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This paper describes simple photolithography-based methods to fabricate multi-level three-dimensional (3D) microstructures without repeated processes using flexible and transparent film substrates such as polyethylene terephthalate (PET). When using a thin transparent film substrate, propagation of UV can be assumed to be near-field diffraction where wave property is more dominant than particle property. Using this phenomena, we patterned multi-level 3D SU-8 microstructures such as master molds for microfluidic neuron culture devices. PET film is compatible with SU-8 photoresist processing steps and exhibited good adhesion with patterned structures that were stable for more than 50 replica moldings. Advantages of the suggested method using PET are (1) simplified and faster fabrication of 3D structures with tunable thickness, (2) no requirement for mask alignment and repeated processes in fabricating multilayer structure, and (3) economical process with substrates that are ∼1000 times cheaper than Si wafers. To use PET based molds for soft lithography, we have designed an isothermal curing jig for replica molding with PDMS. Use of the PET substrate combined with back-side exposure can dramatically reduce fabrication time and cost. We believe that PET films can replace expensive Si based wafers for soft lithography master fabrication. Furthermore, they can be applied to the fabrication of microfluidic devices and complex multi-level high aspect ratio structures as an alternative to conventional photolithography.

Fabrication of functional 3D multi-level microstructures on transparent substrates by one step back-side UV photolithography