Superconducting-normal metal bi-layers for antenna structures of hot-elektron detector

We have developed a quasi-optical version of a superconducting hot-electron detector (HED) suitable for application in the THz frequency range based on a sub-micron bridge made from ultra-thin superconducting (NbN) film embedded into antenna structure. Standard antenna is made from 200 – 300 nm thick gold layer with in-situ deposited buffer layer of getter materials like Ti or Cr. The surface of NbN ultra-thin film is contaminated during previous fabrication steps. Impurities located in an interface between the NbN and buffer layers for gold antenna structure lead to an interface contact resistance worsening the high-frequency performance of HED. At the same time a low resistance of NbN film – buffer layer interface is characterized by a stronger proximity effect between ultra-thin superconducting film and normal metal antenna structure. The critical temperature of the device is reduced and thus its performance is worsened. The superconductivity of ultra-thin NbN films under the antenna structure can be supported by using in-situ deposited Nb or even better NbN layer as a buffer for Au deposition. Two magnetrons (Nb and Au) depositing system additionally equipped with an Ar-ion source for interface cleaning purpose has been built and tested. Results on development of thin film (Nb, NbN, Au) deposition processes will be presented and discussed in terms of critical temperature and temperature dependence of the resistivity of as-deposited films and multi-layer structures.