SEMICONDUCTOR MICROELECTRONICS

AND NANOELECTRONICS PROGRAMS

NISTIR 7010

July 2003

U.S. DEPARTMENT OF COMMERCE

Donald L. Evans, Secretary

Technology Administration

Phillip J. Bond, Under Secretary of Commerce for Technology

National Institute of Standards and Technology

Arden L. Bement, Jr., Director
DISCLAIMER

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

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

imply that the equipment and/or software identified 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 2001. This document is available from

the Semiconductor Industry Association (SIA), 181 Metro Drive, Suite 450, San Jose, CA

95110, phone: (408) 436-6600; fax: (408) 436-6646.

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. We also have included a list of the NSMP

projects that demonstrate the synergism resulting from this matrix-managed

program.
Semiconductor Microelectronics and Nanoelectronics Programs iii

CONTENTS

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

Office of Microelectronics Programs Organization ............................................................ vii

Lithography......................................................................................................................... 1

Metrology Supporting Deep Ultraviolet Lithography ............................................................ 2

Metrology Supporting EUV Lithography .............................................................................. 7

Polymer Photoresist Fundamentals for Next-Generation Lithography ............................... 12

Critical Dimension and Overlay Metrology Program ................................................ 17

Atom-Based Dimensional Metrology ................................................................................. 18

Scanning Electron Microscope-Based Dimensional Metrology .......................................... 22

Optical-Based Dimensional Metrology ............................................................................... 26

Scanning Probe Microscope (SPM)-Based Dimensional Metrology ................................. 31

Electrical-Based Dimensional Metrology ........................................................................... 35

Small Angle X-ray Scattering (SAXS)-Based Dimensional Metrology .............................. 40

Model-Based Linewidth Metrology .................................................................................... 44

Thin Film and Shallow Junction Metrology Program ................................................ 48

Two- and Three-Dimensional Dopant Profiling .................................................................. 49

Gate Dielectric Metrology .................................................................................................. 56

Chemical Metrology of Materials & Particle Contamination ............................................. 68

Thin Film X-ray Metrology for Microelectronics................................................................ 73

Interconnect and Packaging Metrology Program ...................................................... 77

Superconformal Deposition of Copper and Advanced Interconnect Materials .................. 78

Porous Thin Film Metrology for Low-k Dielectric ............................................................. 82

Electron Microscope Tomography of Electronic Materials ................................................ 87

Wire Bonding to Copper/Low-k Semiconductor Devices ................................................... 89

Solders and Solderability Measurements for Microelectronics ........................................... 90

Interconnect Materials and Reliability Metrology ............................................................... 93

Thermal Measurements and Packaging Reliability ............................................................. 99

Atomic Layer Deposition – Process Models and Metrologies ......................................... 104

Dielectric Metrology Supporting Integrated Passive Device Technology ........................ 107

Wafer Characterization and Process Metrology Program....................................... 110

Wafer and Chuck Flatness Metrology ...............................................................................111

Modeling, Measurements, and Standards for Wafer Surface Inspection ......................... 114

High Resolution Microcalorimeter X-ray Spectrometer for Chemical Analysis ............... 119

Thermophysical Property Data for Modeling CVD Processes and for the

Calibration of Mass Flow Controllers ............................................................................... 124

Temperature Measurements and Standards for Rapid Thermal Processing .................... 129
iv National Institute of Standards and Technology

Low Concentration Humidity Standards ........................................................................... 133

Plasma Process Metrology ............................................................................................... 138

Measurements for Vacuum Process Control .................................................................... 143

Test Metrology Program .............................................................................................. 145

Metrology for System-on-a-Chip (SoC) ........................................................................... 146

Nonlinear Device Characterization ................................................................................... 148

At-Speed Test of Digital Integrated Circuits .................................................................... 152

Jitter and Propagation Delay Measurement ..................................................................... 154

Failure Analysis of Integrated Circuits.............................................................................. 156

Manufacturing Support ................................................................................................. 158

NIST/SEMATECH e-Handbook of Statistical Methods .................................................. 159

Engineering Chain Management in the Semiconductor Industry ...................................... 160

Abbreviations and Acronyms....................................................................................... 162

Technical Contacts ........................................................................................................ 166
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 fulfilling 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 five 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 sixty 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 Office 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 identified 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 provided a $12 million budget in fiscal year 2003.
vi National Institute of Standards and Technology

FOSTERING NIST’S RELATIONSHIPS WITH THE INDUSTRY

NIST’s relationships with the SIA, International SEMATECH (ISMT), and the Semiconductor Research Corporation (SRC) are also coordinated through the OMP. Staff from OMP

represent NIST on the SIA committees that develop the International Technology Roadmap

for Semiconductors (ITRS) as well as on numerous SRC Technical Advisory Boards. OMP

staff are also active in the semiconductor standards development work of the ASTM, the

Deutsches Institut für Normung (DIN), 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 identified 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:

Office 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

email: 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

SCACE, Robert I. (Guest Researcher) robert.scace@nist.gov

(301) 975-5198

BUCKLEY, Michele L. (Secretary) michele.buckley@nist.gov

(301) 975-4400

Back Row: Stephen Knight, Robert Scace, Joaquin Martinez de Pinillos; Seated: Michele

Buckley.

Semiconductor Microelectronics and Nanoelectronics Programs 1

LITHOGRAPHY

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 being introduced, and exposure tools operating at 157

nm are in development. Looking beyond the deep ultraviolet, extreme ultraviolet radiation

(EUV) at 13 nm is being investigated, and demonstration tools are being designed and assembled. The overall goal of this task is to support these developments in DUV and EUV.

The areas of emphasis are lens materials, laser calorimetry, radiation detector sensitivity and

damage, EUV lens metrology, and metrology for the development of advanced phoresist

materials for both DUV and EUV.
2 National Institute of Standards and Technology

METROLOGY SUPPORTING DEEP ULTRAVIOLET

LITHOGRAPHY

Technical Contacts:

J. H. Burnett

M. Dowell

“It’s an excellent service

NIST has performed for

the entire industry. The

kind of thing NIST is there

for – to identify issues

before the train wreck

takes place.”

Mordechai Rothschild,

Massachusetts Institute

of Technology’s

Lincoln Laboratory

GOALS

Develop solutions to key 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, characteriza-tion of optical

materials, sources, and detectors, in addition to

advising customers on in-house measurements.

CUSTOMER NEEDS

Increasing information technology requirements

have yielded a strong demand for faster logic

circuits and higher-density memory chips. This

demand has led to the introduction of DUV laserbased 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

measurements at DUV laser wavelengths. Next

generation tools employing F2 (157 nm ) excimer

lasers, projected for insertion into production lines

by 2005, require even higher accuracy measurements. To support these efforts, the National

Institute of Standards and Technology (NIST),

with International SEMATECH support, has

initiated a DUV metrology program focusing on

the characterization of DUV optical materials,

sources, and detectors.

The potential solutions for lithographic systems

are discussed in the 2001 International

Technology Roadmap for Semiconductors on

page 14 of the Lithography Section, and in

Figure 34 on page 15. “Optical lithography is

the mainstream approach through the 90 nm

node,—.” The emphasis is on 193 nm and 157 nm

light source systems.

TECHNICAL STRATEGY

1. Beginning with the first 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, specifically KrF

(248 nm) and ArF (193 nm), and more recently F2

(157 nm) excimer lasers, are the preferred sources

for high-resolution lithography at this time. To

meet the laser metrology needs of the optical

lithography community, we have developed

primary standards and associated measurement

systems at 193 and 248 nm, and are in the process

of developing standards at 157 nm measurements,

Fig. 1.

Figure 1. Excimer laser calorimeter for 157 nm

measurements.

DELIVERABLES: Develop a 157 nm excimer laser

primary standard and calibration service to provide

support for the next generation of optical lithography.

1Q 2003

2. To meet the semiconductor industry’s demands

for more rapid, accurate measurements of excimer

laser power and energy, we are building a system

to determine the nonlinear response of an excimer

laser detector, Fig. 2. This system uses a

correlation method to measure the nonlinear

response of pulsed-energy detectors at 248 nm.

The response of the detector under test is

compared to the corresponding response of a

linear monitor detector as a function of incident

laser pulse energy. This method addresses the

difficulties related to large pulse-to-pulse

instability of most excimer laser and delivers

measurement results with an expanded

uncertainty (k=2) of 0.8 %.
Semiconductor Microelectronics and Nanoelectronics Programs 3

DELIVERABLES: Add capability to perform

nonlinearity measurements at 248 nm. 4Q 2003

3. High-accuracy measurements of the index

properties of UV materials is a requirement for

the design of DUV lithography systems. To meet

this demand NIST has developed methods to

make measurements of the DUV refractive index,

as well as its wavelength, temperature, and stress

dependencies to the high accuracy needed. Index

variations and birefringence have become a

limiting factor in the development of the optics

for lithography systems, especially at 157 nm. To

address this problem we have developed unique

VUV polarimetry and Twyman-Green interferometer systems to measure 157 nm index

variation in lens materials due both to external

stress and grown-in defects.

DELIVERABLES: Characterize index inhomogeneities

of DUV materials near 157 nm at the sub ppm level.

4Q 2003

4. In the course of our measurements we

discovered that in addition to index variations

and birefringence due to material defects and

external stress, there are also index variations and

birefringence intrinsic to the material. The

commonly-accepted assumption that the cubic

symmetry of the crystals used would ensure the

isotropy of the optical properties, in fact breaks

down due to the finite value of the photon

momentum q. This previously-neglected effect

on ultraviolet optics has a 1/λ2

 wavelength

dependence, and is negligible at visible

wavelengths, where the index homogeneity and

birefringence are measured. However, at 157 nm

the effect is large (≈ten times the 157 nm

birefringence specification), and this has serious

implications on 157 nm lithography system design

and performance. This intrinsic birefringence

must be accurately characterized for all materials

considered for optics in 157 nm systems, including

the mixed crystals Ca1-x

BaxF2 we are codeveloping that have the potential of having

negligible intrinsic birefringence.

Figure 2. Schematic diagram of 248 nm nonlinear

response system.

DELIVERABLES: Develop and characterize

Ca1-xSrx

F2 mixed crystals, produced to eliminate

intrinsic birefringence. 4Q 2003

5. We are developing a new method for measuring

the refractive index of transmissive samples to a

high accuracy ≈1 ppm in the DUV and VUV using

a VUV FT spectrometer. This method is directed

to measurements down to 135 nm using synchrotron radiation as a continuum source.

DELIVERABLES: Measurements of VUV index

properties of optical materials important for the

lithography industry, using an interferometric

technique in conjunction with SURF III. 4Q 2003

6. Our efforts for complete characterization of the

optical properties of materials involve measuring

the transmittance, reflectance, surface and bulk

scatter, and surface and bulk absorption. This

characterization is done on one of the beam lines

at the NIST Synchrotron Ultraviolet Radiation

Facility (SURF) which is devoted to material and

detector characterization in the wavelength range

120 nm – 320 nm. We have used this facility to

characterize various samples of calcium fluoride

where the transmittance and reflectance was

measured with an uncertainty of better than 1%.

SURF III acts as the primary standard for both

sources and detectors in the DUV and VUV

spectral region.

DELIVERABLES: Achieve a 0.1% standard uncertainty of UV irradiance from 3 nm to 400 nm, and

enable accurate, direct radiance, and irradiance

comparisons with new as well as existing source

transfer standards. 4Q 2002

7. Monochromatized radiation from SURF III along

with a cryogenic radiometer is used to provide

absolute detector-based radiometric calibrations

in the spectral range from 125 nm to 320 nm with a

standard uncertainty of better than 1%. This

facility has also been used to study the

degradation in diodes induced by exposure to UV

radiation. A wide variety of diodes (Si diodes from

Hamamatsu, nitrided Si diodes from IRD, PtSi, GaN,

GaP, GaAsP, and diamond) were characterized for

spectral responsivity and uniformity mapping,

and the degradation in these diodes at 130 nm

was also measured.

A new facility for characterizing the degradation

of diodes to excimer radiation at 157 has been

completed. This facility allows the measurement

of the spectral responsivity of the devices in the

spectral range from 130 nm to 500 nm along with

the measurement of the reflectivity of the diodes

as the devices are irradiated by the excimer
4 National Institute of Standards and Technology

radiation. This allows identification of potentially

stable diodes for UV irradiance measurements.

The facility can also be used to characterize other

types of detectors such as photochromic films.

DELIVERABLES: Characterize the stability of variety

of semiconductor diodes to excimer radiation at

157 nm. 2Q 2002

8. We plan to use the synchrotron radiation in

conjunction with a cryogenic radiometer to

measure the transmittance, reflectance, surface

and bulk losses, which would lead to a complete

optical characterization of the transmissive

samples. Capability will also be developed to make

polarization dependent measurements in the

spectral range from 125 nm to 320 nm.

DELIVERABLES: Build a state of the art facility for an

accurate and complete characterization of the optical

properties of transmissive materials. 4Q 2002

ACCOMPLISHMENTS

# We have developed a system to characterize

the nonlinear response of a 193 nm excimer laser

detector based on the correlation method. The

method and system solves measurement

difficulties associated with the temporal and

spatial fluctuations of excimer laser pulse energy.

The system has a dynamic range of 30 μJ to 50 mJ

of pulse energy. The typical measurement

uncertainty is 0.8 %. Several DUV detectors were

tested. Using this system, one can easily

determine the origin of a detector’s nonlinear

response, such as those due to the incident pulse

energy, range discontinuities associated with

detector gain, and detector background noise, see

Fig. 3. We have discovered detectors having

nonlinearity as high as 8 %.

Figure 3. Nonlinearity measurement result of a 193 nm

pulsed laser energy detector. CF is the correction for

the detector’s nonlinear response. The response is

measured in four spans, covering 2.5 meter settings.

The large degree of nonlinearity at the low end of the

meter range is due to background effects.

# We have determined the damage thresholds

and lifetimes of several materials using 157 and

193 nm excimer lasers and a beam profile technique

similar to ISO 11254-2, Fig. 4. We made these

measurements to select an appropriate absorbing

material for use in our primary standard laser

calorimeter for 157 nm excimer laser power

measurements. The materials we tested were

nickel-plated sapphire, chemically-vapordeposited silicon carbide (CVD SiC), nickel-plated

copper, and polished copper. Applied pulse

energy densities (or dose) ranged from 80 to 840

mJ/cm2. We determined the applied dose from a

series of laser beam profile measurements. Silicon

carbide had the highest damage threshold: 730 mJ/

cm2 per pulse. For this reason, and for its high

thermal and electrical conductivities, we have

chosen silicon carbide as the absorber material

for the 157 nm calorimeter.

Figure 4. Damage threshold measurements. Beam

profiles were recorded and characterized at 22 planes

along the propagation axis using a CCD camera and

quantum converter. Three energy density distributions at inner- and outer-most positions, together with

their dimensions, are shown. The dimensions are

based on the calculation of the second moments

according to ISO 11146.

# Using a unique UV polarimetry system we

developed, we made the first measurements of an

intrinsic birefringence in CaF2 and BaF2. These

values turned out to be over ten times the 157 nm

lithography birefringence target value, and have

forced all 157 nm system designs to be substantially redesigned. We developed the complete

theory of the effect, now fully accepted, and analyzed its angular dependence. From this we first

suggested a compensation approach based on

combining lenses of different crystal axis orientations. All 157 nm system designs now utilize this

correction approach. We also showed that since

the intrinsic birefringence of CaF2

 and BaF2 have

opposite signs, then a mixed crystal Ca1-x

BaxF2

can in principle be made which has zero intrinsic

birefringence at 157 nm, Fig. 5. We have worked

with crystal growers to develop this material and

characterize its optical properties.
Semiconductor Microelectronics and Nanoelectronics Programs 5

Figure 5. Measured (symbols) and calculated

(curves) intrinsic birefringence of CaF2 . SrF2, and

BaF2. The figure shows opposite signs of the effect for

CaF2, and SrF2 and BaF2.

# Immersion lithography has been identified

as a possible way to extend the commercial

viability of 193 nm lithography technology.

However, various technical issues with immersion

lithography cannot be resolved without accurate

data on the refractive index (n) of the immersion

fluid (water), and its dependencies on temperature

(dn/dT), pressure (dn/dP), dissolved gas content,

and impurities. We have made the first accurate

measurements of these index properties of water

near 193 nm using both the goniometric method

and the interferometric method, with consistency

of results. These values are now being used by

the semiconductor industry for design of

immersion lithography tools.

# The stability of semiconductor diodes under

irradiation from an excimer laser operating at

157 nm has been evaluated. We have built a

facility at SURF III that allows simultaneous

exposure of photodiodes to excimer radiation, see

Fig. 6, and synchrotron radiation. Measurements

of the spectral responsivity can be made in the

spectral range from 130 nm to 320 nm with a

standard uncertainty of less than 1%. The intense,

pulsed laser radiation was used to expose the

photodiodes for varying amounts of accumulated

irradiation whereas the low intensity,

continuously-tunable cw radiation from the

synchrotron source was used to characterize the

photodiodes. The changes in the spectral

responsivity of different kinds of diodes such as

UV silicon, GaP, GaAsP, PtSi, diamond, and GaN

were measured for a large range of total

accumulated dose from an F2

 excimer laser

operating at 157 nm. Differing amounts of

changes were seen in different diodes depending

on the total excimer irradiation dose and they

showed different spectral changes in the

responsivity as well. This yields important

information about the mechanism responsible for

the degradation of photodiodes.

Figure 6. Photodetector damage experiment for

157 nm radiation.

# We have also characterized pyroelectric

detectors which are commonly used for highpower laser application. In the vacuum UV to near

UV range, pyroelectric detectors are commercially

available because of important applications in

areas like semiconductor photolithography.

Instead of calibrating these detectors using highpower radiation, we performed measurements with

low-power UV radiation at beamline 4 of the SURF

III. To accommodate the pulse detection nature

of the pyroelectric detectors, we installed a 10

Hertz tuning fork chopper at beamline 4 and lockin amplifiers were used for detector signal

processing. Several commercial pyroelectric

detectors were tested at our facility. We found

that in most cases, the manufacturer-supplied

amplifiers were too noisy for our light intensity

on the order of one microwatt. Subsequently, a

low-noise amplifier was constructed and installed

near the detection head. In addition, for several

high reflectance pyroelectric detectors, we also

measured the reflectance of the pyroelectric

element to check the internal quantum efficiency

of the detector.

# We have constructed and characterized a

probe that is suitable for accurate measurements

of irradiance in the vacuum ultraviolet spectral

range. Many industrial applications such as UV

curing, photolithography, or semiconductor chip

fabrication require accurate measurement of the

irradiance and will benefit from having such a

stable, accurate UV probe. The probe was

characterized at various wavelengths ranging from

157 nm to 325 nm, encompassing many of the
6 National Institute of Standards and Technology

important industrial application wavelengths. The

principle of measurement of the irradiance is based

on scanning the probe in a light field and

measuring the spectral responsivity on a grid with

regular spacing. Measurement of the spectral

responsivity in the center of the probe along with

the integrated total responsivity yields the

spectral irradiance, Fig. 7. This method can

alternatively be used to calculate aperture areas

as well by measuring the ratio of the total

responsivity and the responsivity in the center.

Figure 7. Measurement of the areal responsivitity of

a UV probe at 157 nm.

COLLABORATIONS

MIT Lincoln Laboratory, Mordechai Rothschild; DUV

detector damage, immersion optical fluid measurements.

RECENT PUBLICATIONS

“A Primary Standard for 157 nm Excimer Laser

Measurements,” C. L. Cromer, M. L. Dowell, R. D. Jones,

D. A. Keenan, S. Yang, 2003 Int’l Conf. on

Characterization and Metrology for ULSI Tech. (March

2003) in press.

“A 193 nm Detector Nonlinearity Measurement System

at NIST,” S. Yang, D. A. Keenan, H. Laabs, M. L. Dowell,

Proc. SPIE Metrology, Inspection, and Process Control

for Microlithography XVII, (February 2003) in press.

“New developments in excimer laser metrology at 157

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

and R. Morton, Proc. SPIE Metrology, Inspection, and

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

Editor, (March 2002) 63.

“Thermal Response and Inequivalence of Pulsed

Ultraviolet-Laser Calorimeters,” D. X. Chen, M. L.

Dowell, C. L., Cromer, Z. F. Zhang, J. Thermophys.

Heat Trans. 16 (January 2002) 36.

“Damage Testing of Partial Reflectors for 157 nm Laser

Calorimeters,” H. Laabs, R. Jones, C. Cromer, M. Dowell,

V. Liberman, Proc. SPIE Laser-Induced Damage in

Optical Materials 2001 (October 2001) 332.

“Pulsed-laser Metrology at NIST,” M. Dowell, Optics

and Photonics News (February 2001) 28.

“The Power of Light: Choosing the Right Detector is

key to accurate beam power measurements,” M. Dowell,

OE Magazine (January 2001) 56.

“New Developments in Deep Ultraviolet Laser

Metrology for Photolithography,” M. L. Dowell, C. L.

Cromer, R. D. Jones, D. A. Keenan, and T. R. Scott,

Characterization and Metrology for ULSI Technology:

2000, D. G. Seiler, A. C. Diebold, T. J. Shaffner, R.

McDonald, W. M. Bullis, P. J. Smith, and E. M. Secula,

Eds. (AIP, New York, 2001), pp. 391-394.

“New Developments in Deep Ultraviolet Laser

Metrology for Photolithography,” M. L. Dowell, C. L.

Cromer, R. D. Jones, D. A. Keenan, and T. R. Scott,

Society of Manufacturing Engineers Technical Note

Paper MS00-236 (2000).

“Intrinsic birefringence in calcium fluoride and barium

fluoride,” Burnett, J. H., Levine, Z. H, Shirley, E. L.,

Phys. Rev. B (in press).

“Optical Properties of Materials,” T. A. Germer, R.

Gupta, L. Hanssen, and E. Shirley, Optics and Photonics

News, 12, 38, (2001).

“Characterization of materials using an ultraviolet

radiometric beamline at SURF III,” P.-S. Shaw, R. Gupta,

T. A. Germer, U. Arp, T. Lucatorto, and K. R. Lykke,

Metrologia, 37, 551 (2000).

Symmetry of spatial-dispersion-induced birefringence and

its implications for CaF2

 ultraviolet optics,” J. H. Burnett,

Z. H. Levine, E. L. Shirley, and J. H. Bruning, Journal of

Microlithography, Microfabrication, and Microsystems

1, pp. 213-224 (2002).

“Measurement of the refractive index and thermo-optic

coefficient of water near 193nm,” J. H. Burnett and S.

Kaplan, to appear in, Optical Microlithography XVI,

Proceedings of SPIE Vol. 5040 (2003).
Semiconductor Microelectronics and Nanoelectronics Programs 7

Technical Contact:

Tom Lucatorto

Charles Tarrio

Steven Grantham

Ulf Griesmann

Figure 1. Diagram of NIST/DARPA EUV Reflectometry Facility, including the sample chamber and

monochromator.

METROLOGY SUPPORTING EUV LITHOGRAPHY

GOALS

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

and detectors used in Extreme Ultraviolet

Lithography (EUVL). (EUVL utilizes radiation at

13.4 nm.)

CUSTOMER NEEDS

An intense international effort is presently

underway to develop EUVL for the patterning of

wafers beginning in 2007. A milestone in the U.S.

effort was the building of an alpha-tool called the

ETS (Engineering Test Stand) that is housed at

the Sandia National Laboratory. While the ETS

has demonstrated the feasibility of EUVL for 70

nm design rules and beyond, much work needs

to be done to improve the throughput, mask

fabrication, and lifetime of the optics.

High resolution imaging with EUV radiation was

not possible until the development of multilayer

EUV mirrors in the mid 80s. This development

has spawned the relatively new field of EUV

optics and its associated set of new metrological

challenges. Among these are: 1) precise EUV

reflectivity maps; 2) EUV dosimetry; 3) EUV

damage characterization; and 4) nanometer level

optical figure measurement.

TECHNICAL STRATEGY

1. PRECISE EUV REFLECTIVITY MAPS

The present NIST/DARPA EUV Reflectometry

Facility is located on a multipurpose beamline on

the NIST Synchrotron Ultraviolet Radiation

Facility (SURF III) storage ring. The beamline can

provide a monochromatic beam of EUV or soft xray radiation in the 3 nm to 40 nm (400 eV to 30 eV)

spectral range. Although primarily designed to

serve the EUVL community by providing accurate

measurements of multilayer mirror reflectivities,

this beamline with its associated sample chamber

has been used for many other types of

measurements since the beamline’s commissioning in early 1993. Among the other

measurements performed recently are grating

efficiencies, photocathode conversion efficiencies, phosphor conversion efficiencies, film

dosimetry, and determination of EUV optical

constants through angle dependent reflectance

measurements. A diagram of the NIST/DARPA

EUV Reflectometry Facility is shown in Fig. 1.

Currently the NIST/DARPA facility is the only

one in the U.S. large enough to measure optics

larger than 200 mm in diameter. A recent

international intercomparison among NIST and

several other labs demonstrated reflectivity

accuracy of 0.3 % and wavelength accuracy of

0.01 nm. Plans are underway to reduce wavelength uncertainty by an order of magnitude.

DELIVERABLES: Full reflectivity map of EUV mirrors

up to 40 cm in diameter and 45 kg mass on an as

needed basis for the EUVL community. Reflectivity

and transmittance measurements of metrology

components for International SEMATECH.

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.

Until recently all NIST-characterized EUV

photodetectors were calibrated on the SURF

storage ring, which is essentially a cw source.

Recently we have designed and built a pulsed

EUV source similar to the source being used in

the ETS to calibrate EUV detectors for characterizing sources as well as wafer-plane dosimeters

to be used in EUVL. A schematic of the pulsed

radiometric facility is shown in Fig. 2.

Two major concerns arise when using solid state

photodiodes for detection of short pulse length

radiation. First, while the average power may be

quite modest, the peak power can be quite high.

For example, a 10 Hz laser with an average power

of 10 mW has a peak power of 100 kW for a 10 ns

pulse. Photodiode saturation may seriously affect

the linearity of EUVL dosimeters, even at fairly
8 National Institute of Standards and Technology

Figure 2. Schematic of Pulsed Radiometric Facility.

low average power levels. We have measured the

limit of the linear operating range for an EUV

sensitive Si photodiode using 532 nm radiation

as a proxy for 13.4 nm; the absorption characteristics in Si are nearly identical. We have found

that the photodiode responsivity is an inherently

non-linear function of pulse energy, but the

responsivity can be fit by a calibration function

having two constant parameters. Use of the nonlinear 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. Defining

the limit of linear operation as the pulse energy at

which there is a loss of responsivity of 2 %, the

total charge collected from a Si photodiode is a

linear function of pulse energy at peak powers of

1 W (for an unbiased device) to 5 W (for a reverse

bias of 10 V). The peak voltage developed is a

linear function of pulse energy from 60 mW

(unbiased) to 250 mW (10 V reverse bias).

Currently, we are extending these results to

determine the effect of the longer pulse lengths

typical of EUVL plasma discharge sources, to

understand the dependence on pulse energy

density, and to explore a range of higher reverse

bias conditions. A second concern is the

equivalence of the responsivity under pulsed

conditions and under the cw conditions of

calibration facilities. We have compared the

measured responsivity of a Si photodiode under

pulsed conditions to the results of a low-power

cw calibration. The low-power limit of the nonlinear pulsed responsivity function is identical to

the low-power cw responsivity with a relative

standard uncertainty of 1 %. Thus, the results of

a cw calibration may be transferred to a pulsed

application if the pulse energy dependence is

properly accounted for.

We have developed a model of the physical

processes that are responsible for the observed

non-linear response function of the photodiodes.

The loss of total collected charge is modeled as a

result of recombination processes in the device

during the electronic readout process. In this

model, the decrease in the peak voltage developed

arises from an increase in the junction capacitance

due to the presence of the photogenerated

electron-hole pairs, which act as a polarizable

dielectric medium. The model correctly predicts

the behavior of both calibration parameters as a

function of reverse bias. To date, we have obtained

results in the reverse bias range from 0 V to 10 V,

and we will shortly be extending our observations

to 150 V.

We are currently working to reduce the uncertainty

of our cw radiometric scale in the neighborhood

of 13 nm wavelength from the current relative

standard uncertainty of 4 % to 1 %. We will accomplish this by operating an absolute cryogenic

radiometer (ACR) on a high-throughput beamline

at the SURF III synchrotron radiation facility and

calibrating working standard photodiodes by

direct comparison to the radiometer. The improved

cw scale will be transferred to the EUVL calibration

facility described above to provide a U.S. national

radiometric scale for detector-based pulse energy

measurements at 13.4 nm wavelength with a

relative standard uncertainty of a few percent.

DELIVERABLES: (1) Commission ACR based EUV

calibration facility 4Q 2003. (2) Finalize model for

pulsed response of photodiode responsivity 4Q 2003.

(3) Determine the feasibility of using LPP-based EUV

calibration facility to calibrate assembled instrumentation packages. 4Q 2003

3. EUV DAMAGE CHARACTERIZATION

The energetic (91 eV) EUV photons impinging on

the mirrors in the vacuum environment of the

stepper induce various reactions that can damage

the multilayer coatings. Some of the reactions can

be attributed to EUV-excited water vapor and EUVdissociated hydrocarbons. (Both a small amount

of residual water and hydrocarbon vapors are

present in the stepper environment.) In fact,

lifetime tests show that the expected lifetime under

present conditions is far short of the year duration

needed for economic operation.

We have recently commissioned a beamline that

is dedicated to the exposure of multilayer optics

under controlled environmental conditions. This

effort has been supported by Sandia National

Laboratory and International SEMATECH.

Multilayer samples with silicon and ruthenium

capping layers, provided by Lawrence Livermore
Semiconductor Microelectronics and Nanoelectronics Programs 9

Figure 3. Reflectivity vs. fluence for multilayer

samples exposed to EUV radiation.

Figure 4. A view of the XCALIBIR interferometer.

Figure 5. Topography of a 300 mm diameter

XCALIBIR reference flat.

National Laboratory, have been exposed for

periods of 60 hours each. The samples resided in

a water rich atmosphere under an average EUV

intensity of 5 mW/mm2. The results are shown in

Fig. 3. The Si capped sample exposed for 60 hours

suffered a degradation in reflectivity of more than

20 % due to oxidation at the surface of the mirror.

In contrast, the Ru-capped sample suffered a

reflectivity degradation of only 2 % under similar

illumination and atmospheric conditions. We are

currently adding the capability of introducing

additional atmospheric components like hydrocarbons to further simulate the conditions present

in a projection optics box. This beamline will

provide a valuable tool for the EUV optics development community by permitting long-term

exposures of various capping layer schemes

under controlled atmospheric conditions.

DELIVERABLES: (1) Install new exposure chamber

4Q 2003; (2) Determine feasibility of multioptic schemes

for increased intensity on sample 3Q 2003; (3) Carry

out exposures on samples provided by the EUVL

community and International SEMATECH.

4. SUB-NM OPTICAL FIGURE

MEASUREMENT

The commissioning of the “eXtremely accurate

CALIBration InterferometeR” (XCALIBIR) at

NIST is now complete and the instrument is fully

functional (Fig 4). The XCALIBIR interferometer

is a multi-configuration precision phase-measuring

interferometer for optical figure measurements of

flat, spherical and aspheric optics that can achieve

the very low measurement uncertainties that are

required for the measurement of EUVL optics.

The interferometer may be operated in either

Twyman-Green or Fizeau configurations. 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 laserinterferometers tracks the movement of the test

mount in the direction of the optical axis. A singlemode 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

fibers 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 changing the fiber core diameter.
10 National Institute of Standards and Technology

Figure 6. Distribution of rms deviations from best

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

Figure 7. Topography of a precision silicon sphere,

stitched from 36 measurements. The figure error is

approximately 60 nm PV.

The 300 mm diameter reference flats for the flat

Fizeau configuration of the interferometer were

calibrated with a 3-flat, 6-position self-calibration

test. Fig. 5 shows the topography of one of the

reference flats. A large number of 3-flat measurements were made to estimate the measurement

uncertainty. For each of the flats A, B, and C the

rms of the difference between the averaged flat

solutions and the individual measurements was

plotted in a histogram (Fig. 6). A (statistical)

measurement rms uncertainty of approximately

0.2 nm is evident.

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 figure measurement of

the entire aspheric surface a number of

overlapping subaperture measurements must

then be combined, or “stitched” together. We have

implemented flexible and robust algorithms for

the stitching of subaperture measurements. To

demonstrate the power of the stitching algorithm

a precision silicon sphere was set up on a rotary

table in XCALIBIR and the surface figure error

was measured with an F/1.3 transmission sphere.

36 surface error measurements were made at 10°

intervals. As shown in Fig. 7, the individual,

overlapping, surface error measurements were

then stitched together to form a map of the

“equatorial region” of the silicon sphere.

DELIVERABLES: Establish capability for measuring

aspheric optics without null optics. (4Q 2003)

COLLABORATIONS

VNL at Sandia National Laboratory, Leonard Klebanoff,

and Mike Malinowsky, Environmental Team.

VNL at Lawrence Livermore National Laboratory,

Eberhard Spiller (consultant), Don Sweeney, Saša Bajt,

and Regina Soufli, EUV Multilayer Development and

Coating Team.

RECENT PUBLICATIONS

“Design and performance of capping layers for EUV

multilayer mirrors,” S. Bajt, H. N. Chapman, N. Nuygen,

J. Alameda, J. C. Robinson, M. E. Malinowski, E. M.

Gullikson, A. Aquila, C. Tarrio, S. Grantham, in preparation

for Proc. SPIE vol. 5037, Emerging Lithographic

Technologies VII.

“At wavelength metrology for EUV Lithography,” C.

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

proceedings of the First International EUV Lithography

Symposium, published on CD by International Sematech,

Austin, TX, (2002).

“Accurate reflectometry for Extreme Ultraviolet

Lithography at the National Institute of Standards and

Technology,” S. Grantham, C. Tarrio, and T. B.

Lucatorto, Journal of Vacuum Science and Technology

20, 3080 (2002).

“Facility for extreme ultraviolet reflectometry of

lithography optics,” C. Tarrio, S. Grantham, T. B.

Lucatorto, Metrologia 40, S229 (2003).

“First results from the improved NIST/DARPA EUV

Reflectometry Facility,” S. Grantham, C. Tarrio, M. B.

Squires, and T. B. Lucatorto, in Proc. SPIE vol. 4688

Emerging Lithographic Technologies VI, p. 348 (2002).

“Response of silicon photodiode to pulsed radiation,” R.

Vest and S. Grantham. Submitted to Applied Optics.
Semiconductor Microelectronics and Nanoelectronics Programs 11

“Characterization of the response of chromium-doped

alumina screens in the VUV using synchrotron radiation,”

K. J. McCarthy, A. Baciero, B. Zurro, U. Arp, C. Tarrio,.

T. B. Lucatorto, A. Morono, P. Martin, and E. R.

Hodgson, J. Appl. Phys. 92, 6541 (2002).

“Uncertainties in Interferometric Measurements of

Radius of Curvature,” Schmitz, T., Davies, A., and Evans,

C., to appear in Proc. SPIE 4451 (2001).

“Interferometric Testing of Photomask Substrate

Flatness,” Evans C. J., Parks R. E, Shao L-Z., Schmitz

T., and Davies A. Proc SPIE Vol 4344, in press.

“Interferometric Metrology of Substrates for VLSI,”

Evans C. J., Davies A., Schmitz T., Parks R. E, and Shao

L-Z., Proc of 2nd EUSPEN International Conference,

Turin, May 2001, pp. 388-92.
12 National Institute of Standards and Technology

POLYMER PHOTORESIST FUNDAMENTALS

FOR NEXT-GENERATION LITHOGRAPHY

Technical Contacts:

E. K. Lin

C. L. Soles

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).

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 specific 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 100 nm structures. The unique measurement

methods we apply include x-ray and neutron

reflectivity (XR, NR), small angle neutron

scattering (SANS), incoherent neutron scattering

(INS), near-edge xray absorption fine structure

(NEXAFS) spectroscopy, quartz crystal

microbalance (QCM), nuclear magnetic resonance

(NMR), atomic force microscopy (AFM), Brillouin

light scattering (BLS), 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 100 nm

structures, (2) the spatial segregation and

distribution of photoresist components, (3) the

transport and kinetics of photoresist components,

environmental contaminants, 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, and (5) the polymer physics

of the developer solution and the dissolution

process. These data are needed to meet the future

lithographic requirements of sub 100 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-amplified (CA)

photoresists, complex and highly tuned

formulations of a polymer film loaded with

photoacid generators (PAGs) and other additives.

Upon exposure of the photoresist film through a

mask, the PAG creates acidic protons. A postexposure bake is then applied and the acid protons

diffuse and catalyze a deprotection reaction on

the polymer that alters its solubility in an aqueous

base developer solution. These reaction, diffusion,

and development processes must be understood

and controlled at the nanometer length scale to

fabricate effectively integrated circuits.

Chemically amplified resists are also deposited

onto bottom anti-reflection coatings (BARCs).

Interactions and component transport between

the BARC and resist layer can lead to loss of

profile control or pattern collapse. Detailed

studies of these interaction and transport

mechanisms are needed to design materials for

the successful fabrication of sub 100 nm

structures.

There are significant challenges in extending this

technology to fabricate the smaller feature sizes

(sub 100 nm) needed to continue performance

increases in integrated circuits. First, new

radiation sources with shorter wavelengths

(193 nm, 157 nm, or EUV) require photoresist films

nearing 100 nm thick to ensure optical transparency and uniform illumination. In these ultrathin

films, confinement can induce deviations in

several key materials parameters such as the

macromolecular chain conformation, glass

transition temperature, viscosity, or transport

properties. Furthermore, the required resolution

for a sub 100 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 confinement will

affect the ultimate resolution in these ultra-thin

photoresist films. Additionally, the material

sources of feature resolution (line-edge and

sidewall roughness) and profile control need to

be identified and understood to ensure the

success of needed patterning technologies.

The requirements for advanced photoresists are

discussed in the 2001 International Technology

Roadmaps for Semiconductors on page 243,

Lithography, “Photoresists need to be developed

that provide good pattern fidelity, good linewidth

control, and low defects at several new

wavelengths. Each of the 130 nm, 90 nm and

65 nm nodes could involve the introduction of a
Semiconductor Microelectronics and Nanoelectronics Programs 13

new wavelength or non-optical lithography, so

each node will require the development of an

entirely new resist platform. As feature sizes get

smaller, defects and polymers will have

comparable dimensions with implications for

filtering of resists.”

TECHNICAL STRATEGY

1. In this project, we use model photoresist

materials to validate the new measurement methods. Model 248 nm photoresist materials have

been used initially to address several important

fundamental questions including the thermal

properties of ultrathin films as a function of film

thickness and substrate type, the conformation

of polymer chains confined in ultrathin films, the

surface concentration of PAGs, the diffusion and

the reaction kinetics of the deprotection reaction,

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 strong basis for understanding the

material property changes that may affect the

development of lithography for sub 100 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: Correlate x-ray reflectivity, bilayer

deprotection reaction rate, and incoherent elastic

neutron scattering <u2> data corresponding to

changes in small molecule diffusion in thin films.

2Q 2003

DELIVERABLES: Develop depth profiling NEXAFS

method to quantify spatial extent of changes in

surface versus bulk chemistry in photoresist films.

2Q 2002

DELIVERABLES: Measure NEXAFS spectra and

developer solution conformation of model 157 nm

photoresist polymers. 3Q 2002

DELIVERABLES: Use SANS to measure the

deprotection path of dispersed PAG molecules in

d-PBOCSt resist films. 2Q 2003

DELIVERABLES: Quantify solvent quality of

photoresist polymers in developer solutions as a

function of added salt and temperature. 4Q 2003

DELIVERABLES: Identify material transport and

chemical interactions leading to the formation of

residual layers at the BARC-resist interface as a

function of processing conditions. 4Q 2003

DELIVERABLES: Obtain crucial insight by using

different polymers and polymer/substrate combinations, we will obtain crucial insight into the dynamical

effects of polymer thin film confinement by studying

different polymers and polymer/substrate combinations with INS and QCM. 4Q 2003

ACCOMPLISHMENTS

# Incoherent neutron scattering was used to

measure the atomic-level dynamics of model

photoresist polymer thin films for the first time.

The local, atomic-level, dynamics of the

photoresist polymer directly affect transport

processes essential to modern photoresists, such

as the diffusion of photogenerated acids and other

small molecules within the polymer matrix. To

date, changes in the local dynamics of polymer

thin films have been inferred from changes in

macroscopic quantities such as the apparent glass

transition temperature, Tg, as a function of film

thickness and substrate interaction energies.

Figure 1. The ratio of the bulk to film diffusion

coefficients are displayed as a function of film

thickness for the bilayer, neutron scattering, and

QCM experiments. All three techniques indicate

similar reductions of mobility at similar length scales.

Direct measurements of the segmental motions

of polymer chains confined to ultrathin films

provide a molecular picture of observed changes

in these macroscopic quantities and insight into

differences in photoresist transport processes in

ultrathin films. QCM measurements of moisture

diffusion rates in ultrathin resist films are

dramatically reduced relative to those in thicker

films. Further, in Fig. 1, three different measurements show similar reductions of mobility at similar

film thickness length scales.
14 National Institute of Standards and Technology

# After photogeneration, acid molecules

catalyze multiple reactions within the photoresist

matrix. A central issue in photoresist design is

quantifying both the number of reactions each

acid can catalyze and the size and shape of the

deprotected volume created upon reaction. The

size and shape of the volume have been identified

in recent computer simulations as a primary

source line edge roughness (LER) formation.

Figure 2. SANS data from the deprotection of

dispersed PAGs after exposure and bake. The data

are consistent with a random walk deprotection

reaction path.

Using SANS and specially deuterated polymers,

we successfully measured characteristics of the

deprotection volume due to acid diffusion paths

in a model photoresist. The general technique is

readily extendible to photoresists with the

appropriate deuterium labeling, independent of

the PAG species. The deprotection volume from

dispersed acid molecules features a diffuse

interface, a result inconsistent with a well-defined

two-phase structure. The data in Fig. 2 are

consistent with models for random walk statistics.

We determined that the deprotection volume has

a radius of approximately 8.5 nm after 120 s of

post exposure bake at 90 oC. Infrared spectroscopy was used to determine the level of deprotection

at 32 % with a 0.7 % mass fraction of PFOS. The

result is consistent with a large diffusion path,

and hence a large number of catalyzed reactions

per acid molecule.

# The deprotection reaction front profile was

measured with nanometer resolution using both

xray and neutron reflectometry on a bilayer

structure prepared with a specially labeled

(deuterated) protected polymer. The upper layer

of the structure is loaded with the PAG. Upon

exposure and baking, the acid diffuses into the

lower layer and catalyzes the deprotection

reaction. The protecting group is deuterated and

is volatile upon reaction. Thus, contrast to

neutrons results from the reaction allowing for

observation of the reaction front. By comparing

the reaction front to the developed film profile,

we obtain important insight into both the spatial

extent of the reaction and the development

process itself. These data are the first available

with this spatial resolution and are critically

needed for the development of process control

over nanometer length scales. We find that the

reaction front broadens with time while the surface

roughness of the developed structure remains

relatively sharp. Additionally, we find that a

broader reaction front results in changes in the

compositional profile upon development in an

aqueous base.

Figure 3. Schematic diagram of the NEXAFS

measurement geometry. Spectra are obtained from

the film surface and bulk simultaneously.

# NEXAFS measurements were used to

measure the surface concentration of photoresist

and BARC components and the surface reaction

kinetics in model photoresist polymers as a

function of common processing conditions. A

significant advantage of the NEXAFS

measurement is the capability of separating

interfacial and bulk signals within the same sample

and experiment in Fig. 3. NEXAFS measurements

of interfacial chemistry are possible because of

the limited escape depth of produced secondary

electrons. By separately observing the electron

and fluorescence yield, the chemistry at the

surface (2 nm) and bulk (200 nm) may be

determined. Different chemistries may be

observed by examining the near-edge xray spectra

of light elements such as carbon, oxygen, fluorine,

and nitrogen. In this way, changes in the surface

chemistry relative to the bulk film can be

investigated as a function of lithographic
Semiconductor Microelectronics and Nanoelectronics Programs 15

processing steps such as exposure and heating.

We have found that fluorinated PAG molecules

preferentially segregate to the film surface. The

relative amount of segregation is dependent upon

the specific polymer. In addition, NEXAFS

analysis of residual layers arising from BARCresist component transport and interactions

enable detailed analysis of potential mechanisms

leading to loss of profile control.

# 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 100 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 un-ionized form to the ionized form, for

instance in the 248 nm material poly(hydroxystyrene). SANS data, in Fig. 4, demonstrate the

origin of the miscibility in aqueous base is due to

the ionization of the photoresist leading to

polyelectrolyte behavior. The identification 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

(tetramethyl ammonium chloride) and inorganic

salts (NaCl, KCl). The addition of salts controls

the observed line-edge roughness, by reducing

the influence of polyelectrolyte behavior. Current

experiments using different developing and

rinsing protocols demonstrate that the pH of the

Figure 4. 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.

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.

COLLABORATIONS

Polymers Division, NIST – Ronald L. Jones, Vivek M.

Prabhu, Chia-Ying Wang, Erin L. Jablonski, Bryan D.

Vogt, Tengjiao Hu, Natsuko Noda, Michael X. Wang,

Francis W. Starr, Patricia M. McGuiggan, David

Vanderhart, Michael Fasolka, Alamgir Karim.

Ceramics Division, NIST – Daniel A. Fischer, Sharadha

Sambasivan.

Center for Neutron Research, NIST – Charles J. Glinka,

Sushil K. Satija, Daniel Neumann, Robert M. Dimeo.

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

Arpan Mahorowala, Karen Temple, Marie Angelopoulos.

IBM Almaden Research Center – Hiroshi Ito.

DARPA – Advanced Lithography Program, Contract

N66001-00-C-8083.

Brewer Science, Inc. – Chelladurai Devadoss, Yubao Wang,

Rama Puligadda, James Claypool.

University of Texas at Austin – Brian C. Trinque, Sean

D. Burns, C. Grant Willson.

University of Akron – Alexsei Sokolov, Ryan Hartschuh,

Yifu Ding, Joon Roh, Alexander Kisliuk.

Sandia National Laboratories – Joseph L. Lenhart.

RECENT PUBLICATIONS

C. L. Soles, J. F. Douglas, E. K. Lin, J. L. Lenhart, R. L.

Jones, W. L. Wu, and D. L. Goldfarb, “Incoherent

Neutron Scattering and the Dynamics of Thin Film

Photoresist Polymers,” J. Appl. Phys. 93, 1978 (2003).

E. K. Lin, C. L. Soles, D. L. Goldfarb, B. C. Trinque, S.

D. Burns, R. L. Jones, J. L. Lenhart, M. Angelopoulos,

C. G. Willson, S. K. Satija, and W. L. Wu, “Direct

Measurement of the Reaction Front in Chemically

Amplified Photoresists with Nanometer Resolution,”

Science, 297, 372 (2002).

E. K. Lin, C. L. Soles, D. L. Goldfarb, B. C. Trinque, S.

D. Burns, R. L. Jones, J. L. Lenhart, M. Angelopoulos,

C. G. Willson, S. K. Satija, and W. L. Wu, “Measurement

of the Spatial Evolution of the Deprotection Reaction

Front with Nanometer Resolution using Neutron

Reflectometry,” Proc. SPIE, 4690, (2002).

R. L. Jones, F. W. Starr, C. L. Soles, E. K. Lin, J. L.

Lenhart, W. L. Wu, D. L. Goldfarb, M. Angelopoulos,

“Chain Conformation in Ultrathin Polymer Resists,”

Proc. SPIE, 4689, (2002).
16 National Institute of Standards and Technology

J. L. Lenhart, R. L. Jones, E. K. Lin, C. L. Soles, W. L.

Wu, D. L. Goldfarb, and M. Angelopoulos, “A

Combinatorial Methodology to Discovering the Material

Factors Controlling Resist Line Edge Roughness, Shape,

and Critical Dimension,” J. Vac. Sci. Tech. B, 20, 2920

(2002).

C. L. Soles, E. K. Lin, J. L. Lenhart, R. L. Jones, W. L.

Wu, D. L. Goldfarb, and M. Angelopoulos, “Thin Film

Confinement Effects on the Thermal Properties of

Model Photoresist Polymers,” J. Vac. Sci. Tech. B., 19,

2690 (2001).

D. G. Goldfarb, M. Angelopoulos, E. K. Lin, R. L. Jones,

C. L. Soles, J. L. Lenhart, and Wen-li Wu, “Confinement

Effects on the Spatial Extent of the Reaction Front in

Ultrathin Chemically Amplified Photoresists,” J. Vac.

Sci. Tech. B., 19, 2699 (2001).
Semiconductor Microelectronics and Nanoelectronics Programs 17

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, and comparisons of different critical dimension and

overlay measurement techniques.

Currently, resolution improvements have outpaced overlay and critical dimension measurement improvements. To maintain cost effectiveness significant advances must be made.
18 National Institute of Standards and Technology

ATOM-BASED DIMENSIONAL METROLOGY

Technical Contact:

R. Silver

GOALS

Provide technological leadership to semiconductor and equipment manufacturers and

other government agencies by developing the

methods, tools, and artifacts needed to apply

leading edge, high-resolution atom-based

dimensional measurement methods to meet the

metrology needs of semiconductor microlithography. One specific 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 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 to allow

transfer to other measurement tools which can

measure the artifacts with dimensions known on

the nanometer scale.

Figure 1. The atomic resolution image above was

measured with a 20 picometer resolution interferometer system mounted on the UHV STM.

CUSTOMER NEEDS

NIST responds to U.S. industry needs for

developing length intensive measurement

capabilities and calibration standards in the

nanometer scale regime. The new class of scanned

probes have unparalleled resolution and offer the

most promise for meeting these future needs of

the microelectronics industry. One important

application of the high-resolution SPM methods

is in the development of 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 new NIST-designed

picometer resolution interferometer.

Figure 2. This recent result utilizes the atomically flat

surfaces as a substrate for nanomanufacturing. The

feature’s CDs are 10 nm and the entire lateral field of

view is 250 nm. The background shows an atomically flat substrate.

The work funded in this project is for the

development of atom-based linewidth standards

to assist in the calibration of linewidth metrology

tools and the development of unique interferometry capabilities which can be used in

conjunction with accurately measured tips to

measure feature critical dimensions. This effort

is intended 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. 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,

the instrument response and the uncertainty in

the edge location within an intensity pattern

becomes a significant issue due to the increased

sensitivity to detailed elements of the edge

detection model. The complexity of these models

and large computer resources required for each

individual computation make the idea of having

samples of known geometry and width, essential.
Semiconductor Microelectronics and Nanoelectronics Programs 19

This project is intended to develop samples of

known geometry and atomic surface structure

which will yield measurements resulting in a

specific number of atoms across the line feature

or between features. These samples will be

measured in the UHV environment and then

stabilized and subsequently transferred to other

instruments (see Fig. 1).

There are three primary applications of this type

of artifact after it has been atomically counted.

The first method is the direct calibration in an

SEM for a product wafer whose geometry and

material is near to the structure of the atomically

counted sample. The second method utilizes an

AFM to calibrate the SEMs with the AFM acting

as an SEM matching tool. In this method, the

AFM is calibrated with an atomically measured

sample of the same geometry as the product wafer

to be measured by the SEMs. This sample only

needs to have similar geometry to the product

wafer, but does require similarity in materials due

to the insensitivity of an AFM to materials,

variations. AFM, that has been calibrated for a

particular geometry by an atomic artifact, can then

transfer that calibration to a product wafer of

similar geometry and any material such as

photoresist, resulting in a calibrated product wafer

which can then be transferred to calibrate an SEM.

The third method uses the number of atoms

between features to determine the feature

spacing. This method can be used to make

magnification and pitch calibration standards

based on the intrinsic crystal lattice.

These methods of atom counting and highresolution 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 and

subsequently re-measured atomically. 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 threedimensional structures in semiconductor

materials. These structures must be prepared in

such materials as to allow the atomic surface

reconstruction of those features such that the

atomic order 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 as small as 10 nm. Develop the process

so it can be implemented in the M-cubed STM

metrology system. Develop the process to write

features for optical methods. 2Q 2003

DELIVERABLES: Work with SEMATECH and EEEL for

the development of improved methods for etching

nanostructures written in silicon. Use RIE etching

techniques. These structures must be prepared in

such materials as to allow the measurements of

those features in silicon. 4Q 2003

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 well characterized tip probes can

then be used to measure the samples with

photolithographically defined and canonically

ordered surfaces on the sub-nanometer length

scale. We are developing the SPM tip etching,

field 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 field emitters.

Our tip preparation methods leave us uniquely

qualified in this arena.

DELIVERABLES: Work with the SEM metrology

project for the testing of nanotips as SEM field

emitters. Assist in automation and tip modeling for

field emitters. 1Q 2003

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 magnification 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 chemical processing fully operational and are currently

working on preparing atomically ordered Si surfaces at significantly reduced temperatures. For
20 National Institute of Standards and Technology

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.

DELIVERABLES: Develop metrology methods for the

new set of wafers specifically for atom-based

dimensional metrology. This new wafer set was

written by ebeam on silicon (111) wafers which are

aligned crystallographically. 3Q 2003

Figure 3. These are atomically flat, chemically

prepared surfaces for use in atom-counting and

nanomanufacturing.

4. The development of a new interferometer

system intended to measure dimensions in the 20

picometer range with high accuracy. The system

should also be capable of use on the STM so

atomic scale measurements can be made with new

levels of accuracy. In addition we are exploring

new applications of field ion microscope (FIM)

calibrated tips in conjunction with the

interferometry to measure CDs of leading edge,

small semiconductor features.

The long term technical objective is the

development of in situ stabilized, atomically

ordered surfaces which 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 magnification calibration samples.

These samples will have been measured either by

direct atom counting or high-resolution interferometry and atomically measured tips.

ACCOMPLISHMENTS

# We have obtained atomically flat surfaces

and are now trying to obtain atomic order, to

routinely obtain the low temperature wet chemical

based methods. Although we had a similar effort

with GaAs, which was successful, we are currently

focused on silicon substrates

# 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

prepared as a journal article.

# We have prepared two publications which

give new insight into the etching process for

fabricating atomically flat silicon surfaces. The

results, seen in figure 3, 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 first UHV transfer was been

demonstrated between the NIST MBE system and

the PED UHV STM. The sample was maintained

in a UHV environment during the entire event.

Further UHV sample manipulation and preparation

for transfer to other systems and long term

storage will be investigated in the future as

required.

# We have developed 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. We are now applying these ideas to FIM

tips used in SEM metrology.
Semiconductor Microelectronics and Nanoelectronics Programs 21

Figure 4. A FIM image showing the atomic order in

a W tip is shown. The schematic on the right shows

the method of atom counting or high resolution tipbased interferometry.

# We have attempted our first direct measure

of the surface atom spacings based on a traceable

interferometer measurement. We have fitted our

UHV STM with a high accuracy sub-angstrom

resolution interferometer. We have closed the

loop and made our first atomic resolution

measurements with full interferometer length basis.

The successful completion of this aspect has

enabled direct distance determination with simple

atomic counting.

DELIVERABLES: Publish the results for the first

demonstration of direct interferometer measurements

of surface atom spacings with a complete unbroken

uncertainty. Present these results at the ASPE

conference and SPIE to make the new interferometry

methods available to the industry. 3Q 2003

# Wafers from the NIST designed metrology

reticle set have been evaluated for use in atombased dimensional metrology. The initial

evaluation is very promising and all four wafer

flows yielded good product wafers. These wafers

contain the prototype test structures, including

a comprehensive set of critical dimension and line

space arrays for general metrology purposes. The

double-etched silicon multi-level features are

intended to provide long term stable artifacts for

calibration with the line arrays being test patterns

for atom-based dimensional metrology work.

# We have produced atomically ordered

surfaces of GaAs at far reduced temperatures and

have worked with patterned GaAs linewidth

samples. The GaAs linewidth features have been

fabricated and we have successfully prepared As

capped samples without damaging the line

geometry or integrity in any measurable way.

These samples have been processed using the

complete atomic surface preparation method,

measured in UHV and then allowed to oxidize to

create a stable surface for measurement in other

tools.

COLLABORATIONS:

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

University of Purdue, Dept. of Physics.

RECENT PUBLICATIONS

“Wet Chemistry of Si samples,” J. Fu, H. Zhou, and R.

