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Electrochemical behavior of copper and cobalt in post-etch cleaning solutions
Microelectronic Engineering 86 (2009) 2038–2044
Contents lists available at ScienceDirect
Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee
Electrochemical behavior of copper and cobalt in post-etch cleaning solutions S. Bilouk a,b,*, L. Broussous a, R.P. Nogueira b, V. Ivanova c, C. Pernel c a b c
ST Microelectronics, 850 rue Jean Monnet, 38926 Crolles Cedex, France UMR 5266 et 5631 CNRS Grenoble-INP, SIMAP/LEPMI, 1130 rue de la piscine, BP 75, 38402 St. Martin d’Hères Cedex, France CEA, LETI, MINATEC, F38054 Grenoble, France
a r t i c l e
i n f o
Article history: Received 14 October 2008 Accepted 16 January 2009 Available online 24 January 2009 Keywords: Self-aligned barrier Corrosion Inhibitor Benzotriazole
a b s t r a c t Cleaning is one of the key steps of the integration of self aligned barriers (SAB) in microelectronic devices for 32 nm technology and below. It is hence important to investigate the impact of different cleaning solutions on the metallic components of SAB, mainly in which concerns their surface stability. In this sense, the electrochemical behavior of copper and cobalt was studied under potentiodynamic conditions in different aqueous solutions of glycolic acid (GA) with and without benzotriazole (BTA) inhibitor. It has been observed that the presence of glycolic acid induces a monotonic increase of copper corrosion and a slight decrease in the case of cobalt. The cobalt dissolution remains nonetheless very active and is shown to be governed by oxygen reduction reaction. The addition of BTA, a well-known corrosion inhibitor for copper has shown to be also effective in the case of Co surfaces, with a ca 15-fold reduction of the intrinsic Co corrosion current density. The possibility of galvanic coupling between both metals, supposed to enhance the Co dissolution, has also been qualitatively investigated and seems not to be a determinant factor in these conditions. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction The copper damascene process is widely established and has brought higher performance to semiconductor devices [1]. However, due to continuously shrinking design rules and hence to increasing current densities, copper electromigration remains an important challenge. To overcome this issue, the integration of CoWP or CoWB self aligned barriers (SAB) for 32 nm technology was proposed [2–5]. The integration of these new materials implies a compatibility study with the various conventional dry and wet treatments used for the patterning of interconnection levels. Particularly, the postvia etch cleaning can strongly impact the integrity of the SAB due to corrosion. In the ideal integration scheme, the SAB is etched in order to minimize the resistance of vias [6]. In this configuration, the copper at via bottom is in contact with the SAB and both are exposed to the cleaning solution (Fig. 1a). This may lead to SAB dissolution not only by spontaneous corrosion processes, but also those induced by galvanic coupling as schematically illustrated in Fig. 1b. Indeed, the simultaneous presence of dissimilar metallic components exposed to an ionic conductive medium can entail galvanic corrosion processes yielding accelerated damage of the less noble of the metallic phases during * Corresponding author. Address: ST Microelectronics, 850 rue Jean Monnet, 38926 Crolles Cedex, France. Tel.: +33 (0)4 38 78 19 43; fax: +33 (0)4 38 78 30 34. E-mail address: [email protected] (S. Bilouk). 0167-9317/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2009.01.035
the post-via etch clean step [7]. The standard potentials of the Cu/ Cu2+ and Co/Co2+ couples are ECu=Cu2þ ¼ +0.34 V/ENH, and ECo=Co2þ ¼ 0.28 V/ENH, respectively. Thus cobalt is expected of undergoing galvanic corrosion when is in contact with copper, enhancing its selective dissolution as it clearly appears in Fig. 1c. On this TEM picture a total withdrawal of Co-alloy after the via opening and the cleaning step is evidenced. The use of organic inhibitors is one of the most widespread approaches to prevent the generalized corrosion of metals occurring in a variety of aqueous solutions [8,9]. Recent studies demonstrated that the corrosion rate of cobalt in a borate buffer solution and in presence of copper ions ðCu2þ Þ is significantly reduced thanks to the formation of a protective Cu(I)–benzotriazole (Cu(I)–BTA) film on the cobalt surface [10]. The 1 H-benzotriazole (1H-BTA), is a well-known and widely used inhibitor of copper corrosion over a wide pH and temperature range [11–14]. It was proposed that benzotriazole forms an insoluble polymeric ½Cuþ BTA n surface film and that the presence of a Cu2 O layer on the copper surface facilitates the formation of this complex [11,15–18]. Schnyder et al. [19] have also shown the corrosion inhibition of WC-Co alloy by BTA in a borate buffer solution by polarization experiments. This indicates that the use of BTA during the post-via etch clean can be considered as a potentially interesting way to slow down the cobalt dissolution through both spontaneous corrosion and galvanic coupling, but to our knowledge, no systematic study has been devoted to the BTA inhibiting effect concerning Co corrosion in acidic media.
S. Bilouk et al. / Microelectronic Engineering 86 (2009) 2038–2044
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Fig. 1. (a) Self-aligned barrier (SAB) configuration in the integration schema; (b) SAB dissolution during the post-via etch clean process; (c) SEM micrograph of opened via where no SAB remains.
