Laser-induced explosive boiling during nanosecond laser ablation of silicon

The surface morphology of single crystal (100) Si wafers irradiated by 266 nm and 1064 laser pulses emitted by a solid state Nd:YAG laser has been investigated. The morphology of the bottom of craters remained flat and almost featureless after 266 nm single or multipulse laser irradiation up to the maximum fluence of 18 J/cm² used in this study. The rims of the craters showed signs of radial liquid flow but it was apparent that the vaporization process was confined to the surface region. A different morphology was observed on the bottom of the craters formed by the 1064 nm wavelength laser pulses. Because this wavelength is absorbed in volume, α < 10⁴ cm^-1, a rather thick liquid Si pool formed at the surface. For laser fluences higher than 3-5 J/cm² evidence of boiling sites were observed on the bottom of the crater, especially for multipulse irradiation. An evolution of surface morphology, from waves towards deep cavities was observed with the increase of pulse number. By analyzing the cavity formation mechanisms, their density and shape, we suggest that they were induced by heterogeneous boiling and not homogeneous boiling.