Aperture edge scattering in focused MeV ion beam lithography and nuclear microscopy: An application for the GEANT4 toolkit

Collimators are widely used to define MeV ion beams. Recent studies have shown the capability of combinations of collimators and lenses to define beams of MeV ions with sub-100 nanometre dimensions. Such nanometre beams have potential applications in MeV ion beam lithography, which is the only maskless technique capable of producing extremely high aspect ratio micro- and nano-structrures, as well as in high resolution MeV ion beam based imaging. The ion scattering from the collimator-edges can be a resolution restricting factor in these applications. Scattering processes at edges are difficult to study using conventional simulation codes because of the complicated geometry. In this work we overview the possibilities and pitfalls in using the GEANT4 toolkit as a simulation tool for studying the behaviour of ions impinging onto, or in close proximity to aperture edges. Results from simulations of scattering of 2 MeV protons incident on a 5 mm in diameter cylindrical beam defining tungsten carbide (WC) aperture blade, such as used in nuclear micro- and nanobeams as object and scraper apertures, are presented. The bubble formation at the aperture edges due to the implantation of H was first believed to cause edge roughening and hence the beam quality degradation. The concentration profile of implanted H in a WC cylinder shows that the bubble formation is not likely to be the edge roughening factor, but rather other mechanisms of beam induced topographical changes such as ion beam induced morphological changes and cracking of hydrocarbons.