They're increasingly finding their way into consumer, industrial, medical, automotive, and computer applications. Plus, we can't overlook their march into the instrumentation, military, and scientific sectors. It's obviously clear that microelectromechanical systems (MEMS) have arrived.
Illuminating this fact were the numerous presentations given at recent conferences like the 2004 Commercialization of MEMS Conference (COMS 2004) in Edmonton, Alberta, Canada. The annual conference was sponsored by the Micro and Nanotechnology Commercialization and Education Foundation (MANCEF).
What are the reasons behind the rapid maturation of MEMS technology? Many observers point to the standardization developments in testing, prototyping, and packaging MEMS ICs; MEMS production and processing advancements; and improvements in software design tools. MEMS devices have become more integrated and are coming down in price to compete in a host of applications (see "The MEMS Market," p. 54). They've grown through multiple levels of integration, from discrete devices that featured separate driver, signal-conditioning, interface, and control electronics to single-chip devices with nearly everything on the same die.
One good example of the latter is a new family of low-cost, low-g accelerometers introduced by Freescale Semiconductor (a wholly owned subsidiary of Motorola). These capacitive MEMS sensors offer tilt, motion, position, shock, and vibration sensing. Targeted applications for the devices are littered throughout the consumer, computer, automotive, industrial, medical, and scientific markets (see "Mini Low-g Accelerometers Sport Five New Functions," electronic design, June 7, p. 44).
Functionally, just about any application is now possible with MEMS devices, including every type of sensor, switch, tunable capacitor, inductor, antenna, transmission line, filter, and resonator. Some of the largest suppliers include Agilent Technologies and Infineon for resonators, Memscap and Taiwan's Wolshin for MEMS filters, and Teravicta and Magfusion for MEMS switches.
MEMS technology is even challenging the familiar reed switch. Memscap developed a surface-mount MEMS magnetic proximity switch that's just 1.8 by 1.8 by 0.8 mm (Fig. 1). It replaces larger reed switches, the smallest of which on today's market are still at least twice as large. On top of being a filter supplier, Memscap happens to be a major foundry for making MEMS devices.
The auto market looms large. One of the fastest growing applications within the MEMS device world comes from the automotive sector. Presently, all cars use dozens of pressure and accelerometer sensors, notably for automatic airbag deployment for the driver and front-seat passenger. Many automakers now use MEMS sensors for extra airbags intended for side-impact deployments, as well as rooftop deployments in case of vehicle rollovers.
"Over 70 potential applications exist for MEMS devices in the automotive market," explains Roger H. Grace, president of Roger Grace Associates, an authority on MEMS technology and markets and past president of MANCEF. "Many of these MEMS devices have found their initial applications in high-end vehicles like Mercedes Benzes and BMWs, where their performance and convenience features outweigh their relatively higher costs compared to low-end cars."
One such application is for adaptive ride control and electronically controlled suspension systems using MEMS low-g accelerometers from VTI Technologies, from 0.5 to 12 g (Fig. 2). "We're the largest supplier to high-end vehicles like Volvo, Mercedes Benz, and Cadillac for electronic control suspension. Three to five such sensors are used in the wheel hub and a reference point like the trunk," says Rick Russell, director of marketing and sales for VITI Technologies. "Auto makers are even considering using these sensors for motor dampening with actuators in the engine mount."
"New applications generally tend to find use in high-end vehicles first. They then migrate to lower-cost and higher-volume vehicle types," says Grace. While present automotive "killer" applications include absolute-manifold pressure (MAP) and airbag accelerometer sensors, Grace foresees large "killer" applications in the future for angular-rate, tire-inflation, wheel-speed, adaptive braking, fuel evaporation, fuel-line, CAM/crankshaft position, X-by-wire, and passenger-seat applications.
MEMS accelerometers made by companies like Analog Devices, Bosch, Dalsa, Delphi-Delco, Denso, Infineon, Motorola, VTI Technologies, and X Fab constitute about 90% of the MEMS automotive market. However, an even larger application looms in tire-pressure monitoring, mandated by the U.S. Government's National Highway Transportation and Safety Administration's (NHTSA's) TREAD (Transportation Recall Enhancement, Accountability, and Documentation) Act. It calls for tire-pressure monitoring systems in all vehicles made after 2006. Auto-industry analyst J.D. Power & Associates predicts that more than 17 million tire-pressure monitoring systems will be in cars by 2007.
Presently, automotive electronics suppliers prefer direct-pressure monitoring methods, where every car tire has a pressure sensor in the wheel hub, over indirect methods. With the indirect method, tire pressures are calculated from parameters other than those of the actual internal tire pressures, and they are thus an estimate rather than an accurate reading. With direct systems, each tire-pressure monitoring system contains a microcontroller and an RF transmitter that relays information to the driver on a front-panel readout.
Melexis has carefully studied tire-pressure monitoring systems, examining indirect, direct, and what it considers "intelligent" tire-pressure monitoring systems. It's concluded that the debate over which approach is best has yet to be decided. One problem is sensor battery power that must operate under extreme temperature conditions and withstand thermal and mechanical shocks.
According to Dirk Leman, Melexis' tire-pressure monitoring system product manager, passive direct tire-pressure monitoring systems are possible at a production cost of $6 per wheel-well sensing/transmitting system, compared to $5 for a battery-based system. In fact, the company demonstrated such a system using "energy harvesting" and 13-MHz magnetic coupling (Fig. 3). Kinetic energy is generated by the tire's motion, or it can be induced by electromagnetic coupling between a wheel-well-mounted RFID antenna and the receiver on the tire's pressure sensor.
According to Siemens VDO Automotive, the estimated average cost per new vehicle to consumers adds up to only $66.33 to implement direct tire-pressure monitoring systems. The company, in a joint effort with Goodyear Tire and Rubber Co., is developing such a system—using RF transmission—for European vehicles. Known as Tire IQ, the system utilizes two high-temperature-resistant (3 V total), lithium-manganese-dioxide (LiMnO2) coin-cell batteries developed by Hitachi Maxell Ltd. that have a lifetime of at least five years and possibly up to 10 years.
VTI Technologies also supplies 8-g MEMS accelerometers for tire-pressure monitoring systems. The company says it will have a three-axis accelerometer next year for many additional automotive and non-automotive applications.
When it comes to the smart home, MEMS' presence is already being felt within the consumer sector. MEMS devices are serving home-theater projection TVs that use digital light processors (DLPs) from Texas Instruments (Fig. 4). Other examples include stereo components, video games, bathroom scales, temperature thermometers, portable blood-pressure monitors, hair dryers, exercise and fitness equipment, washing machines, refrigerators, dishwashers, microwave ovens, toasters, vacuum cleaners, and home-security systems. All that's missing is a network that connects these basic building blocks, making the home even more intelligent.
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