Impact of conductor losses and metal overetching on integrated superconducting coplanar waveguide resonators

In this work, basic guidelines for the development of superconducting coplanar waveguide resonators (CPWR) suitable for quantum computing applications are presented along with simulation and experimental results that highlight the impact of pattern definition. CPWR with resonance frequencies between 5 and 20 GHz were designed, simulated, modeled using an RLC equivalent circuit model and a transmission line model, and fabricated using Al/Si and Al/SiO₂/Si structures. Finite element simulations were done using resistivity values for Al corresponding to temperatures in the range between ambient temperature and the superconducting regime. From the simulations, the impact of conductor losses in the quality factors of the resonators was confirmed. To study the effect of the coupling structures, gap and finger coupling capacitors were considered. Results show that undercoupled and overcoupled conditions can be achieved from those coupling structures. Finally, simulations, models and ambient-temperature measurements were correlated. From this comparison, it was found that differences between experimental and theoretical data can be attributed to an imperfect photolithographic definition of the critical dimensions for Al-based CPWR, pointing out the relevance and impact of fabrication processes for integrated circuits operating in the microwave regime.