Inspection method and apparatus for the inspection of either random or repeating patterns
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United States of America Patent
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Feb 1, 2000
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N/A
app pub date -
Sep 7, 1995
filing date -
Sep 30, 1993
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Abstract
The present invention is a hybrid technique for finding defects on digitized device images using a combination of spatial domain and frequency domain techniques. The two dimensional spectra of two images are found using Fourier like transforms. Any strong harmonics in the spectra are removed, using the same spectral filter on both spectra. The images are then aligned, transformed back to the spatial domain, and subtracted. The resulting spectrally-filtered difference image is thresholded and analyzed for defects. Use of the hybrid technique of the present invention to process digitized images results in the highest-performance and most flexible defect detection system. It is the best performer on both array and random devices, and it can cope with problems such as shading variations and the dark-bright problem that no other technique can address. The hybrid technique of the present invention also uses frequency domain techniques to align the images with fewer errors than spatial domain techniques of similar or lesser complexity. Further, the relative offsets of the pairs of images are determined by frequency domain techniques--and this method may be the most accurate and the least expensive.
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