- Email: [email protected]
Fabrication of reliable Cu-Cu joints by low temperature bonding isopropanol stabilized Cu nanoparticles in air
Accepted Manuscript Fabrication of Reliable Cu-Cu Joints by Low Temperature Bonding Isopropanol Stabilized Cu nanoparticles in Air Yun Mou, Hao Cheng, Yang Peng, Mingxiang Chen PII: DOI: Reference:
S0167-577X(18)31105-4 https://doi.org/10.1016/j.matlet.2018.07.061 MLBLUE 24627
To appear in:
Materials Letters
Received Date: Revised Date: Accepted Date:
25 June 2018 11 July 2018 14 July 2018
Please cite this article as: Y. Mou, H. Cheng, Y. Peng, M. Chen, Fabrication of Reliable Cu-Cu Joints by Low Temperature Bonding Isopropanol Stabilized Cu nanoparticles in Air, Materials Letters (2018), doi: https://doi.org/ 10.1016/j.matlet.2018.07.061
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Fabrication of Reliable Cu-Cu Joints by Low Temperature Bonding Isopropanol Stabilized Cu nanoparticles in Air Yun Moua , Hao Chenga, Yang Penga*, Mingxiang Chena * a
School of Mechanical Science and Engineering, Huazhong University of Science and Technology,
Wuhan 430074, China. *Corresponding authors: [email protected], [email protected].
Abstract: We proposed a novel Cu-Cu bonding approach by low temperature bonding isopropanol (IPA) stabilized Cu nanoparticles (NPs) in air. The synthesized Cu NPs with the diameter size of about 6.5 nm have good anti-oxidative properties, and IPA has low boiling point (160°C) and reductive ability, providing the favorable conditions for low temperature bonding in air. Consequently, reliable Cu-Cu joints with high strength (>25 MPa) are fabricated after bonding at 250°C and 275°C in air, and the bonded Cu NP layers are copper crystals without oxidation. Moreover, the fracture surfaces of Cu-Cu joints also display obvious ductile deformations and sintering characteristics. Keywords: Low temperature bonding; Isopropanol (IPA); Cu nanoparticles (NPs); Oxidation; Sintering; Cu-Cu joints 1.
Introduction The power electronic packaging trends to be miniaturization, high integration, and high power
consumption [1], which results in higher junction temperature (> 250°C), but such temperature is too high for the used Sn based solders. Although Pb-, Au-, Bi-, and Zn-based solders have been applied in power electronic packaging, these solders have their own unavoidable drawbacks, including environmental damage, high cost, inferior conductivity, and high process temperature [2, 3]. Sintering of silver 1
nanoparticle (NP) paste is a novel method to address the above-mentioned problems because of its excellent conductivity and good thermo-stability [4, 5]. However, the high cost and low electro-migration resistance of silver restrict its practical application. Therefore, a low cost and Pb-free bonding material for high temperatures has not been established until now. Recently, Cu NP paste has received great attentions because of its low cost, excellent conductivity, and good electro-migration resistance, has been considered a promising substitute for these solders and silver NP paste [6-8]. Unfortunately, the spontaneous oxidation of Cu NPs significantly reduces conductivity and increases bonding temperature. For this reason, the bonding process must be performed in protective atmosphere. Although Cu-based core-shell NPs contribute to improving anti-oxidative properties [1, 9], it is still difficult to fabricate reliable Cu-Cu joints by using Cu NPs in air, and their cost and synthetic process are more expensive and complicated than Cu NPs. Aiming to solve these problems, we proposed a novel Cu-Cu bonding method by low temperature bonding isopropanol (IPA) stabilized Cu NPs in air. The effect of bonding temperature on shear strength and the evolution of crystal phases and microstructures were investigated. Reliable Cu-Cu joints with high shear strength was achieved at 250°C and 275°C, and their fracture surfaces display obvious ductile deformations without oxidation. 2.
