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1Applied Science and Engineering Progress, 2019
Fabrication of Metallic Nano Pillar Arrays on Substrate by Sputter Coating and Direct
Imprinting Processes
Potejanasak Potejana*
Department of Industrial Engineering, School of Engineering, University of Phayao, Phayao, Thailand
Nopparat Seemuang
Department of Production Engineering, Faculty of Engineering, King Mongkut’s University of Technology
North Bangkok, Bangkok, Thailand
* Corresponding author. E-mail: potejanasak.po@gmail.com DOI: 10.14416/j.asep.2019.09.001
Received: 3 April 2019; Revised: 3 July 2019; Accepted: 11 July 2019; Published online: 17 September 2019
© 2019 King Mongkut’s University of Technology North Bangkok. All Rights Reserved.
Abstract
In this study, an efficient fabrication process of metallic nanostructures was proposed and the feasibility of the
process was verified. This process comprises of direct imprinting and sputter coating techniques. Firstly in this
process, a silicon wafer mother mold of nanopattern is prepared by photolithography and dry etching technique.
The nanopatterns of mother mold are transferred to an acrylic film by hot embossing method. Secondly, a quartz
glass substrate is cleaned in the acetone bath and then by sputter etching for cleaning the contamination on the
surface. Then, a substrate is coated with a gold thin film by the Argon gas sputter coating process. Then, an
acrylic film mold, whose surface has been patterned with the nanopatterns, is used to transfer directly of pillar
pattern onto the gold thin film. As a result, the gold thin films are inflated as nanopillar arrays on the substrate.
This is because of the imprinting load from an acrylic film mold is effective to transfer the nanopillar arrays
onto a gold thin film. The experimental results show that an acrylic film mold is effective to form the nanopillar
arrays on the Au film although the acrylic film mold is softer than Au thin film. Furthermore, the plasmonic
properties of the nanopillar arrays are investigated. It is also found that the plasmonic nanopillar arrays show
good performance as a localized surface plasmon resonance (LSPR)-active substrate. Feasibility of the proposed
process is confirmed by experimental study, and efficiency of the process is discussed.
Keywords: Direct imprinting, Metallic nanopillar arrays, Sputter Coating, Polymer film mold
Research Article
1 Introduction
In recent years, metallic nanostructures have a great
potential to be utilized in various applications in bio-
molecular sensors [1], [2], nanoelectronic devices
[3], [4], display devices [5], and catalyst [6], [7]. For
example, metallic nanoparticles exhibit unique optical
characteristics attributed to LSPR (localized surface
plasmon resonance), which is due to the collective the
oscillation of electrons induced by the electromagnetic
field of incident light. The LSPR characteristic is
evaluated by absorbance spectrum, where a sharp peak
of absorbance spectrum is observed at the wavelength
of the resonant frequency. The peak wavelength of the
absorbance spectrum is mainly dependent on the size,
shape, and alignment of the nanoparticles [8]–[10].
Interestingly, the LSPR resonance frequency is also
dependent on the refraction index of the surrounding
environmental medium. Utilizing this property, it is
expected to be applied to biosensors. Other metallic
Please cite this article in press as: P. Potejana and N. Seemuang, “Fabrication of metallic nano pillar arrays
on substrate by sputter coating and direct imprinting processes,” Applied Science and Engineering Progress,
(2019). DOI: 10.14416/j.asep.2019.09.001
P. Potejana and N. Seemuang, “Fabrication of Metallic Nano Pillar Arrays on Substrate by Sputter Coating and Direct Imprinting Processes.”
2Applied Science and Engineering Progress, 2019
nanostructures such as nanopillars [11]–[13], nanowires
[14], [15], nanodots [16]–[18] and nanorods [19] also
exhibit unique optical properties due to LSPR, and
they are expected to provide excellent performance
as LSPR biosensors. Based on these requirements in
biosensor application [20], [21], efficient methods to
fabricate these nano structures with tunable plasmonic
properties are desired.
