ON CHEMOMECHANICAL 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 oxid e (SiO2), cerium oxide (CeO2), iron oxide (Fe2O3), yttrium oxide (Y2O3 ), copper oxide (CuO), and molybdenum oxide (Mo2O3)] to investigate th eir relative effectiveness in the finishing of uniaxially pressed Si3N 4 bearing balls by magnetic float polishing (MFP) technique. CMP depen ds both on the chemical and the mechanical effectiveness of the abrasi ve and the environment with respect to the workmaterial. Among the abr asives investigated for CMP of Si3N4 balls, CeO2 and ZrO2 were found t o be most effective followed by Fe2O3 and Cr2O3. Extremely smooth and damage-free Si3N4 bearing ball surfaces with a finish R-a of approxima te to 4 nm and R-t of approximate to 40 nm were obtained after polishi ng with either CeOl(2)or ZrO2. Thermodynamic analysis (Gibb's free ene rgy of formation) indicated the feasibility of the formation of SiO2 l ayer on the surface of the Si3N4 balls with these abrasives. This is p articularly so in a water environment which facilitates chemo-mechanic al interaction between abrasive and workmaterial by participating dire ctly in the chemical reaction leading to the formation of a softer SiO 2 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 t o that of SiO2 layer but significantly lower than the hardness of the Si3N4 workmaterial, removal of the SiO2 reaction layer effectively wit hout scratching and/or damaging the Si3N4 substrate is facilitated by the subsequent mechanical action of the abrasives. The chemical reacti on would proceed on a continuing basis only if the passivating layers are removed continuously by subsequently mechanical action. It was fou nd that the CMP ability in an oil-based polishing environment to be ra ther limited. A mechanism similar to the CMP of Si3N4 may be applicabl e to the polishing of silicon wafers, various glasses, and SIC due to similarities in the material removal processes. (C) 1998 Elsevier Scie nce S.A. All rights reserved.