MEMS and Microfabrication Technology
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A microsystem is a miniature device or system that includes a micro mechanism, a microsensor, a microactuator, and a signal processing and control circuit or even an interface, communication, and power supply. The common terminology of microsystems is: MEMS (Micro Electro Mechanical System) or MOEMS (Micro Opto-Mechanical Systems, USA), Micro-Machine (Micro-Machine, Japan), Micro Systems (Micro System, Europe). From the viewpoint of its size, it can be classified into 1 to 10 mm micromachines, 1 to 1 mm micromachined, and 1 to 1 micron nanomachines. Correspondingly, microfabrication technology can also be divided into micron, submicron and nanoscale microfabrication.

The MEMS system mainly includes three parts: a miniature sensor, an actuator, and a corresponding processing unit. As various information of the input signal in the natural world, it is first converted into an electrical signal by a sensor, and after signal processing (including the conversion between analog/digital signals), it acts on the external world through a microactuator. Sensors can convert energy to convert signals such as acceleration and heat into electrical signals that can be processed by the system. Actuators are based on signal processing, and the commands issued by the control circuit automate 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 the form of optical, electrical, magnetic, etc., and output signals for display, or work with other systems to form a more complete system.

1.Application of MEMS

Micro-machinery is undoubtedly a huge application potential in the fields of biomedical and precision instruments, especially in the aerospace airborne equipment where space is small, operation accuracy is high, and functions are highly integrated. It is considered as an emerging technology that can be widely applied in the 21st century. Microsystem applications can be divided into the following four areas:

(1) Micro-components Micro-gears, micro-motors, micro-turbo, micro-optics, micro-bearings, micro-springs, etc. are manufactured through micro-machining technology. They are the basic mechanical parts of the microsystem. The ever-increasing level of micro-machine design and machining can produce increasingly finer micro-components.

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

(3) Microactuator The microactuator requires that the desired action be performed under the drive of the power source. Microactuators commonly used include microvalves, micropumps, microswitches, and microresonators. Microsystem drive commonly used drive methods are: thermal drive, shape memory alloy drive, piezoelectric crystal drive and electrostatic force drive.

(4) Microdevices and systems are more commonly used in medical and surgical equipment such as artificial organs, in vivo administration and sampling micropumps. Micro-robots (see Figure 1). The mini-navigation system, micro-satellite, and micro-aircraft (see Figure 2) in the aerospace field, as well as micro-optical systems, micro-flow measurement control systems, micro-gas chromatography, bio-chips, bionic devices, etc.

2. Microfabrication and its key technologies

With the development of micro-electromechanical systems, micro-manufacturing technology as the key to the realization of MEMS technology has also begun to attract the attention of material science workers and industrial circles in the developed countries. To process precise microelectromechanical devices, microfabrication technology must be available.

At present, the following methods are commonly used:

(1) Photolithography This method first applies a photoresist (photoresist) on the host material, and then utilizes an energy beam with an extremely high limit resolution to perform photolithography through the mask. Exposure (or photolithography). After development, the same fine geometry as the mask pattern was obtained on the resist layer. Finally, using other methods, micro structures can be fabricated on the workpiece material. At present, the main exposure technologies used in lithography include: electron beam exposure technology, ion beam exposure technology, X-ray exposure technology and UV excimer exposure technology.

