTRONIC DEVICES

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 spinpolarized 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.

“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).

Technical Contacts:

J. Unguris

R. Goldfarb
186 National Institute of Standards and Technology 

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 spinbased 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 

Figure 1. Within a few years, critical device 

dimensions will be less than 100 nm. Magnetic fi eld 

based switching of MRAM is no longer feasible at the 

65 nm node and alternative switching mechanisms 

will required. Similar scalability limitations are 

also expected in CMOS devices. Source: NIST 

Electromagnetics Division. 

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 and provide a basis for 

evaluating the potential of nanoscale spintronic 

devices for replacing/augmenting CMOS in the 

deep nanometer regime.

1. Develop method to determine intrinsic current-based switching threshold in all metallic 

MRAM structures.

DELIVERABLES:

Characterize thermally activated current induced 

switching in all-metallic MRAM structures. Complete measurements in 4Q 2005.

Measure high speed switching in all-metallic devices ranging from 50 nm to 150 nm and extrapolate the value of the intrinsic switching threshold. 

Compare extrapolated value to the value measured 

at low temperature. Complete measurements in 2Q 

2006 and submit paper.

2. Determine scaling of magnetic damping parameter with device size.

DELIVERABLES: Measure damping parameter in a 

variety of magnetic nanostructures with dimensions 

between 100 nm and 1 micrometer and compare those 

measurements of damping in continuous fi lms. Complete measurements in 2Q 2006 and submit paper.

3. Characterize noise and current induced 

switching tunnel junction based MRAM devices.

DELIVERABLES: 

Measure and characterize low frequency noise in 

high-quality aluminum oxide based magnetic tunnel 

junctions. Complete measurements in 3Q 2006 and 

submit paper.

Measure threshold for current induced switching in 

tunnel junction based MRAM devices at below 1 ns 

for device sizes ranging from 50 nm to 150 nm. 

Compare results with those obtained from all-metallic based structures and theoretical models. Complete measurements in 4Q 2006 and submit paper.

Although SEMPA is already an established tool 

for imaging magnetic nanostructures in simple 

ferromagnetic structures, 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 

•

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•

•
Semiconductor Microelectronics and Nanoelectronics Programs 187

element directly to the SEMPA probe need to be 

developed. Of course these cleaning procedures, 

which will involve techniques such as ion milling, must not disturb the magnetic structure under 

investigation. Second, since a critical question is 

the behavior of magnetic nanostructures in a functioning device, instrumentation will be developed 

to allow magnetic imaging while the device is 

electrically active i.e., while applying magnetic 

fi elds or measuring magnetoresistance.

1. Image simple static devices that are the same 

size and shape as real devices, but are not electrically active or covered with capping layers.

DELIVERABLES: SEMPA images of magnetic 

nanostructure in patterned devices made from bare 

ferromagnetic fi lms and multilayers. 2Q 2006

2. Develop cleaning procedures to expose 

magnetic material in more complex, multilayer 

device structures where the magnetic structures 

are buried under nonmagnetic layers. Determine 

in-situ ion milling protocols that can remove 

nonmagnetic coatings without disturbing the 

magnetization.

DELIVERABLES: SEMPA images of buried magnetic 

nanostuctures. 3Q 2006

3. Construct custom sample holders to enable 

magnetic imaging of electrically active magnetoelectronic devices. These sample holders 

will allow up to seven different electrical connections per sample without compromising the 

electron microscope’s load lock or goniometer 

operation.

DELIVERABLES: SEMPA images of devices while 

applying magnetic fi elds and measuring magnetoresistance. 4Q 2006

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 thresh-

#

old 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. 

Figure 2. High speed current induced switching of 

all-metallic MRAM structures. The data show that the 

switching frequency is roughly linearly proportional 

to the threshold switching current. Source: 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).

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 

#
188 National Institute of Standards and Technology 

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. While the measurements will be 

limited to giving the average properties of device 

arrays, we expect that the information we gain 

will yield signifi cant insights into the hierarchy of 

damping mechanisms in magnetic nanostructures. 

Measurements are presently underway. We are 

also exploring new measurement methods which 

will allow accurate damping measurements of 

individual magnetic nanostructures.

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. 

Work is presently underway to incorporate these 

tunnel junction barriers into nanometer scale 

MRAM devices in order to investigate both current induced switching and noise in nanoscale 

MTJ devices.

