Stationary beam full-field transmission helium ion microscopy using sub-50 keV He+: Projected images and intensity patterns

A dedicated Transmission Helium Ion Microscope (THIM) for sub-50 keV helium was developed to investigate ion scattering processes and contrast mechanisms to develop new imaging and analysis modalities. Unlike a commercial Helium Ion Microscope (HIM), the in-house built instrument allows full flexibility in experimental configuration. Here, we report transmission imaging and scattering patterns obtained from powder and bulk crystalline samples using a stationary broad-beam as well as convergent-beam illumination conditions in THIM. The scattered He+ ions formed unexpected spot patterns in the far-field for MgO, BN and NaCl powder samples, but not for Si bulk sample. The mechanistic origins of the spot patterns in these samples were investigated. Surface diffraction of ions was excluded as a possible cause because the recorded scattering angles do not correspond to the predicted Bragg angles. Complementary Secondary Electron (SE) imaging in a HIM revealed that these samples charge significantly under He+ ion irradiation. It is suggested that the spot patterns obtained in THIM experiments are artefacts related to sample charging. The results presented here indicate that factors other than channelling, blocking and surface diffraction of ions have an impact on the final scattered intensity distribution in the far-field. Hence, the different processes contributing to the final scattered intensities should be understood in more detail to decouple and study the relevant ion beam scattering phenomena.