Decreasing Ion Energy Research Articles

The dependence on displacement rate and temperature of near-surface void-denuding in self-ion irradiated pure polycrystalline and single-crystal iron

Pure iron has been irradiated with Fe2+ ions in a series of studies to identify neutron-atypical physical processes that influence the depth dependence of void swelling, focusing especially on suppression effects arising from injected interstitials and surface proximity. One paper in this series examined the surface influence in single-crystal Fe irradiated to 50 and 100 peak dpa over a range of temperature (425–525 °C) and ion energy (1.0, 2.5, 3.5, 5.0 MeV) while keeping the peak damage rate at 1.2 × 10–3 dpa/s, although the surface dpa rate was lower but increasing with decreasing ion energy, providing a small range of surface dpa rate. The observed denuded width Δx was modified to incorporate sputtering loss. The activation energy governing the denuding process was found to be EΔx4=1.65±0.03eV, higher than the vacancy migration energy EVm known to be 0.67 eV. This difference was attributed to the effect of dissolved carbon (103 appm) which reduces the effective vacancy mobility and thereby increases the effective migration energy.Since the previous study involved a factor of only 2.83 in near-surface dpa rate it is important to confirm that the dependence of EΔx4 on dpa rate is maintained over a larger range of damage rates. In this study polycrystalline Fe with 140 appm carbon was irradiated with 5 MeV Fe2+ ions to 50, 100 and 150 dpa over a range of peak dpa rates (2.0 × 10–4, 1.2 × 10–3, 6.0 × 10–3 dpa/sec) and temperatures (425, 475, 525 °C). At very high dpa levels sputtering and void growth lead to loss of voids via shrinkage, limiting the upper dose level where this technique can be applied. It was found that the single-crystal and polycrystal specimens yielded essentially identical behavior with EΔx4 = 1.65 eV, validating the application of this activation energy over a wider range of dpa, dpa rate, temperature, and crystal form.

Diagnostics of target-cavity confined plasmas produced by nanosecond pulsed laser ablation of graphite

Ion energy of laser plasmas produced by repeated nanosecond pulsed laser ablation of graphite is measured by time-of-flight (TOF) spectrometry and retarding potential analysis. An Nd:YAG laser with λ=1064 nm wavelength is incident toward the center of a cylindrical graphite target while the target is rotated along its axis. Ion energy distributions are measured via TOF spectroscopy. TOF signals show a decrease in the ion peak current by half followed by shifting of the signals by 4 μs toward later times after 10 000 laser shots. An ion energy decrease of 200 eV was measured from the retarding potential analysis.

Plasma induced damage on AlGaN/GaN heterostructure during gate opening for power devices

During the fabrication of metal oxide semiconductor high electron mobility transistor based on AlGaN/GaN heterostructure, gate patterning is recognized as the most critical step that can lead to electrical degradation of the transistor. In this work, we performed the SiN cap layer plasma etching processes by two fluorine-based plasma processes (SF6/Ar and CHF3/CF4/Ar) with low (≈15 eV) and high (≈260 eV) ion energies. Moreover, we investigate the postetching treatment using a KOH solution in order to restore the quality of the AlGaN barrier surface after etching. The objective of this article is to evaluate the AlGaN barrier surface damage after the listed plasma etching processes and postetching strategies by using quasi-in situ angle-resolved x-ray photoelectron spectroscopy, transmission electron microscopy, and atomic force microscope. Accordingly, it is found that both high ion energy plasma processes lead to a significant stoichiometric change and modification of the AlGaN barrier layer into a 1.5 nm F-rich AlGaNFx subsurface reactive layer. The decrease in ionic energy leads to a decrease in the SiN etch rate and a significant improvement in the SiN/AlGaN etch selectivity (which becomes infinite) for both plasma chemistries. Moreover, the decrease in ion energy decreases the depth of the modification (about 0.5 nm) and reduces the stochiometric change of the AlGaN barrier layer. However, both low and high ion energy SF6/Ar plasma lead to 0.8 eV Fermi level shift toward the valence band. Furthermore, the KOH postetching treatment demonstrates complete and effective removal of the AlGaNFx subsurface reactive layer and restoration of the surface properties of the AlGaN layer. However, this removal leads to AlGaN recesses that are correlated to the thickness of the reactive layer formed during the etching.

