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IEEE Transactions on Applied Superconductivity

Fabrication and Characteristics of All-NbN SQUID Series Array

Qiyu Zhang, Huiwu Wang, Xin Tang, Wei Peng, Zhen Wang

Published Aug 30, 2019• 8 citations
Publisher

Abstract

In this article, an all-NbN superconducting quantum interference device (SQUID) array was developed for future applications by taking advantage of the higher critical temperature of a NbN film compared with Nb. The NbN SQUID array was composed of 100 SQUIDs with equal loop areas connected in series. The array was fabricated on a single-crystal MgO (100) substrate using 3-μm-diameter NbN/AlN/NbN Josephson junctions. The hysteresis and shielding parameters of the SQUID were 0.67 and 0.15, respectively. The SQUIDs in the array were coherently modulated by injecting current in a common flux control line, which resulted in a modulation voltage of 3.54 mV with consistent periodicity and a flux-to-voltage transfer coefficient of 23 mV/Φ0. Using a flux locked loop circuit, the fabricated NbN SQUID array exhibited excellent performance with a low white flux noise of 1.4 μΦ0/√Hz, which included the preamplifier noise.

DOI
10.1109/tasc.2019.2938582

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In this article, an all-NbN superconducting quantum interference device (SQUID) array was developed for future applications by taking advantage of the higher critical temperature of a NbN film compared with Nb. The NbN SQUID array was composed of 100 SQUIDs with equal loop areas connected in series. The array was fabricated on a single-crystal MgO (100) substrate using 3-μm-diameter NbN/AlN/NbN Josephson junctions. The hysteresis and shielding parameters of the SQUID were 0.67 and 0.15, respectively. The SQUIDs in the array were coherently modulated by injecting current in a common flux control line, which resulted in a modulation voltage of 3.54 mV with consistent periodicity and a flux-to-voltage transfer coefficient of 23 mV/Φ0. Using a flux locked loop circuit, the fabricated NbN SQUID array exhibited excellent performance with a low white flux noise of 1.4 μΦ0/√Hz, which included the preamplifier noise.
Superconducting quantum interference filter (SQIF) is an array of superconducting quantum interference devices (SQUIDs) with different loop sizes. Such a device is capable of providing a much higher transfer factor and much higher magnetic field sensitivity with respect to those of single SQUID. Series niobium nitride (NbN) SQIF containing 400 SQUIDs with randomly varied loop areas and epitaxial NbN/AlN/NbN Josephson junctions have been developed, fabricated, and measured at helium temperature. The device shows a nonperiodic voltage response with a single sharp peak at zero magnetic field. The measured magnetic flux to voltage trans fer factor reaches 37.8 mV/Φ0at 4.2 K.
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.
We have fabricated dc-superconducting quantum interference devices (SQUIDs) with all-thin-film point contacts (Nb–Nb nanoconstriction-NbN). The size of the nanoconstriction may be smaller than the superconducting coherence length of bulk Nb. The quasiparticle characteristic of the all-thin-film point contact can be explained using the Blonder, Tinkham and Klapwijk (BTK) quasiparticle current which is applicable to the superconducting quantum point contact. The flux noise of SQUID with 10 pH operated in a flux locked loop is 8 µΦ0/ Hz1/2 at 1 Hz, and 4 µΦ0/ Hz1/2 at frequencies above 10 Hz is limited by the preamplifier.
We have fabricated superconducting quantum interference devices (SQUIDs) with an inductance of 140 pH by using sub- µm Nb/Al–AlO x –Al/Nb edge junctions. The small capacitance of the junction resulted in the very large voltage swing of 210 µV and gradient dV/ dΦ of 940 µV/Φ0. Consequently, the very low flux noise of 0.61 µΦ0/ Hz0.5 corresponding to the energy sensitivity ε of 8.6h at the white noise level was obtained by using a conventional flux-locked loop configuration. The low-frequency noise was also very low, 2.7 µΦ0/ Hz0.5 corresponding to ε= 170 h at 1 Hz. We could obtain very low flux noise for the SQUID which had the practical inductance using only a conventional readout circuit.
User-friendly ultra-high-speed readout electronics for dc superconducting quantum interference devices (SQUIDs) are presented. To maximize the system bandwidth, the SQUID is directly read out without flux modulation. A composite preamplifier is used consisting of a slow dc amplifier in parallel with a fast ac amplifier. In this way, excellent dc precision and a high amplifier bandwidth of 50 MHz are achieved, simultaneously. A virtual 50 Ω amplifier input resistance with negligible excess noise is realized by active shunting, i.e., by applying feedback from preamplifier output to input via a high resistance. The white voltage and current noise levels are 0.33 nV Hz −1 and 2.6 pA Hz −1/2 , respectively. The electronics is fully computer controlled via a microcontroller integrated into the flux-locked loop (FLL) board. Easy-to-use software makes the various electronic settings accessible. A wide bias voltage range of 1.3 mV enables the readout of series SQUID arrays. Furthermore, additional current sources allow the operation of two-stage SQUIDs or transition edge sensors. The electronics was tested using various SQUIDs with input inductances between 30 nH and 1.5 µH. Typically, the maximum FLL bandwidth was 20 MHz, which is close to the theoretical limit given by transmission line delay within the FLL. Slew rates of up to 4.6 Φ 0 µs −1 were achieved with series SQUID arrays. Current noise levels as low as 0.47 pA Hz −1/2 and coupled energy sensitivities between 90 h and 500 h were measured at 4.2 K, where h is the Planck constant. The noise did not degrade when the system bandwidth was increased to the maximum value of about 20 MHz. With a two-stage set-up, intrinsic white energy sensitivities of 30 h and 2.3 h were measured at 4.2 and 0.3 K, respectively.
A superconducting quantum interference device (SQUID) miniaturized into the nanoscale is promising in the inductive detection of a single electron spin. A nano-SQUID with a strong spin coupling coefficient, a low flux noise, and a wide working magnetic field range is highly desired in a single spin resonance measurement. Nano-SQUIDs with Dayem bridge junctions excel in a high working field range and in the direct coupling from spins to the bridge. However, the common planar structure of nano-SQUIDs is known for problems such as a shallow flux modulation depth and a troublesome hysteresis in current–voltage curves. Here, we developed a fabrication process for creating three-dimensional (3-D) niobium (Nb) nano-SQUIDs with nanobridge junctions that can be tuned independently. Characterization of the device shows up to 45.9% modulation depth with a reversible current–voltage curve. Owning to the large modulation depth, the measured flux noise is as low as 0.34 μΦ 0 /Hz 1/2 . The working field range of the SQUID is greater than 0.5 T parallel to the SQUID plane. We believe that 3-D Nb nano-SQUIDs provide a promising step toward effective single-spin inductive detection.