Abstract Body

Al/AlO_X/Al junctions are widely used in superconducting qubits because of their easy fabrication using electron beam lithography and shadow evaporation and the stability of AlO_X tunnel barrier. However, AlO_X is an amorphous oxide, which is well-known decoherence source in superconducting qubits. We have developed superconducting qubits based on NbN/AlN/NbN epitaxial junctions as an alternative Josephson junction to Al/AlO_X/Al junctions to suppress decoherence caused by two-level systems in amorphous AlO_X tunnel barrier. The fabrication process of submicron NbN/AlN/NbN epitaxial junctions has been established and it has been demonstrated that coherence times exceeding 20 us can be achieved using this junction technology [1,2]. However, superconducting quantum circuits using NbN/AlN/NbN tri-layer stacks require an amorphous SiO₂ sacrificed layer deposited by sputtering or plasma-enhanced chemical vapor deposition to isolate the base electrode from the wiring layer, while the amorphous SiO₂ film is a lossy dielectric. To avoid qubit decoherence due to this lossy dielectric, the SiO₂ sacrificed layer must be removed at the final step of the fabrication process. We have so far employed wet etching using a buffered HF solution to remove the SiO₂ layer, but this process sometimes results in etching residue around the qubit that are thought to be composed of silicic oxide, which can cause decoherence. And this wet etching process sometimes leads to stiction due to the surface tension of the rinse solution. To solve this problem, vapor HF etching was introduced to remove the SiO₂ sacrificed layer [3]. The generation of particles composed of silicic oxides was minimized by adopting appropriate etching conditions. Coplanar waveguide (CPW) resonators was fabricated with a signal line width of 10 um and a gap between the signal line and the ground plane of 6 um, where the ground plane was connected by airbridges consisting of NbTiN wiring layer. The resistance between signal line and ground plane in the CPW resonators fabricated with wet etching using buffered HF was below 1 kohm due to the stiction between the signal line and the airbridges, while the resistance for the CPW resonator fabricated with vapor HF was above 10 kohm. This indicates that the airbridges fabricated with vapor HF showed no electrical shorts between the signal lines and the airbridges due to stiction. And we also confirmed that the etching process using vapor HF does not affect the properties of the NbN/AlN/NbN epitaxial junctions at all. These results suggest that vapor HF etching technique is effective for realizing superconducting quantum circuits with multi-layer wiring.

References

[1] W. Qiu et al., Appl. Phys. Exp. 13,126501, 2020.
[2] S. Kim et al., Comms. Mater., 2:98, 2021.
[3] H. Dunsworth et al., Appl. Phys. Lett. 112, 063502, 2018.

Acknowledgment

This work is supported by JST Moonshot R&D (JPMJM2067) and MEXT Q-LEAP (grant no. JPMXS0118068682).