Patent ID: 7508526

Claim:
An apparatus for inspecting a defect in a pattern formed on a mask, comprising: a differential interference optical system having an optical axis, which includes: a first birefringence prism configured to separate light rays into first ordinary and extraordinary light ray components which are separated in a first predetermined direction and are directed to the mask, the first ordinary and extraordinary light ray components emerging from the mask forming a first differential interference image, and a second birefringence prism configured to separate light rays into second ordinary and extraordinary light ray components which are separated in a second predetermined direction different from the first predetermined direction and are directed to the mask, the second ordinary and extraordinary light ray components emerging from the mask forming a second differential interference image; first and second image pickup sensors configured to pick up the first and second differential interference images, respectively; and a defect detecting unit configured to detect a defect in the pattern formed on the mask by comparing the first and second differential interference images with first and second reference images, respectively.