Strings of innovation
you might have played several guitars , but plucking the strings of this guitar may not be easy. Harold Craighead and Dustin Carr at the Cornell University, usa , have developed the world's smallest guitar that is nearly hundredth of a millimetre long. By inventing the guitar, the researchers wanted to show the useful mechanical and electrical properties of silicon chips.
How useful these mechanical properties might be, is now being demonstrated by another team of researchers based at the University of Michigan, usa. Clark Nguyen and his colleagues want to change the little silicon wires into devices such as signal filters that might be used in mobile phones ( The Economist , Vol 345, No 8043).
To fabricate the wires, the researchers use same methods which are devised to create microelectronics. They employ fine beams of electrons to draw the required pattern in an electron-sensitive film on a silicon surface. Then the researchers outline the unexposed regions to leave behind wires suspended between larger silicon pillars. If these tiny strings could somehow be plucked, they would resonate at nearly 10 million times per second.
The Nguyen team has been able to stimulate them electrically. They suspend each wire over an electrode. Electrode is a conductor by which electricity enters or leaves an object, substance or region. Whenever voltage is applied to the electrode, the wire is electrostatically attracted towards it. In this process, the voltage oscillates and then the wire swings according to variation in the voltage.
The devices which show electrical and mechanical properties are known as microelectromechanicals systems ( mems ). The mems can be developed in frequency filters. But they require a more complex construction. For developing the filters, Nguyen and his colleagues suspend two slightly thicker silicon beams. They connect the beams with a very small wire. The two beams are suspended in such a way that one of them can be left swinging by a high-frequency signal running through the electrode. The electrode is placed under it.
Whenever the wire starts vibrating, the other beam swings according to the vibration produced. The frequency range transmitted to the other beam can be controlled by adjusting the dimensions and the tension of the wire. After this, the oscillations of the second beam are converted into electrical signal by reversing the process that leaves the first beam swinging.
It is expected that the new filters would prove to be a boon for the mobile-phone industry. At present, the smallest filter available in the market is about one sq cm. Compared to these filters, the new devices are very small. The filters would consume less energy than the larger electronic filters. The day is not far, when researchers could be able to apply the mems technology for building mobile phones on a single chip.