Fabrication, study of normal metal-insulator-superconductor junctions Al/AlO$_x$ /Nb

The paper presents the design, fabrication and research of structures based on normal metal-insulator-superconductor tunnelling junctions. The morphology of the three-layer Al/AlO$_x$/Nb structure has been analyzed by transmission electron microscopy, transmission scanning electron microscopy and electron diffraction methods. Selective Niobium Etching and Anodization Process technology was used to form tunnel junctions. The fabricated devices, in which the role of superconductor is performed by Nb, are able to work as thermometers in the temperature range of 1.5–8 K. The quality parameter of the fabricated structures was achieved – the ratio $R_d/R_n$($V$ = 0) = 53 at a temperature of 2.8 K, while the theoretically expected value is 54. The design of structures of series and parallel connected tunnel elements has been developed, the peculiarity of such a design is that the area of the normal metal is large, which means that its electronic system will be reliably thermalised, so such a design will be preferable for thermometry. Integrated thermometer structures whose $dR/dT$ is larger than that of a single tunnelling junction have been fabricated. Normal metal-insulator-superconductor structures based on niobium can be used as thermometers, detectors as well as electronic coolers.