Read/Write Mechanism for a Scattered Type Super-Resolution Near-Field Structure Using an AgO x Mask Layer and the Smallest Mark Reproduced

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1 Japanese Journal of Applied Physics Vol. 44, No. 1A, 5, pp #5 The Japan Society of Applied Physics Read/Write Mechanism for a Scattered Type Super-Resolution Near-Field Structure Using an AgO x Mask Layer and the Smallest Mark Reproduced Hiroo UKITA, Yasushi UEDA and Mai SASAKI Faculty of Science and Engineering, Ritsumeikan University, Nojihigashi, Kusatsu-shi, Shiga 525, Japan (Received June 9, 4; accepted September 17, 4; published January 11, 5) A working mechanism for a scattered type super-resolution near-field structure (super-rens) disk using a silver oxide (AgO x ) mask layer has been studied experimentally. The AgO x mask layer has five possible states depending on the laser power: AgO x (as-depo), uniformly dispersed Ag particles (after the initialization of 3.5 mw), Ag cluster (4 5 mw), Ag diffusion ( mw), and a Ag ring structure (greater than 8 mw) for an objective lens numerical aperture of.5, a laser wavelength of 826 nm and a medium velocity of 2 m/s. On the other hand, the GeSbTe recording layer has the following possible states: crystal, halfway amorphous, completely amorphous, and gas bubble associated with Ag particles. For super-resolution read power (4 mw), the mask layer will have a Ag ring structure that increases both the signal carrier to noise ratio and the resolution limit. We improve the resolution limit of 413 nm to 5 nm at the duty ratio of % for the write optical pulse. [DOI:.1143/JJAP ] KEYWORDS: super-rens, near field, super-resolution, Ag cluster, Ag ring, bubble pit, duty ratio, AgO x, GeSbTe, optical data storage 1. Introduction The super-resolution near-field structure (super-rens) is thought to have the capacity for super high-density optical recording beyond the diffraction limit. For the aperture type super-rens using an antimony (Sb) mask layer, the achievement of 9 nm mark length readout 1) and the direct observation 2) of the near-field aperture and the thermal lens 3) formed on a Sb layer have been reported. The write phase change mechanism of the mask and recording layers and a read mechanism beyond the diffraction limit resolution have also been reported. 4) A scattered type super-rens using a silver oxide (AgO x ) mask layer 5 7) has been proposed to significantly improve the carrier to noise ratio (CNR). The small metal particles in the gas bubble pit formed in the write process enhanced the near field (surface plasmons on the particles) as shown in the upper figure of Fig. 1. Ho et al. found that the functional structure 6) of AgO x depends on the write power as shown in the lower figure. The aggregated Ag nanocluster (having high reflectivity due to Ag dots) efficiently scatters the near field and the Ag ring (having high transmission due to a nanoaperture) not only confines the input laser beam so that it is thin, but also enhances the scattering field. Kataja et al. numerically studied the AgO x super-rens phenomena using a 2D finite difference time domain (FDTD) method. 8) They indicated that an AgO x super-rens structure can be produced beyond the diffraction limit resolution when the aperture is surrounded by small Ag particles that are formed and filled with low-index materials such as O 2. The deformed gas bubble associated with metallic nanoparticles was also elucidated for a platinum oxide super- RENS disk. A CNR of over 47 db for a mark length of nm was obtained in an optical disk with the following structure: PC-substrate (.6 mm)/zns SiO 2 (17 nm)/pto 1:6 (4 nm)/zns SiO 2 (4 nm)/aginsbte (6 nm)/zns SiO 2 ( nm). 9,) Recently, Tominaga et al. have demonstrated a huge change in the refractive index generated in a focused laser address: ukita@se.ritsumei.ac.jp 197 Ag Particle 1. Ag cluster structure (High reflectance) Laser beam Bubble formation Ag Particle Bubble Mask layer layer Recording layer 2. Ag ring structure (High transmission) Mask layer layer Fig. 1. Bubble pit formation and functional structures of a scattered type super-rens using AgO x mask layer optical disk. Write model after Kim et al., ) Read model after Ho et al. 6) spot and have proposed a ring aperture formed by a ferroelectric catastrophe 11) in an AgInSbTe recording medium. This new super-resolution aperture can be observed between 35 C and 4 C, resulting in a second phase transition from a hexagonal to a rhombohedral structure. This type of optical disk has the following structure: PCsubstrate (.6 mm)/znsio 2 (13 nm)/aginsbte (4 nm)/ ZnSiO 2 ( nm). 