Influence of Oxygen Flow Rate on the Variation of Surface Roughness of Fused Silica during Plasma Polishing Process

A novel capacitive coupled hollow cathode (CCHC) RF vacuum discharge polishing apparatus was established in house. The plasma was generated by using 13.56 MHz and 100 MHz RF power source respectively. Pre-polished fused silica substrates with average surface roughness of 1.5 nm (rms) were used for the investigation of the plasma polishing process. SF6 was used as the active etching gas, Ar and O2 or their mixture was used as carrying gas. The gas flow of SF6 was fixed at 10 SCCM, the total flow rate of the carrying gases was selected as 110 SCCM. When the carrying gas was pure Ar, the surface roughness of the fused silica substrate increased from about 1.5 nm to 2.0 nm and to 4.1 nm for 13.56 MHz and 100 MHz plasma etching respectively. When the plasma was generated at 13.56 MHz, the surface roughness of the fused silica substrate decreased with the increasing of the gas flow rate of O2, and reached its minimum of 1.04 nm with an O2 flow rate of 50 SCCM. Similar result was obtained when the plasma was generated at 100 MHz, the difference was that the minimum roughness of the substrate was 0.88 nm.