Gregor Hlawacek

Nanometer scale elemental analysis in the helium ion microscope using time of flight spectrometry.

Time of flight backscattering spectrometry (ToF-BS) was successfully implemented in a helium ion microscope (HIM). Its integration introduces the ability to perform laterally resolved elemental analysis as well as elemental depth profiling on the nm scale. A lateral resolution of ≤54nm and a time resolution of Δt≤17ns(Δt/t≤5.4%) are achieved. By using the energy of the backscattered particles for contrast generation, we introduce a new imaging method to the HIM allowing direct elemental mapping as well as local spectrometry. In addition laterally resolved time of flight secondary ion mass spectrometry (ToF-SIMS) can be performed with the same setup. Time of flight is implemented by pulsing the primary ion beam. This is achieved in a cost effective and minimal invasive way that does not influence the high resolution capabilities of the microscope when operating in standard secondary electron (SE) imaging mode. This technique can thus be easily adapted to existing devices. The particular implementation of ToF-BS and ToF-SIMS techniques are described, results are presented and advantages, difficulties and limitations of this new techniques are discussed.

Direct Depth- and Lateral- Imaging of Nanoscale Magnets Generated by Ion Impact

Nanomagnets form the building blocks for a variety of spin-transport, spin-wave and data storage devices. In this work we generated nanoscale magnets by exploiting the phenomenon of disorder-induced ferromagnetism; disorder was induced locally on a chemically ordered, initially non-ferromagnetic, Fe60Al40 precursor film using u2009nm diameter beam of Ne+ ions at 25u2009keV energy. The beam of energetic ions randomized the atomic arrangement locally, leading to the formation of ferromagnetism in the ion-affected regime. The interaction of a penetrating ion with host atoms is known to be spatially inhomogeneous, raising questions on the magnetic homogeneity of nanostructures caused by ion-induced collision cascades. Direct holographic observations of the flux-lines emergent from the disorder-induced magnetic nanostructures were made in order to measure the depth- and lateral- magnetization variation at ferromagnetic/non-ferromagnetic interfaces. Our results suggest that high-resolution nanomagnets of practically any desired 2-dimensional geometry can be directly written onto selected alloy thin films using a nano-focussed ion-beam stylus, thus enabling the rapid prototyping and testing of novel magnetization configurations for their magneto-coupling and spin-wave properties.

A high resolution ionoluminescence study of defect creation and interaction

Helium ion microscopy has been used to investigate the ionoluminescence of NaCl. A 35 keV, sub-nanometer He(+) ion beam was used to generate ionoluminescence. The interaction of ionizing radiation with alkali halides leads to the formation of various crystal defects, in particular so-called color-centers. Their subsequent recombination with charge carriers leads to the emission of visible light. Broad peaks at 2.46 eV and 3.05 eV were measured. We have also investigated the dynamics of defect creation as a function of the beam scanning parameters (current and pixel spacing). The resolution and detection capabilities of ionoluminescence in helium ion microscopy are sensitive to both sample properties and scanning parameters.

Threshold and efficiency for perforation of 1 nm thick carbon nanomembranes with slow highly charged ions

Cross-linking of a self-assembled monolayer of 1,1-biphenyl-4-thiol by low energy electron irradiation leads to the formation of a carbon nanomembrane, that is only 1 nm thick. Here we study the perforation of these freestanding membranes by slow highly charged ion irradiation with respect to the pore formation yield. It is found that a threshold in potential energy of the highly charged ions of about 10 keV must be exceeded in order to form round pores with tunable diameters in the range of 5–15 nm. Above this energy threshold, the efficiency for a single ion to form a pore increases from 70% to nearly 100% with increasing charge. These findings are verified by two independent methods, namely the analysis of individual membranes stacked together during irradiation and the detailed analysis of exit charge state spectra utilizing an electrostatic analyzer.

