Maximizing the penetration in high voltage electron microscopy

Abstract

The crystal orientations which give maximum electron transmission have been determined for a wide variety of materials at 100 to 1000 kv. It is found both experimentally and theoretically that the diffraction conditions for best transmission change considerably as the accelerating voltage is raised above 100 kv. These conditions are tabulated for materials of low, medium and high atomic numbers. The changes are predicted by many-beam dynamical diffraction theory and may be understood in terms of the channelling properties of individual Bloch waves which represent the fast electrons inside crystals. The nature of defect contrast under these conditions is considered, as is the sensitivity of theoretical calculations to absorption and extinction parameters. Penetration of heavy materials at 1000 kv using the crystal orientations given in this paper is considerably greater than previously expected. For example, defects were observed in a gold foil well over one micron in thickness.