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Cu multilayers by annealing process
Materials Science & Engineering A 772 (2020) 138818
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Materials Science & Engineering A journal homepage: http://www.elsevier.com/locate/msea
The enhanced strength and electrical conductivity in Ag/Cu multilayers by annealing process J.Z. Huo a, M.Z. Wei a, *, Y.J. Ma b, Z.H. Cao b, c, **, X.K. Meng b a
Institute of Physics and Electronic Engineering, Linyi University, Shandong, PR China Institute of Materials Engineering, National Laboratory of Solid State Microstructures, College of Engineering and Applied Sciences, Nanjing University, Jiangsu, PR China c College of Materials Science and Engineering, Tech Institute for Advanced Materials, Nanjing Tech University, Nanjing, PR China b
A R T I C L E I N F O
A B S T R A C T
Keywords: Multilayer Hardness Electrical conductivity Thermal stability
In this work, the hardness, electrical resistivity and thermal stability of annealed Ag/Nb multilayers were investigated with individual layer thickness (h) ranging from 3 to 100 nm after annealing at 200–400 � C. For all scales multilayers, the hardness improves 6–16% whereas the electrical resistivity decreases 6–12% after annealing at 200 � C compared with those of as deposited state. At 300 � C, the enhancement shows layer thickness dependent due to the different interfacial structures. The hardness improves about 4–10% and the electrical resistivity decreases 11–28% for multilayers with h � 10 nm. A good combination of high strength and high conductivity has been obtained in annealed Ag/Cu multilayer with h � 10 nm when annealing temperature is not more than 300 � C. Both hardness and electrical conductivity are enhanced by annealing process. The improvement of electrical conductivity mainly comes from the reduction of grain boundaries due to the grain growth. The enhancement of hardness relates with the increment of twin boundaries, the compressive stress and the lack of dislocations after annealing process.
1. Introduction High strength conductors have attracted much attention due to the combination of strength and electrical conductivity, which enables them to be used in high-field magnets [1–4], electrode materials in large-scale integrated circuits [5], interconnect fabrication in microelectronics [6] and so on. Ag–Cu composites are extensively used due to their promi nent properties [7–10]. Recently, many studies on Ag/Cu multilayers have been performed to investigate the strengthening mechanisms and the electrical properties at the room temperature (RT) [11–15]. In fact, the serving environment of Ag/Cu materials usually involves high temperature or thermal-electrical coupled fields. The stabilities of structures and performances are the key factors to influence the usage of the multilayers [16]. It is important to investigate the properties and the structural stabilities at elevated temperatures. In general, high strength is irreconcilable with high electrical con ductivity in composite materials. The electron scattering at interfaces and grain boundaries (GBs) is the main factor reducing the conductivity
of multilayers [17–19]. However, these negative factors for conductivity happen to be the positive factors to improve the strength of the multi layers. Hence, how to coordinate the contradictions between the two aspects, is the key to obtaining high strength conductors. Lu et al. [20] reported that the numerous twins in Cu can greatly increase the strength and lead to an extremely low electrical resistivity (ρ) as well. Zhang et al. [21] reported that the high strength and high electrical conductivity have been obtained in bulk Cu–Ag alloy with nanotwins and nanograins. Valiev et al. [22] reported that high strength can be obtained in Al alloy by nano twined method. And there are many other researches also get the similar results [23,24]. They convey the information that the introduction of twin boundaries (TBs) effectively blocks the dislocation motion while has negligible effect on the electrical conductivity. As we have known, annealing process is a good method to achieve nanotwins in some metal with low stacking fault energy. However, annealing process will bring the grain growth and result in the reduction of strength. Hence, the annealing temperature must be limited in a certain range. In Ag/Cu multilayers, the twins are easily formed in Ag layers due
* Corresponding author. ** Corresponding author. Institute of Materials Engineering, National Laboratory of Solid State Microstructures, College of Engineering and Applied Sciences, Nanjing University, Jiangsu, PR China. E-mail addresses: [email protected] (M.Z. Wei), [email protected] (Z.H. Cao). https://doi.org/10.1016/j.msea.2019.138818 Received 18 August 2019; Received in revised form 11 December 2019; Accepted 11 December 2019 Available online 14 December 2019 0921-5093/© 2019 Elsevier B.V. All rights reserved.
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Materials Science & Engineering A 772 (2020) 138818
Fig. 1. The bright-field cross-sectional TEM and HRTEM images of as-deposited Ag/Cu multilayers with (a) h ¼ 50 nm, (b) h ¼ 20 nm, (c) h ¼ 10 nm and (d) h ¼ 5 nm.
to the low stacking fault energy. In annealing process, the grain growth occurs and the interface/GBs migrate independently. Meanwhile, twining and de-twining processes take place at different annealing temperatures. The migration competition of different inter faces/GBs/TBs at different temperatures will influence the strength and conductivity extensively. Furthermore, the thermal stability of multi layers shows layer thickness dependent. The situation will become complicated with the different scales at different annealing tempera tures. Hence, it is essential to explore the details on the strategy of improving the strength and electrical conductivity of Ag/Cu multilayers at elevated temperatures. In present work, we studied the hardness and the electrical resistivity of Ag/Cu multilayers at elevated temperatures. It has been found that the annealed Ag/Cu multilayers show enhanced hardness and electrical conductivity compared with the as-deposited samples. The structure evolution and the influences of these structures on the properties are discussed.
