Strength and weibull characterization of polysilicon membranes for mems applications

Thin film polysilicon membranes are used in many MEMS (Micro Electro Mechanical Systems) applications such as pressure sensors, accelerometers, and potentially drug deliver)' microsystems for the controlled release of pharmacological agents. The brittle nature of polysilicon makes its strength sensitive to variable surface defects and multiaxial stress states. hence necessitating a consistent probabilistic treatment to assure device reliability and durability. The Weibull and Batdorf probabilistic theories have been shown to be generally applicable at the MEMS scale for polysilicon. To apply these theories in assessing the reliability of complex MEMS devices, the fracture strength distribution and Weibull parameters obtained from specimen geometries and processing similar to these components must be determined. Therefore, to design reliable pressurized drug delivery membranes or pressure sensors, it is best to extract the strength and Weibull parameters from pressure membrane specimens fabricated using similar processing methods. By doing so, similar flaw populations are sampled and used in predicting the probability of failure for the actual devices. The objectives of this work is to: I) present a methodology, using Finite Element Analysis (FEA) and the NASA CARES/Life code, to compute the strength and Weibull parameters from ruptured polysilicon thin film membranes, and 2) using these Weibull parameters as metric to determine which of four microfabrication techniques yields membranes with the highest reliability. Inherent residual stresses due to these processing techniques were included in the FEA simulation to accurately model their fracture behavior. Knowing these strengths and Weibull parameters for polysilicon would then permit their use to probabilistically design more reliable MEMS devices, including other device geometries, within a tolerable probability of failure level.