Lateral scanning white-light interferometry on rotating objects

Abstract

White-light interferometry (WLI) has been widely established as a contact-free measurement method for surface topographies. While the widespread vertical scanning approach provides a height resolution of under 1 nm, it is not suitable for measurements on objects in motion. An alternative for moving surfaces is laterally scanning white-light interferometry (LSWLI). Though LSWLI allows to measure objects in motion, the measurement on rotating, curved objects is still challenging, since the tilt angle between the optical axis of the WLI and the surface normal varies over the field of view, and the height evaluation strongly depends on the tilt angle that needs to be calibrated. For this reason, an enhancement of the signal evaluation is proposed that enables LSWLI measurements on rotating objects, cylinders in particular, without a preceding calibration of the tilt angle distribution in the field of view. This was achieved by enhancing a self-calibration algorithm originally developed for linearly scanned, plane surfaces with spatially resolved time-frequency analysis. Using this self-calibrating algorithm, the local tangent surface angles present in the field of view of the LSWLI are retrieved and used to calculate the micro topography of a cylindrical specimen. These topography data contain no information on the global shape of the object, therefore shape-removal algorithms, as needed for VSWLI measurements, are not necessary. As a byproduct of the angle evaluation, the radius of the cylindrical specimen can be determined in addition. The comparison with VSWLI reference data finally shows that the LSWLI setup is able to produce similar topography results despite the much inferior hardware.