Ž .Sensors and Actuators 72 1999 1–15

Invited Paper

Reflexions on the future of microsystems

Walter Lang )

( )Hahn-Schickard-Gesellschaft— Institute of Micromachining and Information Technology HSG-IMIT , Wilhelm-Schickard-Str. 10,78052 Villingen-Schwenningen, Germany

Received 27 April 1998; revised 15 June 1998

Abstract

The development of a new technology like microsystems is not only a technological step, but also a process of the society incorrelations with many fields of our personal and social life. This paper has the intention of looking at the dynamics of microsystems inthe field put up by technology, economy and society. The following are the three main questions: 1. What are the typical features ofmicrosystems? Where are the advantages and drawbacks? 2. What is known about the microsystems market and its dynamics? Publishedmarket studies are critically reviewed. 3. What are the most important prerequisites for application? Four main topics are identified:availability of production, performance, reliability and cost. q 1999 Elsevier Science S.A. All rights reserved.

Keywords: MEMS; Microsystems; Market; Future

1. Micro electromechanical systems: what it is and whyit is made by us

1.1. Why micro electromechanical systems: demands andpossibilities

The essential of microsystems can be taken directlyfrom the term itself. Two ideas are combined within thisnew technology: the technology of micro structuring andthe system approach of design.

Microsystem technology is the response of the techno-w x 1logical development to a certain need of the society 1,2 .

Fig. 1 shows some important influences: On the left side ofthe figure the challenge is shown, on the right hand sidethe technological possibilities are enumerated.

1.1.1. The challenge: technical demandWhat is the demand of the society concerning new

technologies? At the moment, a large part of the world hasreached a high standard of living combined with an exten-sive exploitation of resources. It is a highly ranked goal inthe technologically developed countries to stabilize and toimprove this standard. In those parts of the world which

) Tel.: q49-7721-943-256; Fax: q49-7721-943-210; E-mail:walter.lang@ imit.uni-stuttgart.de; Internet address: http:rrwww.imit.uni-stuttgart.de

are less rich the most important goal is to increase thetechnological standard in order to reach a situation compa-rable to the one in the first world. Nevertheless, theconsumption of resources can hardly be further increasedwithout causing serious damage to the ecosphere. Fromthis follows that the necessary development can only bemade by an increase of efficiency. Therefore, new techno-logical systems have not only to be more powerful thanexisting ones, but at the same time they must consume lessresources.

What does low consumption mean? If we say, MEMSis concerned with small systems and microsystems, weactually apply a generalized idea of smallness which hasmany facettes:1. General reduction of mass and size.2. Low production cost.3. Low energy consumption.4. Small amount of material consumed for manufacturing.5. Avoiding of expensive, rare or environmentally harmful

material.6. Easy disposal.7. Long lifetime.8. Few and standardized means of communication in order

to avoid complicated wiring and periphery.

1 A number of papers on the future of MEMS is given in the specialŽ .issue of Sensors and Actuators A 56 1996 1–197.

0924-4247r99r$ - see front matter q 1999 Elsevier Science S.A. All rights reserved.Ž .PII: S0924-4247 98 00205-2

( )W. LangrSensors and Actuators 72 1999 1–152

Fig. 1. Influences on the emerging MEMS technology.

There is a great demand for new systems which aresmaller in consumption and, at the same time, more pow-erful in performance. The decision wether MEMS is theappropriate technology for a planned products is takenlooking at the overall net profit taking into account allthese factors.

More and more technical systems are operated close toa theoretical optimum. An example is the motor manage-ment of automotives concerning exhaust. A new enginewhich produces more exhausts than inevitable is no longeraccepted by the society. The laws become more and morerestrictive, sometimes they are even in advance of techno-logical state of the art. That way, the society pushesforward the technological development. Other examplesare house heating and thermal isolation. In order to operatea system in an optimized way, a considerable effort inmeasurement and control is necessary. Therefore, a secondimportant observation about the technological demand maybe stated: There is an increasing need for small andpowerful systems for sensing, analyzing and actuating.This is one reason why at the moment sensorics is a fieldwhich shows strong development dynamics. There is aclear tendency towards intelligent sensors and even com-plete analytical systems. In order to effect control not onlysensors are needed but also actuators. Also in actuators animpressive activity can be observed, but at the moment thishappens more in research and development rather than inproduction. Looking at the development at the moment wemay expect that the production of actuators may soonshow even stronger dynamics than sensors do today.

1.1.2. The response 1: technological possibilitiesWe have discussed the demands on a new technology,

now it is time to have a look at the right hand side of Fig.1. What has the technological world at hand to work out anadequate response to these needs? Of course, there is alarge number of different technologies, ideas, visions and

possible interaction. Among these the synergy of twoelements seems to be very promising with respect to thespecific demands: the first one is an outstanding manufac-turing technology, the micro fabrication technology whichhas been developed for microelectronics, the second one isan important new vision and development idea: the systemapproach.

1.1.2.1. Structuring technologies for microelectronics. Thetremendous change of the technical world during the last20 years was caused to a large degree by the developmentof microelectronics and silicon technology. At the mo-ment, the technical world is largely determined by thesesystems and it is expected that the application of siliconmicroelectronics will keep growing in the next decades.

For the production of integrated circuits, a number oftechnologies have been developed or significantly im-proved.

1. Crystal production: Silicon single crystals are avail-able with high purity, large crystal size and at a relativelylow price.

2. Thin film technology: Thin films for metallization,insulation and a number of functional purposes can bedeposited on silicon substrates with standard technology.

3. Lithography and etching: Silicon and thin films canbe structured by wet chemical etching and by dry etchingusing plasma and ion beam methods. For microelectronics,structure sizes of 0.25 mm are realized in full production.

4. Modeling: The function of microelectronic devices isprecisely predicted by modeling tools. A new microelec-tronic product is developed by modeling, the first silicondevice has to work. This is one of the main reasons for therapid development of microelectronics.

5. Characterization: Quality control is one of the mostimportant parts of production. Automated tools for electri-cal characterization and eventual trimming on the waferlevel are available.

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These technologies have been used for microelectronicswith great success. Why should they not be used for otherdevices like sensors, actuators and microstructures? Microparts in silicon are not restricted to electronic interaction,all other physical interactions work as well and may haveuseful applications. For example, like any material, a pieceof silicon is deformed by mechanical forces. This can beapplied for mechanical sensors: if a membrane is made, itwill be deformed by pressure; if a mass is suspended onbridges, the suspension is deformed due to acceleration ofthe device. In order to realize a pressure sensor or anaccelerometer, the deformation must be measured. Theway to do this was paved when the piezoresistive effectwas discovered and investigated and soon pressure sensorsand accelerometers were realized. For sensor developmentand production this approach shows some very interestingfeatures: well developed material and technology can beused, due to the manufacturing in full wafer technology thedevices can be made at low cost if large numbers areproduced.

In order to realize the small measurement and controlsystems described the systems have to combine sensors,actuators and electronics. This is another strong argumentfor the use of the technologies developed for microelec-tronics: the system integration. The methods developed toproduce electronic systems like, e.g., chip on board tech-nology or multichip modules can as well be used tointegrate electronic chips together with micro mechanicdevices.

1.1.3. The response 2: the full system approachEven more important for MEMS are technologies for

assembly, interconnection, housing and system integration.Microsystem technology means particularly an assemblyof systems. Here, a large number of powerful technologieshas been developed for the production of electronic sys-tems. The most important developments are given below.

