Self-ion Bombardment Research Articles

Comparative study of deuterium retention in irradiated Eurofer and Fe–Cr from a new ion implantation materials facility

A new facility to study the interaction of hydrogen isotopes with nuclear fusion-relevant first wall materials, and their retention and release, has been produced. The new facility allows for implanting a range of gases into samples, including tritium. An accurate study of isotope effects, such as the isotopic exchange in damaged microstructure, has previously been difficult due to a background signal of light hydrogen. This new capability will allow virtually background free measurements using tritium and deuterium. The design and build of this facility are described and commissioning results are presented. Within the UKAEA-led tritium retention in controlled and evolving microstructure (TRiCEM) project, this facility is used for the comparative study of deuterium retention in self-ion irradiated Eurofer steel and Fe–Cr alloy. Self-ion bombardment with energies of 0.5 MeV is used to mimic the defects created by neutrons in fusion power plants and the created traps are then filled with deuterium in the new facility. Implanted samples are analysed using thermal desorption spectrometry (TDS), secondary ion mass spectrometry (SIMS), and transmission electron microscopy. Results on the total deuterium content as a function of time, TDS spectra and SIMS analysis are presented. A comparison of the results for Eurofer and Fe–Cr revealed several differences. While some of them may be due to experimental details like different time delays between exposure and analysis, others, such as deuterium retention as function of dose, might be genuine and require further studies.

Self-ion bombarded Cr films: Crystallographic orientation and oxidation behaviour

Low-energy self-ion bombardment effects on the crystallographic orientation and oxidation behaviours of arc-ion-plated pure Cr films were investigated. Microstructurally compact Cr films as observed by SEM were obtained under ion-bombardment energy of 50–250 eV. Stronger ion bombardment induced crystallographic orientation of Cr (200) and slight self-sputtering effect in the Cr films. Oxidation of the Cr films at 900 °C showed that the stronger self-ion bombardment induced formation of the more compact chromia scales with significantly lowered growth rate, indicating that self-ion bombardment can be an effective approach to improve the oxidation resistance of metal films.

A study to compute integrated dpa for neutron and ion irradiation environments using SRIM-2013

Displacements per atom (dpa), estimated based on the standard Norgett–Robinson–Torrens (NRT) model, is used for assessing radiation damage effects in fast reactor materials. A computer code CRaD has been indigenously developed towards establishing the infrastructure to perform improved radiation damage studies in Indian fast reactors. We propose a method for computing multigroup neutron NRT dpa cross sections based on SRIM-2013 simulations. In this method, for each neutron group, the recoil or primary knock-on atom (PKA) spectrum and its average energy are first estimated with CRaD code from ENDF/B-VII.1. This average PKA energy forms the input for SRIM simulation, wherein the recoil atom is taken as the incoming ion on the target. The NRT-dpa cross section of iron computed with “Quick” Kinchin-Pease (K-P) option of SRIM-2013 is found to agree within 10% with the standard NRT-dpa values, if damage energy from SRIM simulation is used. SRIM-2013 NRT-dpa cross sections applied to estimate the integrated dpa for Fe, Cr and Ni are in good agreement with established computer codes and data. A similar study carried out for polyatomic material, SiC, shows encouraging results. In this case, it is observed that the NRT approach with average lattice displacement energy of 25 eV coupled with the damage energies from the K-P option of SRIM-2013 gives reliable displacement cross sections and integrated dpa for various reactor spectra. The source term of neutron damage can be equivalently determined in the units of dpa by simulating self-ion bombardment. This shows that the information of primary recoils obtained from CRaD can be reliably applied to estimate the integrated dpa and damage assessment studies in accelerator–based self-ion irradiation experiments of structural materials. This study would help to advance the investigation of possible correlations between the damages induced by ions and reactor neutrons.

