Nanoscale metal-silicide films prepared by surfactant sputtering and analyzed by RBS

Surfactant sputtering has been applied to modify the surface structure of Si substrates and to produce ultrathin metal-silicide films with nickel and platinum surfactants, utilizing the steady state coverage of a Si-substrate surface with surfactant atoms simultaneously during sputter erosion by combined ion irradiation and surfactant atom deposition. Si (1 0 0) substrates were eroded using 5 keV Xe-ions and 10–30 keV Ar ions at incident angles of 65° and 70° with fluences of up to 2 × 10¹⁸/cm² under continuous sputter deposition of platinum and nickel from targets irradiated simultaneously by the same ion beam. These surfactant atoms form metal-silicides in the surface near region and strongly modify the substrate sputter yield and the surface nanostructure. Atomic force microscopy and scanning electron microscopy were carried out to observe a transition of surface topography from ripple to relief patterns, granular patterns or smooth surfaces. The Si and metal sputter yield as function of the steady state metal coverage were determined by combination of Rutherford-backscattering spectroscopy (RBS) and profilometry. The composition and the depth distributions of metal-silicide films were analyzed via high-resolution RBS and transmission electron microscopy. We show that RBS results in comparison with SRIM and TRIDYN sputter yield simulations allows us to identify the silicide surface structure on the nanometer scale.     

Ion beam mixing in uranium nitride thin films studied by Rutherford Backscattering Spectroscopy

Thickness, composition, concentration depth profile and ion irradiation effects on uranium nitride thin films deposited on fused silica have been investigated by Rutherford Backscattering Spectroscopy (RBS) using 2 MeV He⁺ ions. The films were prepared by reactive DC sputtering at the temperatures of ?200 °C, +25 °C and +300 °C. A perfect 1U:1N stoichiometry with a layer thickness of 660 nm was found for the film deposited at ?200 °C. An increase of the deposition temperature led to an enhancement of surface oxidation and an increase of the thickness of the mixed U–N–Si–O layers at the interface. The sample irradiation by 1 MeV Ar⁺ ion beam with ion fluence of about 1.2–1.7 × 10¹⁶ ions/cm² caused a large change in the layer composition and a large increase of the total film thickness for the films deposited at ?200 °C and at +25 °C, but almost no change in the film thickness was detected for the film deposited at +300 °C. An enhanced mixing effect for this film was obtained after further irradiation with ion fluence of 2.3 × 10¹⁶ ions/cm².     

Nanostructured carbide surfaces prepared by surfactant sputtering

Nanostructured surface layers of titanium carbide and tungsten carbide were prepared on tetrahedral amorphous carbon (ta-C) films using the surfactant sputtering technique. Surfactant sputtering is a novel ion beam erosion technique, which utilizes the steady state coverage of a substrate surface with foreign atoms simultaneously during sputter erosion by combined ion irradiation and atom deposition. These foreign atoms act as surfactants, which strongly modify the substrate sputtering yield on atomic to macroscopic length scales. The novel technique allows smoothing of surfaces, the generation of novel surface patterns and nanostructures, controlled shaping of surfaces on the nanometer scale and the formation of ultra-thin compound surface layers. We have sputter eroded ta-C films using 5 keV Xe ions under continuous deposition of either tungsten or titanium surfactants. This leads to the steady state formation of a W_xC or a TiC/a-C nanocomposite surface layer of few nm thickness. Depending on the ion angle of incidence the layer is either smooth or nanostructured with a ripple- or dot-like surface topography. We have analyzed the surface topography, the composition and microstructure of the metal-carbon nanocomposites, and compare coverage dependent sputtering yields with SRIM and TRIDYN simulations.     

Erosion and re-deposition processes in JET tiles studied with ion beams

Plasma facing components in fusion reactor chambers will operate under extreme conditions. Among the processes with implications on the material lifetime are erosion and re-deposition due to plasma interactions.This work will address the behaviour of both JET divertor and outer poloidal limiters (OPL) under plasma irradiation. The limiters comprise about 50 pairs of tiles in a poloidal stack, each of which has a plasma facing surface about 25 mm (poloidal) by 350 mm (toroidal) and is about 50 mm thick. The divertor tiles are located at the bottom of the chamber and withstand high fluxes of radiation and heat. Standard carbon-fibre composite (CFC) tiles coated with a thin layer of W overlaid with a 10 μm layer of C were studied with RBS/PIXE to understand the erosion/re-deposition processes occurring in these regions of the reactor chamber. High resolution surface morphology was assessed through SEM with and without tilting of the sample. The retention of hydrogen isotopes in the tiles were studied combining NRA and ERDA techniques – this is mostly ²H from the fuelling gas, but ³H is also present as a result of ²H–²H fusion reactions, and ¹H coming from the atmospheric exposure.     

