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Effects of He+ energy and irradiation temperature on W sputtering yields under fusion-relevant conditions
Journal of Nuclear Materials 470 (2016) 164e169
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Journal of Nuclear Materials journal homepage: www.elsevier.com/locate/jnucmat
Effects of Heþ energy and irradiation temperature on W sputtering yields under fusion-relevant conditions Yunfeng Wu, Weiyuan Ni, Hongyu Fan, Lu Liu, Qi Yang, Baosheng Cao, Dongping Liu* School of Physics and Materials Engineering, Dalian Nationalities University, Dalian 116600, People's Republic of China
h i g h l i g h t s The sputtering yields and roughness of W films irradiated with Heþ at different energy and temperatures have been measured. The sputtering yield increases by about one order of magnitude when E varies from 18e100 eV to 150 eV. The W sputtering yield rapidly increases from 4.06 103 to 1.44 102 W/Heþ with increasing T from 293 to 973 K. CAFM measurements show that plenty of nanometer-sized defects are formed at E 150 eV.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 6 August 2015 Received in revised form 17 December 2015 Accepted 18 December 2015 Available online 24 December 2015
The sputtering yields of W coatings irradiated with Heþ at different irradiation temperatures under fusion-relevant conditions have been measured as the function of Heþ energy ranging from 18 to 300 eV. The sputtering yield of W coatings irradiated at temperature of 673 K increases by one order of magnitude from ~103 to ~102 W/Heþ when Heþ energy varies from 100 eV to 150 eV, indicating the existence of W sputtering threshold. The sputtering yield of W coatings irradiated at Heþ energy of 300 eV rapidly increases from 4.56 103 to 1.44 102 W/Heþ when the temperature increases from 293 to 973 K. Measurements by conductive atomic force microscopy show that plenty of nanometersized defects are formed after Heþ irradiation at a relatively high Heþ energy. The serious etching of W coatings at Heþ energies of 150 eV can be also related to He trapping into defects in the sub-surface layer and their growth, leading to an increase in the thermal instability of W. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Tungsten as one of the best candidates for plasma facing materials in fusion reactors will be subject to the strong bombardments by low-energy Heþ/Hþ [1e3]. The flux and energy of Heþ/Hþ species bombarding the first wall of ITER were estimated as 1020e1022 particles/m2,s and <500 eV, respectively [4]. The bombardments of W by the high-flux and low-energy Heþ/Hþ can result in the serious surface etching. The surface etching of W materials is the major concern because it may cause general destruction of the components and significantly affect the stability of fusion plasmas [5,6]. However, there have few reports about the sputtering yield and
* Corresponding author. E-mail address: [email protected] (D. Liu). http://dx.doi.org/10.1016/j.jnucmat.2015.12.022 0022-3115/© 2015 Elsevier B.V. All rights reserved.
erosion process of W materials irradiated with low-energy Heþ/Hþ under fusion-relevant conditions [7e9]. The sputtered threshold energy of tungsten by Heþ is approximately 105e110 eV, which is typically in the range of expected energies of particles in the baffle region of the ITER divertor [7]. Molecular dynamics simulations have been performed to predict the sputtering behavior of W due to Heþ bombardment when the Heþ energy is varied in the range of 300 eVe1 keV [8,9]. The calculated sputtering yields are in reasonable agreement with experimental data. The sputtering yields were found to depend on surface orientation and incident Heþ energy. However, the dependence of W sputtering yield on W temperature and Heþ energy below 300 eV needs to be investigated. The purpose of this work is to explore the erosion process of W materials irradiated at the Heþ energy close to the W sputtering threshold. How will Heþ energy and irradiation temperature affect the sputtering yield of W materials under fusion-relevant plasma
Y. Wu et al. / Journal of Nuclear Materials 470 (2016) 164e169
conditions? 2. Experimental procedures Ultra-smooth W coatings deposited on a silicon substrate were used to characterize the erosion process and calculate the sputtering yield of W irradiated with low-energy Heþ. A high-vacuum duplex chamber sputtering apparatus was used for the deposition of W coatings in this Lab. It comprised of sputtering deposition chamber and pre-vacuum chamber which are connected through a vacuum valve. One radio frequency (RF) magnetron sputtering target 80 mm in diameter with a permanent magnet was used to deposit the W coating on a silicon substrate. Working gases were introduced into the two chambers controlled by the mass flowmeters. The base pressures for the sputtering deposition chamber and pre-vacuum chamber are lower than 4 104 Pa and 1 101 Pa, respectively. The W coatings were deposited onto Si (100) single crystal samples 2.0 cm 1.0 cm 0.5 mm at room temperature in the sputtering deposition chamber. The purity of the W target was 99.99%. The Si (100) samples were cleaned in acetone and ethanol, followed by a 3% HF solution etching to remove any SiO2 from the surface. Prior to deposition, Si substrates were sputtered by RF bias for 10 min to diminish the surface pollution. Then, W coatings were deposited for 56.5 min and 49.4 min at Ar pressure of 4 101 Pa and deposition power of 60 W. The Si sample was rotated, resulting in the uniform deposition of W coatings. The thickness of two kinds deposited W coatings were 320 nm and 280 nm, respectively. After the deposition, the Si sample was cut into 1.0 1.0 cm2 for Heþ irradiation experiments. Heþ irradiation experiments have been performed by using our material irradiation experiment system (MIES) [10]. Briefly, the MIES consists of a RF plasma source, vacuum chamber, substrate holder, and laser heating system. The RF plasma source was used to generate a Heþ beam. The flux was 1020 Heþ/m2,s, which was comparable with the ITER first wall condition. The energy (E) of Heþ bombarding W specimen is adjustable when W specimen placed on a sample holder is negatively biased in the range of 0 to 280 V. Thus, E is changed from 20 to 300 eV when taking into account the plasma potential of 20 V. When W specimen is floating, the energy of Heþ may be expressed as [11].
