We report on the electrochemical process and nucleation mechanism of the “direct-on-barrier” electroplating of copper on ultrathin cobalt adhesion layer from an alkaline CuSO4 and ethylenediamine (En) bath without additives. Results show that the corrosion current density was much lower in the alkaline bath than that in the commercial acidic bath due to formation of the CuEn22+ species in the aqueous CuSO4-En solution. The CuEn22+ species can well stop the reaction between the copper ions and Co by reducing the potential of copper ions. The current-time transients were not typical of a diffusion-limited nucleation and growth. Pure instantaneous or progressive nucleation of copper on Co layer was not observed. The impact of current density on the orientation, resistance and morphology was studied. Copper film in 250 nm thick was successfully deposited on the 10 nm Co film in the patterned wafers. And Cu was electroplated onto ultrathin Co layer with ∼5 nm thickness and TEM result shows that there is no obvious Co thickness loss.

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