Progress of Video Service and Expectation for Optical Disk
Dynamics of Atomic Displacement Induced by Electronic Excitation in Solids
We discuss some aspects of athermal atomic displacement induced by electronic excitation in solids. Especially recent topics on the spectroscopic study of the ultrafast atomic displacement are reviewed, and the origin of the driving force of the atomic processes is clarified from theoretical model calculation
|Clues to find out superior phase-change materials
|Today, GeTe-Sb2Te3 pseudobinary and Sb-Te binary compounds are most widely used as active materials for rewritable phase change optical disks or non-volatile electronic memories. In thermal equilibrium states, almost all these compounds are crystallized into very complicated structures with long-period layer stackings, which are called homologous series compounds.
Both of these systems also possess, however, the crystalline phases that have simple structures, together with the homologous phases. They are NaCl-type cubic compounds in the former pseudobinary system and A7-type
compounds with small amount of Ge, Ag, and/or Indium in the latter binary system. The compounds of these two groups are very similar in structure to each other; it is the presence of those compounds that enables high-speed phase change from the amorphous phases to the crystalline phases. Our crystallographic researches found out some effective clues for exploiting
more superior phase-change materials with better performance.
|Optical Recording Molybdenum-Oxide Films
Prepared by Pulsed Laser Deposition
|Approximately 30〜50 nm thick MoO3 films were deposited on glass or polycarbonate substrates by PLD using ArF excimer laser (λ＝193 nm). For films deposited on glass substrates, transmittance change between the as-deposited and the annealed (340℃×10min) states was about 40% at the wavelength of 400 nm. This increase in transmittance was presumably caused by a transformation of oxygen-deficient as-deposited state into oxygen-sufficient annealed state through an annealing-induced
reaction involving oxygen absorption, which was confirmed by XRD spectra. The value of CNR for 30 nm thick film grown on polycarbonates was about 50 dB at the write-peak power of 3〜11 mW, that for 40 nm thick film was about 50 dB at the peak
power of 3〜12 mw and that for 50 nm thick film was about 55 dB at the write peak power of 5〜14.5 mW. The value of CNR for 30 nm thick film with the protection layer of 30 nm was 50 dB at the peak power of 9〜17 mW. These results give a characteristic feature of the wide write-power margin.
|Reliability for Lens Impact against Phase Change Recording Layer
in a Near-Field Optical Disk Drive System
|A near-filed optical technology is one of the promising technologies to realize a capacity higher than 100 Gbytes in a 12cm disk. In the near-field optical disk drive system utilizing a solid immersion lens (SIL), the disk surface is required to be so protected by applying the top coat to the phase change recording layer that the system is stable when the SIL collides against the disk surface due to disturbances as dust, shock and the like.
We have developed a SIL impact evaluation system, which contributes to the top coat development. In this system, it is possible to evaluate the top coat performance with nanometer accuracy by unitizing the gap error signal between the SIL and the disk surface before and after the impact.
At first, the impact evaluation system is introduced. Secondly, the criterion of the impact test is proposed. Finally, some evaluation results are reported and the top coat performance will be discussed.
