Prospects for realizing a sub-Å sub-eV resolution EFTEM

The arrangement of a sub-Angstrom ngstrom and sub-eV resolution energy filtering transmission electron microscope (EFTEM) is outlined. This ideal future analytical microscope is a combination of a scanning transmission (STEM) and a corrected fixed-beam tansmission electron microscope (TEM) and operates at voltages between 150 and 300 kV. The ultraresolution EFTEM will consist of a field emission gun followed by a monochromator yielding an energy width below 0.2 eV. The condenser system provides Koehler illumination for the TEM mode and a spot size of about 0.2 nm for the STEM mode. The spherically corrected aplanatic objective lens consists of a coma-free round lens and an integrated hexapole corrector. The formation of the energy loss spectrum is performed by the ultradispersive aberration-free MANDOLINE filter. The filtered intermediate image or the energy loss spectrum, respectively, are imaged onto a CCD array with variable magnification by means of a distortion-free projector system consisting of several quadrupoles and octopoles. For obtaining sub-Angstrom ngstrom resolution the parasitic mechanical and electromagnetic instabilities must be reduced to such an extent that the information limit is pushed below 0.06 nm. All requirements can be met at the present state of technology. (C) 1999 Elsevier Science B.V. All rights reserved.