Diagnostic Techniques for Electrical Discharge Plasma Used in PVD Coating Processes

This article discusses the possibilities of two methods for monitoring Physical Vapor Deposition (PVD) process parameters: multi-grid probe, which makes it possible, in particular, to determine the energy distribution of ions of one- or two-component plasma and spectrum analyzer of the glow discharge plasma electromagnetic radiation signal based on the Prony–Fourier multichannel inductive spectral analysis sensor. The energy distribution curves of argon ions in the low-voltage operation mode of ion sources with closed electron current have been analyzed. With a decline in the discharge current, the average ion energy decreases, and the source efficiency (the ratio of the average ion energy W to the discharge voltage U) remains approximately at the same level of W/U ≈ 0.68, …, 0.71 in the operating voltage range of the source. The spectrum analyzer system can obtain not only the spectra at the output of the sensor, but also the deconvolution of the spectrum of the electromagnetic radiation signal of the glow discharge plasma. The scheme of a spectrum analyzer is considered, which can be used both for monitoring and for controlling the processing process, including in automated PVD installations.

Gap length effect on discharge mode and etching profiles in asymmetric dual frequency capacitive CF<sub>4</sub>/Ar discharges

The capacitive CF<sub>4</sub>/Ar discharges driven by a dual frequency source based on the electrical asymmetry effect (EAE) are studied by using a one-dimensional fluid coupled with Monte-Carlo (MC) model and a two-dimensional trench model. The effects, induced by varying the relative gap distance, on self-bias voltage, electronegativity, ion flux, neutral flux and other plasma characteristics are systematically discussed. In this asymmetric discharge, as the gap distance increases, the absolute value of the self-bias voltage and electronegativity decrease. Meanwhile, the plasma density and absorption power increase accordingly because the effective discharge area expands but the boundary loss is still limited. In addition, both <inline-formula><tex-math id="M72">\begin{document}$ \mathrm{\alpha } $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="9-20210546_M72.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="9-20210546_M72.png"/></alternatives></inline-formula> mode and drift-ambipolar (DA) mode can play their important roles in the discharges with different gap distances, though DA mode is weakened in large gap discharge. Owing to the fact that the self-bias is larger and electronegativity is stronger for the case of smaller gap distance, the sheath expansion electric field at the powered electrode and the bulk electric field heat the electrons, leading the ionization rate to greatly increase near the collapse of the sheath at the grounded electrode. Besides, at the larger gap distance, the maximum value of the ionization rate decreases due to the reduction of electrons with relatively high-energy, and the ionization rate near the grounded electrode is reduced evidently. Moreover, with the increase of the gap distance, the maximum ion energy decreases and the ion energy distribution width becomes smaller due to the reduction of the self-bias voltage. Meanwhile, the etching rate increases a lot since the neutral flux increases significantly near the powered electrode. However, as the gap distance increases to 5 cm, the etching rate stops increasing and the trench width at the bottom becomes narrow because the neutral flux increases greatly compared with ion flux, forming a thick layer of polymer. So, besides separately controlling the ion energy and flux, optimizing the synergistic effect of ion flux and neutral group flux to adjust the etching rate and improve the etching morphology is also an interesting topic in the asymmetric CF<sub>4</sub>/Ar discharges.

Trajectory-dependent electronic excitations by light and heavy ions around and below the Bohr velocity

We present experiments demonstrating trajectory-dependent electronic excitations at low ion velocities, where ions are expected to primarily interact with delocalized valence electrons. The energy loss of H$^+$, H$_2 ^+$, He$^+$, B$^+$, N$^+$, Ne$^+$, $^{28/29}$Si$^+$ and Ar$^+$ in self-supporting silicon membranes was analysed along channelled and random trajectories in a transmission approach. For all ions, we observe a difference in electronic stopping dependent on crystal orientation. For heavier ions, the energy-loss difference between channelling and random geometry is generally found more pronounced, and, in contrast to protons, increases for decreasing ion energy. Due to the inefficiency of core-electron excitations at employed ion velocities, we explain these results by reionization events occurring in close collisions of ions with target atoms, which are heavily suppressed for channelled trajectories. These processes result in trajectory-dependent mean charge states, which strongly affects the energy loss. The strength of the effect seems to exhibit a Z$_1$ oscillation with an observed minimum for Ne. We, furthermore, demonstrate that the simplicity of our experimental geometry leads to results that can serve as excellent benchmark systems for dynamic calculations of the electronic systems of solids using time-dependent density functional theory.

