Positive Ion Beam Research Articles

Carbon Nitride Film Formation by Low Energy Positive and Negative Ion Beam Deposition

AbstractUsing a newly developed ion beam apparatus, PANDA (Positive And Negative ions Deposition Apparatus), carbon nitride films were prepared by simultaneous deposition of mass-analyzed low energy positive and negative ions such as C2-, N+, under ultra high vacuum conditions, in the order of 10−6 Pa on silicon wafer. The ion energy was varied from 50 to 400 eV. The film properties as a function of their beam energy were evaluated by Rutherford Backscattering Spectrometry (RBS), Fourier Transform Infrared spectroscopy (FTIR) and Raman scattering. From the results, it is suggested that the C-N triple bond contents in films depends on nitrogen ion energy.

Carbon Nitride Films Formed using Sputtering and Negative Carbon Ion Sources

AbstractWe report the results of using sputtering and negative carbon ion sources to prepare thin films of carbon nitride. In this work, we compare the structural, tribological, and optical properties of the carbon nitride films that were prepared by two different ion assisted techniques. In the first approach we used a magnetron gun to sputter deposit carbon in a nitrogen atmosphere. The second method utilized a beam of negatively charged carbon ions of 1 to 5 μA/cm2 current density impinging the substrate simultaneously with a positive nitrogen ion beam produced by a Kaufman source. We were able to synthesize microscopically smooth coatings with the carbon to nitrogen ratio of 1 : 0.47. These films possess wear rates lower than 5×10−7 mm3/Nm and friction coefficients in the range of 0.16 to 0.6. Raman spectroscopy revealed that the magnetron sputtered films are more structurally disordered than those formed with the negative carbon ion gun. FTIR showed the presence of the C≡N stretching mode in both types of films. Finally, spectroscopic ellipsometry produced films with dielectric constants as low as 2.3 in the photon energy range from 1.2 to 5 eV.

Simultaneous mass-analyzed positive and negative low-energy ion beam deposition apparatus

We have developed a new concept low-energy ion beam deposition apparatus. This machine can generate mass-analyzed very low energy ion beams with positive and negative charges at the same time. It is possible to deposit these ions not only simultaneously but also alternatively. The name is Taotron and the nickname is PANDA (Positive And Negative-ions Deposition Apparatus). The aiming specifications of this apparatus are: (1) available positive ions are H, B, C, N, O, Si, Fe, etc. and negative ions are H, B, C, O, Si etc., (2) ion energy range covers 10 eV to 20 keV, (3) typical ion beam current is ≥ 10 μA (for 10 eV oxygen ions with both charges) and the beam size is ≥ 10 mm ⊘, (4) the base pressure of the deposition chamber is in the order of 10 −8 Pa and the pressure during deposition is in the order of 10 −6 Pa.

Stone-age mass spectrometry: the beginnings of “SIMS” at RCA Laboratories, Princeton

This paper takes you back almost half a century, to the time when mass spectrometry was young and exciting, and research was exploring new frontiers every day. Our early experiments called “sputtering of surfaces by positive ion beams”, now known as “SIMS”, were carried out with the simple, yet flexible instrumentation available at that time and produced many interesting results that are still valid today. Since commercial equipment did not exist in those days, just about everything had to be designed and constructed in-house - hardware as well as circuitry. Thus progress was slow, but occasional malfunctioning of a component could be readily rectified. It is gratifying to see how SIMS has developed in the interim from our early feasibility studies into a major research effort carried out worldwide in many laboratories.

Static SIMS: Surface charge stabilization of insulators for highly repeatable spectra when using a quadrupole mass spectrometer

AbstractSurface charge stabilization is important to achieve stable static SIMS spectra from insulators. For static SIMS using a quadrupole mass spectrometer, the problem is exacerbated by the narrow spread of ion energies passed by the quadrupole system. Here, for a positive ion beam we study three methods of electron beam surface charge neutralization and compare their effectiveness for large‐area static SIMS analysis. These methods are: a traditional low‐energy flood source aimed at the sample; a 500 eV electron beam aimed at the sample; and a low‐energy source of scattered electrons placed between the sample and the quadrupole‐based static SIMS entrance ion optics. The latter method is shown to be efficient, easy to use and reliable. Variations in the surface potential are considerably reduced but small variations still show an effect leading to variable intensities in different peaks. This variation is reduced a further order of magnitude so that, in the static mode, final scatters for poly(tetrafluoroethene) have standard deviations below 2%. This final reduction is achieved by the application of a triangular waveform potential of 32 V peak‐to‐peak amplitude at 6.5 kHz frequency, to the sample holder and the sample environment. The mass spectra are then equivalent to the results for a mass spectrometer with a 32 eV wide energy acceptance window.

