Development and characterization of a custom-made 3D direct laser writing lithography system based on a visible picosecond laser

Direct laser writing (DLW) refers to a class of maskless microfabrication techniques in which a focused laser beam induces a localized modification in a material. In photopolymer-based DLW, this localized interaction enables the production of two- and three-dimensional structures by inducing the polymerization of a photoresist. In this thesis, a custom-made 3D direct laser writing system based on a visible picosecond-pulsed laser is developed and characterized, with the aim of enabling submicron microfabrication through an accessible and versatile optical platform. The system is based on an upright optical microscope setup equipped with a 532 nm ps-pulsed laser, an imaging arm, a 2-axis motorized stage and a high magnification objective mounted on a motorized holder for adjustable focusing. SU-8 photoresist is employed as the main resist material platform and it is exposed at 532 nm, outside its main ultraviolet absorption band. Under these conditions, the photoinduced polymerization is expected to occur in a non-conventional interaction regime, in which low one-photon absorption and two-photon absorption can both contribute to the writing process, enabling confined 3D polymerization. The influence of laser power, exposure time, scanning speed, and focus position is investigated to define the fabrication window and optimize the writing process. The fabricated structures are characterized by optical microscopy and scanning electron microscopy to evaluate their morphology, feature size and resolution. Overall, this thesis presents and validates the feasibility of a versatile 3D direct laser writing platform, adaptable beyond standard planar substrates to non-conventional geometries, including optical fibers.