A second-order gradiometric superconducting quantum interference device current sensor with cross-coupled structure
Authors: Xu Da , Zhong Qing , Cao Wen-Hui , Wang Xue-Shen , Wang Shi-Jian , Li Jin-Jin , Liu Jian-She , Chen Wei ,   Center for Advanced Measurement Science, National Institute of Metrology, Beijing 102200, China,   Institute of Microelectronics, Tsinghua University, Beijing 100084, China
Published: June 2021

Superconducting quantum interference device (SQUID) has extremely high magnetic field sensitivity, current sensitivity, and can detect a low-noise weak current signal. The SQUID current sensor has become the only option of the readout of low-noise detector, such as transition-edge sensor (TES). In this paper, a second-order gradiometric cross-coupled SQUID current sensor for TES application is developed. According to the requirements for TES detectors, the structure and various parameters of SQUID current sensor are designed. The SQUID loop, input coil and feedback coil of the SQUID current sensor all use the second-order gradiometric structure. All the couple ways between SQUID loop and input coil or feedback coil adopt cross-coupling mode in different planes, which can effectively weaken the parasitic capacitance. A second-order gradiometric cross-coupled SQUID current sensor based on Nb/Al-AlO<i><sub>x</sub></i>/Nb Josephson junction is successfully fabricated on a silicon wafer by optimizing the process. The properties of the second-order gradiometric cross-coupled SQUID current sensor are measured at liquid helium temperature. The bias current of SQUID is 215 μA when the modulation depth of <i>V</i>-<i>Φ</i> modulation curve is maximum. The maximum modulation peak of SQUID is 31 μV. The flux-to-voltage transfer coefficient of SQUID is 108 μV/<i>Φ</i><sub>0</sub>. The input coil current sensitivity is 17 μA/<i>Φ</i><sub>0</sub>, the mutual inductance between SQUID loop and input coil is 117 pH. The current sensitivity of feedback coil is 86 μA/<i>Φ</i><sub>0</sub>, the mutual inductance between SQUID loop and feedback coil is 23 pH. The second-order gradiometric cross-coupled SQUID current sensor has a white flux noise of 2 μ<i>Φ</i><sub>0</sub>/<inline-formula><tex-math id="M3">\begin{document}$ \sqrt{{\rm{H}}{\rm{z}}} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="12-20201816_M3.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="12-20201816_M3.png"/></alternatives></inline-formula> and a white current noise of 34 pA/<inline-formula><tex-math id="M4">\begin{document}$ \sqrt{{\rm{H}}{\rm{z}}} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="12-20201816_M4.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="12-20201816_M4.png"/></alternatives></inline-formula> with 1/<i>f</i> corner frequency around 200 Hz. The result of noise level under the condition without magnetic shielding shows that the SQUID current sensor with second-order gradiometric cross-coupled structure has an excellent capability of weakening the environmental electromagnetic interference. In the future, we will further improve the mutual inductance of the second-order gradiometric cross-coupled SQUID current sensor between SQUID loop and input coil, optimize the size and critical current of Josephson junction, in order to improve the input sensitivity of SQUID device, reduce the current noise level and the 1/<i>f</i> corner frequency, and meet more requirements for TES applications.