Electron-transparent Regions Research Articles

Using Xe Plasma FIB for High-Quality TEM Sample Preparation.

A direct comparison between electron transparent transmission electron microscope (TEM) samples prepared with gallium (Ga) and xenon (Xe) focused ion beams (FIBs) is performed to determine if equivalent quality samples can be prepared with both ion species. We prepared samples using Ga FIB and Xe plasma focused ion beam (PFIB) while altering a variety of different deposition and milling parameters. The samples’ final thicknesses were evaluated using STEM-EELS t/λ data. Using the Ga FIB sample as a standard, we compared the Xe PFIB samples to the standard and to each other. We show that although the Xe PFIB sample preparation technique is quite different from the Ga FIB technique, it is possible to produce high-quality, large area TEM samples with Xe PFIB. We also describe best practices for a Xe PFIB TEM sample preparation workflow to enable consistent success for any thoughtful FIB operator. For Xe PFIB, we show that a decision must be made between the ultimate sample thickness and the size of the electron transparent region.

An optimized TEM specimen preparation method of quantum nanostructures

Electron transparent TEM lamella with unaltered microstructure and chemistry is the prerequisite for successful TEM explorations. Currently, TEM specimen preparation of quantum nanostructures, such as quantum dots (QDs), remains a challenge. In this work, we optimize the sample-preparation routine for achieving high-quality TEM specimens consisting of SrRuO3 (SRO) QDs grown on SrTiO3 (STO) substrates. We demonstrate that a combination of ion-beam-milling techniques can produce higher-quality specimens of quantum nanostructures compared to TEM specimens prepared by a combination of tripod polishing followed by Ar+ ion milling. In the proposed method, simultaneous imaging in a focused ion-beam device enables accurate positioning of the QD regions and assures the presence of dots in the thin lamella by cutting the sample inclined by 5° relative to the dots array. Furthermore, the preparation of TEM lamellae with several large electron-transparent regions that are separated by thicker walls effectively reduces the bending of the specimen and offers broad thin areas. The final use of a NanoMill efficiently removes the amorphous layer without introducing any additional damage.

Blisters formed by D plasma exposure in an electron-transparent tungsten sample

By irradiating a tungsten sample for transmission electron microscopy with deuterium plasma at 230 K, we created a high density of blisters below the plasma-exposed surface. Blisters were even found in the electron-transparent region close to the central perforation. We determined the abundance of blisters depending on the local sample thickness and found that no blisters were formed for thicknesses below about 45 nm for these specific plasma exposure conditions. After removing the approximately 10 nm thick, defect-rich layer at the plasma-exposed surface, which corresponds roughly to the ion implantation zone, by electrochemical polishing, we were able to clearly identify material distortions and dislocation network halos corresponding to blisters by scanning transmission electron microscopy. Compared with unirradiated tungsten of the same grade, we found that in the blister zone the average dislocation density is about 2 orders of magnitude higher, which may, in addition to gas enclosed in blister cavities, explain enhanced deuterium retention in blistered tungsten samples. The proof-of-principle experiments described in this article have the potential to provide constraints for theoretical models for blister nucleation. They also pave the way for direct investigations of the spatial correlation of deuterium-plasma-induced blisters and intrinsic defects.

Focused Ion Beam Preparation of Specimens for Micro-Electro-Mechanical System-based Transmission Electron Microscopy Heating Experiments.

Micro-electro-mechanical systems (MEMS)-based heating holders offer exceptional control of temperature and heating/cooling rates for transmission electron microscopy experiments. The use of such devices is relatively straightforward for nano-particulate samples, but the preparation of specimens from bulk samples by focused ion beam (FIB) milling presents significant challenges. These include: poor mechanical integrity and site selectivity of the specimen, ion beam damage to the specimen and/or MEMS device during thinning, and difficulties in transferring the specimen onto the MEMS device. Here, we describe a novel FIB protocol for the preparation and transfer of specimens from bulk samples, which involves a specimen geometry that provides mechanical support to the electron-transparent region, while maximizing the area of that region and the contact area with the heater plate on the MEMS chip. The method utilizes an inclined stage block that minimizes exposure of the chip to the ion beam during milling. This block also allows for accurate and gentle placement of the FIB-cut specimen onto the chip by using simultaneous electron and ion beam imaging during transfer. Preliminary data from Si and Ag on Si samples are presented to demonstrate the quality of the specimens that can be obtained and their stability during in situ heating experiments.

How do specimen preparation and crystal perfection affect structure factor measurements by quantitative convergent-beam electron diffraction?

