- Email: [email protected]
Investigation of a novel MEMS orthogonal fluxgate sensor fabricated with Co-based amorphous ribbon core
Accepted Manuscript Title: Investigation of a novel MEMS orthogonal fluxgate sensor fabricated with Co-based amorphous ribbon core Authors: Shaotao Zhi, Zhu Feng, Lei Guo, Chong Lei, Yong Zhou PII: DOI: Reference:
S0924-4247(17)31061-0 https://doi.org/10.1016/j.sna.2017.09.045 SNA 10353
To appear in:
Sensors and Actuators A
Received date: Revised date: Accepted date:
6-6-2017 9-9-2017 26-9-2017
Please cite this article as: Shaotao Zhi, Zhu Feng, Lei Guo, Chong Lei, Yong Zhou, Investigation of a novel MEMS orthogonal fluxgate sensor fabricated with Co-based amorphous ribbon core, Sensors and Actuators: A Physical https://doi.org/10.1016/j.sna.2017.09.045 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Investigation of a novel MEMS orthogonal fluxgate sensor fabricated with Co-based amorphous ribbon core
Shaotao Zhi, Zhu Feng, Lei Guo, Chong Lei*, Yong Zhou*
Key Laboratory for Thin Film and Microfabrication of Ministry of Education, Department of Micro/Nano Electronics, School of Electronic Information and Electrical Engineering, Shanghai Jiao Tong University, Dongchuan Road 800, Shanghai 200240, China.
E-mail: [email protected]
Highlights
Micro orthogonal fluxgate sensor with Co-based amorphous ribbon core and three-dimensional solenoid coil was first designed and fabricated.
The Co-based amorphous ribbon core was designed in the shape of meander structure.
The micro orthogonal fluxgate sensor was fabricated by MEMS technology.
A maximum sensitivity of 575 V/T, a wide linear range of ± 480 μT, and an equivalent magnetic noise of 0.20 nT/√Hz @ 1 Hz were achieved.
Abstract In this paper, we present a novel MEMS orthogonal fluxgate sensor fabricated by standard micro fabricated technology. The sensor mainly consists of a three-dimensional solenoid pick-up coil and a meander-shaped Co-based amorphous ribbon core. The experimental results demonstrate that the sensitivity and noise can be optimized by tuning operation conditions with excitation current amplitude and frequency. The fabricated sensor exhibits a maximum sensitivity of 575 V/T, a wide linear range of ± 480 μT, and a perming below 0.8 μT for 90 mA rms sinusoidal excitation current
at 500 kHz frequency. The equivalent magnetic noise is 0.20 nT/√Hz at 1 Hz, and the RMS noise is 1.09 nT in the frequency range of 0.1−10 Hz under the same excitation. In comparison with other micro fabricated fluxgates in similar dimensions, this device possesses relatively high sensitivity and low noise spectral density.
Keywords: Orthogonal fluxgate; Micro sensor; MEMS (Micro Electro-Mechanical System); Amorphous ribbon.
1. Introduction High-precision miniature magnetic sensors are required in many fields, besides the traditional geophysical instruments they also serve for position tracking, aviation safety, bio-medical application and consumer electronics [1-6]. Fluxgate sensors, which are used to measure dc or low frequency magnetic fields, have been the most widely used sensors due to their high linearity, high sensitivity, high temperature stability, and low noise density [7,8]. In order to meet the application requirements for miniature systems, micro fabricated fluxgate sensors have been recently developed and fabricated via MEMS technology [9-12]. Comparing with conventional fluxgate sensors, the micro fluxgate sensors have many particular advantages, such as small size, light weight, low power consumption and easy realization of mass production. Previously reported micro fluxgate sensors are mainly focusing on the parallel configuration, but fewer studies on orthogonal configuration [13-16]. The simplest of orthogonal fluxgate sensors is composed of a pickup coil and a sensing element of single magnetic core in which an excitation current is fed directly, and no excitation coil is needed. In compared with parallel fluxgate, the orthogonal fluxgate sensor has shown better potential for miniaturization due to its simpler structure. However, the reported micro orthogonal fluxgate sensors possess relatively low sensitivity and inferior noise performance due to the low cross-sectional area and poor magnetic properties of magnetic core. For instance, Zorlu et al. [17] presented a micro orthogonal fluxgate with electroplated permalloy core and planar pick-up coils, the sensitivity of the sensor was only 0.51
V/T, and equivalent magnetic noise was 95 nT/√Hz at 1 Hz. Guo et al. [18] developed an orthogonal micro-fluxgate with three-dimensional solenoid coil, the equivalent magnetic noise was 4.1 nT/√Hz at 1 Hz. In order to develop a high performance micro fabricated orthogonal fluxgate sensor, a new MEMS orthogonal fluxgate sensor with Co-based amorphous ribbon core was first designed and fabricated in this paper. Compared with electroplated permalloy, amorphous alloy was more propitious to enhance fluxgate performance due to its higher permeability and excellent property in high frequency [19,20]. Demagnetization effect and eddy current effect limited the increase of the width and the thickness of the core, in order to increase the cross-sectional area, the amorphous ribbon core was designed in the shape of meander structure. The sensor performance such as sensitivity, offset stability, perming error and output noise was deeply investigated.
