Effect of bath temperature for Cu electroless deposition onto acrylon nitril butadiene (ABS) insulating substrate

642 Journal of Chemistry, Vol. 44 (5), P. 642 - 647, 2006 EFFECT OF BATH TEMPERATURE FOR Cu ELECTROLESS DEPOSITION ONTO ACRYLON NITRIL BUTADIENE (ABS) INSULATING SUBSTRATE Received 8 August 2005 MAI THANH TUNG, LAI HUY NAM Dept. of Electrochemistry and Corrosion Protection, Hanoi University of Technology SUMMARY Influences of bath temperature (Tbath) on Cu electroless deposition rate and on morphology, structure, corrosion resistivity of deposited Cu layers were investigated. Results showed that the increasing Tbath from 25 to 70oC resulted an increase of the deposition rate, while the deposition rate decreased as Tbath increased from 70 to 90oC due to the bulk reduction of Cu2+. SEM results indicated that the crystals of deposited layers became finer as Tbath increased. XRD analyses showed that mean grain size Lmean decreased and intensity ratio I(111)/I(200) increased remarkably with increasing Tbath from 25 to 70oC and changes of both Lmean and I(111)/I(200) are less pronounce in the range of Tbath = 70 - 90o. The corrosion resistivity increased remarkably with increasing Tbath from 25 to 70oC and became nearly invariable the range of Tbath = 70 - 90o. These results were explained by the relation between structure and corrosion properties of the electrolessly deposited Cu layers. I - INTRODUCTION Electroless deposition technique has been intensively studied due to its important applications in electronics, surface technology and modern micro- and nanotechnology [1 - 4]. The main advantage of the electroless deposition technique is the possibility to form metal layers on insulating and semiconducting substrates. Among the metals used for electroless deposition, Cu is one of the most important materials due to its high conductivity and good mechanical properties. Therefore, the Cu electroless deposition became the key process in printed circuit boards (PCBs) production and metallization process in plastic industry [1 - 7]. The Cu electroless deposition process is based on the so-called autocatalytic effect of Cu2+ reduction when electrocatalytic metals such as Pt, Pd (activators) are present on the surfaces [6 - 8]. Typically, the Cu electroless deposition bath contains metal sources (Cu2+) and a reducing agent (HCHO) and the deposition occurs following two reactions [6, 7]: Cathode process: Cu2+ + 2e
Cu (1) Anode process: 2HCHO + 4OH-
2HCOO- + 2H2O + 2e (2) (The process (2) is initiated on surfaces of activators Pd, Pt) Since kinetics of the deposition process are controlled by both processes (1) and (2), the deposition rate and structure, morphology, mechanical properties of deposited films are influenced by activators, bath composition and temperature. It has been shown in several studies that bath temperature plays a very important role during the plating process and decides structure and properties of the obtained layers [2, 6 - 8]. 643 In this work, we will show results of study on the influences of bath temperature on deposition rate and structure, morphology, mechanical properties of Cu electrolessly deposited films onto Acrylon Nitril Butadiene (ABS) surface. II - EXPERIMENTAL The electroless deposition was performed on ABS plastic. Prior to the plating, the ABS samples were polished, degreased and rinsed carefully. In order to prepare rough and hydrophilic ABS surfaces, samples were pretreated in etching solution (CrO3 150 g/l, H2SO4 400 g/l, t = 70 oC). The electroless deposition process consisted of 3 steps: sensization, activation and electroless deposition. Solutions and conditions for the processes are given in table 1. Table 1: Solutions and conditions of the Cu electroless deposition steps Step Solution Temperature pH Duration Sensization (BK-PLAC-sens) 1g/l SnCl2.2H2O +surfactance 25 oC - 2 min Activation (BK-PLAC-act) 0,1 g/l PdCl2.2H2O +complex agent 25 oC - 2 min Plating solution (BK-PLAC- plat) 20 g/l CuSO4.6H2O 6 ml/l HCHO (37%) 35 g/l EDTA 26 g/l NaOH 25 - 90oC 11 15 min