Characterization of Nanoporous Materials with Atom Probe Tomography.

A method to characterize open-cell nanoporous materials with atom probe tomography (APT) has been developed. For this, open-cell nanoporous gold with pore diameters of around 50 nm was used as a model system, and filled by electron beam-induced deposition (EBID) to obtain a compact material. Two different EBID precursors were successfully tested-dicobalt octacarbonyl [Co2(CO)8] and diiron nonacarbonyl [Fe2(CO)9]. Penetration and filling depth are sufficient for focused ion beam-based APT sample preparation. With this approach, stable APT analysis of the nanoporous material can be performed. Reconstruction reveals the composition of the deposited precursor and the nanoporous material, as well as chemical information of the interfaces between them. Thus, it is shown that, using an appropriate EBID process, local chemical information in three dimensions with sub-nanometer resolution can be obtained from nanoporous materials using APT.

Sidewall angle tuning in focused electron beam-induced processing.

Structures fabricated using focused electron beam-induced deposition (FEBID) have sloped sidewalls because of the very nature of the deposition process. For applications this is highly undesirable, especially when neighboring structures are interconnected. A new technique combining FEBID and focused electron beam-induced etching (FEBIE) has been developed to fabricate structures with vertical sidewalls. The sidewalls of carbon FEBID structures have been modified by etching with water and it is shown, using transmission electron microscopy imaging, that the sidewall angle can be tuned from outward to inward by controlling the etch position on the sidewall. A surprising under-etching due to the emission of secondary electrons from the deposit was observed, which was not indicated by a simple model based on etching. An analytical model was developed to include continued etching once the deposit has been removed at the exposed pixel. At this stage the secondary electrons from the substrate then cause the adsorbed water molecules to become effective in etching the deposit from below, resulting in under-etched structures. The evolution of the sidewall angle during etching has also been experimentally observed in a scanning electron microscope by continuously monitoring the secondary electron detector signal.

Spectral DQE of the Volta phase plate.

The Volta Phase Plate (VPP) consists of a heated, thin film that is placed in the same plane as the focused diffraction pattern of an electron microscope. A change in surface potential develops at the point irradiated by the intense, unscattered electron beam, and this altered surface potential produces, in turn, a phase shift between the unscattered and scattered parts of the electron wave. While the VPP thus increases the image contrast for weak-phase objects at low spatial frequencies, we report here that it also leads to the loss of an increasing fraction of the signal at higher resolution. The approximately linear dependence (with increasing resolution) of this loss has been quantified at 200 kV and 300 kV, using evaporated-carbon films of different thicknesses as Volta phase plates. In all cases, the loss of signal remains almost independent of variation of the conditions and parameters that were tested. In spite of having done a number of additional, discovery-based experiments, the cause of this loss of signal remains unexplained at this point.