Electrical insulation characteristics of metal-insulator-metal structures using boron nitride dielectric films deposited with low-energy ion impact

Sp2-bonded boron nitride (BN) films are deposited on Ni and Si substrates with mean ion impact energies between a few eV and around 100 eV by surface-wave plasma enhanced chemical vapor deposition and their electrical insulation characteristics are examined for temperatures up to 300 °C. The overall crystallinity and order of sp2 structure in the films increase with decreasing ion energy, while the mass density becomes as high as sintered hexagonal BN only when the ion energy is above ∼50 eV. For a given temperature, the electrical resistivity of the films measured for metal-BN-metal structures increases with decreasing ion energy, and hence increasing crystallinity and order of sp2 structure, almost independent of mass density. A decrease in resistivity with temperature is small below 150 °C, then becomes large above 150 °C with a drastic increase in apparent activation energy for carriers, which is attributed to generation and transport of the carriers that get over higher barrier heights associated with stronger ion impact.

FEATURES OF SiO2 REACTIVE-ION ETCHING KINETICS IN CF4 + Ar + O2 AND C4F8 + Ar + O2 GAS MIXTURES

The effect of Ar/O2 mixing ratio on gas-phase characteristics and SiO2 etching kinetics in CF4 + Ar + O2 and C4F8 + Ar + O2 plasmas was studied under conditions of 13.56 MHz inductive RF discharge. The constant processing parameters were fraction of fluorocarbon component in a feed gas (50%) total gas pressure (6 mTorr), input power (700 W) and bias power (200 W). It was found that the full substitution of Ar for O2 in both gas systems results in non-monotonic (with a maximum at ~ 25% Ar + 25% O2) SiO2 etching rates as well as in monotonically increasing photoresist etching rate with higher absolute values for CF4-containing mixture. The steady-state densities of active species were determined using a combination of plasma diagnostics by Langmuir probes and 0-dimensional (global) plasma modeling. Corresponding results indicated that both gas systems are characterized by quite close parameters of electron and ion components while exhibit sufficient differences in the kinetics of neutral species, especially in the presence of O2. The latter produces opposite changes in F atom density as well as in effective probability of ion-assisted chemical reaction vs. Ar/O2 mixing ratio. Relationships between type of fluorocarbon component and heterogeneous process kinetics were analyzed through the set of gas-phase-related parameters (fluxes, flux-to-flux ratios) characterizing chemical etching pathways for SiO2 and formation/destruction balance for the fluorocarbon polymer film. It was suggested that the transition toward O2-rich plasma in the low-polymerizing CF4 + Ar + O2 plasma suppresses the effective probability for SiO2 + F reaction through decreasing efficiency for oxide bond breaking and desorption of etching products due to decreasing ion energy flux. Oppositely, an increase in O2 content in the high-polymerizing C4F8 + Ar + O2 mixture lifts up the effective reaction probability by decreasing fluorocarbon film thickness and providing better access of F atoms to the etched surface.

Background pressure effects on ion dynamics in a low-power magnetic nozzle thruster

The effect of background neutral density on the ion acceleration region of a magnetic nozzle thruster is investigated through a combination of theoretical models and experiment. Ion velocities are measured in the magnetic nozzle using laser induced fluorescence as the background pressure is raised from 0.98 to 26 μTorr-Xe. It is found that the ion velocity exiting the acceleration region decreases from roughly 12 800 to 10 200 m s−1 as background pressure is increased corresponding to a drop in accelerated ion energy of 37%. Global models are employed to determine discharge properties and to calculate the power consumed by collisions in the plume. It is found that charge exchange collisions in the plume cannot explain the decrease in performance. As an alternative, inelastic electron–neutral collisions in the plume are proposed as the mechanism responsible for the decrease in ion energy, consuming between 1.6% and 39% of the power incident on the plume as background pressure is increased over the range tested. The findings suggest that facility effects can significantly alter plume dynamics and thruster performance even at relatively low background pressures. However, with the validated global models it is possible to make predictions of on orbit ion velocities using data taken at finite background pressures.

