Ceria nanoparticles are used in chemical mechanical planarization (CMP) slurry for their selective removal of oxides over nitrides. This is the result of a reversible redox reaction at the surface of the particles allowing these particles to participate in both the chemical and mechanical aspects of planarization. A key property that determines the ability of ceria to undergo this reaction is the ratio of Ce3+/Ce4+ on the surface of the particles. As this ratio increases, so does the interaction with the oxide surface, resulting in an increased removal rate. Most studies to date focus on how synthesis and particle size effect these ratios but ignore the changes that could arise when introducing the particles to a slurry environment.
In this study, X-ray photoelectron spectroscopy (XPS) was used to measure the ratio of Ce3+/Ce4+ as a function of several slurry properties such as pH, oxidizing agent and surfactant. The effects of these properties were examined in virgin and aged slurries, and as a function of concentration and type.
It appears that slurry age and pH have little effect on the particles, but much depends on the peak fitting method, which is not yet a settled question in the XPS community. We discuss these findings and their implications for the specific design of future slurries.