An array of 100 Al-Al2O3-Cu SIN tunnel junctions in direct-write trilayer technology

We present superconductor-insulator-normal metal (SIN) tunnel junction thermometers made of arrays of 4-100 Al-Al2O3-Cu SIN tunnel junctions fabricated in direct-write technology. The technology is based on in situ evaporation of the superconductive electrode followed by the oxidation and the normal counter-electrode as a first step and deposition of normal metal absorber as a second one. This approach allows one to realize any geometry of the tunnel junctions and of the absorber with no limitation related to the size of the junctions or the absorber, which is not possible using the shadow evaporation technique. Measurements performed at 300 mK showed the high quality of the fabricated tunnel junctions, low leakage currents, and that an Rd/Rn ratio of 500 has been achieved at that temperature. The junctions were characterized as temperature sensors, and voltage versus temperature dependence measurements showed a dV/dT of 0.5 mV K-1 for each single junction, which is typical for this kind of tunnel junction. A temperature resolution of ± 5 μK has been achieved which is much better than the previously reported value of ± 30 μK for this type of thermometer