mi 2pme
1 2 =
E ¼ Te ln
(1)
where Te is electron temperature. mi and me are the mass of ions and electrons, respectively. Our Langmuir probe measurement shows Te ¼ 5 eV. The detailed irradiation conditions are listed in
165
Table 1. A variable-power semiconductor laser is used to heat the backside of W specimen during Heþ irradiation. Heþ irradiations were performed at the Heþ fluence of 1.0 1024 ions/m2. The surface temperature (T) of W specimens measured with an infrared STL-150B pyrometer is adjustable in the range of 300e973 K (see Table 1). Cross-sectional observations of all W specimens were performed by scanning electron microscopy (SEM, Hitachi S-4800). The thickness of W coatings prior to Heþ irradiation is compared to the one of irradiated W coatings. The sputtering yield of W coatings is calculated as the functions of E and T. Previously, CAFM (Veeco DI 3100) has been used to detect the nanometer-sized defects of Heþ-irradiated hydrocarbon films [12], single-crystalline 6HeSiC [13] and polycrystalline W materials [14]. The CAFM technique is nondestructive, and it does not make any damage to the irradiated materials. In the CAFM method, one laser system is used to keep the constant deflection of the PtIr-coated tip in contact with the measured specimen. From CAFM measurements, one can simultaneously obtain the surface topography and current emission images of irradiated materials. To simultaneously obtain the current image of irradiated W materials, a constant voltage (Vtip) is applied between the PtIr-coated tip and the W specimen. The current measurement is very sensitive for detecting nanometer-sized defects at a depth of <30e50 nm from the surface in irradiated materials [14]. All CAFM measurements were performed at Vtip ¼ 2 mV. Tapping-mode AFM is also used to obtain the surface topography of irradiated W specimens, and the surface root-mean-square (RMS) roughness values are derived from the tapping-mode AFM images. In the tapping mode, a silicon tip with its diameter of 2e5 nm is scanned across the surface, which is very sensitive to a change in surface topography. 3. Results The cross-sectional observations by SEM show the dependence of W coating thickness on E in the range of 18e300 eV (Fig. 1). Heþ irradiations were performed at T ¼ 673 K. Prior to irradiation, the thickness of W coatings is 320 nm (Fig. 1(a)). Fig. 1(a) shows that W coating consists of columnar microstructures, and the columnar growth of W coating occurs during deposition. Our XRD analysis shows the predominant (110) peak for the W coating. The coating thickness shows a little dependence on E ranging from 18 to 100 eV (Fig. 1(b)-(d)). However, the thickness of W coatings obviously decreases when E varies from 150 to 300 eV (Fig. 1(e)-(h)). The sputtering yields of W coatings have been obtained as a function of E, as shown in Fig. 2. The sputtering yield remains almost constant (<1 103 W/Heþ) when E is in the range of 18e100 eV. However, the sputtering yield rapidly increases by one order of magnitude
Table 1 Detailed irradiation conditions for W film specimens in MIES. Specimen
Bias (V)
Heþ energy (eV)
W film surface temperature (T) (K)
Heþ fluence (ions/m2)
11 12 13 14 15 16 17 21 22 23 24 25
Floating 30 80 130 180 230 280 280 280 280 280 280
18 50 100 150 200 250 300 300 300 300 300 300
673 673 673 673 673 673 673 293 373 573 773 973
1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0
1024 1024 1024 1024 1024 1024 1024 1024 1024 1024 1024 1024
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Fig. 1. Cross-sectional views of W coatings irradiated at Heþ energy ranging from 18 to 300 eV. Heþ irradiations were performed at the Heþ fluence of 1.0 1024/m2 and W temperature of 673 K.
Fig. 2. Sputtering yields and RMS roughness of irradiated W coatings as a function of Heþ energy. Heþ irradiations were performed at the fluence of 1.0 1024/m2 and W temperature of 673 K.
from <103 to ~102 W/Heþ when E varies from 100 to 150 eV. This indicates that the strong surface sputtering is formed when E is obviously larger than the W threshold energy of 105e110 eV [7]. Then, the sputtering yield slowly increases from 1.0 102 to 1.3 102 W/Heþ when E varies from 150 to 300 eV. The MD simulation predicts that the sputtering yield of W crystal at 300 eV is about 5 103 W/Heþ [8,15]. Our experimental measurement is in reasonable agreement with the simulated data in Refs. [8,15]. We select 5 points on the films randomly, and measure the thickness point by point. The average values of these 5 points are considered as the center point of thickness. The Sputtering yield values are calculated according to the average thickness change of the film after irradiation. Error bars symbols stand for the sputtering yield range of these five points. In a similar way, the RMS roughness of W coating is measured in tapping mode AFM. we select 5 zones on the surface of the W film randomly to carry out AFM experiments. The average RMS roughness is obtained from these measurements. Error bars symbols stand for the difference between RMS roughness values of these five locations. RMS values of W coatings are plotted as a function of E, as shown in Fig. 2. Prior to Heþ irradiation, the RMS value of W coatings is 3.1 nm. When E varies in the range from 18 to 100 eV, the RMS value remains almost constant (2.2e2.6 nm). However, the RMS value significantly increases from 1.4 to 6.2 nm when E increases from 150 to 300 eV, indicating that Heþ sputtering significantly alters the surface microstructures of W coatings.
