Single-step UV diffraction lithography to define a hydrophobic SU-8 interconnected hoodoo structure

Micro-hoodoo (inverse trapezoidal) structures fabricated by silicon etching, soft-lithography, or photolithography have attracted great interest due to their improved hydrophobicity and self-cleaning properties. We present here a simple single-step UV diffraction lithography technique for fabricating micro-hoodoo structures with or without adjustable interconnecting bridges using a negative SU-8 photoresist. The theoretical calculations and the fabrication results revealed that the sizes and sidewall profiles of the micro-hoodoo structures and interconnecting bridges could be precisely controlled by the fabrication conditions, including the pattern-to-pattern spacing, exposure dose, and gap between the mask and the SU-8 surfaces. The theoretical calculations were conducted using an integrated model based on a Fresnel diffraction model for estimating the hoodoo size and an exponent decay model for estimating the sidewall profile. The integrated model agreed well with the fabricated hoodoo sizes and sidewall profiles, and the model provided an explanation for the structural instabilities observed during formation of the interconnecting bridges between hoodoos. The interconnecting bridges made the hydrophobic hoodoo structures sticky toward water, with a water contact angle hysteresis of up to 86.6°. The directional bridge interconnections produced directionally sticky hydrophobic surfaces that successfully mimicked function in butterfly wings to enable directional water removal.