Fabrication of 3-D Structures Utilizing Synchrotron Radiation Lithography

Abstract

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