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NIST

PUBLICATIONS

NIST SPECIAL PUBLICATION 260-119

U.S. DEPARTMENT OF COMMERCE/Technology Administration/National Institute of Standards and Technology

Standard Reference Materials:

Antireflecting-Chromium

Linewidth Standard, SRM 473,

for Calibration of Optical Microscope

Linewidth Measuring Systems

QC

100

.U57

260-

119

1992

Carol F. Vezzetti, Ruth N. Varner, and James E. Potzick
The National Institute of Standards and Technology was established in 1988 by Congress to "assist

industry in the development of technology . . . needed to improve product quality, to modernize

manufacturing processes, to ensure product reliability . . . and to facilitate rapid commercialization . . . of products based on new scientific discoveries."

NIST, originally founded as the National Bureau of Standards in 1901, works to strengthen U.S.

industry's competitiveness; advance science and engineering; and improve public health, safety, and the

environment. One of the agency's basic functions is to develop, maintain, and retain custody of the

national standards of measurement, and provide the means and methods for comparing standards used

in science, engineering, manufacturing, commerce, industry, and education with the standards adopted

or recognized by the Federal Government.

As an agency of the U.S. Commerce Department's Technology Administration, NIST conducts basic

and applied research in the physical sciences and engineering and performs related services. The Institute does generic and precompetitive work on new and advanced technologies. NIST's research facilities are

located at Gaithersburg, MD 20899, and at Boulder, CO 80303. Major technical operating units and their principal activities are listed below. For more information contact the Public Inquiries Desk, 301-975-3058.

Technology Services

• Manufacturing Technology Centers Program

• Standards Services

• Technology Commercialization

• Measurement Services

• Technology Evaluation and Assessment

• Information Services

Electronics and Electrical Engineering

Laboratory

• Microelectronics

• Law Enforcement Standards

• Electricity

• Semiconductor Electronics

• Electromagnetic Fields1

• Electromagnetic Technology1

Chemical Science and Technology

Laboratory

• Biotechnology

• Chemical Engineering 1

• Chemical Kinetics and Thermodynamics

• Inorganic Analytical Research

• Organic Analytical Research

• Process Measurements

• Surface and Microanalysis Science

• Thermophysics2

Physics Laboratory

• Electron and Optical Physics

• Atomic Physics

• Molecular Physics

• Radiometric Physics

• Quantum Metrology

• Ionizing Radiation

• Time and Frequency1

• Quantum Physics1

Manufacturing Engineering Laboratory

• Precision Engineering

• Automated Production Technology

• Robot Systems

• Factory Automation

• Fabrication Technology

Materials Science and Engineering

Laboratory

• Intelligent Processing of Materials

• Ceramics

• Materials Reliability1

• Polymers

• Metallurgy

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Building and Fire Research Laboratory

• Structures

• Building Materials

• Building Environment

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Computer Systems Laboratory

• Information Systems Engineering

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Computing and Applied Mathematics

Laboratory

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• Scientific Computing Environments2

• Computer Services2

• Computer Systems and Communications2

• Information Systems

1 At Boulder, CO 80303.

2 Some elements at Boulder, CO 80303.
Standard Reference Materials:

NIST Special Publication 260-119

Antireflecting-Chromium

Linewidth Standard, SRM 473,

for Calibration of Optical Microscope

Linewidth Measuring Systems

Carol F. Vezzetti

Ruth N. Varner

James E. Potzick

Precision Engineering Division

Manufacturing Engineering Laboratory

National Institute of Standards and Technology

Gaithersburg, MD 20899

U.S. DEPARTMENT OF COMMERCE, Barbara Hackman Franklin, Secretary

TECHNOLOGY ADMINISTRATION, Robert M. White, Under Secretary for Technology

NATIONAL INSTITUTE OF STANDARDS AND TECHNOLOGY, John W. Lyons, Director

Issued September 1992
National Institute of Standards and Technology Special Publication 260-119

Natl. Inst. Stand. Technol. Spec. Publ. 260-119, 48 pages (Sept. 1992)

CODEN: NSPUE2

U.S. GOVERNMENT PRINTING OFFICE

WASHINGTON: 1992

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

Standard Reference Materials (SRM's) as defined by the National Institute of

Standards and Technology (NIST) are well-characterized materials, produced in

quantity and certified for one or more physical or chemical properties. They are used to assure the accuracy and compatibility of measurements throughout the

Nation. SRM's are widely used as primary standards in many diverse fields in

science, industry, and technology, both within the United States and throughout

the world. They are -also used extensively in the fields of environmental and

clinical analysis. In many applications, traceability of quality control and

measurement processes to the national measurement system is carried out through

the mechanism and use of SRM's. For many of the Nation's scientists and

technologist s , it is therefore of more than passing interest to know the details

of the measurements made at NIST in arriving at the certified values of the SRM's

produced. The NIST Special Publication 260 Series is a series of papers reserved

for this purpose.

The 260 Series is dedicated to the dissemination of information on different

phases of the preparation, measurement, certification, and use of NIST SRM's.

In general, much more detail will be found in these papers than is generally

allowed, or desirable, in scientific journal- art icles . This enables the user to assess the validity and accuracy of the measurement processes employed, to judge

the statistical analysis, and to learn details of techniques and methods utilized

for work entailing greatest care and accuracy. These papers also should provide

sufficient additional information so SRM's can be utilized in new applications

in diverse fields not foreseen at the time the SRM was originally issued.

• Inquiries concerning the technical content of this paper should be directed to

the author (s). Other questions concerned with the availability, delivery, price,

and so forth, will receive prompt attention from:

• Standard Reference Materials Program

Bldg. 202, Rm. 204

National Institute of Standards and Technology

Gaithersburg, MD 20899

William P. Reed, Chief

Standard Reference Materials Program

iii
OTHER NIST PUBLICATIONS IN THIS SERIES

McKenzie, R. L., ed., NIST Standard Reference Materials Catalog 1990-91, NIST Spec. Publ. 260 (January

1990)

Michaelis, R. E., and Wyman, L. L., Standard Reference

Materials: Preparation of White Cast Iron

Spectrochemical Standards, NBS Misc. Publ. 260-1

(June 1964). COM74-11061**

Michaelis, R. E., Wyman, L. L., and Flitsch, R., Standard

Reference Materials: Preparation of NBS CopperBase Spectrochemical Standards, NBS Misc. Publ. 260-2 (October 1964). COM74- 11063**

Michaelis, R. E., Yakowitz, H, and Moore, G. A., Stan- dard Reference Materials: Metallographic Characterization of an NBS Spectrometric Low-Alloy Steel Standard, NBS Misc. Publ. 260-3 (October 1964).

COM74- 11060**

Alvarez, R., and Flitsch, R„ Standard Reference Materials: Accuracy of Solution X-Ray Spectrometric

Analysis of Copper-Base Alloys, NBS Misc. Publ. 260-5 (March 1965). PB 168068**

Shultz, J. I., Standard Reference Materials: Methods for die Chemical Analysis of White Cast Iron Standards,

NBS Misc. Publ. 260-6 (July 1965). COM74-

11068**

Bell, R. K., Standard Reference Materials: Mediods for the Chemical Analysis of NBS Copper-Base

Spectrochemical Standards, NBS Misc. Publ. 260-7

(October 1965). COM74-1 1067**

Richmond, M. S., Standard Reference Materials:

Analysis of Uranium Concentrates at die National

Bureau of Standards, NBS Misc. Publ. 260-8

(December 1965). COM74- 11066**

Anspach, S. C, Cavallo, L. M„ Garfinkel, S. B„ Hutchinson, J. M. R., and Smith, C. N., Standard Reference

Materials: Half Lives of Materials Used in die Preparation of Standard Reference Materials of

Nineteen Radioactive Nuclides Issued by die National

Bureau of Standards, NBS Misc. Publ. 260-9

(November 1965). COM74-1 1065**

Yakowitz, H., Viedi, D. L., Heinrich, K. F. J., and

Michaelis, R. E., Standard Reference Materials: Homogeneity Characterization of NBS

Spectrometric Standards II: Cartridge Brass and LowAlloy Steel, NBS Misc. Publ. 260-10 (December

1965). COM74-1 1064**

Napolitano, A., and Hawkins, E. G., Standard Reference

Materials: Viscosity of Standard Lead-Silica Glass,

NBS Misc. Publ. 260-1 1 (November 1966).

Yakowitz, H, Vietii, D. L., and Michaelis, R. E., Stan- dard Reference Materials: Homogeneity Characterization of NBS Spectrometric Standards III: White

Cast Iron and Stainless Steel Powder Compact, NBS

Misc. Publ. 260-12 (September 1966).

Menis, O., and Sterling, J. T., Standard Reference Materials: Determination of Oxygen in Ferrous Materials—SRM 1090, 1091, and 1092, NBS Misc. Publ. 260-14 (September 1966).

Yakowitz, H, Michaelis, R. E., and VieUi, D. L., Stan- dard Reference Materials: Homogeneity Characterization of NBS Spectrometric Standards IV:

Preparation and Microprobe Characterization of W20% Mo Alloy Fabricated by Powder Metallurgical

Mediods, NBS Spec. Publ. 260-16 (January 1969).

COM74- 11062**

Paule, R. C, and Mandel, J., Standard Reference Materials: Analysis of Interlaboratory Measurements on

die Vapor Pressure of Gold (Certification of Standard

Reference Material 745). NBS Spec. Publ. 260-19

(January 1970). PB 190071**

Paule, R. C, and Mandel, J., Standard Reference Materials: Analysis of Interlaboratory Measurements on

die Vapor Pressures of Cadmium and Silver, NBS

Spec. Publ. 260-21 (January 1971). COM74-1 1359**

Yakowitz, H, Fiori, C. E., and Michaelis, R. E., Standard

Reference Materials: Homogeneity Characterization

of Fe-3 Si Alloy, NBS Spec. Publ. 260-22 (February

1971). COM74-1 1357**

Napolitano, A., and Hawkins, E. G., Standard Reference

Materials: Viscosity of a Standard Borosilicate Glass,

NBS Spec. Publ. 260-23 (December 1970). COM71-

00157**

Sappenfield, K. M., Marinenko, G., and Hague, J. L., Standard Reference Materials: Comparison of Redox

Standards, NBS Spec. Publ. 260-24 (January 1972).

COM72-50058**

Hiclio, G. E., Yakowitz, H, Rasberry, S. D., and

Michaelis, R. E., Standard Reference Materials: A

Standard Reference Material Containing Nominally

Four Percent Austenite, NBS Spec. Publ. 260-25

(February 1971). COM74-1 1356**

iv
Martin, J. F., Standard Reference Materials: National

Bureau of Standards-US Steel Corporation Joint Prograin for Determining Oxygen and Nitrogen in Steel, NBS Spec. Publ. 260-26 (February 1971). PB

81176620**

Garner, E. L., Machlan, L. A., and Shields, W. R., Stan- dard Reference Materials: Uranium Isotopic Standard

Reference Materials, NBS Spec. Publ. 260-27 (April

1971). COM74-1 1358**

Heinrich, K. F. J., Myklebust, R. L., Rasberry, S. D., and

Michaelis, R. E., Standard Reference Materials:

Preparation and Evaluation of SRM's 481 and 482

Gold-Silver and Gold-Copper Alloys for

Microanalysis, NBS Spec. Publ. 260-28 (August

1971). COM7 1-50365**

Geller, S. B., Standard Reference Materials: Calibration

of NBS Secondary Standard Magnetic Tape (Computer Amplitude Reference) Using the Reference

Tape Amplitude Measurement '"Process A-Model

2," NBS Spec. Publ. 260-29 (June 1971). COM71-

50282**

Gorozhanina, R. S., Freedman, A. Y., and Shaievitch, A.

