Capillary vs. externally wetted ionic liquid ion sources

Ionic liquid ion sources: characterization of externally wetted emitters

Development of ionic liquid ion source with porous emitter for surface modification

Ionic liquid ion sources (ILIS) have been proposed as a source of alternative ion chemistries for surface etching, focused ion beams, and secondary ion mass spectrometry. These ion sources produce polydisperse ion beams containing several solvated ion species with a distribution of energies, and it is necessary to filter the ion beam to obtain monoenergetic beams with a single ion chemistry as required by nanomanufacturing and analytical applications. In this work, a Wien filter has been designed, built, and tested for use with an ILIS using the liquid 1-ethyl-3-methylimidazolium tris(pentafluoroethyl)trifluorophosphate, or EMI-FAP. comsol multiphysics simulations have been used to validate the design. The full and unfiltered ion beams have been characterized using time-of-flight mass spectrometry and retarding potential analysis, and these measurements confirm that the filter is effective in isolating monomer ions from heavier ion species in the beam.

UltraCold Ion Beam Source. Focused ion beam (FIB) machines are largely used in the semiconductor industry mainly for imaging, milling and deposition of material, at the nanometer scale. To keep up with the trend of making smaller and smaller components, FIBs need to be continuously improved. A FIB is mainly made of an ion source and a focusing column. In this thesis, two concepts of ion sources are presented, modeled and discussed. The ultra-cold ion source (UCIS) is based on creating very cold ion beams (T <1 mK) by near-threshold photo-ionization of a laser-cooled and trapped atomic gas. In our experiment, 85Rb atoms are confined in a magneto optical trap built directly inside an accelerator structure. A spherical portion of the atom cloud is ionized with a 2-step ionization process and the bunched ions are accelerated by either a DC or a pulsed electric field. The liquid-metal ion source (LMIS) is the current state-of-art for focused ion beam technology, having a reduced brightness of 106 A/m2 sr eV. The reduced brightness characterizes the source and is proportional to the ion current and inversely proportional to the beam emittance (closely related with the source temperature). A high brightness is of importance as much as a low energy spread of the ion beam, since the latter is related with the focus ability of the beam. We have already shown that the UCIS can provide much lower energy spread (20 meV) than LMIS (˜ 4.5 eV), see Reijnders et al. (PRL, 2009), but not yet in combination with a high brightness as well. The energy spread is the limiting factor for the final achievable spot size in a FIB based on a LMIS. Chapter 3 and 4 present measurements of the source temperature and of the extracted current to complete the characterization of the UCIS. The ultra low temperature of the source permits collimated bunches to be created at low kinetic energy (down to few eV), which allows using time-dependent fields for accelerating and focusing. With this lens, a transversal ion temperature of (3±2) mK has been measured, as shown in chapter 3. This temperature is an upper limit since it is ten times higher than the expected value. The novel technique of focusing with time-dependent electric field is successfully applied to an ultra cold ion beam. In chapter 4, a dynamic model of the source describing its properties under pulsed operation has been developed and experiments validated it. A maximum average ion current of (13 ± 1) pA has been extracted from the UCIS and this is enough for imaging (1-100 pA are used). Those results correspond to a source reduced brightness of (8 ± 1) × 104 A/m2 sr eV with 0.9 eV energy spread. This result is 12.5 times lower than the brightness of a LMIS. An interesting pushing effect, due to the laser used to excite the atoms in the cloud, is described as well. Particle tracking simulations show that disorder induced heating has an important effect on the brightness, lowering its value by 1-2 orders of magnitude depending on the extracted current. Calculations and simulations of a different ion source, the atomic beam laser-cooled ion source (ABLIS), are presented in chapter 6. The follow-up project is based on a similar idea as an UCIS but aims for production of ion beams from a much higher initial atomic flux. The atoms originate from a Knudsen cell and, after being cooled and compressed transversally with laser cooling and trapping techniques, are photo-ionized and accelerated. The source size is reduced as well with respect to the UCIS. Realistic particle tracking simulations with 87Rb show that disorder induced heating is still very important, but in certain conditions, the so-called pencil-beam regime is reached and the disorder-induced heating does not deteriorate the brightness anymore. Depending on the extracted current, it is possible to reach a reduced brightness of 2 × 107 A/m2 sr eV with an ion current of 22 pA (corresponding to a final aperture diameter df = 1.6 µm) and an energy spread of 0.7 eV can be achieved. This source is likely capable of provide a high brightness and a low energy spread at the same time. The calculations are done assuming a purified 87Rb source. Eventually, it is possible to reach very high current (1 nA), making the ABLIS ideal also for milling applications with FIBs.

