Patent ID: 7911600

Claim:
An apparatus for inspection of a mask blank, comprising: a stage for mounting a reflective mask blank to be inspected; a light source for generating inspection light including the same wavelength as used during mask exposure by means of a mask having patterns formed on the reflective mask blank; an illuminating optics for illuminating a target region on the mask blank with the inspection light supplied from the light source; a dark-field imaging optics for collecting scattered light other than specularly reflected light out of light reflected from the target region to form a magnified image on a predetermined focal plane; an optical branching element for dividing light outputted from the dark-field imaging optics into a first and second light beams; a first image sensor having a plurality of detection pixels, the sensor being located at a position displaced by a predetermined distance from a focal plane of the first light beam along the light traveling direction; a second image sensor having a plurality of detection pixels, the sensor being located at a position displaced by a predetermined distance from a focal plane of the second light beam along a direction opposite to the light traveling direction; and an image processing unit for determining presence or absence of any defects in the mask blank, based on signals from the first and the second image sensors.