Page 53 - Microsensors, MEMS and Smart Devices - Gardner Varadhan and Awadelkarim

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MeMS Materials and their

Preparation

3.1 OVERVIEW

Microelectromechanical system (MEMS) materials may be classified into five main types:
metals, semiconductors, ceramics, polymers, and composites. This chapter first introduces
the basic nature of each type of material and then discusses the different ways in which
they can be prepared. It also provides the reader with a brief overview of the underlying
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physical structure of these materials and how they relate to their material properties . More
advanced readers may wish to omit parts of this chapter as the details given here on the
preparation of electronic-grade silicon (EGS) are also presented in Chapter 4, Standard
Microelectronic Technologies, that discusses the central role of silicon processing within
the fields of microsensors and smart devices.

3.1.1 Atomic Structure and the Periodic Table

To understand the classification of materials, it is necessary to understand the quantum
mechanical structure of an atom and relate it to the nature of the macroscopic bulk
material. Every atom in an element is made up of a small, positively charged nucleus,
which is balanced in charge by the negatively charged electrons surrounding the nucleus.
The positive charge in the nucleus arises from nuclear particles called protons. Each
proton possesses a positive charge that is equal in magnitude to the negative charge
of an electron e, and in a neutral atom the number of protons in the nucleus is equal
to the number of electrons outside the nucleus. The number of electrons around the
nucleus, or the number of protons in the nucleus, of a neutral atom is equal to its atomic
number Z.
If we consider a one-electron atomic model, such as that for hydrogen, then the energy
of its single electron can only have values given by the following equation (Tuck and
E n
Christopoulos (1986)):
2
Z m ee 4 1

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See Moseley and Crocker (1996) for a review of sensor materials.