Abstract
The tribology of microelectromechanical systems (MEMS) and the wear mechanism of materials used to fabricate these devices are of current interest to researchers. Most work to date has concentrated on layers of these materials and little data are available on real, fully functional MEMS. This thesis investigates both the surface and wear properties of a self-assembled monolayer (SAM) coated MEMS test structure fabricated at Sandia National Laboratories, USA. Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), atomic force microscopy (AFM) and transmission electron microscopy (TEM) were used to study the surface chemistry, morphology and bulk structure of the MEMS gears and substrate. Wear behaviour of the MEMS was investigated using a diamond tipped AFM cantilever operated under different loads. Regarding the MEMS test structure, SEM and AFM images show an island-like morphology on the polycrystalline silicon (polysilicon) gear and silicon nitride substrate surfaces but it is not associated with columnar growth according to SEM and TEM cross-sections investigations. AFM wear studies show this island-structure undergoes greater wear than the surrounding areas and will be worn flat at the early stage of wear. XPS and AFM force curve results confirm the presence of the octadecyltrichlorosilane (OTS) SAM on the MEMS surface. The presence of OTS increases the wear resistance of the MEMS surface. Compared with the uncoated LPCVD polysilicon layer and CVD silicon nitride layer, wear resistance of the MEMS gear improves by 2 times while that of the substrate improves by up to 3 times. The wear mechanisms of the selected materials used for MEMS manufacture in the load range from 10 to 70 muN are discussed. Determination of specific wear amounts as a function of maximum contact pressure provides useful information on determining the wear mechanisms of these materials.