Abstract Superconducting circuits based on Josephson junctions have the potential to achieve high speed and ultra-low power consumption, but their integration is limited by the low controllability of Nb-based tunnel junction and the existence of shunt resistors. In this work, we report the fabrication of superconductor/normal metal/superconductor (SNS) Josephson junctions with high reproducibility on oxidized Si substrates. The junctions based on NbN/TaN/NbN trilayers measured at 4.2 K show excellent Josephson properties with a wide range of critical current ( I c ) from 227 to 2000 μ A. The variations of critical current density ( J c ) and characteristic voltage ( V c ) are respectively less than 7.1% and 7.5% in the 2 inch region. The standard deviation of I c is calculated to be less than 1.7% for Josephson arrays with the number of junctions up to 10 000 occupying an area of 0.34 mm 2 . The results provide a guarantee for the applications of NbN-SNS Josephson junctions in superconducting large-scale integrated circuits.
Abstract Superconducting circuits based on Josephson junctions have the potential to achieve high speed and ultra-low power consumption, but their integration is limited by the low controllability of Nb-based tunnel junction and the existence of shunt resistors. In this work, we report the fabrication of superconductor/normal metal/superconductor (SNS) Josephson junctions with high reproducibility on oxidized Si substrates. The junctions based on NbN/TaN/NbN trilayers measured at 4.2 K show excellent Josephson properties with a wide range of critical current ( I c ) from 227 to 2000 μ A. The variations of critical current density ( J c ) and characteristic voltage ( V c ) are respectively less than 7.1% and 7.5% in the 2 inch region. The standard deviation of I c is calculated to be less than 1.7% for Josephson arrays with the number of junctions up to 10 000 occupying an area of 0.34 mm 2 . The results provide a guarantee for the applications of NbN-SNS Josephson junctions in superconducting large-scale integrated circuits.
Abstract We report on the electrical properties of NbN/TaN/NbN Josephson junctions grown on thermally oxidized silicon substrates, along with the design and fabrication of superconducting single-flux-quantum (SFQ) circuits based on these NbN superconductor/normal metal/superconductor (SNS) junctions. The critical current density ( J c ) of the junctions was found to be relatively sensitive to the barrier thickness, decreasing from 108.0 ± 8.1 kA cm −2 for a 15 nm barrier to 12.8 ± 1.9 kA cm −2 for a 30 nm barrier. For a J c of approximately 24.5 ± 2.1 kA cm −2 and a barrier thickness of 25 nm, the NbN SNS junctions are self-shunted and exhibit nonhysteretic current–voltage ( I – V ) characteristics. Especially for junctions with diameter ( φ ) ranging from 0.8 to 1.6 μ m, their critical current ( I c ) falls within the range of 110–450 μ A, making them suitable for SFQ circuits. By considering the impact of excess current and incorporating it as an additional term in the conventional resistively and capacitively shunted junction model, the I – V curves of NbN SNS junctions can be precisely described, successfully minimizing the deviation between simulations and test results. The DC-SFQ and SFQ-DC interface circuits can both operate normally, and the bias margins of cell circuits such as Josephson transmission line, confluence buffer, D flip-flop, and splitter are greater than 40%. Compared to Nb superconductor/insulator/superconductor junctions, their self-shunting characteristics and relatively thick 25 nm barriers can also enhance the integration of circuits and increase the yield to complex circuits.
We have developed resistively shunted NbN junctions to realize superconducting single flux quantum circuits operating at 10 K and/or high speed. The junctions consist of epitaxial NbN/AlN/NbN tunnel junctions and molybdenum (Mo) resistors fabricated on single-crystal MgO substrates. The junction qualities are systematically investigated in a wide range of critical current density (Jc). The gap voltage and the ratio of Rsg/RN were about 5.6 mV and 11 for the junctions with the Jc of 10 kA/cm2, respectively. The overdamped Josephson junctions with parallel Mo resistors having a nominal sheet resistance showed non-hysteretic current-voltage characteristics for the junctions with Jc of 10 kA/cm2.
