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Dot formation with 170-nm dimensions using a thermal lithography technique
Microelectronic Engineering 67–68 (2003) 651–656 www.elsevier.com / locate / mee
Dot formation with 170-nm dimensions using a thermal lithography technique M. Kuwahara a , *, J.H. Kim b , J. Tominaga a a
Laboratory for Advanced Optical Technology, National Institute of Advanced Industrial Science and Technology (AIST), Central 4, 1 -1 -1 Higashi, Tsukuba 305 -8562, Japan b Digital Media R& D Center, Samsung Electronics Co. Ltd., 416, Maetan-3 -dong, Paldal-gu, Suwon City, Kyungki-do 442 -742, South Korea
Abstract We have developed a lithographic technique called ‘Thermal Lithography’ for application during the mastering process of ultra-high density optical disks. A Gaussian distribution of light intensity in a focused laser spot produces a spatially confined hot area that is of far smaller size than the laser spot size. The hot area induces a thermal cross-linking reaction in photoresist film, with the result that minute structures are fabricated after development. In this paper, we describe the use of this technique to fabricate 170-nm size dots in a photoresist film, each separated by a distance of 200 nm, shorter than the diffraction limit of 310 nm given by our optical set-up. We also confirmed that the size and height of the dots are approximately proportional to the incident laser power. 2003 Elsevier Science B.V. All rights reserved. Keywords: Ultra-high density optical disk; Mastering process; Gaussian distribution; Reversible photoresist; Thermal cross-linking
1. Introduction Since 2001, we have been developing a lithographic technique called ‘Thermal Lithography’ for the mastering process of ultra-high density optical disks [1,2]. Using this method, narrow lines and minute dots with 100-nm dimensions were fabricated in a photoresist film at high speed (6 m / s) using a semiconductor red laser source ( l 5 635 nm). A hot spot with sub-wavelength dimensions can be generated in the focused laser spot, since the light intensity in the focused spot has a Gaussian profile. A thermal cross-linking reaction in a special photoresist film (AZ5214E, image reversible type, Clariant Co.) is induced in the tiny hot area. The cross-linked portion is insoluble in developer, with * Corresponding author. E-mail address: [email protected] (M. Kuwahara). 0167-9317 / 03 / $ – see front matter 2003 Elsevier Science B.V. All rights reserved. doi:10.1016 / S0167-9317(03)00127-8
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M. Kuwahara et al. / Microelectronic Engineering 67–68 (2003) 651–656
the result that minute structures are left on the sample surface after the developing process. Conventional techniques using shorter wavelength light [3,4] or electron beams [5,6] are conventionally used to fabricate minute pits; however, these techniques require large and expensive facilities. On the other hand, thermal lithography enables low-cost fabrication because neither special light sources nor a special atmosphere are necessary. The thermal lithography clearly has great potential for practical application to the mastering process for producing pits in ultra-high density optical disks. We were previously able to produce only isolated lines and dots on the land areas of an optical disk substrate. However, structures of this rudimentary nature are not practical for use in optical disks: it is necessary to produce minute, adjacent structures. In this study, we succeeded in producing dots close to each other using a special disk drive tester. The dependence of dot sizes on the incident laser power was also examined.
2. Experimental The cross-sectional structure of our sample is shown in Fig. 1. A ZnS–SiO 2 / Ge 2 Sb 2 Te 5 / ZnS–SiO 2 multilayer was sputtered on a flat polycarbonate disk substrate. Ge 2 Sb 2 Te 5 (GST) is commonly used as a recording material in phase-change optical disks. It plays a role of absorbing incident light, resulting in the heat formation seen in our experiment. After the deposition, a hexamethyldisilazane (HMDS) treatment was carried out to improve the adhesion of the photoresist film and to prevent the film peeling off during development. The AZ5214E photoresist was then spin-coated on the ZnS–SiO 2 top layer after dilution with a commercial solvent to adjust the film thickness to about 30 nm. Sample disks were prebaked for 1 min at 90 8C and flood-exposed to blue light (mercury lamp, l 5 365 nm) according to the image reversal process. During exposure, protons are generated in the film, and work as catalysts in the thermal cross-linking reaction that takes place during annealing. The samples were developed using an organic alkali solution (NMD-W, Tokyo Ohka Kogyo Co., Ltd.) for 30 s and then rinsed in pure water for 30 s, both at room temperature. Finally, the samples were post-baked at 110 8C for 60 s. Fig. 2 schematically shows the experimental set-up. Our sample disk was set in the specially designed optical disk drive tester and rotated at a constant linear velocity of 6 m / s. The optical pick-up device was installed on a mechanical movement with a fine-pitch screw. The position of the laser spot on the sample surface could be controlled using a desktop computer to an accuracy of 40 nm in the disk radial direction. The laser wavelength was 405 nm, and the diffraction limit estimated
Fig. 1. Cross-sectional view of the sample disk.
