Liquid Metal Ion Source Research Articles

Nanotubes stretched in liquid-metal-ion-sources and their influence on isotopic anomalies

The present paper argues that an intense electric field (few $\mathrm{V}∕\mathrm{\AA{}}$) provides an alternative method to stretch matter and to form nanotubes locally. The very high electric field is supplied by a liquid-metal-ion-source (LMIS). Intriguing aspects are displayed by the LMIS mass spectra of some pure elements. The periodicity of pure Ge or Sn LMIS, i.e., series of equidistant peaks such ${{\mathrm{Ge}}_{6n+1}}^{3+}$ with $n=3--8$ or ${{\mathrm{Ge}}_{6n+4}}^{3+}$ with $n=7--14$ or the formation of unexplained ${{\mathrm{Au}}_{8}}^{3+}$ and ${{\mathrm{Au}}_{16}}^{3+}$ ions for the pure Au LMIS, is attributed to the existence of Ge, Sn, or Au nanotubes in operating LMIS. LMIS results on a eutectic ${\mathrm{Au}}_{0.73}{\mathrm{Ge}}_{0.27}$ alloy show the formation of a gold nanotube associated with the strong ${{\mathrm{Au}}_{8}}^{3+}$ emission. The ${{\mathrm{Ge}}_{2}}^{+}$ emitted near the gold nanotube interacts with a larger electric field than in the pure Ge LMIS provoking a bond break in heteroisotope dimers and therefore isotope anomalies in dimer emission.

Study of a liquid metal ion source for external ion injection into electron-beam ion source

A liquid metal ion source (LMIS) has several attractive features as an external injector of primary ions (mostly metallic ions) into electron-beam ion source (EBIS). It does not use a buffer gas and therefore it provides only a very small gas load to the system; its control and operation are simple, power consumption does not exceed 10W, and beam pulses are very stable. A gold-silicon LMIS was supplied by FEI Company (http://www.feibeamtech.com/pages/liquid.html) and tested in a pulsed regime with an ion pulse width of 2ms and frequency up to 5Hz. Total extracted ion current reached 50μA and the normalized emittance of the total ion beam was 0.05πmmmrad. The results of this test, as well as results of experiments in which this ion source is used for injection of Au ions into EBIS, are presented.

Investigation of FIB assisted CoSi 2 nanowire growth

The ion beam synthesis process of CoSi 2 by writing stoichiometric ion implantation and subsequent annealing has been studied. For this a Focused Ion Beam (FIB), equipped with a Co 36Nd 64 alloy liquid metal ion source, was applied. Si(1 0 0) and (1 1 1) wafers were implanted with 60 keV Co 2+ ions in the dose range of 2 × 10 16–2 × 10 17 cm −2. The implantation parameters, like pixel dwell time, relaxation time, dose rate as well as the pixel overlapping factor were investigated. During subsequent annealing CoSi 2 nanostructures with dimensions down to 10 nm have been achieved. To investigate the silicide formation more in detail the annealing process was done in situ in a transmission electron microscope (TEM) on a pre-dimpled and FIB implanted samples of a Si(1 1 1). The formation of the cobalt silicide nano-crystals was monitored by plane-view TEM imaging during a 30 min heat treatment at 600 °C in vacuum.

Energy properties of an EMI-Im ionic liquid ion source

The identity and the energy distributions of positive and negative ions electrostatically extracted from the liquid phase in an ionic liquid ion source (ILIS) are analysed with a time-of-flight mass spectrometer and a multi-grid retarding potential analyzer. Accurate energy measurements using ionic liquids in an externally wetted configuration are reported for the first time. Droplet-free beams are produced using the ionic liquid 1-ethyl-3-methylimidazolium bis(triflouromethylsulfonyl)amide (EMI-Im) in which the solvated ions (EMI-Im)nEMI+ and (EMI-Im)nIm− with n = 0,1,2 are observed. The small ion source size and the energy distribution widths and deficits of a few electronvolts are quite similar to those of liquid metal ion sources, confirming that ILIS can be used in applications requiring highly focusable beams, e.g. sub-micron ion lithography. Measurements also suggest that solvated ions with n ⩾ 1 exhibit post-extraction fragmentation into lighter species at a rate increasing with their original degree of solvation. About 10% of the total beam current is carried away by metastable species that break up almost immediately after extraction while inside the emitter accelerating region.

