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Evaluation of filling behavior on UV nanoimprint lithography using release coating
Microelectronic Engineering 87 (2010) 918–921
Contents lists available at ScienceDirect
Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee
Evaluation of filling behavior on UV nanoimprint lithography using release coating Kazutomo Osari a, Noriyuki Unno a,b, Jun Taniguchi a,*, Ken-ichi Machinaga a, Takeshi Ohsaki c, Nobuji Sakai c a
Department of Applied Electronics, Tokyo University of Science, 2641 Yamazaki, Noda, Chiba 278-8510, Japan Research Fellow of the Japan Society for the Promotion of Science, 6 Ichibancho, Chiyoda-ku, Tokyo 102-8471, Japan c Toyo Gosei Co., Ltd., Photosensitive Materials Research Center, 4-2-1 Wakahagi, Inba-mura, Inba-gun, Chiba 270-1609, Japan b
a r t i c l e
i n f o
Article history: Received 18 September 2009 Accepted 30 November 2009 Available online 5 December 2009 Keywords: Nanoimprint lithography (NIL) UV photocurable resin Hydrogen silsesquioxane (HSQ) Contact angle Kinetic viscosity
a b s t r a c t Ultra violet nanoimprint lithography (UV-NIL), which is able to obtain the nano-scale pattern effectively and quickly, is strongly desired for the next-generation lithography technology. However, it is well known that the higher viscosity UV-curable resin with UV-NIL tends to be the shorter obtained pattern without the sufficient transfer pressure. This phenomenon is caused by the filling behavior of UV-curable resin into the UV-NIL mold, thus, the investigation of the filling behavior is very important. In this study, the filling behavior in UV-NIL was observed by using a ‘‘midair structure mold”, which is able to eliminate the bubble defect. As a result, it is clear that the filling behavior with low transfer pressure was depended on the capillary force in the mold pattern, which is described by the mold aperture size, the mold surface condition and the resin property. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction UV nanoimprint lithography (NIL) is expected to be a next-generation lithography technology used for various fields [1,2]. To fabricate various devices by fulfilling the each requirement, the various UV-curable resins are needed. For instance, transparency, dry etching tolerability and mechanical strength are required. However, it is well known that the obtained pattern using the higher viscosity UV-curable resin tends to be the shorter pattern without the sufficient transfer pressure. This phenomenon is caused by the filling behavior of UV-curable resin into the UVNIL mold, thus, the investigation of the filling behavior is very important. When UV-NIL is carried out using normally mold in air, however, the bubble defect [3] interrupts the observation of the essential filling behavior. Therefore, we fabricated the midair structure mold to observe the filling behavior of the UV-curable resin at atmospheric pressure [4]. In our previous study, the filling behavior into the mold pattern in air was observed clearly with scanning electron microscope (SEM). As a result, the filling behavior was expressed as following;
PC ¼
4c cos h a
ð1Þ
is called ‘‘capillary force” [4,5]. Here, a are lengths of the square aperture size, and c is the surface tension of the liquid, h is contact angle in the channel. Fig. 1 shows a schematic view of the changes * Corresponding author. E-mail address: [email protected] (J. Taniguchi). 0167-9317/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2009.11.175
of the filling behavior by the changes of these parameters. In this study, three kinds of the UV-curable resins which have difference viscosities and surface tension were examined by varying the mold aperture size and the mold surface condition. As a result, this study reveals the value of Pc is depended on the aperture size of the UVcurable resin, as shown in Eq. (1). 2. Experimental apparatus and procedure The midair structure molds were fabricated using electron beam lithography (EBL) with hydrogen silsesquioxane (HSQ), which is a negative-type electron beam resist [6]. First, HSQ was spin-coated at 3000 rpm on a cleaned silicon substrate and baked at 180 °C for 5 min, resulting in a 300 nm film. Next, EBL was carried out at a high accelerating voltage of 30 kV delineate a lines pattern. Subsequently, a second EBL carried out at a low acceleration voltage of 3 kV delineate up lines in a direction at right angles to the first one. Finally, the resist was developed in 5% tetramethylammonium hydroxide (TMAH) for 180 s. The line delineated at 30 kV formed trough beams and the line at 3 kV formed bridges. The aperture sizes of the fabricated midair structure mold were 500 nm, 1000 nm and 1500 nm. Next, we prepared midair structure molds with and without release agent (Daikin Co., Optool DSX 0.1%), which is fluorine silane coupling agent. Then, UV-NIL was carried out at atmospheric pressure using these midair structure molds. PAK-01, PAK-02-TU01, PAK-02 (Toyo Gosei Co., Ltd.), whose kinetic viscosity are 63.5 mPas, 16.0 mPas and 9.30 mPas, respectively, and whose surface tension are 30.6 mN/m, 27.1 mN/m and 29.9 mN/m, respectively, were employed
K. Osari et al. / Microelectronic Engineering 87 (2010) 918–921
Fig. 1. The schematic model of the HSQ channel (a) with release agent, h > 90° and (b)without release agent, h < 90°.