Fig. 3 shows the surface topography (left) and the simultaneously measured current images (right) of W coatings nonirradiated (a), and irradiated at E ¼ (b) 18 eV, (c) 50 eV, (d) 100 eV, (e) 150 eV, and (f) 300 eV. Prior to Heþ irradiation, the surface topography of W coating shows the well consistence with its current image (Fig. 3(a)), indicating that the columnar microstructures which are dense contribute to the electron emission through W coatings. Borders between columnar microstructures are relatively insulating. However, after W coatings were irradiated at E ¼ 18 or 50 eV (Fig. 3(b)-(c)), the edges of microstructures became more conductive due to He trapping. Vacancies and defects occupied by He atoms exist at the edges of microstructures. The surface topography showed the consistence with the current image when W coating was irradiated at E ¼ 100 eV. The nanometer-sized protuberances which are conductive can be formed due to the surface diffusion and coalescence of sputtered W atoms. When W coating was irradiated at E ¼ 150 eV, plenty of nanometer-sized defects were observed from the current image (Fig. 3(e)). The size and density of these defects are quite similar to the ones of defects in W crystal, as reported previously [10]. Both the surface roughness of W coating and sizes of defects were greatly improved due to Heþ irradiation at E ¼ 300 eV (Fig. 3(f)). With increasing E, more He atoms penetrate into the sub-surface layer, and they get trapped into defects, resulting in an increase in the size of defects and surface swelling of W coating. An improvement in the roughness can be due to the surface swelling of W coating. Fig. 4 shows the cross-sectional views of W coatings irradiated at the W surface temperature ranging from 293 to 973 K. Heþ irradiations were performed at E ¼ 300 eV. Prior to irradiation, the thickness of W coatings is 280 nm (Fig. 4(a)). W coatings irradiated with Heþ become thinner and thinner with increasing T from 293 to 973 K (Fig. 4(b)-(f)). The dependence of W sputtering yields on T is shown in Fig. 5. The sputtering yield rapidly increases from 4.06 103 to 1.44 102 W/Heþ when T increases from 293 to 973 K. This shows that an increase in W temperature greatly contributes to W sputtering during Heþ irradiation. The surface roughness is strongly dependent on W surface temperature (Fig. 5). The RMS value slightly decreases from 2.6 to 1.5 nm with increasing T from 293 to 373 K. However, the RMS value rapidly increases from 1.8 to 8.2 nm when T increases from 573 to 773 K. Then, the RMS value turns to decrease to 2.4 nm at T ¼ 973 K. This indicates that W sputtering process can be significantly influenced by surface temperature. Fig. 6 shows the typical surface topography (left) and current images (right) of W coatings
Y. Wu et al. / Journal of Nuclear Materials 470 (2016) 164e169
167
Fig. 3. The surface topography (left) and the simultaneously measured current images (right) of W coatings non-irradiated (a), and irradiated at Heþ energies of (b) 18 eV, (c) 50 eV, (d) 100 eV, (e) 150 eV, and (f) 300 eV. Heþ irradiations were performed at the fluence of 1.0 1024/m2 and W temperature of 673 K.
Fig. 4. Cross-sectional views of W coatings irradiated at W surface temperature of (b) 293 K, (c) 373 K, (d) 573 K, (e) 773 K, and (f) 973 K. Heþ irradiations were performed at the fluence of 1.0 1024/m2 and the energy of 300 eV.
observed from each current image. Both the size and distribution of defects show the dependence on T. The size of defects obviously increases when T varies from 373 to 773 K. However, their size is greatly reduced at T ¼ 973 K. No direct correlation between the surface topography and the distribution of defects can be observed from CAFM measurements.
4. Discussion
Fig. 5. Sputtering yields and RMS roughness of irradiated W coatings as a function of W surface temperature. Heþ irradiations were performed at the fluence of 1.0 1024/ m2 and the energy of 300 eV.
irradiated at T ¼ (a) 373 K, (b) 573 K, (c) 773 K, and (d) 973 K. Nanometer-sized defects which are conductive can be clearly
This study shows that both Heþ energy and temperature strongly influence the sputtering yield of W coatings. Two basic physical processes are formed during Heþ irradiation: (1) Heþ implantation into W, and thermal diffusion of He atoms inside the W surface layer, and (2) surface W sputtering due to the energetic bombardments of Heþ and W atoms escape from the surface. After Heþ bombard W surface and alter surface microstructures, He atoms penetrate into the sub-surface layer of W coating. They prefer to occupy the vacancies and form nanometer-sized defects in the sub-surface layer. Our CAFM measurements confirmed that when E varies from 18 to 50 eV, defects were formed in the edges of
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the sub-surface layer. It is easier for He atoms in W to enter into defects than escape from them [18]. Due to the thermal activation process, W atoms are easily sputtered at an elevated temperature, and sputtered W atoms escape from the surface. Thus, serious etching of W coatings is formed at an elevated temperature. Our measurements show that the W sputtering yield rapidly increases from 4.06 103 to 1.44 102 W/Heþ with increasing T from 293 to 973 K. An increase in W samples temperature can contribute to the thermal diffusion of He atoms in the sub-surface layer. The thermal diffusion of He atoms in the sub-surface layer can lead to the rapid growth of nanometer-sized defects, thus certain surface etching. The surface properties of irradiated W coatings, such as roughness, surface etching, and the distribution of defects are significantly influenced by W surface temperature. However, the effects of W surface temperature on surface roughness and the distribution of defects need to be further studied. 5. Conclusions
Fig. 6. The typical surface topography (left) and current images (right) of W coatings irradiated at W surface temperature of (a) 373 K, (b) 573 K, (c) 773 K, and (d) 973 K. Heþ irradiations were performed at the fluence of 1.0 1024/m2 and the energy of 300 eV.
