Effect of Relative Surface Charge of Colloidal Silica and Sapphire on Removal Rate in Chemical Mechanical Polishing

Many studies have looked at the chemical mechanical polishing (CMP) process of sapphire substrates. However, the research on the processing mechanism of the sapphire substrate is insufficient compared with semiconductor CMP processing. This paper focuses on the effect of the slurry pH on the removal rate of sapphire substrate in CMP. When the pH of the slurry is changed, the possible factors that can influence removal rate of sapphire include the zeta potential, abrasive agglomeration, and hydration reaction layers. The colloidal silica slurry used in this study did not show aggregation at any pH value. However, the zeta potential between the abrasive and the substrate changed remarkably at acidic and basic pH. The attractive force between the abrasive and substrate in acidic conditions is higher than that in basic conditions due to the relative surface charge. A higher attractive force caused by opposite charges makes more of the abrasive participate in the polishing process, which increases the removal rate in acidic conditions. However, the removal rate in basic conditions is higher than that in acidic conditions despite the repulsive relative charges. The reason for the higher removal rate in basic conditions seems to be the easier formation of a hydration layer, which is caused by the higher concentration of [OH]− in basic conditions. If the hydration effect is negligible, then the removal rate strongly depends on the magnitude of relative surface charges, which can be defined by the product of the zeta potentials of the abrasives and the substrate. If the product of the potentials is large, the probability of contact between the abrasives and the substrate is increased. Thus, the removal rate is increased. However, if the hydration layer forms, it plays a dominant role in determining the removal rate.