Abrasive-free formulations and role of abrasives for barrier CMP

4 Abrasive-free formulations and role of abrasives for barrier CMP 

Tantalum (Ta) and tantalum nitride (TaN) are particularly suitable materials for use in the damascene process as adhesion-promoting and/or diffusion barrier layers for copper-based devices. However, the properties of Ta and of TaN differ from those of copper, being considerably more chemically inert, such that abrasive-free polishing compositions useful for the polishing of copper are often unsuitable for the removal of underlying Ta and TaN. Hence, independent chemistries are often developed to clear Ta/TaN without further dishing of interconnect metal (Cu) or dielectric loss. 

Brown et al. [ 11 ] developed a slimy formulation containing potassium periodate, an oxidizer, and silica abrasives for polishing ruthenium (Ru) films at pH 9. Using silica slurries (1—5 wt%) without any oxidizer, the polish rates of Ru films were close to 0 nm/min suggesting that the mechanical action from abrasives was not strong enough to abrade the Ru surface due to its relative hardness compared to silica abrasives. Also, using an abrasive-free solution of potassium periodate (0.01 —0.1 M), the removal rates were also quite low despite the electrochemical reactions at Ru surface. 

Potassium periodate oxidizes the Ru surface into Ru oxides, such as RUO4, Ru04, and Ru04, as shown in the reaction equations below  

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