A large number of bonding and joining techniques isapplied. The classical technology is soldering housed com-ponents on printed circuit boards. For higher integration

Žcomponents with special small housings are applied SMD:.surface mount technology . It is also possible to place

‘naked’ silicon dies like electronic components or mi-crosensors directly on the substrate. This technology is

Ž .known as ‘chip on board’ COB . It allows the highestdegree of integration, i.e., the smallest hybrid systems.

ŽOne method uses chip supports TAB: tape automated.bonding . Other technologies put the chips directly on theŽ .substrate DCA: direct chip attach . Electrical intercon-

Žnects can be made by wire bonding Chip and Wire.Technology . Another way of interconnecting uses bumps

to perform direct interconnection between the chip, whichŽis placed face down, and the substrate Flip Chip Technol-

.ogy .Mounting and placing of components on the substrate is

fully automated including process control by visual obser-

vation or electrical measurement. The same holds for wirebonding.

Concerning substrates, besides printed circuit boards, anumber of new types are used. Ceramic substrates withinterconnects and passive components made by thick filmtechnology are used for hybride integrated electronics. Forcomplex wiring, multilayers are used. New injectionmoulding technologies allow the integration of electrical

Žinterconnects in the moulded plastic part MID: moulded.interconnect devices . Chips can also be mounted on a

plate of silicon which has a structured metallisation forinterconnects. This type of multichip modules is appliedfor large electronic components since a multichip modulehas lower production cost and a higher yield than onesingle overlarge silicon die.

Integration technologies are the technological side ofthe system idea, but there is a theoretical aspect, too:Among science and technology there is a widespreadattitude to apply a system approach in analysis and design.For analyzing a system we have to look at the componentsand at their interaction. During the past decades it turnedout that for complex systems the analysis of the interac-tions reveals new and unexpected phenomena and often isthe essential part for the understanding.

The origin of the system approach is in the scientificresearch considering complex phenomena like thermody-namics far off equilibrium, solid state behavior, biochem-istry, biology and ecology. In technology, system approachmeans a way to plan, to develop and to test new devices.An example is the comparison of a pressure measurementsystem with a classical approach using single componentsand an integrated system. At first a pressure sensor ischosen which could be a metal membrane with thin filmresistors placed on it. This sensor is mounted in a metalhousing which only holds the sensor and no other activeparts. The sensor is connected with some wires to anelectronics. This is made up of standard components placedtogether on an integrated circuit board and is mounted inits own housing. All the single components have beendeveloped and fabricated without any respect to this spe-cial measurement system. They are bought from differentsuppliers according to the specific needs and are combinedto form the system.

The next step of improvement is an integrated pressuresensor. The membrane is placed on a silicon chip, theelectronics is placed on the same chip. A number ofprocess steps like doping, metallization, structuring anddeposition of insulating layers are used simultaneously forthe sensor part and the electronic part. In this case, it is notpossible to plan and develop each single part for itself. Thetechnological process for the piezoresistors on the sensormembrane is developed with respect to the technologyused for the electronics, the same holds for metallizationand insulating layers. Since the electronics is on the samepiece of silicon there will be a thermal interaction whichmust be considered. The sensor is placed on a cube of

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Pyrex glass which is placed in a metal housing. This wayof mounting is chosen in order to minimize thermal stressbetween the silicon and the metal. All the bonding, gluingand soldering processes which are applied for mountinginvolve heating of the sensor and the electronics. Theplaning of the process must consider all these thermal andmechanical crosstalks during fabrication and during opera-tion.

Now it seems that the designer of the integrated systemhas a very difficult job. It is never possible to look at everyfeasible interaction, and soon enough an unforeseencrosstalk will cause lots of trouble in the prototype. How-ever, there is an enormous potential of possibilities to usenew effects and combinations. In any case, for monolithi-cal integration, for hybrid integration and for discretesystems on circuit boards, the system approach can im-prove the quality of the product in several importantpoints.

1. Reliability: Experience shows that interface betweencomponents and subsystems are dangerous points where asystem may fail. In the case of a pressure sensor, thesensor chip itself is generally very reliable. The weak

Žpoints are the mounting e.g., bonding of the sensor chip. Ž .on the holder or the electrical contact e.g., bond wires .

The number of these weak points is reduced by integration.2. Performance and cost: Better adaption of the subsys-

tems on each other allows a better performance of thewhole with the same amount of complexity of the compo-nents. There is no ‘waste’ of component complexity. Animportant point is the adaptation of mounting processes.

3. Volume: Smaller systems save material, energy andresources.

The system approach leads to new methods in severalareas with a close connection to MEMS technology: Sys-tem theory, signal- and information processing, test anddiagnosis, quality management, failure analysis, FMEA,standardization.

1.2. Features of microsystems

1.2.1. An exampleThe general features of technological development can

be observed by looking at the development of the automo-Ž .tive engine: the engines consume less resource fuel , their

Ž .exhaust values get better resource environment and theyŽshould be as cheap as possible in production resource

.money . During the last few years the motor managementhas been improved considerably. At first, the mixing offuel and air was not controlled except for the adjustment ofthe carburetor in rather long intervals. It turned out that theair equivalence ratio is an essential parameter for con-sumption and exhausts. Therefore it must be controlled tokeep its optimum for cold and hot engine, for full powerand for running idle. A sensor has been developed tomeasure the remaining oxygen in the exhausts, this waythe air equivalence ratio is controlled. During driving, the

engine changes its speed range very often, therefore dy-namical control is needed. To do this, a flow sensor is usedwhich measures the air mass flow into the engine. The dataof this sensor are used to control the injected amount offuel. Thermal air mass flow sensors for this purpose havebeen developed on the basis of heated thin wires or asmetallic films on insulating substrates.

An important improvement was obtained this way, butstill car engines did not run in their possible optimum. Thereason why is because the existing sensors are not preciseenough and that they are too slow. Their thermal timeconstant is in the region of 100 ms, which is longer thanone full turn of the engine. It would be better using sensorswith thermal constants around 10 ms as these could re-solve single phases of the cycle like intake and exhaust ofair. The regulations concerning car engines become moreand more strict, not far from this day the use of newsensors will be unavoidable. Therefore, engineers are look-ing for new solutions. Sensors which measure the flowspeed with high dynamical resolution are available on themarket, but they are either not robust enough like in thecase of thin hot wire probes or they are very expensive likelaser anemometers. This is the point where microsystemstechnology enters the stage: the challenge is to construct ameasurement system which is high performing, robust andcan be produced in high numbers at a reasonable cost.

Several institutes and companies have presented mi-w xcrosensors for this purpose 3,4 . Fig. 2 shows a schematic

of such a system. The actual sensing element is a very thin

Fig. 2. An air mass flow sensor as an example for an integratedŽ .measurement system. The silicon chip is sketched in detail lower part .

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Ž .free standing membrane lower part of the figure with anelectrical heater and two thermometers. The sensor ana-lyzes the temperature profile: Due to the flow, the tempera-ture downstream of the heater is higher than upstream.

