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The influence of non-uniform copper oxide layer on tin whisker growth and tin whisker growth behavior in SnAg microbumps with small diameter
Materials Letters 258 (2020) 126773
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/mlblue
The influence of non-uniform copper oxide layer on tin whisker growth and tin whisker growth behavior in SnAg microbumps with small diameter Yuancheng Li, Menglong Sun, Siru Ren, Huiqin Ling, Tao Hang, An-ming Hu ⇑, Ming Li State Key Laboratory of Metal Matrix Composites, Key Laboratory for Thin Film and Microfabrication Technology of the Ministry of Education, School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, China
a r t i c l e
i n f o
Article history: Received 12 June 2019 Received in revised form 22 August 2019 Accepted 3 October 2019 Available online 8 October 2019 Keywords: Electronic materials Semiconductors Tin whisker Copper oxide layer Microbump Twin boundary
a b s t r a c t A non-uniform copper oxide layer with several hundred nanometers thick was observed covering the surface of 10 lm-diameter Cu/SnAg microbump. Tin whiskers were also found to form on the weak spots of this copper oxide layer, where were localized stress relief centers. The HRTEM results reveal the existence of twin grain boundary between tin whisker and neighboring grain. Numerous dislocations at the twin boundary provide paths for the tin atoms to slip into tin whisker. It is the first time that the existence of copper oxide layer and the relationship between copper oxide layer and tin whiskers growth was studied and revealed in micron-level bumps. This present study has significant meaning for 3D electronic packaging as small size microbumps become increasingly prevalent and in which, surface diffusion becomes more important. Ó 2019 Elsevier B.V. All rights reserved.
1. Introduction Tin whisker in electronic devices could cause severe short circuit problems when contacting with neighboring devices [1]. There has been a lot of researches attempting to explain the growth mechanism of tin whisker in lead-free solders [2–4]. Although none of them could offer an absolute convincing mechanism for tin whisker growth, the compressive stress is generally considered as the driving force for tin whisker growth [5,6]. Nevertheless, most of the studies focus on tin whisker growth on thin films or flip-chip solder bumps [7]. Three-dimensional (3D) packaging is currently the most promising way to extend the limit of Moore’s law. The microbumps, with 10 lm in diameter or even smaller, used in 3D packaging are much smaller than ball-grid-array (BGA) (around 100 lm) in flip chip technology [8]. However, few previous researches have studied tin whisker growth in isolated microbump. Sun M. [9] reported a tin whisker growth mechanism from 25 lm-diameter Sn3.5Ag/Cu microbump. Nevertheless, under the decreasing size trend, the microbumps used in industrial life have already been smaller than 20 lm and the size of solder joints could shrink to 1 lm in diameter in the forseeable future. Previous ⇑ Corresponding author. E-mail address: [email protected] (A.-m. Hu). https://doi.org/10.1016/j.matlet.2019.126773 0167-577X/Ó 2019 Elsevier B.V. All rights reserved.
studies could neither catch up with the minimizing rate in electronic industry nor predict the tin whisker growth in micron level bumps. That is the motivation and necessity of our study. 2. Experimental The samples used in this work were prepared following the procedures: (1) sputtering of Cu seed layer on the silicon substrate, (2) lithography was used to spin coat and pattern the photoresist, (3) electrodeposition of Cu and SnAg respectively, (4) reflow and cooling of the SnAg solder caps under flux protection, (5) clean up the flux, (6) the fabricated wafer was then diced to 66 mm square as the samples used in this experiment. Each silicon die contains approximately 80,000 microbumps. In real life application, the microbumps are inevitably under compression introduced by electricity [10] or heat [11]. Once the sample was prepared, in order to stimulate the growth of tin whiskers, force was loaded on top of one silicon dice as long-term compressive stress for 5 days at 25 °C, as shown in Fig. 1(a). The loaded force was 10 kgf, putting the compressive stress on each microbump approximately 16 MPa. Low vacuum scanning electron microscopy (LV-SEM, FEI NOVA NanoSEM 230) was used for morphology observation. The cross section of the unloaded microbump was made by ion beam milling system (Leica EM TIC 3X). A thin film of the cross section contain-
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Y. Li et al. / Materials Letters 258 (2020) 126773
Fig. 1. (a) Schematic drawing of microbumps with 10 lm in diameter under long-term compressive stress. (b, c) SEM image of 10 lm microbumps. (d) EDS mapping scan results of selected solder cap region, as circled in the white square in Fig. 1(c). (e) BSE image of the microbump cross section fabricated by ion beam milling.
ing both the SnAg microbumps and tin whisker with the direction parallel to the whisker growth direction was made using focused ion beam (FIB, GAIA3 GMU Model 2016), which uses highly focused ion beams to precisely scan and cut the sample in order to observe fine features. The fabricated sample was then observed using high-resolution field-emission transmission electron microscopy (HR-TEM, TALOS F200X) imaging to examine and identify the microstructure.
