Electronic Design

MEMS: Size Does Matter

Packaging and testing developments, greater standardization efforts, improved software, and the emergence of MEMS "clusters" worldwide point to a ready-to-explode market.

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.

MORE COST-EFFECTIVE TESTING
Testing and packaging absorb much of a MEMS device's final cost. Developments in these areas have been ongoing, though, with companies like ETEC spearheading the creation of turnkey automated MEMS testing systems.

At the COMS 2004 meeting, for instance, the Fraunhofer Institute for Microelectronics & Systems reported on a cost-effective wafer-level test, calibration, and assembly infrastructure for turnkey integrated pressure sensors. The system achieves accuracies within ±1% and has a throughput of several million sensors per year.

At the same meeting, Scanimetrics discussed an improved wafer-level testing method using wireless probe cards. The company says this dramatically improves wafer-testing effectiveness and lowers testing costs.

Suss-MicroTec achieved a major breakthrough in MEMS testing with the development of next-generation test equipment for MEMS devices at the wafer level (Fig. 5). It offers a standardized means of testing, resulting in lower costs. The equipment enables simulation or measurement using sound, light, pressure, motion, and even fluidics, all in high- and low-temperature environments as well as in humid conditions encountered by working MEMS devices.

One major application for Suss' equipment involves the testing of automotive tire-pressure sensors. Inertial sensors, microbolometers, and even acoustic MEMS speakers are other targeted applications.

SOFTWARE LENDS A HELPING HAND
As mentioned earlier, packaging costs constitute a large portion of a MEMS device's final cost. In fact, it can be as high as 80% of the total product's cost. While packaging developments for MEMS devices are ongoing, software has come to the rescue.

Three-dimensional software for open-tool packaging, like those supplied by Coventor, is making a big difference in reducing packaging costs. These MEMS-specific software packages allow engineers to design MEMS chips with the packaging in mind, well before the chip is placed in a package or even before selecting the package. The software contains representations of molds for various open-tool package types, each described in the software library with regards to geometry and type of material for the package. Consequently, designers can bring MEMS-enabled products to market faster and at a lower cost.

Recently, Coventor announced an automotive "design tool set" software package. It lets MEMS designers build complex devices, such as accelerometers, gyroscopes, and pressure sensors for automotive applications, that require multiphysical domain analyses.

RF APPLICATIONS WILL SPREAD
RF MEMS for microwave and millimeter-wave applications represents one of the most lucrative applications for MEMS devices. In addition to their potential for higher levels of integration and miniaturization, RF MEMS devices offer lower power consumption, higher linearity, and higher Q factors than conventional commercial devices. And, they can be made on many different processes that use silicon, gallium arsenide (GaAs), silicon carbide (SiC), and silicon-on-insulator (SOI) substrates. Over the past couple of years, Agilent Technologies and Infineon have been supplying RF MEMS switches for mobile phones.

RF MEMS switches display excellent RF characteristics, including insertion loss of about 0.1 dB and isolation of about 30 dB in series-configured switches at lower RF frequencies (VHF to about 10 GHz). This makes them attractive candidates for switchable routing in RF front ends, capacitor banks, routing time-delay phase-shifter networks, and electronically reconfigurable antennas.

One of the largest RF MEMS applications will exist in the military and automotive sectors. For military applications, phased-array radar systems are a target—a single phased-array radar system can make use of 500,000 RF MEMS switches. Although not as large a market, automotive applications include radar collision-avoidance systems.

Within a year or two, RF MEMS devices operating at 10 GHz and higher will find applications in high-speed instrumentation front ends and automotive radars. Here, their higher performance levels will offset their relatively higher costs compared to other approaches (often, there are no other approaches). Further down the road lie military radars, smart munitions, and satellite communications.

According to Wicht Technologie Consulting (WTC), large companies like Fujitsu, IBM, Intel, Matsushita, Memscap, and Philips plus others like Discera, Epcos, LG Electronics, and MEMX have already demonstrated the use of RF MEMS switches and expect to sample and mass produce them within the next few years. Research has also shown that high-voltage RF MEMS devices can be integrated on the same substrate as CMOS devices, further broadening RF MEMS applications in wireless communications. On top of that, RF MEMS devices will open up major opportunities in wireless handheld phones and basestations. The development of networks of wireless sensors for homeland security applications will largely impact users of RF MEMS devices, too.

GAZING AT THE CRYSTAL BALL
Expect MEMS devices to continue evolving through higher levels of integration, eventually leading to a single chip that holds not only the MEMS element and its signal-conditioning electronics, but also the control electronics. "There's a definite need to provide higher levels of integration, for both hybrid and monolithic ICs, including communications functionality and intelligence in a single package," says Grace.

New materials will broaden MEMS' commercial appeal. Many of today's microphone and microspeaker MEMS devices are built using condenser-type transducers. However, research has shown that MEMS ferroelectric membranes can be used instead. They're simpler to fabricate and free from polarization-voltage requirements, and they offer a wider dynamic range.

One development indicative of MEMS' growth and maturity is the formation of over 20 MEMS technology clusters in many countries (Canada, China, Germany, France, India, Korea, Mexico, the Netherlands, Taiwan, and the U.K.) as well as many states in the U.S. These clusters, according to Grace, tend to emerge from R&D-centric regions where academic and industrial research labs exist and venture capital funding is available.

Ultimately, whenever MEMS technology is mentioned, nanotechnology follows in the same phrase, which is the next logical step in miniaturization. However, nanotechnology faces many technical hurdles before it can reach commercial success (see "Nanotechnology's Path To Commercialization," p. 52).

NEED MORE INFORMATION?
Agilent Technologies
www.agilent.com

Analog Devices Inc.
www.analog.com

Bosch GmbH
www.bosch.com

Coventor Inc.
www.coventor.com

Dalsa Corp.
www.dalsa.com

Delphi-Delco Electronics
www.delphi.com

Denso Corp.
www.globaldenso.com

Discera Inc.
www.discera.com

Epcos AG
www.epcos.com

Etec Inc.
www.etec-inc.com

Fraunhofer Institute for Microelectronics & Systems
www.ims.fhg.de

Freescale Semiconductorw
ww.freescale.com

Goodyear
www.goodyear.com

Hitachi Maxell Ltd.
www.maxell.com

IBM Corp.
www.ibm.com

Infineon Technologies AG
www.infineon.com

Intel Corp.
www.intel.com

J.D. Power & Associates
www.jdpower.com

LG Electronics Inc.
www.lge.com

Magfusion Inc.
www.magfusion.com

MANCEF
www.mancef.com

Melexis Microelectronic Systems
www.melexis.com

Memscap Inc.
www.memscap.com

Motorola Inc.
www.motorola.com

MEMX Inc.
www.memx.com

NEXUS
www.nexus-mems.com

Roger Grace Associates
www.rogergrace.com

Scanimetrics Inc.
www.scanimetrics.com

Suss-MicroTec AG
www.suss.com

Teravicta Technologies Inc.
www.teravicta.com

Texas Instruments Inc.
www.ti.com

VTI Technologies
www.vti.fi

Wicht Technologie Consulting
www.wtc-consult.de

Yole Développement
www.yole.fr

X-Fab Semiconductor Foundries AG
www.xfab.com

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