Self-aligned Split Gate Electrodes Fabricated on Suspended Carbon Nanotubes
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Self-aligned Split Gate Electrodes Fabricated on Suspended Carbon Nanotubes S.-B. Lee, L.A.W. Robinson, K.B.K. Teo1, M. Chhowalla1, G.A.J. Amaratunga1, W.I. Milne1, D.G. Hasko, and H. Ahmed Microelectronics Research Centre, Cavendish Laboratory, University of Cambridge, Madingley Road, Cambridge, CB3 0HE, United Kingdom 1 Engineering Department, University of Cambridge, Trumpington Street, Cambridge, CB2 1PZ, United Kingdom ABSTRACT We describe the fabrication of self-aligned split gate electrodes on suspended multiwalled carbon nanotube structures. A suspended multiwalled carbon nanotube structure was used as an evaporation mask for the deposition of metal electrodes resulting in the formation of discontinuous wire deposition. The metal deposits on the nanotubes are removed with lift-off due to the poor adhesion of metal to the nanotube surface. Using Al sacrificial layers, it was possible to fabricate self-aligned contact electrodes and control electrodes nanometers from the suspended carbon nanotubes with a single lithography step. It was also shown that the fabrication technique may also be used to form nano-gaped contact electrodes. The technique should prove useful for the fabrication of nano-electromechanical systems. INTRODUCTION Carbon nanotubes (CNTs), with their superior mechanical properties, are proposed as ideal candidates for nano-electromechanical system (NEMS) applications [1-3]. Most of the proposed NEMS applications of CNTs, such as nano-bearing [2], and nano-actuators [3], require the nanotubes to be suspended above the supporting substrate if it is to be realized on a chip. There have been several reported methods of reliably suspending CNTs, such as poly-methyl methacrylate (PMMA) suspended dispersion method [4, 5], where the PMMA holds the nanotube in suspension during contact metal deposition resulting in CNTs being suspended between metal contacts, and the sacrificial layer etching method [6], where after contact fabrication to the nanotubes, the SiOx under the nanotubes are chemically etched away resulting in suspended CNTs. More importantly, for electrical access and control of the suspended CNT structures, a method of placing individual control electrodes with nanoscale precision needs to be investigated. Conventional methods used to apply electric fields to the suspended CNTs are substrate gating [6] and lithographically defining electrodes [4]. In the former method, the distance between the gate and the CNTs can be controlled by controlling suspension height, but it suffers from the fact that it is not possible to apply electric fields locally to individual devices. The latter method gives the possibility of individual access, but suffers from alignment errors, which may lead to shorting. Depositing an insulating layer on the suspended nanotube may prevent shorting. However, this may restrict the mechanical movement necessary for NEMS operation and for electrical device fabrication this may also alter the transport properties of the nanotube [7, 8]. In this paper, we pre
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