Fabrication of sub-micron whole-wafer SIS tunnel junctions for millimeter wave mixers

As part of a program for the development of a space-qualified submillimeter-wave mixer operating in the region of one terahertz, the authors have developed processes for the fabrication of submicron whole-wafer tunnel junctions. Using the self-aligned whole-wafer process (SAWW) with electron beam lithography they have been able to reliably fabricate high-quality (V/sub m/>20 mV) submicron tunnel junctions from whole-wafer Nb/AlO/sub x//Nb structures. In particular, it is shown that the junction quality is independent of size down to 0.3 mu m/sup 2/ junction area. The problems of film stress, anodization, registration for electron beam lithography, and lift-off, which limit the yield of good quality submicron-scale junctions are addressed.