Micromachinery rolling at last?

by George Marsh

Suggestions that we are on the verge of a second Market projections are increasingly bullish. For industrial revolution, based on microsystems example, a survey by Europe’s Network of Excellence (MST), are apt to leave many of us in Multifunctional Microsystems (NEXUS) suggests a unmoved. After all, the prospect of tiny machines, world market of $68 billion by 2005, more than double the level of two years ago (Fig. 1). NEXUS had less than a hairsbreadth in dimension, that can go to to revise earlier projections because of runaway work in optical systems, conventional and RF successes like that of the optical mouse (Agilent electronics, a wide range of sensors, robotics, or even recently shipped its 50 millionth), an our own bodies, has been dangled before us for anticipated breakthrough later in the period for decades. Yet there are signs that microoptoelectromechanical systems (MOEMS), microelectromechanical systems (MEMS) might, at enhanced prospects for RF switching systems, and the last, be about to take off. Could it be that these possible emergence of a market in domestic barely perceptible (to the human eye) syntheses of appliances. Analyst Venture Development Corporation goes further, concluding that the market with micromechanics are finally will grow ‘exponentially’ for the next ten years. The poised to make a big commercial impact? first device to exceed $1 billion in sales is expected to be MEMS-based photonic , within about two years. Current breadwinning applications – desktop ink-jet printers, biomedical pressure sensors/systems, and automotive devices – continue to thrive.

Governments are putting more money into MEMS research and some, like USA and Japan, are making it a national priority. In the US for example, a recent two-year award of $4.35 million to the University of Colorado for research into MEMS and MEMS packaging hints at the scale of federal spending, while Case Western Reserve University – Image above shows a field emission scanning electron in partnership with the Defense Advanced Research Projects micrograph of duplicated MEMS rotors (Courtesy of Dale Batchelor and Phil Russell, Analytical Agency (DARPA), the NASA Glenn Research Center, and the Instrumentation Facility, North Carolina State University.) State of Ohio – is halfway through the $21 million five-year

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Fig. 1 This NEXUS graph shows that total world market for microsystems is expected to grow from $30 billion in the year 2000 to $68 billion by the year 2005.

Glennan Microsystems Initiative. DARPA, sensing practical wafers and chips in batch volumes and at accessible cost. But military utility, has increased funding for MEMS development some industry insiders believe that reliance on CMOS may across the board, to the benefit of national laboratories such have been overdone. as Sandia as well as various universities, and NASA is It is worth a thought that, although it is many years since pursuing space and aeronautics-related developments. means were found to free three-dimensional elements from a Japan has sought to accelerate development through its Si substrate by selectively etching material away from below MITI NEDO ‘Micromachine Technology’ and subsequent and around the desired parts (some examples of typical initiatives. Europe is active at both the national level and MEMS structures are shown in Figs. 2 and 3), it is only in the within European Union (EU) programs such as BRITE-EURAM, last decade that systems produced in this way have achieved ESPRIT, and Europractice. India is among several other real market penetration. Since Analog Devices produced an countries actively developing systems. Much recent funding is integrated, single-chip accelerometer in 1991, this class of focused, above all, on efforts to dissolve finally the barriers sensor has become widely used as a trigger for that stand between what can be achieved at laboratory scale deployment in automobiles. Micro-accelerometers, used in and widespread commercial exploitation. weapons and other systems, as well as a growing range of vehicle applications (for example, inertial brake lights, From CMOS to hybrid? headlight levelling, security devices, and active suspension), Inhibiting issues tend to be those surrounding manufacture, are now a clear success story for MEMS. But history in this in particular economic volume production, compatibility with field is also littered with exciting laboratory demonstrations silicon (Si)-based complementary metal oxide semiconductor that have never scaled up to volume production, much less (CMOS) microelectronics, and device packaging. The MEMS succeeded commercially. community has long sought to leverage efficient processes John Foster, CEO of California-based Innovative Micro developed within microelectronics for producing Si-CMOS Technology (IMT), believes he knows why. “There has been an

