Investigations on the friction force anisotropy of the silicon lattice

State of the art thin film technologies allow silicon, the material of choice in semiconductor applications, to be used for micro-electromechanical systems (MEMS) type microactuators. To investigate the suitability of silicon for these applications, friction force tests for a silicon-silicon interface were performed. For microactuators using friction bearings there is a great need for a general understanding of friction and wear phenomena. Since silicon wafers in general exhibit a single crystalline structure, the investigations included activities regarding the influence of the single crystal silicon's orientation. The test result shows a periodic change in the coefficient of friction depending on the slider's rotational position. For instance, a single crystal silicon disk with a {111} surface crystal orientation exhibits six recurring maxima of the friction force per rotation when tested against a specimen with the same crystal orientation. The contact between wafer and specimen results in a coefficient of friction mu reaching its maximal value of 0.5 every 60degrees of rotation. To find the root cause for this repetitive behavior, the sliding directions for maximal friction values were compared to the wafer's respective crystal orientation. For a {111} silicon wafer, the atoms at the surface are arranged in equilateral triangles. The angle of 60degrees between the atoms in these triangles corresponds with the periodicity of the friction force. It therefore may be concluded that the coefficient of friction follows the crystal structure. Depending on the lattice orientation, the friction force varies by more than 50%. This information is crucial for designing a micro-slide bearing as well as choosing a combination of lattice orientations that yield minimal friction. (C) 2003 Elsevier Science B.V. All rights reserved.