Page 34 - An Introduction to Microelectromechanical Systems Engineering

CHAPTER 2
Materials for MEMS

“You can’t see it, but it’s everywhere you go.”
—Bridget Booher, journalist, on silicon

If we view micromachining technology as a set of generic tools, then there is no rea-
son to limit its use to one material. Indeed, micromachining has been demonstrated
using silicon, glass, ceramics, polymers, and compound semiconductors made of
group III and V elements, as well as a variety of metals including titanium and tung-
sten. Silicon, however, remains the material of choice for microelectromechanical
systems. Unquestionably, this popularity arises from the large momentum of the
electronic integrated circuit industry and the derived economic benefits, not least of
which is the extensive industrial infrastructure. The object of this chapter is to pres-
ent the properties of silicon and several other materials, while emphasizing that the
final choice of materials is determined by the type of application and economics.

Silicon-Compatible Material System

The silicon-compatible material system encompasses, in addition to silicon itself, a
host of materials commonly used in the semiconductor integrated circuit industry.
Normally deposited as thin films, they include silicon oxides, silicon nitrides, and
silicon carbides, metals such as aluminum, titanium, tungsten, and copper, and
polymers such as photoresist and polyimide.

Silicon

Silicon is one of very few materials that is economically manufactured in single-
crystal substrates. This crystalline nature provides significant electrical and
mechanical advantages. The precise modulation of silicon’s electrical conductivity
using impurity doping lies at the very core of the operation of electronic semi-
conductor devices. Mechanically, silicon is an elastic and robust material whose
characteristics have been very well studied and documented (see Table 2.1). The
tremendous wealth of information accumulated on silicon and its compounds over
the last few decades has made it possible to innovate and explore new areas of appli-
cation extending beyond the manufacturing of electronic integrated circuits. It
becomes evident that silicon is a suitable material platform on which electronic,
mechanical, thermal, optical, and even fluid-flow functions can be integrated.
Ultrapure, electronic-grade silicon wafers available for the integrated circuit indus-
try are common today in MEMS. The relatively low cost of these substrates

13