micro actuator production

8/18/2019 Micro Actuator Production

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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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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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