NOVEL MEMS-BASED FABRICATION TECHNOLOGY OF MICRO

SOLENOID-TYPE INDUCTOR FOR MICRO ENERGY APPLICATION

S.Uchiyama1, 2

, A. Toda3, Z.Q. Yang

1, Y. Zhang

1*, M. Hayase

2, H. Takagi

1, T. Itoh

1, R. Maeda

1

1National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Japan

2Tokyo University of Science, Noda, Japan

3Meltex Inc., Saitama, Japan

Abstract: Solenoid configuration is preferred for micro inductor because of high quality factor and low loss but it

is difficult to be realized using conventional MEMS process. In this work, we present a novel MEMS-based

fabrication technology of micro solenoid-type inductor using cylindrical projection lithography. Micro inductor

prototypes were successfully prepared. The minimum feature sizes including average line width and pitch were

reduced to about 18.3 m and 39.4 m, respectively. Electroplating process of copper layer was also successfully

established for tiny capillary substrates. 4.7 and 5.6 m thick copper layers were electroplated on 1 mm-diameter

capillary and the measured resistance per windings was about 0.56 and lower. The prepared prototypes show

attractive potentials in application of micro energy technology such as electric current sensing.

Keywords: MEMS, micro solenoid-type inductor, cylindrical projection lithography.

INTRODUCTION Solenoid-type inductor is one of the most

commonly used electronic components and there have

been being strong interests into its miniaturization

technology. Solenoid-type micro inductor is

particularly interesting for micro energy applications

such as electric current sensing, because it has higher

inductance and lower loss than other types of inductor

[1]. As a matter of fact, the solenoid-type micro

inductor is difficult to be prepared using conventional

MEMS processes, which are mainly for planar

substrates. As a result, most reported MEMS-based

micro inductors are still of planar configurations, and

so they have limited performance and high loss.

Very recently, K. Kratt et al. [2-3] have

successfully prepared solenoid-type micro coils using

an automatic wire bonder. One micro coil with 4

windings and a diameter of 690 m can be fabricated

in 200 ms. Although, their method is very attractive

for practical applications, V. Demas et al had found

that the lithographic coils showed better performance

than wire wound coils when the pitch was shorter than

50.8 µm [4]. Therefore, there are still many efforts on

developing photolithography-based fabrication

technology for micro solenoid-type inductor [4].

Using laser irradiation, T. Kikuchi et al. also

successfully fabricated a platinum grid-shaped

microstructure, a microspring, and a cylindrical

network microstructure with 50-100 m line width [5].

Matsumoto et al. [6] developed a three-dimensional

X-ray lithography method using X-rays from a

synchrotron radiation facility as a light source for

lithographic exposure and an X-ray mask for

patterning cylindrical microcoil structures with a high

aspect ratio. However, these methods are involved of

expensive equipments or limited resolutions. New

photolithography method should be developed for the

fabrication and integration of micro solenoid-type

inductor onto a common substrate on which other

MEMS components and circuits can be easily

fabricated. In this work, we would present a new

cylindrical photolithography method for fabrication of

solenoid-type inductor on a 1 mm-diameter capillary.

Fig. 1 is schematic of the micro inductor to be

prepared.

CYLINDRICAL PROJECTION

LITHOGRAPHY SYSTEM Fig. 2 is illustration of the cylindrical projection

lithography system that was mainly involved of the

movement of cylindrical substrate at the programmed

speed and displacement. In this system, light source is

Fig. 1: Image of solenoid-type inductor.

978-0-9743611-9-2/PMEMS2012/$20©2012TRF 26 PowerMEMS 2012, Atlanta, GA, USA, December 2-5, 2012

an Hg–Xe lamp (LC8, Hamamatsu photonics), which

has a wavelength range of 250-600 nm.

This broad wavelength of incident light results in

chromatic aberration in the focal plane. To remove

this chromatic aberration, a bandpass interference

filter with a central wavelength of 436 ± 10 nm was

inserted between the light source and the shutter. The

mask pattern was projected on to the cylindrical

substrate with the reduction ratio of 1: 2 by passing

through the all optical elements. The He-Ne laser

beam was utilized as a reference to setup and align all

optical elements. The cylindrical substrate could be

moved in X and -direction. The mask could be

moved in X, Z, -direction. Therefore, the whole

system has five degree-of-freedom so that various

patterns could be prepared. Fig. 3 shows the

fabrication sequences of micro solenoid-type inductor

on 1 mm-in-diameter quartz capillary. Metal seed

layers were deposited on the quartz capillary using

sputtering method. Photoresist films on 1 mm-in-

Fig. 2: Schematic of cylindrical projection lithography system.

Fig. 3: Fabrication sequence of micro solenoid-type inductor. (a) Quartz capillary ); (b) Sputtering; (c) Spray

coating of photoresist; (d) exposure; (e) Development; (f) Electroplating of copper; (g) Wet etching; (h) Wet

etching ; (i) Electroplating of copper.

