Atomic Resolution SE Imaging in a 30-200 keV Aberration-corrected UHV STEM

0575442 - ÚPT 2024 GB eng A - Abstract
Hotz, M. T. - Martis, J. - Radlička, Tomáš - Bacon, N. J. - Dellby, N. - Lovejoy, T. C. - Quillin, S. C. - Hwang, H. Y. - Singh, P. - Křivánek, O. L.
Atomic Resolution SE Imaging in a 30-200 keV Aberration-corrected UHV STEM.
Microscopy and Microanalysis. Cambridge University Press. Roč. 29, S1 (2023), s. 2064-2065. ISSN 1431-9276. E-ISSN 1435-8115.
[Microscopy & Microanalysis 2023. 23.07.2023-27.07.2023, Minneapolis]
Institutional support: RVO:68081731
https://academic.oup.com/mam/article/29/Supplement_1/2064/7228064

Images formed using secondary electrons (SE) in a scanning electron microscope (SEM) reveal surface features and can provide life-like images of 3D structures. The spatial resolution is a few nm at beam energies Eo of 1-30 keV, but it can be as good as 4 Å in 30 keV SEMs designed for high resolution. However, this is still not enough to resolve atomic-scale surface structures, or to detect single atoms. SE images obtained in an Aberration-Corrected Scanning Transmission Electron Microscope (AC-STEM) able to form electron probes of 0.8 Å Ø at 200 keV showed that a significant part of the SE signal is sufficiently localized for resolution as good as 1.4 Å, and gave atomically resolved images of surfaces and of single heavy atoms. But the imaging was not completely optimized: 200 keV primary energy results in smaller scattering cross-sections than Eo = 30 keV (typical SEM operating energy), increased knock-on radiation damage, and a slight increase in the SE signal delocalization. Nion HERMES is a monochromated AC-STEM able to form 1 Å electron probes at Eo = 30 keV, using monochromation to overcome resolution loss due to chromatic aberration. An advantage of HERMES for surface-sensitive SE imaging is that its sample chamber typically operates either in or close to ultra-high vacuum conditions (UHV, defined as <7.5×10−10 torr), and clean samples introduced into it stay clean indefinitely. We have therefore set out to investigate SE imaging with this microscope. Our SE detector was designed with the following goals: 1) minimize the deflection and instabilities the detector injects into the primary beam, which traverses it, 2) minimize extra aberrations imparted to the beam, 3) maximize the detector efficiency, and 4) avoid using single O-rings and make the detector bakeable, to preserve UHV environment at the sample. The detector uses the Everhart-Thornley detection principle and can be situated either before or after the sample, or both. The results shown here use the before-the-sample configuration. Secondary electrons travel backwards from the sample's entrance surface through the objective lens, and are directed by an electrostatic deflector towards a scintillator biased up to +10 kV. The resultant photons are channeled into a photomultiplier tube. Applying a small voltage to the sample in a side-entry holder optimizes the efficiency of the SE signal detection, and it can also allow the range of detected SE energies to be varied. Initial tests of the SE detector showed that Au nanoparticles on the back side of a ∼20 nm thick amorphous carbon foil, which are clearly visible in the high-angle annular dark field (HAADF) image, become invisible in the SE image (Fig. 1), thereby confirming that the contribution of back-scattered electrons to the image is small. Comparing simultaneously acquired, unprocessed HAADF and SE images of individual Au particles (Fig. 2) confirmed that Au planes with 2.04 and 2.35 Å spacing were resolved in the SE image, and also that 1.44 Å planes were only visible in the HAADF image, even though the probe size was close to 1 Å. Fig. 3 shows simultaneously acquired HAADF and SE images of BaSnO3. Rather unexpectedly, the contrast in the SE image is reversed: Sn-O columns, whose intensity is weaker than the Ba columns in the HAADF image, become the brightest in the SE image. The explanation is probably that the cross-section for M-shell ionization, which is especially efficient in SE generation, is higher in Sn than in Ba, and that O present in the Sn-containing columns enhances their intensity relative to the Ba columns.
Permanent Link: https://hdl.handle.net/11104/0345233