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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) Invention stage (1945-1960): This phase is mainly marked by the invention of the bipolar transistor in 1947. Since then, this characteristic of semiconductor materials has been more widely used. Smith (CSSmith) and 1945 discovered the piezoresistive effect of silicon and germanium, ie, when an external force acts on a semiconductor material, its resistance will change significantly. The pressure sensor made according to this principle is to attach a strain gauge to a metal thin film, that is, convert the force signal into an electrical signal for measurement. The minimum size at this stage is approximately 1 cm.
(2) The stage of technological development (1960-1970): With the development of silicon diffusion technology, technicians choose the appropriate crystal orientation on the silicon (001) or (110) crystal surface to directly spread the strain resistance on the crystal surface. Then it is processed into a concave shape on the back side to form a thin silicone elastic film called a silicon cup. This type of silicon sensor has the advantages of small size, light weight, high sensitivity, good stability, low cost, and ease of integration. It has realized a metal-silicon eutectic, which offers possibilities for commercial development.
(3) Commercialization and integrated processing stage (1970-1980): The anisotropic etching technology of silicon was applied on the basis of silicon diffusion theory. The diffusion silicon sensor's processing technology was based on the anisotropic etching technology of silicon. The development of silicon anisotropic processing technology [4] can automatically control the thickness of the silicon film, mainly V-groove method, boron automatic suspension method, anodizing automatic termination method and computer control automatic suspension method. Due to the possibility of simultaneous corrosion on multiple surfaces, thousands of silicon pressure membranes can be produced at the same time, achieving an integrated factory processing model and further reducing costs.
(4) Micromachining (1980 - present): Nanotechnology emerged at the end of the last century, making micromachining possible.
Through the micro-machining process, a structural pressure sensor can be machined and controlled by a computer, and its linearity can be controlled in the micrometer range. With this technology, micron-scale trenches, strips, and membranes can be processed and etched, allowing the pressure sensor to enter the micron stage.
2. Research status of pressure sensors at home and abroad The development trends of pressure sensors in the world are mainly in the following directions.
2.1 Optical Fiber Pressure Sensor This is a kind of sensor with many research results, but it is not too much to put into practice. Its working principle is to use the characteristics of deformation and reflected light intensity when the sensitive element is subjected to pressure. A silicon fiber baffle is sandwiched between the diaphragm structure composed of a silicon frame and a gold-chromium film, under pressure, The intensity of the light changes as it passes through the baffle. By measuring this slight change, we can measure the pressure. This type of sensitive element has been used in clinical medicine to measure the pressure inside a coronary artery catheter balloon. It is foreseeable that this kind of pressure sensor will have a good development prospect in microsurgery. At the same time, fiber optic sensors are also rapidly developing in processing and health care.
2.2 Capacitive Vacuum Pressure Sensor E+H's capacitive pressure sensor is composed of a substrate and aluminum oxide (Al2O3) with a thickness of 0.8-2.8mm, brazed together by a self-welding welding ring. The ring has an isolation effect and does not require temperature compensation to maintain long-term measurement reliability and long-term accuracy. The measuring method adopts the principle of capacitance. A capacitor CP on the substrate is located in the center of the diaphragm with the largest displacement, and another reference capacitor CR is located on the edge of the diaphragm. Since the edge is difficult to be displaced, the capacitance value does not change, and the change of CP is related to With regard to the pressure changes applied, the relationship between the displacement of the diaphragm and the pressure is linear. In the event of an overload, the diaphragm is not damaged on the substrate. When it is unloaded, it will immediately return to its original position without any hysteresis, and the overload can reach 100%. 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 Pressure Sensors The emergence of a new semiconductor material, silicon carbide (SiC), has made it possible to fabricate single crystal high temperature sensors. Rober.S.Okojie reported an α(6H)SiC pressure sensor with a running test up to 500°C. The experimental results show that the full-scale output at 23500°C under an input voltage of 5V and a measured pressure of 6.9 MPa. It is 44.66 to 20.03mV, full scale is 20.17%, and hysteresis is 0.17%. After running at 500°C for 10 h, the performance is basically unchanged. The strain temperature coefficients (TCGF) at two points of 100°C and 500°C are 20.19%/°C and -0.11%/°C, respectively. The main advantage of this type of sensor is that the leakage current of the PN junction is small, there is no thermal matching problem, and the temperature rise does not produce plastic deformation and can be processed in batches. Ziermann and Rene reported a pressure sensor made of single-crystal n-type β-SiC material. The pressure sensor has an operating temperature of 573K and is resistant to radiation. At room temperature, the sensitivity of this pressure sensor is 20.2 muV/VKPa.