M. Silver, Applied Physics Letters, in press April 2003.

“A Macro-micro Motion System for a Scanning

Tunneling Microscope,” J. Gilsinn, B. Damazo, R. Silver,

and H. Zhou, World Automation Congress, Proc. From

Intl. Symp. on Robotics and Applications (2002).

“Nanomanufacturing of Atom Based Standards,” NAMT

final report. NISTIR 6850 (2002).

“Calibration of a Prototype Silicon Pitch Artifact

Fabricated by Scanning Probe Oxidation and Anisotropic

Etching,” J. Dagata, F. Chien, W. Hsieh, S. Gwo, J. Jun

and R. Silver. (WERB Complete FY02)

“A Generic Shape Model for STM Tip Geometry,” P. V.

M. Rao, C. Jensen, and R. M. Silver, submitted to

Ultramicroscopy, (2002).

“A New Tunable Diode Laser Method for a Sub-nanometer

Resolution Interferometry,” R. M. Silver, S. Gonda, C.

Jensen, and L. HowardAccepted in Optical Eng. 2003.

“A new design and uncertainty consideration of a

metrology UHV-STM for direct measurements of atom

spacings,” S. Gonda, H. Zhou, J. Fu and R. M. Silver

SPIE Nanotechnology workshop proceedings, NIST

2001.

“Processing atomically flat Si surfaces,” H. Zhou, J. Fu,

S. Gonda and R. M. Silver, SPIE Nanotechnology

workshop proceedings, NIST 2001.
22 National Institute of Standards and Technology

Technical Contacts:

A. E. Vladár

M. T. Postek

“Scanning Electron

Microscopy (SEM) –

continues to provide atline and inline imaging ...

and CD measurements.

Improvements are

needed ... at or beyond

the 90 nm generation...

Determination of the real

3D shape ... will require

continuing advances in

existing microscopy...”

International Technology

Roadmap for Semiconductors,

2001

SCANNING ELECTRON MICROSCOPE-BASED

DIMENSIONAL METROLOGY

GOALS

# Provide the microelectronics industry with

highly accurate SEM measurement and modeling methods for shape-sensitive 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 targets

SEMs as the metrology tool of choice for use in

semiconductor production through at least the

year 2005. The industry needs SEM standard

artifacts, specifically those related to instrument

magnification calibration, performance and the

measurement of linewidth. This entails a

multidimensional program including: artifact

fabrication, understanding the function and signal

generation in the SEM, electron beam interaction

modeling, developing NIST metrology

instruments for the certification 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

100 nm or less with a high degree of precision.

The accuracy of these measurements is also

important, but more so in the development and

pilot lines. The measurements of minimum feature

sizes known as critical dimensions (CD) are made

to ensure proper device operation. The U.S.

industry needs high-precision, accurate, shapesensitive dimension measurement methods and

relevant calibration standards. The SEM

Metrology Project supports all aspects of this

need since scanning electron microscopy is the

major microscopic technique used for this submicrometer metrology.

TECHNICAL STRATEGY

The Scanning Electron Microscope Metrology

Project is a multidimensional project. It is being

executed through several thrusts fully supported

by the semiconductor industry.

1. SEM PERFORMANCE

MEASUREMENT ARTIFACT:

This effort includes 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 performance characteristics of the SEM are particularly important to

precise and accurate measurements on the

semiconductor processing line. NIST has

demonstrated that a critical dimension measured

in a scanning electron microscope functioning

poorly can be 5 nm or more larger than in the same

instrument functioning optimally. For reliable

image sharpness measurements a suitable sample

with small features is needed. NIST is continuously

working on further development of better image

sharpness metrology methods; this includes

cooperation with vendors and users. Currently a

thorough work is underway to assess fundamental imaging properties of CD and other SEMs.

DELIVERABLES: New results to be introduced and

presented at technical meetings as they are worked

out.

2. 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
Semiconductor Microelectronics and Nanoelectronics Programs 23

Figure 1. The RM 8091 SEM Image Sharpness

Reference Material (100 000 x magnification).

Figure 2. The design of the SRM 2120 Linewidth

calibration Standard Reference Material.

methods can deduce the shape of integrated

circuit structures from top-down view images

through modeling and library-based measurement techniques with few nm accuracy. NIST in

several publications 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 longawaited 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 3 mm to down to 70 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 ISMT.

DELIVERABLES: Assessment of the feasibility of

ISMT polySilicon samples to use as SRM 2120

samples. Incorporation of a modified version of the

SRM 2120 structure into ISMT’s new “OMAG” overlay

metrology masks set. 4Q 2003

ACCOMPLISHMENTS

# SEM Performance Measurements – After

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

Reference Material 8091. This sample has 5 nm to

25 nm size structures as is illustrated in Fig. 1.

Fifteen pieces of RM 8091 were delivered to

International SEMATECH (ISMT) for the member

companies, and 75 additional samples have been

delivered to the Office 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 and measurements

methods that give not just repeatable, but accurate

results. These samples are now available to the

public as 2 mm by 2 mm chips and can be mounted

onto 150 mm and 200 mm drop-in wafers, which

are also available form NIST.

# SEM Linewidth Measurement Artifacts –

The development of accurate modeling methods

in progress at NIST have shown excellent results

on polySi samples. From a top-down view, using

our high-accuracy modeling and fitting 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 focusexposure matrix (FEM). The design and the

fabrication of ISMT/NIST mask have been

successfully completed and it is now being tried

on a new high resolution 193 nm lithography tool.

The lines are now well-formed, the resolvable

problem on hand is edge roughness. Experiments

with polySi deposition and etch are underway.

The first wafers will come to NIST just before the

end of summer. These wafers will go through a

qualification process. Full wafers will get

scatterometry and CD-SEM measurements at

ISMT, and cross sectional measurements will be

made at NIST. All these measurements will yield

an excellent database for a decision on how to
24 National Institute of Standards and Technology

Figure 3. Photograph of the NIST nano-tip etcher

tool.

Figure 4. Photograph of the NIST nano-tip.

proceed with this wafer type linewidth reference

sample. It is conceivable that by the end of the

year 2003 the Reference Material 8120 line width

samples, as seen in Fig. 2., will be available. This

work is being carried out in close cooperation with

ISMT.

# Development of Ultra-High Resolution

Nano-Tip 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 replacement.

Nano-tips, by comparison to both conventional

cold field (CFE) and to Schottky field emitters,

offer a substantial increase in brightness (10 x to

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

size. Therefore, in an optimized electron optical

column, substitution of a CFE or Schottky 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.

– Faster scan rate and better charge control.

This work has progressed through:

– New tip preparation and evaluation procedures;

– The Hitachi S-6000 CD-SEM was prepared for

the installation of the nano-tips. This was

facilitated through the help of Hitachi Scientific

Instruments. The SEM currently works to the

original specifications. One sharp tip was

successfully tried with good results.

– Fabrication of a nano-tip assembly, which

matches in geometry the S-6000 CD-SEM field

emitter tip. This new version works with rod-type

tips and greatly improves the speed of tip

exchanges.

The nano-tip fabrication technology (i.e.

sharpening process) is now working reliably, and

the main issue of how to properly mount the

prepared nano-tip in place of the original hairpin

shaped tungsten tip holder has been successfully

solved. With the NIST-developed new fully

computer-controlled etching apparatus, Fig. 3,

new tips are being made and tried in the S-6000

CD-SEM, as can be seen in Fig. 4.

The results so far are promising; the tip life-time is

several months in light use and the resolution is

better than as of the regular tip. Further experiments to fine-tune the electron gun assembly and

explore the full applicability of nano-tips are underway.

COLLABORATIONS

International SEMATECH, Metrology Council.

Naval Research Laboratory; E-beam Lithography.

International Technology Roadmap for

Semiconductors; Microscopy and Metrology Sections.

RECENT PUBLICATIONS

“A New Way of Handling Dimensional Measurement

Results for Integrated Circuit Technology,” A. E. Vladár,

J. S. Villarrubia and M. T. Postek. Proc. SPIE 2003, in

press.
Semiconductor Microelectronics and Nanoelectronics Programs 25

“CD-SEM Measurement of Line Edge Roughness Test

Patterns for 193 nm Lithography,” B. D. Bunday and

M. Bishop, J. S. Villarrubia and A. E. Vladár. Proc. SPIE

2003, in press.

“Applications of image diagnostics to metrology quality

assurance and process control,” A. Starikov; J. A. Allgair;

V. V. Boksha; B. D. Bunday, A. C. Diebold; D. C. Cole; A.

W. Gurnell; D. C. Joy; J. M. McIntosh; J. C. Pellegrini;

R. D. Larrabee, J. E. Potzick; A. E. Vladár; N. P. Smith;

N. T. Sullivan. Proc. SPIE 2003, in press.

“Scanning electron microscope analog of scatterometry,” J. S. Villarrubia, A. E. Vladár, J. R. Lowney, and

M. T. Postek, Proc. SPIE 4689, pp. 304-312 (2002).

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

final report to International SEMATECH, January 2003,

available to member companies at SEMATECH web site.

“Exploring and Extending the Limits of SEMs’

Resolution,” A. E. Vladár, J. S. Villarrubia, and M. T.

Postek, final report to International SEMATECH,

November 2002, available to member companies at

SEMATECH web site.

“A Simulation Study of Repeatability and Bias in the

CD-SEM,” J. S. Villarrubia, A. E. Vladár, and M. T. Postek.

Proc. SPIE 5038, 2003, in press.

“Is low accelerating voltage always required for

semiconductor inspection and metrology,” M. T Postek.

and A. E. Vladár. Proceedings MSA (in press).

“Potentials for high pressure/environmental SEM

microscopy for photomask dimensional metrology”. M.

T., Postek, A. E., Vladár, T. Rice, and R. Knowles, SPIE

2003 (in press).

“Study of Imaging Parameters of the Scanning Electron

Microscope” Vladár, A. E., Postek, M. T., and Radi, Z.,

(in preparation).

“Application of high pressure/environmental scanning

electron microscopy to photomask dimensional

metrology,” M. T. Postek and A. E. Vladár, 2003.

Characterization and Metrology for ULSI Technology

(D. Seiler et al. eds.) AIP Press, New York (in press).

Postek, M. T., Vladár, A. E., Larrabee, R. D. and Jones,

S. N., Report of Investigation. Reference Material 8091

SEM Sharpness Standard. OSRM Publication.
26 National Institute of Standards and Technology

OPTICAL-BASED DIMENSIONAL METROLOGY

Technical Contact:

R. Silver

J. Potzick

T. Doiron

GOALS

Provide technological leadership to semiconductor and equipment manufacturers and other government agencies by developing and evaluating

the methods, tools, and artifacts needed to apply

optical techniques to the metrology needs of semiconductor microlithography. One specific 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 highaccuracy two-dimensional placement metrology.

CUSTOMER NEEDS

Tighter tolerances on CD measurements in

photomask and wafer production place increasing

demands on photomask linewidth accuracy and

on overlay tolerances. NIST has a comprehensive

program to both support and advance the optical

techniques needed to make these overlay and

photomask critical dimension measurements.

In addition, improved two-dimensional

measurement techniques and standards are

needed for measuring and controlling overlay

capabilities of steppers and mask-making tools

under development. Overlay is listed in Table 96

of the 2001 SIA ITRS as a difficult challenge for

both >65 nm and <65 nm processes. Overlay

improvements have not kept pace with resolution

improvements and will be inadequate for ground

rules less than 67 nm. In fact, Table 98b shows

that no known solutions for overlay output

metrology exist beyond the 67 nm node. As

shown in Table 99a, the problems are more acute

for CD mask metrology where the industry is

currently encountering metrology problems

without known manufacturing solutions.

TECHNICAL STRATEGY

There are three main strategic technical components of this project.

1. The first strategy consists of the development

of NIST’s overlay metrology tool, continuous

development of the tool, measurement methods

to obtain uncertainties comparable to or better

than the best industry overlay tools, and standards

to support and calibrate these tools. The technical

strategy for overlay metrology is divided into two

segments: a) instrumentation development and

overlay metrology methodology and b) 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 translate into an artificial 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). A

set of standard artifacts and procedures, under

development at NIST, is designed to assist in

aligning overlay measurement systems and

eliminating TIS. After alignment, the tool must

then be calibrated with standard artifacts to yield

accurate overlay offsets. The measurement

system used in this component is an optical

reflection mode instrument, operational in either

a bright field or confocal mode, with interferometry

on three orthogonal axes also capable of

monitoring the stage tilt. Additional hardware

capabilities include the options to scan the sample

while acquiring data with an on-axis photometer

or high resolution image capture with a full field

CCD data acquisition system. This latter mode

has enabled a detailed study of CCD array

performance and characterization. In this work,

several CCD acquisition systems are being

evaluated and improved edge detection and CCD

array calibration procedures are being developed.

These same methods for two-dimensional CCD

array analysis are now being applied to optics

analysis. Additional investigation of CCD

measurement problems are focused on the detailed

response of typical CCD camera light sensors.

WIS-free standard overlay artifacts have been

fabricated in 200 mm wafers. These overlay

artifacts are for the calibration of industrial optical

overlay tools. The artifacts are being fabricated

in single crystal silicon and will provide an array

of etched silicon three-dimensional targets with

additional targets fabricated using industry

standard process levels. These wafers additionally have an extensive set of characterization targets

and structures developed in close collaboration

with International SEMATECH (ISMT) and

several leading semiconductor manufacturers.
Semiconductor Microelectronics and Nanoelectronics Programs 27

DELIVERABLES: Update the formal qualification

numbers on the overlay microscope, optics, and the

x-y metrology stage. This is largely completed and

the final uncertainties need to be tabulated. The

current, most difficult challenge is the qualification for

calibration of complex overlay target process levels

such as contact-to-poly. 2Q 2002

DELIVERABLES: Use the new metrology reticles

from the ISMT collaboration to make leading edge

overlay calibration targets/wafers. Work with

industry partners to determine designs and which

levels are most appropriate for the silicon fabrication

phase. Calibrate these overlay standards (both

alignment and calibration) for SRM certification.

3Q 2003

2. The ultraviolet transmission microscope,

calibration of NIST Photomask Linewidth

Standard SRM 2059, and the development of

calibration methods to obtain photomask linewidth measurement uncertainties are adequate to

meet industry needs.

The technical strategy for photomask linewidth

standards is similarly 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 to replace the green-light

linewidth calibration system. This new instrument

uses a unique geometry (a Stewart platform) as

the main rigid structure and shows considerable

improvement in vibration characteristics over a

conventional microscope. Higher image

resolution, reduced transmission of UV light

through the chrome, and reduced instrument

vibration will offer improved linewidth

measurement uncertainties.

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 certified on a green

light optical calibration system. Linewidth

uncertainties have been reduced to 40 nm. The

next generation in this line of standards is SRM

2059 (Fig. 2), printed on a standard size 6x6x0.25

inch substrate with calibrated line- and

spacewidths ranging from nominally 0.25 ìm

to 32 ìm and pitch patterns from 0.5 ìm to

250 ìm.

Figure 1. Modeling results for linewidth photomask

samples showing CDs down to 0.25 micrometers.

Figure 2. The new linewidth photomask standard.

In response to customers’ needs for more accurate

photomask feature size measurements, an

industry group was formed for the improvement

of mask metrology through process modeling. The

features on even the highest quality masks exhibit

roughness and runout at the chrome edges,

compromising the definition 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

the relationship between mask-feature metrology

results and the corresponding wafer-feature sizes.

DELIVERABLES: Compare and test the accuracy of

new scattering models for line width evaluation in

transmission. 3Q 2003

DELIVERABLES: Work with SEMI and ISMT in the

development of a new standards specifications

document for overlay metrology. 4Q 2003

3. The third component is the development of twodimensional grid calibration standards and

associated measurement techniques, including
28 National Institute of Standards and Technology

statistical analysis and CCD characterization as

used by industry. These individual technical

strategies for these components are described in

more detail next.

We are approaching the problem of twodimensional measurements from a couple of

directions. The first, and most immediate, is to

develop 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 will be

available as a NIST Standard Reference Material,

# 5001, to standardize 2D measurements in the

semiconductor industry. The effort has three main

parts: development of an industry consensus

standard grid, measurements by state-of-the-art

machines in private industry, and verification of

the measurements using NIST capabilities.

Grids for the SRM have been made and final

measurements are in process. Each measurement

of the grid will have 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.

Verification of the grid measurements is being 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 1D 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

fixtured. Work is now focused on methods to

characterize both of these effects. These studies

provide a complete error budget for SRMs.

To strengthen the foundation of NIST’s claims

for linewidth measurement traceability and to

support the 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.

DELIVERABLES: Calculate asymmetric overlay

targets images upon successful comparison for

modeling results of more standard double etched

silicon structures. Publish and present these results

at SPIE Microlithography. 2Q 2002

DELIVERABLES: Use fully automated focus and

positioning control systems to improve system

performance. Continue to develop comprehensive

analysis capabilities for centerline and edge detection

methods. 3Q 2003

DELIVERABLES: Develop standards and new

mapping software for the new Nikon 5I system.

Measure with a complete uncertainty 2 dimensional

grids with a fully calibrated system. 4Q 2003

DELIVERABLES: Complete the overlay wafer

calibrations and measurements. Continue to work

with ISMT on the development of new standard

wafers and test structures. Deliver the wafers to

the SRM office. 4Q 2003

DELIVERABLES: Complete calibrations of SRM 2059

and the related documentation. Deliver the accompanying documentation to the Office of Standard

Reference Materials for distribution to customers.

3Q 2003

ACCOMPLISHMENTS

# SRM 2800 Microscope Magnification

Standard is a standard-size microscope slide with

calibrated pitch features ranging from 1 ìm to

1 cm. 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 magnification. The SRM may

be used in either transmission or reflection mode

optical microscopes, SEMs, or SPMs. Calibration

is traceable to the meter through NIST Line Scale

Interferometer. Fifty-six units have been calibrated

and delivered to the NIST Office of Standard

Reference Materials.

# NIST is currently commencing calibration for

SRM 2059 Photomask Linewidth Standard,

intended to enable customers to make traceable

measurements of the dimensions of features on

integrated circuit photomasks.

# New image recognition and quantitative

image analysis software has been developed by

lead analyst J. Jun of the Precision Engineering

Division. This comprehensive software package

is based on the Matlab programming language

and tool set and allows the evaluation of

numerous effects on algorithm performance. The

package has been used to quantify feature

roughness and asymmetry effects on overlay

pattern evaluation used in the feedback and

control of lithography stepper tools. It has also

been used in the quantification of algorithm
Semiconductor Microelectronics and Nanoelectronics Programs 29

robustness for sample-to-noise effects. This code

has been used extensively to evaluate correlation

and least-squares.

# Two sets of overlay wafers have now been

received from ISMT. These will be made available

in the very near future as RM 8100 for overlay.

# The overlay metrology project is working

closely with several companies such as

Schlumberger and KLA Tencor to make available

recent important research results on optical

characterization, CCD data acquisition calibration,

and focus and edge detection work. Neil Sullivan

of Schlumberger showed strong interest in

strengthening the collaboration with the overlay

metrology project. NIST development of new

correlation methods and image analysis/

recognition software has enabled the detailed

evaluation of noise, feature roughness, and

feature inhomogeneity effects on repeatability

and robustness of measurement tool

performance, issues of paramount importance in

semiconductor overlay metrology. We have

performed a detailed, in depth study of CCD data

acquisition cameras including the development

of CCD mapping methods

# The NIST Overlay Metrology project leader

has played a significant role in the Overlay

Metrology Advisory Group (OMAG) of ISMT.

This group is developing a comprehensive set of

Figure 3. 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.

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. 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 CCD array performance

and overall optical system characterization and

calibration performance measures have been

adopted.

# The OMAG organized by ISMT has

completed the specification document for the

evaluation and benchmarking of overlay

metrology tools for semiconductor manufacturing. The document will be used for procuring,

testing, and matching tools.

# The first set of two-dimensional grid artifacts,

is known as SRM 5001, and has been received.

These 15 cm (6 in) photomasks have been

measured on a state-of-the-art I-pro metrology

system by the photomask manufacturer. This

effort, to make available traceable, 15 cm (6 in)

feature placement standards involves a 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. The

uncertainty budget for grid measurements has

been developed and peripheral studies on various

items are near completion.

# NIST researchers have made comparisons

between the E. Marx developed optical scattering

code with the Spectel company Metrologia

metrology modeling package. Different material

systems were compared as well as one overlay

feature at different focus positions. New results,

based on the full integration of the NIST scattering

code and Spectel optical microscope model, show

very good agreement. This is an important step

in the effort to provide industry the quantitative

ability to determine sample-dependent effects on

overlay tool performance. Results were presented

at SPIE Microlithography, 2002.

# Near completion for the clean room enclosure

on the overlay metrology tool. This will allow us

to work closely with the industry and make SRMs

directly transferable to a clean room environment.

# Leadership for the industry group for the

improvement of mask metrology through process

modeling has been transferred to ISMT.
30 National Institute of Standards and Technology

COLLABORATIONS:

ISMT, IBM, IVS Schlumberger, KLA-Tencor, Intel,

Motorola, AMD and several other leading manufacturers

or tool vendors.

The fourteen members of The Neolithography

Consortium.

RECENT PUBLICATIONS

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

Larrabee, and M. Davidson, “Calibration Strategies for

Overlay Metrology,” Proc. SPIE 2003.

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

Larrabee, and M. Davidson, “A New Method to Enhance

Overlay Metrology,” Proc. SPIE 2003.

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

Larrabee, B. Russo, and M. Davidson, “Comparison of

Measured Optical Image Profiles of Silicon Lines with

Two Different Theoretical Models,” SPIE Vol. 4689

(2002).

B. Russo, M. Bishop, D. Benoit, and R. Silver, “Overlay

Tool Comparison for sub-130nm Technologies,” SPIE

Vol. 4689 (2002).

R. M. Silver, S. Gonda, L. Howard, and C. Jensen.

“Atomic Resolution Measurements with a New Tunable

Diode Laser-based Interferometer,” To be submitted to

Optical Engineering.

B. Russo, R. Silver et al. “Overlay Benchmarking

Report,” ISMT 2002.

R. Silver, J. Jun, E. Kornegay, “Recent advances in

Overlay Metrology,” Future Fab, 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).

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

“Comparison of Edge Detection Methods using a

Prototype Overlay Calibration Artifact,” Proc. SPIE

Microlithography, Feb. 2001.

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 twodimensional 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 31

Technical Contacts:

T. V. Vorburger

J. A. Dagata

R. Dixson

M. T. Postek

SCANNING PROBE MICROSCOPE (SPM)-BASED

DIMENSIONAL METROLOGY

GOALS

Improve the measurement uncertainty of criticaldimension measurements in the semiconductor

industry through improvements in SPM-based

measurements. The International Technology

Roadmap for Semiconductors (ITRS) identifies

dimensional metrology as a key enabling technology for the development of next-generation

integrated circuits. For example, the goal in 1999

for critical dimension (CD) measurement precision

for isolated lines was ± 2.8 nm; this demand

tightens to ± 0.8 nm by 2011. The technical focus

of this project, development and implementation

of scanned probe microscope instrumentation, is

driven by the anticipated industry needs for

reduced measurement uncertainty, particularly for

existing tools such as the SEM.

CUSTOMER NEEDS

The SEM is the current tool of choice for

inspection and metrology of sub-micrometer

features in the semiconductor industry. SPMs

possess unique capabilities, which may

significantly enhance the performance of SEMs

for in-line critical dimension (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 magnification 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, traceable

pitch/height standards with sub-micrometer pitch

values are not yet available.

TECHNICAL STRATEGY

SPM development is proceeding along two

parallel directions: The first direction addresses

dimensional metrology of SPM specifically

through an in-house research instrument we refer

to as the calibrated atomic force microscope

(C-AFM). This instrument, with metrology

traceable to the wavelength of light for all three

axes of motion, is furthering the design and

development of SPM standards as well as

providing customers with calibrated dimensional

measurements at the nanometer scale. For

example, pitch, height, and width measurement

capabilities of the C-AFM have been evaluated

and validated by internal comparisons. Pitch,

ranging up to 20 μm, has been measured with

standard uncertainties (uc) as low as ≈0.5 nm at

submicrometer scales and relative standard

uncertainties of ≈ 0.1 % at the largest scales. Step

height, ranging from a few nanometers up to

several hundred nanometers, can be measured

with uc on the order of 0.5 %. The width of

submicrometer, nearvertical features, as encountered in CD measurements, can be measured with

uc of ∼10 nm, due mostly to the finite size of the

SPM tip. As part of the project, we have begun to

pursue research in the measurement and

standardization of line edge roughness.

DELIVERABLES: Draft standard for calibration of

AFM z-scales with Si single atom step heights

completed and submitted to ASME and ASTM

Standards Committees. 4Q 2003

DELIVERABLES: Report published demonstrating

improved uncertainty for linewidth measurements

using sharpened tips. 2Q 2004

DELIVERABLES: SRM 2089, a combined pitch and

height standard for AFM, released. 2Q 2005

DELIVERABLES: Reports submitted on two

remaining international Preliminary Key Comparisons

in Nanometrology: 2-D grids, and linewidth. 4Q 2004

DELIVERABLES: Report published demonstrating

improved performance of the C-AFM after integration

of new x-y stage to increase scan area and reduce

motion errors. 1Q 2004

DELIVERABLES: Report published demonstrating

traceable measurements of line edge roughness

using an AFM. 3Q 2003

The second direction addresses increasingly

overlapping demands for dimensional control

during fabrication and subsequent calibration of

nanometer scale features. For this purpose we

employ an SPM-based lithography technique,

pioneered at NIST, to produce grating structures

with linewidths of 20 nm or below. Latent oxide

patterns function as masks for anisotropic wet or
32 National Institute of Standards and Technology

Figure 1. AFM profile of the edge of a prototype

linewidth standard, measured with a nanotube probe.

The calculated slope angle between the green cursors

is approximately 89.6°. Measurement taken by

X. Zhao, Guest Researcher; specimen developed by

M. Cresswell and R. Allen; probe developed by

C. Nguyen, NASA Ames.

dry etching. We are also developing an instrument

which integrates both SPM and probe station

measurement capabilities whereby we are able

to compare SPM-based electrical (capacitance

and surface potential) and topographical

measurements of active device structures simultaneous with traditional current-voltage (I-V)

characteristics measured with a probe station.

This allows us to examine local nanoscale variations at exposed and buried interfaces of critical

dimensioned features in order to identify

processing induced variations which contribute

to linewidth uncertainty in dimensional and

electrical test structures.

DELIVERABLES: Data analysis of optical and SEM

images for a series of six 1-D calibration prototypes

produced by SPM oxidation and anisotropic etching

of silicon. Report in preparation. 2Q 2003

DELIVERABLES: Demonstrate the use of

submicrometer-pitch, 2-D grids produced by SPM

oxidation and anisotropic etching for controlling the

spatial configuration of block co-polymer films.

Second phase using large-scale patterned substrates underway. 2Q 2003

DELIVERABLES: Demonstrate a combined

maskless optical and SPM lithography system for

prototyping sub 100 nm pitch calibration scales

integrated onto 1-cm square chips. 3Q 2003

DELIVERABLES: Demonstrate large-scale replication of sub 100 nm pitch 1- and 2-D features using

SPM lithography, anisotropic etching, and nanoimprint

lithography techniques. 4Q 2003

ACCOMPLISHMENTS

# We are participating in a series of

international Preliminary Key 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 NIST-traceable

measurements of dimensional quantities

important to the semiconductor industry. The

results are submitted for entry in Appendix B of

the Mutual Recognition Arrangement (MRA)

between national measurement institutes (NMIs)

under which the NMIs recognize each other’s

measurement capability, thus helping to eliminate

technical barriers to trade. Appendix B establishes

the technical underpinning of the MRA by

showing the technical bases for mutual

recognition. Comparisons of 1-D pitch, 2-D pitch,

step height, and linewidth measurements are

being carried out. During 2002 we completed the

measurements and the report for the step height

comparison. This was done for steps ranging in

height from 7 nm to 800 nm. The final results

show that NIST’s step height measurements are

among the best in the world. Previous C-AFM

results for 1-D pitch currently appear in Appendix B.

# Another effort involves the study of single

atomic Si steps as fundamental height standards

in the sub-nanometer regime. During 2002 we

submitted a draft procedure for AFM z-calibration

using the single atom steps to the ASTM

Subcommittee 42.14 on STM/AFM and were

invited to develop a draft standard based on the

procedure.

# We are developing techniques to measure

linewidths and edges using AFMs with nanotube

probes. Recent measurements of a prototype

linewidth standard fabricated by colleagues in the

Electronics and Electrical Engineering Laboratory

(See Fig. 1) show leading edges with slopes higher

than 89.5°, thus revealing both the capability of

the measurement technique and the quality of the

prototype.

# We also began work on measurement and

development of physical standards for line edge

roughness (LER), a potential showstopper in the

ITRS. Initial results of LER measured in

cooperation with colleagues in the Building and

Fire Research Laboratory are shown in Fig. 2.
Semiconductor Microelectronics and Nanoelectronics Programs 33

Figure 2. AFM image of a 193 nm lithography test

structure used for LER studies. The image shows a

periodic LER structure with nominal 800 nm spatial

wavelength. Measurements taken by N.G. Orji, Guest

Researcher; specimen developed by B. Bunday,

International SEMATECH.

Figure 3. (a) SEM accessible scale of a 1-D pitch

calibration prototype produced by SPM oxidation and

anisotropic etching of a silicon substrate.

(b) Optically accessible scale with 8 μm pitch.

# Ronald Dixson is currently on detail as

NIST’s first Guest Scientist at International

SEMATECH (ISMT). He is working with ISMT

researcher, Marylyn Bennett, on calibration and

statistical process control techniques to establish

ISMT’s critical dimension AFM as a reference

measurement system (RMS) with close

traceability to the SI unit of length. The RMSAFM is being used primarily for measurements

of CD width, but also for step height and pitch as

well.

# In the integrated SPM probe-station thrust,

we have completed construction and testing of

the combined instrument. Test structures

consisting of silicon FETs realized on silicon-oninsulator substrates are being used as a starting

point for patterning nanodevices and performing

in-situ correlation of the dimensional and electrical

properties of these confined regions. SPM

lithography has been used interactively with

electrical mapping to create successively smaller

active gate regions for detailed studies of local

device scaling. This capability allows us to

examine local nanoscale variations at exposed and

buried interfaces of critical dimensioned features

in order to identify processing induced variations

which contribute to linewidth uncertainty.

# We have also demonstrated large-scale

(100 μm × 100 μm) patterning and anisotropic

etching of sub 50 nm linewidth SPM oxide features

on 110-oriented silicon substrates, as shown in

Figures 3 and 4. A series of six 1-D pitch calibration prototypes were produced and analyzed

using calibration optical and SEM instruments at

NIST. The absolute placement accuracy of the

oxide patterns was determined, allowing an

instrument error map to be constructed. Error

correction will reduce placement errors to well

below 0.1 % over the 100 μm scan range of the

SPM, limited by thermal gradients caused by an

on-board CCD camera. A new series of prototypes

will be produced to demonstrate placement

accuracy in the sub 10 nm range. 2-D grid structures have also been fabricated using these

methods.

COLLABORATIONS

Department of Mechanical Engineering, University of

North Carolina, Charlotte

Physikalisch Technische Bundesanstalt, Braunschweig,

Germany

IBM Almaden Research Center, San Jose, CA

Department of Computer Science, University of North

Carolina, Chapel Hill NC

School of Mechatronic Engineering, Harbin Institute of

Technology, Harbin, China

Departments of Physics and Engineering, University of

Akron, Akron OH

National Institute of Advanced Industrial Science and

Technology, Tsukuba Japan

National Microelectronics Center of Spain, Barcelona

Spain

Center for Measurement Standards, Industrial Technology

Research Institute, Hsinchu Taiwan
34 National Institute of Standards and Technology

Figure 4. (a) Calibrated pitch measurements from

SEM and optical analysis demonstrating 3-4 nm

absolute pattern placement accuracy for etched SPM

oxide features patterned on a silicon substrate.

Department of Physics, National Tsing-hua University,

Hsinchu, Taiwan

International SEMATECH, Austin TX

RECENT PUBLICATIONS

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

“AFM Characterization of Semiconductor Line Edge

Roughness,” Applied Scanning Probe Methods (in press).

N. Sullivan, M. Mastovich, R. Dixson, P. Knutrud, B.

Bunday, P. Fabre, and R. Brandom, “Characterizing CD

SEM Metrology of 193 nm Resists at Ultra Low

Voltage,” Proc. ARCH Interface 2002 Conf., Sept. 2002

R. Dixson, A. Guerry, M. Bennett, T. Vorburger, and B.

Bunday, “Implementation of a Reference Measurement

System Using CD-AFM,” Proc. SPIE 5038, 2003 (in

press).

X. Zhao, T. V. Vorburger, J. Fu, J. Song, and C. V. Nguyen,

“A Model for Step Height, Edge Slope and Linewidth

Measurements Using AFM,” Proc. 2003 ULSI

Conference (to be published).

N. Sullivan, R. Dixson, B. Bunday, M. Mastovich, P.

Knutrud, P. Fabre, and R. Brandom, “Electron Beam

Metrology of 193 nm Resists at Ultra Low Voltage,”

Proc. SPIE 5038 (in press).

F. Perez-Murano, C. Martin, N. Barniol, H. Kuramochi,

H. Yokoyama, and J.A. Dagata, “Measuring electrical

current during scanning probe oxidation,” Appl. Phys.

Lett., May 2003, in press.

C. Harrison and J. A. Dagata, “Lithography with Selfassembled Copolymer Microdomains,” in Self-Assembly

(tent.), I. Hamley, ed., (John Wiley, NY, 2003), in press.

N. Farkas, G. Zhang, E. A. Evans, R. D. Ramsier, and

J. A. Dagata, “Nanoscale Oxidation of Zirconium

Surfaces: Kinetics and Mechanism,” J. Vac. Sci. Technol.

B, in press.

N. Farkas, G. Zhang, K. M. Donnelly, E. A. Evans, R. D.

Ramsier, and J. A. Dagata, “The role of subsurface oxygen

in the local oxidation of zirconium and zirconium nitride

thin films,” Thin Solid Films, submitted.

J. Jun, F. S. Chien, W. F. Hsieh, S. Gwo, R. M. Silver,

A. E. Vladar, and J. A. Dagata, “Calibration of a Prototype

Silicon Pitch Standard by Scanning Probe Oxidation and

Anisotropic Etching,” J. Res. NIST, in preparation.
Semiconductor Microelectronics and Nanoelectronics Programs 35

Figure 1. Christine Murabito using a hot plate to do a

post-exposure bake during the photolithography

process.

Technical Contact:

Michael W. Cresswell

I would like to thank you

and NIST for your

pioneering work on

silicon pocket wafer

technology. We have

now incorporated this

technology in one of our

products. We also

regard very highly your

work on the single

crystal linewidth standard. We see this as the

first and only attempt by

anyone in the world to

produce a linewidth

standard traceable to the

fundamental units of

measure, in the sub100 nm range, for use

in the semiconductor

industry.

This work is of technical

and economic importance to the semiconductor industry, because the

ability to correctly size

ever smaller lateral

dimensions 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.,

Engineering Manager,

VLSI Standards, Inc.

ELECTRICAL-BASED DIMENSIONAL METROLOGY

GOALS

Develop test-structure-based electrical metrology

methods and related reference materials with

emphasis on linewidth and overlay metrologytool calibration; contribute to standards

organizations supporting the development of

metrology standards for the semiconductor tool

industry; target the specific near-term goal of

fabricating a quantity of reference-features with

nominal critical dimensions (CDs) in the range

70 nm to 100 nm having CD uniformities of

1 nanometer per micrometer by June 2003.

CUSTOMER NEEDS

The Semiconductor Industry Association’s

(SIA’s) International Technology Roadmap for

Semiconductors (ITRS) states that it is critically

important to have suitable reference materials for

lithography support available when on-wafer

measurements are made as a new technology

generation in integrated circuit (IC) manufacturing

is introduced, and particularly during

development of advanced materials and process

tools. Each generation of ICs is characterized by

the transistor gate length whose control to

specifications during IC fabrication is a primary

determinant of manufacturing success. The SIA

projects the decrease of gate linewidths used in

state-of-the-art IC manufacturing from present

levels of up to 250 nm to below 70 nm within

several years. Scanning electron microscopes

(SEMs) and other systems used for traditional

linewidth metrology exhibit measurement

uncertainties exceeding ITRS-specified tolerances

for these applications. It is widely believed that

these uncertainties 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 certification has

not been available.

It is also widely believed that the usefulness of

SEM metrology for monitoring wafers in advanced

development and production may become

inadequate at some future IC generation. Thus,

there exists a need for new methodology to meet

future metrology requirements.

TECHNICAL STRATEGY

The technology that the project staff have developed for fabricating linewidth and overlay reference materials is known as the Single-Crystal CD

Reference-Material implementation. Patterning

with lattice-plane selective etches of the kind used

in silicon micro-machining provides reference features with quasi-atomically planar sidewalls over

local length segments of the reference feature,

typically extending to several micrometers. 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 10 micrometers. Essential elements of the

technology implementation include starting silicon wafers having a (110) orientation; alignment

of the reference features to specific lattice vectors; and lithographic patterning with lattice-plane

selective etches of the kind used in silicon micromachining.

The traceability path for dimensional certification

is provided by High-Resolution Transmission Electron Microscopy (HRTEM) imaging. This method

provides nanometer-level accuracy, but is sampledestructive and thus is impractical on a 100 %

basis. The project’s traceability strategy has thus

featured the sub-nanometer repeatability of electrical CD metrology as a secondary reference

means. Low-cost, whole-wafer electrical measurements are effectively calibrated with a limited number of HRTEM lattice-plane image counts made

on a selection of reference features that are replicated on other chips on the same wafer. Typical

reference features are several-hundred lattice

planes wide. A technique of making lattice-plane
36 National Institute of Standards and Technology

“The three-way partnership between International SEMATECH, VLSI

Standards, and NIST is

driving towards creating

a CD standard which is

critically needed as the

industry goes below

65 nm.”

Hal Bogardus, International

SEMATECH

Figure 2. Richard Allen and Michael Cresswell

discuss SEM measurements of prototype reference

materials with Laurie, an International SEMATECH

staff member, in Austin, Texas.

image counts, by automated analysis of HRTEM

phase-contrast images, was developed in order

to minimize the uncertainties of the linewidths of

the standards due to lattice-plane counting.

An important metric of the progress and success

of the project is the level of uncertainty attributed

to the product CD reference features. The units

that were delivered to International SEMATECH

(ISMT) last year, per contractual arrangements,

typically exhibited 14 nm uncertainty. This level

is considered by the industry to be four to five

times higher than what is desired for the roadmap out years. Therefore, the main target of the

project during the current year is reducing uncertainty to 4 nm or better.

Our recent results have indicated that a significant portion of the uncertainty in the certified

linewidth of the finished product is attributable to

variations, which are typically up to tens of nanometers, in the physical linewidth of the features

over segment lengths of typically ten micrometers. Since electrical CD metrology effectively

extracts the average width of a reference feature,

it is insensitive to these physical CD variations.

Consequently, it can contribute commensurate

uncertainties of local linewidths of the finished

product. The current year’s effort has therefore

focused on reducing the macroscopic variations

of the physical CDs of the reference features by

refinements to the starting-material specifications

and to the wafer-fabrication process.

The technical strategy has to be responsive to

industry’s requirement for reference materials to

have the physical properties of standard 200 mm

wafers. Since 200 mm (110) starting material is

virtually 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 wafers to accommodate the test chips. The

result is that finished units are rendered metrology-tool-compatible at an acceptable cost. The

entire fabrication and certification process is

planned to be transferred to a commercial standards vendor.

However, some leading semiconductor manufacturers are now requesting single full-wafer implementations at the 200 mm and 300 mm diameter

levels. Accordingly, the current year’s effort has

also addressed this issue.

In the current year, project researchers have set

up a chip-level reference-material patterning facility at the ISMT facility in Austin. One leading

motivation for this effort was the ready availability of SEM imaging facilities having a capability to

inspect the high volume of reference-material chips

being generated by processing variations. The

latter are directed at reducing single-feature CD

variation to below the several-nanometer level. In

addition, qualified operating staff were also made

generously available. ISMT has been contributing 20 to 40 hours of SEM machine time per week,

with operators. In addition, the project has access to a state-of-the-art dual-beam focused ion

beam (FIB) inspection tool and operator staff for

necessary cross section measurements on an asneeded basis.

A second motivation for setting up the referencematerial patterning facility at the ISMT facility in

Austin was essential proximity to the Advanced

Technology Development Facility (ATDF), which

is being operated by a NIST assignee from the

Manufacturing Engineering Laboratory in ISMT’s

clean room. Our current year’s CD-variation

control effort has depended exclusively on this

one-of-a-kind world-class installation and the

unique skills of the NIST assignee. As stated

previously, the atomic force microscope (AFM)

facility is intended also to serve as an alternative

transfer-calibration tool.

The lithography for the wafers that have been

used during the current year to facilitate our

uncertainty-reduction experiments was conducted

at the Microelectronics Development Sandia

Laboratory at Sandia National Laboratories and

at the Scottish Micro-electronics Centre of the
Semiconductor Microelectronics and Nanoelectronics Programs 37

Figure 3. This image of a S59-SA-G5-4 feature,

patterned by hybrid KOH/TMAH etching, was taken

prior to cleaning. It is estimated to be uniform to

within less than 3 nm over 4 micrometers.

University of Edinburgh. These activities were

funded by ISMT, but the technical direction was

provided by project staff at NIST.

The member companies of the project’s major

client, ISMT, insist that transfer of the SingleCrystal CD Reference-Material (SCCDRM)

technology to a commercial standards manufacturing and marketing company organization be

an integral part of the project. Therefore, ISMT

and NIST have agreed that VLSI Standards, Inc.,

of San Jose, California, be invited to contribute to

the new phase of the project that is described

below. One immediate outcome has been that we

have honored a request to incorporate

scatterometry targets in the next design iteration.

DELIVERABLES: Design improved Single-Crystal

Silicon-on-Insulator and bulk CD reference materials,

procure photomasks and starting materials, and

deliver to ISMT and its contractor for CD referencematerial fabrication. Include scatterometry target

grids in the layout. 2Q 2003

DELIVERABLES: Develop patterning processes

capable of replicating reference features in bulk and

Silicon-on-Insulator (SOI) with line-edge roughness

below 5 nanometers extending up to 10 micrometer

feature lengths. Patterning processes are to include

procedures for removing particles and precipitates

having low-single nanometer dimensions that are

typically produced by the patterning-process

chemistry. In addition, develop decontamination

procedures for removing organic side-wall residues

that are not detectable by electron-beam imaging but

adversely affect AFM transfer metrology. 3Q 2003

DELIVERABLES: Evaluate new vertical etch-stop

implementations to enhance reference-feature

straightness, edge roughness, and side-wall quality.

In particular, design, fabricate, and evaluate buried

boron profiles. Also, replicate reference features in

high-resistivity epitaxial silicon device layers grown

on highly-doped boron substrates. Include evaluation

of stencil-mask based sub-tenth micro-meter

lithography techniques being sponsored by ISMT and

respond to requests by member companies to

develop a single full 200 mm wafer implementation.

4Q 2003

DELIVERABLES: Fabricate and calibrate

scatterometry targets using the single-crystal

reference-material implementation. Invite industry

partners to evaluate performance benefits produced

by alternative starting-material etch stops. Fabricate

a selection of reference materials having features

replicated in metal and polysilicon with built-in rulers

provided by HRTEM illumination. 1Q 2004

ACCOMPLISHMENTS

# A selection of SIMOX (Separation by

Implantation of Oxygen) and bulk (110) 150 mm

wafers was procured, pre-processed, and delivered to Sandia National Laboratories for

lithography on behalf of client ISMT. NIST

project staff also designed a new referencefeature layout and procured the 5X mask that

was used for this Sandia operation. The lot-split

processing was specified by this project and

featured pre-patterning by reactive-ion etching

for reference-feature dimensional uniformity and

plasma etching of the SIMOX-wafer buried oxides

to investigate control of substrate contamination

during SEM inspection. The Sandia work was

funded by ISMT. The finished wafers were

delivered to NIST for dicing prior to being taken

to ISMT for completion of processing at the chip

level.

# A collection of 75 mm (110) wafers was

procured, re-flatted at NIST to meet orientation

requirements, and, after implant pre-processing,

delivered to the Scottish Microelectronics

Centre (SMC) at the University of Edinburgh

for silicon-nitride hard-mask deposition and

patterning by g-line stepper to our specifications.

The lot-split processing was specified by this

project and featured SOI as well as p+np and n+p

buried junctions to isolate the patterned layers

electrically. This work was also funded by ISMT.

A subset of wafers was patterned collaboratively

at the SMC by NIST, SMC, and ISMT staff before

being returned to NIST for dicing and then to

ISMT for pattern-transfer from the hard mask.
38 National Institute of Standards and Technology

Figure 4. This image, as analyzed by FEI Inc.,

indicates a 4 nm CD range over 2 μm of feature

length.

# A facility for patterning individual

reference-feature chips was set up and operated

at ISMT. Processes, including cleaning

protocols, for silicon pattern-transfer and nitride

hard-mask removal at the chip level were

established and documented. Chemical variables

that were optimized for pattern-transfer include

TMAH-etch immersion time and temperature for

the various available substrate boron-doping

levels. The variables that were optimized for

nitride hard-mask stripping include immersiontime and phosphoric-acid temperature and

concentration. Comparisons between straight

TMAH and hybrid TMAH/KOH etching

processes were also investigated and reported.

# Protocols for SEM imaging and evaluation

of large quantities of reference features by ISMT

operators were established and implemented.

The problem addressed was screening and

inspecting thousands of top-down referencefeature images for fabrication-process development purposes and for selecting reference features

for AFM and HRTEM imaging, both of which are

very costly and time-consuming operations. In

order to maintain the inspection budget below

$20K per month, a hybrid optical/SEM inspection

process was implemented and interfaced with a

database that has been designed by project staff.

In addition, image-analysis software was

purchased and modified to allow off-line, largesample, CD extraction from high-density SEM

images of single features. This result was

facilitated through collaboration with Hitachi

Instruments, FEI, Inc., Image-Pro Company, Leeds

Instrument Company, and SEM image-analysis

specialists at NIST/MEL and ISMT.

# Chip-patterning processes to reduce lineedge roughness developed and demonstrated.

The accomplishments listed above were applied

to the identification of a hybrid TMAH/KOH

etching process that reduces average line-edge

roughness from last year’s 14 nm to typically

6 nm on SOI material. This is a very important

result because it is a prerequisite for the reduction

in final-product uncertainty to levels consistent

with requirements of the 70 nm node. Key

contributions to achieving this result were made

by the staff of the Failure-Analysis Laboratory

and numerous other professional staffers at ISMT

in Austin.

# The project’s selected technology-transfer

partner, VLSI Standards, Inc., of San Jose,

California, has formally agreed to support the

project by making its 150 mm wafer clean room,

which is equipped with an i-line stepper, available for performing the lithography for the

project’s next generation of SCCDRMs. The

collaborative agreement, which was recently

concluded, provides for project staff at NIST to

design the reference features which will include a

novel set of scatterometry targets. The NIST

project team has also written the starting-material

specifications, scheduled the pre-processing ion

implants, and taken responsibility for the technical

management of the next phase of the three-way

collaborative project phase. ISMT will provide

the chip-level processing facilities and the AFM

transfer-calibration measurements, and will fund

the HRTEM microscopy.

FY OUTPUTS

COLLABORATIONS

# International SEMATECH member

companies (AMD, Compaq, Conexant, Hewlett

Packard, IBM, Intel, Lucent Technologies,

Motorola, Texas Instruments, Hyundai, Infineon

Technologies, Philips, STMicroelectronics,

TSMC), Development of single-crystal critical

dimension reference materials (Michael W.

Cresswell and Richard A. Allen).

# International SEMATECH, Development of

single-crystal critical dimension reference materials (Michael W. Cresswell and Richard A.

Allen).

# NIST Division 821, Bill Penzes, Development

of electrically calibratable stage micrometer

(Richard A. Allen).
Semiconductor Microelectronics and Nanoelectronics Programs 39

# Photronics, Development of optical/electrical

hybrid critical dimension measurement for

photomasks (Richard A. Allen and Michael W.

Cresswell).

# Sandia National Laboratories Microelectronics Development Laboratory, Sandia

National Laboratories Compound Semiconductor

Research Laboratory, Sandia National

Laboratories Integrated Materials Research

Laboratory, NIST ITL, MSEL, MEL, and

International SEMATECH on Fabrication and

certification of reference materials for linewidth

and overlay metrology (Michael W. Cresswell,

Loren W. Linholm, and Richard A. Allen).

# Sandia National Labs, NIST Divisions 898,

855, and 821, University of Central Florida, and

International SEMATECH, Reference artifacts for

critical dimension measurements (Michael W.

Cresswell, Loren W. Linholm, and Richard A.

Allen).

# University of Edinburgh, U.K., and

International SEMATECH, Process development

for single-crystal critical dimension reference

materials (Michael W. Cresswell, Loren W.

Linholm, and Richard A. Allen).

# VLSI Standards, Development of singlecrystal critical dimension and overlay reference

materials (Michael W. Cresswell and Richard A.

Allen).

# VLSI Standards, Inc., and ISMT, Development of commercial architecture and distribution

plan for single-crystal CD reference materials

(Michael W. Cresswell and Richard A. Allen).

# VTT Finland and George Washington

University, Develop non-contact test structures

and methods (Richard A. Allen, Michael W.

Cresswell, and Loren W. Linholm).

# VTT Finland, Design and fabrication of

capacitance sensors based on co-fired ceramic

technology (Michael W. Cresswell).

STANDARDS COMMITTEE PARTICIPATION

# Electrical Test Structures Task Force, CoChair (Richard A. Allen).

# SEMI International Standards Microlithography Committee, member (Richard A.

Allen).

EXTERNAL RECOGNITION

# Senior Member, IEEE, elected Jun. 2002

(Richard A. Allen).

RECENT PUBLICATIONS

M. W. Cresswell, J. E. Bonevich, R. A. Allen, N. M.

Guillaume, L. A. Giannuzzi, S. C. Everist, C. E. Murabito,

P. J. Shea, L. W. Linholm, “Electrical Linewidth Test

Structures Patterned in (100) Silicon-on-Insulator for

Use as CD Standards,” IEEE Transactions on

Semiconductor Manufacturing, Vol. 14, No. 4, pp. 356-

364, (01-NOV-2001).

R. A. Allen, M. W. Cresswell, C. E. Murabito, W. F.

Guthrie, L. W. Linholm, C. H. Ellenwood, E. H.

Bogardus, “Test Structures for Referencing Electronics

Linewidth Measurements to Silicon Lattice Parameters

Using HRTEM,” Proceedings of the 2002 ICMTS, IEEE

2002 International Conference on Microelectronic Test

Structures, Apr 09-11, 2002, Cork, Ireland, pp. 13-18,

(08-APR-2002).

B. A. am Ende, M. W. Cresswell, R. A. Allen, T. J. Headley,

W. F. Guthrie, L. W. Linholm, E. H. Bogardus, C. E.

Murabito, “Measurement of the Linewidth of Electrical

Test-Structure Reference Features By Automated PhaseContrast Image Analysis,” Proceedings of the 2002

ICMTS, IEEE 2002 International Conference on

Microelectronic Test Structures, Cork, Ireland, Vol. 15,

pp. 1-5, (01-APR-2002).

M. W. Cresswell, E. H. Bogardus, J. V. Martinez de

Pinillos, M. H. Bennett, R. A. Allen, W. F. Guthrie, C. E.

Murabito, B. A. am Ende, L. W. Linholm, “CD Reference

Materials for Sub-Tenth Micrometer Applications,”

Proceedings of the SPIE, SPIE’s 27th Annual

International Symposium and Education Program on

Micro-lithography, Mar 03-08, 2002, Santa Clara,

California, Vol. 4689, pp. 116-127, (01-JUN-2002).

N. M. Guillaume, M. Lahti, M. W. Cresswell, R. A. Allen,

L. W. Linholm, M. E. Zaghloul, “Non-contact Electrical

Critical Dimensions Metrology Sensor for Chrome

Photomasks,” SPIE, 21st Annual BACUS Symposium

on Photomask Technology, Oct 02-05, 2001, Monterey,

California, Vol. 4562, (30-NOV-2001).

R. Schlaf, Y. Emirov, J. A. Bieber, A. Sikder, J.

Kohlscheen, D. A. Walters, M. R. Islam, B. Metha, Z. F.

Ren, T. L. Shofner, B. B. Rossi, M. W. Cresswell, “Using

Carbon Nanotube Cantilevers in Scanning Probe

Metrology,” Proceedings of the SPIE, SPIE’s 27th

Annual International Symposium and Education Program

on Microlithography, Mar 03-08, 2002, Santa Clara,

California, p. 53, (01-JUN-2002).
40 National Institute of Standards and Technology

Technical Contacts:

Ronald L. Jones

Tengjiao Hu

Eric K. Lin

Wen-li Wu

Figure 1. Schematic of the SAXS transmission

geometry. Shown is the x-ray beam (solid line) 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 with

less alignment precision can also be characterized

through measurements at a variety of sample

angles, ω.

SMALL ANGLE X-RAY SCATTERING (SAXS)-BASED

DIMENSIONAL METROLOGY

GOALS

To develop a new SAXS-based methodology to

quickly, quantitatively, and non-destructively

measure critical dimensions and feature shape

with sub-nanometer resolution on production

scale test samples. Quantities of interest include

critical dimension, sidewall angle, and statistical

deviations across large areas. Of particular focus

is delivering an absolute determination of

parameters contributing to line edge roughness,

such as fluctuations in critical dimension over

large areas and correlations in these fluctuations

along a sidewall. In addition to addressing the

metrological needs of future technology nodes,

the sub-Angstrom wavelengths utilized by SAXS

based measurements offer the possibility of

calibration and testing of current metrological

tools based on light scatterometry, SEM, and

AFM.

CUSTOMER NEEDS

The drive to reduce feature sizes to the sub 100

nm regime outlined by the ISMT Roadmap

continues to challenge metrology techniques for

pattern characterization. As pattern sizes

decrease, existing techniques such as CD-SEM

face significant technical hurdles in imaging

quantities such as Line Edge Roughness (LER).

Emerging metrologies based on techniques such

as atomic force microscopy are still being

evaluated, providing a lack of clear definition in

suitable metrology for standardized measurements of both organic and inorganic structures.

To address these issues, NIST is evaluating the

potential application of x-ray scattering as a

metrology tool for both fundamental photoresist

research and in the production of standards

for industrially practiced metrologies such as

CD-SEM.

TECHNICAL STRATEGY

1. The implementation of 157 nm wavelength

exposure tools and resists expected by 2005 will

further restrict the critical dimension (CD) budget,

requiring control of CD to within 10 nanometers.

Further reductions will demand sub-nanometer

precision. These requirements will challenge

traditional methods including CD-SEM, AFM, and

light 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. In contrast to light scatterometry, SAXS is performed in transmission (see

Fig. 1) using a sub-Angstrom wavelength. With

a wavelength more than an order of magnitude

smaller than the pattern size, the patterns are

characterized using methods employed in

crystallographic diffraction. The high energy of

the x-ray allows the beam to pass through a

production quality silicon wafer, and is therefore

amenable to process line characterization. As with

light scatterometry, the measurements are

performed in ambient conditions, minimizing time

required for sample preparation. The current

capabilities of commercially available x-ray sources

and detectors suggest that the technique is

portable to a laboratory scale device capable of

high precision measurements.

While a laboratory scale device is technically

feasible, we have chosen to develop the technique

using a synchotron x-ray source. The synchotron

provides a tunable energy and a wide array of

optical components that enhance our ability to

determine optimal configurations. At present,

however, the high flux of the x-ray source is not

deemed a requirement of the technique.
Semiconductor Microelectronics and Nanoelectronics Programs 41

DELIVERABLES: Provide SAXS measurements of

Critical Dimension and Linewidth with sub-nanometer

precision in a patterned oxide grating. 3Q 2003

DELIVERABLES: Investigate specifications required

for general apparatus, including laboratory scale

device, including wavelength, incident flux, and

detectable background. 3Q 2003

2. Characterization of pattern quality includes

shape factors indicating the slope of the line edge

and curvature. Using the above protocol for

scattering, measurements taken at a series of

angles of incidence allows the reconstruction of

the line shape in a manner similar to microscopic

tomography. We have performed an initial set of

tests using multiple angles on a test grating and

determined an ability to measure sidewall angle

to within 1°. Ongoing analysis and technique

refinement will provide additional shape factors,

allowing determination of more complex shape

information such as sidewall curvature.

