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Fringe fields of the Wien-filter aberration corrector for low-voltage SEM

The resolution of a low-voltage scanning electron microscope can be improved by correcting the chromatic and spherical aberration of the objective lens. Several corrector designs have been proposed, from which the quadrupole-octupole type corrector has been sucessfully constructed (Zach 1995) . We have recently proposed (Mentink 1999) a simplified 6-pole version of the long double­ focussing 12-pole Wien filter of Rose (Rose 1990). An 8-pole design was discussed by Tsuno (Tsuno 1996) . Our design combines the advantages of relatively low electric stability requirements (5-10 ppm) and a round beam with only six mechanical pole pieces: two for a magnetic dipole field, and four electrodes for the additional quadrupole field (sufficient for chromatic aberration correction) and hexapole fields (used for spherical aberration correction). In addition, we have shown that the focussing properties of a Wien filter can be used to match the intrinsic chromatic aberration of the filter with that of any objective lens at all practical working distances, without the need of changing the filter excitation (Steffen 2000). In this case a lens before the filter is excited such that the chromatic aberration of the filter is (de-)magnified such that it exactly matches the chromatic aberration of the objective lens. An alternative is to use the filter itself as (de-)magnifying lens. A small change in filter excitation is required, which seems feasible since only one magnetic dipole field is present in our design.