Fast Neutral Beams Research Articles

Fast and cold negative ion and neutral atom beams from a water spray

High-power lasers are routinely used to generate energetic positively charged ions, and this paper reports on the observation of negative ions in these experiments. A large number of negative ions and neutral atoms at MeV energies was obtained from the interaction of a high intensity laser pulse with a water spray along with positive ions [Abicht et al., Appl. Phys. Lett. 103, 253501 (2013)]. Beams of negative ions and neutral atoms have the same properties as beams of positive ions. However, the mechanism of negative ion formation and acceleration is still under discussion. In order to gain more information about physics of generation negative ions and neutrals, we present a new experiment where all species, positive, negative ions, and neutrals, are spatially separated, and the electron capture and loss of each in water spray is evaluated. The formation of negative ions and neutral atoms of hydrogen and carbon with energies up to 140 keV and 1.2 MeV, respectively, is confirmed. It is suggested that the electrification of spray droplets plays a decisive role in these charge-exchange processes.

Influence of different magnetic configurations on plasma parameters in SPIDER device

The ITER fusion reactor will be heated by fast neutral beams generated by accelerating and neutralizing negative ions, produced in a RF inductively-coupled plasma which expands through a region featuring a magnetic filter. During the current shut down of the SPIDER device (until 2023) some modifications of the device have been planned to improve the system performance. These modifications are based on the experience gained during operation of SPIDER in previous years and include, among others, the addition of further sets of permanent magnets in the expansion chamber of the plasma source and around the RF drivers. The purpose of these modifications is to improve plasma confinement, and thus increase its density and homogeneity. The present paper reports the results of numerical studies of the plasma parameters in SPIDER source with different types of modifications of SPIDER device including new permanent magnets configurations. Analysis are done by means of the numerical code FSFS2D in which a self-consistent two-dimensional fluid description of the source is implemented. In order to partially account for the 3D flow pattern within our 2D model, simulations are run in two different layouts, in the vertical plane with the magnetic filter being perpendicular to the integration domain and in the horizontal plane with the filter field in the integration surface. Simulation results indicate, that the permanent magnets around the RF driver tends to increase the plasma density in the driver.

Applications of the Technology of Fast Neutral Particle Beams in Micro- and Nanoelectronics

Applications of the Technology of Fast Neutral Particle Beams in Micro- and Nanoelectronics

Applications of the Technology of Fast Neutral Particle Beams in Micro- and Nanoelectronics

A brief review of the results of the application of fast neutral particle (FNP) beam sources in the field of the production technology of micro- and nanoelectronic devices published in the literature is presented. Processes such as surface cleaning; sputtering and etching of insulators, simiconductors, and metals; layer-by-layer etching; deposition of thin films directly from beams or by sputtering, and neutral-beam-enhanced deposition; oxidation, nitridation, and fluorination of a near-surface layer; production of the elements of micro- and nanoelectromechanical devices; nanostructuring; and modification of self-assembled organic molecular structures are considered. In addition, the results of investigating a decrease in the degree of degradation of the electrophysical properties of materials and electrical characteristics of device structures when passing from ion-beam technologies to FNP beams are discussed.

Module for dielectric surfaces modification by fast neutral particles beams

In this paper, we describe the module for dielectric and wide-gap semiconductor surfaces modification by fast neutral beam. The module can be used for cleaning, etching or assisting of films deposition. The surface proceeding by neutral beam can prevent an accumulation of surface charge without using current compensation by inserting electrons to the beam or RF power supply. The module beside cathode and anode contains an electrode with floating potential. Insertion of the additional electrode causes electron retention in an electrostatic trap resulting the reducing of the module operating pressure. Moreover, the electrode with floating potential allows increasing the current efficient of the module. An important feature of the module is that neutralization of the ions extracted from the plasma occurs in the cathode potential well. Thereby ions that have not neutralized cannot leave nearcathode region and there are no fast ions in the output beam. Module does not contain sources of the magnetic fields or elements heated by external sources. Module operates with free cooling. Thus, the module does not need water cooling and can be freely moved in the vacuum chamber.