It is obvious, however, that any attempt to slow down or prevent Co corrosion should not be deleterious to the Cu lines. In this sense, it is important to also study the electrochemical behavior of copper in organic acid and in a mixture of organic acid and BTA so as to understand the possible impact of BTA during the cleaning step before introducing it for cobalt protection. Indeed, solutions of diluted organic acids are widely used in microelectronic in the following steps: Chemical and Mechanical Polishing (CMP), preclean of copper surface prior to electroless deposition process and post-etch clean [20,21]. Among the various organic acid that may be suitable for post-etch clean, glycolic acid was selected for its ability to remove CuOx without damaging the copper line beneath [22,23]. In addition, its high solubility in water (up to 70%) facilates its uses in industrial equipments for wafer cleaning. In the present study, the electrochemical behavior of copper and cobalt in post-etch cleaning solutions containing glycolic acid is investigated. The effect of BTA addition to cleaning solutions is also discussed. The electrochemical behavior of copper and cobalt in these solutions is studied by polarization curves which have been also used to assess theoretically the galvanic coupling in the cobalt dissolution. 2. Experimental A three-electrode cell (volume 100 mL) was used with a largearea Pt foil as counter electrode and an Ag/AgCl reference electrode. The working electrodes were made of copper or cobalt cylindrical rods (99.995% and 99.990% respectively) mounted on a Radiometer Analytical CTV 101 rotating disk electrode (RDE). The active area was 0.28 cm2 for copper and 0.196 cm2 for cobalt. The electrode surface was successively polished with 800 and 1200 grit emery papers and with 6.0 to 0.1 lm alumina suspensions. They were then rinsed with deionized water and dried with nitrogen before each experiment. All experiments were performed at room temperature; polarization curves in the potential range ±1 V/OCP with a scan rate of 1 mV s1 were performed using a potentiostat PARSTAT 2273 from Princeton Applied Research with the help of the POWERSUIT software. All measurements were carried out at 1000 rpm. Aqueous solutions were prepared with deionized water. Diluted glycolic acid (GA) solutions with various concentrations: 1%, 2%, 3%
and 4% in weight were used. Diluted BTA solution (0.1% wt) and mixtures of diluted glycolic acid and BTA were also investigated. Since the pH of the glycolic acid solutions is around 2 for all studied concentrations (for example 2.4 for 1% wt and 2.0 for 4% wt), it was interesting to compare the behavior of copper and cobalt in a glycolic acid and in a blank solution (with the same pH = 2) consisting in diluted sulfuric acid. 3. Results and discussion 3.1. Electrochemical behavior of copper and cobalt 3.1.1. Glycolic acid effect The influence of glycolic acid on the electrochemical behavior of copper and cobalt was investigated by electrochemical methods. It was compared to a blank electrolyte with the same pH (pH = 2) to straightforwardly evaluate the contribution coming only from the glycolic acid itself and not from the pH effect. Polarization curves performed after 2 h of immersion in either blank solution or in solutions with different concentrations of glycolic acid from 2% to 4% wt are depicted in Fig. 2A and B for Cu and Co, respectively. For the sake of readability and since glycolic acid induces a mostly monotonic evolution in both cases, the whole ensemble of curves is not presented in the figures. The main parameters issued from these experiments are summarized in Tables 1 and 2 for Cu and Co, respectively. The macroscopic stationary electrochemical behavior of both metals appears to be highly dependent on the acid concentration and it is clearly seen that glycolic acid modifies the oxidation kinetics. In the presence of glycolic acid, the corrosion current of copper (4–12 lA cm2) is higher than in a blank solution at pH = 2 (<2 lA cm2). In addition, it is worth to notice that the corrosion current increases with the acid concentration. The glycolic acid enhances both the cathodic and anodic reactions which results in an increase of the corrosion rate. The pH slightly decreases with [GA] which can enhance proton reduction on copper surface in addition to the dissolved oxygen reduction. The higher aggressiveness is nonetheless not detrimental: according to Faraday’s law, and supposing uniform corrosive attack along the metallic surface, for a 60s exposure to the higher 4% glycolic acid, the measured corrosion current density (12 lA cm2) corresponds to a copper withdrawal of only 2.7 Å.
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S. Bilouk et al. / Microelectronic Engineering 86 (2009) 2038–2044 Table 2 Relevant Co corrosion parameters issued from the polarization curves presented in Figs. 2b and 6b. Solution
pH
Ecorr (V vs Ag/AgCl)
Icorr (lA cm2)
Etch rate (Å min1)
Blank pH 2 GA 1% GA 2% GA 3% GA 4% GA 1%/BTA 0.1% BTA 0.1%
2 2.42 2.28 2.18 2.13 2.5 5.3
0.424 0.270 0.280 0.286 0.288 0.201 0.190
750 380 320 280 220 24 0.6
155 79 66 58 45 5 0.12
dissolution of the cobalt-based self aligned barriers for 32 nm node applications. The anodic behavior of both Cu and Co in these solutions can be tentatively interpreted with the help of thermodynamic data. Glycolic acid can exist in aqueous solutions in two different forms, namely H4C2O3 (HL) and H3 C2 O 3 (L ). The equilibrium between these species is described by:
H4 C2 O3 $ H3 C2 O3 þ Hþ
pKa ¼ 3:8
ð1Þ
which means that H4 C2 O3 predominates at pH values below 3.8 (pKa), while H3 C2 O 3 is prevailing at pH values above 3.8. In addition, glycolic acid may form two soluble complexes with cupric
Fig. 2. Potentiodynamic polarization curves in blank solution (h), in glycolic acid 2% (d) and 4% (D) for (A) copper and (B) cobalt: the curve in continuous line was obtained in deaerated GA electrolyte. Scan rate is 1 mV s1 and X = 1000 rpm.
This characteristic satisfies one of main post-etch cleaning specifications i.e. an etch rate of Cu lower than 10 Å min1 and is in good agreement with industrial process requirements. These results hence confirm that glycolic acid ensures the copper surface etching and cleaning while keeping the bulk corrosion negligible. In the case of Co, the corrosion current values are much higher than those for the Cu in both blank and organic acid containing solutions. Even if the corrosion current density seems to decrease with the concentration of glycolic acid, values remain however high enough to induce a catastrophic dissolution of the cobalt in presence of glycolic acid as well as in the blank solution. In fact, the etch rate varies from 79 Å min1 for a 1% concentration to 45 Å min1 for a 4% concentration which can lead to a pronounced Table 1 Relevant Cu corrosion parameters issued from the polarization curves presented in Figs. 2a and 6a. Solution
pH
Ecorr (V vs Ag/AgCl)
Icorr (lA cm2)
Etch rate (Å min1)
Blank pH 2 GA 1% GA 2% GA 3% GA 4% GA 1%/BTA 0.1% BTA 0.1%
2 2.42 2.28 2.18 2.13 2.5 5.3
0.090 0.120 0.126 0.130 0.134 0.150 0.240
1.6 4 7 10 12 2.5 0.5
0.3 0.8 1.2 1.7 2.7 0.4 0.1
Fig. 3. (a) Potential–pH diagram at 25 °C and 1 atm for the system copper–glycolic acid–water: [Cu2+] = 102 M, glycolic acid – 1%; (b) speciation diagram for the system copper–glycolic acid–water.
S. Bilouk et al. / Microelectronic Engineering 86 (2009) 2038–2044
ions. ½CuH3 C2 O3 þ and CuðH4 C 2 O3 Þ2 are the aqueous copper glycolic acid complexes which are denoted as CuLþ and CuL2 in this paper. According to the potential–pH diagram for the copper–glycolic acid–water system plotted with the help of JSCHESS software [24] and depicted in Fig. 3a, the CuLþ is clearly seen to be the predominant complex present at pH = 2.4 (1% glycolic acid in the electrolyte), which is also confirmed by the speciation diagram built for the system Cu – GA – H2O, Fig. 3b. The presence of these complexes can explain the chelating effect of glycolic acid on copper surface, allowing a slight etch of the metal surface without damaging the metal integrity. As well as in the case of copper, cobalt ions can be also chelated by glycolic acid giving the following two complexes: ½CoH3 C2 O3 þ and CoðH4 C2 O3 Þ2 , respectively denoted as CoLþ and CoL2 . The potential–pH for a 1% GA concentration and the speciation diagrams for the cobalt–glycolic acid–water system are given in Fig. 4a and b. Unlikely to Cu, here CoLþ seems not to be the predominant species at pH = 2.4 and in the cobalt corrosion potential range. Thus, CoLþ can be supposed to be an intermediate component which could enhance cobalt dissolution according to the following steps:
Co þ HL $ ðCoLÞads þ Hþ þ e
ð2Þ
ðCoLÞads $ ðCoLÞþsol þ e
ð3Þ
ðCoLÞþsol þ Hþ ! Co2þ sol þ HL
ð4Þ
Fig. 4. (a) Potential–pH diagram at 25 °C and 1 atm for the system cobalt–glycolic acid–water: [Co2+] = 102 M, glycolic acid – 1%; (b) speciation diagram for the system cobalt–glycolic acid–water.
