Optimizing geometrical design of superhydrophobic surfaces for prevention of microelectromechanical system (MEMS) stiction

Due to the surface smoothness of micromachined structures, strong adhesion forces between these fabricated structures and the substrate can be developed. The major adhesion mechanisms include capillary forces, hydrogen bonding, electrostatic forces and van der Waals forces. Once contact is made, the magnitude of these forces is in some cases sufficient to deform and pin these structures to the substrate, resulting in device failure. This type of failure is one of the dominant sources of yield loss in MEMS. The basic approaches to prevent stiction are increasing surface roughness and/or lowering solid surface energy by coating with low surface energy materials. Combination of micro- and nano-meter scale roughness can dramatically increase the surface roughness. However, in fabrication process, how to optimally design surface geometry with micro-/nano-meter roughness is still not clear. The objectives of this paper are to experimentally study the wetting and hydrophobicity of water droplets on two-tier rough surfaces for comparison with theoretical analyses, and to optimize the surface geometrical design for fabricating stable superhydrophobic surfaces. Two model systems are fabricated: carbon nanotube arrays on silicon wafers and carbon nanotube arrays on carbon nanotube films, to compare wetting on micro-patterned silicon surfaces with wetting on nano-scale roughness surfaces. All surfaces are coated with 20 nm thick fluorocarbon films to obtain low surface energies and to improve the stability of the superhydrophobic surface, formed by plasma enhanced chemical vapor deposition (PECVD). The results show that the microstructural characteristics must be optimized to achieve stable superhydrophobicity on micro-scale rough surfaces. However, the presence of nano-scale roughness allows a much broader range of surface design criteria, decreases the contact angle hysteresis to less than 1deg and establishes stable and robust superhydrophobicity, although nano-scale roughness could not increase the apparent contact angle significantly if the micro-scale roughness dominates. The results of the research could guide the optimized designs of the surfaces for prevention of microelectromechanical (MEMS) stiction