Patent ID: 7072502

Claim:
A method for inspecting a phase shift mask that is used with a photolithographic optical exposure system under a set of exposure conditions, said method comprising: acquiring a plurality of aerial images produced by said phase shift mask using a transmitted light, said plurality of aerial images being acquired in an aerial imaging inspection system and within a process window of said exposure system by emulating said set of exposure conditions; said plurality of aerial images including a first and a second aerial image produced by said phase shift mask; wherein said first aerial image produced by said phase shift mask is in a first out of focus condition, and said second aerial image produced by said phase shift mask is in a second out of focus condition; and comparing said first and said second aerial images to each other to detect phase defects and errors in said phase shift mask, wherein said first and said second aerial images are concurrently produced from the same light transmitted by the mask; and, wherein prior to said comparison step, said first and second aerial images are transformed to simulate a behavior of an exposure system and photoresist.