Materials analysis using channeling and ionoluminescence in a helium ion microscope

Helium ion Microscopy1 is a versatile microscopy technique that provides high resolution imaging and nano-machining in combination with a high surface sensitivity and large depth of focus. It utilizes a narrow beam of He+ ions to achieve a lateral resolution of less than 0.5 nm. Backscattered Helium ions (BSHe) and secondary electrons (SE) can be used to obtain an image of the specimen.
When using crystalline samples channeling of the particles can occur. This effect can be exploited in several ways in the HIM. First of all it is possible to map out the different channeling directions and intensities and thus obtain information on the crystal structure of the sample. A simple geometrical model is introduced that can predict the channeling directions and relative intensities observed in the HIM2. By exploiting channeling and making use of the dechanneling contrast thin surface layers can be made visible in SE as well BSHe images3. We used this to observe composition and structural changes in a 2 ML thin silver layer on Pt(111). Work function differences as small as 40 meV between Ag and Pt rich areas on the surface reveal the position of mono—atomic surface steps. A regular arrangement of areas with reduces the channeling probability reveals the surface reconstruction of the top 2—3 ML which has a periodicity of only 5.8 nm.
Ionoluminescence on the other hand allows to obtain information on defects in the bulk of the material. I will show results obtained for a variety of materials including semiconductors4, rare earth containing perovskites and ionic crystals. The types of defects were identified and the influence of the scanning conditions on the IL signal has been investigated5. We used IL to map out the interaction volume of the beam in NaCl, and demonstrate the possibility of subsurface patterning. In our setup using a 35 keV He+ beam and NaCl only 3 vac/nm-2 are needed to obtain a detectable IL signal6.