Ultra low-loss sputtered NbN films for microwave applications

We present both fabrication method and measurement details regarding low-loss NbN films fabricated at Argonne National Laboratory. We sputter Nb in the presence of N2 gas, with an RF substrate bias, directly onto high-resistivity Si wafers, and by varying N2 flow rates we control the superconducting critical temperature (Tc) to be between 8-16 K. Coplanar waveguide resonators fabricated by etching these films give quality factors greater than 1E6 at 5 GHz when driven at high power, and quality factors of order 1E4 when loaded with single-photon power levels for all values of Tc. Furthermore, in our highest Tc samples, this excellent quality factor persists at temperatures up to ~2 K. We also report on an anomalous temperature-dependent loss mechanism that affects a portion of our samples and consistently turns on at temperatures above 1 K.