Electrostatically actuated nano- electromechanical switches based on carbon nanotubes have been fabricated and tested in a continuing effort to develop high-speed switches for a variety of stationary and portable electronic equipment. As explained below, these devices offer advantages over electrostatically actuated micro- electromechanical switches, which, heretofore, have represented the state of the art of rapid, highly miniaturized electromechanical switches. Potential applications for these devices include computer memories, cellular telephones, communication networks, scientific instrumentation, and general radiation-hard electronic equipment.
A representative device of the present type includes a single-wall carbon nanotube suspended over a trench about 130 nm wide and 20 nm deep in an electrically insulating material. The ends of the carbon nanotube are connected to metal electrodes, denoted the source and drain electrodes. At the bottom of the trench is another metal electrode, denoted the pull electrode (see figure). In the "off" or "open" switch state, no voltage is applied, and the nanotube remains out of contact with the pull electrode. When a sufficiently large electric potential (switching potential) is applied between the pull electrode and either or both of the source and drain electrodes, the resulting electrostatic attraction bends and stretches the nanotube into contact with the pull electrode, thereby putting the switch into the "on" or "closed" state, in which substantial current (typically as much as hundreds of nanoamperes) is conducted.
Devices of this type for use in initial experiments were fabricated on a thermally oxidized Si wafer, onto which Nb was sputter-deposited for use as the pull-electrode layer. Nb was chosen because its refractory nature would enable it to withstand the chemical and thermal conditions to be subsequently imposed for growing carbon nanotubes. A 200-nm-thick layer of SiO2 was formed on top of the Nb layer by plasma-enhanced chemical vapor deposition. In the device regions, the SiO2 layer was patterned to thin it to the 20-nm trench depth. The trenches were then patterned by electron-beam lithography and formed by reactive-ion etching of the pattern through the 20-nm-thick SiO2 to the Nb layer.
A 0.5-nm-thick layer of Fe was deposited, then patterned into catalyst islands for initiating growth of carbon nanotubes by means of photolithography and liftoff. To grow the carbon nanotubes, the workpiece as processed thus far was then placed in a chemical-vapor-deposition furnace, wherein it was exposed to an atmosphere of flowing CH4 and H2 at a temperature of 850 °C for 10 minutes. Next, a layer of Au/Ti was deposited and patterned in a lift-off process to form the source and drain electrodes in contact with the ends of the nanotubes.
Tests have confirmed the expected advantages of these devices over the older electrostatically actuated microelectromechanical switches, which are characterized by response times of ≈1 μs and switching potentials between 60 and 70 V. The present devices are not only smaller but are characterized by response times of a few nanoseconds and switching potentials of a few volts. Hence, the present devices are expected to be better suited for applications in which there are requirements for highly miniaturized, high-speed electronic switches that can be operated from low-voltage (e.g., battery) power sources.
This work was done by Anupama Kaul, Eric Wong, and Larry Epp of Caltech for NASA's Jet Propulsion Laboratory.
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Refer to NPO-43343, volume and number of this NASA Tech Briefs issue, and the page number.