Average deposition rate was determined by mass method, which followed 2 steps: (i) dissolution of deposited Cu film and (ii) determination of the mass (m) of dissolved Cu by chemical analysis. The average deposition rate v was calculated by the equation: 410. .. tAD mv Cu = (3) where v is plating rate (µm/h), DCu is density of Cu (g/cm3), A is total area of sample (cm2), t is deposition time (h). Surface morphologies of the obtained deposited films were analysed using Scanning Electron Microscopy (SEM) (JMS 5410–Jeol equipment). XRD analysis was carried out using Bucker D8 Advance diffractometer. The mean grain size Lmean of the deposited Cu was estimated using the (111) peak broadening according to Sherrer’s equation [5]:   2cos 94.0 × × = eff Cu mean W L (4) where Cu is wavelength of Cu (= 0.1542 nm), Weff is effective full width at half maximum (determined from the Gaussian distribution function of (111) peak), 2 is diffraction angle. Polarization measurements were performed in a conventional three-electrodes electrochemical cell with a saturated calomel electrode (SCE) and a Pt counter electrode. Total surface of working electrode was 2.4 cm2. Before measurements, samples were immersed for 5 minutes in the measuring solution (HCl 0.1N). III - RESULTS AND DISCUSSION Fig.1 displays the influence of bath temperature (Tbath) on the Cu electroless deposition rate. Results show that in the range Tbath = 20 - 70 oC the deposition rate increases with increasing temperature. This behaviour is expected since kinetics of both reactions (1) and (2) are temperature dependent and the rates of reactions (1) and (2) increase exponentially with the factor (-1/Tbath) [2, 3]. However, in the range Tbath = 70 - 90 oC the plating rate decreases with increasing Tbath (Fig. 1). The reason for this is that the reactions (1) and (2) occur not only on the surfaces to form Cu layer, but also in bulk to form Cu particles in the solution since the processes in bulk solution become thermodynamic and dynamic favourable at Tbath 644 > 70oC. As a result, the bulk reactions increase and become dominated, leading to the decrease of the deposition rate. At Tbath > 100 oC, intact Cu deposited layers even cannot be formed on the ABS surface due to the severe reactions in the bulk solution to form dispersed Cu particles [3]. Bath temperature Tbath, oC Figure 1: Influence of bath temperature Tbath on Cu electroless deposition rate v The formation of Cu particles in the solution can also be confirmed by the SEM images (Fig. 2). While the deposited Cu surfaces at Tbath = 25oC, 40 oC, 70oC are clean (Fig. 2a-2c), it can be observed that Cu crystals formed by the bulk reactions with typical size of 2 – 4 µm are adsorbed on the Cu deposited surface at 90oC (Fig. 2d). It is also very interesting to note that the crystal structures of deposited layers are finer as Tbath increases. This result can be explained by the fact that the number of new Cu nuclei increases due to the high nucleation rate at high Tbath, while at low Tbath nucleation energy is low and the growth of the Cu crystals becomes more favorable than the formation of new nuclei [2, 3]. Structures of the electrolessly deposited layers at different Tbath were analyzed using XRD method. Results presented in Fig. 3 show that (111) and (200) textures appear for all deposited films and (111) is the dominated texture. However, the intensity of (111) orientation increases with increasing Tbath (Fig. 3). Table 2 shows the intensity ratios I(111)/I(200) of calculated from peak intensities of XRD patterns. The obtained results indicate that I(111)/I(200) increases with Tbath = 25 - 70oC and no remarkable changes are observed with Tbath = 70 - 90 oC. Table 2 also shows the changes of grain size L calculated by Scherrer’s equation (eq. 4). It is interesting to mention that the grain size also increases strongly with Tbath = 25 oC - 70 oC and changes slightly with Tbath = 70 - 90oC. Fig. 4 shows the polarization curves in HCl 0.1 M of the obtained Cu layers at different Tbath. It can be observed that Cu layers