Fatigue life investigation of the 15 micro-m thick ThELMA Epitaxial Polysilicon

This work deals with the investigation of the fatigue characteristics of the industrial 15 mu m thick ThELMA (TM) epitaxial polysilicon, used by STMicroelectronics for the development of their commercial products. The aim was to supply the customer with the reliability limits of this structural material during the practical usage. Tests were applied to 1 DoF Micro Electro Mechanical System (MEMS) which contains flexure hinges and which is actuated by a rotate comb finger driver. The testing methodology bases on the monitoring of the Device Current (DC), which is proportional to the actuator oscillation. An indirect control of the system operation was then possible and the sample-breaking event was registered by electrical measurements. By acquiring the 2nd or the 1st harmonic of the output signal, either single tests at wafer level or parallel measurements via hardware were taken respectively. The experimental results are here presented and compared to the literature data, based on a critical discussion on the up-to-date fatigue failure mechanisms investigated on MEMS. The computed slope of the relative strength reduction to the number of cycles to failure (S-N curve) is -0.0175. It follows similar trend extracted by literature data performed in different loading conditions of frequency, thus supporting the not time depending failure mechanism theory.