Focused Ion Beam Machining Research Articles

Synthesis and Mechanical Characterisation of an Ultra-Fine Grained Ti-Mg Composite.

The importance of lightweight materials such as titanium and magnesium in various technical applications, for example aerospace, medical implants and lightweight construction is well appreciated. The present study is an attempt to combine and improve the mechanical properties of these two materials by forming an ultra-fine grained composite. The material, with a composition of 75 vol% (88.4 wt%) Ti and 25 vol% (11.4 wt%) Mg , was synthesized by powder compression and subsequently deformed by high-pressure torsion. Using focused ion beam machining, miniaturised compression samples were prepared and tested in-situ in a scanning electron microscope to gain insights into local deformation behaviour and mechanical properties of the nanocomposite. Results show outstanding yield strength of around 1250 MPa, which is roughly 200 to 500 MPa higher than literature reports of similar materials. The failure mode of the samples is accounted for by cracking along the phase boundaries.

Effect of wear of diamond wire on surface morphology, roughness and subsurface damage of silicon wafers

Wear of fixed abrasive diamond wire affects the quality of sliced silicon wafers, necessitating replacement of the costly wire. This paper analyzes the effect of wire wear on the surface morphology, roughness, and subsurface damage of as-cut single crystal silicon wafers. Scanning electron microscopy, Raman spectroscopy, confocal microscopy, focused ion beam machining (FIB), and biaxial flexure are used to evaluate the surface morphology, areal surface roughness, and subsurface damage (cracks). Results show that, with increased wire wear, the wafers exhibit greater evidence of ductile removal, lower surface roughness, fewer but slightly deeper subsurface cracks, and lower average fracture strength.

Light Extraction From Scintillating Crystals Enhanced by Photonic Crystal Structures Patterned by Focused Ion Beam

“Photonic Crystals (PhC)” have been used in a variety of fields as a structure for improving the light extraction efficiency from materials with high index of refraction. In previous work we already showed the light extraction improvement of several PhC covered LYSO crystals in computer simulations and practical measurements. In this work, new samples are made using different materials and techniques which allows further efficiency improvements. For rapid prototyping of PhC patterns on scintillators we tested a new method using “Focused Ion Beam (FIB)” patterning. The FIB machine is a device similar to a “Scanning Electron Microscope (SEM)”, but it uses ions (mainly gallium) instead of electrons for the imaging of the samples’ surface. The additional feature of FIB devices is the option of surface patterning in nano-scale which was exploited for our samples. Three samples using FIB patterning have been produced. One of them is a direct patterning of the extraction face of a $0.8\times 0.8\times 10~\hbox{mm}^3$ LYSO crystal. The second and third were ${{\rm TiO}_2}$ coated LYSO crystal with a $1.4\times 1.4~\hbox{mm}^2$ out coupling face partly covered with high-quality photonic crystals. ${{\rm TiO}_2}$ has high refractive index (around 2.1 - 2.4)–which shows the best results in our light extraction simulations. In this paper, SEM images of the samples will be presented together with measurements of excited crystals which show unrivalled performance for such scintillator samples.

Electrical characterization of FIB processed metal layers for reliable conductive-AFM on ZnO microstructures

We report on the conductive-atomic force microscopy (C-AFM) study of metallic layers in order to find the most suitable configuration for electrical characterization of individual ZnO micro-pillars fabricated by focused ion beam (FIB). The electrical resistance between the probe tip and both as deposited and FIB processed metal layers (namely, Cr, Ti, Au and Al) has been investigated. Both chromium and titanium evidenced a non homogenous and non ohmic behaviour, non negligible scanning probe induced anodic oxidation associated to electrical measurements, and after FIB milling they exhibited significantly higher tip-sample resistance. Aluminium had generally a more apparent non conductive behaviour. Conversely, gold films showed very good tip-sample conduction properties being less sensitive to FIB processing than the other investigated metals. We found that a reliable C-AFM electrical characterization of ZnO microstructures obtained by FIB machining is feasible by using a combination of metal films as top contact layer. An Au/Ti bilayer on top of ZnO was capable to sustain the FIB fabrication process and to form a suitable ohmic contact to the semiconductor, allowing for reliable C-AFM measurement. To validate the consistency of this approach, we measured the resistance of ZnO micropillars finding a linear dependence on the pillar height, as expected for an ohmic conductor, and evaluated the resistivity of the material. This procedure has the potential to be downscaled to nanometer size structures by a proper choice of metal films type and thickness.

