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 developed using conventional substrates (Si and SOI-SIMOX) for making THz spectrometers including SIS receivers and RSFQ logic gates. NbN/MgO/NbN junctions with area of 1 mu m(2), Jc of 10 kA/cm(2) and low sub-gap leakage current (Vm > 25 mV) are currently obtained from room temperature sputtered multilayers followed by a post-annealing at 250 degrees C, Using a thin MgO buffer layer deposited underneath the NbN electrodes, is insuring lower NbN surface resistance values (Rs=7 mu Omega) at 10 GHz and 4 K, Epitaxial NbN (100) films on MgO (100) with high gap frequency (1.4 THz) have also been achieved 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 have 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. Using the planarization techniques developed at the Si-MOS CEA-LETI Facility, me 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, made on large area substrates (up to 8 inches).