Page 14 - MEMS Mechanical Sensors

1.3 Mechanical Transducers 3

while the latter is a methodology. The word “system” is common to both, implying
that there is some form of interconnection and combination of components. As an
example, a microsystem might comprise the following:

• A sensor that inputs information into the system;
• An electronic circuit that conditions the sensor signal;
• An actuator that responds to the electrical signals generated within the circuit.
Both the sensor and the actuator could be MEMS devices in their own right. For
the purpose of this book, MEMS is an appropriate term as it specifically relates to
mechanical (micro) devices and also includes wider areas such as chemical sensors,
microoptical systems, and microanalysis systems.
There is also a wide variety of usage of terms such as transducer, sensor, actua-
tor, and detector. For the purpose of this text, we choose to adopt the definition pro-
posed by Brignell and White [3], where sensors and actuators are two subsets of
transducers. Sensors input information into the system from the outside world, and
actuators output actions into the external world. Detectors are merely binary sen-
sors. While these definitions do not specifically relate to energy conversion devices,
they are simple, unambiguous, and will suffice for this volume.
As we will see in the following, micromachined transducers are generally (but
not exclusively) those that have been designed and fabricated using tools and tech-
niques originating from the IC industry. In general, there are two methods for sili-
con micromachining: bulk and surface. The former is a subtractive process whereby
regions of the substrate are removed; while with the latter technique layers are built
up on the surface of the substrate in an additive manner.

1.3 Mechanical Transducers

The market for micromachined mechanical transducers has, in the past, had the
largest slice of the pie of the overall MEMS market. This is likely to be the case in the
immediate future as well. The main emphasis of this text is on mechanical sensors,
including pressure, force, acceleration, torque, inertial, and flow sensors. Various
types of actuation mechanism, relevant to MEMS, will also be addressed together
with examples of the fundamental techniques used for mechanical sensors. The
main methods of sensing mechanical measurands have been around for many years
and are therefore directly applicable to microsensors. There is, however, a signifi-
cant effect that must be accounted for when considering mesoscale devices (i.e.,
those that fit into the palm of your hand) and microscale devices. This is, of course,
scaling. Some physical effects favor the typical dimensions of micromachined
devices while others do not. For example, as the linear dimensions of an object are
reduced, other parameters do not shrink in the same manner. Consider a simple
cube of material of a given density. If the length l is reduced by a factor of 10, the
3
volume (and hence mass) will be reduced by a factor of 1,000 (l ). There are many
other consequences of scaling that need to be considered for fluidic, chemical, mag-
netic, electrostatic, and thermal systems [4]. For example, an interesting effect, sig-
nificant for microelectrostatic actuators operating in air, is Paschen’s law. This