Impact of Annealing on Superconducting Properties of Tantalum Thin Film

The pursuit for higher-coherence superconducting qubits has led to the adoption of new materials and innovative fabrication techniques. Among the commonly used superconducting materials, tantalum-based or tantalum-capped superconducting qubits have demonstrated the longest lifetimes, outperforming niobium devices even in the presence of their native oxides. Given the losses introduced by the surface oxide, one method to further improve performance is to reduce or eliminate the native oxide on tantalum, either by dissolving through annealing or by removing it with a chemical wet etch. This work examines the effect of annealing on the structural and superconducting properties of tantalum thin films. The chemical composition as a function of annealing temperature was investigated using X-ray photoemission spectroscopy (XPS), secondary ion mass spectrometry (ToF-SIMS), and atom probe tomography (APT), and the results were correlated with electrical transport measurements using standard PPMS methods.  In situ chemical analyses reveal that the native oxide of tantalum is composed of Ta⁵⁺ and Ta²⁺, and annealing leads to the decomposition of the pentoxide, driving oxygen into the film. While oxygen concentration increases within the film with annealing temperature, other impurities, such as carbon and hydrogen, rise to the surface. Although annealing reduces the native oxide, the incorporation of oxygen into the film degrades its superconducting properties, leading to reductions in the transition temperature (Tc), the residual resistivity ratio (RRR), the critical field, and the coherence length of the tantalum film.