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Detailed explanation of MEMS materials and manufacturing processing technology

Microelectromechanical Systems (MEMS) is an industrial technology that integrates microelectronics and mechanical engineering to operate in the micron range. MEMS is known as micromachines in Japan and Micro Systems Technology (MST) in Europe.


MEMS devices are typically between 20 microns and one millimeter in size, and they usually contain a microprocessor and several microsensors inside to obtain information from the outside world. The processing technology of MEMS is adapted from semiconductor processing technology, which is generally used to manufacture electronic devices, to make them practical. MEMS are available in a variety of raw materials and manufacturing technologies, chosen depending on the application, market and other performance requirements.


I. Materials for MEMS




Silicon is the main raw material used to manufacture integrated circuits. Silicon is also a very common raw material for MEMS because there has been a lot of experience in the electronics industry with practical silicon for manufacturing very small structures. The material properties of silicon also have certain advantages. Single-crystal silicon obeys Hooke's law and has almost no elastic hysteresis, so it consumes little energy and has very reliable kinematic properties. In addition, silicon does not break easily, so it is very reliable and can be used for trillions of cycles.


In general, MEMS are produced by stacking layers of material on a substrate and then using lithography and etching methods to allow it to form various desired structures.


2、Polymer materials


Although the electronics industry's experience with silicon processing is extensive and valuable, and offers great economy, pure silicon is still very expensive. Polymer materials are very cheap and have a wide variety of properties. The use of injection molding, embossing, three-dimensional light-curing molding and other technologies can also use polymer materials to manufacture microelectromechanical systems, such systems are particularly beneficial for microfluidic applications, such as portable blood measuring devices.




Metals can also be used to manufacture microelectromechanical systems. Although metal lacks good mechanical properties compared to silicon, it is very reliable within the range of applicability of metal.


Second, MEMS processing technology


①、Traditional mechanical processing method


The traditional machining method refers to the use of large machines to make small machines, and then use small machines to make micro machines. It can be used to process some micro-mechanical devices used in special applications, such as micro-mechanics, micro-table, etc.


Traditional machining methods are represented by Japan, where the focus of MEMS research is on ultra-precision machining, so they are more about miniaturization of traditional machining.


This processing method can be divided into two categories: ultra-precision machining and special microfabrication. Ultra-precision machining uses metal as the processing object, and uses tools with higher hardness than the processing object to cut the object material, and the size of the resulting three-dimensional structure can be less than 0.01mm. This technology includes diamond tool micro cutting processing, micro drilling processing, micro milling processing and micro grinding and lapping processing.


Special microfabrication technology is a cutting process as small as molecule by molecule or atom by the direct action of processing energy. Special machining is the use of electrical, thermal, optical, acoustic and chemical energy forms of energy. Commonly used processing methods include: EDM, ultrasonic processing, electron beam processing, laser processing, ion beam processing and electrolytic processing. Ultra-precision machining and special microfabrication technology processing accuracy has reached micron, sub-micron level, you can batch production of micro-mechanical components such as gears with a modulus of only about 0.02, as well as other processing methods can not manufacture complex microstructure devices.


②, silicon-based MEMS technology


Silicon based MEMS technology, represented by the United States, is the use of chemical etching or IC process technology to process silicon materials to form silicon based MEMS devices. This method is compatible with the traditional IC process and suitable for inexpensive mass production, and has become the mainstream of the current silicon-based MEMS technology.


Current silicon-based micromachining technology can be divided into body micromachining technology and surface micromachining technology.


Body micromachining technology.


Body micromachining technology is a technology for processing silicon substrates. Generally, the use of anisotropic chemical etching, the use of single-crystal silicon corrosion rate of different crystalline direction of the existence of anisotropic characteristics of corrosion to produce different micro-mechanical structures or micro-mechanical parts, the main feature is the corrosion rate of silicon and silicon crystalline direction, adulteration concentration and the applied potential related.