These kinds of metallic structures of nanometer
size are generally fabricated by the conventional
nanofabrication processes, such as ultraviolet
lithography (UVL) [22] or the electron beam
lithography (EBL) [23]. These techniques are useful
for production of well-defined and precise metallic
structures of nanometer size. However, these
processes consist of complicated procedures such as
the resist coating, the pattern drawing, etching and
developing process, where costly equipment and
stringent process control is necessary. In order to
develop the high throughput and low cost of a
fabrication process, many researchers studied the
bottom-up methods such as thermal dewetting [24],
[25] and anodic aluminum oxide (AAO) template
[26]. These processes are able to fabricate a lot of
nanostructures like nanodots and nanoholes by simple
procedures. However, it is difficult to control features
of nano structures by these self-organization processes.
In order to overcome the previous problems, the
authors proposed a new method as a templated direct
imprinting method to supplement limitations for the
current conventional fabrication methods. In order to
fabricate nanostructure arrays of good uniformity and
regularity at low cost of fabrication, the authors aim to
develop the alternative methods by a PMMA acrylic
film mold with groove patterning on the metallic thin
film by means of the direct imprinting method. This
process consists of three steps; a deposition of the Au
thin film on a quartz glass substrate, then patterning
of vertical and horizontal groove grid on the Au thin
film by the direct imprinting method. A soft acrylic
film mold was used to transfer the structure templates
on a surface of gold without lithography and chemical
etching process. This approach possesses a number of
advantages including low-emission, high-throughput,
low-cost fabrication techniques, low stringency and
without dangerous chemical disposals to destroy
the environment are the main issues that need to be
addressed.
2 Experimental Methods
2.1 Experimental procedure for nanopillar arrays
fabrication test
Figure 1 illustrates a newly proposed process for
fabrication of an Au nanopillar array on a quartz glass
substrate. Figure 1(a) Firstly, a quartz glass plate of 1
mm in thickness was cut into the size of 12×12 mm2.
It was cleaned by an ultrasonic cleaner in an acetone
bath for 15 minutes. After the glass substrate was dried
in air, Then the glass plate was placed in a DC sputter
coater, a substrate was subjected to the sputter etching
with Ar gas for 2 minutes to remove the contamination
molecules from the surface of a substrate. Then, a
gold thin film was deposited on a quartz glass plate for
50 nm in thickness. The spatter gas was Argon (Ar),
and its pressure was 15 Pa. The distance between the
specimen and the Au target was 35 mm. The thickness
of the Au film was controlled by adjusting sputtering
time. Figure 1(b) Secondly, the vertical parallel grooves
were formed on the deposited Au thin film by direct
imprinting method using an acrylic film mold. The
acrylic film mold was manually pressed on the surface
of the deposited Au thin film using tweezers as shown
in Figure 2. A rounded tip with the straight type of
tweezers was used in the direct nanoimprinting process.
The radius of the round tip was about 1.3 mm. It was
made of high-quality stainless steel. Figure 1(c) Finally,
an acrylic film mold was rotated in a perpendicular
Figure 1: Proposed process for fabrication of the Au
nanopillar arrays on substrate.
Substrate
Load
Load
Substrate
Substrate
Au thin film
Au pillar arrays
PMMA film mold
PMMA film mold
Grooves
pattern
Substrate
(c)
(a)
(d)
(b)
3
P. Potejana and N. Seemuang, “Fabrication of Metallic Nano Pillar Arrays on Substrate by Sputter Coating and Direct Imprinting Processes.”
Applied Science and Engineering Progress, 2019
direction to transfer the horizontal grooves, Figure 1(d)
then the square pillar pattern was agglomerated on the
Au thin film on a substrate. Morphology and alignment
of nanopillar array were characterized by observation
with atomic force microscopy (AFM, Keyence VN-8010).
2.2 Hot embossing method of a PMMA film mold.
This study involves the use of parallel groove-shaped
as a convenient and easily accessible alternative to
nanoscale features fabricated via direct imprinting
method. For the mother mold fabrication, the silicon
wafer was used as the mother mold materials. The series
of parallel grooves were fabricated by photolithography
and dry etching techniques. A polymer film mold
was fabricated by the following procedure as shown
in Figure 3. Figure 3(a) Firstly, the mother mold was
imprinted on an acrylic film after heated to 110°C. An
acrylic film used in this experiment was poly(methyl
methacrylate) film (PMMA), ACRYPLEN-HBA002P.
