Reduced microwave loss in trenched superconducting coplanar waveguides

Abstract

Reducing the contribution of all sources of microwave loss is important for increasing coherence times in superconducting qubits. In this paper we investigate reducing the loss by systematically removing Si substrate material from the gap region in titanium nitride coplanar waveguides fabricated on intrinsic Si substrates. By exploiting the radial dependence of the etch rate in a parallel plate reactive ion etcher, otherwise identical coplanar waveguides with only the Si gaps etched to varying depth, i.e., trenched, were created in a single TiN film within a single processing step. Measurements at these multiple depths permit the study of the loss reduction in isolation to the unintentional effects caused by any single processing step. When comparing the loss from all trench depths we found that the high power loss was similar, but in the single photon limit the loss was reduced by a factor of two for deeper trenches in agreement with predictions from finite element analysis.