challenges in commercializing mems

T H E M E A R T I C L E

44 IEEE COMPUTATIONAL SCIENCE & ENGINEERING

Challenges in Commercializing MEMS

ERIC PEETERS

Xerox Corporation

♦ ♦ ♦

Several microelectromechanical systems have achieved commercial success. Thebarriers can still be formidable, though, and the path to success is often much

different for MEMS than it was for mainstream semiconductors. Maturing softwarefor comprehensive modeling and design will help in the future.

♦

Adapted with permission from Frontiers of Engineering: Reports on Leading Edge Engineering from the 1996 NAE Symposium on Frontiers of

Engineering. Copyright 1997 by the National Academy of Sciences. Courtesy of the National Academy Press, Washington, D.C.

After initial hype in the early 1980s, a return torealism in the late 1980s, and persistence in theearly 1990s, the field of microelectromechani-cal systems, or MEMS,1 has entered an era of

practical application. The largely academic research ofyesterday is now unmistakably making the transition intoan ever-growing number of industrial and commercialventures.

The entire field of MEMS has been enabled by thebatch fabrication methods established in the semicon-ductor industry. We cannot predict the market successof MEMS-based products, however, by blindly applyingthe economy of scale and other economic models gov-erning semiconductor markets. Although the parallelsare both undeniable and enabling, the successful MEMSventure today is likely to be one that focuses on differ-ences from, rather than parallels with, mainstream semi-conductor markets. It is essential to recognize that themain challenges in commercializing MEMS—on boththe business and technical levels—are different from theclassic semiconductor problems.

Business challengesKey business issues in the MEMS field are time-to-mar-ket, market volume, and infrastructure.

The single most important discriminator between a“MEMS mindset” and the semiconductor “VLSI mind-set” is the lack of a MEMS “transistor.” That is, inMEMS there is no generic element allowing one to buildextremely diverse, function-spanning products by de-signing appropriate interconnection patterns among alarge collection of the generic elements. The ideal“VLSI” paradigm holds almost perfectly for digital cir-cuitry. At the highest level of abstraction, digital circuitsoperate in a single Boolean energy domain (out of theseveral energy domains in which a physical device canoperate—electrical, mechanical, thermal, optical, and soon). The system operation is essentially described interms of a single variable (voltage), and there is, in firstorder, no cross-term coupling between the elements viaother domains.

For analog circuitry this paradigm does not hold quiteas well, because a first-order description of the interac-tion between the elementary circuit elements requires asecond variable (current). The one additional variableintroduces the issues of input and output impedances,mutual loading of circuit elements, linearity, and so on,which accounts for the longer design cycle, the slowertime-to-market, and the higher cost per function char-acteristic of analog circuits. The generic elements areless generic and more application-specific than in digi-

.

JANUARY–MARCH 1997 45

tal circuits. Automated design is a routine toolin digital circuits, but it is challenging researchin analog circuits.

These considerations are taken to an entirelynew level in the context of MEMS-based de-vices, and the VLSI paradigm breaks down.MEMS devices operate not only in the electricaldomain, but by definition also in the mechanicaldomain and often in a third or fourth energy do-main. Typically they are also mutually coupledand analog. For all practical purposes there is nogeneric MEMS component and there never willbe. This should not be a show-stopper, but itdoes have far-reaching consequences on time-to-market, market volume, and infrastructure,as well as on which markets can reasonably beaddressed with MEMS-based products.

Time-to-marketBecause the design and, to a certain extent,

the fabrication of MEMS-based products arelargely application-specific, and because at thehighest level of abstraction MEMS products donot comply with the VLSI paradigm, their de-velopment cycle tends to be long. Also, severalof these cycles are typically still required to meetall design specifications, because we do not yethave a complete set of mature simulation toolsfor coupled-domain modeling. The time-to-market of existing MEMS products is thereforeoften reported to be on the order of 10 years orlonger.2 Development costs for such a productcan only be recovered if a large market exists.

Market volumeObviously the economy of scale that rules the

general semiconductor industry also requireslarge-volume markets because of the high infra-structure investment cost. The subtle differenceis that MEMS, unlike mainstream electronics,requires a large market for the specific end prod-uct, not just for the generic technology. This cri-terion is much harder to meet. A useful analogymight be to consider ASICs—application-spe-cific integrated circuits. The market for an ASICis clearly much smaller than that for a more gen-eral-purpose IC like a Pentium chip. In MEMStoday, everything is like an ASIC.

