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Micro-gear fabrication using optical projection lithography oncopper-clad plastic substrates and electroplating of nickel

Toshiyuki Horiuchi *, Yusuke Furuuchi 1, Ryota Nakamura 2, Katsunori Hirota 3

Tokyo Denki University, 2-2 Kanda-Nishiki-cho, Chiyoda-ku, Tokyo 101-8457, Japan

Available online 14 February 2006

Abstract

Nickel micro-gears were fabricated using optical projection lithography on copper-clad plastic substrates and nickel electroplating.Adopting low numerical-aperture projection exposure, large depth-of-focus was secured, and SU-8 resist moulds with vertical sidewallswere fabricated. In addition, cost was greatly reduced by using copper-clad plastic substrates instead of silicon wafers. After nickel waselectroplated in nickel sulfate solution, nickel gears with modules of 0.0260.04 and pitch diameters of 7601100 lm were stripped fromthe substrates when the resist moulds were dissolved in hot remover. 2006 Elsevier B.V. All rights reserved.

Keywords: Micro-gear; Optical projection lithography; Nickel electroplating; Copper-clad plastic substrate

1. Introduction

Micro-fabrication methods combining lithography andelectroplating are useful for developing various micro elec-trical and mechanical systems (MEMS) such as sensors,actuators, mirror matrixes, chemical reactors, and others.

Though X-ray lithography has been mainly adopted forthese uses [13], low-cost optical lithography is preferablebecause required pattern sizes are generally large[4,5].

On the other hand, silicon wafers are often too expen-sive, and electro-conductive substrates are preferable forthe following electroplating.

For this reason, a new method using optical projection

lithography on copper-clad plastic boards is investigated.After fabricating resist moulds, they are filled with nickelby electroplating, and precise nickel micro-gears are suc-cessfully obtained.

2. Resist mould fabrication

Resist moulds were formed by the developed exposuresystem shown inFig. 1. A camera lens with a low numericalaperture of 0.063 was used as the projection lens to secure alarge depth-of-focus, and a UV light source with spectrumof 290420 nm was prepared for exposing a negative resistSU-8 100 (MicroChem Corp.) having high transmittancefor UV light (Fig. 2).

Light intensity distribution degrades under defocusedconditions. However, the light intensity decreases depend-ing on the absorption through the resist. Therefore, if thebroadening of light intensity distribution curves caused

by the defocus is balanced with the decreasing of the lightintensity caused by the absorption, light intensity curves atvarious depths in the resist cross at one point as shown inFig. 3. Then, the widths of the light intensity curves becomesame at all depths when the exposure slice level is set at thecross point. Therefore, the light intensity contours becomevertical to the substrate.

It is preferable to put the image focal point higher thanthe resist surface to realize this performance. Fig. 4showsexamples of printed line-and-space patterns. They wereprinted well with an aspect ratio of up to 7.

0167-9317/$ - see front matter 2006 Elsevier B.V. All rights reserved.

doi:10.1016/j.mee.2006.01.083

* Corresponding author.E-mail address:[email protected](T. Horiuchi).

1 Present address: Citizen Watch Co., Ltd.2 Present address: Citizen Electronics Co., Ltd.3 Present address: Canon Inc.

www.elsevier.com/locate/mee

Microelectronic Engineering 83 (2006) 13161320
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Isolated line patterns were printed even when the aspectratio was more than 11, as shown in Fig. 5.

Fig. 6 shows the resist moulds of micro-gear with athickness of 200 and 440lm. A regular-size brass gearwas placed on a transparent acrylic plate and used as areticle.

3. Nickel electroplating

Nickel electroplating was investigated next using com-mercially available nickel sulfate solution. Substrates werecut into small pieces with a size of less than 10 mm2, andevery small piece was moved in the solution by a motor(Fig. 7).

Electroplating temperature recommended by the solu-tion vendor was 4565 C. Fig. 8 shows the relationshipbetween the temperature and the electroplating rate.Though the rate was highest at 45 C, we selected 50 Cbecause 45C was the lower limit of the vendors

recommendation.Electroplating rate was much more dependent on the

current as shown inFig. 9, and it was approximately pro-portional to the square root of the current. Therefore, toomuch current is not economical. When the current is largerthan 20 mA, top surfaces of the electroplated nickel gearssometimes became rough and thickness uniformitydegraded. Therefore, the current condition of 15 mA was

0 100 200

Resist thickness (m)

300

=365 nm

=405 nm

0.10

0.20

0.40

1.00

0.80

0.60

timsnarT

cnat

e

Fig. 2. Measured transmittance of SU-8 for exposure light.

