Characterization of Hydroxyls in Surface Oxide of Superconducting Tantalum and Their Mitigation in Quantum Circuits

  1. Ekta Bhatia,
  2. Nicholas Pieniazek,
  3. Aleksandra Biedron,
  4. Sandra Schujman,
  5. Hunter Frost,
  6. Zhihao Xiao,
  7. Jakub Nalaskowski,
  8. Kevin Musick,
  9. Thomas Murray,
  10. and Satyavolu Papa Rao
Recently, tantalum (Ta) has gained attention in superconducting quantum circuits due to the longer coherence times achieved when replacing niobium (Nb) in capacitor pads. Previous literature
shows that surface oxides that form upon ambient exposure on superconducting metals such as Ta, Al, and Nb host two-level system (TLS) defects, which are a leading source of microwave loss and decoherence. While the surface oxides of Nb and Al have been extensively studied, Ta oxides remain less well understood. Using secondary ion mass spectrometry of alpha-Ta films deposited at 300 mm wafer scale, we show for the first time that hydroxyls accumulate in the Ta suboxide region above the underlying Ta. Angle-resolved X-ray photoelectron spectroscopy shows that the surface region is dominated by Ta2O5, with sub-stoichiometric TaOx present in between the Ta2O5 and underlying Ta. The thickness of the tantalum oxide is confirmed by transmission electron microscopy. We demonstrate that [OH] incorporation can be suppressed by replacing the native oxide with an oxide formed during chemical mechanical planarization of alpha-Ta films. Our findings support the hypothesis that TLS defects are non-uniform within the oxide thickness and suggest hydroxyls as a probable molecular origin of these loss channels. Furthermore, we show the feasibility of plasma nitridization as a method to decrease hydroxyl loading on alpha-Ta surfaces. The modulation of hydroxyl content through surface engineering of alpha-Ta can enable the fabrication of more robust, high-coherence superconducting quantum circuits by addressing a potential TLS source.

Tantalum Damascene Coplanar Waveguide Resonators Fabricated Using 300 mm Scale Processes

  1. Ekta Bhatia,
  2. Yingge Du,
  3. Krishna P Koirala,
  4. Chung Kow,
  5. Mingzhao Liu,
  6. Juan Macy,
  7. Tharanga R. Nanayakkara,
  8. Francisco Ponce,
  9. Satyavolu S. Papa Rao,
  10. Drew J. Rebar,
  11. Peter V. Sushko,
  12. Brent A. VanDevender,
  13. Chongmin Wang,
  14. Marvin G. Warner,
  15. and Zhihao Xiao
Surface oxides contribute to losses in superconducting transmon devices resulting in degraded performance. We explore the use of the damascene process to replace the sidewall native
oxide of a device with a metal/substrate interface. We simulate sidewall oxidation by burying an oxide layer during fabrication. We observe a modest improvement between the two types of devices, which is suggestive of a reduction in the surface participation ratio.

Readout-induced suppression and enhancement of superconducting qubit lifetimes

  1. Ted Thorbeck,
  2. Zhihao Xiao,
  3. Archana Kamal,
  4. and Luke C.G. Govia
It has long been known that the lifetimes of superconducting qubits suffer during readout, increasing readout errors. We show that this degradation is due to the anti-Zeno effect, as
readout-induced dephasing broadens the qubit so that it overlaps ‚hot spots‘ of strong dissipation, likely due to two-level systems in the qubit’s bath. Using a flux-tunable qubit to probe the qubit’s frequency dependent loss, we accurately predict the change in lifetime during readout with a new self-consistent master equation that incorporates the modification to qubit relaxation due to measurement-induced dephasing. Moreover, we controllably demonstrate both the Zeno and anti-Zeno effects, which explain suppression and the rarer enhancement of qubit lifetimes during readout.