| In view of recent rapid developments in micromachine devices, there is a growing need to obtain information about fatigue mechanisms at scales relevant to MEMS devices. Such information is of paramount importance to the development of more durable MEMS devices. To investigate the evolution of fatigue damages at the microscale, novel polysilicon fatigue resonators were designed, fabricated, and tested under multiaxial loading, which is a typical loading condition of most MEMS devices. Although this work includes design, fabrication, analysis and testing of the fatigue resonators, this dissertation is focused on analytical and finite element studies of the fatigue resonators. Detailed investigation of the fabrication and testing of the resonators were covered in the papers and theses of my colleagues on this work, Xiantian Sun and Carolyn D. White.; A preliminary stress analysis for the second and third generation fatigue resonators was performed first. Based on the obtained stress and strain field, fourth generation resonators were proposed and fabricated to generate sufficiently high stresses to initiate fatigue. On the other hand, to utilize the available third generation fatigue devices, notches were introduced by a focused ion beam (FIB) to generate high stresses around the notch root. The maximum equivalent von Mises stress resulting from the notches was calculated using the theoretical stress concentration factor. Experimental data obtained with notched specimens are also presented.; Because the fatigue resonators were tested in resonance to initiate fatigue damage, the dynamic characteristics of the resonators are of great importance. The finite element method (FEM) was first employed to analyze the dynamic response of the fatiguing resonators. However, it requires excessive computational time to reach the steady-state. Consequently, a more efficient analytical method based on a simplified second-order differential equation was introduced, and its results were found to be in good agreement with those obtained from the computationally-expensive FEM. With the aid of this analytical method, the nonlinear dynamic behavior of the fatigue resonators was analyzed to determine the effects of damping ratio, loading, and geometry on the dynamic response of the fatigue specimens.; In order to study the fatigue mechanisms of polycrystalline silicon at the microscale, it is essential to have a clear understanding of the variation of the stresses in MEMS devices resulting from different textures. (Abstract shortened by UMI.)... |