Exposure light

Substrate

C

A

B

D

Resist

Position (m)

thgiL

ytisnetni

Appropriate slice level

D

C

B

A

Fig. 3. Principle for obtaining vertical sidewalls.

=290-460 nmNA=0.063Reduction: 1/19Width: 600mm

Depth: 300mmHeight: 970mm

Light guide UV light source

Collective lens

Reticle stage

Projection lens

Wafer stageBase plate

Fig. 1. The exposure system used for fabricating resist moulds.

Fig. 4. Patterns printed in 240-lm thick SU-8 resist: (a) 32-lm line-and-space; (b) 37-lm line-and-space.

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Fig. 5. Isolated patterns with high aspect ratios: (a) width is 24 lm; (b) width is 53 lm.

Fig. 6. Resist moulds of micro-gears: (a) 200 lm thick; (b) 440 lm thick.

Hot plate

Substrate withresist moulds

Nickel

sulfate

Stirring mechanism Nickel anode

Plastic vessel

Fig. 7. Experimental setup for etching.

200

70605040

150

100

50

Temperature (C)

Fig. 8. Plating rate dependence on temperature.

10 100110.1

100

10

Current (mA)

Fig. 9. Plating rate dependence on current.

0

100

200

300

400

500

0 5

Electroplating time (h)

10

Fig. 10. Nickel thickness dependence on electroplating time.

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selected. The optimum current probably depends on thespecimen size. In the above experiments, electroplated areasize was only about 1.05 mm2.

Thickness dependence on electroplating time is shown inFig. 10. Though the thickness increased according to thetime, the plating rate gradually decreased. However, itwas confirmed that nickel was electroplated more than400 lm thick anyway.

4. Resist removal and stripping of gears

Resist moulds were dissolved in SU-8 remover, and elec-troplated nickel parts were stripped from the substrates.SU-8 remover was heated at 80 C in a plastic vessel on ahot plate, and the specimens were soaked in it. When thevessel was swung for 530 min by hands, most of the resistmoulds were dissolved and some of the nickel gears werestripped off from the substrate. Figs. 11 and 12 show thefinished nickel gears and the gear teeth. In some cases,however, the resist was not dissolved and remained at nar-row hollows of the moulds, especially in the center holes.Resist removal and stripping of micro-parts should beinvestigated further.

If these gears were arranged in an appropriate casing

and moved by a motor, micro-gear pump would be real-

ized. Mechanical arrangement of gears in a casing is shownin Fig. 13. The pump casing was fabricated by the samemethod.

5. Conclusion

A low-cost micro-fabrication method using opticalprojection lithography and nickel electroplating was dem-

onstrated. Using optical lithography instead of X-ray

Fig. 11. Fabricated nickel micro-gears. (a) Module is 0.04 and pitch-diameter is 760lm. (b) The gear is supported on a tip of a needle. Module is 0.026and pitch-diameter is 1100 lm.

Fig. 12. Detailed view of the gear teeth: (a) thickness is 200 lm, (b) thickness is 400 lm.

Fig. 13. Nickel gears placed in a pump casing.

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lithography, mask cost and exposure system cost wereextremely decreased. In addition, using copper-clad plas-tic substrates instead of silicon wafers, substrate cost wasalso reduced to less than 1/10.

Applying the new method, micro-gears with modules of0.0260.04, pitch diameters of 7601100lm and thick-

nesses of 200400 lm were successfully fabricated.

Acknowledgments

This work was partially supported by a Grant-in-Aidfrom the Research Institute for Technology, Tokyo Denki

University (Q04M-04) and Tokyo Denki UniversityScience Promotion Fund.

References

[1] E.W. Beckert, W. Ehrfeld, P. Hagmann, A. Maner, D. Munchmeyer,

Microelectron. Eng. 4 (1986) 35.[2] H. Guckel, Proc. IEEE 86 (1998) 1586.[3] C. Yoshimura, Y. Kobayashi, H. Aoki, H. Ueno, S. Sugiyama, Trans.

IEE Jpn. 121-E (2001) 266.[4] K. Hirota, M. Ozaki, T. Horiuchi, Jpn. J. Appl. Phys. 42 (2003) 4031.[5] R. Engelke, G. Engelmann, G. Gruetzner, M. Heinrich, M. Kubenz,

H. Misschke, Microelectron. Eng. 7374 (2004) 456.

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