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TWO DECADES OF MEMS

-- FROM SURPRISE TO ENTERPRISE

Hiroyuki Fujita

Center for International Research on MicroMechatoronics (CIRMM),

Institute of Industrial Science, The University of Tokyo

ABSTRACT

This paper gives a brief overview of MEMSresearch and commercialization in the past two

decades, its present status, and future prospects.The historical development of MEMS technologyis followed in relation to devices enabled bydeveloped technology. For the present status, theimportance of MEMS design and fabricationinfrastructures is discussed in order to help moreMEMS products to be successful in high-endmarket. Two future trends in low cost fabrication,nano miniaturization, and system integration ofheterogeneous functional elements are observed.

1. INTRODUCTION

This paper is the follow up of the one Ipresented at MEMS-97 [1, 2]. This time, I wantto focus more on the overview of the past, presentand future of MEMS than the technologicaldevelopment dealt in my previous paper.

In the past twenty years, MEMS technology hasmatured in the micrometer scale; flexible3-dimensional fabrication, integration withmicroelectronic circuits and operation of variousdevices are successfully demonstrated. I haveexperienced big surprises to see new processes anddevices presented at past IEEE MEMS conferences.Some MEMS products achieved commercialsuccess. Based on such success, many newproducts have been and will be introduced to themarket now. I will touch upon such applicationsand give some consideration on how we canaccelerate the MEMS commercialization. Forwider acceptance of MEMS technology, I believeit is important to provide infrastructures, e.g.MEMS foundry services and a virtual designenvironment, through the collaboration ofacademic, industrial and governmental sectors.Finally, I will discuss future trends of MEMS

research.

2. PAST: ERA OF SURPRISE

The root of MEMS research can be found in theresearch of silicon sensors. A noticeable turning

point from sensor research toward MEMS researchwas the demonstration of micromachined movableparts [3], gears and turbines [4] made on a siliconchip in 1987. Since then, development hascontinued in micromachining processes, materialvarieties, micro actuators, and the application ofMEMS as shown in Fig. 1. The development wasso fast and wide in variety that I and many of you,I guess, were kept surprised watching it.

Micromachining processes [5] are based onsilicon integrated circuits (IC) technology and usedto build three-dimensional structures and movable

parts by the combination of lithography, etching,film deposition, and wafer bonding. The sametechnology base that enabled miniaturization andlarge-scale integration of electronics offers threedistinctive features defining micromachineddevices and systems: miniaturization, multiplicity,and microsystem integration[6]. Miniaturizationis clearly essential. Small parts respond fast,constitute miniature machines that work inextremely shallow spaces, add functionality toportable or wearable devices, and realize tools toinvestigate the nanometric world. Millions of

Fig. 1 Development of MEMS research

sensor

researchers MEMS devices (actuators/structures)

97 99 2

electrical engineers

mechanical engineers

optical engineers

chemists, biologists

communication engineers

MEMS, micromachine

optical MEMS

micro-TAS, bio-MEMS

RF MEMS

98

nano tech.scientists in nano scale

sensor

researchers MEMS devices (actuators/structures)

97 99 2

electrical engineers

mechanical engineers

optical engineers

chemists, biologists

communication engineers

MEMS, micromachine

optical MEMS

micro-TAS, bio-MEMS

RF MEMS

98

nano tech.scientists in nano scale

1-4244-0951-9/07/$25.00 2007 IEEE. MEMS 2007, Kobe, Japan, 21-25 January 2007.1


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such parts can work cooperatively to do thingsimpossible for a single device alone. Thus,multiplicity is one key to successfulmicromechanical systems. The coordination ofthese parts is accomplished by integrating them

with electrical circuits. Furthermore, sensing,optical, fluidic, and biological elements are to beintegrated in multi-functional microsystems in acost-effective manner.

In order to realize those prospects, researchershave improved the fabrication, design andintegration methods of MEMS. As an example,let us see the historical change of technology forsensor fabrication. Pressure sensors, one of thefirst MEMS products, are composed of KOHetched membranes and ion implantedpiezoresistors. Integrated accelerometers depend

on the surface micromachining to have movingmasses and circuits on a chip. Defection of verysmall capacitance change and electrostatic servofeedback enable the sensor of high sensitivity,linearity and wide dynamic range. Angularvelocity sensors (gyroscopes) are intensivelydeveloped recently; the deep reactive etchingtechnology plays a key role to makehigh-aspect-ratio resonating structures of thesensor. Wafer bonding technology is also used tomake a vacuum package for low mechanical loss.

