Page 71 - Microsensors, MEMS and Smart Devices - Gardner Varadhan and Awadelkarim

52 MEMS MATERIALS AND THEIR PREPARATION

e- Primary
electron
Accelerated
© ion
Sputtered
atom

Substrate
Anode

Figure 3.18 Basic components in a physical sputtering unit for depositing materials

of these atoms land on the sample surface and form a thin film. Sputtered films tend to
have better uniformity than evaporated ones, and the high-energy plasma overcomes the
temperature limitations of evaporation. Most elements from the periodic table, including
both inorganic and organic compounds, can be sputtered. Refractory materials can be
sputtered with ease, whereas the evaporation of materials with very high boiling points
is problematic. In addition, materials from more than one target can be sputtered at the
same time. This process is referred to as cosputtering.
The structure of sputtered films is mainly amorphous, and its stress and mechanical
properties are sensitive to specific sputtering conditions. Some atoms of the inert gas can
be trapped in the film, causing anomalies in its mechanical and structural characteristics.
Therefore, the exact properties of a thin film vary according to the precise conditions
under which it was made. Consequently, values given for the bulk material, such as those
given in Appendix F, serve only as an approximate guide to the film values.

3.3 SEMICONDUCTORS

3.3.1 Semiconductors: Electrical and Chemical Properties

Semiconductors are commonly inorganic materials made from elements in the fourth
column (Group IV) of the periodic table. The most important among these elements is
silicon that can be modified in several ways to change its electrical, mechanical, and
optical properties. The use of silicon in solid state and microelectronics has shown a
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spectacular growth since the early 1970s, and this growth pattern is still continuing . Other
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Chapter 1 describes the recent emergence of microtechnologies.