Posted: 9:00 p.m. EST, 3/11/98

CAMBRIDGE, Mass. — With the introduction of its fourth-generation two-axis accelerometer, Analog Devices Inc. (ADI) becomes the first company to push microelectronic machines (MEMS) into consumer realms. The ADXL202 motion-sensing device has been designed into computer-game pads, said James Doscher, marketing manager of the company's micromachined-products division in Cambridge, Mass. The new application will be demo'd at the Electronic Entertainment Expo (E ³ ) show in Atlanta in May, said Doscher.

Other cost-sensitive applications in the works include pedometers and exercise monitors, damping controls for consumer appliances like washing machines, and a variety of tilt sensors. These new applications are a consequence of the die shrink and manufacturing enhancements applied to the company's earlier ADXL50 accelerometer, a device used to trigger air-bag deployment. Indeed, until the arrival of ADXL202, air bags were the primary application of the surface-micromachined capacitive sensors.

ADI has made several advances since ADXL50 began shipping in 1993. It released a low-g version sensitive to arm-and-hand movements in 1995, and a dual-axis accelerator last year. That device puts two accelerometers on one chip — turned 90 degrees from each other — to sense motion a full 360 degrees about an x-y axis.

Belt, arm or headband
The ADXL202 combines separate x-y sensors on a single beam. Running from a 3-V supply, it is sensitive enough, when worn on a belt, arm or headband, to register small motions. The game-pad application that will be revealed at the E ³ show portends new uses in the consumer sphere. "We have the cost structures to make this happen," said Doscher.

ADI's announcement breaks the image of microelectronic machines as expensive devices underwritten by government funding. MEMS detractors believe MEMS require an exotic manufacturing process, and produce miniaturized devices of interest only to scientists and government-funded researchers. MEMS technology etches a mechanical device — a micromachine — into a silicon substrate.

The use of silicon etching encourages miniaturization and allows signal-conditioning circuitry to be implemented on the same chip. But surface micromachining, which builds a 3-D structure with combinations of selective etching and polysilicon deposition, is typically more expensive and difficult to execute than is bulk-etching.

Surface micromachining requires more precision in the formation of mechanical beams and cantilevers. In the absence of high-volume applications to perfect the process, yields could be questionable.

Under Darpa funding, Analog Devices and others had developed mechanical resonators and tunable moving-plate capacitors that could pave the way for single-chip cell phones . These MEMS devices would replace the outboard discrete inductors and capacitors used to tune the radio-frequency sections with on-chip filters. But results in this area were so tentative that experimenters like Texas Instruments Inc. refused to talk about them.

The micromachining process developed by ADI — called "iMEMS," for Integrated Micro Electro Mechanical Systems — etches capacitor plates into a beam that shifts back and forth on one axis in response to motion. There is ordinarily a 1.3-micron gap between the capacitor plates, and the beam rides 1.6 micron above the surface of the substrate. The minimum detectable beam deflection is 0.02 angstrom. The shifting plates register a change in capacitance as small as 1 zF (10 ^-21 F). Collectively, the plates will register 10 fF (10 ^-15 F) as a full-scale response to a 2-g thrust of the motion sensor. The output of this sensor is converted to a PWM output. Even without an A/D converter, said Doscher, the PWM output has a resolution of about 14 bits. The current consumption is only 250 microA for each axis measured. Doscher admitted that some laser trimming of the signal conditioning is necessary to achieve this accuracy, but he views this as a part of the manufacturing "learning curve" that will inevitably disappear. "It's like the early days of making low-offset (25-microV) op amps," he said. "We do those routinely now without trimming."

When outputs of two perpendicular sensors are correlated, the capacitive device becomes a tilt monitor or a low-cost 3-D position sensor. The ADXL202 will likely need to be calibrated to some initial value in the

system that uses it, but ADI feels this can be accomplished by coupling it to a low-end mic rocontroller. The ADXL202 is packaged as a surface-mount IC to encourage integration with other board-level ICs. It will cost less than $10 in volume.

In addition to its $10 million investment in MEMS research, Analog Devices invested more than $50 million to equip a former Polaroid R&D fabrication facility in Cambridge to produce micromachines. ADI believes it will recoup its investment in a very short time. "MEMS will be a significant part of ADI's revenues within five years," said Doscher. If it happens, this will be no small feat, since ADI revenue growth — already at $1.3 billion — shows no sign of slowing down. But $200 million in MEMS — 10 percent of projected 2002 revenues for ADI — is not unreasonable, said Doscher, especially with the potential for selling these devices for consumer applications.

"Micromachining is a 'disruptive technology,' " said Doscher. "It changes the rules of the game. Who knows what kinds of applications it will lead to?"

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