Mechanism of the chemical deposition of nickel on silicon wafers in aqueous solution

Article abstract of DOI:10.1149/1.1391778

The deposition of metallic nickel on n-Si(100) wafers was performed without external potential control in aqueous NiSO₄ solutions of different compositions at pH 8.0. Without giving any catalyzation treatment, the deposition of nickel on hydrogen-terminated Si(100) was confirmed in a conventional electroless plating bath containing NaH₂PO₂ as the reducing agent, sodium citrate as the complexing agent, and (NH₄)₂SO₄ as the buffering agent. The deposition of nickel was found to take place also in a bath without the reducing agent, and even in a simple solution consisting of NiSO₄ and (NH₄)₂SO₄. By using a transmission electron microscope equipped with an energy dispersive X-ray spectrometer, the cross sections of the films deposited from these solutions were examined, which revealed formation of silicon oxide between the Ni deposit and Si substrate. Based on these results, the mechanism of the entire process of electroless Ni deposition on Si is discussed.

Full text of DOI:10.1149/1.1391778

Journal of The Electrochemical Society, 146 (4) 1407-1411 (1999)
1407
S0013-4651(98)06-101-1 CCC: $7.00 © The Electrochemical Society, Inc.
Mechanism of the Chemical Deposition of Nickel on Silicon Wafers in Aqueous Solution
Nao Takano, Naohiro Hosoda, Taro Yamada,* and Tetsuya Osaka*,z
Department of Applied Chemistry, School of Science and Engineering, Kagami Memorial Laboratory for Materials Science and
Technology, Waseda University, Tokyo 169-8555, Japan
The deposition of metallic nickel on n-Si(100) wafers was performed without external potential control in aqueous NiSO solu-
4
tions of different compositions at pH 8.0. Without giving any catalyzation treatment, the deposition of nickel on hydrogen-termi-
nated Si(100) was confirmed in a conventional electroless plating bath containing NaH PO as the reducing agent, sodium citrate
2
2
as the complexing agent, and (NH ) SO as the buffering agent. The deposition of nickel was found to take place also in a bath
4
2
4
without the reducing agent, and even in a simple solution consisting of NiSO and (NH ) SO . By using a transmission electron
4
4 2
4
microscope equipped with an energy dispersive X-ray spectrometer, the cross sections of the films deposited from these solutions
were examined, which revealed formation of silicon oxide between the Ni deposit and Si substrate. Based on these results, the
mechanism of the entire process of electroless Ni deposition on Si is discussed.
©
1999 The Electrochemical Society. S0013-4651(98)06-101-1. All rights reserved.
Manuscript submitted June 29, 1998; revised manuscript received November 15, 1998.
Ϫ3
Various techniques are available for forming metal films on sub-
strates, such as chemical vapor deposition (CVD), sputtering, evapo-
ration, electrodeposition, and electroless deposition. Among these
techniques, electroless deposition has been attracting attention be-
cause of its simplicity in operation and its low cost. In the semicon-
ductor-device industry, attempts have been made to utilize the method
immersion in an aqueous PdCl solution (0.1 g dm of PdCl in
2
2
dilute HCl) for 10 s immediately before immersion in the electroless
deposition bath.
For the chemical deposition of nickel, three types of baths listed
in Table I were employed. Bath A was a typical NiP electroless depo-
sition bath, operated at 80ЊC and pH 8.0 adjusted with NH OH.
The reducing agent, NaH PO , was excluded in baths B and C. Bath
B contained Ni with sodium citrate as the complexing agent for
Ni , and (NH ) SO as the buffering agent. Bath C contained Ni
with only (NH ) SO and no complexing agent. Baths B and C were
both operated at the same condition as bath A. All deposition experi-
ments were performed on a class 100 clean bench.
17,18
4
1,2
of electroless deposition for delineating semiconductor junctions,
2
2
3
4
2ϩ
making ohmic contacts, and micropatterning integrated circuits.
5
,6
2ϩ
2ϩ
In the past several years, filling via holes and trenches have
been attempted for producing ultralarge scale integration (ULSI) in-
terconnects. In this case, electroless deposition is more attractive
than dry processes such as CVD and sputtering because of the sim-
plicity and the via hole filling ability of the former method.