B. (translated by M. C. Selby), Standard Reference

Materials: Standard Samples Issued in die USSR (A

Translation from die Russian), NBS Spec. Publ. 260-

30 (June 1971). COM7 1-50283**

Hust, J. G., and Sparks, L. L., Standard Reference Materials: Thermal Conductivity of Electrolytic Iron SRM

734 from 4 to 300 K, NBS Spec. Publ. 260-31

(November 1971). COM71-50563**

Mavrodineanu, R., and Lazar, J. W., Standard Reference

Materials: Standard Quartz Cuvettes for High Accuracy Spectrophotometry, NBS Spec. Publ. 260-32

(December 1973). COM74-50018**

Wagner, H. L., Standard Reference Materials: Comparison of Original and Supplemental SRM 705, Nar- row Molecular Weight Distribution Polystyrene, NBS

Spec. Publ. 260-33 (May 1972). COM72-50526**

Sparks, L. L., and Hust, J. G., Standard Reference Materials: Thermal Conductivity of Austenitic Stainless

Steel, SRM 735 from 5 to 280 K, NBS Spec. Publ. 260-35 (April 1972.) COM72-50368**

Cali, J. P., Mandel, J., Moore, L. J., and Young, D. S., Standard Reference Materials: A Referee MeUiod for die Determination of Calcium in Serum NBS SRM

915, NBS Spec. Publ. 260-36 (May 1972). COM72-

50527**

Shultz, J. I., Bell, R. K., Rains, T. C, and Menis, O., Standard Reference Materials: Mediods of Analysis

of NBS Clay Standards, NBS Spec. Publ. 260-37

(June 1972). COM72-50692**

Clark, A. F., Denson, V. A., Hust, J. G., and Powell, R.

L., Standard Reference Materials: The Eddy Current Decay Mediod for Resistivity Characterization of High-Purity Metals, NBS Spec. Publ. 260-39 (May

1972). COM72-50529**

McAdie, H. G., Gani, P. D., and Menis, O., Standard

Reference Materials: Selection of Thermal Analysis

Temperature Standards Through a Cooperative Study

(SRM 758, 759, 760), NBS Spec. Publ. 260-40

(August 1972) COM72-50776**

Wagner, H. L., and Verdier, P. H., eds., Standard Refer- ence Materials: The Characterization of Linear

Polyediylene, SRM 1475, NBS Spec. Publ. 260-42

(September 1972). COM72-50944**

Yakowitz, H., Ruff, A. W., and Michaelis, R. E., Stan- dard Reference Materials: Preparation and

Homogeneity Characterization of an Austenitic Iron- Chroniiuni-Nickel Alloy, NBS Spec. Publ. 260-43

(November 1972). COM73-50760**

Schooley, J. F., Soulen, R. J., Jr., and Evans, G. A., Jr., Standard Reference Materials: Preparation mid Use of Superconductive Fixed Point Devices, SRM 767,

NBS Spec. Publ. 260-44 (December 1972). COM73-

50037**

Greifer, B., Maiendial, E. J., Rains, T. C, and Rasberry,

S. D., Standard Reference Materials: Powdered LeadBased Paint, SRM 1579, NBS Spec. Publ. 260-45

(March 1973). COM73-50226**

Hust, J. G., and Giarratano, P. J., Standard Reference

Materials: Thermal Conductivity and Electrical

Resistivity Standard Reference Materials: Austenitic

Stainless Steel, SRM's 735 and 798, from 4 to 1200

K, NBS Spec. Publ. 260-46 (March 1975). COM75-

10339**

Hust, J. G., Standard Reference Materials: Electrical Resistivity of Electrolytic Iron, SRM 797, and Austenitic Stainless Steel, SRM 798, from 5 to 280 K,

NBS Spec. Publ. 260-47 (February 1974). COM74-

50176**

Mangum, B. W„ and Wise, J. A., Standard Reference

Materials: Description and Use of Precision Thermometers for die Clinical Laboratory, SRM 933 and

SRM 934, NBS Spec. Publ. 260-48 (May 1974).

Superseded by NIST Spec. Publ. 260-1 13. Carpenter, B. S., and Reimer, G. M., Standard Reference

Materials: Calibrated Glass Standards for Fission Track Use, NBS Spec. Publ. 260-49 (November

1974). COM74-51 185**

v
Hust, J. G., and Giarratano, P. J., Standard Reference

Materials: Thermal Conductivity and Electrical

Resistivity Standard Reference Materials: Electrolytic Iron, SRM's 734 and 797 from 4 to 1000 K, NBS

Spec. Publ. 260-50 (June 1975). COM75-10698**

Mavrodineanu, R., and Baldwin, J. R., Standard Refer- ence Materials: Glass Filters As a Standard Reference

Material for Spectrophotometry-Selection, Preparation, Certification, Use-SRM 930 NBS Spec. Publ. 260-51 (November 1975). COM75- 10339**

Hust, J. G., and Giarratano, P. J., Standard Reference

Materials: Thermal Conductivity and Electrical

Resistivity Standard Reference Materials 730 and

799, from 4 to 3000 K, NBS Spec. Publ. 260-52

(September 1975). COM75-1 1 193**

Durst, R. A., Standard Reference Materials: Standardization of pH Measurements, NBS Spec. Publ. 260-53 (February 1988, Revision of December 1975

version).

Burke, R. W., and Mavrodineanu, R., Standard Refer- ence Materials: Certification and Use of Acidic Potas- sium Dichromate Solutions as an Ultraviolet

Absorbance Standard, NBS Spec. Publ. 260-54

(August 1977). PB272168**

Ditmars, D. A., Cezairliyan, A., Ishihara, S., and

Douglas, T. B., Standard Reference Materials: Enthal- py and Heat Capacity; Molybdenum SRM 781, from

273 to 2800 K, NBS Spec. Publ. 260-55 (September

1977). PB272127**

Powell, R. L., Sparks, L. L., and Hust, J. G., Standard

Reference Materials: Standard Thermocouple Material, Pt-67: SRM 1967, NBS Spec. Publ. 260-56

(February 1978). PB277172**

Barnes, J. D., and Martin, G. M., Standard Reference

Materials: Polyester Film for Oxygen Gas Transmission Measurements SRM 1470, NBS Spec. Publ. 260-

58 (June 1979). PB297098**

Velapoldi, R. A., Paule, R. C, Schaffer, R., Mandel, J., and Moody, J. R., Standard Reference Materials: A

Reference Method for die Determination of Sodium

in Serum, NBS Spec. Publ. 260-60 (August 1978).

PB286944**

Verdier, P H., and Wagner, H. L., Standard Reference

Materials: The Characterization of Linear

PolyeUiylene (SRM 1482, 1483, 1484), NBS Spec

Publ. 260-61 (December 1978). PB289899**

Soulen, R. J., and Dove, R. B., Standard Reference

Materials: Temperature Reference Standard for Use

Below 0.5 K (SRM 768), NBS Spec. Publ. 260-62

(April 1979). PB294245**

Velapoldi, R. A., Paule, R. C, Schaffer, R., Mandel, J., Machlan, L. A., and Gramlich, J. W., Standard Refer- ence Materials: A Reference Method for die Deter- mination of Potassium in Serum, NBS Spec. Publ. 260-63 (May 1979). PB297207**

Velapoldi, R. A., and Mielenz, K. D., Standard Reference

Materials: A Fluorescence Standard Reference Material Quinine Sulfate Dihydrate (SRM 936), NBS Spec.

Publ. 260-64 (January 1980). PB80-132046**

Marinenko, R. B., Heinrich, K. F. J., and Ruegg, F. C,

Standard Reference Materials: Micro-Homogeneity

Studies of NBS Standard Reference Materials, NBS

Research Materials, and Otiier Related Samples, NBS

Spec. Publ. 260-65 (September 1979). PB300461**

Venable, W. H., Jr., and Eckerle, K. L., Standard Refer- ence Materials: Didymium Glass Filters for

Calibrating die WavelengUi Scale of Spectrophotometers-SRM 2009, 2010, 2013, and 2014, NBS Spec.

Publ. 260-66 (October 1979). PB80-104961**

Velapoldi, R. A., Paule, R. C, Schaffer, R., Mandel, J., Murphy, T. J., and Gramlich, J. W., Standard Refer- ence Materials: A Reference Mediod for the Deter- mination of Chloride in Serum, NBS Spec. Publ. 260-67 (November 1979). PB80-1 101 17**

Mavrodineanu, R., and Baldwin, J. R., Standard Refer- ence Materials: Metal-On-Quartz Filters as a Standard

Reference Material for Spectrophotometry SRM

2031, NBS Spec. Publ. 260-68 (April 1980). PB80-

197486**

Velapoldi, R. A., Paule, R. C, Schaffer, R., Mandel, J., Machlan, L. A., Garner, E. L., and Rains, T. C,

Standard Reference Materials: A Reference Mediod

for die Determination of Lidiium in Serum, NBS

Spec. Publ. 260-69 (July 1980). PB80-2091 17**

Marinenko, R. B., Biancaniello, F„ Boyer, P. A., Ruff, A. W., and DeRobertis, L., Standard Reference Materials: Preparation and Characterization of an Iron- Chromium-Nickel Alloy for Microanalysis, NBS

Spec. Publ. 260-70 (May 1981). PB84-165349**

Seward, R. W., and Mavrodineanu, R., Standard Refer- ence Materials: Summary of die Clinical Laboratory

Standards Issued by die National Bureau of Standards, NBS Spec. Publ. 260-71 (November 1981).

PB82-135161**

Reeder, D. J., Coxon, B., Enagonio, D., Christensen, R.

G., Schaffer, R., Howell, B. F., Paule, R. C, and

Mandel, J., Standard Reference Materials: SRM 900,

Antiepilepsy Drug Level Assay Standard, NBS Spec.

Publ. 260-72 (June 1981). PB81-220758

vi
Interrante, C. G., and Hicho, G. E., Standard Reference

Materials: A Standard Reference Material Containing

Nominally Fifteen Percent Austenite (SRM 486),

NBS Spec. Publ. 260-73 (January 1982). PB82-

215559**

Marinenko, R. B., Standard Reference Materials:

Preparation and Characterization of K-41 1 and K-414

Mineral Glasses for Microanalysis: SRM 470, NBS

Spec. Publ. 260-74 (April 1982). PB82-221300**

Weidner, V. R., and Hsia, J. J., Standard Reference

Materials: Preparation and Calibration of First Surface Aluminum Mirror Specular Reflectance Stan- dards (SRM 2003a), NBS Spec. Publ. 260-75 (May

1982). PB82-221367**

Hicho, G. E., and Eaton, E. E., Standard Reference

Materials: A Standard Reference Material Containing

Nominally Five Percent Austenite (SRM 485a), NBS

Spec. Publ. 260-76 (August 1982). PB83-1 15568**

Furukawa, G. T., Riddle, J. L., Bigge, W. G, and Pfieffer, E. R., Standard Reference Materials: Application of Some Metal SRM's as Thermometric Fixed Points,

NBS Spec. Publ. 260-77 (August 1982). PB83-

117325**

Hicho, G. E., and Eaton, E. E., Standard Reference

Materials: Standard Reference Material Containing

Nominally Thirty Percent Austenite (SRM 487), NBS

Spec. Publ. 260-78 (September 1982). PB83-

115576**

Richmond, J. C, Hsia, J. J., Weidner, V. R., and Wilmering, D. B., Standard Reference Materials: Second

Surface Mirror Standards of Specular Spectral Reflec- tance (SRM's 2023, 2024, 2025), NBS Spec. Publ. 260-79 (October 1982). PB84-203447**

Schaffer, R., Mandel, J., Sun, T., Cohen, A., and Hertz, H. S., Standard Reference Materials: Evaluation by an ID/MS Method of the AACC Reference Method for Serum Glucose, NBS Spec. Publ. 260-80 (October

1982) . PB84-216894**

Burke, R. W., and Mavrodineanu, R., Standard Refer- ence Materials: Accuracy in Analytical

Spectrophotometry, NBS Spec. Publ. 260-81 (April

1983) . PB83-214536**

Weidner, V. R., Standard Reference Materials: White

Opal Glass Diffuse Spectral Reflectance Standards

for the Visible Spectrum (SRM's 2015 and 2016),

NBS Spec. Publ. 260-82 (April 1983). PB83-

220723**

Bowers, G. N., Jr., Alvarez, R., Cali, J. P., Eberhardt, K.

R., Reeder, D. J., Schaffer, R., and Uriano, G. A., Standard Reference Materials: The Measurement of

die Catalytic (Activity) Concentration of Seven En- zymes in NBS Human Serum SRM 909, NBS Spec.

