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Electroplating of copper films on steel substrates from acidic gluconate baths
Applied Surface Science 165 Ž2000. 249–254 www.elsevier.nlrlocaterapsusc
Electroplating of copper films on steel substrates from acidic gluconate baths S.S. Abd El Rehim a,) , S.M. Sayyah b, M.M. El Deeb b b
a Chemistry Department, Faculty of Science, Ain Shams UniÕersity, Cairo, Egypt Chemistry Department, Faculty of Science, Cairo UniÕersity, Beni-Suef Branch, Beni-Suef, Egypt
Received 16 July 1999; accepted 22 December 1999
Abstract Electroplating of thin films of copper onto steel substrate from acidic gluconate bath has been investigated under different conditions of bath composition, pH, current density, and temperature. A detailed study has been made about the effect of these parameters on potentiodynamic cathodic polarization, cathodic current efficiency ŽCCE%., and throwing power ŽTP. of the bath. Fine grained, highly adherent and smooth bright deposits were produced. X-ray diffraction analysis showed that these deposits were produced in one phase with crystalline cubic structure. The optimum conditions are: 10 g ly1 CuSO4 P 5H 2 O, 30 g ly1 C 6 H 11O 7 Na, 10 g ly1 K 2 SO4 , pH s 2.2, I s 2.8 mA cmy2 , and temperature range between 228C and 318C. The TP of the bath is greatly improved by increasing the current density. q 2000 Elsevier Science B.V. All rights reserved. Keywords: Electroplating of copper films; Steel substrates; Acidic guconate baths
1. Introduction Copper electroplating is one of the oldest, protective and decorative metallic coating for steel and other basis metals. Therefore, intensive studies were carried out to obtain copper electroplates suitable for different purposes. One of the most important baths used for electroplating copper was cyanide bath w1x, but due to the environment consideration, cyanide based baths formulation were replaced by noncyanide formulations such as sulphate w2–6x, chloride w4x, pyrophosphate w7x and tartarate bath w1x. )
Corresponding author. Fax: q20-2-483-1836. E-mail address: [email protected] ŽS.S. Abd El Rehim..
Organic additives are added to acidic copper sulphate plating baths to improve the quality of the deposits. The presence of these adsorption compounds in the plating bath results in marked changes in the deposit which can increase brightness, hardness, smoothness, and ductility w8–10x. The effect of thiourea on the electroplating of copper from acidic sulphate bath has been studied w11,12x. This compound acts as brightener and leveler, and in its absence ductile copper deposits are obtained. Thiourea and its copper complexes can adsorb on the cathode surface and increase the overpotential. Surface roughness has been attributed to the fact that at high overpotentials mass transfer limited deposition favours the growth of a deposit with protrusions,
0169-4332r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 Ž 0 0 . 0 0 0 1 5 - 5
250
S.S. Abd El Rehim et al.r Applied Surface Science 165 (2000) 249–254
while at low overpotentials the grain size of the deposit is large w13x. Michailova et al w14x and Peykova et al. w15x examined the effect of some surfactants on the nucleation and growth kinetics of copper from acidic copper sulphate solution. The presence of the surface active compounds affected the rate of nucleus formation in two different ways: directly, through a blocking of active sites on the substrate which will diminish the nucleation rate; and indirectly, by reducing the growth rate of copper grains by increasing the overpotential. The objective of the present study is to develop a cheap and non-toxic alternative to the alkaline cyanide electrolytes, which is a ‘‘medium-acid’’ as it contains a gluconate complexing agent that practically eliminates the cementation processes and allows electrodeposition of copper coatings on steel surface with relatively good adhesion. The proposed bath may be of practical interest for the deposition of a thin striking layer on the steel surface and for addition deposition of copper from high-speed strongly acidic electrolytes, as well as deposition thin copper layer on steel surfaces followed by nickel, chromium, etc., galvanic coatings. In this work, the effects of some plating and operating variables on the characteristics of the electroplating, the quality of the deposits, and on the throwing power ŽTP. of this bath were investigated.
