The high-temperature behaviour of gallium and indium liquid metal ion sources

Energy distribution diagrams and derived data are presented for gallium and indium liquid metal ion sources operated at elevated temperatures. Results for the gallium source confirm that a secondary peak is formed on the energy distribution diagram at source temperatures above 250°C. Contrary to the findings of other research workers, data presented here show that the indium source displays similar characteristics to that of gallium. Off-axis data are also given, showing that secondary peak formation is not limited to the centre of the beam. Present hypotheses propose that secondary peak formation is the result of an increased contribution to emission by free-space field ionisation at elevated temperatures. Data presented here for the gallium and indium sources are discussed and the above hypotheses are examined. It is concluded that a field ionisation mechanism does not satisfactorily explain the form of the high temperature liquid metal ion source energy distributions.     

The influence of substrate geometry on the emission properties of a liquid metal ion source

The effect of needle radius, cone angle and shaft diameter on the threshold voltage and angular intensity — total current relationships for a Ga liquid-metal ion source (LMIS) was investigated. The variation of threshold voltage with needle geometry could be described in terms of the Taylor theory of liquid cone formation by electrostatic fields. The beam energy spread was mainly a function of total source current and was not a sensitive function of emitter geometry. Source angular intensity at a constant total current increased linearly with threshold voltage when the latter was altered due to source geometry.     

Shape of a liquid metal ion source

A model of the liquid-metal ion-source shape consisting of a jet-like protrusion on the end of a Taylor cone shape is shown to be consistent with a field evaporation mechanism of ion formation, fluid dynamic considerations, space charge effects and recent TEM observations. The diameter of the ion emitting area is found to be only a few tens of Å. Self-consistent numerical calculations of electric potential and particle trajectories predict emission characteristics which compare favorably with experimental results.On leave from Trinity Hall, Cambridge, UK     

Measurement of energy spread and angular intensity of a cesium liquid metal ion source

This paper determines the optimum range of total ion current for a cesium liquid metal ion source for use as a focused ion beam (FIB). This range is determined from a figure of merit calculated from measurements of the angular intensity and energy spread of emitted ions. Judging from both the figure of merit and the tail of the energy distribution curves, the total emission current should be set near 1 A for a cesium FIB with a high current density. Assuming that both Cs- and Ga-FIBs have the same diameter, the relative current density of a Cs-FIB is expected to be approximately 80% that of a Ga-FIB.     

Theoretical investigation of liquid metal ion sources: Field and temperature dependence of ion emission

We investigate the field and temperature dependence of the rate of ion formation by important mechanisms in Liquid Metal Ion Sources (LMIS). In addition to field evaporation and field ionization of thermally evaporated neutrals we identify a third mechanism, thermal-field evaporation, which is intermediate between the other two mechanisms. Field (or thermal-field) evaporation is found to be dominant in the normal operating regime of LMIS in agreement with the conclusions of Prewett et al. A jet-like protrusion model of LMIS shape, which is consistent with direct observations, allows high temperatures to be reached with a reasonable power input to the source. Thus a small, but sometimes important, contribution to the total ion current from field ionization of thermally evaporated atoms is expected.     

On-axis energy analysis of69Ga+, Ga2+, and Ga 2 + ions emitted from a gallium liquid metal ion source

On-axis energy distribution measurements of 3 ionic species emitted from a gallium liquid metal ion source are reported. Voltage deficits, FWHM energy spreads, angular intensities and chromatic angular intensities are presented and compared with other published work. Ga⁺ deficit variations with beam current and temperature are discussed in detail. Ga ₂ ⁺ FWHM and deficit changes with source operating conditions are accounted for by a model of droplet fragmentation.     

On the mechanism of liquid metal ion sources

We have developed a theoretical model of liquid metal ion source operation which consistently explains the shape and size of the ion emitting region, the mechanism of ion formation and properties of the ion beam. We find that field evaporation is the main current generating mechanism and that field evaporation and subsequent postionization produce the doubly and higher charged ions. Field ionization of thermally evaporated neutrals may make a significant, but not dominant, contribution to the current of singly charged ions. Our model is consistent with experimental results on energy spread, energy deficit and charge state ratios and we are able to explain the stability of the emitted ion current.     

Current-voltage characteristics of a gas field ion source

Current-voltage characteristics of a gas field ion source (GFIS) have been measured for hydrogen and all rare gases. The parameter set included tip temperature, tip radius and gas temperature and pressure. This investigation has been made to get a complete overview of the field ion currents (FIC) and to estimate the maximum currents in a GFIS, which have been found to a few 100 nA. This estimate allows also a feasibility study of a GFIS, modified by a supertip, a small protuberance on the emitter surface.     

