Surface Texture Evolution of Fused Silica in a Combined Process of Atmospheric Pressure Plasma Processing and Bonnet Polishing

Xing Su,Chenglong Ji,Bo Wang,Yang Xu,David Walker,Duo Li,Hongyu Li,Guoyu Yu

doi:10.3390/coatings9100676

Abstract

The increasing demand for precision optical components invokes the requirement of advanced fabrication techniques with high efficiency. Atmospheric pressure plasma processing (APPP), based on chemical etching, has a high material removal rate and a Gaussian-shaped influence function, which is suitable to generate complex structures and correct form errors. Because of the pure chemical etching, an optically smooth surface cannot be achieved using only APPP. Thus, bonnet polishing (BP) with a flexible membrane tool, also delivering a Gaussian influence-function, is introduced to smooth the surface after APPP. In this paper, the surface texture evolution in the combined process of APPP and BP is studied. The etched texture with increased removal depth of APPP is presented and analyzed. Subsequently, the processed substrates are smoothed by BP. The texture smoothing and the roughness improvement is investigated in detail. The experimental results show that the APPP etched pits coalesce with each other and transform into irregular convex-concave structures, with roughness degraded to about 25 nm arithmetical mean deviation (Ra). The APPP etched texture can be successfully smoothed to 1.5 nm Ra, with 0.2–1 μm material removal of BP.

Highlights

Fused silica has excellent optical properties, mechanical properties and high chemical durability, which make it widely used in advanced optical systems
We focus on the surface texture evolution in the combined process of Atmospheric pressure plasma processing (APPP) and bonnet polishing (BP)
The fused silica substrates were processed by APPP with increased depth

Summary

Introduction

Fused silica has excellent optical properties, mechanical properties and high chemical durability, which make it widely used in advanced optical systems. There are thousands of aspherical lenses and window mirrors made of fused silica in high-power laser projects, such as the National Ignition Facility [1]. The standard polishing tool of the ‘Precessions’ technology is an inflated spherical membrane (the ‘bonnet’) covered with a standard polishing cloth. The compliance of such a tool allows it to conform along a tool path to the varying local topography of aspheric or freeform surfaces, avoiding tool-misfit [3,4,5,6]. The presence of a physical pad in this process is important in “bridging the peaks of micro-roughness”, allowing polishing of ground or rough surfaces, as well correcting

Methods

Discussion

Conclusion