High accuracy form measurement of large optical surfaces

摘要

Surfaces as needed in optical systems, with operating wavelengths ranging from the visible into the EUV region, become larger and often have a length or diameter of 500 mm or 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 accuracy of the measuring system for increasing specimen dimensions basically depends on the method of measurement used and the scaling of different systematic uncertainty components with lateral coordinate values. This is analyzed for flatness and curved surfaces forms 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 showing good-natured scalability to large dimensions. For the measurement of optical flats, the uncertainty for the topography is dominated by the quadratic or spherical contribution in terms of a polynomial description of the surface. 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 measurement 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 extreme costs incurred for large optical components, the potential of LACS is presented.