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Ultrathin Cr added Ru film as a seedless Cu diffusion barrier for advanced Cu interconnects
Applied Surface Science 258 (2012) 7225–7230
Contents lists available at SciVerse ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Ultrathin Cr added Ru film as a seedless Cu diffusion barrier for advanced Cu interconnects Kuo-Chung Hsu a,b , Dung-Ching Perng a,b,∗ , Jia-Bin Yeh a,b , Yi-Chun Wang a,b a b
Institute of Microelectronics, Dept. of Electrical Engineering, National Cheng Kung University, No. 1 University Road, Tainan 70101, Taiwan Center for Micro/Nano Science and Technology, National Cheng Kung University, No. 1 University Road, Tainan 70101, Taiwan
a r t i c l e
i n f o
Article history: Received 4 August 2011 Received in revised form 15 February 2012 Accepted 9 April 2012 Available online 15 April 2012 Keywords: RuCr alloy Diffusion barrier Cu interconnect Cu metallization
a b s t r a c t A 5 nm thick Cr added Ru film has been extensively investigated as a seedless Cu diffusion barrier. Highresolution transmission electron microscopy micrograph, X-ray diffraction (XRD) pattern and Fourier transform-electron diffraction pattern reveal that a Cr contained Ru (RuCr) film has a glassy microstructure and is an amorphous-like film. XRD patterns and sheet resistance data show that the RuCr film is stable up to 650 ◦ C, which is approximately a 200 ◦ C improvement in thermal stability as compared to that of the pure Ru film. X-ray photoelectron spectroscopy depth profiles show that the RuCr film can successfully block Cu diffusion, even after a 30-min 650 ◦ C annealing. The leakage current of the Cu/5 nm RuCr/porous SiOCH/Si stacked structure is about two orders of magnitude lower than that of a pristine Ru sample for electric field below 1 MV/cm. The RuCr film can be a promising Cu diffusion barrier for advanced Cu metallization. © 2012 Elsevier B.V. All rights reserved.
1. Introduction With continuing shrinking of feature size in ultra large scale integrated circuit (ULSI) technology, Cu and low-k dielectric materials have been widely implemented to lower the resistance–capacitance delay effect (R–C delay) and for higher electron migration (EM) resistance [1–4]. However, Cu can react with Si easily and diffuses rapidly in porous dielectric materials, which leads to degradation in electrical performance [5,6]. Therefore, high thermal stability and negligible reaction with Cu and Si are required for a Cu diffusion barrier. Ta/TaN bi-layers are a widely used Cu diffusion barriers in Cu interconnects [7–9]. Ta exhibits negligible solubility with Cu [10] and offers good barrier performance against Cu. A 30 nm Ta/TaN bi-layer barrier is thermally stable up to 850 ◦ C [8]. Unfortunately, a Ta/TaN bi-layer exhibits high resistivity, and this would be unacceptable with regard to achieving lower effective resistivity in future nodes. Moreover, the International Technology Roadmap for Semiconductors [11] (ITRS) indicated that the required diffusion barrier thickness is approximately 2.4 nm, and the aspect ratio (A/R) of the metal line will increase to 1.90 by 2013. With a narrower line width and a higher A/R, using traditional approach, i.e. a thick barrier plus an additional Cu seed layer, is not feasible. Hence, a new barrier material must be developed
∗ Corresponding author at: Institute of Microelectronics, Dept. of Electrical Engineering, National Cheng Kung University, No. 1 University Road, Tainan 70101, Taiwan. Tel.: +886 6 275 7575x62433; fax: +886 6 234 5482. E-mail address: [email protected] (D.-C. Perng). 0169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2012.04.046
that can act as a seedless Cu diffusion barrier with low resistivity and reduced thickness. Ruthenium (Ru), a noble transitional metal, has been widely studied as a seedless Cu diffusion barrier [12,13]. Ru exhibits a low electrical resistivity (bulk = 7.1 -cm) and good adhesion with Cu [12]. In addition, Ru can be directly electroplated onto Cu without an extra Cu seed layer [14,15]. Unfortunately, a pure Ru film exhibits poor barrier performance against Cu [12,16] and poor adhesion to dielectric or oxide layer [17]. The polycrystalline nature of the asdeposited Ru film has been reported that the Ru grain boundaries provided easy paths for Cu diffusion, which causes early failure of the barrier [18]. Recently, many studies have focused on improving Ru barrier performance. Doping other elements, such as N, P, and Ta, into the Ru film to form a Ru alloy barriers have been reported [18–20]. The added elements act as impurities could segregate at the Ru grain boundaries and prohibit Cu penetration through the barrier. Investigations have also shown that a small amount of elements added to the Ru film can alter the microstructure of a pure Ru film from polycrystalline to amorphous-like, and the microstructure is a dominant factor in improving the barrier performance of the Ru-based film. Cr (bulk = 12.5 -cm) has a lower electrical resistivity compared to Ta (bulk = 13.6 -cm) and has no Ru–Cr phase when Cr content is lower than ∼18% [21]. Cr added into Cu film can significantly enhance the adhesion of Cu film to dielectric [22]. The data showed some Cr accumulating at the dielectric interface, suggesting chemical reaction in that region which enhance adhesion. Based on same mechanism, the adhesion of Ru to dielectric is likely to improve by adding Cr to Ru. However, barrier performance of Cr
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added Ru film, especially ultrathin layer, has not yet been studied. In this work, addition of Cr to Ru film, with various atomic composition ratios, were studied. Barrier properties and thermal stability of 5-nm thick Cr added Ru films deposited on a Si substrate with various annealing temperatures up to 800 ◦ C are investigated. Besides the potential adhesion improvement, the results show that the Cr added Ru film could also be a promising candidate as a Cu diffusion barrier for advanced Cu metallization.
