Low-energy scanning electron microscope using a monochromator with double-offset cylindrical lenses

Abstract

The nanoelectronics industry demands continuous improvement in the performance of scanning electron microscopes (SEMs). Extremely low energies of less than 1 keV are required for SEM observations to allow the subsurface and nanoscale information of target specimens to be measured with minimized charge-up and beam damage depths because of the reduced interaction volumes. In this article, the authors propose a new monochromator (MC) structure and investigate its applicability to SEMs operating at such extra-low energies. The proposed MC, which uses double-offset cylindrical lenses, can perform energy filtering in its midsection and form a stigmatic and nonenergy dispersive image at the exit. The energy resolution is expected to be better than 10 meV for a pass energy of 4 keV. The MC has the additional advantage of a simple but robust structure, which is essential for industrial applications. Assuming the use of ideal and high-performance SEM optics, for which the spherical aberration coefficient and the chromatic aberration coefficient are both 1 mm, beam diameters measured in terms of the full width that contains 50% of the beam current (FW50) are calculated at the specimen position. Use of the MC improves the beam diameter dramatically to 4.4 nm, as compared to the diameter of 19.7 nm for the SEM without the MC, at a landing energy of 100 eV. The chromatic aberration contribution also becomes negligible because of the MC. The beneficial effects of the MC with regard to the beam diameter become increasingly prominent at lower landing energies ranging down to 10 eV. A SEM using this MC can generate highly monochromatic (10 meV) electron probe beams with small size (5 nm) and low energy (100 eV), which indicates the additional possibility of a new surface electron microscope that uses phonon signals. Based on this theoretical investigation, the authors conclude that this MC can effectively improve the SEM's performance capabilities in the extra-low-energy region.