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# Autocorrelation Codes for Mask and Wafer Alignment Design

IP.com Disclosure Number: IPCOM000082627D
Original Publication Date: 1975-Jan-01
Included in the Prior Art Database: 2005-Feb-28
Document File: 2 page(s) / 41K

IBM

## Related People

Kolb, GA: AUTHOR [+1]

## Abstract

The autocorrelation codes are useful in the design of mask-to-wafer alignment targets. The group of codes are capable of providing alignment signal-to-noise ratios of from 2:1 to 10:1 or greater depending upon code length. They have the advantage of possessing off-peak maximums of unity or less, which allows for minimum ambiguity at correlation (alignment). The general procedure for synthesizing the codes is as follows: R(O) = a = the number of 1's. Autocorrelation Function R( tau ) = b, for tau > 0, and a > b (in this case b = 1). Since R( tau ) is a symmetrical function, only tau > 0 will be discussed. 1) The first two digits of the sequence (code) are always 11. The last digit is always 1. 2) The first unknown digit, x(i), can be determined by calculating the autocorrelation as: see original.

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Autocorrelation Codes for Mask and Wafer Alignment Design

The autocorrelation codes are useful in the design of mask-to-wafer alignment targets. The group of codes are capable of providing alignment signal- to-noise ratios of from 2:1 to 10:1 or greater depending upon code length. They have the advantage of possessing off-peak maximums of unity or less, which allows for minimum ambiguity at correlation (alignment). The general procedure for synthesizing the codes is as follows: R(O) = a = the number of 1's. Autocorrelation Function R( tau ) = b, for tau > 0, and a > b (in this case b = 1). Since R( tau ) is a symmetrical function, only tau > 0 will be discussed.

1) The first two digits of the sequence (code) are always 11. The last digit is always 1.

2) The first unknown digit, x(i), can be determined by calculating the autocorrelation as:

(Image Omitted)

where: tau = 1, 2, 3, --- X(k+tau) = 0 for k+tau > i z = 1 or 0 If z = 0, then tau = tau +1 and repeat procedure 2) for tau = tau +1. If z = 1, proceed with procedure 3).

3) Test whether:

(Image Omitted)

Unless R( tau ) > b for all tau > 0

4) Repeat procedures 2) and 3) until the number of 1's in the sequence is equal to a.

The table lists each code and its appropriate configuration of 1's and 0's.

The figure illustrates the implementation of a 4:1 code showing the code, the wafer alignment target structure, the scanning slit structure and the output of the photomultiplier tube (autocorrelation function).

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