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Crystallization and annealing effects of sputtered tin alloy films on electromagnetic interference shielding
Applied Surface Science 257 (2011) 3733–3738
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Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Crystallization and annealing effects of sputtered tin alloy films on electromagnetic interference shielding Fei-Shuo Hung a , Fei-Yi Hung b,∗ , Che-Ming Chiang a a b
Department of Architecture, National Cheng Kung University, Tainan 701, Taiwan Institute of Nanotechnology and Microsystems Engineering, Center for Micro/Nano Science and Technology, National Cheng Kung University, Tainan 701, Taiwan
a r t i c l e
i n f o
Article history: Received 18 September 2010 Accepted 19 November 2010 Available online 7 December 2010 Keywords: Electromagnetic interference (EMI) Sn–Al Sn–Cu
a b s t r a c t Sn, Al and Cu not only possess electromagnetic interference (EMI) shield efficiency, but also have acceptable costs. In this study, sputtered Sn–Al thin films and Sn–Cu thin films were used to investigate the effect of the crystallization mechanism and film thickness on the electromagnetic interference (EMI) characteristics. The results show that Sn–xAl film increased the electromagnetic interference (EMI) shielding after annealing. For as-sputtered Sn–xCu films with higher Cu atomic concentration, the low frequency EMI shielding could not be improved. After annealing, the Sn–Cu thin film with lower Cu content possessed excellent EMI shielding at lower frequencies, but had an inverse tendency at higher frequencies. For both the Sn–xAl and Sn–xCu thin films after crystallization treatment, the sputtered films had higher electrical conductivity, however the EMI shielding was not enhanced significantly. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Much research has shown that the effects of electromagnetic interference (EMI) can cause damage and variations to the human body’s DNA structure [1–3]. Relevant literature has shown that people who are exposed to a high electromagnetic environment for a long duration become more susceptible to diseases than normal people [4–6]. Therefore, how to use materials to make the electromagnetic environment safer is not only a very important issue, but also conforms with the objective of environmental protection [7]. Electromagnetic interference (EMI) is a new form of pollution discovered in recent years [8–12]. As of now, many laboratories are investigating the technology of electromagnetic interference (EMI) shielding. Some surface technologies possess better EMI shielding, including conductive films [13–21] and mixed conductive powders [22–27]. Problems such as wear, peeling, oxidation, hard-working and expensive cost have resulted in reduced applications. Sn, Al and Cu not only possess EMI shield efficiency, but also have acceptable costs [28–31] and can be used to make Sn based alloying thin films for EMI applications. Notably, the EMI of tin alloy thin films on material characteristics has not been explored, and the effect of annealing has not been discussed. Because tin alloy thin films have better mechanical properties (higher ductility) and a larger frequency range for EMI shielding,
∗ Corresponding author. Tel.: +886 6 2757575x31395; fax: +886 6 2745885. E-mail addresses: [email protected], [email protected] (F.-Y. Hung). 0169-4332/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2010.11.126
this study uses both Sn–xAl [32] and Sn–xCu [31,33] sputtered films not only to analyze the crystallization characteristics, but also investigate the alloying effects of different additions of elements, thicknesses and annealing treatment so as to further understand the potential for use as an EMI shielding material.
2. Experimental method Sn–40Al and Sn–xCu (x = 20 wt.% and 40 wt.%) sputtered thin films were used to perform the EMI shielding experiment. In the sputtering process, all film was sputtered onto a glass plate of dimensions 4 cm × 4 cm without any treatment. The thickness of the Sn–40Al film was in the range of 380 nm–760 nm. Some Sn–40Al (380 nm) films were annealed at 200 ◦ C for 1 h in a vacuum and are designated 380 nm-H. As for the Sn–xCu specimens, the thickness of the Sn–20Cu film was 460 nm and that of the Sn–40Cu film was 710 nm. Some Sn–Cu films were annealed at 200 ◦ C for 1 h in a vacuum (designated as Sn–xCu–H). In order to understand the characteristics of the films, the surface structures were examined using a scanning electron microscope (SEM: JEOL JSM-7001), XRD and transmission electron micrograph (TEM) to clarify the electromagnetic shield effect on the Sn based film systems. The Elgal set 19A coaxial holder was used for electromagnetic interference shield testing. The range of scan frequency is from 300 k to 3 GHz, and its accuracy is ±10 ppm (25◦ ± 5◦ ). A plane wave was used for vertical firing. The frequency range was controlled from 50 MHz to 3 GHz. Each datum was the average of at least 3–7 test results [32].
