Increasing Ion Energy Research Articles

Effect of Ion Energy on Charge Loss From Floating Gate Memories

Heavy ions typical of the space environment have energies which exceed by orders of magnitude those available at particle accelerators. In this paper we are irradiating state of the art floating gate memories by using both a medium energy (SIRAD) and a high energy (RADEF) facilities. The corruption of stored information decreases when increasing ion energy. The proposed model deals with the broader track found for higher energy ions. Implications for testing procedures and for reliability considerations are discussed.

Simulation of dynamic changes in the surface composition of tungsten under ion irradiation with regard to carbon impurity deposition

Argon ion cleaning of a tungsten surface covered by a carbon film was simulated taking into account the redeposition of graphite. An increase in the argon ion energy from 200 to 1000 eV greatly enhances cleaning. The cleaning rate is lowered when the carbon flux to the surface increases.

Surface modification and chemical sputtering of graphite induced by low-energy atomic and molecular deuterium ions

The surface morphology, and chemical/structural modifications induced during chemical sputtering of ATJ graphite by low-energy (<200 eV/D) deuterium atomic and molecular ions are explored by Scanning Electron Microscopy (SEM), Raman and Auger Electron Spectroscopy (AES) diagnostics. At the lowest impact energies, the ion range may become less than the probe depth of Raman and AES spectroscopy diagnostics. We show that such diagnostics are still useful probes at these energies. As demonstration, we used these surface diagnostics to confirm the characteristic changes of surface texture, increased amorphization, enhanced surface reactivity to impurity species, and increased sp 3 content that low-energy deuterium ion bombardment to steady-state chemical sputtering conditions produces. To put these studies into proper context, we also present new chemical sputtering yields for methane production of ATJ graphite at room temperature by impact of D 2 + in the energy range 10–250 eV/D, and by impact of D + and D 3 + at 30 eV/D and 125 eV/D, obtained using a Quadrupole Mass Spectroscopy (QMS) approach. Below 100 eV/D, the methane production in ATJ graphite is larger than that in HOPG by a factor of ∼2. In the energy range 10–60 eV/D, the methane production yield is almost independent of energy and then decreases with increasing ion energies. The results are in good agreement with recent molecular dynamics simulations.

Ion beam sputtered ultra-thin and nanostructured Ag films for surface plasmon applications

The growth of ultra-thin (10−17nm) Ag films by ion beam sputtering with controlled morphology suitable for surface plasmon applications is demonstrated. A growth rate of 0.02nm/s, by employing Ar ion energies as low as 150eV, is achieved. These conditions result in a surface morphology that consists of oblate particles of size 10–30nm, depending on the ion energy, with surface roughness of 1–3nm. The aspect ratio of the grains decreases from 77 to 20 with increase in incident ion energy. The morphology-induced changes are manifested in the shift of the surface plasmon resonance peak from 440 to 480nm.

Temperature dependence of the chemical sputtering of amorphous hydrogenated carbon films by hydrogen

The temperature dependence of chemical erosion and chemical sputtering of amorphous hydrogenated carbon films due to exposure to hydrogen atoms (H 0) alone and combined exposure to argon ions and H 0 was measured in the temperature range from 110 to 950 K. The chemical erosion yield for H 0 alone is below the detection limit for temperatures below about 340 K. It increases strongly with increasing temperature, goes through a maximum around 650–700 K and decreases again for higher temperatures. Combined exposure to Ar + and H 0 results in substantial chemical sputtering yields in the temperature range below 340 K. In this range the yield does not depend on temperature, but it increases with energy from about 1 (eroded carbon atoms per impinging Ar + ion) to about 4 if the ion energy is increased from 50 to 800 eV. For temperatures above 340 K the measured erosion rates show the same temperature dependence as for the H 0-only case, but they are higher than for H 0-only. The difference between the Ar + and H 0 and the H 0-only cases increases monotonically with increasing ion energy.

高周波プラズマ支援スパッタリング法によるＴｉ薄膜の構造および物性制御

Structure and properties of Ti films deposited by direct current (dc) magnetron sputtering and pulsed dc magnetron sputtering with or without inductively coupled rf plasma assist have been investigated for various discharge pressures. With inductively coupled rf plasma assist, (1) resistivity of Ti films decreased, (2) surface became smooth, and (3) light reflectance increased, in particular, for a discharge pressure of 2.0 Pa, compared to those of Ti films deposited by conventional dc sputtering. By cross-sectional SEM investigation, it was found that Ti film deposited by pulsed dc sputtering with rf-plasma-assist at a pressure of 2.0 Pa became denser than that of Ti film deposited by dc sputtering without rf-plasma-assist at a pressure of 2.0 Pa. It is presumed that a Ti film with a fine columnar structure results from the enhancement in the energy transferred to the surface of a growing film due to the increase in ion fraction and ion energy in the combination of pulsed dc discharge and inductively coupled rf discharge.

