Polishing-pad-free electrochemical mechanical polishing of single-crystalline SiC surfaces using polyurethane-CeO2 core-shell particles

A novel method for electrochemical mechanical polishing (ECMP) of the single-crystalline SiC surface, which has extremely high mechanical and chemical strength compared to conventional electronic materials, is reported. The method does not employ a polishing pad; it comprises electrochemical oxidization of the SiC surface and subsequent removal of the oxide by CeO2 from the polyurethane-CeO2 core-shell particles. The core-shell particles are used to maintain a gap between the polishing plate (cathode) and the SiC wafer (anode), which enables efficient anodic oxidation of the inert SiC surface. The core-shell particles, composed of the elastic polyurethane core covered with an abrasive layer of small and soft CeO2 particles prepared by a simple and low-cost process, can be used to obtain a smooth SiC surface without using a polishing pad. The ratio of polyurethane to CeO2 in the core-shell particles is optimized to obtain core particles that are fully covered with the shell particles without leaving excess CeO2 particles. Using the fabricated core-shell particles, the conventional CMP process is unable to remove the SiC surface without anodization. While a continuous bias during polishing produces a rough SiC surface owing to the oxide film remaining on the treated surface, as confirmed by current measurements and X-ray analysis, a periodically applied bias, whose conditions were determined by the theoretical growth rate and residual thickness of the oxide film, reduces the number of scratches, and a smooth surface with sub-nanometer roughness is obtained. The obtained value of surface roughness is in good agreement with the calculated value determined using conventional grinding theory. Compared to a conventional polishing process with a colloidal SiO2 slurry, the proposed method shows superior polishing efficiency without the need for a polishing pad. SEM observation of the core-shell particles shows that the particles have durability against the strong electric field between the electrodes.