M. Kuwahara et al. / Microelectronic Engineering 67–68 (2003) 651–656
653
Fig. 2. Experimental set-up. The optical pick-up system is mounted on the mechanical movement.
from the wavelength and the numerical aperture (NA 5 0.65) was about l / 2NA 5 310 nm. Pulsed laser light was focused on the GST film and the generated heat reached the AZ5214E film; a thermal cross-linking reaction then occurred in the film. A tapping-mode atomic force microscope (AFM, NanoScope IV, Digital Instruments) was used for the observation of the fabricated structures.
3. Results and discussion Fig. 3 shows AFM images of dots fabricated by pulsed laser heating at a frequency of 6 MHz and duty ratio of 20%. The laser-irradiated length (l) on the disk surface was calculated from the pulse frequency ( f ), the duty ratio (a ) and the disk rotation speed (v) to be l 5 a v /f 5200 nm. The photoresist was repeatedly heated up for 33 ns (20%31 / 6 MHz) in the tangential direction. Fig. 3a–d show the results of using different incident laser peak powers of 11, 10, 9 and 8 mW, respectively. The laser energies per single pulse (Ep ) incident to the sample can be calculated from peak power ( pp ) and pulse frequency ( f ), that is E5pp /f 50.37, 0.33, 0.30 and 0.27 nJ / pulse corresponds to peak powers of 11, 10, 9 and 8 mW, respectively. In this experiment, we moved the position of the laser beam spot in units of 200 nm in the radial direction, which is smaller than the diffraction limit, to form adjacent dots close to each other. The minimum distance between dots is shown to be approximately 220 nm, which is beyond the diffraction limit of 310 nm. It was thus proven that thermal lithography has the ability to fabricate structures beyond the diffraction limit. Fig. 4 shows the dependence of the dots’ diameter and height on incident laser power. Both diameter and height were thought to be proportional to the power in the range between 8 and 11 mW. When using laser pulses of 9 and 12 MHz, both diameter and height showed the same trend as seen in Fig. 4. At a laser power of 7 mW, we found apparent structures, but it was not possible to determine the dot sizes from AFM images. It can be seen in Fig. 4 that the dot height, rather than diameter, appears to decrease rapidly with decreasing laser power. The height of a dot formed at 8 mW is approximately one-third of one formed at 11 mW. On the other hand, the diameter at 8 mW is approximately two-thirds that at 11 mW. This phenomenon may be due to different rates of thermal
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M. Kuwahara et al. / Microelectronic Engineering 67–68 (2003) 651–656
Fig. 3. AFM images of the fabricated dots at different laser powers at a frequency of 6 MHz and 20% duty ratio; (a) 11 mW; (b) 10 mW; (c) 9 mW; (d) 8 mW.
propagation in the lateral and vertical directions in the sample, but it is not possible to tell at this stage. To control the dot size, it is useful to know the dependence of dot formation on pulse frequency. Fig. 5 shows AFM images of dots formed at different pulse frequencies. (a), (b) and (c) indicate 6, 9 and 12 MHz, respectively. The duty ratio and the peak power were 20% and 11 mW. The energies of a single pulse at 6, 9 and 12 MHz correspond to 0.37, 0.24, 0.18 nJ / pulse. The laser run-lengths on the disk surface were 200, 133 and 100 nm, respectively. It was found that dot sizes tend to decrease with increased pulse frequency. Although the experiment was also carried out at 15 MHz, it was difficult to identify any dot formation. We previously simulated the temperature profile induced by a focused laser beam inside the photoresist film in a line formation [2], but our heat profile analysis does not as yet explain the
M. Kuwahara et al. / Microelectronic Engineering 67–68 (2003) 651–656
655
Fig. 4. The dependence of diameter and height of the fabricated dots on the incident laser power at a frequency of 6 MHz and 20% duty ratio. Solid lines are guides for the eye.
formation of the narrow line. It is necessary to elucidate the line-and-dot formation in the context of the temperature profile and the speed of the thermal cross-linking reaction. In future work, we will try to produce a line-and-space structure and dots with a high aspect ratio for application to the mastering process of ultra-high density optical disks.