3. Results and discussions Fig. 1(b) and (c) show the morphology of prepared microbumps. The EDS results, as shown in Fig. 1(d), indicate the existence of 11.64 wt% of copper in the surface of the solder cap. Fig. 1(e) shows the BSE image of the microbump cross section fabricated by ion beam milling. The surface scanning results of the circled area, as presented in Fig. 1(e), indicate the composition of SnAg solder. The thin copper layer on the surface of solder cap was ascribed to copper atoms surface diffusion during reflow process. Liu et al. [12] investigated the diffusion behavior of copper in different pillar size and found that surface diffusion becomes more important than grain boundary and lattice diffusion when the pillar size shrinks. Thus, in small sized bumps, copper layer might influence the formation and growth of microbumps. As shown in Fig. 2(b, c), tin whiskers were clearly formed. Statics show the mean length of tin whisker is 3.04 lm and the mean diameter is 1.87 lm. A typical microbump with tin whisker, as shown in Fig. 2(d), was chosen as the target for FIB experiment along the direction parallel to the tin whisker growth direction. As discussed before, a thin layer of copper oxide existed on the surface of the microbump. Fig. 3(a) shows the microstructure of the copper oxide layer. A detailed image of copper oxide is shown
Fig. 2. SEM images of microbumps after long-term compressive stress. FIB was later applied on a typical microbump with tin whisker in (d).
in Fig. 3(b). The copper oxide layer was found lamellar. The thickness of copper oxide layer ranges from 145 nm to 540 nm in this selected region. Fig. 3(c) and (d) both show that the content of copper in the surface is much higher than that in the solder cap, which supports the point that copper surface diffusion plays an important role in 10 lm-diameter microbump. The existence of Pt was because that it was spurted on the surface of the specimen during the FIB experiment.
Y. Li et al. / Materials Letters 258 (2020) 126773
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tin whisker were reported before [13]. We think that large amount of dislocations formed through recrystallization, in order to store the energy generated from external and internal stress. Fig. 4(b) shows that the interplanar spacings of two grains are both about 0.28 nm, which corresponds with the direction of [2 0 0]. However, the orientation of two grains was different. The intersection angle between two orientation was 12°, indicating twin boundary. Dislocations in the twin boundary were clearly observed. Tin atoms diffuse along the dislocations through the twin boundary, thus tin whisker grows. Although the existence of twin boundary between tin whisker and neighboring grain has been reported in some literature [9,14], to the best of our knowledge, it is the first time that clear and obvious evidence explaining the functionality of the twin boundary in the whisker growth was provided, especially in the microbump with this small diameter (10 lm). 4. Conclusions
Fig. 3. TEM image of loaded microbump with tin whisker (a) overall microstructure and (b) detailed microstructure of copper oxide layer on the surface of the microbump. (c) TEM mapping scan image showing Cu distribution in the microbump. (d) line scan results along AB line across the tin whisker, as marked in (a).
Dense dislocations were found inside the tin whisker single grain, as shown in Fig. 4(a). Similar dislocations microstructure in
Tin whisker growth behavior on 10 lm-diameter Cu/SnAg microbumps under long-term compressive stress was studied. A thin layer of non-uniform copper oxide formed on the surface of the solder cap due to copper surface diffusion. The weak spots of the oxide layer are potential place where tin whisker could grow from. Under internal stress, the twin grain boundary will form on or move to the surface and becomes the nucleation point for further tin whisker growth. The twin boundary creates a large amount of dislocations, which provide diffusion paths for the tin atoms to slip over the twin boundary into the tin whisker. With further shrinking of the microbump diameter, we have reasons to believe that the copper surface diffusion phenomenon will influence the tin whisker behavior more significantly.
Fig. 4. Bright Field TEM image of Sn whisker and neighboring grains. (b) High resolution TEM (HR-TEM) image of the grain boundary between Sn whisker and neighboring grain, as circled in red square in (a). (c) schematic drawings of the morphology of microbump with 10 lm in diameter before and after load.
