Page 68 - Microsensors, MEMS and Smart Devices - Gardner Varadhan and Awadelkarim

METALS 49

Figure 3.16 Hexagonal close-packed (HCP) crystal structure

discussing these different MEMS materials, several material-preparation techniques are
described. These techniques are described within different material sections according
to the frequency of their use in preparing the particular material under consideration.
However, it should be pointed out that several of the preparation techniques described in
the following sections are used to prepare more than one type of material. For instance,
the sputtering technique is described in the metals section; however, it is also used to
deposit semiconductor and ceramic films.

3.2 METALS

3.2.1 Physical and Chemical Properties

Metals are inorganic substances that are composed of one or more metallic elements.
Examples of metallic materials with one element are iron, aluminum, copper, and cobalt.
When a metallic material is composed of two or more metallic elements, it is called an
alloy. Some metallic materials may contain nonmetallic elements that are added inten-
tionally to improve the material's engineering qualities. An example of such a metallic
material is steel, in which the nonmetallic element carbon is added to iron. Metals and
alloys are commonly divided into two types: ferrous metals and alloys that contain high
concentrations of iron and nonferrous metals and alloys that contain no or very low
concentrations of iron.
Single-crystal metals are mostly found in the three simple types of cells: BCC, FCC, and
HCP. Under different conditions of temperature and pressure, different crystal structures
(that is, different unit cells) or phases for the same metal are formed. For example, a bar
of iron at room temperature has a BCC structure. However, if the bar is heated above
4
900 °C, the structure changes to FCC . The BCC iron and the FCC iron are called the
a-phase and y-phase, respectively.

4
The phase change of a material is sometimes used as the sensing or actuating principle of a microdevice. One
example is a shape-memory alloy.