Simulation of a Wien filter as beam separator in a low energy electron microscope

Abstract

The optical properties of a Wien filter as beam separator and the cathode lens of a low energy electron microscope (LEEM) are numerically analyzed. Electro-static and magnetic field distributions are calculated using softwares of the second-order finite element method and the three-dimensional finite difference method followed by direct ray tracing. The calculated resolution of the LEEM is 16 nm under the accelerating voltage of 10 V at the specimen. The simulated results of the direct ray tracing indicate that the alignment of the illumination beam is important to obtain LEEM images, because if the illumination beam is outside the finite area and angle, the beam returns without hitting the specimen and forms a mirror electron microscope image (MEM). The Wien filter beam separator does not introduce noticeable astigmatism, second-order aberrations and dispersion for the image forming beam. However, those are inevitably introduced in the illumination beam.