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Physics of the Solid State

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

Artem M. Chekushkin, Lyudmila V. Filippenko, Fominskiy M. Yu., V. P. Koshelets

Published Jan 1, 2022• 1 citation
Publisher

Abstract

A description of the fabrication technology of high-quality tunnel SIS junctions is presented, with following characteristics: energy gap in superconductors Vg=3.2-3.4 mV, tunnel current density up to J 35 kA/cm2, quality factor Rj/Rn (ratio of subgap resistance to normal-state resistance) up to 30, junction area up to 1 μm2. The SIS junctions are integrated into the NbTiN|SiO2|Al microstrip line. Keywords: superconducting devices, superconductor--insulator--superconductor tunnel junction, plasma etching.

DOI
10.21883/pss.2022.10.54222.49hh

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A description of the fabrication technology of high-quality tunnel SIS junctions is presented, with following characteristics: energy gap in superconductors Vg=3.2-3.4 mV, tunnel current density up to J 35 kA/cm2, quality factor Rj/Rn (ratio of subgap resistance to normal-state resistance) up to 30, junction area up to 1 μm2. The SIS junctions are integrated into the NbTiN|SiO2|Al microstrip line. Keywords: superconducting devices, superconductor--insulator--superconductor tunnel junction, plasma etching.
We discuss fabrication and characteristics of superconductor-insulator-superconductor (SIS) junctions which typically exhibit a 3.5 mV sum-gap voltage. Junctions have a sub-gap to normal state resistance ratio of R/sub SG//R/sub N/=27 for resistance-area products down to R/sub N/A=8 /spl Omega/ /spl mu/m/sup 2/ and high quality junctions have been produced with RNA products as low as 4 /spl Omega/ /spl mu/m/sup 2/. The device structure incorporates a Nb base electrode, a tunnel barrier formed by plasma nitridation of a thin Al proximity layer, and a NbTiN counter-electrode. Results for all Nb junctions with high current density aluminum-nitride barriers are also shown. Nitridation of the aluminum layer is investigated by control of the dc floating potential on a separate rf driven electrode in the vacuum process chamber. Devices are integrated to a mixer antenna structure incorporating NbTiN as a ground plane. The wire circuit layer can be either normal metal or NbTiN. Annealing results show improved I-V characteristics with increased RNA products. Recent receiver measurements employing these junctions exhibit low noise performance up to 900 GHz.
SIS mixers incorporating two-junction, NbTiN tuning-circuits have been designed and fabricated using two different types of tunnel-junctions. The first type of tunnel junction-NbTiN/MgO/NbTiN--has the advantage of a large gap voltage (5 mV), but has a relatively soft I-V characteristic and high specific capacitance as compared to Nb/Al-Ox/Nb tunnel-junctions. The second type of junction--Nb/Al-AlNx/NbTiN, is a hybrid structure, which has many of the advantages of Nb junctions, with a slightly larger energy gap voltage (3.5 mV) and more robust thermal properties. A detailed description of the deposition techniques used in making the AlNx devices will be given in a separate paper. In this paper, we discuss the deposition of high-quality NbTiN films, and the trade-off between stress and quality in these films. We also discuss the deposition details for the NbTiN/MgO/NbTiN junctions. We measured the magnetic-penetration-depth of our NbTiN films with SQUID circuits. Using long resonators coupled to Josephson junctions, we measured propagation velocities for our microstrip-line circuits; this measurement gave us an independent estimate of the NbTiN magnetic-penetration-depth and a qualitative measure of RF-losses in the microstrip-lines.
Nb/Al–AlO x /Nb tunnel junctions are often used in the studies of macroscopic quantum phenomena and superconducting qubit applications of the Josephson devices. In this work, we describe a convenient and reliable process using electron beam lithography for the fabrication of high-quality, submicron-sized Nb/Al–AlO x /Nb Josephson junctions. The technique follows the well-known selective Nb etching process and produces high-quality junctions with V m = 100 mV at 2.3 K for the typical critical current density of 2.2 kA/cm 2 , which can be adjusted by controlling the oxygen pressure and oxidation time during the formation of the tunnelling barrier. We present the results of the temperature dependence of the sub-gap current and in-plane magnetic-field dependence of the critical current, and compare them with the theoretical predictions.
Superconducting qubit circuits require high quality low critical current density Josephson tunnel junctions. We have developed high quality epitaxial NbN/AlN/NbN tunnel junctions with critical current density, Jc, ranging from as low as a few A/cm2to a few tens of kA/cm2. In this work, we measured current-voltage characteristics for junctions with a Jc~ 32 A/cm2in the temperature range from 17 mK to above 6.7 K. All the junctions are in good quality and have a high gap voltage (>; 5.5 mV). We found that the junctions' sub-gap resistances were temperature dependent above 2.5 K and saturated at temperatures below 2.5 K. A junction sub-gap leakage current Irof about 23 pA was measured. The voltage noise power spectral density for 3 junctions with different sizes has been measured at the base temperature of 17 mK with the junctions biased above the gap voltage. The spectral densities clearly show 1/fbehavior at low frequency and are proportional to the junction's linear dimension.