Role of ionic strength in chemical mechanical polishing of silicon carbide using silica slurries

Silicon carbide thin films, though of widespread use in microelectronic engineering, are difficult to process by chemical mechanical polishing (CMP) because of their hardness and chemical inertness. This report discusses the development of slurries based on silica abrasives that resulted in high amorphous SiC (a-SiC) removal rates (RRs). The ionic strength of the silica dispersion was found to play a significant role in enhancing material removal rate, while also providing very good post-polish surface-smoothness. For example, the addition of 50 mM potassium nitrate to a pH-8 aqueous slurry consisting of 10 wt% of silica abrasives and 1.47 M hydrogen peroxide increased the RR from about 150 nm h−1 to about 2100 nm h−1. The role of ionic strength in obtaining such high RRs was investigated using surface ζ-potentials measurements and X-ray photoelectron spectroscopy (XPS). Evidently, H2O2 promoted the oxidation of Si and C to form weakly adhered species that were subsequently removed by the abrasive action of the silica particles. The effect of KNO3 in increasing material removal is attributed to the reduction in the electrostatic repulsion between the abrasive particles and the SiC surface because of screening of surface charges by the added electrolyte.