Defocus in cathode lens instruments

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.