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This reactive scheme, however, does not account for the slight but monotonic reduction of the cobalt corrosion current density shown in Fig. 2B and Table 2 as the GA concentration increases. This apparently contradictory behavior must be ascribed to the cathodic kinetics of Co dissolution as discussed in which follows. In which concerns the cathodic mechanisms, Fig. 2 indicates that the hydrogen reduction is an important component of the cathodic reaction (as should be expected for these acidic medium) since the cathodic current intensity monotonically increases with the acid concentration at higher polarization levels for both Cu and Co. Nevertheless, the concomitant reduction of dissolved oxygen should be also observed at lower polarization levels mainly in the case of Co. Indeed, Fig. 2B shows that in 1% deaerated glycolic acid solution, the corrosion current of cobalt is significantly reduced (3.4 lA cm2 instead of more than 200 lA cm2 – see Table 2) which means that cobalt corrosion in these conditions is a strongly cathodic controlled process. Fig. 5 shows the details of the polarization curves depicted in Fig. 2B in the cathodic region close to the corrosion potential. It clearly appears that before reaching polarization levels high enough for the proton reduction to be dominant, the increase in the GA concentration hinders the cathodic kinetic which is probably related to those decreasing corrosion current densities mentioned before. 3.1.2. BTA effect Fig. 6A presents the polarization curves of copper after 2 hour immersion in a glycolic acid solution 1%, in a mixture of glycolic acid (1%) and BTA (0.1%) or in only 0.1%wt BTA. In diluted solution of BTA 0.1%, the pH of the solution is 5.3 while it is 2.4 in diluted glycolic acid (1%). When only BTA is present, the anodic current is strongly lowered down reflecting the copper dissolution limitation. This is probably related to the formation of a Cu(I)–BTA film which seems to lose its protective nature at more positive potentials (ca +300 mV), beyond which a steep current increase indicates metal dissolution. The corrosion current density is 0.5 lA cm2 i.e eight times lower than in 1% GA solution. When the BTA solution is added to the GA, the pH of the resulting electrolyte decreases to 2.50. In such mixture, an intermediate electrochemical behavior of copper is observed. The anodic inhibition effect of BTA is still clearly present which confirms its action despite the presence of another reactant. However, the inhibition range now is shorter than in presence of BTA alone, which indicates a lower stability of the protective film in this media. In addi-
Fig. 5. Cathodic potentiodynamic polarization curves for Co in glycolic acid at 1% (h), 2% (d), 3% (D) and 4% (r) at the vicinity of the corrosion potential; data for the 2% and 4% solutions are the same than in Fig. 2b.
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S. Bilouk et al. / Microelectronic Engineering 86 (2009) 2038–2044
Fig. 6. Potentiodynamic polarization curves in glycolic acid 1% (h), in 0.1% BTA (d) and in the mixture of 1% glycolic acid and 0.1% BTA (D) for (A) copper and (B) cobalt. Scan rate is 1 mV s1 and X = 1000 rpm.
tion, the value of the current density is 2.5 lA cm2 i.e. five times higher than in the BTA solution. At a first glance, it can be supposed that this decrease of BTA inhibiting action is due to the pH decrease. However, this value (in GA/BTA) is also higher than the value obtained in the blank solution (pH = 2). This means that the observed decrease of BTA inhibiting efficiency should not be ascribed only to the pH change (a decrease from 5.3 to 2.5) but also to GA
etching action. Indeed, glycolic acid cleans the metal surface by smoothly etching it. Thus, it can be supposed that in this case the [Cu(I)BTA] protecting complex is probably partially removed from the metal surface. In this way the conjugated actions of BTA and glycolic acid appear: the Cu surface is less protected than in diluted BTA solution and is slightly etched by the glycolic acid. The same experiments were carried out to determinate the influence of BTA (Fig. 6B) in the cleaning solution on the electrochemical behavior of cobalt. It should be noticed that in the presence of 0.1% BTA, the corrosion current density is much lower (0.6 lA cm2) than in glycolic acid solution 260 lA cm2. This observation allows to consider the BTA as a corrosion inhibitor for Co as well as it has been shown above for Cu. Here, similar to copper, the inhibiting effect of BTA on cobalt still occurs in presence of GA in acidic conditions, although in a less effective way regarding the action of BTA only. Anyway, the corrosion current density is ten times lower than in the GA solution (24 lA cm2 and 260 lA cm2, respectively). It is interesting to notice that the inhibition mechanisms are different for Cu and Co. In fact, unlikely to copper, BTA, when mixed with GA, induces no significant anodic slow down effect in the polarization curves of cobalt. The collapse of the corrosion current in that case is due to a strong decrease in the cathodic activity as seen in Fig. 6B. BTA can hence be supposed to be a cathodic inhibitor of cobalt. Its efficiency is as high as the corrosion mechanism of Co seems to be cathodically controlled as it appeared from the deaerated solution results discussed before (see Fig. 2B). In this sense, a possible mechanism for the BTA inhibition effect is schematically illustrated in Fig. 7: BTA adsorbs on cathodic sites on cobalt surface and inhibits the oxygen reduction hence significantly slowing down cobalt dissolution. It must be noticed however that the corrosion current in the deaerated solution (see Fig. 2B) is still lower than that obtained for the mixed GA – BTA solution (Fig. 6B). This indicates that the blocking effect of BTA should not be absolute. If all cathodic sites were actually blocked by BTA one should expect a corrosion current density close to that in deaerated solution. 3.2. Galvanic coupling In the integration scheme shown in Fig. 1, cobalt and copper are in direct contact in the vias, both physically and by the conductive solutions used during the post-etch clean. A galvanic coupling between these two metals can thus enhance the dissolution of the cobalt self aligned barriers. The quantitative assessment of the actual coupling current is not an easy task. Straightforward measurements can be achieved with the help of zero resistance ammeters. It must be stressed
Fig. 7. Schematic illustration of oxygen reduction on cobalt surface (a) in glycolic acid solution (b) in mixture of glycolic acid and benzotriazole.
