Microelectromechanical Systems Memss

Although microelectromechanical systems do not technically fell under the subject of nanotechnology, it is useftd to briefly discuss them at the beginning of the chapter because they represent a more mature technology, and many of the differences in behavior observed in the micromechanical world could well apply to die nano-regime, thereby providing a basis for the design of nanomachines.

The extensive fabrication infrastructure developed for the manufacture of silicon integrated circuits has made possible the development of machines and devices having components of micrometer dimensions. Lithographic techniques, described in previous chapters, combined with metal deposition processes, are used to make MEMS devices. Microelectromechanical systems involve a mechanical response to

Introduction to Nanotechnology, by Charles P. Poole Jr. and Frank J. Owens. ISBN 0-471-07935-9. Copyright © 2003 John Wiley & Sons, Inc.

an applied electrical signal, or an electrical response resulting from a mechanical deformation.

The major advantages of MEMS devices are miniaturization, multiplicity, and the ability to directly integrate the devices into microelectronics. Multiplicity refers to the large number of devices and designs that can be rapidly manufactured, lowering the price per unit item. For example, miniaturization has enabled the development of micrometer-sized accelerometers for activating airbags in cars. Previously an electromechanical device the size of a soda can, weighing several pounds and costing about $15, triggered airbags. Presently used accelerometers based on MEMS devices are the size of a dime, and cost only a few dollars. The size of MEMS devices, which is comparable to electronic chips, allows their integration directly on the chip. In the following paragraphs we present a few examples of MEMS devices and describe how they work. But before we do this, let us examine what has been learned about the difference between the mechanical behavior of machines in the macro- and micro worlds.

In the microworld the ratio of the surface area to the volume of a component is much larger than in conventional-sized devices. This makes friction more important than inertia. In the macroworld a pool ball continues to roll after being struck because friction between the ball and the table is less important than the inertia of its forward motion. In the mieroregime the surface area: volume ratio is so large that surface effects are very important. In the microworld mechanical behavior can be altered by a thin coating of a material on the surface of a component. We shall describe MEMS sensors that take advantage of this property. Another characteristic of the microworld is that molecular attractions between microscale objects can exceed mechanical restoring forces. Thus the elements of a microscale device, such as an array of cantilevers, microsized boards fixed at one end, could become stuck together when deflected. To prevent this, the elements of micromachines may have to be coated with special nonstick coatings. In the case of large motors and machines electromagnetic forces are utilized, and electrostatic forces have little impact. In contrast to this, electromagnetic forces become too small when the elements of the motors have micrometer-range dimensions, while electrostatic forces become large. Electrostatic. actuation is often used in micromachines, which means that the elements are charged, and the repulsive electrostatic force between the elements causes than to move. We will describe below an actuator, which uses the electrostatic interaction between charged carbon nanotubes. Many of these differences between micromachines, and macromachines become more pronounced in the nanoregime. There are many devices and machines that have micrometer-sized elements. Since this book is concerned primarily with nanotechnology, we give only a few examples of the microscale analogs.

Figure 13.1 illustrates the principle behind a MEMS accelerometer used to activate airbags in automobiles. Figure 13.1a shows the device, which consists of a horizontal bar of silicon a few micrometers in length attached to two vertical hollow bars, having flexible inner surfaces. The automobile is moving from left to right in the figure. When the car suddenly comes to a halt because of impact, the horizontal bar is accelerated to the right in the figure, which causes a change in the separation

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