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Fig. 1. Sputtered thin alloy films: (a) Sn–40Al (380 nm) and (b) Sn–20Cu (460 nm).
3. Results and discussion 3.1. Thin film structure characteristics According to our previous study into the EMI of Sn–xAl powders [32], high Al content coatings offer excellent low frequency EMI shielding. After the annealing treatment, the powders had higher electric conductivity which was able to enhance the EMI shielding. So, the present study selected the Sn–40Al sputtered film to investigate its EMI shielding. Fig. 1(a) shows the surface characteristics of the Sn–40Al sputtered film with 380 nm film thickness. Finer nano-grains can be observed on the films and the surface characteristics of the other thin films are similar. Notably, the grain size of the present thin film was 22–46 nm. On the other hand, Fig. 1(b) is a surface image of the Sn–20Cu film (460 nm) and the average grain size is 65 nm. 3.2. Effects of thickness and annealing on Sn–Al films When the Sn–40Al film thickness was increased from 380 nm to 760 nm (Fig. 2(a)), there was an obvious strengthening effect on the EMI shielding. In fact, increasing the thickness of the film, not only raised its index of crystalline (IOC), but also enhanced the growth of nano-grains because of the longer sputtering duration. In order to avoid the effects of film thickness, grain size and IOC, the 380 nm film was subjected to recrystallization (referred to as 380 nm-H) at 200 ◦ C for 1 h in a vacuum, then cooled to room temperature
Fig. 2. EMI shielding of Sn–40Al sputtering thin films: (a) different thickness and (b) before and after heat treatment.
with a cooling rate of 0.5 ◦ C/min. The EMI shielding of the 380 nmH film and the 380 nm film were compared. The EMI shielding of the 380 nm-H film was higher than that of the 380 nm film (Fig. 2(b)). In addition, a surface image of the 380 nm-H film showed no obvious differences compared with the 380 nm film. Meanwhile the 380 nm-H film had a higher crystallization and lower resistivity. It is clear that the heat treatment (200 ◦ C-1 h) not only raised the electrical conductivity but also improved the high frequency EMI shielding. In order to understand the mechanism of EMI shielding, the X-ray diffraction (XRD) of the films was measured. Fig. 3 shows XRDs of the Sn–40Al specimen. The Al content is 40%. Fig. 4 shows a bright field image of the Sn–40Al film. Only particle-like phases were present in the matrix. The selected area diffraction (SAD) analysis of the matrix and particle-like phases are shown in Figs. 5 and 6. The results indicate that the matrix was a tetragonal -Sn phase and the particle-like phases were cubic ␣Al phases, while no compound phase was observed in this study.
F.-S. Hung et al. / Applied Surface Science 257 (2011) 3733–3738
Fig. 3. XRD of Sn–40Al film.
