Design and development of medium-pressure plasma process for optical substrate finishing: A comparative study with wet chemical etching

Abstract

Plasma polishing is a relatively new technique successfully applied for polishing optical materials due to its numerous advantages. It includes the high quality of the polished surface, reduced surface and sub-surface defects, high processing speed, etc. This paper investigates surface finish on fused silica using two processes, i.e., medium pressure plasma process and wet chemical etching. The experimental result shows that at a 20 mbar chamber pressure and 80 W radio frequency power, the surface finish of coarse machined and lapped fused silica substrate is better than 5 and 10 mbar. The experiments are also conducted using wet chemical etching on coarse machined and lapped polished fused silica substrates using a mixed hydrofluoric and sulfuric acid solution. However, using wet chemical etching, the percentage increase of surface roughness achieved on lapped and coarse machined surfaces is higher than in the plasma process. The percentage increase in surface roughness is 84.8 % and 98.81 % on coarse surfaces and 69.23 % and 892.3 % on lapped surfaces upto 30 μm material depth using plasma process (at 20 mbar pressure) and wet chemical etching, respectively. The surface microstructure of fused silica before and after processing is investigated using field-emission scanning electron microscopy. The result reveals that plasma polishing significantly reduced the micro-cracks and damaged layers. Energy dispersive X-ray results show the presence of elements fluorine, silicon, oxygen, and carbon on the processed surface, showing evidence of plasma reaction. Moreover, an additional element, i.e., sulfur, also appears on the processed surfaces from the solution during wet chemical etching. The sharp diffraction peaks have not been observed in X-ray diffraction patterns, indicating that the material is amorphous fused silica.