Superposition of chromatic error and beam broadening in transmission electron microscopy of thick carbon and organic specimens.

The chromatic error is calculated using our scattering cross sections obtained from contrast experiments and a distribution function of energy losses from Misell and Burge. The assumed ratio of total inelastic and elastic cross sections was 3.5. Monte-Carlo calculations were performed for the multiple scattering problem of thick carbon specimens using these values for single scattering. As expected, a minimum confusion of the chromatic error disc exists at underfocusing. The half width broadening of an edge is in good agreement with experiments at 100 keV if the experimental method of determining the half width is also taken into consideration theoretically. The lateral displacements of electron paths normal to the direction of the electron incidence, which give rise to poorer resolution at the bottom of a thick specimen in scanning transmission electron microscopy, cannot simply be added to the chromatic error in the normal mode of transmission electron microscopy. Calculations show that there is no difference in edge resolution at 100 keV to be expected, in agreement with experiment. With increasing energy, the influence of beam broadening increases relative to the chromatic error. Considering only the chromatic error (1-2 nm), at 1 MeV and optimum defocus details on the top of a 2 micron specimen will be imaged with nearly twice the value of edge half width as they will at the bottom.