2. Experimental work Experimental work was carried out in solutions containing CuSO4 P 5H 2 O, C 6 H 11O 7 Na, and K 2 SO4 . All solutions used were freshly prepared by doubling distilled water and from analytical grade chemicals. The pH was adjusted using sulphuric acid or sodium hydroxide. The experimental setup used was described previously w16x and consisted of rectangular Perspex cell provided with a plane parallel steel sheet cathode and a platinum sheet anode. Each electrode had a dimension of 3 = 3 cm and filled the cross-section of the cell. Before each run, the cathode was mechanically polished with 600-mesh emery paper washed with distilled water, rinsed with ethanol, dried, and weighed. The experiments were conducted at the required temperature "18C with
Table 1 Composition of the copper electroplating solutions Bath no.
Cu-1 Cu-2 Cu-3 Cu-4 Cu-5 Cu-6 Cu-7 Cu-8
Concentration g ly1
pH
CuSO4P5H 2 O
C 6 H 11O 7 Na
K 2 SO4
10 10 10 5.0 15 10 10 10
30 30 30 30 30 0.0 10 50
10 10 10 10 10 10 10 10
4.6 2.2 3.0 2.2 2.2 2.2 2.2 2.2
the help of an air thermostat. The plating duration was 15 min, at the end of that time, the cathode was withdrawn, washed with distilled water, dried, and weighed. The deposits were examined by X-ray diffraction using a Philips diffractometer with CuKa radiation and nickel filter Ž40 kV, 30 mA.. The plating current was supplied by using dc power supply ŽThurby Thandar PL330. and the values of pH were measured by pH meter Ž Jenway 3071.. Potentiodynamic cathodic polarization measurements were recorded by using EG & G Potentiostatr Golvanostat Model 273 A supplied by EG & G Princeton Applied Research. The I–E curves were recorded by a computer software from the same company Model 352 and 270r250. The measurements were carried out in a three-electrode cell, a steel cathode, platinum anode, and a reference saturated calomel electrode ŽSCE.. The composition of the used baths for the electroplating of copper is presented in Table 1.
3. Results and discussion 3.1. Potentiodynamic cathodic polarization curÕes The potentiodynamic cathodic polarization curves for copper electroplating from acidic gluconate baths were recorded under the effect of various variables. Each curve was swept from the rest Žzero current. potential into more negative potentials with the scan rate of 25 mV sy1 . Fig. 1 shows the cathodic polarization curves for copper electroplating from
S.S. Abd El Rehim et al.r Applied Surface Science 165 (2000) 249–254
Fig. 1. Effect of concentration of sodium gluconate on the potentiodynamic cathodic polarization curves for copper electroplating at 308C with scan rate of 25 mV sy1 .
251
for the corresponding gluconate free solution. This inhibition effect of the gluconate ions on the copper electroplating reaction may be due to the adsorption of gluconate ions on the cathode surface andror the complexion of Cu2q with the gluconate ions. It is known that in acidic media ŽpH - 4., Cu2q ions can complex with gluconate ion via a ligand carboxyl group, and the stability of the complex is low ŽpKa s 2.8. and exists as cationic wCuC 6 H 11O 7 xq w18,19x, but on increasing pH, coordinate ion on the secondary alcoholic group is followed by liberation of proton, and formation of chelation ring is possible. This process leads to an increase in the stability of the complex. It is probable that copper–gluconate complexes cannot be reduced so easily as the free Cu2q ions. As the gluconate concentration in the bath is increased, most of the Cu2q ions become complexed. For a given concentration, the rate of transport of copper–gluconate complex is lower than for Cu2q because of the lower value of the diffusion coefficient of a complexed ion w20x. Therefore, the current plateau decreases with increasing gluconate concentration as predicted in Fig. 1 due to a lowering of the uncomplexed Cu2q ions concentration. Fig. 2
sulphate solution under the influence of adding increasing amounts of sodium gluconate. It is obvious that in gluconate free solution ŽCu-6., the current, at first, increases linearly, and then tends to exhibit limiting current plateau with increases in the cathodic potential. It is generally accepted that electroplating of Cu2q ions takes place through two steps w17x:
~ Cu Cu q e ° Cu Cu2qq e q
q
slow step fast step
It was assumed that the first step in this process occurred slowly and the rate was controlled by the equilibrium between Cu2q and Cuq at the electrode surface. However, addition of gluconate ion to the sulphate solution increases the cathodic polarization and decreases the value of limiting current density. The observed changes in the cathodic polarization in the presence of gluconate ions suggest that it must be acting as an inhibitor, which is confirmed by the observation that at any given overpotential, the current density for copper deposition from solutions containing gluconate ions is lower than that found
Fig. 2. Effect of concentration of CuSO4P5H 2 O and pH on the potentiodynamic cathodic polarization curves for copper electroplating at 308C with scan rate of 25 mV sy1 .
