Metal deposition onto a porous silicon layer by immersion plating from aqueous and nonaqueous solutions

Article abstract of DOI:10.1149/1.1498841

Immersion plating of metals (Ag, Cu, Ni) onto a porous silicon (PS) layer from aqueous and nonaqueous solutions has been studied. The modified PS layers after the immersion plating were analyzed by X-ray diffraction and X-ray photoelectron spectroscopy. Fourier transform infrared spectroscopy and scanning electron microscopy were also performed to investigate the structural changes and microstructure of PS samples after the plating process. In both solutions, the deposition of metal oxidizes PS simultaneously to SiO₂. The different deposition behaviors are discussed in terms of different rest potentials of PS in these solutions and electrode potential of each metal. Immersion plating in nonaqueous organic solutions shows that a trace of residual water affects the metal deposition. Based on the results obtained, the mechanism of metal deposition is proposed. The metal deposition proceeds by nucleation and growth via the local cell mechanism. It is also found that metal deposition proceeds very differently on Si wafer and PS surfaces. The different deposition behaviors on both surfaces are discussed.

Full text of DOI:10.1149/1.1498841

C456
Journal of The Electrochemical Society, 149 ͑9͒ C456-C463 ͑2002͒
0
013-4651/2002/149͑9͒/C456/8/$7.00 © The Electrochemical Society, Inc.
Metal Deposition onto a Porous Silicon Layer by Immersion
Plating from Aqueous and Nonaqueous Solutions
F. A. Harraz,a,z T. Tsuboi, * J. Sasano, T. Sakka, * and Y. H. Ogata *
b,
a
a,
a,
aInstitute of Advanced Energy, Kyoto University, Uji, Kyoto 611-0011, Japan
Immersion plating of metals ͑Ag, Cu, Ni͒ onto a porous silicon ͑PS͒ layer from aqueous and nonaqueous solutions has been
studied. The modified PS layers after the immersion plating were analyzed by X-ray diffraction and X-ray photoelectron spec-
troscopy. Fourier transform infrared spectroscopy and scanning electron microscopy were also performed to investigate the
structural changes and microstructure of PS samples after the plating process. In both solutions, the deposition of metal oxidizes
PS simultaneously to SiO . The different deposition behaviors are discussed in terms of different rest potentials of PS in these
2
solutions and electrode potential of each metal. Immersion plating in nonaqueous organic solutions shows that a trace of residual
water affects the metal deposition. Based on the results obtained, the mechanism of metal deposition is proposed. The metal
deposition proceeds by nucleation and growth via the local cell mechanism. It is also found that metal deposition proceeds very
differently on Si wafer and PS surfaces. The different deposition behaviors on both surfaces are discussed.
©
2002 The Electrochemical Society. ͓DOI: 10.1149/1.1498841͔ All rights reserved.
Manuscript submitted November 15, 2001; revised manuscript received March 13, 2002. Available electronically August 2, 2002.
Porous silicon ͑PS͒ is quickly becoming an important and versa-
itself is catalyzing the deposition. In addition, such a technique al-
lows the formation of metal deposits in the form of island-like struc-
ture with simplicity in operation and lower cost. In this case, con-
trolling the size and distribution of the island metal particles could
enhance the catalytic activity of the semiconductor electrodes for
tile electronic material in different fabrication technology. It was
initially observed about 45 years ago as black resist on Si after
1
electropolishing of c-Si. Since the first report on its ability to emit
2
very efficient visible photoluminescence at room temperature, sig-
1
6,17
nificant development has been made toward the understanding and
applicability of such material for electronics and the material has
photoelectrochemical cells, as has been reported.
Furthermore, it
is also possible to obtain patterned metallization of PS structure
since the deposition of some metals can be localized to the illumi-
nation area.
3
been investigated for possible optoelectronic applications.
Another interesting chemical property of PS is its ability to act as
4
a modest reducing agent. In contrast to its optical properties, much
However, in most cases the deposition of metals is accompanied
by a quenching of the photoluminescence, most likely due to the
nonradiative defects generated by the surface modification.12 Mean-
while, the PS luminescence could be retained if the deposited metal
less attention has been given to using PS as both a reducing agent
and a substrate for metal deposition. In addition, it is important to
establish good electrical contact with the volume of the PS structure
for realizing the potential applications in the microelectronics indus-
try. Furthermore, PS layers have also been proposed for potential
application as the host matrix for polymers or metals, which have
interesting optical properties.5 For example, Pt-coated n-type PS
electrodes have shown good solar energy conversion, which is one
1
3
is dissolved by acid treatment. In a different trend of PS formation,
it has been reported that the film formed by chemical etching in the
3ϩ
18
presence of Fe
improved the photoluminescence properties.
Other authors were concerned to deposit various conducting poly-
19,20
mers to PS layers.
Their results showed that after a certain time
6
of the highest for n-Si photoelectrochemical solar cells. Conse-
of immersion in different polymer-containing solutions, the hardness
and the thermal conductivity for the resulting structure were im-
proved without affecting the photoluminescence. For more details,
the different deposition methods, with special attention to the re-
quirements and specific drawbacks of each method, are reviewed in
quently, metal deposition on PS is a subject of interest.
