A new optical heterodyne alignment and gap detection technique for x-ray steppers

Abstract A new optical heterodyne interferometry alignment technique, which measures the displacement and gap between mask and wafer has been proposed. Both the lateral displacement and the proximity gap can be detected using the same optics with two pairs of laser beams with slightly different frequencies. Optical symmetry is essential in this technique. The principle of operation, the optical system and the experimental results are described. The capability for detection of displacement and gap of as small as 11nm and 0.37μm, respectively, was experimentally confirmed.