Electron cooling by arrays of submicron tunnel junctions

We cooled down the electrons of a 3 μm³ copper strip by using an on-chip electronic microcooler made by a superconductor-insulator-normal-insulator-superconductor (SINIS) structure. We obtained an electron temperature drop $\text{ΔT≃−10\hspace{0.05em}}\mathrm{mK}$ at a temperature of the bath slightly above $T_b\text{=500\hspace{0.05em}}\mathrm{mK}$ and we estimated a cooling power slightly larger than 20 pW at this temperature. In contrast to previous experiments, we obtained this result operating with a simple ${}^3\mathrm{He}$ refrigerator. The copper strip temperature change was sensed by measuring the voltage change across a current-biased SINIS junction, with a temperature responsivity $\text{ΔV/ΔT≃−500\hspace{0.05em}μ}\mathrm{V/K}.$ On-chip microcooler and thermometer have been fabricated by two-angle shadow mask evaporation and e-beam lithography.