Phase-Change Media for High-Density Optical Recording
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Phase-Change Media for High-Density Optical Recording
Herman Borg, Martijn Lankhorst, Erwin Meinders and Wouter Leibbrandt Philips Research Laboratories, Prof. Holstlaan 4, 5656 AA Eindhoven, The Netherlands
ABSTRACT Rewritable optical-storage systems are quickly gaining market share in audio, video and datastorage applications. The development of new rewritable optical-storage formats with higher capacity and data rate critically depends on innovations made to the recording media incorporating so-called phase-change materials. These materials allow reversible switching between a low and high reflective state induced by laser heating. In this paper, we highlight phase-change media aspects as optical and thermal design, sputter-deposition, materials optimization, and the development of new recording strategies. Focus is on the speed race in optical recording. INTRODUCTION Over the past few years, the field of optical recording has evolved rapidly. Read-only disc standards (such as CD-Audio and CD-ROM) have been extended with recordable (CD-R) and rewritable (CD-RW) systems, allowing the user to create personal CDs. Simultaneously, highercapacity systems for digital video and data applications have been developed by using shorterwavelength lasers and stronger objective lenses. The market introduction of DVD players is proceeding even faster than the introduction of the CD some 20 years ago, and is currently being followed by rewritable versions. With the introduction of the blue-laser diode [1] and the development of high-numerical-aperture (NA) objective lenses [2], optical-storage research is currently focussing on the development of a third-generation high-capacity recording system for video and data applications. Last year, Sony and Philips presented the rewritable Digital Video Recorder (DVR) format with a user data capacity of 22.5 GB on a 120 mm disc [3], by using a blue laser (?=405 nm) and an NA=0.85 objective lens. The technology used for these rewritable optical-storage systems is phase-change recording. The recording mechanism is based on the reversible phase transition between a crystalline and amorphous state of the recording material, induced by heating with a focussed laser beam. (Sub)micron-sized amorphous marks are written in a crystalline thin-film by using a short laser pulse to locally melt the recording layer. After switching off the laser the molten state cools down rapidly and becomes frozen in the amorphous state. Erasure of recorded marks proceeds via re-crystallization, by heating the recording material to a moderate temperature by applying an erase power level for a longer period of time. The readout mechanism of a phase-change optical disc is similar to that of a read-only disc, as the recorded marks have a lower reflection than their crystalline surrounding. The digital information is contained in the length of the amorphous marks and the crystalline spaces inbetween them. By using a recording strategy, consisting of alternating write-pulse trains with V1.2.1
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