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

Abstract

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 $\hbox {SiO}_{2}$ and ${\hbox {SiN}}_{x}$ 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 ($E_{{\mathrm{gap,SiO}_{2}}}\approx 8\,\hbox {eV}; E_{{\mathrm{gap,SiN}}_{x}}\approx 2.5\,\hbox {eV}$). In these works it was found that few 100 nm thick $\hbox {SiO}_{2}$ films are selectively ablated with a “lift-off” initiated by confined laser ablation whereas the $\hbox {SiN}_{{x}}$ 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 $\hbox {SiO}_{2}$ sample, results show the pulse absorption in the Si substrate, leading to the confined ablation of the $\hbox {SiO}_{2}$ layer by the expansion of the substrate. On the $\hbox {SiN}_{{x}}$ 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.