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diameter capillary were deposited using direct spray

coating. In this work, Shipley S1830 positive

photoresist (Shipley Co. LLC) was used. In addition,

AZ5200 thinner (AZ Electronic Materials) was used

as thinner solvents. AZ5200 thinner mainly consists

of propylene glycol monomethyl ether acetate

(PGMEA).

Two type of electroplating processes were

developed for copper windings. In the Process 01,

copper is used as metal seed layers. Moreover, seed

layers are removed after copper electroplating (Fig. 3

(f) & (g)). On the other hand, in the process 02, gold

is used as seed layers, and after removal of seed layers,

inductor patterns are directly electroplated (Fig. 3 (h)

& (i)).

EXPERIMENTS

Fig. 3 shows the fabrication sequences of micro

solenoid-type inductor on 1 mm-in-diameter quartz

capillary. Metal seed layers were deposited by using

sputtering with rotation of substrates. Photoresist

films on 1 mm-in-diameter capillary were deposited

by using direct spray coating. In this work, Shipley

S1830 positive photoresist (Shipley Co. LLC) was

used. In addition, AZ5200 (AZ Electronic Materials)

was used as thinner solvents. AZ5200 thinner is

mainly based on propylene glycol monomethyl ether

acetate (PGMEA).

Two type of electroplating processes were

developed for copper windings. In the Process 01,

copper is used as metal seed layers. Moreover, seed

layers are removed after copper electroplating (Fig. 3

(f) & (g)). On the other hand, in the process 02, gold

is used as seed layers, and after removal of seed layers,

inductor patterns are electroplated (Fig. 3 (h) & (i)).

RESULTS & DISCUSSION

Fig. 4 shows the fabricated micro inductor prototype

with the pitch of 40 m and the turns of 20. The

averaged feature sizes including line width, pitch, and

thickness along the longitudinal direction were 18.3

µm, 39.4 µm, 4.7 µm, respectively and the measured

value of resistance was 0.56 /turns. Solenoid-type

micro inductors with the same pitch of 40 m but

more coils turns (20-100 turns) were also successfully

prepared on the 1 mm-in-diameter capillary.

However, the copper windings were damaged during

the etching process of the seed layers, as shown in Fig.

4 (b). In addition, it is difficult to fabricate a thicker

inductor because it is difficult to achieve fine patterns

in resist films thicker than 10 µm using the current

cylindrical photolithography system.

In order to solve this problem, instead of

conventional process (Fig. 3 (f) & (g)), a directly

electroplating process is also developed (Fig. 3 (h) &

(i)). Fig. 5 (b) shows the fabricated micro inductor

prototype using the direct electroplating process. The

prepared inductor was of the pitch of about 40 m. Its

line width was about 30.3 µm. The inductor thickness

was about 5.6 µm. Although the growth rate of copper

layer along the sidewall of windings is almost as the

same as that normal to the substrate, fine patterns

were still achieved. Better results could be expected

Photoresist

Copper

Coil

Pad

Pad

(a)

Quartz

CopperCoil

(b)

Pad

Pad

Fig. 4: SEM images fabricated solenoid-type micro

inductor. (a)As electroplated, (b) after removal of

resist and copper seed layer. Table 1: Average feature sizes of prepared solenoid

structure including line width, pitch and thickness.

width pitch thickness

designed values

[µm] 20.0 40.0 6.0

measured values

(Process 01)

[µm]

18.3 39.4 4.7

measured values

(Process 02)

[µm]

30.3 43.3 5.6

28

through optimization of mask pattern. Because gold

seed layer exhibited better compatibility to the whole

fabrication process, more effort would be

concentrated on Process 02 and the latest results

would be presented on the conference.

CONCLUSION One novel MEMS-based fabrication technology of

micro solenoid-type inductor was presented in this

work using the cylindrical projection

photolithography method. Micro inductor prototypes

were successfully prepared and minimum feature

sizes had been reduced down to 20 µm. Two type of

fabrication process have been developed. In the

process 01, a solenoid structure with the averaged

feature sizes including line width, pitch, and thickness

along the longitudinal direction were 18.3 m, 39.4

m, 4.7 m, respectively and the measured value of

resistance was 0.56 /windings. In the process 02, a

solenoid structure with the averaged feature sizes

including line width, pitch, and thickness along the

longitudinal direction were 30.3 m, 43.3 m, 5.6 m,

respectively. The inductor prototypes were of solenoid

configuration so that high inductance and low loss

could be expected, and thus they would be attractive

for micro energy applications.

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Fabrication of Three-Dimensional Platinum

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QuartzCopper

windings

18.3 µm

39.4 µm

(a)

Copper

windings Quartz

30.3 µm

43.3 µm

(b)

Fig. 5: SEM images of fabricated solenoid-type

micro inductor. (a) By using Process 01, (b) by

using Process 02.

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