Micromachine

Micromachine

Micromachine Gearing

Micromachines are fabricated from extremely thin layers of silicon only a few millionths of a meter thick. The silicon layers can be shaped into levers, gears, and other mechanical devices. Micromachine technology is currently used in imaging systems and motion sensors, and is being developed for applications in biomedicine, computers, and telecommunications.

Micromachine, also known as a micro-electro-mechanical system, or MEMS, a miniaturized mechanical device built with the materials and techniques used to make integrated circuits for computers. Micromachines combine sensors, levers, gears, and electronic elements such as transistors to perform various tasks. Micromachines are so small that they are visible only with a microscope. Typical dimensions for MEMS devices are on the order of a few micrometers (a micrometer is one millionth of a meter). In comparison, a typical human hair is 100 micrometers in diameter.

Micromachines have several benefits over larger machines. They are more sensitive, they can move faster, and they use less energy than larger machines do. Micromachines are also cheaper to manufacture and can be easily made in large quantities. MEMS technology is currently used in devices such as air bag sensors and certain types of video screen systems. It is being adapted for uses in many other fields, such as medicine, computers, and communication.

II

HOW MICROMACHINES ARE MADE

Micromachines are constructed by etching or chemically dissolving patterns onto thin slices of silicon wafers. Computers and microscopes are used to control the manufacturing process. MEMS construction has the same advantages of integrated-circuit construction, such as small size, so many can be made at once, and ease of manufacture. Micromachines are also easy and inexpensive to mass-produce (although perfecting the initial design may be expensive). Tens to thousands of identical MEMS devices, such as mirrors, valves, and levers, can be made simultaneously. Some micromachine designs take advantage of the ease of mass production and use thousands or millions of MEMS elements that work together to make a complete system.

III

CURRENT APPLICATIONS FOR MICROMACHINES

Two of the most common applications of MEMS technology can be found in automobile air bag sensors and in certain digital video projection systems. Air bags use tiny sensors that can sense when a car has experienced a sudden impact or crash. In video projection systems that use micromachines, an array of millions of movable micromirrors replaces a conventional video cathode-ray tube to project digital video images.

Air bags use micromachines to detect the sudden change in speed that occurs when a vehicle crashes into an obstacle or another vehicle. The MEMS acceleration sensor consists of a silicon chip containing a few hundred microchip transistors alongside a tiny mechanical spring with a weight attached. The spring and weight are made from a thin film of silicon just 2 micrometers thick. The transistors on the chip convert the motion of the spring and weight into an electrical signal that corresponds to the movement of the weight during a crash. When an impact of sufficient force occurs, the motion of the weight sends an electrical signal through the transistors. The sensor sends this information to a central control unit that inflates the air bag to protect the driver or the front-seat passenger.

Digital video projection systems that are based on MEMS technology use an array of millions of tiny mirrored micromachines. In these systems, the mirrors are created on a rectangular chip about the size of two postage stamps. Each individual mirror is a square of metal 16 micrometers long on each side and roughly 100 nanometers thick (a nanometer is one billionth of a meter). Each mirror rests on a small lever that tilts the mirror in one of two directions, corresponding to an “on” or an “off” digital signal. The mirrors work together to recreate the pixels, or tiny bits, that make up a video image.

Depending on how each mirror is tilted, either it will reflect light or it will not. When the digital signal of an image is sent from a computer to the MEMS array, the mirrors realign to recreate the pixel pattern of the original image. To project an image, light is aimed at the mirrors and reflected into a lens, which magnifies the reflection and projects it onto a screen. The screen can be a television or video monitor screen or the large screen of a movie theater.

IV

NEW APPLICATIONS FOR MICROMACHINES

Research and development in micromachines is growing quickly in many different fields. One of the more promising new applications of MEMS technology is in biomedicine, or the application of biological knowledge to human health. New drug delivery devices such as micropumps are being developed for the controlled, long-term dispensing of drugs such as insulin. Micromachines are being invented to help pharmaceutical companies develop new drugs. Entire miniature chemistry laboratories can be fabricated on the surface of a chip. The MEMS lab-on-a-chip concept allows thousands of drug combinations to be tested all at once, reducing the amount of time needed to test new drugs. MEMS biomedical devices are also being developed to improve electronic surgical scalpels, artificial organs, and artificial limbs.

Micromachines are also being applied to increase the capacity of current communications systems, particularly fiber-optic cables used for connecting long-distance telephone lines. These cables, originally designed to carry voices, are now transmitting graphics, sound, and computer programs over the Internet. In order to obtain more data-carrying capacity, telecommunications systems are transmitting more channels over a single optical fiber using different wavelengths, or colors, for each channel. MEMS micromirror arrays with control and switching functions are being developed to carry out the selection, switching, and redirection of these wavelength channels. When a wavelength channel is switched on or off, it can cause problems with the other wavelengths on the fiber. As a result, other MEMS devices are being developed to keep all the wavelength channels operating normally when switching occurs.

Wireless communications, such as cellular radio telephones, are also beginning to incorporate MEMS devices. MEMS technology is being used to build “smart” antennas that provide maximum reception by responding to changes in communication conditions. Other electronic components that use MEMS technology are being developed to improve the performance and reduce the size of wireless systems.