Chemical-mechanical Polishing of N-polar GaN for GaN HEMT on Diamond Bonding

We report on the optimization of chemical mechanical polishing (CMP) process for N-polar GaN surface. The impact of colloidal slurry pH, slurry concentration and the CMP process variables on the pit density, material removal rate (MRR) and surface morphology were investigated in this work. It was found that the CMP process with the adjusted slurry pH (pH 7) and the reduced concentration of solution exhibited a drastically decreased pit density and the smooth surface morphology (<1 nm) with a MRR of 433 nm / min. Further, the effect of variation in the downforce was investigated on the MRR and surface morphology. At a download force of 3.1 N, a pit-free smooth surface morphology of less than 1 nm was established with a MRR of 45 nm/min. Thus, the optimized CMP process was obtained for N-polar GaN surface in order to obtain a pit-free surface with a smooth surface morphology (RMS roughness < 1 nm), suitable for bonding.