Non-thermal Ablation Effect On Silicon Induced By Femtosecond Laser Irradiation

Ultra-fast lasers have been developed over a half century and have become more accessible and user-friendly in recent years.Compared with longer pulse lasers,ultra-fast pulses are unique in that they are characterized by incredibly high peak intensity and interact with materials faster than the lattice disorder and heat diffusion do.These two features allow ultra-fast lasers to control and manipulate precisely the states of materials.The interaction of femtosecond lasers with silicon has been an active area of material science research,leading to some unexpected phenomena as well as the formation of new materials.Based on these points,we discuss the interaction between femtosecond laser and silicon material under the existing experimental conditions in our laboratory.Then,we present a deep insight into the non-thermal ablation effect and laser-induced periodic surface structures(LIPSS).First,in the study of the femtosecond laser ablation on silicon,the applicability of the photoionization model and the improved double temperature model are discussed.Through the simulation results of finite-difference methods,we analyze the electron density,electron and lattice temperature,and the processes of plasma excitation under the illuminations of single pulse and double pulses,respectively.Besides,the temperature threshold model and the plasma threshold model are compared by monitoring the damage threshold and ablation morphology.The calculated results show that the improved two-temperature model is more accurate than the photoionization model in analyzing complex ablation conditions.When the carrier density reaches the critical value,the excitation of plasma causes the modification of the surface reflectance.In addition,the hole radius are in good agreement with the experimental results.Under the condition that the first pulse induces electron-hole plasma at conduction band,the second pulse energy are partially absorbed by the plasma layer and the silicon is changed from a semiconducting state to a metal-like high-reflective state.Second,the changes of silicon temperature are obtained by the heat conduction process after single pulse femtosecond laser irradiation.The results of quantitative evaluation of the heat accumulation effect during the multi-pulse femtosecond laser ablation silicon are presented for discussion.Numerical evaluations are made using FDTD solution,where the effect of the accumulation of the initial pore structure on the redistribution of the subsequent pulse energy is analyzed.According to experiment data,the surface melting and oxidation will be against to the formation of smooth perforated morphology with the laser fluence of 1~2J/cm2 and the frequency of 10 Hz.Besides,the macroscopic thermal model is constructed to predict the surface temperature during the laser scanning the silicon surface.It's found that the position of the maximum point of laser intensity would be closer to the hole entrance with increasing taper anglebetween the beam and the hole wall.The heat accumulation is not only related to the incident laser fluence and frequency,but also related to the scanning speed.Finally,based on the Sipe-Drude model and the surface plasma(SPP)interference theory,the formation mechanisms of low-spatial-frequency laser-induced periodic surface structures(LSFL)on single-crystalline silicon upon irradiation with single femtosecond laser pulse in air are investigated theoretically.Besides,we propose a refined model of the second harmonic generation ripples spacing theory (?) taking into account the modified femtosecond laser excited silicon refractive index related to the Drude model.And the FDTD method was used to simulate the surface electric field distribution.The results show that the theory for interference of the incident wave with the surface plasmon polariton wave(SPP)and the Sipe-Drude theory are both employed to analyze the formation of LSFL.Detailed calculations show that the Sipe-Drude theory works better for explaining the main features of the ripples on silicon wafer surface.Besides,our results are capable to explain quantitatively the spatial periods of the LIPSSs close to the laser wavelength,their orientation perpendicular to the laser beam polarization,their characteristic fluence dependence,and the period changing in the opposite direction with the incident angle increasing.The period of ripples is wavelength dependent and approximately proportional to the incident laser fluence.The field intensity which is localized in the grooves becomes concentrated on the ridges in between the grooves,leading to the creation of new grooves and thus the HSFLs.