Sub-micron normal-insulator-superconductor tunnel junctions for sensitive thermometry and micro-refrigerators

We report on measurements of normal-insulator-superconductor tunnel junctions at temperatures below 1K. The junctions have been fabricated by shadow mask technique and e-beam lithography and have submicron dimensions. The measurements on our junctions show that their characteristics are very sensitive to temperature changes making them suitable to be used as a sensitive secondary thermometer. To demonstrate this we use this tunnel structure for sensing the electron temperature drop in a copper strip of an electronic micro-refrigerator, that was made by arrays of tunnel junctions, as well.