Spatial Resolution of EFTEM Mapping
Affected by Chromatic Aberration
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In EFTEM mapping, the spatial resolution depends on the chromatic broadening (Δdc) in the objective lens. Chromatic broadening is determined by the energy range (ΔE), the chromatic aberration coefficient of the objective lens Cc, the accelerating voltage E0, and the collection angle (β),

         Chromatic broadening  in PEELS ----------------------- (3384a)

We also can obtain the defocus, Δf, of the electrons that differ by an energy ΔE from those in focus,

         Chromatic broadening  in PEELS ----------------------- (3384b)

For instance, the chromatic broadening (Δdc) is about 2.5 nm at β = 10 mrad, Cc = 1 mm, ΔE = 50 eV, and E0 = 200 keV. This broadening disadvantage does not exist in conventional STEM-based EELS. To minimize it in EFTEM mapping, a small collection angle can be used.

For thick specimens (with multiple scattering) the calculation in Equation 3384a predicts the Cc-effect on the spatial resolution well, while for a thin specimen the characteristic inelastic scattering angle is much smaller, given by,

         θe = γΔE/2E0 ----------------------- (3384c)

where,
          γ -- The relativistic correction factor (=(E0+mec2)/(E0+2mec2), e.g. =0.61 for 300 kV).

θe is normally ≤0.6 mrad. Substituting θe for β in Equation 3384a, we can know that Δdc is normally ≤ 0.2 nm for most microscope configurations.

In EFTEM imaging, the most important contribution for low-loss imaging is the delocalization of the inelastic scattering process itself, while at much higher energy-losses the resolution-limiting parameter is usually the chromatic aberration. For instance, to lower the effect of the chromatic aberration, the accelerating voltage needs to be 300 kV if one expects a resolution better than 0.3 nm for >100 eV energy-loss [1].

In EFTEM imaging mode, the best to do is that the electron beam energy is increased to preserve image focus. With the “Offset” control on EFTEM interface, the value of the energy loss is chosen. By using the high voltage (tension) offset, an ionization edge can be shifted into focus on slit opening position where the zero loss was located previously. If no slit is applied, the other edges also appear at their corresponding energies but blurred in the vertical direction due to the chromatic aberration.

 

 

[1] R.F. Egerton, Electron Energy-Loss Spectroscopy in the Electron Microscope, 2nd Edition, Plenum Press, New York, 1996.

 

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