Indium Liquid Metal Ion Source Research Articles

Fabrication of tungsten tips with controllable shape by a two-step rapid reciprocating electrochemical etching method.

Traditional electrochemical etching methods for the needle of a liquid metal ion source (LMIS) easily produce an exponential profile with an uncontrollable tip length and apex radius. Meanwhile, a ledge forms between the needle tip and the needle rod under the etching of the meniscus, which becomes an obstacle for the flow and replenishment of the liquid metal. This paper proposed a two-step rapid reciprocating etching method, which aims to fabricate LMIS tungsten needles with controllable tip length and apex radius, and also with a smooth transition region between the needle tip and the needle rod. In the first step of rough machining, the needle rapidly reciprocates up and down in the electrolyte and rotates to produce a uniform conical profile. However, an ellipsoidal residual portion is generated concomitantly at the needle tip. In the second step of finish machining, the needle shifts down for a given distance and continues to reciprocate until the sharp tip is formed. The tip length fabricated varied from 0.59 to 5.53mm at different reciprocating strokes. The apex radius ranged from 0.3 to 0.7 µm, and can also be increased to 2 µm by extra reciprocate etching in the electrolyte to meet the LMIS working requirement. A variable named transitivity was defined to quantitatively describe the smoothness of the region between the tip and rod during the etching process. The experimental results showed that a rotation speed of 600 rpm combined with a reciprocating speed of 0.5 mm/s can significantly improve the transitivity of the needle. Those fabricated needle tips have been tested for the indium LMIS and the maximum emission current of the needle tip reached 12 µA.

The beam divergence of an indium LMIS at a distance of 50μm as determined by plasma diagnostic measurements

The current-dependent beam divergence at a distance of 50 μm from an indium-liquid metal ion source is derived from experimental data obtained by measuring the beam spread with a 3D Plasma diagnostic system at a distance of 10 cm from the needle tip. The observed relationship between emission current and beam divergence in vicinity of the emitting needle is used to design a focusing electrode for a field-emission electric propulsion thruster operating at currents up to 150 μA. Another application involves focused ion beam columns which may choose to forego a beam-limiting aperture, such as LMIS-based rapid machining tools with large beam currents.

Minimum emission current of indium‐field emission electric propulsion thruster

PurposeField emission electric propulsion (FEEP) thruster is a type of electric propulsion based on space‐proven indium liquid metal ion sources. The lifetime of FEEP thruster limits its application in space. A better method to improve its lifetime is the reduction of emission current. This paper aims to discuss the minimum emission current of operating FEEP thrusters.Design/methodology/approachIn this work, theoretical models, including fluid‐flow model and ion formation model, are analyzed. Current densities of these models are discussed and the minimum emission current is calculated.FindingsThere are few equilibrium states under low emission current conditions. However, the minimum emission current is the only stable state at which the FEEP thruster can operate.Research limitations/implicationsThis analysis is mainly based on the needle indium FEEP, which is compared indirectly with experiments of gallium.Practical implicationsThis paper attempts to help designers choose appropriate electric parameters to improve the lifetime of FEEP.Originality/valueBy introducing and analyzing theoretical models, this paper calculates the minimum emission current for stable operation of FEEP.

Field emission devices for space applications

AbstractIn the late 1950s, interest arose in the US to use electrostatic droplet sprayers as a new heavy charged particle source for electric space propulsion. Originally, charged droplets were produced from conductive organic liquids. Around 1960, first attempts were made to use liquid metals as the charge material. It was discovered that the device was a very high brightness source of metal ions, which could be focused to spots of nanometer dimensions. This liquid metal ion source (LMIS) has developed into a commercially exploited device for ion microbeam technology. In Europe, the potential of the LMIS as a very high specific impulse ion thruster has been recognized. Since the 1970s, development of cesium and, later, indium ion thrusters has been going on. Presently, the development is at an advanced stage. Contracts have been issued by ESA to equip microsatellites with an attitude control system based on field (ion) emission electric propulsion (FEEP). The first technological test to verify microgravity operation of the indium LMIS was performed in 1991 on the Russian MIR space station, and from 1992 the device has been exploited for active potential control of scientific satellites and for on‐site planetary mass spectrometry in seven space missions. The first gas field‐ionization source was flown as early as 1977 on a sounding rocket and in 1986 to comet Halley. Apart from these field emission devices actually flown, field ion‐ and electron devices are under development or discussion for a number of other space applications. The latest developments and future prospects are discussed. Copyright © 2007 John Wiley & Sons, Ltd.

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

Indium Field Emission Electric Propulsion Microthruster Experimental Characterization

Indium liquid metal ion sources have been flying for more than 10 years on a variety of spacecraft for spacecraft potential control and as the core element of mass spectrometers. Since 1995, a dedicated indium field emission electric propulsion (In-FEEP) thruster has been under development and recently passed a 2000-h endurance test. The In-FEEP thruster is a micropropulsion device for the 1-100 μN thrust range with low thrust noise and high resolution. The latest performance characteristics including direct thrust measurements and beam profiles are summarized. This information is very important for many upcoming missions that require ultraprecise drag-free control such as the Gravity Field and Steady-State Ocean Circulation Mission, LISA, Terrestrial Planet Finder/Darwin, or SMART-2.

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.

Modelling the link between emission current and LMIS cusp length

This paper presents new modelling results on the relationship between emission current and cusp length, for gallium and indium liquid-metal ion sources. The work is based on an improved version of the LMIS modelling program developed by Forbes and Ljepojevic, and achieves enhanced computational speed by means of parallel processing. Good agreement is found between the results derived from numerical simulation and experiment.

Repair of a liquid metal ion emitter by a liquid metal ion emitter

Liquid metal ion source (LMIS) indium emitters which have been poisoned by contamination with backsputtered material were successfully repaired by sputtering off the contamination layer, using a focused indium ion beam from an indium LMIS. It is essential that the repair beam consists of In ions to prevent the emitters from getting poisoned by the repair process itself. Poisoned emitters have excessively high firing voltages, but usually normal operating voltages. Following repair, firing voltage and operating voltage differed by not more than 150 V. Repairs worked equally well if the indium film on the emitter to be repaired was cold or hot (molten). Repairs were performed on fully assembled emitter modules where the emitter tip was recessed behind a stack of beam forming electrodes. Repair times are of the order of 10–15 min.

Ohmic Contacts on p-GaAs Implanted by Indium Liquid Metal Ion Sources

Ohmic Contacts on p-GaAs Implanted by Indium Liquid Metal Ion Sources

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.

Application of a liquid metal ion source to secondary ion mass spectrometry

An indium liquid metal ion source has been incorporated in a quadrupole-based ion microprobe and its performance as a primary ion source for secondary ion mass spectrometry was evaluated. Secondary ion emission of pure elements was studied for 10 keV In+ and 10keV O 2 + bombardment. For most of the elements investigated positive and negative secondary ion yields under In+ impact are comparable to those obtained with O 2 + primary ions. In addition, depth profiles of a Ni/Cr multilayer sample (10 nm single-layer thickness), a shallow oxygen implant in silicon and oxide layers on niobium were obtained using In+ ions for sputter erosion. The results indicate that liquid metal ion sources can be applied successfully in secondary ion mass spectrometry.