MEMS stands for microelectromechanical systems. It is also sometimes known in Europe as MST, for microsystems technology, with a slightly broader definition. But in essence, though, the terms are interchangeable. Despite four decades of development and some commercial deployments — such as pressure sensors and accelerometers to trigger automobile airbags — MEMS is only now showing signs of becoming a field separate from microelectronics. And it appears to be sufficiently large in terms of potential markets to justify investment in startup companies and increased activity by established players.

The signs are there: the breadth and depth of university research leading to commercial startups, renewed interest among mainstream chip makers and, perhaps most significantly, the reaction of the semiconductor equipment makers that have started to make equipment adapted, if not dedicated, to various types of MEMS manufacture.

With a fully functioning silicon MEMS wafer fab costing as little as $60 to $80 million, the barriers to entry into MEMS are considerably lower than those for microelectronics, where a large-scale "economical" wafer fab for digital chips can cost $2 billion. That significantly lower cost might seem like a good thing but the nascent state of MEMS means that the risk of building up overcapacity in manufacturing is even higher in MEMS than in microelectronics.

But for the next few years a limiting factor on growth is likely to be a persistent diversity of component architectures, process technologies and materials. The strong interdependence of design, manufacturing and packaging in MEMS means that a second choke on growth is likely to be the availability of engineers trained in the complexities of MEMS — renaissance engineers who have a breadth of skills and knowledge in process, materials and mechanical and electronic design.

Right now many questions are being asked of the MEMS protagonists. Is silicon set to triumph as the substrate of choice because of the pre-existence of a strong equipment and materials infrastructure? Will it thereby achieve economies of scale and drive out glass and plastic substrates? Even on silicon there are a many alternative process technologies for building MEMS. Which process technologies will succeed? Will each application demand the optimum manufacturing technology? Must so many processes co-exist, even at the expense of requiring many dedicated manufacturing lines? Is MEMS technology development destined to follow the 30-year progress pioneered by microelectronics, albeit at an accelerated rate, or will the differences required by mechanical, optical, fluidic and biochemical devices and their application environments necessarily throw the various subclasses of MEMS onto completely different development paths?

In business terms, does a foundry-and-fabless design-house model make sense? Or will a few of the multitude of agile startups with their own manufacturing capability come to dominate MEMS? Or will a few of the semiconductor giants, perhaps the companies that have shipped the vast majority of MEMS components to date, simply acquire those startups when the time is right, or take the markets from them by dint of deep customer relationships and deep pockets?

Which technical and business visions for the industry will catch the broader imagination and take root? And after four decades of marginal significance why should MEMS make its "big" commercial entrance now at the beginning of the 21st century?

While the industry tries to answer these questions, it must be remembered that contained within MEMS technology are some of the seeds of the "nanotechnology" that will almost inevitably rise up to complement it — and eventually supplant it.

Technology
By definition, MEMS components contain micrometer-dimensioned elements, usually with a moving part, sometimes a solid mechanical one, sometimes a fluid one, and usually integrated together with at least some electronic circuitry. The electronics may be only a piezo-resistor network or a capacitive element to transduce mechanical motion into an electrical signal, but it can be much more. MEMS with fully integrated electronics offer the promise of truly complete "system-on-chip." MEMS encompasses sensors of many types although optical sensors such multipixel CCD or CMOS camera chips are usually omitted.

MEMS are usually manufactured in a fashion similar to integrated circuits, making use of the experience in lithography and etching. Although silicon is a favored substrate for this reason, glass, quartz and plastic substrates are sometimes preferred. MEMS have also evolved natural complementary roles to microelectronics: as sensor-transducers, which can provide signals for electronic processing, and as transducer-actuators, which can move or effect a change in response to that processing. Typical motions are electrostatically operated linear and rotary motors, although steam and internal combustion engines are also being miniaturized.

