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In situ observation of the gold plating process using a photoacoustic technique
409
J. Electroanai. Chem., 278 (1990) 409-414 Elsevier Sequoia S.A., Lausanne - Printed in The Netherlands
Preliminary note
In situ observation of the gold plating process using a photoacoustic technique Sachio Yoshihara, Ryouji Takahashi, Masahiro Odaka, Ikuko Miura and Ei-Ichi Sato Department of Applied Tochigi 321 (Japan)
Chemistry,
Faculty of Engineering.
Utsunomiya
University, Ishii-eho,
Utsunomiya,
Akira Fujis~ma Department of Synthetic Tokyo I13 (Japan)
Chemisity,
(Received 14 November
1989)
Faculty of Engineering,
The University
of Tokyo, Hongo, Bunkyo-ku,
INTRODUCTION
In the past, we have used a photoacoustic technique to study the plating of metals [l-3]. In the present investigation, we have tried to study the plating of gold. Gold is highly resistant to corrosion, and has good electric and thermal conductivity. For these reasons, gold (plating) is used not only for ornamental plating, but also for IC (integrated circuit) parts. In our previous studies [l-3], we reported the possibility of in situ monitoring of copper plating. In that case the difference between the surface structure in the absence and in the presence of brightener could be distinguished by the photoacoustic method. In this report, we examine in situ monitoring of gold plating using a photoacoustic technique.
PAS measurements were carried out on a specimen of oxygen free (99.99%) copper in the form of a 0.5 mm thick plate. The structure of the plated surface was examined ex situ with a scanning electron microscope (SEM, SIGMA-I, Akashi Seisakusho). Electrochemical and PAS measurements were done in a one-compartment acrylic cell with a two-electrode system composed of a copper working electrode and a platinum coated Ti gauze counter electrode. The cell had a flat window for admitting an intense light beam. The temperature of the electrolyte was maintained constant with the aid of a thermostat and a water jacket surrounding the electrolytic cell. Periodic pressure fluctuations, induced by thermal changes in the working electrode, were detected with a piezoelectric detector. A piezoelectric disk (NEPEC 0022-0728/90/$03.50
0 1990 Elsevier Sequoia S.A.
410
Fig. 1. Schematic diagrams of the photo~o~stic cell (a) and the experimental arrangement (b).
NPM N-21, 10 mm diam., 1 mm thick, Tokin) was attached to the rear of the copper electrode using a grease (for high vacuum use) as coupling agent. The light source was an Ar ion laser (Ion Laser Technology, 549OA-S, Multiline). The light beam was chopped m~hanically by a light chopper (NF Electronic Instruments model 5584, chopping frequency 125 Hz). Electrochemical control was carried out with a potentiogalvanostat (Hokuto Denko, HA-301). The electrolyte used for plating was an aqueous solution of 3 X 10m2 mol/dm3 Na,Au(SO,), + 1.4 X 10-l mol/dm3 EDTA + 3.3 X 10-l mol/dm3 Na,SO,. Photoa~ustic sibs-cath~ic charge curves were measured using the galvanostatic method. They were recorded on an X-Y recorder (Yokogawa Electric Works model 3023). The details of the cell and the experimental arrangement are shown in Figs. la and b, respectively.
2.OmAkm’
_*_----
_-______________---------
.O-
__-___ 1. 0mA/cm2
0, SmAIcm2 0
r 1
I
1
2 cathodic charge/
Fig. 2. PA signal (~p~tude)-ethic
3 CQ-I=~
charge curves.
1
4
411 RESULTS
AND
DISCUSSION
We have reported previously that the presence of brightener causes a change in the photoacoustic signal in the case of copper plating on a gold substrate. We therefore suggested the possibility of in situ monitoring of the plating by the photoacoustic method. It was suggested from SEM measurements (ex situ) that in that case the surface roughness affects the PA signal amplitude. Therefore, we can estimate the surface roughness from the PA signal amplitude. In this report, we apply this method to gold plating on a copper substrate. The plating solution for gold was an aqueous solution of sodium gold sulphite. Figure 2
Fig. 3. SEM photographs
of the plated gold at various cathodic charges; current density 2 mA/cm’.
