MEMS: micro technology, mega impact

Microelectromechanical systems (MEMS) are devicesthat have static or moveable components with somedimensions on the scale of a micrometer. For com-parison, a human hair is about 80 m in diame-

ter. MEMS combine microelectronics andmicromechanics, and sometimes micro-optics andmicromagnetics. Many MEMS devices are nowon the market. They are available for use ascomponents in diverse systems with manyapplications. Some MEMS devices are usedprimarily for their planned employment.Systems of micromirrors for transmittingand displaying information are examples.Other MEMS components are finding op-portunistic applications in the hands ofcreative system engineers. Pressuresensors and accelerometers areprime examples. MEMS are alreadyimportant devices in the transpor-tation, communication, analytical andmedical industries. Both the variety of avail-able mechanical microdevices and their applicationswill grow rapidly in the foreseeable future. This article surveyssome of the current and projected applications of MEMS.

The Maturation of MEMSA decade ago, hundreds of MEMS components had beenprototyped and dozens were commercially available. However,

many people then viewed the technology as the classical solutionlooking for a problem to solve. The situation changed markedlyduring the 1990s, when the market for MEMS took off in a man-

ner reminiscent of the growth in sales of integrated circuitsin the 1960s. At present, roughly 100 million MEMS

components are being sold annually. Patents onMEMS are being granted now at the global rate

approaching one per work day; that is, 200 an-nually. While MEMS devices will not be usedas commonly as integrated circuits, theywill be found in a great diversity of productsand installations. Just as most people intechnological societies now own products

with integrated circuits and microlasers,pervasive ownership and use of MEMS is

clearly in prospect. The micromechanical de-vices will both improve the performance of existing sys-

tems and enable entirely new applications.Dozens of companies make and sell MEMS. Now that MEMS

devices are available in greater variety and large numbers, it ispossible for the applications engineer to incorporate them in avery wide variety of products. Most are the systems for whichparticular MEMS were designed, but others are targets of op-portunity. That is, the MEMS components are simply devices

14 8755-3996/01/$10.00 2001 IEEE CIRCUITS & DEVICES MARCH 2001

By David J. Nageland Mona E. Zaghloul

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The Variety and Applications ofMicroelectromechanical Systems DevicesHave Increased Dramatically, Improving

Existing Uses or Enabling Entirely New Ones

that are available for incorporation into whatever systems canuse them.

Figure 1 shows the dollar volume for MEMS products as a func-tion of year as compiled and projected by several studies. It is strik-ing that the compilations vary so widely in their absolute value andin their rate of growth. The variance in values is due to differencesin what was included in the study; for example, are ink-jetprintheads counted as MEMS or not? Some studies also includeproducts enabled by the availability of MEMS devices. However, allthe studies indicate that the production of MEMS is already amulti-billion dollar industry, which doubles in two to four years.The associated growth rates of 17 to 35% are noteworthy.

The first micromechanical device made by modern manufac-turing techniques was demonstrated in the mid-1960s. Signifi-cant commercial production of MEMS started in the 1980s, withthe appearance of pressure sensors for automotive and medicalapplications. Then, in the 1990s, microaccelerometers weremass produced for air bag triggers in cars. Volume production ofmicrofluidic devices also began in that decade. Optical MEMS fordisplays came to market a few years ago, and micromirrors forswitching signals in fiber networks are now entering the volumeproduction stage. MEMS switches for control of radio-frequencyand microwave signals is the next predictable market for MEMSdevices. Devices for dense data storage and many new medicalapplications are also in prospect.

The integration of microelectronics and -mechanics is ahistoric advance in the technology of small-scale systems, andit is very challenging for designers and producers of MEMS. Wewill show examples of monolithic (single substrate) and hybrid(two substrate) MEMS later in this review. Here we pause to ap-preciate the scope and impact of integrating microelectronicsand -mechanics. One way to do this is to consider Fig. 2. It is ar-guably the most important graphic in the entire field of MEMS.We note that the microelectronic, personal computer and in-ternet revolutions all occurred using components withoutmoving parts. The addition of micromachined parts to micro-electronics opens up a large and very important parameterspace to technological development and exploitation. The fig-ure also shows the regions into which both current and futureapplications of MEMS fall. Most commercial MEMS have only afew electronic and mechanical components. Two commerciallyimportant exceptions are the ADXL microaccelerometers fromAnalog Devices and the DMD (Digital Mirror Device) fromTexas Instruments. These will be reviewed below. The variety ofpotential applications shown in Fig. 2 is remarkable. The im-pact of MEMS will increasingly be felt in many industries andby many consumers.

Before getting to the current and projected applications ofMEMS, we consider in the next section the means by which MEMSare designed and made. On a macroscopic scale, carpenters do notmake engines and machinists do not produce buildings. That is,the characteristics and performance of large engineered systemsdepend on the materials and tools used to make them. On a micro-scopic scale, the uses of MEMS similarly depend on the perfor-mance specifications that result from the design and

manufacturing phases, especially the behavior of materials andprocesses at small scales. Hence, the next section surveys thecomputational and factory tools employed to make MEMS. Hav-ing some appreciation of these key aspects of MEMS technologies,we then review diverse applications of MEMS structures, sensors,actuators and systems on a chip. One potential, but uncertain, useof MEMS is their employment for information storage deviceswith terabit-per-square-centimeter data densities, which couldchallenge the dominance of magnetic storage systems.

CIRCUITS & DEVICES MARCH 2001 15

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(SPC 1994)SEMI (1995)SRI Consulting (1997)Intelligent Microsensors

Technology (1996)Venture Development Corp.

(MST) (2000)Venture Development Corp.

(MEMS) (2000)SPC (1999)Japan Micromachine Center

(max.) (1999)Japan Micromachine Center

(min.) (1999)

YearSource: Micromachine Devices Newsletter

1. Plot showing the actual and projected commercial value for themanufacture of MEMS and microsystems from several studies pub-

lished from 1990 to 2000 [1].

