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IEEE Transactions on Applied Superconductivity

Fabrication and Characterization of SQUIDs With Nb/Nb x Si1- x /Nb Junctions

Wenhui Cao, Hua Chen, Tiantian Liang, Jinjin Li, Xiangyan Kong, Tianbao Sun, Qing Zhong, Xueshen Wang, Lanruo Wang, Yuan Zhong

Published May 18, 2018• 5 citations
Publisher

Abstract

Superconducting quantum interference devices (SQUIDs) with 3 μm × 3 μm or 4 μm × 4 μm self-shunted Nb/NbxSi1-x/Nb Josephson junctions are designed and fabricated. By adjusting the Nb content and thickness of the NbxSi1-xbarrier, the parameters of the SQUIDs can be easily tuned. The optimal parameters for the barrier are found to be a thickness of around 15 nm and 7% Nb content. The I-V curve, V-Φ curve, and noise spectrum are measured for the SQUID. The high frequency noise for the SQUID is only about several hundred nΦ0/√Hz. Another advantage for these devices is found, that it is not sensitive to the static electricity.

DOI
10.1109/tasc.2018.2836916

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Superconducting quantum interference devices (SQUIDs) with 3 μm × 3 μm or 4 μm × 4 μm self-shunted Nb/NbxSi1-x/Nb Josephson junctions are designed and fabricated. By adjusting the Nb content and thickness of the NbxSi1-xbarrier, the parameters of the SQUIDs can be easily tuned. The optimal parameters for the barrier are found to be a thickness of around 15 nm and 7% Nb content. The I-V curve, V-Φ curve, and noise spectrum are measured for the SQUID. The high frequency noise for the SQUID is only about several hundred nΦ0/√Hz. Another advantage for these devices is found, that it is not sensitive to the static electricity.
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The authors have designed, fabricated and tested all-refractory DC SQUIDs in Nb-Al/sub 2/O/sub 3/-Nb trilayer technology that have noise performance comparable to the best previously reported for any technology. A variety of SQUID designs were incorporated as part of a trilayer process development test vehicle. SQUID inductance, junction area, and resistive shunt geometry were varied in matrix fashion to give SQUIDs with near-optimum parameter values for a factor of five range in Josephson current density and shunt sheet resistance. The devices were fabricated using a selective niobium anodization with a minimum feature size of 2 mu m. The base electrode and Nb wiring were patterned with dry etching, and the junction areas were defined by anodization: the Ti resistors were patterned with a lift-off process. Current density on different wafers was varied from 400 to 1000 A/cm/sup 2/ with typical junction V/sub m/'s of 60 mV. The shunt sheet resistance was varied in the 1-5- Omega / Square Operator range. The noise was measured with an RF SQUID direct small-signal readout scheme. A 50-pH SQUID with 3- mu m/sup 2/, 16- mu A junctions and 14- Omega shunt resistors was shown to have an ideally low white noise of 1*10/sup -13/ Phi /sub 0//sup 2//Hz, a white to 1/f crossover frequency at 7 Hz, and a noise level less than 6*10/sup -12/ Phi /sub 0//sup 2//Hz at 0.1 Hz.
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