Early stages of femtosecond laser-induced formation of silicon microspikes

Arrays of sharp conical spikes form on crystalline silicon surfaces when irradiated with a train of femtosecond laser pulses in a background of sulfur hexafluoride (SF6); blunter, more irregular microstructures form in vacuum. The tips of the spikes are at the height of the original surface of the wafer, suggesting that the formation process predominantly involves removing material. The spikes are arranged in a quasi-ordered fashion with a characteristic nearest-neighbor separation of a few micrometers; the exact value of this characteristic separation depends on the laser fluence and number of laser pulses. To shed light on the mechanism of microstructure formation, we present images and analysis of the early stages of femtosecond laser microspike development in SF6, and compare these results to microstructure development with femtosecond laser pulses in vacuum and with nanosecond laser pulses in SF6.