| 摘要 |
PURPOSE:To detect the gap between a reference surface and a substrate with high precision by aligning the spot center of a reflected beam from the substrate to the center of one of plural detecting elements on a detection surface. CONSTITUTION:When the position shift of a mask 20 from the focal point of an objective lens 19 as the reference surface is detected, beltlike luminous flux from a laser light source 1 is reflected by an oscillation mirror 6 and split into two by a polarization beam splitter 8 and the polarized beams are rotated by 90 deg. and multiplexed by a beam splitter 12 to strike a mirror 18. The luminous flux is passed through the objective lens 19 to form a spot image on plural marks of the mask 20, and its diffracted light is passed through the lens 19, etc., and split by a half-mirror 24; one beam is split by a polarization beam splitter 26 and detected on photodetection surfaces of photodiode arrays 27 and 28. Further, the mask 20 is moved to and away from the lens 19 to prevent the spot light from deviating from the center of the detecting element.
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