Dispersion optimized white-light interferometer based on a Schwarzschild objective

Abstract

Scanning white-light interferometry (SWLI) provides the capability of fast and high-precision three-dimensional measurement of surface topography. Nevertheless, it is well-known that white-light interferometers more than imaging microscopes suffer from chromatic aberrations caused by the influence of dispersion. Chromatic aberrations lead to systematic measuring errors in SWLI, especially on micro-structures with curved or tilted surface areas. For example, the plane glass plates used in a Mirau-interferometer are a potential source of dispersion. If this influence is not completely corrected for, errors in height measurement occur. In addition, the magnitude of these errors strongly depends on whether the coherence peak's position or the phase of an interference signal is evaluated. This study is intended to overcome these difficulties by a dispersion optimized white-light interferometer. The design corresponds to a Mirau-interferometer, but in order to reduce dispersion phenomena, a reflective Schwarzschild microscope objective is used. For beam splitting a so-called pellicle is positioned in-between the objective and the measuring object. The dominant effect, which limits the accuracy of the interferometer is supposed to depend on multiple reflections from the front and the back side of the pellicle beam splitter. As a consequence, ghost signals were measured in addition to the typical white-light interference signals. This indicates that multiple reflections influence the results and finally limit the accuracy of the interferometer.