as a UV-curable resin. The hold time was 60 s, which was sufficient to complete the formation of the resin pattern and the transfer pressures were 0.2 MPa, 0.5 MPa and 0.8 MPa. After the hold time, UV radiation with an energy density of 2 J/cm2 was focused to the midair structure mold. Then, the midair structure mold was released with freezing the contact angle between the liquid resin and solid mold side walls. Fig. 2 shows the state of transfer after release the mold. 3. Results and discussion Fig. 3 shows the SEM micrograph of the fabricated HSQ midair structure molds. The horizontal lines delineated at 3 kV formed
919
Fig. 2. Nanoimprint process using the midair mold and pattern transfer state after the mold release.
the bridges and the vertical lines delineated at 30 kV formed the through beams after development. Left part of Fig. 4 shows the relationship of the pressures and the transferred pattern heights of PAK-01 with release agent using 1000 nm aperture size mold. When the transfer pressure was 0.2 MPa and 0.8 MPa, the transferred pattern heights were 35 nm and 223 nm, respectively. The larger transfer pressure tended to transfer the higher transferred pattern height with the same mold. On the other hand, right part of Fig. 4 shows the relationship of the pressures and the heights of patterned PAK-01 without release agent using 1000 nm aperture size mold. When the transfer pressure was 0.2 MPa and 0.8 MPa, the transferred pattern heights
Fig. 3. The SEM micrograph of the fabricated midair structure molds.
920
K. Osari et al. / Microelectronic Engineering 87 (2010) 918–921
Fig. 4. SEM image of transferred PAK-01 patterns on glass slides at 0.2 MPa and 0.8 MPa with or without release agent using 1000 nm aperture size molds.
were 258 nm and 274 nm, respectively. The patterned shape using the mold with release agent was rounded, compared to one without release agent. This shape represents the contact angle in the midair structure mold and the value is more than 90°, result in the negative value of Pc. Therefore, the filling of PAK-01 into the mold was prevented by capillary force Pc. In contrast, the patterned shape without release agent was box-shape because the PAK-01 in the mold reached to the silicon substrate due to the positive value of Pc. Consequently, using the release agent, the mold surface condition was changed to the contact angle more than 90° and the UVcurable resin was prevented to fill into the mold, as shown in Eq. (1). Fig. 5 shows the SEM micrograph of the transfer results varying the aperture size of the midair structure mold at a constant
Fig. 6. Result of the PAK-01 transferred pattern height on glass slides by changing the transfer pressure and the aperture size using release agent treated molds.
Fig. 5. The characteristics of transferred PAK-01 patterns on glass slides at 0.5 MPa with release agent by changing the aperture size.
K. Osari et al. / Microelectronic Engineering 87 (2010) 918–921
921
Fig. 7. SEM image of transferred patterns on glass slides at 0.5 MPa with release agent by changing the kinetic viscosity.
