Gallium Focused Ion Beam Research Articles

Combining focused ion beam and atomic layer deposition in nanostructure fabrication

Combining the strengths of atomic layer deposition (ALD) with focused ion beam (FIB) milling provides new opportunities for making 3D nanostructures with flexible choice of materials. Such structures are of interest in prototyping microelectronic and MEMS devices which utilize ALD grown thin films. As-milled silicon structures suffer from segregation and roughening upon heating, however. ALD processes are typically performed at 200–500 °C, which makes thermal stability of the milled structures a critical issue. In this work Si substrates were milled with different gallium ion beam incident angles and then annealed at 250 °C. The amount of implanted gallium was found to rapidly decrease with increasing incident angle with respect of surface normal, which therefore improves the thermal stability of the milled features. 60° incident angle was found as the best compromise with respect to thermal stability and ease of milling. ALD Al2O3 growth at 250 °C on the gallium FIB milled silicon was possible in all cases, even when segregation was taking place. ALD Al2O3 could be used both for creating a chemically uniform surface and for controlled narrowing of FIB milled trenches.

Degradation profiles in aged EPDM water seals using focused ion beam-scanning electron microscopy

The degradation of an ethylene–propylene-diene (EPDM) rubber seal used in a water supply system was investigated using focused ion beam-scanning electron microscopy (FIB-SEM). The EPDM rubber seal was used for 3 years within the temperature range 20–40 °C in a city water system. The accretions present on the surface of the EPDM seal after use were observed by SEM and were found to consist of iron and oxygen atoms based on energy dispersive X-ray spectroscopy (EDS) analysis. A cross-sectional depth image of the EPDM rubber was obtained by FIB-SEM, after slicing the EPDM rubber with a focused Gallium ion beam. Iron and oxygen atoms in the cross-section of the EPDM rubber were detected through EDS. The distribution of iron was comparable to that of oxygen derived from the carbonyl groups generated by the degradation of EPDM, suggesting that iron ions may promote the degradation of natural rubber through catalytic effects.

Aluminum oxide mask fabrication by focused ion beam implantation combined with wet etching

A novel aluminum oxide (Al2O3) hard mask fabrication process with nanoscale resolution is introduced. The Al2O3 mask can be used for various purposes, but in this work it was utilized for silicon patterning using cryogenic deep reactive ion etching (DRIE). The patterning of Al2O3 is a two-step process utilizing focused ion beam (FIB) irradiation combined with wet chemical etching. Gallium (Ga+) FIB maskless patterning confers wet etch selectivity between the irradiated region and the non-irradiated one on the Al2O3 layer, and mask patterns can easily be revealed by wet etching. This method is a modification of Ga+ FIB mask patterning for the silicon etch stop, which eliminates the detrimental lattice damage and doping of the silicon substrate in critical devices. The shallow surface gallium FIB irradiated Al2O3 mask protects the underlying silicon from Ga+ ions. The performance of the masking capacity was tested by drawing pairs consisting of a line and an empty space with varying width. The best result was seven such pairs for 1 μm. The smallest half pitch was 59 nm. This method is capable of arbitrary pattern generation. The fabrication of a freestanding single-ended tuning fork resonator utilizing the introduced masking method is demonstrated.

Characterization of Black Carbon in Fine Aerosol Particles Using High Lateral Resolution TOF-SIMS

Fine aerosol particles were analyzed by time-of-flight secondary ion mass spectrometry with high lateral resolution. After sulfate particles with a diameter of about 1 μm were sputtered by gallium primary ions (a gallium focused ion beam), solid materials with a diameter of about 100 nm were occasionally found inside the particles. Since the mass spectrum for the solid material was almost the same as that of graphite, we concluded that the solids were black carbon. It was also found that the black carbon located at the surface of the sulfate core, and they were usually surrounded by organic matter.

