I. ERTUGRUL et al.: FABRICATION OF MEMS-BASED ELECTROTHERMAL MICROACTUATORS WITH ADDITIVE ...
665–670
FABRICATION OF MEMS-BASED ELECTROTHERMAL
MICROACTUATORS WITH ADDITIVE MANUFACTURING
TECHNOLOGIES
IZDELAVA ELEKTROTERMI^NIH AKTUATORJEV NA OSNOVI
MIKRO-ELEKTRONSKO-MEHANSKIH SISTEMOV, IZDELANIH Z
DODAJNIMI TEHNOLOGIJAMI
Ishak Ertugrul
1,2*
, Nihat Akkus
2
, Huseyin Yuce
2
1
Mus Alparslan University, Faculty of Engineering and Architecture, Department of Computer Engineering, 49250 Mus, Turkey
2
Marmara University Faculty of Technology, Department of Mechatronics Engineering, Goztepe Campus, 34722 Kadikoy – Istanbul, Turkey
Prejem rokopisa – received: 2019-01-30; sprejem za objavo – accepted for publication: 2019-03-27
doi:10.17222/mit.2019.027
This study aimed to fabricate a bidirectional electrothermal microactuator that can be produced with the conventional mi-
cro-electromechanical system (MEMS) fabrication technique, using two-photon polymerization (2PP) and projection mi-
cro-stereolithography (PμSL) methods and to compare these methods with each other. The electrothermal microactuator that can
move in two directions was designed in accordance with determined criteria. Although the same design was used for the 2PP
and PμSL methods, the supporting structures were not produced with the PμSL method. For the PμSL method, the actuator was
produced by removing the supports from the original design. Although 2-μm-diameter supports could be fabricated with the 2PP
method, it was not possible to produce them with the PμSL method. Furthermore, the 2PP method was found to be better than
the PμSL for the production of complex, non-symmetric support structures.
Keywords: MEMS, 2PP, PμSL, thermal actuator, 3D-printer
Avtorji opisujejo {tudijo izdelave dvosmernega elektrotermi~nega aktuatorja s konvencionalno MEMS tehnologijo (angl.: Mi-
cro-Electro-Mechanical System), dvofotonsko polimerizacijo (2PP) in metodami projekcijske mikro-stereolitografije (PμSL) in
nato primerjavo med njimi. Elektrotermi~ni aktuator, ki se lahko giblje v dveh smereh, so oblikovali v skladu z vnaprej
dolo~enimi kriteriji. ^eprav so tako pri metodi 2PP, kot pri metodi PμSL uporabili enak dizajn, podporne strukture niso izdelali
z metodo PμSL. Pri metodi PμSL so aktuator izdelali tako, da so odstranili podpore iz originalnega dizajna. ^eprav se lahko
podpore premera 2 μm izdela z metodo 2PP, le-teh z njo ni bilo mo`no izdelati. Nadalje avtorji ugotavljajo, da je za izdelavo
kompleksnej{ih nesimetri~nih podpornih struktur metoda 2PP primernej{a od metode PμSL.
Klju~ne besede: MEMS, 2PP, PμSL, termi~ni aktuator, 3D-printer
1 INTRODUCTION
It is known that the MEMS fabrication techniques are
highly complex and expensive. The demand for MEMS
is increasing with the advancement of technology. There-
fore, some facilitating and low-cost production tech-
niques are needed.
There are many processing steps and clean-room re-
quirements for the fabrication of traditional MEMS de-
vices. With the development of 3D micro-additive manu-
facturing technologies, the production costs and
processing steps of MEMS devices have been gradually
reduced. According to these advancements, it is possible
to produce MEMSs in atmospheric air without the need
for many operations (e.g., photolithography) and clean
rooms. Additionally, challenging production procedures,
such as Lithografie-Galvanformung-Abformung (LIGA),
used for some materials, are not needed due to 3D mi-
cro-additive manufacturing techniques.
Many companies have used the 2PP method with 3D
devices. In particular, the Photonic Professional GT and
Photonic Professional GT2 products of Nanoschribe
have used the 2PP technique very well. It has been seen
that nano-size production is possible with these devices.
