The hottest MEMS and micro machining technology

2022-08-18
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Micro electromechanical system and micro machining technology

micro system is a micro device or system integrating micro mechanism, micro sensor, micro actuator, signal processing and control circuit, and even interface, communication and power supply. The customary terms of micro system are: MEMS (micro electro mechanical system) or MOEMS (micro opto electromechanical system, United States), micro machine (micro machine, Japan), micro systems (micro system, Europe). From the perspective of its size, it can be divided into 1 ~ 10mm micro machinery, 1 μ Micromachine of M ~ 1mm, 1nm ~ 1 μ M nano machinery. Accordingly, micro machining technology can also be divided into micron level, submicron level and nano level micro machining

The MEMS system mainly includes three parts: micro sensor, actuator and corresponding processing unit. As input signals, all kinds of information in nature are first converted into electrical signals through sensors, and then processed (including the transformation between analog and digital signals) through micro actuators to act on the external world. The sensor can realize the conversion of energy, so that the acceleration and thermal signals can be converted into electrical signals that the system can process. The actuator is based on the signal processing and the instructions sent by the control circuit to automatically complete various functions that people need. The signal processing part can perform signal conversion, amplification, calculation and other processing according to the control circuit. This system can also communicate with the outside world in optical, electrical, magnetic and other forms, and output signals for display, or work together with other systems to form a more complete system

ms application field

micromachine undoubtedly has great application potential in biomedicine, precision instruments, especially Aerospace airborne equipment with small space, high operating accuracy and highly integrated functions. It is considered to be an emerging technology that can be widely used in the 21st century. The application of micro system can be divided into the following four aspects:

(1) micro components the three-dimensional micro components processed by micro machining technology include: Micro gears, micro motors, micro turbines, micro optical devices, micro bearings, micro springs, etc. They are the basic mechanical components of the micro system. With the continuous improvement of the design and processing level of micro machinery, more and more sophisticated micro experimental components can be produced. The data collection, process control and post-processing of experimental data are all completed by computers

(2) micro sensor micro sensor is the most widely used MEMS device. Sensor is a device that transforms energy from one form to another, and provides users with a usable energy output for a specific measurable input. The main types of sensors are: acoustic sensors, biomedical sensors and biosensors, chemical sensors, optical sensors, pressure sensors, thermal sensors, etc

(3) microactuator microactuator requires that it can complete the required actions under the drive of power source. Commonly used microactuators include microvalves, micropumps, microswitches, microresonators, etc. The commonly used driving methods of micro system drive include thermal drive, shape memory alloy drive, piezoelectric crystal drive and electrostatic drive

(4) micro devices and systems are widely used in medical and surgical equipment, such as artificial organs, in vivo drug application and sampling micro pumps. Micro robot (see Figure 1). Micro navigation systems, micro satellites, micro airplanes (see Figure 2), micro optical systems, micro flow measurement and control systems, micro gas chromatographs, biochips, bionic devices, etc. in the aerospace field

2. Micromachining and its key technology

with the development of MEMS, micro manufacturing technology, as the key to the realization of MEMS technology, has also begun to attract great attention from material scientists and industry circles in the world's developed countries. In order to process precise micro electrical components, we must have the corresponding micro machining technology

at present, the following methods are commonly used:

(1) photolithography. This method first coats photoresist (photoresist) on the substrate material, and then uses the energy beam with extremely high limit resolution to expose the photoresist layer through the mask (or lithography). After development, a very fine geometric pattern, which is the same as the mask pattern, is obtained on the resist layer. Finally, by using other methods, micro structures can be fabricated on the workpiece material. At present, the main exposure technologies used in lithography are: electron beam exposure technology, ion beam exposure technology, X-ray exposure technology and UV excimer exposure technology

(2) etching technology etching is usually divided into isotropic etching and anisotropic etching. Isotropic etching can produce micro structures with arbitrary transverse geometry, which are generally several microns high, and are limited to planar structures. Anisotropic etching can produce three-dimensional spatial structures with large depth ratio, and its depth can reach hundreds of microns. ① Chemical anisotropic etching. Chemical etching has a unique characteristic of transverse underetching, which can give full play to the characteristic that the etching speed of materials depends on crystal orientation. Monocrystalline silicon has crystal planes with different crystallization directions, and there are significantly different etching rates between the crystal planes in alkaline solution. A very effective etch stop layer is introduced through silicon controllable doping method to prevent etching and realize selective etching to manufacture microstructures. ② Ion beam etching. Ion beam etching is divided into focused ion beam etching and reactive ion beam etching. When the ion density is in the order of a/cm, focused ion beam etching can produce a beam with a diameter of submicron, which can directly etch the surface of the workpiece, and can accurately control the density and energy of the beam. It transfers momentum to the atoms on the surface of the workpiece material through the human emitted ions to achieve the purpose of eroding the atoms on the surface of the workpiece one by one, so it can achieve nano manufacturing accuracy. Reactive ion beam etching is a physical and chemical reaction etching method. It directs an ion beam of reactive gas directly to the surface of the workpiece, and forms a volatile product that is easy to be processed by ion kinetic energy after reaction. At the same time, the purpose of etching is achieved by reactive gas ion beam sputtering. Therefore, we should pay attention to the selection of submicron micromachining technology when choosing. ③ Laser etching. YAG laser and excimer laser are usually used in laser etching. At present, argon fluoride excimer laser and xenon fluoride excimer laser are commonly used. The far ultraviolet laser beam produced by the argon fluoride excimer laser can etch polymer hard materials such as plastics, which can not only etch extremely fine lines, but also produce no heat. There is no thermal diffusion and scorching around the material where the beam focus acts. The far ultraviolet light produced by this excimer laser has a wavelength of 193nm, a repetition rate of 1Hz or more, and a pulse width of 12ns. A few microns of grooves can be etched with one pulse. Using this laser pulse, the material can be peeled off layer by layer and etched into fine lines. The wavelength of near ultraviolet light produced by xenon fluoride excimer laser is 300nm. Its etching process is that after the silicon wafer placed in chlorine is irradiated by the laser, the chlorine molecule is decomposed into chlorine atoms. At the same time, the electrons on the silicon wafer irradiated by the laser attach to the chlorine atoms to form negatively charged chlorine ions, and then react with positively charged silicon atoms to form a volatile gas of silicon tetrachloride. Silicon tetrachloride is removed through the reactor, Fresh chlorine is provided, so the silicon wafer is corroded, and the required graphics can be obtained without photosensitive glue