In summary, we have experienced theimprovement in etching accuracy and minimumdimension to a few tens of nanometers, and thefreedom of making 3-D shapes. The integrationof actuators/circuits, the replication process of 3-Dstructures and the bonding technology have alsosignificantly advanced. In addition, wafer-scaleencapsulation of MEMS devices is possible bysealing cavities by poly-silicon deposition andepi-growth. Electrical contacts can be routed tothe device through the encapsulation layer.Integrated circuits can be made on the wafersurface after encapsulation [7].

Microactuators are the key devices allowing

MEMS to perform physical functions. Successfuloperation of electrostatic micromotors had a bigimpact to common people as well as scientists [8].Since then, many types of microactuators of sizesfrom 10 micrometers to 1 mm have beensuccessfully operated. Some of them are drivenby force associated with physical fields. Forcecan be generated in the space between stationaryand moving parts using electric, magnetic, andflow fields. There are many design varieties inmicroactuators. You may notice the newfabrication process allows us to realize a new

design. For example, the surface micromachiningenabled the electrostatic rotational motor. Withdeep RIE, vertical comb-drives became verypopular.

Some other actuators utilize active materials

including piezoelectric (PZT, ZnO, AlN, quartz)materials, magnetostrictive materials (TbFe), shapememory alloy (TiNi) and bio molecular motors.Thermal expansion and phase transformations suchas the shape-memory effect and bubble formationcause shape or volume changes. Micromachiningtechnology allows us to make structures in whichwell-controlled field is generated or to deposit andpattern actuator materials. Although theirperformances have improved dramatically, stillthere are a lot of needs for stronger output force,faster response and environmental robustness.

Various MEMS applications have been pursued;that has resulted in some successful products.Accelerometers for automobiles, e.g. an integratedand surface-micromachined accelerometer [9],ink-jet printer heads and MEMS projectiondisplays, e.g. a digital micromirror device [10], aretypical examples. They also provide very goodexamples how MEMS features are beneficial topractical devices. Accelerometers take advantageof electronics integration for good performance.Micromachined channels and integrated heatersenable ink-jet heads to eject ink droplets of a fewpL at around 10 kHz from many nozzles in parallel.Thus, high-definition color pictures are beautifullyprinted. The scaling effect associated withminiaturization, that is faster thermal response dueto decrease in heat mass, is effectively used in thehead. Also parallel processing capability isowing to multiplicity of MEMS. The digitalmicromirror device takes advantages of all threevirtues of MEMS as you may already know well.

Not all products, of course, are successful.MEMS optical switches in 2000 were overheated.After the crash of telecommunication market, veryfew companies that developed MEMS optical

switches have survived. Major reason why thebusiness went wrong was the external cause; themarket environment changed. I may also observethat the technology was too much specific to thetarget. It was difficult to reorient the R&Dactivities because the expensive and sophisticatedtechnology cannot match inexpensive marketsegments. Presently, the MEMS devices havemuch higher performance than those in seven yearsago and 3-D MEMS mirrors for beam steering canrealize inexpensive but highly reliable opticalswitches [11].

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3. PRESENT: ERA OF ENTERPRISE

3.1 EMERGING MEMS PRODUCTS

Following successful cases in inkjet printers,automobile sensors and projection displays, manyMEMS products have been or be introduced to themarket:

(1) Optical MEMS including variable optical

attenuators (VOA) and optical switches.(2) IT sensors including microphones, uncooled

infra-red cameras, TV-game controller sensors,robotic sensors, taste sensors, and odor sensors.

(3) RF MEMS includes RF switches, integratedresonators, and variable capacitors/ inductors witha high-Q factor.

(4) Nanotechnology tools including AFMcantilevers and handling tools for atoms andmolecules.

(5) Micro fluidic systems including DNAanalysis chips, micro reactors, medical diagnosischips and environmental monitoring chips.

(6) Many other products utilizing 3-dimensional

microstructures fabricated by MEMS relatedtechnologies.

Table 1 summarizes the correspondence betweenMEMS products and technologies. You maynotice how the technological development isessential for new successful products.

3.2 CLASSIFICATIONS OF MEMS PRODUCTS

MEMS products are categorized with respect totheir market size. Automobile sensors and printer

heads are in a mass-produced market. Thecurrent market for MEMS display devices is lesslarge as those. Other devices, such as opticalscanners and AFM probes, have only a smallmarket today. One of the features of MEMS is itsbatch production capability; thus, the market ispreferably large.