0.5 MPa with the release coating using PAK-01. When the aperture size was 500 nm, 1000 nm and 1500 nm, the transferred pattern heights were 72 nm, 207 nm and 242 nm, respectively. The transferred pattern heights were higher according to the aperture size increment. The larger aperture size tended to be the smaller Pc absolute value, result in the higher transferred pattern. This result is reasonable, compared to Eq. (1). Fig. 6 shows the relationship between the transfer pressure and the transferred pattern height, by varying the aperture size. The theory values of the capillary force with Eq. (1) are 0.88 MPa at 500 nm, 0.44 MPa at 1000 nm and 0.29 MPa at 1500 nm aperture size, respectively. It appears that a transferred pattern height pressed at under less than Pc value was significantly shorter because of Pc prevention force, compared to initial HSQ thickness. Fig. 7 shows the SEM micrographs of the transfer results at 0.5 MPa with the release coating. The transferred UV-curable resins were PAK-01, PAK-02 and PAK-02-TU01. The transferred pattern heights were 207 nm, 266 nm and 234 nm using PAK-01, PAK-02 and PAK-02-TU01, respectively. Capillary force acts at the interface between the channel surface and the resin, thus, the central part of the resin in channel is movable. Therefore, the transferred pattern height was depended on kinetic viscosities of the resin. As a result, the larger kinetic viscosity of the UV-curable resin tended to be the shorter transferred pattern height. 4. Conclusions The filling behavior of UV-curable resin was observed using a midair structure mold. Using the release agent, the mold surface
condition was changed to the contact angle more than 90° and the UV-curable resin was prevented to fill into the mold. Moreover, the larger aperture size tended to be the smaller Pc absolute value, result in the higher transferred pattern. These results are reasonable, compared to Eq. (1). In addition, the larger kinetic viscosity of the UV-curable resin tended to be the shorter transferred pattern height. Consequently, the elucidation of the filling behavior is very important to be successful in the fine NIL process. Our observation method using the midair structure mold is very useful for this purpose. Acknowledgment This study was supported by the ‘‘Academic Frontier” Project for Private Universities, with a matching fund subsidy from the Ministry of Education, Culture, Sports, Science and Technology (MEXT) 2006-2010. References [1] [2] [3] [4] [5]
Y. Ishii, J. Taniguchi, Microelectron. Eng. 84 (2007) 912. N. Unno, J. Taniguchi, Y. Ishii, J. Vac. Sci. Technol. B 25 (2007) 2361. M. Fukuhara, J. Mizuno, M. Saito, T. Homma, S. Shoji, IEE. J. Trans. 2 (2007) 307. J. Taniguchi, K. Machinaga, N. Unno, N. Sakai, Microelectron. Eng. 86 (2009) 676. E. Delamarche, A. Bernard, H. Schmid, A. Bietsch, B. Michel, H. Biebuyck, J. Am. Chem. Soc. 120 (1998) 500. [6] Y. Matsubara, J. Taniguchi, I. Miyamoto, Jpn. J. Appl. Phys. 45 (2006) 5538.
Contents lists available at ScienceDirect
Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee
Evaluation of filling behavior on UV nanoimprint lithography using release coating Kazutomo Osari a, Noriyuki Unno a,b, Jun Taniguchi a,*, Ken-ichi Machinaga a, Takeshi Ohsaki c, Nobuji Sakai c a
Department of Applied Electronics, Tokyo University of Science, 2641 Yamazaki, Noda, Chiba 278-8510, Japan Research Fellow of the Japan Society for the Promotion of Science, 6 Ichibancho, Chiyoda-ku, Tokyo 102-8471, Japan c Toyo Gosei Co., Ltd., Photosensitive Materials Research Center, 4-2-1 Wakahagi, Inba-mura, Inba-gun, Chiba 270-1609, Japan b
a r t i c l e
i n f o
Article history: Received 18 September 2009 Accepted 30 November 2009 Available online 5 December 2009 Keywords: Nanoimprint lithography (NIL) UV photocurable resin Hydrogen silsesquioxane (HSQ) Contact angle Kinetic viscosity
a b s t r a c t Ultra violet nanoimprint lithography (UV-NIL), which is able to obtain the nano-scale pattern effectively and quickly, is strongly desired for the next-generation lithography technology. However, it is well known that the higher viscosity UV-curable resin with UV-NIL tends to be the shorter obtained pattern without the sufficient transfer pressure. This phenomenon is caused by the filling behavior of UV-curable resin into the UV-NIL mold, thus, the investigation of the filling behavior is very important. In this study, the filling behavior in UV-NIL was observed by using a ‘‘midair structure mold”, which is able to eliminate the bubble defect. As a result, it is clear that the filling behavior with low transfer pressure was depended on the capillary force in the mold pattern, which is described by the mold aperture size, the mold surface condition and the resin property. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction UV nanoimprint lithography (NIL) is expected to be a next-generation lithography technology used for various fields [1,2]. To fabricate various devices by fulfilling the each requirement, the various UV-curable resins are needed. For instance, transparency, dry etching tolerability and mechanical strength are required. However, it is well known that the obtained pattern using the higher viscosity UV-curable resin tends to be the shorter pattern without the sufficient transfer pressure. This phenomenon is caused by the filling behavior of UV-curable resin into the UVNIL mold, thus, the investigation of the filling behavior is very important. When UV-NIL is carried out using normally mold in air, however, the bubble defect [3] interrupts the observation of the essential filling behavior. Therefore, we fabricated the midair structure mold to observe the filling behavior of the UV-curable resin at atmospheric pressure [4]. In our previous study, the filling behavior into the mold pattern in air was observed clearly with scanning electron microscope (SEM). As a result, the filling behavior was expressed as following;
PC ¼
4c cos h a
ð1Þ
is called ‘‘capillary force” [4,5]. Here, a are lengths of the square aperture size, and c is the surface tension of the liquid, h is contact angle in the channel. Fig. 1 shows a schematic view of the changes * Corresponding author. E-mail address: [email protected] (J. Taniguchi). 0167-9317/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2009.11.175
of the filling behavior by the changes of these parameters. In this study, three kinds of the UV-curable resins which have difference viscosities and surface tension were examined by varying the mold aperture size and the mold surface condition. As a result, this study reveals the value of Pc is depended on the aperture size of the UVcurable resin, as shown in Eq. (1). 2. Experimental apparatus and procedure The midair structure molds were fabricated using electron beam lithography (EBL) with hydrogen silsesquioxane (HSQ), which is a negative-type electron beam resist [6]. First, HSQ was spin-coated at 3000 rpm on a cleaned silicon substrate and baked at 180 °C for 5 min, resulting in a 300 nm film. Next, EBL was carried out at a high accelerating voltage of 30 kV delineate a lines pattern. Subsequently, a second EBL carried out at a low acceleration voltage of 3 kV delineate up lines in a direction at right angles to the first one. Finally, the resist was developed in 5% tetramethylammonium hydroxide (TMAH) for 180 s. The line delineated at 30 kV formed trough beams and the line at 3 kV formed bridges. The aperture sizes of the fabricated midair structure mold were 500 nm, 1000 nm and 1500 nm. Next, we prepared midair structure molds with and without release agent (Daikin Co., Optool DSX 0.1%), which is fluorine silane coupling agent. Then, UV-NIL was carried out at atmospheric pressure using these midair structure molds. PAK-01, PAK-02-TU01, PAK-02 (Toyo Gosei Co., Ltd.), whose kinetic viscosity are 63.5 mPas, 16.0 mPas and 9.30 mPas, respectively, and whose surface tension are 30.6 mN/m, 27.1 mN/m and 29.9 mN/m, respectively, were employed
K. Osari et al. / Microelectronic Engineering 87 (2010) 918–921
Fig. 1. The schematic model of the HSQ channel (a) with release agent, h > 90° and (b)without release agent, h < 90°.
as a UV-curable resin. The hold time was 60 s, which was sufficient to complete the formation of the resin pattern and the transfer pressures were 0.2 MPa, 0.5 MPa and 0.8 MPa. After the hold time, UV radiation with an energy density of 2 J/cm2 was focused to the midair structure mold. Then, the midair structure mold was released with freezing the contact angle between the liquid resin and solid mold side walls. Fig. 2 shows the state of transfer after release the mold. 3. Results and discussion Fig. 3 shows the SEM micrograph of the fabricated HSQ midair structure molds. The horizontal lines delineated at 3 kV formed
919
Fig. 2. Nanoimprint process using the midair mold and pattern transfer state after the mold release.
the bridges and the vertical lines delineated at 30 kV formed the through beams after development. Left part of Fig. 4 shows the relationship of the pressures and the transferred pattern heights of PAK-01 with release agent using 1000 nm aperture size mold. When the transfer pressure was 0.2 MPa and 0.8 MPa, the transferred pattern heights were 35 nm and 223 nm, respectively. The larger transfer pressure tended to transfer the higher transferred pattern height with the same mold. On the other hand, right part of Fig. 4 shows the relationship of the pressures and the heights of patterned PAK-01 without release agent using 1000 nm aperture size mold. When the transfer pressure was 0.2 MPa and 0.8 MPa, the transferred pattern heights
Fig. 3. The SEM micrograph of the fabricated midair structure molds.
