Micro Fluxgate

Fluxgate is a sensor that measures weak magnetic fields by using the nonlinear relationship between the magnetic induction intensity and the magnetic field intensity of a high permeability core in the measured magnetic field under the saturation excitation of an alternating magnetic field. Fluxgate sensors, also known as magnetometers, consist of a probe and an interface circuit. They have the characteristics of high resolution (up to 10-11T), wide range of weak magnetic field measurement (below 10-8T), reliability, simplicity, economy, durability, ability to directly measure the components of the magnetic field, and suitability for use in high-speed motion systems. The research on fluxgate sensors began in 1928. A few years later, fluxgate magnetometers that used the magnetic saturation characteristics of magnetic materials appeared. They were used to measure DC or low-frequency AC magnetic fields below 1mT. In 1936, Aschenbrenner and Goubau claimed to have achieved a resolution of 0.3nT. During World War II, fluxgate sensors used for military submarine detection made great progress. Since its introduction, the fluxgate has been continuously developed and improved, and has been widely used in various fields, such as geomagnetic research, geological exploration, oil logging, space magnetic field detection, magnetic navigation, weapons reconnaissance, submarine detection, magnetic material testing and non-destructive testing of materials, and other fields of weak magnetic field detection. In recent years, the fluxgate has also been used in aerospace engineering. For example, it is used to control the attitude of artificial satellites and rockets, and to map the space magnetic field of the "solar wind" of the sun and the interaction of charged particles, the magnetic field of the moon, the magnetic field of the planets, and the graphics of the interstellar magnetic field. NASA is currently formulating an ambitious micro-instrument technology development plan, the main purpose of which is to develop small, low-cost, high-performance spacecraft suitable for the 21st century, and to use MEMS technology to miniaturize certain electromechanical components of the spacecraft payload to greatly reduce the volume and mass of various scientific instruments and sensors and improve the functional density of detectors. The Jet Propulsion Laboratory (JPL) of the United States said that these micro instruments will be the heart of the new micro laboratory, which mainly include: Mars lander, micro accelerometer, micro magnetometer, micro hygrometer, micro weather station, micro seismometer, micro integrated camera, micro imaging spectrometer and micro thruster, etc. It can be seen that the micro fluxgate is in its plan.

The traditional method of manufacturing fluxgate is to mechanically wind the excitation coil and the induction coil on the high permeability core to make a probe, and then connect it with the interface circuit. The fluxgate made by this method is difficult to miniaturize in many aspects such as volume, mass and power consumption. At present, the combination of MEMS technology and semiconductor integrated circuit technology is a breakthrough in the development of miniature fluxgate sensors.

Micro fluxgate The research started relatively late. In the 1990s, scholars in Japan, the United States and some Eastern European countries began to try to use MEMS technology to make micro fluxgates and their systems, and achieved a series of results. Due to the requirements of micromachining technology, micro solid-state sensors must be made on some solid substrate materials. Different substrate materials make the sensor production methods different. According to literature reports, the current manufacturing process of micro fluxgates is mainly divided into three types: First, the fluxgate probe is made by processing PCB boards. In 2000, O. Dezuari et al. in Switzerland published their technology of making micro fluxgate probes using three-layer PCB boards. The fluxgate has a sensitivity of 18V/T at an excitation frequency of 10kHz. At the same time, his compatriots Pavel Kejik et al. also made a fluxgate sensor with a PCB board probe, which has a sensitivity of 55V/T under a frequency excitation of 8.4kHz. The second is to make a fluxgate probe on a non-semiconductor (such as vanadium, glass, etc.) substrate. In 1994, I. Vincueria et al. used planar processing technology to make a micro fluxgate probe on a metal vanadium substrate. The coil is composed of three layers of Ti-Pd-Cu, of which Ti and Pd are made by low-pressure vapor deposition (LPVD) and Cu is made by electroplating. In addition, in 2000, PA Borerson published a micro fluxgate sensor made on glass. The third is to make fluxgate probes and fluxgate systems including interface circuits on semiconductor materials, especially silicon substrates. In 1990, T. Seitz of Switzerland first used microelectronic planar technology to make the world's first micro fluxgate sensor, which integrated the magnetic core and induction coil into one chip. Since then, scientists in Japan, Germany, the United States and Switzerland have also carried out research on micro fluxgates and reported their research results one after another. Since 1993, S. Kawahito et al. in Japan have successively developed single-core, dual-core, and ring-core fluxgate probes as well as micro fluxgate sensor systems integrating probes with interface circuits; R. Gottfried et al. in Germany developed a dual-core fluxgate system with three coils integrated with an interface circuit on a chip in 1996, and conducted a series of studies on the fluxgate of this structure; in 1999, TM Liakopouls et al. in the United States first produced a microstructured long ring-core fluxgate probe; in 2000, L. Chiesi et al. in Sweden published another dual-core integrated micro fluxgate system. There are many differences between the various fluxgate probes and fluxgate systems that people have studied and produced using semiconductor silicon materials, and each has its own characteristics. Compared with the method of making fluxgate sensors on PCB boards or other substrate materials, making fluxgate sensors on semiconductor substrates has incomparable advantages. First, the development of semiconductor microelectronics technology has reached the micron and nanometer levels, which has laid the foundation for further reducing the size of the fluxgate sensor; secondly, the use of semiconductor substrates can not only integrate the probe part of the fluxgate, but also make the fluxgate probe and its interface circuit on the same chip, which greatly reduces the size of the entire instrument. At the same time, it can also reduce the power consumption lost in wiring, discrete devices, etc., and the noise caused by them, further improving the sensitivity and resolution of the fluxgate.

The research directions of micro fluxgate sensors are as follows: ① Systematization, integrating the probe and interface circuit completely on one chip to make a real fluxgate MEMS system; ② Arrayization, making a series of fluxgate probes on one chip as needed can not only improve the performance of the sensor, but also complete certain specific functions, such as making a micro magnetic compass; ③ Using micromachining technology to improve the performance of the fluxgate sensor, especially the performance of the magnetic core; ④ Using computer simulation and emulation software to simulate and optimize the interface circuit of the fluxgate to improve the performance of the circuit; ⑤ Using a computer to simulate and calculate the structure of the micro fluxgate probe to shorten the design cycle, improve research efficiency, and further reduce costs; ⑥ Develop in the direction of practicality and commercialization, thereby promoting the development of related industries.