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506 CAM DESIGN HANDBOOK

15.2 BACKGROUND ON MEMS

The field of microelectromechanical systems (MEMS), as it is called in the United States,
or microsystems technology (MST), as it is called in Europe and elsewhere, deals with
devices that are in the size range of micrometers. To appreciate the size of MEMS, con-
sider the size of traditional macroscale machines, which can be seen with the naked eye
and are manufactured using traditional machining methods. Some critical dimensions of
the macromachinery can be very small (at the micrometer or even nanometer level), espe-
cially when they are used as precision instrumentation. Even in those cases, they are not
called micromachinery or MEMS because in the micro category the general dimensions,
not just the smallest and critical dimensions, are in the size range of microns. In MEMS,
based on the current microfabrication technologies, the smallest dimension can be one-
tenth of a micrometer and the overall size of the device can be of the order of millimeters
and sometimes even a centimeter. In recent years, there has been significant interest in
nanoscale systems. This third category is clearly different from MEMS just as MEMS is
different from macromachinery. In nanoscale devices, the general dimensions are in the
range of nanometers, and the smallest dimension can be as small as a few atoms across.
Thus, this category of devices is sometimes called molecular machinery. MEMS is there-
fore in between macro- and nanoscale devices.
The MEMS technology is more than mere miniaturization of the macroscale devices
and systems. MEMS devices are not only smaller but are also more functional, faster, and
cheaper than their macro counterparts. The reasons for enhanced functionality are twofold.
First, there are advantages when some physical, chemical, and biological devices are
scaled down to the micro level. An example of this comes from numerous ways of mod-
ulating light using microoptomechanical devices comprising mechanical structures with
feature sizes and movements in the range of wavelengths of visible light. Likewise, micro-
chemical reactors make it easier to have an extremely well-controlled environment for the
reaction to take place and also lead the way to distributed microreactor arrays. Second,
complete integration of sensors, actuators, electronics, and components made of movable
solids and fluids is possible to an unprecedented degree. This integration does not refer to
just an assembled final product composed of a variety of subsystems but to a truly inte-
grated product that is conceived, designed, and fabricated in a unified manner. Further-
more, MEMS devices can usually be operated at higher speeds than macrodevices because
of the way in which their inertia, stiffness, and time constants of various phenomena scale
when the size is decreased.
The micron-scale size of MEMS devices can be mainly attributed to the choice of mate-
rial and the fabrication processes used to make them. They could be made, for example,
10 to 100 times larger while still retaining their functional advantages over macrodevices.
However, the MEMS field has its origins in microelectronics, as it was natural to make
micromechanical components using the same fabrication processes used to make inte-
grated chip (IC) circuits. The economical benefits of MEMS devices are a direct conse-
quence of batch fabrication and of the well-developed microfabrication techniques of the
IC industry. As the MEMS field progressed rapidly from the late eighties, traditional micro-
fabrication techniques for MEMS have been substantially enhanced and a number of new
techniques have been developed specifically for MEMS.
The study of MEMS began with applications and is advancing rapidly with an increas-
ing number of applications in a wide range of fields. In the mid-1960s, researchers at West-
inghouse Research Laboratories developed a resonant gate transistor that used gold beams
that are only 100 microns long. Miniaturized pressure sensors using silicon were built even
before that. In the 1960s and 1970s, a number of sensors and devices that used movable
micromechanical elements were developed. Tuning forks, accelerometers, projection dis-