Quantitative study of oxidation mechanism in photoelectrochemical mechanical polishing of difficult-to-process semiconductor wafers

The excellent properties of gallium nitride (GaN), silicon carbide (SiC), and diamond make them the most promising semiconductor materials for the future. However, their extremely stable chemical properties and high hardness lead to a low efficiency in chemical mechanical polishing (CMP). Photoelectrochemical mechanical polishing (PECMP) is an efficient and high-quality machining method for difficult-to-process semiconductor materials, integrating photo, electric, chemical, and mechanical fields. However, the coupling of these fields creates complex mechanisms, making it difficult to quantitatively describe the oxidation mechanism driven by the electric field. As a result, selecting the appropriate applied voltage for specific polishing requirements is challenging. To address this, a detailed analysis of the transfer of electrons and holes at the wafer/solution interface was conducted, and an innovative relationship between variations in the energy field and the wafer surface potential in PECMP was established. For the first time, the Poisson equation was applied to the wafer/solution interface, and a novel theoretical model for the oxidation rate and applied voltage on the wafer surface in PECMP was developed. Specifically, at the voltage threshold, the surface charge type changes from electrons to holes, resulting in a significant increase in hole density. Finally, the model was validated through surface modification and PECMP tests. This research not only presents an innovative theoretical method for determining the applied voltage in photoelectric field-assisted polishing for any semiconductor material but also offers new insights into how surface charge transitions between electrons and holes under varying applied voltages can significantly influence polishing efficiency in photoelectric field-assisted polishing.