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Ion beam deposition of α-Ta films by nitrogen addition and improvement of diffusion barrier property
Thin Solid Films 515 (2007) 4768 – 4773 www.elsevier.com/locate/tsf
Ion beam deposition of α-Ta films by nitrogen addition and improvement of diffusion barrier property Joon Woo Bae a,⁎, Jae-Won Lim b , Kouji Mimura a , Minoru Isshiki a a
b
Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Sendai 980–8577, Japan Materials Development Group, Minerals and Materials Processing Division, Korea Institute of Geoscience and Mineral Resources, 30 Gajeong-dong, Yuseong-gu, Daejeon, 305-350, Republic of Korea Received 4 February 2005; received in revised form 19 September 2006; accepted 16 November 2006 Available online 22 January 2007
Abstract Ta thin films were deposited on Si (100) substrates by an ion beam deposition method at various substrate bias voltages under Ar + N2 atmosphere with different pressure ratios of Ar and N2. The effects of nitrogen pressure in the plasma gas and the substrate bias voltage on the surface morphology, crystalline microstructure, electrical resistivity and diffusion barrier property were investigated. It was found that the fraction of a metastable β-phase in the Ta film deposited at the substrate bias voltage of −50 V films decreased by adding nitrogen gas, while the α-Ta phase became dominant. As a result, the Ta films deposited at the substrate bias voltage of − 50 V under Ar (9 Pa) + N2 (3 Pa) atmosphere showed a dominant α-phase with good surface morphology, low resistivity, and superior thermal stability as a diffusion barrier. © 2007 Elsevier B.V. All rights reserved. Keywords: Ta film; α-phase; Resistivity; Ion beam deposition; Diffusion barrier
1. Introduction Cu interconnects are the most promising candidate for next generation high speed ultra larger scale integrated (ULSI) circuits, because Cu exhibits a lower resistivity and larger electro/stress-migration resistance than conventional Al and Al alloys [1]. However, Cu has disadvantages, such as fast diffusion into Si and SiO2 [2], formation of Cu–Si compounds, and poor adhesion to most dielectric layers [3]. Therefore, an effective diffusion barrier is required to overcome these problems. Various materials such as Ta, W, Mo, Ti, C, Nb, and Cr have been examined as a diffusion barrier interposed between the Cu overlayer and the Si substrate. Among these materials, Ta-based thin films have been shown to be robust diffusion barriers for Cu, because the lattice diffusion of Cu through the Ta layer is very low [4–6]. On the other hand, nitride films of refractory transition metals such as tantalum nitride are also known to be promising candidates for making an excellent diffusion barrier. Ta–N films have better thermal stability, but higher resistivity than that of pure Ta films. ⁎ Corresponding author. E-mail address: [email protected] (J.W. Bae). 0040-6090/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2006.11.190
Ta films deposited by conventional sputtering methods generally contain a metastable tetragonal β-phase rather than a stable b.c.c. α-phase [7]. The β-Ta shows a high resistivity of 200–700 μΩcm and is used for thin film resistors. On the other hand, the resistivity of α-Ta is 15–60 μΩcm, which would be an excellent candidate for a diffusion barrier [7]. Ta–N solid solutions have been attractive among researchers due to their high thermal stability and chemical inertness to copper at elevated temperatures. Yang et al. [8] reported that pure β-Ta with a resistivity of 197 μΩcm was obtained without adding nitrogen, and that the resistivity of the Ta films decreased with increasing nitrogen content and reached a minimum value of 159 μΩcm for 5% nitrogen content. They explained that the change in phase from β-Ta to α-Ta was the reason for the resistivity decrease, because β-Ta has a higher resistivity than that of nitrogen incorporated α-Ta [9]. Lim et al. [10] reported that the resistivity of Ta thin films deposited by applying a substrate bias voltage of − 125 V reached the smallest value because of the increase in the fraction of α-phase in the Ta film. However, there have been no reports that examined simultaneously the effect of the nitrogen addition and applying the substrate bias voltage upon the α-phase stability. Therefore, in this work, the phase changes in Ta films, their crystalline
