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Integration of suspended carbon nanotube arrays

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Integration of suspended carbon nanotube arrays into electronic devicesand electromechanical systemsNathan R. Franklin, Qian Wang, Thomas W. Tombler, Ali Javey, Moonsub Shim,and Hongjie Daia)Stanford University, Department of Chemistry, Stanford, California 94305共Received 29 April 2002; accepted for publication 11 June 2002兲A synthetic strategy is devised for reliable integration of long suspended single-walled carbonnanotubes into electrically addressable devices. The method involves patterned growth of nanotubesto bridge predefined molybdenum electrodes, and is versatile in yielding various microstructurescomprised of suspended nanotubes that are electrically wired up. The approach affordssingle-walled nanotube devices without any postgrowth processing, and will find applications inscalable nanotube transistors 共mobility up to 10 000 cm2/Vs兲 and nanoelectromechanical systemsbased on nanowires. © 2002 American Institute of Physics. 关DOI: 10.1063/1.1497710兴It is desired to electrically contact suspended single-walled carbon nanotubes 共SWNTs兲 for various fundamentalstudies and potentially applications. The fact that a sus-pended nanotube is free of van der Waals interactions with asubstrate makes them appealing for mechanical, electrical,and electromechanical measurements. For instance, Tombleret al. have used suspended SWNTs to investigate how re-versible structural deformation in nanotubes affects theirelectrical properties.1Several postgrowth fabrication meth-ods have been developed for making electrical contact tosuspended nanotubes.1–5One approach involves the growthof a nanotube across a trench, followed by microfabricationprocessing steps to form electrode pads at the two sides ofthe suspended tube.1A second approach involves chemicaletching of the substrate on which a nanotube is resting.2–4These approaches are limited to short (⬍ 0.5␮m) suspendednanotubes since wet processing in solvents and resist solu-tions tend to ‘‘rip’’ suspended nanotubes, due to forces re-lated to viscous fluidic flow or surface tension.Suspended SWNTs 5 to 100␮m have long been synthe-sized previously by patterned chemical vapor deposition共CVD兲 growth on substrates with elevated structures.6,7Inearlier attempts to form electrical contacts to these nanotubesusing the aforementioned methods, we observe that sus-pended nanotubes sag or are swept away after wet processingsteps during microfabrication.Here, we describe an approach to wire up SWNTs elec-trically in a noninvasive manner. The method is general tosuspended nanotubes with arbitrary lengths, but can also beapplied to nanotubes supported on flat substrates. We findthat a refractory metal, molybdenum 共Mo兲 is compatible withhigh-temperature SWNT synthesis. Arrays of Mo electrodepairs are first fabricated on a substrate, SWNTs are thengrown from electrodes to opposing electrodes to formbridges that electrically connect the electrodes. We show thatthis approach affords electrical circuits of SWNTs suspendedover various microstructures. The suspended nanotubes thusgrown can be measured electrically right after growth.Figure 1 shows the process and results for growth ofSWNTs suspended between Mo electrodes on top of twoelevated SiO2terraces. The starting substrate is a p-type sili-con wafer with 2␮m thermally grown oxide. A 50 nm thickMo film is first deposited on the wafer by sputtering 关Fig.1共a兲兴. Subsequently, photolithography and dry etching 共reac-tive ion etching in SF6and C2ClF5for removing Mo notprotected by a photoresist兲 are used to form two opposingMo electrodes on SiO2, followed by the use of 6:1 bufferedHF to etch down the SiO2around the Mo electrodes by 1.5␮m. The photoresist on top of the Mo pattern is then re-moved for the catalyst-patterning step. The substrate iscoated with 1.6␮m thick of poly共methylmethacrylate兲共PMMA兲, patterned with deep ultraviolet light or electronbeam, and developed to form wells in the PMMA film. Analumina-supported iron catalyst suspended in methanol isthen spun onto the substrate followed by lifted off inacetone.8,9This leads to two catalyst islands formed on thetwo opposing Mo electrodes on top of the SiO2terraces 关Fig.1共b兲兴. Other fabrication schemes for such substrate prepara-tion have also been successful. For instance, one can firstpattern catalyst on the Mo film, followed by defining the Moelectrodes, and the subsequent formation of Mo/SiO2ter-races by wet etching.Growth of SWNTs from the patterned catalyst islands iscarried out in a 1 in. CVD system. The growth takes placeunder a 72 mL/min flow of methane 共99.999%兲 anda10mL/min co-flow of hydrogen at 900 °C for 5 min.10Duringheating and cooling of the CVD reactor, a constant flow ofpure H2is used to eliminate the possibility of oxygen impu-rities in the gas oxidizing the Mo electrodes.Using molybdenum as an electrode material is key tothis work. Mo is the only metal found to be compatible withour CVD growth of SWNTs at high temperatures. Other met-als, including gold 共Au兲, titanium 共Ti兲, tantalum 共Ta兲, andtungsten 共W兲 have all failed for various reasons. After CVDgrowth, Au electrodes become discontinuous as gold balls updue to its relatively low melting temperature. Ti and Ta elec-trodes also fail as they become partially etched and thushighly resistive after growth. The etching phenomenon isexplained by the formation of volatile metal hydrides at hightemperatures in an environment containing hydrogen.11Thisa兲Electronic mail: [email protected] PHYSICS LETTERS VOLUME 81, NUMBER 5 29 JULY 20029130003-6951/2002/81(5)/913/3/$19.00 © 2002 American Institute of PhysicsDownloaded 22 Jul 2002 to 171.64.121.112. Redistribution subject to AIP license or copyright, see http://ojps.aip.org/aplo/aplcr.jspchemical incompatibility eliminates hydride-forming metalsas electrode materials for nanotube growth at elevated tem-peratures. Mo and W are among the metals that do not formhydrides and are stable against aggregation at high tempera-tures. However, although W electrodes do survive the CVDgrowth process with high connectivity and conductivity, noSWNTs are found to grow from catalyst patterns on the Welectrodes. The presence of W near the catalyst material ap-pears to inhibit the growth of nanotubes, presumably causedby the high catalytic activity of W towards hydrocarbons,12which interferes with SWNT formation in the CVD process.With Mo electrodes, SWNTs


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