252
S.S. Abd El Rehim et al.r Applied Surface Science 165 (2000) 249–254
3.2. Cathodic current efficiency
Fig. 3. Effect of temperature on the potentiodynamic cathodic polarization curves for copper electroplating from bath ŽCu-2. with scan rate of 25 mV sy1 .
illustrates the influence of CuSO4 concentration in the bath. Increasing the CuSO4 content in the bath decreases the cathodic polarization and increases the limiting current plateau. These results were expected due to an increase in the relative abundance of the uncomplexed Cu2q ions in the solution. At the same time, data of Fig. 2 showed that an increase of pH enhances the cathodic polarization and lowers the limiting current plateau. Increasing the pH while keeping the concentrations of the metal salt and the complexing agent in the bath constant increases the stability of the complex and consequently decreases the ratio free Cu2qrCu–gluconate. These changes lead, thus, to a decrease in the limiting current plateau. The influence of temperature on the cathodic polarization during copper deposition from bath ŽCu2. was examined and the results are shown in Fig. 3. An increase in temperature Ž30–508C. increases the cathodic polarization and reduces the limiting current plateau. This trend could be ascribed to the effect of temperature on the relative abundance of both the free and complexed Cu2q ions in the solution. Elevation of the temperature of the gluconate bath may enhance the formation of the complexed Cu2q ions.
The cathodic current efficiency ŽCCE%. for copper electroplating from gluconate baths were studied under the influence of different plating parameters. Fig. 4 shows the effect of the applied current density on CCE% for copper deposition from different baths. Inspection of the data depicts that the CCE% decreases markedly with increases in the applied current density as the result of simultaneous hydrogen evolution reaction. Such a trend is related to the increase in cathodic overpotential with an increase of current density. The data indicate that CCE% increases with increasing copper sulphate concentration in the bath. These results agree with the cathodic polarization data shown in Fig. 2. On the contrary, the CCE% for copper deposition decreases with the increasing of the gluconate content in the bath. These results are expected due to the inhibiting effect of gluconate ions on the copper deposition reaction. The effect of pH Žfrom 2.2 to 4.6. of gluconate bath on CCE% for copper deposition is given also in Fig. 4. The results display that CCE% increases with the decrease in pH value. It seems that the concentration of the complexed Cu2q ions increases relative to
Fig. 4. Effect of current density on the cathodic current efficiency for copper electroplating from different baths at 308C.
S.S. Abd El Rehim et al.r Applied Surface Science 165 (2000) 249–254
253
Table 3 Throwing power and throwing index of gluconate baths under different conditions Bath no.
Current density mA cmy2
Temperature 8C
TP%
TI
Cu-8 Cu-5 Cu-2 Cu-2 Cu-2
2.8 2.8 2.8 8.3 2.8
30 30 30 30 50
17.6 15 11 81 2.6
1.33 1.29 1.17 6.67 1.01
K 2 SO4 , pH s 2.2, I s 2.8 mA cmy2 , and temperature range between 228C and 318C.
4. Structure of copper deposits Fig. 5. Effect of temperature on the cathodic current efficiency for copper electroplating from bath ŽCu-2. at I s 2.8 mA cmy2 .
