NBN-MGO-NBN JUNCTIONS PREPARED ON ROOM-TEMPERATURE QUARTZ SUBSTRATES FOR QUASI-PARTICLE MIXERS

NbN tunnel junctions are of great interest for THz heterodyne receiver s because their large gap voltage of 5 mV yields an upper frequency li mit of 4 Delta/h = 2.4 THz for quasiparticle mixing, AC losses in NbN films, however, imply that a NbN matching circuit can be used at most to the NbN gap frequency, 2 Delta/h = 1.2 THz. Another issue is the po or thermal conductivity of NbN films which complicates heat extraction from the junction. One solution to these problems are hybride mixer c hips in which NbN junctions are integrated into appropriate supercondu cting or normal metal matching circuits. Both for waveguide mixers and quasioptical mixers quartz substrates are the first choice. We report on a room-temperature process for NbN-MgO-NbN junction fabrication wh ich permits the realization of hybride mixer chips on quartz substrate s. Appropriate plasma conditions for NbN film deposition are created b y employing a second Nb target as a selective nitrogen pump. NbN films on quartz substrates without and with intermediate layers of Nb, Al, Al/SiO2, and Nb/SiO2 have critical temperatures above 15 K and normal state resistivities from 100 to 130 mu Omega cm. In the first step, Nb N junctions with nominal areas of 0.36 mu m(2) and current densities o f 10 kA/cm(2) at 5.5 mV were integrated into Nb films forming the tuni ng circuit and a dipole antenna. The lowest receiver noise temperature in the 350 GHz range was 245 K double side band.