The adsorption mechanism of thiourea (TU) and its effect on Cu electrodeposition were verified using TU derivatization. Contrary to the previously reported behavior of TU as an inhibitor, TU either promoted or inhibited Cu reduction according to the derivatization time. During the short derivatization time, the adsorbed TU with low surface coverage was oxidized to spontaneously reduce Cu2+ to Cu+, thereby, accelerating the Cu deposition. However, TU inhibited the Cu deposition as the coverage of TU-Cu+ increased with the derivatization time. Furthermore, a deterioration in the resistivity of the Cu film, which occurred when TU was added, was largely improved by the derivatization method while maintaining an enhancement in the film hardness. TU derivatization resulted in a 9.2% enhancement in the film hardness and only a 26.0% deterioration in the film resistivity compared to those of Cu films deposited in the absence of TU, which were not simultaneously obtainable upon the direct addition of TU to the deposition bath. (C) 2014 The Electrochemical Society. All rights reserved.