Characterization and growth kinetics of Electroless pure Nickel thin films on Si(001) substrates

Article abstract of DOI:10.1149/1.3263266

We report the results on the growth behaviors of electroless pure nickel thin films on Si(001) substrates from a systematic investigation. Continuous pure Ni films with a polycrystalline structure were successfully plated on the surfaces of Si substrates using the hydrazine-modified electroless Ni deposition processes. The thicknesses of the pure Ni films can be controlled by tuning the electroless plating temperature and time. The deposition rate of the electroless pure Ni films increased exponentially with the plating temperatures. The activation energy of the electroless pure Ni deposition on Si(001) substrate for samples plated at 55-75° C could be calculated from an Arrhenius plot, and it was about 0.95 eV/atom.

Full text of DOI:10.1149/1.3263266

Characterization and Growth Kinetics of Electroless Pure Nickel
Thin Films on Si(001) Substrates
S. L. Cheng and H. C. Peng
J. Electrochem. Soc. 2010, Volume 157, Issue 2, Pages D81-D84.
doi: 10.1149/1.3263266
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Journal of The Electrochemical Society, 157 ͑2͒ D81-D84 ͑2010͒
D81
0013-4651/2009/157͑2͒/D81/4/$28.00 © The Electrochemical Society
Characterization and Growth Kinetics of Electroless Pure
Nickel Thin Films on Si(001) Substrates
S. L. Chengz and H. C. Peng
Department of Chemical and Materials Engineering, National Central University, Chung-Li City,
Taoyuan 32001, Taiwan
We report the results on the growth behaviors of electroless pure nickel thin films on Si͑001͒ substrates from a systematic
investigation. Continuous pure Ni films with a polycrystalline structure were successfully plated on the surfaces of Si substrates
using the hydrazine-modified electroless Ni deposition processes. The thicknesses of the pure Ni films can be controlled by tuning
the electroless plating temperature and time. The deposition rate of the electroless pure Ni films increased exponentially with the
plating temperatures. The activation energy of the electroless pure Ni deposition on Si͑001͒ substrate for samples plated at
55–75°C could be calculated from an Arrhenius plot, and it was about 0.95 eV/atom.
© 2009 The Electrochemical Society. ͓DOI: 10.1149/1.3263266͔ All rights reserved.
Manuscript submitted September 2, 2009; revised manuscript received October 18, 2009. Published December 7, 2009.
wafers were cut into pieces of about 1 ϫ 0.5 cm2. All of these Si
substrates were chemically cleaned by a standard procedure and
then dipped in a dilute HF solution ͑HF:H2O = 1:50͒ to remove the
native oxide layer. Before the electroless Ni plating process, the
cleaned Si surfaces were presensitized by dipping the substrates in a
solution containing 70 g/L stannous chloride ͑SnCl2·2H2O͒ and 70
mL/L hydrochloric acid ͑HCl͒ for 2 min and rinsed with deionized
͑DI͒ water. Then the sensitized Si substrates were treated with a
solution of 0.3 g/L palladium chloride ͑PdCl2͒ and 6.25 mL/L HCl
for 1 min at room temperature to activate the surface of the Si
substrates. Subsequently, the electroless pure Ni deposition process
was carried out by immersing the Pd-activated Si substrates into the
electroless nickel plating baths at 55–75°C for various plating time.
The electroless Ni plating solution was composed of 0.005 M nickel
sulfate ͑NiSO4·6H2O͒ as the Ni metal source and 0.25 M hydrazine
͑N2H4·H2O͒ as the chemical reducing agent. The pH value of the Ni
plating solution was monitored and controlled at 11.5 Ϯ 0.1 by the
addition of ammonium hydroxide ͑NH4OH͒ solution. After the elec-
troless deposition process, the obtained samples were thoroughly
washed with DI water and blown dry with N2 gas.
The surface morphologies of the as-activated samples and as-
synthesized Ni metal films were examined using scanning electron
microscopy ͑SEM͒. Plan-view transmission electron microscopy
͑TEM͒ and selected area electron diffraction ͑SAED͒ analysis were
carried out for microstructure examination and crystallography char-
acterization. The thicknesses of the Ni metal films deposited on
Si͑001͒ substrates at various reaction temperatures and times were
determined by XTEM. Most of the XTEM micrographs were taken
along the ͓110͔ zone axis of the single-crystal Si. Link ISIS energy
dispersion spectrometers ͑EDSs͒ attached to the SEM and TEM
were utilized to determine the chemical compositions of local areas
in the samples.