4. Conclusion We succeeded in fabricating tiny dots in two dimensions beyond the diffraction limit by applying a thermal lithographic technique using a specially designed optical disk drive tester. A minimum
Fig. 5. AFM images of dots fabricated at different incident laser frequencies at a power of 11 mW and 20% duty ratio.
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M. Kuwahara et al. / Microelectronic Engineering 67–68 (2003) 651–656
distance between the dots of approximately 220 nm was achieved. The diameter and height of the formed dots are proportional to the incident laser peak power.
Acknowledgements The authors would like to thank Hisako Fukuda, our technical assistant, for operating the AFM equipment. This work was partly supported by The Ministry of Education, Culture, Sports, Science and Technology under the Nanotechnology Support Project.
References [1] M. Kuwahara, C. Mihalcea, N. Atoda, J. Tominaga, H. Fuji, T. Kikukawa, Microelectron. Eng. 61–62 (2002) 415–421. [2] M. Kuwahara, J. Li, C. Mihalcea, J. Tominaga, N. Atoda, L.P. Shi, Jpn. J. Appl. Phys. 41 (2002) L1022–L1024. [3] T. Ando, H. Nakamoto, M. Yanagi, N. Kimura, Joint International Symposium on Optical Memory and Optical Data Storage 2002, Technical Digest, pp. 30–32. [4] M. Takeda, M. Furuki, T. Ishimoto, K. Kondo, M. Yamamoto, S. Kubota, Jpn. J. Appl. Phys. 39 (2000) 797–799. [5] S. Hosaka, T. Suzuki, M. Yamaoka, K. Katoh, F. Isshiki, M. Miyamoto, Y. Miyauchi, A. Arimoto, T. Nishida, Microelectron. Eng. 61–62 (2002) 309–316. [6] M. Takeda, M. Furuki, M. Yamamoto, Y. Aki, H. Kawase, Joint International Symposium on Optical Memory and Optical Data Storage 2002, Technical Digest, pp. 18–20.
Dot formation with 170-nm dimensions using a thermal lithography technique M. Kuwahara a , *, J.H. Kim b , J. Tominaga a a
Laboratory for Advanced Optical Technology, National Institute of Advanced Industrial Science and Technology (AIST), Central 4, 1 -1 -1 Higashi, Tsukuba 305 -8562, Japan b Digital Media R& D Center, Samsung Electronics Co. Ltd., 416, Maetan-3 -dong, Paldal-gu, Suwon City, Kyungki-do 442 -742, South Korea
Abstract We have developed a lithographic technique called ‘Thermal Lithography’ for application during the mastering process of ultra-high density optical disks. A Gaussian distribution of light intensity in a focused laser spot produces a spatially confined hot area that is of far smaller size than the laser spot size. The hot area induces a thermal cross-linking reaction in photoresist film, with the result that minute structures are fabricated after development. In this paper, we describe the use of this technique to fabricate 170-nm size dots in a photoresist film, each separated by a distance of 200 nm, shorter than the diffraction limit of 310 nm given by our optical set-up. We also confirmed that the size and height of the dots are approximately proportional to the incident laser power. 2003 Elsevier Science B.V. All rights reserved. Keywords: Ultra-high density optical disk; Mastering process; Gaussian distribution; Reversible photoresist; Thermal cross-linking
1. Introduction Since 2001, we have been developing a lithographic technique called ‘Thermal Lithography’ for the mastering process of ultra-high density optical disks [1,2]. Using this method, narrow lines and minute dots with 100-nm dimensions were fabricated in a photoresist film at high speed (6 m / s) using a semiconductor red laser source ( l 5 635 nm). A hot spot with sub-wavelength dimensions can be generated in the focused laser spot, since the light intensity in the focused spot has a Gaussian profile. A thermal cross-linking reaction in a special photoresist film (AZ5214E, image reversible type, Clariant Co.) is induced in the tiny hot area. The cross-linked portion is insoluble in developer, with * Corresponding author. E-mail address: [email protected] (M. Kuwahara). 0167-9317 / 03 / $ – see front matter 2003 Elsevier Science B.V. All rights reserved. doi:10.1016 / S0167-9317(03)00127-8
652
M. Kuwahara et al. / Microelectronic Engineering 67–68 (2003) 651–656
the result that minute structures are left on the sample surface after the developing process. Conventional techniques using shorter wavelength light [3,4] or electron beams [5,6] are conventionally used to fabricate minute pits; however, these techniques require large and expensive facilities. On the other hand, thermal lithography enables low-cost fabrication because neither special light sources nor a special atmosphere are necessary. The thermal lithography clearly has great potential for practical application to the mastering process for producing pits in ultra-high density optical disks. We were previously able to produce only isolated lines and dots on the land areas of an optical disk substrate. However, structures of this rudimentary nature are not practical for use in optical disks: it is necessary to produce minute, adjacent structures. In this study, we succeeded in producing dots close to each other using a special disk drive tester. The dependence of dot sizes on the incident laser power was also examined.
