Film-thickness dependence of 10 GHz Nb coplanar-waveguide resonators
The authors have studied Nb λ/2 coplanar-waveguide (CPW) resonators whose resonant frequencies are 10–11 GHz. The resonators have different film thicknesses, t=0.05, 0.1, 0.2, and 0.3 μm. They measured at low temperatures, T=0.02–5 K, one of the scattering-matrix element, S21, which is the transmission coefficient from one port to the other. At the base temperatures, T=0.02–0.03 K, the resonators are overcoupled to the input/output microwave lines, and the loaded quality factors are on the order of 103. The resonant frequency has a considerably larger film-thickness dependence compared to the predictions by circuit simulators which calculate the inductance of CPW taking into account Lg only, where Lg is the usual magnetic inductance determined by the CPW geometry. By fitting a theoretical S21 versus frequency curve to the experimental data, they determined for each film thickness the phase velocity of the CPW with an accuracy better than 0.1%. The large film-thickness dependence must be due to the kinetic...