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Pressureless low-temperature sintering of plasma activated Ag nanoparticles for high-power device packaging
Materials Letters 256 (2019) 126620
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/mlblue
Pressureless low-temperature sintering of plasma activated Ag nanoparticles for high-power device packaging Hui Fang a, Chenxi Wang a,⇑, Te Wang a, Hong Wang b, Shicheng Zhou a, Yilong Huang a, Yanhong Tian a a b
State Key Laboratory of Advanced Welding and Joining, Harbin Institute of Technology, Harbin 150001, China Academy of Space Electronic Information Technology, Xi’an 710100, China
a r t i c l e
i n f o
Article history: Received 20 July 2019 Received in revised form 29 August 2019 Accepted 2 September 2019 Available online 3 September 2019 Keywords: Nanoparticles Sintering Plasma activation Low-temperature bonding Thermal conductivity Shear strength
a b s t r a c t A controllable plasma-activated method to achieve pressureless sintering of Ag nanoparticles is proposed for low-temperature bonding. O2 plasma activation is employed to effectively decompose organics coated on the nanoparticles prior to sintering, avoiding the loose interfacial microstructures. Therefore, robust bonding Cu-Cu joints with high shear strength (>20 MPa) can be formed at a low temperature of 200 °C without requiring additional pressure. This rapid and controllable activation method significantly enhances the sinterability of the Ag particles, leading to a dense microstructure. The improved thermal conductivity is three times higher than the one prepared without plasma activation. Ó 2019 Elsevier B.V. All rights reserved.
1. Introduction With the application of wide bandgap semiconductor, highpower electronic devices are expected to operate at high temperatures (>250 °C) [1]. Solder alloys are impeded by their limited working temperature range and their inherent drawbacks, such as low-temperature melted Sn-based solders and high-cost Aubased solders [2,3]. Alternatively, Ag nanoparticle paste is a promising material in high-power device packaging because it meets the demand of low-temperature bonding and hightemperature service [4,5]. Ag nanoparticles (Ag NPs) can be sintered at lower temperatures due to the excellent intrinsic Ag properties [6]. However, the organic shells coated on Ag NPs will be decomposed during sintering process, causing a loose microstructure and poor interfacial properties [7]. To remove organics, the sintering process is usually carried out with a long term (>30 min), a higher sintering temperature (250–350 °C) even a larger auxiliary pressure (1–20 MPa) [6,8]. However, these conditions will easily cause irreversible damage on the chips and induce thermal stress to destroy heterogeneous structure. Therefore, it is necessary to develop a controllable bonding process using metal nanoparticles in favor of realizing rapid low temperature pressureless sintering.
⇑ Corresponding author. E-mail address: [email protected] (C. Wang). https://doi.org/10.1016/j.matlet.2019.126620 0167-577X/Ó 2019 Elsevier B.V. All rights reserved.
Oxygen plasma activation is an effective and environmentally friendly method, in which O2 free radicals react with organics and byproducts volatile gas can be extracted by vacuum system [9]. In this paper, O2 plasma activation is proposed to reduce the organics adsorbing on AgNPs, thus achieving compact bonding joints at low temperature without pressure. After removing part of organics, the plasma-activated Ag NPs with higher surface energy might tend to exhibit spontaneous aggregation, growth and sintering behavior. This point of view led to a new way to control the chemical composition of Ag nanoparticle surfaces and lower the sintering temperature. The effects of O2 plasma on Ag NPs are analyzed and the interfacial properties of the sintered joints are investigated to gain insight the mechanisms.
