Effect of photocatalytic oxidation technology on GaN CMP

GaN is so hard and so chemically inert that it is difficult to obtain high MRR in CMP process, thus photocatalytic oxidation technology was adopted in this study to realize efficient removal. Two kinds of N type semiconductor particles, which are TiO₂ and SnO₂, were added to the H₂O₂-SiO₂-based slurry working as the catalysts. By optical excitation, highly reactive photoinduced holes will be produced on the surface of the catalytic particles, which can oxidize OH-and H₂O absorbed on the surface of the catalysts, therefore more OH* will be generated. Compared with SnO₂, TiO₂ has larger specific area and smaller band gap, which are in favor of photocatalytic oxidation reaction. Consequently, GaN MRRs in H₂O₂-SiO₂-based polishing system combined with catalysts are improved significantly, especially using TiO₂, the MRR of which is 122nm/h. XPS analysis shows the variation trend of chemical composition on GaN surface after polishing, with more surface defects removal, the ratio of metallic Ga is lower and the ratio of GaN is higher when using TiO₂, which reveals the planarization process. Besides, physical removal model is proposed to describe the removal mechanism of GaN when using photocatalytic oxidation technology. As more OH* are generated in the slurry, the thickness of the oxide layer increases, which contribute to the mechanical grinding process by SiO₂, hence GaN MRR was improved.