Abstract
The present research work takes the intensive studies on two types of electron lenses (magnetic and electrostatic), in order to obtain the preferable design of a compound lens. The single polepiece magnetic lens models of the same dimension and different structure insulations (iron and air) have been designed and tested. It was found that the lens with iron insulation has been achieved better optical performance. Another study has been carried out on a set of asymmetrical electrostatic lenses having different geometries of the outer electrodes. It was found that the geometrical shape of the outer electrode has a clear effect on the electron optical properties. The electrostatic retarding lens was inserted inside the magnetic lens in front of the specimen due to improvement in the objective focal properties especially at low beam voltages. Furthermore, a good resolving power has been obtained from this type of the compound lens in comparison with that of the traditional pure magnetic lens. The result of the compound lens of the preferable design was compared with those of the published papers; it was found that both results are in good agreement with each other
Highlights
Low- voltage microscopy can produce high - resolution, high - contrast images of nanometerscale surface features
The advantages for which low- voltage operation of the microscope confer are an increase of the secondary electrons emission coefficiens, the reduced penetration of the primary electrons leads to the effects due to the surface layers of the specimen and the ability to examine the insulating materials without charging the artifacts occurring in the images (Tromp, 2000)
The configuration of the compound lens consists of the magnetic lens with an electrostatic immersion lens placed behind the specimen called "Gemini objective lens" was firstly applied by Frosien (Frosien et al, 1989)
Summary
Low- voltage microscopy can produce high - resolution, high - contrast images of nanometerscale surface features. The new type of the electron lenses (compound lens), having an overlapping of the magnetic and electrostatic retarding fields concentrated inside the lens structure (Gemini lens) It is perfectly suitable as an objective lens for low- voltage SEM's, this lens was studied extensively by (Hujazie, 2006). The second part of the work is carried out to investigate the effect of the outer electrode geometry on the performance of an electrostatic lens and the third part is considered as a combination of the asymmetrical magnetic lenses which possess the preferable design previously obtained in part one with that of the asymmetrical two- coaxial cylindrical electrostatic lens of an external conical shape acquires the best lens design in part two This combination of lenses are perfectly suitable as an objective compound lens for the low- voltage SEM which gives a good resolution at low beam energies
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