ULTRASHORT-PULSE LASER-ABLATION OF SILICON - AN MD SIMULATION STUDY

Molecular Dynamics (MD) simulation has been employed in order to study laser ablation of silicon surfaces. The impact of laser pulses with l engths ranging from 5 ps down to 10 fs on a target of approximately 10 0 Angstrom x 100 Angstrom x 50 Angstrom was investigated. The ablation shows a strong dependence on pulse length, on pulse energy and on the number of laser shots. With decreasing pulse length the amount of rem oved particles increases, and with decreasing pulse energy the holes b ecome narrower. Especially in multishot ablations, holes with a diamet er of just a fraction of the focus could be observed. This can be attr ibuted mainly to ablation of atoms from lower areas and their redeposi tion close to the surface, leading to amorphous areas around the ablat ion hole. For pulses of picosecond duration, and even for femtosecond pulses, the main material removal occurs on a timescale of a few ps. I nterestingly, the simulations show two thresholds: the onset of damage at the surface, which depends on the pulse energy but only insignific antly on the pulse length; and the onset of the removal of particles, which shows a strong dependence on the pulse length of the laser.