Ion Beam Instrument Research Articles

Characterization of Fe-Cr alloys irradiated by neutrons at intermediate temperature

A series of Fe-Cr alloys, with 3 at.% - 18 at.% Cr, was irradiated in the Advanced Test Reactor (ATR) at the Idaho National Labs (INL), USA, up to a dose of ∼6.7 dpa at a temperature of ∼456 °C. Transmission electron microscopy (TEM) samples were extracted using a focused ion beam (FIB) instrument, and the resulting microstructural defects, such as voids, dislocation loops, network dislocations, Cr rich precipitates, etc., were characterized using a TEM. It was found that the size and number density of these defects varied widely over the different alloys with varying Cr content. As expected, there were no Cr rich precipitates in samples with Cr up to 9 %, and they started appearing only in samples with 12 at.% Cr and above. The particle size decreased from about 15 nm at 12 % Cr to 8 nm at 18 % Cr, while the number density increased from ∼7e20 /m3 to 6e22 /m3 for the same Cr contents. Grain boundary segregation of Cr, along with a precipitate free zone, was observed in the cases where a boundary was present in the sample. Large voids (>1–2 nm) were almost invisible in the Fe-3at.%Cr sample, while the average void size remained almost constant between 15 and 18 nm for samples with 6–15 at.% Cr and increased slightly at 18 at.% Cr to ∼22 nm. Fe-3 %Cr showed a high density of small voids(<2 nm), estimated to be about 1e23/m3. The number density of large voids increased from ∼0 at 3 % Cr to a peak of ∼6.4e20 /m3 at 12 % Cr, then decreased to about 1.1e20 /m3 at 18 % Cr. The dislocation loops, which appeared in linear arrays in the Fe-9 %Cr sample, were analysed in detail using both invisibility criteria and image simulations using the Oxford University TEMACI software, and it was found that they are most likely to be ½ 〈111〉 type loops on {100} planes. These loops seem shaped like sections of helices in some places and are most likely formed by loops near screw dislocations climbing into a helix and then collapsing into a loop array. Similar dislocation analysis was performed in the other samples as well wherever feasible, and it was found that there is a mixture of ½ 〈111〉 and [100] dislocation loops.

Deployment of Magnetic Sector Secondary Ion Mass Spectrometry Technology on Focused Ion Beam Instruments: From the Initial Concept Idea to the Analytical Add-On System

Deployment of Magnetic Sector Secondary Ion Mass Spectrometry Technology on Focused Ion Beam Instruments: From the Initial Concept Idea to the Analytical Add-On System

Precise Fabrication and Manipulation of Individual Polymer Nanofibers.

Polymer nanofibers hold promise in a wide range of applications owing to their diverse properties, flexibility, and cost effectiveness. In this study, we introduce a polymer nanofiber drawing process in a scanning electron microscope and focused ion beam (SEM/FIB) instrument with in situ observation. We employed a nanometer-sharp tungsten needle and prepolymer microcapsules to enable nanofiber drawing in a vacuum environment. This method produces individual polymer nanofibers with diameters as small as ∼500 nm and lengths extending to millimeters, yielding nanofibers with an aspect ratio of 2000:1. The attachment to the tungsten manipulator ensures accurate transfer of the polymer nanofiber to diverse substrate types as well as fabrication of assembled structures. Our findings provide valuable insights into ultrafine polymer fiber drawing, paving the way for high-precision manipulation and assembly of polymer nanofibers.

Scalable substrate development for aqueous sample preparation for atom probe tomography.

Reliable and consistent preparation of atom probe tomography (APT) specimens from aqueous and hydrated biological specimens remains a significant challenge. One particularly difficult process step is the use of a focused ion beam (FIB) instrument for preparing the required needle-shaped specimen, typically involving a "lift-out" procedure of a small sample of material. Here, two alternative substrate designs are introduced that enable using FIB only for sharpening, along with example APT datasets. The first design is a laser-cut FIB-style half-grid close to those used for transmission-electron microscopy, that can be used in a grid holder compatible with APT pucks. The second design is a larger, standalone self-supporting substrate called a "crown," with several specimen positions that self-aligns in APT pucks, prepared by electrical discharge machining (EDM). Both designs are made nanoporous, to provide strength to the liquid-substrate interface, using chemical and vacuum dealloying. We select alpha brass a simple, widely available, lower-cost alternative to previously proposed substrates. We present the resulting designs, APT data, and provide suggestions to help drive wider community adoption. Lay Description Atom probe tomography (APT) maps the composition of solid materials in three dimensions with sub-nanometre resolution. Preparation of specimens from aqueous specimens remains a significant challenge. To aid with preparation, we introduce here two substrate designs that can hold suitable volumes of liquid for freezing, prior to shaping the final specimen into a sharp needle for APT by using cryogenic focused ion beam milling. We also describe the complete setup that we designed for handling these substrates, including dedicated sample holders. Both substrates are made nanoporous, to provide strength to the liquid-substrate interface, using chemical and vacuum dealloying of alpha brass a simple, widely available, low-cost material. We finally showcase APT data and provide suggestions to help drive wider community adoption. This article is protected by copyright. All rights reserved.