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microstructure, nitrogen concentration, and resistivity as a function of negative substrate bias voltage and nitrogen partial pressure in the plasma gas were studied. The thermal stability of Ta barrier layers was also investigated by using Cu/Ta/Si structures annealed at 550–700 °C for 30 min. 2. Experimental details A non-mass separated ion beam deposition (IBD) system with a radio frequency (RF) sputter-type ion source was used and its details have been described elsewhere [11,12]. The base pressure of 10− 5 Pa was obtained in the deposition chamber using a turbo molecular pump. The silicon (100) wafer substrates were ultrasonically cleaned in acetone and then etched in a 5% HF solution. Targets (Cu: 99.9999%, Ta: 99.99%) were electrochemically polished to remove surface contaminations prior to deposition. A high purity argon (99.9995%) plasma was initially generated in the ion source by applying 13.56 MHz RF power to the copper coil. By applying a negative bias voltage of − 300 Vor − 500 V to the copper or tantalum target, respectively, a copper- or tantalum-rich plasma was generated. The distance between the target and the substrate was 35 mm. Ta films, 100 nm in thickness, were deposited at various substrate bias voltages (0 to −100 V) and nitrogen partial pressures (Ar 9 Pa + N2 0–3 Pa). For the structural analysis of the films an X-ray diffractometer (XRD, Rigaku RINT 2000) was used with Cu Kα radiation generated at 36 kV and 20 mA. The morphology of the samples was observed using field emission scanning electron microscopy (FE-SEM, Hitachi S-4100L) at 15 kV. Resistivity measurements of the Ta films were carried out with the Van der Pauw method. The nitrogen contents of the deposited films were qualitatively analyzed by secondary ion mass spectroscopy (SIMS, ULVAC-PHI 6600) using a Cs+ ion beam (5 kV, 50 nA). To examine the thermal stability of the Cu/ Ta/Si structures, 100 nm Cu films were deposited at the optimum substrate bias voltage of −50 V [13] on the 30 nm thick Ta layer and the Cu/Ta/Si (100) structures were annealed at high temperatures (550–700 °C) for 30 min in a H2 atmosphere. We have examined these films for Cu agglomeration as well as thermal stability. To promote Cu surface diffusion intentionally during annealing, we have used H2 gas instead of an inert gas. The interfacial behavior and resistivity of the samples after annealing were characterized by XRD and the Van der Pauw method, respectively. For the sake of convenience, Ta films deposited at a substrate voltage (a, in Volts) and a nitrogen partial pressure (b, in Pa) will be expressed hereafter as Ta (a,b). 3. Results and discussion To examine the phase changes in the Ta films, XRD patterns were measured on the samples deposited at different nitrogen partial pressures and substrate bias voltages, and the results are shown in Fig. 1. Fig. 1(a) indicates a change in the Ta film XRD patterns with nitrogen partial pressure of the Ta films deposited with at a fixed substrate bias voltage of − 50 V. The phase changes resulting from applying different substrate bias voltages are shown in Fig. 1(b), where the fraction of α phase,
Fig. 1. (a) XRD patterns of the Ta/Si as a function of the nitrogen partial pressure at the substrate bias voltage of −50 V;(b) Changes of (α/α + β ) ratio as a function of the substrate bias voltage and nitrogen partial pressure.
Iα(110) /Iα(110) + Iβ (200), is plotted as a function of nitrogen partial pressure for different bias voltages. The Ta (−50,3) film showed the strongest α-Ta peak. It has been reported that impurities [14], substrate treatment [15], substrate bias voltage [16], substrate temperature [17] and residual stress [18] affect the phase stability. The present results suggest that the addition of nitrogen during the Ta deposition can stabilize the α-phase [19] and the effect becomes dominant with the application of − 50 V substrate bias voltages. It should be noted that the α-phase became dominant by the increase with the N2 partial pressure for all substrate bias voltages and that almost all-pure α-Ta could be obtained with a nitrogen partial pressure of 3 Pa. In order to examine the qualitative nitrogen concentration, SIMS was performed. The depth profiles for nitrogen in the Ta films and the nitrogen ion current (IN) from the Ta films deposited at different substrate bias voltages as a function of nitrogen partial pressure are shown Figs. 2 and 3, respectively. It was found that the IN became larger with increasing nitrogen partial pressure, and became smaller with the application of − 50 V or − 100 V substrate bias voltages. In this paper, we consider the source of N2 in the films deposited without N2 partial pressure. In our previous work [20,21], we found that high concentrations of carbon, nitrogen, and oxygen could be
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Fig. 2. SIMS depth profile of Ta/Si as a function of the substrate bias voltage and nitrogen concentration.
