Proximity effects in all refractory niobium tunnel junctions with a metal back-layer

Josephson devices based on niobium technology are widely used for many applications, so a very high quality is a mandatory characteristic of Nb/Nb Josephson tunnel junctions. The occurrence of proximity effect is the most relevant aspect in determining their quality. Up to now Nb-Al/AlOx/Nb and Nb-Al/AlOx/Al-Nb structures have been studied from both the experimental and theoretical point of view. On the contrary less information is available on Nb/Nb Josephson junctions with a metal back-layer (M-Nb/AlOx/Nb). This is not a minor point in respect to some specific applications. The present paper reports a novel procedure to fabricate high quality Nb/Nb junctions with a back-layer together with preliminary measurements on Al-Nb/AlOx/Nb junctions. (orig.)