Abstract

Ultrafast lasers are highly useful tools in the field of microfabrication. In microfabrication, the laser pulse usually is very short in the pico-second or subpico-second range; therefore, it is very difficult to observe the transient material processing phenomena experimentally. Over the years, the authors have conducted molecular dynamics (MD) simulation to study the ablation process with ultrafast laser irradiation. The MD method has been modified to simulate the laser ablation of metals by updating heat conduction effect by free electrons at each calculation time step. In this study, the previously modified MD is further extended to elucidate the laser ablation phenomena from the viewpoint of phase transformation. In particularly, pulse width dependence of damage threshold fluence, evaporation process and temperature profile in the material are investigated. The main results are summarized as follows: (1) In general, damage threshold fluence is proportional to the square root of the pulse width when the pulse width is relatively long. This is well known as the law of one-half power. However, our simulation results indicate that the threshold fluence becomes higher than the one estimated by the law of one-half power when the pulse width is smaller than ten pico-seconds. (2) In the beginning of laser irradiation, the vicinity of the irradiated surface is superheated before evaporation starts several pico-seconds later. The temperature at the gas and liquid interface is almost equal to the boiling point as known before, but temperature at the liquid and solid interface is much higher than the melting point at the state of thermal equilibrium. (3) There are two types in laser ablation process. One is explosive evaporation which occurs when pulse width is extremely short (see Fig. 1 (a)), and the other is relatively calm evaporation which occurs when pulse width is comparatively long (see Fig. 1 (b)). In the former process, a comparatively large particles scatter, and in the latter process, the size of scattering particles is relatively small. (4) When the fluence is constant, there is an optimal irradiance of the laser pulse to evaporate a unit of atoms. The ratio of the energy used to evaporate atoms to the total laser energy absorbed reaches a maximum and the energy per evaporated atom reduces to a minimum at the optimal irradiating condition.

© 2000 IEEE