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SNOM imaging of very fine pits formed by EB lithography for ultrahigh density optical recording
Microelectronic Engineering 67–68 (2003) 728–735 www.elsevier.com / locate / mee
SNOM imaging of very fine pits formed by EB lithography for ultrahigh density optical recording Sumio Hosaka a,b , *, Hayato Sone a , Yoshitaka Takahashi a , Toshimichi Shintani b , Keizo Kato b , Toshiharu Saiki c a
b
Department of Electronic Engineering, Gunma University, 1 -5 -1 Tenjin, Kiryu 376 -8515, Japan Central Research Laboratory, Hitachi, Ltd., 1 -280 Higashi-koigakubo, Kokubunji, Tokyo 185 -8601 Japan c Kanagawa Academy of Science and Technology, 3 -2 -1 Sakado, Takatsu, Kawasaki 213 -0012, Japan
Abstract The possibility to optically image the very fine pit structure formed by electron beam (EB) writing has been researched using scanning near-field optical microscopy (SNOM). Fine test pit samples were formed in an electron resist (ZEP520) with a minimum pit size of 30 nm 3 160 nm. From experimental results using the pits, a conventional reflection SNOM could not image fine pit structures with a size of less than 100 nm. The technique was improved by coating the metal film on the optical probe and adopting an optical depolarization technique in the SNOM optics efficiently to detect near-field light reflected from the sample surface. We demonstrated that very fine pits with a minimum size of 30 nm were imaged and discussed that reflection type depolarization SNOM has a potential to achieve an ultrahigh density optical reading with 1.2 Tb / in.2 , which is limited by EB fabrication of the pits. 2003 Elsevier Science B.V. All rights reserved. Keywords: EB writing; Near-field optics; Trillion bit recording; Depolarization; SNOM
1. Introduction Magnetic and optical recording densities are drastically increasing. The magnetic recording density will become 50 Gb / in.2 in practice within a few years (1 in. 5 2.54 cm). Also, the optical recording density will increase from 2.9 Gb / in.2 to 15–25 Gb / in.2 within a few years. For optical recording, the technique might be limited by optical diffraction. In order to overcome the limit, we can expect near-field optical recording as a potential solution [1–3]. As a result, we have to develop not only a nanometer-sized optical probe but also to fabricate a nanometer-sized pit pattern. * Corresponding author. Tel.: 1 81-277-30-1721; fax: 1 81-277-30-1707. E-mail address: [email protected] (S. Hosaka). 0167-9317 / 03 / $ – see front matter 2003 Elsevier Science B.V. All rights reserved. doi:10.1016 / S0167-9317(03)00133-3
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Many researchers have studied optical recording to develop ultrahigh density recording with trillion bits / in.2 [4–7]. For fine optical probe, scanning near-field optical microscopy (SNOM) has been introduced instead of conventional optics. Since the introduction of near-field magneto-optical (MO) recording by Betzig et al. [1] and near-field phase change (PC) recording by Hosaka et al. [2], the ultrahigh density recording using near-field optics has been investigated. However, these investigations were carried out using near-field optical writing with a bit size of 60 nm. It is also very important to investigate the limitations of SNOM-based optical reading of fine pits and fabricating of nanometer-sized pit structure using the electron beam (EB) mastering technique. Especially, we are very interested in the SNOM-based optical reading in a pit size region smaller than 60 nm. Furthermore, we have developed the EB mastering system and studied the fabrication of nanometersized pits in electron resist [8,9]. Our study focuses to develop basic technologies such as SNOM-based readout and fine pit fabrication for final goal to achieve 1 Tb / in.2 ROM. In this paper, we will describe the limitations of SNOM-based optical reading of very fine pits using reflection type depolarization near-field optics and fabricating of fine pit patterns with a size of less than 100 nm using field emission electron gun EB writing.
2. Fabrication of very fine pits
2.1. Electron beam writing (recording) system We utilized a conventional electron beam writing (recording) (EBR) system for very fine pits with sizes of less than 100 nm. The EBR system used here is shown in Fig. 1 [8]. This system is constructed based on scanning electron microscopy (SEM) with a high resolution of , 1 nm. Fig. 1 shows the electro-optical column and controller of EB drawing system, which is called the
Fig. 1. Schematic diagram of electron beam drawing system (column and controller parts).
