Minimizing the Surface Roughness for Silicon Ablation with Ultrashort Laser Pulses

The application of ultrashort pulsed lasers for surface patterning enables precise control of the ablation depth. The goal of this study is to find the optimal combination of pulse-to-pulse distance and fluence in order to minimize the surface roughness for Si ablation. A Si wafer with a thickness of 525 μm was irradiated using an ultrafast laser with a pulse duration of 380 fs at a wavelength of 520 nm and a pulse frequency of 200 kHz. An area of 50x50 μm2 was ablated to a depth of about 10 μm using different pulse and line distances dp,l and peak fluences F0. Confocal microscopy was used to investigate the surface profiles and to determine the surface roughness. For a fixed fluence, two pulse distances can be found where the formation of furrows can be suppressed. The global surface roughness minimum of 220 nm was achieved using a pulse and line distance of dp,l ≈ 0.67 w0 and a fluence of F0 ≈ 2.8 J/cm2. The influence of the furrows on the surface roughness is negligible. For fluences above 8 J/cm2, the surface roughness as a function of the pulse distance has two minima. The results also show that locations of these minima increase with fluence.