These results and a relevant report [32] suggest that the solid solution limit of Al is low in Sn matrix, and the Sn–Al matrix not only affects the electric conductivity of the film, but also induces a variation in the EMI shielding. We may say that the uniform distribution of rich Al phases in Sn–Al thin film was able to enhance the EMI shielding. 3.3. EMI shielding of Sn–Cu films Since Sn–Cu specimens have higher electrical conductivity (1.08 × 10−4 ( cm) for Sn–20Cu) and acceptable costs, the present
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study also used Sn–xCu to analyze the EMI mechanism and compared it with the Sn–40AI specimens. According to previous studies [32], Sn based specimens with a high concentration of a second element have excellent EMI shielding in the range of scan frequency from 300 k to 3 GHz. So, high Cu sputtered thin film (Sn–40 wt.% Cu) was selected to investigate the EMI behavior. Fig. 7 shows the EMI shielding of the Sn–40Cu (710 nm) thin film and Sn–40Al (730 nm) thin film at a similar thickness. The EMI shielding of the Sn–40Cu thin film was lower than that of the Sn–40Al thin film. Notably, our previous data revealed that both the annealing treatment and higher film thickness not only increased the electric conductivity, but also enhanced the EMI shielding [31–33]. But, after the annealing treatment, the EMI shielding of the Sn–40Cu film had a tendency to decrease (compare Fig. 7 with Fig. 8), and this result is different from that of the Sn–xAl films (annealed Sn–Al thin film was able to enhance the EMI shielding). Furthermore (Fig. 8), the EMI of the annealed Sn–40Cu film with a greater thickness (710 nm) was still lower than that of the Sn–40Al film (un-annealed and with a smaller thickness of 380 nm). We can be fairly certain that the structure of the Sn–40Cu film had a significant influence on the EMI shielding. In general, the EMI shielding of the films improved after annealing due to the crystallization being raised (the Sn–Al system is a typical case). In the Sn–Cu film, the annealing process stabilized the SnCu compounds (Cu6 Sn5 ; Cu3 Sn) resulting in a decrease in the EMI shielding. It is noteworthy that neither increasing the thickness of the film nor performing an annealing treatment had a significant effect on the EMI shielding of Sn alloying thin film materials. 3.4. How annealing affected the film matrix In Fig. 9, there are three phases (Cu6 Sn5 , Cu3 Sn and -Sn) in the Sn–40Cu film matrix, but no Sn/Al intermetallic compounds (IMC) were observed in the Sn–Al thin film (Figs. 3–6). In other words, the volume rate of these phases (Cu6 Sn5 , Cu3 Sn and -Sn) was close to the EMI shielding. After annealing, the IMCs of the Sn–Cu thin film would increase leading to a decrease in the electrical conductivity (Sn–40Cu: from 7.02 × 10−6 to 2.46 × 10−5 ( cm)) and EMI shielding. So, this study avoided the effects of both IMCs and film thickness by decreasing the Cu content and measuring the EMI shielding before and after the annealing treatment. Under both constant Cu content (Sn–20Cu: lower IMCs) and thickness (Fig. 10), the annealed Sn–20Cu film showed the greater EMI shielding at higher frequencies, but an inverse tendency was found at lower frequencies. One conclusion of the experiment is that the annealing treatment made a greater contribution in low Cu content specimens. Fig. 11 shows bright and dark filled images of the Sn–20Cu film. The needle-like phases are Cu6 Sn5 phases. Fig. 12(a) shows a TEM image of the Sn–40Al film before annealing and many Cu6 Sn5 phases can be observed in the matrix. Notably, the Cu6 Sn5 phases were finer after annealing (Fig. 12(b)). 3.5. EMI mechanism of Sn-alloy films
Fig. 4. BF image of Sn–40 film.
According to our past studies on Sn-based specimens [31–33], the reasons for EMI are not hard to see: (1) -Sn phase is the main EMI shielding mechanism. (2) The complex compounds which result from the annealing possess higher thermal stabilization which promotes the EMI shielding. Comparing the Sn–40Al film (710 nm) with the Sn–40Cu film (730 nm), we found that the Sn–Al film had better EMI shielding (Fig. 7). As for the effect of the matrix [33], it seems safe to say that the -Sn phase content and compound type decided the EMI shielding. The Sn–40Al thin film increased thickness and had an annealed which improved the EMI shielding (Fig. 13(a)). Take Sn–xCu thin films from 20 wt.% Cu to 40 wt.% Cu for example. The annealing induced a phase transformation, resulting in an inverse change at
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Fig. 5. SAED pattern of -Sn with a tetragonal structure.
Fig. 6. SAED pattern of ␣-Al with a cubic structure.
Fig. 7. EMI shielding of Sn–40Cu and Sn–40Al sputtering thin films of identical thickness.
Fig. 8. Comparison of EMI in Sn–40Cu (annealed) and Sn–40Al (un-annealed) sputtered thin films.
F.-S. Hung et al. / Applied Surface Science 257 (2011) 3733–3738
1000
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10
Cu 6Sn 5
Decibel value, EMI/-dB
Cu 3Sn
800
Intensity
Sn 600
400
0
Sn-20Cu: 460 nm -10
-20
200 -30
Sn-20Cu: 460 nm-H
0 20
40
60
2θ
80
100
0
1000
2000
Frequency, F/MHz
3000
Fig. 10. Effect of annealing on the EMI of Sn–20Cu thin films. Fig. 9. XRD of Sn–40Cu thin film.