Investigation into the selectivity of etching various materials by fast neutral particle beams

The results of an investigation into the etching rate and selectivity of silicon substrates, as well as SiO2, W, TiN, TiC, and NbN films, with the use of chemically active compounds CF4, C3F6, and SF6 as working gases, which were obtained using a Neutral-110L source of extended fast neutral particle (FNP) beams developed at the Institute of Physics and Technology of the Russian Academy of Sciences (FTIAN), are presented. The possibility of attaining etching selectivity by FNP beams of the mentioned materials relative to SiO2 in a range of 10–16 is shown. The data are compared with the currently published results of investigations into this region.

Generation of fast neutral beams based on closed drift ion sources

This paper demonstrates the possibility of the creation of a fast neutral beam source based on Radikal ion sources of a different configuration. Our measurements indicate that the output beam of such a source is 100%-neutral independently of the energy of particles and their composition. This effect is achieved with a neutralization channel and an electrostatic deflection system. The high directionality of a beam and the possibility of its long-distance transport have also been demonstrated. Some results of studying the rate of the sputtering of different materials with a neutral Ar0 beam are given.

Alfvén Ion-Cyclotron Instability in a Mirror Trap with Skew Injection of Neutral Beams

The Alfvén ion cyclotron (AIC) instability margin in a mirror trap with skew injection of fast neutral beams into a target plasma is investigated in the present work. The instability is driven by inverse population of trajectories of resonant ions having velocity near the injection velocity. So, the stability margin depends strongly on injection details, in particular an injection angle and angular width. The absolute instability margin analysis as well as WKB-analysis on longitudinal and transversal coordinates are used for the stability threshold studying. Simple estimations relating the wave parameters to injection parameters are presented. The stabilizing effect of strong transversal non-uniformity is shown.

Beams of fast neutral atoms and molecules in low-pressure gas-discharge plasma

Fast neutral atom and molecule beams have been studied, the beams being produced in a vacuum chamber at nitrogen, argon, or helium pressure of 0.1–10 Pa due to charge-exchange collisions of ions accelerated in the sheath between the glow discharge plasma and a negative grid immersed therein. From a flat grid, two broad beams of molecules with continuous distribution of their energy from zero up to e(U + Uc) (where U is voltage between the grid and the vacuum chamber and Uc is cathode fall of the discharge) are propagating in opposite directions. The beam propagating from the concave surface of a 0.2-m-diameter grid is focused within a 10-mm-diameter spot on the target surface. When a 0.2-m-diameter 0.2-m-high cylindrical grid covered by end disks and composed of parallel 1.5-mm-diameter knitting needles spaced by 4.5 mm is immersed in the plasma, the accelerated ions pass through the gaps between the needles, turn inside the grid into fast atoms or molecules, and escape from the grid through the gaps on its opposite side. The Doppler shift of spectral lines allows for measuring the fast atom energy, which corresponds to the potential difference between the plasma inside the chamber and the plasma produced as a result of charge-exchange collisions inside the cylindrical grid.

Fast (tens to hundreds of eV) neutral beams for materials processing

Fast neutral beams (beam energy of tens to hundreds of eV) may be useful for mitigating charging damage that can occur in conventional plasma processing of materials. For neutral beam processing to be viable, however, the beam energy, flux and directionality must be comparable to those in traditional reactive ion etching or reactive ion beam technologies. This paper provides a review of fast neutral beams for materials processing. Neutral beam generation techniques are outlined. Characterization of neutral beams is important to measure the beam flux, energy and angular distributions. Neutral beam materials processing is discussed with emphasis on etching of thin films.