2. Materials and methods Commercial Ag pastes (CT2700R7S, Kyocera) contain Ag flakes/ particles coated with organics, e.g., poly (N-vinylpyrrolidone) (PVP) and ethylene glycol (EG). The standard sintering process of this Ag paste provided by supplier is first keeping the temperature at 150 °C for 30 min to ensure the decomposition of organics, then sintered at 250 °C for 90 min, finally reaching the shear strength of about 20 MPa. In this work, Ag pastes were placed on polished Cu chips (1 mm 1 mm 1 mm) using solder mask to control the same initial thickness of about 85.0 mm. The chips were then settled in a plasma chamber, in which the discharged power are
2
H. Fang et al. / Materials Letters 256 (2019) 126620
fixed at 120–180 W. After activation for different time, the two chips were brought into contact without additional pressure and were heated at 150 or 200 °C in ambient air with a constant heating rate of 5 °C min1. The surface states of Ag NPs were analyzed by Flourier transform infrared spectroscopy (FT-IR, Nicolet is50, Thermo Fisher). Thermogravimetric differential scanning calorimeter (TG/DSC; STA449F3, Netzsch) was used to confirm the organic content in the Ag NPs with a heating rate of 10 °Cmin1. The SEM morphology was observed by scanning electron microscope (SEM, HELIOS NanoLab 600i, FEI). The pore distribution was measured by MATLAB image processing method [10]. The shear strength of the sintered joints was measured by a bond tester (Series 4000, Dage). Because the thickness of Ag layer in this work was thin, indirect measurement method is chosen [11]. The Ag paste was coated on both sides of copper foil (10 10 mm2), and the thermal conductivity of the Ag (k0) was calculated by 0 d k0 ¼ khkkk0 ðhdÞ , where k and k0 are the thermal conductivity of copper foil and whole structure, d and h are the thickness of Ag layer and whole structure. And k and k0 were calculated by k = aqc, where a is thermal diffusivity, q is density, and c is the heat capacity measured by a laser flash conductometer (LFA467, Netzsch).
3. Results and discussion Fig. 1a shows the FTIR spectra of Ag pastes treated with different activation time. The intensities of peaks at 2854 cm1 (assigned to C–H) and 1274 cm1 (assigned to C–N) significantly decreased with the increased activation time, which were related to the breakage of long chain from PVP and the decomposition of pyrrolidone group, respectively. Moreover, the relatively weak peak at 3290 cm1 corresponds to O–H, indicating the reduction of organic EG concentration. Fig. 1b and c show that the obvious weight loss of Ag pastes appeared along with the thermal process. Endothermic peaks presented at approximately 160–170 °C due to the decomposition of organics. The thermal weight loss of 10 s activated Ag paste was about 6%, which was lower than that of non-activation process (8%), attributing to the removal of organics.
Therefore, the activation process can effectively reduce organics in Ag pastes, which might contribute to the sintering process. The activation depth is a critical factor to realize the pressureless low-temperature sintering, as shown in Fig. 2a (see Supplementary Fig. S1 for high resolution SEM images). As output power of plasma equipment raised, the effective activation depth increased from 37.8 mm to 48.9 mm, which totally meets the requirements in practical electronic packaging (<20 mm). Notably, the morphologies exhibited distinct differences in density between the compact upper layer (effectively activated) and loose lower layer (non-activated). Different plasma activation time led to distinguished organics removal from Ag NPs. Fig. 2b exhibited the shear strength of bonding joints obtained by different activation time (black lines: sintering at 150 °C and 200 °C for 20 min) and different sintering time (blue line: sintering at 200 °C after activation for 10 s). When the activation time was short (<10 s), the organics were not completely decomposed, and the sintering ability of Ag paste was poor, which led to fracture failure in the bond zone in shear test. When the activation time exceeded 10 s, the shear strength decreased (black line). The long activation time resulted in high temperature before sintering, making Ag paste pre-sinter into bulk. Moreover, too long activation time would lead to serious oxidation of Ag paste surface, which weakened the bonding strength of Ag-Cu interface. Therefore, the activation time should be strictly controlled at 10 s. When extending the sintering time to 20 min, the shear strength increased to 21 MPa, which was comparable to the value required by supplier and reported studies using similar pressureless sintering process at 200 °C [7,12,13]. Therefore, the interfacial porosity and thermal conductivity of sintered joints were investigated at 200 °C for 20 min, as shown in Fig. 2c and d. The embedded micrograph images of the pore distribution in high resolution for Fig. 2c are as exhibited in Supplementary Fig. S2. Without O2 plasma activation, many large pores were surrounded by the coarse necks, which acted as the main pathway to release the gases decomposed from organics. By comparison, the porosity exhibited a dramatically decreased trend along with the increased activation time. After the plasma activation only for 5 s, the thermal conductivity tripled compared
Fig. 1. (a) FTIR spectra of Ag NPs at different activation time; TG-DSC curves of the Ag NPs (b) before and (c) after activation for 10 s.
