Matthew Weyland

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 1nm) 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.

Magnetite morphology and life on Mars

Nanocrystals of magnetite (Fe3O4) in a meteorite from Mars provide the strongest, albeit controversial, evidence for the former presence of extraterrestrial life. The morphological and size resemblance of the crystals from meteorite ALH84001 to crystals formed by certain terrestrial bacteria has been used in support of the biological origin of the extraterrestrial minerals. By using tomographic and holographic methods in a transmission electron microscope, we show that the three-dimensional shapes of such nanocrystals can be defined, that the detailed morphologies of individual crystals from three bacterial strains differ, and that none uniquely match those reported from the Martian meteorite. In contrast to previous accounts, we argue that the existing crystallographic and morphological evidence is inadequate to support the inference of former life on Mars.

Z-Contrast tomography: a technique inthree-dimensional nanostructural analysis based on Rutherfordscattering

A method for determining the three-dimensional structure of ninorganic specimens using images formed from Rutherford scattered nelectrons, at a spatial resolution of 1 nm in all directions, is described nand illustrated with results from a study of Pd–Ru bimetallic nnanocatalysts supported on mesoporous silica: a 3D animation demonstrating nthe results may be viewed at http://www-hrem.msm. ncam.ac.uk/~mw259/Work/Tomo.html

Three-dimensional imaging of nonspherical silicon nanoparticles embedded in silicon oxide by plasmon tomography

Silicon nanoparticles embedded in silica show promising optoelectronic properties, due to quantum confinement and/or radiative interface states that should correlate with the particles’ average size and shape. Here the authors report the combination of electron tomography with plasmon-filtered microscopy in order to reconstruct the three-dimensional morphology of silicon nanoparticles. They find that particles with complex morphologies and high surface to volume ratios are dominant, rather than the commonly assumed near-spherical structures. These results should affect quantum-confined excitons and the interface density of states. Their findings may help to explain the physical origin of the unusually broad photoluminescence bands and efficiencies.Silicon nanoparticles embedded in silica show promising optoelectronic properties, due to quantum confinement and/or radiative interface states that should correlate with the particles’ average size and shape. Here the authors report the combination of electron tomography with plasmon-filtered microscopy in order to reconstruct the three-dimensional morphology of silicon nanoparticles. They find that particles with complex morphologies and high surface to volume ratios are dominant, rather than the commonly assumed near-spherical structures. These results should affect quantum-confined excitons and the interface density of states. Their findings may help to explain the physical origin of the unusually broad photoluminescence bands and efficiencies.

Three-dimensional imaging of nanovoids in copper interconnects using incoherent bright field tomography

As integrated circuits have shrunk, conventional electron microscopies have proven inadequate for imaging complicated interconnect structures due to the overlap of features in projection. These techniques produce transmission functions with a nonmonotonic dependence of intensity on thickness for common microelectronic materials, making them unsuitable for tomography. We report the use of an incoherent bright field imaging technique in a scanning transmission electron microscope optimized for the three-dimensional reconstruction of thick copper microelectronic structures. Predictable behavior of the signal in samples up to ∼1μm thick allows us to reconstruct and quantify the shape and volume of stress voids within Ta-lined interconnects.

Extending Energy-Filtered Transmission Electron Microscopy (EFTEM) into Three Dimensions Using Electron Tomography

The length scales on which materials microstructures are being formed, grown, and even designed are becoming increasingly small and increasingly three-dimensional. For such complex structures two-dimensional transmission electron microscopy (TEM) analysis is often inadequate and occasionally misleading. One approach to this problem is the modification of electron tomography techniques, developed for structural biology, for use in materials science. Energy-Filtered (EF) TEM elemental distribution images approximate to true projections of structure, and, as such, can be used to reconstruct the three-dimensional distribution of chemical species. A sample holder has been modified to allow the high tilt (+/-60 degrees ) required for tomography and a semiautomatic acquisition script designed to manage energy-loss acquisition. Tilt series data sets have been acquired from two widely different experimental systems, Cr carbides in 316 stainless steel and magnetite nanocrystals in magnetotactic bacteria, demonstrating single- and multiple-element tomography. It is shown that both elemental maps and jump-ratio images are suitable for reconstruction, despite the effects of diffraction contrast in the former and thickness changes in the latter. It is concluded that the image contrast, signal, and signal-to-noise ratio (SNR) are key to the achievable reconstruction quality and, as such, the technique may be of limited value for high energy loss/small inelastic cross section edges.

Nanoscale scanning transmission electron tomography.

Electron tomography enables the study of complex three‐dimensional objects with nanometre resolution. In materials science, scanning transmission electron microscopy provides images with minimal coherent diffraction effects and with high atomic number contrast that makes them ideal for electron tomographic reconstruction. In this study, we reviewed the topic of scanning transmission electron microscopy‐based tomography and illustrated the power of the technique with a number of examples with critical dimensions at the nanoscale.

Chapter 6:Electron Tomography

Transforming promising nanoscience into practical nanotechnology is fraught with a number of major challenges, the foremost among these being, perhaps, the assembly and characterisation of functional nanostructures. While significant progress has been made in the growth of the building blocks of nan...