Growth and structure of alpha-Ta films for quantum circuit integration

Journal Article · 28 January 2025 · Journal of Applied Physics

DOI: https://doi.org/10.1063/5.0245710 · OSTI ID:2507378

Alegria, Loren D.  ^[1]; Abelson, Alex  ^[1]; Kim, Eunjeong  ^[1]; Im, Soohyun  ^[2]; Voyles, Paul M.  ^[3]; Lordi, Vincenzo  ^[1]; DuBois, Jonathan L  ^[1]; Rosen, Yaniv J.  ^[1]

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1. Lawrence Livermore National Laboratory (LLNL), Livermore, CA (United States)
2. University at Buffalo, NY (United States)
3. University of Wisconsin-Madison, WI (United States)

Tantalum films incorporated into superconducting circuits have exhibited low surface losses, resulting in long-lived qubit states. The remaining loss pathways originate in microscopic defects that manifest as two level systems (TLSs) at low temperatures. These defects limit performance, so careful attention to tantalum film structures is critical for optimal use in quantum devices. In this work, we investigate the growth of tantalum using magnetron sputtering on sapphire, Si, and photoresist substrates. In the case of sapphire, we present procedures for the growth of fully-oriented films with α-Ta [1 1 1]//Al₂O₃ [0 0 0 1] and α-Ta [1 −1 0]//Al₂O₃ [1 0 −1 0] orientational relationships and having residual resistivity ratio (RRR) ∼ 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 further demonstrate airbridge fabrication using Nb to nucleate α-Ta on photoresist surfaces. For the films on sapphire, resonators show TLS-limited quality factors of 1.3 ± 0.3 × 10⁶ at 10 mK (for a waveguide gap and conductor width of 3 and 6 μm, respectively). Structural characterization using scanning electron microscopy, x-ray diffraction, low temperature transport, secondary ion mass spectrometry, and transmission electron microscopy reveal the dependence of residual impurities and screw dislocation density on processing conditions. The results provide practical insights into the fabrication of advanced superconducting devices including qubit arrays and guide future works on crystallographically deterministic qubit fabrication.

• Research Organization: Lawrence Livermore National Laboratory (LLNL), Livermore, CA (United States); University of Wisconsin-Madison, WI (United States)
• Sponsoring Organization: National Science Foundation (NSF); US Air Force Office of Scientific Research (AFOSR); USDOE; USDOE National Nuclear Security Administration (NNSA); USDOE Office of Science (SC), Basic Energy Sciences (BES)
• Grant/Contract Number: AC52-07NA27344; SC0020313
• OSTI ID: 2507378
• Report Number(s): LLNL--JRNL-864328; LLNL--LDRD-24-ER-045
• Journal Information: Journal of Applied Physics, Journal Name: Journal of Applied Physics Journal Issue: 4 Vol. 137; ISSN 0021-8979; ISSN 1089-7550
• Publisher: AIP PublishingCopyright Statement
• Country of Publication: United States
• Language: English