Photomask Defect Extraction by Using Difference between a Reference Image and a Test Image after Illumination Adjustment

A new method for extracting a photomask defect by using two optical photomask images, a reference image and a test image, is described in this paper. If the same scale, the same height-to-width ratio, zero parallel translation, and zero rotation between two images are assumed, then a large pixel value of the absolute difference image between the images would indicate the existence of a defect at the pixel. However, not only a true defect but a false defect also causes some large pixel values in the absolute difference image due to illumination difference between the reference image and the test image. To adjust the illumination of the reference image to that of the test image, a dynamic programming is used, and the defect in the test image is segmented.