Acceleration of carbon ion beams by an ultraviolet laser under conditions relevant for ion fast ignition of inertial fusion

The properties of carbon ion beams produced by a 100-kJ, 1-ps, KrF ultraviolet laser under conditions relevant for ion fast ignition (IFI) of DT fusion are numerically investigated using the 2D3V particle-in-cell code, and the possibility of achieving the ion beam parameters required for IFI is tested. The numerical simulations of carbon ion acceleration were carried out for flat carbon targets of various thicknesses (LT) and for various laser beam apertures (dL) on the target, while the laser pulse duration and energy were fixed. It was found that both the radiation pressure acceleration (RPA) mechanism and the sheath acceleration mechanism significantly affects the characteristics of the ion beam, with the RPA dominating in the case of thicker targets (LT ∼ 10–30 μm). The ion beam parameters depend to a significant extent on the target thickness and the distance from the target. The mean and maximum ion energy decrease with the increase of LT from 3 μm to 30 μm, while the ion beam intensity, the beam energy fluence and the total energy of the “useful part” of the ion beam (with an aperture ⩽ 50 μm) reach maximum values at LT ∼ 5–10 μm. For the optimal value of LT and a small distance x from the target (x ∼ LT), the ion beam parameters are close to or higher than what is required for IFI. However, due to the angular divergence of the ion beam, the beam intensity and fluence decrease with an increase in the distance from the target, and at x ⩾ 0.5 mm the beam parameters are below the values required for fusion ignition. To reach these values, higher laser energies and/or more sophisticated schemes of ion acceleration are required.

Radial distribution of C4H8–H2–TMS plasma during plasma‐enhanced chemical vapor deposition of Si‐doped glow discharge polymers

AbstractA silicon‐doped glow discharge polymer (Si‐GDP) was prepared by plasma‐enhanced chemical vapor deposition technique of mixtures of trans‐2‐butene (C4H8), hydrogen (H2), and tetramethylsilane (TMS). The plasma parameters were characterized by mass spectrometry. The film properties of Si‐GDP were investigated by Fourier transform infrared spectroscopy and X‐ray photoelectron spectroscopy. At a TMS flow of 0.15 cm3/min, the radial distribution of the relative intensities of CH fragment ions was uniform, which indicates balanced dissociation and polymerization. As the radial distance increases, the ion energy decreases, and the ratio of sp3/sp2 carbon and the atomic ratio of Si increases. The purpose of this work was to explore correlations between C4H8/H2/TMS plasma parameters and Si‐GDP film properties.

Secondary particle properties for the ion beam sputtering of TiO2 in a reactive oxygen atmosphere

This work deals with the properties of secondary particles, i.e., sputtered target and scattered primary particles, for the ion beam sputtering of TiO2 in a reactive oxygen atmosphere. The angular and energy distributions in dependence on ion beam parameters (ion energy, ion species, or ion mass) and geometrical parameters (ion incidence angle, polar emission angle, or scattering angle) were investigated. The angular distributions of sputtered Ti particles were determined by the collection method, i.e., by growing TiO2 thin films and measuring the thickness, composition, and mass density. The distributions can be described by the superposition of an isotropic and an anisotropic part. The anisotropic part relative to the isotropic part gets more pronounced with decreasing ion energy, increasing ion incidence angle or changing the process gas from Ar to Xe. An energy-selective mass spectrometer was used to measure mass and energy distributions of secondary ions. The most prevalent species were identified to be O+, O2+, Ti+, TiO+, Ar+, and Xe+ ions. The energy distributions of Ti+ and TiO+ ions show a low-energy maximum between 10 eV and 30 eV, followed by an exponential decay. When decreasing the scattering angle, the high-energy tail extends to higher energies. O+, O2+, Ar+, and Xe+ ions show a low-energy maximum between 5 eV and 20 eV, which is followed by a sudden signal drop for scattering angles larger than 90°. For scattering angles smaller than 90°, additional peak(s) at higher energy appear, which are assigned to direct scattering and/or direct sputtering events. The experimental results are compared to calculations based on elastic collision theory and to simulations done with the Monte Carlo code SDTrimSP. The results are also discussed in relation to secondary particle properties for the reactive ion beam sputtering of Ti and to properties of TiO2 thin films grown by reactive ion beam sputter deposition with the same setup and the same parameters. Among others, it was found that sputtering of the TiO2 target exhibits less anisotropy contributions than sputtering of the Ti target.