the concentration of uncomplexed Cu2q with increases in the pH value, and this is reflected in a decrease in CCE%. The effect of temperature in the range from 228C to 508C on the CCE% for copper deposition from bath ŽCu-2. is shown in Fig. 5. A rise of temperature from 228C to 318C has no significant influence on the CCE%. However, a further increase in temperature decreases markedly the CCE%. The data can be related to the effect of temperature on the cathodic polarization. An increase in temperature enhances the cathodic polarization and decreases the limiting current, so a decrease in the CCE% for copper deposition was to be expected. According to the data derived from the experiments, one can conclude that the optimum conditions for copper electrodeposition from acidic gluconate bath on steel substrate are as follows: 10 g ly1 CuSO4 P 5H 2 O, 30 g ly1 C 6 H 11O 7 Na, 10 g ly1
Fine-grained, highly adherent and bright deposits were obtained from the above-mentioned solutions. The adherence tends to decrease with increases in the CuSO4 P 5H 2 O above 20 g ly1 . X-ray diffraction studies were carried out on the as-deposited copper under the optimum conditions. The data given in Table 2 reveal that copper consists of one phase with crystalline cubic structure. The average size of the deposited grains was measured by using scanning
Table 2 X-ray diffraction data of the electroplating copper from gluconate bath ŽCu-2. Ž I s 2.8 mA cmy2 , times15 min at 308C. ŽA.
Ir I0
hk I
Phase
Structure
2.066
100
111
Cu
Cubic
Fig. 6. Effect of concentration of C 6 H 11O 7 Na on the throwing index of gluconate baths at 308C and I s 2.8 mA cmy2 .
254
S.S. Abd El Rehim et al.r Applied Surface Science 165 (2000) 249–254
electron microscope ŽJEOL Model 50 A.. The obtained data reveal that the average size of the grains is about 0.2 mm.
slopes of these lines, i.e. the TI is also listed in Table 3. It is seen that the values of TI and TP vary in a manner almost parallel to each other.
5. TP of gluconate baths
References
The TP of the acidic gluconate baths used for electroplating of copper was investigated by using Haring–Blum cell under various plating conditions. The percentage values of TP were calculated using the empirical Field’s formula at a distance ratio Ž1:3. and the results are recorded in Table 3. Data of Table 3 reveal that an increase in copper content of the bath enhances its TP. This is an unexpected result, since an increase in copper content is accompanied by a decrease in the cathodic polarization. Again an increase in gluconate content of the bath improves TP. Such an effect could be assigned to the corresponding increase in cathodic polarization. An increase in the applied current density strongly improves TP. This could be interpreted on the basis that an increase in current density increases cathodic polarization. On contrary, a rise in the temperature of the bath has an adverse influence on TP even though a rise of temperature is expected to enhance cathodic polarization ŽFig. 3. and electrical conductivity of electrolytes. A graphical method for expressing the results of TP measurements was suggested by Jelink and David w21x. In this method, the metal distribution M was plotted versus the linear ratio L, Žwithin the range from L s 1 to L s 3. on arithmetic coordinates. The reciprocal of the slope of this plot is called throwing index ŽTI. and represents a direct measure of bath TP w22x. A representative linear plot between the metal distribution ratio M and the linear ratio L Ž1:1 to 1:3. are shown in Fig. 6. The reciprocal of the
