Incident Ion Research Articles

Study of atomic sputtering and transport dynamics by Monte Carlo simulation

Magnetron sputtering vacuum coating technology is widely used in Accident Tolerant Fuel (ATF) cladding materials. The growth of thin films is a crucial step in the magnetron sputtering process, closely linked to the energy and angular distribution of atoms sputtered from the target to the substrate. However, experimental investigations of atomic sputtering are challenging due to the difficulty of accurately detecting electron distributions, incident ion distributions, and quantities, especially collisions between sputtered and vacuum chamber gas atoms. In this study, the SRIM/SIMTRA programs based on the Monte Carlo method have been carried out to predict the energy and angular distributions of sputtered atoms from the target surface and to calculate the energy, angle, and number distributions of supering atoms arriving on the substrate after traversing the vacuum chamber. The results of SRIM indicate the sputtered atom distribution in the low-energy region, with the scattering angles exhibiting a ‘heart-shaped’ profile. Additionally, SIMTRA simulation results show that increasing the target-substrate distance (TSD) and vacuum chamber gas pressure leads to significant attenuation of the energy of atoms arriving on the substrate and a tendency for larger incidence angles. These simulation outcomes provide crucial theoretical guidance for optimizing magnetron sputtering coating performance for ATF cladding.

Ion-induced surface reactions and deposition from Pt(CO)2Cl2 and Pt(CO)2Br2.

Ion beam-induced deposition (IBID) using Pt(CO)2Cl2 and Pt(CO)2Br2 as precursors has been studied with ultrahigh-vacuum (UHV) surface science techniques to provide insights into the elementary reaction steps involved in deposition, complemented by analysis of deposits formed under steady-state conditions. X-ray photoelectron spectroscopy (XPS) and mass spectrometry data from monolayer thick films of Pt(CO)2Cl2 and Pt(CO)2Br2 exposed to 3 keV Ar+, He+, and H2 + ions indicate that deposition is initiated by the desorption of both CO ligands, a process ascribed to momentum transfer from the incident ion to adsorbed precursor molecules. This precursor decomposition step is accompanied by a decrease in the oxidation state of the Pt(II) atoms and, in IBID, represents the elementary reaction step that converts the molecular precursor into an involatile PtX2 species. Upon further ion irradiation these PtCl2 or PtBr2 species experience ion-induced sputtering. The difference between halogen and Pt sputter rates leads to a critical ion dose at which only Pt remains in the film. A comparison of the different ion/precursor combinations studied revealed that this sequence of elementary reaction steps is invariant, although the rates of CO desorption and subsequent physical sputtering were greatest for the heaviest (Ar+) ions. The ability of IBID to produce pure Pt films was confirmed by AES and XPS analysis of thin film deposits created by Ar+/Pt(CO)2Cl2, demonstrating the ability of data acquired from fundamental UHV surface science studies to provide insights that can be used to better understand the interactions between ions and precursors during IBID from inorganic precursors.

Atomistic Simulation of Primary Radiation Damage Profiles in Fluorine-Doped Tin Oxide Thin Film Target Using SRIM Code

In this study, we investigated the discrepancy between the numbers of atomic displacements obtained from the Vac.txt method and those obtained from the Vac.dam method using SRIM-2013 operated in full cascade mode. The SRIM simulations also calculated energy partitioning, damage dose and distribution profiles of the implanted He, C and O ions at three different ion energies of 10 keV, 100 keV and 1 MeV. In all the simulations and at each ion energy value, the target thickness was increased until the entire ion beam was absorbed within the target. The simulated fluorine-doped tin oxide (FTO) thin film target stoichiometrically comprised 78.6% Sn, 16.4% O and 5.0% F, and a 6.013 g/cm3 bulk density. To achieve relatively good statistics with compromised computational time, each run followed 10 000 ion histories. The results indicated that, with increasing ion energies, the peaks of vacancy concentrations moved deeper into the material. For He ions, the vacancy profiles have peaks at ∼0.04, ~0.45 and ~2.9 µm at 10 keV, 100 keV and 1 MeV, respectively. Meanwhile, for C and O ions, the peaks were at ∼0.012, ~0.15 and ~1.23 µm for C ions and ∼0.01, 0.12 and 1.2 µm for O ions at 10 keV, 100 keV and 1 MeV, respectively. We also observed that for all the ions and at all ion energies considered, the Vac.txt method predicted more vacancies than the Vac.dam method, and the discrepancies generally increased with increasing Z by an average value of ∼1.21, ∼1.45 and 1.48 for He, C and O ions respectively. In addition, the electronic and nuclear partitioning profiles revealed that most of the energies of the ions were lost in collision with atomic electrons due to ionisation and excitations, and little to nuclear events. The simulations also revealed that nuclear stopping powers increase at low ion energies. This study also indicated that damage dose profiles strongly depend on the type of incident particle and its energy. A comparison of SRIM and iradina calculated vacancies and incident ion distribution profiles generally indicated perfect agreements between the two codes. FTO thin film remains a promising material for thermal control applications in future spacecraft. However, further analyses are still necessary to comprehensively understand its evolution under intense radiation environments comprising various ions and at different energies.