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Optical Switches& Aligners Current MEMS

Future MEMSEnabled

ApplicationsMajority of

Existing MEMS

UltrasonicImagers

2. Plot of VLSI electromechanics by Gabriel showing the number ofelectronic and mechanical components in current and future MEMSdevices [2]. Applications enabled by emerging MEMS are shown by

the gold shaded regions.

There are now manygood references to MEMS.Several recent books offerboth comprehensive anddetailed treatments of thetechnologies germane toMEMS [3-6] . A fewwebsites are also goodgateways to informationon MEMS [7].

From Design to DevicesThe sequential steps common to making most engineered com-ponents also apply to MEMS: design, simulation, fabrication,packaging, and testing. These actions determine both the perfor-mance and the price of devices like integrated circuits andMEMS. Hence, they are considered a necessary prelude to under-standing applications of MEMS.

Design and SimulationMany complex products were engineered before computers be-came widely available in the past two decades. Automobiles, andeven the earliest ICs, were made without CAD software. Now, fac-ile software is necessary not only for the design of systems and de-vices that go into them, but also for the simulation of theirexpected behavior. The relationship of the design and simulationphases of a MEMS development project can be considered by ref-erence to the diagram in Fig. 3. Before common computer usage,only the actions and iterations on the left of the diagram wereavailable. Process design was a matter of applying the design rulesof the fabrication facility to the job at hand. When prototype de-vices were available, they could be packaged and their perfor-mance could be tested. Of course, this sequence is both slow and

expensive. Computers en-abled designers to take thefabrication and design rulesand produce a modelquickly in a form that couldbe accepted and used by thefabrication facility. Such amodel stated the materialsand geometries to be manu-factured, but it said nothingabout how the device would

perform. Even if the device worked, its performance might be offthe specification. Hence, computers are also used for another re-lated, but very different, kind of function, namely performancesimulation. Computational iterations are much faster andcheaper than physical iterations. As noted in Fig. 3, material prop-erties impact the design and simulation phases. That is, they mustbe known adequately for the particular processes and equipmentin the fabrication facility, or else the simulated performance willbe wrong.

There are now severalsoftware packages and suites availablefor the design and simulation of MEMS devices. In a very funda-mental way, they are more complicated than the software for de-sign of either solely electronic or solely mechanical devices. Thisis due to the close coupling of both electrical and mechanical ef-fects within many MEMS. Consider a microcantilever that ispulled down by electrostatic forces. Its simulation has to takeinto account both the flow of electrical charge and mechanicalelasticity in an iterative and self-consistent fashion. Thermal,optical, magnetic, fluidic, and other mechanisms are also activein some MEMS and have to be handled self-consistently in thesimulation phase.

Two basic approaches to the need for specialized software forthe design and simulation of MEMS havebeen taken in the past decade. In the first,CAD design tools and available softwarefrom electronics and mechanics were ap-plied to MEMS. Some of the softwarefrom Tanner Tools VLSI design suite wasused for MEMS, as was the popular me-chanical engineering software fromANSYS. In the second approach, newsuites of software specifically developedfor MEMS were marketed. Most of theminclude electronic, mechanical, and ther-mal effects, and some have other physicalmechanisms. Such software is availablefrom CFD Research Corporation,Coventor (formerly called MicrocosmTechnologies), IntelliSense Corporation,Integrated Systems Engineering, andMEMScaP. These tools vary widely in themechanisms and materials parametersthat they include, the details of designand simulation of devices, and the fabri-

16 CIRCUITS & DEVICES MARCH 2001

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3. Flow diagrams for the design and simulation (green horizontal axis) and for the production,packaging, testing, and use of MEMS devices (red vertical axis). Loops for designing and simula-tion, process design, fabrication and testing, and production and use are shown by the dashed

and solid lines [8].

The MEMS structures and devicesthat result from the sequence of design,

simulation, fabrication, packaging,and testing can be classified into four

major groups.

cation facilities with whichthey interface. The choiceof which software to use forMEMS is still challenging.

Materials and ProcessesMany options for the fabri-cation of MEMS devices arenow available. However,this stage is also a daunting part of producing MEMS. The firstcomplexity has to do with the wide range of materials and associ-ated processes that can be employed.

The fact that the field of MEMS largely grew out of the IC in-dustry has been often noted. There is no doubt that the use of fab-rication processes and associated equipment that were developedinitially for the semiconductor industry has given the MEMS in-dustry the impetus it needed to overcome the massive infrastruc-ture requirements. Less discussed is the additional fact that thefield of MEMS has gone far beyond the materials and processesused for IC production. The situation is indicated schematically inFig. 4. About a half dozen materials, notably silicon and its oxideand nitride, and a similar number of processes, such as lithogra-phy and ion implantation, have generally been employed to makeICs. The set of materials for IC devices is expanding to include, forexample, low dielectric constant materials. However, a relativelysmall number of materials and processes are employed now tomake IC chips, compared to MEMS.

Many MEMS can be made with the same set of materials andprocesses as used for microelectronics. However, one of the hall-marks of the emerging MEMS industry is the use of many othermaterials and processes. Most basically, substrates other than sili-con are being employed for MEMS. Silicon carbide has been dem-onstrated to be a good basis for many mechanisms that canwithstand higher temperature service than silicon. Now, there isearly and strong interest in the use of synthetic diamond sub-strates. MEMS are also being made out of plastic, glass, and ce-ramics. Processes for taking MEMS made out of various materialsand putting them on plastic or steel tapes are also in hand.