|10x-Speed Recording on Dual-Layer Blu-ray Write-Once Media
|This study focused on the speed capability of our dual-layer BD-R. For the purpose we evaluated the dependence on recording speed of the front-layer. We measured readout signal from the media after recording 2T mark & space pattern at 4x, 6x and 10x-speed. First, the problem that the amplitude of 10x was smaller than 4x was observed. We considered that the reason of the deterioration related with mark formation process. Then we tried to calculate recorded mark shapes by the simulation with the parameter of the write strategy and the optical and thermal constant of recording films. As the result, we have obtained improvement of the dependence on recording speed and outlook for 10x-speed recording on dual-layer BD-R.|
|Development of 40 GB Dual-Layer Rewritable HD DVD Media
|We have developed a single-side dual-layer rewritable HD DVD media having a larger capacity of 40 GB (20 GB per layer) for the optical system with the NA of 0.65 and the wavelength of 405 nm, and achieved the good recording characteristics and sufficient high reflectivities. For both layers, the same track pitch of 0.34 μm as HD DVD-RAM was used. We applied more accurate thermal analysis to the dual-layer rewritable media. It was expected that the effect of the cross-erase was very small. By optimizing thermal balance of dual-layer media, the good recording characteristics were obtained for both layers with enough tilt margins. The feasibility of the dual-layer rewritable media of 40 GB user data capacity was shown for HD DVD system.|
|Discussing the Suitability of ZrO2-Based Interface Films for a Dual-Layer
Phase-Change Optical Disk Utilizing a Blue-Violet Laser
|We demonstrated at ISOM2005 that a superior cyclability was realized on the semitransparent recording layer (L1) of the dual-layer Blu-ray Disc by adoption of the ZrO2-based interface films substituting for the GeN-based ones. In this work, we discuss the difference between these two interface materials of an optical disk utilizing a blue-violet laser based on the results of optical and thermal simulations. Consequently, it is clarified that in the case of L1, the position where the temperature takes the maximum not in the Ge-Sb-Te recording film but in the laser-incident-side interface film. It is anticipated that the GeN-based interface film was heated to near or over the decomposition temperature of 700°C in L1 by blue-violet laser heating. Namely, the deterioration found in the optical disk with GeN-based interface films is interpreted to be brought about by the decomposition of the GeN-based film after repetitive recording. For the ZrO2-based material, decomposition and melting do not occur even at 1000°C. This is why L1 with the ZrO2-based interface films shows such a good cyclability of more than 10,000 cycles.|
|ZnO-based material for protective layer of phase-change optical disk with
a wide range of recording speed
|In the low speed recording to the high speed rewritable DVD like an 8x media, some write strategy condition causes high BER (bit error rate) ascribed to the crystal growth in the amorphous mark. It hinders the high speed rewritable DVD from recording with a wide speed-range. Although the Sb-based phase-change material has very low nucleation probability, when the crystallization speed becomes high, accidental nucleation in the amorphous mark happens.
The purpose of our work is to record with a wide speed-range on the higher speed rewritable DVD like a 12x media, avoiding accidental nucleation in the amorphous mark. For the purpose, we applied some materials as an upper protective layer and evaluated recording characteristics. And each material was estimated thermal conductivity. As a result, ZnO-based materials are effective to prevent accidental nucleation, and it does not disturb the high-speed crystallization. It is considered that the high thermal conductivity and crystal acceleration ability of ZnO-based material stretch the range of recording speed.
|Interlayer Crosstalk Reduction Effect by Controlling Backward Reflectivity for Multilayer Discs
|Interlayer crosstalk reduction method by controlling backward reflectivity of information layers was proposed. Reduction of the backward reflectivity brings out reduction of the signal from a ghost spot even if thicknesses of spacer layers are set equal. In this study, the relationships between the interlayer crosstalk and the backward reflectivity were examined for three-layer discs with almost equal spacer layer thicknesses.
In the case of focusing L0 layer in three-layer discs, the readout signal variation were reduced to the range of 95-105% by lowering backward reflectivity of L2 layer, whereas the variation of the disc without controlling reflectivity was 84-119%. In these conditions, the reflectivity ratio of a ghost spot of L2 layer and the spot of L0 layer was reduced from 0.012 to 0.0012. Thus, the influence of ghost spot was suppressed even for the disks designed with the same spacer layer thickness.
|Proposal of an optical switch using phase-change material for future photonic
Recently, the demand for network capacity grows steadily due to the dramatic increase in communication traffic of the Internet. The future large capacity photonic network will require a small-sized, high-speed, and low power dissipation optical node; where optical switches are key elements. Several types of optical switches have been developed using silica, III-V compound semiconductor, silicon, and lithium niobate as a base material. Those switches use the change of refractive index by the thermo-optic effect, the electro-optic effect or the plasma effect of carriers in a semiconductor. However, the amount of the refractive index change is at most 0.005, therefore, the size of the optical switch should be large to obtain phase shift of p. Moreover, the switching time of the optical switch using the thermo-optic effect is usually over several ms and it limits the node performance considerably. In this paper, we propose the optical switch using the phase-change material which has a large refractive index shift from the amorphous structure to the crystalline structure. It should be noted that the switch will have a self-holding characteristics which will reduce the power dissipation of the optical node effectively. The several switch configurations and the simulated results will be reported.
