X. KUANG et al.: FEASIBLE METHOD TO FABRICATE A NICKEL-NANODOT MASK ...
119–123
FEASIBLE METHOD TO FABRICATE A NICKEL-NANODOT
MASK ON A SILICON SUBSTRATE WITH CONVENTIONAL
THERMAL ANNEALING
IZVEDLJIVA METODA KONVENCIONALNE TERMI^NE
OBDELAVE ZA IZDELAVO MASK NA OSNOVI NIKLJEVIH
NANOPIK NA SILICIJEVI PODLAGI
Xuping Kuang1, Jili Tian2, Hui Guo1, Yi Hou1, Huayu Zhang2, Tiejun Liu3
1Shenzhen Selen Science & Technology Co., Ltd, Shenzhen 518055, China
2Shenzhen Key Laboratory of Advanced Materials, Shenzhen Graduate School, Harbin Institute of Technology, Shenzhen 518055, China
3Shenzhen Graduate School, Harbin Institute of Technology, Shenzhen 518055, China
tianjili1987@163.com
Prejem rokopisa – received: 2017-01-12; sprejem za objavo – accepted for publication: 2017-10-26
doi:10.17222/mit.2017.006
In this paper, a simple and feasible method to fabricate a Ni-nanodot mask on a silicon substrate is reported. Without using a
high-cost rapid thermal annealing (RTA) furnace, this method was based on the conventional thermal annealing of a Ni layer
deposited on a Si substrate. In addition, the influences of the Ni-layer thickness and annealing conditions on the size and density
of the Ni nanodots were systematically investigated. The as-prepared Ni nanodots were well distributed, having a uniform size
and a regular spherical shape. Moreover, a Si nanopillar array could be fabricated by inductively coupled plasma (ICP) etching
using the as-obtained Ni-nanodot array mask.
Keywords: Ni nanodot, conventional thermal annealing, ICP, Si nanopillar
V ~lanku avtorji opisujejo enostavno in izvedljivo metodo za izdelavo mask iz Ni-nanopik na Si podlagi. Pri tem niso uporabili
drage pe~i za hitro termi~no obdelavo, temve~ konvencionalni postopek termi~nega `arjenja Ni-plasti nane{ene na Si-podlago.
Sistemati~no so tudi raziskali vpliv debeline Ni-plasti in pogojev `arjenja na velikost in gostoto Ni-nanopik. Le-te so bile
enakomerno razporejene, bile so enake velikosti in pravilne polkrogli~ne oblike. Poleg tega so avtorji ~lanka pokazali, da se
lahko izdelajo matrice sestavljene iz Si-nanostebri~kov s pomo~jo jedkanja z induktivno sklopljeno plazmo (angl. ICP) in
uporabo izdelanih Ni-nanopik.
Klju~ne besede: Ni -nanopike, konvencionalna termi~na obdelava, ICP, Si-nanostebri~ki
1 INTRODUCTION
In the past few decades, there was much interest in
silicon nanostructures due to their potential electronic
and photonic applications such as biosensor,1 gas
detector,2 memory cell,3 photovoltaic crystal structures,4
and field-emission light-emitting devices.5 Typically,
electron-beam (E-beam) lithography together with
inductively coupled plasma (ICP) etching or reactive-ion
etching (RIE) process is the most successful and domi-
nant method for the fabrication of Si nanostructures.6,7
However, we need more accurate lithography when
silicon nanostructures reduce to a small size, particularly
on the nanoscale, which involves high costs and is not
suitable for a large-area fabrication.
Therefore, a simple, cost-effective and non-litho-
graphy method using self-assembled Ni nanodots as the
etching mask has been proposed for many years.8 It is
well known that the self-aggregation of the evaporated
Ni film on Si substrates allows it to function as the
etching mask for the fabrication of high-aspect-ratio Si
nanostructures, as well as Al, Au, and Ag.9,10 Further-
more, J. Zhu et al.11 employed a Ni-nanodot mask to
fabricate GaN-based nanopillar light-emitting diodes.
However, the preparation of Ni nanodots on Si substrates
still has some limitations. Many research groups reported
rapid thermal annealing (RTA) of the Ni layer deposited
on silicon substrates.12–15 Other studies investigated the
introduction of a thin SiO2 buffered layer between the Si
substrate and the Ni film.12,14,15 Recently, I. Levchenko et
al.16 have suggested that Ni-nanodot arrays on a Si sur-
face can be obtained using a plasma-assisted method. In
general, either a high-cost RTA furnace or a complex
process control is required in most cases.