Adsorbate-induced effects on the H− ion collisions with Na/Ag(111) and K/Ag(111) surfaces

The H− ion survival probabilities following on-top collisions with Na adsorbates deposited on Ag(111) at low coverage, are investigated for a wide range of exit angles from 20° to 90° measured from surface, and for various incident ion energies. A wave packet propagation approach is used in these calculations. The survival probabilities exhibit a series of well-defined peaks located at certain exit angles, that are indicative of avoided crossings between the various energy levels involved in the projectile/adsorbate/surface interaction. Both image states and the back-and-forth electronic motion between the ion projectile and the adsorbate/surface system contribute to the electronic population recaptured during the exit trajectory. For ion-surface collisions away from the on-top configuration, but in the close vicinity of adsorbates, a model is proposed to describe the variation of the H− projectile's distance of closest approach to the adsorbate-covered Ag(111) surface in terms of the ion's impact point on surface, e.g., starting from the on-top collision with a single adsorbate and gradually moving away, towards the “clean” surface. The distance of closest approach is a key factor in calculating correctly the ion survival probabilities in the close region around the adsorbate, where the scattered ion fractions are affected the most. Results are shown for H− in interaction with K/Ag(111).

SiO2 sputtering by low-energy Ar+, Kr+, and Xe+ ions in plasma conditions

Modern plasma technologies applied for nm-size devices require localizing ions’ impact within up to atomic layer to avoid uncontrollable damage of underlying structures. The decrease in energy of ions incident on a surface is one of the ways to achieve this. In this work, SiO2 sputtering by Ar+, Kr+, and Xe+ ions at energies of 20–200 eV was studies in the low-pressure ICP-discharge at a plasma density typical for plasma processing. The rf-bias waveform tailoring due to high discharge asymmetry allowed generating and controlling ion narrow energy spectrum with FWHM 5 ± 2 eV. Real time in-situ control over ion composition and flux as well as sputtering rate provided accurate determination of the SiO2 sputtering yield, Y(Ei). It is shown that at ion energy above ∼70 eV, the “classical” kinetic sputtering mechanism prevails. In this case, Y(Ei) rapidly grows with ion energy, while decreasing with the decrease in ion mass. Below ∼70 eV, the change of Y(Ei) is strongly slow down while the sputtering yield still stays high enough (>10−3), demonstrating the plasma impact on the sputtering mechanism. The obtained trends of Y(Ei) under the plasma exposure are discussed in light of possible SiO2 surface modification studied by AFM and angular XPS analysis.

In Situ Uranium Extraction through the Synthesis of the Uranyl Peroxide Studtite Using a Nonthermal Plasma.

Extraction of uranium from water is an essential step in in situ leach (ISL) mining and environmental decontamination. This is often done by precipitating uranium in solution as the uranyl peroxide studtite, [(UO2)(O2)(H2O)2](H2O)2, by adding hydrogen peroxide, which is energy-intensive to produce and hazardous to transport. Here, we present a method for synthesizing studtite, by generating reactive oxygen species in solution using a nonthermal plasma. Precipitation of studtite is observed within 5 min of the onset of plasma treatment as confirmed by X-ray diffraction and Raman spectral analysis. The faradaic efficiency of studtite formation is analyzed to estimate the values of hydrogen peroxide yield, 1.23 molecules per incident ion, and the rate constant of the studtite-forming reaction, 4.44 × 107 M-1 s-1. This work is a proof of concept and identifies significant parameters for the future development of a larger scale, higher throughput system.