Superconductor-normal metal-superconductor (SNS) junctions are well-suited for large-scale integrated superconducting circuits. Potential applications are programmable voltage standards, digital-to-analog converters and large-scale RSFQ circuits. The application of SNS junctions provides some advantages, e.g. a simplified fabrication technology and reduced parasitic inductances. But it requires a technological process with dimensions down to the submicrometer region to achieve sufficiently high characteristic voltages I/sub C/R/sub N/. In our process the normal metal interlayer is titanium. Therefore the sidewall insulation of the junctions can be made by anodization, as it is known from the Nb-Al technology. Nb-Ti-Nb Josephson junctions and junction arrays were fabricated down to the submicrometer range with I/sub C/R/sub N/ values up to 87 microvolt depending on the titanium thickness. Their measured critical currents have a small spread of about 10 percent. Shapiro steps are observed under the influence of microwave irradiation. For the application of such junctions in a programmable Josephson voltage standard a new type of microwave circuit using coplanar strips is proposed and successfully tested in 10 V SIS circuits.
To increase the integration scale of superconductor electronics, we are developing a new node, titled SFQ7ee, of the fabrication process at MIT Lincoln Laboratory. In comparison to the existing SFQ5ee node, we increased the number of fully planarized superconducting layers to ten and utilized NbN and NbN/Nb kinetic inductors to increase the possible inductor number density above a hundred million inductors per cm2. Increasing the Josephson junction number density to the same level requires implementing self-shunted high-JcJosephson junctions. We investigated the properties of Nb/NbNx/Nb trilayer junctions as a potential replacement of high-JcNb/Al-AlOx/Nb junctions, where NbNxis a disordered, overstoichiometric, nonsuperconducting nitride deposited by reactive sputtering. Dependences of theIcRnproduct and Josephson critical current density, Jc on the NbNx barrier thickness and temperature were studied in the thickness range from 5 nm to 20 nm. The fabricated junctions can be well described by the microscopic theory of SNS junctions, assuming no suppression of the energy gap in Nb electrodes near the NbNxinterfaces and a Cooper pair decay length in the NbNx barrier, ξnTcof about 2.3 nm. Current-voltage characteristics of the junctions are well described by the resistively and capacitively shunted junction (RCSJ) model without excess current. In the studied rangeJc2, the Nb/NbNx/Nb junctions have lower values of the specific resistanceRnA, lower IcRn products, and a stronger dependence of theIcRnon temperature than the self-shunted or critically damped externally shunted Nb/Al-AlOx/Nb junctions with the same critical current density; A is the junction area,Icthe junction critical current, and Rnthe effective shunting resistance.
We have fabricated NbTiN/Al-AlNX/NbTiN Josephson tunnel junctions on thermally oxidized Si substrates for their applications to single-flux-quantum circuits operating around 10 K. NbTiN films with a critical temperature Tcof 14.2 K and a root-mean-square value of their surface roughness of 0.45 nm were used as junction electrodes. For a junction barrier, radical-nitrided AlNXwas used to realize high uniformity in the critical current Icof the junctions. We obtained NbTiN junctions with a gap voltage Vg of 4.3 mV and an IcRn. product of 2.6 mV at 4.2 K, where R. is the junction resistance. The critical current density Jc, at 4.2 K was obtained in the range between 0.2 and 6.2 kA/cm2by changing a nitridation time for the AlNXbarrier formation. A series array of 200 junctions with an area of 10 μm × 10 μm exhibited a small I', spread, and the standard deviation 1-σ in Icuniformity was less than 1.6% in the temperature range between 4 K and 10 K.
Abstract Superconducting flux memory devices fabricated with nanoscale junctions and high-inductance materials can maintain high-speed access performance while significantly reducing the area required for superconducting loops. In this work, we present a method for preparing nanoscale NbN-based superconductor/normal-metal/superconductor (SNS) Josephson junctions that combines electron beam lithography with chemical mechanical planarization process. Scanning electron microscope images reveal that the junction has a very round shape, while cross-sectional transmission electron microscopy shows that it has a truncated cone structure, characterized by a narrow top and a wide bottom. The minimum diameter of the junction is 51 nm, with the diameter at the interface between the wiring layer and the top electrode being just 27 nm. These nanoscale junctions exhibit excellent Josephson properties with a critical current density J c of 673 ± 64 kA cm −2 and a characteristic voltage V c of 1.63 ± 0.45 mV at 4.2 K. Furthermore, we designed the superconducting memory cell based on the simulated inductance values of the NbN loop and the nanoscale NbN-based SNS junctions. The cell has a total area of 12.6 μ m × 6.5 μ m and contains a radio-frequency superconducting quantum interference device with one junction in the storage loop and a direct-current superconducting quantum interference device with two junctions in the readout loop. Using the flux-locked and feedback measurement method, a clear flux hysteresis curve can be obtained to verify the read-write functions of the device.