detected in Cu and Ta thin films deposited without negative substrate bias voltage, which suggests that nitrogen can be incorporated in thin films during deposition even in an Ar atmosphere. The nitrogen detected in our Ta film seems to be present in the deposition chamber. On the other hand, by applying a negative substrate bias voltage, the concentration of nitrogen decreased remarkably. This decrease in film nitrogen content with applied substrate bias voltage is attributable to the difference in ion kinetic energy towards the substrate with or without negative substrate bias voltage. When the negative substrate bias voltage is applied, relatively large amounts of Ta+ and Ar+ ions with high kinetic energy bombard the substrate. If nitrogen is physically adsorbed onto the substrate, Ta+ and Ar+ ion bombardment will most likely remove the nitrogen, since Ta and Ar have adsorption energies higher than nitrogen (0.1–0.5 eV) [22]. At a constant substrate bias voltage of − 50 V, the nitrogen intensity increased with nitrogen partial pressure. In Fig. 3, although the nitrogen concentrations are high in the no bias voltage samples, these samples consist almost solely of the βphase, as shown in Fig. 1. This is far from obtaining α-phase Ta film in this study and then a substrate bias voltage of − 50 V is also required to change the phase from β-phase to α-phase. As a result, the film deposited at the substrate bias voltage of − 50 V and the nitrogen content of 3 Pa is expected to be a potential diffusion barrier with a low resistivity. The effect of the substrate bias voltage and nitrogen content on the surface mor-
phology and microstructure, as well as their correlation with the electrical resistivity was examined. Fig. 4(a)–(c) are SEM micrographs that show now substrate bias voltage (with no N2 partial pressure) affected the Ta film surface morphology. For the Ta (0,0) film, small grains and cracks were observed clearly on the surface. These results are similar to the previous Cu–Si (100) case [23], and seem to be due to an insufficient surface migration of low energy particles, because the incident Ta flux is considered to be mainly composed of neutral Ta atoms. However, when the substrate bias voltage of − 50 V was applied
Fig. 3. Intensity of N as a function of the substrate bias voltage and nitrogen partial pressure.
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Fig. 4. SEM surface micrographs of the Ta film as a function of the substrate bias voltage and nitrogen partial pressure. (a) Ta (0,0) (b) Ta (−50,0) (c) Ta (− 100,0) (d) Ta (0,3) (e) Ta (−50,3) (f) Ta (− 100,3).
to the substrate, no cracks were observed, and the surface morphology was remarkably improved as shown in Fig. 4(b). Tagaki [22] reported that ion bombardment during deposition enhances the surface diffusion on the substrate resulting in an improved surface morphology. On the other hand, a further increase in the substrate bias voltage to − 100 V produced a rougher surface, as shown in Fig. 4(c). This is explained by higher energy particles bombarding the surface during the deposition causing resputtering. Fig. 4(d)–(f) show the surface morphologies of Ta (0,3), Ta (− 50,3), and Ta (− 100,3) films, respectively. As N2 gas is added to the plasma gas, at no bias voltage, more surface cracks on the film surface become more pronounced. This is due to the N2 gas diluting the Ar partial pressure in the plasma, which decreases Ta ionization ratio in the plasma. However, applying a substrate bias voltage of − 50 V improved the surface morphology of the film (Fig. 4(e)). Fig. 5 shows the resistivity change of the Ta films as a function of substrate bias voltage and nitrogen partial pressure.
It is clearly seen in the figure that the nitrogen partial pressure and substrate bias voltage strongly affect the resistivity of Ta films. The Ta (0,0) film showed the highest resistivity of 350– 400 μΩcm, which corresponds to that of β-Ta films. Comparing the remarkable resistivity decrease with substrate bias voltage (0 to − 50 V) in Fig. 5 with the change in α-phase fraction from Fig. 1(b), we conclude that the resistivity change is attributable to the increase in the α-phase fraction. The dependence of the resistivity on the nitrogen partial pressure at the substrate bias voltage of − 50 V indicates that the addition of nitrogen made the resistivity lower, with the lowest resistivity (27 μΩcm) obtained for the Ta (− 50,3) film. In Ta films prepared by conventional sputtering methods, the resistivity usually increases with increasing the nitrogen content in the film. However, in this study, the resistivity of the Ta films decreased with increasing the nitrogen content. For this reason, the low resistivity of Ta (− 50,3) is attributed to the stabilization of the α-phase as shown in Fig. 1(a). The slight increase in resistivity
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Fig. 5. Resistivity of the Ta film as a function of the substrate bias voltage and nitrogen partial pressure.
of the Ta (−100,2) and Ta (− 100,3) films compared to the Ta (50,2) and Ta (− 50,3) films also can be explained by the higher alpha-phase formation in the latter to films, since their surface morphology were similar. As the above results show, we found that the Ta/Si structures prepared with a substrate bias voltage of − 50 V had the lowest resistivity and the smoothest surface morphology. Thus, we examined thermal stability of Ta (− 50,3) diffusion barrier in the Cu(100 nm)/Ta(30 nm)/Si structure. Fig. 6 shows the resistivity change of Cu/Ta/Si and Cu/Ta (with N2)/Si structures as a function of annealing temperature in H2 atmosphere for 30 min. The reactive H2 gas was used intentionally to promote the Cu surface diffusion during annealing. All the films were deposited at the substrate bias voltage of − 50 V. It is well known that Cu diffuses fast into Si and forms Cu–Si compounds at temperatures as low as 200 °C, and the formation of Cu–Si compounds increases the resistivity of Cu films. As shown in Fig. 6, the Cu/Ta/Si structures were stable up to 600 °C, whereas a drastic increase in the resistivity was found after annealing above 650 °C, indicating that a severe intermixing or interfacial reactions occurred between the Cu/Ta films and the Si substrate. On the other hand, Cu/Ta(with N2)/Si structures, annealed up to 650 °C did not show any marked resistivity increase. The Cu/Ta (− 50,3)/Si structure, which had
Fig. 6. Resistivity as a function of annealing temperatures in H2 for 30 min.
