Continuous layers of 20 nm of copper have been deposited on a tantalum substrate from the liquid cationic cuprous organic complex [Cu(MeCN)(2)][Tf(2)N]. This type of ionic liquid, with a high concentration of copper(I) ions permits to achieve high nucleation densities. Furthermore, extremely high overpotentials (up to 5.0 V) can be applied without decomposition of the ionic liquid. The deposition of copper has been investigated by cyclic voltammetry (CV), atomic force microscopy (AFM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The resulting copper deposits can be useful as seed layers for aqueous copper filling. (C) 2011 The Electrochemical Society. [DOI: 10.1149/2.035111jes] All rights reserved.