Surface characteristics of fused silica in medium-pressure plasma process

Abstract

In order to obtain an ultra-smooth fused silica surface without surface damage efficiently, medium-pressure plasma processing has been design and developed. In the medium-pressure plasma polishing process, the stable plasma process of the medium-pressure plasma directly impacts surface roughness. The fluorine (F) atom, which is reactive and provided by SF6, O2, and He gas, is a very significant supplementary gas, and those gases play a significant part in the process. The medium-pressure plasma is used for polishing fused silica, and an optical emission spectrometer is used to observe the deviations in the intensity of atoms, ions, and species during processing. The experiment has been conducted at optimum conditions, i.e., gas composition (He:(SF6 + O2)) of 90:10, pressure ratio (SF6/O2) of 1:1, and RF power of 80 W. The surface roughness of the optical material, i.e., fused silica, has been marginally increased from Ra = 1.95 μm to Ra = 2.02 μm at a total pressure of 5 mbar and RF power of 80 W. Field-emission scanning electron microscopy results show that the morphology has been changed after the plasma process. The initial coarse microstructure of fused silica transforms a finer structure through plasma processing under optimal conditions. Energy-dispersive X-ray (EDX) results reveal that the elements Si and O are present at the surface before processing, and Si, O, F, and C, have been observed after processing. After plasma processing, EDX measurement confirms the presence of fluorine elements, demonstrating the interaction of the plasma with the substrate.