7
Dry processes such as physical and chemical vapor deposition
are widely used to deposit metals in the field of Si technology.
However, it appears that such a process leads to poor penetration of
metal inside the pores of the PS due to what is called pore mouth
blockage. However, such blockage is not expected when applying
electrodeposition method8,9 in solution. Under the electrochemical
reaction the charge carriers are provided by the bulk silicon, and it is
expected that the deposition process can take place preferentially at
21
a recent report.
An anodic reaction occurring on the same substrate surface is
important for the immersion plating process to be completed. It is
assumed that the anodic reaction accompanying metal ion reduction
from aqueous solution is the oxidation of Si in the presence of water
the pore bottom, as in the case of the formation of the PS layer. In
14,22
addition, electroless plating10 and immersion plating,
11-14
into SiO2 .
However, due to the lack of water, the oxidation of Si
as wet
or PS is not expected to occur as a counter reaction for metal depo-
sition in nonaqueous solutions.
methods, are expected to be quite effective to deposit various met-
als. In both methods there is, in principle, no great problem of mass
transport inside the pores, which may limit the process. In recent
Methanol ͑MeOH͒, a protic solvent as water, and acetonitrile
͑MeCN͒, an aprotic solvent containing no oxygen atom, were used
as organic solvents in this study. The physicochemical properties of
1
5
work, Belyakov et al. reported that Ag metal could penetrate in-
side the PS layer during the deposition from solution. They also
observed that the structures obtained with chemically prepared metal
contacts show better photoelectric properties and more intense elec-
troluminescence compared to the structures with vacuum-evaporated
contacts.
2
3
water and these solvents are different. Therefore, different condi-
tions for metal deposition may be required. Further, our previous
2
4
work is the first report concerning metal deposition from nonaque-
ous solutions.
With respect to the reaction mechanism during the immersion
In this study the immersion plating method is highly nominated,
where no electrical contact is required as in the case of electroplat-
ing. The method has another virtue over electroless plating since no
catalyst is needed to deposit the metals of interest as the PS surface
11
plating, Jeske et al. have proposed that the reaction proceeds with
injection current multiplication and hydrogen evolution is indispens-
able for deposition. In contrast to this model, chemical processes for
the deposition reaction have been proposed by Tsuboi et al.22 How-
ever, the prior mechanism is put in question because the reaction
with electrons in the conduction band of a p-type silicon cannot
proceed without illumination and gas evolution was never observed
during our experiments. On the other hand, the latter model does not
give information either about the generation of nuclei at the initial
*
Electrochemical Society Active Member.
Present address: Canon, Incorporated, Eltran Business Center, Hiratsuka, Kana-
gawa 254-0013, Japan.
b
z
E-mail: farid@iae.kyoto-u.ac.jp
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Journal of The Electrochemical Society, 149 ͑9͒ C456-C463 ͑2002͒
stage of immersion or how the deposition proceeds after a prolonged
C457
time of exposure. In this article, this information is identified and
thoroughly discussed.
This study reports on the immersion plating of metals ͑Ag, Cu,
Ni͒ onto PS layers from aqueous and nonaqueous solutions, in order
to compare and realize the different deposition behaviors of each
metal and to investigate the effect of residual water in nonaqueous
solutions on the anodic counter reaction of the process. We did not
perform analysis to quantify the metal penetration into PS; therefore,
we do not discuss the pore filling in this study. Another objective of
this work is to clarify the detailed reaction mechanism during the
immersion plating event. In addition, the different deposition behav-
iors of the PS layer and the Si wafer are also discussed.
Experimental
The Si͑100͒ single crystals used in this study were p-type boron-
doped ͑10-20 ⍀ cm in resistivity͒. The starting wafers ͑0.6 mm
thick͒ were cut into 1.3 ϫ 1.3 cm pieces. The PS layers were pre-
pared by electrochemical anodization in a cell made of trifluoroeth-
ylene resin utilizing a current density of 2 mA cmϪ2 for 20 min. A
silicon wafer and a copper current collector were mounted at the
2
bottom of the cell by using an O-ring that allowed 0.785 cm of the
Si crystal to be exposed to the homogeneously mixed electrolyte
composed of 20 wt % HF ͑47 wt % HF:99.5 wt % ethanol
ϭ 1:1.35 in weight͒. Prior to each experiment, the Si wafers were
rinsed in acetone and dipped for a few minutes in 5 wt % HF
solution to remove any native surface oxides. The PS thus prepared
was dried by blowing Ar gas onto it. PS layers about 2 ␮m thick
were produced having porosity in the range of 75-80%. Then PS
was immersed in the solution containing different metal ions at
room temperature. AgClO , AgNO , Ag SO , CuSO , CuCl
Ϫ3
Figure 1. XRD patterns of PS after 1 min immersion in ͑a͒ 5 ϫ 10
M
Ϫ2
Ϫ2
Ag SO , ͑b͒ 10
M CuCl , ͑c͒ 10
M NiCl , and ͑d͒ untreated PS.
