Combined Focused Ion Beam Research Articles

Metrological challenges for reconstruction of 3‐D microstructures by focused ion beam tomography methods

The development of combined focused ion beam and scanning electron microscopes has enabled significant advances in the characterization of the 3-D structure of materials. The repeated removal of thin layers or slices with an ion beam and imaging or mapping the chemical or crystallographic structure of each slice enables a 3-D reconstruction from the images or maps. The accuracy of the reconstruction thus depends on the accuracy with which the slice thickness is measured and maintained throughout the process, and the alignment accuracy of the slices achieved during acquisition or by postacquisition corrections. A survey of papers published in this field suggests that the reconstruction accuracy is not often considered or reported. Using examples from examination of the 3-D structure of hardmetals, issues affecting the accuracy of slice thicknesses and image realignments are examined and illustrated and potential errors quantified by the use of fiducial markers and the expected isotropy of the hardmetal structure itself.

NANOMETER-SIZED PLATINUM-GROUP MINERALS (PGM) IN BASE METAL SULFIDES: NEW EVIDENCE FOR AN ORTHOMAGMATIC ORIGIN OF THE MERENSKY REEF PGE ORE DEPOSIT, BUSHVELD COMPLEX, SOUTH AFRICA: DISCUSSION: TABLE 1

The paper by [Wirth et al. (2013)][1] is of great interest in understanding the mineralogical complexity affecting the crystallization history of platinum group elements (PGE) in mineral deposits such as the Merensky Reef, Bushveld complex, South Africa. A combined focused ion beam and high-

Nanometer-sized platinum-group minerals (PGM) in base metal sulfides: New evidence for an orthomagmatic origin of the Merensky Reef PGE ore deposit, Bushveld Complex, South Africa

We report new results of a combined focused ion beam and high-resolution transmission electron microscopy (FIB/HRTEM) investigation of platinum group element-rich (PGE) samples from the Merensky Reef of the Bushveld Complex in South Africa. Chromitites and pegmatitic melanorite from the Northam and Rustenburg platinum mines contain Ru-Rh-Pt arsenide nanocrystals ( Occasionally the PGM inclusions form clusters, develop a flow texture, or are aligned like pearls on a string. These observations are inconsistent with sub-solidus exsolution and suggest that the nano-scale PGM crystallized before solidification of the sulfide melt and the formation of their BMS hosts. There is evidence from recently performed laboratory experiments on silicate melts with additives of Ru and Pd that discrete Fe-rich Fe-Ru-Pd alloys (nanometer-sized) can indeed crystallize early. These experiments raise the possibility that the PGE-bearing nanocrystal inclusions found in the BMS of the Merensky Reef might represent preserved relicts of an early phase of magmatic PGM that precipitated from the silicate melt and afterwards were collected by the sulfide melt. Combined FIB/HRTEM techniques open up a window through which the nano-scale mineral population can be viewed. PGE previously thought to be present in BMS solid solution might actually be an artefact of inadequate spatial resolution.

Three‐Dimensional Morphological and Crystallographic Investigation of Lamellar Alpha and Retained Beta in a Near Alpha Titanium Alloy by Combination of Focused Ion Beam and Electron Backscattering Diffraction

AbstractWe studied the three‐dimensional (3D) morphology and crystal orientation distribution of coarse primary α, fine second α lamellae, and retained β layers in a near α titanium alloy using a 3D microtomographic electron backscattering diffraction method (3D EBSD). The results demonstrate that primary α lamellae are more like a thick plate in 3D space. The second α lamellae have lower in‐grain (mis)orientation gradients than primary α. The retained β is a continuous network structure and contains many cavities for α lamellae embedding.

Visualization of yeast cells by electron microscopy

In the 1970s, hydrocarbon or methanol utilizable yeasts were considered as a material for foods and ethanol production. During the course of studies into the physiology of yeasts, we found that these systems provide a suitable model for the biogenesis and ultrastructure research of microbodies (peroxisomes). Microbodies of hydrocarbon utilizing Candida tropicalis multiply profusely from the preexisting microbody. β oxidation enzymes in the microbody were determined by means of immunoelectron microscopy. We examined the ultrastructure of Candida boidinii microbodies grown on methanol, and found a composite crystalloid of two enzymes, alcohol oxidase and catalase, by analyzing using the optical diffraction and filtering technique and computer simulation. We established methods for preparing the protoplasts of Schizosaccharomyces pombe and conditions for the complete regeneration of the cell wall. The dynamic process of cell wall formation was clarified through our study of the protoplasts, using an improved ultra high resolution (UHR) FESEM S-900 and an S-900LV. It was found that β-1,3-glucan, β-1,6-glucan and α-1,3-glucan, as well as α-galactomannan, are ingredients of the cell wall. The process of septum formation during cell division was examined after cryo-fixation by high pressure freezing (HPF). It was also found that α-1,3- and β-1,3-glucans were located in the invaginating nascent septum, and later, highly branched β-1,6-glucan also appeared on the second septum. The micro-sampling method, using a focused ion beam (FIB), has been applied to our yeast cell wall research. A combination of FIB and scanning transmission electron microscopy is useful in constructing 3D images and analyzing the molecular architecture of cells, as well as for electron tomography of thick sections of biological specimens.