#

We have systematically measured the magnetic 

structure of prototypical magnetic sensor elements 

as a function of size. The large magnifi cation range 

of SEMPA enables imaging devices that vary 

from millimeter to nanometer dimensions. This 

work quantitatively demonstrated the transition 

from shape dominated, to exchange dominated 

magnetic structure as the size of these structures 

was reduced (Fig. 3).

Figure 3. SEMPA image of a magnetic sensor 

element, and a plot of the amplitude of magnetic 

fl uctuations versus device size. As the device 

gets smaller, exchange interactions prevent the 

magnetization from following the shape of the device 

and the amplitude decreases in agreement with 

micromagnetic (OOMMF) simulations. 

We have successfully imaged MRAM prototypes 

provided by IBM (Fig. 4, page 189). These patterned structures consist of bare ferromagnetic 

fi lms grown on antiferromagnetic fi lms. Instead 

of the magnetization being simply aligned either 

along or opposite to the element axis, SEMPA 

images show considerable fluctuation of the 

magnetization direction within an element. The 

anisotropy dispersion is possibly due to the 

polycrystalline nature of the anitferromagnetic 

fi lm used for exchange biasing, and can lead to 

problematic variations of the magnetic switching 

fi eld for these devices.
Semiconductor Microelectronics and Nanoelectronics Programs 189

Figure 4. SEMPA image of an 0.5 μm MRAM 

element. Although the element is nominally 

magnetized in one direction, variations of the 

magnetization direction within the element are 

seen both in the variations in color (related to 

magnetization direction by the colorwheel) and by 

the overlaid arrow fi eld.

We have imaged spin torque induced domain 

wall motion in nanoscale wires. The spin torque 

associated with the high current density in a ferromagnetic nanostucture can be used to switch 

the magnetic state or to move magnetic domain 

walls. Being able to manipulate the state of a 

magnetic device without applying a magnetic fi eld 

is appealing for high density magnetic memory 

applications. We have prepared magnetic wires 

with different dimensions and investigated how 

the spin polarized current affects domain walls in 

these wires. SEMPA images reveal that the spin 

torques not only push the domain walls along the 

wire, but can also distort the walls, and, in some 

cases, convert the domain walls from one type to 

another (see Fig. 5).

COLLABORATIONS

IBM, MRAM device imaging and thermal induced noise in 

tunnel-junction based MRAM devices.

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.

Cornell University, damping in ferromagnetic systems.

U.S. Army Research Laboratory, imaging magnetic sensors

Figure 5. SEMPA images of current induced domain 

wall transitions in a 300 nm wide ferromagnetic wire. 

The spin torque associated with the current moves 

the domain walls along the wires, but it also causes 

the walls to switch from vortex to transverse wall 

structures.

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

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).
190 National Institute of Standards and Technology 

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 191

BIOELECTRONICS METROLOGY

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 neural 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 are focusing our initial research efforts on 

the study of retinal (neuronal) cells and will later 

progress to other cell systems.

DELIVERABLES: 

Develop an array of micro-electrochemical cells 

and methods to pattern cells on the array for singlecell electrochemical measurements. 3Q 2007

•

Technical Contacts:

M. Gaitan

J. J. Kasianowicz

D. R. Reyes

B. J. Polk
192 National Institute of Standards and Technology 

Microfl uidic and microelectronic components on 

a single platform for delivering precise chemical 

stimuli to single cells. 1Q 2007

Characterization of the single-cell culture response 

to glutamate with PEDF as a neurotrophic factor. 

3Q 2007

Develop integrated rf/microwave structures for 

controlling the temperature of single-cell culture 

chambers and/or for manipulating cells. 4Q 2006

Continuous monitoring of cell growth, development 

and viability in the presence and absence of therapeutic drugs and chemical toxins. 3Q 2007 

ACCOMPLISHMENTS

We developed a method to adhere retinal 

cells on micropatterned polyelectrolyte multilayer 

(PEM) lines adsorbed on poly(dimethylsiloxane) 

(PDMS) surfaces using microfl uidic networks. 

PEMs were patterned on fl at, oxidized PDMS 

surfaces by sequentially fl owing polyions through 

a microchannel network that was placed in contact 

with the PDMS surface. Polyethyleneimine (PEI) 

and poly(allylamine hydrochloride) (PAH) were 

the polyions used as the top layer cellular adhesion material. The microfl uidic network was lifted 

off after the patterning was completed and retinal 

cells were seeded on the PEM/PDMS surfaces. 

