Impedance and Morphological Properties of Electroless Gold on

Impedance and Morphological Properties of Electroless Gold on Industrial Metal Coupons. Anita Sargent, and Omowunmi A. Sadik*. Department of Chemistry...
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Impedance and Morphological Properties of Electroless Gold on Industrial Metal Coupons Anita Sargent and Omowunmi A. Sadik* Department of Chemistry, State University of New York at Binghamton, Binghamton New York 13902-6016 Received September 5, 2000. In Final Form: February 16, 2001 An approach for correlating the bath chemistry during electroless gold deposition with the overall plating quality on industrial wirebond samples is presented. Electrochemical impedance spectroscopy of the reducing agent dimethylamine borane (DMAB) on gold in alkaline solution was used to provide qualitative information on the interfacial adsorption/desorption processes occurring during deposition. Depending on hydroxide and DMAB concentration, impedance data obtained under hydrodynamic conditions at the gold oxide reduction potential (0.184 V vs Ag/AgCl) follow two different equivalent circuit models that do not exhibit contribution from diffusion. Gold deposit quality was assessed with scanning electron microscopy and energy-dispersive spectrometry under a variety of bath compositions. Finally, bath efficiency was evaluated in terms of plating rate obtained from X-ray fluorescence data. The substrate surface/solution interactions are linked to the final quality of the resulting gold layer. This work shows that substrate incompatibility and the ratio imbalance between gold and the reducing agent concentrations produce thin, dull deposits. Maintaining the optimum ratio of DMAB/KOH provides balance in the bath chemistry necessary for efficient plating.

Introduction Electroless gold is frequently used in wirebonding applications as a suitable finish for mounting chips to chip carriers. The bonding pad metal should be of high purity and homogeneity for optimum performance.1-3 Gold is a choice metal for this purpose because it is highly resistant to corrosion and oxide formation, is soft enough for optimal thermosonic wirebond processing, and is capable of withstanding mechanical stresses.4-6 A commonly used electroless gold bath employs KAuCN2 as a gold source, KCN as stabilizer, and dimethylamine borane (DMAB) as a reducing agent in a suitable alkaline media (pH 13-14).7 Plating rates obtained with this bath are typically 1-3 µm/h.8 Industrial applications require that bath quality be periodically assessed for plating rate. Traditionally, this involves the use of metal coupons immersed in the bath for a period of time, after which the plating rate is calculated gravimetrically. The quartz crystal microbalance9,10 and X-ray fluorescence (XRF) are also used to readily determine the deposit thickness. A decreased plating rate could indicate a number of problems: contamination from underlying base metals and/or organic * Corresponding Author: Fax: (607) 777-4478, E-mail: osadik@ binghamton.edu. (1) Harman, G. Wirebonding in MicroelectronicssMaterials, Processes, Reliability and Yield; McGraw-Hill: New York, 1997; Chapter 6. (2) Tummala, R. R.; Rymaszewski, E. J.; Klopfenstein, A. G. Microelectronics Packaging Handbook; Van Nostrand Reinhold: New York, 1989; Chapter 6. (3) Konsowski, S. G., Helland, A. R., Eds. Electronic Packaging of High-Speed Circuitry; McGraw-Hill: New York, 1997; p 64. (4) Ulrich, R.; Wasef, M.; Garrou, P.; Scheck, D.; Im, J.-H. Int. J. Microcircuits Electron. Packag. 1999, 22, 190. (5) Watanabe, H.; Abe, S.; Honma, H. J. Appl. Electrochem. 1998, 28, 525. (6) Okinaka, Y.; Hoshino, M. Gold Bull. 1998, 31, 3. (7) Okinaka, Y. Plating 1970, 57, 914. (8) Gaudiello, J. G. IEEE Trans. Compon., Packag., Manuf. Technol., Part A 1996, 19, 41. (9) Kanazawa, K. K.; Borges, G. L.; Doss, S.; Hildebrand, C. AESF Annual Technical Conference, SUR/FIN 1995, 21. (10) Kanazawa, K. K.; Borges, G. L.; Doss, S.; Hildebrand, C. AESF Annual Technical Conference, SUR/FIN 1994, 623.

