Electroless Co-based capping layers with dimethylamine borane (DMAB) and hypophosphite as reductants on Cu lines were achieved without Pd activation. The oxidation process of electroless Co-based deposition was characterized by cyclic voltammetry and electrochemical impedance spectroscopy. The oxidation of DMAB was greatly suppressed with the addition of hypophosphite during electroless CoBP deposition. At low [OH-], an obvious oxidation peak of hypophosphite was attained due to insufficient formation of BH3OH-, which showed slow charge transfer on the Cu surface; at high [OH-], distinct competition between hypophosphite and OH- and sufficient BH3OH- formed enhanced the oxidation of DMAB, indicating fast charge transfer. The selectivity of electroless Co-based deposition was also strongly influenced by [DMAB], [hypophosphite], and [OH-]. (c) 2007 The Electrochemical Society.