Reactive ion etching in the fabrication of niobium tunnel junctions

A niobium-based tunnel-junction technology has been developed for analog signal-processing applications. Devices of interest would integrate superconductive transmission lines and superconductor/insulator/superconductor (SIS) mixers. Reactive ion etching of niobium patterns with linewidths of 1 μm or less has been investigated with regard to the composition of gases, chamber pressure and power levels. These studies were made on RF-sputtered niobium films having critical-transition temperatures above 9°K. Junction area is defined by reactively ion etching insulating films of silicon monoxide to resolutions of 0.3μm or better. A niobium-oxide tunnel barrier is grown by RF-plasma oxidation. Proper control of the technology has allowed the achievement of critical current densities which are uniform to 10% or better over the silicon substrate. Junction V-I characteristics show low leakage currents with critical current densities to 2 × 10⁴A/cm². Lead is employed for counter-electrodes and indium-gold (or aluminum) for circuit resistors. The robustness and stability of niobium-based junctions is essential in applications which normally require extreme dependability and long standby times.