Abstract: Apparatus 20 for inspecting photomasks 26 and the like by comparison of duplicate die patterns, including improved defect detection. Two-dimensional pixel representations of two die patterns are formed, with pixels having values or black or white or shades or grey, depending upon the features of the die patterns. Defects in the die patterns are found by a defect detector circuit 60 at points of non-agreement between the pixel representations. Two window matrices 130 and 134 of adjacent pixels are defined for corresponding areas of the two die patterns. The center matrix of each window matrix is defined as a comparison matrix 132 and 136. An error value is computed for subsets of the window matrix by summing the squares of the differences between each of the pixel values of each subset and the corresponding pixel values of the opposite comparison matrix. If there is no defect and any misalignment between the two representations minimal, at least one error value will be less than a threshold error value.

Abstract: An automatic inspection system for inspecting holes in a mask including carriage means 30, illumination means 44, optical means 48, photosensitive detector means 46, and signal processing means 56. The mask 34 to be inspected is positioned by the carriage means in a horizontal plane. The optical means projects a focused image of a portion of the mask onto the photosensitive detector means. Photodiodes in the detector means are responsive to light from the illumination means that is transmitted through the holes in the mask. The signal processing means scans the outputs of the photodiodes and stores in memory a digital representation of the mask. The signal processing means performs inspection measurements and comparison tests. A smoothness checker circuit 240 measures the local radius of curvature of each hole at several places and compares the measurements to predetermined curvature limits to detect nicks and sharp protrusion defects.