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.
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.