The traditional practice of using blocking agents 

to prevent the adhesion of cells on unpatterned 

areas was avoided by allowing the PDMS surface 

to return to its uncharged state after the patterning 

was completed. The adhesion of rat retinal cells 

on the patterned PEMs was observed fi ve hours 

after seeding. Cell viability and morphology on 

the patterned PEMs were assayed. These materials 

proved to be nontoxic to the cells used in this study 

regardless of the number of stacked PEM layers. 

Phalloidin staining of the cytoskeleton revealed 

no apparent morphological differences in retinal 

cells compared with those plated on polystyrene 

or the larger regions of PEI and PAH (Fig. 1, 

left panel); however, cells were relatively more 

elongated when cultured on the PEM lines. Cell-

•

•

•

•

#

to-cell communication between cells on adjacent 

PEM lines was observed as interconnecting tubes 

containing actin that were a few hundred nanometers in diameter and up to 55 mm in length (Fig. 1, 

center panel). This approach provides a simple, 

fast, and inexpensive method of patterning cells 

onto micrometer-scale features. 

Figure 2. SEM images of two silver nanopores 

formed by electroplating on micromachined silicon 

micropores.

Figure 1. The left and center panels are images of R28 cells retinal patterned using 

polyelectrolyte multilayers lines adsorbed on poly(dimethylsiloxane). The center panel 

shows a nanochannel that was spontaneously formed by the cells. The right panel is an 

image of cells patterned on a gold surface using the polyelectrolyte multilayers.

We developed a method to create nanofl uidic 

restrictions by electroplating silver on micrometer 

scale pores. Electrodeposition of silver was investigated as a fabrication tool for constricting large 

(103 μm2) vias in silicon substrates while leaving a 

small opening in the center of the via. Silver reduction from ammoniacal silver nitrate was studied at 

electrodes of novel geometry (i.e., the edge of the 

vias) with respect to reduction potential, reduction 

pulse type, and pulse duration. A variety of crystal 

nucleation and growth patterns was observed and 

characterized by scanning electron microscopy 

(Fig. 2). It was found that electroplated silver 

occluded the vias to leave open areas of less than 

1 μm2. Such occlusions might be used as restrictions in microfl uidics systems, forming a type of 

solid-state micropore or nanopore.

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 microrefer-

#

#
Semiconductor Microelectronics and Nanoelectronics Programs 193

ence electrodes. Characterization results 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). A new approach for 

temperature cycling of microfl uidic systems is 

presented that 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). We 

demonstrate that the microwave signal can be 

tuned to most effectively deliver the heat to the 

fl uid at a frequency of 18 GHz (in agreement 

with theory) independent of the ionic strength of 

the solution. 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. This paper suggests that those advantages 

can also be realized in a microfuidic format.

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

S. P. Forry, D. R. Reyes, M. Gaitan and L. E. Locascio 

“Facilitating the Culture of Mammalian Nerve Cells with 

Polyelectrolyte Multilayers,” Langmuir, in press.

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).

#

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.
194 National Institute of Standards and Technology 

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. Sundaresan1, 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 195

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.
196 National Institute of Standards and Technology 
Semiconductor Microelectronics and Nanoelectronics Programs 197

Technical Contacts:

A. R. Hefner, Jr.

M. Y. Afridi

“... 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 development 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 environmental 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 Homeland 

Security and Industrial applications.

CUSTOMER NEEDS

Recent advances in high density CMOS integration and the ability to co-integrate MEMS-based 

sensor devices enable 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 verifi cation strategies for multi-technology 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 designers, SoC manufacturers and 

systems designers. This is generally recognized 

by the chip designers, manufacturers, 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 develop successfully ES-VCs for SoC 

design methodology, the fi rst step in this multistep process is to develop 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 gassensor, including operational amplifi ers, decoders, 

and an analog-to-digital converter (ADC), and a 

microcontroller for control and data processing.

2. The second step is to make ES-VCs compatible with the standard digital SoC design methodology. This approach will require ES-VC to 

incorporate digital interface circuitry and to have 
198 National Institute of Standards and Technology 

the DFT/BIST (Design-For-Test/Built-In SelfTest) functionality required by SoC standards. To 

facilitate this approach we will develop methodologies and standards for adding digital shells to 

ES-VCs and demonstrate them on the gas-sensor 

VC described above. 