Compared with the conventional vacuum deposition methods,
such as thermal and E-beam evaporations, sputtering, and chemical
vapor deposition, electroless wet-chemical plating offers a relatively
facile, low cost, and efficient technique for the deposition of desired
metal and alloy layers onto the surfaces of conducting and noncon-
ducting engineering components with complex geometries.1,2 Dur-
ing the past decades, the wet-chemical deposition of metallic films
by electroless plating has been widely used in many industries, es-
pecially in the production of hard coatings to protect the engineering
components from oxidation and degradation caused by wear and
corrosion.3-6 Among the variety of electroless metallic coatings,
electroless nickel-based films are perhaps the most versatile materi-
als used in engineering due to their unique magnetic and mechanical
properties, as well as good thermal and chemical stabilities.7-10
In addition to the production of protective coatings, many recent
research efforts have been dedicated to the integration of electroless
Ni plating process into the fabrication of electrical contacts and
interconnects in microelectronic and optoelectronic devices.11-14
However, in these studies, borohydride, dimethylamine borane, or
hypophosphite was usually utilized as the chemical reducing agents,
leading to the incorporation of a significant amount of impurities
͑e.g., boron or phosphorus͒ in the electroless Ni layers. Several pre-
vious studies have demonstrated that these impurities would lead to
an increase in the electrical resistivity and decrease in the crystal-
linity and solderability of the deposited Ni films.15-17 These draw-
backs could limit practical applications of electroless Ni in micro-
electronics and packaging. To overcome the aforementioned
limitations, an alternative and effective synthesis route, which is
based on the hydrazine-modified electroless pure Ni deposition pro-
cess, has recently been developed.18-20 Although the improved elec-
troless plating method using hydrazine as the reducing agent has
successfully been applied to produce pure Ni film and
nanostructure,19,21,22 the corresponding studies on the growth kinet-
ics of electroless pure Ni deposition on Si substrates are relatively
scarce. Because the kinetic data are crucial for a fundamental under-
standing of the electroless pure Ni plating process and for a reliable
parameter estimation, it is important to investigate them further un-
der different experimental conditions.
Results and Discussion
Figure 1a shows a typical plan-view SEM micrograph of the
Si͑001͒ substrate after the Sn sensitization and Pd activation treat-
ments. As can be seen in Fig. 1a, dense nanoparticles, several na-
nometers in size, were found to form and disperse on the surface of
the silicon substrate. From the EDS analysis, it is evident that these
nanoparticles produced were found to be composed of pure palla-
dium. The corresponding EDS spectrum of the Pd nanoparticles is
shown in the upper right inset of Fig. 1a. The Pd nanoparticles were
then served as the catalysts in the following electroless nickel plat-
ing process. Figure 1b and c shows the photographs of the Pd-
activated Si substrates before and after electroless Ni deposition,
respectively, using hydrazine as the reducing agent. As can be seen
from the photographs, the as-deposited electroless Ni film exhibits a
bright metal color. The observed bright color suggests that the Ni
film produced was composed of fine grains and its surface morphol-
ogy was flat. A representative XTEM micrograph of the electroless
In this study, we report the results of a cross-sectional transmis-
sion electron microscopy ͑XTEM͒ investigation of the growth kinet-
ics of electroless pure nickel thin films on Si͑001͒ substrates. The
microstructures, crystallinities, and chemical compositions of the
electroless nickel films produced at various synthesis temperatures
and time are studied.
Experimental
Single-crystal, 1–100 ⍀ cm, boron-doped Si͑001͒ wafers were
used as the deposition substrates in the present study. The Si͑001͒
z E-mail: slcheng@ncu.edu.tw
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Journal of The Electrochemical Society, 157 ͑2͒ D81-D84 ͑2010͒
Figure 2. Plan-view SEM micrographs of electroless pure Ni films deposited
at 65°C for ͑a͒ 5 and ͑b͒ 11 min, and at 75°C for ͑c͒ 3 and ͑d͒ 5 min.
rings corresponding to the ͑111͒, ͑200͒, ͑220͒, ͑311͒, and ͑222͒ dif-
fractions of pure Ni phase were detected in the electron diffraction
pattern, indicating that the electroless Ni films were polycrystalline
with a pure face-centered cubic structure. Moreover, from the
bright- and dark-field plan-view TEM micrographs, as shown in Fig.
4a, an equiaxial grain structure can clearly be seen and the typical
grain size of the polycrystalline pure Ni film was measured to be in
the range of 30–80 nm.
Figure 1. ͑Color online͒ ͑a͒ A typical plan-view SEM micrograph of the
Si͑001͒ substrate after the Sn sensitization and Pd activation treatments. The
inset is the corresponding EDS spectrum of the nanoparticles. Photographs
of the Pd-activated Si substrates ͑b͒ before and ͑c͒ after the electroless depo-
sition of pure Ni thin films. ͑d͒ A representative bright-field XTEM micro-
graph of an as-deposited electroless pure Ni film on Si͑001͒ substrate.
nickel deposited on Si͑001͒ substrate is shown in Fig. 1d. For the
samples plated at various temperatures and time, the surface mor-
phologies, crystallinities, and chemical compositions of the electro-
less nickel films were further examined by SEM, TEM, EDS, and
SAED analyses.
