The application of FIB milling for specimen preparation from crystalline germanium
Introduction
Focused ion beam (FIB) milling is a well-established method in the semiconductor industry for failure analysis and transmission electron microscopy (TEM) specimen preparation, especially for silicon-based manufacturing processes (Kirk et al., 1989, Young et al., 1990). The use of a 30 keV FIB, with a gallium source, for specimen preparation of silicon allows electron transparent membranes with a very flat uniform surface and without any significant relief to be readily and rapidly obtained. However, ion implantation in, for example silicon, during FIB processing results in the formation of an amorphous damage layer (typically 30–60 nm thick for 30 keV Ga ions) around the milled surface (Mardinly and Susnitzky, 1998, Venables et al., 1998, Rubanov and Munroe, 2001). Such amorphous damage layers will lead to a rise in intensity of background noise in TEM images and some contrast reduction, especially for high-resolution images. The extent and origins of this damage in silicon has been characterized in detail. In contrast, no systematic study of FIB milling process in germanium has been performed. However, it is known that interaction of the energetic ion beams with germanium during ion implantation results in anomalous morphological instability of the germanium surface. Wilson (1982) observed a complex cellular structure of a germanium surface after 50 keV self-ion bombardment with a dose above 2×1015 ions/cm2. Both Appleton et al., 1982, Holland et al., 1983, using cross-sectional TEM, observed a drastic alteration of the near-surface morphology in ion implanted Ge. Wang and Bitcher (1989) reported a high density of cavities in germanium irradiated with 1.5 MeV krypton ions to a dose of 3×1015 ions/cm2. The implantation dose delivered to the specimen surface during gallium-based FIB processing is in the same range as that used in these earlier studies of germanium, so the employment of FIB milling for germanium may also be expected to lead to strong cellular relief of the specimen surfaces. Indeed, Zhou et al. (2004) recently showed the generation of such a cellular microstructure following milling to doses of up to 1×1017 ions/cm2. As a consequence, the preparation of artifact-free specimens, including high quality TEM samples from germanium using the FIB, may be found to be difficult or even ineffective.
FIB millers, as well as allowing precise accuracy and rapid sectioning, allow imaging of the specimen using either secondary electrons or secondary ions emitted following interaction with the incident ion beam. This unique feature of the FIB allows experiments to be performed with an ion beam irradiation with variable implantation conditions and in situ observation of the affected specimen surface without exposure to the atmosphere.
In this paper, we have used a FIB to irradiate crystalline germanium and perform an in situ study of the morphological transformation of the germanium surface. The effect of milling conditions on the relief of a germanium surface is investigated. The application of the FIB technique for TEM specimen preparation is also briefly discussed. However, a detailed description of the structure and origin of the FIB induced damage in germanium in TEM specimens will be published elsewhere.
Section snippets
Experimental
The samples used in this study were monocrystalline Ge (100) substrates. A FEI×P200 FIB system with a gallium ion source operating at either 10 or 30 keV was used in this work for in situ irradiation and examination of the transformation of the surface. The surface of milled trenches was studied using the FIB secondary electron imaging facilities. All FIB images were recorded with incident angles of either 90 or 45° at a 30 keV ion energy and a 11 pA ion beam current. Image recording was
Results and discussion
The FIB images of trenches milled in a germanium sample at 30 keV using three different beam currents (350, 1000 and 2700 pA) are shown in Fig. 1. It was found that FIB milling produced trenches with very irregular side-walls and bottom-walls. Clearly, the milled surface of the bottom-wall of each trench exhibits a strong cellular relief, which was formed by the interconnection of chaotic continuous ridges. The length of some individual ridges was up to 5 μm and the average thickness of the
Conclusions
The interaction of the energetic gallium ion beam during FIB milling in the preparation of germanium specimens may lead to the formation of artefacts. This is manifested through the formation of both strong cellular relief and ‘curtaining’ type effects. The cellular relief is associated with the development of voids related to the formation of point defects during irradiation. These effects can be reduced, but not eliminated, by the use of either lower beam currents or lower beam energies.
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Current address: School of Physics, University of Melbourne, Melbourne, Vic. 3010, Australia.