All refractory Josephson tunnel junctions fabricated by reactive ion etching

We have developed a novel process for fabricating all refractory Josephson tunnel junctions whose base and counter electrodes are composed of (Nb-NbN) double-layered films. In this process, tunnel junctions have been isolated using a reactive ion etching from a junction sandwich which had been formed on an entire silicon wafer, and also the reactive ion etching has been used for patterning electrodes and insulation layers in devices. Fabricated junctions have yielded good tunneling characteristics with excellent uniformity and reproducibility. The maximum critical current density was widely changed from 300 A/cm²to 8 kA/cm². All refractory single flux quantum(SFQ) memory cells in a 32×32 arrangement have been integrated with a 2.5 μm minimum linewidth on a chip using this process, and an SFQ cell in the memory array has been successfully operated. The maximum deviation of critical currents in the memory array was measured to be ±6.5%.