1632
Ind. Eng. Chem. Res. 1997, 36, 1632-1636
Effect of Cu(OH)2 on Electroless Copper Plating J. Shu, B. P. A. Grandjean, and S. Kaliaguine* Department of Chemical Engineering, Laval University, Ste-Foy, Quebec, Canada G1K 7P4
Although electroless copper deposition has been widely used in the electronic industry, the selection of the bath solutions relies still on some empirical rules. This paper provides a further exploration of the bath compositions for electroless copper plating. It was found that the formation of Cu2O in electroless copper plating is associated with the presence of Cu(OH)2 in the bath. A chemical equilibrium calculation was thus made for the avoidance of Cu(OH)2 in an electroless copper bath. Sodium thiosulfate was found to be an effective stabilizer for electroless copper plating. The plating was further optimized based on a statistical design of sorts, namely, Taguchi’s approach. Effects of individual components on the plating rate were discussed. Introduction Electroless copper plating has been widely used in the manufacture of printed circuit and decorative platings for over 4 decades and is now extended for ultralargescale integration applications (Shacham-Diamand et al., 1995). The plating solutions are often composed of copper sulfate as the metal source, complexing agent such as EDTA, reducing agent HCHO, stabilizer, and NaOH to adjust the pH value. The selection of the bath compositions relies on some empirical rules, mainly established in the late 1950s (Beck, 1984). For example, the ratio of EDTA/Cu should be set at about 2.5 to allow complexing of all cupric ions, whereas pH values are best adjusted between 12.5 and 13.5 (Graham, 1971). So far, various attempts have been made to improve plating quality as well as rate and bath stability (see, for example, Paunovic and Arndt, 1983; Duffy et al., 1983). In electroless copper deposition, the formation of cuprous oxide (Cu2O) in the plating bath or in the deposit has often been a concern. It was proposed that the instability of electroless copper baths mainly results from disproportionation of Cu2O (Lowenheim, 1978; Junginger, 1988). Indeed the presence of small amounts of Cu2O would significantly affect the applicability of copper deposits (Schneble et al., 1971). On the basis of a cyclic voltammetry study, Beck (1984) suggested previously that the formation of Cu2O is through a cathodic partial reaction:
2Cu(OH)2 + 2H+ + 2e- f Cu2O + 3H2O
(1)
The same partial reaction was also suggested by Bindra et al. (1984). Another form of this side reaction during electroless copper plating was also proposed by Shacham-Diamand et al. (1995) and Matsuoka et al. (1995):
2Cu2+ + 5OH- + CH2O f Cu2O + HCOO- + 3H2O (2) It should be mentioned that both expressions (1) and (2) do not take into account the complexation of Cu2+ ions. Practically, one has to add a stabilizer into the plating solution to complex the cuprous species in order * Author to whom correspondence should be addressed. Fax: 1-418-656-5993. E-mail:
[email protected]. S0888-5885(96)00091-7 CCC: $14.00
to increase the electroless copper bath stability. On the other hand, the presence of Cu2O in electroless copper deposits needs confirmation since Nakahara and Okinaka (1975) once argued that Cu2O was not experimentally observed in the copper deposit. The aim of this work is to clarify the formation of Cu2O in relation with the complexed Cu2+ concentration in the electroless bath. A chemical equilibrium calculation was thus made to determine the formation of Cu(OH)2 in the plating solution. Further works on the optimization of electroless copper deposition will also be presented in this paper. Experimental Section The substrate used for copper deposition was 316 L stainless steel disks with an effective surface area of 1 cm2. The specimens were first activated by an electroless Pd precoating to a Pd thickness of about 2 µm. The detailed procedures as well as the physicochemical properties of the Pd deposit were previously reported (Shu et al., 1993a,b, 1994). Unlike the Pd seeds formed by the sensitization procedure, the electrolessly deposited Pd provided stable surface properties and was more active to initiate the electroless copper deposition. The use of this kind of active substrate allowed us to ignore the effect of the activation procedure on the copper deposition. For copper deposition, Pd-deposited specimens were immersed in electroless copper baths with magnetic agitation (650 rpm) at ambient temperature. The deposition was done in air. The baths consisted of copper sulfate, Na2EDTA, formaldehyde, sodium hydroxide, and 10 ppm of sodium thiosulfate. In the presence of catalytically active palladium species, cupric ions could be reduced easily and deposited on the substrate surface. The use of small amounts of sodium thiosulfate in the baths was aimed at replacing the conventional toxic cyanide stabilizer. This additive was found to be effective in stabilizing the electroless Cu bath. Equal volumes of solution A containing CuSO4 and HCHO and solution B containing EDTA and NaOH were mixed to make up the electroless baths in order to reduce the precipitation of copper hydroxide during the bath preparation (Graham, 1971). The pH value of the baths was measured using a pH meter (Fisher, Accumet 810). The deposited Cu content was estimated from the mass gain of the substrate. © 1997 American Chemical Society
Ind. Eng. Chem. Res., Vol. 36, No. 5, 1997 1633
Figure 1. Factorial effects in electroless copper plating (concentrations in mol/L).
