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MEMS chips integrate more intelligence

Posted: 01 Jun 2007 ?? ?Print Version ?Bookmark and Share

Keywords:MEMS chips? automotive sensor? MEMS sensor?

The U.S. National Highway Traffic Safety Administration's requirement that all new passenger vehicles include electronic stability control (ESC) starting in 2012 will save up to 10,000 lives each year, the NHTSA has predicted. Even before that mandate emerged, MEMS chipmaker Freescale Semiconductor Inc. was on the trail of a combo MEMS sensor capable of implementing ESC.

"Electronic stability control is already standard equipment on European automobiles, where it has been very successful," said Dave Monk, MEMS automotive sensor operations manager at Freescale. "For our ESC MEMS chip, we plan to use a combination of low-G sensors and gyros, so we'll have angular momentum outputs in addition to acceleration."

According to Monk, Freescale's ESC chip will capitalize on two trends driving MEMS development: integration of multiple sensing elements into one package and integration of more intelligence, to embed decision-making functions at the point of sensing and to enable communication over sensor networks.

Freescale plans to serve those trends in its automotive MEMS offerings as well as smart multisensor MEMS chips for consumer applications, such as portable media players, cellphones, GPS, medical monitors and E911 locator devices.

Then and now
The company now known as Freescale began its MEMS development work at Motorola's Corporate Research Lab circa 1970. By the early 1980s, Motorola was in production with its first MEMS device, an uncompensated analog pressure sensor based on a piezoresistive transducer that provided a low, 40-60mV output in response to pressure changes.

By 1987, a temperature-compensated version of the sensor using laser-trimmed resistors was in production, and by 1992, a third flavor debuted that integrated bipolar electronics to offer 0-5V outputs.

"All our early MEMS chips were bulk-micromachined from single-crystal silicon," said Monk, adding that all three members of the first family "are still being manufactured and sold today."

Around 1989, Motorola added surface micromachining to its MEMS fabrication repertoire, initially for an accelerometer chip used to trigger airbags. That device, a two-axis MEMS sensor, went into production around 1992. Three-axis versions entered production in 1996.

The company next fielded surface-micromachined capacitive pressure sensors for gauging tire pressure. The first devices hit the market in 2003. In 2005, Freescale (spun out from Motorola in 2004) introduced its first surface-micromachined consumer-grade accelerometer.

Freescale next developed a proprietary MEMS process used to fabricate its most sophisticated MEMS chip yeta three-axis accelerometer announced last year. The proprietary technique uses a silicon-on-insulator wafer to create high-aspect-ratio MEMS accelerometer elements using deep-reactive ion etching (DRIE).

"We use seven processes on our MEMS lines," said Monk. "Three are bulk-micromachined; three are surface-micromachined. The seventh enables a lateral sensor to be mechanically overdamped, plus it gives higher SNR because of larger capacitor plates."

Freescale says its DRIE process yields MEMS mechanical structures that can be 10 times thicker than surface-micromachined structures. "DRIE enables MEMS structures as thick as 25m, compared with 2m for surface-micromachined MEMS structures," said Monk. That makes the DRIE structures "more stable mechanically and increases their capacitance more than an order of magnitude, because capacitance is proportional to the area of the capacitor's plate."

In the future, Freescale plans to advance the dominant MEMS trendsmultiple sensing elements and integration of MEMS dice with smart controllersin automotive as well as consumer apps.

"In automotive applications, there are two places that MEMS sensors with multiple elements are a good fit: tire pressure monitoring and stability control," said Monk. "For tire pressure, we envision putting multiple MEMS sensing elements in a package that not only reads out inflation levels but also uses an accelerometer and an integrated controller" to identify the tire in which it resides.

Multidice package
Freescale plans to include four dice in the package: a MEMS die for pressure sensing, another MEMS die for a three-axis accelerometer, an ASIC die for control and a wireless transmitter die. For consumer devices, Freescale envisions adding a fifth: MCU die.

"In the consumer space, there are going to be a lot of multiple-element MEMS chips used in portable wireless devices like cellphones," said Monk. "We envision using different MEMS elements for camera stabilization, autoscrolling, location sensing, altimeter gauge and power management. All types of existing consumer devices, like media players, can use them. So can sports equipment like golf clubs and baseball bats, and new devices like wireless medical monitors and E911 applications."

Freescale manufactures its MEMS devices in Canada and Japan as well as the United States. Back-end assembly, trim and test functions are performed in South Korea and Malaysia.

Despite its intensive efforts to expand MEMS operations, Freescale claims its MEMS development activity is limited today by a lack of personnel familiar with MEMS fabrication, including engineers capable of doing MEMS layout, verification, packaging, testing, quality control and software coding.

MEMS-savvy systems engineers and product, circuit and RF designers are also in short supply, according to Freescale.

- R. Colin Johnson
EE Times

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