Comparative study of beam-induced carbonaceous deposits in helium-ion and scanning electron microscopes.

In this work, the phenomena of beam-induced contamination in charged beam microscopes (i.e. the scanning electron microscope (SEM) and helium ion microscope (HIM)) was investigated in order to determine its influence on specimen characterisation. This was achieved through a comparative study on the secondary electron (SE) generation mechanisms in both microscopes, which is key contamination formation process. Deposited contamination structures formed by electron and He+ ion irradiation were assessed in relation to specimen properties and beam parameters. Characterised in terms of morphology and composition, their effect on SE image contrast was evaluated. This revealed that the He+ induced contamination behaves initially like electron-induced contamination, depending almost linearly on irradiation dose. However, for larger dose values, the nature of the beam-specimen-hydrocarbon interaction precludes comparison between HIM and SEM due to ion sputtering and implantation. Various specimens were used to evaluate the SE contrast mechanisms. Firstly, image contrast of Ga+ implanted regions in an n-type silicon specimen was analysed. This revealed a similar relationship between, image contrast, Ga+ density and the imaging parameters. The quantification of SE contrast was also significantly influenced by dynamic charging effects. Secondly, SE imaging of a graphene surface was used to provide sub-nanometer information on the morphology of contaminants there on. The contrast of contaminants, i.e. their apparent lateral dimensions, varied with imaging conditions however, a quantitative method to determine the physical size of surface features, regardless of contrast variation, was derived. The HIM was evaluated as a superior SE imaging technique, in terms of image fidelity, due to the finite size of its primary beam probe. Thirdly, a quantitative method for the extraction of graphene work function was determined from SE images of layered graphene specimens. A model based on illumination by both the primary beam and backscattered particles was established to explain the observed layer-dependent SE contrast and work function. Complimentary to the SE imaging study, and to further enhance the specimen characterisation function of HIM, a SE energy filter device was designed and fabricated for use in both HIM and SEM. It was quantified in terms of filtering efficiency and used to generate image based SE energy spectra of electron and He+-ion irradiated specimens. Finally, a dedicated study to understand Raman enhancement of electron-induced carbonaceous deposits was conducted. Its focus was to determine the influence of electron beam energy on the bonding structure of the carbon layers and its effect on their optical properties