4
2
4
4
2
4
7
,8
The deposited specimens were examined by using both a field
emission transmission electron microscope (FETEM; Hitachi, Ltd.,
HF-2000) equipped with an energy dispersive X-ray spectrometer
Furthermore, metal dots of approximately 10 nm diam can be fab-
ricated on Si wafers by electroless deposition, which has a great poten-
Ϫ2
tial for realization of ultrahigh density (over 1 terabit cm ) read-only
(
EDX; Kevex Instruments, Inc., Sigma), and a scanning electron
9
memory (ROM) or random-access memory (RAM) devices. Ion
microscope (SEM; Hitachi, Ltd., S-2500CX). The samples for the
cross-sectional FETEM observation were prepared by a submicro-
meter-scale fabrication technique using a focused ion beam machine
FIB; Hitachi, Ltd., FB-2000A). An electron probe microanalyzer
EPMA; JEOL Ltd., JXA-8600) was used for elemental analysis of
implantation by using a focused ion beam apparatus was found to
10
modify the silicon surface and initiate electroless gold deposition.
We believe that such a phenomenon can be utilized to make small
metal dots on silicon surfaces.
(
(
A project entitled “Wafer-Scale Formation Process of Quantum
Dots” was started under the sponsorship of the Research for the
Future project, The Japan Society for the Promotion of Science. As
part of the initial phase of this project, we have investigated the pos-
sibility of depositing small metal dots on silicon by wet processes.
the deposits.
Results and Discussion
Ni deposition on an uncatalyzed silicon wafer.—The nature of
nickel deposition on Si wafer was first examined. First, NiP elec-
troless plating was performed on a Si wafer with the catalyza-
1
1
Results of our preliminary investigation were reported elsewhere.
Electroless metal deposition on Si wafer was investigated by sev-
1
7,18
tion.
When bath A was used, phosphorus was included in the
2
,5,12-15
deposit because this bath contained NaH2PO2 as the reducing agent.
Therefore, we use the terms NiP and Ni to differentiate the deposits
produced in bath A from those produced from baths B and C,
respectively.
Although the NiP film was clearly formed in bath A, the film
peeled off easily upon rinsing in water. On the other hand, when
plating was done in bath A without catalyzation, a more adherent
eral researchers.
In most of those studies, an HF-containing
In this study, an alkaline aqueous solu-
2
,12-15
electrolyte was used.
tion without FϪ was used instead, and the nickel deposition mecha-
nism on silicon was examined. For the purpose of the mechanistic
study described in this paper, the composition of the electroless
deposition bath was simplified to clarify the mode of participation of
Si in the metal deposition. The electron microscopic analyses were
performed to observe the surfaces and cross sections of specimens.
Experimental
The substrates used were n-type Si(100) wafers (phosphorus-
doped with a resistivity of 8 to 12 ⍀ cm, Shin-Etsu Handotai Co.,
Ltd.). The specimens were 2 ϫ 2 cm square pieces cut out of the
Table I. Bath compositions and operating conditions.
Chemicals (mol dmϪ3
)
Bath A
Bath B
Bath C
1
6
wafer. They were cleaned by the RCA method, which is one of the
standard cleaning procedures for Si wafers. After cleaning, these
wafers were rinsed in distilled water with the resistivity of 1.5 ϫ
NiSO и6H O
0.10
0.15
0.50
0.20
0.10
—
0.50
0.20
0.10
—
0.50
—
4
2
NaH PO иH O
6
6
2
2
2
1
0 ϳ 2.0 ϫ 10 ⍀ cm, which was used in all stages of experiment.
(
NH ) SO
4
2
4
When necessary, catalyzation of Si wafer was performed by simple
Sodium citrate
*
Electrochemical Society Active Member.
E-mail: osakatet@mn.waseda.ac.jp
pH was 8.0 (adjusted with NH4OH).
