Selective niobium anodization process for fabricating Josephson tunnel junctions

TLDR

The authors present a novel process for fabricating refractory superconducting tunnel junctions compatible with both deposited and native oxide barriers. The method forms the full superconductor–barrier–superconductor stack before any patterning, then isolates Josephson junctions by anodizing through the upper electrode while protecting the devices with a photoresist mask. Using this process, Nb–Si:H–Nb junctions were fabricated with a critical‑current × subgap‑resistance product exceeding 10 mV and a critical‑current density that varies by ~50 % across a 2‑inch wafer.

Abstract

A novel process for fabricating refractory sperconducting tunnel junctions is described, which is useful with both deposited and native oxide barriers. The distinguishing feature of the method is that the entire superconductor-barrier-superconductor sandwich is formed before the patterning of any layer. Isolated Josephson junctions are then formed by anodizing through the upper electrode, while the devices themselves are protected by a photoresist mask. Using this process, Nb-Si:H-Nb junctions have been fabricated, whose product of critical current and subgap resistance exceeds 10 mV and whose critical current density varies by about 50% over a 2-in. diameter wafer.