Spectroelectrochemical study of the effect of organic additives on the electrodeposition of tin

Article abstract of DOI:10.1149/1.1554724

Adsorption behavior and the effect of organic additives on tin electrodeposition process were studied utilizing combined electro-chemical and surface enhanced Raman spectroscopy (SERS) techniques. The adsorption behavior of polyethylene glycols with and w

Full text of DOI:10.1149/1.1554724

C202
Journal of The Electrochemical Society, 150 ͑4͒ C202-C207 ͑2003͒
0
013-4651/2003/150͑4͒/C202/6/$7.00 © The Electrochemical Society, Inc.
Spectroelectrochemical Study of the Effect of Organic Additives
on the Electrodeposition of Tin
a
a, ,z
b,
Igor S. Zavarine, Oscar Khaselev * and Yun Zhang *
Lucent Technologies, Electroplating Chemicals and Services, Staten Island, New York 10309, USA
Adsorption behavior and the effect of organic additives on tin electrodeposition process were studied utilizing combined electro-
chemical and surface enhanced Raman spectroscopy ͑SERS͒ techniques. The adsorption behavior of polyethylene glycols with and
without hydrophobic head group was correlated with the morphology of the deposit. SERS experiments suggest that the head
group part of the molecule remains adsorbed at all potentials studied. The potential dependence of the adsorption is primarily due
to the hydrophilic tail in the Triton X100 family of compounds. The head groups of Triton X compounds are not required for
adsorption but significantly enhance it. Triton X also enhances phenolphthalein adsorption on gold or silver surfaces and weakly
interacts with it through some kind of supramolecular interactions. The coadsorption of Triton X100 and phenolphthalein signifi-
cantly changes the grain size and the deposit structure of the electroplated tin as compared to Triton X alone.
©
2003 The Electrochemical Society. ͓DOI: 10.1149/1.1554724͔ All rights reserved.
Manuscript submitted December 19, 2001; revised manuscript received September 25, 2002 Available electronically February 25,
003.
2
Organic additives are commonly used in the electrodeposition of
tin from acidic solutions.1 Surfactants and grain refiners are added
to plating baths to influence the deposit morphology, brightness, and
nature and length of the surfactants used in this study. The major
questions we tried to address in our study were orientation of these
molecules on the electrode surface as a function of applied potential,
effect of the length on adsorption strength and electrochemisty, role
of the hydrophobic head of the molecule, and effect of mutual in-
teraction between Triton X-100 and phenolphthalein.
-4
5
grain refinement. These addition agents are an important compo-
nent of plating baths for the deposition of tin with desirable proper-
ties and morphology suitable for electronic and other practical ap-
plications. While the use of organic reagents in electrodeposition
process is commonplace, the specific activity of the reagents is only
generally understood in terms of adsorption at the cathode surface
during the deposition. The manner in which small amounts of or-
ganic species affects the physical morphology of a metal deposit
remains unclear. Adsorption on the metal surface is believed to be
the main generic way of these additives influencing the electrodepo-
sition process. Numerous experimental works discuss the effect of
additives on metal electrodeposition.6 A major limitation of these
studies is the inability to characterize accurately the adsorption of
ionic and molecular species at the electrode surface during the elec-
trodeposition.
Experimental
The effect of organic additives on the electrodeposition of tin
from aqueous 1 M methanesulfonic acid (CH SO H) and 0.5 M tin
3
3
methanesulfonate (SnCH SO ) electrolyte was characterized by
3
3
electrochemical methods in a standard three-electrode cell. The 0.5
L cell was comprised of a rotating copper disc electrode, platinum
wire counter electrode, and Ag/AgCl reference electrode.
-8
Raman spectra were collected by Renishaw 1000S microscope
equipped with custom build three-electrode spectroelectrochemical
cell. 633 nm unpolarized red excitation and 180° geometry was used
to gather the data. Pt wire was used as a counter electrode and
Ag/AgCl equipped with a Luggin capillary was used as a reference
electrode. Gold and silver working electrodes were roughened using
Surface enhanced raman spectroscopy ͑SERS͒ is a vibrational
6
spectroscopy that provides 10 times stronger Raman signals of the
species adsorbed on a metal surface as compared to normal Raman
spectroscopy. In addition, SERS is highly surface selective, that is,
typically only the molecules ͑or parts of them͒ that are in direct
contact with the surface are enhanced and show up in the spectra.
