Energetic Particle Bombardment Research Articles

Stress evolution in sputtered vanadium-tungsten alloys

While metal alloy films are used in many applications, there remains a rather limited series of studies of their stress evolution during growth as compared to elemental films. Here, we provide measurements of the sputter deposition stress evolution of VxW1-x alloys, which is a system that forms a body centered cubic solid solution across all its compositions. By increasing either the tungsten content or the growth rate, we note an increasing trend towards a compressive stress state. Since the crystallite size is approximately the same for all alloys and growth rates, we conclude that the increasing tungsten content enhances the effect of energetic particle deposition (atomic peening) on the stress facilitating this change. The results suggest that stress can be understood in terms of the same physical processes that have been proposed previously to explain stress in elemental films. These mechanisms, described in the text include effects of non-energetic growth kinetics, microstructural evolution and energetic particle bombardment.

Computational tool for analyzing stress in thin films

A computational model has been developed to analyze calculations of residual stress in thin films determined from in-situ wafer curvature measurements. The model is based on several physical mechanisms that have been proposed to control the stress, including the effect of growth kinetics, grain growth and energetic particle bombardment. From a set of data, the program determines a set of kinetic parameters that can be used to understand the processes controlling stress. These parameters can be used to estimate the stress that will develop under different processing conditions in order to obtain a desired stress state. Several examples are described that illustrate the program's capabilities, including the effects of growth rate, growth temperature and particle energy (via chamber pressure in sputter deposition). Interested researchers may obtain the program and associated documentation to use the program to analyze their own data.

Modelling of physical sputtering properties during tungsten fuzz growth under various impact energies on NAGDIS-II

Fibre-like nanostructure named fuzz can form on tungsten (W) surface under the irradiation of helium (He) plasma. While the W fuzz growth can be suppressed by the bombardment of energetic He particles. The experiments of tungsten fuzz growth and erosion exposed by He plasma with impact energies in the range of 200 ∼ 500 eV have been conducted on the linear divertor simulator NAGDIS-II. To study W physical sputtering properties during fuzz growth on NAGDIS-II, three-dimensional kinetic Monte Carlo code SURO-FUZZ has been upgraded by including the migration of W adatoms and the deposition of sputtered W particles. By means of the renewed SURO-FUZZ, dedicated simulations of W fuzz growth and erosion under various He impact energies have been performed to reproduce the NAGDIS-II experiments. The simulated fuzz layer thicknesses under erosive regimes show a good agreement with the measurements on NAGDIS-II. The W net physical sputtering yield on W fuzzy surfaces increases with He impact energy according to SURO-FUZZ modelling. Further, the relative W net physical sputtering yields of the fuzzy surface to the smooth surface (Yfuzz/Ysmooth) have been studied under different He impact energies, which reveals that the decay trend of Yfuzz/Ysmooth is mainly dependant on the fuzz layer thickness.

Transient energetic particles as the origin of the mid-infrared north polar hotspot of Jupiter

Since it was detected in 1980, Jupiter's 8-μm CH4 north polar hot spot (8CNPHS), ∼20 K warmer than the surrounding polar stratosphere, has been observed for four decades. Unlike normal auroral ovals (i.e., the bright rings of auroral emissions), it is usually filled with bright emission. The causes of both its shape and longevity are not understood, although several mechanisms have been proposed to explain its existence. In order to investigate the deriving mechanism of the 8CNPHS, we have observed the north polar regions near 3 μm, where line emission from another CH4 band as well as a C2H6 fundamental band occur. Using Gemini North/GNIRS in 2013, 2020, 2021, and 2022, we have detected transient 3-μm CH4 and/or C2H6 bright spots within the 8CNPHS, and occasionally no apparent bright spots. By comparing the emission from CH4 with that from C2H6, we demonstrate that the origin of the 8CNPHS must be due to transient and energetic magnetospheric particles, which can penetrate down to the hydrocarbon layers, heating the homopause (∼1 μbar level) and the stratosphere (∼1 mbar level) and energize the 8CNPHS. Based on our observations and analysis, we propose the following three mechanisms for maintaining and containing the decades long warmth of the 8CNPHS: 1) energetic and transient auroral particle precipitations warming the stratosphere of the 8CNPHS, 2) a longer radiative cooling time in the 8CNPHS stratosphere (∼6 months) compared to less than or equal to one month at the homopause, and 3) recently detected polar stratospheric jets likely associated with polar fronts, which resist the free flow of warm gas in the 8CNPHS to the surrounding polar regions. We show that other heating mechanisms proposed so far in the literature, such as Joule heating, polar haze heated by sunlight, etc., are only the secondary mechanisms that follow atmospheric ionization caused by energetic particle bombardment. Our finding of the magnetospheric-ionospheric-stratospheric coupling in the Jovian polar regions open the possibility of quantitatively refining 3-D global circulation models for the atmospheres of giant planets and exoplanets.