voltages. The nanoindentation tests were performed using a TI950 Hysitron Triboindenter with a displacement resolution of 0.1 nm and a loading resolution of 50 nN. A Berkovich tip with 50 nm radius of cur vature was used in all indentation experiments. The maximum inden tation depth was controlled below 15% of the multilayer thickness in order to avoid substrate effect. The electrical resistivity was measured at ambient temperature via four-point-probe technique from the sample surface. 3. Results and discussion The bright-field cross-sectional TEM and HRTEM images of asdeposited Ag/Cu multilayers with h ¼ 50, 20, 10 and 5 nm are pre sented in Fig. 1 (a) - (d), respectively. From TEM images of Fig. 1(a) and (b), we can see the grains of multilayers with h ¼ 50 and h ¼ 20 nm are columnar grains with the average width scaling with h. The interfaces are sharp and flat. The twins exist both in Ag and Cu grains. In partic ular, some twins grow successively along the growth direction, con necting the Ag and Cu layers of the multilayer. The insets in Fig. 1(a) and (b) show the HRTEM morphologies of interface and GBs of multilayers with h ¼ 50 and 20 nm, respectively. We can see the lattice fringes continuing from Ag grain to Cu grain in HRTEM image in Fig. 1(b), indicating the orientation relationship between Ag and Cu grains. While in HRTEM image in Fig. 1(a) there is no orientation relationship be tween the grains. Fig. 1(c) presents the HRTEM and TEM images of multilayer with h ¼ 10 nm. From the TEM image (the inset at the left bottom corner of Fig. 1(c)), the width of columnar grains is much larger than the layer thickness and a great number of stacking faults (SFs) are observed. The HRTEM image shows the strong textures both in Ag and Cu layers. The fringes across the interface in TEM image are turn out to be SFs by the HRTEM image. The Inverse Fast Fourier Transformation (IFFT) (inset at the bottom right corner of Fig. 1(c)) shows the interface
2. Experimental details Nanostructured Ag/Cu multilayers with equal individual layer thickness ranging from 3 to 100 nm were prepared by dc magnetron sputtering on Si substrates at room temperature. Base pressure for the sputtering chamber was kept at less than 1.2 � 10 4 Pa and the sput tering pressure was 1.2 Pa. A Ta barrier layer with 5 nm is first deposited on Si substrate before the preparation of multilayers. The Cu and Ag were deposited alternately on the substrate and the top layer was Ag. Total thickness of the Ag/Cu multilayers was 1000 nm. After deposition, the multilayers were annealed at 200 � C, 300 � C and 400 � C for 30 min in an annealing furnace with high vacuum, respectively. The cross-sectional microstructures were characterized by JEM-2100 transmission electron microscope (TEM) with 200 kV accelerating 2
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Materials Science & Engineering A 772 (2020) 138818
The electrical resistivity data of the multilayers with h ranging from 3 to 100 nm at different annealing temperatures of 200, 300 and 400 � C are shown in Fig. 3. It can be seen that the resistivities of all scales decrease with the increasing annealing temperature. Corresponding to the hardness variation, the tendency of resistivity decreasing varies with the layer thickness at the different annealing temperatures. At 200 � C, the resistivity values of multilayers at all scales decrease about 6–12% (the electrical conductivity improves 6–14%) compared with the as deposited samples. At 300 � C, the resistivity values decrease about 11–28% (the electrical conductivity improves 12–38%) for multilayers with h � 10 nm, and the resistivity of multilayers with h ¼ 3 and 5 nm drop sharply, suggesting a crucial change of the microstructures. When the annealing temperature is up to 400 � C, the resistivities of multilayers at all scales are close to that of pure Ag thin film and much lower than those at RT. Here the pure Ag thin film is prepared at the same condi tions and its resistivity is measured for comparison. From the data of hardness and electrical resistivity, we found that when h � 10 nm, both the strength and the electrical conductivity of Ag/ Cu multialyers annealed within 300 � C are higher than those of as deposited samples. In previous work [11], we reported that a combi nation of high strength and high electrical conductivity was achieved in as deposited Ag/Cu multilayers. In this work, we found that the hardness and resistivity both are further enhanced via the annealing process and show more excellent performance. The properties have close relation ship with the microstructures. So we investigate the morphologies of the annealed multilayers. Considering the hardness and the resistivity show length scale dependent, we choose two representative scales of h ¼ 20 and 5 nm to investigate the different strengthening or weaken effects. The details of cross-sectional TEM and HRTEM morphologies of Ag/Cu multilayers with h ¼ 20 and 5 nm annealed at 200–400 � C are presented in Fig. 4. Fig. 4(a)–(c) show the TEM and HRTEM images of multilayers with h ¼ 20 nm annealed at 200, 300 and 400 � C, respectively. From Fig. 4(a) and (b), we can see that the multilayer maintains lamellar structure when the annealing temperature does not exceed 300 � C. Compared with the as deposited multilayer, the interface of annealed multilayer becomes not clear and distinct at 300 � C. The grains grow with the increasing temperature. At 200 � C, the amount of twins is larger than that of as deposited state. But when the temperature rises to 300 � C, the amount of twins becomes less. It suggests that with the rising annealing temperature, the process of twinning and de-twinning occur successively. The insets in Fig. 4(a) and (b) are the images of the twins, respectively. The lamella thickness of twins is about 2–6 nm. Fig. 4(c) is the TEM image of multilayer after annealing at 400 � C. The lamellar structure disappears and the grains become coarse equiaxed crystals. Fig. 4(d)–(f) show the TEM and HRTEM images of multilayers with h ¼ 5 nm annealed at 200, 300 and 400 � C, respectively. We can see that lamellar structure keeps at 200 � C. When annealing temperature in creases to 300 � C, the grains become coarse equiaxed crystals, and the growth goes on when the annealing temperature is up to 400 � C. From HRTEM image (inserted at right bottom corner) in Fig. 4(d), we can see a great amount of SFs existing in the multilayer. The SAD pattern inserted at top right corner in Fig. 4(d) confirms the microstructure of SFs. Furthermore, some twins can be seen in Fig. 4(e) and (f), showing the results of growth of grains and SFs. In metallic systems with nanocrystalline, annealing usually results in a decrease in hardness due to grain growth. For annealed Ag/Cu mul tilayers, however, the hardness goes up first and then drops down with the increasing annealing temperatures (Fig. 2). Especially at 200 � C, the hardness of all multilayers improve about 6–16% compared with the as deposited state. We take the multilayers with h ¼ 5 and 20 nm as ex amples to explore the reasons. We found that the differences lie in two aspects: the interface structures and the sub-structures such as twins and SFs in grains. By comparing the microstructures of multilayers annealing at 200 � C with the as deposited ones, the amount of twins or SFs in creases obviously. For multilayer with h ¼ 5 nm, the SFs can be seen distinctly increased compared with the as deposited state. For multilayer
Fig. 2. The hardness plotted as a function of individual layer thickness at different annealing temperatures of 200, 300 and 400 � C, respectively.
Fig. 3. The electrical resistivity plotted as a function of individual layer thickness at different annealing temperatures of 200, 300 and 400 � C, respectively.
structure of the square, indicating the orientation relationship between Ag and Cu layers. Fig. 1(d) presents the HRTEM and TEM images of multilayer with h ¼ 5 nm. The TEM image locates at the left bottom corner. The width of columnar grains is large and the wavy interfaces can be observed. From HRTEM image we can clearly see the coherent area and non-coherent area (marked with the blue rectangle), showing a typical semi-coherent relationship. The selected area diffraction (SAD) pattern (inset at the bottom right corner of Fig. 1(d)) indicates the orientation relationship between Ag and Cu layers. Fig. 2 shows the hardness plotted as a function of individual layer thickness at different annealing temperatures of 200, 300 and 400 � C, respectively. From Fig. 2 we can see the hardness data of annealed multilayers change with the annealing temperature and show length scale dependent. At 200 � C, the hardness improves 6–16% compared with the data at RT. With annealing temperature rises from 200 to 300 � C, the hardness of the multilayers decrease differently. At larger scales of h � 10 nm, the hardness decreases slightly, but the values are still higher than those at RT. The hardness data improve about 4–10% compared with the as-deposited samples. At smaller scales of h ¼ 3 and 5 nm, the hardness decreases obviously and the values are lower than those at RT. When the annealing temperature is up to 400 � C, the hardness of the annealed multilayers at all scales drops rapidly. The data are lower than those at RT. 3
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Materials Science & Engineering A 772 (2020) 138818
Fig. 4. The cross-sectional TEM and HRTEM images of annealed Ag/Cu multilayers with h ¼ 20 and 5 nm. (a)–(c) The images of multilayers with h ¼ 20 nm annealed at 200, 300 and 400 � C, respectively. (d)–(f) The images of multilayers with h ¼ 5 nm annealed at 200, 300 and 400 � C, respectively.