The membrane is made from silicon nitride, heaters aremade from metal thin films. The film deposition andstructuring are standard methods from microelectronicstechnology, but not the way it is used. In microelectronics,silicon nitride is used for electrical insulation. Within theflow sensor technology it has two very different functions.First, it forms the flat, free standing and extremely thinŽ .-1 mm membrane which supports the sensor structures.This is possible due to the fact that the nitride is mechani-cally very strong. To obtain a flat membrane which doesnot make wrinkles or waves, the internal stress of the layerhas to be controlled. These requirements on silicon nitridefilms are not known to microelectronics. A typical mi-crosystems technology situation: the material and the pro-cesses are known from electronics technology, but thematerials have to fulfil new requirements which did notplay a major part when the processes were initially devel-oped.

In order to set the membrane free, the silicon below it isremoved using a wet chemical etching process. This is aspecial microsystems process not known to microelectron-ics. While microelectronics technology sticks to the sur-face of the wafers, microsystems technology has to realize3-dimensional structures. There are several ways to do thiswhich have been developed specially for micro mechanics.In addition to the wet chemical etching used here, dryetching by plasma processes recently became more and

w xmore important 5 .The silicon chip described so far is only a small part of

the sensor system described above. Other parts are theelectronics, the housing and the interconnects. The elec-tronics is integrated directly near the sensor. The reasonwhy this is done will be discussed in the following chapter.In the case described here, the electronics is placed on asubstrate together with the sensor. The ceramics holdsseveral layers in thick film technology. Here again, welldeveloped technologies are applied. Hybrid integration ofelectronic components with ceramical substrates and thickfilm technology was developed for the integration of elec-tronic systems with special requirements on reliability andhigh frequency. For microsystems integration, the technol-ogy is adapted and some new steps are developed, espe-cially the processes to mount the sensor chip itself bygluing, soldering or other bonding techniques.

The next important part is the housing. In microelec-tronics the housing has to provide the mechanical protec-tion and has to hold the electrical interconnects. Concern-ing the flow sensor there are more requirements.

1. A flow sensor is by its nature exposed to the medium.The housing has to allow the flow to reach the sensormembrane, simultaneously it has to protect the sensorelectronics.

2. The sensor is mounted somewhere in the front of theengine near to the injection system. This is a roughsurrounding, where temperature drifts occur, pollution offuel and oil is present and mechanical impacts may hap-pen. In order to survive under these circumstances, thesensor housing must be mechanically solid.

3. The flow passes through the sensor system. Thehousing is a part of the flow channel. The form of the flowchannel around the sensor element is very important forthe measurement: The hot film sensor actually measures aflow speed at its surface. It must be guaranteed that thereis a stable correlation of this local flow to the overall massflow in any event. It is important to know about turbu-lence, vortices and other flow phenomena. The fluid dy-namical layout of the flow channel is one of the crucialpoints for the application of the sensor.

Experience of sensor development projects shows thatthe housing and the mounting technology determine thelong term stability and the production cost to a largedegree.

In the case of the sensor system described here thehousing is made by injection molding. This technologyallows a great freedom of design and is not expensive ifproduced in large quantities.

The example shows that, with respect to microelectroni-cal devices, microsystems are very heterogeneous. Herethe system idea has to be considered again: Only if thesensor is planned, laid out and produced as a whole systemincluding sensor chip, electronics and housing it will beeffective in practical application. This shows the impor-tance of modeling for microsystems and the specific prob-lems involved. It is rather straightforward to model themechanics using finite elements, the same applies forthermodynamics while the fluid part is more complex to beperformed. There are very reliable tools for the electronicsas well. The most important task is the interdependencies.Of course it would be possible putting all interactions intoa model but then it cannot be solved any more within areasonable computing time. Therefore it is the task of thenumerical analyst to identify the important interactions andto find a way how to calculate them. Important for thispurpose are system simulating tools.

1.2.2. Integration— why and how?There are many sensor systems which have a sensor in

a housing close to the measurement point and a secondhousing for electronics which is connected to the sensor bywires. What is the reason for have the sensor chip and theelectronics all in one device? Actually there is a number ofgood reasons and the possibility of perform this type ofintegration is considered to be one of the most importantadvantages of microsystems technology.

1. The electric signals of the sensors are often verysmall and frequently voltage differences of mV or currentsof mA are measured. If these weak signals are conductedvia cables, electromagnetic disturbances occur which are

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as large as the signal. In integrated systems, the signals areamplified into an easy-to-handle regime of, e.g., 0–5 V or

Ž4–20 mA or they are coded in the time regime frequency.analogous, impulse width modulation . That way, shielding

of the interconnects is less problematic.2. After processing, chips do not show exactly identical

parameters. The sensitivities of several sensor chips arealways a little different. Requirements on measurementaccuracy are very strict, generally a predefined sensorcharacteristics has to be kept with deviations of 1% or less.In order to achieve this, the singular sensors have to becalibrated individually. Of course, nobody wants to cali-brate a flow sensor within the car engine. When a sensor ismounted, it has to work immediately without any prob-lems. This is achieved by calibrating the single sensorchip. The individual data are stored in the electronics ofthe system using non volatile memory. In the case of apressure sensor, usually four data have to be evaluated andstored: The sensitivity, the baseline offset and their tem-perature coefficients. The way these data are evaluated forthe single sensor is very critical: since calibration is aserial process, it takes time and money. The earlier in theprocess calibration occurs, the less it costs. As much aspossible is done on the wafer level when a fully automatedwafer prober can measure one chip after the other in acomparatively short time. When planning the production ofa microsystems, the strategy of calibration is a criticalpoint determining production cost.

3. Many sensors are active systems, in the example themembrane is heated by electrical current. This heatingmust be controlled, which can be better done by usinglocally an intelligent system.

4. The number of external interconnects is very critical.They make the mounting complex, they generate cost andthey are vulnerable. The best thing to do is to connect thesensor with just 2 or 3 wires only or to make it communi-cate directly with a bus system. In a complicated systemlike a car, where a large number of data of very differentkinds is transported, a reduction of the number of cablesand standardization is important.

5. Integration saves cost and effort for an extra electron-ics housing. Therefore, it may often be the cheaper solu-tion.

Now we know the reasons why we want to build anintegrated system, but how can it be performed? There aretwo basic strategies: Monolithic integration and hybridintegration. In monolithic integration the sensor and theelectronics are located on the same silicon chip. In hybridintegration, several different components are put togetherin one system as described above in the case of the airmass flow sensor. In the beginning of microsystems tech-nology, a lot of effort was put into the development ofmonolithic systems. Within the last decade, a certain dis-enchantment took place and a tendency to hybrid ap-proaches can be observed. The most important argumentsare the following.

1. The development of monolithically integrated sys-tems is very difficult and expensive. For this reason,monolithic integration is feasible in the case of productswhich are sold in very large numbers. In this case mono-lithical integration may have an economical advantage.The threshold is dependant on the specific production;most sensor producers agree that it is well above 1 Milliondevices per year. At the moment there are mainly twodevices which reach this quantity: standard pressure sen-

Žsors with a range around one atmosphere MAP sensors:. w xManifold Absolute Pressure 6,7 and accelerometers withw xa 50 g range for the airbag 8 .

2. Some devices have to be monolithically integratedbecause the number of interconnects would be too large.This is, e.g., the case for the multi mirror projection devicew x9 . The number of active micro mirrors is in the range of 1Million. To address them, multiplexing has to be used,which needs electronics. For this reason, the device has anactive electronic layer below the micro mirrors.

