Effects of polishing pressure and sliding speed on the material removal mechanism of single crystal diamond in plasma-assisted polishing

Plasma-assisted polishing (PAP) was confirmed to be high-efficiency and high-quality when applied to single crystal diamond (SCD) substrates. The effects of polishing pressure applied to the SCD substrate and sliding speed between the polishing plate and SCD substrate in PAP were systematically investigated in this study. Higher polishing pressures or sliding speeds resulted in a higher material removal rate (MRR) of SCD substrate, and the highest MRR achieved 5.3 μm/h. In the case of PAP conducted at low polishing pressures such as 62.5, 81.3 kPa, scanning white light interferometer (SWLI) (84-μm square) and atomic force microscope (AFM) (5-μm square) measurements revealed an atomic-scale smooth surface without the surface texture depending on the crystal direction and the lowest Sq roughness of 0.3 nm (84-μm square) was achieved. Correspondingly, at low polishing pressure of 62.5 kPa, MRRs in PAP along the <100> and <110> directions were nearly identical, suggesting that isotropic removals occurred. In the case of PAP performed at high polishing pressures such as 143.8, 246.9, and 350.0 kPa, SWLI and AFM measurements revealed a rough surface with bands of grooves along <100> direction. Correspondingly, at high polishing pressures of 246.9 and 350.0 kPa, MRRs in PAP along <100> direction were ~11 and ~12 times faster than that in <110> direction, respectively, suggesting that anisotropic removals occurred. Moreover, scanning transmission electron microscopy and angle-resolved X-ray photoelectron spectroscopy measurements confirmed that no damage or non-diamond layer was present on the PAP-processed SCD substrate. However, as the sliding speed varied, both SWLI and AFM measurements demonstrated that Sq roughness remained nearly constant with the same polishing pressure applied to the SCD substrate.