12) A mask layer is not yet produced. In this paper, we present a read/write mechanism of a super-rens optical disk with the structure of PC-substrate (.6 mm)/znsio 2 (17 nm)/ago x (15 nm)/znsio 2 (15 nm)/ GeSbTe (15 nm)/znsio 2 ( nm), along with results obtained from systematic experiments under various read/ write conditions. We also present the smallest mark formation of 5 nm length due to the use of optical pulses with a reduced duty ratio. 2. Initialization Effect To explain the super-rens mechanism directly and clearly, the read/write characteristics have been scrutinized experimentally. The configuration for a super-rens optical

2 198 Jpn. J. Appl. Phys., Vol. 44, No. 1A (5) H. UKITA et al. layer 17 nm 4 nm nm Laser beam (λ=826 nm) NA=.5 PC substrate.6 mm Mask layer (AgOx) 15 nm Recording layer (GeSbTe) 15 nm Fig. 2. Read/write configuration for a super-rens optical disk using AgO x mask layer. Table I. Parameters of materials of super-rens using the AgO x mask layer optical disk used for experiment. Layer Material Thickness (nm) Phase n k PC Sub. Polycarbonate : ZnSiO AgO Mask AgO x 15 Ag particle ZnSiO Crystal Recording Ge 2 Sb 2 Te 5 15 Amorphous ZnSiO disk using an AgO x mask layer is shown in Fig. 2. The optical disk parameters are shown in Table I. The objective lens numerical aperture is NA ¼ :5, the laser wavelength is ¼ 826 nm, and the medium velocity is v ¼ 2 m/s. Therefore, the optical diffraction limit =4NA of our experiments is 413 nm. Figure 3 shows the reproduced signals (normalized with as-depo V i ) at different initializations of laser power Pi for an as-depo medium. The reproduced signal V 1 (mark) at the read power Pr ¼ 1 mw changes in ways such as the following: becomes small negative at Pi ¼ 2:5 mw, becomes maximum positive at Pi ¼ 3:5 mw, (c) becomes nearly zero at Pi ¼ 4 mw, and (d) gradually increases to negative as Pi increases. This variation generated by the written mark is shown in greater detail by the curve mark in Fig. 4. This figure shows the effect of initialization on an as-depo medium of the super-rens disk. The mark reflectivity V 1, normalized with as-depo reflectivity V i, changes from a small negative to zero to positive to Fig. 4. disk. Reflectivity (%) 1 9 Laser power Medium velocity Mark length Write 1.~9. mw V2 (non-mark) V1 (mark) Laser power (mw) 2. m/s 3 nm Read 1. mw Initialization effect on as-depo medium of super-rens optical (c) zero to (d) increases to negative and then becomes saturated as the laser power increases. This phenomenon corresponds to the little decrease in reflectivity due to the little amorphous process in GeSbTe and then its cancellation due to the increase in reflectivity with the AgO x decomposition (Ag particles), fully decomposed Ag particles, (c) cancellation due to the half amorphous process in GeSbTe, (d) the decrease in reflectivity due to the completely amorphous process in GeSbTe. The maximum positive of the laser power of 3.5 mw shows that the AgO x is fully decomposed to Ag particles and O 2, both of which are uniformly dispersed in the mask layer. On the other hand, the non-mark reflectivity V 2 remains constant because thermal interference due to the succcessive laser pulses does not occur for long (3 nm) marks at the duty ratio of 5%. Figure 5 shows the relationship between the CNR for the super-rens readout (Pr ¼ 4 mw) and the initialization laser power Pi. We define Pi ¼ 3:5 mw as the initialization laser power where the CNR for the short mark (under the diffraction limit) reaches its maximum. The maximum CNR corresponds to a reproduced signal maximum at Pi ¼ 3:5 mw in Fig. 4, where fully decomposed Ag particles are dispersed in the mask layer. Figure 6 shows the effect of initialization on the CNR for the super-rens readout, with the write power Pw as a parameter. The CNR with initialization is higher and less dependent on Pw than that without initialization. Moreover, we can detect a mark length of nm (duty ratio 5%) because with initialization the resolution increases. Pi=2.5 mw Pi=3.5 mw nm 4 nm Fig. 3. (c) Pi=4. mw (d) Pi=5.5 mw Effect of initialization laser power Pi on reproduced signals. Initialization power Pi (mw) Fig. 5. Relationship between super-rens CNR (Pr ¼ 4 mw) and initialization laser power Pi.

