METHODS OF TESTING FOR DIFFERENCES BETWEEN A SAMPLE PATTERN WITH A STANDARD USING HOLOGRAPHIC TECHNIQUES

1,248,811. Holography. INTERNATIONAL BUSINESS MACHINES CORP. March 5, 1969 [March 9, 1968], No. 11661/69. Heading G2J. A method of testing for differences between a sample pattern (e. g. a mask for making semiconductor wafers) and a standard pattern comprises forming a hologram 7 from a standard pattern 3 and a reference pattern 8 Fig. 3, illuminating the developed hologram 7h Fig. 4 with light that has passed through the sample pattern 12 so that any change in the resultant reconstructed reference pattern 11a 11b provides an indication of differences between standard and sample patterns. The hologram is formed by successive partial simultaneous exposures of the standard pattern and the reference pattern. The reference pattern 8 preferably comprises a pair of pin holes 8a, 8b, and the standard pattern 3, comprises three known pattern 3a, 3b, 3c, corresponding to the samples to be tested. Pattern 3b need not be used. The reference pattern 8a, 8b is reconstructed as pattern 11a 11b. If desired the reference pattern could be a point having a concentric circle the circumference of which formed the limits of acceptable deviation of the pin hole. In a further embodiment useful where large numbers of samples have to be evaluated the reconstructed image illuminates a digital light deflector which can be measurably adjusted to cancel out the deviation in the sample.