Automatic optical path alignment for a vertical scanning interference profiler

The white-light vertical scanning interferometry (WLVSI) profiler can measure the three-dimensional profile of a minute object with great accuracy. The working principle of the WLVSI is to use the interference patterns measured with a CCD sensor at different objective distance to derive the specimen profile. However, due to the short coherence length nature of the WLVSl profiler, the focusing and leveling operations to bring the specimen surface within the coherence range are very time-consuming. Furthermore, these operations are coupled, for the adjustment of the surface orientation could change the focus condition. An operator has to iterate between focusing and leveling in order to generate a clear image with broad interference stripes. This paper describes an automatic optical path alignment method to focus the image and align the orientation of a specimen surface to perpendicular to the optical axis. Our method first finds the focus point for a specimen by a global search algorithm to establish a starting point. Then, based on a subsequent scanning at several different heights, a rough surface profile is obtained by the depth from focus approach. From this profile, we can fit a plane model to find the major orientation of the specimen. From the surface normal vector, we derive the necessary equations to calculate the rotation angles to restore an oblique surface to perpendicular to the optical axis. After the adjustment, the specimen is refocused again. This implemented automatic alignment system can complete the operations in a few runs.