High-quality sub-micron Nb trilayer tunnel junctions for a 100 GHz SIS receiver

A modified SNIP process was used to fabricate high-quality 0.5, 2, and 4 μm² small-area Nb/AlO_x/Nb trilayer tunnel junctions with current densities as large as 5000 A/cm². The average junction quality factors for the junctions at 4.4 K were V _m(2 mV)=39 mV for J_c=3000 A/cm² and V_m (2 mV)=27 mV for J_c=5000 A/cm². The best values of V_m obtained were 50 mV for J_c=3000 A/cm² and 41 mV for J_c=5000 A/cm². These devices were designed and fabricated for use in a W-band mixer receiver. The substrate was 50-μm-thick fused or crystal quartz. Special methods were developed for handling such thin insulating substrates and patterning films. The fabrication process was self-aligned and used SiO ₂ instead of anodized Nb as the thick insulator. SiO₂ isolated the junction area and defined the opening for contact to the Nb wiring layer. The authors have fabricated series arrays of up to 12 junctions, with individual junction areas of 0.5 μm². The array I-V quality was not degraded compared to that of an individual junction