Focused Ion Beam Fabrication Research Articles

Raman characterization of focused ion beam fabricated lithium niobate film

Lithium niobate (LiNbO3, LN) on insulator (LNOI) has emerged as a promising platform for integrated photonics, due to the strong optical confinement and excellent nonlinear optical and electrical characteristics of LN crystal. Focused ion beam (FIB) as a versatile technique has become a typical way to fabricate LNOI microstructures. However, due to the bombardment of Ga+ ions during fabrication, the LN lattice is damaged, which would deteriorate the performance of LNOI devices. Thus, the recognition of the damage formed by FIB fabrication is necessary for the control and optimization of the properties of LN microstructures. However, previous studies on this issue have not delved into it from a lattice perspective. Here, we conducted an investigation into the damage inflicted on LN film due to FIB fabrication and the subsequent impact of buffered oxide etching (BOE) treatment on this damage using Raman spectroscopy. Our findings indicate that the interaction between Ga+ ions and LN film results in lattice amorphization, as well as a reduction in the ionic bonding and lattice stress within LN. Furthermore, absorption spectra were acquired both before and after BOE treatment, revealing the emergence of additional optical losses attributed to Ga+ ions. Notably, these defects responsible for optical losses are predominantly concentrated near the surface of the milled LN film, and BOE treatment proves efficient in their removal. This study contributes to further understanding of the defect structure in LN film after FIB fabrication, as well as repairing the damage and, thus, improving the performance of LN microstructures.

Molecular dynamics study of the tensile properties of gold nanocrystalline films irradiated by gallium Ions

Understanding the effects of heavy ion irradiation on the tensile properties of metals at the nanoscale is important for the development of nanomechanical and other nanodevices using focused ion beam fabrication. This is both in terms of achieving the expected performance of designed devices and in potentially exploiting changes in mechanical properties to develop novel device concepts. We report investigation of the tensile properties of gold nanocrystalline films bombarded by high-energy Ga ions using molecular dynamics calculations. Ga ion irradiation leads to multiple grains with the redistribution of the grain boundaries (GBs). Importantly, Ga ion irradiation causes the film to become less ductile and more prone to fracture. The main reason is not surface damage caused by the sputtering effect of high-energy particles, but the convergence of GBs to the surface crater. On the other hand, for porous films with micropores in the GBs, irradiation results in the hardening of film and the ultimate tensile strength is larger than that of unirradiated counterpart.

Cryo-electron tomography on focused ion beam lamellae transforms structural cell biology.

Cryogenic electron microscopy and data processing enable the determination of structures of isolated macromolecules to near-atomic resolution. However, these data do not provide structural information in the cellular environment where macromolecules perform their native functions, and vital molecular interactions can be lost during the isolation process. Cryogenic focused ion beam (FIB) fabrication generates thin lamellae of cellular samples and tissues, enabling structural studies on the near-native cellular interior and its surroundings by cryogenic electron tomography (cryo-ET). Cellular cryo-ET benefits from the technological developments in electron microscopes, detectors and data processing, and more in situ structures are being obtained and at increasingly higher resolution. In this Review, we discuss recent studies employing cryo-ET on FIB-generated lamellae and the technological developments in ultrarapid sample freezing, FIB fabrication of lamellae, tomography, data processing and correlative light and electron microscopy that have enabled these studies. Finally, we explore the future of cryo-ET in terms of both methods development and biological application.

Observation of Retained γ Grains in TRIP Steels Using SEM-FIB/EBSD Method and Examination of Stability Evaluation Method

Morphologies of retained γ grains in TRIP steels are a very important factor for understanding the relationship between the microstructure and mechanical properties of TRIP steels. We have investigated a serial sectioning technique using a scanning electron microscope (SEM) equipped with an electron backscatter diffraction (EBSD) and a focused ion beam (FIB) to reconstruct 3D microstructure of TRIP steels. In this paper, FIB fabrication condition dependences of reconstructed 3D structures are presented in TRIP steels and γ-grains stabilities are discussed in terms of ion beam effects. Retained γ-grains are not recognized by EBSD map from a FIB fabricated surface using normal ion accelerating voltage of 30 kV, because γ grains are easily transformed into martensite due to ion irradiation. It was found that the transformation was suppressed by using a low acceleration voltage of 7 kV or less, and the three-dimensional morphology could be observed using low acceleration voltage serial sectioning technique. It becomes possible to evaluate morphology of each phase and we proposed evaluation method of γ stabilities by ion irradiation transformation.