Damage accumulation and annealing in 6H–SiC irradiated with Si+

Damage accumulation and annealing in 6H-silicon carbide (α-SiC) single crystals have been studied in situ using 2.0 MeV He⁺ RBS in a 〈0 0 0 1〉-axial channeling geometry (RBS/C). The damage was induced by 550 keV Si⁺ ion implantation (30° off normal) at a temperature of −110°C, and the damage recovery was investigated by subsequent isochronal annealing (20 min) over the temperature range from −110°C to 900°C. At ion fluences below 7.5 × 10¹³ Si⁺/cm² (0.04 dpa in the damage peak), only point defects appear to be created. Furthermore, the defects on the Si sublattice can be completely recovered by thermal annealing at room temperature (RT), and recovery of defects on the C sublattice is suggested. At higher fluences, amorphization occurs; however, partial damage recovery at RT is still observed, even at a fluence of 6.6 × 10¹⁴ Si⁺/cm² (0.35 dpa in the damage peak) where a buried amorphous layer is produced. At an ion fluence of 6.0 × 10¹⁵ Si⁺/cm² (−90°C), an amorphous layer is created from the surface to a depth of 0.6 μm. Because of recovery processes at the buried crystalline–amorphous interface, the apparent thickness of this amorphous layer decreases slightly (<10%) with increasing temperature over the range from −90°C to 600°C.     

Investigations of the origins of deposited carbon species in gaps between divertor tiles using a kinetic code system

In order to investigate the origins of deposited carbon species in gaps, the simulations have been performed using a kinetic code system. At low plasma temperatures, the deposited carbon species mainly originate from top surfaces of the tile, while at high plasma temperatures the deposited carbon species are basically derived from side surfaces of the tile. A substantial variation of the deposition rate of carbon species originating from side surfaces is obtained due to physical sputtering and topography advantage. The deposited carbon species derived from top surfaces and side surfaces demonstrate different deposition characteristics for physical sputtering and chemical erosion: (i) for deposited carbon species from top surfaces, the deposition ratio for physical sputtering increases evidently and deposition rate virtually increases by one order of magnitude with increasing plasma temperature; and the deposition ratio for chemical erosion reduces correspondingly and deposition rate decreases gradually; (ii) for deposited carbon species from side surfaces, the deposited carbon species principally arise from physical sputtering; the deposition rates for chemical erosion are of the order of magnitude of 10¹⁵ m^?2 s^?1 for all studied plasma temperatures, and the deposition rates for physical sputtering can be two to three orders of magnitude greater than that for chemical erosion.     

Erosion behavior of soft,amorphous deuterated carbon films by heat treatment in air and under vacuum

The erosion of soft a-C:D films by heat treatment in air and under vacuum is studied by ion-beam analysis. When the films are heated in air above 500 K, the film thickness and the areal densities of C and especially D decrease, and oxygen is incorporated in the films. The initial atomic loss rates of carbon and deuterium from the films are 2.6 × 10¹⁷ C atoms cm⁻² h⁻¹ and 4.8 × 10¹⁷ D atoms cm⁻² h⁻¹ at 550 K. However, after D depletion the films show a resistivity against further erosion due to annealing in air. When the films are heated under vacuum erosion starts at about 600 K and all components including D decrease proportionally to the film thickness. Thermal desorption spectroscopy of the films reveals the evolution of C_xD_y type hydrocarbons. Infrared analysis shows that the incorporated oxygen is chemically bonded to carbon. The thermally-activated decomposition of the soft a-C:D films is compared to that of hard a-C:D films and a reaction scheme is suggested.     