S. Bilouk et al. / Microelectronic Engineering 86 (2009) 2038–2044
however that, as it has already been shown [25,26], accurate measurements are dependent on the previous or concomitant knowledge of the relative impedances of the metallic surfaces getting in contact. This quantitative analysis is beyond the aims of the present paper and is matter of ongoing work. It is however interesting, thanks to the polarization curves depicted in Figs. 2 and 6, to proceed to a qualitative analysis of the coupled behavior of Co and Cu submitted to these aggressive environments with or without the presence of BTA as corrosion inhibitor. The central idea is to verify in which extent the Co (lower standard redox potential) surface is prone to be anodically over polarized by the Cu one. By plotting these curves together one can get some information about the joint behavior of the coupled metals which are expected, once in contact, to be held at a mixed potential defined by the intersection between their respective polarization curves. Fig. 8A shows the coupled polarization curves in the case of 1% GA. It is interesting to note that, in spite of a large corrosion potential difference (ca 150 mV) between the two metals, the intercept takes place at the corrosion potential of cobalt, which means that, in service conditions, the Co electrode is actually not expected to be significantly over polarized by the Cu one. In other words, one should not expect an acceleration of cobalt corrosion due to galvanic coupling in these conditions. It is worth noticing however that this is a result issuing from normalized polarization curves i.e. in terms of current densities and are then
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supposed to hold only for the coupling between similar exposed surfaces of both metals. Indeed, as mentioned before, the coupling current is a function of the relative impedance of each electrode which is a direct function of the exposed surface. For the 32 nm technology, the exposed Cu surface is likely to be roughly 2.5 times that of the Co, which should not be enough to constitute a strong increase in the galvanic coupling current. In spite of this favorable result, one should not overlook the fact that Co corrosion currents are already intrinsically very high in these media (see Table 2) so that the addition of BTA seems to be an interesting possibility. Fig. 8B shows the galvanic coupling estimated by the polarization curves in the presence of BTA where it can be seen the same scenario of interception at the cobalt corrosion potential. A slight deleterious effect of the Cu–Co coupling has only be found for higher GA concentrations (4%) but even in that case the current increased by a factor of two, which can also be considered as a restrained effect. 4. Conclusions In the present manuscript we report on the electrochemical behaviors of copper and cobalt studied under potentiodynamic conditions in two different aqueous solutions of glycolic acid: with and without BTA. It has been observed that the presence of glycolic acid induces an increase of copper corrosion and a slight decrease in the case of cobalt which was interpreted in terms of restrained cathodic reaction kinetics. In spite of this slight decrease, the cobalt dissolution remains significant and seems to be governed by oxygen reduction reaction. The incorporation of small concentrations of benzotriazole to the etching solution showed that this well-known copper inhibitor is also efficient for cobalt. The BTA efficiency regarding Cu in GA/BTA mixtures has been shown to be reduced probably due to the etching action of the glycolic acid on the Cu-BTA protective film, which seems to ensure the required controlled cleaning effect of GA on copper surface. At the same time, it has been shown that the cobalt corrosion rate is ten times lower in the presence of BTA than in the pure glycolic acid solution. The qualitative analysis of the coupled Cu–Co behavior in these solutions indicated that no significant increase of the Co corrosion is expected due to galvanic coupling, but these positive results must be confirmed by straightforward measurements carried out with the help of a zero resistance ammeter. This is matter of an ongoing work as well as a sharp physico-chemical characterization of the interfacial phenomena involved in these systems to fully validate this electrochemical study. References
Fig. 8. Potentiodynamic polarization curves in glycolic acid 1% for Cu (h) and Co (d) in (A) 1% glycolic acid and (B) in the mixture of 1% glycolic acid and 0.1% BTA. Scan rate is 1 mV s1 and X = 1000 rpm.
[1] P. Singer, Semicond. Int., May 2004, p. 40. [2] L.G. Gosset, S. Chhun, W. Besling, et al., in: Advanced Metallization Conference 2005 (AMC 2005) Conference Proceedings, 2006, pp. 587–593. [3] J. Gambino, J. Wynne, S. Smith, S. Mongeon, P. Pokrinchak, D. Meatyard, in: IITC Proceedings 2005, pp. 111–113. [4] L. Peters, Semicond. Int., July 2002, p. 63. [5] K.N. Tu, J. Appl. Phys. 94 (2003) 5451. [6] F. Bailly, T. David, D. Eon, R. Bouyssou, T. Chevolleau, O. Joubert, C. Cardinaud, in: Proceedings of International Symposium on Dry Process, November 2007, pp. 95–96 (P1–36). [7] J.W. Oldfield, ASTM STP 979, in: H.P. Hack (Ed.), ASTM International, West Conshohocken (PA), 1988. [8] V.S. Sastri, Corrosion Inhibitors: Principles and Applications, first ed., Wiley, New York, NY, 1998. [9] P.G. Cao, R.A. Gu, Z.Q. Tian, Langmuir 18 (2002) 7609. [10] V. Brusic, G.S. Frankel, A.G. Schrott, T.A. Petersen, B.M. Rush, J. Electrochem. Soc. 140 (1993) 2507. [11] G.W. Poling, Corros. Sci. 10 (1970) 359. [12] R. Babic, M. Meticos-Hukovic, M. Loncar, Electrochim. Acta 44 (1999) 2413. [13] D. Tromans, J.C. Silva, Corrosion 53 (1997) 16. [14] J.L. Guinon, V. Perez-Herranz, J. Appl. Electrochem. 30 (2000) 379.
2044 [15] [16] [17] [18] [19]
S. Bilouk et al. / Microelectronic Engineering 86 (2009) 2038–2044
F. El-Taib Heakal, S. Haruyama, Corros. Sci. 20 (1980) 887. I. Dugdale, J.B. Cotton, Corros. Sci. 3 (1963) 69. D. Tromans, R.H. Sun, J. Electrochem. Soc. 138 (1991) 3235. D. Tromans, J. Electrochem. Soc. 145 (1998) L42. B. Schnyder, C. Stossel-Sitting, R. Kotz, S. Hochstrazzer-Kurz, S. Virtanen, C. Jaeggi, et al., Surf. Sci. 566–568 (2004) 1240–1245. [20] L. Broussous, Q. Hinsinger, S. Favier, P. Besson, Solid State Phenom. 92 (2003) 263. [21] C. Pernel, D. Louis, J. Farkas, J. Vac. Sci. Technol. B 24 (2006) 2467.
[22] D. Rébiscoul, N. Lopez, L. Broussous, D. Louis, G. Passemard, Solid State Phenom. 134 (2008) 333. [23] S. Bilouk, C. Pernel, R.P. Nogueira, L. Broussous, P.H. Haumesser, G. Passemard, ECS Trans. 11 (2) (2007) 395. [24] J. Van der Lee, L. de Windt, CHESS Tutorial and Cookbook/Version 3.0, users manual LHM/RD/02/13, Ecole des mines de Paris, Fontainebleau, 2002. [25] A. Bautista, U. Bertocci, F. Huet, J. Electrochem. Soc. 148 (2001) B412. [26] D. Déforge, F. Huet, R.P. Nogueira, P. Ponthiaux, F. Wenger, Corrosion 62 (2006) 514.