Formation of a three-dimensional bottle beam via an engineered microsphere

In this work, we propose a novel approach to produce three-dimensional (3D) optical trapping with sub-wavelength size through an engineered microsphere, under linear polarization states of an incident light. The engineered microsphere is designed to contain the segmented regions of diffractive patterns and made by focused ion beam fabrication. We simulate and experimentally characterize the focus performance of the engineered microsphere. The emitted light field from the exit surface of the engineered microsphere forms a pair of axially arranged focused beams, and they are connected with a continuous optical field embracing a 3D optical null at the center, forming the so-called optical bottle beam. Experimental results and numerical simulation are in good agreement. Such micro-optics can be used for precise and localized optical trapping.

Focused ion beam fabrication of Janus bimetallic cylinders acting as drift tube Zernike phase plates for electron microscopy

Modern nanotechnology techniques offer new opportunities for fabricating structures and devices at the micrometer and sub-micrometer level. Here, we use focused ion beam techniques to realize micrometer-sized Janus bimetallic cylinders acting as drift tube devices, which are able to impart a controlled phase shift to an electron wave. The phase shift results from the presence of contact potentials in the cylinders, in a similar manner to the electrostatic Aharonov–Bohm effect in bimetallic wires. We use electron Fraunhofer interference to demonstrate that such bimetallic structures introduce phase shifts that can be tuned to desired values by varying the dimensions of the pillars, in particular their heights. Such devices are promising for electron beam shaping and for the realization of electrostatic Zernike phase plates (i.e., devices that are able to impart a constant phase shift between an unscattered and a scattered electron wave) in electron microscopy, in particular, cryo-electron microscopy.

Nano-gap planar metal electrodes: fabrication and I-V characteristics

The nanowires/bars and nano-gap electrodes are vital components for emerging electronics and have wide ranging applications in flat-panel displays, sensors, sub-100 nm transistor circuits, and miniaturized computers/devices. Focused ion beam (FIB) has emerged as a powerful and unique tool for nanofabrication. The research work described here is concerned with (a) the FIB fabrication of planar metallic (copper and gold) nanostructures, (b) their current-voltage (I–V) measurements in situ, and (c) a viable method for extracting the realistic values of emission parameters. The planar electrodes with gap of 80-100 nm are realized by FIB milling of thin metal films. The difficulties faced in objective interpretation of their I-V data (based on known mechanisms) are highlighted. For determining the parameters (namely, effective emission area α eff, apparent work function ф, and the field enhancement factor β), Fowler-Nordheim [ln(I/V2) versus 1/V] plots showing a minimum with straight line of negative slope can be used. The striking findings demonstrated are (i) occurrence of emission from a tiny region (<1 nm2) vis-à-vis physical area (400 μm × 200 nm), (ii) significant lowering of barrier height, and (iii) enhancement of local field due to protrusions present. Typical values of α eff, ϕ, and β deduced are 52.3 Å2, 1.62 eV, and 39.3, respectively for copper planar electrodes (gap ∼100 nm); the corresponding data for the case of gold (gap ∼80 nm) are 29.1 Å2, 1.97 eV, and 12.1, respectively. Moreover, β lowering observed with bias is accompanied by increase in the emission area due to progressive smoothening of protrusions at the cathode surface. The electrodes are found rough/rocky at the nanoscale with protrusions and varying separations at places. These features make the electron emissive region small and pointed with an enhanced local electric field and effectively of a lower barrier height. The current discrepancy in the Child-Langmuir’s space charge regime is attributed to the emission occurring from a restricted area only. These findings are important for futuristic nano-devices like thermo-tunnelling refrigerator, energy harvester, etc.

Evaluation of TEM specimen quality prepared by focused ion beam using symmetry breaking index of convergent-beam electron diffraction.

