Sensors and Actuators A 136 (2007) 62–67
`
`Review
`Trends and frontiers of MEMS
`Wen H. Ko a,b,∗
`
`a Electrical Engineering and Computer Science Department, Case Western Reserve University, Cleveland, OH, USA
`b PEN-TUNG SAH MEMS Research Center, Xiamen University, Xiamen, China
`Received 31 January 2007; accepted 1 February 2007
`Available online 8 February 2007
`
`Abstract
`
`Since 1988, the MEMS has advanced from the early stage of technology development, device exploration, and laboratory research, to the mature
`stage of mass production and applications, as well as launched exploration and research in many new areas. For the trends of MEMS in the next 5–10
`years, people with different background would have different views and projections. This article will present examples to illustrate the suggested
`future trends, as some of us see it.
`From the present MEMS orientation, the future trends of MEMS can be suggested as below:
`
`1. Transfer the traditional useful MEMS to large scale applications, to establish mass markets. This would build up MEMS industries to support
`the sustained MEMS research and development. There are two major directions: (A) Reduce cost, raise yield and efficiency to cultivate mass
`markets. (B) Raise system performance to meet the special needs.
`2. MEMS network. There are needs of having many different functional systems and many similar functional systems working together to perform
`required big tasks.
`3. New materials. Besides silicon and semiconductors, many other materials can be used for MEMS.
`4. Explore new frontiers. Many new Frontiers of research and application were developing. More will be open up. Such as: (A) biological research
`and medical instruments; (B) micro-energy sources—micro-fuel cells, environmental energy converters, remote energy supply techniques,
`etc.; (C) radio frequency and optical/IFR communication; (D) environmental monitoring, and protection; (E) ocean and water-way studies; (F)
`nano-micro-mixed technology.
`
`This article attempts to present the views of some MEMS educators and researchers in an over simplified form. It is hoping that this would
`stimulate more valuable discussions that may be valuable to planners of MEMS development and general readers.
`© 2007 Elsevier B.V. All rights reserved.
`
`Keywords: MEMS; Trends and frontiers
`
`Contents
`
`Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
`1.
`2. Trends and frontiers of MEMS. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
`2.1. Transfer research to applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
`2.1.1. Develop low cost, high efficiency, and mass produced devices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
`2.1.2. Research on high performance, functionality and reliability systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
`2.1.3.
`Integrate related sensors and actuators as a functional chip/system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
`Sensor network and MEMS network . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
`2.2.
`2.3. New MEMS materials and technology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
`2.4. New frontiers of MEMS research and application . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
`2.4.1. Biological research and medical instruments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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`∗
`
`Corresponding author. Tel.: +1 216 368 4081; fax: +1 216 368 6039.
`E-mail address: WHK@cwru.edu.
`
`0924-4247/$ – see front matter © 2007 Elsevier B.V. All rights reserved.
`doi:10.1016/j.sna.2007.02.001
`
`APL1112
`Apple v. Valencell
`IPR2017-00317
`
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`W.H. Ko / Sensors and Actuators A 136 (2007) 62–67
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`2.4.2. RF and optical communication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
`2.4.3.
`Environmental monitoring and protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
`2.4.4. Micro-energy sources. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
`2.4.5. Nano-micro-mixed devices and technology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
`3. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
`Acknowledgements. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
`References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
`Biography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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`1. Introduction
`
`Since 1987, micro-electro-mechanical-systems (MEMS) has
`advanced from the early stage of technology development,
`device exploration, and laboratory research, to the mature stage
`of quantity production, practical applications, and expanding to
`many new areas of exploration and research. As for the trends
`of MEMS research and industrial development, in the next 5–10
`years, people with different background would have different
`views and projections. This paper attempts to outline the views
`of some MEMS educators and researchers in an over simpli-
`fied presentation. The authors hope that this material would
`serve as references to the researchers and the planners of MEMS
`development to stimulate more valuable discussions and studies.