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‘‘nanowriter’’ and made by Hitachi Science Co. and Tokyo Technology Co. The system is available in the high current probe of more than 0.5 nA at a small probe diameter of less than 10 nm, because of the Schottky emission type field emission (SE-FE) electron gun. Noise and drift of the probe current are suppressed within 4% / 10 h. The system promises to fabricate fine pit structure with a size of less than 25 nm (Fig. 2).
2.2. Patterning of fine pits We used a positive type electron resist, the ZEP520 (Nippon Zenon Co.) for patterning of very fine pits. A minimum pattern size with a width of 25 nm has been obtained using a 150 mC / cm 2 dosage as shown in Fig. 2. Here, 200-nm-thick ZEP resist on a silicon substrate was exposed to a 30-keV electron current of 0.5 nA. Fig. 2a and b show the variations of width and developing depth of the pits due to the electron dosage in a case of one-scan writing. From the results, we determine the dosage of 150 mC / cm 2 for perfect writing of the very fine pit pattern of 25 nm width and a resist thickness of
Fig. 2. Exposing condition for fine pit pattern (a) and (b), and SEM image of fine pit array (c) [resist: ZEP520 (100 nm in thickness) / Si, dosage of 150 mC / cm 2 ].
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Fig. 3. SEM images of fine resist pattern written by EB lithography (dose: 50 mC / cm 2 , 30 keV); (a) rectangular pattern drawing (track pitch: 320 nm, pit: 160 mm 3 320 nm), (b) rectangular pattern drawing (200 nm, 100 nm 3 280 nm), (c) line pattern drawing (140 nm, 50 nm 3 200 nm), and (d) line pattern drawing (100 nm, 30 nm 3 160 nm).
100 nm. This system has the potential to fabricate a very fine pit with a width of less than 25 nm as shown in Fig. 2c. Fig. 3 shows test sample SEM images of the fine resist patterns written by electron beam with a probe current of about 1 nA at 30 keV. The exposure dosage was 50 mC / cm 2 . These pit structures correspond to 25 to 500 Gb / in.2 when using 16-level edge modulation, which means 16-level multi-pit-length modulation. Figs. 2c and 3d show that the EBR system has the potential to fabricate a minimum pit size of less than 25 nm. In the experiments described below, we used these pit patterns with a minimum size of 30 nm 3 160 nm.
3. Near-field optical readout
3.1. Conventional reflection type SNOM Using the test sample, we tried to detect near-field light reflected from the very fine pits through the near-field optical aperture using conventional SNOM as shown in Fig. 4. The SNOM system has an etched fiber probe and a shear force control for constant gap between sample and probe using an
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Fig. 4. Scheme of reflection type near-field optical microscopy.
optical deflection. The etched fiber probe has been coated with an aluminium film by oblique evaporation technique to obtain a fine near-field optical aperture. Fig. 5 shows SNOM images of various fine pits of 200 nm 3 400 nm, 160 nm 3 320 nm, 100
Fig. 5. Conventional reflection SNOM images of fine pits patterns with several sizes, (a) track pitch of 400 nm, pit size of 200 nm 3 400 nm, (b) 320 nm, 160 nm 3 320 nm, (c) 200 nm, 100 nm 3 280 nm, and (d) 140 nm, 50 nm 3 200 nm.
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nm 3 280 nm, and 50 nm 3 200 nm. The minimum detectable size is 100 nm 3 280 nm. This means that the near-field optical probe is not sufficiently small. Comparing this result with the conventional optical readout using scanning laser microscopy (SLM) [10], the result is the same as in a case of SLM with a numerical aperture (NA) of 1.4. It can be concluded that the SNOM probe has not only near-field but also far-field optical contributions from our previous results [11]. Furthermore, the light reflected from the probe involves near-field and far-field light. The power of the far-field reflected light is much stronger than that of the near-field reflected light so that we can not detect the near-field reflected light easily.