Materials and methods The IPA stabilized Cu NPs were synthesized by sodium borohydride reduction method. Copper
acetate monohydrate (2.75 g), IPA (12.5 mL), and ethylene glycol (20 mL) were sufficiently stirred together at 5°C for 30 min. Then, sodium borohydride (1.05 g) was dissolved in ethylene glycol (10 mL), and slowly dropwise the solution at room temperature under magnetic stirring. After reaction for 30min, the synthesized Cu NPs were centrifuged by at 10000 rpm for 15 min, and washed by n-hexane to remove 2
unreacted reactants. The purified Cu NPs were treated at 80°C in N2 atmosphere for 120 min to obtain highly concentrated Cu NP paste. Subsequently, the Cu NP paste was uniformly printed on a Cu substrate and another Cu substrate was mounted on the printed Cu NP paste. Under the pressure of 2 MPa, the bonding process was performed from 200°C to 275°C for 10 min in air. 3.
Results and discussion Fig. 1(a) shows transmission electron microscope (TEM) image of Cu NPs. The Cu NPs with the
diameter size of about 6.5 nm have excellent dispersity without hard agglomeration. The crystal structures of Cu NPs were analyzed by selected area electron diffraction (SAED) pattern, as shown in Fig. 1(b). The four fringes are identified as the (111), (200), (220), and (311) crystal planes of the face-centered cubic (FCC) copper crystal. Fig. 1(c) presents the X-ray photoelectron spectroscopy (XPS) survey spectrum of Cu NPs. Carbon, nitrogen, and oxygen elements are detected on the surface of Cu NPs, indicating that IPA is absorbed on the surface of Cu NPs and prevents the oxidation of Cu NPs [10]. The characteristic Cu 2p3/2 peak at 931.7 eV is assigned as copper crystal, and no significant peaks of copper oxide are separated (Inset in Fig. 1(c)). These results demonstrate that the synthesized Cu NPs have good anti-oxidative properties. As shown in Fig. 1(d), the thermogravimetric (TG) curve reveals the copper content of the paste is about 65%, and the differential scanning calorimetric (DSC) curve has an obvious endothermic peak at approximately 200°C due to the volatilization of organics. Hence, the selected bonding temperature can ensure the removal of these organics. Cu NP paste were uniformly printed on glass substrates and bonded at different temperatures in air for 10 min, the corresponding X-ray diffractometer (XRD) results are presented in Fig. 2(a). Only three characteristic peaks of copper crystal are detected, which confirm that the bonded Cu NP layers are not oxidized. It is because that IPA has reductive ability to copper oxide (Supplementary Fig. S1), which 3
forms a reducing atmosphere to reduce or restrain the oxidation during bonding process. Besides, the sandwich structured joints hinder contact between the NP layer and air [6, 8]. Fig. 2(b) shows the shear strength of Cu-Cu joints at different bonding temperatures. The shear strength of Cu-Cu joints is enhanced by increasing the bonding temperature. The shear strength of Cu-Cu joints reaches to 28.3 MPa and 35.1 MPa after bonding at 250°C and 275°C, respectively, which are comparable to that of traditional Pb-Sn solders and Ag NP paste [4, 11]. The bonding mechanism and change of the shear strength of Cu-Cu joints can be explained by sintering theory and small size effect. Increasing bonding temperature enhances the atomic diffusion rate and driving force of the sintering, which contributes to achieving grain boundaries roughening phase transition and reducing the lattice dislocation and surface tension between the substrates and the bonded Cu NP layer to form more reliable Cu-Cu joints [12, 13]. In addition, increasing bonding temperature reduces the amount of different grain boundary facets and forms more stable bonded interface structures, and the stability of the bonded interface structures is related to the adjacent grains [14, 15]. IPA can be removed at low temperature owing to its low boiling point (160°C), which reduces the diffusion barrier of particle sintering. Moreover, Cu NPs have higher diffusion efficiency than the bulk material because of their high grain boundary and surface activation energy [16]. Fig. 3 presents scanning electron microscope (SEM) images of the fracture surfaces of Cu-Cu joints at different bonding temperatures. The grain growth and sintering necks are observed in the fracture surface after bonding at 200°C. Unfortunately, most of these structures only show the morphological changes of the sintered particles under the bonding pressure, and the apparent fracture traces are still not discovered. Many elongated dimples with sharp tips are discovered after bonding at 225°C and 250°C, suggesting that the fracture surfaces of Cu-Cu joints have obvious ductile deformation. After bonding at 275°C, the elongated dimples are obviously broadened, indicating that the reliable joints with higher 4