Fig. 11. Transmission electron micrograph of Sn–20Cu film: (a) BF image and (b) DF image.
Fig. 12. Transmission electron micrograph of Sn–40Cu film: (a) before annealing and (b) after annealing.
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Acknowledgements The authors are grateful to National Cheng Kung University, the Center for Micro/Nano Science and Technology (NCKU Project of Promoting Academic Excellence & Developing World Class Research Center: D99-2700) and NSC 99-2622-E-006-132 for the financial support. References
Fig. 13. EMI shielding characteristics: (a) Sn–Al thin film and (b) Sn–Cu thin film.
different frequencies (Fig. 13(b)). It is obvious that Cu6 Sn5 , Cu3 Sn and -Sn phases were in the matrix of the Sn–xCu films after annealing. Comparing the Sn–20Cu film with the Sn–40Cu film, we found that reduced -Sn phases and increased the compound content did not significantly enhance the EMI shielding. So, the annealing treatment not only promotes the contribution of IMCs, but also decreases the concentration of impurities in the -Sn phases, which improves the EMI shielding. As for the variations in higher frequency EMI shielding after annealing, one explanation [31,33] may be that the Cu3 Sn content of Sn–Cu specimen (there are three phases: Cu6 Sn5 , Cu3 Sn and -Sn) decreases and influences the results. 4. Conclusion For the Sn–Al films, the film thickness increased which increased the electric conductivity and improved the EMI shielding. As for the Sn–Cu films, due to the large number of IMCs and lower content of -Sn, the higher frequency EMI shielding of the annealed Sn–Cu thin films deteriorated. The Sn–Al films had rich-Sn phases and rich Al phases in the matrix which were able to promote the EMI shielding. Their annealing treatment not only provided higher electric conductivity but also increased the high frequency EMI shielding.
[1] C. Polk, E. Postow, Hand Book of Electromagnetic Fields, 2nd ed., CRC Press Inc., 1996, pp. 212–213. [2] C.F. Blackman, J.P. Blanchard, S.G. Benane, D.E. House, J. FASEB 9 (1995) 547–551. [3] C.F. Blackman, S.G. Benane, J.R. Rabinowitj, D.E. House, W.T. Joines, Bioelectromagn.: Sci. Eng. A 6 (1985) 327–338. [4] C.J. Merchant, D.C. Renew, J. Swanson, Exposures to power frequency magnetic field in the home, J. Radiol. Protect.: Sci. Eng. A 14 (1994) 77–87. [5] B.R. Mcleod, A.R. Liboff, Bioelectromagn.: Sci. Eng. A 7 (1986) 177–189. [6] J.M.R. Delgado, J. Leaf, J.L. Monteagudo, M.G. Gracia, J. Anat. 134 (1982) 533– 551. [7] C.M. Chiang, C.M. Lai, A study on the comprehensive indicator of indoor environment assessment for occupants’ health in Taiwan, Build. Environ. 37 (2001) 387–392. [8] N. Day, J. Skinner, E. Roman, S.G. Allen, Lancet 354 (1999) 1925–1931. [9] M. Fechting, A. Ahlbom, Magnetic fields and cancer in children residing near Swedish High Voltage Power Lines, Am. J. Epidemiol. Sci. Eng. A 138 (1993) 467–481. [10] V. Manni, A. Lisi, D. Pozzi, S. Rieti, A. Serafino, L. Giuliani, S. Grimaili, Bioelectromagn.: Sci. Eng. A 23 (2002) 298–305. [11] J. Burnett, Y. Du, Low frequency magnetic interference in high-rise buildings, in: Seventh International IBPSA Conference: Building Simulation, vol. 7, 2001, pp. 327–333. [12] T.S. Tenforde, W.T. Kaune, Health Phys. 53 (1987) 585–606. [13] N. Wertheimer, E. Leeper, Am. J. Epidemiol. 109 (1979) 273–284. [14] A. Ubeda, J. Leaf, M.A. Trillo, J.M.R. Delgado, J. Anat. 37 (1982) 513–536. [15] A.R. Liboff, J. Biol. Phys. 13 (1985) 99–102. [16] D.C. Paine, T. Whitson, D. Janiac, R. Beresford, C. Yang, J. Appl. Phys.: Sci. Eng. A 85 (1999) 12–15. [17] D. Liu, D. Suqiao, Asia-Pacific Conference on Environmental Electromagnetics, vol. 6, 2000, pp. 326–332. [18] J.L. Vossen, Transparent conducting films, Phys. Thin Films 9 (1997) 1–71. [19] R.M. Gresham, Plating Surf. Finishing 13 (1998) 63–69. [20] C.S. Zhang, Q.Q. Ni, S.Y. Fu, K. Kurshiki, Compos. Sci. Technol.: Sci. Eng. A 67 (14) (2007) 2793–2980. [21] T. Wang, G. Chen, C. Wu, D. Wu, Prog. Org. Coat. 59 (2) (2007) 101–105. [22] K. Wenderoth, J. Petermann, Polym. Compos. 10 (1992) 52–56. [23] W.W. Salisbury, Absorbent body for electromagnetic waves, U.S. Patent No. 259,994,410 (1952). [24] R.L. Fante, M.T. McCormack, IEEE Trans. Antenna Propagat. 36 (1998) 1443–1454. [25] J.C. Liu, S.S. Ho, S.S. Bor, IEEE Proc.