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Navigate：Introduction to the working principle and main technical performance of micro magnetic sensor
1 Magnetic sensor and its development
The magnetic sensor converts the magnetic properties of the sensitive components caused by magnetic fields, currents, stress strains, temperature, light, etc. into electrical signals, and detects the corresponding physical quantities in a variety of ways. In fact, the characteristic is that it can be non-contact measurement, the detection signal is almost unaffected by the measured object, and it is resistant to pollution and noise, and can work reliably even under severe environmental conditions, and is durable and durable. Because of this, these sensors are used in many aspects, from health care to the national economy, from health care to everyday life.
The magnetic sensor starts with a guideline using a magnet as a compass. Thereafter, as a component for sensing magnetic field and magnetic flux, a detection coil, a fluxgate magnetometer, a semiconductor Hall element and a magnetoresistive element, and a ferromagnetic thin film anisotropic magnetoresistance (AMR) component were successively developed. There are stress sensors using bulk ferrite cores, temperature sensors using heat-sensitive ferrite cores, fiber-optic current sensors using ferrimagnetic garnet magneto-optical effect, and high-sensitivity superconducting quantum interference devices (SQUID). and many more. In short, there are many types of magnetic sensors, and frequent replacements are frequent.
Magnetic sensors are usually assembled inside machines and equipment for use. Modern machines are rapidly moving toward small, lightweight, versatile, and intelligent, requiring sensors that respond to high-sensitivity and high-speed responses even to changes in physical quantities in tiny spaces. That is, while the sensor itself needs to be small and lightweight, it is also eager to improve its working speed, detection resolution and sensitivity.
Semiconductor large-scale integrated circuit
manufacturing technology, micro-electro-mechanical system (MEMS) manufacturing technology, micro-assembly technology, new materials such as magnetic film, amorphous, multilayer film, nano-magnetic wire and planar coil micro-magnetic device manufacturing process and The continuous improvement of the characterization means lays a solid foundation for the miniaturization and miniaturization of magnetic sensors, and many new high-performance, miniaturized and miniaturized magnetic sensors that apply various new effects are being put on the market. Early-stage AMR film sensors and sensors, the newly launched GMI sensor, SI sensor, SV-GMR sensor, and the upcoming practical application of thin-film fluxgate magnetometers and wireless magneto-micro-sensor arrays are typical examples.
2 new miniature magnetic sensor
2.1 High sensitivity GMI and SI miniature magnetic sensors
The GMI magnetic sensor consists of a low magnetostrictive material and a CMOS integrated circuit, and works with the giant magneto-impedance (GMI) effect of the magnetic material. The so-called GMI effect is that when a high-frequency ("10 kHz" current is applied to a low magnetostrictive amorphous wire or a patterned thin film element, the magnetic permeability and the skin effect of the sensitive element change with the magnetic field by the action of an external magnetic field. As a result, the inductance and the resistance, that is, the impedance changes abruptly. In 1992, Nagoya University professor Maori Jia Nianxiong and others first reported this new effect [1]. In their research, they found that the resistance change rate (△Z/Z) of the quenched cobalt-rich amorphous wire can be 100-300% after proper treatment. Recently, V. Zhukova et al. reported that amorphous filaments with a composition of Co67Fe3.85Ni1.45B11.5Si14.5Mo1.7 were used under optimum conditions (metal core diameter/wire total diameter ρ=0.98 at frequency f=10 MHz). Under the current I = 0.75 mA, the magnetic field induced (ΔZ / Z) max ≈ 615%. [2] According to a report by Professor Arai, a professor at Tohoku University in Japan, a copper conductor (thickness 3 μm, width 0.5 mm) was sandwiched between an amorphous magnetic film (Co73Si12B15 alloy: 2 μm thick, 2 mm wide, 10 mm long) and added between them. In the upper SiO2 insulating layer, when a DC external magnetic field is applied in the longitudinal direction of the element and a carrier current of 10 MHz is passed, an impedance change rate of about 600% and a voltage change amount of 0.8% (A/m) can be obtained.