H. Fang et al. / Materials Letters 256 (2019) 126620
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Fig. 2. (a) Effective activation depth of NPs paste at different discharged power; (b) Shear strength of the joints under different activation time sintered at 150 °C and 200 °C for 20 min (black lines) and after 10 s activation sintered at 200 °C for different time; (c) Porosity and (d) Thermal conductivity of Ag pastes at different activation time.
to that of non-activated samples. Therefore, O2 plasma activation significantly improved the shear strength and thermal conductivity. Fig. 3a and b show the Ag NPs morphologies without and with the plasma activation for 10 s. A part of adjacent particles aggregated together after activation. The lower content of residual organics is beneficial for the diffusion of adjacent Ag particles with
higher surface energy [14]. We also noticed that the temperature of Ag paste increased gradually during activation (see Supplementary Fig. S3), providing a pre-driving force for the diffusion between adjacent nanoparticles. Nevertheless, the activation time should be strictly controlled within 20 s to prevent nanoparticles presintering into bulk. The cross-sectional microstructures of the bonding joints without and with the plasma activation are shown in Fig. 3c and d. Only a few porosity could be detected across the interfaces prepared by the activated Ag paste. It implies that the plasma activation resulted in NPs coated with thinner organic shells, inducing less void formation and a compact interfacial structure. 4. Conclusion A controllable plasma-activated method to achieve pressureless sintering of Ag nanoparticles was developed for low-temperature bonding. After the O2 plasma activation for 10 s, the Ag NPs sintering was realized at 200 °C without pressure, exhibiting reliable bonding joints and dense microstructures. Both shear strength and thermal conductivity were significantly improved. The interfacial porosity dramatically decreased by forming coarse necks at low temperatures. It is noting that the plasma activation could not only effectively remove organics, but also provide pre-driving force for the diffusion of nanoparticles. In the new point of view, this facile and controllable plasma activated sintering process has a great potential in high-power device packaging. Declaration of Competing Interest
Fig. 3. SEM images of Ag NPs without (a) and with (b) plasma activation for 10 s; Cross-sectional images of the bonding joints at 200 °C prepared (c) without and with (d) plasma activation for 10 s.
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.
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H. Fang et al. / Materials Letters 256 (2019) 126620
Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant No. 51975151), the China Postdoctoral Science Foundation (Grant No. 2017M610207), and the Heilongjiang Provincial Natural Science Foundation of China (Grant No. LH2019E041). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.matlet.2019.126620. References [1] X.D. Liu, H. Nishikawa, Scripta Mater. 120 (2016) 80.
[2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14]
V. Chidambaram, J. Hattel, J. Hald, Microelectron. Eng. 88 (2011) 981. S. Kim, K. Kim, S. Kim, K. Suganuma, J. Electron. Mater. 38 (2009) 266. M.A. Asoro, D. Kovar, P.J. Ferreira, Chem. Commun. 50 (2014) 4835. J. Yan, G. Zou, A. Hu, Y.N. Zhou, J. Mater. Chem. 21 (2011) 15981. T.G. Lei, J.N. Calata, L. Guo-Quan, C. Xu, L. Shufang, IEEE T. Compon. Pack. T. 33 (2010) 98. J.F. Yan, G.S. Zou, A.P. Wu, J.L. Ren, J.C. Yan, A.M. Hu, Y. Zhou, Scripta Mater. 66 (2012) 582. T. Morita, E. Ide, Y. Yasuda, A. Hirose, K. Kobayashi, Jpn. J. Appl. Phys. 47 (2008) 6615. K. Choi, S. Ghosh, J. Lim, C.M. Lee, Appl. Surf. Sci. 206 (2003) 355. G.Q. Lu, W.L. Li, Y.H. Mei, G. Chen, X. Li, X. Chen, IEEE Trans. Device Mater. Reliab. 14 (2014) 626. Y.B. Cai, G.J. Wu, J. Jin, New Build. Mater. 38 (2011) 89. C.T. Chen, S. Noh, H. Zhang, C.Y. Choe, J.T. Jiu, S. Nagao, K. Suganuma, Scripta Mater. 146 (2018) 123. S. Wang, M.Y. Li, H.J. Ji, C.Q. Wang, Scripta Mater. 69 (2013) 789. S.C. Fu, M. Zhao, H.Y. Shan, Y. Li, Mater. Lett. 226 (2018) 26.