2. Experimental The cross-sectional structure of our sample is shown in Fig. 1. A ZnS–SiO 2 / Ge 2 Sb 2 Te 5 / ZnS–SiO 2 multilayer was sputtered on a flat polycarbonate disk substrate. Ge 2 Sb 2 Te 5 (GST) is commonly used as a recording material in phase-change optical disks. It plays a role of absorbing incident light, resulting in the heat formation seen in our experiment. After the deposition, a hexamethyldisilazane (HMDS) treatment was carried out to improve the adhesion of the photoresist film and to prevent the film peeling off during development. The AZ5214E photoresist was then spin-coated on the ZnS–SiO 2 top layer after dilution with a commercial solvent to adjust the film thickness to about 30 nm. Sample disks were prebaked for 1 min at 90 8C and flood-exposed to blue light (mercury lamp, l 5 365 nm) according to the image reversal process. During exposure, protons are generated in the film, and work as catalysts in the thermal cross-linking reaction that takes place during annealing. The samples were developed using an organic alkali solution (NMD-W, Tokyo Ohka Kogyo Co., Ltd.) for 30 s and then rinsed in pure water for 30 s, both at room temperature. Finally, the samples were post-baked at 110 8C for 60 s. Fig. 2 schematically shows the experimental set-up. Our sample disk was set in the specially designed optical disk drive tester and rotated at a constant linear velocity of 6 m / s. The optical pick-up device was installed on a mechanical movement with a fine-pitch screw. The position of the laser spot on the sample surface could be controlled using a desktop computer to an accuracy of 40 nm in the disk radial direction. The laser wavelength was 405 nm, and the diffraction limit estimated
Fig. 1. Cross-sectional view of the sample disk.
M. Kuwahara et al. / Microelectronic Engineering 67–68 (2003) 651–656
653
Fig. 2. Experimental set-up. The optical pick-up system is mounted on the mechanical movement.
from the wavelength and the numerical aperture (NA 5 0.65) was about l / 2NA 5 310 nm. Pulsed laser light was focused on the GST film and the generated heat reached the AZ5214E film; a thermal cross-linking reaction then occurred in the film. A tapping-mode atomic force microscope (AFM, NanoScope IV, Digital Instruments) was used for the observation of the fabricated structures.
3. Results and discussion Fig. 3 shows AFM images of dots fabricated by pulsed laser heating at a frequency of 6 MHz and duty ratio of 20%. The laser-irradiated length (l) on the disk surface was calculated from the pulse frequency ( f ), the duty ratio (a ) and the disk rotation speed (v) to be l 5 a v /f 5200 nm. The photoresist was repeatedly heated up for 33 ns (20%31 / 6 MHz) in the tangential direction. Fig. 3a–d show the results of using different incident laser peak powers of 11, 10, 9 and 8 mW, respectively. The laser energies per single pulse (Ep ) incident to the sample can be calculated from peak power ( pp ) and pulse frequency ( f ), that is E5pp /f 50.37, 0.33, 0.30 and 0.27 nJ / pulse corresponds to peak powers of 11, 10, 9 and 8 mW, respectively. In this experiment, we moved the position of the laser beam spot in units of 200 nm in the radial direction, which is smaller than the diffraction limit, to form adjacent dots close to each other. The minimum distance between dots is shown to be approximately 220 nm, which is beyond the diffraction limit of 310 nm. It was thus proven that thermal lithography has the ability to fabricate structures beyond the diffraction limit. Fig. 4 shows the dependence of the dots’ diameter and height on incident laser power. Both diameter and height were thought to be proportional to the power in the range between 8 and 11 mW. When using laser pulses of 9 and 12 MHz, both diameter and height showed the same trend as seen in Fig. 4. At a laser power of 7 mW, we found apparent structures, but it was not possible to determine the dot sizes from AFM images. It can be seen in Fig. 4 that the dot height, rather than diameter, appears to decrease rapidly with decreasing laser power. The height of a dot formed at 8 mW is approximately one-third of one formed at 11 mW. On the other hand, the diameter at 8 mW is approximately two-thirds that at 11 mW. This phenomenon may be due to different rates of thermal
654
M. Kuwahara et al. / Microelectronic Engineering 67–68 (2003) 651–656
Fig. 3. AFM images of the fabricated dots at different laser powers at a frequency of 6 MHz and 20% duty ratio; (a) 11 mW; (b) 10 mW; (c) 9 mW; (d) 8 mW.
