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Effect of a Cu seed layer on electroplated Cu film
Microelectronic Engineering 105 (2013) 18–24
Contents lists available at SciVerse ScienceDirect
Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee
Effect of a Cu seed layer on electroplated Cu film Yan Pan, Yuhong Liu, Tongqing Wang, Xinchun Lu ⇑ State Key Laboratory of Tribology, Tsinghua University, Beijing 100084, China
a r t i c l e
i n f o
Article history: Received 8 May 2011 Received in revised form 10 December 2012 Accepted 17 December 2012 Available online 5 January 2013 Keywords: Sputtering Electroplate Cu film Mechanical property Corrosion
a b s t r a c t Cu films with different thicknesses were electroplated onto Cu seed layers. Cu seed layers with different thicknesses were sputtered on a Ta barrier layer; the thickness of Cu seed layer was varied by changing the sputtering time. To investigate the influence of the Cu seed layers on the performance of the electroplated Cu films, the morphology, grain size, crystallographic orientation, and mechanical and chemical properties of the electroplated Cu films are presented in this paper. As the thickness of the Cu seed layer increases, the grain size increases, and the surface morphology changes from flat to rough to smooth. The adhesion of the Cu seed layers to the substrate increases with increasing thickness of the Cu seed layers but eventually decreases. After the electroplated Cu films are deposited, the morphology, grain size, and crystallographic orientation of the electroplated Cu films are significantly influenced by the seed layers. The hardness of the electroplated Cu film increases with the thickness of the Cu seed layer, and finally reaches a constant value. The adhesion between the Cu film and the substrates is influenced by the Cu seed layer as indicated by the morphology of the film. The Cu film is less corrosion-resistant when its roughness value is higher. Better mechanical and chemical properties are obtained when the thickness of the Cu seed layer is 150 nm. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Cu is widely used in the manufacturing of electronic devices because of its excellent electrical conductivity, resistance to migration and amenability to dual damascene processing [1–3]. The manufacturing of microelectronics presents significant challenges for Cu interconnects with the increase in microelectronic interconnect layers during the integrated circuit (IC) manufacturing process [4]. In the process of IC manufacturing, Cu interconnects are electroplated onto a Cu seed layer that has been sputtered on the barrier layer because this process is less expensive and straightforward than other metal deposition techniques [5,6]. The surface/interface effects that determine the reliability of Cu interconnects [7] have recently become more critically important. The properties of sputtered Cu layers and the interactions between Cu, Ta/TaN and SiO2 have been widely investigated in recent years [8–13]. It has been shown that the out-diffusion of Ta atoms toward Cu layers, which leads to the formation of Cu, Ta and Cu–Ta oxides, may cause the failure of Cu/Ta/SiO2/Si multilayer structures during thermal annealing [14]. Moreover, the Ta/TaN bi-layer structure has much better diffusion barrier properties toward Cu than pure Ta or pure TaN films [15]. Early breakdowns of 100 nm Cu damascene lines can usually be observed with reductions in the thickness of the diffusion barrier Ta and Cu seed layers [16],
⇑ Corresponding author. Tel./fax: +86 10 62797362. E-mail address: [email protected] (X. Lu). 0167-9317/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.mee.2012.12.004
which may be one of the most important obstacles to overcome in the IC manufacturing process. Other researchers have focused on the electrochemical deposition of Cu films [17–19]. The crystal size, texture, and electrical properties of the electroplated Cu produced by optimizing the additives of the electroplating solution, the electroplating conditions and the heat-treatment process have been discussed extensively, both experimentally and theoretically [20,21]. For example, polyethylene glycol (PEG) can improve the wettability of the plating electrolyte on the Cu seed layer and electroplated Cu film, which can influence the Cu interconnect resistance [22]. The types of additives also affect the submicrometer trench-filling process during the Cu-electrodeposition process [23]. Some studies have shown that bis-(3-sulfopropyl) disulfide (SPS) accelerates the Cu bottomup growth process and that Janus green B (JGB) can inhibit Cu deposition at the end of the filling stages, leading to the suppression phenomenon usually caused by high JGB concentrations. Previous research has been less focused on the interaction between the Cu seed layers and the electroplated Cu films. There is little information about the influence of the sputtered Cu seed layer on the microstructure and mechanical properties of the electroplated Cu films. Cu seed layers with different morphologies and grain sizes can be realized by controlling the sputtering time. In this paper, the relationship between the properties of the Cu seed layer and the performance of the electroplated Cu film is investigated in terms of the morphology, grain size, crystallographic orientation, and other properties of electroplated Cu films with different thicknesses.
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Y. Pan et al. / Microelectronic Engineering 105 (2013) 18–24
Table 2 The formulas for the electroplating solution of Cu. The thickness of the Cu film is 1.62 lm after electroplating for 5 min, and the thickness of the Cu film is 3.25 lm after electroplating for 10 min.
2. Experimental 2.1. Cu seed layer preparation A middle-frequency magnetron sputtering system was used to deposit a Ta barrier layer and a Cu seed layer on a SiO2 layer produced by the thermal oxidation of a Si (100) substrate. Impurities on the SiO2 layer were removed by Ar+ ion bombardment. Under controlled conditions (pressure: 2 Pa, bias voltage: 150 V), a 50 nm Ta barrier layer was first sputtered onto the SiO2 layer. Subsequently, Cu seed layers with different thicknesses were sputtered onto the Ta layer. The sputtering process parameters are shown in Table 1. Five identical runs were conducted for each specimen with the same thickness. The average thickness of the Cu seed layers was measured in cross section by ultra-high-resolution scanning electron microscopy (S-5500). The thicknesses of the Cu seed layers are shown in Table 1.
