RING FOCUSED ELECTRON BEAM COLUMN DESIGNS

Abstract

This thesis proposes ring beam focused column designs suitable for two new types of electron cold field emission sources which can in principle act as ring beam electron sources. The first is a ring-shaped micro-fabricated tubular graphene emitter, spanning several microns in diameter but having a nanometer size wall thickness. The second, a single-point emitter made from graphene coated on a nickel wire, is characterized both by its very low work function (around 1 eV) and its wide-angle uniform current emission (over 40°). Direct ray trace simulation methods demonstrate that a ring-beam focused column for the ring-emitter source can be designed to project a magnified/demagnified ring image on to the specimen plane, while for the single-point emitter/annular aperture source, aberration correction units can be designed to act in combination with conventional objective lenses in order to provide high currents (a few to tens of nAs) focused into nano-size probes.