Diverse materials can be used within MEMS devices. Whilealuminum and, recently, copper are the metals used in IC de-vices, micromachining of many other metals and alloys has beendemonstrated. The so-called shape memory alloys are used foractuators in several types of MEMS; for example, microfluidicpumps and valves. Magnetic materials have been incorporatedinto some MEMS devices. Piezoelectric materials are especiallyattractive for MEMS because of their electrical-mechanical reci-procity. That is, application of a voltage to a piezoelectric mate-rial deforms it, and application of a strain produces a voltage.Zinc Oxide and Lead Zirconium Titinate (PZT) are important pi-ezoelectric materials for MEMS. Many other examples of materi-als employed for MEMS could be given. However, the point isclear. Micromechanics are made of many more kinds of materi-als than microelectronics.

Because materials go hand in hand with the processes forproducing and modifying them, the range of processes for mak-

ing MEMS has also wid-ened far beyond thosefound in the IC industry.However, there is some-thing more fundamental atwork when it comes to pro-cesses for making MEMS.Integrated circuits aremonolithic and, despite

having 30 layers in some cases, are made by largely two-dimen-sional thin-film processes that yield what some people call2.5-dimensional structures. By contrast, micromechanical de-vices must have space between their parts so they can move, andthe dimension perpendicular to the substrate is often very fun-damentally necessary for their performance. Development ofprocesses to make micrometer-scale parts that can move relativeto each other was the breakthrough that enabled MEMS. Suchmicromachining processes fall into three major categories,which will now be reviewed briefly.

Surface micromachining involves the build-up ofmicromechanical structures on the surface of a substrate by de-position, patterning, and etching processes. The key step is theetching away of an earlier-deposited and patterned sacrificiallayer in order to free the mechanism. This process was first dem-onstrated about 35 years ago, when a MOSFET with a cantilevermechanical gate was produced [9]. The most common sacrificialmaterial now is silicon dioxide, which is conveniently dissolvedfrom under a moveable part using hydrofluoric acid. Surfacemicromachining has been used to produce an amazing variety ofmicromechanical devices, some of which are now in large-scaleproduction. Microaccelerometers and MEMS angle-rate sensorsare examples.

Bulk micromachining, as the name implies, involves etchinginto the substrate to produce structures of interest. It can be donewith either wet or dry (that is, plasma) processes, either of whichcan attack the substrate in any direction (isotropically) or in pre-ferred directions (anisotropically). Bulk micromachining has twoprimary variants. The first depends on the remarkable propertyof some wet chemical etches to attack single-crystal silicon asmuch as 600 times faster along some crystallographic directionscompared to others. This anisotropic process is called orienta-


ICs

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4. The number of materials and processes employed to make MEMSgreatly exceeds those used to manufacture integrated circuits.

Basically, some condition in the world, be itphysical, chemical, or biological, inducesmotion in the MEMS device that can besensed by any of several mechanisms.

tion-dependent etching (ODE). It was known long before theemergence of MEMS technologies, and it has become a mainstayof the industry. ODE is especially useful for producing thinmembranes that serve as the sensitive element in pressuremicrosensors. It is employed for production of these and othercommercial MEMS devices. The second approach to bulkmicromachining is to use plasma-based etching processes thatattack the substrate, usually silicon, in preferential directions.Deep reactive ion etching (DRIE) is a plasma process that is usedincreasingly to make MEMS. It can produce structures that areover ten times as deep as they are wide.

The third general class of micromachining processes is a col-lection of the numerous and varied techniques that can producestructures and mechanisms on the micrometer scale. Laser-in-duced etching and deposition of materials, electro-etching and-plating, ultrasonic and electron discharge milling, ink jetting,molding, and embossing are all available to the MEMS designer.Wafer-to-wafer bonding to build up MEMS devices, or to vac-uum-package them, is also an important process.

Like integrated circuits, MEMS devices are made using cre-ative combinations of materials and processes noted above.Some remarkable micromechanisms have been demonstrated,largely in academic fabrication facilities, and commercializedusing diverse foundries.

Fabrication FoundriesThe next concern after designing and simulating MEMS, and de-ciding on the materials they will contain and the processesneeded to make them, is which fabrication facility to employ.There is a wide range of choices here also. Sometimes, a standardIC facility can be used with postprocessing of the CMOS or otherchips to remove a sacrificial layer (surface micromachining) orto etch into the substrate (bulk micromachining). For examplethe micro-hot-plate shown in Figure 5 was realized in CMOStechnology made through MOSIS, with a post-processing step ofbulk micromachining to produce a thin unbacked film with a re-sistive heater. The small mass of the heated element permitstemperature changes of 300 C in a few milliseconds. ManyMEMS structures and devices have been produced by suchpostprocessing of CMOS chips.

There are now several foundries specifically for the produc-tion of MEMS. The fact that the design rules in MEMS areroughly two generations behind those in ICs is significant. Thisenables MEMS foundries to buy used equipment from the micro-electronics industry. Mass production of many MEMS now isdone using 150 mm wafers. There are several companies and or-ganizations in the US and abroad that offer fabrication servicesfor MEMS somewhat analogous to what MOSIS does for ICs.They include BFGoodrich Advanced MicroMachines (Ohio),CMP (France), Cronos Integrated Microsystems (the former Mi-croelectronics Center of North Carolina, known as MCNC),CSEM (Switzerland), Institute of Microelectronics (Singapore),IntelliSense (Massachusetts), ISSYS (Michigan), Kionix (NewYork), MEMX (Albuquerque), Sensonor (Norway), StandardMEMS (Massachusetts), Surface Technology Systems (UK), andTronics Microsystems (France).

Most of these foundries have all the facilities in house to pro-duce complete MEMS devices. However, the wide variety of ma-terials and processes that can be designed into MEMS meansthat it is not always possible to find all the needed tools underone roof. Hence, the Defense Advanced Research ProjectsAgency instituted a new type of foundry service two years ago. Itis called the MEMS-Exchange [11]. This organization contractswith diverse industrial and academic fabrication facilities for awide range of services; for example, deposition of zinc oxide pi-ezoelectric materials. The MEMS designer can draw from any ofthem. A completed design and funding are sent to the MEMS-Ex-change, which handles scheduling, production, billing, andother factors, such as the protection of proprietary designs.

