Characterization of Electroplated Cu Thin Films on Electron-Beam-Evaporated Cu Seed Layers

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1 Journal of the Korean Physical Society, Vol. 54, No. 3, March 2009, pp Characterization of Electroplated Cu Thin Films on Electron-Beam-Evaporated Cu Seed Layers Yoojin Song, Jung-Hye Seo and Youn-Seoung Lee Department of Information Communication Engineering, Hanbat National University, Daejon Young-Ho Ryu and Kimin Hong Department of Physics, Chungnam National University, Daejeon Sa-Kyun Rha Department of Materials Engineering, Hanbat National University, Daejon (Received 16 June 2008) Cu and Ti of 20 nm, respectively, were deposited onto p-type Si(100) substrate for use as a seed layer by using electron-beam evaporation. Potentiostatic electrodeposition was carried out using the three-terminal method, with an Ag/AgCl reference electrode, a platinum plate as a counter electrode and the seed layer as a working electrode. The plating electrolyte was composed of CuSO 4, H 2SO 4, HCl and three organic additives (accelerator, suppressor and leveler). The amounts of the additives were changed. The resistivity of the electroplated Cu lms was measured with a four-point probe and the physical properties were investigated using eld emission scanning electron microscopy (FE-SEM), atomic force microscopy (AFM), X-ray diraction (XRD) and X- ray photoelectron spectroscopy (XPS). The increases in the plating concentrations of the organic additives (the increases in the accelerator, the suppressor and the leveler for the high sample were +2, +0.5 and +1 ml/l, respectively as compared with the low sample) led to a predominant Cu (111) texture, an increased lm density and a decreased electrical resistivity of the electroplated Cu lm. PACS numbers: I, Ev, Gh, Mq Keywords: Electroplating, Cu, XRD, XPS, Resistivity, Additives, Surface roughness I. INTRODUCTION According to the recent trend toward the scaling down of electronic device dimensions, the requirement to understand the impact of the microstructure of a metal lm on its electrical performance and reliability is of great importance [1, 2]. Therefore, Cu has been used for metallic interconnects in ultra-large-scale-integrated circuits (ULSI) applications because of its lower resistivity, higher current density and increased scalability compared with Al [3{7]. The resistance of Cu interconnects is known to represent a signicant contribution to the signal delay. The resistivity increase of Cu lines will seriously aect the RC delay and will limit signal propagation in integrated circuits [2,6]. The electroplating process has been proposed for Cu metallization because of its low cost, high throughput, low growth temperature and excellent gap-lling capability [6{11]. While Cu has been successful for fabrica- yslee@hanbat.ac.kr; Fax: tion of narrow metallic lines, there have been interests in variations in the properties with the composition of the plating electrolyte and/or organic additives. In general, organic additives alter the grain growth mechanism and change the microscopic structures, such as the crystalline orientation, the surface roughness and the grain size [10] and these changes in the microscopic structures will aect the electrical properties, such as the resistivity of electrodeposited Cu lms, thus, we have to form of Cu lines or lms with an ideal resistivity (1.67 cm) for the best Cu metallic interconnects. However, the understanding the correlation between electrical properties and the physical properties, including microscopic structures in Cu metallization, is not yet sucient. Organic additives are usually categorized as accelerators, suppressors, levelers and so on. An accelerator is a catalyst having a large diusion length and enhances the plating current density at a specied plating voltage. A suppressor is a surfactant having a short diusion length and prohibits plating, that is, reduces the plating current density [12{18]. A leveler is a material that is preferentially adsorbed to protruding surfaces or

2 Journal of the Korean Physical Society, Vol. 54, No. 3, March 2009 Table 1. Composition of the organic additives for growth of Cu thin lms. The quantities of the additives for the high sample are relatively higher than those for the low sample. Additives Samples Low High Viaform Accelerator (ml/l) 6 8 Viaform Suppressor (ml/l) Viaform Leveler (ml/l) 2 3 corners and enables a reduction in the local growth rate and the formation of a uniform thickness [17{19]. Several additive compositions have been employed in the semiconductor industry because organic additives play a role in the growth of Cu lms [14,18]. In this work, we studied the correlation between the electrical properties and the physical properties, such as the crystalline orientation and the surface roughness, due to a few organic additives. II. EXPERIMENTS The substrate was p-type Si(100) with 20-nm a titanium (Ti) thin lm as an adhesion/diusion-barrier layer on the surface. A 20-nm-thick Cu seed layer was deposited onto the substrate by using electron-beam evaporation. In this paper, we focused on the eect of the organic additives in the growth of electrodeposited Cu thin lms. Therefore, the