| 摘要 |
PURPOSE:To enhance measuring accuracy, by removing a diffraction degree component exerting adverse effect on measurement on the basis of the correlation between diffraction degrees by the operation means incorporated in an interference beam selector. CONSTITUTION:The laser beam emitted from a laser beam source is diffracted by a diffraction lattice 2 and only +1st order, 0th order and -1st order diffracted beams of beam paths P, Q, R are passed through a lens 4 from an aperture 3 to irradiate a specimen 5. A phase modulation plate 6 subjected to sine wave vibration by an oscillator 10 is inserted in the beam path Q. The reflected beam from the specimen 5 is again diffracted by the lattice 2 and the interference of diffracted beams is detected by detectors 7A, 7B. At this time, the -2nd order diffracted beam of the beam path P is contained in the detector 7A and the +2nd order diffracted beam of the beam path R is contained in the detector 7B. These unnecessary components can be removed by synchronously detecting the outputs of the detectors 7A, 7B by lock-in amplifiers 8A, 8B and performing predetermined operation in a secondary diffracted beam removing circuit 9 and measuring accuracy is enhanced.
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