-H 140 (1993) 414–416. [26] E.G. Han, E.A. Kim, K.W. Oh, Synth. Metals 123 (2001) 469–476. [27] G.E. White, Liquid crystalline polymer and multilayer polymer-based passive signal processing components for rf/wireless multi-band applications. U.S. Patent No. 20,070,085,108 (2007). [28] F.Y. Hung, T.S. Lui, H.C. Liao, Appl. Surf. Sci.: Sci. Eng. A 253 (2007) 7443– 7448. [29] Y.S. Kim, K.S. Kim, C.W. Hwang, K. Suganuma, J. Alloys Compd. 352 (2003) 237–245. [30] D.Q. Yu, H.P. Xie, L. Wang, J. Alloys Compd. 385 (2004) 119–125. [31] F.Y. Hung, T.S. Lui, L.H. Chen, N.T. He, J. Alloys Compd. 457 (2008) 171–176. [32] F.S. Hung, F.Y. Hung, C.M. Chiang, T.S. Lui, Mater. Trans. 49 (2008) 655–660. [33] C.H. Wu, F.Y. Hung, T.S. Lui, L.H. Chen, Mater. Trans. 50 (2) (2009) 381–387.
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Crystallization and annealing effects of sputtered tin alloy films on electromagnetic interference shielding Fei-Shuo Hung a , Fei-Yi Hung b,∗ , Che-Ming Chiang a a b
Department of Architecture, National Cheng Kung University, Tainan 701, Taiwan Institute of Nanotechnology and Microsystems Engineering, Center for Micro/Nano Science and Technology, National Cheng Kung University, Tainan 701, Taiwan
a r t i c l e
i n f o
Article history: Received 18 September 2010 Accepted 19 November 2010 Available online 7 December 2010 Keywords: Electromagnetic interference (EMI) Sn–Al Sn–Cu
a b s t r a c t Sn, Al and Cu not only possess electromagnetic interference (EMI) shield efficiency, but also have acceptable costs. In this study, sputtered Sn–Al thin films and Sn–Cu thin films were used to investigate the effect of the crystallization mechanism and film thickness on the electromagnetic interference (EMI) characteristics. The results show that Sn–xAl film increased the electromagnetic interference (EMI) shielding after annealing. For as-sputtered Sn–xCu films with higher Cu atomic concentration, the low frequency EMI shielding could not be improved. After annealing, the Sn–Cu thin film with lower Cu content possessed excellent EMI shielding at lower frequencies, but had an inverse tendency at higher frequencies. For both the Sn–xAl and Sn–xCu thin films after crystallization treatment, the sputtered films had higher electrical conductivity, however the EMI shielding was not enhanced significantly. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Much research has shown that the effects of electromagnetic interference (EMI) can cause damage and variations to the human body’s DNA structure [1–3]. Relevant literature has shown that people who are exposed to a high electromagnetic environment for a long duration become more susceptible to diseases than normal people [4–6]. Therefore, how to use materials to make the electromagnetic environment safer is not only a very important issue, but also conforms with the objective of environmental protection [7]. Electromagnetic interference (EMI) is a new form of pollution discovered in recent years [8–12]. As of now, many laboratories are investigating the technology of electromagnetic interference (EMI) shielding. Some surface technologies possess better EMI shielding, including conductive films [13–21] and mixed conductive powders [22–27]. Problems such as wear, peeling, oxidation, hard-working and expensive cost have resulted in reduced applications. Sn, Al and Cu not only possess EMI shield efficiency, but also have acceptable costs [28–31] and can be used to make Sn based alloying thin films for EMI applications. Notably, the EMI of tin alloy thin films on material characteristics has not been explored, and the effect of annealing has not been discussed. Because tin alloy thin films have better mechanical properties (higher ductility) and a larger frequency range for EMI shielding,
∗ Corresponding author. Tel.: +886 6 2757575x31395; fax: +886 6 2745885. E-mail addresses: [email protected], [email protected] (F.-Y. Hung). 0169-4332/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2010.11.126
this study uses both Sn–xAl [32] and Sn–xCu [31,33] sputtered films not only to analyze the crystallization characteristics, but also investigate the alloying effects of different additions of elements, thicknesses and annealing treatment so as to further understand the potential for use as an EMI shielding material.
