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

Abstract

The success of microelectromechanical systems (MEMS) as a key technology in the 21st century depends in no small part on the solution of materials issues associated with the design and fabrication of complex MEMS devices. The reliable mechanical properties of these thin films are critical to the safety and functioning of these microdevices and should be accurately determined. In order to accomplish a reliable mechanical design of MEMS, a new microtensile test device using a magnetic-solenoid force actuator was developed to evaluate the mechanical properties of microfabricated polysilicon thin films with dimensions of 100–660 μm length, 20–200 μm width, and 2.4 μm 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 Weibull modulus is 10.4–11.7, respectively. Statistical analysis of the specimen size effect on the tensile strength predicated the size effect on the length, the surface area and the volume of the specimens due to microstructural and dimensional constraints. The fracture strength increases with the increase of the ratio of surface area to volume. In such cases the size effect can be traced back to the ratio of surface area to volume as the governing parameter. The test data account for the uncertainties in mechanical properties and may be used in the future reliability design of polysilicon MEMS. The testing of 40 specimens to failure results in a recommendation for design that the nominal strain be maintained below 0.0057.