Optical device for the alignment of two superimposed objects

摘要

This invention provides an optical device for detecting the position of a second mark relative to a first mark by optically scanning the first and second marks on a scanning surface. The first and second marks are scanned by a scanning spot light and the reflecting lights from the first and second marks are detected by a photoelectric detector. The relative positions of the first and second marks are computed on the basis of the detected signals.