Phase composition, microstructure, and physical properties of poly- and single-crystal tantalum nitride layers

TaN_x is presently used in a variety of hard and wear-resistant coatings 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. In this research, a Ta-N growth phase map as a function of growth temperature T _s and nitrogen fraction $fN2$ in mixed Ar/N₂ was established and fundamental physical properties were determined from high-quality single-crystal δ-TaN _x layers grown on MgO(001) by using low energy ion-irradiation in unbalanced-magnetron UHV reactive magnetron sputter deposition. The texture and microstructure evolution behavior in polycrystalline δ-TaN growth as a function of ion-irradiation condition also studied in order to develop an understanding of δ-TaN growth kinetics.; In pure Ar, tetragonal β-Ta is obtained at T_s < 150°C, bcc α-Ta at T_s > 400°C, and the films are two-phase mixtures at intermediate deposition temperatures. The addition of small amounts of N₂ to the discharge, $fN2$ < 0.100, leads to the formation of a series of lower nitrides—bcc TaN_0.1, orthorhombic Ta₄N, hexagonal γ-Ta ₂N, and cubic δ-TaN. The phase boundaries of these nitrides are inclined toward higher $fN2$ values, reflecting reduced N incorporation with increasing T _s. Single-phase δ-TaN_x layers are obtained with $fN2$ = 0.100–0.275 and T_s ≤ 650°C. For a given value of $fN2$ within this range, x decreases with increasing T_s. At T _s > 650°C, two-phase mixtures consisting of δ-TaN_x and hexagonal ϵ-TaN_x are obtained. Layers grown in pure N ₂ are also two-phase; in this case, δ-TaN_x and bct TaN_x.; Fully-dense stoichiometric single crystalline δ-TaN(001)/MgO(001) layers, exhibiting a cube-on-cube epitaxial relationship with the substrate: (001)_δ-TaN||(001)_MgO and [100]_δ-TaN ||[100]_MgO, were obtained at 600°C with $fN2$ = 0.125 and incident ion energy E_i = 30 eV. The room-temperature resistivities, hardnesses, elastic moduli, and relaxed lattice constants of those layers are 185 ± 15 μΩ-cm, 32.9 ± 0.9 GPa, 435 ± 15 GPa, and 0.4351 nm, respectively.; Polycrystalline δ-TaN layers deposited on SiO₂ at 350°C with E_i = 20 eV in mixed Ar+15%N₂ discharges initially exhibit competitive texture evolution until a single texture dominates at t $\gtrsim$ 200 nm. The preferred orientation of 500-nm-thick E_i = 20 eV layers can be selectively and continuously varied from predominantly underdense 111 to nearly complete dense 002 by varying ion-to-Ta flux ratio J_i/J_Ta from 1.3 to ≥7.4. The change in texture is primarily due to an increased steady state atomic N coverage, resulting from collisionally-induced dissociative chemisorption of incident energetic $N+2$ ions, with increasing J_i/J_Ta.