the largest α-phase fraction showed the least increase in resistivity up to the annealing temperature of 650 °C. When the temperature reached 700 °C, the resistivity of all the samples increased, though the Cu/Ta(− 50,3)/Si still showed the smallest increase. To clarify the effect of annealing on the interfacial reaction of Cu/Ta(with/without N2)/Si structures, cross-sectional SEM was performed. Fig. 7 shows cross-sectional views of Cu/Ta/Si and Cu/Ta(− 50,3)/Si structures after annealing at 700 °C in H2 atmosphere for 30 min. In the case of the Cu/Ta/Si structure, Cu atoms diffused along the grain boundaries of the Ta layer into the Ta/Si interface, where a nucleation of Cu3Si took place (Fig. 7(a)). However, the Cu/Ta(−50,3)/Si structure was stable even after annealing at 700 °C, despite the occurrence of agglomeration (Fig. 7(b)). The Cu agglomeration, which resulted from the surface diffusion of Cu atoms, was caused by several driving forces such as residual stress, surface tension, and different crystalline textures between the Cu film and Ta layer [24]. The Cu agglomeration is also thought to be the cause of the resistivity increase rather than an interaction between Cu and Ta. These results indicate that the thermal stability of Cu/Ta/ Si structures was improved by adding nitrogen and applying substrate bias during deposition to form Ta–N solid solutions with the alpha-phase predominant.
Fig. 7. Cross-sectional SEM observations of (a) Cu/Ta/Si and (b) Cu/Ta(−50,3)/ Si structures after annealing at 700 °C in H2 atmosphere for 30 min.
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4. Conclusion In this work, Ta/Si (100) thin films were deposited in an Ar/ N2 gas mixture with different N2 partial pressures and at various substrate bias voltages. It was found that applying a substrate bias voltage of − 50 V and adding nitrogen were effective in producing good surface morphology and low resistivity. These results can be explained by the presence of a large α-phase fraction caused by nitrogen incorporation. When 30 nm thick Ta(with N2) films was used as a diffusion barrier between copper and silicon substrate, the resistivity of Cu/Ta(without N2)/Si samples increased significantly with increasing anneal temperature and was associated with degradation of the interfaces between the layers in the samples. However, essentially no change in resistivity and no evidence of Cu–Si interaction were observed for the Cu/Ta(with N2)/Si samples annealed up to 650 °C, which means that the thermal stability of Cu/Ta/Si structures was improved by forming predominantly alpha-phase Ta–N solid solutions by the addition of nitrogen and substrate bias during deposition. Acknowledgements This work was done as part of the activities of the NanoMetal Project under the auspices of the Japan Research and Development Center (JRCM) and the sponsorship of the New Energy and Industrial Technology Development Organization (NEDO, Ministry of Economics, Trade and Industry). References [1] M.-A. Nicolet, Thin Solid Films 52 (1978) 415.
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[2] J.D. Mcbrayer, R.M. Swanson, T.W. Sigmon, J. Electrochem. Soc. 133 (1986) 1242. [3] Y. Shacham-Diamond, A. Dedhia, D. Hoffstetter, W.G. Oldham, J. Electrochem. Soc. 140 (1993) 2427. [4] K. Holloway, P.M. Fryer, C. Cabral Jr., J.M.E. Harper, P.J. Bailey, K.H. Keleher, J. Appl. Phys. 71 (1992) 5433. [5] S.-Q. Wang, S. Suthar, C. Hoeflich, B.J. Burrow, J. Appl. Phys. 73 (1993) 2301. [6] T. Nakano, H. Ono, T. Ohta, T. Oku, M. Murakami, Proceeding of 1994 VLSI Multilevel Interconnection Conference, 1994, p. 407. [7] H.C. Cook, J. Vac. Sci. Technol. 4 (1967) 80. [8] W.L. Yang, W.-F. Wu, D.-G. Liu, C.-C. Wu, K.L. Ou, Solid-State Electron. 45 (2001) 149. [9] G.S. Chen, S.T. Chen, T.J. Yang, J. Appl. Phys. 87 (2000) 8473. [10] J.-W. Lim, Y. Ishikawa, K. Miyake, M. Yamashita, M. Isshiki, Mater. Trans. 43 (2002) 478. [11] K. Miyake, Y. Ishikawa, M. Yamashita, M. Isshiki, Proceeding of the 2000 International Conference on Ion Implantation Technology, 2000, p. 550. [12] M. Yamashita, J. Vac. Sci. Technol., A 7 (1989) 151. [13] J.-W. Lim, Y. Ishikawa, K. Miyake, M. Yamashita, M. Isshiki, Mater. Trans. 43 (2002) 1403. [14] W.D. Westwood, D.J. Willmott, P.S. Wilcox, J. Vac. Sci. Technol. 9 (1972) 444. [15] P. Catania, J.P. Doyle, J.J. Cuomo, J. Vac. Sci. Technol., A 10 (1992) 3318. [16] P. Catania, R.A. Roy, J.J. Cuomo, J. Appl. Phys. 74 (1993) 1008. [17] R.B. Marcus, S. Quigley, Thin Solid Films 2 (1968) 467. [18] L.A. Clevenger, A. Mutscheller, J.M.E. Harper, C. Cabral Jr., K. Barmak, J. Appl. Phys. 72 (1992) 4918. [19] K. Frisk, J. Alloys Compd. 278 (1998) 216. [20] J.-W. Lim, K. Mimura, M. Isshiki, Jpn. J. Appl. Phys. 43 (2004) 8267. [21] J.-W. Lim, J.W. Bae, K. Mimura, M. Isshiki, Jpn. J. Appl. Phys. 44 (2005) 373. [22] T. Takagi, Thin Solid films 92 (1982) 1. [23] A.Z. Moshfegh, O. Akhavan, Thin Solid Films 370 (2000) 721. [24] J.W. Bae, J.-W. Lim, K. Mimura, M. Isshiki, Mater. Trans. 45 (2004) 877.
