Ion beam milling of optically polished CaF 2 surfaces

Abstract

ABSTRACT Calcium Fluoride roughness evolution caused by ion beam milling has been studied in dependence on the ion milling parameters and different optically polished surfaces, respectively. For polished surfaces with high crystal damage, the roughness is dominated by the uncovering of the sub-surface damage due to the ion beam milling. For smooth surfaces with low damage the roughness is an intrinsic one and the creation of self-organized nanoscale structures can be obtained. Ion milling parameters influence more the intrinsic roughness than the extrinsic. Generally, high ion energies and sputter gases with low atom mass produce rough surfaces. Low ion energies and gases with high mass result in smooth surfaces. Ion bombardment induces a decomposition of CaF 2 in the near surface layer. Surface analytical measurements show that ion sputtering to some extent decompose the CaF 2 surface layer in contrast to the combined action of ion sputtering and low energy electron irradiation. The measured higher VUV absorption after ion milling is caused by the modified sub-surface layer mainly and not by the increased surface roughness. Keywords: Ion beam milling, calcium fluoride, DUV optics