Degradation of the crystalline quality of transmission electron microscopy specimens in silicon prepared with different conditions has been examined using convergent-beam electron diffraction (CBED). The specimens are prepared using focused ion beam (FIB) with different accelerating voltages, Ar-ion milling and crushing method. Symmetry breaking of CBED patterns was quantitatively evaluated by symmetry breaking index S, which has been previously reported. The degradation and inhomogeneity of the FIB specimen were suppressed by decreasing the accelerating voltages of the FIB fabrication in the final process.

Nanoscale mapping of carrier recombination in GaAs/AlGaAs core-multishell nanowires by cathodoluminescence imaging in a scanning transmission electron microscope

Mapping individual radiative recombination channels at the nanoscale in direct correlation with the underlying crystal structure and composition of III–V semiconductor nanostructures requires unprecedented highly spatially resolved spectroscopy methods. Here, we report on a direct one-by-one correlation between the complex radial structure and the distinct carrier recombination channels of single GaAs-AlGaAs core-multishell nanowire heterostructures using low temperature cathodoluminescence spectroscopy directly performed in a scanning transmission electron microscope. Based on an optimized focused ion beam fabrication of the optically active specimen, we directly visualize the radial luminescence evolution and identify four distinct emission lines, i.e., the near band edge and defect luminescence of the GaAs core (819 nm, 837 nm), the emission of the single embedded GaAs quantum well (QW, 785 nm), and the AlGaAs shell luminescence correlated with alloy fluctuations (650–674 nm). The detailed radial luminescence profiles are anticorrelated between QW luminescence and core emission, illustrating the radial carrier transport of the core-shell system. We inspected in detail the low-temperature capture of excess carriers in the quantum well and barriers.

Colossal mid-infrared bulk photovoltaic effect in a type-I Weyl semimetal.

Broadband, efficient and fast conversion of light to electricity is crucial for sensing and clean energy. The bulk photovoltaic effect (BPVE) is a second-order nonlinear optical effect that intrinsically converts light into electrical current. Here, we demonstrate a large mid-infrared BPVE in microscopic devices of the Weyl semimetal TaAs. This discovery results from combining recent developments in Weyl semimetals, focused-ion beam fabrication and theoretical works suggesting a connection between BPVE and topology. We also present a detailed symmetry analysis that allows us to separate the shift current response from photothermal effects. The magnitude and wavelength range of the assigned shift current may impact optical detectors, clean energy and topology, and demonstrate the utility of Weyl semimetals for practical applications.

High frequency in situ fatigue response of Ni-base superalloy René-N5 microcrystals

A novel in situ scanning electron microscope (SEM), high frequency fatigue testing methodology is developed using a combination of laser milling, focused ion beam fabrication and nanoindentation. This methodology is used to investigate crack initiation, propagation, fracture, fatigue life, and the mechanical response of microcantilever samples of a Ni-based superalloy (René-N5) under different cyclic strain amplitudes. The crack initiation and propagation in the microcantilever is monitored by observing changes in the beam's dynamic stiffness and continuous SEM imaging. The dynamic stiffness response of the micro-beams exhibits a transition from softening to hardening at a critical strain amplitude of 7×10−3. Theoretical analysis indicates that this transition corresponds to the stress required to shear γ′ precipitates. SEM imaging reveals the evolution of significant extrusions, intrusions, and slip traces during cyclic loading above this critical strain amplitude. Below this strain amplitude, very little surface roughening is observed. In addition, the measured dynamic stiffness is observed to exhibit two regimes of decrease after crack initiation. These two regimes correspond to short and large crack propagation. Finally, an overall increase in fatigue life is observed when comparing to bulk scale experiments on nickel-base superalloys. It is proposed that this is an inherent size effect in the small-volume, single crystal specimens tested.

Micron-Scale Deformation: A Coupled In Situ Study of Strain Bursts and Acoustic Emission.