Grain growth in thin Al films during deposition from partially ionized vapor

Grain growth in thin Al films during deposition from partially ionized vapor flux with simultaneous self-ion bombardment was studied in this work. The films were deposited at constant ion energy of 940eV and total specific power of 0.4W/cm2 while the deposition time t of 6s to 246s and the resulting substrate temperature (Ts/Tm of 0.35–0.96) were varied. Thin continuous Al films exhibited normal grain growth through the entire experimental range of deposition time without limitation of grain growth by the film thickness effect. Three kinetic stages of the grain growth were observed within 100s of deposition time: the first one exhibits very slow grain growth, accelerated grain growth occurs in the second stage and then it rapidly changes to a retardation and stagnation mode in the third stage. Large average grain sizes Dg up to 11.3μm at film thickness of 1.4μm and integral grain growth rates up to 0.16μm/s were observed in this study. The experimental results were evaluated against various mechanisms of inhibition of grain growth. An estimate of the effective activation energy of the grain growth yields a value of 0.27eV which is lower than that of the bulk Al and much higher than the activation energy of surface self-diffusion on (111)Al monocrystal. The power law Dg=(k t)0.5 gives good match with experimental results in the initial deposition phase preceding the grain growth retardation, while another model that is based on the grain size dependent pinning force adequately explains the entire grain size dependence on time. It is deemed both ion enhanced film/surface interaction and impurities on one side and thermal grooves on another side contribute to the rapid retardation of the grain grooves commencing the second growth stage.

Influence of surface smoothing on normal grain growth in Al films deposited from ionized vapor

Unusually large normal grain growth rates and grain sizes were observed in Al films that were deposited from partially ionized vapor. The average grain size-to-thickness ratio ∼(8–25)h, where h is the film thickness, substantially exceeds those that were reported earlier for thin films and theoretically established limits. The materials thin film microstructure and surface morphology is being changed by ion bombardment during deposition. The roughness of the film surface dramatically increases along with initial growth of grain size up to an average value of 1.8 μm, however, the process abruptly turns to a reverse direction upon further grain growth under a self-ion bombardment beyond the average grain size of 3.5 μm. It is suggested the ion-enhanced smoothing of film surface and thermal grooves is responsible for large grain growth rate and unusually large grains.

How do energetic ions damage metallic surfaces?

Surface modification of structural and functional materials under bombardment by energetic ions is observed under different conditions and can be either an unavoidable effect of the irradiation or an intentional modification to enhance materials properties. Understanding the basic mechanisms is necessary for predicting property changes. The mechanisms activated during ion irradiation are of atomic scale and atomic scale modeling is the most suitable tool to study these processes. In this paper, we present results of an extensive simulation program aimed at developing an understanding of primary surface damage in iron induced by energetic particles. We simulated 25keV self-ion bombardment of Fe thin films with (100) and (110) surfaces at room temperature. A large number of simulations, ∼400, were carried out allow a statistically significant treatment of the results. The particular mechanism of surface damage depends on how the destructive supersonic shock wave generated by the displacement cascade interacts with the free surface. Three scenarios were primarily observed, with the limiting cases being damage created far below the surface with little or no impact on the surface itself, and extensive direct surface damage on the timescale of a few picoseconds. In some cases, formation of large 〈100〉 vacancy loops beneath the free surface was observed, which may explain some earlier experimental observations.

Physical response of gold nanoparticles to single self-ion bombardment

Abstract

Direct evidence for stress-induced texture evolution and grain growth of silver thin films upon thermal treatment and self-ion bombardment

Common failure mechanisms in microelectronics, such as electromigration, creep and fatigue, can be positively influenced by microstructure optimization. In this paper a combination of post-deposition heat treatment and self-ion bombardment is proposed as a valid candidate to gain control over the microstructure of (111) fiber textured thin silver films. Irradiation can induce a strong in-plane texture and hence lead to biaxially textured films through a process of selective grain growth. Moreover, we report microstructural stability of the irradiated regions over a wide range of temperatures (up to 600°C), in contrast to non-irradiated portions of the film, which underwent abnormal growth of the (100) out-of-plane oriented grains, and a consequent texture change, at temperatures as low as 195°C. The thermal stress induced in the film upon heat treatment was quantified in situ and its role in texture change elucidated.

