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Fabrication of composite microstructures comprising more than one type of material has been a fundamental challenge in nanotechnology. In this study, we report a method that can be used for the direct fabrication of complex three-dimensional (3D) polymer/metal hybrid microstructures. The patterning of materials (a polymer and a metal coatable polymer) in selected regions was achieved by a two-photon stereolithography process using a dual-stage scanning process. A precise alignment process was perfected to achieve the coincidence of polymeric and metal-coated polymer components of the microstructure. Selective metallization of the metal coatable polymer microstructure was performed through silver deposition by means of electroless plating. The 3D microcoil was demonstrated and electrically characterized.
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ORIGINAL ARTICLE
3D-Printed Polymer/Metal Hybrid Microstructures
with Ultraprecision for 3D Microcoils
Cheol Woo Ha,
1,2
Prem Prabhakaran,
3
and Yong Son
2
Abstract
Fabrication of composite microstructures comprising more than one type of material has been a fundamental
challenge in nanotechnology. In this study, we report a method that can be used for the direct fabrication of
complex three-dimensional (3D) polymer/metal hybrid microstructures. The patterning of materials (a polymer
and a metal coatable polymer) in selected regions was achieved by a two-photon stereolithography process
using a dual-stage scanning process. A precise alignment process was perfected to achieve the coincidence of
polymeric and metal-coated polymer components of the microstructure. Selective metallization of the metal
coatable polymer microstructure was performed through silver deposition by means of electroless plating. The
3D microcoil was demonstrated and electrically characterized.
Keywords: 3D microcoil, two-photon stereolithography, selective silvering, polymer/metal hybrid microstructures
Introduction
Athree-dimensional (3D) coiled shape is crucial for mi-
crocoils to achieve a reasonable quality factor (Q). However,
the fabrication of functional 3D microcoils has remained dif-
ficult with conventional patterning techniques.
1–6
For these
reasons, the development of alternative direct 3D patterning
techniques to fabricate 3D microcoils, which are useful for
compact wireless communication devices, wireless power
transmission, and small antennas, has attracted much attention.
In previous studies in the literature, 3D microcoils could be
fabricated using automatic wire bonders
7
and inkjet printing.
8
However, the fabrication resolution and feasible shapes were
limited. One of the most promising alternative techniques to
build 3D microstructures is the direct patterning process,
which is based on two-photon polymerization (TPP).
9–12
A
two-photon stereolithography (TPS) process using TPP is an
effective method to fabricate 3D polymer microstructures.
However, additional functionality, such as electrical con-
ductivity, must be achieved to enable these microstructures to
be used in functional nano/microdevices.
Electrical conductivity is a fundamental requirement for
transferring signals and powering microsystems. The fabricated
polymer structure can be transformed into an electrically con-
ductive metal structure through an electroless plating process
that provides uniform coating of various metals (Ag, Cu).
13–16
This allows fabrication of 3D conductive microstructures onto
glass substrates. High functional devices such as electrical
components are composed of hybrid materials, including metal
patterns for electrical conductivity and polymer patterns for
insulation. Therefore, a hybrid material (polymer/metal) pat-
terning technology is needed to fabricate a realizable 3D elec-
trical device that is composed of polymer and metal materials.
In this study, we report an effective approach for directly
fabricating 3D microcoils with desirable electrical functions.
Patterning a hybrid material (a polymer and a metal coatable
polymer) in selected regions is achieved by a two-photon
stereolithography process using a dual-stage scanning pro-
cess (TPS-D). For precise alignment of the polymer and
metal coatable polymer structures to form hybrid material
microstructures, a pattern-to-pattern comparison method is
used for the first time. Selective metallization of the metal
coatable polymer structure is performed through silver de-
position by electroless plating.
13–19,26
This approach enables
the fabrication of a selective metal-coated 3D polymer/metal
microstructure with ultrahigh precision for use in various
nano/microscale devices with electronic, optoelectronic, or
electromechanical functions.
1
School of Mechanical Engineering & Aerospace System, Korea Advanced Institute of Science and Technology (KAIST), Daejeon, Republic
of Korea.
2
Digital Manufacturing Process Group, Korea Institute of Industrial Technology, Siheung-si, Republic of Korea.
3
Department of Advanced Materials, Hannam University, Daejeon, Republic of Korea.
3D PRINTING AND ADDITIVE MANUFACTURING
Volume 6, Number 3, 2019
ªMary Ann Liebert, Inc.