Table 1 Atomic ratio and failure temperature of the Ru and RuCr films. Film composition was measured by EDXS and failure temperature (after 1 min annealing) was defined as the annealing temperature when the sheet resistance of the post-annealed film increased by at least one order of magnitude. Films
Atomic ratio (Ru:Cr)
Failure temperature (◦ C)
Ru RuCr I RuCr II RuCr III
– 10.06:1 1.42:1 0.75:1
575 800 725 700
2. Experimental A p-type Si-(1 0 0) wafer was prepared as a substrate. Prior to film deposition, the substrate was cleaned by a dilute HF to remove native oxide. A 5 nm Cr added Ru film was then deposited on the Si substrate by a co-sputtering system with Ru (purity 99.95%) and Cr (purity 99.95%) targets at 40 W direct-current (DC) power and radio-frequency (RF) power ranging from 5 to 40 W, respectively. The base pressure of the sputtering system was evacuated to below 1 × 10−5 Torr, and the work pressure was set at 1.2 × 10−2 Torr with an Ar (purity 99.9995%) flow rate of 8 standard cubic centimeters per minute. Afterwards, a 100 nm Cu (99.99% purity) film was in situ deposited on the Cr added Ru/Si substrate using a DC power of 80 W. The Ru and Cr composition was adjusted by varying the sputtering power ratio for preliminary barrier studies. For comparison, pure 5 nm Ru was also prepared by sputtering with a DC power of 40 W under same deposition conditions. To evaluate barrier film thermal stability and its barrier performance against Cu diffusion, 5 nm barrier/Si samples were annealed up to 800 ◦ C for 1 min and the Cu/barrier/Si samples were annealed up to 700 ◦ C for 30 min in a vacuum of 1 × 10−5 Torr in a rapid thermal annealing (RTA) system (ULVAC model. MILA5000). The elemental compositions of the barrier films were determined using an energy-dispersive X-ray spectroscopy (EDXS) unit attached to a field emission scanning electron microscopy (FESEM, Hitachi, SU8000). The sheet resistance variation of the Cu/barrier/Si samples with varied annealing temperatures was measured by a four-point probe. A grazing incident angle X-ray diffractometer (GIXRD, Rigaku, D/MAX2500), using CuK␣ radiation ( = 0.1542 nm), was used to identify the microstructure of both barrier/Si and Cu/barrier/Si samples. The inter-diffusion behaviors of Cu/barrier/Si structures were analyzed with an X-ray photoelectron spectroscope (XPS, PHI 5000 VersaProbe) with Al K␣ radiation. FESEM was adopted to observe surface morphologies and cross-sectional images of the Cu/barrier/Si samples pre- and post-annealing. To examine the microstructure and thickness of the barrier film, a high resolution transition electron microscopy (HRTEM, JEOL, JEM-2100F) was used to observe the cross sectional image of the structures. Leakage currents of the Cu/barrier/porous SiOCH/Si (metal/insulator/semiconductor, MIS) structures were measured with a Keithley 2400.
Therefore, RuCr I (the acronym RuCr will be used hereafter) was chosen for further studies against the pure Ru barrier. Fig. 1 shows the sheet resistance results after 30 min annealing of the Cu/barrier/Si system with various annealing temperatures. For both films, the sheet resistance slightly decreases initially with increased temperature, indicating Cu grain growth and defect annihilation of the Cu [23,24]. Grain growth of the Cu film is driven by minimizing surface and interface energies [25]. As the annealing temperature increases further, sheet resistance of the pure Ru film rises slightly after 450 ◦ C annealing, and climbs significantly after annealing at 500 ◦ C. Any sudden rise in sheet resistance could be an indication of forming Cu3 Si. In contrast, no noticeable increase in resistance is observed, even after annealing at 650 ◦ C for RuCr sample. This suggests that a small amount of Cr added to Ru film significantly improves the thermal stability by over 200 ◦ C compared to pure Ru film. Fig. 2a and b shows XRD spectra of the Cu/5 nm Ru/Si and Cu/5 nm RuCr/Si stacks with various annealing temperatures, respectively. The 2 diffraction peaks at 43.31◦ , 50.44◦ and 74.72◦ correspond to Cu (1 1 1), Cu (2 0 0), and Cu (2 2 0), respectively. The broad peaks of the as-deposited Cu film tend to sharpen with elevated annealing temperature, which can be ascribed to significant grain growth and densification of the Cu film [23,26]. Moreover, grain growth can reduce electron scattering at grain boundaries and can improve conductivity, as is seen in the descended sheet resistance in Fig. 1. In the Cu/Ru/Si system shown in Fig. 2a, the slightly broad Cu3 Si peaks first appear after annealing at 500 ◦ C. The formation of Cu3 Si and the disappearance of the Cu peaks imply that Cu reacts extensively with Si. The data shows that pure Ru film fails to hinder Cu from diffusion at 500 ◦ C. In contrast, no obvious Cu3 Si peaks can be detected in the RuCr sample, even after annealing at 650 ◦ C, as shown in Fig. 2b. The RuCr film shows a robust barrier performance to inhibit the Cu and Si reaction. No other compound was detected, which suggests that the Cr added Ru film is a
3. Results and discussion Three different compositions of RuCr films are examined using Cu/barrier/Si structure, and their thermal stability is screened by measuring the sheet resistance at various annealing temperatures. Table 1 summarizes the atomic ratio and failure temperature of each barrier. Failure temperature is defined as the annealing temperature when the sheet resistance of the post-annealed film increased by at least one order of magnitude after 1 min annealing. The short annealing time is for barrier material screening purpose. The data indicates that the lower Cr contented Ru film, RuCr I, had the highest thermal stability. And higher Cr content in Ru film could form RuCr compound, such as Cr3 Ru, at elevated temperature [21].
Fig. 1. Sheet resistance of the Cu/5 nm Ru/Si and Cu/5 nm RuCr/Si stacked structures after annealing at various temperatures for 30 min.
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Fig. 2. X-ray diffraction spectra of (a) Cu/5 nm Ru/Si and (b) Cu/5 nm RuCr/Si structures with various annealing temperatures. The annealing time is 30 min.
thermally stable and effective barrier. With further annealing at 700 ◦ C, Cu3 Si peaks appear which indicates the failure of the RuCr barrier. Overall, the 5 nm RuCr barrier is stable up to 650 ◦ C, which distinctly improves the thermal stability by over 200 ◦ C compared to a pure Ru film. The Ru film exhibits a columnar structure that provides vertical grain boundaries as feasible diffusion paths between the Cu and Si [12]. Therefore, Cu can rapidly diffuse into Si through Ru grain boundaries and further react with Si to form Cu silicide. In contrast, Cr is insoluble with Ru and the addition of Cr probably disrupts the crystallinity of Ru film and therefore enhances its barrier performance. Fig. 3a and b shows the XRD spectra of the 5 nm thick pure Ru and the RuCr films after 1 min annealing, respectively. In the pure Ru film (Fig. 3a), the Ru peak appears at a 2 value of 44.0◦ , which corresponds to Ru (1 0 1). The presence of a Ru peak after 500 ◦ C anneal suggests that the pure Ru film is a microcrystalline structure. The distinct Ru2 Si3 peaks that appear after annealing at 600 ◦ C indicate that Ru reacts extensively with Si. It has been reported that the signature of Ru barrier failure is the Ru2 Si3 formation prior to a significant amount of Cu penetration in a Cu/Ru/Si system. The formation of less dense Ru2 Si3 grains provides the fast diffusion paths and induces Cu penetration [27]. The formation of Ru2 Si3 indicates that the Ru barrier fails at 600 ◦ C, which is 100 ◦ C higher than sample received a 30-min annealing as indicated in Fig. 2a. In contrast, the as-deposited RuCr film (Fig. 3b) had a glassy structure and sustained its amorphous-like microstructure until 700 ◦ C. The insoluble element Cr in Ru might tend to
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Fig. 3. XRD patterns of the 5 nm (a) Ru and (b) RuCr films deposited on a Si substrate with various annealing temperatures for a period of 1 min.
