Cu was electrodeposited directly on W diffusion barrier surface at a constant potential using a neutral Cu-NH3-citrate (Cit) electrolyte for Cu interconnect in Si-based integrated circuit devices. Fundamental study on the nucleation and growth of Cu on blanket Si wafer specimen with top W layer was carried out with changing cathodic deposition potential. The procedure of potentiostatic Cu growth on W was understood using chronoamperometry experiments. Experimental current transient curves were compared with Scharifker and Hills' theoretical curves of instantaneous and progressive nucleation along with SEM observation of Cu deposits. More concentrated electrolytes in presence and in absence of Cu ions were also used for comparison because the Scharifker and Hills' model does not explain adequately Cu nucleation in low concentration supporting electrolyte. A statistical method assisted by image analysis of Cu deposits was used to understand Cu nucleation behavior at low cathodic potentials. Cu reduction from Cu-Cit complexes such as Cu-2(Cit)(2)OH3-, CuCit(-), and Cu-2(Cit)OH at potentials more negative than -0.8 V facilitates the formation of a Cu film on W surface, although additional Cu clusters can be subsequently created on the Cu film for a long deposition time. Consequently, uniform Cu thin films with smooth surface morphology were electrodeposited directly on W diffusion barrier in a neutral electrolyte by selecting appropriate deposition potential and time. (C) 2014 Elsevier Ltd. All rights reserved.