On the origin of the metastable β-Ta phase stabilization in tantalum sputtered thin films

Despite extensive studies since the 1960s, the exact mechanisms underlying phase formation of metastable tetragonal beta-Tantalum (β-Ta) structure commonly found in magnetron sputtered tantalum films have not yet been unraveled. Here, we combine in situ and real-time multiple beam optical stress sensor (MOSS) during sputter-deposition of Ta films together with ex situ X-ray diffraction (XRD) and high resolution transmission electron microscopy (HRTEM) to gain insights into the early growth stages and interface properties. The nucleation conditions were varied by depositing films on different buffer layers, either amorphous (a-Si, a-SiOx, a-SiNx) or crystalline (c-Mo), as well as changing the substrate temperature (Ts = 300–750 K). The influence of the mean deposited energy, in the range of 25–83 eV/atom, as calculated from Monte Carlo simulations, on the resulting stress state and phase formation was also investigated by varying the Ar working pressure (0.12–0.75 Pa) and substrate bias voltage (0 to −190 V). It is shown that the nucleation step is decisive for the stabilization of β-Ta phase on amorphous layers. After the initial formation of an interfacial amorphous Ta(Si) alloyed layer, β-Ta nuclei form due to interface energy minimization and grow columnar with a strong [001] fiber-texture. No transition to the equilibrium bcc alpha-phase (α-Ta) was found with increasing film thickness or variation of the deposited energy. The results are interpreted based on a thermodynamic description of nucleation and growth theory. Such rationale also qualitatively supports experimental findings of α-Ta phase selection with the assistance of substrate heating or by by-passing the nucleation stage upon direct growth in registry on crystalline Mo (110) seed layers.