Nitrogen Plasma Passivated Niobium Resonators for Superconducting Quantum Circuits

  1. K. Zheng,
  2. D. Kowsari,
  3. N. J. Thobaben,
  4. X. Du,
  5. X. Song,
  6. S. Ran,
  7. E. A. Henriksen,
  8. D. S. Wisbey,
  9. and K. W. Murch
Microwave loss in niobium metallic structures used for superconducting quantum circuits is limited by a native surface oxide layer formed over a timescale of minutes when exposed to
an ambient environment. In this work, we show that nitrogen plasma treatment forms a niobium nitride layer at the metal-air interface which prevents such oxidation. X-ray photoelectron spectroscopy confirms the doping of nitrogen more than 5 nm into the surface and a suppressed oxygen presence. This passivation remains stable after aging for 15 days in an ambient environment. Cryogenic microwave characterization shows an average filling factor adjusted two-level-system loss tangent FδTLS of (2.9±0.5)⋅10−7 for resonators with 3 μm center strip and (1.0±0.3)⋅10−7 for 20 μm center strip, exceeding the performance of unpassivated samples by a factor of four.

Optical Direct Write of Dolan-Bridge Junctions for Transmon Qubits

  1. J. T. Monroe,
  2. D. Kowsari,
  3. K. Zheng,
  4. J. Brewster,
  5. D. S. Wisbey,
  6. and K. W. Murch
We characterize highly coherent transmon qubits fabricated with a direct-write photolithography system. Multi-layer evaporation and oxidation allows us to tune the Josephson energy
by reducing the effective tunneling area and increasing the barrier thickness. Surface treatments before resist application and again before evaporation reduce the occurrence of strongly-coupled two-level system fluctuators, resulting in high coherence devices. With optimized surface treatments we achieve energy relaxation T1 times in excess of 80 μs for three dimensional transmon qubits with Josephson junction lithographic areas of 2 μm2.