Optimisation of the FIB induced damage in TEM diamond samples

Abstract

Due to diamond extreme properties, it is very hard to prepare a TEM sample from diamond using traditional methods of preparation including mechanical thinning, ion milling or chemical etching. At the same time diamond can be relatively easily micro‐machined using focused ion beam (FIB) technique. Using this technique a cross‐sectional TEM sample of diamond can be prepared in a few hours. Also, combination of ion implantation and FIB milling allows device fabrication in diamond at micro and nano scales levels. In the last decade FIB milling became essential tool for TEM sample preparation as well as for nanofabrication in diamond. However, Ga FIB milling has an unavoidable result in formation of the damage layers which can significantly reduce the device working areas and limit the applications of the FIB technique for nanofabrication of diamond. The damage layers in the FIB prepared TEM diamond samples can also significantly aggravate the quality of high‐resolution imaging. So, the knowledge of the extent of damage induced in diamond during FIB milling is critical for nanofabrication as well as for TEM imaging. In this work the damage layers after FIB milling of the synthetic single crystal diamond at different ion beam energies were studied using high‐resolution and analytical electron microscopy. TEM image of cross‐section of TEM lamella prepared using 30 keV Ga FIB milling is shown in Fig. 1. Amorphous layer with thickness ~ 16 nm are clearly visible on both sides of TEM lamella. EELS measurements of the carbon K‐edge in the amorphous region shows a prominent feature at 285eV, the p * peak associated with the presence of sp 2 bonding. This indicates the conversion of diamond sp 3 bonds to sp 2 in the amorphous damage area. Electron energy loss spectrum image was taken in STEM mode from central part of cross‐section of TEM lamella shown in Fig.1. Using Gatan Digital Micrograph software the chemical maps for sp 2 and sp 3 bonded carbon were obtained. Fig.2 shows maps for features of carbon K‐edge at 285 eV (sp 2 ) and 290 eV (sp 3 bonding). It is visible from Fig. 1, 2 that TEM lamella prepared from diamond using 30 keV FIB milling contained ~ 20 % of amorphous sp 2 bonded carbon. In case of thinner lamellas (60 nm) prepared using 30 keV FIB milling the fraction of amorphous sp 2 bonded carbon increases to ~50%. The thickness of damage layers with sp 2 bonded carbon in TEM diamond samples can be reduced by using low voltage FIB milling. Fig.3 and Fig. 4 show the HREM images of damage layers in diamond after 30 keV and 2 keV FIB milling. The thickness of amorphous damage layers reduced from 16 to less than 2 nm. Thus, very thin TEM diamond samples with low fraction of amorphous sp 2 bonded carbon could be prepared by using 2 keV FIB milling at final stage of TEM lamella preparation.