Browse Prior Art Database

Semiconductor Positioning System Disclosure Number: IPCOM000077821D
Original Publication Date: 1972-Sep-01
Included in the Prior Art Database: 2005-Feb-25
Document File: 3 page(s) / 57K

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This system automatically aligns a semiconductor chip or wafer with a mask used in projection or contact printing.

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Semiconductor Positioning System

This system automatically aligns a semiconductor chip or wafer with a mask used in projection or contact printing.

In Fig. 1, illumination from lamp 2 is projected by condenser lens 3 through glass mask 4; the image of the mask is projected and focused onto wafer 5 which is mounted on x-y-B positioning table 10. At the edges of the wafer are four sets of bar patterns 7 employed for alignment. Two of the sets are high contrast, containing bright and dark bars imaged by the lens from the glass mask. The other two sets are low contrast, containing bright bars produced by the metallization, silicon, and oxide reflections from the wafer. The high-contrast bars always appear in the same position with respect to the optical system.

A key feature in the position determination, lies in the separation of the high- contrast bar patterns projected by the mask and reflected by the wafer from the low-contrast bar patterns reflected from the wafer. By separating them and keeping them separated even when perfectly aligned, the positions of the individual bar pattern may be estimated relative to each other without the image of one interfering with a position estimate of another. Bar patterns 7 are imaged by projection lens 6 through the half-silvered mirror 8 and relayed by lens; to the central portions of TV cameras 12 and 14.

Each of the patterns preferably comprises five sets of bars encompassing equal areas. The bars within a set have the same width and spacings, but have different widths from the bars in the other sets. Each low-contrast bar pattern on the wafer is indicated below those projected by the mask. Its position relative to the optical system is not known. Initially, the wafer is positioned within +/-1 mil horizontally and +/-1 mil vertically about the nominal value. The pair of alignment patterns preferably fall within an area of 6 mils by 1.6 mils.

The bar patterns selected permit subsequent "matched filter" processing of the video, to build up the signal-to-noise ratio of the output from the bar patterns. At a time after the TV camera readout beam scans over the entire bar pattern a short, high signal-to-noise ratio pulse is produced, indicating the position of the bar patterns relative to the TV scan line.

Each TV camera has two video outputs: a low gain for amplifying the high- contrast pattern and a high gain for amplifying the low-contrast pattern. All of the camera video outputs and the frame and line pulses are fed to selector 16 for video and pulse switching, thereby permitting the use of one set of filters 20, delay lines 22, adder circuitry 24, and threshold detector 25 to be employed in all position estimates.

In operation, the outputs of TV camera 14 are connected for the high-contrast patterns, using the low-gain setting. After data is obtained from camera 14 reflector 16 switches to the pulses and video from camera 12. The...