Publ. 260-83 (June 1983). PB83-239509**

Gills, T. E., Seward, R. W., Collins, R. J., and Webster,

W. C, Standard Reference Materials: Sampling, Materials Handling, Processing, and Packaging of NBS

Sulfur in Coal Standard Reference Materials 2682,

2683, 2684, and 2685, NBS Spec. Publ. 260-84

(August 1983). PB84-109552**

Swyt, D. A., Standard Reference Materials: A Look at Techniques for the Dimensional Calibration of Stan- dard Microscopic Particles, NBS Spec. Publ. 260-85

(September 1983). PB84-1 12648**

Hicho, G. E., and Eaton, E. E., Standard Reference

Materials: A Standard Reference Material Containing

Two and One-Half Percent Austenite, SRM 488, NBS

Spec. Publ. 260-86 (December 1983). PB84-

143296**

Mangum, B. W., Standard Reference Materials: SRM

1969: Rubidium Triple-Point - A Temperature Refer- ence Standard Near 39.30 X, NBS Spec. Publ. 260-

87 (December 1983). PB84- 149996**

Gladney, E. S., Bums, C. E., Perrin, D. R., Roelandts, I., and Gills, T. E., Standard Reference Materials: 1982

Compilation of Elemental Concentration Data for NBS Biological, Geological, and Environmental

Standard Reference Materials, NBS Spec. Publ. 260-

88 (March 1984). PB84-218338**

Hust, J. G, Standard Reference Materials: A FineGrained, Isotropic Graphite for Use as NBS Thermophysical Property RM's from 5 to 2500 K, NBS

Spec. Publ. 260-89 (September 1984). PB85-

112886**

Hust, J. G, and Lankford, A. B., Standard Reference

Materials: Update of Thermal Conductivity and

Electrical Resistivity of Eleetrolytic Iron, Tungsten,

and Stainless Steel, NBS Spec. Publ. 260-90 (September 1984). PB85-1 15814**

Goodrich, L. F., Vecchia, D. F., Pittman, E. S., Ekin, J. W., and Clark, A. F., Standard Reference Materials:

Critical Current Measurements on an NbTi Superconducting Wire Standard Reference Material, NBS

Spec. Publ. 260-91 (September 1984). PB85-

118594**

Carpenter, B. S., Standard Reference Materials:

Calibrated Glass Standards for Fission Track Use

(Supplement to NBS Spec. Publ. 260-49), NBS Spec.

Publ. 260-92 (September 1984). PB85-1 13025**

vii
Ehrstein, J.R., Standard Reference Materials: Preparation and Certification of Standard Reference Materials for Calibration of Spreading Resistance Probes,

NBS Spec. Publ. 260-93 (January 1985). PB85-

177921**

Gills, T. E., Koch, W. F., Stolz, J. W., Kelly, W. R., Paulsen, P. J., Colbert, J. C, Kirklin, D. R., Pei, P.T.S., Weeks, S., Lindstrom, R. M., Fleming, R. F., Greenberg, R. R., and Paule, R. C, Standard Reference

Materials: Methods and Procedures Used at the National Bureau of Standards to Certify Sulfur in Coal

SRM's for Sulfur Content, Calorific Value, Ash Content, NBS Spec. Publ. 260-94 (December 1984).

PB85- 165900**

Mulholland, G. W., Hartman, A. W., Hembree, G. G,

Marx, E., and Lettieri, T. R., Standard Reference

Materials: Development of a 1 |im Diameter Particle Size Standard, SRM 1690, NBS Spec. Publ. 260-95

(May 1985). SN003-003-02665-4*

Carpenter, B. S., Gramlich, J. W., Greenberg, R. R., Machlan, L. A., DeBievre, P., Eschbach, H. L., Meyer, H., Van Audenhove, J., Connolly, V. E., Traliey, N. M., and Zook, A. C, Standard Reference

Materials: Uranium-235 Isotopic Abundance Stan- dard Reference Materials for Gamma Spectrometry

Measurements, NBS Spec. Publ. 260-96 (September

1986). PB87-108544**

Mavrodineanu, R., and Gills, T. E., Standard Reference

Materials: Summary of the Coal, Ore, Mineral, Rock,

and Refactory Standards Issued by the National

Bureau of Standards, NBS Spec. Publ. 260-97 (September 1985). SN003 -003-02688-3*

Hust, J. G., Standard Reference Materials: Glass Fiber- board SRM for Thermal Resistance, NBS Spec. Publ. 260-98 (August 1985). SN003-003-02674-3*

Callanan, J. E., Sullivan, S. A., and Vecchia, D. F., Standard Reference Materials: Feasibility Study for the Development of Standards Using Differential Scanning Calorimetry, NBS Spec. Publ. 260-99

(August 1985). SN003-003-02675-1*

Taylor, J. K., Standard Reference Materials: Handbook

for SRM Users, NBS Spec. Publ. 260- 100 (September

1985). PB86-1 10897**

Mangum, B. W., Standard Reference Materials: SRM

1970, Succinonitrile Triple-Point Standard: A

Temperature Reference Standard Near 58.08 °C, NBS

Spec. Publ. 260-101 (March 1986). SN003-003-

02722-7*

Weidner, V. R., Mavrodineanu, R., Mielenz, K. D., Velapoldi, R. A., Eckerle, K. L., and Adams, B., Standard Reference Materials: Holmium Oxide Solution Wavelength Standard from 240 to 640 nm - SRM

2034, NBS Spec. Publ. 260-102 (July 1986). PB86-

245727**

Hust, J. G., Standard Reference Materials: Glass

Fiberblanket SRM for Thermal Resistance, NBS

Spec. Publ. 260-103 (September 1985). SN003-003-

02687-5*

Mavrodineanu, R., and Alvarez, R., Standard Reference

Materials: Summary of the Biological and Botanical

Standards Issued by the National Bureau of Standards, NBS Spec. Publ. 260-104 (November 1985).

SN003-003-02704-9*

Mavrodineanu, R., and Rasberry, S. D., Standard Refer- ence Materials: Summary of die Environmental Research, Analysis, and Control Standards Issued by the National Bureau of Standards, NBS Spec. Publ. 260-

105 (March 1986). SN003-003-02725-1*

Koch, W. F., ed., Standard Reference Materials: MeUiods

and Procedures Used at the National Bureau of Stan- dards to Prepare, Analyze, and Certify SRM 2694,

Simulated Rainwater, and Recommendations for Use,

NBS Spec. Publ. 260-106 (July 1986). PB86-

247483**

Hartman, A. W., and McKenzie, R. L., Standard Refer- ence Materials: SRM 1965, Microsphere Slide ( 10 [im

Polystyrene Spheres), NBS Spec. Publ. 260-107

(November 1988).

Mavrodineanu, R., and Gills, T. E., Standard Reference

Materials: Summary of Gas Cylinder and Permeation

Tube Standard Reference Materials Issued by the National Bureau of Standards, NBS Spec. Publ. 260-

108 (May 1987).

Candela, G. A., Chandler-Horowitz, D., Novotny, D. B., Marchiando, J. F., and Belzer, B. J., Standard Refer- ence Materials: Preparation and Certification of an

Ellipsometrically Derived Thickness and Refractive

Index Standard of a Silicon Dioxide Film (SRM

2530), NIST Spec. Publ. 260-109 (October 1988).

Kirby, R. K., and Kanare, H. M., Standard Reference

Materials: Portland Cement Chemical Composition

Standards (Blending, Packaging, and Testing), NBS

Spec. Publ. 260-1 10 (February 1988).

Gladney, E. S., O'Malley, B. T., Roelandts, I., and Gills, T. E., Standard Reference Materials: Compilation of

Elemental Concentration Data for NBS Clinical,

Biological, Geological, mid Environmental Standard

Reference Materials, NBS Spec. Publ. 260-111

(November 1987).

viii
Marinenko, R. B., Blackburn, D. H., and Bodkin, J. B., Standard Reference Materials: Glasses for

Microanalysis: SRM's 1871-1875, NISTSpec. Publ. 260-112 (February 1990).

Mangum, B. W., and Wise, J. A., Standard Reference

Materials: Description and Use of a Precision Thermometer for the Clinical Laboratory, SRM934, N1ST

Spec. Publ. 260-113 (June 1990).

Vezzetti, C. F., Varner, R. N. and Potzick, J. E., Standard

Reference Materials: Bright-Chromium Linewidth

Standard, SRM 476, for Calibration of Optical Microscope Linewidth Measuring Systems, NIST Spec.

Publ. 260-114 (January 1991).

Williamson, M. P., Willman, N. E., and Grubb, D. S., Standard Reference Materials: Calibration of NIST

Standard Reference Material 3201 for 0.5 Inch ( 12.65 mm) Serial Serpentine Magnetic Tape Cartridge,

NIST Spec. Publ. 260-115 (February 1991).

Mavrodineanu, R. , Burke, R. W., Baldwin, J. R., Smith,

M. V., and Messman, J. D., Standard Reference Materials: Glass Filters as a Standard Reference Material

for Spectrophotometry and Selection, Preparation,

Certification and use of SRM 930 and SRM 1930,

NIST Spec. Publ. 260-116 (in prep).

Vezzetti, C. F., Vainer, R. N., and Potzick, J. E., Standard

Reference Materials: Antireflecting-Chromium

Linewidtn Standard, SRM 475, for Calibration of

Optical Microscope Linewidth Measuring Systems,

NISTSpec. Publ. 260-117 (January 19921

Williamson, M. P., Standard Reference Materials: Calibration of NIST Standard Reference Material

3202 for 18-Track, Parallel, and 36-Trace, Parallel Serpentine, 12.65 mm (0.5 in), 1491 cpmm (37871

cpi), Magnetic Tape Cartridge, NISTSpec. Publ. 260-

118 (July 1992)

*Send order with remittance to Superintendent of

Documents, U.S. Government Printing Office,

Washington, DC 20102. Remittance from foreign

countries should include an additional one fourth of

the purchase price for postage.

**May be ordered from: National Technical Information Services (NTIS), Springfield, VA 22161.

Contents

1. Introduction 1

2. Physical Characteristics of SRM 473 1

3. Using SRM 473 3

3.1 Special Precautions 3

3.2 Microscope Calibration Procedures 6

4. Calibration of SRM 473 7

4.1 The Measurement System 8

4.2 SRM Calibration Procedure 8

4.3 Specimen and Measurement Axis Alignment 9

4.4 Feature Measurement Sequence 10

4.5 Edge Location Determination 10

4.6 Precision and Accuracy of the SRM Calibration 12

Acknowledgments 14

References 15

Table 17

Figures 18

Appendix: Process Control for SRM 473 Calibrations 27

A. Introduction 27

B. Initialization of Process Parameters 27

C. Procedures for Process Control 29

D. Updating Process Parameters 30

E. Uncertainty Statement for SRM 473 33

Acknowledgments 34

References 34

Tables 35

Figures 36

xi
ANTIREFLECTING-CHROMIUM LINEWIDTH STANDARD, SRM 473,

FOR CALIBRATION OF OPTICAL MICROSCOPE

LINEWIDTH MEASURING SYSTEMS

C. F. Vezzetti, R. N. Varner, and J. E. Potzick

National Institute of Standards and Technology

Gaithersburg, Maryland 20899

ABSTRACT

The precise and accurate measurement of feature dimensions on photomasks, such as those used

in the production of integrated circuits, becomes increasingly difficult as the dimensions

approach the wavelength of the light used to make the measurement. The undesirable effects

of optical diffraction obsure the location of the feature edges. Raggedness and nonvertical walls

along the edges add to the uncertainty of the measurement.

Standard Reference Material SRM 473 was developed for use in calibrating optical microscopes

for measuring linewidths in the range of 0.5 to 30 jiim on antireflecting-chromium photomasks.

The SRM is described, and instructions on its use and precautions concerning its care and

handling are given.

The NIST linewidth measuring system and the procedures used to calibrate the SRM are

discussed. The algorithm used for determining the line edge location incorporates a threshold

criterion derived from analysis of microscope image profiles. The profiles are predicted by

computer modeling based on the theory of partial coherence. The performance of this system

is monitored by measuring line features on a control photomask before and after calibrating each

SRM.

KEY WORDS: accuracy; antireflecting-chromium; calibration; control charts; critical

dimensions; integrated circuits; linewidth measurement; optical microscope; photomask;

precision; semiconductor industry; standard reference material; statistical process control;

threshold; uncertainty.

xii
1. Introduction

The ability to measure and control critical dimensions during the production of integrated circuits

is essential to the semiconductor industry. Many measuring systems claiming high precision are now commercially available for use in determining some of these critical dimensions. As part

of a continuing effort to provide means for calibrating these systems, the National Institute of

Standards and Technology (NIST) has developed Optical Microscope Linewidth Measurement

Standard Reference Materials.

