Holographic freeform micro hole processing using Bessel beams

• Combination of an SLM with an axicon to generate multi-Bessel beams. • Freeform shaped micro holes. • Dynamic adaption of SLM phase holograms for highest flexibility during the process. • Quadratic shaped micro holes with sizes down to 4 μm. • Round micro holes with circularity up to 96 %. Bessel beams are advantageous tools for micro drilling applications due to their diffraction-free properties, their long focal length, and their small focal spot diameter. However, their application in industry is limited because they are challenging to integrate into flexible beam shaping systems. This work is intended to overcome limitations in flexibility by presenting a new approach paving the way to high-aspect ratio freeform holes. Therefore, a spatial light modulator was integrated into an axicon-based setup to merge diffraction-free ablation with the flexibility of holographic spatial beam shaping. A dynamic change of the phase hologram displayed on the spatial light modulator during the process allows for the generation of arbitrary ablation geometries. In contrast to other holographic approaches, the diffraction-free nature of the Bessel beam allows ablation over an extended focal depth without any focus control needed. The applicability of the approach was demonstrated by processing arbitrary hole shapes, such as ellipses, squares, or star-shapes. The hole sizes of the freeform holes range from 4 µm to 80 µm. Furthermore, up-scaling of the process is demonstrated by using the setup for generation of multi-Bessel spots.