Silica, isoelectric point colloids

Abrasive particles are a key component in CMP slurry. The most commonly used abrasive particles include silica, alumina, ceria, zirconia, titania, and diamond. Table 21.1 listed a set of information on each type of abrasive particles such as density, microhardness, and isoelectric points (lEP). It is important to point out that the specific values for these properties depend highly on the preparation techniques and the specific states of the samples. The values listed in the table represent an average of the most commonly reported data. For example, the isoelectric point for silica is a function of the number of hydroxyl groups, type and level of adsorbed species, metal impurity in the solid matrix, and the treatment history of the materials [1]. There are three major types of silica according to their preparation methods fumed, colloidal, and precipitated. The common sources for obtaining these abrasive particles are listed in Table 21.2. As examples, some of the more specific information on... 

The zeta potential of slurry particles was measured as shown in Figure 1. Fumed silica showed a higher isoelectric point (lEP) at which the net charge and electrophoretic mobility is zero, than that of colloidal silica as shown in Figure 1 (a). Also zeta potentials of colloidal silica were around 20mV lower than that of fumed silica. Figure 1 (b) shows the zeta potentials of alumina, ceria and Mn02 particles. Due to their lower hardness than alumina, ceria has been... 

Franks, G.V., Zeta potentials and yield stresses of silica suspensions in concentrated monovalent electrolytes Isoelectric point shift and additional attraction, 7. Colloid Intetf. Sci., 249,44, 2002. 

FIGURE 20.4 Schematic variation of the DLVO theoretical stability domain for a pHjep colloid of pH 2 (critical coagulation concentration, c.c.c. isoelectric point, i.e.p.). The insert shows this theoretical prediction compared to that observed for silica sols. 

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