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Cross-Correlation of Mask and Wafer for Alignment and Process Control

IP.com Disclosure Number: IPCOM000042042D
Original Publication Date: 1984-Mar-01
Included in the Prior Art Database: 2005-Feb-03
Document File: 2 page(s) / 51K

Publishing Venue

IBM

Related People

Kirk, JP: AUTHOR [+3]

Abstract

Alignment precision can be made insensitive to line width variation by the following design of mask and wafer targets. As shown in Fig. 1, the wafer target consists of a pattern of lines whose centers are positioned according to a coded sequence. As shown in Fig. 2, a corresponding mask pattern of narrow slit openings, each of which is narrower than the wafer line widths and preferably at the resolution limit of the projection optics, is spaced according to the same code sequence and is imaged onto the wafer surface. The image of the mask is moved relative to the wafer and, as shown in Fig. 3, two cross-correlation peaks are sensed by detecting light scattered when the mask image correlates with each side of the lines forming the wafer alignment marks.

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Cross-Correlation of Mask and Wafer for Alignment and Process Control

Alignment precision can be made insensitive to line width variation by the following design of mask and wafer targets. As shown in Fig. 1, the wafer target consists of a pattern of lines whose centers are positioned according to a coded sequence. As shown in Fig. 2, a corresponding mask pattern of narrow slit openings, each of which is narrower than the wafer line widths and preferably at the resolution limit of the projection optics, is spaced according to the same code sequence and is imaged onto the wafer surface. The image of the mask is moved relative to the wafer and, as shown in Fig. 3, two cross-correlation peaks are sensed by detecting light scattered when the mask image correlates with each side of the lines forming the wafer alignment marks. Alignment precision, independent of process variation, is obtained by keying the alignment position to an interpolated point midway between the cross-correlation peaks. A precise measure of the line width is obtained from the separation of the cross-correlation peaks. The shape of an individual cross-correlation peaks gives an indication of the line edge profile. A wide variety of coded sequences can be used, such as Barker codes, shift register sequences, etc., depending on the desired signal level, capture range and the amount of wafer and mask area that can be dedicated to alignment and process control. The figures show an example of a 3:1 cod...