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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. Acknowledgements This work is sponsored by the National Basic Research Program of China (973 Program, 2015CB057200), China and the National Natural Science Foundation of China (61376107), China. We also thank the Instrumental Analysis Center of Shanghai Jiao Tong University, for the use of the SEM equipment. References [1] H. Leidecker, J. Brusse, Tin whiskers: a history of documented electrical system failures [J]. 2006.
[2] F.C. Frank, London, Edinburgh, Dublin Philos. Mag. J. Sci. 44 (355) (2014) 854– 860. [3] K.N. Tu, Phys. Rev. B 49 (3) (1994) 2030–2034. [4] Ren, Siru & Sun, Menglong & Jin, Zebin & Guo, Yukun & Ling, Huiqin & Hu, Anmin & Li, Ming, 2019. [5] E. Chason, F. Pei, JOM 67 (10) (2015) 2416–2424. [6] K.N. Tu, Y. Liu, Mater. Sci. Eng. R Rep. 136 (2018) 1–12. [7] J.W. Osenbach, J.M. DeLucca, B.D. Potteiger, A. Amin, F.A. Baiocchi, J. Mater. Sci. Mater. Electron. 18 (2007) 283–305. [8] K.N. Tu, H.Y. Hsiao, C. Chen, Microelectron. Reliabil. 53 (1) (2013) 2–6. [9] M. Sun, M. Dong, D. Wang, H. Ling, A. Hu, M Li, Scr. Mater. 147 (2018) 114–118. [10] K.N. Chiang, C.C. Lee, C.C. Lee, K.M. Chen, Appl. Phys. Lett. 88 (7) (2006) 1–4. [11] Y.C. Liang, C. Chen, IMPACT Conference 2009 International 3D IC Conference – Proceedings, 033708(August) (2009) pp. 158–161. [12] Y. Liu, Y.C. Chu, K.N. Tu, Acta Mater. 117 (2016) 146–152. [13] R. Vallabhaneni, E. Izadi, C.R. Mayer, C.S. Kaira, S.S. Singh, J. Rajagopalan, N. Chawla, Microelectron. Reliabil. 79 (2017) 314–320. [14] C.S. Kaira, S.S. Singh, A. Kirubanandham, N. Chawla, Acta Mater. 120 (2016) 56–67.
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/mlblue
The influence of non-uniform copper oxide layer on tin whisker growth and tin whisker growth behavior in SnAg microbumps with small diameter Yuancheng Li, Menglong Sun, Siru Ren, Huiqin Ling, Tao Hang, An-ming Hu ⇑, Ming Li State Key Laboratory of Metal Matrix Composites, Key Laboratory for Thin Film and Microfabrication Technology of the Ministry of Education, School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, China
a r t i c l e
i n f o
Article history: Received 12 June 2019 Received in revised form 22 August 2019 Accepted 3 October 2019 Available online 8 October 2019 Keywords: Electronic materials Semiconductors Tin whisker Copper oxide layer Microbump Twin boundary
a b s t r a c t A non-uniform copper oxide layer with several hundred nanometers thick was observed covering the surface of 10 lm-diameter Cu/SnAg microbump. Tin whiskers were also found to form on the weak spots of this copper oxide layer, where were localized stress relief centers. The HRTEM results reveal the existence of twin grain boundary between tin whisker and neighboring grain. Numerous dislocations at the twin boundary provide paths for the tin atoms to slip into tin whisker. It is the first time that the existence of copper oxide layer and the relationship between copper oxide layer and tin whiskers growth was studied and revealed in micron-level bumps. This present study has significant meaning for 3D electronic packaging as small size microbumps become increasingly prevalent and in which, surface diffusion becomes more important. Ó 2019 Elsevier B.V. All rights reserved.
1. Introduction Tin whisker in electronic devices could cause severe short circuit problems when contacting with neighboring devices [1]. There has been a lot of researches attempting to explain the growth mechanism of tin whisker in lead-free solders [2–4]. Although none of them could offer an absolute convincing mechanism for tin whisker growth, the compressive stress is generally considered as the driving force for tin whisker growth [5,6]. Nevertheless, most of the studies focus on tin whisker growth on thin films or flip-chip solder bumps [7]. Three-dimensional (3D) packaging is currently the most promising way to extend the limit of Moore’s law. The microbumps, with 10 lm in diameter or even smaller, used in 3D packaging are much smaller than ball-grid-array (BGA) (around 100 lm) in flip chip technology [8]. However, few previous researches have studied tin whisker growth in isolated microbump. Sun M. [9] reported a tin whisker growth mechanism from 25 lm-diameter Sn3.5Ag/Cu microbump. Nevertheless, under the decreasing size trend, the microbumps used in industrial life have already been smaller than 20 lm and the size of solder joints could shrink to 1 lm in diameter in the forseeable future. Previous ⇑ Corresponding author. E-mail address: [email protected] (A.-m. Hu). https://doi.org/10.1016/j.matlet.2019.126773 0167-577X/Ó 2019 Elsevier B.V. All rights reserved.
