外文翻译--微机电系统的未来（MEMS） 英文版

ELSEVIER Sensors and Actuators A 56 (1996) 135-141 Future of microelectromechanical systems (MEMS) Minhang Bao a, Weiyuan Wang b Fudan University, Shanghai. 200433 China b Shanghai Institute of Metallurgy. Shanghai. 200050 China Abstract The development of microelectromechanical systems (MEMS) based on micromachining and microelectronics technologies has been significant for almost a decade. However, it is unrealistic to consider micromachining technology as a micro version of conventional machining technology. As a matter of fact, micromachining technology stemmed from the planar technology of silicon and is basically a two.dimensional processing technology. On the other hand, it is obvious that a micromachine cannot compare with a conventional machine in strength and power. For the successful development of MEMS in the future, a simple rule is suggested by the experience gained in the past few years: try to avoid as much as possible mechanical coupling with the outside world while trying hard to improve the MEMS technology to enhance the mechanical power of the devices. In addition to that, the strategy proven to be correct for the development of solid-state sensors also applies: MEMS devices should mainly be developed for new applications with a vast market. Their substitution for traditional applications should not be considered as a main strategy of development. Based on these arguments, the future development of MEMS devices and technologies is further discussed in the paper. Keyword: Microciectromcchanical systems 1. The development of MEMS On the front cover of the earlier issues of this journal (Sensors and Actuators) there was a subtitle just under the main title which read: international journal devoted to the research and development of solid-state transducers 1. These plain words mean to us that the founder of the journal, Professor Simon Middelhoek, foresaw the emergence of solid-state actuators, and, therefore, micivelecisome, chaaical systems (i.e., MEMS), from the very beginning of the last decade. MEMS are integrated systems consisting of microelectron- ics (IC), microactuators and, in most cases, microsensors. Microelectronics technology has been developing rapidly since 1960 and has been quite mature since the 1970s. Micro- machining technology, the technology for mechanical sen- sors and microactuators, has been developing almost in parallel with microelectronics technology, though the former has lagged far behind in sophistication. At an earlier stage, the development of micromachining was focused on bulk micromachining mainly associated with solid-state pressure transducers. People at the time did not expect too much from the integration of micromechanical structures and microelectronics because bulk-micromachin- 0924-4247/96/$15.00 1996 Elsevier Science $.A. All rights reserved Pii S0924-424 7 ( 96 ) 012/4-5 ing technology as well as solid-state pressure transducers were relatively difficult to integrate with microelectronics. In 1987, the first movable micromechanical parts were fabricated by surface-micromachining technology 2 and a typical microactuator, the electrostatic micromotor, was suc- cessfully operating in the next year 3. Due to the high compatibility between the surface micromachining and microelectronics technologies, the integration between micromachines and microelectronics led to the birth of MEMS in the following years. As there has been no clear definition of MEMS, in this article we consider a typical MEMS device as: (1) A device that consists of a micromachine and micro- electronics, where the micromachines are controlled by microelectronics. Quite often, microsensors are involved in the control system by providing signals to the micro- electronics. (2) A device that is fabricated using micromachining technology and an IC process, i.e. technologies of batch fabrication. (3) A device that is integratedly born, without individual assembly steps for the main parts of the device except for the steps required for packaging. These points mean that a pressure transducer is not consid- ered as a typical MEMS device but as a mechanical sensor as 136 M. Bao. W. Wang/Sensors and Acluators A 56 (1996) 135-141 there is no microclectronics control over micromechanical structures. On the other hand, a micromotor is not a typical MEMS device but a typical part ofMEMS D a microactuator. As MEMS are an integration of micromachining technol- ogy and microelectronics (IC) technology, they emerged soon after the advent of the micromachine in 1987. The devel- opment of MEMS for almost one decade has been significant: various new techniques have been developed, numerous new devices have been designed and fabricated with some of them being commercialized, and research on MEMS has been con- ducted by almost all major universities and research institu- tions, enjoying wide support from industries and government agencies. The field has been described as growing into a credible, dynamic and popular adolescence from an uncertain child in less than a decade 4. But every silver lining has a cloud, and this case was no exception. Some problems developed during the rapid devel- opment too. The rapid appearance of new micromachines stirred up high expectations from the scientific communities and even public news media. Some tended to believe that the most sophisticated electromechanical system, like a robot, can be copied into a micro version and the micromachine can still do a similar job to its macro counterparts. There have aoo been outrageous predictions, like microrobots gathering up radioactive particles at toxic sites, and microsubmarines stalking and attacking viruses and cancerous cells while swimming through the bloodsueam. The real development can hardly match such a development pattern from the news media. Though