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.