2. Design and fabrication Schematic view of the designed micro orthogonal fluxgate sensor is shown in Fig. 1. The micro sensor was fabricated using standard MEMS fabrication processes such as thick photoresist-based UV-photolithography, electroplating and micro assembly techniques. The sensor core was prefabricated by wet etching from 20 μm thick Metglas® 2714A Co-based amorphous ribbon, which was chosen due to its high relative permeability, low coercivity and low magnetostriction. The longitudinal and transverse hysteresis loops of the ribbon were obtained by using a VSM (vibrating sample magnetometer), as shown in Fig. 2. The hysteresis loop shapes had demonstrated the uniaxial anisotropy, and the coercivity values were measured to be about 0.3 Oe for the longitudinal direction, 0.8 Oe for the transverse direction. Three-dimensional solenoid coil was used to pick up the measuring signal. Pick-up coil could be subdivided into three sections during fabrication, which were including the bottom segment, top segment and vias. Vias were used to connect the other two parts, which were fabricated on separate layers. The fabrication of sensor started with sputtering the Cr/Cu seed layer (100 nm) on a cleaned glass substrate. A 25 μm photoresist layer was spin coated onto the seed layer and patterned using lithography for electroplating the bottom segment of coil. Cu was electroplated on it using a standard electroplating bath. A 40 μm photoresist mold with vias was formed on the wafer and Cu was electroplated using the similar process. Photoresist was removed with acetone and the seed layer was removed by reactive ion etching (Fig. 3a). Then, a thin coating of polyimide was spin on the
wafer, which was served as an isolating layer between the coil and the magnetic core (Fig. 3b). After solidification, the pre-etched magnetic cores were glued on the cured polyimide layer and their position was accurately fixed by position mark (Fig. 3c). Next, polyimide was spin again and polished until the vias were exposed (Fig. 3d). The magnetic core electrodes were exposed by a deep etching of polyimide using oxygen-based (O2 + SF6) reactive ion etching (RIE) (Fig. 3e). Remaining copper electrodes and the top segment of the coil were fabricated using the same sequence of lithography, electroplating and polyimide discussed above (Fig. 3f). Finally, the sensor was protected with polyimide. The fabrication process flow of the micro orthogonal fluxgate sensor is illustrated in Fig. 3. The fabricated micro fluxgate sensor is shown in Fig. 4. The pick-up coil had 60 turns, and width of each coil line was 70 μm as well as each gap between the lines was 50 μm. The meandershaped magnetic core was designed with length of 8 mm, width of 250μm, 10 turns, and the gap was 150 μm. Total area occupied by the sensor was 9.4 mm × 6.4 mm, including 1 mm × 1 mm electrodes.
3. Testing system The sensor was tested in the second harmonic mode with an open loop testing system, as seen in Fig. 5. A Tektronix AFG 3022 function signal generator was employed to produce a sine wave excitation signal. A printed circuit board (PCB) power amplifier was employed to amplify the sine signal into the sensor. The power amplifier chip was TI's current feedback operational amplifier OPA561, and an additional series transformer DA101C (turns ratio = 1:1, working frequency < 3 MHz, minimum return loss of 46.8 dB) was used to block DC component of the excitation current. SR844 DSP lock-in amplifiers was used for measuring the second harmonic component of the pickup coil voltage. A manually wound copper solenoid coil with 800 turns was employed to provide DC magnetic field, which was parallel to the axis of the sensing coil of the sensor. Intensity of the DC magnetic field was determined by the InsTek PST3202 programmable DC current source. Both the sensor and the solenoid coil were placed inside a cylindrical shield consisting of six layers of soft magnetic ribbons. In addition, axis of the shield was vertical to the earth’s magnetic field.
4. Results and discussions 4.1. Sensitivity Response curves for positive external fields (0−700 μT) are shown in Fig. 6. The frequency of the excitation signal was from 100 kHz to 600 kHz, and the current was 90 mA rms. It showed that the sensitivity increased with the excitation frequency increasing until 500 kHz was reached at a certain external field. When the excitation frequency was more than 500 kHz, the relationship between output voltage and external magnetic field was tend to nonlinearity that resulting from the frequency dependence of permeability and the parasitic capacitance of the pick-up coil. The changes of the sensitivity with excitation current amplitude are showed in Fig. 7. The sensor sensitivity was increasing linearly with increase of the excitation current amplitude until 90 mA rms. The maximum sensitivity was 575 V/T at 500 kHz. However, when the excitation current was more than 90 mA rms, the magnetic core was almost saturated, and the sensitivity was decreasing slightly due to the degrading relative permeability. The optimum excitation current of 90 mA rms was used for further experiments unless otherwise noted.
4.2. Linear range Fig. 8 shows the output response of the sensor system for 8 repeated measurements in the optimum frequency and amplitude conditions. We observed a relative standard deviation (RSD) below 2% for 8 successive measurements under the same test conditions, indicating good stability and repeatability of the micro orthogonal fluxgate sensor system. The linear fitting result was evident that there was a perfect linear relationship between the output voltage values with the magnetic field values. The linear range was about ± 480 μT, which was determined as the R-squared correlation coefficient of linear fitting was larger than 0.999. Fig. 9 shows the effect of excitation current magnitude and frequency on the linear range of the sensor. The results demonstrated the independence of the linear range on the excitation current amplitude and the excitation frequency. Similar results were found in the previous work [17]. They indicated that the linear operation range of the orthogonal fluxgate sensor had no relation with the excitation conditions, and was directly affected by magnetic core due to the independence of the excitation and detection mechanisms.
4.3. Offset stability
The sensor long-term offset stability was measured by observing the outputs over 12 hours. The sensor was placed in a shielded environment with zero applied field. Fig. 10 shows the offset of this sensor for 500 kHz excitation frequency and 90 mA rms excitation current. Since the magnitudes of the offset changes were similar, only a 1-hour excerpt was shown. After the warming period, the offset changes were about 20 nT band. According to [21], the excessive offset of fluxgate sensors was attributed to the coupling of magnetostriction with stress and temperature variations. Therefore, the near-zeromagnetostrictive alloys were recommended as the preferred core materials.