laminates, consumption of the reducing agent used to drive the chemical reaction, or an inadequate ratio of other chemical bath components needed for optimum plating. In this study, the electroless gold deposition process is examined using industrial wirebond monitors preprocessed as described above. Because the electroless gold plate is the final step in the preparation of suitable wirebonding substrates, we focused on this outermost layer. Electrochemical impedance spectroscopy (EIS) was used to generate qualitative information on the substrate surface/solution interactions occurring during the electroless gold process. The utility of EIS in providing mechanistic and kinetic information has been recently demonstrated in the study of zinc oxide layers under various illumination conditions,11 the oxidation of formaldehyde on gold,12 and the study of electroless Cu and Ni deposition.13-16 To date, we are not aware of any EIS studies on the electroless gold bath using DMAB as reducing agent. Scanning electron microscopy (SEM) and energy-dispersive spectrometry (EDS) were used in assessing bath performance with regards to surface quality and elemental composition on plated wirebond samples. XRF provides the necessary information on plating rate by measuring the thickness of successive layers of metal films. This work reports a novel correlation between the bath chemistry and the overall quality of plated circuitry. Experimental Section Impedance of DMAB on Gold. DMAB solutions were prepared in 0.7 M KOH using deionized water (17.5 MΩ cm) in the concentration ranges of 1.7 × 10-2 to 1.4 × 10-1 M and 8.5 × 10-4 to 1.7 × 10-2 . A gold disk electrode (BAS, 0.0201 cm2) served as the working electrode. Ag|AgCl and Pt wire served as reference and counter electrodes, respectively. An EG&G model 263A potentiostat was used in conjunction with the EG&G model (11) Rudd, A. L.; Breslin, C. B. Electrochim. Acta 2000, 45, 1571. (12) ten Kortenaar, M. V.; Tessont, C.; Kolar, Z. I.; van der Weijde, H. J. Electrochem. Soc. 1999, 146, 2146. (13) Gaudiello, J. G.; Ballard, G. L. IBM J. Res. Dev. 1993, 37, 107. (14) Lo, P.-H.; Tsai, W.-T.; Lee, J.-T.; Hung, M.-P. J. Electrochem. Soc. 1995, 142, 91. (15) Sato, N.; Suzuki, M. J. Electrochem. Soc. 1988, 135, 1645. (16) Gafin, A. H.; Orchard, S. W. J. Appl. Electrochem. 1992, 22, 830.

10.1021/la001267e CCC: $20.00 © 2001 American Chemical Society Published on Web 03/29/2001

Properties of Electroless Gold

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Table 1. Electroless Gold Bath Composition KOH DMAB

40 g/L 5 g/L

KAuCN2 KCN

3 g/L 2.1 g/L

5210 lock-in amplifier for potentiostatic control and impedance analysis. The gold electrode was first polished with coarse alumina, ultrasonicated, polished with fine alumina, ultrasonicated again, and rinsed thoroughly. Prior to impedance analysis, the lowest DMAB concentration was scanned voltammetrically to determine the precise potential location to be used as the applied potential in the impedance scan. An initial cell solution of 100 mL was used, with injections of 3.4 × 10-1 M stock DMAB added to a stirring solution to increase the total DMAB concentration in the cell prior to each impedance scan. After adding stock DMAB solution, the cell was allowed to equilibrate for 2 min prior to running the impedance measurement. The electrode was not removed or disturbed during the course of the experiment. An impedance spectrum was collected at each DMAB concentration within the frequency range of 100 kHz to 100 mHz at an amplitude of 5 mV root mean square under moderate agitation. Electroless Gold Plating of Wirebond Samples. Ten laminate chip carrier (LCC) wirebond monitor samples were obtained from IBM Microelectronics, Endicott, NY, and used as substrates for electroless gold plating. The samples were previously processed at IBM prior to the electroless gold plate as follows: The copper circuitry was coated with electroless nickel to a thickness of 2.92 µm, followed by an immersion gold layer of 7.62 × 10-2 µm. Wirebond samples were cleaned by rinsing with ethanol followed by immersion in 50 °C deionized water with slight agitation for 2 min. This gentle cleaning method was employed to maintain soldermask integrity. The electroless gold bath consisted of KOH, KAuCN2, KCN, and DMAB at the various concentrations listed in Table 1. No other bath additives were used in order to simplify the present study. Wirebond samples were suspended by a thin Teflon string threaded through holes drilled into the sample. Plating time was 45 min at 70 °C under moderate agitation of a solution of 500 mL volume. After careful rinsing and drying of each sample, the samples were analyzed by scanning electron microscopy (SEM, Philips-Electroscan, model 2020) and energy-dispersive spectrometry (EDS Princeton Gamma-Tech IMIX X-ray Microanalysis System) for surface topography and elemental analysis, respectively. Plating thickness was determined by X-ray fluorescence measurements (XRF) using a Fisherscope X-ray System XDVM.

Results Monitoring the plating rate in industrial electroless gold plating tanks ensures suitable deposit quality and high product throughput. For the “die up” LCC applications, wirebond monitors are immersed in industrial electroless gold plating tanks and analyzed by XRF to determine the plating rate. Wirebonding failures on these chip carriers have been mainly attributed to either insufficient electroless gold cover or metallic and organic contamination from poor handling. Though the contribution of contamination from poor handling can be controlled, a better understanding of the electroless chemistry is necessary to compensate for insufficient gold coverage. In a typical LCC processing scheme, electroless Ni plating is initiated on clean copper circuitry using a Pd seed process. The electroless nickel acts as a diffusion barrier to prevent copper from contaminating the ad layers. A relatively thin (0.1 µm) immersion Au layer is then deposited prior to the final electroless gold plate. The immersion Au layer prevents Ni ion accumulation in the bath. Because Ni lies lower in the galvanic series than Au, electroless deposition of gold directly onto the Ni surface initially proceeds as a displacement reaction, generating free Ni ions which severely decreases the electroless gold plating rate.17 The immersion gold also serves as a smooth catalytic surface for the electroless gold plate. The electroless gold serves as a suitable finish

