Correction of Aberrations of a Transmission Electron Microscope

Abstract

One of the most striking problems in electron optics, the correction of resolution limiting aberrations by means of a corrector incorporated into the electron microscope column, has been solved during the last six years by demonstrating the improvement of resolution beyond the theoretical limit of the uncorrected Electron Microscope (EM). At first, in 1995 [1] with the correction of spherical and chromatic aberration of a dedicated Low Voltage Scanning Electron Microscope (LVSEM) and later, in 1997, with the correction of only spherical aberration of a commercially available 200 kV TEM [2]. The correction of spherical aberration of a dedicated Scanning Transmission Electron Microscope (STEM) at 100 keV primary energy has been demonstrated [3] and further improvements can be anticipated within the near future.These achievements could only be obtained due to the emergence of new computer technology and especially CCD-cameras in the case of TEM correctors. These two developments made it possible first to calculate the electron optical components more precisely and hence, to achieve a better understanding of the requirements on the hardware and second, to have a better computer control of the electron microscope and the corrector itself. The combination of these two new technologies made it possible to go towards an automatisation of the alignment. This simplification of the alignment of an even more complex system is achieved by means of a proper combination of image acquisition and dedicated software in order to analyze and measure the aberrations of an electron optical system on one side and on the other to have appropriate tools to compensate these aberrations by computer controlled power supplies [4,5].