Comparing Xe+ pFIB and Ga+ FIB for TEM sample preparation of Al alloys: Minimising FIB-induced artefacts

Ultrastructure of human brain tissue vitrified from autopsy revealed by cryo-ET with cryo-plasma FIB milling

Ultrastructure of human brain tissue has traditionally been examined using electron microscopy (EM) following fixation, staining, and sectioning, which limit resolution and introduce artifacts. Alternatively, cryo-electron tomography (cryo-ET) allows higher resolution imaging of unfixed cellular samples while preserving architecture, but it requires samples to be vitreous and thin enough for transmission EM. Due to these requirements, cryo-ET has yet to be employed to investigate unfixed, never previously frozen human brain tissue. Here we present a method for generating lamellae in human brain tissue obtained at time of autopsy that can be imaged via cryo-ET. We vitrify the tissue via plunge-freezing and use xenon plasma focused ion beam (FIB) milling to generate lamellae directly on-grid at variable depth inside the tissue. Lamellae generated in Alzheimer's disease brain tissue reveal intact subcellular structures including components of autophagy and potential pathologic tau fibrils. Furthermore, we reveal intact compact myelin and functional cytoplasmic expansions. These images indicate that plasma FIB milling with cryo-ET may be used to elucidate nanoscale structures within the human brain.

Local Structural Modifications in Metallic Micropillars Induced by Plasma Focused Ion Beam Processing

A focused ion beam scanning electron microscope (FIB-SEM) is a powerful tool that is routinely used for scale imaging from the micro- to nanometer scales, micromachining, prototyping, and metrology. In spite of the significant capabilities of a FIB-SEM, there are inherent artefacts (e.g., structural defects, chemical interactions and phase changes, ion implantation, and material redeposition) that are produced due to the interaction of Ga+ or other types of ions (e.g., Xe+, Ar+, O+, etc.) with the sample. In this study, we analyzed lattice distortion and ion implantation and subsequent material redeposition in metallic micropillars which were prepared using plasma focus ion beam (PFIB) milling. We utilized non-destructive synchrotron techniques such as X-ray fluorescence (XRF) and X-ray nanodiffraction to examine the micropillars prepared using Xe+ ion energies of 10 keV and 30 keV. Our results demonstrate that higher Xe ion energy leads to higher density of implanted ions within the redeposited and milled material. The mixing of ions in the redeposited material significantly influences the lattice structure, causing deformation in regions with higher ion concentrations. Through an X-ray nanodiffraction analysis, we obtained numerical measurements of the strain fields induced in the regions, which revealed up to 0.2% lattice distortion in the ion bombardment direction.

Toward the development of plasticity theories for application to small-scale metal structures

Experiments are performed on micron-scale single-crystal prototypical structural elements experiencing combined torsion and bending to gather data on their load-carrying capacity in the range of size and strain relevant to micron-scale structures for which little data are available. The observed strengthening dependence on size for the structural elements is in general accord with trends inferred from prior tests such as indentation and pure torsion. In addition, the experiments systematically reveal the strengthening size-dependence of structural elements whose surface has been passivated by a very thin Cr coating, an effect shown to have substantial strengthening potential. A state-of-the-art strain gradient plasticity theory is used to analyze the structural elements over the entire range of size and loading. While the computed trends replicate the experimental trends with reasonable fidelity, the predictive exercise, which is representative of those that will be required in micron-scale structural analysis, brings to light constitutive and computational issues which will have to be addressed before micron-scale plasticity theory can serve as effectively at the micron scale as conventional plasticity does at larger scales.

Fabrication of Specimens for Atom Probe Tomography Using a Combined Gallium and Neon Focused Ion Beam Milling Approach

We demonstrate a new focused ion beam sample preparation method for atom probe tomography. The key aspect of the new method is that we use a neon ion beam for the final tip-shaping after conventional annulus milling using gallium ions. This dual-ion approach combines the benefits of the faster milling capability of the higher current gallium ion beam with the chemically inert and higher precision milling capability of the noble gas neon ion beam. Using a titanium-aluminum alloy and a layered aluminum/aluminum-oxide tunnel junction sample as test cases, we show that atom probe tips prepared using the combined gallium and neon ion approach are free from the gallium contamination that typically frustrates composition analysis of these materials due to implantation, diffusion, and embrittlement effects. We propose that by using a focused ion beam from a noble gas species, such as the neon ions demonstrated here, atom probe tomography can be more reliably performed on a larger range of materials than is currently possible using conventional techniques.

