Electrode surface coarsening in potentiostatic copper electrodeposition

Surface and Coatings Technology, 35 (1988) 39

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45

39

ELECTRODE SURFACE COARSENING IN POTENTIOSTATIC COPPER ELECTRODEPOSITION K. I. POPOV Faculty of Technology and Metallurgy, University of Beograd, 11001 Beograd (Yugoslavia)

Lj. J. PAVLOVI~ Tehnogas, Beograd (Yugoslavia)

M. G. PAVLOVIC Institute for Electrochemistry ICTM, Beograd (Yugoslavia) M. I. CEKEREVAC Metalservis, Institute for Chemical Power Sources, Beograd (Yugoslavia) (Received April 15, 1987)

Summary The increase of electrode surface coarseness during copper electrodeposition is discussed. It is shown that electrode surface coarsening is caused by the differences in deposition current densities at different points, which arises from mass-transport limitations and different deposition rates on different crystallographic planes in mixed-controlled and activationcontrolled deposition respectively.

1. Introduction Any solid metal surface, which represents a substrate for metal deposition, possesses a certain coarseness. It is convenient to define the surface coarseness as the difference in thickness of the metal at the highest and lowest points, above an arbitrary reference plane facing the solution. It seems that the increase of surface coarseness is caused by the differences in diffusion flux to the different points of electrode [1]. Taking a sinusoidal profile for the electrode (2irx\ I \a I

HH0sinI—

(1)

Krichmar [21 obtained the relationship 0257-8972/88/ $3.50

© Elsevier Sequoia/Printed in The Netherlands

40 ri2~M.2\

Ho(t)HoexplI

~.

[\pnFaJL

I

(2)

I tanh(2ir~/a)

for H0 ~ a, or H0(t)

=

H0 exp(!_

(3)

~

\JL Qo,kJ

where

Q=jt

(4)

and

Qo,k

=

pnFa /2irö\ 2irM tanh(\._~~._._) a

(5)

which describes the increase of surface coarseness with deposition time. Simpler mathematics were used in another, independently derived theory of the same phenomenon by Despié and coworkers [3, 41. They found that for any point elevated from the flat surface to a height h, around which lateral diffusion flux can be neglected hh0expl if ö

~>

ITMDCQ p (ö

+

1—f2/f1 2 t nFDC0/j0f1)

h. It is easy to show that, for f2 ~

hhoexp(L

~_

(6)

f

1 (7)

\JL Qo,D!

where

QO,D

Q is given by eqn. (4) and nFpt5 M

(8)

It is seen that, according to both mechanisms, an increase of surface coarseness with increasing current density can be expected for the same quantity of electrodeposited metal. The increase of surface coarseness in electrodeposition of metals was investigated for determined current density and different deposition times [2, 5]. The purpose of this work is to discuss the effect of deposition current density on the surface coarseness increase for the same quantity of electrodeposited metal.

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2. Experimental procedure Copper was deposited on to stationary copper wire electrodes previously plated with bright nickel from 0.2 M CuSO4 and 0.5 M CuSO4 in 0.5 M H2S04 solutions in open cells at room temperature. Doubly distilled water and p.a. chemicals were used. To obtain the same initial substrate in all experiments of one series, copper was deposited from the corresponding solution at 200 mV for 5 mm. Deposition was carried out under potentiostatic control; counter and reference electrodes2 were of pure copper. TheMquantity in deposition from 0.2 CuSO of deposited copper 14 mA from h cm0.5 M CuSO 4 2 inwas deposition and 48 mA h cm 4. The quality of deposit was investigated by scanning electron microscopy (SEM).

3. Results and discussion Polarization curves and Tafel relationships for copper deposition from both solutions are shown in Figs. 1 and 2 respectively. It is seen that deposition enters mixed control at overpotentials larger than 120 mV and 160 mV for 0.2 M CuSO4 and 0.5 M Cu504 solutions respectively. Typical deposits obtained in this work are shown in Figs. 3 and 4. As expected, surface coarseness increases for the same quantity of deposited metal with increasing deposition current density. It is seen that the effect

E 80

O5MCUSO4+0.5MH2S04

—

700

~

60

-

50

—

0

40-

30

—

20

—

to

—

0.2MCuSO4 * 0.5MH2S04

S

4

S

I 100

Fig. 1. Polarization curves.

—•----4-

I 200

300

400

I OVERPOTENTIAL mV

42

E

_ 400

—

uJ ~ 350

—

uJ 0

300

-

250

—

200

—

150

—

—

50

0

02MCuS0~• 0.5MH

•

O.5MCu SO4. 0.5MH2504

2S04

—

•

0

I

I

0.2

05

I 10

Fig. 2. Overpotentials vs. log (current density).

