Patent ID: 8810646

Claim:
A method for detecting defects on a reticle, the method comprising: determining a best focus setting for imaging the reticle; taking a series of images of at least a portion of the reticle utilizing a range of different focus offset values; determining whether the series of images degrade in a symmetrical manner or an asymmetrical manner; determining a first offset and a second offset, the first offset and the second offset both being non-negative focus offsets, wherein the first offset and the second offset are equal when the series of images are determined to degrade in a symmetrical manner, and wherein the first offset and the second offset are different when the series of images are determined to degrade in an asymmetrical manner; obtaining a first image of at least a portion of the reticle, the first image obtained at the best focus setting plus the first offset; obtaining a second image of the at least a portion of the reticle, the second image obtained at the best focus setting minus the second offset; generating a differential image, the differential image representing a difference between the first image and the second image; and identifying a defect on the reticle based on the differential image.