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. PACS 61.25.Mv; 79.70.+q An erratum to this article can be found at .     

Comment on the validity of first-order paraxial theory for electron and ion sources with needle-type or pointed geometry

An essential feature of first-order paraxial theory is the assumption that if the potential along the axis of an axially symmetric system is known, then the potential (fields) near the axis can be obtained by a power-series expansion about points lying on axis. However, in traditional first-order theory, which commonly assumes systems with cylindrical symmetry, only terms up to second-order in the coordinate transverse to the beam axis are retained. In this letter we argue that a consequence of this restriction is that traditional first-order paraxial theory should not be applicable to electron and ion sources with pointed or needle-type geometries. In order to treat non-parallel trajectories which occur in the pointed geometries present, say, in field emission liquid metal ion sources, a modified paraxial theory is suggested which describes two-dimensional particle dynamics.This work has been supported in part by the Division of Materials Research, National Science Foundation, Grant No. DMR-81008829     

Pulsed operation of a liquid metal ion source

Short ion pulses (100 ns FWHM) have been produced from an In liquid metal ion source (LMIS) of the needle type by superimposing short voltage pulses (500 ns) onto a subcritical extraction field. Extractor-pulsed LMIS are thought to be useful as primary ion sources in time-of-flight (TOF) mass spectrometers, particularly for space applications, where extended charge life time and reduced power consumption are of great importance.     

Investigation of a tin liquid metal ion source

In view of its importance in materials research, tin is a metal worth studying in a liquid metal ion source configuration, even if results complement or extend previous work. This is the more so if the new work corrects misconceptions of the past and adds to current thinking. We, therefore, prepared a Sn liquid metal ion source employing a Ni needle to anchor the liquid, cone-shaped, emitter. Source properties, such as the current–voltage curve, the mass spectra of the beam and the energy spread of the main ionic species, were studied in detail. The mass spectra show a considerable amount of Sn clusters, apart from the dominant species, Sn⁺ and Sn⁺⁺. The source was stable down to 1-A emission current, corresponding to an energy spread for the singly charged ions of 7 eV. Theoretical arguments, involving the peak energy deficit of the ion-energy distribution, strongly suggest that both Sn⁺ and Sn⁺⁺ are emitted by direct field evaporation from the liquid surface. The same conclusion is reached from a careful examination of the beam mass spectra of the source. PACS 07.77.Ka; 33.15.Ta; 61.25.Mv     

Computer simulation of liquid metal ion source optics

A numerical calculation of the electric field and current density distribution for a liquid metal ion (LMI) source has been carried out. If a field evaporation mechanism for ion formation is assumed an elongated Taylor cone shape emitter is required to account for the observed total currents. Trajectory calculations including the effect of uniform space charge have been carried out as a function of total current and particle mass. The predicted emission characteristics compare favorably with experimental results for Ga, however the homogeneous space charge model is unable to account for all of the experimentally observed increase in beam divergence with increasing mass and current.     

Electron and ion emission from liquid metal surfaces

The application of positive or negative electric fields to small-area liquid metal surfaces leads to very high brightness DC ion or pulsed electron emission. The stabilization of a cone-shaped structure by the balance of electrostatic and surface tensions forces is described. Electron and ion emission occurs by field emission and field evaporation mechanisms     

Long-life bismuth liquid metal ion source for focussed ion beam micromachining application

Liquid metal ion sources (LMISs) with Ga as ion species are widely used in focused ion beam (FIB) technology for micromachining and surface treatment on the sub-micron and nano-scale. Key features of a LMIS for investigating mechanical properties and 3D-microfabrication of materials are long life-time, high brightness, stable ion current and a highly effective milling ability for the material to be modified. In order to increase the material removal rate, heavier ions than Ga and their clusters should be applied. Bismuth (Bi) is the heaviest, non-radio-active element in the periodic table, is non-toxic and exhibits a low melting point. We have thus produced a long-life (about 1000 h) Bi LMIS with a good beam performance, applicable in any FIB system. Since Bi is the only element in this source, it is not necessary to separate it from other ions by a mass filter. Investigation of the sputtering rate of NiTi shape memory alloys using Ga and Bi LMIS showed that, for the same experimental conditions, the material removal rate with using of Bi_n^k+ ions in a standard FIB machine without a mass separator is about five times larger compared to Ga⁺ ions. This use of Bi as LMIS-species is the ultimate breakthrough in sputtering applications.     

