Structure characterization of fused silica redeposition layer in nanoscale and analysis of impurities

Polishing-induced redeposition layer of fused silica optics seriously limits the laser damage resistance, and currently the focus of research lies in absorbing impurities. This article focuses on structure characterization of fused silica redeposition layer in nanoscale and analysis of impurities. With atomic force microscopy to observe surface morphology of fused silica at different etching time and using secondary ion mass spectroscopy, such redeposition layer structure and impurity distribution are investigated. The results indicate that a large number of microcracks and impurities existing in redeposition layer within 10 nm deep are formed into nanoscale scratches and pits after etching, which decay exponentially with depth. The thickness of the redeposition layer is estimated based on the evolution of nanoscale scratches density and aspect ratio with etching depth, which is basically consistent with the depth of embedded impurities measured by secondary ion mass spectroscopy. Correlation between depth of embedded impurities and distribution of polishing microcracks shows that impurities are more likely to hide in the polishing microcracks.