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/mlblue
Pressureless low-temperature sintering of plasma activated Ag nanoparticles for high-power device packaging Hui Fang a, Chenxi Wang a,⇑, Te Wang a, Hong Wang b, Shicheng Zhou a, Yilong Huang a, Yanhong Tian a a b
State Key Laboratory of Advanced Welding and Joining, Harbin Institute of Technology, Harbin 150001, China Academy of Space Electronic Information Technology, Xi’an 710100, China
a r t i c l e
i n f o
Article history: Received 20 July 2019 Received in revised form 29 August 2019 Accepted 2 September 2019 Available online 3 September 2019 Keywords: Nanoparticles Sintering Plasma activation Low-temperature bonding Thermal conductivity Shear strength
a b s t r a c t A controllable plasma-activated method to achieve pressureless sintering of Ag nanoparticles is proposed for low-temperature bonding. O2 plasma activation is employed to effectively decompose organics coated on the nanoparticles prior to sintering, avoiding the loose interfacial microstructures. Therefore, robust bonding Cu-Cu joints with high shear strength (>20 MPa) can be formed at a low temperature of 200 °C without requiring additional pressure. This rapid and controllable activation method significantly enhances the sinterability of the Ag particles, leading to a dense microstructure. The improved thermal conductivity is three times higher than the one prepared without plasma activation. Ó 2019 Elsevier B.V. All rights reserved.
1. Introduction With the application of wide bandgap semiconductor, highpower electronic devices are expected to operate at high temperatures (>250 °C) [1]. Solder alloys are impeded by their limited working temperature range and their inherent drawbacks, such as low-temperature melted Sn-based solders and high-cost Aubased solders [2,3]. Alternatively, Ag nanoparticle paste is a promising material in high-power device packaging because it meets the demand of low-temperature bonding and hightemperature service [4,5]. Ag nanoparticles (Ag NPs) can be sintered at lower temperatures due to the excellent intrinsic Ag properties [6]. However, the organic shells coated on Ag NPs will be decomposed during sintering process, causing a loose microstructure and poor interfacial properties [7]. To remove organics, the sintering process is usually carried out with a long term (>30 min), a higher sintering temperature (250–350 °C) even a larger auxiliary pressure (1–20 MPa) [6,8]. However, these conditions will easily cause irreversible damage on the chips and induce thermal stress to destroy heterogeneous structure. Therefore, it is necessary to develop a controllable bonding process using metal nanoparticles in favor of realizing rapid low temperature pressureless sintering.
⇑ Corresponding author. E-mail address: [email protected] (C. Wang). https://doi.org/10.1016/j.matlet.2019.126620 0167-577X/Ó 2019 Elsevier B.V. All rights reserved.
Oxygen plasma activation is an effective and environmentally friendly method, in which O2 free radicals react with organics and byproducts volatile gas can be extracted by vacuum system [9]. In this paper, O2 plasma activation is proposed to reduce the organics adsorbing on AgNPs, thus achieving compact bonding joints at low temperature without pressure. After removing part of organics, the plasma-activated Ag NPs with higher surface energy might tend to exhibit spontaneous aggregation, growth and sintering behavior. This point of view led to a new way to control the chemical composition of Ag nanoparticle surfaces and lower the sintering temperature. The effects of O2 plasma on Ag NPs are analyzed and the interfacial properties of the sintered joints are investigated to gain insight the mechanisms.
