**4.3 SR ablation**

In 1996, a technique for direct etching of PTFE using an SR light source was reported (Katoh & Zhang, 1996). Unlike X-ray lithography, direct etching was performed entirely with dry etching and did not use wet etching. With this technology, it is also possible to combine the electroforming technique and moulding process, as in the LIGA process. This technology is referred to as the TIEGATM (Teflon® Included Etching Galvanic Forming) process. Figure 20 shows the process flow of the TIEGATM process. The etching rate of PTFE using SR ablation is very high, 10–100 m/min. Moreover, because the developing process is not necessary, structure collapse is not caused by sticking effects. Sticking is problematic in microfabrication utilizing wet etching, such as lithography. Although fabrication of highaspect-ratio structures is also possible, surface roughness of sidewalls is undesirable, and the taper angle is large compared with that of the LIGA process.

Fig. 20. Process flow of the TIEGATM process

PTFE microfabrication is difficult to achieve. It is impossible to fabricate this material through wet etching with chemicals (acids and alkali) used in numerous microfabrication techniques, such as lithography, because of its excellent chemical resistance. Additionally, when the temperature is increased above the melting point of PTFE (327°C), the viscosity becomes too high for moulding. Moreover, laser ablation, which has been widely used in recent years as an effective tool for direct microfabrication, is also difficult. Because the first absorption band of PTFE is near 160 nm, where little light is absorbed by the UV to infrared (IR) domain, ablation processing using a laser with this domain is not possible. Most laser processing of PTFE is not laser ablation but rather thermal processing, which causes a deformation in structure surfaces. Therefore, lasers with narrower wavelengths, such as vacuum ultraviolet (VUV) pulsed-lasers or ultrashort pulsed-lasers, are used for microfabrication of PTFE. However, the aspect ratio of the structures that can be fabricated is very small, generally <1. For these reasons, laser ablation was developed, replacing lasers

In 1996, a technique for direct etching of PTFE using an SR light source was reported (Katoh & Zhang, 1996). Unlike X-ray lithography, direct etching was performed entirely with dry etching and did not use wet etching. With this technology, it is also possible to combine the electroforming technique and moulding process, as in the LIGA process. This technology is referred to as the TIEGATM (Teflon® Included Etching Galvanic Forming) process. Figure 20 shows the process flow of the TIEGATM process. The etching rate of PTFE using SR ablation is very high, 10–100 m/min. Moreover, because the developing process is not necessary, structure collapse is not caused by sticking effects. Sticking is problematic in microfabrication utilizing wet etching, such as lithography. Although fabrication of highaspect-ratio structures is also possible, surface roughness of sidewalls is undesirable, and

the taper angle is large compared with that of the LIGA process.

Fig. 20. Process flow of the TIEGATM process

**4.2 Microfabrication of PTFE** 

with SR light.

**4.3 SR ablation** 

Fig. 21. (A) is processing mechanism; and (B) is experimental system

Fig. 22. Relation between the beam current and processing depth

We used a 1-mm-thick product from Yodogawa Hu-tech Co., Ltd. for PTFE. The processing mechanism is outlined in Figure 21A. When PTFE is exposed to an SR light source, a photochemical reaction occurs in which the main chain of the material decomposes, forming a fluorocarbon gas (CF3-CnF2n-CF3), and exposed parts are etched. The etching rate generally increases when PTFE is etched in a vacuum chamber and heated during the SR ablation fabrication process. The secession rate of fluorocarbon gas increases if the PTFE temperature is excessively high at this time. Therefore, a vacuum atmosphere at 10-5 torr was used, and a substrate heater was installed in the chamber. To prevent the mask and Be window from becoming contaminated by fluorocarbon gas, a 25-m polyimide film was placed on each (Figure 21B). The processing depth was checked after an exposure of 30 min, and the PTFE surface temperature was increased to 110°C, 140°C, 170°C, and 200°C in succession. Figure 22 shows the relationship between the beam current and processing depth. In general, the TIEGATM process utilizes white light. However, because this experimental system was

Fabrication of 3-D Structures Utilizing Synchrotron Radiation Lithography 333

Chapter 1 (Introduction) described the field of fabrication technologies of high-aspect-ratio structures and 3-D structures and explained the purpose of SR lithography. Chapter 2 (SR Lithography) provided an outline of the LIGA process, fabrication mechanism of SR lithography, processing-depth control, and optimum experimental conditions. Sections 2.2 (Exposure) and 2.3 (Development) described the mechanism of SR lithography and the requirements of the light source, the X-ray mask, and the resist materials. Section 2.4 (Approaches to High-Accuracy Microfabrication) described the resolution, optimum experimental conditions, and micro-loading effect based on both experimental and theoretical values. To achieve high-accuracy microfabrication, all of these are important. Chapter 3 (3-D Fabrication Method) described the fabrication method of 3-D structures utilizing SR lithography. Two fabrication techniques were described in detail: the PCT technique and the pixels exposure technique. In this chapter, the fabrication process and results, as well as the mechanism of 3-D fabrication, were described. Chapter 4 (PTFE Fabrication by SR Ablation) described the 3-D PTFE microstructures fabricated by SR ablation. Advantages of ablation technology and the fabrication mechanism were described. We expect that this research will contribute to elemental technologies in the field of MEMS, the achievement of high functionality, and to the development of high-performance devices

We would like to express our sincere gratitude to our supervisor, Prof. Toshiaki Ohta who is

We would like to thank Dr. Hiroshi Ueno, Dr. Sommawan Khumpuang, Mr. Kazuya Fujioka, Mr. Shinya Fujinawa, and Mr. Shunsuke Kajita for providing us with useful data. We would like to thank Dr. Yasukazu Yamamoto and Mr. Hiroyuki Ikeda for their technical

Ehrfeld, W.; Wood, R.L.; Hessel, V.; Lowe, H.; Schulz, C ; Weber, L (1998), Materials of LIGA

Fujinawa, S.; Kato, F.; Sugiyama, S (2006), Development of fabrication process for shape-

Vol.6037, pp.331-338,, ISSN 0277-786X, Brisbane, Australia, Dec.11-15,2005 Horade, M. & Sugiyama, S (2010), Study on fabrication of 3-D microstructures by

Kato, T. & Zhang, Y (1998), High aspect ratio micromachining by synchrotron radiation

*Technologies*, Springer, Vol.16, Num.8-9, pp.1331-1338, ISSN 0946-7076 Kato, F.; Fujinawa, S.; Li, Y.G.; Sugiyama, S (2007), Fabrication of a spiral microcoil using a

technology, *Microsystem Technologies*, Springer, Vol.5, Num.3, pp.105-112, ISSN

control of three-dimensional submicron structure by synchrotron radiation lithography, *Proceedings of SPIE - The International Society for Optical Engineering*,

synchrotron radiation based on pixels exposed lithography, *Microsystem* 

3D-LIGA process, *Microsystem Technologies*, Springer, Vol.13, Num.3-4, pp.221-225,

direct photo-etching, *Microsystem Technologies*, Springer, Vol.4, Num.3, pp.135-138,

that, so far, have been difficult to realize.

head of SR Center of Ritsumeikan University.

**6. Acknowledgment** 

assistance.

**7. References** 

0946-7076

ISSN 0946-7076

ISSN 0946-7076

designed for use with the TIEGATM process as well as the LIGA process, PTFE was only exposed to the X-ray domain. Therefore, the etching rate and processing depth were low.
