Utilization of complex-valued signals in a white-light scanning interferometer for accurate measurement of a surface profile.

Complex-valued interference signals (CVISs) of a white-light scanning interferometer (WLSI) and a spectrally resolved interferometer (SRI) are obtained from their real-valued interference signals through Fourier transform. First the phase distribution in the CVIS of the SRI indicates a dispersion phase caused by two sides of unequal length in a cubic beam splitter, and the magnitude of the dispersion phase changes linearly along a horizontal direction of the beam splitter. Next the dispersion phase with a different magnitude is subtracted from the spectral phase in Fourier transform of the CVIS of the WLSI. Through inverse Fourier transform of this spectral distribution, a dispersion-free CVIS is obtained, and the position of zero phase nearest to the position of amplitude maximum provides a surface profile measured accurately with an error less than 4 nm after 2π corrections, while a position calculated by the linear component of the spectral phase causes measurement error less than 12 nm.