Polyethylene glycol (PEG) and Janus Green B (JGB) have been widely used as suppressor and leveler, respectively, for copper electrodeposition to fill damascene structures of highly integrated silicon devices. In this study, we investigated fundamental nucleation behavior of copper electrochemically deposited on tungsten diffusion barrier without using a copper seed layer in a citrate-based, neutral electrolyte. Concentrations of PEG and JGB were varied to optimize the nucleation and growth of a thin and uniform copper film on tungsten, ultimately considering the application of this copper electrodeposition method to copper fill of sub-45 nm damascene structures. Important fundamental properties of copper nucleation directly on tungsten such as the nuclei density, size distribution, 3D nucleation mode, and surface roughness were investigated when forming a uniform copper film thinner than 30 nmon tungsten by manipulating additive composition in electrolyte. In consequence, a thin and smooth copper film was electrodeposited resulting from instantaneous nucleation and high area density of copper clusters in the electrolyte including 10 mu M PEG and 10 mu M JGB. (C) 2013 Elsevier B.V. All rights reserved.