Freescale's next-generation high aspect ratio
micro-electromechanical systems (HARMEMS) technology is a proven
technology for airbag sensing applications. The accelerometers have
an advanced transducer design that enhances sensor offset
performance. HARMEMS technology provides over-damped mechanical
response and exceptional signal-to-noise ratio to address customer
requirements. Since the airbag main ECU system is installed in the
vehicle cabin, over-damped HARMEMS technology enables a high degree
of immunity to high-frequency, high-amplitude parasitic vibrations.
HARMEMS technology has also been introduced in dual-axis
accelerometers used in electronic stability control (ESC) to
measure the lateral acceleration of the vehicle.
MEMS Surface Micromachining
In surface micromachining, the MEMS sensors are formed on top of
the wafer using deposited thin film materials. These deposited
materials consist of structural materials that are used in the
formation of the sensors and sacrificial layers that are used to
define gaps between the structural layers. Many of the surface
micromachined sensors use the capacitive transduction method to
convert the input mechanical signal to the equivalent electrical
signal. In the capacitive transduction method, the sensor can be
considered to be a mechanical capacitor in which one of the plates
moves with respect to the applied physical stimulus. This changes
the gap between the two electrodes with a corresponding change in
the capacitance. This change in capacitance is the electrical
equivalent of the input mechanical stimulus.
MEMS Bulk Micromachining
In bulk micromachining, the single crystal silicon is etched to
form three-dimensional MEMS devices. This is a subtractive process
in which the silicon in the wafer is specifically removed using
anisotropic chemistries. Using this bulk micromachining method,
sensors such as piezoresistive pressure sensors have been
manufactured in high volume. In the simplest implementation, the
silicon is selectively etched in certain areas to form a diaphragm.
In an absolute pressure sensor, the silicon wafer is then bonded
with another wafer (either of silicon or glass) to form a
vacuum-sealed cavity below the diaphragm. The diaphragm then
deflects in response to the applied pressure. The transduction
mechanism that has been widely used is the piezoresistive effect.
In piezoresistive materials, the change in the stress causes a
strain and a corresponding change in the resistance. Thus, when
implanted piezoresistors are formed at the maximum stress points of
the diaphragm, the deflection under the applied pressure causes a
change in the resistance. Typically, these piezoresistors are
formed as a bridge network and the voltage applied between two
terminals cause an output voltage to be measured between the other
two terminals.
;){kind=link}