micro-fabrication of crosslinked PTFE

Nuclear Instruments and Methods in Physics Research B 295 (2013) 76–80

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Thermal and radiation process for nano-/micro-fabrication of crosslinked PTFE Akinobu Kobayashi a, Akihiro Oshima b,⇑, Satoshi Okubo a, Hidehiro Tsubokura a, Tomohiro Takahashi a, Tomoko Gowa Oyama a,1, Seiichi Tagawa a,b, Masakazu Washio a a b

Research Institute for Science and Engineering, Waseda University, 3-4-1 Okubo, Shinjuku-ku, Tokyo 169-8555, Japan The Institute of Scientific and Industrial Research (ISIR), Osaka University, 8-1 Mihogaoka, Ibaraki, Osaka 567-0047, Japan

a r t i c l e

i n f o

Article history: Received 30 April 2012 Received in revised form 5 November 2012 Available online 3 December 2012 Keywords: Nano-/micro-fabrication RX-PTFE Electron beam Mold TRaf process

a b s t r a c t Nano-/micro-fabrication process of crosslinked poly(tetrafluoroethylene) (RX-PTFE) is proposed as a novel method using combined process which is thermal and radiation process for fabrication of RX-PTFE (TRaf process). Nano- and micro-scale patterns of silicon wafers fabricated by EB lithography were used as the molds for TRaf process. Poly(tetrafluoroethylene) (PTFE) dispersion was dropped on the fabricated molds, and then PTFE was crosslinked with doses from 105 kGy to 1500 kGy in its molten state at 340 °C in nitrogen atmosphere. The obtained nano- and micro-structures by TRaf process were compared with those by the conventional thermal fabrication process. Average surface roughness (Ra) of obtained structures was evaluated with atomic force microscope (AFM) and scanning electron microscope (SEM). Ra of obtained structures with the crosslinking dose of 600 kGy showed less than 1.2 nm. The fine nano-/micro-structures of crosslinked PTFE were successfully obtained by TRaf process. Ó 2012 Elsevier B.V. All rights reserved.

1. Introduction Perfluorinated polymers such as poly(tetrafluoroethylene) (PTFE), have been widely applied for various industrial fields because of its excellent properties, such as high chemical stability, thermal durability, and low adhesion property, and so on. Moreover, it is confirmed that the crosslinked PTFE (RX-PTFE) has been obtained with irradiation of ionizing radiation such as electron beam (EB) and c-rays under its molten state under oxygen-free atmosphere [1–8]. The properties were improved by network formation, which showed the high optical transparency from ultraviolet (300 nm) to near infrared (900 nm) region, high radiation resistance and high abrasion durability, and so on [4,9]. Generally, the fabrication of PTFE (molding process) is carried out using the metal mold and thermal treatment with high pressure. However, this molding process cannot be applied for nanoand micro-fabrication because of its high viscosity. Nano- and micro-fabrication techniques for PTFE and RX-PTFE are very attractive, however, there is no suitable solvent for the chemical etching to perform wet bulk micro-fabrication. In our previous study, the micro-fabrications of PTFE and RX-PTFE have been studied by direct photo-etching using synchrotron radiation (SR) [10–17] or direct maskless ion beam etching [17–20]. The ⇑ Corresponding author. Tel./fax: +81 6 6879 4309. E-mail address: [email protected] (A. Oshima). Present address: Post-Doctoral Research Fellow of the Japan Society for the Promotion of Science (Quantum Beam Science Directorate, Japan Atomic Energy Agency). 1

0168-583X/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.nimb.2012.11.002

photo-etching using SR could give micro structures of high-aspect ratio higher than 70. On the other hand, in the case of maskless ion beam etching using a focused ion beam (FIB), the self-standing nano-fibers (diameter: 90 nm, aspect ratio: 17) were obtained without any solid debris. It is required through the two processes to obtain the nano-/micro-structures of RX-PTFE in SR/FIB direct etching; one is the crosslinking treatment of PTFE using EB irradiation under selective conditions, and the other one is nano- and micro-fabrication of RX-PTFE using top-down nano-technology. However, nano-scale masks with high accuracy for SR should be very expensive, and also, the spatial resolution could be affected by mask with sufficient thickness for cutting off the X-rays. Furthermore, both SR/ FIB direct etching processes require the long time irradiation and expensive equipment or large facilities. In this study, to obtain the nano- and micro-scale fine structures of RX-PTFE with higher throughputs than SR/FIB direct etching processes, it has been attempted to be carried out as combined process which is thermal and radiation process for fabrication of RX-PTFE (TRaf process).