Generation of Fast Neutral Beams by Ion Neutralization in High-Aspect-Ratio Holes: A Particle-in-Cell Simulation Study

In this paper, a particle-in-cell simulation of ion and neutral-beam extraction through a grid with high-aspect-ratio holes in contact with plasma was developed. Particular emphasis was placed on plasma molding over the holes, ion neutralization along the sidewall of the holes, and the energy and angular distributions of the residual ions and fast neutrals in the beam downstream of the holes. The target application was the generation of fast neutral beams for future charge-free microelectronics manufacturing. The energy and angular distributions of ions at different locations along the hole sidewall showed that ions which neutralize on the top section of the surface of the hole are “bad,” in the sense that these ions yield divergent neutral beams of relatively low energy. Ions that neutralize along the bottom section of the hole sidewall are “good,” in the sense that these ions yield neutral beams that are less divergent and retain more of the energy of the parent ions. A three-grid ion-extraction/neutralization system is proposed to increase the fraction of good ions and produce a higher quality neutral beam.

An improved installation for high-resolution laser spectroscopy on ion beams

For many years, a complex installation has been developed in our laboratory for high-resolution laser spectroscopy of fast ion and neutral beams. Various results have been obtained for different species (H2, O2+, N2+, N2O+, and CO2+). The different spectroscopic techniques applicable to an accelerated beam are presented or recalled in this work to show the performance of the apparatus. Some difficulties in understanding the b4 Σ–g O2+ predissociation process led us to modify the molecular photofragment detection system and make it more efficient, as shown by the results presented. PACS No.: 39.30.+w

The effect of charge exchange with neutrals on the saturation of the spectral line intensities of multicharged ions in plasma

The influence of charge-exchange processes on the spectral line intensities of impurity ions in the edge and core plasmas of fusion devices is considered. It is found that, at a sufficiently high density of neutrals, the rate at which the atomic states are populated through charge exchange becomes independent of the neutral density, which results in the saturation of the spectral line intensities. This effect can substantially limit the efficiency of impurity-ion spectroscopy. Conditions under which the saturation effect manifests itself are examined for both the edge and core plasma regions in the presence of fast neutral beams. The results of calculations for the edge plasma are used to interpret the experimental data from the TORE SUPRA tokamak. It is shown that, in the central plasma region, the intensities of the visible spectral lines associated with the charge exchange of impurity ions in the course of neutral beam injection decrease (rather than increase, as was expected earlier) with increasing ion charge.

Broad fast neutral molecule beam sources for industrial-scale beam-assisted deposition

A new beam-assisted deposition technique is presented featuring fast neutral molecule beams being used instead of conventional ion beams. As compared with ions the neutral beams ensure better treatment stability, reliability, eliminate damages of conductive films with unipolar arcs and substantially reduce the number of beam-induced defects in semiconductors and dielectrics. Fast molecules are being produced as a result of the primary ion beam interaction with slow gas molecules in a charge transfer collision chamber. Acceleration–deceleration of primary ions in two positive space charge layers separated from each other with one accelerating grid allows the beam current density up to 10 mA/cm 2 at any energy of accelerated particles beginning from 10 eV up to several keV. Due to generation of the ion emitter using a cold cathode glow discharge with electrostatic electron confinement, beams of any reactive gas molecules are available. By changing the shape of the accelerating grid and of the electrostatic trap, any beam configurations are available: converging; diverging; or highly coherent beams. Three pilot series of fast molecule sources are developed. Beam cross-section up to 10 4 cm 2 and current up to 10 A lead to industrial-scale beam-assisted deposition at 3×10 −4–6×10 −3 mbar. Negative biasing of a target immersed in a plasma emitter of the beam source allows utilization of secondary electron emission for energy-effective heating of massive substrates and utilization of the target sputtering for metal vapor production. Both processes are inherently unavoidable but still rarely used by-processes of plasma immersion ion implantation.

Energy dissipation of fast neutral beams scattered at glancing angles from crystal surfaces

We provide a model calculation of the contribution of the non-adiabatic response of the substrate electronic degrees of freedom to the energy dissipated by a fast neutral particle scattered under a glancing angle of incidence from a metal surface. These degrees of freedom couple to the monopole charge or static induced dipole of the moving particle. The coupling strength is additionally modulated by the corrugation of the surface electronic density profile. We calculate the mean energy loss and mean number of electron-hole pairs created due to the interaction with an Al(111)-surface, via both types of coupling, of neutral Ar atoms as a function of the parallel velocity of the incident particles in the velocity range 0.02–1 v F (where ν F is the Fermi velocity of electrons in the metal). We find that the corrugation-induced oscillation of the coupling strength between the traversing particle and the surface can provide a significant contribution to the energy loss, irrespective of the nature of the coupling assumed.

