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Superconducting quantum interference device (SQUID) is the most sensitive magnetic flux sensor known, which is widely used in biomagnetism, low-field nuclear magnetic resonance, geophysics, etc. In this paper, we introduce a high-sensitivity SQUID magnetometer, which consists of an SQUID and a flux transformer. The SQUID is first-order gradiometer configuration, which is insensitive to interference noise. The flux transformer includes a multi-turn spiral input coil and a large-sized pickup coil. And the input coil is inductively coupled to the SQUID through mutual inductance. We present an SQUID magnetometer fabricated with Nb/Al-AlO x/Nb Josephson junction technology on a 4-inch silicon wafer at our superconducting electronics facilities. We develop a fabrication process based on selective niobium etching process consisting of five mask levels. In the first two mask levels, the trilayer is patterned by a dry etch to define base electrode, contact pads, and interconnects. The shunt resistor and a dielectric insulating layer are then deposited and patterned by using lift-off and dry etchant, respectively. Finally, the niobium wiring layer is deposited and patterned by using reactive ion etching to define input, pickup and feedback coils. The measurement of the SQUID magnetometer is performed inside a magnetically shielded room. The operating temperature is realized by immersing the SQUID into the liquid helium (4.2 K). Moreover, a superconducting niobium tube is employed to protect the SQUID from being disturbed by external environments. A homemade readout electronics instrument with low input voltage noise is used to characterize the SQUID magnetometer. The results of low-temperature measurements indicate that the magnetometer has a magnetic field sensitivity of 0.36 nT/Φ 0and a white flux noise of 8 μΦ 0/√Hz,corresponding to a white field noise of 2.88 fT/√Hz. This kind of SQUID magnetometer is suitable for multi-channel systems, e.g., magnetocardiography, magnetoencephalography, etc. Although the SQUID process development benefits from the rapid advance of semiconductor process technology, the uniformity of the SQUID on one wafer is fluctuated due to the film deposition. Now, we have realized a best SQUID yield of 50% on a 4-inch wafer. In the future, the SQUID chip yield should be improved by well controlling the optimizing process. The device yield is expected to reach as high as 80%.
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参数 数值 单位 约瑟夫森结Josephson junction 尺寸AJ 4 × 4 μm2 临界电流Ic 8 μA 结电容C[12] 0.56 pF 并联电阻R 10 Ω SQUID 垫圈内边长a 100 μm 线宽ws 300 μm 总电感Ls 350 pH 输入线圈 线圈匝数N 15 — 线宽win 3 μm 线距sin 3 μm 探测线圈 线圈尺寸Ap 15 × 15 mm2 线宽wp 100 μm 
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薄膜 背景真空 Ar流量 Ar气压 恒电流 生长速率 厚度 底层Nb 2.3 × 10–5Pa 10 sccm 0.573 Pa 1.5 A 1.2 nm/s 100 nm Al 3 × 10–5Pa 10 sccm 0.573 Pa 0.3 A 0.25 nm/s 10 nm AlOx 氧气气压：2.6 kPa; 氧化时间：12 h ~2 nm 顶层Nb 2.3 × 10–5Pa 10 sccm 0.573 Pa 1.5 A 1.2 nm/s 100 nm DownLoad:
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参数 数值 单位 临界电流I0 32 μA 正常态电阻Rn 5 Ω 反馈线圈耦合系数1/Mf 4.3 μA/Φ0 最大调制峰峰值Vpp 47 μV 磁通噪声√SΦ(白噪声) 8 μΦ0/√Hz 磁场灵敏度1/Aeff 0.36 nT/Φ0 磁场噪声√SB(白噪声) 2.88 fT/√Hz DownLoad:
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