Focused Ion Beam Sputtering Research Articles

In Situ Focused Ion Beam Redeposition Surface Coatings for Site-Specific, Near-Surface Characterization by Atom Probe Tomography.

Atom probe tomography (APT) enables three-dimensional chemical mapping with near-atomic scale resolution. However, this method requires precise sample preparation, which is typically achieved using a focused ion beam (FIB) microscope. As the ion beam induces some degree of damage to the sample, it is necessary to apply a protective layer over the region of interest (ROI). Herein, the use of redeposition, a (frequently considered negative) side effect of FIB sputtering, is explored as a technique for targeted surface coatings in site-specific, near-surface APT investigations. In addition, the concept of "self-coating" is presented, which is the application of a capping layer using material from the same, or a similar, sample. It is shown to provide a pathway for high-quality coatings, as well as a method of minimizing the field evaporation threshold difference at the cap-sample interface, thus greatly reducing the likelihood of premature fractures. In situ redeposition surface coatings are shown to be versatile, with four materials used in the coating and analysis of two Si-based semiconductors and a Fe-Mn alloy. Several factors are discussed, such as the specimen yield, the capping layer quality, and the ease of ROI identification, all of which demonstrate its effectiveness in routine sample preparation workflows.

Level set simulation of focused ion beam sputtering of a multilayer substrate.

The evolution of a multilayer sample surface during focused ion beam processing was simulated using the level set method and experimentally studied by milling a silicon dioxide layer covering a crystalline silicon substrate. The simulation took into account the redeposition of atoms simultaneously sputtered from both layers of the sample as well as the influence of backscattered ions on the milling process. Monte Carlo simulations were applied to produce tabulated data on the angular distributions of sputtered atoms and backscattered ions. Two sets of test structures including narrow trenches and rectangular boxes with different aspect ratios were experimentally prepared, and their cross sections were visualized in scanning transmission electron microscopy images. The superimposition of the calculated structure profiles onto the images showed a satisfactory agreement between simulation and experimental results. In the case of boxes that were prepared with an asymmetric cross section, the simulation can accurately predict the depth and shape of the structures, but there is some inaccuracy in reproducing the form of the left sidewall of the structure with a large amount of the redeposited material. To further validate the developed simulation approach and gain a better understanding of the sputtering process, the distribution of oxygen atoms in the redeposited layer derived from the numerical data was compared with the corresponding elemental map acquired by energy-dispersive X-ray microanalysis.

Warp-Free TEM Sample Preparation Methods Using FIB/SEM Systems.

Warping is a limiting factor when preparing transmission electron microscopy (TEM) samples using focused ion beam (FIB)/scanning electron microscope (SEM) systems. The conventional FIB sputtering process leaves at least one side of the lamella too thin to provide structural support to offset inherent stresses. As a result, warping can occur impacting imagining and reducing the potential size of lamellae. For example, capturing more than a few back-end metal layers in a 3 μm wide cross-section lamella can be difficult. Frequently, TEM analysts must perform multiple stage adjustments to analyze such a sample. In this paper, two methods are presented that enable FIB/SEM operators to prepare TEM samples where the thinned region of interest is surrounded by thick structures. As a result, these methods reduce warping and enable the fabrication of TEM lamellae not possible by conventional means. For example, these methods have been used to produce a 10 μm wide by 8 μm tall cross-section TEM sample that captured front-end transistors and 14 back-end metal layers. Furthermore, warping was so limited that only one alignment was needed by the TEM analyst to complete the imaging of the sample. The methods, called the horizontal bracing and window methods, make use of the deposition of low-Z amorphous films that are electron transparent in the TEM.

Study of silicon dioxide focused ion beam sputtering using electron microscopy imaging and level set simulation

Sputtering silicon dioxide by the ion beam of gallium ions was studied to obtain the angular dependence of the sputtering yield and estimate the secondary sputtering yield of the redeposited material. For this purpose, rectangular boxes were formed in the SiO2 layer at different incidence angles of the ion beam. The box depths and the distributions of gallium atoms were estimated using the cross-sectional specimens examined by scanning and transmission electron microscopy techniques. The found depth values enabled calculating the sputtering yield angular dependence which was approximated using well-known fitting formulas. It was also demonstrated that for an incidence angle varied from approximately 40°–70° ripple topography is formed on the ion bombarded silicon dioxide. To validate the obtained data characterizing SiO2 sputtering, test symmetrical and asymmetric structures with different aspect ratios were experimentally prepared and simulated using the level set method taking into account the redeposition effect and reflection of incident ions from the sputtered surface. It was established that the performed simulation provides sufficient accuracy in predicting the shape of the focused ion beam fabricated structures in silicon dioxide.