DELIVERABLES: Develop and fabricate a fully digital 

gas-sensor VC in a standard CMOS 0.5-micron technology to demonstrate a standard digital interface and 

SoC Design-For-Test functionality. 4Q 2006 

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 organizations such as OCP-IP, VSIA and 

OSCI to foster large-scale interchange and interoperability of modular digital 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. 3Q 2007

4. 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 gas-sensor 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. 

3Q 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 and Fig. 2). 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 concentrations 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. 

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Figure 1. Micrograph of gas-sensor system to be 

used as demonstration vehicle.
Semiconductor Microelectronics and Nanoelectronics Programs 199

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 demonstrate that the design approach 

was compatible with SoC design methodology. 

The performance of the 8-bit ADC exceeded the 

gas-sensor VC design requirements.

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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.

Figure 4. New microhotplate design implemented in 

standard submicron CMOS technology.

Electrostatic discharge (ESD) protection 

structures were added to the gas-sensor and successfully tested (Fig. 3). These ESD test structures 

are based on multi-fi nger thyristor-type devices 

and are designed to achieve optimum performance 

and reduced area.

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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.

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Methodologies for designing digital interface 

shell functionality for ES-VCs were developed 

and demonstrated by designing the gas sensor 

SoC architecture.

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 microcontroller 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 

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200 National Institute of Standards and Technology 

different CMOS compatible temperature sensors 

and to measure the contact resistance between 

different types of post-processed gas sensor 

electrodes (Fig. 5). This chip also supports the 

extraction of more accurate microhotplate thermal-model parameters. 

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.

RECOGNITION

2005 GEORGE ABRAHAM OUTSTANDING

PAPER AWARD:

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

J. Geist, M. Y. Afridi, A. Varma, A. R. Hefner, “Simple Thermal-effi ciency Model for CMOS-Microhotplate Design,” J. 

Res. NIST (accepted 2006).

M. 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.

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).

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.
Semiconductor Microelectronics and Nanoelectronics Programs 201

AT-SPEED TEST OF DIGITAL INTEGRATED CIRCUITS

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 

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.
202 National Institute of Standards and Technology 

effects include instrument response, impedance 

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. 2Q 2006

Optimize the response of the fi ber optic fi eld probe 

by design and control of the refl ectivity of the integrated MEMS mirrors. 3Q 2006 

Demonstrate the capability to measure the thermal 

fl uctuations of a single cantilever with the integrated probe. 4Q 2006 

Demonstrate the capability to measure time-domain insertion delay of a coaxial cable with 1 ps 

resolution. 4Q 2006 

ACCOMPLISHMENTS

We have designed and tested a prototype 

sinusoidal waveform standard.

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. 

We have demonstrated a calibrated measurement of an on-wafer pulsed waveform up to 

200 GHz using electro-optic sampling. 

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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 (see Fig. 2).

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Figure 2. Measurement system used to verify the 

functionality of our photoconductive switches.

We developed the fabrication process for 

creating the MEMS chips with integrated 45-

degree mirrors and trenches for fi ber alignment 

(see Fig. 3). 

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Figure 3. The Fiber-optic trench with integrated 

45-degree mirror.
Semiconductor Microelectronics and Nanoelectronics Programs 203

We assembled the fi ber optic interferometer 

for the measurement of the defl ection of the cantilevers including the thermal feedback control.

RECENT PUBLICATIONS

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,” accepted for publication 

in IEEE Trans. Microwave Theory Tech. 

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,” accepted for publication 

in IEEE Trans. Instrum. Meas., 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).

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204 National Institute of Standards and Technology 
Semiconductor Microelectronics and Nanoelectronics Programs 205

Technical Contacts:

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

THERMAL MEASUREMENTS AND PACKAGING

RELIABILITY

GOALS

Our goal is to provide the microelectronics packaging industry with information, guidance, and 

tools through technology transfer to characterize 

the behavior of features and interfaces in packaging 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 
206 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.” 

2005 ITRS: one of the fi ve “Diffi cult Interconnect 

Challenges”

“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.” 2005 ITRS: one 

of the fi ve “Diffi cult Interconnect Challenges”

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.