DOI: 10.1089/3dp.2018.0139
165
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Fabrication System for 3D Microstructure
A schematic diagram of the experimental setup is pre-
sented in Figure 1. TPS-D consists of linear motor stages and
piezoelectric stages. For the two-photon absorption phe-
nomena, a femtosecond laser (mode-locked Ti-sapphire la-
ser) was used as a laser beam source; this laser had a
wavelength of 780 nm, ultrashort pulse duration of less than
100 fs, and repetition rate of 80 MHz. A k/2 plate and a
polarizing beam splitter were used to control the laser power.
An optical shutter, which has a maximum frequency of
1.0 kHz, was used for expeditious control of the on/off state
of the laser beam. The femtosecond laser beam was tight-
ly focused using an oil immersion objective lens (100 ·
magnifications, numerical aperture of 1.3). During laser
beam scanning, the laser beam was fixed and tightly focused
at a given position and the specimen was moved three di-
mensionally with piezoelectric stages. The polymer resin
prepared on the substrate was fixed on a dual stage using a jig.
The specimen was positioned at the starting point of the laser
beam path using motor stages with 300 nm resolution. Then,
arbitrary 3D polymer structures were fabricated using the
computer-controlled three-axis piezoelectric stages with
0.1 nm resolution. A high-magnification charge-coupled de-
vice (CCD) camera was used for the adjustment of the fo-
cused laser beam and for real-time process monitoring.
Results
Design of 3D microcoil
The proposed TPS-D can be used for the facile fabrication
of polymer/metal hybrid microstructures that are composed
of polymer structure and metal-coated polymeric structure.
Such 3D electrical structures are difficult to fabricate with
conventional processes. The resonance frequency of the 3D
microcoil allows having flexibility according to the design of
3D microcoil. In this article, we designed the 3D microcoil
with a diameter of 200 lm, a height of 60 lm, and 5 turns.
The inductance and self-capacitance of a coil can be cal-
culated from its dimensions, forming the number of turns
according to the parallel equivalent circuit model using
equations [Eqs. (1–3)].
20,21
L(nH)¼D2N2
0:45DþH(1)
C(pF)¼Der
11:45cosh 1(N1)(sd)
d
(2)
f0(Hz)¼1
2pffiffiffiffiffiffi
LC
p(3)
Here Lis the inductance (nH), Nis the number of turns, D
is the loop diameter (mm), His the height (mm), Cis the self-
capacitance (pF), e
r
is the dielectric constant, sis the turn
spacing (mm), dis the coil diameter (mm), and f
0
is the
resonance frequency (Hz). Using these equations, the theo-
retical inductance and self-capacitance values for the coil can
be calculated. The ideal inductance and self-capacitance are
about 6.67 nH and 0.0062 pF, respectively, and the expected
electromagnetic resonance frequency is 24.79 GHz. This
result shows that the proposed 3D microcoil can be used for
high-performance electrical components such as a wireless
power transmission device.
3D-printed microcoil
We previously reported various techniques for the reali-
zation of complex 3D polymer microstructures using the TPS
process.
11,22–25
In addition, we developed a dual-stage fem-
tosecond laser scanning process for fabricating aligned hy-
brid microstructures that constitute both polymer and metal-
coated polymer microstructures. A schematic diagram of the
experimental setup is presented in Figure 1. The 3D microcoil
was fabricated by TPS-D and selective electroless plating
processes.
A silver-coated coiled structure was fabricated in three
distinct steps. At first a cylindrical structure was fabricated
with SU-8 (MicroChem Co.) and its surface was modified to
make it suitable for silver coating. The second step involved
creating coiled spacer patterns on the cylindrical structure
fabricated from urethane/acrylate photoresists (SCR 500). In
the final step, silver was selectively coated on the SU8 part of
the microstructure by immersing the whole structure in a
silver bath. These processes are described in detail below. As
shown in Figure 2a, micropatterns that act as electrical pads
for the connection with external measuring equipment were
patterned by conventional UV photolithography with SU-8
resin. A drop of SU-8 resin sensitized with a two-photon
chromophore was placed on a cover glass substrate, as shown
in Figure 2b. A drop of SU-8 was prebaked at 95Cfor15min.
With the developed TPS-D with a laser power of 300 mW and
laser scanning speed of 100lm/s, the SU-8 microstructure
aligned with the prefabricated patterns on the substrate was
fabricated, as shown in Figure 2c. This structure acted as a
FIG. 1. Schematic diagram of the developed two-photon
stereolithography system using dual-stage scanning. Color
images are available online.
166 HA ET AL.
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support for the spiral coil structure and the metal-coated part.