2
2
4
2
4
3
2
4
4
2,
Cu(NO ) , (CF SO ) Cu, NiSO , and NiCl were used as metal
3
2
3
3
2
4
2
solutions of NiCl or NiSO ; there was no change either in XRD
2 4
pattern or in color. Copper was deposited during the immersion in
salts. Purification of organic solvents, MeOH and MeCN, was per-
formed by drying over molecular sieves. The residual water was
CuCl and CuSO aqueous solutions, whereas deposition was not
2
5
2
4
measured by the conventional Karl Fischer titration method. The
detected from Cu(NO3)2 solution. Silver deposition resulted from
the immersion in Ag SO , AgClO , and AgNO solutions. Figure 1
Ϫ2
concentration of metal ions prepared was 10 M, unless otherwise
2
4
4
3
specified. The preparation and experiments in nonaqueous solutions
were conducted in a glove box under Ar gas atmosphere. After the
immersion, the PS sample was rinsed in the pure solvent and dried
by blowing Ar gas.
X-ray diffraction ͑XRD͒ patterns were measured with Cu K␣
radiation ͑1.54 Å͒ using a Rigaku RAD 2 C system with an attach-
ment for thin-film measurement. X-ray photoelectron spectroscopy
shows the XRD patterns of the PS samples after 1 min immersion in
Ϫ2
different metal ion aqueous solutions. Dipping the PS in 10
M
NiCl leads to no change in the XRD pattern. However, the pattern
2
Ϫ2
of PS after immersion in 10
M CuCl exhibits two peaks at 43.4
2
and 50.4°, corresponding to Cu͑111͒ and Cu͑200͒, respectively. Fur-
Ϫ3
thermore, the sample exposed to 5 ϫ 10
M Ag SO reveals two
2 4
peaks related to silver deposition.
͑
XPS͒ measurements were performed using a JPS-9010 MC, JEOL.
The different deposition behaviors can be discussed according to
the different electrode potentials of each metal. Half-cell reactions
Fourier transform infrared spectroscopy ͑FTIR͒ spectra were taken
by means of a Nicolet Avatar 360. A Si wafer was used as a refer-
2
6-29
and electrode potentials in different solutions
are summarized in
ence to remove Si bulk absorption. The data were averaged over 64
Ϫ
Ϫ1
Table I. ClO4 is not concerned with the reaction since it is difficult
to be reduced, although it has the highest positive potential. Nickel
has the most negative electrode potential and therefore the reduction
of nickel ions on PS surfaces is not favored. Nickel cannot deposit
on the PS by immersion plating from its simple salts. On the con-
times with a resolution of 4 cm
.
The rest potential of PS in contact with the solutions was mea-
sured using Ag/AgCl as a reference electrode. A potentiostat/
galvanostat, Princeton Applied Research model 273A, was used.
The ohmic contact on the rear side of the samples was made with an
InGa eutectic alloy. The measurements were conducted in the dark.
Current-potential curves using a Si rotating disk electrode ͑RDE͒ in
pure organic solvents were measured using an automatic polariza-
tion system, Hokuto Denko HZ-3000. A three-electrode cell was
used with an Ag/AgCl electrode as a reference and a Pt wire as a
counter electrode. The rotation rate was 2000 rpm and the scan rate
2
ϩ
trary, the potential of Cu /Cu couple is more positive than that of
2
ϩ
Ni /Ni and consequently, copper ions are more easily reduced to
Cu metal. Cu deposits under simultaneous oxidation of PS as con-
firmed by FTIR measurements ͑discussed later͒. Cu was not de-
tected during the immersion in Cu(NO ) aqueous solution although
3
2
the oxidation of PS occurred. Nitrate ion has more positive potential
Ϫ1
2ϩ
Ϫ
3
was 2 mV s . We used 0.1 M tetra-n-butylammonium perchlorate
than Cu . This means that NO ion is thermodynamically easier to
as a supporting electrolyte during the rest potential and current-
potential measurements. Scanning electron microscopy ͑SEM͒,
JSM-5600 JEOL, was used to examine and characterize the mor-
phology of the specimens after the plating process.
2ϩ
be reduced than Cu . In addition, nitrate ion can oxidize silicon,
resulting in absence of Cu deposition. The oxidation of Si in
HF/HNO solution during PS formation by stain etching is well
3
30
known. The same surface oxidation was also observed when we
used NaNO solution.
3
Results and Discussion
The deposition of Ag after dipping PS in aqueous solution of
ϩ
Immersion plating of metals from aqueous solutions.—Metal
deposition on PS was inspected by measuring the XRD pattern.
Nickel deposition was not detected on the PS surface from aqueous
Ag is also related to the positive standard potential of Ag. The
ϩ
2ϩ
potential of Ag /Ag couple is more positive than that of Cu /Cu
couple. The difference has an influence on the metal deposition.
