The mechanism for improved barrier properties against Cu diffusion in electroless deposited Co0.9W0.02P0.08 and Co0.9P0.1 thin films, as compared with the physical vapor deposition of Co, was quantitatively explained. Secondary ion mass spectrometry depth profile measurements were performed on films deposited on Cu substrates after subjecting them to thermal annealing at about half the melting-point of Co. A steady-state mode was observed in the form of the concentration plateaux which originated from a combined contribution of grain-boundary saturation and Cu solubility in the grains. The difference in plateau heights between the samples was attributed to the varying degrees of grain-boundary passivation. For Co0.9W0.02P0.08, the Cu concentration in the grain boundaries was negligible and the solubility between 550 and 700C could be described by CS(at%) ≈ 600 exp[–0.52(eV)/kT]. The higher Cu concentration plateaux in the Co0.9P0.1 films were a result of low non-negligible Cu concentrations at the grain boundaries; estimated to be less than 10at%. The Cu concentration plateaux in physical vapor deposited Co were significantly larger, due to saturation of the grain boundaries.

Improved Diffusion Barriers for Copper Metallization Obtained by Passivation of Grain Boundaries in Electroless Deposited Cobalt-Based Films. A.Kohn, M.Eizenberg, Y.Shacham-Diamand: Journal of Applied Physics, 2002, 92[9], 5508-11

Figure 45

Grain-Boundary Diffusivity of Cu in Electroless Films

(Circles: Co0.9W0.02P0.08, squares: Co0.9P0.1)