The plasma-emitter type ion source

Production, acceleration and diagnostics of high intensity beams

A new high current ion implanter "LUXiON" has been developed for semiconductor production. LUXiON implant several ion species to 300 mm silicon wafer by vertical long ion beam and horizontal mechanical scan wafer handling system. We describe the tool outline, beam result and implant results.

Summary form only given. The MEVVA ion source has been intensively used to provide high current metallic ion beams for film deposition and material modification over the last two decades. At GSI it is using to produce a high particle current to fill up the heavy ion synchrotron SIS to its space charge limit. Many efforts have been made at GSI, to increase the beam brightness, to enhance the fraction of charge state 4 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> for the design ion, to reduce the beam noise, and to improve the beam stability. To deliver the desired ion beam, a multi-aperture triode extraction system is used due to high mass to charge ratio of the design ion. In this paper, a fully three dimensional and interactive code KOBRA3-INP was used to investigate the behavior of the ion beam extracted from a high current MEVVA ion source and the effects of plasma density on extracted high current beam. The beam trajectory and space charge map in the extraction system are presented. Simulation shows that there exists a minimum divergence for the extracted ion beam at the plasma density of about 190 mA/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> , which corresponds to the perveance matched condition. Furthermore, as the plasma density increases, the intensity of the extracted ion beam increases. If the experimental data with an extracted current of about 135-140 mA are used, the experimental data can be reproduced in the simulation with the longitudinal ion energy of 188 eV, the transverse ion energy of 100 eV, and the plasma density of 165 mA/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> under the assumption of the full space charge compensation.

The temperature control system of the large-size plasma grid has been developed to realize the long pulse production of high-current negative ions for JT-60SA. By using this prototype system for the JT-60SA ion source, 15 A negative ions has been sustained for 100 s for the first time, which is three times longer than that obtained in JT-60U. In this system, a high-temperature fluorinated fluid with a high boiling point of 270 degree Celsius is circulated in the cooling channels of the plasma grids (PG) where a cesium (Cs) coverage is formed to enhance the negative ion production. Because the PG temperature control had been applied to only 10% of the extraction area previously, the prototype PG with the full extraction area (110 cm × 45 cm) was developed to increase the negative ion current in this time. In the preliminary results of long pulse productions of high-current negative ions at a Cs conditioning phase, the negative ion production was gradually degraded in the last half of 100 s pulse where the temperature of an arc chamber wall was not saturated. From the spectroscopic measurements, it was found that the Cs flux released from the wall might affect to the negative ion production, which implied the wall temperature should be kept low to control the Cs flux to the PG for the long-pulse high-current production. The obtained results of long-pulse production and the PG temperature control method contributes the design of the ITER ion source.

Microwave ion source for ion implantation

The ion-source and beam-transport characteristics required for industrial application are discussed. Currently, a high-current broad ion beam extracted from multiapertures is most commonly used for large area surface modification and ion beam assisted deposition. The beam quality strongly depends on the space-charge neutrality of the ion beam and nonuniformity of plasma density as well as beam emittance. To produce selective ion species without mass separation, the electron energy for ionization or electron temperature must be controlled. Applications for negative and molecular ions are also discussed. A charging-less ion implantation technique can be realized using negative ions. With molecular ions, ultra-low-energy ion implantation is possible, as is same-depth implantation of multielements.

A vacuum arc ion source provides high current beams of metal ions that have been used both for accelerator injection and for ion implantation, and in both of these applications the degree of space charge neutralization of the beam is important. In accelerator injection application, the beam from the ion source may be accelerated further (post-acceleration), redirected by a bending magnet(s), or focused with magnetic or electrostatic lenses, and knowledge of the beam space charge is needed for optimal design of the optical elements. In ion implantation application, any build-up of positive charge in the insulating targets must be compensated by a simultaneous flux of cold electrons so as to provide overall charge neutrality of the target. We show that in line-of-sight ion implantation using a vacuum arc ion source, the high current ion beam carries along its own background sea of cold electrons, and this copious source of electrons provides a “self-neutralizing” feature to the beam. Here we describe experiments carried out in order to demonstrate this effect, and we provide an analysis showing that the beam is space-charge-neutralized to a very high degree

Pulsed vacuum arcs based on a deuterium-saturated zirconium cathode are widely used for producing deuterium plasma in pulsed ion sources1,2 for different applications, as, for example, neutron flux generation for boron neutron capture therapy. For efficient operation of such ion sources, it is required that the fraction of deuterium ions in the plasma be as high as possible and the fraction of zirconium ions be the least. Here we report on experimental research to maximize the fraction of deuterium ions in the plasma of a vacuum arc and to generate high-current pulsed ion beams with a high deuterium content using a Mevva-V.Ru 3 vacuum arc ion source. Also reported are time-of-flight spectrometry data on the constitution of such beams and on the masscharge state of the plasma as well as comparative data on the fraction of deuterium ions at different durations of discharge pulses ranging from 100 A to 1 kA. At discharge pulse durations of hundred microseconds, the content of deuterium in the plasma coincides with its percentage in the cathode material, and at shorter durations, it can even exceed this value. The physical features that provide the generation of deuterium-rich plasma and its application to produce deuterium ion beams are discussed.

