Microvias with various diameters formed by laser ablation on a printed circuit board (PCB) were used to investigate the filling mechanism of the microvia in copper electroplating. The plating solution consisted of acid copper sulfate, polyethylene glycol (PEG), 3-mercapto-1-propanesulfonate (MPS), and chloride ions. Superfill was obtained using a specific concentration ratio of MPS to chloride ions. According to the examinations of the vias by an optical microscope (OM) for cross-sectional views, a scanning electron microscope (SEM) for top views, and by an energy dispersive spectrometer (EDS) for surface element analyses, chloride ions were verified to be a key factor in determining the superfilling behavior. They play three roles, namely, the electron bridge of Cu2+ reduction, the anchor of suppressor (i.e., PEG + Cl-), and the promoter of accelerator (MPS + Cl-) in copper electroplating. The distribution of the surface coverage of the suppressor and the accelerator adsorbed along the via profile was revealed by tracing the distribution of CuCl crystals generated by synergistic interactions among MPS, Cl-, metallic copper, and Cu2+. Plating results indicate that the adsorptive abilities of the suppressor and the accelerator, and whether a suppressor can displace the accelerator adsorbed on the cathode are convection-dependent. (C) 2005 The Electrochemical Society. All rights reserved.