2. Experimental procedure 2.1. Fabrication of silicon molds The molds for TRaf process were fabricated by EB lithography and plasma dry-etching technique. The EB resist (ZEP520A, ZEON) was spin-coated on 2.0  2.0 cm2 silicon (Si) wafers and baked at

A. Kobayashi et al. / Nuclear Instruments and Methods in Physics Research B 295 (2013) 76–80

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Scheme 1. The fabrication procedure of silicon molds.

180 °C for 3 min. The spin-coating speed and duration time were 2000 rpm and 60 s, respectively. The line and space (L&S) patterns and designed alphabets ‘‘EB’’ were written by EB lithography (JSM-6500F (JEOL) with Beam Draw (Tokyo Technology) system installed at ISIR, Osaka University). After the lithography, the development and reactive ion etching (RIE-10NOU, Samco) were carried out. The fabrication procedure for Si molds is shown in Scheme 1. 2.2. TRaf process Our proposed new process which is thermal and radiation process for fabrication of RX-PTFE (TRaf process) was consisting of three steps, as shown in Scheme 2. First step is spin-coating of PTFE dispersion (FULUONÒ XAD-912, ASAHI GLASS Fluoropolymers, average diameter: 0.25 lm, concentration: 55 wt.%) on fabricated Si mold. The spin-coating speed and duration time were 800 rpm and 60 s, respectively. Second step is crosslinking of PTFE and fabricating of nano-/micro-structures using EB irradiation. The spincoated PTFE was put in an irradiation vessel with a heating device. To melt of the PTFE particle, it was heated up to 360 °C in nitrogen gas atmosphere, and then the temperature was kept within a range of 335 ± 5 °C. The samples were irradiated up to 1500 kGy with a dose rate of 15 kGy/pass (acceleration voltage: 200 kV, current: 1 mA, pass speed: 2 m/min) by the electron accelerator (NHV Corp., CURETRONÒ) installed at RISE, Waseda University, as described in our previous paper [21–26]. Final step is removing nano-/micropatterned RX-PTFE from Si mold. To compare with TRaf process, the conventional thermal process was carried out at 360 °C under in nitrogen gas atmosphere for 15 min. 2.3. Measurements The shrinking ratio of obtained structures was measured by a field emission scanning electron microscope (FE-SEM, S-4500S, HITACHI). Average surface roughness (Ra) was analyzed by an atomic force microscope (AFM, SPA3800, SII NanoTechnology).

3. Results and discussion 3.1. Nano-/micro-fabrication of RX-PTFE Si molds was fabricated using EB lithography and plasma dryetching technique. The obtained structures consisted of linear and curving lines which have the widths from 500 nm to 2.0 lm and the depth was about 270 nm in all etched area, as shown in Fig. 1. Although the nano-/micro-scale fabrications of PTFE film was carried out using the thermal nanoimprint lithography at 360 °C under 40 MPa, imprinting of PTFE could be hardly obtained because of their high viscosities at the molten state. The nano-/micro-scale fabrications of RX-PTFE were carried out using TRaf process. Fig. 2(b)–(f) shows the AFM images of the obtained structures of RX-PTFE with various crosslinking doses. The nano-/micro-scale structures of RX-PTFE could be successfully obtained with TRaf process. In the case of lower crosslinking dose, fibril structures were observed on the fabricated surface, as shown in Fig. 2(b) and (c). The heights of obtained structures were about 180–270 nm. For crosslinking dose of higher than 600 kGy, the fibril structures were hardly observed, and the surfaces became smooth. The height was about 270 nm and it was almost same depth with the Si mold. On the other hand, using PTFE dispersion, the nano-/micro-scale structures of PTFE (non-crosslinking) could be fabricated with thermal process; however, fabricating accuracy was poor, as shown in Fig. 2(a). The relationship between crosslinking doses for TRaf process and Ra on the surface of fabricated etched area and non-etched one were summarized in Table 1. Although Ra of Si mold was less than 0.5 nm, Ra value on surface of PTFE (non-crosslinked) fabricated by the thermal process showed 3.5–4.4 nm. On the other hand, for TRaf process, Ra of etched and non-etched area of obtained structures were decreased with increasing of crosslinking dose until a dose of 600 kGy, and then tended to increase a little. In the case of RX-PTFE (600 kGy), Ra values for etched and nonetched area were 1.2 nm and 0.5 nm, respectively. It is indicating that the surface roughness of RX-PTFE fabricated by TRaf process

Scheme 2. TRaf process.