(2) Etching Techniques Etching is generally classified into isotropic etching and anisotropic etching. Isotropic etching can produce microstructures of any lateral geometry, typically a few microns in height, and is limited to the manufacture of planar structures. Opposite etching can produce a three-dimensional structure with a large depth ratio, which can reach several hundred micrometers in depth. 1 Chemical reverse etching. Chemical etching has a unique lateral under-etching feature that allows the material's etching rate to be fully utilized depending on the crystal orientation characteristics. Monocrystalline silicon has crystallographic planes with different crystal orientations, and there are significantly different etching rates between the crystallographic planes in the alkaline solution. A very effective etch stop layer is introduced through the controlled doping of silicon, preventing the etch from proceeding and enabling selective etching to fabricate the microstructure. 2 Ion beam etching. Ion beam etching is further divided into focused ion beam etching and reactive ion beam etching. Focused ion beam etching, when the ion density is on the order of A/cm2, produces a submicron diameter beam that can directly etch the workpiece surface and precisely control the beam's density and energy. It achieves the purpose of removing the surface atoms of the workpiece one by one by transmitting momentum to the atoms on the surface of the workpiece material. Therefore, it can achieve nano-scale manufacturing accuracy. Reactive ion beam etching is a physical and chemical reaction etching method. It directs a beam of reactive gas ions directly to the surface of the workpiece. After reaction, it forms a product that is volatile and easily processed by the kinetic energy of ions. At the same time, the ion beam sputtering of the reactive gas achieves the purpose of etching. It is a sub-micron micro-processing technology. 3 laser etching. YAG lasers and excimer lasers are commonly used for laser etching. Currently used are argon fluoride excimer laser and xenon fluoride excimer laser. The far-ultraviolet laser beam produced by the ArF excimer laser etch the polymer hard material such as plastic, not only can etch extremely fine lines, but also does not generate heat, and the material is not thermally diffused and burned around the focal point of the beam. Focus phenomenon. The far-ultraviolet radiation generated by this excimer laser has a wavelength of 193 nm, a repetition rate of 1 Hz or more, and a pulse width of 12 ns. One pulse can etch a few microns of trenches. With this laser pulse, the material can be peeled off layer by layer to etch the fine lines. The wavelength of near-ultraviolet light generated by the cesium fluoride excimer laser is 300 nm. The etching process is that after the silicon wafer placed in the chlorine gas is irradiated by the laser, the chlorine molecules are decomposed into chlorine atoms, and at the same time, the silicon wafer is irradiated by the laser. The electrons are attached to the chlorine atom to form a negatively charged chlorine ion and chemically react with the positively charged silicon atom to form a volatile gas of silicon tetrachloride. The silicon tetrachloride is removed through the reactor and provided. Fresh chlorine, so the silicon is eroded, do not need photoresist can get the desired graphics.

(3) LIGA Technology LIGA is an abbreviation generated by three words: German Lithographie (Lithography), Galvanoformung (Electroforming), and Abformung (Injection Molding). It is a rapid microfabrication technology. The geometry processed by LIGA technology is not limited by the material properties and the crystallographic orientation, and it is possible to manufacture micromachines made of various metal materials and plastics. Therefore, there has been a great leap forward in comparison with the processing technology of silicon materials. LIGA technology can produce three-dimensional structures with a large aspect ratio. The vertical dimension can reach several hundred microns and the minimum lateral dimension is 1 μm. Sub-micron size accuracy, but also has a high degree of verticality, parallelism and repeatability. The LIGA technology includes the following three processes: 1 Deep synchrotron X-ray lithography. An X-ray resist (photoresist) with a thickness of up to 0.5 mm fixed on a metal substrate is exposed through synchrotron radiation X-rays through a mask and then developed to form a preliminary template, which is a mask The uncovered portion of the resist layer has the same planar geometry as the mask pattern. 2 electroforming. Electroforming is the deposition of metal on the tire mold to form parts using electrodeposition. The tire mold is the cathode and the metal to be electroformed is the anode. In the LIGA technology, a metal substrate supporting a primary template (resist structure) is used as a cathode, and a microstructured metal material (Ni, Cu, Ag) to be formed is used as an anode. After electroforming, they are completely immersed in a stripping solvent and the primary template is etched away. The remaining metal structure is the desired microstructure. 3 injection molding. The electroformed metal microstructure is used as a secondary template, and the plastic material is injected into the cavity of the secondary template to form a micro-structure member, which is proposed from the metal mold. The formed structure can also be used as a template for electroforming, and LIGA technology can be used for mass production of three-dimensional microstructures.

(4) Sacrificial layer technology The sacrificial layer technology is also called separation layer technology. The sacrificial layer technology uses a chemical vapor deposition method to form micro-components on a silicon substrate, and a separation layer material is added to the voids around the components. Finally, the separation layer is removed by dissolution or etching to separate the micro-components from the substrate. It is also possible to make micromachines that are slightly connected to the substrate.

(5) Epitaxial growth Epitaxial growth is an important means of micromachining. Its characteristic is that the grown epitaxial layer can maintain the same crystal orientation as the substrate, so that various lateral and longitudinal doping distributions and corrosion can be performed on the epitaxial layer. Processing to make various structures.