w1x S. Bharathi, S. Rajendran, V.N. Loganathan, C. Krishna, Am. Electroplat. Surf. Finish. Soc. Ž1996. 263, Orlando, FL, USA. w2x J.O’M. Bockris, M. Enyo, Trans. Faraday Soc. 58 Ž1962. 1187. w3x E. Chassaing, R. Wiart, Electrochim. Acta 29 Ž1984. 649. w4x J. Crousier, I. Bimaghra, Electrochim. Acta 34 Ž1989. 1205. w5x K.I. Popov, M.D. Maksimovic, J.D. Trnjavcev, M.G. Pavlovic, J. Appl. Electrochem. 11 Ž1981. 239. w6x D. Pletcher, I. Whyte, F.C. Walsh, J.P. Millington, J. Appl. Electrochem. 21 Ž1991. 659. w7x H.J. Read, W.P. Minnear, Plating 59 Ž1972. 309. w8x B. Ke, J. Hoekstra, B.C. Sissons Jr, D. Trivick, J. Electrochem. Soc. 106 Ž1959. 382. w9x D.R. Tumer, G.R. Johnson, J. Electrochem. Soc. 10 Ž1962. 798. w10x D. Pletcher, F.C. Walsh, Industrial Electrochemistry, 2nd edn., Blackie A&P, Glasgow, 1993. w11x G. Fabricius, K. Kontturi, G. Sundholm, J. Appl. Electrochem. 26 Ž1996. 1179. w12x D.F. Suarez, F.A. Olson, Appl. Electrochem. 22 Ž1992. 1002. w13x M.J. Armostrong, R.H. Muller, J. Electrochem. Soc. 138 Ž1991. 2303. w14x E. Michailova, I. Vitanova, D. Stoychev, A. Milchev, J. Electrochim. Acta 38 Ž1993. 2455. w15x M. Peykova, E. Michailova, D. Stoychev, A. Milchev, J. Electrochim. Acta 40 Ž1995. 2595. w16x S.S. Abd El Rehim, A.M. Abd El Halim, M.M. Osmam, J. Chem. Technol. Biotechnol. 35A Ž1985. 415. w17x E. Mattsson, J.O’M. Bockris, Trans. Faraday Soc. 55 Ž1959. 1586. w18x R.L. Pecsok, R.S. Juvet, J. Am. Chem. Soc. 77 Ž1955. 202. w19x L.G. Joyce, W.F. Pickering, J. Chem. 18 Ž1965. 783. w20x E.J. Podlaha, C.H. Bonhote, D. Landolt, Electrochem. Acta 40 Ž1995. 2649. w21x V. Jelinek, H.F. David, J. Electrochem. Soc. 104 Ž1957. 279. w22x T.M Shvets, T.N. Amelichina, E.P Zelibo, Poroshk. Metall. 11 Ž1980. 8.
Electroplating of copper films on steel substrates from acidic gluconate baths S.S. Abd El Rehim a,) , S.M. Sayyah b, M.M. El Deeb b b
a Chemistry Department, Faculty of Science, Ain Shams UniÕersity, Cairo, Egypt Chemistry Department, Faculty of Science, Cairo UniÕersity, Beni-Suef Branch, Beni-Suef, Egypt
Received 16 July 1999; accepted 22 December 1999
Abstract Electroplating of thin films of copper onto steel substrate from acidic gluconate bath has been investigated under different conditions of bath composition, pH, current density, and temperature. A detailed study has been made about the effect of these parameters on potentiodynamic cathodic polarization, cathodic current efficiency ŽCCE%., and throwing power ŽTP. of the bath. Fine grained, highly adherent and smooth bright deposits were produced. X-ray diffraction analysis showed that these deposits were produced in one phase with crystalline cubic structure. The optimum conditions are: 10 g ly1 CuSO4 P 5H 2 O, 30 g ly1 C 6 H 11O 7 Na, 10 g ly1 K 2 SO4 , pH s 2.2, I s 2.8 mA cmy2 , and temperature range between 228C and 318C. The TP of the bath is greatly improved by increasing the current density. q 2000 Elsevier Science B.V. All rights reserved. Keywords: Electroplating of copper films; Steel substrates; Acidic guconate baths
1. Introduction Copper electroplating is one of the oldest, protective and decorative metallic coating for steel and other basis metals. Therefore, intensive studies were carried out to obtain copper electroplates suitable for different purposes. One of the most important baths used for electroplating copper was cyanide bath w1x, but due to the environment consideration, cyanide based baths formulation were replaced by noncyanide formulations such as sulphate w2–6x, chloride w4x, pyrophosphate w7x and tartarate bath w1x. )
Corresponding author. Fax: q20-2-483-1836. E-mail address: [email protected] ŽS.S. Abd El Rehim..
