Experiments on the Elastic Size Dependence of LPCVD Silicon Nitride
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Experiments on the Elastic Size Dependence of LPCVD Silicon Nitride Yuxing Ren and David C. C. Lam Department of Mechanical Engineering, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, People’s Republic of China ABSTRACT Recent experimental observations showed significant elastic size effects in small scales. While surface stress theories are used to describe nanometer scale size effects, strain gradient theories describe size effect observed in micron sized epoxy. Size effects in single crystalline silicon and epoxy make it unclear whether there is size dependence of the elastic behaviors of widely used LPCVD silicon nitride thin films in submicron scale. In this paper, submicron thick LPCVD silicon nitride beams were fabricated and bending tests were conducted on the beams. Results showed fluctuating normalized bending rigidities in the beams with different thickness. XPS and XRD analyses were used to analyze the material consistency of the beams. The fluctuations 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 the fluctuations and crystalline phase fractions. 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. 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.
INTRODUCTION In conventional plane strain cantilever elastic bending, the bending rigidity, which contains contributions from geometry and material properties, can be normalized and the normalized bending rigidity depends only on material properties. Recent experiments and simulations showed size effects on elastic properties in a variety of materials in small scales. These effects are delineated from analyses of elastic properties without contributions from geometry. There are theories to describe size effects in small scales and surface stress theories are representative in nanometer scale. Streitz et al [1]and Wolf [2]showed size effect in atomic scale by computational simulations. In experimental observations, Li and Ono et al found that the elastic modulus of silicon cantilevers decreased from 170GPa to 53GPa as the thickness decreased from 300nm to 12nm [3]. The findings are described using surface stress theories. In the researches on carbon nanotubes, analytical models were built and computational simulations were conducted to describe the elastic properties of carbon nanotubes and size dependence of Young’s modulus were predicted [4-7]. The effects induced by surface stress and observed in nanotubes vanish when the structural dimensions are in micron or larger scales. However, experimental observations in elastic bending tests of epoxy micro-cantilever beams revealed that the
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