Fabrication of ultrasmall tunnel junctions by electron beam direct-writing

Fabrication of miniaturized tunnel junctions based on high-melting metals b y 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 tec hnique using e-beam direct-writing lithography in conjunction with material deposition by sputtering. To test the preparation process, we have fabrica ted single electron transistors (SETs) based on the metals Al and Nb, inclu ding mixed Al/Nb samples. For SETs made completely of Nb, we preferred AlOx to the natural oxide NbOx for barrier generation. The yield of functioning samples amounted to about 80%. By means of simple considerations we have e stimated 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.