The evolution of morphology and chemistry in fused silica surface after medium-pressure plasma processing

In order to achieve a process for precision asphere and freeform surfaces, a medium-pressure plasma process (MPPP) is developed for high-rate precision machining of optical materials. Fused silica is commonly used to fabricate optical components such as mirrors, lenses, prisms, and photonic crystals. Plasma polishing, an unconventional method, is employed for the atomic-level removal of material from a substrate’s surface. While polishing the fused silica substrate, the focus lies on investigating process parameters, specifically the radio frequency (RF) power, sulfur hexafluoride/oxygen (SF 6 /O 2 ) ratio, and plasma chamber pressure. The process parameters are maintained throughout all experiments, specifically a consistent sulfur hexafluoride/oxygen (SF 6 /O 2 ) ratio of 1:1 and He:(SF 6 + O 2 ) of 90:10. The surface roughness, elemental composition, and morphology of the fused silica surface are examined by the three-dimensional (3D) optical profiler, energy-dispersive X-ray spectroscopy (EDX), and field emission scanning electron microscopy (FESEM), respectively. After the plasma process, the surface roughness changes marginally without any surface contaminations. However, upon removing the chemically modified layer, the etching process results in the development of a succession of pits. During polishing, the distinct pits merged together, forming a cohesive structure. Further, the contact angle is also examined on silicon dioxide (SiO 2 ) surface before and after plasma process. The contact angle decreases from 89.6° to 23.2° before and after plasma polishing, respectively.