**1. Introduction**

314 Recent Advances in Nanofabrication Techniques and Applications

Videla, F. A.; Torchia, G. A.; Schinca, D. C.; Scaffardi, L. B.; Moreno, P.; Méndez, C.; Giovanetti,

*Journal of Applied Physics*, Vol. 107, No. 11, (June 2010), 114308, ISSN 1089-7550 Villafiorita-Monteleone, F.; Caputo, G.; Canale, C.; Cozzoli, P. D.; Cingolani, R.; Fragouli, D.;

Voltairas, P. A.; Fotiadis, D. I.; Michalis, L. K. (2002). Hydrodynamics of magnetic drug targeting. *Journal of Biomechanics*, Vol. 35, No. 6, (June 2002), pp. 813–821, ISSN 0021-9290 Wang, C.-W.; Moffit, M. G. (2004). Surface-Tunable Photoluminescence from Block

Wang, J.; Ni, X. (2008). Interfacial structure of poly(methyl methacrylate)/TiO2

*Polymer Science*, Vol. 108, No. 6, (June 2008), pp. 3552-3558, ISSN 1097-4628 Wang, M.; Singh, H.; Hatton, T.A.; Rutledge, G. C. (2004). Field-responsive

Wang, R.; Hashimoto, K.; Fujishima, A.; Chikuni, M.; Kojima, E.; Kitamura, A.;

Wang, R.; Hashimoto, K.; Fujishima, A.; Chikuni, M.; Kojima, E.; Kitamura, A.; Shimohigoshi,

*Advanced Materials*, Vol. 10, No. 2, (January 1998), pp. 135-138, ISSN 1521-4095 Wang, R.; Sakai, N.; Fujishima, A.; Watanabe, T.; Hashimoto, K. (1999). Studies of Surface

Weller, D.; Doerner, M. F. (2000). Extremely High-Density Longitudinal Magnetic Recording Media. *Annual Review Materials Science*, Vol. 30, pp. 611-644, ISSN 0084-6600 Wu, F.; Zhang, J. Z.; Kho, R.; Mehra, R. K. (2000) Radiative and Nonradiative Lifetimes of

Xia, H.; Peng, J.; Liu, K.; Li, C.; Fang, Y. (2008). Preparation and Gas Sensing Properties of

Yi, H.; Wu, L.-Q.; Bentley, W. E.; Ghodssi, R.; Rubloff, G. W.; Culver, J. N.; Payne, G. F.

Zangmeister, R. A.; Park, J. J.; Rubloff, G. W.; Tarlov, M. J. (2006). Electrochemical study of

Zhao, X.; Xu, M.; Qin, L.; Xiao, J.Q. (2007). Microstructure and magnetic properties of

Zhou, K.; Yin, J.-J.; Yu, L. (2006). ESR determination of the reactions between selected

Vol. 51, No. 25, (July 2006), pp. 5324–5333, ISSN 0013-4686

102, No. 11, (December 2007), 113913, ISSN 1089-7550

*B*, Vol. 103, No. 12, (March 1999), pp. 2188-2194, ISSN 1520-5207

(December 2010), pp. 18557–18563, ISSN 1520-5827

(December 2004), pp. 11784-11796, ISSN 1520-5827

16, (July 2004), pp. 5505-5514, ISSN 0032-3861

(November 2000), 237-242, ISSN 0009-2614

No. 10, (May 2008), 105405, ISSN 1361-6463

2005), pp. 2881–2894, ISSN 1525-7797

(April 2006), pp. 446–457, ISSN 0308-8146

Vol. 388, No. 6641, (July 1997), pp. 431, ISSN 1476-4687

L. J.; Ramallo Lopez, J. M.; Roso, L. (2010). Analysis of the main optical mechanisms responsible for fragmentation of gold nanoparticles by femtosecond laser radiation.

Athanassiou, A. (2010). Light-Controlled Directional Liquid Drop Movement on TiO2 Nanorods-Based Nanocomposite Photopatterns. *Langmuir*, Vol. 26, No. 23,

Copolymer-Stabilized Cadmium Sulfide Quantum Dots. *Langmuir*, Vol. 20, No. 26,

nanocomposites prepared through photocatalytic polymerization. *Journal of Applied* 

superparamagnetic composite nanofibers by electrospinning. *Polymer*, Vol. 45, No.

Shimohigoshi, M.; Watanabe, T. (1997). Light-induced amphiphilic surfaces. *Nature*,

M.; Watanabe, T. (1998). Photogeneration of Highly Amphiphilic TiO2 Surfaces.