Inter-relationship of stress and microstructure in BCC and ‘beta’ tungsten films

In this work, a series of W films are deposited at different deposition rates (0.2, 0.5, and 1.0 nm/s) and pressures (0.27, 0.47, 0.67, and 1.33 Pa). Comparing the residual stresses between different deposition rates, the stress was found to become more tensile at higher depositions rates over the pressure ranges studied. Films deposited at the three highest pressures were tensile in stress, had small grains (~15 to 20 nm), and stabilized the metastable A15 phase often referred to as β-W. At the lowest pressure, 0.27 Pa, the films were compressive in stress, larger grain sizes (~70 to 90 nm), and primarily stabilized the body centered cubic α-W phase. If a W seed layer was grown under either the α-W or β-W growth conditions, the subsequent W layer adopted the phase state of the seed layer, independent of processing conditions and/or grain sizes, suggesting that the phase state is most likely determined in the initial stages of nucleation. The seed layer experiment also suggest that these layers can promote more controlled grain sizes in thicker β-W films, which has not been observed in previous work. The stress measurements are interpreted in terms of a previously developed kinetic model that includes effects of growth kinetics, microstructural evolution, and energetic particle bombardment.

Phase instabilities in austenitic steels during particle bombardment at high and low dose rates

Disruption of phase stability by energetic particle bombardment is a major challenge in designing advanced radiation-tolerant alloys and ion beam processing of nanocomposites. Particularly, ballistic dissolution susceptibility of different solute nanocluster species in alloys is poorly understood. Here, low dose rate neutron irradiations were conducted on a Fe-Cr-Ni based austenitic steel in the BOR-60 reactor (9.4 × 10−7 dpa/s, 318 °C) followed by accelerated dose rate ion irradiations at multiple temperatures (~ 10−3 dpa/s, 380 – 420 °C). Using atom probe tomography, the stability of radiation-enhanced Cu-rich and radiation-induced Ni-Si-Mn-rich nanoclusters was evaluated. During neutron irradiation, Cu-rich clusters nucleated with their core concentrations progressively increasing with dose, while Ni-Si-Mn-rich clusters formed and evolved into G-phase precipitates. Ion irradiations dramatically altered the nanoclusters. Cu-rich clusters were ballistically dissolved, but Ni-Si-Mn-rich clusters remained stable and coarsened with dose at 400 and 420 °C, highlighting that different nanocluster species in a single microstructure can have innately distinct ballistic dissolution susceptibilities. Solute-specific recoil rates were incorporated into the Heinig precipitate stability model, which shows that in addition to radiation-enhanced diffusion, recovery from ballistic dissolution depends on solute concentration gradient near cluster interfaces. The combined experimental-modeling study quantified the critical temperatures and damage rates where ballistic dissolution dominates for each cluster species.