Improving the IcRn product and the reproducibility of high Tc Josephson junctions made by ion irradiation

A simple model has been proposed to explain the spread in the characteristics of high Tc Josephson junctions made by ion irradiation, assuming that the source of dispersion is the slit’s size variation. Accordingly, increasing ion energy should lead to a significant reduction of inhomogeneities. Test samples have been fabricated using two different beam energies. As predicted, the spread in critical current decreases upon increasing energy. Moreover, since the actual width of the barrier is reduced in this case, the IcRn product increases significantly. These results seem promising for future technological applications.

Structure and properties of Ti–Si–N films with ∼ 10 at.% Si deposited using reactive magnetron sputtering with high-flux low-energy ion assistance

Ti–Si–N films with about 10 at.% Si were deposited onto silicon (100) substrates using inductively coupled plasma-assisted dc magnetron sputtering of Ti 80Si 20 target in a mixture of argon and nitrogen. X-ray diffraction, transmission electron microscopy, scanning electron microscopy, X-ray photoelectron spectroscopy and nanoindentation analysis were employed to investigate the effects of ion flux and ion energy on the structure and hardness of as-deposited films. It was found that (i) high-flux ion irradiation with energy of 20 eV caused microstructure to change from non-assisted amorphous columnar form to one where there are globular nanocrystallites of TiN (nc-TiN) embedded in amorphous Si 3N 4 (a-Si 3N 4) network; (ii) the hardness of this nc-TiN/a-Si 3N 4 nanocomposite film was approximately 35 GPa, significantly higher than that of monolithic TiN film of approximately 20 GPa; (iii) upon increasing ion energy the structure of Ti–Si–N film became columnar again and the hardness of film decreased markedly.

Control of in-plane and out-of-plane texture in shear mode piezoelectric ZnO films by ion-beam irradiation

ZnO polycrystalline films have a strong tendency to grow their c-axis perpendicular to the film surface, even on an amorphous substrate. However, unusual (101¯0) preferred orientations in which the c-axis lies in the substrate plane are often observed when the film is exposed to ion irradiation during its growth. To investigate the effect of ion irradiation on the (101¯0) preferred orientation, ZnO films were fabricated using a 0–1 keV oxygen ion-beam-assisted electron-beam evaporation of zinc. The results clearly indicated that the tendency of (101¯0) preferred orientation was enhanced with increasing ion energy and amount of ion irradiation. This demonstrated that the ion bombardment induced the (0001) preferred orientation to change into a (101¯0) preferred orientation which corresponds to the ion channeling direction. An in-plane preferred orientation was also obtained, probably because of deviations in the incident ion-beam direction from 5° to the substrate surface normal. These in-plane textured (101¯0) ZnO films, fabricated under the ion-beam irradiation of 0.5–1 keV, excited a shear acoustic wave without any longitudinal wave. The highest shear mode electromechanical coupling coefficient was found to be k15=0.16 in the film with ion-beam irradiation of 1 keV. This k15 value corresponds to 60% of that in a ZnO single crystal (k15=0.26).

A transmission matrix model for ion beam energy deposition in stack detectors

The accurate reconstruction of the energy deposition of finite-divergence wide energy spectrum ion beams in multi-material stack detectors calls for substantial computing resources. A transmission matrix model has been developed that uses statistical information extracted from a series of Monte Carlo simulations to build a fast, accurate and flexible algorithm for computing the energy loss of ions in complex heterogeneous detectors. In particular, the ion range and the broadening of the Bragg peak with increasing ion energy were correctly reproduced by the model.

Ion energy increase in laser-generated plasma expanding through axial magnetic field trap

Laser-generated plasma is obtained in high vacuum (10−7 mbar) by irradiation of metallic targets (Al, Cu, Ta) with laser beam with intensities of the order of 1010 W/cm2. An Nd:Yag laser operating at 1064 nm wavelength, 9 ns pulse width, and 500 mJ maximum pulse energy is used. Time of flight measurements of ion emission along the direction normal to the target surface were performed with an ion collector. Measurements with and without a 0.1 Tesla magnetic field, directed along the normal to the target surface, have been taken for different target-detector distances and for increasing laser pulse intensity. Results have demonstrated that the magnetic field configuration creates an electron trap in front of the target surface along the axial direction. Electric fields inside the trap induce ion acceleration; the presence of electron bundles not only focuses the ion beam but also increases its energy, mean charge state and current. The explanation of this phenomenon can be found in the electric field modification inside the non-equilibrium plasma because of an electron bunching that increases the number of electron-ion interactions. The magnetic field, in fact, modifies the electric field due to the charge separation between the clouds of fast electrons, many of which remain trapped in the magnetic hole, and slow ions, ejected from the ablated target; moreover it increases the number of electron-ion interactions producing higher charge states.