Reagent name
Formula
Contents
Blue vitriod Sulfuric acid Hydrochloric acid Additives A Additives B Additives C
CuSO45H2O H2SO4 HCl – – –
200 g/L 40 mL/L 0.2 mL/L 0.5 mL/L 0.2 mL/L 4 mL/L
2.2. Cu electrodeposition Cu electrodeposition was conducted in an electric tank. The anode was a phosphor bronze plate containing 0.05 wt% P. An anode mud could contaminate the electrolyte, resulting in a decrease in the quality of deposited Cu. To prevent such a contamination, an acid-resistant bag was introduced to bind the anode. The electroplating solution formula for Cu is shown in Table 2. The thickness of the Cu film was 1.62 lm after electroplating for 5 min and 3.25 lm after electroplating for 10 min. 2.3. Measuring instruments The surface morphology of the Cu seed layer and the electrodeposited film was investigated with an environment scanning electron microscope (FEI Quanta 200 FEG). The surface roughness (Ra) and the surface topography in a fixed 2 2 lm area were measured using an atomic force microscope (AFM, NanoMan VS, Veeco). BRUKER D8 advanced X-ray diffraction (XRD) analysis using Cu Ka radiation was performed to determine the crystalline structure and preferred orientation. The nanohardness of the electroplated Cu films and the adhesion between the Cu film and SiO2 layer was characterized by the NanoTest™ system from Micro Materials Company. To investigate the corrosion resistance, electrochemical measurements were performed using a CHI660C electrochemical workstation. The counter electrode was platinum, and the reference electrode was a standard calomel electrode. The tip of the reference electrode was placed at a distance of 5 mm from the work electrode. 3. Results and discussion
Fig. 1. X-ray diffraction spectra and crystal grain size of the Cu seed layers with different thicknesses. (1) X-ray diffraction spectra. Seed layer thickness: (a) 25 nm, (b) 50 nm, (c) 100 nm, (d) 150 nm, (e) 200 nm, and (f) 300 nm. (2) Crystal grain size.
Kk b cos h
3.1. Crystallographic orientation and grain size
L¼
The grain sizes of the Cu seed layers sputtered on the oxidized Si substrate with a Ta barrier layer are calculated based on the Scherrer equation [24–27]:
Here, K is a dimensionless constant and can generally be set to unity, k is the X-ray wavelength, h is the Bragg angle, L is the grain size, and b is the width at half of the peak corrected for the
ð1Þ
Table 1 The sputtering process parameters for the Cu seed layers. The sample shelf rotated at a speed of 4 rpm. The sputtering air pressure was 0.8 Pa, the basic bias was 150 V and the sputtering power density was 1.21 W/cm2. Sample code
Sputtering current (A)
Sputtering power density (W/cm2)
Sputtering temperature (°C)
Sputtering (time/min)
Thickness (nm)
Ta Cu
0.6 0.2 0.2 0.2 0.2 0.2 0.2
3.63 1.21 1.21 1.21 1.21 1.21 1.21
130 100 100 100 100 100 100
5 5 10 20 30 40 60
50 25 50 100 150 200 300
a b c d e f
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Y. Pan et al. / Microelectronic Engineering 105 (2013) 18–24
Fig. 2. X-ray diffraction spectra and crystal grain size of the electroplated Cu films with a thickness of 1.62 lm. (1) X-ray diffraction spectra. Seed layer thickness: (a) 25 nm, (b) 50 nm, (c) 100 nm, (d) 150 nm, (e) 200 nm, and (f) 300 nm. (2) Crystal grain size.
instrumentation broadening. Fig. 1(1) shows that the intensity of Cu(1 1 1) increases with the seed layer thickness. The grain size of the Cu seed layer varies linearly with the thickness of the Cu seed layer, as shown in Fig. 1(2). The ability to resist the electrical migration of the Cu(1 1 1) orientation is four times larger than that of the Cu(2 0 0) orientation, and thus the Cu(1 1 1) orientation is more suitable for Cu interconnects [7]. The Cu(1 1 1) orientation of the electroplated Cu film with a thickness of 1.62 lm becomes increasingly dominant with the increase in the thickness of the Cu seed layer, as shown in Fig. 2(1). However, this tendency is not significant in the electroplated Cu films with a thickness of 3.25 lm, as shown in Fig. 3(1), though the Cu(1 1 1) orientation is also more prominent than other orientations. It is assumed that this phenomenon may be related to the properties of the Cu seed layer. It can be observed in Fig. 1(1) that Cu(1 1 1) increases with the seed layer thickness. During the electrodeposition growth process, the electroplated Cu film inherits the characteristics of the seed layer at the initial stage, in which the Cu(1 1 1) orientation is dominant. The XRD results in Fig. 2(2) show that the grain size of Cu(1 1 1) first decreases and then increases irregularly from 60 to 80 nm, whereas the grain sizes of Cu(2 0 0) and Cu(3 1 1) do not change with the various thicknesses of the Cu seed layer, remaining constant at approximately 20 nm. For the electroplated Cu film with a thickness of 3.25 lm, Fig. 3(2) shows that the grain
Fig. 3. X-ray diffraction spectra and crystal grain size of the electroplated Cu films with a thickness of 3.25 lm. (1) X-ray diffraction spectra. Seed layer thickness: (a) 25 nm, (b) 50 nm, (c) 100 nm, (d) 150 nm, (e) 200 nm, and (f) 300 nm. (2) Crystal grain size.