Effects of sample bias on adhesion of magnetron sputtered Cr coatings on SiC

Swelling of SiC at 300 ∘C due to in-service neutron irradiation causes tensile residual stresses in coatings which are expected to adversely affect the performance of coated SiC composite fuel cladding for light water reactors. Matching the coating swelling with the substrate, a solution common for thermal expansion, is not practical in the case of neutron irradiation. Biasing samples during magnetron sputtering deposition induces compressive residual stress which may counteract this. In this study, chromium coatings were deposited on SiC by DC magnetron sputtering with no external heating at bias voltages of –50V, –75V, and –100V. The effects of the bias voltage on morphology, residual stress, microstrain, texture, and adhesion are shown. The low deposition temperature resulted in the coating microstructure evolution following an energetic particle bombardment dominated trend. At the two lower bias voltages knock-on implantation dominated increasing the residual stress and microstrain while at the highest bias voltage, thermal spike migration allowed for defect relaxation. When the knock-on induced compressive residual stress exceeded 0.8 GPa microcrack formation in the SiC substrate decreased coating adhesion. While no microcracks formed at the lowest bias voltage, insufficient atomic mobility during coating growth lead to voids forming in the coating. A balance is needed to form void-free coatings that have high compressive residual stress.

Preliminary estimates of tritium permeation and retention in the first wall of DEMO due to ion bombardment

• Tritium losses in the DEMO first wall were investigated with T gas and ion loading. • Code comparison with TMAP and TESSIM showed good agreement in a DEMO-like scenario. • Hundreds of g of T may be retained in wall. Up to 160 mg T could permeate per day. • With bare EUROFER wall, retention below 100 g but T permeation up to several g/day. • Impact of T losses on T fuel cycle assessed with simplified particle balance model. Tritium self-sufficiency presents a critical engineering challenge for DEMO, requiring efficient breeding and extraction systems, as well as minimizing tritium losses to the surrounding systems, such as plasma-facing components, vacuum vessel, cooling system, etc. Structural and plasma-facing components will act as a tritium sink, as tritium will be accumulated in the bulk of these components due to energetic particle bombardment and may permeate out of the vacuum system. The design of the plasma-facing components will consequently directly influence the plant lifetime, operational safety and cost of any future power plant. Therefore, modeling of tritium retention and permeation in these components is required for the engineering designs of the tritium breeding and safety systems. In this work, the diffusion-transport code TESSIM-X is benchmarked against the well-established TMAP7 code and a comparison with a simplified DEMO-relevant test case is performed. The use of either code for modeling of DEMO conditions is discussed. Following this, TESSIM-X is used to provide a preliminary assessment of tritium permeation and retention in the DEMO first wall, based on the current WCLL (Water Cooled Lithium Lead) and HCPB (Helium Cooled Pebble Bed) breeding blanket designs.

Study of the tungsten sputtering source suppression by wall conditionings in the EAST tokamak

The steady fusion plasma operation is constrained by tungsten (W) material sputtering issue in the EAST tokamak. In this work, the suppression of W sputtering source has been studied by advanced wall conditionings. It is also concluded that the W sputtering yield becomes more with increasing carbon (C) content in the main deuterium (D) plasma. In EAST, the integrated use of discharge cleanings and lithium (Li) coating has positive effects on the suppression of W sputtering source. In the plasma recovery experiments, it is suggested that the W intensity is reduced by approximately 60% with the help of ∼35 h Ion Cyclotron Radio Frequency Discharge Cleaning (ICRF-DC) and ∼40 g Li coating after vacuum failure. The first wall covered by Li film could be relieved from the bombardment of energetic particles, and the impurity in the vessel would be removed through the particle induced desorption and isotope exchange during the discharge cleanings. In general, the sputtering yield of W would decrease from the source, on the bias of the improvement of wall condition and the mitigation of plasma-wall interaction process. It lays important base of the achievement of high-parameter and long-pulse plasma operation in EAST. The experiences also would be constructive for us to promote the understanding of relevant physics and basis towards the ITER-like condition.

