Energy Ion Bombardment Research Articles

Effects of the target-to-substrate distance on the microstructure and properties of TiN coatings fabricated by pulse-enhanced vacuum arc evaporation

Pulse-enhanced vacuum arc evaporation (PEVAE) is promising in surface engineering and produces coatings with better structure and properties. In PEVAE, the target-to-substrate distance (D t-s) affects the substrate ion current and ion bombardment energy which is closely related to the structure and properties of the coatings. In this work, TiN coatings are deposited on high-speed steel (HSS) and silicon plates by the traditional DC mode and PEVAE mode at various D t-s and the substrate current, microstructure, morphology, mechanical properties, and corrosion resistance are evaluated. The results show significant improvements of 42.8%, 33%, and 51.6% for the average substrate current (I Ave.), ion bombardment energy (E bi), and deposition rate (a d) in the PEVAE mode and as D t-s is increased, I Ave., a d and E bi decrease less substantially compared to the DC mode indicating that TiN coatings are still bombarded by more ions despite a larger D t-s. The hardness and wear rate (W R) in the PEVAE mode increase by 41.4 and 56.1%, respectively, and the corrosion resistance and adhesion strength are improved as well. When D t-s is increased, the hardness and corrosion resistance decrease and W R increases in both modes but the effects are less significant in the PEVAE mode.

UV-vis spectroscopic studies of low-energy argon-ion-bombarded ion-exchanged glasses.

The ion-exchanged glasses bombarded with 80 eV, 100 eV, and 120 eV argon ions at room temperature are investigated. The optical and structural properties of ion-exchanged glasses before and after bombardment were analyzed by means of a UV-vis spectrometer, x-ray photoelectron spectroscopy, and electron probe micro analysis, respectively. The optical absorption and transmittance spectra of ion-exchanged glasses appear as obvious changes in the UV-visible region after bombardment. The optical absorption band and transmittance properties of ion-exchanged glasses at about 369-900 nm are less sensitive to the ion bombardment energy than that at about 200-280 nm. The changes in binding energy shift and peak area ratios of non-bridging oxygen and bridging oxygen contributions to the O 1s lines were observed with increasing ion beam bombardment energy. Accompanied with out-diffusion of potassium cations during argon ion bombardment, the peak of potassium cations concentration in the exchanged region decreases and moves into the interior of glasses in different degrees. The results show that variation of structure and optical properties of the ion-exchanged glasses are indicative of alterations of the silicate network structure induced by argon ion bombardment, which provide important information for application of the ion-exchanged glasses.

Copper Oxide Passivation Effect on Electromigration Lifetime of Plasma Etched Copper Lines

The degradation of the copper oxide passivated copper line prepared from a room temperature plasma-based etch process under the electromigration condition has been studied. The copper line surface was oxidized into the copper oxide layer in a parallel-plate plasma reactor operated under the plasma etching or reactive ion etching mode. The surface roughness of the oxide is contributed by the high ion bombardment energy. The lifetime of the sample was shortened by the addition of the oxide passivation layer. It was also decreased with the increase of the stress current density. The sample with the thin bulk copper layer is more resistant to the thermal stress than that with the thick bulk copper layer, which delayed the voids formation in the line breakage process.

Formation of the radio frequency sheath of plasma with Cairns–Tsallis electron velocity distribution

The effect of the non-Maxwellian plasma with enhanced electron tails on the properties of the radio frequency (RF) sheath is studied with a one-dimensional collisionless model, which consists of the sheath model and the equivalent circuit model. In the sheath model, electrons are assumed to obey the Cairns–Tsallis distribution. For various entropic indices q characterizing the degree of electron nonextensivity and parameter α measuring the electron nonthermality state, the electron nonextensivity and nonthermality are found to modify the potential drop across the sheath and the sheath thickness, as well as the spatiotemporal variations of the potential, the ion and electron densities inside the sheath. With the decrease in q and the increase in α, the potential drop across the sheath and the thickness increase at any time in a RF cycle as a result of the increase in superthermal electrons in the non-Maxwellian tail. The dependence of the potential drop across the sheath on q and α is deeply related to the frequency and amplitude of the disturbance current. When the electron nonextensivity and nonthermality are strengthened, the enhancement of the sheath potential drop can cause a significant increase in the ion bombardment energy on the wall, sheath power dissipation, and plasma energy flux to the wall.