Ion beam deposition of α-Ta films by nitrogen addition and improvement of diffusion barrier property Joon Woo Bae a,⁎, Jae-Won Lim b , Kouji Mimura a , Minoru Isshiki a a
b
Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Sendai 980–8577, Japan Materials Development Group, Minerals and Materials Processing Division, Korea Institute of Geoscience and Mineral Resources, 30 Gajeong-dong, Yuseong-gu, Daejeon, 305-350, Republic of Korea Received 4 February 2005; received in revised form 19 September 2006; accepted 16 November 2006 Available online 22 January 2007
Abstract Ta thin films were deposited on Si (100) substrates by an ion beam deposition method at various substrate bias voltages under Ar + N2 atmosphere with different pressure ratios of Ar and N2. The effects of nitrogen pressure in the plasma gas and the substrate bias voltage on the surface morphology, crystalline microstructure, electrical resistivity and diffusion barrier property were investigated. It was found that the fraction of a metastable β-phase in the Ta film deposited at the substrate bias voltage of −50 V films decreased by adding nitrogen gas, while the α-Ta phase became dominant. As a result, the Ta films deposited at the substrate bias voltage of − 50 V under Ar (9 Pa) + N2 (3 Pa) atmosphere showed a dominant α-phase with good surface morphology, low resistivity, and superior thermal stability as a diffusion barrier. © 2007 Elsevier B.V. All rights reserved. Keywords: Ta film; α-phase; Resistivity; Ion beam deposition; Diffusion barrier
1. Introduction Cu interconnects are the most promising candidate for next generation high speed ultra larger scale integrated (ULSI) circuits, because Cu exhibits a lower resistivity and larger electro/stress-migration resistance than conventional Al and Al alloys [1]. However, Cu has disadvantages, such as fast diffusion into Si and SiO2 [2], formation of Cu–Si compounds, and poor adhesion to most dielectric layers [3]. Therefore, an effective diffusion barrier is required to overcome these problems. Various materials such as Ta, W, Mo, Ti, C, Nb, and Cr have been examined as a diffusion barrier interposed between the Cu overlayer and the Si substrate. Among these materials, Ta-based thin films have been shown to be robust diffusion barriers for Cu, because the lattice diffusion of Cu through the Ta layer is very low [4–6]. On the other hand, nitride films of refractory transition metals such as tantalum nitride are also known to be promising candidates for making an excellent diffusion barrier. Ta–N films have better thermal stability, but higher resistivity than that of pure Ta films. ⁎ Corresponding author. E-mail address: [email protected] (J.W. Bae). 0040-6090/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2006.11.190
Ta films deposited by conventional sputtering methods generally contain a metastable tetragonal β-phase rather than a stable b.c.c. α-phase [7]. The β-Ta shows a high resistivity of 200–700 μΩcm and is used for thin film resistors. On the other hand, the resistivity of α-Ta is 15–60 μΩcm, which would be an excellent candidate for a diffusion barrier [7]. Ta–N solid solutions have been attractive among researchers due to their high thermal stability and chemical inertness to copper at elevated temperatures. Yang et al. [8] reported that pure β-Ta with a resistivity of 197 μΩcm was obtained without adding nitrogen, and that the resistivity of the Ta films decreased with increasing nitrogen content and reached a minimum value of 159 μΩcm for 5% nitrogen content. They explained that the change in phase from β-Ta to α-Ta was the reason for the resistivity decrease, because β-Ta has a higher resistivity than that of nitrogen incorporated α-Ta [9]. Lim et al. [10] reported that the resistivity of Ta thin films deposited by applying a substrate bias voltage of − 125 V reached the smallest value because of the increase in the fraction of α-phase in the Ta film. However, there have been no reports that examined simultaneously the effect of the nitrogen addition and applying the substrate bias voltage upon the α-phase stability. Therefore, in this work, the phase changes in Ta films, their crystalline
