Materials analysis by aberration-corrected STEM
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Materials analysis by aberration-corrected STEM
Ondrej L. Krivanek, Neil J. Bacon, George C. Corbin, Niklas Dellby, Andrew McManamaSmith, Matthew F. Murfitt, Peter D. Nellist1 and Zoltan S. Szilagyi Nion Co., 1102 8th St, Kirkland, WA 98033, USA Dept. of Physics, Trinity College Dublin, Dublin, Ireland
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ABSTRACT Electron-optical aberration correction has recently progressed from a promising concept to a powerful research tool. 100-120 kV scanning transmission electron microscopes (STEMs) equipped with spherical aberration (Cs) correctors now achieve sub-Å resolution in high-angle annular dark field (HAADF) imaging, and a 300 kV Cs-corrected STEM has reached 0.6 Å HAADF resolution. Moreover, the current available in an atom-sized probe has grown by about 10x, allowing electron energy loss spectroscopy (EELS) to detect single atoms. We summarize the factors that have made this possible, and outline likely future progress.
INTRODUCTION Adding a Nion spherical aberration (Cs) corrector [1] to a Vacuum Generators (VG) cold field emission (CFEG) scanning transmission electron microscope (STEM) typically improves its resolution by 2-2.5x, to 109 A/(cm2 sr) at 100 kV, i.e. 5-10x brighter than a typical Schottky gun. This gun also delivers an energy spread of 0.30.4 eV under standard operating conditions, compared to 0.5-1.0 eV typical of a Schottky gun. This is beneficial for two reasons: it minimizes the resolution loss due to chromatic aberration, and it improves the quality of electron energy loss spectroscopy (EELS) results.
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Other positive attributes of the VGs are the quality of their stray AC field shielding and their generally good mechanical stability. Many VG STEMs are used without synchronizing the scan to the mains, which typically means that 60 Hz (50 Hz in Europe) stray fields do not move the probe by noticeable amounts. One particular 100 kV VG was routinely achieving resolution around 1 Å while it rested on wooden blocks placed on a concrete floor, its suspension awaiting finishing touches [12]. The same VG has also achieved drift rates of around 1 Å per minute. Furthermore, all VG microscope columns are bakeable to 120-140 °C, and they are easy to modify to all-dry pumping systems in which no hydrocarbon oils or greases are used anywhere in the microscope. As a result, vacuum levels near the sample in the 10-10 torr range are common in VG microscopes, the partial pressure for water is low, and clean samples remain clean and free of contamination essentially for ever even when held at room temperature. Performances of this type are generally not available with more “modern” TEM/STEMs.
Figure 2. Nion UltraSTEM 200 kV scanning transmission electron microscope. The 200 keV cold field emission gun is below the table level, the modular column rises above the table. Gatan Enfina EELS is shown at the top of the column. On the negative side, the VG condensers are difficult to align and drift for over one hour if their current is changed. The objective lens in the 100 kV VG has a large upper bo
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