deposited at higher Tbath have better corrosion resistivity e.g lower corrosion current density icorr and less negative corrosion potential Ecorr (Fig. 4 and table 2). It is very interesting to note that again the changes of icorr and Ecorr are remarkable with Tbath = 25 - 70 oC and are less pronounce as Tbath increases from 70oC to 90oC. This corrosion behaviour can be explained by the correlation with grain size and crystal structure. It has been reported that the deposited layers with lower grain size have lower defect density, meaning that the Cu deposited layers with smaller grain size are more corrosion resistant [2 - 4, 7, 8]. On the other hand, the (111) plane has lowest surface energy among all Cu planes and thereby the (111) texture is less active to corrosive media. Thus, the decrease of grain size and the increase of content of (111) texture result the Pl at in g ra te v, µm /h 645 decrease of icorr as Tbath increases (table 2). The increase of Ecorr as Tbath increases may be explained by the passivation of the deposited Cu layer. (A) 25 C o (A) 40 C o (A) 70 C o (A) 90 C o 2 mµ 2 mµ 2 mµ 2 mµ Cu crystals formed in bulk solution Figure 2: SEM images of Cu electrolessly deposited layers with Tbath of (a) 25oC (b) 40oC (c) 70oC (d) 90oC 2, o Figure 3: XRD patterns of Cu electrolessly deposited layers with Tbath of (a) 25oC (b) 40oC (c) 70oC (d) 90oC In te ns ity ,a .u 646 E, SCE/V (1997) Figure 4: Polarization curves in HCl 0.1M of Cu electrolessly deposited layers with Tbath of (a) 25oC (b) 40oC (c) 70oC (d) 90oC Table 2: Intensity ratio I(111)/I(200), mean grain size Lmean, corrosion potential Ecorr and corrosion current density icorr of electrolessly deposited layers with different Tbath Parameters T = 25oC T = 40oC T = 70oC T = 80oC I(111)/I(200) 2.52 2.67 2.89 2.91 Mean grain size Lmean (nm) 77 62 47 45 Corrosion potential Ecorr (V) (SCE) -0.308 -0.292 -0.277 -0.273 Corrosion current density icorr (A/cm 2) 4.47.10-7 3.71.10-7 3.02.10-7 2.51.10-7 IV - CONCLUSIONS Bath temperature (Tbath) influences on electroless plating rate and on morphology, structure, corrosion resistively of the deposited Cu layers. In the range Tbath = 25 - 70 oC the increasing Tbath results an increase of the deposition rate, while the deposition rate decreases as Tbath increases from 70 to 90 oC due to the bulk reduction of Cu2+. SEM results indicate that the crystals of deposited layers became finer as Tbath increases. XRD results show that intensity ratio I(111)/I(200) increases and mean grain size Lmean decreases remarkably as Tbath increases from 25 to 70 oC and changes of both Lmean and I(111)/I(200) are less pronounce in the range of Tbath = 70 - 90 oC. The corrosion resistively increases also remarkably with increasing Tbath from 25 to 70 oC and less pronounce in the range of Tbath = 70 - 90 oC. These results were explained by the relation between structures and corrosion properties of the deposited layers. Acknowledgements: We thank the Research Fund of Ministry of Education and Training (Project Nr. B-2004-28-152) and VLIR-HUT Research Fund (Project Nr. VLIR- HUT/IUC/PJ10) for the financial support of this work. REFERENCES 1. S. P. Chong, Y. C. Ee, Z. Chen, S. B. Law. Surf. & Coat. Tech., Vol. 198, P. 287 - 290 (2005). 2. C. Marcadal, M. Eizenberg, A. Youn, L. Chen. J. Electrochem. Soc., 149, P. 152 - L og (i /A .c m 2 ) 647 157 (2002). 3. J. Horkan, C. Sambuceti, V. Markovic. IBM J. Res. & Dev., Vol. 28 (6), P. 690 - 695 (1993). 4. P. Andricacos, C. Uzoh, J. Dukovic, J. Horkan, H. Deligiani. IBM J. Res. & Dev., Vol. 56 (7), P. 576 - 582 (1998). 5. M. T. Tung. J. Chem. & App., Vol. 4 (40), P. 32 - 35 (2005). 6. A. Brenner. Electrodeposition of Alloys: Principals and Practice, New York, Academic Press (1963). 7. Y. Okinaka, T. Osaka. Electroless Deposition Processes: Fundamentals and Applications, in Advances in Electrochemical Science and Engineering, Vol. 2, P. 55 - 116, Weinheim, VCH Publishers (1994). 8. Tran Minh Hoang. Electroplating Technology, P. 190 - 199, Hanoi, Science and Technology Publishers (1998).