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The influence of thiourea on copper electrodeposition: Adsorbate identification and effect on electrochemical nucleation
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Thin Solid Films 516 (2008) 3761 – 3766 www.elsevier.com/locate/tsf
The influence of thiourea on copper electrodeposition: Adsorbate identification and effect on electrochemical nucleation Moo Seong Kang a , Soo-Kil Kim b , Keeho Kim c , Jae Jeong Kim a,⁎ a
Research Center for Energy Conversion and Storage, School of Chemical and Biological Engineering, College of Engineering, Seoul National University, Seoul, 151-742, Korea b Fuel Cell Research Center, Korea Institute of Science and Technology (KIST), Seoul 136-791, Korea c Department of Advanced Nano-tech Development, DongbuAnam Semiconductor, 474-1 Sangwoo-ri, Kamgok-myun, Umsung-kun, Chungbuk, 369-852, Korea Received 25 February 2006; received in revised form 18 April 2007; accepted 9 June 2007 Available online 15 June 2007
Abstract The effect of thiourea on copper deposition onto a copper seed layer from an electrolyte composed of CuSO4, H2SO4, deionized water, and thiourea was investigated. Even in the presence of very low concentrations of thiourea, extremely smooth and bright copper deposits were obtained. From the results of X-ray photoelectron spectroscopy, Auger electron spectroscopy, and electrochemical analyses, thiourea was found to react with copper or copper ions leading to the generation of CuS. CuS adsorption onto the copper seed layer seemed to inhibit the initial nucleation of the copper adions, resulting in the formation of smaller Cu grains compared to those forming in the absence of thiourea. CuS was observed to cover all active sites of the 1 cm2 copper seed layer above 0.017 g/L thiourea. The surface roughness as well as the mean grain size of the deposits also approached minimum values above this thiourea concentration. Adsorbed CuS was incorporated into the deposits during electroplating, which was believed to be the major factor for the increased resistivity of the deposits. © 2007 Elsevier B.V. All rights reserved. Keywords: Copper; Electrodeposition; Thiourea; Brightener; Copper sulfide; Nucleation
1. Introduction Copper electroplating has been viewed as a powerful deposition process in ultra large-scale integration (ULSI) interconnection technology [1–3]. It has recently attracted more attention for its use in controlling the film properties of electroplated copper, i.e. surface morphology, resistivity, and crystal orientation. This has been accomplished by using various organic additives such as polyethylene glycol and chloride ions [4–6] as co-suppressors, thiourea (TU) [7,8] and benzotriazole [9,10] as brighteners, and mercapto compounds [11,12] as accelerators. Among them, the control of surface roughness through the addition of TU [13] or other brighteners is one of the key methods used to fill the vias or trenches without voids, since increasing the roughness of the bottom and side walls of the via during electroplating can cause voids or key ⁎ Corresponding author. E-mail address: [email protected] (J.J. Kim). 0040-6090/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2007.06.069
holes. In 0.18 μm complementary metal–oxide semiconductor logic devices, the minimum metal width may be around 0.20 μm. The width will be filled with a barrier metal, seed copper layer, and electroplated copper. Even in a bottom-up process, an electroplated copper roughness of less than 5 nm is desired to fill the gap with an aspect ratio of 3 to 4. Nikolic et al. [14] used TU as a brightener to make a high degree of mirror reflection in copper and zinc coatings. Effects of various additives including TU on electroless copper plating were investigated by Hanna et al. [15]. Gassa et al. [16] reported TU electro-oxidation phenomena on copper electrodes by means of electrochemical impedance spectroscopy. Bolzan et al. [17,18] investigated the complexing mechanism of Au–TU and Cu–TU complex with various potential ranges. Port et al. [19] tried to characterize ligand formation using IR spectroscopy, cyclic voltammetry and rotating disk electrode techniques. In spite of these efforts to identify effect of TU on electrochemistry, more researches should be done in order to verify the possibility of applying TU to ULSI metallization.
