S.K. Saha, V.H. Nguyen, S-C Chen
Lawrence Livermore National Laboratory,
Keywords: multiphoton lithography, direct laser writing, photopolymerization, digital manufacturing
Summary:The availability of scalable nanofabrication processes that can produce arbitrarily complex three-dimensional (3D) structures is critical to transitioning several nano-enabled products from laboratory scale demonstrations to real-world adoption. Two-photon lithography (TPL) is a promising photopolymerization-based submicron additive manufacturing technique due to its ability to generate 3D structures with features on the scale of 100 nm and at a rate significantly higher than high-resolution 2D techniques such as e-beam lithography. TPL relies on nonlinear two-photon absorption to generate features smaller than the diffraction-limited focused light spot. This unique capability of TPL has been leveraged in diverse fields to fabricate functional micro and nanoscale 3D structures for photonic crystals, mechanical metamaterials, micromachines, miniaturized optics, and flexible electronics. Unfortunately, the serial and slow writing scheme of existing TPL implementations makes production scaleup beyond one-off demonstrations impractical. Although past attempts to parallelize this process have been successful in increasing the rate of the process, those implementations have reduced the ability to fabricate complex 3D parts. Specifically, past demonstrations have either printed (i) the same feature into a periodic structure or (ii) an extrusion of an arbitrarily complex 2D plane (without depth resolvability). We have overcome this scalability versus part complexity tradeoff by implementing a projection-based parallel writing scheme for printing of arbitrarily complex 3D parts. Our technique decouples throughput from part-size and increases the rate of printing by a factor of two to three orders of magnitude without compromising the submicron resolution. In our parallelization scheme, an image of an array of individually actuated micro-mirrors is projected onto a plane interior to the photopolymer resist. We have achieved layer thicknesses of less than a micron by ensuring that the pulsed laser beam is both spatially and temporally focused. In addition, we have synthesized custom resists that are index matched to the objective to reduce spherical aberrations during the dip-in printing of tall millimeter scale structures. Here, we present the (i) parallelization scheme based on femtosecond projection optics and (ii) characterization of the parallel writing mechanism.