| 摘要 |
PURPOSE:To remove the influence of variation in the thickness of a member and to improve defocusing detection accuracy by detecting defocusing optically in the form of the contrast ratio of two light and shade pattern projected on the surface of the member. CONSTITUTION:A wafer 12 on an XYZ base 10 is irradiated with an S-polarized laser beam 14 and vertical scattered light is received by a photodetection part 20 to extract a P and an S polarized light component and also move and scan the base 10. Parallel white light is projected on pattern plates 22 and 24, a lens 30 is adjusted to project a pattern of light and shade stripes on the wafer surface, and the intermediate point between both plates at the time of focusing is image-formed on the wafer. Shade parts 22a and 24a of the pattern are arranged alternately. A control circuit 34 controls and drives the base 10 with a signal from the output of a two-dimensional CCD 32 to cancel deviation in focusing. The visual field 32a of the CCD 32 is handled while image pickup areas A and B of the pattern stripes 24a and 22a are divided alternately. The intermediate point between two successive light and shade patterns is image- formed on the wafer surface, so any pattern becomes light images and the mean contrast is nearly 1. Consequently, the focusing is performed stably with high accuracy. |
