Fabrication of high-quality submicron Nb/Al--AlO_x/Nb tunnel junctions

Nb/Al--AlO_x/Nb tunnel junctions are often used in the studies of macroscopic quantum phenomena and superconducting qubit applications of the Josephson devices. In this work, we describe a convenient and reliable process using electron beam lithography for the fabrication of high-quality，submicron-sized Nb/Al--AlO_x/Nb Josephson junctions. The technique follows the well-known selective Nb etching process and produces high-quality junctions with V_m=100 mV at 2.3 K for the typical critical current density of 2.2 kA/cm^2, which can be adjusted by controlling the oxygen pressure and oxidation time during the formation of the tunnelling barrier. We present the results of the temperature dependence of the sub-gap current and in-plane magnetic-field dependence of the critical current, and compare them with the theoretical predictions.