Technical Paper Direct Electroless Silver Plating on Copper Metal from Succinimide Complex Bath Using Imidazole as Reducing Agent Hidemi NAWAFUNE*, Keiko SHIROGUCHI**, Shozo MIZUMOTO*, Yasuhito KOHASHI*** and Keiko OBATA*** *Faculty of Science and Engineering, High Tech. Reseach Center, Konan University (8-9-1, Okamoto, Higashinada-ku, Kobe-shi, Hyogo 658-8501) **Graduate School of Science, Konan University (8-9-1, Okamoto, Higashinada-ku, Kobe-shi, Hyogo 658-8501) ***DAIWA FINE CHEMICALS Co., Ltd. (21-8, Minamifutami, Futami-cho, Akashi-shi, Hyogo 674-0093) Direct electroless silver plating on copper metal from a succinimide complex bath using Imidazole as reducing agent was examined. Though electroless silver plating from this bath is essentially an autocatalytic, re is a problem in properties and adhesion of plated film because of substitution of silver due to dissolution of base copper metal that occurs during first stage of plating. By adding, which is catalytically reacted on copper surface to emit electrons, as second reducing agent, substitution of silver was suppressed, surface condition became dense and adhesion was improved. It has been determined that mixed potential ory can be applied to this reaction based on result of local polarization curve measurement. Key Words ; Electroless Silver Plating, Cyanide Free, Copper Substrate, Deposition Mechanism 1. Introduction Silver has lowest specific resistance, it is comparatively cheap, and excels in soldering among various metal elements, refore, it is widely used as a surface treatment material for general-purpose contact points and various electronic parts. In particular, as miniaturization of electronic parts occurs, it is difficult to apply a plating of uniform thickness for minute parts having a complex shape by electroplating, which has a problem with current distribution uniformity. Consequently, silver plating by electroless is demanded as a new functional material in fields related not only to contact parts but also to optics and semiconductors. Recently, adoption of BGA (Ball Grid Array) type electronic parts and buildup substrate, which can be wired in high density, has increased for use in electronic equipment in which miniaturization and weight-saving are rapidly proceeding. However, buildup substrate tends to increase cost though possible to be wired in a high density compared with a conventional IVH (Interstitial Via Hole) substrate. Moreover, though electroless nickel-phosphorus/gold plating has been conventionally adopted for copper pad of printed wiring substrate to secure reliability of contact point in shielding case, nickel diffuses to surface of gold plating through pinholes, etc., in gold plating layer, and soldering quality deteriorates due to formation of nickel oxide and hydroxide, which causes a decrease in strength of solder bonding part. The adoption of silver plating for printed wiring substrate can solve se problems. Currently, silver plating has been practically applied to main substrate of some models of cellular phones though it is a displacement plating type reaction''. It is considered that its use will expand furr if silver plating by autocatalytic becomes possible in which a predetermined thickness plating can be obtained directly on copper base metal. We have already reported that electroless silver plating bath using an organic nitrogen compound as complexing agent and reducing agent excels in stability and silver film is autocatalytically deposited2'~4'. In this report, possibility of electroless silver plating on copper base metal using this bath was examined, and application of as second reducing agent was examined in order to suppress substitution of silver due to dissolution of copper base metal. 2. Experimental 2. 1 Film thickness, rate, and film appearance The basic bath composition and plating conditions are listed in Table 1. Rolled sheet copper (4 cm2), which was alkaline electrolytically degreased, was used as Table 1 Basic bath composition and plating condition of electroless silver plating.