Temperature was 80ЊC for all baths.
z
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1
408
Journal of The Electrochemical Society, 146 (4) 1407-1411 (1999)
S0013-4651(98)06-101-1 CCC: $7.00 © The Electrochemical Society, Inc.
metallic film was deposited on the wafer, and this film was con-
firmed to consist mainly of nickel and phosphorus by EPMA. In the
latter case, the metallic film did not peel off upon rinsing. From the
SEM images in Fig. 1, the formation of an NiP deposit from bath A
was confirmed on the uncatalyzed wafer. This result suggests that
the nickel deposition in this case did not proceed by the convention-
al electroless plating mechanism, i.e., silicon itself is not a catalyst
for the electroless nickel deposition reaction, and, nevertheless, the
deposition reaction started without catalyzing the silicon.
To understand the initial deposition reaction of electroless NiP
plating on uncatalyzed Si wafer, we prepared baths B and C without
the reducing agent of NaH PO . In bath C, sodium citrate was also
2
2
excluded (see Table I).
Figure 2 shows SEM micrographs of Si(100) after treatment in
bath B. The deposition rate in bath B was much slower than that in
bath A, as clearly seen by comparing the ranges of immersion time
used to obtain deposits shown in Fig. 1 and 2 (see respective figure
captions). Nevertheless, nickel deposition did clearly take place
from bath B, as was confirmed by EPMA. In the case of bath B, the
deposited grains did not grow in size as significantly as did those
from bath A.
From these results, we believe that the mechanism of the initial
deposition of nickel on Si wafer is different for the two different cases,
namely, with and without the presence of the palladium catalyst.
Figure 1. SEM micrographs of NiP deposits produced on RCA-pretreated
Si(100) from bath A (electroless NiP bath) at pH 8.0 and 80ЊC at various
deposition times: (a) 10, (b) 30, and (c) 60 s.
Figure 2. SEM micrographs of Ni deposits produced on RCA-pretreated
Si(100) from bath B (without reducing agent) at pH 8.0 and 80ЊC at various
deposition times: (a) 5, (b) 10, and (c) 30 min.
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Journal of The Electrochemical Society, 146 (4) 1407-1411 (1999)
1409
S0013-4651(98)06-101-1 CCC: $7.00 © The Electrochemical Society, Inc.
due to the electrochemical reduction of Ni2ϩ. It is seen that the Ni
deposition from bath B, which contained sodium citrate, occurred at
more negative potentials than that from bath C, indicating that the
electrodeposition of Ni is retarded by the complexation with citrate
in bath B. This result corresponds well to the chemical deposition
rates indicated in Fig. 2 and 3.
The quantity of Ni deposit obtained from bath C was clearly
greater than that from bath B. Figure 3 shows SEM micrographs of
the Si(100) surface treated in bath C. This result implies that the
reduction of nickel ion is restrained by citrate anions, which have a
2
ϩ
strong ability to coordinate with Ni
.
To clarify this effect of citrate, electrochemical measurements
were performed in the potentiostatic mode with a Ag/AgCl reference
electrode and a Pt wire counter electrode. Figure 4 shows current-
potential curves obtained with a Si(100) electrode in baths B and C.
The dotted line represents background current in the electrolyte con-
In view of the results obtained with baths B and C, it is apparent
that the reaction of the initial Ni deposition from bath A does not
involve the reducing agent, NaH PO , i.e., it should be similar to the
2
2
reaction in baths B and C. After the initial stage, the NiP deposition
in bath A must proceed by the autocatalytic reaction on deposited
metallic Ni involving the reducing agent.
2
ϩ
taining no Ni . The cathodic currents in baths B and C are, hence,
The mechanism of chemical nickel deposition on silicon wafer.—
2ϩ
The electrodeposition of Ni from Ni takes place with the transfer of
electrons supplied by an external source of current. On the other hand,
for the chemical deposition of Ni on Si, an alternate mechanism to
supply electrons must be considered. We hypothesized three different
mechanisms described below.
The first conceivable mechanism is that the nickel deposition
takes place through the displacement of surface hydrogen atoms that
terminate the Si surface. This mechanism is schematized in Fig. 5a.