Evaluation of field emission parameters in a copper nano-tip based diode

Copper nano-tip based diode structure with a gap of ∼120 nm has been fabricated by milling of a thin metallic film with a 30 kV focused gallium ion beam at a current of 100 pA. Its current-voltage characteristics measured at a pressure of ∼10−6 mbar is shown to follow the Fowler-Nordheim (F-N) field emission tunneling above ∼40 V. A simple method has been proposed to evaluate parameters like effective area (Aeff), apparent work function (ϕ), and field enhancement factor (β) of the nano-emitter. The extremely small Aeff, substantial lowering of ϕ, and high β value observed have been explained in terms of changes occurring at the emitter tip with increasing applied field. The formation of metallic nanoparticles over the substrate by local evaporation of cathode material at high currents is also demonstrated.

Development of an Ultra-High Performance Multi-Turn TOF-SIMS/SNMS System “MULTUM-SIMS/SNMS”

A new system incorporating a multi-turn time-of-flight secondary ion/sputtered neutral mass spectrometer (TOF-SIMS/SNMS) with laser post-ionization was designed and constructed. This system consists of a gallium focused ion beam, femtosecond (fs) laser for post-ionization, and multi-turn TOF mass spectrometer. When laser post-ionization was used, the secondary ion signal strengths for several metals increased by up to 650 times, and were greater than the values obtained in conventional TOF-SIMS experiments. Use of the multi-turn mass spectrometer resulted in an increase in mass resolving power with increase in the total TOF. The mass resolving power reached to 23,000 after 800 multi-turn cycles, corresponding to a flight path length of 1040 m. These results indicated that this system is very effective for the analysis of valuable materials such as space samples with high sensitivity, high mass resolving power, and high lateral resolution.

Preparation of magnonic crystals with nanoislands by focused ion beam etching

This paper describes the patterning of gold, cobalt, nickel, cobalt-gold, and nickel-gold nanofilms on the surface of epitaxial garnet ferrite films through sputtering with a focused gallium ion beam. We show that the minimum size and spacing of regularly shaped metallic islands depend not only on the gallium ion beam spot size but also on the thickness of the metallic film. Using Au and Co-Au films less than 10 nm thick as an example, we have demonstrated for the first time the possibility of producing a periodic structure in the form of rectangular metallic islands 70 nm or less in lateral dimensions. Such structures are potentially attractive as two-dimensional magnonic crystals.

Focusing surface plasmons on Er3+ ions through gold planar plasmonic lenses

Gold plasmonic lenses consisting of a planar concentric rings-groove with different periods were milled with a focused gallium ion beam on a gold thin film deposited onto an Er3+-doped tellurite glass. The plasmonic lenses were vertically illuminated with an argon ion laser highly focused by means of a 50× objective lens. The focusing mechanism of the plasmonic lenses is explained using a coherent interference model of surface plasmon-polariton (SPP) generation on the circular grating due to the incident field. As a result, phase modulation can be accomplished by the groove gap, similar to a nanoslit array with different widths. This focusing allows a high confinement of SPPs that can excite the Er3+ ions of the glass. The Er3+ luminescence spectra were measured in the far-field (500–750 nm wavelength range), where we could verify the excitation yield via the plasmonic lens on the Er3+ ions. We analyze the influence of the geometrical parameters on the luminescence spectra. The variation of these parameters results in considerable changes of the luminescence spectra.

AB-INITIO CALCULATIONS OF ELECTRONIC PROPERTIES AND QUANTUM TRANSPORT IN U-SHAPED GRAPHENE NANORIBBONS

Graphene is a promising candidate as a material used in nano-scale devices because of recent developments in advanced experimental techniques. Motivated by recent successful fabrications of U-shaped graphene channel transistors by using the gallium focused ion beam technology, we have performed ab-initio calculations to investigate the electronic properties and quantum transport in U-shaped graphene nanoribbons. The electronic properties are calculated using a numerical atomic orbital basis set in the framework of the density functional theory. The transport properties are investigated using the non-equilibrium Green's function method. The transmission spectra of U-shaped graphenes are analyzed in order to reveal the quantum transport of the systems. We found that the graphene nanoribbons tend to open a band gap when U-shaped structures are formed in both armchair and zigzag cases. The geometrical structures of U-shaped GNRs had enormous influences on the electron transport around the Fermi energy due to the formation of quasi-bound states at zigzag edges. The obtained results have provided valuable information for designing potential nano-scale devices based on graphenes.

