Two-photon lithography is a direct laser write process that enables fabrication of millimeter scale 3D structures with nanoscale building blocks. In this technique, writing is achieved via a nonlinear two-photon absorption process during which two photons are near-simultaneously absorbed at high laser intensities. Due to the high laser intensities, it is essential to carefully select the incident power so that two-photon polymerization (TPP) occurs without any laser damage of the resist. Currently, the feasible range of laser power is identified by writing small test patterns at varying power levels. Herein, we demonstrate that the results of these tests cannot be generalized because the damage threshold power is dependent on the proximity of features and reduces by as much 37.5% for overlapping features. We have identified that this reduction occurs due to a combination of reduced TPP for overlapping features and increased single-photon absorption of the resin after curing. We have captured the damage arising out of this proximity effect via 3D computed tomography images of a non-homogenous part that has varying feature density. Part damage manifests in the form of internal spherical voids that arise due to boiling of the resist at high laser intensities. Herein, we have empirically quantified this proximity effect by identifying the damage threshold power at different writing speeds and feature overlap spacings. In addition, we present a first-order analytical model that captures the scaling of this proximity effect. The scaling laws and the empirical data generated here can be used to select the appropriate writing process parameters so as to correct for proximity effects and prevent part damage during sub-micron additive manufacturing of parts with closely spaced features.

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