Effect of chelating agents on the structure of electroless copper coating on alumina powder

Surface and Coatings Technology 107 (1998) 48–54

Effect of chelating agents on the structure of electroless copper coating on alumina powder W.H. Lin a,*, H.F. Chang b a Department of Professional Basis, Chien Kuo Junior College, Changhua, Taiwan b Department of Chemical Engineering, Feng Chia University, Taichung, Taiwan Received 16 March 1998; accepted 14 June 1998

Abstract Two chelating agents, ethylenediaminetetraacetic acid (EDTA) and triethanolamine ( TEA), were used as complexing agents for copper coated on alumina powder via the electroless copper-plating procedure. Scanning electron microscopy and X-ray diffraction have been used to examine the effect of chelating agents on the surface structure of copper coatings. The experimental results showed that the formation of copper crystallites strongly depended on the complexing capability of the chelating agent. For the EDTA series, it had a stronger complexing capability and a slower copper plating rate, and the copper crystallites were relatively small and uniformly distributed on the alumina surface when compared with those using TEA as a chelating agent. The acidity determined by the chemisorption of ammonia could be reduced more effectively when EDTA was used as a chelating agent. © 1998 Elsevier Science S.A. All rights reserved. Keywords: Alumina powder; Chelating agents; EDTA; Electroless copper coating; TEA

1. Introduction In 1950, a commercial electroless (chemical plating) plating process was developed. This method is used to obtain thin metallic films on a conductive, semi-conductive or non-conductive substrate by simply immersing it into an electrolyte solution in the absence of an external current source. Palladium is generally used as a seeding or a catalyzing agent to provide catalytic nucleating centers on the substrate, and thereafter, the desired metal can be deposited by treating the catalyzed surface with a salt of the metal plus a reducing reagent [1,2]. In this case of electroless copper plating, the chelated copper ions in the plating solution diffuse and adsorb at the nucleating centers of the substrate surface; formaldehyde serves as a reductant to reduce the copper ion to metallic copper. Once copper is deposited on the substrate, the deposited copper itself further catalyzes the electrochemical reaction. Thus, the deposition of copper continues autocatalytically [3,4]. Since the electrochemical reaction proceeds via a redox mechanism, a uniform distribution of active material on a non-conducting support with better physical * Corresponding author. Tel: +886 4 7224676; Fax: +886 4 7256302.

and chemical properties can be expected. Over the past 40 years, the applications of electroless copper plating techniques have been mainly conducted on the fabrications of electronic parts, computer parts, printed circuit boards, etc. [5]. Recently, the application of the electroless plating technique has been extended on the preparation of highly-dispersed supported copper catalyst. It provides a new catalyst preparation method that is mechanistically different from the conventional impregnation method because the former proceeds via a redox mechanism, whereas the latter undergoes an adsorption mechanism. In our previous study, we successfully prepared Cu/Al O catalysts of different copper loadings by the 2 3 electroless plating technique. The results showed that the electroless-plated Cu/Al O catalyst had a higher 2 3 dispersion and a more even spread of copper on the alumina surface than the catalysts prepared by the impregnation method and the precipitation method [6 ]. In this communication, we report the attempts of electroless copper plating of alumina using two different chelating agents, ethylenediaminetetraacetic acid ( EDTA) and triethanolamine ( TEA), and report an investigation of the effects of chelating agents on the structure of electroless copper coating on alumina powder.

0257-8972/98/$ – see front matter © 1998 Elsevier Science S.A. All rights reserved. PII S 02 5 7 -8 9 7 2 ( 9 8 ) 0 0 55 3 - 2

W.H. Lin, H.F. Chang / Surface and Coatings Technology 107 (1998) 48–54

2. Experimental 2.1. Electroless copper plating The electroless copper plating in this study is based on EDTA or TEA as the chelating agent with formaldehyde (HCHO) as the reducing agent. The c-Al O 2 3 (Merck), with a particle size of 80~100 mm, was used as the substrate for the electroless plating process. In order to activate the surface of the substrate, the raw alumina was subjected to alkaline cleaning (1 N NaOH ) at 65–70 °C for 5–15 min, then acid-cleaned (25 vol.% H SO ) at 30–35 °C for 5–15 min, sensitizing 2 4 (20 g l−1 SnCl and 40 ml l−1 HCl solution) at 25–30 °C 2 for 8–10 min, activating (0.25 g l−1 PdCl and 2 0.5 ml l−1 HCl solution) at 40–45 °C for 20–25 min, and finally drying at 110 °C for 15–20 h [7]. The main purpose of the pretreatment is to remove the adsorbed hydrocarbons and contaminants and to provide palladium nucleating centers on the alumina surface that can trigger the catalytic reaction for the electroless plating process [8]. The pretreated alumina was transferred into the chemical copper plating bath, which was prepared according to Table 1. Different copper loadings were obtained by changing the volume of plating solution. The plated alumina was separated by filtration and then washed several times with distilled water. The clean plated alumina was dried at 110 °C for 24 h, and a part of it was calcined at a suitable temperature (300, 400, 500 °C ) to study the surface structure variation resulting from the high-temperature treatment. 2.2. Determination of BET surface area, copper surface area and acidity

