Low-Voltage Scanning Electron Microscope (LVSEM) with a Single-Polepiece Lens

The advantages of the LV SEM are well known. Recently a lot of interesting results from this field were presented which were obtained thanks to development of field emission guns and to the enourmous progress in the computation techniques in electron optics.One of the simplest arrangements of the LVSEM is shown in Figure 1. The Tesla SEM BS 350 with a field emission gun and the TF-W/100-Zr cathode was used for our experiment. The gun provides 10−10 A current in the diffraction limited spot (for the angular density 0.20mA sr−1). If a potential Usp is applied to the specimen the energy E of the electrons that strike the specimen is Ep-eUsp (Ep-primary beam energy, e-elementary charge). The produced secondary (SE) and backscattered (BSE) electrons are accelerated towards the semiconductor detector by the electrostatic field and their energy spectrum extends from eUsp to Ep. The final energy of the SE and BSE can then be sufficient for achieving a reasonable amplification of the semiconductor detector which is directly proportional to the energy of the electrons that strike the detector. We calculated optical properties for a combination of the electrostatic and magnetic lenses of the basic geometry shown in Figure 1 and for an arrangement with the single polepiece lens shown in Figure 2. We particularly investigated coefficients of the chromatic (Cc) and spherical (Cs) aberrations as functions of the ratio of the primary beam energy to the energy of the electrons that strike the specimen Ep/E for some optimum position of the specimen, electropstatic and magnetic field. Our results are shown in Table 1. The coefficients Cs and Cc do not change with the energy Epor E if the ratio Ep/E is maitained the same and aberrations are lower for larger ratios Ep/E, so that the influence of the contribution of the electrostatic lens aberrations is negligible for our geometry. For example, if we require a resolution limit r=2nm and an energy of the electrons that strike the specimen E=300eV, it is possible to calculate that the coefficient of the aberrations must be Cs<0.21mm and Cc<0.14mm for an energy width AE=0.2eV, so that we need the ratio Ep/E≥150 for the arrangement shown in Figure 1 (i.e.Ep≥45keV) and Ep/E≥33 for the arrangement shown in Figure 2 (i.e.Ep≥10keV).The advantages of the combination of the magnetic lens with the electrostatic cathode lens for the high resolution very low energy electron microscopy are well known . We assume that for the LVSEM only a medium electrostatic field strength is admitted at the specimen surface. Nevertheless, our experimental arrangements should certainly be optimized in the future.