Fluctuations in critical dimension result from both

mask related issues, such as overlay or defects,

and factors related to the image development

within the photoresist, such as acid diffusion.

Fluctuations in CD are observed through spreading of the Bragg diffraction peaks, requiring a

precise determination of resolution smearing

effects. Using a 2-D detector, the entire projected

shape of structures such as via pads and contact

holes are obtained in one measurement.

DELIVERABLES: Provide measurements of line

shape in terms of simple trapezoidal model for grating

patterns. 3Q 2003

DELIVERABLES: Provide 2-D measurements of viapad patterns etched in oxide including pitch and

pattern sizes along each dimension in one measurement. 4Q 2003

3. Current Line Edge Roughness (LER) and CD

specification of LER in CD budgets requires high

precision metrology for both quality control by

vendors as well as quality assessment by

customers. Metrologies such as CD-SEM, a top

down technique, measure qualitatively different

quantities compared to that of light scatterometry

and AFM. Our approach is to provide metrology

of sidewall roughness, where sidewall roughness

is defined by deviations from the average sidewall

plane on length scales smaller than the CD. These

measurements can then be connected to varying

definitions of LER through cross measurement

of samples with, for instance, CD-SEM. In the

case of photoresists, we are employing AFM to

provide the appropriate wavevector range, or

frequency spectrum, over which correlations exist.

These data are being used to develop the appropriate neutron and x-ray optical configurations

for sidewall roughness characterization. Our goal

is to produce techniques applicable to a wide

range of roughness types and size scales.

DELIVERABLES: Provide relevant wavevector range

for sidewall roughness scattering measurements.

4Q 2003

DELIVERABLES: Develop metrology of sidewall

roughness based on small angle scattering.

1Q 2004

4. A general technique for pattern characterization

in the microelectronic industry involves the

evaluation of patterns constructed from metals,

oxides, low-k dielectrics, and polymers. The

application of SAXS to this wide range of materials

is possible due to the large disparity in electron

density between all of these materials and air.

Sufficient contrast is also present for some

combinations of materials, including copper

interconnects buried in a low-k matrix. The

transmission geometry allows the probing of

densely packed structures whether buried or

exposed, providing maximum measurement

flexibility. A systematic series of tests will be

performed on structures produced from various

materials from different stages of circuit

production. Plans include measurements of

patterns from initial photoresist at the gate level

through double-damascene and the second metal

deposition.

DELIVERABLES: Direct comparisons of photoresist

patterns and the resulting etched low-k matrix.

1Q 2004

DELIVERABLES: Measurement of a copper pattern in

a low-k matrix. 1Q 2004

ACCOMPLISHMENTS

# We provided the first measurements of critical

dimension using small angle x-ray scattering

(SAXS). Using a series of etched oxide gratings

produced at International SEMATECH (W.

Chism), transmission SAXS was performed using

the ID-9 Beamline at the Advanced Photon Source

(Argonne National Laboratory). The 1-

dimensional Bragg diffraction pattern shown in

Fig. 2 is characteristic of a uniformly spaced

grating, where the orientation of the lines is

perpendicular to the diffraction axis. Prior studies

with neutron scattering have demonstrated the
42 National Institute of Standards and Technology

Figure 2. SAXS transmission scattering measured on

a 2-D detector of a dense line oxide pattern. Shown

are the same sample measured at three orientations:

0.0o (left), 2.5o (middle), 5.0o (right).

Figure 3. 2-D scattering pattern from a repeating

pattern of rectangular “via-pads” etched into the

oxide layer. Diffraction peaks are observed along two

orthogonal axes.

Figure 4. Position of diffraction peak in q-space

(proportional to scattering angle 2θ) as a function of

peak order. The slope is inversely proportional to

repeat distance of the pattern, or pitch. Subnanometer precision is obtained from the 30 orders

produced by the oxide test grating.

effectiveness of using data measured at multiple

angles (designated by the angle ω in the

schematic in Fig. 1). The data below indicate a

dramatic reduction in Bragg scattering as the

sample is rotated from 0 deg (left) to 2.5 deg

(middle) and 5.0 deg (right). In contrast to prior

work with neutron scattering, the beam size is

comparable to current test structures used for

light scatterometry (< 100 μm), with further

reductions possible down to 30 μm using

traditional optical elements to define the beam.

The large number of diffraction peaks is indicative

of the highly correlated order of the repeating

pattern. In addition, the line broadening of each

diffraction spot perpendicular to diffraction axis

is directly related to line roughness.

# SAXS measurements of a series of “via-pad”

patterns etched in oxide (produced by ISMT, W.

Chism) provided a simultaneous characterization

of the complete 2-dimensional pattern projection

as seen in Fig. 3. In contrast to the line patterns

shown in Fig. 2, the shape of the via-pad,

effectively a rectangle with rounded corners,

results in a series of Bragg diffraction peaks along

orthogonal axes. The larger number of peaks along

one axis suggests a higher level of pattern

definition and less deviation from the average

pitch in this direction. In addition, the asymmetry

of the peak spacing suggests the ratio of the

lengths of the sides. As an example, a rectangular

pattern would have equivalent Bragg spacing

along both axes.

# SAXS measurements of test oxide line

gratings have demonstrated sub-nanometer

precision in both average pitch and average line

width. The precision of this measurement is in

part dictated by the number of observable

diffraction orders, which is in turn limited by the

wavelength resolution and pattern quality. Using

a pair of crystal monochromators and two bent

mirrors, the resolution of the SAXS measurement

was increased to produce more than 30 orders of

observed diffraction orders. A linear fit of their

position as a function of their order provides a

slope inversely proportional to the average repeat

distance, or pitch (see Fig. 4). Ongoing studies

will compare these results with measurements from

light scatterometry, atomic force microscopy, and

scanning electron microscopy.
Semiconductor Microelectronics and Nanoelectronics Programs 43

COLLABORATIONS

International SEMATECH, Will Chism, Pattern

production and correlation to light scatterometry.

IBM Yorktown Heights, Arpan Mahorowala, Transfer

of Line Edge Roughness in 193 nm Photoresists to the

Underlying Substrate during RIE.

IBM Yorktown Heights, Dario Goldfarb, Marie

Angelopolous, Contributions to Line Edge Roughness.

Shipley, Inc., Patrick Boulton, George Barclay,

Photoresist Patterning.

Advanced Photon Source, Argonne National Laboratory,

Diego M. Casa, Small Angle X-ray Scattering

Instrumentation Development.

ExxonMobil, Inc., Rainer Kolb, Small Angle X-ray

Scattering Instrumentation Development.

RECENT PUBLICATIONS

R. L. Jones, T. Hu, D. M. Casa, Eric K. Lin, W.-L. Wu,

P. J. Bolton, G. G. Barclay, “Small Angle X-ray Scattering

for sub 100 nm Pattern Characterization,” Appl. Phys.

Lett. Submitted (2003).

R. L. Jones, T. Hu, D. M. Casa, Eric K. Lin, W.-L. Wu,

P. J. Bolton, G. G. Barclay, “Sub-nanometer Wavelength

Metrology of Lithographically Prepared Structures: A

comparison of Neutron and X-ray Scattering,” accepted,

Proc. SPIE (2003).

R. L. Jones, T. Hu, R. Kolb, D. M. Casa, Eric K. Lin, W.-

L. Wu, “3-Dimensional Lineshape Metrology Using

Small Angle X-ray Scattering,” accepted Proceedings of

the International Conference on Metrology and

Characterization in ULSI Technology, (2003).

E. K. Lin, R. L. Jones, W. Wu, J. G. Barker, P. J. Boulton,

and G. G. Barclay, “Structural Characterization of Deep

Sub-Micron Lithographic Structures Using Small Angle

Neutron Scattering,” Proc. SPIE (2002).

E. K. Lin, W. Wu, Q. Lin, and M. Angelopolous, “FeatureShape and Line-Edge Roughness Measurement of Deep

Sub-Micron Lithographic Structures Using Small-Angle

Neutron Scattering,” Proc. SPIE 4344, 414 (2001).

W. Wu, E. K. Lin, Q. Lin, and M. Angelopolous, “Small

Angle Neutron Scattering Measurements of Nanoscale

Lithographic Features,” J. Appl. Phys. 88, 7298 (2000).
44 National Institute of Standards and Technology

MODEL-BASED LINEWIDTH METROLOGY

Technical Contacts:

John Villarrubia

András Vladár

“[The] theoretical work

on the behavior of the

probe-sample interaction

has had a profound

influence on our ability to

quantitatively analyze

probe microscope

images.”

—Joseph Griffith, Bell Labs,

 Lucent Technologies

“The modeling work ...

for electrical CD, AFM,

and CDSEM is of value to

us ... The current crisis

in CD offset uncertainty

may be partially answered by this work.”

—Anonymous industry

 feedback to NIST.

GOALS

The goal of this project is to address the metrology

needs of industry, particularly the U.S. semiconductor industry, for linewidth metrology with

uncertainties of a few nanometers.

CUSTOMER NEEDS

“Lithography metrology continues to be

challenged by rapid advancement of patterning

technology. Critical dimension measurement

capability does not meet precision requirements

which comprehend measurement variation from

individual tool reproducibility and tool to tool

matching...CD measurement must be extended

to line shape determination.” International

Technology Roadmap for Semiconductors,

Metrology Section, p. 7 (2001).

Due to the changing aspect ratios of IC features,

besides the traditional lateral feature size, e.g.

linewidth measurement, full three-dimensional

shape measurements are gaining importance and

should be available inline. Development of new

metrology methods that use and take the full

advantage of advanced digital image processing

and analysis techniques and networked

measurement tools will be needed to meet the

requirements of near future IC technologies.

International Technology Roadmap for

Semiconductors, Metrology Section, p. 6 (2001).

“...the exponential rise in value of each

nanometer, as nominal gate dimensions shrink,

can be estimated... Under these assumptions, the

value of CD control for the 180 nm generation of

microprocessors exceeds $10 per nanometer.”

C. P. Ausschnitt and M. E. Lagus, IBM Advanced

Semiconductor Technology Center, Proc. SPIE

Vol. 3332, p. 212 (1998).

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, which

had approximately $138 billion in worldwide sales

in 2002.1 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

90 nm generation.”

Existing linewidth standards are optical photomask standards, the minimum linewidth of which

is 0.25 μm with a combined expanded uncertainty

of ≈10 nm (coverage factor 2). To support present

and future semiconductor technologies, industry

needs to measure gate widths with total uncertainties, as identified in the International Technology Roadmap for Semiconductors (ITRS), of less

than 10 nm and with measurement repeatabilities

of better than 1 nm. Neither NIST nor any other

national laboratory presently offers a wafer

linewidth measurement service or SRM with this

level of accuracy.

A line’s width must generally be determined from

its image. However, the image is not an exact

replica of the line. The scanning electron

microscope (SEM), scanning probe microscope

(SPM), and optical microscope all have image

artifacts that are important at the relevant size

scales. Physical linewidth determination therefore

requires modeling of the probe/sample interaction

in order to correct image artifacts and identify

edge locations. Barriers to accurate linewidth

determination include inadequate confidence in

existing models, the complexity and consequent

expense of using some models, inadequately

quantified methods divergence, and ignorance of

best measurement practices.

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

1 Computed using statistics compiled by the Material.

Semiconductor Industry Association.
Semiconductor Microelectronics and Nanoelectronics Programs 45

NIST is uniquely positioned to execute such a

project. NIST is a center of expertise in the

instrument models to be tested, with optical, SEM

Monte Carlo, and SPM tip and sample

reconstruction models all having been developed

at NIST. Also, because NIST’s standards function

necessitates concern for accuracy, the NIST measurement tools are among the best characterized

anywhere. For example, transmission optical

measurements can be made here with the same

instrument used to calibrate standard reference

materials, and scale calibrations can be assisted

by the NIST linescale interferometer.

We have been developing a model-based library

method of determining line width and line shape

from top-down SEM images. The top-down

measurement configuration 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 CDSEMs. 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.

Figure 1. The SEM image may be calculated for a

given edge geometry by using a Monte Carlo

algorithm that follows a number of representative

electron trajectories.

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 (Fig. 1). This calculation is repeated

for other choices of edge parameters at discrete

intervals representative of the range of shapes

that one is 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

find the closest match. The corresponding line

shape is assigned to the unknown (Fig. 2). In

practice there may be more than two parameters,

and the library may be interpolated.

In 2002 we reported encouraging results for this

method for polycrystalline Si (poly-Si) samples

imaged in our laboratory SEM (Fig. 3a). We also

established the level of agreement under the

somewhat different imaging conditions in an

industrial CD-SEM (Fig. 3b).

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 longFigure 2. Concept of metrology using a model-based

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.
46 National Institute of Standards and Technology

term standard. However, a capability to measure

resist would also be industrially useful for preetch process control measurements. The effects

of contamination and charging when imaging

non-conducting resists may result in poorer

accuracy for resist samples than for Si samples.

In 2003 we are investigating the applicability of

MBL to resist measurements by performing MBL

vs. cross-section comparisons on the resist test

patterns shown in Fig. 4.

DELIVERABLES: Top-down model-based library

analysis of resist lines will be compared to cross

sections of those lines. The accuracy of the MBL

method on such resist lines will be reported. 4Q 2003

The Fig. 4 pattern will also be fabricated in

polysilicon for MBL vs. cross-section

measurements in that material, for the purpose of

establishing the suitability of MBL for calibrating

wafer linewidth standards.

DELIVERABLES: A candidate linewidth standard (the

NIST 2120 test pattern) will be measured using the

MBL method and compared to cross section images.

The suitability of MBL for calibrating a linewidth

standard will be assessed and reported. 4Q 2003

ACCOMPLISHMENTS

# A new linewidth test pattern (Fig. 4) was

designed and included on an International

SEMATECH mask.

Figure. 3 Agreement between MBL and cross section

measurement: (a) on NIST laboratory SEM (b) on

commercial CD-SEM in a fabrication facility at

International SEMATECH. The MBL results are red

and the cross-section edge assignments blue.

Figure 4. NIST 2090 SEM test pattern. The detail

(shown at the bottom) consists of isolated and dense

(1:1) nominally 100 nm lines suitable for measurement using a variety of tools.

# CD-SEMs differ from the laboratory SEM on

which our initial measurements were performed

in that they employ extraction fields intended to

collect nearly all of the secondary electrons that

escape a sample’s surface. A capability to model

full extraction (the CD-SEM case), no extraction

(as in a laboratory SEM), or intermediate cases

(as in a CD-SEM with low extraction field) was

developed.

# We employed the new modeling capability

to obtain linewidth and line shape information

from top-down CD-SEM images acquired with a

commercial CD-SEM under typical industrial

measurement conditions. We compared these

results to cross-sectional images of the same lines

(Fig. 3b).

# We used the modeling capability to conduct

an industry-requested study of repeatability and

bias in the CD-SEM. The results indicated that

measurement repeatability is most strongly

affected by noise level, electron beam size, sample

sidewall angle, and choice of measurement

algorithm. The results also indicate that sampledependent measurement bias is likely to

contribute a significant component of measurement resolution—a component unmeasured with

current industry tests of instrument precision.

Results were reported to International
Semiconductor Microelectronics and Nanoelectronics Programs 47

SEMATECH and at the SPIE Microlithography

meeting.

# MBL software was installed on a computer

at International SEMATECH. Software to

construct libraries was distributed to interested

CD-SEM manufacturers.

# We reported our progress in developing this

measurement method to International

SEMATECH. We reported also to the

semiconductor metrology community and public

generally via oral presentation at the SPIE

Micrometrology Symposium and publication in

the symposium proceedings.

COLLABORATIONS

International SEMATECH, Benjamin Bunday, Michael

Bishop and Applied Materials, John Swyers: CD-SEM

data acquisition.

International SEMATECH, Benjamin Bunday and

Michael Bishop: linewidth test pattern sample

fabrication.

RECENT PUBLICATIONS

“Scanning electron microscope analog of

scatterometry,” J. S. Villarrubia, A. E. Vladár, J. R.

Lowney, and M. T. Postek, Proc. SPIE 4689, pp. 304-

312 (2002).

“Test of CD-SEM Shape-Sensitive Linewidth

Measurement,” J. S. Villarrubia, A. E. Vladár, and M. T.

Postek, final report to International SEMATECH,

January 2003, available to member companies at

SEMATECH web site.

“Exploring and Extending the Limits of SEMs’

Resolution,” A. E. Vladár, J. S. Villarrubia, and M. T.

Postek, final report to International SEMATECH,

November 2002, available to member companies at

SEMATECH web site.

“A Simulation Study of Repeatability and Bias in the

CD-SEM,” J. S. Villarrubia, A. E. Vladár, and M. T.

Postek, Proc. SPIE 5038, 2003, in press.

“Tip Characterization for Dimensional Nanometrology,”

J. S. Villarrubia, in Applied Scanning Probe Methods,

eds. B. Bhushan, H. Fuchs, and S. Hosaka (Springer Verlag)

2003, in press.
48 National Institute of Standards and Technology

THIN FILM AND SHALLOW JUNCTION 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 films are approaching monolayer thicknesses.

As device dimensions continue to shrink, junctions and critical film thicknesses approach

the realm of several atoms thick, challenging gradient, thickness and roughness metrology

as well as electrical and reliability characteristics. The gate dielectric, traditionally SiO2

, has

been replaced with silicon oxynitrides, and more advanced high dielectric constant materials

are being actively explored.. The overall task is to provide suitable metrology and reference

materials for thin dielectrics and conducting barrier films, including electrical characterization, gradient, thickness and roughness metrology and overall reliability metrology.
Semiconductor Microelectronics and Nanoelectronics Programs 49

TWO- AND THREE-DIMENSIONAL DOPANT PROFILING

Technical Contact:

Greg Gillen (SIMS)

David Simons (SIMS)

Joseph Kopanski (SCM)

Jay Marchiando (SCM)

“SIMS is still a recognized champion in

dopant characterization.

However, profiling of

active dopant components is a must for good

work on dopant characterization today. A

progress in “conventional” 2-D dopant

characterization techniques such as scanning

capacitance microscopy

(SCM) and scanning

spreading resistance

microscopy (SSRM) took

many by surprise. SCM

and SSRM on beveled

cross sections promise

ITRS desired nanometer

and beyond spatial

resolution for 2-D dopant

profiling. Significant

progress (has been)

made in SCM and SSRM

data interpretation.”

Vladimir Ukraintsev,

Texas Instruments

GOALS

To provide industry with the metrology infrastructure needed to measure two- and threedimensional dopant/carrier profiles in the ultrashallow junction regime. The project is divided

into two thrusts (SIMS and SCM):

1) Improve the capabilities for compositional

depth-profiling by defining optimum procedures

for ultra-high depth resolution by Secondary Ion

Mass Spectrometry (SIMS), developing depthprofiling reference materials needed by the

semiconductor industry, and improving the

uncertainty of implant dose measurements by

SIMS.

2) Provide measurement methodologies, theoretical models, and data interpretation software

necessary to make the Scanning Capacitance

Microscope (SCM) a useful two-dimensional

dopant-profiling tool.

CUSTOMER NEEDS

Since source/drain dopant profiles are a critical

factor determining the performance of a transistor,

dopant profiling has always been needed by the

silicon integrated circuit industry. Onedimensional dopant profiles from SIMS or

electrical techniques remain an important process

control tool. As transistors are scaled to eversmaller dimensions, the variation of dopant

profiles in two- and three-dimensions also begin

to influence device operation. Two- and threedimensional dopant profiles 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 depthprofiling conditions and by developing and

making available implant reference materials for

common dopant elements. Scanning Capacitance

Microscopy (SCM) has emerged as a leading

contender to provide two-dimensional carrier

profiles. Relatively accurate 2-D profiles of the

dopant concentration can be obtained when SCM

images are combined with SIMS measurements.

Metrology needs for Doping Technology are

discussed in the 2002 Update of The International

Technology Roadmap for Semiconductors. The

ITRS requirements are for 2-D dopant profile

concentration measurements improving from a

spatial resolution of 2 nm and precision of 4 %

in 2003 to 1 nm and 2 % by 2016. Complete

specifications are given for the short term in Table

100a on page 139 and for the long term in Table

100b on page 140 of the Metrology section.

Three-dimensional dopant profiling is identified

as one of five difficult challenges < 65 nm beyond

2007, in Table 96 on page 134. Requirements

for reference materials and the NIST role are

discussed on pp. 23-24 of the 2001 ITRS, with a

specific reference to dopant profiling. Additional

discussion about dopant profile fabrication and

measurement needs can be found in the Front End

Processes and Modeling and Simulation sections.

TECHNICAL STRATEGY

1. 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 specified. We are working in a

collaboration with Agere Systems to investigate

the parameters that must be controlled to make

highly repeatable dose measurements of As and

P implants in Si with magnetic sector SIMS

instruments. In addition to improved repeatability

for dopants, the ITRS roadmap also requires

increased SIMS detection limits for trace metal

contamination analysis of semiconductor devices.

We are also working on novel methods to enhance

detection limits for common metal contaminants

by increasing the ionization efficiency during the

SIMS sputtering process.

DELIVERABLES: Develop experimental protocols

for high repeatability analysis of As and P implants in

silicon. Develop methods for improving the detection

sensitivity of trace metals by SIMS. 3Q 2002

2. Shrinking semiconductor device dimensions

require increased depth resolution for depth

profiling analysis. Typically, the highest depth

resolution is achieved by using ultra-low energy

primary ion bombardment. However, this

capability is not available on most commercial

SIMS instruments. In instruments where low

energy analysis is available, slow sample erosion

rates can result in prohibitively long analysis

times. We are exploring an alternate approach

that involves using a high energy, focused ion
50 National Institute of Standards and Technology

beam to rapidly mill a beveled cross-section in

the sample under examination. The “altered

layer” produced by the high energy milling beam

can be removed by subsequent bombardment in

a special gaseous cluster ion beam sputtering

instrument. The cross section prepared in this

fashion is then analyzed using high resolution

imaging Auger and SIMS analysis.

DELIVERABLES: Develop new 2-D image depth

profiling technique for high depth resolution SIMS and

Auger depth profiling. 4Q 2003

3. Some thin-film materials such as metals do not

sputter as uniformly as silicon under ion

bombardment. The initial surface topography of

the metal surface or topography induced by the

sputtering process itself often limits the achievable depth resolution. We are exploring a

mechanical thinning method that will allow SIMS

depth profiling from the backside silicon through

the interface with the top layer. This will allow

subtle diffusion effects to be distinguished and

will eliminate degradation in depth resolution

from surface or sputter-induced topography. This

method relies on a commercial polishing system

that allows precise adjustment of the parallelism

between the sample surface and the platen

containing the polishing material. Silicon is

removed by mechanical grinding using a series

of diamond lapping films of decreasing abrasive

size. This approach is being applied to the depth

profiling analysis of blanket films (copper

metallization on silicon) as well as PMOS patterned wafers in collaboration with International

SEMATECH.

DELIVERABLES: Develop sample preparation for

SIMS Backside SIMS Depth Profiling. 4Q 2003

4. Cluster primary ion beam SIMS offers several

advantages for the analysis of semiconductor

materials. Compared to conventional SIMS

analysis using monoatomic primary ions, cluster

bombardment SIMS offers improvements in

depth resolution (Fig. 1), higher sputter rates, and

increased sensitivity for some elemental species.

Also, greater sensitivity for organic species may

greatly increase our ability to detect organic

contamination on silicon surfaces. We are

exploring various applications of this technique

and attempting to develop new, low cost cluster

ion beam sources to promote more widespread

use of the technique.

Figure 1. Computer simulation of subsurface

damage created by a 25 keV Ga+ ion compared to a

cluster projectile. Target is a photoresist.

DELIVERABLES: Continue development of cluster

SIMS technique for depth profiling of semiconductors.

4Q 2003

5. Development of ion implanted reference materials for the semiconductor industry has been given

a high priority by the International SEMATECH

Analytical Lab Managers Working group. Previous ion implant NIST standards include B in silicon (SRM 2137) and As in silicon (SRM 2134),

which was released in FY 2000. Current efforts

are focused on development of a P ion implant

standard in silicon.

DELIVERABLES: Development of ion implant

reference materials. 1Q 2003

The Scanning-Probe Microscope Metrology part

of the project is developing tools that are intended

to enable scanning capacitance microscopes to

function as two-dimensional dopant profiling

tools. This work is divided into three tasks. Task

1 is to develop SCM measurement methodologies.

Best measurement practices are being determined

via collaborative projects with industrial users and

research into the physics of the silicon surface

preparation. Task 2 is to develop theoretical

models of the SCM. The focus of our modeling

effort has been to develop 2-D and 3-D finiteelement solutions of Poisson’s equation for the

SCM geometry. Task 3 is interpretation of SCM

data and technology transfer. Our expertise with

interpreting SCM images is being transferred to

industry through our software program FASTC2D.

The program features an easy to use interface,

rapid profile extraction, and operation in a

Windows environment.

6. Currently, the spatial resolution of 2-D dopant

profiles measured with SCM is limited by the tip

size and by the data interpretation techniques.

One way to overcome the tip size limitation may

be to bevel the sample on a cross-section,

producing magnification both in depth and

laterally. We have begun to investigate SCM

measurements on beveled samples. While it is
Semiconductor Microelectronics and Nanoelectronics Programs 51

straightforward to produce a beveled crosssection, carrier redistribution may raise new

interpretation issues.

The version of FASTC2D currently available

utilizes a calibration curve, determined from a

database of pre-calculated solutions, which can

very rapidly determine a 2-D carrier profile from

an SCM image. When used in conjunction with

SIMS measurements or a reference sample,

relatively accurate profiles can be obtained, see

Fig. 2. We have produced a series of tutorial

samples, Fig. 3, that are intended to be used with

FASTC2D to teach best measurement and data

interpretation procedures.

Figure 2. SIMS and SCM depth profiles of boron in

the P+ region of a test pad on the SCM tutorial

sample (top). SIMS and SCM depth profiles of

arsenic in the N+ region of a test pad on the SCM

tutorial sample (bottom).

Figure 3. Prototype tutorial sample for SCM.

Extended length transistor-like devices for easy cross

sectioning, containing structures both with and

without p-n junctions.

DELIVERABLES: Improve spatial resolution of SCM

measured dopant profiles by developing techniques to

produce precisely beveled samples and techniques

for their interpretation. 4Q 2003

7. A physically accurate 3-D model and

determination of the dopant profile by an inverse

solution are required to meet industrial needs for

2-D dopant profiling to the end of the ITRS. We

have previously developed a quasi-3-D model of

the SCM that predicts the essential behavior of

the SCM measurement. However, for the required

dopant profiling performance goals, a more

rigorous approach is necessary. Towards this end,

the finite element method has been employed to

solve Poisson’s Equation for the SCM geometry

in three-dimensions. Accuracy requirements may

also force the consideration of quantum

mechanical effects, necessitating the solution of

the coupled Poisson and Schrödinger Equations.

We are developing improved 3-D Poisson solvers

using the LaGriT grid management toolbox in

collaboration with Los Alamos National

Laboratories.

DELIVERABLES: Demonstrate optimized 3-D

calculations of the SCM signal across dopant

gradients and junctions. 2Q 2004

8. An inverse solution of the SCM requires

repeated solutions of the forward problem; i.e.,

calculation of the SCM signal from candidate

carrier profiles. The candidate active dopant

profile is adjusted until a dopant profile is found

that yields a calculated SCM signal that agrees

with the measured SCM signal. Project staff have

developed a regression procedure, in Fig. 4, to

do this with a 2-D solver. The final level of

refinement is to use the full 3-D model as the basis

of inverse solutions of the SCM. However, the
52 National Institute of Standards and Technology

Figure 4. SCM regression solution matched to

theoretical dopant profile. Regression solutions

provided enhanced spatial resolution and accuracy

over calibration curve methods.

volume of data to be processed and the time

required for calculation makes this intractable for

routine profile extraction. Practical application

requires finding shortcuts that will achieve the 3-

D result without having to complete the entire

round of 3-D simulations.

DELIVERABLES: Make available to users a

computationally efficient method of determining

carrier profiles from SCM images using inverse

solutions based on 3-D models. Q4, 2005.

ACCOMPLISHMENTS

# High Repeatability SIMS Measurements

– We collaborated with Agere Systems to

investigate the parameters that must be controlled

to make highly repeatable dose measurements of

As and P implants in Si with magnetic sector

SIMS instruments. With optimized settings, we

demonstrated the ability to distinguish As or P

implant doses differing by 5 % with an RSD of

less than 0.5 % in favorable cases. As the level

of required precision for SIMS dopant

concentration measurements continues to

increase, consideration must also be given to more

subtle effects that may affect the measurement.

In particular, the behavior of the secondary ion

detection system (electron multiplier) may

become a significant factor. Typically, the

detector settings used in routine analyses are those

set by the tool manufacturer. However, these are

not always optimal for high precision

measurements. We have found that the detection

sensitivity for elements such as Si and B can

change by a factor of 2 depending on the energy

of the detected secondary ion and the settings of

the detection system. We have developed a system

at NIST that allows for reproducible detector

setup and optimization. We hope this may allow

us to achieve even higher levels of precision for

dopant concentration measurements.

# Enhanced Detection Limits for Trace

Elements – Secondary ion signals in SIMS are

strongly dependent on the local chemistry of the

sample. Oxygen gas backfilling of the analytical

chamber is often used to enhance positive

secondary ion yields for many metals of interest

in semiconductor metrology. In some cases, the

use of halogen containing gases, rather than

oxygen, may lead to further sensitivity

enhancements. To address this issue, pure metal

and ion implant standards have been examined

using freon gas backfilling of the sample chamber.

Preliminary data suggests that Cu and Zn are less

enhanced under freon exposure as compared with

oxygen. The ion yields for Al, Ti, Fe, and Ni are

similar under freon and oxygen, but the ion yields

for V, Cr, Mn, Zr, Nb, Mo, Ag, and under freon

enhancement is several times higher than that of

oxygen. The sputtering rate under freon flooding

is about 2.5 times greater than oxygen flooding

under similar sputtering conditions.

# Bevel Image Depth Profiling – The NIST

magnetic sector SIMS instrument has been

modified to create custom ion-milled bevel cross

sections in multilayer test structures, Fig. 5, using

19.5 keV O2

+ bombardment. The use of an

oxygen primary ion beam was found to minimize

sputter rate variations between layers and to

Figure 5. SIMS Image of SiO2

- secondary ion signals

from a bevel cross-section cut into a depth profiling

reference material of alternating SiO2 delta layers in

Ta205.
Semiconductor Microelectronics and Nanoelectronics Programs 53

reduce sputter-induced topography. Subsequent

removal of the implanted oxygen and subsurface

mixed zone is accomplished using low energy Ar+

bombardment or a gaseous cluster ion beam

sputter system. This approach may provide a

means to produce depth profiles that have higher

depth resolution than is possible using

conventional SIMS analysis using higher energy

primary ion beams.

# Standard Reference Material Ion Implant

Development – A new ion implant reference

material for phosphorus has been developed and

is now available as Standard Reference Material

2133 a phosphorus Implant in Silicon Depth

Profile Standard for SIMS calibration. SRM 2133

is a 1 cm x 1 cm piece of silicon containing a 100

keV implant of P-31 with a certified dose of

9.58E14 atoms/sq cm with an expanded

uncertainty (95 % confidence interval) of

+/- 1.7 %. It was certified by radiochemical

neutron activation analysis that was calibrated

with phosphorus reference solutions. Other ionimplanted SRMs are also available from NIST as

SIMS calibration standards. SRM 2134 is a

100 keV arsenic implant in silicon with a certified

dose of 7.33E14 atoms/sq cm and expanded

uncertainty of 0.38 %, and SRM 2137 is a 50 keV

boron-10 implant in silicon with a certified dose

of 1.018E15 atoms/sq cm and expanded

uncertainty of 3.4 %.

# Polyatomic Primary Ion Beam SIMS –

Studies of polyatomic primary ion beam sources

for high depth resolution SIMS dopant profiling

are continuing. Collaborations with international

SEMATECH have focused on depth profiling of

novel ZrO2

 films, Fig. 6, on both magnetic sector

Figure 6. SIMS depth profile of ZrO2 oxide layer

(6 nm thickness) with SF5

+ depth profiling.

and TOF-SIMS instruments. A new polyatomic

ion source has been designed and constructed that

features an externally removable reaction

chamber that will allow for use either as a standard

duoplasmatron or as an SF5

+ source. A special

lens system was ordered that would allow higher

cluster ion currents to be extracted from the

source.

# Developed a set of tutorial samples – for

2-D dopant profiling with the SCM. These

samples have large source/drain (S/D) spacing

(1.5 ìm), so that any user of SCM should be

able to easily image them and extract 2-D dopant

profiles from the image using the FASTC2D

software. The sample contains four types of

transistors-like structures: the usual CMOS pair

(n+ S/D regions in p-substrate and p+ S/D in a nwell) and a pair with the type of the source/drain

reversed (that is, p+ S/D in the p-substrate and

n+ S/D in the n-well). These samples are to be

used in a tutorial with the FASTC2D software to

teach users how to acquire SCM data suitable for

extraction of quantitative dopant profiles and

proper use of the FASTC2D software, Fig. 7.

Figure 7. Typical SCM image and extracted 2-D

dopant contours for Boron (p-type) implant tutorial

sample.

# Progress in development of SCM 3-D

regression solutions – Developed a methodology

for calculating the SCM signal by using the oxidecapacitance distribution to specify a natural

boundary condition along the probed surface of

the semiconductor sample, so that only the

semiconductor substrate region needs to be

discretized. This approach may lead to an

increase in the speed for which 3-D solutions can

be obtained.
54 National Institute of Standards and Technology

# Developed techniques to optically pump

scanning probe microscopes – Dr. Gyoung-Ho

Buh, guest researcher from the University of

Seoul, Korea has developed techniques to

optically pump scanning probe microscopes. The

technique encompasses a GUI interface written

in Visual C++ to control a laser, lock-in amplifier,

and digital oscilloscope to measure optically

induced capacitance transients. Carrier lifetime

can be deduced from the capacitance transient in

response to a change in illumination. Publications

and a patent application are pending.

# Implemented advanced MOS models in

FASTC2D – Implemented new routines to

calculate the SCM capacitance-voltage curve

database using Fermi-Dirac statistics and either

a classical and quantum mechanical model. This

is the first implementation of a quantum

mechanical correction to the C-V response for

SCM data interpretation. Since SCM usually uses

a very thin oxide this is an important extension

to previous practice.

# Developed software to simulate the dopant

profile exposed on a beveled cross-section –

The BASSAR (for Bevel and Section Simulation

and Reconstruction) software can: 1) simulate

2-D profiles from input model parameters, 2)

predict profiles that would be exposed at specified

cross-sectioning and bevel angles, and 3) recover

the normal profile (to 0th order) from an input

profile measured on a beveled cross-section.

# Completed study on factors influencing

the reproducibility of SCM measurements –

The effects of stray light from the AFM laser and

stray capacitance from sample geometry have

been quantified. Chi Tran, guest researcher from

the University of California, Berkley and Dr. Buh

conducted PSPICE simulations of an RCA style

capacitance sensor confirming several previously

unverified assumptions about the behavior of the

SCM.

COLLABORATIONS

AMD – Metal film depth-profiling by SIMS.

Atolytics Inc, Paul Weiss – Phase II SBIR to develop a commercial version of their scanning microwave microscope.

FM Technologies – High brightness oxygen source for 2D

dopant profiling by SIMS.

Imago Scientific Instruments Corporation, Madison, WI,

Tom Kelly, Evaluation of position sensitive atom probe.

International SEMATECH, Joe Bennett – Thin oxide depthprofiling by SIMS.

Los Alamos National Laboratory, Denise George and Andrew

Kuprat – Development of next generation 3-D Poisson

simulators using the LaGrit grid management toolbox.

Manufacturing Instrumentation Consultant Co – Phase II

SBIR to develop co-axial shielded scanning probe

microscope tips.

North Carolina State University, Fred Stevie – Dose

repeatability of SIMS measurements.

Peabody Scientific – Ion source development for SIMS.

Samsung Electronics Co., Gyoung Ho Buh – Characterization of ultra-shallow junctions and optically pumped SPM

measurements.

University of Queensland, Brisbane, Australia, Tong Yeow

– A PhD student will be a guest researcher at NIST summer

2003 to take data with the SCM and learn about our modeling

effort.

RECENT PUBLICATIONS

G. Gillen, S. Wight, P. Chi, A. Fahey, J. Verkouteren, E.

Windsor and D. B. Fenner, “Bevel Depth Profiling SIMS

for Analysis of Layered Structures,” in Characterization and

Metrology for ULSI Technology 2003, D.G. Seiler et al., eds.,

AIP Press, Accepted for publication 2003.

N. Jin, S.-Y. Chung, A. T. Rice, P. R. Berger, P. E. Thompson,

C. Rivas, R. Lake, S. Sudirgo, J. J. Kempisty, B. Curanovic,

S. L. Rommel, K. D. Hirschman, S. K. Kurinec, P. H. Chi,

and D. S. Simons “Diffusion Barrier Cladding in Si/SiGe

Resonant Interband Tunneling Diodes And Their Patterned

Growth on PMOS Source/Drain Regions,” Accepted for

publication in special issue on Transient Enhanced Diffusion,

J. Appl. Phys. (2003).

G. H. Buh, C. Tran, and J. J. Kopanski, “PSPICE analysis of

a scanning capacitance microscope sensor,” Proceedings of

the 7th Intl. Workshop on Fabrication, Characterization, and

Modeling of Ultra-Shallow Doping Profiles in

Semiconductors, American Vacuum Society, Santa Cruz,

Calf., pp. 201-206 (April 2003).

G. Gillen, “Secondary Ion Mass Spectrometry using Cluster

Ion Beams,” Applied Surface Science 203, 209-213 (2003).

J. F. Marchiando and J. J. Kopanski, “Regression procedure

for determining the dopant profile in semiconductors from

scanning capacitance microscopy data,” J. Appl. Phys. 92,

5798-5809 (2002).

J. J. Kopanski, J. F. Marchiando, and B. G. Rennex,

“Comparison of Experimental and Theoretical Scanning

Capacitance Microscope Signals and their Impact on the

Accuracy of Determined Two-Dimensional Carrier Profiles,”

J. Vac. Sci. Technol. B 20, 2101-2107 (2002).

J. J. Kopanski, “Capacitance Probe Microscopy,” in The

Encyclopedia of Imaging Science and Technology, J. P.

Hornak, ed., John Wiley & Sons, NY (2002).

P. H. Chi, D. S. Simons, J. M. McKinley, F. A. Stevie, and

C. N. Granger, “High Precision Measurements of Arsenic

Implantation Dose in Silicon by Secondary Ion Mass

Spectrometry,” in Characterization and Metrology for ULSI

Technology 2000, D. G. Seiler et al., eds., AIP Press, pp.

682-686 (2001).
Semiconductor Microelectronics and Nanoelectronics Programs 55

G. Gillen, S. Roberson, A. Fahey, M. Walker, J. Bennett and

R. T. Lareau, “Cluster Primary Ion Beam Secondary Ion Mass

Spectrometry for Semiconductor Characterization,” in

Characterization and Metrology for ULSI Technology, D. G.

Seiler et al., eds., AIP Press, pp. 687-691 (2001).

D. E. McBride, J. J. Kopanski, and B. J. Belzer, “Gate Oxide

Formation Under Mild Conditions for Scanning Capacitance

Microscopy,” in Characterization and Metrology for ULSI

Technology, D. G. Seiler et al., eds., A.I.P Press, pp. 657-

661 (2001).

R. L. Paul and D. S. Simons, “Neutron Activation Analysis

for Calibration of Phosphorous Implantation Dose,” in

Characterization and Metrology for ULSI Technology, D. G.

Seiler et al., eds., A.I.P. Press, American Institute of Physics,

pp. 677-681 (2001).

B. G. Rennex, J. J. Kopanski, and J. F. Marchiando,

“FASTC2D: Software for Extracting 2D Carrier Profiles from

Scanning Capacitance Microscopy Images,” in

Characterization and Metrology for ULSI Technology, D. G.

Seiler et al., eds., A.I.P Press, pp. 635-640 (2001).

E.R. Fuoco, G. Gillen, M. B. J. Wijesundara, B. Wallace,

and L. Hanley, “Surface analysis studies of yield

enhancements in secondary ion mass spectrometry by

polyatomic projectiles,” J Phys. Chem B 105(18), 3950-3956

(May 10 2001).

G. Gillen. L. King, B. Freibaum, et al., “Negative cesium

sputter ion source for generating cluster primary ion beams

for secondary ion mass spectrometry analysis,” J. Vac. Sci.

Technol. A 19(2), 568-575 (Mar-Apr 2001).

K. D. Hobart, P. E. Thompson, S. L. Rommel, T. E. Dillon,

P. R. Berger, D. S. Simons, and P. H. Chi, “p-on-n Si

interband tunnel diode grown by molecular beam epitaxy,”

J. Vac. Sci. Technol. B 19(1), 290-293 (Jan-Feb 2001).

RECENT TALKS

J. J. Kopanski, “Two-Dimensional Dopant Profiling of

Semiconductors using the Scanning Capacitance

Microscope,” E. O. Lawrence Berkeley Laboratory Seminar,

Berkeley, CA, May 2, 2003. (INVITED)

G. H. Buh, C. Tran, and J. J. Kopanski, “PSPICE analysis of

a scanning capacitance microscope sensor,” 7th Intl.

Workshop on Fabrication, Characterization, and Modeling

of Ultra-Shallow Doping Profiles in Semiconductors, Santa

Cruz, CA, April 29, 2003.

J. J. Kopanski, J. F. Marchiando, B. G. Rennex, D. Simons,

and Q. Chau, “Towards reproducible SCM image

interpretation,” 7th Intl. Workshop on Fabrication,

Characterization, and Modeling of Ultra-Shallow Doping

Profiles in Semiconductors, Santa Cruz, CA, April 29, 2003.

J. F. Marchiando and J. J. Kopanski, “On calculating the

dopant profile in semiconductors from scanning capacitance

microscopy data,” 7th Intl. Workshop on Fabrication,

Characterization, and Modeling of Ultra-Shallow Doping

Profiles in Semiconductors, Santa Cruz, CA, April 29, 2003.

G. Gillen, S. Wight, P. Chi, A. Fahey, J. Verkouteren, E.

Windsor and D.B. Fenner, “Bevel Depth Profiling SIMS for

Analysis of Layered Structures,” in Characterization and

Meterology for ULSI Technology 2003 Conference, Austin,

TX, April, 2003.

J. J. Kopanski, “Scanning Probe Microscopes for the

Electrical Characterization of Semiconductors,” March 2003

Meeting of the American Physical Society, Austin, TX,

March 4, 2003. (INVITED)

M. Matarazzo and J. J. Kopanski, “High Spatial Resolution

Automatic Resistivity Mapping of Semiconductors,” March

2003 Meeting of the American Physical Society, Austin, TX,

March 4, 2003.

J. J. Kopanski, “Update on NIST’s 2D dopant profiling

project,” SEMATECH Analytical Lab Mangers Working

Group on Two-Dimensional Dopant Profiling, San Jose, CA,

Dec. 13, 2002.

S. Wight, G. Gillen, P. Chi, A. Fahey, A. Roshko, K. Bertness,

“Bevel Crater SIMS for Auger Analysis of Laterally Oxidized

AlGaAs/GaAs Multilayers,” American Vacuum Society 49th

International Symposium, Denver Colorado, Nov. 5, 2002.

D. Simons, E. Steel, R. Marinenko, J. Armstrong, S. Turner,

J. McKinley and F. Stevie, “Si-Ge Interactive Reference

Materials,” 15th Annual SIMS Workshop, Clearwater, FL,

May 1, 2002.

G. Gillen, A. Fahey and S. Wight, “Bevel Depth Profiling

SIMS of Oxidized AlGaAs Superlattice Samples,” 15th

Annual SIMS Workshop, Clearwater, FL, May 1, 2002.

P. Chi and G. Gillen, “Preliminary Evaluation of Positive

Secondary Ion Yield Enhancements Using

Chlorodifluoromethane Backfilling,” 15th Annual SIMS

Workshop, Clearwater, FL, May 1, 2002.

D. Simons, “Development of a Phosphorus-Implant-inSilicon Calibration Standard for SIMS,” 13th International

Conference on Secondary Ion Mass Spectrometry, Nara,

Japan, November 13, 2001.

D. Simons, “Reference Materials for SIMS: Philosophy,

Development, and Results,” AVS 48th International

Symposium and IUVSTA 15th International Vacuum

Congress, San Francisco, CA, October 30, 2001. (INVITED)

D. Simons, “Reference Materials for SIMS - What We Have

Now and What We Need,” 14th Annual SIMS Workshop,

Scottsdale, AZ, May 14, 2001.

A. Fahey and G. Gillen, “Characterization of Electron

Multiplier Post Acceleration using Pulse Height

Distributions,” 14th Annual SIMS Workshop, Scottsdale, AZ,

May 14, 2001.

P. Chi, “Roughness Reduction on Depth Profiling of Cu

Films with Sample Rotation,” 14th Annual SIMS Workshop,

Scottsdale, AZ, May 14, 2001.

J. Kopanski, “Comparison of Experimental and Theoretical

Scanning Capacitance Microscope Signals and Their Impact

on the Accuracy of Determined Two-Dimensional Carrier

Profiles,” Sixth International Workshop on the Fabrication,

Characterization and Modeling of Ultra Shallow Doping

Profiles in Semiconductors,” Napa, CA, April 22-26, 2001.

G. Gillen, “Secondary Ion Mass Spectrometry Using Cluster

Ion Beams,” National Security Agency and University of

Maryland, April 4, 2001. (INVITED)

G. Gillen, “SIMS Depth Profiling with Cluster Primary Ion

Beams,” Surface Analysis 2001 Meeting, Orlando, FL,

March 12-14, 2001.

G. Gillen, “Surface Analysis with Energetic Cluster Ion

Beams,” PITTCON, New Orleans, LA, March 5, 2001.
56 National Institute of Standards and Technology

GATE DIELECTRIC METROLOGY

Technical Contact:

J. Ehrstein

N. Nguyen

J. Suehle

J. Scott

R. Matyi

E. Steel

GOALS

Develop new and improved optical and electrical

measurements, models, data and reference

materials to enable better and more accurate

measurements of select, critical, thin film

parameters for silicon CMOS technology. Major

focus is placed on high-k materials for the gate

dielectric. Additional work is being done on films

and layers for silicon-on-insulator and strained

silicon-germanium layer upgrades of CMOS

device fabrication.

To address needs in composition and thickness

measurements for thin films and interfaces

including high- and low-k materials from gate

dielectrics to polymers. This project develops new

methods and standards as well as characterizes

the accuracy and reliability of existing methods

using analytical electron, x-ray, and laser probes.

This year the project has five main components:

# Apply spectroscopic ellipsometry characterization to novel high-k dielectric materials and

correlate results with measurements from Grazing

Incidence X-ray Photoelectron Spectroscopy

(GIXPS), High Resolution Transmission Electron

Microscopy (HRTEM), X-Ray Absorption Fine

Structure (EXAFS), and Electron Energy Loss

Spectroscopy (EELS).

# Develop GIXPS for the measurement of

silicon oxide and oxynitride films and determining

the uncertainty of this method.

# Characterize the accuracy of HRTEM for

thickness measurements of ultra-thin gate

dielectrics.

 Apply extended EXAFS and EELS to characterize compositional homogeneity and local-scale

structure in ultrathin (< 10 nm) dielectric films.

# To provide electrical and reliability measurement techniques, data, physical models, and

fundamental understanding for ultra-thin silicon

dioxide and alternate gate dielectrics in future

MOS devices. To increase the understanding of

the relationship between the gate dielectric

material/interface properties and device electrical

and reliability measurements.

Test wafer being loaded on wafer prober for longterm dielectric testing.

CUSTOMER NEEDS

The evolving decrease of the gate dielectric film

thickness to an oxide-equivalent value of 2 nm is

identified as a critical front-end technology issue

in the ITRS with effective thickness values of

1.4 nm, or less, being projected in 2004, 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 or metal

silicates. Process control tolerance needs for

dielectric thickness are projected as ±4 % (3 σs),

which translates to less than 0.1 nm for

2 nm films. Requirements for process control

measurements are a factor of ten smaller still.

Spectroscopic ellipsometry (SE) is expected to

continue as a preferred measurement for process

monitoring of future gate dielectric films. IC

fabrication metrology needs not only improved

methods to determine film thickness accurately,

but also: (1) techniques to determine the structure

of the individual films and the interfaces between

them; (2) an improved understanding of the

relationship between physical, electrical, and

optical determinations of film properties; and (3)

mechanisms, for enabling traceability of

measurements to NIST.

In order for SE to meet process control requirements of film thickness and the determination of

film composition and morphology, the optical

properties of these advanced dielectric film

systems must be characterized and understood.

The microelectronics industry has a high demand

for reliable thin film measurement methods that
Semiconductor Microelectronics and Nanoelectronics Programs 57

yield composition and dimension information with

known accuracy and precision.The metrology

section of the ITRS 1999 metrology section

clearly states the needs for “reference materials

and standard measurement method-ology for new,

high-k gate and capacitor dielec-trics with

interface layers, thin films such as interconnect

barrier and low-k dielectric layers, and other

process needs. Optical measurement of gate and

capacitor dielectric averages over too large an

area and needs to characterize interfacial layers.

The same is true for measurement of barrier

layers.”

As semiconductor processing and devices move

to smaller dimensions and new materials, the

characterization of the device thin film thickness,

composition, and interface quality become ever

more critical to device operation and reliability.

The ITRS indicates that the equivalent thickness

of the gate dielectric will need to be 1.0 nm to

1.5 nm by 2004. Due to increased power

consumption, intrinsic device reliability and

circuit instabilities associated with SiO2 of this

thickness, a high permitivity gate dielectric (e.g.,

Si3N4, 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 films in CMOS

technology requires a fundamental understanding

of the relationship between the gate dielectric

material/interface and device electrical and

reliability measurements. “The 2001 ITRS

identifies the gate dielectric as being one of the

most difficult challenges for future device

scaling,” p. 19 of the Front End Processes

Section.

TECHNICAL STRATEGY

This project focuses on the issues of: (1) identifying structural models and developing preferred

optical index dispersion functions or data for

improved ellipsometric analysis of futuregeneration gate dielectric film systems; (2)

correlating optical, electrical, and physical

measurements of thickness, composition, and

interface structure; and (3) developing and

providing the basis for traceability to NIST for

film measurements, particularly for thickness.

Structural and optical models for ellipsometry

A custom-built, high-accuracy spectroscopic

ellipsometer, Fig. 1, with a spectral range of 1.5

eV 6.2 eV and a commercial VUV ellipsometer

which extends the measurement range to 9.5 eV,

are being used for this task. Project staff are

working with International SEMATECH, IC

industry companies, and SRC university staff to

obtain and optically characterize advanced

oxynitrides, oxide/nitride stacks, and metal-oxide,

and more complex compounds such as hafnium

silicon oxynitride and hafnium-aluminum silicate,

Fig. 2.

Figure 1. Researchers align sample on custom-built

high-accuracy spectroscopic ellipsometer.

Since many of these materials have optical bandgaps just below 6 eV, the additional measurement

range obtainable with the VUV ellipsometer

enables the investigation of important processing

related structure in the index of refraction

(dielectric constant) of these materials which

shows up in the absorbing region above the

bandgap energy. Project work is directed at

determining preferred structural models,

spectroscopic index of refraction values, or

preferred optical dispersion functions for each of

these film systems, and, where possible, the

variability of these parameters due to differences

in film fabrication processes. Work is also directed
58 National Institute of Standards and Technology

at extraction of accurate bandgap values and at

interpreting the processing-related structure

found in the dielectric function. Analysis has been

done primarily with software developed by NIST

for spectroscopic ellipsometry. This software

allows maximum flexibility for addition of the

latest published or custom-developed optical

response models as appropriate for each material

system investigated, and has allowed maximum

transparency of analytical procedures employed.

Figure 2. Tauc plots for determining band-gaps of

HfAlO films with differing aluminum concentrations

from analysis of ellipsometrically determined

dielectric function.

DELIVERABLES: Determine mechanism for changes

in dielectric function of HfAlO films vs. anneal

temperature by correlation with physical measurements such as HRTEM and EELS. 4Q 2003

Relation between optical, electrical and

physical measurements of thickness

Through collaborations with International

SEMATECH, IC industry companies, and SRC

university staff, as well as with key researchers

in other parts of NIST, Project staff are leading

and participating in a number of multimethod

comparison studies of various ultra-thin gate

dielectric films. These multimethod studies utilize

techniques such as x-ray and neutron reflectivity,

high resolution TEM, EELS, angle-resolved XPS,

SIMS, C-V and I-V analysis, as well as spectroscopic ellipsometry and reflectivity. Examples of

such comparisons are shown in Fig. 3. and Fig. 4.

The results of these multimethod studies improve

the general understanding of state-of-the-art

(SOA) measurement capability for very thin films,

and also allow Project staff to assess the results

of various optical models being applied to the

analysis of these films with respect to interface

layers and structural composition, morphology,

and uniformity. SOA C-V and I-V measurement

capability for gate films has been established in

the Project. Advanced 1-D analysis software from

commercial and university sources has been

established and benchmarked to determine the

effect of model and algorithm sophistication on

oxide film thickness values calculated from C-V

and I-V data.

Figure 3. Thickness Value Comparison for TEM and

C-V Measurements on 2 nm SiO2 films being studied

in conjunction with NC State Univ. and International

SEMATECH.

Figure 4. Thickness Values from Spectroscopic

Ellipsometry Measurements and Various Analysis

Models Compared with C-V Measurement Results.

DELIVERABLES: Develop evaluation of accuracy of

NIST ellipsometry measurements of oxide thickness

in range 2nm to 8nm based on results of international

study sponsored by ISO CCQM. 3Q 2004

Establish and transfer basis of accuracy for

thin dielectric films

Industry requirements for future thin dielectric

film optical measurements and calibration

standards have been identified at a NISTsponsored workshop. Core ellipsometry

measurement capability is being expanded and

strengthened to meet these requirements. An
Semiconductor Microelectronics and Nanoelectronics Programs 59

investigation has been started into cleaning and

recontamination issues for film calibration

standards to determine whether a NIST

workshop’s expressed goal of 0.015 nm long-term

reproducibility of reference artifact values is

obtainable. Results showing drift in apparent

thickness after thermal desorption of accumulated

residue is shown in Fig. 5. Procedures are being

developed to enable traceability of instrument

accuracy to NIST for suppliers of secondary thinfilm reference materials without requiring volume

production of NIST standard reference materials.

Figure 5. Apparent change thickness change with

time for a 2 nm SiO2 film as determined from

ellipsometry measurements following a thermal

desorption of previously accumulated contamination.

DELIVERABLES: Develop set of alternatives, with

operating requirements, for possible traceability of

optical, or electrical measurements of film thickness

or other properties, consistent with ITRS needs.

4Q 2003

Many of the existing and innovative analytical

procedures for the analysis of thin films such as

x-ray photoelectron spectroscopy (XPS), nonlinear optical spectroscopies (NLO), high

resolution (HRTEM) and analytical electron

microscopy (AEM) with x-ray and electron

energy loss spectroscopies, low voltage scanning

electron microscopy x-ray analysis, and Auger

spectroscopy must be significantly improved to

obtain accurate quantitative composition

measurements on films with thicknesses below

10 nm. Our goals are to develop the necessary

methods, analytical correction procedures,

standard data and materials to determine

realistically achievable accuracy levels for

analysis of thin films by these techniques. Films

for these measurement activities are obtained

from industrial sources and fabricated in-house

by spin coating metalorganic solutions.

Grazing incidence XPS allows both depth and

chemical information to be attained from very thin

(less than 10 nm) films. Using a NIST designed,

built, and patented device on the Brookhaven

synchrotron we have been developing the

necessary instrumentation and data interpretation

procedures to characterize International

SEMATECH silicon oxynitride films, Fig. 6.

One of the more common methods of evaluating

film thickness and composition is through the use

of HRTEM and Analytical TEM, respectively.

But the uncertainty of these approaches has only

recently been looked at quantitatively. Films of

dielectric materials grown on silicon were

obtained from SEMATECH and analyzed to

determine the accuracy of this HRTEM and AEM

approach. Comparisons across many methods

were made to determine the relative accuracy and

precision of these approaches.

