Increasing Active Surface Area to Fabricate Ultra-Hydrophobic Surface by Using “Black Silicon” with Bosch Etching Process

Needle-shaped pillars so-called “Black silicon” (B–Si) were fabricated by etching cleaned silicon wafer with fluorine-based deep reactive ion etching plasma. The B–Si pillar with the pillar size (a) and spacing (b) of 250 nm, and height (h) of 6.47 μm, coated with SiO _x F _y film had water contact angle (WCA) and ethylene glycol contact angle (ECA) of 159.8° and 135.5°, respectively. After coating the pillar with trichloro(1H,1H, 2H,2H-perfluorooctyl)silane (TPFS), the WCA and ECA increased to 166.2° and 161.8°, respectively. At the optimum etching condition, the B–Si pillar with the size a = 376 nm, b = 576 nm, h = 6°47 μm, and the aspect ratio of 14.80 showed the WCA and ECA of 4.25° and 14.77°, respectively. After coating with the TPFS, liquid droplets ran across the sample's surface rapidly and the WCA and ECA could not be measured. Moreover, when the pillar height was increased twice, the WCA and ECA of the B–Si with and without the TPFS coating were greater than 170°, indicating excellent water-and-oil repellency and can be applied for Micro- Electro-Mechanical Systems (MEMS).