Cerium oxide slurries in CMP. Electrophoretic mobility and adsorption investigations of ceria/silicate interaction

The interaction between ceria particles and silicate ions was studied in order to further understand the mechanism of chemical-mechanical polishing (CMP) of silicon-based materials. Electrophoretic mobility (zeta potential) and adsorption measurements were the techniques utilized. In the presence of silicate ions, the zeta potential of ceria particles changed from positive to negative; in addition, the isoelectric point IEP of the particles shifted to lower pH values with increasing silicate addition. The adsorption density of silicate ions on ceria was found to increase as the concentration of silicate ions in the system increased. Also, with increase in pH, silicate adsorption went through a maximum at about pH 9, then decreased rapidly at higher pH values. The adsorption and zeta potential results are rationalized by considering the solution chemistry of the dissolved silicate species, the protonation/deprotonation of ceria surface, and the accompanying surface complexation reactions. These experimental observations demonstrate a strong interaction between ceria particles and silicate ions. The implication is that adsorption of silicate ions on ceria may be directly involved in the removal mechanism of silicon-based materials during the CMP process.