However, the success of a product depends onits added value as well as its market size. Figure2 shows the categorization of MEMS products inboth aspects. The horizontal axis represents themarket size and the vertical axis represents theadded value. All clusters stay above a line withnegative slope. It is natural because a product

wet anisotropicetching

surfacemicromachining

dry anisotropicetching

circuitintegration

nano machining microactuator fluidic device surface

treatment bonding

pressure sensor

accelerometer (servo feedback)

angular speedsensor

ink-jet printerhead

(micro heater)

(hydrophobic

surface)

digitalmicromirrordevice

(anti-sticking)

optical scanner

VOA

electrophoresischip

AFM probe

high

low

small largeMarket sizequantity

failure

success

ValueaddedbyMEMS

- VOA

- Optical scanner- Electrophoresis chip

- AFM probe

- DMD

- GLV

- accelerometer

- pressure sensor

- ink-jet printer

head

high

low


failure

success

ValueaddedbyMEMS

- VOA

- Optical scanner- Electrophoresis chip

- AFM probe

- DMD

- GLV

- accelerometer

- pressure sensor

- ink-jet printer

head

Figure 2 Mapping of MEMS products

high

low


failure

success

ValueaddedbyMEMS

distinctive

MEMS

essential

MEMSMass

production

MEMS

high

low


failure

success

ValueaddedbyMEMS

distinctive

MEMS

essential

MEMSMass

production

MEMS

Table 1 Relation between MEMS products and technology

Figure 3 Categorization of MEMS products

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having a low added-value and a small market cannever be successful. In other words, the regionabove the line is a success region.

The clusters of products in Fig. 2 can beclassified into three groups as shown in Fig. 3;

those are mass-production MEMS, distinctiveMEMS and essential MEMS. Here, thedistinctive MEMS means the use of a MEMSdevice as a key component in the system that has aunique feature because of the MEMS component.The essential MEMS means the MEMS deviceonly with which a system specification can befulfilled. The marketing strategy for eachcategory is very different.

3.3 ISSUES SUFFERING COMMERCIALIZATION

MEMS technology provides a versatilefabrication capability for a wide range of industries.However, MEMS products remain a few in number,probably because the marketing strategy ofsemiconductor devices, namely reducing cost bymass-fabrication, can be applied only to themass-production MEMS. This group includes avery limited number of products. We must breakthe IC manufacturing paradigm [12] for thesuccessful commercialization of the distinctiveMEMS and the essential MEMS. Suchcommercialization, however, bears the followingdifficulties:

(1) Fabrication facilities are too expensive to beinstalled only for small volume production.

(2)Because MEMS fabrication process differsfrom a device to another, a designer should knowmany variations of processes; such high-leveldesigners are very few.

(3)The optimization of a MEMS device requiresmany repetition of design modification and trialfabrication. This makes the development phaselong and costly.

(4)Those who want to utilize MEMS in various

products may not have enough knowledge ofMEMS technology; thus they cannot take the fulladvantages of the technology.

3.4 STRATEGIES TO OVERCOME THE ISSUES

I believe above mentioned issues can beovercome by providing a MEMS manufacturinginfrastructure, namely a MEMS foundry networkand a computer-aided design environmentassociated with a comprehensive material andprocess database.

The MEMS foundry service solves the

fabrication issue. Fabrication cost is reduced bysharing expensive facilities in foundry firms.Following such service in foreign counties, morethan ten Japanese companies offer micromachiningservice now. Each company has, however,

different expertise. Those who use the service forthe first time might need some guidance. Ibelieve an alliance of foundry companies can offerthe comprehensive micromachining capability andrealize a one-stop-service.

MEMS design is more and more important toverify new devices and optimize theirperformances to satisfy the specification ofproducts [13]. The virtual design environment ismost effective to shorten the development time andto reduce its cost. A designer can define a set ofmasks and a micromachining process sequence and

find the final structure with its performancethrough computer simulation. The environmentallows the designer to improve MEMS deviceswith the minimum number of real fabrication. Inorder to have precise matching between thecalculated result and the real one, the software forprocess simulation and mechanical analysis shouldbe optimized to MEMS. The academic sector isstrong in theoretical background and may supplyappropriate solutions. Also, the material databaseshould include the material parameter dependenceon the type and conditions of each process.Foundry companies and equipment venders areexpected to provide such data. A potential clientof a foundry can try how a device works insimulation using detailed parameters matched witha particular foundry and then submit a real order toit, if the simulation result is satisfactory.

There are fairy large amount of knowledgeaccumulated over past two decades on MEMSdevices and fabrication processes. It is notdifficult to provide such knowledge from theacademic sector. A knowledge database equippedwith a good search engine is a very helpful tool fornovice designers and students. In addition, some

popular device structures or process sequencesmay be offered as IP, based on which applicationspecific devices may be developed in a short time.

