| 摘要 |
PURPOSE:To detect the position of a mask or a wafer highly accurately for a short period by moving the formed image point of reflected light flux on the image forming surface approximately vertical to the optical axis of an optical member by a distance corresponding to the movement of an object in a direction vertical to a reference surface and detecting the moving distance. CONSTITUTION:A lens L3 and a prism 4 are arranged on the upper left part of the mask 1 and a prism 5, an optical transmission element which is a photoelectric detection means are arranged on the upper right part. The optical transmission system of the lens L3 and the prism 4 are set up so that parallel laser beams LA made incident on the lens L3 are bent at their optical axis by the prism 4 and focused on a reference surface (rp) fixed between the mask 1 and the wafer 2 through the semitransparent part of the mask 1. Consequently, the images (qm), (qw) of the focused point (qs) on the basis of the surface reflection of the mask 1 and the wafer 2 are formed respectively. The reflected light beams FE, FF forming these images (qm), (qw) are passed through the prism 5 and again focused by the lens L4 to form expanded images QM, QW on the photoelectric element 6.
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