Imaging of fullerene-like structures in CNx thin films by electron microscopy; sample preparation artefacts due to ion-beam milling

The microstructure of CNx thin films, deposited by reactive magnetron sputtering, was investigated by transmission electron microscopy (TEM) at 200 kV in plan-view and cross-sectional samples. Imaging artefacts arise in high-resolution TEM due to overlap of nm-sized fullerene-like features for specimen thickness above 5 nm. The thinnest and apparently artefact-free areas were obtained at the fracture edges of plan-view specimens floated-off from NaCl substrates. Cross-sectional samples were prepared by ion-beam milling at low energy to minimize sample preparation artefacts. The depth of the ion-bombardment-induced surface amorphization was determined by TEM cross sections of ion-milled fullerene-like CNx surfaces. The thickness of the damaged surface layer at 5° grazing incidence was 13 and 10 nm at 3 and 0.8 keV, respectively, which is approximately three times larger than that observed on Si prepared under the same conditions. The shallowest damage depth, observed for 0.25 keV, was less than 1 nm. Chemical changes due to N loss and graphitization were also observed by X-ray photoelectron spectroscopy. As a consequence of chemical effects, sputtering rates of CNx films were similar to that of Si, which enables relatively fast ion-milling procedure compared to carbon compounds. No electron beam damage of fullerene-like CNx was observed at 200 kV.