Progress of the Phase-change Optical Disk Memory
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Progress of the Phase-change Optical Disk Memory Takeo Ohta, Noboru Yamada, Hiroaki Yamamoto, Tsuneo Mitsuyu*1, Takashi Kozaki*2, Jianrong Qiu*3, Kazuyuki Hirao*3 Optical disk systems development center Matsushita Electric Industrial Co., Ltd. Kadoma, Kadoma City, 571-8501 Osaka, Japan *1 The Nakao Laboratory * Matsushita Techno-Research Matsushita Electric Industrial Co., Ltd Yagumo Naka-machi, Moriguchi City, 570-8501 Osaka, Japan 2
*3 Photon Craft Project, ICORP, JST Keihanna-plaza, Hikaridai, Seika-cho, 619-0237 Kyoto, Japan 1. Introduction Optical memory has two recording modes: the photon-mode as a silver halide photograph and the heat-mode as a laser optical disk. Though laser heat-mode recording has the advantage of environmental stability, it has limitations due to thermal diffusion phenomena, which will be discussed in this paper. Optical disk memory has the unique feature of read-only media performance, which is also compatible with the rewritable function and is different from HDD (hard disk drive) technology. Rewritable optical disk technology progressed with the race between the magnetooptical (MO) disks and the phase-change rewritable (PCR) optical disks. With the increasing use of multimedia, phase-change rewritable optical disks are becoming more popular due to their CD (compact disk) and DVD (digital versatile disk) compatibility. In 1968, S. R. Ovshinsky discovered a new memory phenomenon in chalcogenide film materials. This order-disorder phase-change memory effect came to be called the "Ovonic Memory"1). In developing this storage medium, the main issues have been the stability of the film materials, the stability of the reversible cycle characteristics and the recording sensitivity. The author and his colleagues were the first to achieve a breakthrough in these areas, which led to the commercialization of phase-change optical disk products. The first version of the phase-change optical disk product was shipped in 1990 from Matsushita/Panasonic. The PD(phase-change dual) and CD-RW(rewitable) followed, and now a rewritable DVD with 4.7 GB capacity and 3.4 Gbit/in2 density is being produced. Blue laser technology, large numerical aperture lens, volumetric recording and multilevel recording technologies are candidates for the future of high-density phase-change recording technology. This paper describes the phase-change optical disk memory progress of high-density recording, phase-change optical disks with a density of approximately 100 Gbits/in2 and more, and discusses the basic effect of the ultra-short laser pulse of the femto second laser pulse response on phase-change media. 2. Principle of the phase-change overwrite memory 2.1 Overwritable phase-change material The rewritable optical memory phenomenon has been observed in Te81Ge15Sb2S2 composition material2). This material was modified from a Te85Ge15 eutectic composition by adding Sb and S elements. Figure 1 shows the phase diagram of the Ge-Te system. At the eutectic composition, the melting temperature decreases to 375℃. In the early stag
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