sizes of all orientations have similar variations, increasing initially and then decreasing to a constant level. The grain sizes of the Cu film with a thickness of 1.62 lm shown in Fig. 2(2) are larger than the grain sizes of the Cu film with the thickness of 3.25 lm shown in Fig. 3(2) because the electrodeposited film growth process is influenced significantly by the thickness and orientation of the Cu seed layer, especially when the thickness of the electroplated Cu film is only 1.62 lm, which is not sufficient to compensate for the characteristics of the Cu seed layer [7]. 3.2. Surface morphology The SEM micrographs of Cu seed layers with an area of 15 15 lm with different thicknesses prepared by controlling the sputtering time are presented in Fig. 4. At the beginning of the sputtering process, the Cu seed layer becomes increasingly porous with an increase in the layer thickness until the thickness of the Cu seed layer reaches 100 nm, as shown in Fig. 4(c). Then, the number of pores decreases as the layer thickness increases. A Cu seed layer with few pores is shown in Fig. 4(f): when the Cu seed layer thickness is 300 nm. The AFM topography images of the Cu seed layers with an area of 2 2 lm with different thicknesses are shown in Fig. 5. The roughness of the Cu seed layers deposited by magnetron sputtering varies with the thicknesses of the Cu seed layer, from which the values of Ra are shown in Fig. 6. The surface morphologies of the
Y. Pan et al. / Microelectronic Engineering 105 (2013) 18–24
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Fig. 4. Scanning electron micrographs of the Cu seed layers with different thicknesses: (a) 25 nm, (b) 50 nm, (c) 100 nm, (d) 150 nm, (e) 200 nm, and (f) 300 nm.
Cu seed layer seem to be rather rough at the beginning of the sputtering process until the thickness reaches 50 nm. After a longer sputtering time, the surface tends to be become flat, with a constant roughness of approximately 5 nm. Fig. 6 also shows that the roughness of the Cu film with a thickness of 1.62 lm is similar to the roughness of the Cu seed layer. Because the Cu ions absorb onto the Cu seed layer under the influence of the electric fields, they are likely to fill the concave areas and tend to enter the existing grid positions of the Cu seed layer [5,18,19]. The electrically deposited Cu film with a thickness of 1.62 lm maintains the basic
characteristics of the Cu seed layer [19] while the growth of the film with a thickness of 3.25 lm is not affected by the original seed layer. In addition, the Cu films with a thickness of 1.62 lm are rougher than those with a thickness of 3.25 lm. 3.3. Hardness analysis The hardness values of the electroplated Cu films are shown in Fig. 7. The hardness values increase with the thickness of the Cu seed layers until they reach a constant value. This trend is
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Y. Pan et al. / Microelectronic Engineering 105 (2013) 18–24
Fig. 5. Atomic force microscope topographies of the Cu seed layers with different thicknesses: (a) 25 nm, (b) 50 nm, (c) 100 nm, (d) 150 nm, (e) 200 nm, and (f) 300 nm.
Fig. 6. Surface roughness of the Cu seed layers/films for different conditions.
Fig. 7. Hardness values of the Cu films on the seed layers with different thicknesses.
Y. Pan et al. / Microelectronic Engineering 105 (2013) 18–24
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phenomenon is consistent with the various trends in the roughness of the Cu seed layer as its roughness also first increases and then decreases in Fig. 6. It has been reported that a stronger mechanical interlock force will occur between a rougher Cu seed layer and the substrate than a flat Cu seed layer [29,30]. On the other hand, during the growth of the copper seed layer, Cu(1 1 1) textures become dominant with the increasing thickness of the copper seed layer, which results in an increase in adhesion [7]. When copper films are electroplated on the seed layers to thicknesses of 1.62 and 3.25 lm, the adhesion strength between the electroplated copper film and the substrate decreases significantly, and the adhesion strength varies with the thickness of the copper seed layer. The reduction of the adhesion may be induced by the increase in the copper growth stress during electroplating [31,32]. 3.5. Corrosion resistance Fig. 8. Adhesion strength between the Cu seed layers/films and the substrate.
Fig. 9. Corrosion current densities of the Cu films electroplated on different Cu seed layers.
attributed to the fact that the structures of the Cu seed layer change significantly during the sputtering process, as shown in Fig. 5. The Cu seed layers are not compressed at the initial growth stage, leading to a low hardness value. As the sputtered Cu seed layer becomes thicker, the film structure is compressed, and the hardness values increase to a stable value. In contrast, it has been reported that the hardness of the Cu film is high for the dominant Cu(1 1 1) orientation [8,28]. In our study, based on the XRD results and the calculated results of the grain size of Cu(1 1 1) on the Cuelectroplated film, Cu(1 1 1) has obvious advantages for the Cu surfaces with a larger thickness of the seed layer, which results in an increase in the layer hardness. It can also be observed that the 3.25 lm-thick electroplated films are harder than the 1.62 lmthick Cu films. Therefore, the mechanical properties of the films are affected by both the Cu seed layer and the electroplated film. The hardness of the films depends on the properties of the electroplated film to a great extent, especially for deposition thicknesses from 1.62 to 3.25 lm. 3.4. Adhesion analysis The adhesion strength between the Cu films/layers and the substrate is characterized in Fig. 8. The adhesion strength between the Cu seed layer and the substrate, first increases and then decreases with the thickness of the Cu sputtering seed layers. And this