A-SiO_x<Er> active photonic crystal resonator membrane fabricated by focused Ga^+ ion beam

We have fabricated thin erbium-doped amorphous silicon sub-oxide (a-SiOx<Er>) photonic crystal membrane using focused gallium ion beam (FIB). The photonic crystal is composed of a hexagonal lattice with a H1 defect supporting two quasi-doubly degenerate second order dipole states. 2-D simulation was used for the design of the structure and full 3-D FDTD (Finite-Difference Time-Domain) numerical simulations were performed for a complete analysis of the structure. The simulation predicted a quality factor for the structure of Q = 350 with a spontaneous emission enhancement of 7. Micro photoluminescence measurements showed an integrated emission intensity enhancement of ~2 times with a Q = 130. We show that the discrepancy between simulation and measurement is due to the conical shape of the photonic crystal holes and the optical losses induced by FIB milling.

High-Resolution Three-Dimensional Reconstruction of a Whole Yeast Cell Using Focused-Ion Beam Scanning Electron Microscopy

We developed an approach for focused gallium-ion beam scanning electron microscopy with energy filtered detection of backscattered electrons to create near isometric voxels for high-resolution whole cell visualization. Specifically, this method allowed us to create three-dimensional volumes of high-pressure frozen, freeze-substituted Saccharomyces cerevisiae yeast cells with pixel resolutions down to 3 nm/pixel in x, y, and z, supported by both empirical data and Monte Carlo simulations. As a result, we were able to segment and quantify data sets of numerous targeted subcellular structures/organelles at high-resolution, including the volume, volume percentage, and surface area of the endoplasmic reticulum, cell wall, vacuoles, and mitochondria from an entire cell. Sites of mitochondrial and endoplasmic reticulum interconnectivity were readily identified in rendered data sets. The ability to visualize, segment, and quantify entire eukaryotic cells at high-resolution (potentially sub-5 nanometers isotropic voxels) will provide new perspectives and insights of the inner workings of cells.

Properties of SFS heterostructures prepared by a focused-ion-beam technique

We present a study of superconductor-ferromagnet (SF) bilayers and superconductor-ferromagnet-superconductor (SFS) heterostructures of nanometer dimensions prepared by a gallium focused-ion-beam (FIB) technique. The SFS heterostructures were implemented on the basis of high-Tc superconducting YBa2Cu3Ox (YBCO) and ferromagnetic La0.67Sr0.33MnO3 (LSMO) thin films deposited by magnetron sputtering on SrTiO3 (100) single crystal substrates. SFS weak link junctions require weak link dimensions in the range of nanometer size realizable by FIB patterning. On the other hand, the focused gallium ion beam might cause unacceptable degradation of the superconducting and ferromagnetic thin film properties. Our results confirm such influence of the FIB technology. However, protection of the structures by a gold thin film may effectively solve this problem, as is presented in the paper.