3. A state of the art CMOS-process uses about 20photolithographic masks, a sensor process around 6 masks.Therefore, if a large number of wafers is processed, themicro mechanical process will be cheaper than the CMOSprocess. In this case, it is better to put single chipstogether, or else valuable area on the expensive CMOS iswasted. When the sensor process is a back end process tothe electronics, the total amount of masks is even risingwhich makes the process expensive and vulnerable. There-fore it may be preferable to use the CMOS process directly

w xfor the sensor structures 10 .4. The development of a sensor process takes time,

when it is finished the sensor production should work forseveral years to justify the development effort. On theother hand, development cycles of electronic processes arefast. In the case of a monolithically integrated sensor, if anew electronic process is started in a CMOS-line, thesensor processes would have to be modified and redevel-oped as well. Otherwise the old CMOS process has to beperformed only for the sensor product which means addi-tional effort in logistics and quality management.

5. Whenever there are yield problems, a hybrid ap-proach has the advantage of only having to remove thediscard and not the whole system.

6. In microelectronics, there is a tendency to moveaway from extremely large chips towards smaller chips.Several of those are integrated on a silicon substrateŽ .multichip modules . Reasons are development cost andyield. These technologies can be applied for integratedmicrosystems as well.

At present more hybrid systems are being developedand manufactured than monolithically integrated systems.In 1997 approx. 8% of the pressure sensors and approx.12% of the accelerometers were monolithically integratedw x11 .

In any case, for the choice of the appropriate technol-ogy it is very important to realize that as an applied system

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technology, MEMS particularly needs a problem and prod-uct oriented approach, not a technology oriented one. Thebasic idea of microsystems is to chose from the largeamount of different technologies the one which is mostappropriate for the desired product from a pragmatic levelconcerning manufacturability and to integrate it in a sys-tem. Important technologies for microstructuring are notonly silicon micromachining, but also mechanical micro-

Žmachining, high aspect ratio microstructuring Advanced. Žlithography, LIGA and replication methods galvanoform-

.ing, injection moulding, hot forming .

2. Market studies

At the moment there is internationally a large activity inthe research and development of MEMS. Within the lastdecade a lot of money from governments and privateindustry has been spent. There is an emerging market, too:Pressure sensors, accelerometers, thermopile radiation de-tectors and inkjet printing heads are products with a con-siderable turnover and profit. For other products, likeactuators, chemical sensors, flow sensors, microrelays andmicro optic devices the market evolution turns out to belater than expected at the very beginning of MEMS devel-opment. This is the reason why a number of authorsseriously have doubts about the future use of MEMStechnology. The use of MEMS, and especially the amountof money and work put into it, are subject to quitecontroversial discussion. To analyze these topics, we haveto look at two questions.

1. What can we conclude from the data we have todayabout the future market for products involving MEMStechnology. We deal with this question in Section 2.

2. What are the necessary prerequisites for a commer-cial success of MEMS? This will be the topic of Section 3.

2.1. Something about market forecasts

For every industrial branch it is essential to get a feelingof how the market situation will develop in the future. Theproblem in making a market study lies in the difficulty todraw conclusions from existing data to be able to makeplans for the future as good as possible. Of course, thistype of forecast is very dependant on the way the data arecollected and interpreted. The most important methods aregiven next.

( )2.1.1. Extrapolation top down approachThe existing market data are plotted vs. the time and

then extrapolated. The idea is that very different marketsŽ .like bananas, railroads, pressure sensors . . . show similarfeatures in their general development. In order to performthe fit, a numerical model for the development must beaccepted. There are two common models: exponential

growth and logistic curves. Top down methods are appro-priate for homogeneous, well established markets and forlong forecast cycles.

(2.1.2. Empirical market inÕestigation bottom up ap-)proach

ŽHere, a large number of market partners suppliers and.users are questioned about their future needs and plans.

The market analysis is made by addition of the expectedvalues for different products. A bottom up approach allowsto analyse the influence of the individual products indetail. Since the MEMS market is neither homogeneousnor well established, a bottom up analysis seems to be theappropriate method. The art of empirical market studies isthe selection of the participants, the writing of a goodquestionnaire and making a critical evaluation.

The basic law of microelectronics development wasw xgiven by Moore 12 . He stated that the density of single

components on a chip would double within 18 month.Microelectronic development has been showing a strictlyexponential growth for more than 3 decades until now! Atthe moment, there is still no indication for a major devia-tion from the exponential behavior. This is important forMEMS: silicon technologists, having in mind the astonish-ing precision of ‘Moore’s law’, might overemphasize theapplicability of exponential extrapolation.

2.2. Specific problems of MEMS market forecasts

There are several problems involved specifically inMEMS which render the situation complicated.

2.2.1. Unclear quantificationIt is not easy to state a clear quantitative question

concerning the future of microsystems. In microelectron-ics, the leading products are memory and processors. Theirperformance is clearly quantified. Not so in microsystems.The leading products are not clear at the moment. Thequestion ‘how many pressure sensors will be manufac-tured?’ is well defined, but not useful for the future ofMEMS in general. Therefore the question normally raisedis the number of all products involving microsystems andtheir turnover. But then, we run into the general problemthat MEMS are not clearly defined. Shall we include inkjetprinter heads, metal membrane pressure sensors or quartzbased accelerometers or not? The answer may change themarket size by several hundred percent!

2.2.2. CompetitiÕe situationSeveral investigators state that there is an important

difference between microelectronics and microsystems duew xto the different situation concerning competition 13,14 .

Concerning information handling systems, except for spe-cial applications, silicon chips are applied. Wherever asystem needs advanced data handling, silicon chips are

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used. Quite likely this fact is a major reason for the longand constant exponential growth of the chip industry.

For microsystems, the situation is essentially different:there are competitive sensor technologies. Pressure sensorsare fabricated on steel membranes, in ceramics or quartz,accelerometers are made in quartz technology. The com-petitors for MEMS are well established technologies witha longer product experience than MEMS, a good marketpenetration and proved lifetime and stability. For MEMS,quite often the situation is not the introduction of a newdevice idea but the replacement of an existing technologyby a new and more sophisticated one.

This does not only make prediction difficult, but alsothe market introduction. Industrial markets normally be-have in a conservative way. The demands concerningstability and quality are very high, especially in automotiveindustry. Therefore, a new technology has to show aconsiderable advantage in performance and in price to beable to win a market share. In chip industry there is pricecompetition only between different suppliers. In microsensors, price competition takes place between the microsensor and other sensor technologies which often have theadvantage of the product already existing. As always, themanufacturer of the second product for a given demandhas to produce cheaper and has to learn faster than the firstone. To replace a mass product like, e.g., an airbagaccelerometer there must be an overall gain including notonly sensor and system cost, but also general traffic,legislation, assurance and political economy aspects.

