Thin Co(W,P) films, 100-200 nm thick, were electroless deposited on oxidized silicon wafers using sputtered copper or cobalt as catalytic seed layers. The purpose of these films is to encapsulate copper preventing its corrosion or to serve as a diffusion barrier against copper contamination of silicon oxide and silicon in ULSI interconnect and packaging applications. The electroless cobalt layers were integrated with electroless copper and found to function as barriers up to a temperature of 500°C. The microstructure of the barrier film was found to consist of grains of h.c.p. cobalt, ∼ 10 nm in diameter, in which the grain boundaries are most probably enriched by phosphorus and tungsten. It was found that the phosphorus and tungsten impurities stabilize the h.c.p. phase, postponing the transition to the f.c.c. phase by more than 80°C, compared to pure bulk cobalt. The observed good barrier properties can be explained by the nano-sized grains along with the blocking effect of the impurities at the fast diffusion path of the grain boundaries. An advantage of these layers, relative to alternative diffusion barriers for copper, is their low electrical resistivity, 40 uΩ cm.
- Diffusion barrier
- Electroless deposition