Selection and design of ion sources for use at the Holifield radioactive ion beam facilitya)

The Holifield Radioactive Ion Beam Facility now under construction at the Oak Ridge National Laboratory will use the 25 MV tandem accelerator for the acceleration of radioactive ion beams to energies appropriate for research in nuclear physics; negative ion beams are, therefore, required for injection into the tandem accelerator. Because charge exchange is an efficient means for converting initially positive ion beams to negative ion beams, both positive and negative ion sources are viable options for use at the facility. The choice of the type of ion source will depend on the overall efficiency for generating the radioactive species of interest. Although direct-extraction negative ion sources are clearly desirable, the ion formation efficiencies are often too low for practical consideration; for this situation, positive ion sources, in combination with charge exchange, are the logical choice. The high-temperature version of the CERN-ISOLDE positive ion source has been selected and a modified version of the source designed and fabricated for initial use at the facility because of its low emittance, relatively high ionization efficiencies, and species versatility, and because it has been engineered for remote installation, removal, and servicing as required for safe handling in a high-radiation-level ISOL facility. The source will be primarily used to generate ion beams from elements with intermediate to low electron affinities. Prototype plasma-sputter negative ion sources and negative surface-ionization sources are under design consideration for generating radioactive ion beams from high-electron-affinity elements. The design features of these sources and expected efficiencies and beam qualities (emittances) will be described in this report.

Recent advances in polarized ion source development (invited)

Experiments involving polarized beams and/or internal polarized targets are performed or proposed at many laboratories equipped with various type of accelerators and storage rings. Currently, the most powerful sources of polarized atoms and ions base on the atomic beam and the optical pumping methods. The present status of the polarized hydrogen source is discussed, with emphasis on the most recent improvements of the basic techniques allowing for the production of highly polarized, intense, dc or pulsed, positive or negative ion beams.

High-intensity, heavy negative ion sources based on the sputter principle (invited)

Due to their ease of operation, simplicity, long lifetime, and wide range of species capabilities, negative ion sources predicated on the sputter principle are being used for an increasing number of diverse applications. Sources based on this technology have been developed which utilize either direct cesium-surface ionization, or a plasma seeded with cesium to form positive ion beams for sputtering sample materials from which negative ion beams are generated. This article will include a brief review of the fundamental processes underlying negative ion formation by the sputter technique, as well as describe a selected number of recent ion source developments which exemplify the state-of-the art of this technology.

Selection and design of ion sources for use at the Holifield Radioactive Ion Beam Facility (abstract)a)

The Holifield Radioactive Ion Beam Facility now under construction at the Oak Ridge National Laboratory will use the 25-MV tandem accelerator for the acceleration of radioactive ion beams to energies appropriate for research in nuclear physics; negative ion beams are, therefore, required for injection into the tandem accelerator. Because charge exchange is an efficient means for converting initially positive ion beams to negative ion beams, both positive and negative ion sources are viable options for use at the facility. The choice of the type of ion source will depend on the overall efficiency for generating the radioactive species of interest. Although direct-extraction negative ion sources are clearly desirable, the ion formation efficiencies are often too low for practical consideration; for this situation, positive ion sources, in combination with charge exchange, are the logical choice. The high-temperature version of the CERN-ISOLDE positive ion source has been selected and a modified version of the source designed and fabricated for initial use at the facility because of its low emittance, relatively high ionization efficiencies and species versatility, and because it has been engineered for remote installation, removal and servicing as required for safe handling in a high-radiation-level ISOL facility. The source will be primarily used to generate ion beams from elements with intermediate to low ionization potentials. Prototype plasma-sputter negative ion sources and negative surface-ionization sources are under design consideration for generating radioactive ion beams from high-electron-affinity elements. The design features of these sources and expected efficiencies and beam qualities (emittances) will be described in this report.

