Phase chemistry in the Ti-Si-N system: Thermochemical review with phase stability diagrams
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The phase chemistry and thermodynamics of the Ti-Si—N system are reviewed leading to a revision of the ternary phase diagram at 1273 K to include the extended compositional ranges of S— TiN^ and TisSis^^) and the compounds Ti3Si, T15S14, and a— Ti. This same information is presented in the form of phase stability diagrams which plot the stability criteria for each phase as a function of thermodynamic activity of one component and the relative composition of the other two components. Plots in terms of log «si v s Ti/(Ti + N), log aN2 vs Si/(Ti + Si), and log aTi vs N/(Si + N) were constructed. All diagrams are consistent with current knowledge of phase compatibility and, therefore, are topologically correct. Further refinement will be possible as more accurate information on thermodynamics and compositional limits of solid solutions becomes available.
I. INTRODUCTION The titanium-silicon-nitrogen system is technologically interesting because of the properties of several of its compounds. The combination of titanium disilicide and titanium nitride is particularly attractive for use in construction of electrical interconnections in electronic devices.1 While titanium disilicide is used for its high electrical conductivity and thermal expansion compatibility with silicon, titanium nitride acts as a diffusion reaction barrier between TiSi2 and aluminum. Many investigations have examined the formation of TiSi2 and TiN through solid state reaction schemes. This typically involves the deposition of titanium metal thin films onto Si3N4 coated Si, followed by very brief thermal anneals.2^1 Other investigations5'6 have focused on the direct reaction of titanium-rich TiN with silicon. In all cases, complex sequences of phase formation seem to occur which are sensitive to the initial and subsequent conditions. We were interested in rationalizing each observation on the basis of thermochemical considerations. Our analysis is split into two parts. In this paper, we reanalyze aspects of the T i - S i - N system using available thermodynamic and phase equilibria data at 1273 K. The ternary phase diagram initially presented by Beyers et al? has been revised and updated to show more detail of phase compatibility. This includes calculations of each range of stoichiometry for which TiNx is in equilibrium with Si 3 N 4 , Si, and the various titanium silicides. Moreover, we have explicitly included T15S14,
^Current address: BIRL, Industrial Research Laboratory, Northwestern University, Evanston, Illinois 60201. This work is based in part on a Ph.D. Thesis by S. Sambasivan, Arizona State University, Tempe, Arizona, 1990. 2362
J. Mater. Res., Vol. 9, No. 9, Sep 1994
Ti3Si, Ti2N, and a - T i in the ternary diagram as we understand the situation. Previously, these compounds had been omitted from the original diagram. Finally, in this paper we construct phase stability diagrams which plot the thermodynamic potentials (or activities) of one component as a function of the relative atom fractions of the other two components. For instance, we
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