Abstract: Micro-opto-electro-mechanical systems (MOEMS) technology is an emerging technology with applications in optical communication, optical display, data storage, and optical sensing. Optical pressure sensors fabricated using this technology offer unparalleled advantages over traditional pressure sensors. This paper focuses on the structure, working principle, and fabrication process of MOEMS pressure sensors, and briefly introduces the structure and fabrication process of MOEMS pressure sensor arrays. Keywords: Micro-opto-electro-mechanical systems; MOEMS; optical pressure sensor; pressure sensor; array I. Introduction Micro-opto-electro-mechanical systems (MOEMS) are a product of the combination of micro-optics, microelectronics, and micromechanics, and also a synthesis of various technologies such as mechanics, electronics, optics, magnetism, chemistry, and sensing. As a photoelectric signal conversion system, MOEMS applications span multiple fields including optical communication, optical display, data storage, adaptive optics, and optical sensing. The new optical devices made using MOEMS technology have low insertion loss, extremely low crosstalk between optical paths, are insensitive to the wavelength and polarization of light, and are usually made of silicon as the main material, thus making the optical, mechanical and electrical properties of the devices excellent; moreover, due to the modular design, it is more convenient to expand applications. Compared with MEMS systems, the structure of MOEMS systems is not complicated, but the actual manufacturing complexity is higher and the implementation is more difficult. Because the system must leave a light channel, but its core chip must be sealed to prevent sensitive optical devices from being affected by external light. The key is to choose what material to use to make a light guide cover or skylight in the sealed package. At present, although there are many materials to choose from, most skylights use ceramics or metals to ensure good sealing performance. Compared with sensors made using traditional processes, sensors made using MOEMS technology have many advantages, such as being able to work in harsh environments, having a large transmission bandwidth, high sensitivity, and fast response speed; moreover, when optical demodulation technology is applied, mutual interrogation between multiple sensors can be realized [1]. Using wavelength division multiplexing (WDM) technology in a MOEMS sensor array simplifies signal collection and significantly improves efficiency compared to previous methods like electromodulation. II. Classification and Structure of MOEMS Pressure Sensors Optical pressure sensors are a type of MOEMS sensor, also known as fiber optic pressure sensors because they use single-mode or multimode optical fibers. Based on their measurement principles, optical pressure sensors have several main types: optical frequency pressure sensors, optical intensity pressure sensors, phase (interference) pressure sensors, and polarized light pressure sensors. Polarized light pressure sensors can be considered a type of optical intensity pressure sensor. Optical intensity pressure sensors are the simplest in structure, consisting of a light source, fiber bundle, silicon film, and signal receiver. The fiber bundle comprises a transmission fiber and a group of receiving fibers surrounding it. The light source is typically a light-emitting diode (LED). The emitted light beam travels through the transmission fiber to the silicon film, is reflected by the silicon film, and is then transmitted to the signal receiver by the receiving fiber group, ultimately converting it into an electrical signal output. While this type of sensor has relatively low accuracy, its simple structure makes it easy to manufacture. The phase-type pressure sensor is the focus of our introduction. It has two types: one is the built-in Fabry-Perot interferometer and the other is the built-in unbalanced Mach-Zehnder interferometer. The main structure of the Fabry-Perot interferometer (see Figure 1) consists of a transmission fiber, a glass substrate with a cylindrical cavity and a silicon film. The silicon film covers one side of the cavity, and the fiber passes through the glass plate to incident the beam perpendicularly onto the interface between the silicon film and the cavity. There is a semi-permeable membrane on each side of the cavity. This semi-permeable membrane can be a metal film or a dielectric film, thus forming a Fabry-Perot interferometer. When working, the laser beam emitted by the light source is sinusoidally modulated and then irradiates the silicon film. After the silicon film absorbs the light energy, it undergoes local deformation. The deformation period is consistent with the period of the modulated light. Therefore, when the modulation frequency of the light source is consistent with the natural frequency of the micro-deformation of the silicon film, the periodic deformation of the silicon film evolves into resonance [2]. The part of the incident light reflected at the end of the fiber and the part reflected on the surface of the silicon film will form interference. The modulation frequency of the measured interference light intensity is the resonant frequency of the silicon film. When the pressure being measured causes deformation of the thin silicon film, the distance between the two semi-permeable membranes changes. This change leads to a change in the resonant frequency of the entire cavity, establishing a correspondence between the resonant frequency and the measured pressure. The key parameters for this structure are the thickness of the silicon film, the depth of the cavity, and the diameter of the cavity. Correctly selecting these parameters ensures that the sensor can produce a linear response across different pressure ranges. This type of pressure sensor has two major advantages: first, high sensitivity; for example, when the deformation of the silicon film is only 0.25 µm, its reflection coefficient can vary from 0.5 to 0. Moreover, when combined with a high-intensity laser, this sensor can produce very high resolution. The second advantage is its small size, and the third advantage is its insensitivity to source power fluctuations. Of course, this sensor also has some disadvantages, the most obvious being its complex structure and high manufacturing requirements. The two semi-permeable membranes within the cavity must maintain an appropriate distance and must be extremely smooth, all of which pose significant challenges to the manufacturing process. Another type of phase-type optical pressure sensor is the pressure sensor based on an unbalanced Mach-Zehnder interferometer. It consists of a force-sensitive silicon film fabricated on the same silicon wafer and an unbalanced Mach-Zehnder interferometer. One arm (the sensing