Scanning Microscopy


The electron optical properties of an asymmetric magnetic condenser-objective lens are determined not only by its geometric structure and shape parameters, but also by the degree of saturation of its polepieces. A systematic investigation has been carried out for a series of asymmetric magnetic condenser-objective lenses. Results show that, owing to its small bore, the lower polepiece of such an asymmetric condenser-objective lens has considerable effects on the electron optical properties. When this polepiece is appropriately highly saturated, an asymmetric condenser-objective lens possesses low spherical aberration and chromatic aberration coefficients. On the other hand, the large bore of the upper polepiece leads to a more favourable design of the specimen stage and analytical facilities in the electron microscope. Comparison of a series of asymmetric condenser-objective lens suggests that the ratio of bore diameter of the upper polepiece to that of the lower polepiece D1/D2 = 4 is favourable.

Calculation of the localized influence of the axial field distribution on the spherical and chromatic aberration coefficients of the lens is important for investigation of the effects of polepiece saturation on the electron optical properties of the condenser-objective lens, and is very useful for lens design. A method of achieving this is demonstrated.

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