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Nucleation and initial growth of copper electrodeposits under galvanostatic conditions
Surface and Coatings Technology, 52(1992)1—7
Nucleation and initial growth of copper electrodeposits under galvanostatic conditions Pierre Vanden Brande and René Winand Universitd Libre de Bruxelles, Department Metallurgy—Electrochemistry, CP165, 50 avenue Roosevelt, B1050 Brussels (Belgium) (Received November 30, 1990; accepted in final form October 29, 1991)
Abstract First stages of copper electrodeposition on amorphous carbon and polycrystalline silver substrates are studied under galvanostatic conditions. An island growth mode is observed on the two types of substrates but for equivalent conditions the cluster density is higher on silver substrates than on carbon substrates. Dissolution of some copper clusters occurs a short time after the onset of electrolysis. This can be explained by local changes in supersaturation due to current microdistribution at the cathode surface,
1. Introduction Copper electrodeposited thin films are widely used, for instance, in electronics (e.g. printed wiring boards, ductile layers for electrical contacts) and in sheet steel metallization (e.g. corrosion protection layers, ductile intermediate layers). To improve the quality and performance of manufactured items, control of the crystal growth must be accurate. Generally the problem is to keep the film thickness constant and make a completely covering film. When the film thickness is small the first stages of the thin film development (nucleation, growth mode, cluster growth and coalescence stage) play an important role. The nucleation rate and the cluster critical size of the newly formed metallic deposit are functions of the supersaturation which measures the energetic gap between the out-of-equilibrium new phase and the equilibrium bulk metallic phase [1, 2]. In electrodeposition the supersaturation AG is a function of the cathodic overvoltage ~: AG=ze~’j where z is the number of exchanged electrons and e is the electron charge. The cluster critical size, which corresponds to the maximum cluster free energy or to the size for which the probability of cluster growth is higher than the cluster decay probability for a given supersaturation, is proportional to o~3/AG3,where a is the cluster mean surface specific energy. When the supersaturation
0257—8972/92/$5.00
increases, the cluster critical size decreases and, in practice, is less than a few atoms [3, 4]. Information on the nuclei comes generally from the interpretation of the cluster density distribution as a function of time (or the quantity of deposited matter) through a nucleation model. In practice, this model is necessarily atomistic and not capillary because, for very small clusters, thermodynamic properties (e.g. the specific surface energy and the free energy) lose their physical sense [3—6]. The growth mode depends mainly on the deposit— substrate interaction, relative to the deposit cohesion. Let Ea denote the deposit atomic adsorption energy and E~the deposit atomic cohesion energy. Then two very different situations are possible: the first (when Ea < E~) induces an island growth mode (Volmer—Weber), although the second (for which Ea> E~)corresponds to a layer-by-layer growth process (Frank—van der Merwe) [7]. Some authors have observed experimentally the influence of the substrate surface nature on the nudeation of copper deposits by electrolysis [8—lO] and it has been shown that the nucleation rate is a function of the substrate crystallographic orientation, the presence of defects [9] and the contamination [11]. The aim of this paper is to study the first stages of the formation of copper electrodeposits vs. the deposition rate in two situations: the first when Ea < E~,and the second when Ea ~ E~.The first situation is obtained on an amorphous carbon substrate because of the weak bonding energy between carbon and copper. On the contrary, on a polycrystalline silver substrate, the bond between the substrate material and the deposit material is metallic and Ea must be in the same range as E~,so that the second situation can be obtained.
~‘)
1992
Elsevier Sequoia. All rights reserved
1’. to,id’n I3riinth’, R. W(nanil
2
(,U/riino.siaijc
(ii
nucleation and growth
Q
2. Experimental methods 2.1. Galt’ano,static elecirodeposition technique
__________________
__________________
conditions are achieved in a flow— through channel cell which allows good reproducibility for the hydrodynamic conditions (Fig. I). The flow speed is 2 m s in front of the cathode. The electrolyte is a solution of 0.5 M CuSO4 and 20 g I H~SO4deaerated (ialvanostatic
,,,
—
by nitrogin flow Its temperature is rcgulated at 298 K by a secondary water flow circuit. The anode is made of copper. To be able to use transmission electron microscopy (TEM) to study small copper crystallites. a deposition technique on thin film substrates of a layered type was developed (Fig. 2). The first layer is a silver conducting layer of about 500 A. If the substrate surface is made of amorphous carbon, a 100 A carbon layer lies on the silver layer. The mechanical support of the thin film substrate is a glass plate. The thickness of the film
3
,—..--——-— ________________ ________
_________________________
-
_________
/ 6 —‘
-
l ~
Hg. 2. Cross-section of the electrodeposition cell ftr thin film substrates: I, electrolyte channel: 2. thin conducting filni (cathode): 3. anode: 4, anode current lead: 5. cathode current lead: 6. sprino.
r~7:1 substr ite is large Lnough to bi. in equipotcntial under the chosen experimental conditions and small enough to be transparent to the electrons in TEM.
