We propose a means of constructing simplified quaternary phase diagrams based on a thermodynamic approach using known and estimated data for Gibbs free energy of formation. Isothermal, isobaric sections of the condensed phase diagrams are built up as tie lines, tie planes, and tie tetrahedra (representing two-, three-, and four-phase equilibrium, respectively) in a regular tetrahedron. This extends the now well-established methodology for ternary systems described, for instance, by Beyers [J. Appl. Phys. 56, 147 (1984); Mat. Res. Soc. Symp. Proc. 47, 143 (1985)]. The procedure is illustrated by reference to Ti-Si-N-O, Al-Si-N-O, Ti-Al-Si-N and Ti-Al-Si-O, systems which are relevant to interactions occurring at various interfaces during the formation of contacts in integrated circuits. These phase diagrams are then used to predict the stability of—or reactions in—metallization layers and thin-film systems involving four elements. In addition, a method is suggested to estimate unknown free energies of formation from observations of thin-film reactions and stability.

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A thin α-Co layer with an amorphous Si underlayer has been sputter deposited onto a thermal SiO2 substrate, rapid thermal annealed in N2 at 700–1050 °C, and the phases formed examined using Auger electron spectroscopy, transmission electron microscopy, electron diffraction, and sheet resistance measurements. A CoSix film results where x is constant with depth and determined by the relative amounts of Co and Si deposited. With increasing x, phases identified are α- and β-Co containing dissolved Si, Co2Si, CoSi, and CoSi2. At high temperatures, the CoSi2 film agglomerates and thins the underlying oxide probably on account of excess Si in the silicide film. Furthermore, in an N2 atmosphere, the CoSi2 globules are converted into CoSi in accordance with the phase diagram.

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