Fig. 5. The schematic diagrams of microstructure evolution of multilayers with (a) h ¼ 5, (b) h ¼ 10 and (c) h ¼ 20 nm at different annealing temperatures.
with h ¼ 20 nm, the amount of twins increases obviously. The SFs and TBs both can improve the hardness by impeding the dislocation trans mission [25–27]. Hence, the multilayers strengthen at 200 � C. At 300 � C, from the hardness data and morphologies of annealed multilayers, the hardness and thermal stability of Ag/Cu multilayers shows layer thickness dependent. The multilayers with larger layer thickness (h � 10 nm) have better thermal stability and higher hardness compared with the as deposited samples. The thermal stability relates with the interface structure and the atomic diffusion (migration). Ac cording to thermal grooving theory [28], multilayers develop grooves at the triple-point junctions, by diffusive mass transport, the columnar grain boundaries in layers are laterally offset. Thermal grooving at columnar grain boundaries leads to layer pinch-off at elevated temper atures and drives instabilities in polycrystalline multilayers. When the layer thickness is small, the diffusing atoms are more easily migrating across the section because of the short distance, resulting in the pinch-off of the layers. While the layer thickness is larger, the above procedure
becomes more time-consuming and more complicated, and then pro duce better thermal stability. Furthermore, we found that the interface structures and the grains of Ag/Cu multilayers have huge distinctions at different layer thickness. So it produces a much difference in structural evolutions. According to the results of characterization, we depict the schematic diagrams of microstructure evolution of multilayers with h ¼ 5, 10 and 20 nm at different annealing temperatures, as shown in Fig. 5 (a)-(c), respectively. The interface of multilayer with h ¼ 5 nm (Fig. 5 (a)) consists of coherent area and non-coherent area. The non-coherent joints are arched and can be seen as GBs connecting the coherent grains. The arched boundaries bring high grain boundary energy (γgb). When the ratio of γgb/interface energy (γint) is large, significant grooving oc curs where GBs meet the interfaces between layers. So the multilayer with h ¼ 5 nm cannot bear the high temperature annealing and lamellar structure is broken at 300 � C. For multilayers with h ¼ 10 and 20 nm, the interfaces are more straight and SFs and twins distribute in layers, as shown in Fig. 5(b) and (c), respectively. Compared with the columnar 4
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Materials Science & Engineering A 772 (2020) 138818
interfaces. In order to investigate the situation more exactly, taking multilayer with h ¼ 20 nm as example, we made a statistic analysis on the average grain size of Ag and Cu columnar grains, the spacing of twin laminars and the number ratio of twining grains to total grains (δt ¼ ntwin/ntotal) at different annealing temperatures. The data are listed in Table 1. From the statistic data, with the increasing annealing temperature, the columnar grains of Ag and Cu both grow up. Meanwhile, the number ratio of twinning grains to total grains goes up first at 200 � C and then drops a little at 300 � C. The twin spacing keeps unchanged. The grain growth brings the reduction of the amount of high angle GBs, whereas the amount of TBs increases with the increment of δt. The electron scattering at TBs is known to influence the resistivity by an order of magnitude less than does scattering at GBs [20]. Thus the increase of TBs has little effect on the resistivity of multilayers. Through the compre hensive comparison of the size of columnar grains and δt, we suggested that the decrease of resistivity mainly results from the reduction of GBs due to the grain growth. While for the hardness, the cases are much more different. The hardness of multilayer with h ¼ 20 nm annealed at 200 � C and 300 � C both are higher than that of as deposited multilayer. However, the enhancement mechanisms are very different. From Table 1, at 200 � C, the ratio of twinning grains improves from 0.18 to 0.21 compared with the as deposited state. The increase of amount of TBs can enhance the hardness due to its inhibition to the motion of dislocations. At 300 � C, the ratio of twinning grains drops slightly and is lower than that at the room temperature due to the recovery of the annealing twins. However, the hardness is still higher than that of as deposited multilayer. The situation looks complicated. The increase of amount of TBs can explain the hardness enhancement at 200 � C, but it is not valid at 300 � C. We make a further investigation to explore the reasons. The cross-sectional HRTEM images of Ag/Cu multilayers with h ¼ 20 nm at RT, 200 � C and 300 � C are presented in Fig. 6 (a)–(c), respectively. From Fig. 6(a), some SFs can be seen at the interface and GBs. At 200 � C (Fig. 6(b)), the SFs at interface graduate into twin, indicating the increase of twins may come
Table 1 The statistic data on the size of columnar grains, the spacing of twin laminars and δt of multilayer with h ¼ 20 nm at different annealing temperatures. Temperature
The average grain size of Ag (nm)
The average grain size of Cu (nm)
Twin spacing (nm)
δt
RT 200 � C 300 � C 400 � C
19.6 � 3 24.8 � 4 27.8 � 4 ~200 � 20
19.0 � 3 24.8 � 4 27.7 � 4 ~200 � 20
2–6 2–6 2–6 /
18% 21% 16% /
grains, the textures in the multilayer with h ¼ 10 nm is the main reason to protect the laminar structure without destruction. The amount of GBs is reduced greatly due to the formation of textures, and hence reduces the probability of formation of thermal grooving. This leads to a good thermal stability of the multilayer with h ¼ 10 nm. Furthermore, the SFs distribute at the interfaces and grains can impede the migration of atomic diffusion and then improve the thermal stabilities for multi layers. The multilayer with h ¼ 20 nm has a typical polycrystalline structure with nanotwins growing in it. At high temperature, the grooves develop at the triple point of GBs and interface, as shown in Fig. 5(c). The layer pinch-off occurs at 400 � C and result in the coarsening of grains. For multilayers, the hardness has a close relationship with the thermal stability. The interface is the main strengthening factor due to its impeding to dislocation transmission. From the above analysis of thermal stability, the interface of multilayers with h � 10 nm can keep from being destroyed at 300 � C. This is corresponding to length scale dependent of hardness. In composite materials, high strength is irreconcilable with high conductivity [29]. However, in annealed Ag/Cu multilayers, this contradiction is well coordinated by the annealing process. From Fig. 3, the electrical resistivity decreases with the increasing of annealing temperatures. During the annealing processes, the different micro structural variations occur at different temperatures, including growth of columnar grains, twining and de-twining processes and changes of
Fig. 6. The cross-sectional HRTEM images of Ag/Cu multilayers with h ¼ 20 nm at (a) RT, (b) 200 � C and (c) 300 � C. The structures circled in ellipses are SFs and the corresponding FFT patterns are connected with the dashed lines. The IFFT patterns corresponding to the squares are marked, respectively. 5
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Materials Science & Engineering A 772 (2020) 138818
Fig. 7. (a) The XRD patterns of multilayers with h ¼ 20 nm at RT, 200 � C, 300 � C and 400 � C. (b) The residual stress of multilayers with h ¼ 20 nm as a function of temperature.