I
0
/
0
0
2.2.The Production 0/ suhstrate,s different layers which constitute are deposited by vacuum vapour depositionthe on substrate a glass plate. The carbon vapour is produced in a d.c. electric arc while the silver vapour is obtained by heating silver in a molybdenum crucible. With calibrated experimental procedures, a good reproducibility of the substrate surface is achieved. 2.3. .4 nautical techniques
I7
C
t
H ~
~
-
.
-
1-1g. I. Overview of the electrolyte how circuit of the electrolysis cell: I. electrolytic low through channel cell: 2. reference electrode: ilowmeter: 4. electrolyte tank: 5. thermostat: 6. centrifugal pump: 7, bypass: S. nitrogen inlet.
The deposits are directly observed by scanning electron microscopy (SEM) and TEM. On carbon substrates SEM (Fig. 3) is used except for the current density J = 3 A rIm 2 2 below quantity of consumed electricity andthefor J=IOAdm2 below it= Jt=lOCdm 5 C dm 2 On silver substrates SEM is used except for I = 0.68 A dm 2 below Jt = 5 C dm 2 for J = 3 A dm -2 below it = 10 C dm 2 and for J = 10 A dm 2 below it = lOCdrn2. Pictures of the deposits are digitized and introduced in a computer for analysis by a specially developed program. In this way, information is obtained on the evolution of cluster size, cluster surface density and cluster surface distribution. These results are compared with the chemically measured copper deposited quantities (by atomic absorption after dissolution of the deposit in nitric acid). These last results also allow the determination of the current efficiency. .
,
.
P. Vanden Brande, R. Winand
/
3
Galvanostatic Cu nucleation and growth
3. Experimental results 3.1. Copper electrodeposits on amorphous carbon —
i.~ ___
_______________________________________________________
substrates An island growth mode is observed for current densities varying from 0.134 to 10 A dm2. The results will be given extensively for 0.134 A dm2. Relative evolutions being the same, they will mainly be discussed for higher current densities. 3.1.1. Current efficiency The current efficiency Rd vs. the quantity of electricity for a stationary current density presents, in a first zone, a minimum and, in a second zone, a constant value (Figs. 4 and 5). Globally when the current density increases, the current efficiency increases. The generally low observed current efficiency is linked to the low covering ratio ~ as shown in Table I. When the covering ratio reaches 100%, the current efficiency increases to values near 100%. 100
~i)
1OO~m
80
1.8 (Cdm’2)
Fig. 4. Current efficiency Rd vs. the quantity of electricity Jt for copper electrodeposited on an amorphous carbon substrate: El, J= 0.134 A dm2 •, J=0.68 A dm2. Rd(%)
0
200
400
1.1 (Cdm°)
Fig. 5. Current efficiency Rd vs. the quantity of electricity it for copper electrodeposited on an amorphous carbon substrate: El, J= l.bAdm2 x, J=3Adm2.
(hI
Fig. 3. Example of scanning electron micrographs of copper clusters electrodeposited on a carbon substrate: (a) i=0.l34 A dm2, for an electrolysis time of 1512 5; (b) J = 1.7 A dm 2 for an electrolysis time of65s.
4
1’. (widen Brwide. R - I4’inio:d
(,alrano,stat ic
tABLE: I. (‘overing ratio and current efficiency of copper electradeposits on amorphous carbon
25
.1 A rIm
20
2).~
(“.o
I
cot. 5 ca. 13
35 70
3
ca. 90
50
11)0
cot. ((((I
1(1 ‘Current density for a
given
quantity
of
1
electricity of (00 C dm
:
~.
3. 1.2. Superficial density et:olution of clusters The cluster superficial density N vs. the quantity of electricity it decreases in a first zone and reaches a stationary value in a second zone. This phenomenon is
0
Fig. 6.
at
a
current
density
15
cm2(
.
0 -
0
0
40
80
120
ISO
200
1.8
(Cdm2
Fig 6. Cluster superficial density N i-s. the quantity of consumed electricity it for copper eleetrodeposited on an amorphous carbon substrate (J=t).l34Adm
~.
2:
•~i=0.65
A dm2.
An increase of the current density induces an increase of the covering ratio, for a given consumed quantity of electricity as already shown in Table I.
The mean apparent surface of clusters increases as a function of the quantity of electricity (Fig. 7). The covering ratio follows this increase vs. ii (Fig. 8).
L.
400
2)
(Cdm’
Fig. S. Covering ratio 1~i-s. the quantity of electricity it for copper electrodeposiied on an amorphous carbon substrate: . J
of
3.1.3. Evolution of cluster mean size and covering ratio
N003
200
J.t
((.134 A dm
illustrated by 0.134 A dm2
nucleation and growth
T c (%
(00)
(1.134 0.65
(‘it
I
3.1.4. Cluster surftice distribution The nearer-neighbour cluster mean distance d vs. the quantity of electricity ft for a constant current density shows two zones (Fig. 9). The first shows an increase of as a function of ii corresponding to the decrease of the cluster surface density. observed in Fig. 6. As in this figure, the behaviour: second zone in Fig. indicates a stationary the observed constant value of d9corresponds to a constant value of the cluster surface density in this zone. In fact (Fig. 10) we observe that the initial increase of d is. it corresponds to the disappearance of small clusters around bigger islands until the second zone is reached where the density of islands stays constant. In Fig. 10 copper clusters are the small round-shaped black spots and copper islands are the big round- or crystallinc-shaped black spots. These spots have to be distinguished from the diffraction pattern due to the silver
S (~o.m2) _______________________________________
d(~om( 110
1000
100
go 00
Li
70
500 60 50
C
40 0
40
80
l20
l60
J.t(C
200
dm2)
Fig. 7. Cluster mean apparent surface S rs. the quantity of electricity it for copper electrodeposited on an amorphous carbon substrate )i ((.134 A dm2).