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In this work we identified the exact composition of the adsorbates generated from TU by electrochemical and spectroscopic analysis techniques and examined the possibility for applications in the fabrication of semiconductor interconnections by presenting its brightening effect on electroplated copper. 2. Experimental details The seed layer was prepared by 70-nm-thick physical vapor deposition Cu film on p-type (100) oriented silicon wafers with a 40-nm-thick metal organic chemical vapor deposition TiN barrier layer film. Samples (1.5 cm × 1.5 cm) were made by cutting that wafers. Surface pretreatment by immersion into a 1:200 diluted NH4OH : deionized (DI) water solution was performed for 30 s to remove the surface copper oxide on the seed layer. After oxide removal, each sample was rinsed carefully with DI water and dried under a continuous stream of N2. This pretreatment process was found to be quite effective in removing the native copper oxide and maintaining an electrically uniform seed layer [20]. After the pretreatment steps, each sample was placed in the electrodeposition holder which made electric contact between the potentiostat and the sample. When a sample was in the holder, the surface area of samples exposed to the solution was 1 cm2. The standard electrolyte was composed of 1 M H2SO4, 0.05 M CuSO4, and DI water. The constant potential for Cu electroplating was applied by a PAR 263 potentiostat (EG&G Princeton Applied Research Corporation) with respect to a saturated calomel electrode (SCE) at room temperature. We used an electronic-grade Cu wire (1 mm diameter, 3 cm in length) as an anode for copper deposition. All samples were rinsed with DI water and dried in a stream of N2 after electroplating. Annealing was performed at 400 °C for 30 min in a N2 atmosphere using horizontal quartz tube furnace.
Field emission scanning electron microscopy (FESEM, Philips XL-30) and atomic force microscopy (AFM, Digital Instruments Dimension™ 3100) were used to study the surface morphology and roughness. The operating voltage of the FESEM was 25 kV. The AFM operating mode was contact mode and the end of the tip used in the AFM was composed of Si3N4. Cyclic voltammetry and electrochemical impedance analyses using a PAR 5210 Lock-in amplifier (EG&G Princeton Applied Research Corporation) were conducted to verify the adsorption of TU-induced species and determine their effect on cathodic deposition. The frequency range and the AC amplitude of electrochemical impedance analysis were 10 kHz ∼ 0.01 Hz and 5 mV, respectively. In order to identify the adsorbates on the cathodic surface through an examination of the binding state of Cu–S and the stoichiometry among S, C, and N impurities of the deposits, X-ray photoelectron spectroscopy (XPS, Kratos Model AXIS-HS) and Auger electron spectroscopy (AES, Perkin-Elmer Model 660) were applied. Al Kα (1486.6 eV) was used as the X-ray source of the XPS and peak positions were calibrated with respect to C 1s peak at 284.6 eV. In the AES analysis, the accelerating voltage of the electron beam was 10 kV and the sensitivity factors of Cu, S, Ti, N, O and C were 0.307, 1.041, 0.438, 0.930, 0.296 and 0.246, respectively. 3. Results and discussion Fig. 1 shows the dependence of the current–voltage (I–V) plots on the amount of TU in the electrolyte. Even a very small amount of TU affected the I–V plot. The reduced current density and the shift of the cathodic reduction peak in a more cathodic direction were attributed to the generation of new species from the reaction of TU with copper ions that inhibited the cathodic consumption of copper ions.
Fig. 1. Cyclic voltammetry of copper electroplating at various concentration of thiourea.
M.S. Kang et al. / Thin Solid Films 516 (2008) 3761–3766
Nyquist plots without TU are presented in Fig. 2(a) with varying cathodic potential. The curves that corresponded to − 100 mV and −200 mV showed two partially combined semicircles. The diameter ratio of the two semicircles can be used to identify the rate limiting step of copper electrodeposition. As the deposition potential increased from −100 mV to − 200 mV, the first semicircle shrank while the second expanded. The diameter ratio of the first semicircle to the second changed from less than 1 to greater than 1. This indicated that as the deposition potential increased, the deposition process moved from charge transfer limiting conditions to mass transfer limiting conditions. At − 400 mV, the deposition process was almost entirely mass transfer limiting, and the first semicircle was buried within the second one. Since TU was thought to have a much greater effect on the surface charge transfer than mass transfer, an impedance analysis was performed to reveal the effect of TU on surface passivation related to charge transfer resistance. The impedance analysis is shown in Fig. 2 at concentrations of TU ranging from 0.002 to 0.170 g/L and a potential of − 200 mV where the first semicircle was clearly distinguished. The charge transfer resistances increased with the amount of TU, implying that the TU complex blocked the active sites of the working elec-
Fig. 2. Nyquist plot of copper electroplating at (a) different cathodic potential and (b) various concentration of thiourea. Deposition potential of (b) is − 200 mV vs. SCE.