Vol. 52, No.10, 2001 Direct Electroless Silver Plating on Copper Metal from Succinimide Complex Bath Using Imidazole as Reducing Agent 703 substrate, and electroless silver plating was done for a predetermined period. The film thickness and rate were calculated from weight change before and after plating. Surface conditions of deposited film was observed using a scanning electron microscope (JSM-5200 made by Japan Electron Optics Laboratory Co., Ltd. ; hereafter abbreviated SEM). The secondary electron images and reflection electron images were observed at an accelerating voltage of 25 kv, a working distance of 20 mm and a magnification of 3500. 2. 2 Determination of dissolved copper and calculation of substitution amount Copper (Cu2++2 e- > Cu, E =0.340 V) metal has a baser potential than silver (Ag++e--> Ag, E =0.799 V), refore, silver due to substitution reaction can be expected to occur on copper base metal. Because film, which was deposited by substitution reaction, has a large possibility to be a film with poor adhesion which causes easy peeling at interface with substrate, it is necessary to suppress substitution as much as possible. In this study,, which is catalytically reacted on copper surface to emit electron5'', was added to basic plating bath as second reducing agent, and effect was examined. Glyoxylic s of predetermined concentrations were individually added to basic plating bath indicated in Table 1. After soaking copper base metal in each plating bath for a predetermined period, copper ion concentrations in plating bath were determined using RF plasma emission spectrophotometer (SPS-7700 made by Seiko Instruments Inc. ; hereafter abbreviated ICP). The amount of silver due to substitution based on following equation was calculated from amount of copper ion dissolved in plating bath, and amount of silver due to autocatalytic reaction was obtained as difference from total amount of. In above calculation, it was assumed, based on electron configuration, that copper ion is oxidized to 2 valence due to various factors at electrode interface though it is supposed to dissolve as a copper ion of 1 valence during first stage. The substitution reaction of silver (2 mol of Ag is deposited by dissolution of 1 mol of Cu2+) due to dissolution of copper is indicated in following equation. Cu -- Cu2+ + 2 e- 2 Ag+ + 2 e- --~ 2 Ag 2. 3 Local polarization curve In general, it is necessary to measure rate of metal and hydrogen gas and oxidizing velocities of reducing agent to clarify mechanism of electroless plating. Moreover, reaction of electroless plating is interpreted based on mixed potential ory. The mixed potential ory suggests that plating reaction proceeds at a mixed potential (Emp) where oxidizing rate of reducing agent and reduction rate of metal ion becomes equal'. In this study, a local polarization curve was measured by potentiodynamic method to clarify mechanism of this electroless silver plating and to confirm wher or not mixed potential ory is applicable to this process. The HZ-3000 electrochemical system (made by Hokuto Denko Corporation) was used for measurements at potential scanning rate of 10 mv/s. A silver-plated platinum disc of 2 mm in diameter, embedded in a Teflon folder, was used as working electrode, and silver plating of about 3,um in thickness was done at a cathodic current density of 10 ma/cm2 before each measurement. A platinum wire was used for counter electrode and silver-silver chloride electrode (KC1 3.3 mol/l) was used for reference electrode. The electrode potentials in this study are values based on this reference electrode. The solution, which excluded silver nitrate from basic plating bath composition indicated in Table 1, was used for measurement of local anodic polarization curve. Moreover, local anodic polarization curve in each single solution of imidazole, succinimide, and was measured. In measurement of local cathodic polarization curve, solution that excluded imidazole from basic plating bath composition was examined. Nitrogen gas was aerated through solution for 20 minutes before measurements, and oxygen dissolved in solution was eliminated. 3. Results and discussion 3. 1 Film thickness, rate, and amount of dissolved copper by electroless silver plating on copper metal Figure 1 shows relation between plating time, thickness of obtained silver coating and amount of dissolved copper. The film thickness of silver coating increased as plating time increased. On or hand, amount of copper dissolved in plating bath by substitution reaction linearly increased for three hours after start of plating, and n reached almost a constant value. As a result, it is Fig. 1 Relation between plating time and amount of deposited Ag and dissolved Cu substrate.