Soft X-ray fluorescence measurements of irradiated polymer films

Fluorescent soft X-ray carbon Kα emission spectra (XES) have been used to characterize the bonding of carbon atoms in polyimide (PI) and polycarbosilane (PCS) films. The PI films have been irradiated with 40 keV nitrogen or argon ions, at fluences ranging from 1 × 10¹⁴ to 1 × 10¹⁶ cm⁻². The PCS films have been irradiated with 5 × 10¹⁵ carbon ions cm⁻² of 500 keV and/or annealed at 1000°C. We find that the fine structure of the carbon XES of the PI films changes with implanted ion fluence above 1 × 10¹⁴ cm⁻² which we believe is due to the degradation of the PI into amorphous C:N:O. The width of the forbidden band as determined from the high-energy cut-off of the C Kα X-ray excitation decreases with the ion fluence. The bonding configuration of free carbon precipitates embedded in amorphous SiC which are formed in PCS after irradiation with C ions or combined treatments (irradiation and subsequent annealing) is close to either to that in diamond-like films or in silicidated graphite, respectively.     

Laser damage thresholds of ITER mirror materials and first results on in situ laser cleaning of stainless steel mirrors

A laser ablation system has been constructed and used to determine the damage threshold of stainless steel, rhodium and single-, poly- and nanocrystalline molybdenum in vacuum, at a number of wavelengths between 220 nm and 1064 nm using 5 ns pulses. All materials show an increase of the damage threshold with decreasing wavelength below 400 nm. Tests in a nitrogen atmosphere showed a decrease of the damage threshold by a factor of 2–3. Cleaning tests have been performed in vacuum on stainless steel samples after applying mixed Al/W/C/D coatings using magnetron sputtering. In situ XPS analysis during the cleaning process as well ex situ reflectivity measurements demonstrate near complete removal of the coating and a substantial recovery of the reflectivity. The first results also show that the reflectivity obtained through cleaning at 532 nm may be further increased by additional exposure to UV light, in this case 230 nm, an effect which is attributed to the removal of tungsten dust from the surface.     

RBS study of diffusion under strong centrifugal force in bimetallic Au/Cu thin films

The effect of the strong centrifugal force, mega-gravity (MG) on inter-diffusion between Au and Cu thin films was studied by using Au(60 nm)/Cu(500 nm)/α-Al₂O₃ (0 0 0 1) films. The Rutherford backscattering analysis of the Au and Cu depth profiles shows that Cu atoms diffuse through the Au layer, resulting in the formation of the Cu layer on the surface under both the thermal annealing at 220 °C and the application of 0.61 × 10⁶ G at 220 °C. The results indicate that the MG application enhances the layer thickness of the Cu layer on the surface.     

Carbon film growth and hydrogenic retention of tungsten exposed to carbon-seeded high density deuterium plasmas

Tungsten (W) targets have been exposed to high density (n_e ? 4 × 10¹⁹ m^?3), low temperature (T_e ? 3 eV) CH₄-seeded deuterium (D) plasma in Pilot-PSI. The surface temperature of the target was ～1220 K at the center and decreased radially to ～650 K at the edges. Carbon film growth was found to only occur in regions where there was a clear CII emission line, corresponding to regions in the plasma with T_e ? 2 eV. The maximum film thickness was ～2.1 μm after a plasma exposure time of 120 s. ³He nuclear reaction (NRA) analysis and thermal desorption spectroscopy (TDS) determine that the presence of a thin carbon film dominates the hydrogenic retention properties of the W substrate. Thermal desorption spectroscopy analysis shows retention increasing roughly linearly with incident plasma fluence. NRA measures a C/D ratio of ～0.002 in these films deposited at high surface temperatures.     

Nanohardness and structure of nitrogen implanted SixAly coatings post-implanted with oxygen

“Sialons” have several interesting mechanical, thermal and chemical properties which make them candidates for high temperature applications. Solid solutions in the Si–Al–O–N system were synthesised using nitrogen and oxygen implantation into Si₃₃Al₆₇, Si₄₅Al₅₅ and Si₆₇Al₃₃ thin films deposited by DC magnetron sputtering on glassy carbon substrates. Nitrogen has been implanted firstly at 50 and then at 20 keV, oxygen was post-implanted at 50 keV. The different implantation doses ranged from 1 to 10 × 10¹⁷ ions/cm². High depth resolution profiles were obtained using RBS and resonant nuclear reaction, the chemical bonds were investigated using LEEIXS and these results are correlated to the film structure measured by cross section TEM. The TEM micrographs show a columnar structure perpendicular to the substrate surface in unimplanted coatings. Nevertheless, when nitrogen is implanted grain formation is observed and after oxygen post-implantation gas bubbles appear at the film depth where the maximum oxygen concentration is observed. The correlation of these results with RBS and LEEIXS measurements indicates that nitrogen should be enclosed in these bubbles. Nanohardness was also measured. The highest values are observed in samples post-implanted with 1 × 10¹⁷ O/cm² where nanohardness increases from 3 to 10 Gpa.     

