Electron beam lithography fabrication of superconducting tunnel structures

An electron beam lithography technique for fabricating submicron Nb–AlN–NbN junctions has been developed and optimized. An exposure dose, development time, and plasma-chemical etching parameters that would ensure the maximum quality parameter of the $R_j/R_n$ tunnel junctions have been selected. The use of negative resist ma-N 2400 with a lower sensitivity and better contrast as compared with resist UVN 2300-0.5 has made it possible to improve the reproducibility of the structure fabrication process and fabricate the submicron Nb–AlN–NbN tunnel junctions (an area from 2.0 to 0.2 $\mu$m$^2$) with a high current density and a quality parameter of $R_j/R_n$ > 15. The spread of the parameters of the submicron tunneling structures over a substrate and the cycle-to-cycle reproducibility of the structure fabrication process have been experimentally measured.