4.4. Perming error The perming error was investigated by applying a magnetic shock to the sensor by using a current-controlled Helmholtz coil with the amplitude was 10 mT. Then calculating the offset changes from the outputs. Fig. 11 shows the variation of the perming error of the sensor with the excitation current. It was seen that the perming decreased with the excitation current due to the deeply saturation of the core. The measurements showed that, with 90 mA rms excitation current at the frequency of 500 kHz, the sensor showed a perming below 0.8 μT. 4.5. Noise analysis The noise response of the micro orthogonal fluxgate sensor was investigated in a shielded environment. In order to analyze the influence of excitation frequency on noise, the experiments were carried out with an excitation current of 90 mA rms at excitation frequency from 100 kHz to 900 kHz. As depicted in Fig. 12(a), the measurements of voltage noise density at 1 Hz dramatically increased with the excitation frequency increasing. In addition, the magnetic noise spectral density of the sensor was defined as the ratio of the voltage noise density to its corresponding sensitivity around the zero field value. Typically, magnetic noise became low when sensor’s sensitivity was high. As seen in Fig. 12(b), it was shown that the magnetic noise density at 1 Hz descended rapidly at excitation frequency lower than 500 kHz due to the rapid increase in sensitivity. When the excitation frequency was higher than 500 kHz, the magnetic noise increased significantly. Since the voltage noise increased more rapidly than the sensitivity. A relative standard deviation (RSD) of 4% was observed with 8 successive measurements, which indicated the measured result was stability. It was noted that the magnetic noise of the fluxgate sensor was the result of Barkhausen noise in the magnetic core, which occurred when the level of the magnetization fell out of saturation [22,23]. In order to investigate the contribution of Barkhausen noise to the output noise of our sensor,
we plotted in Fig. 13 the magnetic noise spectrum when the sensor was excited by sin current Iex = 50, 70, 90 and 110 mA rms. The excitation frequency was 500 kHz. As we could see in Fig. 13, the noise level decreased if excitation current increased. The noise floor was reduced and the 1/f noise was gradually revealed with the excitation current increasing. For Iex = 90 mA rms the noise floor was about 0.03 nT/√Hz and the noise level at 1 Hz was 0.20 nT/√Hz. We observed that if excitation current Iex = 110 mA rms which was higher than 90 mA rms, then the noise floor looked similar but the 1 Hz noise rose back to 0.27 nT/√Hz. These results indicated that the noise increased due to the rapid increment of Barkhausen noise since the magnetization fell out of saturation. Therefore, any attempt to reduce the noise of the magnetometer should be focused on the magnetic core. The minimum noise for the sensor was 0.20 nT/√Hz, which was achieved for a 90 mA rms excitation current, and the RMS noise was 1.09 nT within 0.1−10 Hz bandwidth. A noise model of orthogonal fluxgate in fundamental mode was described in [23], which achieved a noise level of 2.8 pT/√Hz at 1Hz with a 53 mm long core, and this appeared to be the physical limit for this sensor. Moreover, magnetic annealing treatment on the magnetic core was conducive to further reduce the noise levels [24,25]. Therefore, the micro orthogonal fluxgate sensor still had great potential to reduce noise levels.
5. Conclusion In this work, we have successful designed and fabricated a novel micro orthogonal fluxgate employing Co-based amorphous ribbon as the magnetic core. In comparison with the previously published MEMS orthogonal fluxgate sensors [18,19], the performance of our sensor is highly enhanced owing to higher permeability and cross-sectional area of the magnetic cores, and lower magnetic flux leakage resulting from the three-dimensional solenoid structure of the pick-up coil. The sensor has a maximum sensitivity of 575 V/T, a wide linear range of ± 480 μT, and a perming below 0.8 μT for 90 mA rms excitation current at 500 kHz frequency. The minimum magnetic field noise is 0.20 nT/√Hz at 1 Hz under the same excitation. Moreover, due to the sensor was fabricated using the standard micro assembling method and MEMS technologies, therefore, it can be manufactured in large scale. We can easily say that the micro sensor with such improved characteristics further advances the development of MEMS fluxgate, and is promising for more application fields.
Acknowledgments This work is supported by The National Natural Science Foundation of China (No.61273065), National Science and Technology Support Program (2012BAK08B05), Natural Science Foundation of Shanghai (13ZR1420800), Support fund of Shanghai Jiao Tong University (AgriX2015005) , Support fund of Joint research center for advanced aerospace technology of Shanghai Academy of Spaceflight Technology-Shanghai Jiao Tong University (USCAST2015-2), Support fund of aerospace technology (15GFZ-JJ02-05), the Analytical and Testing Center in Shanghai Jiao Tong University, the Center for Advanced Electronic Materials and Devices in Shanghai Jiao Tong University.
Authors’ biography Shaotao Zhi received master’s degree of optical engineering from Zhejiang University in 2012, and now working toward his PhD at Shanghai Jiao Tong University School of electronic information and electrical engineering. His research interests involve the MEMES technique, thin film fabrication, fabrication of micro fluxgate sensors, and bio-sensing applications for the detection of biomarkers. Zhu Feng received bachelor’s degree of electrical and information engineering from Dalian University of technology in 2016, and now working toward her Ph.D at Shanghai Jiao Tong University School of electronic information and electrical engineering. Her research interests involve micro-biosensor and microsystem of biological detection. Lei Guo received master’s degree of materials science and engineering from Central South University of Science and Technology in 2014, and now working toward his PhD at Shanghai Jiao Tong University School of electronic information and electrical engineering. His research interests involve MEMES technique, thin film fabrication, Co-based and Fe-based ribbons, fluxgate biosensor, micro-inductor and related applications for the detection of beads, biomarkers and bacteria. Chong Lei received PhD degree of electronic science and technology from Shanghai Jiao Tong University Micro-nano Science and Technology Research in 2009, and now working in Shanghai Jiao Tong University. His research interests are in the field of magnetic film, magnetism measure, MEMS magnetic devices and biosensor. Prof. Dr. Yong Zhou received PhD degree of inorganic nonmetallic materials from Shanghai Institute of Optics and Fine Mechanics in 1993, and now working in Shanghai Jiao Tong University Micro-nano Science and Technology Research Institute as a Group leader of Thin Film Electronic Materials and Devices. Research areas include MEMS, X-Ray Lithography
Technique, Micro Actuators, GMI sensors and magnetic biochips Phase change films and phase change memory, and biodevice for the biomedical applications.