The corrosion current densities of the Cu films electroplated on different Cu seed layers are shown in Fig. 9. The corrosion current density of the Cu film with a thickness of 1.62 lm varies with the surface roughness. The reaction area between the Cu film and the slurry increases when the surface is rough, which directly accelerates the process of corrosion [33]. The surface is rough when the seed layer thickness is 50 nm and the 1.62 lm Cu films cannot fully cover the seed layers; thus, the reaction area between the film and the slurry increases, which accelerates the corrosion. For the Cu film with a thickness of 3.25 lm, the corrosion current density variations are not very obvious; it can be deduced that a uniform and dense Cu surface is beneficial for corrosion resistance because of the stabilization of the grain size and surface roughness. The corrosion current density of Cu films with a thickness of 3.25 lm is smaller than those with a thickness of 1.62 lm. This smaller density is due to the greater content of Cu(1 1 1) in the Cu films with a thickness of 3.25 lm than for films with a thickness of 1.62 lm because of the resistance of the Cu(1 1 1) surface to slurry corrosion [34]. 4. Conclusions The influences of the Cu seed layer on the performance of electroplated Cu films are investigated in this paper. The surface morphology, adhesion, and roughness of the copper seed layer have a strong dependence on the thickness of the Cu seed layer. The morphology, grain size, and crystallographic orientation of the electroplated Cu films are significantly influenced by the seed layers. Consequently, the stress and morphology of the film has an influence on the hardness of the electroplated Cu film and the adhesion between the Cu film and the substrates. Acknowledgements The authors acknowledged the financial support from the National Natural Science Funds for Distinguished Young Scholars (Grant no. 50825501) and the National Natural Science Foundation of China (Grant no. 50775122). References [1] R. Rosenberg, D.C. Edelstein, C.K. Hu, K.P. Rodbell, Annu. Rev. Mater. Sci. 30 (2000) 229–262. [2] R.L. Jackson, E. Broadbent, T. Cacouris, A. Harrus, M. Biberger, E. Patton, T. Walsh, Solid State Technol. 41 (1998) 49–59. [3] P.C. Andricacos, C. Uzoh, J.O. Dukovic, J. Horkans, H. Deligianni, IBM J. Res. Dev. 42 (1998) 567–574. [4] J. Rober, S. Riedel, S.E. Schulz, T. Gessner, Microelectron. Eng. 37–8 (1997) 111–119. [5] H. El Sayed, M.T. Greiner, P. Kruse, Appl. Surf. Sci. 253 (2007) 8962–8968. [6] J.C. Ziegler, R.I. Wielgosz, D.M. Kolb, Electrochim. Acta 45 (1999) 827–833.
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[7] L. Haebum, S.D. Lopatin, Thin Solid Films 492 (2005) 279–284. [8] M.O. Bloomfield, D.N. Bentz, T.S. Cale, J. Electron. Mater. 37 (2008) 249–263. [9] S. Shojaei Zadeh, S. Zhang, W. Liu, Y. Yang, S.M. Sadeghipour, M. Asheghi, P. Sverdrup, Thermal characterization of thin film cu interconnects for the next generation of microelectronic devices, in: Therm. Phenom. Electron. Syst. Proc. Intersoc. Conf., 2004, pp. 575–583. [10] C. Detavernier, D. Deduytsche, R.L. Van Meirhaeghe, J. De Baerdemaeker, C. Dauwe, Appl. Phys. Lett. 82 (2003) 1863–1865. [11] H.C. Huang, H.L. Wei, C.H. Woo, X.X. Zhang, Appl. Phys. Lett. 82 (2003) 4265– 4267. [12] K. Yamaguchi, J. Appl. Phys. 89 (2001) 590–595. [13] L. Vanasupa, Y.C. Joo, P.R. Besser, S. Pramanick, J. Appl. Phys. 85 (1999) 2583. [14] K.M. Latt, Y.K. Lee, T. Osipowicz, H.S. Park, Mater. Sci. Eng. B 94 (2002) 111– 120. [15] Q. Xie, X.P. Qu, J.J. Tan, Y.L. Jiang, M. Zhou, T. Chen, G.P. Ru, Appl. Surf. Sci. 253 (2006) 1666–1672. [16] M. Traving, G. Schindler, G. Steinlesberger, W. Steinhogl, M. Engelhardt, Semicond. Int. 26 (2003) 73–78. [17] S. Armini, C.M. Whelan, M. Moinpour, K. Maex, J. Electrochem. Soc. 156 (2009) H18–H26. [18] U.H. Lee, H.J. Lee, H.J. Sohn, T. Kang, Electrochem. Solid-State Lett. 9 (2006) C65–C68.
[19] M. Kang, A.A. Gewirth, J. Electrochem. Soc. 150 (2003) C426–C434. [20] S. Ahmed, D.N. Buckley, S. Nakahara, T.T. Ahmed, Y. Kuo, J. Electrochem. Soc. 154 (2007) D103–D112. [21] K.R. Hebert, J. Electrochem. Soc. 148 (2001) C726–C732. [22] C.W. Liu, J.C. Tsao, M.S. Tsai, Y.L. Wang, J. Vac. Sci. Technol., A 22 (2004) 2315– 2320. [23] M. Hasegawa, Y. Negishi, T. Nakanishi, T. Osaka, J. Electrochem. Soc. 152 (2005) C221–C228. [24] A.L. Patterson, Phys. Rev. 56 (1939) 978–982. [25] K. Pantleon, M. Somers, J. Appl. Phys. 100 (2006). [26] J.I. Langford, J. Appl. Crystallogr. 11 (1978) 10–14. [27] T.H. Dekeijser, J.I. Langford, E.J. Mittemeijer, A. Vogels, J. Appl. Crystallogr. 15 (1982) 308–314. [28] A.K. Siker, A. Kumar, P. Shukla, P.B. Zantye, M. Sanganaria, J. Electron. Mater. 32 (2003) 1028–1033. [29] I.M. Feng, J. Appl. Phys. 23 (1952) 1011–1019. [30] J. Ge, R. Tuominen, J.K. Kivilahti, J. Adhes. Sci. Technol. 15 (2001) 1133–1143. [31] J.M. Paik, I.M. Park, Y.C. Joo, K.C. Park, J. Appl. Phys. 99 (2006). [32] H. Lee, S.D. Lopatin, Thin Solid Films 492 (2005) 279–284. [33] W. Li, D.Y. Li, Acta Mater. 54 (2006) 445–452. [34] P.J. Andersen, M.N. Bentancur, A.J. Moll, M. Frary, J. Electrochem. Soc. 157 (2010) H120.