Calculations using Nernst equations show that the redox potential
2
ϩ
for the Ni (aNi2ϩ ϭ 0.1)-Ni couple is Ϫ0.27 V vs. normal hydrogen
electrode (NHE) at 80ЊC, and that of the hydrogen elimination reac-
•
ϩ
Ϫ
tion (ϵSi–H r ϵSi ϩ H ϩ e ) is Ϫ0.73 V vs. NHE at pH 8.0 and
1
9
8
0ЊC. Although the mechanism of the chemical deposition by gal-
vanic displacement is complex and involves various other factors,
the above calculated potential values indicate that nickel deposition
2
ϩ
by the galvanic displacement between Ni and ϵSi–H is thermo-
dynamically feasible. In this mechanism, it is anticipated that the Ni
atoms produced initially should form a monolayer. Our experimen-
tal results, however, show that the Ni layers formed in bath B or C
were not single monolayers as is clearly seen in Fig. 2 and 3. Thus,
this mechanism can hardly be responsible for the major initial depo-
sition reaction. It is still possible that the Ni deposition at the very
2
ϩ
initial stages occur by displacement of terminating H atoms by Ni
.
The second mechanism we considered is based on the photo-
chemical reaction as represented by the schematic drawing in
Fig. 5b. In this mechanism, Ni is deposited cathodically by the elec-
trons which are made available through the photoexcitation in sili-
Figure 3. SEM micrographs of Ni deposits produced on RCA-pretreated
Si(100) from bath C (without reducing agent and complexing agent) at pH 8.0
and 80ЊC at various deposition times: (a) 5, (b) 10, and (c) 30 min.
Figure 4. Current-potential curves of Si(100) in baths B and C, and back-
ground curve, all recorded at pH 8.0 and 80ЊC with 2 mV s . The Ag/AgCl
reference electrode was also maintained at 80ЊC.
Ϫ1
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1
410
Journal of The Electrochemical Society, 146 (4) 1407-1411 (1999)
S0013-4651(98)06-101-1 CCC: $7.00 © The Electrochemical Society, Inc.
con. Simultaneously, O is generated by the anodic reaction since
diameter in the EDX analysis was 0.5 nm. Figure 6b displays a high-
er magnification FETEM image of the part pointed out by the arrow
in Fig. 6a. Elemental analysis was carried out at five different loca-
tions marked 1 through 5 in Fig. 6b. The results of EDX analysis are
summarized in Fig. 7. It is clearly seen that a significant amount of
oxygen was present between the Si substrate and the Ni deposit.
These results indicate that the white area located between Si and Ni
in Fig. 6a mainly consisted of silicon oxide with a thickness of ca.
20 nm. Such a silicon oxide layer was not found on the bare n-
Si(100) surface prepared by the RCA process, which includes treat-
ment with an aqueous solution of HF. For a specimen immediately
after the HF treatment, it was confirmed by Auger electron spectro-
scopy that there was no silicon oxide layer present on the surface.
From these results, we emphasize that the formation of the sili-
con oxide layer is a direct result of the chemical deposition of nick-
el in the absence of the reducing agent. In Fig. 8, the detailed mech-
anism of nickel deposition is illustrated. After the nucleation of nick-
el (steps 2 and 3), the electrons released by the oxidation of silicon
are supplied to nickel ions through the deposited nickel metal
(step 4), resulting in the growth of crystals of nickel metal. The oxide
layer which was observed between the Ni deposit and the substrate
(Fig. 6a) seems to be produced as a result of the reaction of oxidized
Si with OHϪ ions, which is promoted by Ni ions. These results
show that the reactivity of Si toward the galvanic displacement reac-
tion plays an important role in the chemical deposition of Ni. It is of
interest to note that, very recently, Kim et al. found that the deposi-
2
electrons are injected from hydroxide ions into the valence band of
silicon. To investigate the applicability of this mechanism, experi-
ments were performed in the dark and under illumination of room
light. If this mechanism were operative, Ni deposition would hardly
occur in the dark. However, no difference was noted between the two
cases of with and without illumination. This result shows that the
second mechanism is not valid.
