Scale dependence of tensile strength of micromachined polysilicon MEMS structures due to microstructural and dimensional constraints

The success of microelectromechanical systems (MEMS) as a key technology In the 21st century depends in no small part on the solution of materials Iss ues associated with the design and fabrication of complex MEMS devices. The reliable mechanical properties of these thin films are critical to the saf ety and functioning of these microdevices and should be accurately determin ed. In order to accomplish a reliable mechanical design of MEMS, a new micr otensile test device using a magnetic-solenoid force actuator was developed to evaluate the mechanical properties of microfabricated poIysilicon thin films with dimensions of 100-660 mum length, 20-200 mum width, and 2.4 mum thickness. It was found that the measured average value of Young's modulus, 164 +/- 1.2 GPa, falls within the theoretical bounds. The average fracture strength is 1.36 GPa with a standard deviation of 0.14 GPa, and the Weibul l modulus is 10.4-11.7, respectively. Statistical analysis of the specimen size effect on the tensile strength predicated the size effect on the lengt h, the surface area and the volume of the specimens due to microstructural and dimensional constraints. The fracture strength increases with the Incre ase of the ratio of surface area to volume. In such cases the size effect c an be traced back to the ratio of surface area to volume as the governing p arameter. The test data account for the uncertainties In mechanical propert ies and may be used In the future reliability design of polysilicon MEMS. T he testing of 40 specimens to failure results in a recommendation for desig n that the nominal strain be maintained below 0.0057.