Results and Discussion An electroless plating bath is composed of several components which are necessary to form a metallic coating while maintaining the stability of the bath. In this study, effects of individual bath components on the electroless copper deposition rate were examined in an optimization experiment based on a statistical design of sorts, namely, Taguchi’s approach (1987). An L9 orthogonal array was used for the selection of various concentration variables, as shown in Table 1. The deposition results after 3 h of plating are also included in the table. The results were further analyzed statistically, as shown in the table, where Ki is the summation of corresponding testing results at level i, ki ) Ki/3, and ∆k ) ki,max - ki,min. According to statistics, the parameter ∆k is a measure of the effect of the corresponding factor on the deposition rate (Montgomery, 1991).
Figure 1 shows the dependence of the deposited copper content on each concentration variable. It can be seen that concentrations of Na2EDTA and NaOH are the most important factors controlling the deposition rate of Cu under the present chemical conditions. The Cu deposition was enhanced by increases of [NaOH], [CuSO4], and [HCHO] and a decrease of [Na2EDTA]. This is reasonable since raising [NaOH], [CuSO4], or [HCHO] increases the concentration of reactant species (see below reaction (5)), whereas an excess of EDTA inhibits the reactivity of Cu2+ via complexation. The slight deviation of [CuSO4] and [HCHO] curves from monotonic relationships indicates that there are minor complexing effects among these concentration variables. Indeed, as pointed out by Shacham-Diamond et al. (1995), the reaction orders of the reactants in electroless copper plating depend on the chemical conditions, i.e., the bath composition.
1634 Ind. Eng. Chem. Res., Vol. 36, No. 5, 1997 Table 1. L9 Orthogonal Array of Experiments (Concentration in M) and the Results of Mass Deposited after 3 h results test no.
A [CuSO4]
B [Na2EDTA]
C [HCHO]
D [NaOH]
pH value
kg of Cu/m2
1 2 3 4 5 6 7 8 9
0.0251 0.0251 0.0251 0.0502 0.0502 0.0502 0.0752 0.0752 0.0752
0.0537 0.0806 0.1075 0.0537 0.0806 0.1075 0.0537 0.0806 0.1075
0.107 0.160 0.187 0.160 0.187 0.107 0.187 0.107 0.160
0.300 0.400 0.500 0.500 0.300 0.400 0.400 0.500 0.300
12.73 12.75 12.80 12.94 12.42 12.72 12.87 12.92 11.93
27.5 17.5 35.1 39.0 24.7 13.8 58.3 49.8 12.5 ∑ 278.2
K1 K2 K3 k1 k2 k3 ∆k a
80.1 77.5 120.6 26.7 25.8 40.2 14.4
124.8 92.0 61.4 41.6 30.7 20.5 21.1
91.1 69.0 118.1 30.4 23.0 39.4 16.4
remark
ppta ppta ppta
64.7 89.6 123.9 21.6 29.9 41.3 19.7
ppt denotes precipitate in solutions.
Figure 2. XRD patterns of copper deposits on Pd/SS: (a) from bath 3, (b) from bath 4, and (c) from bath 10.
The above trends provide the basis for predicting concentration effects on the electroless copper deposition. Bright Cu deposits were obtained from homogeneous baths where no Cu(OH)2 precipitate could be observed. In these cases the phase purity of the Cu deposited was confirmed by XRD, as illustrated by the XRD pattern of the Cu deposit from bath 3, shown in Figure 2a. Precipitates of Cu(OH)2 were observed in baths 4, 7, and 8, resulting in deposited copper with a slightly gray color. Small amounts of Cu2O were detected by XRD in the corresponding deposits (Figure 2b), indicating a relation with the presence of Cu(OH)2 in the bath. Cu2O was previously found in the copper deposit from a bath with pH ) 13.0 by Duffy et al. (1983) using Auger spectra. It is known that Cu2O is normally red in color. The present gray appearance might be attributed to the very fine Cu2O particles. Cu2O is catalytically inactive toward further copper deposition
(Bindra et al., 1984). Thus, the codeposited Cu2O would exist in the form of isolated small particles. The presence of Cu2O would significantly affect the properties of Cu deposits, for example, the film ductility. Our experience also showed that the codeposited Cu2O, though in a very small portion, would prevent the formation of Pd-Cu alloys via atomic diffusion at elevated temperature (Shu, 1994). Therefore, Cu(OH)2 precipitation should be avoided in the bath in order to improve the deposit quality. The electroless copper bath is a metastable system where several equilibria exist simultaneously. First, cupric ions should be complexed (reaction (3)) in order to prevent its spontaneous reduction by the reducing agent formaldehyde. Second, the copper deposition needs to be performed in an alkaline medium. Copper hydroxide precipitate might form under certain condi-
Ind. Eng. Chem. Res., Vol. 36, No. 5, 1997 1635
Figure 3. Possibility of Cu(OH)2 precipitate formation in electroless copper baths
tions (reaction (4)). The target electroless reaction is the autocatalytic deposition (reaction (5)).