Formation of tungsten oxide nanowires by ion irradiation and vacuum annealing

Here we reported the fabrication of tungsten oxide (WO3−x) nanowires by Ar+ ion irradiation of WO3 thin films followed by annealing in vacuum. The nanowire length increases with increasing irradiation fluence and with decreasing ion energy. We propose that the stress-driven diffusion of the irradiation-induced W interstitial atoms is responsible for the formation of the nanowires. Comparing to the pristine film, the fabricated nanowire film shows a 106-fold enhancement in electrical conductivity, resulting from the high-density irradiation-induced vacancies on the oxygen sublattice. The nanostructure exhibits largely enhanced surface-enhanced Raman scattering effect due to the oxygen vacancy. Thus, ion irradiation provides a powerful approach for fabricating and tailoring the surface nanostructures of semiconductors.

Modification of graphene by ion beam

Ion induced defect generation in graphene was analyzed using Raman spectroscopy. A single layer graphene membrane produced by chemical vapor deposition (CVD) on copper foil and then transferred on glass substrate was subjected to helium, carbon, nitrogen, argon and krypton ions bombardment at energies from the range 25keV to 100keV. A density of ion induced defects and theirs mean size were estimated by using Raman measurements. Increasing number of defects generated by ion with increase of ion mass and decrease of ion energy was observed. Dependence of ion defect efficiency (defects/ion) on ion mass end energy was proportional to nuclear stopping power simulated by SRIM. No correlation between ion defect efficiency and electronic stopping power was observed.

Modeling of Charging on Unconventional Surface Morphologies of PMMA Substrates During Ar Plasma Etching

The charging on unconventional, rough, surface morphologies of polymeric substrates is investigated. The case study is Ar plasma etching of PMMA surfaces with sinusoidal profiles resembling rough profiles. A modeling framework for the description of charging on plasma “wetted” two-dimensional (2d) surface morphologies is developed. It consists of models for the calculation of the ion and electron trajectories, the local surface charge density, the potential induced by the surface charge, and a surface etching model. The latter is devised by combining experimental measurements and calculations by TRIM code. Etching rate calculations with and without charging are performed; it is shown that charging affects the etching rate mainly due to the decrease of the ion energy. Calculations are performed for sinusoidal profiles of different amplitude (roughness); as the amplitude increases, the ion energy decreases and the angle of ion incidence increases contributing competitively to the etching rate.

Toward site-specific dopant contrast in scanning electron microscopy.

Since semiconductor devices are being scaled down to dimensions of several nanometers there is a growing need for techniques capable of quantitative analysis of dopant concentrations at the nanometer scale in all three dimensions. Imaging dopant contrast by scanning electron microscopy (SEM) is a very promising method, but many unresolved issues hinder its routine application for device analysis, especially in cases of buried layers where site-specific sample preparation is challenging. Here, we report on optimization of site-specific sample preparation by the focused Ga ion beam (FIB) technique that provides improved dopant contrast in SEM. Similar to FIB lamella preparation for transmission electron microscopy, a polishing sequence with decreasing ion energy is necessary to minimize the thickness of the electronically dead layer. We have achieved contrast values comparable to the cleaved sample, being able to detect dopant concentrations down to 1×10(16) cm-3. A theoretical model shows that the electronically dead layer corresponds to an amorphized Si layer formed during ion beam polishing. Our results also demonstrate that contamination issues are significantly suppressed for FIB-treated samples compared with cleaved ones.