3.2. Reflection type depolarization SNOM In order to solve the above technical issues regarding the optical probe and the reflected light, we make use of two new techniques in SNOM as follows: (1) entire metal coating of the fiber probe and (2) use of a depolarization technique. The entire metal coating was used to prevent leaking of the far-field light from the tip of the fiber probe. The small aperture was made by soft touch of the tip onto the sample. The depolarization performed extinguishes the far-field light reflected from the tip and only to detect the light reflected from the sample surface. Reflection type depolarization SNOM [12] is shown in Fig. 6. The system consists of a He–Ne laser source, a Glan–Thompson (G–T) analyzer, l / 4- and l / 2-wave plates, an etched fiber probe with a fine aperture, and a photodiode. In the experiment, we adjusted the phase of the G–T analyzer to eliminate the far-field light reflected from the probe surface. On the other hand, the pit signal can pass through the analyzer to the photo detector because the difference of the phase between probe and sample is about 38 when the gap is controlled to about 10 nm. This eliminates the background signal to improve the signal-to-noise ratio (S /N). Consequently, the near-field light from the sample can be detected more efficiently. The new system achieved high reliability in the observation of fine pits by removing far-field light reflected from the etched SNOM probe. Fig. 7 shows the reflection type depolarization SNOM images of fine pit patterns with sizes of 50 nm 3 200 nm and 30 nm 3 160 nm. It clearly resolves individual pits with very fine sizes. The black
Fig. 6. Scheme of reflection type depolarization near-field optical microscopy.
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Fig. 7. SNOM images of fine pits patterns; (a) track pitch of 140 nm, pit size of 50 nm 3 200 nm, and (b) 100 nm, 30 nm 3 160 nm.
parts correspond to the pits on the sample. The experiment was performed using an incident laser with a wavelength 633 nm and a power of less than 1 mW. Although the pit shape is rectangular with 30 nm 3 160 nm, we can estimate optical probe size from transient region of the pit image. The optical probe size is approximately less than 20 nm from Fig. 7b, while the size includes edge sharpness of the EB fabricated pit. The probe size means that the optical probe can detect very fine pits with a size of 30 nm 3 30 nm and a track pitch of 60 nm. The pit size of 30 nm 3 30 nm with a track pitch of 60 nm corresponds to more than 500 Gb / in.2 using 8-14 coded modulation (EFM) with non-return to zero inverse (NRZI). On the other hand, the pit fabrication limit appears to be 20 nm based on the EB writing patterns. This limit is due to proximity effects with this dense pattern. The limit means that the density of the SNOM-based ROM recording with pits is about 1.2 Tb / in.2 without a high multilevel or multilayer recording method.
4. Summary We have investigated the potential of using an SNOM technique and its density limit to optically image very fine pit structures, which were formed by EB fabrication. Fine test pit samples were formed in electron resist (ZEP520) with the minimum pit size of 30 nm 3 160 nm. As a result, we achieved the following conclusions: 1. Entire metal coating on the probe and depolarization technique are very efficient to detect nanometer-sized small pits. 2. Reflection type depolarization SNOM can detect a minimum pit of 30 nm 3 160 nm. 3. The detectable size of 30 nm corresponds to about 500 Gb / in.2 when using an EFM and an NRZI. 4. The EB writing limit might appear at a size of 20 nm so that the SNOM-based ROM has a potential to achieve an ultrahigh recording density of 1.2 Tb / in.2 without a multilevel or multilayer recording method.
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Acknowledgements The authors would like to thank Dr. T. Matsumoto, Mr. T. Suzuki, Mr. T. Nishida of Hitachi Ltd., for the fabrication of fine pits samples for this experiment and a fruitful discussion.
