Deposition and Characterization of Silicon and Carbon Nitride
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The morphology of the resulting films was analyzed using a Hitachi S5000 FESEM at an accelerating voltage of 15 keV and a Topcon 002B TEM at an operating voltage of 200 keV. The TEM specimens for the C-N-Si/Si(1 11) samples were prepared by scratching particles off the substrate with a tungsten carbide scribe and dispersing these particles on a Cu grid containing a thin, torn carbon support film. No specimen preparation was required for the coated Si needles, due to the microscopic size of the needles (needle length < 200 gtm, tip radius of curvature < 100 nm). The composition of the films was determined by Auger spectroscopy, using a JEOL JAMP-30 high resolution scanning auger microscope. Selected area electron diffraction (SAED) and EELS were used to determine whether the particles were carbon nitride, silicon nitride, graphite, or another form of carbon. The EELS was performed on an ultrahigh vacuum VG 501 scanning transmission electron microscope at an operating voltage of 100 keV. RESULTS AND DISCUSSION SEM revealed that the films were polycrystalline, with faceted particles growing up to 1 gm in size. Figs. la and lb show SEM images of the particles on a Si wafer, viewed normal to and 45' to the wafer surface, respectively. Fig. 2 shows an SEM image of particles encasing a Si needle (the needle is the bright central region in the SEM image). Note that the coating is 1-1.5 gtm thick on the needle shank and about 6 jim thick on the needle tip. The Si needle itself still has its initial shape and does not appear to have reacted significantly in the formation of the coating. An SAED ring pattern for particles scraped off the Si wafer is shown in Fig. 3. Table I lists the interplanar spacings calculated from this ring pattern, along with the predicted spacings for (x- and P-C 3N 4 , and the JCPD spacings for oc-Si 3N4, P-Si 3 N4 , and graphite. As seen from Table I, these measured d-spacings, which were calibrated using a spot pattern from the Cu support grid as a standard, closely match the predicted P-C 3 N4 spacings. However, several of these spacings are also very close to those of ox- and P3-Si 3N 4 , but some of the intense a-Si 3N 4 reflections were not detected in the SAED pattern. HRTEM images (Fig. 4) of particles scraped off the Si wafer show lattice fringes with spacings of 6.64 A, 3.80 A, 4.32 A, 3.13 A, and 2.88 A. The 6.64, 3.80 and 4.32 A spacings correspond to Si 3 N4 . The 3.13 A spacing could correspond to either P3-Si 3 N 4 or 3-C3 N 4 . The 2.88 A spacing matches oa-Si 3N 4 . Thus HRTEM results, combined with the SAED results, show that Si3 N 4 is definitely present in the film. Whether or not P-C3N 4 is also present cannot be determined from just SAED and HRTEM analyses. EELS analysis of particles from this wafer sample found the presence of only Si 3N 4 ; P-C 3 N4 was not detected. However, since the particles were mounted on a C support film for TEM, limiting the EELS analysis to areas hanging over holes in the support film, it is possible that some unexamined particles were carbon nitride
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