NbN tunnel junctions

All-Niobium Nitride Josephson junctions have been prepared successfully using a new processing called SNOP : Selective Niobium (Nitride) Overlap Process. Such a process involves the "trilayer" deposition on the whole wafer before selective patterning of the electrodes by optically controlled Dry Reactive Ion Etching. Only two photomask levels are need to define an "overlap" or a "cross-type" junction with a good accuracy. The properties of the Niobium Nitride films deposited by DC-Magnetron sputtering and the surface oxide growth are analysed. The most critical point to obtain high quality and high gap value junctions resides in the early stage of the NbN counterelectrode growth. Some possibilities to overcome such a handicap exist even if the fabrication needs substrate temperatures below 250 °C.