Subsurface defects detection for the polishing surface of fused silica optics

Accurate detection of subsurface defects is the premise and foundation for research of subsurface damage of fused silica optics, which is crucial to ensure machining quality. The subsurface defects that generated in polishing process, such as hydrolysis layer, defect layer depth and subsurface damage morphology of fused silica components were quantitatively detected by HF acid etching method in this paper. Type and element content of impurities along the depth distribution were studied by X-ray fluorescence spectrometry technology, and the method for determining the depth of subsurface damage was presented. Because there existed hydrolysis layer and sub-surface defects layer, with the increase of etching time, the etch rate showed a decreasing trend and etching rate decreased significantly in the initial stage. When the etching depth exceeded a certain value, the defect layer would be completely removed by etching method, then etch rate remained unchanged. In addition, there were a variety of surface and sub-surface defects in polishing surface, so there were also some differences in subsurface damage morphology, width and depth of the scratches at different etching depths.