The Role of Chemical Modification and Nanostructuring on Adhesion in Microsystems

Microelectromechanical systems (MEMS) consist of devices that are typically fabricated using integrated-circuit microfabrication methodologies for achieving a variety of sensing and actuating functions. Examples of the areas that MEMS technology has impacted significantly include inertial navigation systems, ink jet printing and high-resolution displays. However, due to small dimensions and relatively large surface forces, the device components are particularly vulnerable to irreversible adhesion to neighboring surfaces. This phenomenon, generally called stiction, is one of the major factors limiting the lifetime and reliability of MEMS devices. Organic thin films, known as self-assembled monolayers (SAMs), are a promising strategy to combat stiction. SAMs form via the self-assembly of surfactant molecules on a substrate and allow the tailoring of a number of surface properties. In MEMS technology, SAMs enable the control over adhesion, friction, and electrical properties of interacting surfaces. This work describes an investigation into the growth of the alkylsilane derivative, octadecyltrichlorosilane (OTS) monolayer, and develops techniques for investigating its growth on rough polycrystalline silicon (polysilicon) surfaces. Specifically, islands of the low-temperature condensed phase are adsorbed onto silicon and their surface potential distribution is measured via Kelvin probe force microscopy. Subsequent backfilling experiments, with a high temperature phase or different monolayer species, allow for additional growth characterization, in particular on rough surfaces. The relative performances of two alkylsilane monolayers, namely 1H,1H,2H,2H perfluorodecyltrichlorosilane and dichlorodimethylsilane, are also investigated on MEMS devices to quantify their anti-adhesion performance for impacting polysilicon surfaces and shown to be effective at reducing adhesion. Finally, layer-by-layer conductive thin films fabricated from polypyrrole and sulfonated polystyrene are characterized for their electrical and morphological properties as potential anti-adhesion coatings with charge-dissipating characteristics. A second topic covered is that of the synthesis and characterization of micro- and nanometer scale gecko-inspired dry adhesives. Such materials take inspiration from the remarkable climbing ability of the natural gecko and an understanding of the hierarchical setal structures on their toes. Recent advances in micro-/nanofabrication have enabled the construction of materials that begin to mimic the natural gecko setae. This work has explored methods for fabricating synthetic polymer fibrillar arrays as well as characterizing these materials for "gecko-like" properties, including shear-induced adhesion, minimal interhair adhesion, and residue-free characteristics. The constructions of hierarchical and inorganic silicon nanowire arrays are also covered.