Page 873 - Mechanical Engineers' Handbook (Volume 2)

864 Introduction to Microelectromechanical Systems (MEMS): Design and Application

mation and actuators use information to influence something in the world. Another class
includes systems that integrate both sensors and actuators to provide some useful function.
This classification, like most, is imperfect. For example, some devices that are dominantly
sensors have actuators built into them for self-testing. Airbag triggers are an example. How-
ever, the framework provides a simple but quite comprehensive framework for considering
MEMS devices.
In this chapter we will discuss some aspects of the design of these devices and introduce
the reader to the technology used. In addition, we will discuss the structure of some of those
devices.

2 MICROFABRICATION PROCEDURES
The fact that the field of MEMS largely grew out of the integrated circuit (IC) industry has
been noted often. There is no doubt that the use of fabrication processes and associated
equipment that were developed initially for semiconductor industry has given the MEMS
industry the impetus it needed to overcome the massive infrastructure requirements. However,
it is noted that the field of MEMS has gone far beyond the materials and processes used for
IC production. The situation is indicated schematically in Fig. 1. About a half dozen mate-
rials, notably silicon and its oxide and nitride, and standard microfabrication processes, such
as lithography and ion implantation, oxidation, deposition, and etching, have generally been
employed to make ICs. The set of materials used in IC devices is expanding to include,
for example, low-dielectric-constant materials, polymers, and other nonconventional IC
materials.
Many MEMS can be made with the same set of materials and processes as used for
microelectronics. However, one of the hallmarks of the emerging MEMS industry is the use
of numerous other materials and processes. Most basically, substrates other than silicon are
being employed for MEMS. Silicon carbide has been demonstrated to be a good basis for
many mechanisms that can stand higher temperature service than silicon. Diverse materials
can be used within MEMS devices. While aluminum and, recently, copper are the metals
used in IC devices, micromachining of many other metals and alloys has been demonstrated.
Magnetic materials have been incorporated into some MEMS devices. Piezoelectric materials
are especially attractive for MEMS because of their electrical–mechanical reciprocity. That
is, application of a voltage to a piezoelectric material deforms it, and application of a strain
produces a voltage. Zinc oxide and lead zirconium titinate (PZT) are important piezoelectric
materials for MEMS. Many other examples of materials employed for MEMS could be given.
However, the point is clear. Micromechanics are made of many more kinds of materials than
microelectronics.

MEMS
MEMS
Processes

ICs
Figure 1 The number of materials and processes em-
ployed to make MEMS greatly exceeds those used to man-
Materials ufacture integrated circuits.