飞秒激光打孔硅的孔洞形貌研究

Hole Morphology in Femtosecond Laser Drilling of Silicon

在飞秒激光打孔硅材料过程中,为了得到表面等离子体效应和激光烧蚀形成的孔洞对后续激光能量在孔内分布的影响,建立单脉冲等离子体阈值理论模型及设计连续飞秒激光烧蚀硅材料实验.理论计算得到的损伤阈值为0.21J/cm2,符合实验模型测得的阈值0.20~0.25J/cm2.当载流子密度达到临界值Ncr,等离子体的激发会导致表面反射率短时间内急剧上升.入射激光通量从0.5J/cm2增大到3.0J/cm2,烧蚀深度逐渐增大并趋于约1.1μm,同时脉宽从150fs减小到50fs,烧蚀结构类似于椭圆形烧蚀轮廓.后续激光脉冲辐照在已形成的孔洞上时,基于时域有限差分法,控制光束与孔壁的夹角从79℃到49℃,激光能量越接近孔底中心,越易引发该范围内的等离子体激发;且在不同偏振态光束辐照下,孔底的能量分布不同会造成相应特殊的烧蚀形貌.增大激光通量和减小脉冲宽度获得理想的初始孔洞结构,可使后续脉冲能量集中孔底中心区域,打孔效果更好.

In the femtosecond laser drilling of silicon material,in order to the influence of the surface plasma effect and hole geometries on the redistribution of the follow-up laser energy,the theoretical model and muti-pulse ablation experiment were established.Theoretical calculation of the damage threshold 0.21 J/cm2 is in line with the experimental model measured threshold 0.20~0.25 J/cm2.When the carrier density reaches the critical value Ncr,the excitation of the plasma causes the surface reflectance to rise rapidly.With a larger of laser fluence from 0.5 J/cm2 to 3.0 J/cm2,ablation depth increases to about 1.1 um,and while the pulse width from 150 fs reduced to 50 fs,ablation structure become more similar to the oval ablation profile.What＇s more,the effect of different hole structure on the laser beam propagation inside the micro-hole of silicon wafer is obtained by numerically using FDTD method.It＇s found that the position of the maximum laser intensity point would be closer to the hole enter by decreasing taper angle from 79℃ to 49℃,which finally cause the excitation of plasma more likely.Besides,under different polarization beam irradiation,the energy distribution at the bottom of the hole induced different special ablation structures.The results show that by increasing laser fluence and decreasing pulse width,the ideal initial pore structure can be obtained so that the subsequent pulse energy is concentrated at the bottom center making the drilling efficiency higher.