Molecular dynamics simulation of stress induced by energetic particle bombardment in Mo thin films

Molecular dynamics (MD) simulations are performed to analyze the evolution of defects and stress caused by low-energy (25-400 eV) Ar bombardment of polycrystalline Mo thin films. Simulations were performed under different conditions to explore the role of grain boundaries (GBs), Ar-atom's kinetic energies, and incident directions on defect generation. The results show that the GBs enhance the production of interstitial defects, producing compressive stress at much larger depths than the implantation range. This is attributed to sequences of atomic collisions that knock atoms into GBs instead of a diffusional process. Decreasing the grain size or increasing the kinetic energy of the incoming particles increases the number of interstitials in the film, which increases the compressive stress. The incident angle has little influence on the number of interstitials in the film, but the sputtering yield depends on the polar angle. The stress distribution can be modeled by a superposition of the different defect distributions with the appropriate relaxation volumes.

Measurements and modeling of residual stress in sputtered TiN and ZrN: Dependence on growth rate and pressure

In situ wafer curvature measurements have been conducted on sputter-deposited nitride films (TiN, ZrN) to investigate the effect of growth rate and pressure on stress. For each growth rate, the stress becomes more compressive at lower pressure. At high pressure the stress shows a weak dependence on growth rate, while at low pressure, it becomes more compressive at higher growth rates. The results are interpreted in terms of a kinetic model that includes the effects of growth kinetics and energetic particle bombardment. The overall agreement suggests that the model can reproduce the dependence on processing conditions in multiple nitride systems.

Molecular dynamics simulations of internal stress evolution in ultrathin amorphous carbon films subjected to thermal annealing

The evolution of internal stress in ultrathin amorphous carbon (a-C) films is a complex physical process that is difficult to experimentally analyze due to the very small film thickness (a few nanometers) and the lack of instruments that can perform spatiotemporal stress measurements at sub-nanometer resolutions. Even more challenging is the elucidation of the correlation between internal stress, film activated, and temperature. Molecular dynamics (MD) provides potent computational capability for tracking structural changes activated by stress and temperature at the atomic level. Consequently, the aim of this study was to perform a comprehensive MD analysis that elucidates the origin of internal stress in sub-2-nm-thick a-C films grown on single-crystal silicon under optimal deposition energy conditions and explore its dependence on prevalent structural features (e.g., hybridization state) and temperature. The physical mechanisms of a-C film growth and stress built-up under deposition conditions of energetic particle bombardment and stress relief due to thermal annealing are interpreted in the context of MD results. Simulations of film growth illuminate the correlation between film stress and energy of incident carbon atoms. A significant stress relief occurs mainly in the bulk layer of the multilayered a-C film structure at a critical annealing temperature, which continues to intensify with the further increase of temperature. Simulations of time-dependent variation of stress through the film thickness reveal that the stress relief is a very fast process that accelerates with the increase of temperature. The results of this study provide insight into the spatial and temporal variation of internal stress in ultrathin a-C films due to structure and temperature effects and the film stress-structure interdependence.

A single gas barrier layer of high-density Al2O3 formed by neutral beam-assisted sputtering at room temperature

This paper reveals the formation of high-density Al2O3 thin films at low temperatures for inorganic gas barriers using neutral beam-assisted sputtering (NBAS). The NBAS induces an annealing effect even at room temperature through energetic neut.al particles, leading to an enhancement in the density of Al2O3 thin films. As a result, we obtained a water vapor transmission rate of 1.58 × 10−5 g/(m2·day) using a single layer of Al2O3 thin film at a thickness of 100 nm. In the NBAS, the energetic neutral particle bombardment of the thin film occurs during magnetron sputtering. The high-density Al2O3 gas barrier films formed by the NBAS show applicability to organic microelectronic devices, including organic light-emitting diodes.