Organic additives are added to acidic copper sulphate plating baths to improve the quality of the deposits. The presence of these adsorption compounds in the plating bath results in marked changes in the deposit which can increase brightness, hardness, smoothness, and ductility w8–10x. The effect of thiourea on the electroplating of copper from acidic sulphate bath has been studied w11,12x. This compound acts as brightener and leveler, and in its absence ductile copper deposits are obtained. Thiourea and its copper complexes can adsorb on the cathode surface and increase the overpotential. Surface roughness has been attributed to the fact that at high overpotentials mass transfer limited deposition favours the growth of a deposit with protrusions,
0169-4332r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 Ž 0 0 . 0 0 0 1 5 - 5
250
S.S. Abd El Rehim et al.r Applied Surface Science 165 (2000) 249–254
while at low overpotentials the grain size of the deposit is large w13x. Michailova et al w14x and Peykova et al. w15x examined the effect of some surfactants on the nucleation and growth kinetics of copper from acidic copper sulphate solution. The presence of the surface active compounds affected the rate of nucleus formation in two different ways: directly, through a blocking of active sites on the substrate which will diminish the nucleation rate; and indirectly, by reducing the growth rate of copper grains by increasing the overpotential. The objective of the present study is to develop a cheap and non-toxic alternative to the alkaline cyanide electrolytes, which is a ‘‘medium-acid’’ as it contains a gluconate complexing agent that practically eliminates the cementation processes and allows electrodeposition of copper coatings on steel surface with relatively good adhesion. The proposed bath may be of practical interest for the deposition of a thin striking layer on the steel surface and for addition deposition of copper from high-speed strongly acidic electrolytes, as well as deposition thin copper layer on steel surfaces followed by nickel, chromium, etc., galvanic coatings. In this work, the effects of some plating and operating variables on the characteristics of the electroplating, the quality of the deposits, and on the throwing power ŽTP. of this bath were investigated.
2. Experimental work Experimental work was carried out in solutions containing CuSO4 P 5H 2 O, C 6 H 11O 7 Na, and K 2 SO4 . All solutions used were freshly prepared by doubling distilled water and from analytical grade chemicals. The pH was adjusted using sulphuric acid or sodium hydroxide. The experimental setup used was described previously w16x and consisted of rectangular Perspex cell provided with a plane parallel steel sheet cathode and a platinum sheet anode. Each electrode had a dimension of 3 = 3 cm and filled the cross-section of the cell. Before each run, the cathode was mechanically polished with 600-mesh emery paper washed with distilled water, rinsed with ethanol, dried, and weighed. The experiments were conducted at the required temperature "18C with
Table 1 Composition of the copper electroplating solutions Bath no.
Cu-1 Cu-2 Cu-3 Cu-4 Cu-5 Cu-6 Cu-7 Cu-8
Concentration g ly1
pH
CuSO4P5H 2 O
C 6 H 11O 7 Na
K 2 SO4
10 10 10 5.0 15 10 10 10
30 30 30 30 30 0.0 10 50
10 10 10 10 10 10 10 10
4.6 2.2 3.0 2.2 2.2 2.2 2.2 2.2
the help of an air thermostat. The plating duration was 15 min, at the end of that time, the cathode was withdrawn, washed with distilled water, dried, and weighed. The deposits were examined by X-ray diffraction using a Philips diffractometer with CuKa radiation and nickel filter Ž40 kV, 30 mA.. The plating current was supplied by using dc power supply ŽThurby Thandar PL330. and the values of pH were measured by pH meter Ž Jenway 3071.. Potentiodynamic cathodic polarization measurements were recorded by using EG & G Potentiostatr Golvanostat Model 273 A supplied by EG & G Princeton Applied Research. The I–E curves were recorded by a computer software from the same company Model 352 and 270r250. The measurements were carried out in a three-electrode cell, a steel cathode, platinum anode, and a reference saturated calomel electrode ŽSCE.. The composition of the used baths for the electroplating of copper is presented in Table 1.
3. Results and discussion 3.1. Potentiodynamic cathodic polarization curÕes The potentiodynamic cathodic polarization curves for copper electroplating from acidic gluconate baths were recorded under the effect of various variables. Each curve was swept from the rest Žzero current. potential into more negative potentials with the scan rate of 25 mV sy1 . Fig. 1 shows the cathodic polarization curves for copper electroplating from
S.S. Abd El Rehim et al.r Applied Surface Science 165 (2000) 249–254
Fig. 1. Effect of concentration of sodium gluconate on the potentiodynamic cathodic polarization curves for copper electroplating at 308C with scan rate of 25 mV sy1 .
