Focused Ion Beam Milling Research Articles

Focused Ion Beam Milling as a Method for Machining Explosive Crystals**

AbstractExperimental pore collapse studies have traditionally relied on machining methods that are mechanically active and result in rough surfaces, added stresses, and premature material fractures which are a source of experimental error. This work focuses on the method development and the results of adapting the focused ion beam to mill micro and nano‐scale pores with increased geometrical freedom into explosive crystals. Results include developing a simplified mathematical relationship between the electron beam energy and mill rate, assessments of pore quality, and material and machine‐specific optimal milling parameters.

Cracking mechanism of CA6NM hydro turbine steel in cavitation erosion

Cavitation erosion is a main reason for the damage of hydro turbines. It is important to understand the cracking mechanism of hydro turbine steel in cavitation erosion. In this study, the crack initiation in CA6NM steel after cavitation erosion was examined through electron backscatter diffraction analyses. The crack propagation was investigated adopting focused ion beam milling and transmission electron microscopy. The reversed austenite transformed to strain-induced martensite in cavitation erosion. The lattice parameter of the strain-induced martensite was greater than that of the thermal martensite, which led to stress concentration between the two martensites and caused crack formation and propagation in the region. Finally, approaches aiming to improve the cavitation erosion resistance of hydro turbine parts are suggested.

Basalt Alteration in a CO2–SO2 Atmosphere: Implications for Surface Processes on Venus

AbstractVenus' surface and interior dynamics remain largely unconstrained, due in great part to the major obstacles to exploration imposed by its 470°C, 90 bar surface conditions and its thick, opaque atmosphere. Flyby and orbiter‐based thermal emission data provide opportunities to characterize the surface composition of Venus. However, robust interpretations of such data depend on understanding interactions between the planet's surface basaltic rocks and its caustic carbon dioxide (CO2)‐dominant atmosphere, containing trace amounts of sulfur dioxide (SO2). Several studies, using remote sensing, thermodynamic modeling, and laboratory experiments, have placed constraints on basaltic alteration mineralogy and rates. However, constraints on the effects of SO2‐bearing reactions on basalts with diverse compositions remain incomplete. Here, we present new data from a series of gas‐solid reaction experiments, in which samples of two basalt compositions were reacted in an SO2‐bearing CO2 atmosphere, at relevant Venus temperatures, pressure, and oxygen fugacity. Reacted specimens were analyzed by scanning electron microscopy and scanning transmission electron microscopy using sample cross‐sections produced with focused ion beam milling. Surface alteration products were characterized, and their abundances estimated; subsurface cation concentrations were mapped to show the depth of alteration. We demonstrate that the initial development of reaction products progresses rapidly over the course of 30‐day runs. Alkaline basalt samples are coated by Na‐sulfate (likely thenardite, Na2SO4) and amorphous calcium carbonate (CaCO3) alteration products, and tholeiitic basalt samples are primarily covered by anhydrite (CaSO4), Fe‐oxide (FexOy: likely magnetite, Fe3O4), and other minor phases. These mineralogies differ from previous experiments in CO2‐only atmospheres.

On the preparation and mechanical testing of nano to micron-scale specimens

Over the past two decades, means allowing to probe experimentally the mechanical behavior of materials specimens only a few micrometers or less in diameter have multiplied, largely owing to the widespread adoption and versatility of focused ion beam (FIB) milling for the preparation of small-scale test specimens. Despite its remarkable capabilities, the ion bombardment that is employed during FIB milling operations does not leave machined surfaces unscathed: it may induce a series of alterations in the near-surface microstructure of materials, which vary in severity depending on the material being milled, or milling parameters, or the type of ion used. These alterations in turn can strongly influence the local behavior, and as a result measured mechanical properties, of the milled material. In the first part of this manuscript, we review the different forms that FIB-induced microstructural alterations can take and their influence on small-scale mechanical test results. In the second part, we present alternative strategies that have been used to circumvent the issue. These include the use of entirely different small-scale sample fabrication processes, as well as approaches that do use FIB-milling but put effort in the test design to minimize or avoid the formation of FIB-induced defects in regions where micromechanical test data are collected. The advent of such methods can enhance our understanding of FIB-induced defects on the mechanical behavior of microsamples by comparing the results with those from ion-milled samples. This, in turn, should improve our ability to interpret test data when FIB-milling is the only method available for microsample production.

Study of curtaining effect reduction methods in Inconel 718 using a plasma focused ion beam.

