To address the critical technological bottleneck of achieving atomic-level planarization on single crystal diamond (SCD) surfaces, which is fundamentally limited by the extreme hardness and chemical inertness of the material, the work aims to elucidate the atomic-scale mechanisms governing the micro-morphological evolution of SCD during Plasma-Assisted Polishing (PAP). With Reactive Force Field (ReaxFF) Molecular Dynamics simulations, a comprehensive investigation was conducted on the selective etching behaviors of Nitrogen (N), Oxygen (O), and Argon (Ar) plasmas on complex surface topographies, thereby revealing the underlying physicochemical principles of material removal. A rigorous dual-model approach was employed: a complex (001)-oriented surface model featuring atomic steps, conical asperities, and cylindrical pits was firstly constructed to comparatively analyze the etching behaviors of different plasma species, followed by the establishment of nine distinct two-dimensional sinusoidal rough surface models with varying peak heights (5-20 Å) and periods (5-20 Å) to quantify the relationship between morphological parameters and etching efficiency. The simulations demonstrated that while the fundamental removal modes differed significantly among the species—with Oxygen primarily driving chemical etching to form CO and CO2 gaseous products, Nitrogen inducing surface modification through the formation of C—N bonds and amorphous layers, and Argon relying on physical sputtering and momentum transfer, the spatial selectivity of the etching remained remarkably consistent. All three plasma species exhibited a distinct preference for removing atoms at sharp asperities and atomic step edges while leaving valley regions largely unaffected, indicating that the selective etching process was primarily dominated by the geometric "selectivity" of the diamond surface morphology rather than the specific plasma type. Quantitative analysis of the sinusoidal models revealed that etching efficiency exhibited a strong positive correlation with asperity height and a negative correlation with the period. Specifically, the steepest morphology (Peak 20 Å/Period 5 Å) yielded the highest removal rates and achieved the superior surface quality, reducing the roughness to 8.548 5 Å. The study identifies the core mechanism of this selectivity as a synergistic interplay between macroscopic "geometric shielding" and microscopic "curvature-dependent atomic reactivity". Macroscopically, trajectory analysis confirms a shadowing effect in long-period structures where protruding asperities physical block the valleys, resulting in a non-uniform plasma flux that limits impact frequency in concave regions. More significantly, at the microscopic level, this work establishes that local surface curvature dictates the crystallographic orientation of the exposed facets: steep asperities on the (001) substrate expose side facets approximating (010) or (100) orientations, which possess a high density of dangling bonds (two per atom), rendering them chemically active and easily removed. Conversely, as the surface smoothens and curvature decreases, the exposed facets transition toward (111) orientations characterized by atoms with only single dangling bonds, thereby significantly increasing the energy barrier for removal. These findings elucidate the "self-limiting" nature of the PAP process, where removal rates decay non-linearly as the surface planarizes, providing a theoretical basis for optimizing process parameters to achieve deterministic, damage-free, atomic-scale planarization of hard-brittle materials.
Key words
selective plasma etching /
single crystal diamond /
surface morphology /
molecular dynamics /
atomic-scale removal
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Funding
National Natural Science Foundation of China (52405178, 52350323, 52505171); the Natural Science Foundation of Fujian Province, China（2024J08026, 2024J08132）; the Key Research Project on Education for Young and Middle-aged Teachers in Fujian Province（JZ230005）
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