Figure 1. Laminate chip carrier (LCC) multilayer architecture.

for thermosonic wirebonding. Our studies focus on this layer and the process by which it develops. The multilayered architecture for a LCC is illustrated in Figure 1. Plating Chemistry and the Microscale Processes during Electroless Gold Plating. The ratio of hydroxide to DMAB is very important for electroless gold deposition. Primarily, the reducing agent DMAB needs alkaline media in order to generate the trihydrohydroxy borate ion BH3OH- 18,19

(CH3)2NH‚BH3 + OH- f (CH3)2NH + BH3OH- (1) The boron in BH3OH- has a valency of -3 and can act as an electron donor. The overall electroless gold deposition process in alkaline solution is generally represented as18,19

(CH3)2NH‚BH3 + 4OH- + 3Au(CN)2- f (CH3)2NH + BO2- + 3/2H2 + 2H2O + 3Au + 6CN(2) The formation of the “true” reducing agent, BH3OH-, depends on the pH of the solution as shown in eq 1. BH3OH- is considered a reactive intermediate, which undergoes oxidation at -0.64 V vs SCE.20-24 The change in chemical structure that DMAB undergoes at the gold surface in alkaline media is still an unclear process.25 Some researchers believe that the reducing agent adsorbs to the gold surface within a potential region where it can reduce a previously formed gold monolayer surface oxide.26 Electrochemical quartz crystal microbalance (EQCM) studies indicate that a reactive intermediate adsorbs onto the gold surface in the region where the gold oxide is reduced, resulting in a mass increase concurrent with a cathodic reactivation peak.27 The origin of the reactivation peak and the microscopic process it signifies may provide clues into the interaction between DMAB and the catalytic gold surface during plating. Therefore, studying the adsorption of this intermediate onto the gold surface can help to elucidate the causes of poor gold coverage. Moreover, the gold surface is affected by the hydroxide concentration. At high pH, surface gold atoms interact with OH- species in solution to form a passivating gold oxide species at sufficiently anodic potentials, i.e., ∼0.4 V vs Ag/AgCl. Although oxide forms in neutral or acidic media,28-30 passivation increases with hydroxide concen(17) Mallory, G. O., Hajdu, J. B., Eds. In Electroless Platings Fundamentals and Applications; American Electroplaters and Surface Finishers Society: Orlando, FL, 1990; p 404. (18) Simon, F. Gold Bull. 1993, 26, 14. (19) Canaperi, D. F.; Jagannathan, R.; Krishnan, M. US Patent 5635253, 1997. (20) Pecsok, R. L. J. Am. Chem. Soc. 1953, 75, 2862. (21) Elder, J. P.; Hickling, A. Trans. Faraday Soc. 1962, 58, 1852. (22) Elder, J. P. Electrochim. Acta 1962, 7, 417. (23) Gardiner, J. A.; Collat, J. W.; J. Am. Chem. Soc. 1965, 87, 1692. (24) Gardiner, J. A.; Collat, J. W. Inorg. Chem. 1965, 4, 1208. (25) Homma, T.; Nakai, H.; Onishi, M.; Osaka, T. J. Phys. Chem. B 1999, 103, 1774. (26) Burke, L. D.; Lee, B. H. J. Appl. Electrochem. 1992, 22, 48. (27) Sargent, A.; Sadik, O. A. J. Electrochem. Soc., in press. (28) Bruckenstein, S.; Shay, M. J. Electroanal. Chem. 1985, 138, 131.

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Figure 2. (a) Cyclic voltammogram for 10 mM DMAB in KOH on gold. Scan rate ) 50 mV/s. (b) Effect of DMAB:KOH molar ratio on reactivation peak position for 10 mM DMAB on gold. Concentration of KOH ) (a) 17.7 mM, (b) 44.1 mM, (c) 88.3 mM, (d) 0.442 M, and (e) 0.630 M. Scan rate ) 50 mV/s.

