Sputtering Yield Research Articles

Vacuum DC breakdown characteristics between grid electrodes with ion sputtering

Abstract In ion thrusters, grid electrodes are biased at high voltage to extract and accelerate ions. However, they are susceptible to vacuum breakdown which considerably undermines the ion thruster performance. To optimize the grid electrode design and improve the ion thruster longevity, the impact of ion sputtering on the vacuum DC breakdown characteristics of grid electrodes are systemically examined. The adopted seven-aperture grid electrodes are made of three materials including molybdenum, graphite, and stainless steel, which are exposed to Ar⁺ and Xe⁺ sputtering at energies of 400 eV and 1000 eV for durations ranging from 0.5 to 8 hours. It is found that the initial breakdown voltage, which means the first breakdown voltage after sputtering, generally decreased regardless of the electrode material as the sputtering time increased. However, the breakdown voltage rapidly increases with more repeated discharges. Each kind of electrode material demonstrates unique responses to ion sputtering and breakdowns. In comparison with metal electrodes, the graphite surface relatively easy to appear defects but exhibites the least insulation degradation. The metal surface remains relatively smooth, but breakdown voltage is significantly affected, with electrode materials such as stainless steel failing to recover to their initial insulation levels. Xe⁺ sputtering leads to greater breakdown voltage fluctuations during repeated discharges due to higher sputtering yield. Dark current is measured, and field enhancement factor is calculated to reveal correlations between local electric field and the breakdown voltage. Furthermore, micro particles are identified as major contributor to the first few times breakdown. Based on above observations, two kinds of distinct discharge mechanisms, including the cascade discharge induced by micro particles and the discharge induced by field emission are proposed to interpret experimental data.

Influence of energetic heavy ion sputtering on lifetime of alloy target

When energetic heavy ions are incident on negatively charged structure that collects and deposits ions, ion sputtering will occur. Metal wire is a structure commonly used for accelerating ions, the incidence of continuous high-throughput ions can cause surface loss of metal wire, affecting the service performance and lifespan of the metal wire. The SRIM software commonly used for calculating sputtering yield cannot consider the multi-body interaction problem contained in the alloy crystal structure. so, there is a significant error in calculating the sputtering yield of high-energy ions incident on alloy target. Based on the molecular dynamics method and Langevin temperature control model, the calculation model of ion sputtering parameters of energetic metal ions incident on alloy target is established in this work. The model is used to calculate the sputtering yield under the conditions of intact surface lattice of the target material and long-term incident surface lattice damage. The damages to the cathode metal wire under different incident ion fluences are further calculated, and the cross-sectional characterization of the metal wire is carried under typical working condition. The results show that the discrepancy between the experimental value and the theoretical value is less than 10%, which verifies the accuracy and applicability of the theoretical model. Based on this model, the search direction for sputtering resistant materials is proposed, meanwhile, a theoretical optimization is carried out to improve the service life of metal wire, and a method of using Ni-Ti alloy to improve the service life of metal wires is proposed, which is of great significance for predicting the service life of the metal wire under different conditions.

Focused ion beam (FIB) prepared microtextured microneedle array capable of delivering drugs through punctured skin

Microtextured microneedles are tiny needle-like structures with micron-scale microtextures, and the drugs stored in the microtextures can be released after entering the skin to achieve the effect of precise drug delivery. In this study, the skin substitution model of Ogden’s hyperelastic model and the microneedle array and microtexture models with different geometrical parameters were selected to simulate and analyse the flow of the microtexture microneedle arrays penetrating the skin by the finite-element method, and the length of the microneedles was determined to be 200 μm, the width 160 μm, and the value of the gaps was determined to be 420 μm. A four-pronged cone was chosen as the shape of microneedles, and a rectangle was chosen as the shape of the drug-carrying microneedle. The initial microneedle arrays were prepared by wet etching using monocrystalline silicon as the material due to its high mechanical strength, stability and good biocompatibility. The sputtering yield, energy loss and material damage of single crystal silicon materials during focused ion beam (FIB) processing were calculated using Monte Carlo method and found to have optimum removal efficiency and processing accuracy at 30 keV ion beam voltage. After the initial microneedle arrays were prepared using wet etching, microtextures were prepared on the microneedles with 30° inclination and 45° inclination using FIB technique with processing parameters of 30 keV and 60 nA. Biocompatibility tests showed a cell survival rate of 99.42% for the cell set containing the microneedle array, indicating that the microneedle array was non-toxic to the cells. The viscosity and contact angle of 5-fluorouracil, triamcinolone acetonide and tacrolimus were examined and it was learnt that tacrolimus had the highest viscosity and the lowest contact angle and the configured solution was able to be more present in the microtexture and was more stable and less susceptible to loss. The experiments of drug-carrying skin puncture mice showed that the stained area of tacrolimus was brighter and more uniformly distributed, the stained area of microtextured microneedles was larger than that of ordinary microneedles by 70.52%, the drug-carrying area was larger than the average area of ordinary microneedles by 105.09%, and the drug-carrying effect of microtextured microneedles with a horizontal inclination angle of 30° was more effective.

