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Improved Performance of Integrated Solenoid Fluxgate Sensor Chip Using a Bilayer Co-Based Ribbon Core Chong Lei, Yan Liu, Xue-Cheng Sun, Tao Wang, Zhen Yang, and Yong Zhou Abstract— In this paper, based on the multiwire core design and microelectromechanical system technology, we report improved performance of a microsolenoid fluxgate sensor by integrating a bilayer core of co-based amorphous ribbon. By means of utilizing integrated multilayer core structure and high permeability amorphous ribbon core material, this paper shows how a high-performance microfluxgate chip with small dimensions can be realized. The fabricated sensors exhibit a best sensitivity of 3165 V/T, a power of 183.2 mW and a linear range of 150 µT at 100 kHz. The noise power density is found to be 0.5 nT/Hz1/2 1 Hz and the noise rms level is 2 nT in the frequency range of 25 mHz–10 Hz. Index Terms— Fluxgate sensor, MEMS, amorphous ribbon, bi-layer core.
I. I NTRODUCTION
A
S ROOM-TEMPERATURE vector magnetic field sensors, fluxgate sensors have great advantages in detecting DC or low-frequency AC magnetic fields, such as the Earth magnetic field [1]. MEMS technologies applied in fluxgate sensors, as introduced in [2]–[7], bring the following benefits: small size, light weight, high sensitivity, and integration of signal processing circuits. MEMS micro fluxgate sensors can be used in applications of bionanoparticle detection GPS positioning, nano/pico-satellite attitude control and for game control equipment [8]–[13], etc. Limited by the device dimension and the saturation magnetizing principle of operation, the micro fluxgate sensors bring numerous problems including signal-to-noise ratio or power [14], [15]. The problems, such as power, heating, linear range, can be compensated by some additional means [16]–[19]. Signal-to-noise ratio is the key to determining the application performances of the sensor, which is severely affected by the core material of the sensor [20], [21].
Manuscript received April 5, 2015; revised April 29, 2015; accepted May 10, 2015. Date of publication May 12, 2015; date of current version July 13, 2015. This work was supported in part by the National Natural Science Foundation of China under Grant 61074168 and Grant 61273065, in part by the National Science and Technology Support Program (2012BAK08B05), in part by the Natural Science Foundation of Shanghai (13ZR1420800), in part by the Analytical and Testing Center, Shanghai Jiao Tong University, and in part by the Center for Advanced Electronic Materials and Devices, Shanghai Jiao Tong University. The associate editor coordinating the review of this paper and approving it for publication was Dr. Patrick Ruther. The authors are with the Key Laboratory for Thin Film and Microfabrication, Department of Micro/Nano Electronics, School of Electronic Information and Electrical Engineering, Ministry of Education, Shanghai Jiao Tong University, Shanghai 200240, China (e-mail:
[email protected]). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/JSEN.2015.2432457
Fig. 1. (a) Schematic view and (b) photograph of the micro fluxgate sensor.
Residence time difference (RTD) fluxgate sensor gives an approach to improve detecting resolution of the sensor by following a new signal operational principle [22], [23], while detection limit of the RTD fluxgate sensor is still limited to the intrinsic noise of the core material. Compared to electroplated or sputtered core material, the amorphous soft magnetic alloy ribbons have better magnetic properties, higher working frequency and it is easier to obtain larger thicknesses, which is beneficial to the performances of the sensor. In addition to the properties of the core materials, higher cross-sectional area of the core also can increase the sensitivity to improve the signal-to-noise ratio. Demagnetization effect and eddy current effect limit the increase of the width and the thickness of the core drastically in MEMS micro solenoid fluxgate sensors. A multi-wire amorphous core was applied in the orthogonal fluxgate sensor by Li et al. in order to increase the sensitivity with higher cross-sectional area, and they found that a 16 wire core has sensitivity 65 times higher than the sensitivity of a single wire [24]. Furthermore, Jie et al. [25] and Ripka et al. [26], [27] proved that the larger number of wires core is beneficial to not only higher sensitivity but also lower noise. Comparing a MEMS micro solenoid fluxgate sensor having a single layer core and a sensor having a two-layer core, the total cross-sectional area of the latter is as twice as that of the former. In such a case, a higher sensitivity is obtained due to the two-layer core, while sensor area and coil resistance almost remain unchanged. In this paper, a new MEMS micro solenoid fluxgate sensor with a bi-layer Co-based amorphous ribbon magnetic core is suggested. Co-based amorphous ribbon is chosen as core material due to the advantages of higher permeability, lower saturation induction and zero magnetostriction, which benefits higher sensitivity, lower saturation field, low power and low noise
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LEI et al.: IMPROVED PERFORMANCE OF INTEGRATED SOLENOID FLUXGATE SENSOR CHIP
Fig. 2.
