Decomposition Process of PMMA-based Polymer Using ... - J-Stage

Journal of Photopolymer Science and Technology

Volume 29, Number 4 (2016) 629－631 Ⓒ 2016SPST Communication

Decomposition Process of PMMA-based Polymer Using Atomic Hydrogen Generated by a Tungsten Hot-Wire Catalyst Seiji Takagi1, Takashi Nishiyama1, Masashi Yamamoto2, Eriko Sato1, Tomosumi Kamimura3, Toshiyuki Ogata4, and Hideo Horibe1 1

Graduate School of Engineering, Osaka City University, 3-3-138 Sugimoto, Sumiyoshi-ku, Osaka 558-8585, Japan 2 Department of Electrical and Computer Engineering, National Institute of Technology, Kagawa College, 355 Chokushi-cho, Takamatsu, Kawgawa 761-8058, Japan 3 Department of Electronics, Information and Communication Engineering, Osaka Institute of Technology, 5-16-1 Ohmiya, Asahi-ku, Osaka 535-8585, Japan 4 TOKYO OHKA KOGYO CO., LTD., 1590 Tabata, Samukawa-machi, Koza-gun, Kanagawa 253-0114 Japan Keywords: Atomic hydrogen, Hot-wire catalyst, Poly(methyl methacrylate), Surface structure, Micro structure

1. Introduction Plasma is one of the four fundamental state of matter, flowing solid, liquid and gas states. Plasma is the state in which gas molecules are dissociated ion and electron, i.e. ionized gas, and has attracted much attention in various fields. In semiconductor manufacturing process, plasma etching and ashing are widely utilized. The plasma etching is used to fabricate integrated circuit, being shot the plasma generated by grow discharge of appropriate gas mixture at the sample. The plasma ashing is the process of removing the photoresist after etched Si wafer, and generally used oxygen plasma. Although these processes have some problems such as deterioration of device quality by oxidation of the metal and Si wafer, plasma processing has been applied as useful technique for semiconductor process. In addition, plasma is utilized as surface modification of polymer materials. By plasma treating, the surface is modified of polymers because of functionalization of polymer side chain and the rough surface is formed [1,2]. For example, the polymer surface is amidated or fluorinated by utilizing nitrogen or fluorine gases in plasma treating, and improved the surface wettability and adhesion property. Herein, the microstructure fabricated by plasma treating enhances their properties by increasing specific Received April Accepted May

1, 2016 11, 2016

surface area. On the other hands, we previously demonstrated the photoresist removal method by using atomic hydrogen generated by a tungsten hot-wire catalyst [3-5]. The atomic hydrogen has excellent ability to decompose photoresists and was easily generated by decomposing molecular hydrogen on metal hot-wire catalyst [6,7]. The atomic hydrogen decomposes a photoresist by reduction reaction, thus this process resolves the problem of oxygen plasma ashing process. Additionally, a tungsten hot-wire catalytic method is only generated atomic hydrogen, and is not composed of the accelerated electrons compared with typical plasma method. Therefore, only chemical etching by atomic hydrogen preferentially proceeds to the polymer. From the above, it is expected that atomic hydrogen generated by a tungsten hot-wire catalyst would make it work to advantage in fabrication and control of microstructure on the polymer surface. In this study, we investigated the formation of microstructure on the surface of Poly(methyl methacrylate) – based polymer (PMMA - based polymer), which is a polymer for the ArF photoresist, in decomposition process by using atomic hydrogen generated by a tungsten hot-wire catalyst.

629

J. Photopolym. Sci. Technol., Vol. 29, No. 4, 2016

Thermo monitor

H2/N2 gas

Fused silica

DC Power

Tungsten wire Atomic hydrogen Polyme

Thermo couple

Exhaust

Si Exhaust

Fig. 1. Schematic illustration of the experiment apparatus.

2. Experimental methacrylate) (PMMA, Poly(methyl Sigma-Aldrich Co. LLC., Mw: 996,000), poly(ethyl methacrylate) (PEMA, Scientific Polymer Products, Inc., Mw: 250,000) and poly(methyl acrylate) (PMA, Scientific Polymer Products, Inc., Mw:40,000) were used. Ethyl lactate was used as a solvent for PMMA and PEMA, and N,N-dimethylformamide was used as solvent for PMA. The polymer solutions were spin-coated onto a silicon wafer by using spin coater, and then the wafers were baked for 1 minute on a hot plate at 100 °C. Figure 1 shows a schematic illustration of the generation and irradiation of atomic hydrogen by hot-wire catalytic method. Hydrogen-nitrogen mixed gases (H2/N2=10/90 vol%) flowed from the upper nozzle. The gas flow rate was fixed at 300 sccm using a mass-flow controller. The gas pressure was 32 Pa. The current applied to the tungsten catalyst was 20 A and 30 A. The temperature of tungsten catalyst was measured by two wavelength radiation thermometer (ISR12-L0 from Impac Electronic Corp). When the current is 20 A, the temperature is 1850 °C. When the current is 30 A, the temperature is 2400 °C. The current applied. The distance between the catalyst and the sample substrate was 100 mm. The substrate temperature was controlled by heater and monitored with a thermocouple. Initial substrate temperature was 50 °C, and irradiation time of atomic hydrogen was 3, 6, and 9 minutes. The surface morphology was evaluated by AFM (Digital Instrumental Nanoscope IIIa in tapping mode.) 3. Results and discussion Figure 2 shows the residual film thickness and the substrate temperature on the irradiation time of atomic hydrogen. PMMA and PEMA were 630

