Improving equivalent circuit models of interdigitated capacitors

Interdigitated capacitors are nearly ubiquitous circuits found in everything from telecommunications components to superconducting quantum computers. When the guided wavelength becomes comparable to the length of the interdigitation in the capacitor, common lumped-element models fail to describe how the impedance changes as a function of frequency. This failure limits the frequency range where one can accurately predict the performance of interdigitated capacitors and extract material properties. Here, we develop a multi-mode distributed theory for interdigitated capacitors and quantify the trade-offs between a lumped-element and distributed approach. To test our model, we measured interdigitated capacitors fabricated on (LaAlO₃)_0.3(Sr₂TaAlO₆)_0.7 substrate and interdigitated capacitors with same geometry fabricated on a BaSrTiO₃ thin film.