Proximity effect in parallelized microfabrication using two-photon polymerization

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.