Technical Paper Fig. 3 Effect of on of Ag. Fig. 2 Effects of bath components and plating conditions on rate. considered that silver in this bath is based on autocatalytic reaction. The rates when various factors were individually changed under condition of plating and basic plating bath composition indicated in Table 1 are shown in Figure 2. The rate of silver increased as concentration of silver nitrate, concentration of imidazole, and bath temperature increased. Moreover, it decreased as concentration of succinimide increased. The rate of silver increased as concentration of imidazole increased, and a continuous was not observed in bath which did not contain imidazole. Consequently, it is considered that imidazole worked as a reducing agent. The effect of ph on rate was remarkable, and rate decreased as ph increased. Moreover, an increase in rate due to an increase in bath temperature was observed. Based on se results, a neutral plating condition and low temperature can be used for electroless silver-plating bath when succinimide is complexing agent, imidazole is reducing agent, and bath composition and plating conditions shown in Table 1 are considered to be optimum. 3. 2 Effect of concentration of on amount of dissolved copper and film thickness Glyoxylic of 0.010 mol/l, 0.030 mol/l, and 0.050 mol/l were individually added to basic plating bath indicated in Table 1, and copper base metal was soaked in each plating bath for a predetermined period. The result of examining effect of concentration Fig. 4 Effect of concentration on dissolved Cu substrate. of on amount of deposited silver and dissolved copper is shown in Figures 3 and 4. In Figure 3, effect of concentration of on thickness of obtained silver film is shown, and thickness of silver film obtained from basic plating bath shown in Table 1 is indicated by symbol "0". It is understood that thickness of deposited silver film decreased as concentration of, which was added in basic plating bath, increased. Figure 4 shows effect of concentration of on amount of copper, which was dissolved in plating bath by substitution reaction, amount of copper dissolved from base metal decreased as concentration of increased. Moreover, substitution reaction with copper almost stopped in about three hours after starting plating in basic bath without addition. On or hand, substitution reaction stopped in about
Direct Succinimide Vol. 52, No.10, 2001 Electroless Complex Silver Plating on Copper Metal from Bath Using Imidazole as Reducing Agent 705 Oxidation reaction of Glyoxylic on copperr without with Glyoxylic Glyoxylic Fig. 6 Mechanism oxidation metal. bath, Fig. 5 Effect of autocatalytic on of Ag. two hours after starting plating displacement for acceleration reaction On or nucleation of of nucleation with on Cu substrate. hand, in -added by oxidation reaction of on copper base metal simultaneously occurs with nucleation due to substitution reaction as shown in Figure 6. However, it is considered that and generation of silver nucleus caused by occurs rar than nuclear growth of generated silver nucleus, refore, particle size is small, and masking of base metal is completed in a short time. The addition of into basic for 0.010 mol/l added bath, and in about one hour after starting plating for 0.030 mol/l and 0.050 mol/l added baths. Figure 4 shows amount of copper dissolved in basic plating bath and -added bath by substitution reaction with silver. Figure 5 shows amount of silver deposited by autocatalytic reaction, where amount of silver due to substitution is calculated from this amount of dissolved copper measured by ICP method ; this amount is subtracted from total amount of deposited silver. From this figure, it is understood that silver by substitution reaction was suppressed as concentration of increased. Based on this result, decrease in total amount of silver (Figure 3) obtained from baths can be understood : basic plating bath, 0.010 mol/l -added bath, 0.030 mol/l -added bath, and 0.050 mol/l -added bath, is attributed to decrease in amount of substitution. Such an action of is schematically expressed in Figure 6. In bath without, silver particle is deposited on base metal by substitution reaction of copper with silver ion. It is considered that deposited silver particles behave as nucleus during electroless plating reaction, nuclear growth on surface occurs rar than new nucleation on base metal, refore, particle size increases, and more time is required to mask base plating bath causes decrease autocatalytic silver, which decrease in concentration of liberated in rate of can be due to imidazol formed through azomethyne-forming condensation of imide group with aldehyde group of. Moreover, it was confirmed that electroless copper, which used as a reducing agent, from copper ion dissolved into bath by substitution reaction did not occur in neutral range. From se results, it was understood that added as second reducing agent was effective in suppressing substitution of silver. 3. 3 Appearance of electroless silver plating film on copper metal Figure 7 shows SEM images that describes effect on initial of silver by adding glyox- ylic, which was added to basic plating bath as second reducing agent in order to suppress substitution of silver. Both are surface conditions of silver plating having a film thickness of about 0.5,um. In silver plating film obtained from basic plating bath, which did not include, silver particles, which were deposited during initial stage, were comparatively large. On or hand, deposited silver particles film obtained from -added 51 in bath