arm) of the M-Z interferometer is placed at the edge of the silicon film, while the other arm (the reference arm) is away from the silicon film. In operation, the light beam emitted from the light source is split into two beams of equal intensity by a waveguide. These beams propagate through two interferometer arms of unequal length, resulting in an optical path difference and phase difference. Ultimately, they interfere in another waveguide and are output. When pressure deforms the silicon film, the photoelastic effect changes the effective coefficient of the waveguide mode, thus altering and detecting the output light intensity of the interferometer. The pressure change can be determined by the detected light intensity. Finally, let's introduce the polarized light pressure sensor, which can also be classified as a light intensity pressure sensor. Its working principle is as follows: the incident light becomes linearly polarized after passing through a polarizer, allowing light rays in certain specific directions to pass through. Next, the beam passes through a photoelastic material and exits in a direction perpendicular to the pressure propagation. The exiting light passes through another polarizer, with its propagation direction the same as the first polarizer. When pressure deforms the photoelastic material, the polarization direction of the beam changes, causing a change in the intensity of the beam passing through the second polarizer. The pressure can be measured by detecting the change in emitted light intensity. This type of sensor is easily affected by temperature, so it is mostly used for static pressure measurement. III. Manufacturing process of MOEMS pressure sensor The manufacturing process of pressure sensor based on epper interferometer is relatively complicated [4]. First is the preparation of silicon film. The thickness of the film depends on the pressure range to be measured. Currently, ultra-thin silicon wafers are often used as silicon films because the thickness of ultra-thin silicon wafers has met the requirements for silicon film thickness. Compared with silicon films made by traditional deposition and sacrificial layer processes, it is thinner and of higher quality. It is reported that the thickness of ultra-thin silicon wafers made by a foreign company is only about 2 mm and the size is 4 in. Of course, such ultra-thin silicon wafers also bring many difficulties to the process. Second is the processing of glass. The material of the glass substrate should be heat-resistant glass. The cavity and the channel for embedding optical fiber are made by ultrasonic drilling. At the same time, it has also undergone micromachining processing. The next step is to embed optical fiber in the glass processed by micromachining. An epoxy resin adhesive is used to secure a metal ring around the outer edge of the optical fiber. The metal ring and the end of the optical fiber are polished to make them smooth and flat. The key to this step is the choice of adhesive; the adhesive's reflectivity must be lower than that of the optical fiber to ensure that light does not leak out. Finally, the glass plate with the cavity and the silicon film are bonded together using anodizing. The key fabrication process for pressure sensors based on the M-Z interferometer is to fabricate the interferometer structure and the force-sensitive thin film on the silicon wafer. The interferometer structure is first deposited on the silicon wafer as silicon nitride, then its basic shape is determined by photolithography, and finally obtained by etching. The force-sensitive thin film is first deposited on the silicon wafer as SiO2, then its basic shape is determined by photolithography, and finally obtained by anisotropic etching. From the above description, we can see that the fabrication process of optical pressure sensors is very complex. Therefore, improving the process flow and reducing the cost have become the primary research goals of scientists and engineers engaged in MOEMS worldwide. IV. MOEMS Pressure Sensor Array A sensor array is an organic combination of multiple sensors. It has high accuracy, good repeatability, and can perform distributed measurements. A typical optical pressure sensor array is fabricated using two SOI (silicon-on-insulator) wafers, each consisting of an ultrathin single-crystal silicon layer and an oxide buried layer. During fabrication, a SiO2 layer is first oxidized on the outside of one wafer. Then, photolithography is used to determine the positions of a series of cavities. Next, anisotropic etching is used to etch the cavities, removing excess SiO2 layer. The bottom of the cavity is now the surface of the buried layer. This wafer is then bonded to an untreated wafer, and the single-crystal silicon layer of the upper wafer becomes the force-sensitive film covering the cavities, thus forming the entire sensor array. Figure 2 shows the structure of a MOEMS sensor array. Individual sensors in the array exchange data through the input and output of a single-mode optical fiber. As shown in Figure 2, a silicon-based optical waveguide demodulation network is present on the silicon wafer, and individual sensors are connected through this network to complete information exchange and transmission. Fabricating the array requires very strict control over the film thickness and surface uniformity; therefore, the etching process becomes a crucial step affecting the film properties. Simultaneously, packaging is also a crucial step in array fabrication, requiring the sensor to withstand high temperatures and strong vibrations, and to be protected from minor environmental factors. Currently, scientists are developing a novel hybrid pressure sensor array, unique in that it integrates a Bragg fiber grating into a silicon chip, providing a highly useful platform for composite output technology. However, this device will be subject to many environmental constraints, such as temperature. Optical pressure sensor arrays are frequently used in thrust testing to obtain pressure profiles over a distance, where the number of sensors in the array is quite large. Therefore, multiplexing technology (WDM) will play a significant role. V. Conclusion MOEMS pressure sensors possess excellent static and dynamic characteristics, especially in thrust testing applications. Moreover, with the continuous development and deep integration of microelectronics, micromechanics, and microoptics technologies, MOEMS technology will undoubtedly achieve even greater success, and sensors fabricated using MOEMS technology will undoubtedly elevate sensor technology to a new level. References: [1]. Joseph T. Boyd, Samhita Dasgupta, Howard E. Jackson. MOEMS pressure sensor for Propulsion applications[C]. MOEMS and Miniaturized Systems. Proceedings of SPIE, Vol. 4178. [2]. 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