Quantitative Characterization of LiFePO4 Cathodes Reconstructed by FIB/SEM Tomography

The three-dimensional microstructures of a lab-scale and a high-power LiFePO4 cathode for lithium-ion cells are analyzed by combined focused ion beam (FIB) / scanning electron microscopy (SEM) tomography. The spatial distributions of (a) carbon black as electronic conductor (b) LiFePO4 as active material and (c) pore volume are reconstructed by appropriate image processing methods. The global threshold segmentation procedure is replaced by a refined local threshold method, which accounts for gradients in luminosity even within very large imaged volumes. The precise analysis of the high-power cathode demands for reconstructing a very large volume of 18.15 × 17.75 × 27.8 μm3, caused by the dual length-scale design of LiFePO4, carbon black and pore phase. The microstructure features, (a) electrochemically active surface area and particle size distribution of LiFePO4, (b) shape and particle size distribution of carbon black and (c) porosity and tortuosity of the pore phase are compared between lab-scale and high-power cathode.

FAST INTERACTIVE QUANTIFICATION OF SYNAPSES IN THE CEREBRAL CORTEX

Synapses are key elements in the organization of neuronal circuits. To analyse these structures, it is now possible to obtain serial images from large samples of nervous tissue in an automated manner through combined focused ion beam milling and scanning electron microscopy. However, the identification, 3D reconstruction and quantification of synapses within these samples are labor intensive procedures that require continuous user intervention. We have developed a software tool to achieve the segmentation of synapses in a reconstructed 3D volume of the cerebral cortex, thereby greatly facilitating and accelerating these processes. The tool is interactive, allowing the user to supervise the segmentation process, to modify the appropriate parameters and to validate the results. Some experimental results obtained with our tool in the rat cerebral cortex are also presented.

Characterization of nanoscale features in tapered fractal and photonic crystal fibers

The internal structure of nanostructured air-silica fiber probes have been characterized using a combined focused ion beam and scanning electron microscopy technique. The collapse rate of the air-holes is shown to differ substantially between a regular photonic crystal fiber (PCF) and the quasi-periodic Fractal fiber. The integrity of the Fractal fiber structure is maintained down to an outer diameter as small as 120 nm, whereas the air-holes of the regular PCF begin to collapse when the outer diameter is approximately 820 nm. The observed smallest hole diameter of 10 nm is suggested to be due to physical limits imposed by the molecular structure of silica. These results confirm structural inferences made in previous publications.

Three-dimensional reconstruction of a composite cathode for lithium-ion cells

The three-dimensional microstructure of a porous composite cathode for lithium-ion cells has been analyzed by a combined focused ion beam (FIB)/scanning electron microscopy (SEM) approach. The spatial distributions of 1) carbon black as electronic conductor, 2) LiFePO4 as active material and 3) pore volume were reconstructed by appropriate image processing methods. The representative volume element with a size of 5×5×15μm3=375μm3 (24million volumetric pixels) delivered critical microstructural features as volume fractions, volume-specific surface areas and tortuosity of the three individual phases.

A practical approach to test the scope of FIB-SEM 3D reconstruction

State-of-the-art focused ion beam (FIB) instruments have an ion column for sample modification and an electron column for scanning electron microscopy (SEM). 3D reconstruction of a sample volume can be achieved by serial sectioning using the FIB in combination with high-resolution SEM imaging of each cross-section. Usually, the resolution in the direction in which the sections are milled (z-direction) is much lower than in the plane of the cross-section (xy-direction) itself. Increased sampling in the z-direction can only be achieved by decreasing the distance between single sections. For a constant volume this is equivalent to increasing the number of sections, i.e. time and effort. To perform efficient 3D reconstructions the effect of the reduced sampling in the z-direction to the overall accuracy of the 3D reconstruction has to be known. We tested this approach with FIB conical test structures that were slice-and-view processed and subsequently reconstructed. Using a reference data set with a slice thickness (z-resolution) of 22 nm, data with z-resolutions ranging from 44 nm to 440 nm were created and reconstructed with commercial software. The calculated volumes for the simulated z-resolutions and their deviations from the reference volume are shown. Deviations of up to 35% occur and reach about 10% once the z-resolution was one fifth of the upper diameter of the conical structures.

