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

Abstract

We have developed a combined photolithography and electron-beam lithography fabrication process for sub-µm to µm-size Nb/Al-AlO x /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 µm 2 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 V m > 50 mV and V gap = 2.8 mV at 4.2 K, as well as I c R N 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 T 1 = 26 ns and a dephasing time of T 2 = 21 ns.