A sinusoidal wavelength-scanning interferometer for measuring thickness and surfaces profiles of a thin film has been proposed in which a superluminescent laser diode and an acousto-optic tunable filter are used. The interference signal contains an amplitude Zb of a time-varying phase and a constant phase a. Two values of an optical path difference (OPD) obtained from Zb and a, respectively, are combined to measure an OPD longer than a wavelength. The values of Zb and a are estimated by minimizing the difference between the detected signals and theoretical ones. From the estimated values, thickness and surface of a silicon dioxide film coated on an IC wafer with different thicknesses of 1 mu m and 4 mu m are measured with an error less than 5 nm. (c) 2005 Optical Society of America.