Effect of air on debris formation in femtosecond laser ablation of crystalline Si

The ablation products formed by femtosecond laser irradiation of crystalline Si are investigated in detail in air at 1 atm. Ablation products possess a cotton-wool like morphology accumulated on the target surface in the vicinity of the ablated crater. Web-like links of roundish fine particles are also found in more distant regions. The particles are either single or polycrystalline Si linked with finer particulates and amorphous material. The particles are considered to be the result of a fragmentation and coalescence process of molten Si in the air. Electron beam diffraction indicates a small fraction of crystalline quartz among the debris particles. The cotton-wool-like debris are composed of an oxygen-rich Si compound with an atomic concentration ratio resembling that of SiO2. Ablated matter in vacuum within the peripheral zone of the ablated crater predominantly takes the form of roundish particles with a molten feature. Particles found at a distance of a few millimeters from the crater are nearly spherical and composed of crystalline Si. Particles located far from the crater are likely solidified droplets of liquid Si fragmented from the crater that have cooled at a rate favorable to crystallization during flight.