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

Abstract

TaN x 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 N 2 fraction f N 2 in mixed Ar/N 2 discharges together with the phase composition and microstructure of TaN x 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 f N 2 , even for values > f N 2 *, where f N 2 * is the N 2 fraction corresponding to the maximum rate of N 2 uptake by deposited Ta. Phase composition results are summarized in a phase map plotted as a function of film growth temperature T s (100–800°C) and f N 2 . In pure Ar, the films are tetragonal β-Ta at T s<150°C, bcc α-Ta at T s>400°C, and a mixture of the two phases at intermediate temperatures. α-Ta layers grown on MgO(001) at T s>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 — TaN 0.1, Ta 4N, and Ta 2N — are formed over narrow ranges of f N 2 between 0 and 0.10. This is followed by a wide single-phase field at T s ≤ 650°C corresponding to the growth of metastable B1 NaCl-structure δ-TaN x with x ranging from 0.94 ( f N 2 =0.10) to 1.37 ( f N 2 =0.275). The thermodynamically-stable hexagonal ε-TaN phase is formed at higher growth temperatures. δ-TaN x layers grown on MgO(001) at T s=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-TaN x (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 N 2 are a two-phase mixture of δ-TaN x and body-centered tetragonal TaN x .