2. Experimental method Sn–40Al and Sn–xCu (x = 20 wt.% and 40 wt.%) sputtered thin films were used to perform the EMI shielding experiment. In the sputtering process, all film was sputtered onto a glass plate of dimensions 4 cm × 4 cm without any treatment. The thickness of the Sn–40Al film was in the range of 380 nm–760 nm. Some Sn–40Al (380 nm) films were annealed at 200 ◦ C for 1 h in a vacuum and are designated 380 nm-H. As for the Sn–xCu specimens, the thickness of the Sn–20Cu film was 460 nm and that of the Sn–40Cu film was 710 nm. Some Sn–Cu films were annealed at 200 ◦ C for 1 h in a vacuum (designated as Sn–xCu–H). In order to understand the characteristics of the films, the surface structures were examined using a scanning electron microscope (SEM: JEOL JSM-7001), XRD and transmission electron micrograph (TEM) to clarify the electromagnetic shield effect on the Sn based film systems. The Elgal set 19A coaxial holder was used for electromagnetic interference shield testing. The range of scan frequency is from 300 k to 3 GHz, and its accuracy is ±10 ppm (25◦ ± 5◦ ). A plane wave was used for vertical firing. The frequency range was controlled from 50 MHz to 3 GHz. Each datum was the average of at least 3–7 test results [32].
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Fig. 1. Sputtered thin alloy films: (a) Sn–40Al (380 nm) and (b) Sn–20Cu (460 nm).
3. Results and discussion 3.1. Thin film structure characteristics According to our previous study into the EMI of Sn–xAl powders [32], high Al content coatings offer excellent low frequency EMI shielding. After the annealing treatment, the powders had higher electric conductivity which was able to enhance the EMI shielding. So, the present study selected the Sn–40Al sputtered film to investigate its EMI shielding. Fig. 1(a) shows the surface characteristics of the Sn–40Al sputtered film with 380 nm film thickness. Finer nano-grains can be observed on the films and the surface characteristics of the other thin films are similar. Notably, the grain size of the present thin film was 22–46 nm. On the other hand, Fig. 1(b) is a surface image of the Sn–20Cu film (460 nm) and the average grain size is 65 nm. 3.2. Effects of thickness and annealing on Sn–Al films When the Sn–40Al film thickness was increased from 380 nm to 760 nm (Fig. 2(a)), there was an obvious strengthening effect on the EMI shielding. In fact, increasing the thickness of the film, not only raised its index of crystalline (IOC), but also enhanced the growth of nano-grains because of the longer sputtering duration. In order to avoid the effects of film thickness, grain size and IOC, the 380 nm film was subjected to recrystallization (referred to as 380 nm-H) at 200 ◦ C for 1 h in a vacuum, then cooled to room temperature
Fig. 2. EMI shielding of Sn–40Al sputtering thin films: (a) different thickness and (b) before and after heat treatment.
