**3.1 Advancement in the LIGA process**

326 Recent Advances in Nanofabrication Techniques and Applications

supply new developer to the bottom, and dissolved PMMA tends to remain near the bottom. Figure 14 shows the relationship between line width and processing depth using dosages of 0.03 and 0.05 Ah. Plots show experimental values for line widths of 10, 15, 20 and 25 m, and lines show the processing depth of the domain where the line is wide. Figure 15 shows the percentage of processing depths that reached the domain where the line is wide. As shown in Figures 14 and 15, processing depth decreased with decreasing line width. However, higher percentages of processing depth where the line is wide correspond to longer development time. That is, although developer circulation was reduced, dissolution progressed slowly. When the dosage was low, the percentage was high. Additionally, the processing depth was low, and thus the circulation of the developer was good. However, as shown in Figure 14B and C, when the line width was too narrow, dissolution did not progress slowly. Thus, when narrow line patterns are transferred, it is

necessary to consider the influence of the micro-loading effect.

and (C) is 360 min

(A) (B) (C)

(A) (B) (C)

is 30 min; (B) is 180 min; and (C) is 360 min

**3. 3-D fabrication method** 

Fig. 15. Percentage of processing depths that reached the domain where the line is wide; (A)

This section describes the fabrication of 3-D structures using SR lithography. In this article, two fabrication methods are described in detail: the plain-pattern to cross-section transfer

Fig. 14. Relationship between line width and processing depth; (A) is 30 min; (B) is 180 min;

As mentioned in Section 2.3, a characteristic of SR lithography is that it is possible to fabricate a thick structure, and the processing depth can be controlled by dosage. Thus, it was expected that SR characteristics could be applied to the fabrication technology of 3-D structures with free-form surfaces or sloped sidewalls. Additionally, studies on 3-D processing methods using the LIGA process have been reported previously. "SLIGA" adds a sacrificial layer process to the LIGA process. Although the traditional LIGA process can only fabricate a mechanical component fixed to the substrate, the SLIGA technique enables fabrication involving sensors with moving parts (Ruzzu et al., 1998). For example, fabrication of a high-aspect-ratio microactuator was reported. However, this processing technique produces structures with sidewalls that are vertical to the resist surface. To control the inclination angle of the sidewall, the "skew exposure technique" is used to incline resists for X-rays during exposure (Tsuei et al., 1998; Ehrfeld et al., 1999). However, the structures that can be fabricated are limited, and fabrication of structures with free-form surfaces is not possible. Recently, methods that give energy distributions to the resist surface, such as grayscale mask UV lithography, have been reported. If complicated energy distributions can be given to resist surfaces, it will be possible to fabricate complicated arbitrary 3-D structures.

Fig. 16. PCT technique, black color shows X-ray absorber and corn color shows membrane, energy amount is shown by shading, the energy distribution is deposited in the resist by scanning, and a more complex energy distribution can be given by rotating the mask 90
