Page 67 - Microsensors, MEMS and Smart Devices - Gardner Varadhan and Awadelkarim

48 MEMS MATERIALS AND THEIR PREPARATION

Figure 3.15 Tetrahedral structure of carbon in its diamond state

Each carbon atom has four nearest neighbours forming a tetrahedral bond. This diamond
structure could be visualised as placing an atom at the centre of the cube, two atoms
at the opposite corners of the top face of this cube, and two atoms placed at opposite
corners at the bottom face of the cube, but twisted 90° with respect to the top-face atoms.
This configuration is shown in Figure 3.15, but it is not the unit cell. The tetrahedral
bonds of the four corner atoms to the central atom are very strong and highly directional,
occurring at angles of ~ 109.5°. In essence, the diamond structure can be viewed as two
interpenetrating FCC lattices - one displaced from the other by one-fourth the length and
along a cube diagonal. The diamond structure, which is a special type of cubic structure,
is of particular interest because some of the electronic materials (semiconductors) have
diamond-like crystal structures. Moreover, diamond itself has been used as a functional
material in microdevices.

3.1.3.2 Hexagonal close-packed structure

The hexagonal close-packed (HCP) structure ranks in importance with the BCC and FCC
lattices; more than 30 elements crystallise in the HCP form. Underlying the HCP structure
is hexagonal lattice geometry (see Figure 3.16). To describe hexagonal structures, a few
simple modifications of the Miller indices of directions and planes are required. Instead
of three axes, x, y, and z, four axes are used - three in the horizontal (x, y) plane at
120° to each other, called a 1, a 2, a 3, and the fourth, c, in the z-direction. The use of the
extra axis makes it easier to distinguish between similar planes in the hexagonal structure.
Figure 3.16 shows some planes located using this four-axes reference frame. Using either
three axes (a 1 , a 2, and c) or four axes develops the notation for a direction. It is noted
that a\ and 02 are at 120° even in this instance. Figure 3.16 shows directions specified
using the three-coordinate system.
We have now reviewed all the necessary basic background information that will enable
us to describe different classes of materials. We broadly classify MEMS materials into five
categories: metals, semiconductors, ceramics, polymers, and composites. In the course of