Fatigue and Fracture of Polycrystalline Silicon and Diamond MEMS at Room and Elevated Temperatures
Abstract
A high-resolution Atomic Force Microscopy (AFM)/Digital 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 Young's modulus, Poisson's 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 Poisson's ratio was 30% smaller than polysilicon. The mode I and mixed mode I/II fracture toughness of polysilicon showed 50% scatter due to its polycrystallinity. On the contrary, the mixed mode I/II 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 NASA's 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.
Document Details
- Document Type
- Technical Report
- Publication Date
- Dec 01, 2006
- Accession Number
- ADA464542
Entities
People
- Ioannis Chasiotis
Organizations
- University of Illinois Urbana–Champaign