On the Shift of the Maximum of the Polar Angular Distribution of Sputtered Atoms in the MD Model of the (001) Ni Face Sputtering

Using an up-to-date full molecular dynamics model of sputtering of single crystals, taking into account the incidence of ions on the surface, the peculiarities and mechanisms of atom sputtering during bombardment of the (001) Ni face with 200 eV Ar ions are studied for two target temperatures. It is shown for the first time that with an increase in the energy of sputtered atoms, not only the maximum of their polar angular distribution in the azimuthal direction toward the Wehner spots, but also the maximum of the polar angular distribution integrated over the azimuthal angles shifts first toward the normal to the surface, and then in the opposite direction. The polar distribution integrated over the azimuthal angle is formed by atoms ejecting from the surface at much larger polar angles than in the final (observed) distribution. Both effects are explained in terms of the surface mechanism of single crystal sputtering.

Velocity Effect in Synthesis of Noncircular Nanopores by Etching Tracks of Swift Heavy Ions in Olivine

The velocity effect was studied in the synthesis of nanopores with a noncircular cross section by etching tracks of swift heavy ions in olivine. The developed atomistic model for the etching of olivine irradiated with swift heavy ions predicts the possibility of synthesizing nanopores with a noncircular cross section in it. The model consists of connected blocks that describe the sequential stages of track formation and etching. The TREKIS Monte Carlo model describes the initial electronic and lattice excitations in the nanoscale vicinity of the trajectory of an incident ion. These results are used as initial conditions for molecular dynamics simulation of structural changes along the ion trajectory. The obtained atomic coordinates after cooling of the structurally damaged area serve as the initial data for the original atomistic model of track etching in olivine. The results of the model application show that it is possible to control the cross section of these pores by changing the orientation of the crystal relative to the direction of irradiation. The presented simulation results for Xe ions demonstrate that the size of the resulting pores depends on the velocity of the incident ion, and not only on its linear energy loss.

Analysis and prediction of sputtering yield using combined hierarchical clustering analysis and artificial neural network algorithms

Sputtering is a crucial technology in fields such as electric propulsion, materials processing and semiconductors. Modeling of sputtering is significant for improving thruster design and designing material processing control algorithms. In this study we use the hierarchical clustering analysis algorithm to perform cluster analysis on 17 descriptors related to sputtering. These descriptors are divided into four fundamental groups, with representative descriptors being the mass of the incident ion, the formation energy of the incident ion, the mass of the target and the formation energy of the target. We further discuss the possible physical processes and significance involved in the classification process, including cascade collisions, energy transfer and other processes. Finally, based on the analysis of the above descriptors, several neural network models are constructed for the regression of sputtering threshold E th, maximum sputtering energy E max and maximum sputtering yield SY max. In the regression model based on 267 samples, the four descriptor attributes showed higher accuracy than the 17 descriptors (R 2 evaluation) in the same neural network structure, with the 5×5 neural network structure achieving the highest accuracy, having an R 2 of 0.92. Additionally, simple sputtering test data also demonstrated the generalization ability of the 5×5 neural network model, the error in maximum sputtering yield being less than 5%.

Ion incidence angle-dependent pattern formation on AZ® 4562 photoresist by reactive ion beam etching

Reactive ion beam etching is a key technology in the field of ultra-precise surface engineering. In this process nanopatterns can emerge and alter the functional properties of the surfaces. Therefore, it is necessary to understand which reactive ion beam parameters influence the emergence of these nanopatterns. In this study the influence of reactive ion beam etching on the commercially available photoresist AZ® 4562 is investigated. Atomic force microscopy and scanning electron microscopy reveal the formation of nanopatterns (ripples, triangular features, protrusions, facets) depending on a wide range of ion incidence angles (0°–75°) as well as the etch time. The emerged nanopatterns resemble those known from inorganic materials and therefore, lead to the assumption that local redeposition, surface viscous flow and dispersion plays an important role for the pattern formation on polymer surfaces. Major difference from ion beam erosion with inert species is the absence of nanoholes. Spectroscopic ellipsometry shows that the thickness of the modified surface layer depends on the ion incidence angle but not on the fluence in the investigated range. Using X-ray photoelectron spectroscopy, trends of the chemical composition of the surface/near-surface region were detected, which depend on ion incidence angle and etch time.