Cu2+ + EDTA4- h [CuEDTA]2- (log KMY ) 18.80; Skoog and West, 1976) (3) Cu2+ + 2OH- f Cu(OH)2 (Ksp ) 5.6 × 10-20; Linke, 1958) (4) [CuEDTA]2- + 2HCHO + 4OH- h Cu0 + 2H2O + H2 + 2HCOO- + EDTA4- (5) The conditions of Cu(OH)2 precipitation are related to the simultaneous equilibria of reactions (3) and (4) in the absence of reducing agent:
x[Cu2+]0 (1 - x - y)[Cu2+]0([EDTA]0 - x[Cu2+]0)
)
1 × 1018.8 (mol/L)-1 (6) and
(1 - x - y)[Cu]02+([OH]0 - 2y[Cu2+]0)2 ) 5.6 × 10-20 (mol/L)3 (7) where x and y are the equilibrium conversions for reactions (3) and (4), respectively. [i]0 is the initial concentration of component i. The combined equations (6) and (7) have been solved numerically. The results corresponding to various composition conditions are drawn in Figure 3 as a diagram of [NaOH]/[CuSO4]vs [EDTA]/[CuSO4]. As can be seen, at a constant concentration of CuSO4, there is a boundary between the precipitate region and the homogeneous region in the electroless solution. At the left of each curve, the precipitate of copper hydroxide would form in the electroless bath. The boundary shifts toward the right side with the increase of [CuSO4] in the baths, which means that higher [EDTA]/[CuSO4] ratios should be satisfied with increasing [CuSO4], in order to ensure a homogeneous solution. Figure 3 also reveals that, at any concentration of CuSO4, [EDTA]/[CuSO4] must be more than 1.0 to prevent cupric ions from precipitating. It is known that copper hydroxide could precipitate at pH above 5 in the absence of complexing agent. Another widely accepted fact is that EDTA usually forms 1:1 complexes with most metal ions such as Cu2+. Thus, the value 1.0 of
[EDTA]/[CuSO4] seems to indicate that all Cu2+ must be complexed to avoid precipitation. With the increase of [NaOH] in the baths, this ratio should, therefore, be higher than 1. The calculated results shown in Figure 3 are in good agreement with our experimental observations. Those compositions indicated as a dot in Figure 3 actually correspond to the baths listed in Table 1. It is noteworthy that precipitates were only observed in baths 4, 7, and 8, which are described by dots actually located at the left of their respective curves at [CuSO4] ) 0.05 and 0.075 M. Thus, Figure 3 gives a detailed description of Cu(OH)2 formation conditions in electroless copper baths. In the literature, some authors pointed out empirically that the molar ratio of [EDTA]/[Cu] should be set at about 2.5 to complex all cupric ions. From Figure 3, it can be seen that this ratio depends on the concentrations of NaOH and Cu2+. At a low Cu2+ concentration, [EDTA]/[Cu] could be near 1. In contrast, when Cu2+ concentration is very high, even the EDTA/Cu ratio of 2.5 is not sufficient to obtain a homogeneous solution. From the optimization experiment, the bath composition with the highest plating rate should consist of 0.075 M of CuSO4, 0.054 M of Na2EDTA, 0.50 M of NaOH, and 0.187 M of 37% HCHO. However, upon checking with Figure 3, Cu(OH)2 precipitate could appear in such a bath. Thus, we adjusted the less sensitive factor, [CuSO4], to meet with the condition of no Cu(OH)2 precipitate. Finally, a compromised electroless Cu deposition bath composition was selected, containing 0.025 M of CuSO4, 0.054 M of Na2EDTA, 0.50 M of NaOH, and 0.187 M of 37% HCHO (designated as bath 10). A total of 5 ppm of Na2S2O3‚5H2O was found effective enough to stabilize this bath. A typical copper deposit was obtained and characterized by XRD (Figure 2c), showing an fcc structure with a usual (111) orientation. In summary, this work provides a further understanding of the electroless copper bath composition. The experiment based on Taguchi’s