2.2.3. Technology jumpThere are new devices and features which are not

possible outside the microsystems technology, e.g., micropumps and -valves, micro optic devices and micro mechan-ical devices for data storage. Here, competition is similarto the microelectronics case: these devices are more or lessstanding alone in their specific field. But another problemturns up which is also typical for microsystems. Theapplication of these devices means a technology jump. Thesystems in which they will possibly be used are onlyvaguely known today. One example is microanalysis sys-tems: there is a large demand for distributed local chemicalanalysis with small systems. At the moment the systems donot exist and accordingly there is no market within theexisting technical surrounding. It is clear that they wouldbe applied if they were available. This involves not onlynew technology, but also affords a new organization of thesocial and legal situation. The availability of microanalysissystems will cause a change in the medical treatment andin the organization of analysis and diagnosis. How this willbe done, to which degree and how much time it will takecan hardly be estimated. Similarly, the availability of smalland independent analysis systems for environmental pollu-tion surveillance or for DNA analysis is not just a techno-logical innovation. Most likely the new means of control

will be reflected by new regulations in order to make themuseful for the society.

2.2.4. Definition of system boundaries and pricesMEMS products are components within larger systems.

Generally, the price of the silicon chip is only a very smallpart of the total cost. Therefore it is important which partof the whole product is considered. The price for anaccelerometer silicon chip may be around US$1, the priceof an airbag system around US$100. What figure is used inthe market study? When analyzing the work of differentforecasters, we can find figures between US$2 and US$10for an accelerometer, resulting in very different results forthe total MEMS turnover. For this reason it is very impor-tant to look at the component prices when reading aforecast.

Concerning prices, it is important to know wether thecost of production or the price paid by the costumer iscalculated within a specific study.

2.3. Published MEMS market inÕestigations

During the last years, a number of studies was pub-lished. Fig. 3 shows the results of different studies. Thisreview cannot give a complete overview, but intends toshow the major trends. An overview has recently been

w xgiven in ‘Micro Machining Devices’ 15 .( )In 1994, System Planning Corporation SPC published

w xa study 16,17 covering the years from 1993 onwards withŽ .forecast up to 2000 Table 1 . The methods used by SPC

are interviews with representatives from industry, govern-ment and research as well as marketing and sales person-

w xnel from industry 16 . This study had an important influ-ence on the MEMS community and on other similarinvestigations. Since it was published it is the most fre-quently used and quoted source of information about themicrosystems market. The SPC study shows a very highgrowth rate, reflected by the steep curve in Fig. 3. Lookingat the situation today it seems that the growth rate has to

w xFig. 3. MEMS market studies by different forecasters 15 : SPC: Systemw xPlanning Corporation 16,17 ; SEMI: Semiconductor Equipment and Ma-

w x w x w xterials Intl. 18 ; NEXUS 19,20 ; SRI Consulting 21,22 .

( )W. LangrSensors and Actuators 72 1999 1–15 9

Table 1w xMarket forecast for the year 2000 given by SPC 16,17

Segment

Inertial sensors 2.8 B$ 20%Fluid regulation and control 2.6 B$ 19%Pressure sensors 3.5 B$ 25%Optical switching 2.9 B$ 21%Mass data storage 0.8 B$ 6%Others 1.3 B$ 9%Total 13.9 B$

Ž .All figures are given in B$ Billion $s1,000,000,000$ .For comparison: World semiconductor market in 1996 is 144 B$.

be corrected to a lower value. The market segment distri-bution in the year 2000 is forecasted as given in Table 1.

w xBryzek 11 has analyzed the MEMS market, especiallythe situation of the silicon valley companies. He expects asignificant growth of the existing pressure sensor marketand other emerging applications, his figures are given inTable 2. Especially the non sensing MEMS market whichwas very small in 1995, is expected to grow until the year2005 up to an equal size as the sensors. On the basis of hisdata, assuming a turnover of US$150,000 per employeeand year, Bryzek expects 35,000 new jobs by the year2005 due to the MEMS business.

( )Semiconductor Equipment and Materials Intl. SEMIestimate the world market in the year 2000 to reach

w xUS$9.74 billions 18 . The biggest potential is expected forpressure and inertial sensors.

Recently, the European consortium of Nexus has evalu-ated the world market of microsystems and the position of

w xEurope 19,20 . The methods applied are experts inter-views and extrapolation. Since the material proved to bevery heterogeneous, the authors of the Nexus study investi-gated two kinds of MEMS: Existing microsystems marketsŽe.g., pressure sensors, ink jet heads, hard disk heads,IR-Sensors, visible sensor arrays, accelerometers, magnetic

.sensors, several chemical devices and emerging microsys-Žtems markets e.g., DNA chips, multi mirror devices for

display, micro fluidics components, injection nozzles, mi-croanalytical instruments, coil on chip, inclinometers, lin-ear motors, airbag inflators and switches, electronic nose,atomic force microscope tips, cochlear implants, anticolli-

. Žsion systems, microphones . Looking at the figures Table.3 we see that the existing market covers 97% of the

market in 1996 and 95% in 2002. This shows, that themarket evolution will be driven by the increase of mi-crosystems turnover in the existing fields. The emergingfields will take over long after 2002. Within the existingmarket segments, inkjet heads and hard disk heads are themost important devices, sensor applications are consider-

Ž .ably smaller Table 3b .The figures given by Nexus are much larger than those

given the other forecasters. This is caused by a muchbroader definition of the microsystems and by the applica-tion of the user price. In the NEXUS-study, the value of a

MST product is defined as the ‘‘value associated with itsw xsmallest commercially available component’’ 23 . The

advantage of this procedure is the clear definition of thesystem boundary. The user price is the price in the shopfor a consumer good and the OEM customers price for thecase of OEM. This generates, e.g., a large value of aboutUS$40 for an inkjet printing head. In other studies, ink jetand hard disk heads show less turnover since only themicrostructured parts are included and an assumed produc-tion price is calculated. Both effects together result in afactor of around 10 in turnover.

A much more conservative analysis has been publishedw xby SRI Consulting 21,22 . They apply an end-user needs

based approach taking into account possible competitionwith other technologies. They calculate a worldwideMEMS market of 4.7 B$ in the year 2000. The figuresgiven in Table 4 show that also in this forecast hard discdrives and inkjet printers are the key players. SRIC statethat most other forecasts are over optimistic, mainly due tothe underestimation of the competition with existing tech-nologies. Looking at the figures it strikes that the sensormarket is extremely small. It only shows within the section‘others’, which is US$500 Million in the year 2000.

All market studies emphasize the important ‘leverageeffect’ of microsystems. Microsystems are the core compo-nents of systems. A silicon accelerometer may cost a fewUS$, but the price of an airbag system is, e.g., US$200.The competence in the sensor chip is essential for the gainand defense of the market share of systems. That way, thecompetence in microsystems may control a turnover insystems which is actually more than 10 times higher.

The size of a market is interesting because people wantto earn their living with microsystems. Therefore it isinteresting to estimate the amount of people working onmicrosystems. Nexus calculate a number of 6000 people inEurope. From the turnover the size of the microsystemscommunity can be estimated to be 48,000 people in 1996Ž .USA: 29,000, Japan: 13,000, Europe: 6000 . For the year2002, extrapolation makes a guess of 100,000 peopleworldwide. For the jobs, a leverage effect of a factor ofapproximately 10 can be expected, too.

