Chemical-mechanical polishing performance of core-shell structured polystyrene@ceria/nanodiamond ternary abrasives on sapphire wafer

Driven by electrostatic attraction, Ce4+ ions or/and positively charged detonation nanodiamond (DND) particles can absorb onto negatively charged polystyrene (PS) spherical colloids. Three types of core-shell structured composite abrasives, PS@CeO2, PS@DND and PS@CeO2/DND, can thus be assembled. When PS@CeO2 and PS@DND were used to polish sapphire wafer at pad rotating speed of 120-150 r/min and load pressure of similar to 3 kg, the material removing rate (MRR) exceeded 1.0 mu m h(-1), 10-20 % higher than unitary abrasives. The surface profile roughness (Ra) for wafer polished by these two composite abrasives was respectively 1.25 and 0.63 nm, which is superior to CeO2 (Ra = 1.38 nm) and DND (Ra = 1.29 nm). When using PS@CeO2/DND, the polishing interface area can be increased owing to the combined effect of elastic PS spheres and intensively coated CeO2 and DND. Meanwhile, the synergistic mechanism of sapphire-CeO2 chemical reaction and the strong mechanical abrasion of DND particles benefit the polishing efficiency. MRR for this ternary composite abrasive attained 1.4-1.7 mu m h(-1) while sapphire can be smoothed to a sub-nanoscale roughness.