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Other issues include the three-dimensional nature of micromachinery, which is at odds with planar, two- dimensional integrated electronics (sometimes two or more wafers have to be bonded together to overcome this), the escalating price of electronics-grade Si, and packaging. Packaging three-dimensional topographies in a manner that leaves mobile elements free to move and does not insulate devices from the environment they are supposed to interact with can result in package costs of 20-90% of the total device cost. Final package sizes that dwarf the micron scale of sensor or actuator elements have been another stumbling block. Foster and Heaton believe that MEMS would be better served if it were more widely accepted that device requirements often eclipse the capabilities of CMOS. Not that the two are averse to Si per se; on the contrary, IMT Fig. 2 Field emission scanning electron micrograph of various types of MEMS gears. fabricates directly onto Si devices, such as biomedical (Courtesy of Dale Batchelor and Phil Russell, Analytical Instrumentation Facility, North Carolina State University.) implants, mirrors, and RF MEMS switches, which exhibit no discernible wear and deliver service lifetimes of one billion effort to use CMOS-based technologies to make machines cycles or more. They point out that many structures – simple and there are areas, like accelerometers for , where mirrors, for instance, and optical devices with CMOS is appropriate and efficient. On the other hand,” he electrostatically-operated moving actuators – can be says, “there is no a priori reason why small machines should fabricated straight onto Si with advantage because of the be built that way. It’s like trying to manufacture a car in a material’s virtually infinite mechanical durability below toaster factory. Although they are both made of steel, you’re certain stress levels. much better off making the car in a car factory. Like the steel “Today, we make machines that do important jobs, for in cars, Si in MEMS is not the issue – it’s what you add to it example switching light, that simply don’t wear out,” says that matters.” For a start, Foster explains, MEMS can require a large number of different materials that would not normally be allowed anywhere near a CMOS fab where all potential impurities, except desired dopants, are rigidly excluded. “We are using a range of metals including titanium, tungsten, molybdenum, ruthenium, chromium, gold, and copper for contacts, valving, actuator strips, and other micro elements,” he says. “It’s a culture shock for manufacturing managers to have the multiple machines needed for depositing all these metals and their alloys invade their fabs. But that’s only part of it. After the metals, there are at least as many dielectric materials to reckon with.” Monte Heaton, marketing vice president at IMT adds, “We are dealing with significant portions of the periodic table here. We need ways of etching, depositing, and structuring all these materials, in some cases down to a few atomic layers at a time, onto substrates that Fig. 3 Field emission scanning electron micrograph of a MEMS motor sectioned by a Focused Ion Beam. (Courtesy of Dale Batchelor and Phil Russell, Analytical may or may not be Si.” Instrumentation Facility, North Carolina State University.)

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Fig. 4 Eight microdevices, complete with microfluidic channels and drive motors, fit on a module resting on a soda straw end. (Courtesy of Sandia National Laboratories.)

Foster. “CMOS technology does not readily accommodate, for into a second phase of its evolution. CCMicro-II, coordinated example, iron alloys and other magnetic materials needed for by the UK’s Rutherford Appleton Laboratory (RAL) and electromagnetic actuators,” Heaton adds. “And the complex involving partners SINTEF Electronics and Cybernetics in alloys that make the most reliable and durable Norway and the Fraunhofer Institute of Silicon Technology contacts, valves, mirrors, and other devices are generally not (ISIT) in Germany, will support development of alternative at home with CMOS.” MEMS approaches based on low-cost materials like glass, ‘Force fitting’ micromachinery elements to CMOS is likely, plastics, metals, and ceramics. Having already built up a many claim, to compromise performance and reliability. It presence in Si-based systems, the network fully expects to may be preferable to accept that divergence between the balance this with an alternative material system focus within technologies of microelectronics and micromachinery is three years. inherent, and to accommodate it in a hybrid approach. In a According to Zheng Cui, director of RAL’s Microsystem mobile phone, for instance, all the components, including Technology Centre, the costs of Si and derivative materials, MEMS RF elements, that are currently ‘off-chip’ (about half of along with the associated wafer fabrication techniques, are them) would be combined onto a separate ‘MEMS chip’. often daunting in sectors such as biomedicine, where Optimizing the performance of the resulting chip pair would, production volumes may not be high. “CCMicro-II aims to it is suggested, yield a better solution than trying to achieve offer application-specific technologies in areas where Si everything in a single-chip system. surface or bulk micromachining are not best suited,” explains Cui. “We will be investigating non Si-based technologies such Silicon or not? as micro-injection moulding, hot embossing, LIGA, and laser As the hybrid trend of thought gathers momentum, micromachining in a bid to lower manufacturing costs.” LIGA completely non-Si solutions are being pursued more actively. was developed in Germany and signifies Lithographie, In Europe the Competence Centre for Microsystems Galvanoformung (electroplating), Abformung (molding). (CCMicro), a support network formed two years ago under Other techniques such as laser machining the auspices of the EU’s Europractice initiative, has moved and electrodischarge machining, along with precision

