Gallium Ion Source Research Articles

A Comparative Study of Gallium-, Xenon-, and Helium-Focused Ion Beams for the Milling of GaN.

The milling profiles of single-crystal gallium nitride (GaN) when subjected to focused ion beams (FIBs) using gallium (Ga), xenon (Xe), and helium (He) ion sources were investigated. An experimental analysis via annular dark-field scanning transmission electron microscopy (ADF-STEM) and high-resolution transmission electron microscopy (HRTEM) revealed that Ga-FIB milling yields trenches with higher aspect ratios compared to Xe-FIB milling for the selected ion beam parameters (30 kV, 42 pA), while He-FIB induces local lattice disorder. Molecular dynamics (MD) simulations were employed to investigate the milling process, confirming that probe size critically influences trench aspect ratios. Interestingly, the MD simulations also showed that Xe-FIB generates higher aspect ratios than Ga-FIB with the same probe size, indicating that Xe-FIB could also be an effective option for nanoscale patterning. Atomic defects such as vacancies and interstitials in GaN from He-FIB milling were suggested by the MD simulations, supporting the lattice disorder observed via HRTEM. This combined experimental and simulation approach has enhanced our understanding of FIB milling dynamics and will benefit the fabrication of nanostructures via the FIB technique.

Possible Approaches for Combined Use of Xenon and Gallium Ion Sources for Task Specific Focused Ion Beam Sample Preparation

Possible Approaches for Combined Use of Xenon and Gallium Ion Sources for Task Specific Focused Ion Beam Sample Preparation

Threshold voltage modulation in monolayer MoS2 field-effect transistors via selective gallium ion beam irradiation

Electronic regulation of two-dimensional (2D) transition metal dichalcogenides (TMDCs) is a crucial step towards next-generation optoelectronics and electronics. Here, we demonstrate controllable and selective-area defect engineering in 2D molybdenum disulfide (MoS2) using a focused ion beam with a low-energy gallium ion (Ga+) source. We find that the surface defects of MoS2 can be tuned by the precise control of ion energy and dose. Furthermore, the field-effect transistors based on the monolayer MoS2 show a significant threshold voltage modulation over 70 V after Ga+ irradiation. First-principles calculations reveal that the Ga impurities in the monolayer MoS2 introduce a defect state near the Fermi level, leading to a shallow acceptor level of 0.25 eV above the valence band maximum. This defect engineering strategy enables direct writing of complex pattern at the atomic length scale in a controlled and facile manner, tailoring the electronic properties of 2D TMDCs for novel devices.

Pulsed Laser-Assisted Helium Ion Nanomachining of Monolayer Graphene-Direct-Write Kirigami Patterns.

A helium gas field ion source has been demonstrated to be capable of realizing higher milling resolution relative to liquid gallium ion sources. One drawback, however, is that the helium ion mass is prohibitively low for reasonable sputtering rates of bulk materials, requiring a dosage that may lead to significant subsurface damage. Manipulation of suspended graphene is, therefore, a logical application for He+ milling. We demonstrate that competitive ion beam-induced deposition from residual carbonaceous contamination can be thermally mitigated via a pulsed laser-assisted He+ milling. By optimizing pulsed laser power density, frequency, and pulse width, we reduce the carbonaceous byproducts and mill graphene gaps down to sub 10 nm in highly complex kiragami patterns.

A novel specimen preparation of porous cathode materials in lithium-ion batteries for high-resolution transmission electron microscopy

Micropores and fissures in cathode materials are associated with deterioration of electrochemical performance of lithium-ion batteries. Phase transformation is responsible for anisotropic volume change and pulverization in conjunction with an expansion of the pores and fissures, occurred dominantly at the outermost cathode particles where lithium ions are repeatedly diffused from parent structure to anode and vice versa. The gallium ion sources, in addition, penetrate into the vacuum and etch the particle in the vicinity of pores and fissures during the cross-sectioning process using focused ion beam. Herein, we propose a noble specimen preparation for high-resolution transmission electron microscopy (TEM) by employing epoxy to infiltrate into micropores. The epoxy can retain the agglomerate of cathode particles and avoid ion-beam-induced damage and contamination as well as suppress the side reactions with moisture and oxygen even after exposure to the air for a few months. The well-designed specimen preparation enables in-depth TEM investigation on the microstructure and phase transformation occurred severely at the outermost regions of porous cathode particles.

