Achieving a high Si₃N₄ removal rate and a relatively low SiO₂ removal rate, which is challenging but promising for emerging applications for sub-32 nm node devices, was investigated. This was thought to be impossible as the removal of Si₃N₄ usually follows a two-step mechanism in which silicon nitride is hydrolyzed first and then removed during CMP. Therefore, additives normally used to suppress the SiO ₂ removal rate would tend to suppress the S₃N₄ removal rate as well. However, it was shown that by using a specific type of a cationic polymer in ceria-based dispersions, a low SiO₂ removal rate (<2 nm/min) and relatively high Si₃N₄ removal rate (∼120 nm/min) can be achieved. These results have been extended to processes involved in MEMS and FinFET fabrication, where a polysilicon layer has to be selectively polished/protected with respect to silicon dioxide and/or silicon nitride layers. Several dispersions were identified which yield tunable removal rates of polysilicon (from <2 nm/min to ∼1 µm/min), silicon dioxide (<2 nm/min to ∼500 nm/min) and silicon nitride (<2 nm/min to ∼120 nm/min) films. This has been made possible by using several additives in ceria and silica based dispersions with and without surface functionalization at different pH values.

A fundamental investigation of the interaction of the additive(s) with the abrasives, SiO₂ Si₃N₄, and polysilicon films was also carried out in order to elucidate the removal mechanisms. Zeta potential measurements, UV-Vis Spectroscopy, adsorption isotherms and thermo gravimetric analysis were performed to understand the adsorption behavior of these additives on abrasives and polishing films at different pH values. It was observed that the Ce³⁺ on the surface of the ceria abrasives is reacting with the silicon dioxide and suboxide on the silicon nitride surfaces during polishing. Also, it appears that electrostatic interactions in conjunction with the reactivity of the active sites on the surface of abrasives play a vital role in determining the removal rates of silicon dioxide, silicon nitride and polysilicon films.