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Structure and deuterium retention properties of tungsten layers deposited by plasma sputtering in a mixed atmosphere of D2 and He
Journal of Nuclear Materials 446 (2014) 200–207
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Structure and deuterium retention properties of tungsten layers deposited by plasma sputtering in a mixed atmosphere of D2 and He X.H. Tang a,b, L.Q. Shi a,b,⇑, D.J. O’Connor c, B. King c a
Applied Ion Beam Physics Laboratory, Institute of Modern Physics, Fudan University, Shanghai 200433, PR China Department of Nuclear Science and Technology, Fudan University, Shanghai 200433, PR China c The School of Mathematical and Physical Sciences, University of Newcastle, Australia b
a r t i c l e
i n f o
Article history: Received 13 September 2013 Accepted 9 December 2013 Available online 12 December 2013
a b s t r a c t The influence of the deposition conditions on the surface morphology, crystal structure and deuterium retention of the tungsten layers formed by rf magnetron plasma sputtering in mixed atmosphere of D2, He and Ar, has been carried out. Helium containing deuterated tungsten layers (named He-WDx) on Cu/Si substrate demonstrate serious film damages with zones of cracks, fractures, flaking-off and large surface blisters. However, these kinds of damages do not happen on the He-WDx layers performed on mechanically polished polycrystalline Cu substrates because of larger surface roughness of the substrates. The crystal structure of the W layer greatly changes with the additional He in the layer, and large amounts of defects resulting in lattice expansion and X-diffraction peak broadening were produced in the W crystal. He in the W layer has direct impacts on D retention. Both D and He concentrations vary simultaneously with He fraction, attached negative bias and substrate temperature. Crown Copyright Ó 2013 Published by Elsevier B.V. All rights reserved.
1. Introduction Tungsten is one of the most important candidates for the divertor material in ITER for its excellent material properties such as high threshold energy for sputtering, high melting point and low tritium inventory [1]. Many studies have been devoted to investigating the behavior of hydrogen isotopes trapping in different types of tungsten materials (mainly bulk tungsten) exposed to low energy hydrogen ions or plasma [2–14] in order to understand the tritium (T) inventory problem. Using 1 keV D ion implantation for example, large concentrations of hydrogen isotopes can be trapped in the W near surface achieving a giving a D/W atomic ratio of 0.24 [2]. D retention due to the W surfaces exposed to the plasma strongly depends on the sample damage [3–5] as well as the sample temperature arising from the heat flux of the plasma. Wamper et al. [3] also found that, in ion irradiated tungsten at a peak damage level of 0.6 dpa at 300 °C the deuterium retention reached a D/W atomic ratio of 103, which was two order of magnitude larger than that of undamaged tungsten of 105 D/W . Helium (He) irradiation produced by a D–T fusion reactor is bound to greatly affect the surface morphology and the internal microstructure of tungsten, and therefore it can change hydrogen isotope retention significantly. These effects have been studied in
⇑ Corresponding author at: Applied Ion Beam Physics Laboratory, Institute of Modern Physics, Fudan University, Shanghai 200433, PR China. Tel.: +86 21 65642292; fax: +86 21 65642782. E-mail address: [email protected] (L.Q. Shi).
tungsten (W) [7–11] for He pre-irradiated samples, samples ion irradiated simultaneously by He and D, and the samples exposed to high-density plasma. It was suggested from these studies that He irradiation greatly affects hydrogen isotope behavior in W either by bubble formation or by excitation of a stress field near the bubbles, and that He-defect complexes such as bubbles and dislocations become trapping sites for hydrogen isotopes as well as greatly reducing the diffusion of hydrogen isotopes into the bulk of tungsten. On the other hand, the W co-deposition with deuterium has been studied recently using magnetron plasma sputtering and a linear plasma generator [6,12,13], since the longer pulse and high duty cycle in future fusion reactors will greatly increase the erosion of the tungsten plasma-facing components and cause possibly significant hydrogen isotope retention in re-deposited W layers. Their results indicated that D retention in the deposited W layer was greatly affected by experimental parameters such as the W deposition rate, the incident D particles energy and the substrate temperature. The substrate properties and temperature was also found to have a significant impact on the surface morphology of the W layer. However, there were not sufficient studies on the helium effects on surface morphology and hydrogen isotope retention in tungsten layers deposited under a He/H mixed atmosphere. Studies on the crystal structure of W layers deposited using a He-seeded D plasma are also lacking, as are studies on the behavior of He in those layers. In this paper, we have conducted a study on the W layers codeposited with D by magnetron sputtering in a mixed atmosphere
0022-3115/$ - see front matter Crown Copyright Ó 2013 Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jnucmat.2013.12.008
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of D2, He and Ar. The effect of He on deuterium retention and the surface morphology and crystal structure of the W layers were investigated using Elastic Recoil Detection analysis (ERDA), Scanning Electron Microscope (SEM) and X-ray diffractometer (XRD). The temperature and bias voltage dependence of composition and structure of the above samples were also studied.
2. Experimental 2.1. Specimen preparation Helium containing deuterated tungsten layers (named He-WDx) were formed by radio frequency magnetron sputtering of a pure W target in an atmosphere of D2, He and Ar (Fig. 1). A 70 mm-diameter pure W target (99.99% purity) was mounted on a water-cooled cathode. The He-WDx layers were deposited on polycrystalline copper (Cu) substrates and prepared Cu/Si (around 0.3 lm thick Cu layers deposited on Si wafers) substrates by radio frequency (rf) magnetron sputtering at a constant power of 160 W. Cu was chosen as the substrate material because it does not form an alloy with W and thus avoids any potential mixing at the W–Cu interface, and also because a new technique for plasma facing material W directly deposited on a Cu heat sink is used in some tokomaks [15,16]. The polycrystalline copper substrates were mechanically polished with an Al2O3 metallographic abrasive with particle size down to 1 lm. The substrates after a final treatment of ultrasonically cleaning in ethanol were mounted on a rotating substrate holder facing the target at a distance of 5 cm. The gases used for rf discharge were a mixture of 99.999% purity D2, He and Ar, and the partial pressure of He in the mixture was adjusted by varying the relative flow ratio of QHe/(QD2 + QAr) at the constant flow rates of QD2, QAr = 3.0 SCCM, 1.5 SCCM respectively (SCCM denotes cubic centimeter per minute at standard temperature and pressure). The total pressure of mixed gases in the discharge chamber was monitored by a Baratron pressure gauge (BG), and varied from 1.0 to 1.5 Pa with the He partial pressure ranging from 0.0 to 0.5 Pa by adjusting He flow rate from 0.0 to 2.0 SCCM. Before deposition, the chamber was baked at 100 °C for 3 h and self-cooling for 3 h. The chamber was pumped by a turbomolecular pump and the ultimate vacuum of the system was 2 105 Pa after bake-out and cooling. At low sputtering pressures the plasma sheath is considered to be nearly collision-free, so the measured self-bias voltage (200 V) plus the plasma sheath potential of about 10 V corresponds directly to the incident ion energy (210 eV) on the target. This energy is within the range of particles energy from 10 eV to 10 keV [17–19] that the plasma facing components (PFC) will withstand. In tokomaks, though the in-vessel PFCs are cooled by pressurized
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water at 370 K, the temperature is like to range from 370 K to 1000 K when the transient incidents (such as I ELMs [20] and disruption [21]) happen. Therefore, the substrate holder temperatures used in this experiment were set as room temperature (RT) of approximately 350 K, 423 K, 573 K, 773 K and 1073 K. Heating was provided by a resistive heater and monitored with a thermocouple. The deposition time was set at 1 h for the samples deposited at temperatures equal to or less than 423 K, and at 2 h, 3 h and 4 h for the samples deposited at 573 K, 773 K and 1073 K respectively. The DC bias voltage on the substrate holders was altered in order to change the energy of ions impinging on the growing layer, which corresponds to the effects of ion bombardment into the deposition layers on PFCs surface with various incident energies.