251
for the corresponding gluconate free solution. This inhibition effect of the gluconate ions on the copper electroplating reaction may be due to the adsorption of gluconate ions on the cathode surface andror the complexion of Cu2q with the gluconate ions. It is known that in acidic media ŽpH - 4., Cu2q ions can complex with gluconate ion via a ligand carboxyl group, and the stability of the complex is low ŽpKa s 2.8. and exists as cationic wCuC 6 H 11O 7 xq w18,19x, but on increasing pH, coordinate ion on the secondary alcoholic group is followed by liberation of proton, and formation of chelation ring is possible. This process leads to an increase in the stability of the complex. It is probable that copper–gluconate complexes cannot be reduced so easily as the free Cu2q ions. As the gluconate concentration in the bath is increased, most of the Cu2q ions become complexed. For a given concentration, the rate of transport of copper–gluconate complex is lower than for Cu2q because of the lower value of the diffusion coefficient of a complexed ion w20x. Therefore, the current plateau decreases with increasing gluconate concentration as predicted in Fig. 1 due to a lowering of the uncomplexed Cu2q ions concentration. Fig. 2
sulphate solution under the influence of adding increasing amounts of sodium gluconate. It is obvious that in gluconate free solution ŽCu-6., the current, at first, increases linearly, and then tends to exhibit limiting current plateau with increases in the cathodic potential. It is generally accepted that electroplating of Cu2q ions takes place through two steps w17x:
~ Cu Cu q e ° Cu Cu2qq e q
q
slow step fast step
It was assumed that the first step in this process occurred slowly and the rate was controlled by the equilibrium between Cu2q and Cuq at the electrode surface. However, addition of gluconate ion to the sulphate solution increases the cathodic polarization and decreases the value of limiting current density. The observed changes in the cathodic polarization in the presence of gluconate ions suggest that it must be acting as an inhibitor, which is confirmed by the observation that at any given overpotential, the current density for copper deposition from solutions containing gluconate ions is lower than that found
Fig. 2. Effect of concentration of CuSO4P5H 2 O and pH on the potentiodynamic cathodic polarization curves for copper electroplating at 308C with scan rate of 25 mV sy1 .
252
S.S. Abd El Rehim et al.r Applied Surface Science 165 (2000) 249–254
3.2. Cathodic current efficiency
Fig. 3. Effect of temperature on the potentiodynamic cathodic polarization curves for copper electroplating from bath ŽCu-2. with scan rate of 25 mV sy1 .
illustrates the influence of CuSO4 concentration in the bath. Increasing the CuSO4 content in the bath decreases the cathodic polarization and increases the limiting current plateau. These results were expected due to an increase in the relative abundance of the uncomplexed Cu2q ions in the solution. At the same time, data of Fig. 2 showed that an increase of pH enhances the cathodic polarization and lowers the limiting current plateau. Increasing the pH while keeping the concentrations of the metal salt and the complexing agent in the bath constant increases the stability of the complex and consequently decreases the ratio free Cu2qrCu–gluconate. These changes lead, thus, to a decrease in the limiting current plateau. The influence of temperature on the cathodic polarization during copper deposition from bath ŽCu2. was examined and the results are shown in Fig. 3. An increase in temperature Ž30–508C. increases the cathodic polarization and reduces the limiting current plateau. This trend could be ascribed to the effect of temperature on the relative abundance of both the free and complexed Cu2q ions in the solution. Elevation of the temperature of the gluconate bath may enhance the formation of the complexed Cu2q ions.
The cathodic current efficiency ŽCCE%. for copper electroplating from gluconate baths were studied under the influence of different plating parameters. Fig. 4 shows the effect of the applied current density on CCE% for copper deposition from different baths. Inspection of the data depicts that the CCE% decreases markedly with increases in the applied current density as the result of simultaneous hydrogen evolution reaction. Such a trend is related to the increase in cathodic overpotential with an increase of current density. The data indicate that CCE% increases with increasing copper sulphate concentration in the bath. These results agree with the cathodic polarization data shown in Fig. 2. On the contrary, the CCE% for copper deposition decreases with the increasing of the gluconate content in the bath. These results are expected due to the inhibiting effect of gluconate ions on the copper deposition reaction. The effect of pH Žfrom 2.2 to 4.6. of gluconate bath on CCE% for copper deposition is given also in Fig. 4. The results display that CCE% increases with the decrease in pH value. It seems that the concentration of the complexed Cu2q ions increases relative to
Fig. 4. Effect of current density on the cathodic current efficiency for copper electroplating from different baths at 308C.