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and the acidity of the catalysts in this study were measured by the volumetric chemisorption method using nitrous oxide and ammonia, respectively [7]. For each measurement, approximately 2 g of catalyst were loaded into the sample cell. Before evacuation, the catalyst was reduced at 250 °C for 12 h with a suitable hydrogen flow. The main purpose of the reduction was to reduce the surface copper oxide that may be formed by the reaction with atmospheric oxygen. Continuous degassing was carried out at a temperature of 400 °C and a pressure not exceeding 10−3 Torr for 3 h. After degassing the catalyst cooled down to the adsorption temperature, nitrous oxide decomposition and ammonia adsorption were carried out at 95 and 175 °C, respectively. The decomposition of nitrous oxide molecules on the copper surface is to produce chemisorbed oxygen atoms, thus generating gas-phase nitrogen according to the process: N O +2Cu N +(Cu–O–Cu) , 2 (g) (s) 2(g) (s) where s denotes a surface phase, and the copper surface area can be determined using the following equation: S=10−6×

2n×A 1.7×1019

(m2 g-cat.−1),

where n is the amount of N O (mmol g-cat−1) decom2 posed. The number of copper atoms per m2 of copper surface is 1.7×1019, assuming that the equilibrium plane distribution corresponds to about 25% of (100) planes, 5% of (110) planes and 70% of (111) planes [10]. A is the Avogadro constant, which is equal to 6.02×1023. The acidity of the copper catalyst can be directly determined by the amount of NH (mmol g-cat−1) adsorbed. 3 2.3. XRD and SEM analyses

BET surface area and pore size distribution were determined from multipoint BET isotherms (Micromeritics 2300) using nitrogen as adsorbate at −195 °C [9]. Before each measurement, the sample was degassed at 150 °C for 60 min. The copper surface areas Table 1 Electroless copper-plating baths and operating conditions Components

CuSO · 5H O 4 2 EDTA · 4Na TEA HCHO Pyridine 2-2,dipyridine K4[Fe(CN )6 ] · 3H2O Temperature Time pH

The X-ray diffraction was carried out at room temperature by the continuous position sensitive detection technique (CPS) on a material analysis and characterization (MAC ) MPX3 diffractometer (Japan) using a copper target (1.5405 CuKa radiation). The SEM images were obtained using a Hitachi S-2700 microscope (Japan).

Concentrations EDTA bath

TEA bath

3. Results and discussion

0.04 M 0.08 M — 0.08 M 5 ppm — — 70 °C 30 min 12.5

0.06 M — 0.18 M 0.22 M — 10 ppm 20 ppm 70 °C 5 min 12.5

In electroless copper plating, EDTA has been commonly used as a ligand to chelate copper ions. However, EDTA has a drawback owing to the low deposition rate [11–14]. Paunovic [11] used the galvanostatic transient technique to study the ligand effects in electroless copper deposition and found that a definite correlation could be established between the rate of dissociation of the chelating agents and the rate of copper deposition. He also observed that the rate of copper deposition

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W.H. Lin, H.F. Chang / Surface and Coatings Technology 107 (1998) 48–54