Contents lists available at ScienceDirect
Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee
Electrochemical behavior of copper and cobalt in post-etch cleaning solutions S. Bilouk a,b,*, L. Broussous a, R.P. Nogueira b, V. Ivanova c, C. Pernel c a b c
ST Microelectronics, 850 rue Jean Monnet, 38926 Crolles Cedex, France UMR 5266 et 5631 CNRS Grenoble-INP, SIMAP/LEPMI, 1130 rue de la piscine, BP 75, 38402 St. Martin d’Hères Cedex, France CEA, LETI, MINATEC, F38054 Grenoble, France
a r t i c l e
i n f o
Article history: Received 14 October 2008 Accepted 16 January 2009 Available online 24 January 2009 Keywords: Self-aligned barrier Corrosion Inhibitor Benzotriazole
a b s t r a c t Cleaning is one of the key steps of the integration of self aligned barriers (SAB) in microelectronic devices for 32 nm technology and below. It is hence important to investigate the impact of different cleaning solutions on the metallic components of SAB, mainly in which concerns their surface stability. In this sense, the electrochemical behavior of copper and cobalt was studied under potentiodynamic conditions in different aqueous solutions of glycolic acid (GA) with and without benzotriazole (BTA) inhibitor. It has been observed that the presence of glycolic acid induces a monotonic increase of copper corrosion and a slight decrease in the case of cobalt. The cobalt dissolution remains nonetheless very active and is shown to be governed by oxygen reduction reaction. The addition of BTA, a well-known corrosion inhibitor for copper has shown to be also effective in the case of Co surfaces, with a ca 15-fold reduction of the intrinsic Co corrosion current density. The possibility of galvanic coupling between both metals, supposed to enhance the Co dissolution, has also been qualitatively investigated and seems not to be a determinant factor in these conditions. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction The copper damascene process is widely established and has brought higher performance to semiconductor devices [1]. However, due to continuously shrinking design rules and hence to increasing current densities, copper electromigration remains an important challenge. To overcome this issue, the integration of CoWP or CoWB self aligned barriers (SAB) for 32 nm technology was proposed [2–5]. The integration of these new materials implies a compatibility study with the various conventional dry and wet treatments used for the patterning of interconnection levels. Particularly, the postvia etch cleaning can strongly impact the integrity of the SAB due to corrosion. In the ideal integration scheme, the SAB is etched in order to minimize the resistance of vias [6]. In this configuration, the copper at via bottom is in contact with the SAB and both are exposed to the cleaning solution (Fig. 1a). This may lead to SAB dissolution not only by spontaneous corrosion processes, but also those induced by galvanic coupling as schematically illustrated in Fig. 1b. Indeed, the simultaneous presence of dissimilar metallic components exposed to an ionic conductive medium can entail galvanic corrosion processes yielding accelerated damage of the less noble of the metallic phases during * Corresponding author. Address: ST Microelectronics, 850 rue Jean Monnet, 38926 Crolles Cedex, France. Tel.: +33 (0)4 38 78 19 43; fax: +33 (0)4 38 78 30 34. E-mail address: [email protected] (S. Bilouk). 0167-9317/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2009.01.035
the post-via etch clean step [7]. The standard potentials of the Cu/ Cu2+ and Co/Co2+ couples are ECu=Cu2þ ¼ +0.34 V/ENH, and ECo=Co2þ ¼ 0.28 V/ENH, respectively. Thus cobalt is expected of undergoing galvanic corrosion when is in contact with copper, enhancing its selective dissolution as it clearly appears in Fig. 1c. On this TEM picture a total withdrawal of Co-alloy after the via opening and the cleaning step is evidenced. The use of organic inhibitors is one of the most widespread approaches to prevent the generalized corrosion of metals occurring in a variety of aqueous solutions [8,9]. Recent studies demonstrated that the corrosion rate of cobalt in a borate buffer solution and in presence of copper ions ðCu2þ Þ is significantly reduced thanks to the formation of a protective Cu(I)–benzotriazole (Cu(I)–BTA) film on the cobalt surface [10]. The 1 H-benzotriazole (1H-BTA), is a well-known and widely used inhibitor of copper corrosion over a wide pH and temperature range [11–14]. It was proposed that benzotriazole forms an insoluble polymeric ½Cuþ BTA n surface film and that the presence of a Cu2 O layer on the copper surface facilitates the formation of this complex [11,15–18]. Schnyder et al. [19] have also shown the corrosion inhibition of WC-Co alloy by BTA in a borate buffer solution by polarization experiments. This indicates that the use of BTA during the post-via etch clean can be considered as a potentially interesting way to slow down the cobalt dissolution through both spontaneous corrosion and galvanic coupling, but to our knowledge, no systematic study has been devoted to the BTA inhibiting effect concerning Co corrosion in acidic media.
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Fig. 1. (a) Self-aligned barrier (SAB) configuration in the integration schema; (b) SAB dissolution during the post-via etch clean process; (c) SEM micrograph of opened via where no SAB remains.
It is obvious, however, that any attempt to slow down or prevent Co corrosion should not be deleterious to the Cu lines. In this sense, it is important to also study the electrochemical behavior of copper in organic acid and in a mixture of organic acid and BTA so as to understand the possible impact of BTA during the cleaning step before introducing it for cobalt protection. Indeed, solutions of diluted organic acids are widely used in microelectronic in the following steps: Chemical and Mechanical Polishing (CMP), preclean of copper surface prior to electroless deposition process and post-etch clean [20,21]. Among the various organic acid that may be suitable for post-etch clean, glycolic acid was selected for its ability to remove CuOx without damaging the copper line beneath [22,23]. In addition, its high solubility in water (up to 70%) facilates its uses in industrial equipments for wafer cleaning. In the present study, the electrochemical behavior of copper and cobalt in post-etch cleaning solutions containing glycolic acid is investigated. The effect of BTA addition to cleaning solutions is also discussed. The electrochemical behavior of copper and cobalt in these solutions is studied by polarization curves which have been also used to assess theoretically the galvanic coupling in the cobalt dissolution. 2. Experimental A three-electrode cell (volume 100 mL) was used with a largearea Pt foil as counter electrode and an Ag/AgCl reference electrode. The working electrodes were made of copper or cobalt cylindrical rods (99.995% and 99.990% respectively) mounted on a Radiometer Analytical CTV 101 rotating disk electrode (RDE). The active area was 0.28 cm2 for copper and 0.196 cm2 for cobalt. The electrode surface was successively polished with 800 and 1200 grit emery papers and with 6.0 to 0.1 lm alumina suspensions. They were then rinsed with deionized water and dried with nitrogen before each experiment. All experiments were performed at room temperature; polarization curves in the potential range ±1 V/OCP with a scan rate of 1 mV s1 were performed using a potentiostat PARSTAT 2273 from Princeton Applied Research with the help of the POWERSUIT software. All measurements were carried out at 1000 rpm. Aqueous solutions were prepared with deionized water. Diluted glycolic acid (GA) solutions with various concentrations: 1%, 2%, 3%
and 4% in weight were used. Diluted BTA solution (0.1% wt) and mixtures of diluted glycolic acid and BTA were also investigated. Since the pH of the glycolic acid solutions is around 2 for all studied concentrations (for example 2.4 for 1% wt and 2.0 for 4% wt), it was interesting to compare the behavior of copper and cobalt in a glycolic acid and in a blank solution (with the same pH = 2) consisting in diluted sulfuric acid. 3. Results and discussion 3.1. Electrochemical behavior of copper and cobalt 3.1.1. Glycolic acid effect The influence of glycolic acid on the electrochemical behavior of copper and cobalt was investigated by electrochemical methods. It was compared to a blank electrolyte with the same pH (pH = 2) to straightforwardly evaluate the contribution coming only from the glycolic acid itself and not from the pH effect. Polarization curves performed after 2 h of immersion in either blank solution or in solutions with different concentrations of glycolic acid from 2% to 4% wt are depicted in Fig. 2A and B for Cu and Co, respectively. For the sake of readability and since glycolic acid induces a mostly monotonic evolution in both cases, the whole ensemble of curves is not presented in the figures. The main parameters issued from these experiments are summarized in Tables 1 and 2 for Cu and Co, respectively. The macroscopic stationary electrochemical behavior of both metals appears to be highly dependent on the acid concentration and it is clearly seen that glycolic acid modifies the oxidation kinetics. In the presence of glycolic acid, the corrosion current of copper (4–12 lA cm2) is higher than in a blank solution at pH = 2 (<2 lA cm2). In addition, it is worth to notice that the corrosion current increases with the acid concentration. The glycolic acid enhances both the cathodic and anodic reactions which results in an increase of the corrosion rate. The pH slightly decreases with [GA] which can enhance proton reduction on copper surface in addition to the dissolved oxygen reduction. The higher aggressiveness is nonetheless not detrimental: according to Faraday’s law, and supposing uniform corrosive attack along the metallic surface, for a 60s exposure to the higher 4% glycolic acid, the measured corrosion current density (12 lA cm2) corresponds to a copper withdrawal of only 2.7 Å.
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S. Bilouk et al. / Microelectronic Engineering 86 (2009) 2038–2044 Table 2 Relevant Co corrosion parameters issued from the polarization curves presented in Figs. 2b and 6b. Solution
pH
Ecorr (V vs Ag/AgCl)
Icorr (lA cm2)
Etch rate (Å min1)
Blank pH 2 GA 1% GA 2% GA 3% GA 4% GA 1%/BTA 0.1% BTA 0.1%
2 2.42 2.28 2.18 2.13 2.5 5.3
0.424 0.270 0.280 0.286 0.288 0.201 0.190
750 380 320 280 220 24 0.6
155 79 66 58 45 5 0.12
dissolution of the cobalt-based self aligned barriers for 32 nm node applications. The anodic behavior of both Cu and Co in these solutions can be tentatively interpreted with the help of thermodynamic data. Glycolic acid can exist in aqueous solutions in two different forms, namely H4C2O3 (HL) and H3 C2 O 3 (L ). The equilibrium between these species is described by:
H4 C2 O3 $ H3 C2 O3 þ Hþ
pKa ¼ 3:8
ð1Þ
which means that H4 C2 O3 predominates at pH values below 3.8 (pKa), while H3 C2 O 3 is prevailing at pH values above 3.8. In addition, glycolic acid may form two soluble complexes with cupric
Fig. 2. Potentiodynamic polarization curves in blank solution (h), in glycolic acid 2% (d) and 4% (D) for (A) copper and (B) cobalt: the curve in continuous line was obtained in deaerated GA electrolyte. Scan rate is 1 mV s1 and X = 1000 rpm.
This characteristic satisfies one of main post-etch cleaning specifications i.e. an etch rate of Cu lower than 10 Å min1 and is in good agreement with industrial process requirements. These results hence confirm that glycolic acid ensures the copper surface etching and cleaning while keeping the bulk corrosion negligible. In the case of Co, the corrosion current values are much higher than those for the Cu in both blank and organic acid containing solutions. Even if the corrosion current density seems to decrease with the concentration of glycolic acid, values remain however high enough to induce a catastrophic dissolution of the cobalt in presence of glycolic acid as well as in the blank solution. In fact, the etch rate varies from 79 Å min1 for a 1% concentration to 45 Å min1 for a 4% concentration which can lead to a pronounced Table 1 Relevant Cu corrosion parameters issued from the polarization curves presented in Figs. 2a and 6a. Solution
pH
Ecorr (V vs Ag/AgCl)
Icorr (lA cm2)
Etch rate (Å min1)
Blank pH 2 GA 1% GA 2% GA 3% GA 4% GA 1%/BTA 0.1% BTA 0.1%
2 2.42 2.28 2.18 2.13 2.5 5.3
0.090 0.120 0.126 0.130 0.134 0.150 0.240
1.6 4 7 10 12 2.5 0.5
0.3 0.8 1.2 1.7 2.7 0.4 0.1
Fig. 3. (a) Potential–pH diagram at 25 °C and 1 atm for the system copper–glycolic acid–water: [Cu2+] = 102 M, glycolic acid – 1%; (b) speciation diagram for the system copper–glycolic acid–water.
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ions. ½CuH3 C2 O3 þ and CuðH4 C 2 O3 Þ2 are the aqueous copper glycolic acid complexes which are denoted as CuLþ and CuL2 in this paper. According to the potential–pH diagram for the copper–glycolic acid–water system plotted with the help of JSCHESS software [24] and depicted in Fig. 3a, the CuLþ is clearly seen to be the predominant complex present at pH = 2.4 (1% glycolic acid in the electrolyte), which is also confirmed by the speciation diagram built for the system Cu – GA – H2O, Fig. 3b. The presence of these complexes can explain the chelating effect of glycolic acid on copper surface, allowing a slight etch of the metal surface without damaging the metal integrity. As well as in the case of copper, cobalt ions can be also chelated by glycolic acid giving the following two complexes: ½CoH3 C2 O3 þ and CoðH4 C2 O3 Þ2 , respectively denoted as CoLþ and CoL2 . The potential–pH for a 1% GA concentration and the speciation diagrams for the cobalt–glycolic acid–water system are given in Fig. 4a and b. Unlikely to Cu, here CoLþ seems not to be the predominant species at pH = 2.4 and in the cobalt corrosion potential range. Thus, CoLþ can be supposed to be an intermediate component which could enhance cobalt dissolution according to the following steps:
Co þ HL $ ðCoLÞads þ Hþ þ e
ð2Þ
ðCoLÞads $ ðCoLÞþsol þ e
ð3Þ
ðCoLÞþsol þ Hþ ! Co2þ sol þ HL
ð4Þ
Fig. 4. (a) Potential–pH diagram at 25 °C and 1 atm for the system cobalt–glycolic acid–water: [Co2+] = 102 M, glycolic acid – 1%; (b) speciation diagram for the system cobalt–glycolic acid–water.
