The use of multipole fields for aberration correction in π/2 Wien filters

A Wien filter capable of producing nanospots at the selection slit for electrons with nominal energy has been designed. An accurate version of the boundary element method (BEM) is used to model the shape and field distributions. Using a direct ray tracing simulation, we investigate the geometrical aberrations in the basic filter, in which dipole and quadrupole electric fields and dipole magnetic field are present. The addition of a quadrupole magnetic component as well as of both hexapole components produces a drastic reduction in the aberrations. The analysis of the Rose condition for cancelling second order geometric aberrations shows a close agreement between the present design and the ideal case. (C) 2008 Elsevier B.V. All rights reserved.