Improvement of Thermal Stability And Critical Current Of Superconducting Nb Multi-Layer Wiring

A spiral coil of double-Nb layers is fabricated to investigate the thermal degradation mechanism of Nb multilayer wiring. The superconductivity of the coil with a sputtered SiO₂insulation layer easily deteriorates as a result of surface oxidation where the step coverage is incomplete. Higher critical current and thermal stability are obtained by a combination of planarization using spin-coated SiO₂for the insulation between Nb wiring, surface nitri- dation of Nb, and taper etching of the contact hole. The critical current of the coil with a spin-coated SiO₂film is about twice that of a coil with sputtered SiO₂even though it was baked at 300°C for 30 min for each layer.