920
K. Osari et al. / Microelectronic Engineering 87 (2010) 918–921
Fig. 4. SEM image of transferred PAK-01 patterns on glass slides at 0.2 MPa and 0.8 MPa with or without release agent using 1000 nm aperture size molds.
were 258 nm and 274 nm, respectively. The patterned shape using the mold with release agent was rounded, compared to one without release agent. This shape represents the contact angle in the midair structure mold and the value is more than 90°, result in the negative value of Pc. Therefore, the filling of PAK-01 into the mold was prevented by capillary force Pc. In contrast, the patterned shape without release agent was box-shape because the PAK-01 in the mold reached to the silicon substrate due to the positive value of Pc. Consequently, using the release agent, the mold surface condition was changed to the contact angle more than 90° and the UVcurable resin was prevented to fill into the mold, as shown in Eq. (1). Fig. 5 shows the SEM micrograph of the transfer results varying the aperture size of the midair structure mold at a constant
Fig. 6. Result of the PAK-01 transferred pattern height on glass slides by changing the transfer pressure and the aperture size using release agent treated molds.
Fig. 5. The characteristics of transferred PAK-01 patterns on glass slides at 0.5 MPa with release agent by changing the aperture size.
K. Osari et al. / Microelectronic Engineering 87 (2010) 918–921
921
Fig. 7. SEM image of transferred patterns on glass slides at 0.5 MPa with release agent by changing the kinetic viscosity.
0.5 MPa with the release coating using PAK-01. When the aperture size was 500 nm, 1000 nm and 1500 nm, the transferred pattern heights were 72 nm, 207 nm and 242 nm, respectively. The transferred pattern heights were higher according to the aperture size increment. The larger aperture size tended to be the smaller Pc absolute value, result in the higher transferred pattern. This result is reasonable, compared to Eq. (1). Fig. 6 shows the relationship between the transfer pressure and the transferred pattern height, by varying the aperture size. The theory values of the capillary force with Eq. (1) are 0.88 MPa at 500 nm, 0.44 MPa at 1000 nm and 0.29 MPa at 1500 nm aperture size, respectively. It appears that a transferred pattern height pressed at under less than Pc value was significantly shorter because of Pc prevention force, compared to initial HSQ thickness. Fig. 7 shows the SEM micrographs of the transfer results at 0.5 MPa with the release coating. The transferred UV-curable resins were PAK-01, PAK-02 and PAK-02-TU01. The transferred pattern heights were 207 nm, 266 nm and 234 nm using PAK-01, PAK-02 and PAK-02-TU01, respectively. Capillary force acts at the interface between the channel surface and the resin, thus, the central part of the resin in channel is movable. Therefore, the transferred pattern height was depended on kinetic viscosities of the resin. As a result, the larger kinetic viscosity of the UV-curable resin tended to be the shorter transferred pattern height. 4. Conclusions The filling behavior of UV-curable resin was observed using a midair structure mold. Using the release agent, the mold surface
condition was changed to the contact angle more than 90° and the UV-curable resin was prevented to fill into the mold. Moreover, the larger aperture size tended to be the smaller Pc absolute value, result in the higher transferred pattern. These results are reasonable, compared to Eq. (1). In addition, the larger kinetic viscosity of the UV-curable resin tended to be the shorter transferred pattern height. Consequently, the elucidation of the filling behavior is very important to be successful in the fine NIL process. Our observation method using the midair structure mold is very useful for this purpose. Acknowledgment This study was supported by the ‘‘Academic Frontier” Project for Private Universities, with a matching fund subsidy from the Ministry of Education, Culture, Sports, Science and Technology (MEXT) 2006-2010. References [1] [2] [3] [4] [5]
Y. Ishii, J. Taniguchi, Microelectron. Eng. 84 (2007) 912. N. Unno, J. Taniguchi, Y. Ishii, J. Vac. Sci. Technol. B 25 (2007) 2361. M. Fukuhara, J. Mizuno, M. Saito, T. Homma, S. Shoji, IEE. J. Trans. 2 (2007) 307. J. Taniguchi, K. Machinaga, N. Unno, N. Sakai, Microelectron. Eng. 86 (2009) 676. E. Delamarche, A. Bernard, H. Schmid, A. Bietsch, B. Michel, H. Biebuyck, J. Am. Chem. Soc. 120 (1998) 500. [6] Y. Matsubara, J. Taniguchi, I. Miyamoto, Jpn. J. Appl. Phys. 45 (2006) 5538.