The curtaining effect is a common challenge in focused ion beam (FIB) surface preparation. This study investigates methods to reduce this effect during plasma FIB milling of Inconel 718 (nickel-based superalloy). Platinum deposition, silicon mask and XeF2 gas injection were explored as potential solutions. These methods were evaluated for two ion beam current conditions; a high ion beam intensity condition (30kV-1µA) and a medium one (30kV-100nA) and their impact on curtaining reduction and resulting cross-section quality was assessed quantitatively thanks to topographic measurements done by atomic force microscopy (AFM). XeF2 assistance notably improved cross-section quality at medium current level. Pt deposition and Si mask individually mitigated the curtaining effect, with greater efficacy at 100nA. Both methods also contributed to reducing cross-section curvature, with the Si mask outperforming Pt deposition. However, combining Pt deposition and Si mask with XeF2 injection led to deterioration of these protective layers and the reappearance of the curtaining effect after a quite short exposure time.

Noninvasive Readout of the Kinetic Inductance of Superconducting Nanostructures.

The energy landscape of multiply connected superconducting structures is ruled by fluxoid quantization due to the implied single-valuedness of the complex wave function. The transitions and interaction between these energy states, each defined by a specific phase winding number, are governed by classical and/or quantum phase slips. Understanding these events requires the ability to probe, noninvasively, the state of the ring. Here, we employ a niobium resonator to examine the superconducting properties of an aluminum loop. By applying a magnetic field, adjusting temperature, and altering the loop's dimensions via focused ion beam milling, we correlate resonance frequency shifts with changes in the loop's kinetic inductance. This parameter is an indicator of the superconducting condensate's state, facilitating the detection of phase slips in nanodevices and providing insights into their dynamics. Our method presents a proof-of-principle spectroscopic technique with promising potential for investigating Cooper pair density in inductively coupled superconducting nanostructures.

Demonstration of an integrated-heating load frame for quantitatively assessing microscale tensile properties of copper and Zircaloy-2

Small-scale mechanical testing (SSMT) is of great interest in the nuclear materials community as it permits the use of surrogate irradiation techniques, such as ion beam irradiation when investigating the impact of irradiation damage on mechanical properties. The design of a micro-electro-mechanical-system (MEMS) micro-tensile stage is demonstrated to enable micron-sized specimen testing up to failure under ambient and reactor-relevant temperatures. Copper and Zircaloy-2 microscale specimens, having gauge lengths of 200 μm, gauge widths of 48 μm, and specimen-dependent constant gauge thicknesses of 10 μm to 26 μm, are fabricated using micro-wire electrical discharge machining and focused ion beam milling. From each gauge section, microstructure data is captured prior to testing using electron backscatter diffraction analysis, and an optimized digital image correlation speckle pattern is deposited on the specimen surface. The speckle pattern is tracked during deformation and post-processed to obtain strain-field evolution maps throughout deformation. Strain maps provide a means to account for and explain microstructure-dependent features observed in the captured stress-strain curves. The combination of using micro-wire electrical discharge machining, focused ion beam cleaning, and ion beam induced platinum deposition for micron-sized specimen preparation as well as utilizing microfabrication techniques to fabricate an integrated heating microtensile load frame reported herein serve as a demonstration of SSMT approaches that can be adopted to tackle challenging problems in nuclear materials research, including but not limited to the effects of ion beam irradiation on mechanical performance and providing experimental data to support small-scale modeling efforts of embrittling precipitates or radiation-induced segregation.

Wear mechanisms and material deposition of high-performance polymer composites for hydrogen compression

The transistion to renewable energy production is a main component in achieving the climate targets defined in the 2015 Paris Agreement. Power-to-gas concepts, where electrical energy is stored as hydrogen gas, require the design and operation of state-of-the-art reciprocating compressor technology to realize high pressure ratios and high volumetric flow rates for gas supply demands. The main challenges in designing machines suited for these operating conditions are high thermo-mechanical stresses acting on the piston sealing rings. High shear loads and minimal ring gap sizes demand material pairings with excellent wear resistance and sliding properties. This work investigates the life cycle limiting wear mechanisms present in the piston compressor and develops advanced concepts for piston rings in cylinder tribo-contacts. PI and PPS polymers reinforced with carbon fiber were modified with dry lubricating PTFE to fulfil the requirements of this highly stressed tribo-system. Tribo-tests were performed against steel countersurfaces with a linear oscillating tribometer in pin-on-plate configuration. The failure mechanisms were analysed utilizing scanning electron microscopy on cross-sections cut into the depth of the countersurface material with focused ion beam milling. 2D scanning transmission electron microscopy mapping supported by energy dispersive X-ray spectroscopy were performed to analyze the depth profile of the deposited tribo-layer elements on the wear track.

