Development and Performance Evolution of Medium-Pressure He/SF₆/O₂-Based Plasma and Wet Chemical Etching Process for Surface Modification of Fused Silica

The rising demand for precision optics widely employed in ground and space-based astronomical instruments and other scientific instrumentation requires highly efficient advanced fabrication methods. Due to complex-shaped fused silica substrate surfaces like freeform or aspheres with strong curvatures or very small-sized components, a novel non-contact medium-pressure plasma-based method is developed to finish optical components. The present study critically compares the polished optical surfaces of a prism with a medium-pressure plasma process and wet chemical etching to provide insight into their smoothing. The results show that surface roughness (R_a) increases from 0.54 to 2.61 nm and 0.53 to 0.57 nm at 5 and 20 mbar total pressures, respectively, using a plasma process without surface contamination. However, wet chemical etching increases surface roughness (R_a) from 0.52 to 15.9 nm. The substrates' surface morphology, elemental composition, and surface topography are analyzed using FESEM, EDX, and AFM. Moreover, subsurface improvements are analyzed using Raman spectroscopy analysis.