Substrate etching for quasiparticle effect mitigation: compatibility with superconducting circuits*

We fabricate, measure, and characterize superconducting coplanar waveguide (CPW) resonators located on thin (~100 nm) SiN membranes, where the Si substrate originally supporting the membranes is entirely etched away. This isolates the resonators from the bulk substrate, motivated by the desire to reduce correlated effects from nonequilibrium quasiparticles mediated by phonons in the substrate. We find typical single-photon internal quality factors of these suspended resonators to be ~10⁵, comparable to other resonators on the same chip where the substrate beneath them remains. These two resonator types are further compared in their temperature equilibration rates and their frequency stability versus time, where no detrimental effects are observed when suspending. To evaluate the device quality from other metrics, we additionally probe the devices using XPS, EDS, and TEM. Ultimately, we find that fabricating superconducting circuit components atop thin SiN membranes is compatible with yielding high-Q resonators. As such, this technique utilizing substrate etching is a promising platform to explore the impact of quasiparticles on qubits and their propagation within a chip.