High-accuracy form measurement of large optical surfaces

摘要

Surfaces as needed in optical systems, ranging from the visible even into the EUV region, become larger and often have a length or diameter of 500 mm and more. The form of these surfaces, describing the surface spatial frequency content with components below 1 mm-1, has to be characterized on the nanometer and sometimes even on the sub-nanometer scale. The extendibility of the measuring systems accuracy to large specimen dimensions basically depends on the method of the measurement and the scaling of different systematic uncertainty components with lateral coordinate values. This is analyzed for flatness and sphericity measuring systems, with a focus on the systems for Extended Shear Angle Difference (ESAD) and Large Area Curvature Scanning (LACS) used at PTB. Both are scanning methods working absolute and with a good natured scalability to large dimensions. For the measurement of optical flats the dominant uncertainty of topography is in the quadratic or spherical contribution of the surface in terms of a polynomial description. For calibration flats, as used for large interferometers, this often cannot be measured absolutely with sufficient accuracy. The potential of ESAD and other methods is analyzed with respect to this uncertainty component. Uncertainty considerations and measurement results for large flats are presented. For the form measuremetn of largely extended convex or concave surfaces, where classical interferometric set-ups are not possible due to the lack of a master surface or the extrme costs incurred for large optical components, the potential of LACS is presented.