from the transformation of SFs. This phenomenon is in agreement with the references [30–32] that the twins result from the coincidental overlapping of stacking fault ribbons in nano grains. At 300 � C, we still observed some SFs at GBs. By comparing the IFFT patterns at RT, 200 � C and 300 � C corresponding to the square area at the interfaces, we found that the atomic arrangement in IFFT patterns becomes regular and clear and the dislocations become less with the elevated annealing tempera tures. It is known that the plastic deformation dependent on the motion of dislocations. Recently, some researchers proposed that at small scales, the dislocation starvation leads to strengthening because it requires very high stresses to nucleate new dislocations [33,34]. So the removal of dislocations by annealing renders the multilayer stronger without any other influencing factors. But in fact, the annealing processes are complicated and many other factors should be considered. From Fig. 6(a), the measured interplanar spacing is 0.234 nm, which is very close to the Ag (111) plane (d(111) ¼ 0.236 nm). However, in Fig. 6(c), the measured interplanar spacing of Ag (111) plane is 0.228 nm, suggesting an obvious interplanar contraction effect after anneal ing. Hence we investigated the XRD data to analyze the stress state of the multilayers. The XRD patterns of multilayer with h ¼ 20 nm at RT, 200 � C, 300 � C and 400 � C are plotted in Fig. 7(a). The diffraction peaks of Ag (111) and Cu (111) at different annealing states all shift to high angle. It is consistent with the measurement results of interplanar spacing in Fig. 6. We estimated the residual stress with the measured 2θ in the 2theta scan in the following equation [35,36],
σr ¼
E Δ2θ ð1 þ νÞ 2 tanθ0
Table 2 The calculated values of fs and fρ at different annealing temperatures. Layer thickness (nm) h¼ h¼ h¼ h¼ h¼ h¼
3 5 10 20 50 100
RT
200 � C
300 � C
fs/fρ
fs/fρ
fs/fρ
1/1 1/1 1/1 1/1 1/1 1/1
1.13/1.14 1.08/1.11 1.06/1.07 1.11/1.06 1.11/1.07 1.16/1.11
/ / 1.04/1.12 1.09/1.26 1.04/1.25 1.10/1.38
increase of twins, the compressive stresses and the lack of dislocations. When the annealing temperature rises to 300 � C, it is inferred that the compressive stress state and the dislocation starvation contribute a lot to the hardness enhancement. In order to evaluate the enhancement effect of combined properties of strength and electrical conductivity, the annealed hardness and electrical conductivity are compared with those at RT. The fs and fρ are defined to indicate the enhancement coefficient of strength and elec trical conductivity, respectively. Taking the hardness and electrical conductivity at RT as the basis for comparison, fs and fρ are given by
(1)
fs ¼
σT HT ¼ σ RT HRT
(2)
fρ ¼
1=ρT ρ ¼ RT 1=ρRT ρT
(3)
where σT (HT) and σ T (HRT) represent the strength (hardness) at annealing temperature and RT, respectively. ρT and ρRT represent the electrical resistivity at annealing temperature and RT, respectively. The calculated values of fs and fρ at different annealing temperatures are listed in Table 2. Note that the data at 400 � C are omitted because of the collapse of layer structure. Considering the comprehensive enhancement effect depends on the improvement of their comprehensive properties combined the strength and the conductivity, we defined a comprehensive coefficient fcomp, which can be given by
where σ r is the residual stress, E and ν are the Young’s modulus and Poisson ratio of Ag (Cu), respectively, θ0 is the Bragg angle under stressfree condition, and Δ2θ is the difference between 2θ0 and the measured 2θ. Here EAg ¼ 82.7 GPa, νAg ¼ 0.37, θ0Ag ¼ 19.058� (Ag (111)), ECu ¼ 129 GPa, νCu ¼ 0.34, and θ0Cu ¼ 21.6485� (Cu (111)). The calculated σ r of Ag and Cu at different temperatures are shown in Fig. 7(b). At RT, the residual stresses of Ag and Cu both are small, with compressive stress in Ag grains and tensile stress in Cu grains. After annealing, the stress state of Cu becomes compressive stress. The compressive stresses in Ag and Cu both increase with the improvement of annealing temperatures and reach maximum at 300 � C. It has been well accepted that the stress state affect the strength of the films. Hardness typically increases with compressive stress [37–39]. So the compressive stress state contributes to the hardness enhancement in annealed Ag/Cu multilayer with h ¼ 20 nm. At the same time, we found that the compressive stress also exists in annealed Ag/Cu multilayers at other scales. From above analysis, we found that the hardness enhancement of multilayer with h ¼ 20 nm should come from different aspects including
fcomp ¼
2fs ⋅fρ fs þ fρ
(4)
here if fcomp > 1, it indicates the comprehensive property of the multi layer enhanced by the annealing process. It is noted that we only use this expression to describe the comprehensive enhancement effect of annealed multilayer relative to the deposition state. We substitute the data of fs and fρ of the multilayers at different 6
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Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgement This work was jointly supported by the Natural Science Foundation of Shandong Province (Grant No. ZR2016EMM17), National Natural Science Foundation of China (No. 51671103,51371096) and the priority academic program development of Jiangsu higher education institutions (PAPD). References Fig. 8. The coefficients fcomp plotted as a function of individual layer thickness at annealing temperatures of 200 and 300 � C, respectively.
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annealing temperatures into Eq. (4), and the comprehensive coefficient can be calculated. The coefficients fcomp plotted as a function of h is shown in Fig. 8. From Fig. 8, it reveals that the annealed multilayers have the higher fcomp than the as deposited samples, indicating the strength and the conductivity are enhanced by the annealing treatment withhin 300 � C. 4. Conclusions In this work, the hardness and electrical resistivity of annealed Ag/ Nb multilayers were investigated with h ranging from 3 to 100 nm after annealing from 200 to 400 � C for 30 min. For Ag/Cu multilayers of all scales, the hardness improves about 6–16% whereas the electrical re sistivity decreases 6–12% after annealing at 200 � C. The reduced re sistivity results from the growth of columnar grains and the increase of hardness comes from the annealing twins or SFs in the multilayers. At 300 � C, the enhancement shows layer thickness dependent due to the different interface structures. The hardness improves about 4–10% and the electrical resistivity decreases 11–28% for multilayers with h � 10 nm. But the hardness drops sharply for multilayers with h ¼ 3 and 5 nm due to the collapse of lamellar structures. A good combination of high strength and high conductivity has been obtained in annealed Ag/Cu multilayer with h � 10 nm when annealing temperature is not more than 300 � C. Both hardness and electrical conductivity are enhanced by annealing process. The improvement of electrical conductivity mainly comes from the reduction of GBs due to grain growth and the enhancement of hardness relates with the increment of TBs, the compressive stress and the lack of dislocations after annealing. This investigation provides a basis for controlling the annealing temperature of Ag/Cu multilayers. Author contributions section J.Z. Huo:Methodology, Investigation, Writing - Original Draft; M.Z. Wei:Writing- Reviewing and Editing, Project administration, Funding acquisition; Y.J. Ma:Validation,Formal analysis; Z.H. Cao:Funding acquisition,Resources; X.K. Meng: Supervision.
