Design, Fabrication and Characterization of a Microwave Resonator ...

5.1 First Experimentsname design W S length frequency capacitance substrate[µm] [µm] [mm] [GHz] value materialR41 Yale 5 10 9.759 5.85 overcoupled 6.2 fF SapphireTable 5.4: Description of the resonator R41 on Sapphire.R41 was designed for and measured in the probing station installed at the Walther-Meißner-Institutwhich allows fast measurements down to below 4 K for frequencies up to 20 GHz.At 4 K no change in the spectrum compared to room temperature was visible. The resonatorwas additionally tested at room temperature in a probing station at the Institute for High-Frequency Engineering of the Technical University of Munich. Also here the structuresshowed insulating behavior. With high probability the Wolfram probes of the probing stationare not able to connect to the Niobium, as the latter is insulated due to an oxide layer.After having no success in a probing station, we bonded R41 in the PCB box. We clearlysaw a peak at 6.16 GHz at 4 K with an insertion loss of -15 dB, that is not visible at highertemperatures. We are not sure whether this is the desired resonance peak as we have nophase data to verify the result. In addition the magnitude is much smaller than what wewould expect in the case of a resonance. The peak is broad and has a quality factor ofaround 60. With this coupling capacitance geometry Frunzio et al. [6] measured qualityfactors of about 5 × 10 3 at 4.2 K.41