Fabrication of Superconducting Nb–AlN–NbN Tunnel Junctions Using Electron-Beam Lithography

Abstract

Mixers based on superconductor–insulator–superconductor (SIS) tunnel junctions are the best input devices at frequencies from 0.1 to 1.2 THz. This is explained by both the extremely high nonlinearity of such elements and their extremely low intrinsic noise. Submicron tunnel junctions are necessary to realize the ultimate parameters of SIS receivers, which are used as standard devices on both ground and space radio telescopes around the world. The technology for manufacturing submicron Nb–AlN–NbN tunnel junctions using electron-beam lithography was developed and optimized. This article presents the results on the selection of the exposure dose, development time, and plasma chemical etching parameters to obtain high-quality junctions (the ratio of the resistances below and above the gap Rj/Rn). The use of a negative-resist ma-N 2400 with lower sensitivity and better contrast in comparison with a negative-resist UVN 2300-0.5 improved the reproducibility of the structure fabrication process. Submicron (area from 2.0 to 0.2 µm2) Nb–AlN–NbN tunnel junctions with high current densities and quality parameters Rj/Rn > 15 were fabricated. The spread of parameters of submicron tunnel structures across the substrate and the reproducibility of the cycle-to-cycle process of tunnel structure fabrication were measured.

Terahertz high-sensitivity SIS mixer based on Nb–AlN–NbN hybrid superconducting tunnel junctions

“…SIS junctions with an area of approximately 1 µm 2 were 058501-3 fabricated on Nb/AlN/NbN tri-layer films using photolithography, RIE for etching Nb and NbN, and ion etching to physically remove Al-AlN, and anodization technique to prevent short connection between side part of SIS-junction and aluminum electrode. [15] The dielectric layer for the junction insulation consists of 250 nm of SiO 2 , which is defined by a self-aligned lift-off process. Finally, top microstrip electrodes, 400 nm in thickness, are deposited using DC magnetron sputtering and defined through a lift-off process.…”

Noise performance of a 200–280 GHz mixer with Nb-AlN-NbN superconducting tunnel junctions

“…In figure 2(a), the red curve with blank squares is SIS1's coupling efficiency, the blue curve with blank triangles is SIS2's coupling efficiency, and the pink curve with solid dots is PCTJ's total coupling efficiency, which is 80%-85% at 200-260GHz frequency band. The mixer chips are fabricated at the Kotelnikov Institute of Radio Engineering and Electronics, Russia Academic of Science 9 . Figure 3 show the optical microscope images of the mixer chips.…”

Fabrication of High-Quality Josephson Junctions Based on Nb|Al-AlN|NbN

“…Its sputtering is performed with the help of DC magnetron with Nb target in the atmosphere of Ar:N 2 10:1 gas mixture. The film production modes and their calibration are described in more detail in the work [6,7].…”