Sectioning studies of biomimetic collagen-hydroxyapatite coatings on Ti-6Al-4V substrates using focused ion beam

A biomimetic bone-like collagen-hydroxyapatite (Col-HA) composite coating was formed on a surface-treated Ti-6Al-4V alloy substrate via simultaneous collagen self-assembly and hydroxyapatite nucleation. The coating process has been carried out by immersing sand-blasted, acid-etched and UV irradiated Ti-6Al-4V alloy in type I collagen-containing modified simulated body fluid (m-SBF). The surface morphology and phase composition of the coating were characterized using various techniques. More importantly, dual-beam FIB/SEMs with either gallium ion source (GFIB) or xenon plasma ion source (PFIB) were used to investigate the cross-sectional features of the biomimetic Col-HA composite coating in great details. As a result, the cross-sectional images and thin transmission electron microscopy (TEM) specimens were successfully obtained from the composite coating with no obvious damages or milling ion implantations. Both the cross-sectional SEM and TEM results have confirmed that the Col-HA coating demonstrates a similar microstructure to that of pure HA coating with homogeneously distributed elements across the whole cross section. Both coatings consist of a uniform, crack-free gradient structure with a dense layer adjacent to the interface between the Ti-6Al-4V substrate and the coating facilitating a strong bonding, while a porous structure at the coating surface aiding cell attachment.

Structure-Performance Relations in Multi-Layer Solid-State Li-Ion Electrolytes Using FIB-Tomography

As demands on battery technology approach the limits of standard organic liquid electrolytes, a multitude of new materials are being developed as potential replacements, including solid-state Li-conducting ceramics. However, the major obstacles to integrating solid electrolytes into Li-ion batteries originate primarily from the relatively low ionic conductivity of the electrolyte itself (compared to organic liquid electrolytes) and from the high interfacial impedance associated with poor electrolyte-electrode contact. Several solid electrolytes, including the garnet-like ceramic Li7La3Zr2O12 (LLZ), have partially addressed the first issue by achieving ionic conductivities of ~1 mS/cm, approaching that of organic liquid electrolytes. With regards to high interfacial impedance, surface chemistry modifications at the electrolyte-electrode interface have greatly improved solid-solid contact with lithium metal anodes, though cathode interfaces remain a significant challenge for solid-state batteries. An especially promising development replaces the typical pellet geometry with a variable-porosity multi-layer structure, resulting in dramatically increased electrolyte-electrode interfacial area, thus decreasing interfacial impedance. Such an approach opens the possibility of tailoring the architecture of the electrolyte microstructure to improve the electrolyte-electrode interface and all-solid battery performance. However, what constitutes the ideal microstructure is currently unknown. At the same time, adopting such a complex structure may introduce unforeseen effects that impact overall electrolyte properties, positively or negatively. Li-ion migration through tortuous electrolyte pathways, cycling of electrode material at varying distances from the electrolyte dense layer, and effective pore filling by the electrodes are some of the new variables that must be accounted for when designing and evaluating such structures. While research on non-planar solid electrolytes exists, the focus has been limited to thin film batteries, with little work undertaken for macro-scale systems. Here, we have varied the porosity of high dimension (>100 um thick) LLZ electrolyte porous-dense-porous multi-layers, characterized the physical microstructure of the layers, and investigated how key structural parameters (e.g., tortuosity, LLZ/pore volume fractions, MAZO angle) related to the electrochemical properties. To study the LLZ microstructure, we utilized focused ion beam (FIB) tomography to reconstruct 3D representations of regions in the electrolyte, from which structural parameters were calculated. Samples to be analyzed were epoxy infiltrated and mounted in a Xenon-plasma source FIB/SEM, where we deposited platinum on the area of interest and milled a trench around the area to expose a cross-section of the LLZ (Figure 1, top). We then moved the sample to a Gallium-ion source FIB/SEM to finish polishing the cross-section (Figure 1, bottom), followed by serial milling-and-imaging to produce the set of images necessary for 3D reconstruction. To evaluate electrolyte performance, we relied on electrochemical impedance spectroscopy (EIS) to determine Li-ion conductivities for the different LLZ multi-layer microstructures.

