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 (Tg) of SU‐8. The pillars in the exposed region become highly crosslinked and, therefore, neither soluble nor able to reflow above Tg, 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 bilevel hierarchical structures with short pillars and single‐level, dual‐scaled, high‐aspect‐ratio (up to 7.7) pillars in a microdot array, respectively. Different hierarchical SU‐8 pillar arrays are fabricated by a combination of capillary force lithography and photolithography. After capillary imprinting of SU‐8 pillar arrays and UV exposure through a photomask, two developing approaches are exploited: i) solvent developing and drying to create bilevel hierarchical structures with short pillars, and ii) thermal reflowing to create single‐level, dual‐scaled, high‐aspect‐ratio pillars in a microdot array.