Size effect on the mechanical properties and reliability analysis of microfabricated polysilicon thin films

The increasing use of small microelectromechanical systems (MEMS) and advanced sensors has lead to concern about the failure modes and reliability of these structures. The wider acceptance of MEMS devices depends on the solution of issues associated with materials, design, and fabrication. The reliable mechanical properties of thin films are critical to the safety and functioning of these complex microdevices and should be accurately determined. in order to determine mechanical properties for a reliable design of MEMS, a new microtensile test device using a magnetic-solenoid force actuator was developed. Mechanical properties of microfabricated polysilicon thin films with dimensions of 100 to 660 mum long, 20 to 200 mum wide, and 2.4 mum thick were evaluated. It was found that the average value of Young's modulus, 164 GPa +/- 1.2 GPa, falls within the theoretical bounds. The average fracture strength was 1.36 Gpa with a standard deviation of 0.14 GPa, and the Weibull moduli were between 10.4 and 11.7. Statistical analysis of tensile strength for various specimen sizes predicted the effects of specimen length, surface area, and volume that occurs due to microstructural and dimensional constrains. These test data accounts for the uncertainties in mechanical properties of miniature elements and may be used for design of a reliable polysilicon MEMS. Based on the test results of 40 specimens to failure, it is recommended to maintain the nominal strain below 0.0057 mum/mum for a safe design.