A 200-kV STEM/SEM Produces 1 Å SEM Resolution
Xiao Feng Zhang1, 2 1 Hitachi High Technologies America, 5100 Franklin Drive, Pleasanton, CA 94588-3355 2 Hitachi High Technologies Corporation, Tokyo 105-8717, Japan
xiao.zhang@hitachi-hta.com
Introduction “Second best no more” was the title of an article written
by David C. Joy for Nature Materials [1]. Te article was for highlighting a breakthrough made by a team formed between Brookhaven National Laboratory (BNL, USA) and Hitachi High Technologies Corporation (HHT, Japan). Sub-angstrom secondary electron (SE) images were obtained on a Hitachi HD-2700, which is a combined scanning electron microscope (SEM) and scanning transmission electron microscope (STEM) operating at a maximum accelerating voltage of 200 kV. For the first time, single atoms and atomic lattices on crystal surfaces are unambiguously presented in SE images. Like any standard 30 kV SEM, the 200 kV HD-2700 STEM/
SEM probes the specimen with an electron beam focused into a tiny spot on a specimen. Te electron probe scans across a specimen area, and SE signals emitted from the illuminated specimen area are collected by a high-sensitivity SE detector installed above the specimen position, resulting in images that carry structural information of material surfaces. In terms of the image formation process, the 200 kV SE imaging is similar to the conventional 30 kV SE imaging—the most popular imaging mode for SEM. Te difference is the resolution power. Te 200 kV HD-2700 delivers sub-angstrom (0.8 Å) SE imaging resolution; whereas, the best achievable resolution on a conventional 30 kV SEM is about an order of magnitude worse. Te BNL/ HHT team received a 2010 Microscopy Today Innovation Award because this team, according to the award statement, was the first to observe isolated single atoms with secondary electron imaging, and these observations point to a new mechanism for secondary electron emission. Tis article describes the high-voltage, high-resolution STEM/SEM and shows its benefits for secondary electron microscopy applications.
HD-2700: A Spherical Aberration-Corrected Field-Emission STEM/SEM Te HD-2700 is a 200 kV, field-emission STEM/SEM [2]
equipped with an SE detector above specimen position to collect secondary electrons, while a bright-field (BF) detector and a high-angle annular dark-field (HAADF) detector below specimen position collect electrons transmitted through the specimen (Figure 1). To achieve an ultrahigh resolution in SEM or STEM, a spherical aberration corrector (Cs corrector) is employed in front of the objective lens (Figure 1). Te Cs corrector provides a large flux of electrons in the incident electron probe that is focused into a fine spot at the back focal plane of the objective lens. Tis allows 10 times higher probe current and 2 times finer probe size compared to non-Cs- corrected microscopes. Te fine probe size provides high resolution for all
scanning electron probe-based imaging
modes (SE, BF-STEM, HAADF-STEM), while the high probe current generates sufficient signals for image formation and chemical analysis at each pixel in the scanned specimen area.
26 For the HD-2700 equipped with a Cs corrector, a
cold-field-electron emitter, and a high-resolution pole piece (the one at BNL), an electron probe of 0.8 Å (0.08 nm) diameter can be formed. Tis probe size is small enough to be placed on individual atoms, which is what the BNL/HHT team was doing. Tey held such an electron probe on single uranium atoms and collected HAADF-STEM and SE images simultaneously. Quite unexpectedly, the team found that the SE images showed the same resolution as that of the HAADF images. In other words, the image profile of single uranium atoms was revealed in both SE and HAADF-STEM images [3]. In another observation, a Si (110) surface was imaged as shown in Figure 2. Te well-known Si dumbbells with a 1.4 Å distance between the two projected neighboring Si columns can be clearly distinguished in the SE image.
Applications of High-Voltage, High-Resolution SE Imaging Having achieved SE imaging resolution on an atomic
level, the BNL/HHT team immediately applied the HD-2700 to various materials characterization problems. Figure 3
Figure 1: Schematic illustration of the electron optical system for a Cs-corrected HD-2700 STEM/SEM. A probe-forming Cs corrector is positioned above the objective lens. The SE detector is located above, and the BF and HAADF detectors are located below the specimen position.
doi:10.1017/S1551929511000873
www.microscopy-today.com • 2011 September
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