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Superconducting quantum interference device (SQUID) is one of the most sensitive flux sensors and is critical in fields such as biomagnetism, low-field nuclear magnetic resonance (NMR), and geophysics. In this paper, an integrated magnetoencephalography (MEG) SQUID chip is investigated in detail, which consists of a magnetometer and two gradiometers. The SQUID and pick-up coils are fabricated on different-sized wafers. The SQUID is fabricated on a commercial silicon substrate using micro- and nano-fabrication processes, including thin-film growth, i-line stepper photolithography, and reactive ion etching (RIE). The sub-micron Josephson junction technology is employed to achieve a junction size of 0.7 μm×0.7 μm with a junction capacitance of only 0.05 pF. The pick-up coil is designed as a single-turn coil for a magnetometer and a planar first-order gradient coil for a gradient sensor. The MEG SQUID chips are tested in a well-shielded chamber with the helium-liquid temperature (4.2 K). Customized low-voltage noise readout circuit and source measure units are used to characterize the magnetic field white noise, current-voltage (I-V) characteristics, and voltage modulation amplitude of 171 SQUID channels. The results show that 81% of the SQUID chips exhibit the lower magnetic field noise (< 5 $ {\text{fT/}}\sqrt {{\text{HZ}}} $), and the high modulation amplitudes (in a range of 80–120 μV) with the low working currents of 15–20 μA, yielding a wafer yield rate of 78%. In summary, the SQUIDs show excellent performance in terms of magnetic field white noises, modulation amplitudes, and working currents, which are suitable for the very weak magnetic signal detection. One of future studies will focus on optimizing the SQUID chip fabrication process to minimize performance variations between chips on the same wafer.
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Keywords:
- superconducting quantum interference device /
- magnetometer /
- tradiometer /
- magnetic field noise
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图 1 (a) SQUID晶圆集成示意图; (b) 探测线圈晶圆集成示意图
Figure 1. (a) Wafer of SQUIDs; (b) the wafer of pickup coils.
图 2 SQUID芯片封装示意图 (a) 引线键合; (b) SQUID封装成品图
Figure 2. SQUID chip packaging: (a) The lead bonding; (b) the final SQUID packaging.
图 3 测试电路框图 (a) I-V特性、V-Φ特性测试框图; (b) 噪声测试框图
Figure 3. Block diagrams of the test circuits: (a) Block diagram for I-V and V-Φ characteristic measurements; (b) block diagram for noise measurement.
图 4 (a) 磁强计在不同环境中的磁场噪声谱; (b) 梯度计在不同环境中的磁场噪声谱
Figure 4. (a) Magnetic field noise spectrum diagrams of MAG in different environments; (b) magnetic field noise spectrum diagrams of GRA in different environments.
图 5 单元脑磁SQUID芯片的参数特性曲线 (a) 磁强计与梯度计磁场与磁通噪声图; (b) 磁强计与梯度计的I-V特性曲线; (c) 磁强计与梯度计的最大电压摆幅特性曲线
Figure 5. Parametric characteristic curves of single MEG SQUID chip: (a) Magnetic fields and flux noises of MAG and GRA; (a) the I-V characteristic curves of MAG and GRA; (c) the maximum voltage swing characteristic curves of MAG and GRA.
图 6 磁强计与梯度计不同外加磁场的Ic分布 (a) 磁强计Ic散点图; (b) 磁强计直方图; (c) 梯度计Ic散点图; (d) 梯度计直方图
Figure 6. Distribution of Ic under different external magnetic fields of MAG & GRA: (a) The Ic scatter plot of MAG; (b) the MAG histogram; (c) the Ic scatter plot of GRA; (d) the GRA histogram.
图 7 最大电压摆幅与偏置电流对应图 (a) 磁强计; (b) 梯度计
Figure 7. Plots of maximum voltage swings and bias currents: (a) Magnetometers; (b) gradiometers.
图 8 SQUID芯片低频磁场噪声Z-score谱(对数坐标) (a) 磁强计低频噪声Z-score谱; (b) 梯度计低频噪声Z-score谱(实线表示噪声的均值曲线, 阴影部分表示噪声的波动范围(Mean ±3σ))
Figure 8. Z-score spectrum of low-frequency magnetic field noise of SQUID chip (logarithmic coordinate): (a) Z-score spectrum of low-frequency noise of MAG; (b) Z-score spectrum of low-frequency noise of GRA (The solid line represents the mean noise curve, and the shaded area indicates the noise fluctuation range (Mean ±3σ)).
图 9 磁强计与梯度计磁场噪声 (a) 磁强计磁场噪声分布图; (b) 梯度计磁场噪声分布图; (c) 磁强计与梯度计磁场噪声散点图
Figure 9. MAG & GRA magnetic field noise: (a) The noise distribution plot of MAG; (b) the noise distribution plot of GRA; (c) the noise scatter plot of MAG and GRA.
表 1 SQUID设计参数
Table 1. Parameters for designing a SQUID.
SQUID结构 参数 数值 单位 约瑟夫森结 尺寸 0.7×0.7 μm2 临界电流密度 2 kA/cm2 临界电流 10 μA 结电容 0.03 pF 结电阻 35 Ω βc 1 — 环电感 环结构 1 st order gradiometer — 内边长 20 μm 线宽 84 μm 单loop电感 56 pH Input线宽 2 μm Input线距 2 μm Input匝数 18×2 — Input电感 36 nH 
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表 2 探测线圈设计参数
Table 2. Parameters for designing a pickup coil.
器件类型 参数 数值 单位 磁强计 Pickup内边长 7000 mm Pickup线宽 500 μm Pickup电感 19 nH 梯度计 Pickup内边长 8000 mm Pickup线宽 500 μm Pickup电感 45 nH
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