Characterizing dielectric properties of ultra-thin films using superconducting coplanar microwave resonators

We present an experimental approach for cryogenic dielectric measurements on ultra-thin insulating films. Based on a coplanar microwave waveguide design we implement superconducting quarter-wave resonators with inductive coupling, which allows us to determine the real part ε1 of the dielectric function at GHz frequencies and for sample thicknesses down to a few nm. We perform simulations to optimize resonator coupling and sensitivity, and we demonstrate the possibility to quantify ε1 with a conformal mapping technique in a wide sample-thickness and ε1-regime. Experimentally we determine ε1 for various thin-film samples (photoresist, MgF2, and SiO2) in the thickness regime of nm up to μm. We find good correspondence with nominative values and we identify the precision of the film thickness as our predominant error source. Additionally we demonstrate a measurement of ε1(T) vs. temperature for a SrTiO3 bulk sample, using an in-situ reference method to compensate for the temperature dependence of the superconducting resonator properties.