Characteristic evaluation of typical silicon-based

2022-10-12
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Evaluation of the characteristics of typical silicon-based force sensors

silicon-based force sensors are sensors made of sensitive elements based on silicon (single crystal or polycrystalline) materials and using MEMS technology, whose characteristic parameters change significantly due to external force or stress changes. The force here includes physical quantities such as gravity, tension, pressure, torque, pressure, etc. Force sensors include force sensors, pressure sensors, differential pressure sensors, liquid level sensors, etc

silicon based force sensor is one of the most widely used sensors in industry. Its technical level, product level, application level and industrialization level have received great attention and attention from the industry. It represents the development trend and direction of force sensitive sensors

Figure 1. Silicon capacitance pressure sensor product

silicon piezoresistive pressure sensor

silicon piezoresistive pressure sensor is a sensor made by using the principle of silicon piezoresistive, using the good mechanical and electrical properties of monocrystalline silicon, and injecting the resistance sensitive to pressure into the pressure sensing film through diffusion or ion implantation process to realize the integration of pressure sensing elements and conversion circuits. Its characteristic evaluation is as follows:

good linearity silicon piezoresistive pressure sensor natural granite building plate gb/t18601 ⑵ 001, there is a good linear relationship between input and output. Because silicon piezoresistive pressure sensor usually has four force sensitive resistors to form a full bridge Wheatstone bridge, although the four resistors are subject to lateral pressure, and they also have similar nonlinear characteristics, but when forming a full bridge, the nonlinear characteristics can offset each other, Therefore, nonlinearity can be made very small

the fabrication process is compatible. The fabrication process of the sensor is relatively simple, and the MEMS process is integrated with the IC process, so the cost is relatively low

through the structural design, the performance can be improved. Using the chip structural design, its sensitivity can be very high, and the measurement of micro and low pressure can be realized. 10kA silicon micro pressure sensor can be realized by using "double island structure", and 1ka silicon micro pressure sensor can be realized by using "beam membrane structure". Using the structural design of the silicon piezoresistive pressure sensor, the isolation diaphragm liquid filled packaging can be realized, and the reliability and stability of the silicon piezoresistive pressure sensor can be improved

the process is mature. The manufacturing process of silicon piezoresistive pressure sensor is mature. Manufacturers usually provide temperature compensation networks with constant current and constant voltage power supply

disadvantages in principle, silicon piezoresistive pressure sensor is a physical property sensor based on silicon material. Silicon material is greatly affected by ambient temperature, which will produce great zero temperature drift and sensitivity temperature drift, and has various forms, which is not conducive to improving the stability of devices. At the same time, silicon piezoresistive pressure sensor must be temperature compensated, otherwise it is difficult to be applied in industry; Establishing a complete set of temperature compensation technology not only increases the cost, but also increases human resources. In a sense, it greatly limits the wide application of silicon piezoresistive pressure sensors. Typical technical parameters of silicon piezoresistive pressure sensor are shown in Table 1

Table 1. Typical technical parameters of silicon piezoresistive pressure sensor

note: accuracy includes hysteresis, repeatability and nonlinearity (least square method)

no other explanation, all test values are relative to 25 ℃, 1mA constant current

zero temperature coefficient and sensitivity temperature coefficient are indicators after temperature compensation

capacitive pressure sensor is based on silicon material and adopts capacitance principle, that is, it uses the change of capacitance pole spacing to convert pressure into capacitance change, and is made by MEMS process. The characteristics of the silicon capacitance pressure sensor are evaluated as follows:

the silicon capacitance sensor is a structural sensor with good stability. In terms of detection principle, its stability is better than that of the physical sensor, which ensures the stability of this kind of sensor from the perspective of structural design; The structural process adopts the full hard sealing solid-state process, and the silicon glass metal impulse pipe adopts electrostatic sealing, which reduces the stress, hysteresis and deterioration caused by sealing with glue; The capacitor is insensitive to temperature, and the additional error of temperature is small, so it does not need to carry out complex temperature compensation like silicon piezoresistive devices. Good stability is one of the main reasons why silicon capacitance sensors are well liked by users

