Fabrication of ultrasmall tunnel junctions by electron beam direct-writing

Fabrication of miniaturized tunnel junctions based on high-melting metals by the shadow evaporation technique is rather complicated. The thermal load of the suspended bridge mask during metal evaporation is assumed to be the most serious problem. As an alternative we have developed a preparation technique using e-beam direct-writing lithography in conjunction with material deposition by sputtering. To test the preparation process, we have fabricated single electron transistors (SETs) based on the metals Al and Nb, including mixed Al/Nb samples. For SETs made completely of Nb, we preferred Al0_x to the natural oxide NbO_x for barrier generation. The yield of functioning samples amounted to about 80%. By means of simple considerations we have estimated the tunnel capacitances to be of the order of a few 10^-16 F, the tunnel resistance spread was less than one order of magnitude