**5. Conclusions**

MEMS devices have attracted much attention, and further studies are needed to realize their full potential. In fabrication technology, an elemental technology, microfabrication, developed primarily using a semiconductor process, is in high demand. Recently, MEMS devices have diversified, and microfabrication technologies for the production of highaspect-ratio and 3-D structures are in demand. This article described the fabrication of 3-D microstructures utilizing SR lithography.

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 that, so far, have been difficult to realize.