Modelling the Impact of Argon Atoms on a WO3 Surface by Molecular Dynamics Simulations.

Machine learning potential energy functions can drive the atomistic dynamics of molecules, clusters, and condensed phases. They are amongst the first examples that showed how quantum mechanics together with machine learning can predict chemical reactions as well as material properties and even lead to new materials. In this work, we study the behaviour of tungsten trioxide (WO3) surfaces upon particle impact by employing potential energy surfaces represented by neural networks. Besides being omnipresent on tungsten surfaces exposed to air, WO3 plays an important role in nuclear fusion experiments due to the preferred use of tungsten for plasma-facing components. In this instance, the formation of WO3 is caused by the omnipresent traces of oxygen. WO3 becomes a plasma-facing material, but its properties, especially concerning degradation, have hardly been studied. We employ molecular dynamics simulations to investigate sputtering, reflection, and adsorption phenomena occurring on WO3 surfaces irradiated with Argon. The machine-learned potential energy function underlying the MD simulations is modelled using a neural network (NNP) trained from large sets of density functional theory calculations by means of the Behler-Parrinello method. The analysis focuses on sputtering yields for both oxygen and tungsten (W), for various incident energies and impact angles. An increase in Ar incident energy increases the sputtering yield of oxygen, with distinct features observed in different energy ranges. The sputtering yields of tungsten remain exceedingly low, even compared to pristine W surfaces. The ratios between the reflection, adsorption, and retention of the Ar atoms have been analyzed on their dependence of impact energy and incident end angles. We find that the energy spectrum of sputtered oxygen atoms follows a lognormal distribution and offers information about surface binding energies on the WO3 surface.

Towards Pharmaceutical Particle Imaging in Ultrathick Organic Films: 3D Visualization and Image Distortions Using Gas Cluster Bombardment Secondary Ion Mass Spectrometry

ABSTRACTPolystyrene spheres ranging in diameter from 2, 5, 10, to 20 μm were embedded in five different types of thick organic films (> 50 μm) to serve as model systems to evaluate the use of time‐of‐flight secondary ion mass spectrometry for the localization and particle sizing of pharmaceuticals in orodispersible drug delivery films. It was found that certain films, such as gelatin and ethyl cellulose, are prone to developing micron‐scale topography which can affect the linearity of the sputter yield, which can ultimately affect the maximum analyzable depth. Surprisingly, the sputter yield of the film was more of a determining factor for affecting the measurement of spheres. It was possible to more accurately extract the true dimensions of the buried spheres in faster sputtering films, while slower sputtering films were associated with matrix effects and sputtering artifacts that degraded the quality of the reconstructed image. Signal attenuation was also found to be problematic and quite significant for certain film chemistries, which placed a limit on the size of the particles that could be visualized. Overall, visualization of particles embedded in thick organic films is possible, but the extraction of quantitative data such as size and the amount of material is difficult given the influence of sample dependent sputtering artifacts.

Solar Wind Ion Sputtering from Airless Planetary Bodies: New Insights into the Surface Binding Energies for Elements in Plagioclase Feldspars

Our understanding of the ion-sputtering contribution to the formation of exospheres on airless bodies has been hindered by the lack of accurate surface binding energies (SBEs) of the elements in the various mineral and amorphous compounds expected to be on the surfaces of these bodies. The SBE for a given element controls the predicted sputtering yield and energy distribution of the ejecta. Here, we use molecular dynamics computations to provide SBE data for the range of elements sputtered from plagioclase feldspar crystalline end members, albite and anorthite, which are expected to be important mineral components on the surfaces of the Moon and Mercury. Results show that the SBE is dependent on the crystal orientation and the element’s coordination, meaning multiple SBEs are possible for a given element. Variation in the SBEs among the different surface positions has a significant effect on the predicted yield and energy distribution of the ejecta. We then consider sputtering by H, He, and a solar wind mixture of 96% H and 4% He. For each of these cases, we derive best-fit elemental SBE values to predict the ejecta energy distribution from each of the (001), (010), and (011) cleavage planes. We demonstrate that the He contribution to the sputtering yield cannot be accounted for by multiplying the 100% H results by some factor. Lastly, we average our results over all three possible lattice orientations and provide best-fit elemental SBE values that can be easily incorporated into sputtering yield models.