SERS has been widely used to obtain the information on what spe-
cies are adsorbed, what their orientations are on the surface, and
1
2,13
optimized literature procedures
and 0.1 M KCl solutions in
deionized water ͑Millipore͒. The potential of the electrode was con-
trolled by a EG&G M273 potentiostat. All chemicals were pur-
chased from Aldrich and used as received.
9
how strong is the adsorption. These studies, however, have been
mainly limited to relatively small and simple molecules, which are
normally not used in electroplating. Hope and Brown used SERS to
study adsorption of polyethylene glycol ͑PEG͒ at a polarized copper
1
0
electrode. They found the adsorption to be potential dependent.
Unfortunately, direct measurements of adsorption behavior of or-
ganic molecules on tin electrode by SERS is very difficult because
the enhancement effect is limited for gold, silver, or copper elec-
trodes. Nevertheless, valuable insights can be gained by correlation
of adsorption behavior measured by SERS on gold or silver elec-
trodes with electrochemistry of tin and morphology of tin deposit.
In this paper, we used SERS to study the adsorption of additives
used in tin electroplating. As a model system we used combination
of Triton X-100 and its derivatives with phenolphthalein. Such sys-
tem is known to produce smooth and reflective coating in both pure
tin and tin/lead electroplating.11 Schematic structures of Triton
X-100 and phenolphthalein are shown in Fig. 1. Table I lists the
*
Active Member of the Electrochemical Society.
a
Present address: Cookson Electronics, Orange, Connecticut 06477, USA.
Present address: Technic Advanced Technology Division, Plainview, New
York 11803, USA.
b
Figure 1. Schematic structures of the Triton X family of surfactants and
phenolphthalein.
z
E-mail: okhaselev@cooksonelectronics.com
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Journal of The Electrochemical Society, 150 ͑4͒ C202-C207 ͑2003͒
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Table I. Length and chemical nature of the molecules studied.
Number of ethylene oxide
groups in the tail
Hydrophobic head
Igepal CA520
Triton X-100
Igepal DM970
PEG1
PEG2
PEG2
C Ph
5
10
150
7
14
77
101
ϳ85
8
C Ph
8
(C ) Ph
9
2
none
none
none
none
none
PEG4
PEG-PPG
Results and Discussion
Electrochemistry of tin electrodeposition, Triton X class of
surfactants.—Typical voltammograms for the electrodeposition of
tin in methanesulfonic acid with and without Triton X-100 are
shown in Fig. 2. The onset of reduction current occurs at ca. Ϫ0.42
V. Without Triton X-100, a current density peaked at ϳ0.5 V. No
diffusion limiting current plateau was seen in the curve, and as po-
tential decreased, current density started increasing rapidly again.
Such behavior is usually associated with dendritic crystal growth,
which greatly increases surface area of the electrode. The addition of
Triton X-100 to the solution suppressed the dendritic growth of the
tin deposit and a cyclic voltammogram ͑CV͒ revealed two distinct
peaks. The current in the first peak at ϳ0.5 V did not depend on tin
concentration in the solution and it was reasonable to associate ap-
pearance of this peak with adsorption of Triton X-100 on the grow-
ing tin layer, which strongly inhibited the tin reduction current. This
process can be reversible, as seen from the inset in Fig. 2. The
second peak at Ϫ0.75 V depended strongly on the tin concentration
indicating that it is a diffusion limiting current.
The effect of agitation on the reduction of tin in the presence of
Triton X-100 is shown in Fig. 3. At 100 rpm, in the forward scan
reduction current reached plateau at ca. Ϫ0.75 V and remained al-
most constant up to ca. Ϫ1.2 V. At more negative potentials it
slightly increased due to onset of hydrogen evolution. In the back-
ward scan a current density was significantly lower than in the for-
ward scan. At 400 revolutions per minute ͑rpm͒, the limiting current
density was approximately twice higher than at 100 rpm, consistent
with convective-diffusion considerations. The current density re-
mained constant up to ϳ1 V, then increased slowly with potential. In
the backward scan, current density was slightly higher than in the
forward scan. At 800 rpm, as expected, the limiting current density
in the forward scan further increased, but unexpectedly the differ-
Figure 3. Effect of electrode rotation speed on electroreduction of tin.
ence between the current densities in the forward and backward
scans grew even larger. These results could indicate that effect of
Triton X-100 on reduction current of tin weakened at negative po-
tentials and at higher agitation. In order to further validate this as-
sumption we measured current density transients at constant applied
potential of Ϫ1.2 V at different electrode rotation speeds, see Fig. 4.