Size distribution of sputtered particles from Au nanoislands due to MeV self-ion bombardment

Nanoisland gold films, deposited by vacuum evaporation of gold onto Si(100) substrates, were irradiated with 1.5MeV Au2+ ions up to a fluence of 5×1014ionscm−2 and at incidence angles up to 60° with respect to the surface normal. The sputtered particles were collected on carbon-coated grids (catcher grid) during ion irradiation and were analyzed with transmission electron microscopy and Rutherford backscattering spectrometry. The average sputtered particle size and the areal coverage are determined from transmission electron microscopy measurements, whereas the amount of gold on the substrate is found by Rutherford backscattering spectrometry. The size distributions of larger particles (number of atoms/particle, n⩾1000) show an inverse power law with an exponent of ∼−1 in broad agreement with a molecular-dynamics simulation of ion impact on cluster targets.

Effect of self-ion bombardment on the corrosion behavior of zirconium

In order to study the influences of self-ion bombardment on the aqueous corrosion behavior of zirconium, specimens were implanted by zirconium ions with a fluence range from 1 × 10 15 to 2 × 10 17 ions/cm 2 at about 170 °C, using MEVVA source at an extracted voltage of 50 kV. The valence of the surface layer was analyzed by X-ray photoemission spectroscopy (XPS); the thickness of the oxide film was measured by Auger electron spectroscopy (AES). Three-sweep potentiodynamic polarization measurement was employed to value the aqueous corrosion resistance of zirconium in a 1N H 2SO 4 solution. Glancing angle X-ray diffraction (GAXRD) was employed to examine the phase transformation due to the self-ion implantation in the oxide films. It was found that the aqueous corrosion behavior of zirconium implanted with 5 × 10 16 Zr ions/cm 2 is the best in all the samples. The natural corrosion potentials of all the bombarded samples are more negative than that of the as-received zirconium. The tendency is probably that the natural corrosion potential declines with raising the bombarded fluences. Finally, the mechanism of the corrosion behavior of the self-ion implanted zirconium is discussed.

Method and experimental arrangement for Al thin film deposition from r.f. metal plasma with simultaneous self-ion bombardment

A method, experimental arrangement, and some particular details of the process for Al thin film deposition from r.f. metal plasma are described in this paper. In the plasma deposition system, Al is evaporated by an electron beam, partial ionization of the vapor is realized in r.f. inductive metal plasma, and film deposition is accompanied by simultaneous self-ion bombardment (SSIB) in a high-vacuum environment. Special attention is given to the accurate determination of such basic physical parameters of the process as plasma potential, ion energy, fraction of ions in the total arrival flux, added (normalized) energy, and power density of the ion bombardment. It is concluded that the fraction of secondary neutral energetic atoms in the total flux can be neglected in the calculations of the basic deposition parameters. At the power density in excess of about 0.5 W/cm 2, the substrate temperature can rise up to the Al melting point (660°C) during deposition. An analysis of the main heat sources in the plasma deposition system is presented in the paper. It is found that the results of the computer simulation of the substrate temperature increase caused by ion bombardment have a very good agreement with demonstrated experimental data.

Effect of surface on defect creation by self-ion bombardment of Si(001)

We have studied defect formation and defect distributions in silicon under low-energy (25\char21{}800 eV) self-bombardment of the $2\ifmmode\times\else\texttimes\fi{}1$ terminated Si(001) surface. We applied the classical molecular dynamics technique and collected statistically significant averages to be able to detect defect production trends in the energy dependence. The number of defects created in implantations was found to be a superlinear function of energy at low energies $(l400\mathrm{eV})$ and larger than the defect production in the bulk up to about 1 keV. We have also examined the depth dependence of close-to-surface damage and explored the energy and time dependence of the defect creation mechanisms and the sensitivity of the results to the choice of the model potential.