Contents lists available at SciVerse ScienceDirect
Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee
Effect of a Cu seed layer on electroplated Cu film Yan Pan, Yuhong Liu, Tongqing Wang, Xinchun Lu ⇑ State Key Laboratory of Tribology, Tsinghua University, Beijing 100084, China
a r t i c l e
i n f o
Article history: Received 8 May 2011 Received in revised form 10 December 2012 Accepted 17 December 2012 Available online 5 January 2013 Keywords: Sputtering Electroplate Cu film Mechanical property Corrosion
a b s t r a c t Cu films with different thicknesses were electroplated onto Cu seed layers. Cu seed layers with different thicknesses were sputtered on a Ta barrier layer; the thickness of Cu seed layer was varied by changing the sputtering time. To investigate the influence of the Cu seed layers on the performance of the electroplated Cu films, the morphology, grain size, crystallographic orientation, and mechanical and chemical properties of the electroplated Cu films are presented in this paper. As the thickness of the Cu seed layer increases, the grain size increases, and the surface morphology changes from flat to rough to smooth. The adhesion of the Cu seed layers to the substrate increases with increasing thickness of the Cu seed layers but eventually decreases. After the electroplated Cu films are deposited, the morphology, grain size, and crystallographic orientation of the electroplated Cu films are significantly influenced by the seed layers. The hardness of the electroplated Cu film increases with the thickness of the Cu seed layer, and finally reaches a constant value. The adhesion between the Cu film and the substrates is influenced by the Cu seed layer as indicated by the morphology of the film. The Cu film is less corrosion-resistant when its roughness value is higher. Better mechanical and chemical properties are obtained when the thickness of the Cu seed layer is 150 nm. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Cu is widely used in the manufacturing of electronic devices because of its excellent electrical conductivity, resistance to migration and amenability to dual damascene processing [1–3]. The manufacturing of microelectronics presents significant challenges for Cu interconnects with the increase in microelectronic interconnect layers during the integrated circuit (IC) manufacturing process [4]. In the process of IC manufacturing, Cu interconnects are electroplated onto a Cu seed layer that has been sputtered on the barrier layer because this process is less expensive and straightforward than other metal deposition techniques [5,6]. The surface/interface effects that determine the reliability of Cu interconnects [7] have recently become more critically important. The properties of sputtered Cu layers and the interactions between Cu, Ta/TaN and SiO2 have been widely investigated in recent years [8–13]. It has been shown that the out-diffusion of Ta atoms toward Cu layers, which leads to the formation of Cu, Ta and Cu–Ta oxides, may cause the failure of Cu/Ta/SiO2/Si multilayer structures during thermal annealing [14]. Moreover, the Ta/TaN bi-layer structure has much better diffusion barrier properties toward Cu than pure Ta or pure TaN films [15]. Early breakdowns of 100 nm Cu damascene lines can usually be observed with reductions in the thickness of the diffusion barrier Ta and Cu seed layers [16],
⇑ Corresponding author. Tel./fax: +86 10 62797362. E-mail address: [email protected] (X. Lu). 0167-9317/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.mee.2012.12.004
which may be one of the most important obstacles to overcome in the IC manufacturing process. Other researchers have focused on the electrochemical deposition of Cu films [17–19]. The crystal size, texture, and electrical properties of the electroplated Cu produced by optimizing the additives of the electroplating solution, the electroplating conditions and the heat-treatment process have been discussed extensively, both experimentally and theoretically [20,21]. For example, polyethylene glycol (PEG) can improve the wettability of the plating electrolyte on the Cu seed layer and electroplated Cu film, which can influence the Cu interconnect resistance [22]. The types of additives also affect the submicrometer trench-filling process during the Cu-electrodeposition process [23]. Some studies have shown that bis-(3-sulfopropyl) disulfide (SPS) accelerates the Cu bottomup growth process and that Janus green B (JGB) can inhibit Cu deposition at the end of the filling stages, leading to the suppression phenomenon usually caused by high JGB concentrations. Previous research has been less focused on the interaction between the Cu seed layers and the electroplated Cu films. There is little information about the influence of the sputtered Cu seed layer on the microstructure and mechanical properties of the electroplated Cu films. Cu seed layers with different morphologies and grain sizes can be realized by controlling the sputtering time. In this paper, the relationship between the properties of the Cu seed layer and the performance of the electroplated Cu film is investigated in terms of the morphology, grain size, crystallographic orientation, and other properties of electroplated Cu films with different thicknesses.
19
Y. Pan et al. / Microelectronic Engineering 105 (2013) 18–24
Table 2 The formulas for the electroplating solution of Cu. The thickness of the Cu film is 1.62 lm after electroplating for 5 min, and the thickness of the Cu film is 3.25 lm after electroplating for 10 min.
2. Experimental 2.1. Cu seed layer preparation A middle-frequency magnetron sputtering system was used to deposit a Ta barrier layer and a Cu seed layer on a SiO2 layer produced by the thermal oxidation of a Si (100) substrate. Impurities on the SiO2 layer were removed by Ar+ ion bombardment. Under controlled conditions (pressure: 2 Pa, bias voltage: 150 V), a 50 nm Ta barrier layer was first sputtered onto the SiO2 layer. Subsequently, Cu seed layers with different thicknesses were sputtered onto the Ta layer. The sputtering process parameters are shown in Table 1. Five identical runs were conducted for each specimen with the same thickness. The average thickness of the Cu seed layers was measured in cross section by ultra-high-resolution scanning electron microscopy (S-5500). The thicknesses of the Cu seed layers are shown in Table 1.
