In-situ Transmission Electron Microscopy Study on Niobium Oxide Annealing in Nb Resonator for Quantum Computing*

Niobium is commonly used for superconducting quantum systems as readout resonators, capacitors, and interconnects. The coherence time of the superconducting qubits is mainly limited by microwave dissipation attributed to two-level system defects present at interfaces, such as the Nb/air interface. One way to improve the Nb/air interface quality is by thermal annealing, as shown by extensive studies in 3D cavities. However, it is unclear how the microstructure and chemistry of the oxide layer change during heat treatment. To better understand and optimize device processing at the atomic scale we have developed an in-situ method using an aberration-corrected transmission electron microscope. Combining atomic level imaging and spectroscopy we were able to monitor the niobium oxide layer evolution on Nb thin film during heating from room temperature to ~400 ˚C. Complex structure evolution, including structure, chemistry, and oxidation state of the niobium oxide will be discussed.