Abstract: A system includes multiple optical Fourier transform cells which simultaneously scan a device under test. The illuminated area for each Fourier transform cell is small to provide high resolution, while the number of cells is large to cover a relatively wide area and keep inspection speed high. The advantages of optical computing performed by Fourier transform optics also keeps the inspection speed high because illuminated areas are large when compared to the resolution and Fourier transforms are linear shift invariant so that optical measurements can be performed during scanning. In one embodiment, Fourier transform cells are offset from each other perpendicular to the scan direction by less that the width of an illuminated area. This provides complete coverage during scanning of a device under test. Because the illumination for the Fourier transform is collimated, the system is insensitive to focusing errors due to fluctuations in working distance.

Abstract: High speed pattern and defect detection in flat panel displays, integrated circuits, photo mask reticles, CRT color masks, printed circuit boards, and any other patterned devices, regular or irregular, uses analog optical computing. Using appropriate illumination and optics, the Fourier transform of the image of a device under test is formed. The Fourier transform components of an ideal pattern are compared to the Fourier transform components of a measured pattern, and differences in relative intensities of the spatial components indicate a defect. A spatial separator is used to direct different components of the Fourier transform in different directions for parallel, simultaneous measurement and analysis. Utilizing Statistical Process Control, and properly comparing the different Fourier transform components, the defect is partially classified. Optical image processing is done in real time at the speed of light.