Nanoscale SNS junction fabrication in superconductor-normal metal bilayers

We have developed a reliable and versatile technique for fabricating SNS junctions in a superconductor-normal metal bilayer using a focused ion beam microscope (FIB) in conjunction with an in-situ resistance measurement technique. This technique offers a simple method for creating multi-junction devices (SQUIDs, 3-terminal devices, arrays) with high integration densities. In this paper we discuss recent results from devices created in Nb-Cu tracks by cutting 50 nm trenches in the top Nb layer to weaken the superconducting coupling. Cuts of depths between 60 and 100% of the Nb thickness yield reproducible junctions with current voltage (I(V)) characteristics in accordance with the resistively-shunted-junction (RSJ) model, characteristic voltage I_C R_N~50 μV at 4.2 K and excellent microwave response. A thorough study has been carried out of the effect on device parameters of varying the Cu layer thickness (0-175 nm). In addition transmission electron microscopy (TEM) studies have been carried out on the device structure. A two-channel model of device operation has been developed and related to the results of I^CR_N(T) measurements (down to 350 mK) on selected devices