The scanning electron microscope mirror (SEMM) method is considered as a suitable approach
for the characterisation of the charge trapping ability of insulators. However, the interpretation
of the experimental results is far from being obvious. The aim of this paper is to give some new
insights in this problem. Our description of the mirror plots is conventionally based on a multipole analysis of the charge
distribution and on the radius of curvature approximation. This approach indicates that the trend
of the plots is mainly controlled by the quadrupole contribution. This clearly illustrates the non-
uniqueness of the interpretation since different trapped charge distributions, but with nearly
equal quadrupole terms, can produce rather similar mirror plots. To make the problem unambiguous, a semi-ellipsoidal shell model is used to account for the
implanted space charge. Within this model, we propose a simple method to get the mean density
of trapped charge ρ0. Our Monte Carlo simulations of the charge implantation phase show that ρ0
can strongly differ from the density of traps NT. The reasons for these deviations
are discussed for a significant set of values of NT and of the primary energy
Ep.