The glass transition temperature of an acrylic film is
105°C. The thickness of the film was 75 μm, the pattern
area was 25 mm2 and the hardness of an acrylic film
mold was about 15 HV. Figure 3(b) The mother mold
was continuously cooled to room temperature (RT),
and the polymer film was peeled off from the mother
mold.
The use of an acrylic film mold in the replica molding
helps to overcome the damage to the nanostructures
on the mother mold. The acrylic film can be removed
easily from a mother mold without damaging the
nanostructures on either surface during separation
process because of the toughness and elasticity property.
Moreover, the mother mold can be used for replication
to transfer a pattern to the other acrylic film. The acrylic
film mold was applied to direct imprinting on the Au
thin film coated on a quartz glass substrate.
In order to fabricate an acrylic film mold, Figure 4
shows the silicon wafer mother mold used for the hot
embossing process. The nanostructures were fabricated
by the photolithography and dry etching method on
the mother mold.
Figure 5 illustrates the SEM micrographs of the
vertical cross section for groove patterns on the silicon
wafer mother mold. The dimension parameter of each
nano-groove patterns are summarized in Table 1.
Figure 2: The manually direct imprinting process to
transfer the pattern from an acrylic film mold to Au
thin film by the tweezers.
Figure 3: Hot embossing method of a film mold: (a)
Mother mold imprint on a PMMA film, (b) Peeling
off process.
Figure 4: SEM image of a silicon wafer mother mold
with the parallel groove patterns.
Figure 5: Scanning electron micrographs (SEM)
demonstrating the dimension of groove patterns
fabricated on the silicon wafer.
Au thin film
Acrylic
film mold
Substrate
Tweezers
Indenting direction
Plate
Load
PMMA film mold, RT
PMMA film, 110 C
Mother mold, 110 C
Mother mold, RT
(a) (b)
Distance of peak
Width
Height
P. Potejana and N. Seemuang, “Fabrication of Metallic Nano Pillar Arrays on Substrate by Sputter Coating and Direct Imprinting Processes.”
4Applied Science and Engineering Progress, 2019
Table 1: The dimension parameter of each nano groove
patterns on the mother mold
Pattern Width (μm) Height (μm) Distance of peak (μm)
Grooves 2 3 4
Figure 6 illustrates a photograph of an acrylic film
mold. An acrylic film mold was utilized for a direct
imprinting test. A groove pattern of 5×5 mm2 was
successfully fabricated from the mother mold onto an
acrylic film. Figure 7(a) shows the AFM topography
images and Figure 7(b) shows the height profiles along
with the x-x’ line of the parallel grooves transferred
on an acrylic film. The width of crests was 2 μm, the
average height of groove was about 1 μm and the pitch
of groove pattern was 4 μm, which was nearly the
same as the pitch of groove on the mother mold. The
fabricated groove structure was analyzed by an atomic
force microscopy (AFM, Keyence VN-8010). An
acrylic film mold was utilized for a direct imprinting
test on Au thin film.
3 Results
3.1 Metallic pillar arrays on a substrate
Figure 8 shows the AFM image after the direct imprinting
process by using an acrylic film mold. Results have
shown that the nanopillar protuberances are regularly
aligned onto Au thin film on a substrate. Figure 9(a)
illustrates the height profile of the nano protuberances
on the horizontal cross-section line as an X-X’ and
Figure 9(b) illustrates the height profile of the nano
protuberances on the vertical cross section line as
Y-Y’. A result showed that the distance between pillar
is about 4 μm and the average height is about 90 nm.
This agrees with the distance of parallel grooves on
an acrylic film mold. Thus, it is attributed to plastic
deformation caused by an acrylic film mold. According
Figure 6: An acrylic film mold with a groove pattern
of 5×5 mm2 for direct imprinting experiment.
Figure 8: The AFM image of the Au nanopillar arrays
on a substarte by direct imprinting process.
