“Applications of Coherent Electron Beams

The use of coherent beams for interferometric measurements has gained great popularity in light optics over the last several decades. The availability of coherent electron sources has now opened the door to apply the concept of holographic imaging in many new areas. Off-axis holograms can now be recorded in field emission transmission electron microscopes equipped with the electron optical equivalent of a biprism. This technique allows the accurate retrieval of phase and amplitude of the electron wave, which has been transmitted through a sample. The sensitivity of the phase of the electron wave to electrical potentials makes it possible to map out potential distributions on the specimen with sub-micron resolution. As part of this thesis, off-axis electron holography has been applied to map out the small potential changes, which occur over pn-junctions in doped semiconductor devices. To this end a special alignment of the electron microscope has been devised, and new methods for preparing electron transparent samples of semiconductor devices, specially tailored for electron holography, have been developed. Also, new ways to improve on the existing electron biprism technology have been investigated. The observed voltage signals could be linked to active dopants in silicon by annealing experiments, and the viability of the method for voltage profiling of real-world semiconductor devices has been demonstrated. The use of coherent electron emitters also allows the recording of in-line electron holograms in a lens-less projection microscope at ultra-low beam energies. Such a point projection microscope, which is capable of recording in-line holograms in transmission imaging, has been built. With defect review on silicon wafers as a possible application for in-line electron holography in mind, the feasibility of point projection imaging in a reflection geometry has been demonstrated. In this context the elastic backscattered yield for electrons in different materials and under different geometries has been calculated using Monte Carlo simulations. Several problems, which occur in reflection imaging, are pointed out and possible solutions are presented.