Eutectic liquid in Sol-Gel process for superhydrophobic silica thin films — antistiction of MEMS devices

The creation of superhydrophobic surfaces that mimick lotus leaves is a relatively new research field that has attracted considerable attention. It shows promising applications in self-cleaning, microfluidics, bio-antifouling, and anticorrosion etc. Generally two requirements must be satisfied in order to achieve superhydrophobicity. First, the surface must be hydrophobic with a contact angle greater than 90°. Second, the surface needs to have a two-tiered roughness; that is, it has to display both micro and nano structures. Such surface roughness converts the hydrophobic surface to a superhydrophobic surface. Based on the theory of superhydrophobicity for low surface energy coatings, we describe the preparation of a superhydrophobic antistiction silica coating for MEMS devices. The process uses a novel sol-gel process approach with a eutectic liquid as a templating agent. The eutectic liquid displays negligible vapor pressure and very low melting point (12°C at ambient conditions) to reduce solvent loss during the high speed spincoating process. After a fluoroalkyl silane treatment, superhydrophobicity is achieved on the as-prepared silica thin film. The solvent can be extracted after the gelation and aging processes. Spin speed effect, eutectic liquid:TEOS ratio in the solution were systematically studied in order to optimize the surface roughness to ensure excellent super-hydrophobicity[l]. Comparison of the silica thin films with silicon pillar surfaces showed that superhydrophobicity for the traditional sol-gel derived silica films demonstrated significant improvement, especially under humid conditions. Mechanisms for the reduction of capillary force were discussed. The AFM force curve obtained with a tipless probe showed that the interaction force is greatly reduced on a rough silica superhydrophobic surface. This result offers great potential to reduce stiction failures in MEMS devices.[1].