Al/ti(x)w(1-X) Metal/diffusion-Barrier Systems: Interfacial Reaction Paths and Kinetics as a Function of Composition and Microstructure
Bergstrom, Daniel Brian
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Permalink
https://hdl.handle.net/2142/82871
Description
Title
Al/ti(x)w(1-X) Metal/diffusion-Barrier Systems: Interfacial Reaction Paths and Kinetics as a Function of Composition and Microstructure
Author(s)
Bergstrom, Daniel Brian
Issue Date
1997
Doctoral Committee Chair(s)
Greene, Joseph E.
Department of Study
Materials Science and Engineering
Discipline
Materials Science and Engineering
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Engineering, Electronics and Electrical
Language
eng
Abstract
TiW layers, 165 $\pm$ 25 nm thick, were grown on MgO(001), SiO$\sb2$ and Si(001) substrates by ultra-high vacuum (UHV) magnetron sputter deposition. Al overlayers, 190 $\pm$ 10 nm thick, were then deposited without breaking vacuum. TiW films deposited in Ar were found by Rutherford backscattering spectroscopy (RBS) and Auger electron spectroscopy to be increasingly Ti deficient with increases in the Ti sputtering rate and/or $\rm T\sb{s}$ at a constant W sputtering rate. TRIM calculations and Monte Carlo gas-transport simulations were used, in combination with the experimental results, to show that the Ti loss was due primarily to differential resputtering of the growing film by energetic Ar particles backscattered from the heavier W target. The use of Xe, rather than Ar, as the sputtering gas, greatly reduces both the flux and the average energy of backscattered particles incident at the substrate such that measurable Ti loss is no longer observed. Changes in Al/TiW bilayer sheet resistance $\rm R\sb{s}$ were monitored continuously as a function of time $\rm t\sb{a}$ and temperature $\rm T\sb{a}$ during UHV annealing. In addition, RBS, x-ray diffraction, transmission electron microscopy (TEM), and scanning TEM, in which cross-sectional specimens were analyzed by energy-dispersive x-ray compositional mapping 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. Information from the microchemical and microstructural analyses was used to model the sheet resistance results based upon a novel multi-element equivalent circuit approach which accounts for non-planar reaction fronts. The results show that the monoclinic WAl$\sb4$ phase forms first with W as the dominant diffusing species and that this phase controls the subsequent kinetics of WAl$\sb{12}$ formation. In Ti deficient TiW barriers ($<$0.06 at% Ti) sputter deposited in Ar, the WAl$\sb4$ layer is discontinuous and the activation energy for barrier failure is 2.7 eV. In TiW barriers sputtered in Xe (0.33 at% Ti), the diffusion of Ti to the surface creates a counterflux of vacancies to the Al/TiW interface which assists W diffusion and promotes WAl$\sb4$ formation. The continuous WAl$\sb4$ layer blocks further diffusion, increasing the activation energy to 3.4 eV and increasing the barrier failure temperature by $60\sp\circ$C.
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