Macroscale smoothing mechanism in plasma polishing of polycrystalline diamond : diffusion-controlled etching

Abstract

Atmospheric-pressure inductively coupled plasma (APICP) polishing has emerged as a highly efficient technique for polishing ultra-rough polycrystalline diamond surfaces. Yet, the underlying mechanism, particularly the planarization of protrusions hundreds of micrometers in size, remains unclear. This work reveals the mechanism underlying the macroscale smoothing effect in APICP polishing through a combination of theoretical and experimental studies. Theoretical study investigates the influence of the relative rate between reactant diffusion and chemical reaction through numerical simulation. Simulation results indicate that when the reaction outpaces the diffusion, preferential removal at protrusion peaks occurs due to a steeper gradient of reactant concentrations at protruding regions. This preferential removal leads to protrusion smoothing and rounded edge geometry and is not influenced by crystal orientations. The experimentally measured edge geometry, roughness evolution, and etching rate dependence on crystal orientations were essentially consistent with simulation results, demonstrating the macroscale smoothing in APICP polishing to be a diffusion-controlled process. The excited argon in APICP, rather than oxygen atoms and temperature introduced by the plasma, plays a critical role in macroscale smoothing. These excited species potentially enhance the oxygen-diamond reaction and transit the rate-limiting step into diffusion. The revealed mechanism exhibits strong potential for extension to other semiconductor materials and for inspiration to other non-contact polishing methods. This work advances the mechanistic understanding of APICP polishing, reshaping its processing capability, equipment design, and parameter optimization in industrial manufacturing.