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Chapter 8
Development of a New Nano-Micro Solid
Processing Technology Based on a LIGA
Process and Next-Generation Microactuators
Daiji NODA1 and Tadashi HATTORI1
Abstract The demand of micro-fabrication such as microactuators, microcoils,
smart sensors is continually increasing. Actuators can occupy a large part of the
volume and the weight of an overall system, and therefore required to be reduced
in size. However, there has been little progress in fabricating microactuators using
existing technologies. Micro-fabrication processing and new technologies are
needed in order to form three-dimensional electromagnetic type microactuators.
The LIGA process could be used to fabricate nano- and micro-scale parts for
many applications. Consequently, we fabricated spiral microcoils with a narrow
pitch and high aspect ratio coil lines for an electromagnetic type microactuator us-
ing the LIGA process. We have fabricated coil lines with a width of 10m and an
aspect ratio of 5. We have also estimated the suction force of actuators using these
microcoils. It is very expected that these high aspect ratio microcoils would be ca-
pable of delivering high performance in spite of their miniature size.
8.1 Introduction
Actuators are finding increasing use in a variety of fields and many applications.Therefore, they are one of the most important components in various machines
because the operation of the machine depends on their performance. Recently, ac-
tuators can constitute a large part of the weight of a system, and although demands
have been made for reductions in size and greater sophistication, very little pro-
gress have been fabricate so far. However, the miniaturization of actuators has
made little progress since it requires micro-fabrication, micro-processing, and
other new technologies that are not compatible with traditional machining tech-
niques.
1 Daiji NODA and Tadashi HATTORI
Laboratory of Advanced Science and Technology for Industry, University of Hyogo
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80 Daiji NODA and Tadashi HATTORI
Typical driving power sources for actuators are electrostatic, piezoelectric, elec-
tromagnetic, shape memory alloy (SMA), etc. Among these actuators, we are fo-
cusing on the electromagnetic type actuators driven at a low voltage, with high
power, high efficiency, and low cost. However, the current carrying capacity of
miniature coils is small when current paths of coil lines are microscopic, making itdifficult to obtain sufficient output power. In addition, it is also very difficult to
fabricating process microscopic current paths by means of conventional machin-
ing techniques.
On the other hand, the LIGA (German acronym for Lithographite, Galvanofor-
mung, and Abformung) process [1] could be used to fabricate nano- and micro-
scale parts and devices. The LIGA is a total process for fabricating the master
mold for micro-structured parts using X-ray lithography, electroforming a micro
pattern mold, and molding plastic micro-structure parts [2,3]. For X-ray lithogra-
phy, the NewSUBARU synchrotron radiation facility at our university [4] was
used. This was operated at an energy of 1.0 or 1.5 GeV modes. The X-ray expo-
sure at BL11 of NewSUBARU was carried out with the workpiece held in a spe-
cially manufactured nine parts operation exposure stage [5]. Thus, this X-ray ex-
posure stage makes it feasible to form three-dimensional (3D) structure such as
spiral coil patterns [6-8]. With this technique, it was possible to fabricate high as-
pect ratio coil line structures.
8.2 Design and Simulation of Electromagnetic Actuator
An electromagnetic type actuator including a magnetic circuit was designed with
the aid of calculated by simulation. The simulation was carried out varying the as-
pect ratio of the coil lines.
8.2.1 Design of Electromagnetic Actuator
For the design of the magnetic circuit, we used the type known as an “open frame
solenoid ”, which is open at the sides as shown in Fig.8.1 [6,7]. For the material of
the magnetic core (fixed core and plunger) and the shield parts (yoke) we used the
nickel iron alloy Permalloy 45, because it has the largest permeability of the soft
magnetic metals. Therefore, it can generate a strong magnetic field with a very
small electric current. When a voltage is applied to the coil, a magnetic flux is
formed in a gap, which deforms the magnetic field and produces a suction force
on the plunger.
An acrylic pipe with an outside diameter of 5 mm and an inside diameter of 3 mmwas used as the base material for coil lines fabrication. The pipe material is
PMMA (polymethylmethacrylate) which has the properties of a positive type
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Development of a New Nano-Micro Solid Processing Technology 81
photoresist. Therefore, it could be directly exposed to X-ray lithography to form
high aspect ratio structures on the acrylic pipe surface.
