Improvements in silicon oxide dielectric loss for superconducting microwave detector circuits

Abstract

Dielectric loss in low-temperature superconducting integrated circuits can cause lower overall efficiency, particularly in the 90 to 220 GHz regime. We present a method to tune the dielectric loss for silicon oxide deposited by plasma-enhanced chemical-vapor deposition at ambient temperatures. Deposition in an environment with a higher silane-to-oxygen ratio produces silicon oxide films with a lower loss-tangent and a slightly higher optical index of refraction, while contributing no appreciable change in film stress. We measured the dielectric loss by fabricating a series of Nb- SiO_x-Nb microstrip resonators in the frequency range of 6 to 9 GHz and comparing their temperature dependence to a model of parasitic two-level-system fluctuators. The dielectric loss-tangent of silicon oxide was improved from 6 × 10^-3 for stoichiometric silicon dioxide to 2 × 10^-3 for a more silicon-rich silicon oxide. We present details of the fabrication process and measurements of microstrip resonators.