References [1] P. Ripka, Magnetic Sensors and Magnetometers, Artech House, London, 2001. [2] P. Ripka, M. Janosek, Advances in magnetic field sensors, IEEE Sens. J. 10 (2010) 1108-1116. [3] H.U. Auster, V. Auster, A new method for performing an absolute measurement of the geomagnetic field, Meas. Sci. Technol. 14 (2003) 1013-1017. [4] M.H. Acuna, Space-based magnetometers, Rev. Sci. Instrum. 73 (2002) 3717–3736. [5] A. Forslund, S. Belyayev, N. Ivchenko, et al. Miniaturized digital fluxgate magnetometer for small spacecraft applications, Meas. Sci. Technol. 19 (2008) 0152002. [6] 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. 48 (2012) 3792–3795. [7] F. Kaluza, A. Gruger, H. Gruger, New and future applications of fluxgate sensors, Sens. Actuators A 106 (2003) 48–51. [8] P. Ripka, Advances in fluxgate sensors, Sens. Actuators A 106 (2003) 8–14. [9] P. A. Robertson, Microfabricated fluxgate sensors with low noise and wide bandwidth, Electron. Lett. 36 (2000) 331–332. [10] W.Y. Choi, J.O. Kim, Two-axis micro fluxgate sensor on single chip, Microsyst. Technol. 12 (2006) 352–356. [11] A. Baschirotto, et al., Fluxgate magnetic sensor and front-end circuitry in an integrated microsystem, Sens. Actuators A 132 (2006) 90-97. [12] A. Baschirotto, E. Dallago, P. Malcovati, M. Marchesi, and G. Venchi, A fluxgate magnetic sensor: From PCB to micro-integrated technology, IEEE Trans. Instrum. Meas. 56 (2007) 25–31. [13] P.M. Wu, C.H. Ahn, Design of a low-power micromachined fluxgate sensor using localized core saturation method, IEEE Sens. J. 15 (2008) 308–313. [14] C. Lei, R. Wang, Y. Zhou, Z. Zhou, MEMS micro fluxgate sensors with mutual vertical excitation coils and detection coils, Microsyst. Technol. 15 (2009) 969-972.
[15] C.C. Lu, Y.T. Liu, F.Y. Jhao, J.T. Jeng, Responsivity and noise of a wire-bonded CMOS microfluxgate sensor, Sens. Actuators A 179 (2012) 39-43 [16] T. Heimfarth, M.Z. Mielli, M.N.P. Carreño, Miniature planar fluxgate magnetic sensors using a single layer of coils, IEEE Sens. J. 15 (2015) 2365-2369. [17] O. Zorlu, P. Kejik, R.S. Popovic, An orthogonal fluxgate-type magnetic microsensor with electroplated permalloy core, Sens. Actuators A 135 (2007) 43–49. [18] B. Guo, S. Liu, S. Yang, Orthogonal micro-fluxgate with S-shape excitation wire and 3D solenoid detection coil, Microsyst. Technol. 21 (2015) 1579-1586. [19] K. Goleman, I. Sasada, High sensitive orthogonal fluxgate magnetometer using a Metglas ribbon, IEEE Trans. Magn. 42 (2006) 3276. [20] J. Kubik, L. Pavel, P. Ripka, PCB racetrack fluxgate sensor with improved temperature stability, Sens. Actuators A 130 (2006) 184–188. [21] P. Ripka, M. Pribil, M. Butta, Magnetostriction offset of fluxgate sensors, IEEE Trans. Magn. 51 (2015) 1-4. [22] M. Butta, S. Yamashita, and I. Sasada. Reduction of noise in fundamental mode orthogonal fluxgates by optimization of excitation current, IEEE Trans. Magn. 47 (2011) 3748-3751. [23] M. Butta, I. Sasada, Sources of noise in a magnetometer based on orthogonal fluxgate operated in fundamental mode, IEEE Trans. Magn. 48 (2012) 1508–1511. [24] P. Butvin, M. Janošek, P. Ripka, et al., Field annealed closed-path fluxgate sensors made of metallic-glass ribbons, Sens. Actuators A 184 (2012) 72–77. [25] M. Butta, I. Sasada, Orthogonal fluxgate with annealed wire core, IEEE Trans. Magn. 49 (2013) 62-65.
Fig. 1. Schematic view of micro orthogonal fluxgate sensor
Fig.2. Longitudinal and transverse hysteresis loops of the Co-based amorphous ribbon.
Fig. 3. The fabrication process flow of the micro orthogonal fluxgate sensor.
Fig. 4. Microscopic image of the fabricated micro orthogonal fluxgate sensor
Fig. 5. Block diagram of the testing system
Fig. 6. The sensor response for different levels of the excitation frequency.
Fig. 7. The sensitivity of the sensor with the magnitude of the excitation current.
Fig. 8. The linear fit to the response curve (fex = 500kHz, Iex = 90 mA rms).
Fig. 9. The linear range of the sensor with the excitation current magnitude and frequency.
Fig. 10. Offset drift of the sensor over one hour.
Fig. 11. The perming error of the sensor with the excitation current.