`In the first 10–15 years, besides a few success stories like ink-
`jet printers, automobile sensors, digital mirror displays, MEMS
`activity mainly devoted to the establishment of micro-fabrication
`technology, device design, and the verification of research ideas
`to lay the foundation for this field and extend to explore other
`areas that MEMS may has good potential. In recent years,
`MEMS is maturing and moving gradually to field applications,
`to establish MEMS industries, and to support sustainable MEMS
`research and industrial growth.
`In the early stage of MEMS development, there are many
`important historical events or developmental monuments of gen-
`eral interests. A few selected ones are given below.
`In 1954, a paper, measured the piezoresistivity coefficients in
`germanium and silicon, was published by Professor C. S. Smith,
`on sabbatical leave from Western Reserve University, Cleve-
`land, OH, USA, to work in Bell Telephone Laboratory. These
`data paved the way for today’s piezoresistive sensors design,
`including pressure, displacement, and strain sensors [1].
`In 1959, the paper “There is plenty of room at the bottom”,
`by Professor Richard Feynman, was published [2]. It ushered-
`in the New Era of micro-machining, micro-devices and nano-
`technology.
`From 1960 to 1970, resonant gate transistors [3], accelerom-
`eters [4], pressure sensors and silicon based strain sensors were
`fabricated. Fig. 1 is a resonant gate transistor of 1967 [3].
`In 1981, “Journal of Sensors and Actuators” was published
`and “the First International Conference on Solid-State Sensors,
`Actuators and Microsystems” (Transducers 81) was held in
`Boston, USA.
`In 1982, Professor K.E. Peterson published the paper, “Sil-
`icon as a mechanical material” [5]. The micro-machining
`technique was introduced.
`
`In 1987, the First “MEMS” related Workshop on “Micro-
`Robots and Tele-operators”, was held at Hyannis, MA, USA.
`This and the following workshops in USA and other parts of the
`world, introduced the terms micromachining, micro-systems,
`micro-fabrication-technology, and micro-electro-mechanical-
`systems (MEMS).
`issue of “Journal of Micro-Electro-
`In 1992,
`the first
`Mechanical-Systems” (JMEMS) was published.
`In the last 20 years, intensive laboratory explorations were
`made. A few well known industrial success cases in MEMS
`are:
`
`1. MEMS based inkjet head from Hewlett-packard. These inkjet
`heads occupied a majority of today’s inkjet printer market
`with more than 1 billion US dollars.
`2. Pressure sensors from Nova Sensors of general motor.
`3. Accelerometers and gyroscope from Analog Devices
`4. Digital light processing (DLP) from Texas Instruments. Dig-
`ital light processing (DLP) is a revolutionary way to project
`and display information based on the digital micro-mirror
`device (DMD). The research started in 1976 and it took about
`20 years to finalize and commercialize this product. Today,
`the DLP system was used by more than 40 manufactory world
`wide.
`5. Bio-chips and micro-fluidics devices
`6. MEMS applications in communication – RF, Optical, IFR
`communication components, used in cell phones and Internet
`systems.
`
`Fig. 1. The resonant gate transistor [3].
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`
`2. Trends and frontiers of MEMS
`
`Gauged from the past MEMS development and orientation,
`the future trends of MEMS research and development, as the
`author see it, may be summarized as below.
`
`2.1. Transfer research to applications
`
`Transfer the established traditional MEMS devices and sys-
`tems, which are proved to be useful, to large scale applications,
`to mass markets, thus build up MEMS industries to support sus-
`tained MEMS research and development. In 2005, the MEMS
`components world market is about 5 billions US dollars. As
`MEMS technologies and fundamental research accumulate, and
`the pace of industrial application and business development
`accelerates, the future MEMS market potential will be great.
`At the same time, the rapid advances in integrated circuits and
`the market demand on new products provided MEMS a very
`good opportunity to flourish. In a sense, the New Era of MEMS
`is arriving.
`Fig. 2 is an interesting figure which was presented by Dr. K.E.
