UNIFORM Ti-BARRIER JOSEPHSON JUNCTION ARRAYS FOR AC JOSEPHSON VOLTAGE STANDARD.

NIST researchers have made the first uniform arrays of Josephson junctions using titanium as a junction barrier material. We have focused on using Ti because it can be reactive ion etched with the superconducting Nb electrodes. A considerable effort has been made in experimenting with different etch chemistries to improve the etch anisotropy for etching pentalayer Nb-Ti-Nb-Ti-Nb junction stacks. Two vertically stacked junctions fabricated with this process should have nearly identical areas that very closely match the photoresist dimensions.

Using this new process we recently fabricated arrays of single (unstacked) Nb-Ti-Nb junctions in microwave circuits designed for arbitrary waveform synthesis. These junction arrays are embedded in microwave circuits, which are used to evaluate the junction uniformity. We successfully generated constant-voltage steps in 1000- and 4100-junction series arrays. This is a very good result because it shows that the correct voltage is generated over significant current range for a broad range of frequencies for this large 4100-junction array. This indicates good junction uniformity as well as good microwave uniformity for the entire circuit design. Most of the structure in this circuit is caused by filter resonances], which are also under development. Stacked 2-junction pentalayer arrays are under development.