Investigation of Etching Characteristic of Reactive Ion Etching for Gigantic Telescope Mirror Material

Abstract

In this thesis, the plasma etching characteristic of silicon and fused silica, which are the main elements of mirror materials, were investigated using reactive ion etching. The plasma etching parameters selected for the reactive ion etching process can be utilized to determine the most influential plasma etching parameters, except for operating pressure, in atmospheric pressure plasma etching system, which is considered a promising surface figuring process for meter-scale mirrors, to correct for mid and high spatial frequency error of optical mirror materials to under 20 nm RMS and 2 nm RMS, respectively. The etching rate, the repeatability error of etching rate, and surface roughness are dependent on the plasma etching parameters, such as the gas flow rate, the type of etching gas, RF power, and chamber pressure. The effect of the aforementioned parameters on mirror surfaces were investigated for silicon and fused silica. The etched depth and surface roughness were measured through ZYGO optical profiler. The cost of etching gases consumed during the plasma etching process was calculated to find the cost-effective combination among plasma etching parameters selected. The maximum etching rate of silicon and fused silica was achieved as 782.44 nm/min and 70.56 nm/min, respectively, within the selected plasma etching parameters. The repeatability error of etching rate of ±5~10% and ±1~2% was demonstrated for silicon and fused silica. The RMS surface roughness obtained after reactive ion etching process of silicon and fused silica was 1.8~3.5 nm RMS and approximately 1.5 nm RMS, respectively. The plasma etching parameter to achieve minimum cost of consumed etching gas was found for silicon and fused silica.The goal of the surface roughness that has to be achieved in this thesis was maintained at initial surface roughness (i.e. the surface roughness of before plasma etching process) of silicon (2.9 nm RMS) and fused silica (1.5 nm RMS) and was achieved at all plasma etching parameters selected. The plasma etching parameters selected in this thesis can be utilized for the application of surface figuring of silicon and fused silica and is expected to be applied for the finishing process of optical mirror material in atmospheric pressure plasma etching system.