columnar microstructures. An increase in the internal pressure of defects results in the mutation of crystal lattice. The formation of conductive defects can be due to a high density of W atoms at their edges [14]. Increasing E leads to W sputtering at the surface, and sputtered W atoms may escape from the surface, and deposit on the wall of vacuum chamber. At a relatively low Heþ energy, no obvious sputtering occurs, and the thermal diffusion of He atoms inside the W surface layer plays a dominant role in forming defects. Our measurements show that the sputtering yield increases by about one order of magnitude when E varies from 18 to 100 eV to 150 eV. Serious surface etching occurs at E 150 eV. This indicates that in ITER, decreasing E below 100 eV is important for avoiding the strong etching and general destruction of W component [7]. When E increases from 150 to 300 eV, plenty of defects were observed from the current images. The size of defects is also improved when E increases from 150 to 300 eV. An increase in Heþ energy indicates that more He atoms penetrate into the sub-surface layer, which contributes to the growth of nanometer-sized defects in W. The internal stress of nanometer-sized defects in the subsurface layer can be greatly improved due to the coalescence and swelling of He atoms. The W atoms at surface are weakly bonded at an evaluated surface temperature, which made it easy to escape from the samples. W atoms in defects are relatively easily sputtered during Heþ irradiation, and the inner stress of defects is rapidly released after sputtering. After the W coating was irradiated at E ¼ 300 eV, the growth of nanometer-sized defects in the subsurface layer results in the surface swelling, as shown in Fig. 3(f). Both the thermal diffusion of He atoms inside the W surface layer and W sputtering process are strongly dependent on W surface temperature. At an elevated W temperature, He atoms bombarding the surface prefer to be trapped into the vacancy, or interstitial and substitutional sites in the grain boundaries [16,17]. Many He atoms diffusing in W become nanometer-sized defects in
Both the sputtering yield and surface roughness of W coatings are strongly dependent on the Heþ energy ranging from 18 to 300 eV. When E varies from 100 eV to 150 eV, the sputtering yield increases by about one order of magnitude, indicating that in ITER, decreasing Heþ energy below 100 eV is very important for avoiding the strong etching and general destruction of W component. The sputtering yield also shows the clear dependence on W surface temperature ranging from 293 to 973 K. CAFM measurements show that plenty of nanometer-sized defects are formed at E 150 eV. This study indicates that the surface properties of irradiated W coatings, such as surface etching, roughness, and the distribution of defects can be strongly influenced by W surface sputtering, surface coalescence of sputtered W atoms, and thermal diffusion of He atoms in the sub-surface layer. Acknowledgments Grants from the National Magnetic Confinement Fusion Program (Key project No. 2011GB108011) and National Science Foundation of China (NSF-11175038, NSF-11405023 and NSF-10875025) and Fundamental Research Funds for the Central Universities (DC201501069) are greatly appreciated. References [1] T. Tanabe, J. Nucl. Fusion (5 (Suppl.)) (1994) S129eS148. [2] H. Iwakiri, K. Yasunaga, K. Morishita, N. Yoshida, J. Nucl. Mater. 283e287 (2000) 1134e1138. [3] W.M. Shu, E. Wakai, T. Yamanishi, Nucl. Fusion 47 (2007) 201e209. [4] R.A. Anderl, R.A. Causey, J.W. Davis, R.P. Doerner, G. Federici, A.A. Haasz, G.R. Longhurst, W.R. Wampler, K.L. Wilson, J. Nucl. Mater. 273 (1999) 1e26. [5] S. Kajita, N. Yoshida, R. Youshihara, N. Ohno, M. Yamagiwa, J. Nucl. Mater. 418 (2011) 152e158. [6] S. Kajita, W. Sakaguchi, N. Ohno, N. Yoshida, T. Saeki, Nucl. Fusion 49 (2009) 095005e095006. [7] G. Federici, C.H. Skinner, J.N. Brooks, J.P. Coad, C. Grisolia, A.A. Haasz, A. Hassanein, V. Philipps, C.S. Pitcher, J. Roth, W.R. Wampler, D.G. Whyte, Nucl. Fusion 41 (12R) (2001) 1967e2136. [8] F. Sefta, N. Juslin, K.D. Hammond, B.D. Wirth, J. Nucl. Mater. 438 (2013) S493eS496. [9] F. Ferroni, K.D. Hammond, B.D. Wirth, J. Nucl. Mater. 458 (2015) 419e424. [10] Q. Yang, D. Liu, H. Fan, X. Li, J. Niu, Y. Wang, Nucl. Instrum. Methods Phys. Res. B 325 (2014) 73e78. [11] Michael A. Lieberman, Allan J. Lichtenberg, Principles of Plasma Discharges and Materials Processing, second ed., John Wiley & Sons, Inc., Hoboken, New Jersey, 2005. [12] H. Fan, D. Yang, L. Sun, Q. Yang, J. Niu, Z. Bi, D. Liu, Nucl. Instrum. Methods Phys. Res. B 312 (2013) 90e96. [13] H. Fan, R. Li, D. Yang, Y. Wu, J. Niu, Q. Yang, J. Zhao, D. Liu, J. Nucl. Mater. 441 (2013) 54e58. [14] Q. Yang, H. Fan, W. Ni, L. Liu, T. Berthold, G. Benstetter, D. Liu, Y. Wang, Acta Mater. 92 (2015) 178e188.
Y. Wu et al. / Journal of Nuclear Materials 470 (2016) 164e169 [15] ITER, 2001 Atomic and Plasma-material Interaction Data for Fusion, 7 IAEA, Vienna, 2001. [16] N. Yoshida, H. Iwakiri, K. Tokunaga, T. Baba, J. Nucl. Mater. 337e339 (2005) 946e950.
