Fatigue-induced dynamic pull-in instability in electrically actuated microbeam resonators

The high operating frequencies of microelectromechanical system (MEMS) resonators make their microbeam components usually suffer from cyclic fatigue. In this paper, the fatigue behaviours of polysilicon are investigated by using molecular dynamics (MD) simulations. Our MD simulation results indicate that the damage accumulation in polysilicon includes three processes, which, successively, are the initiation of microvoids, the formation of large cracks due to the coalescence of initial microvoids, and the rapid propagation of cracks. Some intrinsic and extrinsic factors such as the amplitude of fatigue loading, the number of grains and the temperature are found to be able to greatly affect the fatigue behaviours of polysilicon. Moreover, our results also indicate that the damage accumulation in polysilicon can be well described by the conventional damage evolution equation. The damage constitutive equation together with this damage evolution equation is thus employed to construct the continuum mechanical model for theoretically studying the impact of fatigue on the dynamic behaviours of electrically actuated microbeams. The results extracted from this theoretical model show that the fatigue effect can induce the dynamic pull-in failure in microbeams, which thus should be considered in the design and application of MEMS resonators. Specifically, the driving voltage of MEMS resonators should be carefully selected, because it is the major factor controlling the dynamic pull-in time.