tration.31 Therefore, the competition between DMAB and OH- ions for access to the gold surface can influence gold nucleation and growth of the electroless gold layer. Electrochemistry of the Plating Process by CV and EIS. Successful electroless gold plating depends on the catalytic activity of the metal substrate toward oxidation of the reducing agent. Therefore, knowledge of the electrochemical behavior of the reducing agent DMAB on gold is useful from a mechanistic standpoint. The DMAB:KOH molar ratio is important to the efficiency of the electroless gold plate on a macroscale.32 Therefore, a study on the DMAB:KOH molar ratio was undertaken at a microscale level to determine the relevance of this ratio to successful electroless gold plating. Figure 2a shows a cyclic voltammogram of DMAB on gold in 0.7 M KOH electrolyte. The anodic scan typically reveals a large broad DMAB oxidation peak beginning at -0.8, which is later suppressed due to growth of the gold oxide layer at 0.4 V. Upon reversal of the scan, the gold oxide layer is reduced and the surface “reactivates” with respect to DMAB oxidation, causing the anodic current at 0.2 V. This behavior suggests irreversibility of DMAB oxidation. The peak position of this reactivation depends on the molar ratio of DMAB:KOH, as shown in Figure 2b. In general, the reactivation peak increases in height and area with an increase in hydroxide concentration, suggesting that (29) Gordon, J. S.; Johnson, D. C. J. Electroanal. Chem. 1994, 365, 267. (30) Juodkazis, K.; Juodkazyte, B.; Sˇ ebeka, B.; Lukinskas, A. Electrochem. Commun. 1999, 1, 315. (31) Guzman, L.; Segarra, M.; Chimenos, J. M.; Cabot, P. L.; Espiell, F. Electrochim. Acta 1999, 44, 2625. (32) Sargent, A.; Gaudiello, J. G.; Sadik, O. A. SPIE Symp. Ser. 3537 1998, A23, 119.

Sargent and Sadik

a high pH is favorable for DMAB oxidation on gold. At hydroxide concentrations lower than the DMAB concentration, there is either none or very little charge associated with the reactivation peak, since sufficient hydroxide is required to generate the reactive species and thus the reactivation peak. A maximum cathodic charge occurs at a DMAB:KOH molar ratio of 10:100. Beyond this ratio the cathodic charge accompanying reactivation decreases significantly, probably due to enhanced coverage of the gold surface by OH-. Adsorption of OH- induces surface passivation such that the possibility of DMAB oxidation, i.e., reactivation, is lessened due to competition between OH- and BH3OH- for gold sites. We studied the impedance behavior of gold in solutions by varying the molar ratios of DMAB:KOH. A typical electroless gold bath employs DMAB at a concentration of 8.5 × 10-2 M. With this in mind, initial impedance studies of DMAB on gold were conducted within the range 1.7 × 10-2 to 1.4 × 10-1 M DMAB at 70 °C. At this temperature and DMAB concentration, excessive bubble formation at the electrode surface was observed causing high solution resistance. Room-temperature experiments employing the same DMAB concentrations had a similar effect. Employing a lower concentration range of 8.5 × 10-4 to 1.7 × 10-2 M DMAB at room temperature under mild stirring yielded the desired results. The bias potential for the impedance measurements was selected according to the cyclic voltammogram. The primary peak of interest is the reactivation peak at 0.2 V, as this peak only occurs at sufficient hydroxide concentration and may indicate the relative facility of DMAB oxidation on gold. At this potential we believe that a reactive intermediate adsorbs to the surface during the cathodic scan. This is the same potential regime where reduction of the anodically formed gold oxide occurs,27-30 and reduction of the passivating oxide layer allows DMAB oxidation to resume. Therefore, the first impedance measurements were conducted at a bias potential of 0.184 V, just at the foot of the reactivation peak. The impedance spectra of DMAB on gold at this potential are shown in Figure 3a. In this plot, frequencies below 25.12 Hz generated spurious data, so they were omitted for clarity. The low frequency (LF) arcs reveal negative resistances terminating in the second quadrant. The LF loops become more compact with an increase in DMAB concentration. Furthermore, the LF loops terminate at less negative resistances with an increase in DMAB concentration. This is consistent with a decrease in charge-transfer resistance, Rct, with an increase in the concentration of redox active species. The most unique feature of these impedance spectra is the negative resistance associated with the RC loops. The origin of a negative resistive element is attributed to increasing coverage of the surface by an adsorbed intermediate and is also correlated to a negative slope in the polarization curve.33,34 An excellent example of negative Rct is documented by Dobbelaar and de Wit.35 They observed negative values of Rct during the active-passive transition of chromium dissolution. The existence of the negative differential resistance is also explained in theoretically derived equations that describe impedance behavior of electrode reactions involving adsorbed inter(33) MacDonald, J. R., Ed. In Impedance Spectroscopy; John Wiley & Sons: New York, 1987; p 285. (34) Keddam, M.; Lizee, J. F.; Pallotta, C.; Takenouti, H. J. Electrochem. Soc. 1984, 131, 2016. (35) Dobbelaar, J. A. L.; de Wit, J. H. W. J. Electrochem. Soc. 1990, 137, 2038.

