High-stability white-light interferometry with reference signal for real-time correction of scanning errors

White-light interferometry (WLI) for object topography measurements relies on an accurate rate of change of the optical path difference (OPD) between the object and reference beams. However, the motion of the scanner realizing the OPID change is not perfect, and scanning errors directly impact measurement accuracy. We describe a whitelight interferometer that is capable of accounting for these errors and correcting them in real time through the monitoring of the scanner motion. This monitoring is achieved by embedding an additional high-coherence interferometer into the system. Besides allowing for monitoring the scanning progress, this system also provides the means to automatically calibrate the WLI. Significant improvements both in the accuracy and repeatability are demonstrated experimentally. Data analysis is discussed as well. (C) 2003 Society of Photo-Optical Instrumentation Engineers.