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The microfabrication steps of the micro fluxgate sensor [28]. Fig. 4.
Fig. 3.
Cross section of the bi-layer ribbon core.
for the micro fluxgate sensor. The schematic view and microphotography of the sensor is depicted in Fig. 1. II. FABRICATION Fabrication of micro fluxgate sensor is based on MEMS technology, mainly including thick photoresist-based UV lithography and electroplating techniques, and micro assembling method. The micro-fabrication steps of the fluxgate sensors are summarized in Fig. 2 [28]. The solenoid coils comprise two layers of copper (the bottom conductors and the top conductors), patterned by photolithography to form tracks, which are connected together with connection conductors to form a solenoid coil. Inside the coil is a rectangular ring core of Co-based amorphous ribbon. At first, the bottom conductors and connection conductors of copper are electroplated, as seen in Fig. 2 (a). The pre-etched magnetic core chips are glued twice by polyimide on a dried polyimide layer of 5μm thickness (PAE process, pasting after etching) to form a bi-layer laminated core structure, as seen in Fig. 2 (b). As shown in Fig. 3, the interface between the
Block diagram of the test system.
magnetic core and polyimide is uniform and tight, and there is no significant stratification between each polyimide layer. In Fig. 2 (c), polyimide as thick as electroplated copper is spun again and cured hard to electrically insulate top conductor lines and magnetic core, then fine polished until the connection conductors are exposed, as seen in Fig. 2 (d). Finally, the top copper conductors are fabricated to form the excitation and detection coils, as seen in Fig. 2 (e). For more details please refer to our previous paper [28]. The dimension of the sensor is approximately 6.74mm × 9mm (not including pads). The excitation coils have 42 turns and the detection coils have 63 turns. Both the line width and the line space of the coils are 40μm, and the thickness is 30μm. The measured resistance of the excitation coils is 3.18, measured by HP E4194A IMPEDANCE/GAIN-PHASE ANALYZER, which almost agrees with the calculated one, considering poles, connecting conductor lines and electro-deposition defections. The width of magnetic core is 1200μm and 800μm under the excitation coils and the detection coils respectively, which helps to reduce the saturated excitation current based on the law of magnetic circuit. The Co-based amorphous materials with a thickness of approximately 20μm used in this work are commercially available from the Metglas, Inc, 2714A. III. R ESULT AND D ISCUSSION A. Test System and Test Conditions In order to test the sensor’s performances, an open loop test system based on second harmonic operation principle is established, which includes function signal generator, power amplifier circuit, second harmonic biquadratic band pass filter and oscilloscope, as seen in Fig. 4. Fig. 5 shows photograph of the test system. The device output is amplified six times by a band pass filter and then connected to the oscilloscope (Tektronix TDS2014B). Under testing, the device and the
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Fig. 5.
Photograph of the test system.
Fig. 7. Effect of excitation current rms value on the sensitivity of the sensor.
Fig. 6. Characteristic curves of the fluxgate sensors with the bi-layer Co-based ribbon core.
Fig. 8. sensor.
solenoid (providing a uniform test field H parallel to the detection coil axis) are placed inside a magnetic shield with three-layer soft magnetic ribbons. The test field is controlled through the current by a programmable DC power supply. Sine wave is chosen as excitation waveform. The excitation frequency of the fluxgate sensors is chosen at 100kHz, which constitutes a balance between sensitivity, noise, power and interface circuit design. The center frequency of the second harmonic biquadratic band pass filter is set to 200kHz. In Fig. 6, the characteristic curves of the sensor for different excitation currents are shown. The results are discussed as follows.
relative permeability. The results indicate the largest sensitivity of 3165V/T for 240mA-rms excitation current. After being divided by six times (magnification factor), the real largest sensitivity of the sensor is 527.5 V/T. The multiple measurement error of the sensitivity is larger than 7% with respect to the excitation current amplitude of 240mA-rms.
