Vapor-phase HMDS passivation improves performance and ambient stability of niobium superconducting resonators

Native oxide formation (NbOx) at the niobium (Nb) metal–air interface is a source of microwave loss and performance variability in superconducting resonators. We introduce vapor-phase hexamethyldisilazane (HMDS) as a dry surface passivation treatment compatible with standard nanofabrication workflows. In five independent fabrication runs, HMDS-passivated resonators consistently demonstrate improved internal quality factor (Qi) and narrower performance distribution relative to paired unpassivated controls. The Qi improvement occurs across the full microwave power range, suggesting NbOx suppression as a contributing mechanism. Statistical depth profile analysis of Nb oxidation state by electron energy loss spectroscopy corroborates this mechanism, revealing smaller chemical shifts and shallower oxidation depth in passivated specimens relative to the control. Furthermore, re-measurement after 60 days of ambient storage shows that HMDS-passivated resonators retain their Qi, indicating their ambient stability. These results establish vapor-phase HMDS as a surface passivation strategy with strong potential for superconducting quantum devices.