The adsorption-desorption behavior of polyethylene glycol (PEG) on the cathode surface in copper plating solution with and without Cl- was investigated by cathodic polarization measurement and electrochemical impedance spectroscopy (EIS) measurement. Cathodic polarization analyses show that the inhibiting effect of PEG is potential-dependent in the solution with and without Cl-. EIS analyses suggest that the adsorption of PEG is strengthened by the existence of Cl-, and both adsorption and desorption potential range of PEG are widened by the existence of Cl-, which is attributed to the different adsorption mode of PEG. The adsorption of PEG in the solution without Cl- is in the form of PEG-Cu2+ and PEG-Cu+, which adsorb on the cathode surface mainly by two acting forces, the van der Waals force and the electrostatic force. In the solution with Cl-, the adsorption of PEG is mainly in the form of PEG-Cu2+-Cl- and PEG-Cu+-Cl-. It is due to the specific adsorption ability of Cl- on the electrode surface that the adsorption of PEG is greatly strengthened compared with that in the solution without Cl-. Thus, the adsorption and the desorption potential range of PEG are widened, and PEG shows a much stronger inhibiting effect on copper deposition in a wider potential range. (c) 2008 The Electrochemical Society.