Deposition and characterization of tantalum thin films on different seed layers towards their application in superconducting qubits

Tantalum has been demonstrated as a promising material for fabricating superconducting qubits. In particular, Ta-based qubits have been demonstrated to exhibit enhanced coherence times. Most researchers have been focusing on alpha-phase Ta thin-films, which are anticipated to yield best superconducting resonator and qubit performance. While there are several methods to fabricate Ta thin films in the alpha phase, it is not yet fully understood under which conditions a (pure) alpha-phase is formed. We have investigated ~200 nm thick Ta films, which were sputter-deposited on high resistivity silicon (100) substrates at different temperatures, ranging from T=300–900K, and on different seed layers (Al, AlN, Ti, TiN and TaN) with few nanometres thickness in-between Si and Ta. We have characterized these thin films in terms of surface roughness, residual-resistance ratio (RRR), crystal phase composition and superconducting transition temperature. Further, coplanar waveguide resonators with frequencies ranging from 4-8 GHz were fabricated from optimized Ta thin films. The performance of resonators is assessed in terms of internal quality factor and determined at ~10 mK over a range of input power. We measured internal quality factors of up to 2 million in the single-photon power regime, in agreement with a predominant alpha phase of these Ta films.