Properties of Electroless Gold

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Figure 3. Complex-plane impedance plot for DMAB on gold at 0.184 V under moderate agitation in (a) 0.7 M KOH and (b) 7.0 × 10-2 M KOH. Amplitude ) 5 mV root mean square. (c) Complex-plane impedance plot of gold in 0.7 MKOH at 0.184 V under the same conditions. (d) Complex-plane impedance plot for DMAB on gold at -0.7 V under the same conditions.

mediates.36-38 Although the concept of a negative resistance is unfamiliar, the electrochemical phenomena occurring at the reactivation potential can exhibit the same frequency behavior as that of the negative resistance.33 In the area of electroless metal deposition, negative RC loops have previously been observed for oxidation of formaldehyde on gold in studies of electroless copper using formaldehyde as a reducing agent.12 The species interacting with the gold surface at the reactivation potential may contain boron.27 Therefore, it is likely that BH3OH- or an intermediate of another chemical structure containing boron adsorbs to the gold surface at this potential. This behavior is dependent on DMAB concentration, confirming that the adsorbed species originates from DMAB and not the gold surface or hydroxide electrolyte. Furthermore, as the KOH concentration decreases, the tendency toward negative resistive behavior decreases and the termination of the low frequency (LF) data occurs at a higher resistance value, as shown in Figure 3b. In fact, at low hydroxide concentration, there is a clear shift in the impedance spectra from a positive to negative resistive element between 8.5 (36) Macdonald, J. R. J. Electroanal. Chem. 1976, 70, 17. (37) Franceschetti, D. R.; Macdonald, J. R. J. Electroanal. Chem. 1977, 82, 271. (38) Personal communication with J. R. Macdonald, December 2000.

× 10-4 and 1.7 × 10-3 M DMAB. Within this concentration range, a capacitive element dominates. Assuming that adsorption of the reactive intermediate generates the negative resistance and that the intermediate, BH3OH-, participates by donating electrons to the gold/solution interface during the oxide reduction process, parts a and b of Figure 3 suggest that there is a lower limit in DMAB concentration at which the double-layer capacitance, Cdl, is maximum. A maximum Cdl would correlate to a decreased double-layer thickness, which could be the case at very low DMAB or reactive intermediate concentrations. Figure 3c shows the impedance spectrum of gold in pure KOH at 0.184 V, which exhibits a simple charge-transfercontrolled reaction. The spectra obtained in hydroxide electrolyte did not change with additional hydroxide. This suggests that kinetic factors, rather than the diffusion of OH- to the gold surface, predominate the electrode reactions and the reactivation peak at 0.184 V is dependent on hydroxide concentration only in the presence of DMAB and not in hydroxide solution alone. This highlights the importance of the DMAB:KOH molar ratio on the oxidation of DMAB and may provide insight into the catalytic nature of the electroless gold bath, since the plating rate increases with both DMAB and KOH concentration.32 These results suggest that the plating efficiency may be correlated to

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Figure 4. Variation of 1/Rct with DMAB concentration.

the competing adsorption/desorption of DMAB and OHto the autocatalytic gold surface. To determine if the adsorptive behavior is an exclusive property of the gold/DMAB interface at the reactivation potential, we investigated another potential region. Oxidation of DMAB generally commences in the region -0.9 to -0.6 V depending on pH. The impedance spectra behave quite differently at -0.7 V than at the reactivation potential, as illustrated in Figure 3d. Shown here is typical behavior for an electrochemical process undergoing simultaneous kinetic and diffusion control observed over a wide frequency range. This system does not exhibit negative resistance behavior associated with adsorption because DMAB oxidation is just beginning and any reactivation current is negligible. An increase in DMAB concentration results in a lower value of Rct. The relation 1/Rct is proportional to the plating rate according to13

1/Rct ) ipl/K

(3)

where ipl is the exchange current at the mixed-potential where electroless deposition occurs and K ) RT/RnF, where R is the mass transfer coefficient and n is the number of electrons transferred. Although eq 3 is intended for the complete bath containing both gold ion and reducing agent at steady state, it is apparent that an increase in DMAB concentration near its oxidation potential theoretically increases the plating rate. Values of 1/Rct (extrapolated from Figure 3d) increased with DMAB concentration as shown in Figure 4. These results indicate that adsorption is confined to the potential region where reactivation of DMAB oxidation occurs, i.e., where the change in the slope of the i-E curve is negative. Role of Cyanide on the Microscopic Processes in the Electroless Gold Bath. Free cyanide is used in the electroless gold bath as a stabilizer and has good brightening properties. It is well-known that cyanide promotes dissolution of gold.31 Therefore, the electroless deposition of gold via DMAB oxidation must inherently compete with the gold dissolution process. Furthermore, free cyanide ions are generated as the deposition proceeds according to eq 2, and the plating rate gradually declines unless care is taken to remove the free cyanide. Avoiding free cyanide buildup has been a subject of study on the DMABbased electroless gold bath.19 Interestingly, gold dissolution due to cyanide complexation becomes minimal at very high pH values. Some workers reported a 100 µmol/(m2 s) drop in the rate of gold dissolution in the presence of cyanide between pH 12 and 13.31 This was attributed to the adsorption of OH- to the gold surface thus implying an additional advantage to operating the electroless gold bath at high pH. In this work, we studied the effect of

Figure 5. (a) Cyclic voltammogram of 8.5 × 10-4 MDMAB in 0.7 MKOH plus 3.2 × 10-2 M KCN on gold. Scan rate ) 50 mV/s. (b) Complex-plane impedance plot for 8.5 × 10-4 M DMAB in 0.7 M KOH plus 3.2 × 10-2 MKCN on gold at 0.132 V under moderate agitation. Amplitude ) 5 mV root mean square.

