The influence of the molecular weight ͑Mw͒ of polyethylene glycol ͑PEG͒ on the microvia filling by copper electroplating was demonstrated and examined by cross-sectional images using an optical microscope. The electrochemical behavior of PEG of different molecule weights in the copper electroplating was characterized by galvanostatic measurement. In the presence of excess Cl − , the surface coverage of PEG of various Mw adsorbed on the copper surface was characterized by observing the size and distribution of CuCl precipitates using a scanning electron microscope. As PEG Mw was increased, the best filling performance of plating formula was obtained when the PEG Mw ranged from 6000 to 8000 g/mol. Only large PEG amounts whose Mw exceeds 2000 g/mol can effectively polarize the cathode, in turn inducing the catalytic effect of bis͑3-sulfopropyl͒ disulfide on copper deposition, resulting in a synergistic interaction between the suppressor and accelerator on the microvia filling.