Standard Reference Material (SRM) 473 is designed for calibrating optical microscope systems

to measure linewidths on antireflecting-chromium photomasks. It was produced with

conventional technology by a commercial photomask manufacturing facility. In addition to isolated opaque and clear lines for linewidth calibration, this SRM contains line patterns for

checking length scale, adjusting video-type micrometers, and detecting mechanical or optical

nonlinearities. The design of the calibrated pattern is described in section 2.

Section 3 gives information and precautions on the care and use of this SRM to calibrate an

optical linewidth measuring system. Because of the variety of linewidth measuring systems in use today, no attempt has been made to give specific instructions for each type of microscope.

To calibrate the SRM line features, a photometric microscope with lenses selected for least

aberration was modified at NIST. Except for the initial positioning, aligning, and focusing of

the photomask, the entire calibration process is automated. The line features are illuminated

with partially coherent green light (wavelength 0.53 fxm) from a filtered incandescent source. The linewidths are determined from the image profile (image intensity versus position across a

feature). A considerable amount of theoretical work was conducted to establish the location on

the observed image profile that corresponds to the physical edge of a feature. The quality of

the feature edge geometry of a sample from each production batch of SRM photomasks is examined with a scanning electron microscope (SEM). The limit of accuracy of the calibration

measurements given in the certificate data is based on this sampled edge geometry and the

agreement between theoretically modeled and experimentally generated image profiles. Section

4 contains brief descriptions of the NIST linewidth calibration system and the automated

calibration process as well as discussions of the line edge location algorithm, precision, and

accuracy. The process control procedures used in the calibration of this SRM are discussed in

some detail in the Appendix.

2. Physical Characteristics of SRM 473

SRM 473 is made from an antireflecting-chromium photoplate by conventional photolithographic

techniques. The substrate is a quartz plate of a type commonly used for fabricating integrated

circuit photomasks, nominally 127 x 127 x 2.3 mm (5.0 x 5.0 x 0.09 in). The nominal

thickness of the chromium layer is 100 nm.

1
Figure 1 shows the overall pattern on the chromium-coated side of the standard. The three

horizontal and three vertical intersecting lines help locate the basic measurement pattern which

is repeated at eight locations around the center of the standard as shown in figure 2 (a magnified

view of the central area of fig. 1). A pattern identification number (1 through 8) is located

within each basic pattern. Only one of these eight patterns is certified. The certificate

accompanying the SRM gives the number of the certified pattern. The carpet design at the

center of the photomask as well as those within each basic pattern contain no calibrated features.

Figure 3 shows the details of the features in each of the eight identical patterns on the SRM.

The pattern identification number can be seen in the lower right quadrant, just above the carpet

design. The vertical sides of the box surrounding the basic pattern are parallel to the calibrated

line features and may be used to aid in aligning the SRM features to be perpendicular to the

measurement axis.

The calibrated features are arranged in six rows. Row numbers are located at the ends of each row. Each feature within rows 1 through 5 is further identified by a letter, A through L, located

immediately above the feature. Thus, IE refers to the opaque line in row 1 at position E. Row

6 contains a single multiple-line feature with every 5th line elongated and every 10th line

numbered. All rows on the SRM contain a broken horizontal fiducial line which defines the

measurement position on each feature.

Row 1 consists of 12 opaque lines (1A through 1L) on a clear background, and row 2 consists

of 12 clear lines (2 A through 2L) on an opaque background. These opaque and clear lines are used for calibrating optical microscopes used to measure linewidths of isolated lines of either or both polarities. Nominal linewidths of these features range from 0.5 to 30 /xm.

Rows 3 and 4 are intended to be used for calibrating optical microscopes for making line spacing

(pitch) measurements as well as for making initial length scale adjustments when calibrating

linewidth measurement systems. Row 3 consists of five features (3A through 3E), each with

four opaque lines. Certified values are given for the pitch of the two interior (short) lines

1 of

each feature. Nominal pitches for these features range from 2.0 to 6.0 fim. Row 4 contains a

series of nine opaque lines, with certified pitch values given for the six short lines (4A through

4F) only 1

. The values given on the certificate are for the pitches from line 4A to lines 4B

through 4F. Pitches for the other combinations of lines (e.g., 4B to 4E) can be calculated from

the certificate values, giving an array of nominal pitch values from 5.0 to 70 /xm.

The two outer lines of each pattern in row 3 and the three unlettered (long) lines in row 4 serve

as "guard lines" during the photolithographic etching process to equalize proximity effects along

the line edges and are not calibrated.
Row 5 consists of four multiple-line features (5A through 5D) with approximately equal line and

space widths. The widths of the left interior line and central space are certified . Nominal

widths range from 1.0 to 5.0 (xm. These features are useful for adjusting brightness and contrast

of video image-scanning instruments and setting variable-threshold systems to achieve the proper

line-to-space ratio.

The calibrated feature in row 6 is a series of 33 opaque lines, nominally 1.0 wide with 2.0

center-to-center spacing; distances from line 0 to lines 1 through 312 are certified. This

feature is intended to be used as a linear scale in checking for mechanical nonlinearities and

optical distortions in the linewidth measurement system (e.g., the magnification as a function

of position over the field of view) and for checking the resolving power of the microscope

objective.

3. Using SRM 473

The following section provides information on the care and handling of the SRM photomask and

gives basic instructions and precautions on its use for calibrating optical microscope systems for

measuring linewidths of features on antireflecting photomasks or similar artifacts.

3.1 Special Precautions

Contamination or damage can change the measured linewidths, invalidating the NIST calibration.

Particular care should be taken during use to avoid bringing the microscope objective, or any

other object, into contact with the top (chromium-coated) surface of the SRM. It is recommended that users calibrate secondary standards of their own design and use these in

routine calibrations while keeping the NIST standard in safe storage. If this is done, the

secondary standards should be checked periodically against the NIST standard. Also, it may be

advisable for the user to calibrate one or more of the uncalibrated patterns on the SRM for use

in the event that the NIST calibrated pattern is destroyed.

Precautions should be taken to prevent the accumulation of airborne and other contaminants on

the SRM. If cleaning becomes necessary, use only noncorrosive wetting solutions (surfactants)

at room temperature.

For cleaning we recommend the following procedure:

- Soak the SRM for 15 minutes to several hours in a mild solution of commercial

mask cleaner and deionized water.

2The two outer lines of each pattern in rows 5 and 6 serve as "guard lines" during the

photolithographic etching process to equalize proximity effects along the line edges and are not

calibrated.

3
- While the mask is still immersed, brush the coated side gently with a soft lens

brush; stroke parallel to the calibrated line length and in one direction.

- Rinse the mask thoroughly with deionized water.

- Blow away water droplets with a stream of clean dry air or nitrogen at room

temperature.

If the contamination persists, apply a few drops of undiluted mask cleaner directly on the SRM

before repeating the above cleaning process.

Removing fingerprints or other greasy contamination may require rinsing the SRM with alcohol or acetone and repeating the above cleaning process.

Inappropriate use of the NIST linewidth standards can result in inaccurate calibrations and may

invalidate traceability to NIST. The practices most apt to give inaccurate calibrations when

using the NIST linewidth standard include:

a. Using the linewidth standard to calibrate a measurement system that will then be used to measure linewidths on specimens with optical properties that differ significantly from those

of the standard (for example, features on silicon wafers). One important requirement for

accuracy is that the image profile (or diffraction pattern) of the edge have the same shape

for both the standard and the user's specimens. These image profiles will not have the

same shape if the optical properties of the standard and the user's specimens differ.

When calibrating optical measuring systems that use transmitted light, it is especially

important that the transmittance of the chromium film on the standard and the user's

specimen match at the measuring wavelength. The transmittance of SRM 473 is less than

0.2% at a wavelength of 0.53 /xm. Line edge location conditions for photomasks with

transmittance greater than about 0.5% may be significantly different from those of the

SRM.

When calibrating optical measuring systems that use reflected light, the standard and the

user's specimen should match even more closely. The more important properties to match

are the complex reflection coefficient of the patterned metal layer and the substrate, the

thickness of the patterned layer, and the transmittance of the patterned layer.

b. Using the linewidth standard to calibrate a scanning electron microscope. This SRM is designed specifically for use with optical microscopes and, as no criterion has been

established for edge location in an SEM image, this SRM cannot be used to calibrate an SEM for linewidth measurements. Its use in an SEM is further discouraged because the

profile of the feature could change as a result of coating the SRM with an evaporated film

to reduce electrical charging, of deposition of contamination during operation of the SEM,

and of detachment of the chromium during cleaning to remove evaporated films or

4
contaminants. (The substrate of this SRM is quartz and, even when low-voltage SEM

techniques are used it is next to impossible to view the SRM features in the SEM without

first coating the sample).

c. Failing to correct for scattered (or flare) light. Although the chromium pattern on SRM 473

is not highly reflective, it includes isolated features surrounded by various large clear areas and the image profiles may exhibit a moderate component of scattered light which may vary

from feature-to-feature and from the user's specimen. The intensity of the scattered light

should be subtracted from all measured intensity levels before determining the edge location

(see sec. 4.5). This correction has been made in the calibration of SRM 473. For the SRM

user, determination of ^for each feature by using eq (4)(sec. 4.5) would be time consuming

and impractical; however, when, as for this SRM, the transmittance is low (less than

0.2%), ^and/7

are nearly identical and the user may consider all measured /0 intensity to be flare light. Then the correction for flare light can be implemented simply by one of the

following actions: shift the intensity zero level so that /0 = 0; subtract TQ from the

measured intensities; determine the threshold level as a percent of (^, - ^)- If the user cannot make this correction, the reflectance and transmittance of the standard used for

calibration should match the reflectance and transmittance of the user's specimen at the

measuring wavelength.

At the present time, NIST has two other linewidth standards (475 and 476). These two

SRMs have a more limited range of linewidths than SRM 473. SRM 476 is patterned with

bright chromium and SRM 475 is patterned, as is SRM 473, with antireflective chromium.

We recommend that the user: (1) use the SRM that most closely matches the specimens to be measured and (2) make the scattered light correction outlined above.

d. Using the NIST linewidth standards to generate a calibration curve that is then used for

features that are larger than the largest or smaller than the smallest feature on the standard.

The nominal linewidth range of SRM 473 is from 0.5 to 30 and this SRM will not

adequately calibrate a microscope outside of this range. This is especially true for

extensions much below the nominal range where the calibration curve becomes nonlinear.

The user should be aware that all standards have a precision and accuracy of calibration

associated with them and, to this extent, are not perfect. The calibration of a microscope using a standard has an imprecision associated with that calibration and also has an imprecision

associated with the subsequent use of that calibrated microscope to measure an unknown

specimen. Therefore, the accuracy of the user's measurements cannot exceed the accuracy of

the standard. The uncertainty of the final measurement on the unknown specimen is a combination of the accuracy of the standard used for calibration, the precision of the calibration

measurements using the standard, and the precision of the measurements of the unknown

specimen.

5
These and other topics are discussed more fully in the references [1-14]. The need to use good

measurement techniques to achieve the best results with these linewidth standards cannot be

overemphasized. The user who knows more about the potential problems is more likely to make

better use of the linewidth standard.

3.2 Microscope Calibration Procedures

The following procedures are recommended for using this SRM to calibrate optical microscope

systems for measuring linewidths on antireflecting photomasks. It is assumed the user is familiar

with the operation of the microscope system being calibrated; no attempt is made to give detailed

instruction on the use of microscope systems. The steps marked with an asterisk*^ need only

be performed the first time the system is used or after any changes in the measurement system.

Procedures

1. Set up the measurement system for

dimensional measurements; use the

same procedures that will be used

for measuring photomasks.

2. Locate within the microscope field-ofview the specific basic pattern on the

SRM that has been calibrated by NIST.

4c 3. Check the resolving power of the

microscope objective by focusing on row 6.

4. Align the SRM so that lines are measured in a direction perpendicular

to their length.

5. Adjust the measurement system length

scale to give the same reading for the

spacing of appropriate line pair(s)

in row(s) 3 or 4 as the NIST value.

6. * Check for mechanical nonlinearity and/

or optical distortion by measuring the

spacings of the lines in row 6, and compare the results with the NIST values.

Explanatory Notes

Follow manufacturers instructions or

consult reference [1] for recommended

procedures including adjustments for

Kohler illumination.

