HIGH-QUALITY SUBMICRON 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 (mu-m)2 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 4.4K were V(m)(2mV) = 39 mV for J(c) = 3000 A/cm2 and V(m)(2mV) = 27 mV for J(c) = 5000A/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-mu-m 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 SiO2 instead of anodized Nb as the thick insulator. SiO2 isolated the junction area and defined the opening for contact to the Nb wiring layer. We have fabricated series arrays of up to 12 junctions, with individual junction areas of 0.5 (mu-m)2. The array I-V quality was not degraded compared to that of an individual junction.