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EXPOSURE AND FOCUS SCALES FOR OPTICAL LITHOGRAPHY

IP.com Disclosure Number: IPCOM000005657D
Original Publication Date: 1987-Oct-01
Included in the Prior Art Database: 2001-Oct-24
Document File: 2 page(s) / 107K

Publishing Venue

Motorola

Related People

B. R. Stallard: AUTHOR [+6]

Abstract

In checking the critical dimensions (CDs) of a resist image for IC's it is desirable to be able to use the same optical microscope that is used for checking alignment and other aspects of after development inspec- tion. Since this procedure may be unreliable for CDs less than about 1 micrometer, indirect methods for CD inspection are needed. We have invented a new indirect method of CD inspection for optical lithography.

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MOTOROLA Technical Developments Volume 7 October 1987

EXPOSURE AND FOCUS SCALES FOR OPTICAL LITHOGRAPHY

by B. R. Stallard, Y. Bukhman, C. Casteel, A. J. Gonzales, J. N. Helbert, and G. Ziniewicz

Bipolar Technology Center

   In checking the critical dimensions (CDs) of a resist image for IC's it is desirable to be able to use the same optical microscope that is used for checking alignment and other aspects of after development inspec- tion. Since this procedure may be unreliable for CDs less than about 1 micrometer, indirect methods for CD inspection are needed. We have invented a new indirect method of CD inspection for optical lithography.

1. THEEXPOSURESCALE

   An exposure scale consisting of an array of squares 20-50 micrometer on a side having transmission of 50-80 percent and made up of crosses (see FIG. 1) with arms having a width of 1.5 micrometers and length of 0.75-1.5 micrometers are exposed and printed on any unused part of the wafer. The exposures of the squares in the array are adjusted to bracket the known sensitivity of the resist for large area clearing. Submicrometer features in the die are exposed with 1.5 to 2 times the exposure required for large areaclearing in order to com- pensate for the lack of internal proximity effect in the small features. A target square in the exposure scale array is identified empirically so that during development this square just clears when the submicrometer features in the die are correctly sized (see FIG. 2). Once the correlation between the exposure scale reading and the submicrometer CD has been established only the exposure scale need be inspected. Examination of the ex- posure scale with a conventional optical microscope is sensitive enough to distinguish a cleared from uncleared square with only a2-4% difference in exposure. It is also found that submicrometer CDs in direct-write electron beam lithography can be controlled using the exposure scale to about kO.05 micrometer (3 sigma).

II. THE FOCUS SCALE

    Before exposing a wafer lot on an optical stepper it is customary to establish the correct focus and ex- posure settings using a test wafer with a6 by 10 array of dice bracketing the expected range of settings. Historical. ly, best focus has been determined by judging the sharpness of the resist image for the various focus settings represented in the array. This is highly subjective and quite difficult for inexperienced op...