The pattern identification number is located in the box above the carpet

design in the lower right (see fig. 2).

The identification number of the

calibrated pattern is given on the

SRM certificate.

If the objective cannot

resolve clearly the lines in this

feature, use another objective.

The box surrounding the basic pattern

group (see fig. 3) may be used as an

alignment aid.

The line pair(s) chosen should have

spacing in the same range as the

dimensions of the features to be

measured by the user.

For all further measurements, use only

the portion of the field of view corres- ponding to the location where the differ- ences from NIST values are relatively

constant or that portion of the videodisplay which exhibits minimum distortion.

6
Procedures - continued Explanatory Notes - continued

Adjust system contrast, brightness

(on video-type image-scanning systems)

and/or threshold level until the

measured widths of both the line and

space of an appropriate feature in

row 5 agree as closely as possible

with the NIST values. Use these same

settings throughout this measurement

session.

The feature chosen should have widths

within the range of the anticipated

measurements.

Compensate for flare light during this

process and for all subsequent measure- ments (see paragraph c, sec. 3.1).

NOTE: If any changes other than refocusing, repositioning, and adjusting for flare are inadvertently made during the following steps, discard the data and start again with

step 5.

8. Measure and record the widths of the

calibrated features in rows 1 and 2

and/or the pitches in rows 3 and 4.

Derive the calibration curves as described in reference [2],

Use the same focusing criteria

throughout and make all measure- ments in the same direction of

travel. Compensate for flare

light on all photomasks (not only

this standard).

These caibration curves apply only

to this system/operator combination.

The system is now ready for measurement of other antireflecting-chromium photomasks or

artifacts with optical properties similar to SRM 473 (low reflectance and very low trans- mittance) using the same threshold value and flare-light correction procedure. If the user attempts to measure artifacts with chromium layers having transmission much greater than 0.2%,

it may be necessary to measure the phase angle, </>, and use eq (3) to determine a different edge

location threshold. These procedures are beyond the scope of this report.

Repeat the complete calibration procedure on a routine periodic basis and whenever a substantial

change is made in the measurement system. The time between periodic calibrations may have

to be determined empirically.

4. Calibration of SRM 473

All measurements at NIST of the SRM feature dimensions were performed on the automated

optical linewidth system [3] in a laboratory with temperature controlled at 21 ± 2 °C.

Linewidths and pitches are determined from the optical profile data. The uncertainty of the

calibrations is a combination of the uncertainties of the measurement process, of the feature edge

location algorithm, and of the geometry of the physical edge of the measured features.

7
4.1 The Measurement System

The measured system, diagrammed in figure 4, is built around a carefully aligned optical

transmission microscope mounted on a vibration isolation table. The photomask is placed on

a scanning piezoelectric flexure-pivot stage with finely controlled motion in the X (scanning) and

Z (focus) directions; this stage is mounted on another stage with coarse motion leadscrews in

the X and Y directions to allow positioning of the desired feature in the field of view.

The photomask is illuminated from below with Kohler illumination (i.e., each point on the lamp

filament evenly illuminates the entire specimen) from an incandescent source filtered at 530 nm

wavelength with a coherence parameter of 2/3 (0.6 numerical aperture condenser lens and 0.9

numerical aperture objective lens). A 20 x 400 slit is fixed on axis in the image plane in

front of a photomultiplier tube. Image magnification at the slit is 157 times, giving an effective

measurement area on the photomask of 0.127 x 2.55 fxm, which is centered top-to-bottom on

the feature (at the fiducial line). The photomultiplier output is amplified and digitized by a 12-

bit analog-to-digital converter (ADC). Stage motion in the scanning direction is measured by

a laser interferometer with resolution of 125 points per micrometer. All these devices are connected via appropriate control hardware and IEEE-488 bus to a dedicated desktop digital

computer.

4.2 SRM Calibration Procedure

An outline of the overall calibration procedure is charted in figure 5. Before each complete

SRM calibration, selected features on a control photomask are measured and compared with

control history to ensure that the system has not changed or drifted. These selected features

include spacing patterns 3E, 4F, and row 6 which have been independently calibrated by the

Dimensional Metrology Group at NIST [4] to provide traceability to the standard meter. Each

feature on the SRM being calibrated is then measured in sequence and the sequence repeated

nine times. After each SRM calibration is completed, the control photomask is measured again.

All measurements, including the control measurements, are entered into the linewidth database.

After the calibration measurements are completed the database is searched to ensure that the

control was measured before and after the calibration and that these two control measurements were statistically invariate. The database entries for the calibration are combined and examined

statistically: the standard deviation for each feature is calculated, number of measurements

checked, etc. Criteria must be met for each of these statistical factors. If necessary, more

measurements can be made and added to the database.

Once all the above conditions are met, the certificate is printed and the SRM linewidth standard

is released to the Standard Reference Materials Program Office for sale. All of the calibration

database files for this serial number are then stored on one flexible disk along with summary

data. The disk is kept for archival storage along with the printed calibration results for each

measurement, a printed summary of the statistical data, and dark-field illumination micrographs

of the calibrated pattern.

8
4.3 Specimen and Measurement Axis Alignment

The measurement axis is the axis of the interferometer laser beam (actually the geometric center

of the four beams used in this interferometer) and is defined as the X-axis. The scanning axis

is the axis of motion of the piezoelectric scanning stage, and the specimen axis is a line on the

surface of the photomask perpendicular to the length of the linewidth feature being measured.

Ideally these axes would coincide, but in practice it is not possible to locate these axes with great

precision.

In this application the perpendicular distance between parallel lines (the right edge and left edge

of a feature) is measured. It is important to align the specimen axis with the measurement axis,

but slight misalignment of the scanning axis causes no error because only the component of

motion parallel to the measurement axis is measured and this is also the component parallel to the specimen axis. In other words, a scanning axis misalignment will cause the width

measurement to "slide" slightly along the length of the line, but it will always be parallel to the

line's width. Even so the scanning axis is aligned as carefully as possible, first by aligning the

leadscrew stage by moving it back and forth in the Y direction and adjusting its rotation in the

X-Y plane until the interferometer indicates no periodic change in X, and then by geometrically

aligning the piezoelectric stage by eye.

Misalignment of the specimen axis with the measurement axis will lead to a geometric error proportional to 1 - cos(misalignment angle). This alignment is checked by scanning and

measuring the X position of the center of the long vertical fiducial line at the right side of the

pattern (see fig. 3) near its top and bottom ends. The angle of rotation of the specimen can be

calculated from the positions of these centers and the nominal distance between them. After the

specimen has been mounted and aligned by eye, the alignment is checked and readjusted until

the program indicates the specimen is properly aligned. The calibration program will not

commence taking data unless the misalignment angle is less than ±0.1 deg. This allows a maximum cosine error of 1.5 ppm, or 0.105 nm on the longest feature on this photomask. If

the specimen is tilted (i.e., the specimen stage is rotated about the Y-axis), the leading and

trailing edges of the longer patterns will not both be in focus, and this condition will be detected

in the measurements.

A significant potential error source is the Abbe error caused by possible offset between the

measurement axis and the specimen axis in combination with angular motion of the scanning

stage. The Abbe" offset in the linewidth measurement system is less than about 2 mm.

Comparison of pitch measurements made on this apparatus and on the NIST Line Scale

Interferometer (see sec. 4.6) allow compensation for errors of this type. Small random rotations

of the scanning stage contribute to the measured random uncertainty.

9
4.4 Feature Measurement Sequence

A flow chart of the main steps of the feature measurement sequence is given in figure 6. The

feature to be measured is first centered in the field of view, then focused and scanned as the

optical profile position and intensity data are taken and stored as two one-dimensional arrays.

The data are then low-pass filtered to reduce extraneous noise and processed to find the edge

locations. Linewidth is then calculated.

Position and intensity data points are correlated by alternately triggering the interferometer and

the a-d converter to take one reading each in a software loop while the scanning stage is moving.

There may be a few CPU clock cycles delay between the two readings of a data pair, but this

delay is very small (corresponding to 2 nm/ms of delay) and is the same at leading and trailing

edges; thus, it cancels in the linewidth or pitch calculation.

Image profiles such as those in figure 7 are presented on the computer screen during data

acquisition and processing to allow monitoring system operation. After passing several data

quality checks, the results are entered into a database for the SRM being calibrated.

A more detailed description of the measurement sequence and system can be found in reference

PL

4.5 Edge Location Determination

Analysis of optical microscope imaging gives the following equation for image intensity at the

edge of a line [5]:

Tc =Rt d0 + L

+ 2^cos<»), (D

where Tcis the intensity of the light at the threshold point (edge) on the image profile (see fig.

8); I

Q

is the intensity of the light passing through the not-perfectly-opaque chromium layer; Im

is the intensity of the light passing through the clear areas (beyond the diffraction peaks); and

4> is the optical phase difference of the light transmitted through these two areas. Rt is a

theoretically derived ratio, of approximately 0.25, which varies slightly depending on the

coherence factor, viewing slit width, focus, proximity of the next edge, and other imaging

conditions. For the conditions of measurement of this SRM in the NIST calibration system, Rt varies from 0.25 to 0.28 (see last paragraph of this section).

Real microscope images often include some flare light (light scattered off the microscope

components illuminating the otherwise opaque features on the photomask from above or reaching

the image plane by indirect paths). In nonlaser illumination systems, this light is temporally

incoherent with respect to the light comprising the diffraction pattern (image profile) and simply

adds incoherently (intensity-wise) to each intensity of the image profile.

10
To a first approximation the intensity of the flare light is not a function of position across a

feature. Therefore, the effect of the flare light can be incorporated into eq (1) by simply

subtracting its value from each intensity component on the image profile:

h = i - 9 *m

= C - 9 tc = t!

c -if

, (2)

where the prime designates an observed intensity (including the effects of diffraction,

transmission, and flare) and where 7^ is the magnitude of the flare light component in the image

profile for each feature. Substituting into eq (1) gives:

Both <f) and Iy must be known to evaluate the threshold condition. Iy is feature and background

dependent and must be determined for each feature. For this SRM, where the antireflectingchromium layer can be considered to be homogeneous, the transmittance (7>) and <f> can be taken

as constants and I

Q can be expressed as (Tr x 7m). Then, considering that (Im

- I

0) equals (7^-7^) and substituting in eq (2), it can be shown that

If

= [7^-(7rX7^)]/(l-7>). (4)

The transmittance of the SRM was determined by using the linewidth measuring system to measure the intensity of light passing through the chromium near the center of the large

chromium-covered upper-left quadrant of the mask and found to be about 0.18% of the incident

intensity.

There is no known simple method for determining <$>. However, a recent study of image profiles

generated by a computer model [5], developed by D. Nyyssonen, of the NIST microscope

system indicates that linewidth uncertainty from not knowing this phase difference is no greater

than 15 nm if <j> = x/2 is used in determining the threshold intensity. This uncertainty is included in the systematic error budget. The model is based on the theory of partial coherence

and allows variation of image formation conditions such as: linewidth; wavelength of incident

radiation; transmittance and phase of the object and background illumination; and slit width.

Profiles generated by this model agree very closely with profiles generated from the

measurement data. Theoretical profiles were generated for lines and spaces 0.50 to 15 ^m wide

where the transmittance of the "opaque" areas is 0.2% and 4> ranges from 0 to t. The results

of the study also indicate that the relative threshold intensity varied from 25 % to 28 % of (Im

- I0

) over the range of widths simulated, (fig. 9). Therefore, an algorithm for determining

linewidth was implemented that assumes a phase difference of x/2 and iteratively selects the

threshold intensity ratio from this model-generated data according to the linewidth of the feature

being measured.

11
4.6 Precision and Accuracy of the SRM Calibration

The SRM certificate reports the measured linewidths and pitches (spacings). The reported values

are the average of at least nine repeated measurements. The certified linewidths and pitches

have separate uncertainty values because of subtle differences in the way errors affect the

measurement of widths and pitches. The uncertainty statement given is based on two

contributing factors: precision and accuracy.

Process precision is defined as the variability observed in repeated measurements of a single

physical quantity under essentially the same conditions [6] and is intended to describe the

reproducibility of the measuring device, the optical microscope, as well as all of the environmental conditions which may affect the measurement [7]. A numerical value for process

precision cannot be determined until the measuring system is operating in a state of statistical control and the source of variability is shown to be random in nature and stochastically

stationary. When these criteria have been met the process standard deviation quantifies this

random error. The value for the process precision on the certificate of calibration includes the

variability of the control measurements and the variability of the nine repeated SRM

measurements. The details for computing this value are given in the Appendix.

