Lubrication of microelectromechanical systems radio frequency switch contacts using self-assembled monolayers

Contact failures in microelectromechanical systems (MEMS) switches prevent widespread use of MEMS technology for current handling in miniature devices. A self-assembled monolayer (SAM) lubricant was applied to MEMS switch surfaces in this paper as a possible approach for preventing contact failure. Chemical and physical processes on SAM lubricated contact surfaces were investigated at low (10 mu A) and high (1 mA) current using a micro/nanoadhesion apparatus as a switch simulator with in situ monitoring of contact resistance and adhesion force. This was coupled with ex situ analytical analyses of the contacts using x-ray photoelectron spectroscopy (XPS) and micro-Raman techniques. Diphenyl disulfide was chosen as a lubricant due to its thermal stability, enhanced conductivity, and its ability to form a 3.4 angstrom thick SAM on the gold electrode surface. Hot switching experiments were conducted in humid air (45% RH) and dry nitrogen using a MEMS-scale contact force of 200 mu N and 5 Hz frequency. At low current, lubricated contacts failed by growth in both adhesion and contact resistance (R) at about 10(5) cycles. A multi-step degradation mechanism was suggested which includes (1) SAM debonding under electron flow with formation of charged molecular species and dipole molecular structures, (2) migration and trapping of charged molecular species and/or molecular dipoles in the contact zone, (3) decomposition of molecular structures under Joule heating and repeated mechanical impact, and (4) increased R due to carbonaceous film formation that further accelerates thermal decomposition of the SAM. At high current, switch contacts failed immediately due to SAM thermal decomposition. Failure mechanisms and durability were similar in either air or dry nitrogen, indicating a minimum influence of the environment chemistry on the contact processes. This study establishes degradation mechanisms of SAM based lubricants in MEMS electrical contacts and results can be used in designing contact switch lubrication materials. (c) 2007 American Institute of Physics.