Importance

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Abstract

The autofocus system disclosed herein utilizes grazing angle illumination of a spot on the surface of a semiconductor wafer approximately on the axis of the projection lens so that the position of the spot varies as a function of the spacing between the lens and the wafer. The spot is viewed, also at grazing angle, by an optical system which projects an image of the spot towards a set of detectors. The beam is split and the split beams are directed against oppositely facing reflective surfaces on an angularly oscillatable mirror. After reflection from the mirror, the respective split beams are detected so as to generate respective pulse signals corresponding to the passing of the respective spot image past a predetermined position. The relative timing or phasing of the two pulse signals is variable as a function of the relative positions of the lens and wafer but is relatively insensitive to small displacements of the mirror and the timing relationship is employed in a servo loop for adjusting the relative positions of the lens and the wafer.

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First Claim