Improvement of the Elastic Modulus of Micromachined Structures using Carbon Nanotubes
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Improvement of the Elastic Modulus of Micromachined Structures using Carbon Nanotubes Prasoon Joshi1, Nicolás B. Duarte1, Abhijat Goyal1 , Awnish Gupta2, Srinivas A. Tadigadapa1 and Peter C. Eklund2 Departments of 1Electrical Engineering, and 2Physics, Pennsylvania State University, University Park, PA 16802. ABSTRACT Microelectromechanical flexural structures have been fabricated using sandwiched multilayers consisting of bundled singled walled carbon nanotubes(SWNTs) incorporated into silicon nitride (Si3N4) films. The Si3N4-SWNT composite layer was patterned by reactive ion etching followed by release in XeF2 to create freestanding bridge structures. The mechanical stiffness of the micromechanical bridges was monitored via force-displacement (F-D) curves obtained using an Atomic Force Microscope (AFM). Inclusion of SWNTs resulted in an increase in the spring constant of the bridge by as much as 64%, with an average increase of 25%. In a second experiment, micromachined bridges fabricated using dissolved wafer process were coated with debundled SWNTs. The SWNTs suspended in N-methyl-2-pyrrolidinone (NMP) solvent were sprayed locally on each bridge using a piezoelectric print head. Resonance frequency measurements were done in vacuum (~10-4 Torr) on the bridges after successive SWNT depositions. A 20% increase in the resonance frequency of the bridges was observed. The observed increase in stiffness in the first set of experiments as well as the observed increase in the frequency in the second set of experiments can be attributed to the high axial modulus of elasticity (~1 TPa) of the carbon nanotubes. INTRODUCTION There is considerable interest in high frequency mechanical resonators and mechanical structures for RF applications [1]. However, high frequency mechanical resonators can be achieved only via miniaturization of the structures and for frequencies in the 100MHz-1GHz range the size of these resonators start approaching the nanometer scale. This makes the practical implementation of such high frequency resonators quite cumbersome if not impossible. Even using the torsional mode of operation, the micron scaled resonators are typically limited to a maximum frequency of ~ 1 GHz [2, 3]. At the micrometer dimensions one way to achieve further improvements in the resonator characteristics is to use higher stiffness materials. Carbon nanotubes (CNT) have been measured to have very high axial modulus of elasticity ~ 1 TPa [4] and the incorporation of these high elasticity nanotubes into typical thin film materials used in micromechanical structures is expected to improve the elastic properties of these thin films. Motivated by this possibility, we incorporated CNTs into micromechanical structures in two different ways and independently observed an increase in the stiffness of these structures. In this paper we report an observation of the improvement of the stiffness of microelectromechanical structures by the addition of carbon nanotubes.
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EXPERIMENTAL DETAILS PECVD silicon nitride (Si3N4) bridges
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