Ultralow impact energy dynamic secondary ion mass spectrometry with nonfully oxidizing surface conditions

Main accent of this research at ultralow impact energy sputtering is a primary beam species selection to facilitate data interpretation and to improve the quantification of very shallow boron implants. The effects present during the very low energy sputtering with O2+ produce the nonuniform sputtering and ion yield variation within the first nanometer of SIMS depth profiling. The experiments using sputtering beam formed in the ion source supplied with different gas mixture including a variable oxygen fraction were performed. It was concluded that the modification of sputtering conditions by reducing the O2+ content in the sputtering beam is capable of providing an accurate characterization of low energy boron implantation wafers.

Erosion enhancement by impurity entrainment in the highly collisional plasmas of Magnum-PSI

Abstract Extrinsic impurity seeding is planned for ITER to avoid high heat fluxes to the tungsten divertor targets. Nevertheless, excessive sputtering by impurities would lead to a reduction in fusion power output and divertor lifetime. Unlike today's fusion devices, the entrainment of impurities is expected to play an increasing role in ITER's highly collisional near-surface plasma. Impurity entrainment refers to the acceleration of impurities with the plasma flow by collisional drag, leading to elevated impact energies. To investigate impurity entrainment under ITER-like plasma conditions ($n_e > 2 \cdot 10^{20}$ m$^{-3}$, $T_e <$ 5 eV), argon-seeded hydrogen plasmas were formed with the linear plasma generator Magnum-PSI. Doppler shifts in spatially-resolved emission profiles of argon (Ar II - 480.6 nm) and hydrogen (H$_\mathrm{\beta}$ - 486.1 nm) reveal the entrainment of seeded argon towards the upstream hydrogen velocity ($\sim$9 cm from surface), corresponding to $\sim$0.39 of the common-system sound speed. In addition, the sputtering yield of tungsten by argon bombardment for the argon-seeded hydrogen plasma was compared to a pure argon plasma by monitoring tungsten (W I - 400.9 nm) emission. These sputtering experiments indicate further entrainment of the argon impurities to $\sim$0.65 of the common-system sound speed at the sheath edge, leading to a large increase in their impact energy. Extrapolation of the Magnum-PSI results to ITER, using existing SOLPS-ITER simulations to determine the divertor plasma conditions, indicates more than an order of magnitude increase in gross erosion of the divertor targets due to impurity entrainment. However, the net erosion rate could still be kept under control if high re-deposition rates are achieved, which is treated in the companion paper \cite{Cornelissen2023}. The combination of a low sputtering threshold energy and high impact energy of impurities constrains the fine balance between radiative cooling with impurity seeding and avoiding extensive divertor erosion.

Characteristics of [formula omitted]-gallium oxide sputtering etching by focused gallium ion beam in the micro-nano scale fabrication

β−Gallium oxide (β−Ga2O3) is an ultra-wide bandgap (UWBG) semiconductor that is widely recognized as an ideal material for the fabrication of optoelectronic and high-voltage devices. As an advanced sputtering etching technology, focused ion beam (FIB) is increasingly used in the micron and nanoscale processing of semiconductor materials. In this paper, the influence of Ga FIB on β−Ga2O3 sputtering etching is systematically investigated from three parts, namely, the calibration of sputtering yield, the influence of process parameters on etched profiles and the effect of redeposition. Firstly, sputtering etching of β−Ga2O3 substrate was carried out under typical processing conditions with different incident angles, and the key parameter of sputtering yield was precisely calibrated. Further, Yamamura’s model is applied to fit the curve of sputtering yield as a function of incident angle. In addition, the sputtering yield of β−Ga2O3 is compared with those of GaNand Si under the same conditions, revealing the differences between them. Next, through line scan etching and trench structure etching experiments, this study analyzed the maximum etched depth, width, and etched volume variation with ion dose. The evolution of β−Ga2O3 trench structure is described and compared with GaN and GaAs, and from the perspective of redeposition effects, the reasons for the differences between the three in the sputtering etching process are analyzed. Lastly, the study investigated the impact of FIB process parameters on the redeposition effects of β−Ga2O3. Experimental results indicated that significant redeposition effect occurs when etching β−Ga2O3 with large single dwell time. Further analysis identified a specific range of single dwell times under different beam currents that reduce redeposition and prevents the formation of sidewall angles. These findings provide important data support for the high-resolution etching process of β−Ga2O3-based micro-nano devices using FIB technology, and also establish a reliable data foundation for related simulation studies.