Comments on nonlinearity,response time and polarity reversal in a thermal field emission metal whisker diode

In a series of recent experiments, research groups have made absolute frequency measurements with laser beams in the infrared region (μm) using a metal-metal point contact diode for the generation, frequency mixing and detection. At present there are two models which attempt to explain the rectification mechanism of the diode: 1) Tunneling of electrons through an intermediate oxide film from whisker to the metal base, i.e., configuration is considered to be a metal-oxide-metal (MOM) tunneling junction. 2) Rectification and nonlinear processes are the result of a thermal enchanced field emission. Such emission is a consequence of the immersion of the whisker in the laser radiation which results in conduction induced thermionic emission and/or generation of an electric field at the tip necessary for electron tunneling by field emission. The purpose of this comment is: a) to discuss qualitatively the basic difference between MOM and TFE theories as regards the origin of the nonlinearity and rectification properties of the metal point contact junction; b) to review the analyses describing the ultimate frequency response of the device; and 3) to provide a possible explanation for polarity reversal consistent with the TFE mechanism describing the operation of the whisker diode. This research was supported in part by the NATO Research Grants Program, Scientific Affairs, Brussels, Belgium, and under the auspices of the joint projects ESIS (electronic structure in solids) and IRIS (Institute for Research in Interface Sciences) of the Belgian Ministry for Science Policy     

Influence of surface morphology on the field emission properties of planar SiC/Si heterostructures formed by ion beam synthesis

The electron field emission properties of planar SiC/Si heterostructures with various surface morphology formed by high dose C⁺ implantation into Si using a metal vapor vacuum arc ion source were investigated. An implant energy of 35 keV was used with doses of 8×10¹⁷, 1×10¹⁸ and 1.2×10¹⁸ ions/cm⁻² with subsequent annealing in Ar at 1200 °C for various times. X-ray photoelectron spectroscopy and ultraviolet photoelectron spectroscopy showed that a thin stoichiometric SiC surface layer is formed and the surface work function is about 4.5 eV. Atomic force microscopy indicated that the size and density of the densely distributed small protrusions formed on the surface vary with preparation conditions. Results showed that there is an optimum annealing time for the corresponding implant dose at which a remarkably low turn-on field of about 1 V/μm is observed. The density and size of the small protrusions on the surface are believed to be the main factors affecting the field emission properties.     

Stability of liquid-metal ion sources

In this paper we find the parameters (radii, lengths, velocities) of the jets produced in liquid-metal ion sources. The jet stability with respect to developing sausages (Rayleigh instability) is studied. The Rayleight instability is shown to occur for rather large currents (in long jets). The currents critical for the instability initiation are calculated for Ga and Au sources. The results are in accordance with the experimental data available. In the case of Ga, the accordance is achieved only for the hot jet model (TT _m , whereT _m is the melting temperature), when the viscosity is appreciably lower.     

Experimental validation of a mass- efficiency model for an indium liquid-metal ion source

A model is derived linking microdroplet emission of a liquid-metal ion source (LMIS) to the actual current–voltage characteristic and operating temperature. All parameters were experimentally investigated using an indium LMIS, confirming the relationships found. The model allows for the first time the optimisation of a LMIS for low droplet emission at high emission currents. This is very important for application as a thruster, which has been developed at ARC Seibersdorf research. It can be also used to extrapolate droplet emission values along the current–voltage characteristic. Received: 29 January 2002 / Accepted: 7 October 2002 / Published online: 17 December 2002 RID="*" ID="*"Corresponding author. Fax: +43-50550/3366, E-mail: martin.tajmar@arcs.ac.at     

The measurement of energy spectra of neutral particles in low energy ion scattering

An apparatus is described for low energy (0.1–10 keV) ion scattering (LEIS) experiments. A time of flight (TOF) spectrometer is incorporated in the system to be able to measure the energy of particles in the neutral state after scattering. The energy resolution ΔE/E of the TOF spectrometer is discussed and found to be 0.5% (FWHM). This is sufficient for our scattering experiments. An electrostatic analyzer (ESA) is used to measure the energy of scattered ions [ΔE/E=0.5% (FWHM)]. Experiments show that in general the ion dose needed to obtain a TOF spectrum (2×10¹⁰ ions/cm²) is much smaller than the dose needed for an ESA-spectrum (6×10¹³ ions/cm²). The ion spectra measured with the TOF spectrometer, by subtracting the neutral yield from the total yield, as well as with the ESA are found to agree quite well. This provides a way to calibrate the TOF spectrometer. The determination of the ion fraction of scattered particles is discussed [10 keV⁴⁰Ar⁺ on Cu(100), scattering angle 30°]. It is shown that the TOF spectrometer is able to measure light recoil particles (e.g. hydrogen) from a heavy substrate. In the analysing system is, in addition to the TOF spectrometer, also incorporated a stripping cell to measure the energy of neutral scattered particles. An energy spectrum of neutral scattered particles measured with both methods is shown.