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Materials Science & Engineering A journal homepage: http://www.elsevier.com/locate/msea
The enhanced strength and electrical conductivity in Ag/Cu multilayers by annealing process J.Z. Huo a, M.Z. Wei a, *, Y.J. Ma b, Z.H. Cao b, c, **, X.K. Meng b a
Institute of Physics and Electronic Engineering, Linyi University, Shandong, PR China Institute of Materials Engineering, National Laboratory of Solid State Microstructures, College of Engineering and Applied Sciences, Nanjing University, Jiangsu, PR China c College of Materials Science and Engineering, Tech Institute for Advanced Materials, Nanjing Tech University, Nanjing, PR China b
A R T I C L E I N F O
A B S T R A C T
Keywords: Multilayer Hardness Electrical conductivity Thermal stability
In this work, the hardness, electrical resistivity and thermal stability of annealed Ag/Nb multilayers were investigated with individual layer thickness (h) ranging from 3 to 100 nm after annealing at 200–400 � C. For all scales multilayers, the hardness improves 6–16% whereas the electrical resistivity decreases 6–12% after annealing at 200 � C compared with those of as deposited state. At 300 � C, the enhancement shows layer thickness dependent due to the different interfacial structures. The hardness improves about 4–10% and the electrical resistivity decreases 11–28% for multilayers with h � 10 nm. A good combination of high strength and high conductivity has been obtained in annealed Ag/Cu multilayer with h � 10 nm when annealing temperature is not more than 300 � C. Both hardness and electrical conductivity are enhanced by annealing process. The improvement of electrical conductivity mainly comes from the reduction of grain boundaries due to the grain growth. The enhancement of hardness relates with the increment of twin boundaries, the compressive stress and the lack of dislocations after annealing process.
1. Introduction High strength conductors have attracted much attention due to the combination of strength and electrical conductivity, which enables them to be used in high-field magnets [1–4], electrode materials in large-scale integrated circuits [5], interconnect fabrication in microelectronics [6] and so on. Ag–Cu composites are extensively used due to their promi nent properties [7–10]. Recently, many studies on Ag/Cu multilayers have been performed to investigate the strengthening mechanisms and the electrical properties at the room temperature (RT) [11–15]. In fact, the serving environment of Ag/Cu materials usually involves high temperature or thermal-electrical coupled fields. The stabilities of structures and performances are the key factors to influence the usage of the multilayers [16]. It is important to investigate the properties and the structural stabilities at elevated temperatures. In general, high strength is irreconcilable with high electrical con ductivity in composite materials. The electron scattering at interfaces and grain boundaries (GBs) is the main factor reducing the conductivity
of multilayers [17–19]. However, these negative factors for conductivity happen to be the positive factors to improve the strength of the multi layers. Hence, how to coordinate the contradictions between the two aspects, is the key to obtaining high strength conductors. Lu et al. [20] reported that the numerous twins in Cu can greatly increase the strength and lead to an extremely low electrical resistivity (ρ) as well. Zhang et al. [21] reported that the high strength and high electrical conductivity have been obtained in bulk Cu–Ag alloy with nanotwins and nanograins. Valiev et al. [22] reported that high strength can be obtained in Al alloy by nano twined method. And there are many other researches also get the similar results [23,24]. They convey the information that the introduction of twin boundaries (TBs) effectively blocks the dislocation motion while has negligible effect on the electrical conductivity. As we have known, annealing process is a good method to achieve nanotwins in some metal with low stacking fault energy. However, annealing process will bring the grain growth and result in the reduction of strength. Hence, the annealing temperature must be limited in a certain range. In Ag/Cu multilayers, the twins are easily formed in Ag layers due
* Corresponding author. ** Corresponding author. Institute of Materials Engineering, National Laboratory of Solid State Microstructures, College of Engineering and Applied Sciences, Nanjing University, Jiangsu, PR China. E-mail addresses: [email protected] (M.Z. Wei), [email protected] (Z.H. Cao). https://doi.org/10.1016/j.msea.2019.138818 Received 18 August 2019; Received in revised form 11 December 2019; Accepted 11 December 2019 Available online 14 December 2019 0921-5093/© 2019 Elsevier B.V. All rights reserved.
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Fig. 1. The bright-field cross-sectional TEM and HRTEM images of as-deposited Ag/Cu multilayers with (a) h ¼ 50 nm, (b) h ¼ 20 nm, (c) h ¼ 10 nm and (d) h ¼ 5 nm.
to the low stacking fault energy. In annealing process, the grain growth occurs and the interface/GBs migrate independently. Meanwhile, twining and de-twining processes take place at different annealing temperatures. The migration competition of different inter faces/GBs/TBs at different temperatures will influence the strength and conductivity extensively. Furthermore, the thermal stability of multi layers shows layer thickness dependent. The situation will become complicated with the different scales at different annealing tempera tures. Hence, it is essential to explore the details on the strategy of improving the strength and electrical conductivity of Ag/Cu multilayers at elevated temperatures. In present work, we studied the hardness and the electrical resistivity of Ag/Cu multilayers at elevated temperatures. It has been found that the annealed Ag/Cu multilayers show enhanced hardness and electrical conductivity compared with the as-deposited samples. The structure evolution and the influences of these structures on the properties are discussed.