segregate at the grain boundaries and extend its re-crystallization and grain growth to a higher temperature. It then provides no rapid diffusion path or fewer grain boundaries in the RuCr film for Cu to diffuse through. Moreover, no Ru2 Si3 was formed at 700 ◦ C, which suggests that a certain amount of Cr added to Ru film not only raises Ru grain growth temperature, but also delays Ru reacting with Si. As a result, the RuCr film improves thermal stability by over 200 ◦ C, and is a more stable barrier than a pure Ru film against Cu diffusion. Fig. 4 shows the as-deposited and post 1-min annealed surface morphologies of the Cu/Ru/Si and Cu/RuCr/Si stacks. The as-deposited Cu/RuCr/Si film stack has a smooth surface (Fig. 4a) and annealing at 600 ◦ C (Fig. 4b) leads to Cu grain growth which is consistent with the sharper Cu peak in the XRD spectra (Fig. 2b) and a decrease in the sheet resistance (Fig. 1). No sign of apparent reaction between Cu and Si, such as Cu3 Si, was observed. This suggests that the Cu/RuCr/Si system is thermally stable, even after annealing at 600 ◦ C. In contrast, the SEM top-view image shows crystallites on Cu surface were found in the Cu/Ru/Si sample after annealing at 600 ◦ C (Fig. 4c). The cross-sectional image in Fig. 4d demonstrates that this triangular crystallite forms deeply in the Si substrate at 575 ◦ C. An EDXS spot analysis, as shown in Fig. 4e, confirms that this crystallite contains mainly Cu and Si elements, indicating copper silicide was formed. The Cu3 Si developing at a low temperature of 575 ◦ C indicates that a pure Ru film has poor barrier performance, and is thermally stable up to only 550 ◦ C and would reduce as much as 100 ◦ C if 30 min annealing is performed.
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Fig. 4. SEM top view images of (a) the as-deposited and (b) the post 600 ◦ C annealed Cu/RuCr/Si sample; (c) the Cu/Ru/Si sample after annealing at 600 ◦ C; (d) SEM cross-sectional image of the 575 ◦ C annealed Cu/Ru/Si sample; and (e) EDXS spot analysis of a copper silicide crystallite. The annealing time is 1 min.
The element’s inter-diffusion behavior was investigated using the XPS depth profiles for the Cu/Ru/Si and Cu/RuCr/Si stacks. In Fig. 5b, a remarkable intermixing of elements was observed in the Cu/Ru/Si sample after annealing at 500 ◦ C for 30 min, as there is no interface between layers and Cu penetrates deeply into the Si substrate, suggesting that the 5 nm Ru barrier failed. In contrast, as shown in Fig. 5d, there is an apparent sharply decline of Cu and Si concentrations before the interface (or RuCr barrier), indicating the barrier is effective in blocking Cu diffusion, even with annealing at 650 ◦ C. There is no Ru–Cr binary phase when Cr content is lower than ∼18%, as describe earlier. The insoluble Cr might act as impurities that exert a drag force on the boundary motion, which thus constrains or retards Ru grain growth, and therefore leads to a higher re-crystallization temperature [28]. Without easy paths for Cu diffusion, such as grain boundaries, barrier performance is improved. HRTEM was used to examine the microstructures of the pure Ru and RuCr films. Fig. 6a shows the as-deposited Cu/Ru/Si structure with a ∼3 nm native oxide layer present on the Si substrate. The Ru film is approximately 7.8 nm thick and has a columnar-like
microstructure. The inset image is the Fourier transformedelectron diffraction (FTED) patterns from the TEM image of the pure Ru film. The FTED pattern indicates that Ru film has a polycrystalline-like microstructure. The columnar microstructures vertically grown on Si can provide fast diffusion paths for Cu atoms, which contribute to the low temperature failure of the barrier against Cu diffusion. In contrast, and with an intentionally biased comparison, the 1-min 600 ◦ C post-annealed RuCr sample is shown in Fig. 6b. Unlike Ru, no bright lattice dots or rings are observed in the FTED image of the RuCr film, indicating that this film preserves its amorphous-like structure even after annealing at 600 ◦ C, which is consistent with the GIXRD results shown in Fig. 3b, where Ru will not re-crystallize until 700 ◦ C with 1 min annealing. The thickness of the RuCr film is approximately 5.1 nm, which is thinner than the pure Ru sample. However, the thermal stability of this thinner RuCr film is more than 200 ◦ C higher than that of the 50% thicker Ru film. The leakage current densities of the barrier films deposited on a porous SiOCH (p-SiOCH) film, using Cu/barrier/p-SiOCH/Si (MIS) structure, were evaluated after 30 min annealing at 400 ◦ C, and a higher leakage current in the pure Ru barrier sample was observed
K.-C. Hsu et al. / Applied Surface Science 258 (2012) 7225–7230
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Fig. 5. XPS depth profiles of the (a) as-deposited; (b) post 30-min 500 ◦ C annealed Cu/5 nm Ru/Si stacked films. And (c) as-deposited; (d) post 30-min 650 ◦ C annealed Cu/5 nm RuCr/Si stacked films.
(Fig. 7). The leakage current arises mainly from the mobile charges in the p-SiOCH film [29]. Higher leakage indicates that Cu atoms may have penetrated through the Ru barrier and diffused into the p-SiOCH film. In comparison, at low (≤1 MV/cm) electric field, about two orders of magnitude improvement is obtained from the sample using RuCr barrier. This implies that the RuCr barrier
can suppress Cu penetration into the p-SiOCH. As the TEM image (Fig. 6b) and XRD patterns (Fig. 3b) show, the enhancement of barrier performance against Cu penetration can be attributed to the amorphous-like microstructure of the RuCr film. The results indicate that the 5 nm RuCr film is a promising candidate as a diffusion barrier for advanced Cu metallization.