micromotors have been designed and fabri- cated one by one, they run into the common problem of small torque and relatively large friction on the micron scale. People are usually excited simply by the functioning of a micromo- tor, not being able to expect too much in terms ,:f torque/ power output. It seems impractical to put them into use like a conventional machine. So far no micromachine can replace a conventional machine in any practical application. This situation has given rise to a sceptical cloud over the future of MEMS. However, application is always the final driving force for any emerging technology. Starting at the beginning of this decade, the practicai application of a microactuator has been talked about more and more often by the transducer com- munity and more and more efforts have been made to put microactuators into real application. There have been two approaches so far to push forward the application of microactuators. One of the approaches is to make laicroactuators more powerful and stronger. For exam- ple, the force produced by a bimetal structure is large enough for many applications, such as to open and close microvalves. This scheme has been quite successful so that the electrocon- trolled microvalve has been commercialized. Shape memory alloy (SMA) can also produce a large force and has been considered for similar purposes. For micromotors, electro- magnetic motors have been designed and fabricated. The torque can be several orders of magnitude higher than that of an electrostatic micromotor, but there are still quite a lot of problems related to the design and processing. Another approach is to look for some new applications where little force output is required. To do this, direct mechanical coupling between the micromachine and the macro world should be avoided. The interface between the MEMS and the outside world will be through electrical, opti- cal and magnetic signals. This approach has resulted in quite a lot MEMS devices with practical applications； some of them has been commercialized. This successful experience tell us that the future of MEMS is bright if the nature of micromachining technology is respected. 2. The nature of micromachining technology MEMS are the offspring of two modern technologies, the microelectronics and the micromachining technologies. From a technological point of view, there are some affinities between these two technologies. It is well known that micro- electronics (IC) technology stemmed from the planar tech- nology of silicon. As a matter of fact, the application of planar processes of silicon to the formation of mechanical structures gave birth to micromachining technology in the 19/0s. The successful application of planar technology in solid-state pressure transducers promoted the development of micro- machining at the early stage. Basically, there are two main categories of micromachin- ing techniques: bulk micromachining and surface microma- chining. The techniques are called bulk micromachining when the bulk material of the substrate (in general silicon) is involved in the process and as surface micromachining if only the deposited (or plated) films on the surface of the substrate are involved in the machining process. Both types of micromachining technologies have the same virtues as microelectronics technology, i.e., high precision and batch fabrication, but they have the same limitations stemming from planar processing technology. First, the structures made by micromechanical technology can be three dinensional in appearance, but they are two dimensional in essence as they are evolved according to cer- tain rules from planar etching masks. The structures can be made more complicated by repeating the film deposition and masked etching more than once, but the flexibility is still limited by the number of repititions and the processing order starting from the surface of the substrate. Therefore, it is unrealistic to consider micromachining technology as a micro version of conventional machining technology as it has the limitation of the planar process: a basically two-dimensional processing technology not suitable for assembly steps to con- struct a machine from indiv；.dually processed parts. We cannot expect micromachining technology to be as flexible and versatile as conventional machining in the conventional world. Some selective deposition and etching techniques claimed to have real three-dimensional capability are under M. Bao, W. WangSensors and Actuators A 56 (1996) 135-141 137 development 5,6, but it is still too early to fores.e their possible application in practice. Therefore, one simple rule that has to be Ix)me in mind is that all mechanical structures made by conventional mechan- ical technology cannot he copied in micromechanical ver- sions, and large arrays with simple structure are more suitable for micromachining technology than a single machine with complicated structure. On the other hand, it is obvious that a micromachine can hardly compare with a conventional machine in strength and power. The smaller the structure, the smaller the strength and the power output it can provide. In many cases, micromachi- nes even have difficulties in just running in a conventional environment due to the extra-large friction on the micro scale and the interference of dust, humidity, etc., not to mention on the power output to drive a macromachine. For successful development of MEMS, one more simple rule is suggested: try to avoid as much as possible mechanical power output while trying hard to improve the MEMS tech- nology to enhance the mechanical strength and power of the devices. It is important to respect the nature of a new technology so that its potential can be fully explored. As a matter of fact, there have been many successful experiences in the short history of MEMS by making full use of the advantages and avoiding the disadvantages of micromachining technology. 