Formation of Cobalt-BTA Complexes and Their Removal from Various Surfaces Relevant to Cobalt Interconnect Applications

The use of benzotriazole (BTA) during Co film planarization leads to the undesirable formation of insoluble Co-BTA complexes, which can be observed as organic residues on various surfaces after polishing. Here, we investigated the formation of Co-BTA complexes at different pH values and their adsorption and removal from Co, TiN, SiN, and SiO2 film surfaces relevant to Co interconnect applications. The stability constant, log Keq, of Co2+-BTA complex was determined from its spectroscopic properties and log Keq was found to lie between 2.7 and 3.9 in the pH range of 6 ∼ 12, much smaller than that of Co3+-ethylenediaminetetraacetic acid (EDTA) complexes (log Keq = 36.0). Using this data, and recognizing that potassium persulfate (K2S2O8) and EDTA can lead to oxidation and ligand exchange of Co2+-BTA complexes, respectively, we were able to remove Co-BTA complexes adsorbed on all films by exposing them to aqueous solutions of 50 mM EDTA, 1.5 wt% K2S2O8, and 1 mM sodium dodecylbenzenesulfonate (SDBS) at pH 12. SDBS was helpful in lowering ΔEcorr to about 10 mV for the Co/TiN couple as well as the corrosion currents of Co film.