Technical Paper Glyoxylic Omol/L Glyoxylic 0.01Omol/L Glyoxylic 0.030mol/L Glyoxylic 0.050rnol/L Fig. 7 Effect on surface morphology of Ag film by adding. Thickness Ag films : 0.5 um became small while increasing concentration of. This is considered to be reason why it did not require time to mask base metal and suppress substitution reaction. Moreover, deposited silver film did not peel off during tape peeling off testing, which indicated excellent adhesion. 3. 4 Deposition mechanism Concerning mechanism of electroless silver plating in which succinimide was used as complexing agent and imidazole was used as reducing agent, we have already clarified it in a previous report'". In this study, effect when was used as second reducing agent exerted on behavior of silver was examined by polarization measurements using potentiodynamism. The result is shown in Figure 8. The solution, which excluded silver nitrate, a metal salt, from basic plating bath composition indicated in Table 1 was used for measurement of local anodic polarization curve. Moreover, anodic polarization curves of this solution to which 0.010 mol/l was added, and 0.010 mol/l single solution (ph=7 for both solution) were also measured. In measurement of local cathodic polarization curve, solution, which excluded imidazole from basic plating bath composition shown in Table 1, was examined. Curve "e" is anodic polarization curve of single solution, and anodic current flowed in potential range nobler than about -0.2 V. Moreover, an anodic Fig. 8 Anodic polarization and cathodic polarization of Ag electrode at 50 C. Working electrode : disc of 2 mm in diameter
Vol. 52, No.10, 2001 Direct Electroless Silver Plating on Copper Metal from Succinimide Complex Bath Using Imidazole as Reducing Agent 707 current flowed from about -0.2 V in anodic polarization curve "f" for solution which contained 0.010 mol/l in basic plating bath. However, because anodic current at about 0.22 V where mixed potential exists, is almost equal to local anodic polarization curve "a" obtained from basic plating bath, it is considered that anodic current around - 0.2 V of curve "c" is attributed to oxidation reaction of, and anodic current around 0-0.6 V is mainly attributed to oxidation reaction of Imidazole. Based on results of Figure 5 and Figure 8, it is elucidated that effect of, which is second reducing agent, on behavior of silver is negligible. The natural electrode potential of silver electrode in basic plating bath to which 0.010 mol/l had been added was 0.24 V, and it almost coincided with mixed potential obtained from local polarization curves "c" and "d" in Figure 8. Moreover, calculated value of silver rate obtained from current value in this mixed potential almost coincided with actual data of silver rate obtained from this bath. Consequently, it has been determined that mixed potential ory is applicable to this reaction. 4. Conclusion Concerning electroless silver plating from a neutral bath which uses an organic nitrogen compound as complexing agent and reducing agent, plating conditions to deposit silver coating during autocatalytic have been established. The possibility of using a second reducing agent to suppress substitution of silver due to dissolution of copper basis metal was examined. The behavior of silver was electrochemically analyzed by measuring local anodic polarization and local catholic polarization. As a consequence of this study, following results were obtained. 1) The electroless silver plating bath containing succinimide, which is an organic nitrogen compound, as a complexing agent, and Imidazole as a reducing agent, exhibited features that it could be used under neutral and low temperature conditions considering durability of organic resist, and it does not contain cyanide. The silver film was deposited by an autocatalytic reaction from bath, and bath also excelled in stability. Moreover, neir turbidity of plating bath nor abnormal of silver was observed. 2) The silver by substitution reaction with copper could be suppressed by adding as second reducing agent. Moreover, silver film, which was obtained from -added bath, exhibited a dense and smooth surface with excellent adhesion because particle size of deposited silver decreased with an increase in concentration of. 3) The mixed potential obtained from local anodic and cathodic polarization curve in basic plating bath almost coincided with natural electrode potential of silver electrode in plating bath. Moreover, actual silver rate coincided with calculated value of silver rate obtained from current value at mixed potential. From se results, it has been determined that mixed potential ory is applicable for reaction in electroless silver-plating bath used in this study. (Received April 16, 2001; Accepted August 8, 2001) References 1) G. Nakamoto ; Proc. of 9 th Microelectronics Symposium, p. 205 (1999). 2 ) K. Shiroguchi, H. Nawaf une, S. Mizumoto, T. Takeuchi, Y. Kohashi ; Proc. of 14 th JIEP Annual Meeting, p. 241 (2000). 3 ) H. Nawaf une, K. Shiroguchi, S. Mizumoto, Y. Kohashi, K. Obata ; Hyomen Gijyutsu, in press. 4 ) K. Shiroguchi, H. Nawaf une, S. Mizumoto, Y. Kohashi, K. Obata ; Proc. of 103 th Annual Conference of SFJ, p. 163 (2001). 5 ) J. Darken ; PCWCV, B 6/2 (1990). 6 ) H. Honma, M. Komatsu, T. Fujinami ; Hyomen Gijyutsu, 42, 913 (1991). 7 ) M. F. Paunovic ; Plating, 55, 1161(1968).