SIGNAL PERIOD DETECTION METHOD AND PROJECTION ALIGNER EMPLOYING IT

摘要

PURPOSE:To allow accurately controlled exposure on the surface of a wafer or the like within the focal point margin of a projection lens by detecting the period of a periodically variable signal using a specific method thereby detecting the average height and/or the inclination of an exposure region highly accurately and then controlling the posture of a table based on the information thereof. CONSTITUTION:Periodically variable signals are subjected to discrete Fourier transform and a peak position is determined for an approximation curve determined by a sample point providing a maximum value at the sample points in the vicinity of a spectrum corresponding to the period of the signal and at least two points on the opposite sides of the sample point. The peak value is employed as a true peak position in the detection of the period of signal. A coherent light is split into two, for example, and one of them is projected obliquely onto the surface of an object 4. Reflected light from the object 4 interfers with the other coherent light to produce interference pattern information which is employed in the detection of the height and/or the inclination of the object 4.