Research on Electrolyte in Slurry for Photoelectrochemical Mechanical Polishing of GaN Wafers

Gallium nitride (GaN), owing to its excellent properties, has emerged as a promising material for wide range of applications. However, due to the high hardness and outstanding chemical stability of GaN, the efficiency of its chemical mechanical polishing (CMP) is relatively low. Photoelectrochemical mechanical polishing (PECMP) has emerged as a highly effective technique for GaN polishing. However, the development of appropriate slurry for PECMP remains a pressing issue, as achieving an optimal balance among electrical conductivity, transmittance, and particle stability is particularly challenging. This research delves into novel slurry designed for GaN PECMP. A systematic investigation is conducted to explore the impact of electrolyte type, concentration, temperature and pH on the performance of the slurry, and conducted PECMP test to evaluate performance of silica sol mixed electrolyte in PECMP. Results indicate that among the electrolytes tested, monovalent cations, particularly in the case of 0.2 M Li2SO4 provided a favorable balance, enhancing conductivity considerably while largely maintaining high UV transmittance and particle dispersion stability over the experimental period. In contrast, divalent cations induce severe aggregation, degrading polishing performance. pH experiment reveals the stability of silica sol arises from the synergistic interplay of surface electrical properties, electrolyte characteristics, and the intrinsic chemical stability of the material PECMP test confirms that aggregated particles from unstable slurries cause surface scratches and pits. Under optimized conditions (0.2 M Li2SO4, 20 nm silica at 5 wt%), the slurry achieved a surface finish with Sₐ 0.292 nm. These results provide profound insights for the design of advanced PECMP slurries and make a significant contribution to the continuous development of GaN wafer manufacturing technology.