Characterization of NbTiN thin films prepared by reactive DC-magnetron sputtering

Niobium-titanium-nitride (NbTiN) thin films were prepared on quartz substrates by reactive dc-magnetron sputtering with a NbTi alloy target, and their properties, were studied systematically. Properties of the thin films, such as superconducting transition temperature were strongly dependent on variation of cathode voltage, ΔU, which is induced when a small fraction of N₂ is added to Ar during sputter discharge. High quality thin films with transition temperature as high as 15 K and resistivity of ∼100 μΩcm have been obtained at around ΔU=28 V with a total gas pressure of 0.9 Pa. Since gap frequency calculated from the measured critical temperature of 15 K based on BCS theory is about 1.1 THz, these NbTiN thin films are good candidates for wiring layers of SIS mixers as well as SIS junctions in the 1 THz band.