50GB BD-R media with Te-O-Pd recording material was estimated to have a life expectancy of over 500 years according to acceleration test under stressed conditions. This stable recording material will reduce the possibility to lose important data even in sever storage environment.
We have developed dual-layer write-once media with Te-O-Pd based recording films on Blu-ray (BD) format. Recording capacity was 50GB with dual layers on a disk of 120mm in diameter. Rear and Front layers showed jitters of 5.8% and 7.7% at 1x speed, and 6.0% and 8.0% at 2x speed, respectively, which were good enough to satisfy the BD format. Evaluations were carried out with blue-violet laser of 405nm wavelength, objective lens NA of 0.85. Recording linear velocities were 4.92m/s at BD 1x (36Mbps), and 9.84m/s at BD 2x (72Mbps). Characteristics at 4x speed recording were also examined, and it was revealed that carrier to niose ratio at high recording linear velocity of 19.7m/s, which corresponds to BD 4x (144Mbps), was alomst as same as those of 1x and 2x. Recording mechanism was discussed and proposed a model that Te-O-Pd films were not crystallized directly through solid process, but crystallized through melting.
Multi-layer optical recording is a promising technology for increasing a disc capacity using an optical pickup identical to that used for a single-layer optical disc. A capacity of 25 GB - 50 GB is required to record 2 - 4 hours HD-TV program. In this paper, the physical format of the dual-layer phase-change optical disc is studied and experimental results of an advanced dual-layer phase-change optical disc, of which first layer is characterized by a transmittance-balanced structure and prepared by a new replication process, are shown and discussed. The transmittance-balanced structure disc is realized by adopting Ge(Sn)-Sb-Te film that has appropriate optical constants and optimizing the thickness of dielectric layers. The signal of the second-layer in the transmittance-balanced structure disc is able to read and write without any influence of the first-layer. A capacity of over 50 GB is demonstrated by the transmittance-balanced structure disc.
A thin film of Sn-doped and GeTe-rich GeTe-Sb<SUB>2</SUB>Te<SUB>3</SUB> shows characteristics that make it suitable for use in rewritable dual-layer optical disks employing a violet laser. By increasing the GeTe component form Ge<SUB>2</SUB>Sb<SUB>2</SUB>Te<SUB>5</SUB> to Ge<SUB>4</SUB>Sb<SUB>2</SUB>Te<SUB>7</SUB>, and Ge<SUB>8</SUB>Sb<SUB>2</SUB>Te<SUB>11</SUB>, optical changes were increased. By substituting Sn for a proposition of Ge in these compositions, crystallization rates are greatly increased and even a 5 nm-thick film showed a very short laser-crystallization time of less than 50 ns. The material film was successfully applied to Layer 0 of rewritable dual-layer disk: capacity of 27 GB and a 33 Mbps data transfer rate were confirmed for a disk using a conventional 0.6 mm substrate, and 45 GB capacity and the same data transfer rate were obtained for another disk using thin cover layer 0.1 mm thick.
Multi-layer write-once optical disks with tellurium suboxide palladium-doped phase-change recording films were designed and adopted to read/write utilizing a violet laser. A dual- layer medium showed carrier to noise ratios (CNRs) of more than 50 dB within a laser power of 7 mW at a linear velocity of 5 m/s in each the condition of numerical aperture (NA) equals 0.65 and NA equals 0.85, which corresponds to a recording capacity of 27 GB and 45 GB on a dual-layer disk of (phi) 120 mm in diameter, respectively. Furthermore, a quadruple-layer medium showed CNRs of more than 48 dB within a laser power of 12 mW at a linear velocity of 5 m/s in the condition of NA equals 0.85, which corresponds to a recording capacity of 90 GB on a quadruple-layer disk of (phi) 120 mm.
We demonstrated the possibility of high data rate recording on a DVD-RAM disk which utilizes Ge-Sb-Te phase-change materials. To ensure high transfer rate overwriting on the DVD, quadruple speed (44Mbps) recording at a linear velocity of 16.4 m/s was tested using a Sn-added Ge-Sn-Sb-Te material as the recording layer. Double speed (22Mbps) recording on the present 4.7GB DVD-RAM at a linear velocity of 8.2 m/s was also tested. A CNR of more than 53 dB and an erasability of more than 30 dB were obtained at each double, triple and quadruple speeds. In addition, by recording via 8-16 random modulation signals, a jitter of 9 percent or less and a direct overwrite performance of 100,000 cycles were confirmed.