Fig. 12. (a) Voltage noise and (b) magnetic noise density of the sensor measured at 1 Hz.
Fig. 13. The magnetic noise spectrum for Iex = 50, 70, 90 and 110 mA rms at 500 kHz excitation frequency.
S0924-4247(17)31061-0 https://doi.org/10.1016/j.sna.2017.09.045 SNA 10353
To appear in:
Sensors and Actuators A
Received date: Revised date: Accepted date:
6-6-2017 9-9-2017 26-9-2017
Please cite this article as: Shaotao Zhi, Zhu Feng, Lei Guo, Chong Lei, Yong Zhou, Investigation of a novel MEMS orthogonal fluxgate sensor fabricated with Co-based amorphous ribbon core, Sensors and Actuators: A Physical https://doi.org/10.1016/j.sna.2017.09.045 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Investigation of a novel MEMS orthogonal fluxgate sensor fabricated with Co-based amorphous ribbon core
Shaotao Zhi, Zhu Feng, Lei Guo, Chong Lei*, Yong Zhou*
Key Laboratory for Thin Film and Microfabrication of Ministry of Education, Department of Micro/Nano Electronics, School of Electronic Information and Electrical Engineering, Shanghai Jiao Tong University, Dongchuan Road 800, Shanghai 200240, China.
E-mail: [email protected]
Highlights
Micro orthogonal fluxgate sensor with Co-based amorphous ribbon core and three-dimensional solenoid coil was first designed and fabricated.
The Co-based amorphous ribbon core was designed in the shape of meander structure.
The micro orthogonal fluxgate sensor was fabricated by MEMS technology.
A maximum sensitivity of 575 V/T, a wide linear range of ± 480 μT, and an equivalent magnetic noise of 0.20 nT/√Hz @ 1 Hz were achieved.
Abstract In this paper, we present a novel MEMS orthogonal fluxgate sensor fabricated by standard micro fabricated technology. The sensor mainly consists of a three-dimensional solenoid pick-up coil and a meander-shaped Co-based amorphous ribbon core. The experimental results demonstrate that the sensitivity and noise can be optimized by tuning operation conditions with excitation current amplitude and frequency. The fabricated sensor exhibits a maximum sensitivity of 575 V/T, a wide linear range of ± 480 μT, and a perming below 0.8 μT for 90 mA rms sinusoidal excitation current
at 500 kHz frequency. The equivalent magnetic noise is 0.20 nT/√Hz at 1 Hz, and the RMS noise is 1.09 nT in the frequency range of 0.1−10 Hz under the same excitation. In comparison with other micro fabricated fluxgates in similar dimensions, this device possesses relatively high sensitivity and low noise spectral density.
Keywords: Orthogonal fluxgate; Micro sensor; MEMS (Micro Electro-Mechanical System); Amorphous ribbon.
1. Introduction High-precision miniature magnetic sensors are required in many fields, besides the traditional geophysical instruments they also serve for position tracking, aviation safety, bio-medical application and consumer electronics [1-6]. Fluxgate sensors, which are used to measure dc or low frequency magnetic fields, have been the most widely used sensors due to their high linearity, high sensitivity, high temperature stability, and low noise density [7,8]. In order to meet the application requirements for miniature systems, micro fabricated fluxgate sensors have been recently developed and fabricated via MEMS technology [9-12]. Comparing with conventional fluxgate sensors, the micro fluxgate sensors have many particular advantages, such as small size, light weight, low power consumption and easy realization of mass production. Previously reported micro fluxgate sensors are mainly focusing on the parallel configuration, but fewer studies on orthogonal configuration [13-16]. The simplest of orthogonal fluxgate sensors is composed of a pickup coil and a sensing element of single magnetic core in which an excitation current is fed directly, and no excitation coil is needed. In compared with parallel fluxgate, the orthogonal fluxgate sensor has shown better potential for miniaturization due to its simpler structure. However, the reported micro orthogonal fluxgate sensors possess relatively low sensitivity and inferior noise performance due to the low cross-sectional area and poor magnetic properties of magnetic core. For instance, Zorlu et al. [17] presented a micro orthogonal fluxgate with electroplated permalloy core and planar pick-up coils, the sensitivity of the sensor was only 0.51
V/T, and equivalent magnetic noise was 95 nT/√Hz at 1 Hz. Guo et al. [18] developed an orthogonal micro-fluxgate with three-dimensional solenoid coil, the equivalent magnetic noise was 4.1 nT/√Hz at 1 Hz. In order to develop a high performance micro fabricated orthogonal fluxgate sensor, a new MEMS orthogonal fluxgate sensor with Co-based amorphous ribbon core was first designed and fabricated in this paper. Compared with electroplated permalloy, amorphous alloy was more propitious to enhance fluxgate performance due to its higher permeability and excellent property in high frequency [19,20]. Demagnetization effect and eddy current effect limited the increase of the width and the thickness of the core, in order to increase the cross-sectional area, the amorphous ribbon core was designed in the shape of meander structure. The sensor performance such as sensitivity, offset stability, perming error and output noise was deeply investigated.
