1.State Key Laboratory of Functional Materials for Informatics, Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Sciences, Shanghai 200050, China 2.Chinese Academy of Sciences, Center for Excellence in Superconducting Electronics, Shanghai 200050, China 3.University of Chinese Academy of Sciences, Beijing 100049, China
Abstract: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-AlOx/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/Φ0 and a white flux noise of 8 μΦ0/√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%. Keywords:superconducting quantum interference device/ magnetometer/ flux noise
4.SQUID磁强计测试结果器件工作所需的低温环境利用液氦(4.2 K)来实现. 为了获得器件本征性能, 需要对外界环境干扰进行有效屏蔽, 因此本实验一方面利用铌屏蔽筒在低温下实现超导屏蔽, 另一方面将杜瓦等低温测试系统置于磁屏蔽室(MSR)内, 在屏蔽室内利用读出电路调节SQUID最佳工作点, 同时在屏蔽室外完成数据采集. SQUID磁强计测试表征利用自主研制的直读电路完成. 与传统磁通调制读出电路不同的是, 该直读电路利用并联前放技术获得较低的电压噪声[20], 可以实现与SQUID之间的直接放大读出. 读出电路利用磁通锁定环(flux-locked loop, FLL)技术实现SQUID线性化读出[21], 即利用闭环负反馈技术将周期、非线性的SQUID调制信号转化为随外场线性变化的磁传感系统. 图4(a)显示了SQUID磁强计的电流-电压特性测试曲线, 三条曲线分别对应外加磁通nΦ0, (n+1/2)Φ0和(2n+1)Φ0/4. SQUID临界电流为32 μA, 导致此时回滞系数βc > 1, 再通过下文的测试结果分析, 此时器件工作于欠阻尼情况下, 此即所谓的欠阻尼SQUID[9], 具有较大的磁通-电压转换系数$(\partial V/\partial \varPhi)$, 更加适合于直读电路. 图 4 (a)电流-电压特性曲线; (b)不同偏置电流下的电压-线圈电流(磁通)调制曲线, 其中调制周期为4.3 μA/Φ0 Figure4. (a) Current-voltage curves; (b) voltage-coil current (flux) curves under different bias currents with a period of 4.3 μA/Φ0.
图4(b)显示了不同偏置电流下的电压-磁通调制曲线, 最大调制幅度达到47 μV, 使工作点W处的$\partial V/\partial \varPhi =$ 146 μV/Φ0. 利用自制低噪声直读电路, 对该磁强计进行了噪声测试, 噪声曲线如图5所示. 可以看到, 在白噪声段, 器件磁通噪声达到8μΦ0/√Hz. 利用亥姆霍兹线圈构建均匀标定磁场, 对该磁强计进行了有效面积标定. 通过施加若干组已知磁场, 监测SQUID输出, 再通过线性拟合得到1/Aeff =0.36 nT/Φ0. 再根据√SB = √SΦ/Aeff, 获得磁强计等效磁场噪声为2.88 fT/√Hz. 相关测试结果汇总在表3中. 图 5 SQUID磁强计噪声曲线 曲线中出现的杂峰主要是实验室震动干扰导致, 插图显示的是最佳工作点(W)时调制曲线 Figure5. Noise figure of SQUID magnetometer, in which the lines between 10–200 Hz were mainly caused by vibrations in the laboratory. The inset shows the modulation curve with the best working point.
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
表3SQUID磁强计测试结果 Table3.Measured results of SQUID magnetometer
图 1 SQUID磁强计示意图
图 2 (a) SQUID磁强计设计图; (b)等效电路图
图 3 器件工艺流程图
图 4 (a)电流-电压特性曲线; (b)不同偏置电流下的电压-线圈电流(磁通)调制曲线, 其中调制周期为4.3 μA/Φ0
图 5 SQUID磁强计噪声曲线 曲线中出现的杂峰主要是实验室震动干扰导致, 插图显示的是最佳工作点(W)时调制曲线