MEMS is a high-tech technology that is flourishing on the basis of integrated circuit production technology and dedicated micro-electromechanical processing methods. Its research and development mainly focuses on micro-sensors, micro-actuators and micro-systems. Currently, the sensors that dominate the MEMS market have been Form an industry. The various micro-sensors developed by this technology have the advantages of small volume, light weight, fast response, high sensitivity, easy production, and low cost, and can measure various physical quantities, chemical quantities and biomass. Under the multiple roles of market guidance, technology promotion, venture capital, and government intervention, automotive MEMS sensors have developed rapidly and have become a hot spot for investment and development by relevant departments. In high-end cars, about 25 to 40 MEMS sensors are used, and the technology is becoming mature and perfect, which can meet the requirements of harsh automotive environment, high reliability, accurate accuracy and low cost, which greatly promotes the application of electronic technology in automobiles.
Automotive electronic control systems have long been recognized as one of the main applications of MEMS pressure sensors, which can be used to measure intake manifold pressure, atmospheric pressure, oil pressure, tire pressure, etc. Table 1 shows some of the main uses.
The most popular automotive MEMS pressure sensor adopts the piezoresistive force sensitivity principle, which is the largest amount of the existing force sensitive sensors. It has developed several generations of products with an annual output of tens of millions. The sensor uses single crystal silicon as a material, and uses a MEMS technology to form a force sensitive diaphragm in the middle of the material, and then diffuses impurities on the diaphragm to form four strain resistors, and then connects the strain resistors into a circuit by a Wheatstone bridge. To obtain high sensitivity, the output is mostly 0~5V analog, the measurement range depends on the thickness of the force sensitive diaphragm. Many force sensitive chips can be fabricated on one wafer at the same time, which is easy to mass production, and the force sensitive chip is affected by temperature. Performance is compensated by conditioning circuitry.
The pressure sensor for automobiles is called the quality of military products and the price of civilian products. The environmental experiment requirements are extremely stringent (Table 2). After the packaged sensor passes the strict environmental test, it can generally guarantee the stability of 0.1%~0.3% FS, and can maintain the stability of 1% FS even if it is subjected to the most stringent long-term test. Signal conditioning circuits and calibration and packaging are also extremely critical technologies in production, and the technology content exceeds the production of force sensitive chips. For example, pressure sensors that measure fuel injection are in contact with liquid fuel for a long period of time and are subjected to high pressure, assembly and mounting dimensions, involving a large number of structural analysis, mechanical stress testing, dielectric exposure testing, and circuit correction techniques.
At present, the company can provide automotive MEMS pressure sensors, such as American Keller, special measurement, SSI, Philco, Texas Instruments, Bosch, Japan Denso, etc., with an annual output of more than 1 million, which is mass production of automobiles. The demand is also a condition for recycling huge investments. Some manufacturers have developed automotive MEMS pressure sensors with integrated force sensitive chips and peripheral signal processing circuits. It is estimated that the future price will drop to 5-7 US dollars. Bosch uses surface MEMS technology to develop miniature silicon mass flow sensors, which are being evaluated by many automotive manufacturers.
The automotive electronically controlled fuel injection system EFI uses multiple pressure MAP sensors to monitor the absolute pressure of the engine intake differential, improve its power performance, reduce fuel consumption, and reduce exhaust emissions. Micro-silicon piezoresistive MEMS pressure sensors can be used in engine exhaust gas circulation systems instead of ceramic capacitive pressure sensors. Pressure measurement in automotive air-conditioning compressors is also a large market for MEMS. In addition to the existing applications, another promising market is the tire pressure automatic monitoring system. The MEMS pressure sensor is suitable for any type of tire. A small pressure sensitive chip is embedded in the tire sidewall to automatically measure the tire pressure. , temperature, speed and other data, and sent out with a specific code. At present, there are many real-time tire pressure monitoring systems available, so that the tires always maintain good application performance, can improve the safety factor, shorten the braking distance by 5~10%, and can reduce fuel consumption by about 10%. Recently, with the recovery of the Firestone Controversy, the US Congress passed a bill requiring every car to be equipped with a tire pressure monitoring system by 2004. The government's intervention has opened up a broader market for MEMS sensors, many Manufacturers are competing for this.
The micro accelerometer usually consists of a parallel cantilever beam. One end of the beam is fixed on the side frame, and the other end is suspended by a small mass object block (about 10 mg). When there is no acceleration, the mass does not move, and when there is vertical acceleration, Mass motion, sensitive to acceleration, and converted to electrical signals, C/V transition, amplified phase sensitive demodulation output.
According to the detection method, the micro accelerometer has several types of piezoresistive, capacitive, tunnel, resonant, and thermal forms. Wherein, the capacitive micro-accelerometer mass moves downward when there is acceleration, and the distance from the other electrode on the frame changes, and the displacement of the mass movement can be obtained by detecting the change of the capacitance, and the main structure is divided into a cantilever swing type and Comb-shaped folded beam type and mutated into other types. The former structure is relatively simple, and the bulk silicon processing method is also used in the fabrication. The simple pendulum structure consists of upper and lower fixed electrodes and movable sensitive silicon cantilever beam electrodes, anisotropic etching by semiconductor planar process, electrostatic sealing The technology package is completed. The latter can be regarded as the parallel and string combination of cantilever beams, which is much more complicated in design. The micro-machining method is mainly based on surface sacrificial layer technology. The isotropic nature of polysilicon material can ensure the symmetry of micro-mechanical properties. The processing precision is high, and the size of the sensitive portion adopting this structure is made small, achieving monolithic integration with peripheral circuits. Capacitive micro accelerometer has high sensitivity, low noise, small drift and simple structure. It is widely used in automotive airbag systems and anti-skid systems. Its detection range and accuracy are 50g (gravity acceleration), 200°/s. 500mg, 10°/s, 100°/s, 1°/s, the detection range of the micro-accelerometer for airbag system detection collision is ±30~50g, the precision is 100mg, the detection side collision is about 250g or 500g, anti-slip stability The measurement range of the system is ±2g with an accuracy of 10mg.