Table 2w xThe MEMS market according to Bryzek 11

Year

1995 2005

Pressure sensors 1.0 B$ 2.5 B$Inertial sensors 0.4 B$ 0.8 B$Fluidic controls 0.01 B$ 0.1 B$Data storage 0.0 B$ 1.0 B$Displays 0.0 B$ 1.0 B$Biochips 0.0 B$ 0.2 B$Communication 0.01 B$ 1.0 B$Others 0.03 B$ 0.1 B$Total MEMS 1.45 B$ 6.7 B$Total non sensing MEMS 0.05 B$ 3.4 B$

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Table 3w xa. The microsystems market as analyzed by Nexus 19,20

Year Existing market Emerging market Total market

1996 Market 11.7 B$ 0.3 B$ 12 B$Number of items 1.240 Million 60 Million 1.300 Million

2002 Market 32.4 B$ 1.6 B$ 34 B$Number of items 4.400 Million 1000 Million 5.400 Million

w xb. The different market segments as given by Nexus 19,20

Market 1996 Number of items

Ink jet heads 5.2 B$ 100 MillionHard disks heads 4.5 B$ 530 MillionPressure sensors 0.60 B$ 116 MillionAccelerometers 0.24 B$ 24 MillionOthers 1.16 B$Emerging market 0.30 B$Total 12.00 B$

When comparing these forecasts it strikes that the fig-ures are very different. This is caused by the specificallydifficult situation in forecasting microsystems market de-scribed above. All the same, these investigations give animportant insight in possible development as expected bythe different experts.

During the last few years, a number of authors haveseriously criticized microsystems R&D, saying that far toomuch money was invested into a technology which willnot lead to products. That way, the governments of theworld investing in microsystems would waste people’smoney. It must be admitted that the road from R&D toproduction for microsystems turned out to be longer thanexpected some years ago. This is not the fate of microsys-tems alone: all innovative technologies which involvemayor technology jumps were slower than expected at thebeginning. How much money is invested in MEMS re-search? Figures range around US$120 Million to US$200

w xMillion per year worldwide 11,18,24,2,32,33 spent bygovernments. Most important contributors are USA, Japanand Germany. The difference in the numbers is generatedby the fact that money given to universities may becalculated as spend on MST development or as generalresearch and education expenses. A recent study whichcalculates all contributions states that the Germany alone

Table 4The worldwide MEMS market estimated for the year 2000 by SRI

w xConsulting 21,22

Section

Hard disc drives 1.5 B$ 31%Inkjet printers 1.0 B$ 21%Waveguides and switches 0.6 B$ 13%Flat panel displays 0.6 B$ 13%Optical storage 0.5 B$ 11%Other 0.5 B$ 11%Total 4.7 B$

w xspends 112 Million US$ per year on MST 25 . Anothercontribution which is hard to estimate is the amount ofmoney spend in those countries which are just now enter-ing the MEMS field, e.g., Korea, Taiwan and Singapore.Industrial activities are about three times higher. Thismakes a total investment in MEMS technology of roughlyUS$600 Million per year. Calculating a turnover of US$10Billion as a rough average between the different figures

Žgiven above, we find that 15% of the actual turnover 4 B$.in 1998 or, respectively, 6% of the expected turnover is

spent on R&D. This figure looks quite high compared toan average of R&D efforts of 2% to 3%. On the otherhand, new technologies generally take greater R&D ef-forts, rising to more than 20% for high tech areas. For acomparison, Table 5 gives R&D efforts of German indus-

Ž .try R&D effort in percent of the gross income . From thatpoint of view, the amount of money spend on MEMS isnot exaggeratedly high, provided of course, that a turnoverof US$10 billion is indeed realistic.

Table 5R&D efforts in Germany in percent of the national gross product in 1997w x26

Type of economy R&D

Average 2.25%Electrotechnics 22.8%Automotive 22.6%Engine building 22.2%Chemical industry 18.9%Precision mechanics 3.1%Metals 1.7%Plastics 1.3%Energy, Mining 1.1%Building-materials 0.9%Food 0.6%Leather, textiles 0.5%Wood, Paper, Printing 0.4%Misc 3.9%

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3. Prerequisites for application

Forecasts are one way to look at the future of microsys-tems, but there is another which may be even moreimportant: We can think about the following question:‘‘what are the prerequisites for a commercial success ofmicrosystems?’’ Asking this question there are four strik-ingly important topics:1. Availability of production2. Performance3. Reliability4. Cost

3.1. AÕailability of production

3.1.1. Approaching microsystems productionLooking at the scientific papers and the conference

proceedings of the last years, we can find reports aboutseveral hundred prototypes of sensors, actuators and othermicrosystems. Only a very small number of these can bebought on the market, most of them have been processedwithin experimental lines of universities, research institutesand industry. In case somebody wants to buy one of thesedevices in a medium number of pieces, lets say 100,000sensors, it turns out that the line where the prototype hasbeen fabricated is not equipped to be able to make thisnumber of devices. As next, a commercial silicon foundryis searched for who would do the fabrication. Normally aninterested company is found, but their cleanroom equip-ment is different from the one used for the prototype.Furthermore, it often turns out that the technological pro-cess used is good for the realization of a few demonstra-tors, but not for the production of large numbers. There-fore the process would have to be adapted which actuallycauses almost as much cost and effort as a new start fromthe scratch. At this point several product ideas whichlooked very promising at first, had to be given up.

An important aspect of the problem is a hole in possiblesupply lines for medium numbers. When a prototype isdeveloped, the developer may be able to process one batchof wafers or two per year, i.e., several thousands ofdevices. If more than 250,000 are required, a siliconfoundry with a powerful cleanroom may be willing tomake the product development and production. But whocould make 20,000 or 50,000 devices? For a commercialcleanroom the number is too small, for an experimentalline it is too large. On the other hand, the customer wantsto start with medium numbers before ordering hundreds ofthousands. There is a lack between R&D activities andproduction. In order to improve the situation, a well coor-dinated cooperation between research institutes and com-

w xmercial MST manufacturers is desirable 27,28 .This problem does not occur in microelectronics. There

Žare standardized processes in the form of ASIC’s Applica-.tion Specific Integrated Circuits available on the market.

If a company wants a small number of specialized IC’s,

they have to make a layout or have it made by a specialist,then the IC is fabricated using the standard ASIC process.Since several designs are on one mask set, also smallnumbers of individual chips can be fabricated.

Attempts were made to develop a kind of MEMS-ASIC,i.e., a microsystem process which can be used for all kindsof microsystems. It turned out that due to the nature ofmicrosystems this does not work like in microelectronics.The main problem is the diversity of microsystems. Theycover a much larger range of different technologies thanelectronics chips. Within silicon technology, the maintechnological differences are the ways to perform 3-dimen-sional structuring. The important technologies are wet

Ž .chemical etching of silicon anisotropic etching in KOHon the one hand and dry etching with plasma and ion beamprocesses, especially for surface micromachining on theother hand. Furthermore, there are mechanical microma-chining, high aspect ratio micromachining, microreplica-tion, thick film technology and quartz technology. Judgingfrom today, no process will totally dislodge the other one.Therefore, there will be several processes for differentpurposes.

An important MEMS industry has been growing duringthe last 20 years and worldwide there is a number offactories running and several companies which are doingwell on the market. Some of these run dedicated siliconproduction lines only for microsystem purposes, othershave a running electronics fabrication which is partly usedfor microsystems. Production of microsystems in runningelectronic production has some positive features but draw-backs, too.

- The number of devices required in microsystem pro-duction is often rather small compared to the number ofelectronic devices produced. Therefore dedicated microsys-tem lines are sometimes operating below capacity whichincreases the specific cost per wafer. A running electronicproduct as a baseload makes the production more prof-itable.

