Nano-Metric Enhancement of Surface Roughness of Silicon Nitride balls (Si3N4) by Chemical Mechanical Polishing (CMP)

Chemical Mechanical Polishing (CMP) is one of the nano-metric polishing processes of non-metal balls and wafers such as silicon nitride balls and various ceramics. In this process, abrasive particles (Fe2O3, CeO2 and ZrO2) which have lower hardness than workpiece are floated in some fluid. In this state, the workpiece reacts chemically with base fluid (water, air, hydrogen peroxide and or oil), forming a thin layer of silica (SiO2) on the surface of the workpiece. Removal of the thin oxidized surface layer of the workpiece by abrasive particles, as a result of the chemical reaction, is easily done. Effective parameters of Chemical- Mechanical Polishing include 1- density of abrasive particles, 2- spindle speed of milling machine, 3- polishing time and 4- kind of abrasive particles which by altering of each, factors such as 1- surface roughness, 2- sphericity and 3- material removal rate change. In the present research, a laboratory device is designed and manufactured for chemically- mechanically polishing of silicon nitride balls and 24 experiments are designed by Minitab software and done to investigate effects of the mentioned factors on surface roughness and material removal rate. For each of the factors two levels is set, after variance analysis of the experimental results, regression equations of the surface roughness and material removal rate were achieved. By increase of polishing time, spindle speed of milling machine and density of all three kinds of abrasives during experiments, material removal rate increased and surface roughness enhanced. Morphology of surface roughness by X-Ray photo-electron spectroscopy (XPS) was studied. Probability of chemical reactions by Gibbs free energy equations was also investigated. Eventually, approaches for optimized polishing of ceramic balls were proposed due to the experimental conditions and analysis of variance. Comparison of the experimental results showed that the surface roughness of balls polished by the Fe2O3 abrasive particles (Ra= 69 nm) was far better than the other two abrasives, and equals.