Dielectric Encapsulated Niobium Films to Achieve Higher Processing Temperatures

To avoid degradation of circuit performance, fabrication processes for niobium-based superconductive electronics are typically limited to temperatures below 150 °C. In this study, we investigated protective dielectric capping layers that preserve the superconducting properties of Niobium (Nb) wiring at processing temperatures as high as 400 °C. To assess the thermal stability of Nb films, 400 nm thick Nb layers were deposited on oxidized silicon wafers and were either left uncapped or were intentionally capped with selected dielectric materials. The samples were subjected to postdeposition annealing in an argon atmosphere for comparison. Samples were annealed up to 450 °C, and changes in room temperature sheet resistance were used as a proxy for film degradation and checked with cryogenic measurements of samples annealed up to 400 °C. A sharp increase in room temperature sheet resistance, and a resulting decrease in residual resistivity ratio was observed in pristine Nb films above 300 °C. We found that the ex-situ deposition of insulating covering ("cap") reduces the onset of degradation. Notably, silicon oxide (SiO _x ) capping shows significant improvement and silicon nitride (Si _x N _y ) capped samples exhibit minimal changes in resistance across the full annealing range. These results suggest that Si _x N _y encapsulation as a dielectric "cap" preserves superconductive properties and offers an expanded thermal budget for Nb-based superconductive electronics, with implications for multilayer integration and scalable fabrication.