Author: Tromp, R.M.
Paper Title Page
Computation and Measurement of Geometric and Chromatic Aberrations is Critical for the Optimal Design and Use of Aberration Corrected Electron Microscopes, and for Quantitative Understanding of Images  
  • R.M. Tromp
    IBM T. J. Watson Center, Yorktown Heights, New York, USA
  Computation and measurement of geometric and chromatic aberrations is critical for the optimal design and use of aberration corrected electron microscopes, and for quantitative understanding of images obtained with such instruments. Here, I will focus on the correction of spherical and chromatic aberrations of a cathode lens instrument (i.e. Low Energy Electron Microscope ’LEEM- or Photo Electron Emission Microscope ’ PEEM) using catadioptrics, i.e. a combination of electron lenses (dioptrics) and an electron mirror (catoptrics). First-order properties calculated with high precision using Munro’s Electron Beam Software’s MIRDA package are in excellent with detailed experimental results. Theoretical maps of C3 vs Cc as a function of the applied potentials then provide a deterministic method to dial in the desired mirror properties at will. Now it is necessary to measure the resultant aberrations of the full system. Unfortunately, the experimental methods developed for TEM and STEM are not applicable in LEEM/PEEM for a variety of reasons. Spherical aberration (plus defocus and astigmatism) can be measured using so-called micro-spot real-space Low Energy Electron Diffraction, or by measuring image shift as a function of beam tilt. Measuring chromatic aberration is more troublesome as it conventionally requires that defocus be measured as a function of gun voltage. However, the use of magnetic prism arrays to separate in- and outgoing path in LEEM results in changing alignment conditions when gun voltage is changed. However, a novel method first demonstrated using ray-tracing simulations enables us to measure chromatic aberration, even at fixed gun voltage. The chromatically corrected system behaves like a simple (but adjustable) achromat, comparable to the crown/flint optical achromat invented by Chester Moore Hall around 1730.  
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