1 Å Resolution in 200 kV STEM/SEM
understanding SE imaging, which is attributed to the decay of collective electron excitation that is delocalized. A new mechanism has been proposed in which the image width of an atom is ascribed to a point-spread function that is dominated by inelastic scattering with large momen- tum transfer [6].
Figure 4: Simultaneously acquired SE and HAADF-STEM images of a cluster of Au nanoparticles with cross-linked polymer shells on the particle surfaces. The close-packed Au nanoparticles are well imaged in the SE mode, whereas the HAADF-STEM image does not show visible contrast for the polymer shells, therefore gaps appear among the nanoparticles. The images were taken on a Hitachi 200 kV field emission HD-2300A STEM/SEM. (Courtesy of Kvar C.L. Black of Northwestern University)
Conclusion High-resolution 200 kV
or 300 kV correlative TEM/ STEM/SEM systems have been developed to simultaneously characterize interior and sur- face structures at atomic resolution. In particular, sub-angstrom-resolution SE imaging has been achieved on the 200 kV Cs-corrected HD-2700 STEM/SEM. Tis breakthrough in resolution challenges the existing SE imaging mechanism. Tere is no doubt that high-voltage, high-resolution SEM will find value and
in materials industrial science applications,
especially when used cor- relatively with the STEM or TEM imaging modes.
Figure 5: A sequence of SE images recorded using a Hitachi 300 kV HF-3300 TEM/STEM/SEM correlative electron microscope. Gold nanoparticles of 1 nm diameter can be seen on an iron oxide support surface. When heated to 700°C, Au nanoparticles became mobile on the support surface. The two particles indicated by arrows in (b) moved toward each other and eventually coalesced. Terraces on facets of the support surface are also seen. (Courtesy of Dr. Jane Howe of Oak Ridge National Laboratory)
For industrial users, atomic-resolution SEM and sub-
surface metrology are attractive if high sample throughput is feasible at the same time. For example, conventional 30 kV SEMs have been used for metrology and defect inspection in semiconductor device industry because of the easy operation, use of bulk samples, and rapid sample throughput. With the shrinking size of electronic device features, the demand for sub-nanometer resolution could reduce demand for 30 kV SEMs in this field. Tin specimen analysis in TEM or STEM is desirable for its atomic resolution but undesirable for the tedious sample preparation process and the operational complexity, which can slow throughput. Now specimens that are too thick to be imaged by 200 kV or 300 kV transmitted electrons are still useful for atomic-resolution SE imaging in high-voltage, high-resolution STEM/SEM instruments because secondary electrons are emitted only from the surface layers of specimen. Te fact that the individual atoms were clearly imaged using secondary electrons challenged the traditional way of
2011 September •
www.microscopy-today.com
Acknowledgments Te author is grateful to
Dr. Yimei Zhu of Brookhaven National Laboratory and Dr. David C. Joy of Oak Ridge National Laboratory (ORNL) for discussions about the
SEM imaging mechanisms and for sharing opinions about the future of SEM. Tanks are also due to Dr. Jinsong Wu of Northwestern University and Dr. Jane Howe of ORNL for providing experimental data and corresponding discussions.
References [1] DC Joy, Nature Mater 8 (2009) 776–77. [2] H Inada, L Wu, J Wall, D Su, and Y Zhu, J Electron Micros 58 (2009) 111–22.
[3] Y Zhu, H Inada, K Nakamura, and J Wall, Nature Mater 8 (2009) 808–12.
[4] SJ Pennycook, Annu Rev Mater Sci 22 (1992) 171–95. [5] KCL Black, Z Liu, and PB Messersmith, Chem Mater 23 (2011) 1130–35.
[6] H Inada, D Su, RF Egerton, M Konno, L Wu, J Ciston, J Wall, Y Zhu, Ultramicroscopy 111 (2011) 865–76.
[7] JY Howe, LF Allard, WC Bigelow, and SH Overbury, Microsc Microanal 16 suppl 2 (2010) 312–13.
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