Microwave and DC properties of niobium coplanar waveguides with 50-nm linewidth on silicon substrates

Long superconducting niobium coplanar waveguides (CPW) on silicon wafers have been implemented with inner conductor width, thickness and length of about 50 nm×250 nm×3.2 mm, respectively. CPW tapers yield the transition from small to large cross sections for 50 Ω coaxial SMA connectors. Microwave properties are measured at 4.2 K via λ/2-resonators where the inner conductor width w_i is about half the London penetration depth. Due to the dominant kinetic inductance the measured quality factor is close to an evaluated minimum value for w_i<50 nm. The quality factors of such nanolines with niobium are approximately 150 at 10 GHz, i.e., more than two orders of magnitude larger than with aluminum at the same temperature. An unexpected power dependence of the intermodulation amplitudes and the corresponding dc properties are described.