Air Bridge and Vertical Carbon Nanotube Switches for High Performance Switching Applications
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0924-Z06-04
Air Bridge and Vertical Carbon Nanotube Switches for High Performance Switching Applications Anupama B Kaul, Eric W Wong, Larry Epp, Michael J. Bronikowski, and Brian D Hunt Jet Propulsion Laboratory, 4800 Oak Grove Drive, Pasadena, CA, 91109
ABSTRACT Carbon nanotubes are attractive for switching applications since electrostatically-actuated CNT switches have low actuation voltages and power requirements, while allowing GHz switching speeds that stem from the inherently high elastic modulus and low mass of the CNT. Our first NEM structure, the air-bridge switch, consists of suspended single-walled nanotubes (SWNTs) that lie above a sputtered Nb electrode. Electrical measurements of these air-bridge devices show well-defined ON and OFF states as a dc bias of a few volts is applied. The switches were measured to have switching times down to a few nanoseconds. Our second NEM structure, the vertical CNT switch, consists of nanotubes grown perpendicular to the substrate. Vertical multi-walled nanotubes (MWNTs) are grown directly on a heavily doped Si substrate, from 200 – 300 nm wide, ~ 1 µm deep nano-pockets, with Nb metal electrodes to result in the formation of a vertical single-pole-double-throw CNT switch architecture. INTRODUCTION Carbon nanotubes (CNTs) have remarkable mechanical and electrical properties which makes them excellent candidates for the design of nanoelectromechanical systems (NEMS). Nanotube based NEMS have already been demonstrated in applications ranging from nanotweezers,1 memory devices,2 supersensitive sensors3 and tunable oscillators.4 Nanorelays5,6 are another promising application of nanotubes that offer the potential for high performance switching, with high speed operation at low actuation voltages and power. Although theoretical studies on CNT switches have been reported,5,7 results from fabricated devices that characterize nanoscale electromechanical switching have been limited. To date, switching in both SWNTs2 and MWNTs8,9,10 has been with deposited tubes. Here we demonstrate switching results for SWNT air-bridge devices, where the tubes are grown on-chip with patterned catalysts using materials that are compatible with the high temperature CVD synthesis of SWNTs. We also present process development results for vertical MWNTs formed into single-pole-double-throw switch configurations that can enable high density 3-D integration. EXPERIMENT A schematic of our first NEM structure, the air-bridge device, is shown in Fig. 1a. The fabrication details of this structure have been described elsewhere.11 A schematic of the vertical CNT switch is shown in Fig. 1b, where the MWNTs are grown directly on a conducting Si substrate to form a ground contact to the CNT.
DISCUSSION Air-bridge CNT switch An SEM micrograph of a finished air-bridge device is shown in Fig. 2a, where the underlying Nb pull electrode in the nanotrench is contacted by the Au/Ti electrodes labeled “pull”. Also shown are the source and drain electrodes which contact the CNT that bridges the trench. The high magni
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