Novel coplanar waveguide structures for microwave circuits employing high temperature superconductors

Abstract

Superconducting thin film deposition technologies have advanced significantly over recent years leading to the development of high power transmission line and filter structures. The power handling capabilities of such films are however limited by the fact that the current distribution in the line is strongly peaked at its edges. The peak is detrimental since it leads to local suppression of the order parameter and a corresponding increase in the surface impedance. Noting that the width of the peak region is on the order of the separation of the line from the ground plane, we propose novel transmission line geometries which enhance the maximum microwave power which can be input to a superconducting transmission line without driving it normal. In particular the V-shaped conductor-backed coplanar waveguide is shown to redistribute the current more evenly over the cross-section of the waveguide. Since in this concept the deposition and processing techniques for the superconducting film are unchanged from those used in conventional coplanar waveguide (CPW) structures, no further refinements in film growth are required. We also propose using a multi-layer V-shaped CPW line, which has recently been demonstrated in GaAs technology. Further advances in HTS multi-layer techniques are required before an HTS-based version can be realised.