An improved NbN integrated circuit process featuring thick NbN ground plane and lower parasitic circuit inductances

We report on the development of a 10 K, NbN superconductive integrated circuit (IC) technology that utilizes an improved SiO2 interlevel dielectric (ILD) deposition process and a thick NbN ground plane layer to reduce parasitic circuit inductances, The ILD process uses a novel low frequency (40 kHz) substrate bias during spotter deposition of SiO2, which produces very smooth oxide films having a roughness less than 0.1 nm (rms) as measured by atomic force microscopy (AFM). Bias-sputtered SiO2 is used to planarize and to smooth the surface of the NbN ground plane layer in preparation for fabrication of NbN/MgO/NbN tunnel junctions. High current density tunnel junctions ranging from 1000 A/cm(2) to 5000 A/cm(2), fabricated over NbN ground planes up to 1 mu m thick, exhibit low subgap leakage (V-m similar to 15 mV at 10 K) and high sumgap voltage (V-g = 4.4 mV at 10 K). Typical wiring inductance over ground plane has been reduced by, 25% compared to our present NbN foundry process.