(6) Special Microfabrication Technology 1 Micro EDM. There is no essential difference between the principle of micro EDM and ordinary EDM. The key to micro-EDM processing is the production of micro-axis (tool electrode), micro energy discharge power supply, micro-servo feed of tool electrodes, processing state detection, system control, and processing methods. Micro-EDM technology has been used to process micro-apertures with a diameter of 2.5 μm and micro-apertures of 5 μm, and a mini-automotive mold with a length of 0.5 mm, a width of 0.2 mm, and a depth of 0.2 mm can be manufactured and used to manufacture a micro-car. model. Micro gears with a diameter of 0.3 mm and a module of 0.1 mm can be produced. 2 micro electrolytic processing. Electrolytic processing is a manufacturing technology that uses the principle of metal anode electrochemical dissolution to remove materials. Material removal is performed in the form of ion dissolution. This micro removal technology makes electrolytic processing possible with microfabrication. Someone succeeded in controlling the processing gap to less than 10μm by reducing the processing voltage and electrolyte concentration. Using a microfeed and a metal microtube electrode, a 0.17 mm hole was machined on a 0.2 mm nickel plate. 3 micro-ultrasonic processing. With the wide application of hard and brittle materials such as crystalline silicon, optical glass, and engineering ceramics in micro-mechanics, high-precision three-dimensional micro-fabrication of hard and brittle materials has become an important research topic. Currently available methods for hard and brittle material processing include lithography, EDM, electrolytic machining, laser processing and ultrasonic machining. Using ultra-sonic machining technology, micro-holes with a minimum diameter of 5 μm were machined on the engineering ceramics using a work-piece vibration method. 4 Micro laser forming processing. Micro laser forming is different from the traditional special processing methods. The laser forming process does not remove the material, but instead forms the material through the method of material addition. According to different materials and mechanisms of processing, laser forming processing can be divided into a variety of types such as photocuring, selective laser sintering, and layered solid modeling.

(7) Molecular Assemblage Scanning tunneling microscopy (STM) and atomic force microscopy (AFM) have a resolution of 0.01 μm and are the world's most accurate surface morphology viewers. The tip of the stylus can be used to capture molecules or atoms, and can be assembled into a certain structure as required to perform molecular assembly to make micromachinery. A U.S. company manipulated helium atoms in 1991 and spelled out "IBM" on nickel plates (see Figure 3) and the United States map. The Institute of Modern Chemistry of the Chinese Academy of Sciences also spelled out the word "atom" (see Figure 4) and the map of China. Molecular assembly technology is a kind of nano-scale micro-machining technology. It is a method to construct micro-structures and micro-machines from the microscopic point of view of matter.

(8) Integrated Mechanism Manufacturing Technology Recently, a new trend has emerged in micro-machinery, namely the use of microfabrication technology for large-scale integrated circuits to incorporate various sophisticated micro-mechanisms such as micro-actuators and micro-machines. Sensors, microcontrollers, etc. are integrated on one silicon chip. It can change the traditional passive mechanism into an active mechanism, and can also be made into a complete electromechanical integrated micro-mechanical system. The size of the entire system can be reduced to several millimeters to several hundred micrometers.

3. Status and Prospect of Microsystem Research

China’s MEMS research began in 1989. During the national “Eighth Five-Year Plan” and “Ninth Five-Year Plan” periods, it received active support from the National Natural Science Foundation of China, the Ministry of Science and Technology, the Ministry of Education, the Chinese Academy of Sciences, and the General Armament Department. The investment is about 150 million yuan. After more than 10 years of development, China has already established a certain foundation and technical reserves in various micro-sensors, micro-actuators, and prototypes of several micro-systems, such as the Tsinghua University, Peking University, the Institute of Electronics of the Chinese Academy of Sciences and the Ministry of Information Industry. , Nankai University, Shanghai Institute of Metallurgy, Shanghai Jiaotong University, Fudan University, Shanghai University, Southeast University, Zhejiang University, China University of Science and Technology, Xiamen University, Harbin Institute of Technology, Changchun Institute of Light Industry, Dalian University of Technology, Shenyang Instrument and Meter Technology Research Institute, Chongqing University, Ministry of Information Industry 24 electronics, 44 and 26, Xi'an Jiaotong University, Aviation 618, Aerospace 771. Micro-inertial devices and inertial measurement combinations, mechanical micro-sensors and actuators, micro-fluidic devices and systems, biosensors and biochips, micro-robots and micro-operating systems, and bulk silicon microfabrication processes have achieved certain results. The existing technical conditions have initially formed a one-stop system for MEMS design, processing, packaging, and testing, which provides a better platform for the further development of MEMS technology in China. In short, MEMS-based micro-fabrication technology has been highly valued as a high-tech in the world, and combined with nano-technology, it will have a revolutionary impact on the future development of science and technology.

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