Figure 6. Comparison of Si 2p XPS spectra with

angle taken at 1823 eV (top) and 1844 eV (bottom)

from the same sample. The changes in the spectra

reflect changes in the index of refraction across the

Si K edge at 1839 eV.
60 National Institute of Standards and Technology

GIXPS has been employed successfully as a novel

means to analyze the depth, density, and chemical

composition of the ultrathin dielectric layers being

researched for future semiconductor devices.

Serious discrepancies exist among the traditional

methods of measuring these layers below 10 nm

thickness. An important consideration has

therefore been a determination of the accuracy of

this new method and its potential sources of error.

Discrepancies may also exist because of

differences between phenomeno-logical and

discrete (i.e. atomic) measurements. We have

addressed the first question by studying the

dependence of the results obtained from the

GIXPS method on the accuracy of the physical

parameters required as inputs. We are studying

the second question by comparison with scattering

measurements by short wavelength probes like

x-rays and neutrons.

DELIVERABLES: Compare Grazing Incidence X-ray

Photoelectron Spectroscopy (GIXPS) with neutron

reflectivity and Ellipsometry for the measurement of

silicon oxide and oxynitride films and determine the

uncertainty of this method. 3Q 2003

Figure 7. Molecular model of a gate stack (top) and

one frame from an HRTEM simulation movie

(bottom) showing changes in apparent thickness with

changing imaging conditions.

The ability of HRTEM to measure the thickness

of sub-4 nm gate di-electric films is in question,

pending a quantitative understanding of the errors

and uncertainties in the measurement process. The

goal of this work is to quantify the accuracy of

HRTEM as a technique for measuring the

thickness of such films. Because device

performance is very sensitive to the dielectric

thickness, semiconductor device manufacturers

consider this a critical fabrication parameter and

are keenly interested in its accurate measurement.

To address this problem, a suite of computer

modeling tools was developed to build virtual gate

stacks, Fig. 7 and simulate HRTEM micrographs

from these stacks. Using these new tools, several

hundred images of amorphous SiO2

 films between

Si substrates were calculated, covering a range

of imaging conditions and sample parameters.

Combining the apparent thickness from the micrograph (measured value) and the known thickness

from the model (true value), calculation of the

accuracy as a function of the input variables is

possible. A quantitative understanding of the

errors in HRTEM thickness measurements plays

an important role in improving the quality and

yield of semiconductor electronic device

fabrication.

DELIVERABLES: Determine uncertainty budget for

the characterization the accuracy of High Resolution

Transmission Electron Microscopy (HRTEM) for

thickness measurements of ultra-thin gate dielectrics.

3Q 2003

EXAFS is a synchrotron x-ray spectroscopy tool

that determines the average short-range (about

0.5 nm) local structural information around an

atom that has absorbed an x-ray photon. Since

the separate local structure around each atomic

type present in a material can be measured, the

technique is chemically sensitive. We are

investigating the applicability of EXAFS to study

crystallization and chemical bonding in gate

dielectric films.

DELIVERABLES: Measure crystallinity in ZrO2 films

by EXAFS. 1Q 2003

The strategy of this effort will be to obtain or

fabricate both device samples and blanket films,

to perform reliability and electrical characterization of the devices, and to collaborate with other

researchers to perform analytical characterization.

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 first be developed on the simpler

ultra-thin oxide and oxide-nitride dielectrics and

then be applied to the metal oxide and silicate

dielectrics.

There are two main focus areas for this project.

The first focus area investigates the physics of

failure and the reliability testing techniques for

ultra-thin SiO2

 and high dielectric constant gate

dielectrics. The physical mechanism responsible

for “soft” or “quasi” breakdown modes in ultrathin SiO2

 films and its implications for device
Semiconductor Microelectronics and Nanoelectronics Programs 61

reliability will be investigated as a function of

test conditions and temperature. Long-term timedependent-dielectric breakdown tests will be

conducted on SiO2

 films as thin as 1.5 nm at

electric fields close to operating conditions. These

tests will be used to determine the thermal and

electrical acceleration parameters of device

breakdown.

DELIVERABLES: Experimental results of charge

stability and trapping in HfO2 films including defect

generation rates due to constant voltage stress.

2Q 2003

Experiments will be conducted to investigate

Negative Bias Temperature Instability (NBTI) in

p-channel MOSFET devices. NBTI has become

a serious reliability problem in advanced

microelectronic devices with ultra-thin gate

dielectrics.

DELIVERABLES: Complete study of Negative Bias

Instability (NBTI) of p-channel MOSFETs with 1.2 and

2.2 nm thick gate oxides under pulsed bias conditions. Determine time constants of defect generation

and determine effect on circuit operation. 4Q 2003

The understanding generated in this research will

be used to continue generating standard measurements through a NIST coordinated collaboration

between EIA-JEDEC (Electronic Industries

Association Joint Electron Device Engineering

Council) and the American Society for Testing

and Materials (ASTM). Studies on the reliability

of high dielectric constant dielectrics such as

oxide-nitride stacks will also be performed.

DELIVERABLES: A new JEDEC standard including

guidelines for performing CV, GV and charge

pumping measurements on ultra-thin SiO2 and high-k

films for extracting parameters such as Eot, Dit, VFB,

etc. Q3 2004

Figure 8. A low-noise (fA), high temperature

(300 °C) probe station is used for electrical characterization of devices.

The second focus area is to investigate electrical

measurement techniques, procedures and analysis

associated with devices having thin oxide and

alternate gate dielectrics. An example of a

measurement instrument used in this work is

shown in Fig. 8. The electrical measurement

techniques that we are investigating include

capacitance-conductance characterization,

dielectric tunnel and leakage current characterization and defect density measurements such as

charge pumping and conductance. Furthermore,

standard properties and mechanisms/correlations

for these dielectrics including defect centers,

dielectric constant, defect generation rates, and

leakage/tunnel current will be characterized.

High-k gate dielectric films will be obtained from

key industrial and university groups. Electrical

characterization methodologies will be developed

to address various issues associated with these

films, including large leakage currents, quantum

effects, thickness dependent properties, large trap

densities, transient (non-steady state) behavior,

unknown physical properties, and the lack of

physical models.

Examples of measurement problems that are being

addressed include modifying and verifying

electrical defect density measurement techniques,

including conductance-frequency, capacitancevoltage, and charge-pumping.

DELIVERABLES: Studies of the long-term wear-out

and breakdown of advanced high-k dielectrics.

3Q 2004

ACCOMPLISHMENTS

# Brought commercial spectroscopic ellipsometer on-line with the capability of measuring

out to 9.5eV. This instrument has been used

extensively to investigate high-k dielectric

materials. It’s utility has been established by the

ability to acquire enough data at higher energies

to enable evaluation of the optical band-gap of

the materials being studied. In addition, the higher

energy spectral data has revealed significant

structure in the refractive index (dielectric

function) of the materials above the band-gap.

This additional detail is generally attributed to

processing-related effects on the material such as

the onset of crystallization, and the segragation

of ternary compounds into regions of different

chemical compositions. These observations have

led to the development of collaborations with

scientists in other portions of NIST in order to

develop localized structure and composition
62 National Institute of Standards and Technology

information using such techniques as High

Resolution TEM (HRTEM), Electron Energy

Loss Spectroscopy and Grazing Incidence X-ray

Diffraction. It is expected that the additional

information from such techniques will enable the

identification of specific dielectric function

features with specific defects in the desired

uniform amorphous film structure. The result may

be the rapid identification of onset for various

unwanted material defects and so serve as a

valuable process monitoring improvement.

# Investigated a set of Hafnium aluminate and

titanium aluminate dielectric films with a range

of aluminum concentrations from Yale Univ. The

films were measured with the VUV ellipsometer

from 1eV to 9.5eV. The generalized Tauc-Lorentz

dispersion function, recently developed at NIST,

was the primary dispersion function employed in

the analysis. Film thicknesses and dielectric

functions, determined from the analyses, showed

clear trends in dielectic function values and shapes

for the different films in the set, Fig. 9. These

trends, as well as optical bandgaps were found to

correlate very well with the aluminum

concentration as determined by XPS measurements. However there were clear differences in

the two types of films as determined from the

Figure 9. Testing the ability to fit ellipsometry

measurement data of TiAlO films with GTL dispersion functions by comparison with a direct data

inversion technique.

ellipsometric modeling. For the as-grown HfAlO

films, an optical model consisting of two layers

of different film densities was found to be in

excellent agreement with the experimental data.

For the TiAlO films, only one film was needed in

the model to fit the data.

The experimentally determined dielectric function

of HfAlO samples above 80% Al concentration

showed a single peak in the dielectric function

above the bandgap, while the films with lesser

aluminum concentration showed an additional

satellite peak. The optical bandgaps determined

by Tauc-plots for HfAlO are seen to increase,

meanwhile the relative film densities decrease,

with the increase of Al in the films. In addition,

the optical dielectric functions shift to the higher

energy and decrease in their magnitudes as the

films become more insulating. As a result, the Al

appears to be mixed at the atomic level instead of

forming a phase separation between HfO2

 and

Al2O3. For TiAlO, there were similar results

except that the fundamental optical bandgap was

not strongly affected by the amount of Al

incorporated in the films. To further the

understanding of the physical properties in

relation with the dielectric functions, all these

films were subjected to various high temperature

annealing and the optical properties were then

measured and analyzed. The correlations between

the dielectric function, the film structures such as

densities and thicknesses will be established with

the annealing temperatures.

# Completed a study of thickness measurements of 2 nm SiO2 films. A set of wafers,

fabricates by low pressure thermal oxidation at

NC State Univ. was used in the study.

Ellipsometry, capacitance-voltage and HRTEM

were used for the thickness evaluations. Singlewavelength ellipsometry measurements were

taken on planar oxide wafers immediately

following a thermal desorption cycle to remove

accumulated surface contamination. Capacitancevoltage (C-V) measurements were taken on two

wafers from the set that had a large array of

fabricated capacitors, and HRTEM and EELS

measurements were done on capacitor cross

sections from both capacitor wafers. Ellipsometry

measurements were analyzed with a variety of

simple models of the oxide structure, including

the transition interlayer model that had been used

for previous NIST oxide thickness SRMs greater

than 10 nm thick. The capacitance votage data

was analyzed by several of the available quantummechanics based algorithms formulated to correct
Semiconductor Microelectronics and Nanoelectronics Programs 63

for capacitance due to 2-D quantum states in the

silicon substrate and for depletion in the

polysilicon electrode. The purpose of the study

was to determine the consistency of thickness

values determined with the primary optical,

electrical and physical measurements of thickness

used in the industry for research, performance

testing and process control of gate dielectric films.

Results showed a range of thicknesses resulting

from all three techniques. The spread in

ellipsometry and C-V results were primarily due

to the choice of model or algorithm used to

analyze the data while the spread in HRTEM and

EELS results was almost exclusively due to the

roughness of the upper and lower interfaces of

the oxide films. Most of the overall spread was

less than the thickness of one structural unit of

SiO2, however, it is clearly more than needed

within the industry for process control purposes.

# An extensive comparison of the most

advanced Quantum Mechanical CV simulators

was extended in a number of aspects. A systematic

comparison of QM simulators for p-channel (nsubstrate) devices was performed. The number

of 1-D simulators in the test ensemble was

extended to seven. Quantitative differences in the

accumulation capacitance with ultrathin gate

dielectric films were up to 20 % – similar to

previously reported differences for n-channel

devices. Some of the underlying physical and

modeling differences leading to these differences

were identified, investigated and reported; a

complex interplay of a number of factors was

found.

A method to extend the comparison to include

2-D simulators was also investigated and

demonstrated. A model test structure was

developed, refined and calibrated for use with

Medici, a commercial 2-D simulator, to ensure

that a quasi-1-D device was being simulated.

Requests have been received for permission to

use figures from this work in manuscripts,

presentations and a University MOSFET class.

# We performed an analysis of the effects of

errors in physical input parameters (e.g., x-ray

cross-sections, atomic form factors, electron mean

free paths) on the results obtained for films of

different thicknesses. This allowed us to identify

the sensitivity of the results to certain inputs. We

were able to test one of these dependences

experimentally on the tunable NIST beamline X24A at the National Synchrotron Light Source.

By performing the same photoemission

experiment with x-rays of slightly different

energies about the K excitation edge of Si, the

physical performance of the same sample was

dramatically changed. An extensive analysis

showed that the two results for the same sample

differed by 16 %, which is a good indicator of the

accuracy level of the optical parameters used in

the determination.

# A suite of computer modeling tools was

developed to build virtual gate stacks and simulate

HRTEM micro-graphs from these stacks. Using

these new tools, several hundred images of

amorphous SiO2

 films between Si substrates were

calculated, covering a range of imaging conditions

and sample parameters. Combining the apparent

thickness from the micro-graph (measured value)

and the known thickness from the model (true

value) allows calculation of the accuracy as a

function of the input variables. This revealed that

measured thickness depends strongly on defocus

and astigmatism. Using the Tcl scripting language,

a graphical user interface was written to enhance

the tools, improve efficiency, and simplify the

database interface needed to manage the large

amount of data that was generated.

# EXAFS: The instrument at beamline X23A2

at the National Synchrotron Light Source in

Brookhaven was configured in grazing incidence

mode to measure 5 nm thick ZrO2

 gate dielectric

films. Two series of films were measured: atomic

layer chemical vapor deposited (ALCVD) films

annealed at (825 to 900) °C in air, oxygen,

nitrogen or vacuum (courtesy of J. Chang,

UCLA); and spin-coated films annealed in air at

temperatures of (200 to 800) °C (NIST). The

degree of crystallinity in the films was estimated

from the measured Zr-O-Zr bond angle spread

Äè2 in the EXAFS results. ALCVD films

annealed at temperatures comparable to the

thermal budget required for device fabrication

(825 to 900) °C exhibited the same degree of

crystallinity, Äè2

 < 1°, regardless of annealing

atmosphere. Crystallinity in the spin-coated

films decreased from Äè2 > 6° for the asdeposited films (200 °C) to Äè2

 = 2-4° for films

annealed above 600 °C.

# Test samples: ZrO2 thin films with thicknesses ranging from 2.5 nm to 10 nm were

deposited by spin coating metalorganic zirconium

acetate-based solutions onto (100) Si wafers.

Processes were developed for depositing

zirconium silicate films from zirconium acetate/

tetraethoxysilane and zirconium nitrate/
64 National Institute of Standards and Technology

tetraethoxysilane solutions. Silicate films with

thicknesses of 10 nm to 15 nm and SiO2/ZrO2

ratios of 1:1 to 4:1 were fabricated; the

composition of these films is being measured by

EELS.

# New JEDEC document including noisebased test procedures for TDDB standard of sub2 nm films. The document includes new test

methodologies for detecting soft or noisy break

down events in ultra-thin gate dielectrics and is

expected to be effective in detecting breakdown

in films as thin as 1.5 nm. It has been approved

by the JEDEC JC-14.2 Committee and by the

JEDEC Board of Directors. A round-robin

experiment is currently being planned to

investigate the effectiveness of the test procedure.

# Negative bias temperature instability studied

for deep sub-micro p-MOS devices under pulsed

bias stress. Bias Temperature Instability (BTI) has

become a serious reliability problem in deep submicron p-MOSFETs as device dimensions are

continually scaled down. The ∆Vth of NBTI was

observed to be significantly reduced for pulsed

bias repetition frequencies greater than 10 KHz

implying an important role of hole trapping and

detrapping. However, ∆Vth of PBTI was almost

independent of frequency. The ∆Vth did not show

significant channel length dependency. These

results suggested that there are different

mechanisms for ∆Vth of NBTI and NBTI and the

reliability specifications of NBTI could possibly

be relaxed under certain pulsed operation

conditions.

# A study was performed to investigate defect

generation in ultra-thin silicon dioxide films over

large fluence ranges. The defect generation rate

(Pg) during constant voltage stress is investigated

by using short-time voltage pulses over a large

fluence range. It is found that Pg is not constant

as a function of injected charge and the voltage

acceleration of Pg in the linear defect generation

regime is similar to that of the reciprocal of QBD.

The change of carrier capture cross (ó) during

defect generation was speculated as one of the

reasons responsible for the change of Pg value.

However, from this preliminary report, we have

determined that the change of Pg can not be

explained by the change of ó.

# A Visual Basic MOS simulation code was

developed based on first-order approximations

for the QM effects. The code permits us to quickly

evaluate experimental and simulation parameters

on CV characteristics was developed. Comparisons with full Schroedinger-Poisson solvers

(UTQuant) showed good agreement over a large

range of experimental conditions (tox = 1.0 nm

to 2.0 nm, Nsub = 1015 cm-3 to 1018 cm-3). The

results show that a model based on the

modification of the total semiconductor charge

can be used to simulate QM CV characteristics

of polysilicon-gated MOS devices.

The effects of a distribution of interface states

with energy on the CV characteristics can be

simulated. Further work on the impact of bulk

insulator defects (slow states), EOT extraction,

etc. can now begin. The code is currently being

used in collaboration with J. Kopanski in

FASTC2D applications and was transferred to

SEMATECH to simulate the effects of interface

trap density on CV characteristics. A manuscript

detailing the code was prepared and accepted by

Solid State Electronics.

COLLABORATIONS

# Transfer of beta version of NIST-developed

“SE Studio” software for analysis of spectroscopic

ellipsometry data: Applied Materials and Korean

Research Institute of Standards and Science

# Development and characterization of 2 nm

thick oxide reference materials, NC State Univ.,

KLA-Tencor and International SEMATECH;

Measurement traceability experiments for SiO2

and Si3N4 film thickness, VLSI Standards Inc. and

Rudolph Technologies Inc.

# Studies of optical, electrical and physical

measurements and properties of oxynitrides and

high-k dielectric films, NIST Divisions 837, 842,

852; Univ. Maryland, Univ. Minnesota, NC State

Univ., Univ. Texas-Austin, UCLA, Yale Univ;

IBM, Solid State Measurements, Texas

Instruments, International SEMATECH.

# Development and transfer of ellipsometer

techniques and models for analysis, Penn State

University and Univ. Maryland.

# Spectroscopic ellipsometry characterization

of low-K SiO2

 thin films, Division 854 and

International SEMATECH

# We collaborated on measurements on high

quality SiO2/Si- samples by x-ray reflectivity at

high momentum transfer at the Advanced Photon

Source. The Fourier inversion of the reflectivities

yields electron density with depth. Similarly, we

have measured the neutron reflectivity from the

same samples on a cold neutron beamline at the

NIST Reactor. We are in the process of extracting
Semiconductor Microelectronics and Nanoelectronics Programs 65

mass densities from the data with depth. These

determinations allow us to calibrate the electron

densities and mass densities used in layer

thickness fits by GIXPS, which may differ greatly

in thin films from the quoted values in the

literature for bulk compounds. This type of

verification has not been readily available.

# Based on simulated HRTEM micrographs at

three different objective lens defocus values and

three different specimen tilt angles, their results

suggested (surprisingly) that minimum contrast

defocus yields more accurate film thickness

measurements than Scherzer defocus, and that a

25 mrad specimen tilt was more accurate that no

tilt. Also, as expected, eliminating the spherical

aberration of the objective lens improves the

measurement considerably. Because of their

sparse sampling of the parameter space (12

simulations to characterize changes in 6

variables), they were unable to quantify the results

or confirm these promising anomalies. By using

hundreds of simulations chosen to systematically

sample the parameter space, this work seeks to

extend their result and quantitatively characterize

the error of the measurement as a function of the

input variables. Optimum conditions for performing gate dielectric thickness measurements

can then be chosen with confidence, and the

inevitable effects of experimental variation in the

measurement process can be assessed and managed.

# We have demonstrated the use of a multilayer

thin film stack to selectively measure the VR-SFG

signal from buried interfaces through the

manipulation of Fresnel factors. We expect this

technique to be broadly applicable to polymer/

polymer and polymer/dielectric interfaces. The

study of molecular orientation at buried polymer

interfaces and its influence on adhesion has been

demonstrated. The influence of molecular

orientation on the functional properties of a wide

range of transparent media can now be explored.

# Advanced Micro Devices, ultra-thin oxide

reliability (John S. Suehle)

# Agere Technologies, ultra-thin gate oxide

reliability (John S. Suehle)

# Analog Devices, ultra-thin gate oxide

reliability (John S. Suehle)

 CSTL/Process Measurements Division,

microhotplate-based sensor arrays (John S.

Suehle and Michael Gaitan)

# Division 836, 837, 838, Roger Van Zee, et

al., spectroscopic Ellipsometry of molecular

electronics (Nhan V. Nguyen, Curt A. Richter, and

John S. Suehle)

# Divisions 836, 837, 838, Dr. Roger van Zee

et al., Molecular Electronics Competence Project

(Curt A. Richter and John S. Suehle)

# Fairchild Semiconductor, ultra-thin gate

oxide reliability (John S. Suehle)

# George Washington University, microhotplate-based chemical sensors (John S. Suehle)

# IBM, Alternative Gate Dielectrics (Eric M.

Vogel)

# ISO TC202, Microbeam Analysis, Ryna

Marinenko (EPMA), John Henry Scott (AEM),

John Small (SEM), Dale Newbury

(Nomenclature)

# JEDEC JC14.2 Committee on Wafer-Level

Reliability, Dielectric Working Group, Chairman

(John S. Suehle)

# N.C. State University (oxynitrides, nitrides,

ultra-thin SiO2

), alternative gate dielectrics (Eric

M. Vogel)

# National Semiconductor, ultra-thin gate oxide

reliability (John S. Suehle)

# Penn State University, ultra-thin gate oxide

reliability (John S. Suehle)

# SEMATECH, Alain Diebold, Gate Oxide

Metrology Task FEP Z001 (Eric M. Vogel, James

R. Ehrstein, Nhan V. Nguyen, and Curt Richter)

# Sharp Microelectronics, Dr. John Conley,

characterization of Hafnium-oxide dielectric

films, summer 2000 to summer 2003 (James R.

Ehrstein and John S. Suehle)

# Texas Instruments, electrical and reliability

characterization of ultra-thin gate oxides (John

S. Suehle and Curt A. Richter)

# The Pennsylvania State University,

Molecular Electronics (Curt A. Richter and John

S. Suehle)

# University of Delaware, alternative

dielectrics (John S. Suehle)

# University of Maryland, College Park,

microhotplate-based chemical sensors (John S.

Suehle)

# University of Maryland, College Park, ultrathin gate oxide reliability (John S. Suehle and Eric

M. Vogel)
66 National Institute of Standards and Technology

# University of Minnesota, Alternate Gate

Dielectrics, (Eric M. Vogel)

# Yale University, Alternative Gate Dielectrics

(Eric M. Vogel)

STANDARDS COMMITTEE PARTICIPATION

# ASTM F-1 on Electronics, Membership

Secretary and Member of Executive

Subcommittee, (James R. Ehrstein)

# ASTM F-1 on Electronics, Subcommittee

F1.06 on Silicon Materials and Process Control,

Chairman and Ballot Coordinator (James R.

Ehrstein)

# ASTM F-1 on Electronics, Subcommittee

F1.06, Section B on Thin Film Characterization,

Section Chair, (James R. Ehrstein)

# ITRS Conference, SEMICON West 2001,

San Francisco, CA, July 13-18, 2001 (Barbara J.

Belzer)

# JEDEC JC14.2 Committee on Wafer-Level

Reliability, Dielectric Working Group, Chairman

(John S. Suehle)

RECENT PUBLICATIONS

Afridi, M.Y., Berning, D.W., Hefner, A.R., Suehle, J.S.,

Zaghloul, M.E., Kelley, E.F., Parrilla, Z., Ellenwood, C.H.,

“Transient Heating Study of Microhotplates by Using a HighSpeed Thermal Imaging System,” SEMI-THERM

Proceedings 2002, IEEE SEMI-THERM SYMPOSIUM, Mar

12-14, 2002, San Jose, California, pp. 92-98, (01-MAR2002).

Afridi, M.Y., Suehle, J.S., Zaghloul, M.E., Berning, D.W.,

Hefner, A.R., Semancik, S. Cavicchi, R.E. “A Monolithic

Implementation of Interface Circuitry for CMOS Compatible

Gas-Sensor System,” Proceedings of the ISCAS, IEEE

International Symposium on Circuits and Systems, May 26-

29, 2002, Scottsdale, Arizona, Vol. 2, pp. 732-735, (01-JUL2002).

Chandler-Horowitz, D., Ehrstein, J.R., and Nguyen, N. V.,

“Recent Progress In Thin film Thickness measurements by

Ellipsometry,” Submitted to Conference proceedings: 2003

International Conference, Characterization and Metrology

for ULSI Technology, March 2003, Austin, TX

Chism, W., Diebold, A., Canterbury, J., and Richter, C.,

“Characterization and Production Metrology of Thin

Transistor Gate Dielectric Films, Solid State Phenomena,”

vols. 76-77, pp. 177-180 (2001).

Conley, J. F., Jr., J. S. Suehle, A. H. Johnston, B. Wang, T.

Miyahara, E. M. Vogel, and J. B. Bernstein, “Heavy Ion

Induced Soft Breakdown of Thin Gate Oxides,” IEEE Trans.

Nucl. Sci. Dec. 2001.

Cresswell, M. W., Arora, N., Allen, R. A., Murabito, C. E.,

Richter, C. A., Gupta, A., Linholm, L. W., Pachura, D., and

Bendix, P., “Test Chip for Electrical Linewidth of CopperInterconnection Features and Related Parameters,”

Proceedings of the 2001 IEEE International Conference on

Microelectronic Test Structures, Kobe, Japan, March 19-22,

2001, Vol. 14, pp. 183-188.

Diebold, A., Canterbury, J., Chism, W., Richter, C. A.,

Nguyen, N. V., Ehrstein, J. R., and Weintraub, C.,

“Characterization and Production Metrology of Gate

Dielectric Films: Optical Models for Oxynitrides and High

Dielectric Constant Films,” Materials Science in

Semiconductor Processing, Vol. 4., pp. 3-8, 2001.

Ehrstein, J.R., Richter, C.A., Chandler-Horowitz, D., et al,

“Thickness Evaluation for 2 nm SiO2 Films, a Comparison

of Ellipsometric, Capacitance-Voltage and HRTEM

Measurements,” Submitted to Proceedings: 2003

International Conference, Characterization and Metrology

for ULSI Technology, March 2003, Austin, TX

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

C.A., Heh, D. W., Suehle, J., “Energy Distribution of

Interface Traps in High-k Gated MOSFETs,” 2003 VLSI

Tech. Symp.

Heh, D, Vogel, E.M., and Bernstein, J.B., “Defect Generation

in ultra-thin Oxides over Large Fluence Ranges”, Integrated

reliability Workshop Final Report, Lake Tahoe, CA, October

2002.

Jach, T., Dura, J. A., Nguyen, N. V., Swider, J., Cappello,

G., and Richter, C., “Comparative Measurements of SiO2/

Si Films for Thickness less than 10 nm,” Submitted to

Surface and Interface Analysis

Jach, T., Landree, E., “Grazing Incidence X-Ray

Photoemission Spectroscopy and the Accuracy of Thickness

Measurements of CMOS Gate Dielectrics,” Surf. and Inter.

Anal. 31, 768 (2001).

Jach, T., Landree, E., “Grazing incidence X-Ray

Photoelectron Spectroscopy: A Method to Study Gate

Dielectric Films on Si,” J. Surf. Anal. 9, 339 (2002).

Kim, J.Y., Nguyen, N. V., Lee, H.-J., and Yoon, D. L., Kim,

Y.-H., and Kim, H. J. Kim, “Vacuum Ultraviolet

Spectroscopic Ellipsometry Study of Porous Low-k SiOC

Thin Films,” Extended abstract: Fourth International

Conference on Microelelctronics and Interface (AVSICMI’03)

Landree, E., Jach, T., Brady, D., Karemcheti, A., Canterbury,

J., Chism, W., and Diebold, A.C., “Characterization of

Silicon-Oxynitride Dielectric Thin Films Using Grazing

Incidence X-Ray Photoelectron Spectroscopy,”

Characterization and Metrology for ULSI Technology: 2000

International Conference, D. G. Seiler, A. C. Diebold, T. J.

Shaffner, R. McDonald, W. M. Bullis, P. J. Smith, and E.

M. Secula, Eds., CP550, American Institute of Physics

(2001) p. 159-163.

Levine, Z.H., Gao, J.J., Neogi, S., Levin, T.M., Scott, J.H.,

Grantham, S., “Parallax Measurements of Integrated Circuit

Interconnects Using a Scanning Transmission Electron

Microsope,” J. of Applied Physics, vol. 93, number 4, pp.

2193-2197 (2003).
Semiconductor Microelectronics and Nanoelectronics Programs 67

Newbury, D.E., “Diagnostics for Assessing Spectral Quality

for X-ray Microanalysis in Low Voltage and Variable

Pressure Scanning Electron Microscopy,” Microsc.

Microanal. 7 (Supp 2: proceedings) (2001) 702-703.

Nguyen, N. V., Han, J-P, Kim, J. Y. Kim, and Ehrstein, J.

R., Zhu, W., Luo, Z, and Ma, T. P., “Vacuum Ultraviolet

Spectroscopic Ellipsometry of Hafnium Aluminate and

Titanium Aluminate Thin Films,” Extended Abstract

submitted to Fourth International Conference on

Microelelctronics and Interface (AVS-ICMI’03)

Nguyen, N. V., Han, J-P., Kim, J. Y., Zhu, W., Zhu, Zhijiong,

L., and Ma, T. P., “Optical properties of As-Grown and

Annealed Jet-Vapor-Deposited TiAlO and HfAlO

Determined by Vacuum Utraviolet Spectroscopic

Ellipsometry,” Submitted to Proceedings: 2003 International

Conference, Characterization and Metrology for ULSI

Technology, March 2003, Austin, TX

Nguyen, N. V., Richter, C. A., Cho, Y. J., Alers, G. B., and

Stirling, L. A., “Effects of High Temperature Annealing on

the Dielectric Function of Ta2O5 Films Observed by

Spectroscopic Ellipsometry,” Applied Physics Letters, Vol.

77, No. 19, pp. 1-3 (2000).

Richter, C., Vogel, E., Hodge, A., and Hefner, A.,

“Differences Between Quantum-Mechanical CapacitanceVoltage Simulators,” SISPAD Technical Digest 2001 (2001

International Conference of Simulation of Semiconductor

Processes and Devices), 4 p.

Richter, C.A. Suehle J.S., Edelstein, M.D. Kirillov, O.,

van Zee, R.D., “Molecular Electronic Test Structures,”

Bulletin of the American Physical Society, Vol. 47, No. 1,

pp. 285-285, (01-MAR-2002).

Scott, J.H., “Characterization of Dielectric Films by

Spectrum-Line Imaging, Second International Conference

on Microelectronics and Interfaces, Advanced Emerging

Metrology and Nanoscience,” Emerging Metrology, 2001,

Santa Clara, CA, AVS.

Suehle, J.S., “Ultra-Thin Gate Oxide Reliability: Physical

Models, Statistics, and Characterization,” IEEE Trans. on

Electron Dev., Vol. 49, No. 6, pp. 958-971, (01-JUN-2002).

Suehle, J.S. Wang, B., Conley, J.F., Vogel, E.M. Roitman,

P. Weintraub, C.E., A.H. Johnston, J.B. Bernstein,

“Observation of Latent Reliability Degradation in Ultra-Thin

Oxides after Heavy-Ion Irradiation,” Applied Physics Letters,

Vol. 80, No. 7, pp. 1282-1284, (18-FEB-2002).

Vogel, E.M., Heh, D., Bernstein, J.B., “Interaction Between

Low-energy Electrons and Defects Created By Hot Holes in

Ultra-Thin Silicon Dioxide,” Applied Physics Letters, Vol.

80, No. 18, pp. 3343-3345, (06-MAY-2002).

Zhu, B., Suehle, J.S., Chen, Y. and Bernstein, J. B.,

“Negative Bias Temperature Instability of Deep Sub-Micron

p-OSFETs Under Pulsed Bias Stress, “ Integrated Reliability

Workshop Final Report, Lake Tahoe, CA, October 2002.
68 National Institute of Standards and Technology

CHEMICAL METROLOGY OF

MATERIALS & PARTICLE CONTAMINATION

Technical Contacts:

E. B. Steel

C. J. Powell

“Researchers in the

Microanalysis Research

Group continue to drive

the commercialization of

new x-ray detectors for

materials microanalysis.

The new silicon drift

detectors, whose

development is being

supported and encouraged by the group, will

allow unprecedented

high-count x-ray detection. The semiconductor

industry eagerly awaits

the availability of

commercial instrumentation, first for laboratory

analysis applications,

then eventually for

online defect identification and classification.”

Anne Testoni,

KLA-Tencor

GOALS

Develop methods, standard materials and data for

the characterization of the elemental composition,

crystallographic phase, and chemical structure of

microelectronic thin films and particle

contaminants at nanometer spatial resolution. We

develop and test x-ray, Field Emission Gun

Scanning Electron Microscopy (FEGSEM),

Transmission Electron Microscopy (TEM), X-ray

Photoelectron Spectroscopy (XPS), AugerElectron Spectroscopy (AES), Scanning Probe

Microscopy (SPM) and diffraction methods to

characterize the chemical composition and phase

of microelectronic components and particle

contaminants. Standard materials and data are

developed to support these methods and enable

improved accuracy and applicability of these

methods to microelectronic devices.

CUSTOMER NEEDS

The constant introduction of new materials (SiGe,

oxynitride, novel high-κ, low-κ, Cu, etc.) and

decreasing dimension size in semiconductor

fabrication cause increasing demands for new,

selective, quantitative, interface and thin film

metrology tools. Typically, many of these same

measurement tools are applied to identify particle

contaminants in the Fabs. There is a critical need

to attain better speed and accuracy of chemical

information from thin and chemically complex

layered materials. The development of faster

methods with known accuracy and the

development of standards to demonstrate and

maintain the quality of chemical analysis methods

is critical to the continuous advancement of

semiconductor technology.

The challenges and needs are outlined in the

metrology section of the 2001 International

Technology Roadmap for Semiconductors on

page 21. “The rapid introduction of new

materials, reduced feature size, new device

structures, and low temperature processing

continues to challenge materials characterization and contamination analysis. Correlation of

appropriate offline characterization methods with

each other and with inline physical and electrical

methods should be accelerated. Use of

characterization methods to provide more

accurate information such as layer thickness or

elemental concentration will continue.

Characterization methods will continue to move

toward whole wafer measurement capability and

clean room compatibility.”

TECHNICAL STRATEGY

1. SiGe thin film compositional standards – After

the need for SiGe compositional standards was

discussed at the 14th Annual Workshop on

Secondary Ion Mass Spectroscopy (SIMS) in

2001, NIST contacted fabrication and analytical

laboratory facilities to develop a reference

material using an interactive approach. The

typical time-frame for an NIST SRM is many

years to develop, characterize, and bring to

market. The need for SiGe standards was seen as

an immediate need, because the current

calibration method of Rutherford Backscattering

is only accurate to 5–10 % relative and this is

insufficient for reliable device production. NIST

is collaborating with several semiconductor

facilities and analytical laboratories to develop a

suite of SiGe compositional standard films. Using

an interactive data collection mechanism with

collaborators and publishing the data on the web,

a faster approach to standard materials production

is being developed. This will allow materials to

be used and compared even while reference data

are being developed.

DELIVERABLES: Provide two interactive reference

materials (Ge-Si thin films) for analysis by collaborating laboratories. 3Q 2004

2. Reference Data for AES and XPS – chemical

characterization using AES and XPS depends on

reference data for interpretation of measurements

and an understanding of the physics of x-ray and

electron interactions with semiconductor

materials for modeling AES and XPS experiments

with complex structures.

Standard Reference Database (SRD) 20 provides

identification of unknown spectral lines in user

measurements, retrieval of data for selected

elements, retrieval of data for selected

compounds, and retrieval of data by scientific

citation.

SRD 64 provides values of differential elasticscattering cross sections, total elastic-scattering

cross sections, phase shifts, and transport cross
Semiconductor Microelectronics and Nanoelectronics Programs 69

sections for elements with atomic numbers from

1 to 96 and for electron energies between 50 eV

and 300 keV. These data are needed for modeling

the transport of signal electrons in AES and XPS;

in addition, they can be used for modeling

electron transport in resists utilized in electronbeam lithography.

SRD 71 provides values of electron inelastic

mean free paths (IMFPs), generally for electron

energies between 50 eV and 2,000 eV. IMFPs are

needed for quantitative analyses by AES and XPS,

for deducing the surface sensitivity, and for

modeling the transport of the signal electrons.

SRD 82 provides values of electron effective

attenuation lengths (EALs) and related data for

AES and XPS. EALs are needed mainly for

measurements of thicknesses of overlayer films.

Since XPS is being used to an increasing extent

for the characterization of new gate-oxide

materials, EALs have been computed from SRD

82 for zirconium dioxide, zirconium silicate,

hafnium dioxide, and hafnium silicate for

common XPS measurement conditions.

NIST is developing a new database (SRD 100:

Simulation of Electron Spectra for Surface

Analysis (SESSA)) to be used for AES and XPS

analyses of thin-film structures. This database

includes data from SRD 64 and SRD 71 and

additional data describing x-ray and electron

interactions with solids. SESSA will enable

comparisons to be made of measured and

simulated spectra for user-specified specimen

morphologies and analytical conditions.

DELIVERABLES: Develop new database for

interpreting AES and XPS data from thin-film

structures. 2Q 2004

3. New X-ray Technology – Silicon Drift

Detectors: A prototype of a silicon drift detector

energy dispersive x-ray spectrometer (SDD-EDS)

developed by Photon Imaging, Inc. of Los

Angeles under a NIST Phase II SBIR grant is

being tested on a scanning electron microscope

(SEM) at NIST, Fig. 1. Conventional silicon

(lithium) EDS forms the backbone of x-ray

microanalysis systems implemented on more than

25,000 SEMs worldwide. Conventional EDS is

limited to a maximum count rate of approximately

25,000 per second and is usually cooled with

liquid nitrogen. The new SDD-EDS technology

is capable of limiting count rates of 1MHz with

150 eV energy resolution and operates at -70 C,

which is achieved with Peltier cooling. The SDDEDS detection area is also physically large. Individual detector chips are 50 mm2

, and assemblies

of multiple chips are possible. The combination

of a large solid angle and a high-count rate are

expected to lead to increased efficiency for SEM/

x-ray microanalysis. The high count rate

capability of SDD-EDS will be especially

important for elemental mapping, where the

number of photons counted per image pixel

defines the concentration level that can be

detected.

DELIVERABLES: Install and test second generation

SDD detector system on UHV Auger microscope.

Q1 2004

Figure 1. Silicon drift x-ray detector (highlighted

oval) on conventional SEM.

4. New X-ray Technology – MicrocalorimeterEDS: The microanalysis laboratory at NIST

Gaithersburg is working as a Beta-test site for the

NIST-Boulder μcal EDS system (discussed

elsewhere.) The system has been outfitted with a

second-generation Cu-Mo thermometer rather

than the first-generation Al-Ag. Immediately after

installation of the new detector, the NIST Boulder

staff obtained 7 eV resolution on Al. The NIST

Gaithersburg staff was then instructed on the

operation and maintenance of the cryogenic

systems and the adjustment of the cryoelectronics,

including the SQUID array, that are critical to

obtaining optimum performance.

The initial thrust of the research for the application

of this high resolution x-ray detector is in the

development of fundamental x-ray data including

x-ray weights of lines and peak shape and energy

shifts. While the characteristic x-ray energies

above about 3 keV are reasonably well known,

the energy and especially the relative intensity of

lower energy x-ray lines in L and M shell x-rays
70 National Institute of Standards and Technology

are not well known and causes significant

uncertainty and bias when analyzing complex thin

film systems, e.g. W-Si. The characteristic peak

energy can shift and peak shape can change as a

function of chemical bonding allowing the

possibility of chemical compound identification

and mapping using the microcalorimeter. A

feasibility study of a new database for chemical

x-ray data is being initiated.

The development of these standard x-ray data

systems requires that we first establish a reproducible response function in intensity and energy

for the detector.

DELIVERABLES: Test modifications to the microcalorimeter temperature regulation system to determine if

they improve detector stability. 1Q 2004

5. Characterization of particles: We are

developing two areas of metrology for particles

– quantitative chemical analysis using electron

and x-ray spectroscopies and quantifying

morphology through image analysis of electron

microscope images.

The quantitative elemental characterization and

identification of nanometer and micrometer-sized

particles is critical to defect control. In the

chemical analysis of particles with electron-beam

instruments, the size and shape of the particle

often results in large analytical errors associated

with x-ray emission and absorption. Over the

years, several different researchers have

developed correction procedures to minimize the

uncertainties associated with particle analysis.

Although these correction procedures reduce the

effects of particle geometry, elemental concentrations determined from the quantitative analysis

of particles by these methods are often accompanied by errors on the order of 10 % to 50 % relative

compared to 3 % to 5 % relative errors for the

analysis of conventional samples. We are

investigating the advantages and disadvantages

of low voltage electron beam analysis,

schematically shown in Fig. 2.

Figure 2. Schematic results of Monte Carlo

calculation of electron beam interaction with a two

micrometer particle sitting on substrate.

In addition to using chemical composition to

classify particles, the shape and surface texture

of a particle can help to classify or identify it and

may also be indicative of its chemical composition

or the process through which it was created.

Shape and texture are used routinely by humans

viewing objects, while many image analysis

approaches currently only use a handful of

parameters, such as roundness or root mean

square roughness. Since the fractal dimension of

a boundary or surface seems visually related to

its roughness or tortuosity respectively, we are

investigating the applicability of these dimensions

for the description of the shape and texture of

individual grains of particulate matter. These

physical objects by their very nature do not have

true fractal shapes, but we hope that they do

exhibit fractal-like behavior in a limited size range

(the “local fractal dimension” or LFD). After

investigation of some image processing methods

in the literature, we have chosen and refined two,

to characterize the LFD of the projected boundary

of a particle and the LFD of an averaged

brightness profile representing the surface. These

measurements require the particle to be

segmented from background, a tedious task to

perform manually and a problematic one to do

automatically. We have developed operator-aided

segmenting methods that are designed for the

image data sets under study.

DELIVERABLES: Develop an accurate quantitative

analysis procedure for high-angle x-ray emission

spectroscopy to allow simultaneous combined x-ray/

EBSD phase mapping and x-ray elemental analysis.

ACCOMPLISHMENTS

# Electron Probe Microanalysis – Wavelength

Dispersive Spectrometry (EPMA-WDS) was used

to test the heterogeneity of bulk SiGe wafers for

use as primary standards in subsequent

characterization of SiGe thin films on Si. Two

different commercially available SiGe singlecrystal boules (nominal compositions of 14 atomic

% Ge in Si and 6.5 atomic % Ge in Si) were

evaluated for the extent of micro- and

macroheterogeneity. The expanded uncertainties

for the two SiGe wafers and for pure Si and Ge

wafers were determined. The SiGe14 wafer was

found to be a suitable bulk standard with a

measured heterogeneity about twice that of the

pure Si and Ge wafers.

# During the past year, a new version of the

popular NIST X-Ray Photoelectron Spectroscopy

Database (SRD 20) was released, a new version
Semiconductor Microelectronics and Nanoelectronics Programs 71

of the NIST Electron Elastic-Scattering CrossSection Database (SRD 64) was released, and

work advanced on a new NIST database for

Simulation of Electron Spectra for Surface

Analysis (SESSA).

# Calculations have been made of electron

effective attenuation lengths (EALs) for

zirconium dioxide, zirconium silicate, hafnium

oxide, and hafnium silicate using the NIST

Electron Effective-Attenuation Length Database

(SRD 82). These EALs were calculated for the

principal photoelectron lines excited by Al Kα

x-rays and for a range of film thicknesses and

emission angles of practical relevance. The EALs

were compared with the corresponding inelastic

mean free paths to determine the magnitude of

the correction for elastic-scattering effects on

film-thickness measurements. For common

measurement conditions, this correction varied

between 12 % and 20 %.

Figure 3. The two overlapped spectra above

demonstrate the stable (red spectrum) and unstable

(black spectrum) on the same tantalum elemental

standard.

# The NIST microcalorimeter energy

dispersive x-ray spectrometer (μcal EDS) has

been successfully transferred from NIST Boulder

to an analytical microscopy environment at NIST

Gaithersburg. Routine operation and maintenance

of the cryogenic and cryoelectronic systems has

been taken over by Division 837 staff. Resolution

performance of 9 eV or better has been achieved

for short (≈100 s) accumulations. Long term

(>1000 s) measurements have been found to be

subject to instability that limits accurate

measurement of x-ray peaks, in Fig. 3. Division

837 researchers are currently working with NIST

Boulder to diagnose the problem(s) and design

corrective measures to improve the detector

stability during long analysis times.

 The Silicon Drift X-ray Detector has been

used successfully to collect a 256x256 spectrum

image with total mapping time of approximately

650 s. This compares to a collection time of

approximately 2 hours using a conventional

monolithic Si detector. We are actively working

with Photon imaging to develop a faster pulse

processing system.

# We have evaluated the x-ray analysis of

particles at low accelerating voltages (5 keV).

Results indicate that the effects of particle

morphology increase as a result of x-ray

absorption requiring careful alignment of the

particle under the electron beam to minimize the

magnitude of the absorption.

COLLABORATIONS

Agere, Stevie (now at North Carolina State University),

McKinley, SiGe thin film standards

Evans Analytical, Buyuklimanli, SiGe thin film standards

Institute of Physical Chemistry (Warsaw), Jablonski,

development of databases for AES and XPS and calculations

of effective attenuation lengths

Motorola, Christiansen, SiGe thin film standards

Photon Imaging, Iwanczyk, new x-ray detector technology

RECENT PUBLICATIONS

Joy, D. C., and Newbury, D. E., “Low Voltage Scanning

Electron Microscopy,” Microsc. Microanal. 7 (Supp 2:

proceedings) 762-763, (2001).

Levine Z. H., Gao J. J., Neogi S, Levin T. M., Scott J. H.,

Grantham S , “Parallax Measurements of Integrated Circuit

Interconnects Using a Scanning Transmission Electron

Microscope”, Journal of Applied Physics, volume 93,

number 4, pp. 2193-2197 (2003)

Newbury, D. E., “Diagnostics for Assessing Spectral Quality

for X-Ray Microanalysis in Low Voltage and Variable

Pressure Scanning Electron Microscopy,” Microscopy and

Microanalysis 7, Supp. 2 Proceedings, 762-763, (2001).

Powell, C. J., Conny J. M., and Jablonski, A., “Some Issues

in Quantitative X-Ray Photoelectron Spectroscopy and

Auger-Electron Spectroscopy,” in State-of-the-Art

Application of Surf. & Interface Anal. Meth. to Environ.

Mat. Inter. In Honor of J. E. Castle’s 65th year. edited by D.

R. Baer et al. (The Electrochemical Society, Pennington,

2001), Vol. 2001-5, pp. 15-45.

Powell, C. J., and Jablonski, A., “Electron Effective

Attenuation Lengths for Applications in Auger-Electron

Spectroscopy and X-Ray Photoelectron Spectroscopy,”

Surface and Interface Analysis, 33, 211-229 (2002).

Jablonski, A., and Powell, C. J., “The Electron Attenuation

Length Revisited,” Surf. Science Reports 47, 33-92 (2002).

Jablonski, A., and Powell, C. J., “Comparisons of Practical

Attenuation Lengths Obtained from Different Algorithms for

Application in XPS,” Surf. Science 520, 78-96 (2002).
72 National Institute of Standards and Technology

Jablonski, A., Salvat, A., and Powell, C. J., “NIST Electron

Elastic-Scattering Cross-Section Database,” Version 3.0

(NIST Standard Reference Data Program, 2002).

Jablonski, A., and Powell, C. J., “Information Depth and the

Mean Escape Depth in Auger Electron Spectroscopy and Xray Photoelectron Spectroscopy, J. Vac. Sci. Tech. A 21, 274-

283 (2003).

Wagner, C. D., Naumkin, A. V., Kraut-Vass, A., Allison, J.W.,

Powell, C. J., and Rumble, J. R., “NIST X-ray Photoelectron

Spectroscopy Database,” Version 3.3 (NIST Standard

Reference Data Program, 2003).

Powell, C. J., and Jablonski, A., “Measurement of GateOxide Film Thicknesses by X-ray Photoelectron

Spectroscopy, in Proceedings of International Conference

on Characterization and Metrology for ULSI Technology –

2003 (to be published).

Scott, J. H., “Characterization of Dielectric Films by

Spectrum-Line Imaging,” Second International Conference

on Microelectronics and Interfaces, Advanced Emerging

Metrology and Nanoscience, Emerging Metrology (2001).

Small, J. A. and Bright, D. S., “Comparison of High- and

Low-Voltage X-Ray Mapping of an Electronic Device,” in

Proc. of the International Conference on Characterization

and Metrology for Ultra Large Scale Integration Technology,

AIP 550, Seiler, D. G., et. al., eds. 596-600, (2001).

Small, J. A. and Michael, J. R., “Phase Identification of

Individual Crystaline Particle by Electron Backscatter

Diffraction (EBSD),” J. Microscopy, Vol. 201, 1, 59-69,

(2001).

Small, J. A. “The Analysis of Particles at Low Accelerating

Voltages (≤ 10 kV) With Energy Dispersive X-Ray

Spectroscopy (EDS),” J. of Research of NIST, Vol. 107, 6,

555-566, (2002).

Newbury, D. E. “Barriers to Quantitative Electron Probe XRay Microanalysis for Low Voltage Scanning Electron

Microscopy,” J. of Research of NIST, Vol. 107, 6, 605-520,

(2002).

Marinenko, R. B. “NIST Standards for Mircoanalysis and

the Certification Process,” J. of Research of NIST, Vol. 107,

6, 687-692, (2002).
Semiconductor Microelectronics and Nanoelectronics Programs 73

THIN FILM X-RAY METROLOGY

FOR MICROELECTRONICS

Technical Contact:

Richard J. Matyi

GOALS

Provide the semiconductor industry with high

accuracy x-ray based measurement methods,

reference materials, and data for the structural

characterization of simple and complex thin film

structures.

CUSTOMER NEEDS

Thin film thickness, density and interfacial roughness are critical to the performance of conducting

and dielectric layers in semiconductor devices.

Sensitive and accurate measurement of such properties are needed in the process development phase

as well as in subsequent manufacturing practice.

In the process development phase the needs are

for structural properties that can be associated with

the electrical performance characteristics of subsequently patterned device arrays. In the subsequent manufacturing phase the need is principally

to calibrate the responses of on-line measurement

tools that do not deliver first-principles based data.

These tools have been developed to meet the needs

for high measurement throughput and compatibility with the production environment. Such tools

have material-dependent calibration requirements

that lead end users and tool manufacturers to look

for alternatives that are less sensitive to materials

properties and process variability.

TECHNICAL STRATEGY

The sub-nanometer wavelengths and relatively

weak interaction of x-ray probes make them a

nearly ideal means for determining the geometry

of the thin film and multilayer structures that

underlie modern semiconductor manufacturing.

As critical dimensions continue to shrink, the

sizes of structures are becoming far smaller than

optical wavelengths but are approaching the

nanometer size of x-ray wavelengths. Thus x-ray

probes are increasingly more favorable for

advanced metrological applications. This fact has

not been lost on the community of semiconductor

tool suppliers – within the past two years there

has been a significant increase in fab-compatible

x-ray metrology instruments.

Our approach has been to develop advanced xray metrological capabilities including high

performance instrumentation, advanced forms of

modeling and analysis, and working with

Figure 1. A high resolution x-ray reflectometry

system (shown schematically above and in the

photograph below) was designed and optimized for

the routine analysis of complex thin film and

multilayer structures that are of interest to the

semiconductor industry.

suppliers and users of x-ray metrology tools so

that both communities receive the maximum

benefit from these instruments.

Our specific technical goals are the following:

1. We have made advanced x-ray measurement

capabilities available for applications to a

considerable range of structural problems

involving metallic interconnect layers, advanced

dielectrics, and diffusion barrier films (see Fig.

1). By responding to currently urgent problems,

principally, but not exclusively, mediated through

SEMATECH, it has been possible to develop firsthand knowledge of industrial needs as well as of

the level of timely responsiveness needed to

provide useful input to these problems.

DELIVERABLE: Quantitative measurements of

density, thickness, and roughness in thin film

semiconductor materials (4Q 2003)

2. Our program develops and applies novel and

advanced x-ray analytical methods to reveal the

microstructure of thin film and multi-layer
74 National Institute of Standards and Technology

structures produced by modern semiconductor

manufacturing. To achieve this goal we must

maintain close contact with SEMATECH, its

member companies, and the semiconductor

manufacturing environment in general to address

the current critical needs of the industry and to

anticipate future needs with x-ray tools.

The main components of this work are: (a) the

development and application of high-resolution

x-ray scattering techniques; (b) providing a rapid

response to urgent problems identified by

SEMATECH; and (c) working with the x-ray

metrology tool users community to improve the

utilization of x-ray measurement instruments for

all. Both the details and the areas of emphasis of

our program have evolved on the basis of this

experience and guidance received during

presentations to various specialized groups within

the SEMATECH framework. Finally, we are in

contact with commercial developers of x-ray

instrumentation in order to facilitate interactions

with customers through the documentation of best

practices and the characterization of reference

materials.

DELIVERABLE: Enlarge our knowledge base on the

effects of systematic errors on the analysis of high

resolution x-ray reflectometry analyses of semiconductor thin film structure (4Q 2003)

3. There are a variety of x-ray scattering tools

that are not widely known in the semiconductor

industry but have the potential to be very powerful

tools for specific measurement problems. For

instance, in thin film systems such as HfO2, the

development of polycrystalline structures is

undesirable due to the impact of crystalline

component on the electrical behavior of the film

and its effect on measurement tools (e.g.,

ellipsometry). However, the detection and

measurement of the polycrystalline component of

thin films (≈ 3 nm) in a laboratory setting is very

difficult using conventional methods. We have

proposed to explore the use of grazing incidence

x-ray diffraction (GIXD), or diffraction under

conditions of specular reflection, to probe the

crystalline structure of thin HfO2 materials. Other

methods of interest to us is the use of quasiforbidden or strictly forbidden x-ray reflections

as a sensitive probe of damage and strain in ion

implanted structures.

DELIVERABLES:. Demonstrate grazing incidence

x-ray diffraction assessment of crystallinity in HfO2

films (2Q 2003)

ACCOMPLISHMENTS

# High-k dielectric films – We have continued

our program on the structural characteristics of

HfO2 high-k film structures by high resolution xray reflectometry followed by computer fitting of

the reflectometry scans. Fig. 2 below shows a

series of HfO2 layers grown under a variety of

conditions. In all cases, the x-ray reflectivity

measurements showed that the structure assumed

from the nominal growth conditions did a poor

job of describing the experimental data. For

instance, an HfO2 layer with an expected thickness

of 175 Å was found to be only 148 Å thick. All

layers were well described by a stratified structure

with a composition that varied with depth.

X-ray reflectivity calculations are robust; the

theory of x-ray scattering is very well established,

and comparisons between experimental data and

calculated profiles have been proved to be a

reliable method for performing structural

analyses. While there may be questions regarding

the “uniqueness” of x-ray analyses due to the noise

that is inherent in x-ray scattering, we have

emphasized to the semiconductor industry a

simple yet accurate descriptor of the method –

x-ray reflectivity may not be the best technique

to reveal what a structure is, but it is excellent at

showing what it cannot be.

Figure 2. Comparison of experimental and fitted

x-ray reflectometry data from a series of HfO2 films

processed under a variety of conditions.
Semiconductor Microelectronics and Nanoelectronics Programs 75

Figure 3. X-ray reflectometry analyses of Ta/Cu and

TaN/Cu systems.

# Ta/TaN Cu diffusion barriers – We have

examined thin (nominally 70 Å) Ta and TaN

diffusion barriers with thick (≈750 Å) copper

layers by high resolution x-ray reflectometry. Xray methods are particularly useful for the analysis

of complex metal systems, because the x-rays can

“see” a thin buried layer that lies beneath a much

thicker top layer. Data from these samples are

shown in Fig. 3; the rapid oscillations seen at

small angles in the experimental curves arise from

the interference of the phases of the scattered xrays from the thick Cu and the thin underlying Ta

or TaN layers. The model fit of the experimental

data shows excellent agreement between the two

for an assumed structure consisting of a thin

(approximately 15 Å) SiO2 layer at the silicon

substrate, Ta or TaN layers of 96 Å and 88 Å,

respectively, and Cu with a top copper oxide layer.

# Errors in x-ray reflectivity analyses – We

have initiated a research program to define the

effect of a variety of systematic errors on the

reliability of x-ray reflectometry analyses. Fig.

