Reaction paths and kinetics of aluminide formation in AVepitaxial-W(OO1) model diffusion barrier systems

Single-crystal bee W(OO1) layers, 140 nm thick, were grown on MgO(001) substrates by ultrahigh-vacuum (UHV) magnetron sputter deposition at T,= 600 “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 T,= 100 “C without breaking vacuum. Changes in bilayer sheet resistance R, were monitored continuously as a function of time t, and temperature T, 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 WAl, which exhibit a crystallographic relationship with the underlying W layer. bee WA1i2 forms at a later stage and grows conformally to cover both W and WAl,. WAl, and WAl,, 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 R,(T, , 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 WAl, is diffusion limited with an activation energy E, of 3.1 eV while the formation of WAl,, is reaction limited with E,= 3.3 eV.