Accuracy is the closeness of agreement between an observed value and the true value [8]. Pitch

measurements of several features in a control SRM have been independently calibrated by the

Dimensional Metrology Group on the NIST Line Scale Interferometer. Comparison of these

measurements with measurements of the same features made on the NIST optical linewidth

measurement system provides a scale correction factor to be applied to the SRM calibration

measurements. Typically, the scale correction factor is equal or nearly equal to 1.0, indicating

there is no significant scale difference between the two measurement systems. This comparison

adds a small systematic uncertainty to all measurements (see Appendix, sec. E).

The laser interferometer used to make these dimensional measurements is subject to a sinusoidal

nonlinearity along the beam path due to polarization mixing. This leads to a maximum periodic

systematic error of —3.5 nm every one-quarter wavelength (0.16 /im) for the four beam

differential interferometer used here [9]. Since repeat measurements of each photomask feature are made at substantial time intervals, thermal drift in the apparatus insures that these

measurements are randomly distributed over this quarter-wavelength period. Thus this error is random, with deviations from the mean wavelength equally likely to be positive or negative, and

is included in the calculation of process precision.

The index of refraction of air, and hence the interferometer wavelength, depends on its temperature, pressure, and composition (relative humidity, C02 , etc). In this measurement

system this can lead to error in two ways: the index error times the interferometer deadpath

changes the apparent position in the measurement of each edge, and the index error times the

measured length changes the length scale.

12
The deadpath in this system (the minimum distance between the fixed and moving mirrors) is about 1 cm. The measurement of linewidth or pitch, however, is a differential measurement in

that the difference of the positions of the leading and trailing edges is measured by measuring

the positions of both edges within a very short time (35 s for the longest pattern on this SRM);

the line's width is being measured, not its position. A deadpath error can occur then only if the

index of refraction changes in this time interval. Since such changes (caused, for instance, by

convective turbulence) are equally likely to be positive as negative, this type of error is random

and is included in the measured precision. The maximum magnitude of this deadpath error is 30 nm/°C/min, but since the temperature must decrease after it increases, this error is random

and not systematic, relative to the time scale of repeat measurements.

If the temperature is constant but not at the nominal value (20 °C) there will be an index of

refraction error of approximately 1 ppm/°C. The worst case error for a 3 °C temperature offset

is then 0.21 nm. An atmospheric pressure deviation of 30 mm Hg (4000 Pa) from the nominal

760 mm Hg (101325 Pa) changes the index of refraction by 9 ppm, leading to a 0.63 nm error on the longest feature. Deviation from nominal of the other factors listed above results in

similar but much smaller random errors.

In the field of optical submicrometer linewidth metrology, the ideal reference standard with

features which have vertical walls and smooth edges does not exist. Instead, real features have

erratically varying, nonvertical edge geometries and raggedness along their length [10] (see fig.

10) and the resultant uncertainties of the location of the physical edge. To quantify these

systematic uncertainties the feature edge geometry is examined with a scanning electron

microscope. As this examination precludes use of the photomask as an SRM, only one sample

from each photomask production batch is examined.

The SRM measurements reported represent averages over the effective length (2.6 /*m) of the

NIST instrument's viewing slit, positioned at the center of the line. Therefore, both the

uncertainties of the edge location resulting from nonvertical edge geometry and from raggedness

along the length of the line are estimated as averages along the edge of the line.

Typically, the SEM micrographs of the photomask features show that the raggedness along the

length of a line is less than 30 nm and has a spatial period of 100 nm or less.

3 If the user's

measurements of this SRM are also averaged over a length comparable to that of the NIST

viewing slit, uncertainties due to edge raggedness become insignificant.

3 Occasional isolated flaws have been observed during SEM inspection that are considerably

larger than this typical edge raggedness but which are not discernable in the optical microscope

at 1600 X magnification and could be present on the photomasks accepted for calibration. If the

presence of such flaws in the measurement region should cause degradation of focus sharpness or of measurement precision during the calibration of a photomask, that photomask would be

rejected from certification as an SRM. However, it is not known if such flaws would have any

noticeable effect on the measurements.

13
A determination of the uncertainty caused by the nonvertical edge geometry is accomplished by

estimating the difference between the edge location at the top surface and the corresponding edge

location at the substrate level, as illustrated in figure 11. Estimates are made at uniform

intervals along a 1 /xm section of the sample as imaged in an oblique view SEM micrograph.

The uncertainty is determined as the average of these estimated edge location differences. For

width measurements, edge location errors for the right and left edges tend to add; thus the

linewidth uncertainty is twice the edge location uncertainty [11].

The linewidth uncertainty value given on the certificate data sheet for rows 1,2, and 5 includes

the uncertainty resulting from the nonideal edge geometry estimated from the SEM micrographs

(typically around ±50 nm), ±15 nm from not knowing <f> (see sec. 3.4); and ±5 nm of less

quantitatively defined systematic errors.

Because pitch measurements involve measuring the distance from one location (left edge, right

edge, or center) on one feature to the same location on another feature, edge detection errors tend to cancel and are not included in the uncertainty reported for measurements in rows 3, 4,

and 6.

The measurement precision is determined for each SRM photomask as described in the Appendix

and the pooled 95% confidence levels are reported for the width (rows 1, 2, and 5) and pitch

(rows 3, 4, and 6) measurements on the certificate data sheet.

The uncertainty of the width and pitch values reported is a combination of the appropriate

systematic uncertainties and measurement precision. A summary of the error components is given in table 1.

The user is advised to examine the edge properties of the production photomasks to be

measured. If the quality of the edges of the features on the user's photomasks is significantly

inferior to that of this SRM, an additional level of uncertainty should be added to the uncertainty

of measurements made on the user's photomasks.

ACKNOWLEDGMENTS

Parts of this document (especially the procedures in sec. 3) are based on the earlier SRM 474

and SRM 475 Handbook prepared by Diana Nyyssonen and John Jerke.

The photomicrographs used to examine the edge geometry were provided by the Scanning

Electron Microscope Section (Sam Jones, William Keery, and Michael Postek) of the

Microelectronics Dimensional Metrology Group.

Many thanks to Robert Larrabee for his guidance and advice and to Beverly Wright for

typing the numerous revisions of this document.

14
REFERENCES

[1] 1981 Annual Book of ASTM Standards, Part 43, "Standard Practice for Preparing an

Optical Microscope for Dimensional Measurements," Designation F 728-81, American

Society for Testing and Materials, 1916 Race Street, Philadelphia, PA 19103.

[2] Croarkin, C. and Varner, R.N., "Measurement Assurance for Dimensional

Measurements on Integrated Circuit Photomasks," Natl. Bur. Stand. (U.S.) Tech. Note

1164, August 1982.

[3] Potzick, J., "Automated Calibration of Optical Photomask Linewidth Standards at the

National Institute of Standards and Technology," SPIE Vol. 1087, Integrated Circuit

Metrology, Inspection, and Process Control, San Jose, CA, February 1989.

[4] Beers, J., "Length Scale Measurement Procedures at the National Bureau of

Standards," Natl. Bur. Stand. (U.S.), NBSIR 87-3625, 1987.

[5] Nyyssonen, D., "Linewidth Measurement With an Optical Microscope: The Effect of

Operating Conditions on the Image Profile," Applied Optics Vol. 16, August 1977,

pp. 2223-2230.

[6] Ku, H.H., "Statistical Concepts in Metrology - With a Postscript on Statistical

Graphics," Natl. Bur. Stand. (U.S.), Spec. Publ. 747, August 1988, pp. 12.

[7] Croarkin, C, "Measurement Assurance Programs Part II: Development and

Implementation," Natl. Bur. Stand. (U.S.), Spec. Publ. 676-11, 1985, pp. 23.

[8] 1986 Annual Book of ASTM Standards, "Statistical Terminology for Statistical

Methods," Designation E 456-83a, American Society for Testing and Materials, 1916

Race Street, Philadelphia, PA 19103.

[9] "High Performance Motion Control for Precision Equipment," Hewlett Packard, Co.,

1990, p. 34.

[10] Nyyssonen, D. and Larrabee, R.D., "Submicrometer Linewidth Metrology in the

Optical Microscope," J. Res. Natl. Bur. Stand. (U.S.), 92(3), May-June 1987, pp. 189-

190.

[11] Potzick, J., "Practical Photomask Linewidth Measurements," SPIE Vol. 1261-13,

Integrated Circuit Metrology, Inspection, and Process Control, San Jose, CA, 1990.

15
[12] Bullis, W.M. and Nyyssonen, D., "Optical Linewidth Measurements on Photomasks

and Wafers," Chapter 7 in VLSI Electronics: Microstructure Science, Semiconductor

Microlithography, Vol. 3, N.G. Einspurch, Editor, pp. 119-126 (Academic Press, New

York, NY, 1982).

[13] Nyyssonen, D., "Linewidth Calibration for Bright-Chromium Photomask," NBSIR 86-

3357, Natl. Bur. Stand. (U.S.), May 1986.

[14] Jerke, J.M., Croarkin, M.C., and Varner, R.N., "Interlaboratory Study on Linewidth

Measurement for Antireflective Chromium Photomasks," Natl. Bur. Stand. (U.S.),

Spec. Publ. 400-74, 1982.

16
Table 1 . A listing of error sources and their contributions to overall measurement

uncertainty. Errors are determined in a worst-case sense; scale factor uncertainties (in ppm)

are multiplied by the largest dimension measured, giving scale errors of 0.09 nm (systematic)

and 0.40 nm (random) for the largest linewidth (30 /urn) and 0.22 nm (systematic) and 0.92

nm (random) for the largest pitch (70 jum). Net random error is derived from the

measurements.

SUMMARY OF ERROR COMPONENTS

Systematic

Laser wavelength accuracy

Axis alignment

Abb6 error

1

Substrate thermal expansion 1

Refractive index of air

Chrome edge geometry

Chrome transmission magnitude & phase2 Comparison to Standard meter

Other errors TYPICAL SYSTEMATIC UNCERTAINTY2

Linewidth

0.1 ppm

3.0 ppm

0.0 ppm

0.0 ppm

0.0 ppm

50 nm

15 nm

2.0 nm

3 nm

Pitch

0.1 ppm

3.0 ppm

0.0 ppm

0.0 ppm

0.0 ppm

0.0 nm

0.0 nm

3.0 nm

0.0 nm

70 nm TOTAL 3. 1 nm TOTAL

Random

Substrate thermal expansion

Refractive index of air

Deadpath refractive index

Polarization mixing

Vibration and noise

TYPICAL RANDOM UNCERTAINTY3 (from measurements)

1.2 ppm

12 ppm

6.0 nm

3.5 nm

unknown

1.2 ppm

12 ppm

6.0 nm

3.5 nm

unknown

17 nm TOTAL 15 nm TOTAL

! These scale factor errors are removed by comparing the linewidth calibration system

with the NIST Linescale Interferometer, which measures pitch only, giving traceability

to the standard meter (see sec. 4.3, p. 9). 2Typical: depends on photomask batch.

3 One standard deviation for a single measurement. Long-term [6 months, frequent

mounting and dismounting] and short-term standard deviations do not differ

significantly.

17
Figure 1. A view of the overall pattern on SRM 473. The basic measurement pattern is

repeated eight times about the center. The horizontal and vertical lines help locate the

patterns.

Figure 2. An enlarged view of the center of the SRM. The pattern number given with the

serial number on the certificate identifies which basic pattern has been calibrated by NIST.

Pattern No. 1 is in the upper left; pattern No. 8 is in the lower right. Pattern identification

numbers are included within each basic pattern as shown in figure 3.

18
NIST 473

—I—

Figure 3. A view of one basic measurement pattern on the SRM. The individual features are located by reference to an alphanumeric code with numbers identifying the row and

letters designating the position within the row. The broken horizontal lines mark the central

calibrated area of the features. The box surrounding the overall pattern is used to align the

pattern on the measurement system. The pattern identification number can be seen in the

box above the carpet design in the lower right.

Calibration values are given fon widths of opaque lines in row 1 and clear lines in row 2;

center-to-center spacing of the two inner (short) lines of each feature in row 3; center-to- center spacing of line A to lines B through F in row 4; widths of the left inner Gong) line

and the space to its right of each feature in row 5; and center-to-center spacings of line 0 to lines one through 30 in row 6.