The distribution of H2O, CH3OH, and hydrocarbon-ices on Pluto: Analysis of New Horizons spectral images

On July 14, 2015, the New Horizons spacecraft made its closest approach to Pluto at about 12,000 km from its surface (Stern et al., 2015). Using the LEISA (Linear Etalon Imaging Spectral Array) near-IR imaging spectrometer we obtained two scans across the encounter hemisphere of Pluto at 6–7 km/pixel resolution. By correlating each spectrum with a crystalline H2O-ice model, we find several sites on Pluto’s surface that exhibit the 1.5, 1.65 and 2.0 µm absorption bands characteristic of H2O-ice in the crystalline phase. These sites tend to be isolated and small ( ≲ 5000 km2 per site). We note a distinct near-IR blue slope over the LEISA wavelength range and asymmetries in the shape of the 2.0 µm H2O-ice band in spectra with weak CH4-ice bands and strong H2O-ice bands. These characteristics are indicative of fine-grain (grain diameters < wavelength or ∼ 1 µm) H2O-ice, like that seen in the spectra of Saturnian rings and satellites. However, the best-fit Hapke models require small mass fractions (≲10−3) of fine-grained H2O-ice that we can exchange for other refractory materials in the models with little change in χ2, which may mean that the observed blue slope is possibly not due to a fine-grained material but an unidentified material with a similar spectral characteristic. We use these spectra to test for the presence of amorphous H2O-ice and estimate crystalline-to-amorphous H2O-ice fractions between 30 and 100%, depending on the location. We also see evidence for heavy hydrocarbons via strong absorption at λ > 2.3 µm. Such heavy hydrocarbons are much less volatile than N2, CH4, and CO at Pluto temperatures. We test for CH3OH, C2H6, C2H4, and C3H8-ices because they have known optical constants and these ices are likely to arise from UV and energetic particle bombardment of the N2, CH4, CO-rich surface and atmosphere. Finally, we attempt to estimate the surface temperature using optical constants of pure CH4, and H2O-ice and best-fit Hapke models. Our standard model gives temperature estimates between 40 and 90 K, while our models including amorphous H2O-ice give lower temperature estimates between 30 and 65 K.

Atomistic Structure of Low-Resistance Si/GaAs Heterointerfaces Fabricated By Surface-Activated Bonding at Room Temperature

Tandem solar cells consisting of silicon (Si) and III-V compounds are one of the promising candidates for next-generation terrestrial photovoltaic systems, that can surpass the efficiency milestone of 30% for non-concentrating solar cells without using expensive Ge or GaAs substrates. Recently, the surface-activated bonding (SAB) at room temperature (RT), in which surfaces of substrates are activated before bonding by creating dangling bonds via the removal of contaminants under an energetic particle bombardment in high vacuum, is applied to form Si/GaAs heterointerfaces for hybrid triple-junction cells with a high conversion efficiency above 26% [1]. Even though the interface resistance (~100 Wcm2) is low enough for solar cells, it is still higher than the ideal one at defect-free heterointerfaces (~10-4 Wcm2). Accordingly, a comprehensive knowledge of the electrical property at the heterointerfaces depending on their atomistic structure is indispensable to establish the fabrication processes of high-efficiency tandem cells by optimizing the interface structure. In the present work, p-Si/n-GaAs heterointerfaces were fabricated at RT under a SAB condition [1], with the substrates of B-doped (100) p-Si (with a carrier concentration of 2x1014 cm-3) and Si-doped (100) n-GaAs (2x1016 cm-3) which was 5o off from (100) toward [011]. A part of them were then annealed at 673 K for 1 min. Their structural properties were determined by plane-view transmission electron microscopy (TEM) (Fig. 1), as well as cross-sectional scanning TEM (STEM). As-bonded heterointerfaces included an As-deficient crystalline GaAs layer less than about 1 nm thick and an amorphous Si layer about 3 nm thick [2]. Dimples were introduced on the whole GaAs surface at the bonding heterointerface (Fig. 1d), presumably via the introduction of As vacancies during the surface activation process, and they disappeared after 673 K annealing. The density in the amorphous Si layer was slightly lower than in the conventional amorphous Si, presumably due to the introduction of Si vacancies during the surface activation process, and it was increased by 673 K annealing. Those structural changes would result in the reduction of the resistance at the bonded Si/GaAs heterointerfaces [3]. On the other hand, dislocations in GaAs cropping out the heterointerfaces induce large strains at the interfaces (indicated with the arrows in Figs. 1a and 1b), and they would increase the interface resistance even after annealing. Therefore, the resistance should be reduced by suppressing those interface defects, via the optimization of SAB conditions with a defect-free GaAs substrate. In order to confirm the hypothesis, the electronic property of the defects was examined by cathodoluminescence (CL) spectroscopy in a transmission electron microscope. The relationship of the defects and the interface resistance will be discussed in the conference.

