| Summary: | 碩士 === 國立臺灣大學 === 應用力學研究所 === 96 === In this thesis, we made use of thermal phase transition of Ge2Sb2Te5 to investigate the problem of laser induced bit marks in nanoscale composite materials. The composite materials was fabricated as follows. On a pre-grooved polycarbonate substrate, sputtered in order are the upper dielectric layer, thermal phase transition layer, lower dielectric
layer, reflection layer and the protection layer.
In the past, all the previous studies obtained traces of bit marks by processing data from experiments without being able to characterize bit marks in their formation. In this study, we carried out theoretical/ numerical analysis to obtain the temperature history of the thermal layer, and its melting and crystallizing behaviors during the heating period. The size and edge smoothness of the bit marks can be determined very precisely; these qualities are closely related to the sensitivity to jitters in
reading optical disks.
For the study of thermal behavior, we used the three-dimensional heat conduction equation. An improved ADI (alternating-direction- implicit) method was developed to discretize the equation. In discussing laser light through the composite materials, we obtained the layer solution for Maxwell''s equations by matching boundary conditions at the layer interfaces. The coupled optical and thermal equations enable as to study
realistic/practical problems.
Through the analysis in this thesis, we can very precisely determine the factors and parameters that influence reading and writing optical disks. These include the size, shape, and edge smoothness of bit marks, and the temperature control and determination of the applied laser power during the heating process. For example, the temperature in Ge2Sb2Te5 layer should be higher than the melting point appropriately to avoid maintaining the temperature at the melting point for a long time. Otherwise, the edge of bit marks would have a bad quality in smoothness because of the solid-liquid coexistence zone. In addition, if the applied power of laser exceeds the inducing power, the length of bit marks increases rapidly, and thus the applied power should be controlled
appropriately to avoid over-consumption of the storage volume.
In comparison with existing experimental data, we found that the simulated results in the temperature, rate of heat conduction, phase state, and formation time of bit marks are in close agreement, which adds
confidence in our simulations.
The technology of optical data storage is a very important industry in Taiwan. The module presented in this thesis may provide a useful tool that helps design and analysis to improve the quality/performance of
optical storage.
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