Tailored voltage waveforms applied to a capacitively coupled chlorine discharge

Tailored voltage waveform, composed of a fundamental frequency and the second harmonic, is applied to a capacitively coupled chlorine discharge operated in the pressure range 1–50 Pa. The electronegativity is very high and the electron power absorption is mainly due to drift-ambipolar (DA) electron power absorption within the electronegative core. For operating pressures of 1 and 10 Pa the creation of a dc self-bias and asymmetric response is demonstrated and applied to control the ion bombardment energy while the ion flux on the electrodes remains fixed. However, the available range in mean ion energy is rather limited and the range is significantly narrower than typically observed for electropositive discharges. For operating pressure of 50 Pa it is not possible to control the ion bombardment energy by the phase angle between the fundamental frequency and the second harmonic. At the low pressure of 1 Pa the power absorption by ions dominate, while power absorption by electrons is smaller. At higher operation pressure DA power absorption by electrons dominates. The electric field within the electronegative core is high and not constant within the plasma bulk and exhibits the waveform of the discharge driving voltage for both the single frequency and dual frequency operation.

The magnetic asymmetry effect in geometrically asymmetric capacitively coupled radio frequency discharges operated in Ar/O2

Previous studies in low pressure magnetized capacitively coupled radio frequency (RF) plasmas operated in argon with optimized geometric reactor symmetry have shown that the magnetic asymmetry effect (MAE) allows to control the particle flux energy distributions at the electrodes, the plasma symmetry, and the DC self-bias voltage by tuning the magnetron-like magnetic field adjacent to one electrode (Oberberg et al 2019 Plasma Sources Sci. Technol. 28 115021; Oberberg et al 2018 Plasma Sources Sci. Technol. 27 105018). In this way non-linear electron resonance heating (NERH) induced via the self-excitation of the plasma series resonance (PSR) was also found to be controllable. Such plasma sources are frequently used for reactive RF magnetron sputtering, but the discharge conditions used for such applications are significantly different compared to those studied previously. A high DC self-bias voltage (generated via a geometric reactor asymmetry) is required to realize a sufficiently high ion bombardment energy at the target electrode and a reactive gas must be added to deposit ceramic compound layers. Thus in this work, the MAE is investigated experimentally in a geometrically asymmetric capacitively coupled RF discharge driven at 13.56 MHz and operated in mixtures of argon and oxygen. The DC self-bias, the symmetry parameter, the time resolved RF current, the plasma density, and the mean ion energy at the grounded electrode are measured as a function of the driving voltage amplitude and the magnetic field at the powered electrode. Results obtained in pure argon discharges are compared to measurements performed in argon with reactive gas admixture. The results reveal a dominance of the geometrical over the magnetic asymmetry. The DC self-bias voltage as well as the symmetry parameter are found to be only weakly influenced by a change of the magnetic field compared to previous results obtained in a geometrically more symmetric reactor. Nevertheless, the magnetic field is found to provide the opportunity to control NERH magnetically also in geometrically asymmetric reactors. Adding oxygen does not alter these discharge properties significantly compared to a pure argon discharge.

Simulation of very-low energy alkali ion (≤ 10 KeV) induced effects on Al2O3 micro flakes

Objectives: To simulate the Monte-carlo simulation of irradiation of alkali ions (sodium) having very low energy (5 keV and 10 keV) on aluminum oxide micro flakes. Methods/Statistical analysis: We have utilized a simulation process namely SRIM (Stopping of ion ranges in matter), which is based on the binary collision approximation technique. We have fixed our target as an aluminum oxide in the layered structure having a thickness about 65 nm. We have incorporated two different types of ion energy as the input parameters which are normally incident on the targets. We have analyzed ion distributions, recoil distributions, and further ionizations. Findings: The projected average range for 10 keV is significantly found to be higher, almost double than that of 5 keV. The reason behind this increment is due to the high penetration depth because of higher energy. The straggling of 10 keV is higher than that of 5 keV, which is evident from the recoil distribution where the cascade collision has created a large volume of vacancies, which is very high for higher energy. Application/Improvements: This simulation helps us to gather a rich amount of information regarding ion-induced defects, which is highly essential for experiments on aluminum oxide micro flakes. The surface modification after this low ion energy bombardment leads to low detrimental effects which may modify the wetting properties of these flakes Keywords: Montecarlo simulation; SRIM; Aluminium oxide micro flakes; ion induced defects; BCA

