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NIST SPECIAL PUBLICATION 260"1 1 4

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

Standard Reference Materials:

Bright-Chromium Linewidth

Standard, SRM 476, for Calibration

of Optical Microscope Linewidth

Measuring Systems

QC

100

rol F. Vezzetti, Ruth N. Varner, and James E. Potzick

.U57

260-114

1S91

C.2

Standard Reference Materials:

Bright-Chromium Linewidth

Standard, SRM 476, for Calibration

of Optical Microscope Linewidth

Measuring Systems

Carol F. Vezzetti

Ruth N. Varner

James E. Potzick

Center for Manufacturing Engineering

National Engineering Laboratory

National Institute of Standards and Technology

Gaithersburg, MD 20899

U.S. DEPARTMENT OF COMMERCE, Robert A. Mosbacher, Secretary

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

Issued January 1991
National Institute of Standards and Technology Special Publication 260-114

Natl. Inst. Stand. Technol. Spec. Publ. 260-114, 45 pages (Jan. 1991)

CODEN: NSPUE2

U.S. GOVERNMENT PRINTING OFFICE

WASHINGTON: 1991

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

technologists, 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

articles. 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, Acting Chief

Standard Reference Materials Program

Contents

1. Introduction 1

2. Physical Characteristics of SRM 476 . 1

3. Calibration of SRM 476 2

3.1 The Measurement System 2

3.2 SRM Calibration Procedure 3

3.3 Feature Measurement Sequence 3

3.4 Edge Location Determination 4

3.5 Precision and Accuracy 5

4. Using SRM 476 7

4.1 Special Precautions 7

4.2 Microscope Calibration Procedures 8

5. Acknowledgments 11

6. References 11

Figures 13

Appendix: Process Control for SRM 476 Calibration 22

A. Introduction 22

B. Initialization of Process Parameters 22

C. Procedures for Process Control 24

D. Updating Process Parameters 25

E. Uncertainty Statement for SRM 476 28

Acknowledgments 30

References 30

Tables 31

Figures 32

V
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). COM74-1 1063**

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 the Chemical Analysis of White Cast Iron Standards,

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

11068**

Bell, R. K., Standard Reference Materials: Methods 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 the 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 the Preparation of Standard Reference Materials of Nineteen

Radioactive Nuclides Issued by the National Bureau

of Standards, NBS Misc. Publ. 260-9 (November

1965). COM74-1 1065**

Yakowitz, H., Vieth, D. L., Heinrich, K. F. J., and Michaelis, R. E., Standard Reference Materials: Homogeneity Characterization of NBS Spectrometric

Standards II: Cartridge Brass and Low-Alloy Steel,

NBS Misc. Publ. 260-10 (December 1965). COM74-

11064**

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., Vieth, 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 Vieth, D. L., Stan- dard Reference Materials: Homogeneity Character- ization of NBS Spectrometric Standards IV:

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

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

COM74-11062**

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

the Vapor Pressure of Gold (Certification of Standard

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

(January 1970). PB190071**

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

the 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**

Hicho, G. E., Yakowitz, H., Rasberry, S. D., and Michaelis, R. E., Standard Reference Materials: A Standard

Reference Material Containing Nominally Four Per- cent Austenite, NBS Spec. Publ. 260-25 (February

1971). COM74-1 1356**

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

Bureau of Standards-US Steel Corporation Joint Pro- gram for Determining Oxygen and Nitrogen in Steel,

NBS Spec. Publ. 260-26 (February 1971). PB

81176620**

Gamer, E. L., Machlan, L. A., and Shields, W. R., Standard Reference Materials: Uranium Isotopic

Standard Reference Materials, NBS Spec. Publ. 260-

27 (April 1971). COM74-11358**

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 GoldSilver and Gold-Copper Alloys for Microanalysis,

NBS Spec. Publ. 260-28 (August 1971). COM71-

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 the USSR (A

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

30 (June 1971). COM71-50283**

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, Narrow

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 Method for the Determination of Calcium in Serum NBS SRM

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

50527**

Shultz, J. L, Bell, R. K., Rains, T. C, and Menis, O.,

Standard Reference Materials: Methods 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 Method for Resistivity Characterization of

High-Purity Metals, NBS Spec. Publ. 260-39 (May

1972). COM72-50529**

McAdie, H. G., Garn, 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 Polyethylene, 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-Chromium -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 and Use of

Superconductive Fixed Point Devices, SRM 767,

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

50037**

Greifer, B., Maienthal, 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 the 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**

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

Materials: Thermal Conductivity and Electrical Resistivity Standard Reference Materials: Electro- lytic 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 (April 1990, Revision of November 1975

version). 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 1193**

Durst, R. A. , Standard Reference Materials : S tandardization 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 Po- tassium 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: Enthalpy

