Tantalum thin films for superconducting qubit applications: the influence of the substrate-to-metal interface

Tantalum (Ta) has been established as a promising alternative material for the fabrication of superconducting qubits. It is observed that the body-centered cubic (bcc) alpha-phase of Ta is more desirable for superconducting qubit applications than the meta-stable (beta) phase. There are various methods for obtaining bcc-Ta, one of which is to deposit the thin film on a suitable, few nanometers thick seed layer. In this study, we deposited tantalum thin films with different sputtering parameters and on various seed layers on silicon substrates before characterizing their crystallographic and RF electrical properties. XRD measurements indicate preferential growth of bcc-Ta on Nb, TiN and TaN seed layers, if these are thicker than ~5 nm. We have then structured the thin films into coplanar waveguide resonators and determined their internal quality factors (Q_i) at T=100 mK. Indeed, the presence of the seed layers under the Ta thin films affect their RF-performance significantly: Ta on Nb performs best, with an Q_i of 1.4x10⁶ in the single-photon regime, while Ta on TaN shows Q_i values of one order of magnitude less (1.3x10⁵). Intriguingly, adding another few nm thick Ta buffer layer in-between the TaN and the silicon substrate in the latter film architecture results in a Ta thin film on top, which performs noticeably better (Q_i ~ 6.4x10⁵). By estimating the micro-strain in the thin films from XRD-data it can be seen that the best performing Ta thin films also exhibit the lowest strain values, down to 7.4x10^-3.