Special report: Automotive MEMS drive themselves

Getting to three axes
MEMS accelerometers work by measuring the capacitance between a moving mass and the frame of reference (the die). The moving mass is made by etching away everything alongside and underneath it to physically isolate it from the die, except for the care­fully constructed suspension beams. Maxi­mizing the capacitance of the assembly, and therefore the sensitivity and range of the sensor, usually involves creating complicated interdigitated fingers of silicon. The mass is free to move in one axis and is held in position from the bulk silicon by supporting beams that constrain its movement in the perpendicular direction.
 
One ­and two ­dimensional, in­ plane ac­celerometers can be made in this way, but to integrate a 3­D accelerometer a different structure has to be used. By calibrating for the different geometries and sensitivities of the springs, it is possible to produce figures for acceleration in all three axes. Typical low ­g accelerometers measure full­ scale accelerations up to about 16 g, depending on how they are calibrated and trimmed.

The common form of MEMS gyroscope is a vibrating, typically wheel­ shaped structure; when subject to rotation, it allows a transducer to detect a Coriolis term in its equa­tions of motion.

Infrared MEMS
There is also an emerging automo­tive market for microbolometers and CMOS image sensors to en­able heads­up displays with night­vision capabilities, according to Yole's Robin.

The principle of operation for a microbolometer is analogous to that of an image sensor, with an array of pixels that are sensitive to infrared radiation. But the microbolometer requires that the heat­absorbing array be isolated from a support­ing silicon substrate. It is necessary to etch away a sacrificial layer, which can be up to 2 microns deep.

Thus the principle of manufacture for a microbolometer is similar to that for inertial MEMS sensors.
Mainstream adoption of what are effectively night ­vision cameras in cars is likely to be driven, again, by regula­tion. One idea is that information about pedestrians or animals moving in front of a vehicle can be displayed on the windshield as a form of aug­mented reality, to help the driver avoid obstacles—and, in extreme circum­stances, to trigger automated braking.

Pending EU regulations making such pedestrian detection mandatory are ex­pected to kick ­start a market around 2014 or 2015, Robin said.

Two European collaborative research projects (ADOSE and ICU+FNIR) pursu­ing this technology have recently wrapped up their work, and some top­end vehicles from Toyota, BMW and Ford already offer night­vision systems, Robin said.

Scanning­ micromirror optical MEMS devices, used as projection display drivers, may have a role to play in pro­jection heads­up displays, he added.