Machining Simulation in Focused Ion Beam Sputtering

Abstract Focused ion beam (FIB) has been applied to micro/nanometer-scale fabrication to control surface functions with the surface topographies. Although the resolution of the FIB sputtering is in the nanometer-scale range in positioning, the removal shape in the depth direction cannot be controlled numerically. This study presents a removal model to predict the surface profile in the simulation. The removal rate depends on not only the ion beam intensity but also the incident angle onto the surface to be structured. The removal model considers the effects of those two parameters to control the surface profile in sputtering. The removal rates in sputtering of the inclined surfaces at incident angles are associated with a Gaussian distribution. The parameters in the model were identified to minimize the simulation error validated against the sputtering tests. The presented model was applied to simulate the microscale structures on surfaces using the identified parameters. The simulation was validated in comparison with the actual machined shapes.

3-D SRIM Simulation of Focused Ion Beam Sputtering with an Application-Oriented Incident Beam Model

Ion beam sputter etching has been widely used in material surface modification and transmission electron microscope (TEM) sample preparation. Due to the complexity of the ion beam etching process, the quantitative simulation of ion beam sputtering is necessary to guarantee precision in surface treatment and sculpting under different energies and beam currents. In this paper, an application-oriented incident ion beam model was first built with aberrations and Coulomb repulsion forces being considered from the Ga ion source to the sample. The sputtering process of this model on the sample was then analyzed and simulated with an improved stopping and range of ions in matter (SRIM) program. The sputtering performance of this model, the point-like incident beam and the typical Gaussian incident beam was given in the end. Results show that the penetration depth of Ga ions having 30 keV energy in silicon is 28 nm and the radial range is 29.6 nm with 50 pA beam current. The application-oriented model has been verified by our focused ion beam-scanning electron microscopy (FIB-SEM) milling experiment and it will be a potential thermal source in simulating the process of FIB bombarding organic samples.

Monte Carlo simulation of nanoscale material focused ion beam gas-assisted etching: Ga+ and Ne+ etching of SiO2 in the presence of a XeF2 precursor gas.

Elucidating energetic particle-precursor gas–solid interactions is critical to many atomic and nanoscale synthesis approaches. Focused ion beam sputtering and gas-assisted etching are among the more commonly used direct-write nanomachining techniques that have been developed. Here, we demonstrate a method to simulate gas-assisted focused ion beam (FIB) induced etching for editing/machining materials at the nanoscale. The method consists of an ion–solid Monte Carlo simulation, to which we have added additional routines to emulate detailed gas precursor–solid interactions, including the gas flux, adsorption, and desorption. Furthermore, for the reactive etching component, a model is presented by which energetic ions/target atoms, and secondary electrons, transfer energy to adsorbed gas molecules. The simulation is described in detail, and is validated using analytical and experimental data for surface gas adsorption, and etching yields. The method is used to study XeF2 assisted FIB induced etching of nanoscale vias, using both a 35 keV Ga+, and a 10 keV Ne+ beam. Remarkable agreement between experimental and simulated nanoscale vias is demonstrated over a range of experimental conditions. Importantly, we demonstrate that the resolution depends strongly on the XeF2 gas flux, with optimal resolution obtained for either pure sputtering, or saturated gas coverage; saturated gas coverage has the clear advantage of lower overall dose, and thus lower implant damage, and much faster processing.

Molecular dynamic simulation of orientation-dependent effect on silicon crystalline during sputtering process of focused ion beam

In this paper, experiments and molecular dynamic simulation are performed to investigate the crystal orientation-dependent effects in a focused ion beam (FIB) sputtering etching process on silicon crystal planes. The experimental results indicate that different silicon orientation planes have different etched profiles, aspect ratios on microstructures, and sputtering yields under the same process conditions, which brings difficulty in deciding the process parameters for the fabrication of high-resolution nanostructures. Molecular dynamic simulation based on empirical potential is carried out to explain this effect on an atomic scale. The sputtered profiles of different silicon crystal planes after hundreds of impacts by Ga+ ions are analyzed. Both the simulation and experimental results show that with the same FIB etching conditions, structures on an Si (111) plane have the maximum opening width and minimum depth. Compared with the etched profile on an Si (100) plane, trenches on Si (110) have a smaller width, but greater depth. The Si (111) plane has the minimum sputtering yield. Furthermore, simulation results on the distribution of Ga+ ions remaining inside the substrate, number of ions sputtered out of the surface, and common neighbor analysis (CNA) can explain the orientation-dependent effect during FIB sputtering etching.

