On chemo-mechanical polishing (CMP) of silicon nitride (Si3N4) workmaterial with various abrasives

Chemo-mechanical polishing (CMP) studies were conducted using various abrasives [boron carbide (B4C), silicon carbide (SiC), aluminium oxide (Al2O3), chromium oxide (Cr2O3), zirconium oxide (ZrO2), silicon oxide (SiO2), cerium oxide (CeO2), iron oxide (Fe2O3), yttrium oxide (Y2O3), copper oxide (CuO), and molybdenum oxide (Mo2O3)] to investigate their relative effectiveness in the finishing of uniaxially pressed Si3N4 bearing balls by magnetic float polishing (MFP) technique. CMP depends both on the chemical and the mechanical effectiveness of the abrasive and the environment with respect to the workmaterial. Among the abrasives investigated for CMP of Si3N4 balls, CeO2 and ZrO2 were found to be most effective followed by Fe2O3 and Cr2O3. Extremely smooth and damage-free Si3N4 bearing ball surfaces with a finish R-a of approximate to 4 nm and R-t of approximate to 40 nm were obtained after polishing with either CeOl(2)or ZrO2. Thermodynamic analysis (Gibb's free energy of formation) indicated the feasibility of the formation of SiO2 layer on the surface of the Si3N4 balls with these abrasives. This is particularly so in a water environment which facilitates chemo-mechanical interaction between abrasive and workmaterial by participating directly in the chemical reaction leading to the formation of a softer SiO2 layer. Since the hardness of some of the abrasives which were found to be most effective in CMP, namely, CeO2, ZrO2, and Fe2O3 is closer to that of SiO2 layer but significantly lower than the hardness of the Si3N4 workmaterial, removal of the SiO2 reaction layer effectively without scratching and/or damaging the Si3N4 substrate is facilitated by the subsequent mechanical action of the abrasives. The chemical reaction would proceed on a continuing basis only if the passivating layers are removed continuously by subsequently mechanical action. It was found that the CMP ability in an oil-based polishing environment to be rather limited. A mechanism similar to the CMP of Si3N4 may be applicable to the polishing of silicon wafers, various glasses, and SIC due to similarities in the material removal processes. (C) 1998 Elsevier Science S.A. All rights reserved.