Bright Contrast Imaging of Carbon Nanofiber-Substrate Interface using Scanning Electron Microscopy
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0963-Q05-13
Bright Contrast Imaging of Carbon Nanofiber-Substrate Interface using Scanning Electron Microscopy Makoto Suzuki1, Yusuke Ominami1, Quoc Ngo1,2, Toshishige Yamada2, Bill Roth3, Mark Betts3, Alan M. Cassell2, Jun Li2, and Cary Y. Yang1 1 Center for Nanostructures, Santa Clara University, 500 El Camino Real, Santa Clara, CA, 95053 2 NASA Ames Research Center, Moffett Field, CA, 94035 3 Hitachi High-Technologies America, Inc., 5100 Franklin Drive, Pleasanton, CA, 94588
ABSTRACT Scanning electron microscopy (SEM) for imaging the interface between carbon nanofibers (CNFs) and the underlying substrate is presented. By irradiating the electron beam perpendicular to the substrate, bright contrast is observed at the region where a small gap exists between the CNF and substrate. Relationship between CNF diameter and beam energy for the observation of the bright contrast is derived, which can be understood by using the theory of electron penetration into solid. Monte Carlo simulation is performed to reproduce the experimental observation based on our model, and the contrast sensitivity to the gap height is discussed. INTRODUCTION In order to realize future devices with nanoscale building blocks, the most important techniques include precise positioning control and efficient evaluation of their characteristics performed on very large scale integration. While the positioning technique of such structures has been recently improved [1-4], efficient and effective evaluation techniques should also be developed in order to achieve high production yield of devices. Recent studies show that the electrical and thermal properties of carbon nanotubes (CNTs) or carbon nanofibers (CNFs) strongly depend on interfaces with substrates, particularly the thermal dissipation via the support material on which they rest [5-7]. Even though these materials are prepared on substrates for device fabrication, some of them are placed with only a portion of their sidewalls in contact with the substrate. This inhomogeneous interface between nanofiber and substrate can affect the heat dissipation and result in poor device performance and reliability. Here, we present an efficient imaging technique for the characterization of CNF configuration relative to the underlying substrate. With a normal incident beam, SEM image contrast provides information about the interface between CNFs and substrate, thereby providing an efficient interface imaging method without beam tilting. We examine the SEM contrast of CNFs with various diameters by changing the electron beam energy. We show that the contrast mechanism is well explained by considering electron penetration into the fibers.
EXPERIMENT The SEM used for this study is a Hitachi S-4800 field-emission SEM with a wide range of beam energy capability from 30 keV down to 100 eV. Signal detection is performed with a detector placed above the objective lens for efficient secondary electron collection (through-thelens (TTL) detector) and thus the image contrast is formed mainly by secondary electrons (SEs
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