Aluminide formation in polycrystalline Al/W metal/barrier thin-film bilayers: Reaction paths and kinetics

Polycrystalline bcc W layers, 110 nm thick with 011 preferred orientation and an average grain size of 40 nm, were grown on amorphous-SiO2/Si(001) substrates by ultrahigh vacuum (UHV) magnetron sputter deposition at Ts=600 degree C. Al overlayers, 170 nm thick with strong 111 preferred orientation and an average grain size of 120 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, area-averaged and local interfacial reaction paths, as well as microstructural changes as a function of annealing conditions, were determined by x-ray diffraction, Rutherford backscattering spectroscopy, transmission electron microscopy (TEM), and scanning TEM in which compositional distributions in cross-sectional specimens were obtained by energy-dispersive x-ray analysis using a 1 nm diam probe beam. The two tungsten aluminides which form, WAl4 and WAl12, are nucleated essentially immediately with no measurable induction time. WAl4 grains, extensively twinned, increase in size during the initial reaction, then stop growing as competitive growth in the diffusion limited regime favors WAl12. Information from microstructural and microchemical analyses was used to model the Rs(Ta,ta) data in order to determine reaction kinetics and activation energies. The results show that WAl12 growth is limited by W diffusion, with an activation energy of 2.7 eV, to the Al/aluminide interface. copyright 1997 American Institute of Physics