Supported Two-Dimensional Materials under Ion Irradiation: the Substrate Governs Defect Production

Focused ion beams perfectly suit for patterning two-dimensional (2D) materials, but the optimization of irradiation parameters requires full microscopic understanding of defect production mechanisms. In contrast to freestanding 2D systems, the details of damage creation in supported 2D materials are not fully understood, whereas the majority of experiments have been carried out for 2D targets deposited on substrates. Here, we suggest a universal and computationally efficient scheme to model the irradiation of supported 2D materials, which combines analytical potential molecular dynamics with Monte Carlo simulations and makes it possible to independently assess the contributions to the damage from backscattered ions and atoms sputtered from the substrate. Using the scheme, we study the defect production in graphene and MoS2 sheets, which are the two most important and wide-spread 2D materials, deposited on a SiO2 substrate. For helium and neon ions with a wide range of initial ion energies including those used in a commercial helium ion microscope (HIM), we demonstrate that depending on the ion energy and mass, the defect production in 2D systems can be dominated by backscattered ions and sputtered substrate atoms rather than by the direct ion impacts and that the amount of damage in 2D materials heavily depends on whether a substrate is present or not. We also study the factors which limit the spatial resolution of the patterning process. Our results, which agree well with the available experimental data, provide not only insights into defect production but also quantitative information, which can be used for the minimization of damage during imaging in HIM or optimization of the patterning process.

Focused Helium and Neon Ion Beam Modification of High- T C Superconductors and Magnetic Materials

The ability of gas field ion sources (GFIS) to produce controllable inert gas ion beams with atomic level precision opens up new applications in nanoscale direct-write material modification. Two areas where this has recently been demonstrated is focused helium ion beam production of high-transition temperature (high-T C) superconductor electronics and magnetic spin transport devices. The enabling advance in the case of superconducting electronics is the ability to use the GFIS to make features on the small length-scale of quantum mechanical tunnel barriers. Because the tunneling probability depends exponentially on distance, tunnel barriers must be less than a few nanometers wide, which is beyond the limits of other nanofabrication techniques such as electron beam lithography. In magnetism, the GFIS has recently been used to generate chemical disordering and modify magnetic properties at the nanoscale. The strongest effect is observed in materials where ion-induced chemical disordering leads to increased saturation magnetization, enabling positive magnetic patterning. In this chapter, we review the latest results and progress in GFIS ion beam modification of (high-T C) superconductors and magnetic materials.

HIM of Biological Samples

Due to its charge compensation capabilities the imaging of insulating sample is a natural application of Helium Ion Microscopy. The imaging of biological samples often requires complicated sample preparation methods who’s influence on the sample structure is not always fully understood. In this chapter we will present a number of recent studies from the aforementioned field that make use and demonstrate the benefits of Helium Ion Microscopy for these research topics. These examples also demonstrate the large depth of focus that distinguishes the method.

Backscattering Spectrometry in the Helium Ion Microscope: Imaging Elemental Compositions on the nm Scale

The idea of using backscattered helium particles to access chemical information on the surface in a helium ion microscope came up right from the early days of this relatively young imaging technique. From the basic principles of backscattering spectrometry, ion solid interaction and particle detection it became clear rapidly that this attempt will suffer many difficulties in terms of technical realization and physical limitations. This chapter is about describing those difficulties and working out different scenarios of how to apply backscattering spectrometry to the HIM anyways. It will be shown that an actual technical realization exist enabling laterally resolved chemical analysis in a HIM with a resolution down to (55,)nm.

Channeling and Backscatter Imaging

While the default imaging mode in HIM uses secondary electrons, backscattered helium or neon contains valuable information about the sample composition and structure. In this chapter, we will discuss how backscattered helium can be used to obtain information about buried structures and provide qualitative elemental contrast. The discussion is extended to the use of channeling to increase image quality and obtain crystallographic information. As an example, we demonstrate that the period of a dislocation network in a film only two monolayers thick can be obtained with atomic precision.

Developing Rapid and Advanced Visualisation of Magnetic Structures Using 2-D Pixelated STEM Detectors

Transmission Electron Microscopy (TEM) electron diffraction patterns, imaged from the back focal plane of the objective lens, reveals rich information about the structure of materials. The sharpest patterns are obtained using a parallel (semi-convergence angle < 1 mrad) electron beam which typically illuminates a circular region with a diameter of 100 nm. In Scanning Transmission Electron Microscopy (STEM) the electron beam is focused to form a fine probe, potentially with sub-Ångström diameter. Signals generated by the interaction of the probe with a sample are collected by detectors which integrate the scattered electron intensity in the back focal plane for each probe position in a 2-D scan. A key aspect to obtaining high resolution information is that the performance of scanning and detection should be performed rapidly in order to provide live imaging for the user and to also to mitigate the effect of microscope instabilities and specimen drifts.