Polycrystalline bccTixW1−xlayers with mixed 011 and 002 texture were grown on oxidized Si(001) substrates at 600 °C by ultrahigh-vacuum (UHV) magnetron sputter deposition from W andTi0.33W0.67targets using both pure Ar and Xe discharges. Ti concentrations in the 100-nm-thick layers were 0, 6, and 33 at. &percent; depending on target composition and sputtering gas. Al overlayers, 190 nm, thick with strong 111 preferred orientation, were then deposited in Ar at 100 °C with and without breaking vacuum. Changes in bilayer sheet resistanceRswere monitored as a function of timetaand temperatureTaduring subsequent UHV annealing. Thermal ramping of Al/W andAl/Ti0.06W0.94bilayers at 3 °Cmin−1resulted in large (>fourfold) increases inRsatTa≃550 °C,whereasRsin theAl/Ti0.33W0.67bilayers did not exhibit a similar increase until ≃610 °C. Area-averaged and local interfacial reactions and microstructural changes were also followed as a function of annealing conditions. The combined results indicate that Al/W andAl/Ti0.06W0.94bilayer reactions proceed along a very similar pathway in which monoclinicWAl4forms first as a discontinuous interfacial phase followed by the nucleation of bccWAl12whose growth is limited by the rate of W diffusion, with an activation energy of 2.7 eV, into Al. In contrast, the W diffusion rate during the early stages ofAl/Ti0.33W0.67annealing is significantly higher allowing the formation of a continuousWAl4interfacial blocking layer which increases the overall activation energyEa,still limited by W diffusion, to 3.4 eV and strongly inhibits further reaction. We attribute observed increases inWAl4nucleation and growth rates in interfacialAl/Ti0.33W0.67to a “vacancy wind” effect associated with the very rapid(Ea=1.7eV) diffusion of Ti into Al. ©1997 American Institute of Physics.