The Influence of an OTS Self Assembled Monolayer on the Wear Resistant Properties of Polysilicon Based MEMS

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.