**4. PTFE fabrication by SR ablation**

This section describes 3-D PTFE microstructures fabricated by SR ablation. Advantages of ablation technology and the mechanism of fabrication are described.

#### **4.1 PTFE**

PTFE is a fluoroplastic material known for its excellent material characteristics, including insulation against high voltages, resistance to chemicals and creep, and high thermal stability. PTFE was discovered in 1938 by Dr. Roy J. Plunkett, who was a researcher of E.I. du Pont de Nemours and Company, a commercial reality in 1946. PTFE is best known by the DuPont brand name Teflon®. In addition to its primary use in non-stick frying pans and filters, PTFE is also used in a broad range of fields, such as those involving household articles, OA equipment, semiconductors, and cars. Despite its long (> 60-year) history, new uses for PTFE are developed continuously because of its outstanding characteristics. Because PTFE has excellent material characteristics, it is expected to be applicable to MEMS.

Fabrication of 3-D Structures Utilizing Synchrotron Radiation Lithography 331

(A) (B)

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
