Fem Simulation of Micro-Rotating-Structures and Their Applications in Measurement of Residual Stresses in Thin Films
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AND THEIR APPLICATIONS IN MEASUREMENT OF RESIDUAL STRESSES IN THIN FILMS Xin Zhang, Tong-Yi Zhang and Yitshak Zohar Department of Mechanical Engineering, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong. ABSTRACT FEM simulation of micro-rotating-structures was performed for local measurement of residual stresses in thin films. A sensitivity factor is introduced, studied and tabulated from the simulation results. The residual stress can be evaluated from the rotating deflection, the lengths of rotating and fixed beams, and the sensitivity factor. The micro-structure technique was applied to measure residual stresses in both silicon nitride and polysilicon thin films, before and after rapid thermal annealing (RTA), and further confirmed by wafer curvature method. Residual stresses in polysilicon films at different RTA stages were also characterized by micro-Raman spectroscopy (MRS). The experimental results indicate that micro-rotating-structures indeed have the ability to measure spatially and locally residual stresses in MEMS thin films with appropriate sensitivities. 1. INTRODUCTION In recent years, microelectromechanical systems (MEMS) have gained much attention and become a rapidly emerging R&D field in which micro-devices are fabricated using IC technique [1]. Residual stresses, usually induced in thin films during MEMS fabrication, have deleterious effects: a compressive stress can result in film buckling and delamination, whereas an excessive tensile stress may lead to film cracking. In either case, residual stresses do damage MEMS or
reduce their service life. Hence, it is of vital importance to study residual stresses in thin films. The most commonly employed method for measurements of residual stresses in thin films is the wafer curvature technique that yields an average biaxial stress over the entire film. In order to measure local residual stress, a number of new methods using free-standing micro-structures have been developed such as buckling of the micromachined devices, the deformation of various types of released structures, or the shift in the resonance frequency [2-7]. Goosen et al. [7] initially introduced micro-rotating-structures to measure residual stresses in thin films. The structures were designed such that when a film is freed from its underlying sacrificial layer, the residual stress in the film is released and hence the rotating beam is deflected. There are many attractive advantages in utilizing this method. First, residual stress in either tensile or compressive state can be measured. Second, residual stress can be evaluated at different locations on a silicon wafer. Third, this technique is not only applicable to crystalline films, but also suitable for amorphous films. Finally, this technique is compatible with the fabrication process and also allows in-situ residual stress measurements in thin films. Following Goosen et al.'s idea, we continue [8] to develop this technique experimentally and numerically. 2. FEM SIMULATION Micro-rotating-structures
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