19
Computer

0

Photomask Linewidth Calibration Apparatus

Figure 4. Schematic of the NIST automated optical linewidth calibration system. The

photomask is placed on the scanning piezoelectric stage and is illuminated from below with

partially coherent light from a filtered incandescent source. The measuring slit remains fixed

while the image of the feature being measured is scanned past the slit by moving the

photomask. The motion is measured with a laser interferometer and the image intensity at

the slit is monitored with a photomultiplier. The digitized and amplified output of the

photomultiplier and the interferometer output are connected via the IEEE-488 bus to the

computer.

20
Find& Correct

C

Start ^

Measure

Control Mask

No

Measure

SRM Mask

Discard

SRM

Measurements

Find & Correct Problem

Summarize Data

Store Data

Print Certificate

Figure 5. Flow chart outline of the overall calibration procedure for the calibration of SRM

photomasks with the NIST optical linewidth measurement system. First, measurements are made on a control photomask and tested statistically to determine if the system is operating

properly. Then, the SRM photomask is calibrated and the system operation is checked again

by measuring the control photomask. If all tests indicate the system is within statistical

control, a calibration certificate can be printed for the SRM photomask.

21
C — )

Mount Mask

Enter LD.

Set Measurement Series j Set Number of Series I

Find 1st Feature in Series

Center and Focus

Feature

CoOect&Rter

Measurement Data (X&P)

Calculate Width

or Fieri Perform Tests

Find Next

Feature

Print Error

Figure 6. Flow chart of the main steps of the measurement sequence performed by the NIST

optical linewidth measurement system. Each feature on the SRM photomask is centered in

the microscope field of view, focused, and measured in sequence. The sequence is repeated

until each feature has been measured nine times. The optical profile position (X) and

intensity (P) are measured and the data are stored as two one-dimensional arrays.

22
143 151 152 193

• f ^ _

• If

ij

• •

n

ji

y

X

... \

i

*

•

\

\

I \

, —

. * .—,

551 552 553 554 555 5S6 557 55 B

i

I

i

t

I 1-

I

\

i

/W\

A. Profile typical of an opaque line in row

1 . The vertical

fiducials mark the

positions of the line

edges.

B. Profile typical of

a clear line (space) in

row 2. The vertical

fiducials mark the

positions of the space

edges.

C. Profile typical of

a line pair in row 3. The vertical fiducials

mark the positions of

the line centers. The

profile includes the

two outer guard lines as well as the

line-spacing pair.

240 245 25B

Figure 7. Samples of optical profiles (measured light intensity versus position) displayed on the computer screen during the calibration process. The vertical axes are relative light

intensity and the horizontal axes are position in micrometers. The horizontal lines are

fiducials marking the relative intensity levels of the clear and coated areas and the threshold.

23
OPTICAL MICROSCOPE

IMAGE PROFILE

COZ

m

Physical Line

Transparent Substrate

Figure 8. Schematic of the cross section of a vertical

-edged chromium line and

the corresponding optical

profile of its microscope

image. Im is the intensity

of die light passing through

the clear area; Ia

is the

intensity of the light passing

through the chromium; Tc

is

the intensity at the physical

edge (threshold); If

is the

intensity of the flare light.

The prime designates an observed intensity. The

vertical axis is optical

intensity and the horizontal

axis is distance.

29

CO£ 27

LU

25

Opaque lines

Clear spaces

2.0 4.0 6.0 8.0 101©

WIDTH (nm)

Figure 9. Optical intensity [% of {1^1^ at edge location versus linewidth, from a computer

model of the NIST calibration system. Transmittance equals 0.2% and 4> equals x/2 radians.

24
;NIST*473

450.24 #1

092310 £5KV X150K 8.88un

Figure 10. SEM micrographs showing the nonideal nature of line edges on an

antireflecting-chromium photomask. The scale is indicated by the row of eleven dots in the

lower right of each micrograph. The dots are 20 nm apart. Beam energy was 25 keV.

25
Figure 11. Schematic representation (not to scale) of a line edge as seen in an oblique view

SEM micrograph. The uncertainty of linewidth measurements includes the uncertainty of the

edge location resulting from non-vertical physical edge profiles. Determination of this

uncertainty is accomplished by estimating the difference between the edge location at the top

surface (A) and at the substrate level (B). Estimates are made at uniform intervals along a

section of a specimen image in an SEM micrograph. The edge uncertainty reported is the

average of such estimates made by several individuals using several different micrographs.

26
Appendix

Process Control for SRM 473 Calibrations

A. Introduction

The procedures used to assure statistical control of the linewidth SRM measurement system are

defined. A control photomask with the same characteristics as the SRM photomask is used for

measurement process control. Six of the features on the control photomask are measured each

time an SRM photomask is calibrated. The six features are: the nominal 0.6 and 5.0 /xm lines

from row 1; the nominal 1.0 and 20.0 fxm lines from row 2; the nominal 6.2 pitch pattern from

row 3; and the nominal 2.0 (xm line from row 5. These correspond to features IB, II, 2F, 2K,

3E, and 5B as shown on the diagram of a pattern in figure 12. The purpose of the control

photomask measurements is to provide a database that can be used to determine whether or not

the measurement system is in a state of statistical control. There are several factors which may

cause the optical measurement system to be out-of-control. There may be a change in the

measurement system or a change in environmental conditions. This document describes the

initialization of the database of control measurements, use of the database to determine if the

measurement system is in control, and the maintenance of the database over a long period of

time.

B. Initialization of Process Parameters

When the measurement system is ready for performing SRM calibrations, a database is initialized. This database consists of at least 15 sets of repeated measurements of the six

selected features on the control photomask taken over a period of several weeks [15], This

period is representative of the normal operating mode of the optical measurement system. The

six features measured are identified as IB, II, 2F, 2K, 3E, and 5B. These features cover the

extremes of the feature sizes and the range of the feature locations on the photomask. The

database includes not only the measured linewidth or spacewidth but also other pertinent

information such as the date and time of the measurement, feature identification and any other

potentially useful information (temperature, scan rate, etc.).

A plot of the repeated measurements for each feature, measured width or pitch versus time, is made to detect any possible anomalies in the measurement system and to verify that the system

produces stable measurements whose variability is random in nature. The control database is accepted as being representative of the normal operating environment of the measurement system

if no more than 5% of the measurements are suspected outliers (unexplained anomalies). If this

is not the case, an effort is made to determine the cause and appropriate adjustments are made

to the measurement system. The control database is then reinitialized.

The initial control database is used to estimate the mean vector (accepted mean values for each

control feature) and the matrix of covariances between them. These are required elements for

the multivariate Hotelling's T

2

test statistic [16]. The details for computing the estimate of the

mean vector and the matrix of covariances are given below. The use of this test statistic and

updating procedure for this statistic are given in following sections of this document.

27
From the database of control measurements for features IB, II, 2F 2K, 3E, and 5B, a matrix

[X] is constructed, as shown below, of the N initial repeated measurements on the control

photomask. Each of the features has the same number of repeated measurements,

XlB,l XIB,2

XU,l XU,2

X2F,l X2F,2

X2K,l X2K,2

X3E,l X3E,2

X5B,l X5B,2

XIB.N

l 2¥,N

2K,N

X-3E.N

^5B,N

where i = 1,2,....

6

and = 1,2,.. ..N.

(B.l)

The average is computed for each of the features based on the N repeated measurements,

*1B

N

= E

7=1 N

Xll N= E

7 = 1 s

N

X2F

N

= E

7=1

X2E

} N

X2K

N

= E

7=1 N

X3E

N

= E

7=1

X3E

j N

X5B

N

= E

7 = 1

X5B^ N

(B.2)

These values are the elements of the vector of means as denoted below:

M =

IB XI] x2F

X2K

3E

5B

(B.3)

A matrix is computed of the differences of the measured values minus the mean values,

where i = 1,2, ....6 and; = 1,2,....//,

(B.4)

28
and the variance-covariance matrix, S, of size 6x6, of the control database is computed with

elements:

N

where / = 1,2,....

and j = 1,2,.... 6.

(B.5)

The inverse of the variance-covariance matrix is computed and is used in conjunction with future

control measurements to determine if the measurement system remains in a state of statistical

control.

C. Procedures for Process Control

At the beginning of an SRM measurement session the features IB, II, 2F, 2K, 3E, and 5B on

the control mask are measured and the multivariate Hotelling's test statistic T

2

is computed as follows:

where yis a vector of newly determined widths and pitches for the above mentioned features.

The system is in control at a 95% confidence level if

where the F 05 (6,N-6) values are found in table 2. The value (N-6) corresponds to v in

table 2.

At the end of the SRM measurement session, the control is remeasured and the test is repeated.

If the system is still in control the SRM data are summarized and a certificate of calibration

produced. The value of ((N-6)/6(N-l))T2

is saved in the control database and the system is ready

for the next SRM measurements.

If the test indicates the system is not in control, the data are tagged when they are saved in the

control database. The system is then checked to determine the cause of the test failure. A

control chart may be used to determine which feature is causing the problem or to see trends in

the control data. A control chart for each feature is constructed from the control database as follows:

T

2 = [Y - M\' S~

l [Y - M] , (C.l)

N+\

(C.2)

29
The mean, X, and the standard deviation, a, for each feature are computed using the N

repeated measurements from the control database:

N

E xi E (xt - X)2 (C.3)

X = i=land

(N - 1)

Control limits are computed by using the following equations:

X ± a t 915 (N-l) for the 2a limit

and (C.4)

X ± a t gg5 (N-l) for the 3a limit.

The values for t are found in table 3. The value (AM) denotes degrees of freedom, df, in table

3.

Figure 13 is an example of a control chart of the initial 70 measurements of feature IB. Future

measurements are added to the chart. The control limits remain the same until the process

parameters are updated.

If it is determined that the cause of the failure did not affect the SRM measurements (for

example, the control photomask was misaligned), the appropriate adjustments are made and the

control photomask is remeasured. If the test then shows the process is in control, the SRM data

are summarized, a certificate of calibration produced, and the system is ready for the next SRM

measurements.

If it is determined that the cause of the failure may also have affected the SRM measurements

(for example, the air-conditioning unit malfunctioned during calibration), the SRM must be re- measured after the problem has been corrected and the test indicates the system is once again

in control. Major changes to the measurement system dictate reinitialization of the database.

D. Updating Process Parameters

If the measurement system remains unchanged after collecting a minimum of 30 new (good)

sets of control photomask measurements, the process parameters, M, S, and a are updated.

Equation (B.3) is used to compute Af2

, a vector of estimated means for the recently collected

control measurements; eqs (B.4) and (B.5) are used to compute S2

, the corresponding variancecovariance matrix; and eq (C.3) is used to compute a2

, a vector of standard deviations for the

repeated measurements for each feature. In the updating process, values that have been flagged

as out of control are omitted.

30
Before updating the control database, a comparison is made between the two databases, the old versus the new, to determine whether or not there is a significant difference in terms of the mean

vectors and the variance-covariance matrices. The equivalency of variance-covariance matrices

is tested as follows:

let I = N{ + N2 , (DA)

where Nl = number of repeated observations in the control database

and N2 = number of repeated observations in the new set of control

observations.

The new control database will contain both new and old measurements.

Let

_ (NX S{) * (N2S2) (D 2)

I where S± is the variance-covariance matrix of the current control database

and S2is the variance-covariance matrix of additional new control measurements.

Compute the statistic [17]:

D = 0.5^ trace -5)5"

_1

] 2 + 0.5N2 trace [(S

2 -S)S~ l f ; (D- 3)

and test whether:

D < xJ/0.05).

D is distributed as a chi-square random variable with df (degrees of freedom) = 0.5p(p+l)

where p = 6, the number of features measured. The value of X21 (0-05) is 32.67. If D <

32.67, then the differences between the old and new covariance matrices can be attributed to random measurement error at the 95% confidence level. However, if the test fails, (D >

32.67), this suggests that the process has changed in some manner and the cause needs to be

identified and evaluated. If the change is significant, appropriate action must be taken and the

control process re-initialized.

If the covariance matrices are statistically the same, the means are compared. To do this, first a pooled covariance matrix is computed:

[(A^-DSt + (N2 -l)S2] P ~ T2 '

where S{ and S2 are defined in (D.2).