with a cooling rate of 0.5 ◦ C/min. The EMI shielding of the 380 nmH film and the 380 nm film were compared. The EMI shielding of the 380 nm-H film was higher than that of the 380 nm film (Fig. 2(b)). In addition, a surface image of the 380 nm-H film showed no obvious differences compared with the 380 nm film. Meanwhile the 380 nm-H film had a higher crystallization and lower resistivity. It is clear that the heat treatment (200 ◦ C-1 h) not only raised the electrical conductivity but also improved the high frequency EMI shielding. In order to understand the mechanism of EMI shielding, the X-ray diffraction (XRD) of the films was measured. Fig. 3 shows XRDs of the Sn–40Al specimen. The Al content is 40%. Fig. 4 shows a bright field image of the Sn–40Al film. Only particle-like phases were present in the matrix. The selected area diffraction (SAD) analysis of the matrix and particle-like phases are shown in Figs. 5 and 6. The results indicate that the matrix was a tetragonal -Sn phase and the particle-like phases were cubic ␣Al phases, while no compound phase was observed in this study.
F.-S. Hung et al. / Applied Surface Science 257 (2011) 3733–3738
Fig. 3. XRD of Sn–40Al film.
These results and a relevant report [32] suggest that the solid solution limit of Al is low in Sn matrix, and the Sn–Al matrix not only affects the electric conductivity of the film, but also induces a variation in the EMI shielding. We may say that the uniform distribution of rich Al phases in Sn–Al thin film was able to enhance the EMI shielding. 3.3. EMI shielding of Sn–Cu films Since Sn–Cu specimens have higher electrical conductivity (1.08 × 10−4 ( cm) for Sn–20Cu) and acceptable costs, the present
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study also used Sn–xCu to analyze the EMI mechanism and compared it with the Sn–40AI specimens. According to previous studies [32], Sn based specimens with a high concentration of a second element have excellent EMI shielding in the range of scan frequency from 300 k to 3 GHz. So, high Cu sputtered thin film (Sn–40 wt.% Cu) was selected to investigate the EMI behavior. Fig. 7 shows the EMI shielding of the Sn–40Cu (710 nm) thin film and Sn–40Al (730 nm) thin film at a similar thickness. The EMI shielding of the Sn–40Cu thin film was lower than that of the Sn–40Al thin film. Notably, our previous data revealed that both the annealing treatment and higher film thickness not only increased the electric conductivity, but also enhanced the EMI shielding [31–33]. But, after the annealing treatment, the EMI shielding of the Sn–40Cu film had a tendency to decrease (compare Fig. 7 with Fig. 8), and this result is different from that of the Sn–xAl films (annealed Sn–Al thin film was able to enhance the EMI shielding). Furthermore (Fig. 8), the EMI of the annealed Sn–40Cu film with a greater thickness (710 nm) was still lower than that of the Sn–40Al film (un-annealed and with a smaller thickness of 380 nm). We can be fairly certain that the structure of the Sn–40Cu film had a significant influence on the EMI shielding. In general, the EMI shielding of the films improved after annealing due to the crystallization being raised (the Sn–Al system is a typical case). In the Sn–Cu film, the annealing process stabilized the SnCu compounds (Cu6 Sn5 ; Cu3 Sn) resulting in a decrease in the EMI shielding. It is noteworthy that neither increasing the thickness of the film nor performing an annealing treatment had a significant effect on the EMI shielding of Sn alloying thin film materials. 3.4. How annealing affected the film matrix In Fig. 9, there are three phases (Cu6 Sn5 , Cu3 Sn and -Sn) in the Sn–40Cu film matrix, but no Sn/Al intermetallic compounds (IMC) were observed in the Sn–Al thin film (Figs. 3–6). In other words, the volume rate of these phases (Cu6 Sn5 , Cu3 Sn and -Sn) was close to the EMI shielding. After annealing, the IMCs of the Sn–Cu thin film would increase leading to a decrease in the electrical conductivity (Sn–40Cu: from 7.02 × 10−6 to 2.46 × 10−5 ( cm)) and EMI shielding. So, this study avoided the effects of both IMCs and film thickness by decreasing the Cu content and measuring the EMI shielding before and after the annealing treatment. Under both constant Cu content (Sn–20Cu: lower IMCs) and thickness (Fig. 10), the annealed Sn–20Cu film showed the greater EMI shielding at higher frequencies, but an inverse tendency was found at lower frequencies. One conclusion of the experiment is that the annealing treatment made a greater contribution in low Cu content specimens. Fig. 11 shows bright and dark filled images of the Sn–20Cu film. The needle-like phases are Cu6 Sn5 phases. Fig. 12(a) shows a TEM image of the Sn–40Al film before annealing and many Cu6 Sn5 phases can be observed in the matrix. Notably, the Cu6 Sn5 phases were finer after annealing (Fig. 12(b)). 3.5. EMI mechanism of Sn-alloy films
Fig. 4. BF image of Sn–40 film.