The design environment is indispensable toeducation in combination with some hand-onexperience in the clean room. Furthermore, thesimulator should not give only electro mechanicalcharacteristics of a device but also its functionalperformances in its final application, e.g. theoptical loss and cross talk of a MEMS switch, thechemical interaction in a micro fluidic system andthe S-parameters of a RF-MEMS device. A

combined multi-disciplinary analysis software

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should be constructed [14].MEMS opportunities are not only in mass

produced devices but also in key devices of ahigh-added-value system. The latter devices canbe fabricated cost-effectively by the foundry

service after the performance confirmation in thevirtual design environment. As shown in Fig. 4,the infrastructure for MEMS design andfabrication will accelerate the commercializationof MEMS in different fields of application thatrequire small-volume-large-variety production.The collaboration among foundry firms, equipmentand material suppliers, software vendors, academicinstitutions and governmental organizations isnecessary to build the infrastructure.

4. FUTURE OUTLOOK

New research trends are (1) to advance thetechnology further and (2) to realizehigh-performance micro systems with multiple

heterogeneous functions. The first directionincludes miniaturization to nano scales andintroduction of printing and replication technologyto IC-compatible micromachining. The seconddirection will be achieved by the integration ofmany elements for sensing, actuation, informationprocessing and communication into a single smallsystem as well as by the miniaturization of eachelement in the nano scale.

Replication of micro molds has beeninvestigated over twenty years. However, thereare two new developments recently. One is nano

imprinting. Structural size below 100 nm can beobtained with typical aspect ratio of 1-2. Hotembossing is one way for nano imprinting. AlsoUV-cured resin is used to replicate transparentnano molds; this is called photo nano imprinting.The other is the roll-to-roll printing process. Asshown in Fig. 5, the idea is to apply differentprocess on films continuously; these processesinclude sputtering of films, off-set or gravureprinting of patterned ink, hot embossing, andlamination. Although the minimum feature sizeand alignment accuracy are approximately 10micrometers, the technology enables us to processmeters wide and hundreds meters long film withvery low cost. Not only dyes and insulators butalso metal conductors and organic/inorganicsemiconductors can be printed. We may envisionthat thinned silicon IC-chips are surface-mountedat certain places on the sheet.

The minimum feature size of MEMS hasdecreased to a few tens of nanometers owing to thedownsizing of VLSI technology. Furtherminiaturization becomes more and more difficult.In order to overcome the difficulty, a novelapproach, so-called a bottom-up method, will be

incorporated. Functional nano elements areconstructed from atoms and molecules. Carbonnanotubes (CNT) and bio molecules are amongsuch elements. CNT can serve as a transistor, aconductive wire and a nano torsion bar [15].However, it is still out of our capability to build acomplicated system, e.g. an integrated memorychip, by (self) assembling only nano elements, e.g.CNT transistors. I believe the combination ofboth is a solution. Individual nano elements areplaced at proper locations in a structure fabricatedby VLSI/MEMS technologies. The structure

provides interconnections among nano elements to

Figure 4 Effect of infrastructure for MEMScommercialization

Figure 5 Roll-to-roll MEMS fabrication(prospected)

high

low

small largeMarket size

quantity

failure

success

ValueaddedbyMEMS

distinctive

MEMS

essential

MEMS

Mass

productionMEMS

Effect of

infrastructure

high

low


quantity

failure

success

ValueaddedbyMEMS

distinctive

MEMS

essential

MEMS

Mass

productionMEMS

Effect of

infrastructure

high

low


quantity

failure

success

ValueaddedbyMEMS

distinctive

MEMS

essential

MEMS

Mass

productionMEMS

Effect of

infrastructure

sputtering

Embossing Lamination

Film Roll

printing

Film Roll

MEMS on film

Coating

Top layer

Bottom layer

sputtering

Embossing Lamination

Film Roll

printing

Film Roll

MEMS on film

Coating

Top layer

Bottom layer

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integrate their functionalities into a target system.It also serves as an interface between nano andmacro worlds and as a control mechanism ofsystem operation. Figure 6 represents how such asystem, named nanosystem, can be obtained.

Various heterogeneous functions will be inintegrated in a nanosystem; those includeelectronic, mechanical, optical, quantum, chemical,biological, etc. In the same way as pastdevelopment of MEMS technology pushedcommercialization, such new development willlead to new products to solve problems in thefuture society.

