Submicron Nb-Al-oxide-Nb junctions for frequency mixers

A fabrication process for submicron Nb-Al-oxide-Nb tunnel junctions using photoresist lines and optical lithography is described. With 0.6 mu m wide resist lines, junction areas of about 0.4 mu m2 were obtained. At present the maximum current density obtainable is 8500 A cm-2, and for this the best Vm parameters of the submicron junctions are 47 mV at 4.2 K and 150 mV at 2.5 K. Compared with standard photoresist pads for junction definition, photoresist lines are more stable and result in junction areas closer to the areas on the mask. A weak increase of the gap voltage with decreasing junction size is observed. Finally, the performance of a 345 GHz receiver incorporating a two-junction array with integrated filter and tuning circuit is discussed.