The degradation in rewriting phase change optical disc

Abstract

In recent years, the emergence of optical disc storage technology with high density and long lifetime is a major breakthrough in information storage technology. For erasable (or rewritable) discs, such as those using the Magneto-Optical (MO) and the Phase Change effects, the most applicable storage media are made of metal alloys. In particular, the Phase Change Rewritable Media (PCRM), due to its compatibility with existing drives and low cost, has been a rapidly developing direction for mass data storage discs. However, one major drawback associated with using phase change media is the lack of cyclability. The current industrial standard requires the lifetime of the phase change media to have more than 1000 rewrite cycle. This is primarily limited by the degradation of the storage layers and must be improved to ensure reliable data storage. Currently the lifetime of PCRM is significantly shorter than that of MO, which is over 1 million rewrite cycle. Lifecycle optimization is still a major research topic for PCRM. This research project aims to study the degradation of PCRM discs in relation to processing parameters. This report begins with an introduction to PCRM technology, then a review of the literature, the methodology, followed by a discussion on different mechanisms of signal degradation, which includes the works that have been performed in this research project. The report ends with a conclusion summarizing the result of the project. The major tools that have been used in this research include TEM, SEM, AFM and an industrial "Plustec" optical disc tester. The combination of these analytical techniques provides much information on the understanding of the degradation of the recording medium during repetitive over-write. The CD-RW format was chosen for the experiment. Our experiments started with the optimization of the CD-RW disc by varying the different layer thickness of the thin film stack. The signal quality of the disc was tested and analyzed using the Plustec tester. This was to ensure the best recording condition of the media before performing the microanalysis. The TEM analysis was focused on the investigation of the microstructure after 1000 overwriting cycles. The amorphous mark formation by laser irradiation was clearly observed in the TEM images. The shape of the mark became irregular after repeated overwriting. During the sample preparation process, we found that the adhesion between layers degraded significantly after repeated overwriting. SEM analysis had been performed in order to avoid tedious TEM sample preparation. However, the mark could not be seen with good image details. We could only look at the surface morphology. The dielectric layer was also examined using SEM, and the dielectric layer was found to be in good condition even up to ten thousand rewrite cycles. But bubbles were found in the phase change layer after 1000 direct overwrite cycles. AFM was also used to examine the CD-RW samples. It gave excellent resolution with high magnifications and it could provide three-dimensional surface profile. AFM measurement revealed that substrate deformation occurred during the initialization process, and most of the stress was relieved during this process. From our experimental results, we have identified that the major cause of degradation was due to the formation of argon bubbles. The presence of argon in the sputtered film was confirmed by XRF. Such bubbles limited the number of rewrite cycle of the sample to a few thousand times. Nitrogen doping and optimization of sputtering parameters were found to be the key factors in improving degradation in overwriting phase change media.

Date of Award	2 Oct 2002
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Ho Pui Aaron HO (Supervisor)