Ion microscopy based on laser-cooled cesium atoms

We demonstrate a prototype of a Focused Ion Beam machine based on the ionization of a laser-cooled cesium beam and adapted for imaging and modifying different surfaces in the few-tens nanometer range. Efficient atomic ionization is obtained by laser promoting ground-state atoms into a target excited Rydberg state, then field-ionizing them in an electric field gradient. The method allows obtaining ion currents up to 130pA. Comparison with the standard direct photo-ionization of the atomic beam shows, in our conditions, a 40-times larger ion yield. Preliminary imaging results at ion energies in the 1–5keV range are obtained with a resolution around 40nm, in the present version of the prototype. Our ion beam is expected to be extremely monochromatic, with an energy spread of the order of the eV, offering great prospects for lithography, imaging and surface analysis.

Performance Evaluation of Novel Micro-textured Tools in Improving the Machinability of Aluminum Alloy (Al 6063)

Ordered array of dots or grooves induces special functionality to the surface. Two types of textures viz. linear (perpendicular to the chip flow direction) and square were developed on plain WC inserts using focused ion beam machining. The inserts were coated with MoS2 solid lubricants using pulsed DC magnetron sputtering. Dry turning tests were carried out on aluminum alloy (Al 6063) work material. Textured tools are found to be more effective in reducing the cutting forces and sticking behaviour of the work material as compared to the non-textured tools. The novel square textured tools performed better than the linear textured tools in terms of reduced cutting forces and improved surface finish. A reduction of about 30% in cutting forces was observed with square textured tools and that with the linear textured tools was about 20% as compared to the non-textured tools. The reduction in cutting forces and the associated change in sticking behaviour are attributed to the reduction in tool-chip contact area and reduced friction at the tool-chip interface owing to the improved lubrication provided by the interlayer of solid lubricant. Textures are functioning as reservoirs of the solid lubricant which in turn reduces the friction at the chip-tool interface.

High resolution determination of local residual stress gradients in single- and multilayer thin film systems

Residual stresses and stress gradients are of great importance in all thin film systems, as they critically influence the structural stability and functionality, and thus the lifetime, of the concerned devices. In this study, an improved ion beam layer removal method is developed to determine the stress distribution in copper- and tungsten-based thin film systems. Cantilevers were prepared from single, bi- and tri-layer systems with an individual layer thickness of 500 nm using focused ion beam machining. Subsequently, residual stress profiles were determined with a depth resolution of 50 nm, employing the ion beam layer removal method. We observe that the evaluated average film stresses correspond to state-of-the-art X-ray diffraction measurements. However, depending on the layer order, different stress profiles with strong stress gradients evolve, and pronounced changes in residual stress occur across an interface within only few grains. These novel findings have profound implications when addressing the interface adhesion, fracture properties and reliability of novel thin film systems, as well as interface dominated materials in general.

Effects of coating material on the fabrication accuracy of focused ion beam machining of insulators

Focused ion beam (FIB) machining of insulators is a crucial process in the rapid prototyping of nanodevices for optical applications. A conductive material is generally coated on the insulator prior to FIB machining to achieve high fabrication accuracy. In this paper, we report on the effects on machining accuracy of four coating materials: Pt, Ni, Ag, and Co. The dimensional accuracy at channel sidewalls was improved by selecting a coating material that induces charge-carrier generation in a small range. The geometric and electrical characteristics of the FIB-machined surfaces were evaluated to elucidate the association between the fabrication accuracy and the range of charge-carrier distribution.

