Fabrication of high‐quality Nb/AlOx/Nb Josephson junctions

This paper studies an Nb/AlOxNb junction, and the cross-sectional shape of the junction interface and the surface state are observed. It is found that the AlOx film used as the tunnel barrier layer is locally nonuniform and is dependent on the shape of the Nb film forming the lower electrode. This dependence is a factor causing junction characteristic variations. Further, the fabrication method of the Nb electrode film is discussed. It is important to form a flat and dense Nb film with few particle boundary trenches when fabricating the lower electrode film. A high-quality Nb/AlOx/Nb film can be fabricated with Rj/Rnn > 20 and a fluctuation of Imo of ±6%. Also, the Nb/AlOx/Nb junction of 1.5 μm2 with improved junction characteristics is applied to a 3 k logic gate array. The delay time of the 826-stage OR gate is found to be 10.5 ps/gate.