Residual stress determination in a C-C composite consisting of a carbonized elastomer matrix filled with graphite, carbon black and short carbon fibers

In this study, composites obtained through low-temperature carbonization of elastomeric matrix highly filled with graphite, carbon black and short carbon fibers were studied for the purpose of determining residual stresses at different scales using a combination of several complementary methods. The state-of-the-art techniques included X-ray stress analysis using the sin2⁡ψ method, the micro-ring-core technique via Focused Ion Beam milling and Digital Image Correlation (FIB-DIC), the contour method, the strain gauge method, and the hole drilling technique with digital laser speckle pattern interferometry (DLSPI). It was found that the contour method could not be used implemented for residual stress evaluation due to the low electrical conductivity of composite. Moreover, the DLSPI hole drilling method did not reveal any fringes indicating significant residual stress level exceeding a few MPa. The strain gauge method also revealed a narrow residual stress distribution with an average value of approximately zero. In contrast, the X-ray sin2⁡ψ method as well as FIB-DIC technique both returned values of about 150–250 MPa. A hierarchical model of the composite is proposed based on the Davidenkov Type I–II–III stress classification that provides an explanation of these observations.

Analysis of Water Ice in Nanoporous Copper Needles Using Cryo Atom Probe Tomography.

The application of atom probe tomography (APT) to frozen liquids is limited by difficulties in specimen preparation. Here, we report on the use of nanoporous Cu needles as a physical framework to hold water ice for investigation using APT. Nanoporous Cu needles are prepared by electropolishing and dealloying Cu-Mn matchstick precursors. Cryogenic scanning electron microscopy and focused ion beam milling reveal a hierarchical, dendritic, highly wettable microstructure. The atom probe mass spectrum is dominated by peaks of Cu+ and H(H2O)n+ up to n ≤ 3, and the reconstructed volume shows the protrusion of a Cu ligament into an ice-filled pore. The continuous Cu ligament network electrically connects the apex to the cryostage, leading to an enhanced electric field at the apex and increased cooling, both of which simplify the mass spectrum compared to previous reports.

Spin–wave dynamics in perpendicularly magnetized antidot multilayers

Using all-optical time-resolved magneto-optical Kerr effect measurements we demonstrate an efficient modulation of the spin–wave (SW) dynamics via the bias magnetic field orientation around nanoscale diamond shaped antidots that are arranged on a square lattice within a [Co(0.75 nm)/Pd(0.9 nm)]8 multilayer with perpendicular magnetic anisotropy (PMA). Micromagnetic modeling of the experimental results reveals that the SW modes in the lower frequency regime are related to narrow shell regions around the antidots, where in-plane (IP) domain structures are formed due to the reduced PMA, caused by Ga+ ion irradiation during the focused ion beam milling process of antidot fabrication. The IP direction of the shell magnetization undergoes a striking change with magnetic field orientation, leading to the sharp variation of the edge localized (shell) SW modes. Nevertheless, the coupling between such edge localized and bulk SWs for different orientations of bias field in PMA systems gives rise to interesting Physics and attests to new prospects for developing energy efficient and hybrid-system-based next-generation nanoscale magnonic devices.

MicroED structure of the C11 cysteine protease clostripain

Clostripain secreted from Clostridium histolyticum is the founding member of the C11 family of Clan CD cysteine peptidases, which is an important group of peptidases secreted by numerous bacteria. Clostripain is an arginine-specific endopeptidase. Because of its efficacy as a cysteine peptidase, it is widely used in laboratory settings. Despite its importance the structure of clostripain remains unsolved. Here we describe the first structure of an active form of C. histolyticum clostripain determined at 2.5 Å resolution using microcrystal electron diffraction (MicroED). The structure was determined from a single nanocrystal after focused ion beam milling. The structure of clostripain shows a typical Clan CD α/β/α sandwich architecture and the Cys231/His176 catalytic dyad in the active site. It has a large electronegative substrate binding pocket showing its ability to accommodate large and diverse substrates. A loop in the heavy chain formed between residues 452 and 457 is potentially important for substrate binding. In conclusion, this result demonstrates the importance of MicroED to determine the unknown structure of macromolecules such as clostripain, which can be further used as a platform to study substrate binding and design of potential inhibitors against this class of peptidases.

Nanoscale Patterning of Surface Nanobubbles by Focused Ion Beam.