B. Sensitivity The sensitivity of the sensor was measured with the test system at excitation currents of 180, 200, 240, 280, 320mA-rms, respectively. Fig. 7 shows the change of the sensitivity with the excitation current amplitude. The sensitivity of the sensor increases linearly with the excitation current amplitude. When the magnetic core is saturated by an excitation current higher than 240mA-rms, the sensitivity of the sensor decreases rapidly because of the degrading
Effect of excitation current rms value on the linear range of the
C. Linear Range Fig. 8 demonstrates the dependence of the linear range on the corresponding excitation current amplitude. The linear range (coefficient of determination of linear fitting is larger than 0.99) increases linearly with the excitation current amplitude, from 100μT to 250μT. After the magnetic core is oversaturated, the linear range tends to saturate for higher excitation current. This can be explained by considering the invariant maximum magnetic induction Bm in the oversaturated magnetic core (be equal to Bs ) at higher excitation current. D. Key Performance of the Sensor The characteristic curve of the sensor for 240mA-rms excitation current is shown in Fig. 9. The largest sensitivity
LEI et al.: IMPROVED PERFORMANCE OF INTEGRATED SOLENOID FLUXGATE SENSOR CHIP
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Fig. 9. The best characteristic curve of the fluxgate sensor for 240 mA-rms excitation current. The inset shows a linear fit to the curve in the linear range. Fig. 10.
of 3165V/T is located below 53μT. The linear range of the sensor is around 150μT. As seen in the inset, linearity of the sensor is determined by linear fitting for dots in the linear range, with coefficient of determination of 0.99 and max linear error of 6%, which is affected severely by the signal stability of the oscilloscope. At the same time, the sensor shows a power of 183.2mW at an excitation current of 240mA-rms. Despite of the reasonable coil resistance control (30μm thick copper conduct lines) and the excellent magnetic core material (with high permeability and low saturation magnetic induction), the power of the sensor is unsatisfactory. E. Noise Parameters In a practical application, the resolution is the core parameter reflecting the performance of the fluxgate sensor. In order to determine the sensor resolution, the noise has to be measured. The sensor is operated in the open loop mode; therefore the sensitivity around the zero field is used to recalculate the noise of the sensor. The noise of the sensor is analyzed through using FFT transform by Matlab based on long time data acquisition on oscilloscope. The noise power spectrum (with the frequency below 10Hz) of the whole test system containing the sensor and the band pass filter is measured for 240mA-rms excitation current at 100kHz, and the sensor is placed inside the 3 layer shield. Then the noise power spectrum is recalculated from V/Hz1/2 to T/Hz1/2 by using the sensitivity of the sensor. Fig. 10 presents the noise power spectrum of the sensor. The calculated noise is 0.5nT/Hz1/2@1Hz for the sensor (24mA-rms, 100kHz). After recalculating, the noise rms level is 2nT in the frequency range of 25 mHz-10 Hz. In addition to the intrinsic core material noise, the circuit noise and thermal noise, there is additional hysteresis noise brought by a lot of pinning points in AC excitation because of rough morphology on ribbon core surface for the fabricated sensor. Chemical mechanical polishing and magnetic annealing treatment on the magnetic ribbon core is conducive to further improve the
Noise spectrum of the fluxgate sensor.
sensor noise [29]. Koch forecasted the theoretical performance of a noiseless single domain fluxgate and achieved the following experimental result: 1.4pT/Hz1/2@1Hz of ring core and 3.5pT/Hz1/2@1Hz of rod core [30]. Therefore, there is still potential for the micro fluxgate sensor to improve further. IV. C ONCLUSION A new MEMS fluxgate sensor with a bi-layer Co-based ribbon core has been manufactured. The fabricated fluxgate sensor exhibits a best sensitivity of 3165V/T, a power of 183.2mW and a linear range of 15μT. The sensor noise is 0.5nT/Hz1/2@1Hz and the noise rms level is 2nT in 10Hz bandwidth. Compared to the results achieved by our previous sensor design, the signal-to-noise ratio of the sensor has been improved. In the next step, pretreatment optimization on Co-based magnetic ribbon core materials and multi-layer core would be conducive to further improve the performances of the sensor. Besides, impulse excitation method and higher excitation frequency will also be executed. R EFERENCES [1] H. U. Auster and V. Auster, “A new method for performing an absolute measurement of the geomagnetic field,” Meas. Sci. Technol., vol. 14, no. 7, pp. 1013–1017, Jul. 2003. [2] P. A. Robertson, “Microfabricated fluxgate sensors with low noise and wide bandwidth,” Electron. Lett., vol. 36, no. 4, pp. 331–332, 2000. [3] A. Baschirotto et al., “An integrated micro-fluxgate magnetic sensor with front-end circuitry,” IEEE Trans. Instrum. Meas., vol. 58, no. 9, pp. 3269–3275, Sep. 2009. [4] W.-Y. Choi and J.-O. Kim, “Two-axis micro fluxgate sensor on single chip,” Microsyst. Technol., vol. 12, no. 4, pp. 352–356, 2006. [5] W. Pei-Ming, and C. H. Ahn, “Design of a low-power micromachined fluxgate sensor using localized core saturation method,” IEEE Sensors J., vol. 8, no. 3, pp. 308–313, Mar. 2008. [6] A. Baschirotto et al., “Fluxgate magnetic sensor and front-end circuitry in an integrated microsystem,” Sens. Actuators A, Phys., vol. 132, no. 1, pp. 90–97, Nov. 2006.