Effect of grain size and orientation on magnetron sputtering yield of tantalum

The electron beam melting (EBM) technique was employed to prepare ultra-highly pure (99.999 wt%) Tantalum (Ta) cast ingot for application in chips. Subsequently, the Ta cast ingot were forged, rolled, and annealed with different durations to gain three different grain sizes (centimeter scale, 99.8 μm, and 36.7 μm). Sputtering experiments conducted under identical conditions revealed that the rolled target (36.7 μm) film deposition rate was increased by 60.6 % compared to the cast ingot target with a centimeter-scale grain size (columnar crystal). Ta targets with a fine grain size and homogeneous distribution demonstrate superior film deposition performance. The sputtering rate is directly related to the atomic packing density of grains. The (111)-oriented grains of BCC targets (Ta target) exhibit sputtering resistance, and the order of sputtering rate of Ta atoms was S(101) > S(001) > S(111).

Simulations of tungsten fuzz growth and erosion under He/Ar mixed plasma irradiation on LP-MIES

Extrinsic noble impurities seeding has been commonly used to reduce the heat flux on divertor target. However, the physical sputtering yield on the target could be obviously enhanced due to larger mass of noble impurities. On the other hand, it has been reported that tungsten fuzz can suppress the physical sputtering due to the trapping effect on sputtered particles. Hence, it is interesting to check the erosion behaviors of tungsten fuzz under the irradiation of noble impurities. Recently, experiments of He/Ar mixed plasma irradiation on W target were conducted on LP-MIES device. Dedicated simulations have been performed by SURO-FUZZ code to study the impact of Ar flux on fuzz growth and erosion, which can well reproduce the experimental measurements on LP-MIES. For the case of pure He incidence, the growth of porous nanostructure leads to an obvious reduction in the fraction of the implanted He particles, which stabilizes to 15 % when the fuzz layer thickness exceeds 1500 nm. For He/Ar mixed plasma irradiation, the dependence of the position of the net sputtering on the local porosity has been investigated in detail. It is found that the net physical sputtering mainly occurs in the regions with the local porosity larger than 0.6.

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.

A First-Principles Study of the Structural and Thermo-Mechanical Properties of Tungsten-Based Plasma-Facing Materials

Tungsten (W) and tungsten alloys are being considered as leading candidates for structural and functional materials in future fusion energy devices. The most attractive properties of tungsten for the design of magnetic and inertial fusion energy reactors are its high melting point, high thermal conductivity, low sputtering yield, and low long-term disposal radioactive footprint. Despite these relevant features, there is a lack of understanding of how the structural and mechanical properties of W-based alloys are affected by the temperature in fusion power plants. In this work, we present a study on the thermo-mechanical properties of five W-based plasma-facing materials. First-principles density functional theory (DFT) calculations are combined with the quasi-harmonic approximation (QHA) theory to investigate the electronic, structural, mechanical, and thermal properties of these W-based alloys as a function of temperature. The coefficient of thermal expansion, temperature-dependent elastic constants, and several elastic parameters, including bulk and Young’s modulus, are calculated. Our work advances the understanding of the structural and thermo-mechanical behavior of W-based materials, thus providing insights into the design and selection of candidate plasma-facing materials in fusion energy devices.

Radiofrequency sheath rectification on WEST: application of the sheath-equivalent dielectric layer technique in tokamak geometry*

Abstract Radiofrequency sheath rectification is a phenomenon relevant to the operation of Ion Cyclotron Range of Frequencies (ICRFs) actuators in tokamaks. Techniques to model the sheath rectification on 3D ICRF antenna geometries have only recently become available (Shiraiw et al 2023 Nucl. Fusion 63 026024; Beers et al 2021 Phys. Plasmas 28 093503). In this work, we apply the ‘sheath-equivalent dielectric layer’ technique, used previously only on linear devices (Beers et al 2021 Phys. Plasmas 28 103508), in tokamak geometry, computing rectified sheath potentials on the WEST ICRF antenna. Advancing the state of the art in sheath rectification modeling, we compute the sheath potentials not just on the limiters, but also on the Faraday Screen bars. The calculations show a peak rectified DC potential of 300 V on the limiters and 500 V on the Faraday screen. Assuming a typical sputtering yield curve, the RF sheath rectification increases the sputtering yield from the limiters by a factor of 2.6 w.r.t. the sputtering due to the non-rectified thermal sheath.