1
MEMS devices produced with the 2PP method can be
used as micro-fluids,
2
micro-mechanical systems,
3–5
opti-
cal systems,
6,7
cell structures,
8
and biomedical devices.
9,10
The PμSL method is also used by many companies to-
day. The NanoArch™ is a 3D micro-fabrication equip-
ment based on the PμSL technology. Studies that have
been performed with the PμSL method include mi-
cro-sensors,
11,12
micro-fluids,
13
optical systems,
14,15
bio-
medical devices,
16
and mobile phones.
17,18
General trends show that many MEMS devices that
can be produced with conventional MEMS fabrication
techniques can also be manufactured with 3D printers.
Moreover, it is evident that 3D-printer methods are more
advantageous than traditional MEMS methods regarding
the process steps and costs.
In this study, the 2PP and PμSL methods, which are
3D MEMS manufacturing techniques, were examined in
Materiali in tehnologije / Materials and technology 53 (2019) 5, 665–670 665
UDK 620.1:681.527:338.32.053.4 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 53(5)665(2019)
*Corresponding author's e-mail:
i.ertugrul@alparslan.edu.tr

detail, and an electrothermal microactuator was fabri-
cated with these methods. These two methods were com-
pared with regard to the fabrication results. It is thought
that this study will contribute to the literature in terms of
fabricating a microdevice through the use and compari-
son of different methods.
2 MATERIALS AND METHODS
2.1 Bidirectional electrothermal actuator design
2.1.1 Working principle
The actuator in our study is defined as the U-type.
Such actuators are generally composed of both thin and
thick arms and have different designs. A U-type actuator
is shown in Figure 1. In such actuators, voltage is ap-
plied to the pads and the thin arm expands due to warm-
ing. This event is based on the temperature differences
between the thin and thick arms. There is a decrease in
the electrical resistance due to the increase in the
cross-sectional area of the thick arm. This situation
causes a decrease in Joule heating. Due to warming, the
expansion occurs in the thin arm. Thus, the thin arm is
longer. The actuator’s movement occurs for these rea-
sons.
19
The bidirectional MEMS-based electrothermal
microactuator is designed to move in two directions,
right and left. In order for the actuator to move to the
right of this design, DC-1 voltage must be applied as
shown in Figure 2. Thus, if the voltage is connected to
the pad attached to the hot arm while the ground is con-
nected to the pad attached to the flexure arm, the actuator
moves to the right. If the actuator is expected to move to
the left, DC-2 voltage should be applied as shown in Fig-
ure 2. Since our design is symmetrical and the actuator
is incapable of moving to the right and left simulta-
neously, voltages DC-1 and DC-2 are not applied at the
same time. The actuator will move to the right or left ac-
cording to the user request.
2.1.2 Design conditions
The lengths of the arm and flexure are of great im-
portance for the actuator to move to the right and left.
Our design was made in accordance with the following
criteria:
19,20
• The flexure arm must be as thin as possible.
• The flexure arm must not be thinner than the hot
arms as the flexure arm can overheat and cause defor-
mation.
• The flexure must be long enough to allow the actua-
tor to change directions.
• The flexure must be long enough not to deform the
actuator.
As our actuator has a symmetrical structure, dimen-
sioning and designing were performed for a single arm
as shown in Figure 3.
All the dimensions of our actuator are given in Ta-
ble 1. These values were obtained with the measure-
ments of the actuator made with the 2PP and PμSL meth-
ods. The same design was used for both methods.
However, since the supporting structures cannot be pro-
duced with the PμSL method, the measurements related
to the support are given in Section 3.
Table 1: Actuator parameters
Parameter Symbol Value Unit
Hot-arm length Lh
250 μm
Cold-arm length Lc 200 μm
Flexure-arm length Lf
100 μm
Actuator gap Lg
7.5 μm
Hot-arm width wh 3μ m
Cold-arm width wc 6μ m
Flexure-arm width wf
3μ m
Air-gap thickness t 4μ m
Actuator thickness t 3μ m
2.2 2PP technology
The 2PP method has been a widely-used technology
in the production of 3D microstructures in recent years.