increased as a function of a ligand in the order: tartrate, EDTA, quadrol, CDTA complex. Kondo and coworkers [13] examined the effects of addition of various ligands, such as EDTA, nitrilotriacetic acid, N,N,N∞,N∞,-tetrakis(2-hydroxypropyl ) ethylenediamine, N,N,N∞,N∞,-tetrakis(2-hydroxyethyl ) ethylenediamine, TEA, and triisopropanolamine on the electroless copper deposition rate. They found that the maximum deposition rate for the TEA system was 20 times faster than the rate observed in the EDTA system. Furthermore, they proposed a plating model based on Langmuir adsorption kinetics, and the different effects of excess ligands on plating rate were interpreted by the relative magnitude of adsorption equilibrium constant of ligands on the reactive surface and that on the adsorbed complexes [9,10]. Table 2 lists the copper content, BET surface area, copper surface area, acidity, dispersion (%), and diameter of copper crystallites of each coating, prepared with different chelating agents [11]. The copper content varied from 8.44~24.56 wt% for the EDTA series and 7.09~25.92 wt% for the TEA series. The BET surface area of the catalyst was gradually decreased for both EDTA and TEA series, indicating that some copper crystallites might block the micropores of the support and reduce the BET surface area. The deposition rate of electroless copper plating relies upon the complexing capability of the chelating agent, as mentioned above. In the present study of electroless copper coating on powder alumina, the preparation with EDTA required 30 min to complete the plating process, whereas with TEA, it took only 5 min. The major function of chelating agents is to chelate copper ions to form EDTA–Cu or TEA–Cu complexes thus preventing Cu precipitation in alkaline solution. At

the beginning of electroless plating, these complex ions will adsorb on the substrate surface and migrate to the nucleating centers, where palladium catalyzes the reduction of copper ions by formaldehyde to form copper crystallites on the surface. The number of copper crystallites formed depends on the complexing capability of chelating agent. With EDTA, due to its high complexing capability, the decomplexing rate of EDTA–Cu is quite slow, and the number of small copper crystallites is proportional to the time span of plating or to the copper loading. On the contrary, the decomplexing rate of TEA–Cu is very fast because of its low complexing capability, which results in the formation of large copper crystallites. Fig. 1 shows the variation in copper surface area with respect to copper loading for both chelate systems. For the EDTA series, the copper surface area gradually increased with loading, reached a maximum at about 16 wt% of Cu, and declined with further copper deposition. As the palladium triggered the copper deposition at the beginning of plating, the copper surface area was proportional to the number of sparsely distributed fine copper crystallite, which increased with copper loading. Up to a certain loading, the substrate surface was saturated with small copper crystallites, and the copper crystallites had the opportunity to contact each other finally forming a thin film covering the whole substrate. This produced the exposed copper surface area maximum. Further copper loading resulted in clusters of copper agglomerates blocking the micropores of the support and consequently a reduction in the exposed copper atoms. Other investigators also demonstrated that the copper surface area increases with copper loading to a certain value, after which further increases in loading led to a reduction [15]. For the TEA series, however, the copper surface area increased linearly with

Table 2 Characteristics of electroless-plated copper on alumina powder Coating

EDTA-1 EDTA-2 EDTA-3 EDTA-4 EDTA-5 EDTA-6 EDTA-7 TEA-1 TEA-2 TEA-3 TEA-4 TEA-5 TEA-6 TEA-7

Copper loading (wt%)

8.44 13.36 14.46 16.52 19.17 22.92 24.56 7.09 11.40 12.56 17.49 21.38 22.96 25.92

Surface area(m2 g-cat−1) BET

Cu

155.33 142.53 140.11 136.65 126.98 101.23 92.38 132.51 125.97 124.14 113.18 107.46 101.50 99.38

7.03 8.88 9.25 9.68 9.43 8.63 8.01 4.06 4.25 4.35 4.61 4.82 4.90 5.09

aCalculated from Scherrer’s equation using X-ray diffraction data. bCalculated from N O decomposition data. 2

Acidity (mmol g-cat−1)

220.50 200.16 184.75 171.59 159.69 155.55 152.90 265.80 260.87 256.32 248.37 244.51 240.12 238.95

Dispersion (%)

14.94 11.93 11.48 10.52 8.83 6.76 5.85 10.28 6.69 6.22 4.73 4.05 3.84 3.52

˚) Crystallite diameter (A da B

db C

207 205 209 200 198 199 189 428 251 389 165 147 158 233

81 101 105 114 136 178 205 117 180 193 254 297 313 341

W.H. Lin, H.F. Chang / Surface and Coatings Technology 107 (1998) 48–54

Fig. 1. Effect of copper loading on copper surface area.

loading at a slower rate, and no maximum appeared. This is because the number of small crystallites is independent of the copper loading for the TEA series, and the increase in copper surface area is mainly attributable to the increase in the number of three-dimensional copper clusters. Fig. 2 shows the pore-diameter distribution of alumina particles before and after electroless copper plating using

Fig. 2. Pore-diameter distribution of alumina particles before and after electroless copper plating.

