ferro cyanide gold

A study on the surface roughness of electrodeposited silver thin films

using a confocal laser scanning microscope

M. Saitou

Department of Mechanical Systems Engineering, University of the Ryukyus, 1 Senbaru Nishihara-cho Okinawa,

903-0213, Japan

We have investigated the surface roughness of silver thin films electrodeposited by a direct or pulse current technique

using a confocal laser scanning microscope. Our reasech is twofold: (1) In direct current deposition, antimony potassium

tartrate as an additive shows a large effect on the surface appearances of the silver thin film electrodeposited from a silver

ferrocyanide-thiocyanate electrolyte. Phase diagrams of the appearance stability indicate that the silver ferrocyanide-

thiocyanate electrolyte including antimony potassium tartrate allows the silver thin film with the shiny appearance within a

narrow region of cathode potential. The surface roughness of the silver thin film measured with the confocal laser scanning

microscope reveals that the shiny appearance corresponds to the two-dimensional growth that results in the smooth surface

and the black appearance does to the three-dimensional growth characterized by vertical growth. Cyclic voltammetry

curves exhibit that the shiny appearances are related to a rapid increase in the current density within the narrow region of

the cathode potential. (2) In pulse current deposition, the effect of three parameters such as the pulse current amplitude,

current on-time, and current off-time on the surface roughness of the silver thin film deposited from the electrolyte free of

antimony potassium tartrate has been investigated using the confocal laser scanning microscope. An interface width

defined by the root mean square of fluctuations in surface height is found to decrease with the current off-time and to

increase with the current on-time. Only in the case of the sufficient current off-time, the interface width decreases with the

pulse current amplitude. These experimental results indicate that the appropriate choice of the current on-time and off-time

enables the fabrication of the silver electrodeposit with a small surface roughness.

Keywords confocal laser scanning microscope; surface roughness; silver; electrodeposition, shiny appearance

1. Introduction

A silver thin film with a smooth and shiny surface is of importance for decorative, industrial and technological

application [1, 2]. A cyanide electrolyte including a leveling agent has been employed in silver electrodeposition

because the silver cyanide concentration can be easily controlled in a cyanide process. Environmental regulations for

wastewater discharges polluted by cyanide, and cyanide hazards during the process have urged the research and

development of cyanide-free processes in silver electrodeposition. The leveling agent such as antimony potassium

tartrate (APT) [3-5] is also a toxic compound. Hence, low toxic silver electrolytes are required for the environmental

regulations and cyanide hazards.

Terminology such as a shiny, white bright, and black apperance has been conventionally used because of a visual

check that is a simple method to represent the surface appearances of electrodeposits. Unfortunately, using electrolytes

containing cyanide-free silver compounds [3-6] such as silver nitrate, silver sulfate, silver ionide and silver thiosulfate

the silver thin film with the shiny appearance has not been produced.

Leveling and brightening additives in electrodeposition adsorb preferentially at active sites in a surface and suppress

growth perpendicular to a substrate. APT in silver electrodeposition is known to be a brightening agent [7] and reported

to make it possible to form patterns in the silver thin film [8-9]. However, there have been very few studies on the

effect of APT on the surface appearance of the silver thin film.

Pulse electrodeposition is applied to produce a deposit with a finer grain size in comparison with direct current (DC)

electrodeposition. The influence of three pulse-parameters such as an amplitude of pulse current, current on-time, and

current off-time on the electrodeposit has been investigated [10-18]. According to the overall conclusion of the studies,

it is difficult to predict a priori the influence of these parameters because each electrochemical system reacts in a

different way. In fact, the grain size in Zn electrodeposition [18] decreases with the current on-time whereas the grain

size in Ni electrodeposition [14] increases with the current on-time. Surface roughness is usually related to the grain

size of electrodeposited thin films.

In this study, a silver ferrocyanide-thiocyanate electrolyte synthesized by a method referred in [8-9,19-20] is

employed as a low toxic one. Using the silver ferrocyanide-thiocyanate electrolyte including APT, phase diagrams are

made so as to find a cell voltage and current density, which yield the silver thin film with the shiny appearance. An

interface width [21] that characterizes the surface roughness, which is observed with a confocal laser scanning

microscope, is introduced to relate the shiny, white bright, and black surface appearance.