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microstructure, nitrogen concentration, and resistivity as a function of negative substrate bias voltage and nitrogen partial pressure in the plasma gas were studied. The thermal stability of Ta barrier layers was also investigated by using Cu/Ta/Si structures annealed at 550–700 °C for 30 min. 2. Experimental details A non-mass separated ion beam deposition (IBD) system with a radio frequency (RF) sputter-type ion source was used and its details have been described elsewhere [11,12]. The base pressure of 10− 5 Pa was obtained in the deposition chamber using a turbo molecular pump. The silicon (100) wafer substrates were ultrasonically cleaned in acetone and then etched in a 5% HF solution. Targets (Cu: 99.9999%, Ta: 99.99%) were electrochemically polished to remove surface contaminations prior to deposition. A high purity argon (99.9995%) plasma was initially generated in the ion source by applying 13.56 MHz RF power to the copper coil. By applying a negative bias voltage of − 300 Vor − 500 V to the copper or tantalum target, respectively, a copper- or tantalum-rich plasma was generated. The distance between the target and the substrate was 35 mm. Ta films, 100 nm in thickness, were deposited at various substrate bias voltages (0 to −100 V) and nitrogen partial pressures (Ar 9 Pa + N2 0–3 Pa). For the structural analysis of the films an X-ray diffractometer (XRD, Rigaku RINT 2000) was used with Cu Kα radiation generated at 36 kV and 20 mA. The morphology of the samples was observed using field emission scanning electron microscopy (FE-SEM, Hitachi S-4100L) at 15 kV. Resistivity measurements of the Ta films were carried out with the Van der Pauw method. The nitrogen contents of the deposited films were qualitatively analyzed by secondary ion mass spectroscopy (SIMS, ULVAC-PHI 6600) using a Cs+ ion beam (5 kV, 50 nA). To examine the thermal stability of the Cu/ Ta/Si structures, 100 nm Cu films were deposited at the optimum substrate bias voltage of −50 V [13] on the 30 nm thick Ta layer and the Cu/Ta/Si (100) structures were annealed at high temperatures (550–700 °C) for 30 min in a H2 atmosphere. We have examined these films for Cu agglomeration as well as thermal stability. To promote Cu surface diffusion intentionally during annealing, we have used H2 gas instead of an inert gas. The interfacial behavior and resistivity of the samples after annealing were characterized by XRD and the Van der Pauw method, respectively. For the sake of convenience, Ta films deposited at a substrate voltage (a, in Volts) and a nitrogen partial pressure (b, in Pa) will be expressed hereafter as Ta (a,b). 3. Results and discussion To examine the phase changes in the Ta films, XRD patterns were measured on the samples deposited at different nitrogen partial pressures and substrate bias voltages, and the results are shown in Fig. 1. Fig. 1(a) indicates a change in the Ta film XRD patterns with nitrogen partial pressure of the Ta films deposited with at a fixed substrate bias voltage of − 50 V. The phase changes resulting from applying different substrate bias voltages are shown in Fig. 1(b), where the fraction of α phase,
Fig. 1. (a) XRD patterns of the Ta/Si as a function of the nitrogen partial pressure at the substrate bias voltage of −50 V;(b) Changes of (α/α + β ) ratio as a function of the substrate bias voltage and nitrogen partial pressure.
Iα(110) /Iα(110) + Iβ (200), is plotted as a function of nitrogen partial pressure for different bias voltages. The Ta (−50,3) film showed the strongest α-Ta peak. It has been reported that impurities [14], substrate treatment [15], substrate bias voltage [16], substrate temperature [17] and residual stress [18] affect the phase stability. The present results suggest that the addition of nitrogen during the Ta deposition can stabilize the α-phase [19] and the effect becomes dominant with the application of − 50 V substrate bias voltages. It should be noted that the α-phase became dominant by the increase with the N2 partial pressure for all substrate bias voltages and that almost all-pure α-Ta could be obtained with a nitrogen partial pressure of 3 Pa. In order to examine the qualitative nitrogen concentration, SIMS was performed. The depth profiles for nitrogen in the Ta films and the nitrogen ion current (IN) from the Ta films deposited at different substrate bias voltages as a function of nitrogen partial pressure are shown Figs. 2 and 3, respectively. It was found that the IN became larger with increasing nitrogen partial pressure, and became smaller with the application of − 50 V or − 100 V substrate bias voltages. In this paper, we consider the source of N2 in the films deposited without N2 partial pressure. In our previous work [20,21], we found that high concentrations of carbon, nitrogen, and oxygen could be
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Fig. 2. SIMS depth profile of Ta/Si as a function of the substrate bias voltage and nitrogen concentration.
