High Resolution EELS in the IBM Sub-Angstrom STEM P.E. Batson IBM T.J. Watson Research Center, Yorktown Heights, New York 10598 For many years, EELS in the STEM has been used to explore nanometer-sized metal and semiconductor particles, both singly and in systems. These studies showed that the EELS response depended on many parameters, including particle and particle coating compositions, scattering impact parameter, orientation of the particles, and inter-particle spacings. It is interesting to ask whether this behavior extends down to atomic dimensions. Do groups of atoms exhibit phenomena that flow smoothly from nanometer scale objects? A related question arises in considering point and line defects in the bulk of materials. Is the EELS scattering at a dislocation sensitive to the orientation and structure of the core? These and other questions require both a very small electron probe which can be positioned accurately relative to an atomic sized object and high resolution spectroscopy to obtain EELS data at the 100-200 meV level. Aberration correction in electron microscopy holds exciting promise to address these questions. At IBM, the first Nion corrector was installed in the VG Microscopes HB501 STEM in 2001. [1, 2] This system includes a high resolution Wien Filter spectrometer [3] in order to study electronic structure in semiconductors.[4] As described in a previous abstract [5], addition of the corrector required a redesign of the electron optics that couples the specimen scattering into the spectrometer to increase the collection efficiency of the spectrometer system, so that high resolution EELS measurements would be possible with the 0.8 ˚ A sized probe made available by abberation correction. In addition, a monochromator prototype operation was demonstrated before installation of the Nion aberration corrector,[6] but it has not been shown yet that it will be possible to efficiently couple this device into the aberration corrected column. Fig. 1 shows the first zero loss obtained with the new optics. The energy distribution is dominated by the intrinsic shape of the room temperature field emission profile as in the past. This spectrum is acquired using a ccd detector in a somewhat unusual way. In order to preserve the energy calibration, the spectrum is scanned across the full ccd, treating the ccd as a set of many energy selecting slits, producing several hundred separate spectra, which are then shifted to a common energy axis and added up for the final result. This converts positional energy errors into an integrated resolution, and normalizes ccd channel to channel gain variations. Fig. 1 shows that the zero loss can be fitted with a theoretical field emission profile convoluted with a 130 meV Gaussian distribution. This blurring is due to 2nd order aberration in this uncorrected filter and to the non-linear dispersion of the spectrum across the detector, which has not been corrected in this example. When the monochromator is reintroduced to the system, this non-linear dispersion will need correction at the detector to realize the 60-70 meV resolution capability of the spectrometer. Microsc Microanal 12(Supp 2), 2006 Copyright 2006 Microscopy Society of America DOI: 10.1017/S1431927606066438 1336 CD https://doi.org/10.1017/S1431927606066438 Downloaded from https://www.cambridge.org/core. IP address: 54.161.69.107, on 10 Jun 2020 at 14:14:23, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms.