Electrodeposition of Au-Sn Alloys from a Modified ...

Electrodeposition of Au-Sn Alloys from a Modified Non-Cyanide Bath J. L. Cardoso, S. G. dos Santos Filho University of São Paulo, LSI/PSI/EPUSP Av. Prof. Luciano Gualberto, 158, trav. 3, São Paulo, SP, 05508-900, Brasil This work presents improved Au-Sn baths to obtain a potentiostatically deposited Au-Sn alloys, by preventing bath degradation during the electrochemical process. It was studied the Au-Sn bath speciation though a program for chemical equilibrium simulation (CHEAQS Pro). The electrochemical reactions were investigated using Cyclic Voltammetry (CV). Rutherford Backscattering Spectroscopy (RBS) was used to determine the deposits composition and surface roughness. The results indicated that the best bath composition was KAuCl4 (5 g·l-1), SnCl2 ⋅ 2 H2O (5 g·l-1), ammonium citrate (150 g·l-1), l-ascorbic acid (7.5 g l-1) and sodium sulfite (60 g·l-1) to obtain an alloy with 16 at.% Sn. The best potential was defined as –600 mV, due to the largest thickness obtained. Introduction Gold-tin (Au-Sn) solders have been widely used in the optoelectronics and microelectronics industry. Some applications of Au-Sn alloys are flip chip package (1, 2), optical packages (2, 3), optical fiber feedthrough joint (4), and die-attachment for ASIC (2). Their passive alignment characteristic, good thermo-mechanical behavior, fluxless soldering capability, suitable thermal conductivity and thermal expansion coefficient, as well as lead-free feature make Au-Sn solders suitable for high temperature applications (5). Although its price is high, a comparison with others lead-free solders, as Zn-Sn and bismuth alloys, indicates that Au-Sn presents a superior fatigue resistance (2). In addition, its comparatively low eutectic temperature of 278 ºC, relative to other hard solders such as Au–Ge and Au–Si, makes it ideally suited for temperature sensitive applications (1). Conventionally, most Au-Sn alloys are deposited either as solder preforms or via vacuum deposition techniques, such as sputtering or evaporation (2, 5). The disadvantages of those techniques are difficult alignment, thickness control, easy oxidation (preforms), and require expensive equipments (vacuum deposition). An alternative technique, which combines the process control capability with the relatively low cost of preforms, is electrodeposition. Electrodeposition of an alloy solder can be done either sequentially, using separate Au and Sn baths, or simultaneously from a single bath. An electroplating process for simultaneously depositing Au-Sn alloys from non-cyanide baths has been developed by Sun and Ivey (6) and Funaoka et al. (7). An advantage of this co-electrodeposition process is that Sn oxide formation, which can occur during sequential electrodeposition, can be prevented. However, the problem with codeposition is the plating bath stability, once baths currently have a useful lifetime of 2 or 3 days. Keeping Au and Sn separated in two different solutions can be extended the lifetime of the bath. The mixture can be done just before use with the reducing agent, increasing the stability to 12 months (8).

The use of sulfite as a complexing agent to gold for plating baths is common in studies of cyanide-free baths. The Au(I)-sulfite bath was considered unstable, justified by a disproportional reaction from Au(III) and metallic Au, which cause the bath decomposition spontaneously on standing (9, 10). The production of colloidal gold is undesirable in gold plating since particles tend to be included in the deposit and promote the formation of nodules. The concentration of Au(I) is therefore dependent on both, the amount of free sulfite and also on the magnitude of the stability constant of the Au(I), and tends to increase as the pH is lowed. The addition of sulfite in a gold solution also reduces the deposit thickness and roughness. It occurs because the deposition of a metal from a complexed ion requires more energy to promote the nucleation of the film. Others additives are usually found in electrodeposition baths to increase the bath stability or improve some deposits characteristics. The l-ascorbic acid used in Au-Sn baths prevents Sn hydrolysis (6), and it also acts as complexing and reducing agent of Au (11). Ammonium citrate is used to prevent Au hydrolysis (6), as buffer (6) and as complexing agent of Sn (12). The tin deposition from Sn(II) salt occurs from acid baths. The deposition of tin from a chloride-only solution results in non-uniform deposits with poor adherence. The addition of organic additives improves the deposit quality, but is usually difficult to control. However, the addition of only ammonium citrate to the stannous chloride bath complexes the Sn(II) and results in flat, uniform and with good adherence deposits (13). The gold-tin alloy deposition is possible, although the redox potentials are different, because of diminishing of the Au deposition potential, by the gold complex, to a value close to the Sn. Accordingly, the composition of the alloy is dependent on the potential applied. The study of the gold-tin bath is going to define the potentials and the compositions of the alloy. In the present paper, we report the improvements made on the Sun and Ivey (6) Au-Sn bath to obtain a potentiostatically deposited Au-Sn alloy, by preventing bath degradation during the electrochemical process. It was studied the Au-Sn bath speciation though a program for chemical equilibrium simulation (CHEAQS Pro) and the electrochemical reactions were investigated with the aid of Cyclic Voltammetry (CV). Rutherford Backscattering Spectroscopy (RBS) was used to determine the deposits composition and surface roughness. RBS simulation of spectra with SIMNRA 6.04 (14) allowed one to determine the thickness, roughness and density of the gold films deposited at the working electrode. Au-Sn Bath Speciation Study In this study, the equilibrium concentrations of gold and tin complexes were calculated using the CHEAQS Pro 2008.1 (15). This is a general-purpose program for solving chemical equilibria in aqueous solution. The stability constants for the bath parts were included in its library to perform the calculations. These stability constants were obtained from Handbooks of chemistry and some papers (9, 11, 16-18). The results were used to better understand the bath behavior. It is important to consider the activity of each ion during the bath speciation study. It can be explained taking into account that the ions present in an electrolytic solution interact with each other, and this interaction changes the availability of the ions to participate in an electrochemical reaction. From this point, the concentration of an ion can’t reflect the real property of a solution, what is done by the activity. The activity

defines the availability of an ion considering its concentration multiplied by an activity coefficient (19). Experimental Details Solutions were prepared from chemicals of analytical grade or better, and Milli-Q deionized water (18.2 MΩ cm-1), using potassium tetrachloroaurate (III) (KAuCl4), sodium sulfite anhydrous (Na2SO3), ammonium citrate ((NH4)2HC6H5O7), l-ascorbic acid (H2C6H6O6), and stannous chloride (SnCl2 ⋅ 2 H2O). All solutions were fresh, using a stirrer. The Na2SO3, added to the KAuCl4 bath, complexes Au(III) ions, and reduces it to Au(I) by forming AuSO3- and Au(SO3)23- species (8). The formation of the complexes is guaranteed by adding the reagents in the following sequence: gold solution (ammonium citrate, gold salt, sodium sulfite and ascorbic acid), tin solution (ammonium citrate, tin salt), then, both solutions were mixed. The pH was measured using pH tapes, which identify pHs from 0 to 14. All the experiments were performed at room temperature (25°C ± 2ºC). The working electrode employed (or cathode) was a metalized Si wafers with evaporated Ti and Au layers, 17.5 and 122.8 nm thick, respectively. The area of the electrode exposed to the solution was 1 x 1 cm2, delimited with an insulate tape (Scotch, 3M). Electrochemical studies and film deposition were made using a potentiostat (CV-50W, BAS) in the three-electrode configuration. The reference electrode was a silver-silver chloride electrode (Ag/AgCl, E = 0.210 V/SHE, the standard hydrogen electrode). Pt wire was used as an auxiliary electrode (or anode). Potential ranged during the cyclic voltammetry from 200 to –1000 mV, cycled at 10 mV s-1. The electrodeposition processes were limited in 30 min, during constant agitation. This study included baths with different concentration of Sn salt (2.5, 5, 10, 20 g l-1), ammonium citrate (200, 150, 100, 50 g l-1), and l-ascorbic acid (15 and 7,5 g l-1). Potentiostatic electrodepositions were performed at -400, -500, -600, -700, and -800 mV. The simulations of RBS spectra considered the thickness of silicon, titanium, and gold from the working electrode. All the deposited gold films mentioned corresponds only depositions during the cyclic voltammetry. Results and Discussion The pH of gold-tin bath was around 5, kept constant due the high concentration of ammonium citrate. The Au(III) salt solution pH was 3, the gold-sulfide solution pH was 9, the gold-sulfide-l-ascorbic acid pH was 7, and, finally, Sn(II) solution pH was 2. The gold-sulfide solution presented high pH due to sulfide, which produces a weakly OHspecies in water medium (hydrolysis). Cyclic Voltammetry Figure 1 shows the cyclic voltammograms of the gold-tin baths. In Figure 1a, the first cycle corresponds to both depositions of gold and tin, indicated by the reduction peak (-620 mV). No oxidation peak can be observed. The changes occurred at the electrode surface moved the peak to lower potentials and reduced its maximum current at the second cycle. The electrode surface became partially gray, corresponding to the

deposition of tin, and partially yellow, because of the deposition of gold. This result, however, indicates a poor homogeneity of the deposition. During the cyclic voltammetry, it was observed the formation of a brown powder at the electrode surface. The powder formation occurred because of the reduction of gold close to the electrode surface, but not close enough to be incorporated in the electrodeposited film. The color indicated that the composition of the powder may be mainly gold. The powder formation should be avoided during the electrodeposition processes, because of the lost of material, bath degradation, and the incorporation of reduced gold at the film, degrading its density and homogeneity.

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Figure 1. Cyclic voltammograms from baths containing (a) 15 g l-1 and (b) 7.5 g l-1 of l-ascorbic acid, KAuCl4 (5 g l-1), SnCl2·⋅ 2 H2O (5 g l-1), ammonium citrate (200 g l-1), sodium sulfite (60 g l-1). A new bath was prepared, removing the tin solution, and a cyclic voltammetry was done to confirm if the powder formation occurred because of the gold. Again, it was observed the powder formation. L-ascorbic acid is the reducing agent of gold and its concentration may be contributing to the excess of gold reduction and powder formation. The simulation of Au-species in the gold-tin bath, indicated that high concentration of l-ascorbic acid reduces the content of free Au(I) species as AuCl21-, Au(NH3)21- and Au(OH) (aq), which presents high activities (~10-11, ~10-19, ~10-25 respectively) at the bath, although increases the activity of AuSO31- and AuSO33-. These gold-sulfide species presents low activities (~10-35). Less complexation of gold in the bath decreases the stabilization of the bath, promoting the gold reduction. Considering this fact, the concentration of l-ascorbic acid was reduced to half. A new voltammogram was recorded, as indicated in Figure 1b. Again, the first cycle corresponds to both depositions of gold and tin, indicated by the reduction peak (-635 mV), with the same behavior as before. This time, the surface color was more homogenous, presenting a dark gray aspect. No powder formation was observed, and this concentration of l-ascorbic acid (7.5 g l-1) was kept during all the others experiments. RBS Study of Deposits’ Composition After definition of an electrochemical-stabilized bath, potentiostatic depositions were carried on identifying the contribution of each additive to the deposit characteristics. The potential suitable for this study were defined considering the middle point between the

minimum and maximum value of current achieved in the cyclic voltammogram of the corresponding solution. 500 15

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Figure 2. RBS spectra of samples electrodeposited in tin-gold baths containing (a) 200 g·l-1, (b) 150 g·l-1, (c) 100 g·l-1 and (d) 50 g l-1 of ammonium citrate, KAuCl4 (5 g·l-1), SnCl2·⋅ 2 H2O (5 g·l-1), l-ascorbic acid (7.5 g l-1) and sodium sulfite (60 g·l-1). 500 30

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Figure 3. RBS spectra of samples electrodeposited in tin-gold baths containing (a) 2.5 g·l-1, (b) 5 g·l-1, (c) 10 g·l-1 and (d) 20 g l-1 of SnCl2·⋅ 2 H2O, KAuCl4 (5 g·l-1), ammonium citrate (150 g·l-1), l-ascorbic acid (7.5 g l-1) and sodium sulfite (60 g·l-1).

Figure 2 shows the influence of ammonium citrate concentration in the gold-tin alloy deposited from a bath containing 7.5 g l-1 l-ascorbic acid. The values of thickness obtained were 103.5 nm, 111.4 nm, 102.2 nm and 26.6 nm, and the relative concentration of tin was 12 at.%, 16 at.%, 13 at.%, and 8 at.% for concentrations of ammonium citrate of 200 g l –1, 150 g l-1, 100 g l –1 and 50 g l –1, respectively. A maximum thickness and concentration of tin was achieved when the concentration of ammonium citrate was reduced from 200 to 150 g l-1. A further reduction of ammonium citrate concentration reduces both the thickness and the relative amount of tin in the alloy. As it seems that the thickness and tin relative concentration of tin achieved a limit at 150 g l-1. The bath simulation indicated that the citrate concentration has many influences in the baths behavior. The increase of citrate concentration induces an increase in Sn(II)-citrate complex activity, reducing the activity of Sn(OH)22+. According to the redox potential, Sn(OH)22+ (E0 = -0.117 V) is one of the species which have great contribution to the deposition of tin. Au(III)-hydroxides and Au(III)-chlorides activities are also reduced when the concentration of citrate is increased, because of the great increase of Au(I)(NH3)3+ activity. The decrease of citrate reduces the concentration of Sn(II)-hydroxides and Sn(II)chlorides activities. SnCl42- (E0 = -0.19 V) also contributes to the deposition of tin; that is to say, the deposition of tin is inhibited. Next parameter evaluated was the influence of the concentration of the tin in the bath and the amount deposited in the alloy. The results are presented in Figure 3 for a bath containing 150 g·l-1 of ammonium citrate. No powder formation was verified. The concentration of tin was varied in 2.5 g·l-1, 5 g·l-1, 10 g·l-1, 20 g·l-1. The thicknesses obtained were 222.7 nm, 111.4 nm, 218.2 nm, 142.1 nm, and the relative concentration of tin was 13 at.%, 16 at.%, 13 at.%, and 15 at.% , respectively. The concentration of tin that induces more incorporation in the alloy is 5 g·l-1. The increase in the concentration of tin, increase the activity of Au(III)-chlorides, and promotes slight reduction of Au(I) and Au(III)-hydroxides. The activity of Sn(II)chlorides increases with the concentration of tin, and the others species activity presents a slow increase. This slow increase seems to indicate that the increase of the SnCl2 could not greatly contribute to the increase of the tin into the alloy. The last parameter evaluated was the deposition potential. Considering the cyclic voltammogram of the deposition bath, shown in Figure 4, it was defined potentials in the regions where the deposition occurs. The voltammogram presents two reduction peaks (-647 and –866 mV) from mainly gold and tin reduction, and an oxidation peak (-262 mV) from mainly l-ascorbic acid oxidation. Then, it was evaluated the thickness and alloy composition of the films obtained, presented in Figure 5. The values of thickness obtained were 18.3 nm, 111.4 nm, 313.1 nm, 677.2 nm and 39.4 nm, and the relative concentration of tin was 5 at.%, 16 at.%, 15 at.%, 45 at.% and 65 at.% for deposition potentials of –400 mV, –500 mV, –600 mV, –700 mV and –800 mV, respectively. The deposition at –700 and –800 mV result in powder formation. At –800 mV, sulfur from sodium sulfite was incorporated in the film. The increase of the potential resulted in an increase of the deposit thickness and relative concentration of tin until –700 mV. At -400 mV, the deposition rate was very slow, which is typical from low current densities. On the other hand, at –700 mV, the deposition rate was so high that the boundaries of the electrode became black, a typical indication of large density currents. At –800 mV, the deposit has low quantities of gold because its current rate was so high that the gold reduced couldn’t be incorporate at the film (20). –500 mV and –600 mV seems to be equivalent considering the incorporation of

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Figure 5. RBS spectra of samples electrodeposited at (a) –400 mV, (b) –500 mV, (c) –600 mV, (d) –700 mV and (e) –800 mV in tin-gold baths containing KAuCl4 (5 g l-1), SnCl2 ⋅ 2 H2O (5 g l-1), ammonium citrate (150 g l-1), l-ascorbic acid (7.5 g l-1) and sodium sulfite (60 g l-1).

Conclusion We presented improvements made on the Sun and Ivey (6) Au-Sn bath to obtain a potentiostatically deposited Au-Sn alloy by preventing bath degradation during the electrochemical process. It was studied the Au-Sn bath speciation with the aid of a program for chemical equilibrium simulation (CHEAQS Pro). The electrochemical reactions were investigated using Cyclic Voltammetry (CV). Rutherford Backscattering Spectroscopy (RBS) was used to determine the deposits composition and surface roughness. The concentration of l-ascorbic acid was reduced from 15 g l-1 to 7.5 g l-1, reducing the gold powder formation during the electrodeposition. The ammonium citrate was reduced from 200 g l-1 to 150 g l-1, and Sn(II)-chloride was kept constant, to obtain an alloy with 16 at %. Sn. Then, the best potential was defined as –600 mV, due to the largest thickness obtained. Acknowledgments We are grateful to FAPESP (proc. 07/01336-1) and CNPq (proc. 140627/07-3) for the financial support; to LAMFI-IFUSP for the RBS analysis. References 1. J.-W. Yoon, H.-S. Chun, B.-I. Noh et al., Microelectron. Reliab., 48, 1857 (2008). 2. K. Suganuma, S.-J. Kim, and K.-S. Kim, JOM, 61, 64 (2009). 3. J.W. R. Teo, G.Y. Li, M.S. Ling, Z.F. Wang, X.Q. Shi, Thin Solid Films, 515, 4340 (2007). 4. M.W. Beranek, M. Rassaian, C.-H. Tang, C.L.S. John, V.A. Loebs, IEEE Trans. Adv. Package, 24, 576 (2001). 5. J.W. R. Teo, F.L. Ng, L.S. K. Goiet al., Microelectron. Eng., 85, 512 (2008). 6. W. Sun, D.G. Ivey, Mater. Sci. Eng. B, 65, 111 (1999). 7. Y. Funaoka, S. Arai, N. Kaneko, Electrochem., 72, 98 (2004). 8. A. He, Q. Liu, D.G. Ivey, J. Mater. Sci. - Mater. Electron., 17, 63 (2006). 9. T.A. Green, S. Roy, J. Electrochem. Soc., 153, C157 (2006). 10. T. Osaka, Y. Okinaka, J. Sasano, K. Masaru, Sci. Technol. Adv. Mater., 7, 425 (2006). 11. A. Fornaro and N. Coichev, Quimica Nova, 21¸ 642 (1998). 12. J. C. Sherlock and S. C. Britton, Br. Corr. J., 7, 180 (1972). 13. A. He, Q. Liu and D. G. Ivey, J. Mater. Sci.: Mater Electron, 19, 553 (2008). 14. M. Mayer, SIMNRA User's Guide - Report IPP 9/113, Max-Planck-Institut für Plasmaphysik, Garching, Germany (1997). 15. W. Verweij, CHEAQS Pro, Netherlands (2008). 16. J. Lurie, Handbook of Analytical Chemistry, Moscow: Mir Publishers (1975). 17. N. A. Lange, Handbook of Chemistry, New York: McGraw-Hill (1999). 18. M. Pourbaix, Atlas of Electrochemical Equilibria in Aqueous Solutions, USA (1974). 19. A. R. Denaro, Fundamentos de Eletroquímica, Brasil: Editora Edgard Blucher LTDA (1974). 20. R. Winand, Electrichim. Acta, 39, 1091 (1994).