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.