Micromachinery rolling at last?

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 technology (MST), are apt to leave many of us

example, a survey by Europe’s Network of Excellence in Multifunctional Microsystems (NEXUS) suggests a

unmoved. After all, the prospect of tiny machines, less than a hairsbreadth in dimension, that can go to

world market of $68 billion by 2005, more than double the level of two years ago (Fig. 1). NEXUS had to revise earlier projections because of runaway

work in optical systems, conventional and RF electronics, a wide range of sensors, robotics, or even our own bodies, has been dangled before us for decades. Yet there are signs that microelectromechanical systems (MEMS) might, at last, be about to take off. Could it be that these barely perceptible (to the human eye) syntheses of microelectronics with micromechanics are finally poised to make a big commercial impact?

successes like that of the optical mouse (Agilent Technologies recently shipped its 50 millionth), an anticipated breakthrough later in the period for microoptoelectromechanical systems (MOEMS), enhanced prospects for RF switching systems, and the possible emergence of a market in domestic appliances. Analyst Venture Development Corporation goes further, concluding that the market will grow ‘exponentially’ for the next ten years. The first device to exceed $1 billion in sales is expected to be MEMS-based photonic switches, within about two years. Current breadwinning applications – desktop ink-jet printers, biomedical pressure sensors/systems, and automotive devices – continue to thrive.

Image above shows a field emission scanning electron micrograph of duplicated MEMS rotors (Courtesy of Dale Batchelor and Phil Russell, Analytical Instrumentation Facility, North Carolina State University.)

44

July/August 2002

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 – in partnership with the Defense Advanced Research Projects Agency (DARPA), the NASA Glenn Research Center, and the State of Ohio – is halfway through the $21 million five-year

ISSN:1369 7021 © Elsevier Science Ltd 2002

APPLICATIONS FEATURE

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 military utility, has increased funding for MEMS development across the board, to the benefit of national laboratories such as Sandia as well as various universities, and NASA is pursuing space and aeronautics-related developments. Japan has sought to accelerate development through its MITI NEDO ‘Micromachine Technology’ and subsequent initiatives. Europe is active at both the national level and within European Union (EU) programs such as BRITE-EURAM, ESPRIT, and Europractice. India is among several other

From CMOS to hybrid?

wafers and chips in batch volumes and at accessible cost. But some industry insiders believe that reliance on CMOS may have been overdone. It is worth a thought that, although it is many years since means were found to free three-dimensional elements from a Si substrate by selectively etching material away from below and around the desired parts (some examples of typical MEMS structures are shown in Figs. 2 and 3), it is only in the last decade that systems produced in this way have achieved real market penetration. Since Analog Devices produced an integrated, single-chip accelerometer in 1991, this class of sensor has become widely used as a trigger for airbag deployment in automobiles. Micro-accelerometers, used in weapons and other systems, as well as a growing range of vehicle applications (for example, inertial brake lights, headlight levelling, security devices, and active suspension),

Inhibiting issues tend to be those surrounding manufacture, in particular economic volume production, compatibility with silicon (Si)-based complementary metal oxide semiconductor (CMOS) microelectronics, and device packaging. The MEMS community has long sought to leverage efficient processes developed within microelectronics for producing Si-CMOS

are now a clear success story for MEMS. But history in this field is also littered with exciting laboratory demonstrations that have never scaled up to volume production, much less succeeded commercially. John Foster, CEO of California-based Innovative Micro Technology (IMT), believes he knows why. “There has been an

countries actively developing systems. Much recent funding is focused, above all, on efforts to dissolve finally the barriers that stand between what can be achieved at laboratory scale and widespread commercial exploitation.

July/August 2002

45

APPLICATIONS FEATURE

Fig. 2 Field emission scanning electron micrograph of various types of MEMS gears. (Courtesy of Dale Batchelor and Phil Russell, Analytical Instrumentation Facility, North Carolina State University.)

effort to use CMOS-based technologies to make machines and there are areas, like accelerometers for airbags, where CMOS is appropriate and efficient. On the other hand,” he says, “there is no a priori reason why small machines should be built that way. It’s like trying to manufacture a car in a toaster factory. Although they are both made of steel, you’re much better off making the car in a car factory. Like the steel in cars, Si in MEMS is not the issue – it’s what you add to it 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 may or may not be Si.”

46

July/August 2002

Other issues include the three-dimensional nature of micromachinery, which is at odds with planar, twodimensional 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 fabricates directly onto Si devices, such as biomedical implants, mirrors, and RF MEMS switches, which exhibit no discernible wear and deliver service lifetimes of one billion cycles or more. They point out that many structures – simple mirrors, for instance, and optical devices with electrostatically-operated moving actuators – can be fabricated straight onto Si with advantage because of the material’s virtually infinite mechanical durability below certain stress levels. “Today, we make machines that do important jobs, for example switching light, that simply don’t wear out,” says

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 Instrumentation Facility, North Carolina State University.)