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 (Feb- ruary 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 the 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 Polyethyl- ene (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 the Determination of Potassium in Serum, NBS Spec. Publ. 260-63 (May 1979). PB297207**

Velapoldi, R. A., and Mielenz, K. D., Standard Refer- ence 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 Other 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 Calibrat- ing the Wavelength 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 Method for the Determination of Chloride in Serum, NBS Spec. Publ. 260-67

(November 1979). PB80-110117**

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

for the Determination of Lithium in Serum, NBS

Spec. Publ. 260-69 (July 1980). PB80-209117**

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 the Clinical Laboratory

Standards Issued by the 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). PB8 1-220758

viii
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-4 1 1 and K-4 14 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., andPfieffer,

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 Reflectance (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

the 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, SRM488, 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 °C, NBS Spec. Publ. 260-

87 (December 1983). PB84- 149996**

Gladney, E. S., Burns, 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). PB84-2 18338**

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). PB85-1 12886**

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).PB85-115814**

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: Cali- brated Glass Standards for Fission Track Use (Supplement to NBS Spec. Publ. 260-49), NBS Spec. Publ. 260-92 (September 1984). PB85-1 13025**

ix
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 Refer- ence 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 p_m 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., Trahey, 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 Refractory 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 SRMUsers, 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 the 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: Methods and Procedures Used at the National Bureau of

Standards 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

fim Polystyrene Spheres), NIST 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. 260-110 (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, and Environmental Standard

Reference Materials, NBS Spec. Publ. 260-111 (November 1987).

x
Marinenko, R B., Blackburn, D. H., and Bodkin, J. B., Standard Reference Materials: Glasses for Microanalysis: SRM's 1871-1875, NIST Spec. 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, SRM 934, NIST

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

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

Reference Materials: Bright-Chromium Linewidth

Standard, SRM476, for Calibration of Optical Microscope Linewidth Measuring Systems, NIST Spec.

Publ. 260-114 (January 1991).

Hartman, A. W., Doiron, T. D., and Fu, J., Standard

Reference Materials: Certification of NIST SRM

1962, 3 (im Diameter Polystyrene Spheres, NIST

Spec. Publ. 260-115 (January 1991).

Hartman, A. W., Doiron, T. D., and Hembree, G. G.,

Standard Reference Materials: Certification of NIST

SRM 1961, 30 \xm Diameter Polystyrene, Spheres,

NIST Spec. Publ. 260-116 (January 1991).

*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 pur- chase price for postage.

**May be ordered from: National Technical Information Service (NTIS), Springfield, VA 22161
BRIGHT-CHROMIUM LINEWIDTH STANDARD, SRM 476,

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

Standard Reference Material, SRM 476, was developed for use in calibrating optical

microscopes used to measure linewidths in the range of 0.9 to 10.8 \im on bright chromium

photomasks, such as those used in the production of integrated circuits. The SRM, the

measurement system, and the procedures used to calibrate the SRM are described. The

algorithm for determining the line edge location uses 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 the system is monitored by

measuring line features on a control photomask before and after calibrating each SRM.

Precautions concerning care and handling and instruction for the use of SRM 476 to calibrate

optical microscopes for photomask linewidth measurement are given.

KEY WORDS: accuracy; bright chromium; calibration; control charts; critical dimensions;

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

process control; semiconductor industry; standard reference material; 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 measuring 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) 476 is designed for calibrating optical microscope

systems to measure linewidths on bright-chromium photomasks. It was produced using

conventional technology by a commercial photomask manufacturing facility. In addition to isolated opaque and clear lines for linewidth calibration, the 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.

A photometric microscope with lenses selected for least aberration was modified at NIST for

calibrating the SRM line features. Except for the initial positioning, aligning, and focusing of

the photomask, the entire calibration process is automated. Linewidths are determined from

the image profile (image intensity versus position across a feature). The quality of the feature

edge geometry of a sample from each production batch of SRM photomasks is examined

using a scanning electron microscope. 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 3

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.

Section 4 gives information and precautions on using this SRM to calibrate an optical

linewidth measuring system. Because of the variety of linewidth measurement systems in use

today, no attempt has been made to give specific instructions for each type of microscope.

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

detail in the Appendix.

2. Physical Characteristics of SRM 476

SRM 476 is made from a bright chromium photoplate by conventional photolithographic

techniques. The substrate is a borosilicate glass plate nominally 63.5 x 63.5 x 1.5 mm (2.5 x

2.5 x 0.060 in). The nominal thickness of the chromium layer is 100 nm.

Figure 1 shows the overall pattern on the chromium-coated side of the standard. The cross- shaped relatively clear area contains a series of horizontal and vertical lines that are used to 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 central area of fig. 1).