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trode. Above a TU concentration of 0.017 g/L, the charge transfer resistances had almost the same value, which indicated that TU complex had adsorbed to cover the entire surface area of the copper seed and the inhibition and deposition saturated. Due to the increased first semicircles, the second ones were partially superimposed on the first semicircles and expressed as straight lines. In consideration of the practical applications of copper electroplating in semiconductor interconnection, copper was electroplated at − 400 mV at a deposition rate of 100 nm/min and surface-analyzed. Fig. 3 shows the FESEM images of the electroplated copper films of about 400 nm in thicknesses deposited with varying concentration of TU. When compared with TU-free deposits, the number density of the grains was much higher as a result of TU addition due to the inhibition of the lateral growth of the grain and formation of larger grains. For example, the grain density of deposits made with 0.083 g/L added TU as found in AFM images was 310/μm2 while that of TU-free deposits was 44/μm2. The inhibition of crystal growth resulted in finer grained deposits, which are the key properties for yielding a smooth surface of electroplated copper. The relation between TU concentration and surface root mean square (RMS) roughness was characterized using AFM in Fig. 4. For all species, the deposition thickness was set as 400 nm to eliminate any thickness effects on surface roughness. The abrupt change of RMS roughness above 0.05 g/L in Fig. 4 presents the dramatic effect of TU on brightening of electroplated copper films. When the concentration of TU increased above 0.05 g/L which corresponds to 6.6 × 10− 4 M, the surface roughness was converged to about 2 nm. The TU concentration at which the sudden change in mean grain size was observed also corresponded to 0.05 g/L as shown in Fig. 5. The mean grain sizes, calculated from AFM images decreased with increasing TU and the mean grain size of 0.17 g/L TU was about 342 nm2, which indicated a strong inhibition of grain growth by the TU. To identify the exact composition of the adsorbates, XPS and AES analyses were performed; the results are shown in Figs. 6 and 7. The 2p3/2 and 2p1/2 spectra of surface copper (Fig. 6(a)) were at 932.6 eV and 952.4 eV. Since the 2p3/2 binding energy of pure Cu, Cu2S, and CuS partially overlap in the range of 932.5 and 932.8 eV, the 2p3/2 and 2p1/2 spectra of copper did not provide any further information. On the other hand, spectra relating to the 2p3/2 and 2p1/2 of S in the deposited copper surface showed a dependency on the TU concentration as given in Fig. 6(b). Through the addition of TU, the S 2p3/2 peak appearing at 162.4 eV and the shoulder shown at 163.6 eV corresponded to S 2p1/2 peak, which were smaller than 164.0 eV for 2p3/2 and 165.18 eV for 2p1/2 in the case of pure S. Since there are only two S-containing groups that have S 2p3/2 binding energy below 163 eV, sulfide and thiophene, it may be concluded that CuS was generated from the reaction of TU with the copper ions. Fig. 7 presents the AES depth profile of S, N, and C in an electroplated copper film of 400 nm thickness deposited with 0.083 g/L TU. More precise information about the TU-induced adsorbed species can be acquired from Fig. 7. It was seen that
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Fig. 3. FESEM and AFM images of electroplated copper films at various concentrations of thiourea: (a) 0.00 g/L, (b) 0.083 g/L, and (c) 0.17 g/L.
only S was incorporated in the Cu deposits and that the contents of C and N were negligible. The average S content was about 2.07% calculated by averaging the S curve of the AES depth profile from sputter time 0 min to 9 min. S-only incorporation indicated that the adsorbate is not TU itself but the copper sulfide. Copper sulfide is the very species that inhibits surface diffusion of copper adions, which leads to smooth and bright copper deposits. Another interesting point is that copper sulfide had a uniform depth profile from the surface of the copper seed
layer to that of the electrodeposits. This implies that copper sulfides were strongly bound to the copper surface and subsequently entrapped in the deposits during each monolayer formation. The identification of CuS through XPS and AES analyses strongly supported the production of CuS through the reaction of copper ion with TU [21,22], though the exact reaction mechanism remains unclear. The resistivity of the electroplated copper film increased linearly from 2 to 16.5 μΩ cm as the TU concentration increased.