Effects of Carbon Deposition on Deuterium Plasma-Driven Permeation Through Tungsten-Coated RAFM Steel

In steady-state operation of fusion reactors, eroded materials and contaminations, especially carbon (C), may deposit on the surface of plasma-facing components. In this work, the effects of C deposition on hydrogen isotope permeation behavior through tungsten (W)–coated reduced activation ferritic/martensitic (RAFM) steel were systematically investigated by plasma-driven permeation (PDP) measurements in the temperature range of 633 to 893 K. A C deposition layer with thickness of ~200 nm was prepared by magnetron sputtering to simulate the formation of C impurities in the first-wall area of tokamaks. The implantation depth of incident deuterium (D) ions was estimated to be <10 nm at incident energy of 114 eV. Deuterium effective diffusion coefficients (Deff’s) for W-coated RAFM steel with/without a C layer were obtained. It was found that the C layer tended to increase Deff in the low-temperature region of ~675 to 820 K. At high temperature, however, Deff was measured be lower than that without a C layer. Nevertheless, the addition of a C layer had no significant effect on Deff compared to the W coating alone with respect to bare RAFM steels. For steady-state D-PDP flux, it was found that the C layer significantly decreased D permeation flux at low temperature. But, the permeation flux difference between the samples with/without a C layer became smaller with increasing temperature, indicating that the influence of C deposition on D permeation was negligible at high temperature. Similar D-PDP behavior was detected as increasing the incident ion flux by means of increasing plasma discharge power. Surface reemission of absorbed D as well as the D concentration gradient throughout the sample was found to be influenced by C deposition; therefore, D permeation flux changed correspondingly.

Experimental subshell vacancies in a multiply ionised Sn atom by heavy ion impact

The adoption of Heavy Ion PIXE for elemental quantification remains limited by the lack of fundamental atomic parameters needed for the calculation of X-ray production cross section data. Widely adapted theoretical models used for approximating ionisation cross sections, as well as other atomic physical parameters such as X-ray fluorescence yields, are calculated based on single-hole vacancy production post ion-atom impact. This renders the use of conventional ‘protonic’ models invalid for heavy ion-atom collisions. The degree to which Multiple Ionisation (MI) manifests in different heavy ion-atom collision symmetries significantly modifies atomic physical parameters. This is due to the significant number of introduced spectator vacancies in higher subshells, especially where near-symmetry is approached and MI is more pronounced. Knowledge of the mean number of vacancies in upper subshells is thus important for modifying atomic parameters that are needed for the translation of ionisation to X-ray production cross sections. In the present study, MI effects for Sn L-shell X-rays were studied using characteristic X-ray energy shifts measured using a standard PIXE spectrometer induced by Si, Cu and I projectile ions with incident energies up to 0.75 MeV/u. MI satellite distributions were studied using a Wavelength dispersive X-ray spectrometer (WDS) for high resolution PIXE, induced by C and Si projectile ions with incident ion energies up to 1.25 MeV/u. The mean number of subshell vacancies in the M- and N- shell for energy shifted Sn Lα X-ray lines were determined using both standard and high resolution PIXE spectrometers.

Simulation of ion beam sputtering with OKSANA and SRIM: A comparative study

The energy distributions and average energies of sputtered particles are calculated using simulation codes OKSANA and SRIM-2013. Most simulations refer to an amorphous Ni target bombarded by normally incident Ar ions of keV energies. Sputtering of Si, Ti, V and Nb targets is also considered. It is shown that SRIM can strongly overestimate the contribution of fast ejected atoms, especially for targets irradiated with lighter ions. The effect is amplified by using the surface binding energies found to fit the measured sputter yields. It is concluded that the enhanced ejection of fast particles is associated with sputtering due to backscattered ions. The role of the first collision of an incident ion with a target atom is particularly noted. A comparison of the OKSANA calculated results with the data of other codes (TRIM.SP, ACAT) and experimental data showed their reasonable agreement.

Liquid water radiolysis induced by secondary electrons generated from MeV-energy carbon ions.

