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Effect of brightener on copper plating
475
J. Electroanal. Chem., 243 (1988) 4X-419 Elsevier Sequoia S.A., Lausanne - Printed in The Netherlands
Preliminary note EFFECI- OF BRIGHTENER
ON COPPER PLATING
IN SITU INVESTIGATION
SACHIO
YOSHIHARA,
Department of Synthetic Tokyo I13 (Japan) (Received
BY THE PAS TECHNIQUE
MASAHIDE Chemistry,
UENO, YASUO
NAGAE
Faculty of Engineering,
and AKIRA
FUJISHIMA
*
The University of Tokyo, Hongo, Bunkyo-ku,
18th January 1988)
INTRODUCTION
In the field of industrial metal plating, various brighteners are used in order to obtain a smooth plated surface. Recently we reported that the surface structures of the plated metal could be monitored in situ by the PAS (photoacoustic spectroscopy) technique [l]. It was shown that the PA signal amplitude is affected by the surface structure of a plated metal. This is because the PA signal amplitude is proportional to the light absorption of samples, which depends on the surface structure. In this study, we have applied the PAS technique to monitor the surface structure of deposited copper in the presence of a brightener. EXPERIMENTAL
PAS measurements were carried out on gold specimens of 99.99% purity in the form of a 0.3 mm thick plate with an exposed area of about 2.0 cm’. Experiments were done in a one-compartment Pyrex cell with a three-electrode system composed of a gold working electrode, a platinum counter electrode and a saturated calomel reference electrode. The cell had a flat window for admitting an intense light beam. Periodic pressure fluctuations induced by thermal changes in the working electrode were detected with a piezoelectric disk (NEPEC NPM N-21, 10 mm diam., 1 mm thick, Tohoku Metal Industries) which was attached to the rear of the gold electrode with epoxy resin. The light source was an Ar ion laser (Spectra Physics, Model 164). The light beam was chopped mechanically by a light chopper (NF Circuit Design Block,
* To whom correspondence
0022-0728/88/$03.50
should be addressed.
0 1988 Elsevier Sequoia S.A.
CH-353). The measurements were performed with chopped light (125 Hz) of 488 nm wavelength. The photoacoustic signal detected by the piezoelectric detector was fed to a lock-in amplifier (NF Circuit Design Block, LI-574). External acoustic and electric noises were diminished as much as possible by using a shielding box made of iron. Electrochemical measurements were carried out with a potentiostat (Fuso Seisakusho 315A). The total charge passed was determined by a coulometer (TOHO Technical Research, Model 3210). The electrolyte used for plating was a 0.7 mol/dm3 CuSO, + 0.5 mol/dm3 H,SO, aqueous solution. Kuppelight (trade name of Nihon Kagaku Sangyo Co., Ltd.), which is one of the typical brighteners for copper plating, was used. The brightener contained mercaptoalkyl sulfonate for the main brightening effect, a polyoxyalkyl type surface-active agent for supporting the above brightening effect, and safranine-azo dye for the leveling effect. An appropriate amount (about 1.5 cm3 Brightener/l dm3 Plating electrolyte) of this brightener was added to the above electrolyte. Photoacoustic signal-cathodic charge curves were obtained by the galvanostatic method. These curves were recorded on an X-Y-Y’ recorder (Yokogawa Electric Works, Type 3078).
CATHODIC CHARGE / C cm-* Fig. 1. PA signal-cathodic charge curves for various current densities in the presence of a brightener. A is the PA signal amplitude obtained at each cathodic charge; A0 is the initial PA signal amplitude (at zero cathodic charge).
RESULTS AND DISCUSSION
Photoacoustic signal-cathodic charge curves in the presence of the brightener for different current densities are shown in Fig. 1. Comparing them with the same curves reported already in the absence of brightener (shown in Fig. 2), we found that the value for the PA signal amplitude is lower in the presence of brightener. The reason for the decrease of the PAS signal is the higher reflection of light at the plated surface. As mentioned in our earlier report [l], the PA signal is affected by the surface structure of an electrode. In other words, by measuring the PA signal we can estimate the surface structure. Low values of the PA signal amplitude correspond to a smooth surface where the reflection factor is relatively large. For the purpose of obtaining bright surfaces of plating metals, various kinds of brighteners are known which are added to the plating electrolyte in the industrial processes. The brighteners have the role of smoothing the surface and the mechanism of the brightener effect, for example for copper plating, can be described as follows [2-41. The main brightener for this case is called DCA (depolarizing control agent); it promotes copper ion adsorption. The DCA adsorbs uniformly on the
I
1 CATHODIC
1
2 CHARGE
3 /
4
C cme2
Fig. 2. PA signal-cathodic charge curves for various current densities in the absence of a brightener. A and A0 as in Fig. 1.