The fabrication of silicon nanostructures by focused-ion-beam implantation and TMAH wetetching

Local gallium implantation of silicon by a focused ion beam (FIB) has been used to create a maskfor anisotropic tetramethylammonium hydroxide (TMAH) wet etching. The dependence of the etchstop properties of gallium-doped silicon on the implanted dose has been investigated and a dose of4 × 1013 ions cm − 2 has been determined to be the threshold value for achieving observable etchingresistance. Only a thin, approx. 50 nm, surface layer is found to be durableenough to serve as a mask with a high selectivity of at least 2000:1 betweenimplanted and non-implanted areas. The combined FIB–TMAH process has beenused to generate various types of 3D nanostructures including nanochannelsseparated by thin vertical sidewalls with aspect ratios up to 1:30, ultra-narrow(approx. 25 nm) freestanding bridges and cantilevers, and gratings with a resolution of20 lines µm − 1.

Low-Energy Focused Ion Beam Milling Provides Reduced Damage During TEM Sample Preparation

Abstract The combined focused ion beam (FIB) and scanning electron microscope (SEM), known as the DualBeam, is well-known for its unique ability to produce site-specific thin samples starting from bulk and then attaching the section to a transmission electron microscope (TEM) grid, all in-situ. It has been reported that producing a thin sample using a 30 kV gallium FIB creates surface damage several tens of nanometers deep. However, recent DualBeam technology improvements now enable the FIB to produce thin samples with a thickness well below 50 nanometers and deliver a tightly focused ion beam at an energy of 2 kV and below, which dramatically reduces the damage depth to as low as 1 to 2 nanometers in typical materials, such as silicon.

Fast three-dimensional nanoscale metrology in dual-beam FIB–SEM instrumentation

A quantitative surface reconstruction technique has been developed for the geometric characterization of three-dimensional structures by using a combined focused ion beam—scanning electron microscopy (FIB–SEM) instrument. A regular pattern of lines is milled at normal incidence on the sample to be characterized and an image is acquired at a large tilt angle. By analyzing the pattern under the tilted view, a quantitative estimation of surface heights is obtained. The technique has been applied to a test sample and nanoscale resolution has been achieved. The reported results are validated by a comparison with atomic force microscopy measurements.

Microstructure and Solidification Sequence of the Interdendritic Region in a Third Generation Single-Crystal Nickel-Base Superalloy

An analysis of the microstructure of the interdendritic regions in an as-cast third generation single-crystal nickel-base superalloy CMSX-10K has been carried out using automated serial sectioning with sequential focused ion beam (FIB) milling and image capture of the secondary electron image in a combined FIB–scanning electron microscope (SEM). Two interdendritic morphologies with vastly different γ/γ′ spacing were identified; these are (1) fine γ/γ′ morphology and (2) coarse cellular γ′ separated by narrow γ channels. The FIB results indicate that following primary solidification (L → γ), solidification of the interdendritic region commences in areas that show the fine γ/γ′ morphology, (1) on the γ dendrite lobe. This is followed by a gradual transition to growth of the coarse cellular γ/γ′ channel structure, (2) which occur up to the termination of the solidification process.

In-situ focused ion beam (FIB) microscopy at high temperature

Materials Scientists need information on the kinetics of the microstructural evolution processes that determine the finished microstructure, and hence the properties, of any material. E.g. recrystallisation, grain growth and phase changes. Such kinetic information requires reliable discrimination of differently oriented crystallites and/or different crystal phases coupled with useful spatial resolution and temporal resolution (i.e. high frame rates). These imaging results must be realised from a hot and changing specimen, in an instrument that is compatible with that hot specimen and with a practical specimen heater. Focused Ion Beams (FIB) offer strong contrast between crystallites and phases, and hence offer the ability to discriminate between these features even while imaging at fast frame rates, however their compatibility with hot specimens was unproven. Here we report results from a novel combination of FIB with an in-situ heating stage, to produce in-situ, real-time microstructural imaging from a variety of metallic specimens at temperatures up to about 700°C. FIB has additional capabilities, such as site-specific milling, which we show remains practical with hot specimens and which can be very useful with certain materials.