We have just completed our fourth year of applying thermal conductivity measurements using the 

IR microscope to the problem of packaging reliability, and have engendered great interest from 

the Advanced Embedded Passive Technology 

(AEPT) Consortium. As thermal conductivity 

measurements are one of the most sensitive indicators of metal purity, likewise they are one of the 

most sensitive indicators of interfacial integrity. A 

minute increase in interfacial thermal conductivity 

is the fi rst indication of the microcracks and fi ssures that ultimately may result in failure. 

DELIVERABLES: Scanning Probe Microscope (SPM) 

calibration devices for thermal SPM designed and 

fabricated. 2Q 2006

2. As the size of packages gets smaller, heat 

removal becomes a more signifi cant problem. 

The SPM is being used to develop a technique 

to measure thermal conductivity of thin fi lms 

for application to metal interconnect lines. The 

technique also will be able to measure interfacial 

thermal resistance between coatings and substrates. The theoretical work on this development 

is being done in collaboration with Dr. Kevin Cole 

of the University of Nebraska.

DELIVERABLES: Refi ne the theory in conjunction 

with development of increased measurement accuracy. 3Q 2006

3. Carbon nanotubes have potential in heatremoval applications and in wear applications. 

Thermal properties show great promise in this 

area, if production issues can be addressed. We 

are currently measuring carbon nanotubes made 

from different inexpensive processes to determine 

if heat removal applications are economically and 

technologically feasible.
Semiconductor Microelectronics and Nanoelectronics Programs 207

DELIVERABLES: Measure interfacial thermal 

resistance of carbon nanotube-covered copper and 

baseline copper materials. 2Q 2006

4. Thermomechanical fatigue of interconnect 

lines is an important reliability issue as lines 

become smaller. Using thermal scanned-probe 

microscopy, we are measuring temperatures of 

damaged locations on lines and temperature distribution on and around interconnect lines to elucidate failure mechanisms and predict lifetimes.

DELIVERABLES: Measure lines both temporally and 

spatially. Report results in literature. 4Q 2006

5. High speed temperature sensitive parameter 

(TSP) measurements are required for in-situ 

evaluation of the heat transport through the interface 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 High 

Power Electronics program 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. 4Q 2006

6. A limitation of commercially available infrared (IR) thermal imaging systems is their inability to make high speed transient 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 resolution. The new 

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: Apply high speed thermal imaging 

system to characterize localized dynamic heating of 

reduced power FUBs in advanced digital integrated 

circuits. 4Q 2006

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. 4Q 2007

7. 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: Complete development of prototype 

IR transient thermal imaging calibration test chip. 

3Q 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.

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Figure 1. A microdevice with an integral heater and 

sensor for calibration of thermal probe tips.

The change in resistance as a function of 

temperature of the sensor will be used to calibrate 

AFM probe tips. Figure 2 (page 208) shows the 

calibration curve for one of the devices.

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208 National Institute of Standards and Technology 

Feasibility has been shown of measurements 

of electronic interconnect lines using the thermal 

SPM. We are now calibrating the response of the 

probe tip to temperature.

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,” submitted to Metallurgical and Materials 

Transactions April 2006, Paper presented at the 

2006 TMS Annual Meeting, 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. 

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Figure 3. Sample data showing time-resolved 

thermal fl uctuations of an interconnect line during 

A.C. heating.

Figure 4. Comparison of temperature differences 

across thermal-grease-and-copper interfaces and 

carbon nanotubes.

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 

begin developing large amounts of tube-to-tube 

contact, the CNT specimens outperform the copper with thermal grease.

#

Figure 2. Calibration curve from a NIST microdevice showing device 

temperature as a function of sensor resistance.
Semiconductor Microelectronics and Nanoelectronics Programs 209

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.

NIST recently developed and tested a transient thermal calibration test structure with 1 μs 

temporal resolution using micro hotplates 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 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.

Figure 5. Microhotplate thermal response to a 20 μs 

heating pulse (as indicated) measured with both IR 

and electrical methods.

operation and the diode shown in Fig. 6b demonstrates highly non-uniform current conduction 

after 500 hours operation as represented by the 

non-uniform high-speed pulsed temperature map 

(only 50 % of device is conducting). 

As low-degradation rate SiC PiN 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 1000 hours of operation. The pulsed thermal 

image results show that the current is 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 highly variable, 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 shows good current uniformity after 500 hours 

#
210 National Institute of Standards and Technology 

NIST has developed a 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 the heating response measurement with 

the thermal model for a commercial high power 

IGBT half bridge module, commercial six-pack 

IGBT module, and prototype NSF Center for 

Power Electronic Systems Module. Electrothermal simulations of a full three-phase inverter 

have been successfully performed and compared 

with measured results of the power converter 

temperature monitors. 

