Novel Finishing Process Development for Precision Complex-Shaped Hemispherical Shell by Bulk Plasma Processing

Abstract

It is not uncommon that inertial sensor technology demands ‘zero’ surface and subsurface defects on sensing element microstructures, which are polished to very fine surface finish. Non-contact type unconventional finishing techniques are being developed to augment or replace chemo-mechanical polishing (CMP) technique for finishing such microstructures to remove subsurface damage. The aim of this study is to develop an atomistic material removal mechanism by chemical vaporization process. The novelty of this process is combining the merits of low-pressure plasma etching by ion such as isotropic material removal on all the surfaces simultaneously and merits of atmospheric plasma process such as chemical vaporization rather than physical bombardment by ions. This achieves defect-free extremely fine-polished surfaces. A finite element based Comsol^® software package is used to model dielectric barrier excited RF discharge for helium and oxygen gas as processing gas and reactive gas, respectively. The gas composition, pressure, electrode configurations and power of RF excitation are studied with respect to oxygen radical formation and their uniformity of distribution in the chamber. Accordingly, plasma chamber is designed and built with Zerodur material with an optical window to achieve a deterministic process, which is capable of simultaneously polishing entire complex 3D surfaces including cavities where no tool or beam can reach. Plasma is established with helium as processing gas and oxygen and SF₆ are used as reactive gases at medium pressure of 20 mbar. The atomic emission spectroscopy is used to monitor the various oxidation states of silica and established correlation with respect to material removal rate and oxidation states of silica Si II and Si III. Fine-tuning of these parameters is done while polishing hemispherical shell based on atomic emission spectroscopy observations and established process with material removal rate of 0.008 mm³/min. After repeated polishing cycles with cumulative 48 h of material removal, surface roughness (Ra) of 3.6 nm is achieved from as-machined shell of Ra = 903 nm.