| 摘要 |
PURPOSE:To enable accurate detection of a positional deviation of a wafer by checking an intensity of interfering light between diffraction lights the same in angle of diffraction or those different in angle of diffraction to detect a positional deviation with a proper selection of the intensity. CONSTITUTION:Two beams BM1 and BM2 are made to irradiate a wafer W crossing each other, a + or - primary diffraction light from a diffraction grating Gw is turned to an interfering light BT(+ or -1) to be received with a photoelectric element 20a and an output signal Sa thereof is inputted into an intensity detecting section 41. Interfering lights BT(-2, 0) and BT(0, +2) are received with optical elements 20b and 20c and an output signal thereof is inputted into an intensity detecting section 43. An intensity judging section 44 inputs output signals GSa, GSb and GSc of the intensity detecting sections 41 and 43 to compare the GSa with an average intensities GU of the GSb and GSc and outputs a larger one as Q(a, b) to calculate deviation from a reference grating 24 with a positional deviation computing section 45. |
