Materials Issues in Blue Recording
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Materials Issues in Blue Recording
A.E.T. Kuiper and L. van Pieterson Abstract In the development of materials for blue recording, the issues for rewritable discs relate to speed and lifetime stability. Both aspects are connected with the properties of the phase-change recording layer. Higher data rates require faster erasure (i.e., recrystallization) of previously written marks. Increasing the Sb concentration or adding specific dopants to the phase-change material appears to accelerate the crystallization rate. Faster erasure rates should, however, not promote spontaneous recrystallization at room temperature, as this would adversely affect the lifetime stability. This roomtemperature stability can be estimated from an activation energy analysis. Excellent lifetime stability proves attainable by a judicious selection of the composition of the phase-change material. For write-once discs, the choice of the recording material is between a spin-coated dye, a phase-change layer, and an inorganic alloy. While suitable dye materials for blue wavelengths are now being developed, research into the alternative of inorganic bilayers has yielded encouraging results. Thus far, good write-once recording performance has been demonstrated with Cu/Si as the recording medium.The mechanism of mark formation in such a bilayer will be addressed. Promising results obtained with optimized Cu/Si stacks illustrate the attractiveness of this type of write-once material for blue recording. Keywords: optical properties, film, phase transformation.
Introduction From the introduction of the compact disc in 1983, digital optical data storage has experienced major improvements. Three generations of disc formats can be identified: CDs, digital versatile discs (DVDs), and Blu-ray discs (BDs). The principle of all of these optical storage generations is the same: data are stored on a disc as small areas with optical properties that are different from their surroundings. In this way, information can be read as changes in the reflectance by a laser beam that probes the disc. The stepwise increase in data storage capacity in subsequent generations is achieved primarily by reducing the spot size of the focused laser beam (shorter wavelength and a stronger objective lens with a higher numerical aperture, NA), so that smaller data marks can be distinguished (see Table I). For an overview of this development, the reader is referred to the review papers in the list of references.1,2 For both CDs and DVDs, prerecorded, or read-only memory (ROM), discs were the first products introduced to the market. Later, recordable (R) and rewritable (RW)
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discs became available. Very important is the interchangeability among all of these disc types. For the third generation (BDs), just as for CDs and DVDs, three types of storage media will be available: prerecorded, rewritable, and write-once discs. On prerecorded, or ROM, discs, the content is replicated by a mastering process.3 In brief, a glass substrate covered with a photoresist layer is illuminated with a
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