Native ultrastructure of fresh human brain vitrified directly from autopsy revealed by cryo-electron tomography with cryo-plasma focused ion beam milling

Ultrastructure of human brain tissue has traditionally been examined using electron microscopy (EM) following chemical fixation, staining, and mechanical sectioning, which limit attainable resolution and introduce artifacts. Alternatively, cryo-electron tomography (cryo-ET) offers the potential to image unfixed cellular samples at higher resolution while preserving their native structures, but it requires samples to be frozen free from crystalline ice and thin enough to image via transmission EM. Due to these requirements, cryo-ET has yet to be employed to investigate the native ultrastructure of unfixed, never previously frozen human brain tissue. Here we present a method for generating lamellae in human brain tissue obtained at time of autopsy that can be imaged via cryo-ET. We vitrify the tissue directly on cryo-EM grids via plunge-freezing, as opposed to high pressure freezing which is generally used for thick samples. Following vitrification, we use xenon plasma focused ion beam (FIB) milling to generate lamellae directly on-grid. In comparison to gallium FIB, which is commonly used for biological samples, xenon plasma FIB is powerful enough to efficiently mill large volume samples, such as human brain tissue. Additionally, our approach allows for lamellae to be generated at variable depth inside the tissue as opposed to being limited to starting at the surface of the tissue. Lamellae generated in Alzheimer's disease brain tissue and imaged by cryo-ET reveal intact subcellular structures including components of autophagy and potential tau fibrils. Furthermore, we visualize myelin revealing intact compact myelin and functional cytoplasmic expansions such as cytoplasmic channels and the inner tongue. From these images we also measure the dimensions of myelin membranes, providing insight into how myelin basic protein forces out oligodendrocyte cytoplasm to form compact myelin and tightly links intracellular polar head groups of the oligodendrocyte plasma membrane. This approach provides a first view of unfixed, never previously frozen human brain tissue prepared by cryo-plasma FIB milling and imaged at high resolution by cryo-ET.

The 3D Controllable Fabrication of Nanomaterials with FIB-SEM Synchronization Technology

Nanomaterials with unique structures and functions have been widely used in the fields of microelectronics, biology, medicine, and aerospace, etc. With advantages of high resolution and multi functions (e.g., milling, deposition, and implantation), focused ion beam (FIB) technology has been widely developed due to urgent demands for the 3D fabrication of nanomaterials in recent years. In this paper, FIB technology is illustrated in detail, including ion optical systems, operating modes, and combining equipment with other systems. Together with the in situ and real-time monitoring of scanning electron microscopy (SEM) imaging, a FIB-SEM synchronization system achieved 3D controllable fabrication from conductive to semiconductive and insulative nanomaterials. The controllable FIB-SEM processing of conductive nanomaterials with a high precision is studied, especially for the FIB-induced deposition (FIBID) 3D nano-patterning and nano-origami. As for semiconductive nanomaterials, the realization of high resolution and controllability is focused on nano-origami and 3D milling with a high aspect ratio. The parameters of FIB-SEM and its working modes are analyzed and optimized to achieve the high aspect ratio fabrication and 3D reconstruction of insulative nanomaterials. Furthermore, the current challenges and future outlooks are prospected for the 3D controllable processing of flexible insulative materials with high resolution.

