Photochemically combined mechanical polishing of N-type gallium nitride wafer in high efficiency

Abstract

The application of conventional chemical mechanical polishing (CMP) to super inert N-type gallium nitride (GaN) wafer suffers from low material removal rate (MRR). In this work, we described and tested a photochemically combined mechanical polishing (PCMP) strategy to promote the MRR following a photo-assisted (PA) oxidation mechanism. The utilization of ultraviolet (UV)-light to irradiate GaN generates electron-hole pairs, while the use of proper oxidants to extract the conduction band (CB) electrons leads the valence band (VB) holes to be able to oxidize GaN. By means of the PCMP prototype we designed, the features of proper oxidants were investigated and the key issues on MRR and surface roughness (Ra) were clarified. Results show that PCMP generates much higher MRR than CMP, but requires certain special oxidants because common redox oxidants, such as H2O2, are preferentially oxidized by the VB holes. When polishing solution (pH = 1.5–13.5) includes 0.1 M K2S2O8 oxidants and 2 wt% SiO2 abrasives, the MRR reaches 180–254.7 nm/h and the lowest Ra attains 0.76 nm (5 × 5 μm2). Although the photo-corrosion of GaN wafer itself may cause the surface roughening, the enhancement of the mechanical polishing process to MRR can decrease Ra, revealing a key factor in the further optimization of the PCMP system and equipment. In conclusion, the present study confirms that PCMP is a promising approach in polishing GaN wafer in high efficiency.