cyanide on the impedance spectra of DMAB on gold because the cyanide concentration directly influences the gold deposition rate. We tested whether adsorption of cyanide inhibits the reactivation current and, hence, correlates to a lower plating rate (described later for plating industrial LCC samples). The cyclic voltammogram of 8.5 × 10-4 M DMAB in the presence of 3.2 × 10-2 M KCN in Figure 5a shows significantly enhanced anodic current magnitudes compared to that obtained in the absence of cyanide. The anodic peaks at 250 and 400 mV are attributed to gold-cyanide complexes formed from the combination of gold atoms generated during the anodic scan with cyanide in solution.30 Therefore, the increase in cyanide promotes the formation of soluble gold cyanide complexes, leading to gold dissolution. A reactivation peak is still observed on the cathodic scan in the presence of cyanide. However, the reactivation peak exhibits a negative shift in potential and a decreased current density in the presence of cyanide. This implies that DMAB adsorption to the gold surface during reactivation is inhibited by cyanide ion. Figure 5b shows the corresponding impedance spectra of DMAB at the reactivation potential in the presence of 3.2 × 10-2 M cyanide, which does not show the negative resistance discussed earlier. Instead, the impedance spectra exhibit chargetransfer control at high frequencies and inductive behavior (i.e., the loop) at lower frequencies. Similar impedance spectra were obtained within a DMAB concentration range of 8.5 × 10-4 to 1.7 × 10-2 M. This implies that the presence of cyanide inhibits the reactivation of DMAB oxidation at the gold oxide reduction potential. It appears that the concentration of cyanide compared to DMAB is so high that (i) gold dissolution via cyanide adsorption is more

Properties of Electroless Gold

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Table 2. Wirebond (WB) Samples Plated with Electroless Gold sample

a

conditions

concentration (M)

WB 1 WB 2

optimized batha low DMAB

a 4.2 × 10-2 M DMAB

WB 3 WB 4 WB 5 WB 6 WB 7 WB 8 WB 9 WB 10

high DMAB low KCN high KCN high KOH low KOH high KAuCN2 low KAuCN2 optimized bath w/0.1 M NaCl

1.7 × 10-1 M DMAB 1.6 × 10-2 M KCN 6.5 × 10-2 M KCN 1.4 M KOH 7.1 × 10-2 M KOH 2.4 × 10-2 M KAuCN2 5.2 × 10-3 M KAuCN2 a

appearance orange-gold not uniform, showing nickel; dissolution of immersion Au orange-gold yellow-gold grayish-gold; dissolution observed yellow-gold all gray; all immersion gold etched yellow-gold not uniform; dissolution observed yellow-gold

rate by XRF (µm/h) 0.044 0.000 0.081 0.047 -0.037 0.054 -0.102 0.054 -0.034 0.044

40 g/L KOH; 2.1 g/L KCN; 3 g/L KAuCN2; 5 g/L DMAB.

probable than DMAB oxidation via adsorption of DMAB to the gold surface and (ii) gold oxide formation is restricted due to CN- competing with OH- for active gold sites. The reaction is largely under charge-transfer control, and the apparent Rct decreases with increasing DMAB concentration. Macroscopic Processes during Electroless Gold Plating Using Wirebond Samples. To qualitatively assess the effect of bath variables on the final electroless gold deposit on a macroscopic scale, 10 wirebond (WB) monitor samples were plated using the bath compositions listed in Table 2. Samples were analyzed before and after plating using SEM and EDS for elemental analysis. Prior to being plated, SEMs of all 10 wirebond samples reveal a globular electroless nickel/immersion gold surface, the granular surface of which is dictated by the electroless nickel layer underneath. The plating rates obtained in this experiment are lower than expected. Suppression of the plating rate may result from the absence of bath additives and plating enhancers. Without the stabilizers, gold nuclei can form in solution in the presence of dissolved or suspended impurities, such as soldermask material. The incompatibility of certain baths with soldermask material presents an obstacle to metal finishing.39,40 Nonetheless, the plating rates vary in relation to the concentration of bath components according to the overall deposition reaction in eq 2. After the samples were plated, SEM analysis exhibits distinct variety in surface morphology with change in chemical bath parameters. Visual inspection of the resulting gold color of plated surfaces indicates that rapid plating leads to an orange-gold color, whereas sluggish plating yields a yellow-gold deposit. This can be explained on the basis of grain size. A high plating rate tends to initiate fewer nucleation sites with more dense growth of these nuclei, resulting in columnar crystal growth with a rougher appearance (greater light scattering). Low plating rates favor interfacial stability and allow more time for the reducing agent and cyanide to adsorb/desorb from the surface. The result is a denser population of nucleation sites with a smooth growth pattern. The Effect of Chemical Composition on the Macroscopic Plating Processes. The optimized bath is one where electroless gold deposition occurs at a suitably rapid rate without compromising bath stability. A high plating rate will cause uneven plating or unwanted extraneous deposits. We use the optimized bath formulation shown in Table 2. After the electroless gold plate using this formulation (WB 1 in Table 2), the surface topography remains similar to the immersion gold surface, as shown in Figure 6a, except that the deposit exhibits a more grainy appearance due to increased gold thickness. Elemental (39) Okinaka, Y.; Sard, R.; Wolowodiuk, C.; Craft, W. H.; Retajczyk, T. F. J. Electrochem. Soc. 1974, 121, 56. (40) Ali, H. O.; Christie, I. R. A. Gold Bull. 1984, 17, 118.