index the advanced capacitance sensor itself has the characteristics of low power, high impedance, small electrostatic attraction, small movable mass and small heating effect, and can carry out non-contact measurement. The comprehensive performance indexes of silicon capacitance sensor, such as nonlinearity, overload, static pressure and reliability, are better than silicon piezoresistive sensor, ceramic capacitance sensor and metal diaphragm capacitance sensor. It is equivalent to silicon resonant sensor, especially the nonlinear index pursued by users. Generally, the nonlinearity of silicon capacitance sensor is better than 0.05% FS, and the yield is greater than 60%

suitable for mass production, low-cost silicon capacitance sensor is made by MEMS process, and the chip size is 3mm × 3mm, a 4-inch silicon chip can make hundreds of components. The product has good processability and consistent performance, which is suitable for mass production and low-cost operation. Its preparation process is compatible with IC process, and the process equipment does not need to be as expensive and complex as the process equipment of silicon resonant sensor, nor does it need to be made in a single piece as the metal diaphragm capacitance sensor, which ensures that the silicon capacitance sensor has a high performance price ratio

the main problem is that its detection principle is to adopt the change of capacitance pole spacing, which is nonlinear in itself. In order to improve the nonlinearity, more complex core structures have been developed, such as contact type, variable area and variable pole spacing in series. Another problem is to solve the problem of weak capacitance signal detection

typical technical indicators of silicon capacitance pressure sensor are shown in Table 2. Table 2. Technical indicators of silicon capacitance pressure sensor

silicon resonant pressure sensor

silicon resonant pressure sensor is a sensor made by MEMS process, which takes monocrystalline silicon as the basic material and adopts the resonance principle, that is, the natural frequency of the resonant beam changes with the change of the applied axial force. The performance of silicon resonant sensor mainly depends on the quality factor and gauge factor of resonant beam. The mechanical quality factor reflects the energy loss of the resonant beam in the vibration process, while the gauge factor reflects the sensitivity of the resonant beam to pressure at a specific natural frequency point. Generally, the resonant beam is sealed in vacuum to improve the quality factor, while the gauge factor is improved through structural improvement. The following is the evaluation of the characteristics of the silicon resonant sensor:

the output signal is a digital quantity, which can be easily interfaced with the microcomputer without analog/digital conversion. This is because the silicon resonant sensor adopts MEMS technology on the surface of the monocrystalline silicon chip, and makes two H-shaped resonant beams with exactly the same shape and size at the center and edge, and is in the vacuum chamber, which is not in contact with the filling liquid, and ensures that it is not affected by air damping during vibration. The resonant beam converts the pressure and differential pressure signals into frequency signals respectively, so that the sensor directly outputs the frequency signal, simplifying the interface with the digital system

the influence of static pressure can be ignored. When static pressure (working pressure) is added, the deformation of two resonant beams with exactly the same shape, size and material is the same, and the frequency change is also the same, and the deviation caused by static pressure is automatically offset

high measurement accuracy and good stability. The measurement accuracy and stability of the silicon resonant pressure sensor mainly depend on the measurement principle, structural mechanical characteristics, preparation process, materials used and other factors. Because the silicon resonant pressure sensor is equipped with a characteristic correction memory in the capsule assembly, which stores the ambient temperature, input and output characteristics, static pressure and other data of the sensor that uses low-cost technology to improve the performance of plastic packaging materials, Each data point can be corrected and compensated, so as to obtain high accuracy and stability

with excellent one-way overvoltage characteristics, can Japan change the future of cars with its advantages in chemical technology? At present, the isolation diaphragm and capsule body adopt unique Corrugated processing technology. When the unidirectional pressure increases to a certain value, the isolation diaphragm is in full contact with the body to play a protective role

the existing problems are complex in structure, difficult to process and high in cost. From the perspective of performance price ratio, they are not optimal. When using silicon resonant sensor, in order to realize closed-loop self-excited oscillation, we should first solve the problem of weak signal detection of detection sensitive elements. Using the principle of "combined sensitivity" can better solve this problem, but the circuit is relatively complex

typical indicators of silicon resonant pressure sensor are shown in Table 3 (Reference). Table 3. Typical indicators of silicon resonant pressure sensor (Reference)

the above describes three typical silicon-based force sensors. The transmitters produced by these sensors are currently in mass production in China and are widely used in the field of automatic control. The characteristics of the transmitter are closely related to the characteristics of the sensor used. (end)

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