2.2. Ion beam analysis (IBA) Elastic Recoil Detection (ERD) and Rutherford Backscattering detection (RBS) analysis were employed to determine the concentrations of light elements (D and He) and elements heavy than He, respectively, at the NEC 9SDH-2 3 MV Pelletron tandem accelerator of Fudan University. The chamber background pressure during ion beam analysis was less than 1 104 Pa. The D depth profiles in the samples were investigated by ERD using a 4.2 MeV He ion beam with 75° incidence angle with respect to the sample surface normal and a 19 lm mylar foil in front of the detector to absorb scattered He. Similarly, He depth profiles in the samples were investigated by ERD by using a 8.0 MeV carbon (C) ion beam with the same incidence angle but a 10 lm mylar foil in front of the detector to absorb scattered C ions. The accurate determination of the dose was accomplished by placing another Au–Si surface barrier detector to measure the backscattering yield of the incident particles. The thickness and overall composition of the layers was determined by RBS measurements with 4.2 MeV He+ directed perpendicular to the sample surface. The composition of the samples measured by RBS was analyzed by the SIMNRA 6.03 code [22]. For the RBS and ERD measurement, Au/Si surface barrier detectors were placed at the laboratory angles of 165° and 30° relative to the incident beam subtending solid angles of 1.87 103 sr and 1.86 103 sr respectively. The typical current of the analyzing beam was 20 nA, and the dose for acquiring one energy spectrum was less than 1.0 1012 cm2. Typical ERD and RBS analysis results of the He-WDx layers on polycrystalline copper are shown in Fig. 2a–c. All the ERD raw spectra were converted to D or He concentration profiles using the Alegria 1.0 code [23] with an expected systematic error below 10%. In this paper, the mean concentrations of the gaseous atoms (D and He) were used to characterize the composition of the layers, because D and He atoms were distributed homogeneously throughout the whole thickness of the samples.
2.3. SEM and XRD analysis
Fig. 1. Radio frequency magnetron sputtering system for preparing He-WDx layers.
A scanning electron microscope (SEM) was used to explore the plasma irradiation effects and temperature effects on the surface damage (such as flaking, fracture, bubble or blister formation or fissure) of the He-WDx layers performed under different conditions. The evolution of the crystal structure of the He-WDx layers formed under different conditions (including He partial pressure, DC-bias voltage and substrate temperature), was investigated using a X-ray diffractometer (XRD). The measurement was conducted using a Rigaknu D/MAX 2550 V XRD with a Cu Ka source in 2h mode. The incidence angle was set at 1°.
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3.1. Morphology and crystal structure of the He-WDx layers
Cu/Si1 and polycrystalline Cu substrates are given in Fig. 3. While comparing the samples performed on above two kinds of substrates under same deposition conditions, it can be seen that the larger surface roughness of the mechanically polished polycrystalline Cu substrate precludes observation of the film whereas the smooth Si substrate makes damage in the film (cracks, flakes and fractures) obvious. In order to effectively distinguish the W layer deposited under various conditions, therefore, the discussion below is focused on the morphology of the samples deposited on the Cu/Si substrate. Flakes and fracture were observed on the pure W layers deposited at 0.3 Pa and RT in pure Ar atmosphere (Fig 3a), while bubbles or blisters were not present in the image. Since the intermediate Cu layer (black area) was observed on the Si surface after flaking off, it is suggested that the cracking and exfoliation is mostly caused by internal stress in the deposited W films. Thermal stress between the W layer and Cu substrate can make an important contribution to film stress causing film deformation because of the large difference in thermal expansion coefficient (17 ppm/°C for Cu, 4 ppm/°C for W at 293 K). On the other hand, the bombardment of the growing film by energetic ions and backscattered Ar particles from the target can change stresses during the deposition. However, since the target is floated, the ion energy, which is determined by the anode sheath potential (about 10 V) is much less than the scattered particle energy (see Fig. 4) and so the effect of ion bombardment can be ignored compared to that from scattered Ar atoms(mean energy is 44 eV). The histograms of the energy of backscattering ions or atoms (calculated by TRIM [24]) from the W target are given in Fig. 4. The energies of the backscattering Ar ions or atoms range from 0.0 eV to 125 eV with a scattering probability of 0.27. High flux Ar particles can directly penetrate the film surface and compress it. However, because of the high density of Ar atoms in the plasma, the energetic atom/ion bombardment to the growing film is bound to be weakened so that tensile stress arises in the film and leads to film exfoliation. Therefore, the presence of cracking and exfoliation on the surface can be mainly attributed to the effect of residual stresses coming from both the thermal stress and the intrinsic stresses generated by backscattered Ar bombardment. Fig. 3b shows the SEM image of deuterated W layers deposited at 1.0 Pa and RT in gas mixtures of D2 and Ar. Besides the flake-off and fracture, bubbles were observed, while the extent of flaking-off and fracturing was reduced by the introduced D2. As PD2 = 0.7 Pa is more than double PAr = 0.3 Pa and the first ionization energy of D (1312 kJ mol1) is slightly lower than that of Ar (1520 kJ mol1), + the D species (including Dþ 2 and D ) become the primary components in the plasma compared to Ar. Moreover, according to the TRIM calculation results shown in Fig. 4, both the scattering probability and mean scattering energy of D2 are around two times that of Ar. Therefore, deuterium particle bombardment and subsequent trapping become important for film growth. Since the D solubility in W crystal is very small, the ultra-high D flux into growing film will cause a large quantity of gas bubbles in the film. Fig. 3c shows the SEM image of the He-WDx layers deposited at 1.5 Pa (PAr = 0.3 Pa, PD2 = 0.7 Pa, PHe = 0.5 Pa) and RT in mixtures of D2, Ar and He. Introduced He causes serious damage to the W layers. In addition to flake-off and fracture, the bubbles become much bigger and some crack. He ions have approximately the same scattering probability and mean scattering energy as Dþ 2 from the W target but the trapped He atoms do not react with W atoms and have a stronger tendency to aggregate into clusters and then grow into bubbles which blister easily. Also, because of low adhesion
3.1.1. Morphology of the He-WDx layers The surface morphology of the W layers was determined by SEM observation. The SEM images of the W layers deposited on
1 Coefficients of thermal expansion for copper and stainless steel are close in value (about 17 ppm/°C and 11-19 ppm/°C, respectively), while for tungsten this coefficient is equal to 4 ppm/°C. The intermediate Cu layer was used to examine the adhesion stability of W layers deposited onto stainless steel components in the ITER divertor.
Fig. 2. Typical ERD (a and b) and RBS (c) analysis results (a and b are for D and He composition analysis, Fig. 2c is for analysis of other elements) of the same He-WDx layer on a polycrystalline copper substrate. The D and He average concentration of this sample are 8.5 D/W and 13.0 He/W respectively. The specific detection geometry and experimental parameters are shown on the figure.
