Abstract: The retention of 1 keV D+ ions implanted into clean and oxidized single crystalline Be at room and elevated temperatures is investigated by a combination of in situ analytical techniques including temperature programmed desorption (TPD), nuclear reaction analysis, low-energy ion spectroscopy (LEIS) and x-ray photoelectron spectroscopy. For the first time, the whole temperature regime for deuterium release and the influence of thin oxide films on the release processes are clarified. The cleaned and annealed Be sample has residual oxygen concentration equivalent to 0.2 monolayer (ML) BeO in the near-surface region as the only contamination. LEIS shows that Be from the volume covers thin BeO surface layers above an annealing temperature of 1000 K by segregation, forming a pure Be-terminated surface, which is stable at lower temperatures until again oxidized by residual gas. No deuterium is retained in the sample above 950 K. By analyzing TPD spectra, active retention mechanisms and six energetically different binding states are identified. Activation energies (EA) for the release of D from binding states in Be are obtained by modelling the experimental data. Two ion-induced trap sites with release temperatures between 770 and 840 K (EA= 1.88 and 2.05 eV, respectively) and two trap sites (release between 440 and 470 K) due to supersaturation of the bulk above the steady state fluence of 2x1017 cm-2 are identified. None of the release steps shows a surface recombination limit. A thin BeO surface layer introduces an additional binding state with a release temperature of 680 K. Implantation at elevated temperatures (up to 530 K) changes the retention mechanism above the saturation limit and populates a binding state with a release temperature of 570 K.
Abstract: Depth profiles of deuterium trapped in tungsten exposed to a low-energy (approximate to 200 eV/D) and high deuterium ion flux (about 1 X 10(21) D/m(2)s) in clean (We use the term ’clean’ in quotation marks having in mind the impossibility to obtain absolutely clean plasma. In our case the conception ’clean’ D plasma means the plasma without intentionally introduced carbon impurities.) and carbon-seeded D plasmas at an ion fluence of about 2 x 10(24) D/m(2) and various temperatures have been measured up to a depth of 7 mu m using the D(He-3 p)(4) He nuclear reaction at a He-3 energy varied from 0.69 to 4.0 MeV. The deuterium retention in single-crystalline and polycrystalline W increases with the exposure temperature, reaching its maximum value at about 500 K (for ’clean’ plasma) or about 600 K (for carbon-seeded plasma), and then decreases as the temperature grows further. It is assumed that tungsten carbide formed on the W surface under exposure to the carbon-seeded D plasmas serves as a barrier layer for diffusion and prevents the outward transport of deuterium, thus increasing the D retention in the bulk of tungsten. (c) 2008 Published by Elsevier B.V.
Abstract: Beryllium, tungsten and carbon are planned as wall materials for the future international tokamak (ITER). Be and W will be the dominant components and therefore the formation of binary Be-W alloys under plasma action is one of the most important issues in plasma-wall interaction processes at the first wall. The formation of alloys and their reactivity under physical sputtering and chemical erosion constitute a new challenge for solid states physics and chemical reactivity. This article proposes a theoretical study of the first stages of the formation of these alloys based on the first principles DFT method. The tungsten adsorption energy on the basal (0001) beryllium surface is first calculated, and then the barrier to retention into the bulk. For each calculation the electronic structure of the formed compound is analyzed through density of states (DOS) calculation. The results are discussed with respect to experimental observations during the formation of a Be12W alloy layer and changes in the electronic structure of Be during alloying, observed in shifts of the Be 1s core levels.
Abstract: The intermetallic Be - W system is investigated by analysing both a W film on polycrystalline Be and the inverse system, Be films on polycrystalline W. The films are annealed up to 1070 K and the alloy formation is investigated by a combination of Rutherford backscattering spectroscopy (RBS) and x-ray photoelectron spectroscopy (XPS). For the structure analysis and identification of the formed alloys, x-ray diffraction (XRD) measurements are used. In the case of W films on Be, we identify Be12W within the diffusion depth, whereas Be films on W show alloy formation restricted to the film - substrate interface. Both XPS and XRD measurements indicate the formation of Be2W.
Abstract: Beryllium is planned as a plasma-facing material for ITER covering most of the inner wall of the plasma vessel. It is thus subjected to intense fluxes of escaping hydrogen ions from the plasma, which are implanted and retained in the plasma-facing material. As beryllium reacts very quickly with oxygen and especially with water, forming a surface oxide layer even under good vacuum conditions, previous studies of the interaction of deuterium and beryllium had to deal with oxygen contamination as a factor of uncertainty. To rule out the influence of a BeO- covered surface, the retention and release of 1 keV deuterium ions implanted in clean beryllium at room temperature are investigated by temperature programmed desorption (TPD). The surface composition is measured by x-ray photoelectron spectroscopy (XPS). The outline of a retention mechanism is developed by discussing the thermal release behaviour as a function of increasing deuterium fluence.
