Advanced measurement techniques of space-charge induced by an electron beam irradiation in thin dielectric layers

In dielectric materials, various electron beam techniques have been developed recently for space charge measurements. If charges injection is carried out with the primary electron beam of a scanning electron microscope, two methods can be developed, i.e. the mirror mode and the influence charge mode. In the first method, if the secondary electron emission yield σ is less than one, after charge implantation a net negative charge is trapped which acts as a mirror reflecting the electron trajectories corresponding to a smaller scanning energy beam. From this method, we can measure the electric charges trapping properties (amount of trapped charges, trap energy, and spreading of charges). In the second method, during the charge injection the measurement of the current, induced by the trapped charges in the metallic sample holder of the microscope, gives information in real time as function of the injected dose on the trapped charge quantities and on the dynamics of trapping–detrapping phenomena. This article describes how to use a scanning electron microscope to perform such measurements. It is shown that the characterisation of thin oxide layers requires primary energies which generally, because of the electron penetration depth, prohibit using the mirror method (due to net positive charge corresponding to σ>1). This topic is debated and some results carried out, using the influence charge method, particularly on SiO2 oxide layers of different nature are reported. This method provides a better understanding of aging and breakdown processes in thin charged dielectrics.