Phase composition and microstructure of polycrystalline and epitaxial TaNx layers grown on oxidized Si(001) and MgO(001) by reactive magnetron sputter deposition

Abstract TaNx is presently used in a variety of hard coating, wear-resistant, and diffusion barrier applications. However, the Ta–N system is inherently complex with more than 11 reported equilibrium and metastable phases and there has been little systematic study of the synthesis of these materials. Here, we report the results of an investigation of the ultrahigh vacuum reactive magnetron sputtering of Ta as a function of the N2 fraction fN2 in mixed Ar/N2 discharges together with the phase composition and microstructure of TaNx layers grown on MgO(001) and oxidized Si(001). Unlike the Ti–N system, for which TiN is the terminal phase, the abundance of N-rich phases in the Ta–N system results in the film deposition rate R and the N/Ta ratio of as-deposited layers varying continuously with fN2, even for values >fN2*, where fN2* is the N2 fraction corresponding to the maximum rate of N2 uptake by deposited Ta. Phase composition results are summarized in a phase map plotted as a function of film growth temperature Ts (100–800°C) and fN2. In pure Ar, the films are tetragonal β-Ta at Ts 400°C, and a mixture of the two phases at intermediate temperatures. α-Ta layers grown on MgO(001) at Ts>500°C are epitaxial with a strain-driven 45° in-plane rotation with respect to the substrate: (001)α-Ta∣∣(001)MgO with [110]α-Ta∣∣[100]MgO. A series of lower nitrides — TaN0.1, Ta4N, and Ta2N — are formed over narrow ranges of fN2 between 0 and 0.10. This is followed by a wide single-phase field at Ts ≤ 650°C corresponding to the growth of metastable B1 NaCl-structure δ-TaNx with x ranging from 0.94 (fN2=0.10) to 1.37 (fN2=0.275). The thermodynamically-stable hexagonal e-TaN phase is formed at higher growth temperatures. δ-TaNx layers grown on MgO(001) at Ts=550–650°C are epitaxial, exhibiting a cube-on-cube relationship: (001)δ-TaN∣∣(001)MgO with [100]δ-TaN∣∣[100]MgO. The relaxed lattice constant of δ-TaNx(001) layers decreases linearly from 0.4350 nm with x=0.94 to 0.4324 nm with x=1.37. Finally, layers grown in pure N2 are a two-phase mixture of δ-TaNx and body-centered tetragonal TaNx.