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IEEE Transactions on Applied Superconductivity

Geometric Dependence of Washer Inductance for NbN DC SQUIDs

Qiyu Zhang, Huiwu Wang, Guanqun Li, Xin Tang, Jie Ren, Wei Peng, Zhen Wang

Published Apr 18, 2018• 1 citation
Publisher

Abstract

Planar niobium nitride (NbN) dc superconducting quantum interference devices (SQUIDs) magnetometers were developed based on high-quality epitaxial NbN/AlN/NbN Josephson junctions for SQUID applications at higher operating temperatures. To improve the design of NbN dc SQUIDs, we measured the SQUID washer's inductance, which is a key SQUID parameter. A set of NbN dc SQUIDs with washers with different dimensions were implemented. The washer inductances were obtained from the periodic flux variation in the SQUIDs when a control current was injected directly into the SQUID washers; the results agree well with the ones calculated with a numerical simulation in InductEX software. We obtained an empirical expression to estimate the NbN SQUID inductance with washers with different geometric structures. Our results will be useful for designing NbN dc SQUID magnetometers in future.

DOI
10.1109/tasc.2018.2825286

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Planar niobium nitride (NbN) dc superconducting quantum interference devices (SQUIDs) magnetometers were developed based on high-quality epitaxial NbN/AlN/NbN Josephson junctions for SQUID applications at higher operating temperatures. To improve the design of NbN dc SQUIDs, we measured the SQUID washer's inductance, which is a key SQUID parameter. A set of NbN dc SQUIDs with washers with different dimensions were implemented. The washer inductances were obtained from the periodic flux variation in the SQUIDs when a control current was injected directly into the SQUID washers; the results agree well with the ones calculated with a numerical simulation in InductEX software. We obtained an empirical expression to estimate the NbN SQUID inductance with washers with different geometric structures. Our results will be useful for designing NbN dc SQUID magnetometers in future.
Integrated NbN direct current superconducting quantum interference device (DC-SQUID) magnetometers were developed based on high-quality epitaxial NbN/AlN/NbN Josephson junctions for SQUID applications operating at high temperatures. We report the current–voltage and voltage–flux characteristics and the noise performance of the NbN DC-SQUIDs for temperatures ranging from 4.2 to 9 K. The critical current and voltage swing of the DC-SQUIDs decreased by 15% and 25%, respectively, as the temperature was increased from 4.2 to 9 K. The white flux noise of the DC-SQUID magnetometer at 1 kHz increased from 3.9 μΦ0/Hz1/2 at 4.2 K to 4.8 μΦ0/Hz1/2 at 9 K with 23% increase, corresponding to the magnetic field noise of 6.6 and 8.1 fT/Hz1/2, respectively. The results show that NbN DC-SQUIDs improve the tolerance of the operating temperatures and temperature fluctuations in SQUID applications.
Abstract Superconducting quantum interference devices (SQUIDs) with 3D nano-bridge junctions can be miniaturized into nano-SQUIDs that are able to sense a few spins in a large magnetic field. Among all device parameters, the inductance is key to the performance of SQUIDs with 3D nano-bridge junctions. Here, we measured the critical-current magnetic flux modulation curves of 12 devices with three design types using a current strip-line directly coupled to the SQUID loop. A best flux modulation depth of 71% was achieved for our 3D Nb SQUID. From the modulation curves, we extracted the inductance values of the current stripe-line in each design and compared them with the corresponding simulation results of InductEX. In this way, London penetration depths of 110 and 420 nm were determined for our Nb (niobium) and NbN (niobium nitride) films, respectively. Furthermore, we showed that inductances of 11 and 119 pH for Nb and NbN 3D nano-bridge junctions, respectively, dominated the total inductance of our SQUID loops which are 23 pH for Nb and 255 pH for NbN. A screening parameter being equal to one suggests optimal critical currents of 89.6 and 8.1 μ A for Nb and NbN SQUIDs, respectively. Additionally, intrinsic flux noise of 110 ± 40 n Φ 0 /(Hz) 1/2 is calculated for the Nb SQUIDs with 3D nano-bridge junctions by Langevin simulation.