Packaging and TestingThe various systems and materials used for sealed packaging ofelectronic chips certainly are complex. Packaging of MEMS isusually more difficult because of their mechanical motions or


5. Optical micrograph (top) of elements in a micro-hot-plate array, andscanning electron micrograph (bottom) of one of the elements [10].

because of other reasons. Dur-ing the manufacture of ICs,some testing can be done at thewafer level as well as after pack-aging. With most MEMS, per-formance can only be testedafter packaging. Hence, boththe packaging and testing ofMEMS are significantly morecomplicated than the similarsteps for ICs.

Packaging of MEMS is morechallenging than the alreadyquite complex packaging of ICs,for a sequence of reasons. Some MEMS can be sealed in pre-scribed atmospheres in the same kinds of packages as ordinarychips. Microaccelerometers are an example. However, manyMEMS require vacuum packaging in order to avoid air dampingof vibratory structures or deleterious thermal conductivity.Microresonators for frequency standards and uncooled infraredsensor arrays fall in this category. The MEMS that are used tomeasure ambient effects must be open in some way to atmo-spheric conditions. Pressure sensors are an example. They havecompliant seals that transmit pressure, but not humidity orchemicals. However, MEMS designed to measure the latterquantities must be fully open to the atmosphere. Maintenance ofperformance over their shelf lives and during use is a problem.Some MEMS (for example, those mounted on the surface of astructure to control vibration or air flow) are not in separatepackages. They have to be mounted on open surfaces, thus offer-ing another set of challenges.

The testing of MEMS devices is intrinsically more complexthan the testing of ICs because of the integrated electronic andmechanical character of MEMS. That is, both the electronic andthe micromechanical aspects of a chip with moving parts have tobe tested at some point in the prototype and mass-productionstages. For ICs, testing involves only electronic inputs and out-puts, although temperature, radiation, and other effects are alsoof interest. For MEMS, the inputs can also be vibrations andother accelerations, or particular conditions of pressure, humid-ity, and chemical vapor composition, among many other ambi-ent parameters. The production of the required input conditionsfor the testing of diverse MEMS devices is complex and costly. Soalso is the measurement of cross sensitivities; for example, theinfluence of temperature and humidity variations on chemicalsensors. Only limited testing of MEMS can be done at the waferlevel. Testing of fully packaged devices is usually needed. Theability of MEMS to withstand aging of the sensitive componentscan be problematic. The calibration of MEMS sensors is anotherimportant and nontrivial part of their testing.

Diversity of ApplicationsThe MEMS structures and devices that result from the sequenceof design, simulation, fabrication, packaging, and testing can beclassified into four major groups. The first includes passive, that

is, nonmoving structures. The second and third involve sensorsand actuators. These are conceptually reciprocal in that sensorsrespond to the world and provide information, and actuators useinformation to influence something in the world. The fourth classincludes systems that integrate both sensors and actuators to pro-vide some useful function. This classification, like most, is imper-fect. For example, some devices that are dominantly sensors haveactuators built into them for self-testing. Airbag triggers are anexample. However, the taxonomy provides a simple, but quitecomprehensive, framework for considering MEMS devices.

The applications of MEMS are conveniently grouped accord-ing to major industries. This approach also raises questions. Forexample, should the transportation industry, which has verylarge components, be considered as one entity, or else subdi-vided into its units, such as the automotive and aerospace indus-tries? Similarly, the communications industry embraces bothfiber-optic and wireless networks. It is simple and useful togroup applications of MEMS as indicated in Fig. 6.

We will provide an overview of the types of MEMS in each ofthe four major categories, including details on particular appli-cations in some of the large industries shown at the top of Fig. 6.

MEMS StructuresImmobile structures made by micromachining generally pro-vide means to guide the motion of signals or fluids. Opticalwaveguides to channel photons along the surfaces of chips are a


Transportation Communications Analytical and Medical Other

MEMS Structures

MEMS Sensors

MEMS Actuators

MEMS Systems

InfraredImagers

Pressure,Acceleration, &Angular RateAerodynamicFlow Control

Optical & RFSignal Guides

PowerSensorsDisplays,

Optical Switches, &RF Switches & Filters

Micro-Filters,-Channels& -Mixers

Micro-Pumps& -Valves

Point of CareAnalytical Systems

Many

DataStorage

6. Matrix relating the four categories of MEMS to applications arenas. Most of the devices and applicationsthat are listed are already commercially significant. Others are under development.

7. Photograph of the plastic microfluidic element produced byMicronics. It is the size of a credit card [12].

prime example. Waveguides for transmission of electronic sig-nals at microwave frequencies are another. Channels in systemsfor the analyses of both gaseous and liquid samples are alsomicromachined structures with no moving parts. Such struc-tures find applications in the communications, analytical, andmedical industries. Static structures integrated with electronicsform the basis of infrared imagers that are of use in the transpor-tation industry, with other applications for civilian security andmilitary operations. Since these devices are also sensors, theywill be discussed in the next section.

Micromachining is already being used commercially to producepassive channels in microfluidics devices, in which control of themotion of the sample and reagent fluids is accomplished by applica-tion of voltages or externally generated pressures. A few establishedand new companies are offering systems, the so-called lab on achip, for analysis of biomedical materials in research laboratoriesand for clinical medicine. These include Agilent, the formerHewlett Packard division (which is teamed with Caliper Technol-ogies), plus newcomers such as Aclara, Micronics, and Nanogen.Figure 7 shows a microfluidics substrate from Micronics. The ap-proach to DNA, protein, and cellular analyses taken by these com-panies generally involves the use of small plastic or glassmicrofluidics systems for mixing of samples and reagents and forseparations, in some cases. The microfluidic elements are insertedinto desktop machines that provide other needed functions, such ascontrol and display. The microfluidic structures are disposable, sothey avoid sample cross contamination. The small size of the keymicrofluidic elements raises the possibility of having entire systemsthat are handheld. This means that medical analyses can be per-formed at the point of care, rather than taking samples to a centrallaboratory. Only very small samples, a drop of blood, for example,are needed for microfluidic analyzer systems. Cephied, anothermicrofluidics start-up, is designing a handheld blood analyzer con-taining micromachined fluidic elements.

