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

Michael Mousley,Nico Klingner,Olivier De Castro,Olivier Bouton,Santhana Eswara,Gregor Hlawacek,Christoph T Koch,Tom Wirtz

doi:10.3762/bjnano.10.160

Abstract

A dedicated transmission helium ion microscope (THIM) for sub-50 keV helium has been constructed to investigate ion scattering processes and contrast mechanisms, aiding the development of 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 projection imaging and intensity patterns obtained from powder and bulk crystalline samples using stationary broad-beam as well as convergent-beam illumination conditions in THIM. The He+ ions formed unexpected spot patterns in the far field for MgO, BN and NaCl powder samples, but not for Au-coated MgO. The origin of the spot patterns in these samples was 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 the HIM revealed that these samples charge significantly under He+ ion irradiation. The spot patterns obtained in the THIM experiments are explained as artefacts related to sample charging. The results presented here indicate that factors other than channeling, blocking and surface diffraction of ions have an impact on the final intensity distribution in the far field. Hence, the different processes contributing to the final intensities will need to be understood in order to decouple and study the relevant ion-beam scattering and deflection phenomena.

Highlights

The use of helium ions for microscopy and nanoanalysis is gaining popularity due to the availability of the high-brightness gas field ion source (GFIS) [1]
The results presented here indicate that factors other than channeling, blocking and surface diffraction of ions have an impact on the final intensity distribution in the far field
The results show that charging-related effects can significantly contribute to the final intensity distribution of deflected He+ ions and these effects must be removed from the recorded intensity patterns in order to quantify channeling and diffraction effects

Summary

Introduction

The use of helium ions for microscopy and nanoanalysis is gaining popularity due to the availability of the high-brightness gas field ion source (GFIS) [1]. With further reduction in the voltage of Lens 2 (as shown in Figure 2C and 2D), the magnification is increased and the intensity is redistributed from an image of the grid to a deflection pattern.

Results

Conclusion