A theoretical model for caesium plasma discharges

Plasma volume sources have been used very successfully to produce positive or negative ion beams for many applications. The development and testing of a 2.51 plasma source working in pure caesium vapour is described. Measurements of plasma density and electron temperature were made using a planar Langmuir probe and the results are presented. The pure caesium discharge apparently differs from conventional gaseous discharges in the roles of the primary and thermal electrons, due to the large cross sections for collisions with the neutral caesium atom. A model which attempts to describe this behaviour is discussed and compared with the probe data.

Time-of-flight mass spectrometry using mega-electronvolt neutral beams

Neutral carbon, nitrogen and oxygen beams with an energy range of 300 keV–2 MeV (velocity range of 0.2–0.6 cm ns −1) have been used to induce secondary ion emission from solid samples. In this work, results for the ion desorption yield for hydrogen and some organic species emitted by bombardment of neutral and low charge state carbon beams on a phenylalanin target are reported. The positive and the negative ion mass spectra obtained for neutral projectiles show very similar features as those obtained for 1 + −6 + charge state projectiles, However, the ion emission yields corresponding to neutral beams are observed to be slightly smaller than those produced by positive ion beams. The secondary ion desorption yield dependence on the projectile velocity is also analyzed. It is found that the hydrogen desorption yield slowly decreases with the projectile velocity, whereas the yields of the other desorbed ions show a second to third power dependence of the projectile stopping power in the target.

A simple positive/negative surface ionization source

A versatile, spherical-geometry, self-extraction surface ionization source has been designed, fabricated, and characterized. The source utilizes direct-surface ionization to form either positive or negative ion beams resulting from interactions between highly electropositive or electronegative atoms or molecules and a spherical-sector surface ionizer maintained at ∼ 1100°C. For positive ion generation, the surface ionizer is chosen to be a high-work-function metal such as Mo, Ta, or W; while for negative ion formation, a LaB 6 surface ionizer is selected. The design features and performance of the source for each of these modes of operation are discussed in this report.

Rf driven multicusp ion source for pulsed or steady-state ion beam production

An rf driven multicusp source has been operated in both cw and pulsed mode to generate H + and various types of positive ion beams. For inert gas plasmas such as He, Ne, Ar, Kr and Xe, the extractable current density is higher than 1 A/cm 2. When diatomic gases, such as H 2, N 2 and O 2 are used for the discharge, the extracted beam contains almost pure atomic ions. Thus, simple accelerator or plasma processing systems can be constructed without the use of mass analyzing magnets. The same source has also been used to produce steady-state beams of metallic ions.

ECRIS for highly charged ions−Present status and new developments (invited)(abstract)

There are now approximately 40 electron cyclotron resonance ion sources (ECRIS) for highly charged positive ions, either operating or under construction, worldwide. In recent years, the number of operating sources has been doubling about every three years. The present working level for high performance is now a compact, CW 14-GHz source with high magnetic confinement, simplified microwave injection, and a somewhat suppressed first stage. It has become clear that electron injection into the main stage plasma, from electron guns, biased cathodes, or high secondary electron emission surfaces close to the plasma, raises ion extracted currents, and several efforts are under way to understand this phenomena. Much effort has also been devoted to the production of ions from nongaseous materials, with positive ion beams from approximately half the periodic table now having been produced in ECRIS. The afterglow mode may permit intense short pulse beams of highly charged ions for injection into synchrotrons. Present development efforts are centered on understanding the performance scaling with microwave frequency and magnetic confinement, as well as continued development of intense highly charged ions in existing sources and the development of new metallic species.

Observations on charging effects of non-conductive and uncoated materials by ion beam pre-bombardment in scanning electron microscopy

The specimen charging defects of non-conductive materials in scanning electron microscopy are discussed with reference to the surface electric field generated by the illuminating electron beam dose. If the charge density depends on the relaxation time constant as defined by a product of the permittivity and resistivity when known or available, the electric field can be evaluated by the incident dose stored when illuminated by an electron scanning beam. It was found by observation that uncoated or non-conductive materials pre-bombarded by a positive ion beam, which contributes to the generated negative field, together with the charging effects, could be eliminated at the optimum time of neutralization. In the normal process of double fixation and staining of biological specimens, the local electric field produces increased contrast due to polarization effects. The dark and bright images of secondary and backscattered electrons, respectively, can be analysed by taking into account local polarization, in addition to voltage contrast.

