Page 77 - Microsensors, MEMS and Smart Devices - Gardner Varadhan and Awadelkarim

58 MEMS MATERIALS AND THEIR PREPARATION

atoms with a silicon wafer surface under ultrahigh vacuum conditions (~10 –10 torr).
MBE can achieve precise control in both chemical composition and impurity profiles
(if introduced intentionally). Single-crystal multilayer structures with dimensions on the
order of atomic layers can be made using MBE.

3.4 CERAMIC, POLYMERIC, AND COMPOSITE
MATERIALS

Ceramics are inorganic materials that consist of metallic and nonmetallic elements that
are chemically bonded together. Examples of these are alumina (Al 2O 3), salts, such as
sodium chloride (NaCl) and calcium fluoride (CaF 2), and ceramic superconductors such
as YBa 2Cu 3O 6.5.
Most ceramic materials, whether single-crystalline or noncrystalline (glass), have high
hardness and high-temperature strength but tend to be mechanically brittle. Several new
ceramic materials have been developed for engine applications; they are lightweight and
have high strength of hardness, good resistance to heat and wear, reduced friction, and
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insulating properties . These properties are highly desirable for engine applications. An
important application for ceramics is their use in tiles for the NASA space shuttle. These
ceramic tiles thermally protect the aluminum internal structure of the space shuttle during
ascent out of and reentry into the earth's atmosphere. Table 3.8 gives the structure and
uses of some selected engineering ceramics.
Polymers are materials that are organic (containing carbon) compounds, which have
long molecular chains or networks. Examples of polymeric materials include epoxies,
polyesters, nylons, and silicones. The strength and ductility of polymers vary greatly, and
because of their atomic structure, most polymers are poor electrical conductors. In fact,
some polymers are excellent insulators and are used in electrical insulation applications.
In general, polymers have low densities and relatively low softening or decomposition
temperatures. The physical properties of some common polymers are given in Table H.2
of Appendix H.
Because polymers are either long snakelike structures or three-dimensional random
networks, polymeric materials are usually noncrystalline. However, one may still find
some single-crystal polymers. Single-crystal properties are encountered only in linear

Table 3.8 Ceramics and some of their application areas
Ceramic Structure Applications
Material Structural Electrical Optical Magnetic
Silicates of Li, Al, etc. Crystalline X X
Alumina Hexagonal X X X
Zirconia Cubic X
Nonmetallic nitrides, e.g. Si Glass X X
Metallic silicides Tetrahedral X
Spinel Cubic X
Garnet Cubic X

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The physical properties of some common ceramic materials are listed in Appendix H.