plating electrolyte, which was composed of 40 g/l CuSO 4 5H 2 O, 10 g/l H 2 SO 4 and 50 ppm HCl, was xed. We attempted to change the organic additives as shown in Table 1. By comparison with the low sample, the accelerator, the suppressor and the leveler for the high sample were increased by 33 %, 20 % and 50 %, respectively. Potentiostatic electrodeposition was carried out using a conventional three-terminal method, with an Ag/AgCl reference electrode, a 2 cm 2.5 cm platinum plate as a counter electrode and the aforementioned copper seed layer as a working electrode. The plating was carried out with all the electrodes immersed in the plating cell. A potentiostat (SI 1286, Solartron) was used as a power supply for the deposition [19]. The plating voltage was {0.3 V. All lms were deposited for 600 sec at room temperature. The surface roughness and the lm thickness of thin lms were measured by using atomic force microscopy (AFM; VEECO, U.S.A.) and eld emission scanning electron microscopy (FE-SEM; FEI, Netherlands, Model Sirion) in the National Nanofab Center (NNFC), Korea. The sheet resistance was measured with a 4-point probe. The crystal structures of the plated thin lms were determined by using X-ray diraction (XRD; Rigaku, Japan, Model M/Max-IIIA) techniques. The chemical states of Fig. 1. FE-SEM cross-sectional images of electroplated copper lms for (a) additive Low and (b) additive High. the composites were investigated by using X-ray photoelectron spectroscopy (XPS; PHI 5700) with monochromatized Al K ( ev) radiation. The binding energies were calibrated to Cu 2p 3=2 at ev for the Cu lms. For all samples, surface cleaning was performed with Ar + ion for 2 min. III. RESULTS AND DISCUSSION Figure 1 shows the FE-SEM cross-sectional images of the electroplated low and high samples. As shown in Figure 1, the lm thickness for the high sample is thinner than that for the low sample although the deposition time is the same. Judging from the peculiarity of the additives, we expect the growth rate of the lm in high sample to be faster than that in the low sample because in the high sample, the increase in the accelerator, which enhances the plating current density is larger than the increase in the suppressor, which reduces the plating current density. However, we obtained the opposite result. Figure 2 shows the AFM surface images of the electroplated samples and the Cu seed layer. After electroplating, the surface changed drastically from the relatively smooth surface of the pristine Cu seed layer to a rough surface. The rms (root mean square) values of the surface roughnesses were about 0.85 nm in the pristine sample, 6.3 nm in the low sample and 5.6 nm in the high sample. In the case of the high sample, the rms value was

3 Characterization of Electroplated Cu Thin Films on { Yoojin Song et al Fig. 3. XRD patterns of the electroplated Cu lms: (a) full scale and (b) narrow scale. Fig. 2. AFM images of the surface morphologies for (a) the Cu seed layer, (b) the Cu lm plated with additive Low and (c) the Cu lm plated with additive High. The rms roughness values are (a) 0.85 nm, (b) 6.3 nm and (c) 5.6 nm. smaller than that of the low sample. We think that the decrease in the surface roughness is due to an increased eect of the leveler, which reduces the local growth rate and causes a uniform thickness. In general, the average lm growth rate is higher when the lm thickness is not uniform due to local growth. Therefore, we can guess that the opposite result for the growth rate in Figure 1 is due to the eect of the leveler. The sheet resistances, R s, of the electroplated lms, measured by using the 4-point probe, were 22.3 m/ (low sample) and 26.0 m/ (high sample). The resistivity () of the electroplated lms was determined by = R s t, where t is the thickness of the Cu lm determined from the FE-SEM image; the resistivities were 2.54 and 2.25 cm for the low and the high samples, respectively. However, compared with the resistivity of bulk Cu of the highest purity, 1.75 cm [21], the values for the electroplated Cu lms are higher. Physical properties, such as the crystalline structure and the chemical states of the lm can aect electrical properties such as the resistivity. At rst, we checked the crystalline structure of these lms. As shown in Figure 3(a), the Cu (111) texture for all samples is predominant. However, a detailed analysis [Figure 3(b)] showed that the electroplated Cu lms had a poly-crystalline structure; Cu (200) and Cu (220) textures coexisted with Cu (111). In general, the (111) direction of the Cu thin lm is known to aect the resistance of the grown Cu lms strongly [3,4,22]. The relative intensity ratios I 200 /I 111 and I 220 /I 111 are displayed in Figure 4 by using the peaks in Figure 3(b). In Figure 4, the I hkl /I 111 ratio of the low sample is larger than that of the high sample, which means that the (111) direction was more dominant for the high sample. This result agrees well with the above result that the resistivity of the high sample is smaller than that of the low sample. In addition, we can explain the electroplated Cu lms having a larger resistivity than bulk Cu with high purity on the basis of a poly-crystallized Cu lm.