2. Materials and methods Commercial Ag pastes (CT2700R7S, Kyocera) contain Ag flakes/ particles coated with organics, e.g., poly (N-vinylpyrrolidone) (PVP) and ethylene glycol (EG). The standard sintering process of this Ag paste provided by supplier is first keeping the temperature at 150 °C for 30 min to ensure the decomposition of organics, then sintered at 250 °C for 90 min, finally reaching the shear strength of about 20 MPa. In this work, Ag pastes were placed on polished Cu chips (1 mm 1 mm 1 mm) using solder mask to control the same initial thickness of about 85.0 mm. The chips were then settled in a plasma chamber, in which the discharged power are
2
H. Fang et al. / Materials Letters 256 (2019) 126620
fixed at 120–180 W. After activation for different time, the two chips were brought into contact without additional pressure and were heated at 150 or 200 °C in ambient air with a constant heating rate of 5 °C min1. The surface states of Ag NPs were analyzed by Flourier transform infrared spectroscopy (FT-IR, Nicolet is50, Thermo Fisher). Thermogravimetric differential scanning calorimeter (TG/DSC; STA449F3, Netzsch) was used to confirm the organic content in the Ag NPs with a heating rate of 10 °Cmin1. The SEM morphology was observed by scanning electron microscope (SEM, HELIOS NanoLab 600i, FEI). The pore distribution was measured by MATLAB image processing method [10]. The shear strength of the sintered joints was measured by a bond tester (Series 4000, Dage). Because the thickness of Ag layer in this work was thin, indirect measurement method is chosen [11]. The Ag paste was coated on both sides of copper foil (10 10 mm2), and the thermal conductivity of the Ag (k0) was calculated by 0 d k0 ¼ khkkk0 ðhdÞ , where k and k0 are the thermal conductivity of copper foil and whole structure, d and h are the thickness of Ag layer and whole structure. And k and k0 were calculated by k = aqc, where a is thermal diffusivity, q is density, and c is the heat capacity measured by a laser flash conductometer (LFA467, Netzsch).
3. Results and discussion Fig. 1a shows the FTIR spectra of Ag pastes treated with different activation time. The intensities of peaks at 2854 cm1 (assigned to C–H) and 1274 cm1 (assigned to C–N) significantly decreased with the increased activation time, which were related to the breakage of long chain from PVP and the decomposition of pyrrolidone group, respectively. Moreover, the relatively weak peak at 3290 cm1 corresponds to O–H, indicating the reduction of organic EG concentration. Fig. 1b and c show that the obvious weight loss of Ag pastes appeared along with the thermal process. Endothermic peaks presented at approximately 160–170 °C due to the decomposition of organics. The thermal weight loss of 10 s activated Ag paste was about 6%, which was lower than that of non-activation process (8%), attributing to the removal of organics.
Therefore, the activation process can effectively reduce organics in Ag pastes, which might contribute to the sintering process. The activation depth is a critical factor to realize the pressureless low-temperature sintering, as shown in Fig. 2a (see Supplementary Fig. S1 for high resolution SEM images). As output power of plasma equipment raised, the effective activation depth increased from 37.8 mm to 48.9 mm, which totally meets the requirements in practical electronic packaging (<20 mm). Notably, the morphologies exhibited distinct differences in density between the compact upper layer (effectively activated) and loose lower layer (non-activated). Different plasma activation time led to distinguished organics removal from Ag NPs. Fig. 2b exhibited the shear strength of bonding joints obtained by different activation time (black lines: sintering at 150 °C and 200 °C for 20 min) and different sintering time (blue line: sintering at 200 °C after activation for 10 s). When the activation time was short (<10 s), the organics were not completely decomposed, and the sintering ability of Ag paste was poor, which led to fracture failure in the bond zone in shear test. When the activation time exceeded 10 s, the shear strength decreased (black line). The long activation time resulted in high temperature before sintering, making Ag paste pre-sinter into bulk. Moreover, too long activation time would lead to serious oxidation of Ag paste surface, which weakened the bonding strength of Ag-Cu interface. Therefore, the activation time should be strictly controlled at 10 s. When extending the sintering time to 20 min, the shear strength increased to 21 MPa, which was comparable to the value required by supplier and reported studies using similar pressureless sintering process at 200 °C [7,12,13]. Therefore, the interfacial porosity and thermal conductivity of sintered joints were investigated at 200 °C for 20 min, as shown in Fig. 2c and d. The embedded micrograph images of the pore distribution in high resolution for Fig. 2c are as exhibited in Supplementary Fig. S2. Without O2 plasma activation, many large pores were surrounded by the coarse necks, which acted as the main pathway to release the gases decomposed from organics. By comparison, the porosity exhibited a dramatically decreased trend along with the increased activation time. After the plasma activation only for 5 s, the thermal conductivity tripled compared
Fig. 1. (a) FTIR spectra of Ag NPs at different activation time; TG-DSC curves of the Ag NPs (b) before and (c) after activation for 10 s.
