SEM Image Optimisation

At a neutral point of charge $I_B=I_{SE}+I_{BSE}$, however in a general case surface charging will add an additional term. This build up of negative charge via absorbed $e⁻$ or positive charge via ionisation of surface species leads to deviation of scattered electrons by electrostatic forces, deforming the image. This is perceived as a shifting of the image over time as charge builds up.

Each of these currents is a function of the accelerating voltage, as shown schematically below. There exist two equilibrium (one stable and one unstable) points of nul charging: $E_{unstable}$ at lower kV, and $E_{stable}$ at higher kV.

The sign of the surface charge influences electron deviation, but also the amount of scattered electrons reaching the detector. Thus, the foillowing protocol is has been proposed by Dr. Ron Rasch:

• Zoom to high magnification into a small rectangular zone and wait for maximal charging (counting to 10 in ones head)
• Zoom out and check the colour of the previously zoomed area
• If it is darker, increase kV, and if it is lighter decrease it.

For polymer species, this $E_2$ equilibrium point is situated around 1.5 kV of accelerating voltage, or 1.5 keV of electron energy. For Daramic coated with polyethylene glycol (PEG) the experimental optimum kV I found was 1.9 kV.