2. Design and fabrication Schematic view of the designed micro orthogonal fluxgate sensor is shown in Fig. 1. The micro sensor was fabricated using standard MEMS fabrication processes such as thick photoresist-based UV-photolithography, electroplating and micro assembly techniques. The sensor core was prefabricated by wet etching from 20 μm thick Metglas® 2714A Co-based amorphous ribbon, which was chosen due to its high relative permeability, low coercivity and low magnetostriction. The longitudinal and transverse hysteresis loops of the ribbon were obtained by using a VSM (vibrating sample magnetometer), as shown in Fig. 2. The hysteresis loop shapes had demonstrated the uniaxial anisotropy, and the coercivity values were measured to be about 0.3 Oe for the longitudinal direction, 0.8 Oe for the transverse direction. Three-dimensional solenoid coil was used to pick up the measuring signal. Pick-up coil could be subdivided into three sections during fabrication, which were including the bottom segment, top segment and vias. Vias were used to connect the other two parts, which were fabricated on separate layers. The fabrication of sensor started with sputtering the Cr/Cu seed layer (100 nm) on a cleaned glass substrate. A 25 μm photoresist layer was spin coated onto the seed layer and patterned using lithography for electroplating the bottom segment of coil. Cu was electroplated on it using a standard electroplating bath. A 40 μm photoresist mold with vias was formed on the wafer and Cu was electroplated using the similar process. Photoresist was removed with acetone and the seed layer was removed by reactive ion etching (Fig. 3a). Then, a thin coating of polyimide was spin on the
wafer, which was served as an isolating layer between the coil and the magnetic core (Fig. 3b). After solidification, the pre-etched magnetic cores were glued on the cured polyimide layer and their position was accurately fixed by position mark (Fig. 3c). Next, polyimide was spin again and polished until the vias were exposed (Fig. 3d). The magnetic core electrodes were exposed by a deep etching of polyimide using oxygen-based (O2 + SF6) reactive ion etching (RIE) (Fig. 3e). Remaining copper electrodes and the top segment of the coil were fabricated using the same sequence of lithography, electroplating and polyimide discussed above (Fig. 3f). Finally, the sensor was protected with polyimide. The fabrication process flow of the micro orthogonal fluxgate sensor is illustrated in Fig. 3. The fabricated micro fluxgate sensor is shown in Fig. 4. The pick-up coil had 60 turns, and width of each coil line was 70 μm as well as each gap between the lines was 50 μm. The meandershaped magnetic core was designed with length of 8 mm, width of 250μm, 10 turns, and the gap was 150 μm. Total area occupied by the sensor was 9.4 mm × 6.4 mm, including 1 mm × 1 mm electrodes.
3. Testing system The sensor was tested in the second harmonic mode with an open loop testing system, as seen in Fig. 5. A Tektronix AFG 3022 function signal generator was employed to produce a sine wave excitation signal. A printed circuit board (PCB) power amplifier was employed to amplify the sine signal into the sensor. The power amplifier chip was TI's current feedback operational amplifier OPA561, and an additional series transformer DA101C (turns ratio = 1:1, working frequency < 3 MHz, minimum return loss of 46.8 dB) was used to block DC component of the excitation current. SR844 DSP lock-in amplifiers was used for measuring the second harmonic component of the pickup coil voltage. A manually wound copper solenoid coil with 800 turns was employed to provide DC magnetic field, which was parallel to the axis of the sensing coil of the sensor. Intensity of the DC magnetic field was determined by the InsTek PST3202 programmable DC current source. Both the sensor and the solenoid coil were placed inside a cylindrical shield consisting of six layers of soft magnetic ribbons. In addition, axis of the shield was vertical to the earth’s magnetic field.
4. Results and discussions 4.1. Sensitivity Response curves for positive external fields (0−700 μT) are shown in Fig. 6. The frequency of the excitation signal was from 100 kHz to 600 kHz, and the current was 90 mA rms. It showed that the sensitivity increased with the excitation frequency increasing until 500 kHz was reached at a certain external field. When the excitation frequency was more than 500 kHz, the relationship between output voltage and external magnetic field was tend to nonlinearity that resulting from the frequency dependence of permeability and the parasitic capacitance of the pick-up coil. The changes of the sensitivity with excitation current amplitude are showed in Fig. 7. The sensor sensitivity was increasing linearly with increase of the excitation current amplitude until 90 mA rms. The maximum sensitivity was 575 V/T at 500 kHz. However, when the excitation current was more than 90 mA rms, the magnetic core was almost saturated, and the sensitivity was decreasing slightly due to the degrading relative permeability. The optimum excitation current of 90 mA rms was used for further experiments unless otherwise noted.
4.2. Linear range Fig. 8 shows the output response of the sensor system for 8 repeated measurements in the optimum frequency and amplitude conditions. We observed a relative standard deviation (RSD) below 2% for 8 successive measurements under the same test conditions, indicating good stability and repeatability of the micro orthogonal fluxgate sensor system. The linear fitting result was evident that there was a perfect linear relationship between the output voltage values with the magnetic field values. The linear range was about ± 480 μT, which was determined as the R-squared correlation coefficient of linear fitting was larger than 0.999. Fig. 9 shows the effect of excitation current magnitude and frequency on the linear range of the sensor. The results demonstrated the independence of the linear range on the excitation current amplitude and the excitation frequency. Similar results were found in the previous work [17]. They indicated that the linear operation range of the orthogonal fluxgate sensor had no relation with the excitation conditions, and was directly affected by magnetic core due to the independence of the excitation and detection mechanisms.
4.3. Offset stability
The sensor long-term offset stability was measured by observing the outputs over 12 hours. The sensor was placed in a shielded environment with zero applied field. Fig. 10 shows the offset of this sensor for 500 kHz excitation frequency and 90 mA rms excitation current. Since the magnitudes of the offset changes were similar, only a 1-hour excerpt was shown. After the warming period, the offset changes were about 20 nT band. According to [21], the excessive offset of fluxgate sensors was attributed to the coupling of magnetostriction with stress and temperature variations. Therefore, the near-zeromagnetostrictive alloys were recommended as the preferred core materials.