北川 様 (松下)
|Randomness of Amorphous States in Phase-Change Memory
|Observations of shunting device using AIST phase change material
|Reduction of Random Data Bit Error Rate in Super-Resolution using Normal-Resolution Cross-Talk Cancellation
|The concept of Normal-Resolution Cross-Talk (NRCT) is introduced which increases bit error rate of random data in super-resolution. Two methods to suppress NRCT (NRCT cancellation, NRCTC) are proposed; disc design and signal processing. Perspectives of reduction of bit error rate and the possibility of narrow track pitch are discussed.|
|Phase Phase Transition Mastering
|Sony succeeded in materializing BD-ROM discs using the PTM (Phase Transition Mastering) technology. Unlike the conventional mastering process using photon mode reactions, PTM is based on a thermo chemical reaction of inorganic resist. A steep threshold accompanying the thermo chemical reaction results in a material change that is limited to a much smaller region of the resist than the laser spot size for exposure.
Presently, we will introduce how high the recording density we can get with the PTM technology. Through further optimization and improvement of the inorganic resist process, we succeeded in mastering a 12cm-diameter ROM disc with 100GB-capacity per side. Using near field optics, a jitter value of 10.2% was obtained at this density. We also managed to produce very fine groove pattern of 100nm pitch, which corresponded to the 250GB?RE discs. This was done by employing the deep UV laser with wavelength of 266nm which was much shorter than the conventional BLD light source.
We also tried the Reactive Ion Etching after the PTM process in order to get higher aspect patterns for other purposes, not limited to the optical disc use. Our inorganic resist showed enough durability as an etch mask against the CHF3 gas.
|Development of nano-scale lithography apparatus toward high-speed and wide-area
|We have developed nano-fabrication apparatus that enables to make nano-fabrication less than 100nm with high-speed process and large patterned area by combination of visible-light lithography using a semi-conductor laser and the thermal lithography. The thermal lithography technique can achieve a high resolution of 100 nm (one forth of beam diameter). Using optical disc technique, this system offer accurate auto focus. Though the objective lens, Laser beam is controlled to focus accurately on the substrate surface. In addition, laser diode strength is very stable by feedback control. Furthermore, it enables a high writing speed of 3m/s and a wide working area (over 5 inches) using disk rotation system. This system is consist of semi-conductor laser beams with a wavelength of 405 nm and an optical system with an objective lens of a numerical aperture (NA) of 0.85 for focusing the laser beams. For example, Fig.1 is a result of using the system above and the thermal lithography technique to fabricate a SiO2 substrate. In this presentation, we present a potential of the nano-fabrication apparatus and its features.|
|Simulation of read-out signal on the Super-Resolution Disc including Phase-change
|Recently, the Super-Resolution (SR) disc including Phase-Change mask layer and PtOx recording layer was proposed. Four characteristics were observed experimentally for this SR disc. First, large SR read-out signal enhancement was observed. Second, the threshold power where SR effect happened didn’t depend on the signal pit length. Third, the read-out power margin of SR read-out signal was very wide. Fourth, the SR read-out signal depended on the thickness between mask layer and recording layer. These characteristics seem to relate to near field light deeply, and a model such as metal conductive ring has been proposed, but it can’t explain these characteristics completely. So, we performed Scalar analysis and FDTD analysis of the read-out signal on this SR disc. In performing these analyses, we introduced the SR aperture which based on the actual thermal distribution, not introducing metal conductive ring. Because the disc is turning, delay happens between beam lighting and temperature rise, so the SR aperture becomes egg shaped and its center shifts from that of the focused beam. Introducing this SR aperture, we can reproduce the second and third SR characteristic above mentioned even by Scalar analysis. Furthermore, we can reproduce the rest SR characteristic by FDTD analysis.|
|Reflection change associated with melting of Ge-Sb-Te alloy
|Reflected light intensity(IR) for liquid and solid Ge-Sb-Te alloy has been measured. Special sample preparation was carried out for the measurement. Mirror-like surface with oxide free was produced in quartz cell, and a laser light was radiated on the surface for IR measurement. The IR was observed statically as a function of temperature in the vicinity of melting point. The IR decreased rapidly just above the melting point, and the value of IR was found to increase steeply with decreasing temperature. At temperatures lower than the melting point, the IR returned to the same level observed during the initial heating cycle. It means that the reversibility of the optical property for Ge-Sb-Te alloy was completely proven in static observation. Additionally, there is no remarkable change of the observed IR below the melting point, an optical aperture creation mechanism in superresolution readout phenomenon was strongly suggested as the change of optical property with melting phenomenon.|