Dispersive Spectrometry at Terahertz Frequencies for Probing the Quality of NbTiN Superconducting Films

We present the quality measurements of thick (thicker than London penetration depth) NbTiN superconducting films at Terahertz frequencies using a Dispersive Fourier Transform Spectrometer (DFTS). The reflected RF signal from the tested film was measured in time domain, allowing us to separate it from other reflections. The complex conductivity of the film depends on frequency and determines the reflection coefficient. By comparing the film reflection in superconducting state (film temperature below $Tc$) with that of the normal state (film temperature above $Tc$), we characterized the film quality at terahertz frequencies, and directly probed the energy of the superconducting gap of the tested film. The experimental results were fitted using the extended Mattis-Bardeen theory and th obtained film parameters show a good agreement with the literature. In addition to the DFTS, we have also measured the properties of NbTiN film using Time Domain Spectroscopy (TDS). It is shown that both TDS and DFTS provide similar results, and both techniques can be used for the quality control of thick NbTiN films. The superconducting gap determined from the measurements by both DFTS and TDS are in good agreement for both solid and meshed films showing that there is no remarkable degradation in the film quality due to technological processes of lift-off or ion etching.