FABRICATION OF NB / AL-NI / NBTIN JUNCTIONS FOR SIS MIXER APPLICATIONS ABOVE 1 THZ

We discuss the material processing limits of superconductor-insulator-superconductor (SIS) junctions with an energy gap high enough to enable THz heterodyne mixer detection. The focus of this work is a device structure which has Nb as a base layer, a tunnel barrier formed by plasma nitidation of a thin Al proximity layer, and NbTiN as a counter-electrode material. These SIS junctions typically exhibit 3.5 mV sum-gap voltages with the sub-gap to normal state resistance ratio Rsg / R N = 15 for resistance- area products RNA = 20 pm'. This process is developed such that junctions will be integrated to mixer antenna structures incorporating NbTili as both ground plane and wire circuit layers. Run-to-run reproducibility and control of the RNA product is addressed with regard to the conditions applied during plasma nitridation of the Al layer. RF plasma nitriciation of the aluminum is investigated by control of DC floating potential, N2 pressure, and exposure time. Processing is done at near room temperature to reduce the number of variables. Stress in the metal film layers is kept in the low compressive range. Recent receiver results will be discussed in another work presented at this symposium. [1