Copper nucleation on an oxide-coated titanium cathode substrate is described in terms of a "pipe tunneling" mechanism in which electrons transfer through tunneling along a dislocation core line. Morphological examination for early stage of copper nucleation and growth reveals that a synergistic interaction between polyethylene imide (PEI) and chloride ions appears to be effective in producing uniformly-distributed copper nuclei and their subsequent growth in a regulated manner of surface leveling. A suggested account for the synergism is that chloride ions and nitro-aromatic parts of PEI may form cuprous chelated complexes with copper ions.