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Compensation for phase change on reflection in white-light scanning interferometry

White-light scanning interferometry uses a wide-band spectrum light source and observes the short coherent variation of interferometric intensity obtained while moving either the test surface or the reference surface along the optical axis of the interferometric optics. When one employs the scanning interferometry for gauging the step height between two separate surface points that are made of different materials, a significant amount of measurement error occurs due to the phase change on reflection. The phase change varies with wavelength and materials, so its effects on resulting interferograms are not easy to be precisely identified. In this paper, we present a practical method for compensating for the measurement error caused by the phase change on reflection in step height gauging. The method takes a first-order approximation to the wavelength-dependent nonlinear behavior of the phase change and then determines its precise value from the phase information of the Fourier-transformed intensity data in the spatial frequency domain. The method can be realized simply by performing two additional quasi-monochromatic phase-shifting interferometric measurements, or more conveniently by adopting a special form of light source that has two spectral peaks. Experimental results prove that the propose compensation method is capable of reducing the measurement error to an accuracy of +/- 2 nm.