Nanostructuring of InP Surface by Low-Energy Ion Beam Irradiation

The InP nanodots of size 55 to 100 nm and height 25 to 30 nm have been synthesized by low-energy Ar+-ion irradiation with different ion energies. Sizes and size distributions of the dots strongly depend on growth conditions. Rapid thermal annealed (RTA) of the patterned surface shows cluster formation for annealing temperature 400 degrees C and above. Raman investigations reveal optical phonon softening due to correlation length shortening and broadening of the optical modes from the patterned surface. The softening is due to confinement of phonons in embedded nanocrystallites within the patterned surface along with surface nanodots, and broadening is attributed to their size distributions, which increases with increase in ion energy. The lattice damage recovery is observed from the patterned surface subjected to RTA, which exhibits upward shift of the LO and TO phonons due to the presence of complex interfacial stress, associated with the removal of crystal defects with RTA.

Damage to plasmid DNA induced by low energy carbon ions

The damage induced in supercoiled plasmid DNA molecules by 1–6 keV carbon ions has been investigated as a function of ion exposure, energy and charge state. The production of short linear fragments through multiple double strand breaks has been demonstrated and exponential exposure responses for each of the topoisomers have been found. The cross section for the loss of supercoiling was calculated to be (2.2 ± 0.5) × 10−14 cm2 for 2 keV C+ ions. For singly charged carbon ions, increased damage was observed with increasing ion energy. In the case of 2 keV doubly charged ions, the damage was greater than for singly charged ions of the same energy. These observations demonstrate that ion induced damage is a function of both the kinetic and potential energies of the ion.

The stopping forces of small cluster ions of Li and C with MeV energies in carbon foils

In this study, we used a transmission method to measure the stopping forces of Li and C cluster ions with MeV energies incident upon thin carbon foils. Experiments were performed on a 3MV tandem accelerator and a half-covered detector with the self-supported foil was used for measurements. Enhancement of energy loss for the cluster ions in 3 and 5μg/cm2 thin carbon foils was observed. In the 15μg/cm2 carbon foil, the ratio of stopping forces of cluster ions to the one of the single atom seems to approach unity. The cluster effect in stopping forces increases slightly as the ion energy increases.

Effects of target bias voltage on indium tin oxide films deposited by high target utilisation sputtering

Indium tin oxide (ITO) films were deposited by reactive High Target Utilisation Sputtering (HiTUS) onto glass and polyimide substrates. The ion plasma was generated by an RF power source while the target bias voltage was varied from 300 V to 500 V using a separate DC power supply. The deposition rate, at constant target power, increased with DC target voltage due to increased ion energy reaching 34 nm/min at 500 V. All the films were polycrystalline and showed strong (400) and (222) reflections with the relative strength of latter increasing with target bias voltage. The resistivity was lowest at 500 V with values of 1.8 × 10 − 4 Ω cm and 2.4 × 10 − 4 Ω cm on glass and polyimide, respectively but was still less than 5 × 10 − 4 Ω cm at 400 V. All films were highly transparent to visible light, (> 80%) but the NIR transmittance decreased with increasing target voltage due to higher free carrier absorption. Therefore, ITO films can be deposited onto semiconductor layers such as in solar cells, with minimal ion damage while maintaining low resistivity.

Laser-produced copper ion energy spectrum employing Thomson scattering technique

Investigations were performed on laser-assisted ion emission when a copper target was irradiated with a Nd:YAG laser (1.064 μm 10 mJ and 1.1 MW). Charge states present in the ion core emitted from plasma were determined employing Faraday cup arrangement using a four-channel 500 MHz Digital Storage Oscilloscope YOKOGAWA DL 1740. The Thomson parabola scattering (TPS) technique was used to measure the energy of ions emitted from plasma. Singly charged positive ions were found to be in excessive amount. For the energy analysis, the ion beam was collimated using a pinhole arrangement. The collimated beam was then made to pass through a uniform magnetic field of 0.4 T. A solid-state nuclear track detector PM-355 was used to register the ion tracks. The energy of ions, found to be in the range of 9–50 keV. The ion energy spectra d 2 N/dEdΩ show a decreasing trend with increasing ion energy. The empirical relation dN/dE ∞ E −n fits well with the experimental data at higher values of energy, i.e., greater than 20 keV.