(Invited) Surface Activated Bonding Method for Low Temperature Bonding

The surface activated bonding (SAB) method for low temperature bonding their development for heterogeneous and 3D integration is reviewed. The standard SAB method is based on surface bombardment by Ar beam in ultra-high vacuum to clean the surfaces so that they can be bonded very strongly at room temperature without heat treatment. Modifications of the surface activation have been investigated to extend the standard SAB method for various materials and applications. The conventional bonding methods used for heterogeneous and 3D integration, such as anodic bonding, glass frit bonding, soldering, metal thermal-compression (diffusion) bonding, ultrasonic bonding, adhesive bonding, in general require high-temperature heating or annealing at 250-400 °C or even 700-1000 °C to achieve high bonding strength, which may induce great thermal stress, degrade the device reliability, and decrease manufacturing yield, especially in heterogeneous integration of dissimilar materials. Therefore, low-temperature bonding and interconnection technologies are highly desirable especially for heterogeneous integration for 2.5D or 3D applications. Although much effort has been made in recent years to attain a low temperature annealing process, most of the bonding methods including plasma activated bonding do not provide sufficient bond strength at below 200 °C or room temperature for critical applications. The question was if such high-temperature reaction and diffusion are inevitable for bond formation. The most solid materials have high surface energy and cohesive energy. It concludes that a stable bonding can be formed just by contact at room temperature. However, the real surface is covered by oxide layer or certain contamination. Such layers, therefore, should be removed prior to the bonding. The term of the Surface Activated Bonding (SAB) was used for the first time in the article of F. S. Ohuchi and T. Suga (1994) [1], to describe the bonding process which "utilizes the fundamental nature of atomically clean surfaces created by energetic particle bombardment leading to bond formation when two surfaces are brought into contact." Effectiveness of the surface activation for room temperature bonding had been demonstrated since the middle of '80 by several research groups in Japan. The bonding process in UHV for Al-Al and Al-Si3N4 was described in 1992 [2], and Cu-Cu for micro-bonding in 1993. The SAB method was applied successfully to the direct wafer boding of Si-Si in 1996 [3] and then expanded to heterogeneous bonding between semiconductors and metals at room temperature. The SAB method has attracted increasing interest due to its simple process flow, no need for additional intermediate materials for bonding, and compatibility with CMOS technology. However, it has failed to bond some dielectric materials, such as glass, sapphire, and silicon oxide. A modified SAB method using Si nano-intermediate layer has been developed to realize bonding of dielectrics at room temperature. novel surface activation processes have also been proposed for Cu/dielectric hybrid bonding, which is a promising approach to high-density 3D interconnects. This paper reviews the SAB methods used for heterogeneous and 3D integration with new approaches by expanding the versatility of the method. The standard SAB method uses Ar beam bombardment to remove surface adsorption and oxidation layer to realize bonding between semiconductors when two surfaces are brought into contact. It has been studied for bonding of Si-Si, Ge-Ge, and compound semiconductors such as GaAs-Si. It has been successfully applied also to room-temperature wafer-level and chip-level Cu-Cu direct bonding [4]. The standard SAB, however, failed to bond some dielectric materials, such as glass and silicon oxide. Therefore, the modified SAB [5] was developed to solve this problem, by using a Si intermediate layer deposited on the activated surfaces. The modified SAB is now applied to bond not only SiO2 glasses but also polymer films such as PEN and Polyimide, as well as WBG semiconductor wafers with a wide perspective of the applicability on flexible electronics and power electronics. [1] F. S. Ohuchi and T. Suga, Electronic structure of metal/ceramic interfaces fabricated by Surface Activated Bonding, Advanced Materials 93, Trans. Mat. Res. Soc. Jpn., Vol. 16B (1994) 1195-1199. [2] T. Suga, et al., Structure of Al-Al and Al-Si3N4 interfaces bonded at room temperature by means of the surface activation method, Acta metall. mater., 40, Suppl. (1992) S133-S137 [3] H. Takagi, et al., Surface activated bonding of silicon wafers at room temperature, Appl. Phys. Lett. 68 (1996) 2222. [4] A. Shigetou, et al., Bumpless Interconnect of 6-μm-Pitch Cu Electrodes at Room Temperature, IEEE Trans. Adv. Packag., 31, 3 (2008) 473-478. [5] R. Kondou, et al., Room temperature SiO2 wafer bonding by adhesion layer method, IEEE 61st ECTC (2011) 2165-2170.