MEMS SensorsMost MEMS sensors involve moving elements, but not all of them.Detectors with no moving parts for sensing optical, infrared, and

microwave signals are also made by micromachining. We will re-view important infrared sensor arrays that recently appeared onthe market before turning to the more familiar and already com-mercially successful micromechanical MEMS sensors.

The detection of infrared radiation with arrays of small ele-ments enables night vision, which has numerous civil and mili-tary applications, especially for security and safety. Warmobjects, such as humans and animals, emit infrared radiation.There are two basic approaches to imaging infrared radiationwith arrays of small elements. The first involves cooling an arrayof narrow band-gap semiconductor elements to low tempera-tures in order to reduce the thermal noise level and allow mea-surement of weak thermal radiation. The second is to isolate theinfrared absorbing element from surrounding solids that woulddrain energy from the element. Doing so involves etching away asacrificial layer under each element, leaving it suspended by finelegs that conduct relatively little heat from the element. The legsare also the current paths used for measuring the resistance ofthe material on each element that absorbs infrared radiation.The associated heating, actually only a few thousandths of a de-gree, is enough to produce a measurable change in the resis-tance of the element. This MEMS approach has the tremendousadvantage of not requiring a cooling system, so it is simpler andcheaper than semiconductor infrared detector arrays, althoughit is not as sensitive to weak signals.

Honeywell has developed uncooled micromachined thermalimagers. Figure 8 shows both a micrograph of part of an arrayand a schematic showing its construction. In the recent past,Raytheon Systems Company provided micromachined arrays ofuncooled infrared detectors to General Motors for incorporationinto some Cadillac vehicles. The systems permit the driver to seethe thermal images from people or animals in the road far be-yond the range of the headlights.

Micromachined devices with moving parts are at the heart ofmost of the MEMS sensors already prototyped, many of whichare in production. Basically, some condition in the world, be itphysical, chemical or biological, induces motion in the MEMSdevice that can be sensed by any of several mechanisms. For ex-


Silicon Nitride andVanadium Oxide

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Transistor

IRRadiation

50 m

Y-Metal

X-Metal

8. Scanning electron micrograph and schematic of the micromachined pixels in the Honeywell uncooled infrared imaging array. The sacrificiallayer under the pixels was 2.5 m thick [13].

ample, capacitive sensing of the deflection of a thin, small mem-brane is used in various MEMS to measure pressure andultrasound levels. The most important MEMS sensors are for de-tection of pressure and acceleration, with devices for measure-ment of angular rates now moving to large scale production,largely for automotive applications.

MEMS pressure sensors cover a wide range of pressures andhave very diverse applications. Commercial parts can measurepressures from about 1% of one atmosphere to over one thousandatmospheres. Figure 9 shows a variety of devices from one of sev-eral manufacturers. Two major areas of application are in the au-tomotive and medical industries. MEMS pressure sensors areemployed for measurement of the air pressure in engine mani-folds, oil and fuel pressures and the pressure in tires. Medical usesinclude the measurement of blood pressure. This application isbased on the fact that MEMS pressure sensors can be madecheaply and discarded after use to avoid contamination. Suchmeasurements can be done on catheters inserted into the body,because of the small size of the sensors, or outside of the bodywhen a needle and tube permit access to the internal blood pres-sure. Because MEMS pressure sensors are catalog items, they areavailable for a wide variety of uses, many of them well beyond theinitially envisioned applications. These include small weather sta-tions; monitors for heating, ventilation, and air-conditioning sys-tems; sensors for water and other fluid systems; and recreationalengineering projects such as payloads for amateur rockets.

Microaccelerometers, like MEMS pressure sensors, have beenon the market for several years, with over 10 million of each sold an-nually. The first market for MEMS accelerometers was for air bagtriggers in automobiles. Initially, single-axis devices were offered,and then two axis components became available. The next signifi-cant application for the two-dimensional microaccelerometers wasto detect the motion of joy sticks in personal entertainment sys-tems. Packaging of microaccelerometers started with transistorcans. Next came surface-mount packages, and leadless devices arenow available. Figure 10 shows the exterior of a two-axis acceler-ometer with a range of plus or minus twice normal gravity, and amicrograph of the chip. The integration of the single moving ele-ment and its associated electronics in and on the silicon substrateis noteworthy. Figure 11 shows two other commercialmicroaccelerometers in which the manufacturers have used sepa-rate substrates for the electronic and mechanical elements. A re-markable feature of commercial microaccelerometer devices isthe very low mass, on the order of a microgram, for the movingelement. A cube of water 100 m on an edge weighs one micro-gram! Despite this minuscule mass, the sensitive electronics en-able the devices to sense a few thousandths of normal gravity.Prototype MEMS devices have measured accelerations exceed-ing 10,000 times normal gravity.

The microaccelerometers are the best example of MEMScomponents being employed for opportunistic applications.Their ability to measure tilt has led to their incorporation into asensor system for workers who have to lift heavy objects. If theperson bends their back excessively while lifting, the systemsounds a warning. MEMS accelerometers are being used by re-

searchers in sleep studies. A glove equipped with such compo-nents on each finger is under development as a training aid forAmerican Sign Language. Microaccelerometers are being usedfor studies of other motions of the human body, especially insports. They are also being incorporated into sports equipment,including golf clubs and tennis racquets. Amateur scientistshave used them to record accelerations during roller coasterrides and to make backyard seismometers.

