MEMS Diversify and Advance
Peter Singer, Editor-in-Chief -- 10/1/2002 Semiconductor International
At a Glance
Devices such as the lab-on-a-chip and new gyroscopes are leading the way in the high-growth MEMS market.
Microelectromechanical systems (MEMS) range from the mundane to the truly spectacular. At one end of the spectrum are devices such as the precisely machined nozzles used in ink-jet printers (if you can consider a hole in a piece of plastic a "device"). At the other extreme, MEMS are being used in an effort to allow the blind to see: A new research progam at Sandia National Labs aims to attach a MEMS chip to the retina of people's eyes that have been damaged by disease. Tiny cameras will send electrical impulses to the brain that once came from rods and cones (see "MEMS Chip Could See for the Blind"). Between these two extremes, you'll find MEMS almost everywhere: in automobiles, doctor's offices, optical networks, global positioning systems and wireless devices, to name a few. Like mainstream semiconductors, MEMS devices continue to diversify as new applications arise, such as tire pressure sensing. And they continue to advance, not only in terms of complexity and amount of on-board circuitry, but also in the design and manufacturing sophistication required to make them. How big is the MEMS market? That's something of a controversy among MEMS analysts. Are the read-write heads used in hard disk drives considered MEMS devices? How about hearing aids? Ink-jet nozzles? Some say yes, others say no. Marlene Bourne of Reed Electronics Group's In-Stat/MDR (Scottsdale, Ariz.) takes a conservative view. She defines MEMS as micron-sized, chip-level devices such as sensors or valves that can sense the physical environment, or manipulate gases, light or liquid. She says that the market for MEMS devices is expected to grow from $3.9B in 2001 to $9.5B in 2006. Other research groups, such as Europe's NEXUS (Grenoble, France), take a broader view, grouping MEMS with larger systems such as hearing aids and cardiac pacemakers. NEXUS expects this "micro-systems" market to increase from $30B in 2000 to $68B by 2005. So far, the highest-volume applications for MEMS have been ink-jet printer heads and inertial sensing (e.g. accelerometers used for air bags). Both markets are considered to be fairly mature. For example, Analog Devices (Norwood, Mass.) last month celebrated the shipment of its hundred millionth MEMS-based acceleration sensor, noting that the company now has 500 people, three R&D centers and three manufacturing sites dedicated to high-volume MEMS production. In other areas, MEMS devices have shown tremendous promise, but are only beginning to be adopted. Two of the most exciting and fastest growing of these are bioMEMS (e.g. lab-on-a-chip) that can perform DNA analysis and rf MEMS for wireless devices. Also of interest is how MEMS micro-mirror arrays can be used for optical switching, a requisite component in an all-optical network.
1. The world’s smallest microchain drive fabricated at Sandia out of silicon, shown engaging device drive gears. The distance between chain link centers is 50 µm. (Source: Sandia)
Of course, there are myriad other types of MEMS that have been developed. On the sensor side, there are MEMS that measure things such as pressure, humidity, temperature, mechanical stress and various chemicals (most notably glucose). On the actuator side, there are gyroscopes, microphones, valves, micropumps and tweezers. Even miniature motors that work with tiny gears and microchains have been built (Fig. 1). Perhaps the ultimate MEMS is the gas turbine engine built out of silicon by researchers at the Massachusetts Institute of Technology (MIT, Cambridge, Mass.). Smaller than 1 cm3, the engine is complete with a set of fuel plenums, pressure ports, fuel injectors, igniters, fluidic interconnects and compressor and static airfoils.1
2. Detail from the world’s first fully integrated gyroscope for angular rate sensing applications, manufactured by Analog Devices with the same surface micromachining process used to make accelerometers. (Source: Analog Devices)
As exciting as such devices are, however, Bourne says that most of the growth will be in more traditional areas. "Despite the diversity of the devices that are continually being developed and are moving into the market, the real opportunity in the short term is for increased use of inertia and pressure sensors. That's because the price keeps coming down, which opens the door for new applications."
