On the chemo-mechanical polishing (CMP) of Si3N4 bearing balls with water based CeO2 slurry

Among various abrasives investigated for the chemo-mechanical polishing (CMP) of Si3N4 balls (Jiang, 1998), cerium oxide (CeO2) was found to be the most effective polishing medium (even superior to Cr2O3, Bhagavatula and Komanduri, 1996), yielding an extremely smooth and damage-free surface with a finish R-a of approximate to 4 nm and R-t of approximate to 40 nm. In this investigation, the underlying reasons for the superior finish with CeO2 were investigated. Various chemical reactions involved in CMP of Si3N4 balls with CeO2 were investigated (Gibbs free energy minimization) and a mechanism for the CMP is proposed. The two important functions that CeO2 performs in the CMP of Si3N4 are: 1. It participates directly in the chemical reaction (oxidization-reduction reaction) with Si3N4 workmaterial leading to the formation of a thin SiO2 layer, 2. The hardness of CeO2 is closer to that of the thin SiO2 layer formed on Si3N4 but significantly lower than Si3N4 workmaterial (approximate to 1/3). It can thus remove the brittle SiO2 reaction product effectively, without damaging the Si3N4 substrate as no abrasion can take place by CeO2 on Si3N4. The kinetic action, which involves the removal of the reaction products from the interface by subsequent mechanical action of flowing water and CeO2 is critical to CMP. The chemical reaction could proceed on a continuing basis so long as the passivating layers are removed by the mechanical action at the same time. CeO2 is found to be very effective in a water environment (hydrolysis) leading to the formation of additional SiO2 by reacting with Si3N4 thereby enhancing the CMP of Si3N4. Several similarities between polishing of Si3N4 and glass (SiO2) (Cook, 1990), including the polishing environment (CeO2 plus the magnetic fluid, pH value approximate to 6) and the mechanism of polishing were observed. Also, after investigating various reaction species in the CMP of Si3N4 with CeO2 and Cr2O3, the former is found to be much safer from an environmental point of view (Redy and Komanduri, 1998).