412
shows the dependence of the PA signal amplitude on the cathodic charge. It is obvious that the trace of the PA signal amplitude depends on the cathodic current density. Especially for a current density of 2 mA/cm2, the PA signal amplitude gradually increases progressively with the plating. Then we observed the surface at each cathodic charge, 0.5, 1 and 4 C/cm’, using a scanning electron microscope (SEM). The results obtained are shown in Fig. 3. It can be seen seen from this figure that the roughness of the plated surface gradually increases progressively with the plating. This is in accord with our previous view. In short, the surface roughness makes the PA signal amplitude much bigger. The degree of this roughness is, however, of the order of the wavelength of the incident light (visible region). This
Fig. 4. SEM photographs
of the plated gold at various current densities; cathodic charge 4 C/cm2.
413
kind of roughness is very ~port~t for the appearance of the plated surface. Viewed with the naked eye, the surface shown in Fig. 3c had a yellow-brown colour. Figure 4 shows the the difference in the surfaces under different current densities. The location of the steady state value of the PA signal, as shown in Fig. 2, is understandable using the above view. In short, the greater is the roughness of the plated surface, the larger the steady state value of the PA signal amplitude becomes. In Fig. 2, at some current densities, a rapid increase of the PA signal was observed at the beginning of plating. In the previous case (copper plating), these tendencies were also observed. At that time we tried to explain these phenomena as due to the initial roughening of the surface. However, the scanning electron micrograph showed only a thinly plated surface or an island-like plated surface, depending on the current density. So it is rather difficult to identify the first increase with the initial roughening of the surface. It was reported that, at the begirming of plating (underpotential deposition (upd) region), the optical index of the surface changes greatly [4]. The optical index obtained for the upd layer was very different from the value for the bulk layer and from that for the substrate. To be precise, the absorption coefficient obtained for the upd layer showed a much higher value than the one for the bulk layer. In this case, the cathodic charge at the peak of the PA signal amplitude corresponds to scores of plated layers, so it is not a upd layer. We think that this phenomenon is due to a transition optical state from a upd layer to a bulk layer. So, it is not a change in the roughness but a change in the optical index by the first plated layers. In the above experiment we could not estimate the thickness of the plated layer, only its roughness. So we then tried to estimate the thickness by measuring the phase difference between the reference signal from a chopper and the PA signal obtained from a piezoelectnjc transducer attached to the rear of the electrode. The results obtained are shown in Fig. 5. The data were obtained at a current density of 0.5 mA/cm2, and the phase difference was measured at each cathodic charge as the plating progressed (increase of the cathodic charge). As shown in Fig. 5, the phase
cathodic
charge/
Gcrii2
Fig. 5. PA signal (phase)-cathodic charge plot; current density 0.5 mA/cm2.
Fig. 6. Cross-sectional 0.5 mA/cm2.
SEM photographs
of the plated gold at various cathodic charges; current density
difference decreases monotonically as the plating progresses. We then thought that this was due to the increasing thickness of the plated layer. Figure 6 shows cross-sectional scanning electron micrographs of the gold plated layer at each cathodic charge, 1, 4 and 8 C/cm*. Naturally, with increasing cathodic charge the thickness of the plated layer increases. Considering the above results, we concluded that the phase difference of the PA signal increases with increasing thickness of the plated layer. This is due to the relative geometrical distance between the region where the PA signal is generated and the piezoelectric transducer (detector). In short, the distance increases with increasing thickness of the plated layer. At present we are attempting a mathematical analysis of this phenomenon. In conclusion, we have succeeded in estimating the in situ roughness of a plated gold layer from the PA signal amplitude and have also succeeded in estimating the in situ thickness of the plated layer from the phase difference of the PA signal. This method is quite new and useful with regard to controlling the plating. REFERENCES 1 2 3 4
S. Yoshihara, M. Ueno, Y. Nagae S. Yoshihara and A. Fujishima, J. S. Yoshihara, M. Ueno, Y. Nagae W. Visscher, Ext. Abstr. 40th ISE
and A. Fujishima, J. Electroanal. Chem., 243 (1988) 475. Metal Finish. Sot. Jpn., 39 (1988) 527. and A. Fujishima, Electrochim. Acta, 33 (1988) 1685. Meeting, Kyoto, 1989, Vol. 2, p. 1273.
