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Experiments on the elastic size dependence of LPCVD silicon nitride

Authors Ren, Yuxing
Issue Date 2004
Summary In classical elasticity, geometric dependence of the rigidities can be normalized and the normalized rigidities are size independent. Recent experimental observations showed significant elastic size effect in normalized bending rigidity in micron scale epoxy beams. Size effect in elastic behaviors can be described by surface stress theories and strain gradient elasticity theories. Surface effect on elastic properties is significant in nanometer scale, but recognized to be negligible in micron scale. The observed size effect in micron sized epoxy beams was found to agree with strain gradient elastic models. Besides epoxy films, effort is underway to characterize strain gradient behaviors in other material systems. Silicon nitride thin film is a widely used film in MEMS. Comprehensive understanding of the elastic deformation behaviors of silicon nitride thin films is essential for design and analysis of MEMS structures. In this thesis, submicron thick LPCVD silicon nitride thin films were fabricated. XPS and XRD analyses were conducted to determine elemental and micro-structural consistency of the as-fabricated thin films. XPS analyses showed element compositions were similar in films with different thicknesses. However, XRD analyses revealed that films had different crystalline phase fractions. Cantilever beams were fabricated from the films and elastic bending tests on the beams were conducted using nanoindenter to investigate the elastic bending behaviors of the films as a function of thickness. Analyses showed that beams with different thickness had fluctuating normalized bending rigidities. The fluctuations of normalized bending rigidities maybe related to varying crystalline phase fractions in the thin films. The beams were annealed and bending tests were conducted to investigate possible correlation between normalized bending rigidity fluctuations and crystalline phase fractions. Bending tests results showed similar level of fluctuations in normalized bending rigidities before and after annealing while XRD results of the annealed films showed increase in crystalline phase fractions for all thicknesses. This suggested crystalline phase fraction cannot be correlated with the fluctuations in normalized bending rigidities of the beams. Error analyses of experiment setup, geometries of the beams and data analysis were conducted. The fluctuations in normalized bending rigidity in different thickness were within the error range. While LPCVD silicon nitride may have size dependence in the nanometer scale, size dependence of normalized bending rigidity of LPCVD silicon nitride appears to be insignificant in submicron scale. Design of silicon nitride MEMS structures can be modeled with conventional elasticity without considerations for size effects.
Note Thesis (M.Phil.)--Hong Kong University of Science and Technology, 2004
Language English
Format Thesis
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