Fig. 6. TEM cross-sectional micrographs of (a) the as-deposited Cu/Ru/Si stacked films, and (b) the. Cu/RuCr/Si stacked films after 1 min annealing at 600 ◦ C. The inset images are the corresponding FTED patterns of the barriers.
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References
Fig. 7. Leakage current densities of the Cu/barrier/p-SiOCH/Si structure after 30 min annealing at 400 ◦ C.
4. Conclusion We have demonstrated that a 5 nm RuCr film can serve as a barrier against Cu diffusion with high thermal stability. There was no significant inter-diffusion in the Cu/RuCr/Si structure, even after 30 min annealing at 650 ◦ C, which suggests that the RuCr film effectively blocks Cu penetration into Si. The TEM micrographs show that the 5 nm RuCr film has an amorphous-like microstructure. The crystallinity of Ru was disrupted by the small amount of Cr added, and this extended Ru re-crystallization to a higher temperature. The leakage current of the RuCr sample after annealing at 400 ◦ C for 30 min is about two orders of magnitude better than that of the pure Ru barrier for electric field less than 1 MV/cm. The dramatic increase in the sheet resistance and the Cu3 Si peaks, as indicated by the XRD spectra, suggest that the RuCr film failed after 30 min annealing at 700 ◦ C due to Ru in the RuCr film re-crystallized. The RuCr film presented in this work can be a promising candidate as a Cu diffusion barrier for advanced Cu metallization. Acknowledgements The authors would like to thank the CRD thin film group of United Microelectronic Co. for p-SiOCH film deposition. The funding support of the National Science Council of Taiwan under contract no. NSC 99-2221-E-006-138 is acknowledged.
[1] Kajita, T. Usui, M. Yamada, E. Ogawa, T. Katata, A. Sakata, H. Miyajima, A. Kojima, R. Kanamura, Y. Ohoka, H. Kawashima, K. Tabuchi, K. Nagahata, Y. Kato, T. Hayashi, S. Kadomura, H. Shibata, Proceedings of the IEEE International Interconnect Technology Conference, 2003, p. 9. [2] R. Rosenberg, D.C. Edelstein, C.-K. Hu, K.P. Rodbell, Annual Review of Materials Science 30 (2000) 229. [3] C.S. Hau-Riege, Microelectronics and Reliability 44 (2004) 195. [4] V. Sukharev, A. Kteyan, E. Zschech, W.D. Nix, IEEE Transactions on Device and Materials Reliability 9 (2009) 87. [5] C.A. Chang, Journal of Applied Physics 67 (1990) 566. [6] O.R. Rodriguez, W.N. Gill, J.L. Plawsky, T.Y. Tsui, S. Grunow, Journal of Applied Physics 98 (2005) 123514. [7] C.C. Chang, S.K. JangJian, J.S. Chen, Journal of the Electrochemical Society 152 (2005) G517. [8] Q. Xie, X.P. Qu, J.J. Tan, Y.L. Jiang, M. Zhou, T. Chen, G.P. Ru, Applied Surface Science 253 (2006) 1666. [9] Z.H. Cao, K. Hu, X.K. Meng, Journal of Applied Physics 106 (2009) 113513. [10] P.R. Subramanlan, D.E. Laughlln, Bulletin of Alloy Phase Diagrams 10 (1989) 652. [11] The International Technology Roadmap for Semiconductors, 2010 update. Available from www.itrs.net. [12] R. Chan, T.N. Arunagiri, Y. Zhang, O. Chyan, R.M. Wallace, M.J. Kim, T.Q. Hurd, Electrochemical and Solid State Letters 7 (2004) G154. [13] H. Kim, T. Koseki, T. Ohba, T. Ohta, Y. Kojima, H. Sato, Y. Shimogaki, Journal of the Electrochemical Society 152 (2005) G594. [14] O. Chyan, T.N. Arunagiri, T. Ponnuswamy, Journal of the Electrochemical Society 150 (2003) C347. [15] T.P. Moffat, M. Walker, P.J. Chen, J.E. Bonevich, W.F. Egelhoff, L. Richter, C. Witt, T. Aaltonen, M. Ritala, M. Leskelä, D. Josell, Journal of the Electrochemical Society 153 (2006) C37. [16] T.N. Arunagiri, Y. Zhang, O. Chyan, M. El-Bouanani, M.J. Kim, K.H. Chen, C.T. Wu, L.C. Chen, Applied Physics Letters 86 (2005) 083104. [17] C.-C. Yang, T. Spooner, S. Ponoth, K. Chanda, A. Simon, C. Lavoie, M. Lane, C.-K. Hu, E. Liniger, L. Gignac, T. Shaw, S. Cohen, F. McFeely, D. Edelstein, Proceedings of the IEEE International Interconnect Technology Conference, 2006, p. 187. [18] M. Damayanti, T. Sritharan, S.G. Mhaisalkar, H.J. Engelmann, E. Zschech, A.V. Vairagar, L. Chan, Electrochemical and Solid State Letters 10 (2007) P15. [19] D.C. Perng, J.B. Yeh, K.C. Hsu, Applied Surface Science 254 (2008) 6059. [20] C.W. Chen, J.S. Chen, J.S. Jeng, Journal of the Electrochemical Society 155 (2008) H1003. [21] M. Venkatraman, J.P. Neumann, Bulletin of Alloy Phase Diagrams 8 (1987) 109. [22] S.W. Russell, S.A. Rafalski, R.L. Spreitzer, J. Li, M. Moinpour, F. Moghadam, T.L. Alford, Thin Solid Films 262 (1995) 154. [23] C.M. Liu, W.L. Liu, W.J. Chen, S.H. Hsieh, T.K. Tsai, L.C. Yang, Journal of the Electrochemical Society 152 (2005) G234. [24] Y. Wang, F. Cao, Z. Song, C.H. Zhaoa, Electrochemical and Solid State Letters 10 (2007) H299. [25] N.-J. Park, D.P. Field, M.M. Nowell, P.R. Besser, Journal of Electronic Materials 34 (2005) 1500. [26] S.-H. Kim, K.T. Nam, A. Datta, H.-M. Kim, K.-B. Kim, D.-H. Kang, Journal of Vacuum Science and Technology B 21 (2003) 804. [27] M. Damayanti, T. Sritharan, S.G. Mhaisalkar, E. Phoon, L. Chan, Journal of Materials Research 22 (2007) 2505. [28] J.W. Cahn, Acta Metallurgica 10 (1962) 789. [29] T. Fukuda, H. Nishino, H. Yanazawa, Japanese Journal of Applied Physics 43 (2004) 86.
