Apparatus for detecting positional deviation of diffraction grat

Details Apparatus for detecting positional deviation of diffraction grat Apparatus for detecting positional deviation of diffraction grat

1995-09-15

1997-10-28

Font, Frank G.

Optics: measuring and testing

By particle light scattering

With photocell detection

356363, 250548, G01B 902

Patent

active

056822391

ABSTRACT:
Apparatus for detecting a positional deviation of two diffraction gratings of each of first and second pairs of diffraction gratings formed on a substrate, by utilizing an optical heterodyne interference method. The apparatus includes a first light emitter for emitting a pair of coherent light beams having different frequencies close to a first frequency, a second light emitter for emitting a pair of coherent light beams having different frequencies close to a second frequency, different from the first frequency, a four-way radiating unit radiating the two pairs of light beams in four directions and for causing a selected pair of the light beams to be incident on a corresponding one of the first and second pairs of the diffraction gratings such that corresponding beam spots overlap with each other, a splitter for splitting diffracted light beams from the diffraction gratings according to frequencies of the diffracted light beams, to separate a first diffracted light beam, which is diffracted from the first pair of diffraction gratings in a diffraction direction, and a second diffracted light beam, which is diffracted from the second pair of diffraction gratings in the same diffraction direction, a beat signal detector for detecting beat signals respectively corresponding to the split light beams, and a deviation detecting unit for receiving the detected beat signals and for detecting a positional deviation in a predetermined direction of two diffraction gratings of each of the first and second pairs, from the beat signals respectively corresponding to the two diffraction gratings of a corresponding one of the first and second pairs.

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