Towards nanometer-scale surface roughness in plasma polishing of polycrystalline diamond via modulating graphitization and oxidative etching

Atmospheric-pressure inductively coupled plasma (APICP) polishing currently cannot achieve nanometer-scale roughness on polycrystalline diamond (PCD). Poor surface quality necessitates prolonged subsequent fine polishing to achieve the required roughness for applications, thereby compromising the overall processing efficiency. The underlying mechanism of surface quality bottlenecks may originate from the complex phase transition and etching processes of PCD surfaces under the plasma, which remains insufficiently explored. This work aims to systematically investigate the phase transition and etching processes of PCD under APICP to improve the polishing quality. It is demonstrated that beyond the previously established atom-selective etching induced by oxygen atoms in plasma, there also exists graphitization triggered by pure argon plasma and oxidative etching induced by oxygen molecules. Specially designed experiments were carried out to individually investigate these two processes. Results indicate that both processes produce phase transition layers on PCD surfaces and rough diamond interfaces below. In normal APICP polishing, these side processes degrade the final surface quality. By suppressing the side processes using moderate power, enhancing atom-selective etching using a high oxygen flow rate, and avoiding additional oxidative etching after polishing, the surface roughness can be reduced to less than 3 nm ( S a ), pushing the surface quality of APICP polishing from sub-micrometer scale to nanometer scale. This work advances the process understanding of APICP polishing of PCD and promotes efficient and nanometric-roughness polishing of diamond.