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[17] H.B. Zhou, Y.L. Liu, Y. Zhang, S. Jin, G.H. Lu, Nucl. Instrum. Methods Phys. Res. B 267 (2009) 3189e3192. [18] D. Nishijima, M.Y. Ye, N. Ohno, S. Takamura, J. Nucl. Mater. 313e316 (2003) 97e101.
Contents lists available at ScienceDirect
Journal of Nuclear Materials journal homepage: www.elsevier.com/locate/jnucmat
Effects of Heþ energy and irradiation temperature on W sputtering yields under fusion-relevant conditions Yunfeng Wu, Weiyuan Ni, Hongyu Fan, Lu Liu, Qi Yang, Baosheng Cao, Dongping Liu* School of Physics and Materials Engineering, Dalian Nationalities University, Dalian 116600, People's Republic of China
h i g h l i g h t s The sputtering yields and roughness of W films irradiated with Heþ at different energy and temperatures have been measured. The sputtering yield increases by about one order of magnitude when E varies from 18e100 eV to 150 eV. The W sputtering yield rapidly increases from 4.06 103 to 1.44 102 W/Heþ with increasing T from 293 to 973 K. CAFM measurements show that plenty of nanometer-sized defects are formed at E 150 eV.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 6 August 2015 Received in revised form 17 December 2015 Accepted 18 December 2015 Available online 24 December 2015
The sputtering yields of W coatings irradiated with Heþ at different irradiation temperatures under fusion-relevant conditions have been measured as the function of Heþ energy ranging from 18 to 300 eV. The sputtering yield of W coatings irradiated at temperature of 673 K increases by one order of magnitude from ~103 to ~102 W/Heþ when Heþ energy varies from 100 eV to 150 eV, indicating the existence of W sputtering threshold. The sputtering yield of W coatings irradiated at Heþ energy of 300 eV rapidly increases from 4.56 103 to 1.44 102 W/Heþ when the temperature increases from 293 to 973 K. Measurements by conductive atomic force microscopy show that plenty of nanometersized defects are formed after Heþ irradiation at a relatively high Heþ energy. The serious etching of W coatings at Heþ energies of 150 eV can be also related to He trapping into defects in the sub-surface layer and their growth, leading to an increase in the thermal instability of W. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Tungsten as one of the best candidates for plasma facing materials in fusion reactors will be subject to the strong bombardments by low-energy Heþ/Hþ [1e3]. The flux and energy of Heþ/Hþ species bombarding the first wall of ITER were estimated as 1020e1022 particles/m2,s and <500 eV, respectively [4]. The bombardments of W by the high-flux and low-energy Heþ/Hþ can result in the serious surface etching. The surface etching of W materials is the major concern because it may cause general destruction of the components and significantly affect the stability of fusion plasmas [5,6]. However, there have few reports about the sputtering yield and
* Corresponding author. E-mail address: [email protected] (D. Liu). http://dx.doi.org/10.1016/j.jnucmat.2015.12.022 0022-3115/© 2015 Elsevier B.V. All rights reserved.
erosion process of W materials irradiated with low-energy Heþ/Hþ under fusion-relevant conditions [7e9]. The sputtered threshold energy of tungsten by Heþ is approximately 105e110 eV, which is typically in the range of expected energies of particles in the baffle region of the ITER divertor [7]. Molecular dynamics simulations have been performed to predict the sputtering behavior of W due to Heþ bombardment when the Heþ energy is varied in the range of 300 eVe1 keV [8,9]. The calculated sputtering yields are in reasonable agreement with experimental data. The sputtering yields were found to depend on surface orientation and incident Heþ energy. However, the dependence of W sputtering yield on W temperature and Heþ energy below 300 eV needs to be investigated. The purpose of this work is to explore the erosion process of W materials irradiated at the Heþ energy close to the W sputtering threshold. How will Heþ energy and irradiation temperature affect the sputtering yield of W materials under fusion-relevant plasma
Y. Wu et al. / Journal of Nuclear Materials 470 (2016) 164e169
conditions? 2. Experimental procedures Ultra-smooth W coatings deposited on a silicon substrate were used to characterize the erosion process and calculate the sputtering yield of W irradiated with low-energy Heþ. A high-vacuum duplex chamber sputtering apparatus was used for the deposition of W coatings in this Lab. It comprised of sputtering deposition chamber and pre-vacuum chamber which are connected through a vacuum valve. One radio frequency (RF) magnetron sputtering target 80 mm in diameter with a permanent magnet was used to deposit the W coating on a silicon substrate. Working gases were introduced into the two chambers controlled by the mass flowmeters. The base pressures for the sputtering deposition chamber and pre-vacuum chamber are lower than 4 104 Pa and 1 101 Pa, respectively. The W coatings were deposited onto Si (100) single crystal samples 2.0 cm 1.0 cm 0.5 mm at room temperature in the sputtering deposition chamber. The purity of the W target was 99.99%. The Si (100) samples were cleaned in acetone and ethanol, followed by a 3% HF solution etching to remove any SiO2 from the surface. Prior to deposition, Si substrates were sputtered by RF bias for 10 min to diminish the surface pollution. Then, W coatings were deposited for 56.5 min and 49.4 min at Ar pressure of 4 101 Pa and deposition power of 60 W. The Si sample was rotated, resulting in the uniform deposition of W coatings. The thickness of two kinds deposited W coatings were 320 nm and 280 nm, respectively. After the deposition, the Si sample was cut into 1.0 1.0 cm2 for Heþ irradiation experiments. Heþ irradiation experiments have been performed by using our material irradiation experiment system (MIES) [10]. Briefly, the MIES consists of a RF plasma source, vacuum chamber, substrate holder, and laser heating system. The RF plasma source was used to generate a Heþ beam. The flux was 1020 Heþ/m2,s, which was comparable with the ITER first wall condition. The energy (E) of Heþ bombarding W specimen is adjustable when W specimen placed on a sample holder is negatively biased in the range of 0 to 280 V. Thus, E is changed from 20 to 300 eV when taking into account the plasma potential of 20 V. When W specimen is floating, the energy of Heþ may be expressed as [11].