Plastic deformation of micron-scale crystalline materials differs considerably from bulk samples as it is characterized by stochastic strain bursts. To obtain a detailed picture of the intermittent deformation phenomena, numerous micron-sized specimens must be fabricated and tested. An improved focused ion beam fabrication method is proposed to prepare non-tapered micropillars with excellent control over their shape. Moreover, the fabrication time is less compared with other methods. The in situ compression device developed in our laboratory allows high-accuracy sample positioning and force/displacement measurements with high data sampling rates. The collective avalanche-like motion of the dislocations is observed as stress decreases on the stress-strain curves. An acoustic emission (AE) technique was employed for the first time to study the deformation behavior of micropillars. The AE technique provides important additional in situ information about the underlying processes during plastic deformation and is especially sensitive to the collective avalanche-like motion of the dislocations observed as the stress decreases on the deformation curves.

FIB-fabricated complex-shaped 3D chiral photonic silicon nanostructures.

Technologies capable of fabricating complex shaped silicon metasurfaces attract increasing attention. The focused ion beam fabrication technique is considered traditionally as causing thick damaged layers in silicon resulting in a significant rise of the optical absorption loss. We examine the structure of the FIB-fabricated nanostructures on the silicon-on-sapphire (SOS) platform and its optical characteristics before and after thermal oxidation. We show that being thermally oxidised the FIB-patterned silicon subwavelength nanostructure tends to regain its chiral optical features. The impact of the oxidation process on the silicon nanostructure optical behaviour is discussed.

Comparison of mechanical characteristics of focused ion beam fabricated silicon nanowires

In this study, we investigate the effects of focused ion beam (FIB)-induced damage and specimen size on the mechanical properties of Si nanowires (NWs) by a microelectromechanical system (MEMS)-based tensile testing technique. By an FIB fabrication technique, three types of Si NWs, which are as-FIB-fabricated, annealed, and FIB-implanted NWs, are prepared. A sacrificial-oxidized NW is also prepared to compare the mechanical properties of these FIB-based NWs. The quasi-static uniaxial tensile tests of all the NWs are conducted by scanning electron microscopy (SEM). The fabrication process and specimen size dependences on Young’s modulus and fracture strength are observed. Annealing is effective for improving the Young’s modulus of the FIB-damaged Si. Transmission electron microscopy (TEM) suggests that the mechanism behind the process dependence on the mechanical characteristics is related to the crystallinity of the FIB-damaged portion.

Electron Microscopy of the Tin-oxide Nanolayer Formed on the Surface of Sn-Ag-Cu Alloys

Sn-Ag-Cu alloy used in the present study is commercial Sn-3.0Ag-0.5Cu solder ball alloys with a diameter of 400 and 300 μm which were long term atmospheric oxidized for about 6 years (specimen-1) and under high temperature/humidity at 85°C and relative humidity of 85% for 2140 h, respectively. Morphologies and nanostructure of the oxide nanolayers formed on the surface of Sn-Ag-Cu alloys were studied from the interface of the oxide film and the tin substrate by transmission electron microscopy (TEM) to verify the oxidation mechanism. Cross-sectional TEM specimens were prepared using a focused-ion-beam (FIB) micro-sampling technique. Before the FIB fabrication, the specimen surface was coated with carbon (C) and tungsten (W) films. Inhomogeneous thickness of tin-oxide nanolayer formed on specimen-1 and specimen-2 were fluctuated between 20-40 nm and 40-50 nm, respectively. The nanolayer on specimen-1, however, consists of polycrystalline SnO and SnO2, whereas the one on the specimen-2 comprises of polycrystalline SnO2. High resolution (HRTEM) image and fast Fourier transformation (FFT) spectra corresponding to the interface and the substrate areas have confirmed those results. The results verify that at very long atmospheric oxidation Sn was gradually oxidized to be SnO (Sn2+) and then SnO2 (Sn4+), in which SnO is present at the region closed to interface between Sn-substrate and the tin-oxide layer. At high temperature oxidation, however, Sn was completely oxidized to be SnO2.

Focused Ion Beam Fabrication of SU-8 Waveguide Structures on Oxidized Silicon

ABSTRACTThis work deals with SU-8 waveguides and waveguide structures fabricated on an oxidized silicon substrate using ‘Focused ion beam (FIB) lithography’. From our experimentation it seems that FIB method is practically not suitable for fabricating long SU-8 waveguide structures, rather it is more suitable for nanoscale modification of already fabricated waveguides, such as, to fabricate photonic crystal structures.