Deposition of rutile (TiO2) with preferred orientation by assisted high power impulse magnetron sputtering

The effect of energetic ion bombardment on TiO2 crystallographic phase formation was studied. Films were deposited using high-power impulse magnetron sputtering (HiPIMS) assisted by an electron cyclotron wave resonance (ECWR) plasma. The ECWR assistance allows a significant reduction of pressure down to 0.075Pa during reactive HiPIMS deposition and subsequently enables control of the energy of the deposited species over a wide range. Films deposited at high ion energies and deposition rates form rutile with (101) a preferred orientation. With decreasing ion energy and deposition rates, rutile is formed with random crystallite orientation, and finally at low ion energies the anatase phase occurs. It is supposed that particles gain high energy during the HiPIMS pulse while the ECWR discharge is mostly responsible for substrate heating due to dissipated power. However, the energetic contribution of the ECWR discharge is not sufficient for annealing and phase transformation.

Modeling of nanocluster formation by ion beam implantation

A theoretical model was developed to investigate the mechanism of the formation of nanoclusters via ion beam implantation. The evolution of nanoclusters, including the nucleation and growth process known as Ostwald ripening, was rebuilt using numerical simulations. The effects of implantation parameters such as the ion energy, ion fluence, and temperature on the morphology of implanted microstructures were also studied through integration with the Monte Carlo Transport of Ions in Matter code calculation for the distribution profiles of implanted ions. With an appropriate ion fluence, a labyrinth-like nanostructure with broad size distributions of nanoclusters formed along the ion implantation range. In a latter stage, a buried layer of implanted impurity developed. With decreasing ion energy, the model predicted the formation of precipitates on the surface. These simulation results were fully consistent with many experimental observations. With increased temperature, the characteristic length and size of nanostructures would increase due to the high mobility. This theoretical model provides an efficient numerical approach for fully understanding the mechanism of the formation of nanoclusters, allowing for the design of ion beam experiments to form specific nanostructures through ion-implantation technology.

Enhancing the ferromagnetization of graphite by successive 12C + ion implantation steps

12C + ion implantation has been shown to be an efficient way to increase the magnetization of highly oriented pyrolytic graphite, and a saturation magnetization of 17.6 emu g −1 at room temperature has been produced. The implantation was performed in four steps with decreasing ion energy for each step. Increasing the magnetization with the number of implantation steps has shown that the magnetization is correlated to the defect density. The temperature dependence of spin susceptibility calculated according to the electron spin resonance spectra indicates that the magnetism in the sample is intrinsic. The evolution of Raman spectra along with the degree of implantation supplied further evidence to the correlation between in-plane defects and the magnetization of the sample, and explained the decreased magnetization resulting from the final implantation step.

Cesium retention during sputtering with low energy Cs+ and oxygen flooding

AbstractAccurate knowledge of the stationary Cs retention during Ultra Low Energy (ULE; < 150 eV) Cs+ sputtering (of semiconductor substrates) is the first step towards understanding the basics of SIMS. We have investigated the effect of oxygen on the retention capacity for cesium on silicon and germanium targets. The amount of cesium that is retained was directly measured using Inductively Coupled Plasma Mass Spectrometry (ICP‐MS). Even a very small partial pressure of oxygen (in the 10−9 to 10−8 mbar range) will tremendously increase the dose of cesium that is retained at the sample surface. Concurrent with this increased retention (due to Cs‐oxide formation) strong variations in the target sputtering yield and the ionization degree of silicon can be observed.These data combined support the model that the observed target sputtering yield is directly linked to the Cs‐surface coverage and that a cap layer containing a very high amount of Cs can be formed by the ion beam. The sensitivity to oxygen becomes stronger as the primary ion energy decreases, and implies that SIMS measurements with Cs aimed at a very high depth resolution will suffer from drastically reduced erosion rates due to the deposition process.1 Copyright © 2010 John Wiley & Sons, Ltd.