PURPOSE: To heighten the accuracy by storing the elapsed time from a memory start marker to a marker point address in such a manner as to have the corresponding elapsed time to which the marker point address relates to be associated with an unknown wafer beam position.
CONSTITUTION: A wafer 19 is placed between opposite light marker pins 53, 55 for the wafer 19, and aligned with the scanning line of an optical system. The beam 57 is swept across the wafer 19, so that the amplitude of a dispersion signal is extracted at specified positions 57a-57c, and the wafer 19 is covered with a regular array of sampling points 51 provided at uniform intervals. The start of the array takes the marker pins 53, 54 as reference. The marker pins 53, 55 use a standard wafer having a dispersion source. The apparent position of the dispersion source is measured by an initial table of the sampling points, and the difference between the apparent position and the known position is used as an error function for correcting a predetermined value stored in a random access memory.
KENESU PII GUROSU
BURAIAN RESURII
JIYOOJI KUREN
