A new on-chip test structure for real time fatigue analysis in polysilicon MEMS

Fatigue test results on 15 lm thick polysilicon specimens are presented and discussed, both quantitatively and qualitatively. The test structure is a newly designed, electrostatically actuated, MEMS device that allows the execution of on-chip fatigue and fracture tests on polysilicon specimens. The experiments have been carried out through a new analog, low-noise and low-perturbing electrostatic position measurement system for capacitive MEMS sensors. The setup allows for a real time monitoring of MEMS position, from which a macroscopic quantity, the elastic stiffness of the specimen, can be continuously evaluated, provided that the applied force is known. The results obtained in the present research put in evidence the decrease of the elastic stiffness during fatigue life before rupture. In addition, the stress amplitude during the load cycles plays a role on the lifetime of the test devices: larger stress amplitudes around a tensile mean stress reduce the fatigue resistance, mainly when a compressive stress is also present, in good agreement with a Wöhler curve.