ON THE CHEMOMECHANICAL POLISHING (CMP) OF SI3N4 BEARING BALLS WITH WATER-BASED CEO2 SLURRY

Among various abrasives investigated for the chemo-mechanical polishin g (CMP) of Si3N4 balls (Jiang, 1998), cerium oxide (CeO2) was found to be the most effective polishing medium (even superior to Cr2O3, Bhaga vatula and Komanduri, 1996), yielding an extremely smooth and damage-f ree surface with a finish R-a of approximate to 4 nm and R-t of approx imate to 40 nm. In this investigation, the underlying reasons for the superior finish with CeO2 were investigated. Various chemical reaction s involved in CMP of Si3N4 balls with CeO2 were investigated (Gibbs fr ee energy minimization) and a mechanism for the CMP is proposed. The t wo important functions that CeO2 performs in the CMP of Si3N4 are: 1. It participates directly in the chemical reaction (oxidization-reducti on reaction) with Si3N4 workmaterial leading to the formation of a thi n SiO2 layer, 2. The hardness of CeO2 is closer to that of the thin Si O2 layer formed on Si3N4 but significantly lower than Si3N4 workmateri al (approximate to 1/3). It can thus remove the brittle SiO2 reaction product effectively, without damaging the Si3N4 substrate as no abrasi on 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 c hemical reaction could proceed on a continuing basis so long as the pa ssivating layers are removed by the mechanical action at the same time . CeO2 is found to be very effective in a water environment (hydrolysi s) leading to the formation of additional SiO2 by reacting with Si3N4 thereby enhancing the CMP of Si3N4. Several similarities between polis hing of Si3N4 and glass (SiO2) (Cook, 1990), including the polishing e nvironment (CeO2 plus the magnetic fluid, pH value approximate to 6) a nd 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).