3 Jpn. J. Appl. Phys., Vol. 44, No. 1A (5) H. UKITA et al nm 3 Laser power Pw (mw) 4 3 nm Laser power Pw (mw) 3. Write Power Dependence To determine the amorphous level of the recording layer, we measured the write power dependence of signal amplitude V pp =V i and the CNR for the initialized medium. Here, V pp ¼ V 1 V 2, and V i is the as-depo level. They were measured under two conditions: immediately after writing (observed at Pr ¼ 1 mw) and at super-rens readout (observed at Pr ¼ 4 mw). We compared the two signals and estimated the phase level (signal) and the noise level of the medium. Figure 7 shows the relationship between CNR, V pp =V i, and write power Pw at the read power of Pr ¼ 1 mw. Figure 8 shows the relationship between CNR, V pp =V i, and write power Pw at Pr ¼ 4 mw, where mark length is a parameter. Upper figure shows that a dip appears for every mark length. This phenomenon occurs because reflectivity decreases as the half amorphous process 4 Fig. 6. Effect of initialization on CNR for super-rens readout (Pr ¼ 4 mw), with write power Pw as a parameter. Without initialization (Pi ¼ mw), With initialization (Pi ¼ 3:5 mw). Signal amp. (Vpp/Vi) nm nm 6 5 Fig. 7. Relationship between CNR, V pp =V i, and write power Pw for read power of Pr ¼ 1 mw, with mark length as a parameter. CNR (bb) nm nm Signal amp. (Vpp/Vi)14 Fig. 8. Relationship between CNR, V pp =V i, and write power Pw for read power of Pr ¼ 4 mw (super-rens readout), with mark length as a parameter. for middle Pw, but it increases as the Ag cluster process for high Pw. Then the CNR becomes constant for high Pw. This suggests that the noise level increases for high Pw because the reflectivity (signal level) was found to increase with the completely amorphous process as Pw increases, as shown in the lower figure. We also find that the signal remains constant between 5.5 mw and 7.5 mw for a long mark at Pr ¼ 1 mw. This suggests that some changes occurred, which are described later, in the mask layer between 5.5 mw and 7.5 mw write process. In the case of the super RENS readout (Pr ¼ 4 mw), we find that every dip disappars as shown in the upper figure of Fig. 8. Moreover, we find that the the signal increases as Pw increases due to the completely amorphous process, as shown in the lower figure. We confirm that the constant signal level between Pw ¼ 5:5 mw and 7.5 mw, which appeared in the lower figure of Fig. 7, is a result of the reflectivity decreasing as the Ag cluster is decomposed and diffused due to the great heat generated at the continuous read power of Pr ¼ 4 mw. We define the optimum write power as Pw ¼ 8:5 mw, at which the CNR for the shortest mark of nm reaches maximum as shown in the upper figure of Fig Read Power Dependence and Super-RENS Mechanism Figure 9 shows the read power Pr dependence on the CNR of the mark written at the power of Pw ¼ 8:5 mw, where mark length is a parameter. From the figure, it is evident that beyond the diffraction limit, signals (4 nm) appear and the highest CNR of each mark length can be obtained at Pr ¼ 4 mw. On the other hand, CNRs are high and almost independent from Pr for long marks ( 3 nm). All the signals disappear at read powers greater than Pr ¼ 6:5 mw because the amorphous mark and the bubble pit are erased due to the continous large Pr. On the basis of these results, we define the optimum super-rens read power as 4 mw.

4 Jpn. J. Appl. Phys., Vol. 44, No. 1A (5) H. UKITA et al nm λ/4na=413 nm Read power Pr (mw) Fig. 9. Read power dependence on CNR of mark written at power of Pw ¼ 8:5 mw, with mark length as a parameter. Figure shows the read power dependence on the signal spectrum and the amplitude for the mark lengths of nm and nm, where read power Pr as a parameter. Both the signal spectrum and the signal amplitude increase at Pr ¼ 4 mw. On the other hand, we find that noise increases at Pr ¼ 6 mw in the spectrum in the upper right figure and the signal amplitude decreases due to the degradation of the resolution (Ag cluster size increases). On the basis of these results, we propose a model for a super-rens mechanism using a AgO x mask layer. Figure 11 shows the states just after writing (Pr ¼ 1 mw) for both the mask layer and the recording layer, with write 46.4dB 44.1dB 31.6dB MHz MHz MHz Pr=2.mW Pr=4.mW Pr=6.mW nm db 11.6dB 3.9dB Pr=2.mW Pr=4.mW Pr=6.mW nm Fig.. Read power dependence on signal amplitude and spectrum for mark lengths of nm with db/div and 1 MHz/div, and nm with db/div and khz/div, with read power Pr as a parameter. Mask (AgOx) Initialization Ag particle Recording (GeSbTe) As-depo Initialization Ag cluster Ag diffusion Ag ring (c) Pw= mw (d) Pw= mw (e) Pw: greater than 8. mw Fig. 11. Proposed working mechanism of a super-rens using AgO x mask layer.

