Thickness dependency of the lubricity of Ag thin films deposited on Si(111)7×7 surface

Abstract

Tribological properties of Ag films on Si (111) 7×7 surface having a wide range of thicknesses have been studied using a pin-on-plate type tribometer under ultrahigh vacuum (UHV) conditions. The coefficient of friction of thicker Ag films and Si (111) Ag−√3×√3 ordered surface show approximately 0.1, whereas the coefficient of friction of 0.4–8.4 nm thick Ag films is one order lower than that obtained from both the thicker Ag films and the Ag−√3 surface. Auger electron spectroscopy (AES) measurements show that the Ag MNN intensity ratio of the worn track surfaces to non-rubbed surfaces changes depending on the Ag thickness, and sub-nanometer thick films show the highest ratio. Reflection high-energy electron diffraction (RHEED) patterns show that the Ag film deposited on Si (111) 7×7 surface at room temperature forms a two-dimensional fiber crystal structure and gradually changes to a single crystal Ag film as the deposition progresses, whereas the films annealed at 500°C form a √3×√3−R30° super-structure. The mechanism of the lubricity of the Ag film in the nanometer-thickness region is discussed in terms of the crystallographic structure of Ag depending on the film thickness.