Nb/Al-AlOx/Nb edge junctions for distributed mixers

We have fabricated high quality Nb/Al-oxide/Al/Nb edge junctions using a Nb /SiO2 bi-layer film as the base electrode, suitable for use as traveling wa ve mixers. An edge is cut in the bi-layer with an ion gun at a 45 degree an gle using a photoresist mask. The wafer is then cleaned in-situ with a phys ical ion gun clean followed by the deposition of a thin Al (al) film, which is then thermally oxidized, an optional second Al (a2) layer, and a Nb cou nter electrode. It was found that devices with an a2 layer resulted in supe rior electrical characteristics, though proximity effects increased strongl y with a2 thickness. The counter electrode is defined with an SF6 + N-2 rea ctive ion etch, using the Al barrier layer as an etch stop. The Al barrier layer is then either removed with an Al wet etch to isolate the individual devices, or the devices are separated with an anodization process. Various ion gun cleaning conditions have been examined; in addition, both wet and p lasma etch bi-layer edge surface pre-treatments were investigated. It was f ound that edge junctions with large widths (i.e., those more suitable for t raveling wave mixers) typically benefitted more from such treatments. Initi al receiver results at 260 GHz have yielded a DSB noise temperature of 60 K .