(3) LIGA technology Liga is an abbreviation generated by German words lithographie (lithography), galvanoformung (electroforming) and abformung (injection molding). It is a rapid micro manufacturing technology. The geometric structure processed by LIGA technology is not limited by material characteristics and crystallization direction, and it can manufacture micromachines made of various metal materials and plastics. Therefore, compared with the processing technology of silicon materials, it has made a great leap. LIGA technology can produce three-dimensional structures with large aspect ratio. The longitudinal dimension can reach hundreds of microns, and the minimum transverse dimension is 1 μ m。 The dimensional accuracy reaches sub micron level, and has high perpendicularity, parallelism and repetition accuracy. LIGA technology includes the following three processes: ① deep synchrotron radiation X-ray lithography. The X-ray resist (photoresist) with a thickness of up to 0.5mm fixed on the metal substrate is exposed by synchrotron radiation X-ray through the mask, and then developed into a primary template, which is the resist layer of the unexposed part covered by the mask, with the same plane geometry as the mask. ② Electroforming. Electroforming is to deposit metal on the mold by electrodeposition to form parts. The mold is the cathode and the metal to be electroformed is the anode. In LIGA technology, the metal substrate supporting the primary template (resist structure) is used as the cathode, and the microstructure metal material (Ni, Cu, Ag) to be formed is used as the anode. After electroforming, they are completely immersed in the stripping solvent, and the primary template is corroded and stripped, and the remaining metal structure is the required micro junction components. ③ Injection molding. The metal microstructure made by electroforming is used as the secondary template, and the plastic material is injected into the mold cavity of the secondary template to form the microstructure, which is raised from the metal mold. The formed structural parts can also be used as templates for electroforming, and LIGA technology is used for mass production of three-dimensional microstructure parts

(4) sacrificial layer technology sacrificial layer technology is also called separation layer technology. Sacrificial layer technology is to form micro components on silicon substrates by chemical vapor deposition, add separation layer materials to the gaps around the components, and finally remove the separation layer by dissolution or etching to separate the micro components from the substrate. This technology can also manufacture micro machines slightly connected with the substrate

(5) epitaxial technology epitaxial growth is an important means of micromachining. Its characteristic is that the grown epitaxial layer can maintain the same crystal direction as the substrate, so various horizontal and vertical doping distribution and corrosion processing can be carried out on the epitaxial layer to produce various structures

(6) special micro machining technology ① micro EDM. There is no essential difference between the principle of micro EDM and ordinary EDM. The key to realize micro EDM lies in the fabrication of micro axis (tool electrode), micro energy discharge power supply, micro servo feed of tool electrode, machining state detection, system control and machining process method. By using micro EDM technology, the diameter of 2.5 can be machined at present μ Micro axis of M and 5 μ The micro hole of M can be used to make the micro car mold with length of 0.5mm, width of 0.2mm and depth of 0.2mm, and the micro car model is made with it. Micro gears with a diameter of 0.3mm and a modulus of 0.1mm can be made. ② Micro electrochemical machining. Electrochemical machining is a manufacturing technology that uses the principle of electrochemical dissolution of metal anodes to remove materials. Material removal is carried out in the form of ion dissolution. This micro removal technology makes electrochemical machining possible. Someone successfully controlled the machining gap to 10 by reducing the machining voltage and electrolyte concentration μ Below M. Using micro feed and metal microtubule electrode, a small hole of 0.17mm was machined on a nickel plate of 0.2mm. ③ Micro ultrasonic machining. With the wide application of hard and brittle materials such as crystalline silicon, optical glass and engineering ceramics in MEMS, the high-precision three-dimensional micromachining technology of hard and brittle materials has become an important research topic. At present, the main methods that can be used for machining hard and brittle materials are lithography, EDM and electrolysis

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