Finite Element Analysis of Scanning Electron Microscope Illumination System

The design, simulation, and beam dynamics analysis of the integrated illumination system of the scanning electron microscope (SEM) have been reported. The dimensions and capacity of the SEM components, such as the electron gun, electron column, and lenses, were determined using the finite element method. The optimization of the illumination system was performed, and characteristics beam parameters were studied analytically. The gun consisted of a simple diode structure having; cathode, Wehnelt, and anode electrodes. A tungsten cathode of diameter 150 μm was used to emit the electrons thermionically. The electrons beam was generated at 2700 K with acceleration potential of 15 kV. Pair of electromagnetic lenses producing field strengths, 221 mT and 253 mT, respectively, focused the beam at 56.5 mm from cathode, with beam diameter of 9 μm in the post anode region. The calculated value of beam brightness was 2.97 \(\times\) $\times$ 10⁶ A/(mm-rad)². The final beam spot thus obtained was stable and perfectly symmetric along the two transverse directions. Gun assembly and lenses are thermally stable at the operational temperature. The illumination system geometry offers full-beam collimation control with reduced column length, making it suitable for electron diffraction studies, where focusing of the beam onto a sample is desired.