Application and development of MEMS sensors in the

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Application and development of automotive MEMS sensors

I. Introduction

mems is a high-tech that has flourished on the basis of integrated circuit production technology and special MEMS processing methods. Its research and development are mainly focused on three aspects: micro sensors, micro actuators and micro systems. At present, the sensors that dominate the MEMS market have formed 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. They can measure various physical quantities, chemical quantities and biomass. Under the multiple effects of market guidance, technology promotion, venture capital and government intervention, automotive MEMS sensors have developed rapidly, and have become a hot spot for relevant departments to invest and develop first. In high-end vehicles, about 25 to 40 MEMS sensors are used. The technology is becoming more and more mature, which can meet the requirements of harsh automotive environment, high reliability, accurate precision and low cost, and greatly promote the application of electronic technology in automobiles

II. Pressure sensor

automotive electronic control system has been considered as one of the main application fields of MEMS pressure sensor, which can be used to measure intake manifold pressure, atmospheric pressure, oil pressure, tire pressure, etc

the most popular automotive MEMS pressure sensor adopts the piezoresistive force sensing principle, which is the most widely used of several existing force sensing sensors. Several generations of products have been developed with an annual output of tens of millions. This sensor uses monocrystalline silicon as the material, and MEMS technology is used to make a force sensitive diaphragm in the middle of the material. Then impurities are diffused on the diaphragm to form four strain resistors, and then the strain resistors are connected into a circuit in the way of Wheatstone bridge to obtain high sensitivity. Most of its output is 0~5v analog quantity, and the measurement range depends on the thickness of the force sensitive diaphragm. Many force sensitive chips can be made on a chip at the same time, which is easy to mass produce, The performance of force sensitive chip affected by temperature is compensated by conditioning circuit

automotive pressure sensors are known as the quality of military products and the price of civilian products. The environmental test requirements are extremely strict. After the packaged sensor passes the strict environmental test, it can generally ensure the stability of 0.1%~0.3%f.s. and maintain the stability of 1%f.s. even after the most stringent long-term test. Signal conditioning circuit, calibration and packaging are also extremely key technologies in production, and the scientific and technological content exceeds the production of force sensitive chips. For example, the pressure sensor for measuring fuel injection is in long-term contact with liquid fuel and bears high pressure. The design of assembly, fixation and installation dimensions involves a large number of technologies, such as structural analysis, mechanical stress test, medium exposure test, and major road correction of the friction force of the driven needle on the electric inducer

at present, the companies that can provide automotive MEMS pressure sensors include Keller, special measurement, SSI, filco, Texas Instruments, Bosch and Denso, with an annual output of more than 1million, which is the demand for mass production of automobiles and the condition for recovering huge investment. Some manufacturers have successfully developed automotive MEMS pressure sensors with integrated force sensitive chips and peripheral signal processing circuits. It is estimated that the price will drop to $5 to $7 in the future. Bosch has developed a miniaturized silicon mass flow sensor using surface MEMS technology. Many automobile manufacturers are evaluating this new sensor

EFI of automotive electronic fuel injection system should use multiple pressure map sensors to monitor the absolute pressure of engine intake manifold, improve its power performance, reduce fuel consumption and reduce exhaust emissions. Micro silicon piezoresistive MEMS pressure sensor can be used in engine exhaust gas circulation system to replace ceramic capacitive pressure sensor. Pressure measurement in automotive air conditioning compressor is also a big market for MEMS. In addition to various existing applications, another promising market is the tire pressure automatic monitoring system. MEMS pressure sensor is suitable for any type of tire. A small pressure sensing chip is embedded in the tire wall to automatically measure tire pressure, temperature, speed and other data, and send them with specific codes. At present, many real-time tire pressure monitoring systems have been developed, which can keep good application performance of tires, improve the safety factor, shorten the braking distance by 5~10%, and reduce the fuel consumption by about 10%. Recently, with the recovery of the Firestone debate, the U.S. Congress passed a bill requiring every car to be equipped with a tire pressure monitoring system by 2004. The government's intervention has opened a broader market for MEMS sensors, and many manufacturers are competing for it

III. micro accelerometer

the micro accelerometer is usually composed of a parallel cantilever beam. One end of the beam is fixed on the frame, and a small mass object block (about 10mg) is suspended at the other end. When there is no acceleration, the mass block does not move. When there is acceleration in the vertical direction, the mass block moves, is sensitive to acceleration, and is converted into an electrical signal, which is converted to c/v conversion, amplified and demodulated by phase sensitivity

according to the detection method, micro accelerometers are piezoresistive, capacitive, tunnel, resonant, thermal, etc. Among them, the mass block of the capacitive micro accelerometer moves downward when there is acceleration, and the distance from the other electrode on the frame changes. The displacement of the mass block movement can be obtained by detecting the change of capacitance. The main structure is divided into cantilever pendulum and comb shaped folded beam, and variant into other types. The structure of the former is relatively simple, and the bulk silicon processing method is also used in manufacturing. The simple pendulum structure is composed of upper and lower fixed electrodes and movable sensitive silicon cantilever electrodes. The fabrication is completed by anisotropic corrosion of semiconductor plane process and electrostatic sealing technology. The latter can be regarded as the parallel and series combination of cantilever beams, which is much more complex in design. The micromachining method is mainly based on the surface sacrificial layer technology. The isotropic property of polysilicon material can ensure the symmetry of micromachining performance, and the batch processing precision is high. The sensitive part of this structure is made very small to realize the monolithic integration with peripheral circuits. Capacitive micro accelerometer has high sensitivity, low noise, small drift and simple structure. It has been widely used in automotive airbag system and anti-skid system. Its detection range and accuracy performance indicators are 50g (gravitational acceleration), 200 °/s, 500mg, 10 °/s, 100 °/s and 1 °/s respectively. The detection range of micro accelerometer for airbag system to detect collision is ± 30~50g, the accuracy is 100mg, and the detection of side collision is about 250g or 500g, The measurement range of the anti-skid stability system is ± 2G, and the accuracy is 10mg