The MEMS accelerometers from Analog Devices have inte-grated into them capacitor structures that act as electrostatic ac-


9. Photograph of various micromachined pressure sensors sold bySilicon Microstructures, Inc. [14].

10. Right: Photomicrograph of the two-directional Analog Devicesmicroaccelerometer with electronics and mechanics integrated on

one chip. The device package is shown on the left [13].

11. Commercial microaccelerometers of Ford (left) and Motorola inwhich the electronics and mechanics are on separate substrates

bridged by wire bonds [16, 17].

tuators. When a voltage isapplied to them, the mov-ing part of the device de-flects, which produces asignal similar to that gen-erated by an input acceler-ation of a specific value.This feature enables abuilt-in test capability that allows the device to automatically as-say its performance. This is an example of the melding of actua-tors into MEMS devices that are made for sensing.

MEMS ActuatorsThere are many MEMS components whose primary function isto move some small structure to accomplish a useful function.The signals that cause such motion are entirely electronic in thecase of electrostatic actuators. In some cases, electrical energy is

first turned into thermalenergy to induce motionon the micrometer scale.Some MEMS actuators de-pend only on thermal ef-fects. One type is based onthe atomic-scale phasetransformation of shape

memory alloys that automatically assume a form given tothem at one temperature when they are returned to that tem-perature. Whatever the physical origin of the small but effectiveforces in MEMS actuators, interesting and useful changes in po-sitions of a micromechanical structure can result. Some of thesewill be discussed in the rest of this section.

MEMS and optical signals have a natural synergism. Manysmall mirrors for redirecting light beams, and fine-scale grat-ings that can be dynamically reconfigured to analyze opticalspectra, are being demonstrated or manufactured.Micromechanisms with dimensions from a few to a few hundredmicrometers are small enough to be moved easily bymicroactuators and large enough to manipulate small beams oflight. In most cases of commercial interest, the actuation is ac-complished by application of electrostatic forces to nearby partswithin the MEMS device.

The first micromirror device to make it to market is also oneof the most remarkable. It is called the Digital Mirror Device(DMD) made by Texas instruments for display of images. The op-eration can be understood with reference to the schematicshown in Fig. 12. Each picture element of the DMD has three op-erational levels. The lowest is electronic. It contains the CMOSelectronics that control the operation of the element by applyingvoltages that produce electrostatic forces on the movable ele-ments. The second layer is mechanical. It has the torsionalhinges that allow the mirror to be tipped 10 degrees one way orthe other. The top layer is optical, consisting of the aluminizedmirror that reflects the light falling on it either into a beamdump or else through a projection system onto the screen. Thelight originates in a xexon arc lamp and then passes through arotating red-green-blue filter that cycles at 60 Hz. During eachcolor phase of each cycle, the mirrors tilt to pass light to thescreen for none, some, or all of the time. This serves to deter-mine the relative brightness of each color during each cycle.

The DMD has been incorporated into what are called displayengines and sold to manufacturers of conference room projec-tors for several years. It is estimated that some tens of thou-sands of these display engines are sold annually. Now, TexasInstruments is testing their use in movie theaters. This will en-able digital distribution of movies, so that handling of celluloidrolls for distribution of movies to theaters will no longer benecessary. In the future, DMD might be employed in homes as areplacement for the large and heavy tubes in current televisionsets. The bright and high-resolution character of images dis-played using DMD technology is attractive. However, the DMDis in competition with other MEMS display devices and liq-uid-crystal-display technologies.


Mirror 10 degMirror +10 deg

Hinge

Yoke

Landing TipCMOS

Substrate

12. Drawing of two pixels of the Digital Mirror Device manufacturedby Texas Instruments. The torsional hinges are 5 by 1 m in area andabout 100 nm thick. The individual mirrors are 16 m square. Over500,000 of them are found in a single device, making this the systemwith the most moving parts produced in the history of mankind. The

inventor, Larry Hornbeck, and the company received televisionEmmy Awards in 1998 for outstanding achievement in engineering

development [18].

13. Prototype integrated eyeglass display from MicroOptical Engi-neering Corporation. The mirror in the middle of one lens permitsviewing of a video image produced in the structure on the left ear

piece of the glasses [19].

Micromirrors are at the heart ofoptical signal routers for a coming

generation of the Internet.

Another, very differentoptical display might beenabled by the small sizeand high performance ofMEMS actuators. Figure 13is a photograph of a proto-type display integrated intoa pair of eyeglasses. Thevideo signal fed to the left ear piece produces a very small imagethat can be viewed by looking at the small mirror mounted in thecenter of the left lens. The wearer can look at the world in a nor-mal manner through most of the lens, or view colored dynamicimages by looking at the mirror on the one lens. A two-dimen-sional MEMS scanning micromirror is a candidate technologyfor production of the image. This approach to a heads up dis-play is substantially lighter than its competitors. Repair person-nel could view manuals while working on a system such as anautomobile. Voice activation would be used for turning pagesand enlarging the images of diagrams.

Micromirrors are also at the heart of optical signal routersfor a coming generation of the Internet. Optical fibers can carryover 100 signals using different photon wavelengths (colors).Now, however, switching this flood of information requiresconverting the light signals for each color to electronic signals,manipulating them and then regenerating light pulses for con-tinued transmission down other fibers. The great advantage ofMEMS mirrors for fiber network switches is that switching canbe done entirely in the optical domain. There is no need to turnthe optical signals into electrical signals for routing, and thento produce other light pulses for further transmission. This iscalled all optical networking (even though the productionand detection of the light bundles still involves electronics!).The MEMS approach requires less hardware and handles allwavelengths. During 2000, because of the explosive growth ofthe Internet, six small MEMS companies were bought by sup-pliers of hardware for fiber communications systems. Two ofthem fetched $750M, one went for $1.25B, and another got

$3.25B (with all of thesevalues in then- currentstock prices). Clearly,there is exploding interestin MEMS components forf iber-opt ic networks ,driven by the dramaticgrowth in the Internet us-

age. Lucent Technologies has been one of the major developersof MEMS switches for optical networks. Their technology isshown in Fig. 14. Lucent, and other companies makingswitches to transfer optical signals from one fiber to another,are now in early stages of field-testing programs. Major, indeedmassive, usage of such switches is expected to begin in the nextfew years.