The key to the practical use of GMI magnetic sensors is to choose the right magnetic material, and the second is to use the appropriate circuit system for the specific application. At present, Japan's Unitika Co., Ltd. has been able to supply the wire for this sensor in batches by cold drawing the amorphous alloy CoFeSiB (λs=-10-7) into a diameter of 15 to 30 μm, and then performing tension annealing on the surface layer. It induces precise circumferential anisotropy. The Co85Nb12Zr magnetic film is processed into a strip shape as a sensor, and a multilayer structure is formed by using a Co73Si12B15 amorphous magnetic film, a copper conductor, and an SiO2 insulating layer to form an outer iron closed magnetic circuit type sensor. In 1997, T. Kanno et al. explored a CMOSFET sensor circuit that uses pulse current to respond to the magnetoresistance effect; a high-resolution linear sensor uses a negative feedback loop in the sensor electronics, and a high-stability switching sensor uses a positive feedback loop. In 2001, Aichi Steel Co., Ltd. developed a high-density CMOS magneto-impedance sensor integrated circuit chip with a diameter of 30μm and a length of 2mm CoFeSiB amorphous wire. In 2002, it was processed into a CMOS type magnetic impedance with a φ20μm long 1μm CoFeSiB amorphous wire. Sensor integrated circuit chip. Proven to provide low-cost, high-volume GMI miniature magnetic sensor products to the market. The main performance indicators for this product are listed in Table 1 and compared to other commonly used high performance magnetic sensor products.
2.2 SV-GMR sensor and its display
The giant magnetoresistance (GMR) effect was originally found in a Fe/Cr multilayer film with a thickness of several atomic layers (several nm), which was found at 4.2 K plus a 1.6 × 107 A/m magnetic field, [7] The change (ΔR/Ro, ΔR=R11-R1) is as high as 46%, and the single-layer metal film having the AMR effect is only 4 to 6% at the maximum. In 1991, Parkin et al. used a Co/Cu multilayer film to apply a magnetic field at room temperature to achieve a resistance change of 65%. However, the magnetic field required for such a change in resistance is too high to be practical. Later, the so-called SV-GMR structural element consisting of an easy magnetized free magnetic layer (NiFe, etc.) / copper spacer layer / hard magnetic pinning layer (such as Co) / antiferromagnetic exchange coupling layer (FeMn, etc.), and CMOS integration The circuit combination is first used as a read head in a high-density HDD machine, and then a practical high-sensitivity magnetic field sensor is developed. The development of miniature magnetic sensor arrays using multiple SV-GMR components is currently underway.
2.3 Thin film fluxgate magnetometer
Conventional fluxgate magnetometers are commonly used to measure weak magnetic fields from 1nT to 1mT with resolutions up to 0.1nT. They are widely used in aerospace aircraft attitude control, prospecting, archaeology, space magnetic field detection and deep dive exploration and other military activities.
This conventional device is usually composed of two magnetic cores of several centimeters in size and a multi-turn coil. Therefore, it is difficult to miniaturize. In addition, manual adjustment during use requires separate calibration, which is inconvenient for operation and high in cost. To this end, magnetic thin film micro fluxgate devices are being actively developed.
The miniature fluxgate magnetometer is made of microelectronic technology, that is, an excitation coil and a detection coil made of a magnetic film, a micromachining or a standard planar process. P. Ripka et al. electroplated two layers of 4 μm permalloy as a magnetic core on a silicon substrate, and two metal layers processed with 3 μm thick aluminum were sandwiched between the permalloy layers. Using a photolithography process, aluminum is formed into a flat excitation coil and two anti-series detection coils; the permalloy film is photolithographically formed into four strips of length 0.7 mm, symmetrically placed on both sides of the coil, and two of them are excited. The channel closes the magnetic circuit. The entire device is similar to a dual fluxgate sensor with a chip size of only 2.5 x 4 mm2. It has been verified that the noise excited by pulse excitation is 20nTrms, the hysteresis is within 1mT, and the ignition caused by the 6mT magnetic field impact is lower than 5μT.
