Vapor phase lubrication of microelectromechanical systems (MEMS) for space applications

Adsorption, adhesion, and current flow measurements have been performed for a homologous series of n-alkanethiols self-assembled monolayers (SAMs) and fluorine-substituted aromatic thiol SAMs deposited from the vapor phase onto contacting gold surfaces. These studies have been conducted in an effort to screen potential vapor phase lubricant system for radio-frequency (RF) MEMS devices involving gold contacts. A systematic formation n-alkanethiol monolayers on Au was observed based on the X-ray photoelectron spectroscopic analysis with film thickness increases with increasing n-alkanethiol chain length. Close agreement between the theoretical film thicknesses and carbon to gold ratios of the SAMs to the values obtained by XPS analysis were observed, except for the significant deviation of the short-chain films. SAMs of aromatic thiols have been found to generally exhibit a lower degree of molecular order and are loosely packed as compared to n-alkanethiol SAMs.;Adhesion measurements performed with AFM as a function of n-alkane chain length demonstrated that adhesive forces are significantly lower for films composed of long chain molecules, as compared to adhesive forces measured at bare Au surfaces and films composed of short-chain alkanethiols. Results from the adhesion measurement on aromatic thiol SAMs revealed a proportional decrease in adhesion as the number of fluorine substituents is increased: the more fluorine substituents, the lower the adhesion.;Simultaneous measurements of current and applied load between a Au-coated AFM tip and SAM-coated gold substrates employing a combined AFM/STM have demonstrated a chain length dependence as well. For the current flow measurements, the interfaces exposed to vapor phase thiols experienced adsorption on both sides of the interface. For the vapor phase deposition of short-chain thiols, significant decrease in adhesion was observed with the uptake of the thiol, while current flow persisted to near saturation coverage. While a reduction in adhesion was detected with the uptake of intermediate chain molecules, current flow was precluded upon saturation of this alkane thiol film as well. For the aromatic SAMs exemplified by C7H7FS thiol, a significant decrease in adhesion was achieved with continuous current flow exhibited for over 100,000 tip-sample contact cycles.;While SAMs of n-alkanethiol molecules represent an inappropriate lubrication scheme for gold/gold interfaces in MEMS devices that require electrical contact, experimental results further demonstrate that SAMs of fluorinated aromatic thiol molecules are potential candidates as a simultaneous current flow and reduction in adhesion between Au surfaces is provided after several thousand contact cycles.;QCM vapor uptake measurements revealed the irreversible binding of thiol molecules to Au and the subsequent formation of the SAMs. Film density has been found to vary as a function of the chain-length (long chain > short chain); molecular structure (n-alkanethiols > aromatic thiols); number of substituents (less substituents > more substituents).