Optical design of electron microscope lenses and energy filters

Paraxial electron trajectory equations of round lenses and prisms are deriv ed from the Newton-Lorentz equation. Objective lenses of TEM giving the min imum spherical and minimum axial chromatic aberration coefficients are disc ussed. A variety of magnetic lenses for SEM are shown schematically. Trajec tory equations of three kinds of prisms (electro-static prism, magnetic pri sm and Wien filter) are described in a unified form. Optical properties of various prisms are summarized based on these equations. Two filters, the Wi en filter and the omega filter, are reviewed in detail by taking examples o f high-resolution electron energy loss spectroscopy (EELS) instrument and e nergy filtering transmission electron microscopy (EFTEM) instrument, respec tively. The Wien condition is satisfied even at the fringing field regions by using an eight-pole filter. Effects of retarding field in the Wien filter are de monstrated. The dispersion of omega filter is estimated analytically in a s imple omega filter. Not only the design of filter but also the magnificatio n combination of pre- and post-filter lenses determines non-isochromaticity .