Focused Ion Beam (FIB) combined with SEM and TEM: Advanced analytical tools for studies of chemical composition, microstructure and crystal structure in geomaterials on a nanometre scale

In order to make meaningful interpretations of trace element and isotopic information from geomaterials, prior characterization of the material of interest with respect to structural and chemical homogeneity, radiation damage, and microstructure is often required. Transmission electron microscopy (TEM) is a powerful analytical tool that may be used to fully characterise a wide range of synthetic and natural materials. The focused ion beam technique (FIB) is an ideal tool for TEM sample preparation that allows for the fabrication of electron-transparent foils with typical dimensions of 15 × 10 × 0.150 µm from any region of interest (i.e., site-specific) and in virtually any material. FIB sample preparation consumes only a small volume of material (approximately 2300 µm 3 for a standard foil with the dimensions 15 × 10 × 0.150 µm), leaving most of the material essentially unaffected by the sampling process. Modern TEM facilitates the measurement of chemical composition with unsurpassed spatial resolution (e.g., nanometer scale). High-resolution imaging and electron diffraction provide important complimentary structural information. It is the combination of crystal structure information and chemical composition that allows for the complete characterization of the volume of interest. High-resolution elemental mapping, or line scans, may be used to acquire complete energy dispersive X-ray analysis (EDX) spectra of a material pixel by pixel. Subsequent evaluation of the data allows for the correlation of the EDX spectrum with the respective elemental map or line scan with an image, thus illustrating spatially resolved chemical composition variations at the 0.5–1.0 wt.% level. Frequently, minerals enclose sub micrometer- or even nanometre-sized inclusions of minerals, fluids, or silicate melt, which are of great interest to geoscientists. Nanoinclusions (e.g., in diamond or olivine) are of particular interest because they may have preserved their original crystal structure thus providing a geobarometer recording the pressure conditions at the time of formation of the inclusion. FIB/TEM are appropriate techniques to sample and subsequently define the chemical composition and the structural state of mineral inclusion on the nanometre scale. The combination of FIB with an SEM allows for 3D information to be obtained from samples including: 3D imaging (e.g., phase distribution and volume of the individual phases in symplectites); 3D distribution of elements in a specific volume (3D elemental maps); and three dimensional texture analysis using electron backscatter diffraction (EBSD).

Three-Dimensional Orientation Microscopy in a Focused Ion Beam–Scanning Electron Microscope: A New Dimension of Microstructure Characterization

In the present work, we report our recent progress in the development, optimization, and application of a technique for the three-dimensional (3-D) high-resolution characterization of crystalline microstructures. The technique is based on automated serial sectioning using a focused ion beam (FIB) and characterization of the sections by orientation microscopy based on electron backscatter diffraction (EBSD) in a combined FIB–scanning electron microscope (SEM). On our system, consisting of a Zeiss–Crossbeam FIB-SEM and an EDAX-TSL EBSD system, the technique currently reaches a spatial resolution of 100 · 100 · 100 nm 3 as a standard, but a resolution of 50 · 50 · 50 nm 3 seems to be a realistic optimum. The maximum observable volume is on the order of 50 · 50 · 50 lm 3 . The technique extends all the powerful features of two-dimensional (2-D) EBSD-based orientation microscopy into the third dimension of space. This allows new parameters of the microstructure to be obtained—for example, the full crystallographic characterization of all kinds of interfaces, including the morphology and the crystallographic indices of the interface planes. The technique is illustrated by four examples, including the characterization of pearlite colonies in a carbon steel, of twins in pseudonanocrystalline NiCo thin films, the description of deformation patterns formed under nanoindents in copper single crystals, and the characterization of fatigue cracks in an aluminum alloy. In view of these examples, we discuss the possibilities and limits of the technique. Furthermore, we give an extensive overview of parallel developments of 3-D orientation microscopy (with a focus on the EBSD-based techniques) in other groups.

Focused ion beam implantation induced site-selective growth of InAs quantum dots

The site-selective growth of InAs quantum dots (QDs) by a combined focused ion beam (FIB) and molecular beam epitaxy (MBE) process has been demonstrated. An array of FIB modified spots on MBE grown GaAs was fabricated. Thereafter, an in situ annealing step followed by InAs deposition was performed. The InAs QDs were preferentially formed in the holes generated by the FIB. The influences of ion dose, annealing parameters, and InAs amount were investigated. With optimized parameters, the authors observe more than 50% single dot occupancy per holes. Photoluminescence spectra confirm the good optical quality of the QDs.