voltages. The nanoindentation tests were performed using a TI950 Hysitron Triboindenter with a displacement resolution of 0.1 nm and a loading resolution of 50 nN. A Berkovich tip with 50 nm radius of cur vature was used in all indentation experiments. The maximum inden tation depth was controlled below 15% of the multilayer thickness in order to avoid substrate effect. The electrical resistivity was measured at ambient temperature via four-point-probe technique from the sample surface. 3. Results and discussion The bright-field cross-sectional TEM and HRTEM images of asdeposited Ag/Cu multilayers with h ¼ 50, 20, 10 and 5 nm are pre sented in Fig. 1 (a) - (d), respectively. From TEM images of Fig. 1(a) and (b), we can see the grains of multilayers with h ¼ 50 and h ¼ 20 nm are columnar grains with the average width scaling with h. The interfaces are sharp and flat. The twins exist both in Ag and Cu grains. In partic ular, some twins grow successively along the growth direction, con necting the Ag and Cu layers of the multilayer. The insets in Fig. 1(a) and (b) show the HRTEM morphologies of interface and GBs of multilayers with h ¼ 50 and 20 nm, respectively. We can see the lattice fringes continuing from Ag grain to Cu grain in HRTEM image in Fig. 1(b), indicating the orientation relationship between Ag and Cu grains. While in HRTEM image in Fig. 1(a) there is no orientation relationship be tween the grains. Fig. 1(c) presents the HRTEM and TEM images of multilayer with h ¼ 10 nm. From the TEM image (the inset at the left bottom corner of Fig. 1(c)), the width of columnar grains is much larger than the layer thickness and a great number of stacking faults (SFs) are observed. The HRTEM image shows the strong textures both in Ag and Cu layers. The fringes across the interface in TEM image are turn out to be SFs by the HRTEM image. The Inverse Fast Fourier Transformation (IFFT) (inset at the bottom right corner of Fig. 1(c)) shows the interface
2. Experimental details Nanostructured Ag/Cu multilayers with equal individual layer thickness ranging from 3 to 100 nm were prepared by dc magnetron sputtering on Si substrates at room temperature. Base pressure for the sputtering chamber was kept at less than 1.2 � 10 4 Pa and the sput tering pressure was 1.2 Pa. A Ta barrier layer with 5 nm is first deposited on Si substrate before the preparation of multilayers. The Cu and Ag were deposited alternately on the substrate and the top layer was Ag. Total thickness of the Ag/Cu multilayers was 1000 nm. After deposition, the multilayers were annealed at 200 � C, 300 � C and 400 � C for 30 min in an annealing furnace with high vacuum, respectively. The cross-sectional microstructures were characterized by JEM-2100 transmission electron microscope (TEM) with 200 kV accelerating 2
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The electrical resistivity data of the multilayers with h ranging from 3 to 100 nm at different annealing temperatures of 200, 300 and 400 � C are shown in Fig. 3. It can be seen that the resistivities of all scales decrease with the increasing annealing temperature. Corresponding to the hardness variation, the tendency of resistivity decreasing varies with the layer thickness at the different annealing temperatures. At 200 � C, the resistivity values of multilayers at all scales decrease about 6–12% (the electrical conductivity improves 6–14%) compared with the as deposited samples. At 300 � C, the resistivity values decrease about 11–28% (the electrical conductivity improves 12–38%) for multilayers with h � 10 nm, and the resistivity of multilayers with h ¼ 3 and 5 nm drop sharply, suggesting a crucial change of the microstructures. When the annealing temperature is up to 400 � C, the resistivities of multilayers at all scales are close to that of pure Ag thin film and much lower than those at RT. Here the pure Ag thin film is prepared at the same condi tions and its resistivity is measured for comparison. From the data of hardness and electrical resistivity, we found that when h � 10 nm, both the strength and the electrical conductivity of Ag/ Cu multialyers annealed within 300 � C are higher than those of as deposited samples. In previous work [11], we reported that a combi nation of high strength and high electrical conductivity was achieved in as deposited Ag/Cu multilayers. In this work, we found that the hardness and resistivity both are further enhanced via the annealing process and show more excellent performance. The properties have close relation ship with the microstructures. So we investigate the morphologies of the annealed multilayers. Considering the hardness and the resistivity show length scale dependent, we choose two representative scales of h ¼ 20 and 5 nm to investigate the different strengthening or weaken effects. The details of cross-sectional TEM and HRTEM morphologies of Ag/Cu multilayers with h ¼ 20 and 5 nm annealed at 200–400 � C are presented in Fig. 4. Fig. 4(a)–(c) show the TEM and HRTEM images of multilayers with h ¼ 20 nm annealed at 200, 300 and 400 � C, respectively. From Fig. 4(a) and (b), we can see that the multilayer maintains lamellar structure when the annealing temperature does not exceed 300 � C. Compared with the as deposited multilayer, the interface of annealed multilayer becomes not clear and distinct at 300 � C. The grains grow with the increasing temperature. At 200 � C, the amount of twins is larger than that of as deposited state. But when the temperature rises to 300 � C, the amount of twins becomes less. It suggests that with the rising annealing temperature, the process of twinning and de-twinning occur successively. The insets in Fig. 4(a) and (b) are the images of the twins, respectively. The lamella thickness of twins is about 2–6 nm. Fig. 4(c) is the TEM image of multilayer after annealing at 400 � C. The lamellar structure disappears and the grains become coarse equiaxed crystals. Fig. 4(d)–(f) show the TEM and HRTEM images of multilayers with h ¼ 5 nm annealed at 200, 300 and 400 � C, respectively. We can see that lamellar structure keeps at 200 � C. When annealing temperature in creases to 300 � C, the grains become coarse equiaxed crystals, and the growth goes on when the annealing temperature is up to 400 � C. From HRTEM image (inserted at right bottom corner) in Fig. 4(d), we can see a great amount of SFs existing in the multilayer. The SAD pattern inserted at top right corner in Fig. 4(d) confirms the microstructure of SFs. Furthermore, some twins can be seen in Fig. 4(e) and (f), showing the results of growth of grains and SFs. In metallic systems with nanocrystalline, annealing usually results in a decrease in hardness due to grain growth. For annealed Ag/Cu mul tilayers, however, the hardness goes up first and then drops down with the increasing annealing temperatures (Fig. 2). Especially at 200 � C, the hardness of all multilayers improve about 6–16% compared with the as deposited state. We take the multilayers with h ¼ 5 and 20 nm as ex amples to explore the reasons. We found that the differences lie in two aspects: the interface structures and the sub-structures such as twins and SFs in grains. By comparing the microstructures of multilayers annealing at 200 � C with the as deposited ones, the amount of twins or SFs in creases obviously. For multilayer with h ¼ 5 nm, the SFs can be seen distinctly increased compared with the as deposited state. For multilayer
Fig. 2. The hardness plotted as a function of individual layer thickness at different annealing temperatures of 200, 300 and 400 � C, respectively.
Fig. 3. The electrical resistivity plotted as a function of individual layer thickness at different annealing temperatures of 200, 300 and 400 � C, respectively.
structure of the square, indicating the orientation relationship between Ag and Cu layers. Fig. 1(d) presents the HRTEM and TEM images of multilayer with h ¼ 5 nm. The TEM image locates at the left bottom corner. The width of columnar grains is large and the wavy interfaces can be observed. From HRTEM image we can clearly see the coherent area and non-coherent area (marked with the blue rectangle), showing a typical semi-coherent relationship. The selected area diffraction (SAD) pattern (inset at the bottom right corner of Fig. 1(d)) indicates the orientation relationship between Ag and Cu layers. Fig. 2 shows the hardness plotted as a function of individual layer thickness at different annealing temperatures of 200, 300 and 400 � C, respectively. From Fig. 2 we can see the hardness data of annealed multilayers change with the annealing temperature and show length scale dependent. At 200 � C, the hardness improves 6–16% compared with the data at RT. With annealing temperature rises from 200 to 300 � C, the hardness of the multilayers decrease differently. At larger scales of h � 10 nm, the hardness decreases slightly, but the values are still higher than those at RT. The hardness data improve about 4–10% compared with the as-deposited samples. At smaller scales of h ¼ 3 and 5 nm, the hardness decreases obviously and the values are lower than those at RT. When the annealing temperature is up to 400 � C, the hardness of the annealed multilayers at all scales drops rapidly. The data are lower than those at RT. 3
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Fig. 4. The cross-sectional TEM and HRTEM images of annealed Ag/Cu multilayers with h ¼ 20 and 5 nm. (a)–(c) The images of multilayers with h ¼ 20 nm annealed at 200, 300 and 400 � C, respectively. (d)–(f) The images of multilayers with h ¼ 5 nm annealed at 200, 300 and 400 � C, respectively.
