Fusion and Evaporation Process of Metals and Semi-Conductors Due to Ultrafast Laser

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Purpose of this study is to realize advanced microfabrication with ultrafast laser to make the processing phenomena clear. In the present research, fusion and evaporation phenomena of metal and silicon has been elucidated by thermohydrodynamics analysis and molecular dynamics simulation. In thermohydrodynamics analysis, the flow of molten pool accompanied with evaporation was analyzed considering latent heat of vaporization, displacement of gas-liquid interface during evaporation, evaporation recoil pressure and Marangoni force. A hole is generated as a result that the surface of molten pool is lowered by evaporation recoil pressure. Because the flow caused by the evaporation recoil pressure pushes the molten metal away to the surroundings, a rising is generated at circumference of the hole. This flow is maintained after the laser irradiation is finished and the rising grows up. In the molecular dynamics simulation of metal, it was found that there are two types in evaporation form. One is the explosive evaporation that occurs when pulse width is extremely short. The other is the relatively calm evaporation that occurs when pulse width is comparatively long. In the former process, comparatively large particles scatter. In the latter process, the size of scattering particles is relatively small. In the three-dimensional molecular dynamics simulations of silicon, it was found that propagation velocity of shock wave in Si[111] direction is faster than that in Si[100] direction. When the pulse width is comparatively long such as 5 ps, fusion depth in Si(111) surface structure is deeper, because atomic arrangement in Si[111] direction is layered and heat is easier to transferred. On the other hand, when the pulse width is extremely short such as 0.2 ps, the fusion depth is independent of surface structure, because the influence of internal absorption for laser is larger and the effect of heat conduction is much smaller.