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Acceptance Angle Control for Improved Transmission Imaging in an SEM


Jason Holm* and Robert R. Keller National Institute of Standards and Technology , Applied Chemicals and Materials Division , 325 Broadway , Boulder , CO


* jason.holm@nist.gov


Abstract: This contribution presents a simple, cost-effective modular aperture system enabling comprehensive acceptance angle control for STEM-in-SEM imaging. The system is briefl y described, and different ways to use it are explained. To demonstrate the utility of the approach, a few samples are examined using the new system with comparisons to images from traditional SEM detectors. We show that the system enables conventional STEM imaging modes ranging from brightfi eld to high-angle annular darkfi eld (that is, Z-contrast), thin annular detection schemes, and even some non-conventional imaging modes.


Introduction


Recent years have seen rapid growth of transmitted electron imaging techniques within the scanning electron microscope (SEM), representing the emergence of the field known as “transmission SEM,” or t-SEM [ 1 ]. While not entirely new, the resurgence is due in part to recent detector technology advances. Combined sample holder/detectors as well as stand-alone segmented detectors, similar to those used in conventional high-energy scanning transmission electron microscopy (STEM) [ 2 – 6 ], lie at the heart of these advances. In fact, all major SEM manufacturers and accessory vendors now offer various STEM detectors as optional add-ons. As a group, these detectors and their associated imaging techniques are often termed “STEM-in-SEM” or “low-voltage STEM.”


Despite the demonstrated utility of t-SEM imaging [ 7 ], the full promise of the technique has not yet been realized. One restriction is that commercial transmitted electron detectors do not allow users to easily control acceptance angles and select which scattered electrons contribute to images. In conventional SEMs there are no post-specimen lenses to focus electrons, so the desired acceptance angles must be obtained through a combination of detector geometry and camera length (CL). To that end, some commercial STEM detectors consist of arrays that include a central circular element for


brightfi eld (BF) imaging and two or three concentric annular elements to detect electrons forward-scattered through diff erent angular ranges for darkfi eld (DF) imaging. Although these arrays enable some acceptance angle control, compre- hensive control is key to extracting the maximum amount of information from any detector. Area detectors with multiple sensors or direct electron detectors may be promising techniques for selecting electrons forward-scattered through specifi c angles [ 8 ], but they are not yet commercially available for SEMs. T is article demonstrates an economical way to obtain comprehensive acceptance angle control using a commercially available STEM detector with little built-in angular selectivity. T e strategy involves a new modular aperture system and a new sample holder, which together can enable most conven- tional STEM imaging modes in any SEM capable of accepting a transmitted electron detector. T e approach is not specifi - cally limited to STEM-in-SEM but could be applied to other microscopes and devices that select electrons or other particles scattered through diff erent angles.


Materials and Methods Materials . Several sample types were imaged to demonstrate the aperture system utility: bundled single-wall carbon nanotubes (SWCNT) with catalyst particles, multi-wall carbon nanotubes (MWCNT) without catalyst particles, Au and TiO 2 nanoparticles, and exfoliated two-dimensional (2D) zeolites. Mild sonication was used to disperse the carbon nanotubes and nanoparticles in diff erent solvents (SWCNTs in chloroform, MWCNTs in n-methylpyrrolidone, and Au and TiO 2 particles in ethanol.) A small amount of each dispersion was drop-cast onto lacey carbon support grids and allowed to dry in air. Zeolites were prepared elsewhere [ 9 ] and were deposited on an ultrathin carbon/lacey carbon substrate.


Figure 1 : (a) An illustration of the STEM detector showing the diode geometry and layout. (b) An interior view of the SEM chamber showing the detector at its lowest position with the vendor-supplied carousel style holder demonstrating the limited sample maneuverability (~4 mm vertical). (c) An interior view of the SEM chamber showing a new cantilever-style holder for positioning a sample at arbitrary orientation. The inset shows an illustration of the holder on a dove-tail standoff block.


12 doi: 10.1017/S1551929516001267 www.microscopy-today.com • 2017 March


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