Formation of hollow microneedles on silicon surface by doughnut- shaped laser pulses using single- and multi-shot irradiation
Abstract
Our work deals with the formation of tubular-like structures and hollow-core microneedles on the surface of monocrystalline silicon using ultrashort laser pulses. Highly deterministic surface processing is ensured by single-shot ablative modification of the sample surface as well as the multi-shot regime was used to study and modify the final structure shape.
Both regimes were operated at laser fluences above the single-shot laser-induced damage threshold by a doughnut-shaped femtosecond laser pulses with duration of 35 fs (Astrella from Coherent, 800 nm wavelength). At laser fluences slightly above the ablation threshold, well reproducible tubular structures are formed whose height increases with fluence culminating with closing the structure on the top (fig. 1).
Upon multi-pulsed irradiation, the height of the needle structures can be increased as compared to those produced by single pulses but, at certain number of pulses, ablation causes the entire structure to collapse. The origin of observed surface structures was discussed with respect to thermodynamics, hydrodynamics, and material stress theory.