propagation in the lateral and vertical directions in the sample, but it is not possible to tell at this stage. To control the dot size, it is useful to know the dependence of dot formation on pulse frequency. Fig. 5 shows AFM images of dots formed at different pulse frequencies. (a), (b) and (c) indicate 6, 9 and 12 MHz, respectively. The duty ratio and the peak power were 20% and 11 mW. The energies of a single pulse at 6, 9 and 12 MHz correspond to 0.37, 0.24, 0.18 nJ / pulse. The laser run-lengths on the disk surface were 200, 133 and 100 nm, respectively. It was found that dot sizes tend to decrease with increased pulse frequency. Although the experiment was also carried out at 15 MHz, it was difficult to identify any dot formation. We previously simulated the temperature profile induced by a focused laser beam inside the photoresist film in a line formation [2], but our heat profile analysis does not as yet explain the
M. Kuwahara et al. / Microelectronic Engineering 67–68 (2003) 651–656
655
Fig. 4. The dependence of diameter and height of the fabricated dots on the incident laser power at a frequency of 6 MHz and 20% duty ratio. Solid lines are guides for the eye.
formation of the narrow line. It is necessary to elucidate the line-and-dot formation in the context of the temperature profile and the speed of the thermal cross-linking reaction. In future work, we will try to produce a line-and-space structure and dots with a high aspect ratio for application to the mastering process of ultra-high density optical disks.
4. Conclusion We succeeded in fabricating tiny dots in two dimensions beyond the diffraction limit by applying a thermal lithographic technique using a specially designed optical disk drive tester. A minimum
Fig. 5. AFM images of dots fabricated at different incident laser frequencies at a power of 11 mW and 20% duty ratio.
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M. Kuwahara et al. / Microelectronic Engineering 67–68 (2003) 651–656
distance between the dots of approximately 220 nm was achieved. The diameter and height of the formed dots are proportional to the incident laser peak power.
Acknowledgements The authors would like to thank Hisako Fukuda, our technical assistant, for operating the AFM equipment. This work was partly supported by The Ministry of Education, Culture, Sports, Science and Technology under the Nanotechnology Support Project.
References [1] M. Kuwahara, C. Mihalcea, N. Atoda, J. Tominaga, H. Fuji, T. Kikukawa, Microelectron. Eng. 61–62 (2002) 415–421. [2] M. Kuwahara, J. Li, C. Mihalcea, J. Tominaga, N. Atoda, L.P. Shi, Jpn. J. Appl. Phys. 41 (2002) L1022–L1024. [3] T. Ando, H. Nakamoto, M. Yanagi, N. Kimura, Joint International Symposium on Optical Memory and Optical Data Storage 2002, Technical Digest, pp. 30–32. [4] M. Takeda, M. Furuki, T. Ishimoto, K. Kondo, M. Yamamoto, S. Kubota, Jpn. J. Appl. Phys. 39 (2000) 797–799. [5] S. Hosaka, T. Suzuki, M. Yamaoka, K. Katoh, F. Isshiki, M. Miyamoto, Y. Miyauchi, A. Arimoto, T. Nishida, Microelectron. Eng. 61–62 (2002) 309–316. [6] M. Takeda, M. Furuki, M. Yamamoto, Y. Aki, H. Kawase, Joint International Symposium on Optical Memory and Optical Data Storage 2002, Technical Digest, pp. 18–20.