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the most valid option not only where single-chip solutions are essential as with ‘smart’ drug delivery systems and microprobes for use within the human body (porous Si, with its biodegradability, has a role here), but also for all applications where electronic circuitry and processing are needed – which is most. While conceding a possible role for plastics in , he and his department have realized a range of Si-based devices including microprobes for monitoring neural activity, electrothermally-excited microresonators, microcutters for eye surgery, pressure sensors with output circuitry, cantilever devices with piezoresistive readouts, hybrid actuators using thick-film printing on Si substrates, and calibration gratings. Evans, in common with other researchers around the world, champions the superior mechanical properties of Fig. 5 A typical lab-on-a-chip device, fabricated at the Rutherford Appleton Laboratory, polycrystalline Si. Many in the MEMS community are trying based on a combination of polymer and glass. to develop Si micromachining as a potential route to affordable batch fabrication in (IC) mechanical methods like diamond milling, are also likely to foundries. A four-level fabrication process (one ground be part of the mix. plane/electrical interconnect and three mechanical layers) Cui believes that many applications will require patented by Sandia National Laboratories as the ‘Sandia microsystems produced independently of semiconductor Ultra-planar Multi-level MEMS Technology’ (SUMMiT™) is practices and is pleased that a handful of companies, including European player microParts, are now producing plastic MEMS. Indeed, as he points out, Si is positively ruled out for some systems because of its physical properties. For example, key to microfluidic applications such as ‘lab-on-a- chip’ is the passing of a current through a liquid drawn by the capillary action of a narrow channel, which must be non- conductive and therefore not Si. (As examples of microfluidic devices, see Figs. 4 and 5.) Accordingly, such channels must be created in glass, plastic, or ceramic, says Cui. His vision of the prospects for non-Si MEMS is expansive. “Their applications will be much wider than for Si-based counterparts, which have only really penetrated the automotive industry. Microsystems fabricated using the high- volume manufacturing techniques traditionally associated with the plastics industry, for example, and which employ far cheaper materials, will become commonplace.” Not everyone, however, is in favor of a pronounced swing away from CMOS to Si-free technology. For example, Alan Evans from Southampton University’s microelectronics center has not only nailed his department’s colors firmly to the Si mast, but believes that with MEMS we may be embarking on Fig. 6 The transmission and linear rack elevate the mirror located in the lower-right of the a ‘second Si revolution’. He argues that the material remains frame. (Courtesy Sandia National Laboratories, SUMMiT™ Technologies.)

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claimed to overcome difficulties of residual film stress and device topography that have hampered previous efforts, and is being made available to third parties via so-far sole licensee, Fairchild Semiconductor. Demonstrator projects using the technology have yielded working elements such as 2000 µm diameter gears free to rotate on hubs and a complete microengine able to position a hinged mirror via a linear rack (Fig. 6). Fairchild is itself concentrating on two of the most promising application areas for MEMS – RF and optical devices. These sectors offer massive market prospects. Systems which, for example, integrate the RF front ends of wireless systems – including RF microswitches, tunable capacitors, integrated filters plus other moving and static elements – would result in fewer separate components, savings in board real estate, smaller size, and reduced cost. Similarly, although delayed, the market for MOEMS devices – filters, beam Fig. 7 Each of the mirrors in a WaveStar™ LambdaRouter is about as wide as the eye of a splitters, switches, (de)multiplexers etc. – needed for high- standard sewing needle. (Reprinted with the permission of Lucent Technologies, Inc./Bell Labs.) speed optical networks, is expected to grow at a compound annual rate of 50-60% as the broadband communications revolution again gathers pace. Not surprisingly, these sectors produced when Si ingots and wafers are sliced by a wire saw. are attracting industrial players in growing numbers. On the Fabricators normally recover SiC (originally on the saw) from optical side, for example, Microvision, Inc. has begun the kerf for re-use, but dispose of the Si microparticles. fabricating a new MEMS optical scanner 60% smaller than PTMC, who has a patent pending on the technology, previously available devices for electronic imaging. Network compound the particles with thermoplastic feedstock from Photonics recently claimed the world’s first MEMS-based which MEMS components can be formed using metal powder wavelength switch, while tunable Fabry-Perot filters for use injection moulding (MIM) techniques. This established power in trunk optical communications are now available from Solus metallurgy route has proved able to deliver complex tiny . The latter, intended for cost-sensitive parts consistently and in large numbers. applications, are based on Solus’ Compliant MEMS Using the technique could, say its proponents, circumvent technology, which is mainly non-Si but retains some Si-based the costly and labor-intensive IC manufacturing processes elements. MEMS-based switching modules were recently currently employed. The initial investment required would, it supplied by OMM to Lumentis AB for optical management is argued, be tiny compared with that needed for sub-systems. Other players include Lucent (see Fig. 7 and conventional fabrication. It remains a matter for conjecture Cover Story for examples of Lucent’s devices), Agilent, and to what extent this ‘new’ material shares its properties with PHS. IMT, as previously indicated, will be one of those seeking conventional Si. to marry Si-free technology with conventional CMOS fabrication to achieve the best results and believes that this Expectation of a breakthrough ‘dual fuel’ approach will expedite penetration of MEMS into In summary, perhaps there was never any reason to expect markets eager to receive devices that deliver reliable that what has taken forty years to achieve in performance at acceptable prices. microelectronics could be done more quickly with MEMS. One, at first sight surprising, alternative to reliance on However, it appears as though the present industrial and ‘standard’ Si is the use of waste. This has been suggested by research initiatives, including the use of alternative materials PTMC Associates of Singapore, who studied the possible and processes alongside the currently ubiquitous Si-CMOS, application of Si particulate recovered from kerf slurries could be clearing the way for the final breakthrough. MT

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