Extended elastic region in nanocrystalline HCP and FCC metals under nano-tension loading

Nano-scaled mechanical properties have become important for abundant submicron and nano electronic or mechanical devices. However, the nano-scale mechanical tests have not been developed widely. In this study, a new nano-tension test specimen is designed, using the focused ion beam (FIB) machining. The dimension of the nano-tension gage section can vary from 50 to 1000nm. The test can be conducted by the widely available nano-indentation facility with a flat tip. The downward applied load from the flat tip onto the central beam (in compression) can be easily transferred to the two-side tension gage sections (in tension). Various pure metals such as face-centered cubic Al and Cu and hexagonal close-packed Mg, Zr, and Ti are selected for the test run. The yield stress and elastic strain are seen to be exceptionally high. The extended elastic region is carefully examined by reproducible experiments and molecule dynamic simulation. It is not considered to be an artifact by FIB machining, but appears to be a common phenomenon for metallic materials in the nano scale when the dislocation activity is difficult to be fully functioned.

Effects of current on early stages of focused ion beam nano-machining

In this report we investigate the effects of focused ion beam machining at low doses in the range of 1015–1016 ions cm−2 for currents below 300 pA on Si(100) substrates. The effects of similar doses with currents in the range 10–300 pA were compared. The topography of resulting structures has been characterized using atomic force microscope, while crystallinity of the Si was assessed by means of Raman spectroscopy. These machining parameters allow a controllable preparation of structures either protruding from, or recessed into, the surface with nanometre precision.

Controlling the optical fiber output beam profile by focused ion beam machining of a phase hologram on fiber tip.

A phase hologram was machined on an optical fiber tip using a focused ion beam (FIB) system so that a ring-shaped beam emerges from the fiber tip. The fiber used for this work was a commercial single-mode optical fiber patch cable for a design wavelength of 633nm with a germanosilicate core. The ring-shaped beam was chosen to ensure a simple geometry in the required phase hologram, though the Gerchberg-Saxton algorithm can be used to calculate a hologram for an arbitrary beam shape. The FIB machining took approximately 45 min at 30kV and 200pA. The radius of the resulting ring beam was 0.083m at 1m standoff, as compared to 0.1m as was initially desired. Results suggest that this imaging technique may provide a basis for a beam-shaping method with several advantages over the current commercial solutions, having permanent alignment, compactness, and mechanical robustness. However, it would appear that minimizing the speckle pattern will remain a critical challenge for this technique to become widely implemented.

Localized mechanical property assessment of SiC/SiC composite materials

SiC fiber-reinforced SiC matrix composites (SiC/SiC) are under consideration as a structural material for a range of nuclear applications. While these materials have been studied for decades, recently new small scale materials testing techniques have emerged which can be used to characterize SiC/SiC materials from a new perspective. In this work cross section nanoindentation was performed on SiC/SiC composites revealing that both the hardness and Young’s modulus was substantially lower in the fiber compared to the matrix despite both being SiC. Using scanning electron microscopy it was observed that the grain growth of the matrix during formation was radially out from the fiber with a changing grain structure as a function of radius from the fiber center. Focused ion beam machining was used to manufacture micro-cantilever samples and evaluate the fracture toughness and fracture strength in the matrix as a function of grain orientation in the matrix. Additionally microstructural characterization techniques like Raman spectroscopy, X-ray diffraction, and microtomography were used to evaluate differences in the matrix and fibers of the composite.

In situ cracking of silica beads in the SEM and TEM — Effect of particle size on structure–property correlations

Stöber Fink Bohn (SFB) silica is a well-known colloidal system: monodisperse spherical particles are readily obtained by this wet chemical synthesis route. Despite the frequent application of SFB silica, its mechanical properties are not comprehensively understood. Within this account, size dependent mechanical properties of single SFB particles are systematically investigated using in situ SEM and TEM techniques: particle properties are addressed in the size range of 1.5μm down to 200nm. Both, untreated particles and heat treated particles densified at 1000°C are investigated. Structural characterization by vibrational spectroscopy, by solid state nuclear magnetic resonance spectroscopy, by scanning electron microscopy and by focused ion beam machining allows correlating mechanical properties and the internal particle structure. For untreated particles comprised of a weakly cross-linked silica network a high degree of plasticity associated with low Young's moduli and hardness values is found. At large strains only cracking without full fracture of the particles occurs, whereby the crack path is clearly linked to the synthesis conditions: for larger particles obtained from a multistep growth process cracks propagate along weak intraparticle interfaces and are deflected around an inner core structure. After the thermal treatment cross-linking of the silica network is significantly enhanced and Young's moduli comparable to vitreous silica accompanied by increased hardness values are found. Only the smallest particles exhibit cracking; fracture into two or more pieces is distinguished for particles above 500nm. It is found by SEM imaging that the relative occurrence of cracking into several pieces is increasing with size — a behaviour associated with the brittle to ductile transition of silica. By testing a large quantity of particles, statistical significance is ensured.

