Modelling of material removal in sapphire double-sided chemical mechanical polishing

A multi-scale model to investigate material removal mechanisms was developed in double-sided chemical mechanical polishing of sapphire, integrating diffusion analysis, contact mechanics, and experimental validation. A solid-liquid diffusion model with moving boundaries was established to describe concentration variations during polishing. Single-particle contact models were developed for two-body wear (abrasives embedded in pad asperities) and three-body wear (abrasives flowing in grooves with fluid shear). Results show that all contact parameters scale linearly with abrasive diameter. In two-body wear, abrasive deformation and sapphire indentation increase nonlinearly with load, while pad contact radius remains nearly constant. In three-body wear, fluid shear significantly enhances indentation depth, up to fourteen times greater than two-body wear under comparable conditions, with contact angle and rotational speed playing key roles. A coupled material removal model incorporating chemical reaction and mechanical wear was validated through orthogonal experiments. Polishing pressure dominates material removal rate, followed by slurry concentration and pH, while speed and abrasive size have minor effects. Optimal pH (10.5) maximizes chemical softening. Crystal orientation anisotropy is significant: C-plane shows highest removal rate due to lower hardness and higher reactivity. Despite orientation differences, double-sided polishing consistently achieves ultra-smooth surfaces (roughness ∼0.2 nm, PV <2 nm) from initial ∼500 nm. Quantitative validation and sensitivity analysis confirmed proposed model robustness. And the applicability and limitations of the model are also clearly defined. The model provides quantitative guidance for optimizing sapphire CMP processes.