In this paper, we report on a simple and feasible me-
thod to prepare the Ni-nanodot mask on a silicon sub-
strate. In this process, well-distributed Ni-nanodot arrays
were prepared by conventionally annealing Ni films
deposited on the Si substrate using a common tube fur-
nace instead of an expensive RTA furnace. The Ni
nanodots were approximately spherical with a uniform
size and a regular shape. Subsequently, we fabricated a
Si nanopillar by dry-etching the Si substrate using the
as-prepared Ni-nanodot array layer.
Materiali in tehnologije / Materials and technology 52 (2018) 2, 119–123 119
UDK 669-153:67.017 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 52(2)119(2018)
2 EXPERIMENTAL PART
In order to obtain a Ni-nanodot array on the Si sub-
strate, we performed conventional annealing of the Ni
films deposited on a silicon wafer. Figure 1 shows a
schematic illustration of the experimental procedures for
the fabrication of the Ni-nanodot mask on the Si
substrate. Firstly, a very thin Ni layer was deposited on
the Si (111) substrate by sputtering the Ni target using a
JGP-450a radio-frequency magnetron-sputtering system.
Prior to deposition, the silicon substrate was cleaned
during the RCA cleaning process in order to remove dust
particles and native oxide. A 99.999-% pure Ni target
with a 2 in. diameter and the Ar plasma (99.999 %) were
used for the sputtering. The distance between the
substrate and the target was 60 mm. The base pressure
was 4 × 10–4 Pa and the flow rate of Ar was 20 mL/min.
Thin Ni films were deposited at a substrate temperature
of 20 °C using different times. The growth velocity of
the thin Ni films was 0.185 nm/s and the typical film
thickness ranged from 6 nm to 22 nm. Then, the as-
sputtered Ni films were annealed in a conventional tube
furnace. The heating rate was set to 8 °C/min, and the
films were annealed in N2 at a flow rate of 50 mL/min
and different temperatures. After annealing, the samples
were naturally cooled down to room temperature and
taken out for characterization.
For the application of Ni nanodots on the Si sub-
strate, the Si substrate was dry etched in a planar-type
inductively coupled plasma (ICP) etching system (Ox-
ford Plasma System 100, ICP 180) at a rf frequency of
13.56 MHz using the as-obtained Ni nanodots as the
nanomask. The ICP and RF power were 1200 W and 90
W, respectively. The etching gases of SF6 and CHF3 with
a condition of SF6/CHF3 = 7/21 mL/min were introduced
into the reactive chamber through individual electronic
mass-flow controllers. The chamber pressure of 1.07 Pa
remained unchanged during the etching duration of
1 min. After the etching, the samples were taken out for
characterization.
The sample surface and side-view morphology were
investigated with scanning electron microscopy (SEM:
HITACHI S-4700).
3 RESULTS AND DISCUSSION
Figure 2 shows SEM images of the Ni nanodots
self-aggregating on the silicon substrates and creating
X. KUANG et al.: FEASIBLE METHOD TO FABRICATE A NICKEL-NANODOT MASK ...
120 Materiali in tehnologije / Materials and technology 52 (2018) 2, 119–123
Figure 2: SEM images of the samples with different Ni films after annealing: a) 6 nm, b) 8 nm, c) 14 nm, d) 16 nm
Figure 1: Schematic illustration of the experimental procedures for
the fabrication of the Ni-nanodot mask on a Si substrate
as-deposited Ni layers with different thicknesses after the
conventional annealing at 800 °C for 30 min. As ex-
pected, the self-assembly of the Ni nanodots on all the Si
substrates can be clearly observed. However, as shown in
Figure 1, the array pattern of nanoparticles was strongly
affected by the thickness of the Ni films at a given
annealing temperature. The average diameter of the
nanodots became larger and the average distance bet-
ween adjacent nanodots was increased while their
density was reduced with the increasing Ni-film thick-
ness. In the cases of thicker Ni films, the islands became
bigger and the correlation distance became larger due to
the Ni nanoclusters merging together,17,18 thus reducing
the density. Therefore, the process can be controlled by
varying the Ni-film thickness influencing the period, size
and density of the pattern of the Ni nanodots on a silicon
substrate.
In general, the solid-state aggregation of a thin metal
film is a phenomenon, allowing the film aggregated to
spontaneously form specific-shape particles when reach-
ing a certain temperature below the melting point. A thin
Ni film aggregated to form an island during the anneal-
ing is depicted in Figure 3. The driving force is the
minimum interfacial free energy between the film and
the substrate. The relationship between the interfacial
energy of the nanoparticles, taking into account per unit
area (	f), and that of the substrate (	s) is given as
Equation (1):19
	s = 	i + 	f ·cos 
 (1)
where 	i is the value of the unit-area energy between the
island-like particles and the substrate interface. 
 repre-
sents the equilibrium contact angle between the particles
and the substrate. It is clearly seen from the Equation
(1) that the solid-state aggregation of a Ni film cannot
proceed under the condition of 	s > 	i + 	f ·cos 
 as a
thin Ni film aggregates to form island-like particles
when a sufficient driving force is in place.
