Understanding The Formation and Evolution of Surface Oxidation Layers in Nb Thin Films

Surface oxides formed on metal films have a significant impact on the decoherence and stability of superconducting qubits. Niobium (Nb) exhibits a complex native oxide layer, approximately 10 nm thick, believed to harbor structural and chemical defects contributing to coherence loss in Nb-based devices. In this study, Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) was used to assess the influence of different treatments on the surface oxidation layers of Nb thin films. We employed ToF-SIMS for high-resolution chemical mapping and depth profiling, combined with ex situ ¹⁸O₂ annealing experiments. This allowed us to characterize and evaluate the surface oxidation layers of Nb thin films resulting from chemical mechanical polishing (CMP) and accelerated neutral atom beam (ANAB) treatments, comparing them to native oxides formed through air exposure. By combining ToF-SIMS results with XPS and TEM studies, we revealed the differences in oxidation layer thickness and composition, including Nb₂O₅, Nb₂O₃, and NbO, on the surface. Moreover, we demonstrated that ANAB-generated oxidation layers exhibit enhanced resistance to further oxidation when compared to native oxides. This finding suggests a promising strategy for mitigating surface oxidation and improving the performance of devices based on superconducting metals.