*formerly the National Bureau of Standards (NBS)

1
An underlined pattern identification number (1 through 8) is located to the left of each basic

pattern. The accompanying certificate identifies which of these eight patterns was certified by

NIST.

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

Letters A, B, E, F and G designate the row locations, and numbers 0 through 9 designate

column locations. Thus, each feature can be identified by its row and column coordinates;

for example, A-5 refers to the opaque line in row A and column 5. Row A consists of

opaque lines on a clear background, and row B consists of clear lines 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. Row E consists of patterns

with opaque line pairs (the two interior lines of shorter length)* for calibrating the length

scale of optical microscopes by measuring the distance between line centers (pitch). Row F

consists of patterns with two adjacent lines and the intervening space (two interior lines of

longer length and the intervening space)* with approximately equal line and space widths.

These patterns are used in setting the line-to-space ratio (contrast, threshold level, or

linearity) on video image-scanning instruments. The widths of the left interior line and

central space are certified. Row G consists of two patterns, with a series of 10 approximately

equally spaced opaque lines in each pattern, to be used as a linear scale in checking for errors

in the linewidth measurement system (e.g., the magnification as a function of position over

the field of view). The center-to-center distances from the first line to each of the other nine

lines of these patterns are certified. All rows on this SRM contain a broken horizontal

fiducial line which is used to define the measurement position of each pattern.

3. Calibration of SRM 476

3.1 The Measurement System

All measurements of the SRM feature dimensions were performed at NIST in a laboratory

with temperature controlled at 21 °C (± 2°) using the optical linewidth measurement system

diagrammed in figure 4 [1].

The carefully aligned optical transmission microscope is 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 using 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 (.60 numerical aperture condenser lens

and .90 numerical aperture objective lens). A 20 by 400 |im slit is fixed on axis in

*The two outer lines of each pattern in rows E and F serve as "guard lines" during the

photolithographic etching process to equalize proximity effects along the line edges and are

not calibrated.

2
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 by 2.55 [im, 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/micrometer. All these devices are connected via appropriate control hardware and

IEEE-488 bus to a dedicated desktop digital computer.

3.2 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 E-4, E-5, and G-l which have been independently calibrated by the

Dimensional Metrology Group at NIST. Then each feature on the SRM being calibrated is 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. Then 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 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.

3.3 Feature Measurement Sequence

A 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.

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 [1].

3
3.4 Edge Location Determination

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

edge of a line [2]:

Tc =MIo +Im

+ 2^XC0Sd>), (1)

where Tc is the intensity of the light at the threshold point (edge) on the image profile (see

fig. 8); IGis the intensity of the light passing through the chromium layer; Im is the intensity

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

difference between I0 and Im. ^ 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 = 0.27 (see below).

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.

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:

(2)

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

transmission, and flare) and where I

f is the magnitude of the flare light component in the

image profile for each feature. Substituting into eq (1) and solving for Tc gives:

Tc = Rt Wo - If) + dm - If) + 2/(Io-If) dm -If) COS (|>] + I

f

. C

Both (J) and I

f must be known to evaluate the threshold condition. If is feature and

background dependent and must be evaluated for each feature, but this is difficult unless

either I0 or Imis independently known. However if the coated layer is known to be

homogeneous, the phase difference, (j>, and the transmissivity can be taken as constant over

the mask. The transmissivity 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

4
large chromium-covered upper-left quadrant of the mask and was found to be less than 0.1%

of the incident intensity, and IQ can be considered zero. Then, from eq (2), I

f = I0 and eq

(3) becomes:

T

x = R a' - \' ) + l' , (4)

and it is not necessary to determine a value for <j>. This is a significant simplification as there

is no known simple method for determining 4>. A further simplification may be accomplished by shifting the intensity zero level so that I0 = 0

(subtracting out the flare light); then the equation becomes:

Tc = R, 0L )• (5)

A recent study of image profiles generated by a computer model, based on the theory of

partial coherence, indicates that, with the NIST microscope and the SRM measurement

conditions, the ratio Rt varies from 0.25 to 0.28 for ideal (vertical-walled, thin) lines and

spaces 0.75 to 10.0 fim wide (see fig. 9A). By setting Rj equal to 0.27 for determining edge

location throughout the measurement of this SRM, the resulting contribution to linewidth

error (difference from true linewidth) is less than + 0.01 /xm (see fig. 9B).

3.5 Precision and Accuracy

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

values are the average of 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 [3] 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 [4]. 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 an accepted reference

value [5]. Pitch measurements of several features in a control SRM have been independently

calibrated by the Dimensional Metrology Group on the NIST Line Scale Interferometer.

