Energy-dependent competing pathways in the plasma processing of fused silica: from topology-selective relaxation to network disordering

Abstract

Oxygen-plasma processing can attenuate polishing-induced topographic features on fused silica, whereas increasingly energetic bombardment can disorder the near-surface network. Atomic force microscopy, Raman spectroscopy, laser-damage measurements, plasma diagnostics, and molecular dynamics simulations were combined to resolve these competing pathways. The measured plasma and floating potentials define a characteristic experimental bombardment scale, while simulations of monoenergetic neutral O-atom impacts isolate collision-driven surface evolution. At 150 V, a 5-min treatment reduced the root-mean-square height from 0.980 to 0.457 nm. Raman spectra acquired after 10 min at the same voltage showed changes consistent with relaxation of strained local structures. Under a 2-on-1 protocol at 355 nm, the fitted 50% laser-induced damage threshold increased from 10.97 J cm⁻ 2 (95% confidence interval, 10.83–11.12 J cm⁻ 2 ) for untreated silica to 12.39 J cm⁻ 2 (12.16–12.63 J cm⁻ 2 ) after 5 min; extending the treatment to 10 min gave 11.95 J cm⁻ 2 (11.80–12.09 J cm⁻ 2 ). A 10-min treatment at 250 V removed large scratches but generated nanoscale asperities and Raman signatures of greater network distortion. Within the stated model, low-energy impacts produce Edwards–Wilkinson-like smoothing, whereas higher-energy impacts produce Kardar–Parisi–Zhang-consistent nonlinear roughening, accompanied by changes in coordination, ring statistics, and density. Among the conditions tested, 150 V for 5 min produced the lowest measured roughness and the highest fitted threshold, consistent with an energy-dependent competition between topology-selective relaxation and network disordering.