Concerning the Effect of Type of Fluorocarbon Gas on the Output Characteristics of the Reactive-Ion Etching Process

The comparative research of the parameters, steady-state composition, and the effects of heterogeneous interaction in the plasma of fluorocarbon gases CxHyFz with various z/x relations in conditions of an induction RF (13.56 MHz) discharge is carried out. The binary systems CxHyFz + Ar based on CF4 (z/x = 4), CHF3 (z/x = 3), and C4F8 (z/x = 2) are used as the subjects of research. Through the methods of diagnostics and simulation of plasma we found (a) the reasons of the differences in the physical parameters (temperature and density of electrons, energy of ion bombardment) of the plasma in the studied systems; (b) the special characteristics of the kinetics of the plasma-chemical processes determining the steady state concentrations of fluorine atoms and polymer-forming radicals; and (c) the special characteristics of the kinetics of the heterogeneous process forming the output parameters (rate, selectivity, anisotropy) of the reactive ion etching of Si and SiO2.

Study on the influence of magnetic mirror ratio on the erosion of the bottom channel of cylindrical Hall thruster with magnetically insulated anode

A cylindrical Hall thruster with a magnetically insulated anode structure was found experimentally to experience erosion sputtering at the bottom of the ceramic channel. In this study, a Particle-In-Cell numerical simulation was used to analyze the influence of the magnetic mirror effect on the ion current density, ion energy, and deposition power density bombardment at the bottom of the discharge channel. The research findings show that, as the magnetic mirror ratio grows, the stronger magnetic mirror effect confines the ions in the boron nitride channel, leading to a decrease in the ion current density and ion energy bombarding the bottom of discharge bottom. Further statistics show that the deposition power density decreases as the magnetic mirror ratio grows, indicating that increasing the magnetic mirror ratio can effectively alleviate the erosion sputtering at the bottom.

Influence of dry etching on the properties of SiO2 and HfO2 single layers.

A comparative study was performed to investigate how etching methods and parameters affect the properties of SiO2 and HfO2 coatings. SiO2 and HfO2 single layers were prepared by electron-beam evaporation (EBE), ion-beam assisted deposition (IAD), and ion-beam sputtering (IBS). Then, ion-beam etching (IBE), reactive ion etching (RIE), and inductively coupled plasma etching (ICPE) were used to study the influence of ion bombardment energy and chemical reaction on the etching rates and properties of the prepared SiO2 and HfO2 single layers. For SiO2 coatings, chemical reaction plays a dominant role in determining the etching rates, so ICPE that has the strongest CHF3 plasma shows the highest etching rate. Moreover, all three etching methods have barely any influence on the properties of SiO2 coatings. For HfO2 coatings, the etching rates are more dependent on the ion bombardment energy, although the chemical reaction using CHF3 plasma also helps to increase the etching rates to some extent. To our surprise, the ion bombardment with energy as high as 900 V does not change the amorphous microstructure or crystalline states of prepared HfO2 coatings. However, the high-energy ion bombardment in IBE significantly increases the absorption of the HfO2 coatings prepared by all deposition techniques and decreases their laser damage resistance to different extents.

Gas-phase and film analysis of hydrogenated amorphous carbon films: Effect of ion bombardment energy flux on sp2 carbon structures

Hydrogenated amorphous carbon (a-C:H) films comprise nanoclustering graphites (nc-G), fused aromatic rings (nR), and olefinic chain clusters (nC) of sp2-bonded carbons in an sp3 matrix. In this study, the sp2 composition of the nc-G, nR and nC in a-C:H films is found to be determined by the ion bombardment energy flux (ΓEi), which can be estimated as the product of ion bombardment energy and ion flux onto the deposited surface, in plasma-enhanced chemical vapor deposition using a plasma mixture of H2 and CH4 gases with the H radical injection method. The sp2 composition is analyzed using Raman spectroscopy and near-edge X-ray absorption structure spectroscopy. a-C:H becomes increasingly graphitized with increasing ΓEi. The precise control of the sp2 C structure composition can be achieved by controlling the very-high-frequency input power and radio frequency input bias power via the ion flux and ion bombardment energy.