3. Results and discussion
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Fig. 3. SEM images of the W layers deposited at (a and a’). 0.3 Pa and RT in a pure Ar atmosphere, (b and b’). 1.0 Pa (PAr = 0.3 Pa, PD2 = 0.7 Pa) and RT in D2 + Ar gas mixtures, (c and c’). 1.5 Pa (PAr = 0.3 Pa, PD2 = 0.7 Pa, PHe = 0.5 Pa) and RT in gas mixtures of D2, Ar and He, (d and d’). 1.5 Pa (PAr = 0.3 Pa, PD2 = 0.7 Pa, PHe = 0.5 Pa) and 573 K in D2 + Ar/He gas mixtures. The images labeled with letters (a, b, c, d) and with letters and left marking (a’, b’, c’, d’) are the SEM observations for the sample deposited onto Cu thin films on Si and on mechanically polished polycrystalline Cu respectively.
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scales, as shown in Fig. 3d. At higher deposition temperature, both the thermal stress effects and He damage effects become much stronger.
Fig. 4. Histograms of the energy of backscattered particles (including Ar, He and D2) reflected from a W target, calculated by TRIM program. The primary energy of the incident ions was 210 eV and the corresponding scattering probability of the three species, mean energies of the scattered species and back sputtering yield for W are given in the figure.
strength of the W film on the Cu layer, the film can easily flake off from the substrate. When the substrate was heated to 573 K, the size of bubbles observed by SEM increase from 10 lm at RT to more than 20 lm, and the surface of the He-WDx layers became messier, i.e. bubble cracking, film flaking and fracturing at larger
3.1.2. Crystal structure of the He-WDx layers The crystal structure of the He-WDx layers deposited on polycrystalline Cu at various conditions was characterized by XRD measurement shown in Fig. 5. The deposition conditions and the corresponding concentrations (from ERD results) of He and D in layers are labeled in the figure. Here, the samples deposited on polycrystalline Cu were used to characterize the crystal structure, because the cracking and fracturing zone of the samples deposited on the Cu/Si substrate affected the XRD results making it difficult to tell the difference between the samples deposited under various conditions. Fig. 5 shows that the XRD patterns of all the W layers have a strong preferred W (1 1 0) orientation and the intensity in the secondary reflection directions including W (2 0 0) and W (2 1 1) was much less than that of the preferred orientation W (1 1 0). The XRD patterns were analyzed by MDI Jade 6.0, and the Gaussian fitting results for full width at half maximum (FWHM) and Bragg Peak shift [25,26]) of the W (1 1 0) peaks are given in Fig. 6. The W (1 1 0) peak shift toward smaller angle and broadening occurred in all the samples under D and He charging. In order to observe He effects in the films, the Bragg Peak shift and FWHM as a function of the He concentration in the films deposited at various conditions (including He partial pressure, DC-bias and substrate temperature) are also given in Fig. 7a and b, respectively. Fig. 6a shows the W (1 1 0) peak shift and broadening (FWHM) for the samples performed at RT and floating potential. The Bragg peak shifted toward smaller angle with an increasing He partial pressure in the mixed gas at a constant D2 and Ar partial pressure (PAr = 0.3 Pa, PD2 = 0.7 Pa). The Bragg peak FWHM also significantly increased with additional He in the gas mixture. It seems that He concentration in the W layer plays a dominant role in changing the film structure. From Fig. 7, we find that the peak shift was almost dominated by He concentration in the film and increases with increasing He content for samples depositing at different He partial pressures as well as different deposition temperatures except at 600 K. Furthermore, from the relationship of the FWHM
Fig. 5. XRD results for the crystal structure of He-WDx layers with different He and D concentration deposited on polycrystalline Cu under the indicated conditions. The deposition conditions and the He and D concentration for different samples are given in figure.
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Fig. 7. The Bragg peak shift (a) and FWHM (b) as functions of He concentrations in films deposited under different conditions including He partial pressure PHe, attached DC-bias voltage and substrate temperature. The guide lines with arrows label the variable trends of the deposition parameters.
Fig. 6. Gaussian fitting results (including Bragg Peak FWHM and shift) of primary W (1 1 0) peak at 2h = 40.26°, analyzed by MDI Jade 6.0. The variables include He partial pressure PHe (a), attached DC-bias voltage (b) and substrate temperature(c).
with He content, the peak width almost consistently increase with He concentrations for all the deposition conditions including He partial pressure, deposition temperature and bias voltage. The above He effects on the peak shift and broadening are consistent with analysis in Ref. [27]. Helium is an inert gas and does not react with W, therefore has a strong tendency to precipitate into bubbles. Such bubble formation in metals implies the formation of self-interstitials atoms (SIAs). When growing, small bubbles will emit isolated SIAs but, above a given size of bubble, the ejection of dislocation loops is thermodynamically favored. The defects (such as isolated SIAs, dislocation loops and bubbles), according to Krivoglaz [28], induce a shift of Debye–Scherrer (DS) lines towards small angles for an increase of lattice parameters, since SIAs and dislocation loops are defects giving rise to distortions of finite
size they are leading to a lattice parameter shift of the host lattice and to diffuse scattering but have no influence on the widths of the Bragg peaks or DS lines of the host lattice. By way of contrast, emitted SIAs can also be incorporated to dislocation networks, these infinite defects will result in crystal lattice expansion and induce a broadening of the peaks or lines. Therefore, according to Krivoglaz theory, as the dislocation networks density increases in the solid for the additional He and D2, a broadening of diffraction peaks occurs as showed in Figs. 6a and 7b. These phenomena were also revealed on the He irradiated Ti films [29,30]. It should be also mentioned that the dependence of the substrate bias voltage on the crystal structure change of the He-WDx layers shown in Figs. 6 and 7 is complicated. On the one hand, the attached bias voltage will increase the energy of He, D and Ar ions and the flux bombarding the growing layer in anode plasma sheath, thus leading to increase in D and He concentration in the films. But the concentrations rapidly reach a limit at the lower bias voltage (50 V), as shown in Fig. 8b. Similar phenomenon had been demonstrated in a previous study [29]. On the other hand, the increasing Ar ion bombardment with the bias voltage will change film stress greatly, and can decrease vacancies in the film and increase atomic density. In generally, films deposited by evaporation are in tensile stress, and with sputter-deposited films the stress may changes from tensile to compressive due to particle bombardment depending upon the parameters of ion to atom arrival rate ratio and ion energy [31]. In addition, ion bombardment can also
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produce various defects causing crystal lattice expansion and strain. Therefore, increasing the ion bombardment energy by changing the bias voltage could contributes to the peak shift and broadening by three ways: the D and He contents trapped in the films, the stress and defects. They are interrelated and the dominant factor is determined by ion energy and flux as well as the D and He contents in the film. Fig. 6c shows that both the Bragg peak shift and FWHM decrease with increasing temperature. The thermal desorption of D and He during growth and annealing of the films decrease not only the amount of trapped D and He atoms but also the density of crystal defects (including SIAs, bubbles and various dislocations). Thus the sample deposited at higher temperature (especially at 1073 K)
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Temperature (K) Fig. 8. The variation of D and He concentration and growth rate for the samples with (a) He fraction (PHe/(PD2 + PAr)) with a constant D2 + Ar pressure (PD2 + PAr = 0.7 Pa + 0.3 Pa), (b) negative bias voltage and (c) substrate temperature, which corresponds with the results showed in Fig. 5.
