Abstract

Direct laser write (DLW) technique allows three-dimensional (3D) structuring of materials with sub-micrometer scale spatial resolution, and is becoming increasingly widely used for fabrication of 3D photonic crystal structures in negative-tone photoresist via two-photon photopolymerization induced by ultrashort laser pulses. Finely-patterned highly macroporous dielectric structures attached to solid substrate for mechanical, stability are known to exhibit non-uniform shrinkage. Here we investigate physical mechanism of photoresist shrinkage and swelling during wet development. Two kinds of simple structures were designed and fabricated by DLW for this study as shown in Fig. 1(a). Thin photoresist rods with both ends (1) or one end (2) attached to massive supporting walls, exhibit markedly different behavior in ethanol and water, illustrated by Fig. 1(b). In ethanol, rods (1) exhibit expansion evident from their buckling, whereas rods (2) can expand freely and remain straight. In water, both types of rods shrink, and rods (1) become straight again. This behavior is reproduced by cycling between ethanol and water, indicating reversible nature of swelling and shrinkage. The swelling can be tentatively explained by assuming that the developed polymer is microporous and hydrophobic. Swelling in alcohol occurs because alcohol molecules, unlike water molecules, can penetrate the micropores. In water alcohol becomes gradually diluted and eventually removed from the micropores, thus leading to shrinkage. In the future, reversible swelling of photoresist may be exploited for actuation and optical sensing in microfluidic systems.

© 2014 Japan Society of Applied Physics, Optical Society of America