PROCESS AND DEVICE FOR MUTUALLY ALIGNING OBJECTS

摘要

Alignment marks in the form of periodic gratings are scanned by a TV camera, and the relative phase difference of the two scanning signals is electronically determined by digital or analog means. In the former case, a microprocessor computes at least the fundamental wave of the Fourier spectrum of each scanning signal, in order to compute relative alignment from the phase positions of the fundamental waves. If gratings with different grating periods are used on the objects to be mutually aligned, the superimposition of the computed fundamental waves leads to the generation of beats which constitute a highly sensitive measure of alignment errors. For an alternative analog evaluation, one of the scanning signals is delayed to compensate for the difference in scan timing before being fed with the other signal to a phase-sensitive amplifier for determining the phase difference.