Direct Measurements of Fracture Toughness and Crack Growth in Polysilicon MEMS
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Direct Measurements of Fracture Toughness and Crack Growth in Polysilicon MEMS I. Chasiotis1, S.W. Cho2, K. Jonnalagadda1 1 Mechanical and Aerospace Engineering, University of Virginia, Charlottesville, VA 22904 2 Materials Science and Engineering, University of Virginia, Charlottesville, VA 22904 ABSTRACT Direct measurements of Mode-I critical stress intensity factor and crack tip displacements were conducted in the vicinity of atomically sharp edge cracks in polycrystalline silicon MEMS using our in situ Atomic Force Microscopy (AFM)/Digital Image Correlation (DIC) method. The average Mode-I critical stress intensity factor for various fabrication runs was 1.00 ± 0.1 MPa√m. The experimental crack tip displacement fields were in very good agreement with linear elastic fracture mechanics solutions. By means of an AFM, direct experimental evidence of incremental crack growth in polycrystalline silicon was obtained for the first time via spatially resolved crack growth measurements. The incremental crack growth in brittle polysilicon is attributed to its locally anisotropic polycrystalline structure which also results in different local and macroscopic (apparent) stress intensity factors. INTRODUCTION The failure behavior of polycrystalline silicon (polysilicon) for Microelectromechanical Systems (MEMS) in the presence of atomically sharp cracks, as described through fracture toughness, is one of the least explored aspects of the mechanics of this material. Due to the low fracture toughness and the high yield strength of polysilicon, it is expected that the local material characteristics will influence the effective (apparent) critical stress intensity factor, KIc,PolySi [1,2]. The first efforts to measure a value for KIc,PolySi of polysilicon involved notched specimens with finite tip radii [3-5]. A later work [6] showed that the previously reported estimates for KIc,PolySi were considerably higher than the actual values. In that work fracture studies were conducted with the aid of on-chip electrostatically actuated devices that required calibration via a Finite Element (FE) analysis. Advances in experimental fracture mechanics of MEMS have allowed for fabrication of fracture specimens with mathematically sharp cracks [6,7] that provide appropriate test vehicles for accurate measurements of KIc,PolySi. In this work we generated atomically sharp cracks and measured the fracture toughness of polycrystalline silicon using uniaxial tension tests. This approach was combined with our Atomic Force Microscopy (AFM)/Digital Image Correlation (DIC) method [8,9] to obtain the local deformation fields in the vicinity of Mode I cracks on MEMS-scale specimens. In this paper we present measurements of the fracture toughness and crack tip deformation fields in freestanding polysilicon specimens with strong evidence of incremental crack propagation supported by direct crack length measurements. EXPERIMENTS Polysilicon specimens were manufactured at Cronos (former MCNC) via the Multi-user MEMS Processes MUMPs39 and MUMPs41. E
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