Interdigital Capacitive Array of Tree-like Carbon Nanotubes on Silicon-based Membranes for Sensor Applications
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Interdigital Capacitive Array of Tree-like Carbon Nanotubes on Silicon-based Membranes for Sensor Applications A. Ebrahimi1, Y. Abdi1,2, S. Mohajerzadeh1 and S. Paydavosi1 1 Nano-Electronics Center of Excellence, Thin Film Lab, Department of Electrical and Computer Eng., University of Tehran, Tehran, Iran 2 Department of Physics, University of Tehran, Tehran, Iran
ABSTRACT We have grown tree-like vertically-aligned carbon nanotubes (CNTs) on silicon substrate, suitable for highly sensitive interdigital capacitive sensors. As an application, we present a sensitive pressure sensor with branched CNTs as its capacitance plates. After realization of the interdigital structure, the growth of CNTs has been achieved through direct-current plasma enhanced chemical vapor deposition (DC-PECVD) method. A sequential growth and hydrogenation has led to the formation of multiple branched structures of nanotubes. The growth of tree-like CNTs on the interdigitally patterned substrate results in a high overlap between adjacent fingers and consequently a significant response to mechanical variations of the membrane as a result of the applied pressure.
INTRODUCTION Carbon nanotubes have exceptional electrical and mechanical properties due to their strong covalent bonds. Their extraordinary properties have given them many applications in nanotechnology. Thanks to their one-dimensional and pointed structure and their extraordinary electronic properties, carbon nanotubes seem to be suitable candidates in the emerging field of molecular electronics, field emission devices and flat panel displays [1-3]. The extraordinary mechanical properties of CNTs suit them for application as Atomic Force Microscope (AFM) tip or for direct writing on a resist-coated surface. The application of such nanostructures in nanolithography has also been recently reported [4]. We have recently reported the growth of carbon nanotubes in a controllable branched structure which has been achieved by a proper designed sequence of deposition and treatment to realize the growth of smaller diameter CNTs on the already grown structures [5]. In this paper, we have exploited these tree-like CNTs grown on an interdigitally patterned structure as the conducting plates of the capacitive sensor and observed high sensitivity to mechanical movements. A high sensitivity pressure sensor as an application of this structure is reported. The structures of the grown CNTs and the prepared sensors have been investigated using transmission electron microscopy (TEM) and scanning electron microscopy (SEM). Also the capacitance measurement of the device resulted by the applied pressure will be presented.
EXPERIMENTAL DETAILS The growth of CNTs is achieved by a mixture of H2 and C2H2 gases in a CVD reactor. Prior to the growth of CNTs, to form a thin silicon membrane, the back side of the (100) silicon wafers is coated with a Si3N4 layer, acting as the protecting mask against alkali-based wet etching of silicon (KOH 34%, 60 ºC), which is deposited in a low pressure chemical
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