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Defocus in cathode lens instruments.

R M Tromp1, M S Altman2

  • 1IBM T.J. Watson Research Center, 1101 Kitchawan Road, Yorktown Heights, NY 10598, USA; Leiden University, Kamerlingh Onnes Laboratorium, P.O. Box 9504, NL-2300 RA Leiden, Netherlands.

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Summary
This summary is machine-generated.

Accurate defocus measurement in electron microscopy (Low Energy Electron Microscopy - LEEM, and Photo Electron Emission Microscopy - PEEM) is crucial for quantitative analysis. A new method precisely relates sample shift to defocus, improving lens calibration for aberration-corrected instruments.

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Area of Science:

  • Electron Microscopy
  • Surface Science
  • Optics

Background:

  • Quantitative image analysis in Low Energy Electron Microscopy (LEEM) and Photo Electron Emission Microscopy (PEEM) requires precise defocus measurement.
  • Establishing a quantitative relationship between lens excitation and image defocus is essential for Fourier Optics (FO) and Contrast Transfer Function (CTF) simulations.
  • Existing methods like Real-Space Microspot LEED have limitations when insufficient diffracted beams are available.

Purpose of the Study:

  • To develop and validate a new method for accurately measuring defocus in cathode lens instruments.
  • To establish an analytical relationship between sample displacement and objective lens defocus.
  • To apply this method for calibrating aberration-corrected Low Energy Electron Microscopy (LEEM) instruments.

Main Methods:

  • Analytical derivation of the relationship between sample shift along the optical axis and image defocus.
  • Experimental application of the derived method to measure defocus in an aberration-corrected LEEM.
  • Analysis of achromats in the LEEM instrument using the new defocus measurement technique.

Main Results:

  • An analytical model was derived to precisely quantify defocus based on sample shift and lens excitation.
  • The method was successfully applied to measure and analyze defocus in a real LEEM instrument.
  • The technique provides a reliable alternative for defocus determination when other methods are not feasible.

Conclusions:

  • The developed method offers a robust and accurate approach for measuring defocus in LEEM and PEEM.
  • This technique enhances the quantitative capabilities of electron microscopy by improving lens calibration.
  • The findings are particularly relevant for aberration-corrected instruments requiring precise optical parameter control.