Microstructural and Geometrical Factors Influencing the Mechanical Failure of Polysilicon for MEMS
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1052-DD02-01
Microstructural and Geometrical Factors Influencing the Mechanical Failure of Polysilicon for MEMS Krishna Jonnalagadda, and Ioannis Chasiotis Aerospace Engineering, University of Illinois at Urbana-Champaign, 104 S. Wright Street, Urbana, IL, 61801 ABSTRACT The mechanical strength of polycrystalline silicon is discussed in terms of activation of critical flaws, as well as material microstructure and inhomogeneity. The Weibull probability density function parameters were obtained to deduce the scaling of material and component strength and to identify critical flaw populations, especially when two or more flaw types are concurrently active. It was shown that scaling of strength can change for self-similar micronsized features, which limits the applicability of strength data from large MEMS components to small MEMS components. On the other hand, the probability of failure for small components is described by a larger Weibull material stress parameter, which makes uniaxial strength data appropriate for conservative design. Furthermore, according to mode I and mixed mode I/II fracture studies for polysilicon, it is concluded that variation in the local critical energy release rate, owed to microstructural inhomogeneity, accounts for up to 50% scatter in strength (with reference to the minimum recorded value.) Thus, the conditions for the applicability of the Weibull probability density function in polycrystalline silicon are rather weak, because flaws of the same length that are subjected to the same macroscopic stresses are not always critical. Introduction The mechanical strength and toughness of brittle materials for microelectromechanical systems (MEMS) are rather orthogonal properties. Common materials, such as silicon and polysilicon, are not stronger than 3.5-7 GPa [1-3]. At the same time the toughness of polysilicon is on average about 1 MPa√m [4-6]. Other materials, such as ta-C have larger strengths (7-11 GPa) [7] and average mode I toughness of about 4.5 MPa√m [8]. Similarly the strength of SiC is on the order of 3-5 GPa while its toughness reaches 3-4 MPa√m [9-10]. Thus, the strengths of these thin film materials are significant but their toughnesses are low. In polysilicon, mechanical strength is defect-controlled and it is affected by the conditions of deposition, micromachining, and post-processing [11-13]. Recent experiments with single crystal silicon structures suitable for nanoelectromechanical systems (NEMS) pointed out that nanometer-scale smooth surfaces limit the size of detrimental flaws and allow for mechanical strengths that approach the theoretical values [14]. Brittle fracture, on the other hand, is controlled by interatomic bond strengths. Polysilicon falls in this category and its mechanical strength is influenced by local properties and stresses. Device design is conducted according to strength data, due to the lack of appreciable toughness. A design methodology presented in [15] incorporated strength data from uniform tension experiments applied to finite element models so
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