Low-damage optical manufacturing via plasma finishing and figuring

Precision optical components have stringent requirements on surface roughness, form error, and subsurface damage for superior performance. However, conventional grinding, lapping, and polishing processes of fused silica inevitably introduce subsurface damage (SSD) due to the use of abrasives. Thus, this paper proposes an abrasive-free, low-damage manufacturing process for fused silica optical components, which combines inductively coupled plasma (ICP) for SSD recovery and capacitively coupled plasma (CCP) for form error correction. This paper mainly aims to reveal the advantages and challenges of the combined plasma process. The SSD recovery capability of ICP finishing was first verified. The comparison of surface morphology after buffered oxide etch (BOE) etching and CCP etching revealed that extensive surface roughening is caused by plasma etching rather than SSD. Experimental studies on the combination of ICP and CCP demonstrated that ICP finishing can not only recover SSD but also inhibit the surface roughening by plasma etching. The investigation of form error after ICP finishing revealed that the induced form error consists of workpiece distortion and localized deformation with a crater-like structure, affecting the precision and duration of CCP figuring. The combined plasma process was conducted and a low-damage surface with roughness less than Sa 0.3 nm and form error less than RMS 20 nm was achieved.