Defect production in tungsten: A comparison between field-ion microscopy and molecular-dynamics simulations

Molecular dynamics (MD) computer simulations of 20 {endash} 30 keV self-ion bombardment of W were performed and compared to past field-ion microscopy (FIM) studies [M. I. Current {ital et al.}, Philos. Mag. A {bold 47}, 407 (1983)]. The simulations show that the unusually high defect production efficiencies obtained by FIM are a consequence of a surface effect, which greatly enhances defect production compared to that in the crystal interior. Comparison of clustering of vacancies and the formation of interstitial atoms found in the FIM experiments and MD simulations shows overall good agreement. {copyright} {ital 1998} {ital The American Physical Society}

Effect of self-ion bombardment damage on high temperature oxidation behavior of Zircaloy-4

In order to simulate the oxidation behavior of Zircaloy-4 in pressurized water reactors, it was bombarded with 190 keV Zr + ions at liquid nitrogen temperature up to a dose of 8×10 16 ions/cm 2 and was then oxidized in pure oxygen at 923 K and 133.3 Pa. Auger electron spectroscopy (AES) and X-ray photoelectronic spectroscopy (XPS) were employed to investigate the distribution and the valence of oxygen and zirconium ions inside the oxide films before and after bombardment. It was found that: (1) the Zr + ion bombardment enhanced the oxidation of Zircaloy-4 and resulted in the oxidation weight gain at a dose of 8×10 16 ions/cm 2 which was four times greater than that of the unbombarded sample; (2) the valence of zirconium ions in the oxide films was classified as Zr 0, Zr 1+, Zr 2+, Zr 3+ and Zr 4+ and the higher valence of zirconium ions increased after the bombardment; (3) the irradiation oxidation damage function was established and (4) a regression formula describing the relation between the oxidation weight gain and dpa (displacement per atom) was obtained.

Flux ionization of arc-like Ti vapor plasma

Arc-like Ti vapor plasma is formed on the electron beam evaporator by applying low dc voltage to an anode near the evaporator. Deposition rate (R d), substrate ion current density (J s) and anode current (I a) were simultaneously measured by changing: (1) anode voltage (V a) at the constant EB gun power 4.05kW; and (2) source-to-substrate distance (distance r) at V a 30V and I a 60A. With an increase in V a from 15 to 35 V, R d 2.6 nm/s decreases only slightly, while I a and J s increases from 40 to 90 A and from 12 to 40 A/m 2, respectively, at distance r 0.45m. With a decrease in distance r from 0.52 to 0.15 m, R d and J s increase from 2.0 to 40 nm/s and from 24 to 420 A/m 2, respectively, (the measured values being mean values). The calculated mean values for flux ionization increase from 40 to 160% for condition (1), and are approximately 120% in all cases for condition (2). The latter suggests collisionless expansion of Ti ions. At this high flux ionization with no biased substrate, self-ion bombardment is more effective for Ti film densification than Ar ion bombardment.

Angular distribution of mass flux from arc-like Ti vapor plasma

Arc-like Ti vapor plasma that appears symmetrical is formed on the electron beam evaporation source in high vacuum by applying ~ 20 V dc voltage to the anode set diagonally above the source. Angular distributions of both ion and mass fluxes from the plasma at the longitudinal section, including the anode, are investigated. The angular distribution of Ti ion flux from the plasma with 40 V–80 A discharge is unsymmetric (maximum ion current density is 16 Am −2 at θ = 0 ° and source-to-Faraday cup distance 460 mm): the ion current at the anode side (back side) is much higher than that at the front side. However, the ion current decreases near the anode direction. Angular distribution of mass flux from the plasma agrees largely with that of ion flux, but disagrees with that of mass flux from the vacuum evaporation source (cos 3 θ distribution). Therefore, ionization efficiency of Ti vapor in the plasma is considered to be high. Rough surface and small voids are seen at vacuum evaporation, but not at the arc-like discharge evaporation. The cross section fractured in liquid nitrogen is brittle in the former, but ductile in the latter. This shows that self-ion bombardment greatly affects Ti film densification.