At 200 rpm, a current density transient revealed typical behavior
under potentiostatic control, i.e., an initial current density dropped
2
quickly to ϳ50 mA/cm and remained constant throughout the ex-
2
periment. At 400 rpm, after an initial drop to ϳ80 mA/cm , the
current density grew slowly with time for about 50 s and then in-
2
creased sharply to ϳ300 mA/cm . At higher rpm, similar current
density transients were observed. However, time required for current
density to increased sharply, and decreased as rotation speed in-
creased. At 800 rpm, no initial current density drop was seen and a
current rapidly increased with time throughout the experiment. The
morphology of tin deposit formed under such condition is shown in
Fig. 5a.
The effect of the size of the Triton X-type molecule on electro-
chemistry of tin is shown in Fig. 6, which compares CVs of tin
reduction obtained with Igepal DM 970 and Igepal CA 520 instead
of Triton X-100. All molecules have a similar hydrophobic head but
different lengths of hydrophilic tail ͑Table I͒. Note that both curves
are very similar to that measured with addition of Triton X-100,
indicating that the length of the hydrophilic tail has only slight effect
on the kinetics of tin electrodeposition. Similar results were obtained
under agitation conditions.
Figure 2. Voltammograms for reduction of tin with and without addition of
Figure 4. Current density transients for electroreduction of tin at different
Triton X-100.
electrode rotation speeds.
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Journal of The Electrochemical Society, 150 ͑4͒ C202-C207 ͑2003͒
Figure 6. Voltammograms for reduction of tin with addition of Igepal DM
70 and CA 520.
9
the presence of PEG of different lengths and with no agitation. In
general, the curves revealed features similar to those for Triton X
type molecules. In the presence of PEG1 (n ϭ 7) observed current
densities were higher than those for Triton X-100 and cyclic volta-
mmetry showed significant hysteresis. With increase in the molecu-
lar weight of PEG the passivation effect on tin electrodeposition
became stronger but observed current densities remained relatively
high and the quality of the obtained deposit was unsatisfactory. In-
troduction of more hydrophobic propylene oxide units into the body
of the PEG molecule strongly enhanced absorption strength of PEGs
as evidenced by suppression in the tin current. Thus, block copoly-
mer containing ca. 30% of polypropylene glycol ͑PPG͒ strongly
suppressed tin electrodepositon current in both stagnate solution and
agitated at 800 rpm, as seen in Fig. 8. The tin deposits obtained from
this solution were smooth with fine grains, as seen in Fig. 5b.
Effect of phenolphthalein.—Phenolphthalein is used in electro-
plating baths in order to improve the surface morphology of the
11
deposits. Because of its very low solubility in water, phenolphtha-
lein is added to the solution together with surfactants, such as a
Triton X type. Figure 9 shows a CV for reduction of tin in the
presence of both phenolphthalein and Triton X-100. Without agita-
tion, the voltammogram was very similar to that for Triton X-100
used alone. However, at 800 rpm the difference between two sys-
tems became evident in the absence of hysteresis characteristic for
Triton X-100. Moreover, in potentiostatic test at applied potential of
Figure 5. Surface morphology of tin deposit obtained from solutions with
addition of ͑a͒ Triton X-100, ͑b͒ block copolymer of ethylene oxide and
propylene oxide, and ͑c͒ Triton X-100 and phenolphthalein.
Effect of polyethylene glycols.—In order to understand the func-
tional role of the head group on the electrochemistry and elec-
trodeposition we performed similar experiments with PEGs which
lack the above head group. Figure 7 shows CVs for tin reduction in
Figure 7. Voltammograms for reduction of tin with addition of PEG with
different molecular weights.
