Measurement of dielectric losses in amorphous thin films at gigahertz frequencies using superconducting resonators

We have developed a reliable method for the direct measurement of dielectric losses in thin films using a special design of lumped element superconducting resonators. The method presented allows obtaining quantitative values for the losses in the film volume as well as the metal/dielectric interfaces. Numerous different resonator geometries were designed and measured in order to study the losses in dielectric thin films usually used for Josephson junction fabrication, such as Nb2O5, SiO and SiNx, at low temperatures and low gigahertz frequencies. The results show that for such amorphous materials, the bulk losses clearly exceed the interface losses. Furthermore, the frequency dependence of the losses in this working regime was studied for the first time. Our results are in good agreement with the universal law describing the frequency dependence of dielectric losses and suggest that the losses are due to many-body interacting dipoles, which is an important fact for the theoretical modelling of two-level fluctuators. We also investigated the losses in multi-layer films and find that the results are in good agreement with the theoretical expectations, which allows optimization of the dielectric multi-layers used in Josephson junction and qubit fabrication.