In-line holography using a point source

Conventional scanning electron microscopes are now close to the limit of their performance for tasks such as the metrology of sub-micron design rule devices. In order to overcome these limits we are investigating the use of in-line electron holography for device metrology. This device will use ultralow energy electrons (50-250eV) emitted from a nano-tip electron source. Out of focus holograms of a mesh were simulated and reconstructed in the transmission mode of the microscope whereas in the reflection mode a sample consisting of only two points was used. In both operation modes of the microscope it is possible to change the distance from the point source (crossover of a lens) to the sample and the distance from the sample to the detector plane such that the magnification is kept constant. Series of simulated holograms consisting of only a few points reveal the distances resulting in the easiest interpretable images. When in-line holography is performed using electrons, the beams are deflected by the electric field between point source and sample. Ray tracing of the electrons performed by solving the appropriate Laplace equation can help to determine an optimum geometry for the microscope.