The effect of cavities produced by focused ion beam milling on solid state dewetting of thin Au films

The goal of this study was to explore the possibility to control the nucleation and expansion of holes during initial stages of solid state dewetting of thin polycrystalline Au film deposited on sapphire substrate. To this end, we fabricated isolated triangular cavities in the film employing Ga+ ion milling in the focused ion beam instrument. The following dewetting heat treatments indicated that the cavities do promote early nucleation and expansion of dewetting holes, yet these holes develop tortuous shapes dominated by a random distribution of nearby hillocks in the film. Deep holes exhibited a triangular patch of amorphous Al-Ga-O phase at their bottom which retained its size and morphology during subsequent thermal treatments. This amorphous phase caused some delay in holes expansion at high temperatures. We proposed a geometrical model of the retracting metal film edge pinning by the amorphous phase. The observed pinning phenomenon can be utilized for the design of patterned thin films with increased thermal stability.

Detectability of Small True-T Hole Image Quality Indicators by Digital Radiographic Techniques

Image quality indicators (IQIs) are used to ensure quality in radiography. The requirements for hole-type IQIs have changed from MIL-STD-453-C (US DOD 1984) to ASTM E1742 and E1025 (ASTM 2018a, 2018b). For materials thinner than 0.50 in., the 2-2T hole does not represent 2% sensitivity. E1025 introduced true-T hole IQIs, where the 2-2T hole represents 2% sensitivity. Stainless steel true-T hole IQIs were fabricated using drills and a plasma-focused ion beam instrument. They were imaged using digital radiography (DR), computed radiography, and X-ray film to determine the limit of their visibility. DR was able to detect holes on the order of 2 to 3 pixels in diameter. All three techniques were able to detect the 2T hole in the number 4 IQI while DR was able to detect it in the number 3 IQI. These hole sizes are near the limit where geometric magnification would be required. The better sensitivity of DR is probably a result of being able to minimize fixed structural noise. Presently, for materials thinner than 0.50 in., there is the option to use IQIs defined by either E1742 Annex A1 or E1025. It is recommended that inspection procedures require a particular sensitivity percent that would necessitate the use of a specific IQI.

SIMS Performed on Focused Ion Beam Instruments : In-situ Correlative Structural and Chemical Imaging

SIMS Performed on Focused Ion Beam Instruments : In-situ Correlative Structural and Chemical Imaging

In-Plane, In-Series Nanopores with Circular Cross Sections for Resistive-Pulse Sensing.

Resistive-pulse sensing with solid-state nanopores is a sensitive, label-free technique for analyzing single molecules in solution. To add functionality to resistive-pulse measurements, direct coupling of the nanopores to other pores and nanoscale fluidic elements, e.g., reactors, separators, and filters, in the same device is an important next step. One approach is monolithic fabrication of the fluidic elements in the plane of the substrate, but methods to generate pores with circular cross sections are needed to improve sensing performance with in-plane devices. Here, we report a fabrication method that directly patterns nanopores with circular cross sections in series and in plane with the substrate. A focused ion beam instrument is used to mill a lamella in a nanochannel and, subsequently, bore a nanopore through the lamella. The diameter and geometry of the nanopore are controlled by the current and dose of the ion beam and by the tilt angle and thickness of the lamella. We fabricated devices with vertical and tilted lamellae and nanopores with diameters from 40 to 90 nm in cylindrical and conical geometries. To test device performance, we conducted resistive-pulse measurements of hepatitis B virus capsids. Current pulses from T = 3 capsids (∼31 nm diameter) and T = 4 capsids (∼35 nm diameter) were well resolved and exhibited relative pulse amplitudes (Δi/i) up to 5 times higher than data obtained on nanopores with rectangular cross sections. For smaller pore diameters (<45 nm), which approach the diameters of the capsids, a dramatic increase in the pulse amplitude was observed for both T = 3 and T = 4 capsids. Two and three pores fabricated in series further improved the resolution between the relative pulse amplitude distributions for the T = 3 and T = 4 capsids by up to 2-fold.

