Effect of Self-Assembled Monolayer Film Order on Nanofriction
S Sambasivan, S Hsieh, Daniel A. Fischer, Stephen M. Hsu
Friction at the nanoscale has become a significant challenge for microsystems, including MEMS, NEMS and other devices. At nanoscale, lateral loading often causes component breakage and loss of functions in devices, therefore, accurate measurement and understanding of nanofriction are critical in device reliability and durability. Since silicon-based devices enjoy overwhelming cost advantages in manufacturing, the issue of controlling friction on silicon using nanometer thick films has attracted significant interest recently in addition to lubrication by a thin layer of water in rare cases. Due to the small scale of measurements, the Atomic force microscopy (AFM) has been the instrument of choice to measure nanofriction. At the same time, we need detailed surface characterization of the monolayer film structure, this paper explores the use of near edge x-ray absorption fine structure (NEXAFS) spectroscopy and Fourier transform infrared (FTIR) spectroscopy to ascertain the order of the film. A series of n-Alkyltrichlorosilanes self-assembled monolayer films with various chain lengths (C5 to C30) were prepared on silicon (100) surfaces. Nanofriction measurements were conducted using an atomic force microscope (AFM). Results showed that the lowest friction was obtained with a C12 film with higher friction values observed for C5 and C30 films. The film structure and order of organization were probed with a x-ray absorption technique (NEXAFS) in conjunction with a Fourier Transform Infrared spectroscope (FTIR). It was observed that C12, C16, and C18 films were more ordered (molecular orientation of the carbon backbone nearly perpendicular to the surface) then those of C5 and C30 films. The combination of these two techniques provides complementary evidence that film order and organization strongly influence nanofriction.