DETECTION OF INCLINATION OF OBJECT TO BE INSPECTED, PROJECTION ALIGNER AND DETECTION OF CYCLE OF PERIODICALLY CHANGING SIGNAL

摘要

PURPOSE:To measure a basic frequency accurately without generating a discrepancy for determining the inclination of an object to be inspected accurately by using an approximate method wherein a very high approximate accuracy can be obtained when the basic frequency comes near a frequency at a halfway point between two adjacent sampling points. CONSTITUTION:An interference fringe generated by laser reflection light 26'' from an object to be inspected 4 and reference light 27'' is detected on a CCD detector 3 and the detected signal is fast-Fourier-transformed. Based on the data about a spectrum position (DELTA') which gives an interpolated spectrum peak determined by the data about a sampling.point to give the maximum value of a part to correspond to a fringe cycle of the spectrum of the fast-Fourier-transformed signal and a larger point next to the sampling point and the data about a spectrum position (DELTA'') which gives an interpolated spectrum peak determined by the data about the maximum value and two adjacent values, a spectrum position (DELTA) is determined by a weighted mean method. By this method, a cycle or pitch of a periodically changing signal is detected accurately and thereby the inclination of the object to be inspected can be determined accurately.