In-situ deposited anti-aging TiN capping layer for Nb superconducting quantum circuits

The performance of Nb superconducting quantum devices is predominantly limited by dielectric loss at the metal–air interface, where Nb₂O₅ is considered the main loss source. Here, we suppress the formation of native oxides by in-situ deposition of a TiN capping layer on the Nb film. With TiN capping layers, no Nb₂O₅ forms on the surface of the Nb film. The quality factor Q_i of the Nb resonator increases from 5.6 × 10⁵ to 7.9 × 10⁵ at low input power and from 6.8 × 10⁶ to 1.1 × 10⁷ at high input power. Furthermore, the TiN capping layer also shows good aging resistance in Nb resonator devices, with no significant performance fluctuations after one month of aging. These findings highlight the effectiveness of TiN capping layers in enhancing the performance and longevity of Nb superconducting quantum devices.