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This reactive scheme, however, does not account for the slight but monotonic reduction of the cobalt corrosion current density shown in Fig. 2B and Table 2 as the GA concentration increases. This apparently contradictory behavior must be ascribed to the cathodic kinetics of Co dissolution as discussed in which follows. In which concerns the cathodic mechanisms, Fig. 2 indicates that the hydrogen reduction is an important component of the cathodic reaction (as should be expected for these acidic medium) since the cathodic current intensity monotonically increases with the acid concentration at higher polarization levels for both Cu and Co. Nevertheless, the concomitant reduction of dissolved oxygen should be also observed at lower polarization levels mainly in the case of Co. Indeed, Fig. 2B shows that in 1% deaerated glycolic acid solution, the corrosion current of cobalt is significantly reduced (3.4 lA cm2 instead of more than 200 lA cm2 – see Table 2) which means that cobalt corrosion in these conditions is a strongly cathodic controlled process. Fig. 5 shows the details of the polarization curves depicted in Fig. 2B in the cathodic region close to the corrosion potential. It clearly appears that before reaching polarization levels high enough for the proton reduction to be dominant, the increase in the GA concentration hinders the cathodic kinetic which is probably related to those decreasing corrosion current densities mentioned before. 3.1.2. BTA effect Fig. 6A presents the polarization curves of copper after 2 hour immersion in a glycolic acid solution 1%, in a mixture of glycolic acid (1%) and BTA (0.1%) or in only 0.1%wt BTA. In diluted solution of BTA 0.1%, the pH of the solution is 5.3 while it is 2.4 in diluted glycolic acid (1%). When only BTA is present, the anodic current is strongly lowered down reflecting the copper dissolution limitation. This is probably related to the formation of a Cu(I)–BTA film which seems to lose its protective nature at more positive potentials (ca +300 mV), beyond which a steep current increase indicates metal dissolution. The corrosion current density is 0.5 lA cm2 i.e eight times lower than in 1% GA solution. When the BTA solution is added to the GA, the pH of the resulting electrolyte decreases to 2.50. In such mixture, an intermediate electrochemical behavior of copper is observed. The anodic inhibition effect of BTA is still clearly present which confirms its action despite the presence of another reactant. However, the inhibition range now is shorter than in presence of BTA alone, which indicates a lower stability of the protective film in this media. In addi-
Fig. 5. Cathodic potentiodynamic polarization curves for Co in glycolic acid at 1% (h), 2% (d), 3% (D) and 4% (r) at the vicinity of the corrosion potential; data for the 2% and 4% solutions are the same than in Fig. 2b.
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Fig. 6. Potentiodynamic polarization curves in glycolic acid 1% (h), in 0.1% BTA (d) and in the mixture of 1% glycolic acid and 0.1% BTA (D) for (A) copper and (B) cobalt. Scan rate is 1 mV s1 and X = 1000 rpm.
tion, the value of the current density is 2.5 lA cm2 i.e. five times higher than in the BTA solution. At a first glance, it can be supposed that this decrease of BTA inhibiting action is due to the pH decrease. However, this value (in GA/BTA) is also higher than the value obtained in the blank solution (pH = 2). This means that the observed decrease of BTA inhibiting efficiency should not be ascribed only to the pH change (a decrease from 5.3 to 2.5) but also to GA
etching action. Indeed, glycolic acid cleans the metal surface by smoothly etching it. Thus, it can be supposed that in this case the [Cu(I)BTA] protecting complex is probably partially removed from the metal surface. In this way the conjugated actions of BTA and glycolic acid appear: the Cu surface is less protected than in diluted BTA solution and is slightly etched by the glycolic acid. The same experiments were carried out to determinate the influence of BTA (Fig. 6B) in the cleaning solution on the electrochemical behavior of cobalt. It should be noticed that in the presence of 0.1% BTA, the corrosion current density is much lower (0.6 lA cm2) than in glycolic acid solution 260 lA cm2. This observation allows to consider the BTA as a corrosion inhibitor for Co as well as it has been shown above for Cu. Here, similar to copper, the inhibiting effect of BTA on cobalt still occurs in presence of GA in acidic conditions, although in a less effective way regarding the action of BTA only. Anyway, the corrosion current density is ten times lower than in the GA solution (24 lA cm2 and 260 lA cm2, respectively). It is interesting to notice that the inhibition mechanisms are different for Cu and Co. In fact, unlikely to copper, BTA, when mixed with GA, induces no significant anodic slow down effect in the polarization curves of cobalt. The collapse of the corrosion current in that case is due to a strong decrease in the cathodic activity as seen in Fig. 6B. BTA can hence be supposed to be a cathodic inhibitor of cobalt. Its efficiency is as high as the corrosion mechanism of Co seems to be cathodically controlled as it appeared from the deaerated solution results discussed before (see Fig. 2B). In this sense, a possible mechanism for the BTA inhibition effect is schematically illustrated in Fig. 7: BTA adsorbs on cathodic sites on cobalt surface and inhibits the oxygen reduction hence significantly slowing down cobalt dissolution. It must be noticed however that the corrosion current in the deaerated solution (see Fig. 2B) is still lower than that obtained for the mixed GA – BTA solution (Fig. 6B). This indicates that the blocking effect of BTA should not be absolute. If all cathodic sites were actually blocked by BTA one should expect a corrosion current density close to that in deaerated solution. 3.2. Galvanic coupling In the integration scheme shown in Fig. 1, cobalt and copper are in direct contact in the vias, both physically and by the conductive solutions used during the post-etch clean. A galvanic coupling between these two metals can thus enhance the dissolution of the cobalt self aligned barriers. The quantitative assessment of the actual coupling current is not an easy task. Straightforward measurements can be achieved with the help of zero resistance ammeters. It must be stressed
Fig. 7. Schematic illustration of oxygen reduction on cobalt surface (a) in glycolic acid solution (b) in mixture of glycolic acid and benzotriazole.