Gold, copper and gold/copper bimetallic nanoparticles obtained by focused ion beam sputter deposition and rapid thermal annealing

Gold, copper and gold/copper thin layers were obtained in FIB/SEM system by Focused Ion Beam sputter deposition. Layers deposited on SiO2/Si and quartz substrates were subjected to Rapid Thermal Annealing (RTA) in vacuum at 1100 °C. One minute annealing of 20 nm thick films leads to formation of nanoparticles with average size of ∼20 nm but does not change the morphology of 170 nm Au and 180 nm Cu films. Implantation of Ga ions into SiO2/Si substrate before deposition of ∼20 nm Au layers has a strong effect on the nanostructures formed after 1 min annealing. Implantation at 30 kV for 1 and 2 min leads to formation of arbitrary shape nanoislands, which are much larger than the nanoparticles outside of the implanted region. Increase of the implantation time to 4 and 16 min leads to formation of large spheres, the size of which increases with the implantation time. The observed Ga effect is explained by the lower melting temperatures of Au/Ga and Cu/Ga binary alloys and by increased nanoparticle mobility, which favors the coalescence process. Gold nanoparticles with size of ∼20 nm obtained by RTA on control quartz substrate showed local surface plasmon resonance at 535 nm.

Visualizing ion transport mechanisms through oxide scales grown on mixed nickel- and cobalt-base model alloys at 900 °C using FIB-SIMS techniques

A set of five model superalloys with a γ′-precipitate strengthened structure and a nominal composition of Ni-xCo-9Al-8 W-8Cr were oxidized at 900 °C by the two-step oxidation method in normal oxygen and tracer oxygen-18 enriched gas atmospheres. Analysis at a nanoscale resolution by gallium focused ion beam sputtering and quadrupole-based secondary ion mass spectrometry of the surface oxide cross-sections provided evidence of both anion and cation motion during the second oxygen-18 oxidation where parabolic oxidation rate conditions can be assumed. These experiments provide new insights into the mass transport vectors for modeling the important oxidation mechanisms at extreme service conditions in mixed cobalt- and nickel-base alloys.

Monte Carlo simulations of secondary electron emission due to ion beam milling

The authors present a Monte Carlo simulation study of secondary electron (SE) emission resulting from focused ion beam milling of a copper target. The basis of this study is a simulation code which simulates ion induced excitation and emission of secondary electrons, in addition to simulating focused ion beam sputtering and milling. This combination of features permits the simulation of the interaction between secondary electron emission, and the evolving target geometry as the ion beam sputters material. Previous ion induced SE Monte Carlo simulation methods have been restricted to predefined target geometries, while the dynamic target in the presented simulations makes this study relevant to image formation in ion microscopy, and chemically assisted ion beam etching, where the relationship between sputtering, and its effects on secondary electron emission, is important. The authors focus on a copper target and validate the simulation method against experimental data for a range of noble gas ions, ion energies, ion/substrate angles, and the energy distribution of the secondary electrons. The authors then provide a detailed account of the emission of secondary electrons resulting from ion beam milling; the authors quantify both the evolution of the yield as high aspect ratio valleys are milled, as well as the emission of electrons within these valleys that do not escape the target, but which are important to the secondary electron contribution to chemically assisted ion induced etching.

Research on the Process Model for Focused Ion Beam Sputtering Etching Micro/Nanofabrication

Research on the Process Model for Focused Ion Beam Sputtering Etching Micro/Nanofabrication

Apparent beam size definition of focused ion beams based on scanning electron microscopy images of nanodots

In this paper, the new term apparent beam size of focused ion beam (FIB) is introduced and an original method of its evaluation is demonstrated. Traditional methods of measuring the beam size, like the knife edge method, provide information about the quality of the beam itself, but practically, they do not give information on the FIB sputtering resolution. To do this, it is necessary to take into account the material dependent interaction of the beam with the specimen and the gas precursor in the vacuum chamber. The apparent beam size can be regarded as the smallest possible dot that FIB can sputter in a given specimen. The method of evaluating it, developed in this paper, is based on the analysis of a series of scanning electron images of FIB produced nanodots. Results show that the apparent beam size can be up to five times larger than the actual physical size of the beam and it is significantly influenced by the presence of gas precursor. It is also demonstrated that the apparent beam size can be used as a reference value for optimization of the beam step during raster scanning.

Analysis of residual stresses and wear mechanism of HF-CVD diamond coated cemented carbide tools

Chemical vapour deposition (CVD) diamond coated tools have demonstrated their potential for the machining of difficult to machine materials in the last 10 years. The lack of adequate coating adhesion in many cases remains an issue and often leads to spontaneous coating delamination and sudden tool failure however. The work described in this paper was undertaken with the aim of understanding the influence of residual stresses and coating quality on the coating adhesion during the machining of aluminium alloys. Residual stresses were determined using Raman spectroscopy as well as X-ray diffraction analysis and a comparison of the results obtained using the two methods is given. Raman spectroscopy was also implemented to analyse the coating quality. Furthermore, the failure mechanism of CVD diamond coated tools was studied using cross-sections prepared by focused ion beam sputtering.

