Transmission electron microscopy and electron energy loss spectroscopy analysis of ultrathin amorphous carbon films
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Transmission electron microscopy (TEM) and analytic TEM were used to study the microstructure of amorphous carbon (a-C) films of thickness in the range of 5–100 nm deposited on Si(100) by radio-frequency (rf) sputtering. High-resolution cross-section TEM revealed a two-layer structure consisting of the a-C film and an ultrathin interface layer, in agreement with electron energy loss spectroscopy results. The presence of a 35-Å-thick interface layer (regardless of the deposition conditions) indicates that nucleation and initial growth of the a-C films were mainly controlled by the substrate surface condition. Mass-thickness contrast in bright-field TEM images showed an interface layer denser than the a-C film. This layer, believed to consist of Si, a-C, and SiC, enhances the adhesion of the film to the silicon substrate and accommodates the residual stress in the film. High-resolution cross-section TEM images revealed the presence of platelike nanocrystallites (∼35 Å in size) randomly distributed in the a-C film and oriented parallel to the surface. The possible mechanisms leading to the formation of these nanocrystalline structures are discussed in terms of sputtering phenomena occurring during film deposition. I. INTRODUCTION
Diamondlike carbon (DLC) films can be produced by various deposition techniques, such as ion beam,1 massselected ion beam,2 laser ablation,3 filtered cathodic arc,4 and plasma-enhanced chemical vapor deposition.5 Sputtering is the most common process used to deposit DLC films6–9 because of its effectiveness to sputter off various materials and relatively straightforward control of deposition conditions. The microstructure of DLC films greatly affects the mechanical properties. Numerous analytical methods10–12 have been used to characterize the microstructures of DLC films deposited by different techniques including Raman spectroscopy, electron energy loss spectroscopy (EELS), transmission electron microscopy (TEM), and x-ray photoelectron spectroscopy. The above microanalysis studies have shown that both trigonal (sp2) and tetrahedral (sp3) atomic carbon bonds exist in DLC films, and that the film properties are functions of the sp3/sp2 ratio, which is strongly dependent on the deposition method and process conditions. TEM is a powerful technique for investigating the microstructures of thin films. Lioutas et al.13 used TEM to investigate the structure of hydrogen-free amorphous carbon (a-C) films deposited by radio-frequency (rf) magnetron sputtering. Large amounts of sp3 carbon were observed in dark-field cross-section TEM images obtained with the objective aperture on the (111) diamond reflection position. Komninou et al.14 studied the microstructure of a-C films with thickness equal to 200 Å DOI: 10.1557/JMR.2004.0272 J. Mater. Res., Vol. 19, No. 7, Jul 2004
deposited on Si(001) by rf magnetron sputtering using TEM, x-ray diffraction, and x-ray reflectivity and found that the films contained platelet microcrystallites oriented parallel to the surface. Davis et al.15 studied the cross-se
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