Hierarchical colloid-based lithography for wettability tuning of semiconductor surfaces

Hierarchical colloid-based lithography and two-step plasma etching involving mask reduction were used to probe and tune the wettability landscape of Si and GaN surfaces from the hydrophilic to superhydrophobic limits over cm length scales. Hydrophobicity, due to the classical Cassie–Baxter (CB) wetting effect, was observed on Si with surface pillars having pitches below 1 μm. Additional tuning of plasma processing conditions at this critical transition provided additional increases in hydrophobicity and led to a highly repellent, lotus leaf effect. Superhydrophobic surfaces were created within the CB wetting state by varying the extent and duration of plasma-based mask reduction and pattern transfer, achieving a maximum contact angle of 157°. Additional submicrometer topography (310 nm spacing) was added to a nominally Wenzel-impregnated, hydrophilic Si micropillar surface (a diameter of 6 μm) with a second lithography cycle, rendering the surface hydrophobic and robust to aging in ambient conditions. An increase in the contact angle with added hierarchy (46°–88°) was also observed for GaN surfaces, albeit diminished compared to Si owing to the relatively lower initial GaN-water contact angle. Overall, this approach has demonstrated a significant degree of wetting tunability in multiple semiconductor systems using colloidal-based nano- and micro-patterning.