Large Area Surface Ablation and Micropatterning of Transparent Dielectrics with Femtosecond UV Laser Pulses

Abstract

Direct laser ablation is one of the most widespread laser microtechnology used in the industry. Many different types of lasers at a broad range of processing parameters have been tested on various surfaces with the primary goal to achieve highly predictable and regularly patterned geometry. Such a task remains a big challenge for the large band-gap materials, where the best results can be expected in a linear laser absorption regime. Therefore excimer UV lasers and high harmonics of nanosecond solid-state laser are successfully applied for surface patterning tasks [1] , [2] . However, the recent advancement of industrial-grade femtosecond laser systems with high harmonics modules expand the range of usable laser sources for this application. It is well known that shorter pulse processing, due to characteristic pulse timescales smaller than the processing material’s atomic vibrations timescale, has reduced heat-affected zones (HAZ), which causes better ablation precision compared to the longer pulses. This leads to a lower surface roughness of an ablated region, and, most importantly, as the absorption remains linear, the roughness and ablation depth can be directly controlled. The choice of patterning algorithm is also important. i.e., Tanvir et al. introduced a model to fine-tune processing parameters to reproduce a particular surface microstructure [3] , while Žemaitis et al. produced an ablation model which takes into account the decrease in ablation threshold and saturation of the ablation depth with the increasing number of pulses per spot [4] . These models help increase the processing area, however, they still struggle to correctly predict ablation depth and surface roughnesses required for the fabrication of diffractive optical elements, microfluidic chips, hydrophobic surfaces, and other devices