Effect of Proximity of Features on the Damage Threshold During Submicron Additive Manufacturing Via Two-Photon Polymerization

Abstract

Two-photon polymerization (TPP) is a laser writing process that enables fabrication of millimeter scale three-dimensional (3D) structures with submicron features. In TPP, writing is achieved via nonlinear two-photon absorption that occurs at high laser intensities. Thus, it is essential to carefully select the incident power to prevent laser damage during polymerization. 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 depends on the proximity of features and reduces by as much as 47% for overlapping features. We have identified that this reduction occurs primarily due to an increase in the single-photon absorptivity of the resin after curing. We have captured the damage from proximity effects via X-ray 3D computed tomography (CT) images of a nonhomogenous part that has varying feature density. Part damage manifests as internal spherical voids that arise due to boiling of the resist. 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. Based on this model and the experiments, we have identified that the proximity effect is more significant at high writing speeds; therefore, it adversely affects the scalability of manufacturing. The scaling laws and the empirical data generated here can be used to select the appropriate TPP writing parameters.

Two Photon lithography additive manufacturing: Video dataset of parameter sweep of light dosages, photo-curable resins, and structures

“…The first and second videos listed in Table 1 show the printing process of cuboid structures described here [2] . Each cuboid structure has the dimension of 10 µm × 10 µm × 10 µm with a Z-layer spacing of 0.6 µm.…”

Fabrication of Functional Microdevices in SU-8 by Multi-Photon Lithography

“…This is sometimes referred to as the memory effect [ 126 ]. Adjacent exposure also lowers the damage threshold of the material, which some refer to as the proximity effect [ 127 ]. To address the memory effect and improve the resolution of MPL, various schemes have been investigated in acrylate systems.…”

Scaling‐Up of Nano‐Architected Microstructures: A Mechanical Assessment

“…This is a surprising observation as the structures (and thus the density) above and below the stitch line are nominally the same. We hypothesize that non‐uniform shrinkage at the stitch line is due to proximity effects, which leads to reduced oxygen inhibitor concentration within the photoresist at the writing front, which, in turn, increases the degree of conversion and ultimately reduces the extent of shrinkage observed for a given laser power . The abrupt pause and subsequent 5 min rest period of the stitched specimens allow the oxygen inhibitor concentration to return to its (higher) equilibrium value.…”

“…In addition, we also observe non-uniform shrinkage at the stitch line (marked by yellow arrows) as evidenced by the sudden noticeable change in gage width (Figure 2e,f ). We hypothesize that non-uniform shrinkage at the stitch line is due to proximity effects, [32,37,38] which leads to reduced oxygen inhibitor concentration within the photoresist at the writing front, which, in turn, increases the degree of conversion [39] and ultimately reduces the extent of shrinkage observed for a given laser power. We hypothesize that non-uniform shrinkage at the stitch line is due to proximity effects, [32,37,38] which leads to reduced oxygen inhibitor concentration within the photoresist at the writing front, which, in turn, increases the degree of conversion [39] and ultimately reduces the extent of shrinkage observed for a given laser power.…”