Impact of Annealing on Superconducting Properties of Tantalum Thin Films
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 through UHV annealing. In this work, we examine the effect of dissolved oxygen as a function of annealing temperature on the superconducting properties of tantalum thin films. Dissolved oxygen is quantified using atom probe tomography (APT) and secondary ion mass spectrometry (TOF-SIMS) and correlated with electromagnetic transport measurements performed using a Physical Property Measurement System (PPMS). This study observes that annealing decomposes the native oxide and drives oxygen diffusion into the film matrix. Increasing oxygen content decreases the superconducting transition temperature (Tc) and the residual resistivity ratio (RRR). The critical field decreases with the incorporation of oxygen, while the film’s coherence length decreases relatively by 5%. These results, as well as findings from ongoing studies linking fabrication processes to the structural and chemical properties of the superconducting film and Josephson junction, provide insight into effective strategies for improving the performance of superconducting devices.