Optimisation of process parameters for electroless plating of Co–W–P capping layers from an alkali-metal-free bath

Co–W–P thin films were electrolessly deposited on a copper-coated silicon wafer from an alkali-metal-free bath and the effects of pH, temperature and hydrodynamic condition of the electrolyte on the composition, thickness and microstructure of the films were investigated. Compositional analyses using X-ray photoelectron spectroscopy revealed that the surface of the films contained large amounts of hydroxide of cobalt and oxides of cobalt and tungsten. The bulk of the films, however, consisted predominantly of elemental cobalt, tungsten and phosphorus. It was observed that the cobalt content and the thickness of the films increased with increase in the pH, temperature and stirring rate of the solution. On the other hand, both the tungsten and phosphorus contents in the films decreased with increase in the temperature and stirring rate. Microstructural analysis using X-ray diffraction showed that the deposited films were crystalline and contained hexagonal cobalt with a preferred crystallographic orientation of [002]. The grain size of the deposit increased with increase in the pH, temperature and stirring rate of the electrolyte. Scanning electron microscopic examination showed that the deposited films had typical spherical nodular structures with good coverage.