High current liquid metal ion sources are well known and found their first application as field emission electric propulsion thrusters in space technology. The aim of this work is the adaption of such kind of sources in broad ion beam technology. Surface patterning based on self-organized nano-structures on, e.g., semiconductor materials formed by heavy mono- or polyatomic ion irradiation from liquid metal (alloy) ion sources (LMAISs) is a very promising technique. LMAISs are nearly the only type of sources delivering polyatomic ions from about half of the periodic table elements. To overcome the lack of only very small treated areas by applying a focused ion beam equipped with such sources, the technology taken from space propulsion systems was transferred into a large single-end ion implanter. The main component is an ion beam injector based on high current LMAISs combined with suited ion optics allocating ion currents in the μA range in a nearly parallel beam of a few mm in diameter. Different types of LMAIS (needle, porous emitter, and capillary) are presented and characterized. The ion beam injector design is specified as well as the implementation of this module into a 200 kV high current ion implanter operating at the HZDR Ion Beam Center. Finally, the obtained results of large area surface modification of Ge using polyatomic Bi2+ ions at room temperature from a GaBi capillary LMAIS will be presented and discussed.

The emission and deposition of molten gold droplets from needle-tip liquid metal ion sources was investigated. The Taylor cone was identified as the source of droplet emission. An upper limit of 8 μm for the virtual source size was measured. Gold films were deposited on silicon substrates. The deposited gold spots showed a peaked, axially symmetric, distribution with a half angle for droplet emission of ∼2° which was considerably smaller than the ion emission angle of ∼45°. Typical deposition fluxes were ∼5×105 μm3/s/sr with total deposition rates of ∼3×103 μm3/s and on an average droplet diameter of ∼1 μm at an emission current of 150 μA. Smaller diameter droplets are produced with decreasing emission current and angle. Gold films with a fine textured morphology suitable for submicron patterning were obtained at emission currents less than ∼135 μA.

ABSTRACTIonic liquid (IL) ion sources with different emitter tip materials and tip numbers were developed and examined on ion beam characteristics with respect to its ILs wettability. As a result of ion current measurements, the most stable emission current was obtained for the graphite emitter tip and the ion current increased with increase of the tip number. The results indicate that the emitter wettability corresponding to the supplying flow rate and the number of emission site play an important role to stabilize and increase the beam current.

The author reports on an extensive study of energy deficit measurements of the energy distribution of the ion beam emitted by a gallium liquid metal ion source in an attempt to resolve the controversy arising from the unusually wide divergence of findings by different workers. The controversy can be conceivably resolved in terms of misalignment of the liquid cone, whereby different parts of the beam enter the energy analyser at different times. Coupled to this is an alteration of the work function of the retarding electrode of the energy analyser by contaminants, such as the liquid metal itself, or its oxides, altering in turn the value of the critical energy deficit for surface field ionisation. A detailed investigation is also made for the region of emission current below 1 mu A-a region rarely investigated by the vast majority of researchers, but a most important one from the point of view of the emission mechanism, as well as from an applications' standpoint. Finally, the reasons for the scatter observed in energy spread measurements are considered, and only a tentative explanation can be given, related to ion source misalignment; this misalignment arises from the fact that the liquid cone does not always form at the centre of the apex region of the supporting needle.

The optical emission close to the Taylor cone tips of liquid gallium and gold ion sources has been studied as a function of wavelength and ion current. The optical emission may be interpreted as due to excitations caused by binary collisions, among which Ga-Ga+ collisions play a dominant role. The release rate of neutrals from the source inferred from the optical-emission curves is consistent with the idea of contributions to the total ion current from field evaporation and field ionization, with the latter becoming increasingly important at higher currents. It is difficult to account for the origin of the neutrals by thermal evaporation at the tip.