Ion beams and their applications in high-resolution probe formation

The most critical demand for high-brightness ion beams emerged from the realm of industrial applications of focused ion beams and it was met by using liquid metal ion source technology. Although recognized as a successful technology, liquid metal ion sources have major limitations in view of beam species. Noncontaminating ion beam species with beam brightness comparable to liquid metal ion source, i.e., /spl sim/10/sup 6/ A cm/sup -2/ sr/sup -1/ and energy spread of /spl sim/1 eV will be ideal for many cutting-edge applications including ion beam milling, mask repair, and imaging studies. This paper attempts to bring out the essential points in the development of high-resolution probe forming beams. With this specific objective in view, this study is based on an analysis of point sources and then a variety of other sources, under the general classification of "broad" source (primarily plasma sources), are discussed. Several sources in this category including a Penning-type source, a multicusp source, and new-generation high-charged state sources seem to have merits for circumventing the limitations of liquid metal ion sources, although further research is needed to improve the beam brightness and advance the state-of-the art.

Analytical model of a gas phase field ionization source

High resolution focused ion beam technology is based on the use of field ionization sources. The most widely used source by far is the liquid metal ion source (LMIS) and most focused ion beam systems today employ it. The gas phase field ionization source (GFIS) is hardly employed today except in field ionization microscopy, but present day technology would allow its unique properties vis a vis the liquid metal ion source, including smaller virtual source size and energy spread, and the ability to produce ions from H, He, and heavier noble gases, to be exploited. If the GFIS is used as an ion source for a focusing column it would be useful to be able to calculate its emission properties (current versus voltage) as a function of emitter shape (a parameter not variable in a LMIS). Such a calculation had not been done precisely based on realistic emitter shapes. In this work, a theoretical description of the mechanism for ion production in a GFIS is presented. The comparison of the result with the experimental data for a H2–Ir GFIS is given, which shows reasonable agreement. With the present model, when used with the model for the optical properties of the GFIS (to be published), a focused ion beam (FIB) optical designer can make reasonable predictions about the properties of a GFIS-based FIB. Based on the modeling, we predict that a sub 1nm beam of He+ should be possible with ∼1pA current.

Influence of Taylor cone size on droplet generation in an indium liquid metal ion source

Depending on the emission current, a liquid metal ion source (LMIS) produces microdroplets in addition to ions. It is demonstrated that this ratio is directly influenced by the geometry of the Taylor cone. Using analytical models and experiments, we can show that the droplet production rate is directly proportional to the Taylor cone base radius for low impedance emitters. This relationship is of particular importance to optimize emitters for either low or high droplet emission at various emission currents.

Beam Stability Improvement of a Liquid Metal Ion Source

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

Erratum to: Influence of Taylor cone size on droplet generation in an indium liquid metal ion source

Erratum to: Influence of Taylor cone size on droplet generation in an indium liquid metal ion source

CoSi 2 nanostructures by writing FIB ion beam synthesis

A mass separated focused ion beam (FIB) is a very useful tool to fabricate nanostructures by writing implantation within an ion beam synthesis process. In these investigations the IMSA-OrsayPhysics FIB, equipped with a Co 36Nd 64 alloy liquid metal ion source, was applied. Si(100) and (111) wafers were implanted with 60 keV Co ++ ions in the dose range of 2 · 10 16 to 2 · 10 17 cm − 2 . Implantation parameters were investigated, like pixel dwell time, relaxation time (time between two cycles), dose rate as well as the pixel overlapping factor. The subsequent annealing was done in a two step process, namely 600 °C for 60 min and 1000 °C for 30 min in a N 2 ambient. The results obtained by SEM investigations in terms of continuous nanowire structures following the <110> direction and interrupted CoSi 2 pattern in the <100> direction show a clear dependence on the time scale as well as the scanning mode of the irradiation. Structure sizes as small as 10 nm are demonstrated. The formation of CoSi 2 nanostructures is explained by precipitation, Ostwald ripening and coarsening leading to a shrinking of the initial implanted profile.