4.4. Perming error The perming error was investigated by applying a magnetic shock to the sensor by using a current-controlled Helmholtz coil with the amplitude was 10 mT. Then calculating the offset changes from the outputs. Fig. 11 shows the variation of the perming error of the sensor with the excitation current. It was seen that the perming decreased with the excitation current due to the deeply saturation of the core. The measurements showed that, with 90 mA rms excitation current at the frequency of 500 kHz, the sensor showed a perming below 0.8 μT. 4.5. Noise analysis The noise response of the micro orthogonal fluxgate sensor was investigated in a shielded environment. In order to analyze the influence of excitation frequency on noise, the experiments were carried out with an excitation current of 90 mA rms at excitation frequency from 100 kHz to 900 kHz. As depicted in Fig. 12(a), the measurements of voltage noise density at 1 Hz dramatically increased with the excitation frequency increasing. In addition, the magnetic noise spectral density of the sensor was defined as the ratio of the voltage noise density to its corresponding sensitivity around the zero field value. Typically, magnetic noise became low when sensor’s sensitivity was high. As seen in Fig. 12(b), it was shown that the magnetic noise density at 1 Hz descended rapidly at excitation frequency lower than 500 kHz due to the rapid increase in sensitivity. When the excitation frequency was higher than 500 kHz, the magnetic noise increased significantly. Since the voltage noise increased more rapidly than the sensitivity. A relative standard deviation (RSD) of 4% was observed with 8 successive measurements, which indicated the measured result was stability. It was noted that the magnetic noise of the fluxgate sensor was the result of Barkhausen noise in the magnetic core, which occurred when the level of the magnetization fell out of saturation [22,23]. In order to investigate the contribution of Barkhausen noise to the output noise of our sensor,
we plotted in Fig. 13 the magnetic noise spectrum when the sensor was excited by sin current Iex = 50, 70, 90 and 110 mA rms. The excitation frequency was 500 kHz. As we could see in Fig. 13, the noise level decreased if excitation current increased. The noise floor was reduced and the 1/f noise was gradually revealed with the excitation current increasing. For Iex = 90 mA rms the noise floor was about 0.03 nT/√Hz and the noise level at 1 Hz was 0.20 nT/√Hz. We observed that if excitation current Iex = 110 mA rms which was higher than 90 mA rms, then the noise floor looked similar but the 1 Hz noise rose back to 0.27 nT/√Hz. These results indicated that the noise increased due to the rapid increment of Barkhausen noise since the magnetization fell out of saturation. Therefore, any attempt to reduce the noise of the magnetometer should be focused on the magnetic core. The minimum noise for the sensor was 0.20 nT/√Hz, which was achieved for a 90 mA rms excitation current, and the RMS noise was 1.09 nT within 0.1−10 Hz bandwidth. A noise model of orthogonal fluxgate in fundamental mode was described in [23], which achieved a noise level of 2.8 pT/√Hz at 1Hz with a 53 mm long core, and this appeared to be the physical limit for this sensor. Moreover, magnetic annealing treatment on the magnetic core was conducive to further reduce the noise levels [24,25]. Therefore, the micro orthogonal fluxgate sensor still had great potential to reduce noise levels.
5. Conclusion In this work, we have successful designed and fabricated a novel micro orthogonal fluxgate employing Co-based amorphous ribbon as the magnetic core. In comparison with the previously published MEMS orthogonal fluxgate sensors [18,19], the performance of our sensor is highly enhanced owing to higher permeability and cross-sectional area of the magnetic cores, and lower magnetic flux leakage resulting from the three-dimensional solenoid structure of the pick-up coil. The sensor has a maximum sensitivity of 575 V/T, a wide linear range of ± 480 μT, and a perming below 0.8 μT for 90 mA rms excitation current at 500 kHz frequency. The minimum magnetic field noise is 0.20 nT/√Hz at 1 Hz under the same excitation. Moreover, due to the sensor was fabricated using the standard micro assembling method and MEMS technologies, therefore, it can be manufactured in large scale. We can easily say that the micro sensor with such improved characteristics further advances the development of MEMS fluxgate, and is promising for more application fields.
Acknowledgments This work is supported by The National Natural Science Foundation of China (No.61273065), National Science and Technology Support Program (2012BAK08B05), Natural Science Foundation of Shanghai (13ZR1420800), Support fund of Shanghai Jiao Tong University (AgriX2015005) , Support fund of Joint research center for advanced aerospace technology of Shanghai Academy of Spaceflight Technology-Shanghai Jiao Tong University (USCAST2015-2), Support fund of aerospace technology (15GFZ-JJ02-05), the Analytical and Testing Center in Shanghai Jiao Tong University, the Center for Advanced Electronic Materials and Devices in Shanghai Jiao Tong University.
Authors’ biography Shaotao Zhi received master’s degree of optical engineering from Zhejiang University in 2012, and now working toward his PhD at Shanghai Jiao Tong University School of electronic information and electrical engineering. His research interests involve the MEMES technique, thin film fabrication, fabrication of micro fluxgate sensors, and bio-sensing applications for the detection of biomarkers. Zhu Feng received bachelor’s degree of electrical and information engineering from Dalian University of technology in 2016, and now working toward her Ph.D at Shanghai Jiao Tong University School of electronic information and electrical engineering. Her research interests involve micro-biosensor and microsystem of biological detection. Lei Guo received master’s degree of materials science and engineering from Central South University of Science and Technology in 2014, and now working toward his PhD at Shanghai Jiao Tong University School of electronic information and electrical engineering. His research interests involve MEMES technique, thin film fabrication, Co-based and Fe-based ribbons, fluxgate biosensor, micro-inductor and related applications for the detection of beads, biomarkers and bacteria. Chong Lei received PhD degree of electronic science and technology from Shanghai Jiao Tong University Micro-nano Science and Technology Research in 2009, and now working in Shanghai Jiao Tong University. His research interests are in the field of magnetic film, magnetism measure, MEMS magnetic devices and biosensor. Prof. Dr. Yong Zhou received PhD degree of inorganic nonmetallic materials from Shanghai Institute of Optics and Fine Mechanics in 1993, and now working in Shanghai Jiao Tong University Micro-nano Science and Technology Research Institute as a Group leader of Thin Film Electronic Materials and Devices. Research areas include MEMS, X-Ray Lithography
Technique, Micro Actuators, GMI sensors and magnetic biochips Phase change films and phase change memory, and biodevice for the biomedical applications.