strength have been fabricated. In addition, the cracks and voids gradually decrease and the bonding interfaces of Cu-Cu joints become more compact with increasing bonding temperature, as show in Supplementary Fig. S2. The corresponding EDX spectra show that the oxygen contents are very low after bonding in air, and the organic solvents are completely evaporated. Fig. 4 shows TEM images of the bonding interface of Cu-Cu joint at 275°C. Under temperature and pressure, Cu NPs have grown into dense polycrystalline structures with hundreds of nanometers. These polycrystalline structures are connected through sintering necks, and small ray-like grains formed by coalescence are observed in the surfaces of polycrystalline structures. In addition, many voids exist in the bonding interface owing to the volatilization of organics and the coarsening of grains. Although there are many voids in the bonding interface, the interfacial gaps are not found between the sintered NP layer and the substrates. These observations indicate that the metallurgical bonding between the sintered NP layer and the substrates is achieved at 275°C, which ensures the high strength of Cu-Cu joints. Therefore, the IPA stabilized Cu NPs is a promising bonding material for achieving reliable Cu-Cu joints at low temperature in air. 4.
Conclusions In summary, low temperature bonding the IPA stabilized Cu NPs in air was proposed for fabricating
reliable Cu-Cu joints. The shear strength and microstructures of Cu-Cu joints were enhanced by increasing bonding temperature. After bonding at 250°C and 275°C in air, the shear strength of Cu-Cu joints reaches to 28.3 MPa and 35.1 MPa, respectively, and this high strength is derived from the metallurgical bonding between the sintered NP layer and the substrates. Moreover, the bonded Cu NP layers are copper crystals without oxidation, and the fracture surfaces of Cu-Cu joints also display obvious ductile deformations and sintering characteristics. Therefore, the IPA stabilized Cu NPs is a low 5
cost and promising bonding material for low temperature bonding and power electronic packaging. Acknowledgements This work was financially supported by the National Natural Science Foundation of China (51275194 and 51775219) and Fundamental Research Funds for Central Universities (2016JCTD112 and 2017JYCXJJ006), China Postdoctoral Science Foundation (2018M630852), and the Graduates' Innovation Fund, Huazhong University of Science and Technology (5003100033). The authors would like to thank Analytical and Testing Center of Huazhong University of Science and Technology for the support in TEM and SEM measurement. References [1] H. Ji, J. Zhou, M. Liang, H. Lu, M. Li, Ultrason. Sonochem. 41 (2018) 375–381. [2] H. Chen, T. Hu, M. Li, Z. Zhao, IEEE T. Power Electr. 32 (2017) 441–451. [3] X. Liu, H. Nishikawa, Scripta Mater. 120 (2016) 80–84. [4] S. Wang, M. Li, H. Ji, C. Wang, Scripta Mater. 69 (2013) 789–792. [5] J. Li, C. M. Johnson, C. Buttay, W. Sabbah, S. Azzopardi, J. Mater. Process. Tech. 215 (2015) 299–308. [6] Y. Mou, Y. Peng, Y. Zhang, H. Cheng, M. Chen, Mater. Lett. 227 (2018) 179–183. [7] T. Ishizaki R. Watanabe, J. Mater. Chem. 22 (2012) 25198. [8]J. Liu, H. Chen, H. Ji, M. Li, ACS Appl. Mater. Interfaces, 8 (2016) 33289–33298. [9] I. Kim, Y. Kim, K. Woo, E.-H. Ryu, K.-Y. Yon, G. Cao, J. Moon, RSC Adv. 3, (2013) 15169. [10] Y. Hokita, M. Kanzaki, T. Sugiyama, R. Arakawa, H. Kawasaki, ACS Appl. Mater. Interfaces,7 (2015) 19382–19389. [11] M. Maruyama, R. Matsubayashi, H. Iwakuro, S. Isoda, T. Komatsu, Appl. Phys. A-Mater. 93 (2008) 6
467–470. [12] L. S. Shvindlerman, B. B. Straumal, Acta Metall. 33 (1985) 1735–1749. [13] L. K. Fionova, A. V. Andreeva, T. I. Zhukova, Phys. Status Solidi A 67 (1981) K15–K19. [14] B. B. Straumal, S. A. Polyakov, E. Bischoff, W. Gust E. J. Mittemeijer, Interface Sci 9 (2001) 287 [15] L.-S. Chang, E. Rabkin, B.B. Straumal, S. Hofmann, B. Baretzky, W. Gust, Defect Diff. Forum, 156 (1998) 135-146 [16] J. R. Greer, R. A. Street, Acta Mater. 55 (2007) 6345–6349.
