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Fatigue and Fracture of Polycrystalline Silicon and Diamond MEMS at Room and Elevated Temperatures

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Final performance rept., 1 Apr 2003-30 Sep 2006

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A high-resolution Atomic Force Microscopy AFMDigital Image Correlation DIC method was developed to investigate the deformation and fracture of tetrahedral amorphous diamond-like carbon ta-C and polycrystalline silicon polysilicon for microelectromechanical systems MEMS. Polysilicon and ta-C test structures were fabricated at the Sandia National Laboratories SNL and at MCNC-Cronos. Their Youngs modulus, Poissons ratio, strength, and fracture toughness were obtained, many of them for the first time. Compared to polysilicon, ta-C was found to have superior mechanical properties Its fracture toughness and strength were 3.5 times and two times that of polysilicon, respectively. Its elastic modulus was 4.5 times that of polysilicon and its Poissons ratio was 30 smaller than polysilicon. The mode I and mixed mode III fracture toughness of polysilicon showed 50 scatter due to its polycrystallinity. On the contrary, the mixed mode III fracture of the amorphous ta-C was described well by deterministic theories for brittle fracture. The stochastic failure of polysilicon was treated by a finite element model that combined NASAs code CARES Life Ceramics Analysis and Reliability Evaluation of Structures Life. This model provided failure predictions for devices with arbitrary geometries. Finally, the multi-grain nature of polysilicon was found to present a potential risk in the accurate determination of its effective mechanical behavior, especially in MEMS with dimensions equal to a small multiple of the grain size. In general, devices with dimensions larger than 15x15 grains can be described by using the isotropic properties reported in literature and presented in this report.

Subject Categories:

  • Mechanics
  • Crystallography

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