Figure 2a-d shows the plan-view SEM micrographs of Ni films
plated at 65°C for 5–11 min and at 75°C for 3–5 min, respectively.
As seen in Fig. 2, it is obvious that an increase in the electroless
plating temperatures and time leads to a gradual increase in grain
sizes of the electrolessly deposited Ni films. Some of the Ni grains
produced at a higher temperature or at a longer time exhibit faceted
surface morphologies. Similar trends for the electroless deposition
of other metallic layers were reported in previous literatures.23,24
Figure 3a and b shows a typical XTEM micrograph and the corre-
sponding EDS spectrum of an electroless Ni film, respectively. The
EDS analysis clearly demonstrated that the electroless film produced
was entirely composed of pure nickel. The appearance of Cu peak in
the EDS spectrum is attributed to the copper grid used for support-
ing the TEM specimen. The microstructures and crystallinities of the
produced Ni films were investigated in more detail by plan-view
TEM and SAED analysis. Representative TEM micrographs and the
corresponding SAED pattern of the as-deposited Ni film are shown
in Fig. 4a and b, respectively. From the SAED analysis, diffraction
Figure 3. ͑a͒ Bright-field XTEM micrograph and ͑b͒ the corresponding EDS
spectrum of the electroless pure Ni film on Si͑001͒ deposited at 70°C for 9
min.
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Journal of The Electrochemical Society, 157 ͑2͒ D81-D84 ͑2010͒
D83
Figure 5. Bright-field XTEM micrographs of electroless pure Ni thin films
deposited at 60°C for ͑a͒ 9, ͑b͒ 11, and ͑c͒ 13 min, and at 70°C for ͑d͒ 5, ͑e͒
7, and ͑f͒ 9 min.
By measuring the Ni plating rate at different deposition tempera-
tures, the activation energy, Ea, of the electroless pure Ni deposition
can be determined using the Arrhenius relationship
Figure 4. ͑a͒ Bright-field plan-view TEM micrograph and ͑b͒ the corre-
sponding indexed SAED pattern of the electroless pure Ni film deposited at
70°C for 9 min. The inset in ͑a͒ is the corresponding dark-field TEM micro-
graph.
Figure 5a-c and d-f shows the XTEM micrographs of pure Ni
thin films plated at 60°C for 9–13 min and at 70°C for 5–9 min,
respectively. The TEM observations clearly revealed that all the
plated Ni films were dense and continuous and the interfaces be-
tween electroless Ni films and Si substrates were relatively smooth.
The thicknesses of the produced pure Ni films increase with electro-
less plating temperatures and time, ranging from 35 to 95 nm. Simi-
lar electroless deposition behaviors were also observed for other Si
substrates plated at 55, 65, and 75°C for various periods of time in
this study. After a series of XTEM examinations, the thicknesses of
the electroless pure Ni films vs deposition time data for Si͑001͒
substrates plated at temperatures from 55 to 75°C were obtained, as
shown in Fig. 6a. The relation curves shown in Fig. 6a are almost
linear. The result indicates that in the range of plating temperatures
and time studied, the growth of electroless Ni films on Si is reaction-
controlled. From the slopes of the straight lines presented in Fig. 6a,
the average Ni deposition rates at various temperatures could be
readily estimated. An example is shown in Fig. 6b. Furthermore, as
can be seen in Fig. 6b, the plating rate of the electroless pure Ni
deposition increases exponentially with the plating temperatures
from 2.7 to 19.4 nm/min.
Figure 6. ͑a͒ Thickness of the electroless pure Ni film vs the electroless
plating time curves for the Si͑001͒ substrates plated at different temperatures.
͑b͒ The average electroless pure Ni plating rate as a function of electroless
plating temperature.
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Journal of The Electrochemical Society, 157 ͑2͒ D81-D84 ͑2010͒
The grain sizes of the electrolessly deposited Ni films gradually
increase with increasing electroless plating temperatures and times.
From TEM, SAED, and EDS analyses, it can be concluded that the
electroless film produced was entirely composed of pure nickel and
its crystal structure was polycrystalline. The thicknesses of the elec-
troless pure Ni films increase linearly with the deposition time for
silicon substrates plated at 55–75°C. By measuring the Ni plating
rate at different temperatures, the activation energy for the linear
growth of the electroless pure Ni films on blank Si͑001͒ was derived
from an Arrhenius plot to be about 0.95 eV/atom.
Acknowledgment
The research was supported by the National Science Council of
Taiwan.
National Central University assisted in meeting the publication costs of
this article.
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In summary, the present study has demonstrated that dense and
continuous pure Ni thin films were successfully plated on Si͑001͒
substrates by using the hydrazine-modified electroless Ni deposition
processes. The growth kinetics, surface morphologies, crystal struc-
tures, and chemical compositions of the electroless nickel films pro-
duced at various temperatures and times have been investigated.
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