Mechanism of high-spatial-frequency roughness deterioration in plasma chemical vaporization machining induced by process-gas-composition-dependent surface chemistry

In ultra-precision optical manufacturing, figuring ensures form accuracy in low- and mid-spatial-frequency ranges, whereas polishing reduces high-spatial-frequency roughness (HSFR). However, polishing may degrade form accuracy and require figuring/polishing cycles; suppressing HSFR deterioration during figuring is essential for full-spatial-frequency surface control. Non-contact plasma jet chemical etching is attractive for damage-free figuring, and atmospheric-pressure plasma chemical vaporization machining (AP-PCVM) enables vacuum-free material removal. Although CF 4 /O 2 process gas composition is adjusted to increase removal rate, its influence on roughness deterioration and reaction mode remains unclear. Here, plasma-jet diagnostics and surface characterization clarified how increasing the O 2 fraction in the CF 4 /O 2 process gas changes surface reaction mode and roughness evolution. At low O 2 fraction, CF 4 dissociation products formed a carbon-related deposition layer, whose presence was associated with a proposed buffering effect on access of fluorine-containing reactive species to the substrate, consistent with morphology-preserving etching. Excessive O 2 fraction suppressed this layer, accompanied by a transition to pit-forming etching. For single-crystal Si, this transition promoted orientation-dependent etch pits and HSFR deterioration. Optimizing the CF 4 /O 2 ratio preserved this buffering effect, suppressed pit formation, and maintained roughness near the pre-processing level. These findings provide mechanistic insight for process gas design and roughness control by regulating surface reaction mode.