A surface defect inspection system based on the spatial frequency filtering technique (SFFT) has been developed for patterned semiconductor wafers. The SFFT is very powerful in detecting tiny pattern defects and foreign particles on complex but periodic patterns, such as semiconductor memory wafers. In this application a spatial frequency filter, which has an opaque spotty pattern corresponding to the periodic pattern of the test object, is placed at the Fourier transform plane of the imaging lens and blocks the light diffracted by the regular periodic pattern. Thus, only random defects are projected on the image plane and not the regular periodic patterns, which makes the execution of defect detection very quick and easy. The developed system uses a photoplate as the filter, and takes in the filtered image directly by an ITV camera, detecting sub-micron defects in about 30 minutes for a 6-inch memory wafer.
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