Fig. 8.1 Designed model of actuator operation with magnetic circuit
8.2.2 Simulation of the Suction Force of the Electromagnetic
Actuator
We proposed a spiral microcoil with high aspect ratio coil lines. Figure 8.2 shows
images of the coil lines. Conventional wire type coils are limited to coated copper
wire of a few ten of micrometers and aspect ratio of 1. However, in this research,
high aspect ratio type was fabricated using the X-ray lithography technique. In this
model, a magnetomotive force is proportional to squares of current paths. If the
aspect ratio of coil is increased, the cross sectional area of coil lines is also in-
creased allowing a greater current flow. Figure 8.3 shows the calculated results of
the suction force and permitted currents in coils with different aspect ratios. Here,
we used coil parameters as the coil line width of 10 m and the number of coil
turns of 675. The gap between the plunger and the fixed core was 1 mm. When the
aspect ratio is 5, the suction force may be about 25 times greater than for a coil
with an aspect ratio of 1.
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Fig. 8.2 Image of high aspect ratio coil lines
Fig. 8.3 Calculation of suction force and permit current in different aspect ratio
8.3 Fabrication Process for Coil Lines
A spiral microcoil was formed on the surface of the acrylic pipe using X-ray li-thography and metallization techniques. Fabrication process for coil lines is shown
in Fig.8.4. First, a thread structure was formed on the pipe surface using X-ray li-
thography. Next, a thin seed layer of copper to be used as an electrode in electro-
forming was deposited on the pipe by spattering. The pipe was then immersed into
a copper plating bath for electroforming and electroforming carried out until the
spiral groove was filled with copper film. Finally, the plated copper was chemi-
cally etched to remove copper from the surface, but leave copper remaining in the
spiral groove thus forming a coil. The following sections give detailed descrip-
tions of each of the process steps.
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Fig. 8.4 Fabrication process for coil lines by X-ray lithography
8.3.1 X-Ray Lithography
In this experiment, we used an X-ray mask with feature widths of 10 m and 30m. Therefore, screw thread structures for 10 m and 30 m line width were
formed. To make a 3D micro coil line structures, the acrylic pipe was rotated us-
ing a stepping motor and movement of the X-ray mask was controlled by piezo-
electric elements. To expose on the pipe surface, X-ray exposure strategy was im-
plemental, in which the process was divided into 60 steps that is close to
continuity exposure. Thus, the pipe was rotated through an angle of 6 degrees
while the X-ray mask was advanced by just 1/60 of the pitch for each X-ray expo-
sure cycle [7].
After X-ray exposure, the PMMA was developed in GG developer (diethylene-glycolmonobutyether: 60 vol.%, morpholine: 20 vol.%, ethanolamine: 5 vol.%,
distilled water: 15 vol.%) at room temperature to form the screw thread structures
on the pipe surface. The spiral structure of coil lines was observed using a scan-
ning electron microscope (SEM). Figure 8.5 shows the spiral coil lines. In the case
of Fig.8.5a, the aspect ratio of coil lines was about 5 with a width of 10m. In the
case of 30 m lines and spaces pattern, an aspect ratio of 2 was obtained, as
shown in Fig.8.5b. From these figures, we were able to confirm that the joints be-
tween each section of the groove pattern were perfectly aligned. The processing
depth, which determined the aspect ratio of the coil lines, was controlled by the X-ray exposure dose and the development time as shown in Fig.8.6.
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(a) 10 m lines and spaces pattern (b) 30m lines and spaces pattern
Fig. 8.5 SEM images of coil lines with high aspect ratio structure
Fig. 8.6 Relationship between processing depth and development time
8.3.2 Formation of Seed Layer
Since the acrylic pipe is nonconductive, a conductive seed layer is required for
electroforming. A 300 nm thick seed layer was formed on the surface of the pipe by sputtering. In order to obtain a low resistance seed layer, we considered that the
pipe was moved along its axis and sputtering carried out with the pipe in three dif-
ferent positions. As a result, the resistance around the circumference of the pipe
was sufficiently low.