A high intensity cluster ion source for fast beam experiments

A high intensity cluster ion source that can be used for various fast beam spectroscopic studies has been constructed and demonstrated. The cluster ions are grown around seed ions that are generated by electron impact ionization of gas very near the exit of the nozzle (∼1 nozzle diameter distance). The generated cluster ions are coaxially focused and extracted from the pulsed (10 Hz, 200 μs pulse) supersonic expansion through a skimmer by applying a weak (∼1 V/cm) electric field, and injected into a photofragment spectrometer. With the source we have successfully generated well-collimated intense beams of various positive and negative cluster ions, such as H+n, H3O+■ (H2)n, (CO)+n, (N2)+n, (CO2)+n, (NO)+n, He+n, Ar+n, Xe+n, and (CO2)−n, with peaks currents of ∼500 pA at cluster size n=10. Preliminary photofragmentation studies of selected cluster ions have been performed to demonstrate the capability of the source.

Beam emittance growth caused by a nonuniform ion density

An expression for the root-mean-square emittance of an ion beam is derived in terms of the distribution of particle density at the plasma boundary. Three distributions are considered: a uniform plasma, a linear variation due to B×v effects, and a quadratic variation. It is shown that for the nonuniform distributions additional terms enter the expression for beam emittance causing emittance growth. Experimental data, in agreement with the model, are presented for both positive and negative ion beam systems. Applying the model to the data provides values of ion temperature within the ion source and the degree of nonuniformity across the plasma boundary for several different ion source systems.

Intense negative hydrogen ion production in high-power diodes

Some peculiarities of using negative ions for accelerators and CTR are examined. Highpower beams of fast atoms obtained by charge exchange of negative ions can have advantages over positive ion beams for inertial confinement fusion (ICF) since they do not experience any deviations, instabilities, and scattering. H− current up to 7 kA in a 100–200 ns pulse with densities up to 200 Å/cm2 and a voltage of 0.8 MV were obtained in experiments on magnetically insulated diodes at the Lebedev Physical Institute. Diagnostics of high-power H− beams are described. Experimental results on cathode plasma formation by surface discharge on a dielectric initiated by a bipolar prepulse or by laser illumination are presented. The dependence of H− yield on cathode current was determined and experiments on plasma stabilization are described. It is concluded that 1 kÅ/cm2 of H− can be obtained.

The effects on ion beam optics of a spherical aberration at the plasma boundary

Experimental data show that the minimum divergence of a positive helium ion beam does not occur at the beam perveance predicted by linear ion optics. A model is presented that introduces a third-order term into the ion optics equations, in the manner of a spherical aberration to the plasma boundary. Values of the aberration coefficient C are calculated from the data and it is shown that C∝(a/d)−1.9, where a/d is the aspect ratio of the accelerator. The contour of the plasma boundary is plotted and it is shown that the aberration leads to a flattening of the boundary at the extremes of the extraction aperture.

Direct Energy Recovery from Unneutralized Ion Beams in a Negative-Ion-Beam-Based Neutral Beam Injection System

This paper reports on a beam direct energy convertor (BDC) designed to recover unneutralized ion beam energies in a 500-keV negative-ion-based deuterium neutral injection system for the Fusion Experimental Reactor of Japan Atomic Energy Research Institute. A newly developed three-dimensional beam transport code KUNABE-3 is used. Due to approximately equal fractions of unneutralized D{sup +} and D{sup {minus}} beams flowing from the gas neutralizer, electrostatic electron suppression is efficient. Also, magnetic separation and deflection of both species by a 1-kG magnetic field are efficiently applicable. Under suitable energy recovery conditions, perfect collection of both positive and negative ion beams is theoretically achievable, even for a collector voltage of {plus minus}480 kV, resulting in 96% energy recovery efficiency. Within {plus minus}10% deviation from the reference parameters, the designed BDC shows excellent performance for such parameters as magnetic fields, incident beam energies, and gas line densities.