Polysilicon MEMS structures are coated with self-assembled monolayers (SAMs) to reduce stiction and improve wear resistance. This study reports on an octadecyltrichlorosilane (OTS) coated low pressure chemical vapour deposited (LPCVD) polysilicon based MEMS test structure fabricated at Sandia National Laboratories, USA.The surface morphology and OTS layer have been studied by SEM, XPS and AFM. Nanowear properties were investigated using a diamond tipped cantilever AFM. The presence of OTS is confirmed by XPS and AFM measurements and the polysilicon/OTS interface is found to be vulnerable to hydrolysis when stored under laboratory conditions. A comparison is made between the wear resistance of the OTS coated and uncoated surfaces of the MEMS polysilicon components and silicon nitride substrate. Nanowear results were also obtained for CVD polysilicon and silicon nitride layers and a silicon wafer. The presence of the OTS layer was found to enhance the wear properties of the MEMS polysilicon and silicon nitride layers, with an increase in the wear resistance of up to 2 times for the MEMS polysilicon and up to 3 times for the MEMS silicon nitride.

Polysilicon microelectromechanical systems (MEMS) are the subject of intense research activity. This paper reports on the surface chemistry, topography and nanowear properties of a MEMS test structure fabricated at Sandia National Laboratories, studied using XPS and atomic force microscopy (AFM). XPS C 1s and Si 2p spectra from the polysilicon components, silicon nitride substrate and a reference Si wafer are compared. The results confirm the presence of a self-assembled monolayer (SAM) on the MEMS surface. An island-like morphology is found on both polysilicon and silicon nitride surfaces of the MEMS. The islands take the form of caps, being up to 0.5 µm in diameter and 20 nm in height. It is concluded that a dual columnar and equiaxed microstructure develops during growth of these low pressure chemical vapour deposition (LPCVD) layers and the islands are caps to the columnar structures. A 35 µN load applied to the AFM diamond tip leads to a wear depth of 1.4 ± 0.13 nm and 2.1 ± 0.06 nm on the polysilicon and silicon nitride MEMS equiaxed surfaces, respectively. Under the same load, greater wear of the columnar caps on both surfaces is observed. The results suggest that the morphology present on the polysilicon surface will be worn flat during operation and will not adversely affect the wear properties of the polysilicon components.

No abstract