A Nonlithographic Approach for Creating Unstable Hierarchical (Micro-Nano) Superhydrophobic Silicon Surfaces

In this paper, we report on an inexpensive non-lithographic approach to create superhydrophobic silicon surfaces using porous silicon technology. We have used a two-step method to create an unstable hierarchical (micro-nano) superhydrophobic silicon surface. Our technique is a unique combination of a high current density (170mA/cm2) porous silicon formation step followed by a wet etching step in BOE/HNO3. Porous silicon layers, of both n- and p-type wafers were used in these experiments. The contact and rolling angles were measured for: 1) regular porous silicon, 2) porous silicon with hierarchical fractal-shape structure, and 3) hierarchical fractal-shape porous silicon after the wet etching step. For both n- and p-type wafers, the contact angles of regular porous silicon (nonhierarchical) were around 120° with a rolling angle of 90°. With hierarchical structure, the contact angle increased to 135° and after addition wet etching, the contact angle approached 160° (superhydrophobic). Besides, after wet etching step the surface became extremely unstable showing a very low rolling angle (<1°).