References [1] E. Betzig, J.K. Trautman, R. Wolfe, E.M. Gyorgy, P.L. Finn, M.H. Kryder, C.-H. Chang, Appl. Phys. Lett. 61 (1992) 142. [2] S. Hosaka, T. Shintani, M. Miyamoto, A. Kikukawa, A. Hirotsune, M. Terao, M. Yoshida, K. Fujita, S. Kammer, J. Appl. Phys. 79 (1996) 8082. [3] F. Isshiki, K. Itoh, K. Etoh, S. Hosaka, Appl. Phys. Lett. 76 (2000) 804. [4] A. Itoh, N. Ohta, T. Uchiyama, A. Takahashi, M. Mieda, N. Iketani, Y. Uchihara, M. Nakata, K. Tezuka, H. Awano, S. Imai, K. Nakagawa, in: Technical Digest of Cleo / Pacific Rim 2001, Tokyo, 2001, pp. II2–II3. [5] J.P. de Kock, S. Kobayashi, T. Ishimoto, H. Yamatsu, H. Ooki, Jpn. J. Appl. Phys. 35 (1996) 437. [6] S. Hosaka, IEEE Trans. Magn. 32 (1996) 1873. [7] Y. Martin, S. Rishton, H.K. Wickramasinghe, Appl. Phys. Lett. 71 (1997) 1. [8] S. Hosaka, H. Koyanagi, K. Katoh, F. Isshiki, T. Suzuki, M. Miyamoto, A. Arimoto, T. Maeda, Microelectron. Eng. 57 (2001) 223. [9] S. Hosaka, H. Koyanagi, K. Katoh, F. Isshiki, T. Suzuki, M. Miyamoto, Y. Miyauchi, A. Arimoto, T. Nishida, in: Technical Digest of Cleo / Pacific Rim 2001, Tokyo, 2001, pp. II4–II5. [10] S. Hosaka, T. Shintani, H. Koyanagi, K. Katoh, T. Nishida, T. Saiki, T. Hasegawa, Jpn. J. Appl. Phys. 41 (2002) L884. [11] S. Hosaka, T. Shintani, A. Kikukawa, K. Itoh, J. Microsc. 194 (1999) 369. [12] T. Saiki, K. Yusu, K. Ichihara, T. Tadokoro, in: Technical Digest of the 6th International Conference on Near-Field Optics and Related Techniques (NFO-6), Twente University, Netherlands, 2001, p. 122.
SNOM imaging of very fine pits formed by EB lithography for ultrahigh density optical recording Sumio Hosaka a,b , *, Hayato Sone a , Yoshitaka Takahashi a , Toshimichi Shintani b , Keizo Kato b , Toshiharu Saiki c a
b
Department of Electronic Engineering, Gunma University, 1 -5 -1 Tenjin, Kiryu 376 -8515, Japan Central Research Laboratory, Hitachi, Ltd., 1 -280 Higashi-koigakubo, Kokubunji, Tokyo 185 -8601 Japan c Kanagawa Academy of Science and Technology, 3 -2 -1 Sakado, Takatsu, Kawasaki 213 -0012, Japan
Abstract The possibility to optically image the very fine pit structure formed by electron beam (EB) writing has been researched using scanning near-field optical microscopy (SNOM). Fine test pit samples were formed in an electron resist (ZEP520) with a minimum pit size of 30 nm 3 160 nm. From experimental results using the pits, a conventional reflection SNOM could not image fine pit structures with a size of less than 100 nm. The technique was improved by coating the metal film on the optical probe and adopting an optical depolarization technique in the SNOM optics efficiently to detect near-field light reflected from the sample surface. We demonstrated that very fine pits with a minimum size of 30 nm were imaged and discussed that reflection type depolarization SNOM has a potential to achieve an ultrahigh density optical reading with 1.2 Tb / in.2 , which is limited by EB fabrication of the pits. 2003 Elsevier Science B.V. All rights reserved. Keywords: EB writing; Near-field optics; Trillion bit recording; Depolarization; SNOM
1. Introduction Magnetic and optical recording densities are drastically increasing. The magnetic recording density will become 50 Gb / in.2 in practice within a few years (1 in. 5 2.54 cm). Also, the optical recording density will increase from 2.9 Gb / in.2 to 15–25 Gb / in.2 within a few years. For optical recording, the technique might be limited by optical diffraction. In order to overcome the limit, we can expect near-field optical recording as a potential solution [1–3]. As a result, we have to develop not only a nanometer-sized optical probe but also to fabricate a nanometer-sized pit pattern. * Corresponding author. Tel.: 1 81-277-30-1721; fax: 1 81-277-30-1707. E-mail address: [email protected] (S. Hosaka). 0167-9317 / 03 / $ – see front matter 2003 Elsevier Science B.V. All rights reserved. doi:10.1016 / S0167-9317(03)00133-3
