| 摘要 |
A heterodyne interference-type alignment sensor that permits the high-precision matching of detection light eams to suit the pitch of the diffraction grating marks to be detected without introducing undue complexity to the optical system. Light beams L0 of a given wideband width are illuminated onto an acousto-optical modulator to obtain light beams L1 (1), L2 (-1) comprising +/- 1st order beams, which are further directed to diffraction grating marks RM, WM via a relay optical system. The diffracted beams from diffraction grating marks RM, WM are detected by photoelectric detectors. The frequencies f1, f2, applied to the acousto-optical modulator are varied while maintaining a correspondence between the difference between frequencies f1, f2 and the pitch of diffraction grating marks RM, WM. <IMAGE> <IMAGE>
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