Measurement of a concave spherical mirror with 50 mm radius of curvature by three dimensional nanoprofiler using normal vector tracing

Abstract

Freeform optical elements are applied in various fields. In special purpose, ultraprecise aspherical mirrors are necessary for developing third-generation synchrotron radiation and X-ray free electron laser (XFEL) sources. In addition, the optical system of extreme-ultraviolet lithography (EUVL) is composed largely of high-accuracy asymmetric mirrors. The 3D profile measurement with nanometer resolution is essential to produce ultraprecise mirrors. Accordingly, the demand of a 3D profiler with nanometer resolution is increasing. We have developed a nanoprofiler which traces the normal vector of the mirror surface. This measuring method is based on the straightness of laser light and accuracy of rotational goniometer. This machine consists of four rotational stages, one translational stage and optical head which has the quadrant photodiode (QPD) and LASER head at optically equal position. In this measurement method, we conform the incident light beam to reflect the beam by controlling five stages and determine the normal vectors and the coordinates of the surface from signal of goniometers, translational stage and QPD. We calculated the three-dimensional shape from the normal vector and the coordinate of each point by a reconstruction algorithm. We have measured shapes with various radii of curvature. In this report, we measured a figure error of concave spherical mirror with 50 mm radius of curvature. 50 mm is the smallest radius of curvature so far. Generally, the systematic error increases as the radius of curvature decreases. And we discuss show the measurement result of figure error and repeatability.