Beam shaping for large-area laser structuring in a roll-to-roll process

Abstract

Laser structuring is a powerful tool for functionalizing surfaces, e.g., improving the tribological properties. To achieve small structures in the < 2 μm range, microscope objectives are typically used in laser material processing. There are two main challenges to achieve small structures: On the one hand, the limited working distance between the focusing optics and the workpiece results in a comparatively small processing area of a few square millimeters. On the other hand, the depth of field is limited when structuring with microscope lenses due to their large numerical aperture. As a result, the intensity of the laser beam is strongly dependent on the position in the propagation direction, so that the process window for material removal is only a few μm and small deviations disrupt the process. For highly productive large-area laser structuring in a roll-to-roll (R2R) process, the processing area must be enlarged, and the depth of field must be increased at the same time to enhance process robustness. With a given R2R process speed of the moving material of 2 m/min, and a material width of 0.5 m, we want to structure an area of 1 m²/min. The structuring pattern is a hexagonal arrangement of spots with a spot diameter of 1-2 μm and a spot distance of 2 μm. Additionally, we want to achieve a depth of field of 45-50 μm to enhance the process robustness. Given this background, this paper presents an approach in which a laser beam is split into numerous sub-beams and these sub-beams are subsequently shaped in such a way that the depth of field is increased for each individual beam. For beam shaping, a combination of static optical elements is used to transform a uniform into a Bessel-like intensity distribution to achieve a greater depth of field. By a skillful arrangement of the focusing elements, structure sizes of 1-2 μm as well as structure distances of 2 μm are achieved with the given R2R process speed.

Authors 5

1. Nicole Grubert Aachen

   RWTH Aachen University

   Affiliation as printed

   RWTH Aachen Univ. (Germany)

2. Oskar Hofmann Aachen

   RWTH Aachen University

   Affiliation as printed

   RWTH Aachen Univ. (Germany)

3. Mario Hesker Aachen

   RWTH Aachen University

   Affiliation as printed

   RWTH Aachen Univ. (Germany)

4. Jochen Stollenwerk Aachen Fraunhofer Institute for Laser Technology ILT

   Fraunhofer Institute for Laser Technology · RWTH Aachen University

   Affiliation as printed

   Fraunhofer Institute for Laser Technology ILT (Germany)

   RWTH Aachen Univ. (Germany)

5. Carlo Holly Aachen Fraunhofer Institute for Laser Technology ILT

   Fraunhofer Institute for Laser Technology · RWTH Aachen University

   Affiliation as printed

   Fraunhofer Institute for Laser Technology ILT (Germany)

   RWTH Aachen Univ. (Germany)