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

Abstract

The increasing use of microelectromechanical systems (MEMS) and advanced sensors has led 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 reliable MEMS design, 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 μm long, 20 to 200 μm wide, and 2.4 μm 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 constraints. The 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 nominal strain below 0.0057 μm/μm for a safe design.