0
40
80
l20
160
200
Ii
tCdm’2)
Fig. 9. Nearer-neighbour cluster mean distance d ‘s. the quantity of electricity for copper eleetrodeposited on an amorphous carbon substrate (J =0.134 A dm
P. Vanden Brande, R. I’Vinwid
(at
Z.
ltt~
6ali
•.
1inostatic Cu nucleation and ~‘rottth
I
___ I,
(h,
b
5
.5
‘O’
_
.4, I
-
(c)
(d~
Fig. 10. Phases of ihe formation of a copper elecirodeposited 2: (c) SC dm2 layer (d)on9 Candm2. amorphous carbon subsiraic for J = 3 A dm quantities of electricity: (a) tt.6 ( dm 2: tht I C dm
and the following
polycrystalline layer beneath the carbon layer 100 A thick. When the quantity of electricity increases, for a given current density, the histogram presented in Fig. 11 is shifted in the direction of higher values of the nearerneighbour cluster distance. Moreover, these histograms do not correspond to a stochastic cluster distribution because the comparison of each experimental histogram with its Poisson statistical histogram also shows a shift of the experimental histogram in the same direction. This can be interpreted on the basis of the formation of an exclusion zone to the presence of clusters around each island (Fig. 12). The Poisson statistical histogram is calculated by means of a Poisson distribution based on the measured cluster surface density [3]. The presence of exclusion zones is also described in the literature [12—l4].
in current density always induces an increase in cluster density (Fig. 13), and a decrease of the cluster mean size and of the nearer-neighbour cluster mean distance.
3.1.5. Influence of the current density When the current density increases from 0.134 to 10 A dm —2 ,the experimental results are similar. An increase
Fig. II. Nearer-neighbour cluster distance histograms for copper electrodeposited on an amorphous carbon substrate for a current
~ (~) 25
20
15
~°
0
2
4
6
12
14
1618
20
22
24
26
28
30
d ((Cm)
‘
.
.
.
.
density 0.68 A dm ~, 100 of C dm~2.
- -
-
‘
for two quantities of electricity:
•, 20 C dm
-2
6
P. Vanden Brande, R. Winand -
Galrano.static Cu nucleation and gro wi/i
TABLE 2. Comparison between cluster surface density and cluster mean apparent area for copper electrodeposits on silver and carbon substrates at the beginning of the coalescence
p (% ) 25
—
20
I
~
t~.......~.,___..___.._,_________..____________
0
d (tim)
Fig. 12. Comparison between the nearer-neighbour cluster distance histogram obtained for a copper electrodeposit amorphous 2 and it = 20onC an dm’2 with a carbon substrate at J =0.68 A dm stochastic distribution of clusters: ~1I,experimental; U. stochastic,
N (Con2) 1010
100
2 10 0 I
o
2
4
I
6
I
I
8
I
10 J(A.dm
.2
12 )
Fig. 13. Cluster surface density vs. the current density at the beginning of coalescence for copper eleetrodeposits on amorphous carbon substrates
3.2. Copper electrodeposits on polvcri’stalline silver substrates On polycrystalline silver substrates we observe an island growth mode and similar experimental results to those on amorphous carbon substrates [2]. For instance, the current efficiency curves vs. ft (at given J) also show a minimum, but less pronounced than on carbon substrates. For i 0.68 A dm 2 the minimum value of the current efficiency tj is tlmjn 50% on a silver substrate while 1lmin 30% on a carbon substrate. Meanwhile, the surface density of clusters is higher and the cluster mean size is smaller on silver substrates than on carbon substrates, for equivalent current density at the beginning of the coalescence, as illustrated in Table 2. =
.1 (A dm 2)
Silver substrate
Carbon substrate
3
>l2x(O~
N~cm
S(~m~)
~
cluster surface density. This means that an important copper loss occurs during the first stages of the electrodeposition clusters disappear principally in zones lying process. around The bigger islands, the above-mentioned exclusion zones. Later, the cluster density becomes constant and clusters grow until coalescence. The current efficiency reaches l00% only when the covering ratio reaches 100%. It is likely that the copper loss occurs by dissolution during the electrodeposition process. Copper dissolution could tentatively be explained as follows. When an island grows, the current flow lines and the pattern of equipotentials in the electrolyte change. This induces a decrease of the current density and also a decrease of the overvoltage around the island. A small cluster, supercritical outside where no large island is growing, can become undercritical in the vicinity of such a large growing island. This undercritical cluster dissolves in the electrolyte (Fig. 14), resulting in so-called exclusion zones. In copper dissolution, an electron transfer must occur between copper and the solution. It is then necessary to consider the presence of the following copper chemical dissolution mechanism: in agreement with ref. 15, Cu ~ ions are present near the cathode during the reduction process. Then, if the current density decreases locally, the local Cu~ concentration can he lowered by diffusion towards the bulk electrolyte. To come back to equilibrium. Cu~ions must be produced by chemical reaction between Cu°and Cu2 ~‘
Cu°+ Cu- —~2Cu Curreni floss- tones
~-
=
I
=
sire,111l
/ B~gcopper osiarld
Cu~°’
‘.,
“.,
~
~u0
/ IiI
/ /
tc~o-0i Ciusir’r on sos 057000070 -
1’~”
Cu’ in e Clu
4. Discussion
SUBSTRATE ~
Cu
Cu
0 7 -,
C
Hg. 14. Schematic process of the dissolution of a copper cluster in
The observed current efficiency decrease at very short electrolysis durations corresponds to the decrease of the
the exclusion zone of a big copper island. In the exclusion zone. Cu+Cu2’ —‘2 Cu.
