Effect of oxalic acid (complexing agent) on anodic dissolution of Cobalt in hydrogen peroxide solutions: mechanism and kinetic analysis by electrochemical impedance spectroscopy

The anodic dissolution of Cobalt in H2O2 solution is investigated in the presence and absence of a complexing agent, oxalic acid, using various techniques including electrochemical impedance spectroscopy (EIS). Anodic polarization measurements of both the solutions show that active dissolution occurs in the potential range of 0 to 600 mV w.r.t open circuit potential (OCP) and the addition of oxalic acid enhances the dissolution of Co by forming highly soluble Cobalt complexes. The mechanistic reaction pathway of Co dissolution at metal-solution interface is investigated by performing EIS measurements at various overpotentials under anodic conditions. EIS spectra exhibit two loops; capacitance (higher frequency) followed by inductance (lower frequency) at all the overpotentials and it was modeled by a multi-step mechanism with 3 intermediate adsorbed species. The dissolution via both chemical and electrochemical steps is considered in the proposed model. From the parameters obtained, dominance of Co-ad(2+) species on the Co metal surface is observed for both the systems. The oxides/hydroxides formed on the Co surface on the addition of oxalic acid to H2O2 are higher than using only H2O2, thus properly justifying the role of a complexing agent in a CMP slurry. Products formed on exposure of Co to H2O2 and H2O2-oxalic acid solution at pH 9 are analyzed using X-ray photoelectron spectroscopy (XPS) analysis. The results confirm the formation and dominance of Co-oxalate complexes in H2O2-oxalic acid system.