2ϩ
The third mechanism we considered is that the reduction of Ni
proceeds through the galvanic displacement of silicon in the alkaline
aqueous solution. In other words, nickel ion receives electrons re-
20
leased by the electrochemical oxidation of Si as schematically illus-
trated in Fig. 5c. To verify this mechanism, it was thought necessary
to identify the final product of Si after ejection of the electrons. It was
considered that significant information should be obtainable by
examining the cross section of the interface between the silicon sub-
strate and the nickel deposit in the initial stages of the reaction.
To observe the reaction products in detail, the cross section of a
sample obtained from bath C was investigated by FETEM (the accel-
erating voltage used was 200 kV). The minimum electron-beam
diameter of FETEM was 5 nm. The sample for cross-sectional
FETEM observation was prepared by using FIB, and it was fabricat-
ed with a thickness of less than 0.1 ␮m. The images are shown in
Fig. 6a. Note that the black area in the upper part of the image shows
the tungsten which was deposited by CVD to protect the sample sur-
face from ion etching. The white area under the tungsten is the car-
bon layer evaporated to make the surface conductive for scanning
ion beam irradiation. The gray area at the bottom portion of the
image is the Si substrate, and the black area between carbon and Si
is the Ni deposit.
2ϩ
An elemental analysis of this interfacial region was carried out by
FETEM equipped with EDX. The minimum electron-beam spot
Figure 5. Three plausible mechanisms considered for the Ni deposition on
silicon surfaces without reducing agent: (a) displacement of surface H-termi-
nated Si surface, (b) the photochemical reaction, and (c) anodic oxidation of
silicon and cathodic reduction of nickel ion.
Figure 6. (a) Cross-sectional FETEM image of Si(100) obtained from bath
C, (b) higher magnification FETEM image. The numbers represent locations
where elemental analysis was carried out. The minimum electron-beam
diameter used in the FETEM observation was 5 nm.
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Journal of The Electrochemical Society, 146 (4) 1407-1411 (1999)
1411
S0013-4651(98)06-101-1 CCC: $7.00 © The Electrochemical Society, Inc.
Figure 7. Peak height ratios of EDX spectra of Fig. 6. The numbers on
abscissa correspond to those in Fig. 6b. The minimum electron-beam spot
diameter in the EDX analysis was 0.5 nm.
tion of Cu on Si from solutions containing Cu2 ions as an impuri-
ϩ
2
1,22
ty is accompanied by the formation of an SiO layer.
2
On the basis of all of the results described above, we conclude
that the nickel deposition from bath A on uncatalyzed silicon pro-
ceeds in two consecutive steps. In the first step, the reduction of
2
ϩ
Ni is initiated by the mechanism of galvanic displacement with
silicon substrate. In the second step, the nickel deposition proceeds
autocatalytically by the reaction with the reducing agent. We con-
firmed that nickel deposition does take place on n-Si(100) surface
from the simple bath C.
This work is expected to contribute to the development of key
processes for the metallic quantum nanocluster formation required
in the future technology of fabrication of ultrahigh density ROM.
Figure 8. Schematic representation of the mechanism for the Ni deposition
on Si from the bath without the reducing agent.
Conclusion
Shirey, J. Vac. Sci. Technol., B, 12, 3884 (1994).
5
.
C. H. Ting, Abstract 512, p. 720, The Electrochemical Society Extended Abstracts,
Vol. 87-2, Honolulu, HI, Oct 18-23 (1987); C. H. Ting, Electroless Deposition of
Metals and Alloys, M. Paunovic and I. Izumi, Editors, PV 88-12, p. 223, The Elec-
trochemical Society Proceedings Series, Pennington, NJ (1988); C. H. Ting and M.
Paunovic, J. Electrochem. Soc., 136, 456 (1989).
C. H. Ting, M. Paunovic, P. L. Pai, and G. Chin, J. Electrochem. Soc., 136, 462
(1989).
