Sputtered a-silicon tunneling barriers for Nb-Nb Josephson junctions

We have developed an IC-compatible process for fabricating Josephson tunnel junctions, which uses dc magnetron-sputtered Nb films as both base and counterelectrodes, and rf-sputtered amorphous silicon as the tunneling barrier. Optical reflectivity measurements have been used to study the silicon barrier, and to allow precise determination of the barrier thickness. The Josephson current density varies exponentially -- over several orders of magnitude -- with the barrier thickness. The product of the critical Current and subgap resistance Vmis constant over this wide range of current density. The specific capacitance of these Junctions is ∼ 2.5 μf/cm²at a current density of a few hundred A/cm². This is lower than the value for lead-alloy Junctions, ∼ 4.3 μf/cm²[1], and is consistent with the measured thickness and dielectric constant of the a-Si barrier. The performance of logic gates fabricated with these devices will be presented elsewhere in this conference.