7
Figures:
Fig. 1. (a) TEM image, (b) SAED pattern, and (c) XPS survey spectrum of Cu NPs and (d) TG-DSC curve of Cu NP paste. Inset in Fig. 1(c) is XPS Cu 2p3/2 spectrum of Cu NPs.
Fig. 2. (a) XRD patterns of bonded Cu NP layers and (b) Shear strength of Cu-Cu joints at different bonding temperatures.
8
Fig. 3. SEM images of the fracture surfaces of Cu−Cu joints at different bonding temperatures. Insets in figures are the corresponding EDX spectra.
Fig. 4. TEM images of the bonding interface of Cu-Cu joint at 275°C.
9
Highlights:
The isopropanol (IPA) stabilized Cu nanoparticles (NPs) were synthesized in air.
The IPA stabilized Cu NPs have good anti-oxidative property.
Reliable Cu-Cu joints by using IPA stabilized Cu NPs were achieved in air.
The bonding mechanism of the IPA stabilized Cu NPs was proposed.
10
S0167-577X(18)31105-4 https://doi.org/10.1016/j.matlet.2018.07.061 MLBLUE 24627
To appear in:
Materials Letters
Received Date: Revised Date: Accepted Date:
25 June 2018 11 July 2018 14 July 2018
Please cite this article as: Y. Mou, H. Cheng, Y. Peng, M. Chen, Fabrication of Reliable Cu-Cu Joints by Low Temperature Bonding Isopropanol Stabilized Cu nanoparticles in Air, Materials Letters (2018), doi: https://doi.org/ 10.1016/j.matlet.2018.07.061
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Fabrication of Reliable Cu-Cu Joints by Low Temperature Bonding Isopropanol Stabilized Cu nanoparticles in Air Yun Moua , Hao Chenga, Yang Penga*, Mingxiang Chena * a
School of Mechanical Science and Engineering, Huazhong University of Science and Technology,
Wuhan 430074, China. *Corresponding authors: [email protected], [email protected].
Abstract: We proposed a novel Cu-Cu bonding approach by low temperature bonding isopropanol (IPA) stabilized Cu nanoparticles (NPs) in air. The synthesized Cu NPs with the diameter size of about 6.5 nm have good anti-oxidative properties, and IPA has low boiling point (160°C) and reductive ability, providing the favorable conditions for low temperature bonding in air. Consequently, reliable Cu-Cu joints with high strength (>25 MPa) are fabricated after bonding at 250°C and 275°C in air, and the bonded Cu NP layers are copper crystals without oxidation. Moreover, the fracture surfaces of Cu-Cu joints also display obvious ductile deformations and sintering characteristics. Keywords: Low temperature bonding; Isopropanol (IPA); Cu nanoparticles (NPs); Oxidation; Sintering; Cu-Cu joints 1.