5 Jpn. J. Appl. Phys., Vol. 44, No. 1A (5) H. UKITA et al. 1 Reflectivity V1, V2 (%) 8 6 V1 Duty (%) 5 8 V Mark length (nm) Fig. 12. Relationship between reflectivity V 1 (non mark), V 2 (mark), and mark length for super-rens readout (Pr ¼ 4 mw), with optical pulse duty as a parameter. power Pw as a parameter. The working mechanism for the super-rens using the AgO x mask layer is as follows. Both the mask and the recording layer have five possible states depending on the write power Pw: as-depo, Ag particles uniformly dispersed and crystallized (after initialization Pi ¼ 3:5 mw), (c) Ag cluster and half amorphous (Pw ¼ 4{5 mw), (d) Ag diffusion and completely amorphous (Pw ¼ 5:5{7:5 mw), and (e) Ag ring and bubble pit (greater than Pw ¼ 8 mw). The mask layer for the super- RENS readout (Pr ¼ 4 mw) has an Ag ring structure, and the aperture is filled with O 2, which increases both the CNR and the resolution limit. 5. Optical Pulse Duty Dependence To realize a short mark length, we illuminated thermally isolated optical pulses (having a decreased duty ratio). Figure 12 shows the relationship between reflectivity V 1 (non-mark), V 2 (mark), and mark length for the write power of Pw ¼ 8:5 mw with the super-rens readout (Pr ¼ 4 mw); the optical pulse duty ratio is a parameter. The signal amplitude V pp ¼ V 1 V 2 increases as the duty ratio decreases, which means the amorphous level difference between mark and non-mark increases because the difference in temperature increases due to the longer time separation between the laser pulses. Figure 13 shows the relationship between CNR and mark length for the super- RENS readout, with optical pulse duty ratio as a parameter. The reproduced signal mark length becomes shorter as the optical pulse duty ratio decreases, and reaches 5 nm (CNR ¼ 17 db) at the duty ratio of %. 6. Conclusions A working mechanism for a scattered type super-resolution near-field structure (super-rens) using ZnSiO 2 / AgO x /ZnSiO 2 /GeSbTe/ZnSiO 2 is clarified, and the smallest mark of 5 nm is reproduced for the initialization power of Pi ¼ 3:5 mw. The results obtained are summarized as follows: Duty (%) Mark length (nm) Fig. 13. Relationship between CNR and mark length for super-rens readout (Pr ¼ 4 mw), with optical pulse duty as a parameter. (1) The mask layer and the recording layer have five possible states depending on the write power Pw: asdepo, Ag particles uniformly dispersed and crystallized (after initialization), Ag cluster and half amorphous, Ag diffusion and completely amorphous, and Ag ring and bubble pit. (2) The mask layer for the super-rens readout has a Ag ring structure, and the aperture is filled with O 2, which increases both the CNR and the resolution limit. (3) The smallest mark length of 5 nm is reproduced with 17 db by decreasing the optical pulse duty ratio of % under the experimental condition of =4NA ¼ 413 nm. Acknowledgments The authors would like to thank Dr. J. Tominaga of the National Institute of Advanced Industrial Science and Technology for preparation and discussion of the super- RENS optical disk. 1) J. Tominaga, T. Nakano and N. Atoda: Appl. Phys. Lett. 73 (1998) 78. 2) D. P. Tsai and C. W. Yang: Jpn. J. Appl. Phys. 39 () ) J. Wei and F. Gan: Appl. Phys. Lett. 82 (3) ) T. Tagashira and H. Ukita: ISOM 1 TOYAMA Satellite Technical Digest (1) p ) H. Fuji, J. Tominaga, L. Men, T. Nakano, H. Katayama and N. Atoda: Jpn. J. Appl. Phys. 39 () 98. 6) F. H. Ho, H. H. Chang, Y. H. Lin, B. M. Chen, S. Y. Wang and D. P. Tsai: Jpn. J. Appl. Phys. 42 (3) 1. 7) T. Kikukawa, A. Tachibana, H. Fuji and J. Tominaga: Jpn. J. Appl. Phys. 42 (3) 38. 8) K. Kataja, J. Olkkonen, J. Akiko and D. Howe: Jpn. J. Appl. Phys. 43 (4) 16. 9) T. Kikukawa, T. Nakano, T. Shima and J. Tominaga: Appl. Phys. Lett. 81 (2) ) J. Kim, I. Hwang, D. Yoon, I. Park, D. Shin, T. Kikukawa, T. Shima and J. Tominaga: Appl. Phys. Lett. 83 (3) ) J. Tominaga, T. Shima, M. Kuwahara, T. Fukaya, A. Kolobov and T. Nakano: Nanotechnology 15 (4) ) M. Kuwahara, T. Shima, A. Kolobov and J. Tominaga: Jpn. J. Appl. Phys. 43 (4) L1. 8