The effectiveness of tripod polishing and crushing as methods of mechanically preparing transmission electron microscopy specimens of hard brittle inorganic crystalline materials is investigatedviathe example of cerium hexaboride (CeB6). It is shown that tripod polishing produces very large electron-transparent regions of very high crystal perfection compared to the more rapid technique of crushing, which produces crystallites with a high density of imperfections and significant mosaicity in the case studied here where the main crystallite facets are not along the natural {001} cleavage planes of CeB6. The role of specimen quality in limiting the accuracy of structure factor measurements by quantitative convergent-beam electron diffraction (QCBED) is investigated. It is found that the bonding component of structure factors refined from CBED patterns obtained from crushed and tripod-polished specimens varies very significantly. It is shown that tripod-polished specimens yield CBED patterns of much greater integrity than crushed specimens and that the mismatch error that remains in QCBED pattern matching of data from tripod-polished specimens is essentially nonsystematic in nature. This stands in contrast to QCBED using crushed specimens and lends much greater confidence to the accuracy and precision of bonding measurements by QCBED from tripod-polished specimens.

A transmission Kikuchi diffraction study of cementite in a quenched and tempered steel

This is the first transmission Kikuchi diffraction (TKD) study to report the indexing of nano-sized cementite as distinct structures and its orientation relationship with the body-centered cubic matrix in a quenched and tempered steel. Crystallographic analysis via TKD and selected area diffraction returned the well-known Bagaryatskii and Isaichev orientation relationships. However, the indexing of nano-sized cementite via TKD was sensitive to the thickness of the electron transparent region such that TEM remains the most precise method to characterise such precipitates.

Preparation of micro-foils for TEM/STEM analysis from metallic powders

A technique has been developed which facilitates the preparation of electro-polished micro-foil transmission electron microscopy (TEM) specimens, which have previously been machined out of ≈100μm diameter metallic powder particles using a Focussed Ion Beam (FIB) instrument. The technique can be used to create small volume TEM specimens from most metallic powder particles and bulk metal samples. This is especially useful when the matrices are ferritic steels, which are often difficult to image in the electron microscope, since the necessary aberration corrections change as the sample is tilted in the magnetic field of the objective lens.Small samples, such as powder particles, were attached to gold support grids using deposited platinum and were then ion milled to approximately 2μm thickness in a focussed ion beam (FIB) instrument. Subsequently, the specimen assemblies were electropolished for short durations under standard conditions, to produce large (5μm×5μm) electron transparent regions of material. The specimens produced by this technique were free from FIB related artefacts and facilitated atomic resolution scanning-TEM (STEM) imaging of ferritic and nickel matrices containing, for example, yttrium rich oxide nano-dispersoids.

Scanning and transmission electron microscopy investigations of defect arrangements in a two-phase γ-TiAl alloy

Different methods of scanning and transmission electron microscopy (SEM, TEM) were applied on a γ-TiAl alloy TNB-V5 after a thermo-mechanical fatigue test. Electron channelling contrast imaging (ECCI) and electron backscattered diffraction were carried out on bulk specimen. In addition, ECCI and scanning transmission electron microscopy in the SEM were carried out on a TEM foil in the electron opaque and the electron transparent region, respectively. The investigations were completed by transmission electron microscopy in the form of standard bright field imaging as well as by taking corresponding diffraction patterns. The results demonstrate in an impressive way that the ECCI technique applied in scanning electron microscopy can successfully supplement or in some cases replace imaging of dislocation arrangements in TEM.

Preparation of high‐quality ultrathin transmission electron microscopy specimens of a nanocrystalline metallic powder

This article explores the achievable transmission electron microscopy specimen thickness and quality by using three different preparation methods in the case of a high-strength nanocrystalline Cu-Nb powder alloy. Low specimen thickness is essential for spatially resolved analyses of the grains in nanocrystalline materials. We have found that single-sided as well as double-sided low-angle Ar ion milling of the Cu-Nb powders embedded into epoxy resin produced wedge-shaped particles of very low thickness (<10 nm) near the edge. By means of a modified focused ion beam lift-out technique generating holes in the lamella interior large micrometer-sized electron-transparent regions were obtained. However, this lamella displayed a higher thickness at the rim of ≥30 nm. Limiting factors for the observed thicknesses are discussed including ion damage depths, backscattering, and surface roughness, which depend on ion type, energy, current density, and specimen motion. Finally, sections cut by ultramicrotomy at low stroke rate and low set thickness offered vast, several tens of square micrometers uniformly thin regions of ∼10-nm minimum thickness. As major drawbacks, we have detected a thin coating on the sections consisting of epoxy deployed as the embedding material and considerable nanoscale thickness variations.