Quest for high brightness, monochromatic noble gas ion sources

Focused ion beam (FIB) machines are key tools for state-of-the art sample preparation in electron microscopy, for characterization and repair in material sciences, for the semiconductor industry and for nanotechnology in general. Liquid-metal ion sources (LMIS) are widely used in FIB machines because they meet the minimum ion source requirements such as source brightness and reliability. However, in FIB machines, noble gas ion sources are favorable for sputtering, beam-induced etching and deposition, because the implanted ions do not change the electrical behavior of the substrate significantly. There are several efforts by various researchers to develop noble gas ion sources that can be used in FIB machines instead of LMIS. The gas ion sources could not meet the minimum ion source requirements. Therefore, LMIS are still a popular choice among FIB machine users. This review article takes a critical look at the reported efforts in the literature to develop noble gas ion sources for FIB machines.

Development of focused ion beam systems with various ion species

Conventional focused ion beam systems employ a liquid-metal ion source (LMIS) to generate high-brightness beams, such as Ga+ beams. Recently there has been an increased need for focused ion beams in areas like biological studies, advanced magnetic-film manufacturing and secondary-ion mass spectroscopy (SIMS). In this article, status of development on focused ion beam systems with ion species such as O2+, P+, and B+ will be reviewed. Compact columns for forming focused ion beams from low energy (∼3keV), to intermediate energy (∼35keV) are discussed. By using focused ion beams, a SOI MOSFET is fabricated entirely without any masks or resist.

Minimum Emission Current for a Liquid Metal Ion Source

Minimum Emission Current for a Liquid Metal Ion Source

Nanowires with well defined radii formed in operating liquid metal ion sources (LMIS)

For some elements such as germanium or tin, the mass spectra of ions emitted by liquid metal ion sources (LMIS) exhibit periodicities, i.e. series of equidistant peaks with an increase, ν, in the number of atoms between two peaks. We attribute it to the existence of jets in operating LMIS, the upper part of them being cylinders with ν-atom sections. The Ge6m+13+ and Sn6m+13+, m=3 to 8, and Ge6m+43+, m= 9 to 14, observed ions can be explained by this mechanism, here ν=6. We extend this mechanism to bismuth and gold and, in this last case, it allows the interpretation of a yet unexplained Au83+ ion.

On the dynamic response of externally wetted ionic liquid ion sources

The electrostatic extraction of nearly monochromatic solvated ions from externally wetted emitters is possible in the case of some ionic liquids or room temperature molten salts. These compact devices are similar to liquid metal ion sources but positive or negative ion beams can be obtained simply by selecting the appropriate polarity for the power supply. dc operation on a single polarity over relatively long periods of time induces electrochemical degradation of the liquid–metal system making voltage alternation at frequencies of the order of 1 Hz necessary to sustain chemical neutrality. This periodic interruption forces a non-steady state behaviour. In particular, the ion current onset is delayed from a square-shaped voltage input signal by a few milliseconds for tungsten metal emitters wetted with the ionic liquid 1-ethyl-3-methylimidazolium tetrafluoroborate, setting the upper bound for the alternation frequency at about 75 Hz. The dependences of these delays on absolute applied voltages are experimentally explored for this compound, suggesting that viscous drag is the main factor determining the dynamic response.

