Abstract
This thesis investigates the design of quadrupole probe forming lens systems with resolution in the nanometre range. To achieve sub-micron resolution, big improvements must be made in focusing, lens aberration, and reduction of scattering processes. This thesis presents the results of a theoretical investigation into the performance of two-stage magnetic quadrupole lens systems with an intermediate focus. Such systems are capable of far greater demagnification than single-stage systems, but the challenge is to find a two-stage system with an acceptable ratio between demagnification and aberration. The results of a systematic survey of two-stage lens systems using the numerical raytracing technique to accurately calculate the lens aberrations of each system are presented. This thesis also investigates the use of pre-lens electrostatic scanning with magnetic quadrupole lenses. It is shown that in spite of the large thirdorder aberration of quadrupole systems, the use of dog-leg deflection systems in which the beam always crosses the optical axis at the entrance principal plane of the lens, can minimize distortion due to off-axis aberration. An evaluation of the lens aberrations in microbeam and nanobeam systems caused by stray DC magnetic fields is also presented. The relative thickness of the beamline optical elements compared to the curvature of the beam in stray magnetic fields causes aberration where the beam axis differs from the optical axis of the lens system. Numerical raytracing is used to study the influence of stray DC magnetic fields on beam resolution at the sub-micron level.