Views Icon Views Article contents Figures & tables Video Audio Supplementary Data Peer Review Share Icon Share Twitter Facebook Reddit LinkedIn Tools Icon Tools Reprints and Permissions Cite Icon Cite Search Site Citation M. Schmidt, J. König, L. Bischoff, W. Pilz, G. Zschornack; Surface and material analytics based on Dresden-EBIS platform technology. AIP Conf. Proc. 9 January 2015; 1640 (1): 88–93. https://doi.org/10.1063/1.4905403 Download citation file: Ris (Zotero) Reference Manager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentAIP Publishing PortfolioAIP Conference Proceedings Search Advanced Search |Citation Search

High electric fields are used for ion generation in gas field ion sources and liquid metal ion sources. With the preparation of stable microprotrusions on refractory metals, stably emitting gas field ion sources for hydrogen and helium ions with angular intensities of 35 and 20 μA sr−1, respectively, are available with a high chromatic angular intensity. Liquid metal ion sources can stably deliver a variety of ions with a source life in some cases over more than 1000 h. They are an essential element for pattern generation in the nanometer region and other focused ion beam applications.

Andromede project: Surface analysis and modification with probes from hydrogen to nano-particles in the MeV energy range

This paper discusses recent items of progress in understanding liquid-metal ion source (LMIS) behaviour, in the more general context of electrohydrodynamics. (I) Like the Taylor-Gilbert cone, the phenomenon of the vena contracta has been known for several hundred years. It is argued that the cusp-on-a-cone shape of an operating LMIS is a similar phenomenon, except that the forces acting are electrical rather than gravitational, and the pressure in the jet is negative rather than positive. (2) In discussing LMIS stability in engineering terms, mechanisms affecting the jet shape can be classified as giving negative feedback (stabilising), or positive feedback (destabilising). Effects associated with field evaporation and/or space charges give negative feedback, effects associated with pressure changes positive feedback. Attention is drawn to the arguments of Ganan-Calvo about the role of 'supercritical' flow as a stabilising factor against hydrodynamic disturbances. (3) Electrohydrodynamic spraying is superficially very similar to LMIS operation, and both sprayers and ion sources adopt a cusp-on-a-cone (or 'cone-jet') shape. But the accepted theoretical explanations of the driving mechanisms are different. Confirmation is offered that the LMIS story is the correct one for very highly conducting liquids, and that a difference in driving mechanism is plausible.

The properties of liquid-metal ion sources are very sensitive to their tip structure. Because the impregnated-electrode-type liquid-metal ion source has a porous tip, it generates more than 300-μA ion currents of various metals including relatively high vapor-pressure metals such as Li, Cu, Ga, Ge, Ag, In, and Au, from which ions can be stably extracted. The large beam divergence, a common disadvantage in liquid-metal ion sources, can be overcome by designing new lens systems to efficiently transport the ion beam for both high- and low-energy applications. In addition, the ion beam current can be multiplied by using multiple emission cusps; thus, a germanium ion current of 1.3 mA was obtained by a three-point emission source. Therefore, the impregnated-electrode-type liquid-metal ion source can be utilized as a metal ion source for general applications.

The advent of nanostructurization technology has also strongly pushed the development of finely focused ion beams of high intensity. While the liquid-metal ion source, LMIS, has already marked a major breakthrough with regard to the available target current densities during the last decade, the gas field ion source, GFIS, has more recently presented an even more promising solution to the problem of ion-beam processing of macroscopic target areas in reasonable time scales. In particular, this progress has been achieved by controlled generation of a small protrusion, called supertip, on top of a regular field emitter tip. In this way, ion current densities of up to 100 A/cm2 appear feasible for certain gases, such as hydrogen, helium, and neon with spot diameters down to 10 nm. Compared to the LMIS, this means an increase in target current density of more than one order of magnitude. The relevant parameters of our supertip GFIS system, characterizing source and ion optics, are given and some first ionographic applications in the fields of optical patterning and topographical structurization, demonstrating the state of the art, are discussed.