8.3.3 Electroforming of Copper
Following deposition of the seed layer, the acrylic pipe was immersed in an elec-
troplating bath of copper sulfate solution, which included a leveling agent to pro-
mote uniform growth by reducing the electric field strength at the edges of coil
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line structures. Therefore, the thickness of copper at the flute was thicker than on
the convex lines. Figure 8.7 shows a SEM image of a coil lines after copper elec-
troforming. This figure shows that the copper layer was grown up from the bottom
of grooves, completely filling the high aspect ratio structures.
Fig. 8.7 SEM image of coil lines after electroforming
8.3.4 Isotropic Chemical Etching
Isotropic chemical etching of copper using E-process-W etchant was performed
until only the copper in the grooves remained, thus forming the coil lines. The
pipe rotation mechanism was also used to rotate the acrylic pipe in the etchant toensure uniform pipe surface etching. From this result, we produced coil lines by
copper etching until the protrusions of groove structures were exposed, as shown
in Fig.8.8.
Fig. 8.8 SEM image of coil lines after isotropic etching
8.4 Measurements of Suction Force
We also built a measurement system, as shown in Fig.8.1, in order to measure the
suction force of the designed electromagnetic type actuator. This system is a very
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simple structure and it is easy to change the coil [7], as shown in Fig.8.9. The gap
between the plunger and the fixed core was adjusted by an XY stage. Figure 8.10
shows a comparison of the theoretical values by simulation and actual measure-
ment of the suction force generated by a coil with 30 m width and an aspect ratio
of 2. The measured results were in relatively good agreement with the theoreticalvalues. Here, the results include considerable errors where the gap between the
plunger and the fixed core is small because the magnetic flux assumed in the
simulation might be much different from the actual flux. Currently, we have been
carrying out measurements of the suction force by fabricating spiral microcoils
with higher aspect ratio structure produced by X-ray lithography and metallization
techniques.
Fig. 8.9 Measurement system for suction force of actuator
Fig. 8.10 Suction force comparison between measurement and simulation values
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8.5 Development of 1 mm Diameter Microcoil
The outside diameter of the acrylic pipe used was 5 mm. Therefore, the size of the
coil is too big for a microcoil and microactuator. So, we used metal master bars
with diameters of 0.5 to 1 mm for microcoil fabrication. PMMA was applied ontothe master bar using a dipping method [9,10]. The thickness of PMMA determines
the structure of the coil line depth. Thus, this is a very important factor in mi-
crocoil fabrication.
8.5.1 Dipping Method
The dipping method was used in order to obtain a thick layer of photoresist on the
metal bar. Figure 8.11 shows the fabrication process for metal bar and dipping process. The fabrication process is largely identical to that used for the acrylic
pipe, expect the final etching step. The dipping method comprises four steps: dip-
ping, recovery, air drying, and baking. A highly viscous photoresist solution and
control over the centrifugal force were important factors to obtain a thick uniform
coating, and thus enable the production of high aspect ratio coil lines.
Fig. 8.11 Fabrication process and dipping method
8.5.2 Results and Discussions
We were able to control the thickness of PMMA on metal bar by the speed of rota-
tion and concentration of PMMA [9]. In these results, PMMA thickness of morethan 100 m was obtained on metal bar in single coating. Thus, the aspect ratio of
coil lines achieved for 30 m width grooves was greater than 3.
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A spiral coil structure was formed in the PMMA on the metal bar using X-ray
lithography technique. In this case, we used an X-ray mask with 30m lines and
spaces patterns. The diameter of the metal bar was 0.5 mm. Figure 8.12 shows a
SEM image of coil line structures with a pitch of 60 m. This figure shows that
the aspect ratio realized was about 6 because the grooves were narrower than thedesigned width of the coil.