4 Journal of the Korean Physical Society, Vol. 54, No. 3, March 2009 Fig. 4. XRD patterns of the electroplated Cu lms indicated by the I200 /I111 or the I220 /I111 ratio. Fig. 6. XPS Cl(2p) core-level spectra before and after cleaning for the electroplated samples and the Cu seed layer. Fig. 7. Normalized XPS Cu 2p3=2 spectra (a) before and (b) after cleaning for the samples. Fig. 5. XPS S(2p) core-level spectra before and after cleaning for the electroplated samples and the Cu seed layer. In the formation of an electroplated Cu lm, one of the problems is impurities such as cupric sulfate and hydrogen-chloride, which are included in the electrolytes and the organic additives. As shown in Figure 5 and Figure 6, the typical impurities, sulfur S and chlorine Cl, were checked before and after surface cleaning by Ar+ ions. Within the detection level of the XPS analysis, we con rmed that there were no impurities. In order to investigate state variations in the Cu lm by air exposure, we measured the Cu 2p core level spectra [Figure 7]. After air exposure, the surfaces of Cu lms were oxidized and Cu2 O or CuO phases had formed on the surface. In Figure 7(a), we can con rm the formation of a CuO phase ( ev) on the surface of the electroplated Cu lm. However, after surface cleaning, the CuO phase had disappeared completely [Figure 7(b)]. By this result, although the CuO phase on the surface of the electroplated Cu lms is removed by surface cleaning, unfortunately we cannot say that the Cu oxides on the surfaces of Cu lms are removed completely by surface cleaning because the Cu 2p3=2 binding energies and the peak shapes of Cu2 O and Cu are essentially identical [23]. In order to investigate the formation of the Cu2 O phase, we measured the X-ray-induced Cu LMM Auger spectra because the Cu LMM Auger peak is more sensitive to change in the chemical state than the Cu 2p core peak. The change in the chemical state of the Cu site caused by air exposure is shown in the Cu LMM Auger spectra [Figure 8]. For the as-deposited Cu lms (=before cleaning) in Figure 8(a), a broad Cu oxide peak appears strongly at ev, which is assigned to Cu2 O. However, after surface cleaning, the Cu2 O peaks decrease abruptly and metallic Cu peaks ( 568 ev) are clearly seen. In Figure 8(b), the intensity of the metallic Cu peak of the low sample is lower and the peak is broader than that of the other samples. For the low sample, the fact that the Cu2 O phase on the surface remains even after surface cleaning indicates that the oxygen in the air penetrated more deeply than it did for the high sam

5 Characterization of Electroplated Cu Thin Films on { Yoojin Song et al. ACKNOWLEDGMENTS This work was supported by \System IC 2010" project of Korea Ministry of Knowledge Economy. REFERENCES Fig. 8. X-ray-induced Cu LMM spectra (a) before and (b) after cleaning for the samples. ple. A deep penetration of oxygen means that the lm is relatively porous and that the lm density is lower. Therefore, we can suggest that the lm density of the low sample is lower than that of the high sample. IV. CONCLUSION We used surface analysis tools such as SEM, AFM, 4-point probe and XPS to investigate the e ects of additives on Cu lms grown by using the electroplating process. In addition, we tried understanding the correlation between the electrical properties, the physical properties, such as the crystalline orientation and the surface