Low voltage SEM image resolution is predominantly limited by chromatic aberration. In order to strongly reduce this effect, FEI has incorporated a monochromator in its latest XHR SEM system (called the Magellan). This enables the reduction of energy spread to 0.15 eV FWHM. To illustrate the potential of a monochromator for low voltage SEM, figure 1 shows the probe size as a function of energy spread, for accelerating voltages U = 0.5 and 1 kV. Sub-nm resolution Magellan images at low voltage have revealed unprecedented information, such as fine surface details and nanoparticle distributions, without any sample preparation [1].An electrostatic Schottky-FEG module has been developed that can provide a monochromatized beam for XHR imaging, and which can also provide large probe currents for analysis. The three modes of operation of this module are shown in figure 2. The extractor contains a first aperture plane with two apertures, defining an axial and offaxial beam. An electrode below the extractor serves as gun lens. This electrode is segmented, so it can also be used for deflection and stigmation. The second aperture plane has an axial aperture and a small, off-axial slit [2]. The latter is used to monochromatize the off-axial beam. A deflector below the second aperture plane is used to direct either the axial or off-axial beam into the column, where a final beam limiting aperture is used to determine the probe current. Unwanted electrons are blocked.In the first mode of operation, the axial beam is used with gun lens off. The axial aperture in the second aperture plane limits the beam current. In the second mode of operation, the gun lens is turned on to deliver more beam current, which can go up to 22 nA. The third mode (called UC mode) uses the off-axial monochromatized beam. Voltages on the gun lens segments are used to position, focus and stigmate the off-axial beam on the energy selecting slit. The beam is dispersed due to the off-axial traversal through the strong gun lens. The spherical aberration of the gun lens results in coma in the off-axial beam. However, the effect of this coma on the energy spread is negligible for useful probe currents, i.e. < 100 pA. For larger probe currents, the loss of brightness becomes more important than the reduction in energy spread.
The resolution of a low-voltage electron microscope is limited by the chromatic and spherical aberration of the objective lens, see Fig. 1. The design of state-of-the-art objective lenses is optimised for minimal aberrations. Any significant improvement of the resolution requires an aberration corrector. Recently, correction of both Cc and Cs has been demonstrated in SEM, using a combination of magnetic and electrostatic quadrupoles and octupoles (Zach and Haider, 1995). The present paper presents an alternative design, which is based on a purely electrostatic concept, potentially simplifying the ease-of-use of an aberration corrected microscope.In 1936 Scherzer showed that the fundamental lens aberrations of round lenses are positive definite, in absence of time-varying fields and/or space charge. Negative lens aberrations, required for the correction of Cc and Cs, can only be obtained using non-round lenses, e.g. quadrupoles and octupoles (Scherzer, 1947).
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.