Comparison of the properties of a-C:H films deposited from methane and heptane precursors: study of the mechanical, chemical and structural properties

In this work, thin films of hydrogenated amorphous carbon (a-C:H) were deposited on silicon substrates from heptane and methane precursors by the radio frequency plasma-enhanced chemical vapor deposition (PECVD) technique, varying the self-bias voltage from −100 V to −900 V. As a liquid, heptane is safer, easier to handle, to store and stock, and presented advantages over methane as a precursor in the deposition of a-C:H films by PECVD as it allows deposition rates more than 4 times higher than of methane. Diamond-like carbon films deposited from heptane were also harder (22.5 GPa) and presented smaller intrinsic stress (3.5 GPa) than those obtained from methane (18.8 GPa and 4.1 GPa, respectively). In addition, these films have a smaller hydrogen content and a reduced sp2 character. The observed behavior is consistent with the subplantation model, caused by the ion bombardment energy during deposition. The differences observed in the deposition from both precursors are related to the different number of carbon atoms of the parent molecules and lead to a change of structure and hardness that could be closely related to the structural changes observed by Raman spectroscopy. Films deposited from heptane may be promising for applications due to a higher hardness to elastic modulus ratio H/E of approximately 0.13 when compared to films deposited from methane.

Improvement of thermoplastic elastomer degradation resistance by low-energy plasma immersion ion bombardment

Thermoplastic elastomers (TPE) have been used instead of traditional elastomers, since they combine the low cost of raw material with easy processing and recyclability. When used in sealing components, the polyester-based TPE, or COPEs, are most common. Although COPEs have mechanical properties similar to those of elastomers, they have limited resistance to corrosion in chlorinated water. Argon Plasma Immersion Ion Implantation (IIIP) treatments were applied to alter the morphology and chemical composition of the COPE surface, with the goal of increasing its chemical inertia in chlorinated water while preserving the desired bulk properties. The effect of ion bombardment energy on the elemental composition, chemical structure, morphology, topography and mechanical properties of COPEs was evaluated, along with whether changes in such properties affected the degradation resistance of the material in chlorinated water. Treatments were performed for 60 min in radiofrequency argon plasmas (13.56 MHz, 5.0 Pa), with the power of the excitation signal varying from 10 to 150 W. Since variations in signal power changed the self-bias potential of the driven electrode, and samples were positioned at this electrode, the ion bombardment intensity was varied in the different treatments. Immediately after treatments, surfaces became more hydrophilic than the as-received ones, but after being aged in air, some samples became hydrophobic. Dehydrogenation was the main alteration attained in chemical composition, inducing changes in the overall chemical structure. Species removal from less resistant regions promoted creation of nanometric structures randomly distributed on the surface but without promoting changes in the volumetric mechanical properties of COPE. The most pronounced surface changes were observed for the sample treated in plasmas at 150 W, which also presented the highest resistance to chlorinated solution. This improvement suggests an increase in COPE performance in practice.

Self-assembly of nanocrystalline@amorphous core-shell nanostructure in the TA19 alloy film to achieve high strength

Improving the strength of alloy materials and simultaneously retaining high toughness are strongly desirable by various engineering applications. Nanocrystalline@amorphous dual-phase nanostructure consisting of nanocrystalline-cores encapsulated with amorphous-shells has been regarded as one of the most effective routes to achieve high strength yet high toughness, which may overcome the limitation of either single-phase nanocrystalline alloys or single-phase metallic glasses because of reverse Hall-Petch effect or shear-band deformation. Herein, nanocrystalline@amorphous core-shell nanostructure has been successfully assembled in the sputtered TA19 alloy film only by regulating bias voltages (Vb), and then Vb induced evolution of microstructure and mechanical property was further investigated by combined experiments of X-ray diffraction (XRD), atomic force microscope (AFM), scanning electron microscope (SEM), select area electron diffraction (SAED), transmission electron diffraction (TEM) and nanoindentation. The results show that the bombardment of energy ions can promote formation of amorphous zones in TA19 alloy film, successfully achieving ordered core-shell nanostructure at −120 V of Vb, while single-phase nanocrystalline occurs at Vb = −40 V and amorphous dominated structure appears at Vb = −200 V. Maximum hardness value of 11.2 GPa achieves at Vb = −120 V when the ordered core-shell nanostructure is formed with invisible shear bands around indenter, but more amorphous result in disappearance of core-shell nanostructure at Vb = −200 V, which worsens hardness, accompanying with presence of obvious shear bands around indenter. This novel core-shell-like structure succeeded in achieving high strength or hardness yet high toughness by providing strong resistance to hinder movements of both grains and shear bands due to large amount of nanocrystalline@amorphous interfaces. It is suggested that the Vb as a key factor controlling the core-shell-like structure of TA19 films, which may provide a new strategy to improve mechanical properties of alloy films.

