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High Resolution Imaging with SEM

In Scanning Electron Microscopy a beam of high energy electrons is focused to a small area and scanned in a raster pattern across the surface of the sample (Figure 1). By which, electrons interact with the sample surface, either by in-elastically or elastically. 


Figure 1: Electron beam interation with specimen 


        In-elastically scattered electrons


        Elastically scattered electrons

These electrons will lose significant energy during collisions and therefore have low energy when they escape from the sample surface. These electrons are then known as secondary electrons (SE).


These electrons will lose little or no energy during their collisions. They escape with high energy and as there is a higher probability that their escape trajectories will be at a high angle, they are known as backscattered electrons (BSE).


Figure 2: Electron beam interation with specimen

The difference in the energies of the secondary and backscattered electrons allows them to be separated by different detectors to produce different signals and images of the sample being investigated. 


To understand the difference between the images it is appropriate to consider the interaction volume from which each signal is produced (Figure 3).


Figure 3: Electron beam interation with specimen








Due to lower energy, they were generated from a small volume around the point where the electron beam hit the sample. 


Due to higher energy, they were, generated from larger volumes


The volume from which both SE and BSE signals are emitted is also dependent on the energy given to the electrons that were focused onto the surface (referred to as primary electrons). Reducing the energy of the primary electrons (by reducing the accelerating voltage applied to them) reduces the interaction volume, making the signals more surface specific.


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