PORTLAND, Ore. High aspect ratio microelectromechanical systems (MEMS) could harness the physics of inertial mass to enhance the sensitivity, signal-to-noise ratio and mechanical dampening of accelerometers for automobile airbags.
Now that accelerometers for auto electronic stability control (ESC) systems have been mandated in the U.S., Freescale Semiconductor Inc. has transfered its MEMS process from airbag accelerometers to ESC accelerometers, the company announced Wednesday (Sept. 10) in Tokyo.
"Historically, the higher the aspect ratio, the more sensitive a MEMS sensor," said Amy Leong, research director at Gartner Inc. (Stamford, Ct.) "Typically a MEMS sensor's moving element has about a 3-to-1 aspect ratio, but what it unique about Freescale's process is that they achieve a 20-to-1 aspect ratio."
Aspect ratios compare the thickness of moving MEMS structures to their surface area. For instance, a 3-to-1 ratio means the thickness is three times the horizontal width of its surface. Moving MEMS structures that have a low aspect ratio are susceptible to extraneous vibrations. By making their MEMS moving structures thicker, Freescale claims their ESC accelerometers are immune to extraneous vibrations. Freescale's proprietary technique uses a silicon-on-insulator wafer to create high-aspect-ratio MEMS accelerometer elements using deep-reactive ion etching (DRIE).
"We have already proven out our high aspect ratio MEMS process in accelerometers for airbags," said Francois Gilly, product line manager for automotive sensor products at Freescale. "We felt it was time to introduce it into electronic stability control accelerometers."
ESC sensors are already standard equipment on many vehicles in Europe and in several high-end vehicles in the U.S. The National Highway Traffic Safety Administration has mandated ESC on all U.S. passenger cars, trucks and buses by 2012, estimating that up to 9,600 lives will be saved and 238,000 injuries prevented annually.
Automotive market analysts iSuppli (El Segundo, Calif.) estimate that 48 million ESC units will be sold annually by 2012, resulting in sales of $715 million--up from $378 million in 2006.
"For the automotive industry, the safety area is by far the fastest growing segment," said Leong. "ESC system enable cars to be extremely stable regardless of road conditions, and with the traction of all tires balanced, an automobile is not only safer, but also gets better gas mileage."
Ordinarily, an automobile only accelerates and brakes in the forward and backward directions, but when a two-axis accelerometer senses lateral motion in an automobile, the ESC system kicks into mitigate the skid by selectively braking individual tires and reving down the engine.
Freescale claims its ESC accelerometers, which are available in samples now and are slated for mass production by 2009, provide immunity to parasitic vibrations that can confuse ESC systems using standard low aspect ratio accelerometers. Freescale's two-chip solution--one chip with the MEMS element and a DSP alongside it in the same package--can also be customizied with firmware for specific automotive customers.
Freescale also plans to aim its accelerometers at consumer electronics applications to measure shock, vibration, tilt and other movements, for everything from washing machines, seismic detectors and security systems to washing machines, sports equipment and video game controllers.