Introduction The power electronic packaging trends to be miniaturization, high integration, and high power
consumption [1], which results in higher junction temperature (> 250°C), but such temperature is too high for the used Sn based solders. Although Pb-, Au-, Bi-, and Zn-based solders have been applied in power electronic packaging, these solders have their own unavoidable drawbacks, including environmental damage, high cost, inferior conductivity, and high process temperature [2, 3]. Sintering of silver 1
nanoparticle (NP) paste is a novel method to address the above-mentioned problems because of its excellent conductivity and good thermo-stability [4, 5]. However, the high cost and low electro-migration resistance of silver restrict its practical application. Therefore, a low cost and Pb-free bonding material for high temperatures has not been established until now. Recently, Cu NP paste has received great attentions because of its low cost, excellent conductivity, and good electro-migration resistance, has been considered a promising substitute for these solders and silver NP paste [6-8]. Unfortunately, the spontaneous oxidation of Cu NPs significantly reduces conductivity and increases bonding temperature. For this reason, the bonding process must be performed in protective atmosphere. Although Cu-based core-shell NPs contribute to improving anti-oxidative properties [1, 9], it is still difficult to fabricate reliable Cu-Cu joints by using Cu NPs in air, and their cost and synthetic process are more expensive and complicated than Cu NPs. Aiming to solve these problems, we proposed a novel Cu-Cu bonding method by low temperature bonding isopropanol (IPA) stabilized Cu NPs in air. The effect of bonding temperature on shear strength and the evolution of crystal phases and microstructures were investigated. Reliable Cu-Cu joints with high shear strength was achieved at 250°C and 275°C, and their fracture surfaces display obvious ductile deformations without oxidation. 2.
Materials and methods The IPA stabilized Cu NPs were synthesized by sodium borohydride reduction method. Copper
acetate monohydrate (2.75 g), IPA (12.5 mL), and ethylene glycol (20 mL) were sufficiently stirred together at 5°C for 30 min. Then, sodium borohydride (1.05 g) was dissolved in ethylene glycol (10 mL), and slowly dropwise the solution at room temperature under magnetic stirring. After reaction for 30min, the synthesized Cu NPs were centrifuged by at 10000 rpm for 15 min, and washed by n-hexane to remove 2
unreacted reactants. The purified Cu NPs were treated at 80°C in N2 atmosphere for 120 min to obtain highly concentrated Cu NP paste. Subsequently, the Cu NP paste was uniformly printed on a Cu substrate and another Cu substrate was mounted on the printed Cu NP paste. Under the pressure of 2 MPa, the bonding process was performed from 200°C to 275°C for 10 min in air. 3.
Results and discussion Fig. 1(a) shows transmission electron microscope (TEM) image of Cu NPs. The Cu NPs with the
diameter size of about 6.5 nm have excellent dispersity without hard agglomeration. The crystal structures of Cu NPs were analyzed by selected area electron diffraction (SAED) pattern, as shown in Fig. 1(b). The four fringes are identified as the (111), (200), (220), and (311) crystal planes of the face-centered cubic (FCC) copper crystal. Fig. 1(c) presents the X-ray photoelectron spectroscopy (XPS) survey spectrum of Cu NPs. Carbon, nitrogen, and oxygen elements are detected on the surface of Cu NPs, indicating that IPA is absorbed on the surface of Cu NPs and prevents the oxidation of Cu NPs [10]. The characteristic Cu 2p3/2 peak at 931.7 eV is assigned as copper crystal, and no significant peaks of copper oxide are separated (Inset in Fig. 1(c)). These results demonstrate that the synthesized Cu NPs have good anti-oxidative properties. As shown in Fig. 1(d), the thermogravimetric (TG) curve reveals the copper content of the paste is about 65%, and the differential scanning calorimetric (DSC) curve has an obvious endothermic peak at approximately 200°C due to the volatilization of organics. Hence, the selected bonding temperature can ensure the removal of these organics. Cu NP paste were uniformly printed on glass substrates and bonded at different temperatures in air for 10 min, the corresponding X-ray diffractometer (XRD) results are presented in Fig. 2(a). Only three characteristic peaks of copper crystal are detected, which confirm that the bonded Cu NP layers are not oxidized. It is because that IPA has reductive ability to copper oxide (Supplementary Fig. S1), which 3
