Characterization of electroless deposited Co(W,P) thin films for encapsulation of copper metallization

Thin Co(W,P) films, 100-200 nm thick, were electroless deposited on oxidize d silicon wafers using sputtered copper or cobalt as catalytic seed layers. The purpose of these films is to encapsulate copper preventing its corrosi on or to serve as a diffusion barrier against copper contamination of silic on oxide and silicon in ULSI interconnect and packaging applications. The e lectroless cobalt layers were integrated with electroless copper and found to function as barriers up to a temperature of 500 degreesC. The microstruc ture of the barrier film was found to consist of grains of h.c.p, cobalt, s imilar to 10 nm in diameter, in which the grain boundaries are most probabl y 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 degreesC, compared to pure bulk cobalt. The observed good barrier properties can be explained by the nano-sized gr ains along with the blocking effect of the impurities at the fast diffusion path of the grain boundaries. An advantage of these layers, relative to al ternative diffusion barriers for copper, is their low electrical resistivit y, 40 mu Omega cm. (C) 2001 Elsevier Science B.V. All rights reserved.