Improvement of the Optical Performance of the Geometrical Parameters of Snorkel Magnetic Lens

Two models of asymmetrical magnetic objective lens geometries have been designed and studied. These two models are called Pinhole and Snorkel lenses. The comparison between these models was realized by the aid of simulation EOD program (Electron Optical Design), using the finite element method to analyze the magnetic flux density and optical properties at constant excitation (NI=2000A-t). The optimal model (Snorkel Lens) gives the best characteristics that were compatible with the research purposes in details, such as the changing of air gap and the axial polepiece bore diameter on the optical performance of the snorkel magnetic lens. This type of lens was used in low energies scanning electron microscope (SEM), which achieved the lowest values of the aberrations coefficient at long working distance.