Uncertainty of Figure Error Measurement by Non-Contact Three Dimensional Nano-profiler Using Normal Vector Tracing Method

摘要

In recent years, optical elements with freeform or aspherical surface have been required in various fields. For example, X-ray focusing mirrors and EUVL mirrors have such kind of surface. Nano-accuracy three-dimensional measurements are needed to manufacture these optical elements. As general methods to measure the surface form of optical elements, there are CMM and phase shift Fizeau interferometer. However, the uncertainty of the measurement in the case of CMM is approximately 100 nm. Further, the interferometer cannot perform absolute measurement, and it is difficult to evaluate uncertainty. Therefore, there is no method which can measure the form of optical elements with nano-scale uncertainty. To achieve this measurement, we develop a non-contact three-dimensional figure error measurement using normal vector tracing method. The principle of our method is that normal vectors at each measured point are decided by making the incident light beam on the mirror surface and the reflected beam at same point coincide. Our method has already achieved sub-nanometer repeatability. In this paper, we report the result of evaluating uncertainty of our measurement.