`Petersen in “the 13th International Conference on Solid-State
`Sensors, Actuators and Microsystems, 2005” (TRANSDUC-
`ERS ’05) [6]. There are two curves in the figure. The upper one
`is the well-known Moore’s law in IC industry, which predicts the
`transistor density of integrated circuits doubles every 18 months
`and with doubled performance. The MEMS device shown in the
`lower curve approximates the time advance of complexity level
`(electronic and mechanical elements/chip) from pressure sen-
`sor, accelerometer, ink-jet head to digital micro-mirror device
`
`Fig. 2. Moore’s law in MEMS [6] [Courtesy of Dr. Kurt Petersen of SiTime,
`USA].
`
`(DMD). Interestingly, the MEMS curve almost has the same
`trend as Moore’s law. We could not predict whether the devel-
`opment of MEMS will accurately follow this curve but what we
`can tell is that the MEMS field will advance with accelerated
`pace.
`As the MEMS field evolves from research mode to applica-
`tion mode, there are three major directions.
`
`2.1.1. Develop low cost, high efficiency, and mass produced
`devices
`These devices will be parts of practical and functional sys-
`tems used in aerospace, military and scientific research as well
`as automated manufacturing industries and consumer products.
`Such as sensing and controlling devices/systems used in auto-
`
`Fig. 3. The roadmap of accelerometers from analog devices [7] [Courtesy of Dr. M. Judy of Analog Devices, USA].
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`
`mobile, manufactory, building, and everyday life applications.
`For example, Fig. 3 is the evolution roadmap of accelerometers
`made by analog devices [3]. MEMS accelerometers have been
`widely used in air bag system in automobile industry, robotics,
`and everyday consumer products.
`
`2.1.2. Research on high performance, functionality and
`reliability systems
`Efforts may be directed to develop high performance, func-
`tionality, and reliability systems to achieve desired requirements
`for high-end, high-priced applications. For example, develop
`better components used in cell phones and mobile personal com-
`puters, and to explore challenging applications in aerospace,
`military, medical research and environmental monitoring. This
`trend requires MEMS research to integrate with design of ASIC,
`packaging, energy sources and management, and software devel-
`opment to achieve high performance, functional, easy-to-use and
`reliable micro-devices and systems.
`Fig. 4 is a wireless MEMS strain sensor with rf powering
`and wireless data telemetry developed in Case Western Reserve
`University, which monitors real time strain information on rotat-
`ing shafts to understanding material fatigue and increase system
`reliability. The system is powered by rf powering, the strain
`information is sent out wirelessly by data telemetry. The core
`is a capacitive MEMS strain sensor and an interface IC. The
`strain information is first sensed and transferred to capacitive
`variation, which is converted to voltage information by the low
`noise interface IC, and then wirelessly telemetered out after A/D
`conversion. At the same time, the interface IC converts rf cou-
`pled energy into stable dc power and control signal. Fig. 5 is an
`illustration of packing design for easy installation and increased
`reliability. By using the integrated system approach, the high
`performance strain sensor system can measure strain from dc to
`10 kHz, with a dynamic range of ±(0.1–1000) ␮␧ [8].
`
`Fig. 5. Illustration of the wireless MEMS strain sensor system package.
`
`Fig. 6. Single-chip multi-function chemical gas detection systems [9] [Courtesy
`of Dr. A. Hierlemann of Physical Electronics Laboratory, Switzerland].
`
`ical reactions, simultaneously. Fig. 7 is a photo of the sensor chip
`with a size of 7 mm× 7 mm. The system is fabricated in a silicon
`chip to achieve system miniaturization, increased reliability and
`low cost. This is a good example of system integration.
`
`2.2. Sensor network and MEMS network
`
`2.1.3. Integrate related sensors and actuators as a
`functional chip/system
`Fig. 6 is a single-chip multi-modes chemical gas detection
`system developed in Swiss Federal Institute of Technology [8],
`which integrates multiple chemical sensors with CMOS IC on
`the same chip. The system can detect mass change, thermal
`change, capacitive change and temperature change due to chem-
`
`When the unit function MEMS systems established user trust
`and confidence in practical fields, it is time to develop large
`scale MEMS network system. There are cases that need many
`different functional systems working together, such as monitor-
`ing/control systems used in biomedical research on the behavior
`responses of living subjects to various stimulations. There also
`are situations that need many similar functional systems working
`
`Fig. 4. Wireless MEMS strain sensing system [8].
