Time-of-flight secondary ion mass spectrometry in the helium ion microscope.

A helium ion microscope, known for high resolution imaging and modification with helium or neon ions, has been equipped with a time-of-flight spectrometer for compositional analysis. Here we report on its design, implementation and show first results of this powerful add-on. Our design considerations were based on the results of detailed ion collision cascade simulations that focus on the physically achievable resolution for various detection limits. Different secondary ion extraction geometries and spectrometer types are considered and compared with respect to the demands and limitations of the microscope. As a result the development and evaluation of a secondary ion extraction optics and time-of-flight spectrometer that allows the parallel measurement of all secondary ion masses is reported. First experimental results demonstrate an excellent mass resolution as well as high-resolution secondary ion imaging capabilities with sub-8 nm lateral resolution. The combination of high resolution secondary electron images and mass-separated sputtered ion distributions have a high potential to answer open questions in microbiology, cell biology, earth sciences and materials research.

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.

An analytic approach to modeling the optical response of anisotropic nanoparticle arrays at surfaces and interfaces.

Anisotropic nanoparticle (NP) arrays with useful optical properties, such as localized plasmon resonances (LPRs), can be grown by self-assembly on substrates. However, these systems often have significant dispersion in NP dimensions and distribution, which makes a numerical approach to modeling the LPRs very difficult. An improved analytic approach to this problem is discussed in detail and applied successfully to NP arrays from three systems that differ in NP metal, shape and distribution, and in substrate and capping layer. The materials and anisotropic NP structures that will produce LPRs in desired spectral regions can be determined using this approach.

Magnetic properties of granular CoCrPt:SiO 2 thin films deposited on GaSb nanocones.

We report on the effect of microstructure and geometrically induced modifications of the magnetic properties of granular CoCrPt:SiO2 films with weakly interacting magnetic grains deposited on pre-structured GaSb nanocone templates fabricated by an ion erosion technique. By tuning the irradiation conditions, nanocone patterns of different cone sizes were prepared (from 28 to 120 nm in diameter and 32 to 330 nm high, respectively). The influence of the intergranular exchange coupling was also investigated by varying the SiO2 content from 8 to 12 at.%. Deposition of CoCrPt:SiO2 on samples with small nanocones leads to a close magnetic grain packing, which results in the formation of extended magnetic domains larger than the average distance between the GaSb cones. In contrast, on larger nanocones, the magnetic coating grows on the side-walls, with a large separation between neighboring cones, leading to magnetic single-domain regions, which are correlated to the underlying structure. Magnetometry indicates that both remanence and coercivity decrease with increasing cone size and/or SiO2 content due to a combined effect of the angular distribution of the magnetic easy axis of the grains and the intergranular exchange coupling strength.

Phase diagram for nanostructuring CaF(2) surfaces by slow highly charged ions.

The impact of individual slow highly charged ions (HCI) on alkaline earth halide and alkali halide surfaces creates nano-scale surface modifications. For different materials and impact energies a wide variety of topographic alterations have been observed, ranging from regularly shaped pits to nanohillocks. We present experimental evidence for the creation of thermodynamically stable defect agglomerations initially hidden after irradiation but becoming visible as pits upon subsequent etching. A well defined threshold separating regions with and without etch-pit formation is found as a function of potential and kinetic energies of the projectile. Combining this novel type of surface defects with the previously identified hillock formation, a phase diagram for HCI induced surface restructuring emerges. The simulation of the energy deposition by the HCI in the crystal provides insight into the early stages of the dynamics of the surface modification and its dependence on the kinetic and potential energies.

Surface nanostructures by single highly charged ions.

It has recently been demonstrated that the impact of individual, slow but highly charged ions on various surfaces can induce surface modifications with nanometer dimensions. Generally, the size of these surface modifications (blisters, hillocks, craters or pits) increases dramatically with the potential energy of the highly charged ion, while the kinetic energy of the projectile ions seems to be of little importance. This paper presents the currently available experimental evidence and theoretical models and discusses the circumstances and conditions under which nanosized features on different surfaces due to the impact of slow highly charged ions can be produced.

Defect mediated desorption of the KBr(001) surface induced by single highly charged ion impact.

The individual impacts of slow (300 eV/amu) highly charged Xe ions induce nanometer sized pitlike structures on the KBr (001) surface. The volume of these structures shows a strong dependence on the ions potential energy. Total potential sputter yields from atomically flat (001) terraces are determined by imaging single ion impact sites. The dependence of the sputter yield on the ions initial charge state combined with structure formation at low and high-fluence irradiations indicates that agglomeration of defects into complex centers plays a major role in the desorption process induced by the potential energy.

Creation of nanohillocks on CaF2 surfaces by single slow highly charged ions.

Upon impact on a solid surface, the potential energy stored in slow highly charged ions is primarily deposited into the electronic system of the target. By decelerating the projectile ions to kinetic energies as low as 150 x q eV, we find first unambiguous experimental evidence that potential energy alone is sufficient to cause permanent nanosized hillocks on the (111) surface of a CaF(2) single crystal. Our investigations reveal a surprisingly sharp and well-defined threshold of potential energy for hillock formation which can be linked to a solid-liquid phase transition.

Early stage of ripple formation on Ge(001) surfaces under near-normal ion beam sputtering.

We present a study of the early stage of ripple formation on Ge(001) surfaces irradiated by a 1 keV Xe(+) ion beam at room temperature and near-normal incidence. A combination of a grazing incidence x-ray scattering technique and atomic force microscopy allowed us to observe a variation of the symmetry of the surface nanopattern upon increase of the ion fluence. The isotropic dot pattern formed during the first minutes of sputtering evolves into an anisotropic ripple pattern for longer sputtering time. These results provide a new basis for further steps in the theoretical description of the morphology evolution during ion beam sputtering.

Channels of potential energy dissipation during multiply charged argon-ion bombardment of copper.

The dissipation of potential energy of multiply charged Ar ions incident on Cu has been studied by complementary electron spectroscopy and calorimetry at charge states between 2 and 10 and kinetic energies between 100 eV and 1 keV. The emitted and deposited fractions of potential energy increase at increasing charge state, showing a significant jump for charge states q>8 due to the presence of L-shell vacancies in the ion. Both fractions balance the total potential energy, thus rendering former hypotheses of a significant deficit of potential energy obsolete. The experimental data are reproduced by computer simulations based on the extended dynamic classical-over-the-barrier model.

Formation of Ordered Nanoscale Semiconductor Dots by Ion Sputtering.

A formation process for semiconductor quantum dots based on a surface instability induced by ion sputtering under normal incidence is presented. Crystalline dots 35 nanometers in diameter and arranged in a regular hexagonal lattice were produced on gallium antimonide surfaces. The formation mechanism relies on a natural self-organization mechanism that occurs during the erosion of surfaces, which is based on the interplay between roughening induced by ion sputtering and smoothing due to surface diffusion.