Lamellae preparation for atomic-resolution STEM imaging from ion-beam-sensitive topological insulator crystals

Good specimen quality is a key factor in achieving successful scanning transmission electron microscope analysis. Thin and damage-free specimens are prerequisites for obtaining atomic-resolution imaging. Topological insulator single crystals and thin films in the chalcogenide family such as Sb2Te3 are sensitive to electron and ion beams. It is, therefore, challenging to prepare a lamella suitable for high-resolution imaging from these topological insulator materials using standard focused ion-beam instruments. We have developed a modified method to fabricate thin focused ion-beam (FIB) lamellae with minimal ion-beam damage and artifacts. The technique described in the current study enables the reliable preparation of high-quality transmission electron microscope (TEM) specimens necessary for studying ultra-thin surface regions. We have successfully demonstrated that the careful selection of FIB milling parameters at each stage minimizes the damage layer without the need for post-treatment.

Enhanced focused ion beam milling with use of nested raster patterns

The focused ion beam (FIB) instrument is designed to provide the removal of material with nanometer-scale precision. However, one often needs to remove a substantial amount of material to expose the region of interest or prepare a specimen for transmission electron microscopy analysis. The maximum current available on Ga+ FIB sources is less than 100 nA, and this is a limiting factor when removal on the millimeter scale is desired. Any improvement in the removal rate reduces the analysis time and increases the range of samples that can be analyzed. Optimization of ion beam parameters, such as dwell time and overlap, can improve material removal and reduce redeposition. Since sputtering occurs faster at an edge, the use of a nested arrangement of raster patterns to more frequently present an edge to the ion beam was able to improve the removal of material at the region of interest by over 30% in the silicon and polycrystalline copper substrates used for this study. A confocal laser scanning microscope made possible an accurate determination of the material removed.

Xenon plasma focussed ion beam preparation of an Al-6XXX alloy sample for atom probe tomography including analysis of an α-Al(Fe,Mn)Si dispersoid

A Xe plasma Focussed Ion Beam instrument was used to prepare in-situ specimens for Atom Probe Tomography from a bulk sample of an aluminium 6XXX alloy. The nature and distribution of precipitates and solute clusters observed in the alloy are not observed to differ between standard electropolishing methods and Xe plasma preparation. Enabled by site specific specimen preparation, analysis of an α-Al(Fe,Mn)Si dispersoid shows segregation at the phase boundary and in the shell of the dispersoid.

High Sensitivity of Fluorine Gas-Assisted FIB-TOF-SIMS for Chemical Characterization of Buried Sublayers in Thin Films.

In this work, we present the potential of high vacuum-compatible time-of-flight secondary ion mass spectrometry (TOF-SIMS) detectors, which can be integrated within focused ion beam (FIB) instruments for precise and fast chemical characterization of thin films buried deep under the sample surface. This is demonstrated on complex multilayer systems composed of alternating ceramic and metallic layers with thicknesses varying from several nanometers to hundreds of nanometers. The typical problems of the TOF-SIMS technique, that is, low secondary ion signals and mass interference between ions having similar masses, were solved using a novel approach of co-injecting fluorine gas during the sample surface sputtering. In the most extreme case of the Al/Al2O3/Al/Al2O3/.../Al sample, a <10 nm thick Al2O3 thin film buried under a 0.5 μm material was detected and spatially resolved using only 27Al+ signal distribution. This is an impressive achievement taking into account that Al and Al2O3 layers varied only by a small amount of oxygen content. Due to its high sensitivity, fluorine gas-assisted FIB-TOF-SIMS can be used for quality control of nano- and microdevices as well as for the failure analysis of fabrication processes. Therefore, it is expected to play an important role in the development of microelectronics and thin-film-based devices for energy applications.

Practical Aspects of Focused Ion Beam Time-of-Flight Secondary Ion Mass Spectrometry Analysis Enhanced by Fluorine Gas Coinjection

The interest in using high vacuum-compatible time-of-flight secondary ion mass spectrometry (TOF-SIMS) detectors integrated within focused ion beam instruments (FIB) has increased due to the possibility of conducting correlative and/or complementary studies with other techniques such as scanning electron microscopy, energy dispersive spectroscopy (EDS), wavelength dispersive spectroscopy, electron backscatter diffraction, atomic force microscopy, and Raman spectroscopy without breaking vacuum conditions. This work discusses the practical aspects of FIB-TOF-SIMS analysis enhanced by fluorine gas. A series of systematic experiments with high-purity Al and Cu single crystals was conducted. We demonstrate that fluorine-assisted FIB-TOF-SIMS analysis yields up to several orders of magnitude higher SIMS signals due to chemically improved ionization probability of elements. The potential of fluorine gas-assisted FIB-TOF-SIMS analysis and the correlated benefits are demonstrated on a real-life sample based on comparing the results to EDS and TOF-SIMS experiments conducted without any gas. Our studies indicate that the presence of fluorine gas reduces the negative effect of preferential reoxidation of the sample surface. This enables high lateral resolution FIB-TOF-SIMS mapping because the secondary ion signals are not dominated by elements with strongly oxygen-enhanced ionization probability.

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.

