| 摘要 |
PURPOSE:To decrease the diameter of luminous flux on the surface of a photodetect and to improve the S/N at the time of the detection of the gravity center of incident luminous flux by a photodetector by providing a mask with 1st and 2nd physical optical device which provide optical operation. CONSTITUTION:The focal length of a 2nd Fresnel zone plate(FZP) 5 is denoted as fM2 and the distance from the FZP 5 to a luminous flux evaluation surface corresponding to the surface of a photodetecting means 8 is denoted as l. Then when a wafer 3 is displaced from a reference position P1 by a distance dG, the quantity S of the movement of the center of gravity on the surface of the means 8 is obtained from an equation I. Then the center position of gravity when the wafer 3 is at a position P1 is found previously and the deviation of the center position of gravity from the reference point when an interval is measured is found and substituted as a movement quantity S in the equation I to find the deviation quantity dG of the interval from the position P1 of the wafer 3. Thus, the interval between the mask 2 and wafer 3 is measured with high accuracy. |