tends to be more perfect resulting in a sharper Bragg peak and a very small peak shift. But Bragg peak shift and FWHM still exist for the films with very few gas atoms deposited at 1073 K, this phenomenon may attribute to that the films deposited by magnetron sputtering are of low density and few inevitable SIAs or dislocation networks during the growth. 3.2. D retention of the He-WDx layers In W layers deposited by rf magnetron sputtering from a mixed plasma of D2, He and Ar gases, both deuterium and helium were distributed homogeneously throughout the whole thickness with a concentration varying with different deposition conditions, as shown in Fig.8. From Fig. 8a, when the He fraction (PHe/PAr+D2) was varied from 0.0 to 0.5 with constant Ar and D2 partial pressure, the He and D concentrations both increase, from 0 to 13 He/W at.% and 1.5 to 8.5 D/W at.% respectively, and the growth rate changes from 0.67 to 0.83 1015 at cm2 s1. The existence of He in the deposition layer greatly influences D retention. According to the discussion in the previous section on the dependence of the morphology and structure of the samples on various deposition conditions, the addition of He atoms in crystal results in various defects including SIAs, bubbles, dislocation loops and dislocation networks. The finite size defects such as SIAs, bubbles and isolated dislocation loops induce lattice expansion, while dislocations and dislocation networks associated with microstrain fields induce peak broadening. The large density of defects will produce D capture centers in the crystal, resulting in an increase in trapped D in the crystal with increasing He contents. A similar increase in D and He concentration is then expected. In addition, the increasing He fraction in the plasma for constant D2 and Ar partial pressure would slightly intensify the sputtering atom flux of W target and thus increase the growth rate of the samples. Increasing the D and He ion energy by varying the bias voltage can also cause a higher trapping probability for D and He ions in the films because of deeper implantation range. But, increasing the Ar ion energy induce a higher anti-sputtering probability to the growing film as well as ion bombardment-induced desorption of D and He, thus resulting in decrease of deposition rate and a fluctuation in the D and He concentration, as shown in Fig. 8b. The substrate temperature has a key effect on the D and He retention in the W layer. Many previous studies on either D or He retention in the films have indicated its dominant influence [3,6,13]. In Fig. 8c, the D and He concentration and growth rate of the W layers are shown. The growth rate is seen to sharply decrease with the increasing substrate temperature from 350 K to 1073 K. On the one hand, the high substrate temperature causes an increasing thermal desorption of the trapped D (desorbed as form of hydrocarbon CxDy) or He atoms and loss at the film surface which decreases the D and He concentration and so growth rate of the film. On the other hand, the high temperature causes a significant increase in the surface mobility of condensing film atoms on the surface, resulting in an increase in crystal quality. 4. Conclusions Deuterium and helium were simultaneously deposited in W layers during RF plasma sputtering of a W target in a gaseous mixture of D2, He and Ar. The amounts of retained He was found to greatly affect the surface morphology, crystal structure and deuterium retention of the W layers. The SEM observation showed that the W layers deposited on Cu/Si at high He partial pressure cause serious film surface damage, while layers deposited on mechanically polished polycrystalline Cu under same conditions do not show obvious damage. This indicates
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that the surface roughness of substrates greatly affects the surface morphology of the samples because of the increase in the nucleation site density and adhesive force in the rougher sample. In addition, an increased temperature also degrades the quality of the film, producing bubble cracking, flaking and fracturing over larger length scales. The crystal structure of the helium implanted deuterated W layers on polycrystalline Cu were characterized by XRD measurement. Measurements show that the W layers have a preferred W(1 1 0) orientation and He and D charging leads to a Bragg peak position shift to lower angle (corresponding to lattice expansion) and width increase. The He and D content in the layer dominates the change of the peak shift and broadening. The film’s properties are also influenced by the DC bias applied to the substrate during deposition, mainly due to the influence of the He content in the film. The XRD results also show temperature dependence, in that both the Bragg peak width and shift decreased with an increased temperature. This shows that the He-WDx co-deposition layers on polycrystalline Cu at high temperature can be made with greatly decreased crystal defects and trapped gaseous atom density. D and He retention in W layers were analyzed by ERD. Both D and He concentration increased with the increasing He content in the discharge gas mixture. The introduction of He in the films caused a large quantity of defects, producing D capture centers in the crystal, resulting in an increase in D trapping in the crystal. So the trapping of both D and He in the W layer vary in a similar way with changing deposition condition. In conclusion, the He introduction (it will inevitably exist in future fusion reactor) in the re-deposition process makes the W deposition layers (from PFCs material) more complicated. It has great impacts on both D retention and structure property of the W deposition layers. Moreover, the He effects do not limitlessly intensified with He concentration in the layers and there seems to be a saturation value of He concentration at different conditions. From our results, heating is an effective way to ease the He impacts and decrease the D and He retention in W layers. Acknowledgments The authors are grateful to the staff of the tandem accelerator of the Institute of Modern Physics at Fudan University. Our work was supported by the National Magnetic confinement Fusion Science
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Program under contract No. 2010GB104002 and by the National Natural Science Foundation of China No. 10975035. References [1] ITER Physics Basis Editors, ITER Physics Expert Group Chairs and Co-Chairs and ITER Joint Central Team and Physics Integration Unit, Nucl. Fusion 39 (1999) 2137. [2] A.A. Haasz, J.W. Davis, J. Nucl. Mater. 241–243 (1997) 1076–1081. [3] W.R. Wampler, D.L. Rudakov, J.G. Watkins, C.J. Lasnier, J. Nucl. Mater. 451 (2011) s653–s656. [4] V.Kh. Alimov, K. Ertl, J. Roth, K. Schmid, Phys. Scr. T94 (2001) 34–42. [5] A.A. Haasz⁄, M. Poon, J.W. Davis, J. Nucl. Mater. 266-269 (1999) 520–525. [6] G. De Temmerman, R.P. Doerner, J. Nucl. Mater. 389 (2009) 479–483. [7] Y. Ueda, H.Y. Peng, H.T. Lee, N. Ohno, S. Kajita, N. Yoshida, R. Doerner, G. De Temmerman, V. Alimov, G. Wright, J. Nucl. Mater. 442S (2012) 267–272. [8] Y. Ueda, M. Fukumoto, J. Yoshida, Y. Ohtsuka, R. Akiyoshi, H. Iwakiri, N. Yoshida, J. Nucl. Mater. 386–388 (2009) 725–728. [9] M. Miyamoto, D. Nishijima, M.J. Baldwin, R.P. Doerner, Y. Ueda, K. Yasunaga, N. Yoshida, K. Ono, J. Nucl. Mater. 