[EDP2] Digital, Quantum Information Technology, Materials and Fabrication

[EDP2-08] Impact of the fabrication process on internal losses in coplanar waveguide resonator based on nitride superconductors

*Satoru Mima¹, Hirotaka Terai¹, Yuji Hishida¹, Kazuyo Takaki¹, Mai Setoura¹, Shigehito Miki¹ (1. NICT - National Institute of Information and Communications Technology (Japan))

Keywords:

Superconducting qubit,Superconducting resonator

We have developed superconducting qubits based on NbN AlN NbN epitaxial junctions to improve the coherence time by employing an epitaxial AlN tunnel barrier alternative to amorphous AlOx, which is a well-known decoherence source in superconducting qubits. However, our nitride superconducting qubit requires various fabrication steps including deposition of a SiO₂ sacrificial layer, formation of a wiring layer, and final removal of the SiO₂ sacrificial layer, while the Al AlOx Al junction is fabricated by a relatively simple fabrication process using shadow evaporation and lift-off technique. We have successfully observed coherence times of over 20 μs, which is still shorter than 500 μs demonstrated for superconducting qubits based on Al AlOx Al junctions . One of the reasons for the limited coherence time of nitride superconducting qubits may be due to the complexity of fabrication process. In this work, we will report the results of investigating the influence of each fabrication process on the internal loss through the measurement of coplanar waveguide CPW resonators to reveal the decoherence source in our nitride superconducting qubits. CPW resonators fabricated with TiN films on high resistivity Si 100 substrate 20 kΩ·cm or more show an internal quality factor Q_i of over 1 million in the single-photon regime. The impact of each fabrication step, including the resist removal process, etching with CF₄ gas, and SiO₂ film deposition and removal with BHF, was evaluated individually by measuring the Q_i of TiN CPW resonators using a vector network analyzer. It was found that an O₂ ashing process to remove the etching residue significantly degrades the Q_i, while the Q_i is improved by removing the surface oxide layer generated by the O₂ ashing process with buffered HF. More details will be reported in the presentation.