detected in Cu and Ta thin films deposited without negative substrate bias voltage, which suggests that nitrogen can be incorporated in thin films during deposition even in an Ar atmosphere. The nitrogen detected in our Ta film seems to be present in the deposition chamber. On the other hand, by applying a negative substrate bias voltage, the concentration of nitrogen decreased remarkably. This decrease in film nitrogen content with applied substrate bias voltage is attributable to the difference in ion kinetic energy towards the substrate with or without negative substrate bias voltage. When the negative substrate bias voltage is applied, relatively large amounts of Ta+ and Ar+ ions with high kinetic energy bombard the substrate. If nitrogen is physically adsorbed onto the substrate, Ta+ and Ar+ ion bombardment will most likely remove the nitrogen, since Ta and Ar have adsorption energies higher than nitrogen (0.1–0.5 eV) [22]. At a constant substrate bias voltage of − 50 V, the nitrogen intensity increased with nitrogen partial pressure. In Fig. 3, although the nitrogen concentrations are high in the no bias voltage samples, these samples consist almost solely of the βphase, as shown in Fig. 1. This is far from obtaining α-phase Ta film in this study and then a substrate bias voltage of − 50 V is also required to change the phase from β-phase to α-phase. As a result, the film deposited at the substrate bias voltage of − 50 V and the nitrogen content of 3 Pa is expected to be a potential diffusion barrier with a low resistivity. The effect of the substrate bias voltage and nitrogen content on the surface mor-
phology and microstructure, as well as their correlation with the electrical resistivity was examined. Fig. 4(a)–(c) are SEM micrographs that show now substrate bias voltage (with no N2 partial pressure) affected the Ta film surface morphology. For the Ta (0,0) film, small grains and cracks were observed clearly on the surface. These results are similar to the previous Cu–Si (100) case [23], and seem to be due to an insufficient surface migration of low energy particles, because the incident Ta flux is considered to be mainly composed of neutral Ta atoms. However, when the substrate bias voltage of − 50 V was applied
Fig. 3. Intensity of N as a function of the substrate bias voltage and nitrogen partial pressure.
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Fig. 4. SEM surface micrographs of the Ta film as a function of the substrate bias voltage and nitrogen partial pressure. (a) Ta (0,0) (b) Ta (−50,0) (c) Ta (− 100,0) (d) Ta (0,3) (e) Ta (−50,3) (f) Ta (− 100,3).
to the substrate, no cracks were observed, and the surface morphology was remarkably improved as shown in Fig. 4(b). Tagaki [22] reported that ion bombardment during deposition enhances the surface diffusion on the substrate resulting in an improved surface morphology. On the other hand, a further increase in the substrate bias voltage to − 100 V produced a rougher surface, as shown in Fig. 4(c). This is explained by higher energy particles bombarding the surface during the deposition causing resputtering. Fig. 4(d)–(f) show the surface morphologies of Ta (0,3), Ta (− 50,3), and Ta (− 100,3) films, respectively. As N2 gas is added to the plasma gas, at no bias voltage, more surface cracks on the film surface become more pronounced. This is due to the N2 gas diluting the Ar partial pressure in the plasma, which decreases Ta ionization ratio in the plasma. However, applying a substrate bias voltage of − 50 V improved the surface morphology of the film (Fig. 4(e)). Fig. 5 shows the resistivity change of the Ta films as a function of substrate bias voltage and nitrogen partial pressure.
It is clearly seen in the figure that the nitrogen partial pressure and substrate bias voltage strongly affect the resistivity of Ta films. The Ta (0,0) film showed the highest resistivity of 350– 400 μΩcm, which corresponds to that of β-Ta films. Comparing the remarkable resistivity decrease with substrate bias voltage (0 to − 50 V) in Fig. 5 with the change in α-phase fraction from Fig. 1(b), we conclude that the resistivity change is attributable to the increase in the α-phase fraction. The dependence of the resistivity on the nitrogen partial pressure at the substrate bias voltage of − 50 V indicates that the addition of nitrogen made the resistivity lower, with the lowest resistivity (27 μΩcm) obtained for the Ta (− 50,3) film. In Ta films prepared by conventional sputtering methods, the resistivity usually increases with increasing the nitrogen content in the film. However, in this study, the resistivity of the Ta films decreased with increasing the nitrogen content. For this reason, the low resistivity of Ta (− 50,3) is attributed to the stabilization of the α-phase as shown in Fig. 1(a). The slight increase in resistivity
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Fig. 5. Resistivity of the Ta film as a function of the substrate bias voltage and nitrogen partial pressure.
