Lubrication of polysilicon micromechanisms with self-assembled monolayers

Abstract

Here, we report on the lubricating effects of self-assembled monolayers (SAMs) on MEMS by measuring static and dynamic friction with two polysilicon surface-micromachined devices. The first test structure is used to study friction between laterally sliding surfaces and with the second, friction between vertical sidewalls can be investigated. Both devices are SAM-coated following the sacrificial oxide etch and the microstructures emerge released and dry from the final water rinse. The coefficient of static friction, ps, was found to decrease from 2.1 f 0.8 for the SiOl coating to 0.1 1 f 0.01 and 0.10 k 0.01 for films derived from octadecyltrichloro-silane (OTS) and 1 H, 1 H,2H,2H-perfluorodecyltrichlorosilane (FDTS). Both OTS and FDTS SAM-coated structures exhibit dynamic coefficients of friction, pd, of 0.08 f 0.01. These values were found to be independent of the apparent contact area, and remain unchanged after 1 million impacts at 5.6 pN (17 kPa), indicating that these SAMs continue to act as boundary lubricants despite repeated impacts. Measurements during sliding friction from the sidewall friction testing structure give comparable initial p d values of 0.02 at a contact pressure of 84 MPa. After 15 million wear cycles, p d was found to rise to 0.27. Wear of the contacting surfaces was examined by SEM. Standard deviations in the p data for SAM treatments indicate uniform coating coverage.

MEMS Reliability: Infrastructure, Test Structures, Experiments, and Failure Modes

“…For rubbing surfaces, both Senft [82] and Srinivasan et al [83] have observed lowered coefilcient of friction for SAM coated devices. The wear properties are still under investigation.…”

“…Early work by Gabriel [56] using air-dried turbines measured the dynamic COF between polysilicon and silicon near 0.3. Work by Srinivasan et al [68] suggests that nanometer thick surface treatments can act as a boundary lubricant and reduce the dynamic COF between FTS-coated polysilicon surfaces to 0.08. The normal force in Srinivasan's experiments was roughly 5~and total number of cycles was in the 105 to 106 range, which is applicable to what the microengines experience.…”

“…al. [76], where a friction coefllcient of 0.08 was measured between PIWS-treated planar surfaces in air. Frictional behavior in humid air and humid nitrogen was similar (p-O.02 near the end of the 5x105 cycle test), although the displacement did not remain uniform over the course of the test.…”

Frictional Aging and Sliding Bifurcation in Monolayer-Coated Micromachines

“…The FOTAS molecule design includes a tri-amino head group to promote covalent bonding to the silanol surface via a nucleophilic reaction, and a perfluorinated tail group to minimize surface energy. The FOTAS molecule was developed as a replacement for solvent-deposited CF 3 (CF 2 ) 7 (CH 2 ) 2 SiCl 3 , (FDTS) [22] for two reasons. First, the vapor-phase deposition is more reproducible because FDTS is prone to form micelles and multilayers during the liquid solvent deposition [23].…”

Friction behaviour of chemical vapor deposited self-assembled monolayers on silicon wafer

“…The PFOTS monolayer shows high values of coefficient of friction, indicating that the effect of size of fluorine atom influences the friction behaviour even at the micro-scale [15,20,21,26]. Other researchers too have observed high values of friction in the similar class of fluorinated SAM (perfluorodecyltrichlorosilane, FDTS) [27][28][29]. The PFDA monolayer shows high values of friction followed by wear.…”