The high power ion beams used in the next generation thermonuclear fusion reactors require high current negative ion beams accelerated to high energy, with high efficiency. One way to meet these requirements is to merge multiple low current density H− beamlets into a single high current beam. The feasibility of a high current merging preaccelerator was demonstrated in this experiment by merging 19 beamlets of H− ions distributed over a circular area 80 mm in diameter from a Japan Atomic Energy Research Institute negative ion source. H− ions were extracted at a current density exceeding 10 mA/cm2 at the ion source which operates at 0.13 Pa (1 mTorr), with a low arc power density (70 V×250 A). Spherically curved grids (with built-in magnetic electron suppression) were used in the preaccelerator to focus the extracted beamlets into a single 104 mA, 100 keV beam. The merged beam has a diameter of 23 mm and a converging angle of ±30 mrad at the beam envelope. The rms emittance of the 104 mA merging beam was 1.00 π mrad cm, which is a condition acceptable to the electrostatic quadropole accelerator for further acceleration.

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.

Ion source and injector experiments at the HIF/VNL

It is proposed one universal H-/D- ion source design combining the most advanced developments in the field of polarized ion sources to provide high-current high-brightness ion beams with >90% polarization and improved lifetime, reliability, and power efficiency. The new source utilizes high-efficiency resonant charge-exchange ionization of polarized neutral atoms by negative ions generated by cesiated surface-plasma interactions via a multi-spherical negative ion focusing element. Multi-spherical focusing of the negative ions strongly suppresses the parasitic generation of unpolarized H-/D- ions. By incorporating new and novel designs for the dissociator and plasma generator in parallel with the multi-spherical focusing the design can suppress adsorption and depolarization of particles from the polarized beam greatly improving performance over current concepts.

Noble metal nanoparticles (NPs) are important in many applications, including light trapping of photovoltaic cells, photoelectrochemical applications, etc. The present study reports the formation of silver NPs from the as-deposited silver coatings on fused silica substrates by solid-state dewetting induced by high-current intense pulsed ion beam (IPIB) irradiation. We described the effects of IPIB irradiation with different ion beam current densities and numbers of pulses on NP morphology and compared the results with conventional rapid thermal annealing (RTA). IPIB irradiation enables superfast heating (higher than 109 K/s) and cooling, providing a superfast annealing solid-state dewetting mechanism. Our results demonstrate that the sphericity of silver NPs is enhanced after IPIB irradiation relative to RTA-annealed silver NPs. Our results suggest further possibilities of shape and sphericity control of silver NPs with very fast heating/cooling annealing rates.

The converter-type negative ion source currently employed at the Los Alamos Neutron Science Center (LANSCE) is based on cesium enhanced surface production of H(-) ion beams in a filament-driven discharge. In this kind of an ion source the extracted H(-) beam current is limited by the achievable plasma density which depends primarily on the electron emission current from the filaments. The emission current can be increased by increasing the filament temperature but, unfortunately, this leads not only to shorter filament lifetime but also to an increase in metal evaporation from the filament, which deposits on the H(-) converter surface and degrades its performance. Therefore, we have started an ion source development project focused on replacing these thermionic cathodes (filaments) of the converter source by a helicon plasma generator capable of producing high-density hydrogen plasmas with low electron energy. In our studies which have so far shown that the plasma density of the surface conversion source can be increased significantly by exciting a helicon wave in the plasma, and we expect to improve the performance of the surface converter H(-) ion source in terms of beam brightness and time between services. The design of this new source and preliminary results are presented, along with a discussion of physical processes relevant for H(-) ion beam production with this novel design. Ultimately, we perceive this approach as an interim step towards our long-term goal, combining a helicon plasma generator with an SNS-type main discharge chamber, which will allow us to individually optimize the plasma properties of the plasma cathode (helicon) and H(-) production (main discharge) in order to further improve the brightness of extracted H(-) ion beams.

The SPIRAL2 project, currently under construction at GANIL, will include an isotope separator on line based facility for the production and acceleration of radioactive ion beams. A superconducting linear accelerator will accelerate 5 mA deuterons up to 40 MeV and 1 mA heavy ions up to 14.5 MeV/u. These primary beams will be used to bombard both thick and thin targets. We are investigating three different techniques to produce the radioactive ion beams: (1) the neutron induced fission of uranium carbide, (2) the direct interaction of deuterons in a uranium carbide target, and (3) the interaction of a heavy ion beam with a target. All these production systems will be coupled to an ion source. Four kinds of ion sources are foreseen for the ionization of the radioactive atoms: an electron cyclotron resonance ion source, a surface ionization ion source, a forced electron beam induced arc discharge ion source, and a laser ion source depending on the characteristics of the desired radioactive ion beam in terms of intensity, efficiency, purity, etc. A presentation of the SPIRAL2 project and of the different production systems is given.