J. Electroanai. Chem., 278 (1990) 409-414 Elsevier Sequoia S.A., Lausanne - Printed in The Netherlands
Preliminary note
In situ observation of the gold plating process using a photoacoustic technique Sachio Yoshihara, Ryouji Takahashi, Masahiro Odaka, Ikuko Miura and Ei-Ichi Sato Department of Applied Tochigi 321 (Japan)
Chemistry,
Faculty of Engineering.
Utsunomiya
University, Ishii-eho,
Utsunomiya,
Akira Fujis~ma Department of Synthetic Tokyo I13 (Japan)
Chemisity,
(Received 14 November
1989)
Faculty of Engineering,
The University
of Tokyo, Hongo, Bunkyo-ku,
INTRODUCTION
In the past, we have used a photoacoustic technique to study the plating of metals [l-3]. In the present investigation, we have tried to study the plating of gold. Gold is highly resistant to corrosion, and has good electric and thermal conductivity. For these reasons, gold (plating) is used not only for ornamental plating, but also for IC (integrated circuit) parts. In our previous studies [l-3], we reported the possibility of in situ monitoring of copper plating. In that case the difference between the surface structure in the absence and in the presence of brightener could be distinguished by the photoacoustic method. In this report, we examine in situ monitoring of gold plating using a photoacoustic technique.
PAS measurements were carried out on a specimen of oxygen free (99.99%) copper in the form of a 0.5 mm thick plate. The structure of the plated surface was examined ex situ with a scanning electron microscope (SEM, SIGMA-I, Akashi Seisakusho). Electrochemical and PAS measurements were done in a one-compartment acrylic cell with a two-electrode system composed of a copper working electrode and a platinum coated Ti gauze counter electrode. The cell had a flat window for admitting an intense light beam. The temperature of the electrolyte was maintained constant with the aid of a thermostat and a water jacket surrounding the electrolytic cell. Periodic pressure fluctuations, induced by thermal changes in the working electrode, were detected with a piezoelectric detector. A piezoelectric disk (NEPEC 0022-0728/90/$03.50
0 1990 Elsevier Sequoia S.A.
410
Fig. 1. Schematic diagrams of the photo~o~stic cell (a) and the experimental arrangement (b).
NPM N-21, 10 mm diam., 1 mm thick, Tokin) was attached to the rear of the copper electrode using a grease (for high vacuum use) as coupling agent. The light source was an Ar ion laser (Ion Laser Technology, 549OA-S, Multiline). The light beam was chopped m~hanically by a light chopper (NF Electronic Instruments model 5584, chopping frequency 125 Hz). Electrochemical control was carried out with a potentiogalvanostat (Hokuto Denko, HA-301). The electrolyte used for plating was an aqueous solution of 3 X 10m2 mol/dm3 Na,Au(SO,), + 1.4 X 10-l mol/dm3 EDTA + 3.3 X 10-l mol/dm3 Na,SO,. Photoa~ustic sibs-cath~ic charge curves were measured using the galvanostatic method. They were recorded on an X-Y recorder (Yokogawa Electric Works model 3023). The details of the cell and the experimental arrangement are shown in Figs. la and b, respectively.
2.OmAkm’
_*_----
_-______________---------
.O-
__-___ 1. 0mA/cm2
0, SmAIcm2 0
r 1
I
1
2 cathodic charge/
Fig. 2. PA signal (~p~tude)-ethic
3 CQ-I=~
charge curves.
1
4
411 RESULTS
AND
DISCUSSION
We have reported previously that the presence of brightener causes a change in the photoacoustic signal in the case of copper plating on a gold substrate. We therefore suggested the possibility of in situ monitoring of the plating by the photoacoustic method. It was suggested from SEM measurements (ex situ) that in that case the surface roughness affects the PA signal amplitude. Therefore, we can estimate the surface roughness from the PA signal amplitude. In this report, we apply this method to gold plating on a copper substrate. The plating solution for gold was an aqueous solution of sodium gold sulphite. Figure 2
Fig. 3. SEM photographs
of the plated gold at various cathodic charges; current density 2 mA/cm’.
