Controlling Galvanic Corrosion with Oxalic Acid and Imidazole for Chemical Mechanical Planarization of Cobalt-Copper Interface

The work focuses on the investigation of Co/Cu removal rate (RR) selectivity and reduction of galvanic corrosion associated with Co and Cu by using oxalic acid (weak acid) as the complexing agent and imidazole as an inhibitor in hydrogen peroxide (H2O2) and fumed silica-based slurry. The results obtained from dissolution study, polishing experiments and potentiodynamic polarization measurements revealed that the proposed chemistry can achieve a desirable Co/Cu RR selectivity and a significant decrease in corrosion potential of Co and Cu (pH 9) to be used in the semiconductor industry. The corrosion potential difference (CuEcorr - Co-Ecorr) was reduced to 12 mV by using 0.1 wt% H2O2 + 0.02 M oxalic acid + 5 ppm imidazole solution at pH 9. Meanwhile, a removal rate of similar to 147 nm min(-1) for Co and similar to 140 nm min(-1) for Cu was achieved using the same composition in a fumed silica slurry which resulted in Co/Cu selectivity ratio of 1.05:1, which is acceptable for cobalt barrier and copper interconnect CMP. Based on FTIR and UV spectra, the dissolution mechanism in the proposed chemistry is also discussed.