APPLICATIONS FEATURE

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 example, iron alloys and other magnetic materials needed for electromagnetic actuators,” Heaton adds. “And the complex alloys that make the most reliable and durable switch contacts, valves, mirrors, and other devices are generally not at home with CMOS.” ‘Force fitting’ micromachinery elements to CMOS is likely, many claim, to compromise performance and reliability. It may be preferable to accept that divergence between the technologies of microelectronics and micromachinery is inherent, and to accommodate it in a hybrid approach. In a mobile phone, for instance, all the components, including MEMS RF elements, that are currently ‘off-chip’ (about half of them) would be combined onto a separate ‘MEMS chip’. Optimizing the performance of the resulting chip pair would, it is suggested, yield a better solution than trying to achieve everything in a single-chip system.

into a second phase of its evolution. CCMicro-II, coordinated by the UK’s Rutherford Appleton Laboratory (RAL) and involving partners SINTEF Electronics and Cybernetics in Norway and the Fraunhofer Institute of Silicon Technology (ISIT) in Germany, will support development of alternative MEMS approaches based on low-cost materials like glass, plastics, metals, and ceramics. Having already built up a presence in Si-based systems, the network fully expects to balance this with an alternative material system focus within three years. According to Zheng Cui, director of RAL’s Microsystem Technology Centre, the costs of Si and derivative materials, along with the associated wafer fabrication techniques, are often daunting in sectors such as biomedicine, where production volumes may not be high. “CCMicro-II aims to offer application-specific technologies in areas where Si 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 micromachining in a bid to lower manufacturing costs.” LIGA was developed in Germany and signifies Lithographie, Galvanoformung (electroplating), Abformung (molding). Other microfabrication techniques such as laser machining and electrodischarge machining, along with precision

As the hybrid trend of thought gathers momentum, completely non-Si solutions are being pursued more actively. In Europe the Competence Centre for Microsystems (CCMicro), a support network formed two years ago under the auspices of the EU’s Europractice initiative, has moved

July/August 2002

47

APPLICATIONS FEATURE

Fig. 5 A typical lab-on-a-chip device, fabricated at the Rutherford Appleton Laboratory, based on a combination of polymer and glass.

mechanical methods like diamond milling, are also likely to be part of the mix. Cui believes that many applications will require microsystems produced independently of semiconductor 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-achip’ is the passing of a current through a liquid drawn by the capillary action of a narrow channel, which must be nonconductive 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 highvolume 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 a ‘second Si revolution’. He argues that the material remains

48

July/August 2002

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 microfluidics, 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 polycrystalline Si. Many in the MEMS community are trying to develop Si micromachining as a potential route to affordable batch fabrication in integrated circuit (IC) foundries. A four-level fabrication process (one ground plane/electrical interconnect and three mechanical layers) patented by Sandia National Laboratories as the ‘Sandia Ultra-planar Multi-level MEMS Technology’ (SUMMiT™) is

Fig. 6 The transmission and linear rack elevate the mirror located in the lower-right of the frame. (Courtesy Sandia National Laboratories, SUMMiT™ Technologies.)

APPLICATIONS FEATURE

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 splitters, switches, (de)multiplexers etc. – needed for highspeed 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 are attracting industrial players in growing numbers. On the optical side, for example, Microvision, Inc. has begun fabricating a new MEMS optical scanner 60% smaller than previously available devices for electronic imaging. Network Photonics recently claimed the world’s first MEMS-based wavelength switch, while tunable Fabry-Perot filters for use in trunk optical communications are now available from Solus MicroTechnologies. The latter, intended for cost-sensitive applications, are based on Solus’ Compliant MEMS technology, which is mainly non-Si but retains some Si-based elements. MEMS-based switching modules were recently supplied by OMM to Lumentis AB for optical management sub-systems. Other players include Lucent (see Fig. 7 and Cover Story for examples of Lucent’s devices), Agilent, and PHS. IMT, as previously indicated, will be one of those seeking to marry Si-free technology with conventional CMOS fabrication to achieve the best results and believes that this ‘dual fuel’ approach will expedite penetration of MEMS into markets eager to receive devices that deliver reliable performance at acceptable prices. One, at first sight surprising, alternative to reliance on ‘standard’ Si is the use of waste. This has been suggested by PTMC Associates of Singapore, who studied the possible application of Si particulate recovered from kerf slurries

Fig. 7 Each of the mirrors in a WaveStar™ LambdaRouter is about as wide as the eye of a standard sewing needle. (Reprinted with the permission of Lucent Technologies, Inc./Bell Labs.)

produced when Si ingots and wafers are sliced by a wire saw. Fabricators normally recover SiC (originally on the saw) from the kerf for re-use, but dispose of the Si microparticles. PTMC, who has a patent pending on the technology, compound the particles with thermoplastic feedstock from which MEMS components can be formed using metal powder injection moulding (MIM) techniques. This established power metallurgy route has proved able to deliver complex tiny parts consistently and in large numbers. Using the technique could, say its proponents, circumvent the costly and labor-intensive IC manufacturing processes currently employed. The initial investment required would, it is argued, be tiny compared with that needed for conventional fabrication. It remains a matter for conjecture to what extent this ‘new’ material shares its properties with conventional Si.

Expectation of a breakthrough In summary, perhaps there was never any reason to expect that what has taken forty years to achieve in microelectronics could be done more quickly with MEMS. However, it appears as though the present industrial and research initiatives, including the use of alternative materials and processes alongside the currently ubiquitous Si-CMOS, could be clearing the way for the final breakthrough. MT

July/August 2002

49