Characterization of encapsulated superconducting microwave resonators

Surface amorphous oxides in superconducting films, commonly used to fabricate superconducting qubits, have been demonstrated to host two-level systems that can limit their coherence. Encapsulation with other materials can potentially reduce the formation of these lossy oxides and improve qubit performance. In this work, we study the effects of encapsulation of superconducting films with non-superconducting materials. For encapsulation, we explore both in-situ capping techniques, after the growth of superconducting films, as well as alternate methods, involving ex-situ etching and subsequent passivation. We first comprehensively study the surface morphology and crystallinity of these superconducting heterostructures using atomic force microscopy, electron microscopy, X-ray photoemission spectroscopy, and X-ray diffraction. Next, we characterize the critical temperature and residual resistivity ratio of these encapsulated films. Finally, we measure microwave resonator devices fabricated from encapsulated and non-encapsulated films in a dilution refrigerator operating at milliKelvin temperatures. Through this study, we aim to correlate the observed material properties with the DC and microwave behavior of superconducting microwave resonators encapsulated with normal metals.