Reagent name
Formula
Contents
Blue vitriod Sulfuric acid Hydrochloric acid Additives A Additives B Additives C
CuSO45H2O H2SO4 HCl – – –
200 g/L 40 mL/L 0.2 mL/L 0.5 mL/L 0.2 mL/L 4 mL/L
2.2. Cu electrodeposition Cu electrodeposition was conducted in an electric tank. The anode was a phosphor bronze plate containing 0.05 wt% P. An anode mud could contaminate the electrolyte, resulting in a decrease in the quality of deposited Cu. To prevent such a contamination, an acid-resistant bag was introduced to bind the anode. The electroplating solution formula for Cu is shown in Table 2. The thickness of the Cu film was 1.62 lm after electroplating for 5 min and 3.25 lm after electroplating for 10 min. 2.3. Measuring instruments The surface morphology of the Cu seed layer and the electrodeposited film was investigated with an environment scanning electron microscope (FEI Quanta 200 FEG). The surface roughness (Ra) and the surface topography in a fixed 2 2 lm area were measured using an atomic force microscope (AFM, NanoMan VS, Veeco). BRUKER D8 advanced X-ray diffraction (XRD) analysis using Cu Ka radiation was performed to determine the crystalline structure and preferred orientation. The nanohardness of the electroplated Cu films and the adhesion between the Cu film and SiO2 layer was characterized by the NanoTest™ system from Micro Materials Company. To investigate the corrosion resistance, electrochemical measurements were performed using a CHI660C electrochemical workstation. The counter electrode was platinum, and the reference electrode was a standard calomel electrode. The tip of the reference electrode was placed at a distance of 5 mm from the work electrode. 3. Results and discussion
Fig. 1. X-ray diffraction spectra and crystal grain size of the Cu seed layers with different thicknesses. (1) X-ray diffraction spectra. Seed layer thickness: (a) 25 nm, (b) 50 nm, (c) 100 nm, (d) 150 nm, (e) 200 nm, and (f) 300 nm. (2) Crystal grain size.
Kk b cos h
3.1. Crystallographic orientation and grain size
L¼
The grain sizes of the Cu seed layers sputtered on the oxidized Si substrate with a Ta barrier layer are calculated based on the Scherrer equation [24–27]:
Here, K is a dimensionless constant and can generally be set to unity, k is the X-ray wavelength, h is the Bragg angle, L is the grain size, and b is the width at half of the peak corrected for the
ð1Þ
Table 1 The sputtering process parameters for the Cu seed layers. The sample shelf rotated at a speed of 4 rpm. The sputtering air pressure was 0.8 Pa, the basic bias was 150 V and the sputtering power density was 1.21 W/cm2. Sample code
Sputtering current (A)
Sputtering power density (W/cm2)
Sputtering temperature (°C)
Sputtering (time/min)
Thickness (nm)
Ta Cu
0.6 0.2 0.2 0.2 0.2 0.2 0.2
3.63 1.21 1.21 1.21 1.21 1.21 1.21
130 100 100 100 100 100 100
5 5 10 20 30 40 60
50 25 50 100 150 200 300
a b c d e f
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Fig. 2. X-ray diffraction spectra and crystal grain size of the electroplated Cu films with a thickness of 1.62 lm. (1) X-ray diffraction spectra. Seed layer thickness: (a) 25 nm, (b) 50 nm, (c) 100 nm, (d) 150 nm, (e) 200 nm, and (f) 300 nm. (2) Crystal grain size.
instrumentation broadening. Fig. 1(1) shows that the intensity of Cu(1 1 1) increases with the seed layer thickness. The grain size of the Cu seed layer varies linearly with the thickness of the Cu seed layer, as shown in Fig. 1(2). The ability to resist the electrical migration of the Cu(1 1 1) orientation is four times larger than that of the Cu(2 0 0) orientation, and thus the Cu(1 1 1) orientation is more suitable for Cu interconnects [7]. The Cu(1 1 1) orientation of the electroplated Cu film with a thickness of 1.62 lm becomes increasingly dominant with the increase in the thickness of the Cu seed layer, as shown in Fig. 2(1). However, this tendency is not significant in the electroplated Cu films with a thickness of 3.25 lm, as shown in Fig. 3(1), though the Cu(1 1 1) orientation is also more prominent than other orientations. It is assumed that this phenomenon may be related to the properties of the Cu seed layer. It can be observed in Fig. 1(1) that Cu(1 1 1) increases with the seed layer thickness. During the electrodeposition growth process, the electroplated Cu film inherits the characteristics of the seed layer at the initial stage, in which the Cu(1 1 1) orientation is dominant. The XRD results in Fig. 2(2) show that the grain size of Cu(1 1 1) first decreases and then increases irregularly from 60 to 80 nm, whereas the grain sizes of Cu(2 0 0) and Cu(3 1 1) do not change with the various thicknesses of the Cu seed layer, remaining constant at approximately 20 nm. For the electroplated Cu film with a thickness of 3.25 lm, Fig. 3(2) shows that the grain
Fig. 3. X-ray diffraction spectra and crystal grain size of the electroplated Cu films with a thickness of 3.25 lm. (1) X-ray diffraction spectra. Seed layer thickness: (a) 25 nm, (b) 50 nm, (c) 100 nm, (d) 150 nm, (e) 200 nm, and (f) 300 nm. (2) Crystal grain size.