Estimation of needle height using relative parallax between two SIM images in single-column focused ion beam instrument

Gallium liquid metal ion source focused ion beam (FIB) instruments are widely used for cross-sectional sample preparation of specific areas of interest for scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Most FIB instruments are equipped with a metal needle for manipulation of the sample in the sample chamber most commonly as a micro-sampling probe for preparing TEM samples. Automatic needle manipulation has not yet been achieved for a conventional single-column FIB instrument without an SEM column because a reliable method for measuring needle height has not been established yet. This study investigated the use of scanning ion microscope (SIM) image processing to measure needle height from the sample surface in a conventional single-column FIB instrument without an SEM column. The needle height is measured using the relative parallax between two SIM images in which the needle is observed from two directions using a conventional aligner without the use of any special devices. It was demonstrated that the relative parallax was proportional to the needle height. The estimation precision was about several 10 μm, which is sufficient for automatic manipulation and safe touchdown of a needle in practical micro-sampling.

Helium ion microscope - secondary ion mass spectrometry for geological materials.

The helium ion microscope (HIM) is a focussed ion beam instrument with unprecedented spatial resolution for secondary electron imaging but has traditionally lacked microanalytical capabilities. With the addition of the secondary ion mass spectrometry (SIMS) attachment, the capabilities of the instrument have expanded to microanalysis of isotopes from Li up to hundreds of atomic mass units, effectively opening up the analysis of all natural and geological systems. However, the instrument has thus far been underutilised by the geosciences community, due in no small part to a lack of a thorough understanding of the quantitative capabilities of the instrument. Li represents an ideal element for an exploration of the instrument as a tool for geological samples, due to its importance for economic geology and a green economy, and the difficult nature of observing Li with traditional microanalytical techniques. Also Li represents a “best-case” scenario for isotopic measurements. Here we present details of sample preparation, instrument sensitivity, theoretical, and measured detection limits for both elemental and isotopic analysis as well as practicalities for geological sample analyses of Li alongside a discussion of potential geological use cases of the HIM–SIMS instrument.

Single Ion Implantation of Bismuth

Herein, the results from a focused ion beam instrument, designed to implant single ions with a view to the fabrication of qubits for quantum technologies, are presented. The difficulty of single ion implantation is accurately counting the ion impacts. This is achieved here through the detection of secondary electrons generated upon each ion impact. The implantation of single bismuth ions with different charge states into Si, Ge, Cu, and Au substrates is reported, and the counting detection efficiency for single ion implants and the factors that affect such detection efficiencies are determined. It is found that for 50 keV implants of Bi++ ions into silicon an 89% detection efficiency can be achieved, which is the first quantitative detection efficiency measurement for single ion implants into silicon without implanting through a thick SiO2 film. This level of counting accuracy provides implantation of single impurity ions with a success rate significantly exceeding that achievable by random (Poissonian) implantation.

Formation of H3 + in Collisions of H2 + with H2 Studied in a Guided Ion Beam Instrument.

In order to study collisions between ions and neutrals, a new Guided Ion Beam (GIB) apparatus, called NOVion, has been assembled and tested. The primary purpose of this instrument is to measure absolute cross sections at energies relevant for technical or inter- and circumstellar plasmas. New and improved results are presented for forming H3 + in collisions of H2 + with H2 . Between 0.1 eV and 2 eV, our measured effective cross sections are in good overall agreement with most previous measurements. However, at higher energies, our results do not show the steep decline, recommended in the standard literature. After critical evaluation of all experimental and theoretical data, a new analytical function is proposed, describing properly the dependence of the title reaction on the collision energy up to 10 eV.

Experimental and computational analysis of the in situ tensile deformation of 2D honeycomb lattice structures in Ni single crystals

A ~12 μm thick film of single crystal Ni was shaped with a focused ion beam (FIB) instrument to form a set of tensile samples with hexagonal holes of sides ~3 μm long and ~2.5 μm thick arranged in a honeycomb lattice structure. The plane normal of the film was <001> and the tensile axis was parallel to <100>. The samples were then tested in tension in situ inside a scanning electron microscope. The tensile properties, viz., yield strength and peak strength observed in the lattice sample, were higher than those of a standard sample by about 30%, while the strain to fracture decreased drastically. Finite element (FE) modelling, utilising size effect corrections and the modified Gurson model, was performed to elucidate the response of the microlattice, and it was found that the changes in the mechanical properties could be explained by the stress distribution around the hexagonal holes and localised sample size effects. These models were corroborated by observations of the crystal orientation using electron backscatter diffraction (EBSD) maps. The assumption of void formation as a mechanism of failure in the Gurson model was validated experimentally by transmission electron microscopy, which showed nano-voids in the thin bands formed by localised slip.

NanoFab SIMS: High Spatial Resolution Imaging and Analysis Using Inert-Gas Ion Beams

Abstract