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[7] J.-T. Jeng, J.-H. Chen, and C.-C. Lu, “Enhancement in sensitivity using multiple harmonics for miniature fluxgates,” IEEE Trans. Magn., vol. 48, no. 11, pp. 3696–3699, Nov. 2012. [8] M. H. Acuna, “Space-based magnetometers,” Rev. Sci. Instrum., vol. 73, no. 11, pp. 3717–3736, 2002. [9] F. Kaluza, A. Gruger, and H. Gruger, “New and future applications of fluxgate sensors,” Sens. Actuators A, Phys., vol. 106, nos. 1–3, pp. 48–51, Sep. 2003. [10] M. R. Kirchhoff and S. Buttgenbach, “MEMS fluxgate magnetometer for parallel robot application,” Microsyst. Technol., vol. 16, no. 5, pp. 787–790, May 2010. [11] A. Forslund, S. Belyayev, N. Ivchenko, G. Olsson, T. Edberg, and A. Marusenkov, “Miniaturized digital fluxgate magnetometer for small spacecraft applications,” Meas. Sci. Technol., vol. 19, no. 1, p. 015202, Jan. 2008. [12] W. Magnes et al., “Highly integrated front-end electronics for spaceborne fluxgate sensors,” Meas. Sci. Technol., vol. 19, no. 11, p. 115801, Nov. 2008. [13] J. H. Dieckhoff, T. Yoshida, K. Enpuku, M. Schilling, and F. Ludwig, “Homogeneous bioassays based on the manipulation of magnetic nanoparticles by rotating and alternating magnetic fields— A comparison,” IEEE Trans. Magn., vol. 48, no. 11, pp. 3792–3795, Nov. 2012. [14] O. Zorlu, P. Kejik, and R. S. Popovic, “An orthogonal fluxgate-type magnetic microsensor with electroplated permalloy core,” Sens. Actuators A, Phys., vol. 135, no. 1, pp. 43–49, 2007. [15] C.-C. Lu, Y.-T. Liu, F.-Y. Jhao, and J.-T. Jeng, “Responsivity and noise of a wire-bonded CMOS micro-fluxgate sensor,” Sens. Actuators A, Phys., vol. 179, pp. 39–43, Jun. 2012. [16] M. Janosek and P. Ripka, “PCB sensors in fluxgate magnetometer with controlled excitation,” Sens. Actuators A, Phys., vol. 151, no. 2, pp. 141–144, Apr. 2009. [17] J. Kubik, L. Pavel, P. Ripka, and P. Kaspar, “Low-power printed circuit board fluxgate sensor,” IEEE Sensors J., vol. 7, nos. 1–2, pp. 179–183, Feb. 2007. [18] E. Delevoye, A. Audoin, A. Beranger, R. Cuchet, R. Hida, and T. Jager, “Microfluxgate sensors for high frequency and low power applications,” Sens. Actuators A, Phys., vols. 145–146, pp. 271–277, Jul./Aug. 2008. [19] P. Ripka and W. G. Hurley, “Excitation efficiency of fluxgate sensors,” Sens. Actuators A, Phys., vol. 129, nos. 1–2, pp. 75–79, May 2006. [20] M. Butta and I. Sasada, “Sources of noise in a magnetometer based on orthogonal fluxgate operated in fundamental mode,” IEEE Trans. Magn., vol. 48, no. 4, pp. 1508–1511, Apr. 2012. [21] M. Butta and I. Sasada, “Orthogonal fluxgate with annealed wire core,” IEEE Trans. Magn., vol. 49, no. 1, pp. 62–65, Jan. 2013. [22] B. Ando, S. Baglio, V. Sacco, A. R. Bulsara, and V. In, “PCB fluxgate magnetometers with a residence times difference readout strategy: The effects of noise,” IEEE Trans. Instrum. Meas., vol. 57, no. 1, pp. 19–24, Jan. 2008. [23] B. Ando, A. Ascia, S. Baglio, A. R. Bulsara, J. D. Neff, and V. In, “Towards an optimal readout of a residence times difference (RTD) fluxgate magnetometer,” Sens. Actuators A, Phys., vol. 142, no. 1, pp. 73–79, Mar. 2008. [24] X. P. Li, J. Fan, J. Ding, H. Chiriac, X. B. Qian, and J. B. Yi, “A design of orthogonal fluxgate sensor,” J. Appl. Phys., vol. 99, no. 8, p. 08B313, Apr. 2006. [25] F. Jie, N. Ning, W. Ji, H. Chiriac, and L. Xiaoping, “Study of the noise in multicore orthogonal fluxgate sensors based on Ni-Fe/Cu composite microwire arrays,” IEEE Trans. Magn., vol. 45, no. 10, pp. 4451–4454, Oct. 2009. [26] P. Ripka, X. P. Li, and J. Fan, “Multiwire core fluxgate,” Sens. Actuators A, Phys., vol. 156, no. 1, pp. 265–268, 2009. [27] P. Ripka, M. Butta, F. Jie, and