Aberration correction for high-voltage electron microscopy

Abstract

The simultaneous correction of third order aperture aberration and axial chromatic aberration is studied theoretically to determine practical upper limits of the accelerating voltage. Aberrations of a round objective lens are compensated by a quadrupole-octopole corrector similar in design to the Archard, Deltrap and Hardy systems. Severe tolerances on lens stability, large electrostatic fields in the quadrupoles, and chromatic aberration of magnification are the primary factors that adversely affect the performance of these systems at high voltage. The variational calculus is used to compute correctors with low electrostatic fields in the quad­rupoles. When lens stability requirements are also considered, the general configuration of high performance systems is deduced. In addition to internal corrector structure, five parameters including voltage and magnification are used to define a system. Optical and field properties are calculated for a wide variety of these parameters. It is concluded that chromatic correction is feasible for accelerating voltages up to 500 kV. Lens stability requirements become very severe for beam energies greater than the electron rest energy, 511 keV. Tabular results are given for systems designed for resolutions of 1, 0.5 and 0.1 nm. In these examples lens stability requirements are one part in 5 x 105, 106and 5 x 106re­spectively. Configuration changes and their effects on stability and field of view are discussed.