Sub-Doppler spectroscopy of 89Y

Sub-Doppler resolution of atomic transitions is obtained by spatially resolved laser induced fluorescence in a diverging atomic beam created by laser ablation. Resonances are spatially separated by relativistic effects experienced between the probing radiation and the moving particles thus proving that it is a high-quality atomic beam. Fast neutral atomic beams of 89Y having centroid energies up to 12 eV are interrogated with tuned, single-frequency laser light. Narrow, spatially separated points of light move along the probe axis and are imaged by detecting only the zero-velocity components of the fluorescence. Accordingly, the Y 2D3/2→Y 2D03/2 transition centered near 16 146 cm−1 is resolved into its hyperfine components with line widths on the order of 55 MHz and with an equivalent resolution of ±5 MHz (±0.0002 cm−1). The upper level hyperfine interaction constant derived from level splitting is +89±5 MHz. An equivalent collimation ratio of 9 mrad is estimated for the expanding pulsed atomic beam. We propose that laser ablation of selected targets provides a unique source of neutral or ionic ground-state atoms and molecules for spectroscopic studies. Details of these experiments are discussed and analyzed.

Self-calibrating magnetic field diagnostics in beam emission spectroscopy

Magnetic field diagnostics in tokamaks using the motional Stark effect in fast neutral beams have been based on two kinds of polarimetry which we call ‘‘static’’ and ‘‘dynamic.’’ A detailed analysis shows that static polarimetry presents a number of advantages over dynamic polarimetry, provided it is made complete in the sense that a sufficient number of polarization analyzers are installed and different parts of the spectrum are explored to yield full information on the set of unknowns inherent in the problem. A detailed scheme of complete static polarimetry is proposed, including the case where an in-vessel mirror with changing characteristics (coating by impurities) is placed in front of the optical detection system. The main merit of this scheme relies on the fact that it is self-calibrating with respect to both the characteristics of the mirror and the transmission of the different polarization channels, the latter item implying that it is uniquely based on relative measurements of spectra. Further advantages are a greater flexibility with regard to different kinds of diagnostics and the circumstance that the technical equipment is less involved. The above scheme is based on a detection system of moderate etendue exploiting a large spectral domain, which is the regime where static polarimetry usually operates. It is also possible, however, to work with large etendue and a small spectral domain, such as commonly adopted in dynamic polarimetry. Using such a regime, static polarimetry loses the advantages mentioned above but gains, as a new advantage, the benefit of a comparatively lower level of photon noise.

Heating and plasma interactions with beams of energetic neutral atoms

The use of fast neutral atom beams to heat toroidally confined plasmas is discussed. The conditions governing the beam power and energy requirements are given, and the limitations on the beams used to achieve these are discussed. The interaction of these beams with the plasma involves several classical collision processes which are described by a Fokker-Planck equation, all the terms of which have now been checked experimentally. The transfer of momentum to the plasma is described, one particular aspect of which is the creation of a beam-driven current. The existence of this current has now been established both experimentally and theoretically and may allow the construction of a continuously operated tokamak reactor.

A mechanical velocity discriminator for fast neutral molecular beams

The transmission characteristics of a mechanical velocity discriminator are analysed and a discriminator design is described in which the ratio (minimum velocity of transmitted beam)/(rotor speed)=5.99*103 cm rev-1. Preliminary results of its operation in conjunction with a fast caesium atom beam are presented.

Origin of Secondary Electrons Ejected from Gas-Covered Surfaces by Fast Neutral Beams

Secondary electron emission from gas covered metal surfaces, discussing results of neutral beam investigations