| 摘要 |
PURPOSE:To improve the signal detection ratio by a method wherein photoelectric elements correspond to any detected patterns in the diffractive direction to be divided for arrangement in the radial direction so that specified signals may be detected by means of selective combination of each photoelectric element. CONSTITUTION:A photodetector 18 is composed of the photoreceiving surfaces 18b1-b4, 18c1-c4, 18e1-e4 respectively divided to receive signals independently. When mark patterns 12a, 13a, 12b are detected, the sum of signals transmitted from the photoreceiving surfaces 18b1, 18c1, 18d1, 18e1, 18b3, 18c3, 18d3, 18e3 will become the mark signals (A channel) while the sum of signals transmitted from the photoreceiving surfaces 18b2, 18c2, 18d2, 18e2, 18b4, 18c4, 18d4, 18e4 will become the noise signals (B channel). Fig. (a) represents the output if there is a dirt 42 on the scanning line when patterns on a mask and a wafer are scanned, while Figs. (b) and (c) respectively represent the outputs of A channel and B channel. On the other hand, Figs, (d)-(f) represent the signals passing through the differential amplifier processing A-B channels to restrain any noise due to a dirt signal from sounding while augmenting pattern S/N. |
