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

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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.

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Nội dung Text: Effect of bath temperature for cu electroless deposition onto acrylon nitril butadiene (ABS) insulating substrate

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 deposition bath contains metal sources (Cu2+) and a reducing agent (HCHO) and the Electroless deposition technique has been deposition occurs following two reactions [6, 7]: intensively studied due to its important Cathode process: Cu2+ + 2e Cu (1) applications in electronics, surface technology and modern micro- and nanotechnology [1 - 4]. Anode process: The main advantage of the electroless 2HCHO + 4OH- 2HCOO- + 2H2O + 2e (2) deposition technique is the possibility to form metal layers on insulating and semiconducting (The process (2) is initiated on surfaces of substrates. Among the metals used for activators Pd, Pt) electroless deposition, Cu is one of the most Since kinetics of the deposition process are important materials due to its high conductivity controlled by both processes (1) and (2), the and good mechanical properties. Therefore, the deposition rate and structure, morphology, Cu electroless deposition became the key mechanical properties of deposited films are process in printed circuit boards (PCBs) influenced by activators, bath composition and production and metallization process in plastic temperature. It has been shown in several industry [1 - 7]. The Cu electroless deposition studies that bath temperature plays a very process is based on the so-called autocatalytic important role during the plating process and effect of Cu2+ reduction when electrocatalytic decides structure and properties of the obtained metals such as Pt, Pd (activators) are present on layers [2, 6 - 8]. the surfaces [6 - 8]. Typically, the Cu electroless 642 In this work, we will show results of study ABS plastic. Prior to the plating, the ABS on the influences of bath temperature on samples were polished, degreased and rinsed deposition rate and structure, morphology, carefully. In order to prepare rough and mechanical properties of Cu electrolessly hydrophilic ABS surfaces, samples were deposited films onto Acrylon Nitril Butadiene pretreated in etching solution (CrO3 150 g/l, (ABS) surface. H2SO4 400 g/l, t = 70oC). The electroless deposition process consisted of 3 steps: II - EXPERIMENTAL sensization, activation and electroless deposition. Solutions and conditions for the The electroless deposition was performed on processes are given in table 1. Table 1: Solutions and conditions of the Cu electroless deposition steps Step Solution Temperature pH Duration o Sensization (BK-PLAC-sens) 1g/l SnCl2.2H2O +surfactance 25 C - 2 min o Activation (BK-PLAC-act) 0,1 g/l PdCl2.2H2O +complex agent 25 C - 2 min 20 g/l CuSO4.6H2O Plating solution (BK-PLAC- 6 ml/l HCHO (37%) 25 - 90oC 11 15 min plat) 35 g/l EDTA 26 g/l NaOH Average deposition rate was determined by function of (111) peak), 2 is diffraction angle. mass method, which followed 2 steps: (i) Polarization measurements were performed in a dissolution of deposited Cu film and (ii) conventional three-electrodes electrochemical determination of the mass (m) of dissolved Cu cell with a saturated calomel electrode (SCE) by chemical analysis. The average deposition and a Pt counter electrode. Total surface of rate v was calculated by the equation: working electrode was 2.4 cm2. Before m measurements, samples were immersed for 5 v= .10 4 (3) minutes in the measuring solution (HCl 0.1N). DCu . A.t where v is plating rate (µm/h), DCu is density of III - RESULTS AND DISCUSSION Cu (g/cm3), A is total area of sample (cm2), t is deposition time (h). Fig.1 displays the influence of bath temperature (Tbath) on the Cu electroless Surface morphologies of the obtained deposition rate. Results show that in the range deposited films were analysed using Scanning Tbath = 20 - 70oC the deposition rate increases Electron Microscopy (SEM) (JMS 5410–Jeol with increasing temperature. This behaviour is equipment). XRD analysis was carried out using expected since kinetics of both reactions (1) and Bucker D8 Advance diffractometer. The mean (2) are temperature dependent and the rates of grain size Lmean of the deposited Cu was reactions (1) and (2) increase exponentially with estimated using the (111) peak broadening the factor (-1/Tbath) [2, 3]. However, in the range according to Sherrer’s equation [5]: Tbath = 70 - 90oC the plating rate decreases with 0.94 × Cu increasing Tbath (Fig. 1). The reason for this is Lmean = (4) that the reactions (1) and (2) occur not only on Weff × cos 2 the surfaces