Whereas microelectronics exploits the semiconducting property of silicon, MEMS and more elaborate micro-machines also exploit the structural properties: a low coefficient of thermal expansion, a high thermal conductivity and a strength that for its equivalent weight is greater than aluminum. In a complementary role are piezoelectric materials that produce an electrical charge when deformed and, conversely, deform under a voltage — allowing precise electrically controlled motion and actuation.

A four-wave history
The first wave of MEMS commercialization started in the late 1970s and early 1980s with bulk etched silicon structures, and back-etched membranes, used to make pressure sensors. As the thin silicon membrane deforms under pressure, it effects a piezoresistive track laid on its surface and the change is used to transform the pressure into an electronic signal. Subsequent devices include the capacitively sensed moving-mass accelerometer used to trigger airbag deployment in automobiles, and gyroscopes for orientation.

A second wave of commercialization arrived in the 1990s, mainly oriented around the boom in PC and information technology. Texas Instruments introduced video projection based on electrostatically actuated tilting micromirror arrays. The thermally operated inkjet print head remains a high-volume application.

It can be argued that the third wave of MEMS commercialization boomed and busted at the turn of the millenium, with micro-optics as an accompaniment to fiber-optic communications — by way of all-optical switches and related devices. Despite the current deflation in that market, micro-optics will be a strong growth area for MEMS in the long term.

Other applications that could be the inspiration and beneficiary of the fourth wave of commercialization include static and moving devices for radio frequency passives components, silicon-based audio, biological and neural probes, so-called lab-on-a-chip biochemical and drug development systems and microscale drug-delivery systems.

What's up with processes?
The recent upsurge of interest in MEMS has partly come about with the advent of surface micromachining technology, in which a sacrificial layer — material that keeps other layers separated as the structure is being built up — is dissolved in the last step, allowing the creation of suspended thin moving and resonant structures.

But as Bernard Courtois of the TIMA Laboratory (Grenoble, France), a MEMS research group, observed: "There are two ways to manufacture microsystems: develop processeses specific to microsystems or use process that have been developed for microelectronics. Among those later processes, some can be targeted to microsystems, or it is possible to add special process steps to accommodate microsystems within integrated circuits."

Many applications call for departures from traditional electronics manufacturing, in terms of extra steps, back-surface processing, unusual metals and more exotic materials, wafer bonding and so on. Indeed many applications, particularly in the biological and medical area, do not tolerate silicon as a substrate.

In many cases, glass and plastics are preferred, and disposable medical devices are often stamped out in plastic for reasons of cost.

But for many companies and research groups the existence of CMOS, silicon-germanium and gallium arsenide process technologies for microelectronics is a starting point for MEMS development. The integration of MEMS with electronics on the same chip could, in theory, improve performance, efficiency and reliability of the overall circuit and reduce the cost of manufacturing and packaging.

One dominant approach to high integration is post-processing of MEMS structures in reserved areas on top of the microelectronic die by surface micromachining. However, the temperature at which the previously fabricated microelectronics is damaged has to be taken into account. Thus, containing MEMS manufacture within a low temperature regime is a key topic for those pursuing monolithic integration.

To this end, the Interuniversity Microelectronics Center (IMEC; Leuven, Belgium), has developed a polycrystalline silicon germanium (SiGe) deposition technique with a critical temperature of 450 degrees C compared with 800 degrees C for polycrystalline silicon. However, with the lower temperature comes a slower deposition rate. And so a second method with a much higher deposition rate at a slightly elevated temperature of 520 degrees C has been developed. The choice of SiGe is an attempt to intersect with what will become the de facto standard process for high-frequency electronics, but many others are looking at mainstream digital CMOS as their starting point.

Earlier this year, IBM announced that it has developed RF MEMS components using standard production materials on a BiCMOS process technology at temperatures below 400 degrees C. IBM has developed MEMS resonators and filters that could replace discrete passive components in wireless devices.

"There's been a continuing discussion about the integration of CMOS and MEMS over many years," said Roger Grace, an industry consultant specializing in MEMS and microsystems. "But about the only integrated process running in volume today is that for Analog Devices ADXL-50 accelerometer. The same functionality is being created by Motorola using two chips, one for the MEMS and one for the microelectronics with integration at the package level.