`
`Fig. 7. Layout of single-chip multi-function chemical gas detection systems [9]
`[Courtesy of Dr. A. Hierlemann of Physical Electronics Laboratory, Switzer-
`land].
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`
`Fig. 8. A Schematic diagram of MEMS network organization [10].
`
`together to perform a task, such as in the environmental research
`that needs similar systems to cover a large area under study. How
`to design these unit function MEMS systems so that hundreds
`or even thousands of similar and dissimilar systems can operate
`together as a functional network presents many challenges. For
`example, how to supply power to the thousands of unit systems
`in the network, how to program them so that they can commu-
`nicate efficiently to each other and to the command center, etc.
`These will require research in theory/principle, design, technol-
`ogy, and operation software. Fig. 8 is a schematic diagram of
`a possible organization of the MEMS Network, which incorpo-
`rated many unit function sensor-and-actuator-systems to build
`up a large system for designated purpose [10].
`
`2.3. New MEMS materials and technology
`
`Besides silicon and semiconductors, many other materials
`can be used for MEMS. Such as: alloys, mixture of ceram-
`ics, polymers, high temperature materials (SiC, Al2O3), giant
`magneto-resistive, and newly developed crystalline and non-
`crystalline materials, as well as nano-materials. Research to use
`these new materials for MEMS and packaging would be valu-
`able. Fig. 9 is an example of the polymeric material field effect
`transistor [11].
`
`2.4. New frontiers of MEMS research and application
`
`Besides the traditional MEMS, many new frontiers of appli-
`cations were opened up in recent years. Many more will be
`developed. The examples are.
`
`2.4.1. Biological research and medical instruments
`From DNA, protein, cell, organs, system biology studies, to
`biochips, micro-tools, and instrument, there are vast fields of
`research for MEMS in biomedical area. In medical field, from
`monitoring and therapeutic tools and diagnostic/therapeutic
`instruments, to large tele-surgery equipment, artificial organs
`
`and implant instruments, MEMS should play a significant role
`in this attractive field of science and technology.
`
`2.4.2. RF and optical communication
`Cell phones, Internet, video display, etc.
`In rf systems, rf switches, filters, modulators, and oscillators,
`as well as wireless consumer products are needed. In optical
`systems, switches, modulators, attenuators, display units, IFR
`communication, internet devices and control systems, as well
`as manufacturing industry and home monitoring, control, and
`automated devices and systems would be of interest.
`
`Fig. 9. A Polymeric material field effect transistor [11] [Curtsey of Dr. T. Cui
`of University of Minnesota, USA].
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`67
`
`2.4.3. Environmental monitoring and protection
`The oceans and water ways are new territory for exploration.
`The security and protection of society also may become an
`important field of research and development.
`
`2.4.4. Micro-energy sources
`Besides the research in miniaturization of chemical batter-
`ies (mm size Li battery), the micro-fuel cells, nuclear battery,
`micro-combustion engine, supper capacitor, and environmental
`energy converters are being studied. The environmental energy
`converters and remote energy supply techniques may include
`those that use: rf energy, sun energy, sonic and ultrasonic energy,
`temperature difference, body fuel cell, body motion energy, etc.
`These are interesting and worthwhile field of study with sig-
`nificant applications. For example, in the implant systems for
`biomedical research, if the body energy—mechanical, chemical
`or biological, can be converted into electrical energy and stored
`for the use of implant system, then the problem of bulky battery,
`limited life time, and the need to change battery, can be resolved.
`
`2.4.5. Nano-micro-mixed devices and technology
`The Nano-technology and materials are speeding ahead with
`tremendous momentum. The scope of NEMS is expending
`rapidly. The merge of Nano and Micro will bring many pleasant
`surprises to our society. The field is so wide and dynamic that
`there is no way this article can include meaningful discussion
`on this subject. Readers should follow other dedicated articles
`and publications.