3 Applications and markets
The development and application of new magnetic sensors have created enormous economic, technical and social benefits, accelerated the realization of industrial automation, management intensification, office automation and modernization of family life, and accelerated the pace of industrialized society to the information society.
They will play an increasingly important role in various aspects such as traditional industrial transformation, resource exploration and comprehensive utilization, environmental protection, bioengineering, and intelligent transportation control.
3.1 Application and market in traditional industrial transformation
According to online reports, the global market for industrial process control sensors alone reached $26 billion in 1995; in 2001, the market for SHD-GMR heads for computer HDD exceeded 400 billion yen ($3.4 billion). While there are many types of electronic sensors, such as capacitive sensors, surface acoustic wave sensors, etc., that provide good performance for these applications, these platforms require a direct connection between the sensor and the data processing electronics using a tangible connector, or Accurate calibration and adjustment between the sensor and detector is required. If a new miniature magnetic sensor, especially a wireless passive (batteryless) device, is used, this can be eliminated. In this way, the operation is simpler and the reliability is improved, the life of the device is increased, and the cost is reduced.
The use of the new magnetic sensor can significantly improve the measurement and control accuracy. For example, with the GMI magnetic field sensor described above, the detection resolution is the same as that of the commonly used fluxgate magnetometer, but the response speed is doubled, and the power consumption is only the latter. 1%; if Hall device is used, its resolution is only 4A/m, and the required external field is more than 300 times higher than the former; in stress detection, the sensitivity of SI sensor is 2000 times higher than that of common resistance wire, which is semiconductor strain gauge 20 to 40 times. The hydraulic pressure or pneumatic cylinder piston position detection of industrial machine tools widely uses a magnetic sensor consisting of a permanent magnet ring and an AMR element mounted on a piston rod. The detection accuracy is 0.1 mm, and the detection speed can be changed at a high speed and low speed within 0 to 500 mm/s. When using GMI or SV-GMR sensors, the measurement accuracy can be increased by at least one order of magnitude.
Position detection and transmission speed control of CNC machine tools, robots and factory automation related equipment, and optical encoders are still widely used. Because the device is susceptible to dust, oil and smoke, it is extremely poorly used in the processing of automatic welding, paint robots, textiles and steel, wood, plastics, etc. Magnetic encoders using AMR, GMR, and GMI sensitive components do not have the above disadvantages. Therefore, their market demand is growing at an annual rate of more than 30%. The integration of the micro magnetic encoder and the control microcomputer facilitates the simplification of the control system structure and reduces the number of components and the space occupied, which is of great significance in the precision manufacturing and processing industries.
Various precision and ultra-precision line displacement magnetic sensors have been widely used in precision machining machines, special machine tools, semiconductor manufacturing equipment and 3D measuring equipment. A current sensor, a magnetic pole position sensor, or the like using a high-sensitivity, high-speed response magnetic sensor plays an important role in various motor drive and control. In the era of machine tool numerical control, digital magnetic scales help designers achieve closed-loop control. The magnetic scale using the absolute signal output is not disturbed by noise, power supply voltage fluctuations, etc., and does not require home position return. The conversion between manual and numerical control can also be done using the working state magnetic sensitive switch.
3.2 Application in environmental monitoring
The premise of environmental protection is the monitoring of the number of environments (temperature, air pressure, atmospheric composition, noise, .....). A large number of sensors are required here. The aforementioned magnetostrictive amorphous magnetic micro-magnetic sensor can simultaneously measure the temperature and air pressure of a vacuum or a confined space, and can be remotely and remotely accessed without a connector. In food packaging, environmental science experiments, etc., the application prospects are broad.