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different chelating agents. The pores of alumina particles ˚. were highly tortuous and had an average size of 50 A Because the copper deposition started from the outer surface of alumina particles and then gradually penetrated in the pore, the pore diameter shrank gradually, and the surface area decreased linearly. At a high copper loading, the pore shrinkage was larger in the EDTA series than that in the TEA series. This is because small copper crystallites were created in the EDTA series that could penetrate deeper into the pore and block the pore more efficiently, whereas large copper agglomerates formed in the TEA series could only dwell on the outer substrate surface. Fig. 3 shows the X-ray diffraction patterns of the copper-plated alumina from the EDTA series and from the TEA series with almost identical copper loading. The peaks that appeared at 2h=43.5°, 50.5°, and 74.2° represent (111), (200), and (220) planes, respectively. According to Scherrer’s equation [16 ], the crystallite diameter is inversely proportional to the full width at half maximum intensity (FWHM ) of the peak. The FWHM of the peak at 2h=43.5° in Fig. 3(a) is larger than that in Fig. 3(b), indicating that the copper crystallites formed in the EDTA series are smaller than those formed in the TEA series. Table 2 lists the crystallite diameters of both series, calculated from Scherrer’s equation. The crystallite diameters of the EDTA series ˚ , independent of the copper loading. were around 200 A However, the crystallite diameters of the TEA series varied irregularly with copper loading, and ranged from ˚ in diameter, indicating a wide size distribu150 to 430 A tion of crystallites or clusters deposited on the outer surface of the alumina particle. The SEM images of electroless plated copper on alumina are shown in Fig. 4. The surface structure is strongly dependent on the chelating agents as well as the calcination temperature. The variation of the surface morphology of electroless plated Cu/Al O with respect 2 3 to copper loading can be found elsewhere [17]. Fig. 4 shows that the particle size increased with calcination temperatures for both series, due to the sintering effect. For the EDTA series, the copper particles were relatively small and uniformly distributed on the surface of the alumina support when compared with those using TEA as the chelating agent. This observation was also confirmed by the XRD pattern shown in Fig. 3 and the chemisorption data shown in Table 2. Fig. 5 illustrates the dependence of acidity on copper loading. The acidity of bare alumina is 345 mmol of NH per gram. The acidities of both the EDTA and 3 TEA series decreased with copper loading up to a certain value and leveled off with further deposition. In addition, the acidity decreased more markedly with copper loading for EDTA series than that for TEA series. Since the copper crystallites resulting from EDTA are smaller and spread more uniformly than from TEA, the acidity

Fig. 3. X-ray diffraction patterns of Cu/Al O catalysts (a) EDTA-2 and (b) TEA-3. 2 3

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W.H. Lin, H.F. Chang / Surface and Coatings Technology 107 (1998) 48–54

Fig. 4. SEM images of coated alumina subjected to different calcination temperature.

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W.H. Lin, H.F. Chang / Surface and Coatings Technology 107 (1998) 48–54

patterns and chemisorption data showed that the size of copper crystallites strongly depended on the complexing capability of chelating agents. The copper-plating rate of the TEA system was much faster than that of the EDTA system, which resulted in the formation of bulky copper agglomerates. The coated alumina prepared by Cu–EDTA chelates had smaller copper crystallites, a higher copper dispersion, and a more uniform crystallite distribution than those prepared by Cu–TEA chelates. The acidity of the coated alumina could be effectively reduced for the EDTA series.

Acknowledgement

Fig. 5. Effect of copper loading on the acidity of copper-coated alumina.

could be reduced more effectively for the EDTA series. When the electroless plating started, a large portion of activated surface was available for copper deposition, and the copper atoms gradually reduced the acidity of the alumina due to a shielding effect. Thus, the acidity decreased as the copper loading increased. At a higher copper loading, however, it was difficult for the Cu–ligand complex to diffuse deeply into the small micropores due to the bulkiness of the complex, which resulted in insufficient copper plating on the walls of the small micropores, and the acidity could not be significantly reduced. Conversely, Sivaraj et al. [18] prepared a Cu/Al O catalyst using a precipitation method and 2 3 found that the acidity was markedly reduced as shown in Fig. 5. This is because the unchelated copper ions can easily diffuse into small micropores of alumina before precipitation and reduce the acidity of the alumina more efficiently.

4. Conclusions Two chelates, EDTA and TEA, were used as the complexing agents for the electroless copper plating on alumina powder. The SEM images together with XRD

The authors thank Prof. F.L. Wang of Providence University for his kind assistance. The authors also thank the National Science Council of the Republic of China for the financial support of the experimental work (Grant no. NSC 87-2214-E035-002).

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