Surface Activated Bonding -from the Standard SAB to Modified SAB

The surface activated bonding (SAB) was proposedin thelate 1980’s for bonding of metal to metal and to ceramics at room temperature. The standard SAB method is based on surface activation process by bombardment of the surfaces by Ar ions or neutral atoms in ultra-high vacuum to clean and activate them so that they can be bonded spontaneously only by contact without heat treatment.The term of the SAB was used for the first time in the article of F. S. Ohuchi and T. Suga (1994) [1],to describe the bonding process which “utilizes the fundamental nature of atomically clean surfaces created by energetic particle bombardment leading to bond formation when two surfaces are brought into contact.” The standard SAB process for Al-Al and Al-Si3N4 was described in 1992[2],and Cu-Cu for micro-bonding in 1993[3]. The standard SAB method was applied successfully to the direct wafer boding of Si-Si in 1996[4]and then expanded to heterogeneous bonding between semiconductors and metals at room temperature. The SAB method has attracted increasing interest due to its simple process flow, no need for additional intermediate materials for bonding, and compatibility with CMOS technology. Simulations of the electronic structure of direct bonded interfaces reveal that certain electron transfer may occur across the bonded interface depending on the nature of the activated surfaces, resulting into covalent, metallic, or ionic bonding at the interfaces. The surface activation process of the ion bombardment induces various effects on the activated surfaces. For metals, the native oxide is removed and point defects are induced with nano-grain formation observed in some cases. For semiconductors, dangling bonds are formed at the surface which however sometimes contains certain electronic defects and becomes amorphous structure. For ionic materials such as oxides, carbides, and nitrides, certain preferential sputtering effect is observed: for instance, SiC surface after the surface activation shows a Si depletion layer. Those effects could contribute the strong bonding formation at the bonded interface, or sometime cause some restriction in the utilization of the bonded interface. The standard SAB method has failed in bonding SiO2, carbon materials and organic polymers because SiO2 and carbon materials may hardly form dangling bonds, the later becomes amorphous, and polymer materials are carbonized and inert to bond. To overcome the limit of the standard SAB, modified SAB method has been developed in which metal nano-intermediate layer is inserted into the bonded interface so that the electronic polarization is sealed by the metallic layer and the week adhesion is enhanced by boding to the metal layer. By the modified SAB, bonding of glass, SiC, GaN, diamond, and polymer films as well as their combinations have been demonstrated. The conventional wafer bonding technique which is called as fusion bonding, oxide bonding, or hydrophilic bonding, requires a reaction with water and OH- group formation as a prior condition and belongs to a different category of the SAB. However, its combination with the modified SAB provides also a new possibility for low temperature bonding.