Fig. 4. Surface RMS roughness dependence on the thiourea concentration for electroplated copper.
Fig. 5. Mean grain sizes of electroplated copper with various concentrations of thiourea.
M.S. Kang et al. / Thin Solid Films 516 (2008) 3761–3766
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Fig. 6. XPS spectrum of the copper surface electroplated in the presence of various concentration of thiourea showing the binding state of (a) Cu and (b) S. Deposition was performed at −400 mV for 400 s.
Fig. 8 shows the S content in the electroplated copper films according to the concentration of TU. As the TU concentration
increased, the S concentration in the deposits also increased. The increased S content was thought to be responsible for the linear
Fig. 7. AES depth profile of copper film electroplated in the presence of 0.083 g/ L thiourea.
Fig. 8. Sulfur impurities incorporated in the electroplated copper films according to thiourea concentration.
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increase in the resistivity of the deposits. Even after an optimized annealing process of the electroplated copper films [10], the resistivity was reduced only to 6 μΩ cm since the grain growth of the deposits in the presence of TU did not occur. 4. Conclusions TU was found to be an efficient brightener of copper in electrodeposition, leading to high-density, fine-grained, cathodic deposits. Based on XPS, AES, and electrochemical analyses, TU was found to generate CuS by reacting with copper ions, which subsequently blocked the working electrode's surface and reduced its effective surface area. The mean grain sizes of the deposits decreased with increasing TU concentrations, which indicated a strong inhibition of the grain growth by the generated CuS. The CuS seemed to cover the entire 1 cm2 copper seed surface above a TU concentration of 0.017 g/L. CuS is believed to bind strongly to the copper surface and subsequently become entrapped in the deposits during the formation of each monolayer. The increased S contaminants due to entrapped CuS are responsible for the increase in resistivity of the deposits. But, since this contamination problem can be overcome using a process reported by Kang et al. [13], TU is considered as an effective brightener. Acknowledgements This work was supported by KOSEF through the Research Center for Energy Conversion and Storage (RCECS), and by the Institute of Chemical Processes (ICP) and ISRC (Interuniversity Semiconductor Research Center) in Seoul National University.
References [1] P.C. Andricacos, C. Uzoh, J.O. Dukovic, J. Horkans, H. Deligianni, IBM J. Res. Develop. 42 (1998) 567. [2] R.J. Contolini, L. Tarte, R.T. Graff, L.B. Evans, J.N. Cox, M.R. Puich, J.D. Gee, X.C. Mu, C. Chiang, Twelfth International VLSI Multilevel Interconnection Conference, Santa Clara, CA, USA, June 27–29, 1995, Proceedings VMIC, 1995, p. 322. [3] S.P. Muraka, S.W. Hymes, Crit. Rev. Solid State Mater. Sci. 20 (1995) 87. [4] J.J. Kelly, A.C. West, J. Electrochem. Soc. 145 (1998) 3472. [5] J.J. Kelly, A.C. West, J. Electrochem. Soc. 145 (1998) 3477. [6] J.P. Healy, D. Pletcher, J. Electroanal. Chem. 338 (1992) 155. [7] L. Muresan, S. Varvara, G. Maurin, S. Dorneanu, Hydrometallurgy 54 (2000) 161. [8] B. Ke, J.J. Hoekstra, B.C. Sison, D. Trivich, J. Electrochem. Soc. 106 (1959) 382. [9] J.K. Prall, L.L. Shreir, Trans. Inst. Met. Finish. 41 (1964) 29. [10] J.J. Kim, S.-K. Kim, J.U. Bae, Thin Solid Films 415 (2002) 101. [11] D. Josell, D. Wheeler, W.H. Huber, T.P. Moffat, Phys. Rev. Lett. 87 (2001) 016102. [12] T.P. Moffat, J.E. Bonevich, W.H. Huber, A. Stanishevsky, D.R. Kelly, G.R. Stafford, D. Josell, J. Electrochem. Soc. 147 (2000) 4524. [13] M.S. Kang, S.-K. Kim, J.J. Kim, Jpn. J. Appl. Phys. 44 (2005) 8107. [14] N.D. Nikolic, G. Novakovic, Z. Rakocevic, D.R. Durovic, K.I. Popov, Surf. Coat. Technol. 298 (2002) 188. [15] Farid Hanna, Z. Abdel Hamid, A. Abdel Aal, Mater. Lett. 58 (2003) 104. [16] L.M. Gassa, J.N. Lambi, A.E. Bolzan, A.J. Arvia, J. Electroanal. Chem. 527 (2002) 71. [17] A.E. Bolzan, R.C.V. Piatti, A.J. Arvia, J. Electroanal. Chem. 552 (2003) 19. [18] A.E. Bolzan, A.S.M.A. Hasseb, P.L. Schilardi, R.C.V. Piatti, R.C. Salvarezza, A.J. Arvia, J. Electroanal. Chem. 500 (2001) 533. [19] S.N. Port, S. Cere, D.J. Schiffrin, J. Electroanal. Chem. 432 (1997) 215. [20] J.J. Kim, S.-K. Kim, Appl. Surf. Sci. 183 (2001) 311. [21] D.R. Turner, G.R. Johnson, J. Electrochem. Soc. 109 (1962) 798. [22] E.E. Farndon, F.C. Walsh, S.A. Campbell, J. Appl. Electrochem. 25 (1995) 574.
