Surface Technology, 23 (1984) 167 - 172 167

FUNDAMENTAL ASPECTS OF PLATING TECHNOLOGY IV: THE EFFECT OF SIMPLE SALT PLATING BATH COMPOSITION ON THE MORPHOLOGY OF METAL DEPOSITS

K. I. POPOV, Z. P. RODALJEVIC and N. V. KRSTAJIC

Faculty of Technology and Metallurgy, University of Belgrade, Belgrade (Yugoslavia)

S. R. POPOV

Faculty of Mining and Geology, University of Belgrade, Belgrade (Yugoslavia)

(Received February 15, 1984)

Summary

The morphology of copper deposits is correlated with polarization mea- surements in an acid copper plating bath. It seems that on the basis of polarization measurements a simple method can be proposed for the deter- mination of the opt imum plating bath composit ion.

1. Introduction

It was shown recently that the opt imum deposition current density or overpotential can be determined from the upper limit of validity of the Tafel equation [1]. The general polarization equation is given by [2]

Jo(f l - - f 2) j - (1)

1 + qo/JL)f l

and deposition is activation controlled when

.'--Of I ~< 0.1 ~ 1 (2) ]L

or

lni0 L / n< 0,c -To/ (3)

A variation in the bath composi t ion and the plating condit ions causes a change in the opt imum deposit ion overpotential and current density. Obviously, both the opt imum plating condit ions and the bath composi t ion are determined by the maximum deposition overpotential and current density.

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168

At the same time [3, 4], it was shown that the overpotential at the edge of the cathode in a plane-parallel cell can be related to the overpotential at the middle of the electrode (where the current lines are homogeneously dis- tr ibuted) by

~c ,e = ~ c , h + R h l h "~ E (4)

when the anodic overpotential is neglected. It is obvious that this relation is also valid in cells with other geometries where the surface area and the current at the edge are negligible. When the difference between the over- potential and the cell potential is lowered a bet ter current density distribu- tion over the macroprofi le is expected, as shown recently [5]. Hence, it seems that the opt imum bath composit ion can be determined by polariza- tion measurements, and the purpose of this paper is to develop the method for this determination.

2. Experimental details

In the polarization measurements copper was deposited potent io- statically onto a stationary platinum wire electrode (previously plated with copper) placed in the middle of a cylindrical cell (diameter, 6 cm) the surface of which was covered by the anode, a high puri ty copper plate. In the determination of the quality of the deposits silver was deposited onto flat platinum cathodes 1 cm × 1 cm in the same cell. The reference electrode was a high puri ty copper wire. A PAR 175 programmer and a Tektronix 5648 oscilloscope were employed to determine the correct ion for the ohmic drop during the polarization measurements. The solutions used were prepared from pro analyse chemicals and distilled water as follows: 1 M CuSO 4 in 0.4 and 1 M H2SO4 and 0.1, 0.3 and 0.5 M CuSO 4 in 1 M H2SO4. The polarization measurements were performed at room temperature under purified nitrogen. The samples used in the determinat ion of the quality of the deposits were obtained in an open cell. The grain size of the deposit was investigated by scanning electron microscopy.

3. Results and discussion

Overpotent ia l -current density and cell po ten t ia l -cur ren t density rela- tionships are presented in Fig. 1 for deposition from 0.1, 0.3, 0.5 and 1.0 M CuSO4 in 1 M H2SO 4. As expected, because J0 ~ c°'~s [6] for copper deposi- t ion and JL cc C T M [7] for metal deposition under natural convection, an increase in the concentra t ion of copper leads to increases in the maximum overpotential and the current density in the Tafel region. The opt imum overpotentials for deposition from 0.5 M CuSO4 and 1.0 M CuSO4 in 1 M H2SO4 were 140 mV and 150 mV respectively with corresponding current densities of 20 mA cm -2 and 30 mA cm -2. At the same time the difference

169

between the cell potential and the overpotential increases with increasing copper concentration. Hence, a decrease in the grain size of the deposit and a less homogeneous current density distribution on the macroprofile are expected with increasing copper concentration. The same dependences, but for copper deposition from 1 M CuSO4 and 0.4 M H2SO 4, are presented in Fig. 2. The optimum deposition overpotential (150 mV) and current density (31 mA cm -2) are almost the same as for 1 M CuSO4 in 1 M H2SO4, but a significant increase in the cell potential with the decrease in the sulphuric acid concentration is observed. In this way, approximately the same grain size and a more uniform current density distribution over the macroprofile

2O0

100

XV

×

x@ x@

-Q5 0.0 0.5 1.0 1.5 10# J mAcn~ 2 2#

Fig. 1. Cel l potential-current density (x)and overpotential-current density dependences for copper deposition from 0.1 M (v) , 0.3 M (a) , 0.5 M (z~) and 1.0 M (e ) CuSO4 in ; M H2S04.