(b)
Figure 7: (a) AFM topography image of an acrylic
film mold of the width of crests 2 μm parallel grooves
pattern and (b) Height distribution along with the x-x’
line of parallel groove patterns, the average height of
groove was about 1 μm.
(a)
XX’
1 μm
Distance X-X’ (nm)
0 10000
1200
1000
800
600
400
200
020000 30000 40000
Height (nm)
X’
Y’
X
Y
1 μm
5
P. Potejana and N. Seemuang, “Fabrication of Metallic Nano Pillar Arrays on Substrate by Sputter Coating and Direct Imprinting Processes.”
Applied Science and Engineering Progress, 2019
to literature, the hardness of Au film is measured at 22 HV
[27], which is much higher than the hardness of an
acrylic film. Nevertheless, the result shown in Figure 8
indicates that harder gold film was deformed by the
softer acrylic film, and an acrylic film mold is effective
to fabricate a nanopillar pattern on the gold thin film.
Figure 10 illustrates the dimension of the square
area on the top of a gold nanopillar structure. The
results revealed that the average square area was 1.6
× 1.6 μm2. It was also confirmed that a gold film is
agglomerated as the square nanopillar structures on
the substrate by the direct imprinting method using an
acrylic film mold.
3.2 LSPR properties
To investigate the optical property of the fabricated
nanopillars, ultraviolet-visible (UV-vis) extinction
spectra were measured by using a spectrometer (BAS
SEC2000). The wavelength range of the spectrum
was selected over 400–850 nm. Figure 11 illustrates a
comparison of the absorbance spectra between (i) the
gold nanopillar arrays on the quartz glass substrate
fabricated by the direct imprinting process of this
study as shown in a blue line with (ii) the gold thin
film was directly deposited on the glass substrate as
shown in a black line. A result reviewed that, the gold
nanopillar arrays fabricated by direct imprinting has
an absorbance peak appeared on the wavelength of
680 nm. However, the gold thin film has the spectrum
peak appeared on the wavelength of 760 nm. It was
confirmed that the absorbance peak depends on the
dimension and alignment of nanopillar arrays. The
gold nanopillar arrays on substrate exhibit a higher
and steeper absorbance peak. It was clearly shown
that LSPR property was very sensitive to the pattern
of gold nanopillar arrays on a quartz glass substrate.
4 Discussion
In the study, the gold nanoparticles were directly
deposited on the surface of a substrate by a sputter
coating process. The yield stress of Au nanoparticle is
Figure 10: The AFM image of the square area on the
top of Au nanopillar arrays on a substrate by direct
imprinting process.
Figure 11: Extinction spectra of gold nanopillar arrays
on a substrate fabricated by direct imprinting method.
(a)
Distance X-X’ (nm)
0 10000
150
100
50
020000 30000 40000
Height (nm)
Distance Y-Y’ (nm)
0 10000
150
100
50
020000 30000 40000
Height (nm)
(b)
Figure 9: The height profiles of the Au nanopillar
arrays on a substrate by direct imprinting process.
1 μm
Wavelength (nm)
Absorbance (a.u.)
400
0
0.2
0.4
0.6
0.8
1
500 600 700 800
(i) Gold nanopillar arrays on substrate.
(ii) Gold thin film on substrate.
P. Potejana and N. Seemuang, “Fabrication of Metallic Nano Pillar Arrays on Substrate by Sputter Coating and Direct Imprinting Processes.”
6Applied Science and Engineering Progress, 2019
approximately 216 MPa [28], [29]. However, The yield
stress of PMMA acrylic film mold is 42 MPa from
the product description of PMMA, ACRYPLEN™
HBA002P [30]. The result of this study indicates that
the hard gold thin film was deformed by the direct
imprinting process. In spite of the yield stress of an
acrylic film mold is lower than the gold thin film, the
patterns can be transferred by a load of direct imprinting
method onto the gold thin film.
One of the reasons for this possible compression
mechanism is attributed to voids in the gold coating
film. In this study, the height of grooves on an acrylic
film mold was about 1 μm (1000 nm). The thickness
of gold thin film on a substrate was coated for 50 nm.