Focused ion beam sectioning studies of biomimetic hydroxyapatite coatings on Ti-6Al-4V substrates

Biomimetic hydroxyapatite (HA) coating has been used to render titanium implants with excellent osteoconductivity and osseointegration in orthopedic and dental applications. However, to date it has not been possible to reveal the fine details of the coating structure or the coating/substrate interface. In this study, a crack-free biomimetic HA coating has been successfully formed on sand-blasted, acid-etched Ti-6Al-4V discs by simply immersing the discs in a modified simulated body fluid (m-SBF). Dual-beam FIB/SEMs with either gallium ion source (GFIB) or xenon plasma ion source (PFIB) were used to prepare the cross-sections and thin transmission electron microscopy (TEM) specimens of the coating, and the capabilities of GFIB and PFIB were compared. Both the cross-sectional SEM and TEM images confirmed that a well bonded biomimetic HA coating has been formed on the Ti-6Al-4V substrate. The coating exhibits a gradient structure with a dense microstructure adjacent to the titanium substrate to facilitate strong bonding, while there is a porous structure at the surface, which is beneficial to bone cell attachment and subsequent new bone integration.

Ion Beam Characteristics of Liquid Metal (Gallium) Ion Source with Inclusion of Suppressor

Ion Beam Characteristics of Liquid Metal (Gallium) Ion Source with Inclusion of Suppressor

The effect of thin oxide film on protein sample measurement with TOF‐SIMS

AbstractRecently, the biomaterial field has become one of the most important applications of time of flight (TOF) SIMS time‐of‐flight secondary ion mass specmetry (TOF‐SIMS). Although TOF‐SIMS can be applied to insulating samples, electrically conductive substrates such as Si, Au plates and indium‐tin oxide (ITO) glass plates are more useful to detect fragment ions from biomaterials. ITO glass and silicon substrates are often employed for the preparation of biomaterial samples instead of glass plates. In this study, an ITO glass plate and silicon plates with various thicknesses of silicon oxide thin film were compared to investigate the most suitable sample preparation conditions for evaluating protein samples using TOF‐SIMS. Four kinds of substrates, an ITO glass plate, a silicon plate, a silicon plate with an oxide film of 50 nm, and another of 250 nm, were employed as substrates to adsorb a model protein, egg white lysozyme. Each sample was measured five times with TOF‐SIMS with a gallium ion source under static conditions. The TOF‐SIMS spectra of each sample were compared to evaluate the effect of thin oxide films on protein measurement with TOF‐SIMS. The specific peaks of each sample related to lysozyme in the positive ion spectra were selected in comparison to their control samples. As a result, TOF‐SIMS spectra of the silicon plate with a 50 nm silicon oxide film showed the most specific peaks related to lysozyme. In addition, TOF‐SIMS spectra of the ITO sample showed many peaks of fragment ions related to lysozyme in a high‐mass range at which the other samples do not show adequate peaks. Thus, both substrates with the oxide films, silicon oxide and ITO, displayed better results than the silicon plate sample. Copyright © 2010 John Wiley & Sons, Ltd.

Determination of lattice orientation in aluminium alloy grains by low energy gallium ion-channelling

Abstract Polished sections of a fine-grained aluminium, silicon carbide metal matrix composite (MMC) alloy were prepared by sputtering using a low energy gallium ion source and column (FIB). The MMC had been processed by high temperature extrusion. Images of the polished surface were recorded using the ion-induced secondary electron emission. The metal matrix grains were distinguished by gallium ion-channelling contrast from the silicon carbide component. The variation of the contrast from the aluminium grains with tilt angle can be recorded and used to determine lattice orientation with the contrast from the silicon carbide (SiC) component as a reference. This method is rapid and suits site-specific investigations where classical methods of sample preparation fail.

Focused ion beam specimen preparation for off-axis electron holography using Si, Ga, and Au ions

Silicon specimens containing p-n junctions have been prepared by focused ion beam milling using gallium, silicon, and gold ion sources. The specimens have been examined using off-axis electron holography and the total thickness of the electrically ‘inactive’ layer has been measured as 80, 140, and 410±15nm for the Au, Ga, and Si ions, respectively. We also show that the thickness of this damaged layer can be predicted using simulations.

Beam Stability Improvement of a Liquid Metal Ion Source

Previous studies on liquid gallium ion sources used an electrochemically etched tungsten wire with a coil-type heater. Such a structure requires excessive power consumption in the course of heating the liquid metal. In this study, a new structure is proposed that replaces the coil-type heater. It uses a gallium reservoir made of six pre-etched 250 µm tungsten wires that surround the needle electrode. Gallium loading at the reservoir is observed to be much more stable, resulting in an improved beam stability.