2OO

E x

x 0 x 0

i t

"0,5 QO CL5 1.0 1~5 l0 0 j, mAc~ 2.0

Fig. 2. Cell potential-current density (×) and overpotential-current density dependences for copper deposition from 1 M CuSO4 in 0.4 M H2SO 4 (0).

170

<

(a) (b)

(c)

Fig. 3. Copper deposits (18 pm) on platinum obtained at the middle of the electrode from (a) 0.5 M CuSO4 in 1.0 M H2SO4 (deposition overpotential, 140 mV), (b) 1.0 M CuSO4 in 1.0 M H2SO 4 (deposition overpotential, 150 mV) and (c) 1.0 M CuSO4 in 0.4 M H2SO4 (deposition overpotentia], 150 mV).

171

(a) (b)

(c)

Fig. 4. Copper deposits (18 pm) on platinum obtained at the electrode edge from (a) 0.5 M CuS04 in 1.0 M H2SO 4 (deposition overpotential, 140 mV), (b) 1.0 M CuSO4 in 1.0 M H2SO 4 (deposition overpotential, 150 mV) and (c) 1.0 M CuSO4 in 0.4 M H2SO4 (deposition overpotential, 150 mV).

172

are expec t ed with increasing sulphuric acid c o n c e n t r a t i o n because o f an increase in the conduc t i v i t y of the so lu t ion [8] . On the basis o f the above resul ts a depos i t wi th a f iner grain size is e x p e c t e d in depos i t ion f r o m 1 M CuSO4 and the best d i s t r ibu t ion on the m a c r o p r o f i l e is e x p e c t e d in deposi- t ion f r o m 0.5 M CuSO4. This conc lus ion is well c o n f i r m e d by the micro- p h o t o g r a p h s p re sen ted in Figs. 3 and 4. Obvious ly , when the grain size and the d i s t r ibu t ion over the m a c r o p r o f i l e are t a k e n into accoun t , the o p t i m u m ba th c o m p o s i t i o n is 1 M CuSO 4 in 1 M H2SO 4. This c o m p o s i t i o n co r r e sponds to the u p p e r l imit o f the c o p p e r su lpha te and sulphuric acid c o n c e n t r a t i o n s in the acid c o p p e r p la t ing ba th [8 ] .

I t seems on the basis o f the above resul ts t h a t the o p t i m u m pla t ing ba th c o m p o s i t i o n can easily be d e t e r m i n e d f r o m s imple po la r i za t ion measure - men t s .

Refe rences

1 K. I. Popov, N. V. Krstajid and S. R. Popov, Surf. Technol., 20 (1983) 199. 2 J. O'M. Bockris, Mod. Aspects Electrochem., 1 (1954) 190. 3 K. I. Popov, M. D. Maksimovid, D. (~. Totovski and V. M. Naki(~, Surf. Technol., 19

(1983) 173. 4 K. I. Popov, D. C. Totovski and M. D. Maksimovid, Surf. Technol., 19 (1983) 181. 5 K. I. Popov, N. V. Krstajid and S. R. Popov, Surf. Technol., 22 (1984) 245. 6 M. Enyo, Ph.D. Thesis, University of Pennsylvania, Philadelphia, PA, 1960. 7 N. Ibl, Electrochim. Acta, 1 (1959) 3. 8 F. A. Lowenheim, Modern Electroplating, Wiley, New York, 1974, p. 186.

Appendix A: nomenclature

E cell po ten t i a l

fl exp07/~70, c) f2 exp (--~/~7 o, a ) I cu r r en t j cu r r en t dens i ty JL l imi t ing cu r r en t dens i ty J0 exchange cur ren t dens i ty R res is tance ~? ove rpo ten t i a l 2.370 s lope o f Tafel line

S u bscr ip ts a anodic c ca thod ic e edge h h o m o g e n e o u s