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Journal of The Electrochemical Society, 150 ͑4͒ C202-C207 ͑2003͒
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Figure 10. SERS spectra of 0.003 M solution of Igepal 520 in 0.1 M H SO
2
4
at ͑top͒ ϩ0.23 V and ͑bottom͒ at Ϫ0.5 V. Bands marked with a star are
assigned to the phenyl ring moiety, and bands marked with a pound sign
assigned to the isooctane moiety. On the bottom spectrum, only the bands
that changed significantly in intensity are marked.
Figure 8. Voltammograms for reduction of tin with addition of block co-
polymer PEG-polypropylene glycol.
2
Ϫ1.2 V, constant current density of ϳ140 mA/cm was established
throughout the experiment, demonstrating that the mixture of Triton
X-100 and phenolphthalein can provide control for tin electrodepo-
sition in much wider range of current densities than Triton X-100
alone. This can be taken as an indication that the mixture of Triton
X-100 and phenolphthalein is more strongly adsorbed at the surface
of tin electrode than Triton X-100 alone. The surface morphology of
tin deposit obtained from the electrolyte containing both additives is
shown in Fig. 5c. A comparison with Fig. 5a shows a dramatic
improvement brought by phenolphthalein.
Electrochemical data presented so far suggest that the activity of
the organic additives is a strong function of applied potential and
agitation. However, what exactly happens on the surface of the elec-
trode when these parameters vary remains unknown. Adsorption/
desorption is believed to be the main mechanism by which the ad-
ditives influence the observed electrochemistry and morphology of
the obtained deposits. However, classical electrochemistry provides
very indirect ways of measuring the adsorption strength and cover-
age. In order to better understand the connection between the ad-
sorption of the species studied and their effect on electrochemistry
and electrodeposition process, we performed a series of SERS mea-
surements as a function of applied potential.
0
.003 M͒ solution of Igepal 520 in 0.1 M sulfuric acid at OCP
͑ca. ϩ0.23 V͒ and at Ϫ0.5 V vs. saturated calomel electrode. By
comparison of the spectrum at OCP with the bulk Raman spectra
reported in the literature, we were able to assign all the Raman
bands either to the isooctane ͑marked by #͒ or a phenyl ring ͑marked
by a *) part of the molecule. For a number of bands the exact
assignment is impossible because the vibrations originating from the
different parts of the molecule accidentally have the same frequen-
cies. These bands have multiple assignments in Table II. The bands
that reported in the literature to originate exclusively from the poly-
1
4
Table II. Raman bands assignments according to Ref. 12. Bands
assigned exclusively to the polyoxyethylene tail of the molecule
are shown in bold.
Frequency, cmϪ1
Assignment1
3
2
2
1
1
1
1
1
067
927
876
607
466
447
289
241
Ph
C8
C8
Ph
Surface enhanced Raman spectroscopy, Triton X class of
surfactants.—The SERS spectra of the Triton X class of surfactants
could be exemplified by the spectra of Igepal 520 on a gold surface.
Figure 10 shows the SERS spectra obtained using saturated ͑ca.
a
Ph, C , OE
8
Ph, C8
Ph, OE
Ph, OE
Ph
1183
1
129
OE, C8
Ph
OE
OE
Ph
Ph, C8
Ph
C8
1
1
1
1
9
9
9
8
8
8
7
7
7
6
6
5
5
5
4
094
060
040
006
64
22
06
86
68
OE
C , OE
8
37
96
46
Ph, OE
Ph, OE
C8
C8
Ph
Ph
Ph
OE
Ph, OE
Ph
14
79
37
84
66
25
72
Figure 9. Voltammograms for reduction of tin with addition of Triton X-100
and phenolphthalein.
a
OE is oxyethylene unit.
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Journal of The Electrochemical Society, 150 ͑4͒ C202-C207 ͑2003͒
Figure 11. Suggested orientation of Igepal 520 on a gold surface at ͑a͒
ϩ0.23 V and ͑b͒ Ϫ0.5 V vs. SCE. Oxygen atoms are in red; hydrogen atoms
are not shown for clarity.
