Ultimate resolution limits for scanning electron microscope immersion objective lenses

Abstract

This paper sets out to establish resolution limits for electron objective immersion lenses that can be used for high resolution scanning electron microscopy. On-axis aberration coefficients, both spherical (Cs) and chromatic (Cc), are calculated for immersion lenses that are: purely magnetic, of the electric retarding field type, and that have mixed electric-magnetic fields. The aperture lens model is used to derive a general framework by which these lenses can be compared. Simulation results show that the mixed field electric-magnetic immersion lens is expected to provide the best image resolution. For a primary beam landing energy of 1 keV, and for field strength limits set by magnetic saturation and vacuum electrical field breakdown, the lowest on-axis aberration coefficients predicted for this lens lie between 50 μm to 60 μm.