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C458
Journal of The Electrochemical Society, 149 ͑9͒ C456-C463 ͑2002͒
Table I. Half-cell reactions and electrode potentials at 25°C in different solutions.a
Electrode potential ͑V vs. SHE͒b
MeOH
Half-cell reaction
H O
MeCN
—
Ϫ0.34
0.11
—
2
Ni2 ϩ 2e
ϩ
Ϫ
→
→
Ni
Cu
Ϫ0.32
0.28
0.68
0.40
0.78
—
0.41
0.75
—
2
ϩ
Ϫ
Cu ϩ 2e
ϩ
Ϫ
Ag ϩ e
→
Ag
Ϫ
ϩ
Ϫ
NO3 ϩ 4H ϩ 3e
→
→
NO ϩ 2H O
2
Ϫ
ϩ
Ϫ
Ϫ
ClO4 ϩ 2H ϩ 2e
ClO3 ϩ H O
—
—
2
a
The potential is referred to a standard hydrogen electrode ͑SHE͒ in an aqueous solution. In all solutions, pH is set equal to 7 and the activities of Ni2ϩ,
2
ϩ
ϩ
Ϫ
Ϫ
Ϫ
Ϫ2
Cu , Ag , NO , NO, ClO4 , and ClO3 are set equal to 10
.
3
b
References 26-29.
ϩ
trolyte. We measured the rest potential of PS in 10Ϫ2
M
AgClO4-MeOH and MeCN solutions. The Ag/AgCl reference elec-
trode was connected to the electrolyte by means of a saturated
Unlike Cu, Ag was deposited from AgNO solution because Ag
has more positive potential than NO3 ion.
3
Ϫ
The behavior of PS during the immersion in aqueous solutions is
classified into two categories on the basis of XRD and FTIR results:
KNO -agar salt bridge made of a polyethylene tube. The measure-
3
(
i) no change and (ii) silicon oxidation to SiO . The classification
ments were reproducible within a few millivolts. The result is shown
in Table III. The rest potential of PS in the MeOH solution, from
which Ag was deposited, stays at less noble ͑more negative͒ value,
while the value in the MeCN solution shifts to noble direction ͑less
negative͒, indicating the unfavorable Ag deposition. The result of
rest potential is dependent on the electrolyte composition and the
surface site reactivity of the electrode material. We used PS samples
prepared under the same conditions and the same metal ion concen-
tration; hence, the different rest potentials are mainly related to the
solvent. The potential shift in the case of MeCN solution may be due
2
of all samples is summarized in Table II. It is worth noting that the
oxidation of silicon to SiO is indispensable to the metal deposition.
2
Metal deposition from nonaqueous solutions and structural
changes.—All the preparations and plating experiments reported in
this section were conducted in a glove box under Ar gas atmosphere.
We investigated the influence of anhydrous NiSO and NiCl dis-
solved in purified MeOH and MeCN solutions on PS substrates. We
utilized 10
vealed that Ni was not detected in all solutions after the immersion
process. This behavior can also be attributed to the more cathodic
redox potential of Ni in these solutions, as mentioned in the previous
section.
Deposition of Ag onto PS was inspected by measuring XPS and
also visually. Ag deposition was commenced immediately from
MeOH solution, as evident from a metallic silver appearing on the
PS surface. On the contrary, no metal was observed from MeCN
solution. Figure 2 shows the Ag 3d and Si 2p XPS spectra of PS
4
2
Ϫ2
M metal ion concentration. XPS measurements re-
ϩ
to a complex formation with Ag ions. It should be noted that
ϩ
23
MeCN forms a stable complex with Ag ions.
CuSO and CuCl solutions of MeOH and MeCN were prepared.
4
2
Since CuSO is sparingly soluble in these solvents, we prepared a
4
Ϫ3
Ϫ4
concentration of 3 ϫ 10
MeCN solution. We prepared 10
M in MeOH and 2 ϫ 10
M in
Ϫ2
M CuCl solutions. Cu metal
2
was deposited from CuSO -MeOH solutions while only a trace of
4
Cu was observed from CuCl -MeOH solutions. Figure 3 shows the
2
5/2
Ϫ2
Cu 2p3/2 and Si 2p XPS spectra of PS immersed in these CuSO4 and
after 10 min immersion in 10
M AgClO -MeOH and MeCN so-
4
CuCl -MeOH solutions for 10 min. A sharp peak at 932.4 eV cor-
2
lutions. The charge correction was made by setting the binding en-
ergy of C 1s to 284.5 eV as an energy reference. These spectra show
that the Ag metal-related peak appears at 368.5 eV from MeOH
solution, whereas no signal is detected in the case of the MeCN
responding to Cu deposition from CuSO -MeOH solution was de-
4
tected while only a small peak from CuCl was observed. This trace
2
ϩ
solution. The reduction of Ag ions was accompanied by the oxi-
dation of PS: two peaks were detected at 99.4 and 103.0 eV, which
31
can be assigned to Si͑0͒ and SiO , respectively. This behavior is
2
due to the different electrode potentials of Ag in both solutions. The
electrode potential in MeOH solution is more positive than that in
MeCN solution by more than 0.5 V ͑see Table I͒.