4. shows the effect of intentional misalignments

on experimental x-ray reflectivity profiles from

nominally bare silicon and ZrO2

/Si. These results

clearly demonstrate that even small angular missets in sample alignment can have a large effect

on the resultant specular x-ray scan. We are

currently performing theoretical analysis of the

effects of various experimental aberrations on xray reflectivity data. This information should be

useful to the vendors and users of fab-compatible

x-ray reflectivity systems for estimating the

effects of instrumental errors on reflectivity

analyses.

In addition, we have significantly upgraded our

experimental capability for x-ray reflectometry

analyses. Construction and calibration has been

completed on a novel “metrologically-sound” xray reflectometer that will permit quantitative

analyses of the effects of misalignments on x-ray

reflectivity analyses. Furthermore, we have

installed various high-precision sample positioning fixtures that will both improve the accuracy

of our own reflectivity measurements and permit

quantitative analyses of systematic errors in

reflectivity measurements to be performed.

# Grazing incidence x-ray diffraction for

crystallinity in high-k materials – As mentioned

earlier, crystalline inclusions in otherwise

amorphous HfO2 could both alter the electrical

characteristics of this high-k material and

complicate other measurement techniques. To

address this issue, we have developed a grazing

incidence x-ray diffraction (GIXD) capability for

the analysis of crystallinity in very thin films.

Pictured below is a GIXD scan from a HfO2 layer

following a furnace anneal (560 °C, 2 h). The

experimental GIXD scan (see Fig. 5) clearly

shows crystalline peaks that apparently arise from

Figure 4. Effects of misalignment on x-ray

reflectivity scans from nominally bare Si (top) and

ZrO2/Si (bottom).
76 National Institute of Standards and Technology

the monoclinic and ortho-rhombic phases of

HfO2. We are continuing to refine this technique

and use its capabilities to rationalize various

anomalies that have been observed in ellipsometry

analyses of thin HfO2 materials.

PUBLICATIONS AND PRESENTATIONS

Matyi, R. J., “High Resolution X-ray Scattering Methods

for ULSI Materials Characterization,” Proceedings of the

2003 International Conference on Characterization and

Metrology for ULSI Technology, submitted.

Matyi, R. J., “High Resolution X-ray Diffractometry and

Reflectometry Analyses for Semiconductor Materials

Characterization,” Short Course at the 2003 International

Conference on Characterization and Metrology for ULSI

Technology. 3/24/03.

Matyi, R. J. “Accuracy and Precision in High Resolution Xray Reflectometry Analyses of Semiconductor Materials,”

APS March Meeting, 3/4/03.

Matyi, R. J. “Issues in High Resolution X-ray Reflectometry

Analyses of High-k Materials,” Analytical Laboratory

Managers Council Meeting, 2/4/03.

Matyi, R. J., “High Resolution X-ray Reflectometry: Theory,

Practice, Accuracy, Precision,” 202nd Electrochemical

Society Meeting, 10/24/02.

Deslattes, R. D. and Matyi, R. J., “Analysis of thin layer

structures by X-ray reflectometry,” Handbook of Silicon

Semiconductor Metrology (ed. A.C. Diebold), 789 (2001).

Figure 5. GIXD scan from HfO2/Si with superimposed calculated peak positions and intensities for

monoclinic and orthorhombic crystalline HfO2.
Semiconductor Microelectronics and Nanoelectronics Programs 77

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 flip 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 significant component of reliability evaluation.
78 National Institute of Standards and Technology

SUPERCONFORMAL DEPOSITION OF COPPER AND

ADVANCED INTERCONNECT MATERIALS

Technical Contacts:

T. P. Moffat

D. Josell

“We are very excited

about the capabilities of

this tool, and we want to

put it to use ASAP!!! Yet

another example of NIST

innovation helping

industrial competitiveness!!!”

Dennis Buss, Vice President,

Silicon Technology

Development,

Texas Instruments

GOALS

This project is developing solutions to metrology

issues confronting integrated circuit manufacturers in the area of interconnect metallization.

The main effort involves determining the essential

process requirements for superconformal filling

of high aspect ratio features. For electrodeposition

this includes: determining measurement methods

for characterizing electrolytes, quantifying

kinetics for industrially relevant copper electrolytes, and direct assessment of the efficacy of

electrolytes for trench and via filling. This project

also seeks to explore the generality of the

superconformal filling mechanism for metallizations other than copper and processes other

than electro-deposition.

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” fill

high aspect ratio features has made electrodeposited copper the interconnect material of

choice. To support these efforts the National

Institute of Standards and Technology (NIST) has

a program focusing on the role of electrolyte

additives on the superconformal filling process.

Evaluation of prospective electrolytes will be

greatly accelerated by NIST’s recent development

of a predictive capability for describing the

influence of additives on superconformal

deposition.

Interconnect metallization issues are discussed

in the 2001 International Technology Roadmap

for Semiconductors on Interconnect Section,

page 11.

TECHNICAL STRATEGY

1. The semiconductor industry has steadily

worked to reduce interconnect dimensions, while

improving their electrical performance. As a result,

metallization has changed from sputtered

aluminum (copper) alloys to electrodeposited

copper. To meet future industrial needs, we have

developed the metrology and fully disclosed

electrolytes that permit characterization of the

ability of generic electrolytes to fill fine features.

We are also developing electrolytes and

metrology for deposition of advanced interconnect materials as well as alternative processing

schemes such as chemical vapor deposition.

In FY 2003 we have:

# Established the generality of the Curvature

Enhanced Accelerator Coverage (CEAC) mechanism for superfilling by electro-depositing copper,

silver and other metals.

# Developed a new processing method for

eliminating the delay time for superfill to begin

during electrodeposition, thereby increasing the

aspect ratio and decreasing the width of trenches

that can be superfilled.

# Extended the CEAC mechanism to quantitatively predict superfill during chemical vapor

deposition.

# Extended the diagnostic capability of voltammetry for monitoring electrolyte aging effects.

# Demonstrated the utility of using a cation

selective membrane to minimize electrolyte aging

effects.

Our current measurement and modeling effort is

focused on copper and silver metallizations, the

latter metal having the lowest resistivity of any

element, thus having potential on-chip

applications. Superconformal silver deposition

was demonstrated in the past year in the

Metallurgy Division and is the first example of

superconformal feature filling with a metal other

than copper. “Superfilling” is characterized by

bottom-up deposition as shown in Fig. 1.

Figure 1. Superconformal electrodeposition of silver

in vias. After a ~40 s incubation period of conformal

filling, rapid bottom-to-top filling occurs.
Semiconductor Microelectronics and Nanoelectronics Programs 79

DELIVERABLES: We will publish the first complete

account of the electrochemical behavior of a model

electrolyte for superconformal electrodeposition of

copper. The kinetic model details both the adsorption

and consumption dynamics of the catalytic additive.

4Q 2003

Our publications explain the mechanism behind

the process of superconformal “bottom-to-top”

filling (see Fig. 2). They also detail how it can be

fully quantified using only experiments with

planar substrates. Electrochemical and surface

analytical measurements on planar substrates

were used in FY2002 to establish a one-to-one

correlation between catalyst coverage and the

metal deposition rate. In the case of silver

deposition, adsorbed selenium was found to

catalyze the deposition rate. For the thiol/

disulfide-polyether-halide additives used in

copper superfilling, the sulfonate-end group of

the thiol or disulfide molecules was found to

destabilize the passivating polyether surface film

thereby accelerating the local metal deposition

rate.

DELIVERABLES: We will develop software for

modeling superconformal feature filling as well as

codes for describing roughness evolution during film

growth on an arbitrary surface profile. 4Q 2003

Our model for the superconformal filling process

has been implemented in computer code. Three

different electrode shape change algorithms have

been developed. They show excellent agreement

with experimental results. These algorithms are

available from the Metallurgy Division website,

and are currently being examined by Texas

Instruments and IMEC.

http://www.ctcms.nist.gov/~wd15/superfill/

superfill.html

Figure 2. This simulation details the time-dependent

filling of a trench (viewed in cross section). One

observes the initial conformal deposition, bottom-totop superconformal filling, and final creation of an

overfill bump. All these phenomena are also

observed industrially.

DELIVERABLES: We will publish an analysis

quantifying how the mechanism that results in

superconformal filling of fine features also yields

bright deposits on planar surfaces. 4Q 2003

The insight provided by uncovering the CEAC

mechanism has enabled the development of a new

two-step process for superconformal film growth;

in contrast to standard electrodeposition techniques, the catalyst is deposited on the substrate

prior to the metallization step. Specifically, the

patterned substrate is first “derivitized” or

“dosed” with a submonolayer coverage of

catalyst and then transferred for electroplating in

an electrolyte that does not contain the catalyst.

Figure 3. Time evolution of trench filling after

different “derivitization” steps. The concentration of

SPS or MPSA catalyst used for the 30 s surface

derivitization steps are indicated. All specimens were

transferred to a catalyst-free electrolyte for copper

deposition.

For an optimum catalyst coverage, superconformal filling of trenches and vias occurs as shown

in Fig. 3 (50 μmol/L SPS). If the catalyst coverage

is too low or high, conformal or subconformal
80 National Institute of Standards and Technology

deposition occurs, resulting in void formation

during feature filling (0.5, 500 μmol/L SPS,

1mmol/L MPSA). The filling behavior is

completely analogous to that obtained using a

single (conventional) electrolyte containing both

catalytic and inhibiting species. The new process

provides unambiguous verification of the CEAC

mechanism of superconformal film growth.

Restricting the catalyst to the surface prior to metal

deposition also enables the rate differentiation

provided by the CEAC mechanism to be increased

relative to the conventional process. From a

technical perspective, the two step process offers

an interesting solution to the difficult control

issues associated with catalyst destruction and

related aging effects which are known to occur in

the “conventional” single-electrolyte superfilling

process currently used in industry.

DELIVERABLES: We will quantify the effect of iodine

catalyst on superconformal chemical vapor deposition of copper. 4Q 2002

The generality of the CEAC mechanism has now

been extended to include the first quantitative

prediction of superconformal chemical vapor

deposition (CVD). More recently, this work has

enabled a direct comparison of the CEAC

mechanism with detailed experiment results on

iodine catalyzed copper CVD as shown in Fig. 4.

Figure 4. Superconformal filling of vias during iodine

catalyzed chemical vapor deposition. Experimental

via results are from Hynix Semiconductor and model

predictions for a trench are from the Metallurgy

Division of NIST.

2. In collaboration with William Egelhoff (MSEL)

and Lee Richter (CSTL) we have quantified the

impact of the catalyst on the superconformal

filling process through electrochemical

measurements as well as surface studies (XPS,

FTIR).

DELIVERABLES: We anticipate several publication

describing surface analytical measurements of

catalyst evolution during superconformal copper and

silver electrodeposition. 3Q 2002

3. Our publications note the link that can exist

between electrolytes that provide “bright”

deposits and those that yield superconformal

filling of fine features.

DELIVERABLES: We will publish analysis quantifying

how the mechanism that results in superconformal

filling of fine features also yields bright deposits on

planar surfaces. 4Q 2002

ACCOMPLISHMENTS

# We have quantified the kinetics of catalyst

adsorption and consumption during copper

electrodeposition. The derived kinetics have been

used to demonstrate a one to one correlation

between experiments and simulation of

superfilling ~100 nm features.

# We have applied this understanding to

demonstrate silver electrodeposition and copper

chemical vapor deposition.

# A new two step process has been developed

which allows the difficulties associated with

electrolyte aging effects to be avoided and

enables optimum feature filling.

# In a similar manner, we have improved the

stability of the “conventional” superfilling

process by introducing a cation selective

membrane into the electrochemical cell.

We have established the metrology for determining all kinetic parameters required to model

superconformal feature filling from studies of

deposition on planar substrates.

COLLABORATIONS

D. Wheeler, B. Baker, W.E. Egelhoff, Materials Science

and Engineering Laboratory. L. Richter and C. Yang,

Chemical Science and Technology Laboratory, NIST.

ISMT, Christian Witt; fabrication of patterned substrates.

Motorola, Bradley Melnik; Fabrication of patterned

Substrates.

RECENT PUBLICATIONS

B. C. Baker, C. Witt, D. Wheeler, D. Josell and T. P.

Moffat, “Superconfromal Silver Deposition Using

KSeCN Derivatized Substrates,” Electochemical and

Solid-State Letters, 6, C67-C69 (2003).

D. Wheeler, D. Josell and T. P. Moffat, “Modeling

Superconformal Electrodeposition Using The Level Set

Method,” J. Electrochem. Soc., 150, C302-310 (2003).

D. Josell, S. Kim, D. Wheeler, T. P. Moffat, S. G. Pyo,

“Superconformal Deposition by Iodine-Catalyzed

Chemical Vapor Deposition,” J. Electrochem Soc.,150,

C368-C373 (2003).
Semiconductor Microelectronics and Nanoelectronics Programs 81

T. P. Moffat, B. C. Baker, D. Wheeler and D. Josell,

“Accelerator Aging Effects During Copper

Electrodeposition,” Electrochemical and Solid-State

Letters, 6, C59-C62 (2003).

S. G. Pyo, S. Kim, D. Wheeler, T. P. Moffat and D.

Josell, “Seam-free fabrication of submicrometer copper

interconnects by iodine-catalyzed chemical vapor

deposition,” J. Appl. Phys., 93, 1257 (2003).

B. C. Baker, M. Freeman, B. Melnick, D. Wheeler, D.

Josell, T.P. Moffat, “Superconformal Electrodeposition

of Silver from a KAg(CN)2-KCN-KseCN Electrolyte,”

J. Electrochem. Soc., C61-C66 (2003).

T. P. Moffat, D. Wheeler, C. Witt and D. Josell,

“Superconformal Electrodeposition using Derivitized

Substrates,” Electrochemical and Solid-State Letters,

C110-C112 (2002).

D. Josell, B. Baker, C. Witt, D. Wheeler and T. P. Moffat,

“Via Filling by Electrodeposition: Superconformal Silver

and Copper and Conformal Nickel,” J. Electrochem.

Soc., 149, C637-C641 (2002).

T. P. Moffat, B. Baker, D. Wheeler, J.E. Bonevich, M.

Edelstein, D. R. Kelly, L. Gan, G. R. Stafford, P. J. Chen,

W. F. Egelhoff, and D. Josell, “Superconformal

Electrodeposition of Silver in Submicron Features,” J.

Electrochem. Soc. 149, C423 (2002).

Our findings have also been conveyed to U.S. industry,

academia and other national laboratories through more

than eighteen external presentations in the last year.
82 National Institute of Standards and Technology

POROUS THIN FILM METROLOGY FOR

LOW-k DIELECTRIC

Technical Contacts:

Barry J. Bauer

Wen-li Wu

“The SANS/SXR measurements have become

a key metric in our low-k

dielectric materials

characterization and

screening process”

Jeffrey Wetzel –

International SEMATECH

“The NIST x-ray

reflectivity program is

useful, perhaps even

critical, to the industry”

Abner Bello –

Applied Materials

GOALS

In this project, we are developing measurement

methods of morphological characteristics in

porous thin films for low-k dielectric applications.

We work closely with industrial collaborators to

develop and apply these methods to advanced

materials destined for integration in the next

generation of integrated circuits. The unique

measurement methods we apply include x-ray

reflectivity (XR), small angle neutron scattering

(SANS), Rutherford backscattering spectroscopy

(RBS), and forward recoil elastic spectroscopy

(FRES). Our efforts focus on two areas, providing

high quality data and measurements of film

thickness, coefficient of thermal expansion (CTE),

moisture uptake, film connectivity, pore volume,

pore size, and matrix density on films under

development, and devising new measurement

methods to characterize pore size distribution

(PSD), pore connectivity, and matrix homogeneity.

CUSTOMER NEEDS

As integrated circuit (IC) feature sizes continue

to shrink, new low-k interlevel dielectric materials

are needed to address problems with power

consumption, signal propagation delays, and

cross talk between interconnects. One avenue to

low-k dielectric materials is the introduction of

nanometer scale pores into a solid film to lower

its effective dielectric constant. However, the pore

structure of these low-k dielectric materials

strongly affects important material properties

other than the dielectric constant such as

mechanical strength, moisture uptake, coefficient

of thermal expansion, and adhesion to different

substrates. The characterization of the pore

structure is needed by materials engineers to

optimize and to develop future low-k materials

and processes. Currently, there is no clear

consensus among IC chip manufacturers for the

selection of a class of material or a processing

method of nanoporous films. Candidates include

silica-based films, organic polymers, inorganic

spin-on materials, chemical vapor deposited

materials, and several others. With the large

number of possible materials and processes, there

is a strong need for high quality structural data to

understand correlations between processing

conditions and the resulting physical properties.

TECHNICAL STRATEGY

1. The small sample volume of 1 μm films and the

desire to characterize the film structure on silicon

wafers narrows the number of available

measurement methods. A unique suite of

measurement techniques, Fig. 1, has been

developed at NIST using a combination of SANS,

XR, RBS, and FRES to determine important

structural and physical property information about

thin porous films less than 1 μm thick deposited

on a 1 mm thick substrate. These measurements

are performed directly on films supported on

silicon substrates so that processing effects can

be investigated.

The elemental composition of the films is

determined by RBS for silicon, carbon, and oxygen

and FRES for hydrogen. In both techniques, a

beam of highenergy ions is directed toward the

sample surface. The number of scattered particles

is counted as a function of their energy. Fits are

performed on the scattered peaks to compute the

relative fraction of each element. The atomic

composition information is necessary to calculate

the relative contrast factors for x-rays and

neutrons.

The XR experiments are performed at grazing

incident angles on a modified to θ−2θ x-ray

diffractometer at the specular condition. With

the modified configuration, reflectivity fringes can

be observed from films up to 1.2 μm thick. Highresolution XR is a powerful experimental

technique to accurately measure the structure of

thin films in the direction normal to the film surface.

In particular, the film thickness, film quality

(roughness and uniformity) and average film

density can be determined with a high degree of

precision. In addition, changes in the density

profile from processing or post-application

treatments can be determined. The CTE is

determined from measurements of the film

thickness at different temperatures.

The SANS measurements are performed at the

NIST Center for Neutron Research (NCNR). Up

to 10 films are stacked to increase the SANS signal

and the samples are placed in vacuum without

any obstructions between the sample and the

neutron detector. Scattering measurements are

initially performed under ambient conditions to
Semiconductor Microelectronics and Nanoelectronics Programs 83

determine the structural characteristics of the pore

structure. The scattering data are initially

analyzed using a simple random two-phase

description of the film, the Debye model. This

model is appropriate for a class of films having

random pores that makes up a majority of the

samples measured. Other models are applied

where appropriate such as a polydisperse

collection of non-overlapping spherical pores. An

additional method has been developed in which

the Debye model is extended to include more

complex distributions of pore sizes that need not

be spherical in shape.

Figure 1. Three thin film measurements that are

combined for complete characterization.

DELIVERABLES: Measure and report on up to

20 films for pore volume, pore size, and matrix

density associated with ISMT. 3Q 2003.

2. Further developments in the determination of

pore size distributions have focused on the

adaptation of conventional probe molecule

porosimetry methods to the NIST experimental

techniques. For example, XR was done on films

that have been measured in vacuum in a

conventional way, and then exposed to saturated

toluene vapor for several hours, Fig. 2. Capillary

action fills the pores of the film. The XR critical

edge where adsorption first begins gives an

accurate value of the average mass density that

is a combination of the walls and the solventfilled pores. By comparing the results of the

sample in air or vacuum with the toluene results,

one can calculate the amount of toluene adsorbed,

and hence the volume of open pores. Also, XR

oscillations at higher angles provide a measurement of the total film thickness before and after

exposure to toluene and gives a measure of the

solvent resistance and rigidity of the walls.

The toluene infusion results confirm that the open

pores of thin films can be filled by toluene supplied

by saturated vapor and that XR can accurately

measure the amount adsorbed. If the vapor

pressure of the toluene or any other condensable

solvent can be controlled at a partial pressure,

standard porosimetry techniques may be applied

to thin films. Data on the amount of solvent

adsorption for a series of pressures could be

converted to a PSD through the appropriate

thermodynamic analysis.

Figure 2. Schematic of the porosimetry measurement,

increased solvent vapor pressure fills increased pore

sizes.

DELIVERABLES: Identify strengths and weaknesses

in XR porosimetry using varying methods of generating specific partial pressure environments. 3Q 2003

3. The use of deuterated and hydrogenated probe

molecules enables the powerful use of contrast

match methods with the SANS technique. This

technique can be used for characterization of the

porous thin films by filling the pores with various

mixtures of toluene-h8 and toluene-d8. If the pores

are accessible to the solvent and there are

homogeneous walls, the wall density can be

found. If the wall is heterogeneous, the average

wall density could be found with information on

the extent of heterogeneity also being possible.

If closed pores exist that are inaccessible to

solvent, closed pore porosity can be determined.

We have developed a combination of contrast

match SANS and XR porosimetry. A match solvent

mixture at controlled vapor pressures deposits the

contrast match liquid in the pores through capillary

action. SANS would provide an additional

measure of PSD.

DELIVERABLES: Develop measurement methods and

equipment for SANS contrast match technique and

make measurements on IMST samples. 2Q 2003.
84 National Institute of Standards and Technology

4. While very small pores lower the dielectric

constant without significantly sacrificing electrical

or physical properties, it has been speculated that

a small number of very large pores or aggregated

matrix material can form with sizes comparable to

the film thickness. Such large “killer” defects

could cause a chip to fail catastrophically and

can potentially be a major source of chip rejection.

Killer pores can be studied by the use of electron

microscopy, but such methods cannot be used in

a routine manner because the field of view of such

techniques is relatively small and an extensive

number of micrographs would be necessary to

fully examine films. SANS, however, can sample

relatively large areas of a film at once. The major

difficulty is to obtain data at low enough

scattering angles to reveal this information.

The Bonse-Hart-type, perfect channel-cut crystal

diffractometer that came into full operation this

year at the NCNR extends the measurement range

of the SANS instrumentation at the NCNR by more

than one order of magnitude, to over 5000 nm.

This overlaps the range of the 30-m pinhole

collimation SANS instruments so that together

they provide continuous coverage of microstructural features over 4 orders of magnitude

(≈1 nm to >5000 nm). This new capability can be

explored to identify the limits of “killer” pore

detection with SANS.

DELIVERABLES: Measure and characterize pore

structure development as a function of processing

conditions in collaboration with Dow Chemical.

2Q 2003.

5. A practical method of transforming phase size

distributions into density correlation functions

and PSD’s and into SANS or SAXS is being

developed. Previous methods of calculating

scattering from a PSD assume spherical pores,

while important for a certain class of pores, are

inappropriate for the majority of pore structures

that are randomly interconnected. The Debye

methodology assumes a random, interconnected

type of pore structure, but is limited to a single

distribution type. A new method has been

developed to provide information on the size

distributions of the pore and matrix phases based

on the “chord length distribution” introduced by

Tchoubar and Mering.

The transformation of scattered intensity into

phase size distributions is possible by this method

and has been demonstrated for bimodal

distributions. Development of a computational

scheme to fit data is underway.

DELIVERABLES: Develop data fitting methods for

small angle scattering and present results at national

meeting. 2Q 2003.

ACCOMPLISHMENTS

# A project was completed with International

SEMATECH (ISMT) in which 20 thin films have

been characterized for film thickness, CTE,

moisture uptake, film connectivity, pore volume,

pore size, and matrix density by XR, SANS, RBS,

and FRES. Quarterly reports were delivered on

the results and distributed to member companies.

Additional measurements were performed on

specific samples for further analysis using

methods such as additional SANS configurations

or x-ray porosimetry.

# X-ray reflectivity measurements were used

to assess depth-dependent phenomena in

nanoporous thin films. Measurement of electron

density profile can identify gradients in chemical

composition or porosity. Measurements on lowk films exposed to either plasma or electron beam

treatments show modified electron density

profiles. Modeling of SXR data collected in air

indicates that the film is densified near the top

(air-film interface). When the film is exposed to

saturated toluene vapor, an overall increase in

electron density is noted due to the filling of pores

with liquid toluene by capillary condensation.

However, when the film is exposed to saturated

water vapor, the SXR data clearly show that the

topmost portion of the film fills with liquid water,

whereas the lower portion remains essentially

empty, indicating that the top portion is more

hydrophilic. This information is vital to the

screening of low-k materials because solvent

uptake during processing can adversely affect

device performance.

# X-ray porosimetry measurements have been

implemented utilizing the controlled infusion of

toluene into the open pores of a film through mass

flow controllers and computer control. All of the

connected open pores become filled with liquid

toluene through capillary action. The critical edge

measured by XR is used to calculate the total

mass density and, hence, the total amount of

adsorbed solvent and open pore content. This

method allows calculation of the total open pore

porosity that can be compared to the total

combined open and closed pore porosity that is

measured by the previous method that uses a

combination of XR, SANS, RBS, and FRES.

Further, pore size distributions may be extracted

using conventional thermodynamic equations

from obtained absorption/desorption curves.
Semiconductor Microelectronics and Nanoelectronics Programs 85

# A contrast match method using saturated

solvent vapor was developed to provide an

independent SANS measurement of pore volume,

pore size, and matrix density as well as pore

connectivity, and matrix homogeneity. The films

in the SANS cell become saturated by the vapor

and the pores become filled. Several solvent

ratios are used and the SANS results of the

saturated films along with SANS of the films in

vacuum are used to calculate the exact match

composition. The match composition is used to

calculate the mass density of the matrix material

and closed pores. This matrix density measurement is independent of the method that uses

toluene infusion XR. The contrast match method

offers improved accuracy of the final measured

parameters. The matrix heterogeneity and the

closed pore content can also be determined by

the contrast match method. The contrast SANS

method was further advanced by performing

SANS porosimetry measurements to determine

pore size distributions that may be compared with

the x-ray porosimetry data.

# The structural evolution of pore formation

in low-k dielectric thin films (with a deuterated

porogen) at various stages in processing was

investigated using a combination of specular xray reflectivity (SXR) and small angle neutron

scattering (SANS). SXR provides information as

to the porosity and the density of the wall. SANS

data show that the neutron scattering decreases

during processing as the deuterated porogen

degrades and pores are formed. The pore size

and pore size distribution were estimated by fitting

the SANS data to a structural model that

describes the scattering intensity from a

population of polydisperse spheres that includes

hard sphere interactions between the particles

and uses a Schultz distribution to describe the

polydispersity. The porosity was found to

decrease, while the pore size increased, during

processing. A practical method of transforming

phase size distributions into density correlation

functions has been demonstrated. The computations are rapid and can produce density

correlation functions, and hence scattered

intensities to any necessary degree of accuracy.

Phase size distributions other than the exponential

ones described by Debye et al. can be transformed

into density correlation. The transformation of

scattered intensity into model phase size

distributions is possible by this method.

COLLABORATIONS

Polymers Division, NIST – Hae-Jeong Lee, Christopher

L. Soles, Ronald C. Hedden, Da-wei Liu, Michael

Silverstein

Center for Neutron Research, NIST – John Barker,

Charles J. Glinka, Derek L. Ho.

International SEMATECH – John Iacaponi, Youfan Liu

Rohm & Haas – Nick Pugliano

Dow Chemical – Brian Landes, John Lyons, Brandon

Kerr, Jason Niu, Tom Kalantar

Axcelis – Carlo Waldfried, Orlando Escorcia, David Green

LG Chemicals – Minjin Ko

IBM T. J. Watson Research Center – Alfred Grill

Seoul National University – Kookheon Char, Do Yoon.

Lucent Technologies – Shu Yang

University of Michigan - David Gidley

RECENT PUBLICATIONS

Lee, H. J., Lin, E. K., Bauer, B. J., Wu, W. L., Hwang, B.

K., and Gray, W. D., “Characterization of ChemicalVapor-Deposited Low-K Thin Films Using X-Ray

Porosimetry,” App. Phys.Lett., 28(7): 1084-1086 (2003)

Yang, S., Mirau, P. A., Pai, C. S., Nalamasu, O.,

Reichmanis, E., Pai, J. C., Obeng, Y. S., Seputro, J., Lin,

E. K., Lee, H. J., Sun, J. N., and Gidley, D. W., “Nanoporous Ultralow Dielectric Constant Organosilicates

Templated by Triblock Copolymers,” Chem. Mater.,

14(1): 369-374 (2002).

Lee, H. J., Lin, E. K., Wang, H., Wu, W. L., Chen, W.,

Deis, T. A., and Moyer, E. S., “Structural Comparison of

Hydrogen Silsesquioxane Based Porous Low-k Thin Films

Prepared With Varying Process Conditions,” Chem.

Mater., 14: 1845-1852 (2002).

Lin, E. K., Lee, H. J., Lynn, G. W., Wu, W. L., and

O’Neill, M. L., “Structural Characterization of A Porous

Low Dielectric Constant Thin Film with a Non-uniform

Depth Profile,” App. Phys. Lett., 81: 607-609 (2002).

Hedden, R. C., Bauer, B. J., Lee, H. J., and Wu, W. L.,

“Determination of Closed Pore Content in Nanoporous

Films by SANS Contrast Match,” ACS Polymeric

Materials Science and Engineering 2002; 87: 437-438

(2002).

Lee, H. J., Soles, C. L., Liu, D. W., Bauer, B. J., and Wu,

W. L., “Pore Size Distributions in Low-k Dielectric Thin

Films from X-ray Porosimetry,” Journal of Polymer

Science Part B-Polymer Physics 2002; 40(19): 2170-

2177 (2002).

Bauer, B. J., Lin, E. K., Lee, H-J., Wang, H. and W, W-l,

“Structure and Property Characterization of Low-k

Dielectric Porous Thin Films,” J. Electronic Materials,

30(4), p. 304 (2001).
86 National Institute of Standards and Technology

Lin, E. K., Lee, H. J., Bauer, B. J., Wang, H., Wu, W. L.,

and J. T. Wetzel, “Structure and Property Characterization of Low-k Dielectric Porous Thin Films

Determined by X-ray Reflectivity and Small-angle

Neutron Scattering in Low Dielectric Constant Materials

for IC Applications,” edited by P. S. Ho, J. Leu, and W.

W. Lee, Springer Publishing, Inc. (2001).

Lee, H. J., Lin, E. K., Wu, W. L., Fanconi, B. F., Liou, H.

C., Lan, J. K., Cheng, Y. L., Wang, Y. L., Feng, M. S.,

and C. G. Chao, “X-ray Reflectivity Measurements of

N2 Plasma Effects on the Density Profile of Hydrogen

Silsesquioxane Thin Films,” J. Electochem. Soc,148,

F195 (2001).

Kohdoh, E., Baklanov, M. R., Lin, E. K., Gidley, D. W.,

and A. Nakashima, “Comparative Study of Pore Size of

Low-Dielectric Constant Porous Spin-on-glass Films with

Different Ways of Nondestructive Instrumentation,”

Japanese Journal of Applied Physics, 40, L323 (2001).
Semiconductor Microelectronics and Nanoelectronics Programs 87

ELECTRON MICROSCOPE TOMOGRAPHY

OF ELECTRONIC MATERIALS

Technical Contacts:

Z. H. Levine

J. H. Scott

GOALS

Enable the use of three-dimensional imaging for

thick samples using commercial scanning

transmission electron microscopes (STEM).

Typical samples include porous low-k dielectrics,

two-layer 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-k 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 for interconnect are

discussed in the 2001 International Technology

Roadmap for Semiconductors on pages 9-10 and

24 of the Interconnect Section. “Two new

measurement needs [include] the pore size

distribution in porous low-k materials.”

TECHNICAL STRATEGY

1. First, we will upgrade an existing commercial

transmission electron microscope to be able to

obtain high angle STEM images with a full

quantitative understanding of the input and

output signals. Noise reduction is also a key

issue.

2. In parallel, we will develop theoretical and

computational expertise for the understanding of

STEM signals associated with multiple scattering

of electrons.

3. We will obtain a tilt series and perform a

tomographic — analysis of a photonic band gap

system an artificially periodic polymer structure.

4. We will perform a similar analysis on a low-k

dielectric material.

DELIVERABLES: Tomographic study of photonic

band gap material. 4Q 2003

Three-dimensional images of a photolithographically patterned polymer photonic band gap

material will be obtained after the STEM is

upgraded.

DELIVERABLES: Tomographic study of a low-k

dielectric material. 2Q 2004

The porous low-k dielectric will pose more of a

challenge, particularly for alignment, because of

its random nature. Hence, it will be studied

second.

ACCOMPLISHMENTS

# To date, we demonstrated the ability to obtain

three-dimensional information using a scanning

confocal transmission electron microscope on

integrated interconnect samples several

micrometers (Frigo, Levine, and Zaluzec, 2002) as

well as the ability to obtain three-dimensional

information on integrated circuit interconnect

samples using a commercial scanning transmission electron microscope (Levine et al., 2003).

# We simulated the tomographic reconstruction using transmission electron microscopy of

an 8-micrometer square sample of photonic bandgap material, see Fig. 1. This represents an order

of magnitude increase in sample size compared to

Figure 1. Tomographic reconstruction of a slice

through a photonic band gap material from simulated

scanning transmission electron microscope data.

The scale bar is 1 micrometer, which also corresponds to the typical size of the largest samples used

in TEM tomography in the scientific literature.
88 National Institute of Standards and Technology

all TEM-based reconstructions in the scientific

literature (to the best of our knowledge).

COLLABORATIONS

International Sematech, Brendan Foran, preparation of

low-k samples, electron microscopy.

Lucent Technologies, Shu Yang; preparation of photonic

band gap material.

RECENT PUBLICATIONS

S, Frigo, Z. H. Levine, and N. J. Zaluzec., “Submicron

imaging of buried integrated circuit structures using a

scanning confocal electron microscope,” Appl. Phys.

Lett. 81, 2112, (2002).

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 89

WIRE BONDING TO COPPER/LOW-k

SEMICONDUCTOR DEVICES

Technical Contacts:

G. G. Harman

C. Johnson

GOALS

The overall objective is to determine the best

process method/top coating and support

structures for wire bonding to copper/low-k chips.

A two step gold deposition system is being

developed and some thin inorganic films are also

being studied as top-coating surfaces to prevent

oxidation during bonding.

CUSTOMER NEEDS

The advent of copper metallization semiconductor

chips has resulted in a requirement to wire-bond

interconnect them in a manner similar to that used

for 65 billion ICs. The process should be invisible

to the current wire bonding machines. However,

the copper bond pads oxidize, requiring a

protective/bondable coating. When low-modulus

low-dielectric materials lie below the pad, a

support structure is necessary to prevent damage

to the interconnection/dielectric layers. There are

many approaches to solving these problems; the

NIST program is attempting to optimize and

develop new solutions where necessary.

TECHNICAL STRATEGY

1. One approach to protecting the copper is to

coat the bond pads with a bondable gold layer.

For this approach, the first objective is to

determine the diffusion coefficients of copper

through gold, because copper can diffuse to the

surface and will oxidize, preventing a good bond.

Literature values on various gold platings are

contradicting and are being measured using

deposits on actual damascene-process copper

pads. A more classical approach, but also a more

difficult process, using SIMS has been proposed.

We have developed a two step (immersion + autocatalytic gold) deposition process over the

copper-low-k chips. This will be pursued at NIST.

Even without the actual diffusion coefficients, a

pragmatic approach is being carried out by

heating gold coated samples and doing extensive

wire bonding and evaluating with ball shear tests.

A subset of this work will be to determine the

minimum thickness of gold necessary to prevent

copper diffusion/oxidation in normal bonding/

processing temperatures. At the same time several

inorganic coatings for protection are being

evaluated. Sandia has applied thin deposited SiO2

coatings on the copper wafers and they were

evaluated. Results so far indicate that all of our

coatings are nonuniform, but bonding did occur

on the better pads. This evaluation is continuing,

possibly by depositing a thick, uniform coating

and then etching thinner.

2. Another objective is to measure the nanohardness and modulus of the damascene copper

in order to optimize bonding, [hardness should

be minimized (80 Knoop to 100 Knoop)]. This has

been accomplished with a series of annealing

experiments made with copper chips, and

pragmatically verified by actual bonding

experiments. Values of measured nanohardness

have been dependant on the annealing gas (argon

is best) and have been as low as 60 GPa (about

120 unannealed) and the modulus as low as

20 GPa.

Problems have occurred when sawing the bare

copper chips in which the saw particles stick to

the bare copper. several approaches to curing this

problem have been pursued. Copper damascene

process samples have been supplied by International SEMATECH for this effort.

DELIVERABLES: Problems with the two-step gold

deposition will be solved. Some pad lifting during gold

deposition will be studied and an understanding

obtained. The desired thickness of gold deposition

will be determined, and hardness and modulus

studies completed. A nickel strike coating also was

developed.

ACCOMPLISHMENTS

# One paper published, entitled “Wire Bonding

To Advanced Copper-Low-K Integrated Circuits,

the Metal/Dielectric Stacks, and Materials

Considerations,” by George G. Harman and

Christian E. Johnson, Proc. 2001 IMAPS

Symposium, Oct 9-11, 2001, Baltimore, MD, pp.

484-491. This paper won an award as an outstanding paper in the Symposium. An updated version

of this was published in the IEEE Transactions

on Components and Packaging Technologies, V25, Number 4, Dec. 2002, pp. 677-683.

# The two step non-contact gold deposition

process used on some of the original International

SEMATECH (ISMT) wafer copper were etched in

our processing, whereas other copper deposition

methods were satisfactory and bonded well.

Currently, wafers from another source with thicker

copper are being acquired.
90 National Institute of Standards and Technology

SOLDERS AND SOLDERABILITY MEASUREMENTS

FOR MICROELECTRONICS

Technical Contacts:

Frank W. Gayle

Carol Handwerker

Maureen Williams

Christian Johnson

“As a result of NIST’s

involvement, I feel NEMI

has been successful in

responsibly leading the

effort to understand the

implications of lead-free

assembly in a way that is

benefiting the entire

electronics assembly

industry.”

— Edwin Bradley,

Motorola and

National Electronics

Manufacturing Initiative

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 flip-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 specified in the 2000 IPC

Lead-Free Solder, 2001 ITRS, and 2002 NEMI

Roadmaps. Pressure from the European Union

and the Japanese consumer product market to

produce lead-free microelectronics continues to

increase. These industrial needs are addressed

under this NIST project.

Additional needs have been identified through

participation in the National Electronics

Manufacturing Initiative (NEMI) working group

on Pb-free solder alloys. It was learned that

significant industrial problems had arisen due to

contamination of Pb-free solders by the Pb

contained in the protective solder coatings that

are used on copper (Cu) leads. The protective

layer deposited on 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

tin (Sn) tend to grow “whiskers,” however, which

can cause shorts across leads. Thus the

development of Pb-free alloy platings to replace

Pb-containing protective layers is considered

important, and tests which ascertain the tendency

to form whiskers are much needed. NIST is a cochair in the NEMI Sn Whisker Modeling Group

and an active participate in the NEMI Accelerated

Sn Whisker Test Group.

In the 2001 ITRS, Table 74 on page 321

“Assembly and Packaging Difficult Challenges,” the need for Pb, Sb, and Br free

packaging materials is specifically mentioned.

In the 2002 NEMI Technology Roadmaps,

Technology Designs, Environmentally Conscious

Electronics, page 9, states “The NEMI Pb-Free

Assembly and Rework Project is leading the

North American effort to develop a lead-free

second level assembly process utilizing current

manufacturing equipment technology.” In the

Manufacturing Technologies, Board Assembly,

on page 2, it states “In Transition to Pb-free

production, it is important to understand the

long-term reliability performance of Pb-free and

eutectic solder joints.”

TECHNICAL STRATEGY

We are providing the microelectronics industry

with measurement tools and data to address

solder interconnect problems. For example, a

thermodynamic database has been publicly

distributed for modeling the processing behavior

of lead-free solder systems. We also work closely

with industry groups on measurement tools

needed for development of lead-free solders for

use in harsh environments, and provide guidance

for adoption of these solders into assembly

processes through work with industrial standards

organizations.

We will continue working with NEMI to establish

suitability of particular lead-free solder

compositions. In addition, solderability tests will

be developed working with the IPC lead-free

solder sub-committee. Databases for phase

diagrams and thermodynamics critical to solder

development and for mechanical properties of

solder will be expanded and distributed via the

web. In addition, a much-needed guide to

interpretation of thermal analysis data will be

produced.

It is well known that the use of pure Sn protective

deposits has serious problems. Sn whiskers

(filament type whiskers are typically 1 μm diameter

and several mm long) can grow from the Sn plate

and cause electrical shorts and failure. An example

of a Sn whisker is shown in Fig. 1. 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 this project on

Pb-free Sn-rich deposits with alloying additions

that would retard whisker formation. The Sn-Cu

system was chosen for the surface finish, since
Semiconductor Microelectronics and Nanoelectronics Programs 91

Sn-Cu-Ag is likely to be the Pb-free bulk solder

of choice for industrial application. The basic idea

is that the substitution of a different solute for Pb

in the Sn-rich deposit will also retard whisker

growth. A detailed micro-structural comparison

of deposits with high and low whiskering

tendency is being conducted. Sn grain size, shape,

preferred orientation and residual stress will be

measured. The dominant mechanism(s) for whisker growth will be determined.

Figure 1. Typical whisker growing from electrodeposited tin surface.

DELIVERABLES: Perform wetting balance tests for

the IPC solderability task group pilot study on Pb-free

surface finishes and components. 4Q 2003, IPC

solder study).

DELIVERABLES: Expand pages for phase diagram

Metallurgy Division Webbook with emphasis on

solder alloys. Add pages for constituent binary and

ternary systems containing Sb. 3Q 2003.

DELIVERABLES: Perform x-ray and SEM analysis on

electrodeposits for the NEMI Modeling Group to

determine factors that influence Sn whisker

propensity. 4Q 2003.

DELIVERABLES: Make simple phase equilibria

software available for interactive use; make more

complex programs available for downloads.

 4Q 2003.

DELIVERABLES: Prepare draft Best Practice Guide

for Differential Thermal Analysis. 4Q 2003.

ACCOMPLISHMENTS

# NIST has taken a major role working with industry through a NEMI Task Force to identify

and move Pb-free solders into practice. NIST cochairs the NEMI alloy selection group that selected

standard alloy compositions for U.S. microelectronics assembly.

# NIST has developed the database necessary

to calculate multicomponent phase diagrams essential for Pb-free alloy development. The experimental determination of phase diagrams is a timeconsuming, costly task requiring expert interpretation of results. The calculation of phase diagrams

significantly reduces the effort required to determine phase evolution in multicomponent systems

and can provide quantitative information that is

frequently needed in other modeling efforts. During the past year antimony (Sb) containing systems were added to the current NIST thermodynamics database. This expands the applicability

of the database to a wider range of solder alloys.

# An easy-to-use interface for phase equilibria

software for the calculation of liquidus temperature, lever rule equilibrium and Scheil solidification has been developed.

# We are also working in collaboration with IPC

Standards Committees (most closely with members from Celestica, Lucent, Raytheon, Rockwell,

and Shipley-Ronel) to establish reproducible solderability test standards for board level assemblies. Activities include providing benchmark experiments for the wetting balance tests to predict

on-line solderability for a wide range of surface

finishes, lead materials, and solder alloys.

New NIST research to develop an understanding

of whisker formation in Sn-based, Pb-free

electroplated surface finishes complements the

solderability studies. NIST is a co-chair in the

NEMI Sn Whisker Modeling Group and an active

participate in the NEMI Accelerated Sn Whisker

Test Group.

A plating cell using a rotating cathode was used

to control the hydrodynamic conditions during

the electrodeposition of Sn and Sn alloys. We

have characterized the grain size, and surface

topography of Sn and SnCu coatings on a variety

of substrates (amorphous carbon, vapor

deposited Cu and electroplated Cu and Sn).

Residual stress measurements are ongoing using

the cantilever beam technique.

High purity bright Sn electrolytes were prepared

using 18.3 Mohm-cm water. Controlled amounts

of Cu were added to the Sn electrolyte. It was

found that even trace levels Cu in Sn plating bathes

have been identified as a contributory factor to
92 National Institute of Standards and Technology

the propensity of tin whisker growth in electroplated deposits.

In collaboration with NEMI and TMS, NIST cosponsored a full day workshop on Tin Whisker

Formation in Microelectronics for industrial and

academic participates at the TMS meeting held in

San Diego, CA on March 2, 2003.

COLLABORATIONS

IPC; development of solderability test procedures

National Electronics Manufacturing Initiative; Lead-free

solders and reliability.

TMS, The Minerals, Metals & Materials Society;

workshop sponsor with NIST and NEMI

RECENT PUBLICATIONS

Boettinger, W. J., Handwerker C. A., Newbury, B., Pan,

T. Y., and Nicholson, J. M., “Mechanism of Fillet Lifting

in Sn-Bi Alloys,” J. Electronic Materials, 31 (2002) 545.

Boettinger, W. J. and Kattner, U. R., “On DTA Curves

for the Melting and Freezing of Alloys,” Metal. and

Matls. Transactions, 33A (2002) 1779-1794.

Kattner, U. R., “Phase Diagrams for Solder Alloys,”

JOM 54 (12) 2002, 45-51.

Williams, M. E., Johnson, C. E., Stafford, G. R.,

Handwerker, C. A., and Boettinger, W. J., “The

Formation of Whiskers on Electroplated Sn containing

Cu,” Proceedings of the American Electroplaters and

Surface Finishers Society (AESF) SUR/FIN 2002 Annual

International Technical Conference, 31-39.

Lewis, D. J., Allen, S. L., and Notis, M. R., “Ag-Cu-Sn

Eutectic Structure Determination,” J. Electronic

Materials, 31,161-167 (2002).

Josell, D., Wallace, W. E., Warren, J. A., and Wheeler,

D., “Misaligned Flip-chip Solder Joints: Prediction and

Experimental Determination of Force-displacement

Curves,” J. Electronic Packaging, 124 (3), 227-233

(2002)

Josell, D., Wheeler, D., Moffat, T. P., “Superconformal

Electrodeposition in Vias,” Electrochemical and Solid

State Letters, 5 (4) C49-C52 (2002).

Gayle, F. W., “Fatigue-Resistant, High Temperature

Solder,” Adv. Mater. Process., 159 (4) 43-44 (2001).

Gayle, F. W., Becka, G., Syed, A., Badgett, J., Whitten,

G., Pan, T.-Y., Grusd, A., Bauer, B., Lathrop, R., Slattery,

J., Anderson, I., Foley, J., Gickler, A., Napp, D., Mather,

J., and Olson, C. “Lead-Free High-temperature Fatigueresistant Solder,” Final Report, NCMS, August 2001

(published as CD-ROM).

Gayle, F. W., Becka, G., Syed, A., Badgett, J., Whitten,

G., Pan, T.-Y., Grusd, A., Bauer, B., Lathrop, R., Slattery,

J., Anderson, I., Foley, J., Gickler, A., Napp, D., Mather,

J., and Olson, C., “High Temperature Lead-free Solder

for Microelectronics,” Proceedings of The 4th Pacific

Rim Conf. on Adv. Materials and Processing, Honolulu,

HA (2001) in press.

Gayle, F. W., Becka, G., Syed, A., Badgett, J., Whitten,

G., Pan, T.-Y., Grusd, A., Bauer, B., Lathrop, R., Slattery,

J., Anderson, I., Foley, J., Gickler, A., Napp, D., Mather,

J., and Olson, C., “High Temperature Lead-free Solder

for Micro-electronics,” JOM, 53 (6) 17-21 (2001).

Kattner, U. R. and Handwerker, C. A., “Calculation of

Phase Equilibria in Candidate Solder Alloys,” Z.

Metallkd., 92 (7) (2001).

Moon, K. W., Williams, M. E., Johnson, C. E., Stafford,

G. R., Handwerker, C. A., and Boettinger, W. J., “The

Formation of Whiskers on Electroplated Tin Containing

Copper,” Proceedings of The 4th Pacific Rim Conf. on

Adv. Materials and Processing, Honolulu, HA (2001)

pp. 1115-1118.

Moon, K. W., Boettinger, W. J., Kattner, U. R.,

Handwerker, C. A., and Lee, D. J., “The Effect of Pb

Contamination on the Solidification Behavior of Sn-Bi

Solders,” J. Electron. Materials, 30, 45-52 (2001).

Bradley, E., Charbonneau, R., Bath, J., Handwerker, C. A.,

Parker, R., Sohn, J., and Gedney, R., “Lead-free Soldering

Trends in the U.S. NEMI Pb-free Task Group,”

Proceeding of JISSO/PROTEC Forum, Japan (2001)

pp. 134-139.

Handwerker, C. A., Noctor, D., and Whitten, G.,

“Reliability of Lead-free Solders,” Environmentalfriendly Electronics: Lead-free Technology, Hwang, J.S.

(ed.) Electrochemical Publications LTD (2001) pp. 566-

589.

Moon, K. W., Boettinger, W. J., Kattner, U. R.,

Biancaniello, F. S., and Handwerker, C. A., “Experimental and Thermodynamic Assessment of Sn-Ag-Cu

Solder Alloys,” J. Electron. Mater., 29 (10) 1122-1136

(2000).

Moon, K. W., Boettinger, W. J., Kattner, U. R.,

Biancaniello, F. S., and Handwerker, C. A., “The Ternary

Eutectic of Sn-Ag-Cu Solder Alloys,” Proceedings of

2000 SMTA Conference, Edina, MN, Surface Mount

Technology Assoc, 941-944 (2000).
Semiconductor Microelectronics and Nanoelectronics Programs 93

INTERCONNECT MATERIALS AND RELIABILITY

METROLOGY

Technical Contacts:

D. T. Read

R. R. Keller

M. Gaitan

“A microtensile methodology, developed at the

National Institute for

Standards and Technology, has been adopted

and applied in Motorola

to evaluate material

properties of thin films.

This methodology is a

significant part of the

materials technology

development at

Motorola.”

Betty Yeung, David Read,

Yifan Guo, Bill Lytle, and

Vijay Sarihan (Motorola AISL,

Tempe, AZ), “Microtensile

Methodology for Mechanical

Characterization of Thin

Films,” Solid State Technology, June 2002, p. 125.

GOALS

The objectives of this project are: (1) to develop

experimental techniques to measure the reliabilityrelated properties of thin interconnect conducting

and insulating films, including basic tensile

properties, elastic modulus by both static and

dynamic means, residual stresses, fatigue,

fracture resistance, and electromigration and

stress-voiding resistance, 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 film reliability to

microstructure and by developing understanding

of the relationships between various modes of

thin film failure, for example, electromigration and

mechanical fatigue.

CUSTOMER NEEDS

Thin films are an essential component of all

advanced electronic devices. Interconnect

structures built up on ULSI microchips consist

of 7 thin-film layers now, and will soon reach 10

layers (International Technology Roadmap for

Semiconductors, 2001, Interconnect, Table 62a).

These structures are fabricated using adjacent

layers of materials with very different thermal

expansion coefficients, exotic materials such as

nanoporous low-k dielectric, and operate at ever

higher temperatures. According to the Roadmap

(2001), Interconnect, p. 23, “Computer-aided

design (CAD) tools will need to 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 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 National

Electronics Manufacturing Initiative (NEMI)

Roadmap of December 2000 reports a similar need.

A section on pages 227-228 entitled “Key

Simulation Prerequisites” states that “Laboratory infrastructure/experimental expertise is

essential for both model verification as well as

property input evaluations, to have truly effective

simulations.” A following subsection lists critical

simulation areas. Listed first is “ReliabilityMechanics-Physics of Failure (POF), and

associated Mechanical Analysis and Design.”

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 films. This issue

of mechanical modeling is likely to increase in

significance with the growing integration of

interconnect between the chip and the package,

with their disparate material sets.

Because the films are formed by physical vapor

deposition or spin-on deposition, their microstructures, and hence their mechanical properties,

are quite different from those of bulk materials of

the same chemical composition. While the general

principles of conventional mechanical testing are

applicable to thin films, conventional test

equipment and techniques are not. Because

vapor-deposited films are of the order of 1 ìm

thick, the failure loads are of the order of gramforces or less, and the specimens cannot be

handled directly. So, techniques specific to films

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.

Radically different materials and material

technologies are being considered for future ULSI

devices, as the further development of leadingedge lithography increases in const 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

significant 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.
94 National Institute of Standards and Technology

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

influenced by processing conditions specific to

the specimen material at hand.

We study individual thin films 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. Some of our

techniques require the removal of the silicon

substrate beneath the test structure itself, to a

depth of up to 50 ìm. We have developed dry

etching systems that use xenon difluoride to carry

out these processes.

Figure 1. Test chip produced in the 1.2 μm AMI

CMOS process available through the MOSIS service.

Measurement capabilities operating within this

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

electromigration measurements, and resonant

structure measurements. We have developed the

silicon-frame tensile specimen and the piezoactuated tensile tester, which operate successfully

for specimens 100 ìm wide and larger. Because

problems were encountered with specimens

narrower than 100 ìm, a new technique, called

the force-probe tensile test technique, has been

developed. The apparatus includes a tensile

loading system operable within the scanning

electron microscope (SEM). This system has

now been used on specimens as small as 2 ìm

wide. It is anticipated that the magnification 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, film thickness, annealing,

and other relevant variables.

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 Aland 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.

We have begun an effort this year, with the NIST

Metallurgy Division, to address reliability of

advanced interconnect materials with linewidth

less than 100 nm. In addition to the challenges

associated with electrodeposition into extremely

narrow trenches, we expect significant influences

of the interconnect sidewalls on electrical

resistivity and possibly on electromigration

resistance.

We are also developing non-contact optical

methods to measure mechanical properties of thin

films 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 films using fixed-fixed beams. We

are currently developing resonant measurements

for cantilever beams to characterize the elastic

modulus of each layer (Fig. 2).
Semiconductor Microelectronics and Nanoelectronics Programs 95

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 fixedfixed beams used to measure residual strain and

strain gradient. With the residual strains, from

the fixed-fixed beam method, and the elastic

constant, from the resonant method, the residual

stress and its gradient can be calculated.

DELIVERABLES: Our results are being disseminated in conference presentations and peerreviewed articles in archival journals, as well

as in presentations and written reports to the

organizations that have supplied specimens.

Design and fabricate a test chip with improved

cantilever test structures; develop method to

determine elastic modulus from resonant frequency

measurements.

Conduct electrical tests on electrodeposited copper

lines of width 100 nm or less. 4Q 2003.

ACCOMPLISHMENTS

Progress in recent years on measurements of the

mechanical behavior of thin films 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; and results for the same

property by different measurement methods.

These comparisons are necessary to reach our

goals of providing accurate and believable

measurement techniques and an understanding

Figure 3. Comparison of Young’s modulus obtained

from nanoindentation and microtensile tests.

of the results. This year we obtained nanoindentation results on a previously tested aluminum

film and on thick copper films from an outside

collaborator. From the nanoindentation loaddisplacement data, we deduce Young’s modulus

and yield strength of the material in a very small

volume deformed by the nanoindenter tip. The

results obtained from nanoindentation scatter

widely in both Young’s modulus and yield

strength for electron beam-evaporated aluminum;

both values are consistently higher than those

obtained from microtensile tests (see Fig. 3). This

problem of scatter between nanoindentation and

microtensile results is typical in such

comparisons. Nano-indentation results for

polyimide were also higher than microtensile

results, but with less scatter. Work is continuing

with other materials and understanding the causes

of the scattering. The measurements on the thick

copper films, analyzed using plastic-zone-sized

deduced from AFM images, gave a good

correlation with the micro-tensile results shown

below.

DELIVERABLE: The collaborative study on copper

has been presented at a technical conference and

submitted for publication in a technical conference

proceeding.

A key experimental tool for understanding the

sources of mechanical weakness is fractography

of broken specimens. A new scanning electron

microscope, with a field-emission source and an

in-lens detector, is allowing us to obtain very clear

images of the fractures surfaces of microtensile

specimens. An example is shown in Fig. 4. This

aluminum CMOS contact metal had low

elongation, and variable tensile strength with some

very low values.
96 National Institute of Standards and Technology

Figure 4. Fracture surface of tensile specimen of

CMOS contact metal obtained through the MOSIS

service.

Recently, we have been working to demonstrate

the applicability of these techniques to materials

recently introduced in the microelectronics

industry, specifically copper, in the form of both

sputtered thin films and thick electrodeposits. A

microtensile specimen of electro deposited copper

is shown in Fig. 5. We typically find 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 film thickness from, for

example, 1 to 10 ìm, increases the ductility from

around 1 % or less to 5 % or more. Annealing also

changes the tensile properties markedly, as shown

in the stress strain curves (Fig. 6). The changes

with annealing are qualitatively as expected. The

differences between the thin sputtered film and

the thick electrodeposited film are generally

consistent with trends seen for other thin film

materials.