Rewritable DVD (DVD-RAM) 4.7 GBytes products such as DVD-RAM disc, DVD-RAM drive, and DVD Video Recorder have been developed. DVD-RAM can integrate all Audio, Video, Imaging, Computer, and Network applications with write compatibility between DVD-RAM 2.6 GBytes and read compatibility across the entire DVD format family. DVD-RAM disc and drive technologies such as high speed reliable recording, easy operability, secure copy protection functions, and DVD Video Recorder are presented. With the coming of blue light sources, the capacity of the DVD-RAM disc can be upgraded 3 to approximately 5 times, and furthermore could be expanded over 10 times in conjunction with a high-NA optics and a dual-layer disc technology, then 2 to approximately 4 hours high-definition video will be recorded.
Two marked effects are obtained by forming a Ge-N interface layer on either side of Ge-Sb-Te recording layer. One effect is a suppression of atomic diffusion between Ge-Sb-Te layer and protective layers, ZnS-SiO<SUB>2</SUB> representatively, which leads to a significant improvement in overwrite cyclability, and the other is the acceleration of crystallization process which leads to higher speed optical disks. A rapid-cooling type experimental disk with Ge-N layers on both sides of the Ge-Sb-Te recording layer proved to be capable of exceeding 10<SUP>5</SUP> cycle overwrites and a recording data rate 40 Mbps at linear velocity 12 m/s. The recording conditions: bit length 0.28 micrometer and track pitch 0.6 micrometer (L/G method) using laser source with a wavelength 658 nm and a numerical aperture 0.6 correspond to a capacity 4.7 GB/(phi) 120 mm.
By nitrogen doping into a Ge-Sb-Te phase change optical disk's recording layer, we were able to significantly increase its cyclability. For example, our PD attained, at the maximum, 800,000 overwrite cycles through accurate control of nitrogen concentration. We quantified the nitrogen concentration of recording layer using secondary ion mass spectrometry (SIMS) and determined, from the viewpoint of cyclability, signal amplitude and other parameters, the optimum concentration to be around 2 - 3 at.%. From analyses by thermal desorption mass spectrometry (TDMS) and X-ray diffraction (XD) using powder, we found: (1) nitrogen atoms are mainly bound with Ge to create an amorphous phase of Ge-N; (2) as long as the nitrogen concentration remains around 5 at.%, those Ge, Sb and Te atoms which are not bound with nitrogen form NaCl type crystals. We obtained the following model by combining the results of the above analysis. Nitrogen-doped Ge-Sb-Te recording layer is composed of Ge-Sb-Te grains intermingled with a small quantity of amorphous Ge-N, which exists in the form of a thin film penetrating the grain boundary of Ge-Sb-Te. The Ge-N composing this high-melting-point material layer appears to suppress any micro-material-flow that may occur during overwrite.
Achieving disk overwrite at a high-data-rate depends on increasing both the disk revolving speed and recording bit density. This work was done to estimate the possible data transferring rates that can be expected in the near future. It is projected that combining the typical phase-change material Ge-Sb-Te and a `thermally-balanced structure' to decrease over-write distortion, several 10 Mbps will soon be obtained by a red laser and more than 100 Mbps will be obtained by a blue laser.
A rewritable digital video disc recorder, using four optical heads on both sides of a 30 cm disk which improves the data transfer rate up to 128 Mbps for phase change rewritable media, has been developed. We achieved 19 GB large capacity which realizes 20 minute Composite digital video signal recording, with new double sided multi-head MCAV method and high density mark edge recording.
A feasibility of phase change media to an erasable compact disk (CD) was studied. We proposed a quadrilayered optical disk having a recording layer of Ge<SUB>12</SUB>Sb<SUB>39</SUB>Te<SUB>49</SUB> thin film. The ternary material film was crystallized and also amorphized reversibly by laser irradiation of 0.7 microsecond(s) . This transition time was appropriate for the CD which revolves at 1.2-1.4 m/s. A multi-pulse (MP) recording method was also proposed to record EFM signals without teardrop shaped distortions. On the revolving disk, each recording mark of amorphous state was formed by irradiation of 'multi-pulses': a set of one preceding longer pulse and succeeding shorter pulses. Newly developed simple electric circuit for producing the MP waveform was examined to overwrite the single frequency signals of 196 kHz and 720 kHz, alternatively using only one laser beam. As a result, carrier to noise ratio (CNR) of more than 50 dB, secondary harmonic distortion (SHD) of -40 dB, and erasability of 40 dB were obtained. It was observed by a transmission electron microscope that the teardrop shaped distortion of the recording mark was reduced. Eight to fourteen modulation (EFM) signals of the audio CD format were also overwritten successfully and a clear eye-pattern was confirmed. The combination of the GeSbTe phase change media and the MP recording method will be applicable to the erasable compact disk.