- For several processes, microsystem technology hasreduced requirements with respect to electronics. An ex-ample is lithography: in electronics 0.35 mm designrulesare used, microsystem processes very often can be madewith 5 mm designrules. Now it seems feasible to take usedmachines from the electronic production for microsystemprocesses. While this idea looks very convincing at firstsight, it has inherent problems: the process cost is stronglydetermined by the cost of the cleanroom and the labor. Forolder equipment the cleanroom does not become cheaperand the labor cost is higher.

At the moment, most work using wet chemicalanisotropic etching for 3-dimensional structuring is done indedicated microsystem lines. Surface micro machining,which is much closer to the CMOS-process, is often donein electronic factories. Many companies running smalldedicated microsystem lines try to buy several processeswhich need especially expensive equipment from external

( )W. LangrSensors and Actuators 72 1999 1–1512

sources. One example is ion implantation, others are ther-mal oxidation and LPCVD-processes for silicon oxide,nitride and polysilicon. This approach is financially con-vincing, but there is a problem with the wafer size. Elec-tronics technologists use 6 in. wafers with a tendencytowards 8 in. or more. For microsystem applications veryoften 4 in. are sufficient since the number of devices is notso high anyway. Microtechnologists may run into majorproblems to find a supplier for ion implantation and thinfilm deposition who is able to work on smaller wafers.

3.1.2. The foundry processesDuring the last two years, several companies have

started to offer microsystem production services for cus-tomers as a foundry service. Like in ASIC productionseveral designs are placed on one wafer, therefore these

Žservices are called foundry processes or MPC’s Multi.Circuit Processes . These services are very interesting for

the development of prototypes, but also for production insmall, medium and large numbers. The customer plans hisdevice and has to supply a layout. If he is not familiar withtechnology, this part can be done by a specialist, e.g., by aresearch institute. A small number of chips is produced ona MPC-wafer and delivered for testing. In case the produc-tion is wanted, the same process can directly be run at

Žlarger numbers. The delivery time is rather short few.months and the cost is small in comparison to a dedicated

technology run. MPC’s might become the ASIC’s of mi-crosystem technology. Several processes are offered, aselection of which will be briefly described below:

1. R. Bosch offer a surface micro machining processw xwith structural layer of thick polysilicon 29 . This process

is very suitable for inertial sensors like accelerometers orangular rate sensors. Surface micro machining processesfollow a very strict process schematic with a well definedsequence of processes and masks. Therefore they are espe-cially suitable for MPC’s.

2. Another surface micro machining process usingpolysilicon is offered by the MEMS technology applica-tions center in North Carolina under the name of MUMPsŽ . 2Multi User Mems Process . This process has severalpolysilicon layers and is especially useful for freely mov-able parts like, e.g., microhinges. Monolithically integrated

Ž .electronics is available SMARTMUMPs . MEMS tech-nology applications center also offer a high aspect ratio

Žmicromachining process using LIGA technology LIGA-.MUMPs .

3. Analog Devices offer the surface micromachiningprocess developed for their accelerometer ADXL50 under

Ž . 3the name of iMEMS integrated MEMS . The process isespecially recommended for monolithically integrated sys-tems.

2 http:rrmems.mcnc.org.3 http:rrimems.mcnc.orgrimemsrintro.html.

4. A bulk micro machining process is offered by TIMA-Ž . w x 4CMP Circuits Multi-Projets in Grenoble, France 30 .

Anisotropic etching is performed from the front side. Thisprocess allows the processing of free standing bridges andcantilevers which makes it useful for thermal sensors.

In Europe, a network of 5 MST-manufacturing clustersŽ .among them Nr. 1 and 4 of the previous list is foundedby the european community within the programme

w x‘Europractice’ 31 . Together these clusters offer the fullrange of microsystem technologies and services.

3.2. Performance

There is no obvious reason why microsystems shouldshow a better performance than other technologies, butthere are certain circumstances which are very favourable.Under the following circumstances it makes sense to thinkof MEMS as a production technology.

1. The production numbers are very high. As a batchprocessing technology, microsystems become more inter-esting if the numbers of devices are very large since thecost per device will become very low. Cost reductioncontinuous until a large number of batches are run per yearwhich means millions of devices. Unfortunately, mostcustomers do not need that many sensors and the numbersfor actuators and microstructures are even smaller. There-fore, up to now, the real advantage of price reduction isgained only for very few products like MAP pressuresensors, airbag accelerometers or inkjet heads.

2. Functions are not available except by microsystemtechnology. This is a very favourable situation since thereis only competition within one technology, a situationsimilar to CMOS technology. At the moment, there areonly a few examples for this situation. This is not inherentto the technology, but it is a question of time. Totally newfeatures take quite long before they are found and devel-oped. For microfluidic systems it can be expected that thissituation will happen. This is the reason why micro-valves,pumps and fluidic systems may become of extraordinaryeconomical interest on the long run.

3. The devices have to be small in order to be function-able. A situation which occurs, e.g., in watches, camerasand parts of medicine technology. Remembering the en-larged sense of small discussed at the beginning, also smallenergy consumption may be a definitive argument formicrosystems, e.g., in standalone systems which are run onbatteries or solar cells.

4. The performance of the product cannot be achievedwithout monolithic integration of electronics. This is the

Žcase for large arrays of sensors e.g., IR or visible light. Ždetector arrays or actuators e.g., micromirror projection

.devices . Without electronic integration, thousands or mil-lions of bondwires would be necessary and this cannot be

4 http:rrtima-cmp.imag.fr.

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put into practice. Therefore, multiplexing is performed onthe chip level.

3.3. Reliability

At the moment the requirements on reliability of techni-cal systems are generally rising. This refers particularly toall devices that are security-sensitive which refers to largesections of the automotive technology. The society doesnot tolerate products which are mediocre in respect tosecurity and therefore, this trend has led to a number oflaws concerning product liability. This legal situation inreturn, makes high demands on the components. The goalof today’s quality assessment is zero failure, quite oftensuppliers have to guarantee failure rates of few ppm.Several authors claim that microsystem technology has thepotential of high reliability. Indeed there are strong argu-ments in this respect.

1. Microelectronics in silicon and in hybrid systems isconsidered to be a very reliable technology. For example,imagine a very simple calculator made of several thou-sands of relays or vacuum valves. This device would neverwork, because every other second another piece wouldbreak. In the pioneering phase of computers this type ofdevices was built, but they never worked continuously formore than just a few minutes. If all devices are integratedon one chip, the calculator will run for years and years.The reason for this is because transistors have, statistically,a longer lifetime than relays or vacuum valves and becausedevices tend to break down at the interfaces. Soldering ofwires, contacts or bonds are possible sources of trouble.Integration helps to omit these weak points. Furthermore,standardized processes are used on a high level of automa-tion during fabrication which omits operators errors. Thesame applies for microsystems provided that a product ismade with a mature technology.

2. Microsystems are not manufactured at the perfor-mance limit of technology. While etching of structuresbelow 0.25 mm is possible with photolithographic equip-ment and dry etching machines, we often apply designrules of 5 mm.

3. Silicon is a good material for mechanically bentparts. It is a monocrystal and does not show creep orexhaustion.

These are strong arguments indeed, but practically thereare problems with reliability of microsystems.

1. There is a lack of experience because this technologyis still young. An airbag sensor has to work absolutelyreliable still after ten years, but the technology used tomanufacture it has only been developed five years ago.Under these circumstances, it is difficult to convince acustomer.

