In the present work, a novel fabrication technique of inverted trapezoidal microstructures on a silicon substrate is presented. The novel microstructures caused the inherently hydrophilic silicon surface to become hydrophobic without the use of coating or thin film deposition. The microstructures were fabricated on a Silicon On Insulator (SOI) wafer through steps of photolithography, dry/wet etching, and bounding. Starting with a (100) plane SOI wafer, squares were patterned into a nitrite etch mask. Potassium hydroxide (KOH) anisotropic wet etchant was then used to etch the wafer, as a result created pits with flat sloping {111}-oriented sidewalls and a flat (100)-oriented bottom of the silicon device layer (i.e., at SiO2 layer). To get the inverted trapezoids, wafer bonding was carried out after etching, and the handle wafer along with the SiO2 layer were completely etched to expose the small openings of the trapezoids.
SEM images were taken to investigate the surface and to measure the dimensions of the square openings from both sides before and after bonding. Contact angle measurements demonstrated the ability of the novel surfaces to form Cassie wetting states with a liquid droplet, sustaining large contact angles above 90 degrees, whereas bare silicon surfaces were measured to have contact angles of around 40 degrees. Shifts in contact angles were attributed to air entrapment.