Development of CrN precipitates during the initial stages of PIII nitriding of stainless steel thin films

Stainless steel thin films produced by ion beam sputtering (IBS) were used as a model system to investigate the nitrogen diffusion and CrN formation after 10 min of nitrogen plasma immersion ion implantation (PIII) at 350 °C and 450 °C. At 350 °C, a thin nitrided layer of 70 nm is formed, with additional diffusion of nitrogen along grain boundaries and the growth of CrN precipitates along these grain boundaries. For 450 °C, a complete nitriding of the whole 400 nm thick layer was observed, with the lower 75 nm consisting of an expanded phase and the upper 330 nm of a decomposed phase with CrN precipitates formed inside the original grains. Such a layered structure capturing the transformation process has not been observed before. A determination of time–temperature dependencies of this process and the transfer of these results for bulk material should be possible.     

Enhancement of hydrogen isotope retention capacity for the impurity deposited tungsten by long-term plasma exposure in LHD

The stress relieved tungsten samples were placed at three positions, PI (sputtering erosion dominated area), DP (deposition dominated area) and HL (Higher heat load area) during 15th plasma experiment campaign in Large Helical Device (LHD) at National Institute for Fusion Science (NIFS), Japan and were exposed to ～ 6700 shots of hydrogen plasma in a 15th long-term experiment campaign in LHD. Thereafter, the additional deuterium ion implantation to these tungsten samples was performed to evaluate the change of hydrogen isotope retention capacity in the samples by long-term plasma exposure. It was found that the carbon-dominant mixed-material layer with more than 100 nm thickness was formed on a wide area of the tungsten surface. The thicker mixed-material layer was formed on the DP sample, where the deuterium retention was about 21 times as high as that for pure W. The major desorption temperature of deuterium was shifted toward higher temperature side, which was comparable to the trapping characteristic of carbon or irradiation damages.     

Analysis of surface morphological change in PET films induced by tungsten ion implantation

Surface morphological changes and metal nanograin formation of polyethylene terephthalate films with tungsten ion implantation were studied. Tungsten ions were accelerated with a voltage of 40 kV and implanted at fluences from 5 × 10¹⁶ to 2 × 10¹⁷ cm^?2 using a metal vapor vacuum arc implanter. Scanning electron micrographs at the highest fluence show semi-spherical hills, indicating formation of tungsten nanograins on the polymer. The tungsten nanograin formation in the polymer film is confirmed by cross-sectional observation using transmission electron microscopy. Depth profiles of tungsten atoms obtained from energy dispersive X-ray spectra indicate densification and sputtering of the polymer surface layer during implantation. These results indicate that surface morphological change is related with the effects of a critical fluence and tungsten nanograin formation.     

Characterisation of Ni+ implanted PEEK,PET and PI

Polyimide (PI), polyetheretherketone (PEEK) and polyethyleneterephthalate (PET) were implanted with 40 keV Ni⁺ ions at room temperature at fluences ranging from 1.0 × 10¹⁶ to 1.5 × 10¹⁷ ions cm^?2 and with ion current density varying between 4 and 10 μA cm^?2. The depth profiles of the implanted Ni atoms determined by the RBS technique were compared with those predicted by the SRIM and TRIDYN codes. Hydrogen depletion as a function of the ion fluence was determined by the ERDA technique, and the compositional and structural changes of the polymers were characterised by the UV–vis and XPS methods. The implanted profiles differed significantly from those predicted by the SRIM code while the lower fluences were satisfactorily described by the TRIDYN simulation. A significant hydrogen release from the polymer surface layer was observed along with significant changes in the surface layer composition. The UV–vis results indicated an increase in the concentration and conjugation of double bonds.     