Corrosion pitting and environmentally assisted small crack growth

In many applications, corrosion pits act as precursors to cracking, but qualitative and quantitative prediction of damage evolution has been hampered by lack of insights into the process by which a crack develops from a pit. An overview is given of recent breakthroughs in characterization and understanding of the pit-to-crack transition using advanced three-dimensional imaging techniques such as X-ray computed tomography and focused ion beam machining with scanning electron microscopy. These techniques provided novel insights with respect to the location of crack development from a pit, supported by finite-element analysis. This inspired a new concept for the role of pitting in stress corrosion cracking based on the growing pit inducing local dynamic plastic strain, a critical factor in the development of stress corrosion cracks. Challenges in quantifying the subsequent growth rate of the emerging small cracks are then outlined with the potential drop technique being the most viable. A comparison is made with the growth rate for short cracks (through-thickness crack in fracture mechanics specimen) and long cracks and an electrochemical crack size effect invoked to rationalize the data.

A haptic digital tool to assist the design, planning and manufacture of micro- and nanostructures

This work addresses the engineering demands for precise interactive process planning for the manufacture of high precision in the manufacturing of 3D micro- and nanoproducts. It outlines the development of a digital tool based on a haptics-based human–machine interface. This interface is underpinned using an empirical model derived from a series of experimental results obtained from a focused ion beam machine through a series of experiments, which produced the surface topographies of a number of micro- and nanograting arrays; these were systemically analysed to provide an underpinning empirical database, which could be accessed via the haptic process planning system. On accessing these data, highly accurate geometrical models are then input into a haptics human machine interface to emulate the focused ion beam machining process as defined by the customised fabrication parameters. The haptic planning system predicts and simulates the shape and form of the finished product profiles as the haptic device is passed across a virtual substrate. Once the simulation is complete and the outputs verified by the user, the system then automatically outputs a stream file to drive the focused ion beam process to produce the surface topographies required. The simulation results reveal that compared with trial and error method, the haptics-based simulation method is a reliable way to plan and optimise the fabrication parameters in design and manufacture of micro- and nanogratings by focused ion beam machining cost-effectively. This could potentially provide substantial lead time and cost reductions associated with these currently inherently expensive unit cost products as well as demonstrate a novel tool not only for the planning of nanomachining cutting sequences but also for the interactive design of such products. This work also highlights the benefits of an empirical model as a means of underpinning and supporting real-time haptics interaction, something not possible using more numerical-based techniques.

The effects of dwell time on focused ion beam machining of silicon

In this study, the effects of dwell time on Ga+ focused ion beam machining at 30keV for different milling currents were investigated. The surface topographies were analysed using atomic force microscopy (AFM) and the substrate structures were investigated by means of Raman spectroscopy. It has been observed that by increasing dwell time the total sputtering yield was increased even though the total dose was remained the same. Also the silicon damage by ion bombardment is reduced as the dwell time is increased. This is mainly due to catalyst behaviour of Ga inside Si which over a period of hours causes recrystallization of Si at room temperature by lowering the activation energy for crystallization.

G0310201 多結晶純チタン表面に設けた平行溝間およびV字断面溝内微小領域の圧縮塑性変形(材料力学部門一般セッション:変形・ひずみ(2))

The compressive deformation behavior in micro areas of industrial pure titanium was investigated, by using a new method in which the deformation constraints from the neighboring areas were reduced by fabricating micro parallel grooves or V-shaped cross-section grooves, by focused ion beam machining. Uniaxial compression test was performed in the direction parallel to the grooves, and the deformation behavior in the areas between the grooves was observed by scanning electron microscopy. The parallel grooves were fabricated to across the deformation twin that appeared by pre-compression, and then the influence of deformation constraint on twinning was discussed. Meanwhile, the V-shaped cross-section grooves were fabricated on several grains to examine the crystal orientation dependency of the deformation of the micro areas. In both cases, the possibility was found to change the primary deformation mechanism in the areas by fabricating micro grooves.

