Characterization of NbTiN Films for Superconducting Terahertz Circuitry

We have investigated superconducting properties of niobium titanium nitride (NbTiN) films to be used for the lower and upper electrodes of a microstrip line with a SiO₂ insulator on a quartz substrate at Terahertz frequencies. NbTiN films were prepared on a 1-mm thick quartz substrate directly (assumed to be the lower electrode of the microstrip line) and on an about 150-nm thick SiO₂ layer sputtered on a quartz substrate (assumed to be the upper electrode) by reactive dc magnetron sputtering at ambient temperature. The complex conductivity measurement by Terahertz time- domain spectroscopy revealed that both films have similar superconducting properties with critical temperatures of about 14 K and gap frequencies of about 1.2 THz at 5 K. This indicates that all-NbTiN microstrip lines may work well as extremely low-loss transmission lines at Terahertz frequencies.