STEP DETECTOR AND CONTROLLER OF AUTOMATIC FOCUSING MECHANISM USING STEP DETECTOR

摘要

PURPOSE:To detect one point in the step passing period of a measuring instrument by using the measured value of the measuring instrument and the differential value of the measured value in combination. CONSTITUTION:When an air nozzle 7 crosses the edge 2 of a wafer 1 continuously, the output signal of a differential pressure detector 8 draws a gradual increase curve and this signal is inputted from a detour circuit 14 to an adder 13. Further, a differential signal is inputted from a differentiator 11 to the adder 13. The adder 13 sums up the both and the sum is inputted to a comparator 15 and compared with a reference value signal from a setter 16. When the former surpasses the latter, the detection signal of the edge 2 is outputted. The comparator 15 generates its output only at one point, which is invariably the position on condition corresponding to the reference value.