roughness, due to a few organic additives. For the high sample, in which the amounts of the accelerator, the suppressor and the leveler were increased by +2, +0.5 and +1 ml/l, respectively, the resistivity was lower by 11.4 % and the Cu (111) texture was more predominant than it was in the low sample. The XRD I200 /I111 ratio for the high sample was lower by 32 %. In addition, we could infer that the lm density of the high sample was higher than that of the low sample by using an indirect XPS method. The decrease in the surface roughness may be related closely to the increased lm density. The surface roughness and the lm thickness for the high sample were lower by 11 % and 24 %, respectively, compared with the low sample. Increasing the leveler and the accelerator led to the formation of a denser Cu lm with a lower surface roughness and a predominant Cu (111) texture. Finally, we can suggest that the resistivity value of the Cu lms grown by using the electroplating process decreases with increasing Cu (111) texture and with increasing Cu lm density. [1] C. S. Krisha and M. Forrokh, IEEE Trans. Electron Dev. 29, 4 (1982). [2] K. Leaming-Sphabmixay, M. J. Van Olmen, K. J. Moon, Kris Vanstreels, J. D'Haen, Z. Tokeik, S. List and G. Beyer, Microelectronic Engineering 84, 2681 (2007). [3] D. C. Edelstein, G. A. Sai-Halasz and Y.-J. Mii, IBM J. Res. Develop. 39, 383 (1995). [4] C.-A. Chang, J. Appl. Phys. 87, 566 (1990). [5] D. Edelstein, J. Heidenreich, R. Goldblatt, W. Cote, C. Uzoh, N. Lusting, P. Roper, T. McDevitt, W. Motsi, A. Simion, J. Dukovic, R. Wachnik, H. Rathore, R. Schulz, L. Su, S. Luce and J. Slattery, Technical Digest, IEEE International Electron Devices Meeting 773 (1997). [6] J. H. Lee, B. N. Park, S. J. Park and S. Y. Choi, J. Korean Phys. Soc. 33, S112 (1998). [7] W. Bang and K. Hong, Electrochem. Solid-State Lett. 10, J86 (2007). [8] M. K. Lee, H. D. Wang and J. J. Wang, Solid State Electron. 41, 695 (1997). [9] B. N. Park, S. C. Bae, S. C. Son, J. H. Lee, S. Y. Choi, C. G. Suk and J. K. Choi, J. Korean Phys. Soc. 38, 232 (2001). [10] K. Hong, J. Lee, J. Lee, Y.-D. Ko, J.-S. Chung and J.-G. Kim, J. Mag. Mag. Mater. 304, 60 (2006). [11] J. H. Kim, Y. G. Geol and N.-E. Lee, J. Korean Phys. Soc. 51, S187 (2007). [12] J. Lee and K. Hong, J. Kor. Mag. Soc. 15, 198 (2005). [13] A. J. Bard and L. R. Faulkner, Electrochemical Methods, Fundamentals and Applications (John Wiley & Sons, New York, 2001). [14] M. Tan and J. N. Harb, J. Electrochem. Soc. 150, C420 (2003). [15] A. C. West, J. Electrochem. Soc. 147, 227 (2000). [16] K. Hong, J.-K. Kim, S.-K. Lee, S. Park, B. Gyun, Y.-D. Ko, N.-J. Jeon and J.-S. Cung, Phys. Stat. Sol. (b) 241, 1681 (2004). [17] W. Bang, J.-B. Lee, K. Hong, Y.-D. Ko, J.-S. Chung and H. Lee, Phys. Stat. Sol. (a) 204, 4067 (2007). [18] J. Lee, J. Lee, J. Bae, W. Bang, K. Hong, M. H. Lee, S. G. Pyo, S. Kim and J.-G. Kim, J. Electrochem. Soc. 153, C521 (2006). [19] J. Bae, W. Bang, J.-G. Kim and K. Hong, ECS Trans. 2, 33 (2007). [20] D. R. Lide, Handbook of Chemistry and Physics (CRC Press, New York, 2000). [21] S. Vaiday and A. K. Sinha, Thin Solid Films 75, 253 (1981) [22] S.-P. Wang, X.-C. Zheng, X.-Y. Wang, S.-R. Wang, S.M. Zhang, L.-H. Yu, W.-P. Huang and S.-H. Wu, Catal. Lett. 105, 163 (2005). [23] J. F. Moulder, W. F. Stickle, P. E. Sobol and K. D. Bomben, Handbook of X-ray Photoelectron Spectroscopy (Physical Electronics Inc., 1995).