Monocrystalline body-centered cubic 140nm-thick (001) W layers were grown onto (001) MgO substrates by means of ultra-high vacuum magnetron sputter deposition at 600C. Then, 190nm-thick Al over-layers with a strong (001) or (011) preferred orientation and an average grain size of 200nm, were deposited at a temperature of 100C without breaking the vacuum. Changes in the bi-layer sheet resistance were monitored continuously, as a function of time and temperature, during annealing in ultra-high vacuum. Rutherford back-scattering spectroscopic, X-ray diffraction, transmission electron microscopic, and scanning transmission electron microscopic (in which cross-sectional specimens were analyzed at a resolution of 1nm by means of energy-dispersive X-ray analysis) techniques were used to monitor area-averaged and local interfacial reaction paths. It was found that the initial reaction products were discontinuous regions of Al4W with a monoclinic structure. This had a crystallographic relationship to the underlying W layer. Body-centered cubic Al12W formed at a later stage, and grew conformally to cover both W and Al4W. The Al4W and Al12W continued to grow, with W as the primary mobile species, until the Al layer was completely consumed. The results were modelled on the basis of a multi-element equivalent circuit approach which accounted for the observed non-planar nature of the reaction front. The results showed that the growth of Al4W was diffusion-limited, with an activation energy of 3.1eV, while the formation of Al12W was reaction-limited; with an activation energy of 3.3eV.

D.B.Bergstrom, I.Petrov, L.H.Allen, J.E.Greene: Journal of Applied Physics, 1995, 78[1], 194-203