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The effect of size and concentration of ceria abrasive particles (untreated and treated with a poly-acrylic acid (PAA) additive used as a dispersant) on oxide and nitride polish rates for STI CMP was investigated. It was observed that a significantly high oxide polish rate (~ 350-370 nm min-1 at 4 psi down pressure) was obtained for both 60 nm and 45 nm ceria particles at a low concentration of 0.25 to 0.375% by wt. The corresponding nitride polish rates were also relatively high (~80-100 nm min-1). In contrast, for 25 nm ceria particles, both the oxide and nitride polish rates were low (<15 nm min-1) at all the studied concentrations. In case of ceria treated with PAA, the highest oxide polish rate (~250 nm min-1) was obtained for 1% ceria concentration. The addition of proline to this dispersion suppressed nitride polish rates, leading to a (oxide:nitride) selectivity that is as high as ~150:1.