Citric Acid as a Complexing Agent in Chemical Mechanical Polishing Slurries for Cobalt Films for Interconnect Applications

Abstract

A colloidal silica-based slurry (3–10 wt%) containing H2O2 (1 wt%) and citric acid (50 mM) was found to polish chemical vapor-deposited (CVD) cobalt (Co) films with removal rates (RRs) of ∼180–500 nm/min and a dissolution rate (DR) of ∼0 nm/min at pH 8 along with an RMS roughness of ∼0.5 nm and a corrosion current of ∼50 μA/cm2. Our results suggest that, in the presence of H2O2, the Co film surface was covered with a passive film of CoO in acidic conditions and Co3O4 in alkaline conditions. However, in the presence of H2O2 and citric acid in acidic conditions, formation of the soluble complex [Co(C6H5O7)2]3− from abraded Co enhanced the RRs significantly. The roles of H2O2, citric acid, and silica abrasives as well as the pH on the Co material removal process are discussed and a removal mechanism is proposed. Two inhibitors namely, 1, 2, 4- Triazole (TAZ) and Benzotriazole (BTA), were tested in the presence of 50 mM citric acid at pH 8 but were found to be ineffective even at concentrations of 100 mM in reducing the ΔEcorr of Co-Ti couple to minimize galvanic corrosion that is essential when the Co/Ti structure is polished after the removal of bulk of Co and requires further study.