References [1] P. Ripka, Magnetic Sensors and Magnetometers, Artech House, London, 2001. [2] P. Ripka, M. Janosek, Advances in magnetic field sensors, IEEE Sens. J. 10 (2010) 1108-1116. [3] H.U. Auster, V. Auster, A new method for performing an absolute measurement of the geomagnetic field, Meas. Sci. Technol. 14 (2003) 1013-1017. [4] M.H. Acuna, Space-based magnetometers, Rev. Sci. Instrum. 73 (2002) 3717–3736. [5] A. Forslund, S. Belyayev, N. Ivchenko, et al. Miniaturized digital fluxgate magnetometer for small spacecraft applications, Meas. Sci. Technol. 19 (2008) 0152002. [6] 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. 48 (2012) 3792–3795. [7] F. Kaluza, A. Gruger, H. Gruger, New and future applications of fluxgate sensors, Sens. Actuators A 106 (2003) 48–51. [8] P. Ripka, Advances in fluxgate sensors, Sens. Actuators A 106 (2003) 8–14. [9] P. A. Robertson, Microfabricated fluxgate sensors with low noise and wide bandwidth, Electron. Lett. 36 (2000) 331–332. [10] W.Y. Choi, J.O. Kim, Two-axis micro fluxgate sensor on single chip, Microsyst. Technol. 12 (2006) 352–356. [11] A. Baschirotto, et al., Fluxgate magnetic sensor and front-end circuitry in an integrated microsystem, Sens. Actuators A 132 (2006) 90-97. [12] A. Baschirotto, E. Dallago, P. Malcovati, M. Marchesi, and G. Venchi, A fluxgate magnetic sensor: From PCB to micro-integrated technology, IEEE Trans. Instrum. Meas. 56 (2007) 25–31. [13] P.M. Wu, C.H. Ahn, Design of a low-power micromachined fluxgate sensor using localized core saturation method, IEEE Sens. J. 15 (2008) 308–313. [14] C. Lei, R. Wang, Y. Zhou, Z. Zhou, MEMS micro fluxgate sensors with mutual vertical excitation coils and detection coils, Microsyst. Technol. 15 (2009) 969-972.
[15] C.C. Lu, Y.T. Liu, F.Y. Jhao, J.T. Jeng, Responsivity and noise of a wire-bonded CMOS microfluxgate sensor, Sens. Actuators A 179 (2012) 39-43 [16] T. Heimfarth, M.Z. Mielli, M.N.P. Carreño, Miniature planar fluxgate magnetic sensors using a single layer of coils, IEEE Sens. J. 15 (2015) 2365-2369. [17] O. Zorlu, P. Kejik, R.S. Popovic, An orthogonal fluxgate-type magnetic microsensor with electroplated permalloy core, Sens. Actuators A 135 (2007) 43–49. [18] B. Guo, S. Liu, S. Yang, Orthogonal micro-fluxgate with S-shape excitation wire and 3D solenoid detection coil, Microsyst. Technol. 21 (2015) 1579-1586. [19] K. Goleman, I. Sasada, High sensitive orthogonal fluxgate magnetometer using a Metglas ribbon, IEEE Trans. Magn. 42 (2006) 3276. [20] J. Kubik, L. Pavel, P. Ripka, PCB racetrack fluxgate sensor with improved temperature stability, Sens. Actuators A 130 (2006) 184–188. [21] P. Ripka, M. Pribil, M. Butta, Magnetostriction offset of fluxgate sensors, IEEE Trans. Magn. 51 (2015) 1-4. [22] M. Butta, S. Yamashita, and I. Sasada. Reduction of noise in fundamental mode orthogonal fluxgates by optimization of excitation current, IEEE Trans. Magn. 47 (2011) 3748-3751. [23] M. Butta, I. Sasada, Sources of noise in a magnetometer based on orthogonal fluxgate operated in fundamental mode, IEEE Trans. Magn. 48 (2012) 1508–1511. [24] P. Butvin, M. Janošek, P. Ripka, et al., Field annealed closed-path fluxgate sensors made of metallic-glass ribbons, Sens. Actuators A 184 (2012) 72–77. [25] M. Butta, I. Sasada, Orthogonal fluxgate with annealed wire core, IEEE Trans. Magn. 49 (2013) 62-65.
Fig. 1. Schematic view of micro orthogonal fluxgate sensor
Fig.2. Longitudinal and transverse hysteresis loops of the Co-based amorphous ribbon.
Fig. 3. The fabrication process flow of the micro orthogonal fluxgate sensor.
Fig. 4. Microscopic image of the fabricated micro orthogonal fluxgate sensor
Fig. 5. Block diagram of the testing system
Fig. 6. The sensor response for different levels of the excitation frequency.
Fig. 7. The sensitivity of the sensor with the magnitude of the excitation current.
Fig. 8. The linear fit to the response curve (fex = 500kHz, Iex = 90 mA rms).
Fig. 9. The linear range of the sensor with the excitation current magnitude and frequency.
Fig. 10. Offset drift of the sensor over one hour.
Fig. 11. The perming error of the sensor with the excitation current.
Fig. 12. (a) Voltage noise and (b) magnetic noise density of the sensor measured at 1 Hz.
Fig. 13. The magnetic noise spectrum for Iex = 50, 70, 90 and 110 mA rms at 500 kHz excitation frequency.