Tuning the Plasma Etching Mode for the Atomic-Scale Smoothing of Single-Crystal Silicon

Atomic-scale smooth surfaces of single-crystal silicon (Si) are indispensable for cutting-edge applications, such as semiconductor chips, quantum devices, and X-ray optics. Here, we vary the CF₄/O₂ reactant gas ratio to tune the etching mode from isotropic and orientation-selective etching to atom-selective etching in an atmospheric inductively coupled plasma (ICP). At low CF₄/O₂ ratios, the diffusion of the etching species dominates, resulting in isotropic etching. By contrast, the kinetics of ICP etching becomes dominant upon increasing the CF₄/O₂ ratio to between 1:1 and 2:1, inducing orientation-selective etching. Notably, CF₄/O₂ ratios above 2:1 result in atom-selective etching, whereby atoms around rough surface sites can be selectively removed. The atom-selective etching mode was used to achieve an atomically smooth surface with a S_a roughness of 0.14 nm. The results of this study demonstrate that atom-selective etching is an efficient and effective approach for manufacturing Si atomic surfaces.