The small actuators that result from MEMS technology arealso useful for switching microwave-frequency electronic signalson chips. Micromechanisms can be employed to make tunable ca-pacitors and stable resonators for manipulation of such signals.These technologies for the microwave and nearby frequencies arenow under intense development and should come to market inthe next few years. Such devices are expected to have major im-pacts on wireless communication systems and short-range radarsfor automobile collision avoidance, among other products.

A wide variety of MEMS switch designs for high frequencieshas been prototyped. In general, the switches have very low in-sertion losses (when closed) and excellent isolation (when open).Raytheon has developed a capacitively coupled switch, as shownin Fig. 15. Rockwell has produced MEMS metal-to-metalswitches, as well as interdigitated capacitors for use in micro-wave circuits. The University of Michigan has the highest fre-quency MEMS resonators. A device based on a beam with bothends free (similar to a xylophone key) has exhibited resonance at92 MHz. Its vibratory displacement is on the order of ananometer. Other designs based on the expansion and contrac-tion of disc-shaped structures are being developed at Michiganfor operation at frequencies approaching 1 GHz. Few microwave


MEMS Switch

Reflector

Tilted Mirror

Imaging Lens

Fibers

14. Optical micrograph of a sewing needle atop the MEMS mirrors in the optical switch array developed by Lucent Technologies. The schematicon the right shows how tilting the mirrors routes signals from one fiber to another, whatever the wavelength of the light [20].

The entire field of MEMS has turned thecorner in recent years from gradualgrowth to rapid exponential growth.

MEMS devices are on the market now, but this situation willchange markedly in the coming several years because of thewireless revolution.

The final application of microactuators that deserves atten-tion is in the field of microfluidics. Micrometer-scale actuatorshave been used to move pumps and valves in microfluidic sys-tems. The small forces available from most MEMS actuatorslimit the pressures that can be developed and switched and theflow rates, but the performance is adequate for small-scale ana-lytical systems, the so-called lab on a chip.

Integrated MEMS SystemsThe last letter in MEMS stands for systems. This is due partly tothe fact that a MEMS device is quite complex in itself. However,MEMS devices are only components that are used in morecomplex systems. That is, individual sensors or actuators can beused as components and incorporated into larger systems in or-der to perform some useful function. The accelerometer in theair bag sub-system of an automobile and the DMD in a projectionsystem in a theater are examples. However, it is also possible toclosely couple both MEMS sensors and actuators into miniaturesystems all on one substrate. These are called systems on achip. Microfluidics with all the needed functionality on a sub-strate, including pumps and valves, as well as channels, mixers,separators and detectors, are under development for compactanalyzers. These will be relatively evolutionary advances overcurrent microfluidic chips. High-density data storage systemswith both actuation and sensing functions represent a more rev-olutionary example of integrated microsystems.

In the last decade, it was shown that scanning instrumentswith very sharp tips could modify surfaces, as well as make imagesof them, both on an atomic scale. The techniques involve the useof scanning tunneling or atomic force microscopes, so calledproximal probes. The ability to make a dot of several atoms of amaterial (a bit) at a location on a surface raises the possibility ofproducing integrated MEMS devices that could store terabits ofinformation on a square centimeter. Such a density would exceedthe expected capabilities of magnetic storage. Hence, MEMS tech-nology holds the possibility of displacing part of the gianthard-disk market. Thousands of fine tips would be employedwithin a square centimeter. Each tip would be mounted on amicromechanical mechanism that can be independently movedin three dimensions, as sketched in Fig. 16. Tips would first writebits and later sense their presence or absence, analogous to thefunction of the magnetic head in a disk drive. The individual bitswould have dimensions and spacings of a few atom diameters.This nascent technology might be called nano-cuneiform afterthe production of marks in clay tablets by people in Mesopotamia(at about 100 to 1000 bits per square centimeter!). Methods forerasing bits are being developed, so the MEMS data storage de-vices would be comparable in behavior to current magnetic me-dia. This approach to data storage is being pursued by a few othercompanies in addition to IBM. The initial MEMS data storageproducts would offer densities near 30 Gb per square centimeter.It is interesting that, while an entirely new MEMS technology isbeing developed to displace magnetic disk drives, MEMS actua-tors are being incorporated into the arms within such drives to


Top View

SignalPath

Membrane

Dielectric

LowerElectrode

UndercutAccessHoles

Cross Section

High Resistivity Silicon

Buffer Layer

Post

Dielectric Electrode

15. Micrograph (top) and schematic of the Raytheon MEMS micro-wave switch. The electrode under the flexible membrane is the actua-tor. The capacitance of the switch varies from near zero (open) to 3.4

pF (closed). The signal path is about 50 m wide [21].

2-D Cantilever Array Chip Multiplex-Driver

x

z3

z2

z1y

Storage Mediaon xyz Scanner

100 m

16. Millipede technology being developed by IBM Zurich for datastorage. Top: Schematic of the electromagnetically actuated stageunder an array of microcantilevers with fine tips for recording and

reading nanometer-scale bits in the storage medium. Bottom: Photo-micrograph of the microcantilevers [22].

give finer control of the read-write heads, which decreases thetrack spacing and increases storage density.