3. BioMEMS, such as this DNA separation device, are one of the high-growth areas in the MEMS market. (Source: L.R. Huang, Princeton University)
Figures 2 and 3 are examples of the latest MEMS devices. Figure 2 shows a gyroscope, scheduled for introduction this month. The gyroscope has an 8 µg polysilicon mass, suspended 2 µm over the IC substrate, with 5000 fingers spaced by 1.6 µm. The circuitry is so sensitive that it can detect deflection of 0.00016 Å. Figure 3 shows a lab-on-a-chip designed for DNA analysis.2 The device consists of an array of micron-scale posts as the sieving matrix, and relies on integrated microfluidic channels to spatially tune uniform electric fields over the matrix. The microfluidic channels connect to electrolytic buffer reservoirs, where voltages are applied. DNA molecules are injected with a single channel connected to a DNA loading reservoir, and collected in different channels after separation. Asymmetric pulsed fields causes DNA molecules to move in different directions according to their molecular weights. The foundry factor However it is defined, analysts agree on one thing: The MEMS market is growing — exploding really. According to the MEMS Industry Group (MIG, Pittsburgh), there are about 1.6 MEMS devices per person in use today in the United States, and the number is expected to grow to nearly five devices per person by 2004, a compound annual growth rate of 45%. This kind of growth has attracted a lot of attention. In Europe alone, MEMS manufacturing activity employs more than 4000 people working in more than 100 different plants.3 Interest in MEMS is also running high in the Asia-Pacific region, where a number of new foundries dedicated to MEMS have been established over the past year. "Fabs have been popping up in Taiwan, Malaysia, Singapore and even India," Bourne said. Unlike semiconductor foundries, MEMS foundries typically offer design, prototyping and packaging services. That's a critical point, says consultant/analyst Roger Grace of Grace Associates (Mountain View, Calif.), who estimates that there are now 60-80 MEMS foundries in the world. "A good MEMS foundry needs to have design experience. They need to understand the nuances of design and the implications of changes to the design on the electrical and mechanical performance of the device, as well as the yieldability and cost of production issues." On the other hand, the MEMS fabless model is so new that it's not really proven. "At the moment you see a lot of small MEMS producers and small suppliers of equipment," said Peter Podesser, CEO of EV Group (EVG, Schärding, Austria), an equipment supplier focused on MEMS. "If the foundry model really works out, I think there will come a transition similar to the one in the semiconductor industry. A failure of such a model would have some collateral damage to the development of MEMS from the technology side." About half of the companies involved in MEMS today have some level of production capability, Bourne said. "What differentiates this industry is the number of small start-ups that have their own in-house facilities for prototyping and low-level production. It is not strictly a fabless model." Manufacturing evolution The MEMS industry has employed three distinct processing technologies, more or less defined by the height of the finished structure: surface micromachining, bulk micromachining, and a process commonly known as LIGA, an acronym from the German words for lithography (lithografie), electroplating (galvanoformung) and molding (abformung). For most of the highest-volume applications, there has been a tendency to use surface micromachining, which is similar to standard IC processing technology not only because it makes it easier to integrate circuitry with the device, but also makes it possible to use the same equipment developed for IC manufacturing. "We had the strategic mindset that we had when we went into the manufacturing of our MEMS 10 years ago; that we wanted to stay close to the IC industry as far as tool development," said Craig Core of Analog Devices. "We saw a path to be able to use standard poly deposition tools and poly etch tools. We changed our recipes so we got thicker films, but still got to take advantage of the industry's evolution of those standard IC-type tools." In-Stat's Bourne added, "A lot of companies that have in-house fabrication facilities are actually buying used semiconductor equipment. It's perfectly suitable." On the other hand, Core also sees some advantages in bulk micromachining. "Unlike the IC curve, where people are going for smaller and smaller, some of our MEMS are requiring thicker and bigger. We get better sensitivity out of our sensors, and we get flatter structures, which in the optical world is an important criterion." As a result, the distinction between surface and bulk is becoming blurred. "People are finding that bulk micromachining has some advantages, and tool manufacturers are starting to develop tools that look like IC-type tools in terms of wafer handling, but are doing deep silicon etches and processes that are considered more a bulk technique than a surface technique," Core added. Paul Lindner, EVG's group technology manager, said the demand for lower-cost MEMS is a driving force that creates several technology trends in the manufacturing of MEMS, including: The use of larger wafer diameters (200 vs. 150 mm). A higher degree of automation. Packaging at wafer level through bonding vs. chip level. Integration of MEMS and ASIC at wafer level, either on one wafer that is subsequently packaged, or by 3-D interconnect (bonding of MEMS and ASIC wafer). "The strong trend towards a higher degree of automation is reflected in the equipment-installed base," Lindner said. "Where years ago individual process steps were integrated with cassette-to-cassette handling systems, today we install both the lithography steps and the aligned wafer bonding systems fully integrated." Roadmaps for sale If you're looking for a MEMS roadmap, you're in luck — kind of. There are no less than three roadmaps available for sale from three different organizations. NEXUS has one that was issued in late 2000, MIG's was issued in late 2001 and MANCEF's (Micro and Nanotechnology Commercialization Education Foundation) came out just last month. All of these organizations have taken different approaches, with MANCEF's roadmap being the most thorough, said Grace, who also is vice president of MANCEF. It contains 14 chapters addressing the significant factors on the commercialization of microsystems, including foundries, standards, EDA tools, market analysis and reliability. MANCEF was incorporated in December 2000 and has about 350 members representing companies and institutions from North America, Europe, Asia, the Pacific Rim and the Middle East. NEXUS claims to be the largest MEMS/MST network in Europe, and aims to provide "microsystems professionals with access to strategic guidance through reports containing the most up-to-date analysis of markets, technologies, applications, and long-term trends." NEXUS has more than 500 member organizations and 1000 personal members, many of whom are in the United States and Far East. MIG describes itself as "the unifying voice of the MEMS industry" and the trade association representing U.S. MEMS and microstructures industries. The association's mission is "to enable the exchange of non-proprietary information among members; provide reliable industry data that furthers the development of technology; and to work with legislators, policymakers and others toward the greater commercial development and use of MEMS and MEMS-enabled devices."
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MANCEF | MEMS Industry Group | NEXUS-emsto
References
A. Mehra, et al, "A Six-Wafer Combustion System for a Silicon Micro Gas Turbine Engine," J. of Microelectromechanical Systems, December 2000, p. 517. L.R. Huang, et al, "A DNA Prism: The Physical Principles for Optimizing a Microfabricated DNA Separation Device," scheduled to be presented at the International Electron Devices Meeting (IEDM), December 2002. E. Mounier, "A Look at Europe's MEMS Industry ," Semiconductor International , December 2001.