Real-time monitoring of the fatigue damage accumulation in polysilicon microstructures at different applied stresses.

Despite fatigue failure in polysilicon microstructures has been reported by many research groups, its origin is still contentious. In this paper it is presented a systematic fatigue analysis on suitably designed polysilicon microstructures including a 15 μm thick notched specimen with a radius of curvature of 0.5 μm. Different fatigue tests are performed as a function of the applied stress. During each test the elastic stiffness of the structure is real-time monitored to investigate how fatigue effects nucleate and propagate during fatigue lifetime. It is shown that no fatigue effect is evidenced at low applied peak stresses during fatigue life. On the contrary for high applied stresses two different effects seem to be responsible of the device degradation: the presence of a stable damage accumulation is revealed on specimens tested with maximum peak stresses between the 50% and the 60%of the monotonic rupture stress. This degradation begins after approx. 106 cycles and progressively leads to the device failure. For maximum peak stresses beyond the 60% of the monotonical rupture stress the failure happens within a number of cycles lower than 105. In this case the delayed failure is sudden and it is not anticipated by any elastic stiffness decrease prior to failure.