Patent ID: 8335369

Claim:
A method of inspecting a photomask comprising: directing radiation from a radiation source onto a first side of a photomask so that at least a portion of the radiation is transmitted through the photomask; detecting a first photomask image from the transmitted portion of the radiation transmitted through the photomask and perceptible at a second side of the photomask; creating a second photomask image by applying an exposure simulation model to a photomask design; determining, with a comparator, a difference between the first photomask image and the second photomask image, the comparator comprising a computer, wherein determining a difference comprises: determining a first value from a portion of the first photomask image; determining a second value from a portion of the second photomask image; and determining a difference from the first value and the second value; comparing the difference to a predetermined threshold; and storing a coordinate location of one of the portion of the first photomask image and the portion of the second photomask image if the difference between the portions exceeds the predetermined threshold.