Atmospheric Plasma Jet processing for figure error correction of an optical element made from S-BSL7

Abstract

To meet the increasing market demand for optical components, Plasma Jet Machining (PJM) of Borosilicate Crown Glass (BCG), which can be an alternative to Fused Silica, is presented. Surface figure error correction was performed by applying reactive plasma jet etching, where a fluorine containing microwave driven plasma jet is employed to reduce the figure error in a deterministic dwell-time controlled dry etching process. However, some of the glass constituents of BCG cause the formation of a residual layer during surface treatment which influences the local material removal. By heating the substrate to about Ts= 325°C to 350°C during processing, the etching behavior can clearly be improved. Geometric conditions of the optical element nevertheless lead to a characteristic temperature distribution on the substrate surface, which requires an adjustment of the local dwell times in order to obtain the required material removal. Furthermore, the resulting local surface roughness is also influenced by the surface temperature distribution. It is shown that figure error can be significantly reduced by taking the local temperature distribution and resulting local etching rates into account. A subsequent polishing step smoothens roughness features occurring during etching to provide optical surface quality.