studies could neither catch up with the minimizing rate in electronic industry nor predict the tin whisker growth in micron level bumps. That is the motivation and necessity of our study. 2. Experimental The samples used in this work were prepared following the procedures: (1) sputtering of Cu seed layer on the silicon substrate, (2) lithography was used to spin coat and pattern the photoresist, (3) electrodeposition of Cu and SnAg respectively, (4) reflow and cooling of the SnAg solder caps under flux protection, (5) clean up the flux, (6) the fabricated wafer was then diced to 66 mm square as the samples used in this experiment. Each silicon die contains approximately 80,000 microbumps. In real life application, the microbumps are inevitably under compression introduced by electricity [10] or heat [11]. Once the sample was prepared, in order to stimulate the growth of tin whiskers, force was loaded on top of one silicon dice as long-term compressive stress for 5 days at 25 °C, as shown in Fig. 1(a). The loaded force was 10 kgf, putting the compressive stress on each microbump approximately 16 MPa. Low vacuum scanning electron microscopy (LV-SEM, FEI NOVA NanoSEM 230) was used for morphology observation. The cross section of the unloaded microbump was made by ion beam milling system (Leica EM TIC 3X). A thin film of the cross section contain-
2
Y. Li et al. / Materials Letters 258 (2020) 126773
Fig. 1. (a) Schematic drawing of microbumps with 10 lm in diameter under long-term compressive stress. (b, c) SEM image of 10 lm microbumps. (d) EDS mapping scan results of selected solder cap region, as circled in the white square in Fig. 1(c). (e) BSE image of the microbump cross section fabricated by ion beam milling.
ing both the SnAg microbumps and tin whisker with the direction parallel to the whisker growth direction was made using focused ion beam (FIB, GAIA3 GMU Model 2016), which uses highly focused ion beams to precisely scan and cut the sample in order to observe fine features. The fabricated sample was then observed using high-resolution field-emission transmission electron microscopy (HR-TEM, TALOS F200X) imaging to examine and identify the microstructure.
3. Results and discussions Fig. 1(b) and (c) show the morphology of prepared microbumps. The EDS results, as shown in Fig. 1(d), indicate the existence of 11.64 wt% of copper in the surface of the solder cap. Fig. 1(e) shows the BSE image of the microbump cross section fabricated by ion beam milling. The surface scanning results of the circled area, as presented in Fig. 1(e), indicate the composition of SnAg solder. The thin copper layer on the surface of solder cap was ascribed to copper atoms surface diffusion during reflow process. Liu et al. [12] investigated the diffusion behavior of copper in different pillar size and found that surface diffusion becomes more important than grain boundary and lattice diffusion when the pillar size shrinks. Thus, in small sized bumps, copper layer might influence the formation and growth of microbumps. As shown in Fig. 2(b, c), tin whiskers were clearly formed. Statics show the mean length of tin whisker is 3.04 lm and the mean diameter is 1.87 lm. A typical microbump with tin whisker, as shown in Fig. 2(d), was chosen as the target for FIB experiment along the direction parallel to the tin whisker growth direction. As discussed before, a thin layer of copper oxide existed on the surface of the microbump. Fig. 3(a) shows the microstructure of the copper oxide layer. A detailed image of copper oxide is shown
Fig. 2. SEM images of microbumps after long-term compressive stress. FIB was later applied on a typical microbump with tin whisker in (d).
in Fig. 3(b). The copper oxide layer was found lamellar. The thickness of copper oxide layer ranges from 145 nm to 540 nm in this selected region. Fig. 3(c) and (d) both show that the content of copper in the surface is much higher than that in the solder cap, which supports the point that copper surface diffusion plays an important role in 10 lm-diameter microbump. The existence of Pt was because that it was spurted on the surface of the specimen during the FIB experiment.