The measurement of rest potential is quite interesting because
this parameter is characteristic of the anodic and cathodic reactions
taking place on the Si surface. The rest potential is the potential that
PS substrate spontaneously attains when it is immersed in the elec-
Table II. Summary of the results of immersion plating on PS
À2
from aqueous solution. PS was immersed in 10
0 min.
M solution for
1
Solute
Deposited metal
Silicon oxide
NiSO4
NiCl2
None
None
Cu
Cu
None
Ag
None
None
SiO2
SiO2
SiO2
SiO2
SiO2
CuSO4
CuCl2
Cu(NO3)2
AgNO3
AgClO4
Figure 2. Ag 3d and Si 2p XPS spectra of PS after 10 min immersion in
5/2
Ϫ2
Ag
10
M AgClO of ͑a͒ MeOH and ͑b͒ MeCN solutions.
4
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Journal of The Electrochemical Society, 149 ͑9͒ C456-C463 ͑2002͒
C459
Table III. Rest potentials „V vs. AgÕAgCl… of PS immersed in 0.01
a
M of „CF SO … Cu and AgClO in MeOH and MeCN solutions.
3
3
2
4
Organic solvent
System
Rest potential, V
Deposited metal
MeOH
MeCN
MeOH
MeCN
Cu
Cu
Ag
Ag
Ϫ0.20
ϩ0.34
Ϫ0.62
Ϫ0.10
Yes
No
Yes
No
a
0
.1 M tetra-n-butylammonium perchlorate was added as a supporting
electrolyte.
amount of Cu deposition is attributed to the inhibition of Cu depo-
Ϫ
14
sition by Cl ions. Tsuboi et al. have investigated that Cu deposi-
tion from its halide aqueous solution was inhibited as the concen-
tration of halide ions increased. They attributed the inhibition to the
reversible contact adsorption of halogen ions on the PS surface. We
Ϫ
have investigated in detail the effect of Cl ions on Cu deposition
onto PS from a methanol solution,32 and we attributed this inhibition
to the stabilization of Cu͑I͒ species in this solution. From the XPS
measurements it is also revealed that the deposition of Cu was ac-
companied by the oxidation of Si. The oxidation of Si was also
confirmed by measuring FTIR spectra, as explained latter.
Figure 4. Current density-potential curves of Si RDE in ͑a͒ pure MeOH
͑
͒
and ͑b͒ pure MeCN ͑• • • •͒ containing 0.1
M tetra-
n-butylammonium perchlorate as a supporting electrolyte. The rotation rate
Ϫ1
On the contrary, no Cu was detected from MeCN solution con-
and scan rate were 2000 rpm and 2 mV s , respectively. The scans were
taining either CuSO or CuCl . Since we could not prepare the
started at the OCP.
4
2
Ϫ2
10
M CuSO in organic solutions and CuCl seems to inhibit the
4 2
deposition, we used trifluoromethanesulfonic acid copper͑II͒ salt,
(
CF SO ) Cu, to prepare other solutions. The concentration of Cu
pared with that in MeOH solution, ͑see Table I͒ might be related to
the absence of Cu deposition from MeCN solution.
3
3 2
Ϫ2
ions was 10
M. We carried out the immersion plating process
under the same experimental conditions. Copper deposition was in-
spected by XPS, XRD, and visually. Copper was found to deposit
from MeOH solution, whereas no metal was observed from MeCN
solution. We also measured the rest potential of PS in these solu-
tions. The results, given in Table III, show that the rest potential of
In addition to the two factors mentioned ͑rest potential of PS and
redox couple of each metal͒ that affect and determine the deposition
behavior in organic solvents, one should not neglect the influence of
the different physicochemical properties of both solvents. As men-
tioned earlier, MeOH is classified as a protic solvent, whereas
MeCN is aprotic, containing no oxygen atom. MeOH has a greater
Gutmann’s donor number than MeCN ͑20.0 and 14.1,
respectively͒, i.e., MeOH has a higher donor ability ͑basicity͒. As
a consequence, the solvation power of MeOH toward ions is greater
than MeCN and it is quite similar to water ͑MeOH molecules can
donate one and accept two hydrogen bonds͒. In view of these facts,
it is expected that the solvation states of metal ions ͑Cu or Ag͒ in the
two solvents are not similar and consequently, this may also have an
effect on the deposition behavior.
2
ϩ
PS in MeOH solution containing Cu ions stays at less noble po-
tential, ͑Ϫ0.2 V vs. Ag/AgCl͒, indicating the possibility of Cu
deposition, while the value in MeCN solution shifts to the noble
direction, (ϩ0.34 V), explaining the inhibition of metal deposition.