Wettability Conversion on TiO2 Single-Crystal Surfaces. *Journal of Physical Chemistry* 

Band Edge States and Deep Trap States of CdS Nanoparticles Determined by Time-Correlated Single Photon Counting. *Chemical Physical Letters*, Vol. 330, No. 3-4,

Novel CdS-Supramolecular Organogel Hybrid Films. *Journal of Physics D,* Vol. 41,

(2005). Biofabrication with chitosan. *Biomacromolecules*, Vol. 6, No. 6, (November

chitosan films deposited from solution at reducing potentials. *Electrochimica Acta*,

magnetite thin films prepared by reactive sputtering. *Journal of Applied Physics*, Vol.

phenolic acids and free radicals or transition metals. *Food Chemistry*, Vol. 95, No. 3,

Microfabrication of high-aspect-ratio or three-dimensional (3-D) structures is critical for the production of various components for micro electro mechanical systems (MEMS). The term "three-dimensional structure" refers to a structure with a free-form surface or sloped sidewall. This article describes the fabrication of 3-D microstructures using synchrotron radiation (SR) lithography. SR lithography technology is one component of the LIGA process, and it is also called X-ray lithography.

MEMS devices have attracted a great deal of attention, and further studies are needed to realize their full potential. Among fabrication technologies, microfabrication, developed using a semiconductor process, is in high demand. Recently, the demand for MEMS devices has diversified, and microfabrication technologies for the production of high-aspect-ratio and 3-D structures are required to meet this demand.

Microfabrication technologies for the production of high-aspect-ratio structures include deep reactive-ion etching (D-RIE) and deep X-ray lithography in the LIGA process utilizing SR light. In the former, because SR light is highly directional, it is possible to fabricate a structure with a thickness of several hundred to one thousand micrometers. Moreover, because SR light contains X-ray (short wavelength) regions, it is possible to transfer patterns that are ≤ 1 m (diffraction during exposure does not occur readily). Therefore, SR lithography has been used as a fabrication technology for high-aspect-ratio structures. It is possible to fabricate high-aspect-ratio structures using D-RIE. However, because a patterned, indented sidewall called a scallop is formed due to the nature of the process mechanism, it is difficult to fabricate structures with smooth sidewall surfaces. On the other hand, it is possible to fabricate structures with smooth sidewall surfaces using SR lithography, which is discussed in more detail in Chapter 2.

In the field of 3-D microfabrication, techniques such as KOH anisotropic etching of silicon and laser machining have been employed (Tsukada et al., 2005). However, 3-D fabrication using SR lithography was recently achieved, and results have already been reported (Horade & Sugiyama, 2009; Lee & Lee, 2003; Matsuzuka et al., 2005; Mekaru et al., 2007; Sugiyama et al., 2004; Tabata et al., 2000). Nanoscale 3-D microfabrication technology using SR lithography can be used to fabricate high-aspect-ratio structures by exploiting the properties of SR, and free-form structures with inclined sidewall surfaces can be fabricated. Additionally, this article describes 3-D polytetrafluoroethylene (PTFE) microstructures fabricated by SR ablation. Because PTFE is a remarkable material, there are high

Fabrication of 3-D Structures Utilizing Synchrotron Radiation Lithography 317

regions, it is possible to transfer patterns that are ≤ 1 m (diffraction during exposure does not occur readily). Therefore, SR lithography has been used as a fabrication technology for high-aspect-ratio microstructures. Fabrication of Ni structures with a line width of 2 m and an aspect ratio of 100 or a line width of 0.2 m and an aspect ratio of 75 have been reported (Kato et al., 2007; Kondo et al., 2000; Ueno et al., 2000). Although it is also possible to transfer patterns that are ≤ 1 m using electron beam lithography, it has the disadvantage of a long exposure time. On the other hand, because the production of large volumes is possible using electroforming and moulding in the LIGA process, it is a superior technology

In SR lithography, the use of ultra-bright and highly directional SR light sources provides perfect conditions for fabricating structures with the required thickness. Although SR light is spectrally continuous and includes a wide wavelength range, wavelengths of 0.2 to 0.5 nm are most suitable for SR lithography because they reduce the spread of light by Fresnel diffraction in the long-wavelength domain and the generation of secondary electrons inside

Experiments described in this article utilized the superconductivity compact SR source "AURORA" at the SR Centre of Ritsumeikan University in Japan (Figure 2). The properties of SR at AURORA include a wavelength range from 0.15 nm to visible light and an applied electron energy and maximum storage current of 575 MeV and 300 mA, respectively. This light source was adapted for our studies; there are 16 beam lines, 4 of which are used in SR lithography. The light from AURORA penetrates two 200-m beryllium (Be) windows and, within the exposure chamber, uses light with a 0.15- to 0.95-nm wavelength domain. The outline of the beam line is shown in Figure 3. For beam line number 13 (Bl-13), the distance from the light source to the sample is 3.388 m. The exposure environment in the chamber was helium (He) gas at 1 atm to prevent the attenuation of X-rays by N2 or O2 gases and to prevent damage to the mask or resist by heat generated during exposure. Figure 4 shows the wavelength and photon density after penetration of the two 200-m Be windows; the peak

Fig. 2. Superconductivity compact SR source "AURORA" at the SR Centre of Ritsumeikan

resists in the short-wavelength domain, enhancing resolution.

in terms of time and cost.

wavelength was 0.37 nm.

University in Japan

**2.2 Exposure 2.2.1 Light source** 

expectations regarding its application in various devices. PTFE fabrication by SR ablation is discussed in detail in Chapter 4.