P. Vanden Brande, R. Winand
/
Galvanostatic Cu nucleation and growth
7
When the current density increases, the production rate of Cu + increases and the zones where the Cu + concentration is low enough for copper dissolution to occur become smaller. When the deposit is constituted by discrete islands and the covering ratio is lower than 100%, the dissolution effect seems to play an important role because the current efficiency is lower than 100%. When the covering ratio reaches 100%, the current efficiency increases near
an increase of the nucleation rate and a decrease of the cluster dissolution. At constant deposition rate (through the current density) and deposited theoretical copper quantity, the cluster density is higher (before the coalescence stage) on silver substrates than on amorphous carbon substrates. This shows the importance of the substrate surface nature through the value of the adhesion energy Ea compared with the cohesion energy E5 in the deposit.
100%, as is usual in copper electrodeposition. An increase of the current density induces an increase of the cluster surface density and a higher covering ratio, Hence higher current densities can provide thinner covering films. On silver substrates, the cluster surface density is higher and the cluster mean size is smaller than on carbon substrates under equivalent conditions, probably because the higher adhesion energy between copper and silver stabilizes the clusters. On the other hand, when the adhesion energy increases, for a given superSaturation, the nucleation rate increases [2, 7].
When Ea increases, at constant supersaturation, the mean surface specific energy decreases together with the cluster critical size. Hence the nucleation rate increases and the clusters are more stable.
5.
Acknowledgments
a
The authors thank l’Institut pour l’Encouragement la Recherche Scientifique dans l’Industrie et l’Agriculture and the National Fund for Scientific Research, two Belgian governmental organizations, for partial financial support.
Conclusion
In this paper, we have shown that the first stages of copper electrodeposition present an apparent island growth mode on both amorphous carbon substrates and silver substrates. On amorphous carbon substrates we have a Volmer—Weber growth mode because Ea < E5. We have observed, during the beginning of the deposit formation, a cluster dissolution around larger islands in the so-called exclusion zones. This dissolution is probably due to a drop of the local current density and overvoltage in the neighbourhood of the islands, promoting undersaturation of some clusters and oxidation of copper in Cu~ions with electrolyte 2 + ions. It should bybe reaction noted that thethe initial island Cu growth (in the early stages of the deposit formation) was not studied because of the experimental constraints, but the cluster depletion period and the period where the cluster surface density is constant right up to coalescence were well examined. An increase of the current density increases the cluster surface density and the covering ratio, probably through
References I K. J. Vetter, Elektrochem. Kinetik., Springer, Berlin, 1961. 2 P. Vanden Brande, Ph.D. Thesis, Université Libre de Bruxelles, 1990. 3 A. Milchev, Electrochim. Acta, 28 (1983) 947. 4 A. Milchev and S. Stoyanov, i. Electroanal. Chem., 72(1976) 33. 5 A. Milchev and S. Stoyanov, Thin Solid Films, 22 (1974) 255. 6 A. Milchev and S. Stoyanov, Thin Solid Films, 22 (1974) 267. 7 B. Lewis and J. C. Anderson, Nucleation and Growth of Thin Films, Academic Press, New York, NY, 1978. 8 Y. Ueno, N. Kidokoro and M. Tsuiki, J. Electrochem. Soc., 121(2) (1974) 202. 9 R. Sard and R. and Weil,H.Electrochim. Acta, 15 (1970) 1977. 10 T. B. Vaughan J. Pick, Electrochim. 11 V. M. Kozlov, Elektrokhimiya, 23(1987) 853.Acta, 2 (1960) 179. 12 G. Adzic, A. R. Despic and D. M. Drazic, J. Electroanal. Chem., 220 (1987) 169. 13 E. A. Mamontov, L. A. Kurbatova and A. P. Volenko, Elektrokhimiya, 23 (1987) 187. 14 I. Markov, A. Boynov and S. Toschev, Electrochirn. Acta, 18(1973) 377. IS W. J. Lorenz, A. De Agostini and E. Schmidt, Electrochim. Acta. 34 (1989) 1243.