The simplicity and quickness of the fabrication, the pro-
I. ERTUGRUL et al.: FABRICATION OF MEMS-BASED ELECTROTHERMAL MICROACTUATORS WITH ADDITIVE ...
666 Materiali in tehnologije / Materials and technology 53 (2019) 5, 665–670
Figure 3: Bidirectional electrothermal microactuator dimensions Figure 1: U-type actuator
19
Figure 2: Working principle of a bidirectional electrothermal
microactuator

duction of complex structures, and the high resolution al-
low this technology to be preferred over the others.
Photopolymer materials are used with this method.
21
In order to fabricate an actuator with the 2PP method,
a femtosecond (fs) pulsed laser light source is required.
The working principle of 3D microfabrication using the
2PP method is shown in Figure 4. Near-infrared (NIR)
fs pulses produced with a titanium (Ti) sapphire laser are
transformed into visible ones with an optical parametric
oscillator (OPO). In order to adjust the intensity of the
beam, the light source must enter a neutral density filter
(ND). The beam focuses on the sample to initiate poly-
merization. The photosensitive resin is compacted be-
tween two coverslips and mounted on a 3D piezoelectric
scanning layer. The motion of the scanning layer is
pre-programmed with a computer to generate different
microstructures. To analyze the fabrication process, it is
recorded with a charge-coupled-device (CCD) camera.
22
For the 2PP method, the material used in this study
was the IP-S resin. Table 2 gives the properties of this
material.
Table 2: Material properties of the actuator fabricated with the 2PP
method
23,24
Parameter Symbol Value Unit
Density D 1.2 g/cm
3
Young’s modulus E 4.6 GPa
Poisson’s ratio V 0.35 -
Thermal conductivity kp
1.82 W/m.K
Thermal expansion coefficient  0.98 μm/m.K
Specific heat capacity C 29 J/kg.K
Electrical resistivity R 2.9 μ. .m
Refractive index N 1.48 -
2.3 PμSL technology
PμSL is a versatile and cost-effective process that can
be used to rapidly generate microlenses, microgrippers,
microfluidics and microbeams with electrolysis or resin
additives. Materials such as a curable photopolymer,
polymer and nanoparticle composites are used with this
method. This method starts by creating a 3D structure
using computer-based design (CAD) software and then
turns the structure into a set of mask images (digital
mask). The working principle of PμSL is shown in Fig-
ure 5. Each image represents a thin layer of the 3D
structure. During a production cycle, a single image is
displayed on the reflective liquid crystal display (LCD)
panel. The image from the LCD is then reflected on the
liquid surface. The whole layer (usually 5–30 μm thick)
is polymerized. Once the layer has been solidified, it is
re-immersed in the resin to allow a new thin layer of liq-
uid to form. Repeating the loop creates a 3D micro-
structure from a layer stack.
25
The properties of the photosensitive resin, which
were specially developed for this printer and used in the
fabrication of the actuator, are given in Table 3.
Table 3: Material properties of the actuator fabricated with the PμSL
method
26
Parameter Symbol Value Unit
Density D 0.89 g/cm
3
Young’s modulus E 0.9 GPa
Poisson’s ratio V 0.89 -
Thermal conductivity kp 1.12 W/m.K
Thermal expansion coefficient  0.41 μm/m.K
Specific heat capacity C 38 J/kg.K
Electrical resistivity R 1.12 μ. .m
Refractive index N 5.48 -
I. ERTUGRUL et al.: FABRICATION OF MEMS-BASED ELECTROTHERMAL MICROACTUATORS WITH ADDITIVE ...
Materiali in tehnologije / Materials and technology 53 (2019) 5, 665–670 667
Figure 5: Schematic of the working principle of PμSL
25
Figure 4: Working principle of 3D microfabrication with the 2PP
method
22

3 RESULTS AND DISCUSSION
3.1 Fabrication with the 2PP technology
The bidirectional electrothermal microactuator,
whose design is illustrated in Figure 6a, was fabricated
utilizing a 2PP-technology-based Photonic Professional
GT device.
