Two-photon polymerization is a photochemical process usually initiated by tightly focusing an ultrafast laser pulse into a volume of photosensitive photoresists with a high-numerical-aperture objective. Scanning a write voxel" in 3D enables near free-form fabrication, but at a limited speed which is a critical factor for industrial purposes, because generally only a single writing-beam is used. Several strategies have been implemented to improve the fabrication speed, one such strategy is massive parallelization which is the approach used in our PHENOmenon H2020 European project. Massive parallelization can be realized by beam splitting diffractive optical elements which allow simultaneous fabrication with thousands of beams, decreasing the overall fabrication time. A major unexpected obstacle is encountered in massively parallelized fabrication: using several spots simultaneously to polymerize, local changes in the 2PP threshold have been observed. We linked this to the proximity effect. The aim of this study is to understand the proximity effect in parallel microfabrication using simulation to predict its behaviour and different systematic experiments to reduce the proximity effect such as changing photoresist, using thinner photoresist layers to increase oxygen penetration or using higher Numerical Aperture Objectives.
The use of two-photon absorption (TPA) for polymerization, also known as 3D Lithography, Direct Laser Writing, or High-Precision 3D Printing is gaining increasing attraction in industrial fabrication of micro- and nanostructures. Mainly due to its vast freedom in design and high-resolution capabilities, TPA enables the fabrication of designs which are not feasible or far too complicated to be achieved with conventional fabrication methods. TPA is a scanning technology and fabrication in 3D requires axial overwritings. High industrial throughput fabrication can be achieved by intelligent fabrication strategies combined with an excellent material basis. Further boosting the throughput can be achieved by multispot exposure strategies. In this paper, massive parallelization is demonstrated which was realized by using a beam splitting diffractive optical element (DOE). Simultaneous fabrication using commercially available acrylate-based hybrid resin with 121 parallel focal spots arranged as 11 x 11 array is reported. Structures fabricated by a single laser beam and by 121 parallel beams are compared to each other with regard to shape and polymerization threshold. It was found that polymerization is strongly increased when parallel beams are used, especially for the central beams. As a result, polymerization threshold is lower in the center of the 11 x 11 array compared to the edges of the array. Furthermore, structures at the center of the 11 x 11 array are bigger compared to structures at the edges of the array when assigning equal intensity to all diffracted beams. These results are attributed to diffusion of photo initiators, quenchers, and radicals.
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