Nucleation and initial growth of copper electrodeposits under galvanostatic conditions Pierre Vanden Brande and René Winand Universitd Libre de Bruxelles, Department Metallurgy—Electrochemistry, CP165, 50 avenue Roosevelt, B1050 Brussels (Belgium) (Received November 30, 1990; accepted in final form October 29, 1991)
Abstract First stages of copper electrodeposition on amorphous carbon and polycrystalline silver substrates are studied under galvanostatic conditions. An island growth mode is observed on the two types of substrates but for equivalent conditions the cluster density is higher on silver substrates than on carbon substrates. Dissolution of some copper clusters occurs a short time after the onset of electrolysis. This can be explained by local changes in supersaturation due to current microdistribution at the cathode surface,
1. Introduction Copper electrodeposited thin films are widely used, for instance, in electronics (e.g. printed wiring boards, ductile layers for electrical contacts) and in sheet steel metallization (e.g. corrosion protection layers, ductile intermediate layers). To improve the quality and performance of manufactured items, control of the crystal growth must be accurate. Generally the problem is to keep the film thickness constant and make a completely covering film. When the film thickness is small the first stages of the thin film development (nucleation, growth mode, cluster growth and coalescence stage) play an important role. The nucleation rate and the cluster critical size of the newly formed metallic deposit are functions of the supersaturation which measures the energetic gap between the out-of-equilibrium new phase and the equilibrium bulk metallic phase [1, 2]. In electrodeposition the supersaturation AG is a function of the cathodic overvoltage ~: AG=ze~’j where z is the number of exchanged electrons and e is the electron charge. The cluster critical size, which corresponds to the maximum cluster free energy or to the size for which the probability of cluster growth is higher than the cluster decay probability for a given supersaturation, is proportional to o~3/AG3,where a is the cluster mean surface specific energy. When the supersaturation
0257—8972/92/$5.00
increases, the cluster critical size decreases and, in practice, is less than a few atoms [3, 4]. Information on the nuclei comes generally from the interpretation of the cluster density distribution as a function of time (or the quantity of deposited matter) through a nucleation model. In practice, this model is necessarily atomistic and not capillary because, for very small clusters, thermodynamic properties (e.g. the specific surface energy and the free energy) lose their physical sense [3—6]. The growth mode depends mainly on the deposit— substrate interaction, relative to the deposit cohesion. Let Ea denote the deposit atomic adsorption energy and E~the deposit atomic cohesion energy. Then two very different situations are possible: the first (when Ea < E~) induces an island growth mode (Volmer—Weber), although the second (for which Ea> E~)corresponds to a layer-by-layer growth process (Frank—van der Merwe) [7]. Some authors have observed experimentally the influence of the substrate surface nature on the nudeation of copper deposits by electrolysis [8—lO] and it has been shown that the nucleation rate is a function of the substrate crystallographic orientation, the presence of defects [9] and the contamination [11]. The aim of this paper is to study the first stages of the formation of copper electrodeposits vs. the deposition rate in two situations: the first when Ea < E~,and the second when Ea ~ E~.The first situation is obtained on an amorphous carbon substrate because of the weak bonding energy between carbon and copper. On the contrary, on a polycrystalline silver substrate, the bond between the substrate material and the deposit material is metallic and Ea must be in the same range as E~,so that the second situation can be obtained.
~‘)
1992
Elsevier Sequoia. All rights reserved
1’. to,id’n I3riinth’, R. W(nanil
2
(,U/riino.siaijc
(ii
nucleation and growth
Q
2. Experimental methods 2.1. Galt’ano,static elecirodeposition technique
__________________
__________________
conditions are achieved in a flow— through channel cell which allows good reproducibility for the hydrodynamic conditions (Fig. I). The flow speed is 2 m s in front of the cathode. The electrolyte is a solution of 0.5 M CuSO4 and 20 g I H~SO4deaerated (ialvanostatic
,,,
—
by nitrogin flow Its temperature is rcgulated at 298 K by a secondary water flow circuit. The anode is made of copper. To be able to use transmission electron microscopy (TEM) to study small copper crystallites. a deposition technique on thin film substrates of a layered type was developed (Fig. 2). The first layer is a silver conducting layer of about 500 A. If the substrate surface is made of amorphous carbon, a 100 A carbon layer lies on the silver layer. The mechanical support of the thin film substrate is a glass plate. The thickness of the film
3
,—..--——-— ________________ ________
_________________________
-
_________
/ 6 —‘
-
l ~
Hg. 2. Cross-section of the electrodeposition cell ftr thin film substrates: I, electrolyte channel: 2. thin conducting filni (cathode): 3. anode: 4, anode current lead: 5. cathode current lead: 6. sprino.
r~7:1 substr ite is large Lnough to bi. in equipotcntial under the chosen experimental conditions and small enough to be transparent to the electrons in TEM.
