Nb edge junction process for submillimeter wave SIS mixers

The authors describe a junction fabrication process that produces high-quality Nb edge junctions of areas less than 0.2 μm² on thin quartz (0.10 nm) substrates. This process utilizes a 10:1 projection wafer stepper for high-resolution and high-accuracy layer-to-layer registration. Large numbers of junctions were fabricated reliably with high-quality I-V (current-voltage) characteristics and with impedances suitable for use in SIS (superconductor-insulator-superconductor) mixers for submillimeter astronomy. Junctions produced by this process can be stored on the shelf for more than two years with no special precautions taken, and with an impedance change of less than 10%. No failures have been observed after thermal cycling. Currently these junctions are being used in an astronomical receiver that demonstrates state-of-the-art performance in the atmospheric window centered at 345 GHz. With some improvements to the process, it is possible to make junctions with areas sufficiently small for ωRC≈3 at 800 GHz