Silicon dioxide isoelectric point

The pH of a slurry has a profound influence on its colloidal stability and CMP performance. Strong correlations have been established between the particle isoelectric point (lEP) and the optimal pH for slurry stability. The general rule is that the slurry is more stable at a pH that is away from the lEP, so the zeta potential of the particles is greater than 20 mV. The focus of this section is on the influence of pH on the slurry performances such as material removal rate and defectivity. In order to examine the impact of slurry pH on these two important performance features, we first take a closer look at the interaction between abrasive particles and the surface to be polished. There is a vast amount of literature on the interaction between abrasive particles and silicon dioxide surface [26]. The discussion below will focus on the interaction between ceria abrasive particles and the silicon dioxide surface to be polished. The basic principles and conclusions can be easily extended to other pairs of abrasive particles and surfaces. 

The results of the OWLS experiments (see Figure 3) indicate that the PLL-g-PEG polymer spontaneously adsorbed from a pH 7.4 buffered aqueous solution onto metal oxide surfaces. The example shown in Figure 3 involved the adsorption of PLL-g-PEG onto three different metal oxide surfaces, specifically, titanium, niobium, and silicon/titanium. Tbis adsorption process occurred rapidly and resulted in the formation of a layer of adsorbed polymer on the surface. Typically, for Sio4Tio602 smfaces, a layer with an adsorbed areal density of approximately 125 ng/cm formed, and 95% of the final observed mass was reached within the first 5 min. Similar behavior was observed for the two other metal oxide surfaces investigated (that is, niobium pentoxide and titanium dioxide). Although the adsorption kinetics were quite similar, the resulting amount of PLL-g-PEG adsorbed to the surface was different and depended on the characteristic isoelectric point of the metal oxide, as shown in Figme 4 and Table 4. 37,38,46.48,49... 

Maximal polymer adsorption occurs within a pH range between the isoelectric point of the surface and the pA/a of the polymer. Experiments performed at different pH values, as described in section 2.4.2, showed that negligible PLL-g-PEG adsorption is observed at pH values higher than the pA/a of the polymer (pH 10) or lower than the isoelectric point of the metal oxide surface. The experimental data shown in Figure 7 demonstrate that, for a silicon/titanium dioxide surface, PLL-g-PEG adsorption takes place only in the pH range between 3 and 11. Outside of this range, negligible adsorption is observed. 

Electrostatic Forces— The AFM Tip. The most frequently used commercial AFM tip consists of microfabricated silicon nitride. Under normal laboratory conditions, this is, in reality, an ill-defined, hydroxylated melange of silicon oxynitride and silicon dioxide, the precise composition of which depends on a combination of manufacturing conditions and tip history. An isoelectric point of around pH=5 has been firequently observed in force-distance measurements under electrolyte in our laboratory (15), suggesting the oxidized oxynitride composition mentioned above. In order to achieve a better reproducibility, however, we have adopted an oxygen plasma-treatment procedure, which leaves the tip coated with hydroxylated silica, as evidenced by an isoelectric point of around pH=3 (16), as measured by force titration (75). 

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