I
0
/
0
0
2.2.The Production 0/ suhstrate,s different layers which constitute are deposited by vacuum vapour depositionthe on substrate a glass plate. The carbon vapour is produced in a d.c. electric arc while the silver vapour is obtained by heating silver in a molybdenum crucible. With calibrated experimental procedures, a good reproducibility of the substrate surface is achieved. 2.3. .4 nautical techniques
I7
C
t
H ~
~
-
.
-
1-1g. I. Overview of the electrolyte how circuit of the electrolysis cell: I. electrolytic low through channel cell: 2. reference electrode: ilowmeter: 4. electrolyte tank: 5. thermostat: 6. centrifugal pump: 7, bypass: S. nitrogen inlet.
The deposits are directly observed by scanning electron microscopy (SEM) and TEM. On carbon substrates SEM (Fig. 3) is used except for the current density J = 3 A rIm 2 2 below quantity of consumed electricity andthefor J=IOAdm2 below it= Jt=lOCdm 5 C dm 2 On silver substrates SEM is used except for I = 0.68 A dm 2 below Jt = 5 C dm 2 for J = 3 A dm -2 below it = 10 C dm 2 and for J = 10 A dm 2 below it = lOCdrn2. Pictures of the deposits are digitized and introduced in a computer for analysis by a specially developed program. In this way, information is obtained on the evolution of cluster size, cluster surface density and cluster surface distribution. These results are compared with the chemically measured copper deposited quantities (by atomic absorption after dissolution of the deposit in nitric acid). These last results also allow the determination of the current efficiency. .
,
.
P. Vanden Brande, R. Winand
/
3
Galvanostatic Cu nucleation and growth
3. Experimental results 3.1. Copper electrodeposits on amorphous carbon —
i.~ ___
_______________________________________________________
substrates An island growth mode is observed for current densities varying from 0.134 to 10 A dm2. The results will be given extensively for 0.134 A dm2. Relative evolutions being the same, they will mainly be discussed for higher current densities. 3.1.1. Current efficiency The current efficiency Rd vs. the quantity of electricity for a stationary current density presents, in a first zone, a minimum and, in a second zone, a constant value (Figs. 4 and 5). Globally when the current density increases, the current efficiency increases. The generally low observed current efficiency is linked to the low covering ratio ~ as shown in Table I. When the covering ratio reaches 100%, the current efficiency increases to values near 100%. 100
~i)
1OO~m
80
1.8 (Cdm’2)
Fig. 4. Current efficiency Rd vs. the quantity of electricity Jt for copper electrodeposited on an amorphous carbon substrate: El, J= 0.134 A dm2 •, J=0.68 A dm2. Rd(%)
0
200
400
1.1 (Cdm°)
Fig. 5. Current efficiency Rd vs. the quantity of electricity it for copper electrodeposited on an amorphous carbon substrate: El, J= l.bAdm2 x, J=3Adm2.
(hI
Fig. 3. Example of scanning electron micrographs of copper clusters electrodeposited on a carbon substrate: (a) i=0.l34 A dm2, for an electrolysis time of 1512 5; (b) J = 1.7 A dm 2 for an electrolysis time of65s.
4
1’. (widen Brwide. R - I4’inio:d
(,alrano,stat ic
tABLE: I. (‘overing ratio and current efficiency of copper electradeposits on amorphous carbon
25
.1 A rIm
20
2).~
(“.o
I
cot. 5 ca. 13
35 70
3
ca. 90
50
11)0
cot. ((((I
1(1 ‘Current density for a
given
quantity
of
1
electricity of (00 C dm
:
~.
3. 1.2. Superficial density et:olution of clusters The cluster superficial density N vs. the quantity of electricity it decreases in a first zone and reaches a stationary value in a second zone. This phenomenon is
0
Fig. 6.
at
a
current
density
15
cm2(
.
0 -
0
0
40
80
120
ISO
200
1.8
(Cdm2
Fig 6. Cluster superficial density N i-s. the quantity of consumed electricity it for copper eleetrodeposited on an amorphous carbon substrate (J=t).l34Adm
~.
2:
•~i=0.65
A dm2.
An increase of the current density induces an increase of the covering ratio, for a given consumed quantity of electricity as already shown in Table I.
The mean apparent surface of clusters increases as a function of the quantity of electricity (Fig. 7). The covering ratio follows this increase vs. ii (Fig. 8).
L.
400
2)
(Cdm’
Fig. S. Covering ratio 1~i-s. the quantity of electricity it for copper electrodeposiied on an amorphous carbon substrate: . J
of
3.1.3. Evolution of cluster mean size and covering ratio
N003
200
J.t
((.134 A dm
illustrated by 0.134 A dm2
nucleation and growth
T c (%
(00)
(1.134 0.65
(‘it
I
3.1.4. Cluster surftice distribution The nearer-neighbour cluster mean distance d vs. the quantity of electricity ft for a constant current density shows two zones (Fig. 9). The first shows an increase of as a function of ii corresponding to the decrease of the cluster surface density. observed in Fig. 6. As in this figure, the behaviour: second zone in Fig. indicates a stationary the observed constant value of d9corresponds to a constant value of the cluster surface density in this zone. In fact (Fig. 10) we observe that the initial increase of d is. it corresponds to the disappearance of small clusters around bigger islands until the second zone is reached where the density of islands stays constant. In Fig. 10 copper clusters are the small round-shaped black spots and copper islands are the big round- or crystallinc-shaped black spots. These spots have to be distinguished from the diffraction pattern due to the silver
S (~o.m2) _______________________________________
d(~om( 110
1000
100
go 00
Li
70
500 60 50
C
40 0
40
80
l20
l60
J.t(C
200
dm2)
Fig. 7. Cluster mean apparent surface S rs. the quantity of electricity it for copper electrodeposited on an amorphous carbon substrate )i ((.134 A dm2).
