Micro-Lego of 3D SU-8 structures and its application to a re-entrant surface

Deterministic multi-layer micro-scale assembly of SU-8 polymer objects as well as their cohesive interfacial joining strength are investigated. The process of SU-8 photoresist into retrievable individual micro-scale objects is reviewed and the construction of three-dimensional (3D) SU-8 micro-structures via micro-Lego, which involves transfer printing and permanent joining of SU-8 objects on a target site, is demonstrated. Owing to the additive 3D nature of micro-Lego, the demonstrative micro-scale architectures are unattainable through other means such as conventional photolithography. The mechanical joining strength between SU-8 objects assembled by micro-Lego is inspected by probing SU-8 cantilever specimens using a nanoindenter and the results reveal that the joining strength is ~ 0.988 J/m, which is comparable to wafer scale silicon fusion bonding strength. In order to demonstrate a practical application of SU-8 micro-Lego, a SU-8 re-entrant surface is assembled by one step process. It is shown that the SU-8 re-entrant surface can repel a water droplet while SU-8 is natively hydrophilic with a low intrinsic contact angle (73 ± 5°).