Electron beam broadening in electron-transparent samples at low electron energies

Area of Science:

• Materials Science
• Electron Microscopy
• Physics

Background:

• Scanning transmission electron microscopy (STEM) is valuable for imaging weakly scattering and radiation-sensitive materials at low primary electron energies.
• However, electron beam broadening at low energies degrades resolution and limits specimen thickness.
• Understanding beam broadening is critical for interpreting low-energy STEM images.

Purpose of the Study:

• To investigate and quantify electron beam broadening in various materials at low electron energies (15–30 keV).
• To evaluate existing models and develop a new approach for describing beam broadening.
• To compare experimental measurements with simulations based on the electron transport equation.

Main Methods:

• Direct measurement of beam broadening using a multisegmented STEM detector in a scanning electron microscope.
• Experiments conducted on materials with atomic numbers Z between 10 and 32 and thicknesses up to 900 nm.
• Comparison of experimental data with calculations from a new model and simulations solving the electron transport equation.

Main Results:

• Measured beam diameters were well-described by a recent model by Gauvin and Rudinsky.
• The Hurst exponent (H) deduced from measurements was 0.75, deviating from the expected diffusion value of 0.5.
• Simulations using the electron transport equation showed good agreement with experimental data, yielding H = 0.80.

Conclusions:

• The study provides accurate measurements of electron beam broadening at low primary electron energies.
• A recently published model and electron transport equation simulations effectively describe the observed beam broadening.
• The findings are crucial for optimizing STEM imaging of thin and sensitive materials.