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however that, as it has already been shown [25,26], accurate measurements are dependent on the previous or concomitant knowledge of the relative impedances of the metallic surfaces getting in contact. This quantitative analysis is beyond the aims of the present paper and is matter of ongoing work. It is however interesting, thanks to the polarization curves depicted in Figs. 2 and 6, to proceed to a qualitative analysis of the coupled behavior of Co and Cu submitted to these aggressive environments with or without the presence of BTA as corrosion inhibitor. The central idea is to verify in which extent the Co (lower standard redox potential) surface is prone to be anodically over polarized by the Cu one. By plotting these curves together one can get some information about the joint behavior of the coupled metals which are expected, once in contact, to be held at a mixed potential defined by the intersection between their respective polarization curves. Fig. 8A shows the coupled polarization curves in the case of 1% GA. It is interesting to note that, in spite of a large corrosion potential difference (ca 150 mV) between the two metals, the intercept takes place at the corrosion potential of cobalt, which means that, in service conditions, the Co electrode is actually not expected to be significantly over polarized by the Cu one. In other words, one should not expect an acceleration of cobalt corrosion due to galvanic coupling in these conditions. It is worth noticing however that this is a result issuing from normalized polarization curves i.e. in terms of current densities and are then
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supposed to hold only for the coupling between similar exposed surfaces of both metals. Indeed, as mentioned before, the coupling current is a function of the relative impedance of each electrode which is a direct function of the exposed surface. For the 32 nm technology, the exposed Cu surface is likely to be roughly 2.5 times that of the Co, which should not be enough to constitute a strong increase in the galvanic coupling current. In spite of this favorable result, one should not overlook the fact that Co corrosion currents are already intrinsically very high in these media (see Table 2) so that the addition of BTA seems to be an interesting possibility. Fig. 8B shows the galvanic coupling estimated by the polarization curves in the presence of BTA where it can be seen the same scenario of interception at the cobalt corrosion potential. A slight deleterious effect of the Cu–Co coupling has only be found for higher GA concentrations (4%) but even in that case the current increased by a factor of two, which can also be considered as a restrained effect. 4. Conclusions In the present manuscript we report on the electrochemical behaviors of copper and cobalt studied under potentiodynamic conditions in two different aqueous solutions of glycolic acid: with and without BTA. It has been observed that the presence of glycolic acid induces an increase of copper corrosion and a slight decrease in the case of cobalt which was interpreted in terms of restrained cathodic reaction kinetics. In spite of this slight decrease, the cobalt dissolution remains significant and seems to be governed by oxygen reduction reaction. The incorporation of small concentrations of benzotriazole to the etching solution showed that this well-known copper inhibitor is also efficient for cobalt. The BTA efficiency regarding Cu in GA/BTA mixtures has been shown to be reduced probably due to the etching action of the glycolic acid on the Cu-BTA protective film, which seems to ensure the required controlled cleaning effect of GA on copper surface. At the same time, it has been shown that the cobalt corrosion rate is ten times lower in the presence of BTA than in the pure glycolic acid solution. The qualitative analysis of the coupled Cu–Co behavior in these solutions indicated that no significant increase of the Co corrosion is expected due to galvanic coupling, but these positive results must be confirmed by straightforward measurements carried out with the help of a zero resistance ammeter. This is matter of an ongoing work as well as a sharp physico-chemical characterization of the interfacial phenomena involved in these systems to fully validate this electrochemical study. References
Fig. 8. Potentiodynamic polarization curves in glycolic acid 1% for Cu (h) and Co (d) in (A) 1% glycolic acid and (B) in the mixture of 1% glycolic acid and 0.1% BTA. Scan rate is 1 mV s1 and X = 1000 rpm.
[1] P. Singer, Semicond. Int., May 2004, p. 40. [2] L.G. Gosset, S. Chhun, W. Besling, et al., in: Advanced Metallization Conference 2005 (AMC 2005) Conference Proceedings, 2006, pp. 587–593. [3] J. Gambino, J. Wynne, S. Smith, S. Mongeon, P. Pokrinchak, D. Meatyard, in: IITC Proceedings 2005, pp. 111–113. [4] L. Peters, Semicond. Int., July 2002, p. 63. [5] K.N. Tu, J. Appl. Phys. 94 (2003) 5451. [6] F. Bailly, T. David, D. Eon, R. Bouyssou, T. Chevolleau, O. Joubert, C. Cardinaud, in: Proceedings of International Symposium on Dry Process, November 2007, pp. 95–96 (P1–36). [7] J.W. Oldfield, ASTM STP 979, in: H.P. Hack (Ed.), ASTM International, West Conshohocken (PA), 1988. [8] V.S. Sastri, Corrosion Inhibitors: Principles and Applications, first ed., Wiley, New York, NY, 1998. [9] P.G. Cao, R.A. Gu, Z.Q. Tian, Langmuir 18 (2002) 7609. [10] V. Brusic, G.S. Frankel, A.G. Schrott, T.A. Petersen, B.M. Rush, J. Electrochem. Soc. 140 (1993) 2507. [11] G.W. Poling, Corros. Sci. 10 (1970) 359. [12] R. Babic, M. Meticos-Hukovic, M. Loncar, Electrochim. Acta 44 (1999) 2413. [13] D. Tromans, J.C. Silva, Corrosion 53 (1997) 16. [14] J.L. Guinon, V. Perez-Herranz, J. Appl. Electrochem. 30 (2000) 379.
2044 [15] [16] [17] [18] [19]
S. Bilouk et al. / Microelectronic Engineering 86 (2009) 2038–2044
F. El-Taib Heakal, S. Haruyama, Corros. Sci. 20 (1980) 887. I. Dugdale, J.B. Cotton, Corros. Sci. 3 (1963) 69. D. Tromans, R.H. Sun, J. Electrochem. Soc. 138 (1991) 3235. D. Tromans, J. Electrochem. Soc. 145 (1998) L42. B. Schnyder, C. Stossel-Sitting, R. Kotz, S. Hochstrazzer-Kurz, S. Virtanen, C. Jaeggi, et al., Surf. Sci. 566–568 (2004) 1240–1245. [20] L. Broussous, Q. Hinsinger, S. Favier, P. Besson, Solid State Phenom. 92 (2003) 263. [21] C. Pernel, D. Louis, J. Farkas, J. Vac. Sci. Technol. B 24 (2006) 2467.
[22] D. Rébiscoul, N. Lopez, L. Broussous, D. Louis, G. Passemard, Solid State Phenom. 134 (2008) 333. [23] S. Bilouk, C. Pernel, R.P. Nogueira, L. Broussous, P.H. Haumesser, G. Passemard, ECS Trans. 11 (2) (2007) 395. [24] J. Van der Lee, L. de Windt, CHESS Tutorial and Cookbook/Version 3.0, users manual LHM/RD/02/13, Ecole des mines de Paris, Fontainebleau, 2002. [25] A. Bautista, U. Bertocci, F. Huet, J. Electrochem. Soc. 148 (2001) B412. [26] D. Déforge, F. Huet, R.P. Nogueira, P. Ponthiaux, F. Wenger, Corrosion 62 (2006) 514.