The most important driving force for the commercialization of micro-accelerometers comes from the automotive industry. The most successful ones are ADXL05 and ADXL50 series monolithic integrated differential capacitance accelerometers from Analog Devices, Inc., with a monthly output of 2 million. Motorola, USA, mass-produces MMAS40G capacitive accelerometers for automobiles, and chooses dual-chip design and manufacturing technology. The package is a dual-in-line or single-in-line plastic package with an acceleration measurement range of ±40g. EG&G IC Sensors has established a MEMS processing line. It has successfully developed 3255 and 3000 series piezoresistive accelerometers. The 3255 is mainly used in automotive safety systems. The sensitive chips and signal self-care chips are packaged in a surface mount housing. German Bosch and Japan Denso have similar products. Micro accelerometers are replacing the electromechanical accelerometers of the past, and are growing rapidly with the increasing popularity of automotive airbag systems.
The micro-mechanical gyro is a vibrating angular rate sensor, which has attracted much attention in the application development of the automotive field. It is mainly used for GPS signal compensation and automobile chassis control systems for car navigation, and has great application potential.
There are two vibration modes in the micro-mechanical gyro, one is the transverse vibration mode, that is, the driving vibration mode, usually called reference vibration, which generates additional motion under the action of Coriolis force; the other is the normal vibration mode, ie the sensitive vibration mode. The detection of the additional motion reflecting the Coriolis force obtains the angular velocity information contained in the Coriolis force.
According to the materials used, the micro-mechanical gyro is divided into two types: quartz and silicon vibration beam. The quartz material structure has the highest quality factor Q value, the gyro characteristics are the best, and it has practical value. It is the earliest productized, and the American company Delco and BEI adopt MEMS. Technology, mass production of single-axis, three-axis solid-state quartz piezoelectric gyro, can be used in high-end cars, navigation, aircraft, aerospace and other markets. Bosch, Japan's Panasonic's angular rate sensor for cars is priced at $25.
Quartz processing is difficult and costly, and it cannot meet the low cost requirements of automobiles. The silicon material has complete structure and good elasticity, and it is easy to obtain a high-Q micro-mechanical structure. With the appearance of deep reactive ion etching technology, the processing precision of bulk silicon micro-machining technology is significantly improved, and it is made of polysilicon on a silicon substrate. Suitable for mass production, driving and testing are more convenient, and become the mainstream of low-cost research and development. From the structure of the silicon micromachined gyroscope, the vibrating beam structure, the double frame structure, the plane symmetrical structure, the lateral tuning fork structure, the comb tuning fork structure, the beam island structure, etc. are often used, and the driving method for generating the reference vibration is electrostatic driving and pressing. Electric drive and electromagnetic drive, etc., and detection of additional vibration due to Coriolis force detection methods include capacitance detection, piezoelectric detection, and piezoresistive detection. The gyro design of electrostatic drive and capacitance detection is the most common, and some products have been successfully developed.
The performance of the existing silicon micromachined gyro products is not high, and the precision is generally at the level of 0.1 ° / s, which can only meet the requirements of automotive applications, but to obtain a large number of applications, it is also necessary to solve the measurement circuit and package stability, reliability, and price. And so many questions.
The rise of global automotive electronics and automotive computer control systems has driven the development of automotive MEMS sensors. In all systems of the car, almost all of the MEMS can be found. The better the car, the more MEMS are used. There are more than 70 MEMS on the BMW 740i. The German Hella Group offers 250 automotive sensors in the European aftermarket. Many sensors can be replaced by MEMS. According to reports, the sales of automotive MEMS sensors in 2000 was 1.26 billion US dollars, and it is expected to grow to 2.35 billion US dollars in 2004. Some people predict that its market growth will be greater. In short, it will further occupy a larger share in future automotive sensors.
MEMS sensors are high-volume, high-reliability and low-cost, and are particularly suitable for use in automotive electronic control systems. Bosch is proud of its product line integrity, with more than 35 automotive MEMS sensors. Venture capitalists still favor MEMS and are regarded as emerging industries. In the first quarter of 2001, they invested 510 million US dollars in MEMS companies. Some new MEMS factories will soon be put into production, and other semiconductor manufacturers will join in. The US has established a MEMS industry organization. It will further promote its development.
The materials used in automotive MEMS sensors have evolved from monocrystalline silicon, polycrystalline silicon, and quartz crystals to other new materials. For example, giant magnetoresistance materials have been used to develop wheel speed sensors for braking anti-lock braking systems, in order to replace the magnetic Resistance sensor. The production process has developed non-silicon-based micro-EDM, micro-electroforming, laser processing, surface mounting and other technologies, and silicon-based processing technology has also developed.
In China, the basic research and development of MEMS sensors has achieved remarkable results, but it has not yet possessed the capability of mass production, which is quite different from the international industrialization. Domestically produced vehicles will reach a production capacity of 6 million vehicles in 2010, and there are more than 100 kinds of sensors required, and the development prospects are attractive. Starting from market demand, combining various forces to promote the industrialization of automotive MEMS sensors will be an important direction of MEMS, and also a market platform for its technology to transform productivity.
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