mi 2pme
1 2 =
E ¼ Te ln
(1)
where Te is electron temperature. mi and me are the mass of ions and electrons, respectively. Our Langmuir probe measurement shows Te ¼ 5 eV. The detailed irradiation conditions are listed in
165
Table 1. A variable-power semiconductor laser is used to heat the backside of W specimen during Heþ irradiation. Heþ irradiations were performed at the Heþ fluence of 1.0 1024 ions/m2. The surface temperature (T) of W specimens measured with an infrared STL-150B pyrometer is adjustable in the range of 300e973 K (see Table 1). Cross-sectional observations of all W specimens were performed by scanning electron microscopy (SEM, Hitachi S-4800). The thickness of W coatings prior to Heþ irradiation is compared to the one of irradiated W coatings. The sputtering yield of W coatings is calculated as the functions of E and T. Previously, CAFM (Veeco DI 3100) has been used to detect the nanometer-sized defects of Heþ-irradiated hydrocarbon films [12], single-crystalline 6HeSiC [13] and polycrystalline W materials [14]. The CAFM technique is nondestructive, and it does not make any damage to the irradiated materials. In the CAFM method, one laser system is used to keep the constant deflection of the PtIr-coated tip in contact with the measured specimen. From CAFM measurements, one can simultaneously obtain the surface topography and current emission images of irradiated materials. To simultaneously obtain the current image of irradiated W materials, a constant voltage (Vtip) is applied between the PtIr-coated tip and the W specimen. The current measurement is very sensitive for detecting nanometer-sized defects at a depth of <30e50 nm from the surface in irradiated materials [14]. All CAFM measurements were performed at Vtip ¼ 2 mV. Tapping-mode AFM is also used to obtain the surface topography of irradiated W specimens, and the surface root-mean-square (RMS) roughness values are derived from the tapping-mode AFM images. In the tapping mode, a silicon tip with its diameter of 2e5 nm is scanned across the surface, which is very sensitive to a change in surface topography. 3. Results The cross-sectional observations by SEM show the dependence of W coating thickness on E in the range of 18e300 eV (Fig. 1). Heþ irradiations were performed at T ¼ 673 K. Prior to irradiation, the thickness of W coatings is 320 nm (Fig. 1(a)). Fig. 1(a) shows that W coating consists of columnar microstructures, and the columnar growth of W coating occurs during deposition. Our XRD analysis shows the predominant (110) peak for the W coating. The coating thickness shows a little dependence on E ranging from 18 to 100 eV (Fig. 1(b)-(d)). However, the thickness of W coatings obviously decreases when E varies from 150 to 300 eV (Fig. 1(e)-(h)). The sputtering yields of W coatings have been obtained as a function of E, as shown in Fig. 2. The sputtering yield remains almost constant (<1 103 W/Heþ) when E is in the range of 18e100 eV. However, the sputtering yield rapidly increases by one order of magnitude
Table 1 Detailed irradiation conditions for W film specimens in MIES. Specimen
Bias (V)
Heþ energy (eV)
W film surface temperature (T) (K)
Heþ fluence (ions/m2)
11 12 13 14 15 16 17 21 22 23 24 25
Floating 30 80 130 180 230 280 280 280 280 280 280
18 50 100 150 200 250 300 300 300 300 300 300
673 673 673 673 673 673 673 293 373 573 773 973
1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0
1024 1024 1024 1024 1024 1024 1024 1024 1024 1024 1024 1024
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Fig. 1. Cross-sectional views of W coatings irradiated at Heþ energy ranging from 18 to 300 eV. Heþ irradiations were performed at the Heþ fluence of 1.0 1024/m2 and W temperature of 673 K.
Fig. 2. Sputtering yields and RMS roughness of irradiated W coatings as a function of Heþ energy. Heþ irradiations were performed at the fluence of 1.0 1024/m2 and W temperature of 673 K.
from <103 to ~102 W/Heþ when E varies from 100 to 150 eV. This indicates that the strong surface sputtering is formed when E is obviously larger than the W threshold energy of 105e110 eV [7]. Then, the sputtering yield slowly increases from 1.0 102 to 1.3 102 W/Heþ when E varies from 150 to 300 eV. The MD simulation predicts that the sputtering yield of W crystal at 300 eV is about 5 103 W/Heþ [8,15]. Our experimental measurement is in reasonable agreement with the simulated data in Refs. [8,15]. We select 5 points on the films randomly, and measure the thickness point by point. The average values of these 5 points are considered as the center point of thickness. The Sputtering yield values are calculated according to the average thickness change of the film after irradiation. Error bars symbols stand for the sputtering yield range of these five points. In a similar way, the RMS roughness of W coating is measured in tapping mode AFM. we select 5 zones on the surface of the W film randomly to carry out AFM experiments. The average RMS roughness is obtained from these measurements. Error bars symbols stand for the difference between RMS roughness values of these five locations. RMS values of W coatings are plotted as a function of E, as shown in Fig. 2. Prior to Heþ irradiation, the RMS value of W coatings is 3.1 nm. When E varies in the range from 18 to 100 eV, the RMS value remains almost constant (2.2e2.6 nm). However, the RMS value significantly increases from 1.4 to 6.2 nm when E increases from 150 to 300 eV, indicating that Heþ sputtering significantly alters the surface microstructures of W coatings.