Characterization of mineral surfaces using FIB and TEM: A case study of naturally weathered alkali feldspars

Using a focused ion beam (FIB) instrument, electron-transparent samples (termed foils) have been cut from the naturally weathered surfaces of perthitic alkali feldspars recovered from soils overlying the Shap granite, northwest England. Characterization of these foils by transmission electron microscopy (TEM) has enabled determination of the crystallinity and chemical composition of near-surface regions of the feldspar and an assessment of the influence of intragranular microtextures on the microtopography of grain surfaces and development of etch pits. Damage accompanying implantation of the 30 kV Ga+ ions used for imaging and deposition of protective platinum prior to ion milling creates amorphous beneath outer grain surfaces, but can be overcome by coating grains with > 85 nm of gold before FIB work. The sidewalls of the foil and feldspar surrounding original voids are also partially amorphized during later stages of ion milling. No evidence was found for the presence of amorphous or crystalline weathering products or amorphous layers immediately beneath outer grain surfaces. The absence of a leached layer indicates that chemical weathering of feldspar in the Shap soils is stoichiometric, or if non-stoichiometric, either the layer is too thin to resolve by the TEM techniques used (i.e., <=similar to 2.5 nm) or an insufficient proportion of ions have been leached from near-surface regions so that feldspar crystallinity is maintained. No evidence was found for any difference in the mechanisms of weathering where a microbial filament rests on the feldspar surface. Sub-micrometer-sized steps on the grain surface have formed where subgrains and exsolution lamellae have influenced the propagation of fractures during physical weathering, whereas finer scale corrugations form due to compositional or strain-related differences in dissolution rates of albite platelets and enclosing tweed orthoclase. With progressive weathering, etch pits that initiated at the grain surface extend into grain interiors as etch tubes by exploiting preexisting networks of nanopores that formed during the igneous history of the grain. The combination of FIB and TEM techniques is an especially powerful way of exploring mechanisms of weathering within the internal zone beneath outer grain surfaces, but results must be interpreted with caution owing to the ease with which artifacts can be created by the high-energy ion and electron beams used in the preparation and characterization of the foils.

FIB/DualBeam Techniques for Quantitative Analyses

Lucille A. Giannuzzi FEI Company, 5350 NE Dawson Creek Drive, Hillsboro, OR 97124 The use of Focused ion beam (FIB) and combined FIB plus scanning electron microscope (SEM) dual platform instruments have continued to increase in popularity over the past decade in both the physical and biological sciences. FIB based techniques enable site specific regions to be prepared for direct quantitative analysis and/or extracted for quantitative analyses using other analytical instruments in material systems that were once too difficult or unfeasible to observe by other means. In addition, since FIB techniques in general offer higher throughput than conventional methods, more samples may be quantified than previously possible. In this paper, quantitative techniques facilitated by either direct FIB imaging or FIB-based specimen preparation methods are discussed. One of the most popular uses of focused ion beams is for transmission electron microscopy (TEM) specimen preparation. The ubiquitous FIB lift-out techniques allow for site-specific extraction of interfaces, inclusions, failure sites, etc., from a wide range of materials [1]. Therefore, quantitative x-ray energy dispersive spectrometry (XEDS) techniques can be directly applied to e.g., interdiffusion studies [2], or grain boundary segregation effects on grain boundary diffusion [3]. A particular advantage to FIB-based imaging is the channeling contrast generated by ion induced secondary electron imaging which is unsurpassed compared to conventional SEM imaging. Thus, FIB imagining can provide direct capabilities for quantification of microstructural features. This ability to use FIB channeling contrast imaging on a mechanically polished Cu-Ni diffusion couple revealed regions where diffusion induced recrystallization (DIR) occurred at the diffusion couple interface [2]. Site specific TEM specimen preparation of DIR and non-DIR regions allowed for direct quantitative analysis of the Cu-Ni diffusion couple showing an enhancement of lattice interdiffusion due to grain boundary contributions [2]. FIB channeling across grain boundaries was also used to prepare TEM specimens for the quantification of the effects of Bi segregation on the Ni diffusion through Cu twist boundaries [3]. In addition, dopant profiles of p/n junctions within Si-based transistor gates can be quantified using electron holography methods with the utilization of back-side FIB milling plus low energy polishing steps [4]. The basic FIB lift-out techniques have also been employed to prepare samples for quantification in surface science instruments such as x-ray photoelectron spectroscopy [5], Auger electron spectroscopy [6], secondary ion mass spectroscopy (SIMS) [6], as well as 3D atom probe [7,8]. The same techniques used to prepare atom probe specimens can be used to make pillar-shaped TEM specimens for on-axis TEM tomography [9,10].