The purposes of the present paper are to show that using the confocal laser scanning microscope, the shiny

appearance of the silver thin film deposited from the silver ferrocyanide-thiocyanate electrolyte including APT is

Microscopy: Science, Technology, Applications and Education A. Méndez-Vilas and J. Díaz (Eds.)

©FORMATEX 2010 2035

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related to the two-dimensional growth, and that the appropriate choise of the three parameters in pulse current

electrodeposition allows the silver thin film with a small roughness.

2. Experimental setup

Experiments were performed using the two kinds of electrolyte that includes the following components (gL-1

): (1) in

DC electrodeposition, AgNO3, 25.5; K4Fe(CN)6·3H2O, 72; KSCN, 146; KNaC4H4O6·4H2O, 59.3; C8H4K2O12Sb2·3H2O

(APT), 5, 10 and 15; K2CO3, 31.3; (2) in pulse current electrodeposition, AgNO3, 25.5; K4Fe(CN)6·3H2O, 72; KSCN,

146; KNaC4H4O6·4H2O, 59.3; K2CO3, 31.3. The mixed solution of AgNO3, K2CO3 and K4Fe(CN)6·3H2O, which

solution was boiled for 30 minutes, yielded burnt umber precipitates of iron hydroxides. After removal of the iron

hydroxide, the remaining components were added into the solution. Thus, the electrolyte [22] containing APT or that

free of APT was synthesized. The two electrolytes having the pH of about 8.2 were low toxic ones comprising the

complex agent [19-20] Ag(CN)2CNSφ-(φ+1)

where φ has a value between 1 and 2.

In this study, the salt KNaC4H4O6 was employed as a stabilizer. The concentration of Ag+ in the electrolyte was very

low because most of the silver ions were incorporated into the complex agent Ag(CN)2CNSφ-(φ+1)

and very few neutral

complexes were formed by Ag+ and tartaric monoanions [22-25]. Hence, potassium sodium tartrate in the electrolyte

had little influence on surface roughness. In fact, in our preliminary experiment using the solution with or without

potassium sodium tartrate, there was no difference in the surface roughness of silver thin films.

A poly-crystalline copper and carbon plate of 30x10 mm2 each were prepared for a working and counter electrode.

The copper substrate of 99.9 wt% purity had a mirror-like appearance. The two electrodes cleaned by a wet process

were located parallel in a quiescent electrochemical cell at a temperature of 300 K. A fixed voltage or current was

applied with an electric power supply across the electrochemical cell and a resistor. The resistor having the resistance of

50 to 100 Ω, was connected in series with the electrochemical cell to change the cell voltage and current. In DC

electrodeposition, the current through the electrochemical cell and the cell voltage between the cathode and anode

electrode were calculated from the potential drop across the resistor. The cathode potential was measured with a Luggin

capillary with a Ag/AgCl electrode in a KCl solution. On the other hand, in pulse current electrodeposition, a square

wave pulse current having a peak current density Jp of 2 to 24 mAcm-2

, current on-time Ton of 1 to 100 msec, and

current off-time Toff of 1 to 900 msec was applied with the power supply. The cathode potential was measured with the

Luggin capillary and was recorded into a digital storage oscilloscope.

The surface morphology of the silver thin film was observed with the confocal laser scanning microscope (Keyence

VF7500) to an accuracy to 0.01 µm in height. The interface width [21] was calculated from the surface profile of the

surface comprising 512 pixels measured with the confocal laser scanning microscope. The interface width that means

the standard deviation of the surface roughness is defined by

( ) ( ) ,1

2/1

1

2

−= ∑

=

N

i

i hhN

tw (1)

where t is the growth time, N is the number of pixel, hi is the surface height and h is the average height over the lateral

length of the surface profile.

3. Results and Discussion

The charge transfer reaction of the Ag complex agent in silver electrodeposition [26] becomes

( ) ( ) −−−+− ++→+ CNSCNAgeCNSCNAg φφφ 2

12

. (2)

The cell voltage and current through the electrochemical cell were changed by the resistor connected in series with

the electrochemical cell.

3.1 Silver thin film in DC electrodeposition

3.1.1 Phase diagram of surface appearance

Figure 1 shows the phase diagram of the appearance stability for the silver thin film electrodeposited from the

electrolyte including APT of 5 to 10 gL-1

. Terminology such as the shiny, white bright, and black appearance is used to

represent the surface appearance in a conventional fashion. The white bright appearance means the bright but slightly

smoked surface appearance. The shiny appearance is produced within a narrow cell voltage ranging from 1.2 to 1.35 V.

