Antireflecting-Chromium Linewidth Standard, SRM 473, for Calibration of Optical Microscope Linewidth Measuring System
James E. Potzick
This document describes the physical characteristics of Standard Reference Material SRM 473, provides instructions for its use in calibrating optical photomask linewidth measuring systems, and gives information and precautions concerning its care and handling. Standard Reference Material SRM 473 was developed for use in calibrating optical microscopes for measuring linewidths in the range of 0.5 ?m to 30 ?m on antireflecting-chromium photomasks. In addition, it contains pitch (center-to-center) patterns ranging from 2 ?m to 70 ?m. The 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 effects of optical diffraction obscure the location of the feature edges. Raggedness and nonvertical walls along the edges add to the uncertainty of the measurement. This SRM makes possible traceable linewidth measurements by facilitating the evaluation of these and other components of linewidth measurement uncertainty. The NIST linewidth measuring system and the procedures used to calibrate this SRM are discussed. These include the algorithm used for determining the line edge location from the optical intensity data, which incorporates a threshold criterion derived from analysis of microscope image profiles. The profiles are predicted by a numerical model based on the theory of partial coherence. The statistical performance of this system is monitored by measuring line features on a control photomask before and after calibrating each SRM. The factors that affect the calibration uncertainty are explained and evaluated. NIST photomask linewidth SRMs 473, 475, and 476 are available from the Office of Standard Reference Materials, NIST, EM 205, Gaithersburg, Md. 20899. Voice 301-975-6776, FAX 301-948-3730.
Antireflecting-Chromium Linewidth Standard, SRM 473, for Calibration of Optical Microscope Linewidth Measuring System, Technical Note (NIST TN), National Institute of Standards and Technology, Gaithersburg, MD
(Accessed December 1, 2023)