The Role of Electron Scattering in Materials Science

As a probing radiation, electrons serve a unique role for the characterization of matter, complementing those of X rays and neutrons. A comparison of select properties of current state-of-the-art instruments featuring these three distinct radiation sources is shown in Table 1. The small characteristic length scales of electrons, combined with the availability of high brightness sources and powerful electromagnetic lenses, make electrons uniquely suited to the imaging and analysis of individual nanoscale objects, with atomic resolution.
 
 

Table 1. Selected characteristics of state-of-the-art neutron, X ray and electron sources

Radiation	Source Brightness (particles/cm2 /eV/ steradian)	Elastic Mean-Free Path (nm)	Absorption Length (nm)	Minimum Probe Size (nm)
Neutrons	1024	107	108	106
X rays	1026	103	105	102
Electrons	1029	101	102	10-1

 

The Case for Aberration Correction

Although electron microscopes provide the only available means for direct imaging of individual structures at the nanometer and atomic levels, their spatial resolution does not approach the fundamental diffraction limit, because of limiting aberrations of the associated electromagnetic lenses. Electrons with energies as low as one hundred electron volts have wavelengths of Angstrom dimension suitable for atomic-scale imaging of matter; however, electron energies three orders of magnitude higher are used for lattice imaging. Even at these high energies, lenses with very short focal lengths are needed to limit the detrimental effects of lens aberrations. Although recent breakthroughs in spherical aberration correction promise an immediate improvement in spatial resolution and image interpretation for current generation electron microscopy, electron optical imaging in an aberration-free environment would achieve a paradigm shift in the ways that electrons are used to probe matter.
 
 

The significance of improving charged particle lens aberrations is not limited to the field of electron optical imaging. Focused beams of charged particles are used for the synthesis of nanoscale structures with strong scientific and technological ramifications. For example, the ultimate dimensions achievable by projection electron lithography are limited by the same lens aberrations as those that limit electron optical imaging and analysis. Focused ion beam technologies are similarly limited. These "direct write" technologies might someday replace the masked photon-based methods used to fabricate the present generation of computer chips. However, these methods have a more immediate relevance to the creation and scientific investigation of individual nanostructures, since they have the flexibility to allow quick transitions from inspiration to realization without the substantial initial investment required for mask-based technologies. Aberration correction addresses not only the ultimate achievable feature dimensions achievable by direct write methods, but also the ultimate intensity that could be concentrated into a fine probe, and thus the practical utility and writing speed of such instrumentation. Aberration correction would be a truly enabling scientific endeavor, with broad impact.
 
 

Materials Research in an Aberration-Free Environment

For electron optical imaging and analysis, the consequences of overcoming the limitations of lens aberrations are not limited to improved ultimate spatial resolution. A comprehensive approach to aberration correction would enable a paradigm shift in electron optical characterization, which would open to electron scattering the considerable flexibility in experimental design that is currently available at the nation's state-of-the-art X-ray and neutron sources. Ultimately, individual scientists would be able to design custom experiments that could be interfaced to a state-of-the-art electron source specifically designed to address a class of scientific studies. These state-of-the-art electron sources would be developed by instrument scientists in consultation with a diverse potential user base drawn from the greater scientific community. While close collaboration with electron microscope manufacturers is envisioned, the state-of-the-art in electron imaging and analysis would cease to be defined by commercial viability, with its incentive toward the development of all-purpose packaged instruments, and would give way to a science-driven approach, limited only by the imaginations of individuals comprising the scientific community. Scientists would have the flexibility to choose the incident electron wavelength, specimen environment and detector array to best suit their scientific study.
 
 

TEAM: The Next Step

The four Electron Beam Microcharacterization Facilities supported by the Department of Energy (DOE) Office of Basic Energy Sciences (BES) are proposing to lead a broad effort to advance the capabilities of electron beam instrumentation through the development of electron optics, specifically the correction of both spherical and chromatic aberration. A significant advancement in electron microscopy has already been achieved through the correction of spherical aberration, and both TEM and STEM instruments incorporating spherical aberration correction are starting to generate results in laboratories around the world. However, these first-generation aberration-corrected instruments must still utilize very short focal length lenses, which limit the flexibility of the materials research that can be achieved with them. The additional correction of chromatic aberration would allow comparable resolution to be achieved with much larger focal length lenses, thereby increasing the space in the specimen area. It is this advancement which would allow a first step toward realizing the revolutionary vision of materials research in an aberration-free environment. The additional space around the specimen would allow explicit incorporation into the instrument design an interchangeable specimen stage area that could be variously designed to incorporate specialized environmental cells, detector arrays, and ancillary in-situ stimuli, while providing additional degrees of freedom to orient the specimen for characterization. While clearly a variety of multi-purpose stages would be designed specifically for a given instrument, the interchangeable specimen stage paradigm would allow individual members of the scientific community to write proposals for the development of stages that could be interfaced to these instruments to address specific scientific problems, thus leveraging the investment in instrument development with the research potential of the scientific community as a whole.