27
There are supports to the arms and pads of
this design. The number of supports to the arms is 17,
the diameter is 2 μm and the height is 4 μm while the
number of supports to the pads is 3, the diameter is 40
μm and the height is 4 μm. The dimensions of these sup-
ports are given in Table 4. Some unsuccessful experi-
ments were done before this design. Fractures occurred
during the fabrication when the number of supports to
the arms was low. According to the experiments, the av-
erage distance between the supports should be 45 μm in
order to avoid breakage. As the number of supports to
the flexure arm is related to the cold arm, continuous
fractures occurred in experimental studies. A side view
of the actuator on a glass wafer is given in Figure 6b.
According to experimental studies, the maximum value
of the resolution of a 3D printer should be 1 μm. Other-
wise, the supports cannot be fabricated precisely. If the
supports are not printed correctly, they cause collapses
and breaks of the actuator.
Table 4: Dimensions of supports in the actuator
Parameter Symbol Value Unit
Pad-support diameter rps 40 μm
Arm-support diameter ras 2μ m
Support height hs
4μ m
An image of the actuator fabricated with the 2PP
method is given in Figure 7. This image was taken with
the microscope of the 3D printer.
3.2 Fabrication with the PμSL technology
The bidirectional electrothermal microactuator,
whose design is illustrated in Figure 8a, was fabricated
using a PμSL-technology-based NanoArch™ series de-
vice.
28
The production of the supported design with this
method was not possible for two reasons. First, the sup-
ported structures represent the system as a 3D design.
However, it is not possible to produce 3D structures with
the devices of the BMF Technology Company based on
the PμSL technology. Second, these devices have a reso-
lution of 2 μm and can fabricate a minimum thickness of
up to 3 μm. When the support structures are removed, as
I. ERTUGRUL et al.: FABRICATION OF MEMS-BASED ELECTROTHERMAL MICROACTUATORS WITH ADDITIVE ...
668 Materiali in tehnologije / Materials and technology 53 (2019) 5, 665–670
Figure 6: a) CAD design of the actuator. The structures at the top of the design were designed as the support; b) fabrication structure and design
on the glass wafer
Figure 8: a) CAD design of the actuator. The support structures under the actuator are removed; b) for fabrication, the silicon wafer is coated
with a sacrificial layer of silicon nitride
Figure 7: Image of the actuator fabricated with the 2PP method
(photonic professional GT device’s camera)

shown in Figure 8a, it is possible to perform the fabrica-
tion as the actuator design can be introduced to the de-
vice (a PμSL-technology-based 3D printer) in two di-
mensions. The silicon wafer is coated with a sacrificial
silicon nitride layer, used to separate the actuator from
the silicon wafer, as shown in Figure 8b.
The image of the actuator produced with the PμSL
method is shown in Figure 9. This image was taken with
the microscope of the 3D printer.
4 CONCLUSIONS
In this study, a bidirectional electrothermal micro-
actuator, fabricated with the traditional MEMS tech-
nique, was produced using the 2PP and PμSL methods,
which are 3D-printer procedures. These methods are the
most common procedures used with 3D-printer devices.
When the experimental results were examined, it was
observed that 2-μm-diameter supports were produced
with the 2PP method. However, it was not possible to
produce them with the PμSL method. The minimum
structure that could be fabricated with the PμSL method
was 3 μm in length. Structures smaller than 3 μm could
not be fabricated. Besides, the support structures could
not be produced with the PμSL method even when the
diameters of the supports were 3 μm. The support struc-
tures were then removed and a normal actuator produc-
tion was performed. It was determined that the PμSL
method was not suitable for complex structures.
As a result of this study, it was found that the 2PP
method is more appropriate since it allows the fabrica-
tion of more complex 3D structures with smaller dimen-
sions, while the PμSL method is only suitable for simple
2D micro-structures.
Acknowledgment
This study was supported by the Marmara University
Scientific Research Projects Commission under project
number FEN-C-DRP-101018-0536.
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670 Materiali in tehnologije / Materials and technology 53 (2019) 5, 665–670