SIMS analyses on Co:ns-C thin films

Supersonic cluster beam deposition (SCBD) with a pulsed microplasma cluster source (PMCS) was used to deposit cobalt porous carbon-nanocomposite thin films (Co:ns-C), on silicon substrates at room temperature and under vacuum. The films find applications due to their magnetic properties. Analytical images and mass spectra of secondary ions were obtained using the University of Chicago-scanning ion microprobe (UC-SIM). This instrument, using a liquid gallium ion source and a magnetic sector mass spectrometer, makes it possible to obtain images of secondary ions with a lateral resolution of 50 nm. Mass spectra indicate a low levels of contamination at the surface and inside the film and the presence of metallic cobalt and cobalt oxides. The images show the distribution of Co + and CoO − at different depths. We observe the distribution and the relative size of cobalt and cobalt oxide aggregates inside the film. The capability to control the presence of cobalt is relevant for the films magnetic properties.

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.

Modelling the link between emission current and LMIS cusp length

This paper presents new modelling results on the relationship between emission current and cusp length, for gallium and indium liquid-metal ion sources. The work is based on an improved version of the LMIS modelling program developed by Forbes and Ljepojevic, and achieves enhanced computational speed by means of parallel processing. Good agreement is found between the results derived from numerical simulation and experiment.

Angular distributions of current density in liquid Ga-ion sources

Current-voltage (I-V) characteristics and angular distribution of the current density of a liquid gallium ion source were investigated under various emitting tip temperatures and surface flow impedances. It is shown that the I-V slope is significantly influenced by the flow impedance of the liquid gallium on the emitting tip. The slopes of the I-V curve are shown to be steep for an emitting tip with a rough surface, and gentle for a smooth surface. The angular distribution of Ga ions was determined by measuring the current on metal rings concentric with the emission axis and located so as to form a portion of a hemisphere. The current density was found to be depressed near the center compared to the surrounding region. This density distribution persists for various axial locations of the rings. Moreover, the angular profile of current density at fixed axial locations is found to be identical for the same amount of emission current, irrespective of the tip-surface condition or the applied voltage. It shows that the amount of emission current has a strong correlation with the surface shape of the ion emitting Ga-liquid.

Gallium clusters from a liquid metal ion source

Gallium ion clusters Ga+n with n ranging up to 30 have been measured from liquid gallium ion sources using a time-of-flight spectrometric technique. The observations were made as a function of total ion emission current, angle of emission, and temperature of the liquid metal. Under all conditions Ga+ is the overwhelmingly dominate species emitted. The cluster data reveal several remarkable features. First, emission of clusters tends to be very low below 2 μA emission current, to rise steeply with current to 10 or 20 μA and decrease for higher current. Second, within the range examined, clusters with n=8 and 15 have particularly low abundances. Third, the energy distributions of the resolvable clusters are bimodal with characteristic peaks at approximately 30 and 120 eV below the source potential. Fourth, the angular distributions of the clusters are more nearly axial than that of the primary species. The temperature of the liquid metal has no noticeable effect on cluster emission over the range examined. A possible model for cluster formation will be discussed.

走査型イオン顕微鏡によるイオンプローブの形成と二次電子像

Making use of a liquid metal gallium ion source, a scanning ion microscope (SIM) has been developed which produces ion-induced secondary electron images. The gallium ion source was very stable in operation at 30 kV and its mass spectrum and energy distributions were measured. A probe diameter of the gallium ion beam was verified to be less than 0.1 μm by SEM images of a evaporated Cr film which was line-etched in the SIM. The ion-induced secondary electron images were compared with SEM images. The former more strongly reflect surface structure of specimens.

Optical emission: A probe of neutral atoms in liquid-metal ion sources

The optical emission close to the Taylor cone tips of liquid gallium and gold ion sources has been studied as a function of wavelength and ion current. The optical emission may be interpreted as due to excitations caused by binary collisions, among which Ga-Ga+ collisions play a dominant role. The release rate of neutrals from the source inferred from the optical-emission curves is consistent with the idea of contributions to the total ion current from field evaporation and field ionization, with the latter becoming increasingly important at higher currents. It is difficult to account for the origin of the neutrals by thermal evaporation at the tip.