In this paper,a new synergy effect of 2-mercaptobenzothiazole (2-MBT) and polyether (PE) on electroless copper deposition was found,and a complete bottom-up filling of electroless copper,in which the deposition rates of electroless copper were not only inhibited at the surface of the substrate,but also were accelerated in the bottom of the trenched,was achieved in the plating bath with an addition of 2-MBT and PE-3650.The cross-sectional SEM observation indicated that all trenches with different widths ranging from 70 to 520 nm were filled completely by electroless copper and no void was found.This was attributed to three factors,the deposition rate of electroless Cu was accelerated markedly by an addition 2-MBT only; and it was decreased sharply by a combination 2-MBT and PE-3650; the large molecular weight and lower diffusion coefficient of PE-3650 resulted in concentration gradient of PE-3650 in the trenches.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.