AN IMPROVED MODEL FOR GASEOUS AMPLIFICATION IN THE ENVIRONMENTAL SEM

We present a new model for the gas amplification effect used in many e nvironmental scanning electron microscopes, wherein molecular complexi ty is shown to be the critical factor, Monte Carlo simulations, based on experimental electron scattering cross-sections, are used to deduce a predictive model for the amplification process that is superior to the Townsend gas capacitor model. These predictions are compared with experimentally obtained amplification curves, Significantly, it is sho wn that the ionization efficiency of the electrons changes dramaticall y over the gap distance, and a constant value cannot be assumed. Atomi c and molecular excitations affect the amplification process in two wa ys: first, they serve to lower the average kinetic energy of the imagi ng electrons, thereby keeping a greater fraction near the ionization t hreshold energy, Second, molecular normal modes determine the effectiv eness of positive gas ions in producing additional secondaries upon su rface impact. Practical implications such as signal gain and fraction of useful signal as a function of operating conditions are discussed i n the light of the new model, Finally, we speculate on potential new c ontrast mechanisms brought about by the presence of an imaging gas.