415 (2011) S657–S660. [10] M. Miyamoto, D. Nishijima, Y. Ueda, R.P. Doerner, H. Kurishita, M.J. Baldwin, S. Morito, K. Ono, J. Hanna, Nucl. Fusion 49 (2009) 065035. [11] H.T. Lee, A.A. Haasz, J.W. Davis, R.G. Macaulay-Newcombe, D.G. Whyte, G.M. Wright, J. Nucl. Mater. 363–365 (2007) 898–903. [12] K. Katayama, K. Imaoka, T. Okamura, M. Nishikawa, Fusion Eng. Des. 82 (2007) 1645–1650. [13] V.Kh. Alimov, J. Roth, W.M. Shu, D.A. Komarov, K. Isobe, T. Yamanishi, J. Nucl. Mater. 399 (2010). 255–230. [14] X.H. Tang, W.Z. Zhang, L.Q. Shi, Q. Qi, B. Zhang, W.Y. Zhang, K. Wang, J.S. Hu, Nucl. Instrum. Methods Phys. Res. B 305 (2013) 55–60. [15] I. Smid, M. Akiba, G. Vieider, et al., J. Nucl. Mater. 258–263 (1998) 160. [16] G.-N. Luo, X.D. Zhang, D.M. Yao, et al., Phys. Scr. T128 (2007) 1–5. [17] J. Roth, W. Eckstein, M. Guseva, Fusion Eng. Des. 37 (1997) 465. [18] Technical Basis for the ITER-FEAT Final Design Report, IAEA, Vienna, 2001. [19] R. Aymar, Fusion Eng. Des. 49&50 (2000) 13. [20] ITER Physics Basis, Nucl. Fusion 39 (1999) 2137. [21] H. Zohm, Plasma Phys. Control. Fusion 38 (1996) 105. [22] M. Mayer, SIMNRA, Max-Planck-Institut für Plasmaphysik, Garching, Version 6.03. [23] F. Schiettekatte, G.G. Ross, AIP Conf. Proc. 392 (1996) 711–714. [24] J.F. Ziegler, J.P. Briersack, SRIM – The Stopping and Range of Ions in Solids, (2009). [25] M. von Laue, Röntgenstrahlinterferenzen (Akad. Verlagsgesellschaft, Leipzig, 1941), second ed., Frankfurt/M, 1960. [26] F. Rustichelli, Philos. Mag. 31 (1975) 1–12. [27] S. Thiebant, V. Paul-Boncour, A. Percheron-Gue, et al., Phys. Rev. B 57 (1998) 10379. [28] M.A. Krivoglaz, Theory of X-ray and Thermal Neutron Scattering by Real Crystals Plenum, New York, 1969. [29] L.Q. Shi, C.Z. Liu, S.L. Xu, Z.Y. Zhou, Thin Solid Films 479 (2005) 52–58. [30] C.Z. Liu, L.Q. Shi, Z.Y. Zhou, X.P. Hao, B.Y. Wang, S. Liu, L.B. Wang, J. Phys. D: Appl. Phys. 40 (2007) 2150–2156. [31] S.M. Rossnagel, J J Cuomo, Vacuum 38 (1988) 73.
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Journal of Nuclear Materials journal homepage: www.elsevier.com/locate/jnucmat
Structure and deuterium retention properties of tungsten layers deposited by plasma sputtering in a mixed atmosphere of D2 and He X.H. Tang a,b, L.Q. Shi a,b,⇑, D.J. O’Connor c, B. King c a
Applied Ion Beam Physics Laboratory, Institute of Modern Physics, Fudan University, Shanghai 200433, PR China Department of Nuclear Science and Technology, Fudan University, Shanghai 200433, PR China c The School of Mathematical and Physical Sciences, University of Newcastle, Australia b
a r t i c l e
i n f o
Article history: Received 13 September 2013 Accepted 9 December 2013 Available online 12 December 2013
a b s t r a c t The influence of the deposition conditions on the surface morphology, crystal structure and deuterium retention of the tungsten layers formed by rf magnetron plasma sputtering in mixed atmosphere of D2, He and Ar, has been carried out. Helium containing deuterated tungsten layers (named He-WDx) on Cu/Si substrate demonstrate serious film damages with zones of cracks, fractures, flaking-off and large surface blisters. However, these kinds of damages do not happen on the He-WDx layers performed on mechanically polished polycrystalline Cu substrates because of larger surface roughness of the substrates. The crystal structure of the W layer greatly changes with the additional He in the layer, and large amounts of defects resulting in lattice expansion and X-diffraction peak broadening were produced in the W crystal. He in the W layer has direct impacts on D retention. Both D and He concentrations vary simultaneously with He fraction, attached negative bias and substrate temperature. Crown Copyright Ó 2013 Published by Elsevier B.V. All rights reserved.
1. Introduction Tungsten is one of the most important candidates for the divertor material in ITER for its excellent material properties such as high threshold energy for sputtering, high melting point and low tritium inventory [1]. Many studies have been devoted to investigating the behavior of hydrogen isotopes trapping in different types of tungsten materials (mainly bulk tungsten) exposed to low energy hydrogen ions or plasma [2–14] in order to understand the tritium (T) inventory problem. Using 1 keV D ion implantation for example, large concentrations of hydrogen isotopes can be trapped in the W near surface achieving a giving a D/W atomic ratio of 0.24 [2]. D retention due to the W surfaces exposed to the plasma strongly depends on the sample damage [3–5] as well as the sample temperature arising from the heat flux of the plasma. Wamper et al. [3] also found that, in ion irradiated tungsten at a peak damage level of 0.6 dpa at 300 °C the deuterium retention reached a D/W atomic ratio of 103, which was two order of magnitude larger than that of undamaged tungsten of 105 D/W . Helium (He) irradiation produced by a D–T fusion reactor is bound to greatly affect the surface morphology and the internal microstructure of tungsten, and therefore it can change hydrogen isotope retention significantly. These effects have been studied in
⇑ Corresponding author at: Applied Ion Beam Physics Laboratory, Institute of Modern Physics, Fudan University, Shanghai 200433, PR China. Tel.: +86 21 65642292; fax: +86 21 65642782. E-mail address: [email protected] (L.Q. Shi).
tungsten (W) [7–11] for He pre-irradiated samples, samples ion irradiated simultaneously by He and D, and the samples exposed to high-density plasma. It was suggested from these studies that He irradiation greatly affects hydrogen isotope behavior in W either by bubble formation or by excitation of a stress field near the bubbles, and that He-defect complexes such as bubbles and dislocations become trapping sites for hydrogen isotopes as well as greatly reducing the diffusion of hydrogen isotopes into the bulk of tungsten. On the other hand, the W co-deposition with deuterium has been studied recently using magnetron plasma sputtering and a linear plasma generator [6,12,13], since the longer pulse and high duty cycle in future fusion reactors will greatly increase the erosion of the tungsten plasma-facing components and cause possibly significant hydrogen isotope retention in re-deposited W layers. Their results indicated that D retention in the deposited W layer was greatly affected by experimental parameters such as the W deposition rate, the incident D particles energy and the substrate temperature. The substrate properties and temperature was also found to have a significant impact on the surface morphology of the W layer. However, there were not sufficient studies on the helium effects on surface morphology and hydrogen isotope retention in tungsten layers deposited under a He/H mixed atmosphere. Studies on the crystal structure of W layers deposited using a He-seeded D plasma are also lacking, as are studies on the behavior of He in those layers. In this paper, we have conducted a study on the W layers codeposited with D by magnetron sputtering in a mixed atmosphere
0022-3115/$ - see front matter Crown Copyright Ó 2013 Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jnucmat.2013.12.008
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of D2, He and Ar. The effect of He on deuterium retention and the surface morphology and crystal structure of the W layers were investigated using Elastic Recoil Detection analysis (ERDA), Scanning Electron Microscope (SEM) and X-ray diffractometer (XRD). The temperature and bias voltage dependence of composition and structure of the above samples were also studied.
