Effect and mechanism of oxidant on alkaline chemical mechanical polishing of gallium nitride thin films

Abstract

As a wide band gap semiconductor, gallium nitride (GaN) is widely used in kinds of electronic devices. With the improvement of device accuracy, the requirements for GaN surface processing efficiency and quality are getting higher and higher. However, due to the high hardness and high chemical stability of GaN, the processing is very difficult. Chemical mechanical polishing (CMP) is one of the high-precision processing method. The slurry plays a vital role in the GaN CMP process, especially the oxidant. As common oxidants, hydrogen peroxide (H2O2) and potassium persulfate (K2S2O8) are widely used, but their efficiency on typical GaN CMP still need to be promoted. A novel oxidant potassium peroxomonosulfate sulfate (2KHSO5·KHSO4·K2SO4) known as Oxone was also used to promote polishing efficiency and achieve a balance between efficiency and surface roughness (Sq). In order to explore the influence trend and mechanism of these three oxidants on polishing efficiency under different pH conditions and the same pH conditions after optimization on GaN CMP, a series of polishing, electrochemical and related test experiments as X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM) were carried out. The results indicated that the CMP efficiency of these oxidants extremely affected by the pH values and their concentrations in alkaline slurry. The best pH values for CMP efficiency are 10, 10 and 8 for Oxone, H2O2 and K2S2O8, respectively. In addition, comparing the chemical characteristics of H2O2 and K2S2O8 with Oxone, mixed oxidants of H2O2 and K2S2O8 with different proportions were used and the results indicated that the mixed oxidant of H2O2 and K2S2O8 had better CMP efficiency than either H2O2 or K2S2O8. Sorting from largest to smallest, the polishing efficiency was Oxone, mixed oxidant, K2S2O8 and H2O2 while the surface roughness was K2S2O8, mixed oxidant, Oxone and H2O2. Considering typical CMP efficiency and average surface roughness, when Oxone concentration was 0.5 wt% at pH 10, the material removal rate of 247.92 nm/h and surface roughness of 0.547 nm was obtained simultaneously. Such results have important industrial application value for GaN CMP.