X. Li, “Sensitivity and noise of wire-core transverse fluxgate,” IEEE Trans. Magn., vol. 46, no. 2, pp. 654–657, Feb. 2010. [28] C. Lei, J. Lei, Z. Yang, and Y. Zhou, “Improved micro fluxgate sensor with double-layer Fe-based amorphous core,” Microsyst. Technol., vol. 19, no. 2, pp. 167–172, Feb. 2013. [29] P. Butvin et al., “Field annealed closed-path fluxgate sensors made of metallic-glass ribbons,” Sens. Actuators A, Phys., vol. 184, pp. 72–77, Sep. 2012. [30] R. H. Koch and J. R. Rozen, “Low-noise flux-gate magnetic-field sensors using ring- and rod-core geometries,” Appl. Phys. Lett., vol. 78, no. 13, pp. 1897–1899, Mar. 2001.
IEEE SENSORS JOURNAL, VOL. 15, NO. 9, SEPTEMBER 2015
Chong Lei received the M.S. degree in material from the University of Yanshan, Qinhuangdao, China, in 2002, and the Ph.D. degree in microelectronics and solid-state electronics from Shanghai Jiao Tong University, China, in 2009. In 2002, he joined the Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, where he was an Assistant until 2004. Since 2009, he has been with the National Key Laboratory of Nano/Micro Fabrication Technology, Shanghai Jiao Tong University, where he is currently an Assistant Professor with the School of Electronic Information and Electrical Engineering. His research interests are in the field of magnetic film, magnetism measure, microelectromechanical systems magnetic devices, and biosensors. Yan Liu received the bachelor’s degree in microelectronics from Xidian University in 2013. He is currently pursuing the master’s degree at the School of Electronic Information and Electrical Engineering, Shanghai Jiao Tong University. Research interests include designs of giant magneto-impedance (GMI) and fluxgate circuit.
Xue-Cheng Sun received the master’s degree in mechanical engineering from the Nanjing University of Aeronautics and Astronautics in 2013. He is currently pursuing the Ph.D. degree at the School of Electronic Information and Electrical Engineering, Shanghai Jiao Tong University. His research interests involve the fabrication of fluxgate sensors, giant magneto-resistance (GMR) sensor, and biosensing applications for the detection of biomarkers.
Tao Wang received the M.S. degree in physical electronics from the University of Electronic Science and Technology of China in 2010. He is currently pursuing the Ph.D. degree at the School of Electronic Information and Electrical Engineering, Shanghai Jiao Tong University. His research interests include MEMES technique, thin-film fabrication, Fe-based and Co-based magnetic materials, GMI sensor, and GMI-based sensor for the detection of magnetic beads and biomarkers.
Zhen Yang received the master’s degree in materials science and engineering from the Kunming University of Science and Technology in 2011. He is currently pursuing the Ph.D. degree at the School of Electronic Information and Electrical Engineering, Shanghai Jiao Tong University. His research interests involve MEMES technique, thin-film fabrication, Co-based and Fe-based ribbons, GMI biosensor, fluxgate biosensor, microinductor, and related applications for the detection of beads, biomarkers, and bacteria. Yong Zhou received the Ph.D. degree in inorganic nonmetallic materials from the Shanghai Institute of Optics and Fine Mechanics, in 1993. He is currently with the School of Electronic Information and Electrical Engineering, Shanghai Jiao Tong University, as a Group Leader of Thin-Film Electronic Materials and Devices. His research areas include microelectromechanical systems, X-ray lithography technique, micro actuators, GMI sensors and magnetic biochips phase change films and phase change memory, and biodevice for the biomedical applications.
