Fabrication and properties of 1 µm2edge junctions

We describe the construction and characteristics of Nb/Nb2O5/Pb In edge junctions which are intended primarily for use in DC SQUIDs. Our aim has been to make junctions as small as possible whilst using conventional optical lithography and lift-off techniques. The base electrode for these junctions is an anodised layer of Nb, which is patterned by reactive ion etching to produce angled edges on which the junctions are formed. The tunnel barriers are grown by RF plasma oxidation using 5% O2in Ar. A series of test junctions with areas between 1 and 10 μm²has been made to establish the necessary oxidation conditions, which have been found to be markedly different from those needed for the formation of barriers for window junctions. To measure the junction capacitance, we have made small unshunted SQUIDs with self inductances ranging between 3 and 10pH, and from the effect of resonances on their current-voltage (I-V) characteristics and their periodic response to current injected through the structure, we determine the junction capacitance to be 0.22±0.01pFμm^-2, at a critical current density of 15 × 10³Acm^-2We have incorporated similar junctions in simple magnetometer-style SQUIDs of InH inductance, and for these we measure the flux noise as 8 × 10^-6φoHz^-1/2in the white noise region. These junctions will also be used in more complex SQUIDs which have spiral input coils, and, based on present measurements, we expect these to have a current sensitivity of less than 0.2pAHz^-1/2