Abstract: Most present fusion devices use carbon at least partially as a first wall material. Due to high particle loads, especially hydrogen isotopes from the plasma, wall material is eroded and redeposited at different positions. This leads to the formation of surface layers, which are themselves subject to erosion. It is known that carbon exhibits chemically enhanced erosion by low energy hydrogen ions or elevated temperatures. However, room temperature erosion of carbon by keV ions is considered to be governed by a purely kinematic sputtering process. We found that this is not necessarily the case for carbon surface layers with thicknesses of a few nanometers. To investigate the basic mechanisms governing the erosion of carbon, carbon layers in the range of several nm thickness are evaporated on clean metal substrates. The films are irradiated with 1 or 1.3 keV deuterium ions and the surface layer thickness and composition are analysed by X-ray photoelectron spectroscopy (XPS). For our studies we choose carbon films on tantalum, titanium and beryllium to compare the influence of a wide range of substrate atomic masses. Our experimental data is compared to Monte Carlo calculations using the SDTRIM code which takes into account kinematic interactions and adjusts the sample composition dynamically. We discuss the results with respect to the kinematic collision interaction and the ion-induced chemical phase formations. (C) 2007 Elsevier B.V. All rights reserved.
Abstract: Both Be and W are planned as wall materials for ITER. Although these materials will dominate the ITER first wall, and despite the fact that their interaction during operation will be dominated by surface processes, mostly bulk material data on the Be-W binary system are available. This article describes investigations of thin films of Be deposited on W, the inverse system (W films on Be), as well as experiments where W surfaces are exposed to a Be-seeded deuterium plasma. The formed alloy phases Be2W and Be12W are identified by X-ray photoelectron spectroscopy (XPS) and depth profile data both from sputter-XPS and MeV ion beam analysis are presented. (c) 2007 Elsevier B.V. All rights reserved.
Abstract: Beryllium-seeded deuterium plasma is used in PISCES-B to investigate mixed-material erosion and redeposition properties of ITER relevant divertor materials. The beryllium containing plasma simulates the erosion of first wall material into the ITER sol plasma and its subsequent flow towards the carbon divertor plates. The experiments are designed to quantify the behaviour of plasma created mixed Be/C and Be/W surfaces. Developing an understanding of the mixed material surface behaviour is crucial to accurately predict the tritium accumulation rate within the ITER vacuum vessel. The temporal evolution of the plasma interactions with the various mixed surfaces are examined to better understand the fundamental mechanisms in play at the surface and to allow scaling of these results to the conditions expected in the ITER divertor. A new periodic heat pulse deposition system is also installed on PISCES-B to simulate the transient temperature excursions of surfaces expected to occur in the ITER divertor during edge localized modes (ELMs) and other off-normal events. These periodically applied heat pulses allow us to study the effects of transient power loading on the formation, stability and tritium content of mixed-material surfaces that are created during the experiments.
Abstract: Sputtering of tungsten by simultaneous incidence of gaseous and non- volatile ions is an important field of research for nuclear fusion with magnetically confined plasmas. In order to investigate the underlying processes in detail, W layers deposited on graphite and Si substrates have been irradiated simultaneously with beams of 12 keV C-2(-) and 9 keV D-3(+) ions. The dynamics of W sputtering as well as the accumulation of implanted C and D was studied in-situ by ion beam analysis (IBA) using 2.5 MeV He-3(+) ions. In this work, particularly the sputter yield of W and the implantation of C and D as a function of the C fraction in the incident flux is discussed. Comparison of experimental data to TRIDYN simulations reveal a strong contribution of surface roughness to W sputtering and C implantation. In comparison to the influence of roughness, the contribution of chemical effects appears negligible. (c) 2007 Elsevier B.V. All rights reserved.
Abstract: The bimetallic system Be - W is studied after room temperature deposition of Be films on W and annealing experiments up to 1070 K using x- ray photoelectron spectroscopy (XPS). Already at room temperature an intermixing at the interface occurs. The amount of intermetallic compound increases during the annealing experiments, but is limited to similar to 1.2 nm. The surface alloy formation is clearly visible as a binding energy (BE) shift of both core levels Be 1s and W 4f, and in the valence band (VB) region. The surface alloy is stable and the remaining layer thickness is independent of the initial Be layer thickness. Using a combination of sputter depth profiling before and after annealing and Monte Carlo simulation of the sputter process (TRIDYN) the depth scale of Be - W inter- diffusion is determined.
Abstract: The use of silver interconnects enables higher speed for silicon integrated circuits. The formation of Ag interconnects requires sequential deposition of a continuous barrier layer followed by silver deposition and chemical-mechanical polishing (CMP). In this article, various organometallic precursors (hfac)Ag(1,5-COD), (fod)Ag(PEt3) and (hfac) Ag(VTES) for the metal organic chemical vapor deposition (MOCVD) of silver on a TiN adhesion layer were evaluated and their deposition characteristics was studied. It was confirmed that Ag could be deposited at a substrate temperature as low as 180 degrees C with (hfac)Ag(VTES). The silver thin film was deposited at a precursor vaporization temperature of 50 degrees C and a substrate temperature of 220-250 degrees C, resulting in Ag film with resistivity around 1.8-2.0 mu Omega cm. (c) 2005 Elsevier B.V. All rights reserved.