In recent years, rapid single flux quantum (RSFQ) circuits, which are promising for the development of high-speed digital electronics, have attracted widespread interest. Niobium nitride (NbN) films have also attracted significant attention for high-frequency applications in superconducting electronics because of their superior material parameters, including their high critical temperature Tc and large gap voltage Vg. To enable the development of all-NbN SFQ circuits, we have developed a fabrication process based on NbN junctions and inductors made of NbN strip lines. It is important to evaluate the inductance because it can affect the performance of an RSFQ circuit and is essential for the RSFQ design. We measured the inductance properties of both epitaxial and polycrystalline NbN strip lines using a superconducting quantum interference device modulation technique at 4.2 K and obtained the width dependence on the inductance of the NbN films. The penetration depth was then calculated from these measurements. The NbN inductance values were calculated numerically with respect to the penetration depth and were then compared with the measured values from the experiments. It was demonstrated that the differences between these values were within 5%. The results reported here will be useful for the design and fabrication of all-NbN RSFQ circuits.
Reduction of the size of the superconducting quantum interference device (SQUID) ring structure improves its sensitivity to magnetic moments. Nanobridge junctions are advantageous for SQUID miniaturization. However, many planar SQUIDs with nanobridge junctions have shallow flux modulation depths because of their nonideal Josephson current-phase relationships. Here, we fabricated a three-dimensional niobium nitride (NbN) miniaturized SQUID in which the nanobridge junction structures are much thinner than the superconducting ring. We grew a thick superconducting NbN film on a silicon wafer and embedded an insulating slit in the middle of the film. By setting two thin nanobridge junctions across this insulating slit, we were able to manufacture NbN miniaturized SQUIDs that, in principle, can become nano-SQUIDs. The flux modulation depths of these devices reached 36% at 4.2 K. In addition, the fabricated devices show reversible current-voltage characteristics in the absence of any shunting resistance.
Practical thin-film dc superconducting quantum interference devices (SQUIDs) are often built with an integrated input coil which leads to the formation of various parasitic elements. We have designed and fabricated several square washer-shaped dc SQUID test devices with various amounts of parasitic capacitance formed by covering the washer slits with superconducting coverplates of various widths. In the simplest approximation, the parasitic capacitance Cp appears entirely across the Josephson junctions; in a better approximation, Cp divides the total inductance into two parts, an effective SQUID loop inductance L and a small parasitic inductance Lp. For both models, noise-free and noise-rounded current-voltage and voltage-flux characteristics are calculated for various values of the ratios L/Lp and Cp/C, where C is the Josephson junction capacitance. The SQUID dynamics are significantly affected by Cp and Lp. The energy resolution ε at first deteriorates with increasing Cp for Cp/C ≲ 2, but as Cp/C becomes large, ε improves to less than twice the value for Cp/C ≊ 0. The optimal points of operation for large Cp, however, are found at voltages above the low-frequency resonance determined by L and Cp where the flux-to-voltage transfer function is low. Furthermore, for devices with Cp/C ≫ 1 and Lp≠0, the double-loop geometry leads to an additional resonance determined by Lp and C that appears at high frequencies. The energy resolution inside this high-frequency resonance is nearly independent of Cp and comparable to the energy resolution of a device having the same total inductance but with Cp/C ≪ 1. Extensive noise mappings on several different test devices have been carried out using a computer-controlled measurement system. The measured characteristics and noise performance are shown to be in good agreement with the simulations.
We have investigated the electrical properties of high-T c dc-SQUID's formed in a coplanar waveguide, and discussed their applicability to single flux quantum (SFQ) circuits. The planar type Josephson junctions were fabricated in NdBa2Cu3O y (NBCO) superconducting thin films by a narrow-focused ion beam (FIB) irradiation technique. The critical current and normal resistance values of the Josephson junction at 4.2 K were 115 µA and 8 Ω, respectively. The inductance was estimated by applying a direct injection of the control current into the dc-SQUID loop and measuring the periodic voltage response. A loop inductance of 47 pH was obtained for a 25 µm-wide and 20 µm-long dc-SQUID at 4.2 K. It has been clarified that for a stable operation of SFQ logic circuits, a 10 µm-square dc-SQUID's are required.