Optimization of the performance of CVD diamond electron multipliers

Aiming at the production of image-enhancing devices (electron multipliers) to amplify the backscattered electron signal in scanning electron microscopy, a study of the stationary secondary electron emission from CVD diamond films, irradiated by a high-energy primary electron beam, was performed in order to deepen the understanding of the specific material characteristics that appear to control the electronic emission. An extensive analysis of the secondary electron emission yield δ, as a function of the primary electron beam energy, was performed on polycrystalline CVD samples of different thickness. The emission yield was observed to increase up to a maximum value corresponding to a film thickness of about 15 μm and to decrease at larger thickness values. The experimental results are analyzed according to a theoretical model previously developed [P. Ascarelli, E. Cappelli, F. Pinzari, M.C. Rossi, S. Salvatori, P.G. Merli, A. Migliori, J. Appl. Phys. 89 (2001) 689] and extended within this work. We realized that the most important material parameter controlling the secondary electron emission yield as a function of the primary electron energy is the mean escape depth λS of the secondary electrons. The obtained λS values were found to be adequate to explain the film emission performance dependence on its thickness d. Moreover, using the atomic force microscopy technique and developing an algorithm based on a three-points method to identify the mean orientation of crystal facets in space, we observed that the λS(d) (and consequently the secondary electron emission performance) evolution during the growth process may be largely explained by a variation of the film surface mean orientation and a concomitant variation of the film quality.