Method and an apparatus for aligning first and second objects with each

Abstract

A distance between a mask and a wafer is set such that exposure light beams emerged from the mask are converged by the projection lens to be focused on the wafer. Two light beams with same wavelength of the alignment light beam emerge from a first point (b) which is located far away from the mask, and are focused on the wafer or neighborhood of it by the projection lens. Two mask marks of diffraction gratings are formed on the mask and spaced at a predetermined distance from each other. When the alignment light beams are applied to the mask marks, two diffracted light beams of predetermined order emerge individually from the mask marks in such a manner that the respective optical axes of the two diffracted light beams, which are directed oppositely to of the diffracted light beams, intersect each other on the first point. Thus, the diffracted light beams advance as if the diffracted light beams were the two light beams emerging from the first point. Therefore, the two diffracted light beams can be focused on the wafer or neighborhood of it or can be incident on a wafer mark which is a diffraction grating. Thus, rediffracted light beams emerge from the wafer mark and are detected, so that the mask and the wafer are aligned with each other. Accordingly, the alignment can be performed, despite a great diffraction between the wave-lengths of the exposure light beam and the alignment light beam.