Fig. 5. The schematic diagrams of microstructure evolution of multilayers with (a) h ¼ 5, (b) h ¼ 10 and (c) h ¼ 20 nm at different annealing temperatures.
with h ¼ 20 nm, the amount of twins increases obviously. The SFs and TBs both can improve the hardness by impeding the dislocation trans mission [25–27]. Hence, the multilayers strengthen at 200 � C. At 300 � C, from the hardness data and morphologies of annealed multilayers, the hardness and thermal stability of Ag/Cu multilayers shows layer thickness dependent. The multilayers with larger layer thickness (h � 10 nm) have better thermal stability and higher hardness compared with the as deposited samples. The thermal stability relates with the interface structure and the atomic diffusion (migration). Ac cording to thermal grooving theory [28], multilayers develop grooves at the triple-point junctions, by diffusive mass transport, the columnar grain boundaries in layers are laterally offset. Thermal grooving at columnar grain boundaries leads to layer pinch-off at elevated temper atures and drives instabilities in polycrystalline multilayers. When the layer thickness is small, the diffusing atoms are more easily migrating across the section because of the short distance, resulting in the pinch-off of the layers. While the layer thickness is larger, the above procedure
becomes more time-consuming and more complicated, and then pro duce better thermal stability. Furthermore, we found that the interface structures and the grains of Ag/Cu multilayers have huge distinctions at different layer thickness. So it produces a much difference in structural evolutions. According to the results of characterization, we depict the schematic diagrams of microstructure evolution of multilayers with h ¼ 5, 10 and 20 nm at different annealing temperatures, as shown in Fig. 5 (a)-(c), respectively. The interface of multilayer with h ¼ 5 nm (Fig. 5 (a)) consists of coherent area and non-coherent area. The non-coherent joints are arched and can be seen as GBs connecting the coherent grains. The arched boundaries bring high grain boundary energy (γgb). When the ratio of γgb/interface energy (γint) is large, significant grooving oc curs where GBs meet the interfaces between layers. So the multilayer with h ¼ 5 nm cannot bear the high temperature annealing and lamellar structure is broken at 300 � C. For multilayers with h ¼ 10 and 20 nm, the interfaces are more straight and SFs and twins distribute in layers, as shown in Fig. 5(b) and (c), respectively. Compared with the columnar 4
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interfaces. In order to investigate the situation more exactly, taking multilayer with h ¼ 20 nm as example, we made a statistic analysis on the average grain size of Ag and Cu columnar grains, the spacing of twin laminars and the number ratio of twining grains to total grains (δt ¼ ntwin/ntotal) at different annealing temperatures. The data are listed in Table 1. From the statistic data, with the increasing annealing temperature, the columnar grains of Ag and Cu both grow up. Meanwhile, the number ratio of twinning grains to total grains goes up first at 200 � C and then drops a little at 300 � C. The twin spacing keeps unchanged. The grain growth brings the reduction of the amount of high angle GBs, whereas the amount of TBs increases with the increment of δt. The electron scattering at TBs is known to influence the resistivity by an order of magnitude less than does scattering at GBs [20]. Thus the increase of TBs has little effect on the resistivity of multilayers. Through the compre hensive comparison of the size of columnar grains and δt, we suggested that the decrease of resistivity mainly results from the reduction of GBs due to the grain growth. While for the hardness, the cases are much more different. The hardness of multilayer with h ¼ 20 nm annealed at 200 � C and 300 � C both are higher than that of as deposited multilayer. However, the enhancement mechanisms are very different. From Table 1, at 200 � C, the ratio of twinning grains improves from 0.18 to 0.21 compared with the as deposited state. The increase of amount of TBs can enhance the hardness due to its inhibition to the motion of dislocations. At 300 � C, the ratio of twinning grains drops slightly and is lower than that at the room temperature due to the recovery of the annealing twins. However, the hardness is still higher than that of as deposited multilayer. The situation looks complicated. The increase of amount of TBs can explain the hardness enhancement at 200 � C, but it is not valid at 300 � C. We make a further investigation to explore the reasons. The cross-sectional HRTEM images of Ag/Cu multilayers with h ¼ 20 nm at RT, 200 � C and 300 � C are presented in Fig. 6 (a)–(c), respectively. From Fig. 6(a), some SFs can be seen at the interface and GBs. At 200 � C (Fig. 6(b)), the SFs at interface graduate into twin, indicating the increase of twins may come
Table 1 The statistic data on the size of columnar grains, the spacing of twin laminars and δt of multilayer with h ¼ 20 nm at different annealing temperatures. Temperature
The average grain size of Ag (nm)
The average grain size of Cu (nm)
Twin spacing (nm)
δt
RT 200 � C 300 � C 400 � C
19.6 � 3 24.8 � 4 27.8 � 4 ~200 � 20
19.0 � 3 24.8 � 4 27.7 � 4 ~200 � 20
2–6 2–6 2–6 /
18% 21% 16% /
grains, the textures in the multilayer with h ¼ 10 nm is the main reason to protect the laminar structure without destruction. The amount of GBs is reduced greatly due to the formation of textures, and hence reduces the probability of formation of thermal grooving. This leads to a good thermal stability of the multilayer with h ¼ 10 nm. Furthermore, the SFs distribute at the interfaces and grains can impede the migration of atomic diffusion and then improve the thermal stabilities for multi layers. The multilayer with h ¼ 20 nm has a typical polycrystalline structure with nanotwins growing in it. At high temperature, the grooves develop at the triple point of GBs and interface, as shown in Fig. 5(c). The layer pinch-off occurs at 400 � C and result in the coarsening of grains. For multilayers, the hardness has a close relationship with the thermal stability. The interface is the main strengthening factor due to its impeding to dislocation transmission. From the above analysis of thermal stability, the interface of multilayers with h � 10 nm can keep from being destroyed at 300 � C. This is corresponding to length scale dependent of hardness. In composite materials, high strength is irreconcilable with high conductivity [29]. However, in annealed Ag/Cu multilayers, this contradiction is well coordinated by the annealing process. From Fig. 3, the electrical resistivity decreases with the increasing of annealing temperatures. During the annealing processes, the different micro structural variations occur at different temperatures, including growth of columnar grains, twining and de-twining processes and changes of
Fig. 6. The cross-sectional HRTEM images of Ag/Cu multilayers with h ¼ 20 nm at (a) RT, (b) 200 � C and (c) 300 � C. The structures circled in ellipses are SFs and the corresponding FFT patterns are connected with the dashed lines. The IFFT patterns corresponding to the squares are marked, respectively. 5
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Fig. 7. (a) The XRD patterns of multilayers with h ¼ 20 nm at RT, 200 � C, 300 � C and 400 � C. (b) The residual stress of multilayers with h ¼ 20 nm as a function of temperature.
