# Superconducting Sub-Terahertz Oscillator with Continuous Frequency Tuning
## 2 Materials and Methods  
At present, the main elements of superconducting electronics are SIS tunnel structures based on niobium and its compounds. Films made of these materials have high mechanical strength, chemical stability, sustainability to thermal cycling and good adhesion, as well as a sufficiently high critical temperature to operate at liquid helium temperature (T = 4.2 K). The critical temperatures of the films made of niobium and niobium nitride (NbN) are 9.2 K and 15.5 K, respectively. The technology of Nb tunnel junctions with an Al interlayer is based on the well-known fact that a very thin Al layer completely covers the Nb base electrode [43; 44; 45; 46; 47], effectively wetting the columnar microstructure of the Nb film and flattening the surface [45] in spite of its roughness. The tunnel barrier is formed by the oxidation or nitridization of a thin aluminum film (5-7 nm), which covers the niobium surface as a continuous layer and makes it possible to obtain ultrathin tunnel layers (about 1 nm thick). The possibility of fabricating the tunnel junctions with high current density and extremely low leakage, which do not change their characteristics for many years, is the most important factor that ensures the widespread use and practical application of junctions based on niobium. Fabrication technology of multilayer superconducting circuits based on Nb-AlO\({}_{x}\)-Nb junctions made by Selective Niobium Etching and Anodization Process (SNEAP) [46] was developed and implemented at Kotelnikov Institute of Radio Engineering and Electronics of Russian Academy of Science. A number of new technological processes and techniques for manufacturing superconducting tunnel junctions with unique parameters have been developed [47; 48]. Tunnel junctions with high current density allow increasing the operating frequency of SIS receivers and expanding their bandwidth. However, there is a limit to the transparency of the barrier for SIS junctions based on alumina. This limit is of the order of 10-15 \(\mathrm{kA/cm^{2}}\); with a further increase in the current density, a sharp degradation of the quality of the junctions occurs. In order to overcome this limitation, a technology for manufacturing Nb/Al-AIN/Nb SIS tunnel junctions with extremely high transparency of the tunnel barrier by nitridization of the Al surface in an RF plasma discharge with acceptable values of the ratio of the subgap to normal resistance (quality parameter \(\mathrm{Rj/Rn}\)) was developed [49; 50; 51; 52]. Application of NbN as the upper electrode in a three-layer structure made it possible not only to achieve a critical current density up to 70 \(\mathrm{kA/cm^{2}}\), but also to significantly improve the parameters of the tunnel structures. As a result, the value of the gap voltage of the Nb-AIN-NbN junctions as high as 3.7 mV was obtained; the quality parameter \(\mathrm{R_{j}/R_{n}}\) exceeds 20 even at high current densities [4; 29; 52], which indicates the perfect quality of the tunnel barrier. A technology has been developed for manufacturing integrated circuits based on Nb-AIN-NbN tunnel junctions with niobium wiring, which makes it possible to use the advantages of Nb-AIN-NbN junctions in circuits with a topology designed and tested earlier for niobium structures. Using this technology, circuits for studying the linewidth of a superconducting local oscillator, microcircuits of an integrated submm wave spectrometer, and a number of other unique elements and devices were made [4; 18; 19; 20; 21; 22; 53]. For the formation of tunnel structures of submicrometer sizes (down to 100 nm), methods of electron-beam lithography were used [54]. The Nb-AlO\({}_{x}\)-Nb and Nb-AIN-NbN structures were deposited layer by layer in a single vacuum cycle on a silicon substrate, covered with a protective (buffer) layer of Al\({}_{2}\)O\({}_{3}\). Furthermore, by the means of photolithography and plasma etching, the areas of SIS junctions were formed on the surface of the workpiece.