In damascene copper plating with polyethylene glycol (PEG) and bis- (3-sulfopropyl) disulfide (SPS) as the suppressor and the accelerator respectively, the synergistic effects among additives determine the quality of superfilling. In situ surface-enhanced Raman spectroscopy (SERS) and atomic force microscopy (AFM) were combined with the electrochemical tests to investigate the competitive adsorption between PEG and SPS in the electrolyte containing chloride ions. AFM observation performed at -0.25 V (SCE) shows that PEG accumulate at the prominent position of copper surface gradually in the electrolyte without SPS. When SPS were subsequently added into the above solution, SPS displace most of the PEG molecules to adsorb on the surface within short time (no agitation in solution). In the electrolyte in the presence of both PEG and SPS, SERS data demonstrate that SPS are preferentially adsorbed on copper surface at -0.15 V and -0.25 V (SCE). CV and CA tests carried out at 0rpm and 1000rpm indicate that the adsorption competition between PEG and SPS is apparently potential and convection dependent. These results suggested that SPS can combine with both Cu+ and CuCl to generate weakly-adsorbed intermediate in copper deposition/dissolution, while PEG can only be strongly-chemisorbed on copper surface through CuCl. (C) 2012 The Electrochemical Society. [DOI: 10.1149/2.021302jes] All rights reserved.