Explosive change in crater properties during high power nanosecond laser ablation of silicon

Mass removed from single crystal silicon samples by high irradiance (1x10(9 ) to 1x10(11) W/cm(2)) single pulse laser ablation was studied by measuring the resulting crater morphology with a white light interferometric microsc ope. The craters show a strong nonlinear change in both the volume and dept h when the laser irradiance is less than or greater than approximate to 2.2 x10(10) W/cm(2). Time-resolved shadowgraph images of the ablated silicon pl ume were obtained over this irradiance range. The images show that the incr ease in crater volume and depth at the threshold of 2.2x10(10) W/cm(2) is a ccompanied by large size droplets leaving the silicon surface, with a time delay similar to 300 ns. A numerical model was used to estimate the thickne ss of the layer heated to approximately the critical temperature. The model includes transformation of liquid metal into liquid dielectric near the cr itical state (i.e., induced transparency). In this case, the estimated thic kness of the superheated layer at a delay time of 200-300 ns shows a close agreement with measured crater depths. Induced transparency is demonstrated to play an important role in the formation of a deep superheated liquid la yer, with subsequent explosive boiling responsible for large-particulate ej ection. (C) 2000 American Institute of Physics. [S0021-8979(00)04015-9].