A molecular dynamics investigation of fluorocarbon based layer-by-layer etching of silicon and SiO2

A molecular dynamics model is used to understand the layer-by-layer etching of Si and SiO2 using fluorocarbon and Ar+ ions. In these two-step etch processes, a nanometer-scale fluorocarbon passivation layer is grown on the material’s surface using low energy CFx+ ions or radicals. The top layers of the material are then reactive ion etched by Ar+ ions utilizing the fluorocarbon already present on the material surface. By repeating these two steps, Si or SiO2 can be etched with nanometer-scale precision and the etch rate is considerably faster than what traditional atomic layer etching techniques provide. The modeling results show that fluorocarbon passivation films can be grown in a self-limiting manner on both Si and SiO2 using low energy CF2+ and CF3+ ions. The fluorocarbon passivation layer is a few angstroms thick, and its thickness increases with the fluorocarbon ion’s energy. Increasing the ion energy, however, amorphizes the top atomic layers of the material. In addition, the fluorocarbon film becomes F rich with increasing ion energy. Simulations of fluorocarbon passivated SiO2 surface show that Ar+ ions with energy below 50eV etch Si (within SiO2) in a self-limiting manner. Si etching stops once F in the fluorocarbon passivation layer is exhausted or is pushed too deep into the substrate. Oxygen within SiO2 is more easily sputtered from the material surface than Si, and the top layers of SiO2 are expected to become O deficient during Ar+ ion bombardment. Ar+ ion etching of fluorocarbon passivated Si also appears to be self-limiting below 30eV ion energy, and etching stops once F on the material surface is either consumed or becomes inaccessible.

Irradiation effects of low energy helium ions on optical reflectivity of metallic mirror

Polycrystalline molybdenum specimens kept at room temperature and 873K has been irradiated by 1.2keV- and 8keV-He+ up to 3×1022 He+/m2. Optical reflectivity has been measured with a spectrophotometer for the wave length between 190 and 900nm. Surface morphology and chemical analyses has been examined with a scanning electron microscope, an atomic force electron microscope and X-ray photoelectron spectrometer. Loss of metallic gloss and remarkable reduction of reflectivity (190–900nm) occurs by the irradiation of only 1×1022 He+/m2. Reflectivity decreases with increasing ion energy, irradiation temperature and dose of helium ions. At rather low dose, it is reasonable to suppose that the reflectivity reduction may be due to the surface roughening and fine bubble formation. With increasing dose, roughness became smaller, while reflectivity decreased further. The further reduction of the reflectivity at high dose is considered to be due to the porous structure with nano-scale helium bubbles.

Influence of ion irradiation effects on the hydriding behavior of nanocrystalline Mg–Ni films

Mg–Ni films were grown on a silicon substrate using two magnetron sputter deposition sources and simultaneous Ar ion irradiation. X-ray diffraction microstructure and phase composition, EDX elemental composition and atomic force microscopy surface topography analysis showed that under low-energy Ar ion irradiation (bias voltages from 0 to −120 V), the Mg 2Ni phase was dominant and on the contrary with the increase of ion energy (bias voltages from −120 to −200 V), the MgNi 2 phase appeared. The Mg content changed from 63 at% down to 42 at% in films grown under bias voltages of 0 and −200 V, respectively. During hydrogenation at 8 bar, 270 °C for 3 h, films with a dominant phase of Mg 2Ni were transformed into Mg 2NiH 4. Hydrogen in MgNi 2 films was mainly in interstitials and tended to form bubbles.

Tungsten erosion at the ICRH limiters in ASDEX Upgrade

All limiters on the low field side of ASDEX Upgrade are equipped with tungsten coated tiles. The W influx from the limiters was measured with a new spectroscopic setup providing sufficient time resolution to resolve the influx peak during edge localised modes (ELM). The ELM peak is mainly due to an increase of the mean ion energy hitting the W-surface. W influx due to sputtering by fast D+ ions from neutral beam injection was measured for different injection angles and compared to Monte Carlo calculations of the fast ion loss. The erosion depends less strongly on the injection angle than predicted by the code due to an enhancement of the fast ion transport by ELMs. At the antenna limiters of the ion cyclotron resonance heating (ICRH), the influx rapidly increases when the ICRH is switched on, which can be explained by a rise of the average sheath potential at the limiter.