31
Then the statistic is computed:

and tested whether:

z

2 < F 05 (p,e-P -\)

where iVj, N2 and t are defined in (D.l),

A/j is the mean vector for the current database,

M2is the mean vector for the newly collected control data,

and p = 6, the number of measured features.

T

2

is a random variable with an F-distribution with p degrees of freedom in the numerator and

with t-p-\ degrees of freedom in the denominator. The F 05 (p,(-p-l) value is given in table I. If T

2 >F 05 (pJ-p-l), this suggests that there has been a change in the measurement process.

The change needs to be identified and appropriate action needs to be taken to reestablish the

measurement system and begin the process control anew. However, if T

2 <F 05 (pJ-p-l) then

the differences between the old and new mean vectors can be attributed to measurement error

at the 95% confidence level. Since the test for equality of means was only performed if the

hypothesis of equal covariance matrices was not rejected, it can be said that there has been no

statistically discernable change in the measurement process at the 90% confidence level and the

control may be updated to include the new measurements. The covariance matrix is updated as shown in eq (D.4) and the current covariance matrix is:

S = S

p

. (D.6)

The mean vector is updated as shown below:

M = JLJ U. (D.7)

The standard deviation for each feature is updated as follows:

(Wj-l)^ + (N2 -l)a

2

2N2 (D.8)

N{ + N2 - 2

32
E. Uncertainty Statement for SRM 473

The uncertainties for the certified linewidth and pitch values given in the certificate include small

contributions from the measurement precision (random error) and a contribution from the

systematic error. The systematic error for both pitch and linewidth values includes a length

dependent contribution introduced by correcting the measurements to agree with the NIST Line

Scale Interferometer measurements (see sec. 3.5). The systematic error for the linewidth values

has a significant contribution (on the order of ± 0.05 /im) resulting from the edge geometry of

the features (see sec. 3.5). See table 1 for a detailed summary of error components.

Before determining the total uncertainty for the reported certificate values, it is assumed that all the measurements on the SRM and in the control database have been corrected to compensate

for the difference of measurements between the NIST Line Scale Interferometer System and the

optical linewidth measurement system. The correction factor is derived by using the model

given below and ordinary least squares to estimate a and its variance:

where X represents a measurement from the linewidth measurement system,

Y represents a measurement from the line scale measurement system,

and € is the random error of measurement.

Then the uncertainties, UL and Up

, for linewidth and pitch measurements are determined by the

equations below:

The variance of each SRM measurement, j. , is

X = a Y + e (E.l)

3. -2 var(a)

si = xi —— +

J J A 2 a

I n

n-l

(E.2)

where Xj is the average of the yth feature,

var(a) is the estimated error of the slope,

a is the least squares determination of the slope,

and n is the number of repeated measurements.

The variance of the control measurements, bk

, is

A 2 -2 var(a)

°k = c

k —t1 + CVk

a

(E.3)

where c^is the average of the kth control feature

and CVkis the kth diagonal element of the variance-covariance matrix for the

control data.

33
Then the pooled variance from the N repeated measurements in the control database and the n

repeated measurements of the SRM, § , is

P 2 q 2

^2 _ *=i y=i

[

{t,A)

P ~ (N-l)p + (n-1)^

The uncertainty for pitch measurements is

£/

p

= ±(1-96 sp

l\fn + systematic pitch uncertainty) jum (E-5)

and the uncertainty for linewidth measurements is

UL

= ±(1.96 sp

l\fn + systematic linewidth uncertainty) fxm,

where s is determined by using eqs (E.2), (E.3), and (E.4) for pitch and linewidth

measurements on the SRM and in the control database. Typically s = .015 jxm and n = 9

repeat measurements. The factor 1.96 converts the uncertainty to the 95% confidence level.

ACKNOWLEDGMENTS

The authors thank Susannah Schiller for suggesting the statistical tools to assure process control

of the linewidth SRM measurement system and for her review of this documentation.

REFERENCES

[15] Croarkin, C, Measurement Assurance Programs Part II: Development and

Implementation, Natl. Bur. Stand. (U.S.) Spec. Publ. 676-11, 1985, p. 35.

[16] Anderson, T.W., "An Introduction to Multivariate Statistical Analysis," Second ed. John

Wiley and Sons, 1984. Chapter 5, pp. 156-190.

[17] ibid., p. 423.

34
Table 2. Critical Values of F.o5(6,v) of the Table 3. Critical Values of t.975 (df) and t. w5(clf) F-Distribution of the Student's t-Distribution

V F.os(M df '.975 t 995

in1U 3.11 1 10 2.228 3.169

191Z 9 QO£ z.yyo 12 2.179 3.055

14 9Z.040 848 14 2. 145 2.977

1 u 9 941 Z. /4

1

16 2. 120 2.921

IS1

0

9 AA1 Z.001 18 2. 101 2.878 9nzu 9 5QQ zoyy 20 2.086 2.845 99zz 9 </<A z.54y 22 2.074 2.819

Z4 9 CAQ ZOUO 24 2.064 2.797 9A 2.474 26 2.056 2.779

90 1 vl /I c 2.445 28 2.048 2.763 in 9 ylT 1 Z.421 30 2.042 2.750

"19 jz 9 100 z. jyy 32 2.037 2.738 Z.JOU 9 ian 34 2.032 2.728 JO 9Z. J1A4OH 36 2.028 2.719 JO 9 140 38 2.024 2.712 40 9Z. JJU llfi 40 1 A^ 1 2.021 Z. /U4 42 9Z. 194 JZH 42

*"» A 1 O 2.018 ^ £AO 2.698 44 9 111 44 2.015 2.692 4A40 9 9n4 Z.JU4 46 2.013 2.687 48 9 905 Z.Z7J 48 2.011 2.682 sn 9 98A Z.ZOO 50 2.009 2.678

59JZ 9 990 z.z/y 52 2.007 2.674

54 9 999 Z.Z/Z 54 2.005 2.670 JO 9 9A£ Z.ZOO 56 2.003 2.667 JO5ft 9 9An z.zou 58 2.002 *> ZT /TO 2.663 ^n 9 954 Z.Z34 60 2.000 2.660

A9oz 9 940 Z.Z4y 62 1.999 2.657 A40-4 9 944 Z.Z44 £. A 64 1.998 *i zTC C 2.655 00 9 990 z.zjy 66 1.997 2.652

its 00 9 995 L.Lij 68 1.995 ^ zTC

A

2.650 9n/u 9 991 70 1.994 2.648 99/z 9 999 Z.ZZ / 72 1.993

** zT A tt 2.646

94/4 9 994 Z.ZZ4 74 1.993 *i £ A A 2.644

10 9 99A z.zzO 76 1.992 2.642 9S/O 9 919 z.zl / 78 1.991 2.640 enou 9 9 1 yl Z.Z14 80 1.990 2.639

89OZ 9 911 z.zl 1 82 1.989 2.637

8404 9 9AO Z.zUV 84 1.989 2.636

8£00 9 9A£ z.zUo 86 1.988 2.634

8800 9 9A9 88 1.987 2.633 onyu 9 9Ai Z.ZU1 c\r\ 90 1.987 i L*i*\ 2.632 09 9 1 0O z. iyy 92 1.986 2.630 04y4 9 1 09 z. iy / 94 i no z: 1.986 2.629

Of. yo 9 1 05 z. iyj 96 i noc 1.985 2.628 08yo 9 1 09 z. iyj AO98 1 AO A 1.984 2.627

i nn 1UU 9 1Q1 z. iy i 100 1 no a 1.984 2.626

1 f>9 1UZ 9 1 80 z.ioy 102 1.983 2.625

1U4 9 1 89 Z. 10 / 104 1.983 2.624

i aa 1U0 9 i as 106 1.983 2.623

1 na luo 9 1 84 Z. 104 108 1.982 2.622

i in ill! 9 1 89 Z. 1 OZ 1 10 1 ao^ 1.982 2.621 112 2.181 112 1.981 2.620

114 2.179 114 1.981 2.620

116 2.178 116 1.981 2.619

118 2.176 118 1.980 2.618

120 2.175 120 1.980 2.617

oo 2.099 oo 1.960 2.576

35
H-H-H-IH' • a I i 4—Hllll I

• • i

5'

Be

Figure 12. A view of the basic pattern on the SRM 473 control photomask. Features B and

I in row 1, F and K in row 2, E in row 3, and B in row 5 are measured before and after

each SRM is calibrated on the NIST linewidth measuring system. Feature E in row 3, rows 4 and 6 have been independently calibrated on the NIST Linescale Interferometer by the

NIST Dimensional Metrology Group.

+0.04

©

1

o

-0.04

70 Successive Meaurements (188 days)

Figure 13. Control chart of feature 1-B. Vertical axis is variation (pun) from the mean

(solid line); horizontal axis is successive measurements. The dotted lines mark the limits of

the 95% confidence level. Future measurements are added to the chart.

36
NIST-1 14A U.S. DEPARTMENT OF COMMERCE

(REV. 3-90) NATIONAL INSTITUTE OF STANDARDS AND TECHNOLOGY

BIBLIOGRAPHIC DATA SHEET

1. PUBLICATION OR REPORT NUMBER

NIST/SP-260/119

2. PERFORMING ORGANIZATION REPORT NUMBER

3. PUBLICATION DATE September 1992

4. TITLE AND SUBTITLE Antireflecting-Chromium Linewidth Standard, SRM 473, for Calibration of Optical

Microscope Linewidth Measuring Systems

5. AUTHOR(S) Carol F. Vezzetti, Ruth N. Varner, and James E. Potzick

6. PERFORMING ORGANIZATION (IF JOINT OR OTHER THAN NIST, SEE INSTRUCTIONS)

U.S. DEPARTMENT OF COMMERCE ,- Mr,, NATIONAL INSTITUTE OF STANDARDS AND TECHNOLOGY

GAITHERSBURQ, MD 20899

7. CONTRACT/GRANT NUMBER

8. TYPE OF REPORT AND PERIOD COVERED

Final

9. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (STREET, CITY, STATE, ZIP)

Item #6 above.

10. SUPPLEMENTARY NOTES

11. ABSTRACT (A 200-WORD OR LESS FACTUAL SUMMARY OF MOST SIGNIFICANT INFORMATION. IF DOCUMENT INCLUDES A SIGNIFICANT BIBLIOGRAPHY OR

LITERATURE SURVEY, MENTION IT HERE.)

The precise and accurate measurement of feature dimensions on photomasks, such as those used

in the production of integrated circuits, becomes increasingly difficult as the dimensions

approach the wavelength of the light used to make the measurement. The undesirable effects

of optical diffraction obsure the location of the feature edges. Raggedness and nonvertical walls

along the edges add to the uncertainty of the measurement.

Standard Reference Material SRM 473 was developed for use in calibrating optical microscopes

for measuring linewidths in the range of 0.5 to 30 on antireflecting-chromium photomasks.

The SRM is described, and instructions on its use and precautions concerning its care and

handling are given.

The NIST linewidth measuring system and the procedures used to calibrate the SRM are

discussed. The algorithm used for determining the line edge location incorporates a threshold

criterion derived from analysis of microscope image profiles. The profiles are predicted by

computer modeling based on the theory of partial coherence. The performance of this system

is monitored by measuring line features on a control photomask before and after calibrating each

SRM.

12. KEY WORDS (6 TO 12 ENTRIES; ALPHABETICAL ORDER; CAPITAUZE ONLY PROPER NAMES; AND SEPARATE KEY WORDS BY SEMICOLONS)

accuracy; antireflecting-chromium; calibration; control charts; critical dimensions;

integrated circuits; linewidth measurement; optical microscope; photomask; precision;

semiconductor industry; standard reference material; statistical process control;

threshold; uncertainty

13. AVAILABILITY 14. NUMBER OF PRINTED PAGES

X UNLIMITED 48

FOR OFFICIAL DISTRIBUTION. DO NOT RELEASE TO NATIONAL TECHNICAL INFORMATION SERVICE (NTIS). X ORDER FROM SUPERINTENDENT OF DOCUMENTS, U.S. GOVERNMENT PRINTING OFFICE, WASHINGTON, DC 20402.

15. PRICE

X

ORDER FROM NATIONAL TECHNICAL INFORMATION SERVICE (NTIS), SPRINGFIELD, VA 22161. ELECTRONIC FORM * U.S. G.P.O. : 1992-31 1-891 : 60061



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