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Author: Dominique H.W. HUBERT, PhD.
Director, NanoResearch Solutions, FEI Company, the Netherlands.
The explosive growth in nanotechnology has created significant new demand for the high resolution imaging and analysis capabilities of transmission and scanning transmission electron microscopy (S/TEM). Though these are not new techniques, their reputation for difficulty, in both operation and interpretation of results, has historically limited their use to high powered research applications where the speed and cost of analysis were only secondary considerations. All that is changing.
January 30th, 2007
Perspective: New Generation of S/TEMs Will Play a Central Role in Continuing Nanotechnology Revolution
The explosive growth in nanotechnology has created significant new demand for the high resolution imaging and analysis capabilities of transmission and scanning transmission electron microscopy (S/TEM). Though these are not new techniques, their reputation for difficulty, in both operation and interpretation of results, has historically limited their use to high powered research applications where the speed and cost of analysis were only secondary considerations. All that is changing. As nanotechnologies move from development to production, the practical aspects—ease of use, speed of analysis, operator skill—of techniques used to develop new products and control production processes have become just as important as their analytical performance. In response to this need, the latest generation of S/TEMs combines sub-Angstrom imaging and analytical resolution with unprecedented advances in usability.
The traditional impediments to S/TEM analysis fall primarily into three broad categories—sample preparation, instrument operation, and results interpretation. Improvements in the first of these, sample preparation, derive not as much from changes in the S/TEM itself, as from developments in focused ion beam (FIB)-based preparation techniques. S/TEM requires samples thin enough to transmit beam electrons—typically less than 100 nm, but thinner is almost always better. Conventional preparation techniques focused on manual polishing procedures that were difficult, time-consuming, and not site-specific. FIB techniques use the ion beam to sputter material from the sample with nanoscale precision: a thin lamella can be extracted from the bulk at a site defined by the operator. In some applications, such as semiconductors, highly automated routines can take a sample from whole wafer to S/TEM in hours, rather than the days typically needed for manual techniques. FIB techniques have also proven to be powerful for S/TEM sample preparation of soft and even hybrid materials: materials that combine hard and soft phases, where classical mechanical preparation techniques have failed. S/TEM also allows observation of nanostructures dispersed in liquids. Here, automated technologies are also available for vitrification of the liquid medium, to prepare thin vitrified films with the dispersed nanostructures, compatible with S/TEM cryogenic imaging.
The second category, instrument operation, has also seen tremendous improvements. S/TEMs are complex systems requiring precise electron optical alignments that are easily disrupted by myriad influences both internal and external to the instrument. Perhaps the greatest operational improvements can be attributed to fully digital controls and stability, which permit saving and recalling of previously determined alignment conditions. Programmable digital control also allows the automation of lengthy data collection procedures such as tomographic series or wide area surveys, reducing operator fatigue and improving repeatability. Other dramatic improvements result from innovative design solutions such as constant power lenses that allow rapid changes in magnification and operation mode¯switching between the various imaging modes and analytical modes—without waiting for thermal equilibration, highly stable power supplies that reduce internally generated interference, and close attention to hardening against external mechanical and acoustic interference. Another important improvement is the aspect of flexibility: the capability to choose the optimum experimental conditions to obtain the best result from the sample under investigation. The latest generation of S/TEMs offers a wide range of acceleration voltages (80-300kV) to permit imaging and analysis that are tuned for the application and the sample. This flexibility is important in nanotechnology with its wide range of sample classes.
But the improvement likely to have the greatest impact on general acceptance of S/TEM may be the direct interpretation of high resolution images made possible by the use of aberration correctors. Much of TEM's reputation for difficulty derives from the complexity of the results it generates, and a major source of this complexity are the effects of spherical aberration in the magnetic lenses used to focus electrons. The effect may be a simple blurring that limits resolution, or it may be more insidious, such as delocalization, which can create artifacts that obscure real structure or are easily misinterpreted as structure where there is none. Adding further confusion, a double standard has evolved for TEM resolution: point resolution¯which can be directly interpreted from an image, and the information limit—typically several times better than point resolution, but achievable only with extensive data processing and reconstruction. Though aberration correctors have been available for some time, only now are systems becoming available that incorporate them without compromising other aspects of system performance. These corrected S/TEMs eliminate the adverse effects of spherical aberration, providing artifact free images with directly interpretable resolution down to the information limit—what you see is truly what you get.
Taken together, these advances in sample preparation: ease of operation, flexibility and direct image interpretation, promise to improve analytical speed, repeatability and reliability, and to reduce operator skill and training requirements across a broad range of applications. S/TEM can be expected to play an even more intense role in continuing nanotechnology revolution by delivering the understanding of materials down to the atoms.