478 DCA (4
(b) ,
Fig. 3. Model for the smoothing effect of a brightener. For explanation see text.
uneven surface of the substrate (Fig. 3a), and the following copper plating takes place uniformly as shown in Fig. 3b. If uniform adsorption of DCA is assumed, the surface concentration of DCA becomes larger in the concave region than in the flat region. So in the concave region, growth of plated copper will be promoted more at the start of plating. Thus, those regions are filled with a copper layer first and therefore the whole surface becomes smooth. As shown in Fig. 1, under current densities of 40, 10 and 5 mA/cm2 each curve reaches an amplitude which is lower than the one under 40 mA/cm2 in the absence of brightener (Fig. 2). This indicates that in the presence of brightener the surface of plated copper is smooth. In addition, the curves for 40 and 10 mA/cm2 reach almost the same value finally. This means that we can obtain surfaces with the same smoothness independently of the applied current density (40 or 10 mA/cm2), which is important in industrial plating where complicated substrates like the body of a car may be used. Some portion of the substrate may be plated under a relatively higher current density than another portion because of the edge effect. In that case, without a brightener, in the region under high current density, a thicker plated layer will be generated. But in the presence of a brightener, plating will take place uniformly, irrespective of the surface unevenness. Further, by varying the concentration of the brightener, we have obtained the dependence of the PA signal on the cathodic charge. The result is shown in Fig. 4 (current density 40 mA/cm2). In the case of low concentration of the brightener, a characteristic peak is generated. We consider that it is due to the time difference of adsorption for copper and the brightener. That is, the first peak (< 0.1 C/cm2)
479
08
2
1
CATHODIC
CHARGE
/C
3 cme2
4
Fig. 4. PA signal-cathodic charge curves for various concentrations of brightener (current density 40 mA/cm’), A and A, as in Fig. 1. 0, 0.5, 1.0 and 1.5 stand for the ratio volume of brightener added/standard volume.
corresponds to the adsorption of the copper ion and the second peak corresponds to that of the brightener. Investigations into the details of the mechanism are under way. Thus, by the use of this technique, we can check the smoothness of the plated surface quantitatively. In metal plating factories, PAS can be used to obtain in situ information about the plated surface. At present we are trying to measure the PA signal by scanning the surface with a laser beam for the purpose of obtaining an in situ image of the surface structure for plated metals. ACKNOWLEDGEMENTS
We would like to thank Nihon Kagaku Sangyo Co., Ltd., for the donation of the brightener (Kuppelight) and useful information about it, and also Dr. A. Aruchamy for useful discussions. REFERENCES 1 2 3 4
S. 0. J. J.
Yoshihara, M. Ueno, Y. Nagae and A. Fujishima, Electrochim. Kardos, Plating, 61 (1974) 129, 229, 316. Osterwald, J. Schulz-Harder, Galvanotechnik, 66 (1975) 360. Osterwld, Oberfhache-Surf., 17 (1976) 89.
Acta, submitted.
J. Electroanal. Chem., 243 (1988) 4X-419 Elsevier Sequoia S.A., Lausanne - Printed in The Netherlands
Preliminary note EFFECI- OF BRIGHTENER
ON COPPER PLATING
IN SITU INVESTIGATION
SACHIO
YOSHIHARA,
Department of Synthetic Tokyo I13 (Japan) (Received
BY THE PAS TECHNIQUE
MASAHIDE Chemistry,
UENO, YASUO
NAGAE
Faculty of Engineering,
and AKIRA
FUJISHIMA
*
The University of Tokyo, Hongo, Bunkyo-ku,
18th January 1988)
INTRODUCTION
In the field of industrial metal plating, various brighteners are used in order to obtain a smooth plated surface. Recently we reported that the surface structures of the plated metal could be monitored in situ by the PAS (photoacoustic spectroscopy) technique [l]. It was shown that the PA signal amplitude is affected by the surface structure of a plated metal. This is because the PA signal amplitude is proportional to the light absorption of samples, which depends on the surface structure. In this study, we have applied the PAS technique to monitor the surface structure of deposited copper in the presence of a brightener. EXPERIMENTAL
PAS measurements were carried out on gold specimens of 99.99% purity in the form of a 0.3 mm thick plate with an exposed area of about 2.0 cm’. Experiments were done in a one-compartment Pyrex cell with a three-electrode system composed of a gold working electrode, a platinum counter electrode and a saturated calomel reference electrode. The cell had a flat window for admitting an intense light beam. Periodic pressure fluctuations induced by thermal changes in the working electrode were detected with a piezoelectric disk (NEPEC NPM N-21, 10 mm diam., 1 mm thick, Tohoku Metal Industries) which was attached to the rear of the gold electrode with epoxy resin. The light source was an Ar ion laser (Spectra Physics, Model 164). The light beam was chopped mechanically by a light chopper (NF Circuit Design Block,
* To whom correspondence
0022-0728/88/$03.50
should be addressed.
0 1988 Elsevier Sequoia S.A.