to form Cu layer, but also in bulk to where Cu is wavelength of Cu (= 0.1542 nm), form Cu particles in the solution since the Weff is effective full width at half maximum processes in bulk solution become (determined from the Gaussian distribution thermodynamic and dynamic favourable at Tbath 643 > 70oC. As a result, the bulk reactions increase deposited layers even cannot be formed on the and become dominated, leading to the decrease ABS surface due to the severe reactions in the of the deposition rate. At Tbath > 100oC, intact Cu bulk solution to form dispersed Cu particles [3]. Plating rate v, µm/h 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 70oC and no remarkable changes are observed can also be confirmed by the SEM images (Fig. with Tbath = 70 - 90oC. Table 2 also shows the 2). While the deposited Cu surfaces at Tbath = changes of grain size L calculated by Scherrer’s 25oC, 40 oC, 70oC are clean (Fig. 2a-2c), it can equation (eq. 4). It is interesting to mention that be observed that Cu crystals formed by the bulk the grain size also increases strongly with Tbath = reactions with typical size of 2 – 4 µm are 25 oC - 70 oC and changes slightly with Tbath = adsorbed on the Cu deposited surface at 90oC 70 - 90oC. (Fig. 2d). It is also very interesting to note that Fig. 4 shows the polarization curves in HCl the crystal structures of deposited layers are 0.1 M of the obtained Cu layers at different Tbath. finer as Tbath increases. This result can be It can be observed that Cu layers deposited at explained by the fact that the number of new Cu higher Tbath have better corrosion resistivity e.g nuclei increases due to the high nucleation rate lower corrosion current density icorr and less at high Tbath, while at low Tbath nucleation energy negative corrosion potential Ecorr (Fig. 4 and is low and the growth of the Cu crystals table 2). It is very interesting to note that again becomes more favorable than the formation of the changes of icorr and Ecorr are remarkable with new nuclei [2, 3]. Tbath = 25 - 70oC and are less pronounce as Tbath increases from 70oC to 90oC. This corrosion Structures of the electrolessly deposited behaviour can be explained by the correlation layers at different Tbath were analyzed using with grain size and crystal structure. It has been XRD method. Results presented in Fig. 3 show reported that the deposited layers with lower that (111) and (200) textures appear for all grain size have lower defect density, meaning deposited films and (111) is the dominated that the Cu deposited layers with smaller grain texture. However, the intensity of (111) size are more corrosion resistant [2 - 4, 7, 8]. On orientation increases with increasing Tbath (Fig. the other hand, the (111) plane has lowest 3). Table 2 shows the intensity ratios surface energy among all Cu planes and thereby I(111)/I(200) of calculated from peak intensities the (111) texture is less active to corrosive of XRD patterns. The obtained results indicate media. Thus, the decrease of grain size and the that I(111)/I(200) increases with Tbath = 25 - increase of content of (111) texture result the 644 decrease of icorr as Tbath increases (table 2). The explained by the passivation of the deposited Cu increase of Ecorr as Tbath increases may be layer. 2µm o (A) 25 C 2µm o (A) 40 C 2µm (A) 70oC 2µm (A) 90oC 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 Intensity, a.u 2 ,o Figure 3: XRD patterns of Cu electrolessly deposited layers with Tbath of (a) 25oC (b) 40oC (c) 70oC (d) 90oC 645 Log(i/A.cm2) 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 2 -7 -7 -7 Corrosion current density icorr (A/cm ) 4.47.10 3.71.10 3.02.10 2.51.10-7 IV - CONCLUSIONS pronounce in the range of Tbath = 70 - 90oC. These results were explained by the relation Bath temperature (Tbath) influences on between structures and corrosion properties of electroless plating rate and on morphology, the deposited layers. structure, corrosion resistively of the deposited Cu layers. In the range Tbath = 25 - 70oC the Acknowledgements: We thank the Research increasing Tbath results an increase of the Fund of Ministry of Education and Training deposition rate, while the deposition rate (Project Nr. B-2004-28-152) and VLIR-HUT decreases as Tbath increases from 70 to 90oC due Research Fund (Project Nr. VLIR- to the bulk reduction of Cu2+. SEM results HUT/IUC/PJ10) for the financial support of this indicate that the crystals of deposited layers work. became finer as Tbath increases. XRD results show that intensity ratio I(111)/I(200) increases REFERENCES and mean grain size Lmean decreases remarkably as Tbath increases from 25 to 70oC and changes 1. S. P. Chong, Y. C. Ee, Z. Chen, S. B. 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