Alignment apparatus with optical length-varying optical system

Claim 1

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 comprising: an objective for focusing an alignment mark formed on the mask and an alignment mark formed on the wafer; a photodetector for imaging both of the alignment marks; an optical length-varying optical system including at least one prism means disposed for rectilinear movement along a guide between the objective and a focusing position so as to change an optical length of the optical system and to bring each of the alignment marks into focus, the optical system enabling light rays from the alignment marks to enter the at least one prism means an even number of times and to exit from the at least one prism means an even number of times so that the first entrance of the light rays to the at least one prism means and the last exit of the light rays from the at least one prism means may be shifted, the first entrance of the light rays including optical images from the objective and the last exit of the light rays being directed toward the photodetector; a displacement detecting sensor for detecting a magnitude of the movement of the at least one prism means of the optical length-varying optical system; magnification compensation means for compensating magnitudes of displacements of the respective alignment marks detected by the photodetector, from an optic axis by calculation based on the movement magnitude of the at least one prism means detected by the displacement detecting sensor, the optical length and a focal distance of the objective, so as to equalize magnifications of the images; and means to detect a relative displacement magnitude between the mask and the wafer by the use of the images of the equal magnifications compensated by the magnification compensation means; whereby the mask and the wafer are aligned by movement thereof.