Fabrication of Hierarchical Pillar Arrays from Thermoplastic and Photosensitive SU-8

By exploiting the thermoplastic and photosensitive nature of SU-8 photoresists, different types of hierarchical pillar arrays with variable aspect ratios are fabricated through capillary force lithography (CFL), followed by photopatterning. The thermoplastic nature of SU-8 enables the imprinting of micropillar arrays with variable aspect ratios by CFL using a single poly(dimethylsiloxane) mold, simply by tuning the initial film thickness of SU-8 on a substrate. The pillar array is subsequently photopatterned through a photomask, followed by post-exposure baking above the glass transition temperature (T-g) of SU-8. The pillars in the exposed region become highly crosslinked and, therefore, neither soluble nor able to reflow above T-g, whereas the pillars in the unexposed regions can reflow and flatten out. Two developing strategies are investigated after UV exposure of the SU-8 pillar arrays including i) solvent development and drying and ii) thermal reflow to create bile vel hierarchical structures with short pillars and single-level, dual-scaled, high-aspect-ratio (up to 7.7) pillars in a microdot array, respectively.