The Aun cluster probe in secondary ion mass spectrometry: influence of the projectile size and energy on the desorption/ionization rate from biomolecular solids

A Au-Si liquid metal ion source which produces Au(n) clusters over a large range of sizes was used to study the dependence of both the molecular ion desorption yield and the damage cross-section on the size (n = 1 to 400) and on the kinetic energy (E = 10 to 500 keV) of the clusters used to bombard bioorganic surfaces. Three pure peptides with molecular masses between 750 and 1200 Da were used without matrix. [M+H](+) and [M+cation](+) ion emission yields were enhanced by as much as three orders of magnitude when bombarding with Au(400) (4+) instead of monatomic Au(+), yet very little damage was induced in the samples. A 100-fold increase in the molecular ion yield was observed when the incident energy of Au(9) (+) was varied from 10 to 180 keV. Values of emission yields and damage cross-sections are presented as a function of cluster size and energy. The possibility to adjust both cluster size and energy, depending on the application, makes the analysis of biomolecules by secondary ion mass spectrometry an extremely powerful and flexible technique, particularly when combined with orthogonal time-of-flight mass spectrometry that then allows fast measurements using small primary ion beam currents.

Development of nanophosphors—A review

Nanophosphors have been extensively investigated during the last decade due to their application potential for various high-performance displays and devices. These act as a strategic component in almost all displays. Synthesis of nanophosphors can be accomplished in two ways namely, chemical and physical methods. Under chemical methods, different routes such as colloidal, capping, cluster formation, sol–gel, electrochemical, etc., are being followed. Physical methods widely used are molecular beam epitaxy, ionised cluster beam, liquid metal ion source, consolidation, sputtering and gas aggregation of monomers. Chemical precipitation in presence of capping agents, reaction in microemulsions, sol–gel reaction and autocombustion are commonly used techniques for synthesis of nanophosphors. However, the particle size has to be restricted to 3–5 nm to get the real advantage of quantum confinement. In other words, the particle size must be less than twice of Bohr radii of exciton as quantum confinement regime is limited to that size. A brief review of different synthesis techniques employed all over the world for the development of industrially important nanophosphors and extent of particle size reduction achieved is discussed.

The post-ionisation of Pb ions from a molten Sn host field-ion emitter

A tin liquid metal ion source contaminated with lead has been studied. From a detailed analysis of the source mass spectra as a function of emission current, it is established that while Sn+, Sn++ and Pb+ are directly field evaporated from the liquid surface, Pb++ forms by the post-ionisation of Pb+. Moreover, our results for the Pb++/Pb+ ratio are in excellent agreement with those of Komuro obtained with a pure lead LMIS.

Alloy liquid metal ion sources and their application in mass separated focused ion beams

For special purposes like writing ion implantation or ion mixing in the micrometer- or sub-micrometer range different ion species are needed. Therefore alloy liquid metal ion sources (LMISs) are used. The energy distribution of the ions from an alloy LMIS is one of the determining factors for the performance of a FIB column. Different source materials like Au 73Ge 27, Au 82Si 18, Au 77Ge 14Si 9, Co 36Nd 64, Er 69Ni 31, and Er 70Fe 22Ni 5Cr 3 were investigated with respect to the energy spread of the different ion species as a function of emission current, ion mass and emitter temperature. The alloy LMISs discussed above have been used in the Rossendorf FIB system IMSA especially for writing implantation to fabricate sub-micrometer pattern without any lithographic steps. A Co-FIB was applied for the ion beam synthesis of CoSi 2 micro-structures. Additionally, the possibility of varying the current density with the FIB by changing the pixel dwell time was used for radiation damage investigations in Si and SiC at elevated implantation temperatures. Furthermore, a broad spectrum of ions was employed to study the sputtering process depending on temperature, angle of incidence and ion mass on a couple of target materials using the volume-loss method. Especially this technique was used for the fabrication of various kinds of micro-tools.

Indium capillary liquid-metal-ion-source operation in the flow resistance regime

A novel capillary liquid-metal-ion-source (LMIS) was developed that operates in the same flow resistance regime as the previously reported smooth needle-type LMIS. This new LMIS has the advantage that the flow resistance can actually be mechanically designed. Moreover, the operation of this type of source promises better stability during long-term operation compared with needle-type sources. The experimental results obtained agree well with analytical models from Mair.