Superconductivity observed in platinum-silicon interface

We report the discovery of superconductivity with an onset temperature of ∼0.6 K in a platinum-silicon interface. The interface was formed by using a unique focused ion beam sputtering micro-deposition method in which the energies of most sputtered Pt atoms are ∼2.5 eV. Structural and elemental analysis by transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy reveal a ∼ 7 nm interface layer with abundant Pt, which is the layer likely responsible for the superconducting transport behavior. Similar transport behavior was also observed in a gold-silicon interface prepared by the same technique, indicating the possible generality of this phenomenon.

Micro machining for control of wettability with surface topography

A micro fabrication is presented to manufacture hydrophobic surfaces with micro-scale structures. Hydrophobicity is controlled with the shape and the alignment of micro pillars in the structure. The structures are manufactured in large areas at high production rates in the following processes: (1) the structure is fabricated on a tool by focused ion beam sputtering; (2) the reverse structure is formed on a metal plate by incremental stamping using the structured tool; and (3) the structure is transferred onto plastic plates by molding. A consecutive stamping is also proposed to fabricate several structures on a surface accurately with a structured tool, in which the moving pitch of the structured tool is numerically controlled. The effect of the surface topography on hydrophobicity is discussed with measuring contact angles on the structured surfaces in the water droplet tests. Hydrophobicity on the plastic plate is associated with the solid fraction on the structured surface based on the Cassie–Baxter model. A larger contact angle is observed for a smaller solid fraction of the surface.

Large area microcorrals and cavity formation on cantilevers using a focused ion beam

The authors utilize a focused ion beam (FIB) to explore various sputtering parameters in order to form large area microcorrals and cavities on cantilevers. Microcorrals were rapidly created by modifying ion beam blur and overlaps. Modifications of the FIB sputtering parameters affect the periodicity and shape of the corral microstructure. Cantilever deflections show ion beam amorphization effects as a function of the sputtered area and cantilever base cavities with or without side walls. The FIB sputtering parameters address a method for the rapid creation of a cantilever tensiometer with integrated fluid storage and delivery.

Influence of crystal orientation on pattern formation of focused-ion-beam milled Cu surfaces

The erosion profiles of Cu surfaces after focused ion beam sputtering have been investigated as a function of crystal orientation and ion beam incidence. We find that all patterns are aligned with crystallographic axes and have wavelengths of about $0.5$ \ensuremath{\mu}m. The patterns depend strongly on the crystal orientation, typically with similar patterns for neighboring orientations, but may also be influenced by the ion beam direction. For orientations close to ${100}$, we find however that surfaces stay smooth for all incidence angles. The results are discussed in the context of current continuum models and indicate that modifications to the models are required to account for the effect of crystal orientation.

Effects of the Nanostructuring of Gold Films upon Their Thermal Stability

We report results relating to the thermal stability of nanoparticles and show a remarkable effect of nanostructuring of the metal. Au films are nanostructured by focused ion beam sputtering (FIB) to produce isolated areas of metal, which are imaged by atomic force microscopy (AFM). Images of the surface show that, if the islands are made small enough, the metal in the islands is lost by evaporation, whereas the nonfabricated areas outside are relatively stable and the nanoparticles remain present there.

A review of focused ion beam sputtering

This paper reviews the applications of focused ion beam (FIB) sputtering for micro/nano fabrication. Basic principles of FIB were briefly discussed, and then empirical and fundamental models for sputtering yield, material removal rate, and surface roughness were presented and compared. The empirical models were more useful for application compared to fundamental models. Fabrication of various micro and nano structures was discussed. Trimmed atomic force microscope (AFM) tips were tested in measurement and imaging of high aspect ratio nanopillars where higher accuracy and clarity were observed. Micromilling tool fabricated using FIB sputtering was used to machine microchannels. Slicing and dwell time control approaches on FIB sputtering were presented for the fabrication of three dimensional microcavities. The first approach is preferred for practical applications. The maximum aspect ratio of 13:1 of the microstructures was achieved. The minimum size of the nanopore was in the range of 2–10 μm. Cavities of microgear of 70 μm outside diameter were sputtered with submicrometer accuracy and 2–5 nm average surface roughness. The microcavities were then filled with polymer in a subsequent micromodling process. The replicated microcomponents were inspected with scanning electron microscope where faithful duplication of accuracy and surface texture of the cavity was observed.