A new synergy effect of 2-mercaptobenzothiazole (2-MBT) and polyether (PE-3650) on electroless copper deposition was found, and a complete bottom-up filling of electroless copper was achieved in the plating bath. The cross-sectional SEM observation indicated that all trenches with the width ranging from 70 to 520 nm were filled completely by electroless copper and no void was found. The effects of 2-MBT and PE-3650 on the polarization behaviors of the electroless copper plating bath were investigated by the linear sweep voltammetry method and mixed potential theory. (C) 2011 The Electrochemical Society. [DOI: 10.1149/2.018112esl] All rights reserved.