Repeatable switching of the bending direction of ZnO nanoneedles by ion beams

We studied the ion beam bending of ZnO nanoneedles to find the dependence of their bending direction on the ion beam energy and to clarify the bending mechanism. Through gallium focused ion beam (FIB) bending, the stems of the nanoneedles were found to be bent to the direction of the ion beam source for ion beam energies of 30 keV whereas they were bent in the opposite direction at ion energies lower than 20 keV. We found for the first time that the bending direction of ZnO nanoneedles could be changed by repeated switching of the ion beam energy between lower and higher energy levels, and that the thin tip parts of the nanoneedles were bent toward to the ion beam source like the higher energy bending mode during the process of lower energy bending below 20 keV. Through high resolution transmission electron microscopy (HRTEM) observations of the microstructure of a nanoneedle, bent by 30 keV higher energy ion beams, based on the atomic scale, we found that more edge dislocations were created in the rear side, deeper than the central plane of the nanoneedle, than the front side and that each edge dislocation added an extra lattice plane in this region. These observations clearly showed that the bent nanoneedles were plastically deformed by the edge dislocations created by the ion beams.

Fabrication of discrete gallium nanoislands on the surface of a Si(001) substrate using afocused ion beam

A gallium focused ion beam (FIB) has been used to implant Ga at specificsites on the surface of undoped Si(001) substrates. Upon annealing at600 °C, discrete nanoscale surface islands form within the FIB patternedregions when the total Ga ion dose, or fluence, is greater than1.0 × 1016 ions cm − 2. The number of islands depends on the size of the irradiated regionand a single island can be achieved for a FIB milled region that is100 nm × 100 nm.The average sizes of the islands were found to range from 24.5 nm when exposed to a total ion dose of1.2 × 1016 ions cm − 2 to 45 nm fora dose of 3.0 × 1016 ions cm − 2. We have confirmed that these surface islands are metallic Ga by performing aselective chemical etch that removes the islands and by transmission electronmicroscopy characterization. These patterned Ga surface templates could serve asnucleation sites for the lateral arrangement of discrete quantum dot structures.

Toward Local Growth of Individual Nanowires on Three-Dimensional Microstructures by Using a Minimally Invasive Catalyst Templating Method

We present a novel minimally invasive postprocessing method for catalyst templating based on focused charged particle beam structuring, which enables a localized vapor-liquid-solid (VLS) growth of individual nanowires on prefabricated three-dimensional micro- and nanostructures. Gas-assisted focused electron beam induced deposition (FEBID) was used to deposit a SiO(x) surface layer of about 10 × 10 μm(2) on top of a silicon atomic force microscopy cantilever. Gallium focused ion beam (FIB) milling was used to make a hole through the SiO(x) layer into the underlying silicon. The hole was locally filled with a gold catalyst via FEBID using either Me(2)Au(tfac) or Me(2)Au(acac) as precursor. Subsequent chemical vapor deposition (CVD)-induced VLS growth using a mixture of SiH(4) and Ar resulted in individual high quality crystalline nanowires. The process, its yield, and the resulting angular distribution/crystal orientation of the silicon nanowires are discussed. The presented combined FIB/FEBID/CVD-VLS process is currently the only proven method that enables the growth of individual monocrystalline Si nanowires on prestructured substrates and devices.

Influence of film thickness on the optical transmission through subwavelength single slits in metallic thin films

Silver and gold films with thicknesses in the range of 120-450 nm were evaporated onto glass substrates. A sequence of slits with widths varying between 70 and 270 nm was milled in the films using a focused gallium ion beam. We have undertaken high-resolution measurements of the optical transmission through the single slits with 488.0 nm (for Ag) and 632.8 nm (for Au) laser sources aligned to the optical axis of a microscope. Based on the present experimental results, it was possible to observe that (1) the slit transmission is notably affected by the film thickness, which presents a damped oscillatory behavior as the thickness is augmented, and (2) the transmission increases linearly with increasing slit width for a fixed film thickness.

Analysis of Focused Ion Beam Damages in Optoelectronic Devices Fabrication

A study of the damages caused by gallium focused ion beam (FIB) is presented. Potential damages caused by local heating, ion implantation, and selective sputtering are presented. Preliminary analysis shows that local heating is negligible. Gallium implantation is shown to occur over areas tens of nanometers thick. Gallium accumulation as well as selective sputtering during III-V compound milling is expected. Particularly, for GaAs, this effect leads to gallium segregation and the formation of metallic clusters. Microdisks resonators are fabricated using FIB milling of different emission currents. It is shown that for higher emission current, thus higher implantation doses, the cavity quality factor rapidly decreases.

