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 lengths ranging from 5 ps down to 10 fs on a target of approximately 100 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 removed particles increases, and with decreasing pulse energy the holes become narrower. Especially in multishot ablations, holes with a diameter of just a fraction of the focus could be observed. This can be attributed mainly to ablation of atoms from lower areas and their redeposition close to the surface, leading to amorphous areas around the ablation hole. For pulses of picosecond duration, and even for femtosecond pulses, the main material removal occurs on a timescale of a few ps. Interestingly, the simulations show two thresholds: the onset of damage at the surface, which depends on the pulse energy but only insignificantly on the pulse length; and the onset of the removal of particles, which shows a strong dependence on the pulse length of the laser.