Class I and Class II Chitin Synthases Are Involved in Septum Formation in the Filamentous Fungus Aspergillus nidulans

The class II and class I chitin synthases of the filamentous fungus Aspergillus nidulans are encoded by chsA and chsC, respectively. Previously, we presented several lines of evidence suggesting that ChsA and ChsC have overlapping functions in maintaining cell wall integrity. In order to determine the functions of these chitin synthases, we employed electron and fluorescence microscopy and investigated in detail the cell wall of a DeltachsA DeltachsC double mutant (DeltaAC mutant) along with the localization of ChsA and ChsC. In the lateral cell wall of the DeltaAC mutant, electron-transparent regions were thickened. Septa of the DeltaAC mutant were aberrantly thick and had a large pore. Some septa were located abnormally close to adjacent septa. A functional hemagglutinin (HA)-tagged ChsA (HA-ChsA) and a functional FLAG-tagged ChsC (FLAG-ChsC) were each localized to a subset of septation sites. Comparison with the localization pattern of actin, which is known to localize at forming septa, suggested that ChsA and ChsC transiently exist at the septation sites during and shortly after septum formation. Double staining of HA-ChsA and FLAG-ChsC indicated that their localizations were not identical but partly overlapped at the septation sites. Fluorescence of FLAG-ChsC, but not of HA-ChsA, was also observed at hyphal tips. These data indicate that ChsA and ChsC share overlapping roles in septum formation.

Electrolytic preparation of metallic thin foils with large electron-transparent regions

This paper presents a successful technique for the preparation of thin foils for examination in the transmission electron microscope with large regions of electron-transparent material. The technique is also demonstrated to achieve similar large, thin areas from the cross-section of very thin materials down to a sheet thickness of approximately 180 μm.

Unraveling the Organization of the Internal Nuclear Matrix: RNA-Dependent Anchoring of NuMA to a Lamin Scaffold

Using quantitative immunoelectron microscopy we show here that when the nuclear matrix is isolated from rat hepatocytes in the presence of an inhibitor of RNase activity both lamins and the nuclear mitotic apparatus protein (NuMA) preferentially localize within the electron-dense domains of the internal nuclear matrix (INM). After RNA digestion NuMA undergoes a sharp depletion, while labeling by an antibody against lamins A and C within the electron-transparent regions increases, suggesting that a subset of lamin epitopes is masked by the interaction with RNA. We were able to explain this result by visualizing for the first time a thin web of lamin protofibrils which connects the electron-dense regions. Confirmation of these changes has been obtained by immunoblot analysis and confocal microscopy. As RNA digestion results both in the release of NuMA and in the collapse of the INM, we propose that a fraction of nuclear RNA brings about the association of NuMA islands with a lamin scaffold and that this interaction is required to maintain the latter in a state of high molecular dispersion.

Preparation of damage-free glass TEM specimens

A jet polishing technique to chemically thin glass specimens for transmission electron microscopy (TEM) after a preliminary mechanical dimpling step has been developed. Slightly modified commercial equipment is used with automatic optical termination of the polishing process to produce foils exhibiting large, high quality, electron transparent regions.

Identification of weak interfaces in composites using transmission electron microscopy

A new experimental technique was developed to identify crack paths with a resolution of nanometres in fibre-reinforced composites. Cracks were introduced through Vickers indentations on one side of the sample prior to starting the thinning process. Indentations were placed close to the fibres in order to get enough cracks at the fibre/matrix interface in the electron-transparent region of the thinned sample. The technique was used in a Nicalon-fibre Al2O3 matrix composite prior to and after a heat treatment at 1200 degrees C for 1 h. The analysis of the crack paths allowed the identification of the weakest interface in each condition.

On the nature of extended defects in CVD diamond and the origin of compressive stresses

Abstract Diffraction contrast transmission electron microscopy has been used in a detailed investigation of the nature of defects in thick square-faceted CVD diamond films. It is concluded that each crystallite progresses from octahedral growth with {111} facets to truncated octahedra before reaching the final form with large square facets. Open-faced tetrahedral defects are formed on the {111} faces of the octahedra with faults or very thin twins on the three {111} planes inclined to each facet growth surface. By micro-Raman experiments on individual grains of electron transparent regions and electron-energy-loss spectroscopy, it was shown that the defective regions are associated with the incorporation of non-diamond carbon and a model is proposed, based on this information, to account for the very high compressive stresses that have been found in these diamond films.

High Resolution FIB as a General Materials Science Tool

Abstract Focused Ion Beam or FIB systems have been used in integrated circuit production for some time. The ability to combine rapid, precision focused ion beam sputtering or gas-assisted ion etching with focused ion beam deposition allows for rapid-prototyping of circuit modifications and failure analysis of defects even if they are buried deep within the chip's architecture. Inevitably, creative TEM researchers reasoned that a FIB could be used to produce site specific parallel-sided, electron transparent regions, thus bringing about the rather unique situation wherein the specimen preparation device often was worth as much as the TEM itself. More recently, FIB manufacturers have concentrated on improving the resolution and imaging characteristics of these instruments, resulting in a more general-purpose characterization tool. The Micrion 2500 FIB system used in this study is capable of 4 nm imaging resolution using either secondary electron or secondary ions, both generated by a 50 kV liquid metal gallium ion source.

