Image Formation Mechanisms in Scanning Electron Microscopy of Carbon Nanofibers on Substrate M. Suzuki,* H. Kitsuki,* Q. Ngo,* ** T. Yamada,** * K. Gleason,* Y. Ominami,* B. Roth,*** M. Betts,*** A. M. Cassell,** J. Li,** and C. Y. Yang* * Center for Nanostructures, Santa Clara University, 500 El Camino Real, Santa Clara, CA, 95053 ** NASA Ames Research Center, Moffett Field, CA, 94035 *** Hitachi High Technologies America, Inc., 5100 Franklin Drive, Pleasanton, CA, 94588 Nanostructures fabricated on thick substrates (typically a silicon wafer) are building blocks for high-performance electronic devices. While detailed structural analysis using high-resolution electron microscopy usually requires a thinning process to obtain an electron transparent specimen, a non-destructive approach is also required to analyze the structure. A recent study showed that heat dissipation into the substrate from a nanotube device governs its electronic transport characteristics [1]. In such a system, structural analysis that can be performed without any sample modification is favorable since imaging can be performed before, after, and even during the electrical operation of devices to investigate structural change. In this paper, we present two kinds of imaging techniques using conventional scanning electron microscopy (SEM) developed for the characterization of carbon nanofiber (CNF) devices fabricated on a thick Si substrate without any sample modification. Figure 1 shows SEM images of a CNF placed on the Si substrate, captured by a through-the-lens (TTL) secondary electron (SE) detector. With a 1 keV electron beam [Fig. 1(a)], the CNF shows uniform contrast along the fiber with a prominent edge peak at the periphery of the CNF. By using a 30 keV beam [Fig. 1(b)], however, a bright portion appears in the CNF. By tilting the substrate [Fig. 1(c)], it is found that the bright portion corresponds to an area where the CNF is not in contact with the substrate, thus suggesting a useful imaging technique to explore the CNF-substrate interface structure. This technique is important for considering the effect of heat dissipation via the substrate. This unique contrast comes from the fact that, by using the electron beam with sufficiently high energy to penetrate the CNF, SEs emitted from the substrate contribute to the image contrast only when there is a finite gap between the CNF and substrate as shown in Figs. 1(d) and (e) [2]. Figure 2 illustrates a scanning transmission electron microscopy (STEM) technique, using conventional SEM with a TTL detector, which provides the internal structure without specimen thinning. Here we use a constant beam energy of 30 keV, while capturing SEM images as a function of the substrate tilting. At moderate tilt angle of θ = 60˚, a typical SEM image contrast with surface morphology is obtained as shown in Fig. 2(a). Bright contrast of Ni catalyst particle also indicates the contribution of the backscattered electrons as well as SEs generated at the substrate by the largely deflected electrons. At larger tilt angle of 88˚ [Fig. 2(b)], the internal structure of the CNF can be seen, thus a bright-field STEM image contrast is obtained using a conventional TTL detector of SEM without sample thinning or an additional detector below the specimen. The mechanism of this STEM contrast is described briefly in Figs. 2(c)-(e). At moderate to small tilt angles [Fig. 2(c)], the contrast is mainly formed by the SEs from the CNF 580 CD DOI: 10.1017/S1431927607072418 Copyright 2007 Microscopy Society of America Microsc Microanal 13(Suppl 2), 2007