H. Fang et al. / Materials Letters 256 (2019) 126620
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Fig. 2. (a) Effective activation depth of NPs paste at different discharged power; (b) Shear strength of the joints under different activation time sintered at 150 °C and 200 °C for 20 min (black lines) and after 10 s activation sintered at 200 °C for different time; (c) Porosity and (d) Thermal conductivity of Ag pastes at different activation time.
to that of non-activated samples. Therefore, O2 plasma activation significantly improved the shear strength and thermal conductivity. Fig. 3a and b show the Ag NPs morphologies without and with the plasma activation for 10 s. A part of adjacent particles aggregated together after activation. The lower content of residual organics is beneficial for the diffusion of adjacent Ag particles with
higher surface energy [14]. We also noticed that the temperature of Ag paste increased gradually during activation (see Supplementary Fig. S3), providing a pre-driving force for the diffusion between adjacent nanoparticles. Nevertheless, the activation time should be strictly controlled within 20 s to prevent nanoparticles presintering into bulk. The cross-sectional microstructures of the bonding joints without and with the plasma activation are shown in Fig. 3c and d. Only a few porosity could be detected across the interfaces prepared by the activated Ag paste. It implies that the plasma activation resulted in NPs coated with thinner organic shells, inducing less void formation and a compact interfacial structure. 4. Conclusion A controllable plasma-activated method to achieve pressureless sintering of Ag nanoparticles was developed for low-temperature bonding. After the O2 plasma activation for 10 s, the Ag NPs sintering was realized at 200 °C without pressure, exhibiting reliable bonding joints and dense microstructures. Both shear strength and thermal conductivity were significantly improved. The interfacial porosity dramatically decreased by forming coarse necks at low temperatures. It is noting that the plasma activation could not only effectively remove organics, but also provide pre-driving force for the diffusion of nanoparticles. In the new point of view, this facile and controllable plasma activated sintering process has a great potential in high-power device packaging. Declaration of Competing Interest
Fig. 3. SEM images of Ag NPs without (a) and with (b) plasma activation for 10 s; Cross-sectional images of the bonding joints at 200 °C prepared (c) without and with (d) plasma activation for 10 s.
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.
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H. Fang et al. / Materials Letters 256 (2019) 126620
Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant No. 51975151), the China Postdoctoral Science Foundation (Grant No. 2017M610207), and the Heilongjiang Provincial Natural Science Foundation of China (Grant No. LH2019E041). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.matlet.2019.126620. References [1] X.D. Liu, H. Nishikawa, Scripta Mater. 120 (2016) 80.
[2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14]
V. Chidambaram, J. Hattel, J. Hald, Microelectron. Eng. 88 (2011) 981. S. Kim, K. Kim, S. Kim, K. Suganuma, J. Electron. Mater. 38 (2009) 266. M.A. Asoro, D. Kovar, P.J. Ferreira, Chem. Commun. 50 (2014) 4835. J. Yan, G. Zou, A. Hu, Y.N. Zhou, J. Mater. Chem. 21 (2011) 15981. T.G. Lei, J.N. Calata, L. Guo-Quan, C. Xu, L. Shufang, IEEE T. Compon. Pack. T. 33 (2010) 98. J.F. Yan, G.S. Zou, A.P. Wu, J.L. Ren, J.C. Yan, A.M. Hu, Y. Zhou, Scripta Mater. 66 (2012) 582. T. Morita, E. Ide, Y. Yasuda, A. Hirose, K. Kobayashi, Jpn. J. Appl. Phys. 47 (2008) 6615. K. Choi, S. Ghosh, J. Lim, C.M. Lee, Appl. Surf. Sci. 206 (2003) 355. G.Q. Lu, W.L. Li, Y.H. Mei, G. Chen, X. Li, X. Chen, IEEE Trans. Device Mater. Reliab. 14 (2014) 626. Y.B. Cai, G.J. Wu, J. Jin, New Build. Mater. 38 (2011) 89. C.T. Chen, S. Noh, H. Zhang, C.Y. Choe, J.T. Jiu, S. Nagao, K. Suganuma, Scripta Mater. 146 (2018) 123. S. Wang, M.Y. Li, H.J. Ji, C.Q. Wang, Scripta Mater. 69 (2013) 789. S.C. Fu, M. Zhao, H.Y. Shan, Y. Li, Mater. Lett. 226 (2018) 26.