To date, successful MEMS-based products in-clude mechanical sensors for the automotive in-dustry (pressure and acceleration sensors) andthe medical industry (disposable pressure sen-sors), and thermal inkjet printheads for computerprinters. Emerging markets seem to be in opticaldisplays and beam steering and in microfluidics-

handling systems for the medical industry.A common scenario after introduction of

MEMS devices into the mass markets for whichthey were initially developed is cross-pollinationinto a variety of smaller markets such as con-sumer electronics. Examples are the pressuresensors in altimeters, scuba diver wrist watches,and vacuum cleaners, and accelerometers inwashing machines, toys, and golf clubs. Themost viable current approach to entering nichemarkets directly with custom-developed MEMSdevices is to “piggyback” on an established base-line technology, design within the given limitsof that technology, and target such high-marginmarkets as biomedicine or the military. A multi-tude of small MEMS startup companies are pur-suing this avenue.

InfrastructureAgain owing to the inherent application-

specificity of MEMS, a “silicon foundry” con-cept is much more difficult to implement thanit is for mainstream semiconductor work, al-though promising efforts are being undertakenin the United States (DARPA,3 MCNC4), aswell as in Europe (NEXUS)5 and Japan. It isclear, though, that MEMS foundry processeswill never reach the same levels of generality andbreadth of applicability as the standard semi-conductor foundries. In the foreseeable future,the more successful independent MEMSfoundries are likely to be those focusing on aparticular class of devices or applications (suchas inertial sensors or fluidics components or op-tical actuators) rather than those aspiring towide breadth at insufficient depth. [Elsewherein this issue of IEEE CS&E, Karen Markustreats infrastructure issues for MEMS manufac-ture in greater depth. —Ed.]

For practical purposes MEMS development,unlike custom integrated circuit design, is less ofa “software” activity and much more of a “hard-ware” activity. Most of the added value is real-

In MEMS today,

everything islike an ASIC.

◆

◆

.


ized at locations that have a strong internal hard-ware development capability. The balance mayshift in the future, but the scale is unlikely to tip.

Scientific and technologicalchallengesThe main scientific challenges in the MEMS arenaare associated with material properties. The ef-fective properties of the materials used to pro-duce micromechanical components depend notonly on the material used but also on the way thematerial is deposited, as well as on previous andsubsequent treatments. Often the structuralproperties, such as the modulus of elasticity andbuilt-in stress, depend not only on the depositionparameters but on the actual equipment used andpossibly even the history of that equipment.

Predictability and reproducibility can be seri-ous issues depending on the design and materi-als choices. They are typically less of a concernfor single-crystal silicon microstructures becauseof the inherent predictability of this material, butremain important concerns for polysilicon, metal,or polymer structures. Single-crystal silicon istypically the structural material in “bulk micro-machined” parts,1 which are fabricated from the“bulk” of a silicon substrate by deep, crystallog-raphy-dependent wet etching1 or, more recently,by deep reactive ion etching (RIE).6,7 “Deep” inthis context means up to several hundred mi-crons, often all the way through a silicon wafer.

Polysilicon is typically the structural materialin “surface micromachined” parts,1 which arefabricated using sequential deposition and pat-terning of thin “sacrificial” and “structural” films,and subsequent selective chemical dissolution ofthe sacrificial material, leaving the thin structuralmaterial suspended above a substrate. “Thin” inthis context means up to a few microns thick.

Metal is typically the structural material in elec-troformed microparts,8 which are most often fab-ricated by electroplating nickel or copper into

thick, exposed and developed photoresist molds.The parts can be up to tens of micron high usingUV-based lithography and up to several hundredmicrons high using X-ray lithography.

The material issues vary widely, from, for ex-ample, dielectrics-induced stress in crystallinestructures, to deposition-induced stress gradi-ents in polycrystalline structures, to stress fromnonuniform current distributions during theelectroforming of metal parts. Regardless of thespecifics of the fabrication technology used, tol-erant design is currently the best defense. A tol-erant-design strategy can include avoiding mi-crocomponents clamped on more than one end,avoiding dielectrics on stress-sensitive areas, andthe inclusion of stress relief geometries.

The main technological challenges in MEMSare related to multidomain optimization and topackaging. The coupled-domain operation thatis so characteristic of these devices is not only aconcern in MEMS design, but also in fabrica-tion. Combining electrical functionality withmechanical functionality in a single compo-nent/structure/material requires compromisingbetween optima in different domains. For in-stance, the optimum deposition conditions for“electrical” polysilicon can differ from the op-timum conditions for “structural” polysilicon.In general, this has the effect of narrowing the“process windows” in which all variables of thefabrication process simultaneously satisfy the re-quirements for the device being produced.Sometimes no window can be found.

