When a fine probe irradiates an insulator at normal incidence, the surface potential is no longer uniform over the surface but it is a function of the lateral co-ordinate r, V(r,0), as shown in Figure 4565, because charging can induce electrostatic stress. This surface potential is a function of the increase in positive and negative charges and their redistribution into the specimen. The negative charge distribution is proportional to 1 − η0 developed down to the maximum penetration depth of the primary electrons (PEs) and spreads over a more or less spherical volume of approximate diameter R0. η is the backscattering coefficient of electrons. One strongly non-uniform positive charge layer at thickness t results from the lateral distribution of SE1 released at the surface by the primary electrons (PEs), δP, while the other distribution of SE2 is generated by backscattered electrons (BSEs), δB. SE1s are generated within a disc determined by the incident spot diameter, dP, so that the associated positive charges are highly concentrated in a very small volume. Around the impacted disc, the electric field is centrifugal and the surface potential is positive. The whole system is negatively charged when δ0 + η0 < 1. Here, δ is secondary electron emission yield. In such a state, the surface potential V(x) changes the sign with radii (r) and the electric field changes direction. As suggested by Cazaux, Figure 4565 (b) shows the change in direction of the electric filed component (Fr) when δ0 + η0 < 1 (upper and front view of the surface). The approximate shape of the surface potential function, V(r,0) when δ0 + η0 < 1 (green curve). When δ0 + η0 > 1, this function is completely positive and the electric field remains centrifugal. This principle may be applied to explain some ring features obtained under short pulse and focused irradiations. An example is shown in Figure 4565 (a).
Figure 4565. (a) Ring shape feature resulting from the injection of 12 pC into polyethylene at 30 keV (adapted from ). (b) Calculated radial potential function, V(r,0), when δ0 + η0 > 1 (red curve) and δ0 + η0 < 1 (green curve) (adapted from ). (c) Calculated radial field component Fr (open green arrows) induced by a focused probe when δ0 + η0 < 1. (d) Top view of the
surface when δ0 + η0 < 1. Front view with the SE trajectories of the most energetic SE (violet arrow) and the re-attraction of the less energetic SE (yellow arrow).
 Mady, F., Renoud, R., Attard, C., Bigarré, J., Ganachaud, J.-P. & Hourquebie, P.
(2002) Interpretation method for mirror experiments based on a Monte
Carlo implantation model. Eur. Phys. J. Appl. Phys. 20, 41–53.
 Cazaux, J. (2005) Recent developments and new strategies in scanning electron microscopy, Journal of Microscopy, 217, 16–35.
 Cazaux, J. (2004) About the role of the various types of secondary electrons
on the performance of LVSEM. J. Microscopy 214, 341–347.