Next, we performed a metallization process, including electroforming and
photoresist etching. In this case, the metal bar acts as the seed layer for electro-
forming. Therefore, electroforming layer was grown up from the bottom com-
pletely filling the high aspect ratio grooves. Figure 8.13 shows a SEM image of
coil lines with a pitch of 60 m after removing the photoresist. The aspect ratiowas obtained about 2. Figure 8.14 shows a comparison of the size of the fabricated
microcoils. On the right was a coil made using the acrylic pipe as the base material
and on the left was used the metal bar. This figure shows we were able to obtain a
0.5 mm diameter microcoil with high aspect ratio. Therefore, these microcoils are
very expected to have high performance despite their miniature size.
Fig. 8.12 SEM image of coil lines
Fig. 8.13 SEM image of coil lines after resist etching
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Fig. 8.14 Fabricated microcoils comparison metal bar with acrylic pipe
8.6 Conclusions
We have fabricated narrow pitch and high aspect ratio spiral microcoils for an
electromagnetic type actuator using 3D deep X-ray lithography technique and me-
tallization process. Using these techniques, we succeeded in producing a grooved
structure with 10 m in coil line widths with a maximum aspect ratio of about 5.
We also succeeded in electroforming copper in the high aspect ratio structure and
forming a coil line by isotropic copper etching. Therefore, we could obtain mi-
crocoils with high aspect ratio coil lines structures.
In addition, we developed a measurement system to measure the suction force
produced by these electromagnetic type actuators. The results of suction force
measurements enabled us to confirm the results of simulation. These measurement
results were in relatively good agreement with the simulated ones.
We also attempted to fabricate microcoils with diameters of less than 1 millime-
ter. Using a dipping method, photoresist thickness of over 100m were achieved
using a highly viscous solution and controlling the centrifugal force. We suc-
ceeded in producing a spiral microcoil with 30m coil lines width with an aspect
ratio of about 2 using X-ray lithography and metallization techniques.
Using these techniques, we were able to fabricate microcoils with high aspect
ratio coil lines. Thus, it is very expected that electromagnetic type microactuators
with high suction force could be manufactured despite their miniature size.
Acknowledgments This research was partially supported by the Grant-in-Aid for Scientific Re-
search on Priority Area, No. 438, “Next-Generation Actuators Leading Breakthroughs”, from the
Ministry of Education, Culture, Sports, Science and Technology, Japan
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References
1. Becker EW, Ehrfeld W, Hagmann P et al (1986) Fabrication of microstructures with high as-
pect ratios and great structural heights by synchrotron radiation lithography, galvanoforming,
and plastic moulding. Microelectron Eng 4:35-56
2. Mekaru H, Kusumi S, Sato N et al (2005) Development of three dimensional LIGA process to
fabricate spiral microcoil. Jpn J Appl Phys 44:5749-5754
3. Mekaru H, Kusumi S, Sato N et al (2007) Fabrication of a spiral microcoil using a 3D-LIGA
process. Microsyst Technol 13:393-402
4. Ando A, Amano S, Hashimoto S et al (1998) Isochronous storage ring of the New SUBARU
project. J Synchrotron Rad 5:342-344
5. Mekaru H, Utsumi U, Hattori T (2001) Beam line BL11 for LIGA process at the NewSU-
BARU. Nucl Instrum Methods A 467-468:741-744
6. Mochizuki H, Mekaru H, Kusumi S et al (2007) Design of solenoidal electromagnetic micro-
actuator utilizing 3D X-ray lithography and metallization. Microsyst Technol 13:547-550
7. Matsumoto Y, Setomoto M, Noda D, Hattori T (2008) Cylindrical coils created with 3D X-raylithography and metallization for electromagnetic actuators. Microsyst Technol 14:1373-1379
8. Setomoto M, Matsumoto Y, Yamashita S et al (2008) Fabrication of Spiral Micro Coil Lines
for Electromagnetic Actuators. J Adv Mech Des Syst Manuf 2:238-245
9. Noda D, Yamashita S, Matsumoto Y et al (2008) Fabrication of High Aspect Ratio Microcoil
Using Dipping Method. J Adv Mech Des Syst Manuf 2:174-179
10. Noda D, Matsumoto Y, Setomoto M, Hattori T (2008) Fabrication of Microcoils Using X-ray
Lithography and Metallization. IEEJ Trans SM 128:181-185
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