Figure 12. SERS spectra of phenolphthlein at ϩ0.2 V and at Ϫ0.4 V. Bands
marked with a star are assigned to the phenolic moiety, bands marked with
the pound sign are assigned to the lactone ring moiety.
oxyethylene tail of the molecule are at 1064, 1055, 1044, 886, and
SERS of polyethylene glycol on Ag electrode.—Dilute ͑7g/L,
.002 M͒ solution of PEG4 ͑Table I͒ in 0.1 M sulfuric acid shows no
Ϫ1 14
5
64 cm
.
These bands are mostly due to ␯(C-O) and ␯(C-C)
0
stretching vibrations and rocking in the (O-CH -CH ) unit. The
2
2
n
SERS signal on silver or gold electrodes at OCP ͑ϩ0.2 V͒. How-
ever, boosting up the concentration of PEG4 to 0.06 M ͑ca. 200 g/L͒
does lead to the appearance of the Raman bands characteristic of
bulk PEG. Just like with the Triton X family, the SERS intensity was
observed at ca. Ϫ0.4 V. The characteristic ␯(C-O) band at 1064
total absence of all of the above five bands from the Raman spec-
trum observed at OCP suggests that only the short hydrophobic head
of the molecule is close to the gold surface and is therefore strongly
enhanced. The bands at 1466, 1289, 1241, 1129, 868, 837, 796, and
Ϫ1
5
25 cm probably originate from either the isooctane or phenyl
Ϫ1
cm is clearly enhanced at Ϫ0.4 V vs. relatively more positive
ring parts of the molecule ͑or both͒. The long hydrophilic tail is not
in direct contact with the surface. Figure 11a, and subsequently its
Raman bands, are not enhanced and not detected. The same conclu-
sion is reached when examining the SERS spectra obtained on
roughened silver electrode although the band intensity distribution is
different from the one observed on gold electrode. Similar Raman
spectra were obtained for surfactants with longer polyoxyethylene
chains ͑tails͒; at the same head group concentration the observed
intensity of the SERS signal decreased with the increase in the chain
length: n ϭ 5 ӷ n ϭ 8 Ͼ n ϭ 10 ӷ n ϭ 150. This trend sug-
gests lower head group coverage of the surface with the increase in
the hydrophilicity of the molecule. The conformation suggested in
Fig. 11a is in agreement with the Ref. 15. The relationship between
the SERS intensity and the adsorption coverage is known to be
complex and nonlinear. Nevertheless, we believe that for the mol-
ecules of the similar chemical nature the above semiquantitative
conclusions about the adsorption strength and coverage could be
made.
potentials. The main difference between the two compounds is that
Triton X has an anchor group ͑the hydrophobic head͒ whereas PEG4
does not. Therefore, the hydrophobic head group of Triton X is not
required for the adsorption at Ϫ0.5 V but it significantly enhances it,
especially at potentials anodic of Ϫ0.4 V. PEG4 does not form mi-
celles but shows similar behavior of ␯(C-O) bands at the same
potentials as Triton X compounds. Interpretation of the potentials
dependent Raman spectra of Igepal 520 leads us to propose the
following adsorption mechanism of Fig. 11a and b. Immediately, it
suggests that the functional group that fulfills the role of adsorption
is largely the hydrophobic head group. Equally important is that the
hydrophilic tail group participates also in the adsorption at more
negative potentials at which the electrodeposition of tin takes place.
The relative importance of the head and the tail group is potential
dependent, indicating the dynamic nature of the surfactant/surface
interactions.
The apparent potential dependent change in the surface orienta-
tion of Igepal 520 observed by SERS helps to explain the CV in Fig.
Examining spectra of Igepal 520 obtained at Ϫ0.5 V shows that
most of the bands observed at OCP remained either unchanged at all
or enhanced. In addition to the old bands, there are three new bands
1. The orientation shown in Fig. 11b ͑Ϫ0.5 V͒ leads to a much
higher effective surface coverage than that in Fig. 11a ͑0.23 V͒. The
net effect of this molecular ‘‘flip’’ which takes place on a surface is
a stronger adsorption and higher surface coverage which would lead
to the decrease in the observed tin current at ca. Ϫ0.5 V.