Extending service life of hall thrusters: recent progress and future challenges

Hall thrusters are widely applied to space propulsion missions because of characteristics such as high specific impulse, great thrust-to-power ratio, high efficiency, and simple structure. Because the scope of space missions using Hall thrusters has expanded, a higher total impulse is required to extend a north–south station-keeping mission to an orbit transfer mission, requiring Hall thrusters with improved lifetimes. Existing research shows that improving the anti-sputtering capacity of the discharge channel and significantly reducing the ion energy and flux bombardment on the wall of the discharge channel are the best measures to improve the lifetime of Hall thrusters. This study summarizes the lifetime restraining factors of Hall thrusters and analyzes the magnetic shielding technology, wall-less technology, aft-magnetic technology, and cylindrical Hall thruster technology to reduce ion energy and flux bombardment on the discharge-channel wall, adopting low-sputtering yield materials to reduce the wall erosion rate, which can improve the lifetime of Hall thrusters. This paper also identifies the limitations of each technology, such as the erosion of magnetic poles (MS) and ceramic cover plates (CHT), larger plume divergence angle (wall-less), hard-to-select aft-magnetic degree (aft-magnetic technology), low compactness, and great porosity (graphite material), providing a reference for further research.

Increasing charge lifetime in dc polarized electron guns by offsetting the anode

Charge lifetime of strained superlattice GaAs photocathodes in DC guns is limited by ion back bombardment. It needs to be improved at least an order of magnitude to meet the requirements for future colliders such as Electron-Ion Collider (EIC). In this work, we propose and present simulation results for an offset anode scheme to increase charge lifetime in DC guns. This scheme eliminates the bombardment of high energy ions on the cathode and enables maximum usage of the available cathode area. Depending on the size of the available cathode area, this method can increase the charge lifetime by an order of magnitude compared to the current best alternative method. An anode assembly capable of in-vacuum movement is required for this method, which has been designed and fabricated at Brookhaven National Laboratory.

Inducing locally structured ion energy distributions in intermediate-pressure plasmas

Ion energy distribution functions (IEDFs) incident upon material surfaces in radio frequency (rf) capacitively coupled plasmas are coupled to spatial and temporal sheath dynamics. Tailoring the ion energy distribution function within intermediate-pressure plasmas (≈133 Pa, 1 Torr), which find application in surface modification and aerospace industries, is challenging due to the collisional conditions. In this work, experimentally benchmarked 2D fluid/Monte-Carlo simulations are employed to demonstrate the production of structured IEDFs in a collisional (200 Pa 1.5 Torr argon) rf hollow cathode discharge. The formation of structures within the IEDFs is explained by an increase in the Ar+ ion-neutral mean-free-path and a simultaneous decrease in the phase-averaged sheath extension as the rf voltage frequency increases over 13.56–108.48 MHz for a constant rf voltage amplitude (increasing plasma power) and gas flow rate. Two distinct transitions in the shape of the IEDF are observed at 450 V, corresponding to the formation of “mid-energy” (60–180 eV) structures between 40.68 and 54.24 MHz and additional “high energy” (≳180 eV) structures between 81.36 and 94.92 MHz, with the structures within each region displaying a distinct sensitivity to the applied voltage amplitude. Transitions between these energy ranges occurred at lower applied voltages for increased applied voltage frequencies, providing increased control of the mean and modal ion energy over a wider voltage range. The capabitlity to extend the range of access to an operational regime, where the structured IEDFs are observed, is desirable for applications that require control of the ion-bombardment energy under collisional plasma conditions.