Focused Ion Beam Sample Preparation of Non-Semiconductor Materials

AbstractFocused Ion Beam (FIB) systems have been steadily gaining acceptance as specimen preparation tools in the semiconductor industry. This is largely due to the fact that such instruments are relatively commonplace as failure analysis tools in semiconductor houses, and are commonly used in the preparation of cross-sections for imaging under the ion beam or using an electron beam in an SEM. Additionally, the ease with which cross-sectional TEM specimens of semiconductor devices can be prepared using FIB systems has been well demonstrated. However, this technology is largely unknown outside the semiconductor industry. Relatively few references exist in the literature on the preparation of cross-sectional TEM specimens of non-semiconductor materials by FIB. This paper discusses a specific use of FIB technology in the preparation of cross-sectional TEM specimens of non-semiconductor samples that are difficult to prepare by conventional means. One example of such materials is commercial galvannealed steel sheet that is used to form corrosion resistant auto-bodies for the automobile industry. Cross-sectional TEM specimens of this material have proved difficult and time-intensive to prepare by standard polishing and ion milling techniques due to galvanneal's inherent flaking and powdering difficulties, as well as the different sputtering rates of the various Fe-Zn intermetallic phases present in the galvannealed coatings. TEM results from cross-sectional samples of commercial galvannealed steel coatings prepared by conventional ion milling and FIB techniques are compared to assess image quality, the size of the electron-transparent thin regions that can be readily prepared and the quality of samples produced by both techniques. Specimen preparation times for both techniques are reported.

The surface topography of an ion milled WC-Co sample

A tungsten carbide-cobalt (WC-30 wt% Co) thin foil was prepared for transmission electron microscopy (TEM) by electron-spark cutting a thin slice out of bulk material, polishing mechanically both sides of the slice on diamond powder (down to 0.25/~m particle size) and ion milling both sides in a Gatan 600 Dual Ion Mill. The milling process consisted of bombarding simultaneously both sides of the sample with Ar ion beams of 5 keV, impinging on the surfaces at a nominal angle of 15 ° to the sample surface while the sample rotated at approximately 2 rpm. The sample obtained was a 3.2 mm disc with an outer rim approximately 100 ktm thick and a central hole surrounded by a narrow electron-transparent region. Two microcracks radiated from the hole, relieving the residual tensile stresses in the cobalt [11. The sample was held in the ion mill between two plates each with a central hole of 2.3 mm diameter. The surface of the sample which was not covered by these plates was eroded by the Ar beams. The intensity of the beams may be assumed to have had a Ganssian distribution around a central axis. The centre of the beam impinged on the sample at the centre of the resulting hole and its intensity decreased with increasing distance from the hole. Therefore, by examining the topography of the ion milled surfaces at various distances from the hole, i.e. in areas that received various doses of Ar ions, it was possible to acquire information on how the resulting topography developed. The surface topography was examined by atomic force microscopy (AFM) and scanning electron microscopy (SEM). Random areas at various distances from the outer rim of the sample were analysed by both techniques. A Jeol JSM 840 operating at 20 kV was utilized for the SEM studies and an AFM using a PSI autoprobe. The autoprobe was used to produce topographs of the sample, to measure the dimensions of surface features and to measure the average roughness of the sample surface. The Si3N4 AFM tip had a square pyramidal geometry with an apex nominal radius of 40 nm. Fig. 1 is a three-dimensional AFM micrograph of

Ultramicrotomy of nanocrystalline materials.

The methodology and one of the first attempts to produce transmission electron microscopic (TEM) specimens of nanocrystalline metals, alloys and ceramics by ultramicrotomy are presented. Samples of the pure elements Co, Pd; alloys of Y-12 at. % Fe, Al-7 at. % Ag and W-30 at. % Ga; and ZnO ceramic, were found to section successfully to varying degrees. Advantages of sections prepared through ultramicrotomy over ion beam methods include extensive electron-transparent regions of uniform thickness and absence of ion beam damage. Typical artefacts were observed (knife marks, tearing, pull-out, shear lamellae, section curling, and anodic dissolution) but did not impede TEM analysis significantly. A potentially important effect observed was that of a texture development upon sectioning of the Co and Pd samples. It is thought that this unusual phenomenon results from the extremely fine scale of the microstructure and the purity of the Co and Pd samples, and may be enhanced by frictional heating effects and the state of the knife edge.

Preparation of Germanium TEM Specimens by Jet Electropolishing

An improved, jet electropolishing technique for the production of germanium TEM specimens has been developed. The use of slightly modified, commercially available equipment permits rapid, reliable thinning of foils exhibiting large electron transparent regions. The use of automatic termination of polishing produces cleaner foil surfaces than incremental final polishing did previously.