3. The future of MEMS devices As mentioned above, for the future development of MEMS technology, two-fold efforts should be made: one is to improve micromachining technologies continuously and the other is to develop appropriate devices for practical appli- tion according to the nature of the MEMS technologies. The latter is an urgent task at present, lherefore, the first thing we have to address is: what are the appropriate MEMS devices.? As micromachining technologies have the advantage of high-precision low-cost batch production but the limitations of two-dimensional masking, low strength, low power output and high susceptibility to the interference of many environ- ment factors, such as dust, humidity, etc., the future MEMS devices should be mainly packaged independent subsystems consisting of micromachines microelectrouics and, in many cases, microsensors. The coupling between the MEMS devices and the outside world would mainly be via electrical, optical, magnetic and other non-contact signals for power supplies, conxol information, input and output signals. A large array of relatively simple mechanical structures is pref- erable to complicated mechanical structures. Also, the future MEMS devices should be aimed at new applications with a vast market so that the device can be mass produced to explore fully the advantage of a planar process in low-cost mass production. Here the argument made by Professor Simon Middelhoek 7 for silicon smart sensors applies to MEMS too: substitution in an old application does not have the potential to create a large enough market. There- fore, it cannot be considered as a main strategy for future MEMS development. As a matter of fact, the above-mentionod approaches have been proven successful in developing MEMS devices in the past and will be adapted for future devel)pment. A variety of MEMS devices meeting the above-mentioned criteria will be developed in the future, and arc described below. 3. !. MF.MS devices for inertial sensing Silicon accelerometers have been developing rapidly dur- ing the last decade and are considered as the next mass- produced micromechanical sensor after silicon pressm sensors. The most attractive type of microaccelerometer, the fort.e- balanced accelemmeter, is in fact a MEMS device consisting of a beam-mass mechanical structure, a capacitive sensor for the position of the mass, the signal-processing electronics for the sensor and an electrostatic actuator to apply a feedback force to the seismic mass. A variety of force-balanced microaccelerometers have been developed by now, but so far the most successful one is the fully integrated microacceler- ometer, ADXLSO, which was released for production about two yeats ago 8. The mechanical structure of the devices is fabricated by means of polysilicon surface micromachining, and the elec- tronics are fabricated by means of BiCMOS IC technology. Aiming at applications for airbag release control, the opera- tion range of the device is 50g with a single 5 V power supply. The entire microsystem is fabricated on a silicon chip meas- uring 3 mm 3 mm and housed in a TO- I00 can. Though the process is considered quite sophisticated and difficult 9, the developer claimed that they can be marketed at a cost under US$15 apiece. Furthermore, an improved version with an operation range as low as 5g or lg has been announced 10. Similar devices based on SIMOX SOI material 11 and thick epi-polysilicon have also been developed 12. Force-balanced microaccelerometers can be considered as one of the most successful MEMS at present. One reason for their success is that the sensing of acceleration by the seismic mass is through non-contact inertial force and the output is an electrical signal so that the whole system can be hermeti- cally sealed in a package to ensure that the performance of the micromechanical structures would not be hindered by any environmental interferences. The second reason is that accel- erometers can find mass application in a variety of motion- control systems. A notable example is for mass applications in airbag control in automobiles. This kind of application spurred significant interest and investment from industry. Another inertial sensing device, the gyroscope, has a sim- ilar operation pattern to the accelerometer and can also find wide applications in motion control, including automotive applications such as traction control systems and ride-stabi- lization systems, consumer electronics applications such as video camera stabilization and model aircraft stabilization, 138 M. Bao. W. WangSensors and Actuators A 56 (1996) 135-141 computer applications such as an inertial mouse, robotics applications and, of course, military applications. Therefore, the micromechanical gyroscope has been receiving vigorous development efforts in recent years. As the high-speed rotation parts and bearings in a tradi- tional gyroscope are difficult to miniaturize and batch fabri- cate by micromechanical technologies to produce low-cost devices, micromechanical gyroscopes are exclusively of vibrating types, including double-gimbals structure 13, cantilever beam structure 141, tuning-fork structure 15 and vibrating ring structure 16. Among them the vibrating ring device is the most sophisticated one, which is developed by a LIGA-iike post-circuit process for incorporating high- aspect-ratio electroformed metal microstructures with a CMOS circuit for control and readout electronics. Though none of these micromechanical gyroscopes has been commercialized yet, it is quite likely that some form of MEMS gyroscope will be mass produced in the near future. 