Impact of the Buffer Layers and Anodization on Properties of NbTiN Films for THz Receivers

In this article, we present the results of the research aimed at improving the fabrication process of a SIS-mixer for operation at frequencies close to 1 THz. We study the impact of buffer aluminum oxide layer and anodization effects on the properties of superconducting NbTiN films which form the electrodes of the transmission lines in THz-range devices. These layers are traditionally used in technological processes. The measurements of THz response are performed using terahertz time-domain spectrometer at frequencies from 0.3 to 2.0 THz. It was found that the critical temperature, normal-state conductivity just above the transition temperature, superconducting gap value and London penetration depth of the NbTiN film sputtered on aluminum oxide buffer layer are almost the same as of the film sputtered directly onto the substrate. The difference between the parameters is comparable to the measurement uncertainty for NbTiN films with and without additional surface layers of aluminum and anodization.