Alignment apparatus with optical length-varying optical system

Abstract

In an aligner system wherein a mask and a wafer are arranged so as to oppose each other with a predetermined gap therebetween, an alignment apparatus is provided including an objective for focusing an alignment marks formed on the mask and on the wafer, a photodetector for imaging both alignment marks, an optical length-varying optical system, a displacement detector, a magnification compensation arrangement, and a relative displacement magnitude detector. The optical length-varying optical system includes at least one prism arrangement for changing an optical length of the optical system to bring each of the alignment marks into focus. The optical system enables light rays from the alignment marks to enter the at least one prism arrangement an even number of times and to exit from the at least one prism arrangement an even number of times, so that the first entrance of the light rays to the at least one prism arrangement and the last exit of the light rays from the at least one prism arrangement may be shifted. The first entrance of the light rays includes optical images from the objective with the last exit of the light rays being directed toward the photodetector. The displacement detector detects the magnitude of movement of the at least one prism and the magnification compensation arrangement compensates for magnitudes of the displacements of the respective alignment marks from an optic axis whereby relative displacement magnitude between the mask and the wafer are detected by use of the images of equal magnifications and the mask and wafer can be aligned with high sensitivity and high accuracy.