Atomistic structures of 〈0001〉 tilt grain boundaries in a textured Mg thin film

Nanocrystalline Mg was sputter deposited onto an Ar ion etched Si {100} substrate. Despite an ∼6 nm amorphous layer found at the interface, the Mg thin film exhibits a sharp basal-plane texture enabled by surface energy minimization. The columnar grains have abundant 〈0001〉 tilt grain boundaries in between, most of which are symmetric with various misorientation angles. Up to ∼20° tilt angle, they are composed of arrays of equally-spaced edge dislocations. Ga atoms were introduced from focused ion beam milling and found to segregate at grain boundaries and preferentially decorate the dislocation cores. Most symmetric grain boundaries are type-1, whose boundary planes have smaller dihedral angles with {21̄1̄0} rather than {101̄0}. Atomistic simulations further demonstrate that type-2 grain boundaries, having boundary planes at smaller dihedral angles with {101̄0}, are composed of denser dislocation arrays and hence have higher formation energy than their type-1 counterparts. The finding correlates well with the dominance of type-1 grain boundaries observed in the Mg thin film.

On the fabrication of atom probe tomography specimens of Al alloys at room temperature using focused ion beam milling with liquid Ga ion source

In this work, a simple rectangular milling technique was demonstrated to prepare needle shape atom probe tomography (APT) specimens from Al alloys by focused-ion-beam (FIB) milling using Ga⁺ ions at room temperature. Ga has high miscibility in Al owing to which electropolishing technique is preferred over Ga⁺ ion FIB instruments for the fabrication of APT specimens. Although, site specific sample preparation is not possible by the electropolishing technique. This led to the motivation to demonstrate a new rectangular milling technique using Ga⁺ FIB instrument that resulted a significant reduction of Ga⁺ ion impregnation into the specimens. This is attributed to the reduction of milling time (<30 s at 30 kV acceleration voltage) and the use of lower currents (<0.3 nA) compared to the conventional annular milling method. The yield of specimens during field evaporation in APT was also significantly increased from around 8 million ions to more than 86 million ions due to the avoidance of Ga⁺ ion embrittlement. Therefore, the currently demonstrated rectangular milling technique can be used to prepare APT specimens from Al-alloys and obtained accurate compositions of matrix, phases, and hetero-phase interfaces with Ga < 0.1 at%.

Engineering Optically Active Defects in Hexagonal Boron Nitride Using Focused Ion Beam and Water

Hexagonal boron nitride (hBN) has emerged as a promising material platform for nanophotonics and quantum sensing, hosting optically active defects with exceptional properties such as high brightness and large spectral tuning. However, precise control over deterministic spatial positioning of emitters in hBN remained elusive for a long time, limiting their proper correlative characterization and applications in hybrid devices. Recently, focused ion beam (FIB) systems proved to be useful to engineer several types of spatially defined emitters with various structural and photophysical properties. Here we systematically explore the physical processes leading to the creation of optically active defects in hBN using FIB and find that beam-substrate interaction plays a key role in the formation of defects. These findings are confirmed using transmission electron microscopy, which reveals local mechanical deterioration of the hBN layers and local amorphization of ion beam irradiated hBN. Additionally, we show that, upon exposure to water, amorphized hBN undergoes a structural and optical transition between two defect types with distinctive emission properties. Moreover, using super-resolution optical microscopy combined with atomic force microscopy, we pinpoint the exact location of emitters within the defect sites, confirming the role of defected edges as primary sources of fluorescent emission. This lays the foundation for FIB-assisted engineering of optically active defects in hBN with high spatial and spectral control for applications ranging from integrated photonics, to nanoscale sensing, and to nanofluidics.

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.

Large volume tomography using plasma FIB-SEM: A comprehensive case study on black silicon