3.2. MEM$ devices for optical signal processing and display Like an ultraviolet erasable ROM, it is possible to have a MEMs device hermetically sealed in a package but still with the ability to communicate with the outside world via light as well as electrical signals. Therefore, MEMS devices for optical signal processing or display are promising. One of the most sensational MEMS devices so far is the digital micromirror device (DMD) announced at Transduc- ers 93 by Texas Instrument 17. The DMD is a two-level structure on a silicon chip. The ground level is for electronics, an SRAM of 768 576 bit with peripherals for parallel-series signal input and the second level (the upper level) is the micromechanical structure, an array of square micromirrors made of highly reflective aluminium plates. Each mirror is supported diagonally by a pair of springs so that it can be deflected away from its balanced position by means of the electrostatic force applied by the outputs of the memory cell underneath. Therefore, each mirror has two reflection states ( + 10 or - 10), one reflecting the incident light beam onto the screen to form a bright spot and the other reflecting the beam out of the screen. The mirrors act as light switches controlled by the data stream flowing into the SRAM and the DMD can display moving pictures on the screen. Though the DMD with a large ,umber of mirrors is con- sidered very difficult to produce, TI managed to present DMD boards with a chip of 500 000 pixels at Japan Electronics Show 95 181. The MEMS DMD can triple the brightness of the existing liquid-crystal projection display, enabling all- digital processing for high-fidelity colour. The displays are of three types: home TVs, business presentation and audito- rium, and outdoor displays. Three DMD chips are used for professional applications to display separately red, green and blue colours. For home and business use, one chip uses a rotation filter. Recently the company prototyped a 1.3 million pixel DMD with an addressable size at the XGA level of 1280 1024 pixels and the HDTV-level of 1280 960 pixel. It is also reported that the projection display products will be commercialized soon. In addition to the DMD by TI, an infrared chopper for a pyroelectric sensor has been developed by Toyota 191, a micro shutter by Technical University of Berlin 201, a mov- able stage with on chip LED and photo detector by Grr 211, and a MEMS device for optical-fibre control by Mohr et ai. 22. 3.3. MEM$ devices with magnetic signal output A magnetic signal is another non-contact signal. A micro- electromechanical system consisting ef a micromechanical magnetic saucture and control electronics can be hermeti- cally sealed in a package but can still communicate with the outside world through magnetic signals. In this case, there is no mechanical movement inside the package but some form of mechanical action can be caused outside the MEM system through magnetic field. The microclectromagnetic compo- nents are now the microactuators. By now, some forms of microelectromagnetic components have been developed. A fully integrated micromachined toroidal inductor with an Ni- Fe magnetic core has been developed by Grr 1231 and a micromachined solenoid with electroplated permalloy cores has been developed by Toyohashi University 24. A typical example ofa MEMS device with magnetic signal output is the magnetic printing head developed by CSEM 25 . This device integrates an array of 6 80 micreelec- ttomagnets with demultiplexing electronics on a single sili- con chip. The microelectromagnet is composed of a flat 10-tnrn gold coil surrounded by a magnetic circuit of Fi-Ni material with a density of 1000 magnets cm-=. Th small device provides a maximum induction of 1.4 Tesla at the tip of the Fe-Ni magnetic core, the writing pole, which gives out the magnetic signals. It is reported that this magnetic MEMS device has been successfully tested in a magnetic graphic printer. 3.4. MEMS device for microposition control Microactuators can hardly provide mechanical power large enough to drive a conventional mechanical structure, but there have been a variety of microactuators which have proved functional in driving a microactuator to move in an appropriate enviroament. Therefore, MEMS devices for microposition control are promising for the future if massive applications are found for them. One example is the MEMS device for the position cuntxol of a magnetic head for hard discs. As hard discs are the most popular media for data storage, the production volume is huge and the maximum density is progressing every year. Currently the maximum track density is about 5000 tracks per inch (TPI). At the turn of the century, it is projected that the maximum track density will be at least 25 000 TPI, which corresponds to a track pitch of I bun and M. Bao. W. Wang/Sensors and Actuators A 56 (1996) 135-141 139 a tracking accuracy of less than 0.1/m. In another five years, it is projected that the maximum track density will have to be at least 100 000 TPI, which corresponds to a track density of 0.25 tm and a tracking accuracy of less than 0.025 tim. It is well recognized throughout the industry that these levels of performance simply cannot be achieved by an evolution of the existing technologies. The only solution is to add a microactuator as the second stage for microposition control. (The first stage is for track location by a voice coil motor.) The microactuator is driven by the signal from the control electronics according to the off-track error. Many effocts have been made in this area and a silicon- based electromagnetic microactuator has been developed for the purpose of microposition control 26. The millimetre size microactuator consists of a movable platform suspended by four planar microsprings to the frame and driven by an electromagnetic actuator. The platform, microsprings and the frame are made of silicon by bulk-micromachining technol- ogy. The electromagnetic actuator consists of electroplated planar copper coils and permaUoy cores (the conventional technologies for magnetic heads). For further development, control electronics can be integrated with the microactuator to form a st.bsystem. Furthermore, the magnetic hard-disc drive system is an electromagnetic system and the whole system is miniaturizing rapidly. It is conceivable that more and more subsystems can be replaced by MEMS devices 27. Though this kind of MEMS devices for position control will not be independently sealed in a hermetic package, the environment inside the hard-disk drives is good enough for them to work properly. In addition to the application in hard-disk drives, there arc many other possible applications, such as the microposition control for an STM 28 or STM aray 291, or, as racntioned before, for optical controls 20,21 . 