| 摘要 |
Two diffraction gratings having the same grate pitch are respectively attached to a semiconductor wafer and a mask to be used in lithography in production of a semiconductor, such that the diffraction gratings are parallel to each other. When first and second frequency lights respectively having different frequencies, are incident, through a light path adjusting system, upon the mask- and wafer-side diffraction gratings in the directions of the first order diffraction angles symmetric with respect to normal-line directions of the diffraction gratings, the frequency lights first orderly diffracted by the diffraction gratings, interfere with each other, thus forming mask- and wafer-side interference lights. An initial phase difference DELTA phi 0 between the interference lights, is detected. Then, the light path adjusting system is adjusted to shift the interference lights by the same angle theta 1 in the same direction, and a later phase difference DELTA phi 1 between the interference lights thus shifted, is detected. Based on the relation of DELTA phi 1- DELTA phi 0=(2 pi Gtan theta 1)/(P/2), a mask-wafer gap G is detected with high precision according to the change in phase difference ( DELTA phi 1- DELTA phi 0). This improves the precision of setting the gap G in a semiconductor production.
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