Cross-Scale Synergistic Control of Multifields and Motions in Electrochemical Mechanical Polishing (ECMP) for Atomic-Scale GaN Surface

Gallium nitride (GaN), crucial for 5G and electric vehicles, faces ultraprecision polishing challenges due to its hardness, brittleness, and inertness. Achieving efficient, uniform material removal for damage-free atomic-scale surfaces remains difficult, particularly addressing inherent nonuniformity in conventional processes. This study developed an integrated electrochemical mechanical polishing (ECMP) system synchronizing the electric field, pressure field, and abrasive trajectory optimization. The design of a copper-polytetrafluoroethylene (PTFE) composite anode achieved matched electric potential and pressure gradients, resolving removal nonuniformity. Kinematic simulations optimized the polishing head oscillation for complete abrasive coverage. Experimental parameter optimization identified the ideal conditions. The ECMP system attained excellent surface uniformity (material removal uniformity MRU = 0.013), atomic-scale smoothness (surface roughness R_a = 0.118 nm), and high material removal rate (MRR = 926 nm/h), significantly outperforming conventional methods. In addition, the material removal mechanism of GaN in the ECMP process was investigated by atomic force microscopy (AFM) and molecular dynamics (MD) simulations. It was found that etched GaN wafers consistently exhibit greater indentation depths than unetched wafers across all loads, which is attributed to the electrochemically softened oxide layer in material removal. The reductions in Young's modulus in compression and nanohardness, as well as the high intensity of the Ga-O peak in X-ray photoelectron spectroscopy (XPS), also indicate the extensive formation of gallium oxide during electrochemical etching, which was further confirmed by AFM friction experiments. This study provides a theoretical and technical foundation for efficient and damage-free GaN wafer manufacturing, advancing the translation of ECMP technology from laboratory research to industrial applications.