Mechanical properties of thick, surface micromachined polysilicon films

Polycrystalline silicon is the most widely used structural material for surface micromachined microelectromechanical systems (MEMS). There are many advantages to using thick polysilicon films; however, due to process equipment limitations, these devices are typically fabricated from polysilicon films less than 3 μm thick. In this work, microelectromechanical test structures were designed and processed from thick (up to 10 μm) undoped and in situ boron-doped polysilicon films. The elastic moduli of the doped films were 150±30 GPa, and appeared to be independent of film thickness. The thermal oxidation of the polysilicon induced a compressive stress into the top surface of the films, which was detected as a residual stress in the polysilicon after the device fabrication was complete. The average nominal fracture toughness of the polysilicon was 2.3±0.1 MPa √m