Plasma-assisted polishing with self-limiting surface modification for sub-nanometer finishing of AlN ceramics

Advanced ceramics, typically fabricated by sintering single-crystal grains with a binder phase, retain the intrinsic properties of the raw materials while offering low manufacturing cost and high design flexibility in shape and size, making it widely applicable across diverse fields. To fully exploit its potential, sub-nanometer-level smooth surfaces are required. However, intrinsic heterogeneities among grains and between grains and the binder phase lead to anisotropic material removal and grain shed-off during conventional mechanical processing, becoming a critical bottleneck to sub-nanometer-level finishing. In this study, the effects of abrasive size and lapping pressure in conventional mechanical lapping were first investigated. Using small-size abrasives at lower pressure effectively suppressed grain shed-off and reduced surface roughness. However, the attainable smoothness was ultimately limited by material heterogeneity and the threshold pressure required for material removal. To overcome this limitation, plasma-assisted polishing (PAP) that combines self-limiting surface modification with selective removal at ultra-low polishing pressure was developed. Firstly, an easily removable modified layer was formed on the AlN ceramic surface via plasma irradiation. By leveraging the self-limiting reaction characteristics of the modified layer, uniform modification insensitive to grain orientation and phase composition was achieved by controlling modification parameters. Subsequently, this uniform modified layer was selectively removed at a polishing pressure significantly below the conventional removal threshold. Using this approach, a grain-boundary step free and ultra-smooth surface with an average surface roughness S a of 0.86 nm over an 84 μm × 84 μm region was obtained on AlN ceramics. These results demonstrate the feasibility of PAP for achieving sub-nanometer-level finishing of hard-to-machine ceramics represented by AlN, which provides a new processing route for high-power and high-frequency electronic packaging substrates.