A method and apparatus for inspecting a reticle for photolithography is disclosed. Inspection tools are used to obtain multiple patch area images of each patch area of each die of the same set of dies on the reticle. The integral intensity value is determined for each patch area image. A gain is applied to the integral intensity value for each patch region image based on the relative value of the pattern sparse metric of the patch region image and the pattern sparse metric of the other patch region image. To form a difference intensity map of the reticle, the difference between the integral intensity values of each patch of a pair of die including a test die and a reference die is determined. The difference intensity map correlates with feature characteristic deviations depending on the feature edge of the reticle.
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