In most practical cases however, it is the pack-aging of MEMS devices that is by far the maintechnological challenge. Packaging is not a triv-ial matter since the primary purpose of the pack-age, which is protecting the MEMS chip (usu-ally called a die) from the environment, appearsto be in direct conflict with the purpose of thedie, which is sensing or actuating the environ-ment. These interactions with the environmentmust be understood in a mechanical, chemical,thermal, optical and/or other sense, dependingon the energy domains the MEMS device wasdesigned to operate in. In addition, the packag-ing is highly application-specific and often doesnot benefit from the economy of scale, as it isgenerally not based on batch processes. There-fore, with few exceptions, packaging dominatesMEMS production costs. This is a strong in-centive to incorporate as many “packaging”functions as possible in the MEMS die itself,rather than in the package encapsulating it. Ex-amples of “packaging” functions would be over-

An important obstacle in MEMSdesign is the lack of

mature coupled-domainmodeling tools.

◆

◆

.

JANUARY–MARCH 1997 47

range protection, stress decoupling, chemicalpassivation, electrical shielding, and connectors.

Currently, an important obstacle in MEMS de-sign is the lack of mature coupled-domain mod-eling tools. This keeps multiple-iteration “trialand error” design, as well as multiple-iterationprototyping methods, alive. However, modelingtools are recognized as a high priority for the ad-vancement of the MEMS field, by industry andacademia as well as by the funding sources, andprogress is being made rapidly in this area.

Thermal inkjet case studyA success story concerning the introduction ofMEMS technology in a high-volume market in-volves the disposable printheads in the thermalinkjet printer products now dominating the low-end market for color computer printers.9 Partof the reason for this success is that the prob-lems specific to the application were positionedfavorably with regard to the MEMS businessand technical barriers outlined earlier.

Operating principles and Xeroximplementation

Thermal inkjet (TIJ) printing operates by ap-plying a short electrical pulse to a resistive micro-heater, which then rapidly heats a thin layer of inkat the heater surface. During heating, the ink incontact with the heater surface superheats and avapor bubble is nucleated. As the bubble grows,it transmits momentum to the surrounding fluidand ejects ink out the channel nozzle in the formof a well-defined drop. After the completion ofthe heating pulse, the vapor bubble collapses andink refills the channel from the ink reservoir.

Xerox Corporation’s TIJ implementationconsists of bonding a bulk micromachined sili-con channel wafer to a MOS heater wafer withan intermediate polyimide spacer layer, asshown in Figure 1. The channel wafer containsan array of bulk micromachined fluid-flow chan-nels, local ink reservoirs, and ejector nozzles.The heater substrate is a MOS wafer that con-tains polysilicon heater elements, power drivers,and addressing logic.

Business challengesThe large market was “bootstrapped” in this

case by the introduction of color printers thatgave unprecedented performance for their cost.MEMS technology was the enabler for the prod-uct that created the large-volume market re-quired to sustain the MEMS investment. The

market volume has been further sustained overtime because the TIJ printheads are disposable:the MEMS components are embedded in con-sumable supplies. In addition, the conventionaltechnology with which TIJ was competing in thelow-end market—black and white dot-matrixprinters—was clearly far inferior, and inexpen-sive color printers were practically nonexistent.

Time-to-market tends to be reasonable in TIJapplications because the heater wafers can readilybe piggybacked on slightly modified baselineMOS processes and because the fluid-flow path-ways in the channel wafer are relatively uncom-plicated micromachined parts. In addition, thefirst-order coupling between domains (electrical,thermal, fluid dynamics) is fairly well understoodin this case, and mature thermofluidic simulationtools are commercially available. The MEMS-specific infrastructure investment can stay mod-erate because the application does not demandstate-of-the-art submicron line widths or featuresizes on the chip. (The smallest TIJ features aremore on the order of 10 microns.) Depreciatedmanufacturing process lines can be revived andcan easily deliver another full life cycle of service.

Technical challengesImportant material-related challenges in TIJ

are cavitation (pitting caused by the collapse of

Addresslead

Channelwafer

Spacer

Heaterwafer

CommonThin filmresistor

MOS substrate

Figure 1. Thermal inkjet printheads are a success story in the com-mercialization of MEMS devices. The implementation developed byXerox Corp. is shown here.


bubbles) and corrosion. Without protectionfrom cavitation, the lifetime of a polysilicon filmis very limited when exposed to the cyclic andfocused pressure pulses up to 100 atm that oc-cur during vapor-bubble collapse. A tantalumbarrier is deposited over the poly heaters forcavitation protection. Other customizations ofthe heater wafer baseline process satisfy re-quirements of thermal efficiency, heater stability,and protection from the ionic ink environment.Thermal efficiency is achieved through athicker-than-usual field oxide to avoid thermallosses to the substrate, and the MOS circuitry isprotected from the ionic ink environment withan extra polyimide passivation film.