S. Hosaka et al. / Microelectronic Engineering 67–68 (2003) 728–735
729
Many researchers have studied optical recording to develop ultrahigh density recording with trillion bits / in.2 [4–7]. For fine optical probe, scanning near-field optical microscopy (SNOM) has been introduced instead of conventional optics. Since the introduction of near-field magneto-optical (MO) recording by Betzig et al. [1] and near-field phase change (PC) recording by Hosaka et al. [2], the ultrahigh density recording using near-field optics has been investigated. However, these investigations were carried out using near-field optical writing with a bit size of 60 nm. It is also very important to investigate the limitations of SNOM-based optical reading of fine pits and fabricating of nanometer-sized pit structure using the electron beam (EB) mastering technique. Especially, we are very interested in the SNOM-based optical reading in a pit size region smaller than 60 nm. Furthermore, we have developed the EB mastering system and studied the fabrication of nanometersized pits in electron resist [8,9]. Our study focuses to develop basic technologies such as SNOM-based readout and fine pit fabrication for final goal to achieve 1 Tb / in.2 ROM. In this paper, we will describe the limitations of SNOM-based optical reading of very fine pits using reflection type depolarization near-field optics and fabricating of fine pit patterns with a size of less than 100 nm using field emission electron gun EB writing.
2. Fabrication of very fine pits
2.1. Electron beam writing (recording) system We utilized a conventional electron beam writing (recording) (EBR) system for very fine pits with sizes of less than 100 nm. The EBR system used here is shown in Fig. 1 [8]. This system is constructed based on scanning electron microscopy (SEM) with a high resolution of , 1 nm. Fig. 1 shows the electro-optical column and controller of EB drawing system, which is called the
Fig. 1. Schematic diagram of electron beam drawing system (column and controller parts).
730
S. Hosaka et al. / Microelectronic Engineering 67–68 (2003) 728–735
‘‘nanowriter’’ and made by Hitachi Science Co. and Tokyo Technology Co. The system is available in the high current probe of more than 0.5 nA at a small probe diameter of less than 10 nm, because of the Schottky emission type field emission (SE-FE) electron gun. Noise and drift of the probe current are suppressed within 4% / 10 h. The system promises to fabricate fine pit structure with a size of less than 25 nm (Fig. 2).
2.2. Patterning of fine pits We used a positive type electron resist, the ZEP520 (Nippon Zenon Co.) for patterning of very fine pits. A minimum pattern size with a width of 25 nm has been obtained using a 150 mC / cm 2 dosage as shown in Fig. 2. Here, 200-nm-thick ZEP resist on a silicon substrate was exposed to a 30-keV electron current of 0.5 nA. Fig. 2a and b show the variations of width and developing depth of the pits due to the electron dosage in a case of one-scan writing. From the results, we determine the dosage of 150 mC / cm 2 for perfect writing of the very fine pit pattern of 25 nm width and a resist thickness of
Fig. 2. Exposing condition for fine pit pattern (a) and (b), and SEM image of fine pit array (c) [resist: ZEP520 (100 nm in thickness) / Si, dosage of 150 mC / cm 2 ].
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Fig. 3. SEM images of fine resist pattern written by EB lithography (dose: 50 mC / cm 2 , 30 keV); (a) rectangular pattern drawing (track pitch: 320 nm, pit: 160 mm 3 320 nm), (b) rectangular pattern drawing (200 nm, 100 nm 3 280 nm), (c) line pattern drawing (140 nm, 50 nm 3 200 nm), and (d) line pattern drawing (100 nm, 30 nm 3 160 nm).
