Introduction to Nanotechnology

13.1. MICROELECTROMECHANICAL SYSTEMS (MEMSs) 333
an applied electrical signal, or an electrical response resulting from a mechanical
deformation.
The major advantages of MEMS devices are miniaturization, multiplicity, and the
ability to directly integrate the devices into microelectronics. Multiplicity refers to
the large number of devices and designs that can be rapidly manufactured, lowering
the price per unit item. For example, miniaturization has enabled the development of
micrometer-sized accelerometers for activating airbags in cars. Previously an
electromechanical device the size of a soda can, weighing several pounds and
costing about $15, triggered airbags. Presently used accelerometers based on MEMS
devices are the size of a dime, and cost only a few dollars. The size of MEMS
devices, which is comparable to electronic chips, allows their integration directly on
the chip. In the following paragraphs we present a few examples of MEMS devices
and describe how they work. But before we do this, let us examine what has been
learned about the difference between the mechanical behavior of machines in the
macro- and microworlds.
In the microworld the ratio of the surface area to the volume of a component is
much larger than in conventional-sized devices. This makes friction more important
than inertia. In the macroworld a pool ball continues to roll after being struck
because friction between the ball and the table is less important than the inertia of its
forward motion. In the microregime the surface area : volume ratio is so large that
surface effects are very important. In the microworld mechanical behavior can be
altered by a thin coating of a material on the surface of a component. We shall
describe MEMS sensors that take advantage of this propedy. Another characteristic
of the microworld is that molecular attractions between microscale objects can
exceed mechanical restoring forces. Thus the elements of a microscale device, such
as an array of cantilevers, microsized boards fixed at one end, could become stuck
together when deflected. To prevent this, the elements of micromachines may have to
be coated with special nonstick coatings. In the case of large motors and machines
electromagnetic forces are utilized, and electrostatic forces have little impact. In
contrast to this, electromagnetic forces become too small when the elements of the
motors have micrometer-range dimensions, while electrostatic forces become large.
Electrostatic actuation is often used in micromachines, which means that the
elements are charged, and the repulsive electrostatic force between the elements
causes them to move. We will describe below an actuator, which uses the
electrostatic interaction between charged carbon nanotubes. Many of these diffe-
rences between micromachines, and macromachines become more pronounced in
the nanoregime. There are many devices and machines that have micrometer-sized
elements. Since this book is concerned primarily with nanotechnology, we give only
a few examples of the microscale analogs.
Figure 13.1 illustrates the principle behind a MEMS accelerometer used to
activate airbags in automobiles. Figure 13.la shows the device, which consists of a
horizontal bar of silicon a few micrometers in length attached to two vertical hollow
bars, having flexible inner surfaces. The automobile is moving from left to right in
the figure. When the car suddenly comes to a halt because of impact, the horizontal
bar is accelerated to the right in the figure, which causes a change in the separation