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. Typical stress-strain curves for annealed

and as-deposited films, obtained by tensile testing.

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.

 Our a.c. tests continue to reveal damage

phenomena drastically different from that

observed in conventional d.c. electromigration

tests. Last year, we reported observations of slipinduced surface offsets likely due to classical

dislocation activity. TEM experiments will confirm

the role of dislocations, and we expect to begin

such experiments later this year. We have pursued

crystallographic mapping experiments within a

field emission SEM this year instead, 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

significant growth of selected grains in a

polycrystalline interconnect, which subsequently

become more susceptible to surface offset

formation with further cycling (Fig. 7).

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

significantly due to deformation. We are

attempting to construct a model describing the

roles of these factors, including strength of

individual grains (grain size), ease of slip within

individual grains (grain orientation), and

prevalence of dislocation sources (grain boundary

structures, surrounding grain constraints). The
Semiconductor Microelectronics and Nanoelectronics Programs 97

end goal is to offer a scheme for predicting where

an open circuit should be expected, given the

initial grain structure of the interconnect.

Because it is sometimes impossible to get films of

individual materials with the interconnect stack

in a chip that goes through a normal manufacturing process, 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

films in the laminate is to be determined by

differences between the resonant frequencies of

beams with different combinations of metal,

dielectric, and polysilicon. A model has been

developed for a composite cantilever beam.

Preliminary measurements indicate that this

technique can provide accurate values of the

different layers in the interconnect structure, as

well as insight into the behavior of the structure

as a whole (see Fig. 8).

Figure 7. Quasi in-situ a.c. testing sequence in Al-1Si,

showing development of surface offsets and accompanying grain growth after 0, 10, 20, and 40 seconds of

a.c. cycling at approximately 12 MA/cm2. Top four

images obtained by secondary electron imaging.

Bottom four images from same locations, obtained by

automated electron backscatter diffraction.

Figure 8. Elastic modulus of CMOS thin films

extracted from the cantilever resonant frequency

measurements.

COLLABORATIONS

Max-Planck-Institut für Metallforschung, Stuttgart,

Germany, Prof. Eduard Arzt, Dr. Cynthia Volkert

Intel Corp., Chandler, AZ, Dr. Richard Emery

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

Keller, R. R., Mönig, R., Volkert, C. A., Arzt, E.,

Schwaiger, R., and Kraft, O., “Interconnect Failure due

to Cyclic Loading,” Proceedings of the Sixth International

Workshop on Stress-Induced Phenomena in

Metallizations, S. P. Baker, M. A. Korhonen, E. Arzt,

and P. S. Ho, Eds., American Institute of Physics

Conference Proceedings Vol. 612, pp. 119-132, Melville,

NY (2002).

Read, D. T., Cheng, Y. –W., McColskey, J. D., and Keller,

R. R., “Mechanical Behavior of Contact Aluminum

Alloy,” MRS Symposium Proceedings vol. 695, pp.

L6.11.1-L6.11.6. (2002).

Read, D. T., Cheng, Y.-W., and McColskey, J. D.,

“Microtensile Behavior of a Commercial Photodefinable

Polyimide,” Proceedings of the SEM Annual Conference

on Experimental and Applied Mechanics (2002).

Herman, David, Gaitan, Michael, and DeVoe, Don,

“MEMS Test Structures for Mechanical Characterization

of VLSI Thin Films,” Proc. SEM Conference, Portland

Oregon, June 4-6, 2001.

LaVan, D.A., Tsuchiya, T., Coles, G., Knauss, W. G.,

Chasiotis, I., and Read, D. T., “Cross Comparison of

Direct Strength Testing Techniques on Polysilicon

Films,” Mechanical Properties of Structural Films, ASTM

STP 1413, C. Muhlstein and S. B. Brown, Eds., American

Society for Testing and Materials, West Conshohocken,

PA (2001).
98 National Institute of Standards and Technology

Long, G. S., Read, D. T., McColskey, J. D., and Crago,

K., “Microstructural and Mechanical Characterization

of Electrodeposited Gold Films,” Mechanical Properties

of Structural Films, ASTM STP 1413, C. L. Muhlstein

and S. B. Brown, Eds., American Society for Testing and

Materials, West Conshohocken, PA (2001).

Read, D. T., Cheng, Y.-W., and McColskey, J.D., “High

Ductility in Small Tensile Specimens of e-BeamDeposited Aluminum Films,” Scripta Materiala 45

(2001), pp. 583-589.

Read, D. T., Cheng, Y-W., Sutton, M. A., McNeill, S. R.,

Schrier, H., “Proposed Standardization Effort: DigitalImage Correlation for Mechanical Testing,” Proceedings

of the SEM Annual Conference on Experimental and

Applied Mechanics, pp. 365-368 (2001).

Read, D. T., McColskey, J.D., and Cheng, Y.-W.,

“Microscale Test Technique and Results for Aluminum

Thin Films,” Proceedings of the SEM Annual Conference

on Experimental and Applied Mechanics, pp. 442-445,

June 2001.

Shuman, Shivesh, Gaitan, Michael, Joshi, Yogendra, and

Harman, George, “Wire Bond Temperature Sensor,”

IMAPS 2001 Symposium, Baltimore, MD, October 7-

11, 2001.

Yeung, B., Read, D. T., Guo, Y., Lytle, W., Sarihan, , V.,

“Microscale test technique and results for aluminum thin

films,” accepted for International Mechanical

Engineering Conference and Exhibition, November

2001.

RECENT PRESENTATIONS

Geiss, R. H., Roshko, A., Bertness, K. A., and Keller, R.

R., “Electron Backscatter Diffraction for Studies of

Localized Deformation,” Mike Meshii Symposium,

TMS, forthcoming (2003).

Read, D. T., Geiss, R., Ramsey, J., Scherban, T., Xu, G.,

Blaine, J., Miner, B., Emery, R. D., “Nanoindentation

and tensile behavior of copper films,” 2003 Materials

Research Society Spring Meeting, San Francisco, CA,

April 21-25, 2003.

Read, D. T., McColskey, J. D., and Cheng, Y.-W.,

“Microtensile Testing of Thin Films in the Optical and

Scanning Electron Microscopes,” International

Conference on Characterization and Metrology for ULSI

Technology, Austin, TX, March 24-28, 2003 (poster

presentation).

Read, D. T., “Interconnect Materials And Reliability

Metrology,” Government Microcircuit Applications &

Critical Technology Conference 2003, Tampa, FL,

March 31-April 3, 2003.

Mönig, R., Orso, S., Volkert, C. A., Keller, R. R., and

Arzt, E., “AC-Induced Damage in Cu Interconnects,”

Materials Research Society Spring Meeting, San

Francisco, CA, April 1-5, 2002.

Keller, R. R., Mönig, R., Volkert, C. A., Arzt, E.,

Schwaiger, R., and Kraft, O., “Interconnect Failure due

to Cyclic Loading,” Sixth International Workshop on

Stress-Induced Phenomena in Metallizations, Ithaca, NY,

July 25-27, 2001. (INVITED)

Read, D. T., Cheng, Y. –W., McColskey, J. D., and Keller,

R. R., “Mechanical Behavior of Contact Aluminum

Alloy,” Materials Research Society Fall Meeting, Boston,

MA, November 26-30, 2001.
Semiconductor Microelectronics and Nanoelectronics Programs 99

THERMAL MEASUREMENTS

AND PACKAGING RELIABILITY

Technical Contacts:

A. J. Slifka

E. S. Drexler

D. L. Blackburn

A. R. Hefner

“The task of dissipating

the heat from integrated

circuits while maintaining

acceptable junction

temperatures has been a

significant challenge for

semiconductor and

system manufacturers.”

“. . . .We continue to push

the cooling and mechanical limits of electronic

products. Complete

thermal and mechanical

modeling, validated

with measurements,

is needed.”

2001 International Technology

Roadmap for Semiconductors

GOALS

The goals of this project are to:

a) Provide the micro-electronics 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,

which present a reliability concern to the

manufacturer or end-user. 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 allows us to refine the techniques

to make them optimally beneficial to industry and

to demonstrate to the industry at large the

capabilities of the techniques on actual packages

in development, essential for technology transfer.

b) Develop and evaluate methods for measuring

temperatures at and within the chip-level for the

purposes of determining in situ material thermal

properties (e.g., thermal properties of interlayer

dielectrics), device performance (e.g., SOI

devices), and for evaluating the thermal

performance of new architectures and technologies (e.g., 3D integration and innovative

cooling methods for high performance logic).

CUSTOMER NEEDS

The trend in electronics is toward components of

higher density and smaller size using less

expensive materials. Materials used in packaging

are many and display a variety of thermal

responses that are not always compatible. This

makes interfaces particularly vulnerable to

thermomechanical fatigue failures. The program

seeks to offer support and verification of models

conducted or sponsored by the microelectronics

industry. Development of measurement

techniques allowing observation of the package

throughout the thermal cycling is done so that

one can identify the locations and materials in

which deformations are occurring.

The physical size of nearly all electronic devices

is decreasing rapidly and, at the same time, their

capabilities are increasing dramatically. One move

in this direction is the advent of integral passive

components and another is the increasingly

prevalent use of organic (polymeric) conductors

and fillers. These organic materials have a large

coefficient of thermal expansion, which can reduce

the reliability of electronic packaging systems.

We are investigating interfaces between organic

materials and between organics and metals to

determine the initiation of damage and failure

mechanisms in these material systems.

Temperature measurements for microelectronic

devices are more important today than they ever

have been. It has always 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 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.

“Packaging cost is a second area that could be

an obstacle to realizing the potential of advances

in silicon technology. ...the average packaging

share of total product cost will double over the

next 15 years and, more significantly, the ultimate

result will be greatly reduced gross profit

margins, limiting investments in R&D and

factory capacity.” 2000 NEMI Roadmap

The maximum junction temperatures for chip

operation are shown in tables 75 a and b of the

ITRS Assembly and Packaging section. The

criticality of knowing thermal properties of

materials and devices is discussed on page 8 of

the Assembly and Packaging section, page 25 of

the Metrology section, and page 19 of the

Interconnect (Reliability) section. Validated

thermal models are discussed on page 25 of the

Metrology section.
100 National Institute of Standards and Technology

TECHNICAL STRATEGY

1. Our established programs in infrared (IR)

(thermal) microscopy and electron-beam moiré,

and our developing techniques in scanning

thermal microscopy, utilizing the AFM (atomic

force microscope), 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 are the strains in the onchip tungsten vias under thermal loading.” The

first was answered using the IR microscope; the

second has yet to be answered.

We have just completed our third 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 Passives 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 decline in interfacial thermal

conductivity is the first indication of the

microcracks and fissures that may ultimately result

in failure.

DELIVERABLES: Quantitative measurements of

industrial specimens from AEPT using thermal SPM,

laser-heated IR microscopy, and Joule-heated IR

microscopy. 4Q 2003

2. The electron-beam moiré technique offers a

unique capability. Unlike moiré interferometry, it

can quantify strain in the different materials at

both low temperatures and at high temperatures.

It measures thermally induced strains representative of those experienced by the package in

service, rather than residual strains due to

processing. In addition, it can quantify the

accumulation of plastic strain during thermal

cycling, and it is clear from the images in which

materials the strains are occurring. The technique

is designed to look at strains on a local scale, but

is flexible enough to allow comparison of those

local strains at multiple locations across the cross

section.

DELIVERABLES: The results of a collaboration with

the Jet Propulsion Laboratory were presented at the

Fifth Annual Microelectronics Reliability and Qualification Workshop. 1Q 2003

3. Work continues with the AEPT on the embedded resistors. A second test vehicle has been

sent to NIST for strain measurements for embedded resistors that have different sizes and layouts. Two resistor sizes for each of two layouts

will be compared.

DELIVERABLES: Electron beam moiré measurements of next-generation embedded resistor

specimens from the AEPT consortium. 3Q 2003

4. Thermal microscopy and electron-beam moiré

are complementary techniques, each supplying

pieces to the puzzle of how failure occurs.

Combining the data from both techniques will

yield a complete picture of thermomechanical

fatigue.

DELIVERABLES: Analyze and report IR microscopy,

SPM thermal microscopy and electron beam moiré

data from next generation embedded resistor

specimens. 4Q 2003

5. As the size of packages gets smaller, heat

removal becomes a more significant problem. The

SPM is being used to develop a technique to

measure thermal conductivity of thin films for

application first to metal interconnect lines. The

technique will also be able to measure interfacial

thermal resistance between materials. The

theoretical work on this development is being

done in collaboration with Dr. Kevin Cole of the

University of Nebraska.

DELIVERABLES: Report via the literature, on

measurements of a model film, such as gold or

diamond-like-carbon. 4Q 2003

6. The reasons for desiring to know the operating

temperature of a semiconductor device can be

divided into the following four broad categories:

# Predict reliability or operating life of device

# Measure material/device thermal properties

in-situ

# Confirm or determine the operating limits or

thermal performance of a device

# Validate thermal models for chip and device

performance.

A broad range of temperature measurement

techniques will be investigated. These will

include:

# Electrical techniques that use temperature

sensitive device and material parameters (such

as junction voltages, threshold voltages, and

resistivity) as thermometers, optical techniques

such as infrared thermal emission, micro-Raman

spectroscopy, and reflectance, and micro-probes

based upon AFM systems.
Semiconductor Microelectronics and Nanoelectronics Programs 101

DELIVERABLES: Complete modeling of buried

interconnect system thermal performance. 3Q 2003

Develop and fabricate test structures for interconnect thermal metrology. 2Q 2004

Demonstrate temperature measurements of buried

interconnect lines. 3Q 2004

Develop procedure for coating chips with thin high

emissivity paint and characterize the performance

for high speed IR thermal metrology. 4Q 2003

Develop transient thermal IR metrology calibration test

chip using micro hotplates. Demonstrate temporal and

spatial calibration procedure. 3Q 2003

Apply transient IR test system to develop gas sensor

temperature BIST. 4Q 2003

ACCOMPLISHMENTS

# Thermal measurements have continued on

three similar embedded resistor specimens. One

specimen each is being measured using laserheated IR microscopy, Joule-heated IR

microscopy, and thermal SPM. Upon completion

of thermal cycling and testing, results will be

analyzed and the techniques compared. A single

crack appeared in one of the three similar

specimens after 11 thermal cycles, but failure has

not occurred yet. All three specimens have been

thermally cycled 18 times and data analysis has

begun. Figure 1 shows a thermal image from the

SPM and accompanying temperature profile data

used in the analysis. The y-axis scale of the

temperature profile corresponds to 8 oC.

Figure 1. Thermal SPM image and temperature

profile of an industrial integral resistor specimen.

Figure 2. Moiré images at 177 °C from small and

large sized resistors with the same layout.

# Electron-beam moiré tests have been

completed on three of four specimens from

DuPont comparing the strain fields in a printed

circuit board containing ceramic resistors of

various sizes and layouts during thermal cycling.

Figure 2 shows electron-beam moiré images from

two specimens with the same layout, but with

different sized resistors taken at 177 °C.

# In a collaboration with the Jet Propulsion

Laboratory (JPL), the effect of a microwave

process on solder bump reliability was studied.

The program looked at the differences in strain in

solder bumps that have been processed before

thermal cycling, and a control set that were not be

processed.

# Development of a technique for measuring

the thermal conductivity of thin films using the

SPM has begun. Most films used in electronics

and electronic packaging are so thin that a

measurement using even the thermal SPM would

be primarily a measurement of the substrate

material. With collaboration from Dr. Kevin Cole

of the University of Nebraska, work has begun

on a theoretical treatment of the data to allow

measurement of thin films and the interfacial

thermal resistance between the film and substrate.

One application for this technique is measurement

of thermal properties of interconnect lines.

Measurements have begun using a model system,

gold films of various thickness evaporated onto

glass substrates. Figure 3 shows the imaging

contrast available with the technique.
102 National Institute of Standards and Technology

# We have been approached by Cenymer

Corporation to make thermal and other measurements on a diamond-like-carbon coating that they

manufacture. Since the coating is typically around

a micron thick, the thermal SPM is a good tool for

this measurement. Cenymer is also interested in

interface materials to allow adhesion of diamondlike-carbon coatings to various substrates. This

work fits well with our thin-film research,

providing specimens with different, controlled

interfaces to help solve the problem of analyzing

thermal data on thin films and even thinner

adhesion layers.

Figure 3. Thermal SPM image of a 200 nm thick

gold film on glass.

Figure 5a. Drawing of the layout of the IGBT chips

within the module. and b) the vertical structure from

one of the chips to the baseplate of the module.

# We have 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 gate-source

voltage, VGS, at a constant, relatively low current

and at a high anode-cathode voltage as the

temperature sensitive parameter. Under these

circumstances, variations in VGS result from equal

changes in the threshold voltage. The 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). Under the

measurement conditions used, there are concerns

about the sharing of current between the chips in

the package, but by taking these into account,

excellent agreement is obtained between the

measured and simulated temperatures (see Fig. 4).

The module is shown in Fig. 5.

Figure 5b. Drawing of the vertical structure in the

power module from one of the chips at the top to the

baseplate at the bottom.

Figure 4. Electrically measured and computer

simulated measurement of the temperature of IGBT

chip in a power module.
Semiconductor Microelectronics and Nanoelectronics Programs 103

# We have been invited to organize a technical

session on Temperature Measurements of Semiconductor Devices at the 2004 IEEE Semiconductor Thermal Measurements and Management Symposium (SEMITHERM) to be held in March of

2004. The intent is to have presentations from

experts in electrical, optical, and scanning probe

temperature measurements for semiconductor

devices and circuits.

COLLABORATIONS

IBM, Rochester, MN

Arv Sinha, Joe Kuczynski

AEPT Consortium

Dupont, John Felten

MacDermid, Dennis Fritz

Merix, Bob Greenlee

MicroFab, Plano, TX

Virang Shah

SAS Circuits, Littleton, CO

Richard Snogren, Matt Snogren

University of Colorado

Ken Douglass, T. Andrew Winningham

Colorado School of Mines

Ivar Reimanis, Saki Krishnamurthy,

John Berger

University of Nebraska

Kevin Cole

Cenymer Corporation

Charles Partee

Georgia Institute of Technology

George Washington University

University of Maryland

Motorola

Cree, Inc.

Army Research Laboratory

RECENT PUBLICATIONS

Slifka, A. J., Drexler, E. S., “Thermomechanical Effects

in Embedded Passive Materials,” Journal of Electronic

Materials , 31, 4, pp.286-291 (2002).

Drexler, E. S., Snogren, R. C., Snogren, M. C., Felten, J.

J., and Green, P. A., “Strain Measurements In Thermally

Loaded Circuit Boards Containing Embedded Resistors,”

Proceedings of the 52nd Electronic Components

Technology Conference, pp.526–531 (2002).

Slifka, A. J., Drexler, J. W., “Apparent Mobility of

Interfaces in Integral Resistor Material,” Microscopy and

Analysis, pp.19-21, January 2003.

Hefner, A., Berning, D., Blackburn, D., Chapuy, C., and

Bouche, S. A.“High-Speed Thermal Imaging System for

Semiconductor Device Analysis,” Proceedings of the 17th

Annual IEEE Semiconductor Thermal Measurement and

Management Symposium, pp. 43-49 (2001).

Afridi, M. Y., et. al, “Transient Thermal Study of

Microhotplates by Using High-Speed Thermal Imaging

System,” Proceedings of the 18th Annual IEEE

Semiconductor Measurement and Management

Symposium, March 2002.

Blackburn, D. L., “Temperature Measurement of

Semiconductor Devices,” Thermes 2002-Thermal

Challenges in Next Generation Electronic Systems,

Millpress, Rotterdam Netherlands (2002), pg. 23.

J. Rodriguez, Z. Parrilla, M. Veléz-Reyes, A. Hefner, D.

Berning, J. Reichl, and J. Lai “Thermal Component

Models for Electro Thermal Analysis of Multichip Power

Modules,” in conf. record IEEE Industry Applications

Society Meeting, pp. 234-241, October 2002.

D. Berning, J. Reichl, A. Hefner, M. Hernandez, C.

Ellenwood, and J-S. Lai, “High Speed IGBT Module

Transient Thermal Response Measurements for Model

Validation,” to be published in IEEE IAS meeting, October

2003.

Ortiz-Rodríguez, J. M., Hefner, A. R., Vélez-Reyes, M.,

and Gonzalez, J., “Electro-Thermal Modeling of the

IPEM Generation II in SABER,” CPES Annual Industry

Seminar, April 28-29, 2002.
104 National Institute of Standards and Technology

ATOMIC LAYER DEPOSITION –

PROCESS MODELS AND METROLOGIES

Technical Contacts:

J. E. Maslar

D. R. Burgess, Jr.

W. S. Hurst

GOALS

Develop validated process models and in situ

metrologies for atomic layer deposition processes.

CUSTOMER NEEDS

As device critical dimensions continue to shrink,

atomic layer deposition (ALD) is receiving

increased attention as a method of depositing thin

(nanometer-scale), conformal layers, required for

many microelectronics applications. Applications

include high-κ gate dielectric layers, diffusion

barrier layers, copper seed layers, and DRAM

dielectric layers. For some thin layer deposition

applications, the 2001 ITRS identifies ALD as the

dominant solution.

According to the 2001 ITRS, “modeling of thin

film deposition and etch variation and feature

variations across a wafer can provide

tremendous savings in development time and

cost” [2001 ITRS, Modeling and Simulation,

page 2]. In addition, according to the 2001

ITRS, “An important future application of

equipment modeling is the control of

manufacturing equipment based on real time

simulators including sensor design and

simulation” [2001 ITRS, Modeling and

Simulation, page 7]. Hence, wafer-scale

modeling of ALD processes should facilitate

efficient integration of ALD into the fab and aid

in development of ALD diagnostics. However,

validated ALD process models are not generally

available. In fact, the 2002 ITRS Update identifies

ALD modeling as a difficult challenge with

important issues being the availability of accurate

fundamental physical data as well as reaction

mechanisms and reduced models for complex

chemistry. In addition, the 2001 ITRS identifies

experimental validation as a “key difficult

challenge across all modeling areas” [2001 ITRS,

Modeling and Simulation, page 1].

TECHNICAL STRATEGY

This project is in the initial stages. As such, this

project involves only two primary directions:

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 most

important reaction species will be identified as

understanding of a particular ALD reaction

improves, thus facilitating the design of improved

process metrologies.

Ultimately, aspects of both of metrology

development and reaction mechanism

development will be required to create validated

ALD process models. ALD process models would

be created by incorporating the chemical reaction

mechanisms developed here into commercially

available computational fluid dynamics code and

then validating the process model under a range

of parameters using experimental data collected

in the course of this project. Initial reaction

mechanism development will focus on Al2

O3 ALD

deposition.

While these two directions will be closely coupled

for development of a specific 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.

Also, metrologies are being investigated that will

potentially be useful for ALD layer characterization but that would not necessarily provide

data to validate ALD models currently under

development, e.g., metrologies sensitive to

impurity distributions in transition metal oxide

films.

1. The first step in providing validated ALD

process models is evaluating the suitability of

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. Mass

spectrometry is relatively straight-forwardly

integrated with an ALD reactor and, when in situ

diagnostics are applied to ALD reactors, mass

spectrometry is frequently employed. However,

mass spectrometry is only sensitive to volatile
Semiconductor Microelectronics and Nanoelectronics Programs 105

species and, hence, it is sometimes difficult to

relate the species detected to mechanisms of

interest on the growth surface. Because of the

desire to directly probe ALD surface chemistry,

Raman spectroscopy and infrared spectroscopy

are also being investigated as potential ALD

diagnostics. However, it is technically challenging to apply Raman and infrared spectroscopy to

detect species present at monolayer quantities.

For this reason, the suitability of these techniques

is being evaluated using research-grade ALD

reactors with optimized optical accessibility. After

various diagnostics are evaluated in test ALD

reactors, a more industrially-relevant ALD reactor

will be designed and built. Suitable diagnostics

will be integrated into this reactor to provide data

that will aid in chemical mechanism development

and overall process model validation.

DELIVERABLES: Construction of research-grade,

optically-accessible ALD reactors. 3Q 2003

Evaluation of selected optical spectroscopic

techniques for characterization of ALD films.

4Q 2003

2. The calculation, estimation, and dissemination

of fundamental thermochemical and chemical

kinetic properties of organometallic compounds

with potential application to ALD, as well as

chemical vapor deposition (CVD), processes is

on-going. 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

first data activity, which is in coordination with

the Standard Reference Data Program, compiles

and evaluates currently available thermochemical

data of organometallic compounds and related

precursors. These data are available through a

NIST website: http://srdata.nist.gov/ckmechx/.

The second activity supplements available data

by using ab initio and semi-empirical calculations

to develop reaction mechanisms from computed

molecular structures, thermodynamic properties

and spectroscopic properties of Group III and

Group V compounds. The third activity utilizes

the experimental and computed thermochemical

and chemical kinetic data to develop mechanisms

for the decomposition of organometallic

precursors.

DELIVERABLES: Bibliographic and thermochemical

data for aluminum and aluminum oxide deposition and

relevant gas phase precursors. 3Q 2003

Calculated bond dissociation energies and barriers to

reaction for organoaluminum precursors relevant to

aluminum and aluminum oxide deposition. 4Q 3003

ACCOMPLISHMENTS

# ALD Reactor Construction — Researchgrade, optically-accessible ALD reactors and

precursor delivery systems have been designed

and built. Each reactor is optimized for a particular

optical diagnostic and will be used to test the

suitability of that diagnostic to probe ALD

processes. Investigations of various optical

diagnostics have begun.

# Chemical Properties Calculations —

Molecular structure and vibrational frequency

data for Group III (Al, Ga, In) and Group V (N,P,As)

Hydrides have been compiled. These include the

stable molecules (e.g., AlH3

) and radicals (e.g.,

AlH2). Ab initio calculations have been utilized

over a wide range of theories utilizing different

quality basis sets. The ab initio results have

been compared to available experimental data in

order to determine the minimum level of calculation

necessary to give good molecular structures and

vibrational frequencies. Although some of this

work has already been reported in the literature,

no comprehensive set of benchmark calculations

for the whole series has been performed.

Molecular structure and vibrational frequency

data and bond dissociation energies (BDE’s) for

the Indium Methyl Hydrides InHx

(CH3)y have

been computed at the B3LYP/cc-pVTZ level of

theory. The Indium Methyl Hydrides include the

stable molecules InH3, InH2(CH3), InH(CH3)2,

In(CH3)

3

, the radicals InH2, InH(CH3), In(CH3)

2

, and

the closed shell intermediates (with lone pairs)

InH, In(CH3

). Transition state calculations for

abstraction of H atoms from the Indium Methyl

Hydrides (In-H, CH2-H bonds) by H atoms and

CH3 have been computed, as well as a number of

other transition state calculations. These

transition state structures were then used

to determine temperature-dependent rate

expressions for the reactions. A simple

decomposition mechanism has been constructed

based on these data, compared to available

experimental data, and is currently being refined.

Calculated molecular structure, vibrational

frequencies, and BDE’s for the Gallium Methyl

Hydrides have also been carried out.
106 National Institute of Standards and Technology

# Database Website — A website http://

srdata.nist.gov/ckmechx/ 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. This site currently contains thermochemical and bibliographic information of silicon

hydrides and halocarbons important to

semiconductor processes. The plan for this site

is to have it provide all data necessary to support

chemical mechanisms of significance to the

industry. The current version of CKMechX has a

number of capabilities. Over 4,000 enthalpies of

formation are available for more than 1,000 species

with over 4,000 bibliographic citations. These

data were taken from NIST compilations and

evaluations of silicon hydrides, silicon oxyhydrides, hydrocarbons, fluorinated hydrocarbons, and chlorinated hydrocarbons.

# This ALD project relies heavily on experience

obtained in the course of a previous project

investigating silicon thermal CVD via silane

pyrolysis. That CVD project also involved

development of chemical reaction mechanisms

and experimental measurements to support

mechanism development and process model

validation. Multiple CVD process models were

constructed employing the silane pyrolysis

mechanisms, including gas phase nucleation of

silicon particles, developed during this project

and validated using the experimental results

obtained in the course of this project. The different

models employed reaction mechanisms of varying

complexity and reactor geometries of varying

dimensionality. Experimental measurements were

performed in a vertical flow, rotating disk reactor

under various process conditions. Gas

temperature profiles were determined using

rotational Raman spectroscopy. Gas phase silicon

particle spatial distributions were determined with

elastic light scattering. The extent of precursor

decomposition and the chemical composition of

the gas-phase-nucleated particles were

investigated with vibrational Raman

spectroscopy. A website (http://www.cstl.nist.

gov/div836/836.02/cvd/toppage.html) has been

established in order to disseminate the numerical

and experimental results obtained from this

investigation.

COLLABORATIONS

Professor G. W. Rubloff, University of Maryland; ALD

processes and metrology.

RECENT PUBLICATIONS

D. M. Kremer, R. W. Davis, E. F. Moore, J. E. Maslar, D.

R. Burgess, and S. H. Ehrman, An Investigation of Particle

Dynamics in a Rotating Disk Chemical Vapor Deposition

Reactor, J. Electrochem. Soc. 150, G127-G139 (2003).

D. M. Kremer, R. W. Davis, E. F. Moore, and S. H.

Ehrman, A Numerical Investigation of the Effects of GasPhase Particle Formation on Silicon Film Deposition

From Silane, J. Cryst. Growth 247, 333-356 (2003).
Semiconductor Microelectronics and Nanoelectronics Programs 107

DIELECTRIC METROLOGY SUPPORTING

INTEGRATED PASSIVE DEVICE TECHNOLOGY

Technical Contact:

J. Obrzut

“We truly need NIST

expertise to take on this

metrology work and help

guide the participating

people in the proper

direction.”

Tom Newton

Director of PWB

Standards & Technology

IPC Association

Connecting Electronics

Industries

“Your effort to characterize the dielectric

performance of the

materials at high frequencies helped us to

understand these

materials and successfully realize our program

objectives.”

Richard A. Charbonneau,

Project Leader of the

EDC Consortium,

Storage Technologies

GOALS

To develop testing procedures specific to the

needs of the electronics industry needed for the

development of novel materials for Embedded

Passive Devices Technology. The current focus

is on the measurement of broad-band permittivity

at microwave frequencies, dielectric withstanding

voltage, and key structural attributes that control

the properties of organic resins filled with

electronic modifiers. The development of

Integrated Passive Device technology depends

on the availability of suitable measurement

techniques to evaluate materials’ properties and

the functional characteristics of the manufactured

passive devices.

CUSTOMER NEEDS

The continuous reduction in the electrical charge

required to drive logic gates and storage cells

has resulted in tremendous progress in

miniaturization and density of integrated circuits.

However, in modern highspeed electronics, the

functional performance increasingly depends on

passive components, where the dielectric

permittivity and impedance characteristic are the

main factors that control spatial dimensions, time

scale, speed, shape, and amplitude of electronic

signals. Passive devices have not shrunk in size

as rapidly as active devices and tend to occupy

an increasingly larger area and mass fraction of

electronic subassemblies. In order to increase

component density and reduce the parasitic

effects of the associated interconnections,

passive components may be integrated within

substrates. Since integrated passive components

are formed during substrate fabrication, they

require new processing steps in addition to those

that provide only interconnections and new thinfilm integrated passive materials.

Novel dielectric hybrid materials based on

polymers filled with modifiers of their

electromagnetic properties have been identified

by the industry as essential for advancing

miniaturization and functional performance of

high-speed electronics. There are significant

challenges in developing new thin film materials

that exhibit enhanced electrical performance

characteristics, such as impedance and high

dielectric constant, which are needed to operate

at higher microwave frequencies and at increased

voltage strengths. Consequently, new metrology

methods are needed to address the specific

behavior of thin film specimens and for better

fundamental understanding of the relation

between functional performance and the structural

attributes of the materials.

The materials and testing requirements for

capacitors, resistors and are outlined in the 2001

International Technology Roadmap for

Semiconductors. A need for new standard test

methods for embedded materials was identified

by the IPC Embedded Passive Device Standard

Sub-committee D-39.

TECHNICAL STRATEGY

1. The strategy pursued for high frequency

characterization is to incorporate a thin passive

film material into a device that comprises a

capacitive or resistive termination in a

transmission line. Samples that represent recent

development and technological trends in

embedded passive materials are obtained from our

industrial partners. We also use model polymer

composite materials to validate the measurement

methods. Multifunctional acrylate polymers filled

with electronic modifiers such as ferroelectric

ceramics, metal particles, and carbon nanotubes

are used to address fundamental dielectric

properties of the composites. We employ a broadband measurement technique to develop a

systematic methodology for measuring impedance

and the dielectric permittivity of embedded

passive device materials at frequencies of 100MHz

to 10 GHz.

2. We are exploring non-linear dielectric

measurement methodology for testing passive

materials at high electric fields and voltages. Due

to the thin film configuration, nonlinear effects

may be activated at moderate bias levels and

contribute to dielectric breakdown. Nonlinear

dielectric effects may cause rectifying, an

increased repolarization loss, or act as frequency

multipliers, thus altering the nominal impedance

characteristics of the material.

DELIVERABLES: Complete feasibility study of

measurement on high-k films that are currently being

developed by the electronic materials suppliers.

1Q 2004
108 National Institute of Standards and Technology

DELIVERABLES: Develop an AC dielectric withstanding voltage measurement system for embedded

passive materials. Measure impedance of high-k

films as a function of applied high voltage. Determine

both the magnitude and phase of impedance from the

corresponding voltage and current waves. 4Q 2003

3. In addition, NIST staff is chairing the Test

Methods Task Group, D-37d, for the IPC – 4902

Embedded Passive Devices Standard Specification. This Test Methods Subcommittee is responsible for developing test methods which are

specific to the needs of the D-37 Embedded

Passive Devices Committee and its 3 Task Groups

for materials, performance and design. As these

Embedded Passive Devices-specific test methods

are developed, they will be sent to the 7-11 Test

Methods Subcommittee for inclusion in the TM650 manual.

DELIVERABLES: Draft a standard test method

document for IPC. 3Q 2003

4. The relationships between the dielectric

properties and structure in polymer resins filled

with ferroelectric ceramics are explored using these

new measurement methods. Our effort is focused

on local fast relaxation mechanism that we

discovered in dielectric composites.

DELIVERABLES: Identify key materials attributes that

control dielectric properties in polymers filled with

ferroelectric inclusions. 3Q 2003

ACCOMPLISHMENTS

# We have implemented a broadband

technique as a standard test method that enables

dielectric measurements at microwave frequencies

of up to several GHz. The technique is based on

the observation and theoretical analysis of the

fundamental mode propagating at high

frequencies in thin film dielectrics that terminate

a coaxial air-filled transmission line.

# In partnership with IPC, we initiated a

standard test method development and chair the

IPC-D37-d test method sub-committee for

Embedded Passive Devices. We guided the

design of the test protocol and made

arrangements with co-sponsoring member

companies for round robin evaluation.

# Our capability to measure the dielectric

relaxation times in the sub-nanosecond regime

and the relaxation strength for a wide range of

dielectric permittivity values is used to quantify

dispersion, alignment, and structure in hybrid

materials. Using the broadband study, we

demonstrated that the composites of organic

polymer resins filled with ferroelectric ceramics

exhibit a dominant intrinsic high frequency

relaxation behavior. Such dielectric properties

were found beneficial in eliminating the

electromagnetic noise in processors and logic

devices. We have confirmed that the fastest

dielectric relaxation process is controlled by the

di-polar dynamics of the polymer matrix while the

dielectric loss arises from the difference in

coupling between the relaxed and unrelaxed

dipoles. Our numerical models are capable of

quantitatively correlating the dependence of the

real and imaginary parts of the complex permittivity

on the volume fraction of the ceramic filler.

# Suitable experimental set-up and testing

procedures have been demonstrated for

unambiguous determination of the dielectric

withstanding voltage of embedded passive

materials. In contrast to conventional procedures,

the specimen voltage and current are determined

as complex quantities from the corresponding time

resolved voltage waves (Fig. 1). The new testing

procedure represents a compatible extension of

the existing standard test method, but is better

suited for capacitive and resistive thin film

materials.

Figure 1. The specimen voltage wave (circles) and the

wave corresponding to the specimen current

(triangles). Distortions seen in the specimen current

results from re-polarization dynamics, which has a

considerable effect on impedance and contributes to

the overall loss.

The specimen impedance and the loss tangent of

the material can be determined by performing

complex algebra calculations. It was found that

embedded passive materials do not exhibit a

constant impedence characteristic, as is the case

for conventional dielectrics, but that their

impedence characteristic sharply decreases with
Semiconductor Microelectronics and Nanoelectronics Programs 109

increasing voltage. Conventional dielectrics such

as FR-4 exhibit a flat impedance characteristic,

nearly independent of voltage, up to the material

break-down conditions. The break-down

symptoms are associated with HV flashing and

sparking that require further failure analysis. We

concluded that the existing test methods seem to

be adequate for typical dielectrics for which

substantial laboratory and field data have been

accumulated. In contrast, the impedance of

embedded passive materials decreases

continuously with increasing voltage and is

associated with a large increase in the dielectric

loss. This change in impedence cannot be detected

by conventional methods.

With our method, the value of the dielectric break

down can be accurately determined as an applied

voltage at which the specimen impedance

characteristic changes irreversibly. The value of

the dielectric withstanding voltage can be

determined when the specimen impedance drops

below a certain functionally acceptable level or

the phase angle increases above the functional

limit.

COLLABORATIONS

Polymers Division, NIST – A. Anopchenko, K. Kano,

N. Noda

Hokkaido University – R. Nozaki

DuPont — McGregor, G. S. Cox, J. Felten

Shipley — D. Senk

Nortel — R. Sheffield

Gould Electronics — T. Bergstresser

Electro Scientific Industries — K. Fjeldsted

Merix — R. Greenlee

Motorola — R. Crosswell

RECENT PUBLICATIONS

Noda N., and Obrzut J., “High frequency dielectric

relaxation in polymers filled with ferroelectric ceramics,”

Mat. Res. Soc. Symp. Proc., 628, pp. EE.3.8.1-6 (2002).

Obrzut, J., Noda N., and Nozaki, R., “Broadband

Characterization of High Dielectric Constant Films for

Power - Ground Decoupling,” IEEE Trans. on

Instrumentation and Measurement, 51, pp. 829 (2002).

Obrzut J., Anopchenko A., “High Frequency Input Impedance Characterization of Dielectric Films for PowerGround Planes,” IEEE Trans. on Instrumentation and

Measurement, in press (2003).
110 National Institute of Standards and Technology

WAFER CHARACTERIZATION AND PROCESS

METROLOGY PROGRAM

Device scaling has been the primary means by which the semiconductor industry has achieved

unprecedented gains in productivity and performance quantified 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 significant 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 sufficient, 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 difficult. The overall task represented by the projects below reflects

the need for analytical techniques with unparalleled spatial resolution, accuracy, robustness

and ease of use.

Shrinking dimensions of transistors while simultaneously increasing the wafer diameter from

200 mm to 300 mm is placing more stringent requirements on wafer flatness, thickness and

warp, ion and particle contamination.

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 flow meters and

residual gas analyzers. Water contamination at extremely low levels in process gases presents serious manufacturing difficulties. 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.

Detection and accurate sizing of particle contamination continues to challenge semiconductor

manufacturing.
Semiconductor Microelectronics and Nanoelectronics Programs 111

WAFER AND CHUCK FLATNESS METROLOGY

Technical Contacts:

U. Griesmann

R. Polvani

“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

GOALS

Develop measurement support for 300 mm

diameter silicon wafers used in lithography

applications. This project will provide measurement and infrastructural technology to support

the interferometric measurement of thickness

variation and surface flatness in the free form or

chucked condition.

CUSTOMER NEEDS

Decreasing linewidths, and the resulting reduced

depth of focus, combined with larger wafer

diameters for current stepper lithography

applications place ever increasing restrictions on

flatness measurement and the required

measurement uncertainty for thin parallel

windows. For example, the International

Technology Roadmap for Semiconductors (ITRS)

suggests a flatness of less than 90 nm will be

required per die site for the 90 nm node (expected

by 2004). We are focused on meeting customer

requirements for calibrated thickness variation

maps of free form wafers and flatness

measurements of chucked wafers. We are

addressing the need for thickness variation maps

of 300 mm diameter wafers using the NIST

Improved Infrared Interferometer (IR3). There are

two reasons for concentrating on wafer thickness

variation. First, an independent traceable

measurement is required by instrument manufacturers to certify the performance of their

instruments. Second, thickness variation

measurements of silicon wafers can be combined

with models of wafer/chuck interactions to

determine the flatness of low surface area wafer

vacuum chucks, which is difficult to measure

directly. The surface flatness of chucked wafers

can be measured using NIST’s “eXtremely

accurate CALIBration InterferometeR”

(XCALIBIR). XCALIBIR has a 300 mm aperture

for flat measurements and provides a way of

verifying industry models of wafer/chuck

interactions.

TECHNICAL STRATEGY

IR2 is a prototype infrared interferometer built for

NIST by Tropel, Inc., based on a NIST patent

U.S. (5,739,906). The instrument can be used in

several configurations. The collimated wavefront

mode is the current focus of the project. In this

method, the planar infrared wavefront is normally

incident on the wafer. A portion of the beam is

reflected from the front wafer surface, while the

rest passes through the wafer and reflects from

the rear surface. The interference of these two

wavefronts produces fringes and, by wavelength

phase shifting, allows calculation of the wafer

thickness variation. See Fig. 1 below. This

measurement configuration is limited to double

side polished wafers.

Figure 1. Schematic for collimated wavefront wafer

thickness variation measurement.

1. There are two primary limitations to reaching

the desired measurement performance using the

current configuration of IR2. These are the

available measurement aperture and the optical

imaging system. Using the collimated wavefront

measurement technique, the existing measurement

aperture is 150 mm. Although this aperture allows

measurements to be performed and uncertainty

evaluation to proceed, it is not sufficient to reach

the end goal of thickness variation measurements

of 300 mm diameter wafers. Additionally, it has

been experimentally observed that due to largescale geometric distortions in the free form wafers

(e.g., bow or a drum shaped deflection), it is not

possible to obtain data over the full measurement

aperture. Instead, data dropout occurs because

light does not reach the detector from certain

portions of the wafer. This is due to: 1) a long

path length from the objective lens to zoom lens

in the imaging system; 2) non-uniform illumination

over the measurement aperture, which subtracts

from the available dynamic range of the detector;

and 3) an asymmetric design for the objective lens.

To address these needs, a collimator with 300 mm

clear aperture has been designed and built by
112 National Institute of Standards and Technology

Nu-Tek Corp. The infrared interferometer is

currently being upgraded and will be available

for the testing of 300 mm diameter wafers in the

2nd half of 2003.

DELIVERABLES: Complete upgrade to IR2 interferometer. 3Q 2003

2. A component in the evaluation of chucked

wafer non-flatness is the characterization of

interactions between the vacuum chuck and wafer.

We will be continuing collaboration with

Wavefront Sciences, Inc. (WFSI), Albuquerque,

NM 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. XCALIBIR, a general purpose

300 mm aperture phase measuring interferometer

developed at NIST, can now be used to measure

the flatness of chucked wafers. We will compare

our measurements with measurements made on

the WFSI optical measurement tool. The data can

then be used to evaluate the influence of wafer/

chuck interactions on the chucked wafer flatness.

DELIVERABLES: Make flatness measurements with

XCALIBIR of 300 mm diameter wafers to help

characterize interactions between vacuum chucks

and wafers. 3Q 2003

ACCOMPLISHMENTS

# A drift test was carried out using a 150 mm

aperture collimator that captured data from the

central portion of a 200 mm diameter, 750 μm thick,

double side polished wafer. The measured optical

path difference (OPD) was converted to thickness

variation in nm using an assumed, homogeneous

silicon index of 3.5. Total thickness variation (TTV

or GBIR), which includes both the constant wafer

thickness as well as local and/or linear variations

in thickness, was not measured here because the

piston, or constant, term was considered a setup

error and removed during data analysis. Piston

was removed because the OPD between the front

surface and back surface reflections varies with

changes in the angular orientation between the

wafer and collimated source.

# Average of repeatability testing

measurements. The average of 192 phase

measurements recorded at 15 minute intervals over

a two day period is shown in Fig. 2. A peak-tovalley (PV) thickness variation of 1663.0 nm and

root-mean-square (RMS) value of 414.4 nm were

recorded over the measured aperture. Clearly,

this wafer geometry is dominated by wedge, or a

linear variation in thickness across the wafer face

(the wafer is thinner at the top).

Figure 2. Average of repeatability testing measurements.

The (one sigma) pixel by pixel standard deviation

in the 192 measurements is also shown in Fig. 3.

This result, which was dominated by stray light

in the interferometer, suggests repeatability at the

6 nm level for this set of tests. Temperature

sensitivity was also measured; however, no strong

correlation between variation in the results and

temperature existed due to the high stability of

the measurement cavity (i.e., the wafer).

Figure 3. One sigma pixel by pixel standard deviation

of repeatability measurements.

# The result for a thickness variation measurement of a 100 mm diameter, 750 μm thick double

side polished wafer follows and shows a PV thickness variation of 204 nm, Fig. 4, where the thickness variation is manifested as ‘power’, or the

Zernike 0

a2 term, in the phase map. This large

power suggests a gradual thinning radially from

the outside edge to the center of the wafer.
Semiconductor Microelectronics and Nanoelectronics Programs 113

Figure 4. Thickness variation measurement for

100 mm diameter, 750 μm thick double side polished

wafer (piston and tilt removed). Map shows that

wafer is thinner in center by approximately 204 nm.

0

a2

0

a2

Surface reflection measurements of this wafer were

performed using a commercial phase measuring

interferometer in a Fizeau configuration. A front

surface measurement showed a slightly concave

shape with an Zernike coefficient of 4.437

waves (at a wavelength of 632.8 nm). A back surface measurement demonstrated a slightly convex shape with an value of 4.372 waves. The

sag, s, for each surface can be calculated as

. The larger sag for the

front (concave) surface suggests the wafer is thinner in the center, as shown by the IR2

 result. The

sag for the front surface measurement was 5615

nm and 5533 nm for the back surface. The difference of these two gives an estimate of the gross

thickness variation over the approximate measurement aperture of 30 mm (the smaller aperture was

required to obtain reasonable fringe density for

the bowed wafer and allow successful phase unwrapping). This difference is 82 nm.

# A diamond-turned vacuum chuck for 300 mm

diameter wafers was designed and machined at

NIST for chucked wafer flatness measurements

with the XCALIBIR interferometer.

COLLABORATIONS

Wavefront Sciences, Inc, T. D. Raymond; flatness

measurements of free form and chucked wafers on

XCALIBIR.

Virginia Semiconductor, Inc., Stephen Jones; thickness

variation measurements of ultra-flat wafers.

Komatsu Silicon America, Paul Langer; 300 mm diameter

wafer measurements.

RECENT PUBLICATIONS

Griesmann, U., Schmitz, T. L., Soons, J., Polvani, R.,

“New Optical Metrology at NIST for Semiconductor

Lithography,” ASPE Spring Topical Meeting “Progress

in Precision,” April 17-18, 2002, Raleigh, NC (in press).

Parks, R. E., Shao, L.-Z., Davies, A., Evans, C. J.,

“Haidinger Interferometer for Silicon Wafer TTV

Measurement,” Proceedings of SPIE, Vol. 4344,

Metrology, Inspection and Process Control for

Microlithography XV (2001), pp. 496-505.

Evans, C. J., Davies, A., Schmitz, T. L., Parks, R. E.,

Shao, L.-Z., “Interferometric metrology of substrates

for VLSI,” 2nd International Conference of the European

Society for Precision Engineering and Nanotechnology

– euspen, “Torino Incontra” Congress Centre, Torino,

Italy (2001).

Evans, C. J., Parks, R. E., Shao, L.-Z., Davies, A.,

“Interferometric Metrology of Photomask Blanks:

Approaches Using 633 nm Wavelength Illumination,”

NISTIR 6701, December 2000.
114 National Institute of Standards and Technology

MODELING, MEASUREMENTS, AND STANDARDS

FOR WAFER SURFACE INSPECTION

Technical Contacts:

T. A. Germer

G. W. Mulholland

”I would like to thank you

and NIST for the support

that you have provided to

VLSI Standards in the

sizing of polystyrene

latex spheres through the

work of Dr. George

Mulholland. We are very

pleased with the measurements that Dr.

Mulholland has performed, and with the

level of technical support

that the NIST staff has

provided to us. This

work is of technical and

economic importance to

the semiconductor

industry and to VLSI

Standards, because the

ability to correctly size

ever smaller particulate

contaminants on silicon

substrates is key to the

manufacturing yield of

silicon chips. We look

forward to a continuing

technical relationship

between VLSI Standards

and NIST.”

Marco Tortonese, Ph.D.

VLSI Standards, Inc.

GOALS

Provide industry with models, measurements,

and standards for particles and other defects in

order to improve the inspection of wafer surfaces.

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). 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 (ITRS) identifies the detection

and characterization of defects and particles on

wafers to be a potentially show-stopping barrier

to device miniaturization. The roadmap specifies

that by 2005, 40 nm particles must be detectable

on bare silicon, nonmetallic films, and metallic

films, while 100 nm particles must be detectable

on wafer backsides. Currently, no solutions to

this inspection problem exist. While the detection

sensitivity for defects must be increased, the

ability to characterize defects in terms of size,

shape, composition, etc., is critical for yieldlearning. Defects must be characterized

independent of defect location and topology.

With the need to detect smaller defects, the costs

of inspecting wafers are skyrocketing. For new

advances to be implemented in production

environments, improvements in sensitivity must

be achieved without suffering a tradeoff in

throughput and must be cost-effective. The drive

towards in situ sensors for production tools

requires techniques which can be effectively

miniaturized.

In order that wafer manufacturers and device

manufacturers have a common basis for

comparing specifications of particle

contamination, improved standards for particles

are needed. A recent comparison of the

measurements of calibration wafers by thirteen

different SSISs 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 %. By 2005, it

is anticipated that accurate calibration particles

as small as 30 nm will be needed.

By 2005, at the 80 nm node, particle having

diameters 40 nm must be detectable on bare

silicon, nonmetallic films, and metallic films,

while 100 nm must be detectable on the backsides

of wafers. No known solutions exist at this time.

[2002 ITRS, Yield Enhancement, Table 93a]

Figure 1. The Goniometric Optical Scatter Instrument is a state-of-the-art laser scattering facility.

TECHNICAL STRATEGY

There are two major strategies to 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-defined

size and material. Recent work by this group has

demonstrated that the polarization of light

scattered by particle contaminants, subsurface

defects, and microroughness has a unique
Semiconductor Microelectronics and Nanoelectronics Programs 115

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 finding 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 improvement 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

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 and developing

methods for generating other types of monosize

particles.

Specific project elements are defined below:

1. Polarized Light Scattering Measurements –

The Goniometric Optical Scatter Instrument

(GOSI), shown in Fig. 1, 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). We measure the

light scattering properties of well-characterized

samples exhibiting interfacial roughness,

deposited particles, subsurface defects, dielectric

layers, or patterns. The emphasis is on providing

accurate data, which can be used to guide the

development of light scattering instruments, and

to test theoretical models.

DELIVERABLES: Light scattering measurement of

diameter of a 60 nm sphere standard and an

associated uncertainty analysis. 3Q 2003

Demonstrate proof of principle light scattering-based

overlay measurement and assess uncertainty of the

measurement. 3Q 2003

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

efficiently 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, via the world-wide-web, a

version of the SCATMECH library of scattering codes

which contains a theory for scattering by axiallysymmetric particles on surfaces. 1Q 2004

Figure 2. The MHPOSI enables the scattering from

individual particles to be analyzed.

3. Instrument Development – A second

instrument, the Multidetector Hemispherical

Polarized Optical Scatter Instrument (MHPOSI)

(see Fig. 2), complements the capabilities of GOSI

as a prototype for a production-line light

scattering inspection tool. An instrument with

twenty-eight fixed detection elements covering

the scattering hemisphere, MHPOSI enables a

determination of the differential scattering cross

section, for individual particles or defects on a

wafer surface. This instrument can be configured

so that it is blind to interfacial roughness. Together

with an understanding of the light scattering

functions for different imperfections, MHPOSI

has a substantially improved capability for rapidly

detecting and identifying defects, particles, and

microroughness on wafers.

DELIVERABLES: Perform size distribution measurements of the 100 nm sphere standard (NIST SRM

1963) using the MHPOSI system. 3Q 2003

4. Size Distribution Measurements – Differential

mobility analysis (DMA) has been shown to be

capable of making accurate size measurements

for mean particle size for 100 nm monosize

polystyrene latex spheres (PSL). There are
116 National Institute of Standards and Technology

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: Development of particle sizing

calibration facility and documentation of measurement protocol and data analysis. 2Q 2003

Provide uncertainty estimates for moments of the

size distribution for PSL spheres based on DMA

measurements. 4Q 2003

5. Aerosol Generation – An aerosol must be

formed typically from a liquid spray of a particle

suspension before the particles can be sized by

the DMA or deposited on a wafer. Work is in

progress to use a variety of innovative methods

for generating, shaping the size distribution of

the aerosol, and depositing the particles. These

include the electrospray for generating particle

sizes smaller than 60 nm, impactor to remove the

large size fraction of the aerosol and to deposit

the particles, and an electrostatic chamber for

depositing small particle sizes. Work is also in

progress to generate metallic particles from

chemical precursors in a tube furnace.

DELIVERABLES: Recertify SRM 1963 for the size of

the primary sphere. 1Q 2003

Deposit of monosize 65 nm PSL spheres on silicon

wafers for use in light scattering measurements.

3Q 2003

Publication of manuscript on the generation and

sizing of PSL spheres as small as 30 nm using

electrospray. 4Q 2003

6. Resource on Particle Science – Over the past

five 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: Coordinate round robin to assess

the performance of particle deposition systems in

collaboration with the semiconductor industry.

3Q 2003

ACCOMPLISHMENTS

# In January 2001, A NIST-sponsored

workshop entitled “Issues Related to SSIS

Calibration with Polystyrene Spheres” brought

together suppliers of wafers, reference particles,

particle sizing and deposition equipment, and

wafer inspection instruments. This sets the stage

for developing a more responsive particle

program.

# 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 significant finding enables

improved wafer depositions by reducing the

number of contaminant residue particles, the

number of doublets, and the amount of residue

on the particles.

# 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.

Figure 3. Transmission electron microscope images

of 100 nm copper spheres generated by a novel spray

pyrolosis method. These particles are being developed to test light scattering theories for scattering by

particles on surfaces.
Semiconductor Microelectronics and Nanoelectronics Programs 117

# In collaboration with the University of Maryland, developed a method for generating pure

copper spheres (see Fig. 3), with diameters ranging

from 100 nm to 200 nm. These spheres, which

mimic real-world 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 finding establishes

the validity of the light scattering models for

roughness in a dielectric film, 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 film, helping to

establish the limits of validity of the scattering

theory.

# Developed the SCATMECH (see http://

physics.nist.gov/Divisions/Div844/facilities/

scatmech/html/) 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 to the end of January 2002, 647 copies of the

library have been downloaded from the web. In

July 2003, Version 4 will be released, which

contains an accurate theory for the scattering of

light by a spherical particle on a surface and

theories for scattering from roughness and small

defects in dielectric layers.

# 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 sample-tosample variations, which probably result from the

presence of doublet particles. Uncertainties in the

substrate optical properties, the thickness and

optical properties of the substrate oxide, and the

shape of the particle dominate the systematic

uncertainty.

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

George W. Mulholland, Da Ren Chen, and David Y. H.

Pui, “Comparison of the Size Distribution of Polystyrene

spheres Produced by Pneumatic and Electrospray

Nebulization,” in Characterization and Metrology for

ULSI Technology: 2000 International Conference, D.

G. Seiler, A. C. Diebold, T. J. Shaffner, R. McDonald, W.

M. Bullis, P. J. Smith, and E. M. Secula, eds., Proc. AIP

550, 322–326 (AIP, New York, 2001).

Jung Hyeun Kim, Sheryl H. Ehrman, George W.

Mulholland, and Thomas A. Germer, “Polarized Light

Scattering from Metallic Particles on Silicon Wafers,”

in Optical Metrology Roadmap for the Semiconductor,

Optical, and Data Storage Industries, A. Duparré and B.

Singh, Eds., Proc. SPIE 4449, 281–290 (2001).