Contents lists available at SciVerse ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Ultrathin Cr added Ru film as a seedless Cu diffusion barrier for advanced Cu interconnects Kuo-Chung Hsu a,b , Dung-Ching Perng a,b,∗ , Jia-Bin Yeh a,b , Yi-Chun Wang a,b a b
Institute of Microelectronics, Dept. of Electrical Engineering, National Cheng Kung University, No. 1 University Road, Tainan 70101, Taiwan Center for Micro/Nano Science and Technology, National Cheng Kung University, No. 1 University Road, Tainan 70101, Taiwan
a r t i c l e
i n f o
Article history: Received 4 August 2011 Received in revised form 15 February 2012 Accepted 9 April 2012 Available online 15 April 2012 Keywords: RuCr alloy Diffusion barrier Cu interconnect Cu metallization
a b s t r a c t A 5 nm thick Cr added Ru film has been extensively investigated as a seedless Cu diffusion barrier. Highresolution transmission electron microscopy micrograph, X-ray diffraction (XRD) pattern and Fourier transform-electron diffraction pattern reveal that a Cr contained Ru (RuCr) film has a glassy microstructure and is an amorphous-like film. XRD patterns and sheet resistance data show that the RuCr film is stable up to 650 ◦ C, which is approximately a 200 ◦ C improvement in thermal stability as compared to that of the pure Ru film. X-ray photoelectron spectroscopy depth profiles show that the RuCr film can successfully block Cu diffusion, even after a 30-min 650 ◦ C annealing. The leakage current of the Cu/5 nm RuCr/porous SiOCH/Si stacked structure is about two orders of magnitude lower than that of a pristine Ru sample for electric field below 1 MV/cm. The RuCr film can be a promising Cu diffusion barrier for advanced Cu metallization. © 2012 Elsevier B.V. All rights reserved.
1. Introduction With continuing shrinking of feature size in ultra large scale integrated circuit (ULSI) technology, Cu and low-k dielectric materials have been widely implemented to lower the resistance–capacitance delay effect (R–C delay) and for higher electron migration (EM) resistance [1–4]. However, Cu can react with Si easily and diffuses rapidly in porous dielectric materials, which leads to degradation in electrical performance [5,6]. Therefore, high thermal stability and negligible reaction with Cu and Si are required for a Cu diffusion barrier. Ta/TaN bi-layers are a widely used Cu diffusion barriers in Cu interconnects [7–9]. Ta exhibits negligible solubility with Cu [10] and offers good barrier performance against Cu. A 30 nm Ta/TaN bi-layer barrier is thermally stable up to 850 ◦ C [8]. Unfortunately, a Ta/TaN bi-layer exhibits high resistivity, and this would be unacceptable with regard to achieving lower effective resistivity in future nodes. Moreover, the International Technology Roadmap for Semiconductors [11] (ITRS) indicated that the required diffusion barrier thickness is approximately 2.4 nm, and the aspect ratio (A/R) of the metal line will increase to 1.90 by 2013. With a narrower line width and a higher A/R, using traditional approach, i.e. a thick barrier plus an additional Cu seed layer, is not feasible. Hence, a new barrier material must be developed
∗ Corresponding author at: Institute of Microelectronics, Dept. of Electrical Engineering, National Cheng Kung University, No. 1 University Road, Tainan 70101, Taiwan. Tel.: +886 6 275 7575x62433; fax: +886 6 234 5482. E-mail address: [email protected] (D.-C. Perng). 0169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2012.04.046
that can act as a seedless Cu diffusion barrier with low resistivity and reduced thickness. Ruthenium (Ru), a noble transitional metal, has been widely studied as a seedless Cu diffusion barrier [12,13]. Ru exhibits a low electrical resistivity (bulk = 7.1 -cm) and good adhesion with Cu [12]. In addition, Ru can be directly electroplated onto Cu without an extra Cu seed layer [14,15]. Unfortunately, a pure Ru film exhibits poor barrier performance against Cu [12,16] and poor adhesion to dielectric or oxide layer [17]. The polycrystalline nature of the asdeposited Ru film has been reported that the Ru grain boundaries provided easy paths for Cu diffusion, which causes early failure of the barrier [18]. Recently, many studies have focused on improving Ru barrier performance. Doping other elements, such as N, P, and Ta, into the Ru film to form a Ru alloy barriers have been reported [18–20]. The added elements act as impurities could segregate at the Ru grain boundaries and prohibit Cu penetration through the barrier. Investigations have also shown that a small amount of elements added to the Ru film can alter the microstructure of a pure Ru film from polycrystalline to amorphous-like, and the microstructure is a dominant factor in improving the barrier performance of the Ru-based film. Cr (bulk = 12.5 -cm) has a lower electrical resistivity compared to Ta (bulk = 13.6 -cm) and has no Ru–Cr phase when Cr content is lower than ∼18% [21]. Cr added into Cu film can significantly enhance the adhesion of Cu film to dielectric [22]. The data showed some Cr accumulating at the dielectric interface, suggesting chemical reaction in that region which enhance adhesion. Based on same mechanism, the adhesion of Ru to dielectric is likely to improve by adding Cr to Ru. However, barrier performance of Cr
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K.-C. Hsu et al. / Applied Surface Science 258 (2012) 7225–7230
added Ru film, especially ultrathin layer, has not yet been studied. In this work, addition of Cr to Ru film, with various atomic composition ratios, were studied. Barrier properties and thermal stability of 5-nm thick Cr added Ru films deposited on a Si substrate with various annealing temperatures up to 800 ◦ C are investigated. Besides the potential adhesion improvement, the results show that the Cr added Ru film could also be a promising candidate as a Cu diffusion barrier for advanced Cu metallization.
