3D electron microscopy in the physical sciences: the development of Z-contrast and EFTEM tomography

The rapid advances in nanotechnology and the ever decreasing size of features in the microelectronics industry brings with it the need for advanced characterisation with high spatial resolution in two and three dimensions. Stereo microscopy allows some insight into the three-dimensional nature of an object but for true quantitative analysis, one has to turn to tomography as a way to reconstruct a three-dimensional object from a series of two-dimensional projections (images). X-ray tomography allow structures to be imaged at relatively large length scales, atom probe tomography at the atomic level. Electron tomography offers an intermediate resolution (of about 1 nm) with a field of view of hundreds of nm making it ideal for the characterisation of many nanoscale devices. Whilst electron tomography has been used in the biological sciences for more than 30 years, it is only now being applied to the physical sciences. In this paper, we review the status of electron tomography, describe the basis behind the technique and some of the practicalities of recording and analysing data for tomographic reconstruction, particularly in regard to solving three-dimensional problems that are encountered in materials science at the nanometre level. We present examples of how STEM dark-field imaging and energy-filtered TEM can be used successfully to examine nearly all types of specimens likely to be encountered by the physical scientist