CH-353). The measurements were performed with chopped light (125 Hz) of 488 nm wavelength. The photoacoustic signal detected by the piezoelectric detector was fed to a lock-in amplifier (NF Circuit Design Block, LI-574). External acoustic and electric noises were diminished as much as possible by using a shielding box made of iron. Electrochemical measurements were carried out with a potentiostat (Fuso Seisakusho 315A). The total charge passed was determined by a coulometer (TOHO Technical Research, Model 3210). The electrolyte used for plating was a 0.7 mol/dm3 CuSO, + 0.5 mol/dm3 H,SO, aqueous solution. Kuppelight (trade name of Nihon Kagaku Sangyo Co., Ltd.), which is one of the typical brighteners for copper plating, was used. The brightener contained mercaptoalkyl sulfonate for the main brightening effect, a polyoxyalkyl type surface-active agent for supporting the above brightening effect, and safranine-azo dye for the leveling effect. An appropriate amount (about 1.5 cm3 Brightener/l dm3 Plating electrolyte) of this brightener was added to the above electrolyte. Photoacoustic signal-cathodic charge curves were obtained by the galvanostatic method. These curves were recorded on an X-Y-Y’ recorder (Yokogawa Electric Works, Type 3078).
CATHODIC CHARGE / C cm-* Fig. 1. PA signal-cathodic charge curves for various current densities in the presence of a brightener. A is the PA signal amplitude obtained at each cathodic charge; A0 is the initial PA signal amplitude (at zero cathodic charge).
RESULTS AND DISCUSSION
Photoacoustic signal-cathodic charge curves in the presence of the brightener for different current densities are shown in Fig. 1. Comparing them with the same curves reported already in the absence of brightener (shown in Fig. 2), we found that the value for the PA signal amplitude is lower in the presence of brightener. The reason for the decrease of the PAS signal is the higher reflection of light at the plated surface. As mentioned in our earlier report [l], the PA signal is affected by the surface structure of an electrode. In other words, by measuring the PA signal we can estimate the surface structure. Low values of the PA signal amplitude correspond to a smooth surface where the reflection factor is relatively large. For the purpose of obtaining bright surfaces of plating metals, various kinds of brighteners are known which are added to the plating electrolyte in the industrial processes. The brighteners have the role of smoothing the surface and the mechanism of the brightener effect, for example for copper plating, can be described as follows [2-41. The main brightener for this case is called DCA (depolarizing control agent); it promotes copper ion adsorption. The DCA adsorbs uniformly on the
I
1 CATHODIC
1
2 CHARGE
3 /
4
C cme2
Fig. 2. PA signal-cathodic charge curves for various current densities in the absence of a brightener. A and A0 as in Fig. 1.
478 DCA (4
(b) ,
Fig. 3. Model for the smoothing effect of a brightener. For explanation see text.
uneven surface of the substrate (Fig. 3a), and the following copper plating takes place uniformly as shown in Fig. 3b. If uniform adsorption of DCA is assumed, the surface concentration of DCA becomes larger in the concave region than in the flat region. So in the concave region, growth of plated copper will be promoted more at the start of plating. Thus, those regions are filled with a copper layer first and therefore the whole surface becomes smooth. As shown in Fig. 1, under current densities of 40, 10 and 5 mA/cm2 each curve reaches an amplitude which is lower than the one under 40 mA/cm2 in the absence of brightener (Fig. 2). This indicates that in the presence of brightener the surface of plated copper is smooth. In addition, the curves for 40 and 10 mA/cm2 reach almost the same value finally. This means that we can obtain surfaces with the same smoothness independently of the applied current density (40 or 10 mA/cm2), which is important in industrial plating where complicated substrates like the body of a car may be used. Some portion of the substrate may be plated under a relatively higher current density than another portion because of the edge effect. In that case, without a brightener, in the region under high current density, a thicker plated layer will be generated. But in the presence of a brightener, plating will take place uniformly, irrespective of the surface unevenness. Further, by varying the concentration of the brightener, we have obtained the dependence of the PA signal on the cathodic charge. The result is shown in Fig. 4 (current density 40 mA/cm2). In the case of low concentration of the brightener, a characteristic peak is generated. We consider that it is due to the time difference of adsorption for copper and the brightener. That is, the first peak (< 0.1 C/cm2)
479
08
2
1
CATHODIC
CHARGE
/C
3 cme2
4
Fig. 4. PA signal-cathodic charge curves for various concentrations of brightener (current density 40 mA/cm’), A and A, as in Fig. 1. 0, 0.5, 1.0 and 1.5 stand for the ratio volume of brightener added/standard volume.
corresponds to the adsorption of the copper ion and the second peak corresponds to that of the brightener. Investigations into the details of the mechanism are under way. Thus, by the use of this technique, we can check the smoothness of the plated surface quantitatively. In metal plating factories, PAS can be used to obtain in situ information about the plated surface. At present we are trying to measure the PA signal by scanning the surface with a laser beam for the purpose of obtaining an in situ image of the surface structure for plated metals. ACKNOWLEDGEMENTS
We would like to thank Nihon Kagaku Sangyo Co., Ltd., for the donation of the brightener (Kuppelight) and useful information about it, and also Dr. A. Aruchamy for useful discussions. REFERENCES 1 2 3 4
S. 0. J. J.
Yoshihara, M. Ueno, Y. Nagae and A. Fujishima, Electrochim. Kardos, Plating, 61 (1974) 129, 229, 316. Osterwald, J. Schulz-Harder, Galvanotechnik, 66 (1975) 360. Osterwld, Oberfhache-Surf., 17 (1976) 89.
Acta, submitted.