Table 1 Atomic ratio and failure temperature of the Ru and RuCr films. Film composition was measured by EDXS and failure temperature (after 1 min annealing) was defined as the annealing temperature when the sheet resistance of the post-annealed film increased by at least one order of magnitude. Films
Atomic ratio (Ru:Cr)
Failure temperature (◦ C)
Ru RuCr I RuCr II RuCr III
– 10.06:1 1.42:1 0.75:1
575 800 725 700
2. Experimental A p-type Si-(1 0 0) wafer was prepared as a substrate. Prior to film deposition, the substrate was cleaned by a dilute HF to remove native oxide. A 5 nm Cr added Ru film was then deposited on the Si substrate by a co-sputtering system with Ru (purity 99.95%) and Cr (purity 99.95%) targets at 40 W direct-current (DC) power and radio-frequency (RF) power ranging from 5 to 40 W, respectively. The base pressure of the sputtering system was evacuated to below 1 × 10−5 Torr, and the work pressure was set at 1.2 × 10−2 Torr with an Ar (purity 99.9995%) flow rate of 8 standard cubic centimeters per minute. Afterwards, a 100 nm Cu (99.99% purity) film was in situ deposited on the Cr added Ru/Si substrate using a DC power of 80 W. The Ru and Cr composition was adjusted by varying the sputtering power ratio for preliminary barrier studies. For comparison, pure 5 nm Ru was also prepared by sputtering with a DC power of 40 W under same deposition conditions. To evaluate barrier film thermal stability and its barrier performance against Cu diffusion, 5 nm barrier/Si samples were annealed up to 800 ◦ C for 1 min and the Cu/barrier/Si samples were annealed up to 700 ◦ C for 30 min in a vacuum of 1 × 10−5 Torr in a rapid thermal annealing (RTA) system (ULVAC model. MILA5000). The elemental compositions of the barrier films were determined using an energy-dispersive X-ray spectroscopy (EDXS) unit attached to a field emission scanning electron microscopy (FESEM, Hitachi, SU8000). The sheet resistance variation of the Cu/barrier/Si samples with varied annealing temperatures was measured by a four-point probe. A grazing incident angle X-ray diffractometer (GIXRD, Rigaku, D/MAX2500), using CuK␣ radiation ( = 0.1542 nm), was used to identify the microstructure of both barrier/Si and Cu/barrier/Si samples. The inter-diffusion behaviors of Cu/barrier/Si structures were analyzed with an X-ray photoelectron spectroscope (XPS, PHI 5000 VersaProbe) with Al K␣ radiation. FESEM was adopted to observe surface morphologies and cross-sectional images of the Cu/barrier/Si samples pre- and post-annealing. To examine the microstructure and thickness of the barrier film, a high resolution transition electron microscopy (HRTEM, JEOL, JEM-2100F) was used to observe the cross sectional image of the structures. Leakage currents of the Cu/barrier/porous SiOCH/Si (metal/insulator/semiconductor, MIS) structures were measured with a Keithley 2400.
Therefore, RuCr I (the acronym RuCr will be used hereafter) was chosen for further studies against the pure Ru barrier. Fig. 1 shows the sheet resistance results after 30 min annealing of the Cu/barrier/Si system with various annealing temperatures. For both films, the sheet resistance slightly decreases initially with increased temperature, indicating Cu grain growth and defect annihilation of the Cu [23,24]. Grain growth of the Cu film is driven by minimizing surface and interface energies [25]. As the annealing temperature increases further, sheet resistance of the pure Ru film rises slightly after 450 ◦ C annealing, and climbs significantly after annealing at 500 ◦ C. Any sudden rise in sheet resistance could be an indication of forming Cu3 Si. In contrast, no noticeable increase in resistance is observed, even after annealing at 650 ◦ C for RuCr sample. This suggests that a small amount of Cr added to Ru film significantly improves the thermal stability by over 200 ◦ C compared to pure Ru film. Fig. 2a and b shows XRD spectra of the Cu/5 nm Ru/Si and Cu/5 nm RuCr/Si stacks with various annealing temperatures, respectively. The 2 diffraction peaks at 43.31◦ , 50.44◦ and 74.72◦ correspond to Cu (1 1 1), Cu (2 0 0), and Cu (2 2 0), respectively. The broad peaks of the as-deposited Cu film tend to sharpen with elevated annealing temperature, which can be ascribed to significant grain growth and densification of the Cu film [23,26]. Moreover, grain growth can reduce electron scattering at grain boundaries and can improve conductivity, as is seen in the descended sheet resistance in Fig. 1. In the Cu/Ru/Si system shown in Fig. 2a, the slightly broad Cu3 Si peaks first appear after annealing at 500 ◦ C. The formation of Cu3 Si and the disappearance of the Cu peaks imply that Cu reacts extensively with Si. The data shows that pure Ru film fails to hinder Cu from diffusion at 500 ◦ C. In contrast, no obvious Cu3 Si peaks can be detected in the RuCr sample, even after annealing at 650 ◦ C, as shown in Fig. 2b. The RuCr film shows a robust barrier performance to inhibit the Cu and Si reaction. No other compound was detected, which suggests that the Cr added Ru film is a
3. Results and discussion Three different compositions of RuCr films are examined using Cu/barrier/Si structure, and their thermal stability is screened by measuring the sheet resistance at various annealing temperatures. Table 1 summarizes the atomic ratio and failure temperature of each barrier. Failure temperature is defined as the annealing temperature when the sheet resistance of the post-annealed film increased by at least one order of magnitude after 1 min annealing. The short annealing time is for barrier material screening purpose. The data indicates that the lower Cr contented Ru film, RuCr I, had the highest thermal stability. And higher Cr content in Ru film could form RuCr compound, such as Cr3 Ru, at elevated temperature [21].
Fig. 1. Sheet resistance of the Cu/5 nm Ru/Si and Cu/5 nm RuCr/Si stacked structures after annealing at various temperatures for 30 min.
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Fig. 2. X-ray diffraction spectra of (a) Cu/5 nm Ru/Si and (b) Cu/5 nm RuCr/Si structures with various annealing temperatures. The annealing time is 30 min.
thermally stable and effective barrier. With further annealing at 700 ◦ C, Cu3 Si peaks appear which indicates the failure of the RuCr barrier. Overall, the 5 nm RuCr barrier is stable up to 650 ◦ C, which distinctly improves the thermal stability by over 200 ◦ C compared to a pure Ru film. The Ru film exhibits a columnar structure that provides vertical grain boundaries as feasible diffusion paths between the Cu and Si [12]. Therefore, Cu can rapidly diffuse into Si through Ru grain boundaries and further react with Si to form Cu silicide. In contrast, Cr is insoluble with Ru and the addition of Cr probably disrupts the crystallinity of Ru film and therefore enhances its barrier performance. Fig. 3a and b shows the XRD spectra of the 5 nm thick pure Ru and the RuCr films after 1 min annealing, respectively. In the pure Ru film (Fig. 3a), the Ru peak appears at a 2 value of 44.0◦ , which corresponds to Ru (1 0 1). The presence of a Ru peak after 500 ◦ C anneal suggests that the pure Ru film is a microcrystalline structure. The distinct Ru2 Si3 peaks that appear after annealing at 600 ◦ C indicate that Ru reacts extensively with Si. It has been reported that the signature of Ru barrier failure is the Ru2 Si3 formation prior to a significant amount of Cu penetration in a Cu/Ru/Si system. The formation of less dense Ru2 Si3 grains provides the fast diffusion paths and induces Cu penetration [27]. The formation of Ru2 Si3 indicates that the Ru barrier fails at 600 ◦ C, which is 100 ◦ C higher than sample received a 30-min annealing as indicated in Fig. 2a. In contrast, the as-deposited RuCr film (Fig. 3b) had a glassy structure and sustained its amorphous-like microstructure until 700 ◦ C. The insoluble element Cr in Ru might tend to
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Fig. 3. XRD patterns of the 5 nm (a) Ru and (b) RuCr films deposited on a Si substrate with various annealing temperatures for a period of 1 min.
