Processes of Direct Laser Writing 3D Nano-Lithography

Abstract

Direct laser writing three-dimensional nano-lithography is an established technique for manufacturing functional 3D micro- and nano-objects via non-linear absorption induced polymerization process. In this Chapter an underlying physical mechanisms taking place during nano-confined polymerization reaction, induced by tightly focused ultra-short laser pulses, are reviewed and discussed. The specialattention is paid on the effects that directly impact structuring resolution and minimum achievable feature size. Analysis of possible photo-initiation mechanisms as contributing multi-photon absorption and avalanche ionization in pre-polymers under diverse exposure conditions (wavelength, pulse duration) is presented. Feasible structuring of pure (non-photosensitized) and functional nanoparticles doped polymer precursors is justified and benefits of such materials/structures for microoptics, photonics and cell scaffolds are highlighted. The influence of temperature effects (induced by writing process itself or determined by ambient conditions) on polymerization process, observed in different pre-polymers under diverse exposure regimes is outlined. The further adjustment of the structuring resolution is possible via precise control of light polarization and diffusion assisted radical quenching. The work is concluded with a brief outlook on future challenges and perspectives related to refinement of 3D ultra-fast laser lithography fabrication process in the means of application of diverse post-processing methods and research into novel photo-curable materials including inorganic ones.

Fabrication of Tapered 3D Microstructure Arrays Using Dual-Exposure Lithography (DEL)

“…When the wait time was introduced between exposures (on top of the mask rotation time) (Dome to Bullet, Bell to Pillar, Figure 2 F), the resulting microstructures became much taller with a nearly four times (Bullet; height 75 µm), and three times (Pillar; height 110 µm) increase, indicating much deeper SU-8 micro-pockets. This result is likely caused by the free-radical quenching during photopolymerization, as reported in previous studies [ 49 , 50 , 51 , 52 ]. We speculate that through the wait-time (on top of mask rotation time), the free radicals formed during the first exposure could have likely been quenched and reduced the number of free radicals present for further resist polymerization.…”

Challenges and limits of Mechanical Stability in 3D Direct Laser Writing

“…This is of particular relevance for improved manufacturing of delicate structures. Experimental studies in this realm encounter serious challenges as it is not easy to control all parameters that influence the DLW process 27 or to observe this process on the nanoscale in operando. Additionally, difficulties in reliable fabrication of nanoscale specimens in combination with strain measurement complications make mechanical experiments challenging 28 .…”

Direct Laser Writing to Generate Molds for Polymer Nanopillar Replication

“…Direct Laser Writing (DLW) via 2 photon-polymerization (2PP), in which a laser beam is focused into a tiny volume which serves as a building block within UV-curable photoresists, can yield feature sizes below the diffraction limit of the excitation light. 11 Since the advent of the technology, research groups have strived to push the boundaries of achievable resolution and line width through innovative physical and chemical approaches. Kawata et al demonstrated the dramatic effects which could be achieved through the use of additional radical quenchers in the photoresist, which serve to minimize the extent to which created radicals can survive, thereby reducing the effective voxel volume.…”

“…Laser-based direct-write technology using multiple photon absorption can allow spatial confinement of polymerization with minimal topological constraints. Direct laser writing (DLW) via 2-photon polymerization (2PP), in which a laser beam is focused into a tiny volume that serves as a building block within UV-curable photoresists, can yield feature sizes below the diffraction limit of the excitation light . Since the advent of the technology, research groups have strived to push the boundaries of achievable resolution and line width through innovative physical and chemical approaches.…”