Fast ion beams induce damage to deoxyribonucleic acid (DNA) by chemical products, including secondary electrons, produced from interaction with liquid water in living cells. However, the production process of these chemical products in the Bragg peak region used in particle therapy is not fully understood. To investigate this process, we conducted experiments to evaluate the radiolytic yields produced when a liquid water jet in vacuum is irradiated with MeV-energy carbon beams. We used secondary ion mass spectrometry to measure the products, such as hydronium cations (H3O+) and hydroxyl anions (OH-), produced along with ·OH radicals, which are significant inducers of DNA damage formation. In addition, we simulated the ionization process in liquid water by incident ions and secondary electrons using a Monte Carlo code for radiation transport. Our results showed that secondary electrons, rather than incident ions, are the primary cause of ionization in water. We found that the production yield of H3O+ or OH- was linked to the frequency of ionization by secondary electrons in water, with these electrons having energies between 10.9 and 550eV. These electrons are responsible for ionizing the outer-shell electrons of water molecules. Finally, we present that the elementary processes contribute to advancing radiation biophysics and biochemistry, which study the formation mechanism of DNA damage.

Ion energy dependence of helium plasma irradiation effects on the photoelectrochemical properties of tungsten oxide

Abstract Tungsten samples with fuzz nanostructures on the surface were generated using helium plasma with different incident ion energies, and then fuzz tungsten oxide electrodes were prepared by calcination. The photoelectrochemical (PEC) properties and stability of the samples were measured, and the dependence on the incident ion energy was discussed. The mechanism of the fuzz structure to enhance the PEC performance of tungsten oxide was analyzed by scanning electron microscope (SEM), X-ray diffraction (XRD), and electrochemical impedance spectroscopy (EIS). The results show that fuzzy sample fabricated with higher ion energy has greater PEC performance, which is mainly caused by the increase in active surface area.

Effect of low-energy ion modification of microcomposite (1-x)WO3 – xZnO ceramics on improved photocatalytic decomposition efficiency

The main purpose of this work is to study the possibility of using ionic modification of (1-x)WO3 – xZnO microcomposite ceramics in order to elevate their photocatalytic activity, as well as enhance stability to degradation during long-term use and exposure to aggressive environments. The objects of study were (1-x)WO3 – xZnO ceramics obtained by solid-phase synthesis from tungsten and zinc oxides. Moreover, according to X-ray phase analysis data, it was found that variations in the ratio of oxide components during their mechanical activation and subsequent thermal annealing result in the formation of two-phase ceramics with different contents of the zinc tungstate phase. The results of experiments on the photocatalytic decomposition of the organic dye Rhodamine B revealed that two-phase WO3/ZnWO4 ceramics have the greatest efficiency, for which ion irradiation with fluences of 1015 ––5 × 1015 leads to a growth in the decomposition efficiency from 80 to 95 %. At the same time, the dominant effect influencing the photocatalytic decomposition efficiency growth is the change in the band gap of the ceramics, associated with the accumulation of the athermal effect of energy dissipation of incident ions on the change in electron density. The results of assessment of changes in the strength and structural parameters of (1-x)WO3 – xZnO ceramics depending on the time spent in model solutions (to simulate the effects of environments with different acidity levels) indicate a positive effect of ionic modification on enhancing stability to degradation and softening as a result of the accumulation of amorphous inclusions.

Atomic scale smoothing of nanoscale quartz mold using amorphous carbon films

Abstract Surface roughness control of end products is increasingly becoming significant, especially with the miniaturization trends in the semiconductor industry. Ultra-thin amorphous carbon (a-C) films offer a prime solution to optimize surface roughness due to their outstanding characteristics. In this study, hydrogenated a-C films are deposited on two-dimensional quartz plates and three-dimensional quartz molds to evaluate the growth mechanisms and changes in the surface roughness, which is supported by molecular dynamics simulations. Results reveal that surface roughness encounters multiple variations until it reaches stable values. These fluctuations are categorized into four different stages which provide a concrete understanding of various growing mechanisms at each stage. Different behavior of the atoms in the top layers is recorded in the cases of normal and grazing incidents of carbon atoms. Lower surface roughness values are obtained at low-angle deposition. Interestingly, surface smoothing is attained on the sidewalls of the nanotrench mold where the deposition occurs with high incident ion angles.

