Nanodrilling of fused silica using nanosecond laser radiation

Abstract

The fast laser drilling of dielectric surfaces with hole diameters in the sub-μm range and a high aspect ratio is a challenge for laser methods. In this study, a novel laser structuring method for the production of randomly and periodically distributed holes in a fused silica surface will be presented using a self-assembling process. A fused silica surface was covered with a 10nm thick magnetron-sputtered molybdenum film. The metal film was irradiated by a KrF excimer laser (wavelength λ=248nm, pulse duration Δtp=25ns) with low laser fluences (Φ<1J/cm2) and the laser-induced heating resulting in a melting of the metal film and finally in a self-assembled formation of randomly distributed metal droplets due to the surface tension of the metal liquid phase using a top hat beam profile. Furthermore, the usage of a periodically modulated laser beam profile allows the fabrication of periodically distributed droplet pattern. The multi-pulse irradiation of the laser-generated metal droplets with higher laser fluences results in a stepwise evaporation of the metal and in a partial evaporation of the fused silica near the metal droplets. Finally, the laser-induced stepwise evaporation process results in a formation of cone-like holes in the fused silica surface where the resultant holes are dependent on the size of the generated metal droplets and on the laser parameters. The “drilling” process allows the fabrication of holes with a depth up to 1μm at a hole radius in the sub-μm range. Furthermore, the hole density can be adjusted by the laser fluence where a hole density up to ∼1.3μm−2 can be achieved. Furthermore, the process was simulated by finite element method using a simple thermodynamic model and the theoretical results were compared with the experimental findings.