5. CONCLUSION

The MEMS research has shown remarkabledevelopment. Micromachining capability inmicrometer scale has reached its maturity. Suchtechnological advance enabled MEMS commercialproducts. In order to accelerate

commercialization of MEMS, MEMS forhigh-added-value systems should be introduced tomarket as well as mass produced MEMS devices.The cost effective production of various MEMSdevices in rather small quantities will be achievedby efficient design and optimization in MEMSCAE environment and by MEMS foundry service.In the future, MEMS technology will includeprinting/replication processes and will be capableof integrating nano elements into the system.MEMS will evolve into nanosystems that haveheterogeneous multiple functionalities.

REFERENCES

[1] Hiroyuki Fujita, A Decade of MEMS and itsFuture, IEEE Int. Workshop on MEMS, Nagoya,Japan, Jan. 26-30, 1997, pp. 1-8

[2] Hiroyuki Fujita, Microactuators andMicromachines, Proceedings of THE IEEE, VOL.86, NO.8, August 1998, pp.1721-1732[3] Long-Sheng FAN, Yu-Chong TAI, RichardS.Muller, Integrated Movable MicromechanicalStructures for Sensors and Actuators, IEEETransactions on Electron Devices, Vol.35, No.6,June 1988, pp.724-730[4] Mehran Mehregany, Kaighan J. Gabriel,William S. N. Trimmer, Integrated Fabrication ofPolysilicon Mechanisms, IEEE Transactions onElectron Devices, Vol.35, No.6, 1988, pp.719-723

[5] K.E. Petersen, Silicon as a mechanicalmaterial, Proc.IEEE, vol. 70, p. 420, 1982[6] K.J. Gabriel, Engineering microscopicmachines, Sci. Amer., vol. 260, no. 9, pp.118-121,Sept. 1995[7] R. N. Candler W.T. Park, H.M. Li, G. Yama, A.Partridge, M. Luts, T. W. Kenny, Single WaferEncapsulation of MEMS Devices, IEEE Trans. onAdvanced Packaging 26, 227, 2003[8] L.-S. Fan, Y.-C. Tai, R. S. Muller,IC-processed electrostatic micromotors, Sensors& Actuators, vol. 20, pp. 41-48, 1989[9] N. Yazdi, F. Ayazi, K. Najafi, MicromachinedInertial Sensors, Proceeding of IEEE, vol. 86, pp.1640-1659 , 1998[10] P.F. van Kessel, L.J. Hornbeck, R. Meier, M.R.Douglass, A MEMS-Based Projection Display,Proc. IEEE, vol. 86, pp. 1687-1704, 1998[11] Mitsuhiro Yano, Compact and stablecrossconnect 3-D MEMS switches and its systemapplications IEEE/LEOS Interntl Conf. onOptical MEMS, Tkamatsu, Japan, 22-26 August,2004, p. 156-157.[12] Martin A. Schmidt, MEMS and Nano: ThePath to Manufacturing, Proc. of the 23

rd Sensor

Symposium, pp.1-3, 2006[13] S.D. Senturia, Micro System Design,Kluwer Academic Press Publ., Boston, USA, 2000[14] Hideo Kotera, Computer aided engineeringsystem for Micro Electro-MechanicalSystems-MEMS-One-, Proc. of the 23

rd Sensor

Symposium, pp.5-8, 2006[15] A. M. Fennimore, T. D. Yuzvinsky, Wei-QiangHan, M. S. Fuhrer, J. Cumings & A. Zettl,"Rotational actuators based on carbon nanotubes",NATURE 424, p.408, 2003

Figure 6 Creation of nanosystems bycombining bottom-up and top-downtechnologies

MEMS tools and devices

0.1 1 10 100 1000nm

nano technology region

micro reactor

manipulation by localized electric field

multi-probe,

nanogripper

SPM

Laser tweezers

atom/molecule handling

bottom-up approach

from science to

manufacturing technique)

self organization

protein engineeringsupra molecular synthesis

top-down approach(miniaturizationtoward nanoscale)

ultra precision machining

nanolithography

nanomachining

nano system

based on both top-down

and bottom-up approaches


0.1 1 10 100 1000nm


micro reactor


multi-probe,

nanogripper

SPM

Laser tweezers


bottom-up approach

from science to


self organization




nanolithography

nanomachining

nano system




0.1 1 10 100 1000nm


micro reactor


multi-probe,

nanogripper

SPM

Laser tweezers


bottom-up approach

from science to


self organization




nanolithography

nanomachining

nano system




0.1 1 10 100 1000nm


micro reactor


multi-probe,

nanogripper

SPM

Laser tweezers


bottom-up approach

from science to


self organization




nanolithography

nanomachining

nano system



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