Hydride precipitates in Nb thin film superconducting qubits

We report the observation of hydride precipitates within Nb thin films on Si substrates as a possible decoherence source for superconducting qubits. We performed high-resolution transmission electron microscopy (HR-TEM) and electron diffraction analysis on the Nb thin films and observed hydrides in the Nb thin films of superconducting qubit test devices. The size, morphology, and atomic structures of the hydrides within the device structures are illustrated. Hydride precipitates tend to be seen near the surface of the Nb films and exhibit variation in size and morphology from small (~5 nm) irregular shaped precipitates within the interior Nb grains to large (~10-100 nm) grains partially or fully transformed to niobium hydride. Atomic force microscope (AFM) analysis on the surface of the device film at cryogenic temperatures shows unknown structures, implying possible structural changes on the surface of Nb during cool down, possibly related to hydride precipitation. Time-of-flight secondary ion mass spectroscopy (TOF-SIMS) analysis indicates that the hydrogen and other impurity concentration is enriched near the surface, which may play a role in the nucleation of hydrides near the surface regions. Additional investigations of hydrides in various Nb samples imply that hydride formation in Nb is correlated to the grain size, etc. Nb hydrides are non-superconducting and may contribute to the decoherence due to quasiparticles and two-level system (TLS). This study suggests the importance of preventing hydride formation in the Nb films during the Nb thin film growth and fabrication process. Possible pathways to prevent the detrimental hydride precipitates in Nb thin film are also discussed.