Ϫ1
at 1060, 1040, and 566 cm . These new bands are unequivocally
assigned to the polyoxyethylene tail of the molecule. They began to
show up at Ϫ0.2 V, reached a maximum at ca. Ϫ0.5 V, and then
decreased at more negative potentials. The SERS bands of the hy-
drophobic head showed up at any potential studied ͑Ϫ0.8 to ϩ0.4
SERS of phenolphthalein.—Grain refiners are used in electroplat-
ing baths in conjunction with the surfactants to improve the surface
morphology. The presumed mechanism of grain refiners action is
adsorption on high energy sites of the metal surface. Figure 12
shows the SERS of one of the commonly used grain refiners, phe-
nolphthalein, on a roughened gold surface at ϩ0.2 V ͑OCP͒ and at
Ϫ0.4 V. The most striking feature in Fig. 12 is the total absence of
V͒ and reached maximum intensity also at ca. Ϫ0.5 V. The terminal
Ϫ1
␯
(C-OH) band at 886 cm was not observed at all which suggests
that the end of the tail is not in direct contact with the surface at any
potential. The bands at Ϫ0.5 V are shifted toward the lower wave-
Ϫ1
numbers by up to 8 cm as compared to those at ϩ0.23 V. This
Ϫ1
could be explained by larger reduced mass of the molecule due to
stronger interaction with the relatively heavy metal atoms, which in
turn suggests stronger ͑tighter͒ adsorption of the head groups at
Ϫ0.5 V. Larger coverage ͑adsorption͒ normally leads to the positive
shifts in the Raman frequencies because the new molecules would
have to occupy higher energy sites on the metal surface; therefore, it
can not explain the observed negative shifts in Raman frequencies.
Also, the bands at Ϫ0.5 V are considerably broader than the corre-
sponding bands at ϩ0.23 V and, again, suggests stronger metal-to-
surfactant interaction ͑adsorption͒ at Ϫ0.5 V.
normally very strong ␯(CvO) bands at 1740 cm present in the
bulk Raman or IR spectrum of phenolphthalein. This indicates that
the lactone CvO group is not close to the surface at both potentials.
Bands that are enhanced at Ϫ0.4 V are bands associated with the
1
6-18
two phenolic groups.
This data suggest that at Ϫ0.4 V, the adsorption becomes stron-
ger with the molecule moving closer to the metal surface. Bands
from the furan ring ͑lactone part͒ are either too weak to be detected
or totally absent at both potentials. Quinone tautomer form of phe-
nolphthalein is present to some degree at Ϫ0.5 V but not at ϩ0.2 V
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Journal of The Electrochemical Society, 150 ͑4͒ C202-C207 ͑2003͒
C207
as judged by a weak band at 1680 cmϪ1. Similar analysis of the data
obtained on a roughned silver surface suggested different orientation
of phenolphthalein with the lactone group in direct contact with the
metal surface.
the effective coverage is relatively low, which results in a rough tin
deposit. Triton X-100 facilitates adsorption of phenolphthalein and
interacts weakly with it through some kind of host-guest interac-
tions. The coadsorption of Triton X-100 and phenolphthalein modi-
fies the grain size and the deposit structure to impart a uniform and
smooth surface.
SERS of Triton X-100 and phenolphthalein together.—SERS
spectra obtained from a solution containing both Triton X-100 and
phenolphthalein shows only the Raman bands due to phenolphtha-
lein even though the Triton X-100 is present at a much higher con-
centration. This indicates that phenolphthalein adsorbs more
strongly than does Triton X-100. Also, in the presence of Triton
X-100, phenolphthalein bands are much more intense as compared
with an experiment with no Triton X-100. This indicates synergetic
interactions between the two species. In the presence of Triton
X-100, the band positions of phenolphthalein are shifted by ca. Ϫ5
Acknowledgments
The support of this work through the Advanced Technology Pro-
gram of NIST ͑grant 9H3021͒ is greatly appreciated.
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4
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1
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Conclusions
Combined electrochemical and SERS study provided new in-
sights on adsorption behavior of organic additives used for tin elec-
troplating. Triton X-100 is adsorbed on the electrode surface in the
wide range potential. A hydrophobic head group serves as an anchor
of the molecule to the metal surface. At negative potentials, around
Ϫ0.5 V, the molecule turns its long hydrophilic tail to the surface,
increasing the effective surface coverage. The head groups of Triton
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