Fig. 2. Residual film thickness (the left axis) and substrate temperature (the right axis) with the irradiation time of atomic hydrogen at 20 A (close symbol) and 30 A (open symbol) of catalyst current. The polymers used (●) PMMA, (■) PEMA, and (◆) PMA.

proportionally decreased the film thicknesses by the atomic hydrogen irradiation, but the PMA was not observed the film thickness decrease by atomic hydrogen at 20 A of the tungsten current. The reason for the above results is due to the difference of decomposition process of methacrylate and acrylate polymers. It was reported that methacrylate polymers (PMMA and PEMA) are the scission type in the main chain and acrylate polymers (PMA) are the crosslinking type in the main chain, by an irradiation of strong energy [8]. PMA was hardly decomposed as compared with PMMA and PEMA, but barely able to decompose by the irradiation of atomic hydrogen at 30 A of the tungsten current 9 min a)

b)

50 nm

0 nm

c)

d)

100 nm

0 nm

e)

f)

200 nm

0 nm

Fig. 3. AFM images of a,b) PMA at 30 A, c,d) PEMA at 20 A and e,f) PMMA at 20 A (film dimension : 5 µm × 5 µm). Irradiating time of atomic hydrogen were a,c,e) 0 min, b) 6 min, and d,f) 9 min.

J. Photopolym. Sci. Technol., Vol. 29, No. 4, 2016

minutes because of a higher reaction temperature. Figure 3 shows the AFM images of PMMA, PEMA and PMA. PMMA and PEMA were irradiated atomic hydrogen at 20 A of tungsten current and only PMA was irradiated at 30 A. All the film surfaces were flat and smooth before irradiating atomic hydrogen (Figure 3a, 3c, and 3e). After irradiating atomic hydrogen for 9 minutes, PMMA and PEMA were formed dot or needle like microstructure on the surfaces (Figure 3d and 3f). The microstructures were more clearly formed on the PMMA surface, and regularly arrayed in the order of nanometers. By contrast, PMA was not changed the surface after irradiating atomic hydrogen (Figure 3b), although the etching rate of PMA at 30 A of the tungsten current was higher than that of PMMA and PEMA at 20 A from the result in Figure 2. Table 1. Surface roughness (Ra) before and after irradiating atomic hydrogen. Irradiation time (min)

Ra (nm) PMMA

PEMA

PMA

0

0.3

0.3

0.3

3

0.7

0.6

0.4

6

28.0

1.5

0.4

9

27.3

5.5

n.da)

a)

Completely removed.

Table 1 shows the surface roughness (Ra) of PMMA, PEMA and PMA before and after irradiating the atomic hydrogen. Ra after irradiating for 3 minutes was almost the same value as that before irradiating, regardless of the kind of polymer. In the cases of the irradiation for 6 and 9 minutes, Ra of PMMA and PEMA were dramatically increased, but that of PMA at 30 A was not changed and PMA was completely removed by irradiating the atomic hydrogen for 9 minutes while maintaining the flat and smooth surface. As one of the reason why the differences in the surface morphology of methacrylate and acrylate polymers were occurred, it is expected that the fluctuation in crosslinking density causes on the outermost film surface while irradiating. PMA as acrylate polymer, which is classified in terms of the crosslinking type in main chain. So, the

reaction both of degradation and crosslinking may occur for PMA. As the result, the polymer density fluctuation on the outermost film surface while irradiating becomes smaller and the film surface may be uniformity, because the difference of the etching rate in microscopic surface region is smaller. On the other hand, PMMA and PEMA as methacrylate polymers, which are classified in terms of the scission type in main chain. For these polymers, just only the degradation reaction may occur. The etching rate in microscopic surface region is higher, and polymer density fluctuation on the film surface while irradiating atomic hydrogen becomes larger. Consequently, the film surface will be unevenness, and be formed the microstructure. Furthermore, we considered that the difference of microstructure between PMMA and PEMA affects to the glass transition temperature (Tg) of each polymer. The Tg of PEMA (Tg: 65 °C) is lower than that of PMMA (Tg: 100°C). When substrate temperature is near 100 °C (20 A of the tungsten current), chain mobility of PEMA is higher than PMMA, and polymer density of the film surface was partially relieved. From the above, Ra of PEMA becomes smaller than that of PMMA. References 1. K. F. Grythe and F. K. Hansen, Langmuir, 22 (2006) 6109. 2. A. Sakakibara, F. Itoigawa, T. Nakamura, and S. Hayakawa, J. Surf. Finish. Soc. Jpn., 62 (2011) 35-40. 3. H. Horibe, M. Yamamoto, E. Kusano, T. Ichikawa, and S. Tagawa, J. Photopolym. Sci. Technol., 20 (2008) 293. 4. M. Yamamoto, T. Maruoka, A. Kono, H. Horibe, and H. Umemoto, Appl. Phys. Exp., 3 (2010) 026501. 5. M. Yamamoto, H. Umemoto, K, Ohdaira, S. Nagaoka, T. Shikama, T. Nishiyama, and H. Horibe, J. Photopolym. Sci. Technol., 28 (2015) 303. 6. K. F. Grythe, F. K. Hansen, Langmuir., 22 (2006) 6109. 7. A. Sakakibara, F. Itoigawa, T. Nakamura, and S. Hayakawa., J. Surf. Finish. Soc. Jpn., 62 (2011) 35. 8. A. A. Miller, E. J. Lawton, and J. S. Balwit, J. Polym. Sci., 14 (1954) 503.

631