Effects of Ar ion bombardment on the structure and properties of β-quartz

β-quartz is an important component that affects the physical and chemical properties of glass-ceramics. The material removal process of ion beam machining is complicated as one of the ultra-precision machining forms. Therefore, it is important to understand the removal mechanism of ion beam machining materials. The methods of molecular dynamics (MD) and first-principles were adopted to investigate the microscopic mechanism of ion beam bombardment of the β-quartz structure. The damage evolution of structures and the evolution of material properties caused by defects respectively from the atomic and electronic levels. Through MD analysis, we find that the random collision between incident ions and matrix atoms can lead to the formation of sputtered atoms. The incidence of ions will increase the disorder degree of the matrix, and the influence on the disorder degree at different depths of the matrix is different. The degree of disorder increases obviously in the range of 50–70 Å from the surface of the matrix. It has a significant impact on the surface topology of the matrix, the number of defects, surface roughness, and matrix density. The results of first-principle calculation show that the refraction, absorption and other characteristics of the system are significantly different with different doping forms, and the energy loss is most affected by substituted doping. And the substituted doping defects have the most significant effects on the structural energy loss. Among, the Ar-substituted O doping structure leads to higher energy loss, and the peak energy loss intensity reaches 5.146. In contrast, the Ar-substituted Si doping structure causes the least energy loss, the intensity is 2.543. This study provides a theoretical basis for the selection of parameters and the realization of ultra-smooth surface in actual ion beam machining.

STRUCTURAL CHANGES IN CORTICAL BONE AFTER BOMBARDMENT WITH LOW-ENERGY HYDROGEN IONS

Radiobiological studies focused mainly on tracking the effects of photons (X and gamma rays) and high-energy charged particles over low energy ions, which raises interest in studying their biological effects. Therefore, the aim of this study is to investigate the interaction of low-energy hydrogen ions beam with xcortical bovine bone as a biological model. A hydrogen ions beam was used to bombard cortical bone with different fluences; 45, 70 and [Formula: see text][Formula: see text]ions/cm2 at low energy of 5[Formula: see text]keV. The interaction of hydrogen ions with cortical bone was estimated theoretically by the SRIM/TRIM code. The obtained results showed that the average distribution range and the maximum depth for 5[Formula: see text]keV hydrogen ions are 91 and 150[Formula: see text]nm respectively. The energy losses from incident ions are 4.45[Formula: see text]keV for ionization, 170[Formula: see text]eV for phonons and 18[Formula: see text]eV for vacancies. Also, the energy losses from the recoil atoms are 119[Formula: see text]eV for ionization, 234[Formula: see text]eV for phonons and 8[Formula: see text]eV for vacancies. At the same time, the effect of low-energy hydrogen ions bombardment on the structure of bone was studied by X-ray diffraction (XRD) and FTIR.

In situ measurement of electron emission yield at Si and SiO2 surfaces exposed to Ar/CF4 plasmas

Abstract Plasma simulations require accurate yield data to predict the electron flux that is emitted when plasma-exposed surfaces are bombarded by energetic particles. One can measure yields directly using particle beams, but it is impractical to create a separate beam of each particle produced by typical plasmas. In contrast, measurements made in situ, during plasma exposure, provide useful values for the total emitted flux and effective yield produced by all incident particles. Here, in situ measurements were made at thermally oxidized and bare silicon wafers placed on the radio-frequency (rf) biased electrode of an inductively coupled plasma system. The rf current and voltage across the sheath at the wafer were measured, along with Langmuir probe measurements of ion current density and electron temperature. The measurements are input into a numerical sheath model, which allows the emitted electron current to be distinguished from other currents. The effective yield, i.e. the ratio of the total emitted electron flux to the incident ion flux, was determined at incident ion energies from 40 eV to 1.4 keV, for Si and SiO2 surfaces in Ar, CF4, and Ar/CF4 mixtures at 1.33 Pa (10 mTorr). Yields for Ar plasmas are compared with previous work. For SiO2 surfaces in Ar/CF4 mixtures and pure CF4, the yield is dominated by ion kinetic emission, which is the same for all mixtures, and, presumably, for all ions. For Si surfaces in Ar/CF4 and CF4, the yield at high energies can be explained in part by fragmentation of molecular ions, and the yield from Ar+ can be distinguished from the other ionic species. Analytic fits of the yields are provided for use in plasma simulations.