Research status and development trend of pressure sensors

Sensor technology is one of the important technologies of modern measurement and automation systems. From space development to seabed exploration, from production process control to modern civilized life, almost every technology is inseparable from sensors. Therefore, many countries attach great importance to the development of sensor technology. For example, Japan lists sensor technology as one of the six core technologies (computers, communications, lasers, semiconductors, superconductors and sensors). Among all kinds of sensors, pressure sensors have the advantages of small size, light weight, high sensitivity, stability and reliability, low cost, and easy integration. They can be widely used in the measurement and control of pressure, height, acceleration, liquid flow, flow rate, liquid level, and pressure. In addition, it is also widely used in water conservancy, geology, meteorology, chemical industry, medical and health care, etc. Since this technology combines plane technology with three-dimensional processing and is easy to integrate, it can be used to make sphygmomanometers, anemometers, water speed meters, pressure gauges, electronic scales, and automatic alarm devices. Pressure sensors have become the most mature, most stable, and most cost-effective type of sensors among all kinds of sensors. Therefore, technicians engaged in modern measurement and automatic control must understand and be familiar with the research status and development trends of pressure sensors at home and abroad. 1 Development History of Pressure Sensors Modern pressure sensors are marked by the invention of semiconductor sensors, and the development of semiconductor sensors can be divided into four stages [1]: (1) Invention stage (1945-1960): This stage is mainly marked by the invention of bipolar transistors in 1947. Since then, this property of semiconductor materials has been widely used. In 1945, CS Smith discovered the piezoresistive effect of silicon and germanium [2], that is, when an external force acts on a semiconductor material, its resistance will change significantly. The pressure sensor made based on this principle is to stick a strain gauge on a metal film, that is, to convert the force signal into an electrical signal for measurement. The minimum size of this stage is about 1 cm. (2) Technology development stage (1960-1970): With the development of silicon diffusion technology, technicians directly diffused strain resistors on the (001) or (110) crystal plane of silicon by selecting appropriate crystal orientations, and then processed the back side into a concave shape to form a thinner silicon elastic membrane, called a silicon cup [3]. This type of silicon cup sensor has the advantages of small size, light weight, high sensitivity, good stability, low cost, and easy integration. It realizes the metal-silicon eutectic and provides the possibility for commercial development. (3) Commercial integrated processing stage (1970-1980): Based on the silicon cup diffusion theory, the anisotropic etching technology of silicon was applied. The processing technology of the diffused silicon sensor is mainly based on the anisotropic etching technology of silicon, and has developed into an anisotropic silicon processing technology that can automatically control the thickness of the silicon film [4]. The main methods include V-groove method, concentrated boron automatic stop method, anodization automatic stop method and microcomputer control automatic stop method. Since etching can be performed on multiple surfaces simultaneously, thousands of silicon pressure membranes can be produced simultaneously, realizing an integrated factory processing mode and further reducing costs. (4) Micromachining stage (1980-present): The emergence of nanotechnology at the end of the last century made micromachining technology possible. Through micromachining technology, structural pressure sensors can be processed by computer control, and their linear dimensions can be controlled within the micron range. This technology can be used to process and etch micron-level grooves, strips, and membranes, making pressure sensors enter the micron stage. 2 Current research status of pressure sensors at home and abroad From a global perspective, the development trends of pressure sensors mainly focus on the following directions. 2.1 Fiber optic pressure sensor [5] This is a type of sensor with many research results, but not many of them have been put into practical use. Its working principle is to use the characteristics of the deformation of the sensitive element under pressure and the intensity of the reflected light. The diaphragm structure composed of a silicon frame and a gold-chromium film is sandwiched with a silicon fiber baffle. When there is pressure, the intensity of the light will change when it passes through the baffle. By detecting this tiny change, we can measure the pressure. This sensitive element has been applied in clinical medicine to measure the pressure inside the balloon of a coronary artery catheter. It is foreseeable that this pressure sensor will have a good development prospect in microsurgery. At the same time, in the fields of processing and health care, optical fiber sensors are also developing rapidly. 2.2 Capacitive vacuum pressure sensor [6] E + H's capacitive pressure sensor is composed of a substrate and alumina (Al2O3) with a thickness of 0.8 to 2.8 mm, which are brazed together with a self-melting welding ring. The ring has an isolation effect and does not require temperature compensation, which can maintain long-term measurement reliability and lasting accuracy. The measurement method adopts the capacitance principle. A capacitor CP on the substrate is located in the center of the diaphragm with the largest displacement, while another reference capacitor CR is located at the edge of the diaphragm. Since it is difficult for the edge to produce displacement, the capacitance value does not change. The change of CP is related to the change of the applied pressure. The relationship between the displacement of the diaphragm and the pressure is linear. When overloaded, the diaphragm will not be damaged when attached to the substrate. When there is no load, it will immediately return to its original position without any hysteresis. The overload can reach 100%, and even if it is damaged, it will not leak any contaminated medium. Therefore, it has a wide range of application prospects. 