Material Requirements for Reversible Phase Change Optical Recording
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MATERIAL REQUIREMENTS FOR REVERSIBLE PHASE CHANGE OPTICAL RECORDING Kurt A. Rubin, IBM Research Division, Almaden Research Center, 650 Harry Road, San Jose, California 95120 ABSTRACT: The science and technology underlying phase change reversible optical storage, with an emphasis on the media, are reviewed. The transformation kinetics and their effect on writing (amorphization), erasing (crystallization) and long-term data stability are discussed. Phase separation is shown to affect cyclability and cause increased media noise. A readback CNR of 65.0 db was obtained by eliminating grooves as a source of media noise and using a recording layer which crystallizes into a cubic phase. Recording performance at short wavelengths is discussed. Introduction The purpose of this paper is threefold: (1) illustrate the general principals underlying phase change recording, (2) summarize the current state of the art, and (3) indicate where future opportunities are given improvements to the media and the optical head. A complimentary paper reviews the current status of some of the more drive related parameters of phase change[1]. Optical storage technology is based upon focusing laser light with a high quality objective lens to a small spot on the recording media (see Figure 1). The smallest size of the focused spot is limited by diffraction. The FWHM diameter of the light beam (0) with a Gaussian intensity distribution focused on the disk is: 0 = 0.562JNA where X is the wavelength of light and NA is the numerical aperture of the lens. Using a laser operating at a wavelength of 780 nm with a lens NA of 0.55 results in a spot size of 0 = 0.8 RIm. The spacing of the tracks and the areal density of recorded data are also limited by the size of the focused spot. For example, a track pitch and spot spacings of 1.6 iim corresponds to 40 Megaspots/cm 2. The use of an objective lens with a -1 mm working distance results in a large separation between the optical head and the media which prevents the lens from hitting the disk. This separation helps ensure data integrity and together with the ability to actively servo on the track allows the media to be removable, resulting in a drive with essentially unlimited near-line data storage capacity. Successfully implementing erasable phase change recording requires developing media which can be erased after writing. Usually materials which undergo a crystalline-to-amorphous (writing) and an amorphous-to-crystalline (erasing) transition are chosen. In addition, to read the data, the two states must have different optical properties. An example is indicated in Figure 2 which shows, in transmission, written amorphous spots as bright regions on a darker, crystalline background. Among the greatest challenges present in phase change technology are understanding and improving the number of reversible cycles and reducing the media noise. Despite these challenges, there are several reasons why research and development are being pursued. One is that direct overwrite works well, which means that data already
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