Micro-ERDA,micro-RBS and micro-PIXE techniques in the investigation of fish otoliths

Elemental distribution in the otolith of the fresh water fish burbot (Lota lota L.) collected in Hungary was measured with Elastic Recoil Detection Analysis (ERDA), Rutherford Backscattering Spectrometry (RBS) and as a complementary technique, Particle-Induced X-ray Emission (PIXE) with a focussed ion beam of 1.5 × 1.5 μm² spot size. The organic- and inorganic-rich regions of the otolith are distinguished and they are presented as hydrogen and calcium maps at depth regions of 0–70, 70–140 and 140–210 nm. The textured surface of the sample and its porosity were characterized from the effect on the RBS spectra. The oxygen and carbon PIXE elemental maps can also be used to identify the organic- and inorganic-rich regions of the otolith. The calcium map was found to be more homogeneous because the otolith structure is averaged in a larger depth. The trace elements Fe, Zn and Sr were detected only in very low concentration by micro-PIXE.     

Evolution of Zr/Hf/Zr trilayers during annealing studied by RBS

The Zr/Hf system is highly interesting due its various applications, e.g. formation of amorphous ternary alloys, superconductive properties and production of gate oxide layers with high dielectric coefficients by oxidation of Zr/Hf multilayers. In this work Zr/Hf/Zr trilayers with an individual layer thickness of approximately 50 nm were deposited by electron gun evaporation on a substrate consisting of silicon covered by a micrometer thick thermal oxide layer. Samples were subjected to annealing procedures at 500 and 1200 °C in flowing air atmosphere to promote oxidation and Zr/Hf interdiffusion effects.RBS studies of the as-deposited and annealed samples were performed at the van-de-Graaff accelerator of ITN using He⁺ and H⁺ beams with energies between 2.0 and 2.525 MeV in order to study compositional changes induced by the heat treatment. In the case of low-temperature annealing the layer system appears, besides the oxidation process starting from the surface, to be stable. On the other hand, high-temperature annealing leads to an asymmetric Hf-diffusion into the surface and interior Zr-layer provoked by anomalous diffusion due to a phase transition in Zr accompanied by an almost complete oxidation of the layer structure Oxygen and metal depth distributions obtained by RBS in the as-deposited and treated samples are provided.     

Homogeneously size distributed Ge nanoclusters embedded in SiO2 layers produced by ion beam synthesis

500 nm SiO₂ layers were implanted with 450 keV (F=3 × 10¹⁶ at./cm²) and 230 keV (F=1.8 × 10¹⁶ at./cm²) Ge ions at room temperature to obtain an almost constant Ge concentration of about 2.5 at.% in the insulating layer. Subsequently, the specimens were annealed at temperatures between 500°C and 1200°C for 30 min in a dry N₂ ambient atmosphere. Cross-sectional TEM analysis reveal homogeneously distributed Ge nanoclusters arranged in a broad band within the SiO₂ layer. Their mean cluster size varies between 2.0 and 6.5 nm depending on the annealing conditions. Cluster-free regions are always observed close to the surface of the specimens independent of the annealing process, whereas a narrow Ge nanocluster band appears at the SiO₂/Si interface at high annealing temperatures, e.g. ⩾1000°C. The atomic Ge redistribution due to the annealing treatment was investigated with a scanning TEM energy dispersive X-ray system and Rutherford back scattering (RBS).     

Argon plasma irradiation of polypropylene

Polypropylene samples were exposed to argon plasma discharge and the changes of the PP surface properties were studied by different methods. Surface wettability was derived from contact angle measured by standard goniometry and chemical structure of the plasma modified PP was studied using X-ray photoelectron spectroscopy (XPS) and by Rutherford backscattering spectroscopy (RBS), surface morphology and roughness of samples using AFM. Zeta potential of pristine and modified PP was determined with the SurPASS. The presence of incorporated oxygen in the PP surface layer, about 60 nm thick, was observed in RBS spectra. Oxygen concentration is a decreasing function of the depth. With progressing aging time the oxygen concentration on the PP surface decreases. Plasma treatment results in a rapid decrease of the contact angle, which increases again with increasing aging time. In XPS measurement the oxygen containing structures, created by the plasma treatment, were found on the very surface of the modified PP and the zeta potential being changed too. The significant difference in zeta potential between pristine and plasma treated PP clearly indicates that the plasma treatment leads to a more hydrophilic PP surface.