Thomas A. Germer, “Polarized Light Scattering by

Microroughness and Small Defects in Dielectric Layers,”

J. Opt. Soc. Am. A 18(6), 1279–1288 (2001).

Thomas A. Germer, “Large Angle In-Plane Light

Scattering from Rough Surfaces: Comment,” Applied

Optics 40(31), 5708–5710 (2001).

Jung Hyeun Kim, Thomas A. Germer, George W.

Mulholland, and Sheryl Ehrman, “Size-Monodisperse

Metal Nanoparticles Via Hydrogen-Free Spray

Pyrolysis,” Advanced Materials 14(7), 518–521 (2002).

Thomas A. Germer, “Light Scattering from Slightly NonSpherical Particles on Surfaces,” Optics Letters, Opt.

Lett. 27, 1159–1161 (2002).

Junghyeun Kim, Sheryl H. Ehrman, George W.

Mulholland, Eric B. Steel, and Thomas A. Germer,

“Method for Making Spherical Pure Metal Micro and

Nanoscale Powders by Spray Pyrolysis,” United States

Patent filed January 26, 2002.
118 National Institute of Standards and Technology

Jung Hyeun Kim, George W. Mulholland, Sheryl H.

Ehrman, and Thomas A. Germer, “Polarized light

scattering from dielectric and metallic spheres on silicon

wafers,” Appl. Opt. 41 (25), 5405–5412 (2002).

Thomas A. Germer, George W. Mulholland, Jung Hyeun

Kim, and Sheryl H. Ehrman, “Measurement of the 100

nm NIST SRM® 1963 by laser surface light scattering,”

in Advanced Characterization Techniques for Optical,

Semiconductor, and Data Storage Components, A.

Duparré and B. Singh, Eds., Proc. SPIE 4779, 60–71

(2002).

Thomas A. Germer, “Measurement of lithographic

overlay by light scattering ellipsometry,” in Surface

Scattering and Diffraction for Advanced Metrology II,

Z.-H. Gu, and A. A. Maradudin, Eds., Proc. SPIE 4780,

72–79 (2002).

George W. Mulholland. and Michelle K. Donnelly,

“Particle Size Measurements for Spheres With Diameters

of 50 nm to 400 nm,” NISTIR 6935, National Institute

of Standards and Technology, Gaithersburg, November

2002.

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

George W. Mulholland, and Sheryl H. Ehrman, “CoSolvent Assisted Spray Pyrolysis for the Generation of

Metal Particles,” J. Materials Research, in press (2003).

Michelle K. Donnelly, George W. Mulholland, and

Michael R. Winchester, “NIST Calibration Facility for

Sizing Spheres Suspended in Liquids,” to be published in

Characterization and Metrology for ULSI Technology:

2000 International Conference, D. G. Seiler, A. C. Diebold,

T. J. Shaffner, R. McDonald, R. Khosla, S. Zollner, and

E. M. Secula, eds., Proc. AIP 550, 322–326 (AIP, New

York, 2001).

George W. Mulholland, Thomas A. Germer, and John C.

Stover, “Modelling, Measurement, and Standards for

Wafer Inspection,” GOMACTech 03 “Countering

Asymetric Threats,” Tampa, FL, March 2003.
Semiconductor Microelectronics and Nanoelectronics Programs 119

HIGH RESOLUTION MICROCALORIMETER

X-RAY SPECTROMETER FOR CHEMICAL ANALYSIS

Technical Contacts:

K. D. Irwin

G. C. Hilton

S. W. Deiker

S. W. Nam

C. D. Reintsema

J. N. Ullom

L. R. Vale

“[T]his type of resolution,

very simply, was something that I thought was

truly, truly a dramatic

advance. And I really

would like to encourage

the people working on

this at NIST and the

equipment industry to

get this into commercialization as soon as they

possibly can.”

Mark Melliar-Smith,

Former President and

Chief Executive Officer

of SEMATECH

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

near-unity quantum efficiency. 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 efficiency and

statistical quality of existing materials analysis

applications. Further, large-format arrays (up to

1,000 pixels) will make it possible to chemically

analyze smaller features and trace constituents,

and to track rapidly evolving x-ray spectra for inprocess 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)

amplifier. The temperature pulse height gives a

measurement of the energy of the x-ray photon

one to two orders of magnitude more sensitively

than Si-EDS. The detector is cooled to 0.1 K by a

compact adiabatic demagnetization refrigerator.

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 xray detector developed at NIST has been

identified as a primary means of realizing these

detector advances, which will greatly improve inline and off-line metrology tools that currently

use semiconductor energy-dispersive spectrometers (EDS). At present, these metrology tools

fail to provide fast and unambiguous analysis for

particles less than approximately 0.1 mm to 0.3

mm 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.

2001 International Technology Roadmap for

Semiconductors
120 National Institute of Standards and 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, Maryland to be used to study problems

of interest to the semiconductor industry.

TECHNICAL STRATEGY

1. The usefulness of single-pixel x-ray

microcalorimeter EDS in materials analysis has

now been well established in a variety of

demonstrations. Small arrays of x-ray

microcalorimeters are being tested, and the

development of technologies for large-format

arrays is in process. To meet the needs of the

semiconductor industry, it is necessary to make

both single-pixel and array microcalorimeter

systems more widely available. In addition to

microcalorimeters, the system requires novel

superconducting electronics to instrument the

detectors, compact adiabatic demagnetization

refrigerators to simplify cooling to milliKelvin

operating temperatures, and custom roomtemperature electronics and software to process

the output signals. Our goal is to develop new

generations of detector systems, to transfer them

to the Chemical Science and Technology Laboratory (CSTL), see Fig. 1 in NIST Gaithersburg,

Maryland for collaborative use in studying

problems of interest to the industry, and to work

with other partners in disseminating the

technology.

DELIVERABLES: Provide continued support and

upgrades for the single-pixel microcalorimeter

system transferred to CSTL. Prepare for the eventual

transfer of an array microcalorimeter system.

Collaborate with CSTL on using the microcalorimeter

to study problems of interest to the semiconductor

industry. 4Q 2003

2. For many semiconductor materials analysis

problems, further improvements in energy

resolution (beyond that already demonstrated

with these detectors) are not as important as an

increase in the maximum count rate and collection

area. Both the collection area and count rate can

be improved by the implementation of multipixel

arrays of detectors. Small arrays of x-ray

microcalorimeters are now being tested. This work

must be continued with a demonstration of the

addition of x-ray spectra from multiple pixels.

DELIVERABLES: Demonstrate the addition of x-ray

spectra from multiple pixels in a close-packed x-ray

microcalorimeter array to demonstrate the increase in

collection area and count rate achievable with

arrays. 4Q 2003

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: Fabricate an integrated solid-state

tunnel-junction refrigerator and microcalorimeter

x-ray detector that could be coupled to a simple 3

He

refrigerator. 4Q 2003

4. The multiplexed operation of 8 x-ray

microcalorimeters has now been demonstrated

using SQUID multiplexers operated at cryogenic

temperatures. A higher bandwidth system must

now be demonstrated to increase the number of

pixels that can be multiplexed and the maximum

count rate.

DELIVERABLES: Design and fabricated a highbandwidth SQUID multiplexer optimized for highcount-rate microcalorimeter EDS. 3Q 2003

5. Extensive custom room-temperature digital

electronics and firmware has been developed to

drive and provide feedback signals to SQUID

multiplexers, and to acquire data from multiplexed

x-ray microcalorimeters. Improvements are still

necessary to make a useful multiplexed array

system. Dedicated, multi-row address-line driver

boards to switch on multiplexed amplifiers must

be developed, and software must be developed

to analyze the data that is streamed to disk.
Semiconductor Microelectronics and Nanoelectronics Programs 121

DELIVERABLES: Develop dedicated custom

address-line driver electronics and software to

analyze multiplexed array data and create pulseheight spectra. 3Q 2003

ACCOMPLISHMENTS

# This year, we fabricated 8×8 arrays of

microcalorimeters on surface-micromachined

structures (see Fig. 2). These structures consist

of a silicon-nitride membrane freely suspended 2

microns above a solid silicon substrate, and

connected to it by silicon-nitride supports. This

is in contrast to the usual bulk micromachining

technique, in which a silicon nitride membrane is

deposited on a silicon wafer, and all silicon below

it is removed through the entire thickness of the

wafer. Single pixels from the array have been

shown to work. These arrays are now being tested

in a multiplexed SQUID readout system.

Figure 2. A 8x8 array of x-ray microcalorimeters on

suspended silicon nitride platforms created using

surface micromachining at NIST. Each microcalorimeter consists of a microcalorimeter and a 2-μm-thick

Bi absorber.

Figure 3. Microcalorimeter EDS spectrum of NIST

calibration glass K3670 taken by the microcalorimeter system transferred to CSTL in Gaithersburg,

Maryland.

Figure 4. Photograph of an 8x8 array of microcalorimeters. 8 channels of the array are read out in one

output channel with a SQUID multiplexer. The chip

on the left is the microcalorimeter array. The middle

chip is a filter chip. The chip on the right is the

SQUID multiplexer.

# We transferred a complete prototype

microcalorimeter EDS system from EEEL in NIST,

Boulder to CSTL in NIST, Gaithersburg. The

microcalorimeter system has been successfully

implemented on a CSTL analytical scanning

electron microscope in Gaithersburg. We are now

collaborating with CSTL researchers on

upgrading the system. It will eventually be

upgraded to a microcalorimeter array system (see

Fig. 3).

# We have demonstrated the multiplexed

readout of 8 microcalorimeter pixels from an 8×8

microcalorimeter array (see Fig. 4). All eight

channels were multiplexed and read out in one

output channel, and demultiplexed in software.

The energy resolution was measured for pulses

of energy injected in the microcalorimeters due to

Joule heating. No energy resolution degradation

was observed due to multiplexing. We are now

testing the multiplexed array with x-rays. We are

also designing faster SQUID multiplexers to allow

more pixels to be multiplexed in one channel.

# We have begun work on an on-chip solidstate refrigerator to cool x-ray microcalorimeters

from 300 mK to 100 mK (see Fig. 5). 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
122 National Institute of Standards and Technology

systems coupled to the solid-state refrigerator.

This device is a superconducting analog of a

Peltier cooler. Cooling is produced in the devices

by the tunneling of electrons through normalinsulator-superconductor junctions. We have

recently demonstrated cooling from 300 to 175

mK in devices whose cooling power is well

matched to microcalorimeter x-ray sensors. The

solid-state refrigerator is fabricated on a silicon

wafer using conventional thin-film and

photolithographic techniques, has no moving

parts, and operates continuously. A microrefrigerator will be integrated with an x-ray

microcalorimeter this year.

Figure 5. Micrograph of solid-state refrigerator.

Four normal-insulator-superconductor tunnel

junctions cool the central metal island. The island has

dimensions of 15 μm x 40 μm. NIS microrefrigerators

will be integrated with microcalorimeters this year.

Figure 6. The Al-Kα region of a microcalorimeter

EDS spectrum of the multielement NIST K3670 glass.

The acquired spectrum is shown as well as a weighted

least-squares fit of the Al Kα and satellite lines

convolved with a Gaussian instrument response,

yielding an energy resolution of 2.0 eV ± 0.1 eV

FWHM.

Figure 7. SEM photo (top) of an aluminum film

partially covering a sapphire substrate and a falsecolor 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.

# The NIST microcalorimeter EDS holds the

world record for energy resolution for an EDS xray detector of 2.0 eV at 1500 eV, which is over 30

times better than the best high resolution

semiconductor-based detectors currently

available. This energy resolution was measured

using a glass prepared by Dale Newbury of NIST

to use as a test standard for EDS.

# We created a chemical shift map showing the

chemical bonding state of Al in a sample

containing both Al and Al2

O3 (see Fig. 6). An

aluminum film 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

(see Fig. 6). The high energy resolution of the

microcalorimeter allowed the shift to be measured,

resulting in the false-color image in Fig. 7. The

map clearly demonstrates that microcalorimeter

EDS can be used to discriminate the chemical

bonding state using shifts in the positions of xray 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.
Semiconductor Microelectronics and Nanoelectronics Programs 123

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

Irwin, K. D., Beall, J. A., Deiker, S., Hilton, G. C., King,

L., Nam, S. W. , Newbury, D. E., Reintsema, C. D., Small,

J. A., and Vale, L. R., “The Development of

Microcalorimeter EDS Arrays,” Microsc. And

Microanal., Vol. 8, No. suppl. 2, pp. 78-79 (2002)

Irwin, K. D., “SQUID Multiplexers for Transition-Edge

Sensors” (review), Physica C, 368, 203 (2002).

Geer, R. E., Wu, D., Wollman, D. A., “High-Resolution

Energy-dispersive X-ray Spectroscopic Analysis of

Ultrathin Ion Diffusion Barriers Using Microcalorimetery,” Journ. Appl. Phys., 91, 1099 (2002).

Irwin, K. D., Vale, L. R., Bergren, N. E., Deiker, S.,

Grossman, E. N., Hilton, G. C., Nam, S. W., Reintsema,

C. D., and Rudman, D. A., “Time-Division SQUID

Multiplexers,” Proc. 9th International Workshop on LowTemperature Detectors, AIP Conf. Proc. 605, 301

(2002).

Geer, R. E., Wu, D., Wollman, D. A., “High-resolution

EDS Analysis of Ultra-Thin TaSiN Diffusion Barriers

for Cu Metallization Using Microcalorimetry,” Proc.

IEEE 2001 Int’l Interconnect Tech. Conf., 192-194

(2001).

Nam, S. W., Wollman, D. A., Newbury, D. E., Hilton, G.

C., Irwin, K. D., Rudman, D. A., Deiker, S., Bergren, N.

F., Martinis “Progress Towards Arrays of

Microcalorimeter X-ray Detectors,” J. M., Microscop.

& Microanal. 7 (Suppl. 2: Proc.), 1050-1051 (2001).

Deiker, S., Chervenak, J., Hilton, G. C., Irwin, K. D.,

Martinis, J. M., Nam, S., Wollman, D. A., “Transition

Edge Sensor Array Development,” IEEE Trans. Appl.

Supercond. 11, 469-472 (2001).

Hilton, G. C., Martinis, J. M., Irwin, K. D., Bergren, N.

F., Wollman, D. A., Huber, M. E., Nam, S. W.,

“Microfabricated Transition-Edge X-ray Detectors,”

IEEE Trans. Appl. Supercond. 11, 739-742 (2001).

Irwin, K. D., Hilton, G. C., Martinis, J. M., Deiker, S.,

Bergren, N. F., Nam, S. W., Rudman, D. A., Wollman D.

A., “A Mo-Cu Superconducting Transition-Edge

Microcalorimeter with 4.5 eV Energy Resolution at 6

keV,” Nuclear Instrum. and Methods in Physics Research

A 444, 184-187 (2000).

Wollman, D. A., Nam, S. W., Newbury, D. E., Hilton, G.

C., Irwin, K. D., Bergren, N. F., Deiker, S., Rudman, D.

A., Martinis, J. M., “Superconducting Transition-EdgeMicrocalorimeter X-ray Spectrometer with 2 eV Energy

Resolution at 1.5 keV,” Nucl. Instrum. and Methods in

Phys. Res. A 444, 145-150 (2000).

Wollman, D. A., Nam, S. W., Hilton, G. C., Irwin, K. D.,

Bergren, N. F., Rudman, D. A., Martinis, J. M., Newbury,

D. E., “Microcalorimeter Energy-Dispersive

Spectrometry Using a Low Voltage Scanning Electron

Microscope,” J. Microscopy 199, 37-44 (2000).

Nam, S., Wollman, D. A., Hilton, G. C., Chervenak, J.,

Deiker, S., Irwin, K. D., Martinis, J. M., “A Digital

Readout Scheme for Arrays of Microcalorimeter X-ray

Detectors,” Proc. Microscopy and Microanal. 2000,

742-743 (2000).

Wollman, D. A., Newbury, D. E., Nam. S. W., Hilton, G.

C., Irwin, K. D., Rudman, D. A., Deiker, S., Bergren, N.

F., Martinis, J. M., “Microcalorimeter EDS: Benefits

and Drawbacks,” Proc. Microscopy and Microanal. 2000,

738-739 (2000).

Chervenak, J. A., Grossman, E. N., Irwin, K. D., Martinis,

J. M,. Reintsema, C. D., Allen, C. A., Bergman, D. I.,

Moseley, S. H., Shafer, R., “Performance of multiplexed

SQUID readout for Cryogenic Sensor Arrays,” Nuclear

Instruments and Methods in Physics Research A 444,

107 (2000).

Stahle, C. K., McCammon, D., Irwin, K. D., “Quantum

Calorimetry,” Physics Today 52, 32 (1999).
124 National Institute of Standards and Technology

THERMOPHYSICAL PROPERTY DATA FOR MODELING

CVD PROCESSES AND FOR THE CALIBRATION

OF MASS FLOW CONTROLLERS

Technical Contacts:

John Hurly

Michael 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 Hannifin,

Central Engineering.

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 (CVD) and the

calibration of mass flow controllers (MFCs). As

data are acquired, they are promptly being posted

as part of an online database at http://

properties.nist.gov/semiprop (Fig. 1). The gases

and the properties to be studied were identified

by industry representatives. The gases include

process gases, “surrogate” gases used for

calibration, and binary mixtures of process and

carrier gases. The required properties include:

speed-of-sound, heat capacity, density (equation

of state), viscosity, and thermal conductivity.

Industry representatives have also recommended

targets for the accuracy of the data.

CUSTOMER NEEDS

 The Modeling and Simulations section of the

2001 International Technology Roadmap for

Semiconductors lists “Materials Modeling” as

first in a list of “Technology Requirements.” The

Roadmap states that “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 that

contain both experimental and, calculated . . .

are needed.” The Roadmap states that

continuing research is needed to obtain

experimental data for “transport and thermal

constants.” This project will generate transport

and thermodynamic property data for the gases

Figure 1. Sample Web Page from database located at the URL http://properties.nist.gov/semiprop/

Figure 2. Components of a generic mass flow

controller (MFC) and the thermophysical properties

required to model them.
Semiconductor Microelectronics and Nanoelectronics Programs 125

used in semiconductor processing. The data will

be useful for equipment modeling in CVD

processes and the data will also provide a rational

basis for the calibration of MFCs used to meter

process gases.

Figure 2 shows the components of a generic MFC

and the thermophysical properties required to

model their performance. During May 2000, a

workshop entitled “Mass Flow Measurement and

Control for the Semiconductor Industry” was

organized at NIST by Dr. Robert Berg. At the

workshop, representatives of industry identified

the gas properties and their allowable

uncertainties which are required for accurately

modeling MFCs and related equipment. The

workshop’s list of properties is: heat capacity at

constant pressure Cp(T) (±0.1 %), equation of

state ρ(T,p) for predicting gas densities (±0.1 %),

viscosity η(T) (±0.5 %), and thermal conductivity

κ(T) (±0.5 %). The workshop urged that values of

these properties be made available from a

‘standard’ source that is accessible to all of the

industries associated with semiconductor

processing.

TECHNICAL STRATEGY

In the first phase of the work, NIST is 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

standard uncertainties of 0.0001 × u. The initial

results range up to 200 °C and from 25 kPa to

1500 kPa (or to 80 % of the vapor pressure for

condensable gases). As an example, Figure 3

shows the phase diagram of trimethyl gallium.

Each triangle on Figure 3 indicates values of

u(T, p) where a speed-of-sound measurement was

made. The speed-of-sound data were used to

determine the idealgas heat-capacities Cp

0

(T) with

the targeted uncertainty of 0.001×Cp

0

. The

pressure and temperature-dependence of u(T, p)

were correlated with model two-body and threebody intermolecular potentials. These potentials

are used to calculate the virial equation of state

ρ(T, p) and to get first estimates of the viscosity

η(T) and the thermal conductivity κ(T). For gases

where reliable data exist, we verified that results

calculated in this way have uncertainties that are

less than 0.001×ρ, 0.1×η, and 0.1×κ from 200 K to

1000.

In parallel with measuring the speed of sound,

NIST is developing novel acoustic techniques to

measure the viscosity and thermal conductivity

with uncertainties of less than 0.5 % as specified

by the Mass Flow Controller Workshop.

Throughout the project, the results will be made

available to the customers through publications

in professional journals, presentations at

professional meetings, and via an on-line data

base accessible through the internet at http://

properties.nist.gov/semiprop.

DELIVERABLES: Design and fabricate a facility

capable of measuring the speed of sound in the

hazardous gases utilized in semiconductor processing. Completed 2Q 2000.

1. NIST has already developed techniques to

accurately measure the speed of sound in gases

and to determine the ideal-gas heat-capacity and

the equation of state from the u(T, p) data. NIST

used these techniques to determine the

thermodynamic properties of alternative refrigerants. Because the alternative refrigerants are

inert and non-hazardous by design, modifications

are needed to study the reactive, corrosive, and/

or toxic gases used in semiconductor processing.

These modifications address the issues of safety,

sample purity, sample disposal, and materials

compatibility.

DELIVERABLES: Develop and optimize novel

acoustic techniques for measuring the transport

properties of semiconductor process gases.

Viscometer designed, fabricated and optimized by

1Q 2002, thermal conductivity device designed

by 2Q 2002, fabricated by 4Q 2002, and optimized

by 2Q 2003.

Figure 3. Phase diagram of trimethyl gallium. The

vapor pressure is represented by the curved line

which ends at the critical point: Tc ≈ 510 K and

pc ≈ 4.4 MPa. At each triangle, the speed of sound

was measured.
126 National Institute of Standards and Technology

2. Because conventional measurements of the

transport properties of dilute, corrosive gases are

difficult, we are developing acoustical methods

of measurement. As suggested by M. Greenspan, the acoustical viscometer is a double

Helmholtz resonator. The viscosity is deduced

from the viscous damping of gas oscillating at

acoustic frequencies through a tube joining two

chambers. A second resonator will be developed

to obtain the thermal conductivity from

measurements of the thermal losses of similar

oscillations.

DELIVERABLES: Design and fabricate a facility

capable of safely measuring the transport properties

in the hazardous gases utilized in semiconductor

processing. Facility installed by 1Q FY2002,

viscometer installed in facility by 2Q FY2002,

automation software written and tested by

3Q FY2002, design and fabricate hazardous gas

viscometer by 3Q FY2002, install hazardous gas

viscometer by 4Q FY2002, design and fabricate

hazardous gas thermal conductivity device by

2Q FY2003.

3. A second facility to measure the transport

properties will have to be fabricated taking into

account the same issues of safety, sample purity,

sample disposal, and materials compatibility as

for the speed-of-sound facility.

Figure 4. Speed of sound in tungsten hexafluoride as

a function of pressure along isotherms.

DELIVERABLES: Measure the speed of sound in the

semiconductor process gases identified by the

customers. From the speed-of-sound measurements

determine the ideal-gas heat-capacity and equation of

state for each species. Nitrous oxide by 1Q 2002,

Nitric Oxide by 2Q 2002, Octafluoro-cyclobutane by

4Q 2002.

DELIVERABLES: Measure the transport properties in

the semiconductor process gases identified by the

customers. C2F4 by Q3 2002, CF4 by 4Q 2002, SF6 by

1Q 2003, NO by Q2 2003, NO2 by 3Q 2003.

4. Once the two facilities have been fabricated,

calibrated and safety assessments performed the

actual measurements in the semiconductor

process gases identified by the customers will be

performed.

Figure 5. Percent deviation of viscosity measurements

made in the Greenspan viscometer from the reference

values in several gases.
Semiconductor Microelectronics and Nanoelectronics Programs 127

DELIVERABLES: Disseminate the resulting measurements to the customer. Update on-line database by

4Q 2002, publish measurements in Cl2

 by 1Q 2002,

publish measurements in Ga(CH3

)

3

, C2H4O, and NF3 by

3Q 2002, publish measurements in NO and NO2 

by

1Q 2003.

5. The measurements will be disseminated to the

customer through papers in professional journals,

talks given at professional meetings, and on an

on-line database available via the internet.

ACCOMPLISHMENTS

# A facility was built to safely measure the

speed of sound in semiconductor process gases.

The apparatus was calibrated with argon. The

speed of sound was measured in the surrogate

gases CF4, C2F6 and SF6.

# Computer programs were developed for

correlating speed-of-sound data with model,

hard-core Lennard-Jones intermolecular

potentials. Programs were developed to calculate

second and third virial coefficients and transport

properties from the model intermolecular

potentials.

# The speed of sound was measured in the

process gases Cl2

, HBr, BCl3, WF6, Ga(CH3)

3

, NF3,

C2H4O, NO, and N2O throughout the temperature

and pressure ranges listed in Table 1. Figure 4

shows a fraction of the results for WF6. Typically,

the standard uncertainty of the speed of sound

was less than 0.01 %. The ideal-gas heat-capacity

was determined to within 0.1 % from the zeropressure intercept of each isotherm. The slope

and curvature of each isotherm provided

information about each gas’s non-ideality from

which we developed an equation of state to predict

the gas’s densities to within 0.1 %. This year we

will complete measurements in C4

H6 and

octafluoro-cyclobutane.

Figure 6. Monel Greenspan acoustic viscometer for

use with corrosive process gases.

# The Greenspan acoustic viscometer was

developed to measure the viscosity of process

gases. Several viscometer geometries and

acoustic models were tested to optimize the

viscometer’s performance. The performance of

the acoustic viscometer was tested by comparing

the acoustic results for several gases with

reference data from the literature. Figure 5 shows

that the data from the acoustic viscometer are

within 0.5 % of the from reference values. We

have measured the viscosity in the three surrogate

gases CF4, C2F6 and SF6 as well as the hazardous

semiconductor gases N2O and NF3.

# A second generation Greenspan viscometer

has been designed and will be assembled this

year. The new version is being constructed out

of Monel which will allow the study of the

corrosive process gases. Figure 6 shows the parts

of the new resonator before their final assembly.

# A data base available on the internet at the

URL http://properties.nist.gov/semiprop/ has

been developed (Figure 1). This data base is

updated regularly to give our customers

immediate access to our results.

RECENT PUBLICATIONS

Hurly, J. J., “Thermophysical Properties of Gaseous CF4

and C2F6 from Speed-of-Sound Measurements,”

International Journal of Thermophysics, Vol 20, No 2,

455-484, September 1999.

Hurly, J. J., and Moldover, M. R., “Thermophysical

Properties of Process Gases,” Proceedings of SEMICON

West 1999.

Hurly, J. J., “Thermophysical Properties of Gaseous HBr

and BCl 3

 from Speed-of-Sound Measurements,”

International Journal of Thermophysics, Vol.21, No. 4,

805-829, July 2000.

Hurly, J. J., “Thermophysical Properties of Gaseous

Tungsten Hexafluoride From Speed-of-Sound

Measurements,” International Journal of Thermophysics,

Vol. 21, No. 1, 185-206, January 2000.

Hurly, J. J., “A Progress Report: Thermophysical

Properties of Semiconductor Manufacturing Process

Gases,” Proceedings of SEMICON West 2000.

Hurly, J. J., “The Thermodynamic Properties of

Chlorine From Speed-of-Sound Measurements,” Vol. 23,

No. 3, 667-695, May 2002.

Hurly, J. J., “Thermophysical Properties of Nitrogen

Trifluoride, Ethylene Oxide, and Trimethyl Gallium from

Speed-of-Sound Measurements,” Vol. 23, No. 2, 455-

475, March 2002.

Moldover, M. R., Gillis, K. A., Hurly, J. J., Mehl, J. B.,

and Wilhelm, J., “Chapter 10. Acoustic measurements

in gases.”, in Modern Acoustical Techniques for the

Measurement of Mechanical Properties, edited by Moises
128 National Institute of Standards and Technology

Levy and Henry E. Bass, pp 377-427 (Academic Press,

San Diego, USA, 2001); also “Chapter 12: Acoustic

measurements in gases: Applications to thermodynamic

properties, transport properties, and the temperature

scale”, Vol IV, in Handbook of Elastic Properties of

Solids, Fluids, and Gases, edited by M. Levy, H. Bass, and

R. Stern, pp 329-370 (Academic Press, San Diego, USA,

2000).

Wilhelm, J., Gillis, K. A., Mehl, J. B. and Moldover, M.

R., “An improved Greenspan viscometer,” Int. J.

Thermophysics, Int. J. Thermophysics, Vol 21, pp. 983-

997 (2000).

LIST OF TALKS/PRESENTATIONS

Hurly, J. J., “Progress Report: Thermophysical Properties of Semiconductor Manufacturing Process Gases,”

delivered to the Gas Distribution Systems Working

Group, Sunday July 15, at the SEMICON west 2000

conference in San Francisco, CA.

Hurly, J. J., “Thermophysical Properties of Semiconductor Process Gases,” delivered to the European

SEMI Technical Gas Committee, September 21, 2000,

at Oostende Belgium.

Hurly, J. J., “Thermophysical Properties of Semiconductor Process Gases Determined with Acoustic

Techniques,” delivered at the Fourteenth International

Symposium on Thermophysical Properties, June 28,

2000.
Semiconductor Microelectronics and Nanoelectronics Programs 129

TEMPERATURE MEASUREMENTS AND

STANDARDS FOR RAPID THERMAL PROCESSING

Technical Contact:

K. G. Kreider

D. P. DeWitt

B. K. Tsai

GOALS

The goal is to develop the technologies required

to enable the measurement of Rapid Thermal

Processing (RTP) wafer absolute temperatures

with uncertainties of 2 °C at 1000 °C as prescribed

in the International Technology Roadmap for

Semiconductor (ITRS).

Our project, initiated in FY97, has approached this

goal with four objectives: (1) To improve

calibration wafer technology to a 1 °C standard

uncertainty by demonstrating the use of thin-film

thermocouples (TFTCs) in conjunction with wire

thermocouples (TCs) on test wafers, in Fig. 1, RTP

tools; (2) To develop methods for in-tool radiation

thermometer (RT) calibration, which relate TFTC

wafer temperatures to indicated radiance

temperatures; (3) To develop and validate models

to account for wafer emissivity and the effects of

chamber reflected-irradiation on temperatures

determined from model-corrected RTs that are

calibrated against blackbodies; and (4) To

collaborate with the semiconductor industry in

implementing new methods for reliable and

traceable temperature measurements.

Figure 1. NIST thin-film thermocouple calibration

wafer used in lightpipe proximity evaluations after

41 thermal cycles above 800 °C.

CUSTOMER NEEDS

The measurement needs of the semiconductor

manufacturing industry have been stated in the

ITRS. The requirement is for measurement and

control of RTP tools to ± 2 °C at 1000 °C during

processing with calibrations traceable to the

International Temperature Scale of 1990 (ITS-90).

In the 2001 edition of the ITRS in the section on

“Metrology Difficult Challenges” the roadmap

states “Better sensors must be developed for . . .

wafer temperature measurement during RTA.”

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 (20 companies meeting annually

at NIST since 1997). They serve as a bridge

between research and practice, provide advice

on shaping objectives, and generate opportunities

for technology transfer. ISMT has established a

joint effort at UT-Austin, with NIST and several

instrument suppliers, to develop a facility to

evaluate and validate accuracy claims of

commercial thermometry systems. In addition

Applied Materials and Atmel are evaluating the

NIST test wafer.

TECHNICAL STRATEGY

Our strategy is to address three core elements of

our research that will enable the semiconductor

industry to meet the road map requirement: (a)

fabrication of test wafers with improved thin-film

technology for use by our industrial collaborators

to demonstrate in-tool calibration of RTs traceable

to the ITS-90; (b) experimentation on the NIST

test bed and thermal modeling to determine the

effects of wafer emissivity and lightpipe proximity

on in-tool calibration of lightpipe radiation

thermometers (LPRTs); and, (c) calibration and

characterization of LPRTs.

The present scope of the TFTC technology work

includes designing, fabricating, and testing thinfilm thermocouple calibration wafers for use in

industrial RTP tools, defining the effects of various

wafer and film emissivities as well as sensor

arrangement on the temperature measurement,

and establishing the uncertainty of the temperature measurements using the NIST calibration

wafer.

We are using the NIST Test Bed to perform intercomparisons between the TFTCs and LPRTs. The

aims are: (a) to demonstrate calibration procedures

for and establish uncertainties of the LPRTs

against the TFTCs, and (b) to establish uncer-
130 National Institute of Standards and Technology

tainties for model-corrected LPRTs calibrated

against blackbodies. Experimental studies are

being designed for these conditions: a range of

wafer and film emissivities, variable separation

distance between wafer and cold shield, different

distances between the LPRT and wafer, and

variable chamber wall reflectance. These studies

require concurrent efforts to develop and validate

radiation heat transfer models to estimate wafer

effective emissivities that are essential for

establishing uncertainty limits for LPRTs in our

test bed, and in production tools.

DELIVERABLES: Evaluation of wafer emissivity and

paper on LPRT proximity effects on wafer temperatures. 2Q 2003

We are using industrial RTP test beds to establish

the usefulness of the NIST calibration wafer in

the semiconductor processing industry and to

define the benefits of using the NIST calibration

wafer under various industrial conditions.

A cooperative project with ISMT and the

University of Texas at Austin (UT-Austin) uses

their RTP test bed, which has very high uniformity

in wafer temperatures and can be used to confirm

the uncertainties and repeatability of the NIST

calibration wafer. Their test bed also is used to

compare the NIST wafer with commercial

thermocouple test wafers and industrial lightpipe

radiation thermometers.

DELIVERABLES: NIST TFTC calibration wafers for

ISMT test bed and joint report with UT-Austin on LPRT

calibrations. 2Q 2003

Cooperative projects with Applied Materials and

Atmel industries are permitting investigations of

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

thin-film thermocouple calibration wafer has

demonstrated unique capabilities in temperature

measurements and in establishing traceability to

the international temperature scale (ITS-90).

DELIVERABLES: NIST TFTC calibration wafers for

RTP temperature measurements and joint report with

Applied Materials and Atmel on LPRT calibrations.

3Q 2003

Common Interest Group (CIG) meetings have

been held annually since 1997 for the purposes

of assessing and planning project research

directions, and for fostering collaborative work

with equipment suppliers, chip makers, and

instrumentation suppliers. At the 10th International Conference on Advanced Thermal

Processing of Semiconductors (RTP’02, Sept. 25-

27, 2002, Vancouver, Canada) the meeting

addressed two major themes: reports by our

industrial collaborators on test wafer demonstrations and discussions on proposals for

establishing emissivity-standard wafers. At

RTP’03 (Sept. 24-26, 2003, Charleston, South

Carolina), our team will organize two regular

conference sessions under the theme of

traceability of RTP temperature measurements.

Papers will be presented on TC and RT

calibrations methods and thermal modeling. A

panel will discuss plans for an industry-wide

emissivity standards initiative. An important

contribution by NIST to the initiative is the use of

the high temperature properties measurement

facilities to generate a reliable, traceable data base

and to validate optical properties models.

DELIVERABLES: Organize and conduct CommonInterest Group meeting on traceable temperature

measurements at key industry RTP conference.

4Q 2003.

ACCOMPLISHMENTS

Emissivity Effects

We investigated the effect of different silicon

wafer emissivities and the effect of low emissivity

films on RTP wafer temperature measurements

using LPRTs. These tests were performed in the

NIST RTP Test Bed. We used a NIST TFTC

calibration wafer to calibrate the LPRTs in situ.

The measurements of LPRTs viewing Au and Pt

thin-film spots in the center of the wafer were

compared to LPRT radiance temperature readings

that viewed silicon surface with thermal oxide film..

We found differences of up to 36 °C at 900 °C in

the LPRT measurements due to the low emissivity

films. A model of the wafer temperature

measurement was presented to provide insight

into the effects of wafer emissivity on LPRT

measurements in RTP tools.

Thermal Modeling: Emissivity Effects

A thermal model of the wafer and cold shield

enclosure has been developed and employed to

predict the effects of NIST Test Bed-chamber

features on LPRT measurements. The thermal

response of the wafer was predicted for different

thin-film spot, wafer emissivity, and shield

reflectivity. The thin-film spot resulted in a

temperature rise at the center of the wafer and its

magnitude was strongly dependent on the shield

reflectivity and less dependent on the wafer
Semiconductor Microelectronics and Nanoelectronics Programs 131

emissivity. The effects of the lightpipe sensor on

the temperature distribution of the wafer were also

investigated. Due to its low reflectivity compared

with the cold shield, the lightpipe sensor caused

a temperature decrease at its viewing area, which

is a strong function of the shield reflectivity.

Lightpipe Proximity

Our Common Interest Group indicated the need

for a better understanding of the lightpipe

radiation thermometer (LPRT) location on wafer

temperature measurement. The lightpipe is a

sapphire rod of 2 mm diameter surrounded by a

4 mm diameter sapphire sheath and has a very

low reflectivity (0.1) compared to the surrounding

reflecting plates of the cavity facing the wafer.

Shown in the accompanying schematic, Fig. 2, is

the wafer-cold shield enclosure with the light pipe

at a proximity distance p. The effect of the light

pipe is to cause a temperature depression over

the wafer, which can significantly influence the

temperature indicated by the radiation

thermometer.

Figure 2. Schematic of wafer-cold shield enclosure

with lightpipe at proximity distance p and temperature

depression due to lightpipe proximity from model

above.

We designed an experimental and analytical study

to quantify the lightpipe proximity effect and

provide a model for industrial users of LPRTs to

correct their LPRT readings. The experiments

employed thin-film thermocouple wafers, our RTP

test bed, and LPRT readings to characterize the

effects of lightpipe proximity on wafer

temperatures. We used various distances

between the wafer and the reflecting plate and

various distances between the wafer and the tip

of the lightpipe to quantify the wafer temperature

depression.

Figure 3 shows the comparison of the temperature

depression caused by various wafer to lightpipe

distances, p, with the reflecting plate to wafer

distance of s = 10 mm. Dashed and solid lines in

the figure represent the model’s predictions at

6 mm and 3 mm. This temperature depression is

compared to the measurement made with the

lightpipe flush with the reflecting plate. It can be

seen that the wafer temperature can be depressed

more than 20 °C by positioning the lightpipe tip

too close to the wafer. These results were

presented in a paper for the 2003 International

Conference on Characterization and Metrology

for ULSI Technology on March 27. An expanded

paper on the same subject will be presented at

the RTP conference in September.

Figure 3. Change in central radiance temperature

due to the proximity of center lightpipe to the wafer.

Lightpipe Characterization

Our objective was to investigate improved

procedures for characterization and calibration of

sapphire lightpipes using uniform blackbody

sources. We evaluated spectral, spatial, and

temporal characterizations of LPRTs to obtain

critical information on effective wavelength and

field of view, as well as stability information

required for making accurate temperature

measurements and uncertainty assessments. We

reported guidelines for making qualitative and

quantitative inspections of the sapphire

lightpipes. Proper inspections of the lightpipes

can assist the user in identifying lightpipes that

will have poor performance before using them in

RTP tools. We also compared “cold” calibrations

done in less than 5 s and “hot” calibrations taking

about 30 min and discovered that the difference

between “cold” and “hot” calibrations can be as

much as 2.5 °C in some situations. We offered

caution for utilizing information received in factory
132 National Institute of Standards and Technology

calibration reports. A comparison of three vendors

showed that the differences between the factory

calibration and the NIST calibration were as much

as 7.6 °C. We disseminated a list of recommendations for the lightpipe user to assist them

in performing more useful characterizations and

accurate calibrations.

RECENT PUBLICATIONS

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”

in Characterization and Metrology for ULSI Technology,

D. Seiler et al, Ed., March 2003, AIP, NY, in press.

K. G. Kreider, W. A. Kimes, C. W. Myer, D. C. Ripple, B.

K. Tsai, D. C. Chen, and D. P. DeWitt, “Temperature

Measurements and Standards for Rapid Thermal

Processing”, in Proceedings of 8th Symposium on

Temperature: Its Measurement and Control in Science

and Industry, D. C. Ripple et ed., October 2002, AIP,

NY, NY, in press.

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,” in

Proceedings of 8th Symposium on Temperature: Its

Measurement and Control In Science and Industry, D.

C. Ripple et al., Eds, October 2002, AIP, NY, in press.

B. K. Tsai and D. P. DeWitt, “Characterization and

Calibration of Lightpipe Radiation Thermometers for

use in Rapid Thermal Processing,” in Proceedings of 8th

Symposium on Temperature: Its Measurement and

Control In Science and Industry, D. C. Ripple et al., Eds,

October 2002, AIP, NY, in press.

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”. Presented at

the Rapid Thermal and other Short-time Processing

Technology Symposium at the 201st Electrochemical

Society Meeting, Philadelphia, PA, May 15, 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,” in 10th

IEEE International Conference on Advanced Thermal

Processing of Semiconductors-RTP ’02, J. Gelpey, et al,

Eds., Sept. 25-27, 2002, Vancouver, Canada

K. G. Kreider, D. W. Allen, D. H. Chen, D. P. DeWitt, C.

W. Meyer, and B. K. Tsai, “Effects of Wafer Emissivity

on Light Pipe Radiometry in RTP Tools,” Ninth

International Conference on Advanced Thermal

Processing of Semiconductors - RTP’01, pp 163-168,

September 2001, Anchorage, AK.

Y. H. Zhou, and Z. M. Zhang, (University of Florida,

Gainesville, FL), D. P. DeWitt and B. K. Tsai, “Effects

of Radiative Property of Surfaces on Radiometric

Temperature Measurement,” Ninth International

Conference on Advanced Thermal Processing of

Semiconductors - RTP’01, pp 179-188, September

2001, Anchorage, AK.

C. W. Meyer, “Effects of Extraneous Radiation on the

Performance of Light Pipe Radiation Thermometers,”

TEMPMEKO 2001, The Eighth Symposium on

Temperature and Thermal Measurements in Science and

Industry, pp 937, 942, June 19-21, 2001, Berlin,

Germany.

B. K Tsai, and D. P. DeWitt, “Methods Used at NIST to

Characterize and Calibrate Lightpipe Radiation

Thermometers,” TEMPMEKO 2001, The Eighth

Symposium on Temperature and Thermal Measurements

in Science and Industry, pp 931- 936, June 19-21, 2001

Berlin, Germany.

Y. H. Zhou, Y. J. Shen, and Z. M. Zhang (University of

Florida), B. K. Tsai and D. P. DeWitt, “Effect of

Directional Properties on the Radiometric Temperature

Measurement in Rapid Thermal Processing,” Eighth

International Conference on Advanced Thermal

Processing of Semiconductors - RTP’00, pp 94-103,

September, 2000, Gaithersburg, MD.

B. K. Tsai, C. W. Meyer, and F. J. Lovas, “Calibration

and Characterization of Light-Pipe Radiation

Thermometers for the NIST RTP Test Bed,” Eighth

International Conference on Advanced Thermal

Processing of Semiconductors - RTP’00, pp 83-93,

September 2000, Gaithersburg, MD.

Y. H. Zhou, Y. J. Shen, Z. M. Zhang, B. K. Tsai, and D.

P. DeWitt, “Monte Carlo Simulation for Radiometric

Temperature Measurement in Rapid Thermal

Processing,” Proceedings of the ASME Heat Transfer

Division - 2000, J. H. Kim et al., Eds., HTD-Vol 366-5,

pp 187-190 (2000).

K. G. Kreider, D. P. DeWitt, C. W. Meyer, and V. P.

Scheuerman, “Calibration of Light-Pipe Radiation

Thermometers in an RTP Tool at 1000 °C with an

uncertainty of 2 °C,” Eighth International Conference

on Advanced Thermal Processing of Semiconductors -

RTP’00, pp 64-70, September 2000, Gaithersburg, MD.

K. G. Kreider and G. J. Gillen, “High Temperature

Materials for Thin-film Thermocouples on Silicon

Wafers,” Thin Solid Films, 376 (2000) 32-37.
Semiconductor Microelectronics and Nanoelectronics Programs 133

LOW CONCENTRATION HUMIDITY STANDARDS

Technical Contacts:

J. T. Hodges

G. E. Scace

“The LFPG is the ‘Gold’

standard for moisture

generation and after the

round robin experiments

it is possible for the

industry to talk about

moisture measurements

that can be compared to

the standard.”

Suhas Ketkar,

Air Products and Chemicals

GOALS

The primary objective is to establish quantitative

standards enabling the accurate measurement of

trace quantities of water vapor (< 1013 molecules

cm-3). This effort supports the development and

application of commercial humidity sensors used

for gas purity measurements and inline monitoring

and process control – functions that are relevant

to minimizing wafer misprocessing.

CUSTOMER NEEDS

As discussed in the 2001 International

Technology Roadmap for Semiconductors (ITRS)

in the chapter entitled Metrology, the evolution

of sensor-based metrology for integrated

manufacturing requires the development of insitu sensors enabling in-time measurements. In

Table 96 entitled Metrology Difficult Challenges,

the need for robust and accurate sensor

technology and impurity detection in starting

materials is highlighted. Of the known impurities

in processing gases, water vapor is one of the

most ubiquitous and difficult to eliminate. Thus

its measurement and control is often critical to

various semiconductor-related processes.

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 first-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.

Figure 1. NIST Low Frost-Point Humidity Generator.

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, 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

Figure 2. Steady state response of saturator control

thermometers in NIST Low Frost-Point Humidity

Generator.
134 National Institute of Standards and Technology

ideally suited for testing the performance of

various sensing and humidity generation

technologies. To date, it is has been used to

characterize water vapor measurement and

generation systems at the research and

development stage as well as commercial devices.

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 flow rate. We recently

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 efficient and low-cost mechanism

for the dissemination of NIST trace humidity

standards. To meaningfully characterize the longterm stability of this system and its combined

uncertainty, measurements of the system

performance must be taken over an extended time

frame.

DELIVERABLES: Complete system testing and

uncertainty analysis of permeation-tube calibration

system. 3Q 2003

2. The basis for the LFPG as a humidity standard

is knowledge of the temperature-dependent ice

vapor pressure. The most commonly used

correlation is that developed by Wexler of NBS.

This correlation is generally considered valid to

a minimum temperature of -100 °C. Recent

comparisons in our laboratory with independent

humidity generation techniques suggest that the

actual ice vapor pressure may differ from that

predicted by the Wexler correlation by an amount

exceeding the expected uncertainty. Further, to

use the LFPG at temperatures below -100 °C

requires extrapolation of this correlation, with the

risk of large uncertainties. We expect to quantify

important deviations from the Wexler ice-vapor

pressure correlation, and extend its useful range

to -110 °C.

DELIVERABLES: Refine and extend the temperaturedependent ice vapor pressure correlation. 1Q 2004

3. 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, and knowledge of the ice vapor

pressure discussed above. A new strategy for

pmol mol-1 -level humidity generation has been

identified. The approach involves the controlled

dilution of water vapor/gas mixtures produced by

the LFPG, using a flow dilution system similar to

that incorporated within the PTG calibration system

described above.

DELIVERABLES: Extend the LFPG to pmol mol-1 levels

of humidity generation. 2Q 2004

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 rovibronic

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 strengths. 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 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. CRDS

is a cavity-enhanced optical absorption technique

that has high sensitivity, fast response, and probes

a compact well-defined 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 refined

version of CRDS called frequency-stabilized

single-mode cavity ring-down spectroscopy

(FSSM-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. See Figure 3 below.
Semiconductor Microelectronics and Nanoelectronics Programs 135

Using the existing FSSM-CRDS apparatus,

appropriate transfer standard generators and

hygrometers, we will link H2

O transition line

strengths to thermodynamic-based LFPG. Taking

advantage of the high spectral resolution afforded

by FSSM-CRDS, pressure-broadening of these

transition line shapes by various media will also

be quantified.

DELIVERABLES: Link H2O transition line strengths to

LFPG for water vapor concentration measurements

in the range 10 nmol mo1-1 to 100 μmol mo1-1 and

measure N2 pressure broadening coefficients.

4Q 2003

ACCOMPLISHMENTS

# In FY 2003 we constructed a permeation tube

calibration facility (see Fig. 4 above). 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 highsensitivity quartz crystal microbalance (QCM).

The water-containing permeation tubes to be

calibrated are placed inside the temperaturestabilized PTG oven. Water vapor diffuses from

the tube surface into a precisely controlled stream

of purified N2. The diluent gas flow 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. The calibration facility is

now operational and being tested to assess its

combined uncertainty.

Figure 3. Frequency-stabilized CRDS system. Figure 4. New PTG calibration apparatus.

# A new strategy for pmol mol–1 (ppt) -level

humidity generation was recently identified. The

approach, shown in Fig. 5, involves the controlled

dilution of water vapor/gas mixtures produced by

the LFPG, using a flow dilution system similar to

that incorporated within the PTG calibration

system described above.

Figure 5. Proposed dilution system for extending the

LFPG operating range to pmol mol-1 levels.

# The LFPG uncertainty analysis is based on

the uncertainties in temperature and pressure

within the LFPG saturator, ice vapor pressure and

the enhancement factor for mixtures of water vapor

and air. However, this analysis neglects background effects associated with the transient

adsorption and desorption of water vapor from

internal surfaces in the flow manifold located

downstream of the LFPG. For the lowest range

considered, such processes may affect

significantly the water vapor concentration in the

sample gas delivered by the LFPG to test instrumentation. In collaboration with Air Products Inc.,

we quantified the magnitude of this water vapor

background using atmospheric pressure

ionization mass spectrometry (APIMS). Results,

shown in Fig. 6, indicate that background

contribution to H2

O from system components
136 National Institute of Standards and Technology

downstream of the LFPG was less than 0.2 nmol/

mol. These measurements also demonstrated that

linearity deviations of the LFPG output H2

O mole

fraction are less than 0.1 nmol/mol.

Figure 6. APIMS measurement of LFPG background

H2O concentration showing decay to steady state

level.

# We developed a quantitative method for

comparing the outputs of commercial and custom

permeation tube moisture generators (PTGs) to

that of the LFPG. This technique is capable of

resolving fractional differences of approximately

1 %, for PTGs covering the mole fraction range 10

to 100 nmol/mol. An intercomparison of PTG

devices from several customers was completed,

and results comparing two representative PTGs

to the LFPG are summarized in Fig. 7. Uncertainties in flow metering and background water

vapor present in the carrier gas were identified as

limiting effects. This technique was the basis for

newly developed permeation-tube calibration

service discussed above.

Figure 7. Comparison of results for PTG A (circles)

and PTG B (triangles). The dashed curves represent

the respective average percent differences between the

PTG and the LFPG. Error bars represent 1 standard

deviation.

# We measured trace levels of water vapor

generated by the LFPG using a prototype diode

laser hygrometer (DLH). This hygrometer, which

was based upon wavelength modulation

spectroscopy, had a linearity better than 1 % over

the water vapor mole fraction range 3 nmol/mol to

2 μmol/mol and yielded a sensitivity of better than

0.5 nmol/mol when tested against the LFPG. These

experiments and subsequent tests on a beta

system were critical to the development of a similar

DLH that is now commercially available.

# We recently completed the development of a

FSSM-CRDS system, based on a near-infrared

continuous wave diode laser emitting near 936 nm.

The gas sampling system is optimized for highprecision measurements of trace water vapor concentration. The flow system has all-metal seals,

low dead volume, and active mass flow rate and

pressure regulation. Background levels < than

0.5 nmol mol-1 (background equivalent) have been

demonstrated. A FSSM-CRDS spectrum of atmospheric H2

O is shown in Fig. 8, corresponding to a

mole fraction of approximately 8 μmol mol-1 and

concentration of 2.7x1013 molecules cm-3. The spectral resolution of the FSSM-CRDS spectrum is

approximately 1 MHz, and permits high precision

line shape determinations that are not instrumentlimited. To illustrate, systematic deviations from

commonly used Voigt line shapes are observable

with this system, suggesting its utility for fundamental studies of collision-induced line shape

phenomena. Other results indicate that the integrated CRDS spectra scale linearly with H2

O concentration, and transition line strengths can be

reproducibly determined to within 0.5 %. 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 accessible with

the near-ir diode laser used in this system. Finally,

a replica of the FSSM-CRDS system is now used

to characterize the purity of semiconductor source

gases in a molecular beam epitaxy (MBE) facility

at NIST-Boulder.
Semiconductor Microelectronics and Nanoelectronics Programs 137

COLLABORATIONS

Air Products and Chemicals Inc., Seksan Dheandhanoo;

APIMS measurements of trace moisture and

characterization of semiconductor gas purity.

NIST Optoelectronics Division, Kris A. Bertness; CRDS

measurements of H2

O in III-V compound process gases.

Dow Chemical, Linh Le and J. D. Tate; CRDS

measurements for process gas control RH Systems,

Robert Hardy; Characterization of low range chilledmirror hygrometers.

Southwest Sciences Inc, Chris Hovde.; Development of

wavelength modulation laser hygrometer for trace H2

O

sensing.

RECENT PUBLICATIONS

J. T. Hodges, J. P. Looney and R. D. van Zee, “Response

of a ring-down cavity to an arbitrary excitation,” J.

Chem. Phys., 105, 10278-10288 (1996).

G. E. Scace, P. H. Huang, J. T. Hodges, D. A. Olson and J.

R. Whetstone, “The new NIST low frost-point humidity

generator,” National Conference of Standards

Laboratories, Proceedings of the 1997 Workshop and

Symposium, 2, 657-673, July 27-31, 1997, Atlanta, GA.

G. E. Scace, D. C. Hovde, J. T. Hodges, P. H. Huang, J. A.

Silver and J.R. Whetstone, “Performance of a precision

low frost-point humidity generator,” 3rd International

Symposium on Humidity and Moisture, sponsored by

the National Physical Laboratory, Teddington, England,

April 6-8, 1998.

G. E. Scace, D. C. Hovde, J. T. Hodges, P. H. Huang, J. A.

Silver and J.R. Whetstone, “The NIST Low Frost-Point

Humidity Generator: A New Standard for Trace Humidity

Measurement,” Workshop on Gas Distribution Systems,

Semicon West 98, San Francisco, CA, July 12, 1998.

J. T. Hodges, J. P. Looney and R. D. van Zee, “Single

mode cavity ring-down spectroscopy with pulsed lasers,”

1998 International Symposium on Molecular

Spectroscopy, June 15- 19, 1998, Columbus, OH.

Figure 8. High-resolution FSSM-CRDS spectrum of

pressure-broadened H2O rovibronic transition at

10687.36 cm-1. The open circles are the measured

losses, and the red solid line is a Voigt fit to the

measured profile. The blue line represents the fit

residuals.

R. D. van Zee, J. P. Looney, and J. T. Hodges, “Measuring

pressure with cavity ring-down spectroscopy,” in

Advanced Sensors and Monitors for Process Industrial

Environment, W. A. de Groot, ed., SPIE Proc. 3535

(1998).

J. P. Looney, J. T. Hodges and R. D. van Zee,

“Quantitative absorption measurements using cavityringdown spectroscopy with pulsed lasers,” Chapter 7,

in Cavity-Ringdown Spectroscopy-A New Technique for

Trace Absorption Measurements, ed. K. W. Busch,

American Chemical Society Symposium Series No. 720

(1999).

R. D. van Zee, J. T. Hodges and J. P. Looney, “ Pulsed,

single-mode cavity ring-down spectroscopy,” Appl. Opt.,

38, 3951-3960, 1999.

D. C. Hovde, J. T. Hodges, G. E. Scace, J. A. Silver,

“Wavelength-modulation laser hygrometer for

ultrasensitive detection of water vapor in semiconductor

gases,” Appl. Opt., 40, 829-839 (2001).

G. E. Scace, and J. T. Hodges, “Uncertainty of the NIST

low frost-point humidity generator,” Proceedings of the

8th International Symposium on Temperature and

Thermal Measurements in Industry and Science, June

19-21, 2001, Berlin, Germany.

P. H. Huang, G. E. Scace, and J. T. Hodges, “Measuring

technique for quantifying performance of dilution-based

trace moisture generators,” Proceedings of the 8th

International Symposium on Temperature and Thermal

Measurements in Industry and Science, June 19-21, 2001,

Berlin, Germany.

J. G. Cormier, J. T. Hodges and J.R. Drummond, “On the

Application of Cavity Ringdown Spectroscopy to

Measurements of Line Shapes and Continuum

Absorption,” in SPECTRAL LINE SHAPES: 16th

International Conference on Spectral Line Shapes

(Volume 12), AIP Conference Proceedings 645, ed.

Christina A. Back, pp.401-412 (2002).

S. Y. Lehman, K. A. Bertness and J. T. Hodges, “Detection

of trace water in phosphine with cavity ring-down

spectroscopy,” Jour. of Crys. Growth, 250, 262-268

(2003).
138 National Institute of Standards and Technology

PLASMA PROCESS METROLOGY

Technical Contacts:

M. Sobolewski

K. Steffens

E. Benck

“NIST is one of the

leaders in plasma

processing related

research in the US.

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.