According to our past studies on Sn-based specimens [31–33], the reasons for EMI are not hard to see: (1) -Sn phase is the main EMI shielding mechanism. (2) The complex compounds which result from the annealing possess higher thermal stabilization which promotes the EMI shielding. Comparing the Sn–40Al film (710 nm) with the Sn–40Cu film (730 nm), we found that the Sn–Al film had better EMI shielding (Fig. 7). As for the effect of the matrix [33], it seems safe to say that the -Sn phase content and compound type decided the EMI shielding. The Sn–40Al thin film increased thickness and had an annealed which improved the EMI shielding (Fig. 13(a)). Take Sn–xCu thin films from 20 wt.% Cu to 40 wt.% Cu for example. The annealing induced a phase transformation, resulting in an inverse change at
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Fig. 5. SAED pattern of -Sn with a tetragonal structure.
Fig. 6. SAED pattern of ␣-Al with a cubic structure.
Fig. 7. EMI shielding of Sn–40Cu and Sn–40Al sputtering thin films of identical thickness.
Fig. 8. Comparison of EMI in Sn–40Cu (annealed) and Sn–40Al (un-annealed) sputtered thin films.
F.-S. Hung et al. / Applied Surface Science 257 (2011) 3733–3738
1000
3737
10
Cu 6Sn 5
Decibel value, EMI/-dB
Cu 3Sn
800
Intensity
Sn 600
400
0
Sn-20Cu: 460 nm -10
-20
200 -30
Sn-20Cu: 460 nm-H
0 20
40
60
2θ
80
100
0
1000
2000
Frequency, F/MHz
3000
Fig. 10. Effect of annealing on the EMI of Sn–20Cu thin films. Fig. 9. XRD of Sn–40Cu thin film.
Fig. 11. Transmission electron micrograph of Sn–20Cu film: (a) BF image and (b) DF image.
Fig. 12. Transmission electron micrograph of Sn–40Cu film: (a) before annealing and (b) after annealing.
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F.-S. Hung et al. / Applied Surface Science 257 (2011) 3733–3738
Acknowledgements The authors are grateful to National Cheng Kung University, the Center for Micro/Nano Science and Technology (NCKU Project of Promoting Academic Excellence & Developing World Class Research Center: D99-2700) and NSC 99-2622-E-006-132 for the financial support. References
Fig. 13. EMI shielding characteristics: (a) Sn–Al thin film and (b) Sn–Cu thin film.
different frequencies (Fig. 13(b)). It is obvious that Cu6 Sn5 , Cu3 Sn and -Sn phases were in the matrix of the Sn–xCu films after annealing. Comparing the Sn–20Cu film with the Sn–40Cu film, we found that reduced -Sn phases and increased the compound content did not significantly enhance the EMI shielding. So, the annealing treatment not only promotes the contribution of IMCs, but also decreases the concentration of impurities in the -Sn phases, which improves the EMI shielding. As for the variations in higher frequency EMI shielding after annealing, one explanation [31,33] may be that the Cu3 Sn content of Sn–Cu specimen (there are three phases: Cu6 Sn5 , Cu3 Sn and -Sn) decreases and influences the results. 4. Conclusion For the Sn–Al films, the film thickness increased which increased the electric conductivity and improved the EMI shielding. As for the Sn–Cu films, due to the large number of IMCs and lower content of -Sn, the higher frequency EMI shielding of the annealed Sn–Cu thin films deteriorated. The Sn–Al films had rich-Sn phases and rich Al phases in the matrix which were able to promote the EMI shielding. Their annealing treatment not only provided higher electric conductivity but also increased the high frequency EMI shielding.
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