Ultra-precision tactile profilometry using the ISARA 400

Ultra-precision fabrication of complex shaped optical surfaces like aspheres and freeforms for laser-, X-ray- or space applications as well as semiconductor production equipment requires the precise assessment of surface form before and after each production step. Our ultra-precision surface manufacturing methods comprise deterministic sub-aperture techniques like ion beam figuring, plasma jet machining, or corrective polishing, which all make use of the dwell time method [1]. Thus, surface form data need to be acquired on the whole surface as an area mapping in order to accomplish a de-convolution with the tool function yielding the dwell time distribution. In the case of aspheres and freeforms the application of interferometry may be limited due to large departures from best fitting spheres. Ultra-precision measuring systems like NOM at BESSY-II that are specially dedicated for synchrotron optics reach their limits if strongly curved surfaces with some tens of mm (radius of curvature) have to be measured. CMMs and profilometers rather provide both sufficient flexibility and accuracy to map complex surface forms, especially in a production environment. In order to investigate the accuracy and limits of the different ultra-precision measurement systems used for areal surface profiling, we conducted a measurement comparison of different optical parts on the following systems located at IOM: The ISARA400, an ultra-precision coordinate measuring machine, precision scanning optical surface profiler CyberScan CT350S, and D100 interferometer in stitching mode. Furthermore, the obtained results have been compared with cross sectional and areal measurements performed on external ultra-precision measurement systems such as the Nanometer-optic measuring machine NOM (HZB, Berlin), M400 tactile CMM (Carl Zeiss, Oberkochen), and ZYGO interferometer (PTB, Braunschweig).