Fig. 3 shows the surface topography (left) and the simultaneously measured current images (right) of W coatings nonirradiated (a), and irradiated at E ¼ (b) 18 eV, (c) 50 eV, (d) 100 eV, (e) 150 eV, and (f) 300 eV. Prior to Heþ irradiation, the surface topography of W coating shows the well consistence with its current image (Fig. 3(a)), indicating that the columnar microstructures which are dense contribute to the electron emission through W coatings. Borders between columnar microstructures are relatively insulating. However, after W coatings were irradiated at E ¼ 18 or 50 eV (Fig. 3(b)-(c)), the edges of microstructures became more conductive due to He trapping. Vacancies and defects occupied by He atoms exist at the edges of microstructures. The surface topography showed the consistence with the current image when W coating was irradiated at E ¼ 100 eV. The nanometer-sized protuberances which are conductive can be formed due to the surface diffusion and coalescence of sputtered W atoms. When W coating was irradiated at E ¼ 150 eV, plenty of nanometer-sized defects were observed from the current image (Fig. 3(e)). The size and density of these defects are quite similar to the ones of defects in W crystal, as reported previously [10]. Both the surface roughness of W coating and sizes of defects were greatly improved due to Heþ irradiation at E ¼ 300 eV (Fig. 3(f)). With increasing E, more He atoms penetrate into the sub-surface layer, and they get trapped into defects, resulting in an increase in the size of defects and surface swelling of W coating. An improvement in the roughness can be due to the surface swelling of W coating. Fig. 4 shows the cross-sectional views of W coatings irradiated at the W surface temperature ranging from 293 to 973 K. Heþ irradiations were performed at E ¼ 300 eV. Prior to irradiation, the thickness of W coatings is 280 nm (Fig. 4(a)). W coatings irradiated with Heþ become thinner and thinner with increasing T from 293 to 973 K (Fig. 4(b)-(f)). The dependence of W sputtering yields on T is shown in Fig. 5. The sputtering yield rapidly increases from 4.06 103 to 1.44 102 W/Heþ when T increases from 293 to 973 K. This shows that an increase in W temperature greatly contributes to W sputtering during Heþ irradiation. The surface roughness is strongly dependent on W surface temperature (Fig. 5). The RMS value slightly decreases from 2.6 to 1.5 nm with increasing T from 293 to 373 K. However, the RMS value rapidly increases from 1.8 to 8.2 nm when T increases from 573 to 773 K. Then, the RMS value turns to decrease to 2.4 nm at T ¼ 973 K. This indicates that W sputtering process can be significantly influenced by surface temperature. Fig. 6 shows the typical surface topography (left) and current images (right) of W coatings
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Fig. 3. The surface topography (left) and the simultaneously measured current images (right) of W coatings non-irradiated (a), and irradiated at Heþ energies of (b) 18 eV, (c) 50 eV, (d) 100 eV, (e) 150 eV, and (f) 300 eV. Heþ irradiations were performed at the fluence of 1.0 1024/m2 and W temperature of 673 K.
Fig. 4. Cross-sectional views of W coatings irradiated at W surface temperature of (b) 293 K, (c) 373 K, (d) 573 K, (e) 773 K, and (f) 973 K. Heþ irradiations were performed at the fluence of 1.0 1024/m2 and the energy of 300 eV.
observed from each current image. Both the size and distribution of defects show the dependence on T. The size of defects obviously increases when T varies from 373 to 773 K. However, their size is greatly reduced at T ¼ 973 K. No direct correlation between the surface topography and the distribution of defects can be observed from CAFM measurements.
4. Discussion
Fig. 5. Sputtering yields and RMS roughness of irradiated W coatings as a function of W surface temperature. Heþ irradiations were performed at the fluence of 1.0 1024/ m2 and the energy of 300 eV.
irradiated at T ¼ (a) 373 K, (b) 573 K, (c) 773 K, and (d) 973 K. Nanometer-sized defects which are conductive can be clearly
This study shows that both Heþ energy and temperature strongly influence the sputtering yield of W coatings. Two basic physical processes are formed during Heþ irradiation: (1) Heþ implantation into W, and thermal diffusion of He atoms inside the W surface layer, and (2) surface W sputtering due to the energetic bombardments of Heþ and W atoms escape from the surface. After Heþ bombard W surface and alter surface microstructures, He atoms penetrate into the sub-surface layer of W coating. They prefer to occupy the vacancies and form nanometer-sized defects in the sub-surface layer. Our CAFM measurements confirmed that when E varies from 18 to 50 eV, defects were formed in the edges of
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the sub-surface layer. It is easier for He atoms in W to enter into defects than escape from them [18]. Due to the thermal activation process, W atoms are easily sputtered at an elevated temperature, and sputtered W atoms escape from the surface. Thus, serious etching of W coatings is formed at an elevated temperature. Our measurements show that the W sputtering yield rapidly increases from 4.06 103 to 1.44 102 W/Heþ with increasing T from 293 to 973 K. An increase in W samples temperature can contribute to the thermal diffusion of He atoms in the sub-surface layer. The thermal diffusion of He atoms in the sub-surface layer can lead to the rapid growth of nanometer-sized defects, thus certain surface etching. The surface properties of irradiated W coatings, such as roughness, surface etching, and the distribution of defects are significantly influenced by W surface temperature. However, the effects of W surface temperature on surface roughness and the distribution of defects need to be further studied. 5. Conclusions
Fig. 6. The typical surface topography (left) and current images (right) of W coatings irradiated at W surface temperature of (a) 373 K, (b) 573 K, (c) 773 K, and (d) 973 K. Heþ irradiations were performed at the fluence of 1.0 1024/m2 and the energy of 300 eV.