sizes of all orientations have similar variations, increasing initially and then decreasing to a constant level. The grain sizes of the Cu film with a thickness of 1.62 lm shown in Fig. 2(2) are larger than the grain sizes of the Cu film with the thickness of 3.25 lm shown in Fig. 3(2) because the electrodeposited film growth process is influenced significantly by the thickness and orientation of the Cu seed layer, especially when the thickness of the electroplated Cu film is only 1.62 lm, which is not sufficient to compensate for the characteristics of the Cu seed layer [7]. 3.2. Surface morphology The SEM micrographs of Cu seed layers with an area of 15 15 lm with different thicknesses prepared by controlling the sputtering time are presented in Fig. 4. At the beginning of the sputtering process, the Cu seed layer becomes increasingly porous with an increase in the layer thickness until the thickness of the Cu seed layer reaches 100 nm, as shown in Fig. 4(c). Then, the number of pores decreases as the layer thickness increases. A Cu seed layer with few pores is shown in Fig. 4(f): when the Cu seed layer thickness is 300 nm. The AFM topography images of the Cu seed layers with an area of 2 2 lm with different thicknesses are shown in Fig. 5. The roughness of the Cu seed layers deposited by magnetron sputtering varies with the thicknesses of the Cu seed layer, from which the values of Ra are shown in Fig. 6. The surface morphologies of the
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Fig. 4. Scanning electron micrographs of the Cu seed layers with different thicknesses: (a) 25 nm, (b) 50 nm, (c) 100 nm, (d) 150 nm, (e) 200 nm, and (f) 300 nm.
Cu seed layer seem to be rather rough at the beginning of the sputtering process until the thickness reaches 50 nm. After a longer sputtering time, the surface tends to be become flat, with a constant roughness of approximately 5 nm. Fig. 6 also shows that the roughness of the Cu film with a thickness of 1.62 lm is similar to the roughness of the Cu seed layer. Because the Cu ions absorb onto the Cu seed layer under the influence of the electric fields, they are likely to fill the concave areas and tend to enter the existing grid positions of the Cu seed layer [5,18,19]. The electrically deposited Cu film with a thickness of 1.62 lm maintains the basic
characteristics of the Cu seed layer [19] while the growth of the film with a thickness of 3.25 lm is not affected by the original seed layer. In addition, the Cu films with a thickness of 1.62 lm are rougher than those with a thickness of 3.25 lm. 3.3. Hardness analysis The hardness values of the electroplated Cu films are shown in Fig. 7. The hardness values increase with the thickness of the Cu seed layers until they reach a constant value. This trend is
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Fig. 5. Atomic force microscope topographies of the Cu seed layers with different thicknesses: (a) 25 nm, (b) 50 nm, (c) 100 nm, (d) 150 nm, (e) 200 nm, and (f) 300 nm.
Fig. 6. Surface roughness of the Cu seed layers/films for different conditions.
Fig. 7. Hardness values of the Cu films on the seed layers with different thicknesses.
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phenomenon is consistent with the various trends in the roughness of the Cu seed layer as its roughness also first increases and then decreases in Fig. 6. It has been reported that a stronger mechanical interlock force will occur between a rougher Cu seed layer and the substrate than a flat Cu seed layer [29,30]. On the other hand, during the growth of the copper seed layer, Cu(1 1 1) textures become dominant with the increasing thickness of the copper seed layer, which results in an increase in adhesion [7]. When copper films are electroplated on the seed layers to thicknesses of 1.62 and 3.25 lm, the adhesion strength between the electroplated copper film and the substrate decreases significantly, and the adhesion strength varies with the thickness of the copper seed layer. The reduction of the adhesion may be induced by the increase in the copper growth stress during electroplating [31,32]. 3.5. Corrosion resistance Fig. 8. Adhesion strength between the Cu seed layers/films and the substrate.
Fig. 9. Corrosion current densities of the Cu films electroplated on different Cu seed layers.
attributed to the fact that the structures of the Cu seed layer change significantly during the sputtering process, as shown in Fig. 5. The Cu seed layers are not compressed at the initial growth stage, leading to a low hardness value. As the sputtered Cu seed layer becomes thicker, the film structure is compressed, and the hardness values increase to a stable value. In contrast, it has been reported that the hardness of the Cu film is high for the dominant Cu(1 1 1) orientation [8,28]. In our study, based on the XRD results and the calculated results of the grain size of Cu(1 1 1) on the Cuelectroplated film, Cu(1 1 1) has obvious advantages for the Cu surfaces with a larger thickness of the seed layer, which results in an increase in the layer hardness. It can also be observed that the 3.25 lm-thick electroplated films are harder than the 1.62 lmthick Cu films. Therefore, the mechanical properties of the films are affected by both the Cu seed layer and the electroplated film. The hardness of the films depends on the properties of the electroplated film to a great extent, especially for deposition thicknesses from 1.62 to 3.25 lm. 3.4. Adhesion analysis The adhesion strength between the Cu films/layers and the substrate is characterized in Fig. 8. The adhesion strength between the Cu seed layer and the substrate, first increases and then decreases with the thickness of the Cu sputtering seed layers. And this
The corrosion current densities of the Cu films electroplated on different Cu seed layers are shown in Fig. 9. The corrosion current density of the Cu film with a thickness of 1.62 lm varies with the surface roughness. The reaction area between the Cu film and the slurry increases when the surface is rough, which directly accelerates the process of corrosion [33]. The surface is rough when the seed layer thickness is 50 nm and the 1.62 lm Cu films cannot fully cover the seed layers; thus, the reaction area between the film and the slurry increases, which accelerates the corrosion. For the Cu film with a thickness of 3.25 lm, the corrosion current density variations are not very obvious; it can be deduced that a uniform and dense Cu surface is beneficial for corrosion resistance because of the stabilization of the grain size and surface roughness. The corrosion current density of Cu films with a thickness of 3.25 lm is smaller than those with a thickness of 1.62 lm. This smaller density is due to the greater content of Cu(1 1 1) in the Cu films with a thickness of 3.25 lm than for films with a thickness of 1.62 lm because of the resistance of the Cu(1 1 1) surface to slurry corrosion [34]. 4. Conclusions The influences of the Cu seed layer on the performance of electroplated Cu films are investigated in this paper. The surface morphology, adhesion, and roughness of the copper seed layer have a strong dependence on the thickness of the Cu seed layer. The morphology, grain size, and crystallographic orientation of the electroplated Cu films are significantly influenced by the seed layers. Consequently, the stress and morphology of the film has an influence on the hardness of the electroplated Cu film and the adhesion between the Cu film and the substrates. Acknowledgements The authors acknowledged the financial support from the National Natural Science Funds for Distinguished Young Scholars (Grant no. 50825501) and the National Natural Science Foundation of China (Grant no. 50775122). References [1] R. Rosenberg, D.C. Edelstein, C.K. Hu, K.P. Rodbell, Annu. Rev. Mater. Sci. 30 (2000) 229–262. [2] R.L. Jackson, E. Broadbent, T. Cacouris, A. Harrus, M. Biberger, E. Patton, T. Walsh, Solid State Technol. 41 (1998) 49–59. [3] P.C. Andricacos, C. Uzoh, J.O. Dukovic, J. Horkans, H. Deligianni, IBM J. Res. Dev. 42 (1998) 567–574. [4] J. Rober, S. Riedel, S.E. Schulz, T. Gessner, Microelectron. Eng. 37–8 (1997) 111–119. [5] H. El Sayed, M.T. Greiner, P. Kruse, Appl. Surf. Sci. 253 (2007) 8962–8968. [6] J.C. Ziegler, R.I. Wielgosz, D.M. Kolb, Electrochim. Acta 45 (1999) 827–833.