Such debates are commonplace in microelectronics. It is noticeable that analog and mixed-signal microelectronics are frequently integrated at the package level as circuits on separate die. Similarly, intelligent power electronics is frequently implemented as a multichip solution although others tout the benefits of intelligent-power process technologies. The reasons for and against integrating mechanical structures and large amounts of electronics are even more complex.

The situation is compounded by the fact that, unlike microelectronics, where standard packages developed quickly and are the norm (varying, in essence, on pin-count and attach method), MEMS environmental parameters are very diverse. Some packages must exclude light while others must allow it onto the die surface. Some packages must maintain a vacuum over or behind the die, while others must pipe gases or liquids around a chip.

It is recognized that it is impossible to develop a standard package for the diversity of MEMS applications. But it is also highly desirable that for each application category the industry defines a standard package and its road map.

"MEMS designers tend to make the device first and then think about test and packaging," observed consultant Roger Grace. Given that in excess of 95 percent of the cost of a component can be in the test, packaging and final assembly, getting that optimized should be more important than building the most elegant MEMS structure.

Meanwhile, SEMI, the industry group, is starting to put work into standardization of both packaging and manufacturing processes.

Thus, for example, Sandia National Laboratories has developed the Summit V process technology that includes up to five layers of polysilicon and has licensed it and related design kits out to commercial companies such as Coventor Inc. and Ardesta LLC (Ann Arbor, Mich.) a venture capital company. Sandia has licensed its four-layer Summit IV process technology to Fairchild Semiconductor Corp. (South Portland, Maine).

This is part of a deliberate policy of infrastructure commercialization undertaken by Sandia and referred to by Sandia workers at a recent MEMS symposium as the "Visionary's Dilemma, or How to evolve from initial demonstration and to industry standard."

Roger Grace said: "Sandia's policy is to license technology into industry to get the volume of deployment up so they can prove reliability sufficiently for their own low volume applications." The Summit processes are perhaps shown off to best advantage when building gears, chains and micromachines.

"Summit V is an expensive process. Are there enough applications to support Summit V?" Grace asked. "Engineers still want to match the manufacturing process technology to the application."

According to Grace, only a handful of volume applications for MEMS have been established so far. Even Texas Instruments' moving-mirror video projection chip, despite its success in the desk-top projector market, runs at less than 1 million units per year, according to Grace.

"We could see a continuation of custom processes and custom solutions. I don't see them [engineers] compromising."

So is talk of MEMS as a boom market premature?

Marlene Bourne, senior analyst at In-Stat MDR, predicts that the worldwide market for MEMS will grow from $3.9 billion in 2001 to $9.5 billion in 2006, an average growth rate of 19.5 percent. By comparison, the worldwide market for semiconductor chips has been hovering at about $150 billion since 1996, although it is expected to get back to 20 percent growth in 2003.

Nexus, a European industry body, estimated that the market for microsystems was already $30 billion in 2001 and will rise to $68 billion by 2005. This disparity with In-Stat's numbers is due mainly to a broader definition of "microsystems" used by Nexus. Nexus says microsystems can include whole systems, such as heart pacemakers, and extend to polymer, glass, metal and ceramic-based devices.

Bourne said: "I define MEMS by size and as devices with a mechanical function generally fabricated in silicon although not exclusively. It's more to do with the process technology, which could be derived from bulk, surface micromachining or Liga. I am trying to measure components as supplied to OEMs so I am not measuring value at the die level, nor at the end-user product."

Business
"The MEMS industry goes in waves, we've seen pressure sensors, accelerometers and mirror-based optical devices come up," said Bourne. "RF is the next wave; miniature relays and switches. They have extreme volume but extreme price pressure," she said.

Bourne also referred to silicon microphones as an upcoming area. In such devices the traditional microphone diaphragm is made from a thin silicon membrane. Certainly companies such as Akustica Inc. (Pittsburgh) and SonionMEMS A/S (Lyngby, Denmark) are being set up to exploit this possibility.