After the laser scanning process, the sample was postbaked at
95C for 15min. The remaining resin was washed away with
propylene glycol methyl ether acetate (PGMEA) as a solvent,
as shown in Figure 2d. The surfaces of the polymer micro-
structures were functionalized with amine groups by treating
them with 1,3-diaminopropane [NH
2
(CH
2
)
3
NH
2
] in the pres-
ence of butyl lithium for 5 min, after which they were rinsed
thoroughly in ethanol, as shown in Figure 2e.
26
The amine
groups undergo a Michael addition reaction with unreacted
surface epoxides. This reaction creates surface-bound amine-
terminated amides.
27
Amines are capable of reducing gold ions
from solutions of gold salts. Gold nanoparticles were generated
on the surfaces of the SU-8 structures by immersing in aqueous
AuCl
4
-
(5.3 ·10
-4
M aqueous HAuCl
4
for 1 h) and reducing
with NaBH
4
(0.1 M aqueous NaBH
4
for 5 min). These gold
nanoparticles are capable of acting as seeds that can initiate
further metal deposition on the polymeric surfaces under fa-
vorable conditions. For fabrication of the second structure,
which was a spiral insulating coil, modified SCR-500 ( Japan
Rubber Co.) resin was used, as shown in Figure 2f. Unlike
structures fabricated with SU-8 resin, the structures fabricated
from SCR-500 resin can be clearly distinguished from the un-
polymerized resin owing to the refractive index change in the
polymerized structures. This enables the real-time monitoring
of the fabrication process with a CCD camera. The real-time
monitoring capability facilitates the alignment of different
structures (surface-functionalized polymer structures for metal-
coated structures and polymer structures for insulation) with
other structures. As shown in Figure 2g, the center of the spiral
coil structure must be aligned exactly with the center of the
prefabricated supporting structure. Generally, the use of a view-
finder in a CCD camera system with an index line is sufficient
to align the patterns with each other. However, the focused
image varies depending on the condition, such as the amount,
position, and different optical properties of the dropped resin in
this system. Therefore, for a precise alignment of structures, a
pattern-to-pattern comparison method was used to realize the
possibility of direct patterning during the TPS-D process.
Figure 3 shows the pattern-to-pattern comparison method
used in this process. Several primary mark patterns were
fabricated, as shown in Figure 3a and b. After developing the
remaining primary resins (SU-8), another polymer resin
(SCR-500) was dropped. Then, it was possible to fabricate
the secondary mark pattern in the center of the prefabricated
primary mark pattern using a CCD vision system, as shown in
Figure 3d. The errors of the xand yaxes compared with the
primary mark pattern and the secondary mark pattern can be
measured using the CCD vision system and piezoelectric
stages as shown in Figure 3c. Using this method, the xand y
axes on the stages can be aligned to the original positions, and
the aligned primary and secondary mark patterns can be
fabricated, as shown in Figure 3e and f. With this pattern-to-
pattern comparison method, the insulating coil structure with
FIG. 2. Schematic diagram of the fabrication process flow. (a) Polymer (SU-8) micropatterns for electrical pads were
fabricated by conventional UV photolithography. (b) Modified SU-8 resin was dropped onto the prefabricated micro-
patterns. (c) Microfabrication by TPS-D. (d) Removal of the remaining SU-8 resin with PGMEA (propylene glycol
monomethyl ether acetate) as a solvent. (e) Polymer surface treatment to improve the metal coating. (f ) Modified SCR resin
was dropped onto the structure. (g) Microfabrication by TPS-D. (h) Removal of the remaining SCR resin with ethanol. (i)
Silver coating by electroless plating. TPS-D, two-photon stereolithography process using a dual-stage scanning process.
Color images are available online.
3D-PRINTED MICROCOILS FOR WIRELESS COMMUNICATION 167
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the modified SCR-500 resin could be fabricated in alignment
with the prefabricated supporting structure made by SU-8
resin. After the residual SCR-500 resin is washed away with
ethanol, the polymer spiral structure acts as insulation for the
3D metallic microcoil, as shown in Figure 2h. Finally, silver
nanoparticles are selectively deposited onto the functiona-
lized surface of the polymer structure using an electroless
coating process, as shown in Figure 2i. The electroless silver
coating solution was prepared by mixing 12 mL of 33 wt%
aqueous gum Arabic, 1 mL of 1.5 M citric acid, 1 mL of 0.5 M
trisodium citrate, 3 mL of 37 mM aqueous silver lactate, and
3 mL of 0.52 M aqueous hydroquinone.