1,5

I

2) 20 LOGARITHM OF CURRENT DENSITY (mA/cni

does not depend on concentration of electrolyte, being more pronounced with increased quantity of electrodeposited metal. It should be noted that theories of increase in surface coarseness are valid for H 0(t) a and ii ~ i.e. for short deposition times. In this paper, the effect of deposition current density was investigated in a qualitative way and larger quantities of metals were used, assuming that a qualitative picture of the phenomenon will not be changed. In further quantitative investigations, the results which determine the deposition times in which approximation ~ ~ h is valid [6] must be taken into account. Deposits obtained were polycrystalline, which is in accordance with ref. 7 for copper deposition at higher overpotentials. Hence the change of the coarseness of the electrode surface can be ascribed only to the differences in the diffusion fluxes at different points of the electrode. These facts are valid for mixed-controlled deposition as are eqns. (2), (3), (6) and (7). ~ the region of activation control, the situation becomes quite different, as can be seen from Fig. 5. Activation-controlled deposition of copper produces large grains with relatively well defined crystal shapes, as seen from Fig. 5. This can be explained by the fact that the values of exchange current densities on different crystal planes are quite different, whereas the reversible potential is approximately the same for all planes [7]. This can lead to preferential growth of -~

(a)

(c)

iii

~I~ (b)

(d)

Fig. 3. Copper deposits obtained from 0.2 M CuSO

4 in 2.(a) 0.5 MInitial H2S04 surface; by mixed-controlled (b) deposition overpotential 135 quantity electrodeposition; my; (c) deposition of electricity overpotential 14 mA h cm~ 200 mV; (d) deposition overpotential 250 mV; SEM, 60°, magnification 52X.

some crystal planes, because the rate of deposition depends on the orientation, producing deposits like that in Fig. 5. In this way the surface coarseness can be strongly increased. Obviously, the X-ray diffraction analysis of copper deposits obtained in activation-controlled deposition is necessary to confirm this proposition. However, even at low degrees of diffusion control, the formation of large well defined grains is not expected, because of irregular growth caused by mass-transport limitations. This probably causes the formation of polycrystalline deposits with random orientation of grains in mixed-controlled deposition. Hence, it seems that smooth copper cannot be obtained by prolonged electrodeposition from pure sulphate solutions, because of crystallographic and mass-transport limitations in activation-controlled and diffusion-controlled deposition respectively.

iU’~

(a)

(b)

lip— (c)

(d)

Fig. 4. Copper deposits obtained from 0.5 M Cu50

surface; (b) deposition 4 in2.0.5(a)MInitial H2S04 by mixed-controlled electrodeposition; of electricity 48 mA h cm overpotential 180 quantity mV; (c) deposition overpotential 230 mV; (d) deposition overpotential 300 mV; SEM, 60°, magnification 52X.

(a)

(b)

Fig. 5. Copper deposits obtained by activation-controlled

electrodeposition:

(a) 0.2 M

CuSO 2 4 (b) 0.5 MM H2S04, CuSO deposition overpotential 70 mV, quantity of electricity 14 mA h in 0.5 cm 42 inSEM, 0.5 M 60°, H2504, magnification deposition 52X. overpotential 120 mV, quantity of electricity 48 mA h cm

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Acknowledgments Two of us (K. I. P. and M. G. P.) are indebted to the Research Fund of the Republic of Serbia whose material support has made this work possible.

References 1 A. R. Despie and K. I. Popov, Transport-controlled deposition and dissolution of metals, Mod. Aspects Electrochem., 7 (1972) 199. 2 S. I. Krichmar, Elektrokhimiya, 1 (1965) 609. 3 A. R. Despié, J. W. Diggle and J. O’M. Bockris, J. Electrochem. Soc., 115 (1968) 507. 4 J. W. Diggle, A. R. Despié and J.O’M. Bockris, J. Electrochem. Soc., 116 (1969) 1503. 5 K. I. Popov and A. R. Despié, Bull. Soc. Chim. Beograd, 36 (1971) 173. 6 K. I. Popov, M. D. Maksimovjé, D. T. Lukié and M. G. Pavlovié, J. AppI. Electrochem., 10 (1980) 299. 7 A. Damjanovié, Plating, 52 (1965) 1017.

Appendix A: Nomenclature a C0

wavelength of sinusoidal profile

t

bulk concentration diffusion coefficient exp(~/ii0,~) exp(—3~/r~o,a) Faraday constant height of a protrusion initial height of a protrusion local elongation initial amplitude of sinusoidal profile amplitude of sinusoidal profile in time t current density limiting current density atomic or molecular weight number of electrons quantity of electricity defined by eqn. (8) defined by eqn. (5) time

x

coordinate

2~3flOa 2•3fl0,c

thickness of the diffusion layer overpotential slope of the anodic Tafel line slope of the cathodic Tafel line density of the metal

D

Ii [2

F

h H H0 H0(t)

j

JL

M to

Q Q0,D Qo,k

p