MEMS (Micro-Electro-Mechanical Systems) utilizes a manufacturing process combining microelectronics and microfabrication technologies (including silicon bulk micromachining, silicon surface micromachining, LIGA, and wafer bonding) to produce a variety of high-performance, low-cost, miniaturized sensors, actuators, drivers, and microsystems. MEMS is a novel, multidisciplinary technology that has emerged in recent years and is poised to revolutionize future human life. It involves multiple disciplines such as mechanics, electronics, chemistry, physics, optics, biology, and materials science. MEMS pressure sensors are a type of pressure sensor manufactured using MEMS technology. MEMS pressure sensors measure pressure values by combining micromechanical structures and electronic components, converting them into electrical signals for output.
MEMS pressure sensors are manufactured using microelectromechanical systems technology and typically consist of micromechanical structures and electronic components. The micromechanical structures usually include thin films, springs, and diaphragms, while the electronic components include capacitors, resistors, and inductors. When external pressure is applied to the diaphragm of a MEMS pressure sensor, the diaphragm undergoes minute deformation. This deformation causes deformation of the springs or thin films within the micromechanical structures. This deformation leads to minute changes in the capacitance, resistance, and other electronic components within the micromechanical structures. These changes can be measured and amplified by circuitry, converting them into an electrical signal output. By measuring the magnitude of the output electrical signal, the magnitude of the external pressure can be determined.
MEMS pressure sensors are characterized by miniaturization, high precision, high sensitivity, and low power consumption, and are widely used in industries such as manufacturing, medicine, automotive, and aerospace. For example, MEMS pressure sensors can be used to measure the pressure of liquids or gases, as well as physiological signals such as blood pressure and respiration. Due to their high performance and low cost, MEMS pressure sensors have become one of the most important technologies in the field of pressure sensors.
MEMS sensors come in many varieties depending on their function, operating principle, and operating environment. Today, we will mainly introduce one type: MEMS pressure sensors. Current MEMS pressure sensors include silicon piezoresistive pressure sensors and silicon capacitive pressure sensors, both of which are microelectromechanical sensors fabricated on silicon wafers.
(1) The MEMS silicon piezoresistive pressure sensor uses a circular stress cup with a fixed inner wall. Four high-precision semiconductor strain gauges are directly etched onto the surface at the point of maximum stress using MEMS technology, forming a Wheatstone bridge as a force-to-electricity conversion circuit. This circuit directly converts the physical quantity of pressure into an electrical quantity, achieving a measurement accuracy of 0.01%–0.03%FS. The structure of the silicon piezoresistive pressure sensor is shown in Figure 3. The upper and lower layers are glass, with a silicon wafer in the middle. A stress cup is formed in the center of the silicon wafer, and a vacuum cavity is located above the stress silicon film, making it a typical absolute pressure sensor. The side of the stress silicon film in contact with the vacuum cavity is photolithographically etched to create a strain gauge bridge circuit. When external pressure enters the sensor's stress cup through the pressure chamber, the stress silicon film bulges slightly upwards due to the external force, undergoing elastic deformation. This causes a change in the resistance of the four strain gauges, disrupting the original balance of the Wheatstone bridge circuit. The bridge output is a voltage signal proportional to the pressure.
(2) The capacitive pressure sensor uses MEMS technology to manufacture a horizontal grid on a silicon wafer. The upper and lower horizontal grids form a set of capacitive pressure sensors. The upper horizontal grid is displaced downward under pressure, which changes the distance between the upper and lower horizontal grids, and thus changes the capacitance between the plates, i.e., Δpressure = Δcapacitance.
The fabrication process of MEMS sensors requires MEMS technology, based on the well-established microelectronics, integrated circuit, and related fabrication techniques. It shares many similarities with traditional IC processes, such as photolithography, thin film deposition, doping, etching, and chemical mechanical polishing. However, some complex microstructures are difficult to achieve using IC processes and must be manufactured using microfabrication techniques. Microfabrication techniques include bulk silicon microfabrication, surface microfabrication, and special microfabrication techniques. Bulk microfabrication refers to the etching process along the thickness direction of the silicon substrate, including wet etching and dry etching, and is an important method for realizing three-dimensional structures. Surface microfabrication uses thin film deposition, photolithography, and etching processes to deposit structural layers on a sacrificial layer and then remove the sacrificial layer to release the structural layer, thus achieving movable structures. Ceramics are recognized as highly elastic, corrosion-resistant, wear-resistant, and resistant to impact and vibration. Ceramic substrates have stronger, lower-resistivity metal films. The thermal stability of ceramics and their thick-film resistance allow them to operate in a temperature range of -40 to 135 °C, while also offering high measurement accuracy and stability.
MEMS pressure sensors are widely used in automotive electronics, such as TPMS, engine oil pressure sensors, automotive brake system air pressure sensors, automotive engine intake manifold pressure sensors (TMAP), and diesel engine common rail pressure sensors; consumer electronics, such as tire pressure gauges, blood pressure monitors, kitchen scales, health scales, pressure sensors for washing machines, dishwashers, refrigerators, microwave ovens, ovens, and vacuum cleaners, air conditioning pressure sensors, and liquid level control pressure sensors for washing machines, water dispensers, dishwashers, and solar water heaters; and industrial electronics, such as digital pressure gauges, digital flow meters, and industrial batching weighing devices. With the gradual arrival of the Internet of Things era, sensors are now being widely used in many emerging fields, including various smart terminals, smart cars, and biomedicine, and the demand is increasing daily.
