Increased quality factor in superconducting microstrip resonators by selective removal of the gold contact layer

Abstract

We present measurements of the unloaded quality factor of superconducting microstrip resonators at 77 K and 3.86 GHz. The resonators were made of 0.3 μm thick YBa2Cu3O7 films with a transition temperature of 90 K on both sides of ceria-buffered 3 inch sapphire wafers. In particular, we investigated the effect of a 0.3 μm thick gold contact layer on the resonator performance. It was found that the gold layer decreases the quality factor by a factor of almost 5. This result is due to an additional microwave loss in the gold film which can be quantitatively described by the impedance transformation rules for transmission lines. On the basis of the quantitative analysis, we suggest a selective removal of the contact layer by appropriate patterning in order to eliminate the extra loss without deterioration of the low-ohmic galvanic contact to the microwave housing. The experimental results demonstrate the usefulness of the proposed method. Their comparison with the calculated surface current density in the ground plane shows that the contact layer has to be in an area where the current is zero. Furthermore, the results reveal that the surface current density distribution in the ground plane and microstrip depends on the microwave power.