0
40
80
l20
160
200
Ii
tCdm’2)
Fig. 9. Nearer-neighbour cluster mean distance d ‘s. the quantity of electricity for copper eleetrodeposited on an amorphous carbon substrate (J =0.134 A dm
P. Vanden Brande, R. I’Vinwid
(at
Z.
ltt~
6ali
•.
1inostatic Cu nucleation and ~‘rottth
I
___ I,
(h,
b
5
.5
‘O’
_
.4, I
-
(c)
(d~
Fig. 10. Phases of ihe formation of a copper elecirodeposited 2: (c) SC dm2 layer (d)on9 Candm2. amorphous carbon subsiraic for J = 3 A dm quantities of electricity: (a) tt.6 ( dm 2: tht I C dm
and the following
polycrystalline layer beneath the carbon layer 100 A thick. When the quantity of electricity increases, for a given current density, the histogram presented in Fig. 11 is shifted in the direction of higher values of the nearerneighbour cluster distance. Moreover, these histograms do not correspond to a stochastic cluster distribution because the comparison of each experimental histogram with its Poisson statistical histogram also shows a shift of the experimental histogram in the same direction. This can be interpreted on the basis of the formation of an exclusion zone to the presence of clusters around each island (Fig. 12). The Poisson statistical histogram is calculated by means of a Poisson distribution based on the measured cluster surface density [3]. The presence of exclusion zones is also described in the literature [12—l4].
in current density always induces an increase in cluster density (Fig. 13), and a decrease of the cluster mean size and of the nearer-neighbour cluster mean distance.
3.1.5. Influence of the current density When the current density increases from 0.134 to 10 A dm —2 ,the experimental results are similar. An increase
Fig. II. Nearer-neighbour cluster distance histograms for copper electrodeposited on an amorphous carbon substrate for a current
~ (~) 25
20
15
~°
0
2
4
6
12
14
1618
20
22
24
26
28
30
d ((Cm)
‘
.
.
.
.
density 0.68 A dm ~, 100 of C dm~2.
- -
-
‘
for two quantities of electricity:
•, 20 C dm
-2
6
P. Vanden Brande, R. Winand -
Galrano.static Cu nucleation and gro wi/i
TABLE 2. Comparison between cluster surface density and cluster mean apparent area for copper electrodeposits on silver and carbon substrates at the beginning of the coalescence
p (% ) 25
—
20
I
~
t~.......~.,___..___.._,_________..____________
0
d (tim)
Fig. 12. Comparison between the nearer-neighbour cluster distance histogram obtained for a copper electrodeposit amorphous 2 and it = 20onC an dm’2 with a carbon substrate at J =0.68 A dm stochastic distribution of clusters: ~1I,experimental; U. stochastic,
N (Con2) 1010
100
2 10 0 I
o
2
4
I
6
I
I
8
I
10 J(A.dm
.2
12 )
Fig. 13. Cluster surface density vs. the current density at the beginning of coalescence for copper eleetrodeposits on amorphous carbon substrates
3.2. Copper electrodeposits on polvcri’stalline silver substrates On polycrystalline silver substrates we observe an island growth mode and similar experimental results to those on amorphous carbon substrates [2]. For instance, the current efficiency curves vs. ft (at given J) also show a minimum, but less pronounced than on carbon substrates. For i 0.68 A dm 2 the minimum value of the current efficiency tj is tlmjn 50% on a silver substrate while 1lmin 30% on a carbon substrate. Meanwhile, the surface density of clusters is higher and the cluster mean size is smaller on silver substrates than on carbon substrates, for equivalent current density at the beginning of the coalescence, as illustrated in Table 2. =
.1 (A dm 2)
Silver substrate
Carbon substrate
3
>l2x(O~
N~cm
S(~m~)
~
cluster surface density. This means that an important copper loss occurs during the first stages of the electrodeposition clusters disappear principally in zones lying process. around The bigger islands, the above-mentioned exclusion zones. Later, the cluster density becomes constant and clusters grow until coalescence. The current efficiency reaches l00% only when the covering ratio reaches 100%. It is likely that the copper loss occurs by dissolution during the electrodeposition process. Copper dissolution could tentatively be explained as follows. When an island grows, the current flow lines and the pattern of equipotentials in the electrolyte change. This induces a decrease of the current density and also a decrease of the overvoltage around the island. A small cluster, supercritical outside where no large island is growing, can become undercritical in the vicinity of such a large growing island. This undercritical cluster dissolves in the electrolyte (Fig. 14), resulting in so-called exclusion zones. In copper dissolution, an electron transfer must occur between copper and the solution. It is then necessary to consider the presence of the following copper chemical dissolution mechanism: in agreement with ref. 15, Cu ~ ions are present near the cathode during the reduction process. Then, if the current density decreases locally, the local Cu~ concentration can he lowered by diffusion towards the bulk electrolyte. To come back to equilibrium. Cu~ions must be produced by chemical reaction between Cu°and Cu2 ~‘
Cu°+ Cu- —~2Cu Curreni floss- tones
~-
=
I
=
sire,111l
/ B~gcopper osiarld
Cu~°’
‘.,
“.,
~
~u0
/ IiI
/ /
tc~o-0i Ciusir’r on sos 057000070 -
1’~”
Cu’ in e Clu
4. Discussion
SUBSTRATE ~
Cu
Cu
0 7 -,
C
Hg. 14. Schematic process of the dissolution of a copper cluster in
The observed current efficiency decrease at very short electrolysis durations corresponds to the decrease of the
the exclusion zone of a big copper island. In the exclusion zone. Cu+Cu2’ —‘2 Cu.