Akustica is also pursuing the possibility of silicon sound-generation devices.

Although small size would limit output power, such components could be well suited to hearing-aid applications. And the ability to put 255 such silicon loudspeakers on a single die, for example, would not only increase the loudness of the device but could allow 8-bit digital addressing of such miniature "loud" speakers. Similarly, the ability to deploy multiple microphones and microphones in arrays could open up new application areas. Such applications might include directional microphone strips that could be run strung around a space — the interior of an automobile or home for example — so that a captured sound is accompanied by positional information.

"What we are seeing is a strong segmentation of the MEMS field," said Bourne.

And it's true that startups and established chip and systems companies are playing in such areas as transportation, health care, telecoms and consumer electronics. And many of the startups are present in only one application segment. As with microelectronics, MEMS is becoming a horizontal enabling technology that may serve a number of broad vertical markets.

"There has been a lot of interest from the traditional semiconductor manufacturers but that is probably a knee-jerk reaction to problems in their own sector. The process technologies are so different," said Bourne.

But are they? As TIMA and others have pointed out, there are advantages to be gained if you can use a standard process even if it is modified from a baseline IC process.

The difference between silicon MEMS, MOEMS (micro-optical electro-mechanical systems) and conventional IC manufacture is therefore one of degree.

And indeed, this being the case, an entry into MEMS is appealing to smaller chip manufacturers, which have been under assault from the top integrated device manufacturers (IDMs) and the Taiwanese microelectronics foundries. As these smaller chip companies cannot afford the cost of developing deep-submicron process technologies and new factories to house them, they must find niche markets or leave manufacturing altogether.

It is notable that Fairchild Semiconductor and Austria Microsystems (Unterpremstatten, Austria) have added MEMS to the range of devices they can manufacture. And that a Xicor 6-inch wafer fab in Milpitas, Calif., has been acquired by Standard MEMS Inc. (Burlington, Mass.) to be turned over to MEMS manufacture; and that the Plymouth, England, wafer fab of Zarlink Semiconductor was sold to X-Fab Semiconductor Foundries AG (Erfurt, Germany), which characterizes itself as a mixed-signal and MEMS device foundry.

But it is also notable that the vast majority of MEMS sold to date have been made by major semiconductor players such as Motorola, Analog Devices and Texas Instruments and that STMicroelectronics is expanding its interests in the area.

The appeal of MEMS to the larger chip manufacturers, the ones who can still afford to pursue deep-submicron CMOS process technology, is making an increased margin on older process technologies and wafer fabs that have been fully amortized by years of manufacture. In other words, the trailing edge of microelectronics can become the leading edge of silicon MEMS manufacture.

In-Stat's Bourne said: "Yes, a number of conventional IC fabs have said they can do this; but it is not easy. It takes very specialized expertise, which is in short supply and resides in people at the universities and start-up companies."

She added that while there is the development of a foundry business model serving fabless component developers, these tended to be dedicated MEMS foundries such as Standard MEMS, Intellisense and Cronos. "Fifty percent of the startups have in-house fab facilities. You need to have people who understand the process technology." Which, in turn, highlights the need for training.

"For ST the interest in MEMS is quite strategic," said Benedetto Vigna, MEMS business unit manager, STMicroelectronics. "The main reason is that we want to address new markets with silicon by exploiting the different properties of silicon. And because MEMS can be built at about one micron it means we can reuse existing manufacturing facilities."

ST has been a longtime supplier of thermally operated inkjet printer heads to Hewlett Packard. In addition it has developed a range of inertial sensors, including angular and linear accelerometers. It has also shown a thermally operated optical switch developed for Agilent.

And, indicating one direction for the silicon MEMS market to develop, STMicroelectronics struck a deal with Onix Microsystems Inc. (Richmond, Calif.), a heavily backed Californian startup.