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A. Kobayashi et al. / Nuclear Instruments and Methods in Physics Research B 295 (2013) 76–80 Table 1 The relationship between crosslinking dose for TRaf process and Ra of fabricated structures. Dose (kGy)

Si mold Thermal process 105 300 600 1200 1500

Ra (nm) Etched area of structures

Non-etched area of structures

0.5 3.5 3.0 2.9 1.2 2.2 2.3

0.25 4.4 4.4 1.8 0.5 1.4 1.3

3.2. Shrinking ratio

Fig. 1. AFM images of designed character patterns on Si mold.

with a dose of 600 kGy would be more accurate than that of any other crosslinking doses. According to our previous study on radiation induced-crosslinking of PTFE, Ra of spin-coated PTFE has been improved by crosslinking [19], and the morphology was changed from discontinuous stratum of a pile of random crystal fibril phase to continual stratum of visco-elastically amorphous one at the crosslinking dose higher than 500 kGy [2]. Moreover, at the higher crosslinking doses than 500 kGy, the decomposed low molecular segments should be evolved from the highly crosslinked matrix [5,27,28]. Thus, it is guessed that TRaf process with the dose of about 600 kGy would be appropriate to make the fabricating pattern due to the visco-elastically amorphous phase formation and balance of evolved gas from matrix.

The line and space (L&S) patterns on Si molds were used for the evaluation of the shrinking ratio for RX-PTFE in TRaf process. The LER and the shrinking ratio of the obtained structures were observed by FE-SEM. The shrinking ratio was defined by the ratio between the space width of the molds and the line width of the structures. Fig. 3 shows the FE-SEM images of L&S patterns on Si mold. Line width, space width and height of fabricated mold are 670 nm, 400 nm and 270 nm, respectively. The FE-SEM images of the L&S patterns on obtained structures for RX-PTFE are shown in Fig. 4. The nano-scale L&S patterns of RX-PTFE could be successfully obtained with TRaf process. In the case of lower crosslinking dose such as 105 kGy and 300 kGy, the fibril structures were observed on the surface as same as AFM observation, as shown in Fig. 2(b) and (c). For crosslinking dose of more than 600 kGy, the fibril structures were hardly observed, and the shapes became clear, as shown in Fig. 4(d)–(f). On the other hand, the obtained fabricating accuracy was not good for thermal process, as shown in Fig. 4(a). The relationship between the crosslinking dose for TRaf process and the shrinking ratio of obtained structures are summarized in Table 2. The L&S patterns of used Si mold were corresponding to the obtained S&L patterns. The space widths of Si molds were

Fig. 2. AFM images of designed character patterns on Si mold and the obtained structures of PTFE (a) PTFE (0 kGy, Thermal NIL), (b) RX-PTFE (105 kGy), (c) RX-PTFE (300 kGy), (d) RX-PTFE (600 kGy), (e) RX-PTFE (1200 kGy), and (f) RX-PTFE (1500 kGy).

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Fig. 3. FE-SEM images of line & space patterns on Si mold. (Line width, space width and height of fabricated mold are 400 nm, 670 nm and 270 nm, respectively.)

400 nm, 530 nm, 930 nm and 1360 nm. In the case of the thermal process, the obtained line widths which corresponded to the space widths of the molds were 150 nm, 350 nm, 700 nm and 1000 nm, respectively. It was found that obtained lines by the thermal process were rough due to the fibril structures and incomplete filling of PTFE. Moreover, the shrinking ratios at the measured line widths show very large values, especially, the ratio (%) at the initial space width (400 nm) is about 62%. As the average particle size of PTFE used in this study is about 250 nm, PTFE would not be filled completely in the space width of 400 nm region. In fact, when the space width of mold is wider, PTFE should be filled easily, and then the shrinking ratio became a smaller value.

