A study of wear on MEMS contact morphologies

The fabrication of many micro electro mechanical systems (MEMS) is mainly based on silicon and its compounds and, for moving structures such as micromotors, tribological behaviour plays a key role in the performance. In this paper the wear of MEMS-compatible materials has been investigated for a range of contact areas and contact forces typical of MEMS. Special test specimens incorporating a range of micromachined micro structures on their top surfaces have been fabricated in order to simulate those conditions. These single crystal silicon (SCS) micro structures were coated with a range of materials used in MEMS; diamond-like carbon (DLC), silicon nitride (Si₃N₄), silicon dioxide (SiO₂), and doped and undoped polysilicon. A specimen-on-disc arrangement (a development of the macroscopic pin-on-disc) was used for the wear experiments and dead weight loading was applied using micro loads specially calibrated for this purpose. The results show that the wear rates of DLC and SCS sliding on DLC decrease with increasing sliding distance whereas Si₃N₄ and SiO₂ showed approximately linear wear behaviour. The effect of contact morphology and contact pressure was investigated for doped polysilicon sliding on DLC and for doped and undoped polysilicon sliding on Si₃N₄. The results can be attributed to the differing mechanical and chemical properties of the materials leading to wear mechanisms ranging from asperity fracture to asperity deformation.