100 nm. This system has the potential to fabricate a very fine pit with a width of less than 25 nm as shown in Fig. 2c. Fig. 3 shows test sample SEM images of the fine resist patterns written by electron beam with a probe current of about 1 nA at 30 keV. The exposure dosage was 50 mC / cm 2 . These pit structures correspond to 25 to 500 Gb / in.2 when using 16-level edge modulation, which means 16-level multi-pit-length modulation. Figs. 2c and 3d show that the EBR system has the potential to fabricate a minimum pit size of less than 25 nm. In the experiments described below, we used these pit patterns with a minimum size of 30 nm 3 160 nm.
3. Near-field optical readout
3.1. Conventional reflection type SNOM Using the test sample, we tried to detect near-field light reflected from the very fine pits through the near-field optical aperture using conventional SNOM as shown in Fig. 4. The SNOM system has an etched fiber probe and a shear force control for constant gap between sample and probe using an
732
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Fig. 4. Scheme of reflection type near-field optical microscopy.
optical deflection. The etched fiber probe has been coated with an aluminium film by oblique evaporation technique to obtain a fine near-field optical aperture. Fig. 5 shows SNOM images of various fine pits of 200 nm 3 400 nm, 160 nm 3 320 nm, 100
Fig. 5. Conventional reflection SNOM images of fine pits patterns with several sizes, (a) track pitch of 400 nm, pit size of 200 nm 3 400 nm, (b) 320 nm, 160 nm 3 320 nm, (c) 200 nm, 100 nm 3 280 nm, and (d) 140 nm, 50 nm 3 200 nm.
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nm 3 280 nm, and 50 nm 3 200 nm. The minimum detectable size is 100 nm 3 280 nm. This means that the near-field optical probe is not sufficiently small. Comparing this result with the conventional optical readout using scanning laser microscopy (SLM) [10], the result is the same as in a case of SLM with a numerical aperture (NA) of 1.4. It can be concluded that the SNOM probe has not only near-field but also far-field optical contributions from our previous results [11]. Furthermore, the light reflected from the probe involves near-field and far-field light. The power of the far-field reflected light is much stronger than that of the near-field reflected light so that we can not detect the near-field reflected light easily.
3.2. Reflection type depolarization SNOM In order to solve the above technical issues regarding the optical probe and the reflected light, we make use of two new techniques in SNOM as follows: (1) entire metal coating of the fiber probe and (2) use of a depolarization technique. The entire metal coating was used to prevent leaking of the far-field light from the tip of the fiber probe. The small aperture was made by soft touch of the tip onto the sample. The depolarization performed extinguishes the far-field light reflected from the tip and only to detect the light reflected from the sample surface. Reflection type depolarization SNOM [12] is shown in Fig. 6. The system consists of a He–Ne laser source, a Glan–Thompson (G–T) analyzer, l / 4- and l / 2-wave plates, an etched fiber probe with a fine aperture, and a photodiode. In the experiment, we adjusted the phase of the G–T analyzer to eliminate the far-field light reflected from the probe surface. On the other hand, the pit signal can pass through the analyzer to the photo detector because the difference of the phase between probe and sample is about 38 when the gap is controlled to about 10 nm. This eliminates the background signal to improve the signal-to-noise ratio (S /N). Consequently, the near-field light from the sample can be detected more efficiently. The new system achieved high reliability in the observation of fine pits by removing far-field light reflected from the etched SNOM probe. Fig. 7 shows the reflection type depolarization SNOM images of fine pit patterns with sizes of 50 nm 3 200 nm and 30 nm 3 160 nm. It clearly resolves individual pits with very fine sizes. The black
Fig. 6. Scheme of reflection type depolarization near-field optical microscopy.
