| 摘要 |
PURPOSE:To enable the titled detector to accurately detect markers by a method wherein the peak position of the detected output of a reflected beam is detected,and when a plurality of peak positions below the threshold value are detected, the generation of the coincidence pulse emitted from a comparison logic circuit is stopped at that part. CONSTITUTION:A mark position is detected from the scanning position of an electron beam when a coincidence pulse is obtained from a comparison logic circuit 17, and pertaining to the detection of the peak position of a peak position detecting circuit 22, a peak position is detected every time the electron beam passes one side of a threshold value. Peak positions P1-P3, for example, are detected. In this case, as the peak position is only one point of P2 on the part having the threshold value LS or below, the operation of the comparison logic circuit is not prohibited. However, peak positions P1-P5 are detected and as there are two peak positions P3 and P4 on the part having the value less than the threshold value, the operation of the comparison logic circuit is prohibited at these parts. Accordingly, the detected output passes the threshold value even at the positions E3 and E4, but no coincidence pulse is generated from the comparison logic circuit on these positions E3 and E4, and there is no possibility of performing an inaccurate mark detection.
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