segregate at the grain boundaries and extend its re-crystallization and grain growth to a higher temperature. It then provides no rapid diffusion path or fewer grain boundaries in the RuCr film for Cu to diffuse through. Moreover, no Ru2 Si3 was formed at 700 ◦ C, which suggests that a certain amount of Cr added to Ru film not only raises Ru grain growth temperature, but also delays Ru reacting with Si. As a result, the RuCr film improves thermal stability by over 200 ◦ C, and is a more stable barrier than a pure Ru film against Cu diffusion. Fig. 4 shows the as-deposited and post 1-min annealed surface morphologies of the Cu/Ru/Si and Cu/RuCr/Si stacks. The as-deposited Cu/RuCr/Si film stack has a smooth surface (Fig. 4a) and annealing at 600 ◦ C (Fig. 4b) leads to Cu grain growth which is consistent with the sharper Cu peak in the XRD spectra (Fig. 2b) and a decrease in the sheet resistance (Fig. 1). No sign of apparent reaction between Cu and Si, such as Cu3 Si, was observed. This suggests that the Cu/RuCr/Si system is thermally stable, even after annealing at 600 ◦ C. In contrast, the SEM top-view image shows crystallites on Cu surface were found in the Cu/Ru/Si sample after annealing at 600 ◦ C (Fig. 4c). The cross-sectional image in Fig. 4d demonstrates that this triangular crystallite forms deeply in the Si substrate at 575 ◦ C. An EDXS spot analysis, as shown in Fig. 4e, confirms that this crystallite contains mainly Cu and Si elements, indicating copper silicide was formed. The Cu3 Si developing at a low temperature of 575 ◦ C indicates that a pure Ru film has poor barrier performance, and is thermally stable up to only 550 ◦ C and would reduce as much as 100 ◦ C if 30 min annealing is performed.
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Fig. 4. SEM top view images of (a) the as-deposited and (b) the post 600 ◦ C annealed Cu/RuCr/Si sample; (c) the Cu/Ru/Si sample after annealing at 600 ◦ C; (d) SEM cross-sectional image of the 575 ◦ C annealed Cu/Ru/Si sample; and (e) EDXS spot analysis of a copper silicide crystallite. The annealing time is 1 min.
The element’s inter-diffusion behavior was investigated using the XPS depth profiles for the Cu/Ru/Si and Cu/RuCr/Si stacks. In Fig. 5b, a remarkable intermixing of elements was observed in the Cu/Ru/Si sample after annealing at 500 ◦ C for 30 min, as there is no interface between layers and Cu penetrates deeply into the Si substrate, suggesting that the 5 nm Ru barrier failed. In contrast, as shown in Fig. 5d, there is an apparent sharply decline of Cu and Si concentrations before the interface (or RuCr barrier), indicating the barrier is effective in blocking Cu diffusion, even with annealing at 650 ◦ C. There is no Ru–Cr binary phase when Cr content is lower than ∼18%, as describe earlier. The insoluble Cr might act as impurities that exert a drag force on the boundary motion, which thus constrains or retards Ru grain growth, and therefore leads to a higher re-crystallization temperature [28]. Without easy paths for Cu diffusion, such as grain boundaries, barrier performance is improved. HRTEM was used to examine the microstructures of the pure Ru and RuCr films. Fig. 6a shows the as-deposited Cu/Ru/Si structure with a ∼3 nm native oxide layer present on the Si substrate. The Ru film is approximately 7.8 nm thick and has a columnar-like
microstructure. The inset image is the Fourier transformedelectron diffraction (FTED) patterns from the TEM image of the pure Ru film. The FTED pattern indicates that Ru film has a polycrystalline-like microstructure. The columnar microstructures vertically grown on Si can provide fast diffusion paths for Cu atoms, which contribute to the low temperature failure of the barrier against Cu diffusion. In contrast, and with an intentionally biased comparison, the 1-min 600 ◦ C post-annealed RuCr sample is shown in Fig. 6b. Unlike Ru, no bright lattice dots or rings are observed in the FTED image of the RuCr film, indicating that this film preserves its amorphous-like structure even after annealing at 600 ◦ C, which is consistent with the GIXRD results shown in Fig. 3b, where Ru will not re-crystallize until 700 ◦ C with 1 min annealing. The thickness of the RuCr film is approximately 5.1 nm, which is thinner than the pure Ru sample. However, the thermal stability of this thinner RuCr film is more than 200 ◦ C higher than that of the 50% thicker Ru film. The leakage current densities of the barrier films deposited on a porous SiOCH (p-SiOCH) film, using Cu/barrier/p-SiOCH/Si (MIS) structure, were evaluated after 30 min annealing at 400 ◦ C, and a higher leakage current in the pure Ru barrier sample was observed
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Fig. 5. XPS depth profiles of the (a) as-deposited; (b) post 30-min 500 ◦ C annealed Cu/5 nm Ru/Si stacked films. And (c) as-deposited; (d) post 30-min 650 ◦ C annealed Cu/5 nm RuCr/Si stacked films.
(Fig. 7). The leakage current arises mainly from the mobile charges in the p-SiOCH film [29]. Higher leakage indicates that Cu atoms may have penetrated through the Ru barrier and diffused into the p-SiOCH film. In comparison, at low (≤1 MV/cm) electric field, about two orders of magnitude improvement is obtained from the sample using RuCr barrier. This implies that the RuCr barrier
can suppress Cu penetration into the p-SiOCH. As the TEM image (Fig. 6b) and XRD patterns (Fig. 3b) show, the enhancement of barrier performance against Cu penetration can be attributed to the amorphous-like microstructure of the RuCr film. The results indicate that the 5 nm RuCr film is a promising candidate as a diffusion barrier for advanced Cu metallization.