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Fig. 7. SNOM images of fine pits patterns; (a) track pitch of 140 nm, pit size of 50 nm 3 200 nm, and (b) 100 nm, 30 nm 3 160 nm.
parts correspond to the pits on the sample. The experiment was performed using an incident laser with a wavelength 633 nm and a power of less than 1 mW. Although the pit shape is rectangular with 30 nm 3 160 nm, we can estimate optical probe size from transient region of the pit image. The optical probe size is approximately less than 20 nm from Fig. 7b, while the size includes edge sharpness of the EB fabricated pit. The probe size means that the optical probe can detect very fine pits with a size of 30 nm 3 30 nm and a track pitch of 60 nm. The pit size of 30 nm 3 30 nm with a track pitch of 60 nm corresponds to more than 500 Gb / in.2 using 8-14 coded modulation (EFM) with non-return to zero inverse (NRZI). On the other hand, the pit fabrication limit appears to be 20 nm based on the EB writing patterns. This limit is due to proximity effects with this dense pattern. The limit means that the density of the SNOM-based ROM recording with pits is about 1.2 Tb / in.2 without a high multilevel or multilayer recording method.
4. Summary We have investigated the potential of using an SNOM technique and its density limit to optically image very fine pit structures, which were formed by EB fabrication. Fine test pit samples were formed in electron resist (ZEP520) with the minimum pit size of 30 nm 3 160 nm. As a result, we achieved the following conclusions: 1. Entire metal coating on the probe and depolarization technique are very efficient to detect nanometer-sized small pits. 2. Reflection type depolarization SNOM can detect a minimum pit of 30 nm 3 160 nm. 3. The detectable size of 30 nm corresponds to about 500 Gb / in.2 when using an EFM and an NRZI. 4. The EB writing limit might appear at a size of 20 nm so that the SNOM-based ROM has a potential to achieve an ultrahigh recording density of 1.2 Tb / in.2 without a multilevel or multilayer recording method.
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Acknowledgements The authors would like to thank Dr. T. Matsumoto, Mr. T. Suzuki, Mr. T. Nishida of Hitachi Ltd., for the fabrication of fine pits samples for this experiment and a fruitful discussion.
References [1] E. Betzig, J.K. Trautman, R. Wolfe, E.M. Gyorgy, P.L. Finn, M.H. Kryder, C.-H. Chang, Appl. Phys. Lett. 61 (1992) 142. [2] S. Hosaka, T. Shintani, M. Miyamoto, A. Kikukawa, A. Hirotsune, M. Terao, M. Yoshida, K. Fujita, S. Kammer, J. Appl. Phys. 79 (1996) 8082. [3] F. Isshiki, K. Itoh, K. Etoh, S. Hosaka, Appl. Phys. Lett. 76 (2000) 804. [4] A. Itoh, N. Ohta, T. Uchiyama, A. Takahashi, M. Mieda, N. Iketani, Y. Uchihara, M. Nakata, K. Tezuka, H. Awano, S. Imai, K. Nakagawa, in: Technical Digest of Cleo / Pacific Rim 2001, Tokyo, 2001, pp. II2–II3. [5] J.P. de Kock, S. Kobayashi, T. Ishimoto, H. Yamatsu, H. Ooki, Jpn. J. Appl. Phys. 35 (1996) 437. [6] S. Hosaka, IEEE Trans. Magn. 32 (1996) 1873. [7] Y. Martin, S. Rishton, H.K. Wickramasinghe, Appl. Phys. Lett. 71 (1997) 1. [8] S. Hosaka, H. Koyanagi, K. Katoh, F. Isshiki, T. Suzuki, M. Miyamoto, A. Arimoto, T. Maeda, Microelectron. Eng. 57 (2001) 223. [9] S. Hosaka, H. Koyanagi, K. Katoh, F. Isshiki, T. Suzuki, M. Miyamoto, Y. Miyauchi, A. Arimoto, T. Nishida, in: Technical Digest of Cleo / Pacific Rim 2001, Tokyo, 2001, pp. II4–II5. [10] S. Hosaka, T. Shintani, H. Koyanagi, K. Katoh, T. Nishida, T. Saiki, T. Hasegawa, Jpn. J. Appl. Phys. 41 (2002) L884. [11] S. Hosaka, T. Shintani, A. Kikukawa, K. Itoh, J. Microsc. 194 (1999) 369. [12] T. Saiki, K. Yusu, K. Ichihara, T. Tadokoro, in: Technical Digest of the 6th International Conference on Near-Field Optics and Related Techniques (NFO-6), Twente University, Netherlands, 2001, p. 122.