Surface templates fabricated using a focused ion beam for lateral positioning of nanoscale islands on Si (001) substrates

The authors have explored using a gallium focused ion beam (FIB) as a method of integrating lattice mismatched materials with silicon by creating template patterns directly on Si with nanoscale resolution. This is one method for arranging materials at nanoscale dimensions that could potentially provide better properties or new functionalities to overcome limits in current device technologies. The FIB patterned templates are of interest as a means of locally controlling topography at nanoscale dimensions or as a means of locally implanting Ga, the ion source for these experiments, at specific surface sites. The authors have annealed these templates in vacuum to study the effects of ion dosage on local Ga concentration and surface topography. They have also used magnetron sputtering to deposit SiGe on the FIB patterned Si substrates after ex situ cleaning in order to understand how the template influences the resulting surface morphology that evolves. This morphology generally consists of pits and/or islands whose size and location are influenced by the patterning and growth conditions. The templates are characterized using atomic force microscopy (AFM) and transmission electron microscopy characterization. AFM of the resulting nanostructures are also shown after SiGe deposition.

Angular Dependence of the Ion-Induced Secondary Electron Emission for He+ and Ga+ Beams

In recent years, novel ion sources have been designed and developed that have enabled focused ion beam machines to go beyond their use as nano-fabrication tools. Secondary electrons are usually taken to form images, for their yield is high and strongly dependent on the surface characteristics, in terms of chemical composition and topography. In particular, the secondary electron yield varies characteristically with the angle formed by the beam and the direction normal to the sample surface in the point of impact. Knowledge of this dependence, for different ion/atom pairs, is thus the first step toward a complete understanding of the contrast mechanism in scanning ion microscopy. In this article, experimentally obtained ion-induced secondary electron yields as a function of the incidence angle of the beam on flat surfaces of Al and Cr are reported, for usual conditions in Ga+ and He+ microscopes. The curves have been compared with models and simulations, showing a good agreement for most of the angle range; deviations from the expected behavior are addressed and explanations are suggested. It appears that the maximum value of the ion-induced secondary electron yield is very similar in all the studied cases; the yield range, however, is consistently larger for helium than for gallium, which partially explains the enhanced topographical contrast of helium microscopes over the gallium focused ion beams.

Imaging three-dimensional tissue architectures by focused ion beam scanning electron microscopy

In this protocol, we describe a 3D imaging technique known as 'volume electron microscopy' or 'focused ion beam scanning electron microscopy (FIB/SEM)' applied to biological tissues. A scanning electron microscope equipped with a focused gallium ion beam, used to sequentially mill away the sample surface, and a backscattered electron (BSE) detector, used to image the milled surfaces, generates a large series of images that can be combined into a 3D rendered image of stained and embedded biological tissue. Structural information over volumes of tens of thousands of cubic micrometers is possible, revealing complex microanatomy with subcellular resolution. Methods are presented for tissue processing, for the enhancement of contrast with osmium tetroxide/potassium ferricyanide, for BSE imaging, for the preparation and platinum deposition over a selected site in the embedded tissue block, and for sequential data collection with ion beam milling; all this takes approximately 90 h. The imaging conditions, procedures for alternate milling and data acquisition and techniques for processing and partitioning the 3D data set are also described; these processes take approxiamtely 30 h. The protocol is illustrated by application to developing chick cornea, in which cells organize collagen fibril bundles into complex, multilamellar structures essential for transparency in the mature connective tissue matrix. The techniques described could have wide application in a range of fields, including pathology, developmental biology, microstructural anatomy and regenerative medicine.