Growth and Structure of alpha-Ta films for Quantum Circuit Integration

Tantalum films incorporated into superconducting circuits have empirically low surface losses, resulting in long-lived qubit states. Remaining loss pathways originate in microscopic defects which manifest as two level systems (TLS) at low temperature. Defects bound qubit performance, recommending careful consideration of tantalum film structures. Here we investigate the growth of tantalum on sapphire, Si, and photoresist substrates by magnetron sputtering. In the case of sapphire, we present procedures for growth of fully-oriented films with alpha-Ta [1 1 1] // Al2O3 [0 0 1] and alpha-Ta [1 -1 0] // Al2O3 [1 0 0] having residual resistivity ratios ~ 60 for 220 nm thick films. On Si, we find a complex grain texturing with Ta [1 1 0] normal to the substrate and RRR ~ 30. We also demonstrate airbridge fabrication using Nb to nucleate alpha-Ta on photoresist surfaces. Superconducting resonators patterned from films on sapphire show TLS-limited quality factors averaging 1.6 +/- 0.4 x 10^6 at 10 mK. Structural characterization using scanning electron microscopy, X-ray diffraction, low temperature transport, secondary ion mass spectrometry, and transmission electron microscopy describe the dependence of residual impurities and screw dislocation density on processing conditions.