Apparatus with four light detectors for checking surface of mask with pellicle

An apparatus for detecting foreign particles on a surface of a substrate covered with a transparent pellicle to protect said substrate, comprising:

• a supplying means for supplying a light beam passing through said pellicle to the surface of said substrate from an oblique direction, said light beam forming a light spot repeatedly shifting along a scanning line on the surface of said substrate;
• a shifting means for shifting said substrate to vary a position of said scanning line;
• first and second light detectors arranged in a first plane which includes said scanning line and is perpendicular to the surface of said substrate and positioned symmetrically with respect to an incident plane of said light beam passing through a center of said scanning line, said first and second light detectors generating photoelectric outputs, respectively;
• third and fourth light detectors arranged in a second plane parallel to said first plane and positioned symmetrically with respect to said incident plane, said third and fourth light detectors generating photoelectric outputs, respectively;
• an operating means for receiving the photoelectric outputs of said first and second light detectors and for generating an output signal representing a logic sum of said photoelectric outputs;
• and
• a discriminating means for receiving the output signal of said operating means and the photoelectric outputs of said third and fourth light detectors and for discriminating the presence of a foreign particle on the basis of a logic product of said output signal and said photoelectric outputs of said third and fourth light detectors.