P. Vanden Brande, R. Winand
/
Galvanostatic Cu nucleation and growth
7
When the current density increases, the production rate of Cu + increases and the zones where the Cu + concentration is low enough for copper dissolution to occur become smaller. When the deposit is constituted by discrete islands and the covering ratio is lower than 100%, the dissolution effect seems to play an important role because the current efficiency is lower than 100%. When the covering ratio reaches 100%, the current efficiency increases near
an increase of the nucleation rate and a decrease of the cluster dissolution. At constant deposition rate (through the current density) and deposited theoretical copper quantity, the cluster density is higher (before the coalescence stage) on silver substrates than on amorphous carbon substrates. This shows the importance of the substrate surface nature through the value of the adhesion energy Ea compared with the cohesion energy E5 in the deposit.
100%, as is usual in copper electrodeposition. An increase of the current density induces an increase of the cluster surface density and a higher covering ratio, Hence higher current densities can provide thinner covering films. On silver substrates, the cluster surface density is higher and the cluster mean size is smaller than on carbon substrates under equivalent conditions, probably because the higher adhesion energy between copper and silver stabilizes the clusters. On the other hand, when the adhesion energy increases, for a given superSaturation, the nucleation rate increases [2, 7].
When Ea increases, at constant supersaturation, the mean surface specific energy decreases together with the cluster critical size. Hence the nucleation rate increases and the clusters are more stable.
5.
Acknowledgments
a
The authors thank l’Institut pour l’Encouragement la Recherche Scientifique dans l’Industrie et l’Agriculture and the National Fund for Scientific Research, two Belgian governmental organizations, for partial financial support.
Conclusion
In this paper, we have shown that the first stages of copper electrodeposition present an apparent island growth mode on both amorphous carbon substrates and silver substrates. On amorphous carbon substrates we have a Volmer—Weber growth mode because Ea < E5. We have observed, during the beginning of the deposit formation, a cluster dissolution around larger islands in the so-called exclusion zones. This dissolution is probably due to a drop of the local current density and overvoltage in the neighbourhood of the islands, promoting undersaturation of some clusters and oxidation of copper in Cu~ions with electrolyte 2 + ions. It should bybe reaction noted that thethe initial island Cu growth (in the early stages of the deposit formation) was not studied because of the experimental constraints, but the cluster depletion period and the period where the cluster surface density is constant right up to coalescence were well examined. An increase of the current density increases the cluster surface density and the covering ratio, probably through
References I K. J. Vetter, Elektrochem. Kinetik., Springer, Berlin, 1961. 2 P. Vanden Brande, Ph.D. Thesis, Université Libre de Bruxelles, 1990. 3 A. Milchev, Electrochim. Acta, 28 (1983) 947. 4 A. Milchev and S. Stoyanov, i. Electroanal. Chem., 72(1976) 33. 5 A. Milchev and S. Stoyanov, Thin Solid Films, 22 (1974) 255. 6 A. Milchev and S. Stoyanov, Thin Solid Films, 22 (1974) 267. 7 B. Lewis and J. C. Anderson, Nucleation and Growth of Thin Films, Academic Press, New York, NY, 1978. 8 Y. Ueno, N. Kidokoro and M. Tsuiki, J. Electrochem. Soc., 121(2) (1974) 202. 9 R. Sard and R. and Weil,H.Electrochim. Acta, 15 (1970) 1977. 10 T. B. Vaughan J. Pick, Electrochim. 11 V. M. Kozlov, Elektrokhimiya, 23(1987) 853.Acta, 2 (1960) 179. 12 G. Adzic, A. R. Despic and D. M. Drazic, J. Electroanal. Chem., 220 (1987) 169. 13 E. A. Mamontov, L. A. Kurbatova and A. P. Volenko, Elektrokhimiya, 23 (1987) 187. 14 I. Markov, A. Boynov and S. Toschev, Electrochirn. Acta, 18(1973) 377. IS W. J. Lorenz, A. De Agostini and E. Schmidt, Electrochim. Acta. 34 (1989) 1243.