Another common technique is electrochemical etching, which has been developed for electrochemical self-stopping corrosion, which is mainly used for silicon corrosion to prepare thin and uniform silicon film. This technique can be used to produce precise three-dimensional structures of MEMS.


The bulk micromachining technique is achieved by deep etching of silicon and monolithic bonding of silicon wafers, which can control several dimensions to the micron level. Because anisotropic chemical etching can be performed on large wafers, MEMS devices can be produced in batch with high precision, while eliminating the residual mechanical stresses caused by grinding processes, improving the stability and yield of MEMS devices.


Surface micromachining technology.


Surface micromachining is a technology that forms a thin film on the front side of a silicon wafer and processes the film to form a microstructure according to certain requirements, all the processing involves only the film on the front side of the wafer. It was developed in the 1980s by the University of California, Berkeley, and uses polysilicon as the structural layer and silicon dioxide as the sacrificial layer. Surface microfabrication technology is most similar to IC technology, and its main feature is to prepare micro-mechanical structures based on "thin film + precipitation", using lithography, etching and other common IC processes, and finally releasing structural units using selective etching technology to obtain movable two- or three-dimensional structures.


With this technology, silicon dioxide, silicon nitride and polysilicon films can be deposited; evaporation coating and sputtering coating can be used to prepare aluminum, tungsten, titanium, nickel and other metal films; thin film processing generally uses photolithography, such as ultraviolet lithography, X-ray lithography, electron beam lithography and ion beam lithography. The designed micromechanical structure is transferred to the silicon wafer through photolithography, and then plasma etching, reactive ion etching and other processes are used to etch polycrystalline silicon film, silicon oxide film and various metal films to form the micromechanical structure.


This technique avoids the problems of double-sided alignment and back-side etching required for bulk micromachining, and is compatible with the integrated circuit process, and the process is mature enough to generate hundreds of MEMS devices in bulk on a single monocrystalline silicon substrate with a diameter of several tens of millimeters.



Deep etching technology


Deep etching technology refers to deep reactive ions etching into the inside of the silicon chip, etching into a sacrificial layer inside the chip and being eroded away after the etching is completed, so that the structure originally buried inside the chip can move freely.


Deep etching technology is one of the micromachining methods LIGA, which is a comprehensive micromachining technology using simultaneous X-ray deep lithography, microelectroforming and plastic injection replication and other major process steps.


LIGA technology can be used to process various metals, plastics and ceramics to obtain fine structures with large aspect ratios and processing depths of several hundred microns.


LIGA technology has the following features compared to other stereoscopic micromachining technologies.


The ability to produce 3D stereoscopic microstructures with heights of hundreds to 1000 μm, depth-to-width ratios that can be greater than 200, and sidewall parallel deviations in the submicron range.


no restrictions on the lateral shape of the microstructure, and lateral dimensions can be as small as 0.5 μm, with an accuracy of up to 0.1 μm


Wide range of materials to be used, metals, alloys, ceramics, glass and polymers can be used as LIGA processing objects.


The clever combination with micro electroforming and casting plastic can realize the mass replication production with low cost.


The main process steps of LIGA are as follows: after X-ray mask plate making and X-ray deep lithography, micro electroforming is performed to create micro replication molds and use them for micro replication process and secondary micro electroforming, and then micro casting and plastic technology is used for mass production of micro devices.


Since the synchronous X-ray source required by LIGA is relatively expensive, the quasi-LIGA technology is generated on the basis of LIGA, which uses UV light source instead of synchronous X-ray source, and although it cannot achieve the process performance of LIGA processing, it can meet many requirements in microfabrication. The DEM technology, which is jointly developed by Shanghai Jiao Tong University and Peking University and has independent intellectual property rights, is also one of the LIGA technologies. This technology uses inductively coupled plasma deep etching process to replace synchrotron X-ray deep lithography, followed by conventional microelectro-casting and micro-replication processes. This technology has a wide application prospect because it does not require expensive synchrotron X-ray sources and special X-ray mask plates. Translated with www.DeepL.com/Translator (free version)



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