5
Comparison of these measurements with measurements of the same features made on the

NIST linewidth measurement system shows a small scale factor difference. The source of

this difference is not fully understood, but may be attributable, in part, to a small Abbe offset error on the linewidth measurement system. Hence, a correction factor is applied to all the

SRM calibration measurements, using the NIST Line Scale Interferometer results as the

standard, and the variance introduced by this procedure is included in the uncertainty given

for all measurements (widths and pitches).

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 [6]

(see fig. 10). To account for these systematic errors in the uncertainty statement, the feature

edge geometry is examined using a scanning electron microscope (SEM). As this

examination precludes use of the photomask as an SRM, only one sample from each

photomask production batch is examined. The effective length of the viewing slit in the NIST

instrument is 2.6 u.m, and the linewidth reported is the average over this length, positioned at the center of the line. Therefore both the nonvertical edge geometry and the raggedness

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

Differences between the edge locations (and therefore linewidths) at the top surface and at the

substrate were estimated at uniformly spaced intervals along a 1 fim section of the sample as imaged in an oblique view SEM micrograph. The average of these differences is the

estimated uncertainty due to the nonideal edge shape. The uncertainty thus estimated for

SRM 476 is ± 0.015 |im for each edge. The SEM micrographs also show that raggedness

along the length of a line typically is less than 30 nm and has a spatial period of 0.1 u.m or

less. If the user's measurements of the SRM are averaged over a length comparable to that at NIST, then uncertainties due to edge raggedness become insignificant.

For width measurements, edge location errors for the right and left edges tend to add, making

the linewidth uncertainty twice the edge location uncertainty [7]. The resulting level of

inaccuracy, + 0.03 u.m, is larger than the standard deviation of the calibration measurement,

thus showing that the major contribution to the uncertainty of calibration is the nonideal edge

geometry of the patterned features on the SRM.

For a typical series of SRM 476 photomasks the maximum systematic linewidth uncertainty,

for rows A, B, and F, is estimated to be ± 0.05 u.m. This value includes the + 0.03 u.m

uncertainty resulting from the nonideal edge geometry estimated from the SEM

photomicrographs, the ± 0.01 u.m maximum error introduced by use of the fixed value of

27% of the intensity, 1^, as the edge location threshold (see sect. 3.4); and an estimated

maximum 0.01 \im 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 maximum uncertainty reported for row E

and G measurements.

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 A, B, and F)

and pitch (rows E and G) measurements on the certificate data sheet.

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

appropriate systematic uncertainties and measurement precision.

The user is advised to examine the edge properties of the 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.

4. Using SRM 476

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 bright-chromium photomasks or similar

artifacts.

4.1 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.

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 can not 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 contaminants.*

*By using great care and low KV techniques, it is possible to view this SRM in an SEM

without an evaporated film and thereby check the magnification of the SEM by measuring the

spacing (pitch) of the features in rows E and G. However, other NIST SRMs, designed

specifically for calibrating the magnification of SEMs, are recommended for this purpose.

Also, after this SRM has been used in an SEM, its use for optical linewidth system

calibration may no longer be valid.

7
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 de-ionized water.

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 de-ionized 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.

4.2 Microscope Calibration Procedures

The following procedure is recommended for using this SRM to calibrate optical microscope

systems for measuring linewidths on bright-chromium photomasks. It is assumed the user is familiar with the operation of the microscope system being calibrated, and no attempt is made

to give instruction on the use of microscope systems; however, instructions on setting up an

optical microscope with Kohler illumination for dimensional measurements can be found in

reference [8].

8
Procedures

1. Set up the measurement system for

dimensional measurements using the

same procedures that will be used

for measuring photomasks.

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

SRM that has been calibrated by NIST.

3. Check the resolving power of the

microscope objective using feature

G-0. If the objective cannot

clearly resolve the 10 lines, use another objective.

4. Align the SRM so that lines are measured in a direction perpedicular to their length.

5. Adjust the measurement system length

scale using feature E-5 (see fig. 3)

to give the same reading for the

spacing of the two inner lines as the NIST value.

6. Check for mechanical nonlinearity and/

or optical distortion by measuring the

spacings of the 10 lines in feature

G-0 (or G-l) and comparing the results

with the NIST values.

For all further measurements, use only the portion of the field of view

corresponding to the location where

the differences from NIST values are

relatively constant or that portion

of the video-display which exhibits

minimum distortion.

Explanatory Notes

Consult reference [8] for

recommended procedures including

adjustments for Kohler illumination.

Each pattern group has a pattern number

located to the left of Row E. The SRM

Certificate identifies the calibrated

pattern group (see fig. 2).

