An electrochemical approach not involving activation-related processes is presented to grow ultrafine (similar to 3 nm) metallic seeds, subsequently catalyzing the electroless deposition of an ultrathin (20 nm) Co-alloy film with a controlled composition of Co95.5B4.5, Co92.0B8.0, or Co90.1W2.9B7.0 on thermally oxidized SiO2 on Si. Examining thermal stability of Si/SiO2/Co-alloy/Cu samples after 450 degrees C/1 h annealing reveals that the Co-B film is converted into Co2Si by Si that has penetrated SiO2, and is thus insufficient to retard Cu diffusion. However, doping a minor amount of tungsten can substantially strengthen the barrier's thermochemical stability, hence alleviating intermixing of copper, silicon, and cobalt. (c) 2006 The Electrochemical Society.