The xenon plasma focused ion beam and scanning electron microscopy (PFIB-SEM) system is a promising tool for 3D tomography of nano-scale materials, including nanotextured black silicon (BSi), whose topography is difficult to measure with conventional microscopy techniques. Advantages of PFIB-SEM include high material removal rates, precise control of milling parameters and automated slice-and-view procedures. However, there is no universal sample preparation procedure nor is there an established ideal workflow for the PFIB-SEM slice-and-view process. This work demonstrates that specimen preparation, including the orientation of the volume of interest, is critical for the quality of the final reconstructed 3D model. It thoroughly explores three unique configurations incrementally optimized for higher total throughput. All three sampling configurations are applied to a resin-embedded BSi sample to determine the most favourable workflow and highlight each approach's advantages and disadvantages. The reconstructed 3D models of the BSi surface obtained are shown to be qualitatively closer to the topography measured directly by SEM. The height distribution data extracted from the rendered 3D models reveal a higher structure depth compared to that obtained from an atomic force microscopy measurement. Furthermore, the work demonstrates how samples with different rigidity react to long-term ion-beam interaction, as both amorphous (resin) and crystalline (Si) material is present in the tested specimen. This study improves the understanding of sample-beam interaction and broadens the utility of the 3D PFIB-SEM for more complicated sample structures.

3D electron backscatter diffraction characterization of fine α titanium microstructures: collection, reconstruction, and analysis methods

3D electron backscatter diffraction (3D-EBSD) is a method of obtaining 3-dimensional crystallographic data through serial sectioning. The recent advancement of using a Xe⁺ plasma focused ion beam for sectioning along with a complementary metal-oxide semiconductor based EBSD detector allows for an improvement in the trade-off between volume analyzed and spatial resolution over most other 3D characterization techniques. Recent publications from our team have focused on applying 3D-EBSD to understand microstructural phenomena in Ti-6Al-4V microstructures as a function of electron beam scanning strategies in electron beam powder bed fusion additive manufacturing. The microstructures resulting from this process have fine features, with α laths as small as 1 μm interwoven in a highly complex fashion, presenting a significant challenge to characterize. Over the course of these fundamental works, we have developed best-practice 3D-EBSD collection protocols and advanced methods for 3D data reconstruction and analysis of such microstructures which remain unpublished. These methods may be of interest to the 3D materials characterization community, especially considering the lack of standard commercial software tools. Thus, the current paper elaborates on the methods and analysis used to characterize fine titanium microstructures using 3D-EBSD and presents a detailed description of the new algorithms developed for probing the unique features therein. The new analyses include algorithms for identifying intervariant boundary types, classifying three-variant clusters, assigning grains to variants, and quantifying interconnectivity of branched α platelets.

Comparing Xe+ pFIB and Ga+ FIB for TEM sample preparation of Al alloys: Minimising FIB-induced artefacts

Recently, the dual beam Xe+ plasma focused ion beam (Xe+ pFIB) instrument has attracted increasing interest for site-specific transmission electron microscopy (TEM) sample preparation for a local region of interest as it shows several potential benefits compared to conventional Ga+ FIB milling. Nevertheless, challenges and questions remain especially in terms of FIB-induced artefacts, which hinder reliable S/TEM microstructural and compositional analysis. Here we examine the efficacy of using Xe+ pFIB as compared with conventional Ga+ FIB for TEM sample preparation of Al alloys. Three potential source of specimen preparation artefacts were examined, namely: (1) implantation-induced defects such as amophisation, dislocations, or 'bubble' formation in the near-surface region resulting from ion bombardment of the sample by the incident beam; (2) compositional artefacts due to implantation of the source ions and (3) material redeposition due to the milling process. It is shown that Xe+ pFIB milling is able to produce improved STEM/TEM samples compared to those produced by Ga+ milling, and is therefore the preferred specimen preparation route. Strategies for minimising the artefacts induced by Xe+ pFIB and Ga+ FIB are also proposed. LAY DESCRIPTION: FIB (focused ion beam) instruments have become one of the most important systems in the preparation of site-specific TEM specimens, which are typically 50-100 nm in thickness. TEM specimen preparation of Al alloys is particularly challenging, as convention Ga-ion FIB produces artefacts in these materials that make microstructural analysis difficult or impossible. Recently, the use of noble gas ion sources, such as Xe, has markedly improved milling speeds and is being used for the preparation of various materials. Hence, it is necessary to investigate the structural defects formed during FIB milling and assess the ion-induced chemical contamination in these TEM samples. Here we explore the feasibility and efficiency of using Xe+ PFIB as a TEM sample preparation route for Al alloys in comparison with the conventional Ga+FIB.