The features within each pattern

group are located by reference to an alphanumeric grid with the

letters A, B, E, F, and G

identifying the row and the

numbers 0 through 9 identifying

the column (see fig. 3).

The long vertical line, running

from row A to row G at the far

right (see fig. 3), may be used

as an alignment aid.

Do not use any other standards or procedures for this adjustment of

the length scale unless specifically instructed otherwise by the

manufacturer of the instrument.

To differentiate between optical

and mechanical errors, multiple

tests must be made. For example,

repeat the test with interchanged

optics while using the same screw

thread to determine if a given

error pattern is due to mechanical

errors rather than optical distortion. If the errors are small,

restricting the field of view may

eliminate the necessity of

identifying the error source.

9
Procedures Explanatory Notes

7. Adjust the line-to-space ratio on video- The widths of the left inner (long)

type image-scanning micrometer attachments line and the adjacent space between

by measuring the calibrated line and the two inner lines are certified,

space of a feature in row F.

Adjust the system contrast, brightness, Accurate linewidth measurements

and/or threshold until both measured require a fixed relationship between

widths agree as closely as possible a threshold in the image intensity

with the NIST values. profile, which corresponds to the

physical line-edge location, and the

video output or other signal from

which the measurement is obtained.

Subtract out the flare light by setting l'Q A flare light correction should

to zero (see fig. 8 and eq (4) and (5) and be made when measuring linewidths

determine the ratio T^/I^. Use this ratio on all photomasks (not only this

to set the threshold for each subsequent standard),

measurement in this calibration session.

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 A and B.

Use the same focusing criteria For optical systems with Kohler

throughout. illumination, a fine bright band

(or other diffraction effects) may

Do not refocus between setting on the appear along the edge of the

left and right edges of the same line. line image at best focus.

Make all measurement in the same Some mechanical systems exhibit

direction of travel. backlash when their direction of

travel is changed.

9. Measure and record the line spacings

(pitch) of NIST calibrated line pairs

in row E.

10. Derive the calibration curves as These calibration curves apply only

described in reference [9]. to this system/operator combination.

10
The system is now ready for measurement of other bright chromium photomasks or artifacts

with similar optical properties to SRM 476 (high reflectivity and very low transmissivity)

using the same threshold value and flare light correction procedure. If the user attempts to measure artifacts with chromium layers having transmission greater than 0.2%, it may be

necessary to measure the phase angle, <j>, 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.

5. ACKNOWLEDGMENTS

Parts of this document (especially the procedures in sect. 4) 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 and her

word processor for typing the numerous revisions of this document.

6. REFERENCES

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

National Institute of Standards and Technology," SPIE Vol. 1087, San Jose, CA,

Integrated Circuit Metrology, Inspection, and Process Control, (Feb. 1989).

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

Operating Conditions on the Image Profile," Applied Optics Vol. 16, pp. 2223-2230,

(Aug. 1977).

[31 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.

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

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

[5] 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.

li
[6] Nyyssonen, D. and Larrabee, R.D., "Submicrometer Linewidth Metrology in the Optical

Microscope," J. Res. Natl. Bur. Stand. (U.S.), May-June 1987, pp. 189-190.

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

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

[8] 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.

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

on Integrated Circuit Photomasks," NBS Tech. Note 1164, August 1982.

12
Figure 1. A view of the overall pattern on SRM 476. The basic measurement pattern is repeated eight times about the center. The horizontal and vertical lines help locate the

patterns.

"I I I -

- I I I .

Figure 2. A view of the center of the SRM. The pattern number given with the serial

number on the calibration sheet identifies which pattern has been measured by NIST. Pattern

identification numbers can be seen to the left of each basic measurement pattern.

13
R

B

0 1 a 3 H 5G1B9

II -l-l-fl-l-l-ll-l

B I

1 S 3 H S E 1 B 9

E H»H"hH'H-E-|»HHH"H"h

1 2

F F-l

G

Figure 3. A view of one basic measurement pattern on the SRM. The individual lines and

line patterns are located by reference to an alphanumeric grid with the letters identifying the

row and the numbers identifying the columns. The long vertical line on the right is used to align the pattern on the measurement system. The broken horizontal lines mark the central

calibrated area of the features.

Calibration values are given for: widths of opaque lines in row A and clear lines in row B;

center-to-center spacing of the two inner (short) lines of each line pattern in row E; widths of

the left inner (long) line and the space to its right of each line pattern in row F; and center-to- center spacings of lines relative to the first line on the left of each line pattern in row G.

14
I HV Supply

Photomultiplier

Tube

Transmission Optical

Microscope

Photomask

Fixed

Slit

DC Amp

HV Amps

Green Filter v ; s ; s >/ ;tt7\

Laser Incandescent Bulb

I

Computer

PM ADC -

X-Piezo Drive

Z-Piezo Drive

0

i MH

X-Y Leadscrew

Drive

Shutter Control

Laser Interferometer

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 using

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.

