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

本研究的目的是利用超快激光实现先进的微细加工，使加工现象清晰化。本研究通过热流体力学分析和分子动力学模拟阐明了金属和硅的熔化和蒸发现象。在热流体动力学分析中，考虑蒸发潜热、蒸发过程中气液界面位移、蒸发反冲压力和Marangoni力等因素，对蒸发过程中熔池的流动进行了分析。由于蒸发反冲压力使熔池表面下降，从而产生孔洞。由于蒸发反冲压力引起的流动将熔融金属推离到周围，因此在孔的周边处产生隆起。该流动在激光照射结束后保持，并且上升增长。在金属的分子动力学模拟中，发现蒸发形式有两种。一种是脉冲宽度极短时发生的爆炸性蒸发。另一种是当脉冲宽度相对较长时发生的相对平静的蒸发。在前一个过程中，相对较大的颗粒分散。在后一过程中，散射颗粒的尺寸相对较小。在对硅的三维分子动力学模拟中，发现冲击波在Si[111]方向上的传播速度比在Si[100]方向上的传播速度快。当脉冲宽度比较长时，例如5ps，Si（111）表面结构中的熔合深度更深，因为Si[111]方向上的原子排列是分层的并且热更容易传递。另一方面，当脉冲宽度非常短时，如0.2ps，熔化深度与表面结构无关，因为激光的内部吸收的影响较大，而热传导的影响小得多。

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.