It is most plausible that the observed potential shift stems from a
complex formation with Cu ions. It is also reported that MeCN
3
3
23
forms a stable complex with Cu͑I͒ ions. In addition, the negative
value of the potential of Cu2ϩ/Cu couple in MeCN solution com-
It is also of interest to note that Cu was found to deposit much
faster on the PS from MeOH solution than aqueous solution. The
amount of deposited Cu from both solutions for the same Cu ion
concentration and immersion time was estimated by inductively
coupled argon plasma emission spectroscopy ͑ICP͒ technique. The
2
ϩ
potential of Cu /Cu couple in MeOH solution is more positive than
that in aqueous solution. This means that Cu ions are more easily
reduced in MeOH than in aqueous solution thermodynamically.
The average level of residual water was 30 ppm in CuSO and
4
(
CF SO ) Cu-MeOH solutions, while it was around 50 ppm in
3 3 2
CuCl -MeOH solution according to Karl Fischer titration. The con-
2
centration of 50 ppm water content corresponds to about 2
Ϫ3
ϫ 10
M and hence, the water concentration in the organic solu-
tions cannot be neglected. We applied vacuum distillation to im-
prove the residual water content after drying over molecular sieves.
However, further reduction of water content was not attained.
Current-potential curves using a Si RDE were measured to investi-
gate the electrochemical behavior of pure MeOH and MeCN sol-
vents. We used 0.1 M tetra-n-butylammonium perchlorate as a sup-
porting electrolyte during the measurements. The result is depicted
in Fig. 4. It reveals that both solvents exhibit a wide potential win-
dow during the measurements. The oxidation of MeOH starts at
potentials higher than ϩ0.3 V vs. Ag/AgCl. A similar oxidation
Figure 3. Cu 2p and Si 2p XPS spectra of PS immersed in ͑a͒ 3
3
/2
Ϫ3
Ϫ2
34
ϫ 10
M CuSO and ͑b͒ 10
M CuCl -MeOH solutions for 10 min.
potential of MeOH was reported in aqueous solution. The rest
4
2
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C460
Journal of The Electrochemical Society, 149 ͑9͒ C456-C463 ͑2002͒
ing appeared in both solutions. No absorption at 2190-2250 cmϪ1
assigned to O SiH , is observed. The absorption intensities related
,
y
x
Ϫ1
to SiH , namely, at 630-680 cm , are decreased. The disappear-
ance of the absorption at 2090-2150 cm assigned to SiH after
x
Ϫ1
x
the immersion in CuSO -MeOH and aqueous solutions is also ob-
4
served. However, the same absorption band is slightly decreased
after the immersion in CuCl -MeOH solution. It is also of interest to
2
note that the different SiuOuSi peak intensities, Fig. 5a and c,
indicate that in CuCl -MeOH solution PS is more slowly oxidized
2
compared to CuSO -MeOH solution, and this result corresponds
4
well to the result obtained by XPS measurements ͑see Fig. 3͒ in
which only small peak-related Cu deposition is detected due to the
Ϫ
inhibition by Cl ions. Furthermore, the appearance of an absorption
Ϫ1
peak at 1100 cm for all the samples immersed in aqueous and
nonaqueous solutions of Cu ions demonstrates that the oxidation of
PS accompanies the deposition of Cu metal from the solutions.
Mechanism of metal deposition onto PS surface.—In order to
clarify the reaction mechanism of metal deposition onto the PS sur-
face, a series of SEM images of Cu deposition from 0.01 M
(
CF SO ) Cu-MeOH solutions was examined at different immer-
3 3 2
sion times. The SEM images obtained are shown in Fig. 6. At the
early stage of immersion, after 5 s, many Cu particles in the order of
120-180 nm diam were detected on the surface of PS, Fig. 6a. A
prolonged immersion for 30 s produced a size increase of the Cu
crystals, in the range of 500 nm, as clearly observed in Fig. 6b.
Further morphological investigation revealed that the Cu nuclei in-
crease rapidly both in number and size as the deposition time pro-
ceeds. After 5 min immersion in the same solution, the PS surface is
almost completely covered by Cu metal, as clearly seen in the SEM
image of Fig. 6c.
Figure 5. Transmission FTIR spectra of PS after 10 min immersion in ͑a͒
Ϫ3
Ϫ2
3
ϫ 10
M CuSO -MeOH solution, ͑b͒ 10
M CuSO -aqueous solu-
4
4
Ϫ2
tion, ͑c͒ 10
M CuCl -MeOH solution, and ͑d͒ freshly prepared PS.
2
potential of PS in MeOH vs. solution, where Cu and Ag were de-
posited, is Ϫ0.2 and Ϫ0.62 V ͑Ag/AgCl͒, respectively, as shown in
Table III. These results suggest that MeOH is a stable solvent and is
not expected to be involved in the reaction where the reduction of
metal ions occurs. Furthermore, the immersion of PS in all MeCN
solutions containing metal ions, in which the water content was
Copper ions are able to withdraw electrons from the PS sub-
strate, and thus deposition occurs. Here we consider that the reduc-
2ϩ
tion of Cu proceeds through the galvanic displacement of PS in
2ϩ
the plating bath. In other words, Cu gains electrons released by
the electrochemical oxidation of PS to be reduced and deposit on the
surface. A similar reductive deposition of Cu and Ag on PS from
21-26 ppm, led to oxidation of Si, although the solvent does not
43,44
aqueous solutions has been reported.
contain oxygen atoms ͑see Fig. 2, for example͒. Accordingly, we
concluded that the trace of residual water in the organic solutions
strongly affects the deposition process.