2. Experimental 2.1. Specimen preparation Helium containing deuterated tungsten layers (named He-WDx) were formed by radio frequency magnetron sputtering of a pure W target in an atmosphere of D2, He and Ar (Fig. 1). A 70 mm-diameter pure W target (99.99% purity) was mounted on a water-cooled cathode. The He-WDx layers were deposited on polycrystalline copper (Cu) substrates and prepared Cu/Si (around 0.3 lm thick Cu layers deposited on Si wafers) substrates by radio frequency (rf) magnetron sputtering at a constant power of 160 W. Cu was chosen as the substrate material because it does not form an alloy with W and thus avoids any potential mixing at the W–Cu interface, and also because a new technique for plasma facing material W directly deposited on a Cu heat sink is used in some tokomaks [15,16]. The polycrystalline copper substrates were mechanically polished with an Al2O3 metallographic abrasive with particle size down to 1 lm. The substrates after a final treatment of ultrasonically cleaning in ethanol were mounted on a rotating substrate holder facing the target at a distance of 5 cm. The gases used for rf discharge were a mixture of 99.999% purity D2, He and Ar, and the partial pressure of He in the mixture was adjusted by varying the relative flow ratio of QHe/(QD2 + QAr) at the constant flow rates of QD2, QAr = 3.0 SCCM, 1.5 SCCM respectively (SCCM denotes cubic centimeter per minute at standard temperature and pressure). The total pressure of mixed gases in the discharge chamber was monitored by a Baratron pressure gauge (BG), and varied from 1.0 to 1.5 Pa with the He partial pressure ranging from 0.0 to 0.5 Pa by adjusting He flow rate from 0.0 to 2.0 SCCM. Before deposition, the chamber was baked at 100 °C for 3 h and self-cooling for 3 h. The chamber was pumped by a turbomolecular pump and the ultimate vacuum of the system was 2 105 Pa after bake-out and cooling. At low sputtering pressures the plasma sheath is considered to be nearly collision-free, so the measured self-bias voltage (200 V) plus the plasma sheath potential of about 10 V corresponds directly to the incident ion energy (210 eV) on the target. This energy is within the range of particles energy from 10 eV to 10 keV [17–19] that the plasma facing components (PFC) will withstand. In tokomaks, though the in-vessel PFCs are cooled by pressurized
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water at 370 K, the temperature is like to range from 370 K to 1000 K when the transient incidents (such as I ELMs [20] and disruption [21]) happen. Therefore, the substrate holder temperatures used in this experiment were set as room temperature (RT) of approximately 350 K, 423 K, 573 K, 773 K and 1073 K. Heating was provided by a resistive heater and monitored with a thermocouple. The deposition time was set at 1 h for the samples deposited at temperatures equal to or less than 423 K, and at 2 h, 3 h and 4 h for the samples deposited at 573 K, 773 K and 1073 K respectively. The DC bias voltage on the substrate holders was altered in order to change the energy of ions impinging on the growing layer, which corresponds to the effects of ion bombardment into the deposition layers on PFCs surface with various incident energies.
2.2. Ion beam analysis (IBA) Elastic Recoil Detection (ERD) and Rutherford Backscattering detection (RBS) analysis were employed to determine the concentrations of light elements (D and He) and elements heavy than He, respectively, at the NEC 9SDH-2 3 MV Pelletron tandem accelerator of Fudan University. The chamber background pressure during ion beam analysis was less than 1 104 Pa. The D depth profiles in the samples were investigated by ERD using a 4.2 MeV He ion beam with 75° incidence angle with respect to the sample surface normal and a 19 lm mylar foil in front of the detector to absorb scattered He. Similarly, He depth profiles in the samples were investigated by ERD by using a 8.0 MeV carbon (C) ion beam with the same incidence angle but a 10 lm mylar foil in front of the detector to absorb scattered C ions. The accurate determination of the dose was accomplished by placing another Au–Si surface barrier detector to measure the backscattering yield of the incident particles. The thickness and overall composition of the layers was determined by RBS measurements with 4.2 MeV He+ directed perpendicular to the sample surface. The composition of the samples measured by RBS was analyzed by the SIMNRA 6.03 code [22]. For the RBS and ERD measurement, Au/Si surface barrier detectors were placed at the laboratory angles of 165° and 30° relative to the incident beam subtending solid angles of 1.87 103 sr and 1.86 103 sr respectively. The typical current of the analyzing beam was 20 nA, and the dose for acquiring one energy spectrum was less than 1.0 1012 cm2. Typical ERD and RBS analysis results of the He-WDx layers on polycrystalline copper are shown in Fig. 2a–c. All the ERD raw spectra were converted to D or He concentration profiles using the Alegria 1.0 code [23] with an expected systematic error below 10%. In this paper, the mean concentrations of the gaseous atoms (D and He) were used to characterize the composition of the layers, because D and He atoms were distributed homogeneously throughout the whole thickness of the samples.
2.3. SEM and XRD analysis
Fig. 1. Radio frequency magnetron sputtering system for preparing He-WDx layers.
A scanning electron microscope (SEM) was used to explore the plasma irradiation effects and temperature effects on the surface damage (such as flaking, fracture, bubble or blister formation or fissure) of the He-WDx layers performed under different conditions. The evolution of the crystal structure of the He-WDx layers formed under different conditions (including He partial pressure, DC-bias voltage and substrate temperature), was investigated using a X-ray diffractometer (XRD). The measurement was conducted using a Rigaknu D/MAX 2550 V XRD with a Cu Ka source in 2h mode. The incidence angle was set at 1°.
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3.1. Morphology and crystal structure of the He-WDx layers
Cu/Si1 and polycrystalline Cu substrates are given in Fig. 3. While comparing the samples performed on above two kinds of substrates under same deposition conditions, it can be seen that the larger surface roughness of the mechanically polished polycrystalline Cu substrate precludes observation of the film whereas the smooth Si substrate makes damage in the film (cracks, flakes and fractures) obvious. In order to effectively distinguish the W layer deposited under various conditions, therefore, the discussion below is focused on the morphology of the samples deposited on the Cu/Si substrate. Flakes and fracture were observed on the pure W layers deposited at 0.3 Pa and RT in pure Ar atmosphere (Fig 3a), while bubbles or blisters were not present in the image. Since the intermediate Cu layer (black area) was observed on the Si surface after flaking off, it is suggested that the cracking and exfoliation is mostly caused by internal stress in the deposited W films. Thermal stress between the W layer and Cu substrate can make an important contribution to film stress causing film deformation because of the large difference in thermal expansion coefficient (17 ppm/°C for Cu, 4 ppm/°C for W at 293 K). On the other hand, the bombardment of the growing film by energetic ions and backscattered Ar particles from the target can change stresses during the deposition. However, since the target is floated, the ion energy, which is determined by the anode sheath potential (about 10 V) is much less than the scattered particle energy (see Fig. 4) and so the effect of ion bombardment can be ignored compared to that from scattered Ar atoms(mean energy is 44 eV). The histograms of the energy of backscattering ions or atoms (calculated by TRIM [24]) from the W target are given in Fig. 4. The energies of the backscattering Ar ions or atoms range from 0.0 eV to 125 eV with a scattering probability of 0.27. High flux Ar particles can directly penetrate the film surface and compress it. However, because of the high density of Ar atoms in the plasma, the energetic atom/ion bombardment to the growing film is bound to be weakened so that tensile stress arises in the film and leads to film exfoliation. Therefore, the presence of cracking and exfoliation on the surface can be mainly attributed to the effect of residual stresses coming from both the thermal stress and the intrinsic stresses generated by backscattered Ar bombardment. Fig. 3b shows the SEM image of deuterated W layers deposited at 1.0 Pa and RT in gas mixtures of D2 and Ar. Besides the flake-off and fracture, bubbles were observed, while the extent of flaking-off and fracturing was reduced by the introduced D2. As PD2 = 0.7 Pa is more than double PAr = 0.3 Pa and the first ionization energy of D (1312 kJ mol1) is slightly lower than that of Ar (1520 kJ mol1), + the D species (including Dþ 2 and D ) become the primary components in the plasma compared to Ar. Moreover, according to the TRIM calculation results shown in Fig. 4, both the scattering probability and mean scattering energy of D2 are around two times that of Ar. Therefore, deuterium particle bombardment and subsequent trapping become important for film growth. Since the D solubility in W crystal is very small, the ultra-high D flux into growing film will cause a large quantity of gas bubbles in the film. Fig. 3c shows the SEM image of the He-WDx layers deposited at 1.5 Pa (PAr = 0.3 Pa, PD2 = 0.7 Pa, PHe = 0.5 Pa) and RT in mixtures of D2, Ar and He. Introduced He causes serious damage to the W layers. In addition to flake-off and fracture, the bubbles become much bigger and some crack. He ions have approximately the same scattering probability and mean scattering energy as Dþ 2 from the W target but the trapped He atoms do not react with W atoms and have a stronger tendency to aggregate into clusters and then grow into bubbles which blister easily. Also, because of low adhesion
3.1.1. Morphology of the He-WDx layers The surface morphology of the W layers was determined by SEM observation. The SEM images of the W layers deposited on
1 Coefficients of thermal expansion for copper and stainless steel are close in value (about 17 ppm/°C and 11-19 ppm/°C, respectively), while for tungsten this coefficient is equal to 4 ppm/°C. The intermediate Cu layer was used to examine the adhesion stability of W layers deposited onto stainless steel components in the ITER divertor.
