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 orientat ion and an average grain size of 40 nm, were grown on amorphous-SiO2/S i(001) substrates by ultrahigh vacuum (UHV) magnetron sputter depositi on at T-s = 600 degrees C. Al overlayers, 170 nm thick with strong 111 preferred orientation and an average grain size of 120 nm, were then deposited at T-s = 100 degrees C without breaking vacuum. Changes in b ilayer sheet resistance R-s were monitored continuously as a function of time t(a) and temperature T-a during UHV annealing. In addition, ar ea-averaged and local interfacial reaction paths, as well as microstru ctural changes as a function of annealing conditions, were determined by x-ray diffraction, Rutherford backscattering spectroscopy, transmis sion electron microscopy (TEM), and scanning TEM in which compositiona l distributions in cross-sectional specimens were obtained by energy-d ispersive x-ray analysis using a 1 nm diam probe beam. The two tungste n aluminides which form, WAl4 and WAl12, are nucleated essentially imm ediately with no measurable induction time. WAl4 grains, extensively t winned, increase in size during the initial reaction, then stop growin g as competitive growth in the diffusion limited regime favors WAl12. Information from microstructural and microchemical analyses was used t o model the R-s(T-a,t(a)) data in order to determine reaction kinetics and activation energies. The results show that WAl12 growth is limite d by W diffusion, with an activation energy of 2.7 eV, to the Al/alumi nide interface. (C) 1997 American Institute of Physics.