SEM images of the Ni nanodots self-aggregating on
the Si substrate as a function of the annealing tempera-
ture are shown in Figure 4. During this process, the
conventional annealing condition for the 12-nm thick Ni
layer processed for 30 min remains unchanged. It is
clearly seen that the density of the Ni nanoparticles
gradually decreased, but then reversed for the size with
the increasing annealing temperature, which may have
X. KUANG et al.: FEASIBLE METHOD TO FABRICATE A NICKEL-NANODOT MASK ...
Materiali in tehnologije / Materials and technology 52 (2018) 2, 119–123 121
Figure 4: SEM images of Ni nanodots self-aggregating on Si substrates formed by annealing at different temperatures: a) 700 °C and b) 800 °C
Figure 3: Equilibrium shape of a Ni island with isotropic surface
energy on the Si substrate
been caused due to two reasons: (1) the migration of
nickel atoms into the substrate surface was enhanced by
a higher temperature, which means that the nickel atoms
aggregated on the surfaces of large nanoparticles, further
reducing the interfacial energy and achieving the mini-
mum free energy;19 (2) meanwhile, the fusion and
coarsening behavior towards a small size of the Ni
nanoparticles were enhanced, lowering the free energy,
therefore, the small particles eventually merged or grew
into larger ones with the increase in the annealing tem-
perature. As a result of the above, the Ni atoms could not
migrate over long distances and eventually changed into
a strip of particles due to the weaker migration at a lower
temperature, which means that more spherical nanoparti-
cles were formed at a higher temperature. In order to
obtain Ni nanodots with a uniform size and a regular
spherical shape, it is necessary to appropriately increase
the annealing temperature.
The effect of the annealing time on the size and
density of the self-assembled Ni nanodots was also stu-
died. Figure 5 shows SEM images of the self-assembled
Ni nanodots on silicon substrates after annealing at
800 °C for different times. The Ni film initially breaks
into large Ni strips and gradually achieves a spherical
shape with the increasing annealing time. In addition,
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122 Materiali in tehnologije / Materials and technology 52 (2018) 2, 119–123
Figure 6: Cross-sectional SEM image of Si nanopillars formed after
ICP etching for 1 min
Figure 5: SEM images of self-aggregated Ni nanodots on Si substrates formed by annealing for different durations: a) 15 min, b) 30 min, c) 60
min
these small Ni nanodots eventually self-aggregate, en-
larging their average diameter as the annealing time
increases. The reason for this is the fact that a short
annealing duration leads to an insufficient self-aggre-
gation of a Ni film and therefore unevenly sized
nanodots are formed. On the contrary, with the increase
in the annealing time, the aggregation of a Ni film is
sufficient,20 resulting in a fusion of small nanodots into
larger ones. In this way, Ni nanodots with a uniform size
and a regular spherical shape are easily obtained.
Si nanopillar arrays were prepared with the Ni-nano-
dot mask using inductively coupled plasma (ICP) etching
method. The Ni-nanodot mask was obtained by con-
ventionally annealing the Ni film (22 nm) at 800 °C for
1 h. Figure 6 shows a side-view SEM image of the Si
nanopillars formed after the ICP etching for 1 min. It is,
of course, necessary to continue optimizing the Ni mask
and the dry-etching technique in the future. Nevertheless,
this analysis gives us a good prospective of the above-
mentioned method so that we can prepare Ni-nanodot
masks on Si substrates for practical applications.
4 CONCLUSIONS
We report on a simple and feasible method for pre-
paring the Ni-nanodot mask on a Si substrate. The
method was based on the conventional annealing of Ni
films deposited on a silicon substrate using a common
tube furnace instead of a high-cost RTA furnace. The
size of nanodots became larger and their density was
reduced with the increasing Ni-film thickness or anneal-
ing temperature. The Ni film initially broke into large Ni
strips, gradually obtaining a spherical shape with the
increasing annealing time. Well-distributed Ni nanodots
with a uniform size and a regular spherical shape were
formed with this technique. Furthermore, we success-
fully fabricated a Si-nanopilliar array using inductively
coupled plasma etching on the as-prepared Ni-nanodot
mask.
Acknowledgment
This project was supported by the National Natural
Science Foundation of China (Grant Nos. 51472061 and
51172054) and Shenzhen Selen Science & Technology
Co., Ltd.
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