Fig. 2. Typical ERD (a and b) and RBS (c) analysis results (a and b are for D and He composition analysis, Fig. 2c is for analysis of other elements) of the same He-WDx layer on a polycrystalline copper substrate. The D and He average concentration of this sample are 8.5 D/W and 13.0 He/W respectively. The specific detection geometry and experimental parameters are shown on the figure.
3. Results and discussion
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Fig. 3. SEM images of the W layers deposited at (a and a’). 0.3 Pa and RT in a pure Ar atmosphere, (b and b’). 1.0 Pa (PAr = 0.3 Pa, PD2 = 0.7 Pa) and RT in D2 + Ar gas mixtures, (c and c’). 1.5 Pa (PAr = 0.3 Pa, PD2 = 0.7 Pa, PHe = 0.5 Pa) and RT in gas mixtures of D2, Ar and He, (d and d’). 1.5 Pa (PAr = 0.3 Pa, PD2 = 0.7 Pa, PHe = 0.5 Pa) and 573 K in D2 + Ar/He gas mixtures. The images labeled with letters (a, b, c, d) and with letters and left marking (a’, b’, c’, d’) are the SEM observations for the sample deposited onto Cu thin films on Si and on mechanically polished polycrystalline Cu respectively.
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scales, as shown in Fig. 3d. At higher deposition temperature, both the thermal stress effects and He damage effects become much stronger.
Fig. 4. Histograms of the energy of backscattered particles (including Ar, He and D2) reflected from a W target, calculated by TRIM program. The primary energy of the incident ions was 210 eV and the corresponding scattering probability of the three species, mean energies of the scattered species and back sputtering yield for W are given in the figure.
strength of the W film on the Cu layer, the film can easily flake off from the substrate. When the substrate was heated to 573 K, the size of bubbles observed by SEM increase from 10 lm at RT to more than 20 lm, and the surface of the He-WDx layers became messier, i.e. bubble cracking, film flaking and fracturing at larger
3.1.2. Crystal structure of the He-WDx layers The crystal structure of the He-WDx layers deposited on polycrystalline Cu at various conditions was characterized by XRD measurement shown in Fig. 5. The deposition conditions and the corresponding concentrations (from ERD results) of He and D in layers are labeled in the figure. Here, the samples deposited on polycrystalline Cu were used to characterize the crystal structure, because the cracking and fracturing zone of the samples deposited on the Cu/Si substrate affected the XRD results making it difficult to tell the difference between the samples deposited under various conditions. Fig. 5 shows that the XRD patterns of all the W layers have a strong preferred W (1 1 0) orientation and the intensity in the secondary reflection directions including W (2 0 0) and W (2 1 1) was much less than that of the preferred orientation W (1 1 0). The XRD patterns were analyzed by MDI Jade 6.0, and the Gaussian fitting results for full width at half maximum (FWHM) and Bragg Peak shift [25,26]) of the W (1 1 0) peaks are given in Fig. 6. The W (1 1 0) peak shift toward smaller angle and broadening occurred in all the samples under D and He charging. In order to observe He effects in the films, the Bragg Peak shift and FWHM as a function of the He concentration in the films deposited at various conditions (including He partial pressure, DC-bias and substrate temperature) are also given in Fig. 7a and b, respectively. Fig. 6a shows the W (1 1 0) peak shift and broadening (FWHM) for the samples performed at RT and floating potential. The Bragg peak shifted toward smaller angle with an increasing He partial pressure in the mixed gas at a constant D2 and Ar partial pressure (PAr = 0.3 Pa, PD2 = 0.7 Pa). The Bragg peak FWHM also significantly increased with additional He in the gas mixture. It seems that He concentration in the W layer plays a dominant role in changing the film structure. From Fig. 7, we find that the peak shift was almost dominated by He concentration in the film and increases with increasing He content for samples depositing at different He partial pressures as well as different deposition temperatures except at 600 K. Furthermore, from the relationship of the FWHM
Fig. 5. XRD results for the crystal structure of He-WDx layers with different He and D concentration deposited on polycrystalline Cu under the indicated conditions. The deposition conditions and the He and D concentration for different samples are given in figure.
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Fig. 7. The Bragg peak shift (a) and FWHM (b) as functions of He concentrations in films deposited under different conditions including He partial pressure PHe, attached DC-bias voltage and substrate temperature. The guide lines with arrows label the variable trends of the deposition parameters.