As shown in Figure 12, the acrylic grooves with the
height of 1000 nm were directly pressed onto the 50 nm
thickness of coated gold film layer. The gold thin film
had a coarse microstructure in which minute grains of
gold were piled up like a sand layer. After an acrylic
grooves penetrated into the gold thin film at some
indentation depth, then some of the gold clusters have
flowed over into the cavity of an acrylic grooves.
Therefore, the height of gold layer in the cavities were
continuously increased throughout the direct imprinting
process. However for the cross direct imprinting of
this study, it was revealed that the gold pillar structures
with the thickness about 90 nm were formed on the
glass substrate.
Another possible reason for this phenomenon
is the increase of hardness of the PMMA film mold
caused by the imprinting pressure. The width of
the crests of the mold was 2 μm, whereas the space
between crests was 2 μm. The compressive stress
concentrates on the crests, and the stress becomes
double that of the imprinting pressure. Because of
this high compressive stress, the crests would be
compressed elastically, and the density of the crests
would be increased. Therefore, the hardness of the
crests might be increased.
In addition, the alignment accuracy of pillar
structures on the gold thin film is also inherently low
as shown in Figure 10, because of increased sizes
variation of the crests on groove patterns of a PMMA
film mold due to the manually imprinting load by the
tweezers. Therefore, the development of a uniform
direct imprinting technique that can form narrow and
dense pillar arrays is necessary to realize the proposed
fabrication process of metal nanostructures.
The limitation of this direct imprinting process
by a soft material as the acrylic film mold is, in fact,
the reusing of mold. As shown in Figure 13, the crests
of groove patterns were destroyed after the direct
imprinting process by tweezers. Therefore, the drawback
of this study is an acrylic film mold cannot use for the
replication. The advantage of this process is that it does
not require expensive equipment other than a sputter
coater and that it does not use hazardous chemicals,
such as acid or alkali solutions. The fundamental
mechanism of this process is the indentation on metal
films by the low cost of materials such as an acrylic
film mold. For this purpose, it is necessary to find a
polymer material that can achieve a reusable property.
Figure 12: The demonstrating of an indentation behavior
of a PMMA acrylic film mold by direct imprinting
method onto the Au cluster film deposition on a quartz
glass substrate.
Figure 13: The structured polymer surfaces after the
direct imprinting process.
Cracks on the grooves after the direct imprinting.
7
P. Potejana and N. Seemuang, “Fabrication of Metallic Nano Pillar Arrays on Substrate by Sputter Coating and Direct Imprinting Processes.”
Applied Science and Engineering Progress, 2019
5 Conclusions
A new fabrication process of the square shape nanopillar
arrays, which employed the cross-sectional of vertical
and horizontal groove patterning by the combination of
the sputter coating and direct imprinting method was
proposed. A soft acrylic film mold was used instead
of a hard mold for direct imprinting to transfer the
nanostructures onto a surface of Au thin film. Then
direct imprinting of nanopillar arrays on a substrate was
investigated experimentally. The results summarized
as follows.
The proposed process is effective to produce an
ordered Au nanopillar array aligned on a pattern by
an acrylic film mold indentation has been developed.
The indentation process of the direct imprinting was
examined experimentally. A process that employs the
scraping on the behind side of a polymer film mold
with the tweezers. Even though the coated Au layer
on the quartz glass substrate has the hardness (22 HV)
greater than the PMMA film mold (15 HV), it can
be fabricated by direct imprinting method to transfer
the cross-section of vertical and horizontal parallel
grooves by a PMMA acrylic film mold on the Au
thin film. A result has shown that the gold nanopillar
arrays on a quartz glass substrate were fabricated by
direct imprinting process with uniformity of square
shape. Most important of all, the nanostructures can
be fabricated by using a very high throughput process
without using any photo-resist layer, lithography, and
chemical etching in the process.
The absorbance spectra of the gold nanopillar
structures on the quartz glass substrate display a peak
in the visible range wavelength. The absorbance peak
intensity increased when the nanopillar aligned along
with the patterns. It was confirmed that the variation in
the LSPR property is closely correlated to the distribution
and alignment of nanopillar arrays on the substrate.
Acknowledgments
The author would like to acknowledge the financial
support from School of Engineering, University of Phayao.
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