Mechanisms of nodule formation on Ni-Pt targets during sputtering

Ni-Pt alloys are employed in the formation of Ni-Pt silicide to facilitate contact and interconnection functions in semiconductor devices. However, the formation of nodules on the Ni-Pt target during sputtering results in a reduced deposition rate and abnormal discharge, significantly impacting the film quality. This study investigates the nodule formation mechanism on the surface of Ni-Pt targets during sputtering, utilizing field-emission scanning electron microscopy (FE-SEM), energy-dispersive spectroscopy (EDS), and electron backscatter diffraction (EBSD). The findings reveal that defects and inclusion areas in the target exhibit a slower sputtering rate compared to normal areas, ultimately leading to nodule formation. Notably, platinum-rich layers were observed on the surfaces of nodules, particularly those exceeding a height of 100 μm. The variations in target surface concentration, angular distribution, and sputter yield of Ni and Pt atoms, coupled with the blocking effect of the lateral areas of nodules, contribute to the formation and compositional variation of the Pt-rich layers on the nodule surfaces.

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%.

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.

Sputtering of targets in high-intensity heavy-ion beam experiments

Based on available models and experimental data, the sputtering of target atoms in long-term experiments with intense heavy ion (HI) beams has been considered. The experiments on the synthesis of superheavy nuclei (SHN), which are carried out in laboratories around the world, are examples of such experiments encountering the target atoms sputtering in specific conditions. Rotating wheels with a relatively large target annulus area are used in these experiments to reduce the temperature and radiation loads on the target. Large areas of rotating targets allow one to reduce the sputtering yields of target atoms significantly. The question arises about the reliability of these yields in experiments on the synthesis of SHN with Z>118. These nuclei can be produced with extremely low cross sections, requiring HI beam doses exceeding 10 20 particles passed through the target to observe several decay events of the thus synthesized SHN. Estimates based on approximations and simulations considered in this work are presented and compared with some data on actinide target sputtering obtained in experiments performed on the synthesis of SHN.

Multi-physics modeling of tungsten collector probe samples during the WEST C4 He campaign

We describe the results of a multi-scale, multi-physics modeling assessment of SOLPS-ITER, hPIC2, RustBCA and Xolotl, in which five single-crystal tungsten (W) samples were placed in a reciprocating collector probe and exposed to helium (He) plasma in the WEST fusion device. In our models, we considered a pure (100 %) He plasma, as well as one with oxygen (O) present (95% He 5% O) corresponding to the impurity concentration estimated during the C4 He campaign in WEST. Our SOLPS simulations approximately match experimental reciprocating Langmuir probe plasma measurements of plasma density and temperature. Using these plasma parameters as input, hPIC2 and RustBCA predict that the presence of oxygen impurities lead to a 15%–20% decrease in ion and heat fluxes to the surface, and an order of magnitude higher sputtering yields (compared with a pure He plasma). Xolotl predictions for the response of tungsten to plasma surface interactions (PSIs) agree with experimental LAMS analysis, and indicate large near-surface He concentrations, which quickly decay with depth. Our model also shows an increasing role of erosion—in removing the near-surface He—with time. Overall, slightly higher retention is predicted for tungsten exposed to a pure He plasma, with the largest differences in the near-surface gas content caused by the large oxygen-induced erosion. This highlights the important role that impurities play in PSI. Therefore, future work will focus on providing a fully self-consistent description of oxygen (and oxides, etc.) in our models, through multi-species implementation in GITR and inclusion of oxygen and tungsten oxide formation in Xolotl.

Simple dynamical model that leads to sputter cone formation.

We introduce a model for sputter cone formation that includes only the angular dependence of the sputter yield and a fourth-order smoothing effect like surface diffusion. In one dimension, a sputter cone is a particular kind of shock wave that is known as an undercompressive shock. Simulations of our model show that a wide variety of initial conditions lead to the formation of sputter cones and that the opening angle of the cones does not depend on the detailed form of the initial condition. In two dimensions, a sputter cone is a higher-dimensional analog of an undercompressive shock. For two particularly simple choices of parameters, a sputter cone is a four-sided pyramid with rounded edges that is produced by the superposition of two orthogonal, one-dimensional undercompressive shocks.