Thin Solid Films 516 (2008) 3761 – 3766 www.elsevier.com/locate/tsf
The influence of thiourea on copper electrodeposition: Adsorbate identification and effect on electrochemical nucleation Moo Seong Kang a , Soo-Kil Kim b , Keeho Kim c , Jae Jeong Kim a,⁎ a
Research Center for Energy Conversion and Storage, School of Chemical and Biological Engineering, College of Engineering, Seoul National University, Seoul, 151-742, Korea b Fuel Cell Research Center, Korea Institute of Science and Technology (KIST), Seoul 136-791, Korea c Department of Advanced Nano-tech Development, DongbuAnam Semiconductor, 474-1 Sangwoo-ri, Kamgok-myun, Umsung-kun, Chungbuk, 369-852, Korea Received 25 February 2006; received in revised form 18 April 2007; accepted 9 June 2007 Available online 15 June 2007
Abstract The effect of thiourea on copper deposition onto a copper seed layer from an electrolyte composed of CuSO4, H2SO4, deionized water, and thiourea was investigated. Even in the presence of very low concentrations of thiourea, extremely smooth and bright copper deposits were obtained. From the results of X-ray photoelectron spectroscopy, Auger electron spectroscopy, and electrochemical analyses, thiourea was found to react with copper or copper ions leading to the generation of CuS. CuS adsorption onto the copper seed layer seemed to inhibit the initial nucleation of the copper adions, resulting in the formation of smaller Cu grains compared to those forming in the absence of thiourea. CuS was observed to cover all active sites of the 1 cm2 copper seed layer above 0.017 g/L thiourea. The surface roughness as well as the mean grain size of the deposits also approached minimum values above this thiourea concentration. Adsorbed CuS was incorporated into the deposits during electroplating, which was believed to be the major factor for the increased resistivity of the deposits. © 2007 Elsevier B.V. All rights reserved. Keywords: Copper; Electrodeposition; Thiourea; Brightener; Copper sulfide; Nucleation
1. Introduction Copper electroplating has been viewed as a powerful deposition process in ultra large-scale integration (ULSI) interconnection technology [1–3]. It has recently attracted more attention for its use in controlling the film properties of electroplated copper, i.e. surface morphology, resistivity, and crystal orientation. This has been accomplished by using various organic additives such as polyethylene glycol and chloride ions [4–6] as co-suppressors, thiourea (TU) [7,8] and benzotriazole [9,10] as brighteners, and mercapto compounds [11,12] as accelerators. Among them, the control of surface roughness through the addition of TU [13] or other brighteners is one of the key methods used to fill the vias or trenches without voids, since increasing the roughness of the bottom and side walls of the via during electroplating can cause voids or key ⁎ Corresponding author. E-mail address: [email protected] (J.J. Kim). 0040-6090/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2007.06.069
holes. In 0.18 μm complementary metal–oxide semiconductor logic devices, the minimum metal width may be around 0.20 μm. The width will be filled with a barrier metal, seed copper layer, and electroplated copper. Even in a bottom-up process, an electroplated copper roughness of less than 5 nm is desired to fill the gap with an aspect ratio of 3 to 4. Nikolic et al. [14] used TU as a brightener to make a high degree of mirror reflection in copper and zinc coatings. Effects of various additives including TU on electroless copper plating were investigated by Hanna et al. [15]. Gassa et al. [16] reported TU electro-oxidation phenomena on copper electrodes by means of electrochemical impedance spectroscopy. Bolzan et al. [17,18] investigated the complexing mechanism of Au–TU and Cu–TU complex with various potential ranges. Port et al. [19] tried to characterize ligand formation using IR spectroscopy, cyclic voltammetry and rotating disk electrode techniques. In spite of these efforts to identify effect of TU on electrochemistry, more researches should be done in order to verify the possibility of applying TU to ULSI metallization.