GOALS

To provide advanced measurement techniques,

data, and models needed to characterize plasma

etching and deposition processes important to

the semiconductor industry, enabling continued

progress in model-based reactor design, process

development, and process control.

CUSTOMER NEEDS

To fabricate future generations of devices, the

semiconductor industry requires improvements

in plasma etching and deposition processes.

Plasma processes and equipment face

increasingly stringent requirements due to the

need to maintain high device yields at decreasing

feature sizes, the introduction of new dielectric

materials, and the constant pressure to keep

production efficiency high. To meet these

challenges, the International Technology

Roadmap for Semiconductors (ITRS) identifies a

need for better predictive system modeling. To

obtain more reliable predictions of the chemical,

physical, and electrical properties of processing

plasmas, further progress in model development

and validation is required. ITRS also identifies a

need for improvements in intelligent process

monitoring and control, which require the

development of robust and reliable sensors that

are compatible with manufacturing equipment.

TECHNICAL STRATEGY

Our multifaceted program provides numerous

outputs to assist our customers, including

advanced measurement methods, high-quality

Figure 1. One of the inductive, high-density plasma

reactors used in our experimental studies.

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. See Fig. 1 for a photograph of one

of our plasma reactors.

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-defined

conditions in highly-characterized standard

plasma reactors.

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 specific areas:

1. We are currently developing a new optical

diagnostic technique, sub-millimeter wave

spectroscopy, to measure the density and

temperature of important chemical species in

plasmas. Work is progressing on validating the

sensitivity limits of important plasma species and

feed gas contaminants. In addition, spatially

resolved absorption measurements will be made

to map out the species and temperature distributions within the plasma and surrounding gas.
Semiconductor Microelectronics and Nanoelectronics Programs 139

DELIVERABLES: Validate new applications of the

sub-millimeter wave absorption spectroscopy as a

plasma diagnostic. Validate sensitivity estimates of

water vapor contamination in plasma feed gases.

Develop method to separate absorption signal

contributions from the plasma and the surrounding

gas. 4Q 2003

2. Gas phase fluorocarbon radicals have been

identified as species critical in the deposition of

the fluorocarbon polymer layer that forms on wafer

surfaces and provides selective etching of SiO2

over Si during dielectric etch processes. The direct

correlation between gas phase chemistry and

surface composition of the polymer layer under a

wide variety of plasma conditions has not been

adequately measured. To gain a better understanding of the relationship between gas phase

precursors and surface composition, we plan to

couple our existing capabilities in gas phase

measurements with surface measurements. We

will utilize planar laser-induced fluorescence

(PLIF) to determine 2-D density maps of

fluorocarbon radicals in the gas phase. Wafer

surfaces will be probed using IR absorption at

several radial positions to determine how

variations in gas phase chemistry affect the

composition of the fluorocarbon polymer films.

DELIVERABLES: Develop diagnostic technique for

surface measurements and correlate gas phase PLIF

species density maps with spatial variations on

surfaces. 4Q 2004

3. In order to continue the progress in shrinking

semiconductor device dimensions, the ITRS

Roadmap requires that lithographic techniques

switch to shorter wavelength sources. Photoresist

materials which are now commonly used do not

work at these shorter wavelengths, so that entirely

new materials based on very different chemistry

are required. How the new photoresist materials

will interact with etching plasmas is poorly

understood at best. We will be applying a broad

range of diagnostic capabilities (optical emission

spectroscopy, sub-mm absorption spectroscopy,

electrical diagnostics, ion mass spectrometry, ion

energy analysis, microwave interferometry, and

Langmuir probe techniques) to study plasmasurface interactions with new 157-nm photoresists. Initial work will focus on how etching of

photoresist alters the etching plasma. Later work

will examine polymer deposition and other means

by which the plasma affects the photoresist

surface.

DELIVERABLES: Characterize plasma – surface

interactions of 157 nm photoresists with fluorocarbon etching plasmas. 4Q 2004

4. Electrical measurement techniques for use in

process monitoring and control applications are

also under development. These techniques rely

on measurement of the radio-frequency current

and voltage applied to plasma reactors. The rf

measurements are compatible with commercial

reactors and they contain valuable information

about the flux and energy of the ions that bombard

wafers during processing. Values for the total

ion flux and ion energies are obtained by

analyzing the current and voltage signals using

electrical models of plasma sheaths. Tests to

validate the models have largely been completed.

Values for the total ion flux, sheath voltages, and

ion energy distributions obtained from the rf

measurements have been compared against

independent measurements and shown to be in

good agreement. Future development of these

techniques is focused on improving the speed of

the analysis algorithms to allow them to be used

in real-time process monitoring and control

applications, and subsequent demonstration of

the ability of the technique to monitor and

diagnose drift in ion flux and ion energies during

plasma etching processes.

DELIVERABLES: Methods for monitoring total ion

flux and ion energies using rf current and voltage

measurements, with improved analysis speed to

allow real-time process monitoring and control.

4Q 2003

DELIVERABLES: Demonstration and evaluation of

rf-based ion flux and ion energy monitoring during

plasma etch processes. 3Q 2004

ACCOMPLISHMENTS

# We have recently developed and tested a

model-based technique for monitoring ion energy

distributions that relies on rf current and voltage

measurements made outside of a plasma reactor.

The technique is designed for process monitoring

applications in manufacturing where the use of

invasive probes to measure ion energies is not

practical. The method was validated by tests

performed in argon and CF4 discharges at 10-25

mTorr in an inductively-coupled, high-density

plasma reactor, with rf bias applied to the

substrate electrode at frequencies of 0.1-20 MHz.

Plasma potential waveforms and sheath voltage

waveforms obtained from the noninvasive rf

technique agreed well with independent

measurements made using a capacitive probe. Ion

energy distributions from the rf technique were

in good agreement with distributions measured

by a mass spectrometer equipped with an ion
140 National Institute of Standards and Technology

energy analyzer, at grounded as well as rf-biased

electrode surfaces (see Fig. 2).

Figure 2. Ion energy distributions in a high-density

CF4 plasma: measured by a mass spectrometer/ion

energy analyzer system (top) and obtained from noninvasive rf current and voltage measurements

(bottom).

Figure 3. Sub-millimeter wave absorption spectrum

of CHF3 in a high-density, inductively coupled plasma.

# Sub-millimeter wave absorption spectroscopy is being developed as a plasma diagnostic

to identify and monitor species in etching

plasmas. Sub-mm wave spectroscopy can monitor

the crucial chemical species in a plasma and

provide the necessary feedback for understanding plasma processing (see Fig. 3). The

density of radicals in capacitively coupled

fluorocarbon etching plasmas have been

measured for several different fluorocarbon

etching gases in the presence of several different

blanket coated silicon wafers. These

measurements were correlated with etching rates

and FTIR measurements of the downstream

effluents. In situ measurements of water vapor

contamination of the vacuum chamber and

feedgases indicated a sensitivity limit for water

on the order of 7.7x107

 cm-3. A detection

sensitivity of 2x107 cm-3 was demonstrated for SiO,

an etching byproduct of SiO2 surfaces. The

spectral resolution of the sub-mm diagnostic is

so high that it can also be used to determine the

translation temperature of the plasma species

through the Doppler broadening of absorption

line shapes. Gas temperatures are important input

parameters to many plasma models since they

are necessary to relate the measured gas pressure

to the actual particle density in the chamber. Initial

gas temperature measurements made from two

different types of sub-mm sources, a backward

wave oscillator (BWO) and an IR laser photomixer,

were different. We have recently shown that this

difference was caused by frequency jitter in the

IR lasers. With improved frequency stabilization,

gas temperatures measured with both sources are

now in agreement.

# Also this year, we have developed the

capability to measure spatially-resolved 2-D

temperature maps in fluorocarbon plasmas using

planar laser-induced fluorescence (PLIF) of the

CF radical. Several PLIF images are measured,

each probing a different rotational level to give a

2-D map that is related to the CF population in the

probed rotational state. With the relative

population for several rotational levels known at

each location, a rotational temperature map is

calculated and assumed to be equivalent to the

gas temperature under these conditions. We have

measured temperature maps in CF4 plasmas as a

function of pressure and power, with and without

silicon wafers present. Simultaneously, CF

density images were obtained. Under our

conditions, radial variations in temperature from

10 K to 90 K were observed. Axial temperature

gradients were also observed to be quite large,

especially under our highest pressure conditions

(800 mTorr or 107 Pa) (see Fig. 4). The strongest

temperature gradients were consistently found

near the cooled electrode surfaces. These

variations can have strong implications. Species

density measurements that probe a specific

rotational level can be misleading if the population

of the chosen rotational level is not constant within

the temperature range investigated. In addition,

gas density in hotter regions will be lower, and

this must not be interpreted as a chemical effect.
Semiconductor Microelectronics and Nanoelectronics Programs 141

Especially near surfaces, where species density

fluxes are often interpreted as indicating surface

chemistry, one must be aware of the implications

of these temperature effects. In addition,

understanding temperature is important for

modeling, since chemical reaction rates are often

a function of temperature.

Figure 4. Temperature maps of a capacitivelycoupled CF4 plasma at 200 mTorr (26.7 Pa) and

650 mTorr (86.7 Pa).

# Our capactively coupled plasma reactors

have recently been modified to allow operation in

dual-frequency mode. Dual-frequency reactors are

becoming of increasing importance to the

semiconductor industry, particularly in dielectric

etching. New electrical measurement software has

been developed to monitor the applied voltage

and current waveforms, account for reactor stray

impedance, and determine the true values of

current, voltage and power drawn by these

discharges. Unlike reactors that operate at a single

frequencywhere only the fundamental and a

limited number of harmonic components are

presentmeasurements of a dual frequency

reactor need to monitor the fundamental and

harmonics of both frequencies as well as the sum

and difference frequencies. Time-resolved optical

emission measurements of dual-frequency

plasmas have also been performed. Together, the

optical and electrical results provide a well-defined

characterization of the dual-frequency plasmas,

better understanding of their operation, and

validation for models of their behavior.

# Absolute, mass-resolved ion fluxes were

measured in the presence of a silicon water in

inductively coupled plasmas generated in a C4

F6

processing gas. These measurements were made

simultaneously with sub-mm absorption and

optical emission measurements of the neutrals in

the plasma. C4F6 is of interest as an alternative

etching gas with low greenhouse gas emissions.

The results provide insight into the complicated

chemistry of C4F6 plasmas and are useful for

validating and refining plasma simulation codes

used by industry.

# 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 (http://eeel.nist.gov/811/refdata/) (see

Fig. 5). This web site has experienced tens of

thousands of hits throughout its history.

Figure 5. Recommended electron cross section data

for SF6 (shown here) and many other plasma

processing gases are available at http://eeel.nist.gov/

811/refdata/.

RECENT PUBLICATIONS

K. L. Steffens and M. A. Sobolewski, “Effect of changing

the electrode gap on the spatial and electrical properties

of O2/CF4 plasmas,” J. Vac. Sci. Technol. A 21, 241 (2003).

M. Edamura and E. C. Benck, “Transition from E to H

mode discharge in pulse-modulated inductively coupled

plasmas,” J. Vac. Sci. Technol. A 21, 470 (2003).

M. A. Sobolewski, Y. Wang, and A. Goyette,

“Measurements and modeling of ion energy distributions

in high-density, radio-frequency biased CF4

 discharges,”

J. of Appl. Phys. 91, 6303-6314 (2002).

M. A. Sobolewski, “Measuring the ion current in highdensity plasmas using radio-frequency current and voltage

measurements,” J. Appl. Phys. 90, 2660-2671 (2001).

K. L. Steffens and M. A. Sobolewski, “Investigation and

control of spatial characteristics of chamber-cleaning

plasmas,” in AIP Conf. Proc. 550, Proceedings of the

2000 International Conference on Characterization and

Metrology for ULSI Technology, 233-237 (2001).
142 National Institute of Standards and Technology

 M. A. Sobolewski, “Monitoring ion current and ion

energy during plasma processing using radio-frequency

current and voltage measurements,” in AIP Conf. Proc.

550, Proceedings of the 2000 International Conference

on Characterization and Metrology for ULSI Technology,

263-267 (2001).

E. C. Benck and K. Etamadi, “Fiber optic based optical

tomography sensor for monitoring plasma uniformity,”

in AIP Conf. Proc. 550, Proceedings of the 2000

International Conference on Characterization and

Metrology for ULSI Technology, 268-272 (2001).
Semiconductor Microelectronics and Nanoelectronics Programs 143

MEASUREMENTS FOR VACUUM PROCESS CONTROL

Technical Contact:

Robert F. Berg

A. Lee

All in all we are very

pleased and gratified

NIST took the time to

perform this comparison

with us. We learned a

great deal. Our technical

personnel received some

really good exposure to

detailed analysis of flow

comparisons and the

amount of careful

preparation required.

M. Bair,

DH Instruments

GOALS

Develop primary standards for gas flow in the

range from 10-7 to 10-3 mol/s and transfer this flow

measurement capability to the US semiconductor

industry. (10-6 mol/s is 1.3 standard cubic

centimeters per minute.)

CUSTOMER NEEDS

Many industrial processes require the accurate

metering of mass flow rate over the range from

10-7 to 10-3 mol/s. Most prominent are the

manufacturers of semiconductor devices, who

use thermal mass flow controllers (MFCs) to

control a wide variety of toxic, flammable, and

corrosive gases. Participants at an industry

workshop at NIST expected future requirements

for flow measurement accuracies to be better than

1 %, and they identified the need for national flow

transfer standards with uncertainties of 0.1 %.

New primary flow standards and improved flow

measurement techniques must be developed to

meet these needs.

Measurement of gas flow is crucial for the control

of the stoichiometry in plasma etching. The

Critical Dimension reduction efforts discussed

in the 2001 International Technology Roadmap

for Semiconductors imply improvements in

MFCs. “Improvements will be required in the

etch chemistries...”; “...development of multistep etch processes...may translate into the need

to change gas chemistries in the same etch

module....”

TECHNICAL STRATEGY

The flow measurements are based on a diverse

series of primary standards. The first was a

constant-volume (pressure rate-of-rise) primary

standard that we developed to measure flows 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 to 5 atmospheres with an

uncertainty of about 0.05 %. The third primary

standard is gravimetric; flow measurements made

by a transfer standard are integrated and compared

to the weight change of a gas bottle.

Transfer standards allow the primary flow

standards at NIST to be compared to flow meters

at other locations. Although a flow meter

manufacturer typically constructs its own primary

standard, comparisons with NIST allow the

manufacturer to demonstrate proficiency and, if

necessary, provide traceability to NIST. For this

purpose, we have developed a series of very

stable transfer standards based on laminar flow

through a thermostatted duct. The first

generation used a stainless steel, helical duct of

rectangular cross-section. It was used to perform

on-site proficiency tests of industrial flow

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 (see Fig. 1). It has been

used for comparisons with metrological institutes

of other countries as well as manufacturers of

flow meters. A third generation standard is under

construction.

Figure 1. Left: A transfer standard flow impedance

formed from 19 parallel quartz capillaries. Right:

Tightly coiling the flow impedance tested the centrifugal corrections used in the transfer standard’s

hydrodynamic model.

DELIVERABLE: Develop and characterize primary

standards for small gas flow. 2Q 2003

This flowmeter uses optical interferometry to

measure and control the displacement of a piston

of known cross-section. Controlling the piston’s

rate of displacement maintains a constant pressure

in the gas accumulation volume.

DELIVERABLE: Develop and characterize an

improved transfer standard for gas flow. 1Q 2003

The second-generation transfer standard is based

on laminar flow through a quartz capillary

impedance. It operates at higher flow rates with a

smaller slip correction and much smaller

centrifugal effects than the first-generation

transfer.
144 National Institute of Standards and Technology

DELIVERABLE: Investigate new gas flow measurements systems. 4Q 2003

ACCOMPLISHMENTS

# We directed flows of sulfur hexafluoride and

propane through the second-generation transfer

standard and into the constant-pressure primary

flow standard. Both gases have a large density

and a large deviation from ideal gas behavior. The

results led to improvements in the hydrodynamic

model of the transfer standard.

# We used the second-generation transfer

standard to make comparisons of gas flow with

two manufacturers of flow meters. Both

companies used the comparison to evaluate new

flow meter designs.

# We improved the characterization of the

constant-pressure primary flow meter by modeling

the effects of temperature lag and thermal

expansion.

RECENT PUBLICATIONS

R. F. Berg, “Two primary standards for low flows of

gases”, Proceedings of the Fifth International

Symposium on Fluid Flow Measurement, Washington,

DC (2002).
Semiconductor Microelectronics and Nanoelectronics Programs 145

TEST METROLOGY PROGRAM

Lead counts of several thousand per chip and test frequencies in the microwave regime

challenge current test methodologies. 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 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 crucial to develop accurate testing strategies.
146 National Institute of Standards and Technology

METROLOGY FOR SYSTEM-ON-A-CHIP (SOC)

Technical Contacts:

A. R. Hefner, Jr.

M. Y. Afridi

”NIST Microhotplate may

help search for extraterrestrial life.”

Science Daily

“The NIST-developed

microhotplate gas

sensor technology is the

future for homeland

security and military

applications of toxic gas

and trace explosive

detection.”

Ernesto A. Corte,

Vice President,

Thermo Electron Corporation

GOALS

Develop solutions to key metrology issues

confronting the semiconductor System-on-a-Chip

(SoC) industry. These include development of

measurement methods and standards for

characterizing non-digital Virtual Components

(ND-VCs), a critical class of building blocks from

which SoCs are developed. One focus is on

delivering standards to facilitate the incorporation

of multi-technology (MT) VCs including MEMSbased VCs into SoCs. The project activities

include: multi-technology hardware description

language (HDL) model development, VC interface

standards, synthesis and scaling standards for

ND-VCs compatible with digital methodologies,

testing standards and verification standards. The

NIST MEMS-based integrated gas-sensing VC

will be used as a test bed to demonstrate the

viability of these standards. In addition, the

demonstration of general purpose gas-sensing

VC methodologies will be used to facilitate the

adoption these MT-VCs into new Homeland

Security and Industrial applications.

CUSTOMER NEEDS

The driving force in today’s semiconductor

industry is the need to maintain a rate of

improvement of 2x every two years in highperformance components. Historically, these

improvements have relied exclusively on

advances made in semiconductor miniaturization

technology. The 1999 International Technology

Roadmap for Semiconductors (ITRS) suggested

that, “innovation in the techniques used in circuit

and system design will be essential to maintain

the historical trends in performance improvements.” Achievement of this advancement in

circuit and system design techniques is

increasingly becoming dependent on integrating

multiple technologies into a single chip referred

to as System-on-a-Chip (SoC). The design

challenges for SoC-MT devices will be overcome

with the use of platform-based design approaches

that emphasize design reuse, i.e. the development

of ND-VCs that can be used as cost-effective

building blocks for SoC-MT devices.

The challenges for SoC-MT devices are discussed

in the 2001 International Technology Roadmap

for Semiconductors in the Design, System Drivers

and Test and Test Equipment sections.

TECHNICAL STRATEGY

1. The key to developing successful ND-VCs for

SoC-MT devices it to make the ND-VCs, for all

practical purposes, look like digital VCs. The first

step in this multi-step process is to develop the

ability to make the ND-VC devices via a CMOS

compatible process. To demonstrate this capability we have chosen a MEMS microhotplate

based VC, including operational amplifiers and

decoders to process the data.

2. The second step in making the ND-VC look like

a digital VC is to build a digital interface shell

around the ND-VC. Since the ND-VC needs to

interface to the digital circuitry of the system, this

shell will add no additional cost to the system

and having digital circuitry on as many of the

interfaces as possible will greatly simplify the

definition of the ND-VC’s IO. To facilitate this

approach we will develop methodologies and

standards for adding digital shells to ND-VCs and

demonstrate them on the gas sensor VC described

above.

DELIVERABLES: Develop methodologies for

designing digital interface shells for ND-VCs and

demonstrate their viability via our micro hotplate gas

sensor technology. 4Q 2003

3. The predominant design approach used by

industry for SoC devices is top-down design. This

requires that high-level HDL models 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 ND-VCs is at

best poorly developed. To address this need,

high-level models are being developed for NDVCs using Analog and Digital Hardware

Description Languages, and methodologies are

being developed to validate the models.

DELIVERABLES: Develop methodologies and

standards for developing high-level models for

ND-VCs and demonstrate their viability via our micro

hotplate gas sensor technology. 4Q 2005

4. After the high-level simulation of the system in

the top-down design process is successfully

completed, the next step is to synthesize the

individual VCs. Compared to those for digital

VCs, the methodology and standards for ND-VC

synthesis is at best poorly developed. To address

the need for synthesizing ND-VCs, we are
Semiconductor Microelectronics and Nanoelectronics Programs 147

developing standards and metrologies for a nondigital synthesis like process that is compatible

with digital synthesis. This process is based upon

libraries of existing designs and the device design

equations.

DELIVERABLES: Develop methodologies for an

equivalent MT-VC synthesis approach and demonstrate its viability via our microhotplate gas sensor

technology. 2Q 2006

5. Scaling digital circuitry is a key capability used

by digital designers to reduce the costs of their

designs. Since most systems that would use NDVCs will be predominately digital, it is important

that there be an equivalent scaling capability for

the ND-VCs. To address the need for scaling

ND-VCs, we are developing metrologies for nondigital scaling processes.

DELIVERABLES: Develop methodologies and

standards for an equivalent ND-VC scaling approach

and demonstrate its viability via our micro hotplate

gas sensor technology. 3Q 2004

6. The testability of ND-VCs represents another

significant challenge since standards and methodologies for non-digital circuits do not exist. The

most promising approach to address testability

is to use Built-In-Self-Test (BIST) techniques.

To facilitate this approach we will develop

methodologies and standards for adding BIST to

ND-VCs and interface them via the digital shell.

DELIVERABLES: Develop methodologies for built-in

self test of of ND-VC devices and demonstrate their

viability via our micro hotplate gas sensor technology.

4Q 2004

ACCOMPLISHMENTS

# A monolithic micro-gas-sensor system was

designed and fabricated in a standard CMOS

process (Fig. 1). The gas-sensor system incorporated an array of four micro hotplate-based gassensing structures. The system utilized a thin

film of tin oxide (SnO2

) as a sensing material. The

interface circuitry on the chip has digital decoders

to select each element of the sensing array and

an operational amplifier to monitor the change in

conductance of the film. The chip is postprocessed to create micro hotplates using bulk

micro-machining techniques. Detection of gas

concentrations in the 100 parts-per-billion range

was achieved. This represents a factor of 100

improvement in sensitivity compared to existing

MEMS-based micro-hotplate gas sensors.

# A monolithic CMOS four element gas sensor

VC is designed and submitted for fabrication. The

design includes both analog and digital integrated

circuits. The digital part of the circuit has an 8-bit

analog-to-digital converter (ADC). The objective

of the design is to make it compatible with the

SoC design methodology (Fig. 2).

Figure 1. Micrograph of gas-sensor system to be used

as demonstration vehicle.

Figure 2. Layout of the four element gas sensor VC

with analog-to-digital control.

RECENT PUBLICATIONS

Afridi, M. Y., Suehle, J. S., Zaghloul, M. E., Berning,

D. W., Hefner, A. R. Hefner, Semancik, S., Cavicchi,

R. E., “A Monolithic Implementation of Interface

Circuitry for CMOS Compatible Gas-Sensor System,”

ISCAS-2002

Afridi, M. Y.; Suehle, J. S.; Zaghloul, M. E.; Berning,

D. W.; Hefner, A. R.; Cavicchi, R. E.; Semancik, S.;

Montgomery, C. B.; Taylor, C. J., “A monolithic CMOS

microhotplate-based gas sensor system,” IEEE SENSORS

JOURNAL, VOL. 2, NO. 6, pp. 644- 655, DECEMBER

2002.
148 National Institute of Standards and Technology

NONLINEAR DEVICE CHARACTERIZATION

Technical Contact:

Don DeGroot

“Agilent Lightwave

Division would like to

encourage NIST’s

continued work to

develop the nose-to-nose

characterization, so that

the technology can

become more widely

available.”

Dennis Derickson,

Agilent technologies, Inc.

GOALS

Develop and support general methods of characterizing nonlinear components, circuits, and

systems used in digital wireless communications;

refine and transfer these methods through interactions with industrial research and development

laboratories.

Dr. Kate Remley performs large-signal measurements

of an RF power amplifier using the Nonlinear

Network Measurement System.

CUSTOMER NEEDS

Radio-frequency measurements are applied

extensively in the deployment of commercial

wireless communication systems. They are crucial

to all stages of system development, from device

modeling to circuit design and system

performance characterization. NIST’s RF and

microwave measurement teams are addressing the

critical need for accurate measurements of

nonlinear electrical networks and supporting

industrial standards development.

TECHNICAL STRATEGY

The Nonlinear Device Characterization (NDC)

Project is developing and verifying measurementbased descriptions of devices, circuits, and

systems that contain nonlinear elements. The RF

power amplifier is a key nonlinear component with

which engineers are currently contending.

Industrial experts estimate that the RF power

amplifiers account for 60-70 % of base station

costs and 20-30 % of the total wireless link cost.

Traditional microwave circuit design has relied

on the ability to cascade circuit elements through

simple linear operations and transformations, but

engineers lose the ability to predict circuit

performance across operating environments, or

states, when their circuits include a nonlinear

element. Presently, there is a critical need for

fundamental RF measurement techniques to

develop and validate nonlinear models and

commonly applied figures of merit. Contributions

in this area will significantly improve design-cycle

efficiency and trade between manufacturers, and

will eventually facilitate improvements in

communications through the full incorporation

of nonlinear models at the system design level.

The NDC Project recently acquired and

established a new measurement facility known as

the Nonlinear Network Measurement System

(NNMS). The system provides the most general

approach to measuring large-signal responses. It

is a stimulus-response network analyzer that

supplies periodic signals, and then acquires

broadband incident and reflection waveforms at

the device under test. The NIST facility will be

used as a reference system in measurement and

model comparisons. The project team is

developing accurate calibration and measurement

techniques for the NNMS, including validation

of the Nose-to-Nose calibration technique, a

practical and available method of measuring the

phase relations of components in signals with 50

GHz bandwidths. The project team is now refining

the statement of measurement uncertainty in the

Nose-to-Nose method and will apply it to NNMS

measurements.

The Nonlinear Network Measurement System is

being applied first to canonical circuits to compare

general measurements with predictions made by

circuit simulators and new behavioral models. We

have applied these techniques to identify stable

verification circuits that will be used in NISTsponsored interlaboratory comparisons. Second,

the measurement system is being applied to

develop and verify artificial neural network (ANN)

models for nonlinear circuits being developed in

cooperation with the University of Colorado.

NNMS data will be used to train ANN models, to

verify circuit operation and model predictions, and

to validate a circuit optimization approach.

The NDC Project has also started to examine the

link between nonlinear circuit descriptions and
Semiconductor Microelectronics and Nanoelectronics Programs 149

system performance simulations. Through

collaborations with NIST’s broadband standards

development effort, members of the NDC Project

will assemble a measurement system to test the

performance of communication links.

Plans are underway with the University of

Colorado to establish a Joint Research Center for

Nonlinear Electronics in Wireless at Radio

Frequencies (newRF). This Center, funded by

industrial members, will support graduate research

projects. The graduate research assistants and

CU faculty will work with NIST staff on the newRF

projects. The Center will increase the

effectiveness of the NIST facilities while

developing a new class of technical professionals

who have the skills required by industry.

ACCOMPLISHMENTS

# Defined consistent method for defining the

phase relation of signal components that are not

at the same frequency.

# Expanded and clarified definition of largesignal scattering parameters, demonstration

application to nonlinear rf circuit design.

# Expanded numerical simulations of the Noseto-Nose calibrations as the basis for an

uncertainty statement on the NNMS phase

alignment. There are no other methods of

identifying the error terms due to the internal

sampling electronics used by the Nose-to-Nose

equipment. The simulator-based sensitivity study

gives us the basis for a first-level uncertainty

bound.

# Adapted Multiline TRL Calibration for NNMS

and included in commercial software, with Agilent

collaborators. All users of current and future

NNMS-like instruments will have access to this

NIST reference calibration in their daily

measurements.

# Added modulated signal capabilities to the

NIST NNMS. Demonstrated modulated signal

measurements on example RF amplifier.

Conducted multiple modulation calibrations to

study instrument repeatability. NIST can now use

the NNMS to characterize circuits using two-tone

and multi-tone signals, and then compare these

to commonly used figures of merit.

# Designed interlaboratory measurement

comparison for nonlinear network analyzer users.

Designed and fabricated prototype verification

circuits. Formulated initial comparison method

and measures. Performed initial comparisons

between NIST and Belgian research labs. Users

of NNMS and related nonlinear network analyzers

will gain assurance in their ability to identify

nonlinear input-output transfer functions or

extract model parameters.

# Developed nonlinear models for verification

devices. Developed and applied conventional

compact diode models. Developed frequencydomain behavioral modeling strategy with the

University of Colorado. Developed time-domain

behavioral model with guest researcher from K.

U. Leuven, Belgium. The interlaboratory

comparison of nonlinear network analyzers

requires models since we do not have a generalized

nonlinear parameter to compare. The models can

accurately represent the nonlinear transfer

functions of the verification circuits over the

state-space that they are characterized, allowing

participants to find the differences between their

measurements and our model predictions.

# Developed first-generation metrics for

comparing data from nonlinear circuit characterizations. These tools can easily summarize the

differences found in multi-dimensional data sets

common to nonlinear network analysis. The

metrics will be used immediately in the NISTsponsored measurement comparison.

# Developed generalized approach to

measurement-based frequency-domain models of

nonlinear circuits. Demonstrated ANN models to

define and obtain large-signal scattering functions.

This approach is proving useful in improving the

efficiency of nonlinear circuit design and

optimization. It is currently the only definition of

large-signal scattering functions that does not

assume some level of linearization.

# Concluded second phase of a Passive

Intermodulation measurement comparison.

Participants determined how well their

measurements compared to ensemble averages

for characterizations of a verification device with

ultra-low passive intermodulation response.

# In collaboration with Professor K. C. Gupta

of the University of Colorado at Boulder, applied

artificial neural networks (ANNs) to improve the

modeling of on-wafer open-short-load-thru

(OSLT) standards and coaxial line-reflect-match

(LRM) calibrations used for calibrating vector

network analyzers. The new methods will be used

in Time-domain Network Analyzer (DNACal) and

new NNMS software.
150 National Institute of Standards and Technology

# Discovered nonlinear error mechanism in the

Nose-to Nose calibration. Through an innovative

combination of large-signal and small-signal

analyses, showed how nonlinear junction

capacitance in the sampler circuit induces an error

that had been ignored in the initial Nose-to-Nose

analysis.

# Initiated NIST’s Nonlinear Network

Measurement System (NNMS) and conducted

NIST’s first large-signal nonlinear device

verification experiments. Agilent Technologies

delivered the NNMS instrument in fulfillment of a

custom equipment contract and extensive

collaborative research effort with NIST.

COLLABORATIONS

Vrije Universiteit Brussel, Department ELEC. To

develop stochastic calibrations for accurate radiofrequency circuit and device measurements.

University of Colorado, Electrical and Computer

Engineering Department. To develop new

measurement-based behavioral models of

nonlinear RF circuits and devices.

NIST ITL Statistical Engineering Division and

EEEL Electromagnetic Technology Division. To

develop standard nonlinearity, then use it to verify

NNMS calibrations and modeling approaches; to

extend nonlinear circuit models to system

characterization.

K.U. Leuven, Belgium. To develop models for

measurement comparison verification devices,

and to study instrument repeatability.

RECENT PUBLICATIONS

D. C. DeGroot, J. A. Jargon, and R. B. Marks, “Multiline

TRL revealed,” 60th ARFTG Conf. Dig., pp. 131-155,

Dec. 2002.

J. A. Jargon, D. C. DeGroot, K. C. Gupta, and A. Cidronali,

“Calculating ratios of harmonically related, complex

signals with application to nonlinear large-signal

scattering parameters,” 60th ARFTG Conf. Dig., pp. 113-

122, Dec. 2002.

J. A. Jargon, K. C. Gupta, and D. C. DeGroot,

“Applications of artificial neural networks to RF and

microwave measurements,” Int. J. RF Microwave

Computer-Aided Eng., vol. 12, no. 1, pp. 3-24, Jan.

2002.

J. A. Jargon, K. C. Gupta, D. Schreurs, and D. C. DeGroot,

“Developing frequency-domain models for nonlinear

circuits based on large-signal measurements,” Proc.

XXVIIth Gen. Assembly Int. Union Radio Sci., CD-ROM

A1.O.6, Aug. 2002.

A. B. Kos, T. J. Silva, P. Kabos, M. R. Pufall, D. C.

DeGroot, L. Webb, and M. Even, “Design and

performance of an inductive current probe for integration

into the trace suspension assembly,” IEEE Trans. Mag.,

vol. 38, no. 1, pp. 50-54, Jan. 2002.

K. A. Remley, J. A. Jargon, D. Schreurs, D. C. DeGroot,

and K. C. Gupta, “Repeat measurements and metrics for

nonlinear model development,” 2002 IEEE MTT-S Int.

Microwave Symp. Dig., pp. 2169-2172, June 2002.

D. Schreurs, J. A. Jargon, K. A. Remley, D. C. DeGroot,

and K. C. Gupta, “Artificial neural network model for

HEMTs constructed from large-signal time-domain

measurements,” 59th ARFTG Conf. Dig., pp. 31-36,

June 7, 2002.

J. A. Jargon, K. C. Gupta, D. Schreurs, K. A. Remley, and

D. C. DeGroot, “A method of developing frequencydomain models for nonlinear circuits based on largesignal measurements,” 58th ARFTG Conference Digest,

pp. 35-48, Nov. 29, 2001.

K. A. Remley, D. C. DeGroot, J. A. Jargon, and K. C.

Gupta, “A method to compare vector nonlinear network

analyzers,” 2001 IEEE MTT-S Internat. Microwave

Symp. Dig., pp. 1667-1670, May 21-24, 2001.

K. A. Remley, D. F. Williams, D. C. DeGroot, J. Verspecht,

and J. Kerley, “Effects of nonlinear diode junction

capacitance on the nose-to-nose calibration,” IEEE

Microwave Wireless Comp. Lett., vol. 11, no. 5, pp.

196-198 (2001).

J. A. Jargon and K. C. Gupta, “Artificial neural network

modeling for improved coaxial line-reflect-match

calibrations,” Int. J. RF Microwave Computer-Aided Eng.,

vol. 11, no. 1, pp. 33-37, Jan. 2001.

D. F. Williams and K. A. Remley, “Analytic samplingcircuit model,” IEEE Trans. Microwave Theory Techn.,

vol. 49, no. 6, pp. 1013-1019 (2001).

J. A. Jargon, “Measurement Comparison of a LowIntermodulation Termination for the U.S. Wireless

Industry,” NIST Technical Note 1521, Jul. 2001.

J. A. Jargon and K. C. Gupta, “Artificial neural network

modeling for improved on-wafer line-reflect-match

calibrations,” 31st European Microwave Conf. Dig., Sep.

2001.

J. A. Jargon, P. Kirby, K. C. Gupta, L. Dunleavy, and T.

Weller, “Modeling Load Variations with Artificial Neural

Networks to Improve On-Wafer OSLT Calibrations,”

56th ARFTG Conf. Dig., pp. 76-88, Boulder, CO, Nov.

2000.

J. A. Jargon, K. C. Gupta, and D. C. DeGroot, “Artificial

neural network modeling for improved on-wafer OSLT

calibration standards,” Int. J. RF Microwave ComputerAided Engin., vol. 10, no. 5, pp. 319-328, Sep. 2000.

L. Rouault, B. Verbaere, D. DeGroot, D. LeGolvan, and

R. Marks, “Measurements and models of a power

amplifier suitable for 802.16.1,” IEEE 802.16.1 p-00,

Sep. 13, 2000.
Semiconductor Microelectronics and Nanoelectronics Programs 151

P. D. Hale, T. S. Clement, K. S. Coakley, C. M. Wang, D.

C. DeGroot, and A. P. Verdoni, “Estimating the magnitude

and phase response of a 50 GHz sampling oscilloscope

using the “Nose-to-Nose” method,” 55th ARFTG Conf.

Dig., pp. 35-42, June 13, 2000.

D. C. DeGroot, P. D. Hale, M. Vanden Bossche, F.

Verbeyst, and J. Verspecht, “Analysis of interconnection

networks and mismatch in the Nose-to-Nose calibration,”

55th ARFTG Conf. Dig., pp. 116-121, June 16, 2000.

K. A. Remley, D. F. Williams, and D. C. DeGroot,

“Realistic sampling-circuit model for a Nose-to-Nose

simulation,” 2000 IEEE MTT-S Int. Microwave Symp.

Dig., June 11-16, 2000.

J. A. Jargon and D. C. DeGroot, “NIST unveils status of

PIM testing,” Microwaves & RF, pp. 78-81, Jan. 2000.
152 National Institute of Standards and Technology

AT-SPEED TEST OF DIGITAL INTEGRATED CIRCUITS

Technical Contacts:

D.F. Williams

J. Moreland

P. Kabos

J. Kopanski

“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

difficult than ever

before.”

Travis M. Eiles, Ph.D.

Intel Corp.

GOALS

Develop and demonstrate metrology for the atspeed test of digital integrated circuits. The

program will resolve the essential metrology issues

of at-speed digital integrated circuit test. It will

apply its results to characterizing and calibrating

high-impedance probes, develop atomic force

microscopes (AFMs) capable of precisely

positioning field probes above the surface of the

integrated circuit, and push the current on-chip

sampling technologies now being explored by the

industry.

CUSTOMER NEEDS

The semiconductor industry needs accurate

metrology for the at-speed test of digital

integrated circuits (“Grand Challenges,” page 11,

1997 National Technology Roadmap for

Semiconductors). Traditional IC contact probing

technology requires large contact pads

incompatible with the operation and economic

constraints of modern IC designs. Alternative

probing approaches use high-impedance probes,

non-contact probes, atomic-force microscopes,

electron beams, optical beams, or on-chip

samplers that respond to either electric or

magnetic fields near transmission lines in the

circuits. However, while the uncalibrated field

measurements performed by these probing

systems are suitable for field 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 highperformance digital integrated circuits.

Developing characterization and calibration

methods for high-impedance 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.

The need for noninvasive waveform measurements in silicon integrated circuits is discussed

in the Test and Test Equipment section, pages

1-5, 2001 (International Technology Roadmap

for Semiconductors).

TECHNICAL STRATEGY

We will develop calibration artifacts with precisely

known high-frequency voltages and circuits

suitable for characterizing and calibrating highimpedance probes and samplers of all types. We

will focus on fundamental calibration issues:

transforming the response of the probes to the

electric and magnetic fields above the integrated

circuit into accurate voltages and currents inside

the circuit.

We will first apply the characterization and

calibration procedures to conventional highimpedance probes. To facilitate the development

and test of electric and magnetic probes with

nanoscale resolution, we are constructing a

universal AFM test bed for these probes (see

Fig. 1). We will then apply our characterization

procedures to miniature AFM probes suspended

on custom cantilevers designed for high

frequency measurements on the nanoscale.

Finally, we will tie our metrology back to fullycharacterized electro-optic sampling measurements.

DELIVERABLES: Develop complete electrical

characterization for commercial high-impedance

probe, including transfer function, invasiveness, and

noise immunity. 3Q 2003

DELIVERABLES: Build universal AFM test bed.

3Q 2003

DELIVERABLES: Electrically characterize and AFM

probe. 2Q 2004

Figure 1. Universal test bed for electric and magnetic

field probing on a nanoscale.
Semiconductor Microelectronics and Nanoelectronics Programs 153

ACCOMPLISHMENTS

# We have designed and tested a prototype

sinusoidal waveform standard.

# We have constructed a high-speed electrooptic sampling system.

# We have developed, fabricated and tested a

noninvasive AFM scanning probe for measuring

local microwave power (see Fig. 2 at right).

# We have developed a method of characterizing and calibrating conventional highimpedance probes for low-invasive waveform

measurements (see Fig. 3 at right).

# We have applied our high-impedance-probe

characterization method to a probe mounted on

an atomic-force microscope.

RECENT PUBLICATIONS

P. Kabos, H.C. Reader, U. Arz, and D.F. Williams, “Calibrated waveform measurement with high-impedance

probes,” IEEE Trans. Microwave Theory and Tech., February 2003.

M.D. Janezic, D.F. Williams, V. Blaschke, A. Karamcheti,

and C.S. Chang, “Permittivity characterization of low-k

thin films from transmission-line measurements,” IEEE

Trans. Microwave Theory and Tech. January 2003.

T.S. Clement, P.D. Hale, D.F. Williams, and J.M. Morgan, “Calibrating photoreciever response to 110 GHz,”

15th Annual Meeting of the IEEE Lasers and ElectroOptics Society Conference Digest, Nov. 10-14, 2002,

Glasglow, Scotland.

Albrecht Jander, John Moreland, and Pavel Kabos, “Dielectric micromachined calorimeter probe for high resolution microwave power measurements based on

ferromagnetic resonance,” 15th IEEE Int. Conf. on

MicroElectroMechanical Systems, Jan. 20-24, 2002, Las

Vegas, Nevada.

U. Arz, H.C. Reader, P. Kabos, D.F. Williams, “Wideband

Frequency-Domain Characterization of High-Impedance

Probes,” 58th ARFTG Conference Digest, pp. 117-124,

Nov. 29-30, 2001.

D.F. Williams, P.D. Hale, T.S. Clement, and J.M. Morgan

“Mismatch corrections for electro-optic sampling systems,” 56th ARFTG Conference Digest, Nov. 30-Dec. 1,

2000.

U. Arz, D.F. Williams, D.K. Walker, J.E. Rogers, M.

Rudack, D. Treytnar, and H. Grabinski, “Characterization of Asymmetric Coupled CMOS Lines,” 2000 International Microwave Symposium Digest, June 11-16, 2000.

Figure 3. Waveform comparison at the tip of a

commercial high-impedance probe. The red curve

marked with red squares is the expected waveform.

The brown curve marked with triangles is the

measured waveform. The green curve is the measured waveform after correction by the new NIST

algorithm.

Figure 2. A 1 mm long dielectric bimaterial cantilever

AFM probe fabricated at NIST for noninvasively

measuring local microwave power.
154 National Institute of Standards and Technology

JITTER AND PROPAGATION DELAY MEASUREMENT

Technical Contacts:

N. G. Paulter

D. R. Larson

D. I. Bergman

“Accurate characterization of the jitter on a

signal, as well as a

system’s ability to

tolerate jitter are critical... for the designer

and manufacturer of

communication systems

and components.”

Brian Scott,

Agilent Technologies

GOALS

To develop methods for accurately measuring

timing jitter in high bit rate (> 10 GB/s)

telecommunication systems and develop systems

and protocols for measuring and computing

propagation delay through active circuits. This

includes the development of artifact standards,

data extraction and analysis procedures and

algorithms, and measurement systems, each

where necessary. Existing measurement methods

will be examined to determine consistency

amongst them, measurement limitations, and

range of applicability. Measurement services will

be developed for the measurement of different

types of jitter and for propagation delay.

CUSTOMER NEEDS

Jitter is a limiting factor in the performance of highfrequency/high-speed telecommunications and

computer systems. A typical eye diagram

measurement is shown in Fig. 1. In a telecommunications system, where a series of clockrecovery circuits is typically used, jitter degrades

data recovery performance. Jitter measurements

on telecommunications components and

equipment are needed to assure compatibility

between manufacturers and to identify sources

of jitter. For microprocessor chips, the jitter in

the on-chip phase-lock-loop circuit is used by

the manufacturer to sort the microprocessor for

clock speed usage. Large errors in the measurement of clock jitter represent manufacturing yield

loss. Furthermore, for some high-speed

computers, the frequency of the microprocessor

clock is a multiple of the system clock frequency,

and jitter degrades the timing tolerance of the

circuit. The data storage industry is also

Figure 1. Eye diagram for jitter measurements.

concerned with jitter in recovery of data from the

source media.

The calibration of jitter measurement equipment

to an uncertainty of a few picoseconds is a difficult

problem. Several types of jitter measurements

that are important are: transition-to-transition

jitter, jitter relative to a fixed clock, jitter spectrum,

and data dependent jitter. These measurements

are important for determining the source of jitter

and component compatibility. To test

components accurately, jitter generation is

important. Industry has reported to NIST that

they get inconsistent results from jitter

measurements using different techniques and

would welcome a neutral party to evaluate the

limits of the many measurement methods.

The importance of jitter measurement capability

is described in the 2001 International

Technology Roadmap for Semiconductors on

several pages. In particular, on page 11, it states,

“The jitter generated by the transmitter is the

key parameter to guarantee transmitter quality.

Currently, jitter measurement capability on ATE

is in its infancy – there is no instrument available

that simultaneously satisfies the noise floor,

analog bandwidth, and test time requirements

for high performance interfaces.”

Propagation Delay – Propagation delay in active

circuits is defined as the difference between the

occurrence of some event on an output signal

relative to that of the input signal. The

measurement of this type of delay is not

straightforward because the characteristics of the

input signal affect both the characteristics of the

output signal and the time the output signal exits

the circuit. Automated test equipment (ATE) from

different manufacturers produce different input

signals, and this results in different delays for a

given circuit. Furthermore, the ATE results may

not emulate actual delays because the ATE signal

and/or electrical environment may not represent

accurately what is expected by the circuit.

The ability to reproducibly measure propagation

delay is an issue. This will require interaction

with industry to develop an accepted reference

signal and, perhaps, impedance environment.

Furthermore, accurate (picosecond) delay

measurements will require traceability over the

delay ranges (several microseconds) required by

industry. Measurement protocols will also have

to be established.
Semiconductor Microelectronics and Nanoelectronics Programs 155

TECHNICAL STRATEGY

To develop a precision differential delay system

that will be used to measure jitter and propagation

delay in electronic circuits and systems. The

hardware for providing differential delays to

1 μsec will be designed and developed, and the

temperature sensitivity and repeatability of the

differential delay settings will be measured. In

addition, methods for accurately (uncertainties

± 1 ps) calibrating the discrete differential delays

will be developed. A continuously variable

differential delay from 0 ns to 1 ns and methods

for accurately calibrating the delay transfer

function and its temperature sensitivity will also

be developed.

DELIVERABLES: Design and build the medium

differential delay (1 ns to 16 ns) subsystem.

3Q 2003

DELIVERABLES: Design and build the coarse

differential delay (> 10 ns) and fine differential delay

(< 2 ns) subsystems. 4Q 2003

DELIVERABLES: Design calibration methods for and

calibrate the precision differential delay system.

2Q 2004

ACCOMPLISHMENTS

# New project.
156 National Institute of Standards and Technology

FAILURE ANALYSIS OF INTEGRATED CIRCUITS

Project Leader:

David Pappas

GOALS

This project is developing magnetoresistive

arrays and readout electronics that can image

magnetic fields from data storage media and

current distributions in integrated circuits. These

are targeted to failure analysis of integrated

circuits.

CUSTOMER NEEDS

Failure analysis to identify field problems as well

as to diagnose design errors in the development

phase of integrated circuit production have

always been critically important for success.

Rapidly escalating circuit complexity has placed

ever-increasing demands on failure analysis

techniques.

The 2001 International Technology Roadmap

for Semiconductors explicitly states the need for

defect analysis and failure analysis in Table 19,

page 117 in the section titled Test and Test

Equipment. On page 327 in the section on

Assembly and Packaging is the statement,

“Conception and development of tools for rapid

electrical and physical fault isolation of package

and interconnect technologies is critical.”

TECHNICAL STRATEGY

The Magnetic Technology Division has been

developing metrology for magnetic data storage

since 1996. This program has resulted in advanced

measurement techniques for imaging information

stored on magnetic media with high resolution

and relative ease. The nanoscale recording

system (NRS) developed under this program is a

general-purpose instrument that uses read/write

heads similar to those in computer hard-disk or

tape drives to read and write data on magnetic

media. The NRS can image by rastering either the

head, using computer-controlled micrometers

with 50 nanometer resolution, or the storage

medium, using a piezoelectric x-y stage with 1

nanometer resolution. The NRS is being used as

a prototype for forensic analysis of audio tapes.

We have been able to perform high-speed imaging

of tape samples, identify the signatures of erase

and write heads, and reconstruct analog and

digital data.

The NRS is also capable of magnetic field imaging

for non-destructive analysis of integrated circuits

and materials. Failure analysis of very-large-scaleintegration (VLSI) chips at the die level and after

packaging is critical to identifying short circuits

due to defects and design flaws. Inversion of the

magnetic fields above these devices can provide

high-resolution images of the current distributions

noninvasively. In addition, hidden corrosion and

cracks in materials can be imaged using induced

eddy-current magnetic-field mapping. We have

developed both single-element and multipleelement magnetoresistive probes on a scanning

system to measure these fields. The inversion can

isolate currents and defects with very high

accuracy. We have developed 64- and 256-element

linear arrays of multiplexed sensors. The system

has been applied to failures in flip-chip highdensity processors, random-access memory

chips, and failure analysis of fastener materials.

DELIVERABLES: Design, fabricate and test a linear,

eight-element anisotropic-magnetoresistance (AMR)

“comb” array and controller. Extend the comb array

to 64 elements and compare it to on-chip arrays.

3Q 2003

Develop 50 ohm single magnetoresistive probes and

characterize their frequency response. Test linear

arrays with both in-plane and perpendicular

transforms. 3Q 2003

ACCOMPLISHMENTS

Applications in Integrated Circuit Failure

Analysis — Using a scanning magnetic field

microscope,1 we have analyzed the magnetic fields

from defective integrated circuits (Fig. 1).

1

 We worked with the Federal Bureau of Investigation to upgrade its scanning magnetic field

microscope with magnetoresistive sensors. The

microscope is used for forensic authenticity

analysis of magnetic storage tape evidence, for

example, audio cassette and videotapes. The

system originally used a VHS inductive tape head

sensor coupled to a SQUID magnetometer. The

new sensors have higher resolution and

incorporate a second-harmonic modulation and

sense scheme that rejects thermal anomalies. This

allows the sensor to slide in contact with the

sample, giving the maximum possible resolution.

At present, the resolution allows for 2 micrometer

wide tracks with 0.02 micrometer downtrack

resolution. Other enhancements to the system

were vibration isolation and an electronic

triggering method to more closely map the actual

position of the head over the media.
Semiconductor Microelectronics and Nanoelectronics Programs 157

Figure 1. Failure-analysis image of currents in a flipchip random-access memory. The image is

2.5 centimeters by 2.5 centimeters and shows the

currents in the chip as calculated by inverting the

z component of the magnetic field using Maxwell’s

equations.

This application will be focused on helping

industry screen production devices for

anomalous current drain, thereby increasing

enhancing their failure analysis methods.

RECENT PUBLICATIONS

“Localization of currents in integrated circuits”, A. V.

Nazarov, F. D. DaSilva, D. P. Pappas, submitted, Journal

of Applied Physics.

Second Harmonic Magneto-Resistive Imaging to

Authenticate and Recover Data from Magnetic Storage

Media, D. P. Pappas, C. S. Arnold, G. Shalev, C. Eunice,

D. Stevenson, S. Voran, M. E. Read, E. M. Gormley, J.

Cash, K. Marr, J. J. Ryan, Proceedings of SPIE

Conference on Technologies for Law Enforcement,

Boston, MA, in press, Nov. 2000.
158 National Institute of Standards and Technology

MANUFACTURING SUPPORT

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 efficient 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 International 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 flow across the “engineering chain.”
Semiconductor Microelectronics and Nanoelectronics Programs 159

NIST/SEMATECH e-HANDBOOK OF

STATISTICAL METHODS

Technical Contacts:

W.F. Guthrie

A. Heckert

J.J. Filliben

“I am thoroughly enjoying

your Engineering Statistics Handbook. Thanks

so much for adding real

value to the information

available on the

Internet!!!”

Paul Zaremba,

Systems Engineer,

Agilent Technologies

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 efficiently. An online 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 manufacturing 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 field since its inception in 1988.

Two of the more difficult 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.

DELIVERABLES: Develop and release a version of

the e-Handbook on CD for off-line use by customers.

2Q 2003

ACCOMPLISHMENTS

# Since release of the final version, work has

focused on publicizing the e-Handbook, responding to user feedback, and developing a CD version for off-line use. In the last year, 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.

RECENT PUBLICATIONS

NIST/SEMATECH e-Handbook of Statistical Methods,

M. Carroll Croarkin and Paul Tobias, editors, http://

www.nist.gov/stat.handbook/.

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.

Page from a case study in the process modeling

chapter of the Handbook.
160 National Institute of Standards and Technology

ENGINEERING CHAIN MANAGEMENT

IN THE SEMICONDUCTOR INDUSTRY

Technical Contacts:

B. Goldstein

Y. Li

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.

Integration of FICS [Factory Information

Control System] applications with businesslevel software systems provides accurate factory

floor data for supply management, and improved

product tracking. Potential solutions will require

the standardization of technologies that enable

this level of integration. 2001 International

Technology Roadmap for Semiconductors,

Factory Integration (ITRS), p. 17.

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.

2001 ITRS, Design, p. 1.

According to the 2001 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 specifications and

tool interfaces for the design of a System-on-aChip (SoC) would reduce time to market by

eliminating the need for translations. Security

mechanisms must also be incorporated to protect

IP during transfers.

Other information-exchange gaps in design

houses include standard data formats for timing

and power to expedite timing closure, reduce

design iterations, and promote interoperability

among best-of-breed software tools.

The industry trend towards outsourcing is

leading towards silos of expertise, exactly when

technology advances require a multi-disciplinary

approach to problem solving. The production

process is being conducted across fabs – not

only at the traditional point of packaging and test,

but mid-stream in the IC fabrication process. As

the wafers change hands, so must the information

about that wafer which accumulated during its

production life cycle.

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

benefit from NIST’s neutrality, multi-industry

expertise, and broad and detailed knowledge of

the Information Technology (IT) standards

development process.

DELIVERABLES: Contribute to the 2003 Factory

Integration component of the ITRS by co-leading the

Engineering Chain Management subteam. 4Q 2003

Attendance at workshops, conferences, and

standards meetings held by key organizations in
Semiconductor Microelectronics and Nanoelectronics Programs 161

the semiconductor community such as International SEMATECH (ISMT), Semicon-ductor

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 benefit.

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

figures to share their vision of how IT standards

would expedite advancement of new technologies

and business models.

DELIVERABLES: White paper evaluating semiconductor industry’s IT standards needs and priorities as

well as an assessment of potential solutions to the

current challenges based upon cross-industry

solutions facing similar issues. 4Q 2003

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 XMLbased 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: Define potential NIST roles in

providing technical assistance to facilitate current IT

standards development in the semiconductor

industry. 4Q 2003

ACCOMPLISHMENTS

# 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 specific 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
162 National Institute of Standards and Technology

ABBREVIATIONS AND ACRONYMS

AC 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

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

DC direct current

DFT design-for-test

DMA differential mobility analyzer

DRAM dynamic random-access memory

DSP digital signal processing

DUV deep ultraviolet
Semiconductor Microelectronics and Nanoelectronics Programs 163

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 field ion field emission microscope

FIM field ion microscope

FWHM full-width half-maximum

GIXR/SE grazing incidence x-ray reflection/spectrascopic ellipsometry

GIXPS grazing incidence x-ray photoelectron spectroscopy

HRTEM high resolution transmission electron microscope

HSQ hydrogen silsesquoxane

I-V current-voltage

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 flow controller

MMIC millimeter and microwave integrated circuits

MOS metal-oxide-semiconductor

MOSFET metal-oxide-semiconductor field-effect transistor

MUX multiplex

NCMS National Center for Manufacturing Sciences

NDP neutron depth profiling

NGL next generation lithography
164 National Institute of Standards and Technology

NEMI National Electronics Manufacturing Initiative

NIST National Institute of Standards and Technology

NLO non-linear optical

NSOM nearfield scanning optical microscopy

OMAG Overlay Metrology Advisory Group (ISMT)

PED Precision Engineering Division (NIST)

PLIF planar laser-induced fluorescence

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-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

TDR time-domain reflectometry

TEM transmission electron microscope

TFTC thin-film thermocouple

TOF time-of-flight
Semiconductor Microelectronics and Nanoelectronics Programs 165

TMAH tetramethyl ammonium hydroxide

UHV ultra-high vacuum

UV ultraviolet

WMS wavelength modulation spectroscopy

VUV vacuum ultraviolet

XPS x-ray photoelectron spectroscopy
166 National Institute of Standards and Technology

TECHNICAL CONTACTS
Semiconductor Microelectronics and Nanoelectronics Programs 167
168 National Institute of Standards and Technology