SEM-mirror methods and application to insulator characterization

The SEM mirror method (SEMM) is a nondestructive technique developed for insulators characterization. It provides measurements on the charge trapping ability under high electric field with a spatial resolution down to 1 mum. SEMM consists of experimental measurements, ground current and minor plot, and a simulation part using Monte Carlo calculations. The mirror plot gives information on the trapped charge distribution at an equilibrium state. Many experimental results on different kind of materials, e.g. polymers, ceramics, single crystals, have shown that the mirror plot is useful to study insulators properties. But, the charge distribution deduced from the experimental minor plot is not unique and more information about charge buildup is needed to reduce the set of solutions. More recently, the ground current measurement during the injection phase gave complementary results on the charge trapping rate. Under specific conditions, peaks of electron emission occur during injection which have been associated with a local flashover. This result clearly shows that SEMM is able to study insulators under high field stress close to the breakdown strength. To understand the mirror experiments, a more complete simulation of the injection phase is needed. This can be achieved through Monte Carlo simulation of the electron-matter interactions by taking into account charge trapping and detrapping processes under high field. Using these three complementary approaches, it is possible to get a better understanding of charge trapping and detrapping processes in insulators under high electric field with a fine spatial resolution.