Surface nanobubbles forming on hydrophobic surfaces in water present an exciting opportunity as potential agents of top-down and bottom-up nanopatterning. The formation and characteristics of surface nanobubbles are strongly influenced by the physical and chemical properties of the substrate. In this study, focused ion beam (FIB) milling is used for the first time to spatially control the nucleation of surface nanobubbles with 75 nm precision. The spontaneous formation of nanobubbles on alternating lines of a self-assembled monolayer (octadecyltrichlorosilane) patterned by FIB is detected by atomic force microscopy. The effect of chemical vs topographical surface heterogeneity on the formation of nanobubbles is investigated by comparing samples with OTS coating applied pre- vs post-FIB patterning. The results confirm that nanoscale FIB-based patterning can effectively control surface nanobubble position by means of chemical heterogeneity. The effect of FIB milling on nanobubble morphology and properties, including contact angle and gas oversaturation, is also reported. Molecular dynamics simulations provide further insight into the effects of FIB amorphization on surface nanobubble formation. Combined experimental and simulation investigations offer insights to guide future nanobubble-based patterning using FIB milling.

Nanoscale architecture of synaptic vesicles and scaffolding complexes revealed by cryo-electron tomography

The spatial distribution of proteins and their arrangement within the cellular ultrastructure regulates the opening of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors in response to glutamate release at the synapse. Fluorescence microscopy imaging revealed that the postsynaptic density (PSD) and scaffolding proteins in the presynaptic active zone (AZ) align across the synapse to form a trans-synaptic "nanocolumn," but the relation to synaptic vesicle release sites is uncertain. Here, we employ focused-ion beam (FIB) milling and cryoelectron tomography to image synapses under near-native conditions. Improved image contrast, enabled by FIB milling, allows simultaneous visualization of supramolecular nanoclusters within the AZ and PSD and synaptic vesicles. Surprisingly, membrane-proximal synaptic vesicles, which fuse to release glutamate, are not preferentially aligned with AZ or PSD nanoclusters. These synaptic vesicles are linked to the membrane by peripheral protein densities, often consistent in size and shape with Munc13, as well as globular densities bridging the synaptic vesicle and plasma membrane, consistent with prefusion complexes of SNAREs, synaptotagmins, and complexin. Monte Carlo simulations of synaptic transmission events using biorealistic models guided by our tomograms predict that clustering AMPARs within PSD nanoclusters increases the variability of the postsynaptic response but not its average amplitude. Together, our data support a model in which synaptic strength is tuned at the level of single vesicles by the spatial relationship between scaffolding nanoclusters and single synaptic vesicle fusion sites.

Effect of Angle of Incident on Taper Angle in Femtosecond Laser Machining for Fabrication of Cross Section Analysis Specimen

Focused ion beam (FIB) technology is one of the most widely used methods for fabricating crosssectional analysis specimens because of its high precision and characteristics that minimize the occurrence of defects. Demand for large cross-sectional area analysis is increasing to improve product reliability in various industries, but is limited by the low milling speed of FIB. Other potential techniques such as Ar ion milling and plasma FIB have been adopted, but low milling speed for large areas still remains a problem. A promising solution to this issue involves laser machining prior to FIB milling. In laser machining a laser beam is irradiated to remove materials from the target. This technique can provide several orders of magnitude higher material removal rate than FIB, however, tapering of the machined surface and laser induced damage can occur. Removing these defects leads to increased FIB milling time. In this study, the laser parameters including angle of incident (AOI) were optimized to achieve a vertical like sidewall and minimize laser induced defects. Before applying AOI, laser machining parameters were optimized to reduce the angle of the machined sidewall. The taper angle of 2.5° was fabricated using the optimized parameters and application of AOI. Raman spectroscopy, SEM, and EDS analysis were used to measure not only the geometry of the laser machined sidewalls, but laser induced residual stress and defects. These results were then used to calculate the volume of FIB milling required to remove the laser induced damages and achieve vertical sidewalls. The application of AOI can significantly reduce the processing time in the FIB milling compared to the processing time when AOI is not applied.

Rapid microcantilever preparation for conditional fracture toughness evaluation

The Ga+ focused Ion Beam (FIB) or Xe+ Plasma FIB (PFIB) procedure has enabled the ability to make specimens where mechanical tests in the microscale regime can be achieved with high precision. Nevertheless, milling times can be long and shape optimization can be arduous collectively limiting the ability to readily make a sufficient number of test samples. In this work, we present a sample preparation technique for microscale cantilever fabrication which streamlines the milling procedures by using combined electropolishing and FIB milling. Specifically, the procedure mitigates extensive undercutting requirements which reduces the fabrication time for the microcantilever to more than half that time as compared to direct milling methods from making similar geometries directly from the bulk material. This process is discussed along with specific aspects related to the sample alignment of the cantilever to specific fracture planes for single crystals. In addition, we comment on the applicability of this method to polycrystalline samples.