the most important driving force for the commercialization of micro accelerometers comes from the automotive industry. The most successful are the ADXL05 and ADXL50 series monolithic integrated differential capacitive accelerometers of the American simulator company, with a monthly output of 2million. Motorola Corporation of the United States mass produces mmas40g capacitive accelerometers for automobiles. The dual chip design and manufacturing technology is selected, and the package is double in-line or single in-line plastic packaging. The acceleration measurement range is ± 40g. Eg&g IC sensor company of the United States has established a MEMS processing production line, and has successively developed 3255 and 3000 series piezoresistive accelerometers. The 3255 is mainly used in automotive safety systems, and the sensitive chip and signal self-care chip are packaged in a surface mounted shell. Bosch of Germany and Denso of Japan also have similar products. Micro accelerometers are replacing the previous electromechanical acceleration sensors, and are growing rapidly with the increasing popularity of automotive airbag systems

IV. micromachined gyroscope

micromachined gyroscope is a vibrating angular rate sensor. Its application and development in the automotive field has attracted much attention. It is mainly used for GPS signal compensation of automotive navigation and automotive chassis control system. It has great application potential

there are two vibration modes in the micromachined gyroscope. One is the lateral vibration mode, that is, the driving vibration mode, usually referred to as the reference vibration, which will produce additional motion under the action of Coriolis force; The other is the normal vibration mode, that is, the sensitive vibration mode, which detects the additional motion reflecting the Coriolis force and obtains the angular rate information contained in the Coriolis force

according to the materials used, micromechanical gyroscopes are divided into quartz and silicon vibrating beam. The quartz material structure has the highest quality factor Q, the best gyro characteristics and practical value. It is the earliest product. Delco and Bei companies in the United States use MEMS technology to mass produce single axis and three axis solid-state quartz piezoelectric gyroscopes, which can be used in high-end automobile, navigation, aircraft, aerospace and other markets. The price of each angular rate sensor used in automobiles by Bosch of Germany and Panasonic of Japan is US $25

quartz processing is difficult and costly, which cannot meet the low-cost requirements of automobiles. Silicon material has complete structure and good elasticity, and it is easy to obtain high-Q micromechanical structure. With the development of deep reaction ion etching technology, the development prospect of modified plastic industry is broad. The machining accuracy of bulk silicon micromachining technology is significantly improved. It is suitable for mass production with polysilicon on silicon substrate, and it is convenient to drive and detect, which has become the mainstream of low-cost research and development. From the structure of silicon micromachined gyroscope, vibrating beam structure, double frame structure, plane symmetrical structure, transverse tuning fork structure, comb tuning fork structure, beam island structure, etc. are often used. The driving methods used to generate reference vibration include electrostatic drive, piezoelectric drive and electromagnetic drive, while the detection methods to detect the additional vibration caused by Coriolis force include capacitance detection, piezoelectric detection and piezoresistive detection. Electrostatic drive and capacitance detection gyroscopes are the most common designs, and some products have been successfully developed

the performance of the existing silicon micromachined gyroscope products is not high, and the accuracy is generally at the level of 0.1 °/s, which can only meet the requirements of automotive applications. However, in order to obtain a large number of applications, it is necessary to solve many problems, such as the stability, reliability and price of the measuring circuit and packaging

v. industrialization of MEMS sensors

the rise of global automotive electronics and automotive computer control systems has promoted the development of automotive MEMS sensors. In almost all systems on the car, MEMS can be used. The better the car, the more MEMS will be used. There are more than 70 MEMS on the bmw740i. German Hella Group provides 250 kinds of automotive sensors in the European after-sales market, and many sensors can be replaced by MEMS. It is reported that the sales volume of automotive MEMS sensors in 2000 was US $1.26 billion, which is expected to increase to US $2.35 billion in 2004. It is predicted that its market growth will be greater. In short, it will further occupy a larger share in automotive sensors in the future

mems sensors are produced in large quantities with high precision, high reliability and low unit price. They are especially suitable for application in automotive electronic control systems. Bosch is proud of the integrity of its product series. There are more than 35 types of automotive MEMS sensors. Venture capitalists still favor MEMS, which is regarded as an emerging industry. In the first quarter of 2001, they invested US $510million in MEMS companies. Some new MEMS factories will soon be put into production, and other semiconductor manufacturers will join in one after another. The United States has established a MEMS industrial organization, which will further promote its development

the materials used for automotive MEMS sensors have developed from monocrystalline silicon, polycrystalline silicon and quartz crystal to other new materials. For example, the wheel speed sensor of anti lock braking system with large amount of giant magnetoresistance materials is developed to replace the variable magnetoresistance sensor currently used. In terms of production technology, non silicon based micro EDM, micro electroforming, laser processing, surface mounting and other technologies have been developed, and silicon-based processing technology has also developed in a variety of ways

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