Cryogenic microwave characterization of Kapton polyimide using superconducting resonators

Half-wavelength, capacitively-coupled superconducting microstrip resonators have been constructed on 50.8 μm (2 mil) thick flexible Kapton polyimide substrates. The metal stack-up on each side was a 50 nm Ti adhesion layer followed by a 250 nm Nb layer. These resonators yield high quality factors (loaded Q as high as 4110) at 1.2 K in the 2-10 GHz frequency range, implying a loss tangent of less than 0.000275 at 2 GHz. This work provides complex dielectric permittivity information for Kapton materials that have not previously been reported for this temperature (1-6 K) and frequency range. Furthermore it provides confidence that commercially available flexible Kapton is potentially useful as a substrate material for flexible superconducting interconnects or cables, which are of great interest for use in cryogenic electronics systems.