Atomic-Scale Characterization of Microscale Battery Particles Enabled by a High-Throughput Focused Ion Beam Milling Technique.

The cathode materials in lithium-ion batteries (LIBs) require improvements to address issues such as surface degradation, short-circuiting, and the formation of dendrites. One such method for addressing these issues is using surface coatings. Coatings can be sought to improve the durability of cathode materials, but the characterization of the uniformity and stability of the coating is important to assess the performance and lifetime of these materials. For microscale particles, there are, however, challenges associated with characterizing their surface modifications by transmission electron microscopy (TEM) techniques due to the size of these particles. Often, techniques such as focused ion beam (FIB)-assisted lift-out can be used to prepare thin cross sections to enable TEM analysis, but these techniques are very time-consuming and have a relatively low throughput. The work outlined herein demonstrates a FIB technique with direct support of microscale cathode materials on a TEM grid that increases sample throughput and reduces the processing time by 60-80% (i.e., from >5 to ∼1.5 h). The demonstrated workflow incorporates an air-liquid particle assembly followed by direct particle transfer to a TEM grid, FIB milling, and subsequent TEM analysis, which was illustrated with lithium nickel cobalt aluminum oxide particles and lithium manganese nickel oxide particles. These TEM analyses included mapping the elemental composition of cross sections of the microscale particles using energy-dispersive X-ray spectroscopy. The methods developed in this study can be extended to high-throughput characterization of additional LIB cathode materials (e.g., new compositions, coating, end-of-life studies), as well as to other microparticles and their coatings as prepared for a variety of applications.

Effect of oxidation temperature on physical properties of polycrystalline β-Ga2O3 grown by thermal oxidation of GaN in O2 ambient from 900 to 1400 °C

This study aimed to investigate the effect of oxidation temperature on the physical properties of β-Ga2O3 grown by thermal oxidation of GaN in the O2 ambient at high oxidation temperatures from 900 to 1400 °C. Notably, the analysis of the oxidation temperatures ranging from 1100 to 1400 °C remains unexplored in the other studies. The effects of the oxidation temperature on the physical properties were investigated via X-ray diffraction, X-ray photoelectron spectroscopy, and field-emission scanning electron microscopy (FESEM). At all oxidation temperatures, XPS results revealed oxygen deficiency. The top FESEM images indicated disordered nanocolumn structures on the surface of the oxide layer, which increased in abundance with oxidation temperatures exceeding 1100 °C. Energy-filtered high-resolution transmission electron microscopy images and the selected area electron diffraction patterns revealed that the top nanocolumns and the oxidized layer comprised polycrystalline β-Ga2O3. Additionally, the cross-sectional lamellae prepared through focused ion beam milling revealed an increase in the thickness of the β-Ga2O3 layer with oxidation temperature. To account for the variations in the oxide layer thickness, the oxidation temperatures were classified into two ranges of 900–1100 °C and 1100–1400 °C for calculating the activation energy. Moreover, this study assumed that the activation energy required for the oxidation process remained constant within each temperature interval. According to Arrhenius's model, the activation energy from 900 to 1100 °C was calculated to be 286.15 kJ/mol (2.97 eV). Furthermore, the activation energy from 1100 to 1400 °C was determined to be 111.33 kJ/mol (1.15 eV).

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.

Fabrication of nanoscale stencils through focused ion beam milling and dry transfer of silicon-on-nothing membrane with perforations

Nanoscale stencil lithography, providing sub-micrometer resolutions, is being implemented as a reliable patterning technique within the nanotechnology domain. Despite their advantages such as no resist processing, easy manipulation and reusability, patterning using a nanoscale stencil often faces challenges due to the gap between the nanoscale stencil and the substrate. This tends to result in unwanted pattern blurring, typically dimension wider than intended design. To address this issue, we minimize the gap by conformally attaching the nanoscale stencil to the substrate, thereby effectively eliminating a key factor contributing to the blurring effect. The nanoscale stencil is fabricated by forming nanoslits on the 50 nm thick Silicon-on-Nothing (SON) membrane with perforations, using focused ion beam (FIB) milling. The transfer of this stencil onto a substrate enables conformal adhesion due to its 1012 times lower flexural rigidity of the stencil compared to bulk silicon. Upon deposition of chromium and gold through the transferred stencil, a metal pattern array with the full width at half maximum (FWHM) of 43 nm is produced, demonstrating the potential of our approach for fabricating uniform nanoscale patterns with enhanced pattern resolution.