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In this work we identified the exact composition of the adsorbates generated from TU by electrochemical and spectroscopic analysis techniques and examined the possibility for applications in the fabrication of semiconductor interconnections by presenting its brightening effect on electroplated copper. 2. Experimental details The seed layer was prepared by 70-nm-thick physical vapor deposition Cu film on p-type (100) oriented silicon wafers with a 40-nm-thick metal organic chemical vapor deposition TiN barrier layer film. Samples (1.5 cm × 1.5 cm) were made by cutting that wafers. Surface pretreatment by immersion into a 1:200 diluted NH4OH : deionized (DI) water solution was performed for 30 s to remove the surface copper oxide on the seed layer. After oxide removal, each sample was rinsed carefully with DI water and dried under a continuous stream of N2. This pretreatment process was found to be quite effective in removing the native copper oxide and maintaining an electrically uniform seed layer [20]. After the pretreatment steps, each sample was placed in the electrodeposition holder which made electric contact between the potentiostat and the sample. When a sample was in the holder, the surface area of samples exposed to the solution was 1 cm2. The standard electrolyte was composed of 1 M H2SO4, 0.05 M CuSO4, and DI water. The constant potential for Cu electroplating was applied by a PAR 263 potentiostat (EG&G Princeton Applied Research Corporation) with respect to a saturated calomel electrode (SCE) at room temperature. We used an electronic-grade Cu wire (1 mm diameter, 3 cm in length) as an anode for copper deposition. All samples were rinsed with DI water and dried in a stream of N2 after electroplating. Annealing was performed at 400 °C for 30 min in a N2 atmosphere using horizontal quartz tube furnace.
Field emission scanning electron microscopy (FESEM, Philips XL-30) and atomic force microscopy (AFM, Digital Instruments Dimension™ 3100) were used to study the surface morphology and roughness. The operating voltage of the FESEM was 25 kV. The AFM operating mode was contact mode and the end of the tip used in the AFM was composed of Si3N4. Cyclic voltammetry and electrochemical impedance analyses using a PAR 5210 Lock-in amplifier (EG&G Princeton Applied Research Corporation) were conducted to verify the adsorption of TU-induced species and determine their effect on cathodic deposition. The frequency range and the AC amplitude of electrochemical impedance analysis were 10 kHz ∼ 0.01 Hz and 5 mV, respectively. In order to identify the adsorbates on the cathodic surface through an examination of the binding state of Cu–S and the stoichiometry among S, C, and N impurities of the deposits, X-ray photoelectron spectroscopy (XPS, Kratos Model AXIS-HS) and Auger electron spectroscopy (AES, Perkin-Elmer Model 660) were applied. Al Kα (1486.6 eV) was used as the X-ray source of the XPS and peak positions were calibrated with respect to C 1s peak at 284.6 eV. In the AES analysis, the accelerating voltage of the electron beam was 10 kV and the sensitivity factors of Cu, S, Ti, N, O and C were 0.307, 1.041, 0.438, 0.930, 0.296 and 0.246, respectively. 3. Results and discussion Fig. 1 shows the dependence of the current–voltage (I–V) plots on the amount of TU in the electrolyte. Even a very small amount of TU affected the I–V plot. The reduced current density and the shift of the cathodic reduction peak in a more cathodic direction were attributed to the generation of new species from the reaction of TU with copper ions that inhibited the cathodic consumption of copper ions.