Microstructural, mechanical, and corrosion properties of plasma-nitrided CoCrFeMnNi high-entropy alloys

Plasma nitriding is used for materials that are challenging to nitride by conventional techniques. Because of the sputtering effect of positive ions during glow discharge, protective oxide films on the surfaces of materials including stainless steels, aluminum alloys, and titanium alloys, can be effectively removed by ion and energetic particle bombardment; thus, nitrogen mass transfer from the plasma into the component subsurface can be achieved effectively. Plasma nitriding therefore has potential applicability to treat high-entropy alloys (HEAs) containing large amounts of strong oxide-forming elements such as chromium. In this study, the effects of plasma nitriding were investigated on the microstructural, mechanical, and corrosion properties of a CoCrFeMnNi HEA with an fcc structure that is soft and ductile. The HEA was produced by melting pure metals and casting. Direct-current plasma nitriding was then performed in a gas mixture of 25% N2 and 75% H2 for 54 ks at 673–823 K under 200 Pa with an auxiliary cathodic screen. After nitriding, the nitrided samples were examined using scanning electron microscopy, X-ray diffraction (XRD), Vickers microhardness testing, electron probe microanalysis, and glow discharge optical emission spectroscopy (GD-OES). After nitriding, GD-OES revealed that the thickness of the nitriding layer tended to increase with increasing nitriding temperature. XRD analysis revealed that an fcc supersaturated solid solution phase was formed on surfaces nitrided below 723 K, whereas a CrN phase was formed on those nitrided above 723 K. The Vickers microhardness of the nitrided sample surfaces reached approximately 1300 HV. Ball-on-disk wear tests revealed that the wear loss of nitrided samples was considerably lower than that for untreated samples. Finally, the pitting corrosion resistance of samples nitrided at 673 K was higher than that of untreated samples.

Development of a Charge-Implicit ReaxFF Potential for Hydrocarbon Systems.

Molecular dynamics (MD) simulations continue to make important contributions to understanding chemical and physical processes. Concomitant with the growth of MD simulations is the need to have interaction potentials that both represent the chemistry of the system and are computationally efficient. We propose a modification to the ReaxFF potential for carbon and hydrogen that eliminates the time-consuming charge equilibration, eliminates the acknowledged flaws of the electronegativity equalization method, includes an expanded training set for condensed phases, has a repulsive wall for simulations of energetic particle bombardment, and is compatible with the LAMMPS code. This charge-implicit ReaxFF potential is five times faster than the conventional ReaxFF potential for a simulation of keV particle bombardment with a sample size of over 800 000 atoms.

Benefits of energetic ion bombardment for tailoring stress and microstructural evolution during growth of Cu thin films

We have studied the development of intrinsic stress and microstructure of copper (Cu) films deposited under energetic ion bombardment. Stress evolution during growth of ∼150 nm thick Cu films on Si(001) substrates has been investigated by in situ measurements in a high power impulse magnetron sputtering (HiPIMS) process for different substrate bias voltages (from 0 to −160 V) and benchmarked with conventional direct current magnetron sputtering (DCMS). The microstructure and crystal orientation of the studied films have been examined by various ex situ methods. For HiPIMS, we found that the substrate bias voltage (energy of Cu ions) strongly affects the film continuity during the early growth stages and the compressive stress developed during the post-coalescence stage. Contrarily to common expectations, the stress magnitude can be significantly reduced despite the energy increase of the bombarding particles when using HiPIMS. These results are discussed based on a recent kinetic model taking into account the grain size-dependent defect incorporation due to energetic particle bombardment. Reversible stress relaxations are observed upon growth interrupts, with characteristic time constants of tens of seconds, which suggests that the stress and microstructure development are mainly mediated by surface diffusion processes. In addition, polycrystalline films (111)-textured are obtained for negative bias voltages from 0 to −100 V, while for even higher negative bias voltages (up to −160 V), epitaxial growth of Cu(001) is achieved. For the DCMS samples, there is no significant change in film continuity and crystal orientation when varying the bias voltage.