The oxidation of PS and the reduction of metal ions were found
to occur simultaneously and can be explained by the following
14,22
coupled redox reaction
Changes in FTIR spectra after immersion plating.—The trans-
mission mode of FTIR spectra can provide sensitive information
concerning the surface species. However, in order for this technique
to work well, many surface sites must be available. PS layers satisfy
this condition due to the very large surface area, and that is why the
Oxidation
ϩ
Ϫ
Si ϩ 2H O → SiO ϩ 4H ϩ 4e
͓1a͔
͓1b͔
͓2a͔
͓2b͔
2
2
and
structure of the PS surface has been extensively investigated by
transmission FTIR spectroscopy.3
5-38
The spectra were measured on
ϩ
Ϫ
SiH ϩ 2H O → SiO ϩ ͑4 ϩ x͒H ϩ ͑4 ϩ x͒e
x
2
2
freshly prepared samples to investigate the structure changes after
immersion in different solutions.
Reduction
Figure 5 shows the spectra of an as-prepared PS layer and
Cu2 ϩ 2e → Cu
ϩ
Ϫ
PS immersed in CuSO -MeOH and aqueous solutions and
4
CuCl -MeOH solution for 10 min. FTIR spectra of PS and its oxi-
2
or
dized state have been investigated and the band assignments have
3
9-41
ϩ
Ϫ
been proposed.
As-formed ͑fresh͒ PS displays bands associated
Ag ϩ e → Ag
with surface hydrides: the spectra show distinctive triplets at
Ϫ1
Ϫ1
2
090-2150 cm , a sharp absorption at 910 cm , and strong ab-
The oxidation of PS was confirmed by FTIR spectra, and in all cases
the form of silicon oxide is SiO , as revealed by XPS measurements
Ϫ1
sorption at 630-680 cm , Fig. 5d. This absorption is due to SiHx
species. Absence of the absorption at 1100 cmϪ1 due to SiuOuSi
stretching demonstrates that the oxidation does not proceed for the
2
͑peak at about 103 eV in Fig. 3 and 4͒. The formation of a SiO2
layer was also reported as the product of Cu deposition on Si from
solutions containing Cu as traces of impurity.
On the basis of the morphological analysis obtained by SEM and
the information received from XPS and FTIR, it is revealed that the
oxidation of PS and the deposition of Cu occur from the very be-
ginning of immersion. The detailed mechanism of metal deposition
freshly prepared sample; the spectra was measured soon after the PS
2ϩ
45,46
Ϫ1
formation. Further, absorption at 810 cm
corresponding to Si-Fx
stretching does not appear, indicating that the PS layer contains no
fluorine. This result is in good agreement with the result obtained by
sputter depth profile measurements.42
A marked change in the spectra is observed for the PS immersed
͑Cu or Ag͒ is illustrated in Fig. 7. The metal deposition is suggested
2
ϩ
in aqueous and nonaqueous solutions containing Cu . An absorp-
to initiate from a finite number of nucleation sites which is depen-
dent on the properties of PS surface and the plating bath constitu-
Ϫ1
tion extending from 1000 to 1200 cm due to SiuOuSi stretch-
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Journal of The Electrochemical Society, 149 ͑9͒ C456-C463 ͑2002͒
C461
Figure 6. Plan-view SEM images of PS layers after immersion in 0.01 M
(
CF SO ) Cu-MeOH solutions for different periods: ͑a͒ 5 s, ͑b͒ 30 s, and
3 3 2
͑
c͒ 5 min immersion.
ents. The generation of nuclei is observed at the early stage of im-
mersion ͑step 2 and 3͒. As the exposure time proceeds, the electrons
released by the anodic oxidation of PS are supplied to the metal ions
through the deposited metal, resulting in the growth of crystals ͑step
underneath act as local anodes from which the electrons are supplied
to the metal ions through the deposited metal or intergrain bound-
aries. Based on these observations, the deposition process proceeds
by nucleation and growth via the local cell mechanism.
4
͒. Consequently, the immersion plating process proceeds by a
Comparison of metal deposition on silicon wafer and PS
surfaces.—To investigate the possibility of metal deposition onto
p-type Si͑100͒ wafers, immersion plating under the same experi-
mental conditions was carried out in nonaqueous solutions. Metal
plating inspected by the naked eye revealed no deposition. However,
XPS measurements revealed that a minute amount of Cu and Ag
was adhered on the Si surface from MeOH solution accompanied by
oxidation of silicon. A similar cementation process was also reported
nucleation and growth mechanism.
