Tathagata Banerjee

Resist-free shadow deposition using silicon trenches for Josephson junctions in superconducting qubits

  1. Tathagata Banerjee,
  2. Stephen Daniel Funni,
  3. Saswata Roy,
  4. Judy J. Cha,
  5. and Valla Fatemi
Superconducting qubit fabrication innovations continue to be explored to achieve higher performance. Despite improvements to base layer fabrication and processing, resist-based Josephson
junction (JJ) schemes have largely remained unchanged. The polymer mask during deposition causes chemical contamination and limits in situ and ex situ surface preparation, junction materials, and scalability. Here, we demonstrate a resist-free approach to junction fabrication based on etched silicon trenches that is CMOS compatible and easily integrated into existing innovations in qubit base layer fabrication and chemical processing. We fabricate Al-AlOx-Al JJs and qubits using this method, measuring median energy relaxation times up to 184 microseconds. We find minimal contamination at the substrate-metal interface and fluctuations of energy relaxation on a 35 hour timescale that are narrow and normally distributed. The method widens the process window for substrate preparation and new materials platforms.
arxiv pdf

Low-loss Nb on Si superconducting resonators from a dual-use spintronics deposition chamber and with acid-free post-processing

  1. Maciej W. Olszewski,
  2. Jadrien T. Paustian,
  3. Tathagata Banerjee,
  4. Haoran Lu,
  5. Jorge L. Ramirez,
  6. Nhi Nguyen,
  7. Kiichi Okubo,
  8. Rohit Pant,
  9. Aleksandra B. Biedron,
  10. Daniel C. Ralph,
  11. Christopher J. K. Richardson,
  12. Gregory D. Fuchs,
  13. Corey Rae H McRae,
  14. Ivan V. Pechenezhskiy,
  15. B. L. T. Plourde,
  16. and Valla Fatemi
Magnetic impurities are known to degrade superconductivity. For this reason, physical vapor deposition chambers that have previously been used for magnetic materials have generally
been avoided for making high-quality superconducting resonator devices. In this article, we show by example that such chambers can be used: with Nb films sputtered in a chamber that continues to be used for magnetic materials, we demonstrate compact (3 {\mu}m gap) coplanar waveguide resonators with low-power internal quality factors near one million. We achieve this using a resist strip bath with no post-fabrication acid treatment, which results in performance comparable to previous strip baths with acid treatments. We also find evidence that this improved resist strip bath provides a better surface chemical template for post-fabrication hydrogen fluoride processing. These results are consistent across three Si substrate preparation methods, including a \SI{700}{\celsius} anneal.
arxiv pdf