Method and apparatus for aligning two objects, and method and apparatus

Abstract

A first diffraction grating is formed on a mask, and a second diffraction grating is formed on a wafer. Two light beams having slightly different frequencies interfere with each other and are diffracted as they travel through the first diffraction grating, are reflected by the second diffraction grating, and again pass through the first diffraction grating. As a result, they change into thrice diffracted light beams. The diffracted light beams are combined into a detection light beam which has a phase shift .phi..sub.A representing the displacement between the wafer and the mask, or a phase shift .phi..sub.G representing the gap between the wafer and the mask. The detection light beam is converted into a detection signal. The phase difference between the detection signal and a reference signal having no phase shifts are calculated, thus determining phase shift .phi..sub.A or .phi..sub.G. The displacement or the gap is determined from the phase shift. In accordance with the displacement or the gap, the wafer and the mask are aligned to each other, or the gap between them is adjusted to a desired value. Since the detection signal is generated from diffracted light beams, its S/N ratio is sufficiently great. Therefore, the displacement or the gap is determined with high precision. In addition, it is possible with the invention to perform the aligning of the wafer and the mask and the adjusting of the gap therebetween, simultaneously.