EXCIMER LASER ABLATION OF FUSED-SILICA

Ablation of fused silica has been carried out using standard nanosecond excimer laser pulses at 193, 248 and 308 nm, and 500 fs pulses at 248 nm. At 193 nm material of the rear side of a slab can be ablated without affecting the front side. This rear side ablation with an ablation rate of 0.4-mu-m/pulse is obtained at a fluence of about 3 J/cm2. At higher fluences front side ablation takes place, but the ablation rate reaches only 0.3-mu-m/pulse even at 12 J/cm2. Using fs pulses sharp-structured front side ablation with rates up to 0.3-mu-m/pulse (10 J/cm2) is achieved. The highest ablation rates of several-mu-m/pulse are achieved using standard 308 nm pulses. In this case polished samples tend to crack around the irradiated area, whereas samples showing a certain surface roughness can be structured more cleanly. Using standard 248 nm pulses polished samples show a clear two-phase ablation behaviour. Comparison of the results belonging to different wavelengths, pulse durations and surface qualities leads to the conclusion that different ablation mechanisms are involved: The first seems to be based on volume absorption, bond breaking and material expansion, the second appears mainly controlled by damage in relation with the singularity conditions at the surface.