The thickness of the deposited Cu layer on PS was estimated
according to the following formula
t ϭ m/͑a ϫ d͒
͓3͔
where m is the mass of deposited Cu as measured by ICP, a is the
geometrical surface area of the PS, and d is the density of bulk Cu
metal. The estimated thickness after 5 min immersion was found to
be 0.38 ␮m. The interesting question is now to understand how this
thick Cu layer is formed after 5 min immersion. This result cannot
be explained by the general corrosion-type mechanism, since the
deposition reaction is inhibited and ceased after a short time of
immersion, when the PS surface is no longer in contact with the
solution. Therefore, we suggest that the immersion plating process
should proceed via the local cell type scheme rather than the general
corrosion type at this stage, although both the mechanisms may be
possible at the early nucleation and growth stages. The deposited
nuclei act as local cathodes for further crystal growth. On prolonged
immersion the coalescence of the nuclei occurs and hence, the ex-
posed area of the PS substrate is reduced. Then the unoxidized PS
4
7
on c-Si in aqueous solution.
In order to demonstrate the different deposition behaviors, Fig. 8
shows plan-view SEM micrographs of 2 min Ag deposition from
0.01 M AgClO -MeOH solution on bare Si and PS surfaces. As seen
4
in Fig. 8a, Ag crystals, 100-180 nm diam, are deposited on the Si
wafer surface in the form of island-like structures. However, in Fig.
8b it is apparent that the density of Ag nuclei considerably increased
both in number and size when the substrate is PS. These results
show that much more metal plating occurs on the porous surface
compared to that on the Si wafer. In other words, metal deposition
proceeds very differently on bare Si and PS surfaces.
Metal deposition by immersion plating takes place at the open-
circuit potential ͑OCP, no current͒ of the substrate upon dipping in
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C462
Journal of The Electrochemical Society, 149 ͑9͒ C456-C463 ͑2002͒
Figure 7. Schematic representation of the mechanism of metal deposition
onto PS substrate during the immersion plating.
Figure 8. Plan-view SEM micrographs of Ag deposits after 2 min immer-
the plating bath. Since the rest potential is dependent on the electro-
lyte composition and the surface reactivity of the electrode material,
the measurements of the rest potential may provide good informa-
tion concerning the different deposition behaviors between PS and
Si electrodes. We measured the rest potential of both substrates im-
mersed in 0.01 M (CF SO ) Cu and 0.01 M AgClO -MeOH solu-
tions. The results are shown in Table IV. An inspection of the data
shows that the rest potential of PS in both solutions has a more
negative value ͑less noble potential͒ compared to that of Si wafers.
Less noble potential obtained for the PS indicates the promotion of
the anodic reaction, i.e., the oxidation of PS. Historically, it has been
noted since its discovery that PS layers are significantly more
chemically reactive than bulk Si. Upon exposure to hot wet air or
metal ion solutions, the freshly prepared PS samples are oxidized
rapidly. The speed and extent toward oxidation is much higher com-
pared to Si wafers.
sion in 0.01 M AgClO -MeOH solutions on an ͑a, top͒ p-Si͑100͒ wafer and
4
͑
b, bottom͒ a PS substrate prepared on the same wafer type. The deposition
was carried out under identical experimental conditions.
3
3
2
4
ing PS formation.49,50 (v) Quantum confinement caused by the
nanometer-size structure in PS.
Since we treated the Si wafers with 5% HF solution prior to the
immersion plating, the wafer surface is also terminated by hydrogen
atoms. We expect no significant difference in the states of silicon
hydrides between the two surfaces and consequently, this cannot be
1
4
the direct reason for the rest potential shift. Our research group
and others
4
5,51
have observed no major difference in metal deposi-
According to these observations, the question arises as to under-
stand the difference in rest potential values and why metal deposi-
tion is faster on PS than on Si surfaces under the same conditions.
The different behaviors in the deposition between PS and Si wafers
is unclear. A detailed interpretation of the observed results is diffi-
cult; however, some distinguishable points between the two surfaces
can be outlined as follows: (i) PS differs from Si wafers by its high
porosity ͑in this work 75-80%͒. (ii) The specific surface area of PS
Table IV. Rest potentials „V vs. AgÕAgCl… of PS and Si wafer in
a
0
.01 M of „CF SO … Cu and AgClO -MeOH solutions.
3
3
2
4
Metal ion solution
Substrate
Rest potential, V
Cu ion solution
PS
Si
PS
Si
Ϫ0.20
Ϫ0.11
Ϫ0.62
Ϫ0.02
Ag ion solution
2
3 48
is known to be very large; of the order of 200 m /cm . (iii) The
presence of a great amount of silicon hydrides on PS surface. (iv)
The depression of dopant; the dopant dissolves in HF solution dur-
a
0.1 M tetra-n-butylammonium perchlorate was added as a supporting
electrolyte.
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Journal of The Electrochemical Society, 149 ͑9͒ C456-C463 ͑2002͒
C463
tion when they used different types of dopant, and hence, the de-
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Kyoto University assisted in meeting the publication costs of this article.
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