In July 2001 the two companies agreed to co-develop and manufacture chip sets containing MEMS and ASICs for use in Onix's optical switching engines. ST's volume manufacturing capability will provide the chip sets required by Onix.

"In RF MEMS the priority is not so high," said Vigna."We have nothing commercially developed although one of the great areas of interest is mechanical switches to replace gallium arsenide solid-state switches."

According to Vigna there are still issues of reliability for the industry to demonstrate and, "if it is going in a mobile phone you need a very low actuation voltage. The wish is for sub-5 V although the cell phone makers will probably allow us 12 to 15 V. There again if they need 40 V for organic light-mitting displays we might not need to go so low."

With some startup companies, such as MicroLab Inc. (Chandler, Ariz.), claiming to have RF switches ready for the market, it remains to be seen if these devices will remain discrete or be integrated with SoCs.

Waiting for standards
"I still don't see a CMOS-like universal MEMS process on the horizon, so there is a kind of fragmentation at present. But there will be a consolidation; in EDA, in design, in testing. As soon as standards appear a kind of simplification could happen," said ST's Vigna.

"If we could stick to two or three process platforms we could improve time- to-market for MEMS because they would become reliable, through the buildup of experience."

"I believe the most successful approach," Vigna continued, "will be a modular approach with the integration of CMOS and MEMS at the package level. You could sell the MEMS die with bumps for flip-chip integration.

"We are pushing two or three main processes," said Vigna. "The main process is Thelma — for Thick Epitaxial Layer for Micro Accelerometers. This is a 0.8-micron process with a thick polysilicon layer for structures and a thin polysilicon for interconnect."

The technology allows silicon structures to be attached to the substrate in a few points called anchor points, but which are free to move in a plane parallel to the substrate itself. To be compatible with traditional plastic packaging techniques, a cap is placed over the top of the sensor element to prevent the molding from fouling the moving elements.

For micro-actuation a similar process is used but without the cap. Instead, a flexible passivation layer is added. For the work with Onix on micromirrors a third process is used. This is because Thelma's polysilicon layers are not as good as monocrystalline silicon for producing mirror finishes.

Vigna explained "If we could stay on two or three process platforms we could improve time-to-market because the processes become more reliable." Vigna is also of the opinion that most integration will be done at the package level.

So how will design automation tools for MEMS develop?

Vigna's answer is "slowly."

"EDA tools are useful when design becomes a science. But today MEMS are more like analog circuitry. First you need standard processes, then you get standard design tools."

Despite the present diversity of the process technologies some third parties with EDA tools have emerged from such companies as Ansys Inc. (Canonsburg, Pa.) and Coventor Inc. (Cary, N.C.).

And just like the semiconductor equipment makers, the mainstream EDA vendors, such as Cadence Design Systems Inc. (San Jose, Calif.), are starting to treat MEMS as a distinct engineering discipline.

Memscap, itself a provider of MEMS design automation tools, has transformed itself into one of a new generation of dedicated MEMS fabricators. It is pursuing RF and optical applications and has developed a number of processes in support of these applications. More controversially, it has built a dedicated MEMS wafer fab near its original base in Grenoble, France, and supported Walsin Lihwa, a Taiwanese company, to build through licensing agreements.

"The relationship with Walsin is very close. We helped them build their fab. The idea is that we go optical and they go wireless. So that we both get both technologies to market very quickly, and both of us will have second sources," said Jean-Michel Karam, president and chief executive officer of Memscap.

Karam's argument is that building a wafer fab is part of the investment needed to kick-start the MEMS market. "A lot of companies say they have an RF MEMS switch. But before Nokia commits to using RF MEMS they need to know you can supply 100 million units a year. You need to prepare for big volume," he said.

Karam said that the industry will see standardization. "The best way to make money is to minimize the number of processes — hen when you make changes you make them in design and not in the process. Technically you can't generalize this easily," he said.

The outcome, he argued, is that a few processes will become "standardized" for different high-volume applications in each industry sector and once they become standardized, economic pressure will start to drive MEMS designers to use these processes without alteration.