statistical design approach indicates that the concentrations of EDTA and NaOH are the two most pronounced factors affecting the copper deposition rate under the present chemical conditions. Sodium thiosulfate was found to be an effective stabilizer for electroless copper plating. Undesired Cu2O was codeposited from electroless copper baths containing Cu(OH)2 precipitate. To optimize electroless Cu deposition, the favorable combination of components must meet with the condition of no Cu(OH)2 precipitate in the system since the resultant Cu2O codeposit would limit the applicability of the electroless copper deposits. Acknowledgment This work was supported by the Natural Sciences and Engineering Research Council of Canada (NSERC). The authors thank Mr. Y. Lacombe for experimental assistance. Literature Cited Beck, T. R. Kinetic and Mass Transfer Processes in Electroless Copper Plating. In Application of Polarization Measurements in the Control of Metal Deposition; Warren, I. H., Ed.; Elsevier: Amsterdam, The Netherlands, 1984; pp 158-176. Bindra, P.; Roldan, J. M.; Arbach, G. V. Mechanisms of Electroless Metal Plating: II. Decomposition of Formaldehyde. IBM J. Res. Dev. 1984, 28, 679.
1636 Ind. Eng. Chem. Res., Vol. 36, No. 5, 1997 Duffy, J.; Pearson, L.; Paunovic, M. The Effect of pH on Electroless Copper Deposition. J. Electrochem. Soc. 1983, 130, 876. Graham, A. K. Electroplating Engineering Handbook; Van Nostrand Reinhold Co.: New York, 1971; p 502. Junginger, R. Nodule Formation in Electroless Copper Baths. J. Electrochem. Soc. 1988, 135, 2299. Linke, W. F. SolubilitiessInorganic and Metal-Organic Compounds, 4th ed.; American Chemical Society: New York, 1958; Vol. 1. Lowenheim, F. A. Electroplating; McGraw-Hill: New York, 1978; p 405. Matsuoka, M.; Yoshida, Y.; Iwakura, C. The Effects of Aeration and Accumulation of Carbonate Ions on the Mechanical Properties of Electroless Copper Coatings. J. Electrochem. Soc. 1995, 142, 87. Montgomery, D. C. Design and Analysis of Experiments, 3rd ed.; John Wiley & Sons: New York, 1991; pp 414-438. Nakahara, S.; Okinaka, Y. Transmission Electron Microscopic Study of Impurities and Gas Bubbles Incorporated in Plated Metal Deposits. In Properties of Electrodeposits, Their Measurements and Significance; Sard, R., Leidheiser, H., Jr.; Ogburn, F., Eds.; ECS: Princeton, NJ, 1975; p 50. Paunovic, M.; Arndt, R. The Effect of Some Additives on Electroless Copper Deposition. J. Electrochem. Soc. 1983, 130, 794. Schneble, F. W., Jr.; McCormack, J. F.; Zeblisky, R. J.; Williamson, J. D. U.S. Patent 3,615,737, 1971. Shacham-Diamand, Y.; Dubin, V.; Angyal, M. Electroless Copper Deposition for ULSI. Thin Solid Films 1995, 262, 93.
Shu, J. Development of Palladium-based Catalytic Membrane Reactors. Ph.D. Dissertation, Laval University, Quebec, Canada, 1994. Shu, J.; Grandjean, B. P. A.; Ghali, E.; Kaliaguine, S. Simultaneous Deposition of Pd and Ag on Porous Stainless Steel by Electroless Plating. J. Membr. Sci. 1993a, 77, 181. Shu, J.; Grandjean, B. P. A.; Ghali, E.; Kaliaguine, S. Autocatalytic Effects in Electroless Deposition of Palladium. J. Electrochem. Soc. 1993b, 140, 3175. Shu, J.; Grandjean, B. P. A.; Kaliaguine, S. Morphological Study of Hydrogen Permeable Pd Membranes. Thin Solid Films 1994, 252, 26. Skoog, D. A.; West, D. M. Fundamentals of Analytical Chemistry, 3rd ed.; Holt, Rinehart and Winston: New York, 1976; p 274. Taguchi, G. System of Experimental Design; UNIPUB/Kraus International: New York, 1987; Vol. I.
Received for review February 8, 1996 Revised manuscript received January 27, 1997 Accepted February 3, 1997X IE9600912
X Abstract published in Advance ACS Abstracts, March 15, 1997.