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.

COLLABORATIONS

University of Nebraska: Kevin Cole

Cenymer Corporation: Scott Joray

Cynthia Volkert, Forschungszentrum Karlsruhe, Germany

Reiner Mönig, MIT, Cambridge, MA

Glenn Alers, 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

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RECENT PUBLICATIONS

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, Paper presented at the 2006 

TMS Annual Meeting (April 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.

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,” Volume: 2 , pp.1252-1260 (Oct. 2004).

Blackburn, D. L., “Temperature Measurement of Semiconductor Devices – A Review,” Proceedings of Semiconductor 

Thermal Measurement and Management Symposium, pp. 

70-80 (March 2004).

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, 

Anaheim, CA (Feb. 2004).
Semiconductor Microelectronics and Nanoelectronics Programs 211

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.
212 National Institute of Standards and Technology 
Semiconductor Microelectronics and Nanoelectronics Programs 213

Technical Contact:

Y. Li

FACTORY TIME SYNCHRONIZATION STANDARDS

DEVELOPMENT FOR E-MANUFACTURING

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. 

One of the projected near term Factory Integration difficult challenges in the International 

Technology Roadmap for Semiconductors (ITRS) 

is managing the increasing factory complexity 

which results in: 

“Explosive growth of data collection/analysis 

requirements driven by process and modeling 

needs...” 2005 ITRS Edition, Factory Integration, p. 3.

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. 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). 

Quality time stamps ensure the ability to precisely 

pin-point cause-effect relationships.

“A prerequisite to APC deployment is thus acceptable levels of data quality provided by the 

tool, metrology, and sensors. Data quality issues 

include availability, timeliness (of data capture 

and delivery), accuracy, resolution, freshness, and 

contextual richness (including time stamping).” 

2005 ITRS Edition, Metrology, p. 38.

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 Time 

Synchronization Working Group, this project 

will continue to investigate and document key 

industry requirements and work with industry and 

academia to develop effective guidelines and standards to facilitate deployment of distributed clock 

synchronization and accurate time stamping. 

The fi rst approach is to improve understanding 

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.
214 National Institute of Standards and Technology 

DELIVERABLES: Document semiconductor industry 

precision clock synchronization requirements for the 

IEEE 1588 committee. 1Q 2006

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 also completed discussions in a summary of industry requirements in order to provide 

equipment suppliers and chip manufacturers with 

guidance of upcoming needs. Equipment suppliers would like to understand the key application 

needs in order to prioritize the design of their nextgeneration tools to fi t industry timing requirements. A review of current SEMI standards is also 

needed to mitigate confl icts with the upcoming 

time synchronization standard and gaps between 

current standards and new requirements.

DELIVERABLES: Review of current SEMI Information 

and Control Standards and document gaps with current requirements and potential confl icts with upcoming time synchronization standard. 2Q 2006

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, 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 software-based time synchronization protocols in industrial networks. 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.

DELIVERABLES: Conduct research and document 

methodology, results and recommended practices on 

the impact of network node delays on IEEE 1588 precision clock synchronization performance. 4Q 2006

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.

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“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.

COLLABORATIONS

Harvey Wohlwend, Gino Crispieri, International SEMATECH 

Manufacturing Initiative

James Moyne, Engineering Research Center, University of 

Michigan Ann Arbor

Alan Weber, Alan Weber and Associates

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Semiconductor Microelectronics and Nanoelectronics Programs 215

E-DIAGNOSTICS SECURITY

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

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. As cyber-based attacks become 

increasingly sophisticated, defenses must evolve 

to ensure intellectual property remain protected, 

and factory systems would be able to defend 

against attacks and have the mechanisms to rapidly detect and isolate security attacks. Security in 

factory integration systems has become one of the 

latest key focus areas for International Technology 

Roadmap for Semiconductors (ITRS): 

“Cyber infections such as viruses, worms and 

denial of services have made security a key 

focus area with the potential of disrupting factory operations due to downtime and reduced 

manufacturing productivity.” 2005 ITRS, Factory 

Integration, p. 17.

In addition to threats of viruses and worms, robust 

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.

TECHNICAL STRATEGY

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. With 

a plethora of choices available in implementing 

web-based security, it is imperative to select the 

key security concepts necessary to enable a robust 

yet simple web-based security infrastructure for 

the e-diagnostic environment.

