Laser-induced explosive boiling during nanosecond laser ablation of silicon

The surface morphology of single crystal (1 0 0) Si wafers irradiated by 266 and 1064 nm laser pulses emitted by a Nd:YAG laser has been investigated. The morphology of the bottom of craters formed as a result of single or multipulse laser irradiation by the 266 nm wavelength, which is well absorbed by Si (optical absorption coefficient, α ∼106 cm−1), remained flat and almost featureless 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 is apparent that the vaporization process was confined to the surface region. A different morphology was observed at 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 J/cm2 evidence of boiling sites were observed at the bottom of the crater. By analyzing their formation mechanisms, density and shape, we suggest that they were induced by heterogeneous boiling and not homogeneous boiling, also known as phase explosion.