Electrical properties of sub-100 nm SiGe nanowires**Project support by the Swedish Foundation for Strategic Research “SSF” (No. EM-011-0002) and the Scientific and Technological Research Council of Turkey (No. TÜBİTAK).

In this study, the electrical properties of SiGe nanowires in terms of process and fabrication integrity, measurement reliability, width scaling, and doping levels were investigated. Nanowires were fabricated on SiGe-on oxide (SGOI) wafers with thickness of 52 nm and Ge content of 47%. The first group of SiGe wires was initially formed by using conventional I-line lithography and then their size was longitudinally reduced by cutting with a focused ion beam (FIB) to any desired nanometer range down to 60 nm. The other nanowire group was manufactured directly to a chosen nanometer level by using sidewall transfer lithography (STL). It has been shown that the FIB fabrication process allows manipulation of the line width and doping level of nanowires using Ga atoms. The resistance of wires thinned by FIB was 10 times lower than STL wires which shows the possible dependency of electrical behavior on fabrication method.

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.

Study on Platinum Coating Depth in Focused Ion Beam Diamond Cutting Tool Milling and Methods for Removing Platinum Layer.

In recent years, nanomachining has attracted increasing attention in advanced manufacturing science and technologies as a value-added processes to control material structures, components, devices, and nanoscale systems. To make sub-micro patterns on these products, micro/nanoscale single-crystal diamond cutting tools are essential. Popular non-contact methods for the macro/micro processing of diamond composites are pulsed laser ablation (PLA) and electric discharge machining (EDM). However, for manufacturing nanoscale diamond tools, these machining methods are not appropriate. Despite diamond’s extreme physical properties, diamond can be micro/nano machined relatively easily using a focused ion beam (FIB) technique. In the FIB milling process, the surface properties of the diamond cutting tool is affected by the amorphous damage layer caused by the FIB gallium ion collision and implantation and these influence the diamond cutting tool edge sharpness and increase the processing procedures. To protect the diamond substrate, a protection layer—platinum (Pt) coating is essential in diamond FIB milling. In this study, the depth of Pt coating layer which could decrease process-induced damage during FIB fabrication is investigated, along with methods for removing the Pt coating layer on diamond tools. The optimum Pt coating depth has been confirmed, which is very important for maintaining cutting tool edge sharpness and decreasing processing procedures. The ultra-precision grinding method and etching with aqua regia method have been investigated for removing the Pt coating layer. Experimental results show that when the diamond cutting tool width is bigger than 500 nm, ultra-precision grinding method is appropriate for removing Pt coating layer on diamond tool. However, the ultra-precision grinding method is not recommended for removing the Pt coating layer when the cutting tool width is smaller than 500 nm, because the possibility that the diamond cutting tool is damaged by the grinding process will be increased. Despite the etching method requiring more procedures to remove the Pt coating layer after FIB milling, it is a feasible method for diamond tools with under 500 nm width.

A Cross-Sectional TEM Specimen of a Multilayer Thin Film Prepared Using the FIB Technique

A focused ion beam (FIB) mill equipped with a microsampling (MS) unit and combined with transmission electron microscopy (TEM)/scanning TEM-energy dispersive x-ray spectroscopy (STEM-EDXS) is a powerful tool for studies of the functional advanced materials. For the studies, the specimen must be prepared as a thin foil which is tranparent to the electron beam. Focused ion beam is very effective method for fabricating TEM specimen of the cross-sectional thin film with the “lift-out” technique using a tungsten (W)-needle probe as a micromanipulator. A multilayer film of MgB2/Ni deposited on a Si (001) substrate prepared by FIB-MS technique is presented. Before FIB fabrication, the surface of the multilayer film was coated with W-film to prevent the surface from bombardment by the ion beam. A bright field (BF)-STEM image of the multilayer film related to two-dimensional (2D) elemental mapping clearly showed the presence of MgB2-and Ni-nanolayers. The measured experimental spacing between Ni-nanolayers was comparable with the actual specimen design, but the thickness of Ni-nanolayer was not. Unexpected nanostructures of the formation of SiO2 film on the substrate surface and holes within the film were observed.