DELIVERABLES: Research and document key web 

services security technologies to enable a web services security framework and recommended practices. 

3Q 2006

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. The 

framework will include a method for logging 

each transaction to enable auditing of all those 

who accessed the system and track which actions were performed while a user was logged 

Technical Contacts:

Y. Li

K. Brady
216 National Institute of Standards and Technology 

in. Additional information would also be readily 

extracted from the logs to establish a workfl ow 

for tracking equipment maintenance status via the 

Internet. 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.

DELIVERABLES: Design, implement and evaluate a web-based security framework and document 

results and recommended practices in using security 

technologies for web-based e-Diagnostic applications. 

4Q 2006

ACCOMPLISHMENTS

Draft white paper on “Survey of Web Services Security Technologies and Recommended 

Practices.”

Initial software undergoing development by 

NIST and ESIAL 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 217

ENGINEERING CHAIN MANAGEMENT IN THE

SEMICONDUCTOR INDUSTRY

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 hopes to reduce time, 

inventory, and therefore costs.

“It is expected that improvements in manufacturing productivity will play an even greater future 

role in reducing time to money and getting the 

most out of factory investment.” 2004 International Technology Roadmap for Semiconductors 

Factory Integration (ITRS) Overview Update, 

p.52.

The transition to 300 mm fabs and single-wafer 

lots has challenged the semiconductor industry in 

nearly every aspect of production, such as: facility layout; the mix of R&D within a single fab; 

data analysis of small lots; tool to tool interconnect; automated material handling and tracking 

of product, non-product wafers and reticles; the 

division of the production process across a partnership of foundries, designers, production sites, 

distributors and equipment manufacturers. Chip 

manufacturers require greater visibility of end 

product demand to accurately schedule their factories, and require greater technical collaboration 

with designers, OEMs, and foundries to ensure the 

manufacturability of the product being designed 

today using the generation of equipment that will 

be installed once production starts. 

Cost of design is the greatest threat to continuation 

of the semiconductor roadmap. 2003 ITRS, p. 1.

According to the 2003 ITRS, design complexity has been growing exponentially due to the 

increasing density and number of transistors to 

meet performance goals. Intellectual Property 

(IP) reuse helps companies get complex systems 

to market quickly by eliminating redundant design 

effort. Standard formats for IP specifi cations and 

tool interfaces for the design of a System-ona-Chip (SoC) would reduce time to market by 

eliminating the need for translations. Security 

mechanisms must also be incorporated to protect 

IP during transfers. 

An additional factor affecting design 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 2006 

Roadmapping Activity. 4Q 2006

Technical Contacts:

J. Messina

K. Brady
218 National Institute of Standards and Technology 

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 

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 ISMT 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 2006

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: Working with industry create a 

material declaration standard to assist industry with 

complying with the RoHS directive. 2Q 2006

ACCOMPLISHMENTS

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 has been co-leading the Engineering 

Chain Management sub-team of the ITRS Factory 

Information and Control Systems team. Initial efforts are being 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 

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Semiconductor Microelectronics and Nanoelectronics Programs 219

NIST/SEMATECH E-HANDBOOK OF STATISTICAL

METHODS

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 2006

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 8,000 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.

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.

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.

RECENT PUBLICATIONS

NIST/SEMATECH e-Handbook of Statistical Methods, M. 

Carroll Croarkin and Paul Tobias, editors, http://www.nist.

gov/stat.handbook/.

“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.

Technical Contacts:

W. F. Guthrie

A. Heckert

J. J. Filliben
220 National Institute of Standards and Technology 

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
Semiconductor Microelectronics and Nanoelectronics Programs 221

DUV deep ultraviolet

EBSD electron backscatter diffraction

EELS electron energy loss spectroscopy

EDC embedded decoupling capacitance

EDS energy-dispersive spectroscopy

EMC electromagnetic compatibility

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
222 National Institute of Standards and Technology 

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)

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

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

SURF III Synchrotron Ultraviolet Radiation Facility III

TCAD technology computer-aided design

TDDB time-dependent dielectric breakdown
Semiconductor Microelectronics and Nanoelectronics Programs 223

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
224 National Institute of Standards and Technology 

TECHNICAL CONTACTS
Semiconductor Microelectronics and Nanoelectronics Programs 225
226 National Institute of Standards and Technology