15
Mount Mask

Enter I.D. Set Measurement Series) Set Number ot Series

Find 1st Feature in Series

Center and Focus

Feature

Collect & Filter Measurement Data (X&P)

Calculate Width

or Pitch Perform Tests

C

stop )

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

photomasks using 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.

16
Find & Correct Problem Measure

Control Mask

No System in ^Control V

Yes

t

Measure SRM Mask

I

No ^^Measuremern\

Measure

Control Mask

Discard SRM

Measurements

i Yes or Maybe

i k

No Find & Correct Problem

Summarize Data

Store Data

Print Certificate

( Stop )

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 located,

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) data are taken and

stored as two one-dimensional arrays.

17
552 553 554 555

A. Profile of an opaque line in row A.

B. Profile of a

clear line (space)

in row B.

../. /W! i.JWi...

! 1

_

I

1

1 1 -M— ! -f \

!

! fhi'! I!

i i

; ! ]

• ! !

1

:

i :" !

1 i i

1 : , • 1

[ i i

:

i ;

i ! ; 1 : :

; if i~T •:

! t

i

i ]

•

i

\

<

i •

; :

|

: i"

i i

i • i

: ' i

;

1

! 1 •

C. Profile of a

line pair in row E.

The profile includes

the two outer guard

lines as well as the

line spacing pair.

« ! i 1

' :

! j 1 ! J

\

:

1 '

i \ \ —^- - -1 —1- —J— -

- £40 245 250

Figure 7. Samples of optical profiles (measured light intensity versus position) displayed on

the computer screen during the calibration process.

18
OPTICAL MICROSCOPE

IMAGE PROFILE

>

C/D

LU

IPhysical Line

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 the light passing

through the clear area; I0

is the intensity of the light passing through the chromium; Tc is the

intensity at the physical edge (threshold); I

f 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.

19
29

>

H

COz

LU

4.0 6.0 WIDTH (jim)

10.0

Figure 9. A - Optical intensity [% of (Im

-IG

)] at edge location versus linewidth, from a computer model of the NIST calibration system.

0.01

-0.01

/

1

~~ X

I \ ' /

I / — Ol)aque lines \—l

XJ

Clear spaces

0 2.0 4.0 6.0 8.0 10.0 WIDTH (urn)

Figure 9. B - Theoretical linewidth error versus linewidth, assuming 27% of (Im

-IG

) is used

as the threshold to determine edge location of lines and spaces using the NIST

calibration system.

20
126705 S5KV X181K' SI 30um

Figure 10. SEM photomicrographs showing

the nonideal nature of line edges on a

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

lower right of each photomicrograph. In the

upper photomicrograph, the dots are 0.03 \im

apart and in the lower photomicrograph, they

are 0.06 u.m apart. Beam energy was 25 keV

in both cases and the specimens were uncoated.

r mmarn

120706 25KV X5.8..8K 0.60um

21
Appendix

Process Control for SRM 476 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 2.7

and 9.5 p. m lines from row A; the nominal 9.5 and 2.7 [im lines from row B; and the

nominal 4.0 and 10.8 p.m pitch patterns from row E. These correspond to features A-0,

A-l, B-8, B-9, E-4, and E-5 as shown on the diagram of a pattern in figure A.l.

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 determined to be 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 [1A].

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

The six features measured are identified as A-0, A-l, B-8, B-9, E-4, and E-5. 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 width or space but also other

pertinent information such as the date and time of the measurement and feature identification

and any other potentially useful information (temperature, scan rate, etc.).

A plot, measured width or pitch versus time, of the repeated measurements for each feature 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 pertinent 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 [2A], 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.

22
From the database of control measurements for features A-0, A-l, B-8, B-9, E-4, and E-5, 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,

XA0,1 XA0,2 XA0,N

XA1,1 XA1,2 XA1,N

XB8,1 XB8,2 XB8,N

XB9,1 XB9,2 XB9,N

XE4,1 XE4,2 XE4,N

XE5,1 XE5,2 XE5,N

where i = 1,2,....

and j = 1,2 N

(B.l)

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

L A0

XB8 ~

XE4 "

N

E

j=l

N

E

xAOj

N

X

B8j

N

n x

j=l

E4j

N

kAl

XB9

L E5

N

= E

j=l

N

= E

j=l

N

= E

j=l

XAlj

N

V

B9j

N

N

(B.2)

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

XAO

L A1

M =

k B8

L B9

E4

E5

(B.3)

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

23
Z, = X„ - M,

Wheie

. I = JA""« (B.4)

y y

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

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

elements:

N

s

1 T (7 \ (7 \ where 1 = 1 ' 2 ---- 6 (B5) S

* " N=l k.j

(Z]k) andj = 1,2,....6 .

