Selective ablation of thin SiO2 layers on silicon substrates by femto- and picosecond laser pulses

The selective ablation of thin (similar to 100 nm) SiO2 layers from silicon wafers has been investigated by applying ultra-short laser pulses at a wavelength of 800 nm with pulse durations in the range from 50 to 2000 fs. We found a strong, monotonic decrease of the laser fluence needed for complete ablation of the dielectric layer with decreasing pulse duration. The threshold fluence for 100% ablation probability decreased from 750 mJ/cm(2) at 2 ps to 480 mJ/cm(2) at 50 fs. Significant corruption of the opened Si surface has been observed above similar to 1200 mJ/cm(2), independent of pulse duration. By a detailed analysis of the experimental series the values for melting and breaking thresholds are obtained; the physical mechanisms responsible for the significant dependence on the laser pulse duration are discussed.