forms a reducing atmosphere to reduce or restrain the oxidation during bonding process. Besides, the sandwich structured joints hinder contact between the NP layer and air [6, 8]. Fig. 2(b) shows the shear strength of Cu-Cu joints at different bonding temperatures. The shear strength of Cu-Cu joints is enhanced by increasing the bonding temperature. The shear strength of Cu-Cu joints reaches to 28.3 MPa and 35.1 MPa after bonding at 250°C and 275°C, respectively, which are comparable to that of traditional Pb-Sn solders and Ag NP paste [4, 11]. The bonding mechanism and change of the shear strength of Cu-Cu joints can be explained by sintering theory and small size effect. Increasing bonding temperature enhances the atomic diffusion rate and driving force of the sintering, which contributes to achieving grain boundaries roughening phase transition and reducing the lattice dislocation and surface tension between the substrates and the bonded Cu NP layer to form more reliable Cu-Cu joints [12, 13]. In addition, increasing bonding temperature reduces the amount of different grain boundary facets and forms more stable bonded interface structures, and the stability of the bonded interface structures is related to the adjacent grains [14, 15]. IPA can be removed at low temperature owing to its low boiling point (160°C), which reduces the diffusion barrier of particle sintering. Moreover, Cu NPs have higher diffusion efficiency than the bulk material because of their high grain boundary and surface activation energy [16]. Fig. 3 presents scanning electron microscope (SEM) images of the fracture surfaces of Cu-Cu joints at different bonding temperatures. The grain growth and sintering necks are observed in the fracture surface after bonding at 200°C. Unfortunately, most of these structures only show the morphological changes of the sintered particles under the bonding pressure, and the apparent fracture traces are still not discovered. Many elongated dimples with sharp tips are discovered after bonding at 225°C and 250°C, suggesting that the fracture surfaces of Cu-Cu joints have obvious ductile deformation. After bonding at 275°C, the elongated dimples are obviously broadened, indicating that the reliable joints with higher 4
strength have been fabricated. In addition, the cracks and voids gradually decrease and the bonding interfaces of Cu-Cu joints become more compact with increasing bonding temperature, as show in Supplementary Fig. S2. The corresponding EDX spectra show that the oxygen contents are very low after bonding in air, and the organic solvents are completely evaporated. Fig. 4 shows TEM images of the bonding interface of Cu-Cu joint at 275°C. Under temperature and pressure, Cu NPs have grown into dense polycrystalline structures with hundreds of nanometers. These polycrystalline structures are connected through sintering necks, and small ray-like grains formed by coalescence are observed in the surfaces of polycrystalline structures. In addition, many voids exist in the bonding interface owing to the volatilization of organics and the coarsening of grains. Although there are many voids in the bonding interface, the interfacial gaps are not found between the sintered NP layer and the substrates. These observations indicate that the metallurgical bonding between the sintered NP layer and the substrates is achieved at 275°C, which ensures the high strength of Cu-Cu joints. Therefore, the IPA stabilized Cu NPs is a promising bonding material for achieving reliable Cu-Cu joints at low temperature in air. 4.
