Investigation of CMP on GaN substrate for led manufacturing

The relationship between the surface characterization of GaN substrates after chemical mechanical polishing (CMP) and the pH value of CMP slurry under acidic conditions was investigated in detail. Atomic force microscope (AFM) was used for measuring the surface morphology, pit depth distribution, and atomic step-terrace structure. With the decrease of pH value from ~6 to ~2, the material removal rate (MRR) increased, and the pit depth reduced, resulting in smaller roughness (Ra). Based on an overall consideration of MRR and Ra, pH value of ~2 was the most suitable acidic condition for GaN CMP, the corresponding MRR and Ra was ~165nm/h and 0.65nm (scan area 30×30 μm2), respectively. After CMP with slurry under pH=~2, an atomic step-terrace topography on GaN surface was observed clearly, besides, something like residual chemical product was also discovered. X-ray photoelectron spectroscopy (XPS) analysis indicated that the product was the residual oxides of GaN, which could be removed clearly by increasing the abrasive content. Ra was low to 0.06nm (scan area 1×1μm2) when the surface shows no residual chemical products. The detailed polishing process and mechanism of GaN CMP were also discussed in this paper.