Polishing slurries were prepared by

Polishing slurries were prepared by first adding ethylenediaminetetraacetic acid disodium (EDTA-2Na) to deionized (DI) water until dissolved, immediately followed by the mixture with colloidal silica under successive stirring. EDTA-2Na was used as the chelating agent to capture free metal cations in solution so that the bulk concentration of metal cations could keep a low level. The colloidal silica has a diameter of about 110 nm (Fig. 1(a)), which was made by an ion-exchange method. Next, hydrogen peroxide (H2O2) of 30 wt.% was added in as the oxidant. The amount of H2O2 was precisely calculated and controlled. Finally, DI water was added to dilute the slurries to the desired concentration, and a small amount of sodium hydroxide solution was used as a pH adjustion. The final compositions of the slurries are listed in Table 1.