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.     

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.     

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.     

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.     

The emission characteristics of an indium needle-type liquid metal ion source

Operational data for an indium needle-type liquid metal ion source are presented. Detailed comparisons are drawn between these characteristics and those of a similar gallium source. The behaviours of the two sources are found to be strikingly similar, indicating a common mechanism of ion emission.     

Comment on “variational formulation for the equilibrium condition of a conducting fluid in an electric field”

Sujatha et al. have suggested that the Taylor cone hypothesis is wrong and they have derived equations for the equilibrium shape of a conducting fluid in an electric field. We examine their arguments and suggest that their paper may be incorrect in some respects. We find that Taylor's omission of the pressure difference term in the Laplace formula and his use of a single Legendre function for the potential are correct for the situation that he considered and that consequently the Taylor cone hypothesis is justified. The shape of real liquid metal ion sources appears to be well represented by a jet-like protrusion model which approaches a Taylor cone shape in the low current limit.     

Ion-energy distributions in liquid-metal-ion sources

Energy distributions of mass-separated ions emitted from liquid-metal-ion sources using Ni-B-Si and Pt-P-Sb alloys have been measured in a dynamic range of four orders of magnitude. Low-energy tails are observed on the energy-per-unit ion charge for singly-charged ions, in contrast with doubly-charged ions. The higher ion-emission current brings about longer tails. Rather flat tails are also observed on the high-energy side for M⁺ ion species in the Pt-P-Sb alloy LMIS, where the M²⁺ intensity is larger than the M⁺ one. The origins of these energy tails are discussed.     

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.     

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.     

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     

Nonlinear dynamics in the CO-oxidation on Pt single crystal surfaces

The types of dynamical behaviour that can be found in nonlinear systems far from equilibrium are briefly described. Then experimental manifestations of these phenomena in the CO oxidation on Pt are presented which include bistability, oscillations and chaos as well as propagating and stationary spatial patterns. It is demonstrated how a simple model can be refined to account for the observed complex behaviour.     

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.     

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.     

Ionization mechanism in a liquid-metal ion source. Source for high-melting metals

Investigations have been made of the ionization mechanism in a liquid-metal ion source. At the instant of ion current generation the surface of a liquid metal emitter frozen in a highvoltage field exhibits microprotrusions pulled from the liquid metal by the electric field. A double-focusing mass spectrometer identified two components in the ion beam extracted from the aperture in the extractor. One ion component forms at the tip of the emitter and has an energy spread not exceeding a few tens of electronvolts. The other ion component forms in the cathode plasma at the extractor. On the basis of these investigations, a mechanism is proposed for ion formation in a liquid metal ion source and this mechanism is used to produce a modified source design. The source can produce ion beams from high-melting metals and nonmetals, and beams of Ta, W, Mo, C, and Fe ions were obtained. Zh. Tekh. Fiz. 67, 82–87 (November 1997)     

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     

Electrically conducting ion tracks in diamond-like carbon films for field emission

Electrically conducting channels in an insulating carbon matrix were produced by 140-MeV Xe ion irradiation. The high local energy deposition of the individual ions along their pathes causes a rearrangement of the carbon atoms and leads to a transformation of the insulating, diamond-like (sp³-bonding) form of carbon into the conducting, graphitic (sp²-bonding) configuration. The conducting ion tracks are clearly seen in the current mapping performed with an atomic force microscope (AFM). These conducting tracks are of possible use in field emission applications. Received: 4 May 1999 / Accepted: 5 May 1999 / Published online: 1 July 1999     

An exact solution of Laplace's equation for cuspidal geometry

An exact solution of Laplace's equation is obtained for a system of conducting electrodes with cuspidal symmetry. The significance of this result in predicting and verifying the equilibrium configuration of a rotationally symmetric conducting fluid subject to electrostatic stress is discussed.This work was supported in part by the Division of Materials Research, National Science Foundation, Grant No. DMR-8108829     

Effect of ion impinging on the microstructure and field emission of carbon nanotubes

Effects of ion impinging on the microstructure and field electron emission properties of screen-printed carbon nanotube films were investigated. We observed that the plasma treatment modified the microstructure of CNTs along with the remarkable increase of emission site density. With the prolongation of ion impinging time, the emission current falls down first, and then rises up to higher than that of the untreated films. It is proposed that the change of emission characteristics is due to the different emission mechanisms. After the treatment, electrons are emitted predominantly from the nano-nodes on the tube wall instead from the nanotube tips.     

Development of arc ion sources for heavy ion fusion

An overview of the development of arc ion sources for heavy ion fusion is presented. Two approaches to heavy ion fusion (HIF)-the RF linac-storage ring approach and the induction linac approach-are described. RF linac schemes require low emittance and moderate current levels, because the beam is accumulated in storage rings before being focused on target. The induction linac approach requires low emittance and high current, because this is a single-pass approach to HIF and one wishes to limit the number of beams in the machine. The RF scheme generally uses long pulse sources together with a buncher of RFQ. The induction linac approach requires sources in the microsecond pulse length range, with good optics being maintained during the pulse. Emphasis is on the induction linac approach pursued at the Lawrence Berkeley Laboratory