Fatigue analysis of out-of-plane vibration polysilicon cantilever beam under high-cycle vibration loads

It has been proved that polysilicon film will fatigue under high cycle mechanical loads. However, there are still debates on its fatigue mechanism. In this paper, a different configuration is applied to the fatigue test to discover more phenomena. The test structure is a surface-micromachined microcantilever that vibrates out of plane under electrostatic excitation at its resonant frequency. A U-shaped notch is reaction-ion-etched at the root of the structure to induce stress concentration. Fatigue behavior is monitored by recording the shift of the resonant frequency using a laser Doppler vibrometer. Results show that the relative frequency shift reaches up to 1.3%. Compared with the reported data, the concentrated stress (about 1-10 MPa) is two orders of magnitude smaller, and the shift is two orders larger, indicating a very rapid fatigue procedure. This may be due to the greater roughness of the notch surface induced by the RIE process. The details of the mechanism are still under exploration.