Fig. 6. TEM cross-sectional micrographs of (a) the as-deposited Cu/Ru/Si stacked films, and (b) the. Cu/RuCr/Si stacked films after 1 min annealing at 600 ◦ C. The inset images are the corresponding FTED patterns of the barriers.
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References
Fig. 7. Leakage current densities of the Cu/barrier/p-SiOCH/Si structure after 30 min annealing at 400 ◦ C.
4. Conclusion We have demonstrated that a 5 nm RuCr film can serve as a barrier against Cu diffusion with high thermal stability. There was no significant inter-diffusion in the Cu/RuCr/Si structure, even after 30 min annealing at 650 ◦ C, which suggests that the RuCr film effectively blocks Cu penetration into Si. The TEM micrographs show that the 5 nm RuCr film has an amorphous-like microstructure. The crystallinity of Ru was disrupted by the small amount of Cr added, and this extended Ru re-crystallization to a higher temperature. The leakage current of the RuCr sample after annealing at 400 ◦ C for 30 min is about two orders of magnitude better than that of the pure Ru barrier for electric field less than 1 MV/cm. The dramatic increase in the sheet resistance and the Cu3 Si peaks, as indicated by the XRD spectra, suggest that the RuCr film failed after 30 min annealing at 700 ◦ C due to Ru in the RuCr film re-crystallized. The RuCr film presented in this work can be a promising candidate as a Cu diffusion barrier for advanced Cu metallization. Acknowledgements The authors would like to thank the CRD thin film group of United Microelectronic Co. for p-SiOCH film deposition. The funding support of the National Science Council of Taiwan under contract no. NSC 99-2221-E-006-138 is acknowledged.
[1] Kajita, T. Usui, M. Yamada, E. Ogawa, T. Katata, A. Sakata, H. Miyajima, A. Kojima, R. Kanamura, Y. Ohoka, H. Kawashima, K. Tabuchi, K. Nagahata, Y. Kato, T. Hayashi, S. Kadomura, H. Shibata, Proceedings of the IEEE International Interconnect Technology Conference, 2003, p. 9. [2] R. Rosenberg, D.C. Edelstein, C.-K. Hu, K.P. Rodbell, Annual Review of Materials Science 30 (2000) 229. [3] C.S. Hau-Riege, Microelectronics and Reliability 44 (2004) 195. [4] V. Sukharev, A. Kteyan, E. Zschech, W.D. Nix, IEEE Transactions on Device and Materials Reliability 9 (2009) 87. [5] C.A. Chang, Journal of Applied Physics 67 (1990) 566. [6] O.R. Rodriguez, W.N. Gill, J.L. Plawsky, T.Y. Tsui, S. Grunow, Journal of Applied Physics 98 (2005) 123514. [7] C.C. Chang, S.K. JangJian, J.S. Chen, Journal of the Electrochemical Society 152 (2005) G517. [8] Q. Xie, X.P. Qu, J.J. Tan, Y.L. Jiang, M. Zhou, T. Chen, G.P. Ru, Applied Surface Science 253 (2006) 1666. [9] Z.H. Cao, K. Hu, X.K. Meng, Journal of Applied Physics 106 (2009) 113513. [10] P.R. Subramanlan, D.E. Laughlln, Bulletin of Alloy Phase Diagrams 10 (1989) 652. [11] The International Technology Roadmap for Semiconductors, 2010 update. Available from www.itrs.net. [12] R. Chan, T.N. Arunagiri, Y. Zhang, O. Chyan, R.M. Wallace, M.J. Kim, T.Q. Hurd, Electrochemical and Solid State Letters 7 (2004) G154. [13] H. Kim, T. Koseki, T. Ohba, T. Ohta, Y. Kojima, H. Sato, Y. Shimogaki, Journal of the Electrochemical Society 152 (2005) G594. [14] O. Chyan, T.N. Arunagiri, T. Ponnuswamy, Journal of the Electrochemical Society 150 (2003) C347. [15] T.P. Moffat, M. Walker, P.J. Chen, J.E. Bonevich, W.F. Egelhoff, L. Richter, C. Witt, T. Aaltonen, M. Ritala, M. Leskelä, D. Josell, Journal of the Electrochemical Society 153 (2006) C37. [16] T.N. Arunagiri, Y. Zhang, O. Chyan, M. El-Bouanani, M.J. Kim, K.H. Chen, C.T. Wu, L.C. Chen, Applied Physics Letters 86 (2005) 083104. [17] C.-C. Yang, T. Spooner, S. Ponoth, K. Chanda, A. Simon, C. Lavoie, M. Lane, C.-K. Hu, E. Liniger, L. Gignac, T. Shaw, S. Cohen, F. McFeely, D. Edelstein, Proceedings of the IEEE International Interconnect Technology Conference, 2006, p. 187. [18] M. Damayanti, T. Sritharan, S.G. Mhaisalkar, H.J. Engelmann, E. Zschech, A.V. Vairagar, L. Chan, Electrochemical and Solid State Letters 10 (2007) P15. [19] D.C. Perng, J.B. Yeh, K.C. Hsu, Applied Surface Science 254 (2008) 6059. [20] C.W. Chen, J.S. Chen, J.S. Jeng, Journal of the Electrochemical Society 155 (2008) H1003. [21] M. Venkatraman, J.P. Neumann, Bulletin of Alloy Phase Diagrams 8 (1987) 109. [22] S.W. Russell, S.A. Rafalski, R.L. Spreitzer, J. Li, M. Moinpour, F. Moghadam, T.L. Alford, Thin Solid Films 262 (1995) 154. [23] C.M. Liu, W.L. Liu, W.J. Chen, S.H. Hsieh, T.K. Tsai, L.C. Yang, Journal of the Electrochemical Society 152 (2005) G234. [24] Y. Wang, F. Cao, Z. Song, C.H. Zhaoa, Electrochemical and Solid State Letters 10 (2007) H299. [25] N.-J. Park, D.P. Field, M.M. Nowell, P.R. Besser, Journal of Electronic Materials 34 (2005) 1500. [26] S.-H. Kim, K.T. Nam, A. Datta, H.-M. Kim, K.-B. Kim, D.-H. Kang, Journal of Vacuum Science and Technology B 21 (2003) 804. [27] M. Damayanti, T. Sritharan, S.G. Mhaisalkar, E. Phoon, L. Chan, Journal of Materials Research 22 (2007) 2505. [28] J.W. Cahn, Acta Metallurgica 10 (1962) 789. [29] T. Fukuda, H. Nishino, H. Yanazawa, Japanese Journal of Applied Physics 43 (2004) 86.