Fig. 6. Gaussian fitting results (including Bragg Peak FWHM and shift) of primary W (1 1 0) peak at 2h = 40.26°, analyzed by MDI Jade 6.0. The variables include He partial pressure PHe (a), attached DC-bias voltage (b) and substrate temperature(c).
with He content, the peak width almost consistently increase with He concentrations for all the deposition conditions including He partial pressure, deposition temperature and bias voltage. The above He effects on the peak shift and broadening are consistent with analysis in Ref. [27]. Helium is an inert gas and does not react with W, therefore has a strong tendency to precipitate into bubbles. Such bubble formation in metals implies the formation of self-interstitials atoms (SIAs). When growing, small bubbles will emit isolated SIAs but, above a given size of bubble, the ejection of dislocation loops is thermodynamically favored. The defects (such as isolated SIAs, dislocation loops and bubbles), according to Krivoglaz [28], induce a shift of Debye–Scherrer (DS) lines towards small angles for an increase of lattice parameters, since SIAs and dislocation loops are defects giving rise to distortions of finite
size they are leading to a lattice parameter shift of the host lattice and to diffuse scattering but have no influence on the widths of the Bragg peaks or DS lines of the host lattice. By way of contrast, emitted SIAs can also be incorporated to dislocation networks, these infinite defects will result in crystal lattice expansion and induce a broadening of the peaks or lines. Therefore, according to Krivoglaz theory, as the dislocation networks density increases in the solid for the additional He and D2, a broadening of diffraction peaks occurs as showed in Figs. 6a and 7b. These phenomena were also revealed on the He irradiated Ti films [29,30]. It should be also mentioned that the dependence of the substrate bias voltage on the crystal structure change of the He-WDx layers shown in Figs. 6 and 7 is complicated. On the one hand, the attached bias voltage will increase the energy of He, D and Ar ions and the flux bombarding the growing layer in anode plasma sheath, thus leading to increase in D and He concentration in the films. But the concentrations rapidly reach a limit at the lower bias voltage (50 V), as shown in Fig. 8b. Similar phenomenon had been demonstrated in a previous study [29]. On the other hand, the increasing Ar ion bombardment with the bias voltage will change film stress greatly, and can decrease vacancies in the film and increase atomic density. In generally, films deposited by evaporation are in tensile stress, and with sputter-deposited films the stress may changes from tensile to compressive due to particle bombardment depending upon the parameters of ion to atom arrival rate ratio and ion energy [31]. In addition, ion bombardment can also
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produce various defects causing crystal lattice expansion and strain. Therefore, increasing the ion bombardment energy by changing the bias voltage could contributes to the peak shift and broadening by three ways: the D and He contents trapped in the films, the stress and defects. They are interrelated and the dominant factor is determined by ion energy and flux as well as the D and He contents in the film. Fig. 6c shows that both the Bragg peak shift and FWHM decrease with increasing temperature. The thermal desorption of D and He during growth and annealing of the films decrease not only the amount of trapped D and He atoms but also the density of crystal defects (including SIAs, bubbles and various dislocations). Thus the sample deposited at higher temperature (especially at 1073 K)
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Temperature (K) Fig. 8. The variation of D and He concentration and growth rate for the samples with (a) He fraction (PHe/(PD2 + PAr)) with a constant D2 + Ar pressure (PD2 + PAr = 0.7 Pa + 0.3 Pa), (b) negative bias voltage and (c) substrate temperature, which corresponds with the results showed in Fig. 5.
tends to be more perfect resulting in a sharper Bragg peak and a very small peak shift. But Bragg peak shift and FWHM still exist for the films with very few gas atoms deposited at 1073 K, this phenomenon may attribute to that the films deposited by magnetron sputtering are of low density and few inevitable SIAs or dislocation networks during the growth. 3.2. D retention of the He-WDx layers In W layers deposited by rf magnetron sputtering from a mixed plasma of D2, He and Ar gases, both deuterium and helium were distributed homogeneously throughout the whole thickness with a concentration varying with different deposition conditions, as shown in Fig.8. From Fig. 8a, when the He fraction (PHe/PAr+D2) was varied from 0.0 to 0.5 with constant Ar and D2 partial pressure, the He and D concentrations both increase, from 0 to 13 He/W at.% and 1.5 to 8.5 D/W at.% respectively, and the growth rate changes from 0.67 to 0.83 1015 at cm2 s1. The existence of He in the deposition layer greatly influences D retention. According to the discussion in the previous section on the dependence of the morphology and structure of the samples on various deposition conditions, the addition of He atoms in crystal results in various defects including SIAs, bubbles, dislocation loops and dislocation networks. The finite size defects such as SIAs, bubbles and isolated dislocation loops induce lattice expansion, while dislocations and dislocation networks associated with microstrain fields induce peak broadening. The large density of defects will produce D capture centers in the crystal, resulting in an increase in trapped D in the crystal with increasing He contents. A similar increase in D and He concentration is then expected. In addition, the increasing He fraction in the plasma for constant D2 and Ar partial pressure would slightly intensify the sputtering atom flux of W target and thus increase the growth rate of the samples. Increasing the D and He ion energy by varying the bias voltage can also cause a higher trapping probability for D and He ions in the films because of deeper implantation range. But, increasing the Ar ion energy induce a higher anti-sputtering probability to the growing film as well as ion bombardment-induced desorption of D and He, thus resulting in decrease of deposition rate and a fluctuation in the D and He concentration, as shown in Fig. 8b. The substrate temperature has a key effect on the D and He retention in the W layer. Many previous studies on either D or He retention in the films have indicated its dominant influence [3,6,13]. In Fig. 8c, the D and He concentration and growth rate of the W layers are shown. The growth rate is seen to sharply decrease with the increasing substrate temperature from 350 K to 1073 K. On the one hand, the high substrate temperature causes an increasing thermal desorption of the trapped D (desorbed as form of hydrocarbon CxDy) or He atoms and loss at the film surface which decreases the D and He concentration and so growth rate of the film. On the other hand, the high temperature causes a significant increase in the surface mobility of condensing film atoms on the surface, resulting in an increase in crystal quality. 4. Conclusions Deuterium and helium were simultaneously deposited in W layers during RF plasma sputtering of a W target in a gaseous mixture of D2, He and Ar. The amounts of retained He was found to greatly affect the surface morphology, crystal structure and deuterium retention of the W layers. The SEM observation showed that the W layers deposited on Cu/Si at high He partial pressure cause serious film surface damage, while layers deposited on mechanically polished polycrystalline Cu under same conditions do not show obvious damage. This indicates
X.H. Tang et al. / Journal of Nuclear Materials 446 (2014) 200–207
that the surface roughness of substrates greatly affects the surface morphology of the samples because of the increase in the nucleation site density and adhesive force in the rougher sample. In addition, an increased temperature also degrades the quality of the film, producing bubble cracking, flaking and fracturing over larger length scales. The crystal structure of the helium implanted deuterated W layers on polycrystalline Cu were characterized by XRD measurement. Measurements show that the W layers have a preferred W(1 1 0) orientation and He and D charging leads to a Bragg peak position shift to lower angle (corresponding to lattice expansion) and width increase. The He and D content in the layer dominates the change of the peak shift and broadening. The film’s properties are also influenced by the DC bias applied to the substrate during deposition, mainly due to the influence of the He content in the film. The XRD results also show temperature dependence, in that both the Bragg peak width and shift decreased with an increased temperature. This shows that the He-WDx co-deposition layers on polycrystalline Cu at high temperature can be made with greatly decreased crystal defects and trapped gaseous atom density. D and He retention in W layers were analyzed by ERD. Both D and He concentration increased with the increasing He content in the discharge gas mixture. The introduction of He in the films caused a large quantity of defects, producing D capture centers in the crystal, resulting in an increase in D trapping in the crystal. So the trapping of both D and He in the W layer vary in a similar way with changing deposition condition. In conclusion, the He introduction (it will inevitably exist in future fusion reactor) in the re-deposition process makes the W deposition layers (from PFCs material) more complicated. It has great impacts on both D retention and structure property of the W deposition layers. Moreover, the He effects do not limitlessly intensified with He concentration in the layers and there seems to be a saturation value of He concentration at different conditions. From our results, heating is an effective way to ease the He impacts and decrease the D and He retention in W layers. Acknowledgments The authors are grateful to the staff of the tandem accelerator of the Institute of Modern Physics at Fudan University. Our work was supported by the National Magnetic confinement Fusion Science
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