3.5. MEMS devices for chemical or Oiochemical analysis and reaction A chemical analysis system has long been one of the main application targets from the very early development stage of micromachining technology. By the end of the 1970s, Stan- ford University had tried to d:.velop a gas chromatograph in a silicon wafer 30. After two decades research and devet- opment, the technology is more mature than ever before. As vital components of chemical and biochemical systems, a variety of micromechanical valves 31,32 and microme- chanical flow meters 33 have been developed and some of them have been commercialized. Therefore, furtheg devel- opment on chemical or biochemical MEMS is very premis- ing. It h been reported that the gas chromatograph originally developed at Stanford University is in production in a greatly modified form 4. Some work showing the recent interest in this area includes a reaction chamber for DNA amplifica- tion 34, a bioreactor for biological experiments in space 35 and others 36. In the above, some of the most promising areas for future MEMS devices have been listed. Of course0 the future MEMS devices will not be limited to these areas. 4. The future of MEMS technologies Modern MEMS technology is the result of long develop- ment starting from silicon planar processes. In the course of the development, many technologies have been absorbed from o.her fields. In the mean time the techniques disperse from MEb!S technologies to other fields. In the futmc 4velopment of MEMS technology, things should go in a similar way. MF.IS technology will remain an independent developing technology with close ties to other technical fields. The future development would be expected to be as follows. ( ! ) Silicon will remain the most important material while mote and more new materials will be added for additional choice. The reasons for this are: (a) silicon is a material with excellent mechanical and electrical qualities fully understood after many decades investigation； (b) after many yeats development, Si-based processing technologies have matured both for microelectmnics and micromachining； (c) silicon is an exclusive choice as far as the monolithic integration of microelectmnics and micromachines is concerned. The advantages of making use of non-silicon materials in an Si-based process are obvious. They add in more processing flexibility and provide some unique properties to silicon, including magnetic, optical, thermal, dielectric, piezoelectric, etc. Among many non-silicon materials, the electroplated metals axe the most important ones, as they can be processed at room temperature and are comltible with IC technology. (2) When single-sided and double-sided micromachining processing are compared, single-sided processing technology has the advantages oi small size and low cost and will be the mainstream of future development, though bulk microma- chining will still be developed for further applications. In many silgle-sided processing technologies, the polysil- icon in the traditional surface micromachining is not ideal due to its limited thickness and poor controllability in mechanical and electrical properties. LIGA t,-hnology has not gained in popularity due to its high expense and poor compatibility with microele,:.tronics. Technologies with films as thick as tens of microns will be of most interest in the future, such as the pseudo LIGA process 37,38 for metal materials atd the process based on SIMOX SOl 11 , epi- layer 12, BESOI 39-41 and others 42 for thick silicon layers. (3) The connections between MEMS technology and ICs will be strengthened further in the future not only because of the similarity in processing, packaging, applications, but also for lh expensiv, mass-production facilities already existing in the IC ind,stry 43. 140 M. Bao. 14/. Wang/Sensors and Actuators A 56 (1996) 135-141 5. Conclusions and discussion MEMS have been emerging as a new technology in the past decade. They will continue to develop as an independent technology in the future, while their techniques continue to disperse to other technological fields. The strategy being proven successful in developing MEMS is: according to the features of the micromachining, attention has to be paid to develop MEMS devices that communicate with the outside world through non-contact signals, i.e., elec- trical, magnetic and optical signals. Contact mechanical cou- pling between the micromachine and the outside world should be aveided. MEMS devices should be developed for new applications with a vast market. Substitution for traditional applications of electromechanical systems by MEMS should not be con- sidered as a main strategy of development. MEMS technology will be developing in close connection with the IC industry, as sophisticated MEMS devices can only be developed using state-of-the-art micromachining technologies and modem IC technologies and be mass p:o- duced in a standard IC manufacturing plant. 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