18
The silver coating
time was 60 min to achieve a thickness of 100 nm. The silver-
coated samples were rinsed with deionized water and iso-
propanol. The cleaned silver-coated sample was annealed at
200C for 30 min. The measured electrical resistivity was
2.0 ·10
-5
O$cm. Although this resistivity is nearly 12.5
times higher than that of bulk silver (1.59 ·10
-6
O$cm), this
result signifies that electroless silver-coated structures are
good electrical conductors that can be readily used in elec-
tronic devices. The discrepancy between the resistivity of the
silver-coated pattern and the bulk resistivity value could be
mainly due to the electron scattering caused by the surface
roughness (R
rms
*10 nm) and irregular aggregated silver
nanoparticles. Figure 4 shows a fabricated 3D microcoil with
a diameter of 200 lm, a height of 60 lm, and 5 turns. Silver
coating occurs selectively on the SU-8 surface functionalized
with gold nanoparticles, as shown in Figure 4d. The total
fabrication time for the 3D microcoil is about 5 h, including
TPS-D and selective metal coating process.
FIG. 3. Pattern-to-pattern comparison method. (a, b) Primary mark patterns. (c, d) Errors of the xand yaxes compared
with the primary mark pattern and secondary mark pattern can be measured using a CCD vision system and piezoelectric
stages. (e, f) After compensation for errors on xand yaxes, aligned primary and secondary mark patterns can be fabricated.
CCD, charge-coupled device. Color images are available online.
168 HA ET AL.
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To compare the measured resonance frequency with the-
oretical calculations (24.79 GHz), the frequency character-
istics of the fabricated 3D microcoil were measured using a
network analyzer (Agilent, E8364B). Figure 5 shows the
measured jS
11
jvalue of the fabricated 3D coil. A resonance
frequency was detected at 25.4 GHz. The measured return
loss drops to almost -30.03 dB at the resonance frequency.
These results show that the measured resonance frequency
(25.4 GHz) is a reasonable value when considering errors,
such as skin effect and the metal-coated surface roughness
(R
rms
*10 nm). This result shows that the demonstrated 3D
coil can be operated as a small antenna that can be readily used
for high-performance electrical components such as low-
power transfer applications. The new concept of microdevices
for electronic, optoelectronic, or electromechanical applica-
tions can be studied using the wirelessly transferred electrical
signal.
Conclusions
In summary, we have demonstrated a direct 3D fabrication
method for integrating precisely aligned polymer/metal hy-
brid microstructures composed of conductive metal-coated
polymer and nonconductive polymer structures by TPS-D
and selective electroless plating. For precise alignment of the
hybrid material structures, a pattern-to-pattern comparison
method was used. The fabricated 3D microcoil is operated at
25.4 GHz, which is useful for compact wireless communi-
cation devices, wireless power transmission, and small an-
tennas. Therefore, the suggested technique offers remarkable
FIG. 5. This graph shows the measured jS
11
jvalues of the
3D microcoil as a function of frequency.
FIG. 4. (a–c) SEM images of the fabricated 3D microcoil structure with a diameter of 200 lm, a height of 60 lm, and 5
turns. (d) Partially magnified SEM image (area A in (c)) of the 3D microcoil structure showing the hybrid microstructure
consisting of conductive metal-coated polymer and nonconductive polymer structures. 3D, three dimensional. Color images
are available online.
3D-PRINTED MICROCOILS FOR WIRELESS COMMUNICATION 169
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advantages as a direct, high-resolution, 3D metal-patterning
process that is applicable to precise nano/microscale devices
with electronic, optoelectronic, or electromechanical functions.
Acknowledgment
This work was supported by the KITECH internal project
and basic science research program (2017R1C1B5077130)
of the National Research Foundation of Korea.
Author Disclosure Statement
No competing financial interests exist.
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Address correspondence to:
Yong Son
Digital Manufacturing Process Group
Korea Institute of Industrial Technology
113-58 Seohaean-ro
Siheung-si 15014
Gyeonggi-do
Republic of Korea
E-mail: sonyong@kitech.re.kr
170 HA ET AL.
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... TPS has many applications, e.g., the development of microfluidics 79 , sensors 80 , actuators 81 , microbots 82 , and biomedical devices 83 . Ha et al. 84 reported 3D microcoils (Fig. 8b) employing TPS using a dual-stage femtosecond laser scanning process. The coil was made of a hybrid material consisting of a polymer and a "metal-coatable polymer". ...
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