Aluminum Hard Mask Technique for the Fabrication of High-Quality Submicron Nb/Al-AlOx/Nb Josephson Junctions

TL;DR

This paper introduces a novel aluminum hard mask technique for fabricating high-quality submicron Nb/Al-AlOx/Nb Josephson junctions, enabling improved device performance and quantum bit applications.

Contribution

The paper presents a new fabrication process combining photolithography and electron-beam lithography with an aluminum hard mask for high-quality submicron Josephson junctions.

Findings

Junctions down to 0.5 μm² fabricated with high quality

IV characteristics show Vm > 50 mV and Vgap = 2.8 mV at 4.2 K

Phase qubits exhibit T1 = 26 ns and T2 = 21 ns

Abstract

We have developed a combined photolithography and electron-beam lithography fabrication process for sub-\mum to \mum-size Nb/Al-AlOx/Nb Josephson junctions. In order to define the junction size and protect its top electrode during anodic oxidation, we developed and used the new concept of an aluminum hard mask. Josephson junctions of sizes down to 0.5 \mum2 have been fabricated and thoroughly characterized. We found that they have a very high quality, which is witnessed by the IV curves with quality parameters Vm > 50 mV and Vgap = 2.8 mV at 4.2 K, as well as IcRN products of 1.75-1.93 mV obtained at lower temperatures. In order to test the usability of our fabrication process for superconducting quantum bits, we have also designed, fabricated and experimentally investigated phase qubits made of these junctions. We found a relaxation time of T1 = 26 ns and a dephasing time of T2 = 21 ns.