Correlation between in Situ Diagnostics of the Hydrogen Plasma and the Interface Passivation Quality of Hydrogen Plasma Post-Treated a-Si:H in Silicon Heterojunction Solar Cells.

Passivation of the interface defect states is crucial to mitigate the recombination losses in silicon solar cells. In this work, we have investigated the role of hydrogen plasma treatment (HPT) to passivate the interfacial defects between crystalline (c-Si) and hydrogenated amorphous silicon (a-Si:H) in silicon heterojunction (SHJ) solar cells. For the first time, we have found a correlation between the dynamic properties of hydrogen plasma and passivation quality of the films by using in situ optical emission spectroscopy and quasi-steady state photoconductance measurement. The optimum condition for saturation of the dangling bonds by HPT has been studied in detail by tuning the excited hydrogen (H) species and ion bombardment energies by controlling physical parameters like plasma current and chamber pressure. We have investigated the role of annealing after HPT to redistribute the H in the post-treated a-Si:H film and have obtained an i Voc of 755 mV, minority carrier lifetime (τeff) of 4.6 ms, and SRV of 1.5 cm/s on test structures having only an 10 nm intrinsic a-Si:H layer on textured silicon wafers. The H bond configuration at the interface of a-Si:H and c-Si has been investigated by Fourier transform infrared spectroscopy, which demonstrates improved monohydride bonding in the films after HPT derived from the analysis of microstructure parameter and H concentration values. Raman spectroscopy shows the absence of the nanocrystalline fraction after HPT and verifies reduced coordination defects due to annealing after HPT. The proof of concept has been validated by fabricated SHJ solar cells having a Voc of 729 mV and efficiency of 18.7% after HPT, with the best cell efficiency reaching 20.2% after doped layer optimization. The decrease in reverse saturation current and ideality factor after HPT verifies that the improvement in performance is from reduced recombination losses at the interface due to passivation of defects in midgap states.

Effect of DC bias and RF self-bias on the structure and properties of chromium oxide coatings prepared by vacuum cathodic arc deposition

Cr2O3 coatings were prepared by vacuum cathodic arc deposition (VCAD) on cemented carbide YT15 at different voltages of both direct current (DC) and radio frequency (RF) bias. The structure and properties of the coatings were investigated by scanning electron microscope, X-ray diffraction, and nanoindentation. With both DC and RF bias, coatings deposited at zero and low bias voltage exhibit a columnar structure and a preferred orientation (PO) of (104). As the bias voltage increases, the columnar structure starts to disappear and PO gradually becomes (006). Besides, the crystallite size of the coatings declines as the bias voltage increases. Comparing to DC bias, the RF bias leads to smaller crystallite size by virtue of higher energy and density of ion bombardment. The electrical resistivity of the coatings and saturation ion current density during deposition were measured to explain this effect. The hardness of the coatings was significantly enhanced by the bias voltage, particularly by the RF bias. A nano-hardness of 44.7 GPa was found in the coating deposited under −100 V RF bias, which has not been previously reported.

Radio-frequency microplasmas with energies suited to in situ selective cleaning of surface adsorbates in ion microtraps

We have demonstrated a capacitively-coupled, radio-frequency (RF) microplasma inside the 3D electrode structure of an ion microtrap device. For this work, devices with an inter-electrode distance of 340 μm were used. The microplasmas were operated at ΩRF/2π = 23 MHz, in both He and He:N2 gas mixtures, over a range of RF amplitudes (140–220 V) and pressures (250–910 mbar). Spectroscopic analysis of the He I 667 nm and Hα 656 nm emission lines yielded the gas temperature and electron density, which enabled calculation of the mean ion bombardment energy. For the range of operating parameters studied, we calculated mean He+ energies to be between 0.3 and 4.1 eV. While these energies are less than the threshold for He sputtering of hydrocarbon adsorbates on Au, we calculate that the high energy tail of the distribution should remove adsorbate monolayers in as little as 1 min of processing. We also calculate that the distribution is insufficiently energetic to have any significant effect on the Au electrode surface within that duration. Our results suggest that the microplasma technique is suited to in situ selective removal of surface adsorbates from ion microtrap electrodes.