of the Ta (−100,2) and Ta (− 100,3) films compared to the Ta (50,2) and Ta (− 50,3) films also can be explained by the higher alpha-phase formation in the latter to films, since their surface morphology were similar. As the above results show, we found that the Ta/Si structures prepared with a substrate bias voltage of − 50 V had the lowest resistivity and the smoothest surface morphology. Thus, we examined thermal stability of Ta (− 50,3) diffusion barrier in the Cu(100 nm)/Ta(30 nm)/Si structure. Fig. 6 shows the resistivity change of Cu/Ta/Si and Cu/Ta (with N2)/Si structures as a function of annealing temperature in H2 atmosphere for 30 min. The reactive H2 gas was used intentionally to promote the Cu surface diffusion during annealing. All the films were deposited at the substrate bias voltage of − 50 V. It is well known that Cu diffuses fast into Si and forms Cu–Si compounds at temperatures as low as 200 °C, and the formation of Cu–Si compounds increases the resistivity of Cu films. As shown in Fig. 6, the Cu/Ta/Si structures were stable up to 600 °C, whereas a drastic increase in the resistivity was found after annealing above 650 °C, indicating that a severe intermixing or interfacial reactions occurred between the Cu/Ta films and the Si substrate. On the other hand, Cu/Ta(with N2)/Si structures, annealed up to 650 °C did not show any marked resistivity increase. The Cu/Ta (− 50,3)/Si structure, which had
Fig. 6. Resistivity as a function of annealing temperatures in H2 for 30 min.
the largest α-phase fraction showed the least increase in resistivity up to the annealing temperature of 650 °C. When the temperature reached 700 °C, the resistivity of all the samples increased, though the Cu/Ta(− 50,3)/Si still showed the smallest increase. To clarify the effect of annealing on the interfacial reaction of Cu/Ta(with/without N2)/Si structures, cross-sectional SEM was performed. Fig. 7 shows cross-sectional views of Cu/Ta/Si and Cu/Ta(− 50,3)/Si structures after annealing at 700 °C in H2 atmosphere for 30 min. In the case of the Cu/Ta/Si structure, Cu atoms diffused along the grain boundaries of the Ta layer into the Ta/Si interface, where a nucleation of Cu3Si took place (Fig. 7(a)). However, the Cu/Ta(−50,3)/Si structure was stable even after annealing at 700 °C, despite the occurrence of agglomeration (Fig. 7(b)). The Cu agglomeration, which resulted from the surface diffusion of Cu atoms, was caused by several driving forces such as residual stress, surface tension, and different crystalline textures between the Cu film and Ta layer [24]. The Cu agglomeration is also thought to be the cause of the resistivity increase rather than an interaction between Cu and Ta. These results indicate that the thermal stability of Cu/Ta/ Si structures was improved by adding nitrogen and applying substrate bias during deposition to form Ta–N solid solutions with the alpha-phase predominant.
Fig. 7. Cross-sectional SEM observations of (a) Cu/Ta/Si and (b) Cu/Ta(−50,3)/ Si structures after annealing at 700 °C in H2 atmosphere for 30 min.
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4. Conclusion In this work, Ta/Si (100) thin films were deposited in an Ar/ N2 gas mixture with different N2 partial pressures and at various substrate bias voltages. It was found that applying a substrate bias voltage of − 50 V and adding nitrogen were effective in producing good surface morphology and low resistivity. These results can be explained by the presence of a large α-phase fraction caused by nitrogen incorporation. When 30 nm thick Ta(with N2) films was used as a diffusion barrier between copper and silicon substrate, the resistivity of Cu/Ta(without N2)/Si samples increased significantly with increasing anneal temperature and was associated with degradation of the interfaces between the layers in the samples. However, essentially no change in resistivity and no evidence of Cu–Si interaction were observed for the Cu/Ta(with N2)/Si samples annealed up to 650 °C, which means that the thermal stability of Cu/Ta/Si structures was improved by forming predominantly alpha-phase Ta–N solid solutions by the addition of nitrogen and substrate bias during deposition. Acknowledgements This work was done as part of the activities of the NanoMetal Project under the auspices of the Japan Research and Development Center (JRCM) and the sponsorship of the New Energy and Industrial Technology Development Organization (NEDO, Ministry of Economics, Trade and Industry). References [1] M.-A. Nicolet, Thin Solid Films 52 (1978) 415.
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