`
`3. Conclusion
`
`This article selected a few examples to illustrate the suggested
`future trends, as we see it. It is believed that in the next 10 years,
`MEMS field would need more man power to exert larger efforts
`to bring to the human society and our community more pleasant
`surprises and continue to raise our quality of life throughout the
`world.
`
`Acknowledgements
`
`The author like to thank Dr. Kurt Peterson (SiTime Corp.,
`USA), Professor Andreas Hierlemann (Swiss Federal Institute
`of Technology), Professor Roger T. Howe, (Stanford University,
`USA), Professor Tianhong Cui (University of Minnesota, USA),
`Dr. Robert Sulouff (Analog Device Inc., USA) and Professor
`Darrin J. Young (Case Western Reserve University, USA) for
`
`providing their valuable research materials in this article. The
`author also acknowledges the time and effort of Mr. Peng Cong
`in preparing this manuscript.
`
`References
`
`[1] C.S. Smith, Piezoresistance effect in germanium and silicon, Phys. Rev. 94
`(1) (1954) 42–49.
`[2] R.P. Feynman, “There’s Plenty of Room at the Bottom: An Invitation to
`Enter a New World of Physics,” a classic talk gave on December 29th 1959
`at the annual meeting of the American Physical Society at the California
`Institute of Technology (Caltech), published in the February 1960 issue of
`Caltech’s Engineering and Science.
`[3] H.C. Nathanson, W.E. Newell, R.A. Wickstrom, J.R. Davis, The resonant
`gate transistor, IEEE Trans. Electron Dev. 14 (1967) 117–133.
`[4] L. Roylance, J. Angell, A miniature integrated circuit accelerometer, in:
`Digest of 1978 IEEE International Solid-State Circuits Conference, 1978,
`pp. 220–221.
`[5] K.E. Petersen, Silicon as a mechanical material, Proc. IEEE 70 (5) (1982)
`420–457.
`[6] K. Petersen, A New Age for MEMS, in: Digest of the 13th International
`Conference on Solid-State Sensors, Actuators and Microsystems (TRANS-
`DUCERS ’05), vol. 1, June, 2005, pp. 1–4.
`[7] Michael W. Judy, “Evolution of integrated inertial MEMS technology,”
`Solid-State Sensor, Actuator and Microsystems Workshop Digest 0-
`9640024-5-0, Hilton Head Island, South Carolina, USA, June 6–10, 2004,
`pp. 27–32.
`[8] M. Suster, J. Guo, N. Chaimanonart, W.H. Ko, D.J. Young, A high-
`performance MEMS capacitive strain sensor microsystem IEEE, J.
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`[9] C. Hagleitner, A. Hierlemann, D. Lange, A. Kummer, N. Kerness, O. Brand,
`H. Baltes, Smart single-chip gas sensor microsystem, Nature 414 (2001)
`293–296.
`[10] T. Malik, M. Sanjay, Sensor networks: an overview, IEEE Potentials 22 (2)
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`[11] G. Liang, T. Cui, Fabrication and characterization of poly (3,4-
`ethylenedioxy-thiophene) field effect transistors, Solid State Electron. 48
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`
`Biography
`
`Dr. Wen H. Ko received his BS in E.E. from Amoy (Xiamen) University of
`China in l946, and his MS and PhD degrees from Case Institute of Technology,
`Cleveland, Ohio, USA, in l956 and l959, respectively. He has been an assis-
`tant, an associate and a full professor of Electrical Engineering Department
`and Biomedical Engineering Department, at Case Western Reserve University
`(CWRU), Cleveland, OH, USA, since l959, l962 and l967, respectively. He
`becomes a Professor Emeritus in EE of CWRU in July 1993. He is a professor of
`PEN-TUNG SAH MEMS Research Center, Xiamen University, Xiamen, China
`since 1998. He has publications in journals and books, in areas of: solid state
`electronics, micro-sensors and actuators, bio-medical instrumentation, implant
`electronics and control system design. He is a fellow of IEEE and American
`Institute of Medical and Biological Engineering.
`
`