1 ^

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 A-0, A-l, B-8, B-9, E-4 and

E-5 on the control mask are measured and the multivariate Hotelling's test statistic T

2

is computed as follows:

T

2 =

( JL [Y - M]' S"1 [Y - M]

N+l

(C.l)

where Y is a vector of newly determined widths and pitches for the above mentioned

features.

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

(N "

6)

, T

2 * F 05(6,N-6), (C.2)

(N-l)6

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

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))T2is saved in the control database.

If the test indicates the system is not in control, the data is tagged when it is 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:

24
The mean, X, and the standard deviation, a , for each feature are computed using the N

repeated measurements from the control database:

X =

N

i=l

and

i=l

N

E (X. - X)2 (C.3) a = N (N - 1)

Control limits are computed using the following equations:

X ± a t 975(N-l) for the 2 a limit

and (C4)

X ± a t 995(N-l) for the 3a limit.

The value of t is found in table II. The value, (N - 1), denotes degrees of freedom, df, in

table II.

Figure A. 2 is an example of a control chart of the initial 19 measurements of feature A-0.

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

system is ready for 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 M2

, a vector of estimated means for the recently

collected control measurements; eqs (B.4) and (B.5) are used to compute S2

, the

corresponding variance-covariance matrix; and eq (C.3) is used to compute b2

, 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.

25
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 variancecovariance matrices is tested as follows:

let I = Ni + N2 , (D.l)

where Nj = 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 S = (Ni S i) + (N2S2) (d.2)

where S l is the variance-covariance matrix of the current control database

and S2is the variance-covariance matrix of additional new control measurements.

Compute the statistic [3A]:

D = 0.5^ trace [(S rS)S

_1

] 2 + 0.5N2 trace [(S2 -S)S

-1

] 2

; (D-3)

and test whether:

D * Xdf<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 %21 (0.05) is 32.67. If D ^ 32.67, then the differences between the old and new covariance matrices can be attributed to 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:

26
where Sj and S2 are defined in (D.2).

Then the statistic is computed:

(1-p-lXNjNa)

(Mj - M2)' S^CMj - M2); (D.5)

(£-2)p (0

and tested whether:

T

2 * F 05 (p,£-p-l)

where N1? N2 and £ are defined in (D.l),

Mj 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 £-p-l degrees of freedom in the denominator. The F 05 (p,£-p-l) value is given in

table I. If T

2 >F 05 (p,£-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 (p,£-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:

(D.6)

27
The mean vector is updated as shown below:

M = NjMj + N2M2 (D.7)

The standard deviation for each feature is updated as follows:

Aa = (D.8)

N, + N2 - 2

E. Uncertainty Statement for SRM 476

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 due to systematic error. The systematic error for pitch values consists of the variance introduced by

correcting the measurements to agree with the NIST Line Scale Interferometer measurements

(see sect. 3.5). The systematic error for the linewidth values has a significant contribution

(± 0.05 |im) resulting from the finite material edge geometry of the features (see sect. 3.5)

plus a small contribution from the correction factor mentioned above.

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:

X = a Y + e

(E.l)

where X represents a measurement from the linewidth measurement system,

Y represents a measurement from the line scale measurement system,

and g is the random error of measurement.

28
Then the uncertainties, UL and Up

, for linewidth and pitch measurements are determined by

the equations below:

The variance of each SRM measurement, S: , is

.2 -2 var(a) or n-1

nE

i=l

(E.2)

where Xj is the average of the jth 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.

2 The variance of the control measurements, dk

, is

,2 -2 var(a) _

= a —±J. + CVt a

(E.3)

where is the average of the kth control feature

and CVkis the diagonal of the variance-covariance matrix for the control data.

Then the pooled variance from the N repeated measurements in the control database and the n

repeated measurements of the SRM, 3 , is

,2

.2 (N-1) E ak + (n-1) E §; k=l j = l

'

(N-l)p + (n-l)q

(E.4)

29
The uncertainty for pitch measurements is:

Up = ±(1.96 Sp/v/n") urn. (E.5)

The total uncertainty for linewidth measurements is:

UL

= ±(1.96 Sp/v/n + 0.05) \im (E.6)

where spis determined by using eqs (E.2), (E.3), and (E.4) for pitch and linewidth

measurements on the SRM and in the control database.

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.

[1A] Croarkin, Carroll. Measurement Assurance Programs Part II: Development and

Implementation. Natl. Bur. Stand. Spec. Publ. 676-11; 1985. pp. 35.

