Lotus effect surface 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. Once contact is made, the magnitude of these forces is sufficient to deform and attract these structures to the substrate, resulting in device failure. This type of failure is one of the dominant sources of yield loss in microelectromechanical system (MEMS) fabrications. The basic approaches to prevent stiction include increasing surface roughness and/or lowering solid surface energy by coating with low surface energy materials. By nature, the Lotus Effect surface is an excellent model surface of a combined effect of hydrophobicity and micro/nano scale structure topography. Such surfaces have water droplet contact angles of 150° or higher. The intrinsically superhydrophobic surfaces can avoid an attractive capillary force which pulls the MEMS microstructure to the substrate; as such they reduce van der Waals forces as well. To prepare a lotus effect surface, aligned carbon nanotubes (ACNTs) that are perpendicular to the substrate surface are created. The nanotubes were grown in a chemical vapor deposition (CVD) tube furnace system from a vapor-phase mixture of xylene and ferrocene. The ferrocene was the nucleation initiator and xylene as the carbon source. Multiwalled carbon nanotubes of 20-30 nm in diameter were fabricated onto SiO₂ surfaces that were deposited by the plasma enhanced chemical vapor deposition (PECVD) method. The average center-to-center spacing (pitch) between adjacent nanotubes was ∼50 nm. The as-grown vertical nanotubes showed good adhesion to the substrate, which made the nano-scaled roughness possible. The initial water contact angle on the as-grown aligned CNT surface was 155°. To improve the stability of the superhydrophobic surface, the aligned CNTs were modified by fluorinated polymers formed by PECVD. The as-grown CNTs were characterized using scanning electron microscopy (SEM) and high resolution transmission electron microscopy (HRTEM).