目的 为揭示等离子体辅助抛光（Plasma-Assisted Polishing,PAP）过程中单晶金刚石表面微观形貌的演化机理,探明不同种类等离子体对复杂形貌的选择性刻蚀规律,并从原子尺度阐明其物理化学作用机制,以解决金刚石硬脆表面难以实现原子级平坦化的理论难题。方法 采用反应力场分子动力学（ReaxFF MD）模拟方法,构建包含原子阶梯、圆锥凸峰和圆柱凹谷的（001）晶向单晶金刚石复杂表面模型,在室温及低能离子轰击条件下,对比分析氮（N）、氧（O）、氩（Ar）3种等离子体的刻蚀行为差异。进一步建立9种具有不同波峰高度（5~20 Å,1 Å=0.1 nm）和周期（5~20 Å）的二维正弦粗糙表面模型,定量研究氮等离子体刻蚀效率与形貌参数的关联性,并结合原子应变、去除率及粒子撞击通量统计分析微观去除机制。结果 模拟显示,尽管去除方式（化学刻蚀、表面改性或物理溅射）不同,3种等离子体均表现出一致的形貌选择性,优先去除凸峰和阶梯尖端,凹谷区域几乎未受影响。刻蚀效率与波峰高度呈正相关,与周期呈负相关;形貌越陡峭,去除效率越高。其中,波峰20 Å、周期5 Å的模型平坦化效果最佳,刻蚀后表面粗糙度降至8.55 Å。原子结构分析表明,高陡度凸峰侧面暴露出更多具有高悬空键密度的（010）或（100）晶面,反应活性显著高于凹谷区域;同时,长周期模型凹谷处存在明显的粒子撞击遮蔽效应。结论 单晶金刚石的形貌选择性刻蚀机制是宏观“几何遮蔽效应”与微观“表面曲率依赖的原子活性”协同作用的结果。PAP技术对高频高陡度粗糙峰具有极高的去除选择性,但在修正低曲率长周期波纹时存在加工自限制现象。该研究结果明确了表面曲率对原子去除活性的决定性影响,为优化硬脆材料复杂曲面的确定性平坦化工艺参数提供了理论依据。
Abstract
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.
关键词
等离子体选择性刻蚀 /
单晶金刚石 /
表面形貌 /
分子动力学 /
原子尺度去除
Key words
selective plasma etching /
single crystal diamond /
surface morphology /
molecular dynamics /
atomic-scale removal
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基金
国家自然科学基金（52405178,52350323,52505171）; 福建省自然科学基金（2024J08026,2024J08132）; 福建省中青年教师教育重点研究项目（JZ230005）
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