Depth resolved investigation of ion beam induced pattern formation on silicon using X-ray methods

摘要

In the present work, ion beam induced pattern formation on silicon surfaces is investigated, which was induced by off-normal irradiation of Fe ions. It is observed that the self-organized surface patterning takes place when the ion fluence exceeds a certain limit. The influence of the ion parameters on the patterning mechanism was investigated via monitoring of the near surface area where incorporated Fe ions play a dominant role. In a first step, ion beam induced surface density variation was studied as a function of ion parameters. Conventional X-ray reflectivity revealed the formation of a nm sub-surface layer with incorporated Fe atoms. Using X-ray reflectivity, no major dependence of the surface density on the ion fluence could be obtained. Thus, a new powerful technique with higher surface sensitivity was applied based on extremely asymmetrical X-ray diffraction methods. The density information was extracted from the shift of the diffraction peak caused by refraction of the X-ray beam at the air-sample interface. Simulations based on the dynamical theory of X-ray diffraction revealed a decrease of the density for increasing ion fluence in a region close to the surface. It shows that there is a threshold value for ion fluence leading to reduction of surface density. The obtained results reveal that the change of local density contributes to the development of the pattern on the surface.In a second step, Fe-silicide formation in various stoichiometries was investigated in the amorphized surface region of crystalline Si(100) after irradiation with Fe ions. A depth resolved analysis of chemical states of Si and Fe atoms in the near surface region was performed by combining X-ray photoelectron spectroscopy and X-ray absorption spectroscopy using synchrotron radiation. The formation of silicide bonds of different stoichiometric composition changing from an Fe-rich silicide (Fe3Si) close to the surface into a Si-rich silicide (FeSi2) towards to the inner interfaces was observed. This observation shows that the presence of chemically bonded iron close to the surface is an important prerequisite of pattern formation. In a third step, the recrystallization of the pre-formed Fe-silicides of various stoichiometries was investigated below the amorphized surface of crystalline Si(100). A thermal annealing process was applied in the range between room temperature and 800°C. Depth profiling by grazing incidence X-ray diffraction confirmed that a ε-FeSi phase was formed close to the surface changing to a β-FeSi2 phase with lower Fe content at larger depths. Both phases are distributed with different ratios within the Fe-Si layer at smooth and patterned surfaces, corresponding to low and high ion fluences, respectively. In the last step, the obtained results were examined for pattern formation on Si(001) surfaces under normal incident Kr ion bombardment with simultaneous incorporation of Fe atoms. It was verified that the surface patterning takes place only when the incorporated Fe concentration again exceeds a certain limit. For a high Fe concentration the ripple formation is accompanied by the enrichment of Fe atoms at the top part of ripples, whereas no such Fe enhancement is found for a low Fe concentration at samples with smooth surfaces. Modeling of the measured X-ray photoelectron spectroscopy and X-ray absorption spectroscopy spectra reveals the appearance of different silicide phases with a decreasing Fe content from top towards the volume.