Fabrication of Josephson Junctions with alternative materials using hybrid deposition techniques

Research on superconducting quantum materials has primarily focused on Al/AlOx/Al Josephson junctions (JJ), largely due to the ease in fabricating reliable JJs using double angled shadow evaporation methods. However, materials that are promising alternatives (e.g. Ta, NbN, and TiN) require sputter deposition and the use of subtractive processes for JJ fabrication.

We present a novel approach to explore alternative JJ materials using a cluster deposition system at Berkeley Lab’s Molecular Foundry. By integrating electron beam evaporation and sputtering techniques, we have fabricated asymmetric material stacks through systematic modifications of the top and bottom layers of the JJs. In this context, we have investigated Al/AlOx/NbN and Al/AlOx/TiN Josephson junction stacks, where aluminum is deposited using shadow angled e-beam evaporation followed by oxidation, whereas the NbN and TiN were deposited by reactive sputtering without breaking vacuum. The distinct superconducting gaps enable robust gap engineering to mitigate quasiparticle tunneling. In this talk we assess the device performance using comprehensive material and electrical characterization and its implication for superconducting qubits.