The interactions among additives employed in acidic copper plating solution for microvia filling are characterized by galvanostatic measurement (GM) and electron paramagnetic resonance (EPR), respectively. These additives include polyethylene glycol (PEG), chloride ion, 3-mercapto-1-propanesulfonate (MPS), and bis-(3-sulfopropyl) disulfide (SPS). EPR patterns show that MPS can reduce Cu2+ to Cu+ before electrochemical reduction. However, the GMs show that MPS is an inhibiting reagent for copper electrodeposition. An inhibition mechanism of the MPS is proposed according to the EPR patterns. GMs also revealed that the combination of MPS and Cl- could result in a strongly catalytic effect on the Cu2+ reduction rate. This synergistic effect between MPS and Cl- on enhancing copper electrodeposition is attributed to an inner sphere electron transfer net constructed by a competitive coordination of MPS and Cl- on Cu2+ complexes. SPS also exhibits similar electrochemical behavior to MPS. However, the response speed of SPS in enhancing the Cu2+ reduction rate is slower than that of MPS. GM results obtained by adopting two different rotating speeds of the working electrode demonstrate that PEG competes with MPS to grab Cl- through Cu+, and that the competitive adsorption between PEG-Cl- and MPS-Cl- on the cathodic surface is strongly convection-dependent. (C) 2005 The Electrochemical Society. All rights reserved.