2.3 High temperature resistant pressure sensor The emergence of new semiconductor material silicon carbide (SiC) makes it possible to make single crystal high temperature sensors. Rober. S. Okojie reported an α (6H) SiC pressure sensor with a running test of 500 ℃. The experimental results show that under the conditions of an input voltage of 5V and a measured pressure of 6.9MPa, the full-scale output at 23500 ℃ is 44.66~20.03mV, the full-scale linearity is 20.17%, and the hysteresis is 0.17%. After running at 500 °C for 10 hours, the performance remains basically unchanged. The strain temperature coefficient (TCGF) at 100 °C and 500 °C is 20.19%/°C and -0.11%/°C respectively. The main advantages of this sensor are that the PN junction leakage current is very small, there is no thermal matching problem, and there is no plastic deformation when the temperature rises, so it can be processed in batches. Ziermann and Rene reported a pressure sensor made of single crystal n-type β-SiC material. This pressure sensor can work at a temperature of up to 573K and is radiation resistant. At room temperature, the sensitivity of this pressure sensor is 20.2muV/VKPa. 2.4 Silicon micromachining sensor Today, as micromachining technology is gradually improved, silicon micromechanical sensors are increasingly used in the automotive industry. As the size of micromechanical sensors becomes smaller and smaller, the linear dimension can reach 1-2mm, and they can be placed in important organs of the human body for data collection. Hachol, Andrzej;dziuban, Jan Bochenek reported a tonometer that can be used to measure the eyeball, with a diaphragm diameter of 1mm. When the intraocular pressure is 60mmHg, the static output is 40mV, and the sensitivity coefficient is relatively high. 2.5 Pressure sensor with self-test function In order to reduce the debugging and operation costs, Dirk De Bruyker et al. reported a piezoresistive and capacitive dual-element sensor with self-test function. Its self-test function is based on the thermal drive principle. The size of the sensor is 1.2mm ×3mm ×0.5mm, which is suitable for the biomedical field [7]. 2.6 Multi-dimensional force sensor









































The research and application of six-dimensional force sensors is a hot topic in the research of multi-dimensional force sensors. Currently, only a few countries such as the United States and Japan can produce them. In China, Beijing Institute of Technology has pioneered the development of soft optical array tactile sensors combined with piezoelectric layers based on the development of foreign countries. The array density is 2438 tactels/cm2, the force sensitivity is 1g, the structure is very flexible, and it can grasp and identify eggs and steel balls. It is now used in robots to sort items [8]. 3 Development trend of pressure sensors The research field of pressure sensors in various countries in the world today is very broad and has almost penetrated into all walks of life. However, there are mainly the following trends: (1) Miniaturization The market demand for small pressure sensors is increasing. Such small sensors can work in extremely harsh environments and require little maintenance. They have little impact on the surrounding environment and can be placed in various important organs of the human body to collect data without affecting people's normal life. For example, the sensor produced by Entran Company in the United States has a range of 2 to 500 PSI and a diameter of only 1.27 mm. It can be placed in the blood vessels of the human body without causing a significant impact on the blood circulation. (2) Integrated pressure sensors have been increasingly integrated with other measuring sensors to form a measurement and control system. Integrated systems can improve operating speed and efficiency in process control and factory automation. (3) Intelligence Due to the emergence of integration, some microprocessors can be added to the integrated circuit, so that the sensor has functions such as automatic compensation, communication, self-diagnosis, and logical judgment. (4) Another development trend of widespread pressure sensors is that they are expanding from the mechanical industry to other fields, such as automotive components, medical instruments, and energy and environmental control systems. (5) Standardization The design and manufacture of sensors have formed certain industry standards. Such as the ISO international quality system; the ANSI and ASTM standards of the United States, the ГOCT of Russia, and the JIS standards of Japan. 4 Conclusion With the progress of silicon, micromachining technology, ultra-large integrated circuit technology, and material preparation and characteristic research, the mass production of pressure sensors in optical fiber sensors, the application of high-temperature silicon piezoresistive and piezoelectric junction sensors have become possible. Pressure sensors have broad application prospects in the fields of biomedicine, micro-machinery, etc. References: [1] SM Sze. Semiconductor sensor, 1994 chapter IvppIV. [2] CS Smith, piezoresistive effect in germanium and silicon phys. [3] Feng Jingxing. New technology for electrostatic sealing and silicon cup corrosion [J]. Journal of Fuzhou University, 1994. [4] Desnical UV. SanticB. Trap-induced photoconductivity in semi-insulating gallium arsenide. 1989. [5] Bai Shaohong. The development of optical fiber pressure sensor [J]. Industrial Instrumentation and Automation, 1990. [6] Yuelin Wang et al, The structures for electrostatic servro capacitive vacuum sensor, sensors and actuators A 1998. [7] Dirk De Bruyker et al, A combined piezoresitive pressure sensor and function based on thermal actuation sensors A , 1998. [8] Zhang Weixin, et al. Semiconductor Sensors [M]. Tianjin: Tianjin University Press, 1990. Author introduction: Zhang Xin, male, born in 1981, a 2002 master's student at Shandong Agricultural University, research direction: intelligent water-saving agricultural control.