Selective femtosecond laser structuring of dielectric thin films with different band gaps: a time-resolved study of ablation mechanisms

摘要

Ultrashort pulse lasers have been increasingly gaining importance for the selective structuring of dielectric thin films in industrial applications. In a variety of works the ablation of thin SiO_2 and SiN_x films from Si substrates has been investigated with near infrared laser wavelengths with photon energies of about 1.2 eV where both dielectrics are transparent (E_(gap,SiO_2) ≈ 8eV;E_(gap,SiN_x) ≈2.5eV). In these works it was found that few 100 nm thick SiO_2 films are selectively ablated with a "lift-off initiated by confined laser ablation whereas the SiN_x films are ablated by a combination of confined and direct laser ablation. In the work at hand, ultrafast pump-probe imaging was applied to compare the laser ablation dynamics of the two thin film systems directly with the uncoated Si substrate-on the same setup and under identical parameters. On the SiO_2 sample, results show the pulse absorption in the Si substrate, leading to the confined ablation of the SiO_2 layer by the expansion of the substrate. On the SiN_x sample, direct absorption in the layer is observed leading to its removal by evaporation. The pump-probe measurements combined with reflectivity corrected threshold fluence investigations suggest that melting of the Si substrate is sufficient to initiate the lift-off of an overlaying transparent film-evaporation of the substrate seems not to be necessary.