M. J. Desilva and Y. Shacham-Diamand, J. Electrochem. Soc., 143, 3512 (1996).
The NiP deposition on RCA-pretreated and uncatalyzed n-Si(100)
surfaces was confirmed to take place from a conventional electroless
NiP bath as well as from baths containing NiSO but no reducing
agent. It is apparent that the Ni deposition in the bath consisting only
of NiSO and (NH ) SO proceeds by a mechanism different from that
of the usual electroless deposition. The Ni deposition is accompanied
by the formation of silicon oxide. The results of this study can be uti-
lized for the formation of small metal dots on silicon surfaces, which
are needed for the fabrication of ultrahigh density ROM devices.
4
6
.
.
4
4 2
4
7
8. V. M. Dubin, Y. Shacham-Diamand, B. Zhao, P. K. Vasudev, and C. H. Ting, J.
Electrochem. Soc., 144, 898 (1997).
9. C. Y. Chou, J. Magn. Soc. Jpn., 21, 1023 (1997).
0. T. Shiiba, K. Hara, M. Fukasawa, H. Sakuma, M. Takai, T. Osaka, and I. Ohdomari,
in Proceedings of the 43rd Spring Meeting of the Japan Society of Applied Physics
and Related Societies, p. 531, Japan Society of Applied Physics, Tokyo (1996).
1
Acknowledgment
This work was financially supported by the Research for the
Future project “Wafer-Scale Formation Process of Quantum Dots”
of the Japan Society for the Promotion of Science, and by a Grant-
in-Aid for Scientific Research on Priority Area of Electrochemistry
of Ordered Interfaces from the ministry of Education, Science,
Sports and Culture, Japan. The authors thank the experimental sup-
port of Application Technology Dept., Techno Research Laboratory,
Hitachi Instruments Engineering Co., Ltd. for FIB sample prepara-
tion and FETEM analysis.
11. T. Osaka, N. Takano, and S. Komaba, Chem. Lett., 7, 657 (1998)..
1
1
2. L. A. Nagahara, T. Ohmori, K. Hashimoto, and A. Fujishima, J. Electroanal.
Chem., 333, 363 (1992).
3. L. A. Nagahara, T. Ohmori, K. Hashimoto, and A. Fujishima, J. Vac. Sci. Technol.,
A, 11, 763 (1993).
14. P. Gorostiza, J. Servat, and F. Sanz, Thin Solid Films, 275, 12 (1996).
1
5. P. Gorostiza, R. Diaz, J. Servat, F. Sanz, and J. R. Morante, J. Electrochem. Soc.,
44, 909 (1997).
1
1
1
6. W. Kern and D. A. Puotinen, RCA Rev., 31, 187 (1970).
7. T. Osaka and I. Koiwa, Kinzoku Hyoumen Gijutu (J. Surf. Finish. Soc. Jpn.), 34,
330 (1983).
1
1
8. Y. Okinaka and T. Osaka, in Advances in Electrochemical Science and Engineer-
ing, H. Gerischer and C. W. Tobias, Editors, Vol. 3, p. 55, VCH, Weinheim (1994).
9. P. Allongue, V. Costa-Kieling, and H. Gerischer, J. Electrochem. Soc., 140, 1018
T. Osaka assisted in meeting the publication costs of this article.
References
(1993).
1.
2.
3.
4.
H. Iwasa, M. Yokozawa, and I. Teramoto, J. Electrochem. Soc., 115, 485 (1968).
H. Cachet, M. Froment, and E. Souteyrand, J. Electrochem. Soc., 139, 2920 (1992).
M. V. Sullivan and J. H. Eigler, J. Electrochem. Soc., 104, 226 (1957).
J. M. Calvert, G. S. Calabrese, J. F. Bohland, M. Chen, W. J. Dressick, C. S. Dul-
cey, J. H. George, Jr., J. Kosakowski, E. K. Pavelcheck, K. W. Rhee, and L. M.
20. P. M. M. C. Bressers, S. A. S. P. Pagano, and J. J. Kelly, J. Electroanal. Chem., 391,
159 (1995).
21. H. Morinaga, M. Suyama, and T. Ohmi, J. Electrochem. Soc., 141, 2834 (1994).
22. J. S. Kim, H. Morita, J. D. Joo, and T. Ohmi, J. Electrochem. Soc., 144, 3275
(1997).
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