An increse in the concentration of APT makes the region of shiny appearance shifted to a larger current and cell

voltage.


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In order to relate the surface appearance to the interface width defined by Eq. (1), the surface height hi is measured

with the confocal laser scanning microscope to an accuracy to 0.01 µm in height. The copper substrate that appears to

be mirror-like have an average interface width of 0.05µm. Figure 2 shows the optical microscope images of the silver

electrodeposits and surface profile. The average growth rate was 0.35µm min-1

at a current density of 7.5 mAcm-2

and

the typical film thickness in Fig.2 was about 1.6 µm. The interface width of the silver thin film with the shiny

appearance has a smaller value of 0.03 µm than that of the copper substrate. The interface width of the silver thin film

with the white bright appearance has the same value as that of the copper substrates. This indicates that the lateral

growth of the shiny silver thin film makes it possible to have the smaller interface width. On the other hand, the

interface width with the black appearance has a much larger interface width of 0.64 µm. Thus, the interface width is

shown to correspond to the surface appearance determined by the view check. In addition, the two dimensional and

three dimensional growth mode can be related to the shiny appearance and black appearance.

Fig. 1 Phase diagrams of appearance stability for the silver electrodeposits determined with the view check. The symbol, shiny means

the mirror-like and bright appearance. White bright means the bright but slightly smoked or blunt appearance. Black means the color

of black. The concentration of APT in the solution is (a) 5 gL-1 and (b) 10 gL-1.

Fig. 2 Typical microscope images and surface profiles measured with the confocal laser scanning microscope. The electrodeposits

have a film thickness of about 1.6 µm. The average interface width defined by Eq.(1) is (a) 0.05 µm for the white bright appearance,

(b) 0.03 µm for the shiny appearance. The copper substrates have an average interface width of 0.05 µm.

3.1.2 Current-potential curve

The cathode potential measured with the Luggin capillary immersed into the electrolyte in the vicinity of the copper

electrode is shown in Fig.3. It is evident that APT causes a rapid rise of the current density in a region of the symbol A

in Fig.3. The cathode potential in a region of A is almost within a narrow range from 0.55 to 0.65V, which value is

independent of the sweep rate. At the rapid current rise the shiny appearance deposits are produced whereas it is

believed that the rapid increase in the current forms the rough surface owing to the dendrite growth. However, by

repeating the experiments many times we ascertain that the cathode potential in the region of A for the shiny appearance

deposits are obtained

Next, we investigate whether or not the cathode potential ranging from 0.55 to 0.65 V corresponds to the cell voltage

ranging from 1.2 to 1.35 V in Fig. 1, at which the siver thin film with the shiny appearances yields. When the cell

voltage and current density were 1.3 V and 5.5 mAcm-2

in the electrolyte containing APT of 15 gL-1

, the cathode

potential of 0.6V were measured using the Luggin capillary and the silver thin film with the shiny appearance was



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formed. It is concluded that at the cell voltage ranging from 1.2 V to 1.35 V, the cathode potential takes a value ranging

from 0.55 to 0.65V. Thus, the film growth with the shiny appearance occurs in the region A.

These experimental results suggest that the effect of APT on electrodeposition is different from that of other leveling

additives that suppress the growth of electrodeposits owing to their adsorption at active sites in the surfaces. The

leveling additives do not generally cause an increase in the current density. However, APT in the region A at which the

silver thin film with the shiny appearance grows shows the rapid increase in the current density.

Here, according to a simple perturbation theory, we make an attempt for explaining the effect of the rapid rise of the

current density on electrodeposition. A height fluctuation that takes place in the smooth surface is set as δhsinωt where

δh is the amplitude of the height fluctuation, ω is the angular frequency, and t is the time. The height fluctuation formed

under a current i and cathode potential η may develop or decay with time. The fluctuation of the growth rate δv caused

by the height fluctuation is given by

.sinv thdt

dωδδ = (3)

As a change in the current density δ i is proportional to δv, we have

,cos thM

zFi ωωδ

ρδ = (4)

where M is the molecular weight, z is the valence number, ρ is the density and F is the Faraday’s constant. On the other

hand, a Butler-Volmer equation for the cathode potential η has the form:

,exp

=RT

zFii o

ηα (5)

where io is the exchange current density, α is the charge transfer coefficient for the cathode reaction, R is the gas

constant, and T is the temperature. If a change in the cathode potential caused by the fluctuation is small, for |δη|<1,

Using the result of Eq. (5), Eq.(4) yields

.δηδ ih ∝ (6)

Eq. (6) means that the height fluctuation will remain small if the current density i and the cathode potential

fluctuation δη in the system are small. This conclusion is just true for the silver thin film with the shiny appearance. The

fluctuation of the potential in the A region in Fig.3 is estimated at less than 0.1V. The current density is also small in

comparison with the current density in the region B where the deposits with the black appearances grow. Consequently,

as the height fluctuation remains small, the surface apperance remains shiny.

3.1.3 Scaling behavior of the tnterface width

Figure 4 shows the interface width of the silver thin film with the black appearance. The interface width is initially

independent of time, however, at more than 100 s increases with time. The surface appearance at less than 100 s appears

to be shiny. The interface width is known to obey the equation [21]:

( ) ,βttw ∝ (7)

where β is called the growth exponent. The value of β in Fig.4 is 5.5, much larger than 0.5, which indicates that

anomalous growth takes place locally [28-29].

Fig.3 Typical cyclic voltammetry curves for the

electrolyte free of antimony potassium tartrate

and for that including APT of 15 gL-1 .

Fig.4 A log-log plot of the interface width w(t) vs. the growth time

t for the deposits with the black appearance at a current density of

5.6 mAcm-2, a cell voltage of 1.62 V and APT of 15 gL-1.



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3.2 Silver thin film deposited from the electrolyte free of additives by a pulse current technique

Figure 5 shows a typical surface image of a silver thin film of 1 µm in thchness and surface profile measured with the

confocal laser scanning microscope. The silver thin film was electrodeposited from the electrolyte free of APT. The

surface appearance is white bright and the interface width becomes 0.08µm. The silver thin film of 1µm in thickness is

thick enough to use for printed circuit boards and decorative applications.

Fig.5 Microscope image of the silver thin film grown at Jp=12 mAcm-2 , Ton = 9 ms, and Toff=14 ms.The surface profile was

measured with the confocal laser scanning microscope.

3.2.1 Effect of the pulse current amplitude Jp

A plot of the interface width vs. the cathode potential measured at a current on-time of 1 msec and current off-time of 9

msec is shown in Fig. 6(a). The interface width decreases with the pulse current amplitude Jp whereas the cathode

potential increases with Jp. The experimental result seems to be consistent with the studies on nickel [17] and zinc

electrodeposits [18], which explain that the relationship between the fine grain size and high pulse current density is due

to the nucleus radius of the deposit inversely proportional to the cathode potential. The higher current density yields

finer grain sizes because the higher electrode potential increases the free energy available for the formation of new

nuclei and results in a higher nucleation rate. A finer grain of the deposits becomes a smaller interface width w(t) [18].

Fig.6 A plot of the interface width and cathode potential for a variation of the pulse current density. (a) Ton =1 ms and Toff = 9 ms, (b)

Ton =100 ms and Toff = 900 ms.

To confirm the effect of Jp, w(t) was measured for the deposits grown at a much larger current on-time of 100 msec

than 1 msec. Fig. 6(b) shows that the interface width decreases with Jp as well as Fig. 6(a). The film thickness and the

ratio Ton/Toff are the same as those in Fig.6 (a). Irrespective of Jp, w(t) in Fig. 6(b) is larger than that in Fig. 6(a). This is

because of the longer current on-time at which the grain growth occurs.



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However, Figs. 7 (a) and (b) show that an increase in Jp does not always decrease the interface width. The current

on-time and off-time in Fig.7 (a) are 4 msec and 6 msec, and those in Fig.7 (b) 9 msec and 1 msec. As expected, an

increase in Jp causes an increase in the cathode potential. According to Figs.7 (a) and (b), it is not simply said that an

increase in the cathode potential decreases the interface width.

Fig.7 A plot of the interface width vs. the cathode potential for a variation of the pulse current density. (a) Ton=4 ms and Toff = 6 ms.

(b) Ton =9 ms and Toff = 1ms.