24
Y. Pan et al. / Microelectronic Engineering 105 (2013) 18–24
[7] L. Haebum, S.D. Lopatin, Thin Solid Films 492 (2005) 279–284. [8] M.O. Bloomfield, D.N. Bentz, T.S. Cale, J. Electron. Mater. 37 (2008) 249–263. [9] S. Shojaei Zadeh, S. Zhang, W. Liu, Y. Yang, S.M. Sadeghipour, M. Asheghi, P. Sverdrup, Thermal characterization of thin film cu interconnects for the next generation of microelectronic devices, in: Therm. Phenom. Electron. Syst. Proc. Intersoc. Conf., 2004, pp. 575–583. [10] C. Detavernier, D. Deduytsche, R.L. Van Meirhaeghe, J. De Baerdemaeker, C. Dauwe, Appl. Phys. Lett. 82 (2003) 1863–1865. [11] H.C. Huang, H.L. Wei, C.H. Woo, X.X. Zhang, Appl. Phys. Lett. 82 (2003) 4265– 4267. [12] K. Yamaguchi, J. Appl. Phys. 89 (2001) 590–595. [13] L. Vanasupa, Y.C. Joo, P.R. Besser, S. Pramanick, J. Appl. Phys. 85 (1999) 2583. [14] K.M. Latt, Y.K. Lee, T. Osipowicz, H.S. Park, Mater. Sci. Eng. B 94 (2002) 111– 120. [15] Q. Xie, X.P. Qu, J.J. Tan, Y.L. Jiang, M. Zhou, T. Chen, G.P. Ru, Appl. Surf. Sci. 253 (2006) 1666–1672. [16] M. Traving, G. Schindler, G. Steinlesberger, W. Steinhogl, M. Engelhardt, Semicond. Int. 26 (2003) 73–78. [17] S. Armini, C.M. Whelan, M. Moinpour, K. Maex, J. Electrochem. Soc. 156 (2009) H18–H26. [18] U.H. Lee, H.J. Lee, H.J. Sohn, T. Kang, Electrochem. Solid-State Lett. 9 (2006) C65–C68.
[19] M. Kang, A.A. Gewirth, J. Electrochem. Soc. 150 (2003) C426–C434. [20] S. Ahmed, D.N. Buckley, S. Nakahara, T.T. Ahmed, Y. Kuo, J. Electrochem. Soc. 154 (2007) D103–D112. [21] K.R. Hebert, J. Electrochem. Soc. 148 (2001) C726–C732. [22] C.W. Liu, J.C. Tsao, M.S. Tsai, Y.L. Wang, J. Vac. Sci. Technol., A 22 (2004) 2315– 2320. [23] M. Hasegawa, Y. Negishi, T. Nakanishi, T. Osaka, J. Electrochem. Soc. 152 (2005) C221–C228. [24] A.L. Patterson, Phys. Rev. 56 (1939) 978–982. [25] K. Pantleon, M. Somers, J. Appl. Phys. 100 (2006). [26] J.I. Langford, J. Appl. Crystallogr. 11 (1978) 10–14. [27] T.H. Dekeijser, J.I. Langford, E.J. Mittemeijer, A. Vogels, J. Appl. Crystallogr. 15 (1982) 308–314. [28] A.K. Siker, A. Kumar, P. Shukla, P.B. Zantye, M. Sanganaria, J. Electron. Mater. 32 (2003) 1028–1033. [29] I.M. Feng, J. Appl. Phys. 23 (1952) 1011–1019. [30] J. Ge, R. Tuominen, J.K. Kivilahti, J. Adhes. Sci. Technol. 15 (2001) 1133–1143. [31] J.M. Paik, I.M. Park, Y.C. Joo, K.C. Park, J. Appl. Phys. 99 (2006). [32] H. Lee, S.D. Lopatin, Thin Solid Films 492 (2005) 279–284. [33] W. Li, D.Y. Li, Acta Mater. 54 (2006) 445–452. [34] P.J. Andersen, M.N. Bentancur, A.J. Moll, M. Frary, J. Electrochem. Soc. 157 (2010) H120.