FABRICATION OF MULTIPLE HEIGHT MICROSTRUCTURES USING UV LITHOGRAPHY ON TIMED-DEVELOPMENT-AND-THERMAL-REFLOWED PHOTORESIST
Jungkwun Kim, Kangsun Lee, Hongsub Jee, Kwang Oh and Yong-Kyu Yoon
Multiple height pillar arrays as well as various circular or parabolic microstructures are fabricated by the combination of the timed-development-and-thermal-reflow process and an additional ultraviolet (UV) exposure step. First, uncrosslinked photoresist (SU-8) in a confined region such as a via or trench from the initial lithography step is partially developed in a timed manner to obtain a different development depth while a subsequent thermal treatment above the glass transition temperature of the remaining uncrosslinked polymer allows the polymer to reflow to form an uneven but smooth round shape surface profile, which can be well described using the Young-Laplace equation. Second, an additional UV exposure step to the prepared uneven surface by the back side exposure scheme through a transparent substrate with prepatterned Cr patterns is performed to have pillar arrays with various heights. A 2 by 5 micropillar array with gradually changing heights is successfully demonstrated. Also, a microfluidic channel with a triangular cross section is fabricated for particle sorting.
Keywords: multiple height microstructures, thermal reflow and timed development