Agile vs. stable
One might expect STMicro's Vigna to predict that the major semiconductor companies will reap the MEMS harvest in the years to come.

After all, ST has experience of manufacturing in volume, has wafer fabs all over the world in which it could insert MEMS processes and has close relationships with a number of major customers.

"Big companies like dealing with big companies. A big company can sue another big company if things go wrong. Of course startups are more agile, more flexible and focused to only work on MEMS."

But Vigna suggested a third way forward for MEMS business, a way to combine the volume manufacturing capability of a major company with the flexibility and focus of a startup.

"The first big company to spin off its MEMS group could be really successful," said Vigna, although he denied that STMicroelectronics has any plans to set his own MEMS business unit free.

Memsic Inc. (Andover, Mass.), a spin-off from Analog Devices Inc. (Norwood, Mass.) formed in 1999 would provide a good example except that Analog has also retained its own MEMS activities. In fact, Memsic illustrates another trend: the rising significance of Taiwan and China.

Taiwan and China
Memsic was founded by Yang Zhao, a graduate of Beijing University, who served within Analog Devices' micromachined products division between 1993 and 1999.

Zhao had a particular interest in a thermally based accelerometer and persuaded Analog Devices to let him take the technology and develop it in return for an equity position in his startup. Zhao is a passionate believer in the utility CMOS process technology for MEMS and now produces highly integrated accelerometers embedded in complex logic on a single chip.

"We concentrate exclusively on CMOS MEMS. The only way to succeed is to piggyback on the available technology. You should copy as much as possible before you start inventing," Zhao said.

Memsic uses TSMC as a foundry for logic at 0.6-micron design rules before taking the six-inch wafers back and implementing the MEMS structures at its subsidiary in Wuxi, China. This does restrict Memsic to low-temperature post-CMOS processing so as not to damage the electronics on the wafer — but that is the technique that Memsic has developed.

"It means we can take design to any one of a number of fabs and even though designs have to be tailored to suit the processes we would rather do that," said Zhao. "Besides, TSMC does not have a MEMS process so we do it ourselves."

But TSMC is an investor in Memsic, so will TSMC be without a MEMS module for very long?

In the Shanghai district of China there are several 8-inch foundries being built. Tens of billions of dollars are being spent. So China is going to be one center of silicon manufacturing. Taiwanese foundries may look to develop more specialized processes, including ones supporting MEMS to run in their older fabs on their trailing-edge processes.

"We would be interested in taking on more MEMS business, provided the technology that's required is in tune with our basic semiconductor manufacturing technology. . . . I see CMOS technology as a platform on which to put optical devices, MEMS or even carbon nanotubes," said Chenming Hu, chief technology officer of TSMC, in a recent interview with EE Times (http://www.eetimes.com/story/OEG20020517S0044).

Roger Grace said: "Taiwan and China are gearing up to be big in MEMS. They want to do in MEMS what they've done in microelectronics. We need to keep very close eyes on that."

Summary
The future of MEMS is therefore full of ambiguities.

Engineers will continue to pursue hundreds of MEMS ideas on scores of process technologies and packaging solutions, but standardization will throw up some preferred options based on silicon.

Agile and innovative start-ups across the Uniteed States. will drive MEMS applications forward, leaving others in their wake — but Europe's relatively few hit-the-ground running spin-offs will also succeed. Meanwhile, Taiwan and China will repeat in MEMS the success that they have achieved in the world of microelectronics manufacturing.

But do not expect MEMS to suddenly blossom as microelectronics did in the 1960s and 1970s. MEMS remains extremely diverse and difficult. MEMS is microelectronics plus micromechanics in pursuit of hundreds of applications across all industry sectors.

Do expect MEMS markets to turn in better long-term growth than "pure" microelectronics from this point on. As foundry services develop, engineers will increasingly come to make their designs fit the process technology rather than the other way around, making standard EDA tools and design kits economical and initiating a virtuous cycle.