Printing apparatus having an in-focus detector

摘要

A printing apparatus in which the image of a mask is projected onto a wafer by an imaging optical system and printed on the wafer. This apparatus has a detector for detecting the in-focus or lack of focus conditions of the mask image projected onto the wafer. This detector effects the detection by optically detecting the optical spacing between the mask and the wafer. The detection of such spacing is accomplished by causing a detection light to impinge on the wafer through a projection optical system and detecting the amount of displacement of the position of the reflected light in a plane optically parallel of the wafer.