3.2.2 Effect of the current off-time Toff

Figure 8 shows a plot of the interface width w(t) and the cathode potential for a variation of the current off-time. The

silver thin film was grown at a current on-time of 9 msec. An increase in Toff rapidly decreases the interface width,

which tends to saturate at the current off-time of 12 msec. The presence of some adsorbed compound formed through

electrochemical reactions [8-9, 14] is believed to act as a leveling agent during the current off-time even if no leveling

agent is added in the solution. At the current off-time above 12 msec, the interface width reaches a stable value. Figure

8 indicates that it takes at least 12 msec for the adsorbed compound to cover all of the active sites in the cathode. Hence,

there exists a limitation period beyond which the current off-time has no influence of a decrease in the interface width.

In this study, cyanide [30] or thiocyanate ions may become an adsorption compound during the current off-time. The

active site means a site available for nucleation. Hence, the adsorbed ion suppresses nucleation or growth at the active

site and the deposit results in the smooth surface. Anyway, it is shown that in silver electrodeposition the current off-

time has a significant influence on the interface width and the limitation period of the current off-time at which the

effect of the current off time is lost exists.

Fig.8 Dependence of the interface width and cathode

potential on the current off-time at a fixed current on-time

of 9 ms and at a pulse current density of 12 mAcm-2.

Fig.9 Dependence of the interface width and cathode

potential on the current on-time at a fixed current off-time

of 9 ms and at a pulse current density of 12 mAcm-2.



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3.2.3 Effect of the current on-time Ton

Figure 9 shows a plot of the interface width w(t) and cathode potential for a variation of the current on-time. The

interface width initially decreases somewhat and rapidly increases with the current on-time. In Zn electrodeposition

[16], the grain size decreases with the current on-time (hence the interface width also decreases) because an increase in

the current on-time causes a decrease in the cathode potential.

However, in Fig.9, the cathode potential increases with the current on-time. An increase in the cathode potential

seems to cause an increase in the interface width. The current off-time may be so short that the formed compound can

not adsorb at all active sites. In fact, as seen in Fig. 6(b), if the current off-time is long enough for the compound to

adsorb, for instance, at a current on-time of 100 msec, the interface width will decrease.

3.2.4 Duty ratio and interface width

The duty ratio defined by Ton/(Ton+Toff)=1/(1+Toff/Ton) makes the influence of the current on-time and off-time evident.

As seen in Figs. 6 and 7, the interface width holds small for Toff /Ton>>1 and increases for Toff /Ton<1.5.

Figure 10 shows the dependence of the interface width on the duty ratio. The interface width initially holds 0.1±0.04

µm within fluctuations. An abrupt change of the interface width is observed at a ratio of about 50 % in Fig.10. This

rapid increase corresponds to Ton=Toff.

In summary, Ton<10 msec and Toff>12 msec are necessary for a silver deposit with a smaller surface roughness. In

fact, as shown in Fig. 6, the silver electrodeposit with the smooth surface is obtained by the appropriate choice of the

current on-time and off-time.

Fig. 10 A plot of the interface width vs. the duty ratio at a pulse current density of 12 mAcm-2. Here, the duty ratio is defined by

Ton/(Ton+Toff).

4. Conclusions

The effect of APT on the surface appearances of the silver thin films from the silver ferrocyanide-thiocyanate

electrolyte has been investigated to produce the silver deposits with the shiny appearances. The phase diagrams exhibit

the presence of the shiny appearance within a small range of the cathode potential. The interface width of the

electrodeposits measured with the confocal laser scanning microscope reveals that the shiny and black appearances

correspond to two-dimensional growth and three-dimensional growth. The cyclic voltammetry curves show that the

shiny appearances can be related to the presence of the rapid increase in the current density. Hence, the effect of APT

on the surface roughness is different from that of other leveling additives that suppress vertical growth of

electrodeposits owing to their adsorption at the active sites on the surfaces.

The effect of the three parameters, Jp, Toff, and Ton on the surface roughness of the silver thin films has been

investigated using the confocal laser scanning microscope. The experimental results show that (1) the interface width

rapidly decreases at the longer current off-time than the current on-time, (2) the long current on-time increases the

interface width, and that (3) for the sufficient current-off time in comparison with the current on-time, the interface

width decreases with the pulse current density. In this study, the adequate choice of the current on-time and off-time

allows the silver electrodeposit with the small surface roughness.



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ferro cyanide gold

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