On the other hand, for the TRaf process, the roughness of the obtained structures at both line widths was improved by the crosslinking process except for RX-PTFE (105 kGy). Furthermore, the shrinking ratios were decreased with increasing the crosslinking dose till 600 kGy, and tended to saturate and to become almost constant values at each line width. For example, the fabricated line widths of RX-PTFE (600 kGy) were 310 nm, 490 nm, 760 nm and 1160 nm, respectively. In general, the decomposition gasses including the decomposed low molecular weight segments accompanying with radiation-induced chemical reaction were evolved from surface of bulk materials, via diffusion process in medium. It is observed that the mass of PTFE film would be proportionally decreased with increasing of crosslinking dose and the sample shape would be shrank by crosslinking [27]. Moreover, in the fabrication of fiber reinforced crosslinked PTFE by radiation processing [29], the decomposition gasses are evolved from the PTFE matrix and the mass decreases with increasing crosslinking dose. The evolved gasses will produce the micro defects or vacancies by desorption from sample surface. Furthermore, the morphology is changed by crosslinking from crystalline to amorphous at the dose higher than 500 kGy [5,27,28]. In the case of our TRaf process, it is considered that desorption from crosslinked PTFE would be occurred at 1-direction due to surrounding of PTFE enclosed by Si mold. That is, the evolved components in the mold side, which should be decomposition fragments from PTFE, would be reduced by the enclosed mold. For the ability to hardly desorb itself from the mold side, decomposition fragments would be filled up with the produced low molecular weight segments in medium by enclosed mold side and desorption from PTFE. Thus, it is considered that the shrinking ratio of RX-PTFE

Fig. 4. FE-SEM images of Line & Space patterns on obtained structures of RX-PTFE (a) PTFE (0 kGy, Thermal NIL), (b) RX-PTFE (105 kGy), (c) RX-PTFE (300 kGy), (d) RX-PTFE (600 kGy), (e) RX-PTFE (1200 kGy), and (f) RX-PTFE (1500 kGy).

Table 2 The relationship between the crosslinking dose for TRaf process; Line width of mold and fabricated structures and the shrinking ratio of imprinted structures. Si mold

Width (nm) Shrinking ratio Width (nm) Shrinking ratio Width (nm) Shrinking ratio Width (nm) Shrinking ratio

(%) (%) (%) (%)

400 0 530 0 930 0 1360 0

The thermal process

150 62.5 350 34.0 700 24.7 1000 26.5

TRaf process 105 kGy

300 kGy

600 kGy

1200 kGy

1500 kGy

230 42.5 370 30.2 720 22.6 990 27.2

270 32.5 400 24.5 760 18.3 1080 20.6

310 22.5 490 7.5 760 18.3 1160 14.7

330 17.5 490 7.5 790 15.1 1180 13.2

320 20.0 480 9.4 760 18.3 1180 13.2

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would be influenced by used particle size, geometric spatial relationship between mold and PTFE and desorption from PTFE. By the way, RX-PTFE shows the high optical transparency from ultra-violet to near infrared region and the high radiation resistance [9]. Of course, each property such as optical transparency and radiation resistance is depending on network density. For example, the transparency at g-line (436 nm) and i-line (365 nm) of RX-PTFE with 500 kGy whose thickness is 0.5 mm show about 75% and 65%, respectively. For RX-PTFE with 3000 kGy whose thickness is 0.5 mm, the transparency at g-line, i-line and deepUV region (around 250 nm) shows about 93%, 90% and 50%, respectively. Hence, it is expected that RX-PTFE with the crosslinking dose higher than 500 kGy could be applicable for the polymeric flexible molds for UV-NIL from i-line to g-line. From the results mentioned above, it was found that fabricating patterns with the high accuracy of RX-PTFE would be obtained by TRaf process with the crosslinking dose of 600 kGy. It is considered that the accuracy would be depended on not only the mold patterns such as line width, height and aspect ratio, but also the crosslinking doses. It is necessary to select the suitable conditions according to the application. 4. Conclusion The fine nano- and micro-scale structures of RX-PTFE have been successfully fabricated by TRaf process. This was performed by combined process which is thermal and radiation process for fabrication of RX-PTFE (TRaf process). Ra of etched and non-etched area of obtained structures were decreased with increasing the crosslinking dose till 600 kGy, and then tended to increase a little in the higher crosslinking dose. Roughness of the structures at each line was improved by crosslinking except for RX-PTFE (105 kGy). Moreover, roughness was decreased with increasing the crosslinking dose till 600 kGy, and tended to saturate. The shrinking ratio was decreased with increasing the crosslinking dose till 600 kGy, and then tended to be almost constant values at each line width. The Ra and the shrinking ratio of the structures with the crosslinking dose of 600 kGy were less than 1.2 nm, and less than 23%, respectively. Thus, it is guessed that TRaf process with the dose at 600 kGy would be the appropriate fabricating condition due to the change of visco-elastically amorphous and balance of evolved gas from matrix in the PTFE crosslinking reaction. Acknowledgements A part of this work was supported by ‘‘Nanotechnology Network Project’’ (Handai Multi-functional Nanofoundry in Osaka Univ.) and ‘‘Nanotechnology Platform Project [No. F-12-OS-0018 / S-12-OS-0019]’’ (Nanotechnology Open Facilities in Osaka Univ.)

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