Characterization of Flourocarbon SAM coated MEMS tribogauge

摘要

A MEMS tribogauge was used for on-chip and in-situ characterization of nano-tribological phenomena (stiction, friction, and wear of coated polysilicon surfaces). The device was fabricated using the SUMMiT-V process. Measurements were made on sidewall surfaces on the polysilicon-3 layer. The device consists of two orthogonally positioned comb-drive assemblies that are used for both actuation and sensing. One assembly is used to apply a normal load (F_n) to contacting surface, while the other induces a tangential load (F_T). Precise position control is tracked by employing a LabVEEW controlled AD7747 capacitance sense mechanism. The resolution of the characterization apparatus is ±10nm. Three MEMS tribogauge devices are tested; two of them have a chemisorbed layer of self-assembled monolayer (SAM) coatings and one with no SAM coating. The two types of SAM coatings are FOTS and 'Sandia vapor-SAM' (SVSAM). The tribogauge with no FSAM coating is either UV-Ozone or 'air plasma' treated to remove organic contaminants leaving behind -OH bonds on top of the MEMS surface (native oxide, SiO_2). Characterization using the tribogauge for each coating type includes: measurement of baseline stiction force (F_(baseline)~(FOTS), F_(baseline)~(SVSAM)), static and dynamic coefficient of friction (F_s~(FOTS), F_d~(FOTS)), induced stiction force calculated after specific load cycles (F_(induced~(normal)),F_(induced)~(scrub)). Experiments showed that the induced stiction force increases in proportion to the increase in the number of load cycles, indicating degradation of the FSAM coating and topographical changes to the interacting surfaces. The UV-Ozone /air plasma treated pristine tribogauge was used to measure the stiction force of the device with no SAM coating (F_(baseline)~(Cleaned)).