Electroless plated CoWB and CoWPB films were studied for copper cap applications in semiconductor device processing. Both films were found to be polycrystalline but introduction of phosphorus made the CoWPB film less crystalline than CoWB. Films were stable for four cycles of thermal annealing with each cycle consisting of a 2 hour anneal at 400åC. Cu diffused through thin cap films after one thermal cycle but for subsequent cycles no significant increase in copper concentration was observed. It appeared that the solubility limit of Cu in cobalt is reached limiting further diffusion of Cu after the first cycle at this annealing temperature. Electrical data collected for 3.9m snake and comb test structures with CoWB and CoWPB showed less than one order of magnitude increase in leakage and less than a 1.5 % change in resistance.