from the transformation of SFs. This phenomenon is in agreement with the references [30–32] that the twins result from the coincidental overlapping of stacking fault ribbons in nano grains. At 300 � C, we still observed some SFs at GBs. By comparing the IFFT patterns at RT, 200 � C and 300 � C corresponding to the square area at the interfaces, we found that the atomic arrangement in IFFT patterns becomes regular and clear and the dislocations become less with the elevated annealing tempera tures. It is known that the plastic deformation dependent on the motion of dislocations. Recently, some researchers proposed that at small scales, the dislocation starvation leads to strengthening because it requires very high stresses to nucleate new dislocations [33,34]. So the removal of dislocations by annealing renders the multilayer stronger without any other influencing factors. But in fact, the annealing processes are complicated and many other factors should be considered. From Fig. 6(a), the measured interplanar spacing is 0.234 nm, which is very close to the Ag (111) plane (d(111) ¼ 0.236 nm). However, in Fig. 6(c), the measured interplanar spacing of Ag (111) plane is 0.228 nm, suggesting an obvious interplanar contraction effect after anneal ing. Hence we investigated the XRD data to analyze the stress state of the multilayers. The XRD patterns of multilayer with h ¼ 20 nm at RT, 200 � C, 300 � C and 400 � C are plotted in Fig. 7(a). The diffraction peaks of Ag (111) and Cu (111) at different annealing states all shift to high angle. It is consistent with the measurement results of interplanar spacing in Fig. 6. We estimated the residual stress with the measured 2θ in the 2theta scan in the following equation [35,36],
σr ¼
E Δ2θ ð1 þ νÞ 2 tanθ0
Table 2 The calculated values of fs and fρ at different annealing temperatures. Layer thickness (nm) h¼ h¼ h¼ h¼ h¼ h¼
3 5 10 20 50 100
RT
200 � C
300 � C
fs/fρ
fs/fρ
fs/fρ
1/1 1/1 1/1 1/1 1/1 1/1
1.13/1.14 1.08/1.11 1.06/1.07 1.11/1.06 1.11/1.07 1.16/1.11
/ / 1.04/1.12 1.09/1.26 1.04/1.25 1.10/1.38
increase of twins, the compressive stresses and the lack of dislocations. When the annealing temperature rises to 300 � C, it is inferred that the compressive stress state and the dislocation starvation contribute a lot to the hardness enhancement. In order to evaluate the enhancement effect of combined properties of strength and electrical conductivity, the annealed hardness and electrical conductivity are compared with those at RT. The fs and fρ are defined to indicate the enhancement coefficient of strength and elec trical conductivity, respectively. Taking the hardness and electrical conductivity at RT as the basis for comparison, fs and fρ are given by
(1)
fs ¼
σT HT ¼ σ RT HRT
(2)
fρ ¼
1=ρT ρ ¼ RT 1=ρRT ρT
(3)
where σT (HT) and σ T (HRT) represent the strength (hardness) at annealing temperature and RT, respectively. ρT and ρRT represent the electrical resistivity at annealing temperature and RT, respectively. The calculated values of fs and fρ at different annealing temperatures are listed in Table 2. Note that the data at 400 � C are omitted because of the collapse of layer structure. Considering the comprehensive enhancement effect depends on the improvement of their comprehensive properties combined the strength and the conductivity, we defined a comprehensive coefficient fcomp, which can be given by
where σ r is the residual stress, E and ν are the Young’s modulus and Poisson ratio of Ag (Cu), respectively, θ0 is the Bragg angle under stressfree condition, and Δ2θ is the difference between 2θ0 and the measured 2θ. Here EAg ¼ 82.7 GPa, νAg ¼ 0.37, θ0Ag ¼ 19.058� (Ag (111)), ECu ¼ 129 GPa, νCu ¼ 0.34, and θ0Cu ¼ 21.6485� (Cu (111)). The calculated σ r of Ag and Cu at different temperatures are shown in Fig. 7(b). At RT, the residual stresses of Ag and Cu both are small, with compressive stress in Ag grains and tensile stress in Cu grains. After annealing, the stress state of Cu becomes compressive stress. The compressive stresses in Ag and Cu both increase with the improvement of annealing temperatures and reach maximum at 300 � C. It has been well accepted that the stress state affect the strength of the films. Hardness typically increases with compressive stress [37–39]. So the compressive stress state contributes to the hardness enhancement in annealed Ag/Cu multilayer with h ¼ 20 nm. At the same time, we found that the compressive stress also exists in annealed Ag/Cu multilayers at other scales. From above analysis, we found that the hardness enhancement of multilayer with h ¼ 20 nm should come from different aspects including
fcomp ¼
2fs ⋅fρ fs þ fρ
(4)
here if fcomp > 1, it indicates the comprehensive property of the multi layer enhanced by the annealing process. It is noted that we only use this expression to describe the comprehensive enhancement effect of annealed multilayer relative to the deposition state. We substitute the data of fs and fρ of the multilayers at different 6
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Materials Science & Engineering A 772 (2020) 138818
Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgement This work was jointly supported by the Natural Science Foundation of Shandong Province (Grant No. ZR2016EMM17), National Natural Science Foundation of China (No. 51671103,51371096) and the priority academic program development of Jiangsu higher education institutions (PAPD). References Fig. 8. The coefficients fcomp plotted as a function of individual layer thickness at annealing temperatures of 200 and 300 � C, respectively.
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annealing temperatures into Eq. (4), and the comprehensive coefficient can be calculated. The coefficients fcomp plotted as a function of h is shown in Fig. 8. From Fig. 8, it reveals that the annealed multilayers have the higher fcomp than the as deposited samples, indicating the strength and the conductivity are enhanced by the annealing treatment withhin 300 � C. 4. Conclusions In this work, the hardness and electrical resistivity of annealed Ag/ Nb multilayers were investigated with h ranging from 3 to 100 nm after annealing from 200 to 400 � C for 30 min. For Ag/Cu multilayers of all scales, the hardness improves about 6–16% whereas the electrical re sistivity decreases 6–12% after annealing at 200 � C. The reduced re sistivity results from the growth of columnar grains and the increase of hardness comes from the annealing twins or SFs in the multilayers. At 300 � C, the enhancement shows layer thickness dependent due to the different interface structures. The hardness improves about 4–10% and the electrical resistivity decreases 11–28% for multilayers with h � 10 nm. But the hardness drops sharply for multilayers with h ¼ 3 and 5 nm due to the collapse of lamellar structures. A good combination of high strength and high conductivity has been obtained in annealed Ag/Cu multilayer with h � 10 nm when annealing temperature is not more than 300 � C. Both hardness and electrical conductivity are enhanced by annealing process. The improvement of electrical conductivity mainly comes from the reduction of GBs due to grain growth and the enhancement of hardness relates with the increment of TBs, the compressive stress and the lack of dislocations after annealing. This investigation provides a basis for controlling the annealing temperature of Ag/Cu multilayers. Author contributions section J.Z. Huo:Methodology, Investigation, Writing - Original Draft; M.Z. Wei:Writing- Reviewing and Editing, Project administration, Funding acquisition; Y.J. Ma:Validation,Formal analysis; Z.H. Cao:Funding acquisition,Resources; X.K. Meng: Supervision.
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