Processing and characterization of high Jc NbN Superconducting Tunnel Junctions for THz analog circuits and RSFQ

A generic NbN Superconducting Tunnel Junctions (STS) technology has been de veloped using conventional substrates (Si and SOI-SIMOX) for making THz spe ctrometers including SIS receivers and RSFQ logic gates. NbN/MgO/NbN juncti ons with area of 1 mu m(2), Jc of 10 kA/cm(2) and low sub-gap leakage curre nt (Vm > 25 mV) are currently obtained from room temperature sputtered mult ilayers followed by a post-annealing at 250 degrees C, Using a thin MgO buf fer layer deposited underneath the NbN electrodes, is insuring lower NbN su rface resistance values (Rs=7 mu Omega) at 10 GHz and 4 K, Epitaxial NbN (1 00) films on MgO (100) with high gap frequency (1.4 THz) have also been ach ieved ed under the same deposition conditions at room temperature. The NbN SIS has shown good I-V photon induced steps when LO pumped at 300 GHz. We h ave developed an 8 levels Al/NbN multilayer process for making 1.5 THz SIS mixers (including Al antennas) on Si membranes patterned in SOI-SIMOX. Usin g the planarization techniques developed at the Si-MOS CEA-LETI Facility, m e have also demonstrated on the possibility of extending our NbN technology to high level RSFQ circuit integration with 0.5 mu m(2) junction area, mad e on large area substrates (up to 8 inches).