Scale-like cantilever cell traps

The micro-domain provides excellent conditions for performing biological experiments on small populations of cells and has given rise to the proliferation of so-called lab-on-a-chip devices. In order to fully utilize the benefits of cell assays, means of retaining cells at defined locations over time are required. Here, the creation of scale-like cantilevers, inspired by biomimetics, on chemically robust planar silicon nitride (Si3N4) film using focused ion beam machining is described. Using SEM and optical profilometry imaging, regular tilting of the cantilever with almost no warping of the cantilever was uncovered. With Monte Carlo simulation, it was found that the ion implantation in the film was limited to tens of nanometers, and SEM imaging confirmed that the ion beam milling edges were smooth. Finite element analysis showed that the scale-like cantilever was best at limiting stress concentration without difficulty in manufacture and having stresses more evenly distributed along the edge. It also had a major advantage in that the degree of deflection could be simply altered by changing the central angle. From a piling simulation conducted, it was found that a random delivery of simulated particles on to the scale-like obstacle should create a triangular collection. In the experimental trapping of polystyrene beads in suspension, the basic triangular piling structure was observed, but with extended tails and a fanning out around the obstacle. This was attributed to the aggregation tendency of polystyrene beads that acted on top of the piling behavior. In the experiment with bacterial cells, triangular pile up behind the cantilever was absent and the bacteria cells were able to slip inside the cantilever's opening despite the size of the bacteria being larger than the gap. Overall, the fabricated scale-like cantilever architectures offer a viable way to trap small populations of material in suspension.

Ultrathin metallic subtractive color filters based on surface plasmon primitives

Based on the surface plasmon primitives, subtractive color filters fabricated from ultrathin metallic film nanogratings, have many advantages, such as reliability, easy fabrication and high transmittance. In this work, color filter behaviors of films of four metals, i.e., silver, copper, aluminum and nickel, with one-dimensional nanogratings are systematically studied. We obtain their transmission spectra, the chromaticity coordinates and the empirical formulas of the relationship of the wavelength of transmission minimum with film thickness and period. It is found that nickel has a poor ability to select waves, which is not an ideal material for color filter. Silver, copper and aluminum perform better in selecting the wavelength. They have different transmission spectra, specifically, silver has a wide range of wavelength selection, copper is appropriate for the long wavelength filtering, and aluminum for short wavelength filtering. To verify the simulation results, we fabricate some one-dimensional nanogratings with different materials by using a direct-current magnetron sputtering system and focused ion beam machine. The experimentally observed colors for all fabricated Ag, Cu, and Al film nanogratings are consistent with their simulation results. Both simulation and experimental results reveal that different metal materials and different nanostructures exhibit lager differences in color filter behavior. The obtained results here are useful for designing desirable color filters by selecting the appropriate material and nanostructure.

Improving machining performance of single-crystal diamond tools irradiated by a focused ion beam

• Machining performance of the FIB irradiated tool was improved by the heat treatment. • Gallium ions were removed by the proposed heat treatment. • Wear resistance of the FIB affected area was inferior to the non-irradiated area. Tool shape is an important factor determining the shape and accuracy of machined areas in ultra-precision machining. Use of a focused ion beam (FIB) is an effective means to fabricate micro- to submicro-scale tool shapes. However, ion irradiation causes doping and defects in the tool that reduce tool performance. To use FIB machining on a single-crystal diamond tool without degrading tool performance, a combination of 500 °C heat treatment and aluminum deposition was used to remove gallium (Ga) ions induced by ion irradiation. The method was evaluated through machining experiments that showed that irradiation of Ga ions causes work materials to adhere to the tool surface. This adhesion and the resulting rapid tool wear were reduced by heat treatment. The proposed method also improved the transcription ability and wear resistance of the tool so it was capable of producing a surface quality better than or equal to that produced by non-irradiated tools, even over long cutting distances.