SummaryThe variety of commercially available MEMS and their applica-tions have both increased dramatically in recent years. The pro-duction of MEMS is now a multibillion dollar industry, withabout 100 million devices marketed annually. While this indus-try grew out of the microelectronics industry, it is more complexin many important ways. Most fundamentally, it requires the in-tegration of both microelectronics and micromechanics. ManyMEMS involve several closely coupled mechanisms, some ofwhich behave differently on the micrometer spatial scale than onfamiliar macroscopic scales. This complicates both the designand simulation of MEMS. So also does the much wider variety ofmaterials and processes used to make MEMS, compared to mi-croelectronics. Because many MEMS have to be open to the at-mosphere, their packaging, calibration, and testing are allcomplex. Questions about the long-term reliability of MEMS arebeing answered as MEMS devices spend more years in use byconsumers and industries.

Despite such engineering challenges, MEMS offer high per-formance and are small, low power, and relatively cheap. Theyboth improve on some existing applications and enable entirelynew systems. Some of their applications are targeted from theoutset of design, but others are opportunistic. That is, the largenumber of MEMS components on the market make them avail-able to design engineers for a very wide variety of uses.

It is noteworthy that many large companies have strong po-sitions in the manufacture and use of MEMS. Analog Devices,IBM, and Texas Instruments were mentioned above. Being arelatively new field, MEMS technology has also been the basisof many new companies, a significant fraction of which have re-cently been bought by larger companies, especially in the opti-cal networking industry.

The entire field of MEMS has turned the corner in recentyears from gradual growth to rapid exponential growth. MEMSare not likely ever to be so important as integrated circuits. How-ever, they will be a part of the fabric of life in the new century,embedded in many products owned by most people in techno-logical societies.

AcknowledgmentsThis work was sponsored by the MEMS Program in theMicrosystems Technology Office of the Defense Advanced Re-search Project Agency (Contract N66001-98-1-8923). Figureswere generously provided by T.B. Zaban, R&D MagazinesMicromachined Devices Newsletter, a Cahners Publication (Fig.1); Kaigham Gabriel of Carnegie Mellon University (Fig. 2);Bernhard Weigl of Micronics, Inc. (Fig. 7); Barry Cole ofHoneywell, Inc. (Fig. 8); James Knutti and Rick Hagen of SiliconMicromachines, Inc. (Fig. 9); Robert Sulouff of Analog Devices,Inc. (Fig. 10); Larry Hornbeck of Texas Instruments, Inc. (Fig.12); Noa Rensing of MicroOptical Engineering Corp. (Fig. 13);

Saswato Das of Lucent Technologies, Inc. and Laurie Grace (Fig.14); and Charles Goldsmith of Raytheon Company (Fig. 15).

David J. Nagel and Mona E. Zaghloul are research professor andprofessor, respectively, in the Department of Electrical andComputer Engineering at The George Washington University inWashington, DC. They are members of the Institute for MEMSand VLSI in the School of Engineering and Applied Science(E-mail: [email protected] and [email protected]).

References1. S. Marshall (ed.), MicroMachine Devices Newsletter, vol. 5, no. 12, p. 12,

Dec. 2000.

2. K.J. Gabriel, Engineering microscopic machines, Scientific American:Technology in the 21st Century, vol. 273, no. 3, pp. 150-153, Sep. 1995.

3. M. Madou, Fundamentals of Microfabrication. Boca Raton, FL: CRC Press,1997.

4. G.T.A. Kovacs, Micromachined Transducers Sourcebook. Boston, MA:WCB McGraw-Hill, 1998.

5. N. Maluf, An Introduction to Microelectromechanical Systems Engi-neering. Boston, MA: Artech House, 2000.

6. S.D. Senturia, Microsystem Design. Boston, MA: Kluwer, 2001.

7. http://www.memscenter.com/memsc, http://mems.isi.edu/, andhttp://home.earthlink.net/~trimmerw/mems/Stroud_Dbase.html

8. D.J. Nagel, Design of MEMS and microsystems in Design, Test andMicroFabrication of MEMS and MOEMS, B. Courtois et al (Eds.), SPIE vol.3680, pp. 20-29, 1999.

9.H.C. Nathanson et al., The resonant gate transistor, IEEE Trans. ElectronDevices, vol. ED-14, no. 3, pp. 117-133, 1967.

10. J. Marshall, M. Gaitan, M. Zaghloul, D. Novotny, V. Tyree, J-I. Pi, C. Pina,and W. Hansford, Realizing suspended structures on chips fabricated byCMOS foundry process through the MOSIS service, NISTIR 5402, June1994; and C. Zincke, Microelectromechanical heating element structurecharacterization and control, Masters thesis, The George WashingtonUniversity, Oct. 1995.

11. http://www.mems-exchange.org

12. http://www.micronics.net/

13. R.A. Wood et al., High-performance uncooled microbolometer focalplanes, presented at the 1993 IRIS Detector Conference.

14. http://www.si-micro.com/

15. http://www.analog.com/

16. L. Sprangler and C.J. Kemp, ISAAC-integrated silicon automotive accel-erometer, in Tech. Dig. 8th Int. Conf. on Solid-State Sensors and Actua-tors (Transducers95), Stockholm, Jun. 1995, pp. 585-588.

17. http://e-www.motorola.com/sensors/index.html

18. L.J. Hornbeck Current status of the Digital Micro-Mirror Device (DMD)for projection television applications in Tech. Dig. Int. Electron DevicesMeeting, Washington, DC, 1993.

19. http://www.microopticalcorp.com

20. D.J. Bishop, C.R. Giles, and S.R. Das, The rise of optical switching, Sci-entific American, Jan. 2001, pp. 88-94.

21. C.L. Goldsmith, Z. Yao, S. Eshelman, and D. Denniston, Peformance oflow-loss RF MEMS capacitive switches, IEEE Microwave and GuidedWave Lett., vol. 8, no. 8, pp. 269-271, Aug. 1998.

22. http://www.zurich.ibm.com/st/storage/concept.html and P. Vettiger etal., The Millipede - More than one thousand tips for future data storage,IBM J. Res. and Devel, vol. 44, no. 3, pp. 323-340, May 2000. CD


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