2.4 Silicon Micromachined Sensors Micromachining technology is gradually perfecting today. Silicon micromachined sensors are increasingly used in the automotive industry. As the size of micro-mechanical sensors is getting smaller and smaller, the linearity can reach 1 to 2 mm and can be placed in the vital 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 1 mm. When the intraocular pressure is 60mmHg, the static output is 40mV and the sensitivity coefficient is relatively high.
2.5 Pressure sensors with self-test function In order to reduce the debugging and operating costs, DirkDeBruyker et al. reported a piezoresistive and capacitive dual-element sensor with a self-test function. Its self-test function was based on the principle of thermal driving. The size is 1.2mm × 3mm × 0.5mm, suitable for biomedical field [7].
2.6 The research and application of multi-dimensional force sensor six-dimensional force sensor is the focus of multi-dimensional force sensor research. Now only a few countries such as the United States and Japan can produce it. Based on the tracking of foreign development, Beijing Institute of Technology in China has also pioneered the development of a flexible optical array with a piezoelectric layer. The array density is 2438 tactels/cm2, and the force sensitivity is 1g. The flexibility of the structure is very good. Recognition of eggs and steel balls is now used for robot sorting items [8].
3. Development Trend of Pressure Sensors The pressure sensors in various countries in the world have a wide range of research fields and have penetrated into almost every walk of life. However, there are mainly the following trends:
(1) Miniaturization The demand for small pressure sensors is increasing in the current market. Such small sensors can work in extremely harsh environments and require little maintenance and maintenance, and have little impact on the surrounding environment. Can be placed in the body's vital organs to collect information, does not affect people's normal life. For example, a sensor with a range of 2 to 500 PSI produced by the Entran company in the United States has a diameter of only 1.27 mm and can be placed in the blood vessels of the human body without greatly affecting the circulation of blood.
(2) Integrated pressure sensors have been increasingly integrated with other measurement sensors to form measurement and control systems. Integrated systems can increase operating speed and efficiency in process control and factory automation.
(3) Intelligence Due to the emergence of integration, some microprocessors may be added to the integrated circuit, so that the sensor has functions such as automatic compensation, communication, self-diagnosis, and logic judgment.
(4) Another development trend of extensive pressure sensors is the expansion from the mechanical industry to other fields, such as automotive components, medical instruments, and energy environment control systems.
(5) The design and manufacture of standardized sensors have formed a certain industry standard. Such as the ISO international quality system: the United States, ANSI, ASTM standards, Russia's ГOCT, Japan's JIS standards.
4. Concluding remarks With the development of silicon, micro-machining technology, super-integrated circuit technology, and material preparation and characteristics, pressure sensors have become possible for mass production of fiber optic sensors, high-temperature silicon piezoresistive and piezoelectric junction sensors. In the field of biomedicine, micro-mechanics and other fields, pressure sensors have a wide range of applications.
Research Status and Development Trend of Pressure Sensors
Introduction Sensor technology is one of the most important technologies for modern measurement and automation systems. From the development of the universe to exploration of the seabed, from the control of production processes to modern civilized life, almost every technology is inseparable from sensors. Therefore, many countries have The development attaches great importance to, for example, Japan has listed sensor technology as one of the six core technologies (computers, communications, lasers, semiconductors, superconductors, and sensors). In various types of sensors, pressure sensors have the advantages of small size, light weight, high sensitivity, stability, reliability, low cost, and easy integration. They can be widely used in pressure, height, acceleration, liquid flow, flow rate, liquid level, and pressure. Measurement and control. In addition, it is also widely used in water conservancy, geology, meteorology, chemical industry, medical care and health. Because this technology is a combination of planar processing and three-dimensional processing, and it is also easy to integrate, it can be used to make sphygmomanometers, anemometers, water velocity meters, pressure gauges, electronic scales, and automatic alarm devices. Pressure sensors have become the most mature type of sensor with the most stable performance and the most cost-effective type of sensor. Therefore, technical personnel engaged in modern measurement and automatic control must be familiar with and familiar with the research status and development trend of pressure sensors at home and abroad.