Figure 6. (a) SEM micrograph of a wirebond sample surface after electroless gold plate using the optimized bath conditions for WB 1 (conditions as in Table 1). (b) EDS spectrum of the surface of WB 1 after plating (Supporting Information).

analysis by EDS of this surface indicates the presence of both gold and nickel, as shown in Figure 6b (Supporting Information). Because the thickness of the electroless gold is relatively thin on this sample (3.3 × 10-2 µm), the nickel peaks are observed. In general, the thinner the gold deposit, the more pronounced the nickel peaks become with respect to the gold peaks. In this formulation, the DMAB:KOH molar ratio is approximately 1:8. A plating rate of 0.044 µm/h was determined by XRF. Reducing the DMAB concentration to half the original level while maintaining the other bath variables at the optimized value (WB 2) gives a DMAB:KOH molar ratio of 1:17. This process yields a deposit that has experienced intergranular corrosion, as shown in Figure 7. In this case the higher ratio of KOH to DMAB causes some competition between gold dissolution and gold deposition. This result is consistent with the electrode process observed in the impedance spectra (Figure 5b). As discussed earlier, cyanide and DMAB compete at the autocatalytic gold surface for active sites. Due to insufficient DMAB concentration, this bath failed to plate electroless gold and stripped some existing immersion gold from the surface. Doubling the DMAB concentration to 0.17 M (WB 3) almost doubles the rate to 0.081 µm/h. The resulting SEM is similar to that of the optimized bath. Decreasing the cyanide concentration by half (WB 4) slightly increases the plating rate. This is desirable because many industrial manufacturing sites are developing more environmentally conscious methods of pro-

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Figure 7. SEM micrograph of a wirebond sample surface after plating at low [DMAB]; conditions are for WB 2 in Table 1.

Figure 8. SEM micrograph of a wirebond sample surface after plating at high [KCN]; conditions are for WB 5 in Table 1.

cessing and waste removal for toxic compounds such as cyanide. When the cyanide concentration is doubled to that of the optimized value (WB 5), the same corrosion effect described for the low DMAB bath is apparent, as shown in Figure 8. A significant loss of gold was observed; i.e., the substrate was etched at a rate of -0.037 µm/h. The adverse effect of cyanide on this wirebond sample correlates to the suppression of the reactivation peak observed for DMAB oxidation in Figure 5a. The hydroxide medium is crucial to electroless gold performance. Doubling the hydroxide concentration to 1.4 M increases the plating rate from 0.044 to 0.054 µm/h (WB 6). This result is also consistent with the impedance behavior in Figure 3a, as it correlates to a lower Rct. Lowering the hydroxide yields a deleterious effect on the wirebond sample. At low hydroxide levels, the percentage of hydroxide in the total DMAB:KOH molar ratio is not high enough to support gold deposition, failing to plate

Sargent and Sadik

Figure 9. (a) SEM micrograph of a wirebond sample surface after plating at low [KOH]; conditions are for WB 7 in Table 1. (b) EDS spectrum of the surface of WB 7 after plating (Supporting Information).

the sample. Figure 9a shows a wirebond sample plated with the 7.0 × 10-2 M KOH formulation (WB 7). The amount of hydroxide here was 10 times less than the optimized formulation, i.e., a drop in pH from 13.6 to 12.5. Visual inspection of the sample reveals the complete loss of the pre-existing immersion gold. EDS of the sample confirms the absence of gold peaks, as seen in Figure 9b (Supporting Information). Instead, nickel and phosphorus from the nickel-phosphate electroless nickel plate is observed. It is clear that the low hydroxide bath is incapable of producing a gold deposit. This is probably due to a combination of processes: (i) the low hydroxide content is not sufficient to produce the reactive intermediate according to eq 1, and (ii) the low hydroxide content cannot protect the existing surface from cyanide ions, which will complex the existing gold. In effect, the rate of gold dissolution is much higher than the rate of gold deposition or oxidation of reducing agent. This is evidenced by the extremely high gold etching rate of -0.102 µm/h. The lower plating rate at 7.0 × 10-2 M KOH is correlated to the lower tendency for reactivation of DMAB oxidation shown in Figure 3b. The DMAB:KOH molar ratio of this bath is 1:1. This suggests that the formation of the “true” reducing agent, or reactive intermediate, requires more than one hydroxide ion per DMAB molecule. Accordingly, BH3OH- may not be the final optimized structure of the reducing agent. Increasing the concentration of KAuCN2 to twice the optimized value (WB 8) increases the plating rate by 0.01 µm/h. Typically there is a gold concentration above which additional gold provides no significant improvement in the plating rate.7,41 However, because of the very low plating rates achieved in this experiment, the increase in gold concentration increases plating efficiency. The SEM micrographs recorded for wirebond samples after plating with at a low gold concentration solution (WB 9) exhibits a corroded surface. Only spotty coverage of gold is observed. The etching rate for the low gold bath is -0.034 µm/h. Influence of Salt Concentration on the Nature of Plating. The ability of the optimized bath was tested to (41) Iacovangelo, C. D. J. Electrochem. Soc. 1991, 138, 976