The ink cartridge (the packaging) typicallydominates the cost of disposable TIJ printheads.One approach to reducing packaging and inter-connect expense is to integrate addressing andmultiplexing logic onto the ejector chip to min-imize the number of external connections. Thisis the option Xerox decided to use, especially be-cause it also makes the number of external con-nections essentially independent of the arraysize, readily allowing for increasing the numberof ejector nozzles per chip, thus increasing theprint speed.

To successfully introduce MEMS-basedproducts into the marketplace, we must

recognize that the main challenges, both on atechnical and a business level, are sufficientlydifferent from the classic semiconductor prob-lems to require a new basic mindset and ap-proach. As in other industries, computationalscientists and engineers will be able to play animportant role in overcoming these challengesas part of the overall effort to commercializeMEMS. ♦

AcknowledgmentsThe Xerox TIJ printheads presented as a case study weredeveloped through joint effort of J. Becerra, C. Burke, N. Deshpande, D. Drake, A. Fisher, B. Hawkins, D. Ims,G. Kneezel, K. Kubby, M. O’Horo, J. O’Neill, T. Orlowski,R. Proano, I. Rezanka, T. Tellier, S. Vandebroek, I. Vitomirov, and many other members of the XeroxWilson Center for Research and Technology and theXerox Ink Jet Supplies Business Unit, Webster, N.Y.

References1. K. Petersen, “Silicon as a Mechanical Material,”

Proc. IEEE, Vol. 70, No. 5, 1982, pp. 420–457.2. S. Walsh et al., “Infrastructure Considerations for

the Emerging MEMS Markets,” in Micromachiningand Microfabrication Process Technology, K.W.Markus, ed., SPIE Proc. Vol. 2639, SPIE, Belling-ham, Wash., 1995, pp. 114–123.

3. WWW: http://eto.sysplan.com/ETO/MEMS/index.html.

4. WWW: http://mems.mcnc.org/.5. WWW:http://www.vdivde-it.de/it/NEXUS/

NEXUS01.html.6. D. Craven, K. Yu, and T. Pandumsoporn, “Etching

Technology and Applications for ‘Through-Wafer’Silicon Processing,” in Micromachining and Micro-fabrication Process Technology, K.W. Markus, ed.,SPIE Proc. Vol. 2639, SPIE, Bellingham, Wash.,1995, pp. 258–263.

7. J.K. Bhardwaj and H. Ashraf, “Advanced SiliconEtching Using High Density Plasmas,” in Micro-machining and Microfabrication Process Technology,K.W. Markus, ed., SPIE Proc. Vol. 2639, SPIE,Bellingham, Wash., 1995, pp. 224–233.

8. H. Guckel et al., “Fabrication of Assembled Mi-cromechanical Components via Deep X-ray Lith-ography,” Proc. IEEE Workshop on Micro Electro Me-chanical Systems 1991 [Nara, Japan], IEEE, NewYork, 1991, pp. 74–79.

9. T. Courtney et al., “Print Element for Xerox Ther-mal Ink Jet Print Cartridge,” in Color Hard Copy andGraphic Arts III, J. Bares, ed., SPIE Proc. Vol. 2171,SPIE, Bellingham, Wash., 1994, pp. 126–130.

Eric Peeters is a member of the research and technol-ogy staff at the Xerox Palo Alto Research Center in PaloAlto, California. He is responsible for development ofmicroelectromechanical systems technology for appli-cation in commercial document output products. For-merly he was affiliated with the Xerox Wilson Centerfor Research and Technology in Webster, N.Y., wherehe was a member of the thermal inkjet developmentteam. Peeters received the PhD in electrical engineer-ing from the Katholieke Universiteit Leuven, Belgium,for work on process development for 3D silicon mi-crostructures, with application to mechanical sensingdevices. He has written over 30 publications in the ar-eas of silicon micromachining technology, electro-mechanical design of micromachined devices, MEMSCAD, mechanical sensors, sensor interfacing, and bio-medical applications. He is a member of the IEEE.

Peeters can be reached at Xerox PARC, 3333 CoyoteHill Rd., Rm. 1210, Palo Alto, CA 94304; e-mail,[email protected].

.

challenges in commercializing mems

Documents