Overcoming material and size limitations of niobium-based trilayer Josephson junctions using an airbridge process: Part 2 - Material characterization

Niobium-based trilayer Josephson junctions enable superconducting qubits to operate at ~1 K and in the millimeter-wave range. Recent works using Nb-Al-AlOx-Al-Nb junctions have successfully realized high-coherence qubits [1] and a millimeter-wave qubit [2]. To make this process compatible with a wider variety of barrier materials and smaller junction sizes, we have developed an airbridge-like junction process using grayscale electron-beam lithography of resist spacer, permitting junction contact areas down to ~100 nm while eliminating the need for harsh chemical etching. This process also allows for the exploration of promising alternative barrier materials such as HfOx.

In this second part of our two-part talk, we present a detailed material characterization of these novel junctions. We utilize Transmission Electron Microscopy (TEM) coupled with X-ray Energy Dispersive Spectroscopy (X-EDS) to analyze material interfaces and investigate any residues.

[1] Anferov et al., Phys. Rev. Applied 21, 024047 (2024)

[2] Anferov et al., PRX Quantum 6, 020336 (2025)