Y. Li et al. / Materials Letters 258 (2020) 126773
3
tin whisker were reported before [13]. We think that large amount of dislocations formed through recrystallization, in order to store the energy generated from external and internal stress. Fig. 4(b) shows that the interplanar spacings of two grains are both about 0.28 nm, which corresponds with the direction of [2 0 0]. However, the orientation of two grains was different. The intersection angle between two orientation was 12°, indicating twin boundary. Dislocations in the twin boundary were clearly observed. Tin atoms diffuse along the dislocations through the twin boundary, thus tin whisker grows. Although the existence of twin boundary between tin whisker and neighboring grain has been reported in some literature [9,14], to the best of our knowledge, it is the first time that clear and obvious evidence explaining the functionality of the twin boundary in the whisker growth was provided, especially in the microbump with this small diameter (10 lm). 4. Conclusions
Fig. 3. TEM image of loaded microbump with tin whisker (a) overall microstructure and (b) detailed microstructure of copper oxide layer on the surface of the microbump. (c) TEM mapping scan image showing Cu distribution in the microbump. (d) line scan results along AB line across the tin whisker, as marked in (a).
Dense dislocations were found inside the tin whisker single grain, as shown in Fig. 4(a). Similar dislocations microstructure in
Tin whisker growth behavior on 10 lm-diameter Cu/SnAg microbumps under long-term compressive stress was studied. A thin layer of non-uniform copper oxide formed on the surface of the solder cap due to copper surface diffusion. The weak spots of the oxide layer are potential place where tin whisker could grow from. Under internal stress, the twin grain boundary will form on or move to the surface and becomes the nucleation point for further tin whisker growth. The twin boundary creates a large amount of dislocations, which provide diffusion paths for the tin atoms to slip over the twin boundary into the tin whisker. With further shrinking of the microbump diameter, we have reasons to believe that the copper surface diffusion phenomenon will influence the tin whisker behavior more significantly.
Fig. 4. Bright Field TEM image of Sn whisker and neighboring grains. (b) High resolution TEM (HR-TEM) image of the grain boundary between Sn whisker and neighboring grain, as circled in red square in (a). (c) schematic drawings of the morphology of microbump with 10 lm in diameter before and after load.
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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. Acknowledgements This work is sponsored by the National Basic Research Program of China (973 Program, 2015CB057200), China and the National Natural Science Foundation of China (61376107), China. We also thank the Instrumental Analysis Center of Shanghai Jiao Tong University, for the use of the SEM equipment. References [1] H. Leidecker, J. Brusse, Tin whiskers: a history of documented electrical system failures [J]. 2006.
[2] F.C. Frank, London, Edinburgh, Dublin Philos. Mag. J. Sci. 44 (355) (2014) 854– 860. [3] K.N. Tu, Phys. Rev. B 49 (3) (1994) 2030–2034. [4] Ren, Siru & Sun, Menglong & Jin, Zebin & Guo, Yukun & Ling, Huiqin & Hu, Anmin & Li, Ming, 2019. [5] E. Chason, F. Pei, JOM 67 (10) (2015) 2416–2424. [6] K.N. Tu, Y. Liu, Mater. Sci. Eng. R Rep. 136 (2018) 1–12. [7] J.W. Osenbach, J.M. DeLucca, B.D. Potteiger, A. Amin, F.A. Baiocchi, J. Mater. Sci. Mater. Electron. 18 (2007) 283–305. [8] K.N. Tu, H.Y. Hsiao, C. Chen, Microelectron. Reliabil. 53 (1) (2013) 2–6. [9] M. Sun, M. Dong, D. Wang, H. Ling, A. Hu, M Li, Scr. Mater. 147 (2018) 114–118. [10] K.N. Chiang, C.C. Lee, C.C. Lee, K.M. Chen, Appl. Phys. Lett. 88 (7) (2006) 1–4. [11] Y.C. Liang, C. Chen, IMPACT Conference 2009 International 3D IC Conference – Proceedings, 033708(August) (2009) pp. 158–161. [12] Y. Liu, Y.C. Chu, K.N. Tu, Acta Mater. 117 (2016) 146–152. [13] R. Vallabhaneni, E. Izadi, C.R. Mayer, C.S. Kaira, S.S. Singh, J. Rajagopalan, N. Chawla, Microelectron. Reliabil. 79 (2017) 314–320. [14] C.S. Kaira, S.S. Singh, A. Kirubanandham, N. Chawla, Acta Mater. 120 (2016) 56–67.