| 摘要 |
PURPOSE:To enable the highly accurate detection of a gap between a mask and a wafer with low-priced structure by providing a squarer for squaring the photoelectric output signal of a detector, and a low-pass filter for extracting only the basic wave from the output of the squarer. CONSTITUTION:When a monochromatic and coherent light such as He-Ne laser is incident on a mask 10 with diffraction grating, two interference waves are generated by diffraction, and the amplitude of the composite wave is changed by a gap between the mask 10 and a wafer 20. However, when the gap between the mask 10 and the wafer 20 is vibrated minutely by a high frequency vibrator such as a piezoelectric element either from the mask 10 side or from the wafer 20 side and the quantity of light detected by a detector 7 is squared (8) and further let pass a low pass filter 9, only the signal of an envelope, that is to be the basic wave, can be extracted. The change by the gap can be thereby detected with low-priced structure compared to the case of dealing with the amplitude of the composite wave. |
