Mr. Yoshiko Nishi Profile

Yoshiko Nishi

at Hitachi Ltd

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Proceedings Article | 10 January 2002 Paper

Simulation of recrystallization in phase-change recording materials

Yoshiko Nishi, Hidehiko Kando, Motoyasu Terao

Proceedings Volume 4342, (2002) https://doi.org/10.1117/12.453381

KEYWORDS: Crystals, Optical discs, Optical tracking, Thermal effects, Optical storage, Laser crystals, Transmission electron microscopy, Germanium antimony tellurium

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Phase-change optical discs are common media for audio-visual equipment and computer peripherals. In such recording media as the DVD-RAM, amorphous marks are written on the crystalline matrix by controlling the power of laser beam pulses. Due to the need for higher capacity optical disc drives, track pitches are becoming narrower, so that the thermal effects of adjacent tracks are no longer negligible. This is also a problem in the track direction. Achieving higher density in drives requires smaller marks on the media, leading to shorter intervals between pulses. The thermal effects of the following pulses change the shape of the amorphous mark made by previous pulse. That is, the trailing boundary of an amorphous mark is re-crystallized by the heat of the following pulses. Close attention to the re-crystallization of the recording material is necessary to control the mark shapes on phase-change optical discs. We have simulated the movement of the crystalline/amorphous boundary during the writing process. We have also developed the concept of a 're-crystallization-ring' to explain the mark-forming process. The movement of the boundary was traced by drawing arrows to represent the crystallization direction and speed. The trace lines of these arrows matched the streamlines for crystal growth as observed by TEM. Simulation was done for both GeSbTe and Ag-InSbTe recording materials.

Proceedings Article | 18 September 2000 Paper

Mark-forming simulations of phase-change land/groove disks

Yoshiko Nishi, Takeshi Shimano, Hidehiko Kando

Proceedings Volume 4090, (2000) https://doi.org/10.1117/12.399354

KEYWORDS: Finite-difference time-domain method, Electromagnetism, Absorption, Optical discs, Crystals, Optical simulations, Optical tracking, 3D modeling, Analytical research, Diffraction

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The track pitches of optical discs have become so narrow that it is comparable to the wavelength of laser beam. Finite-difference time-domain (FDTD) simulation, based on vector diffraction analysis, can predict the propagation of light more accurately than scalar analysis, when the size of media texture becomes sub-micron order. The authors applied FDTD simulation to land-and-groove optical disc models, and found out that the effects of 3D geometry is not negligible in analyzing the energy absorption of light inside the land- and-groove multi-layered media. The electromagnetic field in the media does not have the same intensity distribution as the incident beam. Furthermore, the heat conduction inside the media depends on the disc geometry, so the beam spots centered on land and groove makes different effects in heating the recording layers. That is, the spatial and historical profile of temperature requires 3D analysis for both incident light absorption and heat conduction. The difference in temperature profiles is applied to the phase change simulator to see the writing process of the marks in land and groove. We have integrated three simulators: FDTD analysis, heat conduction and phase change simulation. These simulators enabled to evaluate the differences in mark forming process between land and groove.

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