columnar microstructures. An increase in the internal pressure of defects results in the mutation of crystal lattice. The formation of conductive defects can be due to a high density of W atoms at their edges [14]. Increasing E leads to W sputtering at the surface, and sputtered W atoms may escape from the surface, and deposit on the wall of vacuum chamber. At a relatively low Heþ energy, no obvious sputtering occurs, and the thermal diffusion of He atoms inside the W surface layer plays a dominant role in forming defects. Our measurements show that the sputtering yield increases by about one order of magnitude when E varies from 18 to 100 eV to 150 eV. Serious surface etching occurs at E 150 eV. This indicates that in ITER, decreasing E below 100 eV is important for avoiding the strong etching and general destruction of W component [7]. When E increases from 150 to 300 eV, plenty of defects were observed from the current images. The size of defects is also improved when E increases from 150 to 300 eV. An increase in Heþ energy indicates that more He atoms penetrate into the sub-surface layer, which contributes to the growth of nanometer-sized defects in W. The internal stress of nanometer-sized defects in the subsurface layer can be greatly improved due to the coalescence and swelling of He atoms. The W atoms at surface are weakly bonded at an evaluated surface temperature, which made it easy to escape from the samples. W atoms in defects are relatively easily sputtered during Heþ irradiation, and the inner stress of defects is rapidly released after sputtering. After the W coating was irradiated at E ¼ 300 eV, the growth of nanometer-sized defects in the subsurface layer results in the surface swelling, as shown in Fig. 3(f). Both the thermal diffusion of He atoms inside the W surface layer and W sputtering process are strongly dependent on W surface temperature. At an elevated W temperature, He atoms bombarding the surface prefer to be trapped into the vacancy, or interstitial and substitutional sites in the grain boundaries [16,17]. Many He atoms diffusing in W become nanometer-sized defects in
Both the sputtering yield and surface roughness of W coatings are strongly dependent on the Heþ energy ranging from 18 to 300 eV. When E varies from 100 eV to 150 eV, the sputtering yield increases by about one order of magnitude, indicating that in ITER, decreasing Heþ energy below 100 eV is very important for avoiding the strong etching and general destruction of W component. The sputtering yield also shows the clear dependence on W surface temperature ranging from 293 to 973 K. CAFM measurements show that plenty of nanometer-sized defects are formed at E 150 eV. This study indicates that the surface properties of irradiated W coatings, such as surface etching, roughness, and the distribution of defects can be strongly influenced by W surface sputtering, surface coalescence of sputtered W atoms, and thermal diffusion of He atoms in the sub-surface layer. Acknowledgments Grants from the National Magnetic Confinement Fusion Program (Key project No. 2011GB108011) and National Science Foundation of China (NSF-11175038, NSF-11405023 and NSF-10875025) and Fundamental Research Funds for the Central Universities (DC201501069) are greatly appreciated. References [1] T. Tanabe, J. Nucl. Fusion (5 (Suppl.)) (1994) S129eS148. [2] H. Iwakiri, K. Yasunaga, K. Morishita, N. Yoshida, J. Nucl. Mater. 283e287 (2000) 1134e1138. [3] W.M. Shu, E. Wakai, T. Yamanishi, Nucl. Fusion 47 (2007) 201e209. [4] R.A. Anderl, R.A. Causey, J.W. Davis, R.P. Doerner, G. Federici, A.A. Haasz, G.R. Longhurst, W.R. Wampler, K.L. Wilson, J. Nucl. Mater. 273 (1999) 1e26. [5] S. Kajita, N. Yoshida, R. Youshihara, N. Ohno, M. Yamagiwa, J. Nucl. Mater. 418 (2011) 152e158. [6] S. Kajita, W. Sakaguchi, N. Ohno, N. Yoshida, T. Saeki, Nucl. Fusion 49 (2009) 095005e095006. [7] G. Federici, C.H. Skinner, J.N. Brooks, J.P. Coad, C. Grisolia, A.A. Haasz, A. Hassanein, V. Philipps, C.S. Pitcher, J. Roth, W.R. Wampler, D.G. Whyte, Nucl. Fusion 41 (12R) (2001) 1967e2136. [8] F. Sefta, N. Juslin, K.D. Hammond, B.D. Wirth, J. Nucl. Mater. 438 (2013) S493eS496. [9] F. Ferroni, K.D. Hammond, B.D. Wirth, J. Nucl. Mater. 458 (2015) 419e424. [10] Q. Yang, D. Liu, H. Fan, X. Li, J. Niu, Y. Wang, Nucl. Instrum. Methods Phys. Res. B 325 (2014) 73e78. [11] Michael A. Lieberman, Allan J. Lichtenberg, Principles of Plasma Discharges and Materials Processing, second ed., John Wiley & Sons, Inc., Hoboken, New Jersey, 2005. [12] H. Fan, D. Yang, L. Sun, Q. Yang, J. Niu, Z. Bi, D. Liu, Nucl. Instrum. Methods Phys. Res. B 312 (2013) 90e96. [13] H. Fan, R. Li, D. Yang, Y. Wu, J. Niu, Q. Yang, J. Zhao, D. Liu, J. Nucl. Mater. 441 (2013) 54e58. [14] Q. Yang, H. Fan, W. Ni, L. Liu, T. Berthold, G. Benstetter, D. Liu, Y. Wang, Acta Mater. 92 (2015) 178e188.
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