[2A] Anderson, T.W. An Introduction to Multivariate Statistical Analysis, Second ed. John

Wiley and Sons, 1984. Chapter 5, pp. 156-190.

[3A] ibid., p. 423.

REFERENCES

30
!

5

o

oo p»tr> o— *e i*o oots noo —« ee ir> t*» —

^. oo m<»«^«n n—o o>oo io ^'tf>«^»n r«s —-^ —ooo

*• . . • — 1 *-*-" — *- *- -* — * — ' — * — * — *— ' — '— ' — ' — " — '— * - —i —I —- —- I •

- ' CN«N<S<N<S<N(NfN»(NCSfSCMCNO»CN<MfN(NCS(N(NCSCMCN(N<N

V

3

> "o —————<NCscs<s<Ncnfnc^rr>cr>'W'w- vv^^v^v^w^*>©>o>©>©t^r^r*r»r^ooooooaoooCT»o*o*ov©v22222~~— —

•a•CU

e

o

-as

«g ^« «srJ cs (N«si r4r4cir>i rie<i rs* <s cmf4oi<n* «n" <r4 cicicicif^cicie*t cicic-t cicicicicicici t-i ciciciin cn cici cici cirsi oi cici c-i cm' c

B."

8 a Oci^vooo©ci^^oo©cj2l^oooc»^N©oo©ciTroaoQci^voooo

31
H

B

0 1 a 3 H 5 6 1 B 9

3 l-n 3

B I

1 3 3 H 5BTB9

E H|| H|| HH|| H-E-l || H| HH»H"h

F

G

1 5

FFigure A.l. A view of the basic pattern on the SRM 476 control photomask. Features 0 and

1 in row A, 8 and 9 in row B, and 4 and 5 in row E are measured before and after each

SRM is calibrated. Features 4 and 5 in row E and feature 1 in row G have been

independently calibrated on the NIST Linescale Interferometer by the NIST Dimensional

Metrology Group.

32
2.52

2.51 -

M

E

s 2 - 5

U

R

E

D 2.49

V

A

U

2 '< 8

E

S

2.47 H

* *

2.46

0 0 20

TIME

30 40

Figure A. 2. Control chart of feature A-0. Solid line is the mean; dotted line is the 99%

level; and the dashed is the 95% level.

The chart includes the first 19 measurements of the feature. Future measurements are added

to the chart.

33

NIST-114A U.S. DEPARTMENT OF COMMERCE

(REV. 3-89) NATIONAL INSTITUTE OF STANDARDS AND TECHNOLOGY

BIBLIOGRAPHIC DATA SHEET

1. PUBLICATION OR REPORT NUMBER

NIST/SP-260/114

2. PERFORMING ORGANIZATION REPORT NUMBER

3. PUBUCATION DATE January 1991

4. TITLE AND SUBTITLE Bright-Chromium Linewidth Standard, SRM 476, 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

NATIONAL INSTITUTE OF STANDARDS AND TECHNOLOGY

GAITHERSBURG, 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)

Same as item #6

10. SUPPLEMENTARY NOTES

DOCUMENT DESCRIBES A COMPUTER PROGRAM; SF-185, FIPS SOFTWARE SUMMARY, IS ATTACHED.

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.)

Standard Reference Material, SRM 476, was developed for use in calibrating

optical microscopes used to measure linewidths in the range of 0.9 to 10.8

micrometers on bright chromium photomasks, such as those used in the

production of integrated circuits. The SRM, the measurement system, and the

procedures used to calibrate the SRM are described. The algorithm for

determining the line edge location uses 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 the system is monitored by measuring line features on a control photomask

before and after calibrating each SRM.

Precautions concerning care and handling and instruction for the use of SRM

476 to calibrate optical microscopes for photomask linewidth measurement are given

.

12. KEY WORDS (6 TO 12 ENTRIES: ALPHABETICAL ORDER: CAPITALI7F OMl v PROPFB NAMES- AND SEPARATE KEY WORDS BY SEMICOLONS)

accuracy; bright chromium; calibration; control charts; critical dimensions;

integrated circuits; linewidth measurement; optical microscope; photomask;

precision; process control; semiconductor industry; standard reference

material; threshold; uncertainty.

13. AVAILABILITY UNLIMITED FOR OFFICIAL DISTRIBUTION. DO NOT RELEASE TO NATIONAL TECHNICAL INFORMATION SERVICE (NTIS). X

X

ORDER FROM SUPERINTENDENT OF DOCUMENTS, U.S. GOVERNMENT PRINTING OFFICE, WASHINGTON, DC 20402. ORDER FROM NATIONAL TECHNICAL INFORMATION SERVICE (NTIS), SPRINGFIELD, VA 22161.

14. NUMBER OF PRINTED PAGES

45

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