Conclusions In summary, low temperature bonding the IPA stabilized Cu NPs in air was proposed for fabricating
reliable Cu-Cu joints. The shear strength and microstructures of Cu-Cu joints were enhanced by increasing bonding temperature. After bonding at 250°C and 275°C in air, the shear strength of Cu-Cu joints reaches to 28.3 MPa and 35.1 MPa, respectively, and this high strength is derived from the metallurgical bonding between the sintered NP layer and the substrates. Moreover, the bonded Cu NP layers are copper crystals without oxidation, and the fracture surfaces of Cu-Cu joints also display obvious ductile deformations and sintering characteristics. Therefore, the IPA stabilized Cu NPs is a low 5
cost and promising bonding material for low temperature bonding and power electronic packaging. Acknowledgements This work was financially supported by the National Natural Science Foundation of China (51275194 and 51775219) and Fundamental Research Funds for Central Universities (2016JCTD112 and 2017JYCXJJ006), China Postdoctoral Science Foundation (2018M630852), and the Graduates' Innovation Fund, Huazhong University of Science and Technology (5003100033). The authors would like to thank Analytical and Testing Center of Huazhong University of Science and Technology for the support in TEM and SEM measurement. References [1] H. Ji, J. Zhou, M. Liang, H. Lu, M. Li, Ultrason. Sonochem. 41 (2018) 375–381. [2] H. Chen, T. Hu, M. Li, Z. Zhao, IEEE T. Power Electr. 32 (2017) 441–451. [3] X. Liu, H. Nishikawa, Scripta Mater. 120 (2016) 80–84. [4] S. Wang, M. Li, H. Ji, C. Wang, Scripta Mater. 69 (2013) 789–792. [5] J. Li, C. M. Johnson, C. Buttay, W. Sabbah, S. Azzopardi, J. Mater. Process. Tech. 215 (2015) 299–308. [6] Y. Mou, Y. Peng, Y. Zhang, H. Cheng, M. Chen, Mater. Lett. 227 (2018) 179–183. [7] T. Ishizaki R. Watanabe, J. Mater. Chem. 22 (2012) 25198. [8]J. Liu, H. Chen, H. Ji, M. Li, ACS Appl. Mater. Interfaces, 8 (2016) 33289–33298. [9] I. Kim, Y. Kim, K. Woo, E.-H. Ryu, K.-Y. Yon, G. Cao, J. Moon, RSC Adv. 3, (2013) 15169. [10] Y. Hokita, M. Kanzaki, T. Sugiyama, R. Arakawa, H. Kawasaki, ACS Appl. Mater. Interfaces,7 (2015) 19382–19389. [11] M. Maruyama, R. Matsubayashi, H. Iwakuro, S. Isoda, T. Komatsu, Appl. Phys. A-Mater. 93 (2008) 6
467–470. [12] L. S. Shvindlerman, B. B. Straumal, Acta Metall. 33 (1985) 1735–1749. [13] L. K. Fionova, A. V. Andreeva, T. I. Zhukova, Phys. Status Solidi A 67 (1981) K15–K19. [14] B. B. Straumal, S. A. Polyakov, E. Bischoff, W. Gust E. J. Mittemeijer, Interface Sci 9 (2001) 287 [15] L.-S. Chang, E. Rabkin, B.B. Straumal, S. Hofmann, B. Baretzky, W. Gust, Defect Diff. Forum, 156 (1998) 135-146 [16] J. R. Greer, R. A. Street, Acta Mater. 55 (2007) 6345–6349.
7
Figures:
Fig. 1. (a) TEM image, (b) SAED pattern, and (c) XPS survey spectrum of Cu NPs and (d) TG-DSC curve of Cu NP paste. Inset in Fig. 1(c) is XPS Cu 2p3/2 spectrum of Cu NPs.
Fig. 2. (a) XRD patterns of bonded Cu NP layers and (b) Shear strength of Cu-Cu joints at different bonding temperatures.
8
Fig. 3. SEM images of the fracture surfaces of Cu−Cu joints at different bonding temperatures. Insets in figures are the corresponding EDX spectra.
Fig. 4. TEM images of the bonding interface of Cu-Cu joint at 275°C.
9
Highlights:
The isopropanol (IPA) stabilized Cu nanoparticles (NPs) were synthesized in air.
The IPA stabilized Cu NPs have good anti-oxidative property.
Reliable Cu-Cu joints by using IPA stabilized Cu NPs were achieved in air.
The bonding mechanism of the IPA stabilized Cu NPs was proposed.
10