Properties of Electroless Gold

determine if it could operate similarly in the presence of 0.1 M NaCl. The chloride ion is a common culprit in many types of corrosion. It was found that the plating rate was not affected by the presence of salt (WB 10). The SEM and elemental analysis indicate similar results to those of WB 1. In summary, visual inspection of WB 2, WB 5, WB 7, and WB 9 reveals gray streaks along with the expected gold deposit (WB 7, all gray). This signifies complete loss of portions of the underlying immersion gold surface, exposing the underlying nickel. Therefore, these bath concentrations were unable to plate electroless gold onto the existing immersion gold. In this respect, the immersion gold layer failed to provide a solid catalytic surface for electroless gold deposition. These results highlight the importance of the autocatalytic surface layer as well as the interrelationship between the chemical components in the bath. Edge Effects. In high-density circuits, the ability of electroless gold to plate onto finely separated lines is of significant value. The successful plating of metal onto small substrates relies on the diffusion of chemical species to the substrate surface.42 At substrate edges or on very small substrates, nonlinear diffusion becomes important. Therefore, depending on bath conditions and the presence of stabilizers, substrate edges typically exhibit plating overgrowth. Uncontrolled electroless gold plating leads to extraneous deposition, causing electrical shorting and rendering the circuit unusable. Therefore, wirebonding pads are strategically placed onto chip carriers to maximize line density while still allowing for errors that may cause electrical shorting. Electroless gold/soldermask edges were examined for WBs 1-10 after plating to compare the effect of bath variables on edge effects. The result for substrate/mask edge using the optimized bath is shown in Figure 10a. Both the metal and soldermask are viewable. Generally, a noticeable amount of uneven deposit resides at the edge. It appears that this deposit is composed of successive layers of metal. It is most likely caused by the electroless Ni during processing and magnified from successive layers of immersion gold/electroless gold. The edge deposit was observed for all WB samples except for WB 7 in the low hydroxide bath (Figure 10b). The edges are more even and three-dimensional growth of the metal layers was severely diminished at low hydroxide concentration. At low hydroxide concentration, the autocatalytic gold surface is more susceptible to attack from the stabilizer (cyanide), notably at the edge of the substrate. This further highlights the gold dissolution role cyanide assumes in the bath, especially at inadequate concentrations of hydroxide. Conclusions The microscale adsorption/desorption processes occurring at the catalytic gold surface directly influences electroless gold plating quality. Impedance spectroscopy of DMAB on gold at the reactivation potential reveals negative resistive behavior attributed to adsorption of the reducing agent to the electrode surface. In the presence of relatively high cyanide concentration, this adsorption was significantly diminished. On a macroscopic level, adsorption of DMAB to the substrate surface is less favored in baths containing low DMAB, high cyanide, low KOH, and low KAuCN2. In these baths, stripping of the preexisting immersion gold and/or failure to plate electroless gold occurs, as indicated by SEM and EDS. The highest plating rates were obtained with high DMAB and KOH (42) van der Putten, A. M. T.; de Bakker, J. W. G. J. Electrochem. Soc. 1993, 140, 2221.

Langmuir, Vol. 17, No. 9, 2001 2767

Figure 10. (a) SEM micrograph of a wirebond pad/soldermask edge plated using optimized electroless gold bath formulation. (b) SEM micrograph of a wirebond pad/soldermask edge plated with the low KOH bath formulation (WB 7).

concentration. It appears that the critical chemical composition for efficient electroless gold plating is determined by the DMAB:KOH molar ratio. Acknowledgment. The authors gratefully acknowledge Roy Magnuson and John Konrad at IBM Microelectronics, Endicott, for helpful discussion and for providing the wirebond samples and XRF thickness measurements. The authors thank Bill Blackburn from the Geology Department for performing the SEM and EDS measurements. Several helpful discussions with J.R. Macdonald and B. A. Boukamp are also gratefully acknowledged. This work was funded by the National Science Foundation through the NYS Center for Advanced Technology and Integrated Electrical & Electronic Center. Supporting Information Available: EDS spectrum of WB 1 after plating (Figure 6b) and EDS spectrum of WB 7 after plating (Figure 9b). This material is available free of charge via the Internet at http://pubs.acs.org. LA001267E