Morphology of Copper Coatings Electroplated in an Ultrasonic Field

Transcription

1 Morphology of Copper Coatings Electroplated in an Ultrasonic Field L. Martins a, J.I. Martins a,b, A.S. Romeira a, M.E. Costa a, J. Costa a, M. Bazzaoui a,b a CISE, Departamento de Engenharia Electrotécnica, Faculdade de Engenharia da Universidade do Porto, Rua Roberto Frias Porto, Portugal b Departamento de Engenharia Química, Faculdade de Engenharia da Universidade do Porto, Rua Roberto Frias Porto, Portugal Keywords: Copper, Electrodeposition, Adhesion, Hardness, Ultrasound. Abstract. This paper reports the study of ultrasonic effects on some physicochemical properties of electroplated copper on iron subtracts from an acid sulphate bath. The bath efficiency for three stirring processes has been also studied as a function of the time and the current density applied. The bath efficiency, the hardness, the brightness and the adherence of the electrodeposits increase with the ultrasonic agitation, in comparison with the same coatings produced from still or magnetic stirred baths. Scanning electron microscopy (SEM) analysis has revealed a compact and more order structure in the coatings performed under ultrasonic waves at high current densities, conversely to free and dusty obtained with still bath. Introduction The electroplated copper coatings over iron substrates are commonly used for many applications such as undercoats to various other platings, bronzed and oxycopper finishing s and building-up printing and marking cylinders for paper and textiles [1]. The physicochemical properties of the coatings are strongly dependent of the applied current density and stirring processes of the bath. A relationship exists between the crystal structure and the mechanical properties of copper deposits from baths without addition agents. Thus, the hardness and tensile properties are straightly connected to the crystal structure. Refinements of structure imply loss in ductility and an increase in the hardness and tensile properties. The use of ultrasounds during the electroplating causes the formation of cavitation bubbles, which collapse on the solid surface and promote the agitation of the solution. These microjects directed towards the surface enhance the momentum, heat and mass transfer into the electrolytic bath [2,3]. So, the diffusion layer near the cathode is decreased [4] and the electrode surface is activated, advancing conditions to obtain good electrodeposits. Zhao et al. [5] reported that the properties of coatings are improved using low ultrasonic power, short irradiation times and high frequencies. Much of this work has been carried out from an industrial standpoint, and was devoted to study the effect of the time of electrolysis, applied current density and type of agitation, on the morphology, hardness, brightness, adherence and electrolytic yield of an electroplated copper coating on steel, seeing its application in the lithographic printing. Experimental All reagents were of analytical grade from Aldrich, Spain: sodium pyrophosphate (Na4P2O7, 98%), sodium metasilicate (Na2SiO3, 97%), sodium carbonate (Na2CO3, 99%), sodium hydroxide (NaOH, 97%), hydrochloric acid (HCl, 37%), triethanolamine ((HOCH2CH2)3N, 98%), sulphuric acid (H2SO4, 98%) and copper sulphate (CuSO4.5H2O, 98%). The steel (ASTM 1015) electrodes (4 x 8 cm 2 ) before each electrolysis have been submitted to the following treatment scheme: a) mechanical polishing with an abrasive paper (1200-grade); b) cleaning and rinsing with water; c) chemical degreasing at 80 ºC during 5 minutes in sodium 844

2 pyrophosphate 8g/L, triethanolamine 1 g/l, sodium carbonate 25 g/l, sodium hydroxide 33 g/l and sodium metasilicate 20g/L solution; d) rinsing with water; e) acid pickling for 1 minute in a 50% v/v hydrochloric acid solution; f) and rinsing with water. The electrolysis was carried out in an acid copper bath (225 g/l CuSO4.5H2O + 50 g/l H2SO4) contained in glass beakers of 1.5 litres of capacity, at 25 ºC of temperature, without stirring, with magnetic stirring and using ultrasonic waves with a frequency of 40kHz. These last trials have been performed with the container standing in water in Ultrasons-H Selecta equipment. Adherence measurements were based on the scribe-grid test. On the coating is scribe a rectangular grid pattern with a hardened steel tool ground to a sharp (30-deg) point with a distance between the scribed lines of approximately ten times the nominal coating thickness, with a minimum of 0.4 mm. If any portion of the coating between the lines breaks away from the substrate, the adhesion is inadequate. Microhardness measurements were conducted using a hardness tester, model 3212 Zwick, America, equipped with a Vickers diamond indenter under a 50-g load. Four measurements were always averaged. Efficiency of the plating was determinate by weighing before and after deposition, allowance being made for initial reproducible weight losses in the pickling step, and taking into account the global charge that went through the electrolytic cell. The copper deposits were morphologically analysed by scanning electronic microscopy (SEM) with a JEOL JSM-35C equipment. Results and discussion As shown in Fig. 1, for a current density of 1.5 A dm -2, the efficiency of plating is strongly affected by the agitation. Without stirring the bath, the efficiency decreases with the electrodeposition time as a consequence of a depletion in the copper ion on the electrode surface. Imposing an agitation the depletion is reduced as a consequence of the increase of mass transfer to the cathode, which is associated with diffusion and convection processes. However, the ultrasound agitation has a better performance (Fig. 2) allowing even to work with higher current densities at practically the same efficiency (98-99%). For current densities higher than 5 A dm -2 the electrodeposits obtained without stirring were burnt. Yield (%) Figure 1: Yield versus electroplating time time (min) Yield (%) Figure 2: Yield versus current density Current density(a/dm 2 )(A/dm 2 ) without agit. with magnetic agit. ultrasonic agit. without agit. with magnetic agit. ultrasonic agit. Fig. 1. Yield vs. electroplating time (j = 1.5 A dm -2 ) Fig. 2. Yield vs. current density (t = 15 min) The adherence of the copper electrodeposits on iron electrodes galvanostatically prepared with different current densities was estimated using the scribe-grid test, which evaluates the quality of the coating adherence. In brief the obtained data are summarized on the Table 1. In the range of the current densities studied, between 1.5 and 5 A dm -2, the electroplated copper under ultrasonic waves is strongly adherent. The coatings become poorly adherents when obtained 845