Fig. 1. Cyclic voltammetry of copper electroplating at various concentration of thiourea.
M.S. Kang et al. / Thin Solid Films 516 (2008) 3761–3766
Nyquist plots without TU are presented in Fig. 2(a) with varying cathodic potential. The curves that corresponded to − 100 mV and −200 mV showed two partially combined semicircles. The diameter ratio of the two semicircles can be used to identify the rate limiting step of copper electrodeposition. As the deposition potential increased from −100 mV to − 200 mV, the first semicircle shrank while the second expanded. The diameter ratio of the first semicircle to the second changed from less than 1 to greater than 1. This indicated that as the deposition potential increased, the deposition process moved from charge transfer limiting conditions to mass transfer limiting conditions. At − 400 mV, the deposition process was almost entirely mass transfer limiting, and the first semicircle was buried within the second one. Since TU was thought to have a much greater effect on the surface charge transfer than mass transfer, an impedance analysis was performed to reveal the effect of TU on surface passivation related to charge transfer resistance. The impedance analysis is shown in Fig. 2 at concentrations of TU ranging from 0.002 to 0.170 g/L and a potential of − 200 mV where the first semicircle was clearly distinguished. The charge transfer resistances increased with the amount of TU, implying that the TU complex blocked the active sites of the working elec-
Fig. 2. Nyquist plot of copper electroplating at (a) different cathodic potential and (b) various concentration of thiourea. Deposition potential of (b) is − 200 mV vs. SCE.
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trode. Above a TU concentration of 0.017 g/L, the charge transfer resistances had almost the same value, which indicated that TU complex had adsorbed to cover the entire surface area of the copper seed and the inhibition and deposition saturated. Due to the increased first semicircles, the second ones were partially superimposed on the first semicircles and expressed as straight lines. In consideration of the practical applications of copper electroplating in semiconductor interconnection, copper was electroplated at − 400 mV at a deposition rate of 100 nm/min and surface-analyzed. Fig. 3 shows the FESEM images of the electroplated copper films of about 400 nm in thicknesses deposited with varying concentration of TU. When compared with TU-free deposits, the number density of the grains was much higher as a result of TU addition due to the inhibition of the lateral growth of the grain and formation of larger grains. For example, the grain density of deposits made with 0.083 g/L added TU as found in AFM images was 310/μm2 while that of TU-free deposits was 44/μm2. The inhibition of crystal growth resulted in finer grained deposits, which are the key properties for yielding a smooth surface of electroplated copper. The relation between TU concentration and surface root mean square (RMS) roughness was characterized using AFM in Fig. 4. For all species, the deposition thickness was set as 400 nm to eliminate any thickness effects on surface roughness. The abrupt change of RMS roughness above 0.05 g/L in Fig. 4 presents the dramatic effect of TU on brightening of electroplated copper films. When the concentration of TU increased above 0.05 g/L which corresponds to 6.6 × 10− 4 M, the surface roughness was converged to about 2 nm. The TU concentration at which the sudden change in mean grain size was observed also corresponded to 0.05 g/L as shown in Fig. 5. The mean grain sizes, calculated from AFM images decreased with increasing TU and the mean grain size of 0.17 g/L TU was about 342 nm2, which indicated a strong inhibition of grain growth by the TU. To identify the exact composition of the adsorbates, XPS and AES analyses were performed; the results are shown in Figs. 6 and 7. The 2p3/2 and 2p1/2 spectra of surface copper (Fig. 6(a)) were at 932.6 eV and 952.4 eV. Since the 2p3/2 binding energy of pure Cu, Cu2S, and CuS partially overlap in the range of 932.5 and 932.8 eV, the 2p3/2 and 2p1/2 spectra of copper did not provide any further information. On the other hand, spectra relating to the 2p3/2 and 2p1/2 of S in the deposited copper surface showed a dependency on the TU concentration as given in Fig. 6(b). Through the addition of TU, the S 2p3/2 peak appearing at 162.4 eV and the shoulder shown at 163.6 eV corresponded to S 2p1/2 peak, which were smaller than 164.0 eV for 2p3/2 and 165.18 eV for 2p1/2 in the case of pure S. Since there are only two S-containing groups that have S 2p3/2 binding energy below 163 eV, sulfide and thiophene, it may be concluded that CuS was generated from the reaction of TU with the copper ions. Fig. 7 presents the AES depth profile of S, N, and C in an electroplated copper film of 400 nm thickness deposited with 0.083 g/L TU. More precise information about the TU-induced adsorbed species can be acquired from Fig. 7. It was seen that
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Fig. 3. FESEM and AFM images of electroplated copper films at various concentrations of thiourea: (a) 0.00 g/L, (b) 0.083 g/L, and (c) 0.17 g/L.
