Reaction paths and kinetics of aluminide formation in Al/epitaxial-W(001) model diffusion barrier systems

Single-crystal bcc W(001) layers, 140 nm thick, were grown on MgO(001) substrates by ultrahigh-vacuum (UHV) magnetron sputter deposition at Ts=600 degree C. Al overlayers, 190 nm thick with strong (001) and (011) preferred orientation and an average grain size of 200 nm, were then deposited at Ts=100 degree C without breaking vacuum. Changes in bilayer sheet resistance Rs were monitored continuously as a function of time ta and temperature Ta during UHV annealing. In addition, Rutherford backscattering spectroscopy, x-ray diffraction, transmission electron microscopy (TEM), and scanning TEM, in which cross-sectional specimens were analyzed by energy-dispersive x-ray analysis with a 1 nm resolution, were used to follow area-averaged and local interfacial reaction paths as well as microstructural changes as a function of annealing conditions. The initial reaction products were discontinuous regions of monoclinic-structure WAl4 which exhibit a crystallographic relationship with the underlying W layer. bcc WAl12 forms at a later stage and grows conformally to cover both W and WAl4. WAl4 and WAl12 continue to grow, with W being the primary mobile species, until the Al layer is completely consumed. Information from the microchemical and microstructural analyses was used to model the Rs(Ta,ta) results based upon a multielement equivalent circuit approach which accounts for the observed nonplanar nature of the reaction front. Reaction kinetics and activation energies were determined. The results show that the growth of WAl4 is diffusion limited with an activation energy Ea of 3.1 eV while the formation of WAl12 is reaction limited with Ea=3.3 eV. copyright 1995 American Institute of Physics