短脉冲激光加工硅膜的传热过程及烧蚀图形的二维计算

2D Calculation of Heating Process and Crater Shape for Ultrashort Lasers Ablation Silicon Films

为研究短脉冲激光辐照硅膜表面后的能量传输过程,基于双温方程的计算方法以及自由电子气理论,建立了求解能量传输方程的二维有限元模型.针对红外以及可见光波段的激光,通过限制硅膜的大小,有效地控制了计算的精度,并得到电子温度与热流的时间以及空间分布.计算结果表明,激光诱导产生的等离子体密度极大地影响了硅膜表面的反射率及光吸收系数;通过分析电子热流密度随时间的变化曲线,得到硅膜内部能量的传输过程;在激光作用过程中,硅膜内部晶格温度始终保持在熔点以下,证明了等离子体密度是激光烧蚀硅膜的主导因素;预测了激光烧蚀的图形,并分析了不同波长的激光烧蚀图形与高斯曲线的关系.

Based on two-temperature model and free electron gas model, a new calculation method, finite element model is fabricated,which provides a new perspective into studying the energy transport process in silicon film irradiated by ultrashort laser pulses. By choosing suitable thickness of silicon films, the 2D spatial and temporal evolutions of the electron temperature as well as carrier density in silicon film irradiated by IR and visible lasers are obtained. The evolutions of complex refractive index and plasma reflectivity are also calculated,through analysis,results show that they are dominated by carrier density. By depicting the electron heat flux evolutions, the energy transport process is analyzed. The distributions of lattice and carrier density are depicted, results show that the lattice temperature is stayed well down below the melting point, and the critical density of carrier density is the dominated factor of ablation. The calculated threshold fluences are validated by comparing study and experimental data. The predicted crater shapes are obtained,which are waiting for experimental validating.