only S was incorporated in the Cu deposits and that the contents of C and N were negligible. The average S content was about 2.07% calculated by averaging the S curve of the AES depth profile from sputter time 0 min to 9 min. S-only incorporation indicated that the adsorbate is not TU itself but the copper sulfide. Copper sulfide is the very species that inhibits surface diffusion of copper adions, which leads to smooth and bright copper deposits. Another interesting point is that copper sulfide had a uniform depth profile from the surface of the copper seed
layer to that of the electrodeposits. This implies that copper sulfides were strongly bound to the copper surface and subsequently entrapped in the deposits during each monolayer formation. The identification of CuS through XPS and AES analyses strongly supported the production of CuS through the reaction of copper ion with TU [21,22], though the exact reaction mechanism remains unclear. The resistivity of the electroplated copper film increased linearly from 2 to 16.5 μΩ cm as the TU concentration increased.
Fig. 4. Surface RMS roughness dependence on the thiourea concentration for electroplated copper.
Fig. 5. Mean grain sizes of electroplated copper with various concentrations of thiourea.
M.S. Kang et al. / Thin Solid Films 516 (2008) 3761–3766
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Fig. 6. XPS spectrum of the copper surface electroplated in the presence of various concentration of thiourea showing the binding state of (a) Cu and (b) S. Deposition was performed at −400 mV for 400 s.
Fig. 8 shows the S content in the electroplated copper films according to the concentration of TU. As the TU concentration
increased, the S concentration in the deposits also increased. The increased S content was thought to be responsible for the linear
Fig. 7. AES depth profile of copper film electroplated in the presence of 0.083 g/ L thiourea.
Fig. 8. Sulfur impurities incorporated in the electroplated copper films according to thiourea concentration.
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increase in the resistivity of the deposits. Even after an optimized annealing process of the electroplated copper films [10], the resistivity was reduced only to 6 μΩ cm since the grain growth of the deposits in the presence of TU did not occur. 4. Conclusions TU was found to be an efficient brightener of copper in electrodeposition, leading to high-density, fine-grained, cathodic deposits. Based on XPS, AES, and electrochemical analyses, TU was found to generate CuS by reacting with copper ions, which subsequently blocked the working electrode's surface and reduced its effective surface area. The mean grain sizes of the deposits decreased with increasing TU concentrations, which indicated a strong inhibition of the grain growth by the generated CuS. The CuS seemed to cover the entire 1 cm2 copper seed surface above a TU concentration of 0.017 g/L. CuS is believed to bind strongly to the copper surface and subsequently become entrapped in the deposits during the formation of each monolayer. The increased S contaminants due to entrapped CuS are responsible for the increase in resistivity of the deposits. But, since this contamination problem can be overcome using a process reported by Kang et al. [13], TU is considered as an effective brightener. Acknowledgements This work was supported by KOSEF through the Research Center for Energy Conversion and Storage (RCECS), and by the Institute of Chemical Processes (ICP) and ISRC (Interuniversity Semiconductor Research Center) in Seoul National University.
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