Vertically Aligned CNTs Embedded in Cr/TiO 2 Membranes For Realization of Ion Sources
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Vertically Aligned CNTs Embedded in Cr/TiO2 Membranes For Realization of Ion Sources Yaser Abdi, Mehdi Sadeghi, Shams Mohajerzadeh, Javad Koohsorkhi, and Bahman Hekmat-Shoar University of Tehran, Tehran, 14395, Iran ABSTRACT: PECVD-grown carbon nanotubes on (100) silicon membranes have been realized and exploited for electron and ion emission applications. The growth of CNT's is achieved by a mixture of hydrogen and acetylene gases in a CVD reactor and a 5-10nm thick nickel is used as the seed for the growth. The presence of a DC-plasma yields a vertical growth of carbon nanotubes. The as-grown nanotubes are encapsulated by means of an insulating TiO2 layer. The formation of a thin membrane is possible by means of a chemical anisotropic etching technique. The membrane is then removed from the back side to fully suspend the CNT-holding TiO2 layer. Upon exposure to a plasma ashing step the nanotubes are partially removed and a both-end opened hollow nanostructure is formed which can be used as a miniaturized ion source. The CNT-holding substrate can be exploited as a grid to extract the ions from an ionization chamber just underneath the membrane. Applying a proper accelerating electric field, positive ions made inside a DC discharge cavity can form a beam-shape emission of ions towards the opposite negative electrode. The beam is well suited for a source of ion lithography. In which, the emission has the ability of direct writing on a photo-resist coated substrates. Preliminary nano-scale dots have been created with sizes between 50 and 80nm. Scanning electron microscopy has been used to investigate the results. INTRODUCTION Carbon nanotubes have emerged as a promising technology for the fabrication of fieldemission devices and displays. [1-3] Over the past decade several techniques have been used for the growth of single and multi-wall carbon nanotubes including thermal catalystbased CVD, laser-vaporization deposition and arc-discharge growth. However, for optimal emission characteristics, vertical CNTs with sharp tips are desired and plasma enhanced chemical vapor deposition (PECVD) has been shown to yield CNTs with suitable morphological and emission properties. [4-6] Carbon nanotubes have been used in nanolithography applications as well as in the fabrication of field-effect transistors operating on a ballistic transport mechanism. The application of CNTs in logic circuits has also been investigated. We have recently reported the vertical growth of carbon nanotubes using a PECVD technique where a mixture of acetylene and hydrogen was used for the deposition of CNTs. [6-8] In this paper, we discuss the application of these CNTs to fabricate a structure suitable for ion emission applicable to nano-scale lithography.
EXPERIMENTAL SET-UP Vertically aligned carbon nanotubes are grown using a PECVD technique with a mixture of C2H2 and H2. P-type (100) Si substrates are used for the growth of CNTs where 5-10nm-Ni is used as the seed for the growth.[6-8] By patterning the nickel seed layer by a standard p
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