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White-light interferometry with reference signal

White light interferometer (WLI) has become a common tool for measuring surfaces with large height range and/or roughness. Typically, the object is scanned through focus, thus varying the optical path difference (OPD) between the object and reference beams. The rate of the OPD change affects the quality and accuracy of the surface measurement. For high quality measurements a scanning device is often enhanced by a closed loop feedback while the scanning speed is assumed to be known and constant. In this paper we describe a white light interferometer that yields excellent results without requiring a high-end scanner. These results are achieved by embedding an additional interferometer with a long coherence length source that provides an interferometric reference signal that is used to monitor the motion of the scanner during each measurement in real time. The information about the scanner motion is then used in a WLI algorithm. This yields significant improvements in both the accuracy and repeatability of topography measurements.