TEM Study of Diamond Films Grown from Fullerene Precursors
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ABSTRACT Transmission Electron Microscope (TEM) techniques are applied to study the microstructure of diamond films grown from fullerene precursors. Electron diffraction and electron energy loss spectra (EELS) collected from the diamond films correspond to that of bulk diamond. Microdiffraction, high resolution images and EELS help determine that the first diamond grains that nucleate from fullerene precursors generally form on a thin amorphous carbon interlayer and seldom directly on the silicon substrate. Grain size measurements reveal nanocrystalline diamond grains. Cross section TEM images show that the nanocrystalline diamond grains are equiaxed and not columnar nor dendritic. The microstructure of small equiaxed grains throughout the film thickness is believed responsible for the very smooth surfaces of diamond films grown from fullerene precursors. INTRODUCTION Diamond films grown by plasma-assisted chemical vapor deposition using fullerene precursors [1,2] have been found to be extremely smooth with root mean square (rms) surface roughnesses of 20-40 nm using both laser reflectance interferometry (LRI) and atomic force microscopy (AFM) measurements [3]. The tribological properties of the films have also been investigated [4) and yield wear rates of 1.8 x 10-8 mm3 /N.m after 2.4 x 106 cycles using Si3N4 balls with 5N loadings were measured. These results indicate two orders of magnitude improvement over diamond films grown by conventional CH 4 /H 2 methodologies, which typically have surface roughness in the micron range due to the micron size crystallites in the deposits. In a preliminary investigation using transmission electron microscopy (TEM) on these diamond films, an array of larger grains within a polycrystalline matrix of much smaller grains (< 20 nm) was reported [5]. The present investigation of microstructure using TEM was undertaken to better understand the nucleation and growth of diamond films grown from fullerenes as compared to films grown from methane. EXPERIMENT Diamond films were deposited in a microwave plasma chemical vapor deposition reactor (ASTeX PDS-17) as previously described [1,2]. The films were grown on single-crystal silicon wafers polished with 0.1 g±m diamond particles to enhance nucleation density. Film growth was monitored in situ using laser reflectance interferometry to determine growth rate and stop growth at the desired thickness. The films were grown with 2 sccm H2 , 98 sccm Ar and C60 vapor (or methane gas), 100 Torr pressure, 1500 W of microwave power, and a substrate temperature of 850 °C. The microstructure of the diamond films was studied using transmission electron microscopy (TEM). To observe and measure the diamond grain size distribution, plan-view TEM specimens were prepared; and to observe the grain size and shape along the direction of film growth, crosssection TEM specimens were prepared. Images and electron diffraction patterns were recorded in the Philips CM30 TEM operated at 300 kV and in the high-resolution JEOL 4000EXII TEM operated at 400 kV (point-t
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