An
optical scanning system and method for detecting anomalies, including pattern defects and particulate contaminants, on both patterned and unpatterned surfaces, using a
light beam, scanning at a
grazing angle with respect to the surfaces, a plurality of detectors and an interchannel communication scheme to compare data from each
detector, which facilitates characterizing anomalies. The
light beam illuminates a spot on the surface which is scanned over a short scan-line. The surface is moved in a manner so that the spot is scanned over its entire area in a serpentine fashion along adjacent striped regions. The plurality of detectors include groups of collector channels disposed circumferentially around the surface, a bright field
reflectivity / autoposition channel, an alignment / registration channel and an imaging channel. The collector channels in each group are symmetrically disposed, in the
azimuth, on opposite sides of the center of the
scan line. The position of the collector channels, as well as the polarization of the beam, facilitates distinguishing pattern defects from particulate contaminants. The bright field
reflectivity / autoposition channel is positioned to receive specularly reflected light that carries information concerning
local variation in
reflectivity, which is used to classify detected anomalies, as well as determine variations in the height of the surface. The alignment / registration channel is positioned to detect a maximum of the light scattered from the pattern on the surface to ensure that the streets of die present on the surface are oriented so as not to be oblique with respect to the
scan line. The imaging channel combines the advantages of a scanning
system and an imaging
system while improving
signal / background ratio of the present system.