Method for aligning first and second objects, relative to each other,

Abstract

According to this invention, a method and apparatus for aligning a mask and a wafer arranged to oppose each other, in a direction along their opposing surfaces, relative to each other, are arranged as follows. A first diffraction grating as a one-dimensional diffraction grating, bars of which extend in a direction perpendicular to an alignment direction, is formed on the mask. A second diffraction grating which has a checkerboard-like pattern, is formed on the wafer. The first diffraction grating is irradiated with laser beam emitted from a light source. Light beams diffracted and transmitted through the first diffraction grating is transferred to the second diffraction grating. Light beams diffracted and reflected by said second diffraction grating are transferred to said first diffraction grating. The light beams are diffracted by and transmitted through said first diffraction grating, again. Is detected, one of the diffracted light beams, which do not propagate along a predetermined plane. The light beams reflected and diffracted by the surface of the first diffraction grating, is transferred only in the predetermined plane. For this reason, the detected diffracted light beam will not interfere with reflected diffracted light beams. The mask and wafer can be precisely aligned relative to each other, in accordance with the intensity of the detected diffracted light beam.