412
shows the dependence of the PA signal amplitude on the cathodic charge. It is obvious that the trace of the PA signal amplitude depends on the cathodic current density. Especially for a current density of 2 mA/cm2, the PA signal amplitude gradually increases progressively with the plating. Then we observed the surface at each cathodic charge, 0.5, 1 and 4 C/cm’, using a scanning electron microscope (SEM). The results obtained are shown in Fig. 3. It can be seen seen from this figure that the roughness of the plated surface gradually increases progressively with the plating. This is in accord with our previous view. In short, the surface roughness makes the PA signal amplitude much bigger. The degree of this roughness is, however, of the order of the wavelength of the incident light (visible region). This
Fig. 4. SEM photographs
of the plated gold at various current densities; cathodic charge 4 C/cm2.
413
kind of roughness is very ~port~t for the appearance of the plated surface. Viewed with the naked eye, the surface shown in Fig. 3c had a yellow-brown colour. Figure 4 shows the the difference in the surfaces under different current densities. The location of the steady state value of the PA signal, as shown in Fig. 2, is understandable using the above view. In short, the greater is the roughness of the plated surface, the larger the steady state value of the PA signal amplitude becomes. In Fig. 2, at some current densities, a rapid increase of the PA signal was observed at the beginning of plating. In the previous case (copper plating), these tendencies were also observed. At that time we tried to explain these phenomena as due to the initial roughening of the surface. However, the scanning electron micrograph showed only a thinly plated surface or an island-like plated surface, depending on the current density. So it is rather difficult to identify the first increase with the initial roughening of the surface. It was reported that, at the begirming of plating (underpotential deposition (upd) region), the optical index of the surface changes greatly [4]. The optical index obtained for the upd layer was very different from the value for the bulk layer and from that for the substrate. To be precise, the absorption coefficient obtained for the upd layer showed a much higher value than the one for the bulk layer. In this case, the cathodic charge at the peak of the PA signal amplitude corresponds to scores of plated layers, so it is not a upd layer. We think that this phenomenon is due to a transition optical state from a upd layer to a bulk layer. So, it is not a change in the roughness but a change in the optical index by the first plated layers. In the above experiment we could not estimate the thickness of the plated layer, only its roughness. So we then tried to estimate the thickness by measuring the phase difference between the reference signal from a chopper and the PA signal obtained from a piezoelectnjc transducer attached to the rear of the electrode. The results obtained are shown in Fig. 5. The data were obtained at a current density of 0.5 mA/cm2, and the phase difference was measured at each cathodic charge as the plating progressed (increase of the cathodic charge). As shown in Fig. 5, the phase
cathodic
charge/
Gcrii2
Fig. 5. PA signal (phase)-cathodic charge plot; current density 0.5 mA/cm2.
Fig. 6. Cross-sectional 0.5 mA/cm2.
SEM photographs
of the plated gold at various cathodic charges; current density
difference decreases monotonically as the plating progresses. We then thought that this was due to the increasing thickness of the plated layer. Figure 6 shows cross-sectional scanning electron micrographs of the gold plated layer at each cathodic charge, 1, 4 and 8 C/cm*. Naturally, with increasing cathodic charge the thickness of the plated layer increases. Considering the above results, we concluded that the phase difference of the PA signal increases with increasing thickness of the plated layer. This is due to the relative geometrical distance between the region where the PA signal is generated and the piezoelectric transducer (detector). In short, the distance increases with increasing thickness of the plated layer. At present we are attempting a mathematical analysis of this phenomenon. In conclusion, we have succeeded in estimating the in situ roughness of a plated gold layer from the PA signal amplitude and have also succeeded in estimating the in situ thickness of the plated layer from the phase difference of the PA signal. This method is quite new and useful with regard to controlling the plating. REFERENCES 1 2 3 4
S. Yoshihara, M. Ueno, Y. Nagae S. Yoshihara and A. Fujishima, J. S. Yoshihara, M. Ueno, Y. Nagae W. Visscher, Ext. Abstr. 40th ISE
and A. Fujishima, J. Electroanal. Chem., 243 (1988) 475. Metal Finish. Sot. Jpn., 39 (1988) 527. and A. Fujishima, Electrochim. Acta, 33 (1988) 1685. Meeting, Kyoto, 1989, Vol. 2, p. 1273.