The use of the Wien filter to eliminate object slit scattering in MeV ion nanobeam systems

Abstract

One of the fundamental limitations in the performance of MeV ion microbeam focusing systems is the effect of ion scattering at the edges of the object aperture. As the aperture is reduced in the search for smaller spot sizes, the fraction of scattered to unscattered beam increases. The scattered beam contains lower energy particles which can be transmitted through the system to create a halo of over-focused particles surrounding the final image. Removal of this halo is critical to achieving small spot sizes, especially in single ion applications. In this paper, we discuss the use of a Wien filter (crossed magnetic and electrostatic fields) to deflect the reduced energy scattered particles and ensure that only ions with the correct energy are accepted into the lens. This paper reviews the beam optics of Wien filter systems and presents calculations of the parameters required to obtain useful energy dispersion.