Formation of titanium silicides by titanium deposition onto silicon with simultaneous self-ion bombardment

Thin films of titanium silicide were formed by evaporative deposition of Ti onto Si with simultaneous self-ion Ti+ bombardment (SSIB PVD process) at energies up to 5.0 keV and specific powers up to Wi = 7.2 Wcm−2. Five typical zones were identified in the resistivity and phase composition vs. Wi plots. The results show the formation of single-phase TiSi2 layers with resistivity values as low as 13 μΩ cm under the conditions of Wi = 3.0–5.0 Wcm−2 and a high ion bombardment intensity. Not all the results can be interpreted by taking into account the thermal effects of ion bombardment only. The resistivity and phase composition of the films are dependent on the intensity and added energy of self-ion bombardment.

Relationship Between The Void and Hillock Formation and The Grain Growth in Thin Aluminum Films

AbstractVoid and hillock formation during annealing was studied depending on the deposition conditions. Aluminum films were deposited onto oxidized silicon substrates by the self-ion assisted technique. The bias 0 or 6 kV was applied to the substrate during deposition. The films were then annealed in vacuum for 1 hour in the temperature range from 150° to 550°C. The structure of the films was investigated by transmission electron microscopy. The void and hillock formation was studied with optical and scanning electron microscopes.It was found that recrystallization and void and hillock formation in the films depend on the bias during deposition. Normal grain growth occurred in the films deposited without bias. Abnormal grain growth was observed in the 6 kV-films. It was also found that the mechanism of stress relaxation during thermal cycling depends on the self-ion bombardment. In the films prepared without bias, stress relaxation proceeds by diffusion creep. In the films deposited at the 6 kV bias, stress relaxation proceeds by plastic deformation.

Relationship Between The Void And Hillock Formation And The Grain Growth In Thin Aluminum Films

AbstractVoid and hillock formation during annealing was studied depending on the deposition conditions. Aluminum films were deposited onto oxidized silicon substrates by the self-ion assisted technique. The bias 0 or 6 kV was applied to the substrate during deposition. The films were then annealed in vacuum for 1 hour in the temperature range from 1500 to 550°C. The structure of the films was investigated by transmission electron microscopy.It was found that recrystallization and void and hillock formation in the films depend on the bias during deposition. Normal grain growth occurred in the films deposited without bias. Abnormal grain growth was observed in the 6 kV-films. It was also found that the mechanism of stress relaxation during thermal cycling depends on the self-ion bombardment. In the films prepared without bias, stress relaxation proceeds by diffusion creep. In the films deposited at the 6 kV bias, stress relaxation proceeds by plastic deformation.

Some particular features of the condensation process, structure and properties of thin metal films caused by self-ion bombardment

This communication contains a brief review and analysis of our previous and recent results on the influence of self-ion bombardment during deposition (ion energy E i = 0–5 keV, specific power W i = 0–6 W/cm 2) on the structure and properties of Nb, Ti x Si y and Al films. The results have been received under ordinary vacuum conditions ∼ 10 −6 Torr and typical condensation rates ∼ 1–10 nm/s. Some particular features of self-ion bombardment effects are noted, which are not typical for such vacuum conditions and/or for other ion-assisted deposition methods: preparation of ultra-thin (10–30 nm) Nb films with superconductive transition temperature T c = 7–8 K and resistance ratio R 300 K R 10 K = 2–3 , formation of perfect textures in Nb and Al films with a disperse angle of 2.5° and 5°, respectively, rapid formation of single-phase TiSi 2 films with low resistivity ∼ 13–15 μΩ·cm, high grain boundary migration rate and large grain sizes (up to 11 μm of the average value) in Al films. The energetic parameters of the deposition such as specific energy, effective condensation rate, and specific power of self-ion bombardment are characterized as critical. The ion bombardment influences the structure and properties of the films by essentially different mechanisms in different ranges of the values of these parameters.