2. There are only a few investigations on the long termbehaviour of MEMS devices. In the scientific papers andon microsystem meetings there is little information about

stability. Long term tests combined with a solid statisticalinterpretation hardly exist.

As it looks now, on the long run reliability mightbecome an advantage of microsystems. But until then a lotof experience has to be gained yet.

3.4. Cost

From the commercial point of view one of the mostinteresting points in microsystem technology is the batchproduction process with the possibility to make a largenumber of devices at low cost. The well known disadvan-tage of this technology is the fact that production lines areexpensive and development costs are high.

A fully equipped CMOS line which is necessary formanufacturing monolithically integrated sensors costs about

Ž .US$30 Millions cleanroom and equipment . The neces-sary turnover to return an investment of this size canhardly be performed with microsystems alone. Microme-chanical parts, i.e., microsystems without integrated elec-tronics can be made in a smaller unit. Presuming that ionimplantation is made externally, the investment is roughlyaround US$4 Millions. To perform hybrid integration, anassembly and thick film line is requested which has veryreduced cleanroom demands and can be installed for aboutUS$1 Millions. As a consequence, monolithically inte-grated systems are produced by large silicon factorieswhich have running electronics lines. Micromechanicalfabrication can be performed by smaller companies andinstitutions.

A very fascinating approach for small and medium-sizecompanies which have experience in the field of electron-ics and sensor systems is the hybrid integration based onexternally fabricated micromachined parts. An example forthis is the assembly of pressure measurement systemsusing silicon pressure sensor dyes which can be boughtfrom large suppliers or the production of specialized in-frared measurement systems equipped with commerciallyavailable thermocouple detectors. There is a wide marketfor this kind of work: diversification on system level ismuch larger than on silicon chip level and, furthermore,the increment value is mainly in the system and not withthe chip. At the moment, this kind of business is muchmore present in the United States rather than in Europe orJapan. Among the MEMS companies in USA there aremany small and medium-size companies which do not runtheir own silicon line. In Europe and Japan microsystemproduction is more an activity of big companies.

If a production line is available, what will be the costfor the fabrication of a dye? Of course, every process has adifferent complexity, another number of masks and layers.Still it is valuable to discuss about ‘rule of thumb’ num-bers. Microelectronic processes became very wellpricedduring the last few years. The cost for CMOS processedsilicon on the world market is 2.5 to 8 cent per mm2. Thiscorresponds to a price of US$750 to US$2100 for a

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processed 8 in. wafer. There is not really an establishedworld-market for micromachined wafers at the moment,therefore prices are hard to investigate and they vary a lot.In principle, micromachined wafers should be less expen-sive than CMOS-wafers since they have less process steps.It can be assumed that this will be the case on the daywhen they are sold in large numbers using well definedprocess sequences. At the momentary situation the onlypossibility to estimate prices is to calculate processes andto compare tenders for chip production. For this reason thenumbers given here should definitively be understood as a

Ž‘rule of thumb’ only. A typical sensor process e.g., apiezoresistive pressure sensor including anodic bonding

. 2and dicing without housing is offered for 35 cent per mmfor a batch of more than 50,000 chips. Surface micro-machined silicon is offered for US$1.80 per mm2 in small

Ž . Ž .batches 10,000 Chips . For large batches 500,000 chipsthe price drops to 30 cent per mm2.

Whenever discussing dye prices we have to bear inmind the ‘leverage effect’. The money is in the system, notin the dye. A hard competition about silicon prices as weobserve it for CMOS technology may be very disadvanta-geous from the point of view of political economy. Bylosing the dye production, a large part of the systembusiness with its important profit may easily be lost aswell.

If we ask whether a micromachining product can beconveniently sold or bought, we always have to look at thequantity and the price simultaneously. In industrial eco-nomics it is common to plot the cost of a product vs. the

w xnumber of sold items. Petersen 13 has drawn this figurefor MEMS Fig. 4. Production is convenient if a priceabove the critical curve can be obtained. The figure shows

Ž .several examples of existing products, too dots .There are three domains in the plot.1. Mass products with a quantity of more than one

million per year. Although prices are low, the productionis convenient due to the large number. This is the sectionof MAP pressure sensors, airbag accelerometer and severalother consumer products.

w xFig. 4. Price per unit vs. production quantity for MEMS products 13 .

2. Medium volumes of 10,000 to 1 Million chips peryear. Here, the chip production is a high economical risksince the development, overhead, sales and distributioncosts are divided by a number which is too small. For thechip producer it may be much more efficient to build thesystem and to gain the value from system production as

w xwell 13 .3. Small numbers below 10,000 dyes per year. In this

case the production is a big economical problem even ifthe customers are willing to pay high prices. The curve isrising very steeply for a small quantity, approaching fig-ures of several hundred dollars per chip below 10,000pieces a year. There must be a strong reason to use amicrosystem solution in these cases. The example productsenhance this statement: For the small number productsŽ . Ž-10,000 pieces per year very high prices US$10 to

.US$100 are achieved. All the same, these examples fallbelow the threshold line.

4. Conclusion

Market forecasts, technological and financial discus-sions are very important, but there is a question beyondthese: does the society need microsystems? If so, they willbe developed and applied. In the end, the society answersthe question with its own means like public opinion,journalism, and money. The personal answer of the authorto this question is clearly a ‘yes!’ There is a social interestin development and production of microsystems due to thefollowing reasons.

1. The leading goal is the global enhancement of socialwelfare and the improvement of lifestyle in a way which iscompatible with the environment. What are the conse-quences for R&D and industrial development within thenext decades? We can find the following fields of interest:environment, medicine, information technology, biotech-nology and automotive. In which of these fields microsys-tems are applied? The list is quite the same. Actually,microsystem technology is able to give an important an-swer to today’s problems and time has come to solve them.

2. Information technology will become even more im-portant than it already is. The information society is real-ity, and for the next decades it will be based on silicontechnology. Microsystems are compatible with this tech-nology, they form an essential part of the informationsociety.

3. There is a great demand for miniaturization in theextended form explained above. More and more systemsare supervised and actively controlled. Concerning energyconsumption and environmental cost the society expectsconvincing solutions. The consumption of resources cannotbe increased, therefore progress has to be made by anincrease of efficiency. The consequently increasing de-mand for sensors, actuators and systems is creating asteady growth of microsystem applications.

( )W. LangrSensors and Actuators 72 1999 1–15 15

Acknowledgements

The following persons have contributed to this workwith discussions and by correcting the manuscript: Dr. U.Rohrer, Munich, Prof. H. Sandmaier, M. Pal-Stumpp andChristine Lang, Villingen-Schwenningen.

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Walter Lang studied physics at Munich University and received hisDiploma in 1982 on Raman spectroscopy of crystals with low symmetry.His Ph.D. in engineering at Munich Technical University was on flame-induced vibrations. In 1987 he joined the Fraunhofer Institute for SolidState Technology in Munich, where he worked on microsystems technol-ogy. In 1995 he became the head of the sensors department at theInstitute of Micromachining and Information Technology of the Hahn-

Ž .Schickard Gesellschaft HSG-IMIT in Villingen-Schwenningen, Ger-Žmany. His areas of work are microsensors especially sensors for flow

.and for angular rate , microswitches and the technology of porous silicon.