S.S. Abd El Rehim et al.r Applied Surface Science 165 (2000) 249–254
253
Table 3 Throwing power and throwing index of gluconate baths under different conditions Bath no.
Current density mA cmy2
Temperature 8C
TP%
TI
Cu-8 Cu-5 Cu-2 Cu-2 Cu-2
2.8 2.8 2.8 8.3 2.8
30 30 30 30 50
17.6 15 11 81 2.6
1.33 1.29 1.17 6.67 1.01
K 2 SO4 , pH s 2.2, I s 2.8 mA cmy2 , and temperature range between 228C and 318C.
4. Structure of copper deposits Fig. 5. Effect of temperature on the cathodic current efficiency for copper electroplating from bath ŽCu-2. at I s 2.8 mA cmy2 .
the concentration of uncomplexed Cu2q with increases in the pH value, and this is reflected in a decrease in CCE%. The effect of temperature in the range from 228C to 508C on the CCE% for copper deposition from bath ŽCu-2. is shown in Fig. 5. A rise of temperature from 228C to 318C has no significant influence on the CCE%. However, a further increase in temperature decreases markedly the CCE%. The data can be related to the effect of temperature on the cathodic polarization. An increase in temperature enhances the cathodic polarization and decreases the limiting current, so a decrease in the CCE% for copper deposition was to be expected. According to the data derived from the experiments, one can conclude that the optimum conditions for copper electrodeposition from acidic gluconate bath on steel substrate are as follows: 10 g ly1 CuSO4 P 5H 2 O, 30 g ly1 C 6 H 11O 7 Na, 10 g ly1
Fine-grained, highly adherent and bright deposits were obtained from the above-mentioned solutions. The adherence tends to decrease with increases in the CuSO4 P 5H 2 O above 20 g ly1 . X-ray diffraction studies were carried out on the as-deposited copper under the optimum conditions. The data given in Table 2 reveal that copper consists of one phase with crystalline cubic structure. The average size of the deposited grains was measured by using scanning
Table 2 X-ray diffraction data of the electroplating copper from gluconate bath ŽCu-2. Ž I s 2.8 mA cmy2 , times15 min at 308C. ŽA.
Ir I0
hk I
Phase
Structure
2.066
100
111
Cu
Cubic
Fig. 6. Effect of concentration of C 6 H 11O 7 Na on the throwing index of gluconate baths at 308C and I s 2.8 mA cmy2 .
254
S.S. Abd El Rehim et al.r Applied Surface Science 165 (2000) 249–254
electron microscope ŽJEOL Model 50 A.. The obtained data reveal that the average size of the grains is about 0.2 mm.
slopes of these lines, i.e. the TI is also listed in Table 3. It is seen that the values of TI and TP vary in a manner almost parallel to each other.
5. TP of gluconate baths
References
The TP of the acidic gluconate baths used for electroplating of copper was investigated by using Haring–Blum cell under various plating conditions. The percentage values of TP were calculated using the empirical Field’s formula at a distance ratio Ž1:3. and the results are recorded in Table 3. Data of Table 3 reveal that an increase in copper content of the bath enhances its TP. This is an unexpected result, since an increase in copper content is accompanied by a decrease in the cathodic polarization. Again an increase in gluconate content of the bath improves TP. Such an effect could be assigned to the corresponding increase in cathodic polarization. An increase in the applied current density strongly improves TP. This could be interpreted on the basis that an increase in current density increases cathodic polarization. On contrary, a rise in the temperature of the bath has an adverse influence on TP even though a rise of temperature is expected to enhance cathodic polarization ŽFig. 3. and electrical conductivity of electrolytes. A graphical method for expressing the results of TP measurements was suggested by Jelink and David w21x. In this method, the metal distribution M was plotted versus the linear ratio L, Žwithin the range from L s 1 to L s 3. on arithmetic coordinates. The reciprocal of the slope of this plot is called throwing index ŽTI. and represents a direct measure of bath TP w22x. A representative linear plot between the metal distribution ratio M and the linear ratio L Ž1:1 to 1:3. are shown in Fig. 6. The reciprocal of the
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