Polishing tool with phyllotactic distributed through-holes for photochemically combined mechanical polishing of N-type gallium nitride wafers

Abstract In this work, we further developed the photochemically combined mechanical polishing (PCMP) method for finishing N-type gallium nitride (GaN) wafers. A core improvement is to design a novel polishing tool with phyllotactic distributed through-holes, through which the wafer surface underneath through-holes can receive ultraviolet (UV)-light for the photochemical oxidation, while the rest parts undergo mechanical polishing. During PCMP, the co-rotation of the wafer and polishing tool allows the wafer surface to undergo the uniform and high-frequency conversion of oxidation and polishing. Based on the designed PCMP system and apparatus, the fundamental issues arising from such an alternate processing mode, which is different from the parallel mode of conventional chemical mechanical polishing (CMP), were investigated. Results show that the technical features of PCMP depend on the nature of the photochemical oxidation of wafers themselves if the mechanical polishing procedure can sufficiently remove oxides in time. The material removal rate (MRR) is inversely proportional to the dislocation density of wafers. Under acidic conditions, the oxidation proceeds by the GaN monocrystal step orientation, allowing PCMP to clear surface/subsurface damages (SSDs) and to prepare step-terrace structures on the wafer surface. When the polishing solution (pH = 1.5) includes 0.1 M K2S2O8 oxidants and 10 wt% SiO2 abrasives, the surface roughness Sa attains 0.21 nm in 10 × 10 μm2, and the MRR reaches 275.3 nm/h. The present study shows that the phyllotactic distributed through-hole array structure designed for polishing tools offers rich possibilities for the innovation of polishing technologies combining with various oxidation approaches.