3 without agitation, while with magnetic stirring the adherence decreases when the current density increases. Adhesion depends so much on the preliminary cleaning of the surface of the basis metal and the mobility of particles being discharged at the surface to regroup and form crystallites corresponding in shape and orientation to the crystals of the cathode. However, the quality of a deposit and its adhesion are also affected by the plating conditions. The obtained results are intimately connected with the structure grain size of the copper electrodeposited on the steel. The lower current densities impose a structure of large crystals, whereas the increase of the current density promotes smaller crystals. However, if the current density attains or exceeds the limit current density the electrodeposits become brittles. The increase of mass transfer by the ultrasonic waves make possible to work at 5 A dm -2 and to maintain a fine crystal structure. Table 1: Effect of solution agitation and current density on adherence of copper electrodeposits by the scribe-grid test. Current density [A dm -2 ] Agitation condition Still Magnetic stirring Ultrasonic waves 1.5 s g e 3.0 ns s e 5.0 ns ns e s = satisfactory; ns = not satisfactory; g = good; e = excellent The hardness of the deposits depends on its grain size, the finer the grain size the greater will be [6]. For one and same grain size, differences in hardness may be due to a different packing of the atoms in the crystal lattice. The hardness of copper electrodeposits obtained at 25º C increases with the current density and the turbulence of the solution, as is shown in Table 2. These results are in agreement with the values reported by Walk et al. [7], and show the possibility of producing a harder deposit of finer crystal structure ( HV) without addition agents. Some of these last compounds can break down in the plating bath giving undesirable products, which are responsible for hard brittle deposits easily broken by bending. Table 2: Effect of solution agitation and current density on hardness (HV) of copper electrodeposits. Current density [A dm -2 ] Agitation condition Still Magnetic stirring Ultrasonic waves Copper electrodeposits on steel electrodes galvanostatically prepared at a current density of 4 A dm -2 during 15 min and under three electrolysis process: i) copper films prepared without bath stirring; ii) copper electrodeposited on iron using bath stirring and iii) films obtained under ultrasonic waves, were imaged by SEM ( Fig. 3). The copper coating surface properties obtained with the three conditions above mentioned are very different. Indeed, in the absence of bath stirring (Fig. 3A), the coating is brittle and nonhomogeneous. Its structure shows some cracks owing to the cathodic reaction of the hydrogen bubbling, in spite of copper comes below hydrogen in the electrochemical series. An increase in the electrolysis current density may accelerate the discharge of hydrogen ions at the cathode. So, its likelihood to be adsorbed on the surface increases, and consequently imposes changes in the physico-chemical properties of the electrodeposits as has been shown in the hardness results obtained with the still bath. Conversely, when the bath was stirred (Fig. 3B), the morphology of the 846

4 (A) (A) (B) (B) (C) (C) Fig. 3: SEM micrographs of copper coated steel obtained at 4 A dm -2 during 15 min A) without bath stirring, B) with bath stirring and C) under ultrasonic waves 847

5 deposit becomes uniform, smooth and presents a globular structure with grains of about 1-3 µm. As has been referred by Vagramyan [8], normal grain growth requires a certain optimum current density. Finally, under ultrasound waves (Fig. 3C), the film is compact and the effect of the typical mechanical action imposed by bubbles cavitation can be seen on the surface of electrodeposited copper. This structure confirms the effect of ultrasonics in the production of harder deposits. Conclusion It may be pointed out that the experimental results have shown that the physico-mechanical properties of electrodeposited copper undergo substantial changes when the process is accompanied by high current densities. Ultrasounds action on copper electrodeposits reduces substantially the polarization phenomena, enhances the efficiency of the plating rate and changes the physicochemical properties of the electrolytic copper. Under the influence of ultrasound it is possible to produce hard, compact and adherent electrodeposits even using high current densities in the electroplating process. Acknowledgements Funding from Fundação para a Ciência e a Tecnologia, Annual Support for CISE, is gratefully acknowledged. References [1] Frederick A. Lowenheim: Modern electroplating, John Wiley & Sons (3 th Ed.), [2] S.A. Perusich and R.C. Alkire: Journal of Electrochem. Soc. Vol. 138 (1991), p.700. [3] F.J. Touyeras, Y. Hihn, M.L.Doche and X. Roizard: Ultrasonics Sonochemistry Vol. 8 (2001), p.285. [4] J.P. Lorimer, B. Pollet, S.S. Phull, T.J. Mason, D.J. Walton and U. Geissler: Electrochim. Acta Vol. 41 (1996), p